Effects of electrode size

Even though R rises inversely with rg, Q decreases with the square hence scales with rg. This is an important result indicating that smaller electrodes can provide access to much shorter time domains. Consider, for example, the effect of electrode size in a system with Cd = 20 tF/cm and k = 0.013 Il cm (characteristic of 0.1 M aqueous KCl at ambient temperature). With rg = 1 mm, the cell time constant is about 30 ts and the lower limit of time scale in step experiments (defined as a minimum step width equal to 10/ uCd) is about 0.3 ms. This result is consistent with the general experience that experiments with electrodes of normal size need to be limited to the millisecond time domain... 

Hydrodynamic boundary layer — is the region of fluid flow at or near a solid surface where the shear stresses are significantly different to those observed in bulk. The interaction between fluid and solid results in a retardation of the fluid flow which gives rise to a boundary layer of slower moving material. As the distance from the surface increases the fluid becomes less affected by these forces and the fluid velocity approaches the freestream velocity. The thickness of the boundary layer is commonly defined as the distance from the surface where the velocity is 99% of the freestream velocity. The hydrodynamic boundary layer is significant in electrochemical measurements whether the convection is forced or natural the effect of the size of the boundary layer has been studied using hydrodynamic measurements such as the rotating disk electrode [i] and - flow-cells [ii]. 

White et al. synthesized nanometric La2Cu04 through three techniques auto-ignition, Pechini method, and coprecipitation (White et al., 2008). The NPs were used to fabricate sensing electrodes for NO, and the effect of electrode microstructure on the sensitivity and response time was studied. The response times of the sensors were exponentially dependent on electrode grain size. Sensors with fine-grained electrodes were able to produce a steady-state and consistent voltage at lower temperatures, which improved their response sensitivity. 

The upper and lower dashed curves correspond respectively to the limits of a conductor (Kl s —> °°) and of an insulator (Kb s = 0) Equation (6) gives a quantitative estimate of the effect of tip size and rate constant on the transient. The faster the electron transfer, the smaller the electrode has to be to perform a transient measurement of the rate constant. 

In-Vivo Percutaneous Implant Experiment. The principle of percutaneous attachment has extensive application in many biomedical areas, including the attachment of dental and orthopedic prostheses directly to skeletal structures, external attachment for cardiac pacer leads, neuromuscular electrodes, energy transmission to artificial heart and for hemodialysis. Several attempts to solve the problem of fixation and stabilization of percutaneous implants(19) have been made. Failures were also attributed to the inability of the soft tissue interface to form an anatomic seal and a barrier to bacteria. In the current studies, the effect of pore size on soft tissue ingrowth and attachment to porous polyurethane (PU) surface and the effect of the flange to stem ratio and biomechanical compliance on the fixation and stabilization of the percutaneous devices have been investigated.(20)... 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

Some authors believe that the inclined line of impedance at low frequencies comes from the pore size distribution of porous materials [171,182], and a few attempts have been made to consider the effect of pore size distributions (PSD) on the impedance of a porous electrode [171,182], although the PSD must contribute considerably to the distributed characteristics [171,182]. The impedance curve in the Nyquist plot is observed to change with the shape of a pore in the intermediate frequency region, despite its similarity to a cylindrical pore at extremely low or high frequencies. Some authors have reported that the real part of the reduced impedance (the ratio of impedance of a pore to electrolyte resistance in a pore) approached one-third at low frequency, irrespective of the shape of a pore [171,182]. The PSD effect is difQcult to take into account, particularly because of the time-consuming calculations required by this method, while a parametric study is difficult because of too many parameters (sizes of different pores), but some analytical solutions are being used to represent the pore size distribution of a porous electrode [171,182]. 

Perez, J, E.R, Gonzalez, and E.A. Ticianelli, Oxygen electrocatalysis on thin porous coating rotating platinum electrodes. Electrochimica Acta, 1998, 44 pp. 1329-1339 Song, H,-K, H,-Y, Hwang, K.-H. Lee, and L.H, Dao, The effect of pore size distribution on the frequency dispersion of porous electrodes. Electrochimica Acta, 2000. 45 pp. 2241-2257 Srikumar, A, T.G. Stanford, and J.W, Weidner, Linear sweep voltammetry in flooded porous electrodes at low sweep rates. Journal of Electroanalytical Chemistry, 1998. 458 pp. 161-173... 

Figure 5.33 presents the effect of particle size on the Nyquist impedance behavior of the electrode [75]. The plot shows that the effect of particle size is opposite to that of the diffusion coefficient, that is, a decrease in the particle radius shifts the appearance of the transition region to higher frequencies. In smaller particles, hydrogen needs to travel only... 

N. Atta, A. Galal, and F. Khalifa, Electrodeposited metals at conducting polymer electrodes I - Effect of particle size and film thickness on electrochemical response, Appl. Surf. Sci., 253, 4273 282 (2007). 

The overall effects of the ttucroflow on endothelial cells have been surtunarized by Yoimg et al. [22]. In other study, Li et al. [23] used a microfluidic platform and an acoustic wave sensor to monitor contractions of heart muscle cells. In addition to the effects of chenucal stimuli, alterafirais in contractions were observed to be a function of electrode size, presence of the nucrochannel plate, and liquid loading onto the sensor. 

In the case of the positively charged needle setup (Figure 31.2b), effects of electrode spacing on alginate bead size, produced with a 22-gauge needle, are shown in Figure 31.4a [19]. 

FIGURE 31.5 Microbead size as a function of applied potential (a and b) effects of electrode spacing and needle size in the parallel-plate setup and (c) multi-needle device. (From Poncelet, D., Bugarski, B., Amsden, B., Zhu, J., Neufeld, R., and Goosen, M.F.A., Appl. Microbiol. Biotechnol, 42 (2-3), 251-255, 1994. With permission.)... 

Song HK, Jung YH, Lee KH, Dao LH (1999) Electrochemical impedance spectroscopy of porous electrodes the effect of pore size distribution. Electrochim Acta 44(20) 3513-3519... 

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