Active layer materials coated with Teflon AF nano-films deposited from solutions in supercritical CO2 for fuel cell applications

In the present work we suggest a new approach to the thin-film design of active layers for electrodes of fuel cells with phosphoric acid electrolyte in a polymer matrix. A fluoropolymer binder is introduced into common Pt@C active layer materials using supercritical (SC) CO2 as a solvent. Unique wetting properties of this non-hazardous and environmentally friendly solvent allow one to deposit highly uniform thin fluoropolymer films on dispersed carbon supports. As a result, well-percolated gas-permeable fluoropolymer phases are produced in active layers already at comparatively small polymer loadings. Teflon AF 2400 was chosen as a stable high-molecular-weight fluoropolymer soluble in SC CO2 with high oxygen permeability and high Tg value. Fluoropolymer-containing active layer materials prepared via the SC CO2 deposition routes were studied by means of cyclic voltammetry and were tested in operating fuel cells using steady state voltammetry and electrochemical impedance spectroscopy. Polarization curves of operating fuel cells indicate that the optimal content of deposited from SC CO2 fluoropolymer in active layer is about 3-5%. Results of impedance spectra fitting yield information used to explain the detected values of optimal loading.