Preparation and properties of ceramic interconnecting materials,La0.7Ca0.3CrO3−δ doped with GDC for IT-SOFCs

One of the challenges for improving the performance and cost-effectiveness of solid oxide fuel cells (SOFCs) is the development of effective interconnect materials. A widely used interconnect ceramic for SOFCs is doped lanthanum chromite. In this paper, we report a doped lanthanum chromite, La_0.7Ca_0.3CrO_3−δ (LCC) + x wt.% Gd_0.2Ce_0.8O_1.9 (GDC) (x = 0–10), with improved electrical conductivity and sintering capability. In this composite material system, LCC + GDC were prepared by an auto-ignition process and the electrical conductivity was characterized in air and in H₂. The LCC powders exhibited a better sintering ability and could reach a 94.7% relative density at 1400 °C for 4 h in air and with the increase of GDC content the relative density increased, reached 98.5% when the GDC content was up to 10 wt.%. The electrical conductivity of the samples dramatically increased with GDC addition until a maximum of 134.48 S cm⁻¹ in air at 900 °C when the materials contained 3 wt.% GDC. This is 5.5 times higher than pure LCC (24.63 S cm⁻¹). For the sample with a 1 wt.% GDC content, the conductivity in pure H₂ at 900 °C was a maximum 5.45 S cm⁻¹, which is also higher than that of pure LCC ceramics (4.72 S cm⁻¹). The average thermal expansion coefficient (TEC) increased with the increase of GDC content, ranging from 11.12 to 14.32 × 10⁻⁶ K⁻¹, the majority of which unfortunately did not match that of 8YSZ. The oxygen permeation measurement presented a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnect material for higher performance and cost-effectiveness for SOFCs.     

Solid oxide fuel cells operated by internal partial oxidation reforming of iso-octane

Two types of solid oxide fuel cells (SOFCs), with thin Ce_0.85Sm_0.15O_1.925 (SDC) or 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) electrolytes, were fabricated and tested with iso-octane/air fuel mixtures. An additional Ru–CeO₂ catalyst layer, placed between the fuel stream and the anode, was needed to obtain a stable output power density without anode coking. Thermodynamic analysis and catalysis experiments showed that H₂ and CO were primary reaction products at ≈750 °C, but that these decreased and H₂O and CO₂ increased as the operating temperature dropped below ≈600 °C. Power densities for YSZ cells were 0.7 W cm⁻² at 0.7 V and 790 °C, and for SDC cells were 0.6 W cm⁻² at 0.6 V and 590 °C. Limiting current behavior was observed due to the relatively low (≈20%) H₂ content in the reformed fuel.     

Effects of silicon concentration in SOFC alloy interconnects on the formation of oxide scales in hydrocarbon fuels

Oxide scale formations on FeCr alloy interconnects were investigated in anode gas (mixtures of CH₄ and H₂O) atmospheres for solid oxide fuel cells. The silicon concentration in FeCr alloy changed the microstructures of oxide scales, elemental distribution and oxide scale growth rates. Oxide scale is composed of the following phases from surface to inner oxides: FeMn spinel, Cr₂O₃, oxide scale/alloy interface and internal oxides of Si and Al. With decreasing the Si concentration from 0.4 to 0.01 mass%, formation of thin Si and Mn layer was observed inside the FeCr alloy. Oxide scale growth rate constants decreased by lowering the Si concentration in FeCr alloy from 4.2 × 10⁻¹⁸ to 2.1 × 10⁻¹⁸ m² s⁻¹ at 1073 K. Diffusivity of Fe and Cr was changed by the concentration of Si in FeCr alloy, which affects the growth rates of oxide scale. The electrical conductivity of oxidized FeCr alloy shows almost same level regardless the Si concentration (in the orders of 10 S cm⁻² at 1073 K).     

