The effect of low concentrations of CO on H2 adsorption and activation on Pt/C: Part 2—In the presence of H2O vapor

CO affects H₂ activation on supported Pt in the catalyst layers of a PEMFC and significantly degrades overall fuel cell performance. This paper establishes a more fundamental understanding of the effect of humidity on CO poisoning of Pt/C at typical fuel cell conditions (80 °C, 2 atm). In this work, direct measurements of hydrogen surface concentration on Pt/C were performed utilizing an H₂-D₂ switch with Ar purge (HDSAP). The presence of water vapor decreased the rate of CO adsorption on Pt, but had very little effect on the resulting CO surface coverage on Pt_S (θ_CO) at steady-state. The steady-state θ_COs at 80 °C for Pt exposed to H₂ (PH₂=1 atm) and a mixture of H₂/H₂O (1 atm H₂, 10%RH) were 0.70 and 0.66 ML, respectively. Furthermore, total strongly bound surface hydrogen measured after exposure to H₂/H₂O was, surprisingly, the sum of the exchangeable surface hydrogen contributed by each component, even in the presence of CO. In the absence of any evidence for strong chemisorption of H₂O on the carbon support with/without Pt, this additive nature and seemingly lack of interaction from the co-adsorption of H₂ and H₂O on Pt may be explained by the repulsion of strongly adsorbed H₂O to the stepped-terrace interface at high coverages of surface hydrogen.     

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo₁₂O₄₀^3–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H₂ is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H₂ gas feeds (0, 10, 50, and 100 ppm CO in H₂). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H₂ gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance R_p appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H₂ by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H₂ fuel gases.     

The effect of low concentrations of tetrachloroethylene on H2 adsorption and activation on Pt in a fuel cell catalyst

The poisoning effect of tetrachloroethylene (TTCE) on the activity of a Pt fuel cell catalyst for the adsorption and activation of H₂ was investigated at 60 °C and 2 atm using hydrogen surface concentration measurements. The impurity was chosen as a model compound for chlorinated cleaning and degreasing agents that may be introduced into a fuel cell as a contaminant at a fueling station and/or during vehicle maintenance. In the presence of only H₂, introduction of up to 540 ppm TTCE in H₂ to Pt/C resulted in a reduction of available Pt surface atoms (measured by H₂ uptake) by ca. 30%, which was not enough to shift the H₂-D₂ exchange reaction away from being equilibrium limited. Exposure of TTCE to Pt/C in a mixed redox environment (hydrogen + oxygen), similar to that at the cathode of a fuel cell, resulted in a much more significant loss of Pt surface atom availability, suggesting a role in TTCE decomposition and/or Cl poisoning. Regeneration of catalyst activity of poisoned Pt/C showed the highest level of recovery when regenerated in only H₂, with much less recovery in H₂ + O₂ or O₂. The results from this study are in good agreement with those found in a fuel cell study by Martínez-Rodríguez et al. [2] and confirm that the majority of the poisoning from TTCE on fuel cell performance is most likely at the cathode, rather than the anode.     

The effect of H2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H₂S). Trace amount of CO and H₂S can cause considerable cell performance losses. A comparison between the effect of CO and that of H₂S on PEMFC performance was made in this paper. Under the same conditions, the H₂S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H₂S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H₂S and can be only partially recovered by reintroducing neat H₂. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.     

Effect and siting of Nafion in a Pt/C proton exchange membrane fuel cell catalyst

This paper explores the effect and siting (location) of Nafion on Pt/C as exists in a PEM fuel cell catalyst layer. The addition of 30 wt% Nafion on Pt/C (Nfn-Pt/C) resulted in a severe loss of BET surface area by filling/blocking the smaller pore structures in the carbon support. Surprisingly, the presence of this much Nafion appeared to have only a minimal effect on the adsorption capability of either hydrogen or CO on Pt. However, the presence of Nafion doubled the amount of time required to purge most of the gas-phase and weakly-adsorbed hydrogen molecules away from the catalyst during hydrogen surface concentration measurements. This strongly chemisorbed surface hydrogen was determined by a H₂/D₂ switch and exchange procedure. Nafion had an even more pronounced effect on the reaction of a larger molecule like cyclopropane. Results from the modeling of cyclopropane hydrogenolysis in an idealized pores suggest that partial blockage of only the pore openings by the Nafion for the meso-macropores is sufficient to induce diffusion limitations on the reaction. The facts suggest that most of the Pt particles are in the meso-macropores of the C support, whereas Nafion is present primarily on the external surface of the C where it blocks significantly the micropores but only partially the meso-macropores.     

Selective CO oxidation on Cu promoted Pt/Al2O3 and Pt/Nb2O5 catalysts

Pt–Cu catalysts supported on Al₂O₃ and Nb₂O₅ were studied for use in selective CO oxidation. The addition of copper enhanced the activity and selectivity of Pt–Cu/Nb₂O₅ at lower temperatures when compared to Pt/Nb₂O₅. On the other hand, copper addition was not beneficial in the case of Al₂O₃ supported catalysts.     

