Self-Assembled Two-Dimensional NiO/CeO2 Heterostructure Rich in Oxygen Vacancies as Efficient Bifunctional Electrocatalyst for Alkaline Hydrogen Evolution and Oxygen Evolution

The development of high-efficiency bifunctional electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline surroundings is essential and challenging for the large-scale generation of clean hydrogen. Herein, a novel self-assembled two-dimensional (2 D) NiO/CeO₂ heterostructure (HS) consisting of NiO and CeO₂ nanocrystals is prepared through a facile two-step approach, and utilized as an enhanced bifunctional electrocatalyst for the HER and OER under alkaline conditions. It is concluded that this 2 D NiO/CeO₂ HS, rich in oxygen vacancies, demonstrates attractive electrocatalytic properties for both the HER and OER in 1 m KOH, including low onset overpotential (η₁), η₁₀ and Tafel slope, excellent durability, as well as large active surface area. Therefore, the self-assembled 2 D NiO/CeO₂ HS is believed to be an efficient bifunctional electrocatalyst toward the HER and OER.     

Phosphorus‐Modified Tungsten Nitride/Reduced Graphene Oxide as a High‐Performance,Non‐Noble‐Metal Electrocatalyst for the Hydrogen Evolution Reaction

Phosphorus‐modified tungsten nitride/reduced graphene oxide (P‐WN/rGO) is designed as a high‐efficient, low‐cost electrocatalyst for the hydrogen evolution reaction (HER). WN (ca. 3 nm in size) on rGO is first synthesized by using the H₃[PO₄(W₃O₉)₄] cluster as a W source. Followed by phosphorization, the particle size increase slightly to about 4 nm with a P content of 2.52 at %. The interaction of P with rGO and WN results in an obvious increase of work function, being close to Pt metal. The P‐WN/rGO exhibits low onset overpotential of 46 mV, Tafel slope of 54 mV dec^?1, and a large exchange current density of 0.35 mA cm^?2 in acid media. It requires overpotential of only 85 mV at current density of 10 mA cm^?2, while remaining good stability in accelerated durability testing. This work shows that the modification with a second anion is powerful way to design new catalysts for HER.     

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。     

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.     

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.     

Porous Molybdenum‐Based Hybrid Catalysts for Highly Efficient Hydrogen Evolution

We have synthesized a porous Mo‐based composite obtained from a polyoxometalate‐based metal–organic framework and graphene oxide (POMOFs/GO) using a simple one‐pot method. The MoO₂@PC‐RGO hybrid material derived from the POMOFs/GO composite is prepared at a relatively low carbonization temperature, which presents a superior activity for the hydrogen‐evolution reaction (HER) in acidic media owing to the synergistic effects among highly dispersive MoO₂ particles, phosphorus‐doped porous carbon, and RGO substrates. MoO₂@PC‐RGO exhibits a very positive onset potential close to that of 20 % Pt/C, low Tafel slope of 41 mV dec^?1, high exchange current density of 4.8×10^?4 A cm^?2, and remarkable long‐term cycle stability. It is one of the best high‐performance catalysts among the reported nonprecious metal catalysts for HER to date.     

钨掺杂的铁镍基层状氢氧化物用于电催化析氧和析氢反应

采用简便的一步水热合成法,在泡沫镍上原位生长微量W~(6+)掺入的Fe_(0.2)Ni(OH)_2双金属层状氢氧化物(LDH),以此来降低铁镍材料的过电势。通过场发射扫描电子显微镜(FESEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)和拉曼光谱(Raman)等分析方法对材料形貌、组成、结构等进行表征,发现钨掺杂使催化剂材料的晶体结构和电子结构发生变化,W_(0.03)Fe_(0.2)Ni(OH)_2LDH表现出优异的电化学析氧(OER)和析氢(HER)性能。电化学测试表明该催化剂在25 mA·cm~(-2)电流密度下OER和HER过电势分别仅有271和208 mV,塔菲尔斜率分别为61和181 mV·dec~(-1)。此外,经过长达20 h计时电位稳定性测试后,材料的催化性能未见明显下降。     

Ultrathin WS2 Nanoflakes as a High‐Performance Electrocatalyst for the Hydrogen Evolution Reaction

Much has been done to search for highly efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), which is critical to a range of electrochemical and photoelectrochemical processes. A new, high‐temperature solution‐phase method for the synthesis of ultrathin WS₂ nanoflakes is now reported. The resulting product possesses monolayer thickness with dimensions in the nanometer range and abundant edges. These favorable structural features render the WS₂ nanoflakes highly active and durable catalysts for the HER in acids. The catalyst exhibits a small HER overpotential of approximately 100 mV and a Tafel slope of 48 mV/decade. These ultrathin WS₂ nanoflakes represent an attractive alternative to the precious platinum benchmark catalyst and rival MoS₂ materials that have recently been heavily scrutinized for the electrocatalytic HER.     

Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface‐Mounted Metal–Organic Frameworks

Metal–organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a “strain modulation” approach has been applied through the use of surface‐mounted NiFe‐MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA cm^?2 at an overpotential of only ≈210 mV. It demonstrates operational long‐term stability even at a high current density of 500 mA cm^?2 and exhibits the so far narrowest “overpotential window” ΔE_ORR‐OER of 0.69 V in 0.1 m KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts.     

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.     

Molybdenum Phosphosulfide: An Active,Acid‐Stable,Earth‐Abundant Catalyst for the Hydrogen Evolution Reaction

Introducing sulfur into the surface of molybdenum phosphide (MoP) produces a molybdenum phosphosulfide (MoP|S) catalyst with superb activity and stability for the hydrogen evolution reaction (HER) in acidic environments. The MoP|S catalyst reported herein exhibits one of the highest HER activities of any non‐noble‐metal electrocatalyst investigated in strong acid, while remaining perfectly stable in accelerated durability testing. Whereas mixed‐metal alloy catalysts are well‐known, MoP|S represents a more uncommon mixed‐anion catalyst where synergistic effects between sulfur and phosphorus produce a high‐surface‐area electrode that is more active than those based on either the pure sulfide or the pure phosphide. The extraordinarily high activity and stability of this catalyst open up avenues to replace platinum in technologies relevant to renewable energies, such as proton exchange membrane (PEM) electrolyzers and solar photoelectrochemical (PEC) water‐splitting cells.     

Hierarchical β‐Mo2C Nanotubes Organized by Ultrathin Nanosheets as a Highly Efficient Electrocatalyst for Hydrogen Production

Production of hydrogen by electrochemical water splitting has been hindered by the high cost of precious metal catalysts, such as Pt, for the hydrogen evolution reaction (HER). In this work, novel hierarchical β‐Mo₂C nanotubes constructed from porous nanosheets have been fabricated and investigated as a high‐performance and low‐cost electrocatalyst for HER. An unusual template‐engaged strategy has been utilized to controllably synthesize Mo‐polydopamine nanotubes, which are further converted into hierarchical β‐Mo₂C nanotubes by direct carburization at high temperature. Benefitting from several structural advantages including ultrafine primary nanocrystallites, large exposed surface, fast charge transfer, and unique tubular structure, the as‐prepared hierarchical β‐Mo₂C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions, as well as remarkable stability.     

Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst

The replacement of platinum with non‐precious‐metal electrocatalysts with high efficiency and superior stability for the hydrogen‐evolution reaction (HER) remains a great challenge. Herein, we report the one‐step synthesis of uniform, ultrafine molybdenum carbide (Mo₂C) nanoparticles (NPs) within a carbon matrix from inexpensive starting materials (dicyanamide and ammonium molybdate). The optimized catalyst consisting of Mo₂C NPs with sizes lower than 3 nm encapsulated by ultrathin graphene shells (ca. 1–3 layers) showed superior HER activity in acidic media, with a very low onset potential of ?6 mV, a small Tafel slope of 41 mV dec^?1, and a large exchange current density of 0.179 mA cm^?2, as well as good stability during operation for 12 h. These excellent properties are similar to those of state‐of‐the‐art 20 % Pt/C and make the catalyst one of the most active acid‐stable electrocatalysts ever reported for HER.     

Molybdenum Carbide‐Oxide Heterostructures: In Situ Surface Reconfiguration toward Efficient Electrocatalytic Hydrogen Evolution

Heterostructured Mo₂C‐MoO_x on carbon cloth (Mo₂C‐MoO_x/CC), as a model of easily oxidized electrocatalysts under ambient conditions, is investigated to uncover surface reconfiguration during the hydrogen evolution reaction (HER). Raman spectroscopy combined with electrochemical tests demonstrates that the Mo^VI oxides on the surface are in situ reduced to Mo^IV, accomplishing promoted HER in acidic condition. As indicated by density functional theoretical calculations, the in situ reduced surface with terminal Mo=O moieties can effectively bring the negative ΔG_H* on bare Mo₂C close to a thermodynamic neutral value, addressing difficult H* desorption toward fast HER kinetics. The optimized Mo₂C‐MoO_x/CC only requires a low overpotential (η₁₀) of 60 mV at ?10 mA cm^?2 in 1.0 m HClO₄, outperforming Mo₂C/CC and most non‐precious electrocatalysts. In situ surface reconfiguration are shown on W₂C‐WO_x, highlighting the significance to boost various metal‐carbides and to identify active sites.     

