Carbon-Neutral Energy Cycle via Highly Selective Electrochemical Reactions Using Biomass Derivable Organic Liquid Energy Carriers

Abstract

Abstract Efficient storage and transport of electric energy is essential to promote the use of renewable energy based electricity. We demonstrate an energy cycle based on highly selective redox reactions between lactic acid (Lac) and pyruvic acid (Pyr), both of which are liquid under ambient conditions and can be obtained from biomass resources, thus realizing a completely low-emission system. As an energy storage device, an electrosynthesis cell (LAEC) for the production of Lac from Pyr was constructed using a membrane electrode assembly (MEA) consisting of a TiO2 cathode catalyst for the electroreduction of Pyr and an IrOx anode catalyst for water oxidation. Our LAEC achieved highly efficient Lac production from 10 M Pyr aqueous solution with Faradaic efficiency (FE) of approximately 100% in the applied voltage range of 1.4–2.4 V, resulting in an energy conversion efficiency of 50% and a current density of −0.4 A cm−2 at 2.0 V. Direct Lac fuel cell (DLAFC) was also constructed and its FE values for the Pyr production reached approximately 100%, enabling direct electronic energy storage in bio-derivative liquid carriers and efficient energy circulation with minimal CO2 emissions.