PE (polymer electrolyte) FCs utilize a polymeric electrolyte. Nafion TM , a perfluorinated polymer with sidechains terminating in sulfonic acid moieties, and its close perfluorosulfonic acid (PFSA) relatives, are currently the state-of-the-art in membranes for PEFCs, satisfying an array of requirements for effective, long-term use in fuel cells. They combine well the important requirements for a membrane in a PEFC, namely: high protonic conductivity, high chemical stability under typical operating conditions, and low gas permeabilities. Typically, thickness of PFSA membranes for PEFCs range between 50 and 175 m. The main source of PFSA membranes is DuPont (USA), where these membranes were invented in the 1960’s and made into a commercial product for the chlor-alkali industry. Other sources of developmental PFSA membranes have been Dow Chemical (USA), Asahi Glass (Japan), and Asahi Chemicals (Japan).
The most important property of ionomeric membranes employed in polymer electrolyte fuel cells is the high protonic conductivity they provide at the current densities typically required in PEFCs. The specific conductivity of fully hydrated PFSA (immersed) membranes is about 0.1 S/cm at room temperature, and about 0.15 S/cm at the typical cell operation temperature of 80 ºC. These high protonic conductivities provide the basis for the high power densities achievable in PEFCs. The dependence of proton mobility in PFSA membranes on water content is, however, quite critical, and demands effective cell and stack design to maintain a high level of water through the thickness of the membrane for the complete range of dynamic operation.
The number of water molecules carried through the membrane per proton is a central factor in determinating the water profiles in the membrane of an operating PEFC. There is an important difference between the electroosmotic drag coefficient, ( ), a characteristic of an ionomeric membrane with fixed water content and flat water profile, and the net water flux through an operating fuel cell. The latter is the resultant of several water transport modes in the cell. For fully hydrated and (immersed) Nafion 1100 membranes, a drag coefficient of 2.5 H 2 O/SO 3 H is measured, whereas for a membrane equilibrated with vapor-phase water the drag coefficient is close to 1.0 H 2 O/H + over a wide range of water contents. The lack of dependence of the drag coefficient on membrane nanostructure suggests that the drag coefficient is determined by the basic elements of the proton transport process; I.e.; via the hydronium ion or complex..
Fuel Cell Technology
Topics 1. A Very Brief History 2. Electrolysis 3. Fuel Cell Basics - Electrolysis in Reverse - Thermodynamics - Components - Putting It Together 4. Types of Fuel Cells - Alkali - Molten Carbonate - Phosphoric Acid - Proton Exchange Membrane - Solid Oxide 5. Benefits 6. Current Initiatives - Automotive Industry - Stationary Power Supply Units - Residential Power Units 7. Future
A Very Brief History Considered a curiosity in the 1800’s. The first fuel cell was built in 1839 by Sir William Grove, a lawyer and gentleman scientist. Serious interest in the fuel cell as a practical generator did not begin until the 1960s, when the U.S. space program chose fuel cells over riskier nuclear power and more expensive solar energy. Fuel cells furnished power for the Gemini and Apollo spacecraft, and still provide electricity and water for the space shuttle.(1)
s this have to do with fuel cells?” By providing energy from a battery, water (H2O) can be dissociated into the diatomic molecules of hydrogen (H2) and oxygen (O2). Figure 1
Basicslectrolysis in reverse.” The familiar process of electrolysis requires work to proceed, if the process is put in reverse, it should be able to do work for us spontaneously. The most basic “black box” representation of a fuel cell in action is shown work below: Figure 2 O2 fuel H2O H2 cell heat
uel Cell Basicshermodynamics H2(g) + ½O2(g) H2O(l) Other gases in the fuel and air inputs (such as N2 and CO2) may be present, but as they are not involved in the electrochemical reaction, they do not need to be considered in the energy 1 Thermodynamic properties at 1Atm and 29 calculations. H2 O2 H2O (l) Enthalpy 0 0 -285.83 (H) kJ/mol Entropy 130.68 205.14 69.91 (S) J/mol·K J/mol·K J/mol·K Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant pressure. Entropy can be considered as the measure of disorganization of a
uel Cell Basicshermodynamicsf the chemical reaction using Hess’ Law: ΔHreaction = ΣHproducts – ΣHreactants = (1mol)(-285.83 kJ/mol) – (0) = -285.83 kJpy of chemical reaction: ΣSproducts – ΣSreactantsmol)(69.91 J/mol·K)] – [(1mol)(130.68 /mol·K) + (½mol)(2 J63.34 /K J gained by the system: = TΔS = (298K)(-163.34 J/K) = -48.7 kJ
uel Cell Basicshermodynamicsfree energy is then calculated by: ΔH – TΔS = (-285.83 kJ) – (-48.7 kJ) = -237 kJdone on the reaction, assuming reversibility a W = ΔGone on the reaction by the environment is: W = ΔG = -237 kJsferred to the reaction by the environment ΔQ = TΔS = -48.7 kJ More simply stated: The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment.
