Symposium CT01—In Situ/Operando Characterization of Solid–Liquid Interfaces for Sustainable Energy, Water and Environment

Photochemical water splitting is very promising because it converts solar energy into chemical energy in the form of molecular hydrogen (H₂), the most energy-rich chemical compound per unit of mass¹. However, this process is severely affected by catalyst degradation at the anode, where the strong oxidizing environment necessary for the oxygen evolution reaction (OER) causes fast platinum degradation, which is the most active catalyst for this reaction². Currently there are no efficient ways to prevent the catalyst degradation, which results in prohibitive industrial costs and prevents the widespread use of this technology to obtain H₂ and compete with fossil fuels³. This happens because it is very difficult to analyze how the chemistry and structure of platinum change at the atomic-scale when subjected to OER electrochemical potential in real-world conditions⁴.
We performed in situ liquid-phase Transmission Electron Microscopy (TEM) analysis to obtain a time-resolved analysis of platinum nanocatalysts subjected to OER electrochemical stress. The results show that surface Pt_xO_y play a key role in the mobility and dissolution of platinum nanocatalysts during the oxygen evolution reaction. Catalyst degradation involves grain aggregation and growth, followed by Pt_xO_y driven platinum dissolution. The electrochemical stress and degradation effects can be minimized by reducing grain boundary density and through application of intermittent electrochemical stress, which can be achieved by applying a pulsed potential as opposed to a continuous fixed potential. These insights allow correlating the catalyst degradation kinetics to the reaction conditions and catalyst properties, enabling rational optimization of variables such as crystallite size, grain boundary density and the electrochemical protocol for reverting the catalyst degradation and enabling industrially sustainable hydrogen generation from water splitting.


References
1. Chu, S. & Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 488, 294–303 (2012).
2. Seger, B. et al. Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-Protected n+p-Silicon Photocathode. Angew. Chemie 124, 9262–9265 (2012).
3. Habisreutinger, S. N. et al. Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells. Nano Lett. 14, 5561–5568 (2014).
4. Klein, K., de Jonge, N. & Anderson, I. Energy-Loss Characteristics for EFTEM Imaging with a Liquid Flow Cell. Microsc. Microanal. 17, 780–781 (2011).

Solid-water interfaces can drive complex chemical phenomena, where processes such ion adsorption become tightly coupled to phenomena such as surface charging and crystal growth. In-situ atomic force microscopy (AFM) provides a powerful tool to visualize these phenomena. Here we apply in-situ AFM to directly image individual aluminum ions at a mica-water interface and show how the mica-water interface drives changes in aluminum speciation (favoring the formation of hydrolyzed aluminum species). We subsequently investigate the assembly of these ions into dynamically fluctuating clusters, and the coalescence of these clusters to form 2D epitaxial aluminum hydroxide films with a persistent network of gaps. The cluster populations prior to film-growth reveal an energy landscape that deviates significantly from classical theories of heterogenous nucleation, but these deviations can be explained when we consider the electrostatic energy associated with forming charged clusters at the solid-water interface. Monte Carlo simulations indicate that these charging effects can remove the classical barriers to crystal nucleation and enable a barrierless process of crystal growth by cluster coalescence.

Interfacial liquid layers play a central role in a variety of phenomena ranging from friction to molecular recognition. Liquids near a solid surface form an interfacial layer where the molecular structure is different from that of the bulk. Yet the molecular-scale understanding of the interactions of liquid water with solid interfaces is unsatisfactory for the lack of high-spatial resolution methods. The presentation is divided in three sections. The first section is an introduction to the relevance of solid-liquid interfaces. The second section, presents the features and capabilities of 3D-AFM [1-2] to image with atomic resolution the three-dimensional interfacial structure of surfaces immersed in aqueous solutions. The third section reports the structure of interfacial water layers on different surfaces from graphene to a few layer MoS₂; from hexagonal boron nitride to pentacene. Those interfaces are characterized by the existence of a 2 nm thick region above the solid surface where the liquid density oscillates (Fig. 1) [3-4]. The distances between adjacent layers for graphene, few-layer MoS₂, h-BN and pentacene are ~0.50 nm. This value is larger than the one predicted and measured for water density oscillations (~0.30 nm). The experiments indicate that on extended hydrophobic surfaces water molecules are expelled from the vicinity of the surface and replaced by several molecular-size hydrophobic layers.

References
[1] D. Martin-Jimenez, E. Chacon, P. Tarazona, R. Garcia, Nat. Commun. 7, 12164 (2016).
[2] T. Fukuma and R. Garcia, ACS Nano 12 11785 (2018).
[3] M.R. Uhlig, D. Martin-Jimenez and R. Garcia, Nat. Commun. 10 2606 (2019).
[4] M.R. Uhlig et al. (submitted)

Electric double layers (EDLs) are key to electrochemical energy conversion and storage applications such as capacitive charging and redox reactions. However, most of the existing spectroscopy and atomistic imaging methods, such as X-ray spectroscopy and electron microscopy, can only probe the binding states and/or the planar distribution of strongly adsorbed species at the electrode interface, therefore the molecular scale structure of EDLs still remains elusive. Based on our recent success on high-speed 3D force mapping [1], here we report a novel technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular scale EDL structure of an ionic liquid under different electrode potential. Our technique overcomes the limitations of existing 3D-AFM methods in measuring highly viscous liquids. From the 3D-AFM images, we observe rich 3D molecular distribution profiles and potential-dependent reconfigurations. In combination with molecular dynamics simulations, we are able to gain a molecular level understanding of the capacitive charging effects. We expect this mechanistic understanding to have profound impacts on the rational design of electrode-electrolyte interfaces for supercapacitors and batteries.

[1] Panse, K.*; Zhou, S.* and Zhang, Y. 3D Mapping of the Structural Transitions in Wrinkled 2D Membranes: Implications for Reconfigurable Electronics, Memristors, and Bio-Electronic Interfaces. ACS Applied Nano Materials 2019, 2, 5779-5786.

There is an invigorated interest in using operando characterization techniques to analyze the electrode-electrolyte interface in various electrochemical materials systems. Various processes like ion adsorption, intercalation and electric double layer (EDL) charging on the electrode surfaces are responsible for the electrochemical performance, but the precise mechanisms remain unclear. Here we present in-situ electrochemical atomic force microscopy measurements to characterize the EDL structure on MoS₂ surfaces. We have specific interest in using MoS₂ as the electrodes due to their promising applications in batteries and supercapacitors.
We obtain atomic-scale 3D images of the electrode-electrolyte interfaces using a force mapping technique. In the current work, we vary the electrode potential and observe a change of the double layer structure. The observed fine molecular features near the electrode surface are unprecedented and reveal the charge storage mechanism of MoS₂ electrodes.

Reference:
Panse, K. S., Zhou, S., & Zhang, Y. (2019). 3D Mapping of the Structural Transitions in Wrinkled 2D Membranes: Implications for Reconfigurable Electronics, Memristors, and Bioelectronic Interfaces. ACS Applied Nano Materials, 2(9), 5779-5786.

The structure and dynamics of the solid/liquid interface are of fundamental interest for various energy related technologies including energy storage, catalysis, lubrication, and many more. Of particular interest are room temperature ionic liquids (IL) which hold the promise of increasing the electrochemical stability windows for electrochemical capacitors and electrostatic gating of functional oxides. In both cases, the structure of the IL electric double layer (EDL) in three dimensions is the key to understand the device performance. Theories describing the EDL have been proposed showing a layering of cations and anion at solid interfaces. However, a full 3D description is lacking and experimental approaches to measure and understand the EDL’s interfacial structures, dynamics and reactivity are scarce. Due to the characteristic sizes of EDLs, this requires characterization techniques capable of resolving nm length scales. Atomic Force Microscopy (AFM) can investigate such length scales in 3D and has been shown to be suitable to image solid-liquid interfaces.

