Development of Manganese Dioxide/Activated Carbon Composite Electrodes for Enhancing Capacitive Deionization Performance

With climate change and growth of the population, water scarcity is being considered as a serious problem. In response, water desalination technologies can resolve this promising issue Capacitive deionization (CDI) is a promising water purification technology. Currently, most of CDI devices rely on the utilization of porous carbon electrodes with high specific surface area to electrostatically adsorb ions. In this case, the ions are stored at the electrode/electrolyte interface by electrical double layer (EDL) formation. However, the porous carbon electrodes based on EDL capacitance may suffer from the limitation of electrosorption capacity on targeted ions. Manganese dioxide (MnO2) is often preferred as an active material in supercapacitors for its low cost, non-toxic, and high theoretical capacitance. Most recently, significant efforts have been made on the development of MnO2-porous carbon composite as the electrodes of supercapacitors. Due to the similar working principle of CDI and supercapacitors, the fast and reversible redox-active of MnO2 can ensure the high capacitive behavior to intercalate the specific cations, as well as making the best use of the large pseudocapacitive charge to pursue higher adsorption capacity. Our main object is to develop the MnO2/activated carbon (AC) composite electrodes for enhancing the salt adsorption capacity of CDI cells through the redox-mediated active material of MnO2. Our study uses the anodic electrodeposition method to tune the thickness of MnO2, making the full utilization of MnO2 for the mix-faradaic reactions and retaining the sufficient conductivity and porosity of AC in CDI. The resultant MnO2/AC composite electrodes are characterized by surface morphology, electrochemical properties, and CDI performance. Material characterization (i.e., Scanning electron microscope, X-ray photonelectron spectrosmeter, and X-ray powder diffraction) confirms the presence of MnO2 coated on the AC surface. Cyclic voltammetry, galvanostatic charge/discharge curve, and electrochemical impedance spectra measurements are conducted to determine the specific capacitance and electrical conductivity of MnO2-AC electrode. A pair of batch-mode CDI cells is further performed at an applied voltage of 1.0 V for desalting 0.01M NaCl solution for capacitive desalination. The MnO2/AC is used as a cathode for the EDL formation and Na-intercalation of MnO2. According to the results, the salt adsorption capacity of MnO2/AC electrode is determined to be 9.3 mg g-1, which is about 1.6-fold higher than the AC electrode (5.7 mg g-1). Consequently, by taking the advantage of combination with fast faradaic reactions and double-layer charging, modification of porous AC materials with MnO2 is an emerging approach to achieve high CDI performance.