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Surface reconstruction generates real active species in electrochemical conditions; rational regulating reconstruction in a targeted manner is the key for constructing highly active catalyst. Herein, we use the high-valence Mo modulated orthorhombic Pr3Ir1−xMoxO7 as model to activate lattice oxygen and cations, achieving directional and accelerated surface reconstruction to produce self-terminated Ir‒Obri‒Mo (Obri represents the bridge oxygen) active species that is highly active for acidic water oxidation. The doped Mo not only contributes to accelerated surface reconstruction due to optimized Ir‒O covalency and more prone dissolution of Pr, but also affords the improved durability resulted from Mo-buffered charge compensation, thereby preventing fierce Ir dissolution and excessive lattice oxygen loss. As such, Ir‒Obri‒Mo species could be directionally generated, in which the strong Brønsted acidity of Obri induced by remaining Mo assists with the facilitated deprotonation of oxo intermediates, following bridging-oxygen-assisted deprotonation pathway. Consequently, the optimal catalyst exhibits the best activity with an overpotential of 259 mV to reach 10 mA cmgeo−2, 50 mV lower than undoped counterpart, and shows improved stability for over 200 h. This work provides a strategy of directional surface reconstruction to constructing strong Brønsted acid sites in IrOx species, demonstrating the perspective of targeted electrocatalyst fabrication under in situ realistic reaction conditions. Regulating surface reconstruction is an important way to construct highly active catalyst for acidic water oxidation. Here the authors report high-valence Mo accelerates surface reconstruction of orthorhombic Pr3Ir1−xMoxO7 by activating lattice oxygen and cations, forming highly active and self-terminated Ir‒Obri‒Mo species with strong Brønsted acidity for acidic oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

The excessive dependence on fossil fuels contributes to the majority of CO2 emissions, influencing on the climate change. One promising alternative to fossil fuels is green hydrogen, which can be produced through water electrolysis from renewable electricity. However, the variety and complexity of hydrogen evolution electrocatalysts currently studied increases the difficulty in the integration of catalytic theory, catalyst design and preparation, and characterization methods. Herein, this review first highlights design principles for hydrogen evolution reaction (HER) electrocatalysts, presenting the thermodynamics, kinetics, and related electronic and structural descriptors for HER. Second, the reasonable design, preparation, mechanistic understanding, and performance enhancement of electrocatalysts are deeply discussed based on intrinsic and extrinsic effects. Third, recent advancements in the electrocatalytic water splitting technology are further discussed briefly. Finally, the challenges and perspectives of the development of highly efficient hydrogen evolution electrocatalysts for water splitting are proposed. [ABSTRACT FROM AUTHOR]

Proton exchange membrane water electrolyzer (PEMWE) technology is of interest in the context of electrocatalytic hydrogen generation from renewable energies. It has the benefits of immediate response, higher proton conductivity, lower ohmic losses, and gas crossover rate. One key step toward to large‐scale application, is the development of highly efficient, durable, and compatible anodic oxygen evolution electrocatalysts in acidic media to decrease the usage of expensive and scarce precious metals. Within this scenario, an in‐depth understanding of oxygen evolution reaction mechanisms including the adsorption evolution mechanism and lattice oxygen evolution mechanism is first provided to aid development of innovative materials and elucidate the origin of catalyst degradation. Second, recent progress in the development of oxygen evolution electrocatalysts in acid media is reviewed with an emphasis on the underlying structure–performance relationships. Third, the current application status and research progress in PEMWEs along with representative examples are discussed. Last, the remaining challenges and promising insights are proposed to inspire future studies on the development of hydrogen production technology from renewable energy. [ABSTRACT FROM AUTHOR]

