S-Edge-rich MoxSy arrays vertically grown on carbon aerogels as superior bifunctional HER/OER electrocatalysts

Integrated Catalyst-Substrate Electrodes for Electrochemical Water Splitting: A Review on Dimensional Engineering Strategy

Water splitting (or, water electrolysis) is considered as a promising approach to produce green hydrogen and relieve the ever-increasing energy consumption as well as the accompanied environmental impact. Development of high-efficiency, low-cost practical water-splitting systems demands elegant design and fabrication of catalyst-loaded electrodes with both high activity and long-life time. To this end, dimensional engineering strategies, which effectively tune the microstructure and activity of electrodes as well as the electrochemical kinetics, play an important role and have been extensively reported over the past years. Here, a type of most investigated electrode configurations is reviewed, combining particulate catalysts with 3D porous substrates (aerogels, metal foams, hydrogels, etc.), which offer special advantages in the field of water splitting. It is analyzed the design principles, structural and interfacial characteristics, and performance of particle-3D substrate electrode systems including overpotential, cycle life, and the underlying mechanism toward improved catalytic properties. In particular, it is also categorized the catalysts as different dimensional particles, and show the importance of building hybrid composite electrodes by dimensional control and engineering. Finally, present challenges and possible research directions toward low-cost high-efficiency water splitting and hydrogen production is discussed.

2 A cm-2 Level Large-Scale Production of Hydrogen Enabled by Constructing Higher Capacity of Interface "Electron Pocket"

The batch production of high-purity hydrogen is a key problem that restricts the progress of fuel cells and the blueprint for achieving carbon neutrality. Transition-metal chalcogenide heterojunctions exhibit certain activity toward electrochemical overall water splitting (EOWS), but their high-current-density catalytic performances are still unsatisfactory due to the slow kinetic progression (H* or *O → *OOH). Inspired by the "electron pocket" theory, we designed a Ni-Mo bimetallic disulfide interface heterojunction electrocatalyst system (NM-IHJ-V) with high electronic storage capacity around the Fermi level (-0.5 eV, +0.5 eV) (e-DFE), which injects more power into the kinetic progression processes of intermediate species in the EOWS process. Consequently, it achieves a superhigh current density of 2 A cm-2 level for EOWS (only 1.98 V voltage is needed), which is 11.23-fold higher than that of the benchmarked Pt/C//IrO2 (178 mA cm-2@1.98 V), as well as an excellent long-term stability of 200 h. Most strikingly, NM-IHJ-V can efficiently produce hydrogen at currents up to 5 A. Our proposed strategy of constructing catalysts to produce hydrogen at superhigh current density through the electron pocket theory will supply valuable insights for the designing other catalytic systems.

A critical review of research progress for metal alloy materials in hydrogen evolution and oxygen evolution reaction

Hydrogen produced by electrolyzing water has attracted extensive attention as an effective way to generate and store new energy by using renewable energy. Electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were the core reactions in the process of hydrogen production by water electrolysis, however, due to the low efficiency of the electrolytic device caused by its slow kinetic reaction and the dependence on noble metal catalysts (platinum and iridium/ruthenium (oxide)), which limited its wide application. The preparation of high-efficiency catalysts with high catalytic activity, stability, low cost and scalability played a vital role in promoting the development of hydrogen production technology from electrolytic water and has become a current research hotspot. Metal alloy catalysts have been widely studied as high-efficiency electrocatalysts. This study introduced and analyzed the mechanism and application of metal alloy catalyst in hydrogen and oxygen evolution reaction, summarized and discussed the progress in the design, preparation and application of metal alloy electrocatalysts. Finally, the strategy and prospect of new high-efficiency electrocatalysts were proposed.

Facile fabrication of self-supporting porous CuMoO4@Co3O4 nanosheets as a bifunctional electrocatalyst for efficient overall water splitting

Research shows that redox complementarity and synergism among the ingredients of heterogeneous catalysts can enhance the performance of the catalyst. In this research, a porous CuMoO₄@Co₃O₄ nanosheet electrocatalyst is prepared, which is uniformly decorated on nickel foam (NF) by hydrothermal reactions and the impregnation method. The CuMoO₄@Co₃O₄ is an efficient bifunctional catalyst with prominent electrocatalytic activity and durability. It requires overpotentials of only 54 and 251 mV to obtain current densities of 10 and 50 mA cm^-2 for the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) in 1.0 mol L^-1 KOH, corresponding to Tafel slope values of 98.8 and 87.4 mV dec^-1, respectively. Furthermore, the CuMoO₄@Co₃O₄ shows excellent stability of 120 h chronopotentiometry at a current density of 100 mA cm^-2 for the HER/OER. Notably, an alkaline electrolyzer (with CuMoO₄@Co₃O₄ as the HER and OER electrodes) can deliver a current density of 10 mA cm^-2 at a low voltage of 1.51 V. The catalytic activity of CuMoO₄@Co₃O₄ can be attributed to the structure of the porous nanosheets and the synergistic effect between CuMoO₄ and Co₃O₄.

