Dynamics and control of active sites in hierarchically nanostructured cobalt phosphide/chalcogenide-based electrocatalysts for water splitting

Electron and surface engineering of Ni2P/MnP4 heterojunction as high performance bifunctional electrocatalyst for amperage-level overall water splitting

Producing hydrogen through electrocatalytic overall water splitting with ampere-level current density is still limited by the high cost and poor stability of electrocatalysts. In this work, a new type Ni2P/MnP4 heterojunction composite material was designed and prepared as bifunctional electrocatalyst. Based on XPS spectra and theoretical calculation, the formation of Ni2P/MnP4 heterojunction successfully modulates the local electronic structure of Ni2P and enhances the ionization of H and Ni by increasing the electron transfer rate. Moreover, the special nanovilli structure and superhydropholic/superaerophobic surface of Ni2P/MnP4 heterojunction accelerates the transfer of electrolyte and gaseous products. Benefiting from these advantages, the as-prepared Ni2P/MnP4/CF not only exhibits superior electrocatalytic performance, which can release 10 mA/cm2 current density with a low overpotential of 69 mV and 247 mV for HER and OER respectively, but also shows admirable stability of continuous overall water splitting to drive 1000 mA/cm2 for 180 h without notable activity degradation. We believe this material possesses outstanding potential for industrial applications, and our strategy may provide a new pathway to design relative materials.

Molybdenum, tungsten doped cobalt phosphides as efficient catalysts for coproduction of hydrogen and formate by glycerol electrolysis

H2 and formate are important energy carriers in fuel-cells and feedstocks in chemical industry. The hydrogen evolution reaction (HER) coupling with electro-oxidative cleavage of thermodynamically favorable polyols is a promising way to coproduce H2 and formate via electrochemical means, highly active catalysts for HER and electrooxidative cleavage of polycols are the key to achieve such a goal. Herein, molybdenum (Mo), tungsten (W) doped cobalt phosphides (Co2P) deposited onto nickel foam (NF) substrate, denoted as Mo-Co2P/NF and W-Co2P/NF, respectively, were investigated as catalytic electrodes for HER and electrochemical glycerol oxidation reaction (GOR) to yield H2 and formate. The W-Co2P/NF electrode exhibited low overpotential (η) of 113 mV to attain a current density (J) of -100 mA cm-2 for HER, while the Mo-Co2P/NF electrode demonstrated high GOR efficiency for selective production of formate. In situ Raman and infrared spectroscopic characterizations revealed that the evolved CoO2 from Co2P is the genuine catalytic sites for GOR. The asymmetric electrolyzer based on W-Co2P/NF cathode and Mo-Co2P/NF anode delivered a J = 100 mA cm-2 at 1.8 V voltage for glycerol electrolysis, which led to 18.2 % reduced electricity consumption relative to water electrolysis. This work highlights the potential of heteroelement doped phosphide in catalytic performances for HER and GOR, and opens up new avenue to coproduce more widespread commodity chemicals via gentle and sustainable electrocatalytic means.

The interfacial synergy of hierarchical FeCoNiP@FeNi-LDH heterojunction for efficient alkaline water splitting

In response to the growing demand for clean, green, and sustainable energy sources, the development of cost-effective and durable high-activity overall water splitting electrocatalysts is urgently needed. In this study, the heterogeneous structure formed by the combination of FeCoNiP and FeNi-LDH was homogeneously dispersed onto CuO nanowires generated by in-situ oxidation of copper foam as a substrate using an electrodeposition method. This multilevel structure exhibits excellent bifunctional properties as an electrode material in alkaline solutions, for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) only 206 mV and 147 mV overpotentials are needed to achieve a current density of 100 mA cm-2 respectively. Full water electrolysis is thus enabled to take place at such a low cell voltage as 1.64 V to reach the current density of 100 mA cm-2, which exhibits a long-term stability of 30 h. These improved electrocatalytic performances stem from the construction of multilevel structures. The X-ray photoelectron spectroscopy suggests that strong electron transfer occurs between heterogeneous structures, thus facilitating the OER and HER process. The dispersion of CuO nanowires not only increases the electrochemically active surface areas but also improves the overall hydrophilic and aerophobic properties. This work highlights the positive effect of multilevel structure in the design of more efficient electrocatalysts and provides a reference for the preparation of other low-cost, high-activity bifunctional electrocatalysts.

