Abstract: Long-term durability testing of polybenzimidazole (PBI)-based polymer electrolyte membrane (PEM) fuel cells was performed using test protocols designed to simulate fuel cell operational situations which may be found in real applications. The fuel cell voltages and phosphoric acid (PA) loss were carefully monitored over thousands of hours and hundreds of cycles. In the typical operating range for high temperature PEM fuel cells (160 °C), the fuel cell voltage degradation rate was 4.9 μV h–1 for steady-state operation. The PA loss rates were generally low and indicated that long-term operation (>10,000 h) was possible without significant performance degradation due to PA loss from the membrane. Dynamic durability tests also showed that the PA loss rate from the membrane electrode assemblies (MEAs) depended on cell operating temperature and load conditions. Under all conditions, the PA loss was a relatively small amount of the total PA in the membrane.

Abstract: Solid oxide fuel cells (SOFCs) are currently the focus of intense investigation given their high chemical-to-electrical energy conversion efficiency and low carbon footprint. In this review, the development of thin film SOFCs, sometimes described as ‘micro-SOFCs', is highlighted and analysed. Opportunities for reduced temperature operation and portable power generation arise from the decreased thickness of the solid electrolyte, as well at the metastable phases and nanoscale-dependent effects that are a consequence of the reduced temperature of fabrication. Challenges such as enhanced cation diffusion along grain boundaries are; however, also observed, potentially impacting the long-term stability of these devices. Recent progress achieved in understanding these and other challenges are reviewed and directions for future work identified.

Abstract: High temperature PEMFCs based on phosphoric acid-doped ABPBI membranes have been prepared and characterised. At 160 °C and ambient pressure fuel cell power densities of 300 mW cm–2 (with hydrogen and air as reactants) and 180 mW cm–2 (with simulated diesel reformate/air) have been achieved. The durability of these membrane electrode assemblies (MEAs) in the hydrogen/air mode of operation at different working conditions has been measured electrochemically and has been correlated to the cell resistivity, the phosphoric acid loss rate and the catalyst particle size. Under stationary conditions, a voltage loss of only –25 μV h–1 at a current density of 200 mA cm–2 has been deduced from a 1,000 h test. Under dynamic load changes or during start–stop cycling the degradation rate was significantly higher. Leaching of phosphoric acid from the cell was found to be very small and is not the main reason for the performance loss. Instead an important increase in the catalyst particle size was observed to occur during two long-term experiments. At high gas flows of hydrogen and air ABPBI-based MEAs can be operated at temperatures below 100 °C for several hours without a significant irreversible loss of cell performance and with only very little acid leaching.

Abstract: This paper discusses the effect of compression pressure on the mechanical and thermal properties of gas diffusion layers (GDL). The stress–strain curve of the GDL revealed one nonlinear and two piecewise linear regions within the compression pressure range of 0–5.5 MPa. The thermal conductivity of the compressed GDL seems to be independent of the compression pressure and was determined to be 1.18 ± 0.11 W m–1 K–1 at room temperature. The thermal contact resistance between the GDL and graphite was evaluated by augmenting experiments with computer modelling. The thermal contact resistance decreased nonlinearly with increasing compression pressure. According to the results here, the thermal bulk resistance of the GDL is comparable to the thermal contact resistance between the GDL and graphite. A simple one-dimensional model predicted a temperature drop of 1.7–4.4 °C across the GDL and catalyst layer depending on compression pressures.

Abstract: BaZr0.8Y0.2O3–δ, (BZY), a protonic conductor candidate as an electrolyte for intermediate temperature (500–700 °C) solid oxide fuel cells (IT-SOFCs), was prepared using a sol–gel technique to control stoichiometry and microstructural properties. Several synthetic parameters were investigated: the metal cation precursors were dissolved in two solvents (water and ethylene glycol), and different molar ratios of citric acid with respect to the total metal content were used. A single phase was obtained at a temperature as low as 1,100 °C. The powders were sintered between 1,450 and 1,600 °C. The phase composition of the resulting specimens was investigated using X-ray diffraction (XRD) analysis. Microstructural characterisation was performed using field emission scanning electron microscopy (FE-SEM). Chemical stability of the BZY oxide was evaluated upon exposure to CO2 for 3 h at 900 °C, and BZY showed no degradation in the testing conditions. Fuel cell polarisation curves on symmetric Pt/BZY/Pt cells of different thicknesses were measured at 500–700 °C. Improvements in the electrochemical performance were obtained using alternative materials for electrodes, such as NiO-BZY cermet and LSCF (La0.8Sr0.2Co0.8Fe0.2O3), and reducing the thickness of the BZY electrolyte, reaching a maximum value of power density of 7.0 mW cm–2 at 700 °C.

