6. Galvanic cells

Question 1

Electrochemistry refers to any reaction in which electrons are transferred between species.

These are also known as what?

Answer 1

Oxidation-reduction reactions, or redox reactions.

Question 2

What are the 2 types of electrochemical cells?

Answer 2

• Galvanic cells, where spontaneous1 reactions convert chemical potential energy to electrical energy.
• Electrolytic cells, where electrical energy is consumed to drive a non-spontaneous reaction to occur.

Question 3

Electrodes are connected by an external wire, which allows electrons to flow from one electrode to another, resulting in the redox reaction occurring.

In a galvanic cell:

• Oxidation occurs at the negative electrode, which is known as the ___.
• Reduction occurs at the positive electrode, which is known as the ___.

Answer 3

• anode
• cathode

Question 4

Another type of galvanic cell is where the two half-cells are connected by a what?

Answer 4

Salt bridge.

Question 5

Galvanic cells convert chemical potential energy into electrical energy, but how is this energy measured?

Answer 5

An external voltmeter will measure the electromotive force (EMF) or voltage difference between the two half-cells in a galvanic cell.

Question 6

This difference in voltage is also known as what?

Answer 6

The cell potential (Ecellø​), and shows the potential of the cell to pull and push electrons through the circuit. This value is measured in units of volts, }V.

Question 7

Before we can calculate the cell potential for a galvanic cell, we need to know what?

Answer 7

The potentials of both the half-cells.

Question 8

When the oxygen is reduced, and the fuel oxidised, the ions produced by these reactions enter the electrolyte.

Answer 8

The electrolyte solution allows the ions produced in each half-reaction to travel through to the other electrode. There, these ions react and form waste products. Typically, the waste from a fuel cell is 2CO₂​ or H_2​O, depending on the fuel used.

Question 9

The alkaline fuel cell uses a concentrated solution of aqueous what as the electrolyte?

Answer 9

Potassium hydroxide (KOH).

Question 10

Oxygen gas is reduced into OH− where?

Answer 10

At the cathode.

To facilitate this reaction, the anode typically contains a nickel catalyst. The half-equation for the reduction is as follows:

• (aq)O2​(g)+2H2​O(l)+4e−Ni catalyst​4OH−(aq)

This reaction consumes oxygen gas, water and electrons, and produces aqueous hydroxide ions.

Question 11

The fuel used by the alkaline fuel cell is what?

Answer 11

Hydrogen gas.

At the anode, the hydrogen gas is oxidised into water. This reaction is also facilitated by a catalyst, typically platinum metal. The oxidation reaction is as follows:

Question 12

The alkaline fuel cell is the most common; however, there is no reason why the electrolyte needs to be a base.Another common type of fuel cell is one that operates under acidic conditions.

The electrolyte is typically what?

Answer 12

Phosphoric acid. Unsurprisingly, this fuel cell is called the phosphoric acid fuel cell.

As you may have guessed from the name, the ions in the electrolyte are protons. These react with oxygen gas at the cathode according to the following reaction:

Question 13

Which fuel cell uses methanol as fuel?

Answer 13

The direct-action methanol fuel cell (DAMFC).

Question 14

Which is more efficient - fuel cells or combustion engines, and why?

Answer 14

Fuel cells are more efficient than combustion engines.

This is because fuel cells directly generate electricity from the redox reaction, whereas combustion engines generate electricity using other physical processes.

Some of this is because combustion engines operate at very high temperatures, meaning that more energy is lost to heat. The fuel cells we have seen operate at or below 100°C, so more of the chemical energy is converted to electricity.

Fuel cells also avoid the nasty pollutants that come from combustion of fossil fuels, such as sulfur and nitrogen compounds that can contribute to acid rain.

Question 15

There are some disadvantages, the main one being the high cost of construction of the cell in the first place. Most of this cost comes from the fact that the electrodes need to be embedded with what to decrease the activation energy of the reaction?

Answer 15

Catalysts.

