Experimental Benchmarking of Redox Flow Cells

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batteries Article Experimental Benchmarking of Redox Flow Cells Adam H. Whitehead * , Alasdair Robertson , Benjamin Martin, Elisha Martin and Emma Wilson Invinity Energy Systems, Unit 1, 10 Easter Inch Road, East Inch Ind. Estate, Bathgate EH48 2FG, UK * Correspondence: awhitehead@invinity.com Abstract: There are increasing numbers of scientific articles dedicated to developments in the field of redox flow batteries. To date it is most common to provide efficiency values as a measure of perfor- mance. However, there are no agreed standard experimental conditions for these measurements, and so their merit as a tool for comparing different innovations among research groups is put into question. In the following manuscript, various experimental precautions are outlined to reduce experimental artefacts. Original experimental measurements on vanadium flow cells, together with data from the literature, are examined to explore efficiencies and two alternative benchmarking metrics: resistivity and self-discharge current density. The sensitivity of these parameters to current density, temperature, flow rate and state-of-charge range are examined, from which it is concluded that resistivity and self-discharge current density exhibit superior properties to efficiencies for quantifying flow battery improvements. Keywords: vanadium redox flow battery; efficiency; performance benchmarking; resistivity; self-discharge; experimental precautions; current density; standard experimental condition 1. Introduction A wide variety of flow battery chemistries and cell designs are under develop- ment [1,2]. Increasing reliance on intermittent power sources, such as wind, photovoltaic and tidal generators, has created a demand for large-scale energy storage systems. Redox flow batteries can easily provide many hours of energy storage and boast very long life- times [3] with low environmental impact [4–6] and favorable economics for high duty cycle application, [7,8] which makes them well-suited to meet this pressing need. To date, vanadium is the most widely adopted chemistry, especially in commercial systems and will be used to exemplify various concepts hereafter. However, it should be understood that the principles can be more widely applied to study the redox flow cells of various chemistries. Efficiency is a widely used measure of performance. Typically, energy efficiency, ηE, voltage efficiency, ηV, and coulombic efficiency, ηQ, are reported [9]. These terms are defined by common convention (e.g., ref. [10]) as: 􏱸 ηE = 􏱸 􏱶 Citation: Whitehead, A.H.; Robertson, A.; Martin, B.; Martin, E.; Wilson, E. Experimental Benchmarking of Redox Flow Cells. Batteries2022,8,207. https:// doi.org/10.3390/batteries8110207 Academic Editors: Pascal Venet, Karim Zaghib, Seung-Wan Song and Seokheun Choi Received: 15 September 2022 Accepted: 29 October 2022 Published: 2 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 􏱸􏱸􏱶 V·j dt􏱸􏱸 discharge 􏱸 􏱸 (1) 􏱸􏱸􏱶 j dt􏱸􏱸 􏱸 discharge 􏱸 ηQ = 􏱸 􏱶 􏱸 (2) 􏱸 jchargedt 􏱸 ηV = ηE (3) ηQ where jdischarge and jcharge are cell current densities on discharge and charge, respectively; V is cell voltage; and the integrals are measured over the entire discharging and charging periods, respectively. It is important that the cycle starts and returns the electrolyte to the 􏱸 V·jchargedt 􏱸 Batteries 2022, 8, 207. https://doi.org/10.3390/batteries8110207 https://www.mdpi.com/journal/batteries

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