Significant reduction in the operating temperature of the Mn(ii)/Mn(iii) oxide-based thermochemical water splitting cycle brought about by the use of nanoparticles

Abstract

Among the many efforts to devise thermochemical cycles to generate H₂ by water splitting, the Mn(II)/Mn(III) oxide based cycle operating at 850 °C is a significant one and involves no toxic and corrosive materials. The essential process in this cycle is the shuttling of Na⁺ ions in and out of Mn oxides. In an effort to bring down the temperature of this cycle, we have found that the use of Mn₃O₄ nanoparticles is particularly effective. Ball milling has been applied to decrease the particle size of commercial Mn₃O₄ to less than 500 nm. Thus the solid state reaction between Na₂CO₃ and Mn₃O₄ nanoparticles occurs at a temperature 200 °C lower than with bulk samples. One of the challenges of this particular cycle lies in its slow H₂ evolution. It has been possible to operate this cycle and generate H₂ at a much faster rate at 750 °C and even at 700 °C by this means. Furthermore, the step involving hydrolysis of NaMnO₂ can be performed at 50 °C instead of 100 °C.