Stability of CoP_x Electrocatalysts in Continuous and Interrupted Acidic Electrolysis of Water

Cobalt phosphides are an emerging earth-abundant alternative to platinum-group-metal-based electrocatalysts for the hydrogen evolution reaction (HER). Yet, their stability is inferior to platinum and compromises the large-scale applicability of CoP_x in water electrolyzers. In the present study, we employed flat, thin CoP_x electrodes prepared through the thermal phosphidation (PH₃) of Co₃O₄ films made by plasma-enhanced atomic layer deposition to evaluate their stability in acidic water electrolysis by using a multi-technique approach. The films were found to be composed of two phases: CoP in the bulk and a P-rich surface CoP_x (P/Co>1). Their performance was evaluated in the HER and the exchange current density was determined to be j₀=−8.9 ⋅ 10⁻⁵ A/cm². The apparent activation energy of HER on CoP_x (E_a=81±15 kJ/mol) was determined for the first time. Dissolution of the material in 0.5 M H₂SO₄ was observed, regardless of the constantly applied cathodic potential, pointing towards a chemical instead of an electrochemical origin of the observed cathodic instability. The current density and HER faradaic efficiency (FE) were found to be stable during chronoamperometric treatment, as the chemical composition of the HER-active phase remained unchanged. On the contrary, a dynamic potential change performed in a repeated way facilitated dissolution of the film, yielding its complete degradation within 5 h. There, the FE was also found to be changing. An oxidative route of CoP_x dissolution has also been proposed.