The flexible design, showed by this system, allows an easy scale up, but it is necessary to develop stable and high energy density redox electrolytes and efficient and low cost membranes or separators.[1] The membrane is a critical component that to a great extent determines the performance of RFB systems for practical applications The membrane is responsible for electronically insulating the negative and positive compartments while allowing the transport of charge balancing ions to complete the circuit. A membrane that exhibits excellent ionic conductivity, low permeability of the active species, and high chemical stability, is thus critically important for the development of RFB technologies.
Perfluorosulfonic polymers such as Nafion are commonly used in different electrochemical devices, like Polymeric Fuel Cell (PEM), electrolyzers or RFB, due to their high proton conductivity, good chemical and thermal stability. However, when used in VRFBs, Nafion membranes suffer from the crossover of vanadium ions which results in decreased energy efficiency.
So far studies are concentrated on the development of novel composite membranes leading to a low self-discharge rate and low costs. In this work, several cation exchange membranes (CEM) to be used in aqueous electrolyte VRFBs were investigated.
Our approach to reduce costs and vanadium-ion crossover was based on the use of sulfonated poly ether ether ketone (SPEEK) due to its low permeability of diffusing specie and high ionic conductivity.
Moreover, to further block vanadium-ion permeability through the membrane, we also evaluated the effect of adding a filler in the polymer matrix. In particular, an organically-modified ceramic material (TiO2-RSO3H) was synthesized by covalently grafting propylsulfonic acid groups on the surface of TiO2 nanoparticles [2] and added into the ion exchange polymer.
To obtained homogeneous composite membranes two different deposition techniques were evaluated: solution casting and air spraying method. Filled and unfilled Nafion membranes were prepared and used as reference. Permeability measurements were carried out using a static H-type glass cell and the vanadium concentration were evaluated by UV-Vis spectrophotometry.
The ionic conductivities of cation exchanged membranes was evaluated by electrochemical impedance spectroscopy. Coulombic and energy efficiencies of the system were studied by galvanostatic charge-discharge cycles using a lab-scale RFB cells.
The overall performance of the RFB system was further analyzed by polarization curve at different state of charge (SOC) and Electrochemical Impedance Spectroscopy (EIS). Changes in the electrolyte composition were studied with cyclic voltammetry (CV) and UV-vis spectroscopy. Early insights on the understanding of battery capacity loss in long-term cycling tests is also reported in this work.
Reference
[1] Aishwarya Parasuramana, Tuti Mariana Lima, Chris Menictasc, Maria Skyllas-Kazacos,
Electrochimica Acta, 101, 2013, 27-40
[2] D. Cozzi, C. de Bonis, A. D’Epifanio, B. Mecheri, A. C. Tavares, S. Licoccia
Journal of Power Sources 248, (2014) 1127-1132