Determination of the entropy change of the electrode reaction by an ac electrochemical–thermal method

Abstract

Abstract An ac electrochemical thermal method with a polyvinylidene difluoride (PVDF) pyroelectric film sensor and the lock-in amplifier technique was developed for determining the entropy change Δ S of an electrode reaction. It was found that the Peltier heat could be determined separately from the polarization heat and Joule heat by use of a lock-in amplifier with the reference frequency the same as the frequency of the ac current. For two reversible electrode reactions the Δ S value for each electrode could be calculated by measuring the amplitudes of the ac electrochemical thermal signal V 0 , ac current I 0 , the phase shift between them and concerning the total entropy change of the whole cell reaction. The Fe(CN) 6 4− /Fe(CN) 6 3− electrode and its Δ S value could be used as a calibration rule to measure the Δ S for other single electrode reactions. The Δ S values for the reduction reactions in the systems of Fe 2+ /Fe 3+ , Cu/Cu 2+ , Br − /Br 2 and H 2 /H + were determined. The reaction mechanisms for Cu/Cu 2+ and Br − /Br 2 electrodes were discussed from the view of entropy changes. Comparing with conventional and absolute ionic entropy systems, it was found that the conventional system is more convenient for practical use.