A New Technology for Efficient, High Yield Carbon Dioxide and Water Transformation to Methane by Electrolysis in Molten Salts

Abstract

This study presents a new green technology for the sustainable utilization of carbon dioxide. Synthetic methane, if produced effi ciently and with low carbon footprint, provides a ready replacement for natural gas in the existing energy, manufacturing, and transportation industries. The fi rst technology for the production of methane from CO 2 and water by molten electrolysis is demonstrated. The technology is more effi cient than alternative renewable methane production by microbial, aqueous, or solid oxide electrolysis, or than various photocatalytic processes. Methane is produced at 1000-fold the rate observed in photoelectrochemical systems, and without noble metals or an external hydrogen source. Added steam and CO 2 continuously renew the electrolyte. CO 2 and H 2 O are directly transformed by electrolysis at 97.4% current effi ciency at a constant current of 250 mA through 20 cm 2 iron and nickel electrodes in a 600 °C alkali carbonate/LiOH electrolyte at low (<2 V) energy. The electrolysis product is comprised of 64.9% methane, 34.8% H 2 , and 0.3% longer (than C1) hydrocarbons.

Syngas production: diverse H2/CO range by regulating carbonates electrolyte composition from CO2/H2O via co-electrolysis in eutectic molten salts

“…37,38 Furthermore, temperatures of over 600 C favor methane formation, achieving a methane yield of 64.9% in a eutectic mixture of carbonates as electrolyte in electrochemically reducing H 2 O/CO 2 . 28 In our previous study, 33 when the temperature was about 550 C, the methane content was less than 25%. Aer conducting a systematic experimental study of experimental data, the range of electrolysis temperatures studied herein was chosen as 500-600 C.…”

“…As interpreted in our previous study, a high electrolysis temperature leads to an increased hydrogen yield due to enhanced reactivity. 28 Due to slower ionic migration, poor conductivity at low electrolytic temperature, difficulty of operation, and corrosion resistant performance at a high operating temperature, an appropriate temperature range was shown to be necessary for stable, continuous, and efficient electrolysis. Furthermore, according to previous theoretical and electrochemical reports, the selective electroreduction of CO 2 to CO likely occurs in Li-Na and Li-K molten salts at #650 C by cyclic voltammetry.…”

“…27 Herein, the CO 2 reduction mechanism at a gold electrode in molten carbonates is investigated using cyclic voltammetry. The present work is a systematic exploration of changes in the H 2 /CO ratio in various binary or ternary carbonates ( 28 with the goal of broadening the H 2 /CO ratio in syngas. During electrolysis, alkali oxides, which are produced from the decomposition of monovalent alkali carbonate and LiOH, can combine with CO 2 and H 2 O to renew the electrolyte.…”

Effect of molten carbonate composition on the generation of carbon material

“…However (in addition to O 2 product), the vacuum removal of the gas favors the low pressures decomposition of the molten electrolyte as: carbonate (molten) / oxide (dissolved) + CO 2 (gas). Different from co-electrolysis of CO 2 and H 2 O system, 24 no methane is detected due to the lack of hydrogen source. The low, but important formation of CO demonstrates that CO 2 splitting into zerovalent carbon isn't the exclusive degradation pathway at temperature of T < $850 C, accompanied by very little CO generation that is presumably to occur at T > $850 C.…”

Capture and electro-splitting of CO2 in molten salts

“…It should be mentioned that, CO 2 capture and conversion was also reported in the mixture of hydroxide and carbonate molten salts. By tuning the electrolysis conditions, syngas (H 2 + CO) or methane can be obtained [38,39] .…”