General Approach to Synthesize Multilayer Graphitic Carbon-Nanotube-Encapsulated NiCo Alloys as Trifunctional Electrocatalysts: Deciphering the Role of N-Dopants

Designing effective multifunctional electrocatalysts with robust activity and durability is crucial for developing different electrochemical energy storage/conversion devices. In this study, we established a general approach to synthesize a multilayer N-doped graphitic carbon nanotube (CNT)-encapsulated NiCo alloy, which exhibits robust trifunctional electrocatalytic activity toward three fundamental electrochemical reactions: oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Remarkably, the synthesized NiCo alloy with melamine as the N-dopant (NiCo-M) exhibits admirable trifunctional activity with a low overpotential of 109 mV for HER and 295 mV for OER to achieve the benchmark current density of 10 mA cm-2 as well as superior ORR performance (onset: 0.96 V; E1/2: 0.856 V) in alkaline medium. More importantly, the NiCo-M electrocatalyst possesses excellent oxygen electrocatalytic activity with a small potential difference (ΔE) of 0.669 V, which is very close to that of the state-of-the-art Pt/C-RuO2 (0.651 V) couple. Such overwhelming performances stem from the presence of sufficient active sites and the formation of a unique N-doped graphitic CNT-like structure, which encourages electronic structure modulation as well as synergism for improved trifunctional electrocatalytic activities. Moreover, the catalyst shows outstanding electrochemical stability with a negligible decay of overpotential or E1/2 values after exhaustive CV cycles for HER, OER, and ORR. This is attributed to the encapsulation of the NiCo alloy into a N-doped multilayer graphitic carbon shell. The present study will offer a general approach to rationally synthesize robust trifunctional electrocatalysts required for fabricating energy-efficient electrochemical devices.