A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery

Summary

We present the protocols for electrochemically evaluating a symmetric non-aqueous organic redox flow battery and for diagnosing its state of charge using FTIR.

Abstract

Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid and deployment of renewable energy technologies. Among the many flow battery chemistries, non-aqueous flow batteries have the potential to achieve high energy density because of the broad voltage windows of non-aqueous electrolytes. However, significant technical hurdles exist currently limiting non-aqueous flow batteries to demonstrate their full potential, such as low redox concentrations, low operating currents, under-explored battery status monitoring, etc. In an attempt to address these limitations, we recently reported a non-aqueous flow battery based on a highly soluble, redox-active organic nitronyl nitroxide radical compound, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). This redox material exhibits an ambipolar electrochemical property, and therefore can serve as both anolyte and catholyte redox materials to form a symmetric flow battery chemistry. Moreover, we demonstrated that Fourier transform infrared (FTIR) spectroscopy could measure the PTIO concentrations during the PTIO flow battery cycling and offer reasonably accurate detection of the battery state of charge (SOC), as cross-validated by electron spin resonance (ESR) measurements. Herein we present a video protocol for the electrochemical evaluation and SOC diagnosis of the PTIO symmetric flow battery. With a detailed description, we experimentally demonstrated the route to achieve such purposes. This protocol aims to spark more interests and insights on the safety and reliability in the field of non-aqueous redox flow batteries.

Introduction

Redox flow batteries store energy in liquid electrolytes that are contained in external reservoirs and are pumped to internal electrodes to complete electrochemical reactions. The stored energy and power can thus be decoupled leading to excellent design flexibility, scalability, and modularity. These advantages make flow batteries well-suited for stationary energy storage applications for integrating clean yet intermittent renewable energies, increasing grid asset utilization and efficiency, and improving energy resiliency and security.^1,2,3 Traditional aqueous flow batteries....

Protocol

Note: All the solution preparations, cyclic voltammetry (CV) tests, and flow cell assembly and tests were carried out in an argon-filled glove box with water and O₂ levels less than 1 ppm.

1. Electrochemical Evaluations of PTIO Flow Cells

1. CV Test
   1. Polish a glassy carbon electrode with 0.05 µm gamma alumina powder, flush it with deionized water, put it in under vacuum at room temperature for overnight, and transfer it into a glove box.
   2. Dissolve silver nitrate (8.5 mg) with MeCN (5 mL) in the glove box, i.e., 10 mM AgNO₃. Add the solution into the glass tub....

Results

The unique advantages of the symmetric PTIO flow battery system are highly ascribed to the electrochemical properties of PTIO, an organic nitroxide radical compound. PTIO can undergo electrochemical disproportionation reactions to form PTIO⁺ and PTIO⁻ (Figure 2a). These two redox pairs are moderately separated by a voltage gap of ~1.7 V (Figure 2b) and can be used as both anolyte and catholyte redox materials in a symmetric bat.......

Discussion

As we demonstrated before,²⁵ FTIR is capable of non-invasively detecting the SOC of the PTIO flow battery. As a diagnostic tool, FTIR is particularly advantageous because of its easy accessibility, fast response, low cost, small space requirement, facility for online incorporation, no detector saturation, and the ability to correlate structural information to investigate molecular evolutions during flow battery operation. Figure 3e illustrates a proposed flow battery device integr.......

Materials

Name	Company	Catalog Number	Comments
PTIO	TCI America	A5440	>98.0%
Tetrabutylammonium hexafluorophosphate	Sigma-Aldrich	86879	electrochemical grade, ≥99.0%
MeCN	BASF	50325685	Battery grade
Silver nitrate	Sigma-Aldrich	204390	99.9999% trace metals basis
Gamma alumina powder	CH Instruments	CHI120
Graphite felt	SGL	GFD3	Vacuum-dry at 70°C for 24 h
Porous separator	Daramic	AA800	Vacuum-dry at 70°C for 24 h
Battery Tester	Wuhan LAND electronics Co., Ltd.	Lanhe	1A current range
Electrochemical Workstation	Solartron Analytical	ModuLab
glove box	MBRAUN	Labmaster SP	oxygen and water levels <1 ppm
ESR spectrometer	Bruker	Elexsys 580	Equipped with an SHQE resonator with microwave frequency ~9.85 GHz (X band) at 2 mW power, with 100 kHz field modulation