High Temperature Proton Conductors Research Articles

Improving the Performance of High Temperature Protonic Conductor (HTPC) Electrolytes for Solid Oxide Fuel Cell (SOFC) Applications

This work investigated the possibility of coupling the high conductivity of cerates and the good chemical stability of zirconates as proton conductor electrolytes for solid oxide fuel cells (SOFCs). Two different approaches are discussed: the synthesis of barium cerate and zirconate solid solutions, and the fabrication of a bilayer electrolyte made of a Y-doped barium cerate pellet covered by a thin protecting layer of Y-doped barium zirconate. The chemical stability of the tailored samples was tested exposing them to 100% CO2 atmosphere at 700°C for 3 h. X-ray diffraction (XRD) analysis was used to investigate the phase composition of the specimens before and after the CO2 treatment. Electrochemical impedance spectroscopy (EIS) measurements were carried out in humidified H2. Hydrogen-air breathing fuel cell experiments were carried out at 700°C.

Exploring Highly Yttrium Doped Barium Zirconate Proton Conductor Electrolytes for Application in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs)

High temperature proton conductor (HTCP) perovskites are gaining growing attention as electrolyte materials for application in intermediate temperature solid oxide fuel cells (IT-SOFCs). In particular, Y-doped barium zirconate (BZY) electrolyte has been recently the subject of several studies because of its good chemical stability. To investigate the influence of Y concentration on the electrical properties of BZY, BaZr1-xYxO3-δ (0.3 {less than or equal to} x {less than or equal to} 0.6) oxides were synthesized using a wet chemistry process to obtain powders with controlled compositional homogeneity and purity at low processing temperatures. The synthesized powders were characterized by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis. Dense electrolytes were obtained pressing the calcined powders into cylindrical pellets and then sintering at 1600 {degree sign}C for 12 h. Electrical conductivity of the sintered pellets was measured as a function of the temperature and the Y concentration by electrochemical impedance spectroscopy (EIS) measurements.

The long term stability of the titanium/hydrogen system as the solid reference electrode in high-temperature proton conductor-based hydrogen sensors

The electrochemical hydrogen sensor comprised of the high temperature proton conductor and the solid reference material of Ti/TiH2 mixture showed the good sensing characteristics at elevated temperatures. However, in the long run, the EMF drift of about 0.2-0.3 mV/h was observed at 973 K. It is thought that the raise of EMF is due to the increase of the reference hydrogen pressure and the lack of the stability of the solid reference material is very critical to cause this phenomenon. In the present study, the long term stability of the solid reference material is investigated by EMF measurement with time.

High-temperature crystal structures and chemical modifications inRbH2PO4

We have used laboratory and synchrotron x-ray diffraction to investigatethe structural and chemical changes undergone by polycrystallineRbH2PO4 upon heatingwithin the 30–250 °C temperature interval. Our data show no evidence of the previously reported onset of partial polymerizationat T = 96 °C (Park et al 2001 J. Phys.: Condens. Matter 13 9411) which was proposed as an explanationfor the high-temperature proton conductivity enhancement in phosphate-based solid acids.Instead, we found that a tetragonal monoclinic polymorphic transition initiates atT≈90 °C. The transition iscomplete at T≈130 °C, andthe new monoclinic RbH2PO4 polymorph is stable upon further heating toT = 200 °C. Moreover, its crystal structure is isomorphic to that of monoclinicCsH2PO4. This remarkable similarity suggests that the microscopic structures anddynamics responsible for the high-temperature superprotonic behavior ofRbH2PO4 could be the same as those of its Cs-based counterpart.

