Repetitive Potential Cycling Research Articles

In situ STM observation of morphological changes of the Pt(111) electrode surface during potential cycling in 10 mM HF solution

In the present study, we have performed in situ STM observation of the surface oxidation-reduction process at Pt(111) surface in 10 mM HF solution under N(2) atmosphere in a stainless-steel chamber. We have for the first time demonstrated the dynamic process of the roughening of the Pt(111) surface during the potential cycles. At E < 0.8 V vs. RHE, no distinct change was observed in the positive-going potential sweep, even though surface oxidation is expected to commence in the so-called butterfly peak region at 0.6 V. At E > or = 0.9 V, tiny spots with a height of 0.08 nm appeared on the terraces, which can be attributed to adsorbed oxygen species rather than Pt ad-atoms. When the electrode potential reached 1.3 V, the electrode surface became bumpy with small corrugations (<0.1 nm) due to oxygen atoms becoming incorporated into the subsurface, without any additional layer formation. In the negative-going potential sweep, the electrode surface was suddenly covered with monoatomic islands, as well as pits, while the tiny spots disappeared at the moment that the electrode potential reached 0.5 V, at which the surface reduction was completed. During the repetitive potential cycles, the formation and growth of the Pt islands were found to occur only around 0.5 V in each negative-going potential sweep back from 1.3 V.

Electrochemical Measurement of the Surface Alloying Kinetics of Underpotentially Deposited Ag on Au(111)

The cyclic voltammetry characterizing underpotential deposition (UPD) of Ag onto Au(111) varies in the literature with respect to the characteristic UPD peaks in both position and number. Rooryck et al. (1) confirmed that the discrepancy in terms of peak position, specifically the initial UPD to which a third of a monolayer of deposition is attributed, is due to a variation in the quality of the surface. Clean, smooth Au(111) surfaces yield a peak position of 0.53 V vs Ag0/Ag+, while rough disordered surfaces yield a peak position of 0.61 V vs Ag0/Ag+. Repetitive potential cycling in the UPD region resulted in a gradual shift in peak position, with time as the deposited Ag alloyed with, and was stripped from the surface leaving vacancies. We provide a methodology for tracking the rate at which UPD Ag alloys with the Au(111) surface without the use of continuous potential cycling. A simple kinetic model is developed for the surface alloying of Ag on Au(111), from which we extract an activation barrier and attempt frequency for this process. Notably, we introduce a novel technique for the inexpensive parallel fabrication of Au(111) single crystals that allowed us to build statistics and ensured reproducibility of our data.

Stability of Pt–Ni/C (1:1) and Pt/C electrocatalysts as cathode materials for polymer electrolyte fuel cells: Effect of ageing tests

Carbon supported Pt and Pt–Ni (1:1) nanoparticles were prepared by reduction of metal precursors with NaBH 4. XRD analysis indicated that only a small amount of Ni alloyed with Pt (Ni atomic fraction in the alloy about 0.05). The as-prepared catalysts were submitted to chronoamperometry (CA) measurements to evaluate their activity for the oxygen reduction reaction (ORR). CA measurements showed that the ORR activity of the as-prepared Ni-containing catalyst was higher than that of pure Pt. Then, their stability was studied by submitting these catalysts to durability tests involving either 30 h of constant potential (CP, 0.8 V vs. RHE) operation or repetitive potential cycling (RPC, 1000 cycles) between 0.5 and 1.0 V vs. RHE at 20 mV s −1. After 30 h of CP operation at 0.8 V vs. RHE, loss of all non-alloyed Ni, partial dissolution of the Pt–Ni alloy and an increase of the crystallite size was observed for the Pt–Ni/C catalyst. The ORR activity of the Pt–Ni/C catalyst was almost stable, whereas the ORR activity of Pt/C slightly decreased with respect to the as-prepared catalyst. Loss of all non-alloyed and part of alloyed Ni was observed for the Pt–Ni/C catalyst following repetitive potential cycling. Conversely to the results of 30 h of CP operation at 0.8 V vs. RHE, after RPC the ORR activity of Pt–Ni/C was lower than that of both as-prepared Pt–Ni/C and cycled Pt/C. This result was explained in terms of Pt surface enrichment and crystallite size increase for the Pt–Ni/C catalyst.

