The interaction of alumina with HCl: An infrared spectroscopy, temperature-programmed desorption and inelastic neutron scattering study

The interaction of HCl with an η-alumina catalyst has been investigated by a combination of diffuse reflectance infrared spectroscopy, temperature-programmed desorption and inelastic neutron scattering (INS) spectroscopy. Infrared spectra provide evidence for dissociative adsorption of HCl and for a process in which hydroxyl groups terminally bound to Al are replaced by chlorine. Temperature-programmed desorption experiments show HCl to desorb over the temperature range 350–>970 K, indicating dissociative HCl adsorption to occur on a wide range of active sites. INS experiments show the residual alumina hydroxyl groups to exhibit an out-of-plane deformation feature, γ(OH), at ca. 200 cm⁻¹, while the in-plane deformation mode, δ(OH), is seen at ca. 1000 cm⁻¹. The formation of new surface hydroxyl groups via the adsorption of hydrogen chloride yields a δ(OH) feature that can be resolved into two bands at 990 and 1050 cm⁻¹. Hydrogen bonding within the alumina/HCl system is responsible for the observation of an Evans transmission window in the infrared spectrum, that occurs via a Fermi resonance interaction between (i) the ν(OH) mode of hydrogen bonded hydroxyl groups and chemisorbed water with (ii) the overtone of the δ(OH) mode of surface hydroxyl groups. The INS technique is able to discriminate among different hydroxyl group bonding geometries on the basis of the local symmetry of the active sites.     

An electrochemical impedance spectroscopy study of fuel cell electrodes

Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)₃²⁺. Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance.     

Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce_0.8Gd_0.2O_1.9 + xBaO (x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175-350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.     

Proton conductors of cerium pyrophosphate for intermediate temperature fuel cell

The crystal structure and proton conductivity of cerium pyrophosphate are investigated to explore its potential electrolyte applications for intermediate temperature fuel cell. Among the CeP₂O₇ thin plates, which are sintered at 300–900 °C, the 450 °C CeP₂O₇ sample exhibits superior proton conductivity under humidified conditions. Its conductivity, measured with impedance spectroscopy, is higher than 10⁻² S cm⁻¹ in the intermediate temperature range, with a maximum value 3.0 × 10⁻² S cm⁻¹ at 180 °C. When 10 mol% Mg is doped on the Ce site of CeP₂O₇, the maximum conductivity is raised to 4.0 × 10⁻² S cm⁻¹ at 200 °C. The Mg doping not only raises the conductivity, but also shifts and widens its temperature window for electrolyte applications. Ce_0.9Mg_0.1P₂O₇ is considered a more appropriate composition, with conductivity >10⁻² S cm⁻¹ between 160 and 280 °C. Accordingly, a hydrogen–air cell is built with the Ce_0.9Mg_0.1P₂O₇ electrolyte and its performance is measured. The fuel cell generates electricity up to 122 mA cm⁻² at 0.33 V using 50% H₂ at 240 °C.     

PEM fuel cells as membrane reactors: kinetic analysis by impedance spectroscopy

Proton exchange membrane fuel cells (PEMFC) are considered as electrochemical reactors, performances of which are regarded in the context of the various effects influencing FC output, such as mass transports, kinetic of electrode reactions and charge transfer in polymer electrolyte membrane (PEM). An experimental approach, involving the employment of impedance spectroscopy (IS), which allows a deep insight into the nature of these effects, is discussed and its applications to the different aspects of PEMFC functioning are reported. As examples of the use of IS in PEMFC studies, the investigations of the membrane conductivity and in situ studies of the anode and the cathode processes during FC operation are presented.     

Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.     

Analysis of high temperature polymer electrolyte membrane fuel cell electrodes using electrochemical impedance spectroscopy

An electrochemical impedance spectroscopy (EIS) study of electrodes in a phosphoric acid loaded polybenzimidazole (PBI) membrane fuel cell is reported. Using EIS, the effect of electrode parameters such as Pt catalyst wt%, acid doping in PBI and PTFE baesd electrodes and catalyst heat treatment on kinetic and mass transport characteristics is characterised. The influence of cell parameters of current load, temperature and oxidant gas on response is demonstrated and interpreted using an equivalent circuit model. For polarisable electrodes under small to medium steady-state current operation, the model was capable of identifying electrodes with the best kinetic or mass transport behaviour and classifying behaviour in terms of relative performance.     

