Proton-conducting Oxide Research Articles

Static disorder in perovskite-type proton-conducting oxides BaSn1-xMxO3-x/2-(y/2)H2O (M = Ga, Sc, In, Y, La): a novel approach based on statistical analysis of numerous DFT simulated structures.

Perovskite-type proton-conducting oxides inherently include defects such as substitutional aliovalent cations, oxide ion vacancies, and interstitial protons. These defects introduce static disorder into their crystal structures. To investigate such disorder in BaSn1-xMxO3-x/2-(y/2)H2O (M = Ga, Sc, In, Y, La), we employ a novel approach based on density functional theory (DFT) simulations. This approach involves the structural optimization of a large number of relatively small, randomly constructed supercells, followed by statistical analysis of their geometric, energetic, and vibrational properties. Our key findings are as follows. The structural disorder becomes more pronounced as the dopant concentration increases, and as the dopant ionic radius increases from Sc to La. The O-H covalent bond lengths, as well as the number densities, of hydrogen atoms display clearly different distributions depending on whether the hydrogen atoms are trapped by aliovalent cations or not. The hydrogen-trapping energies appear to decrease as the geometric similarity of O-H covalent bonds between trapped and untrapped hydrogen atoms increases. Hydrogen atoms simultaneously trapped by more than one dopant atom exhibit considerable stability, potentially hindering proton diffusion at high dopant concentrations. The O-H stretching wavenumber decreases as the O-H covalent bond length increases, showing a very strong and nearly linear correlation between the two. The simulated IR absorption spectra show semi-quantitative agreement with the measured spectra. These new findings demonstrate the effectiveness of the present 'statistical' approach for investigating static disorder.

Improved electrical properties of Ba1−xCaxCe0.8Nd0.2O3−δ solid oxide fuel cells

Among the most effective proton-conducting oxides for use as electrolytes in fuel and electrolysis cells that are all solid-state are barium cerates. The sol–gel auto-combustion synthesis method synthesised electrolytes of the formula Ba1−xCaxCe0.8Nd0.2O3−δ where x = 0.0, 0.02, 0.04, 0.06, 0.08, and 0.10. XRD, SEM, FTIR, and Raman analysis were done to determine the structure, surface morphology, and bond-stretching vibrations. According to Scherer’s formula, the electrolyte materials’ anticipated crystallite sizes ranged from 20.14 to 36.50 nm. Grain sizes were in the range from 2.5 to 4.5 µm. While FT-IR detected bands related to M–O vibrations, the 362.51 cm−1 vibrational mode related to CeO6 octahedra was identified by Raman spectroscopy. Electrical impedance spectroscopy studies confirmed electrolytes had good ionic behavior, with total conductivities increasing with temperature. The bulk conductivity of the studied perovskite oxides was higher than grain boundary conductivity, with compositions with x = 0.10 exhibiting the maximum overall conductivity.

The effect of Zn-doping on electrical properties, hydration, sintering and chemical resistance of hexagonal perovskite Ba7In6Al2O19

A CO2-stable, easily sintered proton-conducting oxide electrolytes based on solid solution Ba7In6Al2–xZnxO19-0.5x with hexagonal structure has been synthesized for the first time. Within the homogeneity region (0 ≤ x ≤ 0.10), there is an increase in unit cell parameters, cell volumes and free cell volumes. The addition of Zn2+ markedly improved the sinterability of the material. The relative density of the ceramics of the doped samples reached 95 % at lower sintering temperatures than the parent phase. The electrical conductivity was studied using electrochemical impedance spectroscopy. Upon doping the oxygen-ion conductivity increased by 0.25 orders of magnitude at 800 °C. Proton transport was predominant below 500 °C for a wet atmosphere (pH2O = 1.92·10−2 atm). The investigated phases Ba7In6Al2–xZnxO19-0.5x are capable of hydration and incorporate up to 1.45 mol H2O vs 0.41 mol H2O for the parent phase. The studied phases exhibit chemical resistance to CO2 under heat treatment at 600 °C. It was shown that solid solution Ba7In6Al2–xZnxO19-0.5x is a promising electrolyte material for intermediate-temperature fuel cells.

