Hydrogen has become a main subject in energetic transition because of its potential as a clean and multifaceted energetic vector. One of his key methods for hydrogen production is water electrolysis, that consists in separating water molecules in hydrogen and oxygen thanks to an electric current. Three types of main electrolyzers are standing out in the market today: the Proton Exchange Membrane electrolysis (PEM), the alkaline electrolysis and the Solid Oxide Electrolyzer Cell (SOEC). This article compares those three technologies in terms of efficiency and energetic yield.
PEM
PEM is used a lot for small scale hydrogen production in real time. It works at low temperatures and uses a conducting polymer membrane of protons to separate gases. The high efficiency of PEM lies in its capacity to be functional with high current densities and to react quickly to asking variations. However, it needs high-cost catalyzers and strict maintenance procedures. The energetic yield of PEM is usually measured in kWh per cubic meters of hydrogen produced. PEM has reached out about 77% electrical system efficiency.
SOEC
The SOEC functions at elevated temperatures and uses a ceramic electrolyte conducting ionic oxides. Even though it needs external heat to work, this kind of electrolyzer can use residual heat of other industrial processes, improving its global efficiency by doing so. SOEC got a high yield and the potential of creating hydrogen in a more durable way by using remaining electricity coming from renewable sources. SOEC systems have reached higher electrical efficiency (84% achieved by Sunfire, although this is not directly comparable with other technologies, given the additional heat input needed in SOEC systems) and can be a promising solution where waste-heat is available, such as in industrial hubs, given their operation requires temperatures higher than 650 °C.
Alkaline electrolysis
Alkaline electrolyzer uses an alkaline solution (usually hydroxide potassium solution – KOH), like electrolyte. Even though less performing at high densities currents compared to PEM, alkaline electrolysis got the asset of using lower costing catalyzers and not to be needing any specific membrane. That makes it an attractive option for high scale applications such as industrial hydrogen production. Its yield is also measured in kWh per cubic meters of hydrogen produced. Industrial size alkaline electrolyzers have an efficiency of 65+% (up to 67%), they generally operate between 60 and 90°C with a current density of 0.2 to 0.4 A/cm2, leading to a stack lifetime of 60.000 to 90.000 hours.
Global efficiencies estimation through the years
Efficiency comparison
The question of comparing the efficiency of any of these technologies is therefore not easy to answer since the efficiency of the various kinds of electrolyzers depends on several factors, particularly current density, water purity, pressure and temperature, definition of the system boundaries and standard conditions. Usually, PEM offers good reactivity and high efficiency at low charge while alkaline electrolyzers are better adapted to industrial applications needing a big production in significant quantities. SOEC got a high yield but its needs in temperatures are currently limiting its big scale utilization.
In terms of energetic yield, the three technologies are presenting variations depending on their technical specifications and working conditions. It’s also important to keep in mind that temperature has a significant effect on the longevity of an electrolyzer : the higher the temperature, the lower the service life. Plus is the higher the current density, the higher the rate of hydrogen production. This means that the specific costs for an electrolyzer increase significantly at lower current density.
SOEs constitute an advanced concept electrolysis at high temperatures (600–900 °C), whose efficiency is higher than PEM electrolyzers and AWEs. As for the practical application, SOEs meet remarkable challenges as concerns the thermal stability of materials, gas mixture, and sealing issues. Hence, SOEs are still at the R&D stage. Compared to SOEs, PEM electrolyzers and AWEs are commercially available. PEM electrolyzers are more efficient and allow for higher current densities than AWEs. One obvious disadvantage of PEM electrolyzers is the high capital cost of their acid-tolerance components. In addition, their shorter lifetimes than AWEs have also hindered their application in large-scale power-to-gas scenarios. In contrast, AWEs are a relatively mature technology that has been developed over 100 years. For commercial AWEs, lifetime can reach up to 15 years. Hence, AWEs are very suitable for large-scale electrolytic hydrogen projects.
To conclude, the choice of the electrolyzer depends on the specific needs of the application, the size of the production and the availability of the resources. While the PEM is ideal for the little scale applications and in real time, alkaline electrolysis and SOEC are more adapted to big scale industrial needs and to remaining renewable energies sources. The research continues in these domains to improve global efficiency and to make hydrogen production more durable and economically viable.
Finally, I would say that in the past few years, new electrolyzer designs have reported very high efficiencies, such as Hysata’s capillary-fed electrolyzer smashing efficiency records of 90%.
Sources
https://www.iea.org/energy-system/low-emission-fuels/electrolysers
https://www.nature.com/articles/s44172-023-00070-7
https://newatlas.com/energy/hysata-efficient-hydrogen-electrolysis/
https://www.level-network.com/wp-content/uploads/2017/02/ITM-Power.pdf