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.     

Solvent effect in the synthesis of nanostructured Pt–Sn/CNT as electrocatalysts for the electrooxidation of ethanol

Pt and Pt–Sn nanoparticles were synthesized and supported onto carbon nanotubes (CNT), the electrocatalytic activity towards the ethanol oxidation reaction was analyzed. The effect of the solvent employed for the synthesis was evaluated. Metal nanoparticles synthesis was made using water (Pt–Sn/CNT-W) or ethanol (Pt–Sn/CNT-E) as a solvent. Pt–Sn/CNT-W material presented only Pt–Sn alloy nanoparticles homogeneously distributed on the carbon nanotubes support. Pt–Sn/CNT-E sample showed non well-dispersed nanoparticles forming agglomerates along the CNTs surface with predominantly Sn⁴⁺ superficial species (SnO₂) as show the XPS, FTIR and electrochemical results. These surface arrangements had important effects on the electrocatalytic properties. Pt–Sn/CNT-W shows higher ethanol electrooxidation activity than the Pt–Sn/CNT-E, which is attributed to: a) the double catalytic effect and the intrinsic electronic mechanism favored by the presence of Sn; b) the good particle dispersion of the bimetallic active phase on the CNT and; c) the absence of SnO₂ species.     

Polyoxometallate-stabilized Pt–Ru catalysts on multiwalled carbon nanotubes: Influence of preparation conditions on the performance of direct methanol fuel cells

A novel catalyst, polyoxometallate-stabilized platinum–ruthenium alloy nanoparticles supported on multiwalled carbon nanotubes (Pt–Ru–PMo₁₂-MWNTs), was synthesized by a microwave-assisted polyol process. The effects of microwave reaction time, microwave reaction power, and pH value of the reaction solution on the electrocatalytic properties of Pt–Ru–PMo₁₂-MWNTs catalysts were also investigated. The polyoxometallate (PMo₁₂) formed a self-assembled monolayer on the surface of the Pt/Ru nanoparticles and MWNTs, which effectively prevented the agglomeration of Pt, Ru nanoparticles and MWNTs, due to the electrostatic repulsive interactions between the negatively charged PMo₁₂ monolayers. Energy dispersive spectroscopy examination and electrochemical measurements showed that the loading content of Pt/Ru and their electrochemical activity vary with the synthesis conditions, such as pH, reaction time, and microwave power. It was found that the a Pt–Ru–PMo₁₂-MWNTs electrocatalyst with high Pt loading content, small crystallite size, and good electrocatalytic activity could be synthesized using a long reaction time, intermediate microwave power, and a pH value of 7. The electrocatalysts obtained were characterized using X-ray diffraction, and scanning and transmission electron microscopy. Their electrocatalytic properties were also investigated by using the cyclic voltammetry technique.     

The effect of Sn on platinum dispersion in Pt/graphene catalysts for the methanol oxidation reaction

Sn-modified platinum catalysts are presently one of the most active catalysts for the room temperature electrooxidation of ethanol at low potentials. In this study, Pt–Sn/graphene catalysts containing different ratios of Pt and Sn were prepared by the solution-phase reduction. Microstructural characterization shows that metallic Pt, Pt–Sn alloy and tin dioxide (SnO₂) nanoparticles are distributed on the graphene sheets in the synthetic process. In terms of the electrocatalytic properties, graphene-supported Pt–Sn catalysts exhibit much higher current densities with increasing Sn proportions. It's proved that the addition of Sn not only decreases catalyst particles growth and agglomeration, but also promotes methanol electrooxidation by geometric effects on expanding Pt's lattice spacing, causing a synergistic effect between Pt and Sn nanoparticles.     

Synthesis and characterization of H5PMo10V2O40 deposited Pt/C nanocatalysts for methanol electrooxidation

Pt/C(a) catalysts are firstly prepared by modified impregnation method. In order to enhance the ability of Pt/C catalysts for methanol electrooxidation, H₅PMo₁₀V₂O₄₀ (PMV) is adsorbed on Pt/C catalysts to obtain the PMV-Pt/C catalysts. The Pt/C(a) and PMV-Pt/C are characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It is shown that Pt particles with small average size are uniformly disperesed on carbon. Cyclic voltammetry and chronoamperometry show that the PMV-Pt/C catalysts exhibit excellent catalytic activity and stability for methanol electrooxidation.     

