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.     

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I₃⁻)/iodide (I⁻) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I₃⁻ on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.     

Investigations under real operating conditions of the electrochemical promotion by O<Subscript>2</Subscript> temperature programmed desorption measurements

The origin of the electrochemical promotion of catalysis (EPOC) was investigated via oxygen temperature-programmed desorption (O₂-TPD) from a polycrystalline Pt film interfaced with YSZ. TPD experiments were carried out under operating conditions similar to those used for catalytic activity measurements. This study has clearly shown that an anodic current generates the migration of “backspillover” ionic oxygen species from YSZ toward the Pt surface. These ionic species act as promoters and enable the formation of weakly adsorbed oxygen species coming from the gas phase which are more reactive and thus responsible for the activity enhancement. The effect of polarization is to carry or to remove the promoting ionic species on the Pt surface. Therefore, electrochemical promotion of catalysis can be considered as an electrically controlled metal support interaction, where the support is an O²⁻ conducting solid electrolyte.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Investigation of the Pt/YSZ interface at low oxygen partial pressure by solid electrochemical mass spectroscopy under high vacuum conditions

The Pt/YSZ interface was investigated at low oxygen partial pressure under high vacuum (HV) conditions at 400 °C. Two different electrochemical techniques were coupled to mass spectrometric gas analysis using a new solid electrochemical mass spectrometric monitoring device. Under cathodic polarization, the lack of oxygen in the gas phase induces the reduction of the YSZ solid electrolyte which acts as oxygen source for the formation of O²⁻ ions migrating to the anode. Under anodic polarization, both platinum oxidation and oxygen evolution reaction are identified. PtO _x is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms. At the Pt/YSZ interface, PtO _x formation is an electrochemical process following a parabolic growth law, while the presence of PtO _x at the Pt/gas interface is related to the diffusion of PtO _x formed at the triple phase boundary towards the Pt/gas interface. It is proposed that the side oxygen evolution reaction stabilizes thermodynamically the PtO _x diffusion toward the gas exposed interface during the anodic polarization.     

The role of TiO<Subscript>2</Subscript> layers deposited on YSZ on the electrochemical promotion of C<Subscript>2</Subscript>H<Subscript>4</Subscript> oxidation on Pt

The electrochemical promotion of Pt/YSZ and Pt/TiO₂/YSZ catalyst-electrodes has been investigated for the model reaction of C₂H₄ oxidation in an atmospheric pressure single chamber reactor, under oxygen excess between 280 and 375 °C. It has been found that the presence of a dispersed TiO₂ thin layer between the catalyst electrode and the solid electrolyte (YSZ), results in a significant increase of the magnitude of the electrochemical promotion of catalysis (EPOC) effect. The rate enhancement ratio upon current application and the faradaic efficiency values, were found to be a factor of 2.5 and 4 respectively, higher than those in absence of TiO₂. This significantly enhanced EPOC effect via the addition of TiO₂ suggests that the presence of the porous TiO₂ layer enhances the transport of promoting O²⁻ species onto the Pt catalyst surface. This enhancement may be partly due to morphological factors, such as increased Pt dispersion and three-phase-boundary length in presence of the TiO₂ porous layer, but appears to be mainly caused by the mixed ionic-electronic conductivity of the TiO₂ layer which results to enhanced O²⁻ transport to the Pt surface via a self-driven electrochemical promotion O²⁻ transport mechanism.     

Electrochemical investigation of platinum electrode in solid electrolyte cell

Electro-oxidation of platinum film electrode deposited on yttria-stabilized zirconia (YSZ) is examined in situ using potential programmed voltammetry at 450 °C in oxygen containing atmosphere. Under prolonged anodic oxidation different sorts of oxidized species are formed which are consumed subsequently during a linear cathodic potential scan resulting in three distinct reduction peaks, one of them being fairly reversible while the two others strongly irreversible. Higher oxidation potential and longer time of polarization favor the irreversible processes. The coexistence of three electrochemical processes is explained with the extension of the triple phase boundary. The rapid first process is identified as formation of PtO_x at the electrode/metal interface, the second process - a much slower, parallel one - is related to the phenomenon of oxygen backspillover at the metal/gas interface, and the slowest third process - consecutive to the first one - is attributed to growth of the PtO_x layer at the electrode/metal interface toward the bulk of the metal by analogy to electro-oxidation of platinum in aqueous liquid electrochemistry.     