Electrochemical promotion of a platinum catalyst supported on the high-temperature proton conductor La 0.99Sr 0.01NbO 4− δ

The recently discovered, high-temperature proton conductor, La 0.99Sr 0.01NbO 4− δ , was used as a support for the electrochemical promotion of a platinum catalyst. Ethylene oxidation was used as a probe reaction in the temperature range 350–450 °C. Moderate non-Faradaic rate modification, attributable to a protonic promoting species, occurred under negative polarisation; some permanent promotion was also observed. In oxidative atmospheres, both the p O 2 of the reaction mixture and the temperature influenced the type and magnitude of the observed rate modification. Rate-enhancement values of up to ρ = 1.4 and Faradaic-efficiency values approaching Λ = −100 were obtained. Promotion was observed under positive polarisation and relatively dry, oxygen-rich atmospheres suggesting that some oxygen ion conductivity may occur under these conditions. Impedance spectroscopy performed in atmospheres of 4 kPa O 2/N 2 and of 5 kPa H 2/N 2 under dry and slightly humidified (0.3 kPa H 2O) conditions indicated that the electrical resistivity is heavily dominated by the grain-boundary response in the temperature range of the EPOC studies; much lower grain-boundary impedances in the wetter conditions are likely to be attributable to proton transport.

High-temperature proton conductors with structure-disordered oxygen sublattice

Data on the thermogravimetry, spectroscopy, and electrical charge transfer as functions of T, aH2O, and aO2 for niobates and tantalates of alkali-earth metals with structure disordering of the oxygen sublattice, which can show high-temperature proton conduction, are summarized. It is shown that in the solid solution series with decreasing x (that is, with the increasing of the oxygen vacancies concentration) the proton conductivity increase, which is caused by the increasing of both the concentration of proton defects formed in the structure (in compliance with the formula Sr6 − 2xM2 + 2x+5O10(OH)2−6x and their mobility. The proton transfer dominates for the compositions with x < 0.15 at temperatures below 550°C. In the solid solutions (Ba1−yCay)6Nb2O11 (0.23 ≤ y ≤ 0.47) characterized by equal concentration of oxygen vacancies, with the increasing of barium content (correspondingly, with the increasing of the lattice parameter) the oxygen-ion conductivity (at aH2O = 3 × 10−5) grows monotonically, which is caused by the decreasing of the oxygen atom migration energy and increasing of their mobility. In this series, the proton conductivity (at aH2O = 2 × 10−2) increased. It was shown, by using IR-spectroscopy and the 1H NMR method, that the protons exist in the complex oxide structure mainly as energy-wise nonequivalent OH− groups: isolated, closely set, and paired, whose quantitative ratios are determined by the coordination preference of the B-sublattice elements.

The study of the voltage drift in high-temperature proton conductor-based hydrogen sensors adopting the solid reference electrode

The long-term stability of the electrochemical hydrogen sensor comprised of the high-temperature proton conductor of CaZr 0.9In 0.1O 3− δ and the solid reference material of Ti/TiH 2/TiO mixture has been investigated. In the long-term stability test, the EMF drift of about 0.2–0.3 mV/h was observed. It may be caused by the extremely low oxygen partial pressure of 10 −31 atm at 973 K evolved at the reference electrode due to the oxygen equilibrium reaction between TiO 2 and Ti 2O 3. It gives rise to n-type conduction at the reference electrode compartment, which then leads to a depletion of protons in the electrolyte as confirmed by the Raman spectra. The origin of the EMF drift is considered to result from the gradual increase in the hydrogen pressure at the reference compartment owing to the released protons from the electrolyte.

Zirconium doping effect on the performance of proton-conducting BaZryCe0.8−yY0.2O3−δ (0.0≤y≤0.8) for fuel cell applications

Abstract High-temperature proton conductors are promising electrolytes for protonic solid oxide fuel cells (H+-SOFCs). In this study, the relationship between the Zr doping content and structure, chemical stability, carbon dioxide resistivity, sinterability and electrochemical properties of BaZryCe0.8−yY0.2O3−δ (BZCYy), 0.0 ≤ y ≤ 0.8, are studied systemically using XRD, CO2-TPD, SEM, EIS and I–V polarization characterizations. Zr doping suppresses carbonate formation, CO2-TPD demonstrates that the formative rate of carbonate over BZCYy are 7.50 × 10−6 and 8.70 × 10−7 mol m−2 min−1 at y = 0.0 and 0.4, respectively. Investigation of sinterability shows that the anode-supported configuration helps the sintering of the thin-film electrolyte. Peak power densities of 220 and 84 mW cm−2 are obtained at 750 and 450 °C, respectively, with BZCY0.4 electrolyte. Due to the favorable chemical stability against CO2 and good sintering in the thin-film configuration, BZCY0.4 is a potential electrolyte material for H+-SOFCs.

