Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study

We describe the adaptation of the recently developed thin-film rotating disk electrode method and its application in a rotating ring disk configuration (RRDE) to the investigation of the oxygen reduction reaction (orr) on a supported catalyst powder (Pt/Vulcan XC 72 carbon). This allows the determination of kinetic data, such as reaction orders or apparent activation energies, for the orr directly without mathematical modeling. Collection experiments reveal a potential and rotation rate independent collection efficiency. RRDE measurements allow, for the first time, the direct determination of the fraction of peroxide production during oxygen reduction on supported catalysts. Finally, comparison of measurements in 0.5 M H₂SO₄ and 0.5 M HClO₄, respectively, reveals a significant effect of (bi)sulfate adsorption on the orr activity. On the basis of the present results, predictions are made on the kinetic limit of the orr in polymer electrolyte fuel cells, in the absence of ohmic and mass transport resistances at 100% utilization.     

Methanol tolerant oxygen reduction on carbon-supported Pt–Ni alloy nanoparticles

The preparation of carbon-supported Pt–Ni alloy catalysts at a 40 wt% total metal loading and with high Ni content within the alloys and their electrocatalysis for the oxygen reduction reaction has been studied. Emphasis is placed on the methanol-tolerant oxygen reduction on as-prepared alloy catalysts and their application in direct methanol fuel cells. It was found that as-prepared alloy catalysts have single-phase disordered structures and small particle sizes with a relatively narrow size distribution even at 40 wt% loading. As compared to pure Pt/C catalyst for oxygen reduction, such alloy catalysts exhibited enhanced electrocatalytic activities in pure acidic electrolyte and significantly enhanced electrocatalytic activities in methanol-containing electrolyte. The high methanol tolerance of Pt–Ni alloy catalysts during oxygen reduction could be ascribed to a lowered activity of methanol oxidation, which may originate from the composition effect and the disordered structure of the alloy catalysts. Fuel cell tests confirmed that as-prepared Pt–Ni alloy catalysts for oxygen reduction are more active than a commercial Pt/C catalyst with the same metal loading and that the maximum activity was found with a Pt/Ni atomic ratio of 2:1, which is similar to results in half-cell tests.     

Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®

The aim of this work is to study kinetic parameters and the mechanism of the oxygen reduction reaction (orr) on platinum nanoparticles supported on carbon, inside Nafion^® (i.e. in PEMFC cathode conditions). Stationary and electrochemical impedance spectroscopy techniques were used to measure exchange current densities, Tafel slopes, and reaction orders with respect to O₂ pressure and H⁺ activity. The platinum nanoparticle size effect was confirmed. A specific low frequency inductive behaviour of the cathode impedance was observed. The latter demonstrates the presence of (at least) two electrochemical steps in the orr mechanism. Secondly, dc and ac modelling of the reaction in a gas diffusion electrode is proposed in order to simulate current–potential curves and impedance spectra. This paper reveals that an ECE mechanism for oxygen reduction proposed by Damjanovic and coworkers on bulk platinum in acidic medium is also valid for that on Pt nanoparticles.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Studies of CO tolerance on modified gas diffusion electrodes containing ruthenium dispersed on carbon

Studies of the CO tolerance for the hydrogen oxidation reaction (hor) on a polymer electrolyte fuel cell were conducted on modified gas diffusion electrodes with CO filtering layers formed by ruthenium dispersed onto carbon (Ru/C) and having platinum dispersed on carbon (Pt/C) in the catalyst layer. Different configurations for gas diffusion electrodes containing Ru hydrated-oxide (RuO_xH_y/C) layers were tested in order to understand the CO oxidation mechanism on these catalysts. Single cell polarization results indicated that a direct contact between Ru distributed on the diffusion layer and the catalyst layer is required to achieve CO tolerance. Results also show that an enhanced activity is achieved for the hor in the presence of CO with PtRu/C in the catalyst layer obtained by an impregnation method. Also, an electrode formed by a RuO_xH_y/C filtering layer in contact with a catalyst layer with PtRu/C showed the highest CO-tolerance. For this system, the cell voltage loss was only 65 mV at 1 A cm^?2 with H₂ + 100 ppm CO, with respect to the value obtained with pure hydrogen.     

