Oxygen vacancy defects modulated electrocatalytic activity of iron-nickel layered double hydroxide on Ni foam as highly active electrodes for oxygen evolution reaction

It has been known that oxygen vacancy defects on the surface of electrocatalysts are beneficial for oxygen evolution reaction (OER), but the promotion effect for OER is still unclear and sometimes controversial. Here, we developed a new and facile method to introduce the oxygen vacancies (O-vacancies) to the surface of iron-nickel layered double hydroxide deposited on Ni foam (FeNi-LDH/NF) via the control of the NaCl contents in the initial FeNi precursor solution. O-vacancies are created by the intercalation and removal of Na+ cations in the FeNi-LDH crystalline structure, achieving the FeNi-LDH/NF electrodes with O-vacancy ratios of 1.3%, 4.2%, 6.8%, 14.2% and 27.4%. O-vacancies enhance the electronic conductivity, surface wettability and H2O adsorption/activation ability of the FeNi-LDH/NF electrodes. Most important, the results show a distinguished volcano-type dependence of the electrocatalytic activity of FeNi-LDH/NF on the O-vacancy ratios for OER. The best performance is obtained on FeNi-LDH/NF electrodes with 6.8% O-vacancies, achieving an overpotential of 177 mV to reach OER 10 mA cm−2 and excellent stability at a high current density of 400 mA cm−2 in 1.0 M KOH solution for over 60 h. The density functional theory (DFT) calculation indicates that excess O-vacancy defects on the surface of FeNi-LDH/NF catalysts would lead to too many delocalized electrons, which in turn deters the adsorption of the water molecule and thus reduces the activity for OER, while low O-vacancies would result in limited active sites for the H2O adsorption for OER. The results demonstrate the intrinsic relationship between the O-vacancy defects and electrocatalytic activity of FeNi-LDH catalysts for OER. The facile and controllable method to introduce O-vacancy defects in FeNi-LDH structure can also be utilized for other catalysts applications.