Constructing Heterogeneous Metal Nodes in Metal-Organic Framework Lamellar Membranes for High Proton Conduction in Fuel Cells.

Abstract

Precisely defining proton transport channels and regulating the chemical microenvironment of proton carriers are critical for high-performance proton exchange membranes (PEMs). Beyond optimizing carrier type and density, modulation of electron density distribution offers an underexplored route to enhance intrinsic proton conductivity. Herein, a series of ZnM-BDC-COOH nanosheets are synthesized by partial substitution of Zn²⁺ with a second metal ion in Zn-BDC frameworks and subsequent -COOH modification. The optimized ZnCu-BDC-COOH achieves a high intrinsic proton conductivity of 361.5 mS cm^-1 over 16 times that of the pristine Zn-BDC (22.2 mS cm^-1) at 80 °C and 98% RH using comb electrodes. Density functional theory and molecular dynamics simulations reveal that the introduced Cu²⁺, with higher electronegativity, withdraws more electrons from adjacent -COOH groups and Zn ions, generating asymmetric electron cloud distribution within the O-H bonds and the heterogeneous Zn₃CuO(COO)₆ nodes. This electron asymmetry facilitates rapid H⁺ release and establishes a strong local potential gradient that lowers the proton transport barrier. The resulting ZnCu-BDC-COOH lamellar membrane achieves a peak power density of 0.92 W cm^-2, outperforming Nafion-117 (0.28 W cm^-2), and maintains good durability over 100 h of open-circuit voltage (OCV) under 80 °C and 100% RH in fuel cells.