Molybdenum‐tungsten Oxide Nanowires Rich in Oxygen Vacancies as An Advanced Electrocatalyst for Hydrogen Evolution

Electrolysis of water is a promising way to produce hydrogen fuel in large scale. The commercialization of this technology requires highly efficient non‐noble metal electrocatalysts to decease the energy input for the hydrogen evolution reaction (HER). In this work, a novel nanowire structured molybdenum‐tungsten bimetallic oxide (CTAB‐D‐W₄MoO₃) is synthesized by a simple hydrothermal method followed with post annealing treatment. The obtained metal oxides feature with enhanced conductivity, rich oxygen vacancies and customized electronic structure. As such, the composite electrocatalyst exhibits excellent electrocatalytic performance for HER in an acidic environment, achieving a large current density of 100 mA cm^?2 at overpotential of only 286 mV and a small Tafel slope of 71.2 mV dec^?1. The excellent electrocatalytic HER performance of CTAB‐D‐W₄MoO₃ is attributed to the unique nanowire structure, rich catalytic active sites and promoted electron transfer rate.     

Bifunctional Electrocatalyst with 0D/2D Heterostructure for Highly Efficient Hydrogen and Oxygen Generation

A novel MoS₂ quantum dots/CoSe₂ nanosheet (MoS₂ QDs/CoSe₂) hybrid with 0D/2D heterostructure has been developed. The CoSe₂ nanosheets (NSs) enable an excellent oxygen evolution reaction (OER) activity with increasing vacancy configuration on one hand, while the MoS₂ QDs serve as an eminent hydrogen evolution reaction (HER) catalyst on the other. By integrating MoS₂ QDs and CoSe₂ NSs, the hybrid exhibits excellent electrocatalytic performances in HER and OER. The unique 0D/2D hetero‐interface increases the exposed active sites and facilitates electron transfer, thereby boosting the electrocatalytic activity. Relatively low overpotentials of 82 mV and 280 mV are required to drive the current density of 10 mA/cm² for HER and OER, with corresponding Tafel slopes of 69 and 75 mV/dec, respectively. As such, this work provides an efficient yet simple approach to construct bifunctional electrocatalysts with enhanced activity and stability.     

Preferential Cation Vacancies in Perovskite Hydroxide for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is an ideal model to study the relationship between the activity and the surface properties of catalysts. Defect engineering has been extensively developed to tune the electrocatalytic activity for OER. Compared to the anion vacancies in metal oxides, cation vacancies are more challenging to selectively generate, and the insight into the structure and activity of cation vacancies‐rich catalysts are lacked. Herein, using SnCoFe perovskite hydroxide as a precursor, abundant Sn vacancies on the surface were preferentially produced by Ar plasma. Sn vacancies could be preferentially produced as confirmed by the X‐ray absorption spectra, probably owing to the lower lattice energy and weaker chemical bonds of Sn(OH)₄. The Sn vacancies promoted the exposure of active CoFe sites, resulting in an amorphous surface layer, modulated the conductivity, and thus enhanced the OER performance.     

酸性和碱性溶液中金属氮碳材料氧还原和氢析出反应的理论研究

单原子催化剂(SAC)由于其低成本和在各种电催化反应中潜在的高催化活性而被认为是铂族金属的有前景的替代材料,但仍然缺乏对不同金属氮碳材料催化剂之间活性差异的原子机理的理解.在此,通过实验和理论研究相结合,研究了非贵金属氮碳材料(Me-N-C,Me = Fe和Co)作为模型催化剂,以探索在普遍的pH值下氧还原反应(ORR...     

A Bifunctional Perovskite Catalyst for Oxygen Reduction and Evolution

La_0.3(Ba_0.5Sr_0.5)_0.7Co_0.8Fe_0.2O_3?δ is a promising bifunctional perovskite catalyst for the oxygen reduction reaction and the oxygen evolution reaction. This catalyst has circa 10 nm‐scale rhombohedral LaCoO₃ cobaltite particles distributed on the surface. The dynamic microstructure phenomena are attributed to the charge imbalance from the replacement of A‐site cations with La³⁺ and local stress on Co‐site sub‐lattice with the cubic perovskite structure.     

