349757 Advanced Electrolyte Membranes for Energy Storage

Monday, November 4, 2013
Grand Ballroom B (Hilton)
Yanxin Li, Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL

Advanced Electrolyte Membranes for Energy Storage

Fluorinated electrolytes are currently the most promising material to serve as electrolyte membrane in fuel cells, due to their excellent proton conductivity and mechanical and chemical properties. However fluorinated materials have a very high synthesis cost, arising from their high reactivity and resultant safety hazards of fluorine. In this study hydrocarbon membranes are investigated to determine if they can replace the uneconomical fluorinated membrane. Given their higher stability in chemical and mechanical performance, aromatic polymers are chosen to make up the membrane in my research.

The membrane is designed to transport ionic charges in devices (flow batteries) which store electric energy produced in other forms, such as solar energy and wind energy. These energy sources have a continuous supply, however, most of the time the energy generated is not equal to the energy needed. To improve the efficacy of solar or wind energy plants, the flow battery is brought in to function as a mediator. When the energy production exceeds the consumption, the excessive energy accumulates in the flow battery in chemical forms for future use instead of being lost and if the production falls short the battery transforms the chemical energy into electricity for direct use.

The redox couple I have selected for the membrane is vanadium-ions. V2+ is oxidized to V3+ at anode and V5+ is reduced to V4+ at cathode, the overall redox reaction is V2+ + V5+--> V3++ V4+. Protons or chloride/sulfate ions are used to balance the charge on either side.

As the battery ages, its electricity storage capacity decreases by a certain amount and my job is to test four properties of the membranes and how they will change based on battery operations.

1)      Conductivity

The membrane’s ability to conduct ions (protons for cation exchange membrane, chloride/sulfate ions for anion exchange membrane). Conductivity dictates how well the electrolyte membrane works and can be used to determine its quality. Conductivity is measured using a 4-point probe.

2)      Vanadium permeability

Theoretically vanadium ions shouldn’t leave their own compartments (i.e. the membrane should be impermeable). But with time, it is inevitable that some vanadium ions will permeate the membrane and approach the other electrode. This ruins the redox reaction since the oxidant and reductant are mixed together. So it is necessary to measure how many vanadium ions move to the other end and then how much the redox reaction will be affected. This is done using a diffusion cell.

3)      Transport number

This is the fraction of each ion (in the total ionic mix in the system) that contributed to ionic current. It allows us to estimate permselectivity and better understand the Donnan exclusion effect. This is measured with a specialized apparatus in Ramani Lab.

4)      Electro-osmotic drag coefficient

As protons or chloride/sulfate ions move through the membrane, they may have some water attached (solvation shells) to them and consequently redistribute H­­2O to the anode or cathode, which causes the vanadium ions to become more or less concentrated. The drag coefficient is the number of water molecules per ion that is transported across the membrane. This is measured using a special apparatus in the Ramani Lab.

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