Executive Summary : | Objective: To design, develop and demonstrate ion selective membrane separator suitable for application in redox flow battery (RFB) like VRFB durable for minimum 1000 cycles. To estimate the properties such as swelling ratio, water uptake, contact angle, permselectivity, conductivity, and selectivity, of optimized membranes for assessing the suitability of RFB. To assess the membrane stabilities and durability under strong acidic and oxidative environments (H2SO4, VO2+). To record the charge-discharge, self-discharge curves for estimating columbic, voltage and energy efficiencies in vanadium redox flow battery (VRFB) with 25-100 cm2 effective membrane area. To record the durability of VRFB for 1000 cycles. Summary: With the increasing need to seamlessly integrate renewable energy with the current electricity grid, which itself is evolving into a more intelligent, efficient, and capable electrical power system, it is envisioned that energy storage systems will play a more prominent role in bridging the gap between current technology and a clean sustainable future in grid reliability and utilization. Redox flow battery technology is a leading approach in providing a well-balanced solution for current challenges. response, etc. Redox flow battery (RFB) technology, now has been considered suitable for large scale energy storage and has recently attracted considerable research interest because of several attractive features including long calendar life, simple design, and capability to withstand fluctuating power supply. Recent R&D of aqueous RFB cell-level components, are namelyelectrolytes, electrodes, and membranes (or separators).An RFB, shown schematically in a generic form in Figure 1, is a type of energy-storage device capable of providing reversible conversion between electrical and chemical energy, typically in two soluble redox couples contained in external electrolyte tanks sized in accordance with application requirements. Unlike traditional batteries that store energy in electrode materials, RFBs are sometimes referred to as regenerative fuel cells as energy is stored in the incoming fuels in the form of two dissolved redox pairs that convert into electricity at the electrodes. More like their solid-state battery relatives, however, is the fact that the redox reactions are reversible, which qualifies the RFB as a secondary battery system. The conversion between electrical energy and chemical (or electrochemical) energy occurs as the liquid electrolytes are pumped from storage tanks to flow-through electrodes in a cell stack. The electrolytes flowing through the cathode and anode often are different and are referred to as the anolyte and the catholyte, respectively. Between the electrodes is an ionic conducting membrane or separator that keeps the two electrolytes from mixing while allowing transport of the charge carrying ions (e.g., H+, Cl-) to maintain electrical neutrality and electrolyte balance.
The membrane or separator in an RFB system is a key component that separates the cathode and anode compartments, while allowing the transport of charged ions (H+, SO42-, etc.) to complete the circuit. Essential requirements for the membrane separator are as follows: 1. The membrane should be stable under highly oxidative operating environment.
2. Minimum resistance to reduce the power loss. 3. High ionic charge character, essential for hydrophilic membrane at the interface between the liquid electrolyte and solid membrane surface to ensure a fast ion transfer.
4. Fast ionic transport and thus highly selective;
5. Minimum transport of active species to reduce capacity and energy losses.
6. Minimum water transport across the membrane to maintain catholyte and anolyte balance.
7. Low cost membrane with high stability and excellent conductivity, important for
commercializing RFB technology. Traditionally, RFB especially VRFB almost exclusively use perfluorinated polymer membranes (such as Nafion) to withstand the strong acidic environment and highly oxidative VO2+ ions in the positive half-cell electrolyte. For VRFBs hydrocarbon-based membranes and separators can be a sensible choice to significantly reduce the capital cost of the systems. The desire to use hydrocarbon-based membranes in VRFBs is mostly driven by cost-reduction efforts. Recently, the development of non-fluorinated membranes has focused on sulfonated aromatic polymers in a hope that the rigid chain and less connected ionic cluster will prevent vanadium ion crossover. In general, these membranes demonstrated better columbic efficiency in the VRFB cell test, indicating lower vanadium ion permeability. However, the long-term durability of these membranes in the V(V) electrolyte jeopardized applicability of these membranes. Another popular approach for reducing the cost of membrane is developing fluoro carbon- aliphatic hydrocarbon composite charged membranes with controlled charged nature and nano- pore character. Despite the excellent chemical stability and proton conductivity, high vanadium ion permeability remains a concern for Nafion membranes, as cross-contamination severely diminishes the available active materials. The desire to use hydrocarbon-based membranes in VRBs is mostly driven by cost-reduction efforts. |
