Water Electrolysis toward Elevated Temperature: Advances, Challenges and Frontiers

Zr doping Co3O4 nanowires mediated adsorption of chloridion for efficient natural seawater electrolysis

Natural seawater electrolysis is emerging as a desirable approach for hydrogen production, but it suffers from long-term instability due to severe chloride corrosion. In this study, Zr doped Co3O4 is proposed for natural seawater oxidation, which requires an overpotential of only 570 mV to drive a current density of 100 mA cm-2, and a sustained natural seawater electrolysis at 10 mA cm-2 for 500h exhibits only 0.78 % decay. For practicability, membrane electrode with a self-developed anion exchange membrane is assembled for overall natural seawater electrolysis, and the produced hydrogen is converted to ammonia for storage by coupling nitrate reduction. Density functional theory (DFT) calculations further reveal Zr replacing an octahedral Co atom introduces four energy levels within the gap and the lower conduction band energy is formed by substituting a tetrahedral Co atom. The highest energy barrier of the second dehydrogenation step (*OH to *O) reaches 1.82 eV and it is slightly reduced to 1.79 eV after Co3O4 is transformed to CoOOH. Zr-adsorbed chloridion sharply increases its absorption energy on Co sites to a positive value of 0.27 eV, which effectively protects Co active sites from chloride attack.

Construction of hydrophilic-and-cationic metalloporphyrin-based polymers for electrocatalytic small molecule activation

The electrocatalytic activation of small molecules for the production of hydrogen, oxygen, and ammonia represents a novel approach towards achieving environmentally sustainable chemical transformations. Covalent organic polymers have attracted extensive research due to their designable structures and well-defined catalytic sites. Herein, we elaborate on the synthesis, characterization, and electrochemical analyses of a class of hydrophilic and cationic polymers (M-MV) with metalloporphyrins serving as the active central nodes. Owing to the inclusion of cationic viologen units, the as-prepared M-MV exhibited excellent hydrophilicity. When the metal centres vary, three metalloporphyrin-based covalent polymers possess specific catalytic properties for distinct electrocatalytic reactions in aqueous solution. The experimental results show that Co-MV is more favorable for the electrocatalytic HER, Ni-MV is more conducive to the electrocatalytic OER, and Cu-MV exhibits the optimal catalytic activity in the electrocatalytic NO2-RR.

Insight into the structural reconstruction of alkaline water oxidation electrocatalysts

The oxygen-evolution reaction (OER) is an indispensable component of various energy storage and conversion electrocatalytic systems. However, the slow reaction kinetics have forced the development of advanced, efficient, and inexpensive OER electrocatalysts to break through the bottleneck of its application. Recently, the structural reconstruction of precatalysts has provided a promising avenue to boost the catalytic activity of electrocatalysts. Structural reconstruction implies atomic rearrangement and composition change of the pristine catalytic materials, which is a very complex process. Therefore, it is very crucial to have a deep understanding of the reconstruction chemical process and then modulate the reconstruction by deliberate design of electrochemical conditions and precatalysts. However, a systematic review of the structural reconstruction process, research methods, influencing factors and structure-performance relationship remains elusive, significantly impeding the further developments of efficient electrocatalysts based on structural reconstruction chemistry. This critical review is dedicated to providing a deep insight into the structural reconstruction during alkaline water oxidation, comprehensively summarizing the basic research methods to understand the structural evolution process and various factors affecting the structural reconstruction process, and providing a reference and basis for regulating the dynamic reconstruction. Moreover, the impact of reconstruction on the structure and performance is also covered. Finally, challenges and perspectives for the future study on structural reconstruction are discussed. This review will offer future guidelines for the rational development of state-of-the-art OER electrocatalysts.

Transition-Metal-Doped Nickel-Cobalt Layered Double Hydroxide Catalysts for an Efficient Oxygen Evolution Reaction

Hydrogen plays a vital role in the global shift toward cleaner energy solutions, with water electrolysis standing out as one of the most promising techniques for generating hydrogen. Despite its potential, the oxygen evolution reaction (OER) involved in this process faces significant challenges, including high overpotentials and slow reaction rates, which underscore the need for advanced electrocatalytic materials to enhance efficiency. Noble metal catalysts are effective but expensive, so transition-metal-based electrocatalysts like nickel-cobalt layered double hydroxides (NiCo LDHs) have become promising alternatives. In this research, a series of NiCo LDH catalysts doped with Fe, Mn, Cu, and Zn were effectively produced using a one-step hydrothermal technique. Among the catalysts, the Fe-doped NiCo LDH exhibited OER activity, achieving a lower overpotential (289 mV) at a current density of 50 mA/cm2, which was far better than the 450 mV of the undoped NiCo LDH. The Mn-, Cu-, and Zn-NiCo LDHs also exhibited lower overpotentials of 414 mV, 403 mV, and 357 mV, respectively, at this current density. The Fe-doped NiCo LDH had a 3D layered nanoflower structure, increasing the surface area for reactant adsorption. The electrochemically active surface area (ECSA), as indicated by the double-layer capacitance (Cdl), was larger in the doped samples. The Cdl value of the Fe-doped NiCo LDH was 3.72 mF/cm2, significantly surpassing the 0.82 mF/cm2 of the undoped NiCo LDH. These changes improved charge transfer and optimized reaction kinetics, enhancing the overall OER performance. This study offers significant contributions to the development of efficient electrocatalysts for the OER, advancing the understanding of key design principles for enhanced catalytic performance.

Corrosion-resistant titanium-based electrodes synergistically stabilized with polymer for hydrogen evolution reaction

The economic and reasonable design of highly stable and corrosion-resistant electrodes is fundamental to achieving the industrial-scale hydrogen productions via water electrolysis, but electrodes' premature failures are often caused by corrosion and stress damage. Therefore, these challenges are successfully solved by utilizing conductive and crack-resistant polyaniline "stabilizer" with a mild chemical plating process to construct the catalytic electrode on a titanium substrate (15 %PANI-NiB@Ti) in the present work. The 15 %PANI-NiB@Ti catalytic electrodes have been in continuous operation for 350 h at the current density of 200 mA cm-2 with the high efficiency of 98.4 % in a 323.15 K environment. With the high economy and universality, the catalytic electrodes have good catalytic performance and reliability in the extreme industrial environments, such as high temperature, air, and high current density. Except for the above advantages, the 15 %PANI-NiB@Ti catalytic electrodes also have good cracking resistance, which provides a novel and feasible approach to the industrial application of transition metal catalytic electrodes.

