Microstructures and electrical properties of zirconium doped barium cerate perovskite proton conductors

In this work, we vary the Zr to Ce ratio to investigate the microstructures and electrical properties of zirconium doped barium cerates. The solid state reaction is used in synthesizing the BaCe_0.8-xZr_xY_0.2O₃ (X = 0.1–0.5). The electron backscatter diffraction (EBSD) is successfully applied to identify the crystal structure of the barium cerates. EBSD results indicate that all samples have the orthorhombic structure. Conductivity measurement results show that for temperatures below 700 °C, Zr-doped barium cerates have higher protonic conductivities than oxygen-ion/electron-hole conductivities. The protonic conductivity increases with the Zr content initially, but decreases after the Zr content is higher than 0.3. The protonic conductivity of BCZY0.3 reaches 8.8 mS/cm at 700 °C in dry hydrogen atmosphere. Stability test results show that, for stable operation in CO₂ atmosphere, the Zr content in barium cerates should be greater than 0.2.     

High temperature fuel and steam electrolysis cells using proton conductive solid electrolytes

High-temperature-type proton conductive solids are favorable materials as electrolytes for fuel cells and steam electrolysis cells for the production of hydrogen gas.An attempt has been made to construct a high temperature fuel cell and a steam electrolysis cell using an SrCeO₃-based solid electrolyte, which we found to be a protonic conductor in the presence of hydrogen or water vapor. Both cells could be operated stably at 800 – 1000 °C. The major limitation of the cell system was the resistance of the solid electrolytes.     

Fabrication and evaluation of stable micro tubular solid oxide fuel cells with BZCY-BZY bi-layer proton conducting electrolytes

Anode-supported proton conducting micro tubular solid oxide fuel cells (MT-SOFCs) with the configuration of Ni–BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY)/BZCY/BaZr_0.8Y_0.2O_3-δ (BZY)/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-BZY have been prepared by a combination of phase inversion method and suspension-coating technique. The obtained Ni-BZCY anode hollow fiber presents a special asymmetrical structure consisting of a sponge-like layer and a finger-like porous layer, which is propitious to anode electrochemical process. Bi-layer electrolytes consisting of 5 μm thick BZCY and 3 μm thick BZY are successfully fabricated by suspension-coating technique. BZY electrolytes are placed at the cathode side, in order to improve the chemical stability against CO₂. The considerable electrochemical performance and good stability in the presence of CO₂ indicate that the construction of BZY-BZCY bi-layer electrolytes is an effective way for the development of stable proton conducting MT-SOFCs.     

An intermediate temperature direct ammonia fuel cell using a proton conducting electrolyte

The operation of fuel cells based on proton conducting BaCeO₃ solid electrolyte doubly doped with gadolinium and praseodymium is reported. Fuel cells were operated for up to 96 h with both hydrogen and ammonia individually as fuel. Results show that the performance of the cell is the same irrespective of the fuel used. The only chemical by-products of the ammonia fuel cell are nitrogen and water. The data shows that a fuel cell incorporating a doubly doped proton conducting electrolyte operated on ammonia fuel is a viable alternative to fuel cells utilizing hydrocarbon fuels as sources of hydrogen.     

High rate all-solid electrochemical capacitors using proton conducting polymer electrolytes

All-solid electrochemical capacitors (EC) utilizing a proton conducting polymer electrolyte and graphite electrodes have demonstrated exceptionally high rate capability. The solid polymer electrolyte-based ECs charge and discharge at sweep rate over 20 V s⁻¹ and exhibit a time constant of 10 ms. This high rate performance is enabled by a proton conducting ternary solid thin film electrolyte composed of silicotungstic acid, orthophosphoric acid, and polyvinyl alcohol. This work shows that solid polymer electrolytes can support high power and high rate energy storage applications.     

