Research Progress and Prospects of Prussian Blue Analogs and Their Derivatives in Small-Molecule Oxidative Coupled Hydrogen Production.

Renewable energy sources are expected to take up an important portion of produced electric energy in the near future. Production of hydrogen by proton exchange membrane water electrolysis (PEMWE) is nowadays considered as one of the most attractive ways to store an unavoidable excess of renewable energy. Produced hydrogen can be used in automotive applications. However, owing to high capital costs and a relatively low efficiency, this technology is still not a competitive alternative to traditional hydrogen production from fossil fuels. In particular issues related to sluggish oxygen evolution reaction (OER) kinetics and low stability of most of the catalysts in acidic environment must be solved1. Despite its high price and scarcity, iridium based oxides are the only materials considered as anode catalysts in the PEM electrolyzers2, as only they can provide the required longevity at relatively low overpotential of the OER. However, even most stable catalysts such as rutile IrO2 undergoes dissolution under conditions of the OER3,4. Therefore, optimization of the electrolyzer performance and design of novel more efficient iridium based catalysts is demanded, which cannot be achieved without a deep understanding of the OER itself and iridium degradation mechanisms. While the OER on iridium based electrodes was numerously discussed in the literature5, data on iridium electrochemical dissolution is limited. In the present work, a detailed and systematic study of iridium based oxides, hydrous Ir, rutile IrO2, etc, corrosion and the OER kinetics in acidic media is performed. Initial dissolution rates of iridium are measured online with the OER using a scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). To obtain additional information on equilibrium concentration of dissolved iridium, long-term measurements in the h-cell with divided anodic and cathodic compartments are performed. The observed differences in the electrochemical activity and stability of iridium hydrous oxide and rutile iridium dioxide are correlated with diversity in chemical structure. Evaluation of surface composition of the oxides triggered by the OER is studied using X-ray photoelectron spectroscopy and its further effect on anode activity-stability relationships is analyzed. On the basis of the obtained results a general mechanism of the involved reactions is proposed and discussed.

