Schroeder's paradox in proton exchange membrane fuel cells: A review

Schroeder's paradox discovered by Schroeder in 1905 refers to the phenomenon that polymers have different maximum water uptake in the liquid and saturated vapor phases. For more than a hundred years, people have often debated whether this phenomenon conforms to thermodynamics. As proton exchange membrane fuel cell (PEMFC) gradually becomes a promising renewable energy utilization device, its impact on the physical properties of the proton exchange membrane has been studied widely. This paper reviews the theory and experiments on Schroeder's paradox over more than 100 years, especially the exploration of perfluorosulfonic acid membranes and PEMFCs in recent decades. Since membrane water content determines the operational performance of the PEMFC, this paper discusses and analyzes the effect of Schroeder's paradox on the PEMFC performance, including mechanical properties, electrical conductivity, and water transport mechanism. The effect of this phenomenon on the non-equilibrium operation of PEMFC has been highlighted, such as cold start-up, because of the different properties of membranes in contact with liquid water and air. This review gives an introduction to critical aspects of Schroeder's paradox to serve governments and organizations to promote the application of PEMFC in different regions.