Synthesis and characterization of terbium doped barium cerates as a proton conducting SOFC electrolyte

A novel proton conductor, BaCe_0.95Tb_0.05O_3−a (BCTb) perovskite was synthesized via the EDTA-citrate acid complexing method, followed by high temperature calcination. The properties of the powders were characterized by thermo gravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and the conductivity measurement using the 4-probe method. In order to obtain the pure perovskite structure, the calcination temperature was elevated at 1000 °C or greater. The electrical conductivities of the BCTb perovskite mainly result from protons and are 6.51 × 10⁻³−2.0 × 10⁻² S cm⁻¹ in the temperature range of 600-850 °C with the activation energy of 41.95 kJ mol⁻¹. A Ni-BCTb∣BCTb∣LSCF(La_0.6Sr_0.4Co_0.2Fe_0.8O_3-α)-BCTb fuel cell was subsequently fabricated via a co-pressing/co-sintering method, followed by slurry-coating of the cathode. With 50 mL min⁻¹ pure hydrogen as fuel and the ambient air as oxidant, the maximum output of the fuel cell has reached to 753 mW cm⁻² at 700 °C. The results demonstrate that BCTb perovskite may be a promising electrolyte for the proton-conducting solid oxide fuel cells (PC-SOFCs) after its chemical stability is improved significantly.     

Effects of co-doped barium cerate additive on morphology,conductivity and electrochemical properties of samarium doped ceria electrolyte for intermediate temperature solid oxide fuel cells

The composite of samarium doped ceria (Sm_0.2Ce_0.8O_2-δ, SDC) and co-doped barium cerate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ, BZCYYb) is prepared by mechanical mixing and investigated as electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Coexistence of SDC and BZCYYb are observed for composite electrolyte by X-ray diffraction after sintering at 1500 °C for 5 h, while the slight deviation of the diffraction peak indicating the element diffusion between two phases. The scanning electron microscope and electron probe microanalyzer results demonstrate that small BZCYYb grains disperse uniformly around the grains of SDC, limiting the growth of SDC grains and decreasing the average grain size of composite electrolyte. Impedance spectroscopy measurement reveals that the grain boundary resistance can be significantly reduced by about an order of magnitude through adding 15–30 wt. % BZCYYb to SDC. Single cells based on the composite electrolyte are fabricated using nickel cermet (Ni-SDC) anode and perovskite (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, LSCF) cathode.Relatively high open circuit voltage (OCV), much lower polarization resistance and encouraging high power density are obtained for cells with composite electrolyte compared to those with single SDC electrolyte. Among all of the samples, single cell based on 15 wt. % BZCYYb-85 wt. % SDC composite electrolyte exhibits the lowest total resistances of 0.641 Ω·cm² and the highest peak power densities of 0.56 W·cm^?2 at 600 °C.     

Effect of strontium and zirconium doped barium cerate on the performance of proton ceramic electrolyser cell for syngas production from carbon dioxide and steam

Syngas has been produced from carbon dioxide (CO₂) and steam using a proton ceramic electrolyser cell. Proton-conducting electrolytes which exhibit high conductivity can suffer from low chemical stability. In this study, to optimize both proton conductivity and chemical stability, barium cerate and doped barium cerate are synthesized using solid state reaction method: BaCeO₃ (BC), Ba_0.6Sr_0.4CeO_3-α (BSC), Ba_0.6Sr_0.4Ce_0.9Y_0.1O_3-α (BSCY), and BaCe_0.6Zr_0.4O_3-α (BCZ). The BC, BSC, and BSCY are calcined at 1100 °C for 2 h and BCZ is calcined at 1300 °C for 12 h, respectively. All samples exhibit 100% perovskite and crystallite sizes equal 37.05, 28.46, 23.65 and 17.46 nm for BC, BSC, BSCY and BCZ, respectively. Proton conductivity during steam electrolysis as well as catalytic activity toward the reverse water gas shift reaction (RWGS) is tested between 400 and 800 °C. The conductivity increases with temperature and the values of activation energy of conduction are 64.69, 100.80, 103.78 and 108.12 kJ mol⁻¹ for BSCY, BC, BSC, and BCZ, respectively. It is found that although BCZ exhibits relatively low conductivity, the material provides the highest CO yield at 550–800 °C, followed by BSCY, BSC, and BC, correlating to the crystallite size and BET surface area of the samples. Catalytic activity toward RWGS of composited Cu and electrolytes is also measured. Additional Cu (60 wt%) significantly increases catalytic activity. The CO yield increases from 3.01% (BCZ) to 43.60% (Cu/BCZ) at 600 °C and CO can be produced at temperature below 400 °C. There is no impurity phase detected in BCZ sample after exposure to CO₂-containing gas mixture (600 °C for 5 h) while CeO₂ phase is detected in BSC and BSCY and both CeO₂ and BaO are observed in BC sample.     

