Multifunctional electrocatalytic activity of Pt-doped lanthanum strontium ferrite

In the current energetic scenario, new technologies must deal with the transition towards clean and renewable energy sources. Among innovative energy systems, highly stable and efficient energy storage and conversion systems still represent a limiting factor that must be overcome as a priority. This problem is closely connected with the development and optimization of new performing materials. Metal oxides with a perovskite structure (ABO3) have been proven to be high-performance and reliable materials in the field of energy applications and for this reason they are still widely investigated. The tunability of the perovskite structure makes these compounds attractive for catalysis and electrocatalysis. A-site modifications, such as aliovalent cation doping and/or Asite deficiency, have a direct impact on oxygen stoichiometry by modifying the crystal structure, the oxidation state of the B-site element, and the physical properties. Instead, B-site cations can deeply affect the electronic conductivity and act as catalytically active sites influencing the redox behavior. The development of such flexible structures represents a crucial step towards the design of symmetric and possibly reversible solid oxide fuel cell/solid oxide electrolyser cell (SOFC/SOEC) systems. Lanthanum strontium ferrites (LSF)-based oxides are among the most widely investigated perovskites for the flexibility and capability to accommodate different substitutional atoms in both A- and B-sites. The partial Sr substitution at the A-site, for instance, is well-known to improve oxygen vacancies in the structure, and the B-site doping with transition metals, such as Co, Ni and Mn, was proven to improve the electrocatalytic activity towards different reactions. Moreover, a high-temperature reduction can give rise to the surface segregation (exsolution) of B-element metal nanoparticles as additional catalytic sites. This structural evolution is entirely reversible upon re-oxidation providing the ability to renew the catalytic sites at each reduction cycle. All these features make LSF-based oxides multi-tasking electrodes for reversible SOFC/SOECs. In this work, low (0.5-5 mol%) B-site Pt-doping in a lanthanum strontium ferrite is presented as a successful approach to enhance the parent perovskite (LSF) properties as both SOC air and fuel electrode. B-site doping (≤ 5%) with platinum group metals (PGM) has been regarded as a clever way to profitably exploit the high catalytic activity of noble metals while limiting the final cost of the electrode material. Moreover, exposing such compounds to the reducing environment of SOC fuel electrodes, a cost-effective perovskite oxide substrate is supplied with highly active noble metal-based nanoparticles, that segregate on the surface in the exsolution phenomenon. Here, 5 mol% Pt-doped La0.6Sr0.4FeO3-δ (La0.6Sr0.4Fe0.95Pt0.05O3-δ) was fully characterized and compared to the undoped La0.6Sr0.4FeO3-δ. Structural investigation techniques, such as X-ray powder diffraction (XRPD), X-ray photoemission spectroscopy (XPS), as well as temperature analyses, such as thermogravimetric analysis (TGA) and O2 temperature programmed desorption (O2-TPD), were used to assess the structural changes and thermal behavior. The electrical conductivity was also measured, and the electrocatalytic properties were assessed in air and different reducing conditions mimicking both air and fuel electrode atmospheres. The electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times (DRT) techniques were employed to evaluate the rate determining step of the oxygen reduction reactions at different pO2 concentrations. The behavior at very low pO2 was pursued through temperature programmed reduction (H2- TPR), assessing the hydrogen consumption, and evaluating the sample morphology by means of field emission scanning electron (FE-SEM) and transmission electron (TEM) microscopies after reduction. The highly reducible nature of Pt cations yields a uniform and well dispersed metal NPs exsolved on the sample surface. Then, La0.6Sr0.4Fe0.95Pt0.05O3-δ was successfully used as symmetric electrode in an all-perovskite reversible symmetric solid oxide cell (r-SSOC), showing remarkable results in both hydrogen fuel cell mode and CO2 electrolysis mode, along with considerably good reversibility and longterm stability in 50% CO2:CO gas mixture. The stability and the oxygen catalytic activity of La0.6Sr0.4Fe0.95Pt0.05O3-δ were also investigated at low temperature by using rotating disk electrode (RDE) technique in alkaline environment. The sample, after experienced 500 °C reducing treatment, showed promising results as bifunctional catalyst both for oxygen evolution (OER) and oxygen reduction (ORR) reaction, compared to undoped La0.6Sr0.4FeO3-δ. Finally, the structural changes carried out by the reduction treatment were also investigated by using soft synchrotron-based X-ray adsorption spectroscopy (XAS). The thesis is divided into two main sections: an introductory part (Part A: Background) and an experimental part (Part B: Results). Part A provides a scientific overview of the energy scenario and the critical materials related issue (paragraph 1). It also provides an up-to-date description of the properties and applications of perovskite oxides in the field of electrochemical energy storage and conversion devices (paragraph 2). Two separate paragraphs (3 and 4) are dedicated to high and low temperature applications, respectively. The focus is on the state of the art of perovskites for high-temperature (> 700 °C) solid oxide fuel cell (SOC) electrodes and for lowtemperature (< 100 °C) oxygen catalysis. The results are displayed into three chapters. Chapter 1 deals with the synthesis and characterization of Pt-doped La0.6Sr0.4FeO3-δ oxides and focuses on the structural and electrochemical properties at high temperatures both in oxidizing and reducing environments. Chapter 2 displays the use of La0.6Sr0.4Fe0.95Pt0.05O3-δ oxide as electrode in the r-SSOCs and the electrochemical investigation of the single cells. Finally, Chapter 3 reports the investigation of La0.6Sr0.4Fe0.95Pt0.05O3-δ for OER/ORR at low temperature. The results were acquired during the 4 months spent abroad as visiting PhD student at Electrochemistry Laboratory (LEC) of Paul Sherrer Institute (PSI) in Villigen (CH) under the supervision of Prof. Emiliana Fabbri.