Energy storage cells and hydrogen fuel cells

In a nutshell

The cell potential of energy storage cells can be calculated using the electrode potentials. Lithium batteries are rechargeable and are commonly seen in phones and laptops. Hydrogen fuel cells can produce electricity by reacting hydrogen and oxygen to form water under acidic or alkaline conditions.

Energy storage cells

Batteries are energy storage cells, which work in a similar way to electrochemical cells. The cell potential of energy storage cells and overall reaction can be determined, if you know the half equations and their electrode potentials.

Example

The $Ni/Zn$ battery has a $Ni(OH)_2$ cathode, a $Zn$ anode and a $KOH$ electrolyte solution.

location	half equation	Voltage
Anode	$Zn(OH)_2+2e^-\leftrightharpoons Zn+2OH^-$	$-1.24\ V$
Cathode	$NiO(OH)+H_2O+e^-\leftrightharpoons Ni(OH)_2+OH^-$	$+0.49\ V$
Overall	$2NiO(OH)+Zn+2H_2O\rightarrow 2Ni(OH)_2+Zn(OH)_2$	$+1.73\ V$

To determine the overall reaction from the half equations, you can determine that both equations have electrons and $OH^-$ ions so they must be either side of the reaction to balance out. The cathode half equation must be doubled so each equation has $2e^-$ and $2OH^-$ to give you:

$2NiO(OH)+2H_2O+2e^-\leftrightharpoons2Ni(OH)_2+2OH^-$

At the anode, $Zn$ must lose electrons so oxidation can occur so the half equation will start with $Zn$ to give:

$Zn+2OH^-\rightarrow Zn(OH)_2+2e^-$

This can now be combined with the cathode to give you the overall equation:

$2NiO(OH)+Zn+2H_2O\rightarrow 2Ni(OH)_2+Zn(OH)_2$

To work out cell potential which is given as $+1.73\ V$ , the following equation must be used:

$E_{cell}=E^\theta_{(reduction)}-E^\theta_{(oxidation)}$ 

$E_{cell}=0.49-(-1.24)$

$E_{cell}=0.49+1.24$

$E_{cell}=+1.73\ V$

Lithium batteries

Rechargeable lithium cells are used in mobile phones and laptops to provide energy. As lithium batteries have reversible reactions, they are recharged by supplying a current which forces the electrons to flow in the reverse direction.

Lithium batteries are initially expensive but they have longer lifetimes and can be reused, so are cheaper in the long-term. Lithium is flammable and reactive, so the batteries have a risk of catching fire if the device overheats. Toxic chemicals are used to produce many energy storage cells so they must be disposed of correctly.

Hydrogen fuel cells

Fuel cells are different to other energy storage cells as the chemicals are stored outside of the cell, whereas for most cells the chemicals will be within the cell in the electrodes. One of the most common fuel cell is the alkaline hydrogen-oxygen fuel cell which can be used as a source of power for electric vehicles.

Hydrogen gas fuel is reacted with an oxidant such as oxygen gas by feeding the two gases into the cell when electricity is required. An alkaline $KOH$ electrolyte is used as a source of hydroxide ions. The overall reaction is between hydrogen and oxygen to form water.

Example

Chemistry; Redox II; KS5 Year 12; Energy storage cells and hydrogen fuel cells

location	half equation
Anode	$2H_2+4OH^-\rightarrow 4H_2O+4e^-$
Cathode	$O_2+2H_2O+4e^-\rightarrow 4OH^-$
Overall	$2H_2+O_2\rightarrow 2H_2O$

Generating electricity in an alkaline Hydrogen fuel cell

1.	At the anode, hydrogen is fed in and it is reacted with hydroxide ions to produce water and electrons.
2.	The anion-exchange membrane that separates the electrodes allows water and hydroxide ions to pass through, only.
3.	Electrons cannot pass through the membrane, so they pass through an external circuit from the anode to the cathode which generates an electric current.
4.	At the cathode, oxygen is fed in and it is reacted with water and electrons from the circuit, forming hydroxide ions.
5.	The hydroxide ions from the anode, can pass through the membrane to react with hydrogen at the cathode.

Electricity can also be generated from hydrogen and oxygen gas using acidic conditions instead. A polymer electrolyte membrane (PEM) is used which only allows the flow of $H^+$ across. This process produces water which is the only waste product involved.

Example

Chemistry; Redox II; KS5 Year 12; Energy storage cells and hydrogen fuel cells

location	half equation
Anode	$H_2\rightarrow 2H^++2e^-$
Cathode	$\frac{1}{2}O_2+2H^++2e^-\rightarrow H_2O$
Overall	$H_2+\frac{1}{2}O_2\rightarrow H_2O$

Generating electricity in an acidic hydrogen fuel cell

1.	Hydrogen is fed in at the anode and split into protons and electrons by the catalytic platinum electrode.
2.	The protons can pass through the PEM which forces the electrons through the electrical circuit, from the anode to the cathode, generating an electric current.
3.	Oxygen is fed in at the cathode which combines with the protons and electrons to form water.

Advantages and disadvantages of hydrogen fuel cells

Advantages

Disadvantages

Produce energy more efficiently than combustion engines which lose energy in the form of heat.

Produce less pollution and only have one waste product (water) compared to

CO_2

and other toxic pollutants which are produced from combustion engines.

No need to be recharged, a constant supply of oxygen and hydrogen will produce electricity. Water waste can also be reused to supply hydrogen and oxygen.

Energy is needed to produce the supply of hydrogen and oxygen which requires electricity, usually from the burning of fossil fuels.

Hydrogen is a flammable gas so needs to be stored and transported with care.