Electrochemical behaviour of redox-robust electrode in contact with protonic electrolyte: Case of double-layered Sr2Fe1.5Mo0.5O6-δ - Ce0.8Sm0.2O2-δ composite

Electrochemical devices based on proton-conducting oxides offer a significant potential for implementation in a variety of high-temperature electrochemical applications. It is a well-known fact that electrochemical devices require active electrodes to function optimally. Over the last few years, Sr2Fe1.5Mo0.5O6-δ has received much attention as an effective redox-robust electrode for oxygen conducting electrolytes. This study presents, for the first time, the results of investigations on a double-layered composite electrode based on Sr2Fe1.5Mo0.5O6-δ in contact with a barium-free proton-conducting electrolyte La0.9Sr0.1ScO3-δ. It was found that the investigated electrode exhibits low polarization resistance about 0.15 and 0.22 Ohm cm2 in wet air and wet hydrogen at 600 °C, respectively. In addition, the electrode exhibits acceptable redox stability and CO2 tolerance. Also, the results of electrochemical impedance spectra analysis and a discussion on the parallel pathways of hydrogen oxidation and oxygen reduction reactions and the nature of rate-determining steps are presented in the manuscript. The obtained results show the high prospects for a Sr2Fe1.5Mo0.5O6-δ - based composite electrode for protonic ceramic electrochemical cells.

Evaluation of the effect of oxygen incorporation on the thermo–chemical–mechanical properties of the proton-conducting oxide BaZr0.8Yb0.2O3-δ

This study aims to evaluate the effects of multiple defect species produced by water and oxygen incorporation on the lattice parameters of BaZr0.8Yb0.2O3-δ (BZYb20) and model its chemical expansion considering these effects. In this study, the lattice constant of BZYb20 was investigated under oxygen partial pressure (pO2 = 1–10−3 bar), water vapor partial pressure (pH2O = 0.0023 bar and dry) and over a wide range of temperatures (473–1173 K) using two approaches: model calculations and high-temperature X-ray diffraction (HT-XRD). HT-XRD measurements revealed that the lattice expansion of BZYb20 changed within 700–900 K in a humidified atmosphere. In a nonhumidified atmosphere, this temperature range shifts lower. High-temperature gravimetric measurements of oxygen incorporation showed that the change in oxygen vacancy concentration was sufficiently small, and the effect of oxygen incorporation on the lattice constant was negligible. High-temperature gravimetric measurements of water incorporation yielded thermodynamic parameters for the hydration reaction. Based on these results, the HT-XRD results were reproduced by fitting a lattice constant model that neglects the effects of oxygen. Therefore, the effect of oxygen incorporation on chemical expansion can be ignored, and the chemical expansion can be estimated using the model developed in this study.

ZnO sintering additive without negative impact on proton-conducting SrHf0.8Sc0.2O3-δ electrolyte

Many proton-conducting oxides are refractory, making it difficult to fabricate dense electrolyte membranes for electrochemical devices. The use of a sintering additive is a common strategy, although it often has a negative effect on proton transport. We found that the addition of ZnO (2 and 4 mol%) successfully contributes to densification of SrHf0.8Sc0.2O3-δ ceramics at 1450 °C without negative consequences for the ionic transport. The chemical stability of the materials, high levels of hydration and proton conductivity were confirmed. ZnO is partially (less than 0.3 mol%) incorporated into the crystal lattice of the perovskite and partially accumulates at grain boundaries gradually volatilizing during high-temperature treatments. As a result, ZnO sintering additive does not influence the bulk and specific grain-boundary conductivities, ionic transference numbers and hydration ability, maintaining the excellent electrolytic properties of SrHf0.8Sc0.2O3-δ ceramics.

Revisiting the ionic conductivity of solid oxide electrolytes: A technical review

Oxygen-ionic and proton-conducting oxides are an important class of functional materials for their applications as electrolytes in various high-temperature electrochemical cells, including solid oxide fuel cells (SOFCs), solid oxide electrolysis...