Here, we demonstrate the use of in-situ AFM techniques to understand processes at the solid/liquid interface and introduce statistical and theory coupled approaches. We will showcase the capabilities for two different systems. First, we will image the EDL of an ionic liquid at a graphite interface and report the direct observation of the structure and properties of topological defects. We will show the existence of structural domains parallel to the solid-liquid interface towards a full picture of the double layer structure and investigate their change with applied bias and shine light on their origin. Then we will apply the technique to IL gated oxide devices where the gating process is explained in terms of the interfacial IL structure. We will show that the transition between both ON and OFF states of the IL-gated amorphous indium gallium zinc oxide transistor is caused by a densification and preferential orientation of counter-ions at the oxide channel surface. This process occurs in three distinct steps, corresponding to regions of different electrical conductivity. In this case, the EDL thickness associated with the flat arrangement of cations at the surface results in extremely high charge density leading to high drain currents.

The work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

Determining atomistic structure at surfaces and interfaces is challenging because metastable surfaces/interfaces are likely accessible under realistic conditions, rendering energy-only searches insufficient, and experimental data often give incomplete information. Therefore, neither theory nor experimental data alone is sufficient to determine these structures. In this talk, we will discuss how we use machine learning to combine experimental and theory-based data to determine surface and interface structures.

A molecular scale insight into ion transport and decomposition is important for understanding deficiencies of the currently used aqueous and non-aqueous electrolytes. In this presentation I will summarize progress made towards improving molecular scale understanding of the structure and electrochemistry for a wide range of aqueous and non-aqueous electrolytes. Double layer properties are obtained from classical MD simulations, while modeling of non-aqueous electrolytes will focus on the competitive solvent and salt reduction at the passivated electrochemical interfaces using Born Oppenheimer Molecular Dynamics (BOMD) simulations using DFT functionals. These BOMD simulations included critical factors needed to realistically represent electrolyte reactivity at electrodes such as explicit description of the substrate – electrolyte interactions; accurate representation of electrolyte structure, ion pairing and aggregation near an electrode; and collection of sufficient statistics from multiple unique simulations that were initiated with differing initial configurations. For example, 20 BOMD simulations starting from different initial conditions using 2.0M LiPF₆ in THF tetrahydrofuran/2-methyl tetrahydrofuran showed no solvent decomposition nor HF formation while only LiF formation was observed as a result of LiPF₆ salt decomposition.¹ The most frequently observed reduction events included a PF₆⁻ coordinated to Li⁺ cations from the electrolyte and LiF surface that lead to anion defluorination and formation of 3LiF and PF₃ gas. The solvent separated LiPF₆ and did not actively participate in reduction. When surface defects in LiF were present near a high population of PF₆⁻ the anions there was a preference for the LiPF₆ reduction and repair the SEI without ether solvent decomposition. Interestingly, a number of fast diffusion events for F^- from the electrolyte | LiF interface to the LiF-lithium metal interface was observed that would be expected to occur during Li stripping indicating that F^- re-arrangement in the thin LiF passivation films should be also considered.¹ Electrolyte reduction at the passivated interfaces from these simulations will be contrasted with other solvents ranging from ethers with mixed salts or carbonates and results from the representative quantum chemistry (QC) calculations performed on the small model electrolyte clusters to estimate oxidation and reduction.
[1] J. Chen, Q. Li, T.P. Pollard, X. Fan, O. Borodin, C. Wang, Materials Today, 39 (2020) 118-126.

Kinetic information, such as the activation energy and transition state, is critical to understanding the reaction. However, the kinetic information of electrochemistry at solid-water interface is challenging to obtain from conventional models of density functional theory (DFT), as they often neglect the presence and/or the dynamics of the surface charge [1] and the solvent configuration, which are further coupled. Here we present a new model that accounts for these effects, by combining hybrid solvation, constant-electron-potential, and slow-growth sampling techniques together. We then apply this model to elucidate the active site structure and the mechanism of electrochemical carbon dioxide reduction catalyzed by single-nickel-atom embedded in graphene, which shows high performance in experiments while is not well understood [2].
[1] D. Kim, J. Shi, Y. Liu, J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b03002
[2] X. Zhao, Y. Liu, J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b13872

Alkali activated cement (AAC) has proposed as a green alternative to ordinary Portland cement (OPC) due to reduced greenhouse gases emissions compared with the production of OPC powder. Here, we aim at computationally assessing the ability of the AAC binder phase to hinder the diffusivity of corrosive ions and hence protect the reinforcing steel in an AAC-based concrete. We have adopted the tobermorite 14Å crystal structure as a model system for the binder phase because it has Ca/Si ratio similar to sodium-containing calcium-alumino-silicate-hydrate (C-(N)-A-S-H gel that forms during alkali activation of blast furnace slag or class C fly ash. We have also incorporated sodium into the tobermorite structure since the hydration phases tend to be produced using sodium-based activators. To describe the interactions between ions we employed a force field that has been widely used to describe inorganic oxides and relies on representing the polarizability of the oxide ion using a core and a shell connected by a harmonic spring. We believe that this polarizability is needed to accurately describe the tobermorite/water interface. Water molecules have been described by a 3-point flexible model. We validated the transferability of the force field by computing several structural and mechanical properties for tobermorite and compared these with experiments and/or other force fields such as CLAYFF.
In this talk, we will present a detailed analysis of the structure, dynamics, and energetics of Na and Cl ions in bulk water and at the interface between water and tobermorite. We will also examine the effect of introducing and increasing the percentage of sodium as a dopant in tobermorite, with a focus on how this affects the behavior of interfacial ions. In our analysis, we rely on computing a set of time and space correlation functions to explain the mechanisms by which AAC can stop the ingress of corrosive ions such as chloride ions. Our work provides a key step in assessing the durability of reinforced concrete structures produced by AACs.

Vapor bubble nucleation plays an important role in a variety of evaporation/condensation based thermal transport processes such as evaporative cooling, boiling, etc. In most practical situations nucleation processes are heterogeneous. While homogeneous nucleation has been studied extensively using theory and simulations, heterogeneous nucleation mechanisms are still being actively researched as it is less amenable to standard theoretical/numerical techniques. In this study, we use molecular dynamics simulations to investigate homogeneous and heterogeneous (flat surfaces and cavities) bubble nucleation in water under negative pressures. The maximum negative pressure sustained by the system is used to determine the propensity of bubble nucleation. Our results indicate that trends in calculated nucleation pressures deviate from classical heterogeneous nucleation theory predictions due to the nanoscopic nature of the nucleated bubbles. Furthermore, we show that nanoscale cavities aid the nucleation process when weak interaction exists between the fluid and solid materials. In this weak interaction regime, we show that nucleation is a multistage process and also demonstrate the existence of an optimal groove geometry for heterogeneous nucleation.

We present the results of first principles calculations aimed at characterizing the electronic structure and charge transfers at heterogeneous interfaces between photo-absorbers, catalysts and water. We discuss the importance of considering realistic structural models at the atomistic level in order to predict the efficiency of solar energy conversion processes, and to tightly integrate theory, computation and experiment.