Hydrogen (H2) generation through electrolysis has attracted much attention because of growing global energy needs and the demand for green energy strategies. Hydrogen generation through water electrolysis is accompanied by two half reaction, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, OER has the limitation of lowering the energy efficiency of the entire cell due to its sluggish reaction problem and high overpotential. Also, commercial OER electrocatalysts are limited to noble metal oxide such as IrO2 and RuO2, which are scarce and have stability problems in alkaline solution. Therefore, there is a growing desire to develop an OER electrocatalyst that is abundant, stable, and highly efficient for the superior performance of overall water electrolysis.The doping, alloy, oxide, and nitride of transition metal materials have been reported as abundant materials that exceed the electrolysis efficiency of noble metals during OER. Among them, transition metal nitrides showed superior OER electrocatalytic performance due to their high electrical conductivity, which is derived from their metallic properties. To boost the electrocatalytic activity of transition metal based materials, numerous efforts have focused on modulating electronic structure related to catalytic activity in OER. The d-band center theory which proposed that the position of d-band center (Ed) has close relationship with the catalytic activity has unveiled insights into designing electrocatalysts for OER. However, most researchers have used ion exchange or doping method for modulating d-band center, which requires an additional process and leads to higher difficulty of designing electrocatalysts.Recently, Prussian Blue Analogues (PBAs) have been widely employed as a self-template material for the energy conversion and storage field such as water electrolysis and batteries. PBAs, which are a type of Metal Organic Frameworks (MOFs) composed of CN groups, have emerged as a promising material due to the following characteristics: 1) numerous interstitial sites with unique 3-dimensional structure due to CN group; 2) advantages of synthesizing various compositions; and 3) chemical stability in solution. Due to these advantages, PBAs have a large specific surface area, fast diffusion kinetics, and structural stability when applied as a precursor to water electrocatalysts. Although PBAs-derived electrocatalysts have high electrical conductivity and electrocatalytic performance, they are prone to aggregate and lose their unique morphology after high temperature process. As a breakthrough of the limitation, polydomamine (PDA) coating has been attracting attention as an encapsulation method to achieve hybridization with carbon materials, as well as enhance electrocatalytic performance. PDA is simply synthesized by the polymerization of Dopamine (DA) in Tris-buffer solution, which is beneficial for mass production. In addition, PDA is advantageous for N-doped carbon formation, because it has high nitrogen content, which improves the charge transfer ability during electrocatalytic reaction. Also, because PDA has thermal stability, agglomeration between particles can be prevented when high-temperature heat treatment is performed. Due to these characteristics of PDA, PDA both prevents the aggregation of PBAs during carbonization at high temperature and improves stability in alkaline solution, while increasing reaction sites by forming a high content of N-doped carbon layer.Herein, we present a versatile but promising strategy to synthesize cobalt nitride nanoparticles encapsulated in N-doped carbon (Co/Co4N@NC) nanoboxes as highly efficient and stable OER electrocatalysts by modulating d-band center through the simple nitridation of self-template PBAs encapsulated by PDA. The synthesized Co/Co4N@NC nanoboxes have mesoporous and hollow structure, which provides a large specific surface area and fast charge transfer pathways. Especially, the d-band center of CoxN nanoparticles is strategically modulated to be more metallic and heterogeneous to favor electrocatalytic reactions. Furthermore, the highly conductive N-doped carbon shell derived from PDA coating enables electrocatalysts to have not only numerous active reaction sites and fast charge transfer ability but also long-term durability in alkaline solution during OER. The Co/Co4N@NC nanoboxes showed a remarkable overpotential value of 262 mV and 408 mV at 10 mA∙cm-2 and 100 mA∙cm-2, respectively, and a low tafel slope of 130 mV∙dec-1, which is superior to RuO2 (284 mA∙cm-2 and 470 mA∙cm-2). In addition, the Co/Co4N@NC nanoboxes maintained an overpotential value of 92.6 % after 24 h of stability test at 10 mA∙cm-2, as well as showing outstanding durability for 100 h and at high current density of 100 mA∙cm-2. The DFT calculation also identifies advantages of metallic cobalt nitride and heterogeneous structure derived from the modulated electronic structure of Co/Co4N@NC. Finally, the in-situ XANES during OER of Co/Co4N@NC is clear evidence that the modulated electronic structure of Co/Co4N@NC improves the OER catalytic activity due to reconstruction of Co2+ into Co3+. Figure 1

Renewable hydrogen generation using the low-temperature proton-exchange membrane water electrolyzer (PEMWE) represents a crucial technology for achieving global zero-carbon emissions. Iridium oxide (IrOx) stands out as a primary platinum group metal catalyst for the anode in electrochemical water splitting due to a balance in activity and stability in acidic environments.1 However, the anode catalyst performance is still predominately hampered by the sluggish kinetics of the oxygen evolution reaction (OER) compared to the hydrogen evolution reaction (HER) at the Pt cathode.2 Therefore,understanding the factors influencing the OER kinetics of Ir-based catalysts is essential for developing strategies to achieve high OER activity in the PEMWE anode electrocatalysts. This presentation will delve into our investigation of how the perfluorosulfonic acid (PFSA) binder affects the anode catalyst OER behaviors, specifically the corresponding kinetics and activity of two commercial Ir-based OER catalysts, IrOx from Alfa Aesar and TiO2-decorated IrOx (IrTiOx) from Umicore, as a function of the PFSA ionomer-to-catalyst (I/C) ratio. We combined the cavity microelectrode (CME) technique, which offers a direct approach to study OER kinetics without the ionomer,3 with the rotating disc electrode (RDE) technique, which allows to assess the impact of PFSA on OER kinetics and activity at varied I/C ratios in an acidic electrolyte. The study provided valuable insights into ionomer dependent OER mechanisms, elucidating the intricate interactions among the ionomer, catalyst, and reaction intermediates.AcknowledgementsThis research is supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the H2NEW Consortium. Argonne National Laboratory is managed by the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.Reference S. M. Alia, M.-A. Ha, G. C. Anderson, C. Ngo, S. Pylypenko and R. E. Larsen, Journal of The Electrochemical Society, 2019, 166, F1243.E. Oakton, D. Lebedev, M. Povia, D. F. Abbott, E. Fabbri, A. Fedorov, M. Nachtegaal, C. Copéret and T. J. Schmidt, Acs Catalysis, 2017, 7, 2346-2352.J. Behnken, M. Yu, X. Deng, H. Tüysüz, C. Harms, A. Dyck and G. Wittstock, ChemElectroChem, 2019, 6, 3460-3467.

