Correlating the In‐Plane and Through‐Plane Proton Conductivity and Equilibrium Water Content of State‐of‐the‐Art Proton Exchange Membranes

Proton conductivity in proton exchange membranes (PEMs) critically determines the efficiency of electrochemical devices. Current studies have primarily focused on in‐plane conductivity at fully saturated hydration levels, with limited attention paid to in‐plane conductivity across all hydration levels and through‐plane conductivity of novel PEMs. To address this gap, the in‐plane conductivity is comprehensively measured across all hydration levels, while a method which effectively minimizes experimental uncertainties is developed to measure the through‐plane conductivity of four high‐performance PEMs. The anisotropy of four polytetrafluoroethylene (PTFE)‐based PEMs is evaluated by the ratios of through‐plane to in‐plane conductivity. Both conductivities increase with hydration level. Due to compression effects, PEMs with thin PTFE‐layers (e.g., the Gore series) show a relatively slower increase in through‐plane conductivity with rising water activity. In contrast, the thick PTFE‐layer PEM (Dongyue‐15) promotes interconnected ion‐conductive pores and exhibits superior compression resistance, resulting in exceptional through‐plane conductivity (18.94 S m −1 at 70 °C and 1 water activity). Empirical equations for in‐plane conductivity, based on equilibrium water content and temperature, are corrected using anisotropy ratios to obtain through‐plane conductivity equations. This study offers key insights into the experimental characterization of heterogeneous PEMs and establishes a theoretical basis for water/thermal management analysis in electrochemical devices.