Reactivity 1.3.5

Fuel cells (Reactivity 1.3.5)

In Reactivity 3.2, you will learn that a primary (voltaic) cell is an electrochemical

cell that converts chemical energy from spontaneous redox reactions into electrical

energy. These reactions are mostly irreversible, and the cells of this type are mainly

used for low-current applications, such as hand torches, long-life fire alarms,

calculators and wall clocks. However, primary cells often contain materials that are

toxic to humans and the environment if they are not disposed of properly. Primary

cells typically contain zinc metal, manganese dioxide and potassium hydroxide.

In a secondary cell, the chemical reactions are reversible, so the battery can be

recharged. Therefore, secondary cells are more environmentally friendly than

primary cells. The lead-acid battery is a type of rechargeable battery, found in the

majority of gasoline-powered vehicles. Electric vehicles are powered by lithium-

ion batteries, another type of secondary cell.

While secondary cells have advantages over traditional primary cells, there are

still problems associated with these types of batteries, such as overheating,

limited lifespan and environmental concerns in terms of their disposal.

A fuel cell is used to convert chemical energy of a fuel directly into electrical

energy. The first fuel cel was invented in 1838 by Welsh scientist, William

Groves. The difference between a fuel cel and a voltaic cell is that in a fuel cell,

the fuel is continuously supplied from an external source, whereas a voltaic cel

contains finite amounts of reactants locked within the cell. Therefore, a fuel cel|

can produce electricity indefinitely, while a voltaic cell stops working when all|

reactants within the cell are consumed.

Hydrogen fuel cell

The hydrogen fuel cell is an electrochemical cell that uses hydrogen

and oxygen gases as fuel. The redox reaction between hydrogen and

oxygen produces water, electricity and heat.

fuel in

The following steps occur in a hydrogen fuel cell:

1. The proton exchange membrane (PEM) selectively allows

hydrogen ions to diffuse between the cathode and anode but

prevents the passage of other ions, molecules and electrons

between these electrodes.

2. Hydrogen gas is oxidized at the anode on the surface of a

platinum-based catalyst. The half-equation for the reaction at the

anode is as follows:

H2(g) → 2H+(aq) + 2e-| excess

fuel out

3. The electrons cannot move through the PEM, and therefore

have to leave the cell through an external circuit, producing the

electrical output of the cell.

4. The protons formed at the anode move across the PEM to the

cathode, where they combine with the oxygen gas and electrons.

Oxygen gas is reduced to form water as a waste product. The

half-equation for the reaction at the cathode is as follows:

0 ( g ) + 4e + 4H+(aq) → 2H,O(i)

electric current|

个

7 个

H +

4 个

unused

air, water, and heat

H2O

7 anode cathode

PEM and electrolyte

A Figure 12 The main components of a

commercial hydrogen cell include an

electrolyte, a proton exchange membrane

(PEM) and the electrodes

437

Reactivity 1 What drives chemical reactions?

The overall redox reaction is:

2H2(g) + 02(g) → 2H20(1)

Hydrogen fuel cells are considered to be a clean energy source because their

only product is water. However, hydrogen gas, H2(g), is not abundant on

Earth and needs to be produced by the following methods, one of which has

environmental implications.

1. Electrolysis of water

The electrolysis of water is the process of using electricity to split water into

oxygen and hydrogen gas. This is the reverse process of the reaction that occurs

in a hydrogen fuel cell.

2H20(1) → 2H,(g) + 02(g)

The electrolysis of water can be powered by renewable energy sources, such as

solar energy. This environmentally friendly source of hydrogen gas only makes up

a small proportion of the global supply.

2. Steam reforming of hydrocarbons

Hydrocarbons, such as natural gas, gasified coal and biomass, diesel and other

liquid fuels, undergo steam reforming resulting in the production of toxic carbon

monoxide gas and hydrogen gas. The general equation for the steam reforming

of alkanes is as follows:

C,H2m+2(g) + nH20(g) = nCO(g) + (2n + 1)H2(g)

Carbon monoxide then reacts with steam to form the greenhouse gas carbon

dioxide:

CO(g) + H20(g) = CO2(g) + H2(g)

The annual production of hydrogen from the various sources is responsible for

carbon dioxide emissions greater than the total annual carbon dioxide emissions

of the United Kingdom. The scientific community continues to search for new

green sources of hydrogen gas.

