Electronic Structure Modulation of Sulfur-Retaining Nickel-Based Electrocatalyst to Improve the Oxygen Evolution Reaction

Green hydrogen production from renewable energy can help overcome difficult energy challenges worldwide. It can help decarbonize hard-to-abate sectors such as steel, chemicals, and ammonia fertilizer. Polymer Electrolyte Membrane (PEM) water electrolyzer serves as a promising technology when coupled with the fluctuating renewable energies for hydrogen production, with the advantages of fast response, dynamic operation, and excellent overload capacity. However, the only technically feasible anode catalysts for PEM are built on scarce iridium and require more than 40 years of annual production for 1 TW-scale. The scarce metal iridium has become a bottleneck limiting the scale development of PEM water electrolysis technology. Therefore, identifying earth-abundant catalysts with high activity and acid tolerance is essential to realizing large-scale hydrogen production via water electrolysis. Various 3d transition metals have been investigated as potential OER catalysts. In 2019, we reported how manganese oxide shows exceptional stability among earth-abundant materials for oxygen evolution reaction (OER) in acid. At the time, non-precious metal catalysts had difficulty maintaining activity for more than a week at 10 mA cm-2. In comparison, manganese oxide (γ-MnO2) was able to electrolyze water for over 11 months at 10 mA cm-2. This was made possible by identifying a stable potential window for γ-MnO2 in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. After that, a binary spinel oxide catalyst (Co2MnO4) was synthesized by mixing stable manganese with active cobalt using a thermal decomposition method. Cobalt oxides were shown to be active for the OER but unstable in acid. By incorporating Mn into the Co3O4 spinel lattice, the lifetime was extended by two orders of magnitude while maintaining the activity. The water electrolysis with Co2MnO4 in acid can be sustained over 1500 hours at 200 mA cm-2. The activation barrier of the obtained spinel Co2MnO4 is comparable to state-of-the-art iridium oxides, which are reported to have activation energies of 25-30 kJ mol-1 in acidic conditions. The catalyst dissolution was experimentally analyzed by spectroscopy methods. It is clarified that the tetrahedral Co is the preferable dissolution site and the dissolution rate is suppressed in Co2MnO4 compared to Co3O4, resulting in outstanding stability. The active working time of Co2MnO4 as a function of current density for the OER reaction and other earth-abundant electrocatalysts reported in the literature were summarized. The marked improvement of the activity and stability in acid makes cobalt manganese spinel materials an intriguing candidate for non-noble metal water electrolysis. Acknowledgments: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO). Figure caption: Long-term stability of Co2MnO4 during the OER in acid. a, Time dependence of the electrochemical potential necessary to perform OER at 100, 200, 500 and 1,000 mA cm-2 geo in H2SO4 (pH 1) and H3PO4 (pH 1), respectively. The stabilities of Co3O4 and γ-MnO2 in H2SO4 (pH 1) are shown for comparison. All the experiments were performed at 25 °C using an FTO substrate except for curve IX, for which a Pt/Ti mesh was used. b, A comparison of the stability of Co2MnO4 with Co3O4 and other Earth-abundant OER catalysts reported in the literature12-28 (Please refer Nat Catal 5, 109–118 (2022)) (upwards and downwards triangles represent lifetime and operation time, respectively). The measurements were performed under acidic conditions. Data points with roman numerals (I to XI) correspond to curves I to XI in a. Diagonal lines indicate the total amount of charge transferred before deactivation. Only studies that report stability at pH < 3 were included in the literature survey. Figure 1

