Understanding the Activity of Single-Atom Catalysts for the Hydrogen Evolution Reaction: An Experimental Study

Abstract

Over the past few years single-atom catalysts (SACs) have gained more traction in the area of heterogeneous catalysis, with the advent of isolated metal atoms stabilized on metal oxides, bulk metal, metal dichalcogenides and graphene-like support materials.1-4 SACs can exhibit increased specific activity compared to bulk, as well as nano and subnano clusters due to their low-coordination environment, quantum size effects and improved metal-support interaction.2,3 Recent examples of SACs in electrochemistry include very active catalysts for the CO2 reduction reaction2, oxygen evolution reaction4 and hydrogen evolution reaction (HER)3. We synthesised a wide range of atomically dispersed first and second row transition metal catalysts on nitrogen-doped graphene and tested them for the electrochemical hydrogen evolution reaction (HER). All metal-containing catalysts showed higher activity than pristine nitrogen-doped graphene. The factor phi, a measure of electron density at the metal active site previously suggested by computational studies as descriptor5 was shown to correlate with the HER data exhibiting a volcano relationship. In line with previous research3 and computational predictions5, cobalt on nitrogen-doped graphene was found to be the most active catalyst with an overpotential at 10mA cm-2 of only a 175mV. Catalysts were characterised using XRD, XPS, TEM, CV and XAFS. Furthermore, high-resolution STEM, as well as in-situ XAFS were used to investigate the active site of the SACs and establish a structure activity relationship for HER over atomically dispersed transition metals on nitrogen-doped graphene. Understanding the structural factors for HER activity is not only important for the rational development of novel catalysts for water splitting but also imperative for other reactions such as the electrochemical CO2 reduction and nitrogen reduction where HER is the main competing reaction depressing rates and efficiencies. Liu, G.; Robertson, A. W.; Li, M. M.-J.; Kuo, W. C. H.; Darby, M. T.; Muhieddine, M. H.; Lin, Y.-C.; Suenaga, K.; Stamatakis, M.; Warner, J. H.; Tsang, S.C.E. Nature Chemistry 2017, 9 (8), 810–816.Genovese, C.; Schuster, M. E.; Gibson, E. K.; Gianolio, D.; Posligua, V.; Grau-Crespo, R.; Cibin, G.; Wells, P. P.; Garai, D.; Solokha, V.; Arrigo, R.: et al. Nature Communications 2018, 9 (1).Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L. G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; et al. Nature Communications 2015, 6, 8668.Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li, M.; Zhao, Z.; Wang, Y.; Sun, H.; et al. Nature Catalysis 2018, 1 (1), 63–72.Xu, H.; Cheng, D.; Cao, D.; Zeng, X. C. Nature Catalysis 2018, 1 (5), 339–348.