Proton conductivities and structures of BaO–ZnO–P2O5 glasses in the ultraphosphate region for intermediate temperature fuel cells

Phosphate glasses are promising materials for electrolytes of intermediate temperature fuel cells, because they have good proton conductivity at 150–250 °C. However, the effects of the glass composition and melting condition on proton conductivities are unclear yet. In this work, the structures of BaO–ZnO–P₂O₅ glasses were investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) and Raman spectroscopy, and the proton conductivities were measured by an AC impedance method. The proton conductivity of 30 mol%ZnO-70 mol%P₂O₅ glass melted at 800 °C reached 1 × 10⁻³ S/cm at 250 °C for. The proton transportation number of the ZnO–P₂O₅ glass was almost unity, confirmed by a hydrogen concentration cell. The power density of 0.4 mW/cm² was obtained for a fuel cell using the ZnO–P₂O₅ glass electrolyte at 250 °C. A branching phosphate structure was transformed into a middle phosphate structure by substituting BaO with ZnO, which caused an improvement in proton mobility.     

Effects of Nb,Mo, and W dopants on the germanate-based apatites as electrolyte for use in solid oxide fuel cells

Rare information is available in the literature on the cell performance of the solid oxide fuel cells (SOFCs) using apatites known for their good electrical conductivity as electrolyte materials. In this study, La_9.5Ge_5.5Nb_0.5O_26.5, La_9.5Ge_5.5Mo_0.5O_26.75, and La_9.5Ge_5.5W_0.5O_26.75 ceramics were prepared and characterized. The results indicated that the La_9.5Ge_5.5Nb_0.5O_26.5 and La_9.5Ge_5.5W_0.5O_26.75 ceramics reported hexagonal phase, while the La_9.5Ge_5.5Mo_0.5O_26.75 ceramic demonstrated triclinic symmetry. Among the apatities evaluated, La_9.5Ge_5.5Nb_0.5O_26.5 sintered at 1450 °C showed the best conduction with an electrical conductivity value of 0.045 S/cm at 800 °C. Button cells of NiO–SDC/La_9.5Ge_5.5Nb_0.5O_26.5/LSCF–SDC were built and revealed good structural integrity. The total ohmic resistance (R₀) and interfacial polarization resistance (R_P) of the cell read 0.428 and 0.174 Ω cm² and 0.871 and 1.164 Ω cm², respectively at 950 and 800 °C. The maximum power densities (MPD) of the single cell at 950 and 800 °C were respectively 0.363 and 0.095 W cm⁻². Without optimizing the anode and cathode as well as hermetic sealing of the cell against the gas, the study found the performance of the single cell with the pure La_9.5Ge_5.5Nb_0.5O_26.5 as its electrolyte material superior to those of the SOFC cells with a YSZ electrolyte of comparable thickness shown in the literature.     

Characterization and evaluation of BaCo0.7Fe0.2Nb0.1O3−δ as a cathode for proton-conducting solid oxide fuel cells

This study characterizes BaCo_0.7Fe_0.2Nb_0.1O_3−δ (BCFN) perovskite oxide and evaluates it as a potential cathode material for proton-conducting SOFCs with a BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) electrolyte. A four-probe DC conductivity measurement demonstrated that BCFN has a modest electrical conductivity of 2-15 S cm⁻¹ in air with p-type semiconducting behavior. An electrical conductivity relaxation test showed that BCFN has higher D_chem and K_chem than the well-known Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃₋δ oxide. In addition, it has relatively low thermal expansion coefficients (TECs) with values of 18.2 × 10⁻⁶ K⁻¹ and 14.4 × 10⁻⁶ K⁻¹ at temperature ranges of 30-900 °C and 30-500 °C, respectively. The phase reaction between BCFN and BZCY was investigated using powder and pellet reactions. EDX and XRD characterizations demonstrated that BCFN had lower reactivity with the BZCY electrolyte than strontium-containing perovskite oxides such as SrCo_0.9Nb_0.1O_3-δ and Ba_0.6Sr_0.4Co_0.9Nb_0.1O₃₋δ. The impedance of BCFN was oxygen partial pressure dependent. Introducing water into the cathode atmosphere reduced the size of both the high-frequency and low-frequency arcs of the impedance spectra due to facilitated proton hopping. The cathode polarization resistance and overpotential at a current density of 100 mA cm⁻² were 0.85 Ω cm⁻² and 110 mV in dry air, which decreased to 0.43 Ω cm⁻² and 52 mV, respectively, in wet air (∼3% H₂O) at 650 °C. A decrease in impedance was also observed with polarization time; this was possibly caused by polarization-induced microstructure optimization. A promising peak power density of ∼585 mW cm⁻² was demonstrated by an anode-supported cell with a BCFN cathode at 700 °C.     

