The hydrogen evolution reaction (HER) from water through electrocatalysis using highly active noble metal-free catalysts as an alternative to precious Pt-based catalysts holds great promise for clean and renewable energy systems. Here, Mo₂C nanosheets well-regulated in N-doped carbon (MCNS/NC)are facilely achieved by in situ confining carburization of a Mo-based inorganic–organic lamellar mesostructure at 900 °C, and demonstrated for the first time as an ultra-efficient and durable HER catalyst. The MCNS/NC displays a very low onset overpotential of ∼0 mV and an overpotential of 19 mV at 10 mA cm⁻² with a small Tafel slope of 28.9 mV dec⁻¹ in acid media, which is remarkably superior to most other transition metal-based catalysts and comparable to commercial Pt/C. The HER kinetics are further studied by EIS and Tafel slope analysis, suggesting the dominant Volmer–Tafel mechanism. An “outside-in” carburization mechanism for the evolution of MCNS/NC is proposed through detailed investigation of the samples annealed at different temperatures. This study highlights the importance of designing well-regulated functional nanostructures combined with a conductive carbonaceous matrix for versatile applications.

In recent years, due to the impact of global warming, environmental pollution, and the energy crisis, international attention and demand for clean energy are increasing. Hydrogen energy is recognized as one of the clean energy sources. Water is considered as the largest potential supplier of hydrogen energy. However, artificial catalytic water splitting for hydrogen and oxygen evolution has not been widely used due to its high energy consumption and high cost during catalytic cracking. Therefore, the exploitation of photocatalysts, electrocatalysts, and photo-electrocatalysts for rapid, cost effective, and reliable water splitting is essentially needed. Polyoxometalates (POMs) are regarded as the potential candidates for water splitting catalysis. In addition to their excellent catalytic properties and reversibly redox activities, POMs can also modify semiconductors to overcome their shortcomings, and improve photoelectric conversion efficiency and photocatalytic activity, which has attracted more and more attention in the field of photoelectric water splitting catalysis. In this review, we summarize the latest applications of POMs and semiconductor composites in the field of photo-electrocatalysis (PEC) for hydrogen and oxygen evolution by catalytic water splitting in recent years and take the latest applications of POMs and semiconductor composites in photocatalysis for water splitting. In the conclusion section, the challenges and strategies of photocatalytic and PEC water-splitting by POMs and semiconductor composites are discussed.

Splitting of water to produce hydrogen and oxygen is a green and effective method to produce clean energy. Finding an efficient water decomposition catalyst is the key step to realize water decomposition. In this work, by choosing from the literature, six polynuclear manganese (Mn) containing polyoxometalates (Mn-POMs) with different Mn-O clusters and oxidation states of Mn, [Mn^IIMn^IIISiW₁₀O₃₇(OH)(H₂O)]⁶⁻ (Mn₂-POM), [Mn^II₃Mn^III(H₂O)₂(PW₉O₃₄)₂]⁹⁻ (Mn₄-POM), [Mn^II₄Mn^III₂Ge₃W₂₄O₉₄(H₂O)₂]¹⁸⁻ (Mn₆-POM-1), [Mn^III₂Mn^II₄(μ₃-O)₂(H₂O)₄(B-β-SiW₈O₃₁)(B-β-SiW₉O₃₄)(γ-SiW₁₀O₃₆)]¹⁸⁻ (Mn₆-POM-4), [{Mn^III₃Mn^IV₄O₄(OH)₂(OH₂)}₂(W₆O₂₂)(H₂W₈O₃₂)₂(H₄W₁₃O₄₆)₂]²⁶⁻ (Mn₁₄-POM), [Mn^II₁₉ (OH)₁₂(SiW₁₀O₃₇)₆]³⁴⁻ (Mn₁₉-POM) were prepared. First, the catalytic performance towards the water oxidation of six Mn-POMs was investigated in solution for the first time. Second, six Mn-POMs were fabricated on the surface of ITO electrode using layer-by-layer self-assembly (LBL) to form the composite films, which were characterized by UV-vis spectroscopy and cyclic voltammetry, and then the catalytic water oxidation performance of the composite films was studied and compared with that in solution via a series of controlled experiments, the results indicate that the Mn-POMs with three-dimensional structures, which contain variable valence Mn-O cluster similar to the structure of photocatalytic active center (PSII) exhibit better catalytic performance.

With the increase in gas population, the demand for clean and renewable energy is increasing. Hydrogen energy has a high combustion conversion energy while water is its combustion product. In recent years, a way to convert water into hydrogen and oxygen has been found by human beings inspired by plant photosynthesis. However, water decomposition consumes a significant amount of energy and is expensive. People expect to obtain a water decomposition catalyst with low cost and high efficiency. This work selected a six-manganese containing polyoxometalate with a similar structure characteristic to photosynthesizing PSII to fabricate with graphite C₃N₄ nanosheets for the construction of composite film (Mn₆SiW/g-C₃N₄NSs) electrode via layer by layer self-assembly technology, which was used for the photo-electrochemical decomposition of water under visible light conditions. The binary composite film electrode displayed good catalytic efficiency. The photoelectric density of the composite electrode is 46 μA/cm² (at 1.23 V vs. Ag/AgCl) and 239 μA/cm² (at 1.5 V vs. Ag/AgCl). Compared with the g-C₃N₄NSs electrode alone, the photoelectric density of the composite electrode increased by 1 time. The reason is attributed to the fact that Mn₆SiW has a similar structure characteristic to photosynthesizing PSII and high electron transferability. The construction of the composite film containing low-cost Mn₆SiW to modify g-C₃N₄NSs can effectively improve the photocatalytic decomposition of water, thus this study provides valuable reference information for the development of low-cost and high-performance photo-electrocatalytic materials.