The effect of acetaldehyde and acetic acid on the direct ethanol fuel cell performance using PtSnO2/C electrocatalysts

PtSnO₂/C with Pt:SnO₂ molar ratios of 9:1, 3:1 and 1:1 prepared by an alcohol-reduction process were evaluated as anodicelectrocatalysts for direct ethanol fuel cell (DEFC). Acetaldehyde, acetic acid and mixtures of them with ethanol were also tested as fuels. Single cell tests showed that PtSnO₂/C electrocatalysts have a superior electrical performance for ethanol and acetaldehyde electro-oxidation when compared to commercial Pt₃Sn/C_(alloy) and Pt/C electrocatalysts. For all electrocatalysts, no electrical response was observed when acetic acid was used as a fuel. For ethanol electro-oxidation, the main product was acetaldehyde when Pt₃Sn/C_(alloy) and Pt/C electrocatalysts were employed. Besides, PtSnO₂/C electrocatalysts led to the formation of acetic acid as the major product. CO₂ was formed in small quantities for all electrocatalysts studied. A sharp drop in electrical performance was observed when using a mixture of ethanol and acetaldehyde as a fuel, however, the use of a mixture of ethanol and acetic acid as a fuel did not affect the DEFC performance.     

Ethanol electro-oxidation on carbon-supported Pt,PtRu and Pt3Sn catalysts: A quantitative DEMS study

The electrocatalytic activity of commercial carbon supported PtRu/Vulcan and Pt₃Sn/Vulcan bimetallic catalysts (E-TEK, Inc.) for ethanol oxidation under well defined electrolyte transport conditions and their selectivity for complete oxidation were evaluated using cyclic voltammetry combined with on-line differential electrochemistry mass spectrometry (DEMS) measurements and compared to the activity/selectivity of standard Pt/Vulcan catalysts. The main reaction products CO₂, acetaldehyde and acetic acid were determined quantitatively, by appropriate calibration procedures, current efficiencies and product yields were calculated. Addition of Ru or Sn in binary Pt catalysts lowers the onset potential for ethanol electro-oxidation and leads to a subtle increase of the total activity of the Pt₃Sn/Vulcan catalyst. It does not improve, however, the selectivity for complete oxidation to CO₂, which is about 1% for all three catalysts under present reaction conditions—incomplete ethanol oxidation to acetaldehyde and acetic acid prevails on all three catalysts. The results demonstrate that the performance of the respective catalysts is limited by their ability for C–C bond breaking rather than by their activity for the oxidation of poisoning adsorbed intermediates such as CO_ad or CH_x,ad species.     

Properties of Pt-based electrocatalysts containing selectively deposited Sn as the anode for polymer electrolyte membrane fuel cells

Sn-promoted Pt-based catalysts were prepared by the chemical vapor deposition (CVD) of Sn on commercial Pt/C and PtRu/C catalysts using Sn(CH₃)₄ as an Sn precursor. The prepared catalysts showed higher CO tolerance than those prepared by adding Sn using an impregnation (IMP) method. This result was obtained because Sn added by CVD was selectively deposited on the Pt and Ru surfaces, instead of on a carbon support, such that the interfacial contact between Pt and Sn was greater in the Sn-CVD catalyst than in the others, as confirmed by in-situ infrared and X-ray photoelectron spectroscopic observations of the catalysts.     

Electrocatalytic activity of Pt–Pd electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells: Effect of supports

Pt–Pd electrocatalysts supported on different types of support including domestic Hicon Black (HB), multi-walled carbon nanotubes (MWCNT) and titania (TiO₂) were prepared by a combined approach of impregnation and seeding, and compared to that prepared using the commercial Vulcan XC-72 (C). Their oxygen reduction reaction (ORR) activities in an acid electrolyte (0.5 M H₂SO₄) and in a single proton exchange membrane (PEM) fuel cell were evaluated. The type of support was found to affect the Pt–Pd electrocatalyst morphology and ORR activity. The Pt–Pd/C electrocatalyst had the smallest Pt particle size, better catalyst dispersion and a higher Pt:Pd M ratio compared to that of other types of supported Pt–Pd electrocatalysts. However, both in the acid solution and in a single PEM fuel cell, the ORR activities of the Pt–Pd/HB and Pt–Pd/CNT electrocatalysts were comparable to that of the Pt–Pd/C one. The ORR pathway of all supported Pt–Pd electrocatalysts were close to the four-electron pathway.     

