Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting

Article abstract of DOI:10.1021/jacs.5b04856

Light driven water splitting was achieved by a tandem dye-sensitized photoelectrochemical cell with two photoactive electrodes. The photoanode is constituted by an organic dye L0 as photosensitizer and a molecular complex Ru1 as water oxidation catalyst o

Full text of DOI:10.1021/jacs.5b04856

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Article
An Organic dye-sensitized tandem
photoelectrochemical cell for light driven water splitting
Fusheng Li, Ke Fan, Bo Xu, Erik Gabrielsson, Quentin Daniel, Lin Li, and Licheng Sun
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b04856 • Publication Date (Web): 01 Jul 2015
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An organic dye-sensitized tandem photoelectrochemical cell
for light driven total water splitting
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Fusheng Li,[a] Ke Fan,[a] Bo Xu,[a] Erik Gabrielsson,[a] Quentin Daniel,[a] Lin Li[a] and Licheng
Sun*[a, b]
[a] Department of Chemistry, KTH Royal Institute of Technology, 110044 Stockholm, Sweden
[b] State Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research Center on Molecular Devices
Dalian University of Technology (DUT), Dalian 116024, P. R. China
KEYWORDS: Photoelectrochemical (PEC) cell, Light driven water splitting, tandem cell, molecular catalyst, organic
dyes.
ABSTRACT: Light driven water splitting was achieved by a tandem dyeꢀsensitized photoelectrochemical cell with two photoꢀ
active electrodes. The photoanode is constituted by an organic dye L0 as photosensitizer and a molecular complex Ru1 as water
oxidation catalyst on mesoꢀporous TiO2, while the photocathode is constructed with an organic dye P1 as photoꢀabsorber and a
molecular complex Co1 as hydrogen generation catalyst on nanostructured NiO. By combining the photocathode and the phoꢀ
toanode, this tandem DSꢀPEC cell can split water by visible light under neutral pH conditions without applying any bias.
(1V vs. Ag/AgCl) for this photoanode. Later, Mallouk et al.
Introduction
demonstrated a photoandode in which porphyrin dyes were
used as photosensitizers to drive IrOX for water oxidation, but
the photocurrent density (less 50 ꢁA cmꢀ2) and stability were
not satisfactory.16 So far, no organic molecule showing better
performance than Ru(bpy)3 based dye for lightꢀdriving water
splitting in PEC device was reported. Herein, we report a
tandem PEC cell for total water splitting under neutral pH
conditions, in which the photoanode is coꢀsensitized by a
simple organic dye L0 and Ruꢀbased catalyst Ru1 on mesoꢀ
TiO2, and the photocathode employs an organic dye P1 and
Coꢀbased catalysts Co1 coꢀsensitized on NiO, as shown in
Scheme 1. This is the first case that organic dyes are employed
as photosensitizers in both photoanode and photocathode of a
tandem molecular PEC device for light driven total water
splitting.
To satisfy our society for the global sustainable energy deꢀ
mands, utilizing solar energy to split water to produce hydroꢀ
gen by photoelectrochemical (PEC) cell is one of the most
promising strategies.1ꢀ3 Molecular catalysts have shown a great
potential for the development of highly efficient water splitꢀ
ting devices due to their easy modification in structures, fine
tuning in redox properties and high catalytic efficiencies.4ꢀ8
Recently, a series of molecular PEC cells for water splitting
have been developed in our group.9ꢀ12 In these devices,
Ru(bpy)3 type photosensitizers were used together with moꢀ
lecular catalysts on TiO2 as the photoanodes, and Pt as the
cathode. In order to avoid the use of expensive metal Pt, a
tandem molecular PEC cell was developed by our group,13
which showed a steady photocurrent density for water splitting
under neutral pH conditions. This study indicates that Ptꢀfree
tandem molecular PEC cell can achieve total water splitting
driven by visible light. However, the high cost of Ruꢀbased
photosensitizer is another limitation for the future largeꢀscale
application of this type of devices. Metalꢀfree organic dyes
have been widely used in dyeꢀsensitized solar cells (DSSCs)
due to their high efficiency, easy modification, tunable elecꢀ
tron transfer process and low cost. They can be considered as
the alternatives for Ru based photosensitizers in PEC
devices.14 Recently, Finke et al. published an encouraging
work in which perylene diimide served as an nꢀtype semiconꢀ
ductor to drive CoOX for water oxidation and a photocurrent of
150 ꢁA cmꢀ2 was obtained,15 but a very high bias was required
Scheme 1. The representation of the photoanode with organic dye
L0 and catalyst Ru1 on TiO2, and the photocathode with organic
dye P1 and catalyst Co1 on NiO.
