Electrochemicaldevices.pdf

Electrochemical energy systems

Batteries and Fuel cells

Dr. Sakthivel Kandaiah

Basics of Electrochemistry

Heterogeneous electron transfer

• Electrode

• Electrode potential • Electrolyte

• Electrochemical cell • Cell voltage

• Charge transfer

Electrode

• Electrically conductive

• Can be inert electrode or reactive

• Charge transfer takes place at the interface

• Ion intercalation electrode used in secondary Li-ion batteries

Electrode

• Anode (positive terminal), Cathode (negative terminal) in electrolytic cell

• Anode ( negative ), Cathode ( positive ) in Galvanic cell

Electrolyte

• Ionically conductive medium

• Electronically insulating

• Can solid, semisolid,(gel), liquid

• Should have high ionic conductivities

• The solvent should have high potential window, specially organic electrolytes have high potential window ( in Li-ion batteries)

Electrochemical cell

• Electrodes and electrolyte combination

• Two, three, four electrode system

• Electrolytic cell – V or I is applied to the cell

Electrochemical cells

Galvanic cell

ΔG < 0

(spontaneous)

Batteries, fuel

cells

Electrolytic cell

Δ G > 0

Potential drop

• By electrode and electrolyte resistance

• Polarisation resistance of cell

• Local cell formation – this happens because of impurities on the same electrode surface.

• Potential drop will result in decrease in

Thermodynamics Electrochemistry

W = V x I x e = E x nF

ΔG = Δ G

_{+ RT ln Q}

Battery findings

• 1770 – Galvani experiment with frog

• 1800 – Voltaic piles with dissimilar metals

• 1830 – Michael faraday work

• W = zit

• W1/W2 = z1/z2 = E1/E2

• F = 96500 C

Energy and power densities

• Significance of Ragone plot ( first slide)

• Higher the energy density (Wh/gm) and power density (W/gm) better will be device

• Supercapacitors has high power , low energy density

• Batteries have medium energy density but less power density

• Fuel cell has highest energy density but low power density

Galvanic cell redox process

8 H+(_{aq)+ MnO}

4-(aq) + 5e- Mn2+(aq) + 4H2O

Daniel cell – primary batteries

Anode

Zn Zn2+

Cathode

Cu2+ _{+ 2e}- _Cu

Cell reaction

Zn + cu2+ _{+ SO}4- _ZnSO4 _{+ Cu}

(Zn/MnO

+ C) Leclanche cell

Cathode

2MnO₂ + 2H₂O + 2e- _{2MnO(OH) + 2OH}

-Anode

Zn Zn2+ _{+ 2e}-

Secondary reaction

NH₄Cl + 2OH- NH₃ + 2Cl- + 2H₂O

Zn2+ _{+ NH}

3 + 2 Cl- [Zn(NH3)2] Cl2

MnO₂ + C cathode was dipped into 20% NH₄Cl

Primary batteries- primary alkaline cells

• Zn/MnO₂ – with KOH as electrolyte

• Anode

Zn + 2OH- _{ZnO + H}

2O + 2e-

• Cathode

2MnO₂ + 2H₂O + 2e- _{2MnO(OH) + 2OH}-

• Alkaline cells have high output capacity and current carrying ability

• Less variation in output capacity

Lithium primary batteries

2 o

• Li/ Ethylene carbonate + Propylene carbonate+ Li+_/organic

suplhides

• Li/ SOCl_{2 ,} Li/ electrolyte/SO_{2 ,} Li/ Electrolyte/I₂+ polyvinyl pyridine

Anode

Li Li+ _{+ e}-

Cathode

MnO₂ + xLi+ _{+ xe}- _Li

xMnO2

Give high energy density Li metal have high capacity

Lead-acid battery

Electrolyte – 20 % H₂SO₄

H2SO4 Concentration decreases with discharging and regained on charging This can tested by specific gravity measurement of H₂SO₄

Cell voltage 1.88 – 2.15 V

PbO₂ + Pb + 2H₂SO₄ 2PbSOdischarging ₄ + 2H₂O

Ni-Cd

Cd / CdO / KOH / NiO or Ni₂O₃ / Ni Other systems with Fe/FeO,

These batteries comes in a discharged state

Electrodes

• Need of porous electrode substrate

• Need of electrocatalysts

• Noble metal electrocatalysts shows high performance

• Stability of electrocatalysts under the given experimental conditions

Electrolyte in H

-O

fuel cell

• Alkaline electrolyte ( 85 % KOH)

• Phosphoric acid electrolyte

• Solid polymer electrolyte conducting protons – proton exchange membranes ( Nafion)

Solid oxide fuel cell

High temperature fuel cell

Solid oxide fuel cell

• Electrolyte - fused mixture of Yttrium dioxide + Zirconium dioxide

• Cells operate 800 – 1000o_C

• Charge transfer by O2- _ions

• Electrode materials should have good oxide ion diffusion coefficient

• _{Cathode -LaMnO3 , Anode – Ni/ZrO2}

• No need for noble metal catalysts, no corrosion problem as incase of liquid electrolyte systems

Other energy resources

• Solar cells

• Tidal power generation

• Nuclear power