Fuel Cell BasicsComponents Anode: Where the fuel reacts or "oxidizes", and releases electrons. Cathode: Where oxygen (usually from the air) "reduction" occurs. Electrolyte: A chemical compound that conducts ions from one electrode to the other inside a fuel cell. Catalyst: A substance that causes or speeds a chemical reaction without itself being affected. Cogeneration: The use of waste heat to generate electricity. Harnessing otherwise wasted heat boosts the efficiency of power-generating systems. Reformer: A device that extracts pure hydrogen from
Molten Carbonate Fuel Cell (MCFC) carbonate salt electrolyte 60 – 80% efficiency ~650˚C operating temp. cheap nickel electrode Figure 5 catylist up to 2 MW The operating temperature is up constructed, too hot for manyto 100 MW designs applications. exist carbonate ions are consumed in the reaction → inject CO2 to compensate
hosphoric Acid Fuel Cell (PAFC) phosphoric acid electrolyte 40 – 80% efficiency 150˚C - 200˚C operating temp 11 MW units have been tested Figure 6 sulphur free gasoline can be The electrolyte a fuel used as is very corrosive Platinum catalyst is very
Proton Exchange Membrane (PEM) thin permeable polymer sheet electrolyte 40 – 50% efficiency 50 – 250 kW 80˚C operating Figure 7 temperature electrolyte will not leak or crack temperature good for home or vehicle use
Solid Oxide Fuel Cell (SOFC) hard ceramic oxide electrolyte ~60% efficient ~1000˚C operating temperature Figure 8 cells output high temp / catalyst can 100 kW up to extract the hydrogen from the fuel at the electrode high temp allows for power generation using the heat, but limits use
Benefits Efficient: in theory and in practice Portable: modular units Reliable: few moving parts to wear out or break Fuel Flexible: With a fuel reformer, fuels such as natural gas, ethanol, methanol, propane, gasoline, diesel, landfill gas,wastewater, treatment digester gas, or even ammonia can be used Environmental: produces heat and water (less than combustion in both
The water transport through Nafion MembraneWater flux due to electroosmotic drag (mol/cm2 s) is: Nw, drag = Iξ(λ)/F.Where: I is the cell current, ξ(λ) is the electroosmotic drag coefficient at agiven state of membrane hydration λ(=N(H2O)/N(SO3H) and F is the Faradayconstant. This flux acts to dehyddrate the anode side of a cell and tointroduce additional water at the cathode side. The buildup of water at the cathode (including the product waterfrom the cathode reaction) is reduced, in turn, by diffusion back down theresulting water concentration gradient (and by hydraulic permeation of waterin differentially pressurized cells where the cathode is held at higher overallpressure). The fluxes (mol/cm2 s) brought about by the latter twomechanisms within the membrane are: Nw,diff = -D(λ)∆c/ ∆z, Nw,hyd = -khyd(λ)∆P/ ∆zwhere D is the diffusion coefficient in the ionomer at water content λ, ∆c/ ∆zis a water concentration gradient along the z-direction of membranethickness, khyd is the hydraulic permeability of the membrane, and ∆P/ ∆z is apressure gradient along z.
The water transport through Nafion MembraneMany techniques have beenintroduced to prevent thedehydration of the anode(including the introduction ofliquid water into the anodeand/or cathode, etc. – which,however, can lead to “flooding”problems that inhibit masstransfer).However, the overall question of“water management,” including theissue of drag as a centralcomponent, has been solved to avery significant extent by theapplication of sufficiently thinPFSA membranes (<100 µm thick) inPEFCs, combined with humidificationof the anode fuel gas stream.
