Superprotonic KH(PO3H)–SiO2 composite electrolyte for intermediate temperature fuel cells

Novel thin film composite electrolyte membranes, prepared by dispersion of nano-sized SiO₂ particles in the solid acid compound KH(PO₃H), can be operated under both oxidizing and reducing conditions. Long-term stable proton conductivity is observed at 140 °C, i.e., slightly above the superprotonic phase transition temperature of KH(PO₃H), under conditions of relatively low humidification (pH₂O ≈ 0.02 atm).     

Enhanced CO2 stability of oxyanion doped Ba2In2O5 systems co-doped with La, Zr

In the solid oxide fuel cell (SOFC) field, proton conducting perovskite electrolytes offer many potential benefits. However, an issue with these electrolytes is their stability at elevated temperatures in the presence of CO₂. Recently we have reported enhanced oxide ion/proton conductivity in oxyanion (silicate, phosphate) doped Ba₂In₂O₅, and in this paper we extend this work to examine the stability at elevated temperatures towards CO₂. The results show improved CO₂ stability compared to the undoped system, and moreover this can be further improved by co-doping on either the Ba site with La, or the In site with Zr. While this co-doping strategy does reduce the conductivity slightly, the greatly improved CO₂ stability would suggest there is technological potential for these co-doped samples.     

Enhanced performance of supercapacitor based on boric acid doped PVA-H2SO4 gel polymer electrolyte system

Proton Conducting gel polymer electrolytes (GPEs) are taking much attention compared to liquid electrolytes for supercapacitor applications because of their physical properties, electrochemical stability and operation over broader temperature window. Among different GPEs PVA/acid blend electrolytes such as PVA/H₂SO₄, has drawn great attention in recent years. In this study, PVA-H₂SO₄-H₃BO₃ GPE was introduced for electric-double layer capacitor (EDLCs) application, in which electrospun free-standing carbon nanofibers are used as electrodes. Introduced PVA-H₂SO₄-H₃BO₃ GPE serves as both separator and the electrolyte in the supercapacitor. Symmetric Swagelok cells including GPEs were assembled via using two electrode arrangements and the electrochemical properties were searched. Electrochemical performance studies demonstrated that PVA-H₂SO₄-H₃BO₃ GPE had a maximum specific capacitance (Cs) of 134 F g^?1 and showed great capacitance retention (%100) after 1000 charge/discharge cycles. Furthermore, PVA-H₂SO₄-H₃BO₃GPE yielded an energy density of 67 Wh kg^?1 with a corresponding power density of 1000 W kg^?1 at a current density of 1 A g^?1.     

Kinetic performance of hydrogen generation enhanced by AlCl3 via hydrolysis of MgH2 prepared by hydriding combustion synthesis

Magnesium hydride (MgH₂) is a very promising hydrogen storage material and it has been paid more and more attention on the application of supplying hydrogen on-board because the theoretical hydrogen yield is up to 1703 mL/g when it reacts with water. However, the hydrolysis reaction is inhibited rapidly by the passivation layer of Mg(OH)₂ formed on the surface of MgH₂. This paper reports that high purity MgH₂ (～98.7 wt%) can be readily obtained by the process of hydriding combustion synthesis (HCS) and the hydrogen generation via hydrolysis of the as-prepared HCSed MgH₂ can be dramatically enhanced by the addition of AlCl₃ in hydrolysis solutions. An excellent kinetics of hydrogen generation of 1487 mL/g in 10 min and 1683 mL/g in 17 min at 303 K was achieved for the MgH₂-0.5 M AlCl₃ system, in which the theoretical hydrogen yield (1685 mL/g) of the HCSed product was nearly reached. The mechanism of the hydrolysis kinetics enhancement was demonstrated by the generation of a large amounts of H⁺ from the Al³⁺ hydrolysis and the pitting corrosion (Cl^?) of the Mg(OH)₂ layer wrapped on the surface of MgH₂ even at a low temperature. In addition, the apparent activation energies for the MgH₂ hydrolysis in the 0.1 M AlCl₃ and 0.5 M AlCl₃ solutions are decreased to 34.68 kJ/mol and 21.64 kJ/mol, respectively, being far superior to that of reported in deionized water (58.06 kJ/mol). The results suggest that MgH₂ + AlCl₃ system may be used as a promising hydrogen generation system in practical application of supplying hydrogen on-board.     

