Understanding the sulphur-oxygen exchange process of metal sulphides prior to oxygen evolution reaction

Corrosion Protection of Rare Earth for Kilowatt-Level Alkaline Seawater Electrolyzer

The competitive adsorption of Cl- caused by anode OH- consumption under high current density is an important factor restricting the development of an alkaline seawater electrolyzer (ASWE). Here, we propose a strategy for rare earth corrosion protection which utilizes oxygen friendly rare earths that do not participate in the reaction to adsorb OH- and maintain the surface environment for stable anodic catalysis in an ASWE. Differential electrochemical mass spectrometry (DEMS) and electrochemical quartz crystal microbalance (EQCM) were used to identify the causes of chlorine corrosion on the high current anode plate of traditional Ni mesh. In situ fluorescence spectra of N-ethoxycarbonylmethyl-6-methoxyquinolinium bromide (MQAE) labeled with a chloride ion fluorescence probe, a rotating ring disk electrode (RRDE), and a time-resolved absorption spectrum were used to test the recognition mechanism of rare earth. Eu2O3 adsorbs OH- to maintain a high current pH environment and inhibits Cl- adsorption oxidation, thereby exhibiting stability for over 1000 h at 500 mA cm-2 current density. Furthermore, Eu2O3/FeNi2S4 was assembled into a kilowatt-level ASWE in 17 chambers with a total area of 1081.5 cm2 and operated stably for over 100 h at a current density of 500 mA cm-2 under industrial conditions of 80 °C and 30% KOH. Technical economic analysis (TEA) indicates that the rare earth corrosion protection strategy can enhance the service life of ASWE and reduce the cost of hydrogen production for profitable seawater hydrogen production, providing a new approach to solve the chlorine oxidation corrosion problem in an ASWE.

Hydrothermal Fabricated Ni3S4-Yb2O3 Heterojunction Electrocatalysts with Charge Redistribution for Efficient Alkaline Oxygen Evolution Reactions

Designing efficient and stable oxygen evolution reaction (OER) catalysts is of great significance for hydrogen production technology through the electrolysis of water. Nickel sulfide (NixSy) has garnered considerable attention as an OER electrocatalyst owing to its high electrical conductivity and distinctive structure. Herein, Ni3S4-Yb2O3 on nickel foam electrocatalysts was fabricated through a hydrothermal synthesis approach to optimize the performance of Ni3S4. The incorporation of Yb2O3 induced charge redistribution within Ni3S4, expanded the active surface area, and enhanced the availability of active sites. Consequently, the Ni3S4-Yb2O3/NF catalyst demonstrates exceptional electrocatalytic performance in 1 M KOH, attaining a current density of 10 mA cm-2 at a minimal overpotential of 148 mV, along with an ultralow Tafel slope of 47.3 mV dec-1. In addition, we have confirmed that another reason for the improved performance is that Yb2O3, as an electron donor, can effectively regulate the defects of the Ni atoms. This work offers a feasible strategy for developing highly effective OER electrocatalysts in alkaline media.

In Situ Reconfigured Heterostructure Active Sites on Transition Metal Sulfides Heterojunction for Accelerated Water Oxidation

Transition metal sulfides (TMSs) are promising noble-metal-free electrocatalysts for electrochemical water splitting due to their distinctive physical and chemical properties, but they usually undergo complicated structure reconfiguration during the oxygen evolution reaction (OER). Precisely controlling the in situ reconfiguration of TMSs for in situ generation of high-activity real active sites still remains a great challenge. Herein, we propose to in situ reconfigure heterostructure active-sites on transition metal sulfides via heterojunction engineering and achieve high OER performances on (Ni,Fe)S2/MoS2 catalysts. The continuous leaching of Mo and S during electrooxidation induces the reconfiguration, and the strong electronic interaction of (Ni,Fe)S2 and MoS2 generates the special Ni(OH)2/NiOOH/FeOOH heterostructure sites via an asynchronous reconfiguration of Fe and Ni. The (Ni,Fe)S2/MoS2 heterostructure catalyst therefore exhibits excellent OER activity (a small overpotential of 228 mV at 100 mA cm-2) and a low voltage in an alkaline water electrolyzer (1.44 V at 10 mA cm-2), outperforming the homogeneous Mo-free NiFe sulfide catalysts with conventional reconfiguration of Ni-doped FeOOH. This work sheds light on the precise structures design under complicated electrochemical reconstruction and broadens the horizon of reconstruction chemistry to design low-cost and efficient electrocatalysts.

Rational Design of Rare Earth-Based Nanomaterials for Electrocatalytic Reactions

Rare earth-based nanomaterials hold great promise for applications in the electrocatalysis field owing to their unique 4f electronic structure, adjustable coordination modes, and high oxophilicity. As a cocatalyst, the location of rare earth elements can alter the intrinsic properties of support, including coordination environments, electronic structure, and structure evolution under applied potentials in a variable manner, to potentially impact catalytic performance with respect to their activity, stability, and selectivity. Therefore, a comprehensive understanding of the effects of rare earth elements' location on local structure and reaction mechanisms is a prerequisite for designing advanced rare earth-based nanomaterials. In this review, the rare earth-based nanomaterials have been categorized into three main groups based upon the location of rare earth elements in the support, namely lattice, surface, and interface structure. We initially discuss recent advances and representing breakthroughs to realize controllable synthesis of rare earth-based nanomaterials. Next, we discuss the state-of-the-art rare earth-based nanomaterials and the structure modulation strategy employed to enhance their catalytic performance. Combined with advanced characterizations, the role of rare earth elements in reaction mechanisms and structure evolution process is also discussed. Finally, we further highlight the future research directions and remaining challenges for the development of rare earth-based nanomaterials in practical applications.

Iron-doped XC-72 enhancing cobalt selenide for high-efficiency oxygen evolution reaction

As an emerging class of anodic catalyst material for water electrolysis and hydrogen production, transition metal selenides exhibit excellent electron transport properties and multiphase structures with varying conductivities. However, their widespread commercial application is hindered by sluggish reaction kinetics, a lack of exposed active sites, and significant volume expansion. Cobalt, a typical transition metal element abundantly found in the Earth's crust, offers sustainability and cost-effectiveness when used as an electrode material. In this work, a series of iron-modified carbon-supported cobalt selenides exhibit excellent catalytic performance in oxygen evolution reaction (OER) testing. Among them, CS/2XC-Fe@2 demonstrates outstanding activity for the oxygen evolution reaction in 1 M KOH, achieving a current density of 10 mA cm-2 at a low overpotential of 261.8 mV. Additionally, density functional theory (DFT) calculations further revealed that the Fe interface with catalytic sites enhances electron transfer capabilities, as evidenced by density of states and charge analysis, which facilitates the intermediate reactions during electrocatalysis by reducing the energy barrier by 0.25 eV. This study contributes to advancing theoretical calculations and structure-activity relationship research on transition metal selenides, potentially offering a pathway for the large-scale synthesis of non-noble metal electrocatalysts.

Enhancing Oxygen Evolution Electrocatalysis in Heazlewoodite: Unveiling the Critical Role of Entropy Levels and Surface Reconstruction

Entropy engineering has proven effective in enhancing catalyst electrochemical properties, particularly for the oxygen evolution reaction (OER). Challenges persist, however, in modulating entropy and understanding the dynamic reconfiguration of high-entropy sulfides during OER. In this study, an innovative in situ corrosion method is introduced to convert low-valent nickel on a nickel foam substrate into high-entropy heazlewoodite (HES/NF), significantly boosting OER performance. By synthesizing a series of low-, medium-, and high-entropy heazlewoodites, the intrinsic factors influence catalyst surface evolution and electrocatalytic activity is systematically explored. Employing a combination of in situ and ex situ characterization techniques, it is observed that HES/NF dynamically transforms into a stable hydroxide oxide (MOOH)-sulfide composite under OER conditions. This transition, coupled with lattice distortion, optimizes the electrostatic potential distribution, ensuring superior catalytic activity and preventing surface sulfide deactivation through the formation of stable HES-MOOH species. This synergy enables HES/NF to achieve remarkably low overpotentials: 172.0 mV at 100.0 mA cm-2 and 229.0 mV at an extreme current density of 300.0 mA cm-2. When paired with a Pt/C cathode, HES/NF exhibits rapid kinetics, outstanding stability, and exceptional water-splitting performance. The scalable, cost-effective approach paves the way for advanced electrocatalyst design, promising breakthroughs in energy storage and conversion technologies.

