Coupled transport and electrochemical characteristics in redox flow batteries

Abstract

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.¹ 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.² 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.³

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.

Figure 1.

High-performance flow battery design by coupling transport and electrochemical characteristics

Current status and existing challenges

In the RFB, since the surface activity of the electrode has a vital influence on the reaction rates of the reactants, significant efforts have been made to enhance the electrode surface catalytic activity and specific surface areas over the past decades.² However, as a complex system coupling multiple processes, focusing only on electrochemical reactivity intensifies the mismatch between the electrochemical activity and mass transport properties, thus leading to limited output power of the battery. In this regard, exploring coupled mass transport and electrochemical performance enhancement strategies at multi-processes has provided a promising approach.² Accompanied by a gradual understanding of the convection-diffusion-reaction processes that the reactants undergo inside the battery, the development of cross-scale optimization on mass transport and electrochemical performance is expected to lead to the achievement of better performance in RFBs.

Since the reactants in the electrolyte will first experience the forced convection process from the flow channel to the inside of the porous electrode, the structure design at the macroscopic level for flow fields and electrodes has gained great attention among researchers. In frontier work on flow fields, to more efficiently design the structure and facilitate the rapid mass transport of the reactants, Wan et al. developed an end-to-end approach to design the flow fields by machine learning, firstly revealing the quantitative design rules of the flow fields.⁴ Through a screening process, the designed flow fields could yield higher electrolyte utilization and evener electrolyte distribution. This work suggested that optimizing the convection process of the electrolyte upon entering the electrodes was critical for accelerating the transport of reactants and replenishing the reactants to the active sites, which was the key to achieving coupled enhancement of mass transport and electrochemical activity. Apart from the fact that the structure of the flow field affects the convective transport process of the electrolyte, the macro-adjustments of the electrode fibers also play an indispensable role. To accelerate the mass transport of reactants in flow-field structure flow batteries, Sun et al. fabricated uniaxially aligned carbon fiber electrodes using an electrospinning method.⁵ Due to the enhanced permeability and reduced tortuosity of the electrode, the convective transport process of the electrolyte was significantly improved, as evidenced by a more homogeneous local current density distribution in the electrode plane and higher limited current densities. To promote the convective transport process with as little impact on the specific surface area of the electrode as possible, Sun et al. followed up by designing aligned carbon fibers interweaved with highly porous nanofibers, which not only provided high permeability due to the reduced tortuosity and large macropores but also furnished a large specific surface area for electrochemical reactions, thus achieving the coupled enhancement of mass transport and electrochemical properties.⁶

After considering the influence of the convective process on electrolyte transport, the ion diffusion process near the electrode-electrolyte interface also deserves attention, which directly affects the reactant ion concentration near the electrode active sites and constrains the electrochemical reaction rate. For example, Wang et al. constructed a gradient-distributed NiCo₂O₄ nanorod-composed graphite felt electrode, which not only enhanced the diffusion process of the electrolyte but also provided more active sites for the reactants.⁷ Meanwhile, the gradient-distributed catalyst structure accelerated the transport of reactants and electrochemical reactions on the membrane side while balancing the rapid refreshing of the electrolyte on the field side, thus achieving an ordered reaction interface. In the latest frontier work, Jiang et al. investigated nano-transport pathways and exposed optimal crystalline planes to integrate the mass transport and catalytic activities of bismuth-based catalysts, which released the intrinsic advantage of the catalysts and unlocked the interfacial dynamics.² By understanding the processes that reactants undergo inside the battery, they realized that the reactants have trouble successfully reaching the active site surfaces without being guided due to long diffusion paths and low drag forces. In this regard, to make the reactants approach the active sites accurately and directly, they decorated vertical-aligned bismuth nanosheets on the electrode surface, providing diffusion highways and a large number of active sites suggested by unsaturated Bi atoms for the reactants, thus further achieving enhanced catalytic and transport properties.

