Symposium ES09—Advanced Materials for the Water-Energy Nexus

Manganese oxides are a structurally rich class of materials and can be prepared as layered, tunneled, or spinel structured materials. The synthetic versatility of these materials is further expanded as they are often prepared with cations as part of the structures including univalent cations such as K⁺, Ag⁺, or Na⁺, Rb⁺, or Cs⁺ or multivalent cations such as, Ni²⁺, Cu²⁺, Co²⁺, Mg²⁺, or Al³⁺. The ability to deliberately tune both the structure and the composition of the materials has led to investigation of their functional behavior for a number of applications as both the parent structure as well as the nature of the cation can be determining factors. The interest in manganese oxide materials is further enhanced by their relatively low cost and toxicity.

This presentation will explore the electrochemistry of manganese oxides in lithium, sodium and magnesium based electrolytes for possible application in electrochemical energy storage devices. Examples of layered, tunneled and spinel manganese oxide structures will be considered. Discussion will include the functional electrochemistry as well as insight into the reduction and oxidation mechanisms.

Highly concentrated aqueous batteries have emerged as potential candidates for large-scale energy storage applications; however, the need for high-cost organic electrolyte salts is a potential obstacle hindering their practical application. Herein, we revisited all the commonly used low-cost inorganic electrolytes and discovered that the use of low-cost super-concentrated aqueous solution result in a high electrochemical stability window of up to 2.7 V by suppressing water dissolution and induces stable solid-electrolyte interphase (SEI) layer formation without involving the reduction of salt anions. The SEI layer, composed of Na₂CO₃ and Na–O compounds, guarantees the excellent electrochemical storage stability of the full-cell composed of a Na₄Fe₃(PO₄)₂(P₂O₇) cathode and NaTi₂(PO₄)₃ anode for over 890 hours. This new-class 2-V Na-ion battery provides remarkable cycle stability and a coulombic efficiency of over 99% at 1C for over 200 cycles.

The Advanced Electrochemical Materials section at the U.S. Naval Research Laboratory has designed and validated a three-dimensional (3D), monolithic Zn electrode architecture that solves the limited rechargeability of Zn. We use a simple emulsion-based protocol to produce moldable, scalable, Zn “sponge” electrodes whose structural characteristics impart: (i) interconnected pathways that maintain long-range electronic conductivity within the electrode volume throughout charge–discharge; (ii) an electrified 3D interface that imparts more uniform current distribution; and (iii) confined internal void volume that controls Zn/Zn²⁺ precipitation/dissolution dynamics and product redistribution.

These Zn “sponge” anodes exhibit unprecedented cyclability at high depths-of-discharge (theoretical DOD_Zn), increased specific capacity relative to conventional powder-bed Zn electrodes, and tens of thousands of cycles with low-DODZn pulse-power profiles, as recently demonstrated in prototype Ni–Zn cells. This breakthrough enables the entire family of alkaline Zn batteries (Ni–Zn, Ag–Zn, MnO₂–Zn, and Zn–air) plus neutral pH zinc-ion batteries to be reconfigured in extensively rechargeable forms, with energy and power characteristics that are competitive with Li-ion batteries. Our second-generation emulsion protocol improves the volumetric density of the sponge thereby concomitantly improving the energy density of the cell while adding mechanical ruggedness to the anode. The potential paths forward for the development and transition of this safer energy-storage technology will be discussed.

Capacitive deionization (CDI) is a promising technique for water treatment. At the heart of every CDI device are the electrodes that adsorb ions from the liquid. Much of the improvement of CDI performance in the past decade has focused on increasing the sorption capacity of the electrodes.

Here, we will present our latest results on optimizing device operation, charge efficiency and thermodynamic efficiency, and the impact of these results on future electrode material research and performance characterization. Specifically, we will break down the overall charge efficiency into double-layer efficiency, flow efficiency, and Coulombic efficiency and show how the choice of material, morphology, and operating condition affects each of these efficiencies, and highlight how measured salt adsorption capacity (SAC) depends on the charge efficiency. We will demonstrate a simple way to optimize operational parameters for a specific separation and show that for a given separation the impact on SAC on performance can be limited. We will show the impact of electrode dimension on the device power consumption and thermodynamic efficiency. Our findings suggest that optimization of shape and morphology of the material play as important a role as the intrinsic capacity of the material for future CDI electrode development.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Capacitive deionization based on carbonaceous electrodes represents an energy-efficient technology suitable for low-salinity brackish water desalination where high recovery rates are critical to economic viability. Substituting insertion compounds for carbonaceous materials in electrochemical desalination electrodes enables higher ion adsorption capacities, excellent energy efficiency in higher salinity feedwaters, and selective removal of target ions. These insertion electrode properties introduce a novel set of process applications and advantages in deploying electrochemical separations, including the selective recovery of high value byproducts from reverse osmosis concentrate and reduced energy consumption and environmental impacts associated with efficient removal of target ions of environmental concern.

Despite the promise of carbon and insertion based electrochemical desalination processes, the durability of the electrode materials when treating complex waters is a documented issue that has not been systematically investigated. This presentation will review our ongoing work to examine the impact of organic fouling on the performance of carbonaceous and insertion-based electrochemical desalination processes, identify key factors controlling the corrosion/degradation of carbonaceous and insertion-based electrodes used in electrochemical desalination, and explore and assess cost-effective strategies for improving the fouling tolerance and minimizing the corrosion or degradation of carbonaceous and insertion based electrodes.

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which typically desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. Unlike reverse osmosis (RO), a portion of the energy applied during desalination can be recovered directly as electrical energy, reducing net energy consumption. Apart from the thermodynamic minimum energy of separation, all energy applied should be recoverable. However, in practice, ion/electronic resistances, inefficient hydraulic design, parasitic reactions, and poor selectivity limit the portion of energy recoverable.
Several material additions and electrode configurations have been proposed to overcome these limitations. Two of the most prominent modifications are the addition of ion exchange membranes (IEMs), promote counter ion flux out of the desalination chamber, and utilizing flowable, carbon slurry electrodes, to increase salt removal capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of IEMs in membrane capacitive deionization (MCDI), the use of slurry electrodes in flow capacitive deionization (FCDI), and the incorporation of functionalized carbon have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Further, evaluation of practical CDI performance has be limited by low reported experimental salinity reduction (the concentration difference between influent and effluent during charging is typically 10 mM or less). Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space.
In this study, a porous electrode (M/F)CDI model, was used to measure the sensitivity of CDI performance to across the design landscape of architectural, material, and operational inputs. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Sensitivity for CDI, MCDI, and FCDI were evaluated by varying eight common input parameters. Sensitivity analyses were conductions using both Monte Carlo and Morris methods utilizing six common performance metrics (salt adsorption capacity [SAC], average salt adsorption rate [ASAR], energy-normalized adsorbed salt [ENAS], charge efficiency [λ or η_CE], and round-trip efficiency/energy recovery [η_RTE], and thermodynamic efficiency [η_T]).
Our sensitivity analyses show that operating current density, electrode specific capacitance, and contact resistance were the parameters which most significantly dictated (M/F)CDI performance. Both CDI and FCDI were out performed by MCDI, due to relatively poor charge and round-trip efficiencies, respectively. We found that increased system performance across most metrics was associated with input parameters which increased charging cycle duration. Lastly, simulated MCDI achieved thermodynamic efficiency comparable to brackish water RO, although tradeoffs exist between water recovery and thermodynamic efficiency.

The molecular dynamics of confined of water is important for many areas of science and applications ranging from energy storage to transport phenomena in geology and biology. In this context, the understanding of hydrogen bonds formation in various environment and criteria governing the molecular mobility at nanoscale is of high importance. Using neutron spectrometer NEAT as a powerful magnifying glass we have investigated molecular dynamics of water in hydrophobic and hydrophilic nanoconfinement in a very broad temperature range of 20-300 K. By an aimed selection of the confining systems we were able to follow the molecular behavior in pores of various sizes and shapes and in environments with variable degrees of water - host interactions. Changing the amount of confined water we have studied the process of hydrogen bonds formation and were able to explain the molecular mechanism of adsorption behavior in selected hydrophobic systems. Thus in zeolite Al(PO₃)₅ with hydrophobic one-dimensional channels of about 1 nm cross section we observed non-monotonous behavior of the mean square displacement as amount of confined water increases. The cooperative interactions between water-water molecules lead to the increase of molecular mobility and the sharp rise in water intake. Guest-host interactions initiate adsorption-induced changes of the host structure, which in the case of Al(PO₃)₅ cause the anisotropic contraction of the zeolite framework. These structural changes can be the reason for a broad isotherm hysteresis observed in this system.

Nanomaterials have attracted significant attention by researchers over the past decade towards developing sustainable technologies. MXenes, a large family of two-dimensional (2D) transition metal carbides and nitrides, are the front runners of 2D material research due to their diverse physiochemical properties. MXenes are synthesized by wet chemical etching of an atomic layer from layered ternary carbide precursors (MAX phases), resulting in terminations (-O, -F, -OH) on the surface. Their unique combination of hydrophilicity and metallic conductivity, enable compatible solution processing of conductive MXene materials by various methods in scalable quantities.
MXenes can be dispersed in aqueous and organic solvents to make colloidal, functional solutions which can be subsequently processed via printing, spray coating, drop casting, or vacuum filtration for the rational design of devices with different architectures. The 2D morphology, diverse transition metal chemistry, and functionality offered by MXene materials has spurred their use in a wide range of applications, specifically energy storage and water purification. The ability of MXenes to spontaneously intercalate a variety of cations and/or large organic molecules has created the platform for efficient storage or removal of guest species. Fast redox reactions at the transition metal-oxide/hydroxide-like surfaces with efficient charge transfer due to high electronic conductivity, makes MXenes a promising candidate as high rate pseudocapacitive materials for energy storage applications. Furthermore, combination of slit-like pores with electronic conductivity, qualifies MXenes as capacitive deionization (CDI) electrodes for efficient and reversible removal of ionic species/organic molecules from brackish water. In addition, membranes composed of Ti₃C₂ nanosheets have already shown potential in selectively sieving gases, such as H₂, and various alkali, alkaline earth ions (Li₊, Na₊, K₊, Mg₂₊, Ca₂₊), metals (Ni₂₊ and Al₃₊), and dye cations (MB₊) based on charge and size without sacrificing water flux rate. The high aspect ratio of the flakes allows for the fabrication of membranes with uniform and narrow 2D nanochannels. Systematic studies of the effect of interlayer spacing and surface functionalization on the transport of ions in confined channels can provide valuable insight to progress use of MXene in the fields of energy storage and water purification.

