Tungsten Carbide-Based Catalysts for Oxygen Evolution Reaction in Alkaline Media

The material design of highly active catalysts for energy storage applications (e.g., metal-air batteries and fuel cells) is crucial for solving global energy problems. The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is used for charging and discharging rechargeable metal-air batteries, respectively. Although precious metal-based catalysts facilitate these reactions, the kinetics of the OER and ORR are rather sluggish due to their multistep electron transfer. It is therefore important to explore the mechanism of the OER and ORR to discover the key factors for highly active catalysts. As demonstrated by Suntivich et al. (2011)[1],[2], when the number of electrons in the eg orbital is close to unity for transition metals, perovskite oxides exhibit maximum OER and ORR activities. In other words, Mn3+ (t2g 3 eg 1 for both surface and bulk), Co3+ (t2g 5 eg 1 for surface), and Ni3+ (t2g 6 eg 1 for both surface and bulk) become OER and ORR active sites for Mn3+, Co3+ and Ni3+ based compounds . However, LaMnO3 exhibits a significantly lower specific OER activity[3] compared with LaCoO3 and LaNiO3 (~6% of LaNiO3 at 1.8 V vs. RHE). Also, Mn3O4 exhibits a low specific OER activity[4] (40% of Mn2O3 at 1.8 V vs. RHE) similar to LaMnO3. It is therefore important to explore what causes degradation of the OER activity of Mn3+-based (t2g 3 eg 1) compounds to improve their catalytic activity. We attempt to improve the performance of Mn3+-based oxides by controlling the degradation factors of their OER activities. To directly compare the OER activities of Mn3+-based oxides containing more than two cation sites, Mn3+ concentrations at the octahedral site and their initial crystal structures were maintained. Mn3-x Co x O4 (0 ≤ x < 1), a series of tetragonally distorted spinel compounds, satisfies this condition, as their octahedral sites remain occupied by only Mn3+ ions . For Mn3+-based oxides with a single cation site, Mn3+ was directly substituted by Co3+.We systematically studied the OER and ORR performances of Mn3+-based oxides by Co3+-substitution. Nanoparticles were prepared for the evaluation of catalytic activity to minimize the influence of geometric factors. Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (ALS Co., Ltd, Japan). In addition, a Pt wire counter electrode, and an Hg/HgO reference electrode (International Chemistry Co., Ltd., Japan) filled with 0.10 M KOH (Nacalai Tesque, Inc., Japan) were used. Electrochemical measurements were conducted with O2 saturation (30 min bubbling O2 gas through the solution) at ~25 °C, where the equilibrium potential of the O2/H2O redox couple was fixed at 0.304 V vs. Hg/HgO (or 1.23 V vs. RHE). During OER current density measurements for each sample, the potential of the sample-modified GC was controlled from 0.3-0.9 V vs. Hg/HgO (1.226-1.826 V vs. RHE) at 10 mV/s. For all measurements, the OER current density was iR-corrected (R = ~43 Ω) using the measured solution resistance, and capacitance-corrected by averaging the anodic and cathodic scans to remove the influence of the current related to the formation of an electrical double layer. With an increase in Co content, signicant improvements in the OER and ORR activities were observed for Mn3+-based oxides. For example, Mn2.1Co0.9O4 exhibited a high specific OER activity (1700 % of Mn3O4 at 1.76 V vs. RHE) and a long-term stability over 100 cycles [5]. The OER activities of Mn3+-based oxides increased linearly with the suppression of the Jahn–Teller distortion. We examined whether the electronic state of Mn3+ changes in favor of enhancing the OER and ORR activities. When the Jahn-Teller distortion of Mn3+O6 octahedra is suppressed due to the increase in Co content, the splitting of the Mn3+ eg orbitals becomes smaller and the electron occupying the Mn3+ eg orbital shifts to a higher energy level. Thus, the overlap of the antibonding Mn3+ eg orbitals with the O 2p orbitals of the oxygen adsorbate becomes stronger. The OER and ORR activities of Mn3+-based oxides should therefore be enhanced due to the stronger binding of OER intermediates to the catalytic surface. We therefore conclude that the suppression of Jahn–Teller distortion enhances the OER and ORR activities of Mn3+-based oxides. Our result suggests a future application of Mn3+-based oxides as bifunctional catalysts for metal-air batteries. [1] J. Suntivich et al., Science, 2011, 334, 1383. [2] J. Suntivich et al., Nature Chemistry, 2011, 3, 546. [3] B. Han et al., Phys. Chem. Chem. Phys., 2015, 17, 22576. [4] A. Ramirez et al., J. Phys. Chem. C, 2014, 118, 14073. [5] S. Hirai et al., RSC Adv., 2016, 6, 2019.

