WUT - MESC - Galvanic Cells II 1 Galvanic Cell Galvanic Cells - INTRODUCTION Energy sources How did...

WUT - MESC - Galvanic Cells II 1

Galvanic Cell

Galvanic Cells - INTRODUCTION

• Energy sources• How did the battery business start?• History of batteries makes history of electric energy

As ELECTROCHEMICAL DEVICE :Electrode reactions

Thermodynamics and kineticsProperties of Materials

As ENERGY SOURCE :Position on energy market

Power supplyTechnology & Economy


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


DIFFERENT CELLS

• galvanic cells – primary and secondary

Chemical substances in electrodes

Expressed as Q

Electrode Reactions

Expressed as UEnergy = U . Q

• Fuel cells

Electrode Reactions

Expressed as U

Stream of reagents

Energy = U . Q

ISOLATED PORTABLE/TRANSPORTABLE

INDEPENDENT FORM ELECTROENERGETICAL NETWORK


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


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


Electrical circuits with batteries

• Management of voltage and current – connecting the batteries

• Ohm’s 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


Electrode potential

• φ= φo + RT/nF ln ( aMe / aMe(n+) )

• Standard potential at unit activity of particles - φo

• + 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


Ared →Box + n e-Anodic reaction

Cathodic reaction Cox + n e- →Dred

Potential φox

Potential φred

Overall cell reaction A + B = C + D With E = Δ φ

Electromotoric force E comes from change in free enthaply of the overall reaction,

Also combining the ΔG with electrical equivalent of energy E = -ΔG /nF

And defining Eo = ΔG o/nF for standard conditions we get Nernst equation :

E = Eo – RT / nF ln K

where K – equilibrium constant of reaction ABCD


Signs + / - in cells - convention

More negative potential on left side : Zn = Zn2+ + 2e φ = - 0.76 V

Less negative to the right : Cu = Cu2+ + 2e φ = 0.34 V

formal scheme for the cell

External connection / Zn / Zn SO4 aq // CuSO4 aq / Cu / external connection

Sign - // sign +

But .....


Structure and functions of electrodes

A/ metallic reactive electrodes (deposition-dissolution, formation of compounds on the surface)

Reagent and current collector(two-in-one) Charge and mass transport – on the surface

B/ inert electrodesmetalls, graphite, semiconductors

Current collector, not a redox reagent Charge and mass transport – on the surface

C/ multi-function, multi-component electrodeselectroactive component (often insulator)electronically conducting matrixother additives with special functions

Charge and mass transport – on triple-contact sites Cond. matrix

Redox active

electrolyte


Various types of batteries


Specyfic energy - Energy density


Typical battery application


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


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


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)


Cells with Zn anode

Cell name Cathode Electrolyte OCV or EMF (V)

Daniell Cu → Cu2+ ZnSO4/CuSO4 1.2 Anode product – soluble Zn saltsLeclanche MnO2→MnO(OH)

(→Mn3O4 possible)

NH4Cl, ZnCl2 1.6

Alkali MnO2→MnO(OH) KOH 1.55 Anode product - ZnO

Zinc-air O2 → O2- (on carbon matrix)

KOH 1.45

Zinc-silver Ag2O → Ag KOH 1.6


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


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


Zn/MnO2 cells


How to get „more” from a single cell?

• Redox potential for Me – Men+ couples

• Apply special conditions of discharge

• Eliminate water from cells

Zn-Zn2+ -0.76 V O2-OH- 0.4 V

Mg-Mg2+ -2.36 V Ag+-Ag 0.8 V

Na-Na2+ -2.92 V MnO2-MnO(OH) app. 0.74 V

Li-Li+ -3.05 V F2 – 2F- 2.87 V

non-aqueous solutions

synthesis in inert atmosphere

Reserve cells

one-time discharge


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

dry electrodes

inactive electrolyte :

-closed in a vessel

-solid salt to be molten


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


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


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


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 3520CCombination with bromides

Thermal dissociation KCl = K+ + Cl-, high conductivities, simple reaction mechanism


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


Solid electrolyte cell Na-S

Anode Na → Na+

Cathode xS → Sx2- , x 3~5

Overall 2Na + xS → Na2Sx

OCV = 2.07 V

Temperature 310 – 350oC

sulphur Tmelt = 118, boil= 444oC

β-alumina Na2O∙11Al2O3 , conducts Na ions σ300 C ca 0.5-0.1 S/cm


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

Insulated enclosure Cooling system

Heat distribution heatersElectricalnetworking


Stationary energy storage Na-S system


Lithium – iodine solid electrolyte cell

• Anode : Li → Li+ + 2e

• Cathode : nI2∙P2VP + 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)


Primary and secondary cells - basic

PRIMARY SECUNDARY

Irreversible use of electrodes Recovery of electrodes – by supplying electrical energy we restore electrode oxidation state and structure

Anodic and cathodic process (redox) related to specified electrodes, run only once

Anodic and cathodic reactions repeat on both electrodes in charge-discharge cycles

Solid metal electrodes (one-way)

Products may be soluble

Substrates and products stay in electrode phase

Redox reaction „all-solid state”

Minimalizing changes in electrode structure and shape


Secondary cells - basic

• Energy density from < 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


Lead-acid accumulator


cycle „negative mass” „positive mass”

discharge

Pb → PbSO4 (oxidation)

Concentration of H2SO4 ↓

PbO2 → PbSO4 (reduction)

Concentration of H2SO4 ↓

charge

PbSO4 → Pb (reduction)

Concentration of H2SO4 ↑

PbSO4 → PbO2 (oxidation)

Concentration of H2SO4 ↑


Phenomena in discharge cycle

• CH2SO4 • PbSO4 – insulator ( ca. 1010 Ώcm)• Vmol PbSO4 > 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. ?


