In contrast to conventional PEMFC type of gas-fed fuel cells the researchers from Daihatsu Motor Co. have introduced the idea of anion-exchange membrane fuel cell with liquid fuels.^[1-3] Switching from acidic proton exchange to alkaline, anion-exchange membranes has many benefits including: fast kinetic towards fuel oxidation and oxygen reduction reaction (ORR), and possible use of less costly non-platinum group metal (non-PRM) catalysts as anode and cathode material for both sides of the membrane electrode assembly (MEA). The liquid fuel of choice was hydrazine hydrate, which has no carbon atoms and thus will not contribute to increased CO₂ levels. The theoretical electromotive force of such direct hydrazine fuel cell is 1.56V and hydrazine hydrate as a fuel can be oxidized by number of cheap catalysts.^[4] The use of alkaline media allows switching to completely non-PGM materials set not only from anode side (oxidation of hydrazine hydrate), but also from cathode (oxygen reduction reaction). The high activity of cathode catalysts was confirmed in both RDE and MEA tests (Figure 1). As synthesized materials were extensively studied by XPS, SEM, TEM, BET and other methods in order to elucidate the structure-to-properties correlations. This paper describes a new class of templated, self-supported Non-PGM catalysts deroved by sacrificial support method (SSM) and their activity in free alkaline electrolyte (KOH) and in anion-exchange MEA. It will address the issues of alkaline ionomer integration with membrane and he catalyst material, the sensitivity of these ORR catalysts to fuel (hydrazine) crossover, and their successful integration with Non-PGM Ni-Zn based anode catalyst in a functional automotive fuel cell stack to power an electric service vehicle demonstrated by Daihatsu Corp.^[5]
1. Sakamoto, T., Deevanhxay, P., Asazawa, K., Tsushima, S., Hirai, S., Tanaka, H. J. Power Sources, 2014, 252, 35-42.
2. Sakamoto, T., Asazawa, K., Sanabria-Chinchilla, J., Martinez, U., Halevi, B., Atanassov, P., Strasser, P., Tanaka, H. J. Power Sources, 2014, 247, 605-611.
3. Sakamoto, T., Asazawa, K., Martinez, U., Halevi, Suziki, T., Arai, S., Matsumura, D., Nishihata, Y., B., Atanassov, P., Tanaka, H. J. of Power Sources, 2013, 234, 252-259.605-611.
4. Serov, A., Kwak, C., Appl. Cat. B: Environmental, 2010, 98, 1-9.
5. Serov, A., Padilla, M., Roy, A.J., Atanassov, P., Sakamoto, T., Asazawa, K., and Tanaka, H., AngewandteChemie Intern. Ed., 2014, 53, 10336 -10339

Platinum is the main materials component of polymer electrolyte fuel cells. The amount of Pt required to achieve stringent performance targets of these cells is still too high. High Pt loadings are needed in particular on the cathode side, where the oxygen reduction reaction (ORR) incurs a major portion of irreversible voltage losses. Recent theoretical and experimental studies show consistently that effectiveness factors of Pt utilization in the present generation of fuel cell catalyst layers are extremely low, in the range of 3%. Efforts in catalyst and electrode design, which aim to reduce the Pt loading, clash with stability requirements; a drastically reduced Pt loading is often accompanied by markedly enhanced degradation rates. The presentation will provide a systematic account of factors that determine ORR activity and stability of Pt in fuel cell catalyst layers. Insights deduced from catalyst layer modeling will be used to rationalize different reaction conditions in the two main catalyst layer designs, viz. typical gas diffusion electrodes and flooded nanoporous electrodes. These reaction conditions have a critical impact on oxide formation at Pt, surface charging state, ORR kinetics and Pt dissolution rate. The fundamental function in this context is the dependence of the total charge at the metal surface, as “seen” by ions in solution, on metal phase potential. Recent efforts in theory and modeling that aim at systematically uncovering this relation will be presented.

