TD and Kinetics

Fuel Cells Chem 360 Monday November 24 RLG 2008-3 1 Global Warming over Past Millenium Monday November 24 • anthropocene climate regime • 20th century: human population quadrupled, energy consumption increased sixteenfold! • global warming from fossil fuel use became dominant factor in climate change • global mean surface temperature is higher today than it’s been for at least a millennium RLG 2008-3 2 Slide from Marty Hoffert NYU RLG 2008-3 3 RLG 2008-3 4 Efficiencies Monday November 24 • How much energy of the fuel is actually utilized to power a car? ~8% RLG 2008-3 5 Efficiencies • There is nothing like an energy crisis… • BUT: efficiency or entropy “crisis”? • This also is not really a crisis Monday November 24 • Scientific perspective: it’s just an implication of the 2nd Law of TD! RLG 2008-3 6 Fuel Cells and Energy Conversion Monday November 24 • Current energy use: • largely based on fossil fuels • dwindling resources (3-fold increase in oil price in recent years) • adverse environmental impact (air pollution, global warming) • dependence on instable geographic regions RLG 2008-3 7 Fuel Cells and Energy Conversion • Need for a new energy infrastructure – humanity’s No. 1 problem! – has to be solved within ~ 50 years Monday November 24 • sustainable infrastructure: decentralized, renewable, clean • Hydrogen as a fuel? – fuel cells are a key enabling technology of a hydrogen economy RLG 2008-3 8 Fuel Cells and Energy Conversion 2 aspects: 1. The fuel and fuelling infrastructure – hydrogen??? Monday November 24 2. The energy conversion device – fuel cells (factor 2.5 in efficiency) RLG 2008-3 9 Hydrogen as a Fuel? Monday November 24 RLG 2008-3 10 Hydrogen as a Fuel? Monday November 24 • The Hindenburg disaster (1937) • – a problem of hydrogen? – maybe, but… • paint (dark iron oxide and reflective aluminum) caught fire (electrical discharge, thunderstorm) • hydrogen burned quickly and safely • all deaths caused by jumping/falling of people RLG 2008-3 11 Hydrogen as a Fuel? Monday November 24 • hydrogen is less flammable than gasoline • highly volatile, burns upward, quickly • non-toxic RLG 2008-3 12 The Fuel Cell Effect History Monday November 24 • Christian Schönbein (1799-1868): Prof. of Phys. and Chem. in Basel, first observation of fuel cell effect (Dec. 1838, Phil. Mag.) RLG 2008-3 13 The Fuel Cell Effect History Monday November 24 • William Robert Grove (1811 – 1896): Welsh lawyer turned scientist, demonstration of fuel cell (Jan. 1839, Phil. Mag.) RLG 2008-3 14 The Fuel Cell Effect History Monday November 24 • Friedrich Wilhelm Ostwald (1853-1932, Nobel prize 1909): founder of the field of physical chemistry, provided much of the theoretical understanding of how fuel cells operate • “I do not know whether all of us realise fully what an imperfect thing is the most essential source of power which we are using in our highly developed engineering – the steam engine” RLG 2008-3 15 The Fuel Cell Effect History • Grove‘s fuel cell,1839 Monday November 24 RLG 2008-3 16 The Fuel Cell Effect History • his “gas chain” from 1842 Monday November 24 RLG 2008-3 17 Friedrich Wilhelm Ostwald – visionary ideas Monday November 24 energy conversion in combustion engines limited by Carnot efficiency unacceptable levels of atmospheric pollution vs. energy conversion in galvanic elements direct generation of electricity highly efficient no pollution Transition would be technical revolution Prediction: practical realization could take a long time Source: Z. Elektrochemie, Vol. 1, p. 122-125, 1894. RLG 2008-3 18 What is a fuel cell? Monday November 24 • Grove's fuel cell was an alkaline type. Fuel cells are typed by the kind of electrolyte they use. • Fuel cells are of two general types: mobile (portable, as in cars, flashlights, boom boxes, lap tops, and cell phones) stationary (fixed, structural - like a power plant). • Fuel cells produce electrical energy from hydrogen without any moving parts. RLG 2008-3 19 What is a fuel cell? Monday November 24 • Alkaline fuel cells - the first kind - used in Apollo space vehicles. • High-temperature AFCs operate at temperatures between 100°C and 250°C. Newer AFC designs operate at lower temperatures of roughly 23°C to 70°C. • These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. RLG 2008-3 20 Alkaline fuel cells • The anode and cathode are made of porous materials. In such materials, like a sponge, the two gasses soak into the materials. • While the electrolyte is not a good conductor, it produces OH- or hydroxyl ions, which facilitate the oxidation reaction and produce electron flow, which generates an electrical current. Monday November 24 RLG 2008-3 21 Alkaline fuel cells Monday November 24 • Hydroxyl ions meet with hydrogen coming in through the porous anode, to produce water. After the water formation, the electrons are released and flow through the anode. The chemical