In situ screen-printed BaZr0.1Ce0.7Y0.2O3−δ electrolyte-based protonic ceramic membrane fuel cells with layered SmBaCo2O5+x cathode

In order to develop a simple and cost-effective route to fabricate protonic ceramic membrane fuel cells (PCMFCs) with layered SmBaCo₂O_5+x (SBCO) cathode, a dense BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) electrolyte was fabricated on a porous anode by in situ screen printing. The porous NiO–BaZr_0.1Ce_0.7Y_0.2O_3?δ (NiO–BZCY) anode was directly prepared from metal oxide (NiO, BaCO₃, ZrO₂, CeO₂ and Y₂O₃) by a simple gel-casting process. An ink of metal oxide (BaCO₃, ZrO₂, CeO₂ and Y₂O₃) powders was then employed to deposit BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) thin layer by an in situ reaction-sintering screen printing process on NiO–BZCY anode. The bi-layer with 25 μm dense BZCY electrolyte was obtained by co-sintering at 1400 °C for 5 h. With layered SBCO cathode synthesized by gel-casting on the bi-layer, single cells were assembled and tested with H₂ as fuel and the static air as oxidant. A high open-circuit potential of 1.01 V, a maximum power density of 382 mW cm^?2, and a low polarization resistance of the electrodes of 0.15 Ω cm² was achieved at 700 °C.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Lithium ion and electronic conductivity in 3-(oligoethylene oxide)thiophene comb-like polymers

The Li-ion and electronic conductivities of a series of p-doped poly(thiophene)s with oligo-ethylene oxide side chains have been determined at room temperature as functions of side-chain length and concentration of LiOTf dissolved in the polymers in order to assess their utility as binders in Li-ion batteries. The lithium triflate concentration was varied from 0.23 to 2.26 mmol LiOTf/g –C₂H₄O– (100 O:Li to 10 O:Li), and the concentration of dissociated Li⁺ was determined from the IR spectra of the polymer solutions. The greatest ionic conductivity, 2 × 10⁻⁴ S cm⁻¹, was attained with intermediate concentrations of added salt that corresponded with the greatest degree of LiOTf dissociation. Li-ion mobilities of 5 × 10⁻⁷ cm² (Vs)⁻¹ were measured for poly(thiophene)s (PT) with short oligo(ethylene oxide) side-chains (E_n), PE₂T and PE₃T, whereas the polymers with longer side chains, PE₇T and PE₁₅T, had Li-ion mobilities about an order of magnitude greater, 5 × 10⁻⁶ cm² (Vs)⁻¹. The electronic conductivity of the polymers heavily doped with NOBF₄ was near 0.1 S cm⁻¹ for PE₂T and PE₃T, but was orders of magnitude smaller for the polymers with longer side-chains. Addition of LiOTf caused the electronic conductivity of PE₂T and PE₃T to drop to that of the longer chain polymers whose conductivities were insensitive to the LiOTf concentration.     

Effect of synthesis conditions on the properties of LiFePO4 for secondary lithium batteries

LiFePO₄ is one of the promising materials for cathode of secondary lithium batteries due to its high energy density, low cost, environmental friendliness and safety. However, LiFePO₄ has very poor electronic conductivity (∼10⁻⁹ S cm⁻¹) and Li-ion diffusion coefficient (∼1.8 × 10⁻¹⁴ cm² s⁻¹) at room temperature. In an attempt to improve electrochemical properties, Li_XFePO₄ with various amounts of Li contents were investigated in this study. Li_XFePO₄ (X = 0.7–1.1) samples were synthesized by solid-state reaction. High resolution X-ray diffraction, Rietveld analysis, BET, scanning electron microscopy, and hall effect measurement system were used to characterize these samples. Electronic conductivities of the samples with Li-deficient and Li-excess in Li_xFePO₄ were 10⁻³ to 10⁻¹ S cm⁻¹. Discharge capacities and rate capabilities of the samples with Li-deficient and Li-excess in Li_XFePO₄ were higher than those of stoichiometric LiFePO₄ sample. Li_0.9FePO₄ samples fired at 700 °C had discharge capacity of 156 and 140 mAh g⁻¹ at 0.1 C- and 2 C-rate, respectively.     

Nanocrystalline tin oxides and nickel oxide film anodes for Li-ion batteries

Tin oxides and nickel oxide thin film anodes have been fabricated for the first time by vacuum thermal evaporation of metallic tin or nickel, and subsequent thermal oxidation in air or oxygen ambient. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements showed that the prepared films are of nanocrystalline structure with the average particle size <100 nm. The electrochemical properties of these film electrodes were examined by galvanostatic cycling measurements and cyclic voltammetry. The composition and electrochemical properties of SnO_x (1<x<2) films strongly depend on the oxidation temperature. The reversible capacities of SnO and SnO₂ films electrodes reached 825 and 760 mAh g⁻¹, respectively, at the current density of 10 μA cm⁻² between 0.10 and 1.30 V. The SnO_x film fabricated at an oxidation temperature of 600 °C exhibited better electrochemical performance than SnO or SnO₂ film electrode. Nanocrystalline NiO thin film prepared at a temperature of 600 °C can deliver a reversible capacity of 680 mAh g⁻¹ at 10 μA cm⁻² in the voltage range 0.01–3.0 V and good cyclability up to 100 cycles.     

