New Developments in Electrochemical Cells
Science Update Programme
Education Bureau, HKSAR &
Department of Chemistry The University of Hong Kong
June 2002
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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References
Batteries
www.nec-tokin.net
www.duracell.com
Fuel Cells
www.fuelcells.com
chem..hku.hk/~fuelcell
Books:
A.J. Bard, L. Faulkner, “Electrochemical Methods”, 2001, Wiley.
Derek Pletcher and Frank C. Walsh, “Industrial Electrochemistry”, Chapman and Hall, 1990.
C.A. Vincent and B. Scrosati, “Modern Batteries : An Introduction to Electrochemical Power Sources”, Butterworth-Heinemann, 1998.
James Larminie and Andrew Dicks, “Fuel Cell Systems Explained”, Wiley, 2000.
Utilities
www.ifc.com
www.gepower.com
Portable Power Sources
www.nokia.com
www.motorola.com
Capacitors
www.nec-tokin.net
www.faradnet.com
Green Energy
www.greenenergy.org.uk
www.greenenergyohio.org
Electric Vehicles
Evworld.com
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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•Electrochemistry, General Chemistry
•Physical Chemistry:Thermodynamics, Kinetics, Transport
•Organic Chemistry
•Inorganic, Solid State Chemistry
•Materials Science
•Basics Physics, Energy, Electricity
•Environmental Science and Ecological/Biological Issues
Can be discussed with different emphasis, at different levels, and platforms.
Multidisciplinary and Integrated Science
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1. Fundamental Theories and Concepts
2. Batteries
3. Fuel Cells
4. Applications
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Fundamentals
Thermodynamics
•Relate Reactivity to Electrode Potential
•Nernst Equation accounts for concentration(activity) effects
•Calculate Electrode Potential from Free Energy
nF
GEEE
oocell
oanode
ocathode
][
[Re]log
0591.0ln
Oxnaa
aa
nF
RTEE
bB
aA
dD
cCo
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-1.66 -0.76 0.0 0.52 1.23 V
Al/Al+3 Zn/Zn+2 H2/H+ Cu/Cu2+ H2O/O2
Electrochemical Activity Series
nF
GEEE o
celloanode
ocathode
oanodeE
ocathodeE
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Fundamentals
Kinetics
•Current Rate of reaction (Faraday’s law)
•Rate (current) described by Tafel Equation
oeq consti
nF
RTEE .ln
RT
EEnF
C
C
RT
EEnF
C
Cii
eq
R
Req
O
Oo
)()1(exp
)(exp
**
or Butler-Volmer Equation (Bard and Faulkner, Wiley 2001)
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Fundamentals
Kinetics
RT
EEnF
C
C
RT
EEnF
C
Cii
eq
R
Req
O
Oo
)()1(exp
)(exp
**
n F E
Free energy
G
Reaction co-ordinate
nF E
R
O + n e-
O*
from Absolute Rate Theory
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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E=Eeq
i
E or
Current into electrolyte
Electrons out of electrode
RT
EEnF
C
C
RT
EEnF
C
Cii
eq
R
Req
O
Oo
)()1(exp
)(exp
**
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Concentration
or pH effect
i
E
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i- i
Ecell CathodeAnode
E
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i- i
Ecell CathodeAnode
E
Ref. electrode
E-Eref
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Fundamentals
Transport and Interfaces
•Rate of supply of raw materials : diffusion of active materials
• Rate of removal of: products including ions, electrons
ionic vs ohmic resistance
•Change of solid interfaces: dentritic growth
•Wetting/non-wetting affects gas transport into electrolyte
•Selectivity of transport, e.g. cationic membrane
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concentration
10 M
KOH
H2SO40.6
ohm-1 cm-1
CH3COOH
KCl
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E
iLim
iLim
i
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EG
nF
Ideal Voltage
Activation
Ohmic Mass-Transfer
Current Density
Cell
Voltage
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Open Circuit Voltage
Equilibrium potential, Standard Potential
Overpotential, underpotential
Polarization (activation, ohmic, concentration)
Capacity mA hr
Energy Density W hr kg-1 , W hr l-1
Power Density W kg-1 , W l-1 , W cm-2
Current Density mA cm-2
Some Terminologies
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•Anode: Oxidation reaction, release electrons to external circuit, negative terminal (galvanic cell)
•Cathode: Reduction reaction, receive electrons from external circuit, positive terminal (galnanic cell)
•Current Collector: continuous electronic conducting solid phase to collect electrons (in anode) and to distribute electrons (in cathode)
•Electrolyte: ionic conducting but electronic insulating, transfer ions from/to electrodes
•Separator: hydrophilic porous sheet material to hold a thin layer of electrolyte, electronic insulation
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•Polymer Electrolyte: polymeric backbone with fixed charge to allow transport of either cation or anion
•Porous Matrix to hold electrolyte: Ceramic, asbestos, “polymers”.