Growing global interest in electric vehicles (EV) in recent years has driven the need for smaller and lighter rechargeable batteries. A rechargeable Zn-air battery is considered as one of the potential candidates for next generation secondary batteries due to its high theoretical energy density. However, at the current state of technology, its further application and commercialization have been limited due to insufficient kinetics for the air electrode reactions, which are oxygen reduction and oxygen evolution (ORR/OER) upon discharging and charging, respectively. Platinum-based materials are known to be the most active bifunctional catalysts in both acidic and alkaline conditions. However, large scale commercial application of Pt has been precluded by its high cost and scarcity. Therefore, increasing efforts have been devoted to developing an efficient, durable and non-precious metal-based bifunctional catalyst to replace Pt-based materials. In this regard, various materials such as transition metal oxides may be applicable as alternative, low cost bifunctional catalysts.
Recently, Mn and Co mixed oxides and spinels have been widely investigated as promising bifunctional catalysts in alkaline media due to their low cost, high abundance, low toxicity, multiple valence states and high catalytic activity. In this study, we report on the synthesis of nanocrystalline Mn-Co oxide/PEDOT catalysts using facile and rapid sequential anodic electrodeposition on glassy carbon electrodes. Free standing Mn-Co oxide rods (10 µm long) are first synthesized and then coated by electro-polymerization of a conducting polymer (PDEOT). The Mn-Co oxide/PEDOT electrodes consist of MnO₂, with partial substitution of Co²⁺ and Co³⁺ ions for Mn⁴⁺ ions. The amorphous PEDOT coating is added to compensate for the poor conductivity of Mn-Co oxide. Structural characterization of as-deposited and cycled electrodes is conducted using XPS, SEM and TEM. The electrocatalytic properties of Mn-Co oxide/PEDOT is studied using rotating disk electrode (RDE) techniques in 1 M KOH electrolytes and Koutecky-Levich modeling. Mn-Co oxide/PEDOT electrode has comparable bifunctional activity towards ORR/OER relative to Pt/C, as a result of abundant defects and high surface area.

Non-aqueous lithium oxygen (Li-O₂) batteries are very attractive energy stores due to their exceptionally high theoretical energy densities. However, one of the major drawbacks of Li-O₂ batteries are the high charging overvoltages (up to 1.5 V) arising from the low electric conductivity of the insoluble discharge product lithium peroxide (Li₂O₂). Redox mediators have been recently proposed as soluble catalysts enabling a significant reduction of the charging overpotentials [1,2]. Here, we show that TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), homogeneously dissolved in the electrolyte, can act as a mobile redox mediator (RM) during the charging process which enables the oxidation of Li₂O₂ particles even without a direct electric contact to the positive electrode [3]. The incorporation of only 10 mM TEMPO provides a distinct reduction of the charging potentials by 500 mV. In addition TEMPO enables a significant enhancement of the cycling stability leading to a doubling of the cycle numbers compared to a regular Li-O₂ cell.
We present systematic results on the electrochemical performance and the catalytic mechanism of dissolved TEMPO in Li-O₂ cells. This includes a detailed investigation on the relevant chemical and physical properties of TEMPO such as the electrochemical stability of TEMPO (using cyclic voltammetry), the diffusion coefficient of TEMPO in diglyme (by chronoamperometry) and the TEMPO dependent oxygen solubility in diglyme (based on a monitoring of the oxygen pressure). Moreover we studied the discharge reactions in a Li-O₂ battery with TEMPO by means of XRD, Raman spectroscopy, ¹H-NMR spectroscopy and SEM. The catalytic influence of TEMPO on the charging reaction was systematically investigated by varying current density and cathode material. Since the charging plateau merely corresponds to the electrochemical oxidation of the mediator at the cathode surface, a parallel monitoring of the cell pressure and differential electrochemical mass spectrometry (DEMS) were applied to proof the subsequent oxidation of Li₂O₂ to O₂. Herein, the predominant formation of O₂ was confirmed for the charging plateau in the presence of TEMPO. In addition, we present a systematic study on the charging mechanism of Li-O₂ cells with redox mediators combining electrochemical methods with a parallel pressure monitoring. As an important result, we show that the redox mediator can form a parasitic shuttle mechanism between both electrodes, which is of crucial importance for the performance of homogeneously catalyzed Li-O₂ cell. In conclusion, we demonstrate that TEMPO is a stable and effective redox mediator for Li-O₂ cells.
[1] Chen et al., Nat. Chem. 2013, 5, 489
[2] Lim et al., Angew. Chem. Int. Ed.2014, 53, 4007
[3] Bergner et al., J. Am. Chem. Soc. 2014, 136, 15054

