# Introducing an Experimental Route to Identify and Unify Lab‐Scale Redox‐Flow Battery Cell Performances via Molar Fluxes and Cell Constants

## TL;DR

This paper introduces a new experimental method to compare and optimize lab-scale redox flow battery performance using molar fluxes and key parameters.

## Contribution

The paper introduces K1critical, ζ, and K2 as novel parameters to standardize and compare lab-scale RFB performance.

## Key findings

- K1critical identifies the optimal operating ratio for efficient charge-discharge cycling.
- The cell constant ζ quantifies the impact of RFB setup on performance.
- K2 enables comparison of idealized operating parameters across different RFB architectures.

## Abstract

Redox flow batteries (RFBs) are a promising technology for grid energy storage based on their high potential for scalability, design flexibility, high efficiency, and long durability, hence great effort has been invested in this area of research. However, due to the large differences in lab‐scale RFB cell design and construction as well their operational performance, fundamental studies on innovative RFB components (e.g., active materials, separators, additives) compare poorly due to the lack of standard setups, settings, and procedures. This work introduces an experimental calibration route for aqueous as well as nonaqueous RFBs based on a simple mass transport model using molar fluxes, enabling one to compare dissimilar lab‐scale RFB cell setups by introducing several RFB parameters: First, K1, which summarizes the operating parameters of an RFB to identify the critical ratio (K1critical) needed for efficient charge–discharge cycling using a simple overvoltage and charge efficiency evaluation; second, the RFB cell constant ζ, quantifying the influence of a lab‐scale RFB setup on its performance; and finally, K2, ultimately enabling full comparison of (idealized) K1critical operating parameters across RFB cell setups.

Introducing an experimental route to identify optimal lab‐scale redox flow batteries (RFB) operating parameters between electrolyte flow rate, redox‐active species concentration and applied current for aqueous and nonaqueous calibration electrolytes via a molar flux concept. K1critical as an RFB‐setup performance evaluating, ζ as an RFB cell‐constant, and K2 as an cell‐architecture independent performance characteristic number are introduced to improve comparability between lab‐scale RFB architectures.