High-Entropy Oxychalcogenide for Hydrogen Spillover Enhanced Hydrogen Evolution Reaction in Proton and Anion Exchange Membrane Water Electrolyzers

The hydrogen spillover phenomenon provides an expeditious reaction pathway via hydrogen transfer from a strong H adsorption site to a weak H adsorption site, enabling a cost-efficient hydrogen evolution reaction (HER) analogous to platinum with moderate H adsorption energy. Here, a high-entropy oxychalcogenide (HEOC) comprising Co, Ni, Mo, W, O, Se, and Te is prepared by a two-step electrochemical deposition for hydrogen spillover-enhanced HER in acidic and alkaline water electrolysis. The anodic-cathodic reversal current enables the co-deposition of cations and aliovalent anions, facilitating a glass structure with multiple active sites for hydrogen spillover. The HEOC exhibits low overpotentials of 52 and 57 mV to obtain a current density of 10 mA cm-2 in acidic and alkaline media, respectively, and long-term stability for 500 h. The electrochemical and analytical approaches elucidate the hydrogen transfer toward Mo/W-O sites in both acid and alkaline HERs. Meanwhile, the other sites act as hydrogen adsorption or water dissociation-derived hydroxide adsorption sites, showing accommodable behavior in acidic and alkaline media. The HEOC exhibits a practically high current of 1 A cm-2 at cell voltages of 1.78 and 1.89 V and long-term stability for 100 h in proton and anion exchange membrane water electrolyzers, respectively.

Structure and Mixed Proton-Electronic Conductivity in Pr and Nb-Substituted La5.4MoO12-δ Ceramics

Lanthanide molybdates are materials known for their mixed proton-ionic conductivity. This study investigates the effects of Pr content and Nb-doping on the crystal structure and electrical properties of the La5.4-xPrxMo1-yNbyO12-δ (x = 0, 1.35, 2.7, 4.05, 5.4; y = 0, 0.1) series. The research focuses on two primary objectives: (i) enhancing the electronic conductivity through the use of Pr4+/Pr3+ redox pairs and (ii) increasing the ionic conductivity through Nb5+ aliovalent doping. The materials were thoroughly characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission and scanning electron microscopy (TEM and SEM), and complex impedance spectroscopy. The average crystal structure of the materials depended significantly on the Pr content. In general, compositions with a higher Pr content crystallize in a cubic fluorite-type structure, whereas those with a lower Pr content stabilize a rhombohedral polymorph. However, detailed TEM studies reveal a more complex local crystal structure characterized by nanodomains and incommensurate modulations. The highest conductivity values were observed in a N2 atmosphere for compositions with an elevated Pr content, with values of 0.17 and 204.4 mS cm-1 for x = 0 and x = 5.4, respectively, at 700 °C, which is attributed to electronic conduction mediated by the Pr4+/Pr3+ redox pair, as confirmed by XPS. These findings highlight the potential of tailored doping strategies to optimize the conducting properties of lanthanide molybdates for specific high-temperature electrochemical applications.

Promoting in-situ stability of hydroxide exchange membranes by thermally conductive network for durable water electrolysis

Hydroxide exchange membrane (HEM) water electrolysis is promising for green hydrogen production due to its low cost and excellent performance. However, HEM often has insufficient stability in strong alkaline solutions, particularly under in-situ electrolysis operation conditions, hindering its commercialization. In this study, we discover that the in-situ stability of HEM is primarily impaired by the locally accumulated heat in HEM due to its low thermal conductivity. Accordingly, we propose highly thermally conductive HEMs with an efficient three-dimensional (3D) thermal diffusion network to promote the in-situ stability of HEM for water electrolysis. Based on the 3D heat conductive network, the thermal conductivity of polymeric HEM is boosted by 32 times and thereby reduce the HEM temperature by up to 4.9 °C in a water electrolyzer at the current density of 1 A cm-2. Thus, the thermally conductive HEM exhibits negligible degradation after 20,000 start/stop cycles and reduces the degradation rate by 6 times compared to the pure polymeric HEM in a water electrolyzer. This study manifests the significance of thermal conductivity of HEM on the durability of water electrolysis, which provides guidelines on the rational design of highly durable HEMs in practical operation conditions for water electrolysis, fuel cells, and beyond.

Turning the Surface Electronic Effect Over Core-Shell CoS2─FexCo1-xS2 Nanooctahedra Toward Electrochemical Water Splitting in the Alkaline Medium

The long-term challenge in overall water splitting is the conflict in the pH condition of electrolytes for achieving efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the same time, in addition to the typical cost issue in catalysts. It hence raises an intense research interest in seeking cost-efficient non-noble metal electrocatalysts as well as compromising electrolyte conditions for electrocatalytic HER and OER. To tackle the problems, various approaches are demonstrated to engineer the electronic effect on the active sites of catalysts for enhancing the activities. In this work, the core-shell CoS2─FexCo1-xS2 nanooctahedra is fabricated with a tunable Fe content over the surface and took them as the model catalyst for systematic studies in alkaline OER and HER. By various X-ray spectroscopies as well as electron microscopy, the results showed that the shells of CoS2─FexCo1-xS2 nanooctahedra formed the {111} surfaces of Fe0.9Co1.0S2 and Fe0.25Co0.75S2 with and without the promotion by OH- anions during the syntheses. Catalyzed by the CoS2, Fe0.25Co0.75S2, and Fe0.9Co1.0S2 {111} surfaces, the results of alkaline OER and HER indicated the Fe0.9Co1.0S2 the most superior activities by virtue of the optimized Fe─Co electronic effect. From the predictions by density functional theory (DFT) calculations in reaction thermodynamics, the energy barriers in OER and HER both follow the order of Fe0.9Co0.1S2(111) < Fe0.25Co0.75S2(111) < CoS2(111). However, FeS2(111) is worse than Fe0.9Co0.1S2(111). From the confirmations by in-situ X-ray spectroscopies in reaction kinetics, the Co sites of Fe0.9Co0.1S2(111) on the core-shell nanooctahedra exhibited much higher activities than those of CoS2(111) under the applied potentials for OER and HER, which reflected the electronic benefits from the existing Fe neighbors.