Hydrogen absorption of catalyzed magnesium below room temperature

Hydrogen absorption of magnesium (Mg) catalyzed by 1 mol% niobium oxide (Nb₂O₅) was demonstrated under the low temperature condition even at −50 °C. The kinetic and thermodynamic properties were examined for MgH₂ with and without Nb₂O₅. By considering the remarkable absorption features at such low temperature, the essential hydrogen absorption properties were investigated under accurate isothermal conditions. As the results, the activation energy of hydrogen absorption for the catalyzed Mg was evaluated to be 38 kJ/mol, which was significantly smaller than that of MgH₂ without the catalyst. The kinetic improvement was also found on the hydrogen desorption process. On the other hand, thermodynamic properties were not changed by the catalyst as a matter of course. Therefore, the Nb₂O₅ addition mainly affects the reaction rates between Mg and hydrogen and shows the excellent catalytic effects.     

Ba1−xCo0.9−yFeyNb0.1O3−δ (x = 0-0.15, y = 0-0.9) as cathode materials for solid oxide fuel cells

Perovskite oxide Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3−δ has been reported as oxygen transport membrane and cathode material for solid oxide fuel cells (SOFCs). In this study, the effects of A-site cation deficiency and B-site iron doping concentration on the crystal structure, thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance of Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.15, y = 0-0.9) have been systematically evaluated. Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.10, y = 0.2 and x = 0.10, y = 0.2-0.6) can be indexed to a cubic structure. Increased electrical conductivity and decreased cathode polarization resistance have been achieved by A-site deficiency. No obvious variation can be observed in TEC by A-site deficiency. The electrical conductivity and TEC of Ba_0.9Co_0.9−yFe_yNb_0.1O_3−δ decrease while the cathode polarization resistance increases with the increase in iron doping concentration. The highest conductivity of 13.9 S cm⁻¹ and the lowest cathode polarization resistance of 0.07 Ω cm² have been achieved at 700 °C for Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ. The composition Ba_0.9Co_0.3Fe_0.6Nb_0.1O_3−δ shows the lowest TEC value of 13.2 × 10⁻⁶ °C⁻¹ at 600 °C and can be a potential cathode material for SOFCs.     

Novel doped barium cerate–carbonate composite electrolyte material for low temperature solid oxide fuel cells

A composite of a perovskite oxide proton conductor (BaCe_0.7Zr_0.1Y_0.2O_3−δ, BCZ10Y20) and alkali carbonates (2Li₂CO₃:1Na₂CO₃, LNC) is investigated with respect to its morphology, conductivity and fuel cell performance. The morphology shows that the presence of carbonate phase improves the densification of oxide matrix. The conductivity is measured by AC impedance in air, nitrogen, wet nitrogen, hydrogen, and wet hydrogen, respectively. A sharp increase of the conductivity at certain temperature is seen, which relates to the superionic phase transition at the interface phases between oxide and carbonates. Single cell with the composite electrolyte is fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. The cell shows a maximum power density of 957 mW cm⁻² at 600 °C with hydrogen as the fuel and oxygen as the oxidant. The remarkable proton conductivity and excellent cell performance make this kind of composite material a good candidate electrolyte for low temperature solid oxide fuel cells (SOFCs).     