A mixed proton–electron-conducting cathode with a Ru nanoparticle catalyst for electrochemical ammonia synthesis based on a proton-conducting BZCYYb electrolyte

Solid oxide proton conductor electrolysis cells, which operate at intermediate temperatures and utilize both heat and electrical potential, have emerged as a promising alternative to the traditional Haber–Bosch process.

Fundamental and technological aspects of thermochemical expansion of proton-conducting oxides: a case study of BaSn1−xScxO3−δ

This work provides insight into the thermal and chemical expansion behaviors of proton-conducting oxides, using Sc-doped BaSnO3 materials as examples. Both fundamental and applied aspects related to thermo-chemical expansion are thoroughly discussed.

High-entropy design in sintering aids for proton-conducting electrolytes of solid oxide fuel cells

Sintering aids are commonly used to improve the sinterability of proton-conducting oxides, whereas the sintering aid elements are being used alone in previous research. In this study, a high-entropy design method for the BaCe0.4Zr0.4Y0.2O3 (BCZY) proton-conducting oxides sintering aid is proposed. In contrast to the use of sintering aids such as Ni, Fe, Cu, Co, and Zn alone, the high-entropy design allows for the simultaneous use of all five elements, resulting in the new composition BaCe0.4Zr0.4Y0.15Ni0.01Cu0.01Co0.01Fe0.01Zn0.01O3 (HE-BCZY). Compared to conventional BaCe0.4Zr0.4Y0.15M0.05O3 (BCZYM, M = Ni, Cu, Co, Fe, Zn) materials, the sinterability of the new HE-BCZY material is significantly greater than that of BCZYM, despite the fact that the concentration of sintering aids is the same. Following sintering at 1300 °C, the HE-BCZY electrolyte membrane is completely dense. In addition, the HE-BCZY electrolyte has a higher conductivity than BCZYM. The low sintering temperature allows the successful preparation of proton ceramic fuel cells (PCFCs) with the HE-BCZY electrolyte, and the fuel cell reaches a high peak power density of 1218 mW cm−2 at 700 °C, which is much higher than that of PCFCs using BCZY-based electrolytes. In addition, the excellent chemical stability of HE-BCZY is maintained under both H2O and CO2 conditions. Consequently, a fuel cell employing the HE-BCZY electrolyte operates without detectable degradation for 200 h. This study demonstrates that although the high-entropy design is seldom used for sintering aids of proton-conducting oxides, it is an intriguing and promising strategy for creating high-performance electrolyte materials with good sinterability, high conductivity, and excellent stability.

Investigation of La0.65Ca0.35FeO3−δ as a Co-Free Cathode for Protonic Ceramic Fuel Cells with Yb-Doped BaZrO3 Electrolyte

In protonic ceramic fuel cells (PCFCs), the use of cobalt-free cathodes is expected to reduce manufacturing costs and improve durability performance. However, cobalt-free cathodes generally tend to have poor power generation performance due to their lower catalytic activity than cobalt-containing cathodes. Therefore, based on a previous study in which higher power generation performance was obtained with SOFCs using La0.65Ca0.35FeO3−δ (LCaF) than with SOFCs using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) containing cobalt, we focused on LCaF as a cobalt-free cathode. In this study, we attempted to apply cathodes with LCaF to anode-supported PCFCs using a BaZr0.8Yb0.2O3-δ (BZYb) electrolyte in order to improve the power generation performance and durability of PCFCs.The proton-conducting oxides used for composite cathodes were investigated. BZYb, which is the same as the electrolyte, was used, but the adhesion between the electrolyte and the cathode was poor. When, BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb), which has good sinterability compared with BZYb, was therefore used, reading to improvement in the adhesion. Moreover, process parameters such as composite ratio and sintering temperature were also optimized. As a result, a maximum power density of 0.4 W cm-2 at an operating temperature of 600 °C was obtained for PCFCs using a composite cathode of LCaF and BZCYYb at a weight ratio of 6:4 sintered at 1100 °C, which is equivalent to PCFCs using a cathode with LSCF. In addition, the voltage drop rate of PCFCs using a composite cathode of LCaF and BZCYYb in the constant current durability test (0.26 A cm-2) at an operating temperature of 600 °C was 4% per 1000 hours, less than 1/10 that of PCFCs using a cathode with LSCF. Based on these results, LCaF is one of the candidate materials of cobalt-free cathode for PCFCs.Acknowledgements: This work is based on the results obtained from a project (JPNP20003) commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Boosting Water Oxidation on Novel Oxygen Electrode Via Surface Metal Exsolution in Protonic Ceramic Electrochemical Cell

