Effect Of Additive Geometry On The Water Retention And Proton Conductivity Of Composite PEMs

Proton exchange membrane fuel cells (PEMFCs) are one of the most promisingelectrochemical energy conversion devices, because of its high power density, fastinitiation, high specific energy and so on. Recently, proton exchange membraneswithout fluorine have attracted great attention due to its low cost, low electro-osmoticdrag coefficient. However, low proton conductivity is one of main problems.Incorporating hydrophilic/hygroscopic inorganic additives, such as silica, intomembrane matrix is a common approach of improving the water-retention in PEMs.Spherical additives have generally been selected. The main advantages ofinorganic spheres are considered to be an increase in the thermal and chemicalstability of membrane, and water molecules. Fibrous particles can also be used inPEMs. However, it is difficult to say which is more effective because of the scarcityof definitive data.In this contribution, three types of PEM are prepared in this work: phosphoricchitosan (P-Chitosan), phosphoric chitosan membranes with proportional spheres(S-P-Chitosan) and phosphoric chitosan membranes with proportional fibers(F-P-Chitosan). The proton conductivities of those membranes range in the followingorder: S-P-Chitosan> P-Chitosan> F-P-Chitosan at low temperature (<100oC);F-P-Chitosan> P-Chitosan> S-P-Chitosan at high temperature (>100oC), due to thestate of water in membranes, when S-P-Chitosan adsorbs a large amount of free waterwhile F-P-Chitosan obtains more bound water. A mechanism depending onNernst-Einstein Equation and Arrhenius Equation was proposed. For those samples,there are four ways used as proton channels: delivered based on vehicle mechanismby free water through their migration; hopping in Grotthuss mechanism among freephosphates; hopping among bound water without the assistance of diffusible vehiclemolecules; hopping among the network of fixed phosphates by electrostaticinteraction. In addition, chains motion affects the rotation of the molecules in hoppingprocess more than in migration one. All results reveal that the geometry of the dopant has a large impact on theproton conductivity of PEMs at different temperature ranges; hence it is essential toselect the “shape” of the dopant in composite PEMs.