Doping Strategies On Perovskite Oxides-Based Materials For Clean Energy And Energy Vectors

Currently, the Earth is facing with an energy and climate crisis arisen from years of excessive utilization of fossil fuels which have led to a huge and dramatic increase of anthropogenic CO2 and depletion of energy natural resources without a parallel development of alternative energy systems. Today, efforts are constantly spent for developing, on large scale, sustainable and renewable energy systems at zero CO2 emission. Therefore, clean energy transition entails CO2 capture, utilisation and storage as energy vectors and/or chemicals. In this scenario, it is of paramount importance the development of innovative technologies like solid oxide cells (SOCs) able to produce green energy and/or convert CO2 into valuable fuels. SOCs work in reversible mode as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). In SOFC mode clean energy is produced by exploiting the oxygen reduction reaction (ORR) at the cathode and the fuel oxidation at the anode. In SOEC mode, by supplying energy the same electrode materials promote the oxygen evolution reaction (OER) and the electrochemical CO2 reduction reaction (CO2RR), respectively [1]. To make SOCs competitive, it is mandatory to design electrocatalyst with suitable electrocatalytic activity, excellent mixed ionic-electronic conductivity and structural durability under working condition. Perovskite-based oxides with nominal formula of ABO3 have been recognized as one of most promising class of electrocatalysts for SOCs application [2]. The present work deals with doping strategies for the fine-tuning of perovskite-based electrode materials able to operate or in SOFC mode or in SOEC mode. The improvement of ORR through co-doping in A site of perovskite-type oxide was explored in the case of barium-doped LaxSryCo0.8Fe0.2O3 oxides obtained by solution combustion synthesis for different La/Ba and Sr/ Ba ratio. The positive effect of Ba was assessed, and a peculiar behaviour was evidenced after treatment at 900 °C in air with the formation of a bi-perovskite heterostructure, where a lanthanum-enriched cubic phase and a bariumenriched rhombohedral phase coexist as evidenced by X-Ray Diffraction (XRD) associated with Rietveld analysis method. Temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS) evidenced available oxygen vacancies on the surface and in the bulk and a lower area specific resistance towards ORR for bi-perovskite materials compared to the single-phase perovskite with the same nominal composition. The electrocatalytic reduction of oxygen molecules from air was investigated with the lesser-common strategy of fluorine-doping in O sites of La0.6Sr0.4Fe0.8Co0.2O3 perovskites by employing two different synthesis method. La0.6Sr0.4Fe0.8Co0.2O2.9F0.1 and La0.6Sr0.4Fe0.8Co0.2O2.8F0.2 were prepared by solution combustion synthesis (SCS) and complexing citrate method. Doping and synthesis strategies are effective in producing single-phase perovskite oxides only in case of La0.6Sr0.4Fe0.8Co0.2O2.9F0.1 as verified by XRD associated with Rietveld analysis method. Electrochemical Impedance Spectroscopy on symmetrical cells La0.6Sr0.4Fe0.8Co0.2O2.9F0.1/Ce0.8Sm0.2O2-x/La0.6Sr0.4Fe0.8Co0.2O2.9F0.1 pointed to area specific resistance (ASR) value of 0.06 Ωcm2 at 800°C for the sample prepared by citrate method and 0.11 Ωcm2 for the sample obtained by SCS. The improvement of CO2RR through co-doping in B site of perovskite-type oxide has been investigated in case of Cu (Ni)-doped La0.6Sr0.4Fe0.8Co0.2O3 and La0.6Sr0.4Fe0.8Co0.2O2.9F0.1 as potential alternative to well-known in literature cermets. Materials were prepared by solution combustion synthesis. Both Ni and Cu are incorporated in the rhombohedral lattice structure of La0.6Sr0.4Fe0.8Co0.2O3 as highlighted by XRD and Rietveld method. H2-TPR profiles show that this class of materials exhibits reduction properties and oxygen mobility in the SOCs working temperature range. Electrochemical Impedance Spectroscopy under pure CO2 atmosphere on symmetrical cells configuration with Ce0.8Sm0.2O2 as electrolyte revealed ASR value as low as 0.8 Ωcm2 at 800 °C and 1.6 Ωcm2 at 700 °C with activation energy as 0.77 eV.