Reaction kinetics of protons and oxide ions in LSM/lanthanum tungstate cathodes with Pt nanoparticle activation

Composite electrodes of La0.8Sr0.2MnO3 (LSM)/La28–xW4+xO54+3x/2 (x = 0.85, “LWO56”) on LWO56 electrolytes have been characterized by use of electrochemical impedance spectroscopy vs. pO2 and temperature from 900°C, where LWO56 is mainly oxide ion conducting, to 450°C, where it is proton conducting in wet atmospheres. The impedance data are analyzed in a model which takes into account the simultaneous flow of oxide ions and protons across electrolyte and electrodes, allowing extraction of activation energies and pre-exponential factors for the partial electrode reactions of protons and oxide ions. One composite electrode was infiltrated with Pt nanoparticles with average diameter of 5 nm, lowering the overall electrode polarization resistance (Rp) at 650°C from 260 to 40 Ω cm2. The Pt-infiltrated electrode appears to be rate limited by surface reactions with activation energy of ∼90 kJ mol−1 in the low temperature proton transport regime and ∼150 kJ mol−1 in the high temperature oxide ion transport regime. The charge transfer reaction, which makes a minor contribution to Rp, exhibits activation energies of ∼85 kJ mol−1 for both oxide ion and proton charge transfer

Tailoring the properties of a-site substituted Ba1-xGd0.8La0.2+xCo2O6-δ

The double perovskite BaGd0.8La0.2Co2O6-δ (BGLC) shows excellent performance as oxygen electrode for Proton Ceramic Fuel Cells (PCFCs) and electrolyzer cells (PCEC), with polarization resistances in wet oxygen of 0.04 and 10 Ωcm2 at 650 and 350 ⁰C, respectively [1]. Compared with other reported PCFC cathodes [2], BGLC performs better both at high and low temperature. The excellent performance of BGLC in proton ceramic cells is rationalized by a suggested partial proton conductivity at intermediate temperatures, supported by significant hydration up to 400°C observed by thermogravimetric studies. However, the chemical stability of BGLC in high steam pressures under PCEC operation remains a concern due to its highly basic A-site. Thus, tailoring the A-site stoichiometry by partial substitution of Ba with La may be a viable route for further optimizing the balance between chemical stability and electrochemical performance. In the literature we find numerous defect chemical models describing REBaCo2O5.5+δ-type double perovskites, but these are typically limited to describing the oxygen non-stoichiometry. Little can be found which relates defect chemistry to electrochemical performance, electrical conductivity or hydration behavior. Thus, this contribution aims to develop a global defect chemical model of the system Ba1‑xGd0.8La0.2+xCo2O6-δ (x = 0-0.5) by investigating its structural and functional properties as a function of Ba-site substitution. The complex structural behavior of Ba-site substituted BGLC is elucidated by combining synchrotron and neutron diffraction data with high temperature XRD to describe the local Co-O environment and the degree of cation and anion ordering as a function of temperature and pO2. The implications of A-site stoichiometry on proton incorporation are further investigated by thermogravimetric hydration studies supported by neutron powder diffraction of dry and deuterated samples. Finally, these properties are linked to oxygen non-stoichiometry, electrical conductivity and electrochemical performance to develop and validate our general defect chemical model for the system Ba1‑xGd0.8La0.2+xCo2O6-δ (x = 0-0.5). Please click Additional Files below to see the full abstract

Defect chemistry of mixed conducting double Perovskites

Barium Gadolinium Lanthanum Cobaltites with the general formula Ba1-xGd0.8-yLa0.2+x+yCo2O6-δ (BGLC) are reported as Mixed Proton and Electron Conducting materials (MPECs), and have been utilized as positrode (positive electrode) materials for Proton Ceramic Electrochemical Cells (PCECs) [1]. A defect chemical model, treating various charge carrying defects in BGLC was published in 2017 [2] and in this work we expand the model to also comprise formation of protons in BGLC. Protons can be incorporated by two different reactions, in a ratio depending on measurement conditions and the oxidation state of the material. Low temperatures and high pO2 leaves BGLC oxidized, and with increasing electron hole concentration, the hydrogenation reaction is promoted with respect to hydration. Hydrogenation is confirmed by use of isothermal Dry-H2O-D2O switches in thermogravimetric measurements, revealing a larger concentration of protons than expected from hydration only (Figure 1, left). The reduction of BGLC by hydrogenation is slowly counteracted by oxygen uptake combined with an expected cation reordering, bringing the material back to its initial oxidation state after equilibration in wet conditions. By combining oxidation and hydration thermodynamics, hydrogenation entropy and enthalpy can be obtained, making it possible to model proton concentrations from hydration and hydrogenation separately by use of advanced defect chemistry (Figure 1, right). Hydration is proposed to be facilitated by a minor concentration of oxygen vacancies in the O-Co-O layers, where acidic vacancies may accommodate basic hydroxyl groups. These vacancies are neighboured by more basic oxide ions in the O-Ba-O and O-Ln-O layers which in turn may accommodate protons. Please click Additional Files below to see the full abstract