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.

Novel Heterostructure-Based CoFe and Cobalt Oxysulfide Nanocubes for Effective Bifunctional Electrocatalytic Water and Urea Oxidation

The development of effective oxygen evolution reaction (OER) and urea oxidation reaction (UOR) on heterostructure electrocatalysts with specific interfaces and characteristics provides a distinctive character. In this study, heterostructure nanocubes (NCs) comprising inner cobalt oxysulfide (CoOS) NCs and outer CoFe (CF) layered double hydroxide (LDH) are developed using a hydrothermal methodology. During the sulfidation process, the divalent sulfur ions (S2-) are released from the breakdown of the sulfur source and react with the Co-precursors on the surface leading to the transformation of CoOH nanorods into CoOS nanocubes. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses reveal that the interactions at the interface of the CF@CoOS NCs significantly altered the electronic structure, thus enhancing the electrocatalytic performance. The optimal catalysts exhibited effective OER and UOR activities, the attained potentials are 1.51 and 1.36 V. This remarkable performance is attributable to the induction of electron transfer from the CoFe LDH to CoOS, which reduces the energy barrier of the intermediates for the OER and UOR. Furthermore, an alkaline water and urea two-cell electrolyzer assembled using CF@CoOS-2 NCs and Pt/C as the anode and cathode requires a cell voltage of 1.63 and 1.56 V along with a durability performance.

Extraordinary Structural Reconstruction of Nanolaminated Ta2FeC MAX Phase for Enhanced Oxygen Evolution Performance

Renewable energy technologies, such as water splitting, heavily depend on the oxygen evolution reaction (OER). Nanolaminated ternary compounds, referred to as MAX phases, show great promise for creating efficient electrocatalysts for OER. However, their limited intrinsic oxidative resistance hinders the utilization of conductivity in Mn+1Xn layers, leading to reduced activity. In this study, a method is proposed to improve the poor inoxidizability of MAX phases by carefully adjusting the elemental composition between Mn+1Xn layers and single-atom-thick A layers. The resulting Ta2FeC catalyst demonstrates superior performance compared to conventional Fe/C-based catalysts with a remarkable record-low overpotential of 247 mV (@10 mA cm-2) and sustained activity for over 240 h. Notably, during OER processing, the single-atom-thick Fe layer undergoes self-reconstruction and enrichment from the interior of the Ta2FeC MAX phase toward its surface, forming a Ta2FeC@Ta2C@FeOOH heterostructure. Through density functional theory (DFT) calculations, this study has found that the incorporation of Ta2FeC@Ta2C not only enhances the conductivity of FeOOH but also reduces the covalency of Fe─O bonds, thus alleviating the oxidation of Fe3+ and O2-. This implies that the Ta2FeC@Ta2C@FeOOH heterostructure experiences less lattice oxygen loss during the OER process compared to pure FeOOH, leading to significantly improved stability. These results highlight promising avenues for further exploration of MAX phases by strategically engineering M- and A-site engineering through multi-metal substitution, to develop M2AX@M2X@AOOH-based catalysts for oxygen evolution.

Precious Metal Free Hydrogen Evolution Catalyst Design and Application

The quest to identify precious metal free hydrogen evolution reaction catalysts has received unprecedented attention in the past decade. In this Review, we focus our attention to recent developments in precious metal free hydrogen evolution reactions in acidic and alkaline electrolyte owing to their relevance to commercial and near-commercial low-temperature electrolyzers. We provide a detailed review and critical analysis of catalyst activity and stability performance measurements and metrics commonly deployed in the literature, as well as review best practices for experimental measurements (both in half-cell three-electrode configurations and in two-electrode device testing). In particular, we discuss the transition from laboratory-scale hydrogen evolution reaction (HER) catalyst measurements to those in single cells, which is a critical aspect crucial for scaling up from laboratory to industrial settings but often overlooked. Furthermore, we review the numerous catalyst design strategies deployed across the precious metal free HER literature. Subsequently, we showcase some of the most commonly investigated families of precious metal free HER catalysts; molybdenum disulfide-based, transition metal phosphides, and transition metal carbides for acidic electrolyte; nickel molybdenum and transition metal phosphides for alkaline. This includes a comprehensive analysis comparing the HER activity between several families of materials highlighting the recent stagnation with regards to enhancing the intrinsic activity of precious metal free hydrogen evolution reaction catalysts. Finally, we summarize future directions and provide recommendations for the field in this area of electrocatalysis.