2D MOF Structure Tuning Atomic Ru Sites for Efficient and Robust Proton Exchange Membrane Water Electrolysis

Developing cost-effective ruthenium (Ru)-based HER electrocatalysts as alternatives to commercial Pt/C is crucial for the advancement of proton exchange membrane water electrolysis (PEMWE). However, the strong hydrogen adsorption of Ru-based catalysts restricts its activity. Herein, a strategy is reported to tune the electronic structure and improve mass transfer by implanting Ru atoms onto the (002) facet of two-dimensional zeolitic imidazolate framework-67 (Ru@LZIF(002)) to optimize the d-band center (εd) of Ru and the hydrogen spillover behavior. Benefiting from the ultrathin nanosheet structure and optimized εd of Ru, the over-strong H adsorption energy is weakened and the electron/mass transfer is facilitated. Ru@LZIF(002) exhibits an overpotential of 9.2 mV at 10 mA cm-2 and a long-lasting stability of 35 days at 100 mA cm-2. The mass activity and price activity of Ru@LZIF(002) is 2.9 and 14.7 times higher than Pt/C, respectively. More impressively, Ru@LZIF(002) delivers a cell voltage of 2.01 V at a high current density of 4 A cm-2 in PEMWE. The excellent long-term durability of 1200 hours operating at 4 A cm-2 with an ultralow decay rate of 7.5 × 10-3 mV h-1 has been achieved, making it promising as a cost-effective alternative to Pt/C catalyst for PEMWE applications.

Porphyrin-Confined Supported Ultrasmall Ir Clusters as Oxygen Evolution Catalysts for Water Electrolysis

Metalloporphyrin ligands themselves can participate in the redox process, making them beneficial in promoting the multielectron catalytic process of the oxygen evolution reaction (OER). However, OER catalysts synthesized by traditional chemical strategies face challenges in water electrolysis. We synthesized high-performance and stable alkaline and acidic OER electrocatalysts loaded with ultrasmall iridium clusters by taking advantage of the attraction and confinement of Ir atoms by the Ir-N bonds formed by the porphyrin cavity. The N in the porphyrin cavity forms an Ir-N bond with Ir so that Ir carries a negative charge and attracts Ir atoms to form ultrasmall Ir clusters above the cavity to adjust the electronic structure of the Ir clusters. The resulting catalyst Tpyp-Ir(IrOX) exhibits a small overpotential (242 and 259 mV) at a current density of 10 mA cm-2 in alkaline and acidic conditions and demonstrates good long-term operational stability. In addition, Tpyp-Ir(IrOX) exhibits a higher transition frequency (TOF) (1.69 O2 s-1 at 300 mV) in 1 M KOH, which is 7 times that of Ir/C.

Hydrogen storage in a sandwich structure by assembly of BNs and MOFs

Hybrid NUS-8/BNs with a sandwich-like structure were synthesized by layered h-BNs and MOF-NUS-8 via assembly. Thanks to the more abundant exposed binding sites and partial ionic bonding properties of BN, the NUS-8/BN exhibits greatly improved H2 adsorption capacity, which is 1.7 times that of h-BN nanosheets.

Electrocatalytic Aromatic Alcohols Splitting to Aldehydes and H2 Gas

Selective electrocatalytic transformation of alcohols to aldehydes offers an efficient and environmentally friendly platform for the simultaneous production of fine chemicals and pure hydrogen gas. However, traditional alcohol oxidation reactions (AORs) in aqueous electrolyte unavoidably face competitive reactions (e.g., water oxidation and overoxidations reactions) for the presence of active oxygen species from water oxidation, causing an unwanted decrease in final efficiency and selectivity. Here, we developed an integrated all-solid proton generator-transfer electrolyzer to trigger the pure alcohol splitting reaction (ASR). In this splitting process, only O-H and C-H bonds can be cleaved at the proton generator (Pt nanoparticles), thereby completely avoiding all competitive reactions involving oxygen active species to give a > 99% selectivity to aldehydes. The as-generated protons are transported to the cathode by a three-dimensional (3D) conducting network (assemblies of ionomers and carbon spheres) for efficient hydrogen production. Unlike the poor selectivity (<22%) and durability (<3 h) of a conventional AOR electrolyzer, this ASR electrolyzer could be continuously operated at a low cell voltage of 1.2 V for at least 10 days to give a high Faradaic efficiency of 80-93% for aldehyde production.

Modulating the Electronic Structure of Cobalt-Vanadium Bimetal Catalysts for High-Stable Anion Exchange Membrane Water Electrolyzer

Modulating the electronic structure of catalysts to effectively couple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential for developing high-efficiency anion exchange membrane water electrolyzer (AEMWE). Herein, a coral-like nanoarray composed of nanosheets through the synergistic layering effect of cobalt and the 1D guiding of vanadium is synthesized, which promotes extensive contact between the active sites and electrolyte. The HER and OER activities can be enhanced by modulating the electronic structure through nitridation and phosphorization, respectively, enhancing the strength of metal-H bond to optimize hydrogen adsorption and facilitating the proton transfer to improve the transformation of oxygen-containing intermediates. Resultantly, the AEMWE achieves a current density of 500 mA cm-2 at 1.76 V for 1000 h in 1.0 M KOH at 70 °C. The energy consumption is 4.21 kWh Nm-3 with the producing hydrogen cost of $0.93 per kg H2. Operando synchrotron radiation and Bode phase angle analyses reveal that during the high-energy consumed OER, the dissolution of vanadium species transforms distorted Co-O octahedral into regular octahedral structures, accompanied by a shortening of the Co-Co bond length. This structural evolution facilitates the formation of oxygen intermediates, thus accelerating the reaction kinetics.

