Piezoelectric harvesting of mechanical energy for redox chemistry

Abstract

Much work has been done in the utilization of mechanical force to enable chemical processes. However, this process is limited to thermal- and deformation-driven reactions. In fact, the transfer of energy in mechanical reactors can be quite inefficient, with energy lost to heat and mechanical deformation. Although these losses diminish at larger scales, small-scale reactions (from a few milligrams to a kilogram) can suffer from unfavorable energy demands. Recent work has sought to harvest unused energy in mechanical reactors by converting it to a flow of electrons through the use of piezoelectric materials, as many economically important reactions rely on the transfer of electrons to enact chemical change. Recent work has shown that the addition of piezoelectric powders to mechanochemical reactions results in enhanced yields for reductive and oxidative chemistry. However, these materials ultimately contaminate the end product and must be removed. Additionally, impacts on a piezoelectric material produce an AC output; limiting this approach's usefulness to irreversible reactions. We have developed a cleaner approach using an external piezoelectric element to either supply or sink electrons during milling. Methylene blue was reduced to leucomethylene blue using our approach. Mechanochemical reaction rates for this reduction were determined with respect to media quantities and sizes with a maximum rate of 7.76 μM s⁻¹. It was found that the conversion rate is linearly dependent on the number of media and geometrically dependent on the size of the media. Our approach allows selective reduction and eliminates contamination of the products with piezoelectric material. Shuttling electrons in a mechanochemical reaction will enable difficult chemistry, such as the reduction of CO₂ or the production of low oxidation state inorganic compounds, to be achieved more easily.