One of the promising strategies to generate hydrogen for a future hydrogen economy is via electrocatalytic water splitting, where renewable electricity would be used to convert water into hydrogen and oxygen on an electrocatalyst within an electrolyzer. Oxygen evolution reaction (OER), the anodic half-reaction of water splitting is, however, kinetically sluggish and requires four electron-proton transfers making the overall reaction energy-intensive. For the economical production of hydrogen, improving the reaction kinetics in both low and high pH is of utmost necessity by designing an active and stable OER catalyst. Precious metals like Ir, Ru, and their oxides are known to be excellent OER catalysts but their large-scale application is hindered due to their low earth-abundancy. To address this issue, we have synthesized two types of OER catalysts with varying Ir wt.% to achieve a higher OER-activity per mass of Ir in the catalyst: Ir/MnO2, Ir nano-confined in the interlayer of layered MnO2 nanosheets and Ir-doped MnO2 nanosheets. In both acidic and basic environments, Ir/MnO2 exhibited better OER activity than Ir-doped MnO2. In 0.5 M H2SO4, 16.7 wt.% Ir/MnO2 and 17.5 wt.% Ir-doped MnO2 exhibited OER overpotentials (η) of 298 mV and 320 mV, respectively, at a current density of 10 mA cm-2. In 1M KOH electrolyte, 15.4 wt.% Ir/MnO2 and 17.5 wt.% of Ir-doped MnO2 exhibited OER η values of 213 mV and 245mV, respectively at 10 mA cm-2. These overpotentials are ~500 mV lower than the pristine MnO2 nanosheets and ~60-120 mV lower than commercial reference catalysts (20 wt.% Ir/C and IrO2) at both low and high pH conditions, respectively. These synthesized catalysts were able to outperform the commercial materials by exhibiting four times higher OER-activity per mass of Ir, in both acidic and basic media. Hence, this synthetic approach may be a method to achieve enhanced OER activity using a lower mass of Ir. The physical characterization of the materials was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS).

Designing efficient and stable electrolcatalysts for the demanding oxygen evolution reaction (OER) in water electrolysis is challenging and we are still lacking fundamental understanding of the underlying processes related to catalyst transformations governing their final activity and stability. Only the combination of operando spectroscopy techniques with electrochemical measurements can promote a deeper understanding on the interplay of different phenomena taking place during OER [1].In this work, we utilize operando techniques like X-ray Absorption Spectroscopy (XAS) and Surface Enhanced Raman Spectroscopy (SERS) [2], in combination with specifically designed electrochemical protocols to study i) the reconstruction/reactivation of active sites in Ir-based OER catalysts for acidic media (Fig. 1a-c) and Ni-based OER catalysts for alkaline media (Fig. 1d) and ii) the identification of the relevant OER active species which will eventually define the final electrocatalyst performance [3]. We show not only the formation of the active species for OER but also the dynamic operando transformation of the Ir- and Ni-based electrocatalysts before the onset of OER but also their structural transformation due to electro-oxidation during OER, processes that lead to the final active phase.The underlying mechanisms studied via our operando analysis provide fundamental understanding of reaction mechanism but also guidance for the design of efficient OER catalysts optimized for their operation in technical devices.

Water electrolysis is a key technology to convert and store sustainable energy into high-energy-dense hydrogen. However, slow kinetics of oxygen evolution reaction (OER) at the anode hinders overall reaction rate of water splitting and delays realization of hydrogen energy society. OER consists of four electron steps and an ideal catalyst has 1.23 V potential barrier for each step, but a real catalyst has a step which requires higher potential than 1.23 V, i.e., overpotential. Therefore, exploring OER catalyst with minimized OER overpotential is an important challenge in increasing water electrolysis efficiency. On the other side, the direct use of abundant seawater as a splitting reactant has advantage in terms of resources and cost. It can also help commercialization of water electrolysis. Since seawater contains chloride which causes undesirable side reactions and accelerated corrosion of the anode material, appropriate OER catalyst is necessary to avoid these problems.Conventional trial-and-error method in catalyst design usually requires many years of R&D and high cost. Computational screening can preemptively narrow down the candidate group of high-efficient catalyst to reduce time and economic cost. NiFe-based layered double hydroxide (NiFe-LDH) is a promising OER catalyst due to the comparable activity to commercial IrO2catalyst and can provide easily tunable metal composition during synthesis process. Its unique layered structure and reversible oxidation state change in redox condition also have arisen interest of many researchers. In this study, starting from the mechanism study of OER, chloride evolution reaction (ClER), and chloride-induced corrosion on this material, we performed DFT calculations to predict which transition metal dopants can enhance the OER activity of NiFe-LDH without accelerating selectivity and corrosion problems in seawater.NiFe-LDH is known to experience its transition to NiFeOOH phase under OER condition. [001] and [110] facets were set as edge and terrace sides of NiFeOOH, respectively, after their surface energies were investigated. Surface Pourbaix diagram showed that the [001] facet has clean termination without any adsorbate while [110] facet has dissociated-H2O covered termination under alkaline OER condition. OER energetic profile showed that the Fe site on the [110] surface is an OER active site with a theoretical overpotential of 280 mV, and seawater conditions do not significantly affect the OER activity itself. ClER mechanism study revealed that it occurs via *ClOH intermediates on [110] metal sites or *Cl intermediates on deprotonated [001] oxygen sites. Energy calculation of surface chlorination and metal dissolution steps showed that gradual chlorination accelerates metal dissolution and [110] facet is more vulnerable to chloride-induced corrosion than [001] facet.As the third metal candidate of NiFe-LDH, 3d to 5d transition metals excluding heavy metals were screened. Gibbs free energy correction terms such as vibrational entropy, zero-point energy and solvation parameter were calculated only for NiFeOOH case, and applied them to other candidates to simplify the screening process. NiFeOOH [001] surface could not be better than NiFeOOH [110] surface due to the inherent low activity of the [001] facet, but interestingly seven dopants could reduce the overpotential of NiFeOOH [110] surface by affecting the Fe active site or being active sites themselves. Since these dopants increased the oxidation ability of the active site, all of them also lowered chloride oxidation potential. However, fortunately, none of their ClER operating potential was lower than their OER operating condition, which means that there still exist potential windows where 100% OER selectivity can be achieved. In the last screening step, dissolution energies of fully chlorinated metal sites were calculated for the seven candidates, and two cases of them showed negative dissolution energy, which indicates very unstable doping state. As a result, five candidates passed through all three screening criteria: OER activity, OER vs. ClER selectivity, and durability against corrosion. Further, the experimental validation has performed on three promising candidates considering the raw material price, and it revealed that their activity is higher activity than NiFe-LDH. In addition, for the two abandoned cases in the third screening step (about metal dissolution), the experimental data showed that they were not seem to be actually doped into NiFe-LDH, indicating that the computational screening of seawater compatibility really worked. This study supports the utility of theoretical screening in electrochemical catalyst design and presents computational approach method to consider seawater compatibility.

