Intrinsic swelling behavior of free-standing nanoporous ionomer-bound carbon films

Nanoporous ionomer-bound carbon films are frequently operated in the wet environments when they applied as electrode materials in advanced devices, such as polymer electrolyte membrane fuel cells, and electro-active polymer actuators. However, the intrinsic swelling behavior is still concealed because of the challenge in obtaining the film in free-standing form due to its inherent brittleness. Here, we report the pure swelling characteristic of the film under various temperature and humidity by attaining the free-standing film. Mismatched strain is adopted for the separation of the films from the coating substrates. The swelling strain of the free-standing films is measured in-situ by a digital image correlation method. Simultaneously, the electrical resistance is measured by a digital multimeter and it is correlated with the microstructural alteration caused by the swelling. Macroscopic swelling is discovered in contrast to the conventional knowledge that the electrode is dimensionally stable due to the absorption of ionomer's swelling into pore spaces. The nanoporous electrodes demonstrate in-plane swelling of 1-4% at 90 %RH depending on the weight fraction of ionomer. In spite of the macroscopic swelling, the ionomer 30 wt% containing electrode is electrically stable but the ionomer 80 wt% electrode shows 10 times increased electrical resistances. The correlated characteristics reveal that the film has a structural transition from decoupling to coupling between the macroscopic swelling and the electrical resistance depending on the ionomer's binding structure.