Abstract: Many new materials have been identified as being of potential interest to solid oxide fuel cell developers. Recent work has suggested that materials of the La2NiO4 composition possess sufficient activity to act as an effective cathode. Whilst there have been many studies of the transport properties of materials of this family the reactivity with electrolyte compositions has received relatively little attention. In this work we have investigated the compatibility of La2NiO4 +d (LNO) with LSGM and ceria gadolinium oxide (CGO) electrolytes. It is clear that under an air atmosphere LNO reacts readily with CGO electrodes with the formation of a higher order Ruddlesden–Popper (Lan + 1NinO3n + 1) phase as one of the reaction products. In contrast, there is no evidence of secondary phase formation with LSGM electrolytes from the obtained diffraction data over a period of 72 h at temperatures up to 1,273 K.

Abstract: In order to improve the reliability of anode-supported solid oxide fuel cells (SOFCs) in terms of tolerance to redox cycles, and to minimise fuel preprocessing for the direct use of readily available hydrocarbons in SOFCs, alternative ceramic-based anode substrate materials and functional anode materials were investigated, with the emphasis on perovskite oxides. Compared to (Sr,La)TiO3–δ and Nb2TiO7–δ, (Sr,Y)TiO3–δ ceramics (SYT, with a Y content of about 7 at.-%) reduced at high temperatures are promising redox-stable anode substrate materials due to their very small dimensional change upon redox cycling (0.14%). Correspondingly, the composite ceramic SYT/YSZ impregnated with ∼5 vol.-% Ni has a good chance of being applied as a redox-stable functional anode material due to its even smaller dimensional change upon redox cycling (<0.05%) and superior electrochemical performance for H2 oxidation (polarisation resistance ∼0.2 Ω cm2 at 800 °C). The small fraction of Ni homogeneously dispersed on the pore walls of the SYT/YSZ ceramic framework significantly enhances the electrocatalytic activity, and should not have a detrimental effect on the redox stability of the electrode. In addition, two mixed conductors with perovskite structure, (La0.8Sr0.2)0.94Al0.5Mn0.5O3–δ and La0.4Sr0.6Ti0.4Mn0.6O3–δ, were also investigated. They show promising electrochemical performance for the oxidation of H2. However, the high-level Mn substitution which seems necessary to achieve a significant electrical conductivity and catalytic activity has a detrimental effect on the chemical stability of these two materials. Consequently, they show relatively large and irreversible chemical expansion and are thus not considered to be qualified functional anode materials.

Abstract: Sr-doped LaMnO3 (LSM) thin film microelectrodes prepared by pulsed laser deposition and photolithography on YSZ (yttria stabilised zirconia) electrolytes are investigated by impedance spectroscopy. Oxygen partial pressure (p), microelectrode diameter and microelectrode thickness are varied in order to achieve a mechanistic interpretation of the impedance spectra. By means of an equivalent circuit based on a continuum model a quantitative analysis of the spectra was possible and allowed a separation of the resistances associated with oxygen exchange at the surface of the LSM electrodes and oxide ion transport through the thin film electrodes. At low oxygen partial pressures (mbar range) the surface-related resistance is rate limiting. On increasing p the transport resistance becomes larger while the surface resistance decreases and a change in the rate-limiting step can be concluded for high p. Capacitances are interpreted in terms of processes at interfaces and stoichiometry changes in LSM (chemical capacitance). “The p dependence of the ionic conductivity and of the chemical diffusion coefficient of LSM is determined.”

Abstract: Zirconium phosphate (ZrP) was incorporated in preformed Nafion 117 membranes by exchange of the ionomer protons with zirconium cationic species and subsequent treatment with phosphoric acid. Reiteration of these treatments under the same reaction conditions allowed to obtain membranes with filler loading in the range of 20–40 wt.-%. 31P MAS NMR investigations showed that the filler is mainly α-ZrP and that the fraction of monohydrogen phosphate groups in the α-type environment increases with the number of treatments. All membranes were characterised by stress–strain mechanical tests at room temperature and turned out to be stiffer than Nafion 117. Through-plane conductivity determinations were carried out as a function of temperature and relative humidity (RH). At 100 °C and RH in the range of 30–90%, the composite membranes were less conductive than Nafion 117, and the higher the filler loading the larger the conductivity decrease occurring with decreasing RH. The conductivity of the composite membranes showed however better long-term stability at high temperature and RH values. Moreover, the partial replacement of the phosphate groups of ZrP with sulphophenylphosphonate groups leads to a significant improvement of conductivity and stiffness with respect to both Nafion 117 and the parent composite membrane.