equation is as follows: Anode + 2H2 +4OH-  Anode + 4e- + 4H2O (oxidation) Cathode + O2 + 4e- +2H2O  Cathode + 4OH- (reduction) AFCs' high performance is due to the rate at which chemical reactions take place in the cell. They have also demonstrated efficiencies near 60 percent in space applications . RLG 2008-3 22 Alkaline fuel cells Monday November 24 • The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2 ). – even the small amount of CO2 in the air can affect this cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. – This purification process is costly. – Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost. RLG 2008-3 23 Alkaline fuel cells • The problem is the reaction: 2KOH + CO2  K 2CO3 + H2O Monday November 24 • Potassium hydroxide is changed to potassium carbonate and water. – air with carbon dioxide cannot be used. – must use pure hydrogen. – need a process to get rid of the carbon dioxide and the water. RLG 2008-3 24 Alkaline fuel cells Monday November 24 • One solution to the water removal is to circulate hydrogen. – But then, what if the gases get mixed (i.e., hydrogen on the cathode and oxygen migrating to the anode)? – This generates a short circuit, because the electrons will not flow through the anode to the cathode, but flow directly to the cathode through the electrolyte. • The next solution is to make the electrolyte solid (to avoid the pumping of the liquid electrolyte with its related problems). RLG 2008-3 25 Alkaline fuel cells Monday November 24 • A variation is to use hydrazine (H2NNH2) or methanol (CH3OH) as fuels. The reaction for methanol is: CH3OH + 6OH-  5H2O + CO2 + 6e• The production of nitrous oxide and carbon dioxide is not exactly a total "clean" environmental optimal situation. RLG 2008-3 26 Alkaline fuel cells Monday November 24 • Why aren’t these being used every where? • Cost!! – is less of a factor for remote locations such as space or under the sea. • To effectively compete in most mainstream commercial markets, these fuel cells will have to become more cost-effective. RLG 2008-3 27 Alkaline fuel cells • AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. – To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours, – has not yet been achieved due to material durability issues. – possibly the most significant obstacle in commercializing this fuel cell technology. • Today production and research is turning away from the alkaline fuel cell to the PEM. RLG 2008-3 28 Proton exchange membrane • Proton exchange membrane - a solid polymer fuel cell using a perfluorosulfonic acid plastic, which allows hydrogen protons to pass through. PEMs operate at temperatures between 50°C and 100°C. RLG 2008-3 29 Proton exchange membrane Monday November 24 • deliver high power density • advantages of low weight and volume, compared to other fuel cells. • use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. • need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. • typically fueled with pure hydrogen supplied from storage tanks or onboard reformers. RLG 2008-3 30 Proton exchange membrane Monday November 24 • operate at relatively low temperatures, around 80°C (176°F). – allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. • requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, – Cost!!! • The platinum catalyst is extremely sensitive to CO poisoning, – Must use an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. – Cost!!! • Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. RLG 2008-3 31 Proton exchange membrane Monday November 24 • Sulfonation is a familiar process used in making detergent, where the chain repels water (hydrophobic) and clings to the dirt. • This is an " acid" fuel cell. – The sulfonated fluoroethylene is called perfluorosulfonic acid. RLG 2008-3 32 Proton exchange membrane Figure 3. Schematic presentation of proton conduction in perfluorosulfonic acid (left) and acid-doped polybenzimidazole (right) membranes. RLG 2008-3 33 Proton exchange membrane • Dupont has the patent on Nafion, and other fluorosulfonate ionomers. – – – – Similar to Teflon tough and chemically resistant. absorb large quantities of water. allow H+ ions to pass through without the electron passing through. • They can be made in thin films. – reduces the distance between the anode and cathode, thus reducing the ohmic resistance, which causes voltage drops. RLG 2008-3 34 Proton exchange membrane The basic structure of Nafion 117 is: CF2=CFOCF2CFOCF2CF2SO3H | CF 3 Monday November 24 Notice in acid fuel cells like this, the acid will corrode the less precious metals. This is a challenge: how to develop electrodes that use cheaper metals. RLG 2008-3 35 Proton exchange membrane Monday November 24 • PEM fuel cells are used primarily for transportation applications and some stationary applications. • Due to their fast startup time, low sensitivity to