How Hydrogen Can be Used

Hydrogen has the potential for providing a clean, efficient fuel source. This raises the question, how is it used? There are two processes that can be used to take hydrogen fuel and create useful work, combustion and in fuel cells. Combustion simply uses the hydrogen in an internal combustion engine or gas turbine instead of a hydrocarbon, like gasoline or jet fuel. A fuel cell is an electrochemical device and is a big reason hydrogen is seen a great potential fuel source.

Combustion

Before going to hydrogen, conventional combustion with hydrocarbons will be discussed to give a comparison.

Under ideal conditions the fuel burns fully on only reacts with the oxygen in the air. Below is the unbalanced chemical equation. This was used instead of an exact equation because the results are the same, just in different quantities.

Hydrocarbon + O2 ==> CO2 + H2O

This is a great, simply process that releases heat which can be converted to work, however this is the ideal case. The actual process is a lot messier because of impurities, incomplete combustion, and because there are other molecules present during combustion, namely nitrogen. The air is mostly nitrogen, which is why some of it reacts.

Hydrocarbon + N2 + O2 ==> CO2 + H2O + CO + N2 + NOx + C + Hydrocarbon

The fuel never completely burns, and other products are released. The products CO and NOx, while being poisonous, contribute to creating smog. The C comes out of the engine as nasty black soot. The unburned hydrocarbons also contribute to health hazards, and represent waste. Even if the fuel does completely burn the result is carbon dioxide and water. CO2 emission is one of the largest concerns with combustion, since it is a greenhouse gas and may be contributing to global warming.

When hydrogen is combusted it does not release carbon dioxide, carbon monoxide, or soot, since there is no carbon in the fuel. This is a great advantage compared to fossil fuels. However, this does not remove the possibility of creating NOx, since nitrogen is still present during combustion. Below is the general chemical reaction.

H2 + N2 + O2 ==> H2O + N2 + NOx

Using hydrogen as fuel cleans up the combustion process a lot, and it does not produce carbon dioxide. However it not have any better efficiency than fossil fuels, about 25 to 30%. In liquid form hydrogen gas 3 times as much energy in combustion as gasoline, but to get the same amount of energy about 3 times the volume is needed. If liquid is going to be used it needs to be extremely cold ( less than -260°C or 435°F). This would need insulation to keep it cold, which would add to the bulk of the fuel tank.

Fuel Cells

A fuel cell uses the chemical energy available, which is usually released in combustion, and converts it directly into electricity. A fuel cell is an electrochemical device, which is similar to a battery. However, it needs a constant supply of fuel to operate. The overall process is fairly simple, but it is not easy to implement.

Two electrodes are separated by an electrolyte (this is an important part and changes the performance a lot depending on what is used). One electrode has hydrogen introduced and the other has an oxygen source, air is usually used. When the hydrogen comes to the surface of the electrolyte, it is changed to an ion, which is allowed to pass through and the electrons are separated and forced to go up the electrode. Hydrogen passes through the electrolyte and combine with oxygen to create the only chemical by-product, water. The electrons that bypassed perform the work needed and are combined with the hydrogen and oxygen to balance the charge. The process also creates some heat, which needs to be removed.

Schematic of a fuel cell with polymer electrolyte membrane (PEM)

One of the important points to notice is that ionic hydrogen is needed. This is accomplished, usually, with a platinum catalyst. This part is essential for the process, otherwise the reaction would happen extremely slow, and adds to the cost of the system.

The efficiency of this device is much higher than that of an internal combustion engine. By comparing the fuel cell to the combustion of hydrogen it is clear the overall chemical process is similar. However, the fuel cell only creates water as an emission. The efficiency of a fuel cell can be twice the value of a typical car engine. Instead of burning the fuel to release heat and trying to get work from the heat, a fuel cell takes the energy from the fuel and converts it directly into electricity.

THere are several types of electrolytes, and they are suited for various applications. The resulting differences tend to be the operating temperature and where it can be used. The table below summarizes the current fuel cells available.

Some of the future systems in development are reversible fuel cells and systems that can use other fuels besides hydrogen directly. The reversible fuel cell can switch from using hydrogen fuel to produce power to using power to electrolyze water. This would be great for storing energy, and would be great as batteries or for cars.