The morphology, crystallinity, and chemical state of well-defined Ir oxide nanoscale thin-film catalysts prepared on Ti substrates at various calcination temperatures were investigated. Special emphasis was placed on the calcination temperature-dependent interaction between Ir oxide film and Ti substrate and its impact on the electrocatalytic oxygen evolution reaction (OER) activity. The Ir oxide films were characterized by scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and cyclic voltammetry. Furthermore, temperature programmed reduction was applied to study the Ir oxide species formed as a function of calcination temperature and its interaction with the Ti substrate. A previously unachieved correlation between the electrocatalytic OER activity and the nature and structural properties of the Ir oxide film was established. We find that the crystalline high temperature Ir oxide species is detrimental, whereas low temperature amorphous Ir oxy-hydroxides are highly active and efficient catalysts for the OER. Moreover, at the highest applied calcination temperature (550°C), Ti oxides, originating from the substrate, strongly affect chemical state and electrocatalytic OER activity of the Ir oxide film.

Porous NiCo2S4/FeOOH nanowire arrays with rich sulfide/hydroxide interfaces enable high OER activity

Pursuing highly active and stable Ru-based catalysts for the oxygen evolution reaction (OER) under acidic conditions is important in advancing proton exchange membrane (PEM) water electrolyzers. Unfortunately, the inadequate stability, especially under a large current density of Ru-based catalysts, still hinders its practical application. Herein, we report a La doping strategy that simultaneously enhances both OER activity and stability of RuO2 in acidic media. The introduction of La into RuO2 induces tensile strain, which effectively weakens the covalency of Ru-O bonds. This structural modification significantly inhibits Ru dissolution, thereby substantially enhancing the stability of RuO2. Meanwhile, La doping modulates the electronic structure of RuO2 and optimizes the adsorption energy of the reaction intermediates, thereby enhancing the electrocatalytic OER activity. Notably, the optimized La0.05-RuO2 electrocatalyst presents an excellent OER performance in 0.5 M H2SO4 electrolyte, which delivers a low overpotential of 190 mV at 10 mA cm-2 and sustains 150 h without obvious decay at 50 mA cm-2. More importantly, a PEM electrolyzer is constructed by using our La0.05-RuO2 as the anode catalyst, which acquires 200 h stability at 1 A cm-2, highlighting its strong potential for practical industrial applications. This work sheds new light on designing high-performance OER catalysts toward PEM electrolyzer applications.

