Humidified Atmospheres Research Articles

Cost-effective methods of fabricating thin rare-earth element layers on SOC interconnects based on low-chromium ferritic stainless steel and exposed to air, humidified air or humidified hydrogen atmospheres

Most oxidation studies involving interconnects are conducted in air under isothermal conditions, but during real-life solid oxide cell (SOC) operation, cells are also exposed a mixture of hydrogen and water vapor. For this study, an Fe–16Cr low-chromium ferritic stainless steel was coated with different reactive element oxides – Gd2O3, CeO2, Ce0.9Y0.1O2 – using an array of methods: dip coating, electrodeposition and spray pyrolysis. The samples underwent oxidation experiments carried out over 100 h in three different atmospheres at 800 °C: air, an air/H2O mixture, and an Ar/H2/H2O mixture. The influence of different atmospheres on the corrosion of the Fe–16Cr steel was determined via oxidation kinetics studies; the corrosion product was evaluated using X-ray diffraction, scanning electron microscopy and area-specific resistance (ASR) measurements. All coated samples exhibited lower parabolic oxidation rate constants than bare steel and most also had lower ASR. The applied modifications were found to be sufficiently effective to allow the investigated low-chromium steel to be considered for application as an interconnect material for SOCs.

Demystifying ionic and electronic transport in prominent oxide ion and proton conducting ceramic electrolytes: Ce0.8Sm0.2O2 and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ

ABSTRACT The development and performance of ceramic electrochemical cells (CECs) depend critically on choosing ceramic electrolytes with optimal ionic and electronic transport properties. In this study, we compare the structural, morphological, and transport characteristics of two representative ceramic electrolytes, oxide ion conducting Ce0.8Sm0.2O2 (SDC) and proton conducting BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb). We demonstrate that SDC exhibits higher bulk conductivity but lower open-circuit voltage due to its higher electron density and larger crystallite size. In contrast, BCZYYb possesses higher proton conduction, lower activation energy (0.35–0.38 eV), and higher open-circuit voltage due to its lower electron density and smaller crystallite size with more grain boundaries. BCZYYb shows enhanced proton conductivity in humidified atmospheres due to the hydration of oxygen vacancies, making it more suitable for low-temperature CEC applications. These insights into ceramic electrolyte conduction mechanisms and microstructure-property relationships can guide the optimization of CECs for energy applications.

Defect chemistry and transportation within La1.7Ba1.3In2O6.85 mixed ionic-electronic conducting membrane: Towards the application of solid oxide fuel cells

La1.7Ba1.3In2O6.85, as a representative material belonging to La2BaIn2O7-based family, is a novel and promising electrolyte material for solid oxide fuel cells (SOFCs). However, due to the co-existence of various mobile charged defects including oxygen vacancy, proton, and electron-hole, the conduction properties and behaviors of La1.7Ba1.3In2O6.85 are complex and not fully understood. In this work, we systematically explore La1.7Ba1.3In2O6.85 with a focus on defect chemistry and transportation. A theoretical model is built to analyze the experimentally measured conductivities of La1.7Ba1.3In2O6.85 at varying temperatures and oxygen partial pressures. Subsequently, the conduction properties and behaviors of La1.7Ba1.3In2O6.85 membrane at open circuit conditions and operation conditions are investigated thoroughly for the first time. Our findings reveal that in reducing and humidified atmospheres, La1.7Ba1.3In2O6.85 behaves as a dual-ion conducting oxide, with oxygen-ionic transport numbers exceeding 0.7. Our work can provide a new and overall understanding of the La2BaIn2O7-based material for SOFCs’ application.