Solar‐driven water splitting is in urgent need for sustainable energy research, for which accelerating oxygen evolution kinetics along with charge migration is the key issue. Herein, Mn3+ within π‐conjugated carbon nitride (C3N4) in form of Mn–N–C motifs is coordinated. The spin state (eg orbital filling) of Mn centers is regulated by controlling the bond strength of Mn–N. It is demonstrated that Mn serves as intrinsic oxygen evolution reaction (OER) site and the kinetics is dependent on its spin state with an optimized eg occupancy of ≈0.95. Specifically, the governing role of eg occupancy originates from the varied binding strength between Mn and OER intermediates. Benefiting from the rapid spin state‐mediated OER kinetics, as well as extended optical absorption (to 600 nm) and accelerated charge separation by intercalated metal‐to‐ligand state, Mn–C3N4 stoichiometrically splits pure water with H2 production rate up to 695.1 µmol g−1 h−1 under simulated sunlight irradiation (AM1.5), and achieves an apparent quantum efficiency of 4.0% at 420 nm, superior to most solid‐state based photocatalysts to date. This work for the first time correlates photocatalytic redox kinetics with the spin state of active sites, and suggests a nexus between photocatalysis and spin theory. [ABSTRACT FROM AUTHOR]

Incorporating g-C 3 N 4 with transition metal phosphides is emerging as a low-cost and robust co-catalyst for hydrogen evolution. The ammonia borane hydrolysis is an efficient method to release H 2 at ambient conditions in the presence of a catalyst. An efficient and cheap catalyst is needed for practical application to achieve this benchmark. For this purpose, a catalyst Ni 2 P/C 3 N 4 is synthesized by hydrothermal method and low-temperature phosphidation. The optimization reveals that the Ni 2 P/C 3 N 4 with 6.5% Ni contents shows the best performance for H 2 release. Furthermore, 2% Pt nanoparticles loading over Ni 2 P/C 3 N 4 boosts the charge transfer and improves activity 5.7-fold compared to Ni 2 P/C 3 N 4 , and the Pt-loaded catalyst is depicted as Pt@Ni 2 P/C 3 N 4. The reaction kinetics reveals that the hydrogen evolution rate accelerates by increasing the amount of Pt@Ni 2 P/C 3 N 4 and AB concentration, and the loading of Pt nanoparticles loaded over Ni 2 P/C 3 N 4 reduces the activation energy significantly. Moreover, the ionic interaction between Pt and Ni 2 P/C 3 N 4 generates Ptᵟ+ and (Ni 2 P/C 3 N 4)ᵟ− active sites which facilitates B–H cleavage and O–H bonds of ammonia borane and water, respectively. Incorporating transition metals phosphide and noble metals supported over g-C 3 N 4 paves the pathway toward the efficient H 2 evolution from ammonia borane, bringing cost-effective modifications to synthesize constructive catalysts. Hydrogen evolution in the presence of Pt@Ni 2 P/CN. [Display omitted] • The best activity is achieved with 6.5% nickel content for synthesized Ni 2 P/C 3 N 4. • The 2% Pt NPs loaded over Ni 2 P/C 3 N 4 reduces the activation energy. • Ptᵟ+ and (Ni 2 P/C 3 N 4)ᵟ− serve as active sites to enhance the catalytic activity. [ABSTRACT FROM AUTHOR]

Schematic diagram: Synergetic effect of Au NPs loading over Ni 2 P & CoP for H 2 evolution. [Display omitted] • The catalysts (Au/Ni 2 P, Au/CoP) are synthesized and optimized. • The XPS reveals that the synergetic effect of Au/Ni 2 P is stronger than Au/CoP. • The activation energy of Au/CoP is less than Au/Ni 2 P. • Active sites for ammonia-borane hydrolysis are Auδ+ and (Ni 2 P & CoP) δ-. The hydrogen evolution from ammonia borane is intriguing but challenging due to its sluggish kinetics. In this regard, the gold nanoparticles amalgamation with metal phosphides is speculated to be more efficient catalysts. Here, the catalysts Au/Ni 2 P and Au/CoP with the high synergetic effect of Au nanoparticles and metal phosphides were synthesized for ammonia borane hydrolysis. The activity of Au/Ni 2 P increases 4.8-fold (i.e., 0.08 to 0.40 L∙h−1) compared to pristine Ni 2 P, and the activity of Au/CoP increases 1.7-fold (i.e., 0.74 to 1.27 L∙h−1) compared to pristine CoP. This reveals that the synergetic effect of Auδ+ and (Ni 2 P) δ- is stronger than Auδ+ and (CoP) δ- which is manifested by XPS analysis. The kinetics exposes that the activation energy of Au/Ni 2 P (45.28 kJ∙mole-1) is greater than Au/CoP (31.45 kJ∙mole-1) and the TOF of Au/Ni 2 P is less than Au/CoP. This research work presents an effective approach for producing active sites of Auδ+ and (Ni 2 P & CoP) δ- for ammonia borane hydrolysis to enhance the H 2 evolution rate. [ABSTRACT FROM AUTHOR]

