C.6: Electrochemistry, rechargeable batteries and fuel cells (AHL)

What are the advantages of nickel-cadmium batteries?

1. Charge quickly 2. Have a high number of charge/discharge cycles.

What are the drawbacks to the widespread use of fuel cells?

1. Considerably more expensive than the gasoline-powered vehicles 2. A lack of hydrogen infrastructure 3. Hydrogen gas must be handled and stored correctly to prevent accidents.

What are the disadvantages of lithium-ion batteries?

1. Expensive when compared to other types of batteries 2. Have been known to explode, causing fires that are very difficult to extinguish

What are the advantages of lithium-ion batteries?

1. Have high energy densities 2. Low discharge rates.

What are the advantages of lead-acid batteries?

1. Simple to manufacture 2. Cheap 3. Low self-discharge rates.

What are the disadvantages of lead-acid batteries?

1. They have low energy densities and the use of lead can cause environmental concerns. 2. Overcharging can lead to the production of hydrogen gas, which is an explosion risk. 3. Lead is a toxic metal that can affect the central nervous system.

What is the cell potential of a lead-acid battery?

2.02 V

What is the overall reaction in a PEM fuel cell?

2H2 (g) + O2 (g) → 2H2O (l)

What is the overall reaction in an alkaline fuel cell?

2H2 (g) + O2 (g) → 2H2O (l)

What is the overall equation for the recharge of a lead-acid battery?

2PbSO4 (s) + 2H2O (l) → Pb (s) + PbO2 (s) + 2H2SO4 (aq)

What is the overall equation for the reaction that takes place in a microbial fuel cell using glucose as a fuel?

C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l)

What are the disadvantages of nickel-cadmium batteries?

Can cause environmental concerns during disposal

What is the anode of a lithium-ion battery composed of?

Composed of lithium atoms absorbed onto a lattice of graphite.

How does a DMFC differ from a PEM fuel cell?

DMFC uses methanol as the fuel source instead of hydrogen gas.

What primarily determines the voltage produced by a battery?

Depends on the nature of the materials used, specifically the electrodes and the electrolyte.

How is Q calculated in the nernst equation?

Divide the concentration of the half-cell at the anode by the concentration of the half-cell at the cathode.

Why were alkaline fuel cells used by NASA in the Apollo mission series?

Due to their high efficiencies and the fact that they produce pure water, which could be used by the astronauts while in space.

What is the half-equation for the oxidation reaction of glucose in a microbial fuel cell?

C6H12O6 (aq) + 6H2O (l) → 6CO2 (g) + 24H+ (aq) + 24e-

How is the current carried in a voltaic cell?

Carried by the mobile ions that diffuse through the electrolyte.

What is a battery?

One or more voltaic cells connected in series.

What happens at the point of equilibrium in a concentration cell?

The electrolyte concentrations in each half-cell are equal.

What are lead-acid batteries commonly used for?

Typically used in motor vehicles due to their ability to produce the large electric current required to 'start' an engine.

What are lithium-ion batteries used for?

Used in portable electronic devices such as smartphones and portable music players

How are the charges on the anode and cathode different when secondary cells are being recharged compared to when they are being discharged?

When secondary cells are being discharged, the anode is negative and the cathode is positive. When being recharged, the anode is positive and the cathode is negative.

What are examples of primary cells?

Zinc-carbon and most alkaline batteries (with the exception of a nickel-cadmium battery, which is an alkaline secondary cell)

What are examples of secondary cells?

Lead-acid, nickel-cadmium, and lithium-ion batteries.

What limits the maximum current produced by a battery?

Limited by the internal resistance.

What is the redox reaction that occurs in a voltaic cell composed of a copper half-cell and a zinc half-cell?

Oxidation of zinc atoms occurs at the anode, where zinc metal is oxidised to Zn2+ ions, and the reduction of copper ions at the cathode, where Cu2+ ions are reduced to copper metal.

What happens at the cathode of a DMFC?

Oxygen gas is reduced to form water.

What happens at the cathode of a microbial fuel cell?

Oxygen gas undergoes reduction to form pure water.

What occurs at the cathode of an alkaline fuel cell?

Oxygen is reduced to form hydroxide ions.

What is the oxidation equation for the discharge of a lead-acid battery?

Pb (s) + HSO4- (aq) → PbSO4 (s) + H+ (aq) + 2e-

What is the overall equation for the discharge of a lead-acid battery?

Pb (s) + PbO2 (s) + 2H2SO4 (aq) → 2PbSO4 (s) + 2H2O (l).

What is the reduction equation for the discharge of a lead-acid battery?

PbO2 (s) + 3H+ (aq) + HSO4- (aq) + 2e- → PbSO4 (s) + 2H2O (l)

What is the oxidation equation for the recharge of a lead-acid battery?