Hydrogen permeation in asymmetric La28 − xW4 + xO54 + 3x/2 membranes

Asymmetric supported La_28 − xW_4 + xO_54 + 3x/2 (La/W ≈ 5.6) membranes were investigated for their hydrogen permeation properties as a function of temperature and feed gas conditions. Dense membranes of thickness 25–30 μm supported on substrates with 25 and 40 vol.% porosity were compared. Above 850 °C under dry conditions, the hydrogen permeation rate was approximately constant as a function of temperature due to low concentration of protons in the material at high temperatures. Under humid conditions and above 960 °C enhanced permeation rates were observed. A hydrogen permeation as high as 0.14 NmL min⁻¹ cm⁻² was recorded at 1000 °C with 10 vol.% H₂ (2.5 vol.% H₂O) as feed gas.     

Synthesis, characterization and application of Li3Fe2(PO4)3 nanoparticles as cathode of lithium-ion rechargeable batteries

This work introduces a new method to synthesize Li₃Fe₂(PO₄)₃ nanoparticles in the nanopowder form and study its electrochemical performance by cyclic voltammetry and battery tests. Li₃Fe₂(PO₄)₃ is synthesized by the gel combustion method based on polyvinyl alcohol (PVA) as gel making agent. The optimum conditions of the synthesis include 8 wt% PVA, 0.34 wt% lithium slat, 1 wt% iron salt, 0.57 wt% ammonium dihydrogen phosphate, ethanol-water 50:50 as solvent, 675 °C combustion temperature and 4 h combustion time. Characterization of the samples is performed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), EDX analysis, XRD patterns, BET specific surface area and DSL size distribution. In the optimum conditions, a nanopowder is obtained that consisting of uniform nanoparticles with an average diameter of 70 nm. The optimized sample shows 12.5 m² g⁻¹ specific surface areas. Cyclic voltammetry (CV) studies show that the synthesized compound has good reversibility and high cyclic stability. The CV results are confirmed by the battery tests. The obtained results show that the synthesized cathodic material has high practical discharge capacity (average 125.5 mAh g⁻¹ approximately same with its theoretical capacity 128.2 mA h⁻¹) and long cycle life.     

Kinetics and thermodynamic behavior of WO3 and WO3:P thin films

 There is a considerable interest in the research and development of materials and devices, that can be used for optical switching of large-scale glazings. Several potential switching technologies are available for glazings, including those based on electrochromic, thermochromic and photochromic phenomena. One of the most promising technologies for optical switching devices is electrochromism (EC). In order to improve the electrochromic properties of tungsten oxide, we have investigated the effect of phosphorous insertion on the electrochromic behavior of oxide films prepared by the sol–gel process.The kinetics and thermodynamics of electrochemical intercalation of lithium into Li_xWO₃ and Li_xWO₃:P films prepared by the sol–gel process were investigated. The standard Gibbs energy for lithium intercalation was calculated. The chemical diffusion coefficients, D, of lithium intercalation into oxide, were measured by galvanostatic intermittent titration technique (GITT), as functions of the depth of lithium intercalation.     

Effect of pyrophosphates as supporting matrices on proton conductivity for NH4PO3 composites at intermediate temperatures

Composite electrolytes of NH₄PO₃/pyrophosphate (NH₄PO₃/ZrP₂O₇, NH₄PO₃/Sr₂P₂O₇, and NH₄PO₃/TiP₂O₇) with various molar ratios were fabricated, and their thermal and electrochemical properties were compared at intermediate temperatures. The XRD pattern of NH₄PO₃/Sr₂P₂O₇ composite was consistent with a mixed phase of crystalline NH₄PO₃ and Sr₂P₂O₇ regardless of the composition ratio, whereas those of the other composites were identical to pyrophosphates. A significant difference in conductivity was observed depending on the supporting matrices of pyrophosphates although each composite contained almost the same molar concentration of NH₄PO₃. Among the composites, NH₄PO₃/ZrP₂O₇ (molar ratio; 1:1) exhibited the highest proton conductivity, which was more than twice that of NH₄PO₃/TiP₂O₇ (1:1). The conductivity of NH₄PO₃/Sr₂P₂O₇ (2:1) composite was 2–3 orders of magnitude lower than that of NH₄PO₃/ZrP₂O₇ (1:1). These results suggest that the surface property of pyrophosphates strongly affects the electrochemical properties of composites. Furthermore, a fuel cell that used NH₄PO₃/ZrP₂O₇ composite as an electrolyte was successfully demonstrated at 300 °C.     

