Enhanced electrochemical activity of Pt nanowire network electrocatalysts for methanol oxidation reaction of fuel cells

In this work, Pt nanowire networks supported on high surface area carbon (Pt NWNs/C) are synthesized as electrocatalysts for direct methanol fuel cells (DMFCs). The electrocatalytic behavior of Pt NWNs/C catalysts for the methanol and adlayer CO oxidation reactions is investigated and the results are compared with the Pt nanoparticles (NPs) supported on carbon (Pt NPs/C). The results indicate that Pt NWNs are characterized by interconnected nanoparticles with large number of grain boundaries, downshifted d-band center and reduced oxophilicity, which results in the enhanced surface mobility of oxygen-containing species such as CO_ads and OH_ads. The enhanced surface mobility of CO_ads and OH_ads in turn facilitates the removal of intermediate CO species during the methanol oxidation. The activity of the Pt NWNs/C electrocatalyst for the methanol oxidation reaction and electrooxidation of adsorbed CO is also evaluated by cyclic voltammetry, CO stripping, and kinetic analysis. The results show that Pt NWNs/C catalysts have a significantly higher electrocatalytic activity for the methanol oxidation reaction as compared to Pt NPs/C catalysts. The enhanced electrocatalytic activity of Pt NWNs/C catalysts is mainly due to the existence of large number of the grain boundaries of the interconnected nanoparticles of the unique Pt NWN structure.     

Characterizing electrocatalytic surfaces: Electrochemical and NMR studies of methanol and carbon monoxide on Pt/C

We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a ¹³CO-adlayer prepared by electrochemical adsorption from low concentration ¹³CH₃OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed ¹³CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface.     

PtRu colloidal catalysts: Characterisation and determination of kinetics for methanol oxidation

The activities of three 30 wt.% PtRu/C catalysts of the same nominal composition (Pt:Ru, 1:1 a/o) were investigated with regard to methanol oxidation. The catalysts were synthesized by Bönnemann's colloidal precursor method using different metal salt precursors and reducing agents. The catalysts were characterized ex situ by energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) and in situ CO stripping and cyclic voltammetry. The activity towards methanol electro-oxidation was checked in steady-state experiments at 22 and 60 °C. The experimental rate data can be described well by a kinetic model, which includes methanol adsorption on the Pt-sites, formation of C-containing adsorbed species, OH_ads formation on the Ru sites and heterogeneous surface reaction between C-adsorbate and OH_ads. The kinetic model parameters were identified from the experimental data and were used to explain the differences in catalytic activity.     

Methanol oxidation at carbon supported Pt and Pt-Ru electrodes: an on line MS study using technical electrodes

Methanol oxidation at technical carbon based electrodes in 0.05 M H₂SO₄ has been investigated by cyclic voltammetry using online MS under the conditions of an acid methanol fuel cell (DMFC). 5% Pt on Norit BRX and 30% Pt/Ru (40/60) on Norit BRX were used as catalysts. It is shown that methanol oxidation at technical electrodes can be characterized by a combination of cyclic voltammetry and mass spectroscopy. The onset potentials and potential dependences of the methanol oxidation rate can be determined directly by monitoring the formation of CO₂. Onset potentials of 0.5V and 0.25 V/RHE have been measured for Pt and Pt-Ru catalysts, respectively. The onset of methanol oxidation can be shifted to even more cathodic potentials (0.2V) if the Pt-Ru electrode reduces oxygen simultaneously. Carbon monoxide gas was also purged into the methanol containing electroyte during measurement in order to investigate the catalyst performance under more adverse conditions. C¹³-labelled methanol was used to distinguish between CO₂. formed from methanol (m/e = 45) and CO-oxidation (m/e = 44). Without CO the use of C¹³-labelled methanol enabled a distinction between methanol oxidation and carbon corrosion. The methanol oxidation at the platinum catalyst is severely inhibited by the presence of CO, shifting its onset to 0.65 V/RHE. In contrast the performance of the Pt-Ru electrode is not seriously affected under these conditions. It is concluded that Pt-Ru is an excellent catalyst for a methanol anode in an acid methanol fuel cell (DMFC).     