Dual MOF-Derived MoS2/CdS Photocatalysts with Rich Sulfur Vacancies for Efficient Hydrogen Evolution Reaction

Cocatalyst plays an important role in efficient charge transfer and separation for photocatalysis. Herein, a MoS₂/CdS photocatalyst with MoS₂ as cocatalyst was designed by using Mo-MOF and Cd-MOF as precursors. Due to the existence of rich sulfur vacancies and 1T phase, MoS₂ shows strong charge capture and transport ability. The photo-generated electrons on conduction band (CB) can be bound by the sulfur vacancy of CdS and effectively transported to MoS₂ through the compact interface between the CdS nanoparticles and 2D large-scale MoS₂. The optimal photocatalyst 1 %MoS₂/CdS exhibited dramatically improved photocatalytic hydrogen production activity, which is 28 times that of pristine CdS and even about 2 times that of 1 %Pt/CdS with same loading amount of noble metal Pt. This work highlights the role of Mo-MOF derived MoS₂ with 1T-2H phases as a sustainable and prospective candidate of cocatalyst for improving charge separation and photocatalytic stability of MoS₂/CdS composites.     

Efficient Hydrogen and Oxygen Evolution Catalysis Using 3D-Structured Nickel Phosphosulfide Nanosheets in Alkaline Media

Water electrolysis offers a zero-carbon route to generate renewable energy conversion systems. Herein, a self-supported nickel phosphosulfide nanosheet (NS) electrocatalyst was fabricated at a low temperature on carbon cloth, which was then subjected to Ar etching to enhance its catalytic activity. Etching resulted in better hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance than other samples, with overpotentials of 103.1 mV (at 10 mA cm⁻²) and 278.9 mV (at 50 mA cm⁻²), respectively. The characterization results confirmed that Ar etching created a thin amorphous layer around the NiPS₃ NSs, which increased the number of active sites and modulated their electronic structures. These 3D-structured NiPS₃ NSs and their subsequent Ar etching process show promise for applications in overall water splitting in alkaline media.     

Earth-Abundant Transition-Metal-Based Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Media

The depletion of fossil fuels has accelerated the search for clean, sustainable, scalable, and environmentally friendly alternative energy sources. Hydrogen is a potential energy carrier because of its advantageous properties, and the electrolysis of water is considered as an efficient method for its industrial production. However, the high-energy conversion efficiency of electrochemical water splitting requires cost-effective and highly active electrocatalysts. Therefore, researchers have aimed to develop high-performance electrode materials based on non-precious and abundant transition metals for conversion devices. Moreover, to further reduce the cost and complexity in real-world applications, bifunctional catalysts that can be simultaneously active on both the anodic (i.e., oxygen evolution reaction, OER) and cathodic (i.e., hydrogen evolution reaction, HER) sides are economically and technically desirable. This Minireview focuses on the recent progress in transition-metal-based materials as bifunctional electrocatalysts, including several promising strategies to promote electrocatalytic activities for overall water splitting in alkaline media, such as chemical doping, defect (vacancy) engineering, phase engineering, facet engineering, and structure engineering. Finally, the potential for further developments in rational electrode materials design is also discussed.     

NiCo2O4/MXene Hybrid as an Efficient Bifunctional Electrocatalyst for Oxygen Evolution and Reduction Reaction