Preparation of a CO-tolerant PtRuxSny/C electrocatalyst with an optimal Ru/Sn ratio by selective Sn-deposition on the surfaces of Pt and Ru

A CO-tolerant PtRu_xSn_y/C electrocatalyst, with an optimal x/y ratio of 0.8/0.2, was prepared by selectively depositing Sn on the metallic surface of PtRu_0.8/C for use as the anode in a polymer electrolyte membrane fuel cell. The CO tolerance of the catalyst was greater when Sn was added by chemical vapor deposition (CVD) than by a conventional precipitation method because most of the Sn added by CVD was located in the vicinity of Pt and Ru surfaces, on which CO molecules were strongly adsorbed. Accordingly, the bi-functional mechanism of CO oxidation, which involved the migration of oxygenated species from the Sn to the adsorbed CO, was expected to be promoted to greater extents in the catalysts prepared by Sn-CVD than those prepared by Sn-precipitation. On the other hand, the ligand-effect mechanism of CO oxidation, which was facilitated by the Pt-Ru alloy formation, was not much affected by the added Sn because the Pt-Ru alloy remained unchanged, particularly when y ≤ 0.2. Among PtRu_xSn_y/C catalysts prepared by Sn-CVD, one prepared by partially substituting Sn for Ru in the PtRu_1.0/C catalyst, e.g., PtRu_0.8Sn_0.2/C, showed higher CO tolerance than one prepared by simply adding Sn to the PtRu_1.0/C catalyst, e.g., PtRu_1.0Sn_0.2/C, which demonstrated the importance of an optimum x/y ratio in the design of the ternary PtRu_xSn_y/C catalysts.     

Pt/C/MnO2 hybrid electrocatalysts for degradation mitigation in polymer electrolyte fuel cells

Pt/C/MnO₂ hybrid catalysts were prepared by a wet chemical method. Pt/C electrocatalysts were treated with manganese sulfate monohydrate (MnSO₄·H₂O) and sodium persulfate (Na₂S₂O₈) to produce MnO₂. The presence of MnO₂ was confirmed by FTIR spectroscopy. Rotating ring–disk electrode (RRDE) experiments were performed on electrodes prepared using the hybrid electrocatalysts to estimate the amount of hydrogen peroxide (H₂O₂) formed during the oxygen reduction reaction (ORR) as a function of MnO₂ content. Pt/C/MnO₂ (5% by weight of MnO₂) hybrid electrocatalysts produced 50% less hydrogen peroxide than the baseline Pt/C electrocatalyst. The hybrid electrocatalysts were used to prepare membrane electrode assemblies that were tested at 90 °C and 50% RH at open circuit with pure hydrogen as fuel and air as the oxidant. The fluoride ion concentration was measured using an ion selective electrode. The concentration of F⁻ in the anode condensate over 24 h was found to be reduced by a factor of 3–4 when Pt/C/MnO₂ replaced Pt/C as the catalyst. Through cyclic voltammetry and RRDE kinetic studies, the lower ORR activity of the acid treated hybrid electrocatalysts was attributed to catalyst treatment with acid during MnO₂ introduction. The activity of the hybrid catalyst was improved by switching to a water-based synthesis.     

Pt catalyst supported on titanium suboxide for formic acid electrooxidation reaction

Pt catalysts supported on titanium suboxide (Ti₄O₇), commercial TiO₂ and carbon black were prepared by a borohydride reduction method, respectively, and used as electrocatalysts for direct formic acid fuel cells (DFAFCs). Transmission electron microscopy (TEM) images show that Pt nanoparticles have a poorer dispersion on Ti₄O₇ compared to that on TiO₂ and carbon black due to the hydrophobicity and high density of Ti₄O₇. Nevertheless, according to cyclic voltammetry (CV) and chronoamperometry (CA) results, it is found that the Pt/Ti₄O₇ catalyst possesses better catalytic activity and stability. Besides the high electrical conductivity, it is suggested from X-ray photoelectron spectroscopy (XPS) analyses that the higher content of metallic Pt caused by the Ti₄O₇ support material also contributes to the better catalytic performance of Pt/Ti₄O₇.     

