The Principle of Indirect Electrolysis

Indirect electrochemical processes are hybrids in a certain sense they combine an electrochemical and therefore heterogeneous electron transfer reaction with a homogeneous redox process. The redox reagent undergoes a homogeneous reaction with the substrate and is subsequently regenerated in its active form at the electrode (see Fig. 1). 

The second possibility for electrochemical regeneration of a redox catalyst consist in its continuous internal retransformation into its active form without isolation, i.e., within the reaction vessel ( in-cell method, see Fig. 3) 

The electron transfer agent, often also called mediator, which is inserted between electrode and substrate performs a homogeneous chemical reaction with the 

A third way of indirect electrolysis is given, if the redox agent which is activated electrochemically, is fixed at the electrode surface and is regenerated there continuously after reaction with the substrate. In this case a separation step is unnecessary (see Fig. 4). 

Such an activation of the electrode surface can, on the one hand, take place in situ by the continuous formation of the active redox agent on the electrode surface during the electrolysis. This is valid, for example, for the nickel(III)oxide hydroxide electrode which is spontaneously formed during anodic polarization of a 

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Figure 33. The principle of indirect electrolysis exemplified by a reduction process (see Egs. 114-IJ9 for a description of the species involved).

The principle of indirect electrolysis using a redox couple is shown in Fig. Ic. Metal ions in acidic solutions are oxidized on the anode from their stable oxidatirui state to the higher... 

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