In order to reach high voltage and high energy density, electrochemical capacitors (ECs) with activated carbon (AC) electrodes mostly implement organic electrolytes. Due to the relatively poor environment friendliness and high cost of these devices, developing ECs with alternative electrolytes is highly demanded. AC electrodes in aqueous solutions with pH close to 7 have been shown recently to display enlarged potential window compared to classical electrolytes as H₂SO₄ or KOH. The window is extended under negative polarization owing to water reduction and production of hydroxyl anions (OH^-), which leads to a pH increase inside the porosity of the AC electrode and consecutive downshifting of the Nernst di-hydrogen evolution potential. Hence, when using stainless steel current collectors and alkali sulfate solutions, practical voltage values of 1.5 V are reached. Besides, enhanced capacitance was demonstrated when hybrid AC/AC capacitors comprising a faradaic positive electrode and an electrical double-layer (EDL) negative electrode are realized by implementing an aqueous solution of potassium iodide. Subsequently, bi-functional electrolytes containing Li₂SO₄ as negative electrode potential range extender and KI as promoter of battery-like positive electrode have been proposed to enhance cell voltage and capacitance, respectively, allowing the energy of the thereof realized hybrid capacitor to approach the value of an EDL capacitor in organic medium. However, an important cornerstone in the development of capacitors using relatively diluted aqueous solutions is the rapid fading of performance when lowering temperature. In this presentation, after showing the state-of-the-art, we present a number of novel strategies to enhance the energy and utilization temperature range of AC/AC capacitors in aqueous media of pH close to neutrality.
By implementing a highly concentrated solution of LiTFSI (20 mol kg^-1), called water-in-salt, the voltage of ECs could be extended up to 1.6-1.7 V owing to the absence of free water. Since all the water molecules are involved in hydrates with lithium, the di-hydrogen evolution potential is lower than in a typical diluted solution (1 mol kg^-1 LiTFSI or Li₂SO₄), causing advantageously a voltage extension by ca. 200 mV. Despite these advantages of water-in-LiTFSI, the solutions display a very low conductivity and the ECs performance quickly fades when lowering temperature down to -20°C.
Recently, we have implemented a new family of concentrated (5 mol kg^-1) aqueous electrolytes based on choline chloride, owing to which symmetric ECs demonstrated excellent performance down to -40°C. Based on this success, we now propose a bi-functional electrolyte containing 5 mol kg^-1 choline nitrate (ChNO₃), as supporting neutral electrolyte providing low potential extension for the negative electrode, and 0.5 mol kg^-1 choline iodide (ChI), as redox active component for the positive electrode. Importantly, this electrolyte is liquid above -40°C, as identified by differential scanning calorimetry. It shows only glass transition upon cooling, which reappears during heating and is followed by cold crystallization and melting. The hybrid EC in aqueous ChNO₃+ChI exhibits a capacitance of 81 F g^-1 at 24°C and still 50 F g^-1 at -40°C. The energy and power performance of the proposed hybrid EC and an EDLC made of the same carbon electrodes in organic electrolyte is comparable in the temperature range from 24°C to -40°C.

Two-dimensional (2D) transition metal carbides (MXenes) are among the most promising electrode materials for pseudocapacitive energy storage. MXenes are particularly of high interest because they can store charge in the bulk of their electrodes through ultrafast intercalation of ions and fast and reversible redox reactions. To date, about 20 different MXenes with a general formula of M_n+1X_nT_x (M is a transition metal, X is carbon and/or nitrogen, n=1,2, and 3, and T_x represents different functional groups present at their surface) are synthesized by selective etching of the A layer atoms (i.e., Al) from MAX phases (i.e., Ti₂AlC), a large group of layered ternary carbides and nitrides.¹ However, from all the available MXene compositions, so far only pseudocapacitive behavior of few MXenes, such as Ti₃C₂T_x, has been systematically studied and pseudocapacitive properties of most other MXene compositions have remained unexplored. The main reason that has hindered such investigations is instability and rapid structural degradation of delaminated (2D) form of other MXene compositions such as V₂CT_x. Here we demonstrate the fabrication of highly stable freestanding films of 2D vanadium carbide (V₂CT_x) MXene with superior pseudocapacitive properties. We show that a variety of cations including Li⁺, Na⁺, K⁺, Mg²⁺, and Al³⁺ can electrochemically intercalate fabricated V₂CT_x in aqueous electrolytes, offering capacitances of over 1300 F cm^-3. Supercapacitors fabricated using V₂CT_x MXene films can deliver gravimetric capacitances of over 200 F g^-1 at a high rate of 100 A g^-1 with a cyclic stability of over hundreds of thousands of cycles, surpassing previously reported Ti₃C₂T_x supercapacitors. Our research describes a new approach for the design and synthesis of high performance layered electrode materials using 2D MXene building blocks.

References
1. Anasori, B., Lukatskaya, M. R. & Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2, 16098 (2017).

Capacitive deionization (CDI) performance metrics can vary widely with operating methods. To date, nearly all CDI control schemes have consisted of constant current or constant voltage operation with a constant flowrate. Such operating methods typically show benefits in either energy or salt removal performance, but not both.

In this work, first, we develop a theory around and experimentally demonstrate new operation modes for CDI which use sinusoidal forcing voltage (or sinusoidal current). We use a dynamic system modeling approach and quantify the frequency response (amplitude and phase) of CDI effluent concentration. Using a wide range of operating conditions, we show that CDI can be approximately modeled as a linear time invariant system. We validated this model with experiments and showed that a sinusoid voltage operation can simultaneously achieve high salt removal and good energy performance. We explore the coupling between electric charge, ionic charge, and mass transport (advection) and demonstrate that sinusoidal voltage forcing functions can achieve resonance-type operation for CDI. Despite the complexities of the system, we find we can describe the resonance mode with the following simple relation: The resonant time period (frequency) is proportional (inversely proportional) to the geometric mean of the flow residence time and the electrical resistance-capacitance (RC) charging time. Operation at resonance results in an optimal balance between absolute amount of salt removed (in moles) and avoidance of dilution of the feed and brine streams. The result is a maximum average concentration reduction for the desalinated water. We further developed our model to generalize the resonant frequency operation, and study and compare with the responses to square and triangular voltage waveforms as two examples. To this end, we developed a general tool that uses Fourier analysis to construct CDI effluent dynamics for arbitrary input waveforms. Using this tool, we show that most of the salt removal (∼95%) for square and triangular voltage forcing waveforms is achieved by the fundamental Fourier (sinusoidal) mode. We show that the frequency content of the higher Fourier modes severely limit their flow efficiency. Hence, such high frequency modes consume additional energy while providing minimal additional salt removed. This deficiency of higher frequency modes further highlights the advantage of DC-offset sinusoidal forcing for CDI operation.

Rechargeable zinc–air batteries are desirable next-generation power sources that promise to deliver high energy density (upwards of 400 Whkg^–1) using aqueous (nonflammable) electrolytes. We have recently addressed a long-standing problem of dendrite formation at the anode by fabricating the Zn as a 3D sponge. We now turn to another great challenge: the development of an efficient, stable, and practical air cathode that can pair with our Zn anode to deliver extended cycle life and achieve specific energies that surpass today’s Li-ion batteries. Cathode efficiency and stability are determined by the electrode architecture (e.g., porosity, hydrophobicity/hydrophilicity, and electronic & ionic transport) and to a greater extent by the incorporation of high-performance electrocatalysts that can reduce molecular oxygen (ORR) during battery discharge and evolve O₂ via water splitting (OER) upon charging. Our approach is to synthesize ORR and OER electrocatalysts as high surface–area aerogels and then use conventional gas diffusion–electrode fabrication techniques to incorporate these oxide nanoparticles within practical carbon architectures. We choose catalyst compositions that are highly active but only contain earthly abundant elements; namely, cryptomelane (α-MnOx) for ORR and nickel ferrite (Ni_yFe_1–yOx) for OER or nickel ferrite alone as a bifunctional OER and ORR catalyst. We assemble Zn–air cells that comprise our Zn sponge anode and aerogel-functionalized composite cathodes and test their cycle stability and ability to deliver practical energy density and power. We will describe results for a series of cathode configurations in which we vary the placement of the ORR and OER catalyst layers and compare cathodes containing a single bifunctional catalyst with those that are made from separate OER and ORR catalysts.

Highly concentrated aqueous batteries have outperformed conventional electrolytes by forming the solid-electrolyte interphase (SEI) layer with anion decomposition and suppressing water activity, thereby expanding the electrochemical stability window. These newly proposed concentrated electrolyte systems enable high-voltage operation of aqueous batteries for a wide selection of electrodes; however, significant economic concerns have delayed the commercialization of aqueous systems, especially regarding the use of expensive organic salts. In the current study, we revisited all the commonly used low-cost inorganic salts and evaluated them for their use in super-concentrated aqueous electrolytes. These super-concentrated electrolytes offer a high electrochemical stability window of up to 2.7 V through suppressing water decomposition with forming a super stable SEI layer without involving the reduction of salt anions. The SEI layer exhibited the extraordinary electrochemical storage stability of the full-cell for over 890 hours. Using this new class of super-concentrated electrolyte, we successfully developed a 2-V Na-ion battery with remarkable cycle stability and a coulombic efficiency of over 99% at 1C for over 200 cycles.

Ion selective nanoporous membranes are an important component for the purification of water using electrodialysis. Electrodialysis uses an applied DC electric field to actively transport dissolved ions through the membranes, purifying the water on one side of the mebrane and concentrating the salt ions on the other side. This process uses a large amount of electrical energy and thus only finds limited application. In order to increase the efficiency of this process the the ionic selectivity and the ionic conductivity of the membranes must be increased. Our goal is to increase these properties of commercial nanoporous membranes by coated with an inexpensive polyelectrolyte thin films using an aqueous Layer-by-Layer (LbL) process. The polyelectrolyte coating changes the surface chemistry of the membranes leading to changes in the selectivity and conductivity. We can further tailor the properties of the membranes using various crosslinking chemistries to impact not only the chemical selectivity and conductivity but also the chemical stability of the polyelectrolyte surface modification. Polyelectrolyte film structure, such as film thickness, is shown to change the ionic selectivity of the membranes. Additionally, the addition of small functional molecules to the LbL deposition solutions will incorporate the molecules into the resulting thin film. These small molecules, upon crosslinking of the polyelectrolyte, lead to an increase in selectivity. Variations in the structure and composition of the polyelectrolyte film, polyelectrolyte deposition parameters and crosslinking chemistries are shown to controllably influence the ionic selectivity and conductivity of the nanoporous membranes as well as adhesion of the polyelectrolyte coating. Continued advances of these polyelectrolyte thin films will enable the widespread use of electrodialysis as an economical water purification technology.


Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Nitrate is a common contaminant in groundwater which is a by-product of agricultural activities. Nitrate is, for example, implicated in so-called blue-baby disease. Thus, a point-of-use, reusable device to selective remove ions is crucial. Ionic exchange resins are the predominant method to selective adsorbs ionic pollutants. However, the regeneration of these resins requires the disposal of significant volumes of chemicals.

Capacitive deionization (CDI) is a promising technique for water desalination. In contrast to ionic exchange resins, CDI requires no chemical to regenerate. CDI relies on electrical charges for adsorption and regeneration onto high surface area activated carbon (AC) electrodes. However, the experimental and theoretical framework for selective removal of ions with CDI is still under development.

In this work, we explored a method to enhance selectivity with a functionalized CDI system. We functionalized active carbon electrodes with ion-selective functional groups. This functionalization combines the high selectivity function of ionic exchange resins with the electrostatic regeneration of CDI. The active electrode was functionalized with cetyl trimethylammonium bromide (CTAB). This cationic surfactant has a quaternary ammonium moiety which acts as a nitrate selective functional group. The counter electrode was functionalized with the anionic surfactant sodium dodecylbenzenesulfonate (SDBS). In this system, the adsorption step occurs at 0 V due to the presence of chemical surface charge, while the regeneration step occurs at a constant (applied) voltage. We also expanded our understanding of the selective electro-adsorption process by developing and validating a well-mixed reactor model. Our model combines a Donnan treatment for electric double layers (EDL) and electrosorption Langmuir-type equilibria.

The experimental set-up consisted of a cell with a radial architecture. A mixture of sodium nitrate and sodium chloride entered the axisymmetric cell stack from the outer periphery. After treatment, the water exited the cell through a channel along the axis to minimize dispersion. We used ion chromatography to measure the concentration of each anion. We measured the intrinsic selectivity (~7.7) of our CTAB functionalized electrode with an ex-situ experiment. We defined and measured cycle selectivity of nitrate versus chloride as a function of the inlet nitrate concentration and regeneration voltage. We observed that cycle selectivity increases with lower values of voltage, with a maximum of ~6.5 at 0.4 V. We compared and validated our multi-species dynamic model with our quasi-steady state experimental data.

In brief, functionalization of AC electrodes provides a selective ion capture system with that can be regenerated with applied voltage. We demonstrated a nitrate-specific functionalization with up to 6.5-fold cycle selectivity. We also validated a dynamic model which incorporates a Donnan treatment for EDL and Langmuir-type equilibria for surface chemistry.

Limitations of capacitive deionization (CDI) technology and future commercialization efforts are intrinsically bound to electrode stability. In this work, the metastable behavior of salt ions and their link to stability degradation is explored. Chemical extraction studies suggest that once mobile species can become metastably trapped in the surface of aged electrodes. To qualitatively explore their impact on CDI devices, a two-electrode voltage sweep test is presented which can simultaneously monitor the active voltage window for CDI between the electrodes without the need for cell disassembly or alteration of the aqueous input feed. This voltage sweep test can be employed in conjunction with long-term operation of CDI cells. Furthermore, annealing treatments in different ambient conditions can assist to give insights into the nature of carbon-based electrode materials and the formation of different functional groups. Preliminary investigations suggest different behavior in the anode and cathode of the CDI during continuous operation and offer insights into strategies for minimizing electrode degradation as the technology gains momentum for future commercialization.

Growing population, increasing water demand and rising energy use over the past few decades stimulated exploration of alternative water and energy resources [1]. For obtaining clean water from seawater by a most energy efficient method, this work proposes the fabrication of carbon nanotube-based membranes and study their application of the membranes on the forward osmosis (FO). Unlike reverse osmosis (RO), FO is an osmotically driven membrane process which can be operated without high hydraulic pressure, thus it has many potential advantages such as low energy consumption and low fouling tendency. Thin-film composite (TFC)-FO membrane separation technology can also be used in many applications, including water desalination, wastewater treatment, food processing and power generation [2]. However, most of the porous substrates for TFC-FO membranes which were made of polymer via the phase inversion method cannot withstand harsh conditions such as strongly acidic, alkaline and organic solutions. In this study, a novel carbon nanotube-based TFC (CNT-TFC) membrane with high hydrophilicity is developed, which possesses high chemical stability, thermal stability and great FO performance. These properties significantly increase the potential of application of the FO membranes in extreme conditions. Besides, the process for the CNT-TFC membrane could be easily scaled up.

The CNT-TFC membrane consists of a hydrophilic supporting layer and an ultrathin active layer. The supporting layer, designated as PET-PDA-CNT, is made of a layer containing polydopamine (PDA)-coated CNTs cast onto a polyethylene terephthalate (PET) nonwoven fabric, and the thickness of the PDA-coated CNT layer is only around 3 µm. The active layer is an ultrathin polyamide (PA) film synthesized by interfacial polymerization (IP) of 1,3,5-benzenetricarbonyl chloride (TMC) in hexane solution and m-phenylenediamine (MPD) with sodium dodecyl sulfate (SDS) in aqueous phase. To improve the hydrophilicity of the supporting layers, PDA thin films synthesized via self-polymerization are coated on the CNT surfaces. Compared with the pristine PET-CNT supporting layer, the contact angle of the PET-PDA-CNT supporting layer drastically decreases from 77.5° (for PET-CNT) to 34.9°. Owing to the great hydrophilicity and thin thickness, the water resistance of the support layer is decreased and the overall membrane permeability is increased. On the other hand, the addition of SDS, an anionic surfactant, as co-solvent in aqueous phase during the IP process reduces the surface tension between two phases. As a result, it forms a looser PA layer than those synthesized by the conventional IP process. The looser PA layer could result in higher water flux and effectively block the reverse solute.

The results show that the TFC membrane with PET-PDA-CNT supporting layer exhibits great FO performance with the water flux of 12.58 Lm^-2h^-1 and the reverse salt flux of 5.26 gm^-2h^-1 when the selective layer is oriented towards the deionized water feed solution using 1 M NaCl solution as draw solution. The water flux of the membrane with PET-PDA-CNT layer is 267% higher than that of the membrane with the pristine PET-CNT layer. Besides, it is found that the CNT-TFC membrane fabricated using 0.1 wt% SDS in the aqueous phase during IP could lead higher water flux while maintaining low reverse salt flux.


References
[1] C. Klaysom et al., Forward and pressure retarded osmosis: potential solutions for global challenges in energy and water supply, Chemical society reviews, 42, 16, (2013): 6959-6989.
[2] S. Zhao et al., Recent developments in forward osmosis: opportunities and challenges, Journal of membrane science, 396, (2012): 1-21.

While electrochemical water splitting is one of the most promising methods to store electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction (OER) catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. However, catalytic activity of current electrocatalysts is usually restricted by high onset potential and limited active sites. Herein, we fabricated three-dimensional (3D) highly ordered mesoporous Ru-Co₃O₄ inverse opal composite materials as excellent electrocatalysts for OER in alkaline solution with high activity and stability. Three-dimensional highly ordered mesoporous Co₃O₄ inverse opal (Co₃O₄ IO) was firstly synthesized using polystyrene beads as hard template. Then, Ru-Co₃O₄ nanocomposites were prepared by a simple ultraviolet reduction method. The as-prepared mesoporous Ru-Co₃O₄ catalysts have ordered mesoporous structure with a high surface area of 90.6 m² g^-1. Three-dimensional highly ordered mesoporous structure can facilitate diffusion and penetration of electrolyte and oxygen. Moreover, the catalysts can also keep catalyst particles in a well dispersed condition with more catalytic active sites. Electrochemical measurements reveal that the ordered mesoporous Ru-Co₃O₄ catalysts exhibit superior performance in alkaline solution with large current of 14.2 mA cm^-2 at 1.65 V vs reversible hydrogen electrode (RHE), low onset potential of 1.51 V vs RHE and excellent long-duration cycling stability.

Faster and energy-efficient condensation of water vapor contained in atmospheric air has emerged as a solution to the demand for water in some regions of the planet. In these deserts or isolated areas the only accessible source of water is atmospheric air. In this way, academic and industrial research works that seeks to develop water collection devices with high efficiency has great relevance for the scientific community. It’s known that droplets coalescence on nanostructured superhydrophobic surfaces may result in droplet jumping from the surface. It can happen due to the release of excess surface energy because the surface roughness is much smaller than the coalesced drop size. In this work, we aim to show that if designed properly the modified carbon nanotubes forest can removal the condensed drop at micrometric length scales during condensation easier than the hydrophobic, hydrophilic and superhydrophilic surfaces. In addition, this behavior occurs, for our surface, at high super saturation level which is an innovative aspect of this work. The Vertically Aligned Carbon Nanotubes were grown on stainless steel pipes. We used a CO₂ laser and a O₂ plasma to perform the post treatments that changed the nanotubes to superhydrophobic and superhydrophilic, respectively. In addition, the CO₂ laser treatment added a second level of roughness in the surface by etching the walls of the nanotubes. Due the low adhesion of the carbon nanotubes with the substrate a Polyethylene layer was deposed on carbon nanotubes avoiding the removal of them. We experimentally demonstrated a 30% higher vapor condensation rate at high supersaturations levels in comparison to the others prepared surfaces.

Although Li-ion batteries have been successful in various applications, their shortcomings with regard to high cost and global maldistribution of raw materials, as well as safety concerns have stimulated alternative rechargeable batteries based on other carrier ions represented by sodium and magnesium ions, targeting grid-scale energy storage systems (ESSs). However, many electrode materials in these emerging systems often suffer from sluggish kinetics due to the larger size or bivalency of carrier ions, limiting electrochemical performance particular in specific capacity and operation voltage. In this talk, I will introduce a new approach of engaging intercalated water in layered cathode materials. The intercalated water improves the performance of the given materials substantially by shielding electrostatic interactions or maintaining the crystal frameworks over repeated cycles. Detailed effects of intercalated water will also be described, along with promising potentials towards aqueous operations. Electron microscopy characterization for in-depth understanding of these materials will also be introduced.