Enhanced acceptor concentration, proton conductivity, and hydration in multicomponent rare‐earth ortho‐niobates

AbstractThe structural properties of (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1−xCaxNbO4−δ (x = 0–0.05) series have been studied by X‐ray powder diffraction at room temperature. The thermal properties were investigated using thermogravimetry, from which the concentration of proton defects was determined. Additionally, dilatometry studies were carried out, from which the thermal expansion coefficients and the phase transition temperature between low‐ and high‐temperature polymorphs were determined. In order to determine the electrical properties, the electrochemical impedance spectroscopy measurements under atmospheres of dry synthetic air and saturated with water vapor and heavy water at 300–800°C temperature range were performed. All the studied materials were single‐phase compositions and were stable in monoclinic I2/c crystal structure at room temperature. The water uptake studies revealed a small mass change under the switch from dry to humidified synthetic air atmospheres, suggesting the formation of proton defects. The phase transition from monoclinic to tetragonal crystal structure in the (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1−xCaxNbO4−δ (x = 0–0.05) series occurs above 650°C. A small amount of calcium dopant increases the total conductivity in all tested atmospheres by two orders of magnitude, for example, from 0.34 μS cm−1 in La0.2Nd0.2Sm0.2Gd0.2Eu0.2NbO4 to 63.7 μS cm−1 in (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)0.95Ca0.05NbO4−δ at 700°C in humid synthetic air atmosphere.

Benchmarking the yttrium content in the low temperature/low humidity electrical properties of yttrium-doped barium cerate

Yttrium-doped barium cerate (BCY) is one of the best proton-conducting ceramics to be used in fuel cells/electrolyzers at intermediate temperatures, due to its high hydration enthalpy and high conductivity. Nonetheless, it has commonly been discarded for these applications due to its instability in the presence of carbonaceous and humidified atmospheres. A recent work has addressed this issue, by suggesting the use of this material in very low water vapor partial pressures (pH2O ∼ 10−4 atm) [Electrochimica Acta 334 (2020) 135625] and at low temperatures (T ≤ 400 °C). Under these conditions, BCY can still offer pure protonic conductivity while also offering suitable stability. In the current work, we provide a comprehensive analysis of the effects of the acceptor dopant concentration (Y2O3) in the structural, microstructural, and electrical properties in a wide compositional series of BaCe1-xYxO3-δ (x = 0.05, 0.10, 0.20 and 0.30) at low levels of humidity in the low temperature range (100–400 °C). The results show that BaCe0.9Y0.1O3-δ (BCY10) offers the highest bulk conductivity in these conditions, while further increase in dopant concentration leads to impairment due to “proton trapping” effects. In contrast, the specific grain boundary conductivity increases with increasing dopant concentration due to a reduction of Schottky barrier height at the space-charge layer. We demonstrate that, by operating at very low temperatures (e.g., below 400 °C), the peak performing, BCY10 material can still operate at very low levels of water vapor partial pressure (pH2O ∼ 10−4 atm), where its stability can be maintained. This work paves the way for the optimization of this material, with a focus on utilizing low humidity levels and low temperatures, where it can offer a competitive advantage over that of the majority of other proton-conducting perovskites for potential applications operating under these conditions.

Improvement of Mechanical Strength and Long-Term Durability on Anode Support for Solid Oxide Fuel Cells by Adding Nanomaterials

This study improves the mechanical strength and long-term durability of Ni-8YSZ, a commonly used anode support(AS) for Solid Oxide Fuel Cells (SOFC). Instead of Ni-8YSZ, Ni-3YSZ is due to its high mechanical strength and low cost. 3YSZ alone had the disadvantage of degradation of mechanical strength in high-temperature, humidified hydrogen atmospheres due to phase transformation. The addition of nano-ZrO2 and nano-WO3 was expected to improve mechanical strength and long-term durability due to the Hall-Petch equation. For uniform dispersion, NiO and 3YSZ powders were ball-milled for 100 hours, and nano-ZrO2 or nano-WO3 was also added. The powders were sintered at 1400 oC for 5 hours. Sintered specimens were reduced at 750 oC in a hydrogen atmosphere for 8 hours. The reduced specimens were reduced for 500 and 1000 hours, respectively, at 900 oC in a humidified hydrogen atmosphere, which is an accelerated atmosphere. Ni-3YSZ with nano-ZrO2 added had greatly improved mechanical strength and long-term durability.

