Electrode roughness

NiOOH is stiU found at very cathodic potentials (-0.9V), along with some electrode roughness. A potentiostatic hold at this potential shows that Ni(OH)2 is very slowly reduced to Ni, with a concomitant decrease in electrode roughness. 

For a typical monomolecular coverage, T — 10 10 mol/cm2, an electrode roughness factor r = 1000 and an extinction coefficient ads = 107 cm2/mol, the light-harvesting efficiency is, in comparison to the preceding case, very high, intimate contact with the semiconductor surface, hence the conditions for charge injection from S into the semiconductor are almost ideal (q9j—>100 per cent). 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

Figure 10.4 Depiction of electrode roughness compared to diffusion layer thickness, vDt. Dotted line indicates approximate boundary of diffusion layer, with (A) diffusion layer thickness greater than surface roughness, resulting in an observed area equal to the projected area, and (B) diffusion layer thickness on the order of surface roughness, resulting in a larger apparent electrode area.

Duyne and co-workers estimated enhancement factors on the order of 105 to 106 for pyridine on rough silver electrodes. The value was obtained from a comparison between surface-enhanced and normal bulk Raman signals from pyridine by taking into account the different number of molecules on the electrode and in solution. The size of the enhancement was found to correlate with the electrode roughness, indicating that enhancement occurs via a strong electromagnetic field. On the other hand, the dependence of the... 

The potential window over which the electrolyte is electrochemically stable may be estimated using a polarizable (blocking) electrode, such as platinum. The current is monitored as a function of the electrode potential, and the zero-current region (or nearly zero-current) defines the domain of electrochemical stability of the electrolyte. Of course, this potential range will depend mainly on the nature and the surface of the electrode (roughness). 

Fig. 10.18. Effects of surface roughness on EHD impedance (amplitude ratio, H(p)IH(p- 0), and phase lag, 9, against scaled frequency, p and comparison with the behaviour of a uniform disc—asymptotic line marked (a) —and an array of UMEs— asymptotic line marked (b). The frequency shift is deduced from the displacement between the two sections of the phase angle diagram where the data superimpose for different n . The modulation frequency, to2, at which the data deviate from that of a uniform electrode, is related to the amplitude of the surface roughness or the spacing between the elements of the UME array. Data from Reference [121], for Fe(CN)i reduction on smooth Pt at 120 rpm 4 240 rpm, and on a rough, Pt-coated silver electrode (roughness scale 5 (im, disc diameter 6 mm) at O 120 rpm + 240 rpm A 500 rpm and x 1000 rpm.

Electrode roughness call also cause variations in current distributions (28). Penetration of current into pores may be limited if the solution resistance of pores is large relative to the polarization resistance. 

Figure 20. (A) The assembly of an integrated lactate dehydrogenase monolayer electrode by the cross-linking of an affinity complex formed between the enzyme and a PQQ-NAD monolayer-modified Au electrode. (B) Cyclic voltammograms of the integrated cross-linked PQQ-NAD / lactate dehydrogenase electrode (roughness factor ca. 15) (a) in the absence of lactate (b) with lactate, 20 mM. Recorded in 0.1 M Tris buffer, pH 8.0, in the presence of 10 mM CaCb, under Ar potential scan rate, 2 mV s . Inset amperometric responses of the integrated electrode at different concentrations of lactate upon application of potential 0.1 V vs. SCE.

Figure 22. (A) The assembly of a nitrate-sensing electrode by the cross-linking of an affinity complex formed between nitrate reductase (cytochrome-dependent, EC 1.9.6.1), NR and an Fe(III)-protoporphyrin reconstituted de novo four-helix-bundle protein. (B) Cyclic voltammograms of the NR-two heme-reconstituted de novo protein-layered Au electrode at nitrate concentrations of (a) 0, (b) 12, (c) 24, (d) 46 and (e) 68 mM. Inset calibration curve for the amperometric response of the electrode at different nitrate concentrations (at E = —0.48 V vs. SCE). Potential scan rate, 5 mV s" 0.1 M phosphate buffer, pH 7.0, under argon electrode roughness factor, ca. 20.

Thus, in the metal/YSZ systems of solid-state electrochemistry, AC-impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-500 pF cm-2 of superficial electrode surface area and of the order 2-10 pF cm-2 when the electrode roughness is taken into account and, thus, the true metal/gas interface surface area is used, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore AC-impedance spectroscopy... 

The importance of measuring the imaginary component of the quartz crystal in order to study metal deposition and dissolution processes has also been noted by the authors of [26,88]. In particularly, in this way they [26] succeeded in separating contributions of mass loading and roughness to QCM response and to characterize the electrode roughness. 

A comparison of the absorption coefficients for CO on different materials requires a normali2ation, taking into account differences in electrode roughness. With the aim of normalization, one can use, again, the band intensity of CO2 produced upon total oxidation of the respective adlayer. The (COads)/(C02) intensity ratios are 0.87 (for Pt), 0.60 (for PtRu), and 0.25 (for Ru) with an estimated error of ca. 8% [62]. These nor-mahzed intensities of COad reflect, in a first approximation, the relative values of the absorption coefficient of CO on the three substrates. 

The experimental smdy of the solid-solid interface is complicated by a further problem. It is often (perhaps usually) observed that, instead of a purely capacitative behavior, the interface shows significant frequency dispersion. Several authors have found excellent agreement of this behavior with the dispersion shown by the constant-phase element (Bottelberghs and Broers [1976], Raistrick et al. [1977]). Although the amount of frequency dispersion is influenced by electrode roughness and other aspects of the quality of the interface (i.e. nonuniform current distribu-... 

The double-layer capacity (Cai), which is a simple electrical capacity, is often represented by a nonlinear capacity or a constant-phase element (CPiidi) so as to adjust the difference of the electrochemical system from the ideal behavior of an electrical system. Indeed, the use of a simple capacity does not facilitate a perfect adjustment of the real and simulated spectra. The origin of this deviation from the ideal is essentially attributed to irregularities on the surface of the electrode (roughness, presence of impurities, variation in thickness or surface composition of the electrode). 

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