Selenium‐Enriched Nickel Selenide Nanosheets as a Robust Electrocatalyst for Hydrogen Generation

To address the urgent need for clean and sustainable energy, the rapid development of hydrogen‐based technologies has started to revolutionize the use of earth‐abundant noble‐metal‐free catalysts for the hydrogen evolution reaction (HER). Like the active sites of hydrogenases, the cation sites of pyrite‐type transition‐metal dichalcogenides have been suggested to be active in the HER. Herein, we synthesized electrodes based on a Se‐enriched NiSe₂ nanosheet array and explored the relationship between the anion sites and the improved hydrogen evolution activity through theoretical and experimental studies. The free energy for atomic hydrogen adsorption is much lower on the Se sites (0.13 eV) than on the Ni sites (0.87 eV). Notably, this electrode benefits from remarkable kinetic properties, with a small overpotential of 117 mV at 10 mA cm^?2, a low Tafel slope of 32 mV per decade, and excellent stability. Control experiments showed that the efficient conversion of H⁺ into H₂ is due to the presence of an excess of selenium in the NiSe₂ nanosheet surface.     

Sulfur‐Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x‐Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co?S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x‐Vo‐S to exhibit much superior OER activity. FeCoO_x‐Vo‐S exhibits a mass activity of 2440.0 A g^?1 at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec^?1, indicative of its excellent charge transfer rate. When FeCoO_x‐Vo‐S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm^?2 and 406.0 mA cm^?2 at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.     

Vanadium‐Doped WS2 Nanosheets Grown on Carbon Cloth as a Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Two‐dimensional transition‐metal dichalcogenides have been widely studied as electrocatalysts for the hydrogen evolution reaction (HER). However, limited active sites and poor conductivity hinder their application. To solve these disadvantages, heteroatom doping has attracted wide attention because it can not only increase the active sites but also affect the intrinsic catalytic properties of the electrocatalyst. Herein, we grew vanadium‐doped WS₂ nanosheets on carbon cloth (V‐WS₂/CC) as an electrocatalyst for HER under acidic and alkaline conditions. With a proper vanadium doping concentration, the electrochemical surface areas of V_0.065‐WS₂/CC were 9.6 and 2.6 times as large as that of pure WS₂ electrocatalyst under acidic and alkaline conditions, respectively. In addition, the charge‐transfer resistance also decreased with moderate vanadium doping. Based on this, the synthesized vanadium‐doped WS₂ nanosheets exhibited good stability with high HER catalytic activity and could reach a current density of 10 mA cm⁻² at overpotentials of 148 and 134 mV in 0.5 m H₂SO₄ and 1 m KOH, respectively. The corresponding Tafel slopes were 71 and 85 mV dec⁻¹. Therefore, our synthesized vanadium‐doped WS₂ nanosheets can be a promising electrocatalyst for the production of hydrogen over a wide pH range.     

Prussian Blue‐Derived Iron Phosphide Nanoparticles in a Porous Graphene Aerogel as Efficient Electrocatalyst for Hydrogen Evolution Reaction

Tailoring of new hydrogen evolution reaction (HER) electrocatalyst with earth abundant elements is important for large scale water splitting and hydrogen production. In this work, we present a simple synthetic method for incorporating iron phosphide (FeP) particles into three‐dimensional (3D) porous graphene aerogel (GA) structure. The FeP formed in porous 3D GA (FeP/GA) is derived from electroactive Fe hexacyanoferrate (FeHCF). The advantage of incorporating FeP, in the porous 3D graphene network enables high accessibility for HER. As synthesized FeP/GA catalyst shows good electrocatalytic activity for HER in both acidic and alkaline solutions. The developed method can be useful for synthesizing metal hexacyanoferrate derived mono/bimetal phosphide catalyst in porous 3D graphene aerogels.     

Boron‐Dependency of Molybdenum Boride Electrocatalysts for the Hydrogen Evolution Reaction

Molybdenum‐based materials have been considered as alternative catalysts to noble metals, such as platinum, for the hydrogen evolution reaction (HER). We have synthesized four binary bulk molybdenum borides Mo₂B, α‐MoB, β‐MoB, and MoB₂ by arc‐melting. All four phases were tested for their electrocatalytic activity (linear sweep voltammetry) and stability (cyclic voltammetry) with respect to the HER in acidic conditions. Three of these phases were studied for their HER activity and by X‐ray photoelectron spectroscopy (XPS) for the first time; MoB₂ and β‐MoB show excellent activity in the same range as the recently reported α‐MoB and β‐Mo₂C phases, while the molybdenum richest phase Mo₂B show significantly lower HER activity, indicating a strong boron‐dependency of these borides for the HER. In addition, MoB₂ and β‐MoB show long‐term cycle stability in acidic solution.