Accurate measurement of the through-plane water content of proton-exchange membranes using neutron radiography

Abstract

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeder's paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. At open circuit and during fuel cell operation, the measured water content indicates that the membrane is operating between the vapor- and liquid-equilibrated states.

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

“…50 For chemical-potential driven flow, there is still debate over the values of the transport coefficients and the possible existence of a humidity-dependent interfacial resistance in the membrane, where there is experimental support on both sides of the issue. [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68] The value of the electro-osmotic coefficient also varies significantly in literature, [69][70][71] and since it scales with current density it plays a critical role in predicting the water balance. Given the above difficulties, many parametric models use an empirical effective electro-osmotic coefficient.…”

“…134 While the above approach predicts the doubling of the Tafel slope, 135,136 it assumes that the adsorbates are site-blockers and that the first electron transfer step is the RDS. To relax these assumptions, a double-trap kinetic model was developed by Wang et al 130,131 In this model, the kinetics are explained with four elementary reaction [66] any of which can become the RDS depending on the potential. Schematically, these elementary reactions represent two adsorption pathways that proceed through: 1) dissociative adsorption (DA) and 2) reductive adsorption (RA):…”

“…As mentioned, there is still a lot of debate over the functional forms of the transport properties of the membrane, as well as the existence of a possible interfacial resistance. [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68]92,93 To account for such an interface, the membrane water-uptake boundary condition is altered to include a mass-transfer coefficient instead of assuming an equilibrium isotherm directly N = k mt (a in − a out ) [40] where in and out refer to the water activities directly inside and outside of the membrane interface (which can be defined from FloryRehner equation in the polymer 94 and the relative humidity in the gas phase, respectively) and k mt is a mass-transfer coefficient that can be a function of local conditions (e.g., humidity, temperature, gas-velocity, etc.). 51,52,54,58,61,62,[64][65][66][67][68]92,93 This approach is the same as including a surface reaction (e.g., condensation) at the membrane interface, [95][96][97] and is not as often used in modeling as perhaps it should be.…”

“…In fact, only a few models assume nonequilibrium [95][96][97] even though, as mentioned there is thought to be an interfacial resistance toward at least water-vapor transport [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68]92,93 and assumingly transport of other species as well. Through-plane conductivity results are inconclusive since those done in vapor normally require a correction for interfacial contact resistance 220 (which can mask any interfacial resistance), and in liquid one does not observe an interfacial resistance.…”

“…54,62,67 The thought is that the morphology of the interface may change depending on its environmental contact. [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67]89,90 However, this contact is confused in the case of the catalyst layer with the membrane since the membrane may be in contact with liquid, vapor, and/or catalyst-layer ionomer. Thus, the question then is how to treat such a complex interface.…”