Experiments

Alternatives to fossil fuels, ranging from hydrogen fuel cells to biofuels, have

arisen from extensive experimentation by numerous scientists over time.

Creativity and imagination play a role in experimental design, interpretation

and conclusions. Once considered an electrochemical curiosity, fuel cells are

now seen as a promising source of renewable energy.

D i r e c t - m e t h a n o l fuel cell

In a direct-methanol fuel cell (DMFC), the source of the hydrogen ions, H+,

is methanol rather than hydrogen gas. Methanol is a liquid fuel that can be

produced from renewable resources through fermentation. Methanol is cleaner

than hydrogen because the method of its production has less impact on the

environment in terms of formation of greenhouse gases. Methanol also has a far

greater energy density (energy per unit volume) than hydrogen gas.

4 3 8

Reactivity 1.3 Energy from fuels

DMFCs use the electrochemical reactions below:

anode half-equation: CH:OH(aq) + H2O(l) → CO,(g) + 6H+(aq) + 6 е-

cathode half-equation: 2010) + 6H*(ag) + 6e → 3H,01)

overall reaction: CH,OH(aq) + 20,(g) → CO(g) + 2H20(1)

The electrochemical reaction produces carbon dioxide, a greenhouse gas.

The reaction at the anode requires a catalyst that contains expensive precious

metals, usually ruthenium and palladium. Another disadvantage is the toxicity of

methanol.

Methanol can also be used to supply hydrogen gas for hydrogen fuel cells.

Steam reforming of methanol at 250 °C produces CO,(g) and H2(g), along with a

small amount of CO(g).

A distinct advantage of DMFCs is their high energy density (figure 14). The slope

of the graph gives the energy per unit volume, showing that the methanol fuel

cell has a much higher energy density than the lithium-ion battery.

30-

energy content /Wh-

25-

15-

0 +

A Figure 13 A portable direct-methanol

fuel cel (DMFC), which can be used to

power laptops and video cameras

— lithium-ion battery

— direct methanol

fuel cell

50 100 150 200

volume/ cm3

A Figure 14 Comparison of the energy density for the lithium-ion battery and the

direct-methanol fuel cel

Practice questions

6. a. Outline the differences between a primary and secondary cel.

b. What are the advantages a hydrogen fuel cell has over the traditional

lead-acid battery.

c. Identify and explain the function of the feature of a hydrogen fuel cell

that involves the passage ofions.

d. A direct methanol fuel cell (DMFC) converts chemical energy to

electrical energy.

i. Deduce the anode and cathode half equations and the overall

chemical equation for this electrochemical cell.

ii. Outline one advantage and one disadvantage of the methanol cell

(DMFC) compared to a hydrogen fuel cell.

4 3 9

Reactivity 1 What drives chemical reactions?

When comparing fuels, the energy density (energy released per unit volume)

and specific energy (energy released per unit mass) can give quite different

pictures (table 2).

F u e l s o u r c e Energy density

/ MJ dm-3

Specific energy

/ MJ kg~

compressed hydrogen 1.9 120

methanol 16 20

liquefied natural gas 21 50

liquid propane 46

gasoline 32 46

A Table 2 Comparing fuels in terms of energy density and specific energy

The specific energy of hydrogen is more than double that of any other fuel in

table 2. In hydrogen, H2, 2.02 g contains 1 mol of fuel, compared with 32.05 g

for 1 mol of methanol, CHOH, or approximately 110g for 1 mol of gasoline.

Therefore, it could be easy to believe that hydrogen is the best fuel choice.

However, fuels need to be stored and transported. The molar volume of a gas

at room temperature and 1 atm pressure is approximately 24 dm?. One mole

of gaseous hydrogen under these conditions would require a 24 dm? storage

tank, which adds to the weight if the device is to be portable, such as in a car.

One mole of methanol would occupy 40.4 cm?, and the same 24 dm? storage

tank could hold over 545 mol of methanol fuel. Even when compressed, the

hydrogen gas occupies a much larger volume, and regulators and compressors

further increase the weight of the storage system. Gasoline offers the highest

energy density but has associated environmental problems.