Integration of Ni Doping and a Mo2C/MoC Heterojunction for Hydrogen Evolution in Acidic and Alkaline Conditions

Mo2C coated with Ni nanoparticles as the cathode catalyst towards efficient hydrogen evolution reaction: an experimental and computational investigation

Although Mo2C and earth-abundant 3d transition metals are regarded as potential catalysts to replace noble metal catalysts for effective hydrogen evolution reaction, their large-scale application is still inhibited by their own defects. Here, a facile thermal treatment method for nonprecious metal catalysts is developed to prepare a porous Ni/Mo2C composite catalyst. The loading density of Ni nanoparticles on the Mo2C surface has an important effect on the activity of the catalyst. By optimizing the Ni doping ratio, the Ni-40/Mo2C-17 sample exhibits the lowest onset overpotential and lowest overpotential at 10 mA cm-2 in both acidic and alkaline electrolytes, compared to other reported Ni- and Mo2C-based catalysts. In addition, theoretical calculations have also confirmed the synergistic effect between Ni nanoparticles and Mo2C, which can balance the thermodynamics between H adsorption and desorption of H2. This work provides an avenue for designing high-performance water-splitting catalytic materials using low-cost species, which exhibit excellent HER activity in a wide pH range.

In situ synthesis of tentacle-like NiC/Mo2C/NF nanorods array with excellent hydrogen evolution reaction at high current densities

The problem limiting the use of hydrogen evolution reactions in industry is the inability of electrocatalysts to operate stably at high current densities, so the development of stable and efficient electrocatalysts is important for hydrogen production by water splitting. By designing a rational interface engineering not only can the problem of limited number of catalytic sites in the catalyst be solved, but also can facilitate electron transfer, thus enhancing the efficiency of water splitting. Here, we designed a two-stage chemical vapour deposition method to construct NiC/Mo2C nanorod arrays on nickel foam to enhance the electrocatalytic ability of the catalysts, which exhibited efficient HER catalytic activity due to their special tentacle-like nanorod structure and abundant heterogeneous junction surfaces, which brought about abundant active sites as well as promoted electron transfer capability. The resulting catalysts provide current densities of 10, 100 and 500 mA cm-2 with overpotentials of 31, 153 and 264 mV, and exhibit excellent stability at current densities of 10 mA cm-2 for 200 h. This discovery provides a new idea for the rational design of catalysts with special morphologies.

Heterometal doping on nickel selenide corrugations for solar-assisted electrocatalytic hydrogen evolution

Since nickel exhibits good binding energy and is inexpensive, it is widely applied as a hydrogen evolution reaction (HER) electrocatalyst. Among all Ni-based materials, nickel selenide (NiSe) shows a unique electronic structure as a semiconductor with good electrocatalytic activity. Herein, we prepare Co-doped NiSe (Ni_1-xCo_xSe) with a structure of uniform corrugations by one-step chemical vapor deposition. For comparison, Fe-doped NiSe (Ni_1-xFe_xSe) and NiSe are also prepared using the same method. In alkaline electrolyte, Ni_1-xCo_xSe shows great HER performance in terms of low overpotential (93 mV@10 mA cm^-2 and 140 mV@50 mA cm^-2) and long-term stability. Moreover, with the assistance of solar energy, the overpotential needed for Ni_1-xCo_xSe is reduced, making Ni_1-xCo_xSe better than most reported NiSe-based HER catalysts. On the other hand, the current density of Ni_1-xCo_xSe is 13 mA cm^-2@93 mV and 63 mA cm^-2@140 mV with illumination, which is 30% and 26% higher than that without solar illumination assistance, respectively. Therefore, we believe that inducing sunlight to electrocatalytic hydrogen evolution in water splitting could be a supplementary footprint toward the utilization of solar energy.

Strategies to improve electrocatalytic performance of MoS2-based catalysts for hydrogen evolution reactions

Electrocatalytic hydrogen evolution reactions (HERs) are a key process for hydrogen production for clean energy applications. HERs have unique advantages in terms of energy efficiency and product separation compared to other methods. Molybdenum disulfide (MoS2) has attracted extensive attention as a potential HER catalyst because of its high electrocatalytic activity. However, the HER performance of MoS2 needs to be improved to make it competitive with conventional Pt-based catalysts. Herein, we summarize three typical strategies for promoting the HER performance, i.e., defect engineering, heterostructure formation, and heteroatom doping. We also summarize the computational density functional theory (DFT) methods used to obtain insight that can guide the construction of MoS2-based materials. Additionally, the challenges and prospects of MoS2-based catalysts for the HER have also been discussed.

