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Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Sources of Energy A favorite form of energy is electricity Where does electricity.

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Presentation on theme: "Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Sources of Energy A favorite form of energy is electricity Where does electricity."— Presentation transcript:

1 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Sources of Energy A favorite form of energy is electricity Where does electricity come from? Even though electricity is a very useful form of energy, there are very few direct sources of electrical energy on earth. (One example is a lightning storm.) Electricity is really a secondary energy source, which we get by converting another type of energy into it. The original source of energy can be Nuclear Wind Sun Hydrodynamic Chemical energy

2 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Current Energy System What’s wrong with our current energy system? The current world energy consumption is 13 TW, or 13 trillion watts. This number is HUGE Niagara Falls worth of energy. Most (85%) of that energy is converted from chemical energy. Most of the chemical energy is coming from the burning of fossil fuels: oil, gas, and coal. Burning fossil fuels generates carbon dioxide, CO 2. Let’s examine a gallon of gasoline. Each gallon of gasoline generates over 1000 gallons of CO 2 gas at atmospheric pressure. That’s more than 17 pounds of CO 2. So every 100 gallons burned creates nearly TON (2000 lbs) of CO 2.

3 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Renewable sources currently make up a small percentage of US energy 3

4 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Carbon Dioxide Emissions Why is CO 2 a problem? All of the fossil fuels that we are burning lead directly to carbon dioxide. Most of this carbon dioxide is being poured directly into the atmosphere, where it adds to the existing CO 2 levels. The CO 2 concentrations in the earth’s atmosphere have already risen by over 25% in the past century. CO 2 is a greenhouse gas. Increasing its concentration in the earth’s atmosphere leads to a warming of the earth. The effect is already being observed, in higher air temperatures, receding glaciers, increase in wildfires, rising sea levels…

5 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Sustainable and Renewable Energy Solutions To ward off significant climate change, changes will need to be made in how we get our energy. Interest in sustainable, renewable, and clean sources of energy. Sustainable energy: one that is not substantially depleted by continued use, does not cause significant pollutant emissions or other environmental problems, doesn’t cause substantial health hazards or social injustices (from Boyle) Renewable energy: energy obtained from the continuous or repetitive currents of energy recurring in the natural environment (Twidell and Weir, 1986) energy flows which are replenished at the same rate as they are “used” (Sorensen, 2000) energy generated from natural resources (Wikipedia)

6 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Sustainable Energy Some ideas that are being pursued include: Wind Solar cells Solar thermal Biofuels Energy from the Ocean in the form of waves or tides Geothermal energy (e.g. Iceland) Clean fuels Some forms of fuel don’t produce as much CO 2. The “gold standard” in a clean fuel is hydrogen (H 2 ). When hydrogen is burned, it produces no CO 2 at all, only water. One of the ways of extracting this chemical energy from hydrogen is to react it with oxygen in a fuel cell.

7 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Transportable Energy Transportable energy In addition to solutions like solar cells, or wind turbines, we need a way to store energy, and to move it around with us. We need portability for many applications (e.g. driving a car) We also need energy on demand (so we can have it even in the dark). That’s why fuels are so desirable—they are a transportable, storable form of energy. One way to store energy is in the form of hydrogen. Remember that hydrogen is considered the cleanest of the “clean” fuels because when it reacts with oxygen, the only product formed is water.

8 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Fuel Cells Getting electrical energy from chemical energy We could just put hydrogen and oxygen together in a reactor, effectively burning the hydrogen, to get energy out. A more efficient way of doing this is to use a fuel cell. A fuel cell directly converts chemical energy (that from reacting H 2 with O 2 ) into electrical energy. It does this by only letting the oxygen contact the hydrogen in a very controlled fashion. A fuel cell is designed like a sandwich Let’s delve further into fuel cells hydrogen flame

9 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Fuel Cells Fuel cells are devices that convert chemical energy into electrical energy Efficiencies are potentially higher than if using the fuels in an engine Current efficiencies are 40-60% Fuel cells are similar to batteries, but with replenishable materials (fuel) Under consideration for both large scale power generation and small scale portable applications (e.g. laptop and cell phone power) Fuel cell Combustion Engine Battery Unlike a battery, a fuel cell is not consumed when it produces electricity Unlike a combustion engine, a fuel cell directly converts chemical energy into electrical, without going via heat and mechanical energy similarities differences Converts fuel into electrical energy Stores energy through an electrochemical system

10 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Pros and Cons of Fuel Cells Advantages: Clean and green Higher potential efficiencies No moving parts Lower particulate emissions Silent, mechanically robust Scaleable, transportable Disadvantages Expensive Fuel availability Power/energy density issues (for portable applications)

11 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Fuel Cell Basics: What is a Fuel Cell? Electrochemical energy conversion device –directly converts chemical energy to electrical energy –fuel can be H 2 or hydrocarbon (e.g. methanol) The “combustion” reaction is split into two electrochemical half reactions O2O2 Fuel cell H2H2 H2OH2O Electricity H 2 +½ O 2  H 2 O

12 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Fuel Cells Some fuel cell reactions: These are basically combustion reactions. As with batteries, the idea is to harness the electrons from the “redox” reaction to produce electrical energy. Fuel cells contain (1) a thin membrane that conducts ions, (2) an anode and (3) a cathode Both the anode and the cathode need to be catalytically active or contain added catalyst in order to break up the H 2 (or hydrocarbon) and O 2. H 2 +½ O 2  H 2 O CH 3 OH + (3/2)O 2 → CO 2 + 2H 2 O Figure from F. Prinz Hydrogen Methanol

