37 research outputs found
Oxygen electrodes for ceramic fuel cells with proton and oxide ion conducting electrolytes
No full textThe 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
Karakterisering av høytemperaturegenskaper i Ca12Al14O33
No full textSammendrag
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
First results from model electrode studies of reversible electrodes delivered to WP3
No full textWe present the results of the first model electrode study, where the geometryindependent relations between polarisation resistance (Rp) of Ba0.5La0.5CoO6-δ (BLC) and T, pO2 and pH2O have been extracted. The results are used as inputs in a model for quantifying the partial contributions to electrode polarization from oxide ions and protons, and the mediation of polarization by partial transport of electrode holes in a non-faradaic current. The model is fitted to – and exemplified through – a detailed analysis of the Rp of a brush-painted 3x3 mm BLC electrode on a BaZr0.4Ce0.4Y0.2O2.9 (BZCY442) electrolyte. The model may be used as input in higher-level system models, where the efficiency losses under static operation over the cell in electrolysis or fuelcell mode can be linked to materials-specific parameters for electrode or electrolyte materials. The model may also incorporate lower-level input from atomistic modelling to give predictive power to materials development and tailoring of functional properties relevant for anodic, cathodic or reversible operation. This delivery is a first step towards a full electrochemical model, where the direct relation between dynamic overpotential and current will be given in an analytical soloution. The study is undertaken on a brush-painted 3x3 mm BLC electrode as a pre-study for the upcoming geometry-resolved model electrode study which will reveal the extension of the reaction zone in x, y and z directions under increasing positive and negative DC bias. The large test-matrix give information of rate-determining reaction steps and the activation energies for protons and oxide ions across these steps for an electrode material with low proton- and high electronic conductivity. Verification of the experimental set-up will form the basis for further electrode characterisation under DC conditions. Impedance characterisation under DC bias is challenging, and the effect of several cell configurations is therefore investigated. We herein report the results of a cell configuration using Pt as counter electrode (CE) with ø 1 cm. We continue with studying the effect of CE area and material, as the counter electrode may be expected to influence the relative transport of protons, oxide ions and electron holes under DC conditions. We will not report on the results of other cell configurations in this document. The results of the study with Pt CE is that the polarisation resistance at the working electrode in reversible operation (zero bias) is dominated by mass transfer for the anodic oxidation of water at high temperatures, and by mass transfer for the cathodic oxygen reduction at low temperatures
Impedance spectroscopy study of Au electrodes on Gd-doped CeO2 (GDC) – Molten Li2CO3+Na2CO3 (LNC) composite electrolytes
No full textWe herein report an impedance spectroscopy study of Au electrodes on Gd-doped CeO2 (GDC) – molten Li2CO3+Na2CO3 (LNC) composite electrolytes in O2 and O2+CO2 atmospheres. Complementary measurements of Au on GDC alone are provided for supporting insight. We find that the adsorption of CO2 on GDC in O2+CO2 atmospheres effectively blocks oxygen adsorption and severely slows oxygen reduction kinetics. The conductivity of the composite is dominated by the GDC phase in the solid-solid temperature region, while the LNC phase dominates above its melting point, and no further enhancement e.g. by interfacial effects are found. The incorporation of LNC melt into GDC results in a significant reduction in the polarisation resistance of Au electrodes in O2 atmospheres, as the melt mediates the reaction by a peroxide mechanism. In O2+CO2 atmospheres, however, the polarisation resistance of Au electrodes on GDC-LNC membranes is significantly higher, higher even than that on GDC. This we assign again to the blocking adsorption of CO2 (or carbonate) on the surfaces of ceria and the sluggish transport and reactions now mediated by carbonate-carried oxide species (CO42−) instead of peroxide species
Proposal for Grand Instrument access WINNER D1.3