Relationships between the surface chemistry, particle morphology, and “internal” crystal structure in the polymorph-rich family of layered MnO₂ (Birnessite or Chalcophanite) have strong implications on terrestrial recycling, and critically influence the abilities of catalytically and electrochemically active variants. Intense research and development efforts are aimed at characterizing and optimizing the interfaces of such nanostructured materials, yet the identity, concentration, and structure of surface species and their influence on underlying structures are often ambiguous: the availability of experimental tools to observe atomic level details in relevent environmental/operating conditions remains limited. We present our investigation of the local and long-range structure of synthetic layered δ-MnO₂, from formation, through dehydration, and through phase transformation, with a combination of neutron and X-ray total scattering, extended X-ray absorption fine structure, and neutron vibrational spectroscopy analyses. Models incorporating quantified turbostratic stacking disorder and interlayer/surface chemistry reveal that interlayer water molecules and hydrogen bonding play key roles in maintaining long range stacking coherence, while Mn vacancies within the MnO₂ layers promote the intercalation of Cu²⁺ into interlayer spaces, forming strongly distorted interfacial octahedra. Hydration levels and vacancy concentration are observed to have a marked influence on the charge/discharge capacity and cycling abilities of samples studied. This example informs structure-property optimization strategies for the wider family of layered transition metal oxide-based energy materials, and highlights a broader theme of our research aimed at extracting structure models from experimental data with the detail needed to guide and validate modern nanoscale theories and improve understanding of geologically and technologically significant materials.

The “dual circuit redox flow battery” takes advantage of a conventional all-vanadium redox flow battery (VRFB) combined with a catalytic hydrogen evolution reactor in a separate external circuit. Depending on demand, the VRFB can be conventionally discharged by an electrical load or by using the catalytic reactor to yield H₂ on the negative side. However, during hydrogen generation protons are consumed and a state-of-charge imbalance is created. Therefore, the corresponding discharge of the positive compartment is necessary to maintain proton and charge balance of the VRFB. Water oxidation is a suitable secondary reaction, providing regeneration of the positive electrolyte (reduction of VO₂⁺ to VO²⁺) and also supplying protons. However, VO₂⁺/VO²⁺ does not possess a potential high enough to spontaneously drive the oxygen evolution reaction; an additional energy input is required. Thus, in this work the reaction is performed electrochemically in a secondary flow cell (V-O₂ cell) to provide a high rate of conversion while minimizing the energy required. In the V-O₂ electrolyzer, which was added to the VRFB with catalytic reactor, water is oxidized to oxygen and VO₂⁺ is reduced to VO²⁺ electrochemically providing state-of-charge and proton rebalancing for the dual-circuit redox flow battery system.
The main advantage of this approach is that there are no additional chemical species. Water oxidation reaction proceeds with the support of a stable and commercially available DSA® electrode. Moreover, from a practical point of view the V-O₂ cell, being actively driven by current can be switched on and off asynchronously with the hydrogen production in the catalytic reactor. Also, in principle, this system can guarantee much higher hydrogen and oxygen purity than a typical electrolyzer, because of the physical, local, and temporal separation on the production of the two gasses.
In this work a V-O₂ cell is proposed and optimized according to its energy input and a final prototype which integrates a catalytic reactor, a VRFB, and the V-O₂ electrolyzer is successfully tested. A laboratory scale cell was investigated in a complete dual-circuit system including a V-O₂ cell. The dual-circuit vanadium redox flow battery coupled with a V-O₂ cell achieves total efficiency between 42 and 62% depending on the current density and stable operation was demonstrated during extended tests.

Capacitive deionization (CDI) is an emerging water desalination technology accomplished by electrosorption of ions at the interface between saline solutions and nanoporous carbon electrodes. By employing the strictly physical process of ion electrosorption, salt adsorption capacities of typically 15-20 mg/g (mg salt per g of the electrode) have been reported. The use of carbon limits the maximum achievable charge storage capacity and restricts the applicability of CDI to low molar media (typically below 20-50 mM). This issue originates from unfavorable co-ion expulsion from carbon nanopores concurrently occurring during counter-ion adsorption. Only once all co-ions have become depleted in the pores, permselective ion removal is accomplished. To transfer CDI from a niche technology to a more largescale use, new electrode materials and cell concepts need to be explored.
The use of ion intercalation or conversion reaction electrodes has recently re-energized the research field for electrochemical water desalination. The presentation will summarize key aspects of the implementation of ion intercalation materials for the deionization of aqueous media with ion concentration levels typical brackish water and seawater. We demonstrate the facile use of pseudocapacitive materials to effectively remove cations and anions from saline media. Such materials, like transition metal carbides (MXene), some transition metal dichalcogenides (e.g., MoS₂), or some metal oxides (e.g., V₂O₅) allow energy efficient desalination in the absence of an ion exchange membrane. These benefits also can be accessed using electrode materials with battery-like charge/discharge behavior, such as TiS₂. With enhanced ion mobility and lowered energetic barriers, there is even an improved desalination performance when increasing the salt concentration from low (10 mM) to high levels (1 M). The adaptation of intercalation materials requires careful considerations for the desalination cell design.

The rising demand of clean & safe drinking water is one of the primary concerns because of the water scarcity, growth of human population, urbanization, & natural & anthropogenic pollution. Ground water & surface water resources are being contaminated by pervasive low molecular weight hydrophilic compounds (LMHCs) which are not effectively removed by current water treatment technologies. The US Environmental Protection Agency (USEPA) is focused on the removal of many classes of LMHCs from drinking water & wastewater, including; disinfection byproducts precursors, pharmaceuticals, personal care products, heavy metals & perfluoroalkylated substances (PFAs). These LMHCs, especially PFAS, do not hydrolyze or degrade in water. Some of these have been reviewed to have adverse health effects while for others the long term exposure effects are still unknown. In order to address removal of these pervasive contaminants, an anion exchange resin polymer (AEP) functionalized Single Walled Carbon Nanotubes (SWCNTs) in the form of NanoResin has been developed. The large surface area of SWCNTs increases the availability of active ion exchange sites of the AEP. These NanoResins have higher & faster adsorption capacity compared to currently commercialized materials like magnetic ion exchange resins, DOWEX, etc.

This NanoResin was synthesized by modified activator regenerated electron transfer atom transfer radical polymerization technique in all aqueous media where the growing hydrophilic polymer brushes gets attached the surface of hydrophobic SWCNTs. The hydrodynamic diameters of the purified AEP & NanoResins measured using dynamic light scattering were 1.660 ± 0.10 nm & 194.9 ± 11.3 nm, respectively. The ratio between D & G bands, measured using Raman Spectroscopy, increased by 100% compared to pristine carbon nanotubes which is indication of covalent functionalization of the AEP. The proton nuclear magnetic resonance of the growing AEP showed 33 % conversion from monomer to polymer in 40 minutes. The scanning electron microscopy of nanoresin films fabricated on mixed ester cellulose membrane for performance testing showed a smooth morphology of mesoporous architecture of SWCNTs with conformal polymer coating. The step height of fabricated film (0.046 mg-NanoResin/cm²) measured under atomic force microscopy was 142.31 nm. The measured membrane resistance of the thickest nanoresin film (0.184 mg-NanoResin/cm²) was determined to be κ_m = 2.1x10¹¹ m^-1 which is smaller than typical ultrafiltration membranes with κ_m = 2x10¹² m^-1 and hence, demonstrated its high water flux capacity.

We demonstrated the removal of several types of pharmaceuticals (viz., tetracycline hydrochloride and carbenicillin disodium), pesticides (viz., bentazon, terbacil, and bromacil), DPBs (viz., bromoacetic acid and choloroacetic acid) and PFAS (viz., perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS)), from water via adsorption capacity measurements. The percentage removal of PFOA and PFOS, for a starting concentration between 2-4 mg/L of contaminant, was 92.7% and 95.8%, respectively. Since, the US EPA has been challenging systems to measure ~95% removal of PFAS at low concentrations, we established adsorption of PFAS at a starting concentration of 3-5 μg/L and removed PFAS upto ~98%. The regeneration studies performed upto 20 cycles, showed a slope of -0.23 ± 0.18% per cycle which demonstrates that the NanoResin could be regenerated without significant degradation. Recently, we have been employing sonochemistry to synthesize Aq-SNR to be more consistent with the principles of green chemistry. NanoResin with 25 min sonication & no reflux showed higher total mass to SWCNTs ratio (2.7) than 10 min sonication & 2 h refluxed NanoResin (2.4). The facile, green synthesis of fast, reliable, high water flux, high retention of solutes & regeneratable system corroborates an effective & sustainable solution to water purification & enhancement of water quality.

The preparation of membranes with high selectivity based on specific chemical properties such as size and charge would impact both the efficiency of the worlds energy supply, the production of clean water and many other separation technologies. We report a flexible synthetic protocol for preparing highly ordered two dimensional nanoporous polymeric materials (termed covalent organic frameworks or COFs) that allow for placing virtually any function group within the nanopores. We demonstrate that membranes, fabricated with this new family of materials with carboxylated pores, are very water permeable, as well highly charged and size selective. In addition we have designed membranes that have nanopores with high water flux that are not fouled by proteins due to the high number of chaqrged groups in the pores.

Electrochemical desalination technologies using cation intercalation reactions within nanoparticulate host compounds present economical opportunities for energy-efficient water treatment. While the theoretical charge-storage capacity of such materials can exceed the levels need to desalinate seawater salinity levels, removal levels at finite-rate operation are limited by transport phenomena from atomic to device scales. In this talk we address this challenge by characterizing and controlling the rate-limiting processes within porous electrodes containing Prussian blue analogue (PBA) nanoparticles, a material with facile kinetics and omni-valent intercalation capability. We first use the potentiostatic intermittent titration technique (PITT) to characterize both thermodynamic equilibrium of and apparent diffusion within PBA nanoparticles (NPs) incorporated within porous electrodes. By combining atomistic modeling with PITT results, we find that the measured variation of equilibrium potential with the degree of intercalation (DOI) results from the microscopic preferences for cation occupation within the PBA host compound, revealing potential mechanisms for crystal-scale diffusion. We also discover that the apparent diffusion coefficient of cations within PBA porous electrodes varies non-monotonically between 10⁻¹¹ cm²/sec (at 50% DOI) and 10⁻¹⁰ cm²/sec (at DOIs of 0% and 100%) that we later show is a signature of electron-conduction limitations in PBA NP agglomerates. Galvanostatic cycling of electrodes with different average NP-agglomerate sizes reveals that two-fold higher rate capability is achievable when agglomerate radius reduces two-fold, despite having the same NP size distribution. By way of comparison with theory and by characterizing the effective conductivity of PBA-NP compacts, we show that apparent cation diffusion in PBAs is limited by electronic conduction through NP agglomerates. Building on these findings we also measure the effective electronic and ionic conductivity and hydraulic permeability of porous electrodes incorporating non-conductive nanoparticles from which we build processing/structure/property relationships to inform materials selection for desalination devices using cation intercalation materials. We subsequently identify optimal electrode formulations by introducing electrochemical Ashby diagrams with guidelines determined using device-level metrics for electrochemical desalination.

