Carbon material CEs

DSC is well known as potentially low-cost photovoltaic devices (Gratzel, 2004); from this perspective, the application of low-cost materials should be important. Low-cost carbon is the second most widely studied material for CEs after metal materials. Carbonaceous materials feature good catalytic properties, electronic conductivity, corrosion resistance towards iodine, high reactivity and abundance (Wroblowa & Saunders, 1973). Since the fact that Kay and Gratzel found good electrocatalytic activity of graphite/carbon black mixture in 1996 (Kay & Gratzel, 1996), various kinds of carbon are studied, such as hard carbon spherules (Huang et al., 2007), activated carbon (Imoto et al., 2003), mesoporous carbon (Wang et al., 2009a), nanocarbon (Ramasamy et al., 2007), single-walled carbon nanotubes (Suzuki et al., 2003), multiwalled carbon nanotubes (MWNTs) (Seo et al., 2010), carbon fiber (Joshi et al., 2010) and graphene nanoplates (Kavan et al., 2011). Nevertheless carbonaceous electrodes which would be superior to Pt were reported only rarely for certain kinds of activated carbon (Imoto et al., 2003). This result is mainly attributed to the poor catalytic activity for I3"/I" redox reaction. In addressing this issue, several optimizing methods have been developed, such as increasing the surface area, functionalizing the carbon materials to get more active sites for I3-/I – redox reaction.

To achieve a comparable activity to platinum, carbon-based CEs must have sufficiently high surface area. Although carbonaceous electrodes have poor catalytic activity for I3-/I – redox reaction, its exceptional surface area and conductivity, such as mesoporous carbon and graphene, have been shown to be quite effective and in some cases even exceeded the performance of platinum. Imoto et al (Imoto et al., 2003) compared several types of activated carbon with different surface areas ranging from 1000 m2 g-1 to 2000 m2 g-1 as the CE catalyst, assessing in addition the activity of several different types of activated carbon, glassy carbon, and graphite. The surface area of the glassy carbon and graphite was three orders of magnitude lower than those of the activated carbon catalysts. In the preparation of the latter electrodes, a certain amount of carbon black was included. In their results, the electrodes consisting of the lower sheet resistance materials, graphite and glassy carbon, gave lower Jsc values and fill factors, indicating the importance of the roughness of the carbon materials in achieving a better performance. They demonstrated an improvement in the Jsc and FF with increasing thickness (>30 pm) of the carbon material. Robert et al (Sayer et al., 2010) used the dense, vertical, undoped MWCNT arrays grown directly on the electrode substrate as CEs and got a greater short-circuit current density and higher efficiency than DSCs with Pt CE. The improved performance is attributed to increased surface area at the electrolyte/counter electrode interface that provides more pathways for charge transport. Prakash et al (Joshi et al., 2010) investigated the electrospun carbon nanofibers as CEs. The results of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements indicated that the carbon nanofiber based CEs exhibited low charge-transfer resistance, large capacitance, and fast reaction rates for triiodide reduction. Joseph et al (Roy-Mayhew et al., 2010) found that functionalized grapheme sheets with oxygen-containing sites perform comparably to platinum. Using cyclic voltammetry, they demonstrated that tuning the grapheme sheets by increasing the amount of oxygen-containing functional groups can improve its apparent catalytic activity.