The composition-dependent hydrogen evolution reaction (HER) activity of Co 2−x Rh x P nanoparticles in acid is reported. The motivation for the current study stems from (1) prior work demonstrating that, although costly, Rh 2 P 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 Co 2−x Rh x P 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 Co 2−x Rh x P for which x ≥ 1.25, where the materials adopt the cubic antifluorite structure, deliver comparable initial catalytic activities to pure Rh 2 P, 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 Rh 2 P, Co 2−x Rh x P systems do not have the stability associated with Rh 2 P 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. Firstprinciples 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 Co 0.75 Rh 1.25 P exhibiting a ΔG 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 Co 2−x Rh x P can vary significantly, even when the morphologies and particle sizes are similar.

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 Rh 2 P. As a means to combine the abundant nature of base metals with the activity and stability of noble metals, the first synthesis of colloidal Ni 2−x Rh x P nanocrystals by arrested precipitation routes is reported along with their composition-dependent activity for electrocatalytic HER and OER. Phase-pure samples of Ni 2−x Rh x P were realized at the Ni-rich (hexagonal, Fe 2 P-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 Ni 1.75 Rh 0.25 P, and for HER, it is Rh 1.75 Ni 0.25 P. 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 Ni 0.25 Rh 1.75 P, 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 "Rh 1.75 Ni 0.25 P" 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 Ni 2 P 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 Co 2−x Rh x P.

The application of Co2‐xRhxP nanoparticles as electrocatalysts for the hydrogen evolution reaction (HER) and overall water splitting in basic media is reported. The experimental design seeks to dilute rhodium with earth‐abundant cobalt as a means to lower the cost of the material and achieve catalytic synergism, as reported for related bimetallic phosphides. The HER activity of Co2‐xRhxP is found to be composition‐dependent, with the rhodium‐rich compositions being more active as compared to their cobalt‐rich counterparts, with overpotentials (η) at 10 mA/cm2geometric of 58.1–63.9 mV vs. 82.1–188.1 mV, respectively. In contrast, Co‐rich Co2‐xRhxP nanoparticles are active for the oxygen evolution reaction (OER) process in basic media, with η= 290 mV for x=0.25. A full water electrolysis cell was created using the most active compositions for OER and HER as the anode and cathode, respectively, generating an overall η= 390 mV. Notably, the cell became more active over a 50 h stability test, increasing by 2 mV/cm2geometric at a constant applied voltage of 1.62 V vs NHE. This enhanced activity correlates with nanoscale phase segregation of Rh in the anode. Thus, the lower overpotential achieved for Co1.75Rh0.25P relative to Co2P, and the augmented activity over time in the former, may be a consequence of restructuring of the anode driven by Rh phase‐segregation. The augmentation in activity at the anode more than compensates for small losses at the cathode.

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 Co2–x Rh x P and Ni2–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, Co2–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 Ni2–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 Ni0.25Rh1.75P phase was found to have comparable geometric activity (overpotential at 10 mA/cmgeo 2) and stability to Rh2P. In contrast, for OER, all of the tested Ni2–x Rh x P phases had similar overpotential values at 10 mA/cmgeo 2, but these were >2x the initial value for Rh2P. However, the activity of Rh2P fades rapidly, as does Ni2P and Ni-rich Ni2–x Rh x P phases, whereas Ni0.25Rh1.75P shows only modest declines. Overall water splitting (OWS) conducted using Ni0.25Rh1.75P as a catalyst relative to the state-of-the-art (RuO2||20% Pt/C) revealed comparable stabilities, with the Ni0.25Rh1.75P system demanding an additional 200 mV to achieve 10 mA/cmgeo 2. 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.