Understanding the processes governing performance and durability of solid oxide electrolysis cells.

Operation of a Ni-YSZ electrode supported Solid Oxide Cell (SOC) was studied in both fuel cell mode (FC-mode) and electrolysis cell mode (EC-mode) in mixtures of H2O/H2, CO2/CO, H2O/H2O/CO2/CO at 750 °C, 800 °C and 850 °C. Although the SOCs are reversible, the polarisation characterisation shows that the kinetics for the reduction of H2O and CO2 is slower compared to oxidation of H2 and CO, and that oxidation/reduction in CO2/CO mixtures is slower than in H2O/H2 mixtures. The kinetic differences are partly related to the polarisation heating and the entropy change. Also the diffusion resistance is larger in EC-mode as compared to FC-mode and the low frequency concentration resistance (which is affected by diffusion), is asymmetric around the open circuit voltage (OCV), and is significantly higher in the EC-mode. Both the increased diffusion resistance and the asymmetric low frequency concentration resistance result in a decreased activity in the EC-mode. Changing the porosity of the support structure shows a significant change in both the diffusion resistance and low frequency concentration resistance when applying current, showing that diffusion limitations cannot be neglected for SOCs operated in the EC-mode. Also the Ni-YSZ TPB resistance is affected by changing the support porosity, indicating that kinetic investigations under current and even at OCV, and the chase for a general expression for "all" Ni-YSZ electrodes may be pointless. The diffusion limitations through the support and active electrode structure create an increased reducing atmosphere at the interface which may be related to the degradation of the cells.

Eliminating degradation in solid oxide electrochemical cells by reversible operation.

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.

Light at the interface: the potential of attenuated total reflection infrared spectroscopy for understanding heterogeneous catalysis in water.

IR spectroscopy has been an important tool for studying detailed interactions of reactants and reaction-intermediates with catalyst surfaces. Studying reactions in water is, however, far from trivial, due to the excessive absorption of infrared light by water. One way to deal with this is the use of Attenuated Total Reflection spectroscopy (ATR-IR) minimizing the path length of infrared light through the water. Moreover, ATR-IR allows for a direct comparison of reactions in gas and water on the same sample, which bridges the gap between separate catalyst investigations in gas and liquid phase. This tutorial review describes recent progress in using ATR-IR for studying heterogeneous catalysts in water. An overview is given of the important aspects to be taken into account when using ATR-IR to study heterogeneous catalysts in liquid phase, like the procedure to prepare stable catalyst layers on the internal reflection element. As a case study, CO adsorption and oxidation on noble metal catalysts is investigated with ATR-IR in gas and water. The results show a large effect of water and pH on the adsorption and oxidation of CO on Pt/Al(2)O(3) and Pd/Al(2)O(3). From the results it is concluded that water affects the metal particle potential as well as the adsorbed CO molecule directly, resulting in higher oxidation rates in water compared to gas phase. Moreover, also pH influences the metal particle potential with a clear effect on the observed oxidation rates. Finally, the future outlook illustrates that ATR-IR spectroscopy holds great promise in the field of liquid phase heterogeneous catalysis.