Enthelpy of Reaction and Efficiency of Galvanic Cells

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In electrochemical energy devices like batteries and fuel cells, efficiency is defined as the ratio of Gibbs energy change to enthalpy change. The Gibbs energy change corresponds to the electrical work done by the cell, while the enthalpy change is used as the energy input. The discussion raises questions about why enthalpy, which includes heat exchanged with the surroundings, is considered the chemical energy input when electrical work is also involved. The relationship between Gibbs energy and enthalpy is clarified by the equation ΔrH⦵=ΔrG⦵+TΔrS⦵, indicating that enthalpy accounts for both work and heat. Understanding the physical interpretation of enthalpy change in this context remains complex, particularly due to the presence of electrical work in electrochemical cells.
Dario56
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Why is Enthalpy of Reaction Used as Energy Input in Galvanic Cells When Defining its Efficiency?
For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as:

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.
 
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Dario56 said:
Summary:: Why is Enthalpy of Reaction Used as Energy Input in Galvanic Cells When Defining its Efficiency?

For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as:

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.
Can you please provide a reference for that efficiency equation?
 
Chestermiller said:
Can you please provide a reference for that efficiency equation?
This equation isn't correct, my mistake. It should be ratio of Gibbs energy change of system divided by enthalpy change of system for reversible cell. There should be no standard reaction Gibbs energy in equation.
Chestermiller said:
Can you please provide a reference for that efficiency equation?
Basically, I am not sure why is enthalpy change of system taken as energy input since I don't know what is physical interpretation of enthalpy change in electrochemical cell. When there is no non-PV work, it is equal to heat exchanged with surroundings at constant p and T. But, in electrochemicall cells we have electrical work, so interpretation of enthalpy change of system isn't as straightforward, so I am not sure why is it taken as energy input?
 
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