Anode adsorption potential

Figure 2.19 Linear sweep voltammograms of a platinum electrode immersed in N -saturated 0.5 M H2SO t showing the anodic stripping of adsorbed CO. The CO was adsorbed from the CO-saturated electrolyte for 10 minutes at the designated potential. The scan rate was 1 mV s The adsorption potential was (a) 0.05 V and (b) 0.4 V vs. NHE. Note the different electrode potential scales for the two plots. From Kunimatsu et at. (1986).

The primaiy emphasis in this review article is to showcase the use of LEISS to examine the outermost layers of Pt-Co alloys in order to correlate interfacial composition with electrocatalytic reactivity towards oxygen reduction. In some instances, it is desirable to compare the properties of the outermost layer with those of the (near-surface) bulk an example is when it becomes imperative to explain the unique stability the alloyed Co under anodic-oxidation potentials. In such cases. X-ray photoelectron spectroscopy and temperature-programmed desorption may be employed since both methods are also able to generate information on the electronic (binding-energy shift measurements by XPS) and thermochemical (adsorption enthalpy determinations by TPD) properties at the sub-surface. However, an in-depth discourse on these and related aspects was not intended to be part of this review article. 

The steady-state surface coverage by the carbon monoxide residue can be studied by anodic stripping voltammetry. By this technique, it is possible to separate the adsorption residue contribution from the bulk electrooxidation process. The micro-flux cell is adapted with a big flask containing the supporting electrolyte, which is used to wash the cell until there is no trace of methanol in solution. The current vs. potential profile mn from the adsorption potential upward is the tripping profile for the oxidation of the adsorbed residue. An example is presented in Figure 2.4. 

In all three MEAs the rate of methanol oxidation was facilitated by the platinum-ruthenium unsupported catalyst, which in the presence of CO as a byproduct of the reaction, exhibit an electrochemical activity higher than pure Pt. However, compared to Pt supported and unsupported catalysts, the electrochemically active surface area of PtRu alloys cannot be determined by hydrogen adsorption using cyclic voltammetiy due to the overlap of hydrogen and oxygen adsorption potentials, and the tendency for hydrogen to absorb in the ruthenium lattice [xvi]. However, under the same operation conditions, cyclic voltammetry can be used for qualitative estimation of the similarity in the PtRu anode layer properties. 

Figure 16. Volcano plots for oxygen reduction in 85% phosphoric acid at 25°C (log/ at 800mV/HE from Ref. 116). Relative AGads( ) is incipient -O or -OH adsorption potential from anodic cyclic voltammetry (Ref. 116).

Peak potential (in voltammetry) — It is the potential at which the maximum current appears in -> linear scan voltammetry (LSV) and several other techniques peak height). It is one of diagnostic criteria for the estimation of electrode kinetics. If the reaction is simple, fast, and - reversible the peak potential is independent of the scan rate, or frequency, or pulse duration. The condition is that the electrode reaction is not complicated by the -> adsorption, the -> amalgam formation, the precipitation of solid phase, the gas evolution, or the coupled chemical reactions. In LSV and cyclic voltammetry (CV) of a simple, reversible, semi-infinite planar diffusion-controlled reaction Oxaq h- e Redaq the cathodic and anodic peak potentials are p,c = ... 

Similarly for each of the other proteins, cyclic voltammograms were recorded after each addition of an aliquot of protein to the buffer solution in the electrochemical cell. The surface charge density was measured over the region which normally corresponds to a monolayer of OH during oxide formation in aqueous solutions, to the anodic end potential of 0.4 V (vs. SCE). Since this is also the rest potential of lysozyme on platinum, the surface charge densities measured to this anodic end potential may reflect the adsorption of a monolayer of protein and allow correlation with results from other experimental techniques. The calculated values of 7"for the three proteins considered in these studies are shown in Fig. 5. Plateau values in surface charge densities can be seen at low bulk concentrations... 

Figure 5. Adsorption isotherms for proteins in phosphate buffer (pH 7.0,299 K) with an anodic end potential of 0.4 V (vs. SCE) , )9-lactoglobulin A a, lysozyme , ribonuclease A. (Reprinted from Roscoe and Fuller, with permission.)...

Surface concentrations of 1.3 mg m" for the plateau value at low bulk protein concentrations and 1.8 mg m at the higher concentrations, calculated from the surface charge density resulting from adsorption of )ff-lactoglobulin at 299 K and pH 7.0 on a platinum electrode with an anodic end potential of 0.4 V, agree very well with values in the literature... 

The surface adsorption behavior of lysozyme with an anodic end potential of 1.0 V over the temperature range 273-343 K (Fig. 21) showed a steady increase of surface charge density with temperature. Similar results were obtained with ribonuclease A, as shown in Fig. 22. Negligible... 

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

An important feature of such films is their low ionic conductivity that restricts cation transport through the film substance. Electronic semiconduction, however, permits other electrode processes (oxidation of H2O to O2) to take place at the surface without further significant film growth. At elevated anodic potentials adsorption and entry of anions, particularly chloride ions, may lead to instability and breakdown of these protective films (Sections 1.5 and 1.6). 

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