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This relative lower performance stems from the large size of the two carbides Mo
2
C,
300 nm; WC, 190 nm [
29
]. Then they synthesized nano-scaled WC and W
2
C using
metal-urea route. The DSCs using the nanoscaled carbides as CEs achieve high PCE
of 6.68 % (W
2
C) and 6.23 % (WC). Compared with the large WC particle, the nano-
scaled WC showed improved catalytic activity [
30
]. Lee et al. prepared polymer-
derived WC (WC-PD) and microwave-assisted WC (WC-MW), which have been
introduced in DSCs as CE catalysts. The DSCs yielded PCE of 6.61 % (WC-PD) and
7.01 % (WC-MW), lower than that of the DSC using Pt CE (8.23 %) [
31
]. More-
over, Ma group did a systematic research on other transition metal carbides, such as
TiC, VC, ZrC, NbC, Cr
3
C
2,
and so on [
32
,
33
]. All of the carbides give high catalytic
activity except ZrC and NbC.
3.2 Nitrides
Gao et al. prepared TiN nanotubes used as CE catalyst in DSC which produced a
PCE of 7.73 % [
34
]. Similar to TiN, MoN, WN, and Fe
2
N were synthesized by
nitridation of the oxide (MoO
2
,WO
3
,Fe
2
O
3
) precursors in ammonia atmosphere,
after which the nitrides were used as CE catalysts into DSCs [
35
]. The DSCs gave
PCE values of 5.57 % (MoN), 3.67 % (WN), 2.65 % (Fe
2
N), and 6.56 % (Pt).
Meanwhile, Ma et al. prepared Mo
2
N and W
2
N films on flexible Ti sheet as CEs
for DSCs which produced PCE of 6.38 % (Mo
2
N) and 5.81 % (W
2
N). In addition,
they also introduced ZrN, VN, NbN, CrN into DSCs, and decent results were
obtained [
32
]. Gao et al. used surface-nitrided Ni foil as the CE in DSCs, resulting
in a PCE of 5.68 %, much lower than that of the Pt-DSC (8.41 %) [
36
], while NiN
with a mesoporous structure showed high catalytic activity and the DSC yielded a
PCE of 8.31 %.
3.3 Oxides
Ma et al. observed that WO
2
and WO
3
can be used as a catalyst for the reduction of
I
3
-
to I
-
[
37
]. The DSC based on WO
3
CE showed a PCE of 4.67 %. WO
2
nanorod showed excellent catalytic activity and the DSC gave a high PCE of
7.25 %, close to that of the Pt-DSC (7.57 %). Recently, they synthesized H-Nb
2
O
5
(hexagonal), O-Nb
2
O
5
(orthorhombic), M-Nb
2
O
5
(monoclinic), and T-NbO
2
(tetragonal) and then used the four niobium oxides as CE catalysts in DSCs [
38
].
The DSCs showed PCE values of 5.68 % (H-Nb
2
O
5
), 4.55 % (O-Nb
2
O
5
), 5.82 %
(M-Nb
2
O
5
), and 7.88 % (T-NbO
2
). Obviously, NbO
2
performs best among the
four niobium oxides and the crystal forms significantly affecting the catalytic
activity. Xia et al. used V
2
O
5
as CE in solid DSCs. The PCE of the DSC using a
10 nm thick V
2
O
5
CE reached 2.0 % [
39
].
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