Alternative Energy Tutorial

Introduction

Fuel cells and batteries are electrochemical cells used to generate an external electrical current. They consist of an anode, where oxidation occurs, a cathode, where reduction occurs, and an electrolyte through which ions can travel between electrodes (see Figure 1 for a schematic of an electrochemical cell). In fuel cells (discussed below), one or both of the reactants are supplied from an external source to the cell. Though technically fuel cells, when the only reactant supplied to the cell is atmospheric oxygen, the cells are considered batteries (zinc/air or aluminum/air cells for example).

Batteries

Batteries can be divided into two types: primary or disposable batteries and secondary or rechargeable batteries. The main advantages of batteries over fuel cells are their availability, portability, low cost, and wide range of operating conditions. Batteries, however, have much shorter life spans and lack the power output of fuel cells. Power outputs of batteries are typically on the order of 100's of watts, whereas fuel cells can provide kilowatt to megawatt outputs, power enough to light a building or fuel a vehicle for hours. Under heavy energy demands, batteries can build up dangerous levels of heat and pressure, degrading the battery and possibly causing leaks of toxic compounds or even explosions. In addition, the limited life of primary batteries and the limited cycle life (number of times it can be recharged) of most secondary batteries necessitates the need for disposal of often dangerous and toxic battery materials. Table 1 summarizes some of the common types of primary and secondary batteries.

Table 1. Some common types of Batteries.
 

  Battery Type Anode Cathode Electrolyte Advantages Disadvantages
Primary Batteries Alkaline Cell Zn MnO2 KOH High energy density, long shelf life, good leak resistance, performs well under heavy or light use. Costlier than zinc-carbon cell but more efficient
Aluminum/Air Cell Al O2 KOH or neutral salt solution Can operate exposed to sea water (neutral salt solution), easily replaceable electrolytes/electrodes Anode quickly degrades, short shelf life, short operational life
Leclanché Cell (Zinc Carbon or Dry Cell) Zn MnO2 NH4Cl or ZnCl2 Cheap and common (oldest available battery type) Poor performance under heavy or continuous use.
Lithium Cell Li Various liquid or solid materials SOCl2, SO2Cl2, or organic solutions Very high energy density, long shelf life, long operational life Poor performance under heavy use, vulnerable to leaks or explosions
Mercury Oxide Cell Zn or Cd HgO KOH Higher energy density than (Zn/MnO2) alkaline cell High cost and being phased out due to toxicity concerns
Zinc/Air Cell Zn O2 KOH Environmentally benign, cheap, very high energy density, and virtually unlimited shelf life Short operational life, low power density
Secondary (rechargeable) Batteries Iron Nickel Cell Fe Ni(OH)2 KOH Long life under a variety of conditions, excellent back-up battery Low rate-performance, slow recharge rate
Lead/Acid Cell Pb PbO2 dilute H2SO4(aq) Low cost, long life cycle, operates well under a variety of conditions. Common car batteries Minor risk of leakage
Lithium Ion Cell C, carbon compounds Li2O, intercalated into graphite LiPF6, LiBF4, related compounds Relatively cheap, high energy density, long shelf life, long operational life, long cycle life Minor risk of leakage
Nickel/Cadmium Cell Cd Ni(OH)2 KOH Good performance under heavy discharge and/or low temperature High cost, can temporary loose cell capacity if not fully discharged before recharging (memory effect)
Nickel/Metal Hydride (NiMH) Cell Lanthanide or Ni alloys Ni(OH)2 KOH High capacity and power density High cost, some memory effect
Nickel/Zinc Cell Zn NiO KOH Low cost, low toxicity, good for high discharge rates Zinc on the electrolyte tends to redeposit unevenly on anode, severely reducing efficiency
Sodium/Sulfur Cell Molten Na Molten S Al2O3 Inexpensive materials, long cycle life, high energy and power High operational temperature lower efficiency, some danger of explosion upon degradation

The primary component materials of a battery are the anode, cathode, electrolyte, and semi-permeable materials. In addition various catalysts have been used to enhance the performance of electrodes. For example, ruthenium(IV) oxide (Sigma-Aldrich Prod. No 238058) is used as a catalyst in a vanadium redox battery system.1 Table 1 summarizes some of the types of electrodes and electrolytes used in common batteries. Many advanced battery designs focus upon new materials for these key components.

