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Electrocatalytic water splitting is limited by kinetics-sluggish oxygen evolution, in which the activity of catalysts depends on their electronic structure. However, the influence of electron spin polarization on catalytic activity is ambiguous. Herein, we successfully regulate the spin polarization of Co3O4 catalysts by tuning the concentration of cobalt defects from 0.8 to 14.5%. X-ray absorption spectroscopy spectra and density functional theory calculations confirm that the spin polarization of Co3O4 is positively correlated with the concentration of cobalt defects. Importantly, the enhanced spin polarization can increase hydroxyl group absorption to significantly decrease the Gibbs free energy change value of the OER rate-determining step and regulate the spin polarization of oxygen species through a spin electron-exchange process to easily produce triplet-state O2, which can obviously increase electrocatalytic OER activity. In specific, Co3O4-50 with 14.5% cobalt defects exhibits the highest spin polarization and shows the best normalized OER activity. This work provides an important strategy to increase the water splitting activity of electrocatalysts via the rational regulation of electron spin polarization. [ABSTRACT FROM AUTHOR]

The visible‐light responsive semiconductor carbon nitride (C3N4) exhibits suitable electronic band structure, rich sources of precursors, and high stability, which is exploited to satisfy energy demands. However, the photocatalytic efficiency of C3N4 is limited because of its low surface area and low separation and transfer efficiency of charge carriers. As Z‐scheme strategy can effectively help C3N4 overcome its instinct shortage, this review will outline the recent significant progress on the development of C3N4‐based all‐solid Z‐scheme photocatalysts for energy‐conversion applications including water splitting and CO2 reduction. In the beginning, the fundamental principles of Z‐scheme will be introduced, especially for all‐solid‐state system. Then, the comprehensive account of the development of C3N4‐based Z‐scheme photocatalysts are discussed. Finally, perspectives on the challenges and future directions at the forefront of Z‐scheme photocatalysts are provided. [ABSTRACT FROM AUTHOR]

Water oxidation or oxygen evolution reaction (OER) electrocatalysis got much attention in the last few years because of its prime role in water splitting, rechargeable metal-air batteries and fuel cells. Therefore, the development of efficient, abundant and economical catalysts for water oxidation reaction is one of the main subjects of present study in renewable energies. This review article summarizes the very recent efforts in the field of OER electrocatalysis along with the faced challenges. The solutions to these challenges also outline with appropriate examples of scientific literatures. Significantly, the present review will provide the standards to evaluate the activity and stability for heterogeneous OER catalysts. It will clearly summarize the future directions and applications, especially the combination of sustainable energy utilization (like triboelectric nanogenerator) with water splitting. The providing study will help to explore and develop better catalysts and units for practical applications and will offer basic understanding of OER process along with the standard parameters to evaluate the performance. [ABSTRACT FROM AUTHOR]

Nickel phosphides are competitive non-noble candidates for alkaline hydrogen evolution reaction (HER), however, their activities are insufficient due to the inert alkaline water dissociation, which hampers application in hydrogen production. Herein, we design a highly efficient electrocatalyst consisting of monocrystalline NiMoO 4 nanorods modified by broccoli-like hierarchical Ni(PO 3) 2 -Ni 2 P coupling with MoO x self-supported catalyst. As HER proceeds, catalytic performance gets better due to the dissolution of MoO x to further expose more nickel sites as Lewis base sites and in-situ formation of hydroxyl-ligands on the surface of loading nanoparticle as Lewis acid sites, which effectively integrate artificial frustrated Lewis pairs (FLPs) to respectively promote adsorption of H* and activation of H 2 O molecules. Moreover, the electron-transfer leads to the downshift of the d -band center of Ni, which helps optimizing desorption ability of H*. Therefore, such reconstructed catalyst exhibits extremely low overpotentials of 11 and 79 mV to afford 10 and 100 mA·cm−2 in 1.0 M KOH with sustainable durability for over 110 h, outperforming most of reported advanced alkaline HER catalysts. Additionally, this catalyst used as cathode for overall alkaline water splitting at 10 and 100 mA·cm−2 with only 1.48 and 1.57 V, showing its broad application prospect in practical large-scale hydrogen production. [Display omitted] • Broccoli-like hierarchical Ni 2 P-Ni(PO 3) 2 nanoparticles are synthesized on vertical monocrystalline NiMoO 4 nanorods. • Dissolution of molybdenum results in more exposed active sites to further facilitate HER under alkaline conditions. • Ni 2 P and hydroxyl-ligands serve as frustrated Lewis pairs (FLPs) to promote desorption of H* and activation of H 2 O. • Reconstructed Ni-P-O catalyst exhibits efficient HER and overall water splitting activities with excellent durability. [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]