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

New Morphology Modifier Enables the Preparation of Ultra-Long Platinum Nanowires Excluding Mo Component for Efficient Oxygen Reduction Reaction Performance

The structural tailoring of Pt-based catalysts into 1D nanowires for oxygen reduction reactions (ORR) has been a focus of research. Mo(CO)6 is commonly used as a morphological modifier to form nanowires, but it is found that it inevitably leads to Mo doping. This doping introduces unique electrochemical signals not seen in other Pt-based catalysts, which can directly reflect the stability of the catalyst. Through experiments, it is demonstrated that Mo doping is detrimental to ORR performance, and theoretical calculations have shown that Mo sites that are inherently inactive also poison the ORR activity of the surrounding Pt. Therefore, a novel gas-assisted technique is proposed to replace Mo(CO)6 with CO, which forms ultrafine nanowires with an order of magnitude increase in length, ruling out the effect of Mo. The catalyst performs at 1.24 A mgPt -1, 7.45 times greater than Pt/C, demonstrating significant ORR mass activity, and a substantial improvement in stability. The proton exchange membrane fuel cell using this catalyst provides a higher power density (0.7 W cm-2). This study presents a new method for the preparation of ultra-long nanowires, which opens up new avenues for future practical applications of low-Pt catalysts in PEMFC.

Aerophobic/Hydrophilic Nickel-Iron Sulfide Nanoarrays for Energy-Saving Hydrogen Production from Seawater Splitting Assisted by Sulfion Oxidation Reaction

Electrolysis of infinite seawater is a promising and sustainable approach for clean hydrogen production. However, it remains a big challenge to accomplish corrosion-resistant and chlorine-free seawater electrolysis at low power input. Herein, the bimetallic nickel-iron sulfide-based electrocatalytic nanoarrays are constructed by a facile hydrothermal sulfidation of redox-etched iron foam (IF), which manifests an effective and reliable strategy for the sulfion oxidation reaction (SOR) to assist alkaline seawater electrolysis for the achievement of energy-saving hydrogen production and value-added sulfion upcycling. The resulting NiFeSx/FeNi3/IF required 0.353 and 0.415 V vs RHE for SOR at current densities of 50 and 100 mA cm-2, which are considerably lower than the theoretical potential of the oxygen evolution reaction (OER, 1.23 V vs RHE). In situ spectroscopy analysis demonstrated efficient sulfion oxidation on the surface of NiFeSx/FeNi3/IF. Furthermore, the NiFeSx/FeNi3/IF-assembled electrolyzer delivered a greatly reduced cell voltage of 0.92 V at 50 mA cm-2 and maintains excellent durability for 30 h, achieving high Faradaic efficiency for both hydrogen production and sulfion degradation. In addition, under natural sunlight (660.4 W m-2), only a 0.947 V voltage of the solar panel smoothly powers the SOR-coupled seawater electrolysis for green hydrogen production and economic sulfur recovery.

Dimensionality-Tailored Ferromagnetism in Quasi-Two-Dimensional MnSe2 for the Magnetoelectrochemical Hydrogen Evolution Reaction in Alkaline Media

The application of an external magnetic field to the cathode shows great promise in facilitating the hydrogen evolution reaction (HER) via water electrolysis. However, the criteria for designing such cathodes are still under investigation. Among various aspects, understanding the effect of different magnetic states of the cathode material is crucial, especially for the HER in alkaline conditions, which possesses different reaction steps compared to that in acidic conditions. Herein, we present MnSe2 as a cathode material for the magneto-electrocatalytic HER in alkaline media, utilizing its dimension-dependent magnetic phase transition. By tailoring its dimensionality, we have achieved room-temperature ferromagnetism in its quasi-two-dimensional (2D) form, whereas its bulk counterpart exhibits paramagnetism. Upon being subjected to a low external magnetic field of 0.4 T at -182 mV (vs RHE) overpotential, quasi-2D MnSe2 exhibited a 120% improvement in current density compared to itself at zero magnetic field, while negligible changes were observed in the bulk material. This performance enhancement under a magnetic field could originate from the higher spin polarization of the ferromagnetic catalyst. This work signifies a conceptual advancement of the catalyst's spin state in magnetically enhanced electrocatalytic reaction kinetics.

Navigating the future of solid oxide fuel cell: Comprehensive insights into fuel electrode related degradation mechanisms and mitigation strategies

Solid Oxide Fuel Cells (SOFCs) have proven to be highly efficient and one of the cleanest electrochemical energy conversion devices. However, the commercialization of this technology is hampered by issues related to electrode performance degradation. This article provides a comprehensive review of the various degradation mechanisms that affect the performance and long-term stability of the SOFC anode caused by the interplay of physical, chemical, and electrochemical processes. In SOFCs, the most used anode material is nickel-yttria stabilized zirconia (Ni-YSZ) due to its advantages of high electronic conductivity and high catalytic activity for H2 fuel. However, various factors affecting the long-term stability of the Ni-YSZ anode, such as redox cycling, carbon coking, sulfur poisoning, and the reduction of the triple phase boundary length due to Ni particle coarsening, are thoroughly investigated. In response, the article summarizes the state-of-the-art diagnostic tools and mitigation strategies aimed at improving the long-term stability of the Ni-YSZ anode.

