Chemical and fuel cells

In a nutshell

Cells contain chemicals which react to produce electricity. The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.

Equations

word equation	symbol equation
$hydrogen \rightarrow two~hydrogen~ions+two~electrons$	$H_2 → 2H^+ + 2e^–$
$four ~hydrogen~ions +oxygen+four~electrons\rightarrow two~water~molecules$	$4H^+ + O_2 + 4e^– → 2H_2O$
$hydrogen+oxygen\rightarrow water$	$2H_2 + O_2→ 2H_2O$

Definitions

Electrode	An electrical metal conductor which is in contact with the electrolyte.
Electrolyte	An ionic compound which is dissolved in aqueous solution (water) or molten (melted) allowing the ions to move freely.
Cell	A cell is a system which produces electricity from chemical reactions.
Battery	A battery is two or more cells connected up together.
Redox	Reduction and oxidation.
Oxidation	Loss of electrons and gain of oxygen.
Reduction	Gain of electrons and loss of oxygen.
Anode	In fuel cells, the anode is the negative electrode. In electrolysis, the anode is the positive electrode.
Cathode	In fuel cells, the cathode is the positive electrode. In electrolysis, the cathode is the negative electrode.

Simple cells

A simple cell produces a voltage and is made up of two different electrodes dipped in an electrolyte.

The two electrodes are made up of different metals.
The greater the difference in reactivity between the two metals, the greater the voltage.
Different electrolytes react with electrodes differently, which affects the magnitude of voltage.

A simple cell consists of a voltmeter connected to a negative electrode and positive electrode to measure the voltage. The two electrodes are immersed in an electrolyte.

Chemistry; Separate chemistry 1; KS4 Year 10; Chemical and fuel cells

1.	Voltmeter
2.	Negative electrode
3.	Positive electrode
4.	Electrolyte

Non-rechargeable vs chargeable batteries

In non-rechargeable batteries:

The reaction is irreversible
Reactants in the electrolyte and electrode get used up after a period of time
The battery cannot be re-charged

Example

Alkaline batteries

In rechargeable batteries, the battery can be recharged by applying an external electrical current to reverse the reaction.

Example

Phone batteries

Hydrogen fuel cells

Definition

Hydrogen fuel cells are electrochemical cells which produce electricity and are used as an alternative to petrol and diesel.

Hydrogen gas from the fuel reacts with oxygen in the air:

Chemistry; Separate chemistry 1; KS4 Year 10; Chemical and fuel cells

1.	Electrical circuit
2.	Anode
3.	Acidic electrolyte
4.	Cathode
A.	Hydrogen fuel fed in at the anode.
B.	Oxygen fed in at the cathode.
C.	Water that is produced can exit at the anode.
D.	Unreacted oxygen exits at the cathode.

PROCEDURE

1.	An acid is used as the electrolyte.
2.	Hydrogen fuel is oxidised at the anode to produce hydrogen ions: $hydrogen \rightarrow two~hydrogen~ions^++two~electrons\newline H_2 → 2H^+ + 2e^–$
3.	The hydrogen ions migrate to the cathode, where they react with oxygen (from the air) in a reduction reaction: $four ~hydrogen~ions^+ +oxygen+four~electrons\rightarrow two~water~molecules\newline4H^+ + O_2 + 4e^– → 2H_2O$
4.	The movement of electrons from the anode to cathode produces an electrical current.
5.	The overall reaction is: $hydrogen+oxygen\rightarrow water\newline2H_2 + O_2→ 2H_2O$

Advantages and disadvantages of hydrogen fuel cells

Advantages	Disadvantages
Do not produce atmospheric pollutants More energy efficient than petrol and diesel, as more energy is released per kilogram of fuel Reduces the use and disposal of batteries	Exists in the gaseous state so it is difficult and expensive to store; it must be stored under high pressure which is dangerous Hydrogen gas is highly explosive and flammable The production of hydrogen gas usually involves energy-intense processes, such as electrolysis, which uses energy from fossil fuels