While oxygen evolution reaction (OER) taking place at the anode during water splitting reaction, is a very promising route towards clean and renewable energy, synthesis of highly efficient OER catalysts from earth-abundant, precious metal-free resources is the most challenging aspect of the field. We have synthesized a series of nanostructured nickel selenides, NiSe, NiSe2, Ni3Se2 and Ni3Se4 as active catalytic materials for the OER in alkaline medium. While Ni has been proven to show promising catalytic activity, the significant improvement in efficiency observed in this series towards OER can also be partly due to the formed nanostructured interface which increases the exposed surface area of the catalyst. These pure nickel selenides were synthesized by hydrothermal reactions and showed low overpotential at 10 mA cm-2 current density, early onset of catalytic activity, as well as high overall current density during the OER electrocatalytic activity in 1.0 M KOH. The overpotentials at 10 mA cm-2 current density for the above series of nickel selenides are 220, 200, 240 and 250 mV, for NiSe, NiSe2, Ni­3Se2 and Ni3Se4, respectively. Additionally, we have also investigated the stability of this catalyst during long-term OER conditions under 1.0 M KOH, which confirmed that these electrocatalysts maintained their structural and compositional integrity under operational condition for extended period of time. In this presentation we will discuss about the effect of nanostructured morphology on electrocatalytic performance and will offer some insight about designing better electrocatalysts incorporating morphology control. The synthesis of nanostructured electrocatalysts, characterization techniques such as XRD, SEM, TEM and STEM, and detailed electrochemical investigations for catalytic activity will be also discussed.

Energy is at the heart of a sustainable society; with increasing focus on noncarbon (non-C) energy sources, hydrogen (H2) fuel is the center of attraction as a clean alternative. Among emerging technologies, electrocatalytic water splitting, especially the oxygen evolution reaction (OER), plays a crucial role in sustainable H2 production. In line with Sustainable Development Goals (SDGs), coordination polymers (CPs) have attracted consideration due to their flexible porosity, high surface area, redox-active metal centers, and structural stability, making them promising OER catalysts. Herein, we report a redox-active Co(II)-based coordination polymer (Co-CP), synthesized using the unsymmetrical bridging ligand (E)-N-(pyridin-4-ylmethylene)-4H-1,2,4-triazol-4-amine (pmta) and terminal carboxylate ligand 2,5-thiophenedicarboxylate (tda2-). The Co-CP exhibits electrocatalytic OER activity in an alkaline medium. To check the performance of OER activity, Ni2+ has been incorporated into the Co-CP framework, to form Ni@Co-CP composites. Optimized OER efficiency is observed for Ni-2@Co-CP, which shows stability of the crystalline framework upon nickel incorporation even under strenuous experimental conditions and exhibits an overpotential of 290 mV at 10 mA cm-2, a Tafel slope of 35 mV dec-1, and a charge transfer resistance of 6.97 Ω. The strategic incorporation of extraneous metal ions into CP frameworks offers a versatile approach for designing advanced electrocatalysts. Such materials not only show great promise for OER but also hold potential for broader energy conversion and storage applications.

