An SOEC electrolytic cell is built up of ceramic materials and is, in principle, a reversed SOFC fuel cell which Risø is developing in conjunction with, among others, Topsoe Fuel Cells.

—Research Professor Mogens Mogensen from the Fuels Cells and Solid State Chemistry Division at Risø

The process in the electrolytic cells corresponds to part of nature’s own photosynthesis, which takes CO₂ out of the air and transforms it into a store of chemical energy in the form of sugar.

High-temperature cells. High-temperature cells are very efficient compared with other electrolysis methods as they produce more oxygen and carbon monoxide from a given amount of electricity. This is because at high temperatures water and carbon dioxide can be split into synthesis gas (hydrogen + carbon monoxide) and oxygen using the heat, and the SOEC cell is thereby self-cooling—the heat which is inevitably produced when electricity runs through something is needed for the electrolytic process. Moreover, it is possible to utilize the heat which is often available as surplus heat from, for example, power stations and industry.

These high-temperature electrolytic cells will be good for large, central plants for manufacturing synthetic fuel from synthesis gas. The catalytic processes which follow the electrolytic process require a complete facility with a catalytic reactor coupled to an electrolytic cell plant because the synthetic hydrocarbons are not stable at such high temperatures (over 650 Degrees C). Such a facility probably needs to exceed 100 MW for it to be financially viable.

—Mogens Mogensen

Work has been conducted on the high-temperature cells for some time in SERC (Strategic Electrochemistry Research Center), where a number of enterprises and research centres are collaborating on the development of these types of electrolytic cells.

Low temperature cells for local production of synthetic fuel. For local production conditions, it is necessary to develop cells which can operate at temperatures in the 200-400 °C range. This way, small, local electrolysis plants can be established, which can be connected directly to a local wind turbine and produce synthetic fuel for the local area. The lower temperature means less heat loss and makes it easier to build small and modular electrolysis plants.

The vision is to be able to build small, modular plants, with one standing beside each wind turbine in the local area.

—Mogens Mogensen

For this to succeed, it is necessary to develop completely new materials. These will be developed within the larger Catalysis for Sustainable Energy (CASE) research initiative, which is developing catalysts to transform local renewable energy into chemical energy. CASE is headed by Professor Jens K. Nørskov from DTU Physics.

The Fuel Cells and Solid State Chemistry Division is working with two electrolyte types. One is a mesoporous ceramic material, which can absorb liquid electrolytes in their nanopores and retain them. The second type is low-temperature proton-conducting materials which uses a solid ceramic electrolyte.

Limestone from the Danish subsoil can be used to produce sustainable synthetic fuels. Source: Risø. Click to enlarge.

Limestone in the production of sustainable synthetic fuels. It is hard and costly to directly separate CO₂ from the atmosphere. Professor Mogens Mogensen therefore envisages the necessary CO₂ coming from other sources such as breweries and second-generation bioalcohol plants, where fermentation produces large volumes of CO₂. Another local possibility is using Denmark’s most widespread raw material, limestone (calcium carbonate). Heating limestone liberates CO₂, leaving quicklime (calcium oxide). Water is mixed—or ‘slaked’—with quicklime, producing slaked lime (calcium hydroxide), whereby most of the heat which was used is again released.

Slaked lime reabsorbs CO₂ from the air relatively quickly. Slaked lime mixed with sand is called mortar, which has traditionally been used as a binding paste in masonry. The wet mortar between the bricks absorbs CO₂ from the air and hardens through the formation of lime to a stone-hard substance that binds the bricks together.

In other words, the lime is part of a carbon cycle. The CO₂ which is released when the lime is burnt is absorbed again when the slaked lime absorbs CO₂ and is thereby converted back to lime. This cycle can be used to manufacture synthetic CO₂-neutral fuel, Mogensen suggests.