Synthesis of Mesoporous Carbon from Okara and Application as Electrocatalyst Support

Carbon materials derived from biomass are economical and simple. Here, a okara‐derived carbon (ODC) was prepared by carbonized cheap and abundant okara at 800 °C in N₂ atmosphere. A high degree of graphitization, mesoporous structure and large specific surface area of ODC were proved by Raman spectroscopy, nitrogen adsorption–desorption isotherms, X‐ray diffraction, Fourier transform infrared spectra and scanning electron microscope. The ODC can be used as support of platinum nanoparticles, and the catalytic performance for methanol electro‐oxidation of its was measured by cyclic voltammetry and CO stripping voltammetry. The results showed that Pt/ODC catalyst had higher electrocatalytic activity and the resistance to poisoning ability toward methanol electrooxidation than the Pt/C catalyst prepared under the same conditions.     

Nickel phthalocyanine-tetrasulfonic acid as a promoter of methanol electro-oxidation on Pt/C catalyst

This paper describes an investigation of the role of nickel phthalocyanine-tetrasulfonic acid (NiPcTs) for methanol electro-oxidation on a Pt/C catalyst. Cyclic voltammetry (CV) revealed that NiPcTs has no catalytic activity in methanol or CO electro-oxidation. However, methanol electro-oxidation occurs faster on a Pt/C catalyst modified with NiPcTs than on the original Pt/C catalyst. CO stripping results demonstrated that NiPcTs promotes electro-oxidation of adsorbed CO (CO_ads) on the Pt/C catalyst, which is likely to be responsible for the enhancement of the methanol electro-oxidation rate. The promotion effect of NiPcTs is attributable to its ability to modify the electron density of the Pt surface. The electron deficiency of Pt⁰ in the NiPcTs-Pt/C catalyst is shown by the shift of the Pt⁰ 4f peak to higher binding energies in the X-ray photoelectron spectrum.     

Activating and deactivating mass transport effects in methanol and formic acid oxidation on platinum electrodes

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Electrochemical quartz crystal microbalance study on carbon oxides adsorption in the presence of electrosorbed hydrogen on Pd alloys with Pt and Rh

CO₂ and CO adsorption on Pd-Pt and Pd-Rh alloys has been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). Adsorbed CO₂ inhibits partially hydrogen adsorption on Pt and Rh surface atoms but does not block significantly hydrogen absorption into alloy bulk. In the presence of adsorbed CO both hydrogen adsorption and absorption are strongly suppressed. On electrodes covered with adsorbed CO the oxidation of previously absorbed hydrogen is significantly shifted into higher potentials. The EQCM response in CO₂/CO adsorption experiments is affected by both the effects connected with the changes in mass attached to the resonator and the non-mass effects including changes in metal-solution interactions and variation of solution density and viscosity in the vicinity of the electrode. Differences in the EQCM behavior suggest that the products of CO₂ and CO adsorption on the alloys studied are not totally identical.     

Methanol oxidation activities of Pt nanoparticles supported on microporous carbon with and without a graphitic shell

Platinum nanoparticles were prepared as catalysts supported on microporous amorphous carbon with and without a graphitic carbon shell. The electro-oxidation of methanol in acidic solutions at room temperature was used as a probe reaction to explore the effect of the carbon structure on catalysis. CO anodic stripping voltammetry and cyclic voltammetry both recorded enhanced performance for the catalyst supported on the carbon with a graphitic shell. Some rationalizations of the possible roles of the graphitic shell are provided.     

