Importance of Protons and Specifically Adsorbing Ions on Changing Capacitance, Space Charge Potential Inside the Solid, and the Interfacial Potential at the TiO2 Aqueous Solution Interface

This paper reports on the effect of protons and specifically adsorbing ions on the capacitance in aqueous systems employing TiO2 porous thin-film electrodes. The influence of surface potential on capacitance was studied through zeta potential (ζ) measurements, open circuit potential (OCP) studies and charge-discharge testing. Both ζ and OCP varies as a function of pH and phosphate adsorption. The slopes of the OCP versus pH curve were close to but lower than the Nernstian value as often reported in the literature. The shift of OCP-OCPpztc (OCP evaluated at the Point of Zero Total Charge) with phosphate adsorption implies that the space charge (charge developed inside the solid oxide material itself) potential, varies with phosphate concentration. We show that the electrochemical capacitance increases with the deposition of TiO2 thin-films on the platinized Ti electrode that directly relates to its ζ with the lowest values in capacitance occurring near the isoelectric pH (pH (I) or pHiep), in the absence of phosphate. We also demonstrate that capacitance can be improved with phosphate adsorption over the pH range of 5 to 9 for aqueous based systems. As expected, capacitance depends on OCP, the voltage range referenced to the OCP, and the rates of adsorbing and desorbing ions. In this context, it would appear that it is easier to electro-adsorb than desorb protons in these porous oxide thin-film electrodes. Capacitance extracted from cyclic voltammetry (CV) conducted over a large voltage range from −0.5 to +0.5 V resulted in a high pseudocapacitance. However, when cycled continuously (5-25 scans) pseudocapacitance decreased from 110 to 65F/g but electrical double layer (EDL) capacitance (OCP to OCP + 0.1 V) increased in the process, depending upon pHiep, between 100-150%. Lastly, the highest value of capacitance (encompassing both Pseudo and EDL capacitance) was 155F/g and obtained over a voltage range of 1 V at a scan rate of 5 mV/s for a pH of 6.