Incorporating flexible sulfonate-imidazolium ion pair-functionalized MOF in cross-linked polyvinyl alcohol membrane for achieving superprotonic conduction

Abstract

Designing high-performance metal–organic frameworks (MOFs) based proton exchange membranes for fuel cells simultaneously with a high-density proton carrier and continuous hydrogen-bonded networks remains a challenge. The present study has endeavored to construct a crystalline Cu-MOF with chemically stable based on flexible sulfonate-imidazolium ion pair ligand in which flexible sulfonic acid chains orderly anchoring on the channel wall. Moreover, the cross-linked polyvinyl alcohol (CL-PVA) was choosing as matrix to enhance the practicality of Cu-MOF synergistically in fuel cells due to its hydrophilic and hydrophobic structure distribution. The CL-PVA/Cu-MOF-X (X = 5, 10, 15, 20 wt%) composite membranes exhibit strong water affinity and thus significantly enhanced water uptake which also along with good mechanical properties. The highly hydrophilic hydroxyl groups in CL-PVA and the sulfonic acid groups in Cu-MOF form a continuous hydrogen-bonded pathway, which result to the superprotonic conduction. Among them, CL-PVA/Cu-MOF-15 shows the highest proton conductivity of 2.4 × 10-2 S cm−1 but the smallest activation energy Ea (41 kJ/mol) at 80 °C and 90 % RH, indicating more easily transportation of H+. The single-cell power density of CL-PVA/Cu-MOF-15 is up to 49.3 mW cm−2, which will open a new avenue for the practical application of proton-conducting MOFs in proton exchange membrane fuel cells.