Electrolysis and fuel cell mode

DTU-researchers in Nature Materials: New way to improve the long-term stability of ceramic electrochemical cells for energy storage

Monday 22 Dec 14

Contact

Christopher R. Graves
Associate Professor
DTU Energy
+4546 77 58 70

Acknowledgement

The article 'Eliminating degradation in solid oxide electrochemical cells by reversible operation' is part of a project supported by the Program Commission on Sustainable Energy and Environment, The Danish Council for Strategic Research, through the SERC project (http://www.serc.dk), contract no. 2104-06-0011, and the Nordic Energy Research Council (NER) project no. 40000.
A research group at DTU Energy at the Technical University of Denmark (DTU) has demonstrated a way to eliminate a major obstacle to using solid oxide electrochemical cells (SOCs) for energy storage. The discovery is described in an article published in the highly respected science journal Nature Materials.

Whereas fossil fuels and nuclear energy sources can supply electricity on demand, solar and wind energy sources supply power only when the sun shines or wind blows.

This supply-demand energy mismatch of renewable energy sources can be countered by the solid oxide electrochemical cell (SOC) technology, as the same SOC be used for storing electricity as chemical fuels (electrolysis mode) and converting fuels to electricity (fuel-cell mode). Hitherto widespread use of SOCs has been hindered by insufficient long-term stability, in particular at high current densities.

A research group at DTU Energy has now found a way to eliminate one of the major degradation effects, describing the discovery in detail in the article 'Eliminating degradation in solid oxide electrochemical cells by reversible operation', which has just been published online on the website of  Nature Materials.

"We haven’t solved all the degradation mechanisms, but we have found a new way to eliminate the one that was until now the biggest of them all"
Christopher Graves, senior researcher, DTU Energy

A flow battery with methane

In the article, the research group consisting of the senior researchers Christopher Graves, Sune Dalgaard Ebbesen, Søren Højgaard Jensen, postdoc Søren Bredmose Simonsen and professor Mogens B. Mogensen, demonstrates how severe electrolysis-induced degradation, previously believed to be irreversible, can be completely eliminated by reversibly cycling between electrolysis and fuel-cell modes.

“A reversible SOC can be considered as a special type of rechargeable flow battery. The main difference is that a conventional battery stores energy in expensive metal atoms (typically Pb, Ni, Li, V and others) whereas a SOC advantageously stores electrical energy as relatively inexpensive fuels such as hydrogen, methane, and other hydrocarbons”, the group explains in the article.

The group recently found that the considerable loss of electrode activity often observed during electrolysis could be circumvented by removing gas-phase impurities present at only parts-per-billion levels in some of the gases supplied to a conventional Ni-yttria-stabilized zirconia (Ni-YSZ) fuel electrode.

This insight and years of research in materials for electrodes has greatly improved electrode performance stability, but an extreme degradation mechanism still occurred during continuous electrolysis operation at high current densities: a severe microstructural damage near the oxygen-electrode/electrolyte interface due to build-up of high internal oxygen pressure, where pressurized O2 bubbles form nanosized pores able to completely separate the grains of the YSZ and/or delaminate the electrode/electrolyte interface.

Completely inhibits deterioration

“We investigated whether this electrolysis-induced performance loss might be decreased by operating the cell reversibly, periodically cycling between fuel-cell and electrolysis modes. Remarkably, we found that reversible cycling completely inhibits the microstructural deterioration”, explains the group in the article.

The periodic cycling between fuel-cell and electrolysis modes allowed the pressurized O2 bubbles to be released from the interfaces in fuel-cell mode before reappearing in electrolysis mode.

Many tests were conducted with cycling the SOC with different time periods: 1 h in electrolysis mode and 5 h in fuel-cell mode, up to 5 hours in each mode. The 1:5 hour cycle proved the best, as the oxygen bubbles had time to dissolve. Not only that, periodically cycling between fuel-cell and electrolysis modes actually decreased the ohmic resistance in the cell after 4000 hours of testing.

“We haven’t solved all the degradation mechanisms, but we have found a new way to eliminate the one that was until now the biggest of them all: the build-up of high internal oxygen pressure leading to severe microstructural damage. Now we will explore reversible operation some more to find its limits and find out how to best take advantage of this kind of phenomenon”, says senior researcher Christopher Graves from DTU Energy.

The article in Nature Materials can be found here