Electrolysis of Lithium-Free Molten Carbonates

Abstract

Electrochemically converting CO 2 /carbonate to functional carbon materials and oxygen in molten carbonates has been viewed as a promising approach to tackling the environmental and energy challenges. Until now, most molten carbonate electrolyzers have focused on lithium-based molten salts due to their relatively low eutectic temperature and thermodynamic feasibility. However, the use of lithium-based molten salts will be constrained by its limited resource. Herein, the electrolysis of lithium-free molten carbonates (MCO 3 −Na 2 CO 3 −K 2 CO 3 , M = Mg, Ca, Sr, and Ba) is systematically studied. Using the low-cost Ni-based inert anode, high value-added carbon and oxygen are prepared in the molten carbonates containing CaCO 3 , SrCO 3 , and BaCO 3 . In addition, the solubilities of alkaline-earth metal oxides (MO) in molten carbonates are measured and the results show that MgO, CaO, and SrO are nearly insoluble and BaO is soluble (8.62 wt % BaO in 10 wt % BaCO 3 − Na 2 CO 3 −K 2 CO 3 , 750 °C) in the corresponding alkaline-earth metal carbonates. Based on the solubility difference of oxides, two different electrolysis mechanisms are proposedfirst, electrochemical reduction of CO 2 if MO is soluble in molten carbonate with the production of C and oxygen; second, electrolysis of carbonate with the production of MO, C, and oxygen.

Capture and electrochemical conversion of CO2 in molten alkali metal borate–carbonate blends

“…Thus far, only metal carbonates and chlorides (and their blends) have been used as molten salt materials with the CO 2 solubility acceptable for electrochemical conversion of CO 2 . 1,4–10…”

“…Therefore, lithium carbonates per se are inefficient as reversible CO 2 sorbents at medium temperatures (below 750 °C) and are commonly replaced by mixed Li, Na, K carbonates as molten salt media in CO 2 electrolysis. 8,10,24,25…”

“…Thus far, only metal carbonates and chlorides (and their blends) have been used as molten salt materials with the CO 2 solubility acceptable for electrochemical conversion of CO 2 . 1,[4][5][6][7][8][9][10] Our group has advanced molten alkali metal borates such as lithium borate (Li 3 BO 3 ) and mixed lithium, sodium borates as reversible CO 2 sorbents due to their large and temperaturereversible sequestration capacity for CO 2 and acid gases, which generally exceeds the capacity of other molten salts. [11][12][13][14][15][16][17][18][19] Thermoreversible reactions of the alkali metal borate glasses with CO 2 are well-documented.…”

“…The formation of lithium oxide (8) is very important for the lithium carbonate recycling, as Li 2 CO 3 is consumed by the electrolysis and needs to be continuously replenished by reaction of excess Li 2 O (formed electrolysis product) with the CO 2 supplied:…”

“…Therefore, lithium carbonates per se are inefficient as reversible CO 2 sorbents at medium temperatures (below 750 °C) and are commonly replaced by mixed Li, Na, K carbonates as molten salt media in CO 2 electrolysis. 8,10,24,25 On the other hand, eutectic salt compositions of lithium and sodium and/or lithium and potassium carbonates that possess lower melting temperatures (<750 °C) are wellknown. 26,27 Electrochemical carbonate ion reduction and carbon formation/deposition reactions from eutectic alkali metal carbonate melts in the 500-750 °C range have been reported to occur on metallic cathodes made of Ni, Pt, W, Ag, Mo, Al, Cu, and steel, as well as glassy carbon electrodes.…”