The protonic ceramic electrochemical cell (PCEC) is an emerging and attractive technology that converts energy to hydrogen using solid oxide proton conductors at intermediate temperatures. At this temperature range, the water oxidation reaction (WOR) and oxygen evolution reaction (OER) are dominant for the overall electrode reaction while the development of electrode materials is limited and primarily reply on the knowledge from the conventional oxygen-ion conducting counterpart. Here, we developed a novel Ruddlesden-popper (R-P) electrode that presented excellent electrocatalytic activity and durability, resulted in higher hydrogen production efficiency. The present study could provide a new perspective for materials design and bring more research opportunities in the field.

(Digital Presentation) (La0.7-xNdx )Sr0.3Co1-YFeyO3- δ (x = 0.35, y = 0.2) a Novel Perovskite Type Air Electrode Material for Protonic Conducting Solid Oxide Cell (H-SOC)

Proton conducting solid oxide cell (H-SOC) is a highly efficient energy conversion device converting electrical energy into hydrogen by splitting water using solid oxide proton conductor at intermediate temperature range (500-700 0C) [1]. To enhance electrochemical performances in terms of efficient hydrogen generation, robust and durable air electrode materials are required to facilitate faster water oxidation and oxygen reduction reaction, which are the key steps for both electrolyser and fuel cell at a lower temperature [2-3]. In the present study, we have developed ‘La’ and ‘Nd’ A-site co-doped strontium cobaltite and cobalt ferrite: La0.35Nd0.35Sr0.3Co0.8Fe0.2O3-δ(LNSCF) a perovskite type cathode material exhibiting higher electrical and electrochemical performances. The electrical and O2- ion conductivity of the material shows metallic nature, and the highest conductivity values are 973 S cm-1 and 0.318 S cm-1 respectively at 400 oC. The higher value of electrical and ionic conductivity is attributed to highly efficient Co2+ to Co3+ and Fe3+ to Fe4+ transition. Electrochemical impedance spectroscopy analysis of the symmetrical cell (LNSCF/Zirconium and Yttrium doped Barium Cerate (BZCY)/LNSCF) revealed the value of Area Specific Resistance (ASR) to be 0.14, 0.47 and 3.06 Ω cm2 at 800,700 and 600 oC respectively. When operated in H-SOFC mode with humidified hydrogen as fuel and Air (3%H2O) as oxidant, the LNSCF based single cell: Ni-BZCY/BZCY (385m)/ LNSCF+BZCY achieves a maximum current density of 522, 383, and 231mA/cm2 at 800, 700, and 600 oC, respectively at 0.002 V. Whereas, in H-SOEC mode a maximum current density of -1380, -862 and -517 mA/cm2 are obtained at 800, 700, and 600 oC, respectively at 2.5 V. The contribution of different ions (O2- and H2+) and electrons (e-) in the electrochemical performances are evaluated using ion-blocking cells. The figure (1) below represents the Current Density versus Voltage (i-V) characteristic attributes for both H-SOFC and H-SOEC.

Assessing the Impact of BaZr0.7Y0.3-YPryO3- d Composition on Water Splitting Kinetics in Composite Pr2NiO4+Δ-BaZr0.7Y0.3-YPryO3- d Electrodes in Proton-Conducting Electrolysis Cells

