Rh₂P Activity at a Fraction of the Cost? Co_2–xRh_xP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media

Abstract

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.

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

“…The origin of the Ni enhancement at the surface noted for both phases may be a function of differing diffusion energies, surface energies (due to preferred faceting and/or dictated by surface ligands used in the synthesis), and/or defects . We note that a similar trend was observed for Co 0.75 Rh 1.25 P: Rh and P are deficient at the surface relative to the base metal, Co, although the differences are less pronounced than for Ni (Co/Rh = 1.3 vs 0.6 ideal; M / P = 3.6 vs 2.0 ideal) …”

“…58 We note that a similar trend was observed for Co 0.75 Rh 1.25 P: Rh and P are deficient at the surface relative to the base metal, Co, although the differences are less pronounced than for Ni (Co/Rh = 1.3 vs 0.6 ideal; M/ P = 3.6 vs 2.0 ideal). 17 After HER catalysis, the only significant change in bulk composition is a decrease of P by 15% (Ni 0.29 Rh 1.71 P 0.88 ), suggesting that overall P losses are not as great for HER relative to OER. However, according to the XPS data, the surface P is completely depleted as it was for Ni 1.75 Rh 0.25 P post OER, consistent with phosphate displacement by hydroxide species during electrochemical cycling in alkaline media.…”

“…Similarly, we found that Co 2 P, a good base-metal catalyst for OER in alkaline media, actually demonstrated an enhancement in activity upon incorporation of small amounts of Rh, despite the fact that Rh 2 P prepared similarly is not particularly active. The OER activity was found to increase over time, associated with phase segregation of Rh, suggesting that sacrificial depletion of Rh may enhance activity of the Co oxide/hydroxide formed under oxidative conditions. − As a means to discern how the interplay between the base metal and noble metal governs activity and stability, we seek here to investigate the compositional effects on water splitting catalysis of the nickel analogue, Ni 2– x Rh x P. Ni was chosen because it is one of the three base metals (along with Co and Fe) known to form layered double hydroxides that are highly active for alkaline OER catalysis and because of prior reports of high HER activity in a phosphided RhNi phase under alkaline conditions …”

Co2‐xRhxP Nanoparticles for Overall Water Splitting in Basic Media: Activation by Phase‐Segregation‐Assisted Nanostructuring at the Anode

“…In our prior work, we demonstrated that introduction of small quantities of Rh into Co 2 P could significantly boost the OER activity in base with a catalytic sweet spot (η=0.29 V at 10 mA/cm 2 geometric ) occurring for the composition Co 1.75 Rh 0.25 P [19] . More recently, we showed that incorporation of Co into Rh 2 P to produce Co 2‐x Rh x P (x≥1.25) can catalyze the HER reaction in acid with initial activities comparable to Rh 2 P (η=0.084‐0.090 V at 10 mA/cm 2 geometric ), but with considerably less of the expensive Rh component [20] . However, unlike Rh 2 P, the Rh‐rich Co 2‐x Rh x P catalyst rapidly destabilized in acid, dropping from a current density of 10 mA/cm 2 geometric to ∼5 mA/cm 2 geometric before leveling off, correlating with leaching of Co from the catalyst.…”

Janus Mo2P3 Monolayer as an Electrocatalyst for Hydrogen Evolution

“…Among the various HER catalysts, transition-metal phosphides (TMPs) have drawn great attention due to their earth abundance, adjustable catalytic activity, and inherent metallicity. Some TMPs such as nickel phosphides, cobalt phosphides, molybdenum phosphides, iron phosphides, Rh 2 P, Co 0.75 Rh 1.25 P, Ni–Fe–P, and Mn–Co 2 P/Ni 2 P have been extensively studied and shown good HER activity. The P atoms in TMPs are the active sites formed because the negatively charged P atoms can capture positively charged protons .…”