UBC Theses and Dissertations
Water transport in proton exchange membranes Castañón, Tatiana Romero
Water transport across Nafion membranes was investigated under activity gradients at atmospheric pressure. The activity gradients across the membrane were controlled by exposing one side of the membrane to dry gas under laminar flow, while maintaining liquid or vapour equilibrium with water on the other side of the membrane. The measurements were made under steady state and transient conditions. The main objective was to identify the rate limiting mechanism among the three major water transport processes in Nafion: diffusion in the bulk, and sorption across both interfaces. The proposed hypothesis was to represent the overall water transport across the membrane as the sum of resistances across the membrane bulk and interfaces. The experimental implementation required new hardware and techniques. A dual chamber cell with temperature, humidity, and pressure control was designed with a gated valve to control the initial starting time for transient measurements. Unlike previously reported work, this design enabled the individual control of membrane thickness, temperature, pressure, relative humidity, and dry gas flow rate in each chamber. The ability to control these variables made the experimental results amenable to theoretical simulations. Two phenomenological models were proposed to separate the contributions of bulk transport and sorption processes. The Varying Diffusion Coefficient model (VDC) was a preliminary effort to generate mass transport coefficients. The Vaporization-Exchange Model (VEM) provided an approximation for the steady state and transient water transport data through the definition of a novel boundary condition that describes the kinetics at the interface, while diffusion is described by Fick’ s law. The VEM yielded interfacial water transport rates:kv= 0.75 cms⁻¹ for liquid-, and kv= 0.63 cms⁻¹ for vapour-equilibrated membranes. Such results contribute to fill the gap for the membrane interfacial kinetics in the fuel cell literature. The analysis with the VEM revealed that interfacial water transport became rate limiting at membrane thickness below Ca. 100 μm. Analysis of transient data with VEM generated bulk diffusivity coefficients: D2- 7x 10⁻¹° m²s⁻¹, for liquid equilibrated membranes at 30-70°C, which agreed with literature data. A case study is presented for Nafion-Si0₂ composite membranes to study the effect of the membrane water content, by addition of silicon dioxide to Nafion membranes. The status of water in the membrane was characterized with long-established techniques, such as vapour sorption, scanning calorimetry, and water uptake measurements. Information from these measurements was coupled with measurements from the dual chamber cell. Experimental results indicated a threshold at 16 wt% of silicon dioxide above which the water transport properties of the composite differed significantly from additive-free Nafion. Analysis with the VEM suggested that the addition of the composite produced structural changes to the polymer matrix.
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