Tuning the d-Band States of Ni-Based Serpentine Materials via Fe3+Doping for Efficient Oxygen Evolution Reaction

The serpentine germanate materials are promising oxygen evolution reaction (OER) electrocatalysts due to their unique layered crystal structure and electronic structure. However, the catalytic activities still need to be improved to satisfy the practical applications. Adjusting the d-band center of metal active site to balance the adsorption and desorption of intermediates is considered an effective approach to improve the OER activity. In this work, an element dopant strategy was proposed to optimize the d-band state of Ni3Ge2O5(OH)(4) serpentine to improve the OER activity. The density functional theory calculations revealed that Fe3+ doping increased the d-band center of the Ni3Ge2O5(OH)(4) serpentine, which optimized the adsorption strength of intermediates on surface Ni and Fe atoms so that the Fe3+ doped Ni3Ge2O5(OH)(4) (Ni2.25Fe0.75Ge2O5(OH)(4)) exhibited much reduced Gibbs free energy changes in the rate-determining step compared with pristine serpentine. Inspired by the theoretical calculations, the NixFe3-xGe2O5(OH)(4) nanosheets with different amounts of doped Fe3+ were designed and synthesized. The structural characterizations indicated that Fe3+ was successfully doped into Ni3Ge2O5(OH)(4) and replaced the Ni2+. The Fe3+ doped NixFe3-xGe2O5(OH)(4) nanosheets showed greatly improved OER activity than Ni3Ge2O5(OH)(4) and Fe3Ge2O5(OH)(4). Further electrochemical analysis illustrated that Fe3+ doping reduced the adsorptive/formative resistance of intermediates and the charge transfer resistance and facilitated the kinetic process of OER. The in situ Raman spectra indicated that the Fe3+ doped Ni3Ge2O5(OH)(4) possesses a more active Ni-O bond than pristine Ni3Ge2O5(OH)(4). This work provides an effective strategy to tune the d-band center of serpentines for efficient electrocatalytic OER.