Have you ever wondered how a plant uses water and carbon dioxide to create glucose and oxygen? This too is an endothermic reaction, an energy producing reaction run in reverse. Normally we would think of using glucose as a fuel, through oxidation we could produce carbon dioxide, water and energy - In fact this is what OUR bodies do to provide us with the energy we need for maintaining all of our bodily functions including THINKING!
Glucose (C6H12O6) + Oxygen (O2) = Water (H20) + Carbon Dioxide (CO2) + ENERGY
To run the reaction in reverse, the plant utilizes a catalyst - CHLOROPHYLL - and the energy from the SUN to aid in the decomposition of water. While the chlorophyllic reaction does produce diatomic oxygen gas, it does not produce the hydrogen in a gaseous form. The hydrogen released from the water is used for the formation of glucose. Could we use such a catalyst for converting Water and Sunlight into Fuel?
Scientists often use Nature as a model for the development of new compounds. One such development, which has been studied extensively in this regard, is a molecule known as Rubippy. The structure of Rubippy is shown below. It is similar in structure to the chlorophyll molecule having a metal center (in chlorophyll it's a magnesium ion, in rubippy it's a ruthenium ion) and an attached system of organic rings (in chlorophyll its a porphyrin derivative, in rubippy its a pyridine derivative). Acting as "relay" channel for the transfer of electrons, Rubippy has shown some potential to do just that - convert water and sunlight into a clean, seemingly inexhaustible, source of energy. However, while rubippy has shown promise in this regard, it is not a commercially viable enterprise because of it's high cost, instability, and low efficiency. If Scientists were able to get Rubippy to work, or created a viable alternative, what would we do about the explosion potential of using Hydrogen Fuel?
Good question! Would you believe that it is possible to do the combustion of hydrogen without letting the oxygen and hydrogen come in contact? This can happen in a FUEL CELL. A fuel cell is like a battery - It utilizes a chemical reaction to produce electricity. A drawing of a hydrogen-oxygen fuel cell is shown below:

The kind of fuel cell shown here are routinely used in the space program. If this technology ever becomes viably available to the common person, the estimated cost of a fuel-cell hydrogen powered car would be less than half that of your current gas-mobile. In addition, it would be simpler, require less maintenance, and be environmentally friendly!
Water is Weird !?
Chemically speaking, water is very weird.