Fuel Cells: Fundamentals, Types, and Fuel

StorageCarly Reed

History 1839

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

1889 Term “fuel cell” coined by Ludwig Mond

1902 J.H. Reid – first to use NaOH in place of acid electrolyte

1952 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 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

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 First AFC developed by Francis Bacon (1930s) In the Apollo missions

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

35% KOH

O2H2

H2O

OH-

Alkaline Fuel Cell

Anode: C/Pt or C/Raney Ni/Pt

Cathode: C/Pt

H2 + 2OH- H2O + 2e-

O2 + H2O + 2e- HO2- + OH-

HO2- + H2O + 2e- 3OH-

1 A/cm2 at 0.7 V

r.t.-80oC

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 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

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 CO3

2-

Polymer Electrolyte Membrane Fuel Cell Used by NASA in Gemini mission

employed polystyrene sulfonate (PSS) polymer (unstable) Nafion – developed by Dupont (1960s)

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

H2 2H+ + 2e- O2 + 2H+ + 2e- H2O2

H2O2 + 2H+ + 2e- H2O

Anode: C/Pt Cathode: C/Pt

N A F I O N

1 A/cm2 at 0.7 V

85-105oC

H2 O2

H+

H2O

Polymer Electrolyte Membrane Fuel Cell

Advantages: Nonvolatile membrane CO2 rejecting electrolyte few material problems

Problems: 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 Cathode: Pt/C

CH3OH + H2O CO2 + 6H+ + 6e-

O2 + 2H+ + 2e- H2O2

H2O2 + 2H+ + 2e- H2O

N A F I O N

85-105oC

400 mA/cm2 at 0.5V at 60oC

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

Advantages: 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: 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 Solving the Crossover Dilemma

Alter thickness of polymer membrane Thinner = decreases ion flow resistance Thicker = decreases MeOH crossover

Cs+ doped membranes Tricolli, University of Pisa, 1998 Lower affinity for H2O

MeOH tolerant cathodes 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 Most commercially developed fuel cell

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 Cathode: Pt/C

H2 – 2e- = 2H+ O2 + 4H+ + 4e- 2H2O

200oC

100%H2PO4

Si matrix separatorPTFE binding

CH4 or H2 O2

H+

H2O

Phosphoric Acid Fuel Cell

Advantages: H2O rejecting electrolyte high temps favor H2O2 decomposition

O2 + H2O +2e- H2O2 Stable H2O2 lowers cell voltage and corrodes electrode

Problems: O2 kinetic hindered CO catalyst poison at anode H2 only suitable fuel low conducting electrolyte

Molten Carbonate Fuel Carbonate 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 Cathode: NiO

CH4 + 2H2O 4H2 + CO2 + 4e-

H2 +CO32- H2O + CO2 + 2e-

O2 + 2CO2 + 4e- 2CO32-

Li2CO3

and Na2CO3

LiAlO3 used to support electrolyte

580-700oC

150 mA/cm2 at 0.8 V at 600oC

H2, CxH2x+2 O2, CO2

CO32-

Molten Carbonate Fuel Cell Advantages:

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: Materials resistant to degradation at high T

Ni, Fe, Co steel alloys better than SS NiO at cathode leeches into CO3

2- 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 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

Cathode = La1-xSrxMnO3

Y doped ZrO2

Anode = NiO-YSZ cermet 800-1000oC

H2 + O2- H2O + 2e- OR

CH4 + 4O2- 2H2O + CO2 + 8e-

O2 + 2e- 2O2-

Interconnector material = Mg or Sr doped lanthanum chromate

1mA at 0.7V

H2, CxH2x+2 O2

O2-

Solid Oxide Fuel Cell Advantages:

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 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.

ApplicationsFuel 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 Pure H2 gas

eliminates reformer eliminates risk of catalyst degradation from

impure fuel space limitations explosive

Liquid H2 highest energy density of any H2 storage

method limited by boiling point (-253oC)

1-2% evaporation each day

Hydrogen Storage: Metal Hydrides A metal alloy exposed to H2 MH

Upon heating H2 released 150-700 cm3/g

“Powerballs” (Powerball Technology Inc) NaH pellets coated in waterproof skin

Hydrogen Storage: Ammonia Borane

S. Shore (1955) Ammonia Borane H3NBH3 Advantages over MH

Air and Water Stable Heat to release H2 19% wt. storage of H2

Developed by Millennium Cell

Hydrogen Storage

Carbon Nanotubes, Glass Microspheres, Zeolites H2 can permeate at elevated P and T At ambient T and P, H2 is trapped in structure Heating releases H2

Hydrogen Storage: Zeolites D. Fraenkel (1977) Tested by Fritz and Ernst (1995)

Cs3Na9(AlO2SiO2)12 Loaded at 2.5-10.0 MPa at 573oC 9.2cm3/g

Fuel Reformation Catalytic steam reformation

Light hydrocarbons and alcohols (highest yield reforming process)

Endothermic Partial oxidation

Heavier hydrocarbons Exothermic (Combustion)

Autothermal reforming Reformed fuel must be treated to remove CO

References 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