Electrochemical Response of Carbon Electrodes Modified with Zeolites

In this work we studied the electrochemical behavior of a glassy carbon electrode modified with natural and synthetic zeolites. Iron exchanged zeolites were set up for the detection of arsenic species. The electrochemical response both in KCl and HCl 0.1 M of the electrodes after modification was investigated by cyclic and differential pulse voltammetry. The electrochemical parameters of the redox pairs Fe3+/Fe2+ and Fe2+/Fe0 were determined for the iron containing electrode and the effect of repetitive potential cycling on the formation of iron oxides was studied. The reactivity of the exchanged iron species in the zeolite and the reduction of the complex As(V)-iron generated in-situ could be demonstrated.

Electrocatalytic oxidation of glucose on Ni and NiCu alloy modified glassy carbon electrode

Nickel and nickel–copper alloy modified glassy carbon electrodes (GC/Ni and GC/NiCu) prepared by galvanostatic deposition were examined for their redox processes and electro-catalytic activities towards the oxidation of glucose in alkaline solutions. The methods of cyclic voltammetry (CV) and chronoamperometry (CA) were employed. The cyclic voltammogram of NiCu alloy demonstrates the formation of β/β crystallographic forms of the nickel oxyhydroxide under prolonged repetitive potential cycling in alkaline solution. It is also observed that the overpotential for O2 evolution increases for NiCu alloy modified electrode. In CV studies, NiCu alloy modified electrode yields significantly higher activity for glucose oxidation compared to Ni. The oxidation of glucose was concluded to be catalyzed through mediated electron transfer across the nickel hydroxide layer comprising of nickel ions of various valence states. The anodic peak currents show linear dependency with the square root of scan rate. This behavior is the characteristic of a diffusion-controlled process. Under the CA regime, the reaction followed a Cottrellian behavior, and the diffusion coefficient of glucose was found to be 1 × 10−5 cm2 s−1, in agreement with diffusion coefficient obtained in CV studies.

Electrocatalytic oxidation of methanol on Ni and NiCu alloy modified glassy carbon electrode

Nickel and nickel–copper alloy modified glassy carbon electrodes (GC/Ni and GC/NiCu) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol in alkaline solutions. The methods of cyclic voltammetery (CV) and chronoamperometry (CA) were employed. The cyclic voltammogram of NiCu alloy demonstrates the formation of β/β crystallographic forms of the nickel oxyhydroxide under prolonged repetitive potential cycling in alkaline solution. In CV studies, in the presence of methanol NiCu alloy modified electrode shows a significantly higher response for methanol oxidation. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol. The anodic peak currents show linear dependency with the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol was found to be 2 × 10 −6 cm 2 s −1 in agreement with the values obtained from CV measurements.

Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells

Carbon supported Pt and Pt–Co nanoparticles were prepared by reduction of the metal precursors with NaBH 4. The activity for the oxygen reduction reaction (ORR) of the as-prepared Co-containing catalyst was higher than that of pure Pt. 30 h of constant potential operation at 0.8 V, repetitive potential cycling in the range 0.5–1.0 V and thermal treatments were carried out to evaluate their electrochemical stability. Loss of non-alloyed and, to a less extent, alloyed cobalt was observed after the durability tests with the Pt–Co/C catalyst. The loss in ORR activity following durability tests was higher in Pt–Co/C than in Pt/C, i.e. pure Pt showed higher electrochemical stability than the binary catalyst. The lower stability of the Pt–Co catalyst during repetitive potential cycling was not ascribed to Co loss, but to the dissolution–re-deposition of Pt, forming a surface layer of non-alloyed pure Pt. The lower activity of the Pt–Co catalyst than Pt following the thermal treatment, instead, was due to the presence of non-alloyed Co and its oxides on the catalyst surface, hindering the molecular oxygen to reach the Pt sites.

