The role of Fe species on NiOOH in oxygen evolution reactions

The Pourbaix diagram of Ni electrodes under reaction conditions presents several metastable NiOxHy phases and Fe doping enlarges the stability area of oxyhydroxo nature. For the Ni only phase water adsorption and intercalation can significantly lower both the surface and interface energies, and even introduce “negative surface energy”. Thus, water can exfoliate layers leading to Fe ion adsorption on inner layers as demonstrated by ab initio molecular dynamics. These single atoms have been carefully speciated (i.e. initially prepared as Fe2+ and Fe3+) and proton coupled electron transfer between the H2O−Fe and lattice oxygen ions has been observed in all ab initio molecular dynamics simulations, which is attributed to the Fe incorporation as no proton coupled electron transfer occurs in free water conditions. Furthermore, twelve possible oxygen evolution reaction mechanisms near Fe ions show that the main active species corresponds to the Ni2+, which is reduced from Ni3+ by H transfer when a Fe2+ adsorbs nearby, the overpotential can be significantly reduced to 0.23 V.