The continual increase in global population, along with a rapid progression towards extensive urbanization and industrialization, serves as the driving force for the upsurge in the development of water desalination technologies. There are only a limited number of ion removal technologies available on the market. Distillation is used for ion streams with high salt concentration, reverse osmosis (RO) is used for both intermediate salt concentrations as well as brackish water, and electrochemical technologies like electrodialysis (ED/EDR) and capacitive deionization (CDI) are used for brackish water with relatively low salt concentrations. In addition to desalination, there is an increasing demand for the selective uptake of ions. Along this line of motivation, CDI has the potential to offer diverse means to accomplish selective ion separation from aqueous solutions.

CDI is a water desalination technology that facilitates adsorption of ions via two oppositely polarized porous electrodes. In a traditional sense, CDI typically utilizes high surface area carbon materials, where the ions are trapped within the electrical double layers (EDLs) formed in the pores of the carbon electrodes [1]. However, in recent times, new hybrid carbon-intercalation electrodes employing materials with redox activity have been introduced to CDI, [2, 3], in which the ionic species are stored in the crystallographic sites of the intercalation host compound (IHC).

Preferential ion electrosorption in capacitive deionization (CDI) has been the primary focal point of a number of contemporary studies. In general, the major influential factors that regulate ion selectivity of these porous electrodes include: (i) operating conditions such as applied cell voltage and initial ion concentration, (ii) ion properties like ion valence, hydrated size, and electronegativity, and (iii) micropore characteristics such as chemical surface groups and pore sizes. Hence, the technological hurdles associated with the development of efficient CDI electrodes with enhanced ion selectivity are manifold. Furthermore, a comprehensive knowledge of the underlying mechanisms and the materials involved is of prime importance to enact successful optimization of relevant process parameters (e.g. voltage, current, flow, cycle time), to attain preferable ion selectivity during the adsorption/regeneration phase.

This talk will be centered on the comparative analysis between conventional CDI electrodes with porous carbons, and the most recent intercalation electrodes [4, 5-6], utilized for the purpose of water desalination and selective ion separations. In the case of the latter, redox-active nickel hexacyanoferrate (NiHCF) nanoparticles embedded in a carbon matrix facilitates the intercalation and de-intercalation of Na⁺ ions, producing fresh water via a cyclic charging and discharging CDI process. Conclusively, alternate materials that can offer potential in advancing ion separation process will be also discussed.

This presentation provides a general overview addressing various material challenges in relation to ion removal within the domain of CDI.

References:
1. M. E. Suss, et al. Energy. Environ. Sci., 2015, 8, 2296-2319.
2. M. Pasta, et al. Nano Lett., 2012, 12, 839-843.
3. P. Srimuk, et al. J. Mater. Chem. A, 2016, 4, 18265-18271.
4. K. C. Smith, J. Electrochem. Soc., 2016, 163, A530-A539.
5. S. Porada, et al. Electrochim. Acta, 2017, 255, 369-378.
6. K. Singh, at al. Phys. Rev. Appl., 2018, 9, 064036

Electrode materials that undergo electrochemical reactions with aqueous ions can be used to desalinate water or harvest energy from salinity gradients (i.e., two waters with different salt concentrations). Early research exploiting this phenomenon relied primarily on capacitive carbon-based electrode materials that interact with salt ions solely thorough electrostatic reactions. These capacitive electrodes charge quickly, but have low salt storage densities, which limits their potential use for water desalination and salinity gradient energy production. One way to overcome this challenge is to use intercalation electrode materials, which insert/release salt ions to/from their crystal lattices. Intercalation materials have far higher theoretical salt storage densities than capacitive materials and recent studies have shown that indeed intercalation electrode materials outperform capacitive materials. Presently, however, potential electrode materials have been screened primarily through empirical means. Here, we will discuss our on-going work to construct a framework for rationally selecting intercalation electrode materials for desalinating water or harvesting salinity gradient energy. To achieve this goal, we are methodically investigating the relationships that exist among the materials’ (1) physical properties, (2) electrochemical properties and mechanisms, (3) power production from salinity gradients, and (4) energy demand for water desalination. I will focus primarily on manganese oxides because their crystal structures and particle morphologies can be readily tuned. I will also discuss lessons learned from our recent work on other intercalation electrode materials: Prussian Blue analogs, metal nitrides, metal oxides, and spinels. The aim of this survey is to identify the class of materials that exhibits the highest cation removal extent and/or rate and to determine the underlying mechanism(s), which will ultimately lead to a framework for optimizing electrode material design.

Given their intrinsic hierarchical micro-/nano-structures, unique chemical/physical properties and tailorable functionality, hydrogels and their derivatives have emerged as an important class of functional materials for many exciting applications beyond their traditional biomedical application. Bottom-up synthetic strategies to rationally design and modify their molecular architectures enable functional hydrogels to address several critical challenges in renewable energy and environmental technologies.

In this talk, I will present recent advances made in nanostructured functional hydrogels as an emerging material platform for significant applications in the technological water-energy nexus, including high-energy-power electrochemical energy storage, solar water desalination and atmospheric water harvesting. I will further illustrate ‘structure-derived multi-functionality’ of this special class of materials.

Ion-conducting membranes are crucial to the operation of many electrochemical processes for clean water and energy production, including batteries, fuel cells, electrolyzers, water purification by electrodialysis, and salinity-gradient energy technologies such as reverse electrodialysis. It is highly desirable in electrochemical processes that the membranes possess both high conductivity and high selectivity toward the ion(s) of interest (counter-ions). However, there is a recognized tradeoff between the two, making it difficult to achieve both high conductivity and high selectivity simultaneously. We have synthesized a series of ion conducting membranes incorporating a novel physical crosslinking mechanism that appear to have significantly increased conductivity with little to no decrease in counter-ion selectivity, compared to covalently-crosslinked membranes. Physical crosslinking also enhances the mechanical properties of the membranes by increasing their ductility. The observed behavior cannot be rationalized via the conventional understanding of the conductivity-selectivity tradeoff, which is based on membrane swelling and its effect on Donnan exclusion. As such, our results suggest that our understanding of the physicochemical factors governing ion sorption and transport in charged membranes is incomplete. In this presentation, we propose an additional physicochemical mechanism to explain our findings and discuss its implications for design and optimization of ion-conducting membranes.

With increase in the world population and climate change, the importance of and demand for the water desalination systems continue to grow. Liquid surface freshwater constitutes only ~0.5% of the water available on Earth, and thus, desalination of brackish and saline water is becoming increasingly important as the water crisis escalates. An emerging method, hybrid capacitive deionization (HCDI), is a derivative of capacitive deionization (CDI) approach for water desalination, in which one carbon electrode is replaced with a redox-active intercalation electrode, resulting in substantial improvements in ion removal capacity over traditional CDI. However in order to keep pace with rising levels of water consumption, the performance of HCDI systems needs to be improved while maintaining low cost and minimizing environmental impact. As a result, novel scalable and cost-efficient materials that exhibit superior performance are sought for.
In this study¹, two crystallographically and chemically distinct layered manganese oxides (LMOs), Na-birnessite and Mg-buserite, were investigated as intercalation electrodes for HCDI. The layers in crystal structures of LMOs consist of edge sharing MnO₆ octahedra, while Na⁺ and Mg²⁺ ions, known as stabilizing ions, occupy the interlayer positions in the Na-birnessite and Mg-buserite, respectively, along with water molecules. The large difference in interlayer spacing of Na-birnessite (7.2 Å) and Mg-buserite (9.7 Å) is attributed to the different amount of structural water residing between the layers: one layer of water in Na-birnessite compared to two layers of water in Mg-buserite. To understand the ion removal performance of these two materials, Na-birnessite and Mg-buserite electrodes were cycled in NaCl and MgCl₂ solutions in an electrochemical cell with the HCDI configuration. The chosen electrode materials and solutions provide a platform to investigate ion dynamics when stabilizing ions in the electrode structure and ions in solution are the same and when they are different. To gain insights into the stability of layered manganese oxides as HCDI electrodes, the cells were stopped after both 200^th ion adsorption step (IAS) and ion release step (IRS), followed by post-mortem analysis of the electrode materials structure and composition. Replacing carbon electrode after 200 cycles with a fresh one, allowed us to evaluate the contributions of capacity decay related to the structural changes in layered manganese oxides and oxidation of carbon during cycling.
Both materials demonstrated excellent initial ion removal performance with the highest capacities of 37.2 mg/g (637 µmol/g) exhibited by Mg-buserite in NaCl solution and 50.2 mg/g (527 µmol/g) exhibited by Na-birnessite in MgCl₂ solution. The performance decay observed over the course of 200 ion adsorption/ion release cycles was attributed to two major phenomena: oxidation of carbon electrode and evolution of the structure/composition of LMO electrodes. The latter involves disorder in stacking of Mn-O layers and changes in the interlayer spacing/interlayer ions reflecting the composition of the solution being desalinated. This work highlights the importance of understanding the interactions between the HCDI electrodes and solutions containing different ions and the structural analysis of redox active material in intercalation electrodes over the course of operation for gaining insight into the fundamental processes governing desalination performance and developing next-generation HCDI systems with long-term electrochemical stability.

