Advances and challenges in intermediate temperature alkaline water electrolysis: A critical review

Hydrogen is key for global decarbonization, with alkaline water electrolysis as the leading method for green hydrogen production. Raising the operating temperature from the current norm of 60 to 90 °C to above 100 °C would improve the electrode kinetics, leading to lower specific energy consumption, and more value of waste heat. Increasing the temperature of potassium hydroxide electrolytes and the product gases results in severe corrosion issues. Thus, the benefits of improved performance come with the cost of more expensive materials. Comprehensive insights into electrolyzer components properties and the stack's design considerations are crucial for assessing the advantages and drawbacks of operating at intermediate temperatures of 100 to 200 °C. This review underscores the substantial emphasis in prior research on thermodynamic and kinetic enhancements, the exploration of catalytic materials, and the examination of different alloys, alongside safety considerations. Unlike previous reviews that primarily focused on materials, this work emphasizes the often-overlooked aspects of electrolyte properties, optimization of stack design, and the techno-economic implications of intermediate-temperature operation. Our analysis indicates that stray currents, gas mixing in the lye circulation loop, and gas crossover through the diaphragm worsen at higher temperatures, despite increased current density. A simplified capital cost analysis for an alkaline electrolysis stack at 120 °C indicates a 19 % increase in stack costs, with the system costs more than doubling if high-nickel alloys are used. Continuous operation is required to offset the higher capital investment and to fully benefit from the lower energy consumption of the intermediate temperature system.