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To make rational design of a tandem DSꢀPEC cell, several
oxide, mass ratio 1:4) and NiO+ITO (mass ratio 1:4) were
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key issues should be considered. (i) the energy gap of the
photoanode and the photocathode should match with each
other. Here we use TiO2 as photoanode material and NiO as
photocathode material, which is similar to the design of tanꢀ
dem pnꢀDSSCs;17 (ii) the excited state of the nꢀtype dye can
inject an electron into the conduction band (CB) of TiO2 from
its lowest unoccupied molecular orbital (LUMO), while the
excited state of the pꢀtype dye can inject a hole into the vaꢀ
lence band (VB) of NiO from its highest occupied molecular
orbital (HOMO); (iii) for the photoanode the dye’s oxidation
potential Eox should be more positive than the onset potential
of water oxidation catalyst (WOC), while for the photocathode
the dye’s reduction potential Ered should be more negative than
the onset potential of hydrogen generation catalyst (HGC); (iv)
According to our previous study,11 the distance between the
dye and the surface of semiconductor should be shorter than
the distance between the catalyst and the surface of semiconꢀ
ductor to facilitate the desired electron/hole injection and
subsequent electron transfer.
Following the above considerations, the WOC Ru1
[Ru(pdc)(pic)3 with pyridineꢀ2,6ꢀdicarboxylic acid (pdc) as an
anchoring group, pic = 4ꢀpicoline] and the organic dye L0
were immobilized on the surface of nꢀtype TiO2 film (8 ꢁm
thickness) for making the photoanode. Considering the organic
dyes may form aggregates (dye island formation) and affect
the device performance, the catalyst Ru1 was adsorbed on
TiO2 first followed by the adsorption of L0. The loading
amount of catalyst Ru1 can be controlled by changing the
concentration of Ru1 and the loading time. Correspondingly,
the HGC Co1 [Co(dmgBF2)2(H2O) with phosphonic acid as an
anchoring group, dmgBF2 = difluoroboryldimethylglyoximato]
and the organic dye P1 were immobilized on the surface of pꢀ
type NiO for the photocathode.
spin coated on FTO glasses, the films were calcinated at
450℃ for 2h, then TiO2+ITO film and NiO+ITO film were
dipped into Ru1 solution, and Co1 solution for 1h respectively,
Ru1@TiO2+ITO and Co1@NiO+ITO electrodes were thus
obtained.
Preparation of photoanode and photocathode
TiO2 and NiO films were prepared according to our previous
reports.10,20 The thicknesses of obtained bare TiO2 film and
NiO film are ca. 8 ꢁm and 1 ꢁm, respectively. The active areas
of the photoelectrodes were 1 cm2. The bare TiO2 film was
dipped into 1 mM Ru1 DCM (dichloromethane) solution for
15 min, after rinsed by MeOH the film was immersed in 1 mM
L0 DCM solution for 1h. The bare NiO film was dipped into 1
mM P1 DCM solution for 5min. After rinsed by MeOH the
film was immersed in 1 mM Co1 for 1 h. The electrodes withꢀ
out loading photosensitizers or catalysts were prepared as well
by using the same procedure as references.
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Photoelectrochemical measurements
Photoelectrochemical measurements were carried out in
phosphate buffer solution (pH 7, 50 mM Sigma Aldrich). In
order to compare with our previous work,21 all tests were
operated under light source of white LED light (λ >400 nm,
Color temperature 6000ꢀ6500 K, light intensity 100 mW cmꢀ2).
To investigate the molecular photoanode and photocathode
separately, conventional PEC cells were constructed by using
photoanode or photocathode as working electrode, Pt net as
counter electrode and Ag/AgCl electrode (3M KCl) as referꢀ
ence electrode. All the PEC cells were degassed by Ar or N2
for 20 min before the photoelectrochemical measurements.
