Proton-conducting Ceramics Research Articles

Proton-Conducting Ceramics for Metal-Supported Solid Oxide Fuel Cells and Electrolysis Cells

Proton conducting oxide electrolyte materials could potentially lower the operating temperature of metal-supported solid oxide cells (MS-SOCs) to the intermediate range 400 to 600 °C. The porous metal substrate provides the advantages of MS-SOCs such as high thermal and redox cycling tolerance, low-cost of structural materials, and mechanical ruggedness. The cell structure for fuel cell and electrolysis operation is shown in Figure 1. To incorporate proton-conductors into metal-supported cell configuration through co-sintering fabrication, a key factor is that the proton conductors must be compatible with reducing atmosphere sintering, as the porous metal substrate can not be fired in oxidizing atmosphere. In this work, viability of co-sintering fabrication of metal-supported proton conducting solid oxide cells is investigated. Candidate proton-conducting ceramics including perovskite-type oxides BaZr0.7Ce0.2Y0.1O3-δ (BZCY), SrZr0.5Ce0.4Y0.1O3-δ (SZCY), and Ba3Ca1.18Nb1.82O9-δ (BCN), pyrochlore-type oxides La1.95Ca0.05Zr2O7-δ (LCZ) and La2Ce2O7 (LCO), and acceptor doped rare-earth ortho-niobates La0.99Ca0.01NbO4 (LCN) were synthesized via solid state reactive synthesis or sol-gel synthesis. These ceramics are sintered at 1450°C in reducing environment alone and supported on Fe-Cr alloy metal support, and their key characteristics such as phase formation, sintering property, and chemical compatibility with metal support are determined. Most electrolyte candidates suffer from one or more challenges identified for this fabrication approach, including: phase decomposition in reducing atmosphere, evaporation of electrolyte constituents, contamination of the electrolyte with Si and Cr from the metal support, and incomplete electrolyte sintering. BZCY is particularly interesting because of its high proton conductivity, but it is found to suffer from reaction with Si and Cr present in the metal support, and loss of Ba via evaporation during sintering. Various approaches to overcome these limitations are proposed, and preliminary assessment indicates that the use of barrier layers, low-Si-content stainless steel, and sintering aids warrant further development. In contrast, La0.99Ca0.01NbO4 is found to be highly compatible with the metal support and co-sintering processing in reducing atmosphere, allowing dense electrolyte and porous electrode made of LCN to be co-sintered on porous stainless steel, Figure 2. A metal-supported cell is fabricated with La0.99Ca0.01NbO4 electrolyte, ferritic stainless steel support, Pt air electrode and nanoparticulate ceria-Ni hydrogen electrocatalyst. The total resistance is 50 Ω∙cm2 at 600 °C. This work clearly demonstrates the challenges, opportunities, and breakthrough of metal-supported proton-conducting solid oxide cells by co-sintering fabrication. Figure 1

Ba0.95Ca0.05Ce0.9Y0.1O3 as an electrolyte for proton-conducting ceramic fuel cells

Comparative studies of the physicochemical and electrochemical properties of BaCe0.9Y0.1O3 and Ba0.95Ca0.05Ce0.9Y0.1O3 sintered ceramics are described. The orthorhombic (Pnma) space group was determined for both investigated samples within the temperature range 25–800 °C, using the X-ray diffraction method. A decrease in the cell volume of Ba0.95Ca0.05Ce0.9Y0.1O3 compared to that of BaCe0.9Y0.1O3 was observed within the investigated temperature range.It was found that the partial substitution of calcium for barium, resulting in Ba0.95Ca0.05Ce0.9Y0.1O3, caused an increase in specific free volume and global instability index compared to the original composition BaCe0.9Y0.1O3. Improvement of the chemical resistance of Ba0.95Ca0.05Ce0.9Y0.1O3 in a CO2 gas atmosphere was observed. It was found that Ba0.95Ca0.05Ce0.9Y0.1O3 exhibited a slightly lower level of electrical conductivity than BaCe0.9Y0.1O3 in air as well as in ArH2 gas atmospheres.The greater chemical resistance of Ba0.95Ca0.05Ce0.9Y0.1O3 in a CO2 gas atmosphere enhances the suitability of the modified material as an electrolyte for fuel cells with proton-conducting ceramics. The electrochemical response of the interface (La0.60Sr0.40)0.95Co0.20Fe0.80O3−δ|Ba0.95Ca0.05Ce0.9Y0.1O3 was also investigated within the applied potential range from −0.05 to −0.5 V in SOFC mode as well as from 0.05 to 0.5 V in SOEC mode); it was found that the current reached a fairly stable value over time.