Much of the recent battery work has focused on lithium-ion batteries, since they are the primary power source for the ever-growing field of small, rechargeable electronic devices. Nickel sulfide, for example, was recently explored as a cathode material for rechargeable lithium batteries.2 Current research is also concerned with some very mundane materials in electrodes. New morphologies of graphite flakes (Sigma-Aldrich Prod. No 332461), as a case in point, have been studied as anode material in lithium-ion batteries.3 Electrolytes are also very important in battery performance. An LiBF4 (Sigma-Aldrich Prod. No 451622) solution, for example in a butyrolactone/ethylene carbonate (Sigma-Aldrich Prod. No B103608 and E26258) solution has proven to be a highly conductive and highly thermally stable electrolyte for lithium-ion batteries.4

Fuel Cells

Fuel cells offer the promise of a clean energy source for stationary power generation. They produce energy from hydrogen, natural gas, alcohol, or other readily available hydrocarbon fuels (see Figure 2). Fuel cells date back to the nineteenth century when Grove, in 1839, first published his work on the generation of electricity by partially immersing two platinum electrodes and separately supplying oxygen and hydrogen to them.5 There is considerable current interest in fuel cells as an environmentally clean alternative to fossil-fuel-burning power sources.

Schematic of a typical polymeric electrolyte membrane (PEM) fuel cell

Figure 2. Schematic of a typical polymeric electrolyte membrane (PEM) fuel cell.

Purely fuel cell powered vehicles are currently being tested as prototypes with plans for eventual commercialization by as early as 2010.6 Some of these vehicles are already in operation in municipal organizations around the world. See Table 1 for some examples.

Table 2. Some currently operational fuel cell vehicles.6 (courtesy of Fuel Cells 2000, www.fuelcells.org, accessed Dec 16, 2008)
 

