Highly Conductive Polybenzimidazole Membranes at Low Phosphoric Acid Uptake with Excellent Fuel Cell Performances by Constructing Long-Range Continuous Proton Transport Channels Using a Metal-Organic Framework (UIO-66).

Abstract

Phosphoric acid (PA)-doped polybenzimidazoles generally require high PA doping levels to achieve high conductivity as high-temperature proton exchange membranes. However, high PA doping levels result in a significant decrease in the mechanical properties of and PA leaching from the membranes. Herein, a Zr-based metal-organic framework material (UIO-66) was introduced into poly[2,2'-(p-oxydiphenylene)-5,5'-benzimidazole] (OPBI) membranes. The composite membranes exhibited long-range continuous proton transport channels when the mass ratio of UIO-66 to OPBI was ≥30 wt %. The long-range continuous proton transport channels endowed the composite membranes with high proton conductivity at low PA doping levels. When the doping of UIO-66 in the OPBI membrane reached 40 wt %, the membrane exhibited the highest proton conductivity (0.092 S cm-1, at 160 °C) at a low PA uptake (73.25%), while the conductivity of the pristine OPBI membrane was 0.050 S cm-1 with a high PA uptake (217.43%). Additionally, in the oxyhydrogen fuel cell test, 40%UIO-66@OPBI membranes exhibited excellent fuel cell performance with a peak power density of 583 mW cm-2 at 160 °C, which is 50% higher than that of the pristine OPBI membrane (374 mW cm-2). A single cell based on 40%UIO-66@OPBI also demonstrated good durability and could remain at about 600 mV after 500 h of operation under a constant load of 200 mA cm-2.