Ce Cation Migration and Diffusivity in Perfluorosulfonic Acid Fuel Cell Membranes

Abstract

A hydrogen pump experiment was utilized to simultaneously determine the migration and diffusivity of Ce ions in perfluorosulfonic acid (PFSA) ionomer membranes over a range of temperatures and relative humidities. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). Competing transport phenomena were decoupled by fitting XRF profile data with our previously-developed one-dimensional model, which was updated with improved conductivity and water uptake relations. Measured transport values showed good agreement with the Einstein relation and Okada transport theory, implying that conductivity can be used to estimate migration and diffusivity of cations in other cation/PFSA systems. Results presented here may be used to populate device-level models in order to further understand the effects of cation transport on fuel cell performance and durability to determine the mitigation controls for cation stabilization.

Morphology and Transport of Multivalent Cation-Exchanged Ionomer Membranes Using Perfluorosulfonic Acid–CeZ+ as a Model System

“…The microscale model used to specify conductivity is based on a molecular description of Stefan–Maxwell friction coefficients that allows self-consistent calculation of any other membrane transport properties . This model has been validated against various systems, and the gradients predicted (Figure S6) match both the direction and shape of those experimentally observed using microprobe and benchtop XRF. , With this in mind, their precision would be refined through further validation, but their outputs are qualitatively accurate. Similar to the conductivity model, the experimentally determined values for κ and ϕ w and the microscale models for α Ce ∞ and t Ce were used to solve eq for τ at each doping and hydration level.…”

“…63 This model has been validated against various systems, 63 and the gradients predicted (Figure S6) match both the direction and shape of those experimentally-observed using microprobe and benchtop XRF. 27,30 With this in mind, their precision would be refined through further validation, but their outputs are qualitatively accurate. Similarly to the conductivity model, the experimentally-determined values for and and the microscale models for Ce ∞ and Ce were used to solve Equation ( 14) for at each doping and hydration level.…”

“…This approach contrasts previous transport studies, where Ce migration coefficients were taken to be independent of f Ce . 3,27,30 At high ϕ w (blue circles), as f Ce increases, both α Ce and t Ce increase because there are additional Ce ions to diffuse and carry current. At low ϕ w (orange squares), α Ce is significantly reduced relative to the high ϕ w case as the hydrophilic network becomes more tortuous and impedes transport.…”

“…Due to the relatively high concentration of H 2 O 2 during PEFC operation and PFSA acidity, modeling suggests that >99% of the Ce ions exist in the hydroxyl radical-scavenging trivalent ion form . These ionic species, though, can transport within the MEA due to gradients in ionic potential, − ion concentration, − and ionomer hydration. , Other factors, such as membrane compression , and degradation/detachment of cation-exchanged polymer fragments, , could also cause undesired cation movement.…”

Doped Ceria Nanoparticles with Reduced Solubility and Improved Peroxide Decomposition Activity for PEM Fuel Cells

“…15 However, when ceria NPs are added to the acidic PFSA membrane or catalyst layer ionomer, Ce dissolves and diffuses through the MEA after exposure to humidified reactant gases, 18,19 resulting in effects similar to direct Ce ion exchange. 20 Upon the introduction of water, concentration, and potential gradients present during cell operation, these Ce ions can mobilize, [21][22][23][24] which can generate losses by diminishing proton and O 2 transport in the cathode CL. [25][26][27][28] The mobilization of Ce 3+ may also cause local depletion of the radical scavenger, leaving ionomer regions vulnerable to radical attack.…”