orientation, and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses. RLG 2008-3 36 Proton exchange membrane Monday November 24 • A significant barrier to using these fuel cells in vehicles is hydrogen storage. – must store the hydrogen onboard as a compressed gas in pressurized tanks. – Hydrogen is a low density fuel – difficult to store enough hydrogen onboard to allow vehicles to travel the same distance as gasoline-powered vehicles • Higher-density liquid fuels such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline can be used for fuel, – vehicles must have an onboard fuel processor to reform the methanol to hydrogen. – increases costs and maintenance requirements. – reformer releases carbon dioxide, but less than that emitted from current gasoline-powered engines. RLG 2008-3 37 Proton exchange membrane Monday November 24 RLG 2008-3 38 RLG 2008-3 39 Phosphoric acid fuel cells • earliest commercial cells developed Monday November 24 – worked best in handling the carbon produced in producing the hydrogen from reforming carbon sources like fossil fuel or methane gas. • PAFC CHP systems operate at around 220°C. • can generate 200,000 watts of electrical power. • have been used for stationary applications with a combined heat and power efficiency of about 80%, • continue to dominate the on-site stationary fuel cell market. RLG 2008-3 40 Phosphoric acid fuel cells • "first generation" of modern fuel cells. • one of the most mature cell types • the first to be used commercially, -over 200 units currently in use. • typically used for stationary power generation • May be used to power large vehicles such as city buses. RLG 2008-3 41 Phosphoric acid fuel cells Monday November 24 • The electrodes are made of carbon paper coated with a finely-dispersed platinum catalyst, which make them expensive to manufacture. – not affected by carbon monoxide impurities in the hydrogen stream. • Phosphoric acid solidifies at a temperature of 40 °C, – makes startup difficult and restrains PAFCs to continuous operation. • at an operating range of 150 to 200 °C, the expelled water can be converted to steam for air and water heating  bonus! RLG 2008-3 42 Phosphoric acid fuel cells Monday November 24 • are 85 percent efficient when used for the cogeneration of electricity and heat, but less efficient at generating electricity alone (37 to 42 percent). – only slightly more efficient than combustion-based power plants, which typically operate at 33 to 35 percent efficiency. • PAFCs are also less powerful than other fuel cells, given the same weight and volume. – As a result, these fuel cells are typically large and heavy. • PAFCs are also expensive. – require an expensive platinum catalyst – costs between $4,000 and $4,500 per kilowatt to operate. RLG 2008-3 43 Direct Methanol Fuel Cells Monday November 24 • DMFCs are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. • do not have many of the fuel storage problems – methanol has a higher energy density than hydrogen (less than gasoline or diesel) – easier to transport and supply to the public using our current infrastructure since it is a liquid • technology is relatively new – research and development are roughly 3-4 years behind that for other fuel cell types. RLG 2008-3 44 Molten carbonate fuel cells • MCFC systems operate at 650°C. • They generate power in the megawatt range. Monday November 24 RLG 2008-3 45 Molten carbonate fuel cells Monday November 24 • currently being developed for natural gas and coalbased power plants for electrical utility, industrial, and military applications. • use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. • Since they operate at extremely high temperatures of 650°C and above, non-precious metals can be used as catalysts at the anode and cathode – reduces costs. RLG 2008-3 46 Molten carbonate fuel cells • Improved efficiency over PAFCs. Monday November 24 – MCFCs can reach efficiencies approaching 60 percent, considerably higher than the 37-42 percent efficiencies of a PAFC. – When the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent. • don't require an external reformer to convert more energy-dense fuels to hydrogen. – Due to the high operating temperatures, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming – reduces cost!! RLG 2008-3 47 Molten carbonate fuel cells Monday November 24 • not prone to carbon monoxide or carbon dioxide "poisoning" – can even use carbon oxides as fuel – Could use gases made from coal. • more resistant to impurities than other fuel cell types – could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen. RLG 2008-3 48 Molten carbonate fuel cells Monday November 24 • The primary disadvantage of current MCFC technology is durability. – The high temperatures and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. – Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance. RLG 2008-3 49 Solid oxide fuel cells • operate between 