The proton uptake process in double perovskite triple ionic-electronic conducting oxides for protonic ceramic cells

The proton uptake mechanism in a triple conducting oxide, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, is comprehensively investigated based on defect chemistry and experimental analyses of mass and conductivity changes under dry and humidified atmospheres.

Improving the Chemical Stability of BaCe0.9Y0.1O3-d (BCY10) Towards Humidification and Carbonation By Microstructural Design

Protonic ceramic fuel cells are rapidly gaining interest as a promising technology for energy conversion. These devices use proton-conducting electrolytes allowing them to operate at considerably lower temperatures (e.g., 400 – 600 °C) than those of the oxide-ion-conducting counterparts (e.g., > 600 °C). This decrease in temperature can bring significant benefits, including shorter start-up times, lower energy inputs to heat the cell up to the operating temperature and greater longevity [1].Owing to its very high equilibrium constant for hydration, yttrium-doped barium cerate, BaCe1−xYxO3−δ (BCY), stands out as being an interesting electrolyte material to use at lower temperatures, due to its high proton conduction under these conditions (e.g., ∼ 10−3 S cm−1 at 400 °C under humidified atmospheres, p H2O ∼ 10−2 atm). Nonetheless, this composition has been, typically, discarded for real world applications due to its very poor chemical stability towards carbonate or hydroxide formation at low temperatures. In this respect, the presence of amorphous intergranular phases in BaCeO3 polycrystalline materials can be highly reactive to acidic atmospheres and have been suggested to be a likely cause of the structural disintegration of this composition [1,2].To try to overcome this limitation, we report a potential solution based on the modification of BaCe0.9Y0.1O3−δ (BCY10) composition by the introduction of small amounts of NiO (1 vol%) during the sintering process. While the addition of transition metals has been widely adopted for enhancing the sinterability of these materials [1], such behaviour has not been previously tested as a method to improve the chemical stability and lifetime of this material.To accomplish this goal, four different samples were pelletised and sintered: the undoped samples, BCY10 sintered at 1450 °C (BCY1450) and 1600 °C (BCY1600); and the NiO-modified samples, sintered at 1450 °C (Ni-BCY1450) and 1350 °C (Ni-BCY1350). All materials were characterised for phase composition by X-ray diffraction (XRD), and for microstructure by scanning electron microscopy, coupled with chemical analysis by Energy Energy-dispersive X-ray spectroscopy (SEM/EDS).The XRD results reveal all samples to be single phase, indexed as the orthorhombic perovskite BaCe0.9Y0.1O3- d. Nevertheless, slight changes of the cell volume were noted upon Rietveld refinement of the XRD data, suggesting that nickel is incorporated in the perovskite structure of BCY10. From the SEM results, it was possible to observe that all samples showed different average grain sizes. Particularly, the NiO-modified samples showed considerably higher grain sizes and levels of densification than that observed for the unmodified BCY10 samples.Electrochemical Impedance Spectroscopy (EIS) was utilised to evaluate the electrical properties of bulk samples in wet O2 (p H2O = 0.033 atm) in the temperature range 100 – 600 °C. It was found that the bulk response is similar for all samples under study, with no notable dependence from any compositional differences. Conversely, the specific grain boundary conductivity, a factor where the grain boundary response is normalised as a function of microstructural differences, was found to be higher in the case of the BCY1450 and Ni-BCY1350 samples, followed by Ni-BCY1450 and BCY1600.To evaluate the chemical stability, fresh samples were exposed to a humidified CO2 atmosphere (p H2O = 0.033 atm) at 400 °C for 30 days and EIS was used to record impedance data. The results revealed that the unmodified BCY10 samples suffered a substantial drop in total conductivity in this timeframe, while the NiO-modified samples (Ni-BCY1350 and Ni-BCY1450) remained stable through the tests.This result is important as it reveals that the simple addition of 1vol% of NiO not only offers its typical role of decreasing the sintering temperature from 1450 to 1350 °C, but also is sufficient for dramatically improving the stability of BCY10 against humidification and carbonation. The work offers a significant advance to the knowledge of this family of materials at low temperatures, potentially opening a way to use a previously discarded proton-conducting perovskite material in real world applications. Acknowledgements The authors acknowledge to FCT for the PhD grant SFRH/BD/130218/2017 and the projects, PTDC/CTM-CTM/2156/2020, PTDC/QUI-ELT/3681/2020, POCI-01-0247-FEDER-039926, POCI-01-0145-FEDER-032241, UIDB/00481/2020 and UIDP/00481/2020 and CENTRO-01-0145-FEDER-022083 - Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).