Synthesis of LiCoO2 thin films by sol/gel process

LiCoO₂ thin films were synthesized by sol/gel process using acrylic acid (AA) as chelating agent. The gel formulation was optimized by varying solvent (ethylene glycol or water) and precursors molar ratios (Li, Co, AA) in order to obtain a dense film for positive electrode of lithium batteries. The gel was deposited by spin-coating technique on an Au/TiO₂/SiN/SiO₂/Si substrate. Thin films were deposited by either single or multistep process to enhance the density of the thin film and then calcined during 5 h at 800 °C to obtain the R-3m phase (HT-LiCoO₂).A chemical characterization of the solution was realized by Fourier Transform Infrared (FTIR) spectroscopy. Thermal decomposition of precursors and gel was studied by Thermo Gravimetric Analyses (TGA). Further investigations were done to characterize rheologic behaviour of the gel and solvents affinity with the substrate. Crystallinity and morphology were analyzed respectively by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM).The formation of R-3m phase was confirmed by the electrochemical behaviour of the gel derived LiCoO₂. Cyclic voltammograms and galvanostatic cycling show typical curve shape of the HT-LiCoO₂.     

Sintering and conductivity of BaCe0.9Y0.1O2.95 synthesized by the sol-gel method

Ceramic powders of BaCe_0.9Y_0.1O_2.95 (BCY10) have been prepared by the sol-gel method. Barium and yttrium acetate and cerium nitrate were used as ceramic precursors in a water solution. The reaction process studied by DTA-TG and XRD showed that calcination of the precursor powder at T ≥ 1000 °C produces a single perovskite phase. The densification behaviour of green compacts studied by constant heating rate dilatometry revealed that the shrinkage rate was maximal at 1430 °C. Sintered densities higher than 95% of the theoretical one were thus obtained below 1500 °C. The bulk and additional blocking effects were characterized by impedance spectroscopy in wet atmosphere between 150 and 600 °C. A proton conduction behaviour was clearly identified. The blocking effect can be related to a space-charge depletion layer of protons in the vicinity of grain boundaries.     

Preparation of supported SrCeO3-based membrane by spin coating method

SrCe_0.9Y_0.1O_3−δ (SCY10) powder with a pure perovskite phase is prepared by solid-state reaction method. NiO is dispersed uniformly in SCY10 powder to fabricate NiO-SCY10 anode substrate. The starting powder, the mixture of SrCO₃, CeO₂ and Y₂O₃, is deposited directly on the green substrate instead of SCY10 powder by spin coating. After co-firing at 1300 °C for 3 h, the starting powder reacts to form SCY10 top layer on the substrate. SEM micrographs show that the top layer is defect-free and adheres well with the anode substrate without any delamination. A single fuel cell is assembled with anode-supported SCY10 membrane as electrolyte membrane and Ag as cathode. The electrochemical property of the fuel cell is tested with hydrogen as fuel in the temperature range of 600-800 °C. The open circuit voltage (OCV) reaches 1.05 V at 800 °C, and the maximum power density is 50 mW cm⁻², 155 mW cm⁻², 200 mW cm⁻² at 600 °C, 700 °C, 800 °C, respectively.     

Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer

A composite oxygen electrode based on Co₃O₄-loaded La_0.8Sr_0.2MnO₃ (LSM)-BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ (BCZYZ) is investigated for steam electrolysis in a proton-conducting solid oxide electrolyzer. The conductivity of LSM is studied with respect to temperature and oxygen partial pressure and correlated to the electrochemical properties of the composite oxygen electrodes in symmetric cells and solid oxide electrolyzers at 800 °C. The optimal Co₃O₄ loading in the composite oxygen electrode is systematically investigated in symmetric cells; loading catalytically active Co₃O₄ significantly enhances the electrode performance, unlike the bare LSM-BCZYZ electrode. Steam electrolysis was then performed using LSM-BCZYZ and 6 wt.% Co₃O₄-loaded LSM-BCZYZ oxygen electrodes, respectively. The Co₃O₄-loaded catalyst significantly improves the electrode process and enhances the current density below a certain applied voltage. The current efficiencies reach approximately 46% with a 10% H₂O/air feed for the oxygen electrode.     

Catalytic mechanism of Nb2O5 and NbF5 on the dehydriding property of Mg95Ni5 prepared by hydriding combustion synthesis and mechanical milling

The catalytic mechanism of Nb₂O₅ and NbF₅ on the dehydriding property of Mg₉₅Ni₅ prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. It was shown that NbF₅ was more efficient than Nb₂O₅ in improving the dehydriding property. In particular, the dehydriding temperature onset decreases from 460 K for Mg₉₅Ni₅ to 450 K for Mg₉₅Ni₅with 2.0 at.% Nb₂O₅, whereas it decreases to 410 K for that with 2.0 at.% NbF₅. By means of X-ray diffraction and X-ray photoelectron spectroscopy, it was confirmed that the interaction between the Nb ions and the H atoms and that between the anions (O²⁻ or F⁻) and Mg²⁺ existed in Mg₉₅Ni₅ doped with Nb₂O₅ or NbF_5. Further, the pressure–concentration-isotherms analysis clarified that these interactions destabilized the Mg–H bonding, and that NbF₅ had a better effect on the destabilization of the Mg–H bonding than Nb₂O₅ contributing to the better dehydriding property of (Mg₉₅Ni₅)2.0−NbF₅.     

An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.     

Variation on anthracite combustion efficiency with CeO2 and Fe2O3 addition by Differential Thermal Analysis (DTA)

Effects of CeO₂ and Fe₂O₃ on anthracite combustion efficiency were investigated using differential thermal analysis (DTA). Based on heat release (Q_D) of anthracite as well as anthracite with CeO₂ and anthracite with Fe₂O₃ additions against α-Al₂O₃ in DTA experiment, effects of additives CeO₂ and Fe₂O₃ on anthracite combustion efficiency were evaluated. Under the same experimental conditions, heat releases of raw anthracite, anthracite with CeO₂ and anthracite with Fe₂O₃ were 11.04 kJ/g, 11.30 kJ/g and 11.42 kJ/g, respectively, indicating that anthracite combustion efficiency was improved by addition of CeO₂ and Fe₂O₃. To confirm the above results, carbon transfer was monitored using Thermogravimetric analysis Fourier transform infrared (TGA-FTIR) and Carbon-Sulfur analyzer during catalytic combustion process. The results indicated that CO₂ emission was increased, whereas CO emission and residual carbon of ash were decreased, being in accordance with the results of DTA. Finally, according to analyses of ignition temperature and catalytic combustion process, the possible mechanism of catalytic combustion of anthracite was proposed.     

Experimentally validated numerical modeling of Eu doped SrCeO3membrane for hydrogen separation

A numerical solution to the defect chemical equations was used to model the defect population in europium-doped strontium cerate (ESC) at vapor partial pressure and oxygen partial pressure range in hydrogen atmosphere. The results of the simulation compared well with the work previously reported in the literature. The numerically simulated defect concentrations were then used to predict the conductivity and hydrogen permeability of ESC membranes as a function of temperature. Uniquely, the model was then validated by comparing the predictions with experimental data for ESC membranes. The results of that exercise showed that the model is in good agreement with the experiment at temperatures high enough that the effects of defect interaction can be ignored; and where the assumption of a dilute solution of defects is valid. The agreement with the experiment further enabled the model to be used to obtain credible predictions for the ambipolar conductivity of ESC and hydrogen flux through ESC as a function of feed side hydrogen partial pressure.     

