π ‐Conjugated Aligned Polar [CO ₃ ] Triangulars Synergizing Hydroxylation Enables Stress‐Enhanced Geometric–Electronic Asymmetries for Hydrogen Piezoevolution

Piezocatalytic water splitting offers a sustainable route for hydrogen evolution, yet is challenged by weak polarity and slow charge separation kinetics under stress. Herein, we report strong polar hydroxylated Bi 2 O 2 CO 3 (BOC) synthesized scalably, characterized by noncentrosymmetric (NCS) planar triangular [CO 3 ] groups and surface hydroxyls, which applies as a robust piezocatalyst for hydrogen evolution. The aligned NCS planar [CO 3 ] triangulars in BOC enable oriented accumulation of dipole moments to produce strong spontaneous polarization, and the intrinsic delocalized π ‐electrons within these structural units construct a conjugation freeway that minimizes charge migration resistance. Further external mechanical stress triggers a highly anisotropic lattice response; specifically, compression along the b ‐axis induces extreme geometric and electronic asymmetries that amplify the interlayer internal electric field (IEF) for charge separation. When synergistically coupled with surface hydroxylation, this stress‐induced structural distortion significantly lowers the work function and interfacial kinetic barrier for electron escape. Consequently, the hydroxylated BOC catalyst achieves an ultrahigh piezocatalytic H 2 evolution rate of 3055 µmol·g −1 ·h −1 and a record mechanical‐to‐hydrogen (MTH) energy conversion efficiency of 0.31% in pure water. It also maintains robust H 2 evolution from real‐world aquatic matrices, including rainwater, seawater, and antibiotic wastewater. This work establishes a polar group design‐oriented paradigm for exploiting advanced piezocatalysts.