Impedance study of methanol oxidation on platinum electrodes

Methanol oxidation on Pt electrodes is studied by ac voltammetry. Data from voltammograms at frequencies from 0.5 Hz to 20 kHz are assembled into electrochemical impedance spectra and analysed using equivalent circuits. Inductive behavior and negative relaxation times are attributed to nucleation and growth behavior. The rate-determining step is proposed to be the reaction of adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites.     

Impedance study of formic acid oxidation on platinum electrodes

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.     

Electrocatalytic oxidation of methanol to soluble products on polycrystalline platinum: Application of convolution potential sweep voltammetry in the estimation of kinetic parameters

Irreversible oxidation of methanol on polycrystalline platinum leading to soluble products has been carried out by fast scan voltammetry, and the reaction has been studied under diffusion controlled process. The conventional analysis of current–potential data, viz. dependence of peak potential on scan rate and peak width measurements, resulted in the estimation of apparent diffusion coefficient of methanol and the anodic transfer coefficient of the electrode reaction. However, from the convolution potential sweep voltammetry, a more accurate and reliable kinetic data were obtained. Under the above conditions, methanol oxidation follows Butler–Volmer rate law with a linear variation of logarithmic heterogeneous rate constant with electrode potential. A constant apparent anodic transfer coefficient independent of electrode potential was observed pointing to the fact that the standard potential of the reaction cannot be determined from the voltammetric experiments. The experimental current–potential curve was compared with a theoretical voltammogram and further oxidation of products at the electrode surface has also been analyzed using limiting convolution current.     

High efficient electrocatalytic oxidation of formic acid on Pt/polyindoles composite catalysts

Four novel composite catalysts have been developed by the electrodeposition of Pt onto glassy carbon electrode (GCE) modified with polyindoles: polyindole, poly(5-methoxyindole), poly(5-nitroindole) and poly(5-cyanoindole). As-formed composite catalysts are characterized by SEM, XRD and electrochemical analysis. Compared with Pt nanoparticles, respectively, deposited on the bare GCE and on the GCE modified with polypyrrole, the four newly developed composite catalysts exhibit higher catalytic activity towards formic acid electrooxidation by improving selectivity of the reaction via dehydrogenation pathway and thus mostly suppressing the generation of poisonous CO_ads species. The enhanced performance is proposed to come from the synergetic effect between Pt and polyindoles and the increase of electrochemical active surface area (EASA) of Pt on polyindoles.     

Copper chloride modified copper electrode: Application to electrocatalytic oxidation of methanol

Copper chloride modified copper (CCMC) electrode was prepared as a new electrode. For the preparation of the modified electrode, the polished copper electrode was placed in 0.1 M CuCl₂ solution for 20 s. In this step, a layer of copper (I) chloride was formed at the surface of copper electrode. Then, the electrode was placed in 0.1 M NaOH and the electrode potential was cycled between −250 and 1000 mV (vs. SCE) at a scan rate of 50 mV s⁻¹ for 5 cycles in a cyclic voltammetry regime until a featureless voltammogram was obtained. Surface physical characteristics of the modified electrode were studied by scanning electron micrographs (SEM). Results showed that considerable amounts of microcrystals have been formed on the copper surface during the modification. Surface elemental analysis of electrode were performed by energy dispersive X-ray (EDX) technique. The results showed that in addition to copper and chloride elements, there is also oxygen at the surface of CCMC electrode. This indicates that a layer of (ClCu)₂O was formed at the surface of the modified electrode. The electrocatalytic activity of the modified electrode for the oxidation of methanol, in aqueous basic solution was studied by using cyclic voltammetry. Results showed that, copper chloride modified electrode can improve the activity of Cu towards the oxidation of this small organic molecule, showing the possibility of attaining good electrocatalytic anodes for fuel cells. The modified electrode shows a stable and linear response in the concentration range of 5 × 10⁻³ to 8 × 10⁻² M with a correlation coefficient of 0.9958.     

