h i g h l i g h t sThermal activation of carbon paper electrodes enhances VRFB kinetic performance. Large increase in surface area is responsible for improved kinetic performance. Significant improvement in depth of charge is achieved. Charge/discharge cycling efficiency of 76% at 200 mA cm À2 is realized. a b s t r a c tThe roundtrip electrochemical energy efficiency is improved from 63% to 76% at a current density of 200 mA cm À2 in an all-vanadium redox flow battery (VRFB) by utilizing modified carbon paper electrodes in the high-performance no-gap design. Heat treatment of the carbon paper electrodes in a 42% oxygen/58% nitrogen atmosphere increases the electrochemically wetted surface area from 0.24 to 51.22 m 2 g À1 , resulting in a 100e140 mV decrease in activation overpotential at operationally relevant current densities. An enriched oxygen environment decreases the amount of treatment time required to achieve high surface area. The increased efficiency and greater depth of discharge doubles the total usable energy stored in a fixed amount of electrolyte during operation at 200 mA cm À2 .

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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.

A printed circuit board (PCB) was implemented for in-plane, two-dimensional distributed current measurements in an all-vanadium redox flow battery (VRFB). A PCB with built-in shunt resistors is a passive method of measuring localized currents in-situ, in real time. It is demonstrated that lateral current spread through non-or partially-segmented flow field plates will produce a distribution that does not properly reflect the distribution within the electrode; this issue is resolved in this work via fully-segmented flow plates. Large current gradients develop when a cell reaches a mass-transport limitation. Based upon the resultant distributions, it is shown that they reflect a combination of electrolyte velocity and local concentration within the electrode. The impact of flow rate and electrode material properties such as wettability, surface area, porosity, and thickness on current distribution are presented. Redox flow batteries (RFBs) are electrochemical energy storage devices that have received much attention in recent years for their capabilities as grid-scale energy storage.1 Comprised of a battery stack and externally-stored liquid electrolyte, these systems provide the advantage of decoupled power and energy capacity. Compared to other large-scale energy storage options such as pumped hydro or compressed air storage, these systems have a small footprint and no geographic limitations.2 These aspects make RFBs an attractive storage system to complement renewable energy sources and potentially cover base-load power needs.Another attractive aspect of these systems is the flexibility of the electro-active species. RFBs operate on coupled redox reactions at electrodes separated by an ion exchange separator. This provides countless options for different chemistries, some of the most common include all-vanadium, vanadium/bromine, bromine/polysulfide, and iron/chromium, among others.3 The all-vanadium chemistry, in particular, utilizes the four stable oxidation states of the transition metal to store electrical energy. This is an attractive choice because the same active species on both half-cells allows the liquid electrolytes to be rebalanced to regain capacity loss due to self-discharge.Published research on these systems has resulted in improved individual cell components such as membranes, 4,5 electrodes, 6,7 and electrolyte. 8,9 In addition, investigations of flow field design 10,11 and operating conditions 12,13 have improved transport, performance, and efficiency of operating cells. However, these improvements are typically quantified with ex-situ experiments, or are validated with an overall cell performance comparison. Ex-situ experiments, while useful, do not always translate directly to an operating cell. On the other hand, performance of an operating cell only provides an overall cell average; thus, any local changes would be undetectable. Localized real-time knowledge of performance within cells can enable insight into optimization of architecture, materials, charging, discharging, operational trans...