Person:

Huskinson, Brian Thomas

Flow batteries are of interest for low-cost grid-scale electrical energy storage in the face of rising electricity production from intermittent renewables like wind and solar. We report on investigations of redox couples based on the reversible protonation of small organic molecules called quinones. These molecules can be very inexpensive and may therefore offer a low cost per kWh of electrical energy storage. Furthermore they are known to rapidly undergo oxidation and reduction with high reversibility under some conditions, suggesting the possibility of high current density operation, which could lead to low cost per kW. We report cyclic voltammetry measurements for 1,4-parabenzoquinone in neutral pH aqueous solution using a three-electrode setup. We report full fuel cell measurements as well, utilizing p-benzoquinone in an acidic solution as a positive electrode material and a hydrogen negative electrode, where current densities in excess of 240 mA (cm^{-2}) have been achieved to date. These initial results indicate that the quinone/hydroquinone redox couple is a promising candidate for use in redox flow batteries.

In this work I describe various approaches to electrochemical energy storage at the grid-scale. Chapter 1 provides an introduction to energy storage and an overview of the history and development of flow batteries. Chapter 2 describes work on the hydrogen-chlorine regenerative fuel cell, detailing its development and the record-breaking performance of the device. Chapter 3 dives into catalyst materials for such a fuel cell, focusing on ruthenium oxide based alloys to be used as chlorine redox catalysts. Chapter 4 introduces and details the development of a performance model for a hydrogen-bromine cell. Chapter 5 delves into the more recent work I have done, switching to applications of quinone chemistries in flow batteries. It focuses on the pairing of one particular quinone (2,7-anthraquinone disulfonic acid) with bromine, and highlights the promising performance characteristics of a device based on this type of chemistry.

As the fraction of electricity generation from intermittent renewable sources—such as solar or wind—grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output(^{1, 2}). In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form(^{3, 4, 5}). Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts(^{6, 7}). Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the (Br_2/Br^-) redox couple, yields a peak galvanic power density exceeding 0.6 W cm^{−2} at 1.3 A cm^{−2}. Cycling of this quinone–bromide flow battery showed >99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals(^8). This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.

We have demonstrated the performance of an aqueous redox flow battery composed of a negative electrode consisting of a redox couple between anthraquinone di-sulfonate and its corresponding hydroquinone, and a positive electrode consisting of a redox couple between hydrobromic acid and bromine. The peak power density is approximately 0.6 W/cm2. After 750 deep cycles, the average discharge capacity retention is 99.84% per cycle and the average current efficiency is 98.35%.

At present, there is an ongoing search for approaches toward the storage of energy from intermittent renewable sources like wind and solar. Flow batteries have gained attention due to their potential viability for inexpensive storage of large amounts of energy. While the quinone/hydroquinone redox couple is a widely studied redox pair, its application in energy storage has not been widely explored. Because of its high reversibility, low toxicity, and low component costs, we propose the quinone/hydroquinone redox couple as a viable candidate for use in a grid-scale storage device. We have performed single-electrode tests on several quinone/hydroquinone redox couples, achieving current densities exceeding 500 mA/cm2, which is acceptable for use in energy applications. We fabricated a full cell using para-benzoquinone at the positive electrode against a commercial fuel cell hydrogen electrode separated by a Nafion membrane. We evaluated its performance in galvanic mode, where it reached current densities as high as 150 mA/cm2. The results from these studies indicate that the quinone/hydroquinone redox couple is a promising candidate for use in redox flow batteries.