Efficient proton conduction of a cerium(iv) metal–organic framework built using 3,5-pyrazoledicarboxylic acid

Abstract

Proton-conducting materials are crucial for advancing energy conversion and storage technologies; however, developing candidates with both high conductivity and structural stability remains a significant challenge. Herein, we report a comprehensive study on DUT-67-Ce-PZDC (1), a cerium(IV)-based metal–organic framework (MOF) constructed from 3,5-pyrazoledicarboxylic acid (H₂PZDC), focusing on its structural features and proton-conducting properties. 1 forms a 3D porous framework with reo topology, assembled from hexanuclear [Ce₆O₄(OH)₄]¹²⁺ clusters bridged by PZDC²⁻ ligands, creating interconnected cubic and octahedral cages. Comprehensive characterization studies confirm that it retains structural integrity in water and at temperatures up to 260 °C. Alternating current (AC) impedance measurements reveal significant temperature- and humidity-dependent proton conductivity, with an optimal value of 1.96 × 10⁻² S cm⁻¹ at 90 °C and 98% relative humidity (RH), surpassing those of most reported cerium-based MOFs and comparable to those of state-of-the-art proton-conducting MOFs. Activation energy calculations (E_a = 0.57–0.71 eV) indicate proton conduction follows the vehicle mechanism, facilitated by hydration ions (e.g., H₃O⁺) and continuous H-bonded networks within the porous framework. The synergistic effects of high-valent Ce⁴⁺ ions (enhanced coordination stability and polarization), dual-functional PZDC²⁻ ligands (proton-binding sites), and interconnected porous structure (water retention and proton transport channels) endow 1 with outstanding performance. This study highlights the untapped potential of Ce⁴⁺-based MOFs as high-performance proton conductors and provides a design paradigm for efficient proton-conducting materials.