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The electrolysis of water to produce hydrogen is an important technique to replace traditional fossil fuel-based hydrogen production. This method efficiently converts electrical energy into chemical energy, it is ostensibly a promising candidate for addressing the energy crisis. Significant effort has been devoted to developing efficient electrocatalysts for water electrolysis. The exploration of suitable catalytic materials for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and other bifunctional electrocatalytic reactions is crucial. Transition metal selenides (TMSes) have emerged as potential HER and OER electrocatalysts because of their unique electronic structures, which are beneficial for charge transfer, tuneable bandgaps, distinctive morphologies, and low-cost. This review discusses the mechanisms and performance comparisons of TMSes in overall water splitting under various pH conditions. From an industrial and commercial perspective, the catalytic performance of TMSes for the HER and OER is not ideal. Methods for preparing electrocatalytic materials and optimizing materials for overall water decomposition and modulation mechanisms have been introduced to improve electrocatalytic performance, such as element doping, carbon composites, bimetallic systems, morphology control, and heterogeneous interface engineering. Finally, the challenges and prospects of TMSes were discussed. [ABSTRACT FROM AUTHOR]

Herein, two-dimensional cobalt-doped MnPSe3 nanosheets (CMPS) were constructed, which served as an outstanding bifunctional catalyst for alkaline seawater splitting, i.e., offering the current density of 10 mA cm−2 with applied overpotentials of 59 and 300 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The assembled two-electrode system of CMPS//CMPS also demonstrated excellent catalytic activity (10 mA cm−2, 1.59 V) and can remain stable for more than 100 h. Moreover, the theoretical calculations showed that CMPS features a suitable H* adsorption beneficial for the HER, as well as a lower free energy barrier favorable for the OER. [ABSTRACT FROM AUTHOR]

In this study, we prepared amorphous iron-cobalt oxide through the dealloying of trimetallic FeCoAl, showing excellent performance in both urea oxidation and hydrogen evolution reactions (UOR and HER) in alkaline seawater. The catalyst demonstrated stable UOR and HER activity during long-term operations due to the abundant active sites and oxygen vacancies. The catalyst required applied potentials of 1.52 and −0.185 V to yield 100 mA cm−2 for the UOR and HER, respectively. Moreover, when used as both the cathode and anode, the electrolyzer required a working voltage of 1.68 V to yield 100 mA cm−2 for urea-assisted hydrogen production. [ABSTRACT FROM AUTHOR]

Water electrolysis represented a promising avenue for the large-scale production of high-purity hydrogen. However, the high overpotential and sluggish reaction rates associated with the anodic oxygen evolution reaction (OER) posed significant obstacles to efficient water splitting. To tackle these challenges, the urea oxidation reaction (UOR) emerged as a more favorable thermodynamic alternative to OER, offering both the energy-efficient hydrogen evolution reaction (HER) and the potential for the treating of urea-rich wastewater. In this work, a two-step methodology comprising nanowire growth and phosphating treatment was employed to fabricate Cu3P nanowires on Cu foam (Cu3P-NW/CF) catalysts. These novel catalytic architectures exhibited notable efficiencies in facilitating both the UOR and HER in alkaline solutions. Specifically, within urea-containing electrolytes, the UOR manifested desirable operational potentials of 1.43 V and 1.65 V versus the reversible hydrogen electrode (vs. RHE) to reach the current densities of 10 and 100 mA cm−2, respectively. Concurrently, the catalyst displayed a meager overpotential of 60 mV for the HER at a current density of 10 mA cm−2. Remarkably, the two-electrode urea electrolysis system, exploiting the designed catalyst as both the cathode and anode, demonstrated an outstanding performance, attaining a low cell voltage of 1.79 V to achieve a current density of 100 mA cm−2. Importantly, this voltage is preferable to the conventional water electrolysis threshold in the absence of urea molecules. Moreover, our study shed light on the potential of innovative Cu-based materials for the scalable fabrication of electrocatalysts, energy-efficient hydrogen generation, and the treatment of urea-rich wastewater. [ABSTRACT FROM AUTHOR]