The effect of Cr deposition and poisoning on BaZr0.1Ce0.7Y0.2O3−δ proton conducting electrolyte

With the development of chromium tolerant electrode materials, the evaluation of the chromium deposition and poisoning on electrolyte is critical significance for the commercial and widespread application of solid oxide fuel cell stacks (SOFCs). The Cr deposition and poisoning on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) proton conducting electrolyte are initially studied, in order to understand and develop the compatibility for proton conducting SOFC (H-SOFCs). The XRD results imply that Cr₂O₃ is not chemically compatible with BZCY and BaCrO₄ is formed at high temperature above 600 °C. To simulate the Cr volatilization from interconnect and poisoning on BZCY surface, the BZCY bar sample is heat-treated in the presence of Cr₂O₃ at 600 °C, 700 °C, and 800 °C for 50 h. It is clear that Cr deposition occurs even at 600 °C by SEM examination. The XPS results indicate the chemical deposition of BaCrO₄ and physical deposition of Cr₂O₃ on BZCY surface at 600 °C but only chemical deposition at 700 °C and 800 °C. The content of Cr deposition increases with the increase of poisoning temperature. Moreover, the proton conductivity of BZCY after Cr deposition reduces after Cr deposition, indicating the Cr poisoning effect of the electrochemical performance of BZCY electrolyte.     

50 mol% indium substituted BaTiO3: Characterization of structure and conductivity

BaTi_0.5In_0.5O_3−δ was prepared by solid state reaction at 1400 °C. Rietveld analysis of high resolution X-ray powder diffraction data indicated phase pure as-prepared material that adopts a cubic perovskite structure with a = 4.1536(1) Å. Thermogravimetric analysis revealed the presence of significant levels of protons in the as-prepared material and 57% of the theoretically achievable protonation was attained on exposure to a humid environment at 185 °C. After hydration the cell parameter increased to 4.1623(1) Å. Electrical conductivity was measured both with fixed and variable frequency ac impedance methods as a function of temperature, oxygen-, water vapour- and heavy water vapour partial pressures. In the temperature range 400–800 °C a slight increase in the total conductivity with increasing oxygen partial pressure is encountered, characteristic of a contribution from p-type charge carriers. The effect of the water vapour pressure on conductivity below 600 °C is much more prominent indicative of dominant proton conduction. At 300 °C the total conductivity in wet O₂ was estimated to be 9.30 × 10⁻⁵ S/cm. At T > 800 °C the material is a pure oxide ion conductor.     

Nonmetal doping strategy to enhance the protonic conductivity in CaZrO3

Hydrogen energy is one of the most developing areas of clean energy due to various advantages of hydrogen compared to traditional fossil fuels. One of hydrogen energy electrochemical devices is proton-conducting solid oxide fuel cells. The obtaining of novel highly proton conductive materials is relevant. Nonmetal doping strategy to improve the protonic conductivity in perovskite-related materials is understudied. The phosphorous-doped perovskite CaZr_0.95P_0.05O_3.025 was obtained for the first time. The possibility for water uptake was proved by thermogravimetry and mass-spectrometry investigations. It was shown that phosphorous doping led to increase in the conductivity values up to 500 times. The composition CaZr_0.95P_0.05O_3.025 demonstrates nearly pure proton transport below 600 °C under wet air. The proton conductivity values are 3.3·10⁻⁶ S/cm at 670 °C and 7.6·10⁻⁷·S/cm at 500 °C. The nonmetal doping strategy is prospective way to enhance electrical conductivity of proton conductors with perovskite structure.     

