Spinel Metal Oxide-Alkali Carbonate-Based, Low-Temperature Thermochemical Cycles for Water Splitting and CO_2 Reduction

Abstract

A manganese oxide-based, thermochemical cycle for water splitting below 1000 °C has recently been reported. The cycle involves the shuttling of Na+ into and out of manganese oxides via the consumption and formation of sodium carbonate, respectively. Here, we explore the combinations of three spinel metal oxides and three alkali carbonates in thermochemical cycles for water splitting and CO_2 reduction. Hydrogen evolution and CO_2 reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe_(3)O_4 > Mn_(3)O_4 > Co_(3)O_4, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li_(2)CO_3 > Na_(2)CO_3 > K_(2)CO_3. While hydrogen evolution and CO_2 reduction reactions occur at a lower temperature on the combinations with the more reactive metal oxide and alkali carbonate, higher thermal reduction temperatures and more difficult alkali ion extractions are observed for the combinations of the more reactive metal oxides and alkali carbonates. Thus, for a thermochemical cycle to be closed at low temperatures, all three reactions of hydrogen evolution (CO_2 reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na_(2)CO_3/Mn_(3)O_4 combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °C.