second half of electrochem Flashcards
(34 cards)
describe lead-acid battery
low energy density, high power density
all in the solid state,
Ecell = 2.04V
a polyethylene membrane acts as a separator and prevents short circuiting of the battery during charging
water hydrolysed during charging so has to be topped up
90% efficiency for discharge
heavy so low specific energy and reduced cyclability
describe nickel metal hydride cells
good specific energy and power
rechargeable
safe, low maintenance
expensive, hard to recycle, high self-discharge rate
KOH as the electrolyte
what is the cathode and its cell potential for nickel metal hydride cells
Ni(OH)2 + OH- -> NiOOH + H2O + e-
cell potential std = 0.25 V
what is the anode and its cell potential for nickel metal hydride cells
H2O + M + e- -> MH + OH-
cell potential = -1.1V
what is the cathode for alkaline cells
MnO2 + H2O + 2e- -> Mn2O3 + 2OH-
what is the anode and its cell potential for alkaline cells
Zn + 2OH- -> ZnO + H2O + 2e-
EAN = -1.28V
what is the cell potential for alkaline cells
1.43V
describe alkaline cells
non-rechargeable
leak KOH
easy to recycle & recover the SM
Describe lithium batteries
high cell potential
high specific energy and power
rechargeable for more cycles than nickel metal hydride cells
very low self-discharge rate
hazardous (explosive H2 gas & Li is very reactive w/ water)
what are the three methods of Li transport in batteries?
1D channels (olivine) , 2D layered, 3D framework (spinel)
for lithium and graphite cell what is the cathode?
CoO2 + Li+ + e- -> LiCoO2
for lithium and graphite cell what is the anode?
LiC6 -> C6 + Li+ + e-
How does the lithium and graphite cell overdischarge?
irreversible formation of Li2O
How does the lithium and graphite cell overcharge?
collapse of the layered structure and irreversible formation of amorphous Co(IV) oxide
What are the electrolytes in Li batteries?
NOT AQ - generally carbonates or ethers to prevent reaction w/ Li
How is LiBF2(C2O4) used in lithium batteries?
to increase cyclability and side reactions - forms a protecting layer on the electrodes to increase stability
what is the point of solid electrolytes? egs?
no leakage, no fire hazard
polymer-based
ceramic - has Li transport channels
what is m in faraday’s law
the no. of moles of reactant or product consumed/formed
what are the products of the chloroalkali process and their uses?
Cl2 - to make plastics and water & silicon purification
NaOH
H2 - energy in fuel cells
positive electrode eqs in the chloroalkali process
2Cl- -> Cl2 + 2e- = 1.36V
2H2O -> 4H+ + O2 + 4e- = 1.23V
why is Cl2 released and not O2 in chloroalkali positive electrode
strong kinetic barrier of overpotential on the Pt electrodes
negative electrode eqs in the chloroalkali process and their potentials
2H2O + 2e- -> H2 + 2OH- = -0.83 V
Na+ + e- -> Na(s) = -2.71V
why is H2 released and not Na reduction in chloroalkali negative electrode
H2 evolution is at less negative potential = favoured, smaller kinetic barrier
angle for philicity
contact angle CA <90