Mechanism of di(methyl)ether (DME) electrooxidation at platinum electrodes in acid medium

The electrooxidation of DME was studied at a bulk platinum electrode. It was shown that the DME adsorption was a slow step in the overall oxidation reaction. The DME adsorption is potential dependent in the hydrogen region of platinum and independent in the double layer region. From low potential scan rate voltammetry and DME stripping experiments, it was shown that the DME oxidation mechanism occurred via several reaction paths. At low potentials, DME oxidation leads to the existence of a positive current plateau. “In situ” Infrared Reflectance Spectroscopy experiments were carried out to identify the intermediate and reaction products of DME adsorption and oxidation at different potentials. CO_L (linearly bonded CO), CO_B (bridge bonded CO), adsorbed COOH species and CO₂ were detected. From these electrochemical and spectro-electrochemical results, it was proposed that some adsorbed DME was hydrolysed and directly oxidized to CO₂ or HCOOH species and some partially blocked platinum sites at the surface forming Pt–CHO and/or Pt–CO. Then, as soon as platinum becomes able to activate water, a bifunctionnal mechanism occurs to form either HCOOH or CO₂ again following two different reaction paths.     

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt_xM/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM_y/C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.     

Templated Pt-Sn electrocatalysts for ethanol, methanol and CO oxidation in alkaline media

In this work, we have investigated an aerosol-derived templated electrocatalyst for electro-oxidation of small organic molecules in alkaline media. Templated Pt-Sn electrocatalysts are compared to templated Pt catalysts both synthesized in an aerosol synthesis technique. In this synthesis approach, mono-disperse silica nanoparticles are used to template the metallic precursors. Structural and compositional analysis of the nanostructured materials are performed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and BET surface area measurements. The aerosol-derived templated electrocatalysts are examined in conjunction with an anion exchange ionomer for ethanol, methanol and CO oxidation in alkaline media. The electrochemical studies include cyclic voltammetry, chronoamperometry and voltammetric adsorption of adsorbed CO.     

Do magnetically modified PtFe/C catalysts perform better in methanol electrooxidation?

Motivated by the demonstrated magnetic field effect on the oxygen reduction reaction in polymer electrolyte fuel cells (PEMFC), a number of PtFe/C catalysts with different magnetic characteristics were prepared and tested for methanol electrooxidation in acidic solutions at room temperature. The catalysts were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray microanalysis (EDX) and vibrating sample magnetometry (VSM). The activity and CO tolerance of the catalysts in the methanol oxidation reaction (MOR) was measured by cyclic voltammetry and anodic CO stripping voltammetry, respectively. Heat treatment transformed the as-prepared PtFe/C from a face-centered cubic (fcc) structure into a face-centered tetragonal (fct) structure. Vibrating sample magnetometer (VSM) measurements confirmed the as-prepared PtFe/C as superparamagnetic, and the heat-treated catalysts as ferromagnetic. The heat-treated ferromagnetic catalysts were, however, low in specific mass activity and showed no improvement in CO tolerance relative to the as-prepared one. These results led us to conclude that the magnetic modification of catalysts through heat treatment have no practical contributions to the catalysis of MOR.     

Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes

The electrochemical oxidation of carbon monoxide and methanol on single-crystal noble metal electrodes has been studied using cyclic voltammetry, chronoamperometry, in situ FTIR spectroscopy, online electrochemical mass spectrometry, and theoretical methods. The oxidation of CO was found to be enhanced by steps and defects. Furthermore, the surface diffusion rate was found to have a significant influence on the kinetics of the oxidation process: for high diffusion rates, such as the oxidation of CO on platinum, the kinetics can be described by a mean field model, while for low diffusion rates, such as CO oxidation on rhodium in sulfuric acid, a nucleation-and-growth model was found to be more suitable. Voltammetric and mass spectrometric measurements on the oxidation of methanol on platinum indicate that steps enhance the overall reaction rate. In general, the selectivity towards the direct oxidation pathway through soluble intermediates was found to be higher in the absence of strongly adsorbing anions. In both perchloric and sulfuric acid, this selectivity was also found to increase with increasing step density. In sulfuric acid, Pt(111) shows the highest relative contribution for the direct pathway of all surfaces studied in that electrolyte. Based on these results, a detailed reaction scheme for the electrochemical oxidation of methanol is presented.     

