Nanostructured electrodes for high-performance and durable solid oxide cells

Electrical energy is essential for maintaining our standard of living. The rapid growth in electricity generation from intermittent renewable solar and wind sources creates a need for affordable large-scale energy storage to balance the supply-demand mismatch in the grid. One promising technology to address this energy storage challenge is solid oxide cells (SOCs).

They can be operated either in electrolysis mode (as solid oxide electrolysis cells, SOECs) to convert H2O and/or CO2 into H2 and/or CO using renewable electricity, or in fuel-cell mode (as solid oxide fuel cells, SOFC) to generate electricity on demand using the fuels produced. However, the widespread commercialization of the SOC technology is still impeded by high cost and insufficient long-term durability.

In particular, the technology of operating SOCs in electrolysis mode is less mature, in which the electrodes of SOC cells often suffer from considerable degradation during continuous electrolysis operation. The focus of this thesis is the development of high-performance and durable SOCs by designing nanostructured electrodes via infiltration.

Efforts are devoted to three aspects: i) improving the performance of electrodes for intermediate-temperature (600–750 °C) operation. Operating SOCs at intermediate temperatures can not only provide choice for using cheaper interconnects and seals thus reducing system cost but also avoid some issues related to materials incompatibility and heat management; ii) enhancing the durability of electrodes, particularly for the fuel electrodes during electrolysis operation under high current densities.

The high current density enables a high gas production rate and lowers the capital costs; iii) the scale-up of the above progress, which is very important for practical applications. Throughout the thesis, the electrochemical performance and durability of the developed electrodes were mainly evaluated on full cells through electrochemical impedance spectroscopy (EIS), polarization curves, and galvanostatic tests.

The analysis of EIS data was performed by fitting with equivalent circuit model using complex-nonlinear-least-squares (CNLS) regression and calculating the distribution of relaxation time (DRT). First, different from most studies on infiltrated SOCs carried out on button cells with a small active area, 12.5 × 12.5 cm2 fuel-electrode-supported SOCs with La0.6Sr0.4CoO3-δ (LSC) infiltrated gadolinia-doped ceria (CGO) oxygen electrodes were prepared.

The electrochemical performance of the resulting SOCs was examined at 4 × 4 cm2 level (active area). The cell delivered favorable performance at 750 °C under high gas/steam utilization, e.g., a power density of 1.08 W cm−2 at 0.6 V in fuel cell mode and a current density of −1.07 A cm-2 at 1.3 V in electrolysis mode.

These results highlight the potential of using infiltration to produce large-size, high-performance SOCs. However, the cell showed significant degradation when operated at −0.5 A cm-2 for steam electrolysis, and the major degradation was found to be from the Ni/ yttria-stabilized zirconia (YSZ) fuel electrode.

The Ni/YSZ electrode is the most commonly used and best-performing fuel electrode in SOCs, while its degradation during electrolysis operation has been an ongoing challenge. Secondly, in order to enhance the durability of Ni/YSZ electrode, an approach of surface modification of the Ni/YSZ electrode by coating nano-sized CGO electrocatalysts via infiltration was developed.

After modification, the cell durability was dramatically improved, reducing degradation rate from 0.565 to 0.024 V kh-1, when tested at −1.00 A cm-2 and 750 °C for steam electrolysis. The mechanisms of the Ni/YSZ electrode degradation and of the mitigation via surface modification at these conditions were discussed and speculated.

To enable a sufficient porosity in the Ni/YSZ electrode structure for infiltration and to avoid the chemical expansion of the CGO barrier layer and the decomposition of oxygen electrode material during reduction, the full cell was pre-reduced with a “two-atmosphere-reduction”, i.e., the fuel electrode side was exposed to reducing atmosphere while the oxygen electrode was exposed to air.

This was achieved using an in-house built test rig which is well suited for the purpose if only a small number of cells is considered, but not well suited for mass production. To fit the upscaling requirement, the aforementioned infiltration approach was further simplified by replacing the complicated “two-atmosphere-reduction” procedure with a facile “one-atmosphere-reduction”, which can well be carried out in furnace during the cooling of the cell after the final sintering step.

Finally, considering that cobalt-containing materials often suffer from various issues when employed as oxygen electrodes, a nanoengineered La0.6Sr0.4FeO3–δ (LSF)-based oxygen electrode was developed by applying a nanoporous hybrid catalyst coating composed of nanoparticles of CGO and PrOx. Different from the conventional infiltration with a precursor of metal nitrate, here a mixture solution of colloidal CGO nanocrystals and Pr(NO3)3 was used for infiltration to enable this designed nanoengineered architecture.

The excellent activity and durability of the resulting hybrid-catalyst-coated LSF electrode were demonstrated for both fuel-cell and electrolysis operation. In particular, when applying this oxygen electrode on a CGO-modified Ni/YSZ fuel-electrode-supported cell, stable operation at 650 °C under −0.5 A cm-2 with cell voltage close to 1.3 V was achieved.

The results achieved in this thesis demonstrate the great potential of boosting the performance and durability of SOCs via surface modification with nano electrocatalysts.