The principal factor restricting large‐scale hydrogen production via water electrolysis is the development of inexpensive and efficient catalysts that attain high current densities to produces hydrogen at low potentials. In this study, a series of iron‐based phosphide catalysts (denoted as: P X ‐doped Fe/NF) were prepared via one‐step electrodeposition from a eutectic solvent to examine their oxygen evolution reaction (OER) or hydrogen evolution reaction (HER) performance. Notably, P 30 ‐doped Fe/NF forms a porous interconnected nanosheet structure, providing a large active surface areas, abundant active sites and low charge transfer resistance. P 30 ‐doped Fe/NF attained the lowest OER (284.15 mV) and HER (158.17 mV) overpotentials at 10 mA cm −2 in 1.0 M KOH of all the tested catalysts. When used in overall water splitting, the catalyst displayed outstanding adaptability to a range of potentials and current densities. This study provides a facile route for the synthesis of bifunctional transition‐metal phosphide catalysts for the electrolysis of water.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the fundamental processes occurring at the air electrode of metal-air batteries and reversible fuel cells. [1,2] However, the performance of those devices is hindered by the sluggish kinetics of both OER and ORR, and by the low stability and high cost of the catalysts. Here, we will present our recent studies on the one-dimensional (1D) La, Ni-based metal oxide [3] bifunctional catalysts which are characterized by excellent OER and ORR electrocatalytic activity and outstanding stability.The catalysts were prepared by hydrothermal method and, by controlling the KOH concentration, 1D hydroxide precursors with different aspect ratios were synthesized. By using graphene (G) as the supporting material, a variety of La, Ni-based metal oxides such as Ni+La2O3/G, LaNiO3, and La2NiO4+NiO/G were synthesized, Figure 1. Electrochemical characterization of the catalysts showed that the aspect ratio of the materials has a significant effect on their electrocatalytic activity. Namely, Ni+La2O3/G nanorods have proven to be electrochemically active for ORR with a half-wave potential (E1/2) of 0.715 V vs RHE, and for OER with 1. 574 V vs RHE at 10.0 mA cm−2 (E10) in 0.1 M KOH. The performance of Ni+La2O3/G is close to that of commercial Pt/C (E1/2=0.830V) which is the most efficient catalyst of ORR, and significantly higher than commercial RuO2 (E10=1.633 V) and IrO2 (E10=1.663 V) for OER. The unique structural, morphological and electrocatalytic properties of the Ni+La2O3/G catalysts will be presented and discussed. 1. Sui, S.; Wang, X.; Zhou, X.; Su, Y.; Riffat, S.; Liu, C.-j., A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells. Journal of Materials Chemistry A 2017, 5 (5), 1808-1825.2. Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J., Rapid room-temperature synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts. Nature chemistry 2011, 3 (1), 79.3. Singh, Sarika, Daria Zubenko, and Brian A. Rosen. "Influence of LaNiO3 shape on its solid-phase crystallization into coke-free reforming catalysts." ACS Catalysis 2016, 7 4199-4205. Figure 1

Lithium-air batteries have become attractive due to their extremely high theoretical energy density. However, conventional lithium-air cells operating with nonaqueous electrolytes suffer from poor cycle life due to the clogging of the porous air cathodes by insoluble discharge products, contamination of the organic electrolyte by moist air, and decomposition of the electrolyte during cycling. Attempts to overcome these problems by developing compatible electrolyte/catalyst combinations have invariably been disappointing. These difficulties could be overcome by adopting a new cell configuration consisting of a lithium metal anode in the conventional organic electrolyte separated by a lithium-ion conducting solid electrolyte from the air electrode in an aqueous catholyte. Accordingly, this presentation will focus on the development of catholytes that can avoid corrosion of the solid electrolyte as well as efficient, inexpensive electroctalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).A buffer catholyte solution with a moderate pH, consisting of phosphoric acid and a supporting salt, has been developed to keep the solid electrolyte stable and reduce the internal resistance and overpotential. With a high operating voltage and the utilization of all three protons of phosphoric acid, the buffer catholyte enables a rechargeable lithium-air cell with high energy density. Further increase in power density has been realized by increasing the solid-electrolyte conductivity and the operating temperature to 40 oC.The biggest challenge with lithium-air cells is the large overpotentials associated with the ORR and OER. Nanocrystalline iridium oxide has been the best OER catalyst, but it is expensive. In this regard, the low-temperature form of lithium cobalt oxide (designated as LT-lithium cobalt oxide) that adopts a lithiated spinel structure has been developed as an inexpensive, efficient electrocatalyst for OER. Its OER activity is much higher than that of the high-temperature form of lithium cobalt oxide (designated as HT-lithium cobalt oxide), which has a layered structure and is used as a cathode in lithium-ion batteries, and even slightly higher than that of iridium oxide. Although the ORR activity of LT-lithium cobalt oxide is low, a combination of high ORR and OER activity could be realized by chemically extracting lithium from the LT-lithium cobalt oxide, demonstrating the lithium-deficient, spinel-type LT-lithium cobalt oxide as a bifunctional catalyst for rechargeable metal-air batteries. The high OER and ORR activities of spinel-type LT-lithium cobalt oxides are attributed to cubane-like subunits and a pinning of the cobalt-3d energy with the top of the oxygen-2p band. Furthermore, a novel 3-D O- and N-doped carbon nanoweb (ON-CNW) has been developed as an inexpensive, metal-free catalyst for ORR. The 3-D nanoweb structure provides an ideal backbone support for catalytically active sites due to the fast electron and mass transport properties compared to 1-D or 2-D structures. Also, the synergistic effect between the O and N groups creates highly active pyridone groups all over the nanoweb surface that significantly improves the ORR activity. With a hybrid lithium-air cell, the ON-CNW exhibits performance close to that of commercial Pt/C. In addition, a novel hybrid lithium-air cell configuration with decoupled ORR and OER electrodes has been developed. With the conventional bifunctional air cathode that supports ORR during discharge and OER during charge, the ORR catalyst and carbon support suffer from degradation under the highly oxidizing conditions during the charge process, resulting in rapid increase in overpotential upon cycling and limiting the cycle life. In contrast, the hybrid lithium-air cell with decoupled ORR and OER electrodes eliminates the degradation problems and leads to high efficiency with good cycle life.