Electrooxidation of methanol on Pt microparticles dispersed on SnO2 thin films

In this work, we have prepared electrodes through the electrodeposition of platinum micro particles on SnO₂ thin films in order to verify the application of this system as a catalyst for the electrooxidation of methanol. The oxide films were prepared through the method of polymeric precursor decomposition and calcinated at 550 °C. The employed oxides have proved to be a good matrix for the dispersion of platinum particles since they present high roughness. The maximum electrooxidation current was attained for a platinum content of approximately 600 μg cm⁻². The chronoamperometric results showed that the current values obtained for the electrooxidation of methanol were up to 10 times higher than the current values obtained with platinized platinum under the same conditions. The possible mechanisms that lead to this enhancement were discussed.     

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.     

Activity of a novel titanium-supported bimetallic PtSn/Ti electrode for electrocatalytic oxidation of formic acid and methanol

Bimetallic platinum–tin nanoparticles were co-deposited on a titanium surface using a simple one step hydrothermal method process. The electrochemical catalytic activity of this titanium-supported nanoPtSn/Ti electrode towards the oxidation of formic acid and methanol in 0.5 M H₂SO₄ was evaluated by voltammetric techniques, chronoamperometric responses and electrochemical impedance spectra (EIS). According to the cyclic voltammograms of the oxidation of both formic acid and methanol, the nanoPtSn/Ti presents high anodic current densities and low onset potentials. Potential-time transient measurements show that the nanoPtSn/Ti exhibits high steady-state current densities for the oxidation of both formic acid and methanol. The EIS data indicate that the nanoPtSn/Ti presents very low electrochemical impedance values, showing that for the oxidation of both formic acid and methanol, low charge transfer resistances are present on the nanoPtSn/Ti catalyst. This confirms the high electrocatalytic activity of the nanoPtSn/Ti for the formic acid and methanol oxidation.     

Electrochemical quartz crystal nanobalance (EQCN) studies of the adsorption behaviour of an enzyme, mandelate racemase, and its substrate, mandelic acid, on Pt

The adsorption behaviour of the enzyme, mandelate racemase (MR), and its substrate, (S)-mandelic acid (MA), was studied at a polycrystalline Pt surface in pH 7.4 phosphate buffer at 294 K using the electrochemical quartz crystal nanobalance (EQCN) technique of simultaneous cyclic voltammetry (CV) and frequency measurements. It was shown that the EQCN frequency measurements did not directly monitor the molar mass of the adsorbed protein at anodic potentials, but instead measured changes in the surface oxide in the absence and presence of adsorbed protein. However, at a potential characteristic of the double layer for platinum, EQCN frequency measurements gave a measure of the extent of solvent displacement by the adsorbed protein. The adsorption process was modelled using the Langmuir adsorption isotherm. The values for the Gibbs energy of adsorption, ΔG_ADS, obtained with these EQCN frequency measurements gave excellent agreement within experimental uncertainty with those obtained from the simultaneous CV measurements for both the enzyme and substrate, and showed the enzyme to have a greater affinity for the surface than the substrate molecule. The maximum surface concentrations from CV and EQCN frequency measurements gave excellent agreement for the two techniques and showed MA to be adsorbed in a tilted orientation with monolayer coverage. On the other hand, the surface concentrations showed the presence of ∼2.5 monolayers of enzyme from charge transfer CV measurements, while the frequency results showed the presence of a densely packed monolayer from the “footprint” of solvent displacement by the adsorbed enzyme molecules on the electrode surface. The two techniques provide valuable complementary information on the interfacial behaviour of molecules.     

The catalysts supported on metallized electrospun polyacrylonitrile fibrous mats for methanol oxidation

Polyacrylonitrile nanofibrous mats coated with continuous thin gold films (Au-PAN) have been fabricated by combining the electrospinning and electroless plating techniques. The Pt particles are electrodeposited on the Au-PAN fibers surface by multi-cycle CV method, and the Au-PAN decorated with Pt (Pt/Au-PAN) shows higher activity toward methanol electro-oxidation. The catalytic peak current for methanol oxidation on the optimum Pt/Au-PAN electrode can reach about 450 mA mg⁻¹ Pt which is much larger than the catalytic peak current for methanol oxidation (118.4 mA mg⁻¹ Pt) on the electrode prepared by loading commercial Pt/C on Au-PAN (Pt/C/Au-PAN). Further experiments reveal that the Pt/Au-PAN electrodes exhibit better stability and smaller charge transfer resistance than Pt/C/Au-PAN electrodes, which indicates that the Au-PAN may be developed as supporting material for catalyst. The microscopy images of the electrodes show that the Pt particles deposited on Au-PAN conglomerate into larger particles, and that the Pt/C catalyst loaded on the Au-PAN also exhibits conglomeration after stability test. The hydrogen adsorption-desorption experiments indicate that the electrochemical surface area of the Pt particles for the both kinds of electrodes has decreased after stability test.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Electrooxidation of methanol and formic acid on PtCu nanoparticles

