most of the dyeing industries use reactive dyes

From this pie chart, we can know that most of the dyeing industries use reactive dyes. But reactive dyes are not environment-friendly. So, we have to move for other dyes due to others issue.

Figure 4: Worldwide consumption for dyes.

Electrochemical reduction by direct process: Generally, the reduction process needs a mediator, but in electrochemical reduction by the direct process, it is not needed. Actually, it does not need any redox mediator. The leuco dye acts as an electron shuttle between the electrode and the surface of the dye pigment. It is generated first to initiate a small amount of reduction which is proceeded by itself.

Figure 5: Mechanism of the direct electrochemical reduction of indigo radical.

Below the mechanism of direct electrochemical reduction of indigo radical is drawn.

Electrochemical reduction by indirect process: This type of reduction process needs a soluble redox mediator to increase the rate of the electron transferring process. Generally, an electrochemical reduction process is an electron transfer process from cathode to the surface of the microcrystals.

Figure 6: Electrocatalytic hydrogenation of indigo dye.