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An in situ, local potential measurement technique was further developed and applied to all-vanadium redox flow batteries to determine the potential distribution within multilayer electrodes of the battery. Micro-scale potential probes enabled in situ measurement of local potential in electrode layers between the cell flow field and membrane. The local redox potentials were recorded for different operating conditions and states of charges. To further analyze the behavior of potential distribution in the through-plane direction, a mathematical model was developed and the species distribution as well as the flux density of any individual component was modeled in terms of contributions from convective, diffusive and electrophoretic fluxes at each operating condition. Good agreement was achieved between the mathematical model prediction and experimental data with maximum error of 8%. Both mathematical simulation and experimental data confirmed the distribution of potential in the through plane direction as a function of discharge current density, predicting the lowest potential in a region close to the flow plate. Redox flow batteries have many benefits including energy efficiency, capital cost, and life cycle costs compared with other gridscale energy storage technologies.1 Multiple chemistries have been developed for redox flow batteries such as iron-chromium, 2 allvanadium, 3 bromine-polysulfide 4 and zinc-bromine. 5 Among these many chemistries, the all-vanadium redox flow battery (VRFB) has been studied in great detail and is relatively mature with several fullsize systems in operation around the world. The VRFB employs the V (I I )/V (I I I ) redox couple at the negative side and the V (I V )/V (V ) redox couple at the positive side, in the form of V O 2+ and V O + 2 . The kinetics associated with reduction and oxidation of vanadium species are known to be very complex 6,7 In this paper, the following simplified set of global half-reactions are adopted for the negative and positive electrodes.One advantage of VRFBs is that in utilizing the same element, vanadium, as active species in both negative and positive electrolytes, crossover of active species does not foul the electrolyte, and results only in decreased coulombic efficiency. Storage capacity can be regained through electrolyte rebalancing. The VRFB also has a wide operating temperature range from −5 • C to 50• C 8 and fast response (around 350 μs) for start-up and switching between charging and discharging processes, using a sulfuric and hydrochloric acid mixture. 9Cost is still the major barrier to VRFB commercialization. Zhang et al.10 estimated that the cell stack is one of the highest cost components with 31% of capital cost from a base case VRFB. One way to decrease the stack cost is to increase the power density of the VRFB during charge-discharge cycles, allowing for smaller stacks for the same power.11 In order to achieve high power density, the cells should operate at high current density with decreased polarization losses (activation, ohmic an...

Grid-scale energy storage systems are of interest as the world increases reliance on renewable energy sources. Redox flow batteries are a type of grid-scale energy storage technology that shows exceptional promise for accommodating the dynamic output of wind and solar power sources. The research community employs dozens of diagnostic techniques to investigate nearly all facets of these devices. Material properties, operational losses, transport, and integrated system properties are studied through the lens of electrochemical, physical, and chemical phenomena that ultimately dictate cost by influencing efficiency, durability, power, and capacity. These diagnostic techniques can, if applied correctly, elucidate not only the types of losses in redox flow batteries, but also tie those losses to fundamental driving forces in such systems so that next generation systems and models can be designed. This review details various diagnostic techniques used in flow battery analysis. The benefits, unique insights, and limitations of these techniques are discussed. Recommendations are also made to assist researchers in identifying the diagnostics that can advance their particular investigations. The review concludes with a summary of opportunities for new diagnostics that are needed to enable solution of persistent issues in redox flow battery research and development.

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