Electrooxidation of methanol and 2-propanol mixtures at platinum single crystal electrodes

In the present work the electrooxidation of methanol, 2-propanol and different mixtures of both alcohols has been studied on the three platinum basal planes in three different electrolytes (H₂SO₄, HClO₄ and NaOH). The results indicate that, like in the case of both individual alcohols, the electrooxidation of the mixture is a structure sensitive reaction and that Pt(1 1 1) leads to higher current densities for some mixture compositions as compared to what could be expected from the contribution of the individual compounds. The effect of the methanol concentration in the mixture points out that 2-propanol is the main reacting fuel at the Pt(1 1 1) surface. Interestingly, the addition of methanol clearly has a positive effect on 2-propanol oxidation. For a specific mixture composition, while results from cyclic voltammetry indicate a modest twofold increase in current density, chronoamperometric results after 10 min experiment show a nearly 200 times higher current density. IR experiments have been performed to gain information about the enhancement mechanism. Nevertheless, we have found that both methanol and 2-propanol seem to follow the same mechanism as they follow in the absence of the other alcohol, and therefore the enhancement could be probably related to a competitive adsorption for the active surface sites.     

High catalytic activity of chemically activated gold electrodes towards electro-oxidation of methanol

Electro-oxidation of methanol has been investigated on activated, rough gold electrodes in alkaline solutions. The electrodes were activated by formation and decomposition of gold amalgam. The oxidation of methanol starts at potentials about 400 mV less positive as compared with smooth poly and single crystal gold electrodes and the oxidation current is much higher. For freshly prepared, activated gold electrodes the oxidation current is similar to that obtained on smooth platinum, however it diminishes with time. The formation of small crystallites, which could trap OH⁻ anions, seems to be the most important factor for this unusual catalytic activity. The dependence of the oxidation process on electrode topography is discussed.     

Study of thiourea adsorption onto polycrystalline gold electrodes

Cyclic voltammetry, differential capacity determinations and chronocoulometry have been employed for quantitative study of thiourea (TU) adsorption at polycrystalline gold/aqueous KClO₄ solution interface. The film pressure and the Gibbs surface excess are determined as functions of potential. The thermodynamic data suggest that the adsorbed TU exhibits distinct surface states which appear when the TU concentration is increased and the potential is more positive.     

Continuous thin gold films electroless deposited on fibrous mats of polyacrylonitrile and their electrocatalytic activity towards the oxidation of methanol

Polyacrylonitrile nanofibers containing different amounts of gold nanoparticles have been prepared by electrospinning technique. By using the gold nanoparticles as seeds, thin continuous gold films have been deposited on the surface of polyacrylonitrile fibers through self-catalyzed reduction of chloroauric acid in solution. The conductivities of the fibrous mats increase with the amount of gold deposited on the fibers increase. The smooth continuous thin gold films tend to form on the fibers surface when the organic fibers contain more gold seeds, while coarse films tended to form on the fibers containing less gold seeds. The electrocatalytic activity of the fibrous mats electrodes towards the methanol oxidation in alkaline medium has been investigated, indicating that these electrodes exhibit higher electrocatalytic activity than pure gold electrode because of their three-dimensional structures. The results also indicate that the mats with smooth gold coating exhibit higher electrocatalytic activity than that with coarse gold coating.     

Electrooxidation of methanol and ethylene glycol on gold and on gold modified with an electrodeposited polyNiTSPc film

