Coupled transport and electrochemical characteristics in redox flow batteries

With widespread public attention to long-duration energy storage technologies, redox flow batteries are attracting increasing interests of researchers due to their intrinsic safety and good design flexibility. Currently, high capital costs are constraining the widespread commercialization of this system, which calls for the further enhancement of the output performance in its power units. The power output in a redox flow battery is greatly influenced by macro-to-micro mass transport and electrochemical reactions, which are coupled with each other and together determine the performance of the battery. Therefore, exploring how to achieve a coupled enhancement of transport and electrochemical properties rather than focusing solely on one aspect is a current area of interest. This perspective emphasizes the importance of simultaneously enhancing the transport and electrochemical properties of flow batteries and points out the challenges in this regard.

Background

Nowadays, the excessive use of fossil energy has caused a series of climate, energy, and environmental issues, prompting human society to make an energy transition. Renewable energy sources represented by wind, solar, and tidal energy need the intervention of energy storage and conversion devices because they face intermittent and fluctuation problems.2 As a new type of electrochemical energy storage system, the redox flow battery (RFB) is an ideal large-scale and long-duration energy storage system because of its advantages of decoupled energy and power, good scalability, long cycle life, and intrinsic safety. However, in commercial applications, the higher manufacturing and maintenance costs faced by RFB are currently the most dominant factor, which meaning it is urgently needed for the size of the stacks to be reduced by improving the output performance of their power units, thus achieving reduced costs.

The RFB consists of external electrolyte storage devices (electrolyte tanks) and a power unit, and in the process of the operation, the reactants in the electrolyte tank will be pumped into the power unit to conduct the electrochemical reaction, thus realizing the mutual conversion between electrical energy and chemical energy.3 Given that the electrochemical reaction site of the reactants is the surface of the porous electrode fibers, a complex multi-scale mass transport process takes place inside the power unit before the reactants reach the active site, which is essential for understanding the physicochemical processes occurring inside the battery. It generally consists of simultaneous convection-diffusion-reaction processes: (1) reactants that are about to go through the electrochemical reactions are refreshed by forced convection between electrode fibers and flow channels; (2) the renewed reactants will pass through the electrode-electrolyte interface to reach the active sites on the electrode by diffusion process; and (3) redox reactions occur, resulting in electron transfer and energy conversion.3 Based on the above principles, to accelerate the electrochemical reaction process of the reactants and achieve the overall performance improvement of the power unit, the following conditions are satisfied to meet: on the one hand, the reactants can undergo rapid convection-diffusion transport processes inside the power unit, where they can be delivered directly to the nearby of the active sites in time, and the products can be expelled immediately. On the other hand, reactants can conduct rapid redox reaction processes on the active site, which requires that the active sites have good electrochemical reactivity and catalytic activity, thus matching the rapid mass transport rates. Figure 1 illustrates the conceptual schematic of a high-performance flow battery through coupling transport and electrochemical characterization. In this regard, how to combine the two and consider them together is the key direction that should be paid attention to nowadays, as it is essential for enhancing the power density of the RFB and achieving stable operation at higher operating current densities.