The Effect of Impurity Cations on the Transport Characteristics of Perfluorosulfonated Ionomer Membranes

Abstract

Ion and water transport characteristics of perfluorosulfonated ionomer membranes are investigated in the mixed cation form of H/Fe, H/Ni, and H/Cu systems. Nafion membranes, which were equilibrated with HCl/FeCl3, HCl/NiCl2, or HCl/CuCl2 mixed aqueous solutions of various mixing ratios, were prepared as test samples, and equilibrium and transport properties were measured systematically. Membrane cationic composition showed that trivalent cations had more affinity than divalent cations. Also larger valence cations caused less water content in the membrane. The membrane ionic conductivity was markedly influenced by counterions, and H+ mobility u H + was altered according to the nature of coexisting cations. In the presence of Cu2+, u H + increased from its inherent value, while in the presence of Fe3+, u H + decreased to a large extent, Ni2+ bringing about nearly no change in u H + . The ionic transference number of H+ was also influenced by coexisting cations in several ways. Despite the unique influence of impurity cations on the mobility of H+, the mobility of impurity cations was not affected by the presence of H+. The interaction between adjacent cationic species in the membrane ion exchange sites, although plausible in general for multivalence cations, appeared to be not specific due probably to the shielding of the cationic charge by water molecules or by sulfonic acid groups. The water transference coefficient t H 2 O as measured by streaming potential measurements showed unique changes with membrane ionic composition, and t H 2 O increased from 2.5 to over 13 by the presence of impurity ions. These impurity ions were found to result in more water molecules dragged than in the case of individual ions, when coexisting with the H+ ion. Overall, it was noted that the water molecules within the influence of impurity cations appeared to play a large role in the H+ movement in the membrane.

PEM fuel cell cathode contamination in the presence of cobalt ion (Co2+)

“…9 and 10, the membrane resistance can also increase in the presence of Co 2+ . In terms of this conductivity effect, Co 2+ may exchange with protons in the membrane, similar to other metals as reported by several researchers [23][24][25], and/or replace protons in the ionomer of the catalyst layer, both effects resulting in low ionic conductivity (R m ) and thus contributing to decreased cell voltage. For example, the AC impedance results in Fig.…”

Oxygen Reduction Reaction Activity of Carbon-Supported Pt-Fe, Pt-Co, and Pt-Ni Alloys with Stabilized Pt-Skin Layers

“…First, the contamination by the metal cations leads to decreases in the proton conductivity, oxygen solubility, and oxygen diffusion rate, resulting in a decreased ORR rate. 12 Second, some metal cations (specifically, Fe 2+ and Cu 2+ ) accelerate the formation of OH radicals from H 2 O 2 , which can degrade the polymer electrolyte and the gasket material; 12,13 H 2 O 2 is generated both via the two electron-reduction of O 2 (side reaction) at the cathode and via the reaction of crossover O 2 (through the membrane) with H 2 adsorbed at the anode. To mitigate the latter damage, the H 2 O 2 yield (the percentage of H 2 O 2 production rate with respect to that of the overall ORR) of the catalyst should be as low as possible.…”

Effect of Co2+ on oxygen reduction reaction catalyzed by Pt catalyst, and its implications for fuel cell contamination

“…The dissolution of Co 2+ may cause fuel cell contamination, leading to performance degradation. Similar to other metal ions [33][34][35],C o 2+ may exchange with H + in both the membrane and the ionomer of the catalyst layer, and/or adsorb on the Pt catalyst surface, and/or change the hydrophobicity of the catalyst layer and/or gas diffusion layer (GDL), resulting in low fuel cell performance.…”