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NANO2021-Nanoteknologi og nye materiale

Functional Grading by Key doping in Catalytic electrodes for Proton Ceramic Cells

Alternative title: Funksjonell gradering i katalytiske elektroder for protonledende keramiske elektrokjemiske celler

Awarded: NOK 6.3 mill.

Project Number:

299736

Project Period:

2019 - 2023

Location:

Partner countries:

Electrochemical cells, such as fuel cells and electrolysers are central components in a green energy future, where intermittent energy from renewable sources is balanced by production and consumption of hydrogen. Production and consumption of hydrogen is, however, at this point mostly based on production from natural gas and consumption by low temperature fuel cells with a demand of expensive Pt catalysts, vulnerable for sulphur poisoning from hydrogen stemming from natural gas. Therefore, a more robust system with cheap and abundant catalysts and high energy conversion efficiency is needed. Proton Ceramic Electrochemical Cells, comprising Fuel Cells and Electrolysers are the potentially most energy efficient alternative, but is currently limited by cell resistance, and by the efficiency of the positive electrode. "Functional Grading by Key doping in Catalytic electrodes for Proton Ceramic Cells" (FunKeyCat) is an effort to bridge the gap between fundamental science and applied research for a leap towards highly efficient electrochemical cells by understanding the effects of functional and mechanical properties of the constituent materials on the efficiency of the cells. Challenges such as cell resistance and catalytic properties of the electrodes will be overcome through studies of how doping of key elements affects ionic and electronic transport in the electrode materials, and how the balancing between these correlates with chemical and thermal expansion, causing mechanical degradation. Functional grading will increase mechanical robustness, minimise cell resistance and maximise electrochemical functionality. FunKeyCat will also explore a new concept of using electric potential and thermal and atmospheric cycling for exsolution and regeneration of oxide nano catalysts to enhance cell durability and performance. In the first two years of the project, several composition ranges of mixed conducting oxides has been synthesized in pursue of the project?s first two objectives: Materials for graded functionality is chosen in the system Ba0.5La0.5Co1-xFexO3-? (BLCF). This system is investigated with respect to hydration, electrical conductivity, electrochemical performance and thermal expansion. The results confirm the hypothesis of the project: That hydration is increased ? and electrical conductivity and thermal expansion is decreased ? by increasing the Fe-content in BLCF. Electrochemical characterisation of BLCF (x = 0, 0.25, 0.5 and 0.75) as separate electrodes shows that substitution of as much as 75% of Fe for Co renders electrochemical performance unaltered over the substitution series in the temperature interval 450 ? 650°C. Materials and procedures for nano-oxide exsolution are examined. The system of Ti-doped LaMnO3 was examined in the first year, and was abandoned due to non-satisfactory exsolution properties. In stead, exsolution was studied in the systems BaLnCo2O6-? (Ln: Gd, Pr, La, Lu, Y) and BLCF. Both these systems show exsolution of nanoparticles. The second hypothesis of the project is thus partly confirmed. Partly because exsolution happens by reduction, and not by oxidation as hypothesized. Exsolution occurs to some extent in stoichiometric compositions by annealing in low pO2, and further in two main situations: By doping with cations of deviating ionic radius ? as in BaLa1-xLuxCo2O6-? and BaGd1-xYxCo2O6-? ? and by synthesis of A-site deficient compositions. Transmission Electron Microscopy (TEM shows that the exsolved nanoparticles contain the red-ox active B-site cation (Co). Exsolution is found in Ba(Gd,La)1-xLuxCo2O6-?, BaGd1-xYxCo2O6-?, Ba0.95PrCo2O6-?, Ba0.5La0.5Co1-xFexO3-? (stoichiometric BLCF) and Ba0.49La0.5Co1-xFexO3-? (understoichiometric BLCF), and the results show increasing exsolution with increasing Co content, lowered pO2 and increasing Ba-deficiency. Spray-coated BLCF electrodes with thicknesses of 5-10 micrometers thickness is produced for graded functionality Co-Fe interdiffusion studies are conducted, and the production parameters for graded electrodes are optimized accordingly. In Gdansk University of Technology (GUT), a 3D printer has been built for deposition of ceramic electrodes, and procedures for filament (Fused Deposition) and gel (extrusion) with laser post-sintering is under development. The last year of the project will be devoted to fulfilling the two first objectives and combine them in meeting the third objective, which is to develop optimised electrodes with graded functionality and exsolved nano-scaled oxide catalysts. Two manuscripts are under preparation: One on gas-phase analysis of oxygen kinetics over oxide catalysts and one on functional properties of the BLCF system. Two articles are published; one on electrocatalytic effect of double perovskites in photoelectrochemical cells, and one on microstructural design of BLCF.

The performance of Proton Ceramic Electrochemical Cells (PCECs) is currently limited by cell resistance, and by the efficiency of the positive electrode (positrode), which in these systems rely on ceramic materials with mixed protonic and electronic conductivity (MPECs). Recent studies have shown the significance of good MPEC materials for improved electrode functionality. The need for both proton conduction for extended electroactive area, and high electronic conduction for good current collection and utilization of the electrode volume is a tough nut to crack. Moreover, the most optimized MPEC materials often suffer from a severe mismatch of Thermal Expansion Coefficient (TEC) with respect to the electrolyte material, causing delamination, reduced active area and increased ohmic contact resistance. Finally, most positrodes are limited by slow surface kinetics, causing electrode polarization. FunKeyCat will produce graded electrodes with designed functional properties by co-doping MPECs with key elements for shifting the equilibrium of protons and electron holes throughout the electrode thickness, and at the same time ensure a graded TEC mismatch. In the intermediate temperature range, catalysts will be needed to increase the rates of the surface mass transfer reaction. FunKeyCat will explore decoration of electrode surfaces by catalyst nanoparticles by in situ exsolution of nano-scaled oxides, based on thermodynamic and defect chemical principles. The outcome of the project will be fully integrated, highly catalytic electrodes with superior current collection properties and nano-scaled microstructures. The electrodes will exhibit regenerative catalytic properties after long-term degradation, improved functionality, increased thermomechanical and chemical stability, and the manufacturing process will ensure scalability for industrial processing at higher TRLs. The project will start at TRL2 and end at TRL4, where button cells will be manufactured and tested.

Activity:

NANO2021-Nanoteknologi og nye materiale