Solution combustion synthesis of Ce0.6Mn0.3Fe0.1O2 for anode of SOFC using LaGaO3-based oxide electrolyte

This paper describes the potential of solution combustion synthesis (SCS) method for preparing Ce_0.6Mn_0.3Fe_0.1O₂ (CMF) as the anode material for solid oxide fuel cells (SOFC). The stability, crystallinity, morphology, and surface area of the products were depended on the fuel ratio used in SCS as investigated by TGA, XRD, SEM, and BET, which correspondingly influenced their electrochemical properties. The SCS-derived products were directly used for preparing anodes by sintering the screen-printed powders on the electrolyte membrane, and were evaluated from power generation performance, which were compared with the conventional solid-state-reaction (SSR) sample. Significantly, under configuration of the cell of CMF/La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃/Sm_0.5Sr_0.5CoO₃ using humidified hydrogen gas as a fuel and O₂ as an oxidizing agent, the maximum power densities obtained were 1.23 W/cm² at 1000 °C for the SCS product (CMF1) obtained at ? = 0.5. This value was higher than 1.09 W/cm² for the SSR-derived sample under the same evaluation conditions. The results appealed benefits of SCS method for preparing CMF as the anode material with high power generation performance for SOFC, due to its large surface area and nanosized grains, in which fuel ratio was a key parameter for its synthesis.     

Homogeneous combustion of fuel-lean H2/O2/N2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H₂/O₂/N₂ mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gas-phase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p ? 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.     

Conductivity of porous Sm2O3-doped CeO2 as a function of temperature and oxygen partial pressure

Porous samples of Sm₂O₃-doped CeO₂ (samaria-doped ceria, SDC) of composition Sm_0.15Ce_0.85O_2−δ were made by conventional ceramic processing and sintering in air at 1400 °C. Crystal structure and microstructure of the samples were characterized, respectively, by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrical conductivity was measured using a four probe DC method over a temperature range from 200 °C to 800 °C, and over a wide range of oxygen partial pressures corresponding to testing in oxygen and in nearly dry hydrogen. Conductivity rapidly stabilized at any given temperature consistent with the attainment of thermodynamic equilibrium corresponding to the imposed conditions. At and below 300 °C, the conduction was predominantly due to oxygen ion transport. At and above 400 °C, however, significant electronic conduction occurred in reducing atmospheres. The ionic transference number of SDC at 400 °C in hydrogen is only ∼0.4. This result shows that the electrolytic domain of SDC at and above 400 °C is rather narrow. These results also suggest that SDC (and possibly other rare earth oxide-doped CeO₂) is not a suitable electrolyte without a thin electron blocking layer such as yttria-stabilized zirconia (YSZ).     

Polypropylene-supported and nano-Al2O3 doped poly(ethylene oxide)-poly(vinylidene fluoride-hexafluoropropylene)-based gel electrolyte for lithium ion batteries

A new gel polymer electrolyte (GPE) is reported in this paper. In this GPE, blending polymer of poly(ethylene oxide) (PEO) with poly(vinylidene fluoride-hexafluoropropylene) (P(VdF-HFP)), doped with nano-Al₂O₃ and supported by polypropylene (PP), is used as polymer matrix, namely PEO-P(VdF-HFP)-Al₂O₃/PP. The performances of the PEO-P(VdF-HFP)-Al₂O₃/PP membrane and the corresponding GPE are characterized with mechanical test, CA, EIS, TGA and charge-discharge test. It is found that the performances of the membrane and the GPE depend to a great extent on the content of doped nano-Al₂O₃. With doping 10 wt.% nano-Al₂O₃ in PEO-P(VdF-HFP), the mechanical strength from 9.3 MPa to 14.3 MPa, the porosity of the membrane increases from 42% to 49%, the electrolyte uptake from 176% to 273%, the thermal decomposition temperature from 225 °C to 355 °C, and the ionic conductivity of corresponding GPE is improved from 2.7 × 10⁻³ S cm⁻¹ to 3.8 × 10⁻³ S cm⁻¹. The lithium ion battery using this GPE exhibits good rate and cycle performances.