Problem 42 - Chemistry The Science in Context

Question

Fuel Cells The electrolyte in an electricity-generating device called a fuel cell consists of a mixture of \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\) heated to \(650^{\circ} \mathrm{C} .\) At this temperature the ionic solids melt. Explain how this mixture of molten carbonates can conduct electricity.

Step-by-Step Solution

Answer

Answer: The mixture of molten carbonates (Li2CO3 and K2CO3) conducts electricity in a fuel cell operating at 650°C due to the mobility of ions in their molten state. At this temperature, both ionic compounds melt, allowing their positive (Li+ and K+) and negative (CO3^2-) ions to move freely. In the presence of an electric field, the positive ions migrate towards the negative electrode (cathode), while the negative ions migrate towards the positive electrode (anode). The flow of ions constitutes an electric current, allowing the mixture to conduct electricity in the fuel cell.

Step 1: Understanding the Components of the Mixture

The electrolyte consists of a mixture of lithium carbonate (Li2CO3) and potassium carbonate (K2CO3). Both of these compounds are ionic in nature, which means they consist of positive (cations) and negative (anions) ions that are attracted to each other.

Step 2: Melting of Ionic Compounds

When the temperature is increased to 650°C, the ionic solids Li2CO3 and K2CO3 melt. In their molten state, the ions in these ionic compounds are free to move around. This mobility of ions is essential for the conduction of electricity.

Step 3: Conduction of Electricity in Molten State

In the molten state, the positive ions (Li+ and K+) and the negative ions (CO3^2-) are free to move. In the presence of an electric field, the positive ions migrate towards the negative electrode (cathode), and the negative ions migrate towards the positive electrode (anode). This flow of ions constitutes an electric current. Since the mixture of molten Li2CO3 and K2CO3 allows the flow of ions, it can conduct electricity in the fuel cell.

Key Concepts

Molten Carbonate Electrolyte

Fuel cells contain an electrolyte, which is crucial for conducting electricity. A common type is the molten carbonate electrolyte, a mix of lithium carbonate (\( \mathrm{Li}_2\mathrm{CO}_3 \)) and potassium carbonate (\( \mathrm{K}_2\mathrm{CO}_3 \)). At high temperatures, such as 650°C, these ionic compounds melt and become liquid.

• The melting process breaks the rigid crystal structure of the ionic solids.
• The ions become free to move in the liquid state, unlike in a solid.

The flow of these ions between electrodes is what makes the electrolyte conductive, allowing the fuel cell to generate electricity. Molten carbonate fuel cells are particularly efficient in converting energy due to this mechanism.

Ionic Conductivity

Ionic conductivity refers to the ability of an ionic compound to conduct electricity. This is especially prominent in molten or aqueous states.
In an ionic compound, electricity is conducted when ions can move freely. There are several factors that contribute to ionic conductivity:

• Temperature: Higher temperatures can increase ionic mobility by providing energy to overcome electrostatic attractions.
• State of the Compound: Conductivity significantly improves when the ionic compound is in a molten or liquid state.
• Concentration of Ions: More ions available generally mean better conductivity.

In a molten carbonate electrolyte, the high temperature ensures the ions are mobile enough to carry electric current effectively, providing a steady flow between the electrodes in a fuel cell.

Electricity Conduction in Ionic Compounds

The conduction of electricity in ionic compounds relies on the movement of charged ions. Unlike metals, where electron flow facilitates conduction, ionic compounds conduct electricity through the movement of ions.
In the case of a fuel cell using molten carbonates:

• Positive Ions (Cations): Lithium ions (\( \mathrm{Li}^+ \)) and Potassium ions (\( \mathrm{K}^+ \)) move toward the cathode.
• Negative Ions (Anions): Carbonate ions (\( \mathrm{CO}_3^{2-} \)) move toward the anode.

When an electric field is applied, these oppositely charged ions migrate in opposite directions, which constitutes an ionic current. This movement is what makes molten ionic compounds excellent conductors of electricity. It is the balance of these ion flows that ensures efficient electricity generation in fuel cells.