Biofuels are fast advancing as alternative sources of renewable energy due to their non-polluting features and cost-competitiveness in comparison to fossil fuels. However, in order to fast-track their development, focus is shifting towards the use of technologies that will maximize their yields. Nanoparticles are gaining increasing interest amongst researchers due to their exquisite properties, which enable them to be applied in diverse fields such as agriculture, electronics, pharmaceuticals and food industry. They are also being explored in biofuels in order to improve the performance of these bioprocesses. This review critically examines the various studies in literature that have explored nanoparticles in biofuel processes such as biohydrogen, biogas, biodiesel and bioethanol production, towards enhancing their process yields. Furthermore, it elucidates the different types of nanomaterials (metallic, nanofibers and nanotubes) that have been used in these bioprocesses. It also evaluates the effects of immobilized nanoparticles on biofuels such as biodiesel, and the ability of nanoparticles to effectively suppress inhibitory compounds under certain conditions. A short section is included to discuss the factors that influence the performance of nanoparticles on biofuels production processes. Finally, the review concludes with suggestions on improvements and possible further research aspects of these bioprocesses using nanoparticles.
The depletion of hydrocarbon fuel reserves, the increase in environmental pollution, and unstable energy prices have triggered a search for clean and sustainable energy resources [1–5]. Biofuel development initiatives are widely being implemented in many countries in order to mitigate these challenges [6–9]. They are classified into two groups i.e. primary and secondary biofuels. The primary biofuels are directly produced from plants, forests, animal waste and crop residues [10–12]. Secondary biofuels are produced from a combination of biomass feedstocks and microorganisms, and are further categorized into three groups i.e. first generation, second generation, and third generation biofuels [13,14]. First generation biofuels are generated from edible crops such as corn, wheat, sugarcane, barley, sorghum, sunflower oil, etc. Second generation biofuels are synthesized from biomass residues such as wheat straw, grass, jatropha, miscanthus, cassava, corn cob, etc [15–17]. These biofuels are considered a viable option because they do not a pose a threat to food security, deforestation, water shortages, and other social challenges [18,19]. Meanwhile, the third generation biofuels use various types of microalgal species [15,20]. Over the last decade, research focusing on third generation biofuels has also intensified because microalgae can thrive under diverse growth conditions and can produce different types of renewable fuels such as biohydrogen, biodiesel, and biogas [21,22]. Biofuels such as biohydrogen, biodiesel, bioethanol and biogas are receiving increasing attention amongst researchers because they are environmentally friendly, use diverse feedstocks that are accessible, non-edible and cheap [23–25]. Biohydrogen is gaining increasing prominence over contending biofuel technologies due to its characteristics which include: (i) high energy content (120 kJ/g) that is approximately 3 times greater than that of fossil fuels i.e. sub-bituminous coal (25–35 kJ/g), gasoline (41.2 kJ/g), and diesel (42.9 kJ/g), (ii) its carbon-sequestration abilities, (iii) its ability to utilize diverse feedstocks including organic effluents, (iv) its ability to use diverse bacteria which are found in various environments, (v) it can be produced at ambient temperature and pressure, thus making it feasible for its largescale production, and (vi) the process offers the simplest way of producing hydrogen energy . The process is highly dependent on various operational conditions such as substrate concentration, pH, temperature and hydraulic retention time .