Chlorine-free alkaline seawater electrolysis for hydrogen production

A new process for chlorine-free seawater electrolysis is proposed in this study. The first step of the process is separation of Mg2+ and Ca2+ ions from seawater by nanofiltration. Next, the NF permeate is dosed into the electrochemical system. There it is completely split into hydrogen and oxygen gases and NaCl precipitate. The electrochemical system comprises an electrochemical cell operated at elevated temperatures (e.g. ≥ 50 °C) and a settling tank filled with aqueous NaOH solution (20–40 %wt) that operates at lower temperatures (e.g. 20–30 °C). High concentration of hydroxide ions in the electrolyzed solution prevents anodic chlorine evolution, while the accumulated NaCl precipitates in the settling tank. Batch electrolysis tests, performed in NaCl-saturated NaOH solutions, showed absolutely no chlorine formation on Ni200 and Ti/IrO2RuO2TiO2 anodes at [NaOH] > 100 g/kgH2O. Three long-term operations (9, 12 and 30 days) of the electrochemical system showed no Cl2 or chlorate (ClO3−) production on both electrodes operated at current densities of 93–467 mA/cm2. The Ni200 anode was corroded in the continuous operation that resulted in formation of nickel oxide on the anode surface. On the other hand, the system was successfully operated at 467 mA/cm2 with Ti/IrO2RuO2TiO2 electrodes in NaCl-saturated solution of NaOH (30 %wt) for 12 days. During this period no formation of Cl2 and ClO3− has been observed and precipitation of NaCl occurred only in the settling tank. The performance of the system was stable during the operation as indicated by the insignificant fluctuations in the applied cell potentials and measured constant concentrations of NaOH(aq) and NaCl(aq) in the electrolyte solution. During 12 days of operation at ≈ 470 mA/cm2 about 1.2 m3 of H2 and ≈150 g of solid NaCl were produced in the system. Electrical energy demand of the electrolysis cell was 5.6–6.7 kWh/m3H2 for the current density range of 187–467 mA/cm2.