CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Performance degradation studies on PBI/H3PO4 high temperature PEMFC and one-dimensional numerical analysis

Five hundred hours continuous aging test at constant discharge current (640 mA cm⁻²) was performed on PBI/H₃PO₄ high temperature PEMFC unit cell, electrochemical techniques-linear sweep voltammetry (LSV) and AC impedance measurement were used to investigate the changes of electrochemical surface area (ESA) and high frequency resistance (internal resistance) with time. Initial experimental results showed that during 500 h continuous aging test the main reason for cell performance degradation is the decrease of ESA caused by sintering. In addition, a one-dimensional mathematical model was constructed, the concentration distributions of cathode reactant gases (O₂ and gaseous H₂O) were calculated and polarization curves recorded during aging test were simulated based on the model, the simulated polarization curves compare well with the experimental results.     

A polymer electrolyte membrane for high temperature fuel cells to fit vehicle applications

Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H₃PO₄ were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35.19 MPa. After doping with the phosphoric acid, the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0.3 S cm⁻¹ at a relative humidity 8.4% and temperature of 180 °C with a 300% H₃PO₄ doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1.2 W cm⁻² at cell voltages >0.4 V and current densities of 3.0 A cm⁻². The PTFE/PBI/H₃PO₄ composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 °C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs.     

Synthesis,performance and action mechanism of carbon black/Ag3PO4 photocatalysts

Novel carbon black (CB)/Ag₃PO₄ compound photocatalysts were synthesized from a hydrothermal method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and electrochemical methods. The photocatalytic oxidation ability of CB/Ag₃PO₄ was evaluated through methyl orange degradation experiments under visible light irradiation. The CB/Ag₃PO₄ showed higher photocatalytic activity than pure Ag₃PO₄. It was indicated the compound catalysts could absorb and utilize more optical energy to improve the photocatalytic activity, which was attributed to the ability of carbon black to accelerate the electron-hole separation. In particular, the methyl orange photocatalytic degradation rate over the 7?mg/L CB/Ag₃PO₄ was 1.6 times that of Ag₃PO₄. And the results of the cyclic experiment show that the photocatalyst has good stability. Moreover, the mechanism about the photocatalytic activity of CB/Ag₃PO₄ compounds was investigated. In this photocatalytic reaction, ?OH was the major active substrate responsible for the visible-light-driven degradation.     

High temperature proton exchange membrane fuel cell using a sulfonated membrane obtained via H2SO4 treatment of PEEK-WC

A method for the sulfonation of PEEK-WC, a glassy poly(ether ether ketone) with sulphuric acid is presented. Depending on the reaction time, polymers with ion exchange capacity (IEC) from 0.30 to 0.76 meq_H⁺/g are obtained, as determined by titration with NaOH solutions. The thermal properties of the polymers were studied by differential scanning calorimetry, showing that the glass transition temperature increases with increasing degree of sulfonation, from 224 °C for pure PEEK-WC to 246 °C for the polymer having an IEC of 0.76 meq_H⁺/g. The sulfonated polymers were used to prepare proton exchange membranes for possible application in fuel cells. Dense membranes were prepared by solvent evaporation, using DMA as the solvent. The transport properties of the membranes were determined in terms of water uptake and permeability for hydrogen and oxygen. Electrochemical characterization was performed by measuring cell voltage and power density curves as a function of current density at different working temperatures and the results were compared with those of a commercial Nafion membrane. A power density of 284 mW/cm² was obtained for S-PEEK-WC membrane at 120 °C in H₂/air fuel cell, slightly above the corresponding value found for Nafion.     

Facile modified synthesis and intermediate temperature electrical properties of Sn0.95Al0.05P2O7/KSn2(PO4)3 composite electrolyte

In this study, Sn_0.95Al_0.05P₂O₇ and a novel dense Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite electrolytes were synthesized. The structural characterization of X–ray diffraction (XRD) and microstructual properties of scanning electron microscopy (SEM) were carried out. The XRD results indicated that an in-situ reaction between Sn_0.95Al_0.05P₂O₇ and inorganic melt salt take place to form the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite. The intermediate temperature electrical properties were determined by using impedance spectroscopy, oxygen concentration cell and hydrogen concentration discharge cell. Finally, the H₂/O₂ fuel cell using the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ as electrolyte membrane was constructed and the obtained maximum power output densities were 67.7 mW cm^?2 and 142.1 mW cm^?2 at 650 °C and 700 °C, respectively.     

