Multilayer Structure Membranes with Sulfonated Hydrocarbon Methanol Barrier for Direct Methanol Fuel Cells

Sulfonated poly(ether ether ketone) (SPEEK) membranes were synthesized and investigated in direct methanol fuel cells (DMFCs). Single-layer SPEEK membrane with a sulfonation degree in the range from 40 to 60% showed good properties for DMFC applications. DMFC performance of pure SPEEK membranes with higher sulfonation degree (60%) decreased as temperature was increased from 60 to due to significantly increased methanol crossover, which was induced by severe membrane swelling in the methanol solution. Multilayer structure membranes, composed of a central methanol barrier layer and two more proton conductive surface layers, were developed through a solution-casting procedure to suppress methanol crossover in DMFCs. A thin layer of SPEEK with a lower sulfonation degree (41%) and lower methanol permeability was used as the central methanol barrier layer; higher sulfonation degree (60%) SPEEK or recast Nafion membranes, with higher proton conductivity, were applied on the two outer surfaces as the conductive layers. The methanol barrier layer effectively slowed down methanol crossover, while the proton-rich conductive layers on the membrane surfaces facilitated proton transfer at the membrane/electrode interface. The DMFC performance was greatly improved by using the multilayer structure membrane. Especially at DMFC, significant cell voltage gain was obtained by applying a thick 41% sulfonation degree SPEEK methanol barrier between two pieces of 60% sulfonation degree SPEEK layers, compared with that of the same thickness single-layer SPEEK membrane with 60% sulfonation degree.