**8. Influence of fouling**

#### **8.1. Membrane fouling**

The most important component of AFC is a membrane which acts as a physical separator and ion selective in passing protons. Moreover, it also hinders the passage of other materials and prevents the crossover of oxygen from the cathode to the anode. Microbes grow on the surface of the membrane causing membrane fouling when AFC is operated for a long term. Membrane fouling occurs when organic foulants such as extracellular polymeric substances aggregate on the surface of the membrane. The negatively charged sulfonate groups in the membrane are prone to this type of fouling especially at low pH [41]. This bond eventually contributes to the formation of a strong biofouling layer on the membrane.

#### **8.2. Biofouling**

Oxygen reduction reaction occurs on the exposed area of catalyst and its framework present in three-phase boundaries. Over potential of this is efficiently reduced by commonly used expensive catalyst. The latest development in low cost catalyst like carbon based cathode delivers equivalent performance due to abundant pores and larger specific area. However, the main drawbacks of this porous structure are their low resistance to biological fouling. Therefore, ionic membranes and separators are used in AFC to reduce this effect on proton exchange layers.

Biofouling is caused by the bacteria attached to the surface of catalyst layer that releases extracellular polymeric substances. Biofouling on catalyst layer is similar to biofilm on membranes and separators. It is a thick layer developed on carbon based cathode that increases the diffusion resistance responsible for the declined performance during the long term. Further, it also decreases the activity of dopants on the surface of catalyst layer by the combined effect of biofilms with salt deposition. This was evident from the research of Zhang et al. [42] in which improved power density of cathode increase up to 29% was observed after removing the fouling by weak hydrochloric acid. But there are not clear and sufficient demonstrations regarding the individual effect of biofouling located on the surface of the catalyst layer and inside the layer.