Hydrogen pump using a high-temperature proton conductor for nuclear fusion engineering applications

In a nuclear fusion plant, the development of hydrogen processing technologies is an important issue since hydrogen isotope gas must be utilized as fuel. High-temperature proton conductors are favorable materials for hydrogen processing systems in nuclear fusion engineering. Possible applications of proton conductors include hydrogen pumps and hydrogen sensing. For one of these protonic functions, we have proposed applying a hydrogen pump to hydrogen recovery in the hydrogen processing system. In this paper, as a feasibility study, a one-end closed tube of SrZr 0.9Yb 0.1O 3 − α proton conductor was evaluated as a hydrogen pump at elevated temperatures. As a result, hydrogen gas could be extracted from a mixture of gases including hydrogen, water vapor and methane, simulating the gases present during hydrogen processing in a fusion device, while methane and water vapor could be decomposed to hydrogen gas on the platinum electrode. This result indicates that a hydrogen pump using a proton conductor is quite useful in the hydrogen recovery system for the exhaust gas of a fusion device.

Influence of ZnO addition on the properties of high temperature proton conductor Ba1.03Ce0.5Zr0.4Y0.1O3−δ synthesized via citrate–nitrate method

Abstract The effect of various amounts of ZnO addition on the densification behavior, electrical conductivity and chemical stability of high temperature proton conductor Ba 1.03 Ce 0.5 Zr 0.4 Y 0.1 O 3− δ synthesized through citrate–nitrate method was investigated. Small amount of ZnO could considerably promote the densification process and the grain growth of Ba 1.03 Ce 0.5 Zr 0.4 Y 0.1 O 3− δ . Samples with 0.5 wt.% ZnO achieved a relative density of 95.6% at 1250 °C, great higher than that without ZnO addition (50.4%). Field emission scanning electron microscopy observation revealed that ZnO addition led to the segregation of Y 2 O 3 from the matrix. Part of ZnO existed at the grain boundary, its role was to form a eutectic melt to promote the densification process. A small part of ZnO dissolved into the lattice, which should be mainly responsible for the increase of bulk conductivity of Ba 1.03 Ce 0.5 Zr 0.4 Y 0.1 O 3− δ with increasing ZnO level. ZnO also enhanced the grain boundary conductivity and increased the stability of Ba 1.03 Ce 0.5 Zr 0.4 Y 0.1 O 3− δ , especially against boiling water.

High Temperature Protonic Conduction and Crystal and Defect Structures in La1-xBa1+xGa1-yMgyO4-d

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Laser processed protonic ceramics

High temperature protonic conductors of SrCe 0.9Y 0.1O 3− δ , Sr 3Ca 1+ x Nb 2− x O 9− δ and BaCe 0.85Y 0.15O 3− δ were fabricated by laser processing. Laser float zone method and pulse laser deposition were used to fabricate dense high temperature protonic ceramic rods and films. Melt growth processing by laser float zone produced textured microstructures with cellular characteristics. Directional solidified SrCe 0.9Y 0.1O 3− δ contained an aluminium rich inter-granular phase, whereas Sr 3Ca 1+ x Nb 2− x O 9− δ exhibits a cellular microstructure with a decreasing Ca/Nb ratio from the cell center to the cell boundary. A low Ca/Nb ratio is detrimental to proton transport. Nano-domains caused by oxygen cage tilting were observed in both compositions. Sr 3Ca 1+ x Nb 2− x O 9− δ also exhibits stoichiometric domains with an ordered distribution of Nb 5+ and Ca 2+ cations. These domains were surrounded by non-stoichiometric domains containing a random cation distribution, which preserves the global stoichiometry and charge neutrality. Electrical impedance spectroscopy revealed similar protonic transport to published data for sintered compositions. Crystalline BaCe 0.9Y 0.1O 3− δ films were fabricated on porous substrates at deposition temperatures ranging from 400 to 950 °C. Crystalline films 1–10 μm thick were deposited by excimer laser on porous Al 2O 3 and BaZrO 3 substrates. Single-phase BaCe 0.85Y 0.15O 3− δ films with a columnar growth morphology are observed with preferred crystal growth along the [1 0 0] or [0 0 1] direction. The film electrical conductivity was measured perpendicular to the film growth direction. Impedance measurements show the importance of matching crystal symmetry between substrate and film. High protonic conducting films were achieved by deposition on BaZrO 3.