In addition to the continuous optimization of the convection-diffusion-reaction process, it is also important to understand the distribution of the reactants in the battery, which allows deeper insight into the mechanism of the coupled processes between mass transport and electrochemical reactions. In this regard, employing appropriate characterization methods to recognize the reactant flow inside the battery and its redox reactions is necessary. The local current density distribution, reflecting the intensity and distribution of electrochemical reactions within electrodes, is strongly connected to local reactant concentration distribution and the electrochemical property of the electrode. Accurately measuring the local current density distribution or local reactant concentration distribution can provide a theoretical basis for understanding the coupled phenomena inside the battery, and provide a foundation for optimizing the key components. Nowadays, some experimental methods have been developed and investigated, and these can basically be divided into four categories, namely, the segmented cell method, potential probe method, fluorescence microscopy method, and optical refraction index detection method, in which the segmented cell method can also be subdivided into the resistor network method, hall sensor method, and printed circuit board method. In detail, the segmented cell method and potential probe method enable the measurement of local current density distribution on a macroscopic scale. For example, Clement et al. employed the printed circuit board method to calculate the current of each segment, obtaining the current density distribution across the electrode plane.⁸ Meanwhile, the fluorescence microscopy method and optical refraction index detection method enable local current density measurements on a meso-microscopic scale. For instance, Andrew et al. utilized the fluorescent properties of organic redoxes to visualize electrochemical reactions within a single flow channel.⁹ Chen et al. proposed a reflection absorption imaging method to map the concentration distribution of products, which was based on the chromogenic reaction and enabled real time in-situ visualization of different areas across the electrode surface, thus providing a deep understanding of the spatial distribution difference and reaction kinetics inside the electrode.¹⁰

Given the progress that has been made, as previously mentioned, in the aspect of the coupled optimization of convection-diffusion-reaction processes, the above work provides a feasible direction for unlocking the problem of mutual constraints between mass transport and electrochemical activity, i.e., simultaneous enhancement of both by constructing cross-scale mass transport channels and increasing the catalytic activity within the channels/at the electrode surfaces. However, more accurate modeling and mechanism explanations of the coupled processes are still lacking; thus, there are still limitations in giving theoretical guidance for the optimization of key components. Furthermore, the current regulation and optimization strategies are not sufficient to support scaling up and large-scale applications, which impacts the performance improvement of commercial stacks or systems. Additionally, in terms of characterization methods, the currently developed characterization methods have some application limitations, such as insufficient accuracy, limited runtime, restricted large-scale applications, etc., which leads to some constraints on the in-depth study of the coupled phenomena and their changes during long cycles.

Future directions

Considering the current progress that has been made in realizing the coupled enhancement of mass transport and electrochemical properties, there are still some aspects that need to be explored by those researchers in this field.

(1) To gain a deeper understanding of this coupled phenomena, more accurate models at different scales and for various transport-reaction processes need to be constructed so that the relationship between the reactants and various key parameters, as well as important components in the convection-diffusion-reaction processes, can be described in detail, thus providing theoretical guidance for subsequent optimization. In addition, advanced computer technologies such as deep learning can be used to carry out more comprehensive and perfect learning and exploration.

(2) In terms of the modification of key components, the modification of microscopic transport channels on the electrode surface, the exploration of catalytic properties, and the novel flow-field structure design are the current directions that should be focused on, and the scaling-up method and their applicability to large-scale stacks need to be considered in order to narrow the performance gap between commercial stacks and laboratory-scale batteries. Additionally, since the flow channels and electrodes are indispensable to each other and coupled to affect the battery performance, it is recommended to consider the design of the two in combination rather than only one aspect so that the optimized structures can match each other and work together.

(3) On the aspect of characterization methods, future characterization tools will need to be innovatively designed and optimized in terms of precision, accuracy, lifetime, and system integration so that they can be applied not only to lab-scale batteries but also to large-scale commercial power stacks for accurate measurements. In addition, attention should be paid to the combined utilization of advanced computer technology and cross-disciplinary characterization devices to improve analysis efficiency and promote the development of experimental methods.

Funding and acknowledgements

The work described in this paper was supported by the National Natural Science Foundation of China (no. 52106265); the Natural Science Foundation of Tianjin Province, China (no. 23JCZDJC01090); the Guangdong Major Project of Basic and Applied Basic Research (2023B0303000002); and High level of special funds (G03034K001).

Declaration of interests

The authors declare no competing interests.

Published Online: April 18, 2025