Molten Salt Assisted Ni-Doped α-FeOOH on Regulating Spin State for Efficient Oxygen Evolution Reactions

alpha-FeOOH is a promising catalyst for oxygen evolution reaction (OER). However, owing to the high spin state (HS, t2g3eg2) of Fe3+ in typical alpha-FeOOH structure, the bonding strength of oxygen-containing intermediates are usually too strong. In this work, the successful synthesis of intermediate spin state (IS) Fe3+ in alpha-FeOOH structure on iron foam (IF) by introducing Ni atom is reported. Refined structural analysis and theoretical calculations reveal that doped-Ni atom enlarged the d orbital splitting energy, thus converting the crystal field stable HS Fe3+ state to intermediate spin state. The IS Fe3+ optimized the adsorption energy of oxygen-containing intermediates and reduced the reaction energy barrier of rate-determining step (RDS, O* to OOH*). Moreover, the doped Ni reduced the generation of Fe4+ during OER, combined with the amorphous layer formed by the molten salt method, inhibiting the dissolution of Fe. The as-obtained catalyst exhibited excellent OER activity with an overpotential of 178 mV at 10 mA cm-2 and a Tafel slope of 27 mV dec-1, which outperforms those of the state-of-the-art iron oxyhydroxides catalysts. In this paper, a fast and efficient method for the construction of a low-cost and high-activity iron-based OER catalyst is provided.