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Hydrogen is readily obtained from renewable and non‐renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non‐renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost‐effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic‐integrated solar‐driven water electrolysis. Energy‐driven hydrogen production via water splitting with thermal, electrical, photonic and biochemical energy and their combined forms such as thermoelectrolysis, biophotolysis, and photoelectrolysis are summarized in this review. There are focuses on recent advances in water splitting with the use of renewable energy for photocatalytic and electrocatalytic hydrogen production such as photovoltaic‐integrated solar driven water electrolysis.

The Co₄O₄ cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)₂ state may be captured in a Co(III)₂(IV)₂ cubane. We demonstrate that the Co(III)₂(IV)₂ cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)₂(IV)₂ cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)₂ dimer. The exchange coupling in the cofacial Co(IV)₂ site allows for parallels to be drawn between the electronic structure of the Co₄O₄ cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)₂ center on O–O bond formation.

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. This review presents varieties of representative hydrogen evolution reaction (HER) electrocatalysts benefited from intrinsic and extrinsic design strategies and gives insight into classical/novel descriptors and reaction mechanism to provide the audience with a broad and basic understanding. Moreover, the progress on water‐splitting technology is also discussed. Some invigorating perspectives on the challenges and future directions at the HER field are provided.

HighlightsFour main strategies for improving the hydrogen evolution reaction (HER) performance of Ru-based catalysts were summarized.The source of HER activity of Ru-based catalysts is discussed in terms of catalytic mechanism.The current states, challenges and prospects were specifically provided for Ru-based catalysts.The investigation of highly effective, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is a prerequisite for the upcoming hydrogen energy society. To establish a new hydrogen energy system and gradually replace the traditional fossil-based energy, electrochemical water-splitting is considered the most promising, environmentally friendly, and efficient way to produce pure hydrogen. Compared with the commonly used platinum (Pt)-based catalysts, ruthenium (Ru) is expected to be a good alternative because of its similar hydrogen bonding energy, lower water decomposition barrier, and considerably lower price. Analyzing and revealing the HER mechanisms, as well as identifying a rational design of Ru-based HER catalysts with desirable activity and stability is indispensable. In this review, the research progress on HER electrocatalysts and the relevant describing parameters for HER performance are briefly introduced. Moreover, four major strategies to improve the performance of Ru-based electrocatalysts, including electronic effect modulation, support engineering, structure design, and maximum utilization (single atom) are discussed. Finally, the challenges, solutions and prospects are highlighted to prompt the practical applications of Ru-based electrocatalysts for HER.

As a promising substitute for fossil fuels, hydrogen has emerged as a clean and renewable energy. A key challenge is the efficient production of hydrogen to meet the commercial-scale demand of hydrogen. Water splitting electrolysis is a promising pathway to achieve the efficient hydrogen production in terms of energy conversion and storage in which catalysis or electrocatalysis plays a critical role. The development of active, stable, and low-cost catalysts or electrocatalysts is an essential prerequisite for achieving the desired electrocatalytic hydrogen production from water splitting for practical use, which constitutes the central focus of this review. It will start with an introduction of the water splitting performance evaluation of various electrocatalysts in terms of activity, stability, and efficiency. This will be followed by outlining current knowledge on the two half-cell reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), in terms of reaction mechanisms in alkaline and acidic media. Recent advances in the design and preparation of nanostructured noble-metal and non-noble metal-based electrocatalysts will be discussed. New strategies and insights in exploring the synergistic structure, morphology, composition, and active sites of the nanostructured electrocatalysts for increasing the electrocatalytic activity and stability in HER and OER will be highlighted. Finally, future challenges and perspectives in the design of active and robust electrocatalysts for HER and OER towards efficient production of hydrogen from water splitting electrolysis will also be outlined.

It is of vital importance to design efficient and low-cost bifunctional catalysts for the electrochemical water splitting under alkaline and neutral pH conditions. In this work, we report an efficient and stable NiCo 2 S 4 /N, S co-doped reduced graphene oxide (NCS/NS-rGO) electrocatalyst for water splitting, in which NCS microspheres are composed of one-dimentional (1D) nanorods grown homogeneously on the surface of NS-rGOs). The synergetic effect, abundant active sites, and hybridization of NCS/NS-rGO endow their outstanding electrocatalytic performance for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both alkaline and neutral conditions. Furthermore, NCS/NS-rGO employed as both anode and cathode in a two-electrode alkaline and neutral system electrolyzers deliver 10 mA/cm 2 with the low cell voltage of 1.58 V in alkaline and 1.91 V in neutral condition. These results illustrate the rational design of carbon-supported nickel-cobalt based bifunctional materials for practical water splitting over a wide pH range.

