Fuel Cell Technology. SJSU E-10 October 25, 2007 Professor W. Richard Chung Department of Chemical and Materials Engineering San Jose State University. Pictures of Battery. Cathode. Anode. What is A Fuel Cell?.
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Fuel Cell Technology SJSU E-10 October 25, 2007 Professor W. Richard Chung Department of Chemical and Materials Engineering San Jose State University
Pictures of Battery Cathode Anode
What is A Fuel Cell? • A fuel cell is an electrochemical energy conversion device. It produces electricity from external supplies of fuel (anode side) and oxidant (cathode side). • A fuel cell is similar to a battery in that an electrochemical reaction is used to create electric current. The charges can be released through an external circuit via wire connections to anode and cathode plates of the battery or the fuel cell.
Fuel cells are different from batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system.
The major difference between fuel cells and batteries is that batteries carry a limited supply of fuel internally as an electrolytic solution and solid materials (such as the lead acid battery that contains sulfuric acid and lead plates) or as solid dry reactants such as zinc /carbon powders found in a flashlight battery. • Fuel cells have similar reactions; however, the reactants are gases (hydrogen and oxygen) that are combined in a catalytic process. Since the gas reactants can be fed into the fuel cell and constantly replenished, the unit will never run down like a battery.
Fuel cells are named based on the type of electrolyte and materials used. The fuel cell electrolyte is sandwiched between a positive and a negative electrode. • Because individual fuel cells produce low voltages, fuel cells are stacked together to generate the desired output for specific applications. The fuel cell stack is integrated into a fuel cell system with other components, including a fuel reformer, power electronics, and controls. Fuel cell systems convert chemical energy from fossil fuels directly into electricity.
A fuel cell consists of an anode, cathode, and electrolyte A fuel cell is consisted of two electrodes (an anode and a cathode) that sandwich an electrolyte (a specialized material that allows ions to pass but blocks electrons). Fuel cells could power cleaner buses and cars and could provide electricity, heat and hot water to a home, but key engineering and economic obstacles remain which delay widespread adoption of the technology.
The fuel (hydrogen) enters the fuel cell, and this fuel is mixed with air, which causes the fuel to be oxidized. • As the hydrogen enters the fuel cell, it is broken down into protons and electrons. In the case of PEMFC and PAFC, positively charged ions move through the electrolyte across a voltage to produce electric power. (charging mode) • The protons and electrons are then recombined with oxygen to make water, and as this water is removed, more protons are pulled through the electrolyte to continue driving the reaction and resulting in further power production. • In the case of SOFC, it is not protons that move through the electrolyte, but oxygen radicals. In MCFC, carbon dioxide is required to combine with the oxygen and electrons to form carbonate ions, which are transmitted through the electrolyte.
Common Types of Fuel Cell • Metal hydride fuel cell Aqueous alkaline solution (e.g. potassium hydroxide) • Electro-galvanic fuel cell Aqueous alkaline solution (e.g., potassium hydroxide) • Direct formic acid fuel cell (DFAFC) Polymer membrane (ionomer) • Direct borohydride fuel cell Aqueous alkaline solution (e.g., sodium hydroxide) • Direct methanol fuel cell Polymer membrane (ionomer) • Protonic ceramic fuel cell H+-conducting ceramic oxide • Solid oxide fuel cell (SOFC) O2--conducting ceramic oxide (e.g., zirconium dioxide, ZrO2)
Fuel Cells • There are four primary fuel cell technologies. These include phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). The technologies are at varying states of development or commercialization. Fuel cell stacks utilize hydrogen and oxygen as the primary reactants. • However, depending on the type of fuel processor and reformer used, fuel cells can use a number of fuel sources including gasoline, bio-diesel, methane (hydrocarbons), methanol (alcohol), natural gas, “waste material” and solid carbon. • Air, chlorine and chlorine dioxide are reactants.
PEMFC • Proton exchange membrane (PEM) fuel cells work with a polymer electrolyte in the form of a thin, permeable sheet. This membrane is small and light, and it works at low temperatures (about 80 degrees C, or about 175 degrees F). Other electrolytes require temperatures as high as 1,000 degrees C.
