Rh2P Activity at a Fraction of the Cost? Co2-xRhxP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media

The composition-dependent hydrogen evolution reaction (HER) activity of Co2-xRhxP nanoparticles in acid is reported. The motivation for the current study stems from (1) prior work demonstrating that, although costly, Rh2P nanoparticles are highly active and stable toward the HER process and (2) the expectation that diluting Rh sites with Co will result in catalytic synergism while also lowering the overall cost of the material. Here, we establish that the HER activity of Co2-xRhxP nanoparticles in acidic media is composition-dependent, with Rh-rich electrocatalysts showing superior activity as compared to those that are Co-rich. Additionally, compositions of Co2-xRhxP for which x >= 1.25, where the materials adopt the cubic antifluorite structure, deliver comparable initial catalytic activities to pure Rh2P, suggesting that the crystal structure of the material may play a more significant role in driving the overall HER activity than the composition. Despite comparable activity to Rh2P, Co2-xRhxP systems do not have the stability associated with Rh2P but undergo a drop from 10 to 5 mA/cm(2) within the first hour of stability testing, associated with Co loss from the surface. In cases where Pt is used as the counter electrode, no such drop in current density is observed, despite the loss of Co, with Pt transfer to the working electrode compensating for the Co depletion. First- principles calculations based on density functional theory show that both the hydrogen binding energies and the Gibbs free energies of hydrogen adsorption increase linearly with x, with Co0.75Rh1.25P exhibiting a Delta G value that is closest to zero, suggesting that this composition is the most active for HER in this series. Double-layer capacitance data, from which electrochemical surface area (ECSA) data for all of the compositions are computed, are used to demonstrate that the quality and quantity of active sites among different compositions of Co2-xRhxP can vary significantly, even when the morphologies and particle sizes are similar.

Automated summary

The composition-dependent hydrogen evolution reaction (HER) activity of Co2-xRhxP nanoparticles in acid is reported. The study shows that Rh-rich electrocatalysts have superior activity compared to Co-rich ones, and despite comparable activity to Rh2P, Co2-xRhxP systems lack stability. Calculations suggest that Co0.75Rh1.25P is the most active composition in the series.