Preparation, Characterization, and Electrocatalytic Properties of Cobalt Oxide and Cobalt Hexacyanoferrate Hybrid Films

AbstractPolynuclear mixed‐valent films of cobalt oxide and cobalt hexacyanoferrate (CoOCoHCF) have been deposited on electrode surfaces from a solution of Co2+ and Fe(CN)63− ions by repetitive potential cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance measurements demonstrate the steady growth of modified film. The effect of type of monovalent cations as well as acidity of the supporting electrolyte on film growth and redox behavior of resulting film was investigated. In pure supporting electrolyte, electrochemical responses of modified electrode resemble with that of a surface immobilized redox couple. The hybrid film electrodes showed electrocatalytic activity toward oxidation of NADH, hydrazine and hydroxylamine. The feasibility of using our modified electrodes for analytical application was also explored.

Zinc Oxide/Zinc Hexacyanoferrate Hybrid Film‐Modified Electrodes for Guanine Detection

AbstractAn electroactive polynuclear hybrid films of zinc oxide and zinc hexacyanoferrate (ZnO/ZnHCF) have been deposited on electrode surfaces from H2SO4 solution containing Zn(NO3)2 and K3[Fe(CN)6] by repetitive potential cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) measurements demonstrate the steady growth of hybrid film. There are two redox couples present in the voltammograms of hybrid film and it is obvious in the case of pH 2. Surface morphology of hybrid film was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Energy dispersive spectrometer (EDS) data confirm existence of zinc oxide in the hybrid film. The effect of type of monovalent cations on the redox behavior of resulting film was investigated. In pure supporting electrolyte, electrochemical responses of modified electrode resemble with that of a surface immobilized redox couple. The electrocatalytic activity of ZnO/ZnHCF hybrid film was investigated towards guanine using cyclic voltammetry and rotating disc electrode (RDE) techniques. Finally, feasibility of using ZnO/ZnHCF hybrid film‐coated electrodes for guanine estimation in flow injection analysis (FIA) was also investigated.

Impedance study of electropolymerized films of polyNi(II)-macrocycles

Glassy carbon electrodes modified by electropolymerization of two water-soluble compounds, Ni(II)tetrasulphophthalocyanine (NiTSPc) and Ni(II)tetrasulphophenylporphyrin (NiTSPP), and of two water-insoluble compounds, Ni(II)tetraaminophthalocyanine (NiTAPc) and Ni(II)tetraaminophenylporphyrin (NiTAPP), have been studied by impedance measurements. While in the tetrasulpho films the Ni(II)/Ni(III) process was operative, the tetraamino films required activation by repetitive potential cycling before the peaks of this process appeared in the CV, and even then the charge was a fraction of that shown by the tetrasulpho films. The Nyquist plots of all these films shown in the potential range of the Ni(II)/Ni(III) process the semicircle typical of a charge transfer, the charge-transfer resistance decreasing very much with increasing potential. The uncompensated resistance of the GC electrode was barely affected by the films, showing that they were very porous. The impedance at low frequencies of the thick polyNiTSPP film, the thin polyNiTSPc film, and also the activated polyNiTAPc film fitted the Warburg model, the diffusing species being probably the hydroxyl ions involved in the process.

Electropreparation of Poly(benzophenone-4) Film Modified Electrode and Its Electrocatalytic Behavior Towards Dopamine, Ascorbic Acid and Nitrite

The oxidation of benzophenone-4 (2-hydroxy-4-methoxybenzophenone-5-sulfonic acid) at glassy carbon electrode gives rise to stable redox active electropolymerized film during repetitive potential cycling between 0 to 1.3 V (Ag/AgCl). Cyclic voltammogram of poly(benzophenone-4) film shows a redox couple with well-defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 51 mV/pH. The electrocatalytic behavior of poly(benzophenone-4) film modified electrode towards oxidation of dopamine, ascorbic acid and reduction of nitrite was investigated. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(benzophenone-4) film compared to bare glassy carbon electrode. For dopamine, the overpotential was reduced about 180 mV. Feasibility of utilizing poly(benzophenone-4) film coated electrode in analytical estimation of dopamine, ascorbic acid and nitrite was also demonstrated.