Fe doping intensifies the built-in electric field for tailoring the reconstruction of sulfides towards efficient oxygen evolution

The traditional view of sulfides as stable active centers has hindered the development of a clear structure-activity relationship and the rational design of high-performance oxygen evolution reaction (OER) catalysts. In this study, we focus on regulating sulfide reconstruction and have synthesized a Fe-Ni3S4/Cr2O3 pre-catalyst. Under the combined influence of the built-in electric field (BIEF) at the heterogeneous interface and Fe doping, both the sulfide reconstruction process and the electronic structure of the reconstructed product, namely Fe-NiOOH, were effectively tuned. The enhanced BIEF induced by Fe doping generated electron-rich regions on the sulfide surface, stabilizing the reconstruction process. Fe doping into the sulfide induced the incorporation of Fe into NiOOH, modulating the electronic states near the Fermi level of the metal-oxygen bond and subsequently activating the lattice oxygen mediated mechanism (LOM) of Fe-NiOOH, which serves as the true active center. Additionally, the BIEF optimized OH- diffusion dynamics and the energy consumption of hydroxyl deprotonation, reducing the energy barrier of the rate-limiting step of the LOM process, further enhancing OER activity. Remarkably, Fe-Ni3S4/Cr2O3 demonstrated excellent OER activity and commercial viability. This work offers a new perspective on the regulation of reconstruction products of pre-catalyst, providing fresh insights for the design of efficient OER catalysts.

Iodine-Induced Redirection of Active Sources in Cu-Based Catalysts during Efficient and Stable Water Oxidation

Enhancing the mechanistic regulation of the oxygen evolution reaction (OER) is crucial for developing efficient and stable electrocatalysts. However, the dynamic variation of surface structure during the electrocatalytic process limits the accurate identification of the active source and underlying reaction mechanism. Herein, we report an iodine-doping strategy to direct the reconstruction of active species in CuS catalysts toward an unconventional oxygen vacancy oxidation mechanism, thereby overcoming the activity and stability limitations. Mechanistic analysis indicates that the electronic manipulation, weak coordination of Cu-S bonds, and lattice distortion induced by iodine-doping facilitate the thermodynamically favorable Cu2+ to Cu3+ oxidation during OER. The decisively formed oxygen vacancies are emphasized as a genuine active source to promote hydroxyl adsorption, with hypervalent Cu species acting as auxiliary sites to accelerate deprotonation by strengthening Cu-O covalent. Consequently, the optimal iodine-doped CuS exhibits a reduced overpotential of 189 mV at 10 mA cm-2 and superb stability prolonging to 1250 h. When used as a bifunctional electrode in a membrane electrode assembly electrolyzer, it also exhibits a low voltage of 1.65 V at 1 A cm-2, with electrolysis durability of 480 h and a low hydrogen cost of US$1.70/kg H2, outperforming the 2026 targets set by the U.S. Department of Energy.

Ligand-rich oxygen evolution electrocatalysts reconstructed from metal-organic frameworks for anion-exchange membrane water electrolysis

Organic ligands in metal-organic frameworks (MOFs) play an indispensable role in the reconstruction and catalysis during the alkaline oxygen evolution reaction (OER). However, it is still a big challenge to maintain a high content of ligands in MOF-reconstructed OER electrocatalysts and to study the interaction between ligands and derived (oxy)hydroxides. Herein, a ligand-rich trimetallic amorphous electrocatalyst is fabricated through a two-step mechanochemical and electrochemical reconstruction strategy. Experimental and theoretical studies clearly reveal that the d-π interaction between delocalized π-electrons on the benzene ring of ligands and derived (oxy)hydroxides, can trigger the charge transfer from ligands to the active metal centers, thus optimizing the adsorption energy of the oxygen-containing intermediates and enhancing the OER performance. Moreover, an anion-exchange membrane water electrolyzer using such ligand-rich OER electrocatalyst can be operated steadily at 1.69 V and 55 °C under an industrial-level current density of 500 mA cm-2 for over 200 h. This work provides novel insights into the role of organic ligands in alkaline OER electrocatalysis, with the potential to facilitate the production of green hydrogen at industrial-level current densities.

Modulating Support Effect in High-Entropy Sulfide via La Single-Atom for Boosted Oxygen Evolution

Reduced energy barrier induced enhanced oxygen evolution reaction (OER) kinetics can be achieved by implementing an efficient electrocatalyst. Herein, positive effect of lanthanum (La) single-atom modified hollow carbon sphere (HCS) support on OER activity of high-entropy sulfide (HES) material (FeCoNiCrCuAl)S has been reported. Briefly, La single-atom boosts the aggregation of electrons at adjacent Fe, Co, Ni, Cr, and Cu sites and dissipation of electrons at Al site in HES material, facilitating reconstruction of electronic structure and down-shifting their d-band center away from Fermi level, resulting in reduced adsorption energy of OER intermediates. As developed (FeCoNiCrCuAl)S@La-HCS depicts high OER performance with an overpotential of only 297 mV at 100 mA cm-2, surpassing (FeCoNiCrCuAl)S@HCS (324 mV) and commercial RuO2 catalyst (419 mV). This work provides an insight into the integration of single atom with high-entropy sulfide toward efficient oxygen evolution.

Updating the sub-nanometric cognition of reconstructed oxyhydroxide active phase for water oxidation

Unveiling structure-activity correlations at the sub-nanoscale remains an essential challenge in catalysis science. During electrocatalysis, dynamic structural evolution drives the ambiguous entanglement of crystals and electrons degrees of freedom that obscure the activity origin. Here, we track the structural evolution of Ni-based model pre-catalysts (Ni(OH)2, NiS2, NiSe2, NiTe), detailing their catalytically active state during water oxidation via operando techniques and theoretical calculations. We reveal the sub-nanometric structural difference of NiO6 unit with a regular distortion in the reconstructed active phase NiOOH, codetermined by the geometric (bond lengths) and electronic (covalency) structure of the pre-catalysts on both spatial and temporal scales. The symmetry-broken active units induce the delicate balance of the p and d orbitals in NiOOH, further steering the modulation of catalytic intermediate configurations and mechanisms, with improved performance. This work recognizes the fine structural differences of the active phases from the sub-nanometer scale, and quantitatively explains their influence on activity. Our findings provide a more intuitive design framework for high-efficiency materials through targeted symmetry engineering of active units.

10,000-h-stable intermittent alkaline seawater electrolysis

Seawater electrolysis powered by renewable electricity provides an attractive strategy for producing green hydrogen1-5. However, direct seawater electrolysis faces many challenges, primarily arising from corrosion and competing reactions at the anode caused by the abundance of halide ions (Cl-, Br-) in seawater6. Previous studies3,6-14 on seawater electrolysis have mainly focused on the anode development, because the cathode operates at reducing potentials, which is not subject to electrode dissolution or chloride corrosion reactions during seawater electrolysis11,15. However, renewable energy sources are intermittent, variable and random, which cause frequent start-shutdown operations if renewable electricity is used to drive seawater electrolysis. Here we first unveil dynamic evolution and degradation of seawater splitting cathode in intermittent electrolysis and, accordingly, propose construction of a catalyst's passivation layer to maintain the hydrogen evolution performance during operation. An in situ-formed phosphate passivation layer on the surface of NiCoP-Cr2O3 cathode can effectively protect metal active sites against oxidation during frequent discharge processes and repel halide ion adsorption on the cathode during shutdown conditions. We demonstrate that electrodes optimized using this design strategy can withstand fluctuating operation at 0.5 A cm-2 for 10,000 h in alkaline seawater, with a voltage increase rate of only 0.5% khr-1. The newly discovered challenge and our proposed strategy herein offer new insights to facilitate the development of practical seawater splitting technologies powered by renewable electricity.