IN-SITU FT-IR SPECTROSCOPIC STUDIES OF FUEL CELL ELECTRO-CATALYSIS: FROM SINGLE-CRYSTAL TO NANOPARTICLE SURFACES

The work presented in this article shows the power of the variable temperature, in-situ FT-IR spectroscopy system developed in Newcastle with respect to the investigation of fuel cell electro-catalysis. On the Ru(0001) electrode surface, CO co-adsorbs with the oxygen-containing adlayers to form mixed [CO + (2 × 2)-O(H)] domains. The electro-oxidation of the Ru(0001) surface leads to the formation of active (1 × 1)-O(H) domains, and the oxidation of adsorbed CO then takes place at the perimeter of these domains. At 20°C, the adsorbed CO is present as rather compact islands. In contrast, at 60°C, the CO_ads is present as a relatively looser and weaker adlayer. Higher temperature was also found to facilitate the surface diffusion and oxidation of CO_ads. No dissociation or electro-oxidation of methanol was observed at potentials below approximately 950 mV; however, the Ru(0001) surface at high anodic potentials was observed to be very active. On both Pt and PtRu nanoparticle surfaces, only one linear bond CO adsorbate was formed from methanol adsorption, and the PtRu surface significantly promoted both methanol dissociative adsorption to CO and its further oxidation to CO₂. Increasing temperature from 20° to 60°C significantly facilitates the methanol turnover to CO₂.     

国外燃料电池的开发与研究

文章简要叙述了燃料电池的工作原理,介绍国外关于燃料电池研究与开发的几种方法。     

Power generation/energy storage by a fuel cell/battery system: Regeneration of the MnO2 positive electrode with gaseous oxygen

Fuel cell/battery (FCB) systems are promising power generation/energy storage systems because of their bi-functionality as fuel cells and as secondary batteries. We investigated the required charging after the discharged manganese dioxide (MnOOH) by oxygen gas under the rest condition and during the fuel cell operation mode using manganese dioxide as a positive electrode for the FCB system. Electrochemical characterization was performed using cyclic voltammetry and galvanostatic measurements. Additionally, changes in the crystal structure and the chemical functional groups during the electrode reactions were monitored by X-ray diffractometry and Fourier transform infrared spectroscopy. The results indicated that MnOOH formed via the electrochemical discharge of manganese dioxide (MnO₂) and that the oxyhydroxide can be chemically transformed back to MnO₂ with gaseous oxygen (O₂). The recharged MnO₂ can be used as the cathode in a fuel cell with an O₂ supply and it can also be electrochemically discharged without an O₂ supply. In addition, we confirmed that MnO₂ does not convert to Mn₃O₄ during the charge/discharge cycles if the redox reaction is maintained within a restricted range where a homogeneous process exists between MnO₂ and MnOOH. The results in this study suggest that the FCB system can be constructed using MnO₂ as the positive electrode and a metal hydride (MH) as the negative electrode, which can be rapidly charged to more than 70% of the theoretical capacity within 10 min using pressurized H₂ and electrochemically discharged, in an alkaline electrolyte. This system possesses a high-power generation efficiency, a high-energy density and a high load-following capability.     

Impact of acid containing montmorillonite on the properties of Nafion membranes

The counter-ions of montmorillonite have been exchanged for ammonium cations containing either a sulfonic acid or a carboxylic acid in order to improve the performances of sulfonated membranes in direct methanol fuel cell. These layered silicates have been dispersed within Nafion^® by solution mixing. Comparison with conventional organo-modified montmorillonite (Cloisite 30B) shows that the incorporation of carboxylic acid in the clay galleries improves the filler dispersion and, consequently, the methanol barrier properties. Moreover, the negative impact of Cloisite 30B on the ionic conductivity is restricted.     

Polybenzimidazole containing benzimidazole side groups for high-temperature fuel cell applications

A polybenzimidazole (PBI) containing bulky basic benzimidazole side groups, poly[2,2′-(2-benzimidazole-p-phenylene)-5,5′-bibenzimidazole] (BIpPBI), was prepared via the condensation polymerization of 3,3′-diaminobenzidine tetrahydrochloride dihydrate with 2-benzimidazole terephthalic acid in PPA. BIpPBI was found to be soluble in aprotic polar solvents without the addition of inorganic salts, such as lithium chloride, and the BIpPBI film also showed very good acid retention capability as well as very high proton conductivity. The maximum acid content of the BIpPBI film was approximately 81 wt.% and the proton conductivity value of the acid-doped BIpPBI membrane was 0.16 S cm⁻¹ at 180 °C and a 0% relative humidity. For comparison, the maximum proton conductivity of the most commonly used polymer for the high-temperature fuel cell membrane, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (mPBI) membrane, is approximately 0.06 S·cm⁻¹ at 180 °C under anhydrous conditions at a 65 wt.% acid content, which is the maximum acid content that a mPBI membrane can have.     

Oxidative reforming of biomass derived ethanol for hydrogen production in fuel cell applications

Oxidative reforming of biomass derived ethanol over an inexpensive Ni–Cu/SiO₂ catalyst has been carried out with respect to solid polymer fuel cell (SPFC) applications. Two types of runs were performed, either under diluted conditions (with helium as diluent) or under conditions corresponding to an on-board reformer. Selectivities of ethanol reforming have been analyzed as a function of operating parameters: reaction temperature, H₂O/EtOH molar ratio and O₂/EtOH molar ratio of the feed to the reformer. The hydrogen content and the CO₂/CO_x molar ratio in the outlet gases were used as parameters to optimize the operating conditions in the reforming reactor. The tests carried out at on-board reformer conditions evidenced that an H₂O/EtOH molar ratio=1.6 and an O₂/EtOH molar ratio=0.68 at 973 K allow a hydrogen rich mixture (33%) that can be considered of high interest for SPFC. Furthermore, the use of oxygen decreases the production of methane and coke which increases in turn the lifetime of the catalyst. The stability of this catalyst has been fully demonstrated by long time runs.     