The electrocatalytic conversion of water‐to‐hydrogen powered by renewable energy is one of the promising strategies to unravel the energy and environment crises. However, a such process usually costs large energy consumption owing to the low efficiency of anodic oxygen evolution reaction (OER), which is another vital half reaction of the overall water splitting (OWS) system. Herein, we designed and fabricated hollow nitrogen‐doped carbon nanocages loaded with Ni3N and Co nanoparticles derived from polyoxometalate‐encapsuled metal‐organic frameworks. The catalyst exhibited appealing dual functions for both OER and hydrogen evolution reaction (HER), outperforming the Ni‐free and solid samples. Notably, the designed catalyst also showed much enhanced electrochemical performance for glycerol/glucose oxidation reaction when compared with OER. The home‐made two‐electrode electrolyzer, coupled HER with polyalcohol or glucose oxidation reaction, only needs cell voltages of 1.125 and 1.557 V to reach 10 mA cm−2, respectively, which are much smaller than that in 1 M KOH (2.087 V), demonstrating a large amount of energy saving for hydrogen production. [ABSTRACT FROM AUTHOR]

Aqueous rechargeable Zn—gas batteries are regarded as promising energy storage and conversion devices due to their high safety and inherent environmental friendliness. However, the energy efficiency and power density of Zn—gas batteries are restricted by the kinetically sluggish cathode reactions, such as oxygen evolution reaction (OER) during charging and oxygen reduction reaction (ORR)/carbon dioxide reduction reaction (CO2RR)/nitrogen reduction reaction (NRR)/nitric oxide reduction reaction (NORR) during discharge. In this review, battery configurations and fundamental reactions in Zn—gas batteries are first introduced, including Zn—air, Zn-CO2, Zn-N2, and Zn-NO batteries. Afterward, recent advances in active site engineering for enhancing the intrinsic catalytic activities of cathode catalysts are summarized. Subsequently, the structure and surface regulation strategies of cathode materials for optimizing the three-phase interface and improving the performance of Zn—gas batteries are discussed. Finally, some personal perspectives for the future development of Zn—gas batteries are presented. [ABSTRACT FROM AUTHOR]

The fabrication of Zn‐CO2 batteries is a promising technique for CO2 fixation and energy storage. Herein, nitrogen‐doped ordered mesoporous carbon (NOMC) is adopted as a bifunctional metal‐free electrocatalyst for CO2 reduction and oxygen evolution reaction in the near‐neutral electrolyte. The ordered mesoporous structures and abundant N‐dopings of NOMC facilitate the accessibility and utilization of the active sites, which endow NOMC with excellent electrocatalysis performance and outstanding stability. Especially, a nearly 100% CO Faradaic efficiency is achieved at an ultralow overpotential of 360 mV for CO2 reduction. When constructed as an aqueous rechargeable Zn‐CO2 battery using NOMC as the cathode, it yields a high peak power density of 0.71 mW cm−2, a good cyclability of 300 cycles, and excellent energy efficiency of 52.8% at 1.0 mA cm−2. [ABSTRACT FROM AUTHOR]

Hollow materials with a sophisticated structure are promising for various applications with boosted performances and innovative properties. Herein, we report an in situ transformation strategy using multi-layered MOFs as templates to fabricate multi-shelled hollow NiZnCoFe layered double hydroxides (LDHs), which outperformed the double- and single-shelled hollow LDHs and commercial IrO2 in the oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