The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are key chemical transformations ubiquitous in a number of renewable energy technologies including fuel cells, unitized regenerative fuel cells (URFCs), and water electrolyzers. Critical to these technologies is the development of catalysts that can effectively drive these reactions in a stable manner while minimizing or eliminating precious metals. This paper will focus on several aspects of materials and device development for these technologies. First, manganese oxide based materials will be discussed as bi-functional, non-precious metal catalysts for the ORR and the OER in alkaline media. We will then describe their integration into fuel cell and URFC devices that employ alkaline anion exchange membranes (AEMs). These are technologies that could potentially couple to renewable, intermittent electricity such as wind and solar to provide grid-scale storage. Finally, we will discuss our catalyst development efforts for the ORR in an acidic environment, namely our approach to tailor Pt-based core-shell nanoparticles with greater mass activity and stability than commercial standard Pt/C nanomaterials.

Improving oxygen electrochemistry is a ‘holy grail&’ problem with major impacts across many technologies. Here we present discovery of a catalyst system offering reversible oxygen electrochemistry. The catalyst delivers chemically and electrochemically reversible two-electron ORR and OER. We will present electrochemical and other experimental evidence supporting this claim.

The High Throughput Experimentation (HTE) project of the Joint Center for Artificial Photosynthesis performs accelerated discovery of new earth-abundant photoabsorbers and electrocatalysts. We will describe several new screening instruments for high throughput (photo-)electrochemical measurements and summarize the discovery pipelines. This approach will be illustrated using the high throughput discovery, follow-on verification, and device implementation of a new quaternary metal oxide catalyst. Discovering improved electrocatalysts for the oxygen evolution reaction (OER) is of great importance for efficient solar fuels generation, regenerative fuel cells, and recharging metal air batteries. We report a new Ce-rich family of active catalysts composed of earth abundant elements, which was discovered using high-throughput methods to produce 5456 discrete compositions in the (Ni-Fe-Co-Ce)Ox composition space. The activity and stability of this new OER catalyst was verified by re-synthesis and extensive electrochemical testing of samples in a standard format in 1.0 M NaOH. Characterization of selected compositions by XRD, XPS, SEM, TEM, EDS, XRF mapping, and EXAFS both as-synthesized and after electrochemical testing, reveal the importance of nanostructure to the observed electrochemical performance. The discovery of additional electrocatalysts by expansion of the composition space investigated and of new composition spaces tested for OER activity and stability under acidic conditions will be reported.

Non-noble metal electrocatalysts (NNMEs) with a high performance are urgently needed to rival Pt-based electrocatalysts for fuel cells and metal-air batteries. Exploration of unique synthetic approaches is the key to the creation of highly efficient NNMEs yet remains rare. Herein, we report an unprecedented synthetic route that combines ionic self-assembly (ISA) of iron (III) porphyrin (FeP) with traditional pyrolysis, leading to flower-like electrocatalysts. Remarkably, the original morphology of nanoflowers of self-assembled FeP was nearly retained after carbonization, enabling highly desired manipulation over the size, shape and surface area closely related to the performance of NNMEs. The obtained flower-like electrocatalysts exhibited a high oxygen reduction reaction (ORR) activity, excellent durability and methanol tolerance in both alkaline and acidic solution. This approach opens up a new avenue to produce high performance NNMEs with controllable structural features that are difficult or impossible to be achieved prior to this study.