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.

Orbital Occupancy Modulation to Optimize Intermediate Absorption for Efficient Electrocatalysts in Water Electrolysis and Zinc-Ethanol-Air Battery

Spin engineering is a promising way to modulate the interaction between the metal d-orbital and the intermediates and thus enhance the catalytic kinetics. Herein, an innovative strategy is reported to modulate the spin state of Co by regulating its coordinating environment. o-c-CoSe2 -Ni is prepared as pre-catalyst, then in situ electrochemical impedance spectroscopy (EIS) and in situ Raman spectroscopy are employed to prove phase transition, and CoOOH/Co3 O4 is formed on the surface as active sites. In hybrid water electrolysis, the voltage has a negative shift, and in zinc-ethanol-air battery, the charging voltage is lowered and the cycling stability is greatly increased. Coordinated atom substitution and crystalline symmetry change are combined to regulate the absorption ability of reaction intermediates with balanced optimal adsorption. Coordinated atom substitution weakens the adsorption while the crystalline symmetry change strengthens the adsorption. Importantly, the tetrahedral sites are introduced by Ni doping which enables the co-existence of four-coordinated sites and six-coordination sites in o-c-CoSe2 -Ni. The dz2 + dx2 -y2 orbital occupancy decreases after the atomic substitution, while increases after replacing the CoSe6 -Oh field with CoSe6 -Oh /CoSe4 -Td . This work explores a new direction for the preparation of efficient catalysts for water electrolysis and innovative zinc-ethanol-air battery.

Microenvironment reconstitution of highly active Ni single atoms on oxygen-incorporated Mo2C for water splitting

The rational design of efficient bifunctional single-atom electrocatalysts for industrial water splitting and the comprehensive understanding of its complex catalytic mechanisms remain challenging. Here, we report a Ni single atoms supported on oxygen-incorporated Mo2C via Ni-O-Mo bridge bonds, that gives high oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional activity. By ex situ synchrotron X-ray absorption spectroscopy and electron microscopy, we found that after HER, the coordination number and bond lengths of Ni-O and Ni-Mo (Ni-O-Mo) were all altered, yet the Ni species still remain atomically dispersed. In contrast, after OER, the atomically dispersed Ni were agglomerated into very small clusters with new Ni-Ni (Ni-O-Ni) bonds appeared. Combining experimental results and DFT calculations, we infer the oxidation degree of Mo2C and the configuration of single-atom Ni are both vital for HER or OER. This study provides both a feasible strategy and model to rational design highly efficient electrocatalysts for water electrolysis.

MOF-on-MOF-Derived Ultrafine Fe2P-Co2P Heterostructures for High-Efficiency and Durable Anion Exchange Membrane Water Electrolyzers

The alkaline hydrogen evolution reaction (HER) in an anion exchange membrane water electrolyzer (AEMWE) is considered to be a promising approach for large-scale industrial hydrogen production. Nevertheless, it is severely hampered by the inability to operate tolerable HER catalysts consistently under low overpotentials at ampere-level current densities. Here, we develop a universal ligand-exchange (MOF-on-MOF) modulation strategy to synthesize ultrafine Fe2P and Co2P nanoparticles, which are well anchored on N and P dual-doped carbon porous nanosheets (Fe2P-Co2P/NPC). In addition, benefiting from the downshift of the d-band center and the interfacial Co-P-Fe bridging, the electron-rich P site is triggered, which induces the redistribution of electron density and the swapping of active centers, lowering the energy barrier of the HER. As a result, the Fe2P-Co2P/NPC catalyst only requires a low overpotential of 175 mV to achieve a current density of 1000 mA cm-2. The solar-driven water electrolysis system presents a record-setting and stable solar-to-hydrogen conversion efficiency of 20.36%. Crucially, the catalyst could stably operate at 1000 mA cm-2 over 1000 h in a practical AEMWE at an estimated cost of US$0.79 per kilogram of H2, which achieves the target (US$2 per kg of H2) set by the U.S. Department of Energy (DOE).

