Fuel Cells
Why Polymers for Fuel Cell Membranes?
(Polymer electrolytes) Only polymers have the required combination of: -toughness -ease of thin film fabrication (out of it) -can be modified to give proton conduction (specifically with SO3H groups) -conduct at lower temperatures than ceramics -have conductivities that are approaching the same range as liquid electrolytes
Research into the use of polyphosphazenes as fuel cell membranes: Proton Conducting Functional Groups -
-Carboxylic acid -Sulfonic acid -Phosphonic Acid -Sulfonimide groups
Types of PEM Fuel Cells
-Direct hydrogen -Reformed hydrogen -Direct methanol
Solid Oxide Fuel Cell
-Electrolyte is a hard, non-porous ceramic-typically yttrium or scandium oxide stabilized zirconium oxide -Operates near 1,000 deg.C. -No precious metal catalysts needed -Not poisoned by CO or sulfur compounds -Electrodes: Porous Nickel Anode, LaMnO3 cathode -Anode: H2 and CO in; water, CO2, and heat out -Anode rxn: H2 + O2^(2-) --> H2O + 4 e- -Cathode: Air in; heat out -Cathode rxn: O2 + 4 e- --> 2O2^(2-) -Overall rxn: H2 + O2 --> H2O + 4 e-
Benefits of Fuel Cells
-Eliminate urban air pollutants -Reduce CO2 emissions by at least 50% -Twice as efficient as internal combustion engine -Reduced need for lubricants - fewer moving parts -Less noise and maintenance
Proton-conducting fuel cell membranes
-Hydrogen fuel cell -Direct methanol fuel cell
Polyphoshazene with SULFONIC ACID as proton conducting functional group for fuel cells
-Incorporation into polyphosphazenes is fairly easy -strong acid - high proton dissociation
Polyphoshazene with CARBOXYLIC ACID as proton conducting functional group for fuel cells
-Incorporation into polyphosphazenes is known -Weak acid - low proton dissociation
Advantages of Fuel Cells Over Other Power Sources
-Lightweight -Simplicity of design -Invariant operation - H2/O2 cell produces only heat and pure H2O - environmentally benign -Polymeric membranes - reduced corrosion and minimal electrolyte leaching as compared to liquid electrolytes
Polyphoshazene with PHOSPHONIC ACID as proton conducting functional group for fuel cells
-New chemistry for polyphosphazenes -Moderately strong acid - moderate proton dissociation
Polyphoshazene with SULFONIMIDE GROUPS as proton conducting functional group for fuel cells
-New chemistry for polyphosphazenes -Very strong acid - high proton dissociation
Direct hydrogen PEM fuel cells
-No CO2 emissions, but onboard fuel storage and refueling are challenging (someone else is making it elsewhere and shipping it)
Proton exchange membrane fuel cells
-Operate at relatively low temperatures -High power densities -Can vary output quickly -Suitable for quick startup applications -Current PEMFC's are based on Nafion, a poly(perfluorosulfonic acid) membrane with an upper temp limit of 80-100 deg.C.
Challenges with Proton Conduction Membranes
-Permeability of polymer membrane to methanol -Attack by free radicals on polymer membranes -Morphology of membranes strongly affected by ratio of hydrophilic to hydrophobic components and this has a significant effect on proton conduction -Loss of water above ~80-90 deg.C. which prevents high temp use and employment of inexpensive catalysts
Disadvantages of Current IEMFCs
-Requires reforming of fuel to H2 -Expense a) high catalyst loadings - Pt b) Nafion membrane -Dehydration of membrane forces operation at low temperatures
Low Temp Fuel Cells: Fabrication and Operational Issues
-Use and cost of precious metal catalysts (Pt, Ru) -Deposition of the catalyst at the electrodes to maximize surface area (smallest possible particles for raster rxn time) -Adhesion btwn the electrodes and the polymer membrane -Even distribution of the gaseous fuel across the whole membrane -Size of the individ. fuel cells to be incorporated in a multi-fuel cell construct -Temp control (cooling and heat recirculation) -Polymer membrane breakdown (oxygen radicals, dehydration) -Catalyst poisons in the feedstock -Very few commercial membranes available at reasonable cost
Reformed hydrogen PEM fuel cells
-can use gasoline, methane, or methanol, but generate CO2, have added weight and complexity, and partial oxidation leads to higher emissions (making H2 on site)
Nafion
-current standard -(know chemical structure) -fails above 80 deg.C. b/c of loss of adsorbed water -low concentration of SO3H grps -highly permeable to methanol "bi-phased polymer" **Fluorine for hydrophobicity, sulfonic acid for hydrophilicity. Leads to a *phase separated* system in the presence of water**
Alkaline Fuel Cells
-differ in that they depend on the transport of *anions* from the cathode to the anode -they electrolyte is an aqueous soln of potassium hydroxide in an inert porous matrix instead of an ion conductive polymer or molten acid. -fuels: hydrogen and oxygen -products; water and electrical power -Pure H2(g) enters at the anode and combines there w/ hydroxide ions to give e-'s under the influence of an inexpensive catalyst such as microparticulate silver or more expensive Pt. -The water formed at the anode migrates back to the cathode to regenerate hydroxide ions -At the cathode, pure O2(g) reacts w/ potassium ions and water to regenerate potassium hydroxide. -Developed for the NASA Apollo and Space Shuttle missions -Temperatures of operation range from -65 deg. C. to 220 deg.C. -System is very sensitive to impurities, especially CO2
Molten Carbonate Fuel Cell
-functions through the transport of anions from the cathode to the anode -Anode: Hydrogen or methane enters system; Water and CO2 out -Anode rxn: H2 + CO3^2- --> H2O + CO2 + 2e- [CO + CO3^2- --> 2CO2 + 2e-] -Cathode: Oxygen and CO2 enter; CO32- out -Cathode rxn: O2 + 2CO2 + 4e- --> CO3^2- -Overall rxn: H2 + O2 --> H2O + e- -650 deg.C. operating temp -LiAlO2 porous ceramic matrix -Li carbonate + K or Na carbonate electrolyte -The hydrocarbon fuel does not need to be refined -60% efficiency -Not sensitive to CO or CO2 poisoning -Non-precious metal catalysts
Direct methanol PEM fuel cell
-liquid methanol can be stored and supplied easily using existing infrastructure, but CO2 is produced, and few methanol-impermeable membranes exist
Molten Phosphoric Acid Fuel Cell
-most highly developed for stationary applications (ex: power stations) -fuels: hydrogen and oxygen -Porous carbon electrodes w/ Pt catalyst -Anode rxn: H2 --> 4 H+ + 4 e- -transport of protons and e-'s to the cathode takes place through a *liquid* electrolyte of molten phosphoric acid. -Cathode rxn: O2 (g) + 4H+ + 4 e- --> 2H2O (oxygen enters @ the cathode, and water and heat produced) -Overall: 2H2 + O2 --> 2H2O -High Temp Operation: 150-200 deg.C.