Developing affordable and efficient electrocatalysts toward water electrolysis under industrial conditions is crucial for large-scale production of green hydrogen. In this regard, we propose a facile and mild method to construct crystalline-amorphous composites of metal phosphate (MPi) and NiS on nickel foam (NF) for practical water electrolysis. The as-prepared electrodes exhibit excellent performance, achieving a remarkable current density of 1000 mA cm–2 at the ultralow overpotential of 345 and 223 mV for FePi-NiS/NF (oxygen evolution reaction, OER) and NiCoPi-NiS/NF (hydrogen evolution reaction, HER), respectively. The composites can undergo an in-situ transformation into highly active and targeted species under high current density, which possess a unique pore structure interconnected by nanosheets that provides an abundance of catalytic sites and open channels for efficient bubble diffusion. When operated under industrial conditions (6 M KOH, 70 °C), the assembled electrode requires only 1.712 V to attain a current density of 1000 mA cm−2. The electrodes also demonstrate exceptional performance in alkaline electrolysis with an anion exchange membrane (AEMWE) when scaled up to a larger size (area: ≈ 25 cm2). Specifically, they achieve a substantial current of 12.5 A at 1.87 V with a high energy efficiency of 79.2 % and remarkable durability over 30 h under industrial electrolysis conditions (1 M KOH, 50 °C), outperforming benchmark Pt/C/NF || IrO 2 /NF electrodes with energy saving of 0.215 kWh Nm–3. This work provides a cost-effective and efficient strategy for designing and constructing stable and active catalysts for water electrolysis under harsh industrial conditions. [Display omitted] • Self-supported crystalline-amorphous composites are prepared by a universal method. • FePi-NiS/NF and NiCoPi-NiS/NF exhibit excellent OER and HER performance, respectively. • The in-situ formed active species play an important role in water splitting. • The assembled AEMWE (area: ≈ 25 cm2) can be operated efficiently and stably under industrial conditions. [ABSTRACT FROM AUTHOR]

Ni-based alloys are considered as the efficient catalyst for hydrogen-storage materials decomposition. Herein, we applied an in-situ melt-quenching method to dope Mo in Ni-based alloy for catalytic hydrogen evolution from hydrogen-storage materials. Importantly, Mo doped Ni-based catalyst exhibits more than 6 times higher TOF value than that of pure Ni both in AB hydrolysis and hydrazine decomposition, because Mo acts as an electron donor to improve the reducibility of Ni. Hydrogen evolution kinetics were studied over a range of temperatures (303–353 K) and initial feed concentrations (catalyst/hydrogen-storage materials (wt/wt) ratios = 0.2–10). Under optimal reaction conditions, the H 2 evolution rate reaches 1.92 mol H 2 /(mol cat min) and 0.05 mol H 2 /(mol cat min) in the hydrolysis of ammonia borane and decomposition of hydrazine, which are 6.42 and 6.44 times higher than undoped Ni catalyst, respectively. And the apparent activation energy of ammonia borane hydrolysis and hydrazine decomposition were evaluated to be 26.66 ± 3.31 kJ/mol and 40.01 ± 3.38 kJ/mol, respectively. Image 1 • Mo-doped Ni exhibits 6 times higher H 2 evolution rate than undoped one. • H 2 evolution promotion was determined by XPS and H 2 -TPD. • The apparent activation energy of H 2 evolution are evaluated. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting provides a promising pathway to produce hydrogen, but the oxygen evolution reaction (OER) with sluggish kinetics is the bottleneck. The design and synthesis of high efficiency catalysts is the key to overcome the OER barrier. Herein, the surface-modified amorphous FeOOH on NiFe LDHs (FeOOH-a@NiFe LDHs) was synthesized by using a simple hydrothermal and electrodeposition method. The amorphous FeOOH on NiFe LDHs can provide more active sites for the adsorption and activation of *H 2 O and facilitate the interfacial electron transfer, which significantly accelerates the OER kinetics. The FeOOH-a@NiFe LDHs show low overpotentials of 237 mV and 252 mV to achieve 50 mA cm−2 and 100 mA cm−2 respectively, and the activity maintains above 90% after long-term testing at a high current density of 100 mA cm−2 for 500 h. Moreover, the water electrolysis device with the FeOOH-a@NiFe LDHs‖Pt/C(20%) as electrode requires only a low cell votage of 1.48 V to achieve 10 mA cm−2. This work provides an efficient OER catalyst which could serve as a potential catalyst for industrial electrocatalytic water splitting. [Display omitted] [ABSTRACT FROM AUTHOR]

