Currently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in-situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes.
The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and impedance measurements. The PEFC measurements, which are time consuming, have been compared to fast ex situ testing in 3% H2O 2 solution. For the direct assessment of membrane degradation micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. By using low degrees of grafting, below about 35%, the durability of this type of membrane can be increased. Degradation in the fuel cell was found to be highly localised. It was found that in situ measurements in the PEFC alone are not sufficient. Measurement of the cell resistance via impedance is not always a reliable indicator of changes in membrane resistance because other resistance changes in the cell can easily interfere and cannot be separated from those caused by the membrane. Micro-Raman is an ideal complementary method to in situ testing, but it is time consuming. For fast pre-screening of membrane durability mass loss measurements during exposure to 3% H2O2 solution combined with the determination of changes in the IEC can be performed.
Performance losses due to flooding of gas diffusion layers (GDLs) and flow fields as well as membrane dehydration are two of the major problems in PEFC. In this investigation, the effect of GDL on the cell water management in PEFC is studied using segmented and single cell experiments. The behaviour of four different commercial GDLs was investigated at both high and low inlet humidity conditions by galvanostatic fuel cell experiments. The influence of varying reactant humidity and gas composition was studied. The results at high inlet humidity show that none of the studied GDLs are significantly flooded on the anode side. On the other hand, when some of the GDLs are used on the cathode side they are flooded, leading to increased mass transfer losses. The results at low inlet humidity conditions show that the characteristics of the GDL influence the membrane hydration. It is also shown that inlet humidity on the anode side has a major effect on flooding at the cathode.
A two-dimensional, non-isothermal, two-phase model of a polymer electrolyte fuel cell (PEFC) is presented. The model is developed for conditions where variations in the stream-wise direction are negligible. In addition, experiments were conducted with a segmented cell comprised of net flow fields. The, experimentally obtained, current distributions were used to validate the PEFC model developed. The PEFC model includes species transport and the phase change of water, coupled with conservation of momentum and mass, in the porous backing of the cathode, and conservation of charge and heat throughout the fuel cell. The current density in the active layer at the cathode is modelled with an agglomerate model, and the contact resistance for heat transfer over the material boundaries is taken into account. Good agreement was obtained between the modelled and experimental polarization curves. A temperature difference of 6°C between the bipolar plate and active layer on the cathode, and a liquid saturation of 6% at the active layer in the cathode were observed at 1 A cm-2.
The Molten Carbonate Fuel Cell (MCFC) converts chemical energy into electrical energy and heat. Since the working temperature is high, less expensive materials can be used compared to low temperature fuel cells. However, the components of the fuel cell still need to be improved. The dissolution of the NiO cathode has, for a long time, been a problem for the Molten Carbonate Fuel Cell (MCFC) and this area is still the focus for MCFC component research. In this study, solubility measurements for a NiC) cathode material doped with magnesium and iron are carried out and the electrochemical performance of this cathode material is tested under the standard conditions of the MCFC over 2,000 hours and compared with the performance of a standard NiO cathode. After operation, nickel precipitation in the matrices is investigated. It is concluded that a NiO cathode with magnesium and iron could be a viable candidate material for the MCFC.
Stainless steel bipolar plates (BPPs) are the preferred choice for proton exchange membrane fuel cells (PEMFCs); however, a surface coating is needed to minimize contact resistance and corrosion. In this paper, Ni–Mo and Ni–Mo–P coatings were electroplated on stainless steel BPPs and investigated by XRD, SEM/EDX, AFM and contact angle measurements. The performance of the BPPs was studied by corrosion and conduction tests and by measuring their interfacial contact resistances (ICRs) ex situ in a PEMFC set-up at varying clamping pressure, applied current and temperature. The results revealed that the applied coatings significantly reduce the ICR and corrosion rate of stainless steel BPP. All the coatings presented stable performance and the coatings electroplated at 100 mA cm−2showed even lower ICR than graphite. The excellent properties of the coatings compared to native oxide film of the bare stainless steel are due to their higher contact angle, crystallinity and roughness, improving hydrophobicity and electrical conductivity. Hence, the electroplated coatings investigated in this study have promising properties for stainless steel BPPs and are potentially good alternatives for the graphite BPP in PEMFC.
