Manganese oxide based thermochemical hydrogen production cycle

A MnO/NaOH based three-step thermochemical water splitting cycle was modified to improve the hydrolysis of α-NaMnO2 (sodium manganate) and to recover Mn(III) oxides for the high-temperature reduction step. Sodium manganate forms in the reaction of NaOH with MnO that releases hydrogen. The hydrolysis of α-NaMnO2 to manganese oxides and NaOH is incomplete even with a large excess of water and more than 10% sodium cannot be removed prior to the high-temperature reduction step.When mixed oxides of manganese with iron were used in the cycle, the NaOH recovery in the hydrolysis step improved from about 10% to 35% at NaOH concentrations above 1M. Only 60% sodium was removed at 0.5M from the mixed oxides whereas more than 80% can be recovered at the same NaOH concentration when only manganese oxides are used. A 10:1 Mn/Fe sample was cycled through all steps three times to confirm that multiple cycles are possible. The high-temperature reduction was carried out for 5h at 1773 K under vacuum and the conversion was about 65% after the 3rd cycle.Since sodium carryover into the high-temperature reduction cannot be avoided, even with the energy intensive hydrolysis step, a modified two-step cycle without low-temperature sodium recovery is proposed where α-NaMnO2 is reduced directly to MnO at 1773 K under vacuum. On a laboratory scale, about 60% of the sodium that volatilized at the high temperatures was trapped with a water-cooled cold finger and conversions were stable at about 35% after three completed cycles.

► Sodium recovery of MnO/NaOH-based thermochemical cycle improved by adding iron.
► A hydrolysis-free cycle with high-temperature reduction of α-NaMnO2 is proposed.
► Multiple cycles with both hydrolysis-free and Fe/Mn based process were carried out.