Hydrogen and value-added products yield from hybrid water electrolysis: A critical review on recent developments

Realizing tailored catalytic performance on ternary FeP-Ni5P4-CoP in-situ confined Prussian blue analogue framework for anion exchange membrane water electrolysis.

Water electrolysis is extensively researched as a next-generation energy storage method to overcome the intermittency of renewable energy. Electricity generated from renewable sources such as solar and wind power can be converted into pure green hydrogen through water electrolysis. Despite the potential of green hydrogen as a renewable energy storage/carrier medium, the high overpotential resulting from the slow kinetics of oxygen evolution reaction (OER) and the use of expensive noble metals make cost-competitive hydrogen production a significant challenge. Anion exchange membrane water electrolysis (AEMWE) is one method for hydrogen production, offering economic advantages over proton exchange membrane systems as it allows for the use of relatively inexpensive transition metal catalysts like Ni, Fe, and Co due to its alkaline environment.Nickel phosphide (Ni2P)-based catalysts have emerged as highly promising candidates to accelerate the electrocatalytic OER. In particular, the introduction of Fe into Ni2P has been found to induce charge transfer, optimizing the electronic structure and accelerating the OER process. OER catalysts based on transition metal phosphides often exhibit a core-shell structure with phosphide at the core and oxide at the shell during OER measurements. This core-shell structure enhances OER performance through the oxide shell, complementing the deficient electrical conductivity of the phosphide core, resulting in superior catalytic activity and durability. From this perspective, transition metal phosphates are also being studied as electrocatalysts for OER. Phosphates, such as PO3 -, not only promote oxygen adsorbate adsorption but also induce a distorted local metal center geometry that favors OH- adsorption and further oxidation.To address these challenges, this study develops a facile and scalable synthesis of iron-doped nickel phosphide-phosphate (Fe-doped Ni2P-POx) nano-hybrid system as a superior noble-metal free OER powder-catalyst. The utilization of self-filling and pyrolysis approaches facilitates economically viable production of the highly porous phosphide catalyst with a high specific surface area and rough surfaces. X-ray diffractometer, X-ray photoelectron spectroscopy, and electron paramagnetic resonance elucidates the electronic structure modulation, resulting in phosphide-phosphate nano-hybrid structures. Consequently, the catalyst demonstrates excellent OER activity in 1 M KOH, with a significantly low overpotential of 283 mV at 20 mA cm-2, a small Tafel slope of 28.4 mV dec-1, and a superior exchange current density of 8.22 mA cm-2, surpassing state-of-the-art PGM catalysts. Furthermore, its durability over 20 hours indicates its excellent stability, mass transport properties, and mechanical robustness in alkaline media, underscoring its potential as an efficient OER catalyst to facilitate electrocatalytic hydrogen production. Figure 1

Prussian blue analogs (PBAs) are promising catalysts for green hydrogen production. However, the rational design of high-performing PBAs is challenging, which requires an in-depth understanding of catalytic mechanism. Here FeMn@CoNi core-shell PBAs were employed as precursors, together with Se powders, in low-temperature pyrolysis in an argon atmosphere. This synthesis method enabled the partial dissociation of inner FeMn PBAs that resulted in hollow interiors, Ni nanoparticles (NPs) exsolution to the surface, and Se incorporation onto the PBA shell. The resulting material presented ultra-low oxygen evolution reaction (OER) overpotential (184mV at 10mA cm-2 ) and low Tafel slope (43.4mV dec-1 ), outperforming leading-edge PBA-based electrocatalysts. The mechanism responsible for such a high OER activity was revealed, assisted by DFT calculations and the surface examination before and after the OER process.The exsolved Ni NPs were found to help turn the PBAs into Se-doped core-shell metal oxyhydroxides during the OER, in which the heterojunction with Ni and the Se incorporation were combined to improve the OER kinetics. This work shows that efficient OER catalysts could be developed by using a novel synthesis method backed up by a sound understanding and control of the catalytic pathway. This article is protected by copyright. All rights reserved.