A novel layered perovskite cathode for proton conducting solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate proton conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite demonstrates advanced electrochemical properties based on doped ceria electrolyte. This research fully takes advantage of these advanced properties and develops novel protonic ceramic membrane fuel cells (PCMFCs) of Ni-BZCY7|BZCY7|SBSC. The results show that the open-circuit potential of 1.015 V and maximum power density of 533 mW cm⁻² are achieved at 700 °C. With temperature increase, the total cell resistance decreases, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that SBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni-BZCY7|BZCY7|SBSC cell is a promising functional material system for next generation SOFCs.     

New proton conducting polymer blends and their fuel cell performance

Novel polymer blends based on aromatic polyethers with pyridine units have been prepared for their use as electrolytes after being doped with phosphoric acid for high temperature PEM fuel cells. They exhibit very good film-forming properties, mechanical integrity, high modulus up to 230 °C, high glass transition temperatures (up to 260 °C) and high thermal stability up to 400 °C. In addition to the above required properties, these novel materials show high oxidative stability and acid doping ability, enabling proton conductivity in the range of 10⁻² S cm⁻¹ at 130 °C. The preparation and fuel cell testing of membrane electrode assemblies, demonstrated very promising performance, and an initial study has shown the positive effect of humidity on the measured conductivity.     

Low temperature anode-supported solid oxide fuel cells based on gadolinium doped ceria electrolytes

The utilization of anode-supported electrolytes is a useful strategy to increase the electrical properties of the solid oxide fuel cells, because it is possible to decrease considerably the thickness of the electrolytes. We have successfully prepared single-chamber fuel cells of gadolinium doped ceria electrolytes Ce_1−xGd_xO_2−y (CGO) supported on an anode formed by a cermet of NiO/CGO. Mixtures of precursor powders of NiO and gadolinium doped ceria with different particle sizes and compositions were analysed to obtain optimal bulk porous anodes to be used as anode-supported fuel cells. Doped ceria electrolytes were prepared by sol–gel related techniques. Then, ceria-based electrolytes were deposited by dip coating at different thicknesses (15–30 μm) using an ink prepared with nanometric powders of electrolytes dispersed in a liquid polymer. Cathodes of La_1−xSr_xCoO₃ (LSCO) were also prepared by sol–gel related techniques and were deposited on the electrolyte thick films. Finally, electrical properties were determined in a single-chamber reactor where propane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Flux rate values of the carrier gas and propane partial pressure were determinants for the optimization of the electrical properties of the fuel cells.     

Supercapacitive properties of activated carbon electrode using ammonium based proton conducting electrolytes

In this study, we demonstrated the usefulness of proton conducting electrolytes (such as ammonium thiocyanate (NH₄SCN) and ammonium nitrate (NH₄NO₃)) for electrochemical energy storage devices using activated carbon (AC) as the electrode material. The cyclic voltammetry analysis revealed the presence of rectangular shaped cyclic voltammograms indicating the presence of electrical double layer capacitance in AC electrode using NH₄SCN and NH₄NO₃ electrolytes. The mechanism of charge-storage in AC electrode using the proton conducting electrolytes has been studied in detail using electrochemical impedance spectroscopy (Nyquist and Bode plots). The galvanostatic charge-discharge analysis revealed that a maximum specific capacitance of AC electrode using NH₄SCN and NH₄NO₃ electrolytes was found to be 136.75 mF cm^?2 and 113.38 mF cm^?2 at a current density of 0.5 mA cm^?2. This study would open a new avenue for the use of ammonium based proton conducting electrolytes for supercapacitor applications.     