Phosphorus-doped nickel cobalt oxide (NiCo2O4) wrapped in 3D hierarchical hollow N-doped carbon nanoflowers as highly efficient bifunctional electrocatalysts for overall water splitting

Properly design and fabricate capable electrocatalysts with 3D hierarchical hollow framework to realize cost-effective and efficacious overall water splitting (OWS) are particularly meaningful for the large-scale arrangement of pivotal energy technology. In this study, P-doped NiCo2O4 nanoparticles encapsulated in N-doped carbon hierarchical hollow nanoflowers (P-NiCo2O4@NCHHNFs) were constructed using the hydrothermal-pyrolysis-phosphorization approach. This fascinating architecture can not merely serve as a conductive pathway for electron transfer, but at the same time effectively inhibited the aggregation and corrosion of the NiCo2O4 nanoparticles. Additionally, the P doping not only regulates electronic structure configuration to boost the intrinsic activity of the catalyst, but also enhance electrochemical surface areas to reveal more accessible active sites. Attributing to these characteristics, the as-prepared P-NiCo2O4@NCHHNFs exhibit preeminent electrocatalytic performance with low overpotentials of 283 mV and 162 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) (at 10 mA cm-2), respectively. Specifically, by using the P-NiCo2O4@NCHHNFs as bifunctional catalysts, a low potential of 1.56 V (at 10 mA cm-2) is sufficient to drive overall water splitting with splendid durability. This study proposed an innovative strategy for the conceiving and fabricating high-performance catalysts via heteroatom-doping.

Inhibiting Dissolution of Active Sites in 80 °C Alkaline Water Electrolysis by Oxyanion Engineering

Commercial alkaline water electrolysers typically operate at 80 °C to minimize energy consumption. However, NiFe-based catalysts, considered as one of the most promising candidates for anode, encounter the bottleneck of high solubility at such temperatures. Herein, we discover that the dissolution of NiFe layered double hydroxides (NiFe-LDH) during operation not only leads to degradation of anode itself, but also deactivates cathode for water splitting, resulting in decay of overall electrocatalytic performance. Aiming to suppress the dissolution, we employed oxyanions as inhibitors in electrolyte. The added phosphates to the electrolyte inhibit the loss of NiFe-LDH active sites at 400 mA cm-2 to 1/3 of the original amount, thus reducing the rate of performance decay by 25-fold. Furthermore, the usage of borates, sulfates, and carbonates yields similar results, demonstrating the reliability and universality of the active site dissolution inhibitor, and its role in elevated water electrolysis.

Self-assembled PtNi layered metallene nanobowls for pH-universal electrocatalytic hydrogen evolution

Compared with layered materials such as graphite and transition metal disulfide compounds with highly anisotropic in-plane covalent bonds, it is inherently more challenging to obtain independent metallic two-dimensional films with atomic thickness. In this study, PtNi layered metallene nanobowls (LMBs) with multilayer atomic-scale nanosheets and bowl-like structures have been synthesized in one step using structural and electronic effects. The material has the advantage of catalyzing pH-universal hydrogen evolution reaction (HER). Compared with Pt/C, PtNi LMBs exhibited excellent HER activity and stability under all pH conditions. The overpotentials of 10 mA cm-2 at 0.5 M H2SO4, 1.0 M phosphate buffer and 1.0 M KOH were 14.8, 20.3, and 34.0 mV, respectively. Under acidic, neutral and alkaline conditions, the HER Faraday efficiencies reach 98.97%, 98.85%, and 99.04%, respectively. This study provides an example for the preparation of unique multilayer nanobowls, and also provides a basic research platform for the development of special HER materials.

Unveiling Key Interface Characteristics of Ni/Yttria-Stabilized Zirconia Solid Oxide Cell Electrodes in H2O Electroreduction Using Operando X-ray Photoelectron Spectroscopy

Nickel/yttria-stabilized zirconia (YSZ) composites are the most commonly used fuel electrodes for solid oxide cells. While microstructural changes of Ni/YSZ during operational conditions have been thoroughly investigated, there is limited knowledge regarding Ni/YSZ surface chemistry under working conditions. In this study, we examine the interaction between Ni/YSZ electrodes and water vapor under open circuit and polarization conditions, utilizing near ambient pressure soft and hard X-ray photoelectron spectroscopies. Miniature cells with conventional porous Ni/YSZ composite cermet cathodes were modified to facilitate the direct spectroscopic observation of the functional electrode's areas close to the interface with the YSZ electrolyte. The results highlight dynamic changes in the oxidation state and composition of Ni/YSZ under H2 and H2O atmospheres. We also quantify the accumulation of impurities on the electrode surface. Through adjustments in the pretreatment of the cell, the correlation between the nickel surface oxidation state and the cell's electrochemical performance during H2O electroreduction is established. It is unequivocally shown that nickel surface oxidation in H2O electrolysis favors NiO over Ni(OH)x, providing critical insights into the mechanism of Ni-phase redistribution within the electrode during long-term operation. Depth-dependent photoemission measurements, combined with theoretical quantitative simulations, reveal that NiO and Ni phases are uniformly mixed on the surface during H2O electrolysis. This differs from the conventional expectation of a NiO-shell/Ni-core configuration in gas phase oxidation. These findings provide crucial insights into the surface chemistry of Ni/YSZ electrodes under conditions relevant to H2O electrolysis, elucidating their impact on the electrochemical performance of the cell.

Multiple-perspective design of hollow-structured cerium-vanadium-based nanopillar arrays for enhanced overall water electrolysis

It is critical and challenging to develop highly active and low cost bifunctional electrocatalysts for the hydrogen/oxygen evolution reaction (HER/OER) in water electrolysis. Herein, we propose cerium-vanadium-based hollow nanopillar arrays supported on nickel foam (CeV-HNA/NF) as bifunctional HER/OER electrocatalysts, which are prepared by etching the V metal-organic framework with Ce salt and then pyrolyzing. Etching results in multidimensional optimizations of electrocatalysts, covering substantial oxygen vacancies, optimized electronic configurations, and an open-type structure of hollow nanopillar arrays, which contribute to accelerating the charge transfer rate, regulating the adsorption energy of H/O-containing reaction intermediates, and fully exposing the active sites. The reconstruction of the electrocatalyst is also accelerated by Ce doping, which results in highly active hydroxy vanadium oxide interfaces. Therefore, extremely low overpotentials of 170 and 240 mV under a current density of 100 mA cm-2 are achieved for the HER and OER under alkaline conditions, respectively, with long-term stability for 300 h. An electrolysis cell with CeV-HNA/NF as both the cathode and anode delivers a small voltage of 1.53 V to achieve water electrolysis under 10 mA cm-2, accompanied by superior durability for 150 h. This design provides an innovative way to develop advanced bifunctional electrocatalysts for overall water electrolysis.

