Development of high-performing oxygen electrode materials for reversible Solid Oxide Cells

Reaching a sustainable carbon-free energy scenario in the next few years will require the development of reliable renewable energy sources able to effectively supply the growing energy demand. In this scenario reversible solid oxide cells (rSOC) had gained a relevant interest within the most promising technologies to enable the forthcoming energy transition. rSOC are scalable electrochemical devices capable of meeting the future electrical energy production and storage working as solid oxide fuel cells (SOFC) generating electrical power or converting the electrical power into useful chemical compounds (power-to-gas, P2G) in solid oxide electrolysis mode (SOEC). However, despite the outstanding potential of this electrochemical system, many scientific and technological issues must be carefully addressed to ensure a cost competitive, high performing and long-lasting cell technology able to meet the society’s green energy ambitions and needs. Among these issues it is well established that the oxygen electrode is one of the main sources of losses in the system overall energy balance. In this thesis, the development of innovative oxygen electrode materials and architectures was pursued to enhance the low performance of state-of-the-art materials. Different approaches have been followed aiming at the enhancement of the electrode activity and stability by: i) the design of electrode architecture, ii) the synthesis of materials with engineered point defect chemistry coupled with computational quantum mechanical modelling, and iii) the deep understanding of the oxygen red-ox reaction mechanism. Particular attention was paid towards the electrode microstructure trying to propose a solution for the degradation phenomena occurring in SOC under operation conditions for the oxygen electrode material Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). The proposed methodology consisted in the in-situ synthesis of the electrocatalyst inside a porous electrolyte scaffold which successfully produced an active well-dispersed nano-sized thin film coating covering the whole backbone. This electrocatalytic thin-film, well bonded to the robust electrolyte porous structure showed peculiar properties well beyond that those typically achieved with state-of-the-art BSCF-based electrodes. In addition, the prepared system showed high stability within the considered time scale in harsh cathodic and anodic current conditions. A second study was devoted to the electrochemical activity enhancement of the novel crystallographic structure AA’Co2O5+δ. The study of such structure focused on the introduction of point defects by substituting calcium in the barium site of SmBaCo2O5+δ (SBCO). This strategy would potentially lead to faster oxygen mobility and superior electrochemical performance. The incorporation of calcium successfully increased the SBCO electrochemical performance with the best results for the SmBa0.8Ca0.2Co2O5+δ (SBCCO). To gain further insights on the oxygen vacancy formation, density functional theory (DFT) calculations were performed for both structures SBCO and SBCCO. The energy landscape for stable structures was analysed and correlated with their oxygen deficiencies; DFT demonstrated how calcium incorporation has a beneficial effect in the formation of oxygen vacancies. Aiming at a better understanding of the SBCCO electrode behaviour a dedicated part was focused on the implementation of a Kinetic model. Based on the experimental interpretation, a reaction mechanism was proposed. Coupling the experimental and modelling allowed to identify a double-phase boundary (DPB) path driven kinetics. The reaction steps were identified by analysing the recorded electrochemical impedance spectroscopy (EIS) spectra at variable working conditions and applying distribution of relaxation times (DRT) analysis together with equivalent circuit modelling (ECM). The proper description of the kinetic mechanism was then developed with an ad hoc physics-based model for SBCCO electrode simulation. The robustness of the proposed model has then been proven with the aid of direct current measurements. In the last chapter, the development of a new family of oxide materials called High Entropy Oxides (HEO) was carried out. The aim of using this kind of material relies on the entropy stabilization reached by the incorporation of multiple cations in the perovskite structure, which potentially leads to highly stable oxides suitable for SOC. In this section, different materials containing multiple cations were deeply studied and characterised to identify their properties and the best synthesis route.