Hydrogen is considered one of the most important methods for decarbonizing industry and transport. The number of new hydrogen initiatives has increased rapidly in recent years. Nevertheless, according to the International Energy Agency (IEA), faster action is needed to achieve net zero emissions by 2050 and meet the goals of the Paris Agreement.
Although there is a wide range of CO2-free and low-carbon technologies to produce hydrogen, with the main “clean” production method being electrolysis. This process splits water molecules into hydrogen and oxygen using renewable sources such as solar and wind.
Bloomberg estimates that annual supplies of low-carbon hydrogen could increase 30-fold by 2030. While “blue hydrogen” from traditional fossil fuel synthesis and carbon capture will play an important role, analysts expect more than half of it to be produced through electrolysis.
Among the available technologies, alkaline electrolysis currently has the largest share of installed capacity at 60%, followed by proton exchange membrane (PEM) electrolyzers, which now account for about 30%, according to the latest IEA study. Global Hydrogen Report.
Recently, however, solid oxide electrolysis cells (SOECs) have been attracting increasing attention as an additional route to produce carbon-free hydrogen. So what are SOECs and how can they help increase hydrogen production capacity?
What different types of electrolysis are there today?
The most common production method, alkaline electrolysis, consists of two electrodes – an anode and a cathode – with a porous separator (diaphragm) between them. The most commonly used electrolyte for hydrogen production is an alkaline solution of potassium hydroxide (KOH) in water. When electricity is applied to the electrodes, water molecules separate at the negatively charged cathode and release hydrogen. Alkaline electrolysis is known as a low-temperature technique and is typically operated at 50-80 °C.
Although alkaline electrolyzers are a mature technology with a long history of success in areas such as sodium hydroxide production, efficiency problems can arise when synthesizing hydrogen because the gas is not completely retained in the porous separator.
PEM uses a similar technological approach at low temperatures, but with polymer membranes as separators that can better retain the gases produced during the reaction.
Finally, SOECs use the same basic process but require much higher temperatures. SOEC systems typically need to reach temperatures of 800–1,000°C to function properly, although some start at 600°C. To cope with these high temperatures, they use ceramic cells as separators.
What advantages do SOECs offer?
As the reaction in the SOEC fuel cell progresses, the power requirement decreases and part of the required energy can be provided by the high operating temperatures. According to Goldman Sachs Research, efficiencies of up to 85% are currently achievable with the technology; MHI’s SOEC development goal is 90%. With alkaline and PEM electrolysis, the electrical efficiency is in the upper range of around 70-75%.
“Due to this high efficiency, SOECs can theoretically produce more hydrogen per kW than any other type of electrolyzer,” explains Dr. Kenichiro Kosaka, chief engineer and senior manager in Mitsubishi Heavy Industries’ Energy Systems Technology Strategy Department and leading MHI’s electrolyzer development.
In the past, alkaline electrolyzers and PEMs have been found to achieve longer lifetimes than SOECs, but Kosaka points out that theoretically there is not much difference between SOECs and the other technologies, but much depends on the electrolyte material used.
Kosaka explains that the yttrium-stabilized zirconia used in MHI’s SOECs has lower conductivity than other electrolytes, such as gadolinium-doped ceria, but does not undergo the same degree of decomposition at high temperatures.
MHI is developing SOEC technology at its Carbon Neutral Park in Nagasaki and operates a test module at Takasago Hydrogen Park, the world’s first integrated validation facility for technologies from hydrogen production to power generation. The company’s work on SOECs draws heavily on its experience with solid oxide fuel cells (SOFCs), which use the same electrochemical processes but in reverse.
How far has the commercialization of SOECs progressed?
In 2023, only about 1% of green hydrogen will be synthesized by SOECs, the IEA says. But commercialization is progressing, including the commissioning of a green hydrogen production plant at a refinery in the Netherlands and a system at a NASA research center in California.
MHI’s goal is to bring large modular SOECs with an overall efficiency of 90 percent to market by the end of the decade, designed for plants with an output of several hundred megawatts.
Access to rare elements is an important factor in the commercialization of electrolysis. Here, too, says Kosaka, SOECs have an advantage. PEM is based on two precious metals, platinum and iridium, which are among the rarest in the world. SOECs do not need these, and the impact on demand for other rare metals – such as nickel and zirconium – is likely to be minimal due to the higher efficiency of SOECs, Goldman Sachs Research suspects.
“Overall, we believe that SOECs are the most suitable technology for large-scale hydrogen plants. Although they are still in the development phase, SOECs have the advantage of high efficiency and the fact that they are derived from SOFCs, which can be considered a mature technology at this stage,” says Kosaka.
Expansion of the electrolysis technology mix
According to the IEA, there are also other electrolysis technologies in various stages of development, including anion exchange membrane (AEM) electrolyzers. With companies like MHI and others involved in preparing AEM for commercial use, these are also likely to mature in the coming years.
However, green hydrogen production will not be a one-off, and the choice between different technologies will depend on the specific circumstances of each project.
For example, SOECs are particularly suitable for large-scale applications with a stable power supply, Kosaka explains. This could come from non-intermittent sources such as geothermal, hydropower or nuclear energy, or a combination of these with renewable energy.
McKinsey predicts that clean hydrogen could meet up to 73-100% of total hydrogen demand by 2050 if the most ambitious decarbonization scenarios are implemented. To achieve this, CO2Industries with high resource intensity such as steel, cement and heavy transport, as well as green energy producers, will need all the technologies they can get.
The path to decarbonisation is challenging and although green hydrogen is a viable solution, it must be deployed much faster to meet sustainable energy commitments.
