DossierThis paper is a part of the hereunder thematic dossier
published in OGST Journal, Vol. 70, No. 5, pp. 791-902
and available online here
Cet article fait partie du dossier thématique ci-dessous
publié dans la revue OGST, Vol. 70, n°5, pp. 791-902
et téléchargeable ici

DOSSIER Edited by/Sous la direction de : D. Uzio

IFP Energies nouvelles International Conference / Les Rencontres Scientifiques d’IFP Energies nouvelles

PHOTO4E – Photocatalysis for energy
PHOTO4E – Photocatalyse pour l’énergie
Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5, pp. 791-902
Copyright © 2015, IFP Energies nouvelles

791 > Editorial
M. Fontecave, A. Fécant and D. Uzio

799 > Solar Production of Fuels from Water and CO2: Perspectives and Opportunities fora Sustainable Use of Renewable Energy
Production solaire de carburants à partir de l'eau et de CO2 : perspectives etopportunités pour une utilisation durable de l’énergie renouvelable

R. Passalacqua, G. Centi and S. Perathoner

817 > Effect of Post-Synthesis Treatments on the Properties of ZnS Nanoparticles:

An Experimental and Computational Study
Effet des traitements après-synthèse sur les propriétés de nanoparticules de ZnS :une étude expérimentale et computationnelle

853 > Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium

Cellule photocombustible recyclable pour l’utilisation d’eau acide en tant que
milieuY. Ogura, M. Yoshiba, and Y. Izumi

863 > Solar Hydrogen Reaching Maturity

L’hydrogène solaire arrive à maturité

J. Rongé, T. Bosserez, L. Huguenin, M. Dumortier, S. Haussener and J.A. Martens

877 > Design of Compact Photoelectrochemical Cells for Water Splitting

Conception de cellules photoélectrochimiques compactes pour la décomposition
de l’eau

T. Bosserez, J. Rongé, J. van Humbeeck, S. Haussener and J. Martens

E. Balantseva, B. Camino, A.M. Ferrari and G. Berlier

831 > Comparative Study on The Photocatalytic Hydrogen Production from Methanol
over Cu-, Pd-, Co- and Au-Loaded TiO2
Étude comparative de production d’hydrogène par photocatalyse à partir deméthanol et à l’aide de différentes phases actives (Cu, Pd, Co et Au)
supportées sur TiO2
P.P.C. Udani and M. Rønning

841 > Photocatalytic Conversion of Carbon Dioxide Using Zn–Cu–Ga Layered Double

Hydroxides Assembled with Cu Phthalocyanine: Cu in Contact with Gaseous Reactant
is Needed for Methanol Generation
Conversion photocatalytique du dioxyde de carbone par des hydroxydes doubles
lamellaires de Zn–Cu–Ga promus par la phtalocyanine de Cu : nécessité du contact
entre le Cu et le réactif gazeux pour la synthèse du méthanol

S. Kawamura, N. Ahmed, G. Carja and Y. Izumi

891 > Simultaneous Production of CH4 and H2 from Photocatalytic Reformingof Glucose Aqueous Solution on Sulfated Pd-TiO2 Catalysts
Production simultanée de CH4 et H2 par réformage photocatalytique d'une
solution aqueuse de glucose sur un catalyseur Pd-TiO2 sulfaté

V. Vaiano, G. Iervolino, G. Sarno, D. Sannino, L. Rizzo, J.J. Murcia Mesa,
M.C. Hidalgo and J.A. Navío

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5, pp. 853-862
Ó Y. Ogura et al., published by IFP Energies nouvelles, 2015
DOI: 10.2516/ogst/2015031
IFP Energies nouvelles International Conference
Rencontres Scientiﬁques d'IFP Energies nouvelles
PHOTO4E – Photocatalysis for energy
PHOTO4E – Photocatalyse pour l’énergie

Recyclable PhotoFuel Cell for Use of Acidic Water
as a Medium
Yuta Ogura, Mao Yoshiba and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522 - Japane-mail: tyojuryoku@hotmail.co.jp - yubari.com@ae.auone-net.jp - yizumi@faculty.chiba-u.jp* Corresponding author

