Mesoporous carbons as low temperature fuel cell platinum catalyst supports

Platinum catalysts supported on ordered mesoporous carbons (OMC) are described. The mesoporous carbon support, CMK3 type, was synthesised as an inverse replica of a SBA-15 silica template. The platinum catalysts (i.e. Pt 20 wt% and Pt 10 wt%, respectively), obtained through a conventional wet impregnation method, have been investigated to determine their structural characteristics and electrochemical behaviour. The electro-catalytic performance towards the oxygen reduction reaction (ORR) was compared to those of commercial Pt/C-Vulcan XTC72R (E-Tek) catalysts with the same Pt wt%, under the same experimental conditions. The two catalyst samples have allowed the effect of the variation of both the Pt to Nafion and Pt to the supporting carbon ratios to be studied. Electrochemical tests have been carried out in three different systems: a catalyst ink deposited on a glassy carbon rotating disk electrode (RDE), a gas diffusion electrode (GDE) in a three-electrode cell with H₂SO₄ as the electrolyte and a complete PEM single fuel cell. The first results indicate that the OMC performs slightly less well than commercial carbon supports, mainly in the complete fuel cell system. The data from the cell tests indicate a less effective distribution of Nafion on the OMC surface which, probably, decreases the platinum utilisation and the proton conductivity.     

质子交换膜燃料电池用催化剂及其电极的研究进展

膜电极(MEA)是质子交换膜燃料电池(PEMFC)的核心技术。膜电极包含的催化剂层、材料和结构等对PEMFC的性能影响很大。催化剂面层上供三相(质子、电子、气体)用的通道对于电池使用时的催化作用是必不可少的。介绍了近几年催化剂的研究进展,看重对三相通道进行了详细叙述。也回顾了一些用于改善催化剂活性的其他方法,如阴极催化、合金催化剂,根据这些进展,对今后的研究方向提出了建议。     

Thermal degradation of the support in carbon-supported platinum electrocatalysts for PEM fuel cells

The cathode catalyst layer in proton exchange membrane (PEM) fuel cells can contain nanometer-sized platinum particles dispersed on a high surface area carbon. In order to assess support stability, samples of carbon-supported catalysts were held at elevated temperatures under dry air conditions. The samples were weighed at regular intervals. These tests showed that the platinum particles were able to catalyze the combustion of the carbon support at moderate temperatures (125-195 °C). As the temperature increased, the rate of carbon combustion increased. The amount of carbon that was lost after extended oven exposure at a constant temperature was shown to depend on both the temperature and platinum loading. A simple first-order kinetic model was able to describe the results. With further work on a range of different carbon supports, this work is expected to help develop more stable catalyst supports for PEM fuel cells.     

Graphene as catalyst support: The influences of carbon additives and catalyst preparation methods on the performance of PEM fuel cells

The reduction of the platinum amount for efficient PEM (polymer electrolyte membrane) fuel cells was achieved by the use of graphene/carbon composites as catalyst support. The influences of the carbon support type and also of the catalyst preparation method on the fuel cell performance were investigated with electrochemical, spectroscopic and microscopic techniques. Using pure graphene supports the final catalyst layer consists of a dense and well orientated roof tile structure which causes strong mass transport limitations for fuels and products. Thus the catalysts efficiency and finally the fuel cell performance were reduced. The addition of different carbon additives like carbon black particles or multi-walled carbon nanotubes (MWCNT) destroys this structure and forms a porous layer which is very efficient for the mass transport. The network structure of the catalyst layer and therefore the performance depends on the amount and on the morphology of the carbon additives. Due to optimizing these parameters the platinum amount could be reduced by 37% compared to a commercial standard system.     

Nafion functionalized carbon nanohorns as catalyst support for proton exchange membrane fuel cells

Carbon nanohorns (CNHs) are synthesized by DC arc-discharge method in helium atmosphere. The synthesized CNHs are heat treated and then functionalized with Nafion. Platinum is reduced onto Nafion functionalized CNHs through ethylene glycol reduction method. Pt/Nafion-CNHs catalysts are prepared by varying deposition times from 1 to 7 h. Detailed characterization of CNHs and the catalyst is performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, X-ray diffraction (XRD) and thermo gravimetric analysis (TGA). Membrane electrode assemblies are prepared by decal method and electrode performance is analyzed using hydrogen pumping technique. SEM and TGA analysis of soot indicates relatively high level purity of CNHs. TEM and XRD analysis of catalyst with 6 h deposition time reveals that Pt nanoparticles are uniformly deposited on multiwall CNHs surface with particle size of 3–4 nm. Hydrogen pumping studies reveals that Nafion functionalized CNHs act as good catalyst support without hindering electron transfer at electrodes. Hence, these graphitic carbon supports have potential application for high power density applications.     