Facile synthesis of V-doped CoP nanoparticles as bifunctional electrocatalyst for efficient water splitting

The development of highly active and stable electrocatalysts for OER is urgent due to the slow kinetics and high overpotential of oxygen evolution reaction (OER) [1]. To date, the most promising catalysts for OER are IrO2 based catalysts with ordered porous structures, where a templating method was generally used to ensure the high utilization of the catalysts. However, the synthesis route for such catalysts is generally of high technique complexation, where the templates need to be effectively removed after synthesis [2,3]. Hence, a simple template-free method that leads to the formation of highly porous IrO2 catalysts with high surface area is highly attractive. Herein, a satisfactory template-free ammoniating method was developed in which NH3 ligand was successfully kept until pyrolysis treatment, thus acting as the pore-forming agent to achieve highly porous IrO2. The specific and electrochemical surface areas of as-prepared IrO2 catalysts increased with the increasing quantity of NH3·H2O additive. Specifically, IrO2 (1:100)-450 °C catalyst synthesized with the highest ratio of NH3 ligand exhibited high specific surface area of 363.3 m2 g-1 as metal oxide. Furthermore, IrO2 (1:100)-450 °C showed remarkable electrocatalytic activity for acidic OER in the whole potential window, which can be ascribed to the high catalysts utilization. As shown in Figure 1, the over-potential for the IrO2 (1:100)-450 °C catalysts to attain current at 10 mA cm-2 was only 282 mV, which was shifted negatively by 31 mV and 46 mV in comparison to the home-made IrO2 (without NH3) and commercial IrO2 (CM) catalysts, respectively. Thus, the NH3 mediating method was found a superior method for increasing catalysts utilization, where the IrO2can be conveniently synthesized in large scale. Figure 1. CV curves (Left, the inset shows the specific surface area) and LSV curves (Right, the inset shows the TEM of IrO2 (1:100)-450°C) of IrO2 (1:100)-450°C, IrO2 (1:10)-450°C, IrO2-450°C, and IrO2(CM) catalysts. Acknowledgments This work is supported by National Basic Research Program of China (973 Program, 2012CB932800), National Natural Science Foundation of China (21433003, 21373199), the Science & Technology Research Programs of Jilin Province (20150101066JC, 20160622037JC).

