Experimentally Investigating the Thermodynamic Adsorption Limits of Carbon-Based Electrodes for Capacitive Deionization (CDI)

Capacitive Deionization (CDI) is an electrochemical water purification process through which an electric potential is applied across carbon-based electrodes. The goal for this study is to improve the absorption limits for electrodes and predict maximum performance relative to thermodynamic limits. By calculating the enthalpy and entropy we can understand the energy levels, system capacity, and nature of the adsorption process (physisorption vs. chemisorption). Much of this work can be done computationally but further experimental verification is needed to validate the modeling. Each electrode was tested to obtain the removal of salt at variable concentrations and temperatures using a thermal flow cell, capable of heating to maximum tested temperature of 50°C. The experiments pushed saline solution through the 10x10 cm thermal cell at 5mL/min, where it would be charged and discharged to 1.2V and –1.2V respectively. The results confirmed previous trends where higher concentrations would increase capability for salt removal, but only up to a certain maximum value. Upon testing a range of temperatures and concentrations, enthalpy and entropy adsorption characteristics for the adsorption can be predicted, generally suggesting physisorption (low enthalpy) for low salt removal. Moving forward, the goal is to maintain the experiments and continue with more concentrations of saline solutions and alternative voltage values.