Photocatalytic water splitting is promising for hydrogen energy production using solar energy and developing highly efficient photocatalysts is challenging. Defect engineering is proved to be a very useful strategy to promote the photocatalytic performance of metal‐based photocatalysts, however, the vital role of defects is still ambiguous. This work comprehensively reviews point defective metal‐based photocatalysts for water splitting, focusing on understanding the defects' disorder effect on optical adsorption, charge separation and migration, and surface reaction. The controllable synthesis and tuning strategies of defective structure to improve the photocatalytic performance are summarized, then the characterization techniques and density functional theory calculations are discussed to unveil the defect structure, and analyze the defects induced electronic structure change of catalysts and its ultimate effect on the photocatalytic activity at the molecular level. Finally, the challenge in developing more efficient defective metal‐based photocatalysts is outlined. This work may help further the understanding of the fundamental role of defect structure in the photocatalytic reaction process and guide the rational design and fabrication of highly efficient and low‐cost photocatalysts. [ABSTRACT FROM AUTHOR]

Photo‐electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n‐type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge‐carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included. A brief overview of water oxidation cocatalysts‐assisted photo‐electrochemical (PEC) water splitting is provided. The water oxidation half reaction is always the most challenging step in the water splitting process. With the existence of cocatalysts to improve stability, reduce recombination, and enrich reaction sites, the performance of the PEC water oxidation half reaction and even the entire water splitting process will be significantly enhanced. [ABSTRACT FROM AUTHOR]

Photo‐electrochemical (PEC) oxygen evolution is the bottleneck of PEC hydrogen production, for which a cocatalyst is critical to enhance the efficiency. In article 1801505, Lun Pan, Ji‐Jun Zou, and co‐workers review the recent advances of cocatalyst design and fabrication for PEC oxygen evolution, analyze their roles and work mechanism comprehensively, and provide guidance for exploring more efficient cocatalysts. [ABSTRACT FROM AUTHOR]

As a clean, efficient, and renewable energy source, hydrogen has always been recognized as a favourable replacement of fossil fuel. A primary challenge is an efficient generation of hydrogen to fulfil the requirements of hydrogen on a commercial scale. The electrocatalytic process of HER (hydrogen evolution reaction), as primary phase in water electrolytic process for H 2 production, has undergone comprehensive observation from recent decades. Electrolytic water splitting presents a promised route to attain efficient hydrogen generation concerning energy conversion and storage, with electrolysis or catalysis playing a pivotal role. The advancement of catalyst or electrocatalysts that are effective, enduring and economical is necessary prerequisite for realizing the intended electrolytic hydrogen generation from water splitting for applicable considerations, embodying the primary emphasis of this article. In this extensive review, we initially summarize the basics of the Hydrogen evolution reaction and examine the latest cutting-edge progress in economical and highly efficiency catalysts utilizing both non-noble and noble metals. Moreover, the recent breakthroughs over the preceding years in electrolytic HER employing more affordable and widely available nanoparticles with a specific center of attention on economical and non-platinum electrocatalysts rooted in metal free (MF) and transition metal composite catalysts are deliberated here. • A Comprehensive Review on HER. • Fundamentals of HER. • HER Mechanism. • Evaluation Methodologies of HER Electrocatalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Au@Pt decorated Zn-CdS composites are fabricated for promoted charge separation. • Photodeposition leads to formation of Au-S bond and act as charge transfer channel. • The composites show synergetic effect of SPR and charge transfer channel. • Surface Pt serves active sites for rapid surface reaction. • Au@Pt/Zn-CdS exhibits lower charge transfer resistance and higher PEC activity. Photoelectrochemical (PEC) water splitting is a promising approach to solve the current energy and environmental crises. Herein, we developed a "one stone, two birds" strategy to construct Au@Pt decorated ZnO/CdS heterojunctions (ZCAP) with Au-S bond and surface plasmon resonance (SPR) effect for promoted PEC water splitting. The optimized ZCAP structure exhibited the highest photocurrent density of 4.23 mA/cm2 at 1.23 V vs. RHE and hydrogen evolution rate of 72.3 μmol·cm−2·h−1, which is magnitude higher than pure ZnO. In-depth investigations indicate that SPR effect can afford hot electrons, whereas Au-S bonds over interface act as transmission bridge for facilitated electron transfer, the synergy of them achieves efficient charge separation and light utilization. Meanwhile, surface Pt nanoparticles serve as active sites for rapid charge transfer and promoted surface reaction. This article illustrates that the synergy of SPR and electron transfer bridge is an efficient strategy for promoted PEC performance. [ABSTRACT FROM AUTHOR]

