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Biofuel, CO2 Reduction,
Bio-inspired Constructs,
Biomemitic Catalysts,
Methanogenesis |
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Fuel Cells, Borohydrides, Catalysts, Ionic Liquids,
Si-Electroplating |
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| II-IV-V Semiconductors, Nanowires, Metal Complexes |
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| Certification, Fuel Cells, Training, Outreach |
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| Know-How, Library, Education |
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| AZ Solar, Forums, Media |
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Specific Energy of a Fuel Cell vs Batteries
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Fuel Cells are the highest energy density direct electrical generators
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Basic PEM Fuel Cell Operation
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Hydrogen spontaneously oxidized at anode, which is the source of :
- electron current from anode to cathode in external circuit with load
- protons flow from anode to cathode through membrane
- to completes current loop
Oxygen spontaneously reduced at the cathode, oxygen is sink of :
- electrons and protons
- to complete reaction forming water and electrical energy
Benefits of increasing fuel cell operating temperature to 150oC :
- increased current at a given voltage (increased power density)
- reduced catalyst poisoning problems (e.g., CO from reformates)
- reduced size of radiator
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High Temperature Proton Conducting Membranes
for High Temperature Polymer Electrolyte Membrane (PEM) Fuel Cells
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Polymer Membranes for High Temperature PEM Fuel Cells
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Objective:
Improve the power density, efficiency and durability of PEM fuel cells
Approach:
- First, use PEMEAS phosphoric acid loaded poly benzimidazole (PBI)
- In parallel, develop new, low cost polymeric membranes that can operate more efficiently up to 200°C at low relative humidity (<25%)
Benefits:
Increasing PEM fuel cell operating temperature, e.g., to 150°C
- Increases fuel cell power output
- Reduces susceptibility of catalyst to CO poisoning
- Simplifies the system design
- water management,
- parasitic losses from compressors and humidifiers
- Radiation of waste heat
- Whereas conventional PEM FCs are limited to operation at 80°C
- Membrane (Nafion) loses moisture which dramatically reduces the proton conductivity at higher temperatures
- Nafion membrane is costly, $2500/lb
- New membranes are being developed with:
- e.g., NASA ORMoSIL membranes, ASU Protic Ionic-Salt membranes (PIMs)
- Good proton conductivity at 120°C with little or no humidification
- Potential for higher power densities
- Eliminate need for rehydration of membrane (less weight and complexity)
- New membranes projected to cost of < $100/lb
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DIAGRAM OF A PEM FC STACK |
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Why HT PEM Technology ? |
- Lower temperature operation (170oC) than turbine or solid oxide fuel cell (> 800oC)
- Smaller size than low temperature fuel cell due to smaller radiator
- Quiet
- Lower cost fuel cell membrane
- High efficiency system due to good thermal integration to fuel source (reformers)
- Shock and vibration tolerant due to construction with plastics and metals
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Categories of PEM Fuels |
1. Hydrocarbon Fuel (natural gas, gasoline, coal) Solid Oxide Fuel Cell (SOFC)
Large Power utility, 10 mega Watt
T = 700oC, Ceramics are brittle → problems with shock, vibration and sealing cells
2. Impure H2 (reformed hydrocarbons) High Temperature PEM Fuel Cell
Residential Power, 100 Watt to 100 kilo Watt
T = 160 to 190oC, No humidification (PEMEAS)
3. Pure H2 (water electrolysis) Polymer Electrolyte Membrane (PEM) Fuel Cell
Automotive&Backup utility, 1-50 kiloWatt
T = Room temp to 140oC, cost, storing H2 gas is bulky & hazardous
4. Activated hydrocarbon (Methanol/water fuel) Liquid fed PEM-FC
Hand – Carried Portable Power, 1-100W
T = RT, unstable catalyst, high catalyst cost
5. Chemical storage of hydrogen (Borohydride/water fuel) PEM-FC
Hand – Carried Portable Power, 1- 100W
T= RT, stable, borohydride is non-flammable, non-toxic, cost effective
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High Temperature PEM Fuel Cell is the fuel cell of choice.
Shock and vibration tolerant, small radiator.
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Comparison of PEM Fuel Cells
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Advantages of High Temp PEM Fuel Cell
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PEMEAS company has demonstrated long term operation
- >10,000 hours at 0.6 volt at 170oC with no humidification
- Performance degradation rate of < ~ 0.0045 mV/h
Tolerant to shock and vibration due to plastics construction
Simple system
- No need to hydrate membrane for proton conductivity
- No liquid water management issues at 170oC
- All gas phase reactants and products
- Simpler fluid handling therefore HIGHER SYSTEM EFFICIENCY
- Small radiator to reject waste heat and condense by-product water
No crossover issues
- High Fuel utilization
- No need to recirculate water
High temperature effect on catalyst activity
- Better CO tolerance (up to 2% CO with no performance loss)
Modest Pt catalyst loadings
- Most of the fuel cell cost is for platinum (~1mg of Pt per cm2 of cell, $250/kW)
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For more information, please, contact professor Don Gervasio |
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