Abstract — We reported a new PhotoFuel Cell (PFC) comprising two photocatalysts for use of acidic
water as a recyclable medium. Nitrogen and oxygen ﬂow was required in the photoanode andphotocathode, respectively. In this study, we developed a gas-circulating PFC that needs no gas
supply from outside. In the gas-circulating PFC, the reverse reaction of water oxidation at the anodewas prevented by the gas ﬂow of photogenerated O2 from the anode to the cathode inside the PFC.
The gas-circulating PFC accommodated an organic solvent layer over the aqueous electrolyte forthe anode, and also a vent hole in the upper part of the Proton-Conducting Polymer (PCP) ﬁlm.
O2 transferred from the anode electrolyte to the organic solvent due to the solubility difference
between the HCl solution and organic solvent. O2 transfer from the gas phase in the anode to that in
the cathode was achieved by the vent hole in the PCP ﬁlm due to the pressure difference due to theprogress of the reaction. By the addition of a hexane layer to the anode of the PFC, it wasdemonstrated to achieve a photocurrent value of 69.7 lA per 1.3 cm2 of photocatalysts. However, inthe stability tests for more than 7 h, the small amount of remaining O2 in the electrolyte (2.85 lmol L1)
exhibited serious effects on the PFC performance. The ISC , VOC and PMax values of the gas-circulating
PFC were 29.2 lA, 1.18 V and 6.10 lW, that were 40%, 74% and 44%, respectively, of those for a
N2 and O2 ﬂow-type PFC. Apparently, photocurrents were dramatically suppressed by the reverse
reaction at the photoanode in the extended tests for the gas-circulating PFC.
Résumé — Cellule photocombustible recyclable pour l’utilisation d’eau acide en tant que milieu— Nous décrivons une nouvelle cellule photocombustible (PhotoFuel Cell, PFC) comprenant deux
photocatalyseurs utilisant de l’eau acide recyclée. Un ﬂux d’azote et d’oxygène a été nécessaire dans
respectivement la photoanode et la photocathode. Dans cette étude, nous avons développé une PFC à
circulation de gaz qui n’a pas besoin d’être alimentée en gaz de l’extérieur. Dans la PFC à
circulation de gaz, la réaction inverse d’oxydation de l’eau à l’anode a été évitée par le ﬂux de gaz
d’O2 photo-généré de l’anode à la cathode à l’intérieur de la PFC. La PFC à circulation de gaz
présentait une couche de solvant organique au-dessus de l’électrolyte aqueux pour l’anode, etégalement un oriﬁce d’aération dans la partie supérieure du ﬁlm polymère conducteur de proton(Proton-Conducting Polymer, PCP). L’O2 était transféré de l’électrolyte de l’anode au solvant
organique, à cause de la différence de solubilité entre la solution d’HCl et le solvant organique.
Le transfert d’O2 de la phase gazeuse à l’anode vers celle dans la cathode a été réalisé par l’oriﬁced’aération dans le ﬁlm PCP, du fait de la différence de pression due à la progression de la réaction.
En ajoutant une couche d’hexane à l’anode de la PFC, il a été démontré que l’on atteignait une valeurde courant photo-électrique de 69,7 lA par 1,3 cm2 de photocatalyseurs. Toutefois, dans les tests destabilité, pendant plus de 7 h, la faible quantité d’O2 restant dans l’électrolyte (2,85 lmol L1) a eu desThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5854

effets importants sur les performances de la PFC. Les valeurs Isc, Voc et PMax de la PFC à circulation de gaz
ont atteint 29,2 lA, 1,18 V, et 6,10 lW, soit 40%, 74% et 44%, respectivement de celles d’une PFC de type
à ﬂux de N2 et d’O2. Apparemment, les courants photo-électriques ont été considérablement suppriméspar la réaction inverse à la photoanode dans les tests prolongés pour la PFC à circulation de gaz.

INTRODUCTION
Fossil fuels have been utilized as the essential energy source
for industrialization. Industrial CO2 emissions have led to anincrease in the level of atmospheric CO2 concentration(400 ppm), and the effects of this increase on global warming cannot be underestimated. The development of renewable energy as a replacement for fossil fuels has been slow
[1, 2].
Among the renewable energies, solar energy has thegreatest potential. Although the Silicon Solar Cell (Si SC)
has been commercialized, the technology that can convert
solar energy to electricity often needs subsidies to spread
more widely [3]. Other types of SC, e.g. the dye-sensitized
SC and compound SC [4], and Fuel Cells (FC) that usehydrogen/methanol fuel [5-8], potentially obtained using
solar energy [9], have been extensively investigated. However, all the requisites (sustainability, durability and an electromotive force of 1-3 V per cell) have not been fully
satisﬁed.
Recently, a new device was demonstrated: a PFC utilizing
two photocatalysts of TiO2 and silver(0/I)-doped TiO2 [10]on an electrode ﬁlm, both immersed in acidic solutions separated by a PCP ﬁlm [11]. Platinum is not used [5-8, 12-16],
and only inexpensive materials were used in the PFC for sustainability. The PFC is similar to photoelectrochemical cells(PEC) consisting of a photoanode and a photocathode toform O2 and H2, respectively, from water [17, 18] and also
PEC consisting of a photoanode to oxidize water [19] ororganic dye fuel [20] and a photocathode to reduce protons.
The PFC is different from these PEC in that the medium,water, is recycled in the cell and no fuel is needed except
for natural light [21].
Although PFC enable higher electromotive force (1.8 V)[11, 21] and use only acidic water as a medium and inexpensive catalysts, PFC need a gas supply to promote surface
reactions at both the anode and cathode. At the anode, O2
gas needs to be evacuated by N2 ﬂow to prevent a reverse
surface reaction:
O2 þ 4Hþ þ 4e ! 2H2 O