Effect of pretreatment on Pt–Co/C cathode catalysts for the oxygen reduction reaction

Carbon supported Pt and Pt–Co electrocatalysts for the oxygen reduction reaction in low temperature fuel cells were prepared by the reduction of the metal salts with sodium borohydride and sodium formate. The effect of surface treatment with nitric acid on the carbon surface and Co on the surface of carbon prior to the deposition of Pt was studied. The catalysts where Pt was deposited on treated carbon the ORR reaction preceded more through the two electron pathway and favored peroxide production, while the fresh carbon catalysts proceeded more through the four electron pathway to complete the oxygen reduction reaction. NaCOOH reduced Pt/C catalysts showed higher activity that NaBH₄ reduced Pt/C catalysts. It was determined that the Co addition has a higher impact on catalyst activity and active surface area when used with NaBH₄ as reducing agent as compared to NaCOOH.     

Decoration of carbon-supported Pt catalysts with Sn to promote electro-oxidation of ethanol

Carbon-supported Pt/Sn catalysts were prepared by decorating carbon-supported Pt with Sn, by decorating carbon-supported Sn with Pt, and by co-deposition of Pt and Sn on carbon. All of the Pt/Sn catalysts exhibited greatly enhanced activities for ethanol oxidation relative to Pt alone, with the Sn decorated Pt catalyst showing the best performance. The latter catalyst was shown by XPS to contain no metallic Sn, emphasizing the importance of surface Sn oxide species in the activation of Pt for ethanol oxidation. Decoration of Sn with Pt is shown to be a feasible way to increase Pt utilization.     

Ethanol electrooxidation on Pt/C and Pd/C catalysts promoted with oxide

This research aims to investigate Pd-based catalysts as a replacement for Pt-based catalysts for ethanol electrooxidation in alkaline media. The results show that Pd/C has a higher catalytic activity and better steady-state behaviour for ethanol oxidation than that of Pt/C. The effect of the addition of CeO₂ and NiO to the Pt/C and Pd/C electrocatalysts on ethanol oxidation is also studied in alkaline media. The electrocatalysts with a weight ratio of noble metal (Pt, Pd) to CeO₂ of 2:1 and a noble metal to NiO ration 6:1 show the highest catalytic activity for ethanol oxidation. The oxide promoted Pt/C and Pd/C electrocatalysts show a higher activity than the commercial E-TEK PtRu/C electrocatalyst for ethanol oxidation in alkaline media.     

Effect of Ni on Pt/C and PtSn/C prepared by the Pechini method

Different compositions of Pt, PtNi, PtSn, and PtSnNi electrocatalysts supported on carbon Vulcan XC-72 were prepared through thermal decomposition of polymeric precursors. The nanoparticles were characterized by morphological and structural analyses (XRD, TEM, and EDX). XRD results revealed a face-centered cubic structure for platinum, and there was evidence that Ni and Sn atoms are incorporated into the Pt structure. The electrochemical investigation was carried out in slightly acidic medium (H₂SO₄ 0.05 mol L⁻¹), in the absence and in the presence of ethanol. Addition of Ni to Pt/C and PtSn/C catalysts significantly shifted the onset of ethanol and CO oxidations toward lower potentials, thus enhancing the catalytic activity, especially in the case of the ternary PtSnNi/C composition. Electrolysis of ethanol solutions at 0.4 V vs. RHE allowed for determination of acetaldehyde and acetic acid as the reaction products, as detected by HPLC analysis. Due to the high concentration of ethanol employed in the electrolysis experiments (1.0 mol L⁻¹), no formation of CO₂ was observed.     

Controlled synthesis of carbon-supported Pt3Sn by impregnation-reduction and performance on the electrooxidation of CO and ethanol