Using hydrazine oxidation reaction (HzOR) to replace the oxygen evolution reaction is an effective way to decrease the overpotential of the anodic reaction in overall water splitting (OWS), facilitating cost‐effective and safe hydrogen production. Herein, Rh2S3/N‐doped carbon hybrids (Rh2S3/NC) are first reported as novel and efficient bifunctional electrocatalysts for hydrazine‐assisted hydrogen generation over a wide pH range. Specifically, Rh2S3/NC exhibits low overpotentials for the hydrogen evolution reaction (HER) in alkaline (38 mV), neutral (46 mV), and acidic (21 mV) electrolytes, to reach the current density of 10 mA cm−2, and maintains the activities over 70 h. Meanwhile, Rh2S3/NC also displays competitive HzOR performance at all‐pH electrolytes. Thus, serving as a bifunctional electrocatalyst for both HER and HzOR, Rh2S3/NC shows overwhelming‐Pt/C performance in three electrolytes, and can save over 93.3%, 85.2%, and 78.3% energy consumption compared to the corresponding OWS system. Moreover, theoretical calculations confirm that Rh2S3/NC owns low free‐energy changes of the H adsorption and the dehydrogenation of adsorbed NHNH both of which are beneficial to enhance catalytic activity. This work develops a novel bifunctional electrocatalyst with free pH‐dependent condition for the hydrazine‐assisted electrolysis system to furtherly reduce the cost of massive industrial H2 production. [ABSTRACT FROM AUTHOR]

Solar‐driven electrochemical overall CO2 splitting (OCO2S) offers a promising route to store sustainable energy; however, its extensive implementation is hindered by the sluggish kinetics of two key reactions (i.e., CO2 reduction reaction and oxygen evolution reaction (CO2RR and OER, respectively)). Here, as dual‐functional catalysts, Co2FeO4 nanosheet arrays having high electrocatalytic activities toward CO2RR and OER are developed. When the catalyst is applied to a complete OCO2S system driven by a triple junction GaInP2/GaAs/Ge photovoltaic cell, it shows a high photocurrent density of ≈13.1 mA cm−2, corresponding to a remarkably high solar‐to‐CO efficiency of 15.5%. Density functional theory studies suggest that the Co sites in Co2FeO4 are favorable to the formation of *COOH and *O intermediates and thus account for its efficient bifunctional activities. The results will facilitate future studies for designing highly effective electrocatalysts and devices for OCO2S. [ABSTRACT FROM AUTHOR]

Development of highly active and stable electrocatalysts for overall water splitting is important for future renewable energy systems. In this study, porous Mn‐doped FeP/Co3(PO4)2 (PMFCP) nanosheets on carbon cloth are utilized as a highly efficient 3 D self‐supported binder‐free integrated electrode for the oxygen evolution and hydrogen evolution reactions (OER and HER) over a wide pH range. Specifically, overpotentials of 27, 117, 85 mV are required for the PMFCP nanosheets to attain 10 mA cm−2 for HER in 0.5 m H2SO4, 1.0 m phosphatebuffered saline (PBS), and 1.0 m KOH, respectively. In addition to the excellent performance for HER electrocatalysis, PMFCP nanosheets were also efficient electrocatalysts for the OER. Thus, the PMFCP nanosheets can serve as anodes and cathodes for overall water splitting (OWS). The OWS working voltages to attain 10 mA cm−2 are found to be 1.75, 1.82, and 1.61 V in acid, neutral, and alkaline electrolytes, respectively. Chronopotentiometric tests show that the PMFCP electrode can maintain its excellent pH‐universal OWS activity for more than 30 000 s. This work also provides new insights into developing high‐performance electrocatalysts for water splitting over a wide pH range. The improvement in electrochemical performance by introduction of Mn dopant and nano‐holes offers new opportunities in the development of effective electrodes for other energy‐related applications. Splitting the difference: Porous Mn‐doped FeP/Co3(PO4)2 nanosheets are prepared as a highly efficient and stable 3 D self‐supported binder‐free integrated electrode for the hydrogen evolution and oxygen evolution reactions over a wide pH range. The material also exhibits strong catalytic activity and stability for overall water splitting at all pH values. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) is a critical step in water splitting to produce hydrogen. In this report, Au/NiCo2O4 nanorod arrays with high activity for OER have been fabricated by a facile hydrothermal method followed by a reduction process. The Au/NiCo2O4 nanoarrays exhibited OER activity that was almost four times higher than that of Ir/C (at 1.75V vs. RHE), and a small Tafel slope (63 mV?decade-1). Moreover, this Au/NiCo2O4 hybrid-array electrode showed good stability in alkaline solution, which is required of an active anode for water electrolysis. [ABSTRACT FROM AUTHOR]