New ionic conductor: Ba-deficient Ba3Y4O9 with Zr substitution

To lower operating temperatures of solid oxide fuel cells (SOFCs), the development of ion-conducting oxides with high conductivity and durability is desired. In this work, we investigated Zr-substituted “Ba3Y4O9” as an ionic conductor at intermediate temperatures and found that the Zr substitution for Y dramatically improves the phase stability in humidified atmospheres at 300–800 °C. The total electrical conductivity of nominally 20 mol% Zr-substituted Ba3Y4O9 sample is about 0.1 mS/cm at 500 °C in both dry H2 and O2 atmospheres. Because an increase in the conductivity in a humidified atmosphere was not observed, the conducting carriers may be oxygen ions. Besides, in the Zr-substituted “Ba3Y4O9” samples, we observed that BaO-rich phase coexists with the main phase whose composition is estimated to be Ba:(Y + Zr) ~ 2:3. Therefore, the main conducting phase might be Ba-deficient Ba3Y4O9. The mechanism of the ionic conduction and the improvement of chemical stability has not been revealed yet due to the lack of crystallographic information about the Ba-deficient phase. While we are now working on further investigation, we promptly report the characteristic of the new compound.

Rich-Quench-Lean model comparison for the clean use of humidified ammonia/hydrogen combustion systems

Ammonia and hydrogen are examples of zero-carbon fuels of high interest for implementation in gas turbine technologies. However, large emissions of nitrogen oxides are still a major detrimental for the implementation of these technologies. Therefore, various techniques have been presented as potential solutions to mitigate this problem. Rich-Quench-Lean systems combined with humidified atmospheres are amongst the most promising with the reduction of emissions as a consequence of recombinations of species and lower combustion temperatures. However, limited scrutiny exists around the chemical progression of species in systems like these whilst being fuelled with ammonia blends. Furthermore, any chemical study currently faces a challenge for the selection of a chemical kinetic mechanism due to the great variety of available mechanisms for ammonia combustion, each with different characteristics for the resolution of this fuel. Thus, a Chemical Reactor Network (CRN) has been developed to numerically assess an industry scale humidified Rich-Quench Lean system, utilising five of the most used chemical kinetic models in humidified ammonia combustion whilst informing developers of the differences between those selected. The results displayed significant differences between the mechanisms as the flame progresses. Sensitivity analyses of [OH] and [NH3] displayed similar reactions having opposing effects for these two species at various points of the burner. Quantitative Reaction Path Diagrams (QRDP) for NO showed both similarities and differences between the mechanisms in terms of paths taken and rates of production.

Yttria-stabilized barium zirconate surface reactivity at elevated temperatures

Material changes in yttrium-doped barium zirconate, BaZr0.8Y0.2O3–x, were studied using in situ Raman spectroscopy and ex situ x-ray photoelectron spectroscopy analysis. During in situ Raman analysis, samples were heated to temperatures of 300–600 °C and exposed to both dry and humidified H2 atmospheres. At the lower temperatures (300–450 °C), a new vibrational peak appears in the Raman spectra during exposure to humidified H2. The appearance of this feature is reversible, dependent on previous sample history, and possibly results from new, secondary phase formation or lattice distortion.

Ion Mobility-Mass Spectrometry of Iodine Pentoxide-Iodic Acid Hybrid Cluster Anions in Dry and Humidified Atmospheres.

