Hydrogen power, mainly in the form of hydrogen fuel cells has been getting much attention as of late. The promise of a device that uses a fuel that is virtually limitless and non-polluting has captured the interest of world-leading energy companies, the automotive industry, and governments. Fuel cell technology holds so much promise that a notion of a "hydrogen economy" where hydrogen is our primary form of energy has developed.
What is a Fuel Cell?
Simply put, a fuel cell is a device that uses hydrogen fuel and oxygen from the air to produce electricity, heat and water. Fuel cells operate much like batteries converting chemical energy to electrical energy, except that they need a continuous supply of fuel in order to function and they do not store energy.
Where do we Obtain Hydrogen?
Hydrogen is the most abundant element in the universe. However, pure hydrogen is very difficult to come across on our planet. The majority of hydrogen is locked with water or hydrocarbon fuels. As a result, there is ongoing research into the most effective way of obtaining pure hydrogen. Several current production methods include:
Steam reforming: Steam reforming involves burning natural gas in order to obtain hydrogen.
CH4(g) + H2O(g) → CO(g) + 3H2(g) + energy
Carbon Monoxide (Water Shift Gas Reaction): In this process, oxygen from a water molecule is stripped and bonded to carbon monoxide, freeing up hydrogen.
CO(g) + H2O(g) → CO2(g) + H2 + energy
Electrolysis: Hydrogen can be made via the electrolysis of water. However, because the electricity consumed during this process tends to be more valuable than hydrogen, very little hydrogen is produced using this method. About 4% of hydrogen is produced from electrolysis.
2H2O(aq) → 2H2(g) + O2(g)
Anaerobic Digestion: In this process, biogas (composed mainly of methane, carbon dioxide, and water vapour) is produced, which in turn is converted to hydrogen fuel. However, the conversion process does generate some greenhouse gases.
Types of Fuel Cells
Proton Exchange Membrane (PEM) Fuel Cells: PEM fuel cells use a solid polymer membrane (a thin plastic film) as an electrolyte. PEM fuel cells run at fairly low temperatures. Low temperatures allow the cell to have faster start-up times and enable it to quickly adapt to changes in demand for power. PEMs are also compact and produce a powerful electric current for their size. These aspects make PEMs suitable for running automobiles.
Solid Oxide Fuel Cells: Best suited to large central electricity generating stations. Its hard ceramic electrolyte allows the cell to operate at extremely high temperatures using relatively impure fuels. However, there may also be stability and reliability issues.
Alkaline Fuel Cells: Alkaline fuel cells are some of the oldest designs, dating back to the 1960s. It was long used by the National Aeronautics and Space Administration in the Apollo spacecraft and other space shuttles to power electrical components. The cell is susceptible to contamination, having to use pure hydrogen and oxygen. The high cost of alkaline fuel cells has also limited its use.
Regenerative Fuel Cells: A recent development where water is separated into hydrogen and oxygen using a solar-powered electrolyzer. The hydrogen and oxygen are then passed through the fuel cells which generates electricity and water. The water is reused and the process repeats. Thus, regenerative fuel cells are basically self-contained units.
How a PEM Hydrogen Fuel Cell Works
A PEM fuel cell consists of four main parts:
a. Anode: the anode is the negative post that conducts electrons freed from the hydrogen molecules so they may be used in an external circuit.
b. Cathode: the cathode is the positive post that conducts electrons back from the external circuit to the catalyst where the atoms combine to form water.
c. Catalyst: the catalyst is a special material which assists with the reactions of oxygen and hydrogen.
d. Electrolyte: the electrolyte is a proton exchange membrane that conducts positively charged ions and blocks electrons.
The process first begins with hydrogen gas being fed to the anode where a catalyst separates hydrogen into electrons and protons Since the electrons cannot pass through the electrolyte, they are conducted through the anode and through the external circuit, powering the load. The electrons are eventually conducted to the cathode and the remaining hydrogen protons pass through the electrolyte.
Anode Half-reaction: H2(g) → 2H+ + 2e-
On the cathode side the fuel cell, oxygen is split into two atoms. The oxygen atoms have a strong negative charge, causing them to attract the two positive hydrogen ions. Thus, the oxygen atoms and hydrogen ions will combine, along with the electrons that were conducted to the cathode, forming a water molecule. This process repeats, continuing to produce power.
Cathode Half-reaction: 2H+ + ½O2(g) + 2e- → H2O(g)
Total Combined Reaction: H2(g) + ½O2(g→ H2O(g)
Fuel cells are a very promising solution to power our homes, offices and cars. As the technology matures, fuel cells will become more and more important in our daily lives. Combined with other renewable energy sources, like the sun and wind, where energy cannot be constantly produced, hydrogen can become an energy carrier, storing energy until it is needed. Whatever role hydrogen and fuel cells assume in the future, they are certain to be integral in maintaining our energy needs.