The H 2 evolution in the presence of a catalyst Pt@Ni 2 P under visible light irradiation. [Display omitted] • The highest activity is achieved by an optimized 2% Pt@Ni 2 P catalyst. • The visible light irradiation increases the TOF and reduces the activation energy. • The visible light accelerates the electron transfer from Ptδ+ to Ni 2 Pδ−. Ammonia borane is a promising hydrogen storage material that can be handled easily and release hydrogen at room temperature in the presence of a catalyst. Attaching noble metal to transition metal phosphide is needed to improve the dehydrogenation from ammonia borane. Here, a catalyst Pt@Ni 2 P is reported to enhance the hydrogen evolution rate in dark and visible light. The incorporation of Pt with Ni 2 P exhibit partially positive Ptδ+ and partially negative Ni 2 Pδ− as manifested by X-ray photoelectron spectrum, which serve as active sites for ammonia borane and water, respectively. Moreover, visible light irradiation further accelerates the electron transfer from Ptδ+ to Ni 2 Pδ− as exhibited by electrochemical impedance spectroscopy and transient photocurrent spectra. Subsequently, Pt loading over Ni 2 P plays a vital role in improving H 2 evolution rate via ammonia borane hydrolysis. With the incorporation of 2% Pt, the hydrogen generation rate from ammonia borane hydrolysis is increased by 4.0-fold in dark, and visible light irradiation further increases the activity by 5.7-fold compared with pristine Ni 2 P. The visible light enhances the turn-off frequency from 4.7 min−1 to 10.3 min−1 and reduces the activation energy from 52.0 kJ · mole−1 to 43.4 kJ · mole−1 compared with dark. The performance of Pt@Ni 2 P is also outstanding compared with reported catalysts and is expected to be used economically in practical applications. [ABSTRACT FROM AUTHOR]

Graphitic carbon nitride (g-C 3 N 4 ) has emerged as a promising photocatalyst, but poor charge separation and low surface area limit its activity. Here, we report a hydrothermal method to generate hydrogen bonded supramolecular complex via water-based homogeneous supramolecular assembly, which is a promising precursor to fabricate porous and oxygen-doped g-C 3 N 4 . The hydrothermal treatment provides a homogeneous environment for hydrolysis of melamine to produce cyanuric acid and reaction of cyanuric acid with remained melamine to create the in-plane ordering and hydrogen bonded supramolecular complex. The complex can template uniform nanoporous structure and also provide an opportunity for O-doping in the g-C 3 N 4 network upon calcination in air. The resulted g-C 3 N 4 (GCN-4) possesses high surface area, well-defined 3D morphology and oxygen-dopant in the lattice. Subsequently, the visible light absorption, charge separation, and wettability are considerably enhanced. This catalyst exhibits higher hydrogen evolution rate by 11.3 times than the bulk g-C 3 N 4 under visible light irradiation, with apparent quantum efficiency of 10.3% at 420 nm. [ABSTRACT FROM AUTHOR]