Methanol oxidation at Pt/MoO x /MoSe2 thin film electrodes prepared with exfoliated MoSe2

This paper presents results on the dispersion of Pt on MoO _x /MoSe₂ electrodes, which were prepared by an intercalation–exfoliation technique. The Pt/MoO _x /MoSe₂ electrodes were tested for methanol oxidation by cyclic voltammetry. Thin films of MoSe₂ can be oxidized electrochemically to form a MoO _x layer. Compared with pure platinum, Pt/MoO _x /MoSe₂ electrodes gave lower oxidation potentials and higher current densities. We believe that methanol adsorption and subsequent dehydrogenation occur at the Pt atoms, while the OH_ads nucleation occurs at Mo sites. The presence of the adsorbed —OH groups on Mo sites may catalyze CO oxidation to CO₂. This surface reaction is necessary for the improved electrocatalytic behavior of Pt/MoO _x /MoSe₂ electrodes.     

微波法制备的Pt/C催化剂对甲醇和CO的电催化氧化

用微波间断升温法制备了3种Pt/C催化剂,运用循环伏安和线行扫描方法测试甲醇和吸附态CO在不同方法制备的Pt/C催化剂上的电催化氧化情况。发现在酸性溶液中,对于相同Pt载量的Pt(2)和Pt(3)催化剂,Pt(3)具有较小的Pt平均粒径及较高的电催化活性;对于具有较高Pt载量的Pt(1)催化剂,具有最小的平均粒径和最高的电催化活性。     

Generation of carbon dioxide from glycerol: Evidences of massive production on polycrystalline platinum

In this work we investigate the glycerol electrooxidation reaction on polycrystalline platinum in acid media. Cyclic voltammetry shows the existence of multiple oxidation peaks, which are related with a complex electrooxidation mechanism. We follow the voltammetric response of Pt in the presence of glycerol by using FTIR in situ. Results show that during glycerol electrooxidation massive amounts of CO₂ are produced. The production of CO₂ begins at low potentials and depends on the previous formation of adsorbed CO. This pathway is accelerated at high potentials and seems to be the main responsible for the rising of the currents observed in the cyclic voltammogram for potentials up to 1.0 V. Moreover, there is a parallel pathway involving the production of a carboxylic acid (probably glyceric acid), but the relative magnitudes of CO₂ bands and acid bands makes clear that the production of CO₂ is the dominant feature of the spectra, suggesting that glycerol can be a suitable candidate for use in direct alcohol fuel cells.     

Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells

The carbon nanotube (CNT) synthesised by the template carbonisation of polypyrrole on alumina membrane has been used as the support for Pt-WO₃, Pt-Ru, and Pt. These materials have been used as the electrodes for methanol oxidation in acid medium in comparison with E-TEK 20 wt% Pt and Pt-Ru on Vulcan XC72R carbon. The higher electrochemical surface of the carbon nanotube (as evaluated by cyclic voltammetry) has been effectively used to disperse the catalytic particles. The morphology of the supported and unsupported CNT has been characterised by scanning electron micrograph and high-resolution transmission electron micrograph. The particle size of Pt, Pt-Ru, and Pt-WO₃ loaded CNT was found to be 1.2, 2, and 5 nm, respectively. The X-ray photoelectron spectra indicated that Pt and Ru are in the metallic state and W is in the +VI oxidation state. The electrochemical activity of the methanol oxidation electrode has been evaluated using cyclic voltammetry. The activity and stability (evaluated from chronoamperometric response) of the electrodes for methanol oxidation follows the order: GC/CNT-Pt-WO₃-Nafion>GC/E-TEK 20% Pt-Ru/Vulcan Carbon-Nafion>GC/CNT-Pt-Nafion>GC/E-TEK 20% Pt/Vulcan carbon-Nafion>Bulk Pt. The amount of nitrogen in the CNT plays an important role as observed by the increase in activity and stability of methanol oxidation with N₂ content, probably due to the hydrophilic nature of the CNT.