Facet-Selective Hydrogen Evolution on M₂P (M=Ni, Co, Fe) Single Crystals

Transition metal phosphides (TMPs) have recently emerged as promising alternative electrocatalysts to the rare Pt metal for the hydrogen evolution reaction (HER). To meet the demands for practical activity and stability, various strategies have been applied to these materials, such as doping, alloying, phase mixing, and modifying substrates through different synthesis routes. However, there has been a lack of reports and understanding of how HER activity can change depending on the different surface terminations of the catalyst’s crystal structure. It is important because some facets could exhibit several orders of magnitude higher activity, as observed in other electrochemical reactions, e.g., CO₂ reduction. To conduct this type of study, single crystals could be ideal candidates as they have well-defined surface areas and ensure consistent bulk composition. In this presentation, we will introduce metal flux methods for growing mm-sized single crystals of three M₂P compounds (M = Ni, Co, Fe) which are guided by in situ powder X-ray diffraction experiments. HER activity trend studies on the grown crystals showed drastic differences in the activity of two different facets – basal [0001] plane and side [10-10] plane of M₂P, which is further supported by H-adsorption calculations. Lastly, the same facets of the three M₂Ps would be compared to understand the activity orders among them. These findings provide guidance for shape-selective growth of nanocrystals of M₂P to maximize the advantages of unique facet activity.