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Galvanic Cells - INTRODUCTION Energy sources How did the

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Title: Galvanic Cells - INTRODUCTION Energy sources How did the


1
Galvanic Cells - INTRODUCTION
  • Energy sources
  • How did the battery business start?
  • History of batteries makes history of electric
    energy

Galvanic Cell
As ELECTROCHEMICAL DEVICE Electrode
reactions Thermodynamics and kinetics Properties
of Materials
As ENERGY SOURCE Position on energy
market Power supply Technology Economy
2
Electrical power generation
  • Fuel combustion heat effect mechanical
    energy generating electricity
  • CHEMICAL ENERGY indirectly into
    ELECTRICAL
  • Renewable energy source ( wind, water,
    geothermal) transformation of work to electric
    energy
  • Galvanic, fuel, fotovoltaic cells
  • CHEMICAL ENERGY directly into ELECTRICAL

3
DIFFERENT CELLS
  • galvanic cells primary and secondary
  • Fuel cells

Electrode Reactions Expressed as U
ISOLATED PORTABLE/TRANSPORTABLE
INDEPENDENT FORM ELECTROENERGETICAL NETWORK
4
Some milestones in history
1780 L. Galvani animal electricity 1800 A.
Volta pile (battery of zinc and silver discs,
separated by cloth wet with salty solution)
1866 G. Leclanche zinc MnO2 cathode battery
1859 G. Plante lead acid accu made of Pb
plates, 1881 Faury et al pasted plates
instead of solid Pb
5
Transformation from isolated current sources to
electrical network
  • Electromagnetic induction discovered by Faraday
    about 1840
  • Electromechanical generator Siemens about 1857
  • T. A . Edison electric bulb 1879, lighting
    system in NY, Ni-Fe accumulator
  • DC contra AC Edison contra Westinghouse, first
    big power plant in America Niagara Falls
    advantages of supplying energy with AC

6
Electrical circuits with batteries
  • Management of voltage and current connecting
    the batteries
  • Ohms law in simple DC circuit external
    resistance (load),internal resistance( ohmic drop
    on battery components), polarisation resistance
    (ohmic drop on reaction)
  • E I ( Rinter Rpol Rload)
  • Energy and power
  • Energy Q U I t U (m / k) U
  • Power energy produced/consumed in time unit

7
Electrode potential
  • f fo RT/nF ln ( aMe / aMe(n) )
  • Standard potential at unit activity of particles
    - fo
  • deviation from standard due to non-unit
    activity (concentration)
  • Can not be measured directly
  • Electrode reaction
  • Transport of charge or charge and mass over phase
    boundary electrode electrolyte
  • Phases electrode fragment of condensed phase
    electronically conductive
  • electrolyte ionically conducting
    space
  • Observed effects of electrode reaction
  • Change of oxidation grade of an atom in a
    molecule / ion in solution
  • Accompanying changes creation / decomposition
    of a phase
  • changes in phase structures

8
Potential fox
Ared ?Box n e-
Anodic reaction
Potential fred
Cathodic reaction
Cox n e- ?Dred
Overall cell reaction
A B C D
With E ? f
Electromotoric force E comes from change in free
enthaply of the overall reaction, Also combining
the ?G with electrical equivalent of energy E
-?G /nF And defining Eo ?G o/nF for standard
conditions we get Nernst equation E Eo RT
/ nF ln K where K equilibrium constant of
reaction ABCD
9
Signs / - in cells -
convention More negative potential on left side
Zn Zn2 2e f - 0.76 V Less negative
to the right Cu Cu2 2e
f 0.34 V
formal scheme for the cell External connection /
Zn / Zn SO4 aq // CuSO4 aq / Cu / external
connection Sign
- // sign But
.....
10
Structure and functions of electrodes A/
metallic reactive electrodes (deposition-dissolut
ion, formation of compounds on the surface)
Reagent and current collector(two-in-one)
Charge and mass transport on the surface
B/ inert electrodes metalls, graphite,
semiconductors Current collector, not a redox
reagent Charge and mass transport on
the surface C/ multi-function, multi-component
electrodes electroactive component (often
insulator) electronically conducting
matrix other additives with special functions
Charge and mass transport on
triple-contact sites
11
Various types of batteries
12
Specyfic energy - Energy density
13
Typical battery application
14
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15
Zn/MnO2 Cells
  • Leclanche type electrolytes lightly acidic or
    neutral
  • anodic reaction product Zn salts soluble
    in the electrolyte
  • ( NH4Cl, NH4OH, ZnCl2 ? complexes of Zn with
    OH- and Cl-
  • Alkaline electrolyte concentrated KOH
  • anodic reaction product solid ZnO the
    composition ot the electrolyte does not change
  • Different anodic mechanism ? different yields
    of the cells
  • in alkaline cells the maximum current density
    is higher

