As the world’s population increases, energy consumption will increase with it. To meet this demand there is a need to develop alternate energy sources. Although renewable and nuclear energy are growing by 2.5% a year, fossil fuels are still projected to make up at least 80% of the global energy supply in next 20 years. There are uncertainties in estimates of fossil fuel reserves, but the potential for debilitating and possibly devastating environmental impacts arising from the combustion of fossil fuels creates a second imperative for developing alternate energy sources. Of the available renewable sources, the sun is, by far, the largest in availability and is the most likely long-term solution as the dominant primary, carbon-neutral energy source. Harvesting energy directly from sunlight by photovoltaics (PVs) is a very attractive and desirable way to solve the rising energy demand. The solar energy harvested in PV devices can be stored in external batteries. The most efficient and lightweight batteries are typically Li+
ion batteries, but current state-of-the-art Li+
ion battery technology is unable to meet global energy storage demands, with a storage capability of only ~1.03 A-h/g, along with limited stability of many charge/discharge cycles.
Artificial photosynthesis offers a way to overcome these challenges. The goal of artificial photosynthesis is to make high energy chemical fuels from solar energy. The targets are hydrogen from water splitting or CO2
reduction to carbon-based fuels. Solar water splitting provides a sustainable and environmentally benign route for the production of H2
, which can be used as a clean fuel. A promising approach is available by using photoelectrochemistry. The results of a recent analysis suggest that photoelectrochemical cells (PEC) operating with a solar efficiency of 10% could replace fossil fuels as the world’s primary energy source.
PECs are actually solar cells that produce electrical energy or hydrogen in a process similar to the electrolysis of water. Tandem photoelectrochemical systems that use sunlight to generate hydrogen from water at efficiencies greater than 12% have been known for over 15 years. Unfortunately, the high cost in materials and manufacturing, as well as the long term stability of some of these systems, makes large scale implementation of these particular cells unlikely. Bias-free visible light driven water-splitting DS-PECs have become an interesting due to their cost-effective, and promising method. In these devices, low cost dye sensitizers are used together with molecular catalysts on TiO2 as the photoanodes, and Pt as the cathode. In this system, the dye sensitizer will generate electron by light absorption and at the same time facilitate the oxidation of water by molecular catalysts producing O2, as it works very well in dye-sensitized solar cells (DSCs). The photogenerated electrons migrate through an external circuit to the counter electrode, where proton reductions occur to generate H2.