In industrial settings, the role of technology for the efficient management and storage of electric power is as crucial as that of electricity generation technology. There is a limit to the simultaneous production and consumption of variable renewable energy or surplus electricity generated by a power plant. As such, the development of storage technologies for those energy sources has been considered more important in recent years. Although energy storage technologies through energy storage systems (ESSs), such as battery systems and supercapacitors, has been actively researched until now, the demand for storage technologies with higher capacity and longer durations continues to rise.
Definition and methods of hydrogen production through water electrolysis
Water electrolysis technology converts electrical energy from eco-friendly energy sources into high-energy density hydrogen or gaseous organic compounds. One of its benefits is the possible production of eco-friendly hydrogen. Moreover, on-site hydrogen production methods can reduce hydrogen transport and storage costs drastically.
The key to hydrogen production via water electrolysis lies in the generation of hydrogen and oxygen through water splitting. Major technologies for this include both alkaline water electrolysis and proton exchange membrane or polymer electrolyte membrane (PEM) electrolysis. The latter is well suited for operation with variable renewable energy, as it shows the least decline in hydrogen production efficiency in terms of power fluctuations. In recent years, studies of hybrid water electrolysis utilizing the advantages of the two electrolysis technologies have also been conducted in earnest.
Figure 1 - Water electrolysis
Advantages and limitations of water electrolysis technologies
Currently, hydrogen is mostly produced from fossil fuels, such as coal, oil, and natural gas, through hydrogen separation. While this offers affordable manufacturing costs, greenhouse gases emitted during the production result in environmental pollution. In contrast, water electrolysis ensures an eco-friendly hydrogen production process through water splitting.
Besides the environmental implications, the cost of electricity should be considered, as it accounts for the largest portion of the hydrogen production costs. As such, research on the development of highly efficient water electrolysis systems is urgently needed. Current hydrogen production via water electrolysis requires high-priced noble metal catalysts, such as platinum and iridium, hindering its commercialization. In particular, green hydrogen produced through water electrolysis costs 9,000 to 10,000 KRW per kilogram. This is much more expensive than gray hydrogen, which costs 1,500 to 2,000 KRW per kilogram and is produced through petroleum refining or natural gas reforming processes. Furthermore, the hydrogen production efficiency of alkaline water electrolysis utilized by South Korean companies is at approximately 68%, which is much lower than the world’s highest efficiency of about 80%. This low water electrolysis efficiency is the key factor in the increased price of green hydrogen. Therefore, high-efficiency, low-cost catalytic materials must be sought to increase the water electrolysis efficiency and secure the economic feasibility of green hydrogen.
Importance of catalytic materials
During water electrolysis, an oxygen evolution reaction occurs at the anode as hydrogen evolves simultaneously at the cathode. Theoretically, when a voltage of 1.23 V is applied, water should be decomposed into hydrogen and oxygen. However, different external factors, such as the resistance of an electrode/electrolyte interface and non-uniform electrolyte concentration, require more energy (overpotential) than the theoretical decomposition voltage. As such, the overpotential at the anode and the cathode needs to be reduced to improve water electrolysis efficiency, and in doing so, catalytic materials play a key role.
Figure 2 - Catalysis
Current status of catalyst research
A wide range of studies is being conducted to search for a substitute for the high-end noble metal-based catalytic materials. Above all, the facilitation of surface adsorption and charge transfer is the most important to ensure effective catalytic water splitting. Usually, catalytic properties can be improved by controlling the surface electronic structures and nanostructures of relatively low-priced transition metals.
The water splitting reaction can be accelerated through the formation of multiple adsorbents onto which the reactants, particularly the hydroxide ion (OH-) and hydrogen ion (H+), can be adsorbed by changing the surface electronic structures of catalytic materials. Moreover, diverse defects can be artificially formed by adding a third element that the catalytic material does not have. The mixture of highly conductive carbon materials and existing catalytic materials can also improve charge-transfer ability, thereby accelerating the hydrogen production reaction.
Further studies on substitutes for noble metal-based catalytic materials are expected to be vigorously conducted in the future. In particular, government and private-led research and development projects will be carried out. In conjunction with the development of catalytic materials with high activity and durability, large-surface area electrode manufacturing technology applicable to industrial water electrolysis systems must be developed, along with system control techniques for securing hydrogen generation stability, to successfully commercialize water electrolysis systems. As previously stated, commercialization can be achieved through more cost-effective hydrogen energy, thereby advancing the realization of the hydrogen economy.
Assistant Professor of the Department of Energy Engineering, Konkuk Institute of Technology (KIT)