These catalysts are usually metals, such as platinum, nickel, copper and rhodium. These can be expensive (especially platinum and rhodium), which increases the cost of construction.

Furthermore, alkaline and acidic fuel cells (the cheapest ones to manufacture) require a constant supply of hydrogen gas.

Question 16

What did NASA use to fuel space missions in the 60s?

Answer 16

Nuclear power was immediately ruled out, as a leak of the radioactive fuel would have almost certainly been fatal to the crew. Using combustion of fossil fuels was also ruled out, as this would require oxygen, and produce toxic fumes.

This put the engineers in a difficult position, as they needed a fuel source that was efficient and would not pose a danger to the crew. Luckily, the engineers were aware of alkaline fuel cells, and decided to include these!

Question 17

What living things can be used as a fuel cell?

Answer 17

Bacteria feed by reducing oxygen and oxidising a food source. Since fuel cells work by redox principles, some clever researchers managed to combine the two, creating a microbial fuel cell (MFC).

The bacteria oxidised acetate ions (CH3​COO−) which are present in the decaying organic matter into carbon dioxide and hydrogen ions, according to the following reaction:

Question 18

Fuel cells are electrochemical cells that transform chemical energy to electrical energy via what?

Answer 18

Spontaneous redox reactions.

They consist of an anode and a cathode, connected by a conducting wire. These electrodes are usually constructed of graphite meaning they are inert, and have catalysts embedded in and on their surface.

Both of these electrodes are held in one cell, separated by an electrolyte solution. The porous nature of these electrodes allows the ions formed by the oxidation and reduction reactions to enter the electrolyte solution.

Question 19

In both fuel cells and combustion reactions, a constant stream of what is required?

Answer 19

Fuel and oxygen.

Question 20

We have already established pipelines and filling stations for fossil fuels for internal combustion engines. However, similar infrastructure for hydrogen fuel cells has not been fully established yet.

If we want the widespread adoption of hydrogen as the primary fuel source for powering our vehicles and providing electricity, it would require a massive expenditure in infrastructure—that is, changes to what?

Answer 20

Pipelines and filling stations, as well as improvements to hydrogen storage methods.

Question 21

Hydrogen is a gas at room temperature, and boils at −252°C. This low boiling point means that hydrogen must be kept at around 690 atm to keep it as a liquid at room temperature—meaning a considerable amount of energy is required to liquefy it!

What would this mean for filling stations?

Answer 21

This means that filling stations would need to be able to pump liquid hydrogen; which will be an expensive endeavour!

To overcome this issue, researchers are investigating the potential of storing hydrogen in ‘materials-based storage’ methods. That is, the potential of hydrogen being adsorbed to the surface of metal hydrides, either as hydrogen molecules or atoms. Or, the potential of storing H₂​ as a gas in the gaps of a crystal lattice of metal hydrides.

Question 22

The products of hydrogen fuel cells are water and heat.

It is natural to think fuel cells are ‘zero emission’ devices and are environmentally neutral. However, the hydrogen needed for fuel cells must come from somewhere.

If the hydrogen for fuel cells comes from renewable energy sources, hydrogen fuel cells can be labelled as ‘zero emission’ devices. However, more often than not, the hydrogen comes from what?

Answer 22

Fossil fuels.

At present, around 95% of hydrogen produced comes from fossil fuels (i.e. natural gas, oil and coal). This process is called steam reforming, in which fossil fuels are turned into H₂​ and CO. In this process, steam reacts with a fossil fuel at high temperatures whilst in the presence of a nickel catalyst.

Question 23

Why are galvanic cells called half-cells?

Answer 23

The name half-cell implies that we need two of them for something interesting to happen, and this is true. We can connect two of these half-cells together by placing a salt bridge in the electrolyte solutions and a conducting wire between the two electrodes.

Question 24

Batteries are small, enclosed galvanic cells.

As you most likely know, batteries eventually go flat. How can we explain this in chemical terms?