Study of proton conductivity of composite metal-ceramic materials based on neodimium tugstates using a four-electrode technique with ionic probes

Ceramic solid proton electrolytes La0.99Ca0.01NbO4, Nd5.5WO11.25- Nd5.5W0.5Mo0.5O11.25-δ were synthesized and their proton conductivity was measured in the temperature range of 300–650 °С in an atmosphere of dry and humid air. It is shown that solid electrolytes have a high proton conductivity ∼10−4 S/cm at 500 °С. Dense metal-ceramic composites containing phases of metal Ni0.5Cu0.5 and oxides Nd5.5WO11.25 or Nd5.5W0.5Mo0.5O11.25-δ with the total conductivity typical of metals were obtained using hot pressing technique an argon atmosphere. An experimental determination of the contribution of the ionic (in this case proton) conductivity to the overall conductivity of the material comprises is an actual problem in investigation of the properties of metal-ceramic materials. The values of proton conductivity can be estimated from the results of studying the hydrogen permeability of membranes and the diffusion of hydrogen, but these methods are rather complicated in instrumentation. Therefore the use of relatively simple and accessible electrical measurement methods to solve this problem is very relevant. In this paper, the partial proton conductivity of the composite materials mentioned above was first measured using a 4-electrode cell with ion probes made of a ceramic proton conductor La0.99Ca0.01NbO4 in an atmosphere of moist hydrogen and at the temperature range of 300–650 °C. In the low-temperature region, the values of the partial proton conductivity measured in the 4-electrode cell are in good agreement with those obtained by standard complex impedance analysis for the pure ceramics not containing metal. At high temperatures, the values obtained by two independent techniques differ. This can be explained by the contribution of the electrochemical reaction proceeding at the interface between the ion probe and the metal phases and accompanied by the dissolution of atomic hydrogen in the metal. In general, the measured value of the ionic conductivity can be either underestimated or overestimated in comparison with the real one, depending on the rate of chemical reactions occurring at the electrodes. Nevertheless, in a limited temperature range, the use of four-electrode measurements with ionic (proton) probes allows one to obtain correct results.

Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes

This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry. Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr1−xYxO3−δ) and oxidation/reduction of La1−xSrxCo0.2Fe0.8O3−δ. While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally. This was motivated by the discrepancy of some values in literature. The conclusions are that the heating rate has little to no effect on the obtained values. Models for the expansion coefficients of a composite material are presented and include the effect of porosity. A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors. Finally, the methods of mitigation of the thermal mismatch problem are discussed.

Insight into the dehydration behaviour of scandium-substituted barium titanate perovskites via simultaneous in situ neutron powder thermodiffractometry and thermogravimetric analysis

Hydration-dehydration cycles are critical to the mechanical performance of ceramic proton conductors. The development of in situ methods is desirable in order to study their structural response under conditions that mimic the operating ones. Neutron powder diffraction studies combined with simultaneous thermogravimetric analysis were performed on the hydrated forms of two members of the oxygen deficient perovskite BaTi1−xScxO3−δ series, with x = 0.5 and x = 0.7. Rietveld analyses agreed with in situ gravimetric data, allowing correlation of occupancy factors of the oxygen site to hydration levels and other structural data. Dehydration is an activated process that impacts on structural parameters and the level of Sc substitution was found to control the structural response during in situ dehydration, with higher Sc content leading to significantly greater volume contraction. This was rationalised by the chemical expansion due to hydration of oxygen vacancies within the x = 0.5 sample being anomalously small. Furthermore, the behaviour of the x = 0.5 system revealed an unexpected cell expansion during the early stages of dehydration, suggesting the hydration level may influence the thermal expansion coefficient (TEC).