Abstract: The effects of inhomogeneous compression of gas diffusion layers (GDLs) on local transport phenomena within a polymer electrolyte membrane (PEM) fuel cell were studied theoretically. The inhomogeneous compression induced by the rib/channel structure of the flow field plate causes partial deformation of the GDLs and significantly affects component parameters. The results suggest that inhomogeneous compression does not significantly affect the polarisation behaviour or gas–phase mass transport. However, the effect of inhomogeneous compression on the current density distribution is evident. Local current density under the channel was substantially smaller than that under the rib when inhomogeneous compression was taken into account, while the current density distribution was fairly uniform for the model which excluded the effect of inhomogeneous compression. This is caused by the changes in the selective current path, which is determined by the combination of conductivities of components and contact resistance between them. Despite the highly uneven current distribution and variation in material parametres as a function of GDL thickness, the temperature profile was relatively even over the active area for both the modelled cases, contrary to predictions in previous studies. However, an abnormally high current density significantly accelerates deterioration of the membrane and is critical in terms of cell durability. Therefore, fuel cells should be carefully designed to minimise the harmful effects of inhomogeneous compression.

Abstract: The layered perovskite GdBaCo2O5 + δ (GBCO), recently proposed for intermediate temperature solid oxide fuel cell applications, was investigated and compared with Ba0.5Sr0.5Co0.8Fe0.2O3 – δ (BSCF) cathode material using La0.9A0.1Ga0.8Mg0.2O2.85 (A=Sr,Ba) as electrolytes. Area-specific resistance was measured by impendance spectroscopy in symmetrical cells. The cobaltites were prepared by a modified citrate sol–gel route and tested as cathode materials for doped lanthanum gallate-based cells using dry H2 as fuel and air as oxidant, rendering power density values of 180 and 240 mW cm–2 at 1,073 K (1 mm thick pellets) for GBCO and BSCF fuel cells, respectively.

Abstract: Low-molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ·OH radicals produced by UV irradiation of aqueous H2O2 solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5-dimethylpyrroline-N-oxide (DMPO), N-tert-butyl-α-phenylnitrone (PBN) and 2-methyl-2-nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ·OH and of carbon-centred radicals (CCRs), and allowed the determination of the ·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H2O2. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting 19F nuclei; in addition, oxygen-centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from 19F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.

Abstract: Sb-doped SnP2O7 were prepared and characterised with XRD, FTIR, SEM and EIS. The preparation parameters were optimised taking into consideration the influence of H3PO4 content in the reactants and PmOn impurities. Conductivities of SnP2O7 with different Sb doping levels were studied from 100 to 300 °C in un-humidified air. The conductivity of 20 mol-% Sb-doped SnP2O7 was greater than 0.1 S cm–1 at 300 °C. The effect of heat treatment was identified as an important factor in the preparation of the proton conductors. The time dependence of the conductivity demonstrated Sb-doped SnP2O7 as a promising intermediate temperature solid proton conductor.

Abstract: An inorganic filler prepared by impregnation of phosphotungstic heteropolyacid on zirconia (HPW/Zr) was developed to be inserted into a perfluorosulphonic polymer matrix for a polymer electrolyte fuel cell (PEFC) operating at a medium temperature (80–120 °C) and low relative humidity (RH). Two different phosphotungstic acid (PWA) loadings (30 and 45% w/w) were anchored on a nanopowdered ZrO2. Such compounds were characterised by different techniques: differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and porosity and surface area by Brunauer–Emmett–Teller (BET), to verify the introduction and anchorage of PWA on ZrO2. Two composite Nafion membranes were prepared and characterised in terms of chemical–physical characteristics and electrochemical tests. Thermogravimetric analysis (TGA) provided evidence that HPW/Zr had been incorporated into composite membranes and it was not eluted. A good proton conductivity of about 6 × 10–3 S cm–1 at 120 °C and 25% RH was recorded. Accelerated in situ ageing tests highlighted a good electrochemical stability (more than 150 cycles at 90 °C with dry gases) of the composite membranes with a slow decay and a reasonable integrity of the analysed membrane-electrodes assembly (MEA). Finally, a post-mortem SEM–EDX analysis on MEAs confirmed the presence of HPW/Zr in the membrane after the in situ testing.

