Electrode Structures to Improve the Performance of Redox Flow Batteries
Authors: Hoyt Nathaniel C , Wainright Jesse S. , Savinell Robert F. , McCrabb Heather , Taylor E. J. 
Published: February 2020

Flow batteries have many features that show promise for large scale energy storage such as independent sizing of power and energy capacity, potentially low cost, and inherently greater safety as compared to other electrochemical energy storage technologies. Conventional redox flow batteries face practical challenges from non-uniform pressure drops across the stack, mass transfer limitations, and high manufacturing expenses related to electrode and bipolar plate material costs and component alignment challenges. For example, for a vanadium flow battery, the combined electrode and bipolar plate costs are estimated to be about 15% of the system cost, and for the All-Iron Flow battery these costs account for about 45% of the system costs.
By replacing the typical porous electrode material with electrodes that are coupled to thin metallic bipolar plates possessing specially engineered structures such as arrays of posts, pyramids and/or pillars, acceptable pressure drops can be realized while still increasing reactant mass transfer rates. This eliminates costs associated with the porous electrode material and component alignment, thus decreasing the impact of the stack cost for the manufacturing of redox flow batteries.
In this presentation we will describe our approach to address this challenge, and specifically discuss the collaboration and interaction of our university/industry team.
Researchers at Faraday Technology are developing an electrochemical etching process based on pulse reverse waveforms (anodic/cathodic pulses) capable of 1) fabricating structures of interest for electrode/bipolar plates for enhanced reactant mass transfer, 2) fabricating structures with varying aspect ratios from inlet to outlet as an approach to maintaining acceptable pressure drop, and 3) achieving process scalability analogous to pattern plating of printed circuit boards (PCB). Research by Faraday is being carried out to understand the effects of process parameters (anodic on-time, anodic peak voltage, cathodic on-time, cathodic peak voltage, and off-time) and electrolyte flow velocities on the etch factor which thereby results in a library of waveform process parameters that effectively form a set of design rules. With this shared knowledge base, researchers at CWRU are hypothesizing manufacturable flow-field/electrode designs within the design rule constraints and performing initial performance evaluations using computational fluid dynamic models. Joint discussion of the results from modeling and of the practical aspects of manufacturing and implementation allows the team to down-select to a smaller group of electrode designs for further experimental evaluation.
These electrode designs are then fabricated by Faraday using pulse reverse electrochemical etching. Discussions between both groups determine the selection of materials for construction. The electrode structures are then transferred to CWRU for performance evaluation.
CWRU’s evaluation protocol includes 1) coating electrodes for surface protection and electrochemical activity, 2) measurement of pressure drop vs electrolyte flow rate, and 3) characterizing mass transfer and kinetics with test reactions such as the ferric/ferrous redox couple.
In the first phase of the project, Faraday and CWRU identified two “model” electrode/bipolar plate structures to demonstrate manufacturing feasibility and simulation viability: an in-line and a staggered post pattern (Figure 1). CWRU simulations demonstrated reasonable agreement with the measured limiting current densities for the two different structures over a wide range of flow rates in experimental evaluations (Figure 2).
The project is now exploring more detailed designs with higher performance targets. The interaction and open collaboration of this approach will be absolutely necessary for the success of this strategy to reduce the cost of these types of flow battery components.

Acknowledgments

The authors gratefully acknowledge the support of the Department of Energy Office of Electricity and Dr. Imre Gyuk.

Figure 1: Etched flow field pattern on a metal plate

Figure 2: Comparison of limiting current density vs flow rate: simulation and experimental data