13 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Oxidation and Reduction Reactions We are interested in a class of reactions that involve electron transfer at the atomic scale. These are called “Redox” reactions The overall chemical reaction is broken up into two electrochemical half reactions Oxidation: Electrons are lost from a species Reduction: Electrons are gained by a species H 2  2 H e - ½ O H e -  H 2 O Zn  Zn e - 2 e - + Cu 2+  Cu examples

14 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Oxidation and Reduction Reactions In an electrochemical device (such as a fuel cell or battery), the electrochemical half reactions take place at electrodes. The electrode is conductive, i.e. it needs to conduct charge. Anode: the electrode where oxidation takes place Cathode: the electrode where reduction takes place Whether the anode and cathode are positively or negatively charged depends on the type of device. For a galvanic cell (produces electricity), the anode is negative For an electrolytic cell (consumes electricity), the anode is positive

15 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Anode Air inFuel in Flow structure Porous electrode ElectrolyteCathode Schematic of a Fuel Cell The steps in the fuel cell process are: 1.Deliver reactant (transport) 2.Electrochemical reaction at both anode and cathode (requires catalyst too) 3.Movement of ions through the electrolyte; movement of electrons through the external circuit 4.Remove product (transport)

16 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells A fuel cell is just a battery with replenishable electrode materials Anode E 0 = 0 (SHE) Cathode E 0 = V Cell E 0 = V Fuel Cells Compare with a battery (Daniel Cell)

17 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Membranes Properties desired for membrane electrolyte: High ionic conductivity (minimizes resistive losses) Low electronic conductivity (minimizes current losses) Chemical stability in both oxidizing (anode) and reducing (cathode) environments) Low fuel crossover Mechanical strength and manufacturability Categories: liquid, solid, polymeric

18 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells PEMFCPAFCAFCMCFCSOFC Name Proton exchange membrane Polymer electrolyte membrane Phosphoric acid fuel cell Alkaline fuel cell Molten carbonate fuel cell Solid oxide fuel cell ElectrolytePolymer Membrane Liquid H 3 PO 4 (Immobilized) Liquid KOH (Immobilized) Molten Carbonate Ceramic Charge Carrier H+H+ H+H+ OH - CO 3 2- O 2- Operating Temperature 80 0 C200 0 C C650 0 C C CatalystPlatinum NickelPerovskites (Ceramic) Cell Components Carbon- based Stainless- based Ceramic- based Fuel Compatibility H 2, Methanol H2H2 H2H2 H 2, CH 4 H 2, CH 4, CO State-of-Art of Fuel Cells

19 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells The Proton Exchange Membrane (PEMFC) The membrane must conduct protons (hydrogen ions, H + ) but not electrons (otherwise would short circuit) Most common membrane for PEM fuel cells is Nafion (Dupont), a polytetrafluoroethylene (Teflon) with sulfonic acid (SO 3 - H + ) functional groups Fixed charge sites (SO 3 - ) act as temporary centers where the moving ions can be accepted or released. H + ions move by detaching from from sulfonic acid sites and forming hydronium complexes (H 3 O + ) with water Nafion relies on liquid water humidification of the membrane to transport protons Therefore, water management (humidification) systems are necessary. Temperatures must be kept below o C so won’t dry out. Nafion

20 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Phosphoric acid fuel cell (PAFC) First commercial fuel cell type Liquid H 3 PO 4 electrolyte in SiC matrix Operated at o C; expelled water used as steam for space and water heating Used for stationary applications with a combined heat and power efficiency of about 80%; electrical power efficiency alone is ~40% PAFC’s dominate the on-site stationary fuel cell market; 200 kW and 300 kW plants

21 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells The Solid Oxide Fuel Cell (SOFC) O2O2 Anode Porous nickel/YSZ cermet Cathode Porous mixed- conducting oxide Solid ceramic electrolyte YSZ H 2 +O 2-  H 2 O + 2e - H2H2 ½ O 2 + 2e -  O 2- O 2- e-e- Advantages Solid electrolyte Doesn’t need humidification Fuel flexibility (H 2 and simple hydrocarbon) Non-precious metal catalyst (at high T, perovskites are used as catalyst ) Relatively high power density YSZ (yttria-stabilized zirconia) cubic fluorite structure

22 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Y 3+ substitutes for Zr 4+ ions Creates oxygen vacancies! For every 2 Y 3+ ions substituting for Zr 4+ ions there is a O 2- vacancy created (charge neutrality) Oxygen and vacancy exchange Membrane conductivity is proportional to the concentration of O 2- vacancies But too much Y doping leads to vacancy-vacancy interactions which decreases mobility Maximum conductivity occurs at about 8% doping

23 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells The losses in voltage from the ideal output voltage are referred to as ‘‘polarizations’’ Activation Losses Ohmic Losses Concentration Losses Ideal output voltage Current Density vs Voltage: Polarization Curve Energy losses associated with the electrode reactions (Surface reaction kinetics) Energy losses from electronic impedances (electrodes, contacts, and current collectors) and ionic impedances (from electrolyte) Energy losses associated with mass transport limitations (reactants and/or products

24 Stacey Bent, Stanford UniversityLecture on Sustainable Energy; Fuel Cells Fuel Cells Honda FCX Clarity zero-emissions fuel cell vehicle (shown with Jamie Lee Curtis) Vehicle uses a PEM fuel cell stack Will these compete with electric cars?


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