1. Electrochemistry, rechargeable batteries and fuel cells
Energy topic 6
Electrochemistry, rechargeable batteries and fuel cells

2. Metals
Normally move randomly but in an electric circuit they move orderly

4. Batteries
Batteries also have some internal resistance as ions pass through the electrolyte The electromotive force (E) is the total energy that is made available by the chemical reactions in the cell per unit charge. As some of this energy is lost this value will be less that V

6. Concentration and Ecell
Consider the following redox reaction:
Zn(s) + 2H+ (aq) Zn2+(aq) + H2(g)
E°cell = 0.76 V
DG°= -nFE°cell < 0
(spontaneous)
What if [H+] = 2 M?
Expect driving force for product formation to increase.
Therefore DG decreases, and Ecell increases
How does Ecell depend on concentration?

7. Concentration and Ecell (cont.)
Recall, in general:
DG = DG° + RTln(Q)
However:
DG = -nFEcell
-nFEcell = -nFE°cell + RTln(Q)
Ecell = E°cell - (RT/nF)ln(Q)
Ecell = E°cell - (0.0591/n)log(Q)
The Nernst Equation

8. Concentration and Ecell (cont.)
With the Nernst Eq., we can determine the effect of concentration on cell potentials.
Ecell = E°cell - (0.0591/n)log(Q)
Example. Calculate the cell potential for the following:
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
Where [Cu2+] = 0.3 M and [Fe2+] = 0.1 M

9. Concentration and Ecell (cont.)
• First, need to identify the 1/2 cells
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
Cu2+(aq) + 2e- Cu(s)
E°1/2 = 0.34 V
Fe2+(aq) + 2e- Fe(s)
E°1/2 = V
Fe(s) Fe 2+(aq) + 2e-
E°1/2 = V
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V

10. Concentration and Ecell (cont.)
• Now, calculate Ecell
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V
Ecell = E°cell - (0.0591/n)log(Q)
Ecell = 0.78 V - ( /2)log(0.33)
Ecell = 0.78 V - ( V) = V

11. Concentration and Ecell (cont.)
• If [Cu2+] = 0.3 M, what [Fe2+] is needed so that
Ecell = 0.76 V?
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V
Ecell = E°cell - (0.0591/n)log(Q)
0.76 V = 0.78 V - (0.0591/2)log(Q)
0.02 V = (0.0591/2)log(Q)
0.676 = log(Q)
4.7 = Q

12. Concentration and Ecell (cont.)
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
4.7 = Q
[Fe2+] = 1.4 M

13. Concentration Cells Consider the cell presented on the left.
The 1/2 cell reactions are the same, it is just the concentrations that differ.
Will there be electron flow?

14. Concentration Cells (cont.)
Ag+ + e- Ag E°1/2 = 0.80 V
• What if both sides had 1 M
concentrations of Ag+?
• E°1/2 would be the same;
therefore, E°cell = 0.

15. Concentration Cells (cont.)
Anode:
Ag Ag+ + e- E1/2 = ? V
Cathode:
Ag+ + e Ag E1/2 = 0.80 V
Ecell = E°cell - (0.0591/n)log(Q)
0 V 1
Ecell = - (0.0591)log(0.1) = V

16. Concentration Cells (cont.)
Another Example:
What is Ecell?

17. Concentration Cells (cont.)
Ecell = E°cell - (0.0591/n)log(Q) e- 2
Fe2+ + 2e Fe
2 e- transferred…n = 2
anode
cathode
Ecell = -(0.0296)log(.1) = V

18. Summary DG = DG° + RTln(Q) DG = -nFEcell
Ecell = E°cell - (0.0591/n)log(Q)
• None of these ideas is separate. They are all
connected, and are all derived directly from
thermodynamics.

19. Question

21. Question

22. Solution

23. Questions

26. Applications using Batteries

27. Battery Convert stored chemical energy into electrical energy
Reaction between chemicals take place
Consisting of electrochemical cells
Contains
Electrodes
Electrolyte

28. Electrodes and Electrolytes
Cathode
Positive terminal
Chemical reduction occurs (gain electrons)
Anode
Negative terminal
Chemical oxidation occurs (lose electrons)
Electrolytes allow:
Separation of ionic transport and electrical transport
Ions to move between electrodes and terminals
Current to flow out of the battery to perform work
Think of questions to ask the class
Different kinds of batteries, chemistry behind them
Why one type over another in various applications
Biggest restrictions, thing that need improving

29. Battery Overview Battery has metal or plastic case
Inside case are cathode, anode, electrolytes
Separator creates barrier between cathode and anode
Current collector brass pin in middle of cell conducts electricity to outside circuit

30. Primary Cell One use (non- rechargeable/disposable)
Chemical reaction used, can not be reversed
Used when long periods of storage are required
Lower discharge rate than secondary batteries 
Use:
smoke detectors, flashlights, remote controls
Electrochemical reactions are non-reversible materials in the electrodes are utilized, therefore cannot regenerate electricity

31. Secondary Cells Rechargeable batteries
Reaction can be readily reversed
Similar to primary cells except redox reaction can be reversed
Recharging:
Electrodes undergo the opposite process than discharging
Cathode is oxidized and produces electrons
Electrons absorbed by anode

32. Nickel-Cadmium Battery
Anode: Cadmium hydroxide, Cd(OH)2
Cathode: Nickel hydroxide, Ni(OH)2
Electrolyte: Potassium hydroxide, KOH
The half-reactions are discharging:
Cd+2OH- → Cd(OH)2+2e-
2NiO(OH)+Cd+2e- →2Ni(OH)2+2OH-
Overall reaction discharged:
2NiO(OH) + Cd+2H2O→2Ni(OH)2+Cd(OH)2

