Sol-Gel SiO 2 -Polymer Hybrid Heteropoly Acid-Based Proton Exchange Membranes
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Sol-Gel SiO2-Polymer Hybrid Heteropoly Acid-Based Proton Exchange Membranes F. J. Pern,1 and J. A. Turner,1 Fanqin Meng,2 and A. M. Herring2 1
Hydrogen and Electricity, Systems and Infrastructure Group, National Renewable Energy Laboratory, Golden, CO 80401, USA. 2 Department of Chemical Engineering, Colorado School of Mines, Golden, CO 80401, USA.
ABSTRACT Heteropoly silicotungstic acid (STA)-based, cross-linked hybrid proton exchange membranes for intermediate-temperature (80o-120oC) fuel cell applications were fabricated by incorporating STA in a host matrix of binding silane, SiO2 sol gel, ethylene methacrylate copolymer containing glycidyl methacrylate groups (PMG), and molecular cross-linker. The STA loading level relative to the sum of the PMG and a cross-linker exceeded 100 weight%. Upon curing at 145o under pressure, the composite membranes were nearly 100% cross-linked and were flexible, showed high thermal and chemical stability against Fenton’s reagent, and exhibited a break-in behavior during cyclic voltammetric (CV) scans. The best proton conductivity, which was obtained from CV results, decreased from ~8-15 mS/cm at 80oC and 100% relative humidity (RH), to ~1.5-2 mS/cm at 100oC and 46% RH, and ~0.25-0.8 mS/cm at 120oC and 23% RH.
INTRODUCTION Current sulfonic acid (SFA)-based proton exchange membranes (PEMs) such as Nafion® films perform well in fuel cells with a proton conductivity of 0.1 S/cm when humidified sufficiently with moisture at temperatures under 80oC. Their performance decreases as the temperature increases and humidity level decreases, which can severely limit their use. A great deal of effort has been invested in recent years to develop PEMs that can perform at higher temperature and low relative humidity, such as 120oC and 25% RH, in automotive applications to meet the requirements as discussed by Wieser [1]. The general approaches are either to improve the Nafion materials [2,3] or modify the Nafion with different oxides, heteropolyacids (HPA), or solid acids [4-6]. Other materials such as H3PO4-doped PBI or sulfonated polymers are also being developed [7]. Heteropolyacids are known to have high proton conductivity and high thermal stability at temperatures >250oC, and their potential applications as hightemperature PEMs have received increased attention in recent years [8-10]. The primary objective of this work is to continue our previous effort [11] to develop low-cost, highperformance, HPA-based, cross-linked non-Nafon PEM, as opposed to the physical blending of HPA in a polymer host, which can be subject to water leaching. This paper presents our recent advances in the formulation, weight loading of HPA, mechanical strength, thermal and chemical stability, and proton conductivity of the hybrid composite membranes developed. EXPERIMENTAL DETAILS Silicotungstic acid (STA) of Keggin-type H4SiW12O40 (W12-STA) was purchased from Aldrich, and lacunary H8SiW11O39 (W11-STA) and K8SiW10O36 (KW10-STA) were synthesized
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by literature method [12, 13]. Gamma-m
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