On the Characterization of Membrane Transport Phenomena and Ion Exchange Capacity for Non-Aqueous Redox Flow Batteries

Abstract

The development of high-performance membrane materials for non-aqueous redox flow batteries (NAqRFBs) could unlock a milestone towards widespread commercialization of the technology. Understanding of transport phenomena through membrane materials requires diagnostic tools able to monitor the concentrations of redox active species. While membrane characterization in aqueous media focused the attention of the scientific community, dedicated efforts for non-aqueous electrolytes remain poorly developed. Here, we develop new methodologies to assess critical membrane properties, namely ion exchange capacity and species transport, applied to NAqRFBs. In the first part, we introduce a method based on 19F-NMR to quantify ion exchange capacity of membranes with hydrophobic anions commonly used in non-aqueous systems (e.g., PF6 - and BF4 -). We find a partial utilization of the ion exchange capacity compared to the values reported using traditional aqueous chemistry ions, possibly limiting the performance of NAqRFB systems. In the second part, we study mass transport with a microelectrode placed on the electrolyte tank. We determine TEMPO crossover rates through membranes by using simple calibration curves that relate steady-state currents at the microelectrode with redox active species concentration. Finally, we show the limitations of this approach in concentrated electrolyte systems, which are more representative of industrial flow battery operation.

A Method for Quantifying Crossover in Redox Flow Cells through Compositionally Unbalanced Symmetric Cell Cycling

“…S1) whereby the plateau currents are proportional to concentration. 16 Voltammograms were recorded on a CH Instruments 760e potentiostat at 10 mV s -1 using a 25 μm diameter gold working electrode (BASi), a platinum wire counter electrode (BASi), and a Ag/AgCl reference electrode (BASi). Prior to use, the microelectrode was polished on a MicroCloth pad containing an aqueous slurry of 0.05 μm alumina powder (Buehler Ltd.), rinsed with deionized water, and gently dried with lens paper.…”

“…15 Membrane/separator properties are often measured through a suite of ex situ techniques to quantify permeability and conductivity. [15][16][17] While these properties offer a means to compare different materials, flow cell testing is necessary to evaluate transport under representative RFB conditions which feature high current densities and time-varying species concentrations. One solution is to simply test the membrane/separator in a flow cell with the redox chemistry of interest and report metrics such as ratedependent coulombic efficiency, voltaic efficiency, and capacity retention.…”

A method for quantifying crossover in redox flow cells through compositionally unbalanced symmetric cell cycling

“…Leached species concentrations (Table S1) were measured using microelectrode voltammetry (Figure S1) whereby the plateau currents are proportional to concentration. 16 Voltammograms were recorded on a CH Instruments 760e potentiostat at 10 mV s -1 using a 25-μm diameter gold working electrode (BASi), a platinum wire counter electrode (BASi), and a Ag/AgCl reference electrode (BASi). Prior to use, the microelectrode was polished on a MicroCloth pad containing an aqueous slurry of 0.05 μm alumina powder (Buehler Ltd.), rinsed with deionized water, and gently dried with lens paper.…”

“…15 Membrane / separator properties are often measured through a suite of ex situ techniques to quantify permeability and conductivity. [15][16][17] While these properties offer a means to compare different materials, flow cell testing is necessary to evaluate transport under representative RFB conditions-high current densities, time-varying species concentrations. One solution is to simply test the membrane / separator in a flow cell with the redox chemistry of interest and report metrics such as rate-dependent coulombic efficiency, voltaic efficiency, and capacity retention.…”

An electrochemical quartz crystal microbalance (EQCM) based on microelectrode arrays allows to distinguish between adsorption and electrodeposition

“…These differ from the MEAs extensively used in the life sciences, where the electrodes are read out separately, thereby mapping out a current distribution, for instance underneath a neural network. [24][25][26] "Microelectrodic behavior" in the following denotes the consequences of convergent diffusion at the electrodes.…”