Rechargeable redox flow batteries: flow fields, stacks and design considerations

Redox Flow Batteries: How to Determine Electrochemical Kinetic Parameters

Redox flow batteries (RFBs) are promising energy storage candidates for grid deployment of intermittent renewable energy sources such as wind power and solar energy. Various new redox-active materials have been introduced to develop cost-effective and high-power-density next-generation RFBs. Electrochemical kinetics play critical roles in influencing RFB performance, notably the overpotential and cell power density. Thus, determining the kinetic parameters for the employed redox-active species is essential. In this Perspective, we provide the background, guidelines, and limitations for a proposed electrochemical protocol to define the kinetics of redox-active species in RFBs.

Development of Flow Fields for Zinc Slurry Air Flow Batteries

The flow field design and material composition of the electrode plays an important role in the performance of redox flow batteries, especially when using highly viscous liquids. To enhance the discharge power density of zinc slurry air flow batteries, an optimum slurry distribution in the cell is key. Hence, several types of flow fields (serpentine, parallel, plastic flow frames) were tested in this study to improve the discharge power density of the battery. The serpentine flow field delivered a power density of 55 mW∙cm−2, while parallel and flow frame resulted in 30 mW∙cm−2 and 10 mW∙cm−2, respectively. Moreover, when the anode bipolar plate material was changed from graphite to copper, the power density of the flow frame increased to 65 mW∙cm−2, and further improvement was attained when the bipolar plate material was further changed to copper–nickel. These results show the potential to increase the power density of slurry-based flow batteries by flow field optimization and design of bipolar plate materials.

Integrated Photo-Responsive Batteries for Solar Energy Harnessing: Recent Advances, Challenges, and Opportunities

Photo-responsive batteries that enable the effective combination of solar harvesting and energy conversion/storage functionalities render a potential solution to achieve the large-scale utilization of unlimited and cost-effective solar energy and alleviate the limits of conventional energy storage devices. The internal integration of photo-responsive electrodes into rechargeable batteries with the simplest two-electrode configuration is regarded as a reliable and appealing strategy for highly-efficient and low-cost utilization of solar energy by simplifying the device architecture and improving the energy efficiency. This progress report provides a brief review on photo-responsive batteries with integrated two-electrode configuration that can achieve solar energy conversion/storage in one single device. The basic device architecture, operating principles and practical performance of various photo-responsive systems based on solar energy harvesting in various batteries including Li ion batteries, Li-S batteries, Li-I batteries, dual-liquid redox batteries, Li-O₂ batteries, non-Li anode-O₂ /air batteries are summarized and discussed. Finally, the future opportunities and challenges regarding the two-electrode photo-responsive batteries are proposed.

Preparation and Carbon-Dependent Supercapacitive Behaviour of Nanohybrid Materials between Polyoxometalate and Porous Carbon Derived from Zeolitic Templates

An electrochemical cell combining the energy storage characteristics of the chemical redox reaction and a physical capacitor effect presents advantages including high energy and power densities, and long durability. In this study, we prepared nanohybrid materials between polyoxometalate (POM) and porous carbon, which have different porous structures and pore sizes, using different zeolitic templates. The POM molecules were loaded inside the porous carbon, and these POM/carbon nanohybrid materials were used as cathode active materials for lithium–ion batteries (LIBs). The performance of these molecular cluster batteries (MCBs) was significantly dependent on the porous carbon. Operando X-ray absorption fine structure (XAFS) and ⁷Li solid-state nuclear magnetic resonance (NMR) measurements of the POM/carbon-MCBs revealed that three-dimensional porous carbon with high surface areas can improve the performance. The results highlight the remarkable performance of porous carbon with a three-dimensionally-linked pore network structure as an additive for supercapacitors to realise high-performance energy storage devices.

In situ mapping of activity distribution and oxygen evolution reaction in vanadium flow batteries

Understanding spatial distribution difference and reaction kinetics of the electrode is vital for enhancing the electrochemical reaction efficiency. Here, we report a total internal reflection imaging sensor without background current interference to map local current distribution of the electrode in a vanadium redox flow battery during cyclic voltammetry (CV), enabling mapping of the activity and reversibility distribution with the spatial resolution of a single fiber. Three graphite felts with different activity are compared to verify its feasibility. In long-term cyclic voltammetry, the oxygen evolution reaction is proved to enhance activity distribution, and homogeneity of the electrode and its bubble kinetics with periodic fluctuation is consistent with the cyclic voltammetry curve, enabling the onset oxygen evolution/reduction potential determination. Higher activity and irreversibility distribution of the electrode is found in favor of the oxygen evolution reaction. This sensor has potential to detect in situ, among other processes, electrochemical reactions in flow batteries, water splitting, electrocatalysis and electrochemical corrosion.

Enhanced Reactant Distribution in Redox Flow Cells

Redox flow batteries (RFBs), provide a safe and cost-effective means of storing energy at grid-scale, and will play an important role in the decarbonization of global electricity networks. Several approaches have been explored to improve their efficiency and power density, and recently, cell geometry modification has shown promise in efforts to address mass transport limitations which affect electrochemical and overall system performance. Flow-by electrode configurations have demonstrated significant power density improvements in laboratory testing, however, flow-through designs with conductive felt remain the standard at commercial scale. Concentration gradients exist within these cells, limiting their performance. A new concept of redistributing reactants within the flow frame is introduced in this paper. This research shows a 60% improvement in minimum V³⁺ concentration within simulated vanadium redox flow battery (VRB/VRFB) cells through the application of static mixers. The enhanced reactant distribution showed a cell voltage improvement by reducing concentration overpotential, suggesting a pathway forward to increase limiting current density and cycle efficiencies in RFBs.