http://lawrencekok.blogspot.com

Prepared by Lawrence Kok

Tutorial on Secondary Cells, Hydrogen and Microbial Fuel Cell, Thermodynamic Efficiency.

Lithium ion

Types voltaic cell

NH4CI and ZnCI2

Alkaline cellDry cell Nickel cadmium cell

Primary cell (Non rechargeable)

MnO2 and KOH

Secondary cell (Rechargeable)

Lead acid battery

Fuel cell

H2 fuel cell- alkaline electrolyte H2 fuel cell- acidic electrolyte Direct Methanol fuel cell Microbial fuel cell (MFC)

H2H2 O2O2

H2O

CH3OH O2

CO2 H2O

C6H12O6

O2

- No pollution- Fuel is constantly supply- Convert fuel H2 /organic fuel to electricity

Longer life timeRechargeable

Diff rechargeable vs fuel cellRechargeable battery are

reversible fuel cell irreversible – need supply

fuel

Similarity rechargeable vs fuel cellBoth convert chemicalto electrical – redox rxn

(spontaneous)vs

Battery/Fuel cell

Advantage Disadvantage

Lead acid High amt chargeHigh energy

densityCheap

HeavyLead/H2SO4 pollution

Nickel cadmium Longer lifetimeQuick recharge

timeLow resistance

Cadmium toxicExpensive

Memory effect

Lithium ion Low densityHigh voltage –

3.7VLonger lifetimeHigh recharge

cycleNon toxic

ExpensiveExplosive expose to

heat

Fuel Cell More efficientHigh energy

densityNo pollutionLow density

H2 explosiveDiff to store/transport

gasExpensive

Low storage density

Microbial Fuel Cell

Safe and renewable

Treatment waste

Low energy

Battery/Fuel cell

Energy density/MJdm-1

Specific energy/MJkg-1

H2 fuel cell 2 120

Methanol fuel cell

16 20

Liquid natural gas

21 50

Liquid propane 27 46

Gasoline 32 46H2 highest specific energy( ratio energy to mass)1 mol H2 – 2g1 mol methanol – 32g1 mol propane - 44 g1 mol octane/gasoline – 114g

Comparison in terms of energy density/specific energy Advantage/disadvantage of battery/fuel cell

H2 lowest energy density(ratio energy to vol)1 mol H2 – 24000 cm3

1 mol methanol – 40.4 cm3

1 mol propane - 76 cm3

1 mol octane/gasoline – 162 cm3

Comparison in terms of energy density/specific energy

Zinc carbon alkaline nickel/cad Li/Fe Li/Mn nickel/MH Li/ion Li/polymer fuel cell Zinc air

Types voltaic cell

Discharging

Pb + PbO2 + 2H2SO4 →2PbSO4 + 2H2O

Lead acid battery

(-ve) (Anode) - OxidationPb + SO4

2- → PbSO4 + 2e−

+ ve (Cathode)- ReductionPbO2 + 4H+ +  SO4

2- + 2e− → PbSO4 + 2H2O

(-ve) (Cathode)- ReductionPbSO4 + 2e → Pb + SO4

2−

+ ve (Anode)- OxidationPbSO4 + 2H2O → PbO2 + 4H+ +

 SO42- 2e− 

Charging

Electrolyte H2SO4

Nickel cadmium battery

Discharging(-ve) (Anode) - Oxidation

Cd + 2OH- → Cd(OH)2 + 2e−

+ ve (Cathode)- Reduction2NiO(OH) +  2H2O+ 2e− →  2Ni(OH)2 +

2OH-

Charging

nickel (III) oxide hydroxide

(-ve) (Cathode) - ReductionCd(OH)2 + 2e− → Cd + 2OH-

+ ve (Anode)- Oxidation 2Ni(OH)2 + 2OH- → 2NiO(OH)+

2H2O+ 2e−  

net eqn

2NiO(OH)+ Cd + 2H2O+ →  2Ni(OH)2 + Cd(OH)2

net eqn

NiO(OH)

Cd

Electrolyte KOH

Lithium ion battery

(-ve) (Anode) - OxidationLi → Li+ + e−

+ ve (Cathode)- ReductionLi+ +  MnO2 + e− →  LiMnO2

Discharging

Lithium in Graphite layer – prevent lithium for oxidizing to oxide (reactive)