Photocatalytic properties of redox-treated Pt/TiO2 photocatalysts for H2 production from an aqueous methanol solution

The influence of redox-treated Pt/TiO₂ photocatalysts on H₂ production is investigated. Catalyst characterizations are performed by TEM, XPS, XRD, BET, and UV–vis/DR spectroscopy techniques. In terms of production rate, the oxidation treatment shows higher reactivity than the reduction treatment. The reduction treatment allows the formation of metallic Pt(0), which more easily catalyzes the transition of TiO₂ from the anatase to the rutile phases. Reduction-treated Pt/TiO₂ photocatalysts have lower S_BET values than oxidation-treated Pt/TiO₂ photocatalysts due to the higher percentage of TiO₂ in the rutile phase. Combining the results of XPS and optical analyses, PtO/TiO₂ shows a higher energy band gap than metallic Pt(0)/TiO₂, indicating that oxidation-treated Pt/TiO₂ is more capable of achieving water splitting for H₂ production. According to the results of this study, the oxidation treatment of Pt/TiO₂ photocatalysts can significantly enhance the reactivity of photocatalytic H₂ production because of their homogenous distribution, lower phase transition, higher S_BET, and higher energy band gap.     

Effect of H2O2 on Pt electrode dissolution in H2SO4 solution based on electrochemical quartz crystal microbalance study

The effect of H₂O₂ on the Pt dissolution in 0.5 mol dm⁻³ H₂SO₄ was investigated using an electrochemical quartz crystal microbalance (EQCM). For the potential cycling at 50 mV s⁻¹, the Pt weight irreversibly decreases in a N₂ atmosphere with H₂O₂, while only a negligible Pt weight-loss is observed in the N₂ and O₂ atmospheres without H₂O₂. The EQCM data measured by the potential step showed that the Pt dissolution in the presence of H₂O₂ depends on the electrode potential and the H₂O₂ concentration. For the stationary electrolysis, the Pt dissolution occurs at 0.61–1.06 and 1.06–1.36 V vs. RHE. It should be noted that the Pt dissolution phenomenon in the presence of H₂O₂ is also affected by the potential scanning time. Based on these results, H₂O₂ is considered not only to contribute to the formation of Pt-oxide causing the cathodic Pt dissolution, but also to participate in the anodic Pt dissolution and the chemical Pt dissolution.     

The size and valence state effect of Pt on photocatalytic H2 evolution over platinized TiO2 photocatalyst

Photocatalytic hydrogen production from water or organic compounds is a promising way to resolve our energy crisis and environmental problems in the near future. Over the past decades, many photocatalysts have been developed for solar water splitting. However, most of these photocatalysts require cocatalyst to facilitate H₂ evolution reaction and noble metals as key cocatalysts are widely used. Consequently, the condition of noble metal cocatalyst including the size and valence state etc plays the key role in such photocatalytic system. Here, the size and valence state effect of Pt on photocatalytic H₂ evolution over platinized TiO₂ photocatalyst were studied for the first time. Surprisingly, it was found that Pt particle size does not affect the photoreaction rate with the size range of several nanometers in this work, while it is mainly depended on the valence state of Pt particles. Typically, TOFs of TiO₂ photodeposited with 0.1–0.2 wt% Pt can exceed 3000 h⁻¹.     

Properties and H2 production ability of Pt photodeposited on the anatase phase transition of nitrogen-doped titanium dioxide

The effect of calcination temperature on the properties and H₂ production ability of nitrogen-doped (N-doped) titanium dioxide (TiO₂) photodeposited with 0.2 wt% Pt (platinum) was studied. The increase in crystallinity of pre-calcinated N-doped TiO₂ initiated at temperatures higher than 131 °C transformed the morphology from anomalous nanostructure to texture composed of nanoparticles and enhanced the specific surface areas. At 200-400 °C, the anatase peaks gradually became sharper and the visible light absorption region decreased due to the growth of crystallites and the decrease of N-doping content, respectively. Maximum H₂ production was reached when N-doped TiO₂ was calcined at 200 °C followed by Pt photodeposition. The maximum condition is brought about by the formation of textures consisting of nanoparticles and a broad absorption region, thus creating superior active sites for photocatalytic H₂ production.     