As an electrocatalyst for water electrolysis, nickel oxide (NiO) has received significant attention due to its cost-effectiveness and high reactivity among non-noble-metal-based catalytic materials. However, NiO still exhibits poor alkaline hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics compared to conventional noble metal-based catalysts. This is because NiO has a strong interaction with protons for the HER and too low free energy of the OH* state, resulting in slower rate-determining step (RDS) kinetics for the OER. To address these issues, adding a dopant is suggested as an efficient method to modify the electron structure of the NiO electrocatalyst favorably for each reaction kinetics. In this context, we demonstrate that Bismuth (Bi), due to its higher electronegativity than that of Nickel (Ni), induces a positive charge on Ni sites. This enhances the catalytic activity by reducing the number of excessive cation interactions with the NiO electrocatalyst. Moreover, as the Bi ratio increases, the Ni reaction sites in NiO become more positively charged, and these changes in the electronic structure directly impact the free energy of the reaction mechanism. Particularly, it is confirmed that for the HER, Bi additives increase the proton-adsorbed free energy toward a near-zero value and, additionally, decrease the free energy difference of the second step considered as the RDS in the OER, as calculated by density functional theory. The positive effects of Bi in both the HER and the OER are demonstrated in practical electrochemical evaluations of half/single cells. Notably, the Bi-containing catalysts Bi05:NiO and Bi02:NiO exhibit remarkable alkaline HER and OER kinetics, showing performance improvements of 97.0% and 21.9%, respectively.

The chemical coupling of molybdenum carbide (Mo2C) to cobalt (Co) promotes oxygen evolution reaction (OER) kinetics on the Co surface by making the surface more electrophilic. Here, to gain a deeper understanding of the effects of the surface electrophilic properties on the OER kinetics of Co and to obtain high OER activity, Fe and Ni are additionally incorporated into Co nanoparticles that are coupled with Mo2C nanoparticles (Co‐Mo2C). Considering the oxidation states of Fe (Fe3+), Co (Co2+/Co3+), and Ni (Ni2+) ions, Fe and Ni are expected to affect the electronic structure of Co in the opposite direction. Lewis acidic Fe3+ doping makes the Co surface oxide more electrophilic, promoting the formation of OER‐active CoOOH by strongly attracting hydroxide ions (OH−). Thus, the OER kinetics is facilitated on the Co surface of Fe‐doped Co‐Mo2C, resulting in a significantly lower overpotential for the OER. On the other hand, the Ni2+ doping makes the Co surface oxide less electrophilic, leading to an increase in the overpotential for the OER. Tailoring the electrophilic properties of the Co surface is presented as a key parameter in the design of a Co‐based OER catalyst for alkaline water electrolysis.

The production of hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. As spin-dependent kinetics exist in OER, the spin alignment in active OER catalysts is critical for reducing the kinetic barriers in OER. It is effective to facilitate the spin polarization in ferromagnetic catalysts by applying external magnetic field, which increases the OER efficiency. However, more active OER catalysts tend to have dynamic open-shell orbital configurations with disordered magnetic moments, without showing an apparent long-range interatomic ferromagnetism; thus controlling the spin alignment of these active catalysts is challenging. In this work, we report a strategy with spin pinning effect to make the spins in active oxyhydroxides more aligned for higher intrinsic OER activity. Such strategy bases on a controllable reconstruction: ferromagnetic oxides with controlled sulfurization can evolve into stable oxideFM/oxyhydroxide configurations with a thin oxyhydroxide layer under operando condition. The spin pinning effect is found at the interface of oxideFM/oxyhydroxide. The spin pinning effect can promote spin selective electron transfer on OER intermediates to generate oxygens with parallel spin alignment, which facilitates the production of triplet oxygen and increases the intrinsic activity of oxyhydroxide by ~ 1 order of magnitude. Under spin pinning, the spins in oxyhydroxide can become more aligned after magnetization as long-range ferromagnetic ordering is established on the magnetic domains in oxideFM. The OER kinetics are facilitated accordingly after magnetization, implying that the spin pinning effect is involved in the rate-determining step and this step is spin dependent. The spin polarization process in OER under spin pinning is also believed to be sensitive to the existence of active oxygen ligand (O(-)) in oxyhydroxide. When the O(-) is created in 1st deprotonation step under high pH, the spin polarization of ligand oxygens will be facilitated, which reduces the barrier for subsequent O-O coupling and promotes the O2 turnover.