No full textThis deliverable reports on the proposal developed by UiO and SINTEF to access Grand Instrument, as defined in WINNER plan. The targeted study is summarized here. The near-ambient pressure scanning photoelectron microscope (NAP-SPEM) setup at Elettra will be used to perform operando studies of model electrodes based on BaGd0.3La0.7Co2O6-δ (BGLC), which is one of the best air/steam electrode materials available for Proton Ceramic Electrochemical Cells. The reduction and oxidation of H2O and O2 happen at the gas-electrode-electrolyte triple-phase boundary (tpb) and at the BGLC surface. The extension of the reaction zone from the tpb and outward on the electrode surface is not yet known. The red-ox reactions are facilitated by Co, which provide electrons and electron holes to the cathodic and anodic half-reactions, respectively. Thus, Co is expected to deviate from its +3 oxidation state at the surface during operation. Interdigitated model electrodes of BGLC deposited on a BaZr0.4Ce0.4Y0.2O3 (BZCY) electrolyte will be studied. The oxidation state of Co across 70-100 μm wide electrode strips will be monitored by NAP-SPEM while performing Electrochemical Impedance Spectroscopy (EIS) under anodic and cathodic bias in a mixture of O2 and H2O gas at elevated temperatures (400 – 600°C). The reaction zone will be mapped based on Co oxidation state, and while the electrostatic potential will give a uniform distribution of Co-species over the electrode, we predict a gradual change over the reaction zone. Our hypothesis is that the extension of the electrochemically active area of the electrode is changed as a function of applied current density, which calls for analysis tools with the spatial resolution of the SPEM technique. The potential outcome of the experiments will be a ground-breaking progression in the understanding of high temperature electrodes, electrode reactions and -materials
Additive manufacturing of Proton-Conducting Ceramics by robocasting with integrated laser postprocessing
No full text[EN] A hybrid system combining robocasting and NIR laser postprocessing has been designed to fabricate layers of mixed proton-electron conducting Ba0.5La0.5Co1-xFexO3-delta ceramic. The proposed manufacturing technique allows for the control of the geometry and microstructure and shortens the fabrication time to a range of a few minutes. Using 5 W laser power and a scanning speed of 500 mms(-1), sintering of a round-shaped layer with an 8 mm radius was performed in less than 2 s. The single phase of the final product was confirmed by X-ray diffraction. Various ceramic-to-polymer weight ratios were tested, showing that various porosities of microstructures of similar to 30 - 35 % and similar to 19 % can be obtained with 2:1 and 4:1 loading respectively.The project FunKeyCat is supported by the National Science Centre, Poland under the M-ERA.NET 2, which has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No 685451, from the Research Council of Norway (Grant n degrees 299736) , and the Spanish Government (M-ERA.NET PCI2019-103742) . The authors also express gratitude to Piotr Okoczuk, MSc Eng, for performing the confocal microscopy measurements and to Robert Tylingo, Ph.D., D.Sc. Eng., for conducting the viscosity measurements and for their contribution to this work.Pospiech, J.; Nadolska, M.; Cieslik, M.; Sobczyk, T.; Chmielewski, M.; Mielewczyk-Gryn, A.; Strandbakke, R.... (2024). Additive manufacturing of Proton-Conducting Ceramics by robocasting with integrated laser postprocessing. Applied Materials Today. 40. https://doi.org/10.1016/j.apmt.2024.102398S4
Tailoring oxide nanoparticle exsolution in La0.5Ba0.5-yCo1-xFexO3-[delta]
No full text[EN] We show that oxide nanoparticles (NPs) exsolve on La0.5Ba0.5-yCo1-xFexO3-¿ (x = 0-1, y = 0 or 0.01) in oxidizing conditions. The phenomenon occurs only in Co-containing materials and depends on pO2 and pH2O pressures. Under dry conditions, the smallest NPs average about 30 nm, with 200-300 NPs/mu m2 at pO2= 5 x 10-5 atm. For pO2= 1 atm, NP size increases to 100-200 nm, and population drops to a few to about 20 NPs/mu m2 depending on A-site nonstoichiometry and x. In humid conditions, the smallest NPs around 50 nm, with a peak of 100 NPs/mu m2 exsolve for pO2= 1. Transmission electron microscopy shows that exsolved NPs in La0.5Ba0.5-yCoO3-¿ are Ba-Orich. We propose defect chemistry models, indicating that exsolution is driven by oxidation reactions forming A-site vacancies, increasing exsolved material with higher pO2. We suggest that adsorbed water under humid conditions blocks nucleation sites, altering observed trends.Project FunKeyCat is supported by the National Science Centre, Poland, The Research Council of Norway (FunKeyCat 299736) , and the Spanish Government (PCI2019-103742, RYC2021-033889-I) under the M-ERA.NET 2, which has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no 685451. We thank the support of the Electronic Microscopy Service of the Universitat Politècnica de València.Balcerzak, D.;López-García, A.;Carrillo-Del Teso, AJ.;Balaguer Ramirez, M.;Serra Alfaro, JM.;Norby, T.;Strandbakke, R.... (2025). Tailoring oxide nanoparticle exsolution in La0.5Ba0.5-yCo1-xFexO3-[delta]. Journal of the European Ceramic Society. 45(10). https://doi.org/10.1016/j.jeurceramsoc.2025.117347S451
Energetics of formation and stability in high pressure steam of barium lanthanide cobaltite double perovskites