Enhancement of the electrocatalytic O2 reduction on Pt–Fe alloys

The design of high performance cathode electrocatalysts is essential for polymer–electrolyte fuel cells, which are now attracting enormous interest as a primary power source for zero-emission electric vehicles. We have discovered a significant enhancement of electrocatalytic activity of Pt by alloying with Fe, and found a maximum activity at ca. 50% Fe content, which results in 25 times higher activity than pure Pt activity. It was confirmed experimentally at Pt–Fe bulk alloys that the alloy catalyst surface consists of a pure Pt skin-layer (<1 nm in thickness) that is modified in the electronic structure by that of the bulk alloy. The enhancement could be well explained by the 5d-vacancy of the surface, but not by Pt interatomic distance or roughening of the surfaces.     

Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction activity evaluated by rotating disk electrode

The dependence of oxygen reduction reaction (ORR) activity on the loading level of Pt electrocatalysts highly dispersed on carbon black (Pt/CB) has been investigated. We present a standard method for the evaluation of ORR activity at Pt/CB by a rotating disk electrode (RDE), in which Pt/CB powder was attached on a glassy carbon (GC) disk uniformly, followed by coating a thin Nafion film. We describe the optimized protocols for preparing a suspension of the Pt/CB in an ethanol aqueous solution, controlling vapor pressure during drying process, and coating very thin (0.05 μm) Nafion film. The area-specific activity for the ORR in air-saturated 0.1 M HClO₄ electrolyte solution was found not to be influenced by the Pt-loading level on CB from 19.2 to 63.2 wt%, when the amount of Pt/CB on GC and the dispersion state were optimized.     

Kinetics of CH3OH oxidation on PtRu/C studied by impedance and CO stripping voltammetry

The kinetics of the CH₃OH oxidation reaction at 60 °C on well-alloyed platinum–ruthenium supported on carbon (Pt₅₀Ru₅₀/C) was studied by electrochemical impedance spectroscopy and compared with carbon-supported platinum (Pt/C). The reaction rate of the overall CH₃OH oxidation increased with increasing electrode potential for both Pt/C and PtRu/C. In the case of Pt/C, when the electrode potential was E ? 450 mV vs. RHE only a capacitive behavior was observed. Resistive and pseudo-inductive types of behavior were evident above 500 and 600 mV vs. RHE. In the case of PtRu/C, a similar change in behavior was observed, except that the two types of behavior were observed at 200 mV lower electrode potentials than for Pt/C. Correlation of the impedance data with pre-adsorbed carbon monoxide (CO_ad) stripping voltammetry allowed the understanding of the methanol oxidation reaction. The change in the reaction rate of the oxidation of CH₃OH to CO_ad as a function of the electrode potential as well as the promotional effect of Ru was evident from a change in the frequency where the frequency deviated from the ～90° phase angle. The change in the reaction rate of the oxidation of CO_ad to CO₂ as a function of the electrode potential as well as the alloying with Ru was evident from a change in the frequency where the phase angle approached zero.     

An XPS study on binary and ternary alloys of transition metals with platinized carbon and its bearing upon oxygen electroreduction in direct methanol fuel cells

The present X-ray photoelectron spectroscopic study on carbon-supported Pt, Pt–Cr, Pt–Co, Pt–Ni, Pt–Co–Cr and Pt–Co–Ni electrocatalysts suggests that while Pt–Cr/C, Pt–Co/C and Pt–Ni/C binary alloy electrocatalysts are surface rich in platinum, both Pt–Co–Cr/C and Pt–Co–Ni/C ternary electrocatalysts happen to be surface rich in base metals. Among these electrocatalysts, Pt–Co/C electrocatalyst is found to possess a minimum amount of platinum oxides. Accordingly, Pt–Co/C exhibits a higher electrocatalytic activity towards oxygen-reduction reaction in the solid-polymer-electrolyte direct methanol fuel cells.     

