To survive an interminable flight, I was engaged in a favorite pastime: reading C&EN.
I knew nothing about redox flow batteries and thus found your article “Electricity Storage Goes with the Flow” (C&EN, Feb. 8, page 26) very enjoyable. Then I saw (gulp!) the structure of methyl viologen. As a biochemist, I have used it as a redox cycler by that name, but I have also spent several decades working on it as a pesticide chemist as the herbicide paraquat.
When I put on my Sierra Club hat, paraquat is a green herbicide, but as a toxicologist, I should warn you that it probably has killed more people than all other pesticides combined. It is taken up by a transport system in the lung where it generates a variety of reactive oxygen species and burns the lungs; people normally drown several weeks later of pneumonia.
There is no antidote. Most deaths now are suicides and murders, with a few accidents thrown in. It was banned in the U.S. but now is reregistered with a sophisticated formulation that smells terrible, is brightly colored, solidifies on contact with stomach acid, and is a powerful emetic. Compared with many of the materials we work on in my lab, paraquat has a toxicity that is minor, but when it is used on a large scale by people who do not appreciate its danger, the outcome of an accident could be terrible. The paraquat in the battery will not be formulated for safety.
Paraquat is also a very cheap redox agent, so it can be used on a massive scale. I am sure it comes with a cautionary data sheet, but the people building and using these batteries will not be toxicologists. Because paraquat binds so tightly to soil particles and is not bioavailable, its use in batteries probably presents little environmental risk. However, its high toxicity if consumed in even tiny amounts presents a health hazard that requires adequate warnings regarding accidents or end-of-life issues with the proposed batteries. In working some as a toxicologist with the computer industry, I saw highly trained people become very lax over some toxic chemicals. Anytime we have a high-volume use for a highly toxic material, I worry about a problem years downstream.
Editor’s note: To assist educators, C&EN and ACS released an infographic illustrating new National Fire Protection Association guidance for conducting demonstrations and experiments, as well as a video showing how to conduct flame tests using aqueous salt solutions. The resources are available at cenm.ag/labdemo.