How do hydrogen fueled cars work?

by Gareth Lewis (Class of '05)

Fuel cells have been in the news a lot lately. According to news reports, we may soon be using the technology to generate electrical power for our cars. The technology is appealing to people because it offers a means of making power more efficiently and with less pollution. But how does it do this is the question?
What is a Fuel Cell?

A fuel cell is an electrochemical energy conversion device, or a device that converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity. Another electrochemical device that we are all familiar with is the battery. A battery has all of its chemicals stored inside, and it converts those chemicals into electricity. This means that a battery eventually dies and it needs to be thrown out or recharged. With a fuel cell, chemicals are constantly flowing into the cell so it never dies. Most fuel cells in use today use hydrogen and oxygen as the chemicals. A hydrogen car uses hydrogen as its primary source of power for locomotion. There are two main methods: combustion or fuel cell conversion. Fuel cell conversion is what has been in the news most recently and is expected to power the cars of the future.

In fuel cell conversion, the hydrogen is turned into electricity through fuel cells, which then powers electric motors. The proton exchange membrane fuel cell (PEMFC) is one of the most promising technologies. It transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy. A stream of hydrogen is delivered to the anode side of the membrane-electrode assembly. At the anode side it is catalytically split into protons and electrons. This oxidation half-cell reaction is represented by:

H₂ _ 2H⁺ + 2e^-

The newly formed protons filter through the polymer electrolyte membrane to the cathode side. The electrons travel along a circuit to the cathode side of the membrane-electrode assembly. This creates the current output of the fuel cell. Meanwhile, a stream of oxygen is delivered to the cathode side of the membrane-electrode assembly. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit for form water molecules. This reduction half-cell reaction is represented by:

4H⁺+ 4e^- + O₂ _ 2H₂O

How a fuel cell works: Simplified

At a glance these equations look relatively simple. They are however not easily done and not easily explained, as you just read. This is a simple way of explaining the reactions. On one side (Anode side), hydrogen is channeled through to the anode. An anode is the electrode in a device that electrons flow out of to return to the circuit. At the same time oxygen from the atmosphere is channeled into the other side called the cathode side. A cathode is the electrode at which electrons go into a cell, tube or diode whether driven externally or internally. A reaction in the anode side causes the hydrogen to split into positive ions (H⁺) and negatively charged electrons (e^-). A membrane allows only the positive ions to pass into the cathode side. The electrons travel along an external circuit to the cathode. This external circuit is what creates the electrical current that powers the engine. When the electrons and the positive ions combine with the oxygen it creates water.

Chemistry of a Fuel Cell
Anode side:
2H₂ => 4H⁺ + 4e^-
Cathode side:
O₂ + 4H⁺ + 4e^- => 2H₂O
Net reaction:
2H₂ + O₂ => 2H₂O

References:

• Adams, Victor. "Fuel Cells." Physics World. July 1998. Physics Web. 4 November. 2005.
• Ford, Royal. "Out Of Thin Air." Boston Globe 31 October. 2005, Business. ed. : E6+E4.
• "Fuel Cell." Article. 15 November. 2005. Wikipedia. 4 November. 2005.
• "Fuel Cells Basics, how they work." Basics. The Online Fuel Cell Information Resource. 4 November. 2005.