Low temperature fuel cells
-must first convert hydrocarbons or alcohols to hydrogen, which is then consumed in the cell to give water as an end product **Low temp fuel cells are crucially dependent on the performance of ion conducting polymer membranes**
Purpose of a fuel cell
-purpose is to convert fuels directly to electricity without the intermediate use of heat engines. In this way, it is possible to reduce air pollution and noise and increase the efficiency of power generation
What is the most serious hurdle to the development of commercially viable moderate temp fuel cells?
-the lack of suitable proton conduction membranes
The principle of a fuel cell
-to obtain electrical energy, one of the two hydrogen protons must be separated from the atom. Anode (-) --> Catalyst --> Cathode (+) 1.) Hydrogen protons flow across a catalyst... 2.)...where they react with oxygen... 3.) ...which produced electrical energy to power the car catalyst = electrolyte solution
High temperature fuel cells
-used for stationary applications (power stations) -can use hydrocarbon fuels directly
Proton-conducting fuel cell membranes: Main challenges of Direct Methanol Fuel Cell
1 Reduction of methanol crossover while maintaining high proton conduction (prevent methanol from crossing membrane) 2. Stability of membrane above 100 deg.C. 3. Produce an appropriate membrane at moderate cost 4. A number of fuel cells require the use of precious metal catalysts (ex: Pt)
Requirements for Polymer Fuel Cell Membrane:
1. Good conductor of protons above 80 deg.C. 2. Stable at temps up to 150-200 deg.C. 3. Prevents hydrogen or methanol crossover 4. Mechanically robust 5. Good adhesion to electrodes 6. Inexpensive
Hydrogen/Air-Polymer Electrolyte Fuel Cell
1. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channeled to the cathode on the other side of the cell. 2. At the anode, a Pt catalyst causes the hydrogen to split into +ive hydrogen ions (protons) and negatively charged electrons. 3. The Polymer Electrolyte Membrane (PEM) allows only the +charged ions to pass through it to the cathode. The -charged e-'s must travel along an external circuit to the cathode, creating an electrical current 4. At the cathode, the e-'s and +charged hydrogen ions combine with oxygen to form water, which flows out of the cell
Proton-conducting fuel cell membranes: Main challenges of Hydrogen Fuel Cell
1. Stability of membrane above 80 deg.C. 2. ***Water retention above 80 deg.C. 3. Cost of existing membrane materials
Where will the hydroen come from?
1. The water plus methane reaction (by far the cheapest) 2. Electroylsis
Weakness of most polymers for fuel cell applicatoins
1. They contain carbon atoms in the backbone and are, therefore, decomposed by oxidizing agents such as HO* free radicals 2. They are limited by the types of side groups and density of functional grps (such as SO3H, PO3H2, etc.) 3. Tend to be soluble in organic solvents at high temps and to allow the transmission of organic vapors and liquids such as methanol
Direct Methanol Fuel Cells (DMFCs)
Anode||Proton Exchange Membrane||Cathode -Oxidation rxn: CH3OH + H2O --> 6H+ + 6 e- +CO2 -Proton Exchange Membrane -Reduction rxn: 3/2 O2 + 6H+ + 6e- --> 3H2O
Electrolysis
Hydrogen electrode (cathode)||Polymer electrolyte membrane||Oxygen electrode (anode) (uses DC Power) Reduction rxn: 4H+ + 4e- --> 2H2 Oxidation rxn: 2H2O --> 4H+ + 4e- + O2 (water--> oxygen; water,heat) In polymer electrolyte membrane, H+ and H2O move toward the cathode
Current Limitations of DMFCs -
Membranes -Require temps below 100 deg.C. Dehydration (loss of H2O means there's no way for the protons to get from one side to the other for conduction) Loss of physical integrity -Hampered by methanol crossover CH3OH + 3/2 O2 --> CO2 + 3H2O Decreased performance and efficiency
Fuel Cell Fabrication
Metal end unit with gas-transport grooves ----Carbon fiber cloth with catalyst (catalyst deposited on the surface) ----Proton-conducting membrane ----Carbon fiber cloth with catalyst (catalyst deposited on the surface)----Metal end unit with gas-transport grooves
Advantages of High Temp Operation
higher temperature --> increased rxn rates 1.) catalyst -reduce loading -replace with cheaper catalyst 2.) use of non-reformed hydrocarbon fuel - e.g. methanol