For the advancement of electrochemical energy conversion and storage technologies, bifunctional electrocatalysts are crucial for efficiently driving both the oxygen evolution (OER) and reduction reactions (ORR). Cobalt-based spinel oxides are a class of promising bifunctional electrocatalysts. However their low electrical conductivity and stability may hinder further improvement. A novel composite material composed of NiCo₂O₄ nanoparticles integrated with emerging two dimensional MXene nanosheets (NiCo₂O₄/MXene) was developed. The successful integration of NiCo₂O₄ with MXene brings about a number of attractive structural features. This includes synergistic effects between NiCo₂O₄ and MXene, highly accessible surface areas, complete exposure of numerous active sites, and excellent electronic conductivity, all of which collectively contribute to the desirability of composite material for OER and ORR. The synthesized NiCo₂O₄/MXene composite showed extraordinary OER electrocatalytic activity with a lower overpotential of 360 mV at a current density of 10 mA/cm², and a small Tafel slope of 64 mV/dec compared to NiCo₂O₄, MXene and NiCo₂O₄+MXene (physically mixed). Additionally, NiCo₂O₄/MXene displays an ORR limiting current density of −4 mA/cm² and exhibited highest onset potential and half wave potential of 0.92 V and 0.72 V vs. RHE, respectively, for the ORR in alkaline media compared to NiCo₂O₄, MXene and NiCo₂O₄+MXene (physically mixed).     

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^? process, while oxygen can be fully reduced to water by a 4 e^?/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^?. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.     

Carbon Nanotube Supported Molybdenum Carbide as Robust Electrocatalyst for Efficient Hydrogen Evolution Reaction

Molybdenum carbide is considered to be one of the most competitive catalysts for hydrogen evolution reaction (HER) regarding its high catalytic activity and superior corrosion resistance. But the low electrical conductivity and poor interfacial contact with the current collector greatly inhibit its practical application capability. Herein, carbon nanotube (CNT) supported molybdenum carbide was assembled via electrostatic adsorption combined with complex bonding. The N-doped molybdenum carbide nanocrystals were uniformly anchored on the surfaces of amino CNTs, which depressed the agglomeration of nanoparticles while strengthening the migration of electrons. The optimized catalyst (250-800-2h) showed exceptional electrocatalytic performance towards HER under both acidic and alkaline conditions. Especially in 0.5 M H₂SO₄ solution, the 250-800-2h catalyst exhibited a low overpotential of 136 mV at a current density of 10 mA/cm² (η₁₀) with the Tafel slope of 49.9 mV dec⁻¹, and the overpotential only increased 8 mV after 20,000 cycles of stability test. The active corrosive experiment revealed that more exposure to high-activity γ-Mo₂N promoted the specific mass activity of Mo, thus, maintaining the catalytic durability of the catalyst.     

Electrospun Cu-Deposited Flexible Fibers as an Efficient Oxygen Evolution Reaction Electrocatalyst

Developing oxygen evolution reaction (OER) catalysts with high activity, long-term durability, and at low cost remains a great challenge. Herein, we report the high activity of fibrous Cu-based catalysts. The synthesis process is simple and scalable. Electrospinning method was selected to synthesize fibrous polymer substrates (Poly(vinylidene fluoride-co-hexafluoropropylene, PVdF-HFP), which are then covered by Cu via electroless deposition. Cu-deposited PVdF-HFP with different microstructures having smooth and roughened surfaces were also synthesized by drop-casting and impregnation method, respectively, to emphasize the importance of the microstructures on OER activity. The OER activity and durability were studied by linear sweep voltammetry, chronoamperometry, and Tafel slope analysis. The Cu/PVdF-HFP fibrous catalysts exhibit significantly improved OER activity and durability compared with Cu plate as well as Cu-deposited PVdF-HFP with different microstructures. The unique fibrous structure provides better mass transport, diffusion, and large active surface area. In addition to the advantages of the fibrous structure, attenuated total reflection infrared (ATR-IR) and ex situ X-ray photoelectron spectroscopy revealed that the improved specific activity for Cu/PVdF-HFP fiber can be attributed to the synergistic effect between Cu and Cu/PVdF-HFP (electron transfer from Cu to PVdF-HFP) at the Cu|PVdF-HFP interface, which results in optimized reaction energetics for the OER.     

