Negative impact of poly(acrylic acid) on proton conductivity of electrospun catalyst layers

The electrospun catalyst layers (E-spun CLs) have attracted great attention recently owing to their decent performance and durability compared to conventional coated CL fabricated by scrape coating. However, proton transport in E-spun CLs is poorly understood, hindering further performance improvements. We unravel herein that the high molecular weight (MW) polymer used to form nanofibers, usually Poly(acrylic acid) (PAA), is a key factor affecting the proton conductivity. Nuclear magnetic resonance (NMR) unravels strong interaction between the ionomer and PAA. The proton conductivity of ionomer membranes with varying PAA content under different ionomer volume fractions and relative humidity (RH) was quantitatively assessed using the four-probe electrochemical impedance spectroscopy (EIS). E-spun CL exhibited lower proton conductivity than coated CL or pseudo catalyst layer (PCL) at the same ionomer volume fraction due to the detrimental effects of PAA. The proton transport tortuosity of E-spun CL increased with RH, likely attributed to the significant swelling ratio of PAA. The proton transport tortuosity of E-spun CL displayed less sensitivity to the ionomer volume fraction, which can be attributed to the homogeneous distribution of the ionomer within the nanofibers. Our work implies that the key to enhancing proton conductivity of E-spun CLs is to remove PAA completely, or to replace PAA with alternatives that would not impair proton transport.