Automaker Vehicle Type Year Engine Type Fuel Cell Size/
type
Fuel Cell Mfr. Range (mi/km) MPG Equiv.* Max. Speed Fuel Type
BMW Series 7 (745 h) (Sedan) 2000 ICE (fuel cell APU) 5kW/PEM UTC 180mi 300km N/a 140 mph Gasoline/
Liquid hydrogen
Limited intro in 2000 (Munich Airport Hydrogen Vehicle Project)
Daihatsu MOVE EV - FC (micro van) 1999 Fuel cell/ battery hybrid 16kW/ PEM Toyota N/a N/a N/a Methanol
  MOVE FCV – K II (mini vehicle) 2001 Fuel cell/ battery hybrid 30 kW/ PEM Toyota 75mi 120km N/a 65mph 105km/h Compress. hydrogen @ 3600 psi
Japan road testing started in early 2003.
Daimler- Chrysler NECAR 1 (180 van) 1994 12 fuel cell stacks 50kW/ PEM Ballard 81mi 130km N/a 56mph 90km/h Compress. hydrogen @ 4300 psi
  NECAR 2 (V-Class) 1996 Fuel cell 50kW/ PEM Ballard 155mi 250km N/a 68mph 110km/h Compress. hydrogen @ 3600 psi
  NECAR 3 (A-Class) 1997 2 fuel cell stacks 50kW/ PEM Ballard Mark 700 Series 250mi 400km N/a 75mph 120km/h 10.5 gal. of Liquid methanol
First methanol reforming FCV
  NECAR 4 (A-Class) 1999 Fuel cell 70kW/ PEM Ballard Mark 900 Series 280mi 450km N/a 90mph 145km/h Liquid hydrogen
  Jeep Commander 2 (SUV) 2000 Fuel cell/ (90 kW) battery hybrid 50kW/ PEM Ballard Mark 700 Series 118mi 190km 24 mpg (gas.
equiv.)
N/a Methanol
Jeep Commander 1 came out in 1999.
  NECAR 4 - Advanced (California NECAR) 2000 Fuel cell 85kW/ PEM Ballard Mark 900 Series 124mi 200km 53.46 mpg equiv. (CaFCP est.) 90mph 145 km/h 4 lbs. (1.8kg) of Compress. hydrogen @ 5,000 psi
  NECAR 5 (A-class) 2000 Fuel cell 85kW/ PEM Ballard Mark 900 Series 280mi 450km N/a 95mph 150km/h Methanol
  DMFC go-cart (one-person vehicle) 2000 Fuel cell 3kW/ DMFC Ballard Mark 900 Series 9.3mi 15km N/a 22mph 35km/h Methanol (directly)
6kW DMFC built by DC and Ballard is largest in world
  NECAR 5.2 (A-class) 2001 Fuel cell/ battery hybrid 85kW/ PEM Ballard Mark 900 Series 300mi 482km N/a 95mph 150km/h Methanol
Awarded a road permit for Japanese roads. Completed CA – DC drive.
  Sprinter (van) 2001 Fuel cell 85kW/ PEM Ballard Mark 900 Series 93mi 150km N/a 75mph 120km/h Compress. Hydrogen @ 5,000 psi
Delivered to Hamburg parcel service, Hermes as part of the W.E.I.T. hydrogen project
  Natrium (Town & Country Mini Van) 2001 Fuel cell/ (40 kW) battery hybrid 54kW/ PEM Ballard Mark 900 Series 300mi 483km 30 mpg equiv. 80mph 129km/h Catalyzed chemical hydride - Sodium Boro-
hydride
Uses Millennium Cell’s ‘Hydrogen on Demand’ system with a 53 gallon fuel tank
  F-Cell (A-class) 2002 Fuel cell/ battery hybrid 85kW/ PEM Ballard Mark 900 Series 90mi 145km 56 mpg equiv. 87mph 140km/h 4 lbs. (1.8kg) of Compress. hydrogen @ 5,000 psi
60 fleet vehicles in US, Japan, Singapore, and Europe starting in 2003 – small fleet in Michigan operated by UPS.
  Jeep Treo 2003 Fuel cell N/a N/a N/a N/a N/a N/a
Unveiled at Tokyo Motor Show – drive by wire technology
ESORO Hycar 2001 Fuel cell/ battery hybrid 6.4kW/ PEM Nuvera 224mi 360km N/a 75mph 120km/h Compress. Hydrogen
Switzerland’s first FCV
Fiat Seicento Elettra H2 Fuel Cell 2001 Fuel cell/ battery hybrid 7kW/ PEM   100mi 140km N/a 60mph 100km/h Compress. Hydrogen
Next generation due in 2003 w/ Nuvera fuel cells
  Seicento Elettra H2 Fuel Cell 2003 Fuel cell/ battery hybrid N/a Nuvera N/a N/a N/a Compress. Hydrogen
Being investigated for use in Milan, Italy, where gasoline and diesel fueled vehicles are banned on smoggy days.
Ford Motor Company P2000 HFC (sedan) 1999 Fuel cell 75kW/ PEM Ballard Mark 700 Series 100mi 160km 67.11 mpg equiv. (CaFCP est.) N/a Compress. Hydrogen
First FCV by Ford
  Focus FCV 2000 Fuel cell 85kW/ PEM Ballard Mark 900 Series 100mi 160km N/a 80mph 128km/h Compress. hydrogen @ 3,600 psi
  TH!NK FC5 2000 Fuel cell 85kW/ PEM Ballard Mark 900 Series N/a N/a 80mph/ 128km/h Methanol
