In situ Ru K-edge EXAFS of CO adsorption on a Ru modified Pt/C fuel cell catalyst

The Ru-CO bond of CO adsorbed on a Ru modified Pt/C fuel cell catalyst has been directly probed by in situ EXAFS at the Ru K-edge, providing evidence of a CO:metal surface atom ratio greater than 1:1 and that CO is adsorbed at bridging sites associated with Ru atoms at the surface of the catalyst nanoparticles. This result illustrates the limitations of single crystal models as representations of the bonding of adsorbed species at nanoparticle surfaces.     

Performance of highly dispersed Pt/C catalysts for low temperature fuel cells

A novel technique based on intermittent microwave heating (IMH) is used to prepare highly dispersed Pt/C catalysts. It has been proved that more than 60% Pt on carbon can be prepared by one-step procedure. The average Pt clusters on carbon are less than 5 nm with very narrow size distribution. The catalysts prepared by the present method show better performance in low temperature fuel cells.     

Pb and Sb modified Pt/C catalysts for direct formic acid fuel cells

PtPb/C and PtSb/C bi-metallic catalysts were synthesized by chemical deposition of Pb or Sb on a commercial 40% Pt/C catalyst. The performances of catalysts with a range of compositions were compared in a multi-anode direct formic acid fuel cell in order to optimize compositions and evaluate the statistical significance of differences between catalysts. The catalytic activity for formic acid oxidation increased approximately linearly with adatom coverage for both PtPb/C and PtSb/C, to maxima at fractional coverages of ca. 0.7. At a cell voltage of 0.5 V, the currents at the optimum Pb or Sb coverages were ca. 8 times higher than at unmodified Pt/C. CO-stripping results indicate that the presence of Pb or Sb facilitates the oxidation of adsorbed CO. In addition, both metals appear to produce electronic effects that inhibit poison formation on the modified Pt surface.     

Comparative study on pressure swing adsorption system for industrial hydrogen and fuel cell hydrogen

In order to improve the design of PSA system for fuel cell hydrogen production, a non-isothermal model of eight-bed PSA hydrogen process with five-component (H₂/N₂/CH₄/CO/CO₂=74.59%/0.01%/4.2%/2.5%/18.7% (vol)) four-stage pressure equalization was developed in this article. The model adopts a composite adsorption bed of activated carbon and zeolite 5A. In this article, pressure variation, temperature field and separation performance are stimulated, and also effect of providing purge (PP) differential pressure and the ratio of activated carbon to zeolite 5A on separation performance in the process of producing industrial hydrogen (CO content in hydrogen is 10 μl·L^-1) and fuel cell hydrogen (CO content is 0.2 μl·L^-1) are compared. The results show that Run 3, when the CO content in hydrogen is 10 μl·L^-1, the hydrogen recovery is 89.8%, and the average flow rate of feed gas is 0.529 mol·s^-1; When the CO content in hydrogen is 0.2 μl·L^-1, the hydrogen recovery is 85.2%, and the average flow rate of feed gas is 0.43 mol·s^-1. With the increase of PP differential pressure, hydrogen recovery first increases and then decreases, reaching the maximum when PP differential pressure is 0.263 MPa; With the decrease of the ratio of activated carbon to zeolite 5A, the hydrogen recovery increases gradually. When the CO content in hydrogen is 0.2 μl·L^-1 the hydrogen recovery increases more obviously, from 83.96% to 86.37%, until the ratio of activated carbon to zeolite 5A decreases to 1. At the end of PP step, no large amount of CO₂ in gas or solid phase enters the zeolite 5A adsorption bed, while when the CO content in hydrogen is 10 μl·L^-1, and the ratio of carbon to zeolite 5A is less than 1.4, more CO₂ will enter the zeolite 5A bed.     

The efficiency of methanol conversion to CO2 on thin films of Pt and PtRu fuel cell catalysts

Yields were determined for the CO₂ produced upon the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ at the following four fuel cell catalyst systems: Pt black, Pt at 10 wt.% metal loading on Vulcan XC-72R carbon (C/Pt, 10%), PtRu black at 50 at.% Pt, 50 at.% Ru (PtRu (50:50) black), and PtRu at 30 wt.% Pt, 15 wt.% Ru loading on Vulcan XC-72R carbon (C/PtRu, 30 wt.% Pt, 15 wt.% Ru). Samples were electrolyzed in a small volume (50 μl) arrangement for a period of 180 s keeping the reactant depletion in the cell below 1%. The dissolved CO₂ produced was determined ex situ by infrared spectroscopy in a micro-volume transmission flow cell. For the PtRu materials, the efficiencies for CO₂ formation were near 100% at reaction potentials in the range between 0.4 V (versus the reversible hydrogen electrode (RHE), V_RHE ) and 0.9 V_RHE. At the Pt catalysts, the yields of CO₂ approached 80% between 0.8 and 1.1 V_RHE and declined rapidly below 0.8 V_RHE.     

