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As a carbon‐free energy carrier, hydrogen has become the pivot for future clean energy, while efficient hydrogen production and combustion still require precious metal‐based catalysts. Single‐atom catalysts (SACs) with high atomic utilization open up a desirable perspective for the scale applications of precious metals, but the general and facile preparation of various precious metal‐based SACs remains challenging. Herein, a general movable printing method has been developed to synthesize various precious metal‐based SACs, such as Pd, Pt, Rh, Ir, and Ru, and the features of highly dispersed single atoms with nitrogen coordination have been identified by comprehensive characterizations. More importantly, the synthesized Pt‐ and Ru‐based SACs exhibit much higher activities than their corresponding nanoparticle counterparts for hydrogen oxidation reaction and hydrogen evolution reaction (HER). In addition, the Pd‐based SAC delivers an excellent activity for photocatalytic hydrogen evolution. Especially for the superior mass activity of Ru‐based SACs toward HER, density functional theory calculations confirmed that the adsorption of the hydrogen atom has a significant effect on the spin state and electronic structure of the catalysts. A facile movable type printing method is developed to synthesize precious metal‐based single‐atom catalysts, where C3N4 and nitrogen‐doping carbon derived from melamine and polydopamine are used as template and support, respectively. These precious metal‐based single‐atom catalysts can present efficient catalytic activity toward various hydrogen reactions containing hydrogen evolution reaction, photocatalytic hydrogen evolution, and hydrogen oxidation reaction.

Developing low‐cost and efficient catalysts for sustainable hydrogen (H2) production to the reliance on precious metal is an important trend in the future development of catalysts. Herein, a simple in‐situ one‐step hydrothermal strategy is employed to modify the outer layer of Ni3S2 crystals with amorphous MoS2 to construct core‐shell heterostructures and heterogeneous interfaces, which promotes the chemisorption of intermediates, including hydrogen and oxygen, and realizes the coupling enhancement of hydrogen‐evolution reaction (HER) and oxygen‐evolution reaction (OER) in alkaline water electrolysis process. In 1.0 M KOH electrolyte, the overpotentials of the electrodes are 78 mV (HER) and 245 mV (OER) at a current density of 10 mA cm−2, respectively. At the same time, the electrode has excellent stability for more than 100 h at a current density of 100 mA cm−2, due to the amorphous structure. In addition, when used as an anode and cathode to form an electrolyzer, a cell voltage of only 1.5 V is required to produce a current density of 10 mA cm−2. This study demonstrates that the constructed amorphous heterostructured interface synergistically promotes the dissociation of water and the adsorption of intermediates, providing a deep insight on how to accelerate the development of efficient catalysts. Developing low‐cost and efficient catalysts for sustainable hydrogen (H2) production is an important trend. Herein, a simple in situ one‐step hydrothermal strategy is employed to modify the outer layer of Ni3S2 crystals with amorphous MoS2 to construct core‐shell heterostructures and heterogeneous interfaces, which promotes the chemisorption of intermediates, including hydrogen and oxygen, and realizes the coupling enhancement of HER and OER in the alkaline water electrolysis process.

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.

HighlightsThe crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy.The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER.The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Hydrogen production from water splitting using renewable electric energy is an interesting topic towards the carbon neutral future. Single atom catalysts (SACs) have emerged as a new frontier in the field of catalysis such as hydrogen evolution reaction (HER), owing to their intriguing properties like high activity and excellent chemical selectivity. The catalytic active moiety is often comprised of a single metal atom and its neighboring environment from the supports. Recent published reviews about electricdriven HER tend to classify these SACs by the species of active center atom, nevertheless the influence of their neighboring coordinated atoms from the supports is somehow neglected. Thus we classify the SACs for HER through the type of supports, highlighting the electronic metal-support interaction and their coordination environment from support. Then, we put forward some structural designing strategies including regulating of the central atoms, coordination environments, and metal-support interactions. Finally, the current challenges and future research perspectives of SACs for HER are briefly proposed.

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.