Answer 24

In the half-cell where reduction is occurring, the metal ion in the electrolyte is being reduced into its elemental form. The electrode in the other half-cell is being oxidised into its ionic form.

When the reaction proceeds, these reactants are consumed. Eventually, there are not enough reactants present in the cells for the overall redox reaction to occur. This results in a flat battery!

Question 25

How can some batteries can be recharged?

Answer 25

This is achieved by providing an electrical current to the cell. This forces the redox reactions that have occurred in each cell to go in reverse, so that species which were oxidised are reduced, and species which were reduced are oxidised. After this, the spontaneous redox reaction which generated electricity can happen again. However, most batteries cannot be recharged, and are simply discarded. Since some common batteries contain dangerous chemicals, including heavy metals, this can have a negative impact on the environment.

Question 26

The most obvious difference between fuel cells and galvanic cells is what?

Answer 26

The constant supply of fuel. This allows the fuel cell to produce electricity as long as there is a supply of fuel—somewhat like a combustion engine in a car. This comparison is somewhat illuminating, as fuel cells are currently being investigated as an alternative power source to combustion engines in motor vehicles. We can say that galvanic cells store a certain amount of chemical potential energy, and can only produce that amount of electrical energy. This is in contrast to fuel cells, which don’t store energy; they only convert it from chemical potential energy into electricity.

Question 27

The type of electrolyte used in each cell is also quite different. Why?

Answer 27

Galvanic cells require the ion of the metal electrode to be present in the electrolyte. These can be quite expensive and are usually toxic to the environment. This toxicity means that they need to be carefully disposed of. Since most galvanic cells cannot be recharged, the safe disposal of these cells is currently an expensive problem the world is dealing with. Since fuel cells don’t need to be disposed of, they don’t run into this issue, and can simply be repaired. If a fuel cell is no longer required, it can be recycled into its constituent parts and reused. This means there is comparatively little risk of toxic heavy metal waste entering the environment from the use of fuel cells.

Question 28

Due to the fact that fuel cells can be refueled, they can be used in situations where galvanic cells would not be a good solution. Two examples of this are what?

Answer 28

The powering of motor vehicles, and as a power generator for buildings. Both of these situations require a constant supply of energy, which is currently usually provided through the combustion of fossil fuels. However, there is research into using stacks of fuel cells as power sources instead. This is suitable because fuel cells are able to continue producing electricity as long as the fuel is constantly being provided. This is analogous to the way the already existing combustion engines work, so is no extra work for the user. Using fuel cells is also much more efficient and environmentally friendly. If galvanic cells were used rather than fuel cells, they would produce electricity until the electrolyte solution or electrode was consumed. At this point, the cell would need to be replaced or recharged. As stated previously, they are also less environmentally friendly, as the disposal of galvanic cells can leak heavy metals into the environment. If they are rechargeable, they need to consume electricity which needs to be generated somewhere else.

Question 29

Fuel cells do have some disadvantages when compared to galvanic cells. What are they?

Answer 29

Structurally, the catalyst embedded in and on the surface of the electrolyte is usually a heavy metal, such as platinum. You probably know that platinum is exceedingly expensive, so the construction of a fuel cell is usually more expensive than producing a galvanic cell. Fuel cells also require (very unsurprisingly) a fuel source to function. This increases the size of the power supply, and therefore the size of the electronic device as a whole. This means that using a fuel cell to replace the galvanic cells in your watch, or remote control is probably impractical.

Question 30

Despite these disadvantages, fuel cells are a very promising prospect. True or false?

Answer 30

True. ## Footnote For example, there is active research into molecular manufacturing, which allows the catalysts to be more effectively dispersed on the fuel cell electrodes. In some cases, the mass of platinum can be reduced by 80%, while maintaining the efficiency of the cell. This means the high initial cost of the fuel cells can be reduced, making them more affordable. Also, when fuel cells eventually become old and are no longer used, they can easily be recycled into new cells. This means that fuel cells have very little additional impact on the environment, after they have initially been constructed.