33. Nickel-Cadmium Battery
Advantages:
This chemistry is reliable
Operate in a range of temperatures
Tolerates abuse well and performs well after long periods of storage
Used in electronics and toys
Disadvantages:
It is three to five times more expensive than lead-acid
Its materials are toxic and the recycling infrastructure for larger nickel-cadmium batteries is very limited

34. Lead-Acid Battery Anode: Porous lead Cathode: Lead-dioxide
Electrolyte: Sulfuric acid, 6 molar H2SO4
Discharging anode:PbO2(s)+4H+(aq)+SO42-+2e- → PbSO4(s)+ 2H2O  cathode: Pb(s) + SO42-(aq) → PbSO4(s) + 2e- 
During charging  anode: PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- 
cathode: PbSO4(s) + 2e- → Pb(s) + SO42-(aq)

35. Lead-Acid Battery
The lead-acid cells in automobile batteries are wet cells
Deliver short burst of high power, to start the engine
Battery supplies power to the starter and ignition system to start the engine
Battery acts as a voltage stabilizer in the electrical system
Supplies the extra power necessary when the vehicle's electrical load exceeds the supply from the charging system

36. Lead-Acid Battery Advantages: Disadvantages:
Batteries of all shapes and sizes, available in
Maintenance-free products and mass-produced
Best value for power and energy per kilowatt-hour
Have the longest life cycle and a large environmental advantage
Ninety-seven percent of the lead is recycled and reused in new batteries
Disadvantages:
Lead is heavier compared to alternative elements
Certain efficiencies in current conductors and other advances continue to improve on the power density of a lead-acid battery's design

37. Lithium-Ion Battery Anode: Graphite Cathode: Lithium manganese dioxide
Electrolyte: mixture of lithium salts
Lithium ion battery half cell reactions
CoO2 + Li+ + e- ↔ LiCoO2 Eº = 1V
Li+ + C6+ e- ↔ LiC6 Eº ~ -3V
Overall reaction during discharge
CoO2 + LiC6 ↔ LiCoO2 + C6
Eoc = E+ - E- = 1 - (-3.01) = 4V

38. Lithium-Ion Battery Ideal material
Low density, lithium is light
High reduction potential
Largest energy density for weight
Li-based cells are most compact ways of storing electrical energy
Lower in energy density than lithium metal, lithium-ion is safe
Energy density is twice of the standard nickel-cadmium 
No memory and no scheduled cycling is required to prolong battery life 

39. Lithium-Ion Battery Advantages: Disadvantages:
It has a high specific energy (number of hours of operation for a given weight)
Huge success for mobile applications such as phones and notebook computers
Disadvantages:
Cost differential
Not as apparent with small batteries (phones and computers)
Automotive batteries are larger, cost becomes more significant
Cell temperature is monitored to prevent temperature extremes
No established system for recycling large lithium-ion batteries

40. What is a hydrogen fuel cell?
Hydrogen fuel cells (HFCs) are a type of electrochemical cell.
HFCs generate electricity by reduction and oxidation reactions within the cell.
They use three main components, a fuel, an oxidant and an electrolyte.
HFCs operate like batteries, although they require external fuel.
HFCs are a thermodynamically open system.
HFCs use hydrogen as a fuel, oxygen as an oxidant, a proton exchange membrane as an electrolyte, and emit only water as waste.

41. How do they work?
Fuel (H2) is first transported to the anode of the cell
Fuel undergoes the anode reaction
Anode reaction splits the fuel into H+ (a proton) and e-
Protons pass through the electrolyte to the cathode
Electrons can not pass through the electrolyte, and must travel through an external circuit which creates a usable electric current
Protons and electrons reach the cathode, and undergo the cathode reaction

43. Cathode half- reaction:
Acidic
Oxidation
At the anode of the cell, a catalyst (platinum powder) is used to separate the proton from the electron in the hydrogen fuel.
Anode half-reaction:
2H2  4H+ + 4e-
Eo = 0.00V
Reduction
At the cathode of the cell, a second catalyst (nickel) is used to recombine the protons, electrons, and oxygen atoms to form water.
Cathode half- reaction:
4H+ + O2 + 4e-  2H2O
Eo = 0.68V
In electrochemistry, the Eocell value (energy) of a fuel cell is equal to the Eo of the cathode half-reaction minus the Eo of the anode half-reaction. For a hydrogen fuel cell, the two half reactions are shown above. So to calculate the energy of one fuel cell, we need to subtract the anode energy from the cathode energy. For a HFC, the Eocell = 0.68V – 0.00V which equals 0.68V

44. Alkaline

45. Uses of hydrogen fuel cells
There are many different uses of fuel cells being utilized right now. Some of these uses are…
Power sources for vehicles such as cars, trucks, buses and even boats and submarines
Power sources for spacecraft, remote weather stations and military technology
Batteries for electronics such as laptops and smart phones
Sources for uninterruptable power supplies.

46. Problems regarding hydrogen fuel cells
Lack of hydrogen infrastructure
Need for refueling stations
Lack of consumer distribution system
Cost of hydrogen fuel cells
2009 Department of Energy estimated $61/kw
Honda FCX Clarity costs about half a million dollars to make
Carbon cost of producing hydrogen
Problems with HFC cars
Short range (~260 miles)
Warm up time (~5 minutes)

47. Methanol fuel cell

48. Microbial fuel cell
MFCs are bioelectrical devices that harness the natural metabolisms of microbes to produce electrical power directly from organic material

49. MFC Basics
Oxygen Poor
Oxygen Rich

51. MFC

52. MFC