Ionic liquids (ILs) have gained considerable attention in the last two decades owing to their unique and tunable physico-chemical properties (e.g., low vapor pressure, high thermal stability), which have made them potentially useful for a range of applications, including batteries, fuel cells, catalysis. Ionic liquids are particularly attractive in lubrication, since their properties make them suitable for components working under extreme conditions, such as those found in engines, spacecraft, and micro-electromechanical systems. When ILs are used as lubricants, the interface between the IL and the surfaces of the components in relative motion plays a pivotal role in controlling the friction and wear response. Despite the scientific weight of published studies on the tribology of ILs, remarkably little is still known about the underpinning lubrication mechanism of ILs. The development of a fundamental understanding of the mechanism by which ILs reduce friction and/or wear requires shedding light on the processes occurring at nanoscale asperities within macroscale contacts. This constitute a significant challenge since observing and understanding the nanoscale mechanisms at play is inhibited by the hidden nature of the buried interface and the challenge of performing observations at the nanometer scale.
Here, we used atomic force microscopy (AFM) to visualize and quantify the processes occurring at sliding interfaces in situ, in single-asperity nanocontacts¹. The AFM experiments, in which a diamond tip was slid on steel in phosphonium phosphate IL (PP-IL) at high contact pressure (>5 GPa), indicated a significant friction reduction only after the removal of the native surface oxide from steel. Even though the AFM experiments allowed for the identification of changes in topography and friction in situ while sliding in PP-IL, they could not provide any information about the composition and structure of the regions scanned by AFM. The analysis of these regions is a challenging surface science problem owing to their limited lateral dimensions and the small thickness of the surface material modified by the mechanical action of AFM tips. To address this challenge and elucidate the origin of the friction reduction observed during AFM experiments, laterally-resolved ex situ analyses of the surface chemistry of steel were performed by synchrotron-based X-ray photoemission electron microscopy (X-PEEM), low energy electron microscopy (LEEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The analytical results indicated that the mechanically-induced exposure of metallic iron during AFM tests carried out in PP-IL leads to an increase in surface coverage of adsorbed phosphate anions together with a change in surface potential. These surface modifications are proposed to be caused by a change in surface roughness and adsorption configuration of phosphate anions on metallic iron compared to their configuration on iron oxide, which lead to the formation of a densely packed, lubricious boundary layer only on metallic iron.
The findings of this work² not only shed new light on the lubrication mechanism of ILs in general, but also provide guidance for engineering ILs with the aim of tuning their lubricating properties by controlling their interfacial structures with metal and metal oxides. These outcomes can enhance sustainable development through the reduction of the economic (e.g., reduced fuel expenditure) and environmental (e.g., less pollution) impact of tribology, while being a key factor in the attempt of achieving the challenging environmental objective of reducing greenhouse gas emissions.

References
1. Gosvami, N.N.; Bares, J.A.; Mangolini, F.; Konicek, A.R.; Yablon, D.G.; Carpick, R.W. Science 2015, 348, 102-106.
2. Li, Z.; Dolocan, A.; Morales-Collazo, O.; Sadowski, J.T.; Celio, H.; Chrostowski, R.; Brennecke, J.F.; Mangolini F. Advanced Materials Interfaces 2020, 17, 2000426

Scanning electrochemical microscopy (SECM) is a powerful tool for nanoscale imaging of heterogeneous surfaces. Emergent applications of this technique in studies of interfacial properties of materials and electrocatalysts require near-atomic scale spatial resolution and the capacity for measuring currents produced by inner-sphere electrochemical processes that typically deactivate nanoelectrode surface. In this paper we present some new approaches to high-resolution electrochemical imaging of surface reactivity and active site characterization. One of them is based on mediated electron transfer at nanoelectrodes that can be used as SECM tips for reactivity mapping and localized kinetic measurements of inner-sphere electrochemical processes, such as hydrogen evolution reaction and oxygen reduction to hydrogen peroxide. Tunneling mode SECM experiments at flat samples are aimed at establishing the relationship between the electrochemical tunneling signal and local surface properties and developing single entity voltammetry for characterization of individual catalytic nanoflakes. Finally, we discuss SECM experiments in which a TEM finder grid is used as a substrate to enable multi-technique imaging of the same nanoscale portion of the catalytic surface and elucidate the nature of the active sites by correlating the electrochemical reactivity maps with atomic scale structural and bonding information obtained by TEM techniques.

In the early stage of lithium ion batteries (LIBs) cycling, the reaction of negative electrode with electrolyte will result in the formation of a thin film on the electrode surface.[1] This film is referred to solid electrolyte interphase (SEI), the stability of which is essential for practical LIBs. The most used negative electrode material in LIBs is graphite, due to its stability and low working potential.[2] Previous studies indicated that in graphite, the basal to edge plane ratio, particle size, pore size, degree of crystallinity and surface chemical composition affected the formation of SEI and its stability [2-5]. The defects in graphite is also a very important factor for SEI formation but has been rarely studied. Here, we investigated the SEI formation on an almost perfect graphite-HOPG (Highly Oriented Pyrolytic Graphite) and a synthetic defective carbon film - GUITAR (pseudo-Graphite from University Idaho Thermolyzed Asphalt Reaction) through operando electrochemical atomic force microscopy (EC-AFM). It was found that the defects on GUITAR promoted the electrolyte decomposition and SEI formation at a higher potential. Furthermore, the SEI particles evenly formed and densely packed on the GUITAR surface with a particle size of 172 nm ± 83 nm and a thickness of about 50 nm. The SEI thickness on this defective electrode surface is very similar to that on HOPG (about 40-50 nm). It is also noteworthy that on HOPG, the formed SEI only partially covered the electrode surface, along with the graphite layer delamination from the step edge and bulging on the basal plane. This study showed direct evidence of how structural defects affect SEI nucleation and growth upon cycling.

1. Agubra, V.A. and J.W. Fergus, The formation and stability of the solid electrolyte interface on the graphite anode. Journal of Power Sources, 2014. 268: p. 153-162.
2. An, S.J., et al., The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling. Carbon, 2016. 105: p. 52-76.
3. Tsubouchi, S., et al., Spectroscopic Characterization of Surface Films Formed on Edge Plane Graphite in Ethylene Carbonate-Based Electrolytes Containing Film-Forming Additives. Journal of The Electrochemical Society, 2012. 159(11): p. A1786-A1790.
4. Joho, F., et al., Relation between surface properties, pore structure and first-cycle charge loss of graphite as negative electrode in lithium-ion batteries. Journal of Power Sources, 2001. 97-98: p. 78-82.
5. Li, M., et al., Cycle and rate performance of chemically modified super-aligned carbon nanotube electrodes for lithium ion batteries. Carbon, 2014. 69: p. 444-451.

The solid-electrolyte interphase (SEI) is a complex structure that forms on battery anodes as a result of decomposition reactions of solvent and electrolyte prompted by the high electrode polarizations. Understanding the SEI formation and its resulting properties is key to fabricating batteries more effectively, safely, and ensuring their sustained performance. In particular, in-situ analysis of the first stages of SEI formation would allow us to better understand the ultimate fates of electron transfer and ion transport processes on these important structures. In this study, we explored the first stages of formation of the SEI using versatile redox and ionic probes based on scanning electrochemical microscopy (SECM). Specifically, we were interested in exploring how these new probes could be used to compare the behavior of Li⁺ based SEI’s to those formed by Na⁺ and K⁺, which are of great interest for emerging batteries.

For ionic measurements, the principle is based on SECM probes that integrate mercury micro disk-well electrodes on which alkaline ions can be detected by means of fast-scan anodic stripping voltammetry. We unambiguously demonstrated the capability of these probes to detect alkali-ion fluxes at an electrode undergoing SEI formation and subsequent ion intercalation. Surprisingly, reversible Li⁺ fluxes were observed on SEI’s formed at the edge plane of highly oriented pyrolytic graphite (HOPG), which we ascribe to the presence of reversible redox species on the SEI.[1] Further using a combination of redox imaging using sub-micron sized SECM probes and these new ion-sensitive probes, we elucidated key differences in the reactive evolution of the SEI for the Li⁺, Na⁺ and K⁺ systems. We will describe various correlations observed between the observed fluxes, electron transfer rates, and the presence of organic and inorganic structures observed at the corresponding SEIs. SECM mapping revealed aspects of surface reactivity that are lost during averaging in other electrochemical techniques, and allows a correlation of all reactive species, including ions and electrons, in complex interfacial structures.

[1] Gossage, Z.T.; Hui, J.; Zeng, Y.; Flores-Zuleta, H.; Rodríguez-López, J. Probing the reversibility and kinetics of Li⁺ during SEI formation and (de)intercalation on edge plane graphite using ion-sensitive scanning electrochemical microscopy. Chem. Sci. 2019, 10, 10749-10754.