Heat treatment and potential cycling effects on surface morphology, particle size, and catalytic activity of Pt/C catalysts studied by 13C NMR, TEM, XRD and CV

Pt particles of 20% Pt/C samples grew both by repetitive potential cycling and heat treatment. Average particle size variation is linearly proportional to the number of potential cycles while it is proportional to an exponential function of heating temperature. HR-TEM results show rough and irregular Pt surfaces in repetitively potential cycled samples and well-defined cubo-octahedrons without many defects in heat treated samples. This difference of surface morphology plays a main role to produce opposite electrocatalytic activity variation versus particle size of the samples prepared by potential cycling and heat treatment. Our results demonstrate that in terms of noble metal usage Pt particles with an average size of ∼5 nm is the most efficient catalyst if the particle shapes are well defined cubo-octahedrons. In that aspect, heat treatment is better than potential cycling to prepare catalysts.

Electrochemical synthesis of a novel conducting polymer: The poly[(3-pyrrolyl)-octanoic acid

A new pyrrole derivative in which an alkyl chain is functionalized with a carboxyl terminal group, (β-pyrrolyl)-octanoic acid, has been electrochemically characterized in propylene carbonate and its irreversible oxidation used to prepare conducting polymer films on platinum. Under potentiostatic control, the data points to an instantaneous nucleation and tri-dimensional growth mechanism. FTIR spectroscopy was used to prove the presence of the carboxyl acid group in the polymeric matrix, crucial feature for future biomolecule immobilization and use of this new polymer in the fabrication of effective biosensors. The voltammetric responses of the polymer modified electrodes present two redox reversible waves which relative importance is film thickness dependent; in all studied cases good stability during repetitive potential cycling was observed.

Micromolar determination of sulfur oxoanions and sulfide at a renewable sol–gel carbon ceramic electrode modified with nickel powder

The sol–gel technique was used to fabricate nickel powder carbon composite electrode (CCE). The nickel powder successfully used to deposit NiO x thin film on conductive carbon ceramic electrode for large surface area catalytic application. Repetitive cycling in potential range −0.2 to 1.0 V was used to form of a thin nickel oxide film on the surface carbon composite electrode. The thin film exhibits an excellent electro-catalytic activity for oxidation of SO 3 2−, S 2O 4 2−, S 2O 3 2−, S 4O 6 2− and S 2− in alkaline pH range 10–14. Optimum pH values for detection of all sulfur derivatives is 13 and catalytic rate constants are in range 2.4 × 10 3–8.9 × 10 3 M −1 s −1. The hydrodynamic amperometry at rotating modified CCE at constant potential versus reference electrode was used for detection of sulfur derivatives. Under optimized conditions the calibration plots are linear in the concentration range 10 μM–15 mM and detection limit 1.2–34 μM and 0.53–7.58 nA/μM (sensitivity) for electrode surface area 0.0314 cm 2. The nickel powder doped modified carbon ceramic electrode shows good reproducibility, a short response time (2.0 s), remarkable long term stability, less expense, simplicity of preparation, good chemical and mechanical stability, and especially good surface renewability by simple mechanical polishing and repetitive potential cycling. This sensor can be used into the design of a simple and cheap chromatographic amperometry detector for analysis of sulfur derivatives.

Cathodic deposition of molybdenum and vanadium mixed oxyhydroxide films from V-substituted polymolybdophosphate

We report a new electrochemical route for fabricating molybdenum and vanadium mixed oxyhydroxide films on Au electrode from Keggin-type vanadium-substituted polymolybdophosphate. The process involves a potentiodynamic reduction of aqueous 9-molybdo(VI)-3-vanadophosphosphate(V) ([PMo 9V 3O 40] 6−) in the potential region between 0 and −0.7 V versus Ag/AgCl. The resulting Mo V oxyhydroxide film electrode gave a stable redox behavior in Na 2SO 4 electrolyte of pH 3. X-ray photoelectron spectroscopy revealed that this results from oxidation/reduction of Mo 5+/Mo 6+ in the film which accompanies extraction/insertion of protons for charge compensation. The deposited V ions remained in the film upon repetitive potential cycling without affecting their oxidation state. Voltammetric data in the presence of sodium nitrite showed electrocatalytic activity of the Mo V oxyhydroxide toward the electroreduction of nitrite.