Poisoning of Pt/C catalysts by CO and its consequences over the kinetics of hydrogen chemisorption

The initial rate of hydrogen dissociation was studied as a function of irreversible CO coverage at 353 K on 30 wt.% Pt/carbon catalysts (Pt/C) prepared according to different processes. The Pt/C catalysts exhibit similar Pt dispersion (D  0.07) and mean Pt particles size (dp  16 nm). The turnover frequency (number of hydrogen molecules dissociated per CO-free surface Pt atom) was determined as a function of CO coverage from 0.0 to 0.8. The evolution of TOF as a function of CO coverage is in agreement with the model of CO adsorbing on low coordination sites (edges, corners) and then spreading across the faces to grow islands as Brandt suggested in the past (R.K. Brandt, M.R. Hughes, L.P. Bourget, K. Truszkowska, R.G. Greenler, Surf. Sci. 286 (1993) 15–25). At high CO coverage (0.8), TOF depends on the process by which the Pt/C catalyst was prepared. In particular, a Pt/C elaborated according to a colloidal process exhibits a low sensitivity to CO poisoning with an increase of TOF by one order of magnitude.     

Preferential CO oxidation over Pt/3A zeolite catalysts in H2-rich gas for fuel cell application

Even traces of CO in the hydrogen-rich gas fed to proton exchange membrane fuel cells (PEMFC) poison the platinum anode electrode and dramatically decrease the cell power output. In this work, several 1%Pt catalysts, all on 3A-type zeolite support, were prepared starting from different precursors and employing different methods: cation exchange (CE), wet impregnation (WI) and incipient wetness impregnation (IWI). The catalysts were characterised in terms of platinum dispersion and tested under realistic conditions in the quest of a catalyst for the removal of CO via the CO preferential oxidation (CO-PROX) reaction. The best catalytic performance was shown by the 1%Pt–3A catalyst prepared by IWI starting from Pt(NH₃)₄Cl₂ as a precursor, thanks to its highest platinum dispersion.     

Well-dispersed surfactant-stabilized Pt/C nanocatalysts for fuel cell application: Dispersion control and surfactant removal

A simple surfactant-stabilized method was investigated for the preparation of well-dispersed platinum nanoparticles supported on carbon black (Pt/C), using 3-(N, N-dimethyldodecylammonio) propanesulfonate (SB12) as the stabilizer. First, TEM analysis demonstrated that Pt dispersion can be improved by the increase of molar ratio of SB12 to Pt precursor. Moreover, pH environment plays a crucial role in Pt dispersion, and the optimal dispersion with an average Pt particle size of 2.2 nm was obtained under neutral or slightly alkaline environment. Pt dispersion mechanism was shown to involve the electrosteric stabilization of Pt nanoparticles by the zwitterionic surfactant SB12, which is highly pH-dependent. At pH ≥ 7, a stable electrosteric repulsion exists between the Pt particles covered by SB12, where the positively charged part is adsorbed on the particle surface and the negatively charged part (SO₃⁻) and the bulky alkyl chain (C₁₂H₂₅) are pointed away from particles. At pH < 7, protons H⁺ directly interact with Pt particles or SO₃⁻ groups of SB12, resulting in the destruction of the electrosteric stabilization and the following agglomeration of Pt nanoparticles. Furthermore, XPS and cyclic voltammetry showed that the surfactant on Pt particles can be efficiently removed by ethanol wash without any destruction on the dispersion and particle size of Pt, when compared to heat treatment and centrifugation. Electrochemical measurements showed that the ethanol-washed pH-controlled Pt/C catalyst has higher electrochemical surface area and catalytic performance than the commercial one.     