Abbe’s diffraction limit prevents traditional optical spectroscopy to directly access the nanoscale structures and phenomena. One way to bypass the optical diffraction limit is through the detection of photothermal expansion from light absorption with atomic force microscopy (AFM). The combination of AFM and infrared excitation provides the ability to perform chemical-sensitive imaging and vibrational spectroscopy nanoscale spatial resolution. The peak force infrared (PFIR) microscopy is one of the emerging AFM-based infrared microscopies with spatial resolutions at sub 10 nm. In this presentation, we will describe the recent development of the PFIR microscopy into the liquid phase to address the needs for in situ characterization of materials at the liquid/solid interface. We equip the liquid-phase peak force infared (LiPFIR) microscope with the capability of controlling fluid compositions during the measurement so as to initiate physical transformation and chemical reactions. LiPFIR microscopy is capable of tracking the polymer surface reorganization in fluids and detect the product of click chemical reaction in the aqueous phase. We also measure the hyperbolic phonon polaritons of hexagonal boron nitride submerged in water to reveal its dispersion relations in the fluid phase. As a biological application of the LiPFIR microscopy, the budding site of yeast cell wall particles is imaged in water. The super-resolution, label-free, non-destructive chemical imaging and spectroscopy capabilities of LiPFIR will facilitate investigations of chemical compositions and transformations at the liquid/solid interface.

Electrochemical interfaces play a key role in governing the behavior of lithium batteries. However, access to these interfaces is limited due to the presence of liquid and the need for sealing the battery cells in high vacuum electron microscopes. Advanced in liquid-cell and cryo-holders now enables microscopiests to investigate the local chemical and structural changes in battery interfaces. This presentation encompass the efforts of PI in understanding the role of solid-liquid interfaces in controlling the electrochemical reactions kinetics and thermodynamics in lithium batteries. The first part of this presentation provides an overview on the challenges of
high-energy density rechargeable lithium-oxygen (Li-O₂) batteries due to their potential to surpass conventional lithium ion battery. This system is troubled by sluggish kinetics during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). We investigated the discharge/charge behavior in Li-O₂ batteries using in situ liquid-cell transmission electron microscopy (TEM). During ORR, Li₂O₂ particles formation is controlled by Li⁺ diffusion in the electrolyte and Li₂O₂ particles nucleate at the carbon electrode-electrolyte interface. Li₂O₂ nucleation and growth were also observed within the electrolyte where there was no direct contact with the carbon electrode. These Li₂O₂ particles exhibit growth following O₂^- diffusion-limited kinetics, which indicate the existence of non-Faradaic disproportionation reaction of intermediate LiO₂ into Li₂O₂. Our in-situ liquid TEM work depicts the ORR/OER fundamentals in Li-O₂ battery system under factors controlling the discharge and charge process. We expect findings here to guide both the electrode design and the electrolyte screening aiming for enhancement of the ORR and OER kinetics in Li-O₂ battery cells. We also show that the use of cryo-TEM can uncover electrochemical interfaces in polymer lithium cells. In particular, we were successful in understanding the composition and structure of SEI as a function of polymer electrolyte and lithium salts.

Liquid electroanalytical measurements performed inside the transmission electron microscope (TEM) and X-ray microscope (XRM) are becoming more common and are used to study a wide range of electrochemical reaction-systems at the nanoscale [1-4]. The capability to apply stimuli such as electrical, heating and electrochemical measurements has already started to provide new insights on the dynamics and structural changes during nanoparticles synthesis [1,4], lithium charge and discharge [2], crystal growth [3], and metal corrosion [1]. However, the inability to acquire practical quantitative information in TEM/XRM, mimicking bulk behavior, has prevented researchers from interpreting data accurately and reliably. First, the hardware components are not optimized to perform in the reduced scale environment of the TEM/XRM [5]. Second, the chips configurations such as sizes and aspect ratios of different electrodes suitable for various electroanalytical measurements are poorly designed. Here, we present a newly developed electrochemical operando platform to obtain real-world bulk electrochemical analysis during liquid-phase TEM/XRM microscopy.

The operando liquid cell platform typically consists of two microfabricated chips sandwiched with electron transparent SiNx membranes for encapsulating liquid and viewing in the microscope. We integrated newly developed bulk-sized electrochemical counter (CE), and reference electrodes (RE) with optimized electrochemistry chips equipped with a specialized working electrode (WE) surfaces with customizable chemical compositions, such as platinum, gold, carbon, and industrially used amorphous oxide films. This new configuration allows true quantitative measurements of electrochemical processes with details resembling the complete cycle of traditional bench-top bulk electrochemistry. To illustrate this capability, we present cyclic voltammetry (CV) studies of two model electrochemical systems: electrodeposition and stripping of 01.M CuSO₄ and redox cycling of 20 mM K₃Fe(CN)₆/20 mM K₄Fe(CN)₆ in 0.1M KCl solutions. In the former case, the copper deposition and stripping occur at the working electrode at distinct redox peaks in liquid cells. The use of an actual RE such as Ag/AgCl in the newly developed cell avoids using a pseudo-reference electrode (e.g., chip-patterned metal electrode), which is commonly used in traditional liquid cell microscopy. The bulk RE presents an opportunity to replicate data reminiscent of bulk-scale electrochemistry, particularly concerting the drift in the thermodynamic potential that severely affects electrochemical studies using pseudo-reference electrodes [6]. In the latter case, the current-potential (I-V) relationship in the in-situ cell shows similar Tafel slopes at the same potentials to the large electrode setup tests validating the high electrochemical fidelity of the in-situ cell [7]. The electrochemical data from the new liquid electrochemistry cell can be used to correlate the TEM/XRM structural and chemical analysis with the actual bulk behavior of these electrochemical reactions in a true operando condition. The work presented here highlights that with a suitable hardware system, the bulk behavior of the electrochemical processes can be observed and measured quantitatively [8].

References
[1] F.M. Ross in “Liquid Cell Electron Microscopy,” Cambridge University Press (2016).
[2] J. Lim et al., Science 353 (2016), p. 566.
[3] Nielsen et al., Science, 2014, 345 (6201), 1158-1162
[4] Chee et al., Micros. Microanal., 2014, 20 (2), 462-468
[5] E. Fahrenkrug et al., Journal of Electrochemical Society 164 (2017), p. H358.
[6] D. Grujicic and B. Pesic, Electrochimica Acta 47 (2002), p. 2901.
[7] N. Frenzel, J. Hartley, and G. Frisch, Phys. Chem. Chem. Phys., 19 (2017), p. 28841.
[8] KK, DHA, and NS acknowledge funding from the Department of Energy, Office of Basic Energy Sciences, SBIR Grant # DE-SC0009573.

Liquid cell electron microscopy has attracted a lot of interest and significant process has been made nowadays. In this talk, I will present the recent work in my group on the study of electrode-electrolyte interfaces with electrochemical liquid cell development. It is known that the formation of high quality solid-electrolyte interphase (SEI) is important to limiting lithium dendrite growth. However, how SEI may be modified during lithium deposition is hard to resolve due to challenges in in-situ investigation of the SEI with fine details. With electrochemical liquid cell TEM, we were able to directly observe that lithium dendrite growth was suppressed when a poly(diallyldimethylammonium chloride) (PDDA) cationic polymer film was applied on the electrode. Chemical mapping of the deposits provided remarkable details of SEI on individual nanogranules. It showed that lithium fluorides are uniformly distributed within the inner SEI layer, arising from the instantaneous reaction of the deposited lithium with PF₆^- ions accumulated by the cationic polymer film, thus the dendritic growth of lithium was prohibited. The ability to directly measure SEI chemistry at the nanoscale down to the individual grains in-situ and unveil its correlation with lithium deposition behavior opens future opportunities to explore unsolved mechanisms in batteries.

Developing new energy materials for batteries, solar cells, catalysts and gas storage requires understanding their structural evolution across multiple length and time scale. Over the past 15 years, the Cui group has been developing a set of electron microscopy tools to realize this purpose including in-situ electrochemical cell, in-situ gas reaction, in-situ mechanical indentation and cryogenic electron microscopy (cryo-EM). In this presentation, he will discuss how these advanced electron microscopy techniques impact energy materials. 1) New generation of battery materials are accompanied by large volume and structure change and instability of interphase. He has developed in-situ electrochemical cells and in-situ mechanical deformation as powerful techniques for establishing the relationship of structural change with electrochemical performance, which provides fundamental guidelines for materials design. Cui has demonstrated the most important case on Si anodes: from fundamental understanding, materials design to realizing commercial success of high energy density batteries. 2) Many energy materials are highly sensitive to environment. Studying their reaction in-situ by environmental electron microscopy could provide insights on reaction mechanisms. Li metal reaction with gases as an important case will be presented. 3) Many energy materials are not stable under electron beam. Advanced cryo-EM recently developed in structure biology could be utilized and further developed for materials science. Cui pioneered the cryo-EM towards battery materials and obtained the first atomic resolution images of Li metal and solid electrolyte interphase. With cryo-EM he has answered many important fundamental questions which puzzled the battery field for a long time. He also demonstrated the power of cryo-EM to study a wide range of energy materials including metal-organic frameworks, perovskite and electrocatalysts.