LiMnO2 Li

-+Charging

(-ve) (Cathode) - ReductionLi+ + e− → Li

+ ve (Anode)- OxidationLiMnO2 → Li+ +  MnO2 + e−

Li +  MnO2  →  LiMnO2 net eqn

Discharging/Charging possible - insoluble PbSO4/PbO

reversible

Discharging/Charging possible - insoluble Cd(OH)2/Ni(OH)2

reversible

Lithium in MnO2 lattice – prevent lithium for oxidizing to oxide (reactive)

2H2 + O2 → 2H2O

H2 fuel cell- alkaline electrolyte

(-ve) (Anode) - Oxidation2H2 +  4OH- →  4H2O + 4e−

+ ve (Cathode)- Reduction2H2O +  O2 + 4e− → 4OH-

net eqn

net eqn

CH3OH +  1.5O2  →  CO2 + 2H2O

net eqn

Fuel cell

Electrolyte KOH

O2H2

H2 fuel cell- acidic electrolyte

(-ve) (Anode) - Oxidation2H2 →  4H+ + 4e−

+ ve (Cathode)- Reduction4H+ +  O2 + 4e− → 4H2O

2H2 + O2 → 2H2O O2H2

PEM – made of Teflon allow H+ ion to flow

Proton Exchange Membrane

Electron flow in external circuit

Direct Methanol fuel cell

H2O

H2O

(-ve) (Anode) - OxidationCH3OH + H2O →  CO2 + 6H+ + 6e−

+ ve (Cathode)- Reduction6H+ +  1.5O2 + 6e− → 3H2O

O2

H2OCO2

CH3OH

Proton Exchange MembranePEM – made of Teflon allow H+ ion to flow

Carbon oxidized to CO2 - C (-2) to C (+4)

Catalyst – platinum used anode/cathode

Catalyst – platinum used anode/cathode

Catalyst – platinum used anode/cathode

CH3COOH

Microbial fuel cell (MFC)

(-ve) (Anode) - Oxidation C6H12O6 + 6H2O →  6CO2 + 24H+ +

24e−

+ ve (Cathode)- Reduction24H+ +  6O2 + 24e− →  12H2O

net eqn

net eqn

net eqn

Fuel cell

O2CO2

CH3COOH + 2O2 → 2CO2 + 2H2O CH3COOH

Electron flow in external circuit

C6H12O6

O2CO2

Proton Exchange MembranePEM – made of Teflon allow H+ ion to flow

C6H12O6 + 6O2 →  6CO2 + 6H2OMicrobial/bacteria in anode, anaerobicoxidized organic/fatty acid/ alcohol to CO2/H2OElec and H+ produced when oxidized

(-ve) (Anode) - Oxidation CH3COOH + 2H2O →  2CO2 + 8H+ +

8e−

+ ve (Cathode)- Reduction8H+ +  2O2 + 8e− →  4H2O

Microbial fuel cell (MFC) – Ethanoic acid

O2CO2

Electron flow in external circuit

Bacteria GEOBACTER in anode, anaerobicoxidized ethanoic acid to CO2/H2OElec and H+ produced when oxidized

Microbial fuel cell (MFC) – Ethanoic acid

(-ve) (Anode) - Oxidation CH3COOH + 2H2O →  2CO2 + 8H+ +

8e−

+ ve (Cathode)- Reduction8H+ +  2O2 + 8e− →  4H2OCH3COOH + 2O2 → 2CO2 +

2H2O

Bioremediation to break down oil spill/organic waste

H2 fuel cell- acidic electrolyte

2H2 + O2 → 2H2Onet eqn

Fuel cell

(-ve) (Anode) - Oxidation2H2 →  4H+ + 4e−

+ ve (Cathode)- Reduction4H+ +  O2 + 4e− → 4H2O

O2H2

PEM – made of Teflon allow H+ ion to flow

Proton Exchange Membrane

H2O

Catalyst – platinum used anode/cathode

Thermodynamic Efficiency fuel cell

%100..... energyinputtotalenergyoutputusefulefficiencyMax

%100.

sys

sys

HG

efficiencyMax

Ratio of Gibbs Free energy to Enthalpy change of rxn

2H2 (g) + O2 (g) → 2H2O (l)

∆Hf = -285.8kJ mol-1

∆Gf = -237.1 kJ mol-1

%83%1008.2851.237.