The design and fabrication of efficientand inexpensive electrodesfor use in the oxygen evolution reaction (OER) is essential for energy-conversiontechnologies. In this study, high OER performance is achieved usingnovel hierarchical ZnxCo3–xO4nanostructures constructed with smallsecondary nanoneedles grown on primary rhombus-shaped pillar arrays.The nanostructures have large roughness factor, high porosity, andhigh active-site density. Only a small overpotential of ∼0.32V is needed for a current density of 10 mA/cm2with a Tafelslope of 51 mV/decade. The nanostructures are also found to performsignificantly better than pure Co3O4and a commercialIr/C catalyst and to perform similarly to the best OER catalysts thathave been reported for alkaline media. These merits combined withthe satisfactory stability of the nanostructures indicate that theyare promising electrodes for water oxidation. [ABSTRACT FROM AUTHOR]

The development of non-precious metal catalysts with excellent bifunctional activities is significant for air–metal batteries. ABO3-type perovskite oxides can improve their catalytic activity and electronic conductivity by doping transition metal elements at B sites. Here, we develop a novel Sm0.5Sr0.5Co1−xNixO3−δ (SSCN) nanofiber-structured electrocatalyst. In 0.1 M KOH electrolyte solution, Sm0.5Sr0.5Co0.8Ni0.2O3−δ (SSCN82) with the optimal Co: Ni molar ratio exhibits good electrocatalytic activity for OER/ORR, affording a low onset potential of 1.39 V, a slight Tafel slope of 123.8 mV dec−1, and a current density of 6.01 mA cm−2 at 1.8 V, and the ORR reaction process was four-electron reaction pathway. Combining the morphological characteristic of SSCN nanofibers with the synergistic effect of cobalt and nickel with a suitable molar ratio is beneficial to improving the catalytic activity of SSCN perovskite oxides. SSCN82 exhibits good bi-functional catalytic performance and electrochemical double-layer capacitance. [ABSTRACT FROM AUTHOR]

Water splitting requires development of cost‐effective multifunctional materials that can catalyze both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) efficiently. Currently, the OER relies on the noble‐metal catalysts; since with other catalysts, its operation environment is greatly limited in alkaline conditions. Herein, an advanced water oxidation catalyst based on metallic Co9S8 decorated with single‐atomic Mo (0.99 wt%) is synthesized (Mo‐Co9S8@C). It exhibits pronounced water oxidization activity in acid, alkali, and neutral media by showing positive onset potentials of 200, 90, and 290 mV, respectively, which manifests the best Co9S8‐based single‐atom Mo catalyst till now. Moreover, it also demonstrates excellent HER performance over a wide pH range. Consequently, the catalyst even outperforms noble metal Pt/IrO2‐based catalysts for overall water splitting (only requiring 1.68 V in acid, and 1.56 V in alkaline). Impressively, it works under a current density of 10 mA cm−2 with no obvious decay during a 24 h (0.5 m H2SO4) and 72 h (1.0 m KOH) durability experiment. Density functional theory (DFT) simulations reveal that the synergistic effects of atomically dispersed Mo with Co‐containing substrates can efficiently alter the binding energies of adsorbed intermediate species and decrease the overpotentials of the water splitting. [ABSTRACT FROM AUTHOR]

The development of water-separation-performing, noble-metal-free, and cost-effective electrocatalysts is of the utmost importance. This work outlines the preparation of ordered, macroporous MoS 2 –Fe 2 Mo 3 O 8 /MoO 2 electrocatalysts with heterogeneous structures using pyrolysis and sulfidation methods. The resulting macroporous and heterogeneous structures feature more active sites and facilitate ion diffusion, thereby effectively improving the overall performance. The catalyst demonstrated excellent performance in both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in a 1 M potassium hydroxide solution, requiring overpotentials of 213 and 60 mV, respectively, to achieve a current density of 10 mA cm−2. Moreover, the MoS 2 –Fe 2 Mo 3 O 8 /MoO 2 -based electrolyzer only requires an output voltage of 1.49 V, exhibiting superior performance and long-term stability compared to commercial catalysts. This study introduces a novel approach for achieving a heterogeneous macroporous structure that can enhance electrocatalysis. [ABSTRACT FROM AUTHOR]