In this paper, Pr0.7Sr0.3Co1-xRuxO3 perovskite oxides were synthesized by the sol-gel method as bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotentials of PSCR0.05 against HER and OER at 10 mA cm-2 were 319 and 321 mV in alkaline medium, respectively. The Tafel slopes of HER and OER were 87.32 and 118.1 mV/dec, respectively. PSCR0.05 showed the largest electrochemical active area, the smallest charge transfer resistance, and excellent long-term durability. Meanwhile, the PSCR0.05 electrocatalyst was applied for overall water splitting and its cell voltage was maintained at 1.77 V at 10 mA cm-2. The super-exchange interaction between adjacent RuO6-CoO6 octahedra in perovskite made of PSCR0.05 contains sufficient active sites (such as Co2+/Co3+, Ru3+/Ru4+, and O22-/O-). The increase of surface oxygen vacancy and active site is the main reason for the improvement of difunctional catalyst performance. In this work, the electrocatalytic performance of perovskite-type oxides was further optimized by the method of A- and B-site cationic doping regulation, which provides a new idea for perovskite-type bifunctional electrocatalysts.

Developing cost-effective, highly active, and robust electrocatalysts for oxygen evolution reaction (OER) at high current density is a critical challenge in water electrolysis since the sluggish kinetics of the OER significantly impedes the energy conversion efficiency of overall water splitting. Here, a 1D nanothorn-like Nb-CoSe2 /CC (CC=carbon cloth) structure was developed as an efficient OER catalyst. The optimized Nb-CoSe2 /CC catalyst exhibited remarkable OER performance with the low overpotentials of 220 mV at 10 mA cm-2 and 297 mV 200 mA cm-2 and a small Tafel slope (54.1 mV dec-1 ) in 1.0 m KOH electrolyte. More importantly, the Nb-CoSe2 /CC electrode displayed superior stability after 60 h of continuous operation. In addition, cell voltages of 1.52 and 1.93 V were required to achieve 10 and 500 mA cm-2 for the electrolyzer made of Nb-CoSe2 /CC (anode) and the Pt/C (cathode). Density functional theory (DFT) calculations combined with experimental results revealed that incorporating niobium into the CoSe2 could optimize the adsorption free energy of the reaction intermediates and enhance the conductivity to improve the catalytic activity further. Additionally, the super-hydrophilicity of Nb-CoSe2 /CC resulting from the surface defects increased the surface wettability and facilitated reaction kinetics. These results indicate that Nb-CoSe2 /CC intrinsically enhances OER performance and possesses potential practical water electrolysis applications.