Understanding how lithium metal behaves in liquid vs. solid electrolytes is key for developing stable, high-energy density lithium-metal batteries. This talk will discuss my group’s efforts in understanding the interface of Li metal when in contact with liquid and solid electrolytes. I will first discuss our work in understanding how the lithium/electrolyte interface evolves in liquid-electrolyte batteries at low temperatures. Lithium metal is an attractive anode material for low-temperature batteries since it overcomes sluggish diffusion of Li⁺ in graphite. However, most electrolytes exhibit extremely low Coulombic efficiency (CE) for lithium metal cycling at low temperatures. We have developed an ether-based electrolyte system with carbonate additives that substantially improves the CE of lithium metal cycling down to -60 °C. Lithium metal deposited at low temperatures in this and other electrolytes shows a clear reduction in grain size with decreasing temperature, which is correlated with lower CE. Cryo-TEM and X-ray photoelectron spectroscopy investigation of the solid-electrolyte interphase (SEI) shows that this tailored electrolyte allows for greater inorganic content in the SEI at low temperatures, which enables a compact and conducting passivation layer. Next, I will present my group’s work using in situ methods to understand the evolution of lithium interfaces in solid-state batteries. For NASICON-structured L_1+xAl_xGe_2-x(PO4)₃ (LAGP), electrochemical experiments combined with multi-modal in situ investigation of interfacial reactions reveal how the formation of the interphase is linked to electrochemical degradation. In situ transmission electron microscopy (TEM) shows that the reaction of LAGP with lithium is similar to a conversion reaction, in which lithium insertion causes amorphization and volume expansion of ~130%. In situ X-ray tomography experiments of operating LAGP-based cells reveal that the growth of the interphase causes fracture of the SSE, and quantification of the crack network shows that the extent of fracture with time is directly correlated to impedance increases within the cell. Based on this knowledge, we have found that interphase growth trajectories can be modulated through the deposition of interfacial protection layers, which can extend cycling stability of symmetric cells by almost two orders of magnitude. Finally, operando synchrotron X-ray tomography experiments have been developed and used to directly probe lithium/SSE interface dynamics using other SSEs. Together, the insights gained through these studies highlight the similarities and differences between lithium interfaces in liquid- and solid-electrolyte batteries, and building on this knowledge will be key for development of improved lithium metal batteries.

This talk addresses the use of vibrational spectroscopy to evaluate the reactivity associated with CO₂ reduction electroyzers and solid state batteries. In the first área, electrodeposition of CuAg, CuSn, or CuZn alloy films yields high surface area catalysts for the active and selective electroreduction of CO₂ to multi-carbon hydrocarbons and oxygenates. Alloy films containing Ag exhibit the best CO₂ electroreduction performance, with the Faradaic efficiency for C₂H₄ and C₂H₅OH production reaching nearly 60 and 25%, respectively, at a cathode potential of just –0.7 V vs. RHE and a total current density of ~–300 mA/cm². Alloy films containing Sn exhibit greater efficiency for CO production relative to either Cu along or CuAg at low overpotentials. In-situ Raman and electroanalysis studies suggest the origin of the high selectivity towards C₂ products to be a combined effect of the enhanced stabilization of the Cu₂O overlayer and the optimal availability of the CO intermediate due to the Ag or Sn incorporated in the alloy. Sn-containing films exhibit less Cu₂O relative to either the Ag-containing or neat Cu films, likely due to the increased oxophilicity of the admixed Sn. CuZn films exhibit enhanced production of oxygenated products, particularly ethanol. Additionally, modification of the Cu electrode with certain polymers yeilds substantially enhanced reactivity, due in part to control of the Cu₂O layer.
Relevant to batteries, we discuss solid electrolytes (SEs) which have become a practical option for lithium ion and lithium metal batteries due to their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates whose excellent ionic conductivities at room temperature approach those of commercially-utilized electrolytes. Spectroscopic and structural studies on these materials lead to new formulations exhibiting advantageous properties.

Plasmonic based Raman spectroscopic methods such as Surface and Tip- Enhanced Raman Spectroscopy (SERS and TERS) are extremely sensitive techniques with high interfacial selectivity.^1-3 However, applying these methods to obtain insights on the underlying interfacial phenomena at the electrode-electrolyte interphase in a functioning battery is challenging. In this regard, we have undertaken a systematic approach by first studying solvation properties of lithium-ion battery electrolytes using gold nanoparticle monolayer as a gap-mode SERS substrate. Specifically, we have chosen Lithium hexafluorophosphate (LiPF₆) salt in carbonate solvents -Ethylene Carbonate (EC) and dimethyl- carbonate (DMC). SERS studies shows the solvation shell surrounding Li-cation is different in at the bulk of the electrolyte than at the interface (close to the gold electrode). We have compared SERS results with normal Raman and FT-IR spectroscopy that captures the ensemble average. Insitu time resolved SERS studies showed dynamic changes in the Li-cation coordination close to the interface. Our findings here provide a unique high surface sensitive platform for studying the electrolytes molecules both qualitatively and quantitatively at interface. This work will create further interest in other areas where solid/liquid interface is essential, such as water desalination, heterogeneous catalysis, electrophoresis, corrosion, mass transport across bio membranes,

Acknowledgement
This work was supported by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy through the Vehicle Technology Office

Direct Operando Observation of Double Layer Charging and Early Solid Electrolyte Interphase Formation in Li-Ion Battery Electrolytes, N. Mozhzhukhina et.al. J. Phys. Chem. Lett. 2020, 11, 4119−4123
Probing Electrolyte Solvents at Solid/Liquid Interface Using Gap-Mode Surface-Enhanced Raman Spectroscopy, G. Yang, J. Nanda et al. J. Electrochem. Soc. 166 (2) A1-A10 2019
Solvation Structure of Lithium-ion Battery Electrolytes using Gap Mode Surface Enhanced Spectroscopy, G. Yang, I. L. Ivanov, R. E. Ruther, R.L. Sacci, V. Subjakova, D. L. Hallinan and J. Nanda, ACS Nano. 12, 10159−10170 (2018)

Carbon dioxide (CO₂) emissions during cement manufacturing are increasing daily, and the cement industry accounts for 7% of global CO₂ emissions (WBSCD, 2018). Research on cement hydration mechanism is the key for both understanding the physicochemical properties of the cement matrix and designing sustainable and durable construction materials. However, often it is difficult to characterize the initial hydration reactions of cement clinker (tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetra-calcium aluminate) that take place at the same time. Furthermore, it is difficult to identify the exact phase transformations during the cement hydration processes. Most physicochemical characterization methods applied to cement (BET, MIP, XRD, etc.) are performed on solid-state samples that have been prepared with hydration stoppage, dehydration, or pulverization, which could significantly affect the conditions. Also, the low crystallinity of the major hydrates such as calcium silicate hydrates(C-S-H) poses significant characterization challenges when traditional methods such as XRD are used. In this study, we used in-situ and operando confocal Raman spectroscopy to understand the early stage hydration kinetics of Ordinary Portland cement. In particular, 4% of the CaCO₃ powder was added to the mix to accelerate the hydration; we monitored the ettringite formation, which is closely related to the cement paste setting and its rheological properties. We spatially resolved and mapped the sulfate phase transformation with a high spatial (1-micron) resolution and an excellent signal-to-noise ratio.



WBSCD. (2018). Technology roadmap low-carbon transition in the cement industry. World Business Council for Sustainable Development and International Energy Agency.

