Decoupling bulk composition and alloying effects in Ni-Mo materials for electrocatalytic hydrogen evolution at near-neutral pH

Ni-Mo electrocatalysts are extensively studied for the hydrogen evolution reaction (HER). This includes HER at near-neutral pH conditions relevant to, e.g., microbial electrosynthesis (MES). So far, the role of the Ni:Mo ratio in bulk for determining HER performance remains unclear, especially for commercially available powdery electrocatalysts. Here, we examined commercial Ni-Mo powders with different nominal Ni:Mo ratios to assess how composition and mass loading influence HER performance. Despite differences in bulk Ni:Mo ratios, no significant variation in HER performance was observed at equal mass loadings. In contrast, increasing the electrocatalyst mass loading from 0.35 ± 0.04 to 1.30 ± 0.07 mg cm −2 consistently improved HER performance, decreasing the overpotential at −1 mA cm −2 from 463 ± 50 to 210 ± 9 mV, with an intermediate value of 367 ± 33 mV at a loading of 0.61 ± 0.02 mg cm −2 . Comprehensive characterization by XRD, SEM, EDX, and XPS shows that the Ni-Mo powders are not true Ni-Mo alloys but rather physically mixed phases with notable Ni- and Mo-rich segregation. These results demonstrate that the bulk Ni:Mo ratio does not accurately reflect the catalytic structure and hence cannot be used alone to assess structure-function relationships for HER performance. This study underscores the importance of thorough characterization, even of commercial electrocatalysts, and highlights the significance of surface structure over nominal bulk composition for Ni-Mo-based HER electrocatalysts.