Abstract: In this work, the ethanol steam reforming (ESR) reaction has been studied by using a dense Pd-Ag membrane reactor (MR) by varying the water/ethanol molar ratio between 3:1 and 9:1 in a temperature range of 300-400°C and at 1.3 bar as reaction pressure. The MR was packed with a commercial Ru-based catalyst and a constant sweep gas flow rate in counter current mode was used. The influence of the temperature and the feed molar ratio on different parameters such as the ethanol conversion, the hydrogen production, the hydrogen yield and the CO-free hydrogen recovery has been evaluated. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA. 10.1002/fuce.200700018

Abstract: A series of hybrid proton-conducting membranes with an interpenetrating polymer network (IPN) structure was designed with the direct ethanol fuel cell (DEFC) application in mind. In these membranes, glutaraldehyde crosslinked poly(vinyl alcohol) (PVA) were interpenetrated with the copolymer of 2-acrylamido-2-methyl-propanesulphonic acid (AMPS) and 2-hydroxyethyl methacrylate (HEMA) crosslinked by poly(ethylene glycol) dimethacrylate (PEGDMA). Silica from the in situ sol–gel hydrolysis of tetraethyl orthosilicate (TEOS) was uniformly dispersed in the polymer matrix. The membranes fabricated as such had ion exchange capacities of 0.84–1.43 meq g–1 and proton conductivities of 0.02–0.11 S cm–1. The membranes exhibited significantly lower fuel permeabilities than that of Nafion. In a manner totally unlike Nafion, fuel permeabilities were lower at higher fuel concentrations, and were lower in ethanol than methanol solutions. These behaviours are all relatable to the unique swelling characteristics of PVA (no swelling in ethanol, partial swelling in methanol and extensive swelling in water) and to the fuel blocking and swelling suppression properties of silica particles. The membranes are promising for DEFC applications since a high concentration of fuel may be used to reduce fuel crossover and to improve the anode kinetics for a resultant increase in both the energy and power densities of the fuel cell.

Abstract: Direct borohydride fuel cell (DBFC) is one of the most exciting energy technologies that solve the hydrogen storage and safety issues by using aqueous solution of KBH4 or NaBH4. Here, we present a membraneless DBFC with perovskite-type oxide LaNiO3/C-catalysed cathode. A significant finding from the electrochemical experiments is that it obviously shows that the existence of ions has almost no negative influence on the discharge performances of the LaNiO3-catalysed cathode. Therefore, the DBFC is designed without using an ion exchange membrane. The maximal power density of 127 mW cm–2 is obtained at 65 °C under atmospheric pressure. A 500 h life test shows that the DBFC has good stability.

Abstract: The bipolar plate is one of the most imperative components of proton exchange membrane fuel cells (PEMFC) which consumes up to 80% of weight and near about 50% of the total cost of the cell. Development of cost-effective composite bipolar plate with high electrical conductivity and high mechanical strength is both technically and economically demanding. In this paper, a low-cost advanced composite bipolar plate is developed by bulk moulding compression (BMC) technique. It is clear from the experiments that by increasing the matrix volume fraction, bulk density and electrical conductivity of a composite bipolar plate decrease but shore hardness increases. Test results clearly show that best overall properties are achieved when a constant volume fraction of polymer matrix and natural graphite is reinforced with synthetic graphite, carbon black and carbon fibre. This bipolar plate was found to have high conductivity, less porosity and high mechanical strength. The I–V characteristics in single cell test exhibited more uniform power density at both higher and lower current densities

Abstract: The effect of methanol crossover on the fuel utilization of a passive direct methanol fuel cell (DMFC) was reported. The results revealed that the Faradaic efficiency decreased from 46.9 to 17.4% when methanol concentration increased from 1.0 to 8.0 mol L–1 at the lower current density 11.1 mA cm–2. However, the Faradaic efficiency increased from 14.7 to 31.3% when methanol concentration increased from 1.0 to 8.0 mol L–1 at a higher current density of 44.4 mA cm–2. On the other hand, although the amount of methanol was increased, the Faradaic efficiency did not change, obviously due to the uniform methanol crossover and methanol diffusion at the same methanol concentration and constant current.

