Electrode electrolysis with active

Thus far in our discussion of electrolysis, we have encountered only electrodes that were inert they did not undergo reaction but merely served as tlie surface where oxidation and reduction occurred. Several practical applications of electrochemistry, however, are based on active electrodes—electrodes that participate in the electrolysis process. Electroplating, for example, uses electrolysis to deposit a ihin layer of one metal on another metal in order to improve beauty or lesistarKe io corrosion (Figure 20.29 T). We can illustrate the principles of electrolysis with active electrodes by describing how to electroplate nickel on a piece of steel. 

If we look at the overall reaction, it appears as if nothing has been accomplished. During the electrolysis, however, we are transferring Ni atoms from the Ni anode to the steel cathode, plating the steel electrode with a thin layer of nickel atoms. The standard emf for the overall reaction is E°en = E (cathode) — E°enickel atoms from one electrode to the other. In Chapter 23 we will explore further the utility of electrolysis with active electrodes as a means of purifying crude metals. 

In an earlier note (p. 9) we mentioned the occurrence of overvoltage in an electrolytic cell (and overpotentials at single electrodes), which means that often the breakthrough of current requires an Uappl = Eiecomp r] V higher than Ehack calculated by the Nernst equation as this phenomenon is connected with activation energy and/or sluggishness of diffusion we shall treat the subject under the kinetic treatment of the theory of electrolysis (Section 3.2). 

It is very simple to determine the value of = T/Tm for a strongly adsorbed substance in electrolysis with a dropping mercury electrode. If a much smaller amount of substance is sufficient for complete electrode coverage than available in the test solution, then the surface concentration of the surface-active substance T is determined by its diffusion to the electrode. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

The glow electrolysis technique (electrolysis with an anode immersed in the solution and the cathode above the surface) at 600-800 V dc and 300-500 mA converts a solution of starch into ethylene, methane, hydrogen, and both carbon mono- and dioxides.323 Electrochemical methods for converting polysaccharides and other biomass-derived materials have been reviewed briefly by Baizer.324 These methods are mainly oxidations along a potential gradient, which decreases the activation energy of the reactants. Starch in 5 M NaOH solution is oxidized on platinum electrodes to carboxylic acids with an activation energy of about 10 kcal/mol. In acidic media oxidation takes place at C-l followed by decarboxylation and oxidation at the C-2 and C-6 atoms.325... 

Figure 5.2.4 Evolution of the diffusion field during chronoamperometry at an electrode with active and inactive areas on its surface. In this case the electrode is a regular array such that the active areas are of equal size and spacing, but the same principles apply for irregular arrays, (a) Short electrolysis times, (b) intermediate times, (c) long times. Arrows indicate flux lines to the electrode.

Consider an electrode covered with a film that has continuous pores or channels from the solution to the electrode (Figure 14.4.1, process 6). We can ask how the electrolysis of a species in solution at such an electrode differs from that at the bare (unfilmed) electrode. The answer depends upon the extent of coverage of the electrode by the film, the size and distribution of the pores, and the time scale of the experiment. The situation is complicated, because the pores can have different dimensions and degrees of tortuosity, and their distribution within the film may not be uniform. Thus, theoretical treatments of such films often use idealized models. The theory for electrodes of this type is closely related to that for ultramicroelectrode arrays (Section 5.9.3), which, however, often involve a better-defined geometry and uniform distribution of active sites (81, 82). 

SECTION 20.9 An electrolysis reaction, which is carried out in an electrolytic cell, employs an external source of electricity to drive a nonspontaneous electrochemical reaction. The current-carrying medium within an electrolytic ceU may be either a molten salt or an electrolyte solution. The products of electrolysis can generaUy be predicted by comparing the reduction potentials associated with possible oxidation and reduction processes. The electrodes in an electrolytic ceU can be active, meaning that the electrode can be involved in the electrolysis reaction. Active electrodes are important in electroplating and in metaUuigical processes. 

Write appropriate half-equations to describe the following examples of electrolysis. Explain and justify these half-equations by reference to standard electrode potentials, the nature of the electrode (inert or active) and the concentration of the electrolyte (dilute or concentrated) a lithium iodide, Lil(aq), with graphite electrodes... 

Current Efficiency. The theoretical electrochemical equivalents representing the materials produced or consumed in the electrolysis of sodium chloride or potassium chloride brines are given in Table 4. In practice, the yield is ca. 95 - 97% of the theoretical value, owing to side reactions at the electrodes and in the electrolyte. With activated titanium anodes, the yield is largely independent of the distance between the electrodes. 

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