Hydrogen is the smallest, lightest element, whose atoms consist of one proton and one electron. It is also the most abundant element in the universe, fueling the fusion furnaces of most stars, like our own. On Earth, hydrogen is relatively abundant, but not in its pure form, it is bound in other molecules, like water (H2O), natural gas (CH4), organic biomass, and minerals. As a result, energy must be invested to produce pure hydrogen. The energy used determines how environmentally friendly the use of hydrogen as a fuel ultimately becomes.
Hydrogen as an Energy Carrier
Molecular hydrogen, H2, is the most energy dense fuel available, containing approximately three times as much energy per unit weight as gasoline. When reacted with oxygen, hydrogen combusts to produce only water vapor. The process is completely carbon free. This reaction can take place as a simple combustion process, inside modified conventional internal combustion engines or turbine engines, or it can happen chemically within a fuel. The advantage to the former technolgies is that they are already well established. The advantage to fuel cells, the latter technology, is that the process can be more than twice as efficient.
There are currently two commercially established technologies for hydrogen production. The overwhelmingly dominant method is steam reforming of natural gas. This process requires natural gas as a feedstock and mixes that gas with steam at high temperatures, typically exceeding 800°C. The product gases are CO2 and H2 after an intermediate shift reaction to oxidize carbon monoxide, CO, to carbon dioxide, CO2. This process accounts for nearly 90% of the hydrogen currently produced in the United States, the vast majority of which is used in petroleum refining and the production of fertilizers.
The second established technology is electrolysis. This is achieved by placing an electric potential across an electorlyte solution that will conduct electricity. If the potential is high enough, typically at least 1.5 V (1.23 V plus over potential), water will split into hydrogen and oxygen, with hydrogen collecting at the anode and oxygen collecting at the cathode. Optimally, this process can achieve approximately 80% efficiency, which is about the highest limit. Electolyzers of this efficiency are available on the market currently. The limiting factor is a source of electricity cheap enough to make the process economically feasible.