Investigation of Low-Temperature Proton Transport in Nafion Using Direct Current Conductivity and Differential Scanning Calorimetry

Abstract

The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membrane's water content per acid group, , and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above ϳ 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with . Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity.

Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

“…The water content within the membrane consists of chemically bound water, l B , and free water, l F [24,[40][41][42]51]. The water activity within the membrane is approximately equal to the mole fraction of the free water within the membrane, i.e.…”

“…This relationship is referred to as non-affine deformation and has been shown to occur for PFSA membranes by a number of studies in literature [11,[18][19][20][21][22]29,[33][34][35]. Despite the growing body of literature on the sorption behavior [7,8,10,24,34,35,38,40,[44][45][46][47][48][49][50] and nano-structural modeling and characterization [11,13,[17][18][19][20][21][22]28,29,31,[33][34][35][36]39,49,51,52] of PFSA ionomers, the relationship between the water uptake and nanostructure, temperature, mechanical properties and membrane pretreatment is not well established. Understanding of these issues requires a general, yet fundamental model, which is the subject of this study.…”

Modeling Water Management in Polymer-Electrolyte Fuel Cells

“…Mass-transport limitations increase due to both the presence of ice reducing the available open pore volume and product water build-up. Ionic conductivity is reduced due to the presence of ice in the membrane, 322 and the ORR is hindered both by the Arrhenius dependence on temperature (see equation 15) as well as reduced proton activity. 323 To counter the difficulties of cold start, system designers use both procedural strategies and materials design.…”

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…167 Achieving such a startup is difficult in practice due to potential flooding, sluggish reaction kinetics, durability loss, and importantly, rapid ice crystallization that is counteracted by the heating of the cell due to the reaction waste heat and ohmic heating. 168 Typically, the cell potential decays rapidly at low temperatures and/or high current densities due to ice formation at the reactive area of the cathode. [162][163][164][165][166]169,170 The traditional method to provide successful cold-start is to purge the system of water on shutdown, thus providing a sink in the membrane for the water production during startup and allowing for the heating of the stack before irreversible flooding and ice formation occur.…”