Oxygen electrodes are playing a key role in electrochemical energy conversion devices such as fuel cells and water electrolyzers. In both acidic and alkaline environment, both the oxygen reduction and oxygen evolution reaction (ORR and OER), respectively, are limiting the overall energy/voltage efficiency due to its sluggish kinetics. [1, 2]
Whereas in acidic environment, mainly precious metals are used to catalyze the ORR (e.g., Pt or its alloys) or the OER (e.g., IrO₂ ), the variety of possible catalysts in alkaline electrolyte is significantly increased and also many metal oxide based systems can be employed. Generally the oxygen reduction or evolution mechanisms are only partly understood independent of the electrolyte environment and material used.
In this contribution, some new light will be shed on the electrocatalysis of oxide based systems used in acidic and alkaline environment for the ORR and OER. Similarities and differences as compared to noble metal based systems will be provided supplemented with theoretical considerations.
References
[1] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal., 2012, 2 (5), 864-890
[2] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Cat. Sci. Tech., 2014, 4, 3800-3821

Electrochemical water splitting driven by a renewable energy source such as wind or solar, is a promising method to produce hydrogen. Already an important chemical commodity in industry, hydrogen may also potentially be used as a carbon-free fuel. However, the efficiency of electrochemical water splitting is severely limited by the high overpotential costs required for the oxygen evolution half reaction (OER). In order to develop active nonprecious metal-based electrocatalysts for the OER, a better understanding of the activity of transition metal catalysts is needed. Previous studies have shown that depositing transition metal catalysts on metal supports leads to formation of different oxide phases and significantly higher activities.^{1, 2}
In this work we investigate the interaction between manganese oxide (MnO_x) and noble metals to determine the effect on oxidation state and catalytic activity. Beginning with a study on the interaction of MnO_x with gold (Au), we characterize the catalyst using SEM and ex situ L-edge x-ray absorption spectroscopy (XAS) to determine the morphology and oxidation state.³ As a surface sensitive technique, ex situ XAS provides information on the oxidation state of the surface atoms of the MnO_x nanoparticles both alone and in the presence of Au. Electrochemical characterization of this system shows that adding Au to MnO_x greatly increases the activity for the OER and results in an order of magnitude higher turnover frequency compared to MnO_x without Au. Finally, in situ XAS studies provide insight into the oxidation state of the MnO_x both with and without Au under OER operating conditions. Expanding this study to characterize the interaction of MnO_x with other noble metals provides insight to trends with respect to the effect on OER activity.
REFERENCES:
1. Yeo, B.S. & Bell, A.T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J Am Chem Soc133, 5587-5593 (2011).
2. Yeo, B.S. & Bell, A.T. In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen. The Journal of Physical Chemistry C116, 8394-8400 (2012).
3. Gorlin, Y. et al. Understanding Interactions between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation. J Am Chem Soc136 (2014).

The oxygen evolution reaction (OER) is regarded as a major bottleneck in the overall water splitting process due to the slow transfer rate of four electrons and the high activation energy barrier for O-O bond formation. For the decades, Ir, Ru and Pt based inorganic materials have presented efficient catalytic activity with high turnover frequency (TOF) under mild conditions. However, its scarcity and high cost still inspire to develop inexpensive and sustainable catalysts. The cubane Mn₄CaO₅ clusters inside biological photosystem II (PS II) can oxidize water using a much smaller overpotential than required by manmade catalysts. Inspired by this attractive feature of Mn₄CaO₅ cluster, intense effort has been dedicated to develop non-precious metal-based water oxidation catalysts. Unfortunately, except for several rare-earth metal-based catalysts, there have been few candidate catalysts that operate under near neutral condition until now.
In this study, we discovered a new Mn-based catalyst that efficiently performs water oxidation catalysis. For conventional manganese oxide catalysts, up to now, the significantly reduced activity under neutral conditions is an unresolved issues. The instability of Mn (III) species at neutral pH is considered as a critical factor in this degradation. Here, we made the breakthrough of a Mn-based water oxidation catalysis that operates under neutral conditions using well-designed monodisperse manganese oxide nanoparticles. Facile surface treatment method was newly adopted and oxidized Mn(III) species were intentionally generated and stabilized on the manganese oxide surface. Various electrochemical methods and in-situ XAS analysis revealed the superior catalytic performance of partially oxidized manganese oxide NCs.

