Grain‐Boundary Engineering for Electrocatalysis: From Precise Structural Analysis to Reaction‐Pathway Regulation

Amid efforts toward carbon neutrality and rapid renewable energy development, CO 2 /CO electroreduction and water electrolysis have emerged as critical pathways for carbon resource recycling and green hydrogen production. However, CO 2 and CO electroreduction reaction (CO 2 RR/CORR) remain limited by complex multielectron‐transfer pathways, insufficient product selectivity, and poor stability at high current densities. Meanwhile, the hydrogen and oxygen evolution reactions (HER/OER) face challenges associated with noble‐metal dependence, sluggish kinetics, and structural instability. Grain boundaries are interfacial defects that can feature low‐coordinated atoms, local strain, and reconstructed electronic structures, thus offering a promising platform for designing catalysts with enhanced activity, selectivity, and stability. This Review examines the fundamental concepts, classifications, and structure‐activity relationships of grain boundaries. It further outlines synthesis strategies for grain‐boundary‐rich materials and discusses advanced characterization approaches ranging from morphological identification and GB quantification to in situ/operando tracking. The discussion subsequently focuses on the applications and catalytic mechanisms of grain boundaries in the CO 2 RR, CORR, HER, and OER. Representative mechanisms involve regulating intermediate adsorption, promoting C–C coupling, optimizing charge transfer, stabilizing active valence states, and improving the reaction microenvironment. By integrating synthesis, characterization, catalytic applications, and mechanistic insights, this Review provides a theoretical framework for the precise design, mechanistic understanding, and practical deployment of grain‐boundary‐based catalysts.