Activity and Stability of Iron-Based Spinel Oxides for Oxygen Evolution Reaction

In order to reduce the overpotential of oxygen evolution reaction (OER) and improve energy utilization efficiency of water splitting, spinel oxides have attracted considerable interests in OER electrocatalysis due to their potentially both high activity and high stability. Inspired from highly active Fe sites identified in Fe-contained (oxy)hydroxides, there are particular interests in developing Fe-based spinel oxides, yet correlations between their surface structures and OER activities still remain elusive. In addition, the electrochemical stability of spinel oxides has not received adequate concerns as usually deemed stable in alkaline solution. Recent studies have shown a substantial metal ion dissolution (e.g. Fe dissolution in Fe-containing metal oxides) during OER electrocatalysis, yet a fundamental insight into the metal dissolution process, its effect on the OER stability, and an efficient way to control the metal dissolution remain largely unaddressed. Herein, we reveal the atomic and electronic structures (particularly the metal-oxygen hybridization) of a series of Fe-based spinel oxide nanoparticles (XFe 2 O 4 , X=Mn 2+ , Fe 2+ , Co 2+ , and Ni 2+ ) by using aberration-corrected scanning transmission electron microscope, electron energy loss near edge spectroscopy, in-situ Raman spectroscopy combined with density functional theory (DFT) calculations. We show that substituting different divalent cations in the octahedral sites results in increasing metal-oxygen hybridization and thereby increasing activity in the order of Mn 2+ <Fe 2+ <Co 2+ <Ni 2+ , with the most active Ni-Fe spinel oxide even superior to a benchmark IrO 2 catalyst. DFT calculations suggest that the divalent cation X 2+ in the octahedral sites dominantly contributes to the metal-oxygen hybridization, suggesting that it is the divalent cation rather than the trivalent Fe3+ that constitutes the active site of the Fe-based spinel oxides. We further compared the electrocatalytic stability of the Fe-based spinel oxide by using combined microscopic and spectroscopic methods by using in-situ electrochemical quartz crystal microbalance (EQCM) and identical-location transmission electron microscopy (IL-TEM). Distinctly different extents of metal dissolution were observed in the four spinel catalysts, showing significant metal dissolution (Mn/Fe) in MnFe 2 O 4 and Fe 3 O 4 whereas much less metal dissolution (Fe) in CoFe 2 O 4 and NiFe 2 O 4 . Moreover, the Fe dissolution under potential-dynamic condition in the spinel oxides also shows quite different potential dependences, contributing to their different catalytic stabilities. Based on these results, we further developed a robust strategy to suppress Fe dissolution and thus improve the stability of the Fe-based spinels by intentionally adding Fe ions in the electrolyte.