PbSO4 (s) + 2H2O (l) → PbO2 (s) + 3H+ (aq) + HSO4- (aq) + 2e-

What is the reduction equation for the recharge of a lead-acid battery?

PbSO4 (s) + H+ (aq) + 2e- → Pb (s) + HSO4- (aq)

What is the Nernst equation used for in concentration cells?

To calculate the voltage produced by a concentration cell

What does ΔH refer to in the thermodynamic efficiency equation for fuel cells?

ΔH refers to the energy released during the combustion of one mole of fuel

What is a concentration cell?

A concentration cell has the same electrodes in each half-cell but the concentration of the electrolyte in each half-cell is different.

What is a fuel cell?

A device that converts chemical energy into electrical energy and requires a constant source of fuel to produce an electric current.

What is a Proton Exchange Membrane (PEM) fuel cell?

A fuel cell that uses hydrogen gas and oxygen gas to produce an electric current with the only byproduct being pure water.

What happens at the anode of a DMFC?

A mixture of methanol and water is oxidised to form carbon dioxide, hydrogen ions and electrons.

What is the anode and cathode in a nickel-cadmium battery?

A nickel-cadmium battery has a cadmium (Cd) anode and a nickel oxide hydroxide, NiO(OH), cathode.

What is a proton exchange membrane in a fuel cell?

A semipermeable layer that allows the passage of hydrogen ions, or protons, but prevents mixing of the reactants. Certain types of fuel cells contain this membrane.

What is a voltaic cell?

A type of electrochemical cell composed of two half-cells, each containing a metal strip in a solution of its own ions, connected by an external circuit and a salt bridge.

What is a microbial fuel cell?

A type of fuel cell that converts chemical energy to electrical energy using bacteria that live inside the anode.

What is a direct methanol fuel cell (DMFC)?

A type of fuel cell that uses methanol as the fuel source.

What is a secondary cell?

A type of voltaic cell that can be recharged. The chemical reactions that occur within a secondary cell are reversible.

What is a primary cell?

A type of voltaic cell that can only be used once and cannot be recharged. It will continue to produce a current until the material at the anode is used up.

What is the electrolyte in an alkaline fuel cell?

An aqueous solution of potassium hydroxide (KOH).

What happens at the anode and cathode during the recharge of a lead-acid battery?

At the anode, PbSO4 (s) is converted to PbO2 (s) and H+ ions, and at the cathode, PbSO4 (s) and H+ ions are converted to Pb (s) and HSO4- ions.

What happens at the anode and cathode during the discharge of a lead-acid battery?

At the anode, lead is oxidised to form lead(II) sulfate, and at the cathode, lead(IV) oxide is reduced to form lead(II) sulfate. The electrons flow in the external circuit, producing an electric current.

What happens at the anode and cathode when a nickel-cadmium battery is being recharged?

At the anode: 2Ni(OH)2 (s) + 2OH− (l) → 2NiO(OH) (s) + 2H2O (l) + 2e− At the cathode: Cd(OH)2 (s) + 2e− → Cd (s) + 2OH− (aq) Overall equation: 2Ni(OH)2 (s) + Cd(OH)2 (s) → Cd (s) + 2NiO(OH) (s) + 2H2O (l)

What happens at the anode and cathode when a nickel-cadmium battery is being discharged?

At the anode: Cd (s) + 2OH− (aq) → Cd(OH)2 (s) + 2e− At the cathode: 2NiO(OH) (s) + 2H2O (l) + 2e− → 2Ni(OH)2 (s) + 2OH− (l) Overall equation: Cd (s) + 2NiO(OH) (s) + 2H2O (l) → 2Ni(OH)2 (s) + Cd(OH)2 (s)

What reactions take place during the discharge of a lithium-ion battery?

At the anode: Li (graphite) → Li+ (electrolyte) + e- At the cathode: Li+ (electrolyte) + e- + CoO2 (s) → LiCoO2 (s)

What reactions take place during the recharge of a lithium-ion battery?

At the anode: LiCoO2 (s) → Li+ (electrolyte) + e- + CoO2 (s) At the cathode: Li+ (electrolyte) + e- → Li (graphite)

How does a concentration cell produce an electric current?

By decreasing the concentration of the solution in the concentrated half-cell and increasing the concentration of the solution in the dilute half-cell.

What is the net ionic equation for a concentration cell composed of two copper half-cells with different electrolyte concentrations?

Cu(s) + Cu2+(aq) ⇌ Cu2+(aq) + Cu(s)

Which direction does the electron flow in a voltaic cell?

From the negative anode to the positive cathode

How do fuel cells differ from primary cells?