•Gel/Paste electrolyte: immobilize electrolyte but allow ionic transport
•Molten Salt Electrolyte:e.g. Carbonates
•Solid Oxide Electrolyte: oxide ion mobiliity at elevated temperature
Batteries
A. Volta, 1880
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Primary Batteries: Zn/C
Alkaline
Zn/HgO
Li metal
Secondary Batteries: Lead Acid
(Rechargeable) Ni-Cd
Ni-MH
Li ionHybrid of Battery and Fuel Cell: Zn-Air
Al-Air
(Regenerative Fuel Cells)
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Zinc/Carbon (Leclanché 1880s)
•Cathode: 2 MnO2 + H2O + 2e- Mn2O3 + 2OH-
•Anode: Zn Zn2+ + 2e-
•Overall: 2 MnO2 + Zn + H2O Mn2O3 + Zn2+ + 2OH-
G=-257 kJ mol-1 , Eo = 1.55 V
• electrolyte: moist NH4Cl/ZnCl2/MnO2/C powder
• current collectors: graphite rod and zinc
•Capacity 6 A hr, energy density 80 Whr kg-1
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Zinc/Carbon (Leclanché 1880s)
Zinc can anode (-ve)
Carbon rod current collector (+ve)
MnO2 based positive paste
separator
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Batteries
Lead/Acid
•Cathode: PbO2 + 4H+ +SO42- + 2e- 2H2O + PbSO4
•Anode: Pb + SO42- PbSO4 + 2e-
•Overall: PbO2 + Pb + 4H+ + 2SO42- 2PbSO4 + 2H2O
G= -394 kJ mol-1 , Eo = 2.05 V
• electrolyte: aqueous H2SO4
• current collectors: both Pb
•Capacity: 2.7 Ahr, Energy density 30 Whr kg-1
•cell voltage> 1.23 V, Electrolysis of water kinetically hindered
Discharge reactions
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-0.3505 0.0 1.23 V 1.698
Pb/PbSO4 H2/H+ H2O/O2 PbSO4/PbO2
Possible Electrode Pairs?
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Nickel/Cadmium
•Cathode: 2NiO(OH) + 2H2O + 2e- 2Ni(OH)2 + 2OH-
•Anode: Cd + 2OH- Cd(OH)2 + 2e-
•Overall: 2NiO(OH) + Cd + 2H2O 2Ni(OH)2 + Cd(OH)2
G= -283 kJ mol-1 , Eo = 1.48 V
• electrolyte: aqueous KOH
• current collectors: Ni foam and peforated nickel sheet
•Capacity: 4 Ahr, energy density: 33 Whr kg-1
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Nickel/Metal hydride
•Cathode: NiO(OH) + H2O + e- Ni(OH)2 + OH-
•Anode: MH + OH- M + H2O + 2e-
•Overall: MH + NiO(OH) M + Ni(OH)2
•Metal hydride: AB5 e.g. LaNi5 or AB2, e.g. TiMn2 , ZnMn2
• electrolyte: aqueous KOH
• current collectors: Ni foam and peforated nickel sheet
•Capacity: 4 Ahr, energy density: 80 Whr kg-1
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Nickel/Metal Hydride
Overcharging
•Cathode: 2 OH- H2O + ½O2 + 2e-
•Anode: charge reserve M + H2O + 2e- MH + OH-
•Oxygen dissolves to Anode: 2MH + ½ O2 2M + H2O
Prevent gassing and build up of pressure
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Lithium Ion
•Cathode: xLi+ + LiM2O4 + xe- Li1+xM2O4
M=Mn,Ti
xLi+ + LiMO2 + xe- Li1+xMO2
M=Co, Ni
•Anode: LiC6 x Li+ + x e- + Li1-xC6
•Overall: C6 + LiMO2 LixC6 + Li1-xMO2
• LiMn2O4 G= -287 kJ mol-1 , Eo = 2.97 V
•Energy density > 100 Whr/kg
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Batteries
Lithium Ion
•Aprotic Solvent
•Gel
•Polymer (lower weight)
Electrolyte
•Li in graphite lattice
•Lower activity but safer than Li metal
Anode
•Solid Structures for storing Li
•Spinels, Olivines, rhombohedral NASICON
Cathode
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Batteries and Fuel Cells
Batteries Recharge Intermittent Closed system Mostly solid High power density
Fuel Cells ReFuel Continuous Open system Mostly Gas/Liquid Fuel High energy density Micro to Mega Watts
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Fuel Cells Efficient conversion of Chemical
Energy to useful energy (without losing to heat, mechanical linkages)
Environmentally friendly Flexible: from micro to mega Materials and Nanotechnology
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Fuel Cells Classification according to electrolyte Alkaline Fue Cells Proton Exchange Membrane (PEM) Phosphoric Acid Molten Carbonate Solid Oxide Electrolyte
