π-Conjugated Aligned Polar [CO₃] Triangulars Synergizing Hydroxylation Enables Stress-Enhanced Geometric-Electronic...

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 Bi₂O₂CO₃ (BOC) synthesized scalably, characterized by noncentrosymmetric (NCS) planar triangular [CO₃] groups and surface hydroxyls, which applies as a robust piezocatalyst for hydrogen evolution. The aligned NCS planar [CO₃] 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 H₂ 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 H₂ 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.