Graphene 4 Redox Flow Battery
Graphene offers step change in VFRB performance boosting economic viability
Graphene modified materials offer a step change improvement of Vanadium RFB via improved selectivity, mechanical performance and contact resistance to boost economic viability
Vanadium Redox Flow Batteries (VFRB) have been postulated as a solution for energy storage demand and load balancing at the grid scale due to the ability to store extensive amounts of energy across the various oxidation states of the vanadium chemistries used. However, VFRB suffer from high power related losses due to internal cell resistances and relatively low efficiency (versus Li-Battery (LiB)) due to crossover of active species through the cell. The use of graphene modification of the cell components will address this and lead to a step change in both efficiency and power (via access to higher current densities). The main benefit of VFRB is that their power and energy rating is decoupled; i.e. capacity is determined via tank size, power is determined via stack performance. This is especially for larger capacities where low energy related costs (£/kWh) could benefit its market penetration. In addition, the VRFB makes use of a larger part of its gross capacity and superior stability (vs. LiB) with cell lifetime of 10,000 cycles have been realised without significant aging (lead acid: ~2,000 cycles; LiB: ~5,000 cycles). Finally, the technology is very resistant against deep discharge. However, lower efficiencies and comparatively high power related costs at smaller (household) scale (£/kW) are disadvantages of the VRFB, both of which will be addressed by this project. The first issue that this proposal could tackle is the graphene modification of a polymeric proton exchange membrane (PEM) which suffer from inadequate mechanical performance and crossover of water and other reactive species through the membrane. This has led to either use of thick supported proton exchange membranes with high resistivity which also contribute significantly to system cost. Another alternative is via incorporation of anion exchange membranes which suffer from instability to some of the species present within VFRB (i.e. VO2+), reducing the lifetime of this component. Graphene has been proven to work as a perfect barrier for all molecules, including Helium but will allow the passage of protons (H+) through (Geim et al, 2014) due to the low energy barrier of the monolayer material. It is also used extensively as a material to improve the mechanical performance across a wide range of polymeric composite materials. Incorporation of graphene will improve the mechanical performance of the PEM, allowing thinner membranes. This will have a dual effect of reducing the resistance losses associated with the PEM and improving the efficiency of the cell (both Energy and Coulombic Efficiencies) (see image 1a). The second issue this project will address is the lower current and power densities of VFRB (vs. LiB), which can affect the ability to connect to advanced grids. This is seen as a problem innate with the use of carbon felt electrode materials which result in poor cell compression, high contact resistance and non ideal volumetric power density. However the use of alternative materials with adequate compression strength, permeability and flow (e.g. metal foams) is limited as these materials do not possess the necessary electrochemical activity and/or chemical stability. As previously mentioned, graphene has unrivalled barrier properties. Through the development of a graphene coated metal electrode it is hoped that higher cell compression can be used whilst retaining both suitable fluid flow, electrode activity and imparting chemical stability. This would allow higher cell compression to be achieved, decrease cell resistance and increase the current density by an order of magnitude (see image 1b) The proposal will be carried out collaboratively at the University of Manchester and Chester University. The Graphene Engineering Innovation Centre is a new facility at the University of Manchester (UoM) and is ideally placed to develop the incorporation of graphene within PEM materials and has the remit to accelerate the commercial application of graphene materials. As such it is equipped with pilot scale equipment suitable to direct transfer to industrial partners as well as access to the fundamental understanding of graphene nanocomposites through the fundamental knowledge base present within UoM. Chester University have the capability to test and support VFRB capability from bench scale single cell capability (TRL2-4) through scale-up to microgrid application and testing at the Thornton Energy Centre. Geim et al; Nature volume 516, pages 227–230
Graphene is the strongest known material and also offers multifunctionality including chemical resistance, unsurpassed barrier properties and can act as a proton conductivitor. Use of graphene within the membrane system should address the efficiency losses of VRFB through production of relatively thin graphene membranes with lower resistance, improved mechanical properties and improved selectivity to the crossover of active species (see image - a). Alongside this the development of a graphene coated metal electrodes will allow higher cell compression, retaining both suitable flow and electrode activity. The graphene will act as a protective barrier for the metal foam against the corrosive environment whilst doubling up as an active material for the vanadium redox reaction. This would allow higher cell compression to be achieved, decreasing cell resistance, increasing volumetric power density through an increase in the current density of an order of magnitude (see image - b). A combination of these advances could enable the widespread application of VFRB energy storage systems at the grid scale which can be operated at higher current density with lower efficiency losses over long term energy storage cycles.
Long term energy storage Grid scale energy storage