Feeding PEM fuel cells

This paper discusses the laws of the fuel consumption in a polymer electrolyte fuel cell and its related performance. Based on simplifying but realistic approximations these laws are presented in analytical form. We verify these laws with space resolved current-voltage measurements in a single channel fuel cell. The results give a clear picture of the inside-the-cell distribution of performance associated with reactant consumption. This picture helps to rationalize the modes of optimal fuelling and is lessening for design of the flow-fields.     

Mechanical milling of catalyst support for enhancing the performance in fuel cells

In this work, we studied the characteristic variations of catalyst supports caused by mechanical milling and their electrochemical application in fuel cells. Two different catalyst supports, carbon black (XC-72R) and K20 (mesoporous carbon), were crushed and dispersed by mechanical milling using a bead mill. The bead mill operated with 0.3 μm zirconia beads at the rate of 3500 rpm for 30 min. The secondary particle size of the crushed catalyst supports ranged from around 0.1 μm to 10 μm. The secondary particle size of the catalyst supports after crushing represents a decrease of approximately 10% compared with that of raw catalyst supports. To confirm the role of the catalyst supports in the direct methanol fuel cell (DMFC), Pt and Ru were loaded onto these catalyst supports using an impregnation method. In the single cell test, Pt-Ru/XC-Bead and PtRu/K20-Bead showed power densities of 135 mW/cm² and 144 mW/cm² under air at 60 °C, respectively. The performance values of these catalysts, which were fabricated using reformed catalyst supports, were 10% to 20% higher than those of raw catalyst supports. As a result, the catalyst supports crushed by the bead mill helped to improve the electrochemical performance of the direct methanol fuel cell.     

Ordered mesoporous carbons with controlled particle sizes as catalyst supports for direct methanol fuel cell cathodes

The ordered mesoporous carbons (OMCs) with various primary particle sizes were synthesized and the effect of the particle size of the OMC supports on their performance for the oxygen reduction reaction (ORR) in direct methanol fuel cells was investigated. The ordered mesoporous silica (OMS) templates with particle sizes of 100, 300, and 700 nm (OMS-100, -300, and -700) were synthesized by changing the synthesis pH and Na content in the silica source, sodium silicate. The OMCs with similar particle sizes and morphologies (OMC-100, -300, and -700) were faithfully replicated by using the corresponding OMSs as templates and phenanthrene as a carbon source. Structural characterizations revealed that three OMCs exhibit uniform mesopores of 4–5 nm and BET surface areas of 600–800 m² g⁻¹. The Pt nanoparticles of ca. 3 nm were supported onto these OMCs and the resulting Pt/OMC catalysts were tested for the ORR. The three OMC supported catalysts exhibited the catalyst utilization efficiencies and ORR activities of similar range, with the values of Pt/OMC-300 catalyst being slightly higher than the other two catalysts.     

Quantitative characterization of catalyst layer degradation in PEM fuel cells by X-ray photoelectron spectroscopy

A quantitative analysis of catalyst layer degradation was performed with X-ray photoelectron spectroscopy (XPS). XPS is quantitative, surface-sensitive, and is able to distinguish different bonding environments or chemical states of fuel cell catalyst layers and polymer electrolyte membrane. These capabilities have allowed us to explore the complex mechanisms of degradation during fuel cell operation. The elemental surface concentrations of carbon, fluorine, oxygen, sulfur, and platinum on the catalyst layer surface were measured before and after fuel cell operation, and the different chemical states of carbon and platinum were identified. Both XPS analysis and scanning electron microscopy revealed that the ionomer on the catalyst layer degraded or decreased in concentration after fuel cell operation. Ionomer degradation was characterized by a decrease of CF₃ and CF₂ species and an increase in oxidized forms of carbon (e.g. CO and CO), and an increase in less- and non-fluorinated forms of carbon (e.g. CF and graphitic), consistent with overall reduction of fluorine by about 22%. The surface concentration of fluorine and platinum also reduced from 50.1% to 38.9% and from 0.4% to 0.3%, respectively. The concentration of oxidized forms of carbon and platinum increased after fuel cell operation. The surface-sensitive XPS technique should prove useful for the quantitative monitoring of catalyst layer degradation mechanisms over the lifetime of fuel cells.     

Two-dimensional analysis of PEM fuel cells

This study reports a two-dimensional numerical simulation of a steady, isothermal, fully humidified polymer electrolyte membrane (PEM) fuel cell, with particular attention to phenomena occurring in the catalyst layers. Conservation equations are developed for reactant species, electrons and protons, and the rate of electrochemical reactions is determined from the Butler–Volmer equation. Finite volume method is used along with the alternating direction implicit algorithm and tridiagonal solver. The results show that the cathode catalyst layer exhibits more pronounced changes in potential, reaction rate and current density generation than the anode catalyst layer counterparts, due to the large cathode activation overpotential and the relatively low diffusion coefficient of oxygen. It is shown that the catalyst layers are two-dimensional in nature, particularly in areas of low reactant concentrations. The two-dimensional distribution of the reactant concentration, current density distribution, and overpotential is determined, which suggests that multi-dimensional simulation is necessary to understand the transport and reaction processes occurring in a PEM fuel cell.     