Ag3PO4的制备及其复合材料研究进展

能源与环境已成为当前亟需解决的问题,半导体光催化技术因具有氧化降解有机污染物完全、不产生二次污染、易操作等优点,在环境治理和新能源开发领域成为热点研究课题之一。Ag3PO4因具有高量子效率、可见光响应及较高的光催化效率等特点而引起了广大研究者的关注,并具有广泛的应用前景。本文总结了Ag3PO4的制备方法、不同形貌Ag3PO4的可控合成及其复合材料的研究进展。     

Pt-MoOx-carbon nanotube redox couple based electrocatalyst as a potential partner with polybenzimidazole membrane for high temperature Polymer Electrolyte Membrane Fuel Cell applications

A redox couple based electrocatalyst comprising of Pt-Multi Wall Carbon NanoTube (Pt-MWCNT) promoted with molybdenum oxide (MoO_x, 2 < x < 3) nanoparticles was prepared. The objective was to effectively organize the Pt-MoO_x interface on the smooth MWCNT surface to overcome the practical difficulties associated with establishing such interface with Pt dispersed on carbon morphologies possessing surface irregularities. The present study revealed the importance of stringent controlling of the additive level for maintaining a balanced bifunctional behavior of the catalyst combination through the synergistic effects by the components and the need of a proton conducting membrane operable at high temperature to get better output from the Polymer Electrolyte Membrane Fuel Cell (PEMFC) systems. An indigenously developed polybenzimidazole (PBI) membrane was used to fabricate a membrane electrode assembly (MEA) as it can be operated at higher temperatures compared to that of Nafion membranes. MoO_x additive level was carefully controlled by monitoring the active Pt area by cyclic voltammetry. All prepared electrocatalysts were characterized by using HRTEM, XRD and XPS to get information on dispersion and morphology, crystalinity and oxidation state of different elements, respectively. The system prepared with 5% MoO_x addition with respect to Pt (hereafter Pt-MoO_x(5%)-MWCNT) displayed balanced active Pt area and excellent oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities. Rotating Disk Electrode (RDE) system was extensively utilized to understand the ORR kinetics and the favorable role of MoO_x as the promoter in the reaction. The kinetic current (j_k) measured at 0.02 V vs. Hg/Hg₂SO₄ electrode from the Koutecky-Levich plots was 9 times higher and the apparent activation energy during single cell evaluation was 27 kJ/mol lower for the MoO_x promoted system, compared to the system without the additive. A higher operating temperature significantly favored the cell performance by a combined effect of enhancement in proton conductivity of the PBI membrane and possible kinetic benefit by the well postulated oxygen spill over effect by the MoO_x type systems in some combinations involving such systems.     

Effect of CO2, HCO3 and CO3 on oxygen reduction in anion exchange membrane fuel cells

The effect of carbonate and bicarbonate anions on the oxygen reduction reaction was investigated in four alkaline solutions (pH ∼ 14) on a Pt disk type electrode with varying concentrations of carbonate and bicarbonate. The addition of carbonate and bicarbonate had two primary effects on the observed voltammetric behavior: i) The Tafel slope shifts positive with increasing carbonate/bicarbonate concentration, indicating that the carbonate anions may compete for surface adsorption sites; and ii) The dissolved oxygen concentration and diffusion coefficient are depressed with increasing anion concentration. Finally, adding CO₂ to the cathode stream of an anion exchange membrane fuel cell caused an improvement in the device performance under fully hydrated conditions, suggesting that the fuel cell was operating at least partially under the carbonate cycle.     

Curing temperature effect on smokeless fuel briquettes prepared with molasses and H3PO4

Chars from the co-carbonisation of a low-rank coal and olive stones have been used to prepare environmental acceptable smokeless fuel briquettes. The blend was a mixture of char, molasses and H₃PO₄. This acid was added to favour the polymerisation of the binder. The effect of the curing temperature on the physico-chemical features of the briquettes was studied by Fourier Transform Infrared Spectroscopy and Temperature Programmed Decomposition followed by Mass Spectrometry. The presence of H₃PO₄ as well as the curing process at 200 °C of temperature, contribute to the formation of carboxylic acids which lead to the production of briquettes with adequate mechanical properties.     

Solid state protonic conductor NH4PO3–(NH4)2Mn(PO3)4 for intermediate temperature fuel cells

A new proton-conductive composite of NH₄PO₃–(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm⁻¹ was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²–10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.     