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2

The direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl- upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co2 P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200-2000 mA cm-2 over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO4 3- and Fe(CN)6 3- generated by the electrode play synergistic role in repelling Cl- via electrostatic repulsion and dense coverage, which reduced Cl- adsorption by nearly 5-fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.

Unveiling the Structural Self-Reconstruction and Identifying the Reactive Center of a V, Fe Co-Doped Cobalt Precatalyst toward Enhanced Overall Water Splitting by Operando Raman Spectroscopy

The development of efficient and stable bifunctional electrocatalysts based on non-noble metals for water electrolysis is both urgent and challenging. However, unresolved issues remain regarding the challenge of identifying the active phase and gaining a comprehensive understanding of its surface reconstruction and functionality throughout the reaction process. In this study, we have combined doping and heterostructure construction by a one-step electrodeposition and a subsequent activation treatment to synthesize Fe, V co-doped Co3O4/Co(OH)2 and Co/Co(OH)2 heterointerfaces (referred to as A-Co60Fe1.1V). These heterointerfaces, composed of Co/Co(OH)2 and Co3O4/Co(OH)2, are proposed to facilitate charge transfer process during catalysis. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the introduction of V and Fe dopants increases the valence state of Co centers in Co3O4 and Co(OH)2. Further operando Raman spectroscopy reveals that Co(OH)2 and Co3O4 with the high-valence Co centers remain stable during the hydrogen evolution reaction (HER) process. These high-valence Co centers are believed to promote the crucial water dissociation step and therefore enhance the overall HER catalysis. On the other hand, during the oxygen evolution reaction (OER), Fe, V co-doping leads to an earlier formation of the active CoOOH species, while Fe doping can further help stabilize the more reactive β-CoOOH species instead of the less reactive γ-CoOOH. As a result, the A-Co60Fe1.1V catalyst exhibits significantly improved catalytic activity for both HER and OER that it requires low overpotentials of 51 and 250 mV, respectively, to attain a current density of 10 mA cm-2. Moreover, when utilized as both the cathode and anode in alkaline water electrolysis, the A-Co60Fe1.1V catalyst can operate at a mere 1.54 V voltage while maintaining 10 mA cm-2, surpassing the majority of non-noble metal catalysts. Remarkably, it also exhibits stability for at least 40 h at ∼100 mA cm-2.

Prussian-Blue-Analogue-Derived Ultrathin Co2P-Fe2P Nanosheets for Universal-pH Overall Water Splitting

The great interest in large-scale electrochemical water splitting toward clean hydrogen has spurred large numbers of studies on developing cost-efficient and high-performance bifunctional electrocatalysts. Here, a Prussian-blue-analogue-derived method is proposed to prepare honeycomb-like ultrathin and heterogeneous Co2P-Fe2P nanosheets on nickel foam, showing low overpotentials of 0.080, 0.088, and 0.109 V for the hydrogen evolution reaction (HER) at 10 mA cm-2 as well as 0.290, 0.370, and 0.730 V for the oxygen evolution reaction (OER) at 50 mA cm-2 in alkaline, acidic, and neutral electrolytes, respectively. When directly applied for universal-pH water electrolysis, excellent performances are achieved especially at ultralow voltages of 1.45 V at 10 mA cm-2, 1.66 V at 100 mA cm-2, and 1.79 V at 500 mA cm-2 under alkaline conditions. In situ Raman spectroscopy measurements demonstrate that the excellent HER performance can be attributed to heterogeneous Co2P-Fe2P while the ultrahigh alkaline OER performance originates from reconstruction-induced oxyhydroxides.

Synthesis of Fe Atom-Doped Monodisperse Co2P Nanorods with a Dual-Ligand Strategy for Excellent Electrocatalytic Hydrogen Evolution Performance

Cobalt phosphide has been widely used in various catalytic reactions due to its excellent catalytic activity and stability. In contrast to the conventional synthesis of Co2P nanorods using expensive and toxic trioctylphosphine (TOP), this study employs a dual-ligand strategy to prepare iron-atom-doped monodisperse Co2P nanorods. The strategy involves the use of triphenylphosphite (TPOP) as a cost-effective and relatively less toxic strong ligand, alongside hexadecylamine (HDA) as a weaker ligand. The resultant atom-doped Co2P nanorods exhibited a large aspect ratio, providing a plentiful supply of active sites for electrocatalytic hydrogen evolution. In both alkaline and acidic electrolytes, achieving a current density of 10 mA cm-2 required overpotentials of 91 and 141 mV, respectively, with the optimal Co:Fe molar ratio of 1:0.2. The introduction of Fe atoms through doping increased the electron density at the Co atom sites, thereby enhancing H adsorption. This research offers a cost-effective and relatively low-toxicity method for the controlled fabrication of monodisperse transition-metal phosphide nanorods, enabling efficient catalytic reactions.

