Abstract
In this work a variety of modifications of porous glass membranes are investigated considering their application in vanadium redox-flow batteries. Through 3D-printing technology a cell assembly was established that made replicable long-term testing of single glass membranes with a predefined geometry possible. Native as well as modified porous glass membranes with pore diameters in the range of 2 nm to 50 nm and thicknesses varying from 300 μm to 500 μm are electrochemically characterized (cf. Chapter IV). Pore surface modifications included the chemical bonding of sulfonic acid groups, propyl-N, N, N-trimethylene-ammonia-groups and propyl-pyridinium groups (cf. Chapter III). Membranes with large pore diameters and thus high porosity showed the highest performance concerning achievable current densities. However, self-discharge and selectivity are affected negatively in these membranes (cf. Chapter V). A newly developed point system identified the best performance of membranes with a thickness of 300 μm, a mean pore diameter between 5 nm and 10 nm and finely dispersed silica particles throughout the pore system. This corresponds with the membrane type 503FD. With this type a mean charging power density of 54.1 mA cm-2 at 1.8 V and a mean discharging power density of 71.7 mA cm-2 at 0.8 V was achieved during cycling. The maximum power density reached 75.3 mW cm-2 at a current density of 100.1 mA cm-2 and a voltage of 752 mV during discharge starting from a SoC of 100 %. Furthermore very high Coulomb Efficiencies (CE) of 98.1 % are possible with such membranes. Further surface modification of this membrane type does not lead to improvements of the performance. Although surface modifications contribute in reducing the self-discharge of the battery through cation crossover, other electrochemical characteristics suffer considerably. Given the additional work and expense of the process, modification of this membrane type is negligible (cf. Chapter VI). The overall performance of the investigated porous glass membranes and their modifications are still well below those of the referenced system of Nafion™ 117. This is explained by the relatively low porosity of the porous glasses. If it was possible to establish large-size, thin and stable glass membranes with high porosities and mean pore diameters in the range well below of 10 nm at reasonable prices, their longevity and aging resistance as well as the large variety of possible chemical surface modifications might lead to drastic performance and efficiency gains. Therefore porous glass membranes might become a real competition for the widespread use of polymer membranes in vanadium redox-flow batteries. The great potential of porous glasses in electrochemical applications is still to be developed in further studies. Their biggest advantage being that their chemical properties can be adopted to their very purpose through chemical modification. In summary the chemical stability and electrochemical performance of porous glass membranes provide very good preconditions for various long-term applications under the harsh conditions of redox flow-battery systems.
Published Version
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