Support influence on Ni-Cu catalysts behavior under ethanol oxidative reforming reaction

The ethanol oxidative reforming reaction was performed with Ni-Cu catalysts on different supports. The results indicated that Ni-Cu/Nb₂O₅ and Ni-Cu/ZnO were the most appropriate catalysts for the reaction regarding activity, stability, and selectivity for hydrogen production. Ni-Cu/Nb₂O₅ catalysts have strong acidity (at 600 °C), while ZnO has very low acidity. Ni-Cu/Ce_0.6Zr_0.4O₂ catalysts, which only have weak acidity (at 250 °C), presented poor stability and hydrogen selectivity. This shows that acidity has no influence on hydrogen production.     

Doping effects on complex perovskite Ba3Ca1.18Nb1.82O9−δ intermediate temperature proton conductor

In this work, the effects of Ce doping on the Ca and Nb ions in complex perovskite Ba₃Ca_1.18Nb_1.82O_9−δ (BCN18) proton conductor have been evaluated. It has been found that cerium ions can be doped into both the Ca and Nb sites to form a single-phase complex perovskite structure when the sintering temperature is 1550 °C. Ce ions substituted with Nb ions enhances the electrical conductivity, especially the grain boundary conductivity. The highest conductivity has been obtained for a composition of Ba₃Ca_1.18Nb_1.62Ce_0.2O_9−δ, possessing a conductivity of 2.69 × 10⁻³ S cm⁻¹ at 550 °C in wet H₂, a 78% enhancement compared with BCN18 (1.51 × 10⁻³ S cm⁻¹). The chemical stability tests show that Ce-doped BCN18 samples remain single phase after treated either in boiling water for 7 h or in pure CO₂ for 4 h at 700 °C. This work has demonstrated a new direction in developing intermediate temperature proton conducting materials that possess both high conductivity and good stability.     

Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO4

The sintering properties, crystal structure and electrical conductivity of La_1−xCa_xNbO_4−δ (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025), prepared by a solid-state reaction, have been investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and electrochemical impedance spectroscopy (EIS). In 2.5% Ca-doped samples, a small amount of impurities Ca₂Nb₂O₇ were observed from the XRD patterns. Impedance spectra show that the grain boundary resistance increases with increasing Ca content, while the bulk resistance remains essentially constant below 550 °C. Despite the higher degree of grain growth observed for higher Ca doping levels, the total conductivity of the La_1−xCa_xNbO_4−δ series decreases with increasing Ca content from 0.5 to 2.0 mol%. The activation energy for the total conductivity decreases with increasing Ca content from 0.71 eV (x = 0) to 0.54 eV (x = 0.01) for the high temperature tetragonal phase, then it increases to 0.60 eV for x = 0.02. For the monoclinic phase, the activation energy exhibits similar trend except La_0.995Ca_0.005NbO_4−δ shows the lowest value of 1.26 eV. The Ca and Nb content present at the grain boundaries for La_0.99Ca_0.01NbO_4−δ are much higher than that on the grain surface, as determined from the EDS analysis. These results imply that the solubility of CaO in LaNbO₄ is in the range from 0.5 to 1.0 mol%. By increasing the sintering temperature from 1500 °C to 1550 °C, the proton conductivity of the Ca-doped LaNbO₄ was improved with enlarged grain size due to a reduction in the resistive grain boundary contribution.     

Oxidation behavior of ferritic stainless steel containing Nb,Nb–Si and Nb–Ti for SOFC interconnect

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO₂ at the outermost oxide scale and as both Nb₂O₅ and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb₂O₅, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb₂O₅ precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb₂O₅ and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale.     