Highly N-Doped Fe/Co Phosphide Superstructures for Efficient Water Splitting

Developing an inexpensive bifunctional electrocatalyst for overall water splitting is critical for acquiring scalable green hydrogen and thereby realizing carbon neutralization. Herein, an "all-in-one" method is developed for the fabrication of highly N-doped binary FeCo-phosphides (N-FeCoP) with hierarchical superstructure, this delicately designed synthesis route allows the following merits for benefiting water splitting electrocatalysis in alkaline, including high N/defect-doping for mediating the surface property of the as-made N-FeCoP, binary Fe and Co components exhibiting strong coupling interaction, and 3D hierarchical superstructure for shortening diffusion length and thereby improving reaction kinetics. Electrochemical measurements reveal that the N-FeCoP sample exhibits very low overpotentials for initiating the hydrogen and oxygen evolution reactions. Remarkably, overall water splitting can be promoted on N-FeCoP using a commercial primary Zn-MnO2 battery. The developed synthesis strategy may potentially inspire the preparation of other N-doped metal-based nanostructures for broad electrocatalysis.

Dense Crystalline/Amorphous Phosphides/Oxides Interfacial Sites for Enhanced Industrial-Level Large Current Density Seawater Oxidation

Designing high-efficiency and low-cost catalysts with high current densities for the oxygen evolution reaction (OER) is critical for commercial seawater electrolysis. Here, we present a heterophase synthetic strategy for constructing an electrocatalyst with dense heterogeneous interfacial sites among crystalline Ni2P, Fe2P, CeO2, and amorphous NiFeCe oxides on nickel foam (NF). The synergistic effect of high-density crystalline and amorphous heterogeneous interfaces effectively promotes the redistribution of the charge density and optimizes the adsorbed oxygen intermediates, lowering the energy barrier and promoting the O2 desorption, thus enhancing the OER performance. The obtained NiFeO-CeO2/NF catalyst exhibited outstanding OER catalytic activity, with low overpotentials of 338 and 408 mV required to attain high current densities of 500 and 1000 mA cm-2, respectively, in alkaline natural seawater electrolytes. The solar-driven seawater electrolysis system presents a record-setting and stable solar-to-hydrogen conversion efficiency of 20.10%. This work provides directives for developing highly effective and stable catalysts for large-scale clean energy production.

Lamella-heterostructured nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride electrodes as stable catalysts for oxygen evolution

Developing robust nonprecious-metal electrocatalysts with high activity towards sluggish oxygen-evolution reaction is paramount for large-scale hydrogen production via electrochemical water splitting. Here we report that self-supported laminate composite electrodes composed of alternating nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride (FeCo/CeO_2-xN_x) heterolamellas hold great promise as highly efficient electrocatalysts for alkaline oxygen-evolution reaction. By virtue of three-dimensional nanoporous architecture to offer abundant and accessible electroactive CoFeOOH/CeO_2-xN_x heterostructure interfaces through facilitating electron transfer and mass transport, nanoporous FeCo/CeO_2-xN_x composite electrodes exhibit superior oxygen-evolution electrocatalysis in 1 M KOH, with ultralow Tafel slope of ~33 mV dec^-1. At overpotential of as low as 360 mV, they reach >3900 mA cm^-2 and retain exceptional stability at ~1900 mA cm^-2 for >1000 h, outperforming commercial RuO₂ and some representative oxygen-evolution-reaction catalysts recently reported. These electrochemical properties make them attractive candidates as oxygen-evolution-reaction electrocatalysts in electrolysis of water for large-scale hydrogen generation.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Mild construction of an Fe-B-O based flexible electrode toward highly efficient alkaline simulated seawater splitting