Kinetics investigation of the oxygen evolution reaction on the characteristic facets of γ-Cu3V2O8

Although the lithium-oxygen (Li-O2) battery brings hope for the improvement of high-energy rechargeable batteries, the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics become the major stumbling block. Herein, the incorporation of a plasmonic silver cathode as an advanced strategy to promote ORR and OER kinetics due to strong local surface plasmon resonance (LSPR) is introduced. Chronoamperometry results revealed that the highly energetic electrons and holes excited by LSPR of silver nanostructure facilitated ORR and OER kinetics ascribe to the emission of hot carriers in femtosecond time scale. Furthermore, a relatively rare discharge voltage 3.1 V is obtained, correspondingly, the charge plateau also decline to 3.3 V, the energy efficiency of Li-O2 battery by a 23% increase in comparison with a commercial 5% Pt/C catalyst (discharge and charge plateau of 2.75 and 3.61 V). Additionally, the improvement in the efficient charge transfer manner result in a reversible spherical Li2O2 which further improve the ORR and OER kinetics. The LSPR strategy represents a critical step toward developing fast kinetics and high energy efficiency Li-O2 batteries.

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.

Late first-row transition metal oxides, based on cobalt, nickel and iron [1] are reported to be the most active non-precious catalysts for oxygen evolution reaction (OER) in basic solution. Experimental and computational studies in the past decade have been focusing on elucidating OER mechanisms [2-3] and identifying activity and stability descriptors [3-6], where perovskites family (ABO3-δ) with immense structural, chemical and electronic flexibility associated with vast selections of A-site and B-site metal ions and oxygen deficiency [7] has been used to develop design principles of OER activity and stability. Recent works [6-8] have shown that lowering charge-transfer gap or increasing metal-oxygen covalency in perovskites can enhance the OER kinetics, by reducing the energetic barriers associated with electron transfer on the surface of metal oxides including the most active catalysts. However, reducing the charge-transfer gap also lowers the Fermi level on the absolute energy scale, making it below the OER redox potential in the basic solution for the most active catalysts and rendering weaker surface hydroxide affinity [6]. Consequently, the OER kinetics on highly covalent/active metal oxides are limited by proton transfer. Moreover, increasing the covalency typically moves the O 2p band closer to the Fermi level, rendering less energy penalty for the creation of oxygen vacancies and thus leading to surface or bulk instability at OER potentials [5]. Here we explored the substitution of A-site ions with high electronegativity or Lewis acidity in the cobalt perovskites to maintain high Co-O covalency by the inductive effect [9], and tune the surface acid-base chemistry by introducing highly Lewis acidic A-site ions to facilitate OER kinetics. Bismuth-substituted strontium cobalt perovskite, Bi0.2Sr0.8CoO3-δ, was shown to exhibit record OER specific activity in alkaline solution, exceeding those of other Co-based perovskite oxides reported to date, including SrCoO3-δ [8], at high current densities (> 1 mA cm-2 oxide). In addition, neither structural or chemical changes have been found for Bi0.2Sr0.8CoO3-δ, indicative of greater structural stability than other highly covalent oxide catalysts, e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ [4]. The enhanced OER kinetics and high surface stability can be attributed to the stronger affinity towards hydroxide ions to facilitate surface deprotonation due to the presence of strong Lewis acidic surface Bi3+ ions, and the lowered O 2p-band center relative to the Fermi level upon bismuth substitution into the perovskite structure, respectively. This work exemplifies a novel strategy to facilitate the OER kinetics of highly active oxide catalysts by leveraging the inductive effect associated with rational metal substitution to maintain high metal-oxygen covalency and strengthen hydroxide affinity without the expense of surface stability.