High conductivity and chemical stability of BaCe1−x−yZrxYyO3−δ proton conductors prepared by a sol–gel method

High-temperature proton conductors are promising as electrolytes for intermediate-temperature solid oxide fuel cells. Among them, BaCeO3-based materials have high proton conductivity but rather poor chemical stability. In contrast, barium zirconates are rather stable, but have poorly reproducible densities and conductivities. In this study, the investigation of BaCe1−x−yZrxYyO3−δ solid solutions (x = 0, 0.10, 0.20, 0.30, 0.40; y = 0.15, 0.20) was undertaken, with the final aim of finding a composition having both high conductivity and good stability. The influence of the modified sol–gel Pechini synthetic approach on the powder morphology, and of a barium excess on the densification were demonstrated. Single-phase perovskite powders were prepared and high density pellets were obtained at temperatures lower than those commonly employed. Stability tests demonstrated that the Zr introduction into doped barium cerate greatly enhanced the chemical stability, particularly for Zr ≥ 20%. The proton conductivities, measured in a humidified H2/Ar atmosphere by impedance spectroscopy, were only slightly influenced by the Zr amount. Overall, BaCe1−x−yZrxYyO3−δ solid solutions having Zr ≈ 20–40% and Y ≈ 15–20% showed good chemical stability and high conductivity.

Hydrogen diffusion in high temperature proton conducting ceramics

BaCeO3 or SrCeO3-based perovskites doped with a rare earth are high temperature protonic conductors (HTPC) envisioned as electrolytes for fuel cells working at intermediate temperature (400–600°C). In these ceramics, the proton conductance is hampered by microstructural defects that act as barriers for hydrogen diffusion. Respective contributions of bulk and grain boundaries to overall conductivity is usually evidenced via impedance measurements but further information on hydrogen transport relevant for improvement of microstructure design can be obtained with nuclear microanalysis, based on the use of MeV light ions microbeam. We report here a contribution of ion beam microanalysis to the study of hydrogen transport in BaCe0.9Y0.1O3. ERDA hydrogen profiling performed on partially hydrated samples at 200 and 500°C reveals concentration gradients from which diffusion coefficients have been derived with the help of a simple Fickian diffusion model.

Conductivity studies of dense yttrium-doped BaZrO 3 sintered at 1325 °C

High-temperature proton conductors have wide applications in the areas of fuel cells, electrolysis and hydrogen separation. Barium zirconate-based materials are of interest due to their good stability and high protonic conductivity. The reported conductivity of these ceramic materials is generally less than 10 −2 S/cm, even at high temperatures. This is not high enough for an electrolyte-supported device to achieve an ASR of less than 0.2 Ω cm 2 therefore thin film electrolytes are required for successful application. As BaZrO 3-based materials have to be sintered at temperatures as high as 1700 °C, this makes it difficult to find a suitable supporting electrode which will not undergo significant chemical reaction with the BaZrO 3-based electrolyte during fabrication of the required electrode supported electrolyte. In this paper, proton-conducting BaZr 0.8Y 0.2O 2.9 was successfully sintered at 1325 °C with a relative density of 96% via addition of 1 wt% ZnO. Fabrication of electrochemical cells using proton-conducting BaZr 0.8Y 0.2O 2.9 as the electrolyte thus becomes possible. The formula of the 1 wt% ZnO added sample is Ba 0.97Zr 0.77Y 0.19Zn 0.04O 3− δ which exhibits a tetragonal structure with space group P4/ mbm (127); a=5.9787(1) Å, c=4.2345(1) Å, V=151.36(1) Å 3. It was found that a solid solution was formed for a limited range of Zn doping. Conductivity has been studied as a function of atmosphere (air, dry and wet 5% H 2/Ar) with the changes in bulk and grain boundary on changing atmosphere being monitored as a function of time. The total conductivity of Ba 0.97Zr 0.77Y 0.19Zn 0.04O 3– δ is 1.0×10 −3 S/cm above 600 °C therefore it may be used as a proton-conducting thin film electrolyte for efficient electrochemical devices at such temperatures. The grain boundary resistance is insignificant at high temperature for the well-sintered sample.