1. B.W. Byles, B. Hayes-Oberst, E. Pomerantseva, ACS Appl. Mater. Interfaces 10 (2018) 32313.

Management and treatment of hypersaline brines, e.g., produced water from oil and gas extraction, zero liquid discharge effluent, and flue gas desulfurization wastewater, are of growing environmental importance. Current practice of using thermally-driven methods to treat hypersaline brines is highly energy-intensive and costly. Here, we present two novel technologies for hypersaline desalination: cascading osmotically mediated reverse osmosis (COMRO) and temperature swing solvent extraction (TSSE). The first technology, COMRO, utilizes the novel design of bilateral countercurrent reverse osmosis stages to lessening the osmotic pressure difference across the membrane, thereby depressing the hydraulic pressure needed. Our studies indicate COMRO can treat hypersaline brines or achieve high recovery desalination with moderate hydraulic pressures well within practical limits. Crucially, the specific energy requirement of COMRO is 2.1-3.8 kWh per cubic meter of product water, approximately an order of magnitude lower than the prevailing thermal methods of, for example, mechanical vapor compression and thermal crystallizer. The second technology, temperature swing solvent extraction, is membrane-less and not based on evaporative phase-change. The technology utilizes a low-polarity solvent that is immiscible with aqueous solutions to extract water from hypersaline brines. Because water solubility of the solvent is highly temperature-sensitive, a mild temperature swing supplied by a low-grade heat source, such as industrial waste heat, shallow-well geothermal, and low-concentration solar collectors, causes the aqueous phase to demix from the solvent, yielding product water. We demonstrate the potential of TSSE to desalinate hypersaline brines up to 235,000 ppm TDS (approximately 7 times seawater salinity). The development of energy-efficient technologies can offer more sustainable solutions for the intensifying environmental concerns of hypersaline brines.

Forward osmosis (FO) is a membrane-based separation process that uses the osmotic pressure gradient between a concentrated draw solution and a feed stream to drive water flux across a semi-permeable membrane. The primary requirement for draw solutions is to find a mixture with enough osmotic potential to power the trans-membrane transfer. Other challenges include selecting a draw solute that may be easily and economically removed and re-generated. While FO has achieved some market success, substantial RDD&D work remains in order for this method to compete with RO and traditional thermal desalination techniques.
Ionic liquids (ILs) have a number of desirable attributes, such as negligible vapor pressure, high ionic conductance, and often high thermal and chemical stability. The physicochemical properties of ionic liquids can be tailored by chemical modification of the cation and/or anion, leading to a vast number (>10¹⁴) of distinct ionic liquid combinations. A subclass of ionic liquids undergo a thermoresponsive liquid-liquid phase transition of either an upper critical solution temperature (UCST) or lower critical solution temperature (LCST). Such thermoresponsive IL-based mixtures have opened up new potential applications such as protein extraction, metal ion extraction, and forward osmosis draw solutes for water purification.
Currently, there exists disagreement in the literature about the molecular interactions responsible for the IL-water LCST transition. This study attempts to obtain insight in the molecular mechanism of IL-H₂O phase separation in LCST aqueous mixtures of tetrabutylphosphonium 2,4-dimethylbenzenesulfonate [P₄₄₄₄][DMBS] across temperatures ranges in both the fully miscible and phase separated regions.
In IL-H₂O LCST mixtures, the IL forms loosely held aggregate structures that grow in size leading up T_c. Minor chemical modification of the anion that results in the disappearance of LCST transition to a fully miscible aqueous mixture versus temperature leads to aggregates shrinking as the temperature increases, the opposite behavior versus the fully miscible phase of the LCST IL. Radial distribution functions from Molecular Dynamics (MD) simulations support the observation of aggregation phenomena in the IL-H₂O mixture through the LCST. A local molecular structure of the ions is derived from multi-dimensional NMR experiments in conjunction with reported MD simulations. In the fully miscible phase region, negligible shifts among the IL protons are observed whereas water’s hydrogen bonding network shows considerable shifts. Additionally, we observe that the anion’s protons response to temperature changes can be divided into two groups: those that respond to the intermolecular environment and those that respond the intramolecular environment, determined by sulfonate ionic functional group.
The rate of phase change between the homogeneous mixture and the separated IL/water biphasic mixture upon heating or cooling should be rapid but may require facilitation via proper engineering. An example of LCST ILs continuous flow FO reactor will be presented and discussed. This scheme could radically reduce the energy-intensity of water purification.
Acknowledgements
This work was supported by funding from the Assistant Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies Office of the US Department of Energy under the USDOE contract number DEAC02-05CH11231 for Lawrence Berkeley National Laboratory. Work conducted at Idaho National Laboratories was performed under Contract No. DE-AC07-05ID14517. J.F.D. acknowledges the support of Dr. Kevin Wilson and the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division of the U.S. Department of Energy.

Capacitive deionization (CDI) is a fast-emerging water desalination technology, most often applied to brackish water desalination and water softening.[1,2] CDI cells desalt feedwaters by storing ions in electric double layers within micropores of electrically charged carbon electrodes. CDI cells have demonstrated a size-based ion selectivity wherein smaller ions are preferentially electrosorbed over larger ions due to ion volume exclusion interactions.[3] Selective ion removal is a highly desirable feature in water treatment systems, as selectivity mechanisms can be leveraged towards preferential and energy efficient removal of toxins and other undesirable ions. We here demonstrate that the size-based selectivity exhibited by microporous CDI electrodes can be substantially boosted through addition of chemical charge to micropores via surface functionalization.[4] We develop a theory of size-based ion storage in micropores including the effect of chemical charge,[5] which predicts such enhancements and demonstrates they are a result of enhanced counterion crowding in functionalized micropores. Desalting an electrolyte consisting of competing potassium and lithium ions, we demonstrate that a surface functionalized (oxidized) cathode increases the selective removal of the smaller potassium ion. We observe that the largest enhancements are measured at lower cell voltages, for example from a selectivity factor of ~1 for typical electrodes to 1.8 for surface functionalized electrodes at 0.4 V charging voltage. This counter-intuitive result is demonstrated to be due to significant cathode chemical surface charge degradation occurring at higher cell voltages, a phenomenon which has not been previously reported upon in the CDI literature to our knowledge. Model results show that achieving stabilized surface groups at high electrode potentials can lead to further enhancements in size-based selectivity factors achievable by CDI electrodes

References
[1] ME Suss, S Porada, M Biesheuvel, J Yoon, V Presser, “Water desalination via capacitive deionization: What is it and what can we expect from it?” Energy & Environmental Science, 2015, 8, 2296-2319
[2] ME Suss, V Presser, “Water desalination with energy storage materials”, Joule, 2 (1), 10-15, 2018.
[3] ME Suss, “Size-based ion selectivity by micropore electric double layers in capacitive deionization electrodes”, submitted to Journal of the Electrochemical Society, 164 (9), E270-E275, 2017.
[4] E Guyes, T Malka, ME Suss. “Enhancing ion size-based selectivity of capacitive deionization electrodes via surface functionalization”, in preparation, 2018.
[5] PM Biesheuvel, HVM Hamelers, ME Suss. "Theory of water desalination by porous electrodes with immobile chemical charge.” Colloid and Interface Science Communications, 9, 1-5, 2015.

The selective removal of toxic or troublesome ions from otherwise usable water has the potential to dramatically reduce the cost of water treatment. Capacitive deionization (CDI) removes ions from solution using charged electrodes and has demonstrated significant energy savings compared with reverse osmosis for removal of salt from low salinity or brackish water. By tailoring electrode properties such as pore morphology and surface functionality, as well as cell operating parameters, selective separation of ions can be achieved. Lawrence Livermore National Laboratory’s unique flow-through electrode CDI platform has demonstrated remarkable selectivity for removal of toxic nitrate in the presence of chloride and sulfate ions, as well as tunable preferential absorbance of monovalent over divalent cations based on electrode material properties. In this talk, we will present our latest results including the development of a multi-scale model (FE + QMD) to help elucidate the mechanism(s) for ion selectivity in CDI.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Document release number: LLNL-ABS-760441

Driven by climate change, population growth, development, urbanization, and other factors, water crises represent the greatest global risk in the coming decades. Advances in materials represent a powerful tool to address many of these challenges. Understanding—and ultimately controlling—interfaces between materials and water are pivotal.¹ In this presentation, we will lay out the challenges and present several examples of work based on materials science strategies for addressing applications in water. In each instance, manipulation of interfacial properties using either atomic layer deposition (ALD) or sequential infiltration synthesis (SIS) provides novel functionality, ranging from selective transport to energy transduction to pollution mitigation.

¹J. Appl. Phys. 124 (2018) 030901

This presentation will describe the synthesis of a library of perovskite oxides with the composition La_1-xSr_xBO_3-δ (x = 0-1; B[KJP1] = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH with the highest specific capacitance of 492 F g^-1 at 5 mV s^-1 for La_0.2Sr_0.8MnO_2.7. We find that greater oxygen vacancy content (δ)-caused by systematic incorporation of Sr²⁺-linearly increases the surface-normalized pseudocapacitance for the three B-site transition metal series investigated, but the slope of the trends is controlled by the B-site element. Furthermore, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral to alkaline aqueous electrolytes with a maximum voltage window of 2.0 V in 1 M KOH. An asymmetric, perovskite pseudocapacitor’s cell voltage can be increased by increasing the voltage difference between the perovskites’ different transition metal redox potentials in the B-site. Asymmetric capacitors constructed from SrCoO_2.7, La_0.2Sr_0.8MnO_2.7, and BM-Sr₂Fe₂O₅ were made with the BM-Sr₂Fe₂O₅//SrCoO_2.7 combination performing the best with a high energy density of 31 Wh kg^-1 at 450 W kg^-1 and power density of 10,000 W kg^-1 at 28 Wh kg^-1.

The flow efficiency is an underappreciated loss mechanism inherent to cyclic capacitive deionization (CDI) operation. This efficiency factor ultimately limits performance and desalination depth when attempting industrially-relevant separations. The flow efficiency parameter is defined to reflect the fact that desalinated water inside a cell is partially lost to re-salination if desorption is carried out immediately after adsorption. This efficiency is distinct from, and yet multiplicative with, other highly-studied current efficiencies. Flow efficiency losses can be minimized by flowing more feed solution through the cell; however, this also results in less effluent concentration reduction due to mixing with the feed. We show that the flow efficiency concept is a key tool for identifying loss processes occurring during desalination, understanding the performance of different operation modes, and recognizing the key scaling factors that underlie the operational response.