The work function of BiVO 4 is modulated from smaller to larger than that of PCN by controlling the oxygen vacancy, which switches the direction of interfacial band bending in BiVO 4 /PCN heterojunction. Accordingly, the photoinduced charge transfer mechanism is changed from type-II to direct Z-scheme for superior solar-driven pure water splitting. [Display omitted] Nature-inspired artificial Z-scheme photocatalyst offers great promise in solar overall water splitting, but its rational design, construction and interfacial charge transfer mechanism remain ambiguous. Here, we design an approach of engineering interfacial band bending via work function regulation, which realizes directional charge transfer at interface and affords direct Z-scheme pathway. Taking BiVO 4 as prototype, its oxygen vacancy concentration is reduced by slowing down the crystallization rate, thereby changing the work function from smaller to larger than that of polymeric carbon nitride (PCN). Consequently, the photoinduced charge transfer pathway of BiVO 4 /PCN is switched from type-II to Z-scheme as evidenced by synchronous illuminated X-ray photoelectron spectroscopy (XPS) and femtosecond transient absorption spectroscopy. Specifically, the direct Z-scheme BiVO 4 /PCN shows superior photocatalytic performance in water splitting. This work provides deep insights and guidelines to constructing heterojunction photocatalysts for solar utilization. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Pt decorated Ni(OH) 2 composite is highly effective in activating and cleaving H 2 O. • The composite is in-situ grown on semiconductors via photoinduced deposition. • Pt/Ni(OH) 2 -C 3 N 4 achieves an AQE of 1.8% at 420 nm in overall water splitting. • Pt/Ni(OH) 2 enables TiO 2 to split pure water. Abstract Photocatalytic overall water splitting is one of the ultimate goals in solar conversion and the activation and cleavage of H 2 O molecule is the initial and often rate-determining step in this reaction. Through DFT calculations, we designed Pt cluster decorated Ni(OH) 2 nanoparticles (Pt/Ni(OH) 2) as robust cocatalyst capable of activating H 2 O and dissociating HO H bonds. Then the unique structure was fabricated through in-situ photo-oxidation of Ni 2 P to Ni(OH) 2 on semiconductor like C 3 N 4 and subsequent selective photo-deposition of Pt on Ni(OH) 2 surface. Pt/Ni(OH) 2 -C 3 N 4 is much more active in photocatalytic HER, OER and overall water splitting compared with Pt, Ni(OH) 2 and spatially separated co-loaded Ni(OH) 2 -Pt on C 3 N 4. And Pt/Ni(OH) 2 -C 3 N 4 achieves an AQE of 1.8% at 420 nm in overall water splitting by using only 0.3 wt% Pt, superior to most solid-state photocatalytic systems to date. Moreover, Pt/Ni(OH) 2 composite enables TiO 2 to split pure water in good stoichiometry. This work emphasizes the importance of H 2 O activation and may pave the way for enabling single semiconductors to efficiently split pure water. [ABSTRACT FROM AUTHOR]

Photocatalytic water oxidation suffers from sluggish kinetics and remains the bottleneck for water splitting. Here, using CeO 2 nanorods as model photocatalyst we studied the critical role of oxygen vacancies in photocatalytic water oxidation. First CeO 2 nanorods with similar morphology but different concentration of oxygen vacancies were fabricated by one-step hydrothermal method with in-situ reducing treatment. The optical absorption, charge transfer efficiency, and photocatalytic activity in oxygen generation were found closely dependent on the concentration of oxygen vacancies. Then density functional theory calculations were conducted to unveil the role of oxygen vacancies and understand the water oxidation mechanism. It was found the presence of oxygen vacancies narrows the bandgap and modulates the electronic structure for accelerating the charge transfer, in good agreement with the experimental observations. The overall oxygen generation pathway was screened and the oxygen vacancies were found to lower the barrier energy for the rate limiting step of O O bond formation and restrain the reverse reaction of O and H, thus the O 2 generation kinetics on oxygen-defective CeO 2 are improved significantly. This study provides in-depth understanding on the critical role of oxygen vacancies in photocatalytic water oxidation and is helpful for designing highly efficient photocatalyst to overcome the bottleneck of water splitting. [ABSTRACT FROM AUTHOR]

• Amorphous NiO x brings unique urbach tail absorption in near infrared region. • The disorder feature gives rich active site to boost rate-limiting water oxidation. • Ultrathin structure and external electron capture greatly prolong carrier lifetime. • It enlightens pure water splitting up to 680 nm with an AQE of 6.2 % at 420 nm. Harvesting near infrared light for water splitting opens up a new avenue in solar conversion but suffers from the low photon energy. Towards this, amorphous a-NiO x is reported to exhibit strong long-wavelength urbach tail absorption and provide abundant oxidation active sites due to its disordered structure. And its intrinsic two-dimensional ultrathin structure shortens the migration distance of photoinduced carriers. This enlightens water splitting under 680 nm red light when combined with electron captures like reduced graphene oxide. Furthermore, a-NiO x ||C 3 N 4 heterojunction is constructed to extend the light response from UV to near infrared region, together with a built-in-electric field to facilitate interfacial charge separation. The unique structure achieves stoichiometric water splitting under simulated sunlight with an apparent quantum efficiency of 6.2 % at 420 nm, outperforming most state-of-art photocatalysts. This work may bring new opportunities in modulating amorphous materials for artificial photocatalysis. [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]