[Display omitted] • An effective strategy was developed to generate and stabilize Ni3+ on Co 3- x O 4 /NiO. • Co 3- x O 4 /NiO with abundant Ni3+ as active site has high OER activity and stability. • OER mechanism was studied by using DFT + U calculation. • Co 3- x O 4 /NiO can provide suitable binding energy for oxygen-containing intermediates and accelerate OER reaction kinetics. Electrocatalytic water splitting has received widespread attention. However, due to the slow reaction kinetics and complex electron transfer process, the oxygen evolution reaction (OER) occurring at the anode has become a bottleneck. Herein, the metal-defective Co 3- x O 4 was selected as the electron-acceptor carrier and Co 3- x O 4 /NiO was synthesized by a simple two-step method. The results show that Co 3- x O 4 /NiO has a low overpotential of 240 mV and 320 mV at 10 mA·cm−2 and 100 mA·cm−2, a low Tafel slope of 64 mV/dec, and the electrochemical surface area (ECSA) of Co 3- x O 4 /NiO is as high as 1033.3 cm2. Moreover, the decrease in activity after the 60 h stability testing is negligible. Combining experiments and theoretical calculations, the high activity and stability of Co 3- x O 4 /NiO is attributed to the tunable electronic structure, and more electrons are transferred from NiO to Co 3- x O 4 which leads to the in-situ generation of more Ni3+ as active sites. The Co 3- x O 4 /NiO with abundant of Ni3+ can effectively regulate the oxygen-containing intermediates (OH*, O* and OOH*) with appropriate binding energy, reduce Gibbs free energy change of rate-limiting step, and accelerate OER reaction kinetics. This work provides an efficient strategy to generate and stabilize Ni3+ active sites and confirms the key catalytic role of Ni3+ in the OER process. [ABSTRACT FROM AUTHOR]

[Display omitted] • The TM (Ti, Mn, Fe, Co, Ni, Cu) doping can induce magnetism in monolayer WSe 2. • The spin-resolved anti-bonding states filling influence the HER activity of TM-WSe 2. • The Ni/Cu-WSe 2 possesses optimal HER property with lower Δ G H* (-0.20 and 0.06 eV). Transition metal dichalcogenide monolayers are an emerging family of electrocatalysts, in which Tungsten diselenide (WSe 2) has attracted attention for Hydrogen evolution reaction (HER). Here, the magnetism and HER performance of 3 d transition metal atoms doping WSe 2 (TM-WSe 2) are comprehensively studied using density functional theory. The hybridization between the localized TM-3 d and the delocalized Se-4 p or W-5 d states leads to the ferromagnetic/antiferromagnetic coupling between the TM spins and Se or W spins, which induces magnetism in WSe 2. With the analysis of the integrated-crystal orbital Hamilton population (ICOHP), the ICOHP spin up values of TM-WSe 2 are decreased compared to the pure WSe 2 , which improves the interaction of Se-H, thereby the H atom is easier to be adsorbed on the Se1 site. While all the ICOHP spin down values of TM-WSe 2 are increased, this indicates the weak interaction of Se-H, which promotes the desorption of H 2 molecule. Thus, the synergistic effect of spin up and spin down anti-bonding states filling makes TM-WSe 2 possess an enhanced HER performance. Moreover, the Ni/Cu-WSe 2 possesses optimal HER performance with lower Δ G H* (-0.20 and 0.06 eV). Therefore, our results provide a primary understanding for the relationship between the spin up/down anti-bonding states filling and HER electrocatalytic performance. [ABSTRACT FROM AUTHOR]