Sintering aid (ZnO) effect on proton transport in BaCe0.35Zr0.5Y0.15O3‐δ and electrode phenomena studied by distribution function of relaxation times

AbstractIn this work, we have employed an evolutionary programming technique to analyse the impedance spectroscopy (IS) of a ceramic proton conductor BaCe0.35Zr0.5Y0.15O3‐δ (BCZY) to study its proton transport properties as well as the electrode reactions kinetics at the Pt|BCZY interface. The distribution functions of relaxation times (DFRT) obtained from the impedance analysis were used to evaluate the physical parameters such as relaxation time constant, resistance, reactance/capacitance, conductivities, activation energies, grain‐boundary space charge potential Φ(0, T), and electrode polarization resistance. The effect of ZnO sintering aid (2 wt%) on charge‐transfer processes at bulk, grain boundaries and Pt|BCZY interface were thoroughly studied. Φ(0, T) is increased and grain‐boundary conductivity (σgb) is decreased by an order of magnitude due to the presence of Zn at the grain boundaries, whereas bulk properties appear to be unchanged. Variation in σgb with temperature is explained by correlating the grain‐boundary capacitance (Cgb), relaxation time (τgb), space charge effect, and the defect associations (eg, Y‐OH). The anodic functions, for example, hydrogen oxidation kinetics at the electrode (Pt) surface and Pt‐BCZY interface, are explained through the symmetrical cell study and the behavior of relaxation time constants (τ) with temperature.

Galvanic hydrogen pumping performance of copper electrodes fabricated by electroless plating on a BaZr0.9-xCexY0.1O3-δ proton-conducting ceramic membrane

Fuel electrodes on proton-conducting ceramics, such as BaZr0.9-xCexY0.1O3-δ (BZCY), must be stable at high temperatures in reducing and hydrocarbon-containing gases. This work investigated thin Cu-based electrodes (~1 μm) deposited by electroless plating on tubular substrates composed of a dense BZCY film (~25 μm) on a porous Ni-BZCY support. Ru, Pd, and Cu were studied as activation catalysts and the performance of those electrodes (Ru/Cu, Pd/Cu, and Cu/Cu) was evaluated during the galvanic hydrogen pumping at 700 °C in reducing and hydrocarbon-containing feeds. The hydrogen flux was measured with a stoichiometric titration experiment and the highest flux (3.1 NmL·cm−2·min−1) was obtained with Ru/Cu at 400 mA·cm−2. The metric to compare the electrodes was the power required to pump 1 NmL H2·min−1 at 40 mA·cm−2. Those numbers for Ru/Cu, Pd/Cu, and Cu/Cu electrodes in a 10% H2 in Ar feed were 34, 22, and 268 mW/(NmL H2·min−1). Due to the order of magnitude higher power requirement for the pure Cu electrode, only the Ru/Cu and Pd/Cu electrodes were investigated further and their performance was found to deteriorate in the presence of a predominantly methane-feed. The post mortem microstructural analysis showed that the Ru/Cu and Pd/Cu electrodes did reorganize significantly under bias and coke formation was only observed on the BZCY membrane.

CO2 Reduction on the Pre-reduced Mixed Ionic-Electronic Conducting Perovskites La0.6 Sr-0.4 FeO3-δ and SrTi0.7 Fe0.3 O3-δ.

The activity of the pre-reduced perovskites La0.6 Sr0.4 FeO3-δ (LSF64) and SrTi0.7 Fe0.3 O3-δ (STF73) for the CO2 reduction to CO was investigated with special focus on the reactivity of oxide-dissolved hydrogen. This is of particular interest in hydrogen solid-oxide electrolysis cell (H-SOEC) technology, where proton-conducting ceramics are used and the reaction 2e- +2H+ +CO2 →CO+H2 O is of central importance. To clarify if hydrogen dissolved in LSF64 and STF73 partakes in the CO2 reduction, temperature-programmed reduction (TPR) in H2 , followed by temperature-programmed reoxidation (TPO) in CO2 and, moreover, temperature-programmed desorption (TPD) of ad- and absorbed species were utilized. The experiments reveal that 50 mol % of the CO2 is converted by hydrogen dissolved in STF73 and reacts quantitatively. On the other hand, LSF64 converts less than 20 mol % of CO2 via dissolved hydrogen and a residual of bulk OH is still detectable after CO2 -TPO.