Solvothermal synthesis of La9.33Si6O26 nanocrystals and their enhancing impacts on sintering and oxygen ion conductivity of Sm0.2Ce0.8O1.9/La9.33Si6O26 composite electrolytes

La_9.33Si₆O₂₆ (LSO) nanocrystals, quasi-spherical in shape and of an average size ca. 45 nm, were synthesized via a solvothermal process and used to mix with Sm_0.2Ce_0.8O_1.9 (SDC) ultrafine powder to fabricate SDC–LSO composite electrolytes for the applications in IT-SOFCs. The microstructures and phase components of LSO nanocrystals and SDC–LSO composite electrolytes were characterized by XRD, TEM and HRTEM and, in particular, the sintering performance and oxide ion conductivity of SDC–LSO composites with different SDC/LSO volume ratios were studied. It has been found that the LSO nanocrystals are homogeneously dispersed in the as-sintered SDC–LSO composites and no impurity phases due to chemical reactions can be detected between SDC and LSO particles by XRD, but the sintering performance is remarkably improved with a temperature reduction by 100–250 °C, compared to that for the individual constituent phases. Moreover, the oxide ions conductivity of SDC–LSO composites can be conspicuously enhanced with the sample SL7525 (SDC/LSO = 0.75/0.25) showing the highest enhancement by 118%, i.e. 1.62 times that of SDC at 800 °C. The SDC/LSO hetero-interfaces with high energy and appropriate residual thermal stresses in the SDC–LSO composite microstructures are considered responsible for their improved sintering performance and significant enhancements in oxide ions conductivity.     

Influence of Pr substitution on defects,transport, and grain boundary properties of acceptor-doped BaZrO3

We report on effects of partially substituting Zr with the multivalent Pr on the conductivity characteristics of acceptor (Gd) doped BaZrO₃-based materials. BaZr_0.6Pr_0.3Gd_0.1O_3−δ was sintered 96% dense at 1550 °C with grains of 1–4 μm. The electrical conductivity was characterised by impedance spectroscopy and EMF transport number measurements as a function of temperature and the partial pressures of oxygen and water vapour. H₂O/D₂O exchanges were applied to further verify proton conduction. The material is mainly a mixed proton–electron conductor: the p-type electronic conductivity is ∼0.004 and ∼0.05 S/cm in wet O₂ at 500 and 900 °C, respectively, while the protonic conductivity is ∼10⁻⁴ S/cm and ∼10⁻³ S/cm. The material is expectedly a pure proton conductor at sufficiently low temperatures and wet conditions. The specific grain boundary conductivity is essentially equal for the material with or without Pr, but the overall resistance is significantly lower for the former. We propose that replacing Pr on the Zr site reduces the grain boundary contribution due to an increased grain size after otherwise equal sintering conditions.     

High-temperature oxidation process analysis of MnCo2O4 coating on Fe-21Cr alloy

Even though the operation temperature of solid oxide fuel cells (SOFCs) stacks has been reduced (∼750 °C), stainless steel interconnect within the stacks still requires protection by high conductive coatings to delay the growth of oxide scales and reduce chromium evaporation. Manganese cobaltite spinel protective coating with a nominal composition of MnCo₂O₄ was produced on Fe-21Cr stainless steel. Electrical, microstructural and compositional analysis were performed to investigate the interfacial reaction of MnCo₂O₄ protective coating with the stainless steel substrate during 750 °C oxidation process. The spinel coating not only acts as a barrier to Cr outward transport, but also improves the electrical conductivity of the alloy interconnect during long-term oxidation. The coated alloy demonstrates good electrical conductivity with an area specific resistance (ASR) of about 5 mOhm cm² after oxidation for 1000 h at 750 °C, which is about 1/4 of the ASR of bare Fe-21Cr alloy. The reduction of ASR might be caused by the fact that Cr migrated from the steel substrate interact with MnCo₂O₄ coating and generated Mn-Co-Cr spinel phase, which has higher electrical conductivity than that of Cr₂O₃.     