Water Transport (& Interface Reactions)in Nafion Membrane of the PEM Fuel Cell
Solid Oxide Fuel Cell SOFC Air side = cathode: High oxygen partial pressure O2 H2 + 1/2O2 H2O 1 conductance = σ µH2 d H2O Fuel side= anode: H2 + H2O= low oxygen partial pressure
Electromotive Force (EMF) SOFC Chemical Reactions in 2 separated compartements: - Cathode (Oxidation): ½O2 + 2e- O2- - Anode (Reduction): H2 + O H2O + 2e 2- - ∆G = Free Enthalpie z = number of charge carriers F = Faraday Constant EMF of a galvanic Cell: ∆G0= Free Enthalpie in (1) EMF = ∆Gr /-z F standart state R = Gas Constant a ( H 2O )SOFC: ½O2 + H2 H2O (2) ∆G = ∆G0 + RT ln a( H 2 )a (O2 ) 0.5 difference of ∆G between anode und cathode RT p ( O2 ) K Nernst Equation: EMK = ln 4 F p ( O2 ) A
Elektrochemische Potential SOFC Oxygen ions migrate due to an electrical and chemical gradient ∆µ (O 2− ) = ∆µ (O 2− ) − 2 F ∆ϕ % Electrochemichal Chemical Electrical Potential Potential PotentialDriving force for the O2- Diffusion through the electrolyte are thedifferent oxygen partial pressures at the anode and the cathodeside: ∆µ (O 2− ) % σi ∆µ (O 2− ) ji = ionic current ji = − % σi= ionic conductivity 2F
engl. Open Circuit Voltage (OCV) SOFC σi∆µ (O ) = ∆µ (O ) − 2 F ∆ϕ % 2− 2− ji = − ∆µ (O 2− ) % 2F What happems in case : ∆µ % (O 2− ) = 0 ji = 0 No currentOCV Electrical potential difference = chemical potetial
Leistungs-Verluste SOFC Under load decrease of cell voltage and internal losses U(I) = OCV - I(RE+ RC+RA) - ηC - ηAOCV (RE+ RC+RA) Ohmic resistancescell voltage U(I) [V] ηC Non ohmic resistances= ηA over voltages cell current I [mA/cm2]
Überspannungen SOFC Over voltages exist at interfaces of • Elektrolyte - Cathode • Elektrolyte - Anode Reasons: •Kinetic hindrance of the electrochemical reactions •Bad adheasion of electrode and electrolyte •Diffusion limitations at high current densities
Leistungs-Verluste SOFC OCV 1 (RE+ RC+RA) cell voltage U(I) [V] ηC ηA 2 cell current I [mA/cm2] 3 (1) Open circuit voltage (OCV), I = 0 (2) SOFC under Load U-I curve (3) Short circuit, Vcell = 0 (2) 0.5 1.0 Zellspannung [V] 0.4 0.8 Leistung [W/cm 0.3 0.6 0.2 0.4 2 ] 0.1 0.2 (1) 0.0 900°C in Luft/Wasserstoff (3) 0.0 0.0 0.5 1.0 1.5 2.0 2 Stromdichte [A/cm ]
How to determine the electrical conductance SOFC Iinput Umeasured Electrical resistance: U ∆L R = f (T ) = = I A *σ Electrical conductivity: U : voltage [V] I : current [A] σ0 Ea R : resistivity [ohm] σ= log( − ) ∆L : distance between both T kT inner wires [cm] A : sample surface [cm2] 1 σ : conductivity [S/m] σ T vs. ⇒ Ea Ea : activation energy [eV] T T : temperature [K] K : Boltzmann constant
SOFC Design SOFC Tubular design i.e. Siemens-Westinghouse design Segment-type tubular design Planar design i.e. Sulzer Hexis, BMW design
Tubular Design – Siemens-Westinghouse SOFC Why was tubular design developed in 1960s by cathode Westinghouse? interconnection • Planar cell: Thermal expansion mismatch cathode between ceramic and (air) support structures leads to problems with the gas sealing tubular design air flow anode (fuel) was invented Advantages of tubular design: • At cell plenum: depleted air and fuel react heat is generated incoming oxidant can be pre-heated. • No leak-free gas manifolding needed in this
Tubular Design – Siemens-Westinghouse SOFC To overcome problems newcathode Siemens-Westinghouse „HPD- (air) SOFC“ design: New: Flat cathode tube with ligaments anode (fuel) electrolyte Advantages of HPD-SOFC: • Ligaments within cathode short current pathways decrease of ohmic resistance • High packaging density of cells Siemens-Westinghouse shifted from compared to tubular design basic technology to cost reduction and scale up. Power output: Some 100 kW can be produced.
Planar Design – Sulzer Hexis SOFC interconnect Advantages of planar cathode (air) design: • Planer cell design of bipolar electrolyte plates easy stacking no anode (fuel) long current pathways • Low-cost fabrication methods, i.e. Screen printing and tape casting can be used. Drawback of tubular design: • Life time of the cells 3000- 7000h needs to be improved by optimization of mechanical and electrochemical stability of used materials.