Several techniques have been developed to measure in situ surface reactions at the gas/solid and liquid/solid interfaces from vacuum to ambient pressure environments. Probing chemical reactions in liquid environments become more complex with the addition of electrolytes, gases, and resulting interface transformations. Surface vibrational spectroscopy measures the effect of adsorbed surface species at the liquid/solid interface that can determine intermediate steps in environmental and electrochemical processes, that involve surface catalytic reactions. Our group has developed a new vibrational technique that simultaneously measures the air/liquid/solid interface using in situ polarized modulated infrared reflection absorption spectroscopy (PM-IRRAS). This technique was validated using a well-known system of alkanethiol adsorption on gold at the air/ethanol/gold interface in non-aqueous solutions and applying a model of a three-phase system.
We present our investigation of the in situ iron interfacial oxidation from the influence of cations on the formation of inorganic scale, from corrosion. Iron interfaces are ubiquitous in soil, dust, and used as earth-abundant heterogeneous catalysts that undergo spontaneous redox reactions in the presence of oxygen and water, two key reactions in the corrosion mechanism. Chloride ions in electrolytes are found to catalyze the reaction in complex aqueous environments leading to different oxidation rates and results in different mineral scale growth. In this study, different alkali and alkaline chloride electrolyte solutions were studied on the surface of iron to measure the influence of cations on surface corrosion. The electrolytic solutions are shown to either increase or decrease the rate of oxidation, producing different mineral scale from the reaction with air. These findings are corroborated with X-ray photoelectron spectroscopy and in situ liquid atomic force microscopy measurements, connecting oxidation states and surface morphology with surface vibrational signatures. These studies demonstrate how in situ PM-IRRAS reveals surface redox mechanisms that impact complex chemistry in the water and mineral cycles, electrochemical catalysis, and material degradation.

Here we report a new operando experimental technique to determine the local pH at various depths within an operating CO₂ reduction gas diffusion electrode (GDE). There is currently a lack of experimental data and understanding of the reaction conditions at the catalyst surface in CO₂ reduction GDEs - the thickness of the water layer and the local pH near the catalyst. Being able to observe these conditions in situ during GDE operation is important for understanding how the pH and local water quantity effect the selectivity and activity of the device. Measuring these quantities is typically difficult because of the opaque nature of materials used in GDEs and the short relevant length scales. We use a two-color fluorescent pH sensitive dye, DHPDS, with a pK_a of 7.33 and 8.53. The dye is dissolved in the electrolyte and excited by a 458 nm and 488 nm laser consecutively in a confocal microscope. The emission is collected from the two separate excitations and the ratio of the intensity is linearly proportional to the pH. The confocal allows us to excite the dye at specific planes in z, allowing us to see the pH at up to a 40um depth within the porous GDE. As a control we measured what the pH is at the surface of the electrode with no applied potential, and found that the local pH was greater than 9 while the bulk electrolyte (100mM KHCO₃ CO₂ saturated) had a pH of 6.8, when a potential -0.5V vs RHE is applied. This increase is due to the fact that protons are consumed during the CO₂ reduction reaction. The next step in the experiments is to characterize the pH at a range of potentials with operando microscopy observation of local reaction conditions in the pores of the electrode and to determine the uniformity of catalytic activity in the lateral direction, within the catalyst layer plane. We will also report results of experiments with several heterogeneous CO₂ reduction catalysts including Ag, Cu and their alloys, supported on different types of GDE materials with various hydrophobicity and structure. We anticipate that this technique will yield insights that inform future GDE design to optimize wetting and that knowledge about local pH will enhance our knowledge of catalytic mechanisms and control of catalytic selectivity and energy efficiency.

Corrosion is an interfacial process that has a profound impact on society. While the mechanism of iron corrosion has been known for centuries, we haven’t been able to visualize corrosion at the molecular scale due to the spatial and temporal limits of current microscopies and the long time scale of corrosion to develop larger microscale features. We demonstrate that fluorogenic molecules that “turn-on” upon redox reactions can sense the corrosion of iron at the single molecule scale. We first observe the cathodic reduction of non-fluorescent resazurin to fluorescent resorufin in the presence of iron in bulk solution. We show that the fluorescence signal is directly related to the amount of electrons that are available due to corrosion progression and can be used to quantify the catalyzed increase in the rate of corrosion by NaCl. By using modern fluorescence microscopy instrumentation we detect real-time, single-molecule “turn-on” of resazurin by corrosion, overcoming the previous limitations of microscopic fluorescence corrosion detection. Analysis of the total number of individual resorufin molecules shows heterogeneities during the progression of corrosion that are not observed in ensemble measurements.

The novel properties of 2D materials, such as graphene, are proposed to enable exciting applications in diverse areas such as sensing, electronics, and mechanics. However, the thinness of 2D materials and their sensitivity to environmental conditions present challenges when examining the fundamental liquid/solid interactions. For instance, it has been shown that the wettability, surface friction, and electrical conductivity of graphene on silicon dioxide can be tuned by adding bond terminations like hydrogen and fluorine to the graphene lattice. However, graphene's wettability, which is typically characterized using contact angle measurements, has also been shown to vary with its supported substrate—a phenomenon known as wetting transparency. This wetting transparency can further be complicated by the adsorption of volatile organic compounds (VOCs) that lower the surface energy of the substrate, raising the contact angle. Thus, to fully leverage the exciting properties of 2D materials for novel applications, we must develop a more rigorous understanding of the interactions between the 2D material, its supporting substrate, and its surroundings.

In this work, we used microgoniometry to examine the wettability of supported bare graphene and its functionalized forms, including fluorinated, hydrogenated, and hydro-fluorinated graphene. The diameter (50 – 100 microns) of the droplets produced using our microgoniometry apparatus eliminates the effects of gravity on the contact angle measurements, leading to high-fidelity results. The graphene samples were functionalized with hydrogen and/or fluorine bond terminations by indirect exposure to hydrogen plasma and xenon difluoride (XeF₂). We examined a range of substrates, including silicon dioxide, sapphire, gold, copper, and Parylene C. A combination of UV/Ozone exposure and plasma cleaning ensured that the graphene and substrates were cleaned properly to minimize the effects of residues from the graphene transfer process and volatile organic compounds. The effect of the cleaning and quality of the graphene was verified using XPS, ToF-SIMS, and Raman spectroscopy. We also systematically examined the effect of VOCs by exposing the samples to ambient conditions for specific amounts of time. The results indicate that the wetting transparency of graphene depends on the specific bond termination, underlying substrate, and exposure to VOCs. Our results offer guidance to those seeking to use graphene in novel applications where the liquid/solid interaction is of paramount importance, such as liquid phase sensors, actuators, and surface modifiers.

Artificial photosynthesis is a promising route for efficient conversion of solar energy to chemical fuels. To positively affect the status quo, polycrystalline, yet defective and heterogeneous, semiconductor materials are excellent candidates for targeting high efficiency, as well as low production cost, and long lifetimes of the device. However, a typical conundrum in this field is related to the fact that efficient materials are not durable, whereas durable materials show poor efficiency. In this context, characterization of materials transformation and charge transport mechanism is critical to enable design and development of new functional systems. We have established a suite of characterization techniques, including in situ/operando characterizations to provide insights into the materials chemical transformation in artificial photosynthesis. For instance, we use conductive and electrochemical atomic force microscopy (AFM) to elucidate functional variations and structural changes at the nanoscale, and we utilize scanning transmission X-ray microscopy (STXM) to provide insights into the electronic structure and chemical composition of functional photoelectrochemical materials. Better understanding of the material behavior during operating conditions can lead to ultimate optimization of fuel cell efficiency.

Heterogeneous electrochemical processes, including photoelectrochemical water splitting to evolve hydrogen using electrocatalyst-coated semiconductors, are driven by the accumulation of charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. Conventional electrochemical techniques measure/control potentials at the conductive substrate or semiconductor ohmic contact, but are unable to isolate processes and electrochemical potentials at the surface during operation. I will present our recent work demonstrating that the nanoelectrode tip of an atomic-force-microscope cantilever can effectively sense the surface electrochemical potential of electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe₂O₃ and BiVO₄. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight.