Effect of fuel thermal pretreament on the electrochemical performance of a direct lignite coal fuel cell

Proceedings of the 20th International Conference on Solid State Ionics SSI-20The impact of fuel heat pretreatment on the performance of a direct carbon fuel cell (DCFC) is investigated by utilizing lignite (LG) coal as feedstock in a solid oxide fuel cell of the type: lignite | Co–CeO2/YSZ/Ag | air. Four LG samples are employed as feedstock: (i) pristine lignite (LG), and differently heat treated LG samples under inert (He) atmosphere at (ii) 200 °C overnight (LG200), (iii) 500 °C for 1 h (LG500) and (iv) 800 °C for 1 h (LG800). The impact of several process parameters, related to cell temperature (700–800 °C), carrier gas type (He or CO2), and molten carbonate infusion into the feedstock on the DCFC performance is additionally explored. The proximate and ultimate analysis of the original and pretreated lignite samples show that upon increasing the heat treatment temperature the carbon content is monotonically increased, whereas the volatile matter, moisture, sulfur and oxygen contents are decreased. In addition, although volatiles are eliminated upon increasing the treatment temperature and as a consequence more ordered carbonaceous structure remained, the heat treatment increases the reactivity of lignite with CO2 due mainly to the increased carbon content. These modifications are reflected on the achieved DCFC performance, which is clearly improved upon increasing the treatment temperature. An inferior cell performance is demonstrated by utilizing inert He instead of reactive CO2 atmosphere, as purging gas in the anode compartment, while carbonate infusion always results in ca. 70–100% increase in power output (15.1 mW cm− 2 at 800 °C). The obtained findings are discussed based also on AC impedance spectroscopy measurements, which revealed the impact of LG physicochemical characteristics and DCFC operating parameters on both ohmic and electrode resistances.The authors would like to acknowledge financial support from the European project “Efficient Conversion of Coal to Electricity — Direct Coal Fuel Cells”, which is funded by the Research Fund for Carbon & Steel (RFCR CT-2011-00004).Peer reviewe

Mixed proton and electron conducting double perovskite anodes for stable and efficient tubular proton ceramic electrolysers.

[EN] Hydrogen production from water electrolysis is a key enabling energy storage technology for the large-scale deployment of intermittent renewable energy sources. Proton ceramic electrolysers (PCEs) can produce dry pressurized hydrogen directly from steam, avoiding major parts of cost-driving downstream separation and compression. However, the development of PCEs has suffered from limited electrical efficiency due to electronic leakage and poor electrode kinetics. Here, we present the first fully operational BaZrO3-based tubular PCE, with 10 cm(2) active area and a hydrogen production rate above 15 Nml min(-1). The novel steam anode Ba1-xGd0.8La0.2+xCo2O6-delta exhibits mixed p-type electronic and protonic conduction and low activation energy for water splitting, enabling total polarization resistances below 1 Omega cm(2) at 600 degrees C and Faradaic efficiencies close to 100% at high steam pressures. These tubular PCEs are mechanically robust, tolerate high pressures, allow improved process integration and offer scale-up modularity.The work leading to these results has received funding from the Research Council of Norway (grant 236828) and from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement 621244 ('ELECTRA') and Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement 779486 ('GAMER'). This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe research.Vøllestad, E.; Strandbakke, R.; Tarach, M.; Catalán-Martínez, D.; Fontaine, M.; Beeaff, D.; Clark, DR.... (2019). Mixed proton and electron conducting double perovskite anodes for stable and efficient tubular proton ceramic electrolysers. Nature Materials. 18(7):752-759. https://doi.org/10.1038/s41563-019-0388-2S75275918