It is essential to construct self-supporting electrodes based on earth-abundant iron borides in a mild and economical manner for grid-scale hydrogen production. Herein, a series of highly efficient, flexible, robust, and scalable Fe-B-O@FeB_x modified on hydrophilic cloth (denoted as Fe-B-O@FeB_x/HC, 10 cm × 10 cm) are fabricated by mild electroless plating. The overpotentials and Tafel slope values for the hydrogen and oxygen evolution reactions are 59 mV and 57.62 mV dec^-1 and 181 mV and 65.44 mV dec^-1, respectively; only 1.462 V is required to achieve 10 mA cm^-2 during overall water splitting (OWS). Fe-B-O@FeB_x/HC maintains its high catalytic activity for more than 7 days at an industrial current density (400 mA cm^-2), owing to the loosened popcorn-like Fe-B-O@FeB_x that is firmly loaded on a 2D-layered and mechanically robust substrate along with its fast charge and mass transfer kinetics. The chimney effect of core-shell borides@(oxyhydro)oxides enhances the OWS performance and protects the inner metal borides from further corrosion. Moreover, the flexible Fe-B-O@FeB_x/HC electrode has a low cost for grid-scale hydrogen production ($2.97 kg^-1). The proposed strategy lays a solid foundation for universal preparation, large-scale hydrogen production and practical applications thereof.

In Situ Generation of Vertically Crossed P-Cu3Se2 Ultrathin Nanosheets Derived from Cu2S Nanorod Arrays for High-Performance Supercapacitors

Transition-metal selenides (TMSs) have great potential in the synthesis of supercapacitor electrode materials due to their rich content and high specific capacity. However, the aggregation phenomenon of TMS materials in the process of charging and discharging will cause capacity attenuation, which seriously affects the service life and practical applications. Therefore, it is of great practical significance to design simple and efficient synthesis strategies to overcome these shortcomings. Hence, P-doped Cu₃Se₂ nanosheets are loaded on vertically aligned Cu₂S nanorod arrays to synthesize CF/Cu₂S@Cu₃Se₂/P nanocomposites with a unique core-shell heterostructure. Notably, the Cu₂S precursors can be rapidly converted into Cu₃Se₂ nanorod arrays in situ in just 30 min at room temperature. The unique core-shell heterostructure effectively avoids the aggregation phenomenon, and the doped P elements further enhance the electrochemical properties of the electrode materials. Therefore, the as-prepared CF/Cu₂S@Cu₃Se₂/P electrode exhibits a high areal capacitance of 5054 mF cm^-2 (1099 C g^-1) at 3 mA cm^-2 and still retains 90.2% capacitance after 10 000 galvanostatic charge-discharge (GCD) cycles. The asymmetric supercapacitor (ASC) device assembled from synthetic CF/Cu₂S@Cu₃Se₂/P and activated carbon (AC) possesses an energy density of 41.1 Wh kg^-1 at a power density of 480.4 W kg^-1. This work shows that the designed CF/Cu₂S@Cu₃Se₂/P electrode has broad application prospects in the field of electrochemical energy storage.

Interface Engineering on Amorphous/Crystalline Hydroxides/Sulfides Heterostructure Nanoarrays for Enhanced Solar Water Splitting

Developing highly efficient and stable noble-metal-free electrocatalysts for water splitting is critical for producing clean and sustainable energy. Here, we design a hierarchical transition metal hydroxide/sulfide (NiFe(OH)_x-Ni₃S₂/NF) electrode with dual heterointerface coexistence using a cation exchange-induced surface reconfiguration strategy. The electrode exhibits superior electrocatalytic activities, achieving low overpotentials of 55 mV for hydrogen evolution and 182 mV for oxygen evolution at 10 mA cm^-2. Furthermore, the assembled two-electrode system requires voltages as low as 1.55 and 1.62 V to deliver industrially relevant current densities of 500 and 1000 mA cm^-2, respectively, with excellent durability for over 200 h, which is comparable to commercial electrolysis. Theoretical calculations reveal that the hierarchical heterostructure increases the electronic delocalization of the Fe and Ni catalytic centers, lowering the energy barrier of the rate-limiting step and promoting O₂ desorption. Finally, by implementing the catalysts in a solar-driven water electrolysis system, we demonstrate a record and durable solar-to-hydrogen (STH) conversion efficiency of up to 20.05%. This work provides a promising strategy for developing low-cost and high-efficiency bifunctional catalysts for a large-scale solar-to-hydrogen generation.

