Theis fights rampaging C-atoms on electrolysis cells

A growing share of our energy consumption needs is covered by sustainable energy sources, but the wind does not always blow and the sun does not always shine. To store surplus energy from wind mills and solar panels from times with abundance for use in times with deficit we need a way to convert the electrical energy into chemical energy, which can be easily stored and used in e.g. the transportation sector.

Solid oxide electrochemical cells in the form of electrolysis and fuel cells (SOEC and SOFC) are a promising technology that enables us to store surplus energy. The two types of cells are the same cell used differently. In electrolysis, electricity is supplied to the cell and splits water (H₂O) or carbon dioxide (CO₂) into hydrogen (H₂) or carbon monoxide (CO) fuel gases. By reversing the process and using the same cell as a fuel cell, you can convert the chemically bound energy in gases, such as natural gas, directly to electricity in an efficient way.

The combination of electrolysis cells and fuel cells is therefore well suited for storing surplus electricity, but a number of mechanisms limit the efficiency and life of the electrochemical cells.

30-year-old Theis Løye Skafte from DTU Energy chose to identify and tackle some of those issues in his PhD project "Lifetime Limiting Effects in Pre-Commercial Solid Oxide Cell Devices", which he has just defended.

Carbon growth is the big problem

"I wanted to identify and provide an overview of the restrictive mechanisms and I quickly chose to focus on one of the biggest constraints: carbon growth in the cells. My research and my experiments show that the problem of solid carbon formation in electrochemical cells is greater than initially assumed," Theis explains.

The fuel-electrode in electrolysis and fuel cells is typically made of nickel in a well-proven and efficient cell design, but during electrolysis the C-atoms that are energized can form deposits of carbon that at first inhibits the effectiveness and ultimately destroys the cell.

"First, I created an overview of the problem, then I zoomed in to the specific mechanism and in the end, I realized that if you cover the nickel electrodes with ceria after the damage has occurred, you can repair the cell. It then occurred to me that the same method could be used to significantly reduce the degradation of the electrode and thus increase the lifetime of the cells," Theis says.

Cerium oxide (CeO₂), also known as Ceria, is an oxidized form of the metal cerium, which is also used in catalysts in cars while ceria doped with Gd₂O₃, gadolinia, can be used in energy components such as fuel cells and conversion sensors. Doped means that foreign atoms have been added.

Cells with Ceria are more tolerant to Carbon growth

During his study Theis and one of his supervisors, Chris Graves, visited a Stanford University research group that are experts in the fundamental reactions of, among other things, Ceria. Here Theis found out why carbon forms more readily on nickel than on ceria.

"Using x-rays from a synchrotron, we were able to see that C-atoms were captured by oxygen on the surface of ceria before graphite was formed," Theis explains.

This makes cells made of ceria more carbon tolerant, something Theis also showed in his candidate project, but scientists have not been able to explain the exact cause of the higher stability until now.

"Unfortunately, it's hard to use ceria to increase the carbon tolerance in today’s commercial cells, as there simply is not enough space in the electrode for it," he says.

When carbon is formed on nickel, the resulting carbon nanofibres grow strong enough to tear the cell into pieces. Theis has shown that if you catch the problem in time, you can use ceria to restore the contact that was lost when the carbon pushed the particles apart.

"Since the metal nickel is very mobile and tends to clump together at the high temperatures at which the cells operate, the resistance in the cells also rises even during normal operation without carbon growth. I realized that this problem could also be corrected using the same method that repaired the cells damaged by carbon deposition, namely adding ceria after the damage occurred. Surprisingly, it also appeared that this method stabilizes the electrode so that further degradation of the electrode is virtually eliminated," says Theis.

More studies await ahead

"The measurements look very promising and it also seems to work in commercial fuel cell stacks, but it has to be tested further and there are a number of questions still to be answered," Theis says.

He was a physics student at the University of Copenhagen before studying as a candidate at DTU's Master's Program in Sustainable Energy. An experimental course on high temperature fuel cells caught his attention.

"I could see the benefits of saving power by turning it into useful gases, so I got job as student-assistant at DTU Energy, and later I became Industrial PhD at DTU Energy and Topsøe Fuel Cell A/S. It was a good combination as I gained knowledge from both business and university," says Theis, who has now been employed as a postdoc at DTU Energy.

“I chose to stay at DTU Energy to develop new cell designs and new types of electrodes. The current electrodes are well-proven and works, but if we are to scale up this technology we need totally new and more stable designs, that degrades less and doesn’t suffer from carbon growth”, says Theis Løye Skafte.