Plasma-Engraved Lattice-Matched NiO/NiFe2O4 Heterostructure with Ample Oxygen Vacancies for Efficient Water Electrolysis and Zn-Air Batteries

Heterogeneous interface and defect engineering offer effective pathways to accelerate oxygen evolution reaction (OER) charge transfer kinetics and motivate optimal intrinsic catalytic activity. Herein, we report the lattice-matched NiO/NiFe2O4 heterostructure with ample oxygen vacancies (Vo-NiO/NiFe2O4) induced by a feasible hydrothermal followed by calcination and plasma-engraving assistant technique, which shows the unique porous microflower arrangement of intertwined nanosheets. Benefitting from the synergetic effects between lattice-matched heterointerface and oxygen vacancies induce the strong electronic coupling, optimized OH-/O2 diffusion pathway and ample active sites, thus-prepared Vo-NiO/NiFe2O4 presents a favorable OER performance with a low overpotential (261 mV @ 10 mA cm-2) and small Tafel slope (39.4 mV dec-1), even surpassing commercial RuO2 catalyst. Additionally, the two-electrode configuration water electrolyzer and rechargeable zinc-air battery assembled by Vo-NiO/NiFe2O4 catalyst show the potential practical application directions. This work provides an innovative avenue for strengthening OER performance toward water electrolysis and Zn-air batteries via the interface and vacancy engineering strategy.

Optimizing Ionomer Distribution in Anode Catalyst Layer for Stable Proton Exchange Membrane Water Electrolysis

The high cost of proton exchange membrane water electrolysis (PEMWE) originates from the usage of precious materials, insufficient efficiency, and lifetime. In this work, an important degradation mechanism of PEMWE caused by dynamics of ionomers over time in anode catalyst layer (ACL), which is a purely mechanical degradation of microstructure, is identified. Contrary to conventional understanding that the microstructure of ACL is static, the micropores are inclined to be occupied by ionomers due to the localized swelling/creep/migration, especially near the ACL/PTL (porous transport layer) interface, where they form transport channels of reactant/product couples. Consequently, the ACL with increased ionomers at PTL/ACL interface exhibit rapid and continuous degradation. In addition, a close correlation between the microstructure of ACL and the catalyst ink is discovered. Specifically, if more ionomers migrate to the top layer of the ink, more ionomers accumulate at the ACL/PEM interface, leaving fewer ionomers at the ACL/PTL interface. Therefore, the ionomer distribution in ACL is successfully optimized, which exhibits reduced ionomers at the ACL/PTL interface and enriches ionomers at the ACL/PEM interface, reducing the decay rate by a factor of three when operated at 2.0 A cm-2 and 80 °C. The findings provide a general way to achieve low-cost hydrogen production.

Chromium-Induced High Covalent Co-O Bonds for Efficient Anodic Catalysts in PEM Electrolyzer

The proton exchange membrane water electrolyzer (PEMWE), crucial for green hydrogen production, is challenged by the scarcity and high cost of iridium-based materials. Cobalt oxides, as ideal electrocatalysts for oxygen evolution reaction (OER), have not been extensively applied in PEMWE, due to extremely high voltage and poor stability at large current density, caused by complicated structural variations of cobalt compounds during the OER process. Thus, the authors sought to introduce chromium into a cobalt spinel (Co3O4) catalyst to regulate the electronic structure of cobalt, exhibiting a higher oxidation state and increased Co-O covalency with a stable structure. In-depth operando characterizations and theoretical calculations revealed that the activated Co-O covalency and adaptable redox behavior are crucial for facilitating its OER activity. Both turnover frequency and mass activity of Cr-doped Co3O4 (CoCr) at 1.67 V (vs RHE) increased by over eight times than those of as-synthesized Co3O4. The obtained CoCr catalyst achieved 1500 mA cm-2 at 2.17 V and exhibited notable durability over extended operation periods - over 100 h at 500 mA cm-2 and 500 h at 100 mA cm-2, demonstrating promising application in the PEMWE industry.

Silver Nanowires Cascaded Layered Double Hydroxides Nanocages with Enhanced Directional Electron Transport for Efficient Electrocatalytic Oxygen Evolution

Designing and fabricating highly efficient oxygen evolution reaction (OER) electrocatalytic materials for water splitting is a promising and practical approach to green and sustainable low-carbon energy systems. Herein, a facile in situ growth self-template strategy by using ZIF-67 as a consumable layered double hydroxides (LDHs) template and silver nanowires (AgNWs) as 1D conductive cascaded substrate to controllably synthesize the target AgNWs@CoFe-LDH composites with unique hollow shell sugar gourd-like structure and enhanced directional electron transport effect is reported. The AgNWs exhibit the key functions of the close connection of CoFe-LDH nanocages and the support of the directional electron transport effect in the composite catalyst inducing electrons directionally moving from CoFe-LDH to AgNWs. Meanwhile, the CoFe-LDH nanocages with ultrathin nanosheets and hollow structural properties show abundant active sites for electrocatalytic oxygen generation. The versatile AgNWs@CoFe-LDH catalyst with optimized components, enhanced directional electron transport, and synergistic effect achieves high OER performance with the overpotential of 207 mV and long-term 50 h stability at 10 mA cm-2 in an alkaline medium. Moreover, in-depth insights into the microstructure, structure-activity relationships, identification of key intermediate species, and a proton-coupled four-electron OER mechanism based on experimental discovery and theoretical calculation are also demonstrated.