Steam reforming of ethanol over Co3O4/CeO2 catalysts prepared by different methods

In this paper, Co₃O₄/CeO₂ catalysts for steam reforming of ethanol (SRE) were prepared by co-precipitation and impregnation methods. The catalysts prepared by co-precipitation were very active and selective for SRE. Over 10%Co₃O₄/CeO₂ catalyst, ethanol conversion was close to 100% and hydrogen selectivity was about 70% at 450 °C. The catalysts were characterized by X-ray diffraction, temperature-programmed reduction (TPR) and BET surface area measurements. The preparation method influenced the interaction between cobalt and CeO₂ evidently. The incorporation of Co ions into CeO₂ crystal lattice resulted in weaker interaction between cobalt and ceria on catalyst surface. In comparison with catalysts prepared by impregnation, more cobalt ions entered into CeO₂ lattice, and resulted in weaker interaction between active phase and ceria on surface of Co₃O₄/CeO₂ prepared by co-precipitation. Thus, cobalt oxides was easier to be reduced to metal cobalt which was the key active component for SRE. Meanwhile, the incorporation of Co ions into CeO₂ crystal lattice was beneficial for resistance to carbon deposition.     

Fabrication and photocatalytic performance of one-dimensional Ag3PO4 sensitized SrTiO3 nanowire

The 1D Ag₃PO₄ sensitized SrTiO₃ nanowires are prepared by simple route of electrospinning-in situ deposition technique. The results of the thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive Spectrometer (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–Visible diffuse reflectance spectroscopy (UV–Vis) indicate that the Ag₃PO₄ nanoparticles has been deposited on the surface of the SrTiO₃ nanowires successfully. Experimental results showed that compared with pure SrTiO₃, the as-prepared 1D Ag₃PO₄ sensitized SrTiO₃ nanowires exhibit obvious enhancement of photocatalytic performance and stability. Especially, the Ag₃PO₄/SrTiO₃ (3AS sample) had a satisfactory photocatalytic activity for degrading methylene blue (MB) more than 98% under visible light irradiation. As to pure SrTiO₃ and Ag₃PO₄, only 9.8% and 49% of MB was decomposed after 35?min irradiation respectively. Furthermore, the mechanism of the enhancing photocatalytic activity could be ascribed to the nano-heterojunction of the Ag₃PO₄/SrTiO₃, the visible light response of the Ag₃PO₄, and the 1D structure of the nanowires.     

Partial oxidation of ethanol on Ru/Y2O3 and Pd/Y2O3 catalysts for hydrogen production

The partial oxidation of ethanol was investigated over Ru and Pd catalysts supported onto yttria over a wide range of temperatures (473–1073 K). The product distributions obtained over these catalytic systems were correlated with diffuse reflectance infrared spectroscopy analyses (DRIFTS). Results showed that reaction route depended strongly on the type of metal. The decomposition of ethoxy species to CH₄ and CO or oxidation to CO₂ was promoted by Pd, and the acetaldehyde desorption was predominant over Ru in the low temperature region. Furthermore, the acetate and carbonate formation prevailed over Pd, which explained the lower acetaldehyde selectivity. The presence of CH₄ and CO₂ at high temperature is assigned to the decomposition of acetate species via carbonates over Pd-based catalysts. Ru was more suitable system for H₂ production than Pd by achieving a selectivity of about 59%.     

O2 reduction at the Pt/Nafion interface in 85% concentrated H3PO4

The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion® in concentrated 85% H₃PO₄. The effects of Nafion® coatings of different thickness on O₂ electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion® film thickness while the diffusion limiting current decreases with increasing Nafion® film thickness. A O₂ concentration profile model for the Pt/Nafion®/bulk electrolyte has been established, and this model can be used to explain the O₂ reduction polarization results. The performance of Nafion®-modified, high surface area Pt/carbon air cathodes for use in the H₂–air concentrated phosphoric acid fuel cell was also studied.     