Protonic-ceramic electrolysis cells (PCECs), with proton-conducting oxide electrolytes, provide high current densities between 500-700°C because electrolytes such as BaCe0.8-xZrxY0.1Yb0.1O3-δ (BCZYYb) have protonic conductivities in this temperature range. The protonic conductivities of BCZYYb are almost an order of magnitude higher than oxide-ion conductivities in conventional solid-oxide electrolytes. To convert such high ionic fluxes in PCECs into efficient hydrogen production, robust positive electrodes (positrodes) must be developed with reduced overpotentials associated with water-splitting kinetics and proton incorporation [1]. The positrodes with Ruddlesden-Popper oxides like Pr2NiO4+δ(PNO) have demonstrated effective water-splitting kinetics as well as mechanical and chemical compatibility with barium zirconate electrolytes [2]. As a mixed conductor, PNO conducts polarons, oxide ions, and protonic defects (OH●) into the crystal structure [3]. However, its relatively low proton and oxide-ion conductivities limit charge transfer reaction near three-phase boundaries with the electrolyte, which increases positrode overpotentials. To improve the surface conductivity in PNO-based positrodes, our team, in collaboration with West Virginia University [4], has explored the impact of very thin overlayers of mixed ion-conducting BaZr0.7Y0.15Pr0.15O3- d (BZYP) on PNO positrode layers. Electrochemical characterization showed that the BZYP overlayers reduce the area-specific resistance of the PNO positrodes by ≈50% at 550-700°C: this suggests the extension of the electrochemically active surface area required for the water-splitting reaction beyond three-phase boundaries. The present study focuses on optimizing composite PNO-BZYP porous positrodes in conventional negatrode-supported PCECs to understand the water-splitting reaction mechanism for efficient electrolysis.Composite PNO-BaZr0.7Y0.3-yPryO3- d (BZYP) positrodes were fabricated for a range of Y/P cation ratios. The 22 mm diameter negatrode-supported button cells included 1 mm thick porous negatrodes of BaCe0.4Zr0.4Y0.1Yb0.1O3-δ(BCZYYb4411)-NiO and a spray-coated 5-10 μm thick BCZYYb4411 electrolytes as shown in figure 1. These negatrode-electrolytes were sintered at 1550°C for 6 h. The PNO and BZYP positrode powders were synthesized by solid-state reaction at 1350°C and 1500°C, respectively. PNO was identified as an orthorhombic crystal structure, and BZYP as a tetragonal crystal structure. Reactivity tests PNO and BZYP (1:1 wt ratio) at 1050°C, 1150°C, and 1250°C for 5 h showed no phase change. After negatrode-electrolyte sintering, both single-phase PNO (reference), and composite PNO-BZYP slurries were painted on the exposed electrolyte layers and sintered at 1150°C for 5 h to provide porous positrodes.Electrochemical impedance spectroscopy (EIS) and current-voltage measurements on BZYP-PNO positrode were compared with PNO positrode in the temperature range of 550-700°C. During these measurements, steam, and oxygen partial pressures were varied by 3-50%. EIS measurements informed the resistances associated with the different processes in the water-splitting reaction mechanism. The impact of both proton/oxygen ion and electron conductivity on the water-splitting kinetics could be identified based on different BZYP compositions by investigating the effects of temperatures, steam, and oxygen partial pressures. Different Y/P cation ratios in BZYP supported the investigation of how different (proton/oxygen ion and electron) conductivities in the BZYP impact the electrochemical water-splitting reaction. Figure 1

Solid-Oxide Amperometric Sensor for Hydrogen Detection in Air

An amperometric sensor based on CaZr0.95Sc0.05O3−δ (CZS) proton-conducting oxide for the measurement of hydrogen concentration in air was designed and tested. Dense CZS ceramics were fabricated through uniaxial pressing the powder synthesized by the solid-state method and sintering at 1650 °C for 2 h. The conductivity of CZS was shown to increase with increasing air humidity, which indicates the proton type of conductivity. The sensor was made from two CZS plates, one of which had a cavity was drilled to form an inner chamber, that were then pressed against each other and sealed around the perimeter to prevent gas leaking. The inner chamber of the sensor was connected with the outer atmosphere via an alumina ceramic capillary, which acted as a diffusion barrier. The sensor performance was studied in the temperature range of 600–700 °C in the mixtures of air with hydrogen. The sensor signal, or the limiting current, was found to linearly increase with the hydrogen concentration, which simplifies the sensor calibration. The sensor demonstrated a high sensitivity of ~60 μA per 1% H2 at 700 °C, a fast response, high reproducibility, good selectivity, and long-term stability.

