Research

Life on Earth has naturally evolved through a reliance on photosynthesis, which utilizes the stream of sunlight energy. Originally, the energy contained in most traditional fossil fuels came from sunlight. In fact, the energy the Earth receives from the Sun in one hour exceeds the amount of energy the entire globe consumes in the course of 1 year. In 1 hour, approximately 1.2 × 10⁵ TW of power reaches the Earth’s surface. Solar conversion systems with 10% efficiency covering about 0.16% of the land on Earth could generate about 20 TW, which is equal to almost twice the world’s fossil fuel consumption rate. Therefore, the technology to harness the Sun’s energy and make it usable is of critical importance for all humankind.

There are many ways to convert sunlight into different useful outputs. Although other forms of transmissible energy, such as electricity, are appealing and highly useful, there are some associated technical difficulties, such as complex requirements for storage and transport. Solar energy may be stored as potential energy (e.g., pumped hydroelectric power and compressed air), kinetic energy (e.g., flywheels), electric charge (e.g., batteries and supercapacitors), and thermal energy (e.g., seasonal thermal energy storage). Each of these methods has a high cost of deployment, short time span, and/or low energy density. By contrast, energy stored by chemical means has tremendous advantages in terms of storage time and facile mobility.

One of the most important chemical fuels is hydrogen. Hydrogen is an excellent candidate as a substantial energy carrier due to its high energy density by mass (143 MJ/kg) and environmental friendliness. The process of obtaining energy from hydrogen is clean; that is, H₂ enters into an oxidation reaction to produce water without emitting CO₂. This reaction can be described as follows:

H₂+ 1⁄2 O₂ → H₂O

This reaction can be converted to electricity using fuel cells. In contrast to low-efficiency liquid-based fossil fuel engines, the efficiency of fuel cells can reach up to ~70%. In addition, internal combustion engines and turbines can also run on hydrogen to provide motive power.

Artificial conversion of solar energy into chemical fuels is one approach to overcoming the forecasted energy famine. The main challenge facing the development of artificial photosynthesis technology is to achieve highly efficient and stable solar-to-fuel conversion. A critical component of this technology is the exploitation of various semiconductor photocatalysts that can lead to high efficiency and stable solar-to-fuel conversion. Metal-nitride nanowire structures have recently emerged as a material of choice for solar-to-fuel conversion. Compared to conventional oxide-based photoelectrodes, metal-nitride nanowires offer several distinct advantages, including a tunable energy bandgap across nearly the entire solar spectrum, conduction and valence band edges that can straddle water redox and hydrocarbon potentials under deep visible and near-infrared light irradiation, and high-efficiency charge carrier separation and extraction. Such nanowire arrays also exhibit a high level of stability in aqueous solution. Moreover, multiband InGaN nanowire photocatalysts and photoelectrodes have been developed to effectively harvest a large part of the solar spectrum.

In addition, using solar energy to synthesize CO₂ into chemical fuels is a promising technique for overcoming environmental problems and possibly addressing the significant challenges of future energy demand and storage. In this process, CO₂ will be chemically transformed into hydrocarbons, such as CH₄, CHOOH, CH₃OH and/or other high energy density carbon-based fuels. These carbon-based fuels can be readily stored in the form of liquid or solid. Unlike H₂ fuel, carbon-based fuels do not require a complete overhaul in the existence infrastructure. This allows for gradual depart from fossil flues and hence gradual change in infrastructure. Moreover, hydrogen is relatively unsuitable for transportation due to its gaseous state at room temperature which results in low volumetric energy density. On the other hand, the energy density contained in the carbon-based fuels is relatively high compared to H₂. Although the reduction of CO₂ can be accomplished using electricity in electrochemical cells, there is no overall energy gain achieved through this process. Biological photosynthesis is an inspiring process to make fuels from CO₂ using sunlight.