  Advanced Focus FCV 2002 Fuel cell/ battery hybrid 85kW/ PEM Ballard Mark 900 Series 180mi 290km ~50 mpg equiv. N/a 8.8 lb. (4kg) Compress. H2 @ 5,000 psi
~40 fleet vehicles introduction Germany, Vancouver & CA in 2004
  GloCar Concept Only 2003 Fuel Cell N/a N/a N/a N/a N/a N/a
Powered by fuel cells, it uses LED lights to change body panel colors, intensity, and frequency.
General Motors/ Opel EV1 FCEV 1997 Fuel cell/ battery hybrid N/a N/a N/a N/a N/a Methanol
0-60 mph in 9 seconds
  Sintra (mini-van) 1997 Fuel cell 50kW/ PEM N/a N/a N/a N/a N/a
Wants to be 1st automaker to sell 1 million FCVs profitably
**Hydrogenics works with GM on FC development Zafira (mini-van) 1998 Fuel cell 50kW/ PEM Ballard 300mi 483km 80 mpg equiv. 75mph 120km/h Methanol
GM has ceased efforts regarding methanol (2001)
  Precept FCEV Concept only 2000 Fuel cell/ battery hybrid 100kW/ PEM GM** 500mi 800km (est.) 108 mpg equiv. (est.) 120mph 193km/h Hydrogen (stored in metal hydride)
These are concept projections
  HydroGen 1 (Zafira van) 2000 Fuel cell/ battery hybrid 80kW/ PEM GM** 250mi 400km N/a 90mph 140km/h 16 gal. of Liquid hydrogen
GM plans to sell 75kW hydrogen stationary fuel cell generators in 2005
  HydroGen 3 (Zafira van) 2001 Fuel cell 94kW/ PEM GM** 250mi 400km N/a 100mph 160km/h Liquid hydrogen
Being used by FedEx Corp. in Tokyo, Japan from 6/2003 – 6/2004
  Chevy S-10 (pickup truck) 2001 Fuel cell/ battery hybrid 25kW/ PEM GM** 240mi 386km 40 mpg 70 mph Low sulfur, clean gasoline (CHF)
GM has partnership with Toyota on reforming
  AUTOnomy Concept only 2002 Fuel cell N/a N/a N/a Proj. 100 mpg N/a N/a
GM’s 2010 FCV concept Freedom of Design
  Hy-Wire Proof of Concept 2002 Fuel cell 94kW/ PEM GM** 80mi 129km ~41 mpg (gas equiv.) 97mph 160km/h 4.4 lbs.(2kg) Compress. h2 @ 5,000 psi
Uses HydroGen3’s powertrain, so range & mpg theoretically could = HydroGen3
  Advanced HydroGen 3 (Zafira van) 2002 Fuel cell 94kW/ PEM GM** 170mi 270km ~55 mpg (gas equiv.) ~100mph 160km/h 6.8lbs. (3.1kg) Compress. h2 @ 10,000 psi
1st FCV to incorporate 10,000 psi tanks (by Quantum). 6 placed in Washington DC.
  Diesel Hybrid Electric Military truck 2003 Fuel cell APU 5kW/ PEM Hydro-
genics
N/a N/a N/a Low pressure metal hydrides
Turbo diesel ICE/battery hybrid with PEM FC APU. Under eval. for US Army’s new fleet of 30,000 light tactical vehicles.
GM (Shanghai) PATAC Phoenix (Mini van) Oct. 2001 Fuel cell/ battery hybrid 25kW / PEM Shanghai GM** 125mi 200km N/a 70mph 113km/h Compress. Hydrogen
Seventh FCV prototype out of China
Honda FCX-V1 1999 Fuel cell/ battery hybrid 60kW/ PEM Ballard Mark 700 Series 110mi 177km N/a 78mph 130km/h Hydrogen (stored in metal hydride)
  FCX-V2 1999 Fuel cell 60kW/ PEM Honda N/a N/a 78mph 130km/h Methanol
Honda has strict focus on pure hydrogen FCVs (2001)
  FCX-V3 2000 Fuel cell/ Honda ultra capacitors 62kW/ PEM Ballard Mark 700 Series 108mi 173km N/a 78mph 130km/h 26 gal. of Compress. hydrogen at 3600 psi
  FCX-V4 2001 Fuel cell/ Honda ultra capacitors 85kW/ PEM Ballard Mark 900 series 185mi 300km ~50 mpg (gas equiv.) 84mph 140km/h 130 L (3.75kg) Compress. H2 @ 5,000 psi
Completed Japanese road testing - 1st FCV to receive CARB & EPA emission certs.
  FCX 2002 Fuel cell/ Honda ultra capacitors 85kW/ PEM Ballard Mark 900 series 220mi 355km ~50 mpg (gas equiv.) 93mph 150km/h 156.6 L Compress. hydrogen @ 5000 psi
LA (5 total) Japan’s Cabinet Office (1) leasing at $6500/mo. each (12/2/02)
  Kiwami concept 2003 Fuel cell N/a N/a N/a N/a N/a Hydrogen
Unveiled at Tokyo Motor Show
Hyundai Santa Fe SUV 2000 Ambient pressure Fuel cell 75kW/ PEM UTC Fuel Cells 100mi 160km N/a 77mph 124km/h Compress. Hydrogen
  Santa Fe SUV 2001 Ambient pressure Fuel cell 75kW/ PEM UTC Fuel Cells 250mi 402km N/a N/a Compress. Hydrogen
2003 – 2004 limited intro. to power utilities & research ins