Electrocatalytic water splitting driven by renewable energy input to produce clean hydrogen (H 2 ) has been widely considered a prospective approach for a future hydrogen-based society. However, the development of industrial alkaline water electrolyzers is hindered due to their unfavorable thermodynamics with high overpotential for delivering the whole process, caused by sluggish kinetics involving four-electron transfer. Further exploration of water electrolysis with low energy consumption and high efficiency is urgently required to meet the ever-growing energy storage and portfolio demands. This review emphasizes the strategies proposed thus far to pursue high-efficiency water electrolysis systems, including from the aspects of electrocatalysts (from monofunctional to bifunctional), electrode engineering (from powdery to self-supported), energy sources (from nonrenewable to renewable), electrolytes (from pure to hybrid), and cell configurations (from integrated to decoupled). Critical appraisals of the pivotal electrochemistry are highlighted to address the challenges in elevating the overall efficiency of water splitting. Finally, valuable insights for the future development directions and bottlenecks of advanced, sustainable, and high-efficiency water splitting systems are outlined. Graphical abstract

Developing cost-effective, efficient and bifunctional electrocatalysts is vital for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) application. The catalytic activity of electrocatalysts could be optimized by reasonable electronic structure regulation and increasing active sites. Herein, we report the design and fabrication of Mo-doped nickel sulfide/hydroxide heterostructures (Mo-NiS/Ni(OH) 2 ) as a multisite water splitting catalyst via straightforward solvothermal and in-situ growth strategy. Based on foreign metal doping and interface interaction, the electronic conductivity of heterostructures is improved and the charge transfer kinetics across the interface is promoted, which are demonstrated by the theoretical calculations. Mo-NiS/Ni(OH) 2 electrocatalyst is endowed with high electrocatalytic performance for water splitting and remarkable durability in alkaline electrolyte. It exhibits the low overpotential of 186 and 74 mV at 10 mA·cm −2 for OER and HER, respectively. Importantly, after continuously working for 50 h, the current densities of HER and OER both show negligible degeneration. Even, the resulting Mo-NiS/Ni(OH) 2 better catalyzes water splitting, yielding a current density of 10 mA·cm − 2 at a cell voltage of 1.5 V and outperforming Pt/C-IrO 2 couple (1.53 V). This result demonstrates that transition metal doping and heterogeneous interface engineering are useful means for conventional catalyst design.

HighlightsFree-standing bimodal porous interpenetrating-phase FeNiZn and FeNi3 intermetallic heterostructure is in situ built on NiFe foam by dealloying way.FeNiZn/FeNi3@NiFe exhibits superior electrocatalytic activities toward both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with very low overpotentials and prolonged catalytic durability in alkaline medium.The strong synergy between FeNiZn alloy and FeNi3 intermetallic generates modulated electron structure and thus well optimizes the intermediates chemisorption toward OER and HER.The sluggish kinetics of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) generate the large overpotential in water electrolysis and thus high-cost hydrogen production. Here, multidimensional nanoporous interpenetrating-phase FeNiZn alloy and FeNi3 intermetallic heterostructure is in situ constructed on NiFe foam (FeNiZn/FeNi3@NiFe) by dealloying protocol. Coupling with the eminent synergism among specific constituents and the highly efficient mass transport from integrated porous backbone, FeNiZn/FeNi3@NiFe depicts exceptional bifunctional activities for water splitting with extremely low overpotentials toward OER and HER (η1000 = 367/245 mV) as well as the robust durability during the 400 h testing in alkaline solution. The as-built water electrolyzer with FeNiZn/FeNi3@NiFe as both anode and cathode exhibits record-high performances for sustainable hydrogen output in terms of much lower cell voltage of 1.759 and 1.919 V to deliver the current density of 500 and 1000 mA cm−2 as well long working lives. Density functional theory calculations disclose that the interface interaction between FeNiZn alloy and FeNi3 intermetallic generates the modulated electron structure state and optimized intermediate chemisorption, thus diminishing the energy barriers for hydrogen production in water splitting. With the merits of fine performances, scalable fabrication, and low cost, FeNiZn/FeNi3@NiFe holds prospective application potential as the bifunctional electrocatalyst for water splitting.

The development of efficient, low-cost, for water splitting, particularly those stable, non-noble-metal electrocatalysts that can catalyze both the hydrogen evolution reaction （HER） at the cathode and oxygen evolution reaction （OER） at the anode, is a challenge. We have developed a facile method for synthesizing CoSe2 nanoparticles uniformly anchored on carbon fiber paper （CoSe2/CF） via pyrolysis and selenization of in situ grown zeolitic imidazolate framework-67 （ZIF-67）. CoSe2/CF shows high and stable catalytic activity in both the HER and OER in alkaline solution. At a low cell potential, i.e., 1.63 V, a water electrolyzer equipped with two CoSe2/CF electrodes gave a water-splitting current of 10 mA.cm-2. At a current of 20 mA-cm-2, it can operate without degradation for 30 h. This study not only offers a cost-effective solution for water splitting but also provides a new strategy for developing various catalytic nanostructures by changing the metal-organic framework precursors.