Nanometer-scale clusters form from vapor-phase precursors and can subsequently grow into nanoparticles during atmospheric nucleation events. A particularly interesting set of clusters relevant to nucleation is hybrid iodine pentoxide-iodic acid clusters of the form (I2O5) x(HIO3) y as these clusters have been observed in coastal region nucleation events in anomalously high concentrations. To better understand their properties, we utilized ion mobility-mass spectrometry to probe the structures of cluster anions of the form (I2O5) x(HIO3) y(IOα)- ( x = 0-7, y = 0-1, α = 1-3), similar to those observed in coastal nucleation events. We show that (I2O5) x(HIO3) y(IOα)- clusters are relatively stable against dissociation during mass spectrometric measurement, as compared to other clusters observed in nucleation events over continental sites, and that at atmospherically relevant relative humidity levels (65% and less) clusters can become sufficiently hydrated to facilitate complete conversion of iodine pentoxide to iodic acid but that water sorption beyond this level is limited, indicating that the clusters do not persist as nanometer-scale droplets in the ambient.

A Reversible Protonic Ceramic Cell with Symmetrically Designed Pr₂NiO4+δ-Based Electrodes: Fabrication and Electrochemical Features.

Reversible protonic ceramic cells (rPCCs) combine two different operation regimes, fuel cell and electrolysis cell modes, which allow reversible chemical-to-electrical energy conversion at reduced temperatures with high efficiency and performance. Here we present novel technological and materials science approaches, enabling a rPCC with symmetrical functional electrodes to be prepared using a single sintering step. The response of the cell fabricated on the basis of P–N–BCZD|BCZD|PBN–BCZD (where BCZD = BaCe0.5Zr0.3Dy0.2O3−δ, PBN = Pr1.9Ba0.1NiO4+δ, P = Pr2O3, N = Ni) is studied at different temperatures and water vapor partial pressures (pH2O) by means of volt-ampere measurements, electrochemical impedance spectroscopy and distribution of relaxation times analyses. The obtained results demonstrate that symmetrical electrodes exhibit classical mixed-ionic/electronic conducting behavior with no hydration capability at 750 °C; therefore, increasing the pH2O values in both reducing and oxidizing atmospheres leads to some deterioration of their electrochemical activity. At the same time, the electrolytic properties of the BCZD membrane are improved, positively affecting the rPCC’s efficiency. The electrolysis cell mode of the rPCC is found to be more appropriate than the fuel cell mode under highly humidified atmospheres, since its improved performance is determined by the ohmic resistance, which decreases with pH2O increasing.

Structures, Phase Fields, and Mixed Protonic-Electronic Conductivity of Ba-Deficient, Pr-Substituted BaZr0.7Ce0.2Y0.1O3-δ.

The BaZr0.7Ce0.2Y0.1O3-δ-BaPrO3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce0.2Zr0.7)1- xPr xY0.1O3-δ, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N2) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH2O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N2 due to the creation of protonic defects and the consumption of holes.

Characterisation and heat treatment of chloride-contaminated and humidified PuO2 samples

To mimic the reactions that have occurred between plutonium dioxide (PuO2) and degraded polyvinyl chloride during prolonged storage, batches of PuO2 powders with varying specific surface area (4–38 m2 g−1) were exposed to dry hydrogen chloride gas forming a surface-sorbed exchangeable chloride species that was readily leached by dilute caustic solution. One batch was also humidified under a 95% relative humidity atmosphere. Heat treatment at temperatures between 100 and 950 °C steadily reduced the leachable chloride content at least when the initial leachable chloride surface coverage was calculated to be > 0.5 monolayer (ML). Variations between samples are attributed to differences in surface area as well as production route and period of interim storage in air or humidified atmospheres. The data are consistent with surface species and reactions proposed in the literature for HCl adsorption on to TiO2. This indicates that it is interactions between hydrogen chloride/chloride and water/hydroxyl species on the PuO2 surface that are key to understanding the behaviour of these materials when heated in a furnace. These data will inform the design of a future thermal stabilisation process for chloride-contaminated PuO2.