High performance fuel electrodes fabricated by electroless plating of copper on BaZr0.8Ce0.1Y0.1O3-δ proton-conducting ceramic

The stability of copper at high temperatures in reducing and hydrocarbon-containing atmospheres makes it a good candidate for fabricating fuel electrodes on proton-conducting ceramics, such as BaZr0.9-xCexY0.1O3-δ (BZCY). In this work, the electrochemical performance of Cu-based electrodes fabricated by electroless plating (ELP) on BaZr0.8Ce0.1Y0.1O3-δ is studied with impedance spectroscopy. Three activation catalysts (Pd, Ru, and Cu) are investigated and ELP is compared to a commercial Cu paste (ESL 2312-G) for electrode fabrication. The area specific resistances (ASR) for Pd, Ru, and Cu activations at 700 °C in moist 5% H2 in Ar are 2.1, 3.2, and 13.4 Ω cm2, respectively. That is a 1–2 orders of magnitude improvement over the commercial Cu paste (192 Ω cm2). Furthermore, the ASR has contributions from electrode processes and charge transfer at the electrode/electrolyte interface. Additionally, the morphology of the as-fabricated electrode is unaffected by the activation catalyst. However, heat treatment at 750 °C in H2 for 24 h leads to sintering and large reorganization of the electrode fabricated with Cu activation (micron sized pores seen in the tested sample), while Pd and Ru activations are immune to such reorganization. Thus, Pd and Ru are identified as candidates for future work with improvements to charge transfer required for the former, and better electrode processes required for the latter.

(Invited) Advances in Proton Ceramic Fuel Cells, Steam Electrolyzers, and Dehydrogenation Reactors Based on Materials and Process Optimizations

Proton-conducting ceramics such as Y-doped BaZrO3-based perovskites form basis for hydrogen-fueled proton ceramic fuel cells (PCFCs) and proton ceramic electrolyzers (PCEs) for steam. Like the PCFC benefits from running undiluted hydrogen with superior fuel utilization without circulation, the PCEs produce directly dry hydrogen at the balanced total pressure of the electrolyzer while oxygen is diluted to lower pressure with the feed of steam, lowering the hazard of pressurized oxygen. The use of waste or renewable heat to generate and superheat the steam allows operation at high electrical efficiencies.Operation at high steam pressures reduces p-type electronic conduction at the steam+oxygen electrode and the use of novel mixed proton-electron conducting double perovskites lowers the overpotential and hence p-type electronic conductivity further, altogether yielding acceptable faradayic efficiencies of the electrolyzer. The presence of protons and significant protonic conduction in certain double perovskites in combination with their high electronic conductivity can be rationalized by recent results of studies of their structure and defect chemistry. Proton-conducting ceramics also offer unique opportunities for dehydrogenation of hydrocarbons including natural gas along different schemes and for different products, as suggested by Iwahara three decades ago. With materials and reactor design optimizations as well as process adaptions we now see emerging demonstrations of such use of proton-conducting ceramics.

Effect of Preparation Conditions on the Polymorphism and Transport Properties of La6–xMoO12−δ (0 ≤ x ≤ 0.8)

In this work, La6–xMoO12−δ (0 ≤ x ≤ 0.8) materials are investigated as a new family of ceramic proton conductors. Different polymorphic phases with rhombohedral and cubic structure are obtained, depending on the cooling rate applied during the synthesis process. The materials have been thoroughly characterized by different techniques, including X-ray powder diffraction, transmission electron microscopy, and X-ray photospectroscopy in order to fully understand the structural features of the samples. Thermogravimetric analysis and impedance spectroscopy in dry/wet gases (N2, O2, and 5% H2–Ar) confirmed the existence of proton conductivity at low temperature. Under a reducing atmosphere, the materials are mixed ionic-electronic conductors. The sample prepared at the fastest cooling rate exhibits cubic structure and higher n-type electronic conductivity compared to those prepared at a slower cooling rate with rhombohedral structure.