The electrooxidation of methanol and ethylene glycol on Au and on polynickeltetrasulphophthalocyanine-modified gold (polyNiTSPc/Au/Q) electrodes in a pH 11 carbonate/hydrogen carbonate buffer electrolyte was studied by cyclic voltammetry (CV) and with an electrochemical quartz crystal microbalance (EQCM). Au shows negligible activity for methanol oxidation, in agreement with the fact that methanol does not adsorb on Au, since it does not affect the potential at which the mass increase due to Au oxidation starts. On the contrary, ethylene glycol (EG) is electrooxidized on Au at a significant rate, probably because it adsorbs rather strongly on Au, as evidenced by the positive shift by 0.35 V of the mass increase attending Au monolayer oxidation. The polyNiTSPc-modified Au electrodes are hydrophobic, as inferred from the disappearance of the large mass decrease in the double-layer region typical of the bare Au electrode, a decrease which is due to the desorption of the water molecules adsorbed at the negative potential limit. On the polyNiTSPc/Au/Q electrodes the current at the positive potential limit (at which only Ni(III) exists) of methanol oxidation, and the peak current of EG oxidation (at a potential at which only Ni(II) exists), show a Langmuir dependence on the concentration, which indicates that in both cases the reactive species are adsorbed, and that their oxidation rate is much lower than the adsorption rate. As is well known, the oxidation of Ni(II) to Ni(III) in the polyNiTSPc film is accompanied by a large mass decrease, due mostly to the expulsion of water from the film by the hydrophobic Ni(III). This mass decrease is independent of the scan rate, but in the presence of methanol or ethylene glycol it increases with increasing scan rate, which indicates that the oxidation of the alcohols involves a chemical reaction of the alcohols with Ni(III) ions, the extent of which decreases with increasing rate. Consequently, the amount of the hydrophobic Ni(III) will increase, and so will the mass loss. The mass decrease of Ni(II) oxidation decreases with increasing concentration of methanol or ethylene glycol, which again shows that there is a chemical reaction between the alcohols and Ni(III) ions, since the concentration of the latter would decrease with increasing alcohol concentration.     

Mechanism and electrocatalysis in the direct methanol fuel cell

A unified treatment of the mechanism of electrocatalysis of methanol oxidation on platinum and platinum-containing alloys is put forward. The effect of various alloys is shown to be interpretable within this overall model, and methods for the systematic improvement of the rate of electro-oxidation of methanol are described. Finally, some recent results for model direct methanol fuel cells are given, showing that critical performance parameters for commercial exploitation are now achievable with modern catalytic formulations and cell designs.     

Spectroelectrochemical study of the adsorption of acetate anions at gold single crystal and thin-film electrodes

Acetate adsorption at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in-situ infrared spectroscopy. External reflection measurements, performed with gold single crystal electrodes, are combined with Surface Enhanced Infrared Reflection Absorption Spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) carried out with sputtered gold thin-film electrodes. Theoretical harmonic IR frequencies of acetate species adsorbed with different geometries on Au clusters with (1 1 1), (1 0 0) and (1 1 0) orientations have been obtained from B3LYP/LANL2DZ, 6-31 + G* calculations. The theoretical and experimental results confirm that, irrespective of the surface crystallographic orientation, bonding of acetate to the surface involves the two oxygen atoms of the carboxylate group, with the OCO plane perpendicular to the metal surface. DFT calculations reveal also that the total charge of the metal cluster-acetate supermolecule has small effect on the vibrational frequencies of adsorbed acetate species. Both the external and the internal reflection measurements show the co-adsorption of acetate and perchlorate anions. Step-scan measurements carried out with the gold thin-film electrodes have allowed the monitoring of the time-dependent behaviour of perchlorate, acetate and water bands in potential step experiments. Acetate adsorption under those conditions is shown to involve perchlorate desorption and to follow a Langmuir-type kinetics. The step-scan spectra also show the rise and decay of transient water structures with parallel time-dependent shifts of the background intensity in the infrared spectra.     

Self-assembled gold nanoparticles modified ITO electrodes: The monolayer binder molecule effect

The fabrication of gold attached organosilane-coated indium tin oxide Au_NPs-MPTMS/ITO and Au_NPs-APTES/ITO electrodes [MPTMS = 3-(mercaptopropyl)-trimethoxysilane, APTES = 3-(aminopropyl)-triethoxysilane, ITO = indium tin oxide] was carried out making use of a well-known two-step procedure and the role played by the -SH and -NH₂ functional groups in the two electrodes has been examined and compared using different techniques. Information about particle coverage and inter-particle spacing has been obtained using trasmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) whereas, bulk surface properties have been probed with UV-vis spectroscopy, CV and electrochemical impedance spectroscopy (EIS). The catalytic activity of the two electrodes has been evaluated studying the electrooxidation of methanol in alkaline conditions. The results obtained show that the NH₂ functionality in the APTES binder molecule favours the formation of isle-like Au nanoparticle aggregates that lead to both a higher electron transfer and electrocatalytic activity.     