Determination of O2 generation
The electrolyte in the PEC device was thoroughly degassed
by N2. The volumes of the solution and the headspace in the
working compartment were measured. To evaluate oxygen
generation, 0.5 mL gas phase of the headspace was transferred
into a gas chromatography (GC) using a Hamilton Samꢀ
pleLock syringe. GCꢀ2014, Shimadzu Molecular sieve 5A,
TCD detector, nitrogen as the carry gas was used to measure
the H2 evolution, and with helium as a carrier gas was used to
measure the O2 evolution.
Experimental Section
Materials
All chemicals and solvents, if not stated, were purchased
from Sigma Aldrich and used without further purification;
water used in syntheses and measurements was deionized by
MilliꢀQ technique. 4ꢀhydroxyꢀ2,6ꢀpyridinedicarboxylic acid
and 4ꢀmethylpyridine were purchased from TCI Development
Co., Ltd. cisꢀRu(DMSO)4Cl2 and Co(dmgBF2)2(H2O)2 were
prepared according to published methods. 18,19 Synthetic routes
of Ru1 and Co1 can be found in supporting information.
IPCE measurements
Incident photon to current conversion efficiency (IPCE)
spectra was obtained by illumination of the photoꢀ
electrodes with light of a specific wavelength (from 370
nm to 650 nm) and measuring the resulting shortꢀcircuit
current. The currents were recorded using a computerꢀ
controlled setup consisting of potentiostat (EG&GPAR
273). The illumination was supplied by a Xenon light
source (Spectral Products ASBꢀXEꢀ175) and calibrated
using a certified reference solar cell (Fraunhofer ISE).
General Electrochemical Methods
All electrochemical measurements were carried out using an
Autolab potentiostat with a GPES electrochemical interface
(Eco Chemie). An Ag/AgCl in 3 M KCl and a platinum foil
were used as the reference electrode and the counter electrode,
respectively. Using this reference electrode, all the potentials
were converted to the NHE by using [Ru(bpy)3]2+/[Ru(bpy)3]3+
couple (Halfꢀwave potential E1/2=1.26 V vs. NHE) as an
internal reference. E1/2 was determined by cyclic voltammetry
as the average of the anodic and cathodic peak potentials
(E1/2=(Epa+Epc) /2). To measure the electrochemical properties
of catalysts on the metal oxide films, mix films were used due
to the nature of TiO2 and NiO. TiO2+ITO (ITO= indium tin
The specific wavelength was controlled by
monochromator (Spectral Products CM110).
a
Results and discussion
From cyclic voltammetry curves of Ru1@TiO2+ITO and
Co1@Ni+ITO electrodes (Figure S4 and S5), the onset potenꢀ
tials of WOC and HGC can be determined, the Eonset of Ru1
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for water oxidation is around 1.2 V vs. NHE, and the Eonset of
toanode, it produced a remarkable average photocurrent of
ca. 300 ꢁA cmꢀ2. For the L0@TiO2 photoanode without the
catalyst Ru1, it only produced ca. 30 ꢁA cmꢀ2 photocurrent
density under the same conditions, while for Ru1@TiO2
electrode only an indistinct photocurrent can be achieved
(Figure S8). Ru1 have a strong absorption of visible light
(Figure S6), but it will be hard for Ru1 to inject 4 electrons
into TiO2 and generate RuV species for water oxidation.
The significant increase of photocurrent confirms the highꢀ
ly catalytic activity of Ru1 for water oxidation, and the
electron transfer from the catalyst to the oxidized dye
should occur as anticipated. In comparison to our previous
study on similar PEC device using the catalyst Ru1 and
Ru(bpy)3 based photosensitizer as photoanode (giving
Co1 for hydrogen generation is around ꢀ0.54V vs. NHE. The
organic dyes L0 and P1 have been reported in DSSCs.22ꢀ24
Corresponding optical and electrochemical properties of the
dyes and the catalysts are shown in Table 1.
According to the electrochemical properties of the dyes, the
catalysts and the semiconductors,17,25 a schematic energy diaꢀ
gram was illustrated in Scheme 2. After illumination of the
photoanode, the excited state of dye L0 can inject an electron
into the CB of TiO2 and the photoꢀgenerated L0+ can oxidize
the catalyst Ru1, leading to water oxidation after repeating
multiꢀelectron transfer processes. Meanwhile, at the photoꢀ
cathode, the excited state P1 can inject hole into the VB of
NiO and the formed P1ꢀ which can reduce the catalyst Co1 for
eventual hydrogen generation.