Leaching-Reconstruction Engineering of Anions on Ferronickel Phosphate Promotes the Enhancement of the Oxygen Evolution Reaction

Electrochemical reconstruction typically generates powerful active sites for the oxygen evolution reaction (OER). However, engineering effective reconstruction strategies to manipulate the in situ formation of desired catalytically active surfaces, generate powerful active sites, and enhance their catalytic performance remains a challenge. Herein, leveraging the oxidation-potential-assisted precipitation etching, a heterostructure of NiFeOOH/NiFe phosphate was meticulously engineered to achieve highly efficient OER. During the electrochemical reconstruction, the leaching of inactive PO43- species in NiFe phosphate facilitates the exposure of more Ni and/or Fe species and creates more pores, thereby contributing to the formation of a NiFeOOH layer on the surface of NiFe phosphate. The resultant NiFeOOH/NiFe phosphate exhibits excellent OER activity with an overpotential of 205 mV at 50 mA cm-2 in an alkaline electrolyte. The theoretical calculations reveal that the heterostructure of NiFeOOH/NiFe phosphate weakens the thermodynamic barrier from *O to *OOH, thus enhancing the OER activity. The present proof-of-concept study introduces a leaching engineering approach to facilitate further exploration and development of highly efficient energy-related applications.

A Deep Insight into the Microscopic Dynamics of the Electrode-Electrolyte Interface under Extreme Operating Conditions

Understanding the interfacial dynamics during operation is critical for electrochemistry to make great advancements. However, breakthroughs on this topic under extreme conditions are very scarce. Here, as an example, we employ operando Raman spectroscopy to decode the interfacial dynamics of titanium electrolysis using a tailored instrument. Direct spectral evidence not only confirms the two-step reduction pathway and the key intermediate (TiF52-) in molten fluorides with high-temperature and strong-corrosion conditions but also unravels the origins of the undesirable shuttling effect of TiF52-, which are the sluggish reduction kinetics and outward diffusion behavior of TiF52-. Moreover, an insightful atomic scenario of the electric double layer (EDL) under varied potentials has been established. These quantitative understandings guide us to design economical-feasible regulation protocols─the rational combination of a high-concentration, low-valence Ti-ion electrolyte with appropriate applied potential. Impressively, the current efficiency is greatly promoted from 27.7 to 81.8% using our proposed protocols. Finally, this work also demonstrates a bottom-up technological research paradigm for extreme electrochemistry based on mechanism insights rather than phenomenological findings, which will accelerate the advancement of extreme electrochemistry.

Real-Time Detection of Dynamic Restructuring in KNixFe1- xF3 Perovskite Fluorides for Enhanced Water Oxidation

Mechanistic understanding of how electrode-electrolyte interfaces evolve dynamically is crucial for advancing water-electrolysis technology, especially the restructuring of catalyst surface during complex electrocatalytic reactions. However, for perovskite fluorides, the mechanistic exploration for the influence of the dynamic restructuring on their chemical property and catalytic mechanism is unclear due to their poor conductivity that makes the definition of electrocatalyst structure difficult. Herein, for oxygen evolution reaction (OER), various operando characterizations are employed to investigate the structure-activity relationships of the KNixFe1- xF3@NF. Adding iron to the KNixFe1- xF3 structure increases metal vacancies, enhancing electrochemical reconstruction. For reconstructed KNixFe1- xF3 structure, the results from operando Raman, operando X-ray diffraction, operando UV-vis spectroscopy, and differential electrochemical mass spectrometry reveal that the surface Ni sites act as catalytic centers within the amorphous Ni(Fe)OOH active layer, and the incorporation of Fe activates oxidized oxygen ions during water oxidation. Theoretical calculations support this by demonstrating the optimized adsorption-free energy of oxygenated intermediates. Consequently, the KNi0.5Fe0.5F3@NF achieves an overpotential of 281 mV to reach OER current of 150 mA·cm-2 and maintains stable operation for 200 h. These results highlight a promising pathway to tuning OER mechanisms in perovskite fluorides and offer a new perspective for developing high-efficiency and durable OER catalysts.

Quantum Spin Exchange Interactions Trigger O p Band Broadening for Enhanced Aqueous Zinc-Ion Battery Performance

The pressing demand for large-scale energy storage solutions has propelled the development of advanced battery technologies, among which zinc-ion batteries (ZIBs) are prominent due to their resource abundance, high capacity, and safety in aqueous environments. However, the use of manganese oxide cathodes in ZIBs is challenged by their poor electrical conductivity and structural stability, stemming from the intrinsic properties of MnO2 and the destabilizing effects of ion intercalation. To overcome these limitations, our research delves into atomic-level engineering, emphasizing quantum spin exchange interactions (QSEI). These essential for modifying electronic characteristics, can significantly influence material efficiency and functionality. We demonstrate through density functional theory (DFT) calculations that enhanced QSEI in manganese oxides broadens the O p band, narrows the band gap, and optimizes both proton adsorption and electron transport. Empirical evidence is provided through the synthesis of Ru-MnO2 nanosheets, which display a marked increase in energy storage capacity, achieving 314.4 mAh g-1 at 0.2 A g-1 and maintaining high capacity after 2000 cycles. Our findings underscore the potential of QSEI to enhance the performance of TMO cathodes in ZIBs, pointing to new avenues for advancing battery technology.

Bioinspired artificial antioxidases for efficient redox homeostasis and maxillofacial bone regeneration

Reconstructing large, inflammatory maxillofacial defects using stem cell-based therapy faces challenges from adverse microenvironments, including high levels of reactive oxygen species (ROS), inadequate oxygen, and intensive inflammation. Here, inspired by the reaction mechanisms of intracellular antioxidant defense systems, we propose the de novo design of an artificial antioxidase using Ru-doped layered double hydroxide (Ru-hydroxide) for efficient redox homeostasis and maxillofacial bone regeneration. Our studies demonstrate that Ru-hydroxide consists hydroxyls-synergistic monoatomic Ru centers, which efficiently react with oxygen species and collaborate with hydroxyls for rapid proton and electron transfer, thus exhibiting efficient, broad-spectrum, and robust ROS scavenging performance. Moreover, Ru-hydroxide can effectively sustain stem cell viability and osteogenic differentiation in elevated ROS environments, modulating the inflammatory microenvironment during bone tissue regeneration in male mice. We believe this Ru-hydroxide development offers a promising avenue for designing antioxidase-like materials to treat various inflammation-associated disorders, including arthritis, diabetic wounds, enteritis, and bone fractures.

Built-in electric field guides oxygen evolution electrocatalyst reconstruction

Creating a built-in electric field (BIEF) in catalysts represents an effective strategy to promote electron transfer and induce asymmetric charge distribution, thereby facilitating surface dynamic reconstruction under oxygen evolution reaction (OER) conditions. This review summarizes recent advancements in the field of OER electrocatalysts, with a focus on regulating the work function of components to tailor the BIEFs to guide surface reconstruction processes. It also discusses the importance of surface reconstruction in improving electrocatalytic performance and the influence of BIEFs on the reconstruction of catalysts. By analyzing various strategies for manipulating electric fields for guiding surface reconstruction of OER electrocatalysts, along with numerous representative examples, this review highlights how these techniques can enhance catalytic activity and stability. The findings underscore the potential of engineered BIEFs as a powerful tool in the design of next-generation electrocatalysts, paving the way for more efficient energy conversion technologies.