No full text[EN] This study concerns energetics of formation and the stability in high water partial pressure of BaLnCo(2)O(6-delta), (Ln = La, Pr, Nd, and Gd) (BLnC) and BaGd1-xLaxCo2O6-delta, where x = 0.2, 0.5, and 0.7 (BGLC) double perovskite cobaltites. Those materials are extensively studied due to their potential applications as a positrode in electrochemical devices. Therefore, their stability under such conditions is a key issue. All investigated materials are thermodynamically stable relative to binary oxides and exhibit strongly exothermic enthalpies of formation. Moreover, BaGd0.3La0.7Co2O6-delta and BaGd0.8La0.2Co2O6-delta remain the main perovskite structure up to 3 bars of water vapor at 400 degrees C. At higher steam pressure, reaching 10 bar at 300 degrees C, the partial decomposition to constituent oxides and hydroxides was observed. The BGLC compounds exhibit higher negative formation enthalpies in comparison to single-Ln compositions, which does not translate into higher chemical stability under high steam pressures since the BLnC series retained the main perovskite structure at higher temperatures as well as in higher water vapor pressures.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. We also acknowledge Solaris National Radiation Centre Poland for access to the PIRX (XAS/PEEM) beamline time (proposal no 201036). Dr Chiu C. Tang at beamline I11 at Diamond Light Source, Didcot, UK is gratefully acknowledged SR-PXD measurements. The calorimetry at Arizona State University received financial support from the U.S. Department of Energy, Office of Basic Energy Sciences, grant DE-SC0021987.Mielewczyk-Gryn, A.; Yang, S.; Balaguer Ramirez, M.; Strandbakke, R.; Sorby, MH.; Szpunar, I.; Witkowska, A.... (2023). Energetics of formation and stability in high pressure steam of barium lanthanide cobaltite double perovskites. Dalton Transactions. 52(17):5771-5779. https://doi.org/10.1039/d2dt03989cS57715779521
Effects of sintering additives on defect chemistry and hydration of BaZr0.4Ce0.4(Y,Yb)0.2O3−δ proton conducting electrolytes
No full text[EN] The effects of NiO, ZnO, and CuO sintering additives (0.5, 1.0, or 2.0 wt%) on the sintering behaviour, effective acceptor concentration, and hydration thermodynamics are examined for BaZr0.4Ce0.4Y0.1Yb0.1O3_delta and BaZr0.4Ce0.4Y0.2O3_delta proton conducting electrolytes. Thermogravimetry of hydration shows that the sintering additives - except for 0.5 wt% CuO - lead to a decrease in the effective acceptor concentration, and the decrease per mole sintering additive is the largest for NiO. The absence of typical secondary phases such as BaY2NiO5 and the homogeneous distribution of Ni determined from elemental mapping imply that sintering additives dissolve into the perovskite lattice. Exsolution of metallic Ni and Cu upon reduction leads to a substantial recovery of the effective acceptor concentration. Subsequent oxidation is accompanied by a repeated decrease in the effective acceptor concentration as the sintering additives appear to re-dissolve. Defect chemical reactions are proposed to explain the observed results and these are supported by energetics from first principles calculations. Overall, NiO has the highest positive impact on densification and grain growth, and a relatively small amount of 0.5 wt% NiO or CuO is preferable to optimise both sintering and hydration.This study has received European Union's Horizon 2020-Societal Challenges Research and Innovation funding under grant agreement No 838077 (eCOCO2 project). The Research Council of Norway is acknowledged for computational resources provided through the Uninett Sigma2 project nn4604k and for support to the Norwegian Center for Transmission Electron Microscopy (NORTEM) national infrastructure project 197405.r Center for Transmission Electron Microscopy (NORTEM) national infrastructure project 197405.Dayaghi, AM.; Polfus, JM.; Strandbakke, R.; Pokle, A.; Almar-Liante, L.; Escolástico Rozalén, S.; Vollestad, E.... (2023). Effects of sintering additives on defect chemistry and hydration of BaZr0.4Ce0.4(Y,Yb)0.2O3-δ proton conducting electrolytes. Solid State Ionics. 401. https://doi.org/10.1016/j.ssi.2023.116355S40
Electrical transport in a molten-solid V2O5–ZrV2O7 composite
No full textMolten-solid composite oxides are candidates as oxygen transport membranes (OTMs) at intermediate temperatures (500–700 °C). Effects of the constituent phases and interphases on surface reactions and transport processes in these composites are elusive. Here we contribute fundamental insight to such materials systems, applying electrochemical impedance spectroscopy (EIS) and electromotive force (emf) measurements to investigate the electrical conductivity characteristics of a 30 mol% V2O5–ZrV2O7 composite with a eutectic melting point at ∼670 °C. When V2O5 melts and increases the V2O5 volume percolation, the electrical conductivity increases by a factor of 10 and the activation energy increases from 0.21 to ∼0.7 eV. The oxygen red-ox reaction at the surface changes from being rate limited by charge transfer processes to mass transfer processes as a consequence of fast oxygen exchange in molten V2O5 as compared to the all-solid composite. These effects coincide with the ionic transport number rising from essentially zero to ∼0.4, reflecting a significant increase in the relative oxide ion conductivity. Oxygen permeation across a 30 mol% V2O5–ZrV2O7 membrane was estimated to be in the same order as for several dual-phase membranes, but one magnitude lower than for single-phase mixed conducting membranes at intermediate temperatures
