Electrocatalytic Activity of the Brewer Intermetallic Phases Mo6Co7, W6Co7, and W6Fe7 for Hydrogen and Oxygen Evolution Reactions

In pursuit of understanding the electrocatalytic activity trends within Brewer intermetallic phases comprising early and later transition metals, we synthesized three binary crystalline intermetallics, namely, Mo6Co7, W6Co7, and W6Fe7. The intermetallics exhibit an activity trend of Mo6Co7 > W6Co7 > W6Fe7 for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in electrochemical water splitting. The Mo6Co7 demonstrates stable electrocatalytic activity over 24 h under a high current density of 80 mA cm–2 in HER, which highlights its potential as a robust electrocatalyst for hydrogen production. Our observations strongly suggest that the electrocatalytic performance is predominantly governed by intrinsic catalytic active metal sites within the Mo6Co7 structure. Density functional theory (DFT) calculations suggest that for HER, the later transition metals such as Co and Fe are more active than the early transition metals of Mo and W, reflecting the activity order of Co > Mo > W and Co > Fe. The density of states (DOS) diagram indicates that Mo6Co7 exhibits the highest charge density at the Fermi level, surpassing both W6Co7 and W6Fe7. In terms of the rate-determining step (RDS) for the OER, the calculated overpotentials align with an activity trend observed in experiments, where Mo6Co7 demonstrates greater activity compared to W6Co7 and W6Fe7. Our results provide a foundation for tailoring intermetallics to optimize electrocatalytic efficiency, stability, and activity for HER and OER in water splitting.