Rechargeable Zn-H2O hydrolysis battery for hydrogen storage and production

Reactive metals hydrolysis offers significant advantages for hydrogen storage and production. However, the regeneration of common reactive metals (e.g., Mg, Al, etc.) is energy-intensive and produces unwanted byproducts such as CO2 and Cl2. Herein, we employ Zn as a reactive mediator that can be easily regenerated by electrolysis of ZnO in an alkaline solution with a Faradaic efficiency of >99.9 %. H2 is produced in the same electrolyte by constructing a Zn-H2O hydrolysis battery consisting of a Zn anode and a Raney-Ni cathode to unlock the Zn-H2O reaction. The entire two-step water splitting reaction with a net energy efficiency of 70.4 % at 80 °C and 50 mA cm-2. Additionally, the Zn-H2O system can be charged using renewable energy to produce H2 on demand and runs for 600 cycles only sacrificing 3.76 % energy efficiency. DFT calculations reveal that the desorption of H* on Raney-Ni (-0.30 eV) is closer to zero compared with that on Zn (-0.87 eV), indicating a faster desorption of H* at low overpotential. Further, a 24 Ah electrolyzer is demonstrated to produce H2 with a net energy efficiency of 65.5 %, which holds promise for its real application.

Roles of Wettability and Wickability on Enhanced Hydrogen Evolution Reactions

Bubble dynamics significantly impact mass transfer and energy conversion in electrochemical gas evolution reactions. Micro-/nanostructured surfaces with extreme wettability have been employed as gas-evolving electrodes to promote bubble departure and decrease the bubble-induced overpotential. However, effects of the electrodes' wickability on the electrochemical reaction performances remain elusive. In this work, hydrogen evolution reaction (HER) performances are experimentally investigated using micropillar array electrodes with varying interpillar spacings, and effects of the electrodes' wettability, wickability as well as bubble adhesion are discussed. A deep learning-based object detection model was used to obtain bubble counts and bubble departure size distributions. We show that microstructures on the electrode have little effect on the total bubble counts and bubble size distribution characteristics at low current densities. At high current densities, however, micropillar array electrodes have much higher total bubble counts and smaller bubble departure sizes compared with the flat electrode. We also demonstrate that surface wettability is a critical factor influencing HER performances under low current densities, where bubbles exist in an isolated regime. Under high current densities, where bubbles are in an interacting regime, the wickability of the micropillar array electrodes emerges as a determining factor. This work elucidates the roles of surface wettability and wickability on enhancing electrochemical performances, providing guidelines for the optimal design of micro-/nanostructured electrodes in various gas evolution reactions.

Stable Anion Exchange Membrane Bearing Quinuclidinium for High-performance Water Electrolysis

Anion exchange membranes (AEMs) are core components in anion exchange membrane water electrolyzers (AEM-WEs). However, the stability of functional quaternary ammonium cations, especially under high temperatures and harsh alkaline conditions, seriously affects their performance and durability. Herein, we synthesized a 1-methyl-3,3-diphenylquinuclidinium molecular building unit. Density functional theory (DFT) calculations and accelerated aging analysis indicated that the quinine ring structure was exceedingly stable, and the SN2 degradation mechanism dominated. Through acid-catalyzed Friedel-Crafts polymerization, a series of branched poly(aryl-quinuclidinium) (PAQ-x) AEMs with controllable molecular weight and adjustable ion exchange capacity (IEC) were prepared. The stable quinine structure in PAQ-x was verified and retained in the ex situ alkaline stability. Furthermore, the branched polymer structure reduces the swelling rate and water uptake to achieve a tradeoff between dimensional stability and ionic conductivity, significantly improving the membrane's overall performance. Importantly, PAQ-5 was used in non-noble metal-based AEM-WE, achieving a high current density of 8 A cm-2 at 2 V and excellent stability over 2446 h in a gradient constant current test. Based on the excellent alkaline stability of this diaryl-quinuclidinium group, it can be further considered as a multifunctional building unit to create multi-topological polymers for energy conversion devices used in alkaline environments.

In situ self-reconstructed hierarchical bimetallic oxyhydroxide nanosheets of metallic sulfides for high-efficiency electrochemical water splitting

The advancement of economically efficient electrocatalysts for alkaline water oxidation based on transition metals is essential for hydrogen production through water electrolysis. In this investigation, a straightforward one-step solvent method was utilized to spontaneously cultivate bimetallic sulfide S-FeCo1 : 1/NIF on the surface of a nickel-iron foam (NIF). Capitalizing on the synergistic impact between the bimetallic constituents and the highly active species formed through electrochemical restructuring, S-FeCo1 : 1/NIF exhibited remarkable oxygen evolution reaction (OER) performance, requiring only a 310 mV overpotential based on 500 mA cm-2 current density. Furthermore, it exhibited stable operation at 200 mA cm-2 for 275 h. Simultaneously, the catalyst demonstrated excellent hydrogen evolution reaction (HER) and overall water-splitting capabilities. It only requires an overpotential of 191 mV and a potential of 1.81 V to drive current densities of 100 and 50 mA cm-2. Density functional theory (DFT) calculations were also employed to validate the impact of the bimetallic synergistic effect on the catalytic activity of sulfides. The results indicate that the coupling between bimetallic components effectively reduces the energy barrier required for the rate-determining step in water oxidation, enhancing the stability and activity of bimetallic sulfides. The exploration of bimetallic coupling to improve the OER performance holds theoretical significance in the rational design of advanced electrocatalysts.

Spatial configuration of Fe-Co dual-sites boosting catalytic intermediates coupling toward oxygen evolution reaction

Oxygen evolution reaction (OER) is the pivotal obstacle of water splitting for hydrogen production. Dual-sites catalysts (DSCs) are considered exceeding single-site catalysts due to the preternatural synergetic effects of two metals in OER. However, appointing the specific spatial configuration of dual-sites toward more efficient catalysis still remains a challenge. Herein, we constructed two configurations of Fe-Co dual-sites: stereo Fe-Co sites (stereo-Fe-Co DSC) and planar Fe-Co sites (planar-Fe-Co DSC). Remarkably, the planar-Fe-Co DSC has excellent OER performance superior to stereo-Fe-Co DSC. DFT calculations and experiments including isotope differential electrochemical mass spectrometry, in situ infrared spectroscopy, and in situ Raman reveal the *O intermediates can be directly coupled to form *O-O* rather than *OOH by both the DSCs, which could overcome the limitation of four electron transfer steps in OER. Especially, the proper Fe-Co distance and steric direction of the planar-Fe-Co benefit the cooperation of dual sites to dehydrogenate intermediates into *O-O* than stereo-Fe-Co in the rate-determining step. This work provides valuable insights and support for further research and development of OER dual-site catalysts.

