Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process.

Carbon nanotube arrays (CNTAs) embedded with transition metal oxides (TMOs) have been considered as effective bifunctional catalysts. However, the hydrophobic nature of CNTAs usually limits the wetting of aqueous solutions containing inorganic precursors of TMOs, which results in poor decoration of TMOs on it. Herein, we report a facile strategy to tune the wettability of 3D CNTAs by controlling the nitrogen-doping degree. Particularly, superhydrophilic nitrogen-doped CNTAs (N-CNTAs) were obtained by using pyridine as the reactant, which allows uniform anchoring of cobalt oxide nanoparticles (NPs) on their surface through a simple self-priming freeze-drying assisted immobilization method. The heteroatom-doping and uniformly distributed cobalt oxide NPs endow the obtained hybrid material (Co₃O₄/N-CNTAs) with excellent OER and ORR performances, which also demonstrated their high efficiency as bifunctional catalysts for Zn–air batteries.