Green hydrogen production via water splitting is widely regarded as a promising sustainable energy solution. Manganese metal can serve as a secondary energy carrier for hydrogen, offering a cost-effective pathway for integrated energy fixation, storage and transport. In this study, a novel low-temperature electrochemical-thermochemical loop for water-splitting was thus proposed based on the Mn-MnSO₄ redox pair, i.e., the hybrid manganese cycle. This new system incorporates an electrolysis step and an ion recovery step operated within similar temperature ranges, enabling tight integration, uninterrupted hydrogen supply, and efficient heat exchange with the “Power-to-Gas” model or low-cost and multiple-form energy exportation with the “Power-to-Manganese-to-Gas” model. The mechanism of the hybrid manganese cycle was first proposed and verified. The multi-faceted optimisations of the electrolysis step were then conducted through experiments and machine learning (ML), involving electrolyte combinations, additives, cathode materials, power supply, electrolyser design, fluid circulation, and catalysts. In addition, the ultrasound detection was employed to investigate the electrodeposition and detachment of manganese. Finally, the kinetic modelling of ion recovery and its factors were explored.

The results indicate that 316L stainless steel is more suitable as a cathode base material than graphite, brass, or nickel. When employed, the optimal catholyte conditions were determined to be a temperature of 40 °C, a pH value of 2.86, and a manganese ion concentration of 1.64 mol/L. The most effective concentration of sulphuric acid solution for use as the anolyte was found to be 25.25 wt.%. Potassium sulphate concentrations of 0.8 mol/L in the catholyte and 0.2 mol/L in the anolyte are identified as the optimal additive levels. The cell voltage governs the current density ratio between the hydrogen evolution reaction (HER) and the manganese electrodeposition reaction (MEDR), while applying a superimposed pulse effectively reduces energy consumption—a 50% duty cycle at 50 Hz was found to be optimal. A 2.5 mm gap between the proton exchange membrane (PEM) and the anode is recommended to prevent manganese oxidation and maintain low impedances. RuO-IrO₂-Ti shows better oxygen evolution reaction (OER) catalytic performance compared to Ta₂O₅-IrO₂-Ti, while Pt is able to catalyse the co-evolution of the HER and MEDR. However, Pt is not favourable for manganese desorption, so stainless steel can be used as an alternative cathode material, operating with different models. A peristaltic pump speed of 100 rpm ensures stable electrolyte circulation and system output, achieving an overall energy conversion efficiency (OECE) exceeding 50%. The concentrations of manganese sulphate and potassium sulphate significantly affect the reaction order and rate constants during the ion recovery step, thereby influencing the reaction kinetics in a non-linear manner. In contrast, temperature and manganese metal size primarily impact the rate constant without altering the reaction order and are thus considered more effective parameters for modulating the ion recovery rate.