Fuel cells need a constant source of fuel to produce an electric current, while primary cells are used once and then discarded when the battery is no longer able to produce an electric current.

What happens at the anode of a PEM fuel cell?

Hydrogen gas is oxidised to form hydrogen ions and electrons.

What occurs at the anode of an alkaline fuel cell?

Hydrogen gas is oxidised to produce water and electrons.

What is the function of the polymer electrolyte in a PEM fuel cell?

It allows the passage of hydrogen ions, but prevents mixing of the reactants.

How is the voltage produced by a voltaic cell calculated?

It can be calculated using the standard electrode potentials of the half-cells, and the equation EƟcell = EƟred − EƟox

What is the one main advantage of a secondary cell?

It can be used again and again rather than being discarded after a single use.

What is the function of the salt bridge in a voltaic cell?

It completes the circuit and prevents a build-up of charge by allowing ions to migrate between the two half-cells.

How many cells are typically in a lead-acid battery?

It has six cells in series, producing a total of 12V

What is the cathode of a lithium-ion battery made of?

It is a lithium cobalt oxide complex (LiCoO2).

What is the composition of a single lead-acid cell?

It is composed of a lead anode, a lead(IV) oxide cathode, and sulfuric acid (H2SO4) electrolyte.

What is the advantage of using methanol as a fuel source in DMFCs?

It is easier to store and transport than compressed hydrogen gas used in PEM fuel cells, which have the same function as DMFCs.

What is a drawback of using hydrogen as fuel for fuel cells?

It is highly flammable and must be handled and stored correctly to prevent accidents.

What happens to a battery that is no longer able to produce an electric current?

It must be disposed of, which can cause harm to the environment.

What is the catalyst used in most fuel cells?

Platinum (Pt)

What is the main difference between primary and secondary cells?

Primary cells are non-rechargeable and must be disposed of after a single use, while secondary cells are rechargeable and can be used multiple times.

What is a disadvantage of some types of secondary cells?

Some contain toxic metals such as lead and cadmium.

What happens at the anode of a microbial fuel cell?

The bacteria breaks down organic matter to produce hydrogen ions, electrons, and carbon dioxide.

When does a concentration cell cease to produce an electric current?

The electrons flow from the dilute half-cell to the concentrated half-cell until an equilibrium is reached

What is the electromotive force (EMF) of a cell?

The energy produced per unit charge.

What is the basic method of operation for all fuel cells?

The fuel is oxidised at the anode using a catalyst, usually platinum (Pt). Electrons flow in the external circuit from the anode to the cathode, producing an electric current. At the cathode, oxygen gas is reduced, producing pure water.

What happens at the cathode of a PEM fuel cell?

The hydrogen ions combine with oxygen gas and electrons to produce pure water.

What is the advantage of a PEM fuel cell over other types of fuel cells?

The only byproduct of a PEM fuel cell is pure water, making it environmentally friendly.

What is internal resistance in a voltaic cell?

The opposition from the electrolyte to the flow of electric current.

What is the thermodynamic efficiency of a fuel cell?

The ratio of the electrical energy produced (ΔG) to the heat produced by the combustion of the fuel (ΔH).

How can the thermodynamic efficiency of a fuel cell be calculated?

Thermodynamic efficiency = ΔG / ΔH × 100 %

What happens to the lithium atoms at the anode during discharge?

They are oxidised, producing lithium ions and electrons.

What happens to the electrons in a microbial fuel cell?

They are transferred to the anode by the bacteria where they move in the external circuit to the cathode.

What is the advantage of using biodegradable substrates as fuel in microbial fuel cells?

They break down harmful organic pollutants and produce an electric current at the same time.

What are the advantages of fuel cells over internal combustion engines?

They don't produce carbon monoxide, carbon dioxide or oxides of nitrogen during their everyday operation and are more efficient than petrol and diesel engines --> allows a country to be less dependent on imported crude oil

Where do the lithium ions migrate during discharge?

They migrate in the polymer electrolyte to the cathode during discharge, undergoing reduction

What happens to the protons in a microbial fuel cell?

They pass through the proton exchange membrane (PEM) from the anode to the cathode.

What is a potential drawback of using certain types of fuel cells such as direct methanol and microbial fuel cells?

They produce carbon dioxide, which is a potent greenhouse gas.

What makes fuel cells more expensive than primary and secondary cells?

They require metal catalysts such as platinum

What type of battery were nickel-cadmium (NiCd) batteries once used extensively in?

They were once used extensively in devices such as cameras and portable power tools.

What is the equation for the net ionic reaction that occurs in a voltaic cell composed of a copper half-cell and a zinc half-cell?

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

What does ΔG refer to in the thermodynamic efficiency equation for fuel cells?

ΔG refers to the work that can be done by the cell.