Atmospheric water harvesting has received recent interest owing to the abundance of water in the atmosphere. This approach is distinct from state-of-the-art dewing approaches because it can efficiently operate in extreme conditions, that is at low relative humidity and temperature, when the moisture content of air is low. AQSOA zeolites (Z01 and Z02) are aluminophosphate-based microporous molecular sieves that are stable in water and have been extensively studied for adsorption-based refrigeration.¹ Their pore filling mechanism, i.e., Type IV or V isotherm behavior, leads to a sharp uptake at a low relative humidity (5-15% RH) which is indicative of uniform micropore size and a relatively low adsorption enthalpy. In addition, the adsorption enthalpy and the temperature required to desorb the water vapor is lower compared to other zeolites (e.g., 65 degrees C for AQSOA Z01).² In this work, we characterized AQSOA zeolites for water harvesting. One challenge of using solid adsorbents is the relatively slow vapor transport via diffusion in the bulk material. The vapor transport occurs at two length scales: through the bulk material (intercrystalline) and to the pores within an individual crystal (intracrystalline). In a practical device, the intercrystalline transport resistance of AQSOA zeolites can have a significant influence on the kinetics because it occurs over a length scale roughly three orders of magnitude larger than the intracrystalline vapor transport. Transport resistance is also highly dependent on the packing porosity of the adsorbent, which can inhibit mass transfer at high packing densities. Therefore, we fabricated composite adsorbent layers using AQSOA zeolites and metal foam to facilitate both heat and vapor transport. To study the mass transfer characteristics of AQSOA zeolites in this composite layer, we performed adsorption, desorption, and vapor condensation under controlled pressure and temperature. We compared our experimental results to our detailed computational models which capture the diffusive processes at these different length scales. Our results indicate the promise of using composite AQSOA zeolite layers, and can be extended to design other composite adsorbent materials for water harvesting.

[1] Kayal, S.; Baichuan, S.; Saha, B. B. Adsorption Characteristics of AQSOA Zeolites and Water for Adsorption Chillers. Int. J. Heat Mass Transf. 2016, 92, 1120–1127.

[2] Kim, H.; Cho, H. J.; Narayanan, S.; Yang, S.; Furukawa, H.; Schiffres, S.; Li, X.; Zhang, Y.-B.; Jiang, J.; Yaghi, O. M.; et al. Characterization of Adsorption Enthalpy of Novel Water-Stable Zeolites and Metal-Organic Frameworks. Sci. Rep. 2016, 6, 19097.

Efficient catalytic water oxidation is an important reaction for the development of solar hydrogen as a source of green energy. Extraction of high energy electrons from water for the preparation of fuel leaves O₂ as a byproduct. Nature boasts the only truly effective catalyst for this reaction, the oxygen evolving complex (OEC), a tetramanganese-calcium-oxo cluster. The exploration of solid-state manganese oxides inspired by (or possibly a primordial precursor to) this inorganic cluster is of interest for the chemical and electrochemical oxidation of water, and while promising catalysts have been discovered, an understanding of their active site structures and mechanisms has been elusive, precluding incremental improvements in design. This lecture is focused on the birnessite phase of MnO₂, typically viewed as a poor water oxidation catalyst. The Center for Complex Materials at Temple University is undertaking a combined experimental and theoretical approach where Density Functional Theoretical (DFT), and Molecular Dynamics (MD) computations inform and guide experimental approaches as to understand the functioning of, and thereby identify ways to improve the catalytic activity of these materials. Modification of birnessite by enrichment with Mn(III) defect sites, intercalated interlayer Co or Ni ions, and alloying of the layers with Co(III) have turned this relatively poor catalyst into one competitive with excellent IrO-, cobalt-oxide-, and double-layer-hydroxide-based catalysts. The discoveries have been guided by MD simulations which show enhanced geometric frustration of water enhances electron transfer rates, and DFT calculations which describe geometric features that optimize band structure for beneficial electronic conductivity and charge separation.

In the last decade, remarkable efforts have been devoted to the development of novel approaches and materials to convert solar energy directly into storable fuels and other chemicals. One approach to achieving lower cost, more sustainable photocatalysts is to use amorphous materials. Since high temperature treatments for crystallization are not required, amorphous materials can be potentially much cheaper to synthesize and easier to adapt for large-scale preparation. However, for efficient solar energy conversion, it has been commonly accepted that highly crystalline materials are required for effective electron-hole pair generation and separation. In photocatalytic systems, photogenerated carriers must diffuse to the electrolyte interface to catalyze redox reactions at the surface of the photocatalyst; hence, avoiding recombination at defects in the bulk is important. On the other hand, several recent studies have shown that amorphous or disordered materials can display higher photocatalytic activity than crystalline counterparts, particularly in high surface area, nanostructured, and/or mesoporous materials. This can be explained by the fact that small nanoparticles or mesoporous materials have a small bulk volume and large accessibility to the electrolyte. This means that photogenerated carriers only have to diffuse a short distance before they can reach the electrolyte interface where the catalytic reactions occur.

In this work, amorphous titania-based photocatalysts are synthesized using a facile, UV-light mediated method and evaluated as photocatalysts for hydrogen evolution and nitrogen reduction to ammonia. The photocatalysts are prepared through the direct injection of a titanium alkoxide precursor into a water/methanol mixture, with subsequent hydrolysis, condensation, and polycondensation to form TiO_x(OH)_y species under UV-irradiation. The resulting amorphous titania materials exhibit an overall higher hydrogen evolution rate compared to a crystalline TiO₂ reference (P25) due to the highly porous structure and high surface area (~500 m²/g). Synchrotron pair distribution function data show that the short-range local structure of the freshly synthesized materials is brookite-like, but that ageing and re-crystallization to anatase occurs after storage in ambient conditions.

Under photocatalytic reaction conditions, the formation of surface Ti³⁺ species in the amorphous photocatalyst was observed, as indicated by the deep blue coloration of the materials, is thought to play an important role on the chemical reactivity of the catalyst. The Ti³⁺ species are also used to reduce protons to H₂ in the absence of light irradiation or reduce Pt²⁺ to form Pt nanoparticles. The results show that this is a facile approach for the preparation of high surface area titania photocatalysts containing Ti³⁺ species with good photocatalytic activity and unique porous morphology to overcome the limitations presented by increased likelihood of charge carrier recombination in amorphous materials.

An important challenge for the membrane community is to mimic natural protein channels that outperform, by orders of magnitude, man-made systems based on pore size and coarse chemical selectivity. To mimic protein channel pumping on a robust engineering membrane platforms, applied bias can be used to actuate charged gatekeepers and induce ionic pumping. Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates [1,2] 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow [3,4]. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices for nicotine addiction [5]. Electrophoresis and electroosmosis phenomena are discussed in detail through CNT conduits. Another approach is to mimic natural protein channel transport cycles with binding/transport/release/reset events. Porous alumina (AAO) membranes have top and bottom electrodes coated with thin Au layers with pore dimension tuned to match protein dimensions. At this thin layer at pore entrances, Ni-NTA is able to bind to hys-tag residue on target protein, as is commonly employed in chromatography. A binding voltage pulse attracts anionic target protein to top electrode and blocking the pore, while repelling the cationic imidazole release agent. The second voltage cycle attracts cationic release agent to top of membrane while pumping anionic target protein to bottom permeate and resetting the pumping cycle. The separation efficiency of 1cm2 of membrane was comparable to convention 1cm3 volumes used in chromatography [6]. Further modeling of system kinetics shows order of magnitude increase in throughput with sub-second cycle times enabled with high imidazole concentrations in a continuous cross flow operation.

1 “Nanoscale hydrodynamics: Enhanced flow in carbon nanotubes” Majumder, M.; Chopra, N.; Andrews, R; Hinds, B.J Nature 2005, 438, 44.
2 ‘Mass Transport through Carbon Nanotube Membranes in three different regimes: ionic diffusion, gas, and liquid flow’ Mainak Majumder, Nitin Chopra, B.J. Hinds ACS Nano 2011 5(5) 3867-3877
3 ‘Highly Efficient Electro-osmotic Flow through Functionalized Carbon Nanotubes Membrane’ Ji Wu, Karen Gerstandt, Mainak Majunder, B.J. Hinds, RCS Nanoscale 2011 3(8) 3321-28
4 “ Programmable transdermal drug delivery of nicotine using carbon nanotube membranes” J. Wu, K.S. Paudel, C.L. Strasinger, D. Hamell, Audra L. Stinchcomb, B. J. Hinds Proc. Nat. Acad. Sci. 2010 107(26) 11698-11702.
5 “Electrophoretically Induced Aqueous Flow through sub-Nanometer Single Walled Carbon Nanotube Membranes” Ji Wu, Karen Gerstandt, Hongbo Zhang, Jie Liu, and B. J. Hinds Nature Nano 2012 7(2) 133-39
6 “Dynamic Electrochemical Membranes for Continuous affinity protein separation”, Z. Chen, T. Chen, X. Sun and B.J. Hinds. Advanced Functional Materials 2014 24(27) 4317-23

The growing demand for clean water supplies is driving the need for an innovative approach of water desalination. Developing a method for treating water with high salinities is possible with membrane distillation (MD). Additionally, MD is very attractive for pairing with solar energy due to the low temperature requirements which integrate well with solar thermal. Integration of a membrane distillation system with a photovoltaic (PV) system will result in the co-production of electricity and clean distillate; and therefore, improve the economics of MD. Such a hybrid system will directly absorb thermal energy in the membrane for desalination while taking advantage of the spectrally selective nature of PV to generate electricity. At the top of the system is a PV cell that will filter the visible light wavelengths and transmit the remaining ultraviolet and infrared to a membrane doped with absorbing nanoparticles located at the core of the system. This provides the means to integrate this system but also provides an avenue for increasing the temperature polarization coefficient and improving the membrane performance. The proposed membranes will be fabricated using a multi-step phase inversion approach that will allow for controlled distribution of nanoparticles across the membrane. The model developed here can be used to systematically evaluate the hybrid design where heat is provided from PV cell waste heat and selective transmission and absorption directly into the membrane. The overall project would lead to increased use of renewable energy for desalination while improving the ability to use MD for desalination purposes. Additionally, the spectral optical properties of the membranes and potential PV cell materials are integrated into the model for realistic device performance predictions.

Insertion compound materials, which can attract significantly higher volumetric ion concentrations compared to high surface area activated carbons commonly used for CDI, are appealing and still underexplored. These materials posses stable redox potentials that can be leveraged to avoid electrolysis losses and commensurate pH shifts, and can have excellent long-term stability depending on operational conditions. They are also inherently ionically selective in that insertion compound will attract some species and reject others during electrochemical biasing. In this talk, we seek to comprehensively address benefits and challenges of using insertion compounds for CDI in different water concentrations, pH values, and compositions. Specifically, our presentation will explore (a) the origins of electrolysis and other losses in CDI systems and how insertion compounds might be used to overcome these issues, (b) the behavior of two different compound materials in solutions with a wide range of concentrations and compositions, and (c) comparative assessment of deionization devices that used static vs. slurry flow electrodes. Multiple insertion and phase change compounds will be discussed, and their performance in salt concentrations ranging from 10 to 1000 ppM will be discussed using half cell and full CDI device – level data. Results suggest that while better voltage control and volumetric ion densities are possible, overpotential losses and transport asymmetry issues can be problematic, particularly in solutions of lower concentration. The ability for insertion compounds to selectively remove one ionic species over another will also be addressed and contextualized.

