Correlating electrocatalytic oxygen reduction activity with d-band centers of metallic nanoparticles

The establishment of correlating the electronic structures with the activity of non-noble-metal electrocatalysts toward the oxygen reduction reaction (ORR) plays a pivotal role in designing high-performance cathodic electrocatalysts. Here, a procedure of metallic Fe-family nanoparticles (NPs), such as nitrides (Co_5.47N, Fe₄N and Ni₃N) and zero-valence metals (Co and Ni) NPs encapsulated into three-dimensional porous nitrogen-doped carbon (NCC) has been successfully demonstrated by the combination of sol-gel processing and subsequent nitridation process. The density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) valence band experiments, and the electrochemical measurements suggest that the ORR activities of encased Fe-family metallic NPs evidently increase as the d-band centre values move away from the Fermi level as the following order: Ni₃N<Ni<Co<Fe₄N<Co_5.47N, showing that the d-band centre value can be a good criterion to screen and evaluate the electrocatalysts. The optimized Co_5.47N phase leads to an exceptional ORR activity with a nearly close 4e^- reaction pathway (half-wave potential (E₁^/₂): −176 mV vs. saturated calomel electrode (SCE), n: 3.8–4.0, HO₂^- yields: 7.3–8.1%), comparable to those of the state-of-the-art Pt/C in alkaline medium. Moreover, the Co_5.47N/NCC also has a superior long-term stability and tolerance to MeOH crossover effect to commercial Pt/C.