FUEL CELL

A Brief History

Sir William Grove is recognized as the “Father of the Fuel Cell”. In 1839, William Grove, a British jurist and amateur physicist, first discovered the principle of the fuel cell. Grove described the first experiment in which electricity was generated by supplying hydrogen and oxygen to two platinum electrodes immersed in a dilute sulfuric acid electrolyte solution (Fig. 1).

Fig. 1. William Grove's drawing of an experimental "gas battery" from an 1843 letter	Fuel Cell was a term coined in 1889 by Ludwig Mond and Charles Langer. They attempted to build the first practical device using air and industrial coal gas. The first successful cell devices resulted from inventions in 1932 by engineer Francis Bacon. He improved on the expensive platinum catalysts employed by Mond and Langer. Harry Karl Ihrig of Allis-Chalmers Manufacturing Company demonstrated his famous 20-horsepower fuel cell-powered tractor. In the late 1950’s NASA began experimenting with the technology to develop a power source for space travel. Fuel cell technology is also used in NASA’s space shuttle program and is projected for use in the new international space station.
WORKING PRINCIPLE OF THE FUEL CELLS
Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. It works on the same principle as a battery but is continually fed with fuel, usually hydrogen. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in Figure 2.
In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode) compartment; the electrochemical reactions take place at the electrodes to produce an electric current. Fuel cells are silent and reliable (no moving parts) but are expensive to produce. The purpose of a fuel cell is to generate electricity through an alternative, non-polluting electrochemical process. Output voltage is increased by adding fuel cells in series called stacking		Fig. 2. Schematic diagram of the fuel cell
Types of Fuel Cell
Fuel cell systems are usually classified acording to electrolyte used. Alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and proton exchange membrane fuel cell are various types systems that were developed during the past few decades. Alkaline fuel cells They use an aqueous solution of alkaline potassium hydroxide soaked in a matrix as the electrolyte. This is advantageous because the cathode reaction is faster in the alkaline electrolyte, which means higher performance. 2H2 (g) + 4OH^-(l) ®2H2O (l) + 4e^- anode reaction O2 (g) + 2 H2O (l) + 4e^- ®4OH^-(aq) cathode reaction 2H2 (g) + O2 (g) ® 2H2O (l) cell reaction Phosphoric acid fuel cells Phosphoric acid fuel cells (PAFC) are the oldest type whose origins extend back to the creation of the fuel cell concept. Phosphoric acid that is used as electrolyte in these cells is contained in a thin silicon carbide matrix. Many different acids have been used in order to boost performance such as sulfuric and perchloric acids, but when the temperature increases above 150 degrees Celsius, high rates of oxygen reduction are possible which enable phosphoric acid to perform best. The temperature allows the cell to tolerate 1-2% CO and a few PPM of sulfur in the reactant stream which benefits the steam reforming process by reducing the requirement of pure hydrogen input to the anode. H2 (g) ® 2H^+ (aq) + 2e^- anode reaction 1/2O2 (g) + 2H^+ (aq) + 2e^- ® H2O cathode reaction 2H2 (g) + 1/2O2 (g) ® H2O cell reaction Molten Carbonate Fuel Cell The molten carbonate, which operates at 600 degrees Celsius can use CO as a fuel input on the cathode side but needs hydrogen on the anode. Although the high temperature allows carbon in the cell, sulfur can poison the cell in small quantities (~1ppm). Carbonate ions are produced at the cathode and flow across the membrane to react with hydrogen and form two electrons, water and carbon dioxide. H2 (g) + CO2^- 2 ® CO2 (g) + H2O + 2e^- anode reaction 1/2O2 (g) + CO2 (g) + 2e^- ® CO2^- 3 cathode reaction 2H2 (g) + 1/2O2 (g) ® H2O cell reaction Solid Oxide Fuel Cell A solid oxide system usually uses a hard ceramic material of solid zirconium oxide and a small amount of ytrria, instead of a liquid electrolyte, allowing operating temperatures to reach 1,800 degrees F or 1000 degrees C. Power generating efficiencies could reach 60% and 85% with cogeneration and cell output is up to 100 kW. aH2 (g) + bCO (g)+ (a+b) O^2- ® bCO2 (g) + aH2O + 2(a+b) e^- anode reaction ½(a+b) O2 (g) + 2(a+b) e^- ® (a+b) O^2- cathode reaction ½(a+b) O2 (g) + aH2 (g) + bCO (g) ® aH2O(g) + bCO2 (g) cell reaction Proton Exchange Membrane Fuel Cell These cells operate at relatively low temperatures (about 175 degrees F or 80 degrees C), have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications. The proton exchange membrane is a thin plastic sheet that allows hydrogen ions to pass through it. The membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum) that are active catalysts. The electrolyte used is a solid organic polymer poly-perflourosulfonic acid. The solid electrolyte is an advantage because it reduces corrosion and management problems. Hydrogen is fed to the anode side of the fuel cell where the catalyst encourages the hydrogen atoms to release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen has been fed. At the same time, the protons diffuse through the membrane (electrolyte) to the cathode, where the hydrogen atom is recombined and reacted with oxygen to produce water, thus completing the overall process. 2H2 --> 4H+ + 4e^-anode reaction 4e- + 4H+ + O2 --> 2H2O cathode reaction 2H2 + O2 --> 2H2O cell reaction