Structure and Electrical-Transport Relations in Ba(Zr,Pr)O3-δ Perovskites.

Members of the perovskite solid solution BaZr1-xPrxO3-δ (0.2 ≤ x ≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3̅c) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3 to BaPrO3 is Pm3̅m → R3̅c → Imma → Pnma. The structural data indicate that Pr principally exists as Pr4+ on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250-900 °C in dry and humidified (pH2O ≈ 0.03 atm) N2 and O2 atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (σ ≈ 0.08 S cm-1 at 920 °C in dry O2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

Life Testing of Ni-YSZ Fuel Electrodes under Electrolysis, Fuel Cell, and Current Switching Operation

Most studies of solid oxide cell degradation during electrolysis or reversible current-switching operation have focused on the oxygen electrode.1 However, one recent study has shown that Nickel/yttria-stabilized-zirconia (Ni-YSZ) fuel electrodes coupled with YSZ electrolytes may also degrade when operated under applied electrolysis current.2 Most studies of Ni-YSZ degradation have focused on fuel cell operating conditions, including the effect of temperature3 and fuel gas composition.4 Currently, relatively little is known about the electrolysis operating conditions, e.g., temperature and current density, that lead to degradation, and no studies have been reported to date on current switching operation. In the present work, a degradation study of Ni-YSZ electrodes on YSZ electrolytes was carried out using a symmetric cell geometry. The cells are tape-cast co-fired structures consisting of thin YSZ electrolytes sandwiched between thick Ni-YSZ electrodes. This geometry is meant to mimic the structure of Ni-YSZ anode-supported cells, and has the advantages that: (1) the life tests are simplified compared to full cells, with no gas sealing required; (2) unlike full cells, there is no need to separate out responses from an oxygen electrode in impedance spectra; (3) in current-switched testing, both Ni-YSZ electrodes tend to evolve in the same way, such that the cells remain electrochemically symmetrical; and (4) in dc-current testing, results are obtained on both fuel cell and electrolysis operation, as one electrode operates under anodic conditions and the other operates under cathodic conditions. The Ni/YSZ|YSZ|Ni/YSZ cells were tested at 800 °C in 97% H2 – 3% H2O for up to 1000 h, with current density ranging from 0.2 to 1.2 A/cm2. Electrochemical impedance spectroscopy measurements were made periodically with the current set to zero, during short interruptions of the life tests. At 0.2 A/cm2, there was little change in the ohmic or polarization resistance during the tests, and post-test SEM microstructural evaluation showed no measurable change. At the higher current density of 0.8 A/cm2, on the other hand, substantial microstructural and electrochemical changes were observed. For current-switching operation, the ohmic resistance increased slightly, while the polarization resistance showed a decreasing tendency. The main microstructural change was the observation of nanoparticle formation in the Ni-YSZ electrode functional layers. The nanoparticles contain Ni, Yittria and zirconia, which potentially increased the triple phase boundary density, possibly explaining the decrease of polarization resistance. For dc current operation, the ohmic resistance increased relatively rapidly, while the polarization resistance dropped slowly. Nanoparticles were observed on both electrodes after the test, but with different morphologies. In addition, cross-sectional SEM images showed intergranular fracture within the YSZ electrolyte, due to grain boundary voids that formed during the dc current test, presumably explaining the observed electrolyte resistance increase. Reference 1 Call, A. V., Railsback, J. G., Wang, H. Q. & Barnett, S. A. Degradation of nano-scale cathodes: a new paradigm for selecting low-temperature solid oxide cell materials. Phys Chem Chem Phys 18, 13216-13222 (2016). 2 Chen, M. et al. Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current. J Electrochem Soc 160, F883-F891, doi:10.1149/2.098308jes (2013). 3 Kennouche, D., Chen-Wiegart, Y. C. K., Cronin, J. S., Wang, J. & Barnett, S. A. Three-Dimensional Microstructural Evolution of Ni-Yttria-Stabilized Zirconia Solid Oxide Fuel Cell Anodes At Elevated Temperatures. J Electrochem Soc 160, F1293-F1304, doi:10.1149/2.084311jes (2013). 4 Matsui, T., Kishida, R., Muroyama, H. & Eguchi, K. Comparative Study on Performance Stability of Ni-Oxide Cermet Anodes under Humidified Atmospheres in Solid Oxide Fuel Cells. J Electrochem Soc 159, F456-F460, doi:10.1149/2.053208jes (2012).