Regulation of Polymer Configurations Enables Green Solvent-Processed Large-Area Binary All-Polymer Solar Cells With Breakthrough Performance and High Efficiency Stretchability Factor

With the great potential of the all-polymer solar cells for large-area wearable devices, both large-area device efficiency and mechanical flexibility are very critical but attract limited attention. In this work, from the perspective of the polymer configurations, two types of terpolymer acceptors PYTX-A and PYTX-B (X = Cl or H) are developed. The configuration difference caused by the replacement of non-conjugated units results in distinct photovoltaic performance and mechanical flexibility. Benefiting from a good match between the intrinsically slow film-forming of the active materials and the technically slow film-forming of the blade-coating process, the toluene-processed large-area (1.21 cm² ) binary device achieves a record efficiency of 14.70%. More importantly, a new parameter of efficiency stretchability factor (ESF) is proposed for the first time to comprehensively evaluate the overall device performance. PM6:PYTCl-A and PM6:PYTCl-B yield significantly higher ESF than PM6:PY-IT. Further blending with non-conjugated polymer donor PM6-A, the best ESF of 3.12% is achieved for PM6-A:PYTCl-A, which is among the highest comprehensive performances.

Mild Construction of "Midas Touch" Metal-Organic Framework-Based Catalytic Electrodes for Highly Efficient Overall Seawater Splitting

Mild construction of highly efficient and durable practical electrodes for overall water splitting (OWS) at industrial-grade current density is currently a significant challenge. Herein, metal-organic framework (MOF) materials are grown in situ on the surface of carbon cloth (CC) at 25 °C, and quickly "interspersed" by cobalt-boron (Co-B) via electroless plating for 30 min to obtain a highly efficient and stable CoB@MOF@CC self-supporting electrode. Owing to the large specific surface area, abundant active sites, and porous structure, the MOF-based CC modified by bamboo leaf-like ultrathin CoB has remarkable electrochemical catalysis efficiency. The CoB@MOF@CC electrode exhibits excellent performance during the hydrogen evolution reaction (η₁₀  = 57 mV, η₅₀₀  = 266 mV) and oxygen evolution reaction (η₁₀  = 209 mV, η₅₀₀  = 423 mV) in alkaline simulated seawater, and is durable for 2500 h at 500 mA cm^-2 . The OWS performance is obviously enhanced by employing the prepared electrode, which only requires 1.49 V to achieve 10 mA cm^-2 and is durable for over 360 h at industrial-grade current densities in alkaline high-salt, real seawater, rainwater, and urea electrolytes.

Heterogeneous Ni3P/Ni nanoparticles with optimized Ni active sites anchored in N-doped mesoporous nanofibers for boosting pH-universal hydrogen evolution

Developing low-cost, environmentally friendly and efficient non-precious metal electrocatalysts as alternatives to noble metals for the hydrogen evolution reaction (HER) is highly essential for the sustainable advancement of green hydrogen energy. Herein, a novel heterostructured Ni₃P/Ni nanoparticle anchored in nitrogen-doped mesoporous carbon nanofibers (Ni₃P/Ni@N-CNFs) is prepared by a facile solid-phase calcination protocol. The results demonstrated that benefiting from the intensive electronic coupling effect at the interface of the Ni₃P/Ni heterostructure, the electron configuration of the Ni active site is optimized and thus the favorable HER activity. Furthermore, the N-doped carbon nanofiber scaffold with an extensive mesoporous structure endows Ni₃P/Ni@N-CNFs with abundant electrochemically active sites together with excellent conductivity and stability, contributing to fast electron/mass transport. As expected, the resultant Ni₃P/Ni@N-CNF electrocatalyst exhibited exceptional HER catalytic properties under universal pH conditions, driving a current density of 10 mA cm^-2 at pretty low overpotentials of 121 mV, 145 mV and 187 mV in acidic, basic and neutral solutions, respectively, and retaining the catalytic stability for over 60 h. This intriguing work represents a fresh perspective for designing and exploiting highly advanced phosphide electrocatalysts for green hydrogen fuel production.

