*5.3.3.1 H3PO4*

The use of H3PO4 as an activation agent has been shown to improve the porosity and adsorptive properties of activated carbon due to cross-linking reactions with H3PO4 [48]. The products of the reactions between H3PO4 and organic substances in biomass materials diffuse through the char matrix, which can lead to the enlargement of existing pores or the generation of new pores [49].

#### *5.3.3.2 KOH*

According to Ji et al. [50], the primary reaction between KOH and amorphous carbon at the activation temperature is:

$$4\text{KOH} + \text{C} \rightarrow \text{K}\_2\text{CO}\_3 + \text{K}\_2\text{O} + 2\text{H}\_2.\tag{1}$$

However, during the KOH activation process, various side reactions occur as described by Raymund et al. [51]:

$$2\text{KOH} \rightarrow \text{K}\_2\text{O} + \text{H}\_2\text{O}.\tag{2}$$

$$\text{C} + \text{H}\_2\text{O}^{\text{Steam}} \rightarrow \text{H}\_2 + \text{CO}.$$

$$\text{CO} + \text{H}\_2\text{O} \rightarrow \text{H}\_2 + \text{CO}\_2.$$

$$\text{K}\_2\text{O} + \text{CO}\_2 \rightarrow 2\text{K}\_2\text{CO}\_3.$$

$$\rm K\_2O + H\_2 \to 2K + H\_2O.$$

$$\rm K\_2O + C \to 2K + CO.\tag{3}$$

$$\rm K\_2CO\_3 + 2C \to 2K + 3CO.$$

The steam produced during KOH activation plays a crucial role in the creation of pores by converting amorphous carbon to carbon monoxide. Moreover, the activation process results in the formation of K2CO3, which serves as an activation agent to enhance the porosity and surface area of KOH-treated biomass.

*5.3.3.3 ZnCl2*

The activation of biomass char using ZnCl2 involves inhibiting the formation of tar and other liquids that can clog the pores of the biomass material [47]. This process allows for the free movement of volatiles through open pores and their release from the carbon surface, which leads to an increase in the porosity and surface area of the activated carbon. Moreover, ZnCl2 activation converts the carbon skeleton of the activated material from aliphatic to aromatic form through cellulose dehydration, as demonstrated by Yorgun et al. [52].
