The effect of synthesis methods on activity measurements for transition metal phosphides (TMPs) for the hydrogen evolution reaction (HER)

A cheaper, earth abundant alternative to platinum, the best catalyst for the hydrogen evolution reaction (HER), is needed. Recent studies have examined transition metal phosphides (TMPs) as a potential solution. Currently, synthesis methods and activity measurements for TMP catalysts vary widely across research groups, and, critically, their active site is not yet known – making intrinsic activity comparisons difficult. This work developed two different synthesis methods: 1) nanoparticles via incipient wetness on high surface area carbon supports and 2) thin films made by replacing the carbon supports with Ti foil. The TMPs used were CoP, Co₂P, FeP, Fe₂P, MoP, and Ni₂P. The double layer capacitance (C [subscript dl]) technique was used to find the electrochemically active surface area (ECSA). The carbon supports contributed heavily to the ECSA, so measuring the C [subscript dl] due only to the TMP is extremely difficult. This convoluted the nanoparticles intrinsic activity assessment – making results inconsistent and irreproducible. In contrast, the C [subscript dl] measurements for the thin films better represented the TMP C [subscript dl], because the Ti foil contributed a lower relative amount of C [subscript dl]. Hence, the thin film synthesis method is recommended when assessing intrinsic activity. Using the thin films, the effects of acidic vs. alkaline electrolyte were studied. From acidic to alkaline electrolyte, results showed MoP, CoP, and Co₂P activity slightly decreased, while Fe₂P, Ni₂P, and FeP activity all drastically decreased. Despite different magnitudes of activity change, the activity trend across all TMPs remained the same in acidic and alkaline environments except for MoP. A dilution series confirmed the importance of measuring activity in the mass-independent activity regime when comparing activity across studies with different mass loadings. Finally, Scanning Electron Microscopy was used to begin studying the microscale morphology of the thin films. I suggest future work study the stability of TMPs in acidic and alkaline electrolyte, the effects of electrolyte cation size, and morphological and stoichiometric changes under reaction conditions. Finally, I conclude with a list of ‘Best Practice’ reminders focused on the transparent and comprehensive reporting that will be needed for the field to compile data across groups and reveal the activity trends of TMPs for the HER.