Surface Cladding Engineering via Oxygen Sulfur Distribution for Stable Electrocatalytic Oxygen Production

Inevitable leaching and corrosion under anodic oxidative environment greatly restrict the lifespan of most catalysts with excellent primitive activity for oxygen production. Here, based on Fick' s Law, we present a surface cladding strategy to mitigate Ni dissolution and stabilize lattice oxygen triggering by directional flow of interfacial electrons and strong electronic interactions via constructing elaborately cladding-type NiO/NiS heterostructure with controlled surface thickness. Multiple in situ characterization technologies indicated that this strategy can effectively prevent the irreversible Ni ions leaching and inhibit lattice oxygen from participating in anodic reaction. Combined with density functional theory calculations, we reveal that the stable interfacial O-Ni-S arrangement can facilitate the accumulation of electrons on surficial NiO side and weaken its Ni-O covalency. This would suppress the overoxidation of Ni and simultaneously fixing the lattice oxygen, thus enabling catalysts with boosted corrosion resistance without sacrificing its activity. Consequently, this cladding-type NiO/NiS heterostructure exhibits excellent performance with a low overpotential of 256 mV after 500 h. Based on Fick's law, this work demonstrates the positive effect of surface modification through precisely adjusting of the oxygen-sulfur exchange process, which has paved an innovative and effective way to solve the instability problem of anodic oxidation.

Rational Design of Diatomic Active Sites for Elucidating Oxygen Evolution Reaction Performance Trends

Diatomic catalysts, especially those with heteronuclear active sites, have recently attracted significant attention for their advantages over single-atom catalysts in reactions with relatively high energy barrier, e.g. oxygen evolution reaction. Rational design and synthesis of heteronuclear diatomic catalysts are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report macrocyclic precursor constrained strategy to fabricate series of transition metal (MT, Ni, Co, Fe, Mn, or Cu)-noble (MN, Ir or Ru) centers in carbon material. One notable performance trend is observed in the order of Cu-MN Collapse

Key Words

• Diatomic catalysts
• Macrocyclic precursor constrained strategy
• Oxygen evolution reaction
• Performance descriptor
• Transition-noble metal centers

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MESH Headings Collapse

Grants

• 22172170 National Natural Science Foundation of China
• 2022YFA1504002 National Key R&D Program of China
• 2023YFB3813001 National Key R&D Program of China
• XDB0600300 Strategic Priority Research Program of the Chinese Academy of Sciences
• 22F22358 JSPS-KAKENHI
• 22ZR1471600 Science and Technology Commission of Shanghai Municipality

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Affiliation(s)

Nanfeng Xu Laboratory of Advanced Spectro-electrochemistry and Li-ion Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
Yuxiang Jin State Key Lab of High-Performance Ceramics and Superfine microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China
Qiunan Liu The Institute of Scientific and Industrial Research (ISIR-SANKEN), Osaka University, Osaka, 567-0047, Japan
Meng Yu State Key Laboratory of Advanced Chemical Power Sources, College of Chemistry, Nankai University, Tianjin, 300071, China
Xiao Wang State Key Laboratory of Advanced Chemical Power Sources, College of Chemistry, Nankai University, Tianjin, 300071, China
Chao Wang Laboratory of Advanced Spectro-electrochemistry and Li-ion Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
Wei Tu Clean Nano Energy Center, State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China
Zhirong Zhang Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China
Zhigang Geng Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China
Kazu Suenaga The Institute of Scientific and Industrial Research (ISIR-SANKEN), Osaka University, Osaka, 567-0047, Japan
Fangyi Cheng State Key Laboratory of Advanced Chemical Power Sources, College of Chemistry, Nankai University, Tianjin, 300071, China
Erhong Song State Key Lab of High-Performance Ceramics and Superfine microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China
Zhangquan Peng Laboratory of Advanced Spectro-electrochemistry and Li-ion Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
Junyuan Xu Laboratory of Advanced Spectro-electrochemistry and Li-ion Batteries, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China

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MOF-Derived Co(Fe)OOH Slab and Co/MoN Nanosheet-Covered Hollow-Slab for Efficient Overall Water Splitting

The development of economical, efficient, and stable nonprecious metal electrocatalysts presents a crucial approach to achieving alkaline overall water splitting and generating renewable hydrogen. This work presents a simple method for the synthesis of transition metal oxyhydroxides and nitrides derived from the MOF template with different morphological structures for efficient overall water splitting. Co(Fe)OOH slab array is obtained by the electro-activation of Fe-doped Co-MOF precursor, which is usually regarded as the real active substance in the alkaline OER process. Doping the Co(Fe)OOH with Fe alters the local electronic structure of the Co sites, leading to a notable improvement in OER performance, which shows an overpotential of 209 mV at 10 mA cm-2 and demonstrates excellent stability. On the other hand, Co/MoN nanosheet-covered hollow-slab heterostructure is prepared by the nitrogenization of Na2MoO4-etched Co-MOF template, which displays superior HER performance due to its abundant electrochemical active sites and rapid interfacial electron transfer, achieving an overpotential of 37 mV at 10 mA cm-2 while sustaining good stability. Notably, utilizing Co(Fe)OOH as the anode and Co/MoN as the cathode in the alkaline electrolyzer results in a cell voltage of 1.49 V at 10 mA cm-2, while demonstrating a remarkable long-term stability of 100 h. This work provides a facile way to construct an efficient alkaline electrolyzer for overall water splitting by controlling the structures of MOF derivatives.

Promoted surface reconstruction of pentlandite via phosphorus-doping for enhanced oxygen evolution reaction

The pentlandite Fe5Ni4S8(abbreviated as FNS) is not efficient for water splitting because of its inferior performance for the oxygen evolution reaction (OER). This issue originates from the low activity and instability of FNS during the OER process but can be solved through appropriate doping. Herein, a P-doping strategy based on annealing in the presence of NaH2PO2as a phosphorus source upstream was employed on FNS to enhance its activity and stability toward OER. The results demonstrated fine-tuned electronic structures of Fe and Ni in FNS through P-doping, resulting in suppressed Fe leaching,improved electrical conductivity of FNS, and easier formation of NiOOH on the surface of the catalyst. In turn, these features enhanced the OER activity and stability. The optimal P-doped FNS catalyst FNSP-40 exhibited a 4-fold greater electrochemical surface area compared to that of FNS, accompanied by an overpotential of 235 mV at 10 mA cm-2. The optimized FNSP-40 catalyst was used as an anode, and platinum-decorated FNS was used as a cathode. This combination demonstrated an electrolysis performance with a cell voltage of 1.57 V, reaching a current density of 100 mA cm-2,which indicates efficient operation. The advantages of P-doping engineering were also verified in simulated seawater with enhanced OER performance. Overall, the proposed strategy looks promising for the fabrication of pentlandite-structured catalysts for efficient alkaline water and seawater oxidation.

Importance of Morphology of Layered Double Hydroxide in Electrochemical Energy Storage and Catalysis

The development of nanomaterials for energy storage and conversion has always been important. Layered double hydroxide (LDH) is a promising material due to its high capacity, tunable composition and easy synthesis. In this work, the morphology of NiCo-LDH is tuned with surfactants including sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), and investigated the correlation between morphology and electrochemical properties. NiCo-LDH-SDS with a layered structure exhibited a specific capacitance of 1004 C g-1 at 1 A g-1, which is higher than that of the needle-like NiCo-LDH-CTAB (678 C g-1) and the rod-like NiCo-LDH (279 C g-1). Meanwhile, NiCo-LDH-SDS and NiCo-LDH-CTAB showed a reduction of 36 and 19 mV, respectively, in their overpotentials at 10 mA cm-2 compared to NiCo-LDH. Contact angle and adhesive force measurements proved the influence of morphology on the interfacial properties that layered structure is favorable for the timely detachment of the bubbles. Therefore, rational morphology regulation of LDH can effectively alter the gas-liquid-solid interface and thereby accelerate the reaction kinetics. The connections between morphologies, bubbles releasing and electrochemical performance are well established in this work, which can be applied in the investigation of nanomaterials for energy-related activities, especially the ones concerning bubbles releasing processes.