In response to compounding challenges to global water-energy systems, researchers are investigating advanced membrane structures to reduce the energy costs associated with separations and fluid manipulations. Janus membranes are an emerging class of separation materials in which the two sides of a membrane are engineered to have opposing properties. Examples of such properties include wettability, surface charge, and catalytic activity. Janus membranes have been studied for oil/water separations, ionic rectification, and efficient water aeration. The Janus property interface’s placement within the membrane is critically important in dictating how the membrane behaves. However, many fabrication methods rely on interfacial reactions that are highly sensitive to processing conditions and offer limited control of the property interface position. A method for fabricating Janus membranes providing tunable control of the property interface position would enable optimization of the Janus structure for advanced fluid manipulations.

We have used atomic layer deposition (ALD) of hydrophilic Al₂O₃ to achieve Janus membranes on hydrophobic polypropylene (PP) ultrafiltration membranes.¹ While ALD is normally considered a conformal material growth technique, two factors in the PP membrane system cause a gradient deposit to form. The trimethylaluminum precursor used to grow Al₂O₃ has a low sticking probability on PP, and the mean free path of the precursor vapor molecules far exceeded the pore path lengths in the tortuous high-aspect-ratio PP membrane. The combination of PP membrane poor surface chemistry for ALD nucleation and physical dimensions that yield Knudsen flow, coupled with physical masking that limited vapor exposure to one membrane face, are what allowed for Janus membrane fabrication.
ALD processing parameters including the number of cycles, duration of each vapor exposure, and purge time between co-reactant vapors, were found to influence the resulting gradient of Al₂O₃ coverage within the membrane interior. Using energy-dispersive x-ray spectroscopy (EDS) on the membranes prepared in cross-section, the concentration profiles of aluminum as a function of depth in the membrane were measured. These data demonstrated clear control over the shape and position of the concentration profile.

The ALD-derived Janus membranes were used in an aeration experiment. The membranes were secured supporting a column of water above pressurized air. Air coated the hydrophobic surface of the untreated PP membranes and was released as large bubbles. In contrast, the hydrophilic surface of the PP membranes was poorly wet by the emerging air bubbles. This served to reduce the adhesion force between the air and the membrane surface, allowing buoyancy to release the bubbles at a much smaller size at the same air flow rate for rapid gas diffusion into the water. These ALD-derived Janus membranes offer a platform for study on the role of gradient wettability interfaces in novel fluid manipulations. Applying this technique to other materials such as TiO₂ may introduce catalytic activity as an additional functionality.

References:
1) R. Z. Waldman, H.C. Yang, D. J. Mandia, P. F. Nealey, J. W. Elam, S. B. Darling, Adv. Mater. Interfaces 2018, 5, 1800658.

Transition metal oxides are of interest as aqueous ion intercalation materials in neutral-pH electrolytes for alkali ion batteries and redox-active desalinators because of their high capacity and ion intercalation potentials, but few possess high stability in aqueous electrolytes. Understanding the behavior of transition metal oxides, particularly the more stable manganese-based oxides, could enable the design of high capacity, safe, and long-life aqueous electrochemical energy storage. The P2 layered Na⁺manganese-rich oxides widely studied in non-aqueous electrolytes have higher theoretical capacities and voltages for Na⁺intercalation than other manganese-based oxides. While they have previously been shown to be redox active in aqueous electrolytes, this work presents the first systematic study of the factors controlling the interlayer chemistry and redox behavior of these P2 materials in aqueous electrolytes. We show the intertwined effects of applied potential and composition on the interlayer chemistry and redox activity of P2 oxides by investigating compositions previously shown to exhibit a range of moisture stability: Na_0.62Ni_0.22Mn_0.66Fe_0.10O₂(NaNMFe), Na_0.61Ni_0.22Mn_0.66Co_0.10O₂(NaNMCo), Na_0.64Ni_0.22Mn_0.66Cu_0.11O₂(NaNMCu), and Na_0.64Mn_0.62Cu_0.31O₂(NaMCu). When cycled beyond a critical Na⁺content, all compositions experience an irreversible loss of Na⁺in the first anodic cycle and subsequently intercalate water to form a birnessite-like phase. The composition determines the potential at which this critical Na⁺content occurs, with the Cu-substituted materials exhibiting greater retention of the P2 phase. The behavior in aqueous electrolytes is characterized using ex situtransmission electron microscopy and X-ray diffraction (XRD) to understand the structural effects of the presence of water, and in situsynchrotron XRD to understand the phase transformation accompanying the intercalation of water. In addition, the effect of electrolyte water concentration on the intercalation behavior and potential window is studied using water-in-salt electrolytes. Overall, these results present a study of the factors that determine the stability and performance of P2 oxides in aqueous electrolytes, and offer guidance for designing transition metal oxides compatible with aqueous electrolytes.

Here we will present new tools that allow atomic scale characterization of samples relevant to the water energy nexus, providing valuable information about, for example, nanoconfinement or liquid/solid interface chemistry.
Custom adaptations to the local electrode atom probe (LEAP) allow for the transfer of atom probe samples, under ultra-high vacuum and cryogenic temperatures, between a purpose built glovebox, an SEM/FIB with cryo-stage and EBSD/TKD characterization capabilities, and the atom probe analysis chamber. Examples of the capabilities of this new environmentally controlled sample preparation and characterization suite are to be presented for the first time. The current research is focused on the phenomenon of hydrogen trapping within steel samples susceptible to hydrogen embrittlement and the corrosion of magnesium alloys. However, the potential application is much broader and could be used for the study of electrocatalytic reactions. Of particular note is the potential of the current workflow to capture and characterize metal/liquid interfaces.
Atom probe tomography (APT) is a characterization technique recognized for its ability to provide near atomic resolution in three dimensions, providing mass-to-charge-state ratios for each identified ion. A recent adaptation to a local electrode atom probe, at the University of Oxford, enabled vacuum transfer of samples between a catalytic reaction cell and the atom probe analysis chamber. This adaptation has been used to analyze magnesium alloys after controlled gas exposure, without the potential for further reactions to occur within incidental atmospheres encountered during sample transfer. Through this study we were able to observe a correlation between the presence of hydrogen within the magnesium alloys and an increased rate of oxidation. This study highlights the ability of APT to characterize properties such as nanoconfinement and interlayer chemistry, key concerns of the water-energy nexus.

Beyond-Li ion (BLI) alkali ion-based batteries are rising in interest among researchers because of their utilization of more abundant, cost-effective charge carriers, including Na⁺ and K⁺ ions, compared to traditionally used Li⁺ ions. However, because such systems utilize electrochemically cycling ions with larger ionic radii, achieving fast diffusion and high insertion rates of the BLI carriers into traditional, close-packed electrode materials is challenging. As such, these new systems require the development of novel electrode materials with high capacity, rapid charge transfer, and stable behavior over extended cycling. Materials with open, layered crystal structures have proven themselves among the most reliable electrode materials for Na-ion and K-ion based systems, exhibiting high performance in these emerging systems. Tuning and control of interlayer spacing and chemical composition in open layered structures, can be accomplished via simple wet chemical modification approaches. Such tailoring has the capability to increase ion movement and insertion as well as electrochemical stability, which may lead to improvements in electrochemical performance of these electrode structures.

Layered vanadium pentoxide (V₂O₅) phases are promising cathode materials for BLI systems as vanadium can be present in its highest oxidation state, 5+, and can undergo multiple reductions to a 3+ state, allowing for the transfer of up to two electrons per vanadium ion. One family of vanadium oxide phases that demonstrates advanced electrochemical performance in Li-ion and BLI Na- and K-ion systems is the bilayered δ-M_xV₂O₅ family, where M represents alkali (Li⁺, Na⁺, K⁺) and alkali-earth (Mg²⁺ and Ca²⁺) ions.[1-3] The bilayered structure is built from double layers of VO_x polyhedra which are separated by a large interlayer spacing. When no ions are intercalated within this interlayer spacing, the δ-V₂O₅ phase exhibits an interlayer spacing of 11.5 Å which is stabilized only by intercalated water molecules. In this non-ion containing structure, capacity typically decays over extended cycling, due to lattice breathing and the gradual breakdown of the lamellar stacking of the V-O layers. Ion-containing δ-M_xV₂O₅ phases can be synthesized via chemical pre-intercalation and allow for the tunability of the interlayer spacing from 9.65 to 13.44 Å depending on the nature of the inserted ion.[3] Further, synthesis of the electrode materials via chemical pre-intercalation approach can lead to increased capacities and electrochemical stability in Li-ion, Na-ion, and K-ion cells.

This presentation will focus on the chemical pre-intercalation approach as a means to improve electrochemical performance of bilayered vanadium oxide electrodes in Na- and K-ion systems. Electrochemical performance of δ-M_xV₂O₅ (M = Li, Na, K, Mg, Ca) in Li-ion cells will also be presented as a reference. This chemical pre-intercalation technique will be demonstrated to improve electrochemical performance through two modes: (1) pre-intercalation of charge-carrying into the bilayered phase can lead to tailored ion transport and increase overall specific capacities, and (2) pre-intercalation of electrochemically inactive ions to stabilize the bilayered structure and improve capacity retention through BLI ion cycling. While this pre-intercalation may lead to the partial reduction of the oxidation state of vanadium present in the structure, high discharge capacities over 200 mAh/g are observed in all three ion-based systems. A detailed study of the mechanism of charge storage and the effect of charge-carrying ion size on demonstrated capacities and electrochemical stability in Li-ion, Na-ion and K-ion batteries will also be discussed.

[1] M. Clites, B.W. Byles, E. Pomerantseva, Journal of Materials Chemistry A, 4 (2016).
[2] M. Clites, J.L. Hart, M.L. Taheri, E. Pomerantseva, ACS Energy Letters, 3 (2018).
[3] M. Clites, E. Pomerantseva, Energy Storage Materials, 11 (2018).