Oxygen-Ion and Proton Transport of Origin and Ca-Doped La2ZnNdO5.5 Materials

Oxygen-ionic and proton-conducting oxides are widely studied materials for their application in various electrochemical devices such as solid oxide fuel cells and electrolyzers. Rare earth oxides are known as a class of ionic conductors. In this paper, La2ZnNdO5.5 and its Ca-doped derivatives La2Nd0.9Ca0.1ZnO5.45 and La2ZnNd0.9Ca0.1O5.45 were obtained by a solid-state reaction route. Phase composition, lattice parameters, and hydration capability were investigated by X-ray diffraction and thermogravimetric analyses. The conductivities of these materials were measured by the electrochemical impedance spectroscopy technique in dry (pH2O = 3.5 × 10−5 atm) and wet (pH2O = 2 × 10−2 atm) air. All phases crystallized in a trigonal symmetry with P3m1 space group. The conductivity difference between undoped and calcium-doped samples is more than two orders of magnitude due to the appearance of oxygen vacancies during acceptor doping, which are responsible for a higher ionic conductivity. The La2Nd0.9Ca0.1ZnO5.45 sample shows the highest conductivity of about 10−3 S∙cm−1 at 650 °C. The Ca-doped phases are capable of reversible water uptake, confirming their proton-conducting nature.

Probing Local Environments of Oxygen Vacancies Responsible for Hydration in Sc-Doped Barium Zirconates at Elevated Temperatures: In Situ X-ray Absorption Spectroscopy, Thermogravimetry, and Active Learning Ab Initio Replica Exchange Monte Carlo Simulations

Proton-conducting oxides, specifically heavily Sc-doped barium zirconate perovskite, have attracted attention as electrolytes for intermediate-temperature protonic ceramic fuel cells because of their high proton conductivity and high chemical stability against carbon dioxide in that temperature regime. Hydration is a key reaction for incorporating protons by filling oxygen vacancies, VO, with hydroxyl groups and activating proton conduction in the perovskite. However, probing the local environment of oxygen vacancies responsible for hydration is challenging because the behavior depends on the temperature and water partial pressure, which necessitates in situ observations and calculations of the local environments at elevated temperatures. To obtain such information, we combined in situ X-ray absorption spectroscopy (XAS) for both the Sc and Zr K-edges, thermogravimetry, X-ray diffractometry, and active learning ab initio replica exchange Monte Carlo (RXMC) simulations in undoped and 20–40 at % Sc-doped barium zirconates at and below 800 °C. The presence of oxygen vacancies adjacent to Sc and Zr in the dehydrated samples and the hydration of these oxygen vacancies under a wet atmosphere were probed by in situ XAS for Sc and Zr pre-edges at elevated temperatures. Here, the microscopic hydration linearly responds to the macroscopic degree of hydration. RXMC sampling further supports the presence of Sc-VO-Zr and Sc-VO-Sc environments. An initial hydration occurs in the Sc-VO-Zr environment at and above 600 °C, but the Sc-VO-Sc environment contribution is greater at higher degrees of hydration. The Zr-Vo-Zr environment is the least abundant among them for the whole temperature range examined and thus has a negligible impact.

Microwave sintering coupled with sintering aids for proton-conducting oxide membranes

The BaCe0·4Zr0·4Y0·2O3 (BCZY) was modified with the NiO sintering aid and prepared by a microwave sintering (MVS) method as the electrolyte for proton-conducting solid oxide fuel cells (H–SOFCs). The incorporation of NiO into the BCZY lattice improves the sinterability of the material, leading to the increase of the shrinkage from 3% to 26% after the high-temperature sintering. The dense BaCe0·4Zr0·4Y0·15Ni0·05O3 (BCZYN) pellet can be obtained after sintering at 1500 °C, whereas the Ni-free BCZY is porous after sintering at the same condition. By further using the MVS method, the BCZYN can be densified at 1300 °C, which is 200 °C lower than conventional sintering. Furthermore, the microwave-sintered BCZYN has a higher conductivity than the conventionally sintered one due to the larger grain size. Using the MVS method in the H–SOFCs fabrication, the high porosity of the anode substrate can be retained. In contrast, the cell prepared by the conventional sintering (CS) method has a relatively dense anode, which is detrimental to gas fuel transportation. As a result, the microwave sintered cell exhibits an output of 1010 mW cm−2 at 700 °C, which is significantly larger than that for the conventionally sintered cell, being 535 mW cm−2 at the same testing condition, suggesting the advantage of using the MVS method for the fuel cell fabrication.