%100.

efficiencyMax

HG

efficiencyMaxsys

sys

2H2 (g) + O2 (g) → 2H2O (g)

∆Hf = -241.8kJ mol-1

∆Gf = -228.6 kJ mol-1

%95%1008.2416.228.

%100.

efficiencyMax

HG

efficiencyMaxsys

sys

STHG

Liquid water – Entropy lower (order)∆S sys = ∆S(product – reactant) is higher

∆G = ∆H - T∆S∆G = less negativeEfficiency = Less

Gas produced–Entropy gas high (disorder)∆S sys = ∆S(product – reactant) is lower

∆G = ∆H - T∆S∆G = more negativeEfficiency = More

STHG

H2 fuel cell- acidic electrolyte

Click here making H2 fuel cell

H2 fuel cell- acidic electrolyte

2H2 + O2 → 2H2Onet eqn

Fuel cell

(-ve) (Anode) - Oxidation2H2 →  4H+ + 4e−

+ ve (Cathode)- Reduction4H+ +  O2 + 4e− → 4H2O

O2H2

PEM – made of Teflon allow H+ ion to flow

Proton Exchange Membrane

H2OCatalyst – platinum used anode/cathode

Thermodynamic Efficiency

CH3OH + 1.5O2 → CO2 + 2H2O

∆Hf = -726 kJ mol-1

∆Gf = - 685 kJ mol-1

%94%100726685.

%100.

efficiencyMax

HG

efficiencyMaxsys

sysSucrose, C12H22O11 used as substrate in MFCi. Where do the bacteria live in fuel cellii. Oxi number carbon in –ve electrodeiii. Oxi number oxygen in + ve electrodeiv. Overall redox rxn

Direct Methanol fuel cell

CH3OH

CO2

O2

H2O

C12H22O11 + 12O2 →  12CO2 + 11H2O

Oxi C = 0 Oxi C = +4Oxi O = 0

Oxi C = 0 to +4 (oxidized)48 electron lost

(-ve) electrodeC12H22O11 + 13H2O → 12CO2 + 48H+ + 48e–

Oxi O = -2

Oxi O = 0 to -2 (reduced)48 electron gain

(+ve ) electrode48H+ + 12O2 + 48e– → 24H2O

Microbial fuel cell (MFC)

Bacteria in anode (-ve) –oxidation of substrate

- +

C12H22O11 + 12O2 →  12CO2 + 11H2O

Eθ value DO NOT depend surface area of metal electrode.E cell = Energy per unit charge. (Joule)/CE cell- 10v = 10J energy released by 1C of charge = 100J energy released by 10C of charge Eθ – intensive property– independent of amt – Ratio energy/charge

Increasing surface area metal will NOT increase E cell

Total Energy increase ↑

Total Charge increase ↑

Current increase ↑

BUT E cell remain SAMEE cell = (Energy/charge)

tQI

tIQ

Q up ↑ – I up ↑

i. Nature of material, further apart oxidising / reducing in std electrode potential, more voltage produceii. Quantity material, surface area and total number of elec moving iii. Large thick plates increase surface area- quantity of material (work/energy) increase, current/charges increase BUT NO VOLTAGE CHANGEPlacing in series increase voltage

i. State factors that determine voltage of batteryii. Outline what determine , total energy/work/current a battery can doiii. Explain the effect of large surface area, battery have on voltage and work

Surface area increase ↑

E/Voltage remain SAME

Reactive lithium form oxide layer on metal which decrease the contact with

electrolyteHow lithium ion battery overcome this

problem

Mixing lithium with graphite at anode. and lithium with MnO2 lattice at cathode, prevent oxidation of lithium metal

LiMnO2Li - graphite

Oxi Li = +1, deduce oxi number of Mn in mixed

oxide LiMnO2, and show Mn has been reducedLi +(polymer) + MnO2 + e- →

LiMnO2

Oxi Mn = +4

Oxi Li = +1 Oxi O = -2

Oxi Mn in LiMnO2

+1 + Mn + 2(-2) = 0Oxi Mn = +3

Oxi Mn = +4 to +3 ( reduced )

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/http://spmchemistry.onlinetuition.com.my/2013/10/electrolytic-cell.htmlhttp://www.chemguide.co.uk/physical/redoxeqia/introduction.htmlhttp://educationia.tk/reduction-potential-tablehttp://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s23-electrochemistry.html

Prepared by Lawrence Kok

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