Gas-Sensing Properties of Co9S8 Films Toward Formaldehyde, Ethanol, and Hydrogen Sulfide

The chemiresistive gas-sensing properties of pristine Co9S8 film are little known despite its potential as a promising gas sensor material due to its intrinsic characteristics. In this study, a pristine polycrystalline Co9S8 film (approximately 440 nm in thickness) is fabricated by depositing a Co3O4 film followed by sulfidation to investigate its gas-sensing properties. The prepared Co9S8 film sensor is found to exhibit high responsiveness towards formaldehyde (HCHO), ethanol (C2H5OH), and hydrogen sulfide (H2S) at operating temperatures of 300 °C and 400 °C, with strong concentration dependence. On the other hand, the sensor shows very low or no responsiveness towards hydrogen (H2), acetone (CH3COCH3), and nitrogen dioxide (NO2). These results enhance our understanding of the intrinsic gas-sensing properties of Co9S8, aiding in the design and fabrication of high-performance chemiresistive gas sensors based on Co9S8.

Theoretical insights on potential-dependent oxidation behaviors and antioxidant strategies of MXenes

Significant efforts have been devoted to investigating the oxidation of MXenes in various environments. However, the underlying mechanism of MXene oxidation and its dependence on the electrode potential remain poorly understood. Here we show the oxidation behavior of MXenes under the working conditions of electrochemical processes in terms of kinetics and thermodynamics by using constant-potential ab initio simulations. The theoretical results indicate that the potential effects can be attributed to the nucleophilic attack of water molecules on metal atoms, similar to that taking place in the Oxygen Evolution Reaction. Building upon these findings, we deduced the oxidation potential of the common MXenes, and proposed antioxidant strategies for MXene. Finally, we demonstrated that MBenes, the boron analogs of MXenes, may undergo a similar nucleophilic attack in water and inferred that molecule-induced Walden inversion is widely present in material reconstructions. This work contributes to a fundamental understanding MXene stability at the atomic level, and promotes the transition in materials discovery from trial-and-error synthesis to rational design.

Dynamic evolution processes in electrocatalysis: structure evolution, characterization and regulation

Reactions on electrocatalytic interfaces often involve multiple processes, including the diffusion, adsorption, and conversion of reaction species and the interaction between reactants and electrocatalysts. Generally, these processes are constantly changing rather than being in a steady state. Recently, dynamic evolution processes on electrocatalytic interfaces have attracted increasing attention owing to their significant roles in catalytic reaction kinetics. In this review, we aim to provide insights into the dynamic evolution processes in electrocatalysis to emphasize the importance of unsteady-state processes in electrocatalysis. Specifically, the dynamic structure evolution of electrocatalysts, methods for the characterization of the dynamic evolution and the strategies for the regulation of the dynamic evolution for improving electrocatalytic performance are summarized. Finally, the conclusion and outlook on the research on dynamic evolution processes in electrocatalysis are presented. It is hoped that this review will provide a deeper understanding of dynamic evolution in electrocatalysis, and studies of electrocatalytic reaction processes and kinetics on the unsteady-state microscopic spatial and temporal scales will be given more attention.

Magnetically Enhanced Oxygen Evolution Reaction in Mild Alkaline Electrolytes by Building Catalysts on Magnetic Frame

Under large current densities, the excessive hydroxide ion (OH) consumption hampers alkaline water splitting involving the oxygen evolution reaction (OER). High OH concentration (≈30 wt.%) is often used to enhance the catalytic activity of OER, but it also leads to higher corrosion in practical systems. To achieve higher catalytic activity in low OH concentration, catalysts on magnetic frame (CMF) are built to utilize the local magnetic convection induced from the host frame's magnetic field distributions. This way, a higher reaction rate can be achieved in relatively lower OH concentrations. A CMF model system with catalytically active CoFeOx nanograins grown on the magnetic Ni foam is demonstrated. The OER current of CoFeOx@NF receives ≈90% enhancement under 400 mT (900 mA cm-2 at 1.65 V) compared to that in zero field, and exhibits remarkable durability over 120 h. As a demonstration, the water-splitting performance sees a maximum 45% magnetic enhancement under 400 mT in 1 m KOH (700 mA cm-2 at 2.4 V), equivalent to the concentration enhancement of the same electrode in a more corrosive 2 m KOH electrolyte. Therefore, the catalyst-on-magnetic-frame strategy can make efficient use of the catalysts and achieve higher catalytic activity in low OH concentration by harvesting local magnetic convection.

The breakthrough of oxide pathway mechanism in stability and scaling relationship for water oxidation

An in-depth understanding of electrocatalytic mechanisms is essential for advancing electrocatalysts for the oxygen evolution reaction (OER). The emerging oxide pathway mechanism (OPM) streamlines direct O-O radical coupling, circumventing the formation of oxygen vacancy defects featured in the lattice oxygen mechanism (LOM) and bypassing additional reaction intermediates (*OOH) inherent to the adsorbate evolution mechanism (AEM). With only *O and *OH as intermediates, OPM-driven electrocatalysts stand out for their ability to disrupt traditional scaling relationships while ensuring stability. This review compiles the latest significant advances in OPM-based electrocatalysis, detailing design principles, synthetic methods, and sophisticated techniques to identify active sites and pathways. We conclude with prospective challenges and opportunities for OPM-driven electrocatalysts, aiming to advance the field into a new era by overcoming traditional constraints.

In Situ Transformed CoOOH@Co3S4 Heterostructured Catalyst for Highly Efficient Catalytic OER Application

The deprived electrochemical kinetics of the oxygen evolution reaction (OER) catalyst is the prime bottleneck and remains the major obstacle in the water electrolysis processes. Herein, a facile hydrothermal technique was implemented to form a freestanding polyhedron-like Co3O4 on the microporous architecture of Ni foam, its reaction kinetics enhanced through sulfide counterpart transformation in the presence of Na2S, and their catalytic OER performances comparatively investigated in 1 M KOH medium. The formed Co3S4 catalyst shows outstanding catalytic OER activity at a current density of 100 mA cm-2 by achieving a relatively low overpotential of 292 mV compared to the pure Co3O4 catalyst and the commercial IrO2 catalyst. This enhancement results from the improved active centers and conductivity, which boost the intrinsic reaction kinetics. Further, the optimized Co3S4 catalyst exhibits admirable prolonged durability up to 72 h at varied current rates with insignificant selectivity decay. The energy dispersive X-ray spectroscopy (EDX) and Raman spectra measured after the prolonged OER stability test reveal a partial transformation of the active catalyst into an oxyhydroxide phase (i.e., CoOOH@Co3S4), which acts as an active catalyst phase during the electrolysis process.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Atomic Manipulation to Create High-Valent Fe4+ for Efficient and Ultrastable Oxygen Evolution at Industrial-Level Current Density

Manipulating the electronic structure of a catalyst at the atomic level is an effective but challenging way to improve the catalytic performance. Here, by stretching the Fe-O bond in FeOOH with an inserted Mo atom, a Fe-O-Mo unit can be created, which will induce the formation of high-valent Fe4+ during the alkaline oxygen evolution reaction (OER). The highly active Fe4+ state has been clearly revealed by in situ X-ray absorption spectroscopy, which can both enhance the oxidation capability and lead to an efficient and stable adsorbate evolution mechanism (AEM) pathway for the OER. As a result, the obtained Fe-Mo-Ni3S2 catalyst exhibits both superior OER activity and outstanding stability, which can achieve an industrial-level current density of 1 A cm-2 at a low overpotential of 259 mV (at 60 °C) and can stably work at the large current for more than 2000 h. Moreover, by coupling with commercial Pt/C, the Fe-Mo-Ni3S2∥Pt/C system can be used in the anion exchange membrane cell to acquire 1 A cm-2 for overall water splitting at 1.68 V (2.03 V for 4 A cm-2), outperforming the benchmark RuO2∥Pt/C system. The efficient, low-cost, and ultrastable OER catalyst enabled by manipulating the atomic structure may provide potential opportunities for future practical water splitting.