Well-defined Ni 3 N nanoparticles armored in hollow carbon nanotube shell exhibit excellent bifunctional hydrogen electrocatalysis. [Display omitted] Alkaline H 2 -O 2 fuel cells and water electrolysis are crucial for hydrogen energy recycling. However, the sluggish kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) in an alkaline medium pose significant obstacles. Thus, it is imperative but challenging to develop highly efficient and stable non-precious metal electrocatalysts for alkaline HOR and HER. Here, we present the intriguing synthesis of well-defined Ni 3 N nanoparticles armored within an N-doped hollow carbon nanotube shell (Ni 3 N@NC) via the conversion of a hydrogen-bonded organic framework (HOF) to metal-organic framework (MOF), followed by high-temperature pyrolysis. As-developed Ni 3 N@NC demonstrates exceptional bifunctionality in alkaline HOR/HER electrocatalysis, with a high HOR limiting current density of 2.67 mA cm−2 comparable to the benchmark 20 wt% Pt/C, while achieving a lead in overpotential of 145 mV and stronger CO-tolerance. Additionally, it achieves a low overpotential of 21 mV to attain a HER current density of 10 mA cm−2 with long-term stability up to 340 h, both exceeding those of Pt/C. Structural analyses and electrochemical studies reveal that the remarkable bifunctional hydrogen electrocatalytic performance of Ni 3 N@NC can be ascribed to the synergistic coupling among the well-dispersed small-sized Ni 3 N nanoparticles, chain-mail structure, and optimized electronic structure enabled by strong metal-support interaction. Furthermore, theoretical calculations indicate that the high-efficiency HOR/HER observed in Ni 3 N@NC is attributed to the strong OH− affinity, moderate H adsorption, and enhanced water formation/dissociation ability of the Ni 3 N active sites. This work underscores the significance of rational structural design in enhancing performance and inspires further development of advanced nanostructures for efficient hydrogen electrocatalysis. [ABSTRACT FROM AUTHOR]

Ni-MoN nanosheet-assembled microspheres are developed as efficient bi-functional catalyst towards urea oxidation and hydrogen evolution reactions. [Display omitted] Electrocatalytic water splitting is a promising technology for sustainable hydrogen (H 2) production; however, it is restricted by the kinetically sluggish anodic oxygen evolution reaction (OER). Replacing OER with urea oxidation reaction (UOR) with low thermodynamic potential can simultaneously improve the energy efficiency of H 2 production and purify urea-containing wastewater. Here we report a facile assembly-calcination two-step method to synthesize heterogeneous Ni-MoN nanosheet-assembled microspheres (Ni-MoN NAMs). The nanosheet-assembled structure and the synergistic metallic Ni-MoN heterogeneous interface endow the Ni-MoN NAMs with good OER (1.52 V@10 mA cm−2), UOR (1.28 V@10 mA cm−2), and hydrogen evolution reaction (HER, 0.16 V@10 mA cm−2) activity. The two-electrode urea electrolysis cell with Ni-MoN NAMs as both the cathode and anode requires an extremely low cell voltage of 1.41 V to afford 20 mA cm-2, which is 0.3 V lower than that of the water electrolyzer, paving the way for energy-saving H 2 production. [ABSTRACT FROM AUTHOR]

Water splitting is an environmentally friendly method of hydrogen generation. However, it is severely limited by the slow anodic oxygen evolution reaction (OER). Iron-nickel layered double hydroxides (FeNi LDH) are promising electrocatalysts for OER, but their intrinsically low electrical conductivity and activity limit the practical applications. Herein, chromium-doped FeNi LDH nanoarrays in situ vertically grown on the surface of the Ti 3 C 2 T x MXene (Cr-FeNi LDH/MXene) are successfully synthesized. Remarkably, the robust interaction and electrical coupling between Cr–FeNi LDH and MXene, as well as conspicuous charge transfer and the oxygen vacancies optimizing the adsorption free energy of intermediates, equip the nanocomposites with brilliant catalytic activity and stability toward OER. Thus, the optimized Cr–FeNi LDH/MXene shows a considerable boost in the OER, which affords low overpotential (232 mV at 10 mA cm−2) and excellent durability. This work offers a new path to designing highly efficient and earth-abundant catalysts for water splitting and beyond. [Display omitted] • In situ grown Cr doping FeNi LDH on MXene was facilely synthesized. • There are abundant oxygen vacancies in Cr–FeNi LDH/MXene. • Synergy between oxygen vacancies and MXene improves the OER kinetics and activities. • Cr-FeNi LDH/MXene exhibited excellent OER activity and stability. [ABSTRACT FROM AUTHOR]

