Exploring the alternative MnO-Na2CO3 thermochemical cycle for water splitting, A. Bayón, V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, Journal of CO₂ Utilization, 42, 101264, 2020, Online version,  https://www.sciencedirect.com/science/article/abs/pii/S2212982020305035

Abstract

Thermochemical cycles for H2O and CO2 splitting using CeO2 have demonstrated high efficiency (> 5%) and fast kinetics. Alternative multistage cycles based on stoichiometric oxides, such as Mn-Na, can potentially provide much larger efficiencies under less stringent operation conditions. However, these cycles consist of three stages operating at very different temperatures (from 50 °C to 1500 °C) and, accordingly, the overall energy yield of the cycle critically depends on efficient heat recovery. Despite of this, recently the Mn3O4–Na2CO3 cycle has shown a remarkable H2 production and high stability. To further enhance hydrogen generation rate, in this work we explore the feasibility of the novel cycle based on MnO, which can potentially increase the yield per cycle. Several samples of MnO with different physicochemical characteristics were investigated to establish the influence on the oxide physicochemical properties on hydrogen generation. A maximum conversion of 47 %, with a productivity of 20.1 μmol H2 min−1 g−1 were obtained using the MnO sample with relatively large surface area. As expected, this productivity is notably better than that obtained with Mn3O4 under identical conditions. Furthermore, the results obtained here reveals that low temperature hydrolysis under CO2 atmosphere can improve the Na extraction by more than 10 %. This enhances the cyclability of the process and it may allow coupling hydrogen production with CO2 capture. Furthermore, the thermal reduction of the solid product recovered after the carbonation stage yields nearly pure MnO, confirming experimental the feasibility of completing this alternative cycle.