A hybrid electrochemical-thermochemical loop for efficient hydrogen production based on the Mn/MnSO4 redox pair

This study reports the reaction mechanism and electrolyte optimisation aspects of a novel low-temperature water-splitting system developed for the efficient production of hydrogen based on the Mn–MnSO₄ redox pair. The system incorporates an electrolysis step and an Mn²⁺ ion recovery step for splitting water in a cyclic operation. Two steps operate within similar temperature ranges, enabling tight integration and efficient heat exchange. The optimisation of electrolytes for the electrolysis step was first carried out in a proton-exchange membrane (PEM) H-cell. The experiments were figured out using a three-factor case study based on the factorial design approach, incorporating temperature, concentration, and pH value as the main variables. Subsequently, machine learning models were employed to analyse the data and predict the best pairing of electrolytes by systematically exploring the critical ratio of conductivity to potential. The results showed that at a cell voltage of 5.0 V and 40 °C, the ratio of importance between the conductivity and MEDR potential is 1:9 for the catholyte, while the anolyte ratio of importance between the conductivity and OER potential is 6:4. Accordingly, the optimal electrolyte composition was found to be a combination of MnSO₄ solution (1.64 mol/L; pH 2.86) with H₂SO₄ (25.25 wt%). Also, a remarkable corresponding current efficiency of 99.25 % was achieved with an overall energy conservation efficiency of 40.15 %. The proposed cycle is the first of its kind developed based on the chemical looping principle and can be potentially applied for large-scale continuous green hydrogen production at a low-levelized cost.