Rapid complete reconfiguration induced actual active species for industrial hydrogen evolution reaction

Rational regulation of electrochemical reconfiguration and exploration of activity origin are important foundations for realizing the optimization of electrocatalyst activity, but rather challenging. Herein, we potentially develop a rapid complete reconfiguration strategy for the heterostructures of CoC₂O₄ coated by MXene nanosheets (CoC₂O₄@MXene) during the hydrogen evolution reaction (HER) process. The self-assembled CoC₂O₄@MXene nanotubular structure has high electronic accessibility and abundant electrolyte diffusion channels, which favor the rapid complete reconfiguration. Such rapid reconfiguration creates new actual catalytic active species of Co(OH)₂ transformed from CoC₂O₄, which is coupled with MXene to facilitate charge transfer and decrease the free energy of the Volmer step toward fast HER kinetics. The reconfigured components require low overpotentials of 28 and 216 mV at 10 and 1000 mA cm⁻² in alkaline conditions and decent activity and stability in natural seawater. This work gives new insights for understanding the actual active species formation during HER and opens up a new way toward high-performance electrocatalysts.

Rational regulation of electrochemical reconfiguration and exploration of activity origin are important for electrocatalysis. Here, a novel CoC₂O₄@MXene tubular catalyst is rationally designed to achieve rapid complete reconfiguration engineering during the hydrogen evolution reaction process.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived O_d-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni⁴⁺ and Fe⁴⁺ species, which generate oxygen-bridged Ni^IV-O-Fe^IV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free O_d-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-Ni^IV-O-Fe^IV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Interfaces Decrease the Alkaline Hydrogen-Evolution Kinetics Energy Barrier on NiCoP/Ti3C2Tx MXene

Heterointerfaces can adjust the adsorption energy with intermediates in the transition state for a much decreased kinetics energy barrier (E_a). One typical transition metal phosphide, NiCoP grains (∼5 nm in size), was anchored on a Ti₃C₂T_x MXene monolayer (∼1 nm in thickness) to boost the kinetics toward alkaline hydrogen evolution reaction (HER). General electrochemical experiments at different temperatures give a small E_a of 31.4 kJ mol^-1, showing a 22.1% decrease compared to its counterpart NiCoP nanoparticles (40.3 kJ mol^-1). Impressively, the overpotential of NiCoP@MXene dramatically decreases from 71 mV to 4 mV at 10 mA cm^-2 when the temperature increases from 25 °C to 65 °C. On a single NiCoP@MXene sheet, scanning electrochemical microscopy (SECM) tests also give a very close value of E_a = 31.9 kJ mol^-1, with a relative error of ∼1.6%. Density functional theory (DFT) calculations confirm the interface between NiCoP and MXene can effectively decrease the energy barrier of water dissociation by 16.0%. The three kinds of studies on macro, micro/nano, and atomic scales disclose the interfaces can reduce the kinetics energy barrier about 16.0-22.1%. Besides, the photothermal effect of MXenes can easily raise the catalyst temperature under vis-NIR light, which has been applied in practical scenarios under sunlight for energy savings.

Construction of Petal-Like Ag NWs@NiCoP with Three-Dimensional Core-Shell Structure for Overall Water Splitting

High-efficiency, good electrical conductivity and excellent performance electrocatalysts are attracting growing attention in the field of overall water splitting. In order to achieve the desirable qualities, rational construction of the structure and chemical composition of electrocatalysts is of fundamental importance. Herein, petal-like structure Ni0.33Co0.67P shells grown on conductive silver nanowires (Ag NWs) cores as bifunctional electrocatalysts for overall water splitting were synthesized through a facile hydrothermal method and phosphorization. The resultant three-dimensional core-shell petal-like structure Ag NWs@Ni0.33Co0.67P possesses excellent catalytic activities in alkaline conditions with the overpotential of 259 mV for the oxygen evolution reaction (OER), 121 mV for the hydrogen evolution reaction (HER) and a full cell voltage of 1.64 V to reach the current density of 10 mA cm-2. Highly conductive Ag NWs as cores and high surface area petal-like Ni0.33Co0.67P as shells can endow outstanding catalytic performance for the bifunctional electrocatalyst. Thus, the synthetic strategy of the three-dimensional core-shell structure Ag NWs@Ni0.33Co0.67P considerably advances the practice of Ag NWs toward electrocatalysts.