Iron Catalyzed Cascade Construction of Molybdenum Carbide Heterointerfaces for Understanding Hydrogen Evolution

The intercrystalline interfaces have been proven vital in heterostructure catalysts. However, it is still challenging to generate specified heterointerfaces and to make clear the mechanism of a reaction on the interface. Herein, this work proposes a strategy of Fe-catalyzed cascade formation of heterointerfaces for comprehending the hydrogen evolution reaction (HER). In the pure solid-phase reaction system, Fe catalyzes the in situ conversion of MoO₂ to MoC and then Mo₂ C, and the consecutive formation leaves lavish intercrystalline interfaces of MoO₂ -MoC (in Fe-MoO₂ /MoC@NC) or MoC-Mo₂ C (in Fe-MoC/β-Mo₂ C@NC), which contribute to HER activity. The improved HER activity on the interface leads to further checking of the mechanism with density functional theory calculation. The computation results reveal that the electroreduction (Volmer step) produced H* prefers to be adsorbed on Mo₂ C; then two pathways are proposed for the HER on the interface of MoC-Mo₂ C, including the single-molecular adsorption pathway (Rideal mechanism) and the bimolecular adsorption pathway (Langmuir-Hinshelwood mechanism). The calculation results further show that the former is favorable, and the reaction on the MoC-Mo₂ C heterointerface significantly lowers the energy barriers of the rate-determining steps.

Ni-Directed biphase N-doped Mo2C as an efficient hydrogen evolution catalyst in both acidic and alkaline conditions

The development of efficient and low-cost catalysts is of great significance for the future application of the electrocatalytic hydrogen evolution reaction (HER). Herein, a series of Ni,N co-doped Mo₂C nanostructures (Ni_x-Mo₂C/N) with different Ni content levels are fabricated. The phase-directing effect of Ni on Mo₂C/N is observed, which is in charge of the phase transformation of Mo₂C/N from an α- to a β-phase. At the optimized Ni-doping level, biphase Ni₁₅-Mo₂C/N exhibits outstanding HER activity under both acidic and alkaline conditions. In particular, under alkaline conditions, Ni₁₅-Mo₂C/N delivers an overpotential of only 105.0 mV, accompanied by a low Tafel slope of 44.96 mV dec^-1. The performance is comparable to commercial 20% Pt/C and higher than most state-of-the-art Mo₂C-based catalysts as well.

Strongly Coupled Nitrogen-Doped Mo2C@CoNi Alloy Hybrid Architecture toward Efficient Hydrogen Evolution Reaction

Development of high-efficiency electrocatalysts for water splitting is a promising channel to produce clean hydrogen energy. Herein, we demonstrate that the combination of nitrogen-doped Mo₂C and CoNi alloy to form a hybrid architecture is an effective way to produce hydrogen from electrochemical water splitting. Benefiting from a combination of mechanisms, the optimized N-Mo₂C@CoNi-650 shows remarkable hydrogen evolution reaction (HER) activity with small overpotentials of 35, 123, and 220 mV to reach the current density of 10, 50, and 100 mA cm^-2 in alkaline media, respectively, outperforming most previously reported HER electrocatalysts. The efficient electrocatalytic performance is ascribed to the highly exposed active sites, fast reaction kinetics, and improved charge-transfer steaming from the synergistic effect between each component. This work presents a new insight into designing and preparing highly efficient electrocatalysts toward the HER.

An ingeniously assembled metal-organic framework on the surface of FeMn co-doped Ni(OH)2 as a high-efficiency electrocatalyst for the oxygen evolution reaction

To overcome the problem of the sluggish kinetics of the oxygen evolution reaction (OER), it is of great significance to develop an efficient and stable non-noble metal-based OER catalyst for electrocatalytic energy conversion and storage. Herein, a complex of a metal-organic framework and hydroxide is synthesized by performing a ligand etching strategy on FeMn co-doped Ni(OH)₂ nanosheets in situ grown on nickel foam (FeMn-Ni(OH)₂@MOF/NF). Benefiting from the unique sheet-on-sheet hierarchical structure, multi-metal active nodes and two active materials grown in situ, the resulting FeMn-Ni(OH)₂@MOF/NF demonstrated brilliant OER activity with an overpotential of 199 mV to achieve a current density of 10 mA cm^-2 and long-term stability. This research will provide a new strategy for the design of high-performance OER electrocatalysts.