Abstract: The multiwalled carbon nanotubes (MWCNTs) are treated with hydrofluoric acid (HF) aqueous solution so as to make enlarged micropores on the nanotube walls. Normally, there are no chemical bonds between the metal nanoparticles and the support. Therefore, the large contact area between metal nanoparticles and the support would decrease the possibility of nanoparticle agglomeration or their detachment from the support. The obtained large micropores to which the meal nanoparticles are attached can efficiently achieve the above advantages. In the experiment, the catalytic activity and the stability of Pd supported on HF treated MWCNTs (Pd/MWCNTHF) catalyst are evaluated by potential cycling for ethanol oxidation. The voltammetric data suggest that the obtained enlarged micropores on the treated nanotube walls can anchor the metal nanoparticles so effectively that it can overcome the agglomeration or detachment from the surface of the MWCNT. Pd/MWCNTHF catalyst shows improved stability.

Abstract: The proton conduction in immobilised imidazole systems has been investigated in order to support the design of new membrane materials for polymer electrolyte membrane fuel cells (PEMFC) In the experimental part of this work, proton conductivities are measured via impedance spectroscopy The simulation and modelling are performed combining molecular dynamics simulations and energy barrier calculations; the analysis is done via the proton jump energy barrier, collision ratio and radial distribution function The dependence of the proton mobility on the temperature, spacer length and the density of conducting groups per area is presented Donors and acceptors groups approach to each other within a distance from 28 to 3 A where the energy barrier for a proton transfer is very low, which favours the proton jump under the studied conditions The proton conductivity increases with increase in the spacer length The simulation results are in good agreement with the proton conductivities presented

Abstract: A planar anode-supported solid oxide fuel cell (SOFC) has been tested to investigate gas tightness of the electrolyte and the applied seals. Gas leaks reduce the efficiency of the SOFC and it is thus important to determine and minimise them. Probe gases (He and Ar) and a Quadrupole Mass Spectrometer were used to detect both internal (through electrolyte) and external (through seals) gas leaks. The internal gas leak through the electrolyte was quantified under different conditions, as was the external leak from the surroundings to the anode. The internal gas leak did not depend on the pressure difference between the anode and the cathode gas compartment, and can thus be described as diffusion driven. External leaks between the surroundings and the anode, but not the cathode gas compartment was observed. They were influenced by the pressure difference and are thus driven by both concentration and pressure gradients. The measured gas leaks deduced from the probe gas experiments and the total leak calculated from the deviation between the Emf defined by the gases and the cell OCV (which contains all gas leaks as well as effects of electronic leaks) were compared. Good agreement between the two measures was observed.

Abstract: The feasibility of applying single-chamber solid oxide fuel cells (SC-SOFCs) to power generators for exhaust energy recovery was investigated using both model and actual exhaust gases. In the experiments with model exhaust gases, a single cell, Ni-Ce0.8Sm0.2O1.9/YSZ/La0.8Sr0.2MnO3, was operated in a mixture of gases containing ppm levels of CH4, C2H6, C3H8, C4H10 and O2. The cell performance was considerably affected by the molar ratio of total hydrocarbons to O2, the operating temperature and the gas flow rate. The optimal operating conditions of the SC-SOFC were found to be similar to those found in actual exhaust from gasoline engines. Thermal and mechanical loading performance tests demonstrated high tolerance towards thermal cycling and breakage of the electrolyte. Performance tests with and without a gas separator suggested that there is no requirement for a gas separator in an actual exhaust. In the experiments with actual exhaust gases, a 12-cell stack was installed to a 250-cm3 engine. The open circuit voltages (OCVs) were between 5 and 8 V and independent of the number of revolutions, but were lower than the values expected from the model exhaust results. This was considered to be due to the deviation of the actual exhaust gases from the model gases. Nevertheless, the stack performance was reproducible and stable in the range from 1,500 to 5,500 rpm. The resultant peak power reached above 1 W at 4,500 rpm.