The demanding multi-electron transfer process renders the oxygen evolution reaction (OER) a bottleneck for achieving efficient clean hydrogen generation via water electrolysis.[1] Over the past decades, two main categories of catalysts, namely, homogeneous molecular and heterogeneous catalysts, have been implemented for the OER. However, due to the sluggish reaction kinetics and the aggressive reaction media of the OER, the structural integrity of both homogeneous molecular and heterogeneous catalysts faces the dramatic challenges. This calls for a thorough understanding and close monitoring of OER catalysts under their operando reaction conditions.Over the last years, we have been combining a variety of in situ/operando spectroscopy approaches with computational studies toward the comprehensive understanding of our designed catalysts at the atomic level. With this information in hand, we established a full identification of catalytically active species and sites for several systems, some of which are discussed here.First, inspired by nature’s {Mn4CaOx} OER complexes, we recently reported on the design of a tetramer Cu-bipyridyl complex for the OER.[2] Structural characterizations demonstrated a new defect-cubane structure of our designed complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3). We found that this Cu-bipyridyl complex can further undergo structural transformations into two unique complexes under different solution conditions, namely Cu-dimer and Cu-monomers, as revealed by in situ UV-vis and ERP characterizations as well as electrospray ionization mass spectrometry. Specifically, the Cu-monomers can be only formed in presence of carbonate buffer (pH 10.5). Otherwise, the structural transformation into a Cu-dimer complex [Cu2(pyalk)2(OAc)2(H2O)] is dominant under solution conditions. Furthermore, electrochemical characterizations revealed an overpotential of 960 mV to reach a current density of 0.1 mA/cm2 of our designed Cu-dimer catalysts, which is comparable with significant Cu-based OER electrocatalysts. To gain in-depth insights into their conversion processes, postcatalytic characterizations of Cu-based molecular catalysts were carried out based on X-ray photoelectron/absorption (XAS/XPS) spectroscopy appraoches. The results showed that nanosized Cu-oxide-based species were formed in situ in Cu-based molecular catalysts after the OER. Our study highlights the crucial role of the structural integrity of molecular catalysts in solutions for their efficient design.In parallel, we explored the structural transformations of heterogeneous electrocatalysts during the OER. As a typical example, we developed a cost-effective and high-performance NiFe-based coordination polymer (referred to as NiFe-CP) as OER electrocatalyst, which is being investigated as the best-known bimetallic combination for the OER.[3] A central element of our study is the monitoring of true catalytically active species. Results from spectroscopic characterizations revealed a kinetic restructuring of NiFe-CPs into NiFe (oxy)hydroxides during the OER. To further improve the OER activity, we introduced a facile NaBH4-assisted reduction strategy to prepare low-crystalline reduced NiFe-CP (denoted as R-NiFe-CP) OER electrocatalysts with rich structural deficiencies. These catalysts can maintain a very low overpotential of 225 mV at 10 mA/cm2 for over 120 h without any performance decline, outperforming many recent reported bimetallic OER electrocatalysts. As revealed by XAS characterizations and density functional theory (DFT) calculations, engineering of structural deficiencies not only tunes the local electronic structure but also optimizes the rate-determining step towards facilitated OH- adsorption. Noteworthy, the true OER active sites of R-NiFe-CPs originate from the in situ reconstructed Ni-O-Fe motifs. However, fundamental questions, as to (a) the role of engineered structural deficiencies in the generation of active species and (b) facilitating the formation of catalytically active dual oxygen-bridged moieties, need to be answered. Combination of time-resolved operando XAS monitoring and DFT calculations enables the tracking and understanding of the kinetic changes of active species and sites under the operando reaction conditions. We found that the OER of R-NiFe-CPs relies on the in situ formation of crucial high-valent NiIV-O-FeIVO moieties.[4] Furthermore, an anionic engineering strategy through heteroatom sulfur incorporation was carried out to obtain S-R-NiFe-CP showing faster OER kinetics. Importantly, engineered sulfur content promotes the generation of catalytically active S-NiIVO-FeIVO motifs prior to the OER. This offers a lower onset potential to trigger the OER of S-R-NiFe-CPs compared to sulfur-free R-NiFe-CPs. Moreover, our results also suggest a dual-site mechanism pathway of S-R-NiFe-CPs during the OER, in which the O-O bond formed atop the S-NiIVO-FeIVO moieties. Such an anionic modulation strategy for promoting the formation of catalytically active structural moieties and for optimizing the OER kinetics opens an avenue to optimize a wide range of heterogeneous catalysts for the OER.[1] Zhao, Y. et al. Chem. Rev. 2023 , doi.org/10.1021/acs.chemrev.2c00515.[2] Adiyeri Saseendran, D. P. et al. Chem. Comm. 2023, In Revision.[3] Zhao, Y. et al. Adv. Energy Mater. 2020, 10, 2002228.[4] Zhao, Y. et al. ACS Nano 2022, 16, 15318-15327.