Internal Reforming of Methane Fuel within Proton-Conducting Ceramic Anode Supports

The objective of this work is to advance our understanding of hydrocarbon internal-reforming processes within the anode supports of proton-conducting ceramic fuel cells. This work is centered on an exciting new class of protonic-ceramic materials that are emerging from the laboratory to play important roles in the commercial sector. While proton-conducting ceramics have been studied since the early 1980s, the unique properties of these materials are only now being harnessed to address societal challenges in electricity generation, energy storage, and chemicals synthesis. Protonic-ceramic fuel cells (PCFCs) have demonstrated remarkable power density and stable operation under hydrocarbon fuels at operating temperatures that are well below that of the more-well established oxygen-ion conducting ceramics. Performance of recent unit-cell PCFC stacks based on barium cerate-zirconates developed at the Colorado Fuel Cell Center exceeds 300 mW / cm2 at 0.78 V during operation under direct methane-steam fuel at 550 ºC. Such performance brings confidence that protonic ceramics can play a meaningful role in efficient electricity generation. These protonic-ceramic fuel cells have demonstrated remarkable stability under methane-steam fuel mixtures. Under these conditions, no upstream fuel processor is used to convert the methane fuel into synthesis gas (H2, CO, CO2, H2O, etc.). Rather, the methane-steam fuel feed is reformed within the PCFC anode support. This internal reforming brings many benefits, including higher efficiency, improved thermal uniformity, reduced thermal stress, and simpler, lower-cost balance-of-plant components. Such internal reforming is common is oxygen-ion conducting ceramics like yttria-stabilized zirconia (YSZ). However, YSZ-based devices operate at significantly higher temperatures (~ 800 ºC), where the chemical kinetics of methane-steam reforming are more favorable to high conversion. In contrast, PCFCs operate at temperatures at or below 550 ºC, where reaction kinetics are substantially slower. This lower-temperature operation under direct hydrocarbon fuels brings questions about the effectiveness of the internal reforming process. Inadequate reforming can lead to inefficient utilization of the parent hydrocarbon fuel, lower cell performance, and deleterious solid-carbon deposit formation. To explore these questions in more depth, we have developed unique experimentation that decouples the thermochemical phenomena of heterogeneous methane-steam reforming from the electrochemical processes underway during fuel-cell operation and electricity generation. The “Separated Anode Experiment” has been developed to study heterogeneous chemistry and gas-transport processes in solid-oxide fuel cell anodes. This experiment decouples gas-transport and hydrocarbon internal-reforming processes from electrochemistry by simulating operational conditions within an anode support in the absence of a dense electrolyte. The planar anode support is compression-sealed within two alumina manifolds into which flow channels have been machined. Feed gases cross-diffuse and heterogeneously react within the tortuous morphology of the anode. Exhaust-gas compositions are quantified to understand what gases reach the triple-phase boundaries at the anode-electrolyte interfaces, bringing insight to the conditions at which charge transfer occurs. A detailed computational model aids in interpretation of the experimental results. In this presentation, the performance of protonic-ceramic anode supports will be presented from the perspective of internal-reforming effectiveness. Cell morphology and operating conditions that promote efficient internal reforming will be presented. Figure 1

Reversible Protonic Ceramic Electrochemical Cells for Power Generation and Green Fuel Production

A Reversible Protonic Ceramic Electrochemical Cell (RPCEC) is a device based on proton-conducting ceramics that can operate in both fuel cell and electrolysis cell modes. Compared to reversible electrochemical devices based on oxygen-ion conducting ceramics, RPCECs offer several potential advantages including lower temperature operation, improved coking resistance, and broad fuel flexibility. In fuel cell mode, RPCECs can efficiently convert the chemical energy contained within a wide variety of fuels (including hydrogen, various hydrocarbons, methanol, ethanol and ammonia) to electricity at intermediate temperatures (300 °C-600 °C). In electrolysis mode, RPCECs can operate with applied power from a renewable source (e.g. solar) to produce a number of versatile chemical fuels including hydrogen (via water electrolysis), ammonia (via solid-state electrochemical ammonia synthesis), and even certain hydrocarbon and/or liquid fuels. In this work, we will overview our progress towards fuel-flexible, intermediate-temperature RPCEC devices with promising performance, stability, and efficiency in both electricity-production and fuel-production modes.

