Tuning Mass Transport in Electrocatalysis Down to Sub‐5 nm through Nanoscale Grade Separation

Abstract Nano and single‐atom catalysis open new possibilities of producing green hydrogen (H 2 ) by water electrolysis. However, for the hydrogen evolution reaction (HER) which occurs at a characteristic reaction rate proportional to the potential, the fast generation of H 2 nanobubbles at atomic‐scale interfaces often leads to the blockage of active sites. Herein, a nanoscale grade‐separation strategy is proposed to tackle mass‐transport problem by utilizing ordered three‐dimensional (3 d ) interconnected sub‐5 nm pores. The results reveal that 3 d criss‐crossing mesopores with grade separation allow efficient diffusion of H 2 bubbles along the interconnected channels. After the support of ultrafine ruthenium (Ru), the 3 d mesopores are on a superior level to two‐dimensional system at maximizing the catalyst performance and the obtained Ru catalyst outperforms most of the other HER catalysts. This work provides a potential route to fine‐tuning few‐nanometer mass transport during water electrolysis.