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Table 1. Optical and Electrochemical Properties of the Dyes and Catalysts
Dyes and Catalysts
λabs (ε/Mꢀ1 cm1)
λem
Eox(HOMO)
/ V vs. NHE
E0-0
/ eV
LUMO
/ V vs. NHE
/ nm
/ nm
P1
L0
348(34720);481(57900)
373 387(36000)
618
509
1.32
1.37
2.25
2.90
– 0.93
ꢀ1.53
Ru1
Co1
Ru1 onset potential for water oxidation on TiO2 is around 1.20 V
Co1 onset potential for hydrogen generation on NiO is around ꢀ0.54 V
photocurrent density of ca. 100 µA cmꢀ2),13 the higher phoꢀ
tocurrent density obtained from the photoanode
L0+Ru1@TiO2 indicates that this simple organic dye L0
based device exhibits much better device performance than
the expensive Ru(bpy)3 photosensitizer based device.
Additionally, compared with photoanodes developed by
Finke and Mallouk,15,16 our photoande L0+Ru1@TiO2
shows advances of low applied bias potential and high
current density.
First, linear scan voltammetry (LSV) of L0+Ru1@TiO2 unꢀ
der illumination (Figure 1) showed that the photocurrent rapidꢀ
ly increased with the applied potential from ꢀ0.25 V to ꢀ0.15 V
(vs. NHE), and reached a plateau at E > ꢀ0.1 V with a photoꢀ
current density of 0.42 mA cmꢀ2. This value is significantly
higher in comparison to the current densities under dark condiꢀ
tion, indicating that the working electrode is indeed photoacꢀ
tive. From LSV, the relationship between the applied bias
potential and photocurrent can be found, here 0 V vs. Ag/AgCl
as the applied bias was selected to measure the photocurrent of
L0+Ru1@TiO2 electrode.
Figure 1. LSV measurements of the WEs under light illuꢀ
mination (light intensity 100 mW cmꢀ2), scan rate = 50 mV sꢀ
1, in a three-electrode PEC cell with Pt as counter electrode,
Ag/AgCl as reference electrode, operated in a 50 mM pH 7.0
phosphate buffer solution.
Scheme 2. Schematic energy diagram for L0, P1, Ru1, Co1,
FTO, TiO2 and NiO (all data shown at pH 7).
Transient current responses to on˗off cycles and full time
photocurrent under illumination were then studied and
results are shown in Figure 2. For L0+Ru1@TiO2 phoꢀ
In a long term illumination experiment the photoanode
L0+Ru1@TiO2 showed a relatively slow decay of photocurꢀ
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rent (Figure 2 and S10). It’s well known that the stability of
trode. LSV experiments on the assembled P1+Co1@NiO
molecular PEC cells is usually poor.10ꢀ12 One of the main reaꢀ
sons is that molecular catalysts or dyes could detach from the
metal oxide surface in the presence of electrolyte solution,
which leads to a drastically decay of the photocurrent. In our
case, pyridineꢀ2,6ꢀdicarboxylic acid was used as the anchoring
group of the catalyst Ru1 which is exceptionally strong in
comparison to the commonly used carboxylic acid and phosꢀ
phonic acid,13,26 no obvious desorption of the catalyst Ru1
from the surface of TiO2 can be observed with pH ranging
from pH 1 to pH 14. The organic dye L0 is insoluble in water,
which also can hinder the desorption from the surface of TiO2.
Due to these reasons, our photoanode L0+Ru1@TiO2 shows a
much better stability.
photocathode show that, under illumination, the photocurrent
rapidly increased with the applied potential from 0.4 V to ꢀ0.1
V (vs. NHE), and reached a plateau at E < ꢀ0.1 V with a phoꢀ
tocurrent density of ca. ꢀ45 ꢁA cmꢀ2 (Figure 3).
The transient current responses to on˗off cycles show that at
ꢀ0.2 V vs. Ag/AgCl applied potential the P1+Co1@NiO phoꢀ
tocathode can produce an average photocurrent of ca. ꢀ35 ꢁA
cmꢀ2, while the reference photocathode P1@NiO without the
catalyst Co1 produced only ꢀ4 ꢁA cmꢀ2 current density under
the same condition (Figure 4), while for Co1@NiO electrode
only an indistinct photocurrent can be observed (Figure S12).