Complex perovskite oxides are considered as the promising candidates of bifunctional catalysts for high catalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). As another class of oxide-based catalyst, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF5582) is a widely known perovskite oxide as a strong candidate of catalyst in solid oxide fuel cell (SOFC) cathode materials, metal-air batteries and other energy related systems. We report herein the heat treating effect of the BSCF5582 in oxygen atmosphere at 950 °C as a function of annealing time on the electrocatalytic performance, compared with pristine BSCF5582. The pristine BSCF5582 sample was calcined at 1050 °C for 5h in air (BSCF5582), and the heated BSCF5582 sample was prepared by heat-heating the pristine BSCF5582 in oxygen atmosphere at 950°C as a function of annealing time (O₂-BSCF5582). The heat treatment effect of the complex Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF5582) perovskite in oxygen atmosphere at 950°C (O₂-BSCF5582) on the electrocatalytic performances of ORR and OER was investigated. During heat-treatment in oxygen atmosphere, the crystallinity of the overall cubic perovskite structure was enhanced, and, most of all, a nanoscale thick surface layer, which is located between ~30 nm thick amorphous surface layer and particle matrix, was not any more observed. The electrocatalytic performance of BSCF5582 catalyst both in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was improved significantly. The mechanism of such microstructural- and electrochemical- improvement was explained by correlating with the effect of monitoring the surface chemistry and structure in this paper.

Perovskite (ABO₃) based mixed metal oxides were widely studied over last several decades as electrocatalysts for high-temperature fuel cell electrodes due to their mixed electronic/ionic conductivities at high temperatures (>700^oC). The perovskites formed of lanthanide (at A-site)-transition element (on B-site) metal complex have been recognized as efficient electrocatalysts for Oxygen Reduction Reactions (ORR) and/or Oxygen Evolution Reactions (ORR) in alkaline electrolyte. However, the electrochemical performance and stability of these catalysts depends on the choice of the type of A- and B-cations. Additionally, oxygen deficiencies created in mixed perovskite-phase (AA&’BB&’O_3-δ) by partial substitution of less valent cations on A- and/or B-site are believed to contribute significantly towards ORR/OER due to their oxygen-exchange redox behavior. In this regard, mixed perovskites formed by praseodymium (Pr) and samarium (Sm) based nickel-cobaltites such as PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ were considered as electrocatalysts for ORR and OER.
Praseodymium and samarium based mixed-perovskites with x= 0.1, 0.5, 0.9 have been synthesized using a modified nitrate-glycine Pechini method and heat-treated in air at 900°C and 1200°C. X-ray diffraction studies of the resultant materials show that a combination of mixed metal oxide structures such as perovskite, layered Ruddleston-Popper (A₂BO_4-δ) and spinel (ABO₃) depend on heat-treatment temperatures and relative composition of Ni and Co. For example, spinel and individual metal oxides were yielded at x=0.1, whereas a combination of perovskite, layered Ruddleston-Popper phases were developed at x=05 and 0.9. A combination of Temperature Programmed Reduction (TPR) and Thermo-Gravimetric (TG) analysis was further employed to understand the structural changes and accurate oxygen content in these materials as a function of the heat-treatment temperature. Scanning Electron Microscopy (SEM) and BET single point specific surface area (SSA) were used to determine morphology of the materials. In order to improve electronic conductivity, the perovskite-graphene composites were synthesized by dispersing metal oxides onto graphene platelets (90 wt. %). Electrochemical performance of these mixed-perovskite/graphene composites towards ORR and OER were studied in three-electrode configuration in alkaline medium. The structure dependent electrochemical behavior of these composites as a function of relative composition of Ni_x-Co_1-x (x= 0.1, 0.5 and 0.9) for the applications of alkaline fuel cells and metal-air battery cathode materials will be presented.

