Role of Noble- and Base-Metal Speciation and Surface Segregation in Ni2–xRhxP Nanocrystals on Electrocatalytic Water Splitting Reactions in Alkaline Media

Abstract

Transition-metal phosphides have proven to be surprisingly active electrocatalysts for electrochemical water splitting, but the nature of the “active” catalyst depends strongly on the solution pH, the identity of the metals, and whether the reactions are anodic [oxygen evolution reaction (OER)] or cathodic [hydrogen evolution reaction (HER)]. In order to understand the origin of this activity, the synthesis of well-defined, compositionally controlled precatalysts is needed, as are detailed catalytic studies and physicochemical characterization/activity assessment of catalysts at different stages. While base-metal phosphides of Ni and Co have the advantage of being earth-abundant, in alkaline media, they are less active and less stable than noble-metal phosphides such as Rh2P. As a means to combine the abundant nature of base metals with the activity and stability of noble metals, the first synthesis of colloidal Ni2–xRhxP nanocrystals by arrested precipitation routes is reported along with their composition-dependent activity for electrocatalytic HER and OER. Phase-pure samples of Ni2–xRhxP were realized at the Ni-rich (hexagonal, Fe2P-type) end (x = 0.00, 0.25, 0.50) and Rh-rich (cubic, antifluorite-type) end (x = 1.75, 2.00). When assessed in terms of current density normalized to electrochemical surface area (ECSA) at a fixed potential, the most active precatalyst for OER is Ni1.75Rh0.25P, and for HER, it is Rh1.75Ni0.25P. Evaluation of X-ray photoelectron spectroscopy, transmission electron microscopy/energy-dispersive spectroscopy and ECSA data before and after 10 h stability runs were performed. The data reveal surface compositions to be considerably richer in Ni and poorer in Rh and P relative to the bulk composition, particularly for Ni0.25Rh1.75P, where the surface ratio of Ni/Rh is nearly 2:1 and increases to 4:1 after HER catalysis. In all cases, surface phosphorus is completely depleted post catalysis, suggesting a sacrificial role for phosphide under alkaline conditions. Moreover, the activity of “Rh1.75Ni0.25P” for HER decreases over time, even as the ECSA continues to rise, attributed to a decrease in the more active and stable Rh sites relative to Ni on the surface. In contrast, the enhancement in OER activity of Ni2P with 12.5% Rh incorporation is attributed to restructuring upon phase segregation of Rh, suggesting that the noble metal may also play a sacrificial role and not directly participate in OER catalysis. The roles of minority noble metals (Rh) in base-metal phosphides for OER and of minority base metals in noble-metal (Rh) phosphides for HER are discussed in light of related data on Co2–xRhxP.