Electrochemical modeling of ammonia-fed solid oxide fuel cells based on proton conducting electrolyte

An electrochemical model was developed to study the NH₃-fed and H₂-fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-H). The modeling results were consistent with experimental data in literature. It is found that there is little difference in working voltage and power density between the NH₃-fed and the H₂-fed SOFC-H with an electrolyte-support configuration due to an extremely high ohmic overpotential in the SOFC-H. With an anode-supported configuration, especially when a thin film electrolyte is used, the H₂-fed SOFC-H shows significantly higher voltage and power density than the NH₃-fed SOFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH₃-fed SOFC-H is found much higher than the H₂-fed SOFC-H, as the presence of N₂ gas dilutes the H₂ concentration and slows down the transport of H₂. More importantly, the cathode concentration overpotential is found very significant despite of the thin cathode used in the anode-supported configuration. In the SOFC-H, H₂O is produced in the cathode, which enables complete fuel utilization on one hand, but dilutes the concentration of O₂ and impedes the diffusion of O₂ to the reaction sites on the other hand. Thus, the cathode concentration overpotential is the limiting factor for the H₂-fed SOFC-H and an important voltage loss in the NH₃-fed SOFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SOFC-H.     

Evaluation of apatite silicates as solid oxide fuel cell electrolytes

Apatite-type silicates have been considered as promising electrolytes for Solid Oxide Fuel Cells (SOFC); however studies on the potential use of these materials in SOFC devices have received relatively little attention. The lanthanum silicate with composition La₁₀Si_5.5Al_0.5O_26.75 has been evaluated as electrolyte with the electrode materials commonly used in SOFC, i.e. manganite, ferrite and cobaltite as cathode materials and NiO-CGO composite, chromium-manganite and Sr₂MgMoO₆ as anode materials. Chemical compatibility, area-specific resistance and fuel cell studies have been performed. X-ray powder diffraction (XRPD) analysis did not reveal any trace of reaction products between the apatite electrolyte and most of the aforementioned electrode materials. However, the area-specific polarisation resistance (ASR) of these electrodes in contact with apatite electrolyte increased significantly with the sintering temperature, indicating reactivity at the electrolyte/electrode interface. On the other hand, the ASR values are significantly improved using a ceria buffer layer between the electrolyte and electrode materials to prevent reactivity. Maximum power densities of 195 and 65 mWcm⁻² were obtained at 850 and 700 °C, respectively in H₂ fuel, using an 1 mm-thick electrolyte, a NiO-Ce_0.8Gd_0.2O_1.9 composite as anode and La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ as cathode materials. This fuel cell was tested for 100 h in 5%H₂-Ar atmosphere showing stable performance.     

Novel structured gadolinium doped ceria based electrolytes for intermediate temperature solid oxide fuel cells

Novel three-layered intermediate temperature solid oxide fuel cell (SOFC) electrolytes based on gadolinium doped ceria (GDC) are developed to suppress the electronic conductivity of GDC, to improve the mechanical properties of the cell and to minimize power loss due to mixed conductive nature of GDC. Three different electrolytes are fabricated by sandwiching thin YSZ, ScSZ and ScCeSZ between two relatively thick GDC layers. An electrolyte composed of pure GDC is also manufactured for comparison. NiO/GDC and LSCF/GDC electrodes are then coated on the electrolytes by a screen printing route. SEM results show that it is possible to obtain dense and crack free thin layers of YSZ, ScSZ and ScCeSZ between two GDC layers without delamination. Performance measurements indicate that interlayered thin electrolytes act as an electronic conduction barrier and improve open circuit voltages (OCVs) of GDC based cells.     

