Calorimetry of the Electrical Double Layer of Surface-Modified Capacitive Carbon Electrodes

Abstract

Intrusion of seawater into freshwater reserves threatens the availability of potentially potable water. To supply the world's population with sufficient drinking water, an extra desalination step is required. Capacitive Deionization (CDI) is a desalination method using an electric field applied between two electrodes. Onto these electrodes an Electrical Double Layer (EDL) is formed after adsorption of counterions. Electric measurements inform about the charging and discharging kinetics of the EDL and about the number of ions stored in the EDL. In this thesis, it is shown that more information about the EDL can be obtained by performing calorimetric experiments. The combination of the electrical and calorimetric experiments makes it possible to calculate the internal energy of the EDL. Previous calculations of the internal energy showed that the internal energy of the EDL is not zero at the open circuit potential. The proposed interpretation was that the adsorbed ions contribute an adsorption enthalpy to the internal energy of the EDL. In the work presented here, the effect of charged chemical groups on the internal energy of the EDL was investigated by chemically modifying electrodes by oxidation or, amination giving the electrodes a negative or positive surface charge. The potential of zero internal energy of the modified electrodes shifted from 200 mV vs Ag/AgCl for the oxidized electrode to 730 mV vs Ag/AgCl for the aminated electrode. Alternatively, the surface charge was altered by varying the pH of the electrolyte, which led to (de)protonation of chemical surface groups. The resulting potential at which the internal energy per unit charge was zero, shifted from 290 mV vs Ag/AgCl at pH 2 to -140 mV vs Ag/AgCl at pH 12. The latter results reveal that the more positive the chemical surface charge, the more positive the potential at which the internal energy is zero. The chemical modifications indicate that the surface charges are not the only factor that influences the internal energy of the system since the chemical modifications could have altered other chemical properties of carbon which could have influenced the internal energy of the EDL.