Performance of a solid oxide fuel cell utilizing hydrogen sulfide as fuel

The electrochemical performance of a hydrogen sulfide solid oxide fuel cell having the configuration H₂S, Pt/(ZrO₂)_0.92(Y₂O₃)_0.08/Pt, air has been examined at atmospheric pressure and 750–800°C, using both pure and 5% H₂S anode feed streams. The performance of the cell is higher when using diluted H₂S feed compared with pure H₂S feed: current densities up to 100 mA cm⁻² and power densities up to 15.4 mW cm⁻² have been achieved using diluted H₂S gas (5%) at 800°C. However, the platinum anode degrades over time in H₂S stream due to the formation of PtS. Electrochemical oxidation of H₂S on the Pt anode significantly accelerated its degradation. Polarization and impedance spectroscopy measurements show that at low current density (i) electrochemical reaction is the major cause of polarization in the fuel cell. Ohmic loss due to the resistance of the electrolyte material and the electrical connecting wire is a major part of cell polarization at high i.     

High-rate electrochemical partial oxidation of methane in solid oxide fuel cells

This paper describes results on direct-methane solid oxide fuel cell (air, LSM-YSZ|YSZ|Ni-YSZ, CH₄) operation for combined electricity and syngas production. Thermodynamic equilibrium predictions showed that efficient methane conversion to syngas is expected for SOFC operating temperature >700 °C and O²⁻/CH₄ ratios of ≈1. A simple thermal analysis was used to determine conditions where the cell produces enough heat to self-sustain its operating temperature; relatively low cell voltage and O²⁻/CH₄ ratios > 1 were found to be useful. Fuel cells operated at T ≈ 750 °C, V ≈ 0.4 V, and O²⁻/CH₄ ≈ 1.2 yielded electrical power output of ∼0.7 W cm⁻² and syngas production rates of ∼20 sccm cm⁻². Stable cell operation without coking for >300 h was achieved.     

Operation of ceria-electrolyte solid oxide fuel cells on iso-octane–air fuel mixtures

Reduced-temperature solid oxide fuel cells (SOFCs) – with thin Ce_0.85Sm_0.15O_1.925 (SDC) electrolytes, thick Ni–SDC anode supports, and composite cathodes containing La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and SDC – were fabricated and tested with iso-octane/air fuel mixtures. An additional supported catalyst layer, placed between the fuel stream and the anode, was needed to obtain a stable output power density (e.g. 0.6 W cm⁻² at 590 °C) without anode coking. The Ru-CeO₂ catalyst produced CO₂ and H₂ at temperatures <350 °C, while H₂ and CO became predominant above 500 °C. Power densities were substantially less than for the same cells with H₂ fuel (e.g. 1.0 W cm⁻² at 600 °C), due to the dilute (≈20%) hydrogen in the fuel mixture produced by iso-octane partial oxidation. Electrochemical impedance analysis showed a main arc that represented ≈60% of the total resistance, and that increased substantially upon switching from hydrogen to iso-octane/air.     

Effects of addition of TiO2 nanoparticles on mechanical properties and ionic conductivity of solvent-free polymer electrolytes based on porous P(VdF-HFP)/P(EO-EC) membranes

To enhance the performance (i.e., mechanical properties and ionic conductivity) of pore-filling polymer electrolytes, titanium dioxide (TiO₂) nanoparticles are added to both a porous membrane and its included viscous electrolyte, poly(ethylene oxide-co-ethylene carbonate) copolymer (P(EO-EC)). A porous membrane with 10 wt.% TiO₂ shows better performance (e.g., homogeneous distribution, high uptake, and good mechanical properties) than the others studied and is therefore chosen as the matrix to prepare polymer electrolytes. A maximum conductivity of 5.1 × 10⁻⁵ S cm⁻¹ at 25 °C is obtained for a polymer electrolyte containing 1.5 wt.% TiO₂ in a viscous electrolyte, compared with 3.2 × 10⁻⁵ S cm⁻¹ for a polymer electrolyte without TiO₂. The glass transition temperature, T_g is lowered by the addition of TiO₂ (up to 1.5 wt.% in a viscous electrolyte) due to interaction between P(EO-EC) and TiO₂, which weakens the interaction between oxide groups of the P(EO-EC) and lithium cations. The overall results indicate that the sample prepared with 10 wt.% TiO₂ for a porous membrane and 1.5 wt.% TiO₂ for a viscous electrolyte is a promising polymer electrolyte for rechargeable lithium batteries.     

