Morphology-/microstructure-regulated nickel/nitrogen-co-doped monodisperse hollow carbon spheres for controlled electrochemical CO2 reduction to syngas.

Monodisperse carbon materials have attracted increasing attention in the fields of catalysis, adsorption, and energy storage. However, the preparation of monodisperse microstructure-controlled carbon spheres still faces tremendous challenges due to the complexity of involved polymerization/carbonization processes and the difficulty of controlling high monodispersity and regulating microstructures, which are vital for ensuring a uniform packing, determining consistent physical and chemical properties and targeting applications. Here, we report an approach to address these issues by co-doping nickel and nitrogen into an aminophenol-formaldehyde resin (APFR) polymer framework. Such Ni,N-co-doping regulates the morphology and microstructure of the obtained monodisperse carbon spheres. The study elucidates how nickel nitrate and ammonia affect the morphology and microstructure of APFR spheres and the retention of the APFR shape after carbonization. The study also unveils the morphological and microstructural evolution from smooth to rough surface, solid to hollow structure, micropore to hierarchical structure, as well as the changes in sphere size and surface area. Moreover, a synergistic microstructural transformation mechanism of Ni2+ coordination and NH3 regulation is inferred. The resulting optimized Ni,N-co-doped monodisperse hollow carbon spheres are used as a catalyst for electrochemical CO2 reduction to syngas. The results show that highly dispersed Ni particles embedded in N-doped hollow carbon spheres display a high electrochemical performance for CO2 reduction to CO, with a tunable CO/H2 ratio of 0.35-3.48 applying a potential range of -1.1 to -0.8 V (vs. RHE), and long-term stability for CO selectivity. This work describes a promising candidate for methanol conversion, syngas fermentation and Fischer-Tropsch reactions by syngas.