New Design Concept of Ni‒Doped-Ceria Cermet Anode for Solid Oxide Fuel Cells: Redox-Induced Self-Modification

Recently, the strong chemical interaction between Ni and GDC (Gd2O3‒CeO2) was reported for the Ni‒GDC cermet anode in solid oxide fuel cells (SOFCs); CeO2 species migrated on the Ni surface after the exposure to the reducing atmosphere. This surface modification of nickel affected the catalytic property for carbon deposition and CO adsorption. Generally, such a chemical interaction is promoted by the oxidation-reduction (redox) treatment. In this study, then, the influence of redox treatment on the chemical interaction between Ni and doped ceria was studied to design the cermet anode with high tolerance to carbon deposition. The Ni‒GDC cermet subjected to the redox treatment showed high tolerance to carbon deposition. Furthermore, the redox treatment did not affect the mechanism of electrochemical hydrogen oxidation in humidified hydrogen atmospheres.

Chemical surface exchange of oxygen on CeO2-δ in an O2/H2O atmosphere.

The chemical surface reaction rate constant controlling the change of oxidation state of undoped ceria, kChem, was measured at 1400 °C in the range of (∼0 ≤ (pH2O/atm) ≤ 0.163(9)) and (10-3.85 ≤ (pO2/atm) ≤ 10-2.86) via the electrical conductivity relaxation method. In humidified atmospheres, kChem is fully described as the sum of kChem,O2 and kChem,H2O, which are, respectively, the rate constants for oxidation by O2 and by H2O alone. Using measurements under appropriately controlled gas conditions, the total rate constant is found to follow the correlation kChem/cm s-1 = 10-(1.35±0.07) × (pO2/atm)0.72±0.02 + 10-(3.85±0.03) × (pH2O/atm)0.36±0.03 where the pO2 and pH2O values of relevance are explicitly those of the final gas condition. The results suggest that at such high temperatures, the concentrations of surface adsorbed species are too low to influence the independent reaction pathways.

Electrochemical Behaviors for Ag, LSCF and BSCF as Oxygen Electrodes for Proton Conducting IT-SOFC

The electrochemical behaviors of Ag, La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF), and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) as potential cathodes for proton-conducting intermediate temperature solid oxide fuel cell (IT-SOFC) are studied using symmetrical cells based on a leading proton conducting electrolyte of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) at 450–650°C. The influences of different testing conditions including oxygen and water (H2O) content as well as cell configuration (i.e., symmetrical cells versus full cells) on the electrochemical behaviors are systematically investigated. It was found that pure LSCF performs rather poorly like Ag and gives very high interfacial resistance, suggesting sluggish electrode reaction and greatly hindered oxide ion transport between LSCF cathode and BZCYYb electrolyte even under dry condition. In comparison, the LSCF-BZCYYb composite and pure BSCF both show much higher activity toward oxygen electrode reaction, and BSCF outperforms the LSCF composite cathode, suggesting BSCF behaves like a mixed oxide ion-electron conductor in dry atmospheres and a mixed proton-electron conductor in humidified atmospheres. The implication of the experimental observations especially in terms of the similarity and differences in electrochemical behaviors between different electrode materials to the overall cathode reaction mechanism for proton conducting IT-SOFC is discussed, and the directions for further improving the cathode performance for proton conducting IT-SOFC are pointed out.