Ni1−xCux alloy-based anodes for low-temperature solid oxide fuel cells with biomass-produced gas as fuel

Ni–Cu alloy-based anodes, Ni_1−xCu_x (x = 0, 0.05, 0.2, 0.3)–Ce_0.8Sm_0.2O_1.9 (SDC), were developed for direct utilization of biomass-produced gas in low-temperature solid oxide fuel cells (LT-SOFCs) with thin film Ce_0.9Gd_0.1O_1.95 electrolytes. The alloys were formed by in situ reduction of Ni_1−xCu_xO_y composites synthesized using a glycine-nitrate technique. The electrolyte films were fabricated with a co-pressing and co-firing technique. Electrochemical performance of the Ni_1−xCu_x–SDC anode supported cells was investigated at 600 °C when humidified (3% H₂O) biomass-produced gas (BPG) was used as the fuel and stationary air as the oxidant. With Ni–Cu alloys as anodes, carbon deposition was substantially suppressed and electrochemical performance of the cells was sustained for much longer periods of time. For example, the power export of a Ni–SDC supported cell was only 50% of the initial value (200 mW cm⁻² at 0.5 V) after 20 min, while Ni_0.8Cu_0.2–SDC supported cells could maintain 90% of the initial power density (250 mW cm⁻² at 0.5 V) over a period of 10 h. The improved performance of the Ni–Cu alloy-based anodes is worth considering in developing SOFCs fueled directly with dilute hydrocarbons such as gases derived from biomass.     

RuxCrySez electrocatalyst loading and stability effects on the electrochemical performance in a PEMFC

The present paper presents a study of the Ru_xCr_ySe_z chalcogenide electrocatalyst based on physical–chemical characterization through scanning electron (SEM), atomic force (AFM) microscopy and energy dispersion elemental analysis (EDS), thermal stability using differential scanning calorimeter (DSC), electrochemical kinetics towards the oxygen reduction reaction (ORR) in acid media by rotating ring-disk electrode (RRDE) and single and three-stack membrane-electrode assembly (MEA) performance as a function of catalyst loading (10%, 20% and 40% W from 0.2 to 2 mg cm⁻²). Results indicate an electrocatalyst with chemical composition of Ru₆Cr₄Se₅. AFM images showed 80–160 nm nanoparticle agglomerates. Good thermal stability of the cathode Ru₆Cr₄Se₅ was established after 100 h of continuous operation. The electrochemical kinetics study (RRDE) resulted in a electrocatalyst with high activity towards the ORR, preferentially proceeding via 4e⁻ charge transfer pathway towards water formation (i.e., O₂+4H⁺+4e⁻→2H₂O), with a maximum of 2.8% H₂O₂ formation at 25 °C. Finally, MEA tests revealed a maximum power density of 220 mW cm⁻² with a catalyst loading of 20 wt% at 1.6 mg cm⁻².     

Effect of cathode current-collecting layer on unit-cell performance of anode-supported solid oxide fuel cells

According to the characterization of the microstructure and properties of solid oxide fuel cells (SOFC) electrodes, it is essential to verify various processing variables to control microstructural parameters, such as particle size, composition and spatial distribution of the constituent phases of the electrode in order to reduce the ohmic and diffusional polarization losses of the unit-cell performance. From this viewpoint, a current-collecting layer with controlled microstructure very effectively enhances the unit-cell performance by reducing the ohmic and polarization resistance of the cathode. The maximum power density of a 5 cm × 5 cm unit-cell with the controlled current-collecting layer is ∼1.5 W cm⁻² at 750 °C, while a unit-cell without the layer is much lower, viz., 0.9 W cm⁻².     

