Fuel Cell: Components & Operation

Fuel Cell: Components & Operation

Learning Objectives

• Understand the fundamental operating principle of fuel cells and how they convert chemical energy into electrical energy using hydrogen and oxygen.
• Identify and explain the function of each primary component of a fuel cell, including the anode, cathode, catalyst, and electrolyte membrane.
• Examine the concept of fuel-cell stacks and understand why multiple cells are connected in series to achieve usable voltage levels.

A fuel cell is an electrochemical energy-conversion device that, unlike conventional cells, relies on a continuous supply of fuel—typically hydrogen and oxygen—which it converts into water while generating electricity in the process. Because the only waste product is water, fuel cells are considered to be ‘environmentally friendly’, and a potential source of energy for many applications, including electric vehicles.

As fuel cells do not rely on a chemical electrolyte for their operation, they do not require re-energizing and will provide a continuous supply of electrical energy, as long as they supplied with fuel.

We tend to think of fuel cells as ‘new technology’ but, surprisingly, fuel cells are not new at all. In fact the first fuel cell, credited to the Welsh judge and physicist, Sir William Grove (1811–1896), was developed as long ago as 1839. Grove already knew that, by passing an electric current through water, its molecules could be separated to produce hydrogen and oxygen gas and, so, he thought that it might be somehow possible to reverse that process, and produce electricity and water. He was correct, and the result of his experiments was a primitive device which he called a ‘gas voltaic cell’ – a name which, around 50 years or so later, was changed to ‘fuel cell’.

Fuel Cell Components

There are four basic components to any fuel cell:

Anode 

This is the negative electrode of the cell. It has microscopic channels etched into its surface that distributes the hydrogen gas over the surface of a catalyst. It also conducts the electrons, liberated from the hydrogen molecules, to the external load.

Cathode

This is the positive electrode of the cell. It, too, has microscopic channels etched into its surface which distributes oxygen over the surface of the catalyst. It ‘receives’ electrons back from the external circuit, where they recombine with the hydrogen ions, as well as the oxygen, to form water.

Catalyst

A ‘catalyst’ is defined as any material that accelerates the rate of a chemical reaction without, itself, being consumed by that reaction. In this case, it is a porous material, usually manufactured from tiny particles of platinum deposited onto one side of a layer of carbon, which encourages the reaction of hydrogen and oxygen to take place at the electrodes.

Electrolyte

This term is not used in the same sense as it is in a conventional electrochemical cell. Rather, it’s a membrane or ‘selective barrier’ which, in this case, allows the passage of positively-charged hydrogen ions from the anode to the cathode, while blocking the passage of electrons.

Fuel Cell Operation

Figure 1 shows the basic construction of a fuel cell. Hydrogen molecules (H2) enter the cell, under pressure, at the anode side of the cell. Here, they come into contact with the platinum surface of the catalyst, where they separate into two positively-charged hydrogen ions (H+) and two electrons (e-). The electrons are conducted to the external load via the anode.

At the same time, oxygen molecules (O2) enter at the cathode side of the cell, where they are forced through the catalyst, and separate into two, negatively-charged, oxygen ions (O-). Each of these attracts two positively-charged hydrogen ions through the electrolyte membrane, with which they combine – together with two electrons returning from the external circuit – to form water (H2O).

The potential difference developed between the cathode and anode by this process is relatively low. For a PEMFC-type fuel cell, it is typically less than 1 V so, in order to achieve a useful potential difference, several identical fuel cells must be connected in series to form, not a battery, but what is termed a ‘fuel-cell stack’. Unfortunately, fuel cells tend to have a relatively-high internal resistance and, so, any heavy electrical load can cause a significant internal voltage drop and reduction in terminal voltage.

Types of Fuel Cell

There are various types of fuel cell, and they are all classified according to the type of electrolyte they use. These include:

• Alkaline fuel cell (AFC)
• Direct-methanol fuel cell (DMFC)
• Molten-carbonate fuel cell (MCFC)
• Phosphoric-acid fuel cell (PAFC)
• Polymer-exchange membrane fuel cell (PEMFC)
• Solid-oxide fuel cell (SOFC)

The characteristics of these different types of fuel cell differ in terms of their energy per unit mass, operating temperature, efficiency, etc, making some better for certain applications than for others. For example, the PEMFC type appears to be particularly suited for use by electric vehicles, while most of the others appear more suited as static power supplies – eventually, perhaps, for off-grid residential use.

Fuel Cells Future

At the turn of the century, it was widely-believed that the fuel cell would transform the world: replacing the need to burn fossil fuels, and providing a clean and inexhaustible supply of ‘green’ energy – particularly for electric vehicles. But, unfortunately, this hasn’t proved to be the case.

In fact, it’s likely to be some time before we start to see the widespread use of fuel cells and, presently, the biggest problem is their cost – normally expressed in ‘pounds (or dollars, etc.) per kilowatt hour’.

Although hydrogen is a plentiful (it’s the third most-abundant element on the earth’s surface) and clean fuel, it is considered to be ‘energy neutral’ – that is, it takes almost as much energy to produce it as it delivers!

So, at the moment, fuel cells are prohibitively expensive to manufacture and to operate. In fact, it is claimed that the fuel-cell industry has yet to turn a profit!

Fuel Cell FAQs