燃料电池的新进展

主要叙述了国内外燃料电池的新进展,通过分析讨论对国内外燃料电池新进展有较全面的了解。     

Aspects of the anodic oxidation of methanol

This paper describes some aspects of recent investigations into the anodic oxidation of methanol. Methanol has long been proposed as an anode fuel for a fuel cell, chiefly because of its ease of carriage, distribution and manipulation. However, methanol is very much more difficult to oxidise anodically than hydrogen, the more conventional anode fuel, and this has hampered development of commercial direct methanol fuel cells. Platinum-ruthenium catalysts are the most active discovered to date. Some advances in electrocatalysis of the methanol reaction by non-noble materials are discussed.     

Membrane performance comparison in a proton exchange membrane fuel cell (PEMFC) stack

A 5-cell proton exchange membrane fuel cell (PEMFC) stack with different types of membrane electrode assemblies (MEAs) was tested to compare their performances and electrochemical characteristics. The experimental data were obtained with a stack of 5 cells and active area of 125 cm². The stack consisted of different Nafion^® and hydrocarbon membranes with the same types of electrocatalyst. The membranes were installed in different cells and in the same stack. Polarization and voltage measurement data were obtained to compare their performances at different temperatures and anode humidity conditions. Also, impedance spectroscopy data were obtained in similar manner to compare the differences in their resistance.     

Polybenzimidazole gel membrane for the use in fuel cell

Thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA) is investigated by studying the gel morphology, thermodynamics of the gelation, and gelation kinetics utilizing test tube tilting and UV-Vis spectroscopy techniques. Gelation kinetics studies reveal that both the gelation rate and critical gelation concentration (C^∗_t=∞) are function of gelation temperature (T_gel) and the molecular weight of PBI. Highly dense fibrillar network morphology with large number of longer and thinner fibrils is obtained for higher gel concentration and higher molecular weight PBI. Both the gel melting (T_gm) and gelation (T_gel) temperature depend upon the gelation concentration and molecular weight of PBI. The presence of self-assembled chains of PA molecules, which help to produce the PBI crystallites, is observed from the thermodynamical study. I.R. and Raman studies prove the presence of strong hydrogen bonding interaction between the PBI and the PA molecules, and the free PA molecules in the gel network. The gelation occurs in two-step processes which include a slow rate determining conformational transition from coil to rod and followed by aggregation of rod via crystallization. The PA loading of PBI membrane obtained from the PBI-PA gel is significantly high compared to the conventional imbibing process membrane. The PBI gel membrane displays very high thermal and mechanical stabilities. The high acid loading and superb thermo-mechanical stability are due to the gel network structure of the membrane. The proton conductivity of the membrane at 160 °C and 0% relative humidity (RH) is ∼0.1 S cm⁻¹, which is higher than the reported values in the literature for the PBI. The activation energy of the proton conduction is 14-15 kJ/mol indicating faster proton transfer by hopping process inside the gel network.     

Radiation-induced graft polymerization of functional monomer into poly(ether ether ketone) film and structure-property analysis of the grafted membrane

Radiation-induced graft polymerization of sulfo-containing styrene derivatives into crystalline poly(ether ether ketone) (PEEK) substrates was carried out to prepare thermally and mechanically stable polymer electrolyte membranes based on an aromatic hydrocarbon polymer, so-called “super-engineering plastics”. Graft polymerization of the sulfo-containing styrene, ethyl 4-styrenesulfonate (E4S) into a high crystalline PEEK substrate (degree of crystallinity: 32%) hardly progressed, whereas graft polymerization into a low crystalline PEEK substrate (degree of crystallinity: 11%) gradually progressed, achieving a grafting degree of more than 50% after 72 h. Oxygen radicals appeared in the ESR spectra of irradiated PEEK films, indicating that graft polymerization initiates from the phenoxy radicals generated by scission of PEEK main chains and proceeds so as to yield block type grafts. The PEEK-based polymer electrolyte membrane (PEM) converted by aqueous hydrolysis of grafted films exhibited mechanical strength (100 MPa), being 88% of the original PEEK substrates. These mechanical properties of PEEK-based PEM are much higher than those of graft-type fluorinated PEM reported previously and almost three times higher than that of Nafion (35 MPa). Wide- and small-angle X-ray scattering (WAXS and SAXS) indicated that the graft polymerization was accompanied with recrystallization of the amorphous phase of PEEK substrate, the well known solvent-induced recrystallization of amorphous PEEK solids, to form a weak lamellar structure with 8 nm spacing. Complementary SAXS and small-angle neutron scattering (SANS) observations clearly showed that the graft-type PEEK membranes possessed ion channel domains with the average distance of 13 nm, being larger than that of Nafion. Furthermore, there was a micro-structure in the ion channels with the average distance of 1.8 nm.