Summary PEM Operation

Two major groups of performance models have been proposed. The first group considers the membrane as a homogeneous mixture of ionomer and water. The second group involves approaches that consider the membrane as a porous medium. Water vapor equilibrates with this medium by means of capillary forces, osmotic forces resulting from solvated protons and fixed ions, hydration forces, and elastic forces. In this scenario, the thermodynamic state of water in the membrane should be specified by (at least) two independent thermodynamic variables, namely, chemical potential and pressure, subdued to independent conditions of chemical and mechanical equilibrium, respectively. The homogeneous mixture model is the basis of the so-called 

The most complete description of variations in local distributions and fluxes of water is provided by combination models, which allow for concurrent contributions of diffusion and hydraulic permeation to the water backflux that competes with the electro-osmotic drag. The main conclusions from these models for membrane water management under operation are 

In summary, design, integration and performance optimization of advanced PEM needs systematic experimental-theoretical efforts to focus on studying the effects of chemical modifications of the base ionomer, thereby, increasing the ion exchange capacity (to improve transport properties without sacrificing stability) and reducing thickness. 

---