Alkaline electrolysis is a proven technology with several large scale facilities for hydrogen production realized and operated reliably for decades. Nevertheless, its broader deployment is hindered by the relatively high cost for hydrogen production. To overcome this obstacle, it is necessary to improve cell efficiency, increase the production rate, and decrease capital cost. Since conventional alkaline electrolysis technology has reached maturation, only small incremental improvements can be expected. To achieve a drastic step forward, we have developed a new generation of alkaline electrolysis cells that can operate at elevated temperature and pressure, producing pressurized hydrogen at high rate (m³ H₂/hžm² cell area) and high electrical efficiency.

The concept relies on the development of corrosion resistant high temperature diaphragms, based on mesoporous ceramic membranes where aqueous KOH is immobilized by capillary forces, in combination with gas diffusion electrodes that overcome mass transport limitations at large production rates. Raising the operating temperature offers a means to drastically improve performance, as both ionic transport and reaction kinetics are exponentially activated with temperature. Indeed, we have demonstrated alkaline electrolysis cells operating at 200-250 °C and 20-50 bar at very high efficiencies and power densities. This enables high production rates near the thermoneutral voltage, thereby overcoming the need for cooling.

This work will provide an overview of the exploratory technical studies undertaken so far. Two electrochemical test stations have been established to carry our experiments at elevated pressures (up to 99 bar) and temperatures (up to 300 °C). The conductivity of aqueous KOH was investigated at elevated temperatures to establish the optimum concentration at 200-250 °C. An optimum value of 0.84 S cm^-1 was established at 200 °C for 45 wt% aqueous KOH immobilized in mesoporous SrTiO₃. Gas diffusion electrodes were developed using metal foams loaded with different non-precious metal electrocatalysts in order to reduce the overpotentials for oxygen and hydrogen evolution. Small cells have been fabricated and operated at current densities of up to 1.75 A cm^-2 and 3.75 A cm^-2 at cell voltages of 1.5 V and 1.75 V at 200 °C at 20 bar, corresponding to electrical efficiencies of almost 99 % and 85 %, respectively. Long-term operation at 200 °C was successfully demonstrated for 400 h, suggesting relatively stable cell performance. Finally, low-cost production methods have been utilized for a first scale-up of the cell size from 1 cm² to 25 cm². Efforts are currently directed towards the investigation of the intrinsic activity of mixed oxides for the oxygen evolution reaction at elevated temperatures and pressures, and towards establishing the required infrastructure for testing of 25 cm² cells.