Application and prospect of molybdenum carbide in electrocatalytic hydrogen production

With the depletion of petroleum, coal and other traditional fossil energy and the worsening of environmental problems, the traditional energy structure system based on fossil energy is facing unprecedented crisis and challenge. Hydrogen, with its high energy density, excellent combustion performance, clean and pollution-free, is the best alternative to green energy from traditional fossil energy. However, the utilization of hydrogen energy depends on the development of hydrogen production technology to a large extent. At present, industrial hydrogen production technology mainly consists of petrochemical catalytic cracking and natural gas steam reforming to produce hydrogen, which does not meet the development demand of “green and sustainable development” from the perspective of comprehensive utilization of environment and energy. In recent years, with the new power generation technology (such as solar power, wind power, nuclear power, hydropower, geothermal power, etc.) of the optimization and upgrade of continuous development and power grid system, the advantage of the electrolysis of water hydrogen production technology by building up, even by many scientists and entrepreneurs known as “the most ideal industrial hydrogen production methods”, and the core problem of the technology is efficient, stable, non-corrupt hydrogen production (including oxygen) for the development of electric catalyst. At present, platinum-based catalyst is the most effective electrocatalyst in the electrocatalytic hydrogen production process, because this kind of catalyst has the lowest overpotential and high stability in the electrolytic water hydrogen production process. However, the high price and low storage capacity of platinum seriously restrict the wide application of this kind of catalyst in hydrogen production from electrolysis water and the development of the hydrogen production process. Therefore, finding a cheap and substitutable high activity electrocatalytic hydrogen production catalyst is the core problem in the development of hydrogen production technology. The latest research shows that the pretransition metal carbide exhibits high catalytic activity and stability in the electrocatalytic hydrogen production. Among them, molybdenum carbide (MoxC) has been widely studied as one of the best alternative catalysts in recent years because of its high stability, simple synthesis method and wide application range of pH.

The application of molybdenum carbide materials in this field has made rapid development, and the main research can be divided into four directions :(1) to improve the dispersability of molybdenum carbide, and make use of nanotechnology to expose as many active sites as possible in the electrocatalytic process; (2) Improve the porosity of catalyst materials and accelerate the mass transfer and diffusion process (electrolyte and hydrogen) in the catalytic process; (3) Due to the poor conductivity of molybdenum carbide, a large part of research work has been done to improve the conductivity of catalysts by introducing other conductive carriers in the electrocatalytic hydrogen evolution process, such as graphene, CNT, etc.; (4) To change the electronic structure of molybdenum carbide through doping, so as to realize the regulation of its hydrogen evolution property. According to the general classification of the development direction and combined with the work of the research group in this field, the synthesis of molybdenum carbide and its application in the electrocatalytic hydrogen production are reviewed and reviewed briefly.

Molybdenum carbide has d electron structure similar to precious metals such as platinum, which is called “noble metal-like catalyst” and can be used as hydrogenation/dehydrogenation catalyst. The traditional synthesis method of molybdenum carbide is “gas-solid synthesis method”, that is, MoO3 is obtained by carbonization at high temperature in the environment of CH4/H2 mixture. This method has poor controllability, and the synthesized molybdenum carbide has a large particle size (micron level). In addition, this method is more dangerous, involving “gas-solid polyphase reaction”, resulting in “longitudinal difference” of the synthesized catalyst.

The synthesis process of molybdenum carbide (MoxC) reported at the present stage inevitably USES high temperature (~900℃), on the one hand inevitably causes the sintering and agglomeration of MoxC particles. On the other hand, high synthesis temperature will cause the collapse of pore structure of the catalyst, so that the catalyst generally has a small specific surface area (< 50=” >2/g). The above technical bottlenecks seriously restrict the exposure of active sites and the diffusion of reaction products and reactants of MoxC catalyst in the electrocatalytic hydrogen production process, and greatly affect the activity of this kind of electrocatalyst.

Because of the poor conductivity of molybdenum carbide itself, the transfer rate of electrons in the electrocatalysis process is also very important for the reaction. Therefore, our research group synthesized the nano–mo2c electrocatalytic hydrogen evolution catalyst supported by reduced graphene by combining the method of “secondary transformation of organic and inorganic hybrid compounds” with graphene with good conductivity. Under acidic conditions, the electrochemical activity was as follows: when the current density was 10mA/cm2, the overpotential drop was only ca.120mV[4].

On the basis of the discovery of the high hydrogen evolution activity of molybdenum carbide nanoparticles [3,4], the project team further adjusted the composition of moox-amine organic and inorganic hybrid nanowires, and developed moc-mo2c heterogeneous nanowires through the “controllable carbonization” strategy. The synergistic effect on the MOC-MO2C nanometer interface is utilized to simultaneously promote proton reduction (Volmer step) and hydrogen adsorption desorption (Heyrovsky/Tafel step) to improve catalytic activity. In acidic and alkaline conditions, the hydrogen evolution overpotential (10) of moc-mo2c heterogenous nanowires was 126mV, respectively, and the catalytic activity remained unchanged for more than 20 hours. This work provides a new method for the electronic properties of the modified molybdenum active center and the kinetics of hydrogen production, and provides a reference for the further study of the catalytic mechanism.

Based on the regulation principle of the electronic structure of molybdenum carbide, the project team continued to regulate the electronic structure of Mo2C by using Co, which is abundant in D electrons, as the doping element. In the process of constructing moox-amine organic and inorganic hybrid nanowires precursor, Co was introduced, and then the CO-doped Mo2C electrocatalyst could be obtained through high temperature carbonization. Studies have shown that the introduction of Co can increase the electron cloud density near the Fermi level of Mo2C, weaken mo-H, and thus improve the hydrogen evolution performance of Mo2C. In acidity and alkalinity, its hydrogen evolution overpotential (10) is 140mV and 118mV, respectively, and the catalytic activity remains unchanged for more than 20 hours.

The research direction of molybdenum carbide in electrocatalytic hydrogen evolution reaction basically conforms to the “three elements” of traditional construction of high efficiency catalyst, namely: high intrinsic activity and large exposed active site, fast mass transfer diffusion and diffusion process, and high heat transfer (electron transfer, electrocatalytic process) rate. In recent years, molybdenum carbide has developed rapidly in the electrocatalytic hydrogen evolution reaction. Combined with the development process of molybdenum carbide in the electrocatalytic hydrogen evolution process, the future of molybdenum carbide in this reaction process may be in the following aspects :(1) the development of efficient new synthesis methods; (2) Development of molybdenum carbide hydrogen evolution device; (3) Digging and understanding of structure-activity relationship; (4) Construction of multi-component catalyst, etc..