Nickel-dimethylglyoxime complex modified graphite and carbon paste electrodes: preparation and catalytic activity towards methanol/ethanol oxidation

Nickel-dimethylglyoxime complex (abbreviated as Ni(II)(DMG)₂) modified carbon paste and graphite electrodes were prepared by mixing Ni(II)(DMG)₂ with graphite paste, and coating Ni(II)(DMG)₂ to the graphite surface. It is necessary to cycle the electrode potential to a high value (e.g. 0.8 V versus SCE) for the preparation of the modified electrodes. The electrochemical reaction was originally assumed to be a one-electron process converting Ni(II)(DMG)₂ to [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻. [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻ showed a strong catalytic activity toward electro-oxidation of methanol and ethanol. The electrocatalytic oxidation currents consistently increase with the increase in Ni(II)(DMG)₂ loading, OH⁻, and alcohol concentrations. Rotating disk electrode results obtained with a Ni(II)(DMG)₂ coated graphite disk electrode showed that the electrocatalytic oxidation of alcohol is a 4-electron process producing formate anion (methanol oxidation) or acetate anion (ethanol oxidation). A mechanism for the electrocatalytic oxidation of methanol/ethanol was proposed, and a rate-determining step was also discussed.     

CO and methanol electrooxidation on Pt/Ir/Pt multilayers electrodes

This work describes the CO and methanol electrooxidation over an Ir/Pt bilayer electrodeposited on a platinum polycrystalline substrate. In the blank acidic solution it was observed that the electrochemical behavior of both the polycrystalline Pt and Pt/Ir/Pt nanostructured electrodes were very similar. The electroactive area, calculated using the hydrogen desorption method, are the same for both materials. In order to investigate the effect of the thickness change of Ir interlayer, two different samples were prepared. One with 1 Ir monolayer and the second with 3 monolayers thick. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 160 and 180 mV for Pt/Ir/Pt 3:1 and 1:1 ML, respectively, compared to polycrystalline Pt. Besides, for methanol electrooxidation, the Pt/Ir/Pt electrodes presented an increase of 170% in the peak current density compared to polycrystalline Pt. These results are in agreement with the calculated activation energies which were 31.5, 39.0 and 43.5 kJ mol⁻¹ for Pt/Ir/Pt 1:1, 3:1 ML and polycrystalline Pt electrodes, respectively. Using the electrochemical impedance spectroscopy, surprisingly, the Pt/Ir/Pt electrodes, did not exhibit the inductive arc which means that the poisoning of the electrode surface is not important in this case.     

Study on electroactive and electrocatalytic surfaces of single walled carbon nanotube-modified electrodes

An investigation of the electrocatalysis of single-walled carbon nanotubes modified electrodes has been performed in this work. Nanotube-modified electrodes present a surface area much higher than the bare glassy carbon surfaces as determined by capacitance measurements. Several redox probes were selected for checking the reactivity of specific sites at the carbon nanotube surface. The presence of carbon nanotubes on the electrode improves the kinetics for all the reactions studied compared with the bare glassy carbon electrode with variations of the heterogeneous electron transfer rate constant up to 5 orders of magnitude. The most important effects are observed for the benzoquinone/hydroquinone and ferrocene/ferricinium redox couples, which show a remarkable improvement of their electron transfer kinetics on SWCNT-modified electrodes, probably due to strong π–π interaction between the organic molecules and the walls of the carbon nanotubes. For many of the reactions studied, less than 1% of the nanotube-modified electrode surface is transferring charge to species in solution. This result suggests that only nanotube tips are active sites for the electron transfer in such cases. On the contrary, the electroactive surface for the reactions of ferrocene and quinone is higher indicating that the electron transfer is produced also from the nanotube walls.     

Electrocatalysis of gold nanoparticles/layered double hydroxides nanocomposites toward methanol electro-oxidation in alkaline medium

A nanocomposite based on layered double hydroxides (LDHs) and gold nanoparticles (AuNPs) was prepared via hydrothermal treatment followed by a reduction procedure. The AuNPs were obtained in Mg-Al LDHs, and they maintained good stability. The electrocatalytic activities of AuNPs/LDH-modified glassy carbon electrodes for methanol oxidation in alkaline medium were investigated in detail. Under the same conditions, the modified electrode exhibited higher electrocatalytic activity than both the pure AuNPs-modified electrode and LDH-modified electrode. The role of the AuNPs and LDHs in this composite system was explored by cyclic voltammetry and chronoamperometry, respectively. Further studies demonstrated that the promoting effect of LDHs could be due to its strong adsorption and partly to the discharge of OH⁻ during methanol oxidation. This work indicates that LDHs is expected to be a good supporting material in the development of methanol anode catalysts.     