16
Zn electrode and redox cycling
  • Solid Zn anode Zn 2e-? Zn2 in solution 2e-
    ? Zn as powder, needles
  • (? due to specyiic features of
    electrocrystallization of metals)
  • Volumen of anode ? electrical contact within
    the anode ?
  • Powder Zn anode Zn 2e- ? ZnO ( in
    OH-solution) 2e- ? Zn as powder

  • discharge (work) charge
  • Zn metallurgical foil - 100 material as energy
  • complex structure (Zn conducting matrix
    glue) - part of electrode useless as energy
    source

17
MnO2 cathode
  • MnIVO2 H2O ? MnIIIO(OH) OH-
  • (other compounds of MnIII possible)
  • OH- ion takes part in the anodic reaction
    formation of Zn complexes
  • At higher load (high current density) possible
    limitation of anode kinetics due to low
    concentration of comlexing ions
  • Valid for Leclanche type ( Zn complex salts
    soluble in the electrolyte)

18
Cells with Zn anode
19
Zn - air
  • A Zn ? Zn2 (as ZnO) 4e-
  • C O2 2 H2O 4e- ? 4 OH-
    EMF 1.65 V
  • Cathode reaction on inert catalytic electrode (
    graphite catalyst binder)
  • Oxygen supply forced by underpressure in cathode
    space
  • Slow kinetics of oxygen electrode main
    limitation for current value
  • Parasitic processes Zn O2
  • OH- CO2
  • loss of water

20
Electric vehicles
  • zero-emission buses and vans on tests in USA
    and Germany
  • Repleceable anodic casette of Zn with KOH
    (gelled)
  • Ca. 200 Wh/kg and 90 W/kg at 80 d.o.c.
  • Supercapacitor in hybrid system to boost
    accelaration
  • External regeneration of anodes

21
Zn/MnO2 cells
22
Zn/MnO2 cells
23
How to get more from a single cell?
  • Redox potential for Me Men couples
  • Apply special conditions of discharge
  • Eliminate water from cells

Reserve cells
one-time discharge
non-aqueous solutions
synthesis in inert atmosphere
24
Reserve (activated) cells
  • Separated elements
  • Signal to make contact electrolyte electrodes
    closing the circuit inside the cell
  • Activation on signal (decision) or by event
    (water flow, emergency)
  • No or poor activity if energy demand
    intermittent
  • Very long storage time (no parasitic reactions
    and self-discharge)
  • Energy supply short time, but high current
    densities

inactive electrolyte -closed in a vessel -solid
salt to be molten
dry electrodes
25
Reserve cells - examples
  • Mg anode reactions
    Mg 2 H2O
  • Mg(OH)2
    2H 2e Mg(OH)2
    H2
  • (Mg
    covered with MgO Mg open
    to water,
  • layer,
    proton recombinates no
    contribution to current
  • with OH
    from cathode space) drawned from the
    cell
  • Both reactions take place, H2 evolution wastes
    part of electrode, but
  • Gas bubbling ? intensive stirring ? quick
    transport ? high current

26
Reserve cells examples cont.
  • Cathodes in Mg cells
  • 2 AgCl 2e ? Ag 2 Cl-
  • 2 CuCl 2e ? Cu 2 Cl-
  • other simple salts PbCl2 , CuSCN, Cu2I2
  • Overall reaction Mg PbCl2 MgCl2 Pb
  • Electrolytes sea-water, simple salts specific
    for best cathode rate
  • construction composite cathodes, mechanical
    separation of electrodes, soakable separators for
    electrolyte

27
Water and gas activated batteries - applications
  • Air-sea rescue systems
  • Sono and other buoys
  • Lifeboat equipment
  • Diverse signals and alarms
  • Oceanographic and meteo eq.
  • And many others, including military

28
Molten salts and thermal batteries
  • Main parts of a thermal battery

Anodes Li alloys Li(20)Al, Li(40)Si (melt
higher than Li 181 and 600/7090C
resp.) Cathodes Ca, K, Pb chromates, Cu, Fe,
Co sulfides, V2O5, WO3 Electrolyte molten
LiCl-KCl eutectic 3520C Combination with
bromides Thermal dissociation KCl K Cl-,
high conductivities, simple reaction mechanism
29
Thermal batteries applications
  • Pyrotechnic heat source squib, burned serves as
    inter-cell conductor
  • Insulation ceramic, glass, polymers depends
    on time of discharge
  • (salt must be kept molten !)
  • Voltages single OCV 1.6 V (Li/FeS2) , to ca.
    3 V (Ca/K2Cr2O7)
  • Activated life-time minutes, in special
    constructions hours
  • Energy density 2 35 Wh/kg
  • High currents possible
  • Applications mainly military

