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                                                                                                                   Graphic showing working of Fuel Cell http://americanhistory.si.edu/fuelcells/basics.htm

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

Working Diagram Of Fuel Cell Figure 3

Types of fuel cells Temp.°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 Closer to commercialization 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.