Improving the catalytic activity of the anode catalyst is an important task in direct methanol and formic acid fuel cell development. In the present work, catalytic activity of shape-controlled PtCu nanoparticles toward methanol and formic acid oxidation was investigated. The results show that the addition of Cu to Pt increases the catalytic activity of both reactions. In addition, the shape of PtCu nanoparticles plays an important role on improving the reactivity of both reactions. Cubic PtCu nanoparticles are more active for methanol oxidation while spheres are better for formic acid oxidation. The present study demonstrates controlling shape of Pt alloy catalysts is an effective way of improving catalytic activity. Likely mechanisms of the activity enhancement are briefly discussed.     

Electrochemical characterization of the electrooxidation of methanol,ethanol and formic acid on Pt/C and PtRu/C electrodes

Methanol, ethanol and formic acid electrooxidations in acid medium on Pt/C and PtRu/C catalysts were investigated. The catalysts were prepared by a microwave-assisted polyol process. Cyclic voltammetry and chronoamperometry were employed to provide quantitative and qualitative information on the kinetics of methanol, ethanol and formic acid oxidations. The PtRu/C catalyst showed higher anodic current densities than the Pt/C catalyst and the addition of Ru reduced the poisoning effect.     

The role of water in the initial steps of methanol oxidation on Pt(2 1 1)

We report results of quantum-chemical calculations within the framework of density functional theory for the oxidation of methanol on the (2 1 1) face of a platinum single crystal. Similar to the reaction pathway on the low indexed (1 1 1) crystal face water plays an important role as found from energy minimization calculations: the adsorption of methanol on charged and uncharged surfaces is strongly enhanced by the formation of a hydrogen bond to a coadsorbed water molecule. The methanol is adsorbed via a methyl hydrogen atom preceding scission of one of the CH-bonds as the first reaction step. In the presence of additional water, e.g., from a liquid phase, the onset of the oxidation reaction is favored by a coadsorbed neighboring water molecule which forms a hydrogen bond with the methanol OH-group. At a minimum number of adjacent water molecules (n≥2) the CH-bond as well as the OH-bond are cleaved on charged surfaces. The protonic charge stemming from the dissociation of the methanol hydroxyl group is delocalized inside the aqueous cluster and formaldehyde is formed as an intermediate product.     

Effect of cations in solution on the oxidation of methanol on the surface of platinum electrode

The effect of metal cations in solution on the oxidation of methanol on the electrode surface of platinum is a neglected aspect to direct methanol fuel cell (DMFC). In this paper, a smooth platinum electrode absorbing metal cations as the working electrode was applied to investigate the methanol oxidation with the cyclic voltammetry (CV) in 1.0 mol L⁻¹ H₂SO₄. From the analysis of experiment, it is found that the cations, Li⁺, Ce⁴⁺, Mn²⁺, Ni²⁺, Cu²⁺, have some negative effect on the catalytic oxidation of methanol on the surface of platinum. The degree of the effect from different cations was analyzed.     

The role of platinum oxide in the electrode system Pt(O2)/yttria-stabilized zirconia

The existence and role of platinum oxide in the solid state electrode system Pt(O₂)/yttria-stabilized zirconia is discussed. Covering and porous model-type Pt film electrodes on YSZ single crystals are investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and in situ scanning photoelectron microscopy. The formation of Pt oxide and its amount strongly depend on the experimental conditions, such as temperature, oxygen partial pressure, and oxygen flux towards the electrode during anodic polarization. Electrode activation and deactivation processes can be explained by formation and decomposition of Pt oxide, which is reducing or inhibiting the oxygen exchange rate.     

