C.6 Electrochemistry, Rechargeable Batteries and Fuel Cells (HL)

Question 1

What is a battery?

Answer 1

A battery is a portable electrochemical source made up of one or more voltaic (galvanic) cells connected in series.

Question 2

How is charge carried in voltaic cells?

Answer 2

In voltaic cells charge is carried by free moving ions which take a finites time to diffuse through the cell to the electrodes this causes internal resistance which limits the maximum current.

Question 3

What does the maximum voltage a cell can generate depend on?

Answer 3

The maximum voltage tha a cell can generate depends on the nature of the material from which it is constructed.

Question 4

What does the total amount of work that can be obtained by the cell depend on?

Answer 4

The quantity of the material from which it is constructed.

Question 5

What is a primary cell?

Answer 5

A cell in which the redox reaction is non reversible, they cannot be recharged.

Question 6

What is a rechargeable cell?

Answer 6

A cell in which the redox reaction can be reversed using an input of electricity allowing the cell to be used multiple times.

Question 7

What is a fuel cell?

Answer 7

A cel that has a continuous supply of reactants from an external source. Chemical energy contained in fuel eg hydrogen is converted directly to electricity using an oxidising agent eg oxygen.

Question 8

What is a typical fuel cell?

Answer 8

Typical fuels used in fuel cells are hydrogen and methanol, with oxygen gas as the oxidising agent. The electrolyte between the two electrodes may be alkane eg KOH or acidic eg H3PO4 invoking the moment of hydroxide moons or protons respectively. Current is generated by the flow of the electrons from the anode (negative) through an external circuit to the cathode (+)

Question 9

How does a proton exchange membrane (PEM) fuel cell work?

Answer 9

Hydrogen gas passes through a porous carbon anode containing platinum where it is oxidised to H+ ions. The electrons produced flow through the external circuit and the protons pass though a proton exchange membrane (PEM) to the cathode (also contains platinum) combining with oxygen and electrons to form water.

Question 10

How does microbial fuel cells (MFCs) work?

Answer 10

The MFC is essentially a PEM fuel cell where bacteria such as Geobacter species are attached to the anode. Carbohydrates and other substrates present in the waste water are oxidised by the bacteria at the anode to carbon dioxide, protons and electrons (which flow through the external circuit to the cathode)

Question 11

What is the formula for the thermodynamic efficiency of a cell?

Answer 11

Thermodynamic efficiency = Delta G/Delta H

Question 12

What are the similarities between fuel cells and rechargeable batteries?

Answer 12

Both convert chemical energy to electrical energy
Both compose and anode, cathode and an electrolyte
Both use separated reduction and oxidation reactions to drive the flow of electrons through an external circuit

Question 13

What are the difference between fuel cells and rechargeable batteries?

Answer 13

Fuel cells have an external source of fuel, rechargeable batteries contain the energy source
Fuels cells will not run out if the supply of fuel and oxidant is maintained. Rechargeable batteries have to be recharged by connecting to a supply of electricity
Fuels cells can generate more electricity

Question 14

What is the size and voltage of fuel cells?

Answer 14

Size depends on use, single cells are light but stacks can be heavy and bulky, high power to mass ratio
0/6-0/8V per cell - used in a stack with several joined in series

Question 15

What is the size and voltage of lead acid batteries?

Answer 15

Large and heavy, not portable, low power to mass ratio

2V per cell usually used as a battery with 6 cells in series

Question 16

What is the size and voltage of nickel cadmium batteries?

Answer 16

Small and light used in portable devices. Intermediate power to mass ratio
1.2V per cell

Question 17

What is the size and voltage of lithium ion batteries?

Answer 17

Small and light used in a variety or portable devices. Highest power to mass ratio of rechargeable batteries.
About 3.7V per cell

Question 18

When is the Nernst equation used?

Answer 18

The Nernst equation is used to determine the electrode potential of a half cell when the conditions are non standard.

Question 19

What is the Nernst equation?

Answer 19

E = E* - (RT/nF)lnQ

E is the electrode potential 
E* is the standard electrode potential of the cell
R is the gas constant
T is the temperature in K
n is the number of electrons transferred
F is the Faraday constant 
Q is the reaction quotient

Question 20

What is a concentration cell?

Answer 20

It is possible to create a voltaic cell using two half cells contacting the same materials as long as the concentration of the electrolyte is different in each.