Carbon Aerogels as Electrocatalysts for Sustainable Energy Applications: Recent Developments and Prospects

Carbon aerogel (CA) based materials have multiple advantages, including high porosity, tunable molecular structures, and environmental compatibility. Increasing interest, which has focused on CAs as electrocatalysts for sustainable applications including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO₂ reduction reaction (CO₂RR) has recently been raised. However, a systematic review covering the most recent progress to boost CA-based electrocatalysts for ORR/OER/HER/CO₂RR is now absent. To eliminate the gap, this critical review provides a timely and comprehensive summarization of the applications, synthesis methods, and principles. Furthermore, prospects for emerging synthesis, screening, and construction methods are outlined.

Double MOF gradually activated S bond induced S defect rich MILN-based Co(z)-NiMoS for efficient electrocatalytic overall water splitting

Herein, cactus like nanorods with rich S defects and functional group MILN-based Co(z)-NiMoS are synthesized through a facile method. First, we prepared MIL-88B precursor to give a fairly dispersed frame structure, and then Coⁿ⁺ was doped into disulfides, which are rich in sulfur bonds, and the imidazole group was cleverly connected into graphitized carbon via self-etching of ZIF-67. The doping of Coⁿ⁺ and functional groups makes tiny changes in the sulfide lattice, which promotes the unsaturation degree of the S bond and activates it gradually. The prepared semi frame sulfide with a unique structure, on the one hand, ensures the hydrophilicity and multiple active specific surface area, which lays superior functions in morphology. On the other hand, coupling metals that have strong valence change ability and functional groups by active S bonds play an important role in the process of electrocatalytic reaction. Amazingly, disintegration and self-reconstruction of MILN-based Co(z)-NiMoS occurs during oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to the activation of the S bond, which provides a new perspective for the overall water splitting mechanism. The electrochemical results show that the MILN-based Co(z)-NiMoS electrocatalyst exhibits overpotentials of HER, OER, and overall water splitting (OWS) to be 169 mV, 170 mV, and 1.466 V, respectively, making it a promising electrode material for OWS applications.

Single transition metal atom embedded antimonene monolayers as efficient trifunctional electrocatalysts for the HER, OER and ORR: a density functional theory study

Highly efficient, stable and cost-effective electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) have been pursued for several decades. Herein, by employing density functional theory (DFT), a wide range of transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Ir, Pt and Au) atoms anchored on antimonene (Sb monolayer) with a single Sb vacancy as single-atom catalysts (SACs) were investigated for their HER, OER and ORR performance. The results indicate that the defective Sb monolayer can be stable. Some TM@Sb monolayers show excellent stability and good electrical conductivity, beneficial for electron transfer during electrocatalytic reactions. The Ir@ and Pt@Sb monolayers exhibit excellent HER performance, both with about -0.01 eV of ΔG_*H. The d band centre of the TM@Sb monolayer can be used to describe the binding strength between substrates and intermediates directly. The best OER electrocatalyst is the Pt@Sb monolayer, which shows an overpotential (η_OER) of 0.48 V. In contrast, the best ORR electrocatalyst is the Ag@Sb monolayer with an η_ORR of 0.50 V, followed by Pd@, Rh@, Cd@ and Pt@Sb monolayers. Compared with pristine antimonene, only the noble metal atom could improve its OER and ORR performance effectively, and the Pt@Sb monolayer can be a trifunctional electrocatalyst for the HER/OER/ORR. Therefore, our calculations highlight a new type of SAC based on antimonene, which can be useful for energy conversion and storage.

Understanding the air stability of defective MoS2and the oxidation effect on the surface HER activity

The defective single layer MoS₂(SL-MoS₂) with high defect concentrations has shown promising electrocatalytic potential, but it is also highly reactive with gas molecules. The study of electro-chemical activity on gas doped defective SL-MoS₂is of importance yet still scarcely discussed. Herein, we performed density functional theory calculations to study the adsorption and chemical activity of four major air molecules on the defective SL-MoS₂under different defect concentrations, and evaluated the influence on the hydrogen evolution reaction activity. The N₂and CO₂molecules are in physisorption states, H₂O molecule is in molecular chemisorption state, while O₂can be strongly captured and dissociated into atomic O^*, which repair the S-vacancy and form O-doped structure. Further study showed that compared to the inert S surface of pure MoS₂, the O incorporation greatly enhance the surface reactivity. Using H adsorption as the test probe, the adsorption of H becomes stronger with the increasing oxygen concentration. We further unravel the electronic origins underlying the catalytic activity. The lowest unoccupied electronic states are shown to correlate linearly with the activity, and thus can be used as an electronic descriptor to characterize the electrocatalytic activity.

