Porosity Engineering Within Ni─N─C Hollow Spheres for Ampere-Level CO₂ Reduction Electrocatalysis

CO₂ electroreduction typically requires large overpotentials to sustain high reaction rates, which at current densities approaching 1 A cm^-2 inevitably intensify competition from the hydrogen evolution reaction (HER), making it difficult to simultaneously sustain high product selectivity (>95%). In this study, hollow Ni─NC (H-Ni-NC) was synthesized using nanoscale silica sphere templates and atomic Zn as a sacrificial pore-former within the carbon shell. The optimized H-Ni-NC achieves a current density of -1.0 A cm^-2 with over 95% CO Faradaic efficiency in a flow cell. The high performance was attributed to the hollow sphere architecture enriched with gas-permeable through-pores. Notably, the Zn dosage selectively modulates the shell through-porosity, without altering hollow sphere macrostructure or the active-site structure, thereby enabling a systematic investigation of porosity effects. Finite-element simulations provide a qualitative framework suggesting a trade-off between reactant transport and active surface area with increasing porosity in hollow spherical catalysts. This work underscores that engineering of the nanoscale mass-transport environment surrounding active sites plays a critical role in designing high-efficiency electrocatalysts.