Ionic and electronic transport of dense Y-doped barium stannate ceramics for high-temperature applications

The search for new oxide materials with pronounced proton transport is of great importance for the fabrication of electrochemical devices capable of realizing various energy conversion processes with high efficiency. The state-of-the-art proton-conducting oxides based on Ba(Ce,Zr)O3 have been proposed as the most promising electrolytes for such devices, although they suffer from a number of drawbacks. Here, we report an in-depth analysis of the transport properties of Y-doped barium stannates as relatively new proton-conducting compounds. In detail, the single-phase BaSn1–xYxO3–δ (0 ≤ x ≤ 0.4) ceramics were successfully prepared and their electrochemical properties were analysed using a number of characterization methods, including conductivity and thermo-EMF measurements depending on oxygen partial pressure as well as electrochemical impedance spectroscopy. The data obtained clearly indicate that the Y-doping degree affects the transport properties of BaSn1–xYxO3–δ. The weakly doped stannates (0 ≤ x ≤ 0.15) are identified as pronounced n- and p-type electron conductors, while the heavily doped stannates (0.2 ≤ x ≤ 0.4) exhibit a higher ionic conductivity and a wider electrolytic domain boundary, which allows them to be considered as new potential electrolytes with good grain boundary conductivity for application in both protonic ceramic fuel and electrolysis cells.

Anodic Reaction in Syngas-Fueled Proton-Conducting Solid Oxide Fuel Cells

Anode-supported proton-conducting solid oxide fuel cells (PC-SOFCs) fabricated with two representative proton-conducting oxides, BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) and BaZr0.8Y0.2O3-δ (BZY), were compared to obtain the insight into the electrochemical performances when fueled with syngas at 700°C and the correlation between the anode thickness (0.4, 0.8, and 1.6 mm) and operational stability. We have demonstrated that, in stability tests, the BCZYYb cells exhibited significantly higher maximum power density (MPD) than the BZY cells when operating on H2, 1.22 and 0.48 W/cm2 for the BCZYYb and BZY cells, but the BCZYYb cells degraded more rapidly than the BZY cells when operating on syngas. In addition, decreasing the anode thickness significantly enhanced the stability of BCZYYb cells operating on syngas, a reduction of 81, 76, and 71% in MPD for 1.6, 0.8, and 0.4 mm anode after 30 min. The electrochemical impedance spectra and X-ray diffraction patterns indicated that the rapid degradation of cerate-based cells with syngas could be mainly attributed to the considerable increase in polarization resistance due to the phase decomposition of the electrolyte powder in the anode. Heterogeneous catalysis was performed to study the catalytic reaction of the H2–CO mixture over anode powders prepared with the two proton-conducting oxides (Ni-BCZYYb and Ni-BZY) in a fixed bed reactor, and the CO conversion and selectivity to CO2 and CH4 were determined. For all anode powders, continuous CO2 production was initially observed with CH4 formation, and no significant difference in catalytic activity trends was observed between both anode powders. An increased residence time substantially decreased the normalized CO2 yield, which was associated with the potential secondary reaction of CO2 with H2, CH4, or perovskite oxide. Based on the results of heterogeneous catalysis with both anode powders, the observed cell degradation on both cells during operation with syngas may be primarily attributed to carbon coking due to CO disproportionation; however, more than 4 times rapid degradation of the cerate-based cell when operating with syngas was clearly demonstrated to be attributed to the decomposition of the electrolyte powder in anode by the resulting CO2 from the catalytic reaction of the H2–CO mixture.