Laminar flame speed studies of lean premixed H2/CO/air flames

Laminar flame speeds of lean premixed H₂/CO/air mixtures were measured in the counterflow configuration over a wide range of H₂ content at lean conditions. The values were determined by extrapolating the referenced flame speed to zero stretch rate using the non-linear extrapolation method to reduce the systematic error. Detailed calculation of laminar flame speed was also conducted using PREMIX code coupled with three different kinetic models. In general, simulation results agreed well with the experimental data. Both the experimental and calculation results revealed that the laminar flame speeds of lean premixed H₂/CO/air mixtures increased with H₂ content significantly when H₂ content was small (?15%) and gradually when H₂ content was large (>15%).     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

The influence of the CO inhibition effect on the estimation of the H2 purification unit surface

The CO inhibition effect on H₂ permeance through commercial Pd-based membranes was analysed by means of permeation measurements at different CO compositions (0–30% molar) and temperatures (593–723 K) with the aim to determine the increase of the membrane area in order to compensate the H₂ flux reduction owing to the CO inhibition effect. The permeance of H₂ fed with carbon monoxide was observed to decrease with respect to the case of pure hydrogen. At 647 K the H₂ permeance of a pure feed of 316 μmol m⁻² s⁻¹ Pa^−0.5 reduces progressively until 275 μmol m⁻² s⁻¹ Pa^−0.5 when 15% or more of CO is present in the system, until it reaches a plateau at 20%. The inhibition effect occurring when CO is present in the feed stream reduces with the progressive temperature increase; the reduction of the permeance decreases exponentially by 23% at 593 K and by 3% at 723 K with 10% of CO. The inhibition effect is seen to be reversible. An H₂ flux profile in a Sieverts' plot shows the effect produced by the increase of the CO composition along the Pd-based membrane length. The H₂ flux profile allows the area of a Pd-based membrane to be evaluated in order to have the same permeate flow rate of H₂ when it is fed with CO or as a pure stream. Moreover, a qualitative comparison between the H₂ flux profiles and a previously proposed model has been carried out.     

CO2 and H2O gas conversion into CO and H2 using highly exothermic reactions induced by mixed industrial slags

This communication reports conversion phenomena in which CO₂ and H₂O gases are transformed into CO and H₂, respectively, when exposed to a mixture of molten CaO-rich metallurgical slag and V₂O₃-rich gasifier slag. On reaction, CO₂ and H₂O are thermodynamically driven to become CO and H₂, respectively, by giving up oxygen over the formation of calcium orthovanadate in the slag. The concept was experimentally investigated with a synthetic slag heated to 1500 °C (an assumed slag tap-out temperature in the metallurgical industry) in a CO₂ saturated atmosphere. On heating, a rapid drop in oxygen partial pressure occurred between 1405 °C and 1460 °C, where 97% of CO₂ transformed to CO. Potential industrial applications with the H₂O-to-H₂ conversion are then explored using detailed process computations. If the process is made economically viable, CO₂ and H₂O could be converted into products that are environmentally and industrially attractive and that have the potential for energy savings and greenhouse gas reduction in a process.     

Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H₂/CO/N₂/CO₂/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4–1.0). This is mainly due to the high reactivity of H₂ leading to high production rate of H and OH radicals. Reactions  and  play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction.     

Carbon materials for H2 storage

In this work a series of carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The materials used were different types of commercial carbons: carbon fibers, carbon cloths, nanotubes, superactivated carbons, and synthetic carbons (carbon nanospheres and carbon xerogels). Their textural properties (i.e., surface area, pore size distribution, etc.) were related to their hydrogen adsorption capacities. These H₂ storage capacities were evaluated by various methods (i.e., volumetric and gravimetric) at different temperatures and pressures. The differences between both methods at various operating conditions were evaluated and related to the textural properties of the carbon-based adsorbents. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. Furthermore, hydrogen storage capacity seems to be proportional to surface area, especially at 77 K. The micropore size distribution and the presence of narrow micropores also notably influence the H₂ storage capacity of carbons. In contrast, morphological or structural characteristics have no influence on gravimetric storage capacity. If synthetic materials are used, the textural properties of carbon materials can be tailored for hydrogen storage. However, a larger pore volume would be needed in order to increase storage capacity. It seems very difficult approach to attain the DOE and EU targets only by physical adsorption on carbon materials. Chemical modification of carbons would seem to be a promising alternative approach in order to increase the capacities.     

CO electrooxidation study on Pt and Pt-Ru in H3PO4 using MEA with PBI-H3PO4 membrane

CO electrooxidation on Pt and Pt-Ru in H₃PO₄ was studied in the temperature range 120-180 °C using CO-N₂-H₂O gas mixtures of controlled composition. On Pt and Pt-Ru the voltammetry curves exhibited Tafel behavior in a wide potential range with a slope of 80-100 mV per decade. Replacement of Pt with Pt-Ru on the anode resulted mainly in a shift of CO electrooxidation voltammetry curves by approx. −0.3 V. Reaction order in respect to water vapor pressure was found close to unity with both electrocatalysts. Reaction order in respect to CO partial pressure was found negative, close to zero. Values of apparent activation energy of CO electrooxidation on these electrocatalysts were nearly equal, E_a app = 110 ± 15 kJ mol⁻¹. The results were interpreted within the framework of Langmuir-Hinshelwood mechanism. An equation, which describes the observed features of CO electrooxidation on Pt and Pt-Ru, was suggested. Comparing results of the present study with results of earlier studies of CO tolerance of Pt and Pt-Ru electrocatalysts, it was concluded that CO electrooxidation can hardly play a significant role in CO tolerance of PEM FC with PBI-PA membranes.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.