500°C and 1000°C. • SOFCs generate power from 2,000 watts to several megawatts. Monday November 24 RLG 2008-3 50 Solid oxide fuel cells Monday November 24 • SOFCs use a hot, hard, non-porous ceramic compound as the electrolyte. • Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. • SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity. – In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent. RLG 2008-3 51 Solid oxide fuel cells Monday November 24 • High temperature operation removes the need for precious-metal catalyst, – reducing cost. • It also allows SOFCs to reform fuels internally, – enables the use of a variety of fuels – reduces the cost associated with adding a reformer to the system. • The high temperature makes reforming methane easier for producing the hydrogen needed for the process. RLG 2008-3 52 Solid oxide fuel cells • the most sulfur-resistant fuel cell type; Monday November 24 – can tolerate several orders of magnitude more sulfur than other cell types. • not poisoned by carbon monoxide (CO), which can even be used as fuel. – allows SOFCs to use gases made from coal. RLG 2008-3 53 Solid oxide fuel cells Monday November 24 • High-temperature operation has disadvantages. – results in a slow startup – requires significant thermal shielding to retain heat and protect personnel – acceptable for utility applications but not for transportation and small portable applications. – place stringent durability requirements on materials. – development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology. RLG 2008-3 54 Solid oxide fuel cells Monday November 24 • Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 800°C that have fewer durability problems and cost less. • Lower-temperature SOFCs produce less electrical power, however, and stack materials that will function in this lower temperature range have not been identified. RLG 2008-3 55 Regenerative fuel cells Monday November 24 • Regenerative fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. • However, regenerative fuel cell systems can also use electricity from solar power or some other source to divide the excess water into oxygen and hydrogen fuel by electrolysis. • This is a comparatively young fuel cell technology being developed by NASA and others. RLG 2008-3 56 RLG 2008-3 57 RLG 2008-3 58 RLG 2008-3 59 Problems with fuel cells Monday November 24 The closed circuit in the fuel cell measures the kinetic energy, rather than the potential energy. Voltage drops in the fuel cells are caused by: • Polarization – as protons move from the anode to the cathode, charge builds up. – positive charge repels further proton movement, until the proton is united with oxygen and electrons arriving at the cathode to neutralize this polar charge. RLG 2008-3 60 Problems with fuel cells Voltage drops in the fuel cells are caused by: • Fuel crossing over to the other electrode. – This happens when oxygen shows up at the anode and hydrogen gas shows up at the cathode. – the electron does not flow from anode to the cathode, but ends up flowing from cathode to cathode. – This is called a short circuit. No work is done and all of the energy is entropy. RLG 2008-3 61 Problems with fuel cells Monday November 24 • Voltage drops in the fuel cells are caused by: • Transport holes – when fuel is consumed at the electrode, until more fuel arrives at the site, a reaction "hole" exists where no electrons split off from protons on the anode. – causes a drop in the potential and kinetic energy. – Pressure and concentration of pure gas relieve this problem. RLG 2008-3 62 Problems with fuel cells • Voltage drops in the fuel cells are caused by: • Ohmic losses – This is normal resistance to electron flow. – The more electrons one tries to ram through the circuit, the lower the voltage drops. – Voltage = current * resistance RLG 2008-3 63 Problems with fuel cells Monday November 24 • Needless to say, the entropy results in production of heat. If you were to try to pass a lot of current through a resisting circuit at high voltage, you would get the "flash bulb" effect. The circuit would overload, smoke, and ultimately burn. The solution in fuel cells is to use metals with good conductivity and low resistance. The electrolyte provides resistance, so make it thin and reduce the distance of anode and cathode. RLG 2008-3 64 Problems with fuel cells Monday November 24 • The problems mentioned above with voltage drop are solved by making the cell parts smaller. This is a good thing, according the Robert G. Hockaday, founder of Energy Related Devices, Inc., because fuel cell production can use the similar production schemes used in computer chip manufacture. – Can you see the day when Intel makes fuel cells? • He blew up his mother's stove in the 1970's baking his fuel cell electrodes. http://www.energyrelatedevices.com/ourtechs.htm RLG 2008-3 65 Monday November 24 RLG 2008-3 66