Iridium-Based Hexacyanometallate Thin Films in Aqueous Electrolytes

Thin films of metal-hexacyanoiridium(III) (MHCI), KxMy[Ir(CN)6]z, where M = Ru, Fe were electrochemically prepared and used as surface modifiers for glassy carbon electrodes. The redox behaviour of the counter/central ions of these films was investigated in aqueous electrolytes using cyclic voltammetry, chronocoulometry and electrochemical impedance spectroscopy. An electrochemical synthesis of zeolite-like films of KxMy[Ir(CN)6]z and KxFey[Fe(CN)6]z (Prussian blue) was also undertaken using the cyclic voltammetric technique. Porous multifilm assemblies of Prussian blue and MHCI were formed either by the direct electrodeposition of Prussian blue over MHCI or Prussian blue over MHCI during repetitive potential cycling, or by electrochemically-driven insertion-substitution methods. In acidic aqueous KCl, iron hexacyano iridate (FeHCI) displays two redox waves with formal potential Eo f ≈, 0.35 and 0.6 V vs. Ag/AgCl. The electrochemical behaviour of FeHCI was compared with that of related hexacyanometallate compounds, such as KRux[Ru(CN)6]y, KRux[Fe(CN)6]yand KFex[Fe(CN)6]y. In addition, evidence for the catalytic behaviour of MHCIfilms towards the reduction of iodate, IO3 -, is reported.

Iridium-Based Hexacyanometallate Thin Films in Aqueous Electrolytes

Thin films of metal-hexacyanoiridium(III) (MHCI), KxMy[Ir(CN)6]z, where M = Ru, Fe were electrochemically prepared and used as surface modifiers for glassy carbon electrodes. The redox behaviour of the counter/central ions of these films was investigated in aqueous electrolytes using cyclic voltammetry, chronocoulometry and electrochemical impedance spectroscopy. An electrochemical synthesis of zeolite-like films of KxMy[Ir(CN)6]z and KxFey[Fe(CN)6]z (Prussian blue) was also undertaken using the cyclic voltammetric technique. Porous multifilm assemblies of Prussian blue and MHCI were formed either by the direct electrodeposition of Prussian blue over MHCI or Prussian blue over MHCI during repetitive potential cycling, or by electrochemically-driven insertion-substitution methods. In acidic aqueous KCl, iron hexacyano iridate (FeHCI) displays two redox waves with formal potential E o f ≈ 0.35 and 0.6 V vs. Ag/AgCl. The electrochemical behaviour of FeHCI was compared with that of related hexacyanometallate compounds, such as KRux[Ru(CN)6]y, KRux[Fe(CN)6]y and KFex[Fe(CN)6]y. In addition, evidence for the catalytic behaviour of MHCI films towards the reduction of iodate, IO3 – , is reported.

Structure‐Dependent Electrochemical Behavior of Thienylplatinum(II) Complexes of N,N‐Heterocycles

Abstracttrans‐[Pt(MeCN)(PPh3)2(2‐thienyl)]BF4 (1) serves as a convenient precursor to bifunctional mononuclear trans‐[Pt(PPh3)2(η1‐N‐N)(2‐thienyl)]BF4 [N‐N = pyrazine (2); 2‐chloropyrazine, (3)] and dinuclear trans,trans‐[Pt2(PPh3)4(μ‐N‐N)(2‐thienyl)2](BF4)2 [(N‐N = 4,4′‐bipyridine (4); 4,4′‐vinylenedipyridine (5)] complexes. The nuclear selectivity is conveniently controlled by the choice of the heterocyclic ligands or spacers. Both structural types 3 and 5 were confirmed by single‐crystal X‐ray crystallographic analyses. Their solution identities were established by positive‐ion Electrospray Mass Spectrometry (ESMS). The electroactivities of these complexes were studied by cyclic voltammetry (CV). Continuous CV scans of 4 and 5 revealed variations in the redox waves with the number of scans. While the initial oxidative scan exhibited only a broad, irreversible wave, further cycling showed the growth of two additional redox couples up to about the tenth cycle. The peak currents of these redox couples began to decay with prolonged potential cycling beyond the tenth cycle. These findings are consistent with the formation of electroactive oligomers/polymers, and this conclusion is supported by visible thin film formation on the electrodes. In contrast, the mononuclear complexes (2 and 3) do not show such behavior. The films formed were further studied by repetitive potential cycling and XPS. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)

