Speaker: Joren Vos MSc
Title: Calorimetry of Capacitive Porous Carbon Electrodes in Aqueous Media
Abstract: One application of microporous carbon electrodes is in the capacitive deionization of salty water, to generate drinking water. Typically, the opposite carbon walls of a micropore are merely 1 nm apart, and they are covered by a dielectric layer of adsorbed water molecules whose capacitance dominates the capacitance of the electrode. We realized an electrochemical setup, based on a three-electrode setup, consisting of a working electrode, counter electrode and reference electrode. Said setup allows us, via thermodynamic experiments, to characterize the average electric potential experienced by ions inside such pores. From the electrical work and the heat production associated with charging up the electrode, we calculate the change in internal energy of the electrode, and after subtraction of the change in energy stored in the dielectric water layer, what remains is the average potential energy of the ions in the pores. Experiments using different salts indicate that the electric potential of large or strongly hydrated ions (lithium, calcium, magnesium, lanthanum) is relatively insensitive to changes in applied potential, probably because these ions cannot enter deeply into the dielectric adsorbed water layer, for steric reasons. In contrast, smaller or weakly hydrated ions (sodium, potassium, rubidium, cesium, chloride) are more sensitive to the applied potential. In other words, our measurements offer new experimental insight into the “electric double layer” inside electrode micropores. We also show a method to determine the time-dependent heat production via gradual polarization. By lowering the charging speed, we eliminate the irreversible heat, leaving only the reversible heat and showing that this reversible heat is the same as found by our earlier far out-of-equilibrium measurements.
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