Date of Award
Doctor of Philosophy (PhD)
Anion exchange and bipolar membrane fuel cells generate electrical energy directly from chemical fuels and have attracted considerable interests as alternate power sources for large market applications, such as transportation (hydrogen fuel cells) and unmanned vehicles (sodium borohydride fuel cells). Anion exchange membrane (AEM), generally composed of a polymer with covalently tethered ionic groups, is the central component of the fuel cell serving as the electrolyte, conducting hydroxide ions from cathode to anode, where fast ionic conduction is directly related to power output. However, AEMs currently used in fuel cells (H2 fuel cells and sodium borohydride fuel cells) exhibit ion poor transport properties at operation conditions of fuel cells, such as low hydroxide ion conductivities at 80 °C. Another concern with the use of AEMs in fuel cells is their stabilities in strong alkaline environment. AEMs suffer Hoffmann elimination, where the cation is cleaved and a direct nucleophilic (SN2) reaction where the cation is completely cleaved too.This dissertation targets on solving these two major obstacles for using AEMs as electrolytes in alkaline electrochemical energy storage and conversion. To solve the first problem, a triblock copolymer-based polymer (polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene, SEBS) has been developed to synthesize AEMs with high ion conductivities by engineering a phase-separated morphology. To address the second problem, novel cation groups such as imidazolium-based cation and phosphonium-based cation have been incorporated in AEMs. Also, by attaching a six-carbon alkyl spacer between polymer backbone and cation group, alkaline stability of such AEM has been investigated.Once the degradation mechanism of AEMs is well understood and AEMs with high ion conductivities have been developed, this dissertation focuses on developing such AEMs electrolyte for different electrochemical systems such as direct sodium borohydride fuel cells (DBFCs) and redox flow batteries (RFBs).Besides the concern of ionic conductivities and alkaline stabilities of AEMs in alkaline environment, another obstacle for the development of DBFC is the crossover of fuels and oxidants. We demonstrate a pH-gradient-enabled macroscale bipolar interface (PMBI) to effectively separate the anolyte and catholyte of the DBFC. The PMBI configuration enabled significantly enhanced performance in a DBFC as compared to either an all-anion-exchange or an all-cation-exchange configuration (330 mA/cm2 at 1.5 V and 630 mWpeak/cm2 at 1.0 V). The PMBI-type electrodes provide a new and fascinating design to engineer fuel-cell membrane electrode assemblies (MEAs). The bipolar-interface configuration also holds significant applicability in the field of water electrolysis, where the membrane could separate an acidic H2-evolution cathode (a very fast reaction) from an alkaline O2-evolution anode (possible on non-Pt-group metals) in a system wherein the benefits of the best electrodes of present acidic/alkaline water electrolyzers are combined.RFBs are promising candidates for large-scale energy storage systems, since the capacity, power and energy density parameters can be designed independently, which facilitates a convenient way of modification even after installation. AEMs play an important role as separators in some of RFBs where a degree of reactant isolation is required between the anolyte and catholyte compartments. A primary goal for membrane development in RFBs is to limit the diffusion of active species, while maintaining high oxidative stability in related species. A vanadium-cerium redox flow battery (V-Ce RFB) employing SEBS-based AEM as the separator yields an energy efficiency of 86% at a current density of 50 mA/cm2 with a 10% drop in capacity over 20 charge/discharge cycles. In contrast, a V-Ce RFB using Nafion®212 as the separator has an energy efficiency of 80% and a 40% drop in capacity over 20 charge/discharge cycles. The observed capacity fade is primarily due to cation intermixing between the anodic and cathodic compartments – much better permselectivity has been obtained with the AEM separator. After 60 charge-discharge cycles (350 hours of operation), the ion exchange capacity and ionic conductivity of the AEM drops by about 20%. There has been no observed change in mechanical properties. The oxidative stability of the AEM has been evaluated ex situ by immersion in 1.5 M VO2+ + 3 M H2SO4 for 500 hours - the ionic conductivity remained constant over this timeframe.
Vijay K. Ramani
Pratim Biswas, Palghat Ramachandran, Peng Bai, Julio D'Arcy,