Electrophoretic Deposition of Zirconia Thin Film on Nonconducting Substrate for Solid Oxide Fuel Cell Application

Electrophoretic deposition (EPD) of 8 mol% yttria‐stabilized zirconia (YSZ) electrolyte thin film has been carried out onto nonconducting porous NiO‐YSZ cermet anode substrate using a fugitive and electrically conducting polymer interlayer for solid oxide fuel cell (SOFC) application. Such polymer interlayer burnt out during the high‐temperature sintering process (1400°C for 6 h) leaving behind a well adhered, dense, and uniform ceramic YSZ electrolyte film on the top of the porous anode substrate. The EPD kinetics have been studied in depth. It is found that homogeneous and uniform film could be obtained onto the polymer‐coated substrate at an applied voltage of 15 V for 1 min. After the half‐cell (anode + electrolyte) is co‐fired at 1400°C, a suitable cathode composition (La_0.65Sr_0.3MnO₃) thick film paste is screen printed on the top of the sintered YSZ electrolyte. A second stage of sintering of such cathode thick film at 1100°C for 2 h finally yield a single cell SOFC. Such single cell produced a power output of 0.91 W/cm² at 0.7 V when measured at 800°C using hydrogen and oxygen as fuel and oxidant, respectively.     

Electrochemical properties of Ti/SnO2-Sb2O5 electrodes prepared by the spray pyrolysis technique

Cyclic voltammetry for the ferri-ferrocyanide redox couple has been used to study the Ti/SnO₂-Sb₂O₅ electrode prepared by spray-pyrolysis under different conditions. The obtained results shows that increasing the preparation temperature and the duration of coating deposition results in a decrease of i _p and an increase of E _p for the ferri-ferrocyanide couple. This deviation from reversibility has been attributed to the formation of a titanium oxide layer at the Ti/coating interface. Concerning oxygen evolution at the Ti/SnO₂-Sb₂O₅ anodes, a mechanism is proposed in which water is discharged at the anode forming hydroxyl radicals which are further oxidized to form dioxygen. Finally, a generalized mechanism for oxygen evolution at oxide electrodes has been proposed.     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

Electrochemical investigation of O2-exposed Pd electrodes supported on YSZ

Cyclic voltammetry and steady state polarization measurements were performed for the electrochemical characterization of the O₂(g), Pd/YSZ system. The effect of oxygen partial pressure was investigated in the temperature range of 350–400 °C and it was found that under polarization phase transformations can take place. Application of negative potential values, lower than a threshold value of ?0.6 V, can result in the decomposition of PdO, while the reduced species can be re-oxidized during a linear anodic potential scan. The results suggested that during the first minutes of cathodic polarization, the PdO reduction reaction dominated versus the oxygen reduction reaction, while it becomes more significant as the oxygen concentration in the gas phase increased within the range 0–6 kPa.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

纳米TiO_2-Pt修饰电极上甲醇的电催化氧化研究

用电化学法合成前驱体Ti(OEt)4,经直接水解法制备纳米TiO2膜,通过直接在纳米TiO2膜上电沉积Pt微粒得到纳米TiO2 Pt复合催化电极。扫描电子显微镜(SEM)和X射线衍射(XRD)分析结果表明,纳米TiO2的晶形为锐钛矿型,粒径约30nm,电沉积纳米Pt粒子(平均粒径约60nm)均匀地分散在纳米TiO2膜表面。循环伏安和计时电位测试表明,纳米TiO2 Pt修饰电极对甲醇的电氧化具有高催化活性和稳定性,Pt载量为0 68mg/cm2时,室温下甲醇氧化电流达到190mA/cm2,是纯Pt电极上的7 6倍。     

Fe3O4–carbon black nanocomposite as a highly efficient counter electrode material for dye-sensitized solar cell

Iron oxide–carbon black (Fe₃O₄–CB) nanocomposite has been proposed as a cost–effective alternative counter electrode (CE) to the conventional Pt in a dye sensitized solar cell (DSSC). The Fe₃O₄–CB nanocomposite at three different weight ratio (1:1, 1:2 and 2:1) was prepared by a simple solution mixing process and the material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning and transmission electron microscopy techniques. The performance of the three Fe₃O₄–CB nanocomposite CEs was assessed with respect to Fe₃O₄, CB and Pt in a conventional DSSC consisting of N719 dye sensitized TiO₂ (P25) photoanode in contact with an electrolyte containing the I⁻/I₃⁻ redox couple. The electrocatalytic activities of the various CEs towards triiodide (I₃⁻) reduction were analyzed by cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization analysis. The photocurrent voltage (J–V) characteristics of the DSSCs assembled with various counter electrodes were assessed at a light intensity of 80 mW cm⁻². The DSSC assembled with Fe₃O₄–CB nanocomposite (1:2) CE showed the best photovoltaic performance in terms of a high power conversion efficiency of 6.1% which is superior to that of sputtered Pt (4.1%). The simple preparation, excellent electrocatalytic activity and low-cost nature allow the Fe₃O₄–CB nanocomposite to be a promising alternative CE to Pt for use in DSSCs.     