Anodic Hydrazine Oxidation Assists Energy‐Efficient Hydrogen Evolution over a Bifunctional Cobalt Perselenide Nanosheet Electrode

Water electrolysis is a promising source of hydrogen; however, technological challenges remain. Intensive efforts have focused on developing highly efficient and earth‐abundant electrocatalysts for water splitting. An effective strategy is proposed, using a bifunctional tubular cobalt perselenide nanosheet electrode, in which the sluggish oxygen evolution reaction is substituted with anodic hydrazine oxidation so as to assist energy‐efficient hydrogen production. Specifically, this electrode produces a current density of 10 mA cm^?2 at ?84 mV for hydrogen evolution and ?17 mV for hydrazine oxidation in 1.0 m KOH and 0.5 m hydrazine electrolyte. An ultralow cell voltage of only 164 mV is required to generate a current density of 10 mA cm^?2 for 14 hours of stable water electrolysis.     

Temperature Ramp Strategy for Regulating Oxygen Vacancies in NiCo2O4 Nanoneedles Towards Enhanced Electrocatalytic Water Splitting

Oxide-based systems often suffer from higher overpotentials compared to transition metal sulfides and phosphides for the electrochemical hydrogen evolution reaction (HER). Interestingly, the generation of oxygen vacancy/defect has been seen as the strategy for further activating transition metal oxides (NiCo₂O₄ as a model system) for an electrochemical water-splitting process. Herein, we employ the temperature ramp strategy (ambient air calcination) for the generation of oxygen vacancies in NiCo₂O₄ (NCO) towards the tuning of electrocatalytic enhancements. The NiCo₂O₄ synthesized at temperature ramp rates of 2 °C/min (NCO-2), 5 °C/min (NCO-5), and 10 °C/ min (NCO-10) depicts contrasting structural features and varying Ni : Co : O surface composition. The decrease in the crystallite size and converse trend in the particle strain were observed from NCO-2 to NCO-10. Interestingly, the surface Ni : Co : O ratios of 1 : 0.78 : 3.6, 1 : 0.81 : 3.3, and 1 : 0.69 : 2.8 for NCO-2, NCO-5, and NCO-10, respectively, were observed. The reduced relative oxygen ratio in the latter implies the generation of an ample amount of oxygen vacancy defects. HER performance depicts a consistent trend with enhanced oxygen defect concentration with the overpotential requirement of 700, 647, and 597 mV for NCO-2, NCO-5, and NCO-10, respectively, for the generation of a cathodic current of 25 mA cm⁻². The same trend in an electrocatalytic enhancement is observed for other cathodic currents.     

PtM/MxBy (M=Ni,Co, Fe) Heterostructured Nanobundles as Advanced Electrocatalyst for Hydrogen Evolution Reaction

Optimizing the electronic and synergistic effect of hybrid electrocatalysts based on Pt and Pt-based nanocatalysts is of tremendous importance towards a superior hydrogen evolution performance under both acidic and alkaline conditions. However, developing an ideal Pt-based hydrogen evolution reaction (HER) electrocatalyst with moderated electronic structure as well as strong synergistic effect is still a challenge. Herein, we fabricated boron (B)-doped PtNi nanobundles by a two-step method using NaBH₄ as the boron source to obtain PtNi/Ni₄B₃ heterostructures with well-defined nanointerfaces between PtNi and Ni₄B₃, achieving an enhanced catalytic HER performance. Especially, the PtNi/Ni₄B₃ nanobundles (PtNi/Ni₄B₃ NBs) can deliver a current density of 10 mA cm⁻² at the overpotential of 14.6 and 26.5 mV under alkaline and acidic media, respectively, as well as outstanding electrochemical stability over 40 h at the current density of 10 mA cm⁻². Remarkably, this approach is also universal for the syntheses of PtCo/Co₃B and PtFe/Fe₄₉B with outstanding electrocatalytic HER performance.     

