**11. Challenges**

Algae fuel cells are not without limitations. They need high cost infrastructure and energy for harvesting and growth. Another problem associated with microbial fuel cells is the pH membrane gradient which reduces cell voltage and power output. This problem is caused by acid production at the anode, alkaline production at the cathode and the nonspecific proton exchange through the membrane. The high cost of membrane commonly used in laboratories as a proton-permeable membrane would also limit the applications [52]. In addition, the slow rate of oxygen reduction at cathode electrodes is also a major limiting factor for power generation.

Need for improved engineering on downstream algae biofuel processing from AFC for sustainable energy production is another challenge. It includes effective strategies for nutrient circulation and light exposure in designing photo-bioreactors that are reasonably cheap for large-scale deployment in low-cost systems. The secondary challenge related to this is the extraction of crude algae oil which is mostly addressed from the engineering side. The extraction technologies which are successfully demonstrated are relatively expensive. On the other hand, challenges associated with the management of algae bio-oil conversion to usable liquid fuels need improved catalysts similar to petroleum crude.
