Potential cycling

The pc-Au/propylene carbonate (PC) + NaC104 interface has been studied by Nguyen Van Huong.481 A flame-annealed (02 + H2) pc-Au sphere was used. Before each experiment the pc-Au electrode was cleaned in an NaC104 aqueous solution by a few potential cycles involving oxidation-reduction of the surface until the i,E and C,E curves exhibited stable character. The C,E curves were recorded in the interval 15 150... 

After introduction of the working electrode to the spectroelectrochemical cell, continuous potential cycling was performed to obtain a clean surface before each... 

The hysteresis in contact angles caused the hysteresis of the deformation of the droplet put on the substrate during the potential cycle. When the potential was... 

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

The dissolution rate during potential cycling was reported to be around 3.0-5.5 ng/cm per cycle, with the upper potential limit between 1.2 and 1.5 V and various potential scanning rates [Johnson et al., 1970 Kinoshita et al., 1973 Rand and Woods, 1972 Wang XP et al., 2006]. The predominant forms of dissolved Pt were... 

TABLE 9.4 A Comparison of Surface Area and the Catalytic Activity Data for Pt/C and Au/Pt/C Before and After 30,000 Potential Cycles from 0.6 to 1.1 V under the Oxidizing Conditions of the O2 Reduction Reaction. Reprinted with Permission from Zhang et al. [2007b]... 

Mitsushima S, Kawahara S, Ota K-1, Kamiya N. 2007a. Consumption rate of Pt under potential cycling. J Electrochem Soc 154 B153-B158. 

Yasuda K, Taniguchi A, Akita T, loroi T, Siroma Z. 2006b. Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling. Phys Chem Chem Phys 8 746-752. 

Figure 10.4 Area-normalized CL spectra of Pt4/7/2 for the pure Pt (dotted Une), Pt5gCo42 (solid line), and PtgoRu4o (dashed line) alloys with respect to p (a) as-prepared (h) after electrochemical stabilization. The samples were thin film pure Pt or Pt-based alloys (diameter 8 mm and thickness 80 nm) prepared on Au disks by DC sputtering. Electrochemical stabilization of Pt58 C042 was performed by repeated potential cycling between 0.075 and 1.00 V at a sweep rate of 0.10 V s in 0.1 M HCIO4 under ultrapure N2 (99.9999%) until CV showed a steady state. PtgoRu4o was stabilized by several potential cycling between 0.075 and 0.80 V at 0.10 V s in 0.05 M H2SO4 under ultrapure N2. (From Wakisaka et al. [2006], reproduced by permission of the American Chemical Society.)...

A similar inhibition was found also for electrochemical CO oxidation. In COad stripping experiments, numerous potential cycles up to IV were necessary to remove all COad from a smooth Ru(OOOl) surface [Zei and Ertl, 2000 Lin et al., 2000 Wang et al., 2001]. CO bulk oxidation experiments under enforced mass transport conditions on polycrystalline Ru [Gasteiger et al., 1995] and on carbon-supported Ru nanoparticle catalysts [Jusys et al., 2002] led to similar results. Hence, COad can coexist with nonreactive OHad or Oad species on Ru(OOOl) at lower potentials (E < 0.55 V) [El-Aziz and Ribler, 2002]. 

The ventricular action potential is depicted in Fig. 6-2.2 Myocyte resting membrane potential is usually -70 to -90 mV, due to the action of the sodium-potassium adenosine triphosphatase (ATPase) pump, which maintains relatively high extracellular sodium concentrations and relatively low extracellular potassium concentrations. During each action potential cycle, the potential of the membrane increases to a threshold potential, usually -60 to -80 mV. When the membrane potential reaches this threshold, the fast sodium channels open, allowing sodium ions to rapidly enter the cell. This rapid influx of positive ions... 

Thick anodic iridium oxide films are formed by repetitive potential cycling between properly chosen anodic and cathodic limits [89]. The coloration (bleaching) transition is reflected in the cyclic voltammogram by a significant increase (decrease) of the electrode pseudo-capacity at a potential around 0.7 Vsce in acid electrolytes. At potentials above 0.7 V the thick film appears dark blue, while below 0.7 V the film is almost clear. 

Emrich et al.112 using X-ray photoelectron spectroscopy (XPS) data, and Lundgren and Murray,113 using CY, EDAX, XPS, elemental analytical, and spectroelectrochemical measurements, both confirmed /-PB as the initially deposited form with a gradual transformation to s-PB on potential cycling. Other support for the /-PB to s-PB transformation comes from an... 

With respect to the UHV-based techniques capable of providing chemical analysis, such as ESCA, AUGER, etc., several such studies have been performed. However, these studies were, by and large, performed on very thick oxide layers, formed after anodic oxidation of the Pt for many hours. Results from these studies thus have little bearing on the nature of the oxides formed on potential cycling. Part of the reason why these studies used such thick films lies in the considerable difficulty of detecting the thin oxide films formed during a potential sweep, even with relatively sensitive techniques. 

The potential cycling of the him between the insulating and (semiconducting forms can be repeated many times without the loss of the electroactivity of the film. 

FIGURE 6.13 SEM images of Au/CFME (electro-deposition time 30 s) (a), Cys/Au/CFME (b) and Cys/Au/CFME after reductive desorption of cysteine by potential cycling from —0.4 to —1.3V at lOOrnVs-1 in 0.5 M KOH (c). (Reprinted from [158], with permission from Elsevier.)... 

M.L. Hitchman and S. Ramanathan, Evaluation of iridium oxide electrodes formed by potential cycling as pH probes. Analyst 113, 35-39 (1988). 

Fig. 32. Cyclic current-potential curve for Au(l 11), covered with an octadecanethiol SAM, in 0.1 M H2SO4 + 1 mM CUSO4. Scan rate 10 mV s-1. The C18-SAM blocks Cu deposition up to very high overpotentials. Inset Current response of a bare Au(lll) electrode for the same potential cycle [98],...

---