Solid Oxide Fuel Cells ( SOFCs ) • A solid oxide fuel cell (SOFC) is a device that converts gaseous fuels (hydrogen, natural gas, gasified coal) via an electrochemical process directly into electricity. Air is supplied to the cathode (air electrode) At the cathode, the O2 molecules are ionized
Several advantages make SOFC more attractive than Hydrogen fuel cells for some applications • SOFCs are over 60% efficient (conversion of fuel to electricity) making them the most efficient fuel cell currently being developed. • The efficiency makes them a good candidate for a distributed power source (generator or power plant). • Because they are solid, SOFCs are quieter than other types, making them good for indoor applications. • The reactions in SOFC require a high temperature. The advantage is that this creates a by-product of heat. • There are no liquids that cause safety and environmental problems. http://www.ztekcorporation.com/sofc_200kw.htm
In SOFCs oxygen ions react with H2 from the fuel The ionized oxygen diffuses across the electrolyte At the anode, the O2- ions react with H (in the fuel). H2 + O2- H2O + 2e- The reaction produces electrons to do work and water.
The anode and cathode are porous while the electrode is dense The anode and cathode must be porous to allow the air and fuel in The electrolyte must be dense to prevent the air and fuel from mixing. F. Tietz, H.-P. Buchkremer, and D. Stöver, “Components manufacturing for solid oxide fuel cells,” Solid State Ionics, 152-153 (2002) 373-381.
Nanomaterials play a key role in the SOFCs Large surface areas (nanopores) are needed for oxygen transport within the air electrode, as well as hydrogen and water transport within the fuel electrode. Nanoporosity (i.e. pores less than 100 nm in size) can be attained by careful processing of nanosized ceramic particles. Desired Cathode Structure The anode would have the same structure, but the gas that would flow through is hydrogen.
Ceramics are needed to allow for the ionic conduction • The most common electrolyte material is ZrO2 (zirconia), which is doped with small amounts of Y2O3 (yttria). This material is known as yttria-stabilized zirconia (YSZ). This is a ceramic material - a compound of a metal with a non-metal. • Ni/YSZ cermet (ceramic-metal composite) is used as the anode material because of its low cost. It is also chemically stable at high temperatures and its thermal expansion coefficient is close to that of the YSZ electrolyte. • The cathode is based on a (La1-ySry) MnO3-d (LSM) perovskite material. J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, and L.J. Gauckler, “Fabrication of thin electrolytes for second-generation solid oxide fuel cells,” Solid State Ionics, 131 (2000) 79-96.
There are a number of materials related research items to improve SOFCs Thinner layers are being designed to minimize ohmic resistance. Manufacturing processes are being optimized for lower temperatures to avoid undesirable mixing. New materials of higher performance are being investigated S.C. Singhal, “Advances in solid oxide fuel cell technology,” Solid State Ionics, 135 (2000) 305-313. A.J. McEvoy, “Thin SOFC electrolytes and their interfaces - a near-term research strategy,” Solid State Ionics, 132 (2000) 159-165.
SOFC -- passing oxygen ions across a solid electrolyte. SOFC has a ceramic cathode that ionizes oxygen. The cathode needs to be porous to allow air in. The oxygen ions diffuse across a solid, dense, ceramic electrolyte. At the anode, the oxygen ions react with hydrogen to form water and electrons. The electrons can not flow through the electrolyte so they leave through the load. SOFC are being investigated for power plants and generators. They have high efficiency, are quiet, and have no liquids that can be unsafe or dangerous to the environment.
Fuel Cell Applications • Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. • A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts, and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability.
