Platinum Electrode Surface

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

The surface of the platinum electrode can be studied conveniently in the potential range between 0 and 1.7 V (RHE), where in inert solutions (not containing substances able to be oxidized or reduced), the surface is ideally polarizable. At a more negative potential, cathodic hydrogen evolution starts, whereas at more positive potentials, oxygen is evolved anodicaUy. 

1 Electrochemical Methods for Investigating the Surface of Platinum Electrodes 

When an electrode is ideally polarizable, aU of the current through it is nonfaradaic (charging current) and depends on the properties of the electrode surface  

FIGURE 10.9 Galvanostatic charging curve for a platinized platinum electrode in 0.1 M H2SO4 solution (1) anodic scan, (2) cathodic scan. 

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Walton D J, Phull S S, Chyla A, Lorimer J P, Mason T J, Burke L D, Murphy M, Compton R G, Ekiund J C and Page S D 1995 Sonovoltammetry at platinum electrodes surface phenomena and mass transport processes J. Appl. Electrochem. 25 1083... 

Nakabayashi, S., Sugiyama, N., Yagi, 1. and Uosaki, K. (1996) Dissociative adsorption dynamics of formaldehyde on a platinum electrode surface onedimensional domino Chem. Phys., 205, 269-275. 

Claviher J, Orts JM, Gomez R, Pehn JM, Aldaz A. 1996. Comparison of electrosorption at activated polycrystaUine and Pt(531) kinked platinum electrodes. Surface voltammetry and charge displacement on potentiostatic CO adsorption. J Electroanal Chem 404 281-289. 

BaldeUi S, Markovic NM, Ross PN, Shen YR, Somoijai GA. 1999. Sum frequency generation of CO on (111) and polycrystalUne platinum electrode surfaces Evidence for SFG invisible surface CO. J Phys Chem B 103 8920-8925. 

Fig. 8.10. Potentiodynamic i-v profile for a platinum electrode surface in 0.5 M H2S04 solution, showing the effect of potential cycling on cleanup of the surface. The two lines shown are for the first lower" and the twentieth higher sweep. (Reprinted from H. Angerstein-Kozlowska, in Comprehensive Treatise of Electrochemistry, E. Yeager, J. O M. Bockris, B. E. Conway, and S. Sarangapani, eds., Vol. 9, p. 28, Plenum, 1984.)...

As described in the theoretical section of this book, an electrochemical reaction consists of different steps, and each of these steps (transport and/or charge-transfer steps) can be rate determining. In this section, it is explained why it is not possible to obtain a purely transport-controlled oxidation reaction for sulphite as outlined in the previous section. This is caused by the platinum electrode surface condition that has a large influence on the electron-transfer rate. Therefore, the electrochemical behaviour of the electrode surface itself is described first and limited to observations made during oxidation of dithionite and sulphite. 

The condition of the platinum electrode surface can play an important role in the kinetics of electrochemical reactions, but for the kinetics of dithionite oxidation it was found that the platinum surface condition did not seriously interfere (see section 6.3). 

Immobilise the glucose oxidase onto the platinum electrode surface with the cross-linking method as described previously for the immobilisation of enzymes on screen-printed electrodes [2],... 

When voltammetric measurements are utilized, the half-wave potential (EU2) approximates the condition where [(sol)nH30(aq)] = Ph at the platinum-electrode surface. With the further approximation that the activity of the solvent, [sol]", is unity, Eq. (8.15) is simplified to... 

E. Katz, T. Lotzbeyer, D. Schlereth, W. Schuhmann, and H.-L. Schmidt, Electrocatalytic oxidation of reduced nicotinamide coenzymes at gold and platinum electrode surfaces modified with a monolayer of pyrroloquinoline quinone. Effect of Ca2+ cations, J. Electroanal. Chem. 373, 189-200 (1994). 

Strength on activity coefficients, since the activities, rather than the concentrations of the participating species are the quantities that determine the potential due to the redox couple. These criteria are difficult to meet and have led to the skeptical outlook that most Eh measurements are not amenable to quantitative interpretation (2,42,43). Contamination of platinum electrode surfaces by oxygen in aerated waters (, ), by sulfur in anaerobic waters (4 ) and by iron in surface sediments (45) may cause errors in the measured values. Furthermore, many Eh measurements are thought to be mixed potentials ( ). For these reasons, most Eh measurements have been used only in a qualitative sense. 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

The ultimate reference for pH (and Eh) measurement is the hydrogen electrode, which is formed by bubbling H2(g) over a platinum electrode surface (cf. Parsons 1985, and Chap 2). The electrode half-cell reaction is... 

Energy losses of 5.5 eV electrons on an oxidized platinum electrode surface... 

The technique employs a triple-step potential waveform, typically as illustrated schematically in Fig. 21(a), although others are possible [81]. The potential Ex is such that the platinum electrode surface is rapidly oxidized, any contaminants being desorbed. The electrode is then stepped to a potential E2, which is large and negative enough to reduce the oxidized surface and allow adsorption of the analyte. Stepping the potential to E3 oxidises the species on the electrode surface. The analyte is detected in the chronoam-perometric response of the electrode to this last step. Figure 21(b) illustrates schematically the chronoamperometric behaviour in the absence of adsor-... 

A platinum electrode surface can be contaminated by coatings of oxides, sulfides, and other impurities. 

Approaches to interpretation of results based on quantum mechanical calculations have been described [806]. The interface between a platinum electrode surface and a room temperature ionic liquid (l-butyl-3-methyUmidazolium tetrafluorobo-rate) in the presence of adsorbed CO has been probed with SFG [807]. The position of the electrode potential of zero charge pzc could be estimated and reorientation of adsorbed ions as a function of appUed electrode potential could be deduced. 

FIGURE 11.2. Dependence of the signal for H2O2 and ascorbic acid oxidation on the thickness of the polypyrrole film. Values are normalized by the response at the bare platinum electrode surface. (From Ref. 101). 

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