Measuring Redox Flow Battery Kinetics on Single Carbon Fibres
Authors: Landon-Lane Leatham Harry , Downard Alison , van Haren Wessel , Marshall Aaron Timothy 
Published: February 2020

Vanadium redox flow batteries (VRFB) provide large scale energy storage with a long lifecycle, by storing redox couples in large tanks and converting chemical to electrical energy at a solid electrode. VRFB have a higher levelized cost of storage (LCOS), than pumped hydro energy storage (PHES) and Li-ion batteries, preventing wider adoption (1). Improving the LCOS of VRFB would enable greater levels of renewable power generation into electricity grids.
In VRFB, the cost of energy storage ($kWh-1) is independent of the cost of power ($kW-1) (2). Typically, VRFB have V2+/V3+ in the anolyte and V4+/V5+ in the catholyte, both dissolved in sulfuric acid (3). If the V4+/V5+ couple was replaced by Fe2+/Fe3+, the storage cost would be reduced as iron chloride is significantly cheaper than vanadium pentoxide (4). Using iron will lower the cell operating potential and lead to chemical cross-over, negatively impacting the performance (4). Thus, efforts to reduce the cost of VRFB should be focussed on the cost of the cell unit that produces power (3). These cells typically use carbon felt electrodes and if the power density of the can be increased, the cell and hence battery cost will also decrease (4).
It has been shown that treating carbon fibres can improve the kinetics of redox reactions, with carbonyl and hydroxyl groups (5), nitrogen doping (6) and metal catalysts (7) all credited with improving kinetics. The experimental methods used to calculate kinetic parameters can vary significantly and may lack the precision required to identify why different treatments alter performance. In addition, the morphology and structure of carbon felt complicates the measurement of kinetic parameters due to complex flow behaviour through the felt.
In this work, single carbon fibres are used as electrodes to precisely investigate the effect different surface modifications have on electrode kinetics, for both vanadium and iron redox reactions. This should enable greater control over surface functionalization and more accurate testing of kinetic parameters.
For planar electrodes, the standard rate constant can be found from the variation of the peak separation during cyclic voltammetry at different scan rates (8). For micro-fibre electrodes, clear current peaks are not always observed, thus a numerical model has been developed using the underlying Nernst, Butler-Volmer and diffusion equations, to enable the determination of the kinetic parameters. The validity of the model is confirmed by using the standard rate constant calculated from a planar gold electrode, in a solution of Fe2+/Fe3+ in sulfuric acid, and comparing the simulated CV for a gold micro-electrode, to an experimentally recorded CV, under the same conditions.
This approach allows chemical, thermal and electrochemical treatments of the carbon fibre electrodes to be precisely carried out and the effect measured on individual carbon fibers, enabling quick and efficient screening of different activation methods. If a treatment method leads to improved kinetic activity, the increase in current density of an operational cell can be estimated based on a two-dimensional stationary model (9) and then compared to experimental data from a small-scale test cell.

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