Mazda Demio (compact passenger car) 1997 Fuel cell/ ultra capacitor hybrid 20kW/ PEM Mazda 106mi 170km N/a 60mph 90km/h Hydrogen (stored in metal hydride)
  Premacy FCEV 2001 Fuel cell 85kW/ PEM Ballard Mark 900 Series N/a N/a 77mph 124km/h Methanol
Awarded road permit for Japanese roads in 2001 – undergoing public road testing
Mitsubishi SpaceLiner Concept only 2001 Fuel cell/ battery hybrid 40kW/ PEM N/a N/a N/a N/a Methanol
Commercial target date in 2005
  Grandis FCV (mini-van) 2003 Fuel cell/
battery hybrid
68kW PEM Daimler Chrysler/ Ballard 92mi 150km N/a 87mph 140km/h Compress. Hydrogen
Will be launched in Europe in 2004
Nissan R’nessa (SUV) 1999 Fuel cell/ battery hybrid 10kW/ PEM Ballard Mark 700 Series N/a N/a 44mph 70km/h Methanol
Partnership with Renault for gasoline fueled FCV until 2006
**Made prototypes w/ each fuel cell stack Xterra (SUV) 2000/ 2001 Fuel cell/ battery hybrid 85kW/ PEM Ballard Mark 900 Series & UTC Fuel Cells** 100mi 161km N/a 75mph 120km/h Compress. Hydrogen
  X-TRAIL (SUV) 2002 Fuel cell/ battery hybrid 75kW/ PEM UTC Fuel Cells (Ambient pressure) N/a N/a 78mph 125km/h Compress. hydrogen @ 5,000 psi
Approved for Japanese Public road testing – limited marketing later in 2003
  Effis (commuter concept) 2003 Fuel cell/
battery hybrid
N/a N/a N/a N/a N/a N/a
Unveiled at Tokyo Motor Show
PSA Peugeot Citron Peugeot Hydro-Gen 2001 Fuel cell/ battery hybrid 30kW/ PEM Nuvera 186m 300km N/a 60mph 95km/h Compress. Hydrogen
PSA Peugeot Citron Peugeot Hydro-Gen 2001 Fuel cell/ battery hybrid 30kW/ PEM Nuvera 186m 300km N/a 60mph 95km/h Compress. Hydrogen
  Peugeot Fuel Cell Cab "Taxi PAC" 2001 Fuel cell/ battery hybrid 55kW/ PEM H Power 188mi 300km N/a 60mph 95km/h 80 Liters Compress. hydrogen @ 4300 psi
  H2O firefighting Concept only 2002 Battery/ fuel cell APU N/a N/a N/a N/a N/a Catalyzed chemical hydride - Sodium Boro-
hydride
Uses Millennium Cell’s ‘Hydrogen on Demand’ system
Renault EU FEVER Project (Laguna wagon) 1997 Fuel cell/ battery hybrid 30kW/ PEM Nuvera 250mi 400km N/a 75mph 120km/h Liquid hydrogen
Partnership with Nissan on gasoline fueled FCV
Suzuki Covie Concept only 2001 Fuel cell N/a GM N/a N/a N/a N/a
  Mobile Terrace 2003 Fuel cell N/a GM N/a N/a N/a Hydrogen
Unveiled at Tokyo Motor Show
Toyota RAV 4 FCEV (SUV) 1996 Fuel cell/ battery hybrid 20kW/ PEM Toyota 155mi 250km N/a 62mph 100km/h Hydrogen (stored in metal hydride)
  RAV 4 FCEV (SUV) 1997 Fuel cell/ battery hybrid 25kW/ PEM Toyota 310mi 500km N/a 78mph 125km/h Methanol
  FCHV-3 (Kluger V/ Highlander SUV) 2001 Fuel cell/ battery hybrid 90kW/ PEM Toyota 186mi 300km N/a 93mph 150km/h Hydrogen (stored in metal hydride)
Toyota is developing a Japanese residential 1kW stationary fuel cell system for 2005
  FCHV-4 (Kluger V/ Highlander SUV) 2001 Fuel cell/ battery hybrid 90kW/ PEM Toyota 155mi 250km N/a 95mph 152km/h Compress. Hydrogen @ 3,600 psi
Completed Japanese road testing
  FCHV-5 (Kluger V/ Highlander SUV) 2001 Fuel cell/ battery hybrid 90kW/ PEM Toyota N/a N/a N/a Low sulfur, clean gasoline (CHF)
Partnered with GM on gasoline CHP reforming technology
  FCHV (Kluger V/ Highlander SUV) 2002 Fuel cell/ battery hybrid 90kW/ PEM [122 hp] Toyota 180mi 290km N/a 96mph 155km/h Compress. hydrogen @ 5,000 psi
3 leased to UC Irvine, 3 to UC Davis & 4 to Japanese gov’t agencies (12/2/02) for 30 months at $10K/mo. each. 6 more to be leased to Japanese local gov’ts and private co.’s
  FINE-S Concept only 2003 Fuel cell N/a N/a N/a N/a N/a N/a
Toyota’s freedom of design concept
VW EU Capri Project (VW Estate) 1999 Fuel cell/ battery 15kW/ PEM Ballard Mark 500 Series N/a N/a N/a Methanol
Involved Johnson- Matthey, ECN, VW, and Volvo
  HyMotion 2000 Fuel cell 75kW/ PEM N/a 220mi 350km N/a 86mph 140km/h 13 gal. Of Liquid Hydrogen
  HyPower 2002 Fuel cell/ super capacitors hybrid 40kW/ PEM Paul Scherrer Institute 94mi/ 150km N/a N/a Compress. Hydrogen