The ammonia you use to clean and disinfect your kitchen floor starts off as nitrogen, a gas that makes up almost 80 percent of Earth’s atmosphere. But the conversion requires the breaking of a strong chemical bond in a high-heat, high-pressure industrial process known as the Haber-Bosch process. In nature, however, bacteria convert nitrogen gas to ammonia with a nitrogenase enzyme, whose active center is molybdenum, an abundant, nonprecious metal. By mimicking this biological nitrogen fixable process, my group has recently developed a series of new catalysts that can produce ammonia, rocket fuels, and power heavy-duty vehicles in a more sustainable way. The key is anchoring single metal atoms in a nitrogen/oxygen/carbon-coordinated environment to form a so-called single-atom catalyst (SAC). Unlike the Haber-Bosch process, which consumes massive amounts of energy and emits significant quantities of carbon dioxide to the atmosphere, these cheap, single-atom catalysts can be incorporated into modular and compact electrolyzing cells to produce commodities (ammonia, methane, and formate), as well as high-value-added chemicals like pharmaceuticals, with renewable solar or wind power. This emerging technology will not only make the traditional chemical production process greener but can also pave the way for a decentralized chemical industry.

In the past decades, the renewable energy storage and energy conversion systems, such as fuel cells, water electrolysis, and metal-air batteries, have attract great attention. Oxygen evolution reaction (OER) is a key half reaction of water splitting to produce clean fuels. However, the sluggish kinetics of OER has significantly limited the performance and commercialization of such energy conversion devices. Up to now, the most efficient OER catalysts are still noble metal and metal oxides of Ruthenium (Ru) and Iridium (Ir), which are not cost-effective catalysts and unstable under high potentials. Recently, single atom catalysts have been used to improve the surface-to-volume ratio to increase OER catalytic activity. In our work, Ir single-atom catalysts supported by CoO_x amorphous nanosheets (ANSs) for OER. Experimental results show that Ir single-atoms are anchored by abundant surface-absorbed O in CoO_x ANSs. The Ir single-atom catalysts possess ultrahigh mass activity that is 160-fold of commercial IrO₂. The OER of IrCoO_x ANSs reached a record-low onset overpotential of less than 30 mV. The in-situ X-ray absorption spectroscopy reveals that the Ir-O-Co pairs directly boosted the OER efficiency and enhanced the Ir stability.

The world's main source of energy is fossil fuels, but fossil fuels are finite resources and can also irreparably harm the environment. According to the United States Energy Information Administration, the burning of fossil fuels was responsible for 76 percent of the US greenhouse gas emissions. Fossil fuels not only pollute the environment, but also affect human health. My research focuses on making a catalyst that is economical, efficient and above all friendly to the environment. The oxygen reduction reaction (ORR) is an important reaction for energy conversion systems, such as fuel cells. The fuel cells generate electricity directly by electrochemically reducing oxygen and oxidizing fuel in water as the only by-product. Onion-like carbon (OLC) are used as a catalytic support for fuel cell applications due to their high conductivity and high surface-to-volume ratio. The combination of carbon compounds with conductive polymers results in new materials and devices with possible practical applications. In recent decades, several studies have been conducted on nitrogen doped structures in carbon materials for the ORR. In my research we use nanodiamonds (NDs) to dope them with the polymerization of the aniline monomer. Then, through a pyrolysis process, we convert the ND/PANI particles into N-OLC. These particles are then polymerized using the aniline and pyrrole monomers to dope the OLCs with nitrogen. These particles are characterized using the following techniques, Raman, Fourier-transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and X-ray Diffraction (XRD). Then the synthesis is carried out with the non-precious metals. The non-precious metals that we use are iron and cobalt. ORR experiments were performed with the FeCo-N-OLC particles. In future work we will characterize these samples using synchrotron techniques.

The electrochemical reduction of CO₂ using electrical energy derived from renewable sources (wind and solar) offers a potentially attractive means for producing carbon-based chemicals and fuels, particularly if the CO₂ can be recovered from the atmosphere. Accomplishment of this goal requires two types of catalysts – an anode catalyst that facilitates the oxidation of water to O₂ and either H⁺ or OH^-, depending on the pH of the electrolyte, and a cathode catalyst that facilitates the reduction of CO₂ to hydrocarbons and oxygenated products. Extensive research has found that IrO₂ is the best catalyst for the oxygen evolution reaction (OER) in acidic electrolytes and FeNiOOH is the best catalyst for this reaction in basic electrolytes. On the other hand, Cu is the only metal that promotes the electrochemical reduction of CO₂ to hydrocarbons and oxygenates with high faradaic efficiency. This talk will focus on experimental techniques that have been used to characterize IrO₂, FeNiOOH, and Cu under working conditions, i.e., in situ. The technique that will be discussed include infrared spectroscopy, X-ray absorption spectroscopy, ambient-pressure X-ray photoelectron spectroscopy, and X-ray diffraction. We will see that use of these in situ techniques enables acquisition of critical information about the composition and structure of the working catalyst and, in some cases, the nature of adsorbed species on its surface under working conditions.

Lithium metal anode has great advantages such as extremely high theoretical specific capacity (3860 mA h g−1), low density (0.59 g cm−3) and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode) and is considered as the ideal anode for rechargeable lithium batteries. Major challenges it faces include lithium dendrite formation and incompatibility with state-of-the-art electrolyte. Solid-electrolyte-interphase (SEI) plays a key role in determining the reversibility of lithium metal anode and engineering of SEI holds great promises for addressing the previously mentioned challenges. Understanding the properties of SEI is of great importance and has been the focus of many research. In this talk, three questions related to this topic will be discussed based on the results obtained from synchrotron-based scattering experiments. 1. Is LiH really an SEI component? 2. How come an ionic insulator like LiF can be considered as a good SEI component? 3. Is it possible to quantify crystalline/amorphous components in SEI? Strengths and limitations of currently available techniques for studying SEI will also be discussed.

Lithium-sulfur (Li-S) batteries are attractive candidates for energy storage in electric vehicles and grid-scale storage due to their high energy density and low-cost potential. Sulfur, the charge product in Li-S batteries, was believed to be solid, while we recently discovered that sulfur can stay in a super-cooled state as liquid sulfur. However, how the sulfur state (liquid or solid) affects Li-S battery performance is not clear. Here we demonstrate that liquid and solid sulfur provide very different areal capacities through in situ study of electrochemical sulfur generation. We report distinct growth behaviors of sulfur on two-dimensional (2D) layered materials: on the basal plane, only liquid sulfur accumulates; at the edge sites, liquid sulfur accumulates if the thickness of the 2D materials is small, while solid sulfur nucleates if the thickness is large. Based on our understanding of the edge-induced sulfur crystallization, we control the sulfur state (liquid or solid) and demonstrate much larger areal capacities from liquid sulfur compared to solid sulfur in the same charge time period. This work correlates the sulfur states with their electrochemical performance and provides insights on electrode designs for Li-S batteries and the application of 2D materials in Li-S batteries.

Understanding the origins of failure and limited cycle life in lithium-ion batteries (LIBs) requires quantitative linking capacity-fading mechanisms to electrochemical and chemical processes. This is challenging in real systems where capacity is lost during each cycle to both active material loss and solid electrolyte interphase (SEI) evolution. In this talk, I will describe the model system-based approach that we have adopted that combines precision electrochemical measurements of the Coulombic efficiency (CE) and x-ray measurements of the SEI layer and active materials loss. By contrasting these independent quantities, we obtain insight into the SEI growth and evolution. I will discuss how we have used X-ray reflectivity (XRR) to obtain nanoscale insight into solid electrolyte interfaces (SEI) on model anode surfaces that implicate electrochemically formed LiF as playing a major functional role in the SEI. I will also describe how XRR tracks the thickness of a-Si thin films and when this is compared to the CE, we can quantify SEI growth over several cycles. The methodology we are adopting allows to quantitatively track the desirable and undesirable electrochemical processes.