Monitoring of water dissolution into high temperature protonic conductors

Water dissolution into a doped strontium cerate ceramics was monitored with Raman spectroscopy. Since a Raman band at 630 cm − 1 is observed when the ceramics possess oxygen vacancies, and a relative intensity of the band to that of the main band at 345 cm − 1 ( I 630/ I 345) is proportional to its oxygen vacancy concentration, the parameter I 630/ I 345 was examined for the understanding of water dissolution behavior into the lattice. The parameter for quenched samples after annealing decreased gradually with an increase in water vapor partial pressure, P(H 2O) during annealing due to water dissolution, resulting in disappearance of some oxygen vacancies. An in situ monitoring of water dissolution into this ceramics was also performed employing an atmosphere-controlled system.

Hydrogen separation from syngas using high-temperature proton conductors

Hydrogen pumps using high-temperature proton conductors have been examined as a candidate means of hydrogen separation from syngas. Durability to CO2 is a main concern for the alkali-earth-containing perovskites and was tested for a few typical compositions. Ce-excluded and cerate–zirconate solid solution electrolytes, SrZr0.9Y0.1O3-α, and BaCe0.6Zr0.3Nd0.1O3-α were stable in CO2-containing atmospheres, whereas they showed poor overpotential characteristics when platinum electrodes were used. It is demonstrated that the hydrogen pumping properties can be much improved for the case of SrZr0.9Y0.1O3-α by the use of palladium anode and SrCe0.95Yb0.05O3-α interlayer for the cathode.

Carbon deposition during propane dehydrogenation in a fuel cell

Carbon deposition in a high temperature proton-conducting fuel cell for selective propane dehydrogenation to propylene with co-generation of electrical power was investigated. Comparison of carbon deposition was made for catalytic propane dehydrogenation under an open circuit condition and electro-catalytic conversion of propane to propylene on the anode catalyst of synthesized chromium(III) oxide during fuel cell operation conditions with current flow. Carbon deposition under the fuel cell operating conditions was much less than that under the open circuit conditions. Chromium(III) oxide catalyst modified by potassium showed a better resistance to carbon formation under the open circuit conditions but was similar to the unmodified catalyst under current flow conditions.

Ceramic Proton and Mixed Proton-Electron Conductors in Membranes for Energy Conversion Applications

Ceramic high temperature proton conductors offer in principle great advantages over oxide ion conductors and “proton conducting” polymers as electrolytes in fuel cells. This is related to the absence of product water on the anode, and simplified overall water management. The state-of-the-art proton conducting perovskite oxide materials are too basic to be stable in operation with reformed fossil fuels, and new classes of more stable proton conducting oxides have been discovered—so far with more moderate proton conductivities, though. Also new mixed proton-electron conducting oxides are being uncovered, and it is shown how they can be utilized as membranes in fossil fuelled power plants with CO2 sequestration.

Nanoionics phenomenon in proton-conducting oxide: Effect of dispersion of nanosize platinum particles on electrical conduction properties

High-temperature proton conductors are oxides in which low-valence cations are doped as electron acceptors; the incorporation of water molecules into the oxides results in the formation of protonic defects that act as charge carriers. Since the protons thus formed are in equilibrium with other electronic defects, electrons and holes, the oxides possibly have different proton-conduction properties at and near boundaries when they are in contact with another phase. In this paper, we present our recent experimental observation of a marked change in the electrical properties of a proton conductor upon the dispersal of fine platinum particles in the oxide. First, the material shows extremely low electrical conductivity in comparison with the original proton-conducting perovskite. Second, there was a threshold amount of platinum at which such a drop in conductivity occurred. A percolation model is employed to explain these experimental results; the fine platinum particles dispersed in the proton-conducting oxide wears highly resistive skin that is formed due to shifts in defect equilibriums, which prevents ionic/electronic conduction. The experiments suggest that the ion-conducting properties of oxides can be varied by introducing interfaces at a certain density; nanoionics is a key to yielding enhanced and/or controlled ionic conduction in solids.