Combining Electrochemical, Fluid Dynamic, and Imaging Analyses to Understand the Effect of Electrode Microstructure and Electrolyte on Redox Flow Batteries
Authors: Simon Benedict A. , Gayon Lombardo Andrea , Pino Catalina , Wood Charles E. , Tenny Kevin M. , Greco Katharine , Cooper Samuel J. , Forner-Cuenca Antoni , Brushett Fikile R. , Kucernak Anthony R. J. , Brandon Nigel P 
Published: December 2020

Understanding the interplay between electrode microstructure and cell performance of electrochemical devices is important both for modelling and experimental design. Redox Flow Batteries (RFBs) are an electrochemical energy storage technology with potential for grid-scale energy storage applications, although costs need to be further reduced to be competitive. One pathway to lowering the costs involves increasing the power density of each cell, such that fewer cells are required. This can be achieved in a variety of ways, including by improving the design of the electrode microstructures. The aim of this work is to better understand the relationship between electrode microstructure and RFB performance and, ultimately, to make inferences about electrode utility.
The performances of a variety of commercially available carbon electrodes are examined via a series of commonly used microstructural and electrochemical analyses (Figure a-c). [1] We present a comprehensive study of pore-scale mass-transport processes occurring in each of the electrodes and rationalize their effect on the overall cell performance. A matrix of electrochemical tests were carried out in a flow-through RFB cell using incremental flow rates (Figure b-c) and two non-aqueous TEMPO electrolytes with distinct viscosity and diffusivity properties. Scanning electron microscopy (SEM) was used to image the electrodes and large 3 mm samples of each were scanned using X-ray computed tomography (Figure a). A customized segmentation technique was subsequently developed that resamples the image data to ensure the fiber dimensions agree with SEM images, improving the validity of the various extracted metrics. From these images, calculations and electrochemical tests, several microstructural parameters were extracted and a pore network model was used to calculate the permeabilities of the electrodes. A 1D model was developed across half the symmetric membrane electrode assembly and, using the parameters extracted from XCT, fit to galvanostatic polarization measurements to obtain the mass transfer coefficients at each unique operating condition – these were found to be in the range of and m s-1. Transport processes and distributions through the cell are analysed and correlations between dimensionless Reynolds and Sherwood numbers are subsequently discussed, as demonstrated in previous work. [2, 3]
Given a ‘good performance’ is associated with a system being able to reach a high current with a low overpotential, the following findings were made. Firstly, while volume-specific surface area, porosity and tortuosity are useful descriptors for evaluating porous media, they are not good indicators of performance in these systems. Instead the permeability and, by extension, the ease with which convective transport can occur is found to be directly correlated with performance in this system. It is asserted that systems using solvents with higher viscosity have a reduced performance due, in part, due to regions in the electrode being starved of new electrolyte. Consistently, across all electrodes and in each electrolyte a better performance is observed when a higher flow rate is used. This effect is, in part, attributed to the reduction in size of the boundary layer at the electrode surface. Finally, in agreement with previous works, [1, 4] the carbon cloths tested in this work are shown to have higher mass-transfer coefficients and permeabilities than the carbon papers and we conclude that the best performing electrode is the thicker carbon cloth, which has the lowest resistance to convective flow, highest permeability, and highest mass-transfer coefficients in both solvents.
[1]: A. Forner-Cuenca, E. E. Penn, A. M. Oliveira and F. R. Brushett, Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries. J. Electrochem. Soc., 166, 2230-2241 (2019).
[2]: Y. A. Gandomi, D. S. Aaron, T. A. Zawodzinski and M. M. Mench, In Situ Potential Distribution Measurement and Validated Model for All-Vanadium Redox Flow Battery. J. Electrochem. Soc., 163, A5188-A5201 (2016).
[3]: K. M. Tenny, A. Forner-Cuenca, Y.-M. Chiang and F. R. Brushett, Comparing Physical and Electrochemical Properties of Different Weave Patterns for Carbon Cloth Electrodes in Redox Flow Batteries. J. Electrochem. En. Conv. Stor., 17, 041108 (2020).
[4]: M. A. Sadeghi, M. Aganou, M. D. R. Kok, M. Aghighi, G. Merle, J. Barralet and J. Gostick, Exploring the Impact of Electrode Microstructure on Redox Flow Battery Performance Using a Multiphysics Pore Network Model. J. Electrochem. Soc., 166, A2121-A2130 (2019).

Figure 1