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The daunting energy challenges in the 21 st century are a result of over-reliance on limited fossil fuels coupled with everincreasing energy demand. Among the solutions is the development of technologies and infrastructures to help in the smooth transition to alternative and renewable energy sources. Nanotechnology, a combination of chemistry and engineering, is viewed as the new candidate for clean energy applications. It involves the manipulation of nanoscale structures to integrate them into larger material components and systems. In comparison to bulk materials, nanomaterials have high surface areas and are expected to exhibit higher activities. They also demonstrate better stability and durability and are more cost-effective with high recycling potential. This paper reviews selected recent advances in the development of nanotechnology in the emerging solar energy and biofuel fields. Special emphases are given to studies on photovoltaics (including Schottky junction solar cells, organic solar cells, quantum dot-sensitized solar cells, and earth-abundant Cu 2 ZnSnS 4 materials) and artificial photosynthesis. As for the biofuel section, a review on the use of nanotechnology in transesterification, gasification, pyrolysis, and hydrogenation, as well as in the reforming of biomass-derived compounds is given. As these technologies become more mature, efficient, and economical, they could eventually replace traditional fossil fuels.

Increased demand in transportation fuels, environmental concerns, and depletion of fossil fuels require the development of efficient conversion technologies for second-generation biofuels. The main objective of this work is to efficiently liquefy biomass into energy-dense biocrude. A novel two-step process is proposed in which acidic subcritical water followed by alkaline supercritical water media are utilized for the liquefaction. The concept is tested with switchgrass. The first step is carried out at 200 °C in acidic subcritical water to liquefy hemicelluloses to biocrude while avoiding the repolymerization reactions that would otherwise produce char. In the second step, the remaining unliquefied biomass (biomass-H) is subjected to supercritical water at 380 °C with Ca(OH) 2 as catalyst for minimizing the formation of char, enhancing lignin solubilization and therefore increasing liquefaction of the remaining polysaccharides toward biocrude. The proposed two-step liquefaction produces a significantly higher amount of biocrude, as compared to the traditional one-step process. The yield of biocrude from the proposed process is 40% on mass basis and 67% on energy basis of the feedstock biomass.

The challenges with thermal gasification of biomass to produce syngas include high transportation and drying costs of biomass and the need for high-temperature (700À1000 °C) gasifiers and conditioning of the produced gases. Some of these challenges can be addressed by first converting biomass into high-energy-density biochar before transportation and then hydrothermal gasification of biochar at the site of the FischerÀTropsch plant for liquid fuel synthesis. In this work, a highenergy-density (>27 MJ/kg) biochar is first produced via hydrothermal carbonization of switchgrass at 300 °C, and then the biochar is gasified in hydrothermal medium at 400À650 °C. The carbon gasification efficiency in hydrothermal medium is much better than that in the thermal medium. For example, at 550 °C, only 5.9% carbon gasification was achieved in the thermal medium, as compared to 23.8% in hydrothermal medium. The addition of 25 wt % K 2 CO 3 catalyst enhances the hydrothermal gasification to 43.8%. The gasification can be further enhanced if the biochar is passivated with a small amount of Ca(OH) 2 when producing from biomass. With the use of Ca(OH) 2 passivation during hydrothermal carbonization and the use of K 2 CO 3 catalysis during hydrothermal gasification, a high carbon gasification efficiency of 75% is achieved at 600 °C, using short reaction times of 5 and 30 min, respectively. With the recycle of the alkali catalyst and passivation agent, the hydrothermal process can provide an attractive alternative to thermal gasification.

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