Only minute quantities of hydrogen occur naturally in the atmosphere, but it can be produced efficiently in a process called steam reforming. When methane (CH4), the main constituent of natural gas, is heated up to 700-1100°C in the presence of water (H2O), two reactions occur. First, CH4 and H2O combine to form hydrogen (H2) and carbon monoxide (CO). Then, the CO reacts with more H2O to form CO2 and more H2. The net result is that methane and water are made into hydrogen and carbon dioxide. The second reaction is exothermic (it gives off heat), so that heat can be used for part of the heat needed to drive the first reaction. The rest of the heat comes from burning some of the methane. The CO2 has to be sequestered using one of the methods discussed in Chap. 2.

Large factories for steam reforming already exist in the petroleum industry because hydrogen is needed for taking the sulfur out of gasoline and for producing ammonia and fertilizers. These sources supply the hydrogen for initial tests of hydrogen cars. There are other possible ways to produce hydrogen. The classical way is direct hydrolysis of water. An electrolyte is added to the water to make it conduct electric current. Two electrodes45 in the form of plates are then put into the solution, and a DC voltage is applied between then. Water molecules are bro­ken up into hydrogen and oxygen, and they bubble out separately at each elec­trode. The efficiency of the process depends on the electrolyte and electrode design, but in any case is quite low. If the energy used to produce the electricity is counted, the energy content of the hydrogen is perhaps a third of the energy used to produce it by electrolysis. Even that may be worth it if the original energy source is nonpolluting, such as a fission or fusion power plant. Pricewise, it is estimated that 1 kg of hydrogen costs $7-$9 to make by hydrolysis, compared with $4-$5 by steam reforming. The nuclear industry has plans to demonstrate hydro­gen production at $1.50/kg by 2015.50 One kilogram of hydrogen has about the same energy as 1 gallon of gasoline, but these prices cannot be compared directly with the price of gasoline because cars use and carry hydrogen and gasoline in completely different ways.

There are several new ideas on hydrogen generation without producing CO2 also. One is to use dye-sensitized solar cells plus a catalyst to get hydrogen directly from sunlight. Another is to perform artificial photosynthesis by growing algae. The most advanced is a system to run a hydrogen fuel cell backwards, using solar electricity to make hydrogen rather than using hydrogen to make electricity. In the Compagnie Europeenne des Technologies de l’Hydrogene (CETH) in France, a machine called the GenHy5000 Water Electrolyzer has successfully done this [32]. About the size of a refrigerator, the hydrolyzer produces H2 at the rate of 5,000 L/h at atmospheric pressure using electricity with 62% efficiency. It has run continu­ously for 5,000 h, but efficiency will drop with intermittent use. When powered by rooftop solar cells, the hydrogen can be generated and stored at 10-atm pressure for later use. For automobile refueling stations, higher pressures will be required. The hydrogen can be allowed to build up pressure as it is generated. A smaller model has run at 30 atm for a total of 10,000 h. Its other data are: voltage 1.7 V, current 1 A/ cm2, temperature 90°C, and power consumption 4 kWh/m3 of H2. The noble-metal content in the catalysts is 1.5-3 mg/cm2, and the hydrogen is 99.99% pure. Though this is a fuel cell run backwards, years of research have yielded valuable data on fuel cells in general: what materials to use, how to make them, how long they will last, and how they can be contaminated. In particular, it was found that the catalyst layers are best deposited directly on the membrane, and a method was devised to do this using frequency-modulated electric pulses.51

In spite of the problems with the fuel cell, prototype hydrogen cars costing millions of dollars have been made. The Honda FCX, for instance, is sleek, normal­looking passenger car with a 100-kW fuel cell stack weighing 148 lbs (67 kg) and occupying 57 L (2 cu feet). Four kilos of hydrogen are stored at 5,000 psi in a 170-L (6 cu feet) tank. A matching 100-kW (134 HP) electric motor runs on a lithium-ion battery charged by the fuel cell. The relation between kilowatts and horsepower (HP) will be found in Box 3.7. The mileage is stated to be 60 miles/kg of H2, and the range is 240 miles (386 km). The car could be leased at $600/month, but full production is not expected before 2020.

Box 3.7 Kilowatts and Horsepower

Kilowatts and horsepower are both units of energy relevant to electric cars. A kilowatt (kW) is approximately four-thirds of a horsepower (HP), and 1 HP is about three-fourths of a kW. The exact values are as follows:

1 kW = 1.341 HP 1 HP = 0.746 kW 1 W-hr=4.8 HP-sec 50 W-hrs = 241 HP-sec