Banca de DEFESA: ALLAN JEDSON MENEZES DE ARAÚJO

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : ALLAN JEDSON MENEZES DE ARAÚJO
DATE: 05/11/2021
TIME: 09:00
LOCAL: Defesa por videoconferência, presidente localizado Secretaria Integrada de Pós-Graduação CT
TITLE:

OPTIMISATION OF ELECTROLYTES AND OXYGEN ELECTRODES FOR REVERSIBLE SOLID OXIDE CELLS

KEY WORDS:

SOFC/SOEC; Gd-doped ceria (CGO); calcium cobaltite (C349); barium cobaltite (BCO); electrochemical impedance spectroscopy (EIS).

PAGES: 241
BIG AREA: Engenharias
AREA: Engenharia de Materiais e Metalúrgica
SUBÁREA: Materiais Não-Metálicos
SPECIALTY: Cerâmicos
SUMMARY:

Reversible solid oxide cells (r-SOCs) are promising and efficient technologies for converting electricity into fuel and fuel back into electricity. Such devices are targeted to operate in a wide range of temperatures and oxygen partial pressures. Hence, the cell components require materials that possess high levels of performance with good stability. In this respect, due to its high oxide-ion conductivity at intermediate temperatures, the gadolinium-doped ceria oxide, Ce_1-xGd_xO_2-δ (CGO), is one of the most promising electrolyte choices. However, its poor sinterability is a challenge to be faced to obtain high-performance devices. Hence, to try to overcome this limitation, a soft chemical synthesis route known as “proteic sol-gel synthesis” is used to prepare CGO ceramics exhibiting excellent sinterability at 1350 °C. Materials are fully characterised by their structural, chemical, microstructural, and electrical properties. Higher bulk conductivity is demonstrated for a sample with 10 mol% Gd when compared to one with 20 mol% Gd and explained by the formation of defect associates  with lower mobility. In contratst, similar specific grain boundary conductivities, calculated based on the brick-layer model, are observed as a possible result of the comparatively low sintering temperature. A Mott-Schottky model is used to determine the space-charge characteristics, where the lower barrier height of the sample with 20 mol% Gd is suggested to be due to the increased segregation of  species to the grain boundaries, attenuating the depletion of oxide-ion vacancies near the grain boundary core.

Another critical component is that of the oxygen electrode. The misfit calcium cobaltite electrode, [Ca₂CoO_3-δ]_0.62[CoO₂] (C349), has emerged as an interesting option for possessing a thermal expansion coefficient similar to that of the CGO electrolyte, a feature that could benefit this compound among the state-of-the-art electrodes. Nonetheless, its poor oxygen-ion conductivity precludes its use as oxygen electrode. Therefore, in the current work, we attempt to address this problem by providing a thorugh investigation of its electrochemical behaviour, experimenting different strategies to improve its performance. It is demonstrated that the C349 electrode requires a higher volume of solid fraction close to the electrolyte to maximise the ionic current into the bulk. Such an improvement can be achieved by increasing the number of deposited electrode layers due to the better packaging of the grains.

In a different perspective, an active interlayer made of Ce_0.8Pr_0.2O_2-δ (CPO, + 2 mol% Co) is used between the C349 electrode and the CGO electrolyte to improve the electrode performance. A combination of three different approaches using the distribution function of relaxation times (DFRT) analysis reveals the existence of parallel reaction paths. At higher temperatures, the pathway through the C349 is expected, given by its high electronic conductivity and sufficient ionic conductivity. However, at lower temperatures, the CPO + Co interlayer pathway becomes increasingly predominant due to its higher ionic conductivity compared to the C349.

C349/CGO and C349/CPO composite electrodes are also compared. The DFRT analysis demonstrates a preferential series pathway for the oxygen reaction in the CGO-based electrode, being notably faster on the CGO particles. In contrast, parallel pathways are suggested for the CPO-based electrode, with comparatively better performance at lower temperatures and under more oxidising conditions, due to the higher electronic conductivity of the CPO phase in these conditions. When used as composite matrix instead of an active interlayer, CPO provides a greater enhancement of electrode performance. Electrochemical measurements under applied polarisation indicate the CGO/C349 composite electrode offers a promising potential for r-SOCs, performing better as an anode in electrolyser mode.

As a final part of this thesis, we study an alternative layered structure, the promising barium cobalt oxide (Ba₂Co₉O₁₄ – BCO), as a r-SOC electrode. We firstly study the electrochemical behaviour of a 40 vol% CGO composite, which is found to provide a much lower polarisation resistance under cathodic polarisation. On the other hand, its electrochemical performance is slightly impaired under anodic polarisation. In a second work, we study attempt further optimisations by evaluating the influence of volume fraction of the CGO phase in the composite, with the composition being optimised with 50 vol% CGO due to an increased triple-phase boundary (TPB) length.

Overall, the current thesis provides a systematic investigation on the development of r-SOC materials with promising performances, trying to carefully address the fundamental mechanisms involved. The work, therefore, provides a solid advance on the research of these components for r-SOCs; a topic that is still in its infancy.

 

BANKING MEMBERS:
Presidente - 1300987 - CARLOS ALBERTO PASKOCIMAS
Interno - 1350249 - RUBENS MARIBONDO DO NASCIMENTO
Externo à Instituição - DOMINGO PÉREZ-COLL
Externo à Instituição - ANDREI KAVALEUSKI
Externo à Instituição - FRANCISCO JOSÉ ALMEIDA LOUREIRO
Externo à Instituição - DUNCAN PAUL FAGG