Efficient and cost-effective catalysts are essential to drive the oxygen evolution reaction (OER) in sustainable hydrogen production through water splitting. In this study, we introduce an innovative strategy aimed at constructing heterogeneous structure that form abundant Fe 2 O 3 /NiSe 2 interfaces on the FeOOH surface. The resulting catalyst exhibits extraordinary performance with an excellently low overpotential of 169 mV at 10 mA cm−2. Notably, this catalyst also demonstrates impressive long-term stability in alkaline seawater. Compared to traditional heterogeneous catalysts with core-shell structures, Fe 2 O 3 /NiSe 2 possesses a closely heterogeneous interface, which plays a role in modulating the interface electron. Supported by complementary spectroscopy and theoretical calculations, it has been further demonstrated that the distinctive Fe-O-Ni-Se structure can effectively modulate the electronic state of Ni, thereby enhancing the adsorption of oxygen-containing intermediates and facilitating oxygen desorption. Overall, this research presents a promising avenue for enhancing the electrocatalytic performance by amplifying active sites at interfaces. [Display omitted] • Dense heterojunction with abundant interface, thereby increasing the availability of numerous active sites. • Catalysts with high-density heterogeneous interfaces showcasing an excellently low overpotential of 169 mV at 10 mA cm−2 and stable electrocatalysts for OER. • Dual-modification from both Fe and Se results in a moderate oxidation state of Ni heightens the binding energy with oxygen-containing intermediate. [ABSTRACT FROM AUTHOR]

Alkaline exchange membrane fuel cells are impeded by the lack of cost-effective, highly efficient catalysts for the sluggish hydrogen oxidation reaction (HOR). Herein, single-atomic Pd sites supported by ordered porous N,S-doped carbon are synthesized, exhibiting remarkable alkaline HOR performance. This catalyst exhibits an ultrahigh anodic current density and mass-specific kinetic current of 2.01 mA cm–2 and 27,719 A g Pd –1 (at an overpotential of 50 mV), respectively, not only outperforming the Pt/C counterpart but also making it among the best reported HOR catalysts. Furthermore, this catalyst exhibits a negligible activity decay during long-term electrolysis and a good CO tolerance capability. Experiments and theoretical calculations indicate that the synergistic effect from single Pd sites and heteroatom doping (N and S) weakens the binding energy of H ad intermediates, thereby accounting for its superior HOR activity. This study provides a guideline for developing single-atomic site catalysts for highly efficient, stable alkaline HOR. [Display omitted] Single-atomic Pd sites supported on ordered porous N,S-doped carbon are reported as a high-efficiency and stable catalyst for alkaline hydrogen oxidation with an anodic current density and mass-specific kinetic current of 2.01 mA cm–2 and 27,719 A g Pd –1, respectively, both of them are record-level among previously reported Pd-based HOR catalysts. Experiments combined with theoretical calculations reveal that synergistic effects from atomically dispersed Pd–N 4 sites and the N/S dopants can optimize the adsorptive behavior of H ad species. • Single Pd atoms supported on ordered porous N,S-doped carbon have been synthesizedfor alkaline hydrogen oxidation. • The catalyst achieves the ultrahigh mass-specific kinetic current of 27,719 A g Pd –1 at ɳ = 50 mV. • Synergistic effects from single Pd–N 4 sites and N/S dopants can optimize the adsorptive/desorptive behavior of H ad species. [ABSTRACT FROM AUTHOR]