Abstract: The application of the electrophoretic deposition (EPD) technique to the preparation of dense La0.8Sr0.2Ga0.8Mg02O2.8 (LSGM) electrolyte films for intermediate temperature solid oxide fuel cells (IT-SOFCs) was investigated. Suspensions of LSGM were prepared in acetone + I-2 + H2O dispersion media. The effects of water and iodine content, of the applied voltage, and of powder loading on the EPD rate were systematically studied using metallic substrates (Pt and stainless steel). This allowed to identify the suitable set of EPD process parameters that were used to deposit LSGM films on tape-cast composite electrodes, composed of lanthanum-doped ceria (La0.4Ce0.6O2, LDC), polyvinylidene difluoride (PVDF) and carbon powders. After EPD, dense and crack-free 15 mu m thick LSGM films were obtained on porous LDC by co-firing in air at 1,490 degrees C. Line profile analyses performed by energy dispersive X-ray spectroscopy (EDS) did not reveal any interdiffusion of ions across the LSGM/LDC interface. The chemical and structural compatibility of LSGM with LDC was also checked by heat treating a mixture of the two powders (1:1 weight ratio) using the same thermal cycle as that of the LDC/LSGM bi-layer co-firing at 1,490 degrees C. EPD has thus proven to be a viable way for manufacturing anode-supported LSGM electrolyte films.

Abstract: The influence of ionomer content on the performance of direct methanol fuel cells (DMFC) is studied. The performance is studied as a function of the (nafion/carbon, N/C) ratio on the anode and cathode. The performance of the DMFC has been found to increase as a function of the N/C ratio. The ionomer content seems to influence the catalyst layer resistance in the anode and cathode and also the methanol oxidation potential on the anode. On the cathode, catalyst utilisation is seen to be affected by the ionomer content. Cyclic voltammetry (CV) is used to study the effect of ionomer content on Platinum (Pt) utilisation on the cathode. Theoretical calculations are used to study the catalyst layer resistance on the anode and cathode as a function of ionomer content. Findings indicate that ionomer content plays only a partial role in characterising the performance of a DMFC.

Abstract: New polymer electrolytes based on aromatic polyethers bearing main chain pyridine groups have been synthesised. They possess good mechanical properties, high thermal and oxidative stability and can be used as electrolytes for high temperature PEM fuel cells. A study of the influence of the chemical structure on the properties of these materials with respect to their ability to be used in high temperature PEM fuel cells was made, while a blend of the current reported polymers is tested in single cell giving encouraging preliminary results.

Abstract: The mobility of an excess proton has been studied in systems of immobilised imidazole under different conditions using quantum mechanical approaches coupled to a molecular mechanics force field in molecular dynamics. The system is a simple model for imidazole covalently bound to a mesoporous material. Such materials are intended to be used as an additive to a polymer electrolyte membrane for fuel cells. This theoretical work is focused on dynamic properties of the proton transport. The diffusion constant of the system is found to be in the order of magnitude of 10–9 m2 s–1 and it is verified that the proton diffusion increases with increase in temperature or density. Further, the proton transport mechanism is investigated.

Abstract: In this paper, experimental investigations on the influence of operational parameters on PEM fuel cell cold start are presented. The effect of current density, stack impedance at 1 kHz prior to start, as well as gas flow rate, gas pressure, coolant flow rate and surrounding subfreezing temperature are studied. The experimental apparatus is briefly described. It includes a main unit at room temperature and a smaller separate unit in a climatic chamber. Low current density, high impedance prior to start, moderate subfreezing temperature (–5 °C), high gas flow rate, low gas pressure and low coolant flow rate are found to have a positive impact on the cold start performance. Combining these parameters, self start-up of the fuel cell without additional energy is achieved at –5 °C in 30 min. The whole set of observations leads to the following hypotheses on freeze mechanism: in the first phase, dry membranes and low current lead to a transient phase of membrane humidification. Then, in the second phase, ice clogging of the active layers occurs. In the third phase, a variable quantity of the produced water reaches the gas diffusion layers and channels.

Abstract: Electrical properties of grain boundaries, free of any secondary phases, have been investigated in cubic nanostructured zirconia ceramics. Pure 8YSZ powders were synthesised by spray pyrolysis process. These powders were used to perform dense nanostructured ceramics using two compaction techniques: cold isostatic pressing (CIP) and hot uniaxial pressing (HUP) in an oxidising atmosphere. Grain sizes ranging from 25 to 242 nm and relative densities from 93 up to 98% were obtained. Electrical contribution of the bulk and the blocking effect due to grain boundaries were determined by impedance spectroscopy and analysed according to the brick layer model. The additional blocking effect is a decreasing function of the grain size but its magnitude reaches a plateau quite insensitive to the grain size below 100 nm. This indicates that the recorded electrical contribution originates from an intrinsic effect. This can be related to a dominant electrical contribution of space charge layers located at the grain boundaries area of nanoscale grain sizes.