First-principles study of sulfur vacancy concentration effect on the electronic structures and hydrogen evolution reaction of MoS2

Defect engineering has been widely used in experiments to modulate the electrocatalytic properties of molybdenum disulfide (MoS₂). However, the effect of vacancy concentration on the vacancy distribution, electronic properties, and hydrogen evolution reaction (HER) activity remains elusive. Herein, we perform density functional theory (DFT) studies to investigate defective MoS₂ with different numbers of sulfur vacancies. In the case of low S-vacancy concentration, the vacancies prefer to agglomerate rather than being dispersed, while at the higher-vacancy concentration, the combination of local point defect and clustered vacancy chain is preferred. The coupling between S-vacancies leads to decreased band gap and increased Mo-H adsorption strength with increasing vacancy concentration. The optimal HER activity is identified to occur below vacancy concentration of 12.50%. Our work provides an atomic-level understanding about the role of S-vacancies in the HER performance of MoS₂, and offers useful guidelines for the design of defective MoS₂ and other TMDs electrocatalysts.

Vanadium Substitution Steering Reaction Kinetics Acceleration for Ni3N Nanosheets Endows Exceptionally Energy-Saving Hydrogen Evolution Coupled with Hydrazine Oxidation

Designing highly active transition-metal-based electrocatalysts for energy-saving electrochemical hydrogen evolution coupled with hydrazine oxidation possesses more economic prospects. However, the lack of bifunctional electrocatalysts and the absence of intrinsic structure-property relationship research consisting of adsorption configurations and dehydrogenation behavior of N₂H₄ molecules still hinder the development. Now, a V-doped Ni₃N nanosheet self-supported on Ni foam (V-Ni₃N NS) is reported, which presents excellent bifunctional electrocatalytic performance toward both hydrazine oxidation reaction (HzOR) and hydrogen evolution reaction (HER). The resultant V-Ni₃N NS achieves an ultralow working potential of 2 mV and a small overpotential of 70 mV at 10 mA cm^-2 in alkaline solution for HzOR and HER, respectively. Density functional theory calculations reveal that the vanadium substitution could effectively modulate the electronic structure of Ni₃N, therefore facilitating the adsorption/desorption behavior of H* for HER, as well as boosting the dehydrogenation kinetics for HzOR.

Boosting oxygen evolution through phase and electronic modulation of highly dispersed tungsten carbide with nickel doping

Exploring efficient, stable, and earth-abundant electrocatalysts for oxygen evolution reaction (OER) is of great significance for clean and renewable energy conversion technologies. In this work, in situ uniform Ni-doped tungsten carbide (Ni/WC_X) nanoparticles (~3 nm) on carbon nanofibers (Ni/WC_X-CNFs) that were to function as efficient OER catalysts were developed. Both the composition and electronic state of tungsten carbide (WC_X: W-WC-W₂C) could be regulated through varied Ni coupling. Owing to the synergistic effect between Ni and WC_X, the reaction kinetics were facilitated, resulting in improved OER activity with low overpotentials of η₁₀ = 350 mV (modified glassy carbon electrode) and η₁₀ = 335 mV (self-supporting electrode). This work opens a facile territory for the development of cost-effective and highly promising OER electrocatalysts for use in real life applications.

Fabrication of hierarchical SrTiO3@MoS2 heterostructure nanofibers as efficient and low-cost electrocatalysts for hydrogen-evolution reactions

The construction of low-cost, high-performance electrocatalysts instead of platinum catalysts is critical to solving the energy crisis. Here, using simple electrospinning and hydrothermal methods, new MoS₂ nanosheets on SrTiO₃ nanofibers (NFs) and 2D SrTiO₃@MoS₂ heterostructure NFs are synthesized. In addition, SrTiO₃@MoS₂ heterostructure NFs are compared with bare SrTiO₃ NFs and MoS₂ nanosheets. Importantly, the prepared SrTiO₃@MoS₂ heterostructure shows better hydrogen-evolution reaction performance than other MoS₂-based electrocatalysts with an overpotential of 165 mV at 10 mA cm^-2, a Tafel slope of 81.41 mV dec^-1, and long-term electrochemical durability of 3000 cycles. Therefore, the present work strongly demonstrates the positive synergy between SrTiO₃ NFs and layered MoS₂, and also provides a strategy for preparing low-cost and high-activity water-decomposition electrocatalysts.

Support interactions dictated active edge sites over MoS2-carbon composites for hydrogen evolution

The rational design and synthesis of MoS₂-based electrocatalysts with desirable active sites for the hydrogen evolution reaction have been actively pursued. Herein, we demonstrate a microwave-assisted steam heating method for the rapid and efficient synthesis of lamellar MoS₂-based materials with favorable exposed active edge sites. Based on this new strategy, we have further separately introduced reduced graphene oxide (rGO) and carbon nanotubes (CNTs), two typical carbon allotropes widely used to boost the electrocatalytic activity of MoS₂, to comparatively assess the support interactions and their effects on the electrocatalytic activity of MoS₂. It was found that as compared to rGO, the CNTs afford favorable support interactions, which not only benefit to suppress the oriented in-plane growth of MoS₂ to maximize the exposed edge sites but also ensure the maintainence of their intrinsic activity, thereby synergistically facilitating the exertion of the potential of MoS₂ for HER. Our work conceptually highlights the importance of the support interactions for taming the active edge sites of MoS₂ and is expected to inspire the rational design of layered metal dichalcogenide-based electrocatalysts with favorable active edges for HER.