Influence of toluene contamination at the hydrogen Pt/C anode in a proton exchange membrane fuel cell

For fuel cells run on hydrogen reformate, traces of hydrocarbon contaminants in the hydrogen gas may be a concern for the performance and lifetime of the fuel cell. This study focuses on the influence of low concentrations of toluene on the adsorption and deactivation chemistry in a proton exchange membrane (PEM) fuel cell. For this purpose cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques were employed. Results from adsorption and desorption (by oxidation or reduction) experiments performed in a humidified nitrogen or hydrogen flow in a fuel cell test cell with a mass spectrometer system connected to the outlet are presented. The influence of adsorption potential, temperature, and humidity are discussed. The results show that toluene adsorbs on the catalyst surface in a broad potential window, up to at least 0.85 V versus RHE at 80 °C. Adsorbed toluene oxidizes to CO₂ with peak potentials above 1.0 V for temperatures below 95 °C. Some desorption of toluene (or reduced products) may take place at potentials below 0 V. In a hydrogen flow, toluene contamination in per mille concentrations leads to a continuous growth of the charge transfer resistance, while a 10-fold dilution of the toluene concentration resulted in a low and constant charge transfer resistance even for longer exposures. This indicates that a competition between toluene and hydrogen may take place on the active platinum surface at the anode.     

Properties of Pt/C catalyst modified by chemical vapor deposition of Cr as a cathode of phosphoric acid fuel cell

Cr-modified Pt/C catalysts were prepared by the chemical vapour deposition (CVD) of Cr on Pt/C, and their performance as a cathode of phosphoric acid fuel cell (PAFC) was compared with the case of catalysts containing Cr added by impregnation (IMP).The catalyst prepared by CVD showed a higher activity for oxygen reduction reaction (ORR) than one prepared by IMP. There was an optimum amount of Cr that yielded the maximum mass activity of the catalyst because the gain in the intrinsic activity due to the promotional effect of Cr was counterbalanced by the loss of exposed Pt surface area as a result of the Cr introduction. Nevertheless, the activity increase at the optimum amount of Cr was greater for the CVD catalyst than for the IMP catalyst. Also, the optimum amount of Cr to yield the maximum activity was smaller for the former catalyst [Cr/Pt]_CVD = 0.6, than for the latter, [Cr/Pt]_IMP = 1.0.The enhancement of the Pt catalyst activity by Cr addition is attributed to two factors: changes in the surface Pt-Pt spacing and the electronic modification of the Pt surface. The formation of a Pt-Cr alloy, as confirmed by X-ray diffraction, decreased the lattice parameter of Pt, which was beneficial to the catalyst activity for ORR. X-ray photoelectron spectroscopy results showed that the binding energies of Pt electrons were shifted to higher energies due to Cr modification. Accordingly, the electron density of Pt was lowered and the Pt-O bond became weak on the Cr-modified catalysts, which was also beneficial to the catalyst activity for ORR.The promotion of oxygen reduction on Cr-modified catalysts was confirmed by measuring the cyclic voltammograms of the catalysts. All the above changes were made more effectively for catalysts prepared by CVD than for those prepared by IMP because the former method allowed Cr to interact more closely with the Pt surface than the latter, which was demonstrated by the characterization of catalysts in this study.     

Nanoscale graphite-supported Pt catalysts for oxygen reduction reactions in fuel cells

The nanoscale graphite particles were prepared and the Pt catalysts supported on such graphites were developed for oxygen reduction in the polymer electrolyte membrane fuel cells. Catalytic activity and carbon corrosion of the developed catalysts were evaluated using rotating disc electrode techniques and results were compared with those of a state-of-the-art commercial E-TEK Pt catalyst supported on carbon black Vulcan XC72. The results showed that the particle distribution and the structure of the developed Pt nanoparticles supported on the nanoscale graphite were similar to those of the commercial catalyst. The accelerated degradation testing results showed that the electrochemical active surface area losses after 1500 cycles were 46.92% and 62.2% for the developed catalyst and the commercial catalyst, respectively, while mass activity losses were 45.3% and 84.2%, respectively. The temperature-programmed oxidation results suggest that the developed catalysts had better corrosion resistance than the commercial catalyst. The developed Pt catalysts had similar catalytic performance to the commercial catalyst; however, the developed catalysts had much better corrosion resistance than the commercial catalyst. Overall, nanoscale graphite can be a promising electrocatalyst support to replace the currently used Vulcan XC72 carbon black.     

The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell

Durability of Pt/C oxygen reduction reaction (ORR) catalyst remains one of the primary limitations for practical application of proton exchange membrane (PEM) fuel cells. In this work, the effects of relative humidity and oxygen partial pressure on platinum catalyst degradation were explored under potential cycling. At 60 °C, the loss rates of Pt mass and catalyst active surface area were reduced by about three and two times respectively when the relative humidity was lowered from 100% to 50%. The effects of oxygen partial pressure on cathode degradation were found to be insignificant. Cyclic voltammetry studies showed a slight increase in Pt electrochemical oxidation by water when the humidity increased from 50% RH to 100% RH. The rates of Pt dissolution were only slightly affected by change in humidity, and the accelerated catalyst degradation was ascribed to the increased Pt ion transport in the more abundant and larger water channel networks within the polymer electrolyte. Based on the parametric study results from our previous cathode degradation model, it was estimated that the diffusivity of Pt ions at fully humidified conditions was three times that of the value at 50% RH and 60 °C.     

