Hydrogen Electrolysis Eagerly Awaited: Hydrogen Electrolysis Comes of Age
Turning electricity into gas: a closer look at the key technology of the hydrogen revolution. Whether alkaline, PEM or high-temperature electrolysis, the oldest electrochemical process is to make the world of tomorrow possible. By 2021, projects with over 200 gigawatts of electrolysis capacity will be on the agenda. But what is behind these figures? What technologies are there, and where is the journey heading now?
Where is there such growth? By 2040, a recent study estimates, the total capacity of hydrogen electrolysis plants installed worldwide should increase by a factor of 1.000. 213.5 GW of capacity should be in operation around the globe by then — in 2021 it will be just 0.2 GW. Europe is the focus of interest here: currently, about 85 per cent of the planned production capacities are located in the “old world”, the lion's share in Germany. However, there is a catch: planned does not mean built. Only time will tell how many plants will actually be built. The figures for 2030 are more concrete: Germany is planning 9 GW, the Netherlands 6 GW and the UK wants to build 4 GW of electrolysis capacity, with an upward trend.
Also on the rise: the size of the plants: while electrolysers with up to 10 MW capacity are currently common, by 2025 this could be 100 to 500 MW, believes the market research company Aurora Energy. In most cases, the electricity for H2 production is to be generated from wind energy, plus solar power and a small proportion of plants that use grid electricity. An unprecedented success story for a reaction that most know from their first chemistry lesson at grammar school. But the big time for water electrolysis could still be coming — as an electricity storage system and buffer for the energy transition.
Workhorse of the H2 Economy: How Does Alkaline Electrolysis Work?
According to a study by the Boston Consulting Group, volatile renewable energies could increase the global demand for energy storage to a total of 330 GW by 2030. 'Green' hydrogen, i.e. hydrogen produced with emission-free electricity, could play a key role in this.
But how do you turn electricity into gas? Precisely via electrolysis, the electrochemical splitting of the water molecule into its components hydrogen and oxygen. But while the underlying reaction is basically always the same, the implementation as a technical process differs significantly. Even whether a basic or acidic medium is used in the electrolysis cells makes a significant difference: alkaline electrolysis, the workhorse among the water electrolysis processes, uses up to 40-percent potassium hydroxide solutions to obtain hydrogen with a high degree of purity. The electrodes, cells and membranes are comparatively inexpensive and stable over the long term, the efficiencies are high. These properties - as well as the fact that no rare precious metals are needed for the electrodes - make alkaline electrolysis a promising candidate for the production of green hydrogen.
Too Slow for Grid Stabilisation?
But where there is light, there is also shadow: the cells react sluggishly and have a significantly reduced efficiency in the (already low) partial load range — both important requirements if the technology is to work with excess electricity and contribute to load-controlled grid stabilisation.
The developers are therefore paying special attention to the membrane that separates the half-cells from each other: on the one hand, it must be as permeable to ions as possible, and on the other hand, it must be chemically resistant, whereby operating temperatures of 80 °C and more are not uncommon. In addition, the electrical resistance of the cells decreases the closer the two electrodes “move together”, whereby the thickness of the diaphragm remains the minimum possible “zero distance”.
The Membrane Does it: Hydrogen Via PEM Electrolyser
But it also works the other way round: proton exchange membrane electrolysis (PEM electrolysis for short) is basically the "reverse" of the fuel cell. It works in an acidic environment, whereby positive hydrogen ions - protons - migrate through a gas-tight PTFE membrane to the cathode. There, they pick up an electron and form highly pure hydrogen molecules (in practice, post-purification is dispensed with), while oxygen is separated on the anode side and can be used. Since the electrodes are directly attached to the membrane, which also serves as the electrolyte, the PEM electrolyser only requires pure, distilled water. Due to the aggressive reaction conditions, platinum is used as the cathode material (usually on a carbon carrier), and precious metals such as iridium or metal oxides are used on the anode side. In contrast to alkaline electrolysis, however, there is practically no liquid water at the cathode.
Partial Load or Overload? No Problem for PEM!
PEM electrolysis not only achieves high current densities, power densities and efficiencies, but can also be operated in the partial load range without any problems. PEM electrolysers can even be overloaded for short periods, which makes them ideal for "cushioning" voltage peaks in the power grid. In contrast to the proven alkaline electrolysers, however, the technology is still relatively young and was long limited to small niche applications.
That's over now: In the wake of the hydrogen boom, more and more large-scale projects are being announced, such as a 24-megawatt electrolyser that Linde is building together with ITM Power in Leuna. The plant was the world's largest PEM electrolysis project at the beginning of 2021 and is to supply industrial customers with green hydrogen from 2022. In doing so, the partners want to benefit from an existing pipeline network in the region.
Hydrogen by Solid Oxide: Is This The Future?
OEC solid oxide electrolysis is hot in the truest sense of the word: since the reaction takes place at very high temperatures (500 is 850 °C), less energy has to be used to break down the water molecules. This process is also basically a “backwards” fuel cell, in which the half-cells are separated by a solid oxide instead of a membrane. In view of the high operating temperature, an ion conductor such as zirconium dioxide is used as the electrolyte, which must first be heated when the cells are started up. And even extraterrestrially: in 2020, MIT sent a solid oxide electrolyser to the red planet with the Mars Rover to test whether the process would be suitable for oxygen production in space.
What Can Chlorine Electrolysis Do for Hydrogen?
That leaves chlor-alkali electrolysis: Hydrogen is also a by-product in the production of chlorine from brine - and plant engineers have decades of experience with this. The technology is relatively mature and has efficiencies of up to 80 per cent, experts explain.
Most electrolysis plants are supplied in prefabricated standard modules that can be easily transported, installed and interconnected to form various plant sizes up to several hundred megawatts or gigawatts. Chlorine electrolysis can also be used for grid stabilisation and meets all the requirements for the primary control energy market, such as the requirement to be able to switch between maximum and minimum load within 30 seconds or less.
Breakthrough in Sight? These Stumbling Blocks are Slowing Down Electrolysis
So everything is ready for the energy transition? Not quite, says Hanns Koenig, Head of Commissioned Projects at Aurora Energy Research: “The success of green electrolysis hydrogen will depend on two key factors,” says the energy expert. “One is the cost of electricity, which is the largest part of the cost of production. And secondly, the CO2 balance, which is decisive for whether the gas can be considered climate-friendly.” And here Germany is suddenly only in the middle of the pack: firstly, the energy costs are comparatively high, and secondly, the CO2 intensity is not yet at the level of, say, Norway, Sweden or France.
“Only in these countries will electrolysers powered by mains electricity probably comply with the relatively strict limits that the EU plans to set for the 'sustainable' hydrogen label by 2030,” says Koenig. The alternative to reducing the carbon footprint would of course be to decouple electrolysis from the grid and supply it directly with green electricity from wind, sun and water.
How Much Hydrogen Does That Give Again?
But there is another factor that, according to industry experts, determines the economic viability of the process: The possible current density, on which the amount of hydrogen produced directly depends. But since the electrodes and cells begin to corrode at too high voltages, this cannot be increased at will - instead, larger or more electrolysis cells must be produced.
In general, the possible current density of PEM electrolysers (1,000 to 2,000 mA/cm2) is much higher than that of typical alkaline electrolysis cells (200 to 500 mA/cm2), but they are catching up in leaps and bounds. The developers even believe that they will be able to double the current densities in the medium term by improving the catalytic properties of the electrodes, the electrode design and the separator, as well as by increasing the pressure and temperature. Experts expect that this could increase the efficiency to almost 90 per cent while the cell voltage remains roughly the same.