A Little Nickel Goes a Long Way: Ni Incorporation into Rh 2 P 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 Rh 2 P/C nanoparticles, reported to be active and stable toward both the HER and OER. Previously, we investigated base-metal-substituted Rh 2 P, specifically Co 2- x Rh x P and Ni 2- x Rh x P, 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- x Rh x P 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- x Rh x P 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 0.25 Rh 1.75 P phase was found to have comparable geometric activity (overpotential at 10 mA/cm geo 2 ) and stability to Rh 2 P. In contrast, for OER, all of the tested Ni 2- x Rh x P phases had similar overpotential values at 10 mA/cm geo 2 , but these were >2 x the initial value for Rh 2 P. However, the activity of Rh 2 P fades rapidly, as does Ni 2 P and Ni-rich Ni 2- x Rh x P phases, whereas Ni 0.25 Rh 1.75 P shows only modest declines. Overall water splitting (OWS) conducted using Ni 0.25 Rh 1.75 P as a catalyst relative to the state-of-the-art (RuO 2 ||20% Pt/C) revealed comparable stabilities, with the Ni 0.25 Rh 1.75 P system demanding an additional 200 mV to achieve 10 mA/cm geo 2 . In contrast, a Rh 2 P||Rh 2 P OWS cell had a similar initial overpotential to RuO 2 ||20% Pt/C, but is unstable, completely deactivating over 140 min. Thus, Rh 2 P 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.