Electrochemical catalysts for hydrocarbon combustion

Two series of electrochemical catalysts were prepared from sputtered Pt thin films onto two kinds of electrolyte membranes, 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ), an O²⁻ conducting oxide and Na₃Zr₂Si₂PO₁₂ (NASICON), a Na⁺ one; respectively. The thickness of the Pt films varied from 8 to 120 nm. Therefore, the Pt loading was extremely low. The catalytic activity of Pt/YSZ and Pt/NASICON systems has been investigated between 200 and 500 °C for propane and propene, respectively. In spite of the low Pt loading, the Pt/YSZ electrochemical catalysts exhibited high activity for propane combustion. Furthermore, the catalytic activity can be in-situ controlled by applying electrical polarisation with high Faradaic efficiency (10³). The catalytic rate of propene deep oxidation on Pt/NASICON electrochemical catalyst was found to be limited by the number of active sites, which is low on very thin Pt films. Moreover, initial anodic polarisation indicate that Na⁺ ions are already present on the top surface of Pt, probably proceeding from the preliminary stabilisation treatment of Pt in the reactive mixture. Nevertheless, polarisation allows the tuning of the catalytic activity of the electrochemical catalysts for propene oxidation. Finally, for both kinds of electrochemical catalysts, our results have evidenced that the measurement of the open-circuit voltage during catalytic process can be an indicator of the hydrocarbon conversion.     

NO reduction performance of Rh paste catalyst on YSZ under steady state and forced oscillation electropromotion conditions

The technique of cyclic voltammetry was applied in conjunction with on-line catalytic product analysis to investigate the electrochemical promotion of NO reduction by C₃H₆ in presence of O₂ on Rh catalyst-electrode films on YSZ at temperatures 350–490 °C. Cyclic linear potential sweep amperometry under catalytic reaction conditions leads to cyclic non-Faradaic electrochemical modifications in the CO₂ formation and NO reduction rates which are compared to those obtained under steady state potentiostatic operation.     

Oxygen storage in O2/Pt/YSZ cell

The phenomenon of electrochemical promotion of catalysis (EPOC) was initially characterized as fully reversible, i.e. the catalyst restores its initial activity after current interruption. However, it has been recently demonstrated that after prolonged anodic polarization an unusual promoted activity is observed for a certain time after current interruption. This phenomenon has been reported as permanent electrochemical promotion of catalysis (P-EPOC).In this work the oxygen storage reported as responsible of P-EPOC has been investigated by transient electrochemical techniques using an O₂(g)Pt/YSZ cell. A model has been proposed involving place interchange of Pt and O species in Pt/YSZ system. This seems to be induced by the strong lateral interaction of Pt–O surface dipoles and by increasing electric field at the Pt/YSZ interface. Such a rearranged oxide, so-called “phase oxide” can have a lower free energy than the initial monolayer oxide. This cooperative interaction of Pt and O species can lead to further thickening of this “phase oxide” especially at high temperature and potentials (currents). Furthermore, as the charge involved in this oxide thickening shows a t^1/2 dependency, the process seems to be diffusion controlled.     

Characterisation of polypyrrole electrosynthesised on aluminium

Cyclic voltammetry has been used to investigate the role of the anions and the solution pH on the redox properties of polypyrrole electrosynthesised onto aluminium in alkaline solutions. The polymer film was characterised by SEM and IR spectroscopy. Moreover, the influence of the cathodic potential on the redox process was analysed for films formed in acid and alkaline solutions. The electrochemical response was compared with that obtained on vitreous carbon electrode. The differences in the shape and evolution of the voltammograms during cycling have been explained by a different participation of the ions. The best redox activity of the polymer is obtained in NO₃⁻ due to the fact that this anion could limit the anodic dissolution of Al. The other anions analysed cannot be incorporated into the polymer matrix. As a consequence, the transport of OH⁻ occurs, which provokes a significative Al dissolution. It is expected that the Al³⁺ ions generated during the electrodissolution are incorporated into the polymer in order to equilibrate the charge of the film.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

A composite poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]-dioxepine) and Pt film as a counter electrode catalyst in dye-sensitized solar cells

A composite poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]-dioxepine) and platinum (PProDOT-Et₂/Pt) film was prepared for using as a counter electrode (CE) catalyst in a dye-sensitized solar cell (DSSC). Four composite films were prepared by electropolymerization of ProDOT-Et₂ on indium tin oxide (ITO) conducting glass, followed by Pt sputtering for 10, 30, 120, and 720 s. The Pt content in the composite film was verified by energy dispersive X-ray spectroscopy (EDX). The composite films possessed three-dimensional (3D) porous structures, as determined by scanning electron microscopy (SEM). The DSSC with the composite film that was subject to 10 s of Pt deposition (PProDOT-Et₂/Pt-10 s) exhibited the highest solar to electricity conversion efficiency (η) of 6.68%, while the cells with the bare polymer film (PProDOT-Et₂) and Pt that was sputtered for 720 s (s-Pt-720 s) demonstrated efficiencies of 4.76% and 6.43%, respectively. The cell photovoltaic parameters were substantiated through dark current, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) analyses. Incident photon-to-current conversion efficiency (IPCE) curves were used to explain the cell photocurrent behaviors.