The paper discusses experimental features relevant to the synthesis of carbon-supported Pt₃Sn nanosized particles by impregnation-reduction of the salt precursors in carbon. Colloidal techniques are proposed as the most suitable ones for obtaining carbon-supported nanosized Pt₃Sn particles. In most cases, the electrocatalysts obtained have a wide range of Pt and Sn phases, including bimetallic ones. The synthesis of similar materials by impregnating readily available precursors such as SnCl₂ and H₂PtCl₆ yields Pt-enriched catalyst precursors. In order to obtain electrocatalysts with the desired Pt:Sn = 3 atomic stoichiometry, it is necessary to eliminate chloride ions prior to thermal treatments. Microscopy characterization and thermal stability studies of the fresh and treated bimetallic materials reveal that if such ions are present, Sn is eliminated as volatile SnCl_x species at around 120-130 °C. Chloride elimination is achieved by ageing the catalyst precursor in water to ensure the complete hydrolysis of the SnCl₂ precursor. This treatment should be performed once SnCl₂ has been deposited on the carbon to avoid the formation of large Sn-oxide aggregates. A further thermal treatment in hydrogen results in the formation of the desired Pt₃Sn intermetallic phase. The performance of the Pt₃Sn/C samples in the CO and ethanol electrooxidation reaction has been studied by means of electrochemical techniques. The electrocatalysts prepared by the impregnation-reduction approach match the performance of the state-of-the-art Pt₃Sn samples prepared by colloidal techniques.     

Effect of preparation conditions on the performance of nano Pt‒CuO/C electrocatalysts for methanol electro-oxidation

Nano PtCuO particles were deposited on Vulcan XC-72R carbon black using the impregnation and microwave irradiation methods. The prepared catalysts were characterized by XRD, TEM and EDX analyses. TEM images indicated that the microwave method provides homogeneously distributed catalyst particles in smaller size, compared to the one prepared by the impregnation method. The electrocatalytic activity of Pt?CuO/C electrocatalysts was investigated to oxidize methanol in 0.5 M H₂SO₄ solution by applying cyclic voltammetry and chronoamperometry techniques. The oxidation current density of Pt?CuO/C electrocatalyst, prepared by the microwave method, showed two folds increment with a potential shift in the negative direction by 69 and 36 mV at the first and second oxidation peaks, respectively, relative to those at the catalyst prepared by the impregnation method. The effect of varying methanol concentration on the resulting oxidation current density of Pt?CuO/C electrocatalysts was studied. Some kinetic information about the reaction order with respect to methanol and Tafel slope values was calculated. Slower current density decay was observed in the chronoamperogram of Pt?CuO/C electrocatalyst, prepared by the microwave method, reflecting a lower degree of surface poisoning.     

Nitrogen-doped carbon coated ZrO2 as a support for Pt nanoparticles in the oxygen reduction reaction

A new nitrogen-doped carbon (CN_x) support for Pt electrocatalysts was prepared by carbonizing polypyrrole on the surface of ZrO₂ (ZrO₂@CN_x) at high temperature. Well-dispersed Pt nanoparticles were easily formed on the ZrO₂@CN_x. The electrocatalyst was characterized by FT-IR, XRD, TEM, XPS. The electrochemical performances indicate that the presence of ZrO₂ modified the electro-structure of Pt on the catalyst surface and that ZrO₂@CN_x had superior oxygen reduction activity compared to a nitrogen-doped carbon coated carbon (C@CN_x).     

The effect of CuO on a Pt−Based catalyst for oxidation in a low-temperature fuel cell

Electrocatalytic oxidation of methanol, ethanol, and formic acid has currently attracted research attention for low-temperature fuel cells. However, the efficiencies of these fuel cells mainly depend on the electrocatalytic activities of Pt-based anodic catalysts due to the problems of low kinetics for small organic molecule electro-oxidation. An anode catalyst can be developed by the addition of some metal oxides into a Pt-based catalyst, which can effectively promote the electro-oxidation of fuels based on small organic molecules. In this work, a nanocomposite catalyst consisting of multi-wall carbon nanotubes (CNTs), copper oxide (CuO) and Pt nanoparticles was synthesized and used to improve fuel cell oxidation. Due to its low cost and oxophilic character, the metal oxide can play a major role in the oxidation of CO. The synthesis of xPt?yCuO/CNT electrocatalysts was executed through two steps: supporting of CuO nanoparticles on CNTs by the alcothermal method followed by Pt loading onto the prepared CuO/CNT by chemical reduction. The as-prepared catalysts were physically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical measurements. The results demonstrate that CuO is well dispersed onto the CNTs and that this oxide can further interact with the active Pt present on the as-prepared catalyst composites. The activity of various xPt?yCuO/CNT electrocatalysts was determined by cyclic voltammetry (CV), where x and y are the mass ratios of Pt and CuO, respectively. The presence of CuO was found to significantly contribute to enhanced electroactivity towards oxidation reactions. The 1Pt–3CuO/CNT electrocatalyst is a capable catalyst for improving low-temperature fuel cell applications.