Compared to our previous work where P1 and
[Co(dmgBF2)2(H2O)2] were encapsulated on NiO film,21 the
photostability of the present device was significantly improved.
This means the phosphonic acid anchoring group in the cataꢀ
lyst Co1 benefits for the photostability of this PEC device.
After 90 min illumination, a photocurrent density of ꢀ20 ꢁA
cmꢀ2 maintains (Figure S13). The photoꢀgenerated hydrogen
gas was confirmed by GC, 0.082 C charges passed the elecꢀ
trode. 0.29 ꢁmol H2 was detected, giving a Faraday efficiency
of 68%.
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Figure 2. The transient current responses to on˗off cycles and full
time photocurrent of illumination (light intensity 100 mW cmꢀ2)
on photoanodes under an applied bias potential of 0 V vs.
Ag/AgCl in a three-electrode PEC cell with Pt as counter elecꢀ
trode, operated in a 50 mM pH 7.0 phosphate buffer solution.
During 60 min illumination, bubbles are formed on the phoꢀ
toanode surface (inset of Figure 2 and Figure S9), the photoꢀ
generated oxygen gas was confirmed by gas chromatography
(GC), 0.46 C charges passed through the electrode (Figure
S10), 0.87 ꢁmol O2 was detected by GC, and the faraday effiꢀ
ciency was calculated to be 73%. More interestingly, with
long time illumination on photoanode in twoꢀelectrode system
(L0+Ru1@TiO2 0.4 cm2 as working electrode and Pt net as
counter electrode) without applying any bias, the PEC cell can
still generate oxygen which was detected by Clarkꢀelectrode
(Figure S11). For the photoanode with L0 alone (L0@TiO2),
only 15 nmol mLꢀ1 oxygen was generated after 30 min illumiꢀ
nation. For the photoanode with Ru1 alone, almost no oxygen
can be detected. In contrast, for L0+Ru1@TiO2 electrode 140
nmol mLꢀ1 oxygen was obtained after 30 min illumination.
These results clearly prove that the lightꢀdriven water oxidaꢀ
tion is successfully achieved by assembly with catalyst Ru1
and organic photosensitizer L0 on TiO2.
Figure 3. LSV measurements of the WEs under light illumination
(light intensity 100 mW cmꢀ2), scan rate = 50 mV sꢀ1, in a three-
electrode PEC cell with Pt as counter electrode, Ag/AgCl as
reference electrode, operated in a 50 mM pH 7.0 phosphate buffer
solution.
With both functional photoanode and photocathode in hands,
we have prepared a tandem PEC cell with two-electrode
configuration. The working electrode (WE) is the
P1+Co1@NiO photocathode, and the counter electrode (CE)
is the L0+Ru1@TiO2 photoanode. The direction of the light
illumination on photoꢀelectrode was essential to the perforꢀ
mance of the PEC cell. Three configurations were performed
by different illumination ways, as shown in Figure 5. In con-
figuration 1, both electrodes are simultaneously illuminated
and the best performance was obtained.
For preparing the photocathode, a cobalt complex Co1 with
an anchoring group was employed as the hydrogen generation
catalyst and organic dye P1 was used as photosensitizer, and
they were immobilized on NiO film. Since the NiO film used
here was very thin (1 ꢁm in thickness), P1 was adsorbed beꢀ
fore Co1 to make sure more dyes can be loaded on the elecꢀ
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cell is thicker than the NiO film (1 ꢁm), the light can go
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through NiO film first and more remaining light can reach the
TiO2 film. Considering configuration 1 is the best illuminaꢀ
tion way for PEC cell in our case, thus configuration 1 was
therefore selected to test the performances of this tandem PEC
cell.
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From LSV experiments on P1+Co1@NiO/L0+Ru1@TiO2
PEC cell, it was found that the photocurrent rapidly increased
with the changing of applied potential from 0.8 V to 0.6 V (vs.
