Page 156 - Multifunctional Photocatalytic Materials for Energy

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0.16% to 2.2% using 30% graphene material, which may be attributed to either the
improved electrolyte conductivity or the electrolyte diffusion length. Thus it is clear
that graphene material containing electrolytes were limited by the catalytic activity
of graphene for the reduction of triiodide, which resulted in a short-circuit current in
the cell.

7.4.5 Graphene as counter electrode in DSSC
Generally, a thin layer of platinum metal deposited on a transparent conductive oxide
substrate can be used as a counter electrode, and its function is to reduce the oxidized
redox couple in the electrolyte. In DSSC applications, considerable efforts have fo-
cused on optimizing photoanode materials, dyes, and electrolytes. Platinum deposited
on counter electrodes have been widely investigated because of their facile fabrication
and high activity, mainly for the iodide/triiodide mediators. The high cost of plati-
num metal and high electro-catalytic activity toward the iodide/triiodide redox couple
causes degradation over time and strongly reduces the efficiency of the cell. Therefore
it is essential to develop Pt-free counter electrodes, and an extensive investigation is
being conducted to find other counter electrode materials, such as carbon-based mate-
rials, cobalt sulfide, and so on.
In this regard, graphene-based materials have triggered huge interest among re-
searchers because of their excellent transparency, high conductivity, and high surface
area. To date, several graphene-based materials have been applied as counter elec-
trodes in DSSC applications in order to develop Pt-free electrodes.
Zhang et al. reported the use of a graphene nanosheet as counter electrodes in
DSSCs and achieved solar conversion efficiencies between 0.71% and 2.94%, de-
pending on the calcination temperature (from 350°C to 450°C) [124]. In another
study, functional graphene nanosheets used as counter electrodes delivered a high
efficiency of 5%, which is just 10% lower than Pt-based electrodes. In addition,
functionalized graphene sheet-based inks cast on a nonconductive plastic substrate
were used as a counter electrode and delivered comparable efficiency. In addition,
the approach to combine graphene with different conducting polymers, including
poly (3,4-Ethylenedioxythiophene) (PEDOT), has attracted considerable attention in
terms of designing novel carbonaceous electrocatalysts [125]. In particular, graphene-
PEDOT has been a successful combination; however, the capacity of the combined
material to substitute Pt, the easiness of the deposition procedure, the value of the
charge transfer resistance, the stability, and the transparency of the obtained electro-
catalyst film may vary according to the graphene synthesis and deposition method.
In this regard, electrochemically exfoliated graphene was combined with PEDOT to
obtain a successful counter electrode as an efficient Pt substitution [126]. The electro-
chemically prepared HRG displayed high-quality and fewer defects and was success-
fully transferred on FTO electrodes to produce thin transparent films. The films were
combined with PEDOT and were used as counter electrodes in DSSCs. The synergy
between the two materials was remarkable, and the obtained hybrid electrocatalyst
outperformed standard Pt nanoparticle electrocatalysts. Also, HRG/macrocyclic iron
(Fe) complex hybrid materials have been used in the production of counter electrodes