Characteristics of proton conducting membrane based on crosslinked sulfonated polyimide

Abstract

Fuel cells have received enormous attention as novel electrical energy conversion systems applicable to automotive propulsion, distributed power generation, and power source of portable electronics. There are several types of fuel cells clarified by operation temperature or the electrolyte used. Direct methanol fuel cell (DMFC) is one of them using proton exchange membranes as the electrolyte and operated at low temperature (below 100℃). DMFC has been developed as a power source for portable electronic devices but there are major problems that must be solved for successful commercialization such as slow reaction and methanol crossover. Research objective of this paper is to develop an alternative membrane that can substitute for Nafion, the current state-of-the-art proton exchange membrane. To obtain membranes that have significantly reduced methanol permeability and water transport, crosslinked membranes based on sulfonated polyimide was prepared. Crosslinked membranes, c-sPI-ODA and c-sPI-PODA, showed different properties from non-crosslinked membranes, nc-sPI-ODA and nc-sPI-PODA. Crosslinking improved physical properties such as mechanical properties and thermal stability. Also, crosslinked membranes showed decreased water uptake and this resulted in reduction of methanol crossover. Proton conductivity of both noncrosslinked and crosslinked membranes showed similar values except for the case that reduction of water uptake was significant. This is because there is limitation in the effect of water uptake on proton conductivity. In unit cell operation using crosslinked membranes, the OCV increased due to reduced methanol crossover but cell performance is poor due to increased cell resistance.