 

Oxidation/Reduction reaction in a fuel cell

Figure 3. Oxidation/Reduction reaction in a fuel cell.

The hydrogen used in fuel cells can be supplied directly or indirectly from a fuel reformer that converts alcohol, natural gas, or other hydrocarbon fuels into hydrogen. Since the primary exhaust of fuels cells is water, they offer the promise of an environmentally friendly power source.

Table 3 provides a comparison of some currently available fuel cells. In addition, some fuel cells currently under development include:

  • Regenerative fuel cells. In these cells, water is converted to hydrogen and oxygen by a solar-powered electrolyzer. The cell then coverts this fuel into electricity, heat, and water. The water is then recycled into the electrolyzer. NASA is currently spearheading this technology.
  • Zinc-air fuel cells (ZAFC). In ZAFCs, atmospheric oxygen is passed through a gas diffusion electrode and converted to water and hydroxyl ions. The ions then travel through an electrolyte to a zinc anode where it reacts to form zinc and electricity. Much like a rechargeable battery, the zinc electrode can be regenerated. Unlike a battery, however, this process takes only about five minutes.
  • Protonic Ceramic Fuel Cells (PCFC). These cells use high operating temperatures (700 °C) and ceramic electrolytes with high protonic conductivity. As a high temperature cell they have similar advantages to MCFCs while also integrating many of the advantages of PAFCs such as high proton conduction. In addition, they electrochemically oxidize fossil fuels at the anode, eliminating a hydrogen producing step.

Table 3. A comparison of the most common fuel cell types.
 

Fuel Cell Type Electrolyte Availability Operating Temp. Efficiency Advantages Disadvantages
Phosphoric Acid (PAFC) Phosphoric acid soaked in a matrix Currently available 150-200 ºC 40%, 85% cogener-
ation
Can use impure H2 as fuel. Can tolerate up to 1.5% CO at operating temp. Uses expensive Pt as catalyst, relatively low current generation and large size and weight
Proton Exchange Membrane (PEM) Polyperfluoro-
sulfonic acid
Under develop-
ment, prototypes in use
80 ºC   High power density, can quickly vary output (good for vehicles), solid electrolyte Sensitive to fuel impurities
Molten Carbonate (MCFC) Carbonate solution Under develop-
ment, prototypes in use
650 ºC 60%, 85% cogener-
ation
High operating temperature, therefore, no expensive noble metal catalysts and can operate on cheap fuels. High operating temperature accelerates corrosion of cell components
Solid Oxide (SOFC) Yttria-stabilized zirconia, or more recently, lanthanide doped ceria Under develop-
ment, prototypes in use
1000 ºC 60%, 85% cogener-
ation
High operating temperature, therefore, no expensive noble metal catalysts and can operate on cheap fuels High operating temperature accelerates corrosion of cell components
Alkaline KOH(aq) soaked in a matrix Used by NASA on space missions for decades 150-200 ºC up to 70% Aqueous electrolyte promotes fast cathode reaction and high performance High cost
Direct Methanol Fuel Cell (DMFC) Similar to PEM, however, uses methanol directly Under develop-
ment, prototypes in use
50-100 °C 40% Due to the low operating temperature, good for small portable devices Problems with fuel passing over the anode with producing electricity