Application of yttrium doped barium cerate for improvement of the dual membrane SOFC design

Recently a new design of solid oxide fuel cell (SOFC) named dual membrane fuel cell (dmFC) has been introduced and proved. It is based on the introduction of three independent compartments for hydrogen, oxygen and water, which eliminate the dilution of the fuel or the oxidizer respectively in the SOFC construction or in its proton conducting (pSOFC) modification. Due to the registered sufficient mixed ionic conductivity of BaCe_0.85Y_0.15O_2.925 (BCY15), a simplified modification in respect to shaping technology and construction is introduced. It's performance is better than the one of pSOFC cell produced with the same material and shaping procedure: 76 mW/cm² (dmFC thickness 1.1 mm) against 71 mW/cm² (pSOFC thickness 0.66 mm) at 700 °C. Impedance measurements carried out down to 1 mHz ensure information about the water formation and behaviour in the separate central membrane compartment of the dmFC design, which brings to performance improvements, registered also in electrolyser mode of operation.     

Electrochemical testing of suspension plasma sprayed solid oxide fuel cell electrolytes

Electrochemical performance of metal-supported plasma sprayed (PS) solid oxide fuel cells (SOFCs) was tested for three nominal electrolyte thicknesses and three electrolyte fabrication conditions to determine the effects of electrolyte thickness and microstructure on open circuit voltage (OCV) and series resistance (R_s). The measured OCV values were approximately 90% of the Nernst voltages, and electrolyte area specific resistances below 0.1 Ω cm² were obtained at 750 °C for electrolyte thicknesses below 20 μm. Least-squares fitting was used to estimate the contributions to R_s of the YSZ bulk material, its microstructure, and the contact resistance between the current collectors and the cells. It was found that the 96% dense electrolyte layers produced from high plasma gas flow rate conditions had the lowest permeation rates, the highest OCV values, and the smallest electrolyte-related voltage losses. Optimal electrolyte thicknesses were determined for each electrolyte microstructure that would result in the lowest combination of OCV loss and voltage loss due to series resistance for operating voltages of 0.8 V and 0.7 V.     

Measurement of hydrogen and oxygen chemical potential in yttria doped barium cerate (BCY) electrolyte of anode-supported protonic ceramic fuel cells

Yttria doped barium cerate (BCY) electrolyte, Ni + BCY anode supported protonic ceramic fuel cells were fabricated with Pt reference electrodes embedded in a thin (∼40 microns) electrolyte layer. The embedded electrodes function as selective probes exchanging only electrons with the BCY so that the voltage measurements (ΔV) using the embedded probes through the electrolyte correspond to a change in the reduced negative electrochemical potential of electrons (Δφ  ). Using this method, the corresponding change in hydrogen and oxygen chemical potential (ΔμH₂$\Delta {\mu }_{{\text{H}}_2}$, ΔμO₂$\Delta {\mu }_{{\text{O}}_2}$) or partial pressure of hydrogen and oxygen (ΔpH₂$\Delta p_{{\text{H}}_2}$, ΔpO₂$\Delta p_{{\text{O}}_2}$) were determined on the basis of the local equilibrium assumption, allowing us to investigate ionic and electronic transport properties through the BCY electrolyte. The results indicate that the pH₂$p_{{\text{H}}_2}$ and pO₂$p_{{\text{O}}_2}$ change mainly occurs across the middle electrolyte region while the electrolyte regions close to the anode and the cathode showed very small variation. The present work revealed that the BCY electrolyte consists of three major parts with different transport properties; 1) mixed ionic-electronic conduction in the electrolyte close to the anode side (reducing atmosphere), 2) predominantly ionic conduction in the middle region, 3) mixed ionic-hole conduction in the electrolyte close to the cathode side (oxidizing atmosphere).     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.