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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.

Proton Ceramic Electrochemical Cells, comprising 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. Present hydrogen technology is 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. Efficiency of PCECs is limited by cell resistance and kinetic properties of the positive electrode. "Functional Grading by Key doping in Catalytic electrodes for Proton Ceramic Cells" (FunKey Cat) relates electrochemical efficiency to functional properties of electrode materials with mixed protonic and electronic conductivity by doping of key elements affecting ionic and electronic transport. This will affect chemical and thermal expansion, causing mechanical degradation. Functional grading will increase mechanical robustness, minimize cell resistance and maximize electrochemical functionality. FunKeyCat also explore exsolution of oxide nano catalysts to enhance cell durability and performance. Several composition ranges of mixed conducting oxides have been synthesized and tested in the project’s first two objectives: Materials for graded functionality is the system Ba0.5La0.5Co1-xFexO3-d (BLC – BLCF – BLF), investigated with respect to hydration, electrical conductivity, electrochemical performance, thermal expansion, surface kinetics and stability in steam. The results show that hydration is increased – and electrical conductivity, surface kinetics and thermal expansion is decreased – by increased Fe content. Electrochemical characterisation shows that electrode polarization resistance (Rp) resemble the trend of measured proton concentrations for (x = 0, 0.25, 0.5, and 0.75) at low temperatures where proton conductivity is not insignificantly small. This is reflected in a shift to lower slope value for log (Rp) vs 1/T at lower T due to increased proton concentration in BLF (x = 1) – that lowers Rp at low T. Oxygen surface exchange experiments showed that the oxygen exchange coefficient is two orders of magnitude lower for BLF than for BLC (x = 0) at 450°C. The results indicate that slow oxygen kinetics can be counteracted by increased partial proton conductivity and thus increased active surface area. At higher temperatures – where proton conductivity in BLF is negligible – the electrodes with higher Co-content (x = 0, BLC) shows the lowest polarisation resistance. Surface kinetics is also characterized for mixed proton and electron conductor BaGd1-xLaxCo2O6-d (BGLC37, x = 0.7 and BGLC82, x = 0.2). All BGLC and BLC-BLCF compositions are synthesized stoichiometric and with Ba-deficiency, and gas-phase analysis confirm that Ba-deficiency gives significantly improved surface kinetics (BGLC82) and exsolution of nano catalysts (all Ba deficient compositions). Stability tests in high steam pressures also confirm increased stability with Ba deficiency. Thermo-gravimetry confirms hydration of Ba-deficient BGLC37, and electrochemical measurements of this composition will be completed and published after the project is ended. Exsolution of nano catalysts has been shown for the systems BaLnCo2O6-d (Ln: Gd, Pr, La, Lu, Y) and BLCF. The second hypothesis of the project is thus confirmed, although exsolution happens by reduction, and not by oxidation as hypothesized. Exsolution may occur in stoichiometric compositions by annealing in low pO2, and further in two main situations: By doping with cations of deviating ionic radius and by synthesis of A-site deficient compositions. Exsolution is found in Ba(Gd,La)1-xLuxCo2O6-d, BaGd1-xYxCo2O6-d, Ba1-xPrCo2O6-d, and Ba-deficient BLCF, which shows increasing exsolution with increasing Co content, lowered pO2 and increasing Ba-deficiency. In the third objective, electrodes with graded functionality and exsolved nano-catalysts are manufactured. Graded BLCF electrodes with thicknesses of 5-10 micrometers in each layer with increasing Fe towards the electrolyte interface and increasing Co towards the electrode surface have been produced, and Fe-Co interdiffusion electrodes have been produced from BLF and BLC. In Gdansk University of Technology (GUT), 3D printed BLCF electrodes on Ba(CeZr)O3 electrolytes have been manufactured, and these are now ready for electrochemical characterization. So far, two articles are published, one is submitted, and nine manuscripts are under preparation.

The results of FunKey Cat have significant impact on the field of research and development of more robust and efficient PCEC systems. Improvements of PCEC electrode efficiency are enabled based on kinetic- and hydration studies, and robustness and durability are enabled through tailoring of thermal expansion and chemical stability. Demonstration of operation in stack systems is under development in the EU-Project HEU PROTOSTACK – with SINTEF, Norwegian company CoorsTek Membrane Sciences, and multiple European partners – that utilise the project findings to manufacture electrodes with higher stability in steam for high temperature electrolysers, comprising proton conductivity and faster surface kinetics by cobalt oxide exsolution. Ceramic powders developed in FunKey Cat for PCEC electrodes are currently produced by Norwegian company CerPoTech. Electrode kinetics study with development of a mathematical description of the processes and currents over electrode and electrolyte is also implemented in two EU projects (FCH-JU WINNER, HEU PROTOSTACK). The project has thus paved way for scientific progress embedded in higher TRL European projects, and brings potential for further development of machine learning through the extensive datasets and experimental trends on stability and hydration properties. In a larger perspective, the results and improvements of PCEC technology will support the European Strategic Energy Technology Plan (SET-Plan), which will accelerate the development and deployment of low-carbon technologies and bring promising new zero-emissions energy technologies to market. Green hydrogen production from steam electrolysis is a pivotal technology for balancing intermittent renewable energy and reducing CO2 emissions from transportation and in a range of industrial processes, including decarbonization of steel and cement industry. Increased efficiency and lifetime of PCEC systems are key factors for deployment of the technology, and the successful outcomes of FunKey Cat – with improved materials and predictive trends for further improvements in efficiency and stability – give competitive advantages to one of the most promising sustainable technology for hydrogen production. The results support the Norwegian government's hydrogen strategy, which is to reduce costs of sustainable hydrogen production.

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.

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NANO2021-Nanoteknologi og nye materiale