Stability and electrochemical regeneration of activated carbon electrodes for capacitive deionization

Abstract

The stability and electrochemical regeneration of activated carbon electrodes used for Capacitive Deionization have been investigated. To assess the electrode state, electrochemical properties such as capacitance and potential at the point of zero charge (EPZC) were monitored and complemented by elemental composition, wettability and bulk porosity analysis. Long-term cycling was performed with electrolytes containing dissolved organic matter (DOM) and iron (II) under anaerobic and aerobic conditions. It was found that DOM, at concentrations of up to 40 mg L-1, had a marginal impact on capacitance loss and relocation of the EPZC. On the other hand, the scaling nature of iron was apparent from early cycling stages, in waters containing as little as 0.2 mg Fe2+ L-1. In all cases performed, elemental composition analysis demonstrated that incorporation of oxygen to the surface occurred predominantly during the first 15 to 20 cycles. Specifically, the oxygen content increased by a factor of four. Over this time frame, the contact angle decreased from approx. 130˚ to 34˚, on average; which indicated an increase in electrode wettability. In addition, cycling experiments with simple electrolytes revealed improved electrode stability as cycling time increased and as pH decreased. Degradation tests conducted using a two-electrode cell demonstrated the effect of reaction coupling between the processes of carbon corrosion and oxygen reduction. As a result, electrode decay occurred at an accelerated rate. Electrochemical regeneration was proven successful at recovering the electrode capacitance but not at regressing the EPZC. Their corresponding response surfaces were mapped through a 32-factorial design of experiments and revealed a significant potential dependence. Further tests revealed a point of maximum recovery near a potential...