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a roomtemperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm−2). A structural investigation using X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layeredcrystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue for the design of novel high-performance catalysts.

Water electrolysis is a vital electrochemical process that utilizes electrical energy to decompose water into high-purity hydrogen and oxygen for energy storage and conversion technologies. The oxygen evolution reaction (OER), the anodic half-reaction in water electrolysis, is kinetically sluggish and necessitates substantial overpotentials to proceed efficiently. This kinetic limitation significantly hampers the overall energy efficiency and power output of the process. Although Pt-based and Ir/Ru-based materials have been employed as benchmark OER catalysts due to their high catalytic activity, their widespread application is constrained by the high cost, scarcity, and degradation in catalytic performance over time. Additionally, single precious metal catalysts often fail to achieve the required activity for OER under industrial conditions. Consequently, extensive research efforts have been devoted to developing cost-effective and efficient non-precious metal-based catalysts for OER.In this study, three-dimensional (3D) porous phosphorized nickel-copper catalysts are synthesized through a sequential process involving electrodeposition, electroetching, and rapid phosphorization. The resulting catalysts exhibit exceptional OER performance, achieving a current density of 10 mA cm-2 at an overpotential of 250 mV. The NiCu-supported catalysts, coated with a thin metal phosphide layer, demonstrating superior activity compared to other tested materials. Furthermore, long-term stability tests reveal no significant degradation in catalytic performance or noticeable structural and compositional changes, highlighting their robustness. These findings suggest that 3D porous phosphorized nickel-copper catalysts with excellent activity and stability hold great potential for practical applications in water electrolysis.

Porous bimetallic cobalt-iron phosphide nanofoam for efficient and stable oxygen evolution catalysis

The development of acidic oxygen evolution reaction (OER) electrocatalysts with high activity, extended durability, and lower costs furthers the development and utilization of proton-exchange membrane (PEM) water electrolyzers. Although iridium-based catalysts are currently the primary OER catalysts used within PEM electrolyzers, ruthenium has a higher activity, lower cost, and higher global supply compared to iridium; however, ruthenium-based catalysts generally have lower stability compared to iridium-based catalysts. Within acidic OER catalysts, stability remains a significant challenge for non-iridium materials, particularly under the highly corrosive environment of highly acidic conditions and high anodic potentials encountered during PEM water electrolysis. As an approach to obtain high OER activity and improved stability, we investigated the effect of titanium substitution within rutile ruthenium oxide as a model system of oxide structure that combines a highly OER active, unstable element (Ru) with a highly stable but OER-inactive element (Ti). Titanium-substituted ruthenium oxides, Ru1-xTixO2, at different Ti substitution ratios were synthesized via wet-chemistry and subsequent thermal treatments. X-ray diffraction analysis and high resolution transmission electron microscopy images support Ti is substituted within the RuO2 structure. X-ray photoelectron spectroscopy of core and valence bands shows Ti substitution alters the surface electronic structure. From rotating disk electrode measurements, Ti substitution lowers the OER mass activity, OER specific activity, and electrochemical surface area. In addition to affecting OER activity, Ti substitution increases the OER stability and lowers Ru dissolution. Density functional theory (DFT) calculations of the titanium-substituted ruthenium oxides show that effects of Ti substitution on the reaction energies and activation energies are highly dependent on the site. Theoretical analysis supports that specific sites may predominately act as catalytic sites for the OER, while other sites influence metal dissolution. Further understanding how the structure of bimetallic oxides influences OER activity and stability provides a pathway to electrocatalysts with higher activity, improved stability, and lower cost.

