Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.185
Publication date: 23rd September 2021
A range of transition metal-based materials such as metal chalcogenides, carbides, and phosphides as well as their alloys has emerged as earth-abundant electrocatalysts alternative to Pt for HER in either acidic or alkaline in recent years.[1,2,3] Despite the high electrochemical activity obtained with these types of materials, a fundamental understanding of the “real” active sites and the reaction mechanism is usually a challenge due to their characteristic high reactivity,[4] which makes it difficult to rely on ex-situ characterisation to elucidate the species and structure responsible for the performance.
We have been recently working on series of transition metal sulphides and selenides which exhibit high HER activity. Interestingly, we often observed that these materials also require a period of activation at the beginning of the electrochemical test in order to achieve high HER activity. In this contribution, we will discuss the activation process of α-NiS electrodes during the alkaline HER process. Based on operando X-ray absorption and Raman spectroscopies on α-NiS catalyst, we elucidate a complete phase transition from α-NiS to a mixed phase of Ni3S2 and NiO during HER operation. The synergistic effect of Ni3S2/NiO boosts a highly active HER with an overpotential of 85 for 10 mA cm-2 current density. Combined NAP-XPS and density functional theory (DFT) calculation reveal that Ni in NiO was identified as sites for OH* adsorption and S in Ni3S2 as sites for H* adsorption. The phase transition identified in our work highlights the significance of identifying the intrinsic active sites under realistic reaction conditions. Therefore, our study opens a new avenue for designing transition metal sulfide catalysts via the promotion of a phase transition with high activity.
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