Organic electrochemical transistors (OECTs) are a novel device architecture, emerging as a potential platform for biosensors and neuromorphic computing. In an OECT, volumetric doping (gating) of the active semiconductor is achieved through ingress of electrolyte ions, under potential control. This opens unique transduction modalities, due to the mixed (ionic and electronic) modes of conduction. This also challenges established paradigms for the operation of traditional, organic field-effect transistors as operation involves the dynamic swelling of the active layer by both solvent and ion. Thus, quantitative understanding of operation (and process-structure-function relationships) requires in-situ measurements of the films under potential control and in contact with electrolyte. Poly(3- (methoxyethoxyethoxymethyl)thiophene) (P3MEEMT) has emerged as an interesting, prototypical OECT material, differing from the classic OFET material, regioregular poly(3-hexyl)thiophene (P3HT), by the replacement of the non-polar alkyl side chains with oligoethylene glycol (OEG).[1] The OEG side chains facilitate ingress of aqueous electrolytes and introduce rich thermal processing behavior. Combining DSC with in-situ thermal GIWAXS we develop a detailed understanding of the thermal processing of P3MEEMT films. Annealing at (115 to 125)° C, above a crystal to liquid crystal transition, results in distinct semicrystalline morphology with predominantly face-on, pi-stacked domains with coherence length, lc, along the lamella (100) direction up to 30 nm. Annealing above the liquid crystal melt results in a nearly isotropic crystal orientation distribution, characterized by significantly shorter lc. The water vapor swelling of the thermally processed films is studied by both in-situ ellipsometry (total film swelling) and GIWAXS (crystalline domain swelling). The volumetric swelling varies with processing and at all times significantly exceeds the swelling of crystalline material, indicating the majority of the water is in the amorphous regions. Volumetric swelling does not exhibit simple Flory-Rehner behavior with a constant χ. Using a novel ‘rolling drop’ electrode, we are able to characterize the electrochemical doping of the films, as a function of both initial thermal processing and of final water content, from contact with liquid water to equilibration with room relative humidity. The crystalline material exhibits changes similar to those observed for vapor phase doping of P3HT[2]: a small expansion of the (100) lamella separation and a contraction of the π-π separation. The (100) expansion and π-π collapse occur at potentials consistent with the transistor threshold, at doping densities of order 4 10²⁰ cm^-3.
[1] L.Q. Flagg, et al. J Am Chem Soc 2019, doi: 10.1021/jacs.8b12640
[2] E.M. Thomas, et al. Adv Func Mat 2018, doi: 10.1002/adfm.201803687

Electrochemistry involves the redox of solids and molecules. In many solids, like electrodes for batteries and electrocatalysis, bulk redox processes occur due to the (de)insertion of ions like lithium and proton. These processes are usually heterogeneous and is controlled by local reaction and/or diffusion kinetics. In this talk, I will present characterization methods to measure nanoscale redox processes and correlate local composition to kinetics using LiFePO4 and Co(OH)2 as model redox-active solids.

For electrochemical systems such as batteries and fuel cells, the liquid/solid interfaces are critical parts where many important reactions take place. It is critical to understand the interfacial changes for the better design of efficient energy systems. In the past years we have used various operando synchrotron-based X-ray techniques to study the atomic and electronic structure, chemistry and compositions of numerous electrochemical interfaces fuel cells, electrolyzers, lithium- and magnesium-batteries. In my talk, I will mainly focus on two examples. One is our efforts on using operandoX-ray absorption spectroscopy (XAS) to study catalyst restructuring in many electrochemical reactions such as oxygen evolution reaction for water splitting and electrochemical CO₂ reduction. The second example will be focus on our recent works on aqueous sodium-ion batteries using the combination of operando X-ray diffraction and XAS.

Marine antifouling coatings represent an environmentally benign solution to the adhesion of marine organisms that markedly reduces fuel efficiency on ships. The success of these coatings is determined by molecular-scale interactions that take place between the polymer surface, water, and marine organisms. Because polymer surfaces restructure in response to the surrounding environment, in situ characterization is crucial for providing an accurate understanding of the surface chemistry in ambient conditions. To evaluate the surface chemistry of membrane-relevant polymers and their interactions with interfacial water (i.e. water sorption), we present synchrotron Ambient Pressure XPS (APXPS) studies performed on polymer surfaces in contact with water vapor. Amphiphilic side chains incorporated into these polymers showed surface-directing capabilities in conditions ranging from vacuum to the liquid interface present at 20 Torr water vapor, with side chains contributing more to the surface signal than predicted from the theoretical bulk composition. Depth probing in conditions up to 800 mTorr also indicates that these side chains are saturated at the top surface. Furthermore, substantial water sorption at 20 Torr water vapor suggests a surface with a less well-defined water–polymer interface that may aid the coating’s performance as an antifouling material. These findings may be useful in designing and characterizing coatings that leverage favorable interactions with water for marine antifouling applications.

Water interactions with polymer surfaces play an important role in nearly all aspects of life including cellular functions, electrochemistry and water purification. Yet the precise understanding and quantification of such interactions at a molecular level is still incomplete due to difficulty of operating many surface specific techniques under in situ conditions. To fill this gap, we use ambient pressure Tender X-ray Photoelectron Spectroscopy (APXPS [1-4]). Tender-APXPS combines the chemical specificity, high surface sensitivity and quantitative analysis of the surface composition of traditional XPS and allow studies at pressures up to 20 Torr. In this study, interaction of water vapor with model styrenic polymer thin film surfaces were investigated in situ from UHV up to 100% relative humidity (RH) with APXPS to understand the effect of functional groups, interaction types and counter ions. Our results suggest that the interaction of water with polymer surfaces is mediated by polar and charged functional groups. Additionally, we show that water sorption on polyelectrolytes is highly dependent on the counterion and the fraction of dissociated ionic groups.[5] This talk will also discuss the counterion specific potential developments on the polyelectrolyte/solution interface using the facile advantage of XPS to carry information on local potentials. We believe that these findings will provide direct insight into the critical role of side-chain and counterion chemistry in polymer-water interactions while also demonstrating the potential of APXPS with elemental and potential sensitivity to give valuable information to guide the design and control of future membrane-relevant materials for water and energy applications.

References:
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Silicon is a promising candidate for an anode material of next-generation lithium batteries because of its high abundance, negative redox potential (0.35 V vs Li+/Li), and potentially high capacity density (4200 mAh g−1). However, the details of the lithiation/delithiation reactions of the Si electrode in all-solid-state batteries have remained unclear. Transferring such air-sensitive samples from the battery testing environment to the measurement apparatus may significantly change the structure and chemical state of samples due to undesired side reactions. Furthermore, post-processing ex situ approaches using multiple samples may result in misinterpretation due to the variation of samples. Thus, in situ reaction analysis is essential to clarify the mechanism of lithiation/delithiation processes.
X-ray photoelectron spectroscopy (XPS) enables to determine the elemental composition, oxidation states, and electronic structure of sample surfaces in a vacuum chamber. The reaction process can be tracked in a stepwise manner without any influence of variation and inhomogeneity of samples if XPS can be applied to a certain position of the same battery sample under bias application conditions. Such in situ XPS allows us to assign each spectral feature properly to reaction products and to correlate the effect of reaction products on the reversibility of charge/discharge reactions.
Here, we have developed an in situ XPS apparatus equipped with a bias application system and applied it to the electrochemical lithiation/delithiation reactions of an amorphous Si electrode sputter-deposited on a Li6.6La3Zr1.6Ta0.4O12 (LLZT). Upon the first lithiation, a broad Li peak appears at the Si surface, and peaks corresponding to bulk Si and Si suboxide significantly shift to lower binding energy, showing the formation of lithium-silicide and lithium-silicates due to the lithiation of Si and native oxide. Quantitative analysis of electrochemical response and photoelectron spectra determines the composition of lithium-silicide to be Li3.44Si. Peak fitting of the broad Li peak shows the formation of Li2O and Li2CO3 due to side reactions. After the delithiation, the peak corresponding to Li3.44Si phase shifts to higher binding energy to form Li0.15Si phase, while lithium-silicates, Li2O, and Li2CO3 remains as irreversible species. Thus, electrochemical reactions accompanied with lithiation/delithiation processes are successfully observed.
Reference
R. Endo, T. Ohnishi, K. Takada, and T. Masuda, “In Situ Observation of Lithiation and Delithiation Reactions of a Silicon Thin Film Electrode for All-Solid-State Lithium-Ion Batteries by X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 2020, 11, 6649−6654.