Electrical properties and hydrogen flux performance of Sr(Ce0.6Zr0.4)1−xYxO3−δ ceramic proton conductors

In this study, Sr(Ce0.6Zr0.4)1−xYxO3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, SCZY) perovskite proton-conducting ceramics were synthesized using the citrate-ethylenediaminetetraacetic acid complexing method. The effect of the Y concentration on the crystal structure, electrochemical performance and hydrogen flux of the SCZY samples in air and a H2 atmosphere were investigated. The sintered SCZY samples showed a main perovskite crystal structure with several SrY2O4 impurity phases when x ≥ 0.20. The grain size of SCZY samples when x = 0.25 and 0.30 was smaller than that of other samples. The conductivity of all samples increased as the temperature increased in both air and H2; however, the conductivity of SCZY in H2 was lower than that in air. The results of AC impedance, demonstrated that resistance mainly derived from the charge transfer, which was apparently higher than the bulk and grain boundary resistance. Sr(Ce0.6Zr0.4)0.85Y0.15O3-δ exhibited low charge transfer resistance in both air and the H2 atmosphere and had a high hydrogen flux of 0.34 ml min−1 cm−2 μm−1 at 800 °C. The structures of all SCZY samples were stable in 80 °C water.

Influence of the Activation Catalyst on the Polarization Resistance of Electroless Cu Electrodes on BaCe0.1Zr0.8Y0.1O3-δ proton-Conducting Ceramics

Copper (Cu) electrodes appear as potential methane dehydroaromatization (MDA) electrodes on BaCexZr0.9-xY0.1O3- δ (BCZY) proton-conducting ceramics [1]. When applied from paste, the electrodes suffer from poor adhesion, and therefore high polarization resistance. In this work, 1 micron thin Cu electrodes were deposited onto BaCe0.1Zr0.8Y0.1O3-d (BCZY18) specimens by electroless plating, using three different activation catalysts: Pd, Ru and Cu [2]. Impedance spectra were recorded on electrolyte based symmetric cells Cu/BCZY/Cu in the temperature range 750-600°C, in dry and moist reducing atmospheres. The area specific resistances (ASR) of these three electrodes were found to be lower in moist gases and were two orders of magnitude lower than that measured on painted Cu electrodes (see figure) . At 700°C, in 3% moist forming gas, ASR of 2.9, 3.8 and 22.7 Ω∙cm2 were calculated on the Cu electrodes with the Pd, Ru and Cu activation catalyst respectively. The microstructure of the electrodes was evaluated before and after electrochemical testing. In all cases, nano-pores were present on the as-deposited electrodes. After exposure to dry and moist reducing atmosphere at high temperatures, the microstructure of the Pd/Cu and Ru/Cu did not change significantly while major reorganisation was observed for the Cu/Cu electrode. [1] Morejudo SH, Zanón R, Escolástico S, Yuste-Tirados I, Malerød-Fjeld H, Vestre PK, et al. Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor. Science 2016;353:563–566 [2] Patki N, Ricote S, Way JD, Fabrication of reducing atmosphere electrodes (fuel electrodes) by electroless plating of copper on BaZr0.9-xCexY0.1O3-δ – a proton-conducting ceramic, submitted to International Journal of Hydrogen Energy in November 2016 Figure 1

Operando Investigation of the Chemical Interaction of Water Vapor with Ceramic Proton Conductors