Hydrogen generation via electrolysis is rapidly gaining international interest for energy storage applications due to the ability of this technology to cross-link different infrastructures such as the electrical grid, transportation, and chemical processing. For example, generating hydrogen from peak wind power which would otherwise be a stranded resource can reduce the cost of hydrogen for fuel cell vehicles, in a completely green alternative to reformation of natural gas. Electrolysis based on ion exchange membranes offers several advantages vs. traditional liquid electrolyte systems that use concentrated potassium hydroxide. Proton exchange membrane (PEM)-based electrolysis technology is commercially mature, and can provide higher turndown capability, lack of corrosive electrolyte, and system simplicity and ease of maintenance. PEM electrolysis is already cost competitive on an equal output capacity basis vs. other sources of hydrogen for industrial applications, but overall lifecycle cost needs to be reduced for energy markets.
The oxygen evolution reaction is a key efficiency loss in electrolyzers, typically contributing over 300 mV of overpotential in proton exchange membrane systems. In addition, the catalyst loading is very high, due to the lack of stability of most catalyst supports in acidic environments at electrolysis potentials. While catalyst cost is not currently a key driver in the overall system cost, as other costs are decreased through system scale up and improvement in other processes, improving catalyst utilization needs to be improved in parallel to meet overall cost targets. Therefore, research must focus not only on composition of the catalyst but also electrode structure and application method. Proton has shown that catalyst composition, process conditions, and electrode formulation can all improve performance vs. current commercial baselines. Catalyst loadings also have the potential to be significantly decreased without loss in performance.
Anion exchange membrane (AEM) technology is still immature, but could enable much less expensive materials of construction in the cell stack, such as the bipolar plate and catalyst. Recent work in AEMs has improved durability and provided promise for eventual systems based on this technology as well. In alkaline environments, there should be a substantially larger choice of stable and high activity catalysts for the oxygen evolution reaction. Catalysts based on nickel and other first row transition elements for oxygen evolution in alkaline solution are well known. However, translation from solution data to membrane electrode assemblies with solid electrolyte has been challenging. This talk will describe advancements and potential directions in these areas for electrolysis based on both PEM and AEM systems. Progress and next steps needed in this area of study will also be described.

Nanoparticulate metal-oxide catalysts are among the most prevalent systems for alkaline water oxidation. However, comparisons of the electrochemical performance of these materials has been challenging due to the numerous methods of attachment and catalyst loadings used in the literature. Herein, we have identified a conventional drop-casting method that ensures adhesion of the catalysts and consistently applied to a variety of metal oxides for water oxidation reactions in alkaline condition. The nanoparticulate materials were characterized using XRD, XPS, BET, and SEM. The activity and Tafel slopes of the nanoparticulate catalysts attached by this method have been compared directly to similar systems synthesized by different methods such as electrodeposition, sputtering, etc. A brief discussion of the differences and similarities in the surface area as measured by BET and determined from the double-layer capacitance of the system is also provided.

Fuel cells, including proton-exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), have recently been considered as potential power sources for both electric vehicles and portable electronics since they could potentially fulfill the requirements of high energy and power density, high efficiency and low or zero emission simultaneously. One current bottleneck for building high energy-conversion efficiency fuel cells lies on the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode side.^[3] Therefore, development of efficient electrocatalysts for ORR becomes the focal task and main challenge in fuel cells research. Traditionally, platinum (Pt) and Pt-based alloys have been intensively investigated as the most active ORR electrocatalysts. However, Pt-based electrocatalysts suffer from various limiting factors including prohibitive cost, element scarcity and the declining activity caused by methanol crossover, which highly hinder the scalable applications of fuel cells. Therefore, it is highly desirable to develop efficient ORR electrocatalysts based on non-precious metals and with high methanol-tolerance.
Mixed valence oxides involving transition metals have been considered as an important class of possible alternatives that exhibit high ORR catalytic activity in alkaline medium.^[15] In particular, cobalt-based oxides have recently attracted growing interests due to their potentially high activity and relatively easy preparation. For example, Dai et al. fabricated the Co₃O₄/N-doped graphene hybrid structure and observed an enhanced catalytic activity for ORR.^[17] Recently, the partial substitution of cobalt with low cost and environmentally benign elements such as Ni, Cu and Mn has been shown potentially advantageous, while the progress is still quite slow. Moreover, the substituted metal oxides can only deliver much lower mass activity compared with Pt-based materials in most of previous studies. Undoubtedly, it will be of great significance to develop substituted metal oxide electrocatalysts having comparable catalytic performance with Pt-based materials.
In this work, we develop a cost-effective and environmentally benign solution phase method to synthesize hybrid nanosheets by growing well crystalline NiCo₂O₄ nanoparticles on reduced graphene oxide (rGO) sheets through based on a polyol process together with a subsequent thermal annealing treatment. Remarkably, the NiCo₂O₄-rGO hybrid nanosheets exhibit very high catalytic activity for ORR in terms of high current density and low over-potential. More importantly, the NiCo₂O₄-rGO hybrid nanosheets possess very high methanol-tolerance, which is very important for practical applications. The high quality NiCo₂O₄-rGO hybrid nanosheets could find promising use as a noble-metal-free and methanol-tolerant electrocatalyst for fuel cells.