Oxidation kinetics and phase evolution of a Fe–16Cr alloy in simulated SOFC cathode atmosphere

The oxidation behavior of a Fe–16Cr alloy containing a small amount of Mn oxidized in air for up to 500 h within the temperature range of 650–850 °C was examined. Two consecutive oxidation stages were found and these obeyed the parabolic rate law with various rate constants. Formation and growth of Cr₂O₃, MnCr₂O₄, surface nodules and oxide spallation were found to be responsible in the oxidation stages accordingly to various situations. The thin film X-ray diffraction, SEM and EDX confirmed the duplex oxide microstructure with MnCr₂O₄ on top of Cr₂O₃, Cr and Mn diffusion in Cr₂O₃ is considered to be responsible for the formation of each layer, respectively. The estimated area specific resistance (ASR) suggests the possibility of using this alloy as the interconnect material in reduced temperature SOFCs, however, surface modification to enhance its oxidation and spallation resistances is desired.     

Study of dc conductivity and battery application of polyethylene oxide/polyaniline and its composites

The polymer electrolyte based on polyethylene oxide (PEO) complexed with conducting polyaniline (PANI) and salts of AgNO₃ and NaNO₃ has been prepared in different weight percentage ratios. The complexation is confirmed by infra-red and X-ray diffraction studies. Conductivity (dc) measurements are carried out using a two-probe technique in the temperature range 30–80 °C. Electrochemical cell parameters for battery application at room temperature are also been determined. The electric conductivities are 1.5 × 10⁻⁵ S cm⁻¹ at 30 °C and 5.5 × l0⁻² S cm⁻¹ at 80 °C for a PEO:PANI (50:50) composite. The conductivity increases with increasing weight percentage of polyaniline in polyethylene oxide, which may be due to a strong hopping mechanism between the ether group of polyethylene oxide and conducting polyaniline. Samples are fabricated for battery application in configurations of Na:(PEO + PANI):(I₂ + C + sample) and their experimental data are measured using the Wagner polarization technique.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

Behavior of lanthanum-doped ceria and Sr-, Mg-doped LaGaO3 electrolytes in an anode-supported solid oxide fuel cell with a La0.6Sr0.4CoO3 cathode

Anode-supported solid oxide fuel cells (SOFCs) with lanthanum-doped ceria (LDC)/Sr-, Mg-doped LaGaO₃ (LSGM) bilayered or LDC/LSGM/LDC trilayered electrolyte films were fabricated with a pure La_0.6Sr_0.4CoO₃ (LSC) cathode. The behaviors of the two electrolytes in cells were investigated by using scanning electron microscopy, impedance spectroscopy and cell performance measurements. The reactions between LSGM and anode material can be suppressed by applying a ca. 15 μm LDC film. Due to the Co diffusion from the LSC cathode to the LSGM electrolyte during high temperature sintering, the electronic conductivity of the LDC electrolyte cannot be completely blocked with an LSGM layer below 50 μm, which leads to open-circuit potentials of these cells of ca. 0.988 V at 800 °C. The electrical conductivities of LDC and LSGM electrolytes in the cells under operation conditions are obtained from the dependence of the cell ohmic resistance on the electrolyte thickness. The electrical conductivity of LDC electrolyte is ca. 0.117 S cm⁻¹ at 800 °C on the bilayered electrolyte cells with a 50 μm LSGM layer. The bilayer electrolyte cells with a 25 μm LDC layer at 800 °C, had a cell ohmic resistance two-stage linear dependence on the LSGM layer thickness, which showed the electrical conductivity of ca. 1.9 S cm⁻¹ for the LSGM layer below 50 μm and 0.22 S cm⁻¹ for the LSGM layer above 100 μm. With a LDC/LSGM/LDC trilayered electrolyte film for the anode-supported cell, an open-circuit potential of 1.043 V was achieved.     

P(DMS-co-EO)/P(EPI-co-EO) blend as a polymeric electrolyte

A new polymer electrolyte comprising the blend of poly(dimethylsiloxane-co-ethylene oxide) (P(DMS-co-EO)), and poly(epichlorohydrin-co-ethylene oxide) (P(EPI-co-EO)), with different concentrations of LiClO₄ is described. The polymer electrolyte was prepared by a solution-cast technique. The electrochemical properties were studied by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry techniques. The maximum ionic conductivity (σ=1.2×10⁻⁴ S cm⁻¹) was obtained for the P(DMS-co-EO)/P(EPI-co-EO) 15/85 and 20/80 blends with 6 wt.% LiClO₄. These same films had a wide electrochemical stability, higher than 5 V at room temperature. A stable passive layer at the interface between the polymer electrolyte and lithium metal was formed within the first few days and maintained during the follow storage period. UV-Vis absorption spectra of the blends showed a transparent polymer electrolyte in the visible region.