Synthesis of 3D IrRuMn Sphere as a Superior Oxygen Evolution Electrocatalyst in Acidic Environment

The design of a three-dimensional structure for an Ir-based catalyst offers a great opportunity to improve the electrocatalytic performance and maximize the use of the precious metal. Herein, a novel wet chemical strategy is reported for the synthesis of an IrRuMn catalyst with a sphere structure and porous features. In the synthetic process, the combined use of citric acid and formamide is requisite for the formation of the sphere structure. This method leads to a favorable 3D IrRuMn sphere structure with many fully exposed active sites. Furthermore, an alloying noble metal, such as Ir or Ru, with the transition metal leads to enhanced oxygen evolution reaction (OER) activity. The doping of a transition metal, such as Mn, is an interesting example, because it exhibits stability and activity in both acidic and alkaline media. For the OER, the IrRuMn sphere catalyst exhibits an overpotential of 260 mV at a current density of 10 mA cm⁻² in strongly acidic 0.1 m HClO₄, which is superior to that of a commercial IrO₂/C catalyst. This approach provides a novel way to synthesize an Ir-based multimetallic spherical electrocatalyst, which exhibits exceptional efficiency for the acidic OER. It will pave the way for new approaches to the practical utilization of PEM electrolyzers.     

二硫化钼析氢电催化剂

电解水与一次可再生能源耦合,可同时提供洁净制氢方式与先进的能源转化技术,有望在未来清洁能源经济中扮演重要角色,而实现这一美好愿景的关键在于研发高活性、低成本的析氢/析氧电催化材料。二硫化钼(MoS₂)是颇具代表性的非贵金属析氢电催化材料,纵观其研究历程,先导性理论预测与材料设计、先进制备与表征技术的应用均在改性研究中发挥了至关重要的作用,这也从一个侧面折射出当代电催化剂的研究模式与发展趋势。本文按照重要发现与进展的时间顺序,梳理了MoS₂析氢电催化剂的发展历程,重点论述了增多边缘活性位、提高导电性、构筑基面活性位等改性策略的实施方法、效果与机理,最后从全领域总结了MoS₂析氢电催化剂的研究启示并展望其未来发展趋势。     

Electrocatalysts Based on Transition Metal Borides and Borates for the Oxygen Evolution Reaction

Electrochemical water splitting is a clean and sustainable process for hydrogen production on a large scale as the electrical power required can be obtained from various renewable energy resources. The key challenge in electrochemical water splitting process is to develop low-cost electrocatalysts with high catalytic activity for the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. OER is the most important half-reaction involved in water splitting, which has been extensively studied since the last century and a large amount of electrocatalysts including noble and non-noble metal-based materials have been developed. Among them, transition metal borides and borates (TMBs)-based compounds with various structures have attracted increasing attention owing to their excellent OER performance. In recent years, many efforts have been devoted to exploring the OER mechanism of TMBs and to improving the OER activity and stability of TMBs. In this review, recent research progress made in TMBs as efficient electrocatalysts for OER is summarized. The chemical properties, synthetic methodologies, catalytic performance evaluation, and improvement strategy of TMBs as OER electrocatalysts are discussed. The electrochemistry fundamentals of OER are first introduced in brief, followed by a summary of the preparation and performance of TMBs-based OER electrocatalysts. Finally, current challenges and future directions for TMBs-based OER electrocatalysts are discussed.     

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.     

N,P‐Codoped Carbon Networks as Efficient Metal‐free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions

The high cost and scarcity of noble metal catalysts, such as Pt, have hindered the hydrogen production from electrochemical water splitting, the oxygen reduction in fuel cells and batteries. Herein, we developed a simple template‐free approach to three‐dimensional porous carbon networks codoped with nitrogen and phosphorus by pyrolysis of a supermolecular aggregate of self‐assembled melamine, phytic acid, and graphene oxide (MPSA/GO). The pyrolyzed MPSA/GO acted as the first metal‐free bifunctional catalyst with high activities for both oxygen reduction and hydrogen evolution. Zn–air batteries with the pyrolyzed MPSA/GO air electrode showed a high peak power density (310 W g^?1) and an excellent durability. Thus, the pyrolyzed MPSA/GO is a promising bifunctional catalyst for renewable energy technologies, particularly regenerative fuel cells.