POTENTIAL SUSTAINABLE ENERGY STRATEGIES : We can think of strategies that involve the efficient use of energy as well as the use of renewable and sustainable (carbonneutral) energy sources. For example, an incandescent light bulb is about 2% efficient in its conversion of energy to light.8 2%! To overcome this, a new generation of efficient light sources, such as compact fluorescent bulbs (Figure 1) and LEDs (light emitting diodes), have been developed. A second example comes from the internal combustion engine (ICE), or from the thermodynamic standpoint, heat engines, in general. A Carnot cycle analysis shows that an ICE engine has a theoretical efficiency of about 45−50%, depending on actual operating temperatures. (Recall that the Carnot efficiency η is given by: η = (TH − TC)/TH, where TH and TC are the hot and cold temperatures of the appropriate reservoirs.) In reality, a typical gasoline car engine is about 25% efficient in its energy use.9 Put in different terms, 75% of the energy content (recall ability to do work; strictly speaking, this refers to free energy) of the fuel is wasted! This inefficiency has ushered the development of diesel engines, hybrid systems, and fuel cells for automotive applications. Fuel cells convert chemical energy (often with hydrogen as fuel and oxygen as oxidizer) directly to electricity and thus are not constrained by the Carnot cycle limitations. In principle, a fuel cell could exhibit efficiencies around 90%, or even higher.10 (It is worth mentioning that contrary to popular perceptions, hydrogen is not an energy source, rather, it is an energy storage medium. In addition, any analysis must also account for the way hydrogen is produced.) While the adoption of fuel cells for automotive applications has seen tremendous advances in the recent past, they are still not available in the market. However, recent developments at GM, Toyota, and other automobile manufacturers suggest that they may reach the market in limited quantities by 2015. Turning to renewable and sustainable sources of energy, the ones most frequently mentioned are solar, wind, and tidal. Of these, there is no doubt that the most promising is solar. In fact, the amount of energy from the sun hitting Earth in 1 h is more than the total global consumption of energy in one year! This was evident to Edison who in 1931, in a conversation with Henry Ford and Harvey Firestone, said (in a prophetic way), “I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.” 11 In essence, the sun is the only source that can realistically provide the energy (in a carbonneutral way) that we need now and into the future. Solar energy utilization requires two basic aspects, energy capture/ conversion and energy storage. Solar energy capture can be achieved through the use of photovoltaic devices. This, however, will require significant advances so that the average cost per kWh is price competitive with other sources. Currently, we are only about a factor of 2 to 3 times away from that (depending on regional availability, price of competing energy systems and installation costs). Because of diurnal variations in solar irradiation and fluxes, these technologies can only operate for part of the day. Thus, in order for solar energy to truly represent a primary energy source, we will need to develop high-performance, cost-effective electrical energy storage technologies on a vast scale. While there have been great advances in this field,12 viable systems in terms of performance, costs and lifetime are currently not available. An alternative is to use solar energy to generate fuels (often referred to as “solar fuels”). Of particular interest and importance has been the use of solar energy to achieve the photodecomposition of water into hydrogen and oxygen. This requires the integration of light absorbers to generate and separate electrons, and holes that are subsequently used to drive hydrogen and oxygen evolution at suitable catalytic surfaces that are, in turn, separated by a membrane. (Conceptually, these systems represent fuel cells run in reverse.) In addition, there is significant interest in the reduction of CO2 to generate carbon-based fuels. This approach has the attractive feature of using CO2 as a feed. (However, it should be noted that this would be carbon-neutral because one would simply recycle the CO2.) In either case, the generated fuels would be stored and used when needed, for example, in a fuel cell.