By Mark Miltenburg, Member-at-Large for the GCI
Batteries are not usually thought of as very “green” technologies. They typically employ toxic heavy metals and scarce elements, and they are not often recycled very effectively [1, 2]. However, their increasing usage in applications such as cell phones and electric vehicles has led to these factors being overlooked compared to improvements in cost and performance.
I recently came across a paper in the journal Nature on a new redox-flow battery . Redox-flow batteries (RFBs) are stationary energy storage devices primarily used as on-site redundant power sources, and could potentially be used to store energy from intermittent renewable sources like solar or wind.In this paper, the authors improve upon existing RFBs in three key areas: cost, safety, and scalability. This directly addresses at least 3 of the 12 principles of green chemistry as explained below .
Traditional RFBs use expensive and non-scalable materials like lithium or vanadium. This paper replaces these materials by employing inexpensive and environmentally friendly polymers for use as active materials. These polymers were able to approach conventional vanadium-based RFBs in performance, while being shown to be significantly less toxic than vanadium compounds. This greatly improves the cost and scalability of RFBs for future applications. This hits principle number 4: Chemical products should be designed to preserve efficacy of function while reducing toxicity.
The authors also were able to replace the electrolyte, usually based on sulfuric acid, with a non-corrosive, non-toxic sodium chloride solution. This is very important for RFBs due to the sheer size of RFB installations, which contain several tonnes of electrolyte. This checks off principle number 12: Substances and the form of a substance used in a chemical process should be chosen to minimize potential for chemical accidents, including releases, explosions and fires.
In addition, the authors were able to replace one of the most expensive elements of traditional RFBs. The use of Nafion, a perfluorinated polymer used to make conducting membranes, is common due to the chemical stability required by sulfuric acid electrolytes. However, Nafion accounts for 40% of the cost of a reaction cell. The authors replaced Nafion membranes in their battery with inexpensive cellulose-based dialysis membranes, which were compatible with the milder sodium chloride electrolyte. On top of the cost benefit, this touches on principle number 10: Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products. Nafion is very stable and does not easily break down, while cellulose naturally degrades into polysaccharides or glucose.
I really enjoyed this paper, as it demonstrated to me that you are able to make significant improvements to existing technologies by making them greener. In the case of RFBs, a spin-off company is licensing this technology already, making use of the cost and safety benefits brought about by green chemistry. It is one more example that sustainable thinking does not necessarily have to hurt a company’s bottom line.
 Image created by Nick B, distributed under a CC BY-SA 3.0 license. https://commons.wikimedia.org/wiki/File:Redox_Flow_Battery_English.png.
 Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998, p.30.