d -Electron Descriptors for Nitrate Reduction Reaction Activity in Co-Based Heusler Phase Electrocatalysts

The tunability of electronic structure in intermetallics opens the door to systematic studies in catalysis. Here, we experimentally investigate various descriptors for the electrocatalytic nitrate reduction reaction using 9 Heusler phases as catalysts (Co2VSn, Co2MnAl, Co2MnGe, Co2MnSn, Co2FeGe, Co2MnSb, Ni2MnSn, Co2FeSi, and Co2FeAl). In contrast to previous studies with Co-based Heusler phases and oxide perovskites for OER, we find that generally, the total valence electron count, rather than eg filling, provides a better prediction of the activity. However, with period 5 Y elements, such as Sn and Sb, at high valence electron counts, Y-site d-band centers must also be considered. Reaction mechanism analysis by DFT also clarifies the role of X/Y sites. Hence, a single descriptor alone is not sufficient to rationalize catalytic activity across the full spectrum of Heusler phase compositions. So far, valence electron control has relied on single metal or binary alloy compositions for nitrate reduction; however, these Heusler phases help decouple the element identity and valence count while also serving as a guide for more catalysts based on ternary intermetallics. This work thus establishes multi-descriptor guidance for a wide group of catalyst compositions that were previously relatively unexplored.