Enhancing anti-chlorine corrosion of Ni3S2 by Mo-doping for mimic seawater electrolysis

Designing highly active electrocatalysts that can resist chloride ion (Cl-) corrosion during seawater electrolysis is still a challenge. Here, Mo-doping is introduced to synchronously improve the electrocatalytic activity and anti-chlorine corrosion of Ni3S2 toward the efficient overall seawater splitting. With commercial nickel-molybdenum foam (NMF) as the reactive substrates, Mo-doped Ni3S2 columnar arrays (Mo-Ni3S2/NMF) are fabricated via a one-step hydrothermal process, which expose abundant active sites with the ameliorated surface electronic configurations toward the enhanced binding with *OH (* denotes an active site) but the weakened one with *Cl. As expected, they afford the excellent bi-functionality for both oxygen and hydrogen evolution reactions (OER and HER), with the remarkably improved anti-corrosion to Cl- at anode as compared to pristine Ni3S2. In alkaline mimic seawater (1.0 M NaOH + 0.5 M NaCl), Mo-Ni3S2/NMF requires 330 mV (for OER) and 209 mV (for HER) overpotentials at the current density of ±100 mA cm-2, and a low cell voltage of 1.52 V at 10 mA cm-2 for overall seawater splitting. This work highlights a feasible strategy to explore highly active and stable electrocatalysts for sustainable H2 production.

MOF-derived Co-Ni layered double hydroxides/polyethyleneimine modified chitosan micro-nanoreactor for high-efficiency capture of uranium from seawater

The preparation of powder adsorbent into microsphere adsorbent is one of the effective methods for the industrialization of uranium extraction from seawater. Herein, a MOF-derived Co-Ni layered double hydroxides/polyethyleneimine modified chitosan micro-nanoreactor (DNPM) was prepared by a simple method in this work. The microstructure and chemical structure of DNPM were comprehensively characterized. The pH value, adsorption time, initial solution concentration, temperature, competitive ions, regeneration performance, and bed column heights were investigated for the adsorption performance of DNPM by batch adsorption and fixed-bed column continuous adsorption experiments. When the contact time was 8 h, the initial concentration was 150 mg/L, and the pH value was 6, the adsorption capacity of DNPM was 334.67 mg/g. The uranium adsorption by DNPM fits with the pseudo-second-order kinetic and Langmuir models, which was a spontaneous and endothermic process. In addition, DNPM has good adsorption selectivity and reusability. The fixed-bed column continuous adsorption experiment shows that the adsorption capacity increased with the increase of bed column height. The adsorption mechanism can be attributed to coordination chelation and electrostatic interaction. In general, this work provides an effective strategy for developing environmentally friendly uranium adsorbent that can be industrially used.

Sulfion oxidation assisting self-powered hydrogen production system based on efficient catalysts from spent lithium-ion batteries

Converting spent lithium-ion batteries (LIBs) and industrial wastewater into high-value-added substances by advanced electrocatalytic technology is important for sustainable energy development and environmental protection. Here, we propose a self-powered system using a home-made sulfide fuel cell (SFC) to power a two-electrode electrocatalytic sulfion oxidation reaction (SOR)-assisted hydrogen (H2) production electrolyzer (ESHPE), in which the sulfion-containing wastewater is used as the liquid fuel to produce clean water, sulfur, and hydrogen. The catalysts for the self-powered system are mainly prepared from spent LIBs to reduce the cost, such as the bifunctional Co9S8 catalyst was prepared from spent LiCoO2 for SOR and hydrogen evolution reaction (HER). The Fe-N-P codoped coral-like carbon nanotube arrays encapsulated Fe2P (C-ZIF/sLFP) catalyst was prepared from spent LiFePO4 for oxygen reduction reaction. The Co9S8 catalyst shows excellent catalytic activities in both SOR and HER, evidenced by the low cell voltage of 0.426 V at 20 mA cm-2 in ESHPE. The SFC with Co9S8 as anode and C-ZIF/sLFP as cathode exhibits an open-circuit voltage of 0.69 V and long discharge stability for 300 h at 20 mA cm-2. By integrating the SFC and ESHPE, the self-powered system delivers an impressive hydrogen production rate of 0.44 mL cm-2 min-1. This work constructs a self-powered system with high-performance catalysts prepared from spent LIBs to transform sulfion-containing wastewater into purified water and prepare hydrogen, which is promising to achieve high economic efficiency, environmental remediation, and sustainable development.

Facile Synthesis of Co Nanoparticles Embedded in N-Doped Carbon Nanotubes/Graphitic Nanosheets as Bifunctional Electrocatalysts for Electrocatalytic Water Splitting

Developing robust and cost-effective electrocatalysts to boost hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) is crucially important to electrocatalytic water splitting. Herein, bifunctional electrocatalysts, by coupling Co nanoparticles and N-doped carbon nanotubes/graphitic nanosheets (Co@NCNTs/NG), were successfully synthesized via facile high-temperature pyrolysis and evaluated for water splitting. The morphology and particle size of products were influenced by the precursor type of the cobalt source (cobalt oxide or cobalt nitrate). The pyrolysis product prepared using cobalt oxide as a cobalt source (Co@NCNTs/NG-1) exhibited the smaller particle size and higher specific surface area than that of the pyrolysis products prepared using cobalt nitrate as a cobalt source (Co@NCNTs/NG-2). Notably, Co@NCNTs/NG-1 displayed much lower potential -0.222 V vs. RHE for HER and 1.547 V vs. RHE for OER at the benchmark current density of 10 mA cm-2 than that of Co@NCNTs/NG-2, which indicates the higher bifunctional catalytic activities of Co@NCNTs/NG-1. The water-splitting device using Co@NCNTs/NG-1 as both an anode and cathode demonstrated a potential of 1.92 V to attain 10 mA cm-2 with outstanding stability for 100 h. This work provides a facile pyrolysis strategy to explore highly efficient and stable bifunctional electrocatalysts for water splitting.