Two-dimensional transition-metal-based carbides (or nitrides), so-called MXenes, that can be derived from the three-dimensional MAX phases, have attracted considerable attention throughout the past couple of years. The particular structure together with their hydrophilic and metallic nature make them promising candidates for a plethora of applications, such as sensors, electrodes, and catalysts. Obviously, the respective chemical and physical properties are highly dependent on the chemical composition, stoichiometry, and surface structure of the MXene. Here, we introduce a new member of the MXene family, V 4 C 3 T x (T representing the surface groups), based on the chemical exfoliation of the 413 MAX phase V 4 AlC 3 by treatment with aqueous hydrofluoric acid. X-ray powder diffraction data together with scale-bridging electron microscopy studies prove the successful removal of aluminum from the MAX phase structure. The electrocatalytic activity for the hydrogen evolution reaction of this new MXene is tested in acidic solution over the course of 100 cycles. Interestingly, we find a significant improvement of the catalytic performance over time (i.e., the overpotential required to achieve a current density of 10 mA cm −2 decreases by almost 200 mV) that we assign to the removal of an oxide species from the surface of the MXene, as shown by XPS measurements. Our study provides crucial experimental data of the electrocatalytic activity of MXenes together with the evolution of its surface structure that is also relevant for other transition-metal-based MXenes in the context of further potential applications. KEYWORDS: MAX phase, MXene, V 4 AlC 3 , V 4 C 3 T x , hydrogen evolution reaction, electrocatalysis, carbides

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In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono- and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. On the basis of the structural analysis by X-ray diffraction, X-ray-induced photoelectron spectroscopy, and transmission electron microscopy, it is concluded that MeN sites seem to dominate the HER activity of these catalysts. There is a strong relation between the amount of MeN sites that are formed and the energy of formation related to these sites integrated at the edge of a graphene layer, as obtained from density functional theory (DFT) calculations. Our results show, for the first time, that the combination of two metals (Co and Mo) in a bimetallic (Co,Mo)-N-C catalyst allows hydrogen production with a significantly improved overpotential in comparison to its monometallic counterparts and other Me-N-C catalysts. By the combination of experimental results with DFT calculations, we show that the origin of the enhanced performance of our (Co,Mo)-N-C catalyst seems to be provided by an improved hydrogen binding energy on one MeN site because of the presence of a second MeN site in its close vicinity, as investigated in detail for our most active (Co,Mo)-N-C catalyst. The outstanding stability and good activity make especially the bimetallic Me-N-C catalysts interesting candidates for solar fuel applications.

Cobalt hydroxide species are at the origin of OER activity of multiheteroatom doped carbon catalysts.

Today, metal-N- as well as metal-S-doped carbon materials are known to catalyze the hydrogen evolution reaction (HER). However, especially N- and S-co-doped catalysts reach highest activity, but it remains unclear if the activity is related to MN or MS (M=metal) sites. In this work we apply a simple method for multi-heteroatom doping and investigate the effect of cobalt content on the HER in acidic medium. The CoN and CoS sites were evidenced on the basis of structural characterization by Raman, X-ray induced photoelectron spectroscopy, and TEM. The presence of sulfur enables the formation of a larger number of CoN sites. Structure-performance relationship proves that the HER activity is dominated by CoN rather than CoS sites. The most active catalysts also exhibit an excellent stability under galvanostatic conditions making them of interest for electrolyser application.

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