NHE), and reached a plateau at E < 0.6 V with a photocurrent
density of ca. ꢀ70 ꢁA cmꢀ2 (as shown in Figure 6). It is clearly
shown this tandem PEC cell can work without any applied
bias. Transient current responses to on˗off cycles and full time
photocurrent under illumination without bias were studied
(Figure 7). First, the photocurrent density of P1+Co1@NiO/Pt
in twoꢀelectrode setup was much lower (5 ꢁA cmꢀ2, Figure 7
blue) than the threeꢀelectrode setup one (35 ꢁA cmꢀ2, Figure 4
red). This behavior can be explained as follows, in twoꢀ
electrode setup, the valence band of NiO is located around 0.5
V vs. NHE, which is not high enough to drive water oxidation,
that’s why the P1+Co1@NiO/Pt PEC cell almost does not
work. While, in threeꢀelectrode setup, the Pt counter electrode
can get extra potential from electrochemical workstation for
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Figure 4. The transient current responses to on˗off cycles and
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full time photocurrent of illumination (light intensity 100 mW
cmꢀ2) on photocathodes under an applied potential of ꢀ0.2 V vs.
Ag/AgCl in a three-electrode PEC cell with Pt as counter
electrode, operated in a 50 mM pH 7.0 phosphate buffer soluꢀ
tion.
water
oxidation.
Second,
for
the
tandem
cell
(P1+Co1@NiO/L0+Ru1@TiO2), the photocurrent density
Figure
5. The transient current responses to on˗off cycles of a tandem PEC device in two-electrode setup with different directions of the light
illumination (without any applied potential).
When the light is illuminated from the P1+Co1@NiO side
(configuration 2), the tandem PEC cell exhibits better perꢀ
formance than that of configuration 3 where the light is illuꢀ
minated from the L0+Ru1@TiO2 side. The reason is probably
due to the overlap of the absorption regions of L0 and P1
(Figure S6 and S7). As the TiO2 film (8 ꢁm) used in this PEC
shows a significant enhancement (ca 70 ꢁA cmꢀ2) compared to
P1+Co1@NiO/Pt. This enhancement was due to the replaceꢀ
ment of Pt by L0+Ru1@TiO2 photoanode, in this case, the
photoꢀgenerated electrons and holes can flow in this PEC cell
as shown in Scheme 2. The difference between
P1+Co1@NiO/Pt and P1+Co1@NiO/L0+Ru1@TiO2 PEC
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cells indicates that a good photoanode can not only provide
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protons for the hydrogen generation halfꢀreaction, but also can
assist the charge flow between two electrodes, which means
good design of photoanode is quite necessary and important
for tandem PEC cells.
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Figure 7. The transient current responses to on˗off cycles and full
time photocurrent of illumination on photoelectrodes in twoꢀ
electrode setups without any bias, P1+Co1@NiO as WE, Pt or
L0+Ru1@TiO2 as CE. Operated in a 50 mM pH 7.0 phosphate
buffer solution (light intensity 100 mW cmꢀ2)
.
Figure 6. LSV measurements of the WEs under light illuminaꢀ
tion (light intensity 100 mW cmꢀ2), scan rate= 50 mV sꢀ1, in a
two-electrode PEC cell as configuration 1.
With the Faraday efficiency, the performance of the tandem
PEC cell can be assessed by the corresponding solarꢀtoꢀ
hydrogen conversion efficiency ηSTH with equation (1),28ꢀ30
where Jop is the effective operating current density measured
during device operation (70 ꢁA cmꢀ2), V is the water splitting
potential required (1.23 V), Vbias is the bias voltage that can be
added in series with the two electrodes (0 V), Plight is the inciꢀ
dent light power (100 mW cmꢀ2), ηF is Faraday efficiency
(55%). According to equation (1), ηSTH of this tandem device is
calculated to be 0.05%.
From full time photocurrent measurement on this tanꢀ
dem PEC cell, we found a relatively slow photocurrent
decay (ca 60% photocurrent remained after 10 min illuꢀ
mination). Photoꢀgenerated hydrogen was collected and
measured by GC from the tandem PEC cell
(
P1+Co1@NiO/L0+Ru1@TiO2) with twoꢀelectrode
setup and without external bias for 100 min illumination
(using a twoꢀcompartment cell divided by a glass frit as
shown in Figure S1). 0.33 ꢁmol H2 was produced with
0.117 C (Figure S2) of charges passed through the elecꢀ
trodes, which corresponds to 55% Faraday efficiency.
