Degradation of Ni-YSZ and Ni-GDC fuel cells after 1000 h operation: Analysis of different overpotential contributions according to electrochemical and microstructural characterization

Solid Oxide Fuel Cell (SOFC) technologies are emerging as potential power generation units with limited environmental impacts. However, the main challenges towards large scale commercial applications are high costs and low lifetime compared to currently used technologies. The present study aims at understanding degradation mechanisms in SOFCs through both experimental and modelling approaches. For this purpose, two state of the art fuel cell configurations based on Ni cermet fuel electrode (either YSZ-Yttrium Stabilised Zirconia or GDC-Gadolinium Doped Ceria), YSZ electrolyte and LSCF (Lanthanum Strontium Cobalt Ferrite oxide) air electrode were chosen. The cells were tested for 1000 hours with H2 rich mixture as fuel feed and air as oxidant. Cells were characterised at several H2/H2O ratios and temperatures with air or oxygen fed to the air electrode using different techniques. These allowed the identification of kinetic parameters to be implemented in an in-house 2D Fortran based model. The model was able to successfully simulate global cell behaviour as a function of local features, and it was validated with experimental I-V curves recorded prior and post durability operation. Moreover, post-mortem microstructure characterisation was also performed to fine-tune the model towards a more accurate prediction of the degradation influence on cell performance.

Colloidal Processing of Y0.08Zr0.92O2/La0.80Sr0.20MnO3 Semi-Cells Using a Sr-Doped Lanthanum Manganite Synthesized by a Citrate Route

In this paper, the interface between yttria stabilized zirconia (Y0.08Zr0.92O2, YSZ) electrolyte and Sr-doped lanthanum manganite (La0.80Sr0.20MnO3, LSM) cathode for solid oxide fuel cells (SOFCs) is studied. For such a purpose, the combination of a suitable synthesis route for obtaining fine powders and simple aqueous colloidal shaping routes is proposed. The synthesis of nanosized particles of La0.80Sr0.20MnO3 by a citrate route and their full characterization, including the colloidal stability and the densification and phase development determined by X-ray diffraction and electron microscopy at different temperatures, is reported. In a second step, YSZ tapes were obtained by aqueous tape casting and used as substrates for the preparation of LSM coatings by dip-coating using aqueous slurries. YSZ tapes were used either in the green state or after a pre-sintering treatment. Co-sintering at 1350 °C led to a sharp interface with excellent adhesion, also achieved when coating pre-sintered tapes. In both cases, the substrates are dense and the coatings are porous, with thicknesses of 85 and 60 μm for green and pre-sintered tapes, respectively. No diffusion of Zr and Y occurs at the LSM layer, but some diffusion of La and Mn towards the YSZ layer takes place.

Activation of Porous Pt Electrodes Deposited on YSZ Electrolyte by Nitric Acid Treatment

The effect of nitric acid treatment on the electrochemical performance of porous Pt electrodes deposited on YSZ (abbreviation from yttria stabilized zirconia) electrolyte was investigated. Two identical symmetrical Pt/YSZ/Pt cells with porous Pt electrodes were fabricated, after which the electrodes of the first cell were kept as sintered, while those of the second cell were impregnated with HNO3 solution. The electrochemical behavior of the prepared electrodes was studied using impedance spectroscopy and cyclic voltammetry. Significant reduction of the polarization resistance of the HNO3-treated electrodes was revealed. The observed enhancement of the electrochemical performance of porous Pt electrodes was assumed to be caused by adsorption of NOx-species on YSZ and Pt surfaces, which promotes oxygen molecules dissociation and transport to the triple phase boundary by the “relay-race” mechanism. The obtained results allow for considering the nitric acid treatment of a porous Pt electrode as an effective way of electrode activation.

Joining YSZ electrolyte to AISI 441 interconnect for solid oxide cells using the Ag interlayer: Enhanced mechanical and aging properties

Conventional Ag-CuO braze can lead to two electrolyte/interconnect joining issues: over-oxidation at the steel interconnect and hydrogen-induced decomposition of CuO. This work demonstrates that a pure Ag interlayer, instead of Ag-CuO braze, can join YSZ electrolyte to AISI 441 interconnect in air. Reliable joining between YSZ and AISI 441 can be realized at 920 °C. A dense and thin oxide layer (~2 μm) is formed at the AISI 441 interface. Also, an interatomic joining at the YSZ/Ag interface is detected by TEM observation. Obtained joints display high shear strengths (~86.1 MPa), 161% higher than that of joints brazed by Ag-CuO braze (~33 MPa). After aging in reducing and oxidizing atmospheres (800 °C/300 h), joints remain tight and dense, indicating a better aging performance. This technique eliminates the CuO-induced issues, which will extend lifetimes for SOFC/SOEC stacks and other ceramic/metal joining applications.

Analysis of the effect of ionic conductivity of electrolyte materials on the solid oxide fuel cell performance

SOFC solid electrolytes are known for their ionic conductivity characteristics, which increase with increasing SOFC operating temperature. Using COMSOL Multiphysics numerical simulation, analysis of SOFC power performance with yttria-stabilized zirconia (YSZ) and lithium sodium carbonate – gadolinium-doped ceria ({LiNa}2CO3-GDC) electrolytes was conducted to determine the potential of these electrolytes in their application in SOFC. The ionic conductivity of YSZ was differentiated based on the mole value of the yttria content, namely 8, 8.95, 10 and 11.54 mol. Meanwhile, GDC varied based on the (LiNa)2CO3 content such as 7.8, 10, 16.8 and 30 %. With the numerical model, the calculation error is an average of 7.32 % and 6.89 % for the experimental power and voltage values. In SOFC with the YSZ electrolyte, it was found that the power output can increase 26.4–35 times with an increase in operating temperature from 500 °C to 750 °C. SOFC with 8YSZ can produce the highest power compared to other YSZ, which is 123 A/m2 at a current of 198 A/m2 with an operating temperature of 500 °C and 3,440 A/m2 at a current of 5,549 A/m2 with an operating temperature of 750 °C. Whereas in SOFC with the GDC electrolyte, it was found that the power output can increase 18.6–22.6 times with an increase in operating temperature from 500 °C to 750 °C. SOFC with 30 % (LiNa)2CO3-GDC produced the highest power compared to other GDC, which is 231 A/m2 at a current of 444 A/m2 with an operating temperature of 500 °C and 5,240 A/m2 at a current of 10,077 A/m2 with an operating temperature of 750 °C. YSZ also showed the potential for an increase in power output as the SOFC temperature increases above 750 °C, while the 30 % variation (LiNa)2CO3-GDC shows a limited increase in ionic conductivity at 750 °C

Complexions at the Electrolyte/Electrode Interface in Solid Oxide Cells

Rapid deactivation presently limits a wide spread use of high-temperature solid oxide cells (SOCs) as otherwise highly efficient chemical energy converters. With deactivation triggered by the ongoing conversion reactions, an atomic-scale understanding of the active triple-phase boundary (TPB) between electrolyte, electrode and gas phase is essential to increase cell performance. Here we use a multi-method approach comprising transmission electron microscopy and first-principles calculations and molecular simulations to untangle the atomic arrangement of the prototypical SOC interface between a lanthanum strontium manganite (LSM) anode and an yttria-stabilized zirconia (YSZ) electrolyte. We identify an interlayer of self-limited width with partial amorphization and strong compositional gradient, thus exhibiting the characteristics of a complexion that is stabilized by the confinement between two bulk phases. This offers a new perspective to understand the function of SOCs at the atomic scale. Moreover, it opens up a hitherto unrealized design space to tune the conversion efficiency.