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燃料電池發電的原理
正極 ﹕氧氣 ( 氧化劑 )
負極﹕燃料 ( 氫氣﹐酒精﹐ 葡萄糖等 )
電能
CxHyOz ===> CO2 + H2O + e-
O2 + e- ===> H2O
負極
電解液
正極
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Fuel Cells
Chemical Energy Electrical Energy
thermal
Work
Heat
G
H
H T S
H
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EG
nF
Ideal Voltage
Activation
Ohmic Mass-Transfer
Current Density
Cell
Voltage
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Diversity of Technology andMaterials Problems in Fuel Cells
Fuel Oxidant Catalyst Container Control Transport Storage
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Fuels: HydrogenMetalsNatural GasSmall Hydrocarbons(methanol, glucose)
Oxidant: airoxygenhalidesoxides
Catalysts: platinummetalsmetal oxidesmacrocycles
Catalyst Support: Porous CarbonCeramic MatrixMolecular SievesPolymer
Container and Movable Parts:AlloysCeramicPolymers
Transport/Electrolyte:Proton Exchange MembranesPTFE (Teflon)Solid Electrolyte
Storage: Metal Hydride
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Fuels Hydrogen H2+2OH- 2H2O +2e-
2e- +½ O2+H2O 2 OH-
Methanol CH3OH + H2O CO2 + 6H+ +6e-
6e- +1½ O2+6H+ 3H2O
Aluminium Al + 4OH- Al(OH)4- +3e-
4e- +O2+2H2O 4 OH-
Borohydride NaBH4 + 8 OH- NaBO2 + 6H2O + 8e-
Methane (natural gas) Octane : demonstrated in SOFC half cell
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Thermochemistry
cH (kJmol-1)
cG (kJmol-1)
n E kJ/kg kJ/cm3
Hydrogen - 285 - 237 2 1.23 118500 0.011 Methane - 890 - 818 8 1.06 51125 17.31 Methanol - 726 - 702 6 1.21 21938 17.37 Glucose -2808 -2865 24 1.23 15916 24.57 Octane -5471 -5297 50 1.10 47907 66.10
June 2002 Electrochemical Cells, K.Y. Chan, HKU
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Micro and Nanostructured Electrodes:
Catalyst Support: High Surface Carbon Size Effects of Catalysts Controlled Porosity Controlled Wetting Maxinum Gas-Liquid-Solid Interface Minimize ohmic resistance Minimize ionic resistance
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Scanning Tunneling Spectroscopy
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Catalysts
Platinum is the most important for both anode and cathode
Platinum can be replaced by Ag, Mn, Co, only for oxygen reduction in alkaline medium
Platinum subject to CO poisoning (impure H2) Binary/Ternary system, macrocycle,
bifunctional Stability/Life of nanometals
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01020304050607080901000
20
40
60
Pe
ak C
urre
nt D
en
sity (mA
/cm
2 )
Co atom % in Co/Pt
Maximum peak current density at 52.5~77.6% Co, one order of magnitude higher than that of pure Pt particles. One possible role of cobalt in promoting the catalysis of platinum, is the removal of COad COOHad intermediates.
)2( eHCOOHHCOOH ad
)4(2 eHCOCOOH ad
)3( 22 eHOHCOCOCOOHHCOOH adad
)5(2 eHCOOHCO adad
Chi et al., Catalysis Letters, 71 (2001) 21.
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Catalysts
Oxygen Cathode is most limiting and is present in most fuel cells
Non-platinum cathode catalyst can tolerant cross over effect.
At high temperature, no precious metal or no catalysts is needed in MCFC and SOFC
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Performances of different air cathode
-10 0 10 20 30 40 50 60 70-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
10mA/cm2 constant current discharge(186mA CCD)Fuel: 95% ethanol/7M KOH 1:11 Membrane added
Performance Comparision of air cathodesAC-51, AC-65, AC-75
Po
ten
tia
l(V
)H
gO
Time(min.)
AC51KMnO4 AC65Ag AC75Co
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Gas Diffusion Electrodes
H2H+
e-
Chan et al. , Electrochimica Acta, 32 (1987), 1227;33 (1988) 1767.
Tang and Chan, Electroanal. Chem. 334 (1992) 65.
