Abstract
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 CoPx in water electrolyzers. In the present study, we employed flat, thin CoPx 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 CoPx (P/Co>1). Their performance was evaluated in the HER and the exchange current density was determined to be j 0=−8.9 ⋅ 10−5 A/cm2. The apparent activation energy of HER on CoPx (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 CoPx dissolution has also been proposed.
Highlights
Electrocatalysts play a critical role in the performance of electrochemical water splitting devices.[6,7] High energy efficiency for water splitting can only be achieved if suitable electrocatalysts are employed to drive the hydrogen evolution (HER) and oxygen evolution reactions (OER)
We studied the temperature-dependent kinetics from which the apparent activation energy for HER over CoPx in acidic electrolyte has been derived
CoPx films were thermally prepared by phosphidation of smooth plasma enhanced atomic layer deposition (PE-ALD) fabricated Co3O4 films with an initial thickness of 20 nm
Summary
Electrocatalysts play a critical role in the performance of electrochemical water splitting devices.[6,7] High energy efficiency for water splitting can only be achieved if suitable electrocatalysts are employed to drive the hydrogen evolution (HER) and oxygen evolution reactions (OER). The main criteria of a good HER catalyst are the ability to moderately bind both. H2.[8] Platinum is the best-studied and best-performing catalyst for HER and requires only a very small overpotential, even at high current densities in acidic solutions. The search for active and stable earth-abundant electrocatalysts to replace precious metals has triggered a lot of research efforts. Addition of phosphorus leads to an increase in performance of those metals by stabilization of the metal d-orbitals and a small metal to P charge transfer
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