HighlightsSynthesis protocols, design engineering, theoretical calculations, and energy applications for metal–organic frameworks (MOFs)-derived electrocatalysts are systematically analyzed.Synthesizing methods of MOF-derived catalysts and their oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction electrocatalysis are discussed.The current status, ongoing challenges, and potential future outlooks of MOFs-derived electrocatalysts are highlighted.The core reactions for fuel cells, rechargeable metal–air batteries, and hydrogen fuel production are the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), which are heavily dependent on the efficiency of electrocatalysts. Enormous attempts have previously been devoted in non-noble electrocatalysts born out of metal–organic frameworks (MOFs) for ORR, OER, and HER applications, due to the following advantageous reasons: (i) The significant porosity eases the electrolyte diffusion; (ii) the supreme catalyst–electrolyte contact area enhances the diffusion efficiency; and (iii) the electronic conductivity can be extensively increased owing to the unique construction block subunits for MOFs-derived electrocatalysis. Herein, the recent progress of MOFs-derived electrocatalysts including synthesis protocols, design engineering, DFT calculations roles, and energy applications is discussed and reviewed. It can be concluded that the elevated ORR, OER, and HER performances are attributed to an advantageously well-designed high-porosity structure, significant surface area, and plentiful active centers. Furthermore, the perspectives of MOF-derived electrocatalysts for the ORR, OER, and HER are presented.

Dedicating to the exploration of efficient electromagnetic (EM) absorption and electromagnetic interference (EMI) shielding materials is the main strategy to solve the EM radiation issues. The development of multifunction EM attenuation materials that are compatible together EM absorption and EMI shielding properties is deserved our exploration and study. Here, the graphene-wrapped multiloculated NiFe 2 O 4 composites are reported as multifunction EM absorbing and EMI shielding materials. The conductive networks configurated by the overlapping flexible graphene promote the riched polarization genes, as well as electron transmission paths, and thus optimize the dielectric constant of the composites. Meanwhile, the introduction of magnetic NiFe 2 O 4 further establishes the magnetic-dielectric synergy effect. The abundant non-homogeneous interfaces not only generate effective interfacial polarization, also the deliberate multiloculated structure of NiFe 2 O 4 strengthens multi-scattering and multi-reflection sites to expand the transmission path of EM waves. As it turns out, the best impedance matching is matched at a lower filled concentration to achieve the strongest reflection loss value of −48.1 dB. Simultaneously, green EMI shielding based on a predominantly EM absorption and dissipation is achieved by an enlargement of the filled concentration, which is helpful to reduce the secondary EM wave reflection pollution to the environment. In addition, the electrocatalytic properties are further examined. The graphene-wrapped multiloculated NiFe 2 O 4 shows the well electrocatalytic activity as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which is mainly attributed to the interconnected structures formed by graphene and NiFe 2 O 4 connection. The structural advantages of multiloculated NiFe 2 O 4 expose more active sites, which plays an important role in optimizing catalytic reactions. This work provides an excellent jumping-off point for the development of multifunction EM absorbing materials, eco-friendliness EMI shielding materials and electrocatalysts.

HighlightsThe process of machine learning is introduced in detail.Recent developments in machine learning for low-dimensional electrocatalysts are briefly reviewed.Future directions and perspectives for machine learning in hydrogen evolution reaction are critically discussed.Efficient electrocatalysts are crucial for hydrogen generation from electrolyzing water. Nevertheless, the conventional \"trial and error\" method for producing advanced electrocatalysts is not only cost-ineffective but also time-consuming and labor-intensive. Fortunately, the advancement of machine learning brings new opportunities for electrocatalysts discovery and design. By analyzing experimental and theoretical data, machine learning can effectively predict their hydrogen evolution reaction (HER) performance. This review summarizes recent developments in machine learning for low-dimensional electrocatalysts, including zero-dimension nanoparticles and nanoclusters, one-dimensional nanotubes and nanowires, two-dimensional nanosheets, as well as other electrocatalysts. In particular, the effects of descriptors and algorithms on screening low-dimensional electrocatalysts and investigating their HER performance are highlighted. Finally, the future directions and perspectives for machine learning in electrocatalysis are discussed, emphasizing the potential for machine learning to accelerate electrocatalyst discovery, optimize their performance, and provide new insights into electrocatalytic mechanisms. Overall, this work offers an in-depth understanding of the current state of machine learning in electrocatalysis and its potential for future research.

The key challenge for scalable production of hydrogen from water lies in the rational design and preparation of high-performance and earth-abundant electrocatalysts to replace precious metal Pt and IrO 2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Although atomic M-N-C materials have been extensively studied in heterogeneous catalysis field, the insufficient antioxidant capacity of carbonous substrates hinders their practical application in OER. Developing highly active and stable OER electrocatalysts is the key for electrochemical water splitting. This review presents feasible design strategies for fabricating carbon-free single-site catalysts and their applications in HER/OER and overall water splitting. The constitutive relationships between structure, composition, and catalytic performance for HER and OER are detailly discussed, providing ponderable insights into rationally constructing high-performance HER and OER electrocatalysts. The perspectives on the challenges and future research orientations in single-site catalysts for electrochemical water splitting are suggested.