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Insight Into the Catalytic Activity of MXenes for Hydrogen Evolution Reaction

An efficient catalyst is critical to facilitate the low overpotential and fast kinetics since the splitting of water is a difficult process.

The production of hydrogen through electrochemical water splitting has been extensively investigated as it is one of the most promising ways to alleviate the energy shortage and environmental problems. An efficient catalyst is critical to facilitate the low overpotential and fast kinetics since the splitting of water is a difficult process. Although precious metal platinum (Pt) can catalyze hydrogen evolution reaction (HER) with a high exchange current density and small Tafel slope, high cost and scarcity greatly hamper its widespread utilization. To ensure the large-scale and sustainable production of H2, it is essential to develop low-cost, earth-abundant, and highly active alternatives.

Among various potential alternatives, ultrathin two-dimensional (2D) based catalysts have attracted extensive attention owing to their large surface area, efficient charge transfers and outstanding mechanical flexibility. Recently, a new family of 2D layered materials, namely "MXenes", have been extensively studied both experimentally and theoretically, including transition metal carbides, nitrides, and carbonitrides. This kind of materials have potential applications in various fields, such as Li or non-Li ion batteries, hydrogen storage, supercapacitors, CO oxidation, and thermoelectric materials. In addition, MXenes also show excellent potential as catalysts for HER.

In previous work, transition metal-promoted V2C MXene as an effective HER catalyst was explored. Subsequently, experimental work showed that Mo2CTx exhibited excellent HER activity. However, more efforts are focused on evaluating the HER performance of MXenes (e.g., the Gibbs free energy of H adsorption (?GH) and the exchange current densities (i0)) rather than the intrinsic origin of the diverse HER activity. A general understanding at atomic level is still lacking and important to the development of practical HER electrocatalysts.

To provide an in-depth understanding on the variation in HER activity of MXenes, Jinlan Wang's group from Southeast University have explored the relationship between the HER activities and the intrinsic electronic structures based on 20 MXenes (M2NO2 and M2CO2, M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W). This paper was reported in Science Bulletin (2018, 63(21)), entitled "Insight into the catalytic activity of MXenes for hydrogen evolution reaction." 

Firstly, calculation of electrical properties shows that all of the M2NO2 catalysts are metallic while several carbide MXenes with O-terminated surfaces are semiconductors. Secondly, the HER activity of M2NO2 is investigated under standard conditions by using the Gibbs free energy change for hydrogen adsorption (?GH) as the descriptor. It is found that Ti2NO2 and Nb2NO2 possess ultra-high HER with the Gibbs free energy of hydrogen adsorption of 0 and 0.02 eV, respectively, stand out with excellent HER activity under low hydrogen coverage and the Volmer-Heyrovsky mechanism is preferred. Finally, a Fermi-abundance model is proposed as a good descriptor to understand variation of the activity in different Mxenes. It unravels that HER activities of 20 MXenes actually relies on the occupied p electronic states of surface O atoms below the Fermi level.

This model is of good generality, which can be applicable to MXene and other 2D Materials. Therefore, it may be a good descriptor for HER activity of 2D materials. This study not only provides new promising catalysts for HER, but also builds up insightful guideline to the design of highly active HER electrocatalysts.

(Source: Science Direct)

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