The dual active sites reconstruction on gelatin in-situ derived 3D porous N-doped carbon for efficient and stable overall water splitting

The exploration of bifunctional electrocatalysts with high activity, stability, and economy is of great significance in promoting the development of water splitting. Herein, a dual active sites heterostructure NiCoS/NC was designed to be derived in situ on 3D N-doped porous carbon (NC) using gelatin as a nitrogen and carbon source. The characterization of experiments suggests that nanoflower-like Ni2CoS4 (abbreviated as NiCoS) was randomly distributed on the NC substrate, and the sheet-like NC formed a highly open porous network structure resembling a honeycomb, which provided more accessible active sites for electrolyte ions. In addition, the special nanostructures of the catalyst materials help to promote the surface reconstruction to the real active substance NiOOH/CoOOH, and the double active sites synergistically reduce the overpotential of OER and improve its kinetics. DFT (Density-functional theory) calculations reveal the electronic coupling of NiCoS/NC in atomic orbitals, modulation of electrons by the heterointerface and N-doping, and synergistic effect of dual active sites improving the inherent catalytic activity. The NiCoS/NC composite electrocatalyst exhibited a 177 mV small OER overpotential and a 132 mV small HER overpotential with Faraday efficiencies as high as 96 % and 98 % at 10 mA cm-2 current density. In the two-electrode system, it also requires only an ultra-low voltage of 1.52 V to achieve a 10 mA cm-2 current density, and it shows excellent long-term water splitting stability. This provides a new idea for the development of transition metal-based bifunctional electrocatalysts.

Dynamic Active Sites in Electrocatalysis

In-depth understanding of the real-time behaviors of active sites during electrocatalysis is essential for the advancement of sustainable energy conversion. Recently, the concept of dynamic active sites has been recognized as a potent approach for creating self-adaptive electrocatalysts that can address a variety of electrocatalytic reactions, outperforming traditional electrocatalysts with static active sites. Nonetheless, the comprehension of the underlying principles that guide the engineering of dynamic active sites is presently insufficient. In this review, we systematically analyze the fundamentals of dynamic active sites for electrocatalysis and consider important future directions for this emerging field. We reveal that dynamic behaviors and reversibility are two crucial factors that influence electrocatalytic performance. By reviewing recent advances in dynamic active sites, we conclude that implementing dynamic electrocatalysis through variable reaction environments, correlating the model of dynamic evolution with catalytic properties, and developing localized and ultrafast in situ/operando techniques are keys to designing high-performance dynamic electrocatalysts. This review paves the way to the development of the next-generation electrocatalyst and the universal theory for both dynamic and static active sites.

Organic-inorganic hybrid interfaces with π-d electron coupling for preventing metal and sulfur leaching toward enhanced oxygen evolution reaction

Transition metal sulfides (TMSs) catalysts with high catalytic oxygen evolution reaction (OER) activity have been extensively studied, especially Fe and Co-based sulfides. Fe and Co active sites with a strong synergistic effect, which can adjust the electron density distribution and effectively improve the electrocatalytic OER activity. However, TMSs have poor stability in alkaline environment caused by metal ions and sulfur elements are facilitated to dissolve. In this work, TMSs was modified by polyaniline (PANI) to inhibit the precipitation of iron, cobalt, and sulfur elements and enhance its stability under alkaline conditions. Moreover, π-d structure can also be formed by the coating of PANI, which can further adjust its own electronic structure on the basis of stabilizing the TMSs structure, so as to improve the electrochemical performance, rendering them to stably operate at harsh environment for more than 90 h. These findings offer new guidance for improving the electrocatalytic stability of TMSs.

Elevating Oxygen Evolution using Iron Phthalocyanine Infused Vanillic acid Electrocatalyst

Oxygen evolution reaction (OER) is the bottle neck step in water splitting reaction towards the realization of hydrogen based clean energy production and storage. Metal air batteries and polymer electrolyte membrane fuel cells (PEMFC) are the alternative green energy systems that utilise O2 and H2 in the production of continuous and high energy output without the utilization of carbon based fuels which are the major sources of pollution. Transition metal based N4 organics are explored extensively as oxygen electrocatalysts i. e., OER and oxygen reduction reaction (ORR) catalysts because of their ease of synthesis, tuneable properties, low cost and high performance with long term stability. Here, vanillic acid functionalized iron phthalocyanine (FeVAPc) was synthesised and characterised by various spectroscopic techniques. The novel FeVAPc exhibited good thermal stability and was coated on Ni foam for OER studies. The scanning electron microscopy images showed net-work like surface morphology and the X-ray photoelectron spectroscopy indicated the presence of Fe in +3 oxidation state. The Ni/FeVAPc demonstrated excellent electrocatalytic activity for OER with overpotential of 312 mV at 10 mA.cm-2 current density in 1.0 M KOH electrolyte. The designed organic based catalyst exhibited lesser Tafel slope value which is nearer to the benchmark catalyst, IrO2. The proposed catalyst exhibited good stability as phthalocyanines are highly stable and do not undergo decomposition even in strong acidic and basic corrosive media. Integration of FeVAPc onto the Ni foam resulted in higher mass activity, lower charge transfer resistance, high active surface area leading to enhanced conductivity and activity. The fabricated Ni/FeVAPc is an appropriate cost-effective, efficient and stable catalyst for OER towards industrial applications.

Iron, Tungsten Dual-Doped Nickel Sulfide as Efficient Bifunctional Catalyst for Overall Water Splitting

Developing low-cost and highly efficient bifunctional catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is a challenging problem in electrochemical overall water splitting. Here, iron, tungsten dual-doped nickel sulfide catalyst (Fe/W-Ni3S2) is synthesized on the nickel foam, and it exhibits excellent OER and HER performance. As a result, the water electrolyze based on Fe/W-Ni3S2 bifunctional catalyst illustrates 10 mA cm-2 at 1.69 V (without iR-compensation) and highly durable overall water splitting over 100 h tested under 500 mA cm-2. Experimental results and DFT calculations indicate that the synergistic interaction between Fe doping and Ni vacancy induced by W leaching during the in situ oxidation process can maximize exposed OER active sites on the reconstructed NiOOH species for accelerating OER kinetics, while the Fe/W dual-doping optimizes the electronic structure of Fe/W-Ni3S2 and the binding strength of intermediates for boosting HER. This study unlocks the different promoting mechanisms of incorporating Fe and W for boosting the OER and HER activity of Ni3S2 for water splitting, which provides significant guidance for designing high-performance bifunctional catalysts for overall water splitting.

Ceria-Optimized Oxygen-Species Exchange in Hierarchical Bimetallic Hydroxide for Electrocatalytic Water Oxidation

The utilization of rare earth elements to regulate the interaction between catalysts and oxygen-containing species holds promising prospects in the field of oxygen electrocatalysis. Through structural engineering and adsorption regulation, it is possible to achieve high-performance catalytic sites with a broken activity-stability tradeoff. Herein, this work fabricates a hierarchical CeO2/NiCo hydroxide for electrocatalytic oxygen evolution reaction (OER). This material exhibits superior overpotentials and enhanced stability. Multiple potential-dependent experiments reveal that CeO2 promotes oxygen-species exchange, especially OH- ions, between catalyst and environment, thereby optimizing the redox transformation of hydroxide and the adsorption of oxygen-containing intermediates during OER. This is attributed to the reduction in the adsorption energy barrier of Ni to *OH facilitated by CeO2, particularly the near-interfacial Ni sites. The less-damaging adsorbate evolution mechanism and the CeO2 hierarchical shell significantly enhance the structural robustness, leading to exceptional stability. Additionally, the observed "self-healing" phenomenon provides further substantiation for the accelerated oxygen exchange. This work provides a neat strategy for the synthesis of ceria-based complex hollow electrocatalysts, as well as an in-depth insight into the co-catalytic role of CeO2 in terms of oxygen transfer.

