Full Papers
ꢁ
ence of appropriate catalysts anytime and anywhere, as neces-
sary.
The amounts of generated Ce4+ and IO4 increased steadily,
and relatively high faraday efficiency was observed (Ce4+
:
For the case in which 1m HClO4 aqueous solution was used
as the analyte, the photocurrent properties were lower than
those using 1m H2SO4 aqueous solution, and no oxidation
product other than O2 was observed, as shown in a previous
report.[5a] Eventually, O2 production was observed with a high
faraday efficiency [h(O2)] of 95% (Figure S7).
ꢀ40–50%, IO4ꢁ: ꢀ50%; Figure 7) even under a prolonged
2ꢁ
photoelectrochemical reaction, as was the case for S2O8 pro-
duction. A slight amount of H2O2 (<3%) was also generated in
the oxidation of IO3ꢁ to IO4ꢁ, and O2 was observed as an oxida-
tion product other than Ce4+, IO4ꢁ, and H2O2. Notably, Ce4+
ꢁ
and IO4 were produced and accumulated at a considerably
The production and accumulation of Ce4+ and IO4ꢁ, which
are value-added oxidation products other than S2O82ꢁ, were
also challenged, because they are also produced by an oxida-
tion reaction with a standard electrode potential that is posi-
tively higher than that required for O2 production, as shown in
Equations (5) and (6).
low voltage (0.4–1.2 V) by using the WO3 photoelectrode pos-
sessing excellent oxidizing capability. These oxidation products,
in addition to S2O82ꢁ, are important for diverse applications, in-
cluding pollutant treatment and organic transformations by
utilizing high oxidation power. To the best of our knowledge,
there are no reports of these products by using a photoelec-
trode system.
Ce3þ ! Ce4þ þ eꢁ ðCe4þ=Ce3þ ¼ þ1:70 V vs: RHEÞ
IO3ꢁ þ H2O ! IO4ꢁ þ 2 Hþ þ 2 eꢁ
ð5Þ
ð6Þ
To achieve efficient accumulation of H2 and high-value-
added oxidation products without external bias, a tandem
photoelectrode system comprising a WO3 photoelectrode and
a DSSC with FT89 dye was designed and investigated for the
ꢁ
ðIO4ꢁ=IO3 ¼ þ1:65 V vs: RHEÞ
2ꢁ
production of S2O8 in 1.0m H2SO4 aqueous solution. Given
ꢁ
The time course of Ce4+ and IO4 generation upon irradia-
tion of simulated solar light from the back side of the WO3
photoelectrode is shown in Figure 7A,B; a steady photocur-
rent (Ce4+: 0.2 mA, IO4ꢁ: 1.0 mA) was maintained below 1.2 V.
that the DSSC in this tandem system was set to act with light
through the WO3 photoelectrode, the photocurrent properties
of the system with the WO3 photoelectrode were decreased
relative to those without the photoelectrode, as shown in Fig-
ure S8. However, excellent photocurrent properties were also
observed in the DSSC of the tandem system with the WO3
photoelectrode, because the FT89 dye absorbs light over
a wide spectrum up to the near-IR wavelength region.[4c] In
fact, a wide incident photon-to-current efficiency (IPCE) spec-
trum with high efficiency was observed in the region of l=
500 to 900 nm, in which WO3 does not absorb (Figure 8); this
suggests the balance of light absorption between the DSSC
and the WO3 photoelectrode is an important factor that affects
the performance of this tandem system.
Figure 9 shows the photocurrent properties of the tandem
system comprising the DSSC and the WO3 photoelectrode in
1.0m H2SO4 aqueous solution. The intersection of the I–V
curves of the WO3 photoelectrode and the DSSC represents
the photocurrent properties of this tandem photoelectrode
Figure 7. A) Time course of Ce4+ and h(Ce4+) in 1.0m Ce(ClO4)3 dissolved in
1.0m HClO4 aqueous solution as anode and 1.0m HClO4 aqueous solution as
cathode, and B) time course of IO4ꢁ and h(IO4ꢁ) in 0.2m NaIO3 aqueous solu-
tion under simulated solar light irradiation from the back side (Ce4+: 0.2 mA,
IO4ꢁ: 1.0 mA in the range of 0.4 to 1.2 V).
2ꢁ
system. Further, the solar-to-H2 and S2O8 conversion efficien-
cy (STHS O 2ꢁ) can be calculated from Equation (9).
2
8
1.0m Ce(ClO4)3 in 1.0m HClO4 aqueous solution was used on
the anode side and a 1.0m HClO4 aqueous solution was used
on the cathode side in the photoelectrochemical production of
Jopt ꢃ 2:12 ꢃ hðS2O82ꢁÞ
ð9Þ
STHS O 2ꢁ ¼ ½
ꢂ ꢃ 100
2
8
Int
Ce4+, and a 0.2m NaIO3 aqueous solution was used as a bipolar
ꢁ
analyte for IO4 production. In addition, the values of h(Ce4+
)
This equation indicates that the conversion efficiency is
a function of the photocurrent density measured by using this
and h(IO4ꢁ) were calculated by using Equations (7) and (8),
which take the reactions depicted in Equations (5) and (6) into
consideration.
2ꢁ
tandem system (Jopt), the faraday efficiency of S2O8 produc-
tion [h(S2O82ꢁ)], and the intensity of irradiated simulated solar
light (Int: 100 mWcmꢁ2). The photocurrent performance of
0.90 mAcmꢁ2 at 0.72 V for this tandem system was measured
by using an ammeter/voltmeter. These values agree well with
the intersection values in the I–V graph of Figure 9. The
STHS O 2ꢁ of this system was estimated to be 1.9%, as the
hðCe4þÞ¼ ½amount of generated Ce4þꢂ
ð7Þ
ꢃ100=½amount of generated electronsꢂ
2
8
h(S2O82ꢁ) in 1.0m H2SO4 aqueous solution was approximately
100% (Figure 5). H2 was also produced at the Pt counter elec-
trode. However, for this tandem system, an intersection was
hðIO4ꢁÞ¼ ½amount of generated IO4ꢁꢂ
ð8Þ
ꢃ100=½amount of generated electrons=2ꢂ
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