Rechargeable nickel–cadmium (Ni–Cd) batteries can both deliver current to an external circuit (discharge mode) and accept current from a charging device (charge mode). For a particular Ni–Cd cell, the standard Gibbs free-energy change for the overall redox reaction is −157kJ mol−1 when the cell is discharging and +157kJ mol−1 when it is being recharged.
Answer the following questions about the thermodynamics and operation of this battery.
(a) For each mode of operation (discharge and charge), state the sign of ΔG and explain what this sign indicates about the spontaneity of the reaction occurring inside the battery.
(b) Identify whether the battery functions as a galvanic cell or an electrolytic cell in each mode. Provide reasoning that connects the spontaneity to the classification.
(c) During the charging process, energy from an external power supply is stored in the battery. Describe the forms of energy involved and the energy conversion that takes place.
(d) Explain why the same half-reactions that are spontaneous during discharge become non-spontaneous during charge, even though the chemical species and concentrations are identical at the start of each process.