Electrochemical study of benzene on Pt of various surface structures in alkaline and acidic solutions

The electrochemical behaviour of benzene on platinum electrodes (polycrystalline and single-crystal electrodes) has been studied in acidic and alkaline solutions. In acid solutions the reduction of benzene to cyclohexane takes place in all the platinum surface structure employed, however it does not occur in alkaline media (0.1 M NaOH). In this case, the hydrogen adsorption-desorption processes displace the adsorbed benzene from the electrode surface. The oxidation of benzene is also affected by the pH of the electrolyte and also by the surface structure of the platinum electrode used. In acid solutions, this oxidation at higher potentials (1.4 V vs. RHE) yields CO₂, benzoquinone and α,β-unsaturated esters or lactones, however in alkaline media carbonate anions coming from CO₂ and salts of carboxylic acids have been detected by in situ FTIR spectroscopy using a platinum polycrystalline electrode. Bulk electrolysis of benzene solutions using a platinum electrode in acid and alkaline media was performed in order to confirm the results obtained by spectroscopic measurements.     

Au/Pt nanoparticle systems in methanol and carbon monoxide electroxidation

Different Au/Pt bimetallic systems have been synthesised by following the approach suggested by Brust. The nanoparticles have been anchored to glassy carbon surface through a place-exchange reaction involving dithiol molecules. The resulting modified electrode consists of heterogeneous nanostructured Au and Pt patchwork. The different nanoparticles systems developed have been employed for the electroxidation of methanol and carbon monoxide in alkaline aqueous media. The results show that the electrocatalytic activity of the bimetallic systems is enhanced with respect to the single monometallic NP systems.     

Electrocatalytic oxidation of acetaldehyde on Pt alloy electrodes

Reaction intermediates occurring during the oxidation of acetaldehyde were investigated by in situ infrared reflectance spectroscopy (SPAIRS and SNIFTIRS techniques). These measurements, showing that acetaldehyde was transformed into CO and CO₂ successively, allowed us to choose a suitable potential program to oxidize electrocatalytically the reactant on binary and ternary platinum alloy electrodes with two compositions (Pt/Os and Pt/Ru/Os). IR results were useful to set a potential pulse program which allowed to adsorb dissociatively the organic compound. The analysis of the electrolysis products was performed by high performance liquid chromatography (HPLC). Acetic acid, formic acid and carbon dioxide were determined as the main oxidation products of acetaldehyde.     

In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were conducted to analyze the electrochemical oxidation of CO dissolved in bulk electrolyte solution on a polycrystalline Pt electrode during linear sweep (cyclic) voltammetry (CV) in 1% and 100% CO-saturated 0.1 M HClO₄. A CO adlayer was first formed on Pt at 0.05 V vs. the reversible hydrogen electrode (RHE) during CO bubbling in each electrolyte, followed by the CV measurement. In 1% CO-saturated 0.1 M HClO₄, a well-developed prepeak was found, showing an onset and a peak at around 0.25–0.3 V and 0.42 V, respectively. A major contribution of bridge-bonded CO, CO(B), to the prepeak is concluded based on a decrease in the corresponding band intensity. In 100% CO-saturated 0.1 M HClO₄, the onset of bulk CO oxidation takes place around 0.25–0.3 V, which is closely associated with a band intensity decrease of CO(B), whereas atop (linear) CO, CO(L), did not exhibit intensity change in this potential region. This suggests that vacant sites made available upon oxidation of CO(B) serve as active sites for bulk CO oxidation. The oxidation of CO(B) at such low potentials is interpreted in terms of an adsorption energy on Pt that is lower than that for CO(L) and also of the specific structure of an adlayer that consists of intermixed CO(L) and CO(B). The bulk CO oxidation becomes diffusion-limited by dissolved CO above ca. 0.72 V, where we observed hardly any infrared spectral features ascribed to reaction intermediates. During a negative-going scan back to 0.05 V from 1.2 V, a steep decrease of the bulk CO oxidation current takes place around 0.66 V, at which preferential adsorption of CO(L) is observed. A rigid CO(L) island formation is strongly suggested from its high CO stretching frequency vs. its very small initial coverage and from its subsequent dependence on potential, with a linear Stark shift characterized by a slope of −29 cm⁻¹/V. Such an island formation is in marked contrast to the adlayer structure with intermixed CO(L) and CO(B) initially formed at 0.05 V during CO bubbling. It is concluded that the Pt surface saturated with the CO adlayer formed initially at 0.05 V exhibits a low overpotential for bulk CO oxidation owing to its adlayer structure with intermixed atop and bridge-bonded CO.