High dispersion and electrocatalytic properties of Pt nanoparticles on SWNT bundles

Platinum (Pt) nanoparticles were electrochemically dispersed on single-walled carbon nanotube (SWNT) bundles by electro-reduction of the Pt(IV) complex formed on the SWNT surface. The structure and elemental composition of the resulting Pt/SWNT composite were characterized by transmission electron micrograph (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The electrocatalytic properties of the Pt/SWNT electrode for methanol oxidation have been investigated by cycle voltammetry (CV). A high electrocatalytic activity and excellent stability of the Pt/SWNT electrode can be observed. This may be attributed to the high dispersion of platinum nanoparticles and the particular properties of the SWNT supports. The results imply that the Pt/SWNT composite has good potential applications in fuel cells.     

Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts

The kinetics of methanol electro-oxidation on carbon-supported particulate Pt and Pt + Ru catalysts is studied on PTFE-bonded electrodes made of these materials. The electrochemical characterization is carried out both galvanostatically and potentiostatically in sulphuric acid, and special attention is paid to the effects of low methanol concentrations in the range from 5.0 mM to 2.0 M. Furthermore, the effects of reaction temperature, catalyst loading and electrolyte concentration are discussed. A steady-state adsorption model which explains the observed polarization behaviour is presented. The model highlights the subtle balance between the initial methanol adsorption-dehydrogenation and the subsequent oxidative removal steps as well as the important roles of water and surface intermediates in the oxidation reaction. Rough estimates are given for the rate constants and activation energies of the individual reaction steps.     

Oxygen reduction reaction selectivity of RuxSey in formic acid solutions

Carbon-supported and non-supported cluster-like Ru_xSe_y were synthesized in aqueous media. The structure and morphology were analyzed by XRD and TEM, respectively. Particles dispersed uniformly on the support with an averaged size of 2.1 nm were obtained. The oxygen reduction reaction (ORR) in presence of HCOOH was investigated by means of rotating disk electrode at 25 °C on Ru_xSe_y nanoclusters. The kinetics of HCOOH oxidation for non-supported and carbon-supported Ru_xSe_y catalysts was analyzed. A reaction order of 1/1.4 and 1/1.6 was determined for Ru_xSe_y and Ru_xSe_y/C, respectively. This reaction order indicates that the reaction mechanism is similar to Pt/C. The chalcogenide catalyst showed a high tolerance and selectivity towards the ORR in electrolytes containing up to 0.1 M HCOOH. At the highest formic acid concentration (5 M) the potential, at e.g. current density of 0.2 mA cm⁻², shifts to negative for non-supported and carbon-supported Ru_xSe_y by 0.24 V and 0.28 V, respectively. The results reported in this work are the basis for overcoming the negative impact of fuel cross-over in microfluidic formic acid fuel cells that currently use Pt as cathode catalyst.     

Mixed platinum–gold electrocatalysts for borohydride oxidation prepared by the galvanic replacement of nickel deposits

Mixed platinum/gold coatings were prepared by partial galvanic replacement of nickel layers that had been electrodeposited onto glassy carbon electrode substrates. The process (termed “transmetalation”) involves the spontaneous replacement of surface Ni atoms by Pt and Au upon immersion of the former metal into equimolar mixed chloroplatinic and chlorolaurate acid solutions. The multimetallic deposits on glassy carbon substrates, PtAu(Ni)/GC, were characterised by SEM/EDS, XRD, AES and electrochemical techniques. Smooth films with a bulk composition enriched in Au and minimum quantities of Ni left were obtained. Pt and Au were found to be alloyed and both Pt and Au to co-exist with Ni down to the core of the layers. The PtAu(Ni) deposits displayed typical Pt and Au surface electrochemistry indicating the formation of a continuous noble metal shell; the estimated electroactive surface areas point again to an excess of Au on the surface. The activity of the PtAu(Ni)/GC electrodes towards the oxidation of borohydride was studied by voltammetry at a rotating disc electrode, RDE, and compared to that of bulk Pt and Au. As expected, the Pt–Au alloy catalyst showed behaviour intermediate to that of its pure components. However, in the limiting current potential range the alloy tended more to bulk Au behaviour (number of electrons was closer to eight) while at low overpotentials it resembled the catalytic activity of pure Pt.     

Cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using carbon paste electrodes

The present work relates to the cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using solid carbon paste electrodes. The cyclic voltammetric data of the solid catalysts, which were reproducible, were in agreement with those of electro-deposited Pt and PtSn electrode systems. The reversibility of the H-adsorption–desorption system in representative 0.3 Pt–Al₂O₃ monometallic catalyst was confirmed by the difference between the anodic and cathodic peak potentials. The unit ratio of the cathodic to anodic peak currents observed, indicated the equality of the transfer coefficient in the kinetics for the respective processes. The O₂-adsorption–desorption reaction was found to be irreversible as expected. The lower area of the curve for H-adsorption–desorption in the case of bimetallic catalysts gives an indication for the formation of alloys. The use of a carbon paste electrode in studying the cyclic voltammetric behavior of heterogenous catalysts yielded qualitative information about the activity of the catalysts. Good correlations were found between cyclic voltammetric parameters and activity results in both monometallic and bimetallic catalysts.     

Self-assembly of and charge transfer at N-docosyl-N′-methyl viologen on electrode surfaces

Self-assembled molecular films were formed on the electrode surfaces of glassy carbon (GC), platinum (Pt), gold, silver and indium–tin oxide in aqueous electrolyte solutions of N-docosyl-N′-methyl viologen (C₂₂VC₁). The temperature dependence of voltammetric responses showed a higher stability with higher surface coverage and with a Pt than a GC surface. Charge transfer reactions of the solution redox species of Ru(NH₃)₆³⁺ and Fe(CN)₆^4? at the irreversibly self-assembled C₂₂VC₁ ∣ GC interface were found to take place by an interplay of direct penetration of solution species through the self-assembled molecular layer of C₂₂VC₁ and of cross reaction between solution and surface bound redox agents.     

Electrocatalytic behavior of Pt-modified polyaniline electrode for methanol oxidation: Effect of Pt deposition modes

Platinum dispersed in polyaniline film is a better catalyst than bulk platinum for methanol electrooxidation in sulfuric acid solution. The effect of deposition modes of platinum particles on their electrocatalytic activity has been studied. Several electrodeposition methods, including constant potential, cyclic potential and double potential step, have been applied to characterize the performance of different electrodeposition modes. Besides the variation of the electrodeposition methods, rotating disk electrode technique is applied to disperse Pt particles under the control of convection–diffusion condition. The results show that the enhanced electrocatalytic activity may be due to the dispersion of Pt particles in the polyaniline film matrix and the synergistic effects between the dispersed Pt particles and the polyaniline film. Variables related to reaction kinetics, such as mass transport of species, concentration of methanol and acidity of solution, are investigated and reaction mechanism for methanol oxidation is discussed.     

High dispersion and electrocatalytic properties of platinum nanoparticles on surface-oxidized single-walled carbon nanotubes

Platinum nanocatalysts supported on surface-oxidized single-walled carbon nanotubes (Pt/o-SWNT) were prepared by the aqueous solution reduction of H₂PtCl₆ with HCHO. The nanocatalysts contain 10 wt% Pt (by EDS) on o-SWNTs. TEM images show that the Pt particle size is in the range of 5–10 nm. X-ray diffraction (XRD) analysis indicates the presence of platinum in the fcc phase and the average size of the particles calculated from the XRD peak widths agreed well with TEM results. For comparison of electrochemical performance, platinum nanocatalysts supported on purified SWNTs (Pt/p-SWNT) and on Vulcan XC-72 (Pt/C) were also prepared by the HCHO reduction method. The cyclic voltammetry results indicate that Pt/o-SWNT catalyst displays a higher performance than Pt/p-SWNT and Pt/Vulcan XC-72. The difference may be due to the disentanglement of the SWNT ropes when treated with nitric acid and the oxidation of SWNTs caused more surface functional groups resulting in a higher density of Pt particles.     