A sulfophenylated polysulfone (PSU-sph), carrying 0.8 sulfonic acid units per repeating unit of the polymer, is evaluated as a membrane electrolyte for DMFC applications. The liquid uptake, methanol transport characteristics, electrolyte conductivity, and fuel cell performance are investigated. The methanol transport and DMFC performance results are compared to those of Nafion(R) 117. The PSU-sph membrane investigated shows superior qualities with regard to methanol crossover, with a methanol permeability of approximately 25% compared to that of Nafion(R). The conductivity measured to be 15% compared to that of Nafion(R). However, this could not fully account for the internal resistance of the cell, implying that the contact resistance between the electrodes and electrolyte is higher when PSU-sph is used, probably because the electrodes are developed for use with Nafion(R) membranes. The stability of the PSU-sph membrane seems promising, with very low degradation observed over a period of 72 hours. It was concluded that although the mass transport properties of the PSU-sph membrane sample investigated were superior, it could not match the performance of Nafion(R) 117 in a DMFC application. However, a higher degree of sulfonation may have a significant positive effect on cell performance. The results also showed that a fully intergrated MEA is needed to fully assess new membrane materials.
Sulfonated polysulfone (sPSU) membranes were analysed by nuclear magnetic resonance (NMR) cryoporometry, conventional gravimetric water uptake measurements as well as by differential scanning calorimetry (DSC). NMR cryoporometry is based on the relation between the pore size and the melting point depression of the pore-filling liquid, i.e. water in fuel cell membranes; thus providing a relation between the amount of molten water and the temperature shift, i.e. the pore size, in hydrated membranes. An sPSU membrane with high ion-exchange capacity (IEC 1.45 mequiv. g –1) possessed a significant amount of large pores after hydrothermal pretreatment at 80
In order to investigate the possibility of increasing the reactivity for oxygen reduction reaction (ORR) of the cathode in a PEMFC a series of Pt/C catalysts was prepared using water-in-oil microemulsions for synthesizing Pt nanoparticles. The Pt nanoparticles were deposited on porous carbon support (Vulcan XC-72 or a mesoporous carbon) and the catalysts were processed into MEAs. The MEA samples were evaluated and compared with a commercial sample and with Pt/C catalyst samples prepared using a conventional direct impregnation method. The mesoporous carbon support investigated as a potential alternative to Vulcan XC72 has a very high specific surface area and a narrow pore size distribution. The materials were characterized with XRD, TEM, SEM-EDX, N-2 sorption and steady state polarization. It was found that it is possible to increase the ORR reactivity using the microemulsion route for formation of Pt nanoparticles. It was concluded that the MEA processing conditions for the mesoporous carbon support have to be modified to reach improved ORR reactivity, likely due to the large differences in specific surface area, porosity and conductivity compared to the Vulcan carbon.
Graphitised carbon nanofibres (G-CNFs) show superior thermal stability and corrosion resistance in PEM fuel cell environment over traditional carbon black (CB) and carbon nanotube catalyst supports. However, G-CNFs have an inert surface with only very limited amount of surface defects for the anchorage of Pt catalyst nanoparticles. Modification of the fibre surface is therefore needed. In this study Pt nanoparticles have been deposited onto as-received and surface-modified G-CNFs. The surface modifications of the fibres comprise acid treatment and nitrogen doping by pyrolysis of a polyaniline (PANI) precursor. The modified surfaces were studied by FTIR and XPS and the electrochemical characterization, including long-term Pt stability tests, was performed using a low-temperature PEMFC single cell. The performance and stability of the G-CNF supported catalysts were compared with a CB supported catalyst and the effects of the different surface treatments were discussed. On the basis of these results, new membrane electrode assemblies (MEAs) were manufactured and tested also for carbon corrosion by in situ FTIR analysis of the cathode exhaust gases. It was observed that the G-CNFs showed 5?times lower carbon corrosion compared to CB based catalyst when potential reached 1.5?V versus RHE in simulated start/stop cycling.