Structural properties of mixed conductor Ba1−xGd1−yLax+yCo2O6−δ

BaGdLaCoO (BGLC) compositions with large compositional ranges of Ba, Gd, and La have been characterised with respect to phase compositions, structure, and thermal and chemical expansion. The results show a system with large compositional flexibility, enabling tuning of functional properties and thermal and chemical expansion. We show anisotropic chemical expansion and detailed refinements of emerging phases as La is substituted for Ba and Gd. The dominating phase is the double perovskite structure Pmmm, which is A-site ordered along the c-axes and with O vacancy ordering along the b-axis in the Ln-layer. Phases emerging when substituting La for Ba are orthorhombic Ba-deficient Pbnm and cubic LaCoO-based R3̄c. When La is almost completely substituted for Gd, the material can be stabilised in Pmmm, or cubic Pm3̄m, depending on thermal and atmospheric history. We list thermal expansion coefficients for x = 0-0.3, y = 0.2.The research has been supported by the National Science Centre Poland (2016/22/Z/ST5/00691), the Spanish Ministry of Science and Innovation (PCIN-2017-125, RTI2018-102161 and IJCI-2017-34110), and the Research Council of Norway (Grant no. 272797 “GoPHy MiCO”) through the M-ERA.NET Joint Call 2016. The authors acknowledge the skilful assistance from the staff of the Swiss–Norwegian Beamline (SNBL) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Dr. Cheng Li at POWGEN, SNS, Oak Ridge, US and Dr. Chiu C. Tang at beamline I11 at Diamond, Didcot, UK are gratefully acknowledged for PND and SR-PXD measurements, respectively

Fru Pigalopp som feministisk aktivist? En feministisk diskursanalyse av Vivian Zahl Olsens tegninger

Denne oppgaven handler om Vivian Zahl Olsens tegninger av fru Pigalopp, alle hentet fra boken og tv-serien om Travelt-folket fra 1974. Fru Pigalopp er fremstilt som en bestemt, men morsom figur som gjennom fortellingen observerer og kommenterer fenomener hun opplever gjennom sin egen versjon av en sommerferie. Ferien viker ikke nevneverdig fra en vanlig 70-talls ferietur, men fru Pigalopp reagerer på ting som vanligvis oppfattes som helt normale og hverdagslige. Opplevelsene holder hun sammen med sin egen livsfilosofi som gjenspeiles i tegningenes spenn mellom alvor og sprell. Den «pigaloppske» livsfilosofien blir utdypet gjennom oppgaven da essensen blir billedliggjort gjennom tegningene. Vivian Zahl Olsens tegninger er fargerike og fantasifulle og bærer en naivitet som er interessant å se som motpol til den underliggende kompleksiteten som tegningene kun antyder i overflaten. Det teoretiske grunnlaget har tatt for seg redegjørelser av ulike feministiske diskurser som tegningene er knyttet opp imot og et underliggende spørsmål for oppgaven har vært: Er fru Pigalopp relevant som et nytt og progressivt symbol for kvinnebevegelsens perspektiver? Feminismen som metode poengterer at de ikke finnes mono-kausale sannheter, men at man må se på hele nettverk av diskurser for å skape helthetlige syn der forskjellige forklaringsmodeller konstituerer en slags virkelighet. Analysens hovedoppgave ble å synliggjøre hvorvidt det var mulig å lese tegningene som en type feministisk aktivisme. Tegningene tar for seg en representert sosial verden i møte med fenomener knyttet til 70-tallet. Iscenesettelser av subjektet og fremmedgjøring er begreper som går igjen. Ved å lese Vivian Zahl Olsens tegninger gjennom en feministisk diskursiv analyse forbi kunstnerens eller oppdragsgivers intensjon ble resultatet at tegningene viser både feministisk aktivisme på et grunnleggende plan og at tegningene fremstiller kvinnen forbi en tillært femininitet. I dette lå det også en mulighet for en frigjøring av kvinnefremstillinger og kjønnsrollediskursen. 70-årene ble desenniet som inneholdt store endringer sosialt, kulturelt og politisk, og dette representeres i tegningene av fru Pigalopp. Hennes feministiske aktivisme ble tydeliggjort gjennom å se hvordan linjer, farger og former kom til syne over komplekse og mangfoldige sammenhenger. Analysen viser at fru Pigalopps aktivisme ikke fremstilles med opposisjon som virkemiddel. Der 70-tallsfeminismen var preget av kamp, er fru Pigalopps feministiske aktivisme preget av et autonomt utgangspunkt der frihet fra patriarkatets objektiviserende forenklingssystemer kom til syne. Gjennom dette tydeliggjøres et nytt perspektiv innenfor 70-tallsfeminismen