The effect of pretreatment of Vulcan XC-72R carbon on morphology and electrochemical oxygen reduction kinetics of supported Pd nano-particle in acidic electrolyte

The influence of chemical pretreatment of carbon support for oxygen reduction on palladium nano-particles in acidic electrolyte was studied. Vulcan XC-72R carbon as catalyst support for palladium nano-particles was pretreated with 5% HNO₃, 0.07 M H₃PO₄, 0.2 M KOH and 10% H₂O₂. The effect of treatment on the properties of the carbon support was studied by N₂ adsorption and X-ray photoelectron spectroscopy (XPS). It was found that chemical treatment significantly changed the surface chemical properties and surface area of the carbon support. The surface area and pore volume of 5% HNO₃ and 10% H₂O₂ treated carbon supports were drastically decreased due to the oxidative nature of treatment. Ethylene glycol (EG) reduction method was used to synthesise 20% Pd on pr-treated and un-treated carbon supports. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. It was observed that by using a mild reducing agent, namely EG, well-dispersed and nano-size Pd particles could be achieved during catalyst synthesis. The electrocatalytic activity of different Pd/C catalysts towards the oxygen reduction reaction (ORR) was examined by cyclic voltammetry (CV) on a rotating ring-disc electrode (RRDE) and compared with E-Tek 20% Pd/C catalyst under identical experimental conditions. The kinetics of ORR on these electrocatalysts predominantly involved a four-electron step reduction with the first electron transfer being the rate-determining step. However, the observed specific activity, mass activity and amount of hydrogen peroxide produced during ORR were greatly influenced by the pretreatment employed for carbon support.     

Using phosphomolybdic acid (H3PMo12O40) to efficiently enhance the electrocatalytic activity and CO-tolerance of platinum nanoparticles supported on multi-walled carbon nanotubes catalyst in acidic medium

A simple approach is reported for utilizing phosphomolybdic acid, H₃PMo₁₂O₄₀, (HPMo) to enhance the electrocatalytic activity and carbon monoxide (CO) tolerance of platinum electrocatalysts supported on multi-walled carbon nanotubes (MWCNTs) in acidic medium. Pt/MWCNTs catalyst is prepared by the ethylene glycol reduction method, and then physically mixed with HPMo for electrochemical characterizations in terms of alcohol and CO electro-oxidation using cyclic voltammetry (CV), chronoamperometry and electrochemical impendence spectra (EIS). The results show that HPMo can efficiently enhance the electrocatalytic activity and CO-tolerance of Pt/MWCNTs catalyst. The effect of HPMo wt.% on the electrocatalytic activity towards methanol electro-oxidation of Pt/MWCNTs is also investigated by CV. The best results appear at the HPMo wt.% = 30%. We further investigated the reason of Pt/MWCNTs + HPMo composite catalyst with good stability.     

Electrocrystallization of calcium carbonate on carbon-based electrodes

Calcium carbonate was deposited by electrochemical reduction of oxygen to hydroxyl ions at various carbon-based electrodes. Although some vaterite was observed during earlier stages of the electrodeposition, the predominant polymorph during later stages was calcite. The average crystal size reached a value of 15 μm after 10 h at a glassy carbon electrode but the crystal growth rate was substantially accelerated when oxygen was catalytically reduced. The same average size of the calcite crystals in this case (Pt/C electrode) was reached within a period of 1.5 h. Efficient removal of CaCO₃ from water was demonstrated when using a porous aerogel carbon electrode and a potential sufficiently negative to promote reduction of water molecules within the pores.