Submitted by Guided byRiken Mehta Prof. H.P.PATEL

INTRODUCTION

Fuel cell produce electricity by utilizing an electrochemical reaction to combine hydrogen ions with oxygen atoms.

Fuel cell that uses fuel to create chemical reactions that produce either hydrogen- or oxygen- bearing ions at one of the cell’s two electrodes.

The result is an electric current flowing between both electrodes plus the generation of waste heat and water vapor.

CONSTRUCTION

The anode, the negative post of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit.

The cathode, the positive post of the fuel cell, has channels etched into it that distribute the oxygen to the surface of the catalyst. Fig-1

The electrolyte is the proton exchange membrane. only conducts positively charged ions. The membrane blocks electrons.

The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of

platinum.

FUEL CELL PRINCIPLES

The basic principle of a fuel cell is a chemical reaction between hydrogen and oxygen that produces power and heat.

The picture, next to, shows the principle. Hydrogen and oxygen (air) is supplied on each side of a cell.

When hydrogen is lead to the first catalyst layer, the anode, the hydrogen molecules are split into their basic elements, a proton and an electron.

Fig-2

The protons migrate through the electrolyte membrane to the second catalyst layer, the cathode. Here they react with oxygen to form water.

At the same time the electrons are forced to travel around the membrane to the cathode side, because they can not pass the membrane.

This movement of electrons thus creates an electrical current.

WORKING

Fig-3

this Fig. shows the process that goes on inside an individual fuel cell.

The red Hs represent hydrogen molecules (H2) from a hydrogen storage tank. The orange H+ represents a hydrogen ion after its electron is removed.

The yellow e- represents an electron moving through a circuit to do work (like lighting a light bulb or powering a car). The green Os represent an oxygen molecule (O2) from the air, and the blue drops at the end are for pure water- the only byproduct of hydrogen power.

Types of Fuel Cells

Fuel cells are classified primarily by the kind of electrolyte they employ.

• Polymer Electrolyte Membrane (PEM) • Phosphoric Acid • Direct Methanol • Alkaline • Molten Carbonate • Zinc air fuel cell• Protonic ceramic fuel cell • Solid Oxide • Regenerative (Reversible)

POLYMER ELECTROLYTE MEMBRANE

These cells operate at relatively low temperatures (<80°C). They are one of the most promising technologies. This is the type of fuel cell that will end up powering cars, buses and maybe even your house. They typically require very pure hydrogen as the fuel.

Fig-4

PHOSPHORIC ACID

This type of fuel cell is available for commercial use today. More than 200 fuel cell systems have been installed all over the world - in hospitals, hotels, an airport terminal and even in schools. The operating temperature is about 200°C.

Fig-5

DIRECT METHANOL 

• These cells are similar to the Proton Exchange Membrane fuel cells in that they both use a polymer membrane as the electrolyte. However, in the Direct Methanol Fuel Cell, liquid methanol is used as a fuel rather than gaseous hydrogen.

ALKALINE 

• This is one of the oldest designs. It has been used in the space program since the 1960s. AFC may well be one of the first to be commercialised for non-portable applications. This is mainly because they offer the possibility of running without Pt and hence AFCs can be made very cheap.

Fig-6

MOLTEN CARBONATE

• These fuel cells are best suited for large scale power generation, e.g. to replace power stations. They operate at 600°C so the water produced is emitted as very high temperature steam. This steam can be used to power a turbine and so generate even more electricity.

Fig-7

ZINC AIR FUEL CELL

In a typical zinc / air fuel cell, there is a gas diffusion electrode (GDE) a permeable membrane that allows atmospheric oxygen to pass through, a zinc anode separated by electrolyte, and some form of mechanical separators. After the oxygen has converted into the hydroxyl ions and water, the hydroxyl ions will travel through the electrolyte, and reaches the zinc anode. Here, it reacts with the zinc, and forms zinc oxide. This process creates an electric current.

PROTONIC CERAMIC FUEL CELL

• This is similar to the solid oxide fuel cell, but would operate at slightly lower temperature (500-700°C). The electrolyte allows the passage of protons (Hydrogen ions, H+).

• Similar to Solid Oxide Fuel Cells.

• Not very developed, still in research stage.

Solid Oxide 

These fuel cells are best suited for large-scale power generation that could provide electricity for factories or towns. This type of fuel cell operates at very high temperatures (up to 1000°C)

Fig-8

Regenerative (Reversible) Fuel Cells

Regenerative fuel cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser.

The hydrogen and oxygen are fed into the fuel cell which generates electricity and water. The water is then re-circulated back to the solar-powered electrolyser and the process begins again.

These types of fuel cells are currently being researched by NASA and others worldwide.

Technology comparison

ADVANTAGES

Zero Emissions

High efficiency Rapid load-following

Low temperatures Refueling time

Reduced number of energy transformations

DISADVANTAGES

Contaminants sensitivity

High-cost catalyst

New technology

Ice

Lack of infrastructures

FUEL CELL APPLICATIONS

Applications for Portable and Battery Substitution

Applications for Transportation

Applications for Distributed Power

CONCLUSIONS

• Fuel cells are still a few years away from commercialization on a large scale. It is very difficult to tell which fuel and which technology will be predominant in the future. There are some problems to be solved in the SOFC and the DAFC. If these can be solved then these will become the predominant fuel cells being developed in the future.