Periodic Reporting for period 3 - SElySOs (Development of new electrode materials and understanding of degradation mechanisms on Solid Oxide High Temperature Electrolysis Cells.)
Berichtszeitraum: 2018-11-02 bis 2020-05-01
The high temperature Solid Oxide Electrolysis (SOE) technology has a huge potential for future mass production of H2 (& synthesis gas) and shows great dynamics to become commercially competitive against other electrolysis technologies (AEL, PEMEL), which are better established but more expensive and less efficient. The consortium of SElySOs took advantage of the opportunity for a 4-years duration project and focused on understanding of the degradation and lifetime fundamentals on both of the SOEC electrodes, for minimization of their degradation and improvement of their performance and stability, mainly, under H2O electrolysis. In brief, the prime objective of SELySOs is to improve and/or develop new more efficient electrodes and understand the reaction mechanisms and processes that cause degradation on both SOEC electrodes, by combining experiments with theoretical modeling over an extended range of operating conditions. This is accomplished by using a targeted number of modified SoA Ni-based and perovskite-type electrocatalysts both for the H2O & O2 electrodes. Moreover, the technical goal of the project is to identify the key design parameters and acquire the necessary knowledge to guide the development of new SOECs less prone to degradation with improved performance & stability.
In summary, the project`s key objectives were: (•) Development of efficient, durable fuel (cathode) electrodes tolerant to degradation for their use in H2O and H2O/CO2 applications. This was approached through the preparation of two “families” of SOE cathode materials: (a) modified SoA Ni-based (e.g. Ni/GDC) cermets with: Ni-X bi-metallic or Ni-X-Y tri-metallic electrocatalysts (X=Au, Mo, Fe) and (b) perovskite structured catalysts of the general type LaSrCr1–xMxO3 (M=Mn, Fe, Co, Ni), (•) Development of SOEC air electrodes from new materials with reduced degradation, which are based on (i) nickel oxide perovskite materials (e.g. Pr2NiO4) and (ii) cobalt oxide perovskite materials (e.g. SrYCoMnO3), (•) Investigation of the reaction mechanisms on the SoA air electrode, in order to further understand the degradation mechanism of the existing cells, (•) Advanced (ex-situ, in-situ) physicochemical characterization of the applied electrocatalysts in order to understand the effect of different materials and operational conditions on the performance and stability of the proposed electrodes, (•) Development of detailed mathematical model/s accounting for all reactions taking place in a SOEC for the H2O electrolysis and H2O/CO2 co-electrolysis processes, as well as a model for the SOE cell degradation kinetics, (•) Identification of the best performing electrodes (both for the H2O and the O2 electrodes) in terms of efficiency and maximum lifetime for SOEC applications, dealing mainly with H2O electrolysis, (•) Fabrication of single cells (electrolyte or electrode-supported) by applying SoA and novel (Thermal Spraying) fabrication techniques, (•) Scaled up preparation of the best performing electrodes and long-term operation of the best performing combination of electrodes (Fuel electrode and O2 electrode) under H2O electrolysis conditions, (•) Development of 2 demonstration short stack/s, comprising cells with the best performing combination of electrodes and long-term operation under H2O electrolysis conditions.
Targeted modifications of Ni-based materials by using a commercially available NiO/GDC cermet with Au, and/or Mo and Fe via the D.P. and/or D.CP. methods, resulted in binary and ternary materials for use as modified Ni-based SOEC cathodes. A thorough, comparative study for performance assessment of 4 different Ni-metal free fuel electrode materials, namely LaSrCrM, where M=Fe, Ni, Co, Mn, was done for the two processes of interest, the H2O electrolysis and the CO2/H2O co-electrolysis. Surface and bulk characterization of the developed fuel and air powders/electrodes was thoroughly performed. There were also significant results from in-situ characterization on cells (e.g. in-situ XPS), combined with electrochemical measurements.
In regards to the model/s development and validation a multi-dimensional, stationary, isothermal model of solid oxide steam-CO2 co-electrolysis (SOcoE) cell was successfully developed in agreement with the project aims. The model was thoroughly validated, and its physical validity demonstrated, in the broad range of temperature and gaseous reaction mixture composition. One key result is that heterogeneously catalyzed chemical reactions play a key role in the final composition of the produced gas mixture. Finally, a 0-D model analysis of the degradation of the symmetrical cell with LNO-GDC oxygen electrode was also performed.
In the case of the Ni-based fuel cathodes, the cells comprising the Au-Mo modified electrodes exhibited quite better electrochemical performance, compared to the SoA Ni/GDC. In this respect, the nominated Ni-based fuel electrode for the H2O electrolysis was the 3Au-3Mo-Ni/GDC. Regarding the Ni-metal free electrodes, the Fe substituted chromite (LSCrF) electrode was the proposed fuel electrode for H2O & H2O/CO2 co-electrolysis. Finally, in the case of the new developed air electrodes for SOECs two are the proposed candidates for oxygen electrodes and these are PNCO-20 and LPNO. The nominated samples/electrodes were used for: (•) the preparation and testing (for H2O electrolysis) of “combinatorial” button cells, comprising as well the proposed O2 electrodes, (•) the preparation of larger cells for long-term stability tests and (•) the stack/s preparation and testing. Moreover, “Three-layer” and “five-layer” free standing solid oxide cells were manufactured exclusively by Thermal Spray.