Silica-sol-templated mesoporous carbon as catalyst support for polymer electrolyte membrane fuel cell applications

Mesoporous carbon (MC) samples having especially high specific surface area, pore size, and pore volume (e.g. pore volume in excess of 4 cm³ g⁻¹) were prepared and their suitability as Pt catalyst supports in polymer electrolyte membrane fuel cells was examined. Pt particles on the MC support were slightly larger than those on commercial samples of Pt on carbon black, and they showed a greater tendency to agglomerate on the MC support than on carbon black. Ex situ cyclic voltammetry gave values for electrochemically active surface area that were about half that for a commercial Pt-on-carbon black sample. Preliminary attempts to prepare thin-film electrodes from Pt/MC samples with a Nafion binder using conventional ink formulations failed, probably because much of the Nafion electrolyte was taken up inside support pores and was not available to bind the support particles together. An alternate approach involving painting of catalyst inks directly onto gas diffusion layers was used to prepare membrane electrode assemblies (MEAs) from Pt/MC samples, which were tested using single-cell test hardware. Performance of these Pt/MC sample MEAs was compared with that prepared by decal transfer method with commercially obtained Vulcan XC-72R supported Pt catalyst. The reasons for the lower performance of Pt/MC were discussed.     

Homogeneity analysis of square meter-sized electrodes for PEM electrolysis and PEM fuel cells

Electrodes for polymer electrolyte membrane electrolyzers and fuel cells are manufactured by coating a catalyst dispersion, consisting of precious metal, ionomer and solvents, onto a substrate that is subsequently dried. One target of current research is to produce square meter-sized electrodes, but so far the homogeneity that can be achieved in this scaling is unclear. To quantify the achievable homogeneity of an electrode, manufactured by means of slot die coating in a roll-to-roll pilot plant, this study focuses first on the selection of an appropriate substrate by investigating thickness, basis weight and surface free energy distribution at the square meter scale. Afterward, a dispersion is coated on the selected substrate, dried and investigated with respect to thickness and basis weight distribution. Among the investigated substrates, Kapton has the smallest scatter in terms of thickness and basis weight. The subsequent coating results in a precious metal loading of 1.10 mg cm\(^{-2}\), with a scattering of 5.5% that can be further reduced to 4.5% when edge effects can be prevented. These results are now available for further research in which it is necessary to investigate whether or not these fluctuations affect the achievable electrochemical efficiencies of electrodes.     

Catalyst electrode preparation for PEM fuel cells by electrodeposition

The preparation of catalyst electrodes by electrodeposition for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) has been studied. This work looks at the potential to apply the electrodeposition technique, in the forms of direct current (DC) and pulse plating electrodepositions, to prepare Pt and Pt–Co alloy catalysts for membrane electrode assemblies (MEAs). The preparation of the non-catalyst layer was found to be important for the electrodeposition of Pt catalysts. The activities of the electrodeposited catalysts, both pure Pt and Pt–Co alloy, produced by pulse plating are substantially higher than that of the Pt catalyst produced by DC electrodeposition. The improvement in electroactivity towards the ORR of the electrodeposited catalysts produced by pulse plating is likely due to the finer structures of electrodeposited catalysts which contain smaller catalyst particles compared to those produced by DC electrodeposition. A maximum performance towards the ORR in PEMFCs was achieved from the catalysts prepared by pulse plating using a charge density of 2 C cm⁻², a pulse current density of 200 mA cm⁻², a 5% duty cycle and a pulse frequency of 1 Hz.     

Corrosion-resistant lightweight metallic bipolar plates for PEM fuel cells

Corrosion resistant treated metal bipolar plates with higher rigidity and electrical conductivity than graphite were developed and tested for PEM fuel cell applications. Six replicas of single cells were used three of which were made of graphite composites bipolar plates and the other three of the treated metallic plates. A Membrane Electrode Assembly (MEA) with 5.55 cm² active electrode areas, 0.3 mg cm^–2 Pt loading and Nafion membrane 115 was fitted to each cell and operated under identical conditions. The experimental testing was conducted at room temperature (20 °C). The average value of the data obtained for the three graphite cells was plotted. Similarly, the average value of the data obtained for the three treated metal cells was plotted on the same graph for comparison. Generally, the treated metal bipolar plate provided at least 12% saving in hydrogen consumption in comparison to graphite. This is attributed to the lower bulk and surface contact resistance of the metal used in this study in relation to graphite. The results of lifetime testing, conducted at room temperature under variable loading showed no indication of power degradation due to metal corrosion for at least 1500 hours.     

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.