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


Ni-Cd accumulator

cycle „masa ujemna” „masa dodatnia”

discharge

Cd → Cd(OH)2

(oxidation)NiOOH → Ni(OH)2 (reduction)

charge

Cd(OH)2 → (reduction) Ni II(OH)2 → Ni IIIOOH (oxidation)


(further electrolysis after charging effects in evolution of O2)

((further electrolysis after charging effects in evolution of H2)


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 > QNi


Basic secondary cells

Ni-Cd

•Pocket electrode construction of electrodes•Sintered plates

Pb acid

•Pasted plates•Tubular positive plates•Plante’ design


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

Compresed powderNiSO4→Ni(OH)2

CdSO4 →Cd(OH)2

Encapsulated in steel/Ni pocket

Sintered platePorous Ni plate

Impregnated with Ni , Cd saltsTransformed to hydroxides „in situ”


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


Scheme for reaction mechanism at Me electrode

charge discharge overcharge

Me-H

H2O OH- OH- H2O H2O O2

Hads H2Hads Hads

Reversibility of electrode reaction, catalytic for H adsorption and H-O2 recombination


Hydrogen absorbing alloys

• A – metal forming stable hydrides

• B – weak hydrides, catalyst, resistance to corrosion, control Hads pressure

Class (basis) Components Storage Ah/kg Remarks

AB5

(LaNi5)

A: Mischmetall, La, Ce, Ti

B: Ni, Co, Mn, Al

≈ 300 Mostly used

AB2

(TiNi2)

A: Zr, Ti

B: Ni, Fe, Cr, V

≈ 400 „Ovonic” alloys

• Nickel - catalyst for H2 dissociation,, regulator for Zr, Ti, V hydride formation,


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


Lithium cells

Atomic mass LITHIUM ZINC

Standard potential (V) -3.05 -0.76

Melting point (oC) 181 419

Density (kg/m3) 534 7100

Elchem. equivalent (Ah/g) 3.86 0.82

•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?

Anodic reaction : Li = Li+ + 1e


Anode

Metallic Li (foil) Intercalation : Li – Li+ in matrix

Stable passivation layer on discharge

Charge : mossy, dendritic deposit – corrosion of fresh Li

internal shortcutting

Main application – primary cells

Rechargeable –

attempts with polymer electrolytes

Capacity: 3.86Ah/g, in accu < 1 Ah/g

Carbon materials : coke, graphite etc.

6 – 12 C atoms take 1 lithium atom into the structure

First cycle – formation of SEI

(Solis Electrolyte Interface)

portion of Li used for reaction with electrolyte

Some transition metal compounds

Capacity: 0.372 Ah/g


Irreversible loss of capacity on first cycle, electrode : artificial graphite


Cathodes• Redox potentials in 0 – 1 V range - OCV of Li cells from 3 to 4 V

Solid: MexOy

Reduction of Me ion to lower oxidation state, like MnIVO2 – MnIIIO2

Topotactic reaction

Insertion of Li+ into host structure

Some other: V2O5, (CF)n, TiS2

Capacities: 0.31(MnO2), 0.86(CF) Ah/g

Soluble

SO2 + 2e → S2O42-

( in solution, + Lisalt ex. LiAlCl4)

Thionyl chloride:

SOCl2 + 4e → S + SO2

Sulfuryl chloride:

SO2Cl2 + 2e → SO2

(solvents for Li salt)

Capacities : ≈ 0.4 Ah/g


Layered structure of LiCoO2

Carbon layers in regular graphite


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, processableMore stable against Li

Conductivities : 1e-7 –1e-4 S/cm

Gel2 in 1 : polymer matrix immobilizing liquid electrolyte


Solution Ionic conductivity (20oC) S/cm

1M H2SO4 10-1

Nafion® foil (H+) 10-2

1M LiBF4 in acetonitrile 10-3

PEO-LiClO4 complex 10-6


Step-wise intercalation of Li into graphite, observed as voltage plateaux


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×ΔU• Power density• Shelf life


General discharge profile - elements

• Discharge of a galvanic cell


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


Discharge profiles

1. Flat – minimal change in reactants and products2. Step-wise – change in reaction mechanism and potential3. Sloping - composition, internal R ... Change continouosly


Continuous and intermittent discharge

Possibilty for partial recovery of voltage during pause


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

WUT - MESC - Galvanic Cells II 1 Galvanic Cell Galvanic Cells - INTRODUCTION Energy sources How did...

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Transcript of WUT - MESC - Galvanic Cells II 1 Galvanic Cell Galvanic Cells - INTRODUCTION Energy sources How did...