Fuel cells have the same basic components as batteries: anode, cathode, and electrolyte. Yttria-stabilized zirconia (Sigma-Aldrich Prod. No 572349 and 572322) is one of the most commonly used electrolytes in solid oxide fuel cells.5 Lanthanide-doped ceria (Sigma-Aldrich Prod. No 572330, 572357, 572365, or 572381) is also gaining favor as a fuel cell electrolyte due to its improved properties at lower temperatures.8 Furthermore, doped cerium oxide electrolytes exhibit an ionic conductivity three to five times greater than that of yttria stabilized zirconia. Platinum black (Sigma-Aldrich Prod. No 205915) and platinum-based alloys are the most common electrodes for fuel cells. Recent work by Scherer and coworkers have recently developed a good model to examine the surface area effects of platinum electrodes as well as glassy carbon (Sigma-Aldrich Prod. No 484164) electrodes.9

Catalysts for Fuel Cells

Platinum is the most common catalyst for fuel cells, however, due to its high cost it is often doped with palladium, ruthenium, cobalt/nitrogen complexes, or more recently iridium or osmium. In addition to its high cost, platinum is also quite rare. In fact, there is not enough platinum in the world to equip every vehicle in use today with a traditional (Pt-catalyst) proton exchange membrane (PEM) fuel cell. For this reason, new catalysts, doped-platinum catalysts, and new platinum-deposition techniques are all being developed to reduce the amount of platinum needed for fuel cell catalysts.

Conducting Polymers

The discovery over twenty five years ago of relatively high electrical conductivity (~10+3 S/cm) of doped polyacetylene10 sparked extensive research in the application of conjugated polymers in such diverse fields as electronics, energy storage, catalysis, chemical sensing, biochemistry, and corrosion control.11,12 Unfortunately, the conducting polymers were found to be unstable in air and difficult to process. Significant advances in improving the desired electrical, optical, and mechanical properties, while simultaneously enhancing processability and stability, have been realized by cross-disciplinary collaborations between chemists, physicists, materials scientists, and engineers.

Polyaniline is becoming the conducting polymer of choice in many applications for several reasons: its electronic properties are readily customized, it exhibits excellent chemical stability, and is the least expensive of the conducting polymers.

Polythiophenes have been studied extensively for use in light emitting diodes, among other applications, due to the chemical variability offered by substitution at the 3- and 4- positions. The regularity of the side-chain incorporation strongly affects the electronic band gap of the conjugated main chain and is critical to device performance.13 Aldrich offers highly regiocontrolled alkylsubstituted polythiophenes (P3HT): almost completely regioregular head-to-tail (HT) P3HT (Sigma-Aldrich Prod. No 698997 and 698989) and regiorandom (1:1 HT/HH) P3HT (Sigma-Aldrich Prod. No 510823).14

High-Purity Inorganics

Aldrich maintains the highest standards for quality control and quality assurance. High-purity materials are rigorously analyzed by a variety of techniques including trace metals analysis by ICP, which can detect impurities an order of magnitude below ppm levels. Fuel cells and batteries often require high-purity components. For example, the electrolytes in low-temperature rechargeable batteries can be from alkyl carbonates and high purity lithium salts such as LiPF6 (Sigma-Aldrich Prod. No 450227) and LiAsF6 (Sigma-Aldrich Prod. No 308315).15

High-purity inorganics also find significant industrial usage. More than 60% of the industrially used cadmium is in Ni-Cd batteries, of which 75% is found in cellular phones. Much of the remainder of this portion is also used in the telecommunications industry as materials in emergency power supplies for electronic telephone exchanges.

Liquid Electrolytes

The type of electrolyte used for a fuel cell depends upon the choice of fuel cell (see Table 2). The key role of the electrolyte is to create a medium through which ions can move between the anode and the cathode. Electrolytes can also act as a kind of filter, preventing undesirable ions or electrons from disrupting the desired chemical reactions.

