Fuel cells

Fuel cell history First demonstrated in principle by British Scientist

Sir Willliam Robert Grove in 1839.

Grove’s invention was based on idea of reverse electrolysis.

What is a fuel cell

Creates electricity through electrochemical process

Operates like a battery

Emits heat and water only

Battery A battery is essentially a can full of

chemicals that produce electrons. Chemical reactions that produce electrons are called electrochemical reactions.

Battery has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative.

Working of a battery

Working of Battery

Electrons collect on the negative

terminal of the battery. Normally some type of load like a motor or bulb is connected using wire from positive terminal of the battery to its negative terminal

Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place.

Reactions inside Zinc/carbon battery Take a jar filled with sulfuric acid (H2SO4). Stick a zinc rod in it. The acid molecules break up into three ions: two H+ ions and one SO4-- ion. The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to

become Zn++ ions. The Zn++ ions combine with the SO4-- ion to create ZnSO4, which dissolves

in the acid. The electrons from the zinc atoms combine with the hydrogen ions in the

acid to create H2 molecules (hydrogen gas). We see the hydrogen gas as bubbles forming on the zinc rod.

Now stick a carbon rod and connect a wire between zinc and carbon rods The electrons flow through the wire and combine with hydrogen on the

carbon rod, so hydrogen gas begins bubbling off the carbon rod. There is less heat. You can power a light bulb or similar load using the

electrons flowing through the wire. The electrons go to the trouble to move to the carbon rod because they find

it easier to combine with hydrogen there. There is a characteristic voltage in the cell of 0.76 volts. Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up and the battery "dies."

Fuel Cell And battery A fuel cell is an electrochemical energy

conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.

A battery has all of its chemicals stored inside, and it converts those chemicals into electricity too. This means that a battery eventually "goes dead" and you either throw it away or recharge it.

With a fuel cell, chemicals constantly flow into the cell so it never goes dead -- as long as there is a flow of chemicals into the cell, the electricity flows out of the cell. Most fuel cells

Parts of fuel cells

There are 4 main parts• Anode• Cathode• Catalyst• Proton exchange membrane

The Anode The anode is the negative post of the

fuel cell. It conducts the electrons that are

freed from the hydrogen molecules so that they can be used in an external circuit.

It has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst

The Cathode The cathode is the positive post of the fuel

cell. It has channels etched into it that distribute

the oxygen to the surface of the catalyst. It also conducts the electrons back from the

external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.

The Catalyst The catalyst is a special material that

facilitates the reaction of oxygen and hydrogen.

It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen.

The platinum-coated side of the catalyst faces the PEM.

The Proton Exchange Membrane

The electrolyte is the proton exchange membrane.

This is a specially treated material that only conducts positively charged ions.

The membrane blocks electrons.

Fuel Cell Theory A fuel cell consists of two electrodes - Anode and Cathode.

Hydrogen and Oxygen are fed into the cell.

Catalyst at Anode causes hydrogen atoms to give up electrons leaving positively charged protons.

Oxygen ions at Cathode side attract the hydrogen protons.

Cont….. Protons pass through electrolyte membrane.

Electrons are redirected to Cathode through external circuit.

Thus producing the current - power

Fuel cell working

Graphic showing working of Fuel Cell

                                                                                                                 

http://americanhistory.si.edu/fuelcells/basics.htm

The Chemistry of a Fuel cell Pressurized hydrogen

gas (H2), enters the fuel cell on the anode side

Oxygen gas (O2) is forced through the catalyst on the Cathode side

This reaction in a single fuel cell produces about 0.7 volts

Anode side:2H2 => 4H+ + 4e-

Cathode side:O2 + 4H+ + 4e- => 2H2O

Net reaction:2H2 + O2 => 2H2O

Working Diagram Of Fuel Cell

Figure 3

Types of fuel cellsTemp.°C Application

Alkaline (AFC) 70-90 Space Phosphoric Acid 150-210 Commercially available

(PAFC) Solid Polymer 70-90 Automotive

application (PEMFC) Moltan Carbonate 550-650 Power generation

(MCFC) Solid Oxide 1000-1100 Power generation

(SOFC) Direct Methanol 70-90 Under development

(DMFC)

Alkaline Fuel Cell Used in spacecraft to provide drinking

water and electricity Electrolyte: Aqueous solution of

alkaline potassium Hydroxide Output of 300w -5KW Power generation efficiency of about

70% Too expensive for commercial

applications

Phosphoric Acid Fuel cell

Used in hospitals, nursing homes and for all commercial purposes

Electrolyte: Liquid Phosphoric acid Catalyst: platinum Electrical efficiency of 40% Advantages :using impure hydrogen

as fuel and 85% of the steam can be used for cogeneration

Contd …

Disadvantages: uses expensive platinum as catalyst

Large size and weight Low power and current Existing PAFC’s have outputs of

200kw and 1Mw are being tested

Proton Exchange Membrane Cells Also called as Solid Polymers and used for

quick startup in automobiles, light duty vehicles and potentially to replace rechargeable batteries

Electrolyte :Solid organic polymer poly-perflourosulfonic acid.

Catalyst: Metals (usually platinum) coated on both sides of membrane act as catalyst

Advantages: Use of solid electrolyte reduces corrosion and management problems

Contd..

Disadvantages: Sensitive to fuel impurities

Cell outputs generally range from 50 to 250 kW.

Molten Carbonate Fuel cell Majorly used for electric utility

applications Electrolyte: Liquid solution of lithium,

sodium and/or potassium carbonates. Catalyst: Inexpensive metals can be

used as catalyst other than Platinum Advantages: High operating

temperature allow for inexpensive catalysts

Contd.. Higher efficiency and flexibility to use more

type of fuels like carbon monoxide, propane, marine gas due to high temperatures

Disadvantage: Higher temperature enhances corrosion and breakage of cell components

High fuel to electricity generation of about 60% or 85% with cogeneration.

10 kw’s -1 mw’s MCFCS have been tested

Solid Oxide Fuel Cell

Highly promising fuel cell Used in big, high-power applications

including industrial and large-scale central electricity generating stations

Some developers also see SOFC use in motor vehicles

Power generating efficiencies could reach 60% and 85%

Cont..

Two Variations One type of SOFC uses an array of

meter-long tubes, and other variations include a compressed disc that resembles the top of a soup can

Closer to commercialization Demonstrations of tubular SOFC

technology have produced as much as 220 kW

Direct Methanol Fuel Cells Similar to the PEM cells in that they both

use a polymer membrane as the electrolyte The anode catalyst itself draws the

hydrogen from the liquid methanol, eliminating the need for a fuel reformer.

Efficiency of about 40% typically operate at a temperature between

120-190 degrees F

Cont.. Relatively low range Attractive for tiny to mid-sized

applications, to power cellular phones and laptops

Higher efficiencies are achieved at higher temperatures

Major problem: Fuel crossing over from the anode to the cathode without producing electricity.