Heterostructure Engineering of NiCo-LDHs for Enhanced Energy Storage Performance in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are considered a promising device for next-generation energy storage due to their high safety and low cost. However, developing high-performance cathodes that can be matched with zinc metal anodes remains a challenge in unlocking the full potential of AZIBs. In this study, a typical transition metal layered double hydroxides (NiCo-LDHs) can be in situ reconstructed to NiCo-LDHs/Ni(Co)OOH heterostructure using an electrochemical cycling activation (ECA) method, serving as a novel cathode material for AZIBs. The optimized ECA-NiCo-LDHs cathode demonstrates a high capacity of 181.5 mAh g-1 at 1 A g-1 and retains 75% of initial capacity after 700 cycles at 5 A g-1. The abundant heterointerfaces of the NiCo-LDHs/Ni(Co)OOH material can activate additional active sites for zinc-ion storage and accelerate ion diffusion. Theoretical calculations also suggest the heterostructure can boost charge transfer and regulate ion-adsorption capability, thereby improving the electrochemical performance. Additionally, the flexible AZIBs device exhibits good service performance. This study on interface engineering introduces a new possibility for utilizing LDHs in AZIBs and offers a novel strategy for designing electrode materials.

Direct Electroplating Ruthenium Precursor on the Surface Oxidized Nickel Foam for Efficient and Stable Bifunctional Alkaline Water Electrolysis

Water electrolysis to produce hydrogen (H2) using renewable energy is one of the most promising candidates for realizing carbon neutrality, but its reaction kinetics is hindered by sluggish anodic oxygen evolution reaction (OER). Ruthenium (Ru) in its high-valence state (oxide) provides one of the most active OER sites and is less costly, but thermodynamically unstable. The strong interaction between Ru nanoparticles (NPs) and nickel hydroxide (Ni(OH)2) is leveraged to directly form Ru-Ni(OH)2 on the surface of a porous nickel foam (NF) electrode via spontaneous galvanic replacement reaction. The formation of Ru─O─Ni bonds at the interface of the Ru NPs and Ni(OH)2 (Ru-Ni(OH)2) on the surface oxidized NF significantly enhance stability of the Ru-Ni(OH)2/NF electrode. In addition to OER, the catalyst is active enough for the hydrogen evolution reaction (HER). As a result, it is able to deliver overpotentials of 228 and 15 mV to reach 10 mA cm-2 for OER and HER, respectively. An industry-scale evaluation using Ru-Ni(OH)2/NF as both OER and HER electrodes demonstrates a high current density of 1500 mA cm-2 (OER: 410 mV; HER: 240 mV), surpassing commercial RuO2 (OER: 600 mV) and Pt/C based performance (HER: 265 mV).

Engineering interfacial sulfur migration in transition-metal sulfide enables low overpotential for durable hydrogen evolution in seawater

Hydrogen production from seawater remains challenging due to the deactivation of the hydrogen evolution reaction (HER) electrode under high current density. To overcome the activity-stability trade-offs in transition-metal sulfides, we propose a strategy to engineer sulfur migration by constructing a nickel-cobalt sulfides heterostructure with nitrogen-doped carbon shell encapsulation (CN@NiCoS) electrocatalyst. State-of-the-art ex situ/in situ characterizations and density functional theory calculations reveal the restructuring of the CN@NiCoS interface, clearly identifying dynamic sulfur migration. The NiCoS heterostructure stimulates sulfur migration by creating sulfur vacancies at the Ni3S2-Co9S8 heterointerface, while the migrated sulfur atoms are subsequently captured by the CN shell via strong C-S bond, preventing sulfide dissolution into alkaline electrolyte. Remarkably, the dynamically formed sulfur-doped CN shell and sulfur vacancies pairing sites significantly enhances HER activity by altering the d-band center near Fermi level, resulting in a low overpotential of 4.6 and 8 mV at 10 mA cm-2 in alkaline freshwater and seawater media, and long-term stability up to 1000 h. This work thus provides a guidance for the design of high-performance HER electrocatalyst by engineering interfacial atomic migration.

Operando Spectroscopic Monitoring of Metal Chalcogenides for Overall Water Splitting: New Views of Active Species and Sites

Metal-based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X-ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure-activity relationships of low-crystalline cobalt sulfide (L-CoS) catalysts toward overall water splitting. The operando results for L-CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S-Co-Co-S moieties as the true catalytically active species. In comparison, during the acidic HER, L-CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co-S-Co configurations. Further operando characterizations highlight the crucial formation of high-valent Co4+ species in L-CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co-LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity.

Interface-Engineered NiFe/Ni-S Nanoparticles for Reliable Alkaline Oxygen Production at Industrial Current: A Sulfur Source Confinement Strategy

Using powder-based ink appears to be the most suitable candidate for commercializing the membrane electrode assembly (MEA), while research on the powder-based NPM catalyst for anion exchange membrane water electrolyzer (AEMWE) is currently insufficient, especially at high current density. Herein, a sulfur source (NiFe Layered double hydroxide adsorbedSO 4 2 - ${\mathrm{SO}}_4^{2 - }$ ) confinement strategy is developed to integrate Ni3S2 onto the surface of amorphous/crystalline NiFe alloy nanoparticles (denoted NiFe/Ni-S), achieving advanced control over the sulfidation process for the formation of metal sulfides. The constructed interface under the sulfur source confinement strategy generates abundant active sites that increase electron transport at the electrode-electrolyte interface and improve ability over an extended period at a high current density. Consequently, the constructed NiFe/Ni-S delivers an ultra-low overpotential of 239 mV at 10 mA cm-2 and 0.66 mAcm ECSA - 2 ${\mathrm{cm}}_{{\mathrm{ECSA}}}^{ - 2}$ under an overpotential of 300 mV. The AEMWE with NiFe/Ni-S anode exhibits a cell voltage of 1.664 V @ 0.5 A cm-2 and a 400 h stability at 1.0 A cm-2.

Hydrogen Evolution in Neutral Media by Differential Intermediate Binding at Charge-Modulated Sites of a Bimetallic Alloy Electrocatalyst

The energy barrier to dissociate neutral water has been lowered by the differential intermediate binding on the charge-modulated metal centers of Co85Mo15 sheets supported on Ni-foam (NF), where the overpotential for hydrogen evolution reaction (HER) in 1 M phosphate buffer solution (PBS) is only 50±9 mV at -10 mA cm-2. It has a turnover frequency (TOF) of 0.18 s-1, mass activity of 13.2 A g-1 at -200 mV vs. reversible hydrogen electrode (RHE), and produces 16 ml H2 h-1 at -300 mV vs. RHE, more than double that of 20 % Pt/C. The Moδ+ and Coδ- sites adsorb OH*, and H*, respectively, and the electron injection from Co to H-O-H cleaves the O-H bond to form the Mo-OH* intermediate. Operando spectral analyses indicate a weak H-bonded network for facilitating the H2O*/OH* transport, and a potential-induced reversal of the charge density from Co to the more electronegative Mo, because of the electron withdrawing Co-H* and Mo-OH* species. Co85Mo15/NF can also drive the complete electrolysis of neutral water at only 1.73 V (10 mA cm-2). In alkaline, and acidic media, it demonstrates a Pt-like HER activity, accomplishing -1000 mA cm-2 at overpotentials of 161±7, and 175±22 mV, respectively.

