CO₂-free electric power circulation via direct charge and discharge using the glycolic acid/oxalic acid redox couple

Abstract

The establishment of an efficient electric power distribution method is the key to realising a sustainable society driven by renewableenergy-based electricity, such as solar photovoltaics, wind turbine, and wave electricity, in view of supply instability. Here, we demonstrate an electric power circulation method that does not emit CO 2 and is based on the glycolic acid (GC)/oxalic acid (OX) redox couple.Direct electric power storage in GC ensures considerably high energy density storage and good transportability through OX electroreduction with significantly high selectivity (498%) using pure anatase-type titania (TiO 2 ) spheres under mild conditions in the potential region of À0.5 to À0.7 V vs. the RHE at 50 8C. The most desirable characteristic of this electroreduction is the suppression of hydrogen evolution even in acidic aqueous media (Faraday efficiency of 70-95%, pH 2.1). We also successfully generated power without CO 2 emissions via selective electrooxidation of GC with an alkaline fuel cell. † Electronic supplementary information (ESI) available: Experimental methods, Schemes S1 and S2, and Fig. S1 to S10. See An excessive increase of CO 2 in the atmosphere is regarded as the most probable cause of global warming. A substantive transition from fossil-based systems to systems operated by electricity that is generated using renewable energy, i.e., ''renewable electricity'', seems to be the optimal answer to this environmental issue. A lack of efficient distribution techniques for unstably supplied and unevenly distributed renewable electricity is one of the fundamental impediments to its practical use. Thus, electric power storage in high-energy chemicals, called ''energy carriers'', has received much attention due to their efficient storage and on-demand supply of renewable electricity. Here, we demonstrate direct electric power charge using an alcohol/carboxylic acid redox couple. Highly energetic and transportable glycolic acid, an alcoholic compound, was successfully produced by electroreduction of oxalic acid, a dicarboxylic acid, on ubiquitous TiO 2 catalysts with high efficiency and selectivity (70-95% Faraday efficiency and 498% selectivity). Furthermore, we succeeded in electric power generation via the selective electrooxidation of glycolic acid to oxalic acid without CO 2 emission-specifically, carbon-neutral power generation. These results are the first experimental proof of concept for a carbon-neutral energy circulation system based on charging/discharging electric power using an alcohol/carboxylic acid redox couple.

Hydrogenation of oxalic acid using light-assisted water electrolysis for the production of an alcoholic compound

“…Therefore, the suppression of hydrogen evolution reaction (HER) over a TiO 2 electrode is critically important to achieve a high Faradaic efficiency for OX reduction, which has been discussed in our previous report. 33 The TiO 2 electrode used in this work also exhibited high activities for OX reduction but low HER activities under the acidic conditions, as reported in Nature Materials. 53 Based on these results, we succeeded in the first production of an alcoholic compound, GC, from a carboxylic acid, OX, via water oxidation using light energy through an electrochemical reaction system.…”

Impact of TiO2-II phase stabilized in anatase matrix by high-pressure torsion on electrocatalytic hydrogen production

“…Figure 4(b) indicates that the amount of hydrogen is considerably enhanced by the inclusion of TiO 2 -II grains and the best electrocatalytic activity for hydrogen evolution is achieved on the sample with 18 wt% TiO 2 -II, whereas the conversion of oxalic acid on the three TiO 2 catalysts seems almost similar. Considering that the energy position of conduction band bottom predominantly influences the electrocatalytic oxalic acid conversion [23][24][25], the observed activities on TiO 2 catalysts for oxalic acid conversion are plausible because the conduction band energy levels are reasonably the same for the three samples. Taken altogether, it is concluded that the presence of some amounts of TiO 2 -II as the second phase in the anatase matrix can improve the electrocatalytic activity for hydrogen evolution, but the best fraction of TiO 2 -II phase for electrocatalytic activity still needs to be optimized in future studies.…”

“…It should be noted that the group of authors recently reported the electrochemical hydrogenation of oxalic acid (divalent carboxylic acid) to produce glycolic acid (monovalent alcoholic compound) and found that the catalytic activity on TiO 2 considerably depends on the crystal structure, i.e. anatase exhibits extremely high reducibility for oxalic acid but rutile does not [23,24]. The difference in catalytic activity was attributed to the reducibility of electrons introduced from electrode to the conduction band bottom of TiO 2 , i.e.…”

Catalytic transformation of functionalized carboxylic acids using multifunctional rhenium complexes

“…The functional group transformation (FGT) and subsequent reduction of CAs hence represent an important research subject to furnish alcohols for platform/fine chemicals 1–11 , biofuels 11–13 , and electric power-storage materials 14 . Among those FGTs, the catalytic C–H bond functionalization of CAs 15–17 , followed by hydrogenation 18–23 , could potentially widen the diversity of alcohols thus available.…”