(Jop[mAcm−2 ]×(Vwater splitting −Vbias )×ηF
)
(1)
ηSTH
=
P
light[mWcmꢀ2 ]
These
observed
Co1@NiO(in
Co1@NiO/L0
Faraday
threeꢀelectrode
Ru1@TiO2(in twoꢀelectrode setup)
efficiencies
setup)
for
and
P1+
+
A monochromatic incident photonꢀtoꢀelectron conversion
efficiency (IPCE) measurement was performed, due to the
limitation of experimental conditions, the IPCE of Configura-
tion 1 is very challenging to measure. As shown in Figure 5,
Configuration 2 displays a comparable performance to Con-
figuration 1, so the IPCE of this tandem PEC cell was perꢀ
formed by using Configuration 2. As shown in Figure 8, the
tandem PEC cell (P1+Co1@NiO/L0+Ru1@TiO2) shows an
IPCE of 25.2% at 380 nm (maxabs of L0), 3.9% at 480 nm
(maxabs of P1). This IPCE indicates both L0 and P1 contribute
for the photonꢀtoꢀelectron conversion in this tandem PEC cell,
this is an advantage of using the concept of dyeꢀsensitized
photoelectrodes with different dyes to achieve a broader abꢀ
sorption of visible light.
It’s known that NiO has a poor hole mobility as pꢀtype semꢀ
iconductor,31 and short hole diffusion length, leading to fast
charge recombination.32 At the same time, it’s difficult to
prepare thicker NiO films to load more dyes and catalysts.31,33
Our IPCE data also indicates our photoanode can convert
more light into electrons than that of photocathode. And the
photocurrent generated by P1+Co1@NiO is much lower than
that of L0+Ru1@TiO2 (Figure 2 and 4), these indicate that the
NiO based photocathode is the bottleneck of this tandem cell.
However it is believed that with the development of new pꢀ
P1
+
are similar to those reported in literature.27 However, we
believe that the Faraday efficiency was underestimated in
our case, although twoꢀcompartment cell divided by glass
frit was used, some of the molecular O2 and H2 generated
in each compartment can be dissolved in the solution, and
then diffuse to the other compartment for respective reꢀ
duction and oxidation, resulting in the low observed Faraꢀ
day efficiency.
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We acknowledge the financial support of this work by the Sweꢀ
type semiconductors and new routes to prepare thicker films,
better PEC devices can be prepared with this type of tandem
DSꢀPEC cell design.
dish Energy Agency, the Knut and Alice Wallenberg Foundation,
the Swedish Research Council, the National Natural Science
Foundation of China (21120102036, 91233201), the National
Basic Research Program of China (973 program, 2014CB239402)
and China Scholarship Council.
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Figure 8. The IPEC spectra of Configuration 2 in 50 mM pH 7
phosphate buffer, in two-electrode setup without applying any
bias voltage (data points are from average of three independent
experiments).
Conclusion
In summary, an nꢀtype organic dye L0 coꢀabsorbed with a
molecular water oxidation catalyst Ru1 on TiO2 was used for
preparation of a photoanode, and visible light driven water
oxidation by using this photoanode was successfully achieved.
The photoanode L0+Ru1@TiO2 can produce a remarkable
average photocurrent of 300 ꢁA cmꢀ2 under pH 7 neutral conꢀ
ditions. A hydrogen generation catalyst Co1 coꢀsensitized with
an organic dye P1 on NiO was used for a photocathode. Both
photocurrent and photoꢀstability of this photocathode were
improved compared to previous reported systems. A tandem
DSꢀPEC cell was designed and prepared by connecting the
above mentioned photoanode and photocathode. For the first
time, a metal free organic dyes sensitized tandem PEC cell can
split water by visible light with IPCE of 25% at 380 nm under
neutral pH conditions without bias. These results provide a
new guidance for the design of molecular PEC cells, leading
to a great promise to construct lowꢀcost Ptꢀfree devices for
artificial photosynthesis in the future.
ASSOCIATED CONTENT
Supporting Information
Synthesis of molecules, NMR, MS, IR, UVꢀVis and electrochemꢀ
ical measurement details are included in the supporting inforꢀ
mation. This material is available free of charge via the internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Prof. Licheng Sun, KTH
Eꢀmail: lichengs@kth.se
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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E.; Abe, T. J. Phys. Chem. C 2008, 112, 16134.
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S.; Odobel, F. Inorg. Chem. 2009, 48, 8245.
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Contents artwork
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