Transition metal phosphides (TMPs) have recently emerged as a new class of pre-catalysts that can efficiently catalyze the oxygen evolution reaction (OER). However, how the OER activity of TMPs varies with the catalyst composition has not been systematically explored. Here, we report the alkaline OER electrolysis of a series of nanoparticulate phosphides containing different equimolar metal (M = Fe, Co, Ni) components. Notable trends in OER activity are observed, following the order of FeP < NiP < CoP < FeNiP < FeCoP < CoNiP < FeCoNiP, which indicate that the introduction of a secondary metal(s) to a mono-metallic TMP substantially boosts the OER performance. We ascribe the promotional effect to the enhanced oxidizing power of bi- and tri-metallic TMPs that can facilitate the formation of MOH and chemical adsorption of OH⁻ groups, which are the rate-limiting steps for these catalysts according to our Tafel analysis. Remarkably, the tri-metallic FeCoNiP pre-catalyst exhibits exceptionally high apparent and intrinsic OER activities, requiring only 200 mV to deliver 10 mA cm⁻² and showing a high turnover frequency (TOF) of ≥0.94 s⁻¹ at the overpotential of 350 mV.

In this article, we for the first time report the synthesis and electrocatalytic properties of ruthenium cobalt phosphide hybrid clusters for the hydrogen evolution reaction (HER). Two types of catalysts are investigated: wet chemical reduction of Ru³⁺ on pre-formed cobalt phosphide (CoP) nanoparticles results in ruthenium–cobalt phosphide side-by-side structures (Ru/CoP); while phosphorizing chemically reduced RuCo alloy leads to the formation of hybrid ruthenium cobalt phosphide (RuCoP) clusters. Compared to pristine Ru clusters, both Ru/CoP and RuCoP show significantly improved HER performance in both acidic and alkaline solutions. In particular, the hybrid RuCoP clusters demonstrate a considerably low overpotential (η₁₀) of 11 mV at −10 mA cm⁻² and a high turnover frequency (TOF) of 10.95 s⁻¹ at η = 100 mV in 0.5 M H₂SO₄. Even in 1.0 M KOH the excellent HER activity of the RuCoP clusters remains, with a very low η₁₀ of 23 mV and exceptionally high TOF value of 7.26 s⁻¹ at η = 100 mV. Moreover, the RuCoP catalysts can sustain galvanostatic electrolysis in both acidic and alkaline solutions at −10 mA cm⁻² for 150 h with little degradation, showing better catalytic stability than the state-of-the-art commercial Pt/C catalysts. Our density functional theory (DFT) calculations indicate that the RuCoP hybrid exhibits a hydrogen adsorption energy very close to that of Pt and water and –OH adsorption energies distinct from pristine Ru, which reasonably explain the experimentally observed excellent HER activities and highlight the importance of synergistic coupling with cobalt phosphide to boost the HER performance of ruthenium.

Microstructural engineering is an effective approach to improving electrocatalytic activity of a catalyst. Shape-controlled hollow nanostructures represent a class of interesting architectures for use in electrocatalysis, given that they may offer large surface area, preferably exposed active sites, reduced diffusion pathways for both charge and mass transport, as well as enhanced catalytic activity due to the nano-cavity effect. Herein, we for the first time report the synthesis of hollow cobalt phosphide nanoparticles with a well-defined octahedral shape (CoP OCHs) via multi-step reactions. When used as pre-catalysts for the oxygen evolution reaction (OER) in an alkaline solution, the as-obtained CoP OCHs not only show high apparent catalytic activity requiring an overpotential of only 240 mV to deliver the benchmark current density of 10 mA cm⁻², but also exhibit an exceptionally high catalyst surface-area-based turnover frequency (TOF) of 17.6 s⁻¹ and a catalyst mass-based TOF of 0.072 s⁻¹ at a low overpotential of 300 mV, demonstrating excellent intrinsic catalytic activity. Moreover, the CoP OCHs also show high electrocatalytic activity for the methanol oxidation reaction (MOR), outperforming most metal phosphide based MOR catalysts reported so far. The synthetic strategy reported here can be readily extended to prepare other hollow shape-controlled metal phosphide catalysts.