Fe,Co co-implanted dendritic CeO2/CeF3 heterostructure@MXene nanocomposites as structurally stable electrocatalysts with ultralow overpotential for the alkaline oxygen evolution reaction

Exploring low-cost, high-activity, and structurally stable nonprecious metal electrocatalysts for sluggish oxygen evolution reaction (OER) is paramount for water electrolysis. Herein, we successfully prepare a novel Fe,Co-CeO2/CeF3@MXene heterostructure with Fe-Co dual active sites and oxygen vacancies for alkaline OER using an energy-free consumption co-deposition method. Impressively, Fe,Co-CeO2/CeF3@MXene achieves an ultralow overpotential of 192 mV and a long-term stability of 110 h at 10 mA cm-2 without structural changes, thereby outperforming the commercial IrO2 (345 mV). In addition, Fe,Co-CeO2/CeF3@MXene exhibits much superior activity (271 mV) and durability to IrO2 (385 mV) in the real seawater OER. Wind- and solar energy-assisted water electrolysis devices show their promising prospects for sustainable green hydrogen production. Characterization techniques and theoretical calculations reveal that the Fe,Co co-implanted CeO2/CeF3 heterostructure effectively degrades the energy barrier of the OER and optimizes the adsorption strength of *OH, *O, and *OOH intermediates. It exhibits the dual coupling mechanism of the adsorbed evolution and lattice oxygen mechanisms, which synergistically improves the OER performance. This work provides a facile and efficacious strategy for synthesizing a new class of heterostructures to achieve significant enhancement in the activity and stability of OER catalysts.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

High-entropy engineering with regulated defect structure and electron interaction tuning active sites for trifunctional electrocatalysis

High-entropy alloy nanoparticles (HEANs) possessing regulated defect structure and electron interaction exhibit a guideline for constructing multifunctional catalysts. However, the microstructure-activity relationship between active sites of HEANs for multifunctional electrocatalysts is rarely reported. In this work, HEANs distributed on multi-walled carbon nanotubes (HEAN/CNT) are prepared by Joule heating as an example to explain the mechanism of trifunctional electrocatalysis for oxygen reduction, oxygen evolution, and hydrogen evolution reaction. HEAN/CNT excels with unmatched stability, maintaining a 0.8V voltage window for 220 h in zinc-air batteries. Even after 20 h of water electrolysis, its performance remains undiminished, highlighting exceptional endurance and reliability. Moreover, the intrinsic characteristics of the defect structure and electron interaction for HEAN/CNT are investigated in detail. The electrocatalytic mechanism of trifunctional electrocatalysis of HEAN/CNT under different conditions is identified by in situ monitoring and theoretical calculation. Meanwhile, the electron interaction and adaptive regulation of active sites in the trifunctional electrocatalysis of HEANs were further verified by density functional theory. These findings could provide unique ideas for designing inexpensive multifunctional high-entropy electrocatalysts.

Cations induced in situ electrochemical amorphization for enhanced oxygen evolution reaction

Surface reconstruction is widely existed on the surface of transition metal-based catalysts under operando oxygen evolution reaction (OER) condition. The design and optimize the reconstruction process are essential to achieve high electrochemical active surface and thus facilitate the reaction kinetics, whereas still challenge. Herein, we exploit electrolyte engineering to regulate reconstruction on the surface of Fe2O3 catalysts under operando OER conditions. The intentional added cations in electrolyte can participate the reconstruction process and realize a desirable crystalline to amorphous structure conversion, contributing abundant well-defined active sites. Spectroscopic measurements and density functional theory calculation provide insight into the underlying role of amorphous structure for electron transfer, mass transport, and intermediate adsorption. With the assistant of Co2+ cations, the enhanced current density as large as 17.9 % can be achieved at 2.32 V (vs RHE). The present results indicate the potential of electrolyte engineering for regulating the reconstruction process and provide a generalized in-situ strategy for advanced catalysts design.

Ir Single Atoms Boost Metal-Oxygen Covalency on Selenide-Derived NiOOH for Direct Intramolecular Oxygen Coupling

This investigation probes the intricate interplay of catalyst dynamics and reaction pathways during the oxygen evolution reaction (OER), highlighting the significance of atomic-level and local ligand structure insights in crafting highly active electrocatalysts. Leveraging a tailored ion exchange reaction followed by electrochemical dynamic reconstruction, we engineered a novel catalytic structure featuring single Ir atoms anchored to NiOOH (Ir1@NiOOH). This novel approach involved the strategic replacement of Fe with Ir, facilitating the transition of selenide precatalysts into active (oxy)hydroxides. This elemental substitution promoted an upward shift in the O 2p band and intensified the metal-oxygen covalency, thereby altering the OER mechanism toward enhanced activity. The shift from a single-metal site mechanism (SMSM) in NiOOH to a dual-metal-site mechanism (DMSM) in Ir1@NiOOH was substantiated by in situ differential electrochemical mass spectrometry (DEMS) and supported by theoretical insights. Remarkably, the Ir1@NiOOH electrode exhibited exceptional electrocatalytic performance, achieving overpotentials as low as 142 and 308 mV at current densities of 10 and 1000 mA cm-2, respectively, setting a new benchmark for the electrocatalysis of OER.

Surface reconstruction of pyrite-type transition metal sulfides during oxygen evolution reaction

Reconstruction universally occurs over non-layered transition metal sulfides (TMSs) during oxygen evolution reaction (OER), leading to the formation of active species metal (oxy)hydroxide and thus significantly influences the OER performance. However, the reconstruction process and underlying mechanism quantitatively remain largely unexplored. Herein, we proposed an electrochemical reaction mechanism, namely sulfide oxidation reaction (SOR), to elucidate the reconstruction process of pyrite-type TMSs. Based on this mechanism, we evaluated the reconstruction capability of NiS2 doped with transition metals V, Cr, Mn, Fe, Co, Cu, Mo, Ru, Rh, and Ir within different doped systems. Two key descriptors were thus proposed to describe the reconstruction abilities of TMSs: USOR (the theoretical electric potential of SOR) and ΔU (the difference between the theoretical electric potential of SOR and OER), representing the initiation electric potential of reconstruction and the intrinsic reconstruction abilities of TMSs, respectively. Our finding shows that a lower USOR readily initiate reconstruction at a lower potential and a larger ΔU indicating a poorer reconstruction ability of the catalyst during OER. Furthermore, Fe-doped CoS2 was used to validate the rationality of our proposed descriptors, being consistent with the experiment findings. Our work provides a new perspective on understanding the reconstruction mechanism and quantifying the reconstruction of TMSs.

Biological upcycling of nickel and sulfate as electrocatalyst from electroplating wastewater

Upcycling nickel (Ni) to useful catalyst is an appealing route to realize low-carbon treatment of electroplating wastewater and simultaneously recovering Ni resource, but has been restricted by the needs for costly membranes or consumption of large amount of chemicals in the existing upcycling processes. Herein, a biological upcycling route for synchronous recovery of Ni and sulfate as electrocatalysts, with certain amount of ferric salt (Fe3+) added to tune the product composition, is proposed. Efficient biosynthesis of bio-NiFeS nanoparticles from electroplating wastewater was achieved by harnessing the sulfate reduction and metal detoxification ability of Desulfovibrio vulgaris. The optimal bio-NiFeS, after further annealing at 300 °C, served as an efficient oxygen evolution electrocatalyst, achieving a current density of 10 mA·cm-1 at an overpotential of 247 mV and a Tafel slope of 60.2 mV·dec-1. It exhibited comparable electrocatalytic activity with the chemically-synthesized counterparts and outperformed the commercial RuO2. The feasibility of the biological upcycling approach for treating real Ni-containing electroplating wastewater was also demonstrated, achieving 99.5 % Ni2+removal and 41.0 % SO42- removal and enabling low-cost fabrication of electrocatalyst. Our work paves a new path for sustainable treatment of Ni-containing wastewater and may inspire technology innovations in recycling/ removal of various metal ions.