Oxide (CeO2, NiO, Co3O4 and Mn3O4)-promoted Pd/C electrocatalysts for alcohol electrooxidation in alkaline media

This study investigated Pt/C, Pd/C and oxide (CeO₂, NiO, Co₃O₄ and Mn₃O₄)-promoted Pd/C for electrooxidation reactions of methanol, ethanol, ethylene glycol and glycerol in alkaline media. The results show that Pd/C electrocatalysts alone have low activity and very poor stability for the alcohol electrooxidation. However, addition of oxides like CeO₂, NiO, Co₃O₄ and Mn₃O₄ significantly promotes catalytic activity and stability of the Pd/C electrocatalysts for the alcohol electrooxidation. The Pd-Co₃O₄ (2:1, w:w)/C shows the highest activity for the electrooxidation of methanol, EG and glycerol while the most active catalyst for the ethanol electrooxidation is Pd-NiO (6:1, w:w)/C. On the other hand, Pd-Mn₃O₄/C shows significantly better performance stability than other oxide-promoted Pd/C for the alcohol electrooxidation. The poor stability of the Pd-Co₃O₄/C electrocatalysts is most likely related to the limited solubility of cobalt oxides in alkaline solutions.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

Mechanochemical synthesis of kaolin-KH2PO4 and kaolin-NH4H2PO4 complexes for application as slow release fertilizer

A milling process to reduce kaolin to amorphous phase in the presence of KH₂PO₄ or NH₄H₂PO₄ and allow mechanochemical (MC) reaction for incorporation of KH₂PO₄ and NH₄H₂PO₄ into the kaolin structure was investigated in this work. Mixtures of kaolin and KH₂PO₄ and NH₄H₂PO₄ in separate systems were prepared by milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotational speeds from 200 to 700 rpm were performed to evaluate incorporation of KH₂PO₄ and NH₄H₂PO₄ and release of K⁺, NH₄⁺ and PO₄³⁻ ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC process was successfully applied to incorporate both KH₂PO₄ and NH₄H₂PO₄ into the amorphous kaolin structure. Release of K⁺ and PO₄³⁻ ions from the system (kaolin-KH₂PO₄) when dispersed in water for 24 h reached only up to 10%. Under similar conditions for the system (kaolin-NH₄H₂PO₄), release of NH₄⁺ and PO₄³⁻ ions reached between 25 and 40%. These results indicated that the MC process can be developed to allow amorphous kaolin to act as a carrier of K⁺, NH₄⁺ and PO₄³⁻ nutrients to be released slowly for use as fertilizer.     

Selective CO removal in a H2-rich stream over supported Ru catalysts for the polymer electrolyte membrane fuel cell (PEMFC)

We prepared various Ru catalysts supported on different supports such as yttria-stabilized zirconia (YSZ), ZrO₂, TiO₂, SiO₂ and γ-Al₂O₃ with a wet impregnation method. We applied them to the selective CO removal in a hydrogen-rich stream via the preferential CO oxidation (PROX) and the selective CO methanation simultaneously. Among them, Ru/YSZ showed the highest CO conversion especially at low temperatures. Several measurements: the N₂ physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), the CO chemisorptions, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR), the temperature-programmed desorption (TPD) of CO₂ with mass spectroscopy and the transmission electron microscopy (TEM), were conducted to characterize the catalysts. No linear correlation can be found between the amount of CO chemisorbed at 300 K and the PROX activity. On the other hand, the facile activation of O₂ appeared to be closely related to the high PROX activity, judging by the TPO experiment. In addition, the strong adsorption of CO₂ suppressed the low-temperature PROX activity. Ru/YSZ can be easily oxidized and also reduced at low temperatures. It is found that Ru/YSZ uptakes only small amounts of CO₂, which can be desorbed at low temperatures. Ru/YSZ can reduce the high inlet CO concentration to be less than 10 ppm even in the presence of H₂O and CO₂.     

Preparation,structural and luminescent properties of YAl3(BO3)4:Dy phosphor for white light-emission under UV excitation

A series of YAl₃(BO₃)₄ phosphors doped with different concentrations of Dy₂O₃ (0.1≤x≤5 mol%) were prepared by solid-state reaction method. The crystallization process of the precursor has been examined by differential thermal analysis (DTA) measurements. The phase purity and surface morphological features were characterized by X-ray diffraction (XRD) and scanning electron microscopic (SEM) investigations. The YAl₃(BO₃)₄ nanocrystals obtained were found to be about 45 nm in size and have the trigonal structure with some agglomeration. Fourier transform infrared (FTIR) and energy dispersive X-ray spectra (EDS) measurements were carried out to understand the compositional and elemental analysis. The characteristics emission peaks of Dy³⁺ ion corresponding to the transitions of ⁴F_9/2→⁶H_15/2 at 485 nm and ⁴F_9/2→⁶H_13/2 at 576 nm were observed in the emission spectra. The luminescence quenching noticed at higher Dy₂O₃ concentrations is due to the exchange interaction among the excited Dy³⁺ ions.