30
Solid electrolyte cell Na-S
Anode Na ? Na Cathode xS ? Sx2- , x
35 Overall 2Na xS ? Na2Sx OCV 2.07
V Temperature 310 350oC sulphur Tmelt 118,
boil 444oC ß-alumina Na2O11Al2O3 , conducts
Na ions s300 C ca 0.5-0.1 S/cm
31
Solid electrolyte cell Na-S
  • Can be used as rechargeable cell
  • Applications stationary energy storage, motive
    power
  • Working with high-temperature cells
  • warm-up on
    start
  • keep warm
    at intervals in operation
  • manage
    excessive heat during operation (ohmic and
    reaction)
  • Construction of stacks electrical and heat
    management

32
Stationary energy storage Na-S system
33
Lithium iodine solid electrolyte cell
  • Anode Li ?
    Li 2e
  • Cathode nI2P2VP 2e ?
    (n-1)I2P2VP 2 I- (poly-2-vinylpyridine)
  • Overall 2Li I2 ? 2
    LiI
  • LiI thin layer on contact between Li and
    cathode, ionically conducting
  • OCV ca 2.8 V
  • Discharge rates 1 2 µA/cm2 (very low)

34
Primary and secondary cells - basic
35
Secondary cells - basic
  • Energy density from lt 20 (Pb) , 35 (NiCd), 75
    (NiMeH) to 150 Wh/kg (Li-ion)
  • Cycling life 220-700 (Pb) 500
    2000 (Ni-Cd)
  • Voltage 2 V (Pb)
    1.2 V (Ni-Cd)
  • Flat discharge profiles
  • Poor charge retention (shelf life of Ni-Cd
    fully discharged, Pb must be kept charged because
    of sulfation of plates)
  • Vented constructions evolution of H2 / O2
  • Tight closure of cells oxygen recombination (
    at end of charge oxygen developing in anodic
    process diffuses to cathode and oxidates surplus
    of cathode material no overpressure
  • Valve-Regulated-Lead-Acid
    sealed Ni-Cd

36
Lead-acid accumulator
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38
Phenomena in discharge cycle
  • CH2SO4
  • PbSO4 insulator ( ca. 1010 ?cm)
  • Vmol PbSO4 gt Vmol Pb, PbO2
  • worse porosity
  • diffusion of the electrolyte into the
    structure impaired
  • R int
  • What happens with
  • current density at U const ?
  • Voltage at I const. ?

39
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41
Alkaline accumulators
  • Ni Cd , Ni Fe, Ni MeH ( 1.2V)
  • Ag Zn ( 1.5V)
  • Ni Zn (1.6V)
  • Cathode Ni
  • NiIII OOH H20 e-
    Ni(OH)2 OH-
  • Anode Cd
  • Cd 2(OH-)
    Cd(OH)2 2e-
  • Ag-Zn Ag2O H2O 2e 2Ag 2 OH-
  • Zn 2(OH-)
    Zn(OH)2 2e

42
Ni-Cd accumulator
43
(further electrolysis after charging effects in
evolution of O2)
((further electrolysis after charging effects in
evolution of H2)
44
Oxygen and hydrogen formation in cells
  • Reactions possible in water solution
  • Equilibrium potentials E (H/H2) 0V , E
    (OH-/O2) 0.4 V
  • BUT overpotentials due to phenomena at
    gas-solid electrode phase boundary make true
    potentials higher
  • For different metals the hydrogen evolution
    potential grows from
  • Pt - Ni - Ag - Zn - Cd -
    Pb (and compounds)
  • Still, at the end of charge/discharge cycle
    co-evolution of gases in cells occurs
  • In effect overpressure inside the cell, - H2 i
    O2
  • oxygen recombination electrodes not
    equivalent in charge, ex. QCd gt QNi

45
Basic secondary cells
46
Technology of electrode masses in Ni-Cd
  • Electrodes prepared in discharged state Ni(OH)2
    and Cd(OH)2 as
  • Additives graphite ,- mass Fe Ni (? Cd
    crystallization)
  • Formation of plates several charge-discharge
    cycles
  • Assembly and hermetic closure
  • Separators ionic conductivity and oxygen
    diffusion (thickness ca0.2 mm)
  • For O2 recombination higher capacity of - mass
    (Cd) fully charged Ni mass O2evolution
    diffusion Cd oxidised to CdO, no possiblity of
    H2 formation