The key to improving water oxidation is to develop efficient and earth-abundant catalysts for the oxygen evolution reaction (OER). Herein, a new amorphous cobalt iron borate supported on 3D carbon paper integrated electrode is reported as a precatalyst for the OER, which was synthesized by using a one-pot hydrothermal method. An optimum OER activity was obtained at 25 % Fe doping by screening the compositions of the Co and Fe. The best synthesized catalyst needs an overpotential of 227 mV to deliver a current density of 10 mA cm-2 and also exhibits a long-term durability of 24 h. Impressively, we find that CoFe oxyhydroxide was formed in situ in the OER process, which serves as the real catalytic active species for OER. Moreover, the direct conversion from CoFe borate to CoFe oxyhydroxide is reported for the first time in metal borate OER catalysts. The discovery in this work, that is, that metal borate as a precursor can efficiently catalyze the OER in alkaline media, significantly widens the family of OER catalysts.

With the ever-increasing global energy demand, the development of renewable energy technologies is crucial to fulfilling the energy requirements in a sustainable manner. Hydrogen has emerged as a promising energy carrier that can provide emission-free energy for end-use applications. Among the several pathways of hydrogen production, green hydrogen production by electrochemical water splitting using renewable energy is an emission-free, efficient energy conversion technology. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur at the cathode and anode, respectively. Water splits into hydrogen and oxygen at the thermodynamic potential of 1.23 V; however, overpotentials are observed experimentally due to the kinetic barriers, which increases the required potential for water splitting. Catalysts lower the required overpotentials, making the process more energy efficient. A significant issue with large-scale hydrogen production by water electrolysis is that the state-of-the-art catalysts for HER and OER are based on precious metals and thus are limited by their high cost and net production capacity. Thus, there is a lot of interest in developing noble-metal-free electrocatalysts based on earth-abundant materials that can efficiently lower the associated overpotentials for the two halves of the water-splitting reaction.A large number of late transition metal-based ternary complex oxides have been investigated for their catalytic performance towards water splitting, where fine-tuning of the properties using compositional engineering has been demonstrated. Binary sulfides, selenides, and phosphides have also been explored as candidates for water splitting. Recently, early transition metal-based ternary complex materials have emerged as a novel class of electronic materials with applications in infrared optics and photovoltaics. However, similar chalcogenide compositions based on late-transition metals remain underexplored.We have synthesized a series of LaMS3 (M = Mn, Fe, Co, Ni) materials by high-temperature sulfurization of corresponding ternary oxides, using carbon disulfide (CS2) as the sulfurizing agent. The LaMS3 compounds are isostructural and crystallize in the hexagonal P 63 space group. When tested for catalytic performance towards water splitting, they demonstrate good activity towards both HER and OER, with LaNiS3 (LNS) being the most active material, and a periodic activity trend is observed, where LNS > LCS >> LFS > LMS. For OER in alkaline electrolyte, LNS demonstrates an overpotential of 373 mV at 10mA/cm2 in 1.0 M KOH, and a low Tafel slope value of 48 mV/dec. The long-term stability was tested using chronopotentiometry measurements, and LNS shows excellent stability, with negligible change in overpotential over several hours. For HER testing in both acidic and alkaline electrolyte, LNS again seems to be the most active material, and a periodic activity trend in the HER activity of LaMS3 materials, similar to the OER activity trend, is observed. LNS has an overpotential of 340 mV in 0.5 M H2SO4 and 362 mV in 1.0 M KOH, indicating that LNS is catalytically active over a wide range of pH levels. LNS also shows excellent long-term stability towards HER in both acidic and alkaline electrolyte. We also compared the activity of LaMS3 materials with their ternary oxide counterparts from which they were synthesized, and the LaMS3 compounds outperform the LaMO3 compounds, demonstrating lower overpotentials and Tafel slope values for both HER and OER, with the HER overpotentials for LaMO3 materials being > 250 mV higher compared to LaMS3, for both acidic and alkaline electrolytes. The catalytic activity of the LaMS3 series towards both HER and OER, along with their excellent long-term stability, makes these materials promising candidates for bifunctional electrocatalysts for water splitting while offering a wide compositional range that allows for further optimization of their performance.

ZnS-stabilized single atoms for highly-efficient water electrolysis