Long-term stability investigation of the Ni-metal-free fuel electrodes aimed at the target of 1000 hours of operation. The performance of two cells, one with LSCrF/GDC//3YSZ//GDC/LSCoF/LNF and of a second with LSCrF/GDC//3YSZ/LSM-YSZ/LSM was examined and compared under 23.5% H2O/He electrolysis in the absence of H2 under steady-state operation with constant current density of 0.25 A cm-2 at 900˚C.
The enhancing effect of the 3Au-3Mo-Ni/GDC (modified Ni-based) fuel electrode was further verified with the use of the new O2 electrodes in the form of the so called “combinatorial” cells.
The electrochemical performance and long term stability of 8YSZ electrolyte supported 5x5 cm² single cells, comprising the best performing developed electrodes, was also investigated. During the long term stability tests of 1700, 1150 and 850 hours, respectively, for 3 different large-sized (active area of 20 cm2) single cells, at -0.3 A/cm2 and 900C, the lower degradation rate was observed for the cell comprising the modified 3Mo-3Au-Ni/GDC/8YSZ/GDC/LSCF single cell.
In regards to the stack manufacture and long-term operation, there are comparative data between one stack built using large (active area 42 cm2) TRL4 ESCs with 3Au-3Mo-Ni/GDC electrode and of another stack with similar size Ni/GDC electrode, which were operated at 835C, 0.5 A/cm2. The degradation rate for the stack with modified cells was 300 mV/kh, whereas for the Ni/GDC stack it was 700 mV/kh. Conclusively, this kind of element-doping may be an important method to extend the life-time of SOECs.
Management and dissemination activities were realized through various web tools, newsletters, presentations to the wide public, as well as 8 Open Access publications and 89 participations in conferences.
In regards to the model/s development and validation a multi-dimensional, stationary, isothermal model of solid oxide steam-CO2 co-electrolysis (SOcoE) cell was successfully developed in agreement with the project aims. The model was thoroughly validated, and its physical validity demonstrated, in the broad range of temperature and gaseous reaction mixture composition. One key result is that heterogeneously catalyzed chemical reactions play a key role in the final composition of the produced gas mixture. Finally, a 0-D model analysis of the degradation of the symmetrical cell with LNO-GDC oxygen electrode was also performed.
In the case of the Ni-based fuel cathodes, the cells comprising the Au-Mo modified electrodes exhibited quite better electrochemical performance, compared to the SoA Ni/GDC. In this respect, the nominated Ni-based fuel electrode for the H2O electrolysis was the 3Au-3Mo-Ni/GDC. Regarding the Ni-metal free electrodes, the Fe substituted chromite (LSCrF) electrode was the proposed fuel electrode for H2O & H2O/CO2 co-electrolysis. Finally, in the case of the new developed air electrodes for SOECs two are the proposed candidates for oxygen electrodes and these are PNCO-20 and LPNO. The nominated samples/electrodes were used for: (•) the preparation and testing (for H2O electrolysis) of “combinatorial” button cells, comprising as well the proposed O2 electrodes, (•) the preparation of larger cells for long-term stability tests and (•) the stack/s preparation and testing. Moreover, “Three-layer” and “five-layer” free standing solid oxide cells were manufactured exclusively by Thermal Spray.
Long-term stability investigation of the Ni-metal-free fuel electrodes aimed at the target of 1000 hours of operation. The performance of two cells, one with LSCrF/GDC//3YSZ//GDC/LSCoF/LNF and of a second with LSCrF/GDC//3YSZ/LSM-YSZ/LSM was examined and compared under 23.5% H2O/He electrolysis in the absence of H2 under steady-state operation with constant current density of 0.25 A cm-2 at 900˚C.
The enhancing effect of the 3Au-3Mo-Ni/GDC (modified Ni-based) fuel electrode was further verified with the use of the new O2 electrodes in the form of the so called “combinatorial” cells.
The electrochemical performance and long term stability of 8YSZ electrolyte supported 5x5 cm² single cells, comprising the best performing developed electrodes, was also investigated. During the long term stability tests of 1700, 1150 and 850 hours, respectively, for 3 different large-sized (active area of 20 cm2) single cells, at -0.3 A/cm2 and 900C, the lower degradation rate was observed for the cell comprising the modified 3Mo-3Au-Ni/GDC/8YSZ/GDC/LSCF single cell.
In regards to the stack manufacture and long-term operation, there are comparative data between one stack built using large (active area 42 cm2) TRL4 ESCs with 3Au-3Mo-Ni/GDC electrode and of another stack with similar size Ni/GDC electrode, which were operated at 835C, 0.5 A/cm2. The degradation rate for the stack with modified cells was 300 mV/kh, whereas for the Ni/GDC stack it was 700 mV/kh. Conclusively, this kind of element-doping may be an important method to extend the life-time of SOECs.
Management and dissemination activities were realized through various web tools, newsletters, presentations to the wide public, as well as 8 Open Access publications and 89 participations in conferences.