Measuring the ion-specific heat of electrical double layer formation in porous carbon
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The reversible heat of electrical double layer (EDL) formation in porous carbon has been investigated several times since 2006, typically by measuring the combined heat production of the anode and the cathode in an electrochemical cell. In this work, the heat of EDL formation is instead determined at one electrode. Our aim is to use this approach to determine the EDL formation heat for one type of counterion at a time. To measure the EDL formation heat at one electrode, two calorimetric setups are built, both of which contain a three-electrode cell in which only the heat generated at the working electrode is measured. The first setup uses thermometers to conduct a temperature difference measurement, while the second measures the heat directly using a heat flux sensor. The reversible heat, which is the heat of EDL formation, can be separated from the irreversible heat by comparing the total measured heat during charging and discharging. Results obtained with the temperature difference measurement setup show that the heat of EDL formation is higher with chloride or fluoride counterions than with sodium counterions. This is ascribed to different ionic radii giving rise to different entropic changes during EDL charging. A simple, exclusively entropic model is developed based on this concept. It is capable of reproducing the experimental results with reasonable ionic radii and micropore volume as input. Accounting for the ionic radii appears to be necessary to explain the difference in EDL formation heat between sodium counterions and chloride or fluoride counterions.