With a hydrogen fuel cell, the endgame is to make electricity, and to get that electric current from inside the cell to the outside where it can do useful work. The nature of electricity requires that the current flow from inside the cell through a load (the electric motor that powers a vehicle) and return to the cell in order to complete the circuit. Fuel cells require both hydrogen as the fuel and oxygen (to complete the chemical conversion) from the atmosphere to operate.
There are several types of fuel cells and each one operates slightly differently from the other--but in general terms, the principle works like this: Hydrogen atoms enter the cell at the anode (negative electrode), where a catalyst (at the electrolyte) causes a chemical reaction that strips off their electrons (e-). These newly ionized hydrogen atoms (h+) now have a positive electrical charge. The negative electrons (e-) become the DC (direct current) electricity that flows through wires and cables to provide power to the load (motor). If the load requires AC (alternating current) electricity, these negative electrons must first flow through an inverter that converts them from DC to AC. Oxygen enters the cell at the cathode (positive electrode) and in some cell types (such as the Proton Exchange Membrane fuel cell), it combines there with the negative electrons (e-) returning from the load and the hydrogen ions (h+) that have traveled from the anode through the electrolyte. In other types of fuel cells, the oxygen combines with the electrons (e-) and then goes through the electrolyte to the anode and picks up the hydrogen ions (h+).
No matter which side of the cell (anode or cathode) the oxygen and hydrogen combine on, the result is pure water and some latent heat. Because fuel cells chemically convert (instead of burning) hydrogen, there are no byproducts of combustion--such as NOX. Electricity (for work), water and heat (which can be recaptured and used for work) are the only byproducts of fuel cell operation. It's a win-win, and therein lies the clean, pure beauty of fuel cells.
Types of Fuel Cells
Proton Exchange Membrane (PEM)- Efficiency is 40 to 50 percent at about 175 degrees F. Cell output ranges from 50 to 250 kW. The electrolyte is a flexible polymer. Their relatively low operating temperature and flexible electrolyte make them ideal for automotive use.
Alkaline- Operate at about 70 percent efficiency at temperatures between 300 and 400 degrees F. Cell output range is 300W to 5kW and they use a liquid electrolyte of potassium hydroxide (KOH) and water and can potentially leak.
Molten Carbonate- Operate at about 60 to 80 percent efficiency at temperatures of about 1200 degrees F. Cell output is about 200 MW. Carbonate ions from the electrolyte are depleted in the reactions and require the injection of additional carbon dioxide.
Phosphoric Acid- Efficiency ranges from 40 to 80 percent at about 300 to 400 degrees F. Cell output is around 200 kW. The phosphoric acid electrolyte is corrosive to internal cell parts.
Solid Oxide-Operational efficiency is about 60 percent at temperatures of 1800 degrees F. Cell output is up to 100kW. The Solid electrolyte is prone to cracking.
Learn More About Fuel CellsLearn More about Fuel Cell History and What Makes a Fuel Cell a Fuel Cell
Photo Gallery: Hydrogen Fuel Cell Working Diagrams