Spinel Metal Oxide-Alkali Carbonate-Based, Low-Temperature Thermochemical Cycles for Water Splitting and CO₂ 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 CO₂ reduction. Hydrogen evolution and CO₂ reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe₃O₄ > Mn₃O₄ > Co₃O₄, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li₂CO₃ > Na₂CO₃ > K₂CO₃. While hydrogen evolution and CO₂ 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 (CO₂ reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na₂CO₃/Mn₃O₄ combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °C