Fuel Cells: Fundamentals,
Types, and Fuel Storage
Carly Reed
History
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1839
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1889
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J.H. Reid – first to use NaOH in place of acid electrolyte
1952
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Term “fuel cell” coined by Ludwig Mond
1902
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
Sir William Grove – “Gas Voltaic Battery”
Two Pt strips surrounded by closed tubes containing H2 and O2 in
dilute H2SO4
Produced H2O and electricity, but very inconsistent
Alkaline fuel cell developed by Francis Bacon - later used in
Apollo space missions
1960-1965

First successful application achieved with space technology
during NASA Apollo space program
Interest in Fuel Cells
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Development of fuel cells has lagged behind:
 Higher cost
 Materials problems
 Operational inadequacies
During the 20th century as need for electricity increased,
primary fuel sources were still so abundant
Currently, with a desire to decrease:
 Dependence on fossil fuels and foreign oil supplies
 Emissions of NO2, NO3, SO2, CO2 and their effects on
ozone levels, acid rain, and global warming
Fuel cells with renewable energy sources
High electrical efficiency
Fuel Cells: Components and Functions
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MEA = membrane
electrode assembly
(electrolyte and
electrodes)
Anode = fuel
electrode; electronic
conductor and
catalyst
Cathode = air
electrode; electronic
conductor and
catalyst
Electrolyte =
oxygen-ion
conductor, electron
inhibitor
Fuel Cells: Types

Fuel cell types can be divided in two ways:


Low v. High Temperature
Electrolyte Types
 Alkaline
 Polymer Electrolyte Membrane (Proton
Exchange Membrane)
 Direct Methanol
 Phosphoric Acid
 Molten Carbonate
 Solid Oxide
Alkaline Fuel Cell

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First AFC developed by Francis Bacon (1930s)
In the Apollo missions
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85% KOH
200-230oC
Ni anode and NiO cathode
Acidic fuel cells had been used, but alkaline had
faster oxygen reduction kinetics
Fuel cells were used to provide electricity, cool the
ship, and provide potable water
Alkaline Fuel Cell
Anode: C/Pt or
C/Raney Ni/Pt
Cathode: C/Pt
r.t.-80oC
H2
1 A/cm2 at 0.7 V
O2
H2O
OH35%
KOH
O2 + H2O + 2e-  HO2- + OHH2 + 2OH-  H2O + 2e-
HO2- + H2O + 2e- 3OH-
Alkaline Fuel Cell

Advantages:
 Low cost electrolyte solution (KOH 30-35%)
 Non-noble catalyst withstand basic conditions
 O2 kinetics faster in alkaline solution
 OH- v. H2O
Alkaline Fuel Cell

Problem Areas and Solutions:
 Catalysts
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Pt – expensive
Raney Ni – wettability; chemical composition
- Y. Kiros, Pt/Co alloys; similar ability to reduce O2
- E.D. Geeter et. al testing Ag and Co to replace Pt
 Pure gases only
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CO32- builds up in electrolyte and clogs pores
CO2 + 2OH-  CO32- + H2O
Fe sponges can be inserted to absorb CO2
Circling electrolyte can slow build up of CO32-
Polymer Electrolyte Membrane Fuel Cell

Used by NASA in Gemini mission
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Nafion – developed by Dupont (1960s)
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employed polystyrene sulfonate (PSS) polymer (unstable)
Currently used in most PEMs
Polytetrafluoroethylene (PTFE) backbone with a
perfluorinated side chain that is terminated with a sulfonic
acid group
More stable, higher conductivity
The Dow Chemical Company

Developed a polymer similar to Nafion
 Shorter side chain and only one ether oxygen
 No longer available
Polymer Electrolyte Membrane Fuel Cell

Chemical structure of Nafion

Hydration of membrane dissociates proton of
acid group
Solvated protons are mobile in polymer and
provide conductivity

Polymer Electrolyte Membrane Fuel Cell
Anode: C/Pt
85-105oC
H2
Cathode: C/Pt
O2
H+
H2O
NAFION
O2 + 2H+ + 2e-  H2O2
H2  2H+ + 2e-
H2O2 + 2H+ + 2e-  H2O
1 A/cm2 at 0.7 V
Polymer Electrolyte Membrane Fuel Cell
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Advantages:
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Nonvolatile membrane
CO2 rejecting electrolyte
few material problems
Problems:
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Slow O2 kinetics
Hydration of membrane is difficult (30-60%)
 Formed at cathode, but difficult to keep in
membrane
 Too little = dehydration and loss of ion transport
 Solutions
- Humidify gases
- Impregnate Nafion with SiO2 or TiO2
Direct Methanol Fuel Cell
Anode: Pt/Ru/C
400 mA/cm2 at 0.5V
at 60oC
85-105oC
Cathode: Pt/C
N
A
F
I
O
N
O2 + 2H+ + 2e-  H2O2
CH3OH + H2O CO2 + 6H+ + 6e-
H2O2 + 2H+ + 2e-  H2O
Direct Methanol Fuel Cell


Pt catalyst have highest activity for MeOH
oxidation thus far
Ru enhances MeOH catalytic activity
OH- forms at lower voltage

CO blocks sites on Pt surface, Ru helps oxidize
to CO2
Direct Methanol Membrane Fuel Cell
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Advantages:
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Direct fuel conversion – no reformer needed, all positive
aspects of PEMFC
CH3OH – natural gas or biomass
Existing infastructure for transporting petrol can be
converted to MeOH
Problems:

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High catalyst loading (1-3mg/cm2 v. 0.1-0.3 mg/cm2)
CH3OH hazardous
Low efficiency (MeOH crossover – lowers potential)
Direct Methanol Membrane Fuel Cell
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Solving the Crossover Dilemma
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Alter thickness of polymer membrane
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Cs+ doped membranes
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Thinner = decreases ion flow resistance
Thicker = decreases MeOH crossover
Tricolli, University of Pisa, 1998
Lower affinity for H2O
MeOH tolerant cathodes
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Mo2Ru5S5 – N. Alonso-Vante, O. Solorza-Feria
 Higher oxygen reduction activity in presence of
MeOH
(Fe-TMPP)2O – S. Gupta, Case Western, 1997
 High oxygen reduction, insensitive to MeOH
Phosphoric Acid Fuel Cell
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Most commercially developed fuel cell
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Mainly used in stationary power plants
More than 500 PAFC have been installed and tested
around the world
Most influential developers of PAFC

UTC Fuel Cells, Toshiba, and Fuji Electric
Phosphoric Acid Fuel Cell
Anode: Pt/C
200oC
CH4 or H2
O2
H+
PTFE binding
100%
H2PO4
H2 – 2e- = 2H+
Cathode: Pt/C
Si matrix
separator
H2O
O2 + 4H+ + 4e-  2H2O
Phosphoric Acid Fuel Cell
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Advantages:
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H2O rejecting electrolyte
high temps favor H2O2 decomposition
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O2 + H2O +2e-  H2O2
Stable H2O2 lowers cell voltage and corrodes electrode
Problems:
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O2 kinetic hindered
CO catalyst poison at anode
H2 only suitable fuel
low conducting electrolyte
Molten Carbonate Fuel Carbonate
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Developed in the mid-20th century
Developed because all carbonaceous fuel
produce CO2
Using CO32- electrolyte eliminates need to
regulate CO32- build up
Molten Carbonate Fuel Carbonate
Anode: Ni/Al or Ni/Cr
580-700oC
H2, CxH2x+2
O2, CO2
CO32-
150 mA/cm2 at
0.8 V at 600oC
Li2CO3
and
Na2CO3
CH4 + 2H2O  4H2 + CO2 + 4eH2 +CO32-  H2O + CO2 + 2e-
Cathode: NiO
LiAlO3 used to
support
electrolyte
O2 + 2CO2 + 4e-  2CO32-
Molten Carbonate Fuel Cell
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Advantages:
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Higher efficiency (v. PEMFC and PAFC) (50-70%)
Internal reforming (H2 or CH4)
No noble metal catalyst (High T increases O2 kinetics)
No negative effects from CO or CO2
Problems:
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Materials resistant to degradation at high T
 Ni, Fe, Co steel alloys better than SS
NiO at cathode leeches into CO32- reducing efficiency
or crossing over causing short circuiting
 Dope electrode and electrolyte with Mg
 Kucera and Myles (LiFeO2 or Li2MnO3 stabilize)
Solid Oxide Fuel Cell
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1899 Nernst observed conduction in
various types of stabilized zirconia at T >
600oC
1937 Baur and Preis demonstrated a fuel
cell based on zirconium oxide
Solid Oxide Fuel Cell
Anode = NiO-YSZ cermet
800-1000oC
H2, CxH2x+2
O2
O2-
1mA at 0.7V
Y doped
ZrO2
H2 + O2-  H2O + 2e- OR
CH4 + 4O2-  2H2O + CO2 + 8e-
Cathode = La1-xSrxMnO3
Interconnector
material = Mg
or Sr doped
lanthanum
chromate
O2 + 2e-  2O2-
Solid Oxide Fuel Cell
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Advantages:
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Solid electrolyte eliminates leaks
H2O management, catalyst flooding, slow O2
kinetic are not problematic
CO and CO2 are not problematic
Internal reforming - almost any hydrocarbon or
hydrogen fuel
Problems:

Severe material constraints due to high T
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Stainless steal at lower temperatures
Alloyed metal or Lanthanum Chromite material
Fuel Cell Stacks
Individual Cell
0.5-1.0V

Increase system voltage
by stacking cells

Cells’ voltages are added
in series; current constant
over all cells

Interconnects act as flow
channels for gases and
connects anode of one cell
to cathode of the next.
Must be gas tight and
made from conducting
material.

Applications
Fuel cells are being developed for
application in:
Stationary power plants
 Automobiles
 Portable electronics

To enable mobile power source, fuel must
also be portable
Hydrogen Storage: Gas and Liquid
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Pure H2 gas
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eliminates reformer
eliminates risk of catalyst degradation from
impure fuel
space limitations
explosive
Liquid H2
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highest energy density of any H2 storage
method
limited by boiling point (-253oC)

1-2% evaporation each day
Hydrogen Storage: Metal Hydrides
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A metal alloy exposed to H2  MH
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Upon heating H2 released
150-700 cm3/g
“Powerballs” (Powerball Technology Inc)
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NaH pellets coated in waterproof skin
Hydrogen Storage: Ammonia Borane
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S. Shore (1955)
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Ammonia Borane H3NBH3
Advantages over MH
Air and Water Stable
 Heat to release H2
 19% wt. storage of H2

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Developed by Millennium Cell
Hydrogen Storage
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Carbon Nanotubes, Glass Microspheres, Zeolites

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H2 can permeate at elevated P and T
At ambient T and P, H2 is trapped in structure
Heating releases H2
Hydrogen Storage: Zeolites
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D. Fraenkel (1977)
Tested by Fritz and Ernst (1995)
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Cs3Na9(AlO2SiO2)12
Loaded at 2.5-10.0 MPa at 573oC
9.2cm3/g
Fuel Reformation
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Catalytic steam reformation
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Light hydrocarbons and alcohols (highest yield reforming
process)
Endothermic
Partial oxidation
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Heavier hydrocarbons
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Exothermic (Combustion)
Autothermal reforming
Reformed fuel must be treated to remove CO
References
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Carrette, Linds. Friedrich, K. Stimming, Ulrich. Fuel Cells: Principles, Types,
Fuels, and Applications. Chemphyschem 2000, 1, 162-193
Winter, Martin. Brodd, Ralph. What Are Batteries, Fuel Cells, and
Supercapacitors? Chem. Rev. 2004, 104, 4245-42969
Kee, Robert J. Zhu, Huayang. Goodwin, David G. Solid-oxide fuel cells with
hydrocarbon fuels. Proceedings of the Combustion Institute 2005, 2379-2404
Groves, W.G. Philos Mag (14) 1939 127-130
E.D. Geeter, M.Mangan, S.Spaepen, W. Stinissen, G. Vennekens. J. Power
Sources 1999, 80, 207
Y. Kiros. J. Electrochem. Soc. 1996, 41, 2595
Mauritz, Kenneth. Moore, Robert B. The State of Understanding Nafion Chem.
Rev. 2004, 104, 4535-3585
Tricoli, V. Journal of the Electrochemical Society 1998, 145 (11), 3798-3801
Alonso-Vante, N. Tributsch, H. Solorza-Feria, O. Electrochim. Acta 1995, 40,
567.
Gupta, S. Tryk, D. Zecevic, S.K. Aldred, W. Guo, D. Savinelli, R.F. J.Appl.
Electrochem. 1998, 28,673
Status of Carbonate Fuel Cells J. Power Sources 56 (1995) 1-10
Fraenkel, D. Shabtai, J. Encapsulation of hydrogen in molecular sieve zeolites
JACS 1977 7074-7076
Fritz, M. Ernst,S. Int. J. Hydrogen Energy 1995, 20 (12) 967
Shore, Sheldon JACS 1956 78 (2) 502-503