Kinetic and mechanistic evaluation of tetrahydroborate ion electro-oxidation at polycrystalline gold

The anodic oxidation of tetrahydroborate ion is studied in NaOH at stationary and rotating polycrystalline Au disk electrodes. Linear sweep and cyclic voltammetry are applied varying the scan and rotation rate from 0.005 to 51.200 V s⁻¹ and from 52.3 to 314.1 rad s⁻¹, correspondingly. The effects of variation of BH₄⁻ and NaOH concentrations as well as of the potential limits of the ranges studied have been initially followed. Most of the experiments have been carried out with 10.9 mM NaBH₄ in 1.04 M NaOH at 293 K in the potential range from −1.300 to 0.900 V (vs. Ag/AgCl). It is found that 6 electrons are exchanged in the overall oxidation transformation. The kinetic analysis of the processes determining the two anodic peaks recorded under static conditions at scan rates lower than 0.500 V s⁻¹ shows that 1.4 electrons are exchanged in the potential range of the first one (at ca −0.5 V), while the rate of the second one (at ca +0.3 V) is determined by a quasi-reversible 1-electron transfer reaction. A kinetic evidence for the participation of surface bound intermediates in the electro-oxidation process is provided. Two additional well outlined anodic peaks are recorded in the aforementioned potential range under specific experimental conditions. A quasi-8 electron mechanism involving four oxidation and hydrolysis steps is advanced to explain the experimental results. It accounts for the involvement of borohydride oxidation species and the Au⁺/Au³⁺ mediator couple.     

Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells

The electrochemical oxidation of BH₄⁻ in 2 M NaOH on Pt and Au (i.e. catalytic and non-catalytic electrodes, respectively, for BH₄⁻ hydrolysis accompanied by H₂ evolution) has been studied by cyclic voltammetry, chrono-techniques (i.e., potentiometry, amperometry, coulometry) and electrochemical impedance spectroscopy. In the case of Pt the cyclic voltammetry behaviour of BH₄⁻ is influenced by both, the catalytic hydrolysis of BH₄⁻ yielding H₂ (followed by electrooxidation of the latter at peak potentials between −0.7 and −0.9 V versus Ag/AgCl, KCl_std) and direct oxidation of BH₄⁻ at more positive potentials, i.e., between −0.15 and −0.05 V. Thiourea (TU, 1.5×10⁻³ M) was an effective inhibitor of the catalytic hydrolysis associated with BH₄⁻ electrooxidation on Pt. Therefore, in the presence of TU, only the direct oxidation of BH₄⁻ has been detected, with peak potentials between −0.2 and 0 V. It is proposed that TU could improve the BH₄⁻ utilization efficiency and the coulombic efficiency of direct borohydride fuel cells using catalytic anodes. The electrooxidation of BH₄⁻ on Pt/TU is an overall four-electron process, instead of the maximum eight electrons reported for Au, and it is affected by adsorbed species such as BH₄⁻ (fractional surface coverage ∼0.3), TU and possibly reaction intermediates.     

Giant multilayer electrocatalytic effect investigation on Pt/Bi/Pt nanostructured electrodes towards CO and methanol electrooxidation

This work describes CO and methanol electrooxidation over Bi/Pt 1.2:1.2 or 10:10 ML electrodeposited on a bulk platinum substrate. It could be observed in a blank solution that the same features for bulk Pt and Pt/Bi/Pt MM and the hydrogen region were overlaid. The electroactive areas, calculated using the hydrogen desorption method, are the same for both bulk Pt and Pt/Bi/Pt MM electrodes. This is in agreement with AFM images and RMS values, which were the same for bulk Pt and Pt/Bi/Pt MM electrodes. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 138 and 197 mV for Pt/Bi/Pt 10:10 and 1.2:1.2 ML, respectively, in comparison to bulk Pt. Moreover, for methanol electrooxidation the Pt/Bi/Pt 1.2:1.2 ML electrodes presented an enhancement of 315% and 22 times in the peak current density compared to bulk Pt using cyclic voltammetry and chronoamperometry techniques respectively. Using electrochemical impedance spectroscopy, it was possible to observe the lowest resistance charge transfer for Pt/Bi/Pt 1.2:1.2 ML compared to Pt/Bi/Pt 10:10 ML and bulk Pt respectively. We designate the Pt/Bi/Pt MM electrodes as a giant multilayer electrocatalytic (GME) system, due to their enhanced electrocatalytical properties.     