Plasticizers and Binders

The use of plasticizers in commercial polymer formulations to decrease Tg and the internal viscosity, and to increase bulk flexibility is a well-established practice in a multitude of industrial applications. In fact, the "new car smell" enjoyed by many car owners results mainly from the phthalate plasticizer vaporized in the closed car interior, and actually advertises the deterioration of the vinyl upholstery. To improve the permanence of the plasticizer higher-molecular-weight phthalates are commonly used for modern car interiors. A number of criteria are considered in choosing a plasticizer, including cost, compatibility, stability, ease of processing, and permanence. In addition to the aforementioned uses, a growing body of research has emerged over the past two decades on the application of plasticized polymers in areas that involve properties not usually associated with polymers. For example, the introduction of oligomeric poly(ethylene glycols) (PEG) and derivatives as plasticizers, to effect a significant increase in ionic conductivity as solid polymer electrolytes (SPEs), for use in high energy density batteries and other solid-state electrochemical devices.16-18

Cellulose triacetate membranes, plasticized with 2-nitrophenyl octyl ether (Sigma-Aldrich Prod. No 365130), are used as materials for separations. They are impermeable to metal cations, but allow anion exchange19 and are also remarkably permeable to neutral, mono- and disaccharides.20 Highly efficient photorefractive polymer composites can be formed using 9-ethylcarbazole (Sigma-Aldrich Prod. No E16600) (ECZ) as a plasticizer in guest-host polymers.21

Solid Polymeric Electrolytes

NASA's jet propulsion laboratory is currently investigating SPEs formed by reacting lithium salts (e.g. LiClO4, LiPF4, and LiCF3SO3) with cyanoresins for rechargeable batteries and electrochemical cells. Specifically, SPEs would be used as separators between carbon composite anodes and cathodes. It has been proposed that these batteries would have an energy density of 80 W·h·lb-1 and be viable for 1000 recharge cycles. Polyacrylonitrile (Sigma-Aldrich Prod. No 181315), polyvinyl pyrrolidone (Sigma-Aldrich Prod. No 234257 and 856568), and polyethylene (Sigma-Aldrich Prod. No 427772 and 427799) have dielectric constants between 4 and 5 and lithium-ion conductivities between 10-6 and 10-5 S·cm-1. Unfortunately these room-temperature conductivities are too low for effective power generation. SPE's formed from amorphous cyanoresins such as cyanoethyl polyvinyl alcohol (CRV), cyanoethyl pullulan (CRS), and cyanoethyl sucrose (CRU) have dielectric constants as high as 20 or more and lithium-ion conductivities 100 times greater than conventional SPEs.15

Aldrich also carries a complete line of Nafion® resins. Nafion® resins are perfluorinated ion-exchange materials composed of carbonfluorine backbone chains and perfluoro side chains containing sulfonic acid groups. Solid polymer fuel cells for pulse power delivery are based on Nafion® solid polymeric electrolytes.22
 

References

  1. Fabjen, C. et al. Electrochimica Acta 2001, 47, 825.
  2. Han, S.-C. et al. J. Alloy. Compounds 2003, 351, 273.
  3. Wang, H.; Yoshio, M. Mat. Chem. Phys. 2003, 76.
  4. Takami, N. et al. J. Power Sources 2001, 677.
  5. Grove, W. Phil. Mag. 1839, 14, 127.
  6. Chem. and Ind., 2002, 23, 16.
  7. Gorte, R. J. et al. Adv. Mat. 2000, 12, 1465.
  8. Minh, N. Q. J. Am. Ceram. Soc. 1993, 76, 563.
  9. Paulus, U.A. et al. J. Electroanalytical Chem. 2003, 541, 77.
  10. Chiang, C.K. et al. Phys. Rev. Lett. 1977, 39, 1098.
  11. Liu, G.; Freund, M.S. Macromolecules 1997, 30, 5660, and references therein.
  12. Jasty, S.; Epstein; A.J. Polym. Mater. Sci. Eng. 1995, 72, 565, and references therein.
  13. Berggren, M. et al. Nature 1994, 372, 444.
  14. For characterization and solid-state properties, see Chen, T-A. et al. J. Am. Chem. Soc. 1995, 117, 233.
  15. http://www.nasatech.com/Briefs/Dec02/NPO19116.html (accessed Feb. 11, 2003).
  16. Hardy, L.C.; Shriver, D.F. J. Am. Chem. Soc. 1985, 107, 3823.
  17. Chintapalli, S.; Frech, R. Macromolecules 1996, 29, 3499.
  18. Lauter, U. et al. ibid. 1997, 30, 2092.
  19. Hayashita, T. et al. Chem. Lett. 1996, 37.
  20. Riggs, J.A.; Smith, B.D. J. Am. Chem. Soc. 1997, 119, 2765.
  21. Hendrickx, E. et al. Macromolecules 1998, 31, 734.
  22. Lakeman, J. et al. J. Power Sources 1997, 65, 179.