Toyota FCHV PEM FC fuel cell vehicle The world's first certified Fuel Cell Boat (HYDRA), Karl-Heine Kanal in Leipzig, Germany Micro-fuel cell developed by Fraunise ISE for use in applications such as cellular phones A hydrogen fuel cell public bus accelerating at traffic lights in Perth, Western Australia
“Warsitz Enterprises' portable fuel cell power unit” A fuel cell powers a laptop computer Tomorrow, hydrogen's use as a fuel for fuel cells will grow dramatically-for transportation, stationary and portable applications. (PlugPower 5-kW fuel cell (large cell), H2ECOnomy 25-W fuel cell (small silver cell), and Avista Labs 30-W fuel cell).
Fuel cells provide heat and power at the Anchorage mail processing center Hydrogen fueling station at California Fuel Cell Partnership General Motor’s EcoFlex car is showing its HydroGen4 at the Frankfurt auto show in fall 2007
The reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
History • The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine".Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The fuel cell he made used similar materials to today's phosphoric-acid fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the 'Grubb-Niedrach fuel cell'. http://en.wikipedia.org/wiki/Fuel_cell
History (cont’d) • GE went on to develop this technology with NASA, leading to it being used on the Gemini space project. This was the first commercial use of a fuel cell. It wasn't until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks).
Fuel Cell Efficiency The efficiency of a fuel is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells.
A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system un-reacted, constituting an additional loss.)
Major Fuel Cells California Energy Commission: http://www.energy.ca.gov/distgen/equipment/fuel_cells/fuel_cells.html
Process • GE's Russell Hodgdon shows a polymer electrolyte in 1965. To speed the reaction a platinum catalyst is used on both sides of the membrane. Hydrogen atoms are stripped of their electrons, or "ionized," at the anode, and the positively charged protons diffuse through one side of the porous membrane and migrate toward the cathode. The electrons pass from the anode to the cathode through an exterior circuit and provide electric power along the way. At the cathode, the electrons, hydrogen protons and oxygen from the air combine to form water. For this fuel cell to work, the proton exchange membrane electrolyte must allow hydrogen protons to pass through but prohibit the passage of electrons and heavier gases. Efficiency for a PEM cell reaches about 40 to 50 percent. An external reformer is required to convert fuels such as methanol or gasoline to hydrogen. Currently, demonstration units of 50 kilowatt (kw) capacity are operating and units producing up to 250 kw are under development.
Dr. Russell Hodgdon of GE demonstrated a polymer electrolyte in 1965
Fuel Cell Lab Exercise Charging Syringe Distilled Water Discharging Mode Discharging Mode
Future Developments • Low temperature fuel cell stacks proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC) and phosphoric acid fuel cell (PAFC) make extensive use of catalysts. Impurities poison or foul the catalysts (reducing activity and efficiency), thus higher catalyst densities are required. • Not all geographic markets are ready for SOFC powered m-CHP appliances. Currently, the regions that the lead the race in distributed generation and deployment of fuel cell m-CHP units are the EU and Japan • Although platinum is seen by some as one of the major "showstoppers" to mass market fuel cell commercialization companies, most predictions of platinum running out and/or platinum prices soaring do not take into account effects of thrifting (reduction in catalyst loading) and recycling.
Thank You! Q&A
Q1. The major difference between fuel cells and batteries is: • Fuel cells generate hydrogen gas, whereas batteries consume stored solid or liquid • Fuel cells generate oxygen gas, whereas batteries consume electricity • Fuel cells consume hydrogen gas, whereas batteries consume stored solid or liquid • Fuel cells consume oxygen gas, whereas batteries consume stored solid or liquid • Fuel cells consume water, whereas batteries consume stored solid or liquid
Q2. Which of the following is the by-product of a fuel cell reaction? • Water • Hydrogen • Oxygen • Electrolyte • All of the above
Q3. Which of the following is not a reactant of a fuel cell? • Bio-diesel • Methane • Methanol • Air • Gasoline
Q4. Which of the following describes the function of a fuel cell in a discharge mode? • Air is supplied to the cathode side • Air is supplied to the anode side • Water is supplied to the cathode side • Water is supplied to the anode side • Hydrogen gas is supplied to the cathode side
Q5. The efficiency of a fuel cell is often determined by the: • Current drawn • Amount of reactants consumed • Voltage supplied • Operating temperature • Amount of oxidants used