Karakterisering av høytemperaturegenskaper i Ca12Al14O33

Sammendrag Denne oppgaven beskriver arbeidet med å karakterisere høytemperaturegenskapene til Ca12Al14O33, også kalt Mayenitt, 12CaO•7Al2O3 eller bare C12A7. Det er rapportert at Mayenitt inneholder hydridioner etter reduksjon i høy temperatur, og at disse kan fungere som elektrondonorer ved fotoeksitasjon under UV-bestråling. Ca12Al14O33 har en mikroporøsporøs kubisk struktur, hvor Ca12Al14O322+ danner et gitter med seks sub-nanometer bur inneholdende ett O2-ion. Dette oksygenionet er løst bundet sammenlignet med oksygenionene i gitteret, og dette gjør det enkelt å redusere stoffet, slik at oksygen kan erstattes av lokaliserte elektroner på oksygenplass. Ca12Al14O33 har høy oksygenioneledningsevne i oksiderende, tørr atmosfære, og er rapportert som en blanding av elektronisk leder og protonleder i reduserende atmosfære. Imidlertid dekomponerer stoffet i reduserende, tørr atmosfære ved T > 1100 °C. Mayenitt ble syntetisert ved bruk av sitratmetoden, og det ble preparert porøse og tette prøver til bruk ved henholdsvis termogravimetriske målinger og ledningsevnemålinger. Prøvene ble karakterisert både før og etter målingene ved røntgendiffraksjon og scanning elektronmikroskopi. Det ble gjort termogravimetriske målinger av vannopptak som funksjon av temperatur, pO2 og pH2O, og termodynamiske data for hydratiseringsreaksjonen ble utledet. Termogravimetriske målinger ble også benyttet for å teste stabilitet over lengre tid i reduserende, tørr atmosfære. Det ble videre gjort ledningsevnemålinger som funksjon av pO2, temperatur og pH2O. Ledningsevnen ble i tillegg målt over lengre tid i reduserende, tørr atmosfære for å teste effekten av en eventuell dekomponering. Det ble også gjennomført impedansspektroskopi i oksiderende og reduserende atmosfære. Resultatene viste at Ca12Al14O33 i hovedsak har elektronisk ledningsevne i reduserende atmosfære og ionisk ledningsevne i oksiderende atmosfære ved T > 900 °C. Termodynamiske data for hydratiseringsreaksjonen ble beregnet til -223 kJ•K-1 for ∆H og -120 J•K-1•mol-1 for ∆S. Aktiveringsenergi for elektronisk ledningsevne ble beregnet til 2,6 eV på bakgrunn av målte data. Det ble bekreftet ved røntgendiffraksjon og scanning elektronmikroskopi at prøven hadde gjennomgått en faseovergang i løpet av målingene

Oxygen electrodes for ceramic fuel cells with proton and oxide ion conducting electrolytes

