Batteries

Question 1

Name some energy storage systems for transport use? (Inc +ve and -ve)

Answer 1

Petrol, diesel, biofuel - high energy density, established supply network, rapid start up, CO2 footprint, unsecure supply
Batteries and fuel cells - greener, wide range of supply sources, limited supply network, poor energy density, poor lifetime

Question 2

Name some energy storage for the power grid (inc +ve and -ve)

Answer 2

Fission/fusion, oil, gas - high energy density, established network, rapid response, CO2 footprint, security supply, waste disposal, flexible use reduces lifetime
Hydro, geothermal, renewables - renewable, geographically limited, varying power, poor transport

Question 3

Name a type of energy storage technique

Answer 3

Hydroelectric power - pump water into dam, release when needed

Question 4

Name some requirements for batteries

Answer 4

Low density, high voltage, high charge density, fast responding, long lifetime, cheap, readily available materials (rarer = better performance)

Question 5

Why are there limited energy storage techniques?

Answer 5

Means exiting a material to higher energy state and preventing it from decomposing
Too high = spontaneous decomposition, too low = never decompose

Question 6

Describe SEP

Answer 6

Standard electrode potentials - only possible to measure Pd across electrodes so 0 is defined as hydrogen half cell reaction
Positive SEP means wont oxidise, negative will oxidise
More -ve or +ve will react stronger

Question 7

Describe the structure of a voltaic pile and half cell reactions

Answer 7

Electrolyte (Eg brine) separates a Zn and Cu player
Half cell reactions:
Zn -> Zn(2+) + 2e-
2H+ + 2e- = H2
Reaction stops when all 2H+ are used up, Cu is inert as it has the most positive SEP

Question 8

Describe the Daniell cell structure and half cell reactions

Answer 8

Zn anode in sulphuric acid, surrounded by porous container (allows e- through) with Cu sulphate in a Cu cathode around
Half cell: Zn = Zn(2+) + 2e- = -0.76V
Cu(2+) + 2e- = Cu = 0.34V
EMF = 1.1V

Question 9

Describe a dry cell structure and half cell reactions

Answer 9

Metal cap, cathode in magnesium oxide, surrounded by NH4Cl paste with Zn casing
Half cell: Zn = Zn(2+) + 2e-
2MnO2 + 2e- + 2NH4Cl = Mn2O3 + 2NH3 + H2O + 2Cl -> EMF = 1.26V

Question 10

Compare dry cell, voltaic pile and daniell cell

Answer 10

Dry cell is more portable, more lightweight and cheaper but Zn casing so full reaction not achieved

Question 11

Describe an alkaline battery’s structure and half cell reactions

Answer 11

Metal connector in a Zn and KOH powder, with ion conductor separating a MnO2 layer, metal cap (cathode) surrounding cell, all encased in insulator
Half cell: Zn + 2OH- = ZnO + H2O +2e-, 2MnO2 + H2O + 2e- = Mn2O3 + 2OH-

Question 12

What were the advantage of an alkaline battery?

Answer 12

Reactants separated = better shelf life
Recombining half reactions = linger life span
However, still non-rechargeable

Question 13

What is the structure and half cell reactions of a Li battery?

Answer 13

Swiss roll structure - Li and FeS2 separated by electrolyte means more complete reactions possible
Half cell: 2Li + 1/2FeS2 = Li2S + Fe/2

Question 14

Compare a lithium battery to an alkaline battery

Answer 14

Replace Zn with Li and MnO2 with FeS2 (longer lifetime)
Li is monovalent, Zn is divalent (Zn creates twice e-), although Zn and MnO2 are both smaller, half cell restrooms means smaller Li battery needed for same energy (= greater charge and longer lifetime)

Question 15

How do rechargeable batteries store energy?

Answer 15

In a meta stable state, activation energy stops battery from discharging too quickly (too high and battery won’t discharge)
To recharge battery = Eact + energy between charge and discharge

Question 16

Describe a lead-acid battery

Answer 16

Pb and PbO2 separated by dilute acid
Voltage applied in reverse direction (into anode) = recharges battery
Pb can branch over acid centre & short battery
Each cell produces 2V, very bulky and rechargeable limit of 350

Question 17

Describe a NiCd battery

Answer 17

Still roll structure with layers of NiCd, safety spring on top to release H that’s released via charging
Produces 1.2V and a recharge limit of 2000, if not fully charged or discharged can create charge memory

Question 18

Define charge memory

Answer 18

When a battery isn’t discharge or recharged fully so over time charge capacity is reduced

Question 19

Describe a nickel metal hydride battery structure and the limitations and advantages

Answer 19

Ni and metal hydride in Swiss roll structure
Metal selection is limited by hydride need (can’t be too stable = difficult recharge, not stable = self-discharge)
Use RE alloy with polymer based spectators, as high capacity and self-discharge limited. Very high charge density

Question 20

Describe lithium ion batteries

Answer 20

Much higher capacity and voltage than lithium battery
A Li salt separates a Li ion intercalated Graphite layer and a layered lithium compound (different compounds used = different half cells) - then Swiss roll
e- can’t travel through Li salt so travels round external circuit, Li+ goes through salt.

Question 21

What determines if a battery can be rechargeable?

Answer 21

If it is a reversible reaction or not

Question 22

How can lithium ions become unstable (combust)? And what is the most common Li ion battery (inc half cell)?

Answer 22

Salt is a source of fire risk if gets too hot - thermal run away
Most common is LiCO battery, half cell:
LiC6 = C6 + Li+ + e-

Question 23

What are the benefits of a Li ion battery?

Answer 23

Thin film structure, very large charge density = 1200 recharges with no charge memory