High temperature and pressure alkaline electrolysis

Alkaline electrolyzers have proven to operate reliable for decades on a large scale, but in order to become commercially attractive and compete against conventional technologies for hydrogen production, the production and investment costs have to be reduced. This may occur by increasing the operational temperature and pressure to produce pressurized hydrogen at high rate (m³ H₂·h^-1·m^-2 cell area) and high electrical efficiency.
This work describes an exploratory technical study of the possibility to produce hydrogen and oxygen with a new type of alkaline electrolysis cell at high temperatures and pressures. Two measurement systems were built to perform measurements under high pressures and at elevated temperatures of up to 95 bar and 250 °C, respectively. The conductivity of aqueous KOH and aqueous KOH immobilized in a porous SrTiO₃ structure were investigated at elevated temperatures and high concentrations of the electrolyte using the van der Pauw method in combination with electrochemical impedance spectroscopy (EIS). Conductivity values as high as 2.9 S cm^-1 for 45 wt% KOH aqueous KOH and 0.84 S cm^-1 for the immobilized KOH of the same concentration were measured at 200 °C. Porous SrTiO3 was used for immobilization of aqueous KOH solutions. Electrolysis cells with this electrolyte and metal foam based gas diffusion electrodes were successfully demonstrated at temperatures up to 250 °C at 40 bar. Different electro-catalysts were tested in order to reduce the oxygen and hydrogen overpotentials. Current densities of 1.1 A cm^-2 and 2.3 A cm^-2 have been measured at a cell voltage of 1.5 V and 1.75 V, respectively, without noble metal catalysts. Electrical efficiencies of almost 99 % at 1.1 A cm^-2 and 85 % at 2.3 A cm^-2 were obtained.