Preparation of platinum-based electrode catalysts for low temperature fuel cell

We have studied systematically the effects of synthesis parameters in both precipitation and colloidal methods to obtain highly dispersed Pt/carbon catalyst and compared the characteristics of prepared catalysts with commercial ones. The average Pt particle size at optimum condition for 10–60 wt.% Pt/carbon was in the range 1.7–3.8 nm which was about 70–80% of the commercial catalysts at the same Pt loading. The Pt surface area was also 20–40% higher than those of the commercial catalysts. The activities of prepared catalysts, measured by a single cell unit, were comparable with those of commercial ones.     

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.     

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.     

Efficient usage of highly dispersed Pt on carbon nanotubes for electrode catalysts of polymer electrolyte fuel cells

A Pt-deposited carbon nanotube (CNT) shows higher performance than a commercial Pt-deposited carbon black (CB) with reducing 60% Pt load per electrode area in polymer electrolyte fuel cells (PEFCs) below 500 mA/cm². K₂PtCl₄ and H₂PtCl₆·6(H₂O) are used for the Pt deposition onto multi-walled CNTs (MWCNTs), which are produced by the catalytic decomposition of hydrocarbons. The electric power densities produced using the Pt/CNT electrodes are greater than that of the Pt/CB by a factor of two to four on the basis of the Pt load per power. CNTs are thus found to be a good support of Pt particles for PEFC electrodes. TEM images show 2–4-nm Pt nanoparticles dispersed on the CNT surfaces. These high performances are considered to be due to the efficient formation of the triple-phase boundaries of gas–electrode–electrolyte. The mechanisms of Pt deposition are discussed for these Pt-deposited CNTs.     

Influence of hydrogen contamination by mercury on the lifetime of the PEM-type fuel cell

The subject of this study is the influence of traces of mercury present in the hydrogen originating e.g. from the amalgam technology of brine electrolysis on the lifetime of PEM-type fuel cell. Accelerated tests were used in order to record deterioration of laboratory fuel cells’ performance. The power output decrease observed was only partly reversible. As was proven by XPS spectroscopy, the cell performance deterioration originates from the interaction between mercury and the platinum catalyst. The lifetime of the fuel cell was assessed on the basis of the experiments performed as 7000 h for an average mercury concentration in the hydrogen of 10 μg N m⁻³. This is a sufficient value to permit utilization of the hydrogen from this source as a fuel for the fuel cell. It should be kept in the mind, however, that this value was obtained on the basis of the accelerated durability tests.     

Pt surface kinetics for a PrOx reactor for fuel cell feedstream processing

Rate expressions for the oxidation of carbon monoxide and hydrogen over a Pt catalyst are achieved using a combination of surface modeling and experimental measurements. The conditions studied are those expected within a preferential oxidation (PrOx) reactor in a fuel processor for a fuel cell stack. Convenient rate expressions for subsequent reactor modeling in a companion paper are derived from a set of elementary rate expressions using the approximation of near saturation of the catalyst surface by carbon monoxide and near partial equilibrium of adsorption/desorption of carbon monoxide and hydrogen. These rate expressions require only two Arrhenius kinetic parameters and satisfactorily represent our measured rates over the nearly three orders of magnitude variation in carbon monoxide and oxygen concentrations between 80 and 200 ^°C.     

Pt/C anodic catalysts with controlled morphology for direct dimethyl ether fuel cell: The role of consecutive surface

Two types of Pt nanowires (NWs)/C catalysts with different aspect ratios and one type of Pt nanoparticles/C catalyst are successfully synthesized, and DME electrochemical performance on different extent consecutive surfaces is investigated. The morphology and crystallization are confirmed with electron microscopes and XRD. The electrochemical tests show that the nanowire catalysts, especially the one with higher aspect ratio, possess higher electrochemical surface areas, higher absorption capacity of DME, higher CO tolerance, higher electron transfer coefficient, and higher activity towards DME electrooxidation than those of the nanoparticle catalyst. The results prove that the consecutive surface favors for direct dimethyl ether fuel cell (DDFC) anodic catalyst, which are contributive to the study of the mechanism of DME electrooxidation on Pt surface and designing an effective catalyst for anodic DDFC.