Photo-electrochemical (PEC) seawater splitting using semiconductors is a promising means of directly producing hydrogen from seawater occupying approximately 97% of Earth’s water. The n-type perovskite oxynitride semiconductors with a general formula AB(O,N)3 (A=Ca, Sr, Ba or La; B=Ti, Nb or Ta) are capable of absorbing intensive visible light up to 750 nm wavelength, thermodynamically leading to high solar-to-hydrogen conversion efficiency of 10% or above.1 Also, their band positions straddle various oxidation and reduction potentials including the water redox potential to produce value-added energy resources such as H2, HCOOH, and NH3.2 Based on the favorable optical properties, the water splitting activity using the oxynitrides has been significantly increased in strong alkaline electrolytes, although it is still low in neutral electrolytes.2-3 Oxygen evolution reaction (OER) is driven via four-electron oxidation. Its kinetics is commonly very sluggish in neutral aqueous solutions, which the significant overpotential is required to catalyze it. Thus, applying alternative oxidations to OER, such as chlorine evolution reaction (CER), may be a feasible approach to efficient hydrogen production in neutral seawater electrolytes.4 In terms of thermodynamic potential, OER (E 0 = 1.23 VNHE) is more oxidative than CER (E 0 = 1.36 VNHE). However, CER driven via two-electron oxidation can be catalyzed faster than OER kinetically in neutral electrolytes.Herein we first present sunlight-driven seawater splitting activity of perovskite AB(O,N)3 largely improved in neutral environments and also discuss CER activity depending on various pH values as an alternative oxidation to OER. Highly-crystalline, porous SrNbO2N particles grown on Nb substrate was prepared by bottom-up fabrication including oxidation of Nb substrate and flux-assisted nitridation.5 The advanced fabrication caused less-defective oxynitride with a large surface area. The resulting SrNbO2N/Nb photoanode produced significant OER photocurrent in 0.2 M NaPi electrolyte at a neutral pH 6.4, and the photocurrent was three times increased by adding 0.5 M NaCl to catalyze CER simultaneously. Moreover, the onset potential for seawater splitting including CER was shifted to a lower potential by approximately 0.1 VRHE than that for water splitting leading to OER exclusively. These results clearly demonstrate that CER was preferentially catalyzed during neutral seawater splitting compared with competitive OER. It also implies that CER kinetics over the SrNbO2N was faster than OER kinetics despite its unfavorable thermodynamic potential.The BaTaO2N/Ta photoanode, more photoactive and stable than the Nb-based oxynitrides, was synthesized in the similar manner. The seawater splitting of the oxynitride at different pH values was further investigated, and the quantitative analysis of the resulting products, namely, H2, O2, and ClO− was also performed to determine the operating conditions suitable for efficient and stable seawater splitting. Moreover, different electro-catalysts to improve the selectivity to CER were evaluated in the artificial seawater. The enhanced seawater splitting activity and selectivity to CER of the AB(O,N)3 in artificial seawater will be discussed in detail in a presentation. References J. Seo, H. Nishiyama, T. Yamada, K. Domen, Angew. Chem. Int. Ed., 2018, 57, 2.J. Seo, K. Domen, Mater. Chem. Front., 2024, 8, 1451.J. Seo, T. Hisatomi, M. Nakabayashi, N. Shibata, T. Minegishi, M. Katayama, K. Domen, Adv. Energy Mater., 2018, 1800094.C.R. Lhermitte, K. Sivula, ACS Catal., 2019, 9, 2007V-H. Trinh, J. Seo, ACS Sustain. Chem. Eng., 2023, 11, 1655