Determination of Anion Transference Number and Phosphoric Acid Diffusion Coefficient in High Temperature Polymer Electrolyte Membranes

Abstract

The passage of an electrical current through phosphoric acid doped polymer membranes involves parasitic migration of the acid, which imposes a critical issue for long-term operation of the high temperature polymer electrolyte membranes fuel cell (HT-PEMFC). To elucidate the phenomenon, a three-layered membrane is constructed with embedded micro reference electrodes to measure phosphoric acid redistribution in a polybenzimidazole based membrane. Under a constant load, a concentration gradient develops due to the acid migration, which drives the back diffusion of the acid and eventually reaches a steady state between migration and diffusion. The acid gradient is measured as a difference in local ohmic resistances of the anode-and cathode-layer membranes by electrochemical impedance spectroscopy. The phosphoric acid diffusion coefficient through the acid doped membrane is about 10 −11 m 2 s −1 , at least one order of magnitude lower than that of aqueous phosphoric acid solutions. The anion (H 2 PO 4 − ) transference number is found to range up to 4% depending on current density, temperature and atmospheric humidity of the cell, implying that careful control of the operating parameters is needed in order to suppress the vehicular proton conduction as a degradation mitigation strategy.

Transference Number Determination in Poor-Dissociated Low Dielectric Constant Lithium and Protonic Electrolytes

“…The observed discrepancy can be here obviously attributed to the inherent incompatibility of the method with the non-ideal associated systems. In addition to that, the combination of the said approach with the theoretical considerations was presented in [ 68 ] where it was applied to m-PBI membranes doped with the phosphoric acid operating in elevated temperatures. Whereas no direct determination of t + was described here the diffusivity of the phosphoric acid moieties, and therefore, anions were determined as a function of the water vapor pressure for two different operational temperatures of the material.…”

From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides

“…295,296 Moving away from this range leads to an increasing contribution from vehicular migration of phosphoric oxoacid species in electric fields, and ultimately redistribution of the acid. [297][298][299][300][301] From a technological point of point, the redistribution of the acid is a formidable challenge and represents a major degradation mode when the H3PO4-PBI system is used as electrolyte in fuel cells. 5,302 Theoretical studies by means of density functional theory calculations suggest that headhead coupled polybenzimidazoles binds the acid stronger, which would reduce the acid mobility, 303 but it remains to be confirmed experimentally.…”

Characterization methodology for anode starvation in HT-PEM fuel cells

“…Fuel starvation, which refers to hydrogen undersupply (H 2 starvation), occurs during startup and shutdown due to the rapid load variation or as a consequence of inhomogeneous H 2 distribution in a single cell or between assemblies of cells [17]. In addition, an uneven distribution of phosphoric acid in the the HT-PEM FC [18,19], during MEA manufacturing and/or the activation process, can also flood the catalyst and results in local hydrogen undersupply [20].…”