47
Nickel/Metal Hydride
  • Anode 2 NiO(OH) 2 H2O 2e ? Ni(OH)2 2
    OH-
  • Cathode H2 2 (OH-) ? 2 H2O 2e
  • Hydrogen stored as hydride in metallic phase,
  • Capacity of metal hydride electrode c. 0.4 Ah/g
    -- comparable with Cd and Ni sintered plates
    0.3-0.5 Ah/g

48
Scheme for reaction mechanism at Me electrode
overcharge
charge
discharge
H2O
O2
H2O
OH-
OH-
H2O
Hads
Hads
Hads
H2
Me-H
Reversibility of electrode reaction, catalytic
for H adsorption and H-O2 recombination
49
Hydrogen absorbing alloys
  • A metal forming stable hydrides
  • B weak hydrides, catalyst, resistance to
    corrosion, control Hads pressure
  • Nickel - catalyst for H2 dissociation,,
    regulator for Zr, Ti, V hydride formation,

50
Some details on production of alloys
  • Ni mass traditional, new technologies for MeH
    electrode powder
  • Ovonic alloy example main components
    Zr-Ti-V-Ni Cr, Mn, Co, Fe...
  • Preparative technics electric arc or inductive
    oven, Ar atmosphere
  • Production of powder hydrogenation of cast
    alloy (volume expansion crushing of a piece),
    followed by mechanical pulverisation
  • Sintered plates MeH powder Ni, Ni(CO)5
    resin ?
  • pressing and sintering under vacuum

51
Lithium cells
Anodic reaction Li Li 1e
  • Reactivity of metallic lithium reduces most
    substances (even Teflon)
  • Stable passivation key to electrode stability
  • What shall we do with excess lithium?
  • Transport and consume in cathode reaction
  • Why not leave lithium cations in the
    electrolyte?

52
Anode
53
Irreversible loss of capacity on first cycle,
electrode artificial graphite
54
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55
Cathodes
  • Redox potentials in 0 1 V range - OCV of Li
    cells from 3 to 4 V

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Carbon layers in regular graphite
Layered structure of LiCoO2
58
Electrolytes Conductivity, Li transference
number Electrochemical and thermal stability
  • Liquid organic
  • Aprotic
  • Protective passivation layer on Li
  • Li salts solute and dissociate
  • Appropiate physical features
  • stable non-toxic, nonflammable
  • Conductivities 1e-3 S/cm

Polymer Li conduction via coordination sites on
polymer chains (ex. Poly(ethylenoxide) Solid
foils, processable More stable against
Li Conductivities 1e-7 1e-4 S/cm
Gel 2 in 1 polymer matrix immobilizing liquid
electrolyte
59
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63
Step-wise intercalation of Li into graphite,
observed as voltage plateaux
64
Parameters and definitions
  • EMF or OCV
  • nominal voltage (accepted as typical for a
    certain battery)
  • End (cut-off) voltage
  • Theoretical capacity comes from amount of
    active materials
  • Rated capacity
  • Energy density (Watthour/l) and specyfic energy
    (Watthour/kg) theoretical E Q EMF,
    practical E Q?U
  • Power density
  • Shelf life

65
General discharge profile - elements
  • Discharge of a galvanic cell

66
C - rate
  • Charge / discharge current of a battery, given as
  • I (amper) Cn (amperhours) . M (multiply or
    fraction of C)
  • !!! Traditional convention, but units are
    uncorrect!!!
  • However, most producers and studies use this
    measure !!!
  • Ex. For a 250 mAh rated battery (declaration of
    producer)
  • 1C rate 250 mA
  • 0.1C rate 25 mA and so on
  • We can compare batteries at equal C-rates or
    study discharge for a given battery at different
    C-rates

67
Discharge profiles
  • Flat minimal change in reactants and products
  • Step-wise change in reaction mechanism and
    potential
  • Sloping - composition, internal R ... Change
    continouosly

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Continuous and intermittent discharge
Possibilty for partial recovery of voltage during
pause
70
Discharge
  • Discharge mode constant current / resistance /
    power
  • (time to reach cut-off U may differ)
  • Electrode design f (type of service)
  • Max. quantity of active material max. energy
    supply
  • Max. electrode surface high discharge rate
    (current, power)
  • Possibility of partial restoration of voltage
    stand-by intervals

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