Alcoholic Compounds as an Efficient Energy Carrier

Abstract

We introduce a power circulation system using redox reactions of glycolic acid (GC), a monovalent alcoholic compound, and oxalic acid (OX), a divalent carboxylic acid for the efficient circulation of renewable electricity. Electric power is efficiently accumulated in GC via four-electron reduction of OX on TiO2 electrodes. Mapping for phases of TiO2 using electron energy-loss spectroscopy revealed that anatase-type TiO2 particles exhibit superior catalytic activities, i.e. highly selective electroreduction of OX to produce GC, compared to rutile-type ones. GC was successfully produced on porous TiO2 spheres purely composed of the anatase phase under mild conditions in the potential region of −0.5 to −0.7 V versus the RHE at 50 °C with high efficiency and selectivity (70–95% Faradaic efficiency and >98% selectivity). Direct transformation of solar energy into chemical energy in GC was also achieved using a photo-assisted electrochemical cell employing oxide semiconductor photoelectrodes as the anode for water oxidation and TiO2 cathode for OX reduction. A liquid flow-type electrolyzer that continuously produces GC from OX was constructed by employing a polymer electrolyte electrolyzer, named a polymer electrolyte alcohol electrosynthesis cell (PEAEC). Porous anatase TiO2 directly grown on Ti mesh (TiO2/Ti-M) was newly fabricated as the cathode electrode having favourable substrate diffusivity. A membrane electrode assembly composed of the TiO2/Ti-M, Nafion and an IrO2 supported on a gas-diffusion carbon electrode (IrO2/C) was applied to the PEAEC. The PEAEC achieves a maximum energy conversion efficiency of 49.6% or a continuous 99.8% conversion of 1 M OX, which is an almost saturated aqueous solution at room temperature. Electronic power generation via electro-oxidation of GC was demonstrated. The catalytic activity test on various metal materials revealed that Rh, Pd, Ir and Pt have preferable features as a catalyst for GC electro-oxidation, and Pt exhibits the highest catalytic activity. A carbon-supported Pt catalyst (Pt/C) in alkaline conditions, especially in LiOH aq., showed higher activity, durability and product selectivity for electro-oxidation of GC rather than those in acidic media. These efforts will contribute to efficient utilization of renewable electricity.