The protons are the key players in ceramic proton conductors. But they are very elusive and had to detect. I will shows a suite of experiments, ex situ, in situ and operando, where the protons are shown as structural elements at low temperatures where they do not yet constitute charge carriers. They actually constitute OH groups in the crystal lattice. With increasing temperature, the protons participate with the lattice vibrations while and OH-bond breaking takes place at some transition temeprature where the protons become charge carriers. We have observed this on the Y-doped BaZrO3 and BaCeO3 model proton conductors with quasi elastic neutron scattering under ambient and under high temperature high pressure conditions. The phonon modes as determined from high presssure Raman spectroscopy propel the protons and cause a net protonic charge transfer via phonons. The changes in the chemical bond environment were observed with ambient pressre x-ray spectroscopy, particular changes in the oxygen core lecvel spectra and the Ce, Y and Zr resonant valence band spectra under humidified high temperature conditions. We are able to sketch the changes in bond breaking at exactly the temperature where the ptoton conductivty sets on. [1] Q. Chen, F. El Gabaly, F. Aksoy Akgul, Z. Liu, B.S. Mun, S. Yamaguchi, A. Braun, Observation of 8.238 oxygen vacancy filling under water vapor in ceramic proton conductors in-situ with ambient pressure XPS; Chem. Mater., 2013, 25 (23), pp 4690–4696. [2] Q. Chen, J. Banyte, X. Zhang, J. P. Embs, A. Braun; Proton Diffusivity in Spark Plasma Sintered BaCe0.8Y0.2O3-δ: in-situ combination of quasi-elastic neutron scattering and impedance spectroscopy; Solid State Ionics 252 (2013) 2–6. [3] Q. Chen, T.-W. Huang, M. Baldini, A. Hushur, A. Pomjakushin, S. Clark, W. Mao, M. Manghnani, A. Braun, T. Graule, The Effect of Compressive Strain on the Raman Modes of the Dry and Hydrated BaCe0.8Y0.2O3 Proton Conductor, J. Phys. Chem. C, 2011, 115 (48), pp 24021–24027.

Importance of the Formation of Dy4+ in BaZr0.9Dy0.1O3-δ proton-Conducting Ceramics in Oxidizing Atmospheres

Yttrium doped barium zirconate, BaZr1-xYxO3-δ, has been one of the most studied high temperature proton conductors for the last two decades [1]. Several research groups focused on the choice of the trivalent dopant, which is necessary to create the oxygen vacancies required for the hydration of the material. Han et al. studied for example Sc, Sm, Eu and Dy dopants [2-4] and reported the presence of Dy4+in oxidizing atmospheres. In this work, we are interested in the comparison of structural and electrical properties between the well-known BZY10 (BaZr0.9Y0.1O3-δ) and BZDy10 (BaZr0.9Dy0.1O3-δ). A wide combination of experiments was used to confirm the presence of Dy4+ in oxidizing atmosphere, including high temperature X-ray diffraction, Raman spectroscopy, SQUID magnetometry and electron spin resonance (ESR or EPR). Figure 1 summarizes the BZDy10 lattice parameter in dry oxidizing atmosphere as a function of temperature and shows a decrease in the lattice parameter when increasing the oxygen partial pressure. This is explained by the difference in Dy radii in VI coordination: 0.78 Å and 0.912 Å for Dy4+ and Dy3+respectively. Conductivity measurements as a function of pO2in dry and moist conditions were performed for both BZY10 and BZDy10. The two materials behaved quite similarly in moist reducing atmosphere (i.e., pure ionic conduction), while significant differences were noticed in oxidizing atmospheres. [1] Fabbri E., Pergolesi D. and Traversa E., “Materials challenges toward proton-conducting oxide fuel cells: a critical review ”, Chem. Soc. Rev 39, 4355-4369 (2010) [2] D. Han, K. Shinoda, T. Uda, Dopant site occupancy and chemical expansion in rare earth-doped barium zirconate, J. Am. Ceram. Soc. 97 (2014) 643-650 [3] D. Han, T. Uda, Y. Nose, T. Okajima, H. Murata, I. Tanaka and K. Shinoda, Tetravalent Dysprosium in a Perovskite-Type Oxide, Advanced Materials 24 (2012) 2051-2053 [4] D. Han, K. Shinoda, S. Sato, M. Majima, T. Uda, Correlation between electroncutive and structural properties of proton conductive acceptor doped barium zirconate, J. Mater. Chem. A, 13 (2015) 1243-1250 Figure 1

Electrode kinetics at the Pt, O2|La0.9Sr0.1ScO3-δ interface

Most of the ceramic proton conductors reveal considerable hole conductivity in an oxidizing ambient. This results in a necessity of taking into account the nonfaradaic current when studying the electrode kinetics in electrochemical cells with high-temperature protonic electrolytes. The present paper is an attempt to elucidate the electrode kinetics of oxygen reaction at a Pt, O2|La0.9Sr0.1ScO3-δ (LSS10) interface. The polarization conductivity has been determined as a function of temperature (600–900 °C) and oxygen partial pressure (2.1 × 104 > pO2 > 1 Pa) by means of impedance spectroscopy using the elementary model in terms of an equivalent electrical circuit with the bypass resistance of hole transport in the electrolyte. Possible routes of oxygen reaction at the Pt, O2|La0.9Sr0.1ScO3-δ interface have been discussed, and diffusion of the electron holes in LSS10 was proposed as the rate-determining stage of the electrode process.