Planar Design – BMW SOFC Air channel bipolar plate Cathode current collector cathode electrolyte anode porous metallic substrate Fe-26Cr-(Mo, Ti, Mn, Y2O3) alloy bipolar plate Fuel channel Application Batterie replacement in the 20-50 µm Plasma spray BMW cars of the 7-series. 5-20 µm Plasma spray 15-50 µm Plasma spray Power output: 135 kW is aimed.
Current InitiativesAutomotive Industry Most of the major auto manufacturers have fuel cell vehicle (FCV) projects currently under way, which involve all sorts of fuel cells and hybrid combinations of conventional combustion, fuel reformers and battery power. Considered to be the first gasoline powered fuel cell vehicle is the H20 by GM: GMC S-10 (2001) fuel cell battery hybr low sulfur gasoline fue 25 kW PEM 40 mpg 112 km/h top speed Figure 9
Current InitiativesAutomotive Industry Fords Adavanced Focus FCV (2002) fuel cell battery hybrid 85 kW PEM ~50 mpg (equivalent) 4 kg of compressed H2 @ Figure 10 5000 psi Approximately 40 fleet vehicles are planned as a market introduction for Germany, Figure 11 Vancouver and California for
Current Initiatives Automotive IndustryChrysler NECAR 5 (introduced in 2000) 85 kW PEM fuel cell methanol fuel reformer required 150 km/h top speed Figure 12this model completed a California to Washingpermit for Japanese roads
Current InitiativesAutomotive IndustryMitsubishi Grandis FCV minivan fuel cell / battery hybrid 68 kW PEM compressed hydrogen fuel 140 km/h top Figure 13 speed Plans are to launch as a production vehicle for Europe in 2004.
Current InitiativesStationary Power Supply Units More than 2500 stationary fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, and an airport terminal, providing primary power or backup. In large-scale building systems, fuel cells can reduce facility energy service costs by 20% to 40% over conventional energy service. Figure 14 A fuel cell installed at McDonald’s restaurant, Long Island Power Authority to install 45 more fuel
Current InitiativesResidential Power Units There are few residential fuel cell power units on the market but many designs are undergoing testing and should be available within the next few years. The major technical difficulty in producing residential fuel cells is that they must be safe to Residential install in a home, and be easily fuel cells maintained by the average homeowner. are typically the size of a large deep freezer or furnace, such as the Plug Power 7000 Figure 15 unit shown If a power company was here, and cost to install a $5000 - $10 residential fuel cell power unit in a 000. home, it would have to charge the homeowner at least 40 ¢/kWh to be
Future “...projections made by car companies themselves and energy and automotive experts concur that around 2010, and perhaps earlier, car manufacturers will have mass production capabilities for fuel cell vehicles, signifying the time they would be economically available to the average consumer.” Auto Companies on Fuel Cells, Brian Walsh and Peter A commercially available fuel cell Moores, posted on www.fuelcells.org power plant would cost about $3000/kW, but would have to drop below $1500/kW to achieve widespread market penetration. http://www.fuelcells.org/fcfaqs.htm Technical and engineering innovations are continually lowering the capital cost of a fuel cell unit as well as the operating costs, but it is expected that mass production will be of the greatest impact to affordability.
Future internal combustion obsolete? solve pollution problems? common in homes? better designs? higher efficiencies? cheaper electricity? reduced petroleum dependency?
References (1) FAQ section, fuelcells.org (2) Long Island Power Authority press release: Plug Power Fuel Cell Installed at McDonald’s Restaurant, LIPA to Install 45 More Fuel Cells Across Long Island, Including Homes, http://www.lipower.org/newscenter/pr/2003/feb26.fuelce ll.html (3) Proceedings of the 2000 DOE Hydrogen Program Review: Analysis of Residential Fuel Cell Systems & PNGV Fuel Cell Vehicles, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/2 8890mm.pdf Figures 1, 3 http://hyperphysics.phy- astr.gsu.edu/hbase/thermo/electrol.html 4 – 8 http://fuelcells.si.edu/basics.htm 10 http://www.moteurnature.com/zvisu/2003/focus_fcv/focus _fcv.jpg 11 http://www.granitestatecleancities.org/images/Hydrogen_F uel_Cell_Engine.jpg 12 http://www.in.gr/auto/parousiaseis/foto_big/Necar07_2883. jpg 13 http://www3.caradisiac.com/media/images/le_mag/mag138/o eil_mitsubishi_grandis_big.jpg 14 http://www.lipower.org/newscenter/pr/2003/feb26.fuelce