A Little Nickel Goes a Long Way: Ni Incorporation into Rh₂P for Stable Bifunctional Electrocatalytic Water Splitting in Acidic Media

In acidic media, many transition-metal phosphides are reported to be stable catalysts for the hydrogen evolution reaction (HER) but typically exhibit poor stability toward the corresponding oxygen evolution reaction (OER). A notable exception appears to be Rh₂P/C nanoparticles, reported to be active and stable toward both the HER and OER. Previously, we investigated base-metal-substituted Rh₂P, specifically Co_2–xRh_xP and Ni_2–xRh_xP, for HER and OER as a means to reduce the noble-metal content and tune the reactivity for these disparate reactions. In alkaline media, the Rh-rich phases were found to be most active for the HER, while base-metal-rich phases were found to be the most active for the OER. However, Co_2–xRh_xP was not stable in acidic media due to the dissolution of Co. In this study, the activity and stability of our previously synthesized Ni_2–xRh_xP nanoparticle catalysts (x = 0, 0.25, 0.50, 1.75) toward the HER and OER in acidic electrolyte are probed. For the HER, the Ni_0.25Rh_1.75P phase was found to have comparable geometric activity (overpotential at 10 mA/cm_geo²) and stability to Rh₂P. In contrast, for OER, all of the tested Ni_2–xRh_xP phases had similar overpotential values at 10 mA/cm_geo², but these were >2x the initial value for Rh₂P. However, the activity of Rh₂P fades rapidly, as does Ni₂P and Ni-rich Ni_2–xRh_xP phases, whereas Ni_0.25Rh_1.75P shows only modest declines. Overall water splitting (OWS) conducted using Ni_0.25Rh_1.75P as a catalyst relative to the state-of-the-art (RuO₂||20% Pt/C) revealed comparable stabilities, with the Ni_0.25Rh_1.75P system demanding an additional 200 mV to achieve 10 mA/cm_geo². In contrast, a Rh₂P||Rh₂P OWS cell had a similar initial overpotential to RuO₂||20% Pt/C, but is unstable, completely deactivating over 140 min. Thus, Rh₂P is not a stable anode for the OER in acidic media, but can be stabilized, albeit with a loss of activity, by incorporation of nominally modest amounts of Ni.