The role of monovalent cations in activating cathodic reactions for enhanced hydrogen evolution using Mn/MnSO4 redox pair

This study investigates the optimisation of the electrochemical performance of a novel Mn/MnSO₄ redox cycle using monovalent cations, including Li⁺, Na⁺, and K⁺, to enhance hydrogen generation. The reaction mechanism of cations in the simultaneous hydrogen evolution reaction (HER) and manganese electrodeposition reaction (MEDR) was confirmed through a systematic analysis of the co-evolution stages to maximise the cell electrochemical performance. The results indicated that the effects of cations primarily depend on the differences between the activation of ion pairing/bridging and the inactivation of surface blocking for the HER and MEDR, along with the influence of mass-to-charge ratio, ion size, conductivity, ion distribution, and concentration polarisation. A high concentration of cations can efficiently boost the cell performance and current density due to the enhancement of HER, even though it simultaneously leads to the inhibition of MEDR, presenting high energy efficiency and productivity, but low manganese CE. Optimal performance was achieved by adding K⁺ cations using 0.8 mol/L potassium sulphate (K₂SO₄) solution to MnSO₄, with a pH of 2.86, which resulted in 7.16 % improved current efficiency. In addition, it was found that potassium cations can make the electrodeposited manganese metal more easily detached from the electrode and cause a lower corrosion current density, which is in favour of production. The proposed system and approach offer the advantages of reducing specific energy consumption by 7.23 % compared to the conventional cells, providing new insights into the electrochemical behaviour of redox pairs mediated water splitting systems for the next generation of scalable, low-cost PEM electrolysis systems for sustainable hydrogen production.