Evaluation of unique catalysts of the iron group metals with activity in the OER region similar to that of scarce metals is of great importance to achieve sustainable production of H2 on a large scale. Herein, we report the unique nanosheets of nickel iron hydroxy carbonate hydrate (NiFeHCH) which were prepared through a wet-chemical route within 1 h of reaction time, acting as an efficient electrocatalyst for the oxygen evolution reaction (OER) in both alkaline and near-neutral media. The NiFeHCH was prepared with different concentrations of Fe in different ratios: 1:0.2, 1:0.4, 1:0.6, 1:0.8, and 1:1. Among them, nanosheets of NiFeHCH (1:0.2)were found to have superior OER activity, which required an overpotential of 250 mV to reach 20 mA cm-2 with a very low Tafel slope value of 39 mV/decade in 1 M KOH. Nanosheets with other ratios also had comparable OER activity with overpotentials ranging from 256 to 290 mV with Tafel slope values from 39 to 49 mV/decade. Nanosheets of NiFeHCH electrocatalysts screened for the OER in 1 M NaHCO3 (pH ∼8.5) required overpotentials for all of the ratios ranging from 389 to 507 mV at 10 mA cm-2 and Tafel slope values from 155 to 205 mV/decade, of which nanosheets of NiFeHCH (1:0.4) showed better activity by requiring an overpotential of 389 mV at 10 mA cm-2 and Tafel slope value of 155 mV/decade. With these fruitful advantages, these prepared nanosheets of NiFeHCH can be a better alternative to scarce metals for industrial water electrolysis.

Polymer-assisted approach to LaCo1-xNixO3 network nanostructures as bifunctional oxygen electrocatalysts

Construction of δ-FeOOH/NiMn2S4 heterointerface for efficient alkaline oxygen evolution reaction

Most active electrocatalysts include platinum-group metals (PGMs) and their alloys for electrochemical water splitting, but they are strongly limited by material cost and scarcity. [1] On the other hand, transition metal nitrides overcome this bottleneck and show high electrical conductivity, making them catalytically active for water electrolysis. The high conductivity is based on the incorporation of nitrogen atoms into the unit cell of metal, leading to their d-band contraction with electron density enhancement below Fermi level. [2] For instance, metallic nickel nitride (Ni3N) has been widely reported as a promising electrocatalyst material to accelerate the oxygen evolution reaction (OER) by in-situ forming a nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layer. [3] The Ni3N materials can be prepared by single step soft-urea route to obtain pure crystal phase [4] or a composite with carbon [5], which rises the following two research questions: 1. What are the optimal synthesis parameters of the soft-urea route to control the structure of Ni3N? 2. What is the relationship between structure and OER activity for Ni3N materials?In this work, we used the soft-urea route to prepare Ni3N materials by varying the ratios of NiCl2, urea and ethanol as precursor chemicals. The precursors were made into a gel by magnetic stirring having metal:urea ratio of 1:5, and then heated up to 400 ºC in a tubular furnace with constant flow of argon. The as-prepared Ni3N were then characterized by X-ray diffraction (XRD), elemental analysis (CHN), transmission electron microscopy (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) and X-ray absorption spectroscopy (XAS).Based on the quantitative Rietveld refinement, the formation of pristine Ni3N materials is strongly controlled by the annealing temperature. In addition to the Ni3N phase, we observed the formation of the fcc Ni phase (0 – 10 wt.%) as a minor phase. The TEM images show individual particles of below 100 nm in size. Rotating disc electrode (RDE) technique was employed to establish the electrochemical OER activity of Ni3N materials by sweeping the potential from 1.3 to 1.8 VRHE at 10 mV s-1 in Ar-saturated 0.1 M KOH. Polycrystalline Ni and Ni particles are taken as reference materials. We observed an improvement of the OER activity as the overpotential at 10 mA cm-2 (normalized by the geometric surface area) reduced from 379 mV (poly-Ni) to 358 mV (Ni3N). Furthermore, in-situ XAS studies were performed to better understand the nature of the redox reactions between Ni species and the in-situ formation of nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers before and during the OER as a function of the applied potential. Based on the in-situ XANES data, we correlated the structure of Ni3N/fcc-Ni/carbon composite with observed OER activity under alkaline conditions.To summarize, we were able to prepare a composite of Ni3N/fcc-Ni/carbon material with improved OER activity via soft-urea route. The in-situ formation of the layered nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers as the catalytically active OER species were probed by in-situ XANES data.