How does the fuel cell (part 1)

Introduction

It is possible that lots of news about fuel cells have come to their attention lately. According to various news reports, it is likely that in the near future we make use of new technology to generate electrical energy we consume in our cars and homes. The technology is extremely interesting for us all and in all stages of life because it provides the means to obtain energy more efficiently and less pollution. But how does this happen?

In this article, we review all slightly technologies fuel cell, the existing and emerging. We will detail how it works and discuss its potential applications.

From the technical point of view, a fuel cell is a device for electrochemical energy converter. The fuel cell converts the chemicals hydrogen and oxygen into water, and generates electricity while doing it. The fuel cell should be called a fuel cell, but the term cell eventually prevailed.

Another electrochemical device that we know well is the battery. A battery has all the chemical elements within it, and also convert them into electricity. This means that the battery eventually dies, forcing us to throw it away or recharge it.

In the case of the fuel cell, chemicals constantly flow into the cell. It never dies. Since there are chemicals flowing, the electricity will flow from the fuel cell. Currently, most fuel cells use hydrogen and oxygen.

The fuel cell will compete with many other types of converters of energy, including gas turbines at the power plant generating electricity from your city, the gasoline engine of your car and the battery of your laptop. Combustion engines like the turbine and the gasoline engine burn fuels and use the pressure created by the expansion of the gases to do mechanical work. Batteries convert chemical energy back into electrical energy, when this becomes necessary. Fuel cells make these tasks more efficiently.

The fuel cell provides voltage DC (direct current) that can be used to power motors, lamps and other electrical appliances.

There are many types of fuel cells, each with a chemical process responsible for its operation. They are generally classified by the type of electrolyte they use. Some types of fuel cells work well in stationary generation of electricity. Others may be useful for small portable applications or for powering cars.

The fuel cell membrane for proton exchange (PEMFC) is one of the most promising technologies. This is the type of fuel cell that will end up powering cars, buses, and perhaps even our homes. The PEMFC uses one of the simplest reactions of the fuel cell. First, let's look at what constitutes a fuel cell membrane proton exchange (PEM):

Figure 1 can be seen that there are 4 basic elements of a PEMFC:

• The anode, negative pole of the fuel cell that plays several roles. It conducts the electrons released from hydrogen molecules to be used in the external circuit. It has channels that disperse the hydrogen gas equally over the surface of the catalyst.
• Meanwhile the cathode, the positive pole of the fuel cell, has channels that distribute the oxygen on the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with hydrogen ions and oxygen to form water.
• The electrolyte is the proton exchange membrane. This specially treated material resembling plastic wrap common kitchen and only conducts positively charged ions. The membrane blocks electrons.
• The catalyst is a special material that facilitates the reaction between oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or porous fabric. The catalyst is rough and porous so that the maximum surface area of platinum is exposed to hydrogen and oxygen. The platinum-coated side of the catalyst faces the PEM.

In Figure 2, the pressurized hydrogen gas (H2) entering the fuel cell on the anode side. This gas is forced through the catalyst. The H2 molecule when in contact with the platinum catalyst is divided into 2 H + ions and 2 electrons. The electrons are conducted through the anode, make their way through the external circuit (doing useful work, like putting an engine) and return to the cathode side of the fuel cell.

Meanwhile, on the cathode side of the fuel cell, oxygen gas (O2) is being forced through the catalyst, where it forms 2 oxygen atoms. Each of these atoms has a strong negative charge. This negative charge attracts 2 H + ions across the membrane, which combine with one oxygen atom and the 2 of electrons from external circuit to form a molecule of water (H2O).

This reaction, occurring in a single fuel cell produces only about 0.7 volts. To raise the tension to a reasonable level, many separate fuel cells must be combined to form a battery of fuel cells.

PEMFCs operate at a fairly low temperature (about 176 degrees Fahrenheit or 80 degrees Celsius), meaning that they can warm up quickly and do not require expensive containment structures. Constant improvements in engineering and materials used in these cells have increased the energy density so that devices the size of small suitcases have been able to move cars.

Problems of fuel cells:

In the last section, we saw that a fuel cell uses oxygen and hydrogen to produce electricity. The oxygen required for a fuel cell comes from the air, then in a PEM fuel cell, the common atmospheric air is pumped into the cathode. However, hydrogen is not so readily available. There are some limitations that make it impractical for most applications. For example, there is a hydrogen pipeline coming to your house, nor is it possible to use hydrogen bombs at the gas station.

Hydrogen is difficult to be stored and distributed. It would be much more convenient if fuel cells could use fuels available more easily. This problem is solved by a device called a reformer that converts hydrocarbon fuels into hydrogen or alcohol, which can then be used to power the fuel cell. Unfortunately, reformers are not perfect, generate heat and produce other gases besides hydrogen. They use various devices to purify the hydrogen, but still can not produce pure hydrogen, reducing the efficiency of the fuel cell.

Natural gas, propane and methanol are considered as the fuel most likely application. There are already many homes and buildings serviced by natural gas or propane tanks, so that these fuels are the most likely use for fuel cells at home. Methanol is a liquid fuel with properties similar to gasoline (easy to transport and distribute) and therefore a likely candidate to supply power the cars of fuel cells.

Objectives of fuel cells:

Reducing pollution is one of the priority goals of the fuel cell. In comparison to a car fuel cell with a petrol and a battery is easy to see how fuel cells can improve the efficiency of cars.

As the 3 types of cars have the same components (tires, transmission, etc..), Let us set aside these aspects and compare the efficiencies to the point where mechanical energy is generated. We will start the car fuel cell.

As the fuel cell is powered with pure hydrogen, potentially, it could have an efficiency of 80%. That is, it converts 80% of the energy contained in hydrogen into electrical energy. However, as we saw in the previous section, is difficult to store hydrogen in the car. When we add a reformer to convert methanol to hydrogen, the overall efficiency drops to a range between 30 and 40%.

We still need to convert the electrical energy into mechanical work. This is done by the electric motor and inverter. Reasonable efficiency for the motor / inverter is about 80%. So we have 30 to 40% efficiency in converting methanol into electricity and 80% efficiency in converting electricity into mechanical energy. This gives an overall efficiency between 24% and 32%.

Go to the second part.

Source: Karim Nice, translated by HowStuffWorks Brazil.