Electrocatalytic water splitting using energy from sunlight represents a promising strategy for clean, low-cost, and environmentally friendly production of H₂. Unfortunately, the oxygen evolution reaction (OER) at the anode is kinetically slow and represents the bottleneck of this process. Transition metal oxides are good candidates for the anode in electrochemical water splitting. Inspired by recent computational work on b-NiOOH, which is the active phase during the charging and discharging process in alkaline batteries, we performed density functional theory calculations with the inclusion of the Hubbard-U correction on selected surfaces of pure and Co-doped b-NiOOH to calculate the energetics of the OER. We explored different reaction mechanisms of the OER for different Co doping levels of the b-NiOOH surface and different surface unit cell sizes. Our results indicate that the most likely reaction mechanism depends on the amount of Co doping. We find that doping the b-NiOOH surface with only 25% Co decreases the overpotential from 0.28 to 0.18 V. We also find that the theoretical overpotential, and which step is the potential limiting step, depends on the size of the surface unit cell selected in the calculations. This work highlights how optimizing the binding energies of the various intermediates (O, OH and H₂O) on the Ni and Co surface sites, may be key to reducing the overpotential.

The oxygen evolution reaction (OER), known for its slow kinetics, is a limiting reaction in several clean energy technologies including rechargeable metal-air batteries, electrolysis cells and solar fuel production.¹ For this reason, considerable effort has been devoted to identify efficient, stable catalysts with earth-abundant first-row transition metals, especially Ni, Co, Fe and Mn.^{2, 3} Fe is particularly well-known for its incorporation as a dopant/impurity into nickel-based OER catalysts while the OER activity of Fe itself is not well understood. ^{4, 5}
Our previous studies show that trace Fe impurities in KOH electrolyte dramatically increase the activity of Ni- and Co- (oxy)hydroxides as well as bare Au substrates. This indicates the importance of Fe, but the real active site in mixed Ni-Fe and Co-Fe (oxy)hydroxide catalysts remains unclear.^{4, 5} To help identify the active site and understand the role of Fe, we examine the intrinsic activity and stability of iron (oxy)hydroxide. By using a quartz crystal microbalance to measure the electrode mass in situ during catalysis, we find that the OER activity of iron (oxy)hydroxide is strongly dependent on film thickness with thinner films exhibiting substantially higher per metal activities than thick films, in some cases approaching those of the Ni-Fe (oxy)hydroxides. This is likely due to the poor conductivity of iron (oxy)hydroxide, with the increase of film thickness, the total resistance increases, resulting in potential drop in transferring electrons to each catalyst site. This hypothesis is confirmed by using an interdigitated array electrode to measure the conductivity in situ. We find the conductivity for FeOOH to be strongly potential dependent and negligible (< 10^-7 S cm^-1) until ~ 400 mV overpotential. We also report the activity of FeOOH on different substrates in order to understand how the substrate affects activity⁶ in ultra thin films that are not substantially affected by conductivity limitations.
1. N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A., 2006, 103, 15729-15735.
2. L. Trotochaud, J. K. Ranney, K. N. Williams and S. W. Boettcher, J. Am. Chem. Soc., 2012, 134, 17253-17261.
3. R. Subbaraman, D. Tripkovic, K.-C. Chang, D. Strmcni