Eliminating degradation in solid oxide electrochemical cells by reversible operation

Abstract

One promising energy storage technology is the solid oxide electrochemical cell (SOC), which can both store electricity as chemical fuels (electrolysis mode) and convert fuels to electricity (fuel-cell mode). The widespread use of SOCs has been hindered by insufficient long-term stability, in particular at high current densities. Here we demonstrate that severe electrolysis-induced degradation, which was previously believed to be irreversible, can be completely eliminated by reversibly cycling between electrolysis and fuel-cell modes, similar to a rechargeable battery. Performing steam electrolysis continuously at high current density (1 A cm(-2)), initially at 1.33 V (97% energy efficiency), led to severe microstructure deterioration near the oxygen-electrode/electrolyte interface and a corresponding large increase in ohmic resistance. After 4,000 h of reversible cycling, however, no microstructural damage was observed and the ohmic resistance even slightly improved. The results demonstrate the viability of applying SOCs for renewable electricity storage at previously unattainable reaction rates, and have implications for our fundamental understanding of degradation mechanisms that are usually assumed to be irreversible.

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…Clearly, this type of failure can be considered a severe degradation mechanism. However, it was recently discovered that periodic reversal of the polarisation can avoid the damage 51,52 , blurring the line between passivation and degradation. The cathodic polarisation periods relieve the built-up oxygen pressure, but it is not yet clear if this must occur before or after structural deterioration has started --afterwards implies that nano/microcracks that have begun to form are sintered back together during cathodic periods 51 .…”

“…However, it was recently discovered that periodic reversal of the polarisation can avoid the damage 51,52 , blurring the line between passivation and degradation. The cathodic polarisation periods relieve the built-up oxygen pressure, but it is not yet clear if this must occur before or after structural deterioration has started --afterwards implies that nano/microcracks that have begun to form are sintered back together during cathodic periods 51 . There is some disagreement in literature about the precise mechanisms of cathodic activation 51,53 and anodic degradation, which seem to depend critically on the details of the interface in a particular cell, e.g.…”

“…The cathodic polarisation periods relieve the built-up oxygen pressure, but it is not yet clear if this must occur before or after structural deterioration has started --afterwards implies that nano/microcracks that have begun to form are sintered back together during cathodic periods 51 . There is some disagreement in literature about the precise mechanisms of cathodic activation 51,53 and anodic degradation, which seem to depend critically on the details of the interface in a particular cell, e.g. the thermal history, nano/microstructure, and composition (enrichment/deficiency of different cations and presence of secondary phases).…”

Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability

“…Even though reversible operation of SOC (i.e. fuel cell mode operation) has shown advantageous for the performance of SOC [4,5]; it should be emphasized that at higher electrolysis current densities irreversible degradation phenomena takes place which are not likely to be recoverable by SOFC operation of the SOC [6,7]. Long-term stability needs to be improved even considering promising results such as reported by Tietz, Corre and Hjalmarsson [8][9][10][11] if SOFC systems, and later SOEC systems, are to operate profitable for several years.…”

“…Page 19 of 24 4. Discussion SOEC will typically be operated at rather high temperature (700-900 ºC) and high partial pressure of steam (e.g.…”