Proton conductivity and fuel cell performance of organic–inorganic hybrid membrane based on poly(methyl methacrylate)/silica

Organic–inorganic hybrid membranes based on poly(methyl methacrylate) (PMMA)/silica have been synthesized using a sol-gel technique for use in polymer electrolyte fuel cells (PEFCs). The properties of these membranes were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. The results indicate that these membranes are formed through hydrogen bonds between the carbonyl group of PMMA and the uncondensed alcohol functional groups of the inorganic clusters. The proton conductivity of these membranes is on the order of 10⁻¹ S cm⁻¹, and the 60PMMA–30SiO₂–10P₂O₅ membrane displays the highest proton conductivity of 3.85 × 10⁻¹ S cm⁻¹ at 90 °C and 50% RH. The performance of a fuel cell using these membranes was tested. A maximum power density of 370 mW cm⁻² is obtained at 80 °C, and the current density at 0.4 V remains almost unchanged during the 100-h test time under the test conditions. This class of hybrid membranes is an extremely promising material for use in PEFCs.     

Mechanochemical preparation,sintering aids and hybrid microwave sintering in the proton conductor Sr0.02La0.98Nb1-xVxO4-δ, x = 0, 0.15

Mechanochemical preparation of Sr_0.02La_0.98Nb_1-xV_xO_4-δ was demonstrated for values of x = 0 and 0.15. Crystallinity could be improved by calcining at 1073 K. On cooling, the high temperature scheelite phase was retained to room temperature. Several novel sintering additives for LaNbO₄ materials have been tested. The most successful were Cu₃Nb₂O₈ and CuV₂O₆, which reduced the temperature of maximum shrinkage rate to temperatures ∼1173 K. The additive CuV₂O₆ was shown to increase densification and promote grain growth. A mechanosynthesised sample of Sr_0.02La_0.98Nb_0.85V_0.15O_4-δ + 2 mol% CuV₂O₆ could be densified to ∼ 90% that of the theoretical by hybrid microwave sintering at 1168 K for 5 min. The scheelite phase in this material was retained to room temperature, against the normal thermodynamic tendency. Impedance spectroscopy in wet and dry, nitrogen and oxygen atmospheres suggested that the bulk conductivity of this material is unaffected by the sintering aid, whereas the grain boundary conductivity was impaired and exhibited n-type conductivity behaviour, characteristic of the additive. The concentration of this promising additive should be reduced in further work.     

Periodic Nafion-silica-heteropolyacids electrolyte t for PEM fuel cell operated near 200 °C

Periodic ordered Nafion-silica-heteropolyacids electrolyte is synthesized through a facile multiphase self-assembly between the positively charged silica, negatively charged HPW acids (H₃PW₁₂O₄₀) and Nafion ionomers. The results exhibit uniform nanoarrays with long-range order of the electrolyte. The well-ordered proton conducting sites make the proton move through the membrane freely with low humidity dependence of proton transportation through the electrolyte. The Nafion-Silica-HPW electrolyte displays desirable conductivity at both low and elevated temperature. The proton conductivity of Nafion-Silica-HPW electrolyte at absolutely dry condition of 200 °C is 0.044 Scm⁻¹. At low temperature of 75 °C, the proton conductivities of the electrolyte were 0.029 Scm⁻¹ and 0.093 Scm⁻¹ at absolute dry (0 RH%) and full humidifying condition (100 RH%), respectively.     

LaNi0.6Fe0.4O3-Ce0.8Sm0.2O1.9-Ag composite cathode for intermediate temperature solid oxide fuel cells

LaNi_0.6Fe_0.4O₃ (LNF), LNF-Sm_0.2Ce_0.8O_1.9 (SDC), and LNF-SDC-Ag cathodes on SDC electrolytes were investigated at intermediate temperatures using AC impedance spectroscopy. Results show that adding 50 wt.% SDC into LNF yields a significant low area specific resistance (ASR) which was found to be 0.92 Ω cm² at 700 °C. Infiltrating 0.3 mg/cm² Ag into LNF-50 wt.% SDC can improve the electronic conductivity and oxygen exchange reaction activity, and thereby remarkably decrease the ASRs. The ASR value of the LNF-SDC-Ag cathode is as low as 0.18 Ω cm² at 700 °C, and 0.46 Ω cm² at 650 °C. The long-term test shows that the LNF-SDC-Ag cathode may be a promising candidate for solid oxide fuel cells operating at temperatures lower than 650 °C.