Novel Fuel Electrodes for Solid Oxide Cells

As global energy demand rises, there is an urgent need for the transition from primary fossil fuels to more environmentally friendly energy sources due to the severe environmental impacts of fossil fuels. Although renewable energies such as wind and solar have been increasingly utilized, their intermittent nature presents challenges for consistent power supply. It requires effective large-scale, long-term energy storage solutions. Among different approaches, "Power-to-X", which proposes converting renewable electricity into fuels like hydrogen, has garnered significant attention. Green hydrogen, generated through renewable-based Power-to-X technology, can be stored, transported, and even processed into other fuels and chemicals. Among the Power-to-X technologies, electrolysis plays a significant role, particularly Solid Oxide Cells (SOC) technology. While SOCs offer dual functionality(fuel cell mode/electrolysis mode) and high efficiency, their lack of long-term stability hinders the commercialization of the technology. One important component of the SOCs that dictates both the performance and durability is the fuel electrode. The state-of-the-art fuel electrode in SOCs is Ni-YSZ. However, this electrode faces several challenges, including redox instability, susceptibility to coking, and sulfur poisoning.

Therefore, the objective of this project is to develop a durable SOC fuel electrode with high performance. Recently, particular attention has been given to A-site deficient doped SrTiO₃ electrode materials due to their high stability and good charge transport properties. When a second transition metal is added to the B-site (e.g. Fe, Ni, Cu), this may exsolve under reduction providing catalytically active particles on the material surface, which strongly boosts electrode performance. Therefore, this thesis focuses on a class of A-site deficient La-doped SrTiO₃-based material La_0.49Sr_0.31Ti_0.94Fe_0.03Ni_0.03O₃ (LSFNT). We mainly devote to four aspects: i) characterization of electrical properties of LSFNT: a systematic investigation of the electrical properties of LSFNT with different microstructures; ii) enhancement of electrode performance by electrocatalyst integration: improvements in the LSFNT electrode performance through exsolution and/or infiltration of the electrocatalyst phase; iii) optimization of electrocatalyst infiltration process: identification and selection of different infiltration process parameters to achieve stable and high-performance fuel electrodes; iv) development of novel fuel electrode materials with the capability of exsolution of electrocatalytically active phases: exploitation of experimental and numerical approaches.

The structural investigations revealed that LSFNT maintained a cubic perovskite structure under varied oxygen partial pressures. Notably, upon exposure to pure hydrogen, Ni_1-xFe_x nanoparticles were observed to exsolve. The electrical transport measurements of the pre-reduced LSFNT exhibited an electrical conductivity of 10 S/cm at 850°C, notably higher than its in-situ reduced sample, 0.9 S/cm after 700 h of reduction. A key finding was that LSFNT showed mixed ionic electronic conductivity (MIEC). Specifically, the LSFNT ionic conductivity was measured as 0.054 S/cm at 850°C exceeded that of 8% Yttria Stabilized Zirconia and was comparable to gadolinium-doped ceria (CGO), highlighting its potential as an electrode backbone material for SOCs.

The electrochemical analysis revealed that the LSFNT electrode, after reduction and the exsolution of NiFe nanoparticles, exhibited a high polarization resistance of 36 Ω cm² at 880°C in 3% steam/97%H₂. In order to improve the electrode performance, CGO as an additional electrocatalyst was integrated into the LSFNT backbone. The introduction of CGO showed a remarkably reduced polarization resistance of 0.14 Ω cm² at 850°C in a 3% steam/97% H₂ mix. Moreover, in scalability tests using a standardized cell configuration, the polarization resistance contributed by the fuel electrode at 850 and 800 °C is 0.056 Ω cm² and 0.079 Ω cm², respectively, which was comparable to the Ni-YSZ electrode.

Furthermore, the amount of the electrocatalyst and the integration process parameters were optimized. Optimal results were achieved with a load of 2.18 mg/cm² of CGO, yielding a polarization resistance of 0.14 Ω cm² at 850 °C in a 3% H₂O/H₂ atmosphere. Excessive loading of CGO led to the reduction of electrode porosity and resulted in increased diffusion losses. Comparative evaluation of LSFNT electrode with LST showed that the exsolved NiFe nanoparticles in LSFNT enhanced the electrochemical performance, particularly at low temperatures, e.g., 650 °C. The order of the heat treatment processes, specifically the sequence of infiltration of CGO and exsolution of NiFe nanoparticles, significantly impacted the electrochemical performance of LSFNT. The most favorable method was CGO infiltration at 350 °C followed by NiFe nanoparticle exsolution at 950°C for 5 hours. Comparative analysis indicated a superior performance of CGO-infiltrated LSFNT electrodes over Ni-infiltrated ones, attributed to CGO mixed conductivity. The synergistic benefits of Ni-CGO co-infiltration manifested primarily when infiltration was performed after exsolution. The durability measurements of LSFNT fuel electrodes showed ΔR_P= 38 mΩ cm²/kh and further strategies to stabilize the performance are needed.

As observed, the amount of exsolved NiFe nanoparticles in LSFNT was not sufficient to achieve competitive fuel electrodes for SOCs. This motivated the search for alternative materials with improved exsolution capability, preferably at lower temperatures. A series of B-site doped perovskite materials with the capability of exsoluting electrocatalytically active nanoparticles - Sr_0.95Ti_0.3Fe_0.65Cu_0.05O_3-δ (STFC) and Sr_0.95Ti_0.3Fe_0.65Ni_0.4Cu_0.01O_3-δ(STFCN) - were designed and synthesized. The structural characterization showed phase pure compounds. The investigations on exsolution conditions showed that a higher copper concentration facilitated more efficient exsolution at lower temperatures, with STFC samples demonstrating nanoparticle exsolution at 600°C, producing an average particle size of 37 nm and a high population. The study also introduced a nucleation and growth model to elucidate the exsolution mechanism. The predictability of this model was validated by the close resemblance of predicted morphologies to actual SEM observations, providing guidelines for future designs of the exsolution materials in SOCs.

In general, this thesis revealed the mixed electronic and ionic conductive nature of LSFNT, highlighting its suitability as a robust backbone for solid oxide cells. Simultaneously, this thesis demonstrated that the CGO infiltration process was found to be crucial for achieving low polarization resistance and enhancing overall cell performance. Furthermore, the thesis explored new materials with improved electrocatalytic activity, indicating the potential for more efficient SOC operation at lower temperatures through the tailored design and exsolution of nanoparticles.