Electrochemical and spontaneous deposition of ruthenium at platinum electrodes for methanol oxidation: an electrochemical quartz crystal microbalance study

PtRu electrodes with Ru surface concentration ranging from 20 to 50% were prepared by electrolysis of Ru(NO)(NO₃)₃ at a constant potential and/or by spontaneous Ru deposition performed at open circuit potential from a RuCl₃ solution. The amount of either spontaneously or electrochemically deposited ruthenium on the platinum electrode was determined by means of an electrochemical quartz crystal microbalance (EQCM). The effect of the Ru surface concentration on the rate of methanol electrooxidation was also investigated and correlated to the EQCM measurements.     

Electrochemical characterization of different screen-printed gold electrodes

Voltammetric characterization of electrochemical systems is very important with the aim of optimising a required application. In this case, different gold screen-printed electrodes have been evaluated. Although the use of carbon has widespread, gold is still in the beginning. Two different substrates: polycarbonate and alumina have been evaluated. Differences in morphology are clear when observed by SEM. Potassium ferricyanide, p-aminophenol, indigo carmine, silver nitrate and ferrocene have been chosen as model analytes. Potential scans can be made at scan rates as high as 2000 mV s⁻¹ which could result very advantageous. Proportionality between peak current and concentration maintains at this scan rate as demonstrated with ferricyanide. Sensitivity is increased and linear dynamic ranges are widened when alternative electrochemical techniques (SWV and ACV) are employed. In the silver system in basic media, catalysis-mediated reduction is only possible on alumina electrodes since the potential window is very narrow in polymeric electrodes. The precision (intra and interelectrode) has been thoroughly evaluated. Reusability is possible since adequate RSDs are obtained when measurements are performed in different drops without cleaning in-between. On the other hand, low cost of fabrication makes disposability of electrodes an advantageous characteristic that avoids tedious cleaning treatments.     

Electro-oxidation of methanol on copper in alkaline solution

The electro-oxidation of methanol on copper in alkaline solutions has been studied by the methods of cyclic voltammetry, quasi-steady state polarization and chronoamperometry. It has been found that in the course of an anodic potential sweep the electro-oxidation of methanol follows the formation of Cu^III and is catalysed by this species through a mediated electron transfer mechanism. The reaction also continues in the early stages of the reversed cycle until it is stopped by the prohibitively negative potentials. The process is diffusion controlled and the current-time responses follow Cottrellian behavior. The rate constants, turnover frequency, anodic transfer coefficient and the apparent activation energy of the electro-oxidation reaction are reported.     

Molecular-level design of binary self-assembled monolayers on polycrystalline gold electrodes

In this study, we followed up, by measuring the reductive desorption patterns, the time-dependent growth of the self-assembled monolayer (SAM) of a thiol compound (typically cysteine) formed on polycrystalline Au (poly-Au) electrode. Cysteine molecules appeared to adsorb preferentially and consecutively at the Au(1 1 0), Au(1 0 0) and then at the Au(1 1 1) surface domains of the poly-Au. A 95% surface coverage of cysteine (Γ_cysteine) was attained after 5 s, 120 s and more than 300 s for the Au(1 1 0), Au(1 0 0) and Au(1 1 1) domains, respectively, of the poly-Au electrode. The electrochemical reduction of molecular oxygen (O₂) in O₂-saturated 0.5 M KOH was utilized as a probing reaction for the extent of the compactness of the SAM. A binary SAM of two thiols (cysteine and cystamine) has been successfully designed over the poly-Au electrode at which a precise positioning of cysteine was achieved, i.e., cysteine was attached to the Au(1 1 1) domains, while cystamine to the Au(1 0 0) and Au(1 1 0) domains. SEM images for the electrodeposited Ag over the cysteine sub-SAM/Au electrode gave a possible mapping of the Au(1 1 1) domains of the poly-Au electrode.     

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.