Low-Cost Pt-Free Counter Electrode Catalysts in Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 77

(0.62 %) CEs. The highest catalytic activity of the SNTs for regeneration of the
I 3 - /I - redox couple was caused by the one-dimensional nano-feature which pro-
vides better electron transport. Moreover, the impurities (iron, amorphous carbon)
in SNTs can influence the catalytic performance dramatically. For example, the
DSC using purified SNTs CE yielded a PCE of 1.46 %, while the DSC using raw
SNTs CE gave a poor PCE of 0.57 %. This difference caused by catalyst poisoning,
because the catalytic sites was occupied by impurities. The purification process in
turn introduced many oxygen-function groups that formed new catalytic sites.
Graphene is a single layer of two-dimensional graphite with advantages of high
conductivity, transparency, hardness, and corrosion resistance, and it has become a
hot research topic in various fields. Grätzel et al. used graphene to fabricate optically
transparent CE for DSCs [ 13 ]. It was observed that graphene CE was more suitable
for ionic liquid solvent than the traditional organic solvent. The R ct value of the ionic
solvent is much smaller than the traditional solvent, which indicates that the
mechanism for regeneration of I 3 - /I - in the graphene surface is determined by
solution events rather than viscosity. The catalytic activity of graphene is propor-
tional to the content of active sites (edge defects and oxide groups). Finally, they
suggested that graphene might be a promising substitute for Pt and also for the
expensive FTO conductive layer. Aksay et al. found the catalytic activity was
correlated with the concentration of the oxide group and the C/O ratio strongly
influenced the catalytic activity [ 14 ]. When the C/O ratio was up to 13, graphene CE
showed the highest catalytic activity, and the DSC gave a PCE of 5.0 %, close to that
of the DSC using Pt CE (5.5 %). Jeon et al. synthesized graphene by reducing
graphite oxide. It was found that the catalytic activity increased as the number of
oxygen functional groups decreased [ 15 ]. Combining with previous results [ 14 ], we
thought that there existed an optimum number of oxygen functional groups for
graphene to achieve optimal catalytic activity. The PCE values for the DSCs in the
above mentioned works were all less than 6 %, leaving much room for improvement
by modifying the concentration of the oxygen functional groups or lattice defect.
Nevertheless, to fully evaluate these statements requires more research.
Ma group compared the properties of nine kinds of carbon materials containing
Ca, Cb, conductive carbon (Cc), carbon dye (Cd), Cf, CNTs, Ordered mesoporous
carbon (Com), discard toner (Cp), and C 60 at the same conditions [ 16 ]. Com is made
of rows of well-ordered carbon walls with a width of approximate 10 nm. This
carbon wall configuration forms many channels in the CE body which can increase
the contact area of the electrolyte and the CE surface and this configuration can
promote electrolyte diffusion. The DSC using Com CE yielded a high PCE of 7.5 %,
the same as the DSC using Pt CE. The traditional carbon materials (Ca, Cb, Cc,
CNTs, and Cf) showed decent catalytic activity and the DSCs yielded PCE values
from 6.3 % to 7.0 %. Cd showed catalytic activity as high as Com, and the DSC
gave a PCE of 7.5 %. Even the DSC using Cp CE showed a decent PCE of 4.3 %.
When impure C 60 CE was used in DSC, a low PCE of 2.8 % was achieved. As
predicted, C 60 should perform as effectively as the other carbon materials due to its
superior electrical conductivity, stability, and other attributes. The low activity may
be caused by impurities, which may result in catalyst poisoning. They indicated that