Governing Principles in Hydration of Mixed Conducting Oxides

GoPHy MiCO addresses one of the main challenges in the development of new efficient energy systems based on Proton Ceramic Fuel Cells and Electrolysers (PCFCs / PCECs), namely the identification of new Mixed Proton and Electron conducting Ceramics (MPECs). These are essential for the oxygen/steam electrodes, but only few are identified, and the knowledge on these materials systems is limited. By studying a large matrix of double perovskites with different transition metal B-sites and lanthanide cations on distinguishable A-sites, and by systematic substitutions of these, trends in structure, oxidation, hydration, electron structure and conductivity has been established. The project outcome will bring significant contributions to the development of proton ceramic fuel cells and electrolysers and to the ongoing implementation of hydrogen-based energy systems. Methodology employed comprises Powder Neutron Diffraction (PND), conductivity measurements, electrochemical measurements, Secondary Ion Mass Spectrometry (SIMS), Scanning Transmission Electron Microscopy (STEM), thermogravimetry (TG), ab initio atomistic modelling (DFT) and various X-ray based techniques (XPS, XAS, XANES, SR-XRD). Five rounds of composition series' with increasing cation complexity has been investigated. Partners are UiO and IFE (NO), GUT (PL) and ITQ/CSIC (ES). So far, the project has synthesized 152 samples of 80 unique compositions within the five composition ranges of increased cation complexity. Based on screening for hydration properties, the composition matrix has been narrowed down to one series of double perovskites with good hydration properties, and the main hypothesis has been improved and strengthened. Key parameters correlating with hydration are identified, especially the effect of closed-shell elements, which are pivotal for proton stability. As a consequence of the identified parameters, and by conducting an iterative work process, several new MPECs have been identified. The work has been divided into six work packages (WPs). In WP2-5, the generated results have been continuously analysed and pushed back into WP1, where selections of the forthcoming composition ranges has been decided based on this continuous feedback. A technical committee of the main investigators in each institution has had weekly video meetings in the first two years, and every other week the last year. In these meetings, the results of WPs 2-5 have been discussed and forthcoming composition ranges selected. Due to COVID-19, project meetings for the General Assembly (GA) in 2020 have been arranged by video. So far, seven peer-reviewed articles are published in WP6, and six are under preparation. New MPECs are found through substitutions of closed-shell elements on A(I), A(II) and B-sites in the perovskite structure. Closed-shell elements investigated and successfully contributing to proton stability has been Sr and Ba for A(I), La, Gd, Lu og Y for A(II), and Ti and Zn on B-site (Gd is regarded as closed-shell based on its half-filled 4f-shell in inoized state). In this way, the compositions BaGd0.8Lu0.2Co2O6-d (BGLuC82), Ba0.5Sr0.5Gd0.8La0.2Co2O6-d (BSGLC5582), BaGd0.8Y0.2Co2O6-d (BGYC82) , BaGd0.8La0.2Co1.8Ti0.2O6-d (BGLC82:Ti) and BaGd0.8La0.2Co1.8Zn0.2O6-d (BGLC82:Zn) have all been identified as new MPECs. The project has also conducted a fundamental study of the hydration of BaGd1-xLaxCo2O6-d (BGLC, x = 0, 0.2, 0.3, 0.5, 0.7, 0.8, 1), revealing incorporation mechanisms and proton concentrations by use of thermo-gravimetric measurements with isotope exchange between H2O og D2O. Combined use of STEM and TG has shown that water uptake in oxidising conditions is combined with oxidation and structural transition from A-site order to A-site disorder. Under inert conditions, proton incorporation is mainly by hydrogenation (reduction). The project has paved way for new and advanced measurement techniques, especially for conductivity measurements, where measurements of partial proton conductivity has been performed as outlined in the project description by use of Hebb-Wagner measurements. This is the first directly measured partial proton conductivity in materials with high electronic conductivity, such as these cobalt-based MPECs. The results show that the partial proton conductivity is two orders of magnitude lower than in the standard proton conducting ceramics typically applied as electrolytes. These results give important information on to which extent these MPECs increase the electroactive area in PCFC/PCEC-electrodes. Electrochemical studies are performed by use of Electrochemical Impedance Spectroscopy on model electrodes of selected MPECS. The results are under analysis, and a separate publication on the effect of mixed proton and electron conductivity on electrochemical performance is under preparation.

GoPHy MiCO har løftet studiet av Mixed Protonic and Electron Conducting oxides (MPECs) til et nytt nivå. Hovedhypotesen er revidert gjennom en dypere forståelse av vekselvirkningen mellom protonstabilitet og ladningsforhold i kovalente og ioniske bindinger. Resultatet er identifikasjon av strukturelle faktorer for stabilisering av protondefekter i lagdelte MPECs, relatert til ladnings(elektron)overføringen mellom anion og kation, hvordan denne styres av relative elektroniske energinivåforskjeller, og effekten av bindingsvinkler og -lengder. Kationsubstituentenes påvirkning er avdekket. Det har fremkommet at kationer med tomt, halvfullt og fulle elektronskall kan stabilisere protoner, noe som har avdekket flere nye MPECs. Ett M-Era.Net prosjekt, FunKey Cat, er innvilget og igangsatt for videreutvikling av resultatene i GoPHy MiCO. To EU FCH JU prosjekter, GAMER og WINNER er hhv igangsatt og innvilget, hvor resultater fra GoPHy MiCO er sentrale i både oppskalerte og fundamentale studier.

GoPHy MiCO addresses one of the main challenges in the development of new efficient energy systems based on Proton Ceramic Fuel Cells and Electrolysers, namely the identification of ceramics with mixed protonic and electronic conductivity. These are essential for the oxygen/steam electrodes, but only few and mediocre ones are identified. By systematically studying a set of double perovskites with different cations on distinguishable A-sites, and by systematic substitutions of these and also the B-site cation, trends in structures, oxidation, and hydration behaviour and conductivity will be established. The project outcome will potentially bring significant contributions to the ongoing implementation of hydrogen energy systems. Methodology to be employed comprises Neutron Powder Diffraction, electrochemical methods impedance spectroscopy, thermogravimetry, and ab initio atomistic modelling. Partners are UiO and IFE (NO), GUT (PL) and ITQ/CSIC (ES).