The overall aim of this work is to contribute to a better understanding of the reactions taking place at the oxygen electrode in proton ceramic fuel cells (PCFCs) and, moreover, to develop new materials with improved performance for this electrode. PCFCs and their cathode reactions are the main focus of the study, but these reactions are often running in parallel with reactions associated with other charge carriers that also need to be addressed. Most proton conducting ceramics exhibit also transport of oxide ions, and although small at intermediate temperatures, the relative contribution from partial oxide ion conductivity increases with temperature and eventually dominates at higher temperatures. Hence, characterization of the performance of a PCFC cathode may at higher temperatures in reality be affected by or even directly reflect the cathode reactions of an oxide ion conducting solid oxide fuel cell (SOFC) rather than that of a PCFC. The crossover between SOFCs and PCFCs with respect to the oxygen electrode reactions is emphasized in this work. The first manuscript presents status and challenges of PCFC research undertaken in Norway by the start of 2010. The work comprises manufacturing of single cells and cell stacking, focusing on the performance, the mechanical and thermal properties, as well as, the chemical stability of the different PCFC component materials. State-of-the-art cathode material at that time, La0.8Sr0.2MnO3 (LSM), showed a polarization resistance (Rp) of 30 Ωcm2 800°C on proton conducting Ca doped LaNbO4 electrolyte, revealing the necessity for a significant improvement in the cathode performance. New materials had to be found and their microstructural design optimized, based on the requirements specific for the proton conductor oxygen reaction. Reaction kinetics, with particular emphasis on the features specific for the PCFC oxygen electrode is investigated in manuscripts II, III and V. In manuscript IV, the experimental conditions are such that the SOFC reactions dominate the electrode process. The electrolyte/electrode interfacial exchange of protons instead of oxide ions distinguishes PCFCs from oxide ion conducting SOFCs and entails that water is formed on the oxygen side of the electrolyte. A major challenge with the PCFC cathode candidate materials studied so far is the confinement of the electrochemical process to pass the electrolyte / electrode / gas triple phase boundary (tpb), instead of utilizing the whole electrode area as for the best mixed conducting SOFC electrodes. The challenges related to tpbs as a bottleneck are addressed by microstructural improvements. Moreover, a novel material with simultaneous transport of electrons and protons is introduced that will enable also the PCFC cathode reactions to occur over the electrode surface, thereby extending the tpb reaction zone. The effect of water formation on the cathode reaction is studied in detail on a Pt model electrode. The results show higher reaction rates upon increased water vapor partial pressure, pH2O. Since the Pt electrode is rate limited by surface diffusion both under dry and wet conditions, the pH2O effect is explained by the formation of surface hydroxyls with high surface mobility relative to the adsorbed oxide ions which dominate under drier conditions. The presence of surface hydroxyls is confirmed by X-ray photoelectron spectroscopy. Water is looped in the oxygen reaction series, acting both as reactant and product. In manuscript V and in the results part of the thesis it is shown that ambient water vapor gives the same positive effect for the mixed conducting electrodes BaGd0.8La0.2Co2O6-δ (BGLC) and BaPrCo2O6- (BPC) when operated on a BaZr0.7Ce0.2Y0.1O3 (BZCY72) proton conducting electrolyte. At higher temperatures where BZCY72 is mainly oxide ion conducting, water vapor on the other has an adverse effect on the electrode reaction rate for the same mixed conducting electrodes. With the mixed oxide ion-p-type electron conductor La2NiO4+δ (LNiO) as electrode and La27.16W4.85O55.27, (La/W ≈ 5.6; LWO56) as electrolyte (manuscript IV), the electrode performance was independent of pH2O under conditions where oxide ion conductivity dominates in the electrolyte (above 700°C). Three well-established routes to improve the electrode microstructure were followed in this work; (i) addition of nano-sized catalysts by infiltration, (ii) improvement of the functional layer close to the electrolyte and (iii) manufacturing of composite electrodes by mixing electrode and electrolyte materials. The two first methods showed promising results: Addition of Pt nanoparticles in the LSM electrode lowered significantly the polarization resistance; from 260 to 40 Ωcm2 at 650°C. Characterization of the microstructure of BGLC and BPC electrodes showed that a fine-grained functional layer was successfully manufactured. The composite electrode approach did, however, not prove to enhance the performance of an electrode rate limited by surface reactions. The materials investigated in this work range from well-known pure electron conductors such as Pt and LSM, used first and formerly for the detailed characterization of the electrode reactions, via the promising mixed conducting candidate LNiO, to the novel mixed conducting double perovskites BGLC, BPC and their B-site iron-substituted variants BaGdCo1.8Fe0.2O6-δ (BGCF) and BaPrCo1.4Fe0.6O6-δ (BPCF). For Pt and LSM, high capacitance processes like surface diffusion is limiting the overall electrode reaction rate. For the mixed conducting electrodes LNiO and BGLC, the oxide ion transfer is shown to happen through the electrode interior. The latter also shows indications of partial bulk proton conductivity concluded based on the pH2O dependencies encountered for Rp and supported by hydration of the material at low temperatures with a hydration enthalpy of -50 kJ/mol. Bulk proton transport would facilitate the low temperature PCFC cathode reaction and widen the triple phase reaction zone improving the electrode performance. The behavior of these mixed conducting double perovskites, especially BGLC but possibly also BPC, with polarization resistances measured to 0.05 and 0.09 Ωcm2 at 650°C for BGLC and BPC, consequently gives indications of the first established mixed proton / electron conducting materials with sufficient electrochemical performance on a proton conducting electrolyte. To account correctly for mixed conductivity in the electrolyte is challenging when studying electrode reactions. In manuscript III and V, a model for the separation of the measured polarization resistance into the contributions from more than one charge carrier is developed. The model accounts also for the effect of parallel non-faradaic current during high temperature measurements under oxidizing conditions. The results of the modelling show that the measured polarization resistance for the system investigated here and reported above for 650°C is underestimated by approximately one order of magnitude. The same underestimation would apply to any other oxygen electrode measured on BZCY72 if the effect of electrolyte p-type partial conductivity was not properly addressed. In a running fuel cell or electrolyzer cell, the fuel-side reducing conditions are expected to induce a blocking layer for electronic conductivity in the electrolyte. The "true" polarization resistance will therefore be higher when the partial short circuit is absent. At lower temperatures, this effect of parallel non-faradaic current is less pronounced during half-cell electrode characterization. BGLC exhibits a total polarisation resistance for proton transport of only 10 Ωcm2 at 350°C, with an activation energy of 50 kJmol-1 ascribed mainly to the surface electrode reaction. Based on this, there is reason to believe that further improvements of the cathode performance can be achieved by enhanced microstructural processing, such as infiltration of BGLC in a BZCY backbone