Thermal Evolution of BaO-CuO Flux as Sintering Aid for Proton Conducting Ceramic Fuel Cells

The eutectic melt of BaO-CuO flux is known to be a potential sintering aid for Ba(Zr,Y)O₃ (BZY) electrolyte for proton-conducting ceramic fuel cells (PCFCs). A density of BZY higher than 97% of theoretical density can be achieved via sintering at 1300℃ for 2 h using a flux composed of 28 mol% BaO and 72 ㏖% CuO. In the present study, chemical and structural evolution of BaO-CuO flux throughout the sintering process was investigated. An intermediate holding step at 1100℃ leads to formation of various impurity compounds such as BaCuO 1.977 , Ba 0.92 Cu 1.06 O 2.28 and Cu 16 O 14.15 , which exhibit significantly larger unit cell volumes than the matrix. The presence of such secondary compounds with large lattice mismatch can potentially lead to mechanical failure. On the other hand, direct heating to the final sintering temperature produced CuO and Cu₂O as secondary phases, whose unit cell volumes are close to that of the matrix. Therefore, the final composition of the flux is strongly affected by the thermal history, and a proper sintering schedule should be used to obtain the desired properties of the final product.

(Invited) Proton-Conducting Ceramics for Fuels Synthesis

This presentation is centered on an exciting new class of proton-conducting ceramic materials that are emerging from the laboratory to play important roles in the commercial sector. While proton-conducting ceramics have been studied since the early 1980s, the unique properties of these materials are only now being harnessed to address societal challenges. Faculty and staff at the Colorado School of Mines (CSM) are actively developing protonic ceramics for use as fuel cells for efficient electricity generation (PCFCs), electrolyzers for energy storage (PCECs), and membrane reactors for fuels upgrading (PCMRs). Doped barium cerate-zirconate materials (BaCexZr1-x-yYyO3-d) are the focus of these efforts at CSM. The mixed ionic-electronic conductivity of these protonic ceramics can be exploited for developing cost-effective, novel solutions and new products. Many previous approaches used to advance solid-oxide fuel cells are equally relevant for tuning the properties of protonic ceramics to meet the diverse needs of these different applications. In this talk, we will present several such efforts now underway at CSM. As part of an ARPA-E-funded program, our team recently demonstrated a fuel-cell stack based on BaCe0.2Zr0.6Y0.2O3-d (BCZY26) protonic-ceramic materials. To the authors’ knowledge, this is the first reported result of a PCFC stack. The effort included a ten-fold scale up in cell size, and integration of these cells into a stack package. A three-cell stack achieved a power density of 180 mW cm-2 at 0.78 V under methane fuel at 550 ºC. While this PCFC performance is encouraging, we found that similar protonic-ceramic materials demonstrated inefficient operation in electrolysis mode. Heightened electronic conductivity was observed under reverse bias, likely due to cerium reduction. By reducing the cerium content to below the percolation threshold, greatly improved Faradaic efficiency was confirmed. The team is now developing PCEC stacks using BaCe0.1Zr0.8Y0.1O3-d (BCZY18) electrolytes; 95% Faradaic efficiency was observed at 120 mA cm-2 and 600 ºC. Electronic conduction is undesirable in fuel cells and electrolyzers; however, it may prove advantageous for hydrogen-separation membranes and membrane reactors. In an effort to increase flux through “passive” hydrogen-separation membranes, CSM is developing protonic-ceramic composites to increase membrane electronic conductivity. By mixing the BCZY material with an electronic conductor, a two-phase membrane with more balance between protonic and electronic conduction can be realized. Both ceramic (Ce0.8Y0.2O2-d) and metallic (Cu) electronic conductors are being explored as the second phase. While at an early stage of development, these two-phase hydrogen-separation membranes have demonstrated some of the highest thickness-normalized hydrogen fluxes ever observed through ceramic materials. In this presentation, these efforts will be reviewed, with latest results presented.