Unraveling Ce dopant evolution: the formation of NiFeOOH/CeO₂ for enhanced water oxidation

Cerium (Ce) is often used as a dopant and promoter in the development of high-performance oxygen evolution reaction (OER) electrocatalysts. Although it has been confirmed that the majority of catalysts would undergo electrochemical reconstruction during the OER, a thorough understanding of the structural evolution of the Ce dopant in this process is lacking. Herein, an amorphous Ce-doped NiFe hydroxide (Ce-FeNiO) is successfully synthesized via a one-step room temperature corrosion method. Preliminary characterization reveals that Ce doping affects the morphology of NiFe hydroxide and introduces more high-valent Fe³⁺ and Ni³⁺ ions. Interestingly, by combining the characterization results of fresh and post-OER catalysts, it can be confirmed that Ce-FeNiO undergoes significant electrochemical reconstruction, where NiFe hydroxide transforms into NiFeOOH, and the Ce dopant self-assembles into CeO₂ nanoparticles embedded within the NiFeOOH structure. Furthermore, theoretical calculations verify that the integration of CeO₂ into NiFeOOH can regulate the d-band center and optimize the adsorption of oxygen-containing intermediates. Consequently, the genuine NiFeOOH/CeO₂ active species displays a low overpotential of 221 mV at 10 mA cm^-2 and long-term stability over 100 h at 500 mA cm^-2 in 1.0 M KOH for the OER. This work provides fundamental insights into the structural reconstruction of Ce-doped OER electrocatalysts.