Low-Cost Pt-Free Counter Electrode Catalysts in Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 79

was also observed by Biallozor et al. through CV test [ 22 ]. Apart from PEDOT,
Poly (3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b] [1, 4] dioxepine) (PProDOT-Et 2 )
also can be used as CE catalyst for DSCs [ 23 ]. PProDOT-Et 2 film was directly
deposited on FTO substrate by electropolymerization with various charge capac-
ities (10, 20, 40, 80, 120, 160, 200 mC cm -2 ). When the deposited charge capacity
reached 40 mC cm -2 , the PProDOT-Et 2 film produced the largest active surface
area, contributing to the high catalytic activity similar to Pt. The DSC using this
polymer CE yielded a PCE of 5.20 %.
Wu et al. prepared polypyrrole nanoparticles (PPy) with particle size ranging
from 40 to 60 nm [ 24 ]. The DSC using PPy CE showed a PCE of 7.66 %, higher
than the photovoltaic performance of the DSC using Pt CE (6.90 %). Xia et al.
fabricated PPy CEs on FTO glass with vapor phase polymerization (VPP) and
electropolymerization (EP) techniques [ 25 ]. The PPy particle (VPP) size is
100-150 nm and the PPy (EP) has a larger particle size of 200-300 nm. The DSCs
using these PPy CEs yielded PCE of 3.4 % (VPP) and 3.2 % (EP), slightly lower
than Pt-DSC (4.4 %). In addition, PPy sphere with a uniform size of 85 nm was
used as CE catalyst for DSCs and decent PCE was obtained [ 26 ]. Zhao et al.
prepared transparent polyaniline (PANI) CE on FTO glass for DSCs CE [ 27 ]. At
front illumination, the DSC showed a PCE of 6.54 %; at rear illumination, a PCE
of 4.26 % was achieved. Moreover, Wu et al. fabricated mesoporous PANI
thorough an oxidative polymerization of aniline monomer as CE for DSCs and a
PCE of 7.15 % was obtained [ 28 ].
Organic polymer CEs own the advantages of transparency, high catalytic
activity, easy availability, and low cost. However, the stability (chemical stability,
thermal stability, and photo stability) may be an adverse property for practical
application. Reports on stability tests for polymer CEs are still rare.
3 Transition Metal Compounds
Early transition metal carbides, nitrides, oxides, sulfides, and phosphides exhibit
Pt-like catalytic behavior. These materials have been used in the fields of CO 2
methanation, ammonia synthesis, dehydrogenation, hydrogenation, methanol oxida-
tion, photocatalysts, gas sensors, electrochromic devices, field emitters, among others,
as a replacement for the noble metal. Very recently, these compounds have been
proposed to use as CE catalysts in DSCs. Next, we will give a detailed introduction.
3.1 Carbides
Ma group introduced tungsten carbide (WC) and molybdenum carbide (Mo 2 C)
into DSCs as CE catalysts. The DSCs using WC and Mo 2 C as CEs gave PCE of
5.35 % (WC) and 5.70 % (Mo 2 C), lower than the PCE of the Pt-DSC (7.89 %).