• The solid-acid structure was introduced on surfaces of Co 3 O 4 and CoNiFeO x. • The SO 4 2− modification triggers Co4+ and intermediate spin state of Co3+ (t 2 g 5 e g 1). • Additional adsorption of SO 4 2− breaks adsorption-energy scaling relation. • CoNiFeO x -SO 4 exhibits superior performances in OER. Adjusting the electronic structure of electrocatalyst active sites can optimize the adsorption of intermediates to boost the kinetics of oxygen evolution reaction (OER), while this modulation is limited by adsorption-energy scaling relations of three intermediates (*OH, *O and *OOH). This work introduced the solid-acid structure into electrocatalysts through surface bonding SO 4 2− (like Co 3 O 4 and CoNiFeO x). DFT calculations reveal the essence of solid acid modification is that triggers the production of Co4+ and intermediate spin Co3+ (t 2 g 5 e g 1), then lifts metal d-band level and oxygen p-band level. Notably, the hydrogen bonding between SO 4 2− group and ∗OOH provides additional adsorption contribution, thereby breaking the adsorption-energy scaling relation of OER. Modified by SO 4 2−, the CNF-SO 4 shows remarkably high performance with overpotential of 231 mV (at 10 mA cm-2) and low Tafel slop of 33.8 mV·dec-1. This work provides an effective solid-acid-mediated strategy for electronic structure regulation and adsorption-energy scaling relation breaking in OER. [ABSTRACT FROM AUTHOR]

• All-in-one Pt·Ni(OH) 2 /C 3 N 4 photocatalyst realizes one-step overall water splitting. • Pt·Ni(OH) 2 /C 3 N 4 achieves a prominent AQE of 4.2% at 420 nm and excellent stability. • Both H 2 O activation and charge transfer are dramatically enhanced. • The backward recombination of H 2 and O 2 is suppressed. Overall water splitting is vital in solar-hydrogen conversion. In addition to charge separation, the regulation of surface kinetics and suppression of backward reaction become particularly crucial. Herein, an all-in-one Pt·Ni(OH) 2 /C 3 N 4 photocatalyst is proposed by integrating Pt@Ni(OH) 2 composited nanowires and isolated Pt clusters on C 3 N 4. In this heterostructure, Pt@Ni(OH) 2 with rich coordinatively unsaturated sites effectively boost O 2 evolution, and Pt-O-Ni interaction retards O O bond cleavage thus inhibiting backward H 2 O regeneration. Meanwhile isolated Pt forms an Schottky junction with C 3 N 4 for electron transfer and proton reduction. Consequently, Pt·Ni(OH) 2 /C 3 N 4 achieves stoichiometric water splitting with H 2 /O 2 evolution of 1330/632 μmol g−1 h−1, and an outperforming AQE of 4.2% at 420 nm. Our work manifests that accelerating charge migration, inhibiting backward reaction and tuning surface kinetics are dominant in water splitting, for which a rational design of robust redox pathways is necessary. [ABSTRACT FROM AUTHOR]

• MnO x -decorated 3D porous C 3 N 4 is fabricated by NaCl-templated method. • C 3 N 4 possesses donor–acceptor system by Na ions and cyano groups. • MnO x modulate holes transfer and provide oxidation sites. • C 3 N 4 shows quick charge separation and stronger reduction capability. • C 3 N 4 achieves 18-fold higher H 2 production rates of bulk C 3 N 4 under visible light. Carbon nitride is an intriguing visible-light photocatalyst for H 2 production but suffers from low surface area and fast charge recombination. Here, we report a simple yet efficient approach to fine tune the porous structure, modify the electronic structure and carrier behaviors of C 3 N 4 through in-situ growth of MnO x by NaCl template-assisted strategy. This NaCl template method simultaneously constructs 3D porous structure for high surface area, induces Na coordination with N atoms and accompanying cyano group to form an internal donor-acceptor system for efficient charge transfer and separation. Along with the MnO x growth, the band gap of C 3 N 4 is narrowed for stronger visible light absorption and the conduction band level is negatively shifted for stronger reduction capability. Thus, the resulted C 3 N 4 (MSCN) shows much improved activity of about 18 times higher than bulk C 3 N 4 in photocatalytic hydrogen evolution reaction under visible light irradiation. [ABSTRACT FROM AUTHOR]