Electronic circuit: continuous solid phase
Ionic circuit: Continuous electrolyte phase
Materials flow circuit: feed of reactancts
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Single air cathode
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Electrolyte Alkaline electrolyte (first deployed
for Apollo mission) Phosphoric Acid 180 C Polymer Electrolyte Cross Over Stability (CO2 removal in alkaline) Solid Oxide (YSZ, doped Ceria) Shunt Current / Leak Current
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SOFC Electrolyte
Ytrium Stabilized Zirconia Doped Ceria (Cerium Oxide) O2- conductivity at 600~800 C
ZrCe
Ce
Ce
Y Y
O2-
O2-
O2-
O2- O2-
O2-O2-
O2-
O2-
CeCe
Ce Ce
CeCe
Ce
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Stack Design
Manifold for fuel feed Manifold for oxidant feed Electronic circuit Ionic circuit Water transport Temperature, humidity control
June 2002 Electrochemical Cells, K.Y. Chan, HKU
52H+
e-
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Product Name: Fuel Cell StackFuels usable: Glucose, methanol, ethanol,NaBH4 No. of Fuel Cells: 10 in SerialOpen Circuit Voltage: 4.0-9.0VPower Output: 0.5-1.0WApplication: Stationary or Portable( Mobile phone or toy cars )
Applications Demonstrated:Radio(Voice of Glucose);Portable CD player;Mobile Phone(GSM).
Stack Design
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Electronic circuit: continuous solid phase with minimum electrical resistance to electronically connect anode and cathode through external circuit.
•Ionic circuit: to complete the other half of the “charge circuit”. Continuous electrolyte phase connecting cathode and anode, but electronic insulating. Maintain balance of ions for anodic, cathodic reactions.
•Materials flow circuit: feed of reactancts to and removal of products from anode/cathode.
•Avoid shunt current, leak current in multiple cells
•Avoid short circuit of cathode and anode
•Avoid breaking electrochemical window of electrolyte
June 2002 Electrochemical Cells, K.Y. Chan, HKU
Stationery Power Utilities10~100 kW100~500 kWhrONSY (IFC), Fiji SOFC (Westing House, Honey Well)Load LevellingPower DistributionLife
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June 2002 Electrochemical Cells, K.Y. Chan, HKU
Electric Vehicles10~100 kW
100~500 kWhr
Battery vs Fuel Cells
Hybrid with ICE and capacitor
Costs: 7 times normal costs
Startup time
Direct/Reformer
Fueling Station Infrastructure
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Transportation Fuel CellBallard Power Systems
1st Generation Fuel Cell Transit Bus2nd Generation Fuel Cell Transit Bus
ChryslerFuel Cell Vehicle Model.
Coval H2 PartenersT-1000 Neighborhood Truck.
Daimler-BenzThe NECAR 3 Three Generations of NECAR VehiclesThe NEBUS
DaimlerChryslerJeep Commander Hybrid Fuel Cell Concept
Energy PartnersThe "Gator" Utility VehicleThe "Genesis" Golf CartThe "Green Car"
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Ford Motor CompanyThe P2000 Prodigy Hydrogen Fuel Cell VehicleThe P2000 - Platform for a Fuel Cell Vehicle.
General MotorsFuel Cell Engine Model
H Power CorporationFuel Cell Bus50w PEM Fuel CellFuel Cell BicycleFuel Cell Wheelchair
Humboldt State University's Schatz Energy Research Center
The Kewet (Danish 2-Seater)Fuel Cell Golf Carts
International Fuel CellsThe Georgetown University Fuel Cell Bus
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MazdaThe Fuel Cell DemioThe Demio - On the RoadUnder the Hood of the Demio
OpelThe Fuel Cell SintraThe Fuel Cell Zafira
Siemens AGPEM Fuel Cell Powered Forklift
ToyotaThe Fuel Cell RAV 4
Volkswagen/VolvoThe Fuel Cell Golf (coming soon)
ZevcoThe Fuel Cell Taxi Cab (London)
Updated January 8, 1999
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June 2002 Electrochemical Cells, K.Y. Chan, HKU
Portable Power Sources10~100 kW
100~500 kWhr
Battery vs Fuel Cells
Safety (H2 , MeOH, caustic electrolyte), Open vs Closed System
Volume vs Weight
Refueling Vs Recharging
June 2002 Electrochemical Cells, K.Y. Chan, HKU
Special ApplicationsSpace/DefenceCommunicationEnergy Storage for SolarEnergy VectorBiomedicalEnery Recovery from WasteMarine and Remote Power Sources
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Energy Vector
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Fuel Cells Running on Biogas from Garbage
(Kajima Co. Japan)
67% CH4
33% CO2140kg/day
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June 2002 Electrochemical Cells, K.Y. Chan, HKU
Demo Fuel Cells0.02 ~ 10 W
H2 , MeOH, Glucose, alcohols, NaBH4
PEM, Alkaline
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World’s first glucose FC Demonstration Kit(HKU-002, Version 3)
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Typical Performance of HKU-001
0 50 100 150 200 2500.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
Area of anode: 12cm2Area of cathode: 35cm2
Vo
ltag
e(V
)
Time(min)
Model: FC-001
2g Glucose, 30mL 1M NaOH