Electrochemical synthesis of zeolite-like ruthenium-based hexacyanometalates multi-film assemblies

Electrochemical synthesis of zeolite-like films of K + x Ru y [Fe(CN) 6] z (RF) or ruthenium hexacyanoferrate and K + x Ru y [Ru(CN) 6] z (RR) or ruthenium hexacyanoruthenuate, and related compounds have been achieved using cyclic voltammetric technique. Formation of porous multi-film assemblies of Prussian Blue (PB) and RF was carried out by either direct electrodeposition of PB over RF or RF over PB during repetitive potential cycling, or by electrochemically driven insertion–substitution methods. The cyclic voltammetric studies indicate that the method of preparation has no effect on the overall electrochemical (EC) behavior of the formed assemblies. In all studied cases, the EC of all redox centers in the multi-films was observed regardless of their order in the assembly. Substitution of a counter ion In 3+ or Cu 2+ results in less stable assemblies. Studies indicate that whenever Ru 3+ serves as the counter ion in hexacyanometalates (HCM), the redox potential of the complex center ([M z+ (CN) 6]) is shifted to a more negative potential. Some of these multi-film assemblies demonstrated catalytic activities in the oxidation of hydrazine.

An examination of electrochemical and gas phase deposition of silver on Pt{1 0 0}-(1×1) and Pt{1 0 0}-hex-R0.7°

The growth of silver on reconstructed and unreconstructed Pt{100} has been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and cyclic voltammetry (CV). In agreement with earlier studies, the underpotential deposition (upd) of silver gave rise to two adsorption states, one corresponding to the first silver monolayer (0.43 V versus Ag+∣Ag) and the other associated with completion of a second monolayer (0.02 V versus Ag+∣Ag) in which the electrovalencies of Ag were estimated to be 1 and 0.77, respectively. However, in addition, it was found that the peak intensities derived from each of these adsorption sites were a sensitive function of the underlying surface morphology. That is, a more disordered silver deposit was formed when upd occurred onto Pt{100}-hex relative to the (1×1) phase. The on-set of oxide formation at less positive potentials was also a characteristic of the more disordered silver phase. These results are in accordance with previous investigations of copper and palladium growth on reconstructed and unreconstructed Pt{100}. It is also demonstrated that the upd peak at 0.43 V versus Ag+∣Ag corresponds to incomplete stripping of the silver monolayer, the residual first monolayer of silver forming an oxide phase which could be detected after immersion using AES analysis as reported also by Wieckowski and coworkers (Surf. Sci. 335 (1995) 44) for silver upd on Pt{111}. Another striking feature of the Ag/Pt{100} electrochemical system was its propensity to form surface alloys after repetitive potential cycles as reported previously by Aberdam et al. (Surf. Sci. 239 (1990) 71). However, for a defect-free Pt{100}-(1×1) surface no alloy formation took place. It is concluded that step and kink sites are a necessary prerequisite for alloying to proceed at an appreciable rate and that each of these defects generates a Ag–Pt alloy of differing surface composition. Evaporated films of silver prepared in UHV on reconstructed and Pt{100}-(1×1) surfaces each gave rise to Stranski–Krastanov growth. LEED revealed that the hex reconstruction was completely lifted after one monolayer of silver had been deposited. No overlayer superstructures other than (1×1) were observed in the range of silver coverages studied (θAg<5 monolayers).