ð1Þ

On the other hand, at the cathode reactant O2 should be
supplied for the reaction (1) to proceed. Therefore, PFC
need gas cylinders to supply these gases for effective

power generation. This situation limits the design of compact enough PFC that can be accommodated in portableelectronic devices.
Herein, we developed a PFC in which gas circulates internally. By circulating formed O2 gas from the anode to thecathode, the reverse reaction at the anode could be prevented. The concept of gas-circulating PFC is shown in
Figure 1a, Test 3. The PFC has two photocatalysts (TiO2and Ag-TiO2) immersed in HCl solution. The electrolyte at
the anode and that at the cathode (Fig. 1b2, 3) are separated
by a PCP (Fig. 1b4, 5). As the mechanism of gas circulation,
the PFC designed in this study comprises an organic solvent
phase on an anode electrolyte (Fig. 1b1) and a vent hole in
the upper part of the PCP ﬁlm (Fig. 1b4, 5). The organic solvent can dissolve a greater amount of O2 gas than water can.
O2 gas circulated in the PFC as follows. O2 generated by
the anode reaction transferred preferentially into the organic
solvent due to the difference in O2 solubility between the
HCl solution and the organic solvent. Then, the dissolved
O2 in the organic solvent moved to the upper gas phase
due to the increased O2 concentration in the solvent in thecompartment of the anode, and then to the upper gas phasein the compartment of the cathode through the vent hole in
the PCP ﬁlm due to the pressure difference between theO2 formed at the anode and O2 consumed at the cathode(Fig. 1a, 1b1). Finally, the gaseous O2 that moved to thecathode side reached the photocatalyst on the cathode andwas photoreduced.
1 EXPERIMENTAL SECTION
1.1 Sample Preparation of the Photoelectrodes
TiO2 powder [1.00 g; P25, Degussa; anatase/rutile phases =7/3; Brunauer-Emmett-Teller (BET) surface area (SBET) =60 m2 g1] was suspended in 3.0 mL of deionized water(<0.06 lS cm1) and then stirred well. The obtained suspension was dried at 373 K for 24 h and heated in air at 673 K
for 2 h. The resultant powder was suspended in 75 lL of deionized water and placed on an Indium Tin Oxide (ITO, thickness
1.2-1.6 lm)-coated Pyrex glass plate. The TiO2/ITO/Pyrex
was dried at 373 K for 18 h and heated in air at 573 K for
30 min. The amount of TiO2 deposited on the ITO-coated glass
plate was 5.0 mg and covered an area of 1.3 cm2.

Y. Ogura et al. / Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium

V
e
R

855

1)

–

A

2

A hole (S = 18 mm )of Nafion (Tests 3, 5) N2 (Test 1)O2 (Test 1)

2H2O + 4h+

O2 + 4H+ + 4e–+

O2 + 4H

2H2O

Hexane (Tests 3, 4)

2)

3)

4)

5)

O2
H+

TiO2
Aq HCl

Ag–TiO2
Aq HCla)b)

Figure 1a) Components of the PFC comprising TiO2 and Ag–TiO2 photocatalysts used for Tests 1-5. Oxidation by photogenerated holes at the anode andreduction by photogenerated electrons take place. Remaining electrons at the anode ﬂow to the cathode and combine with the remaining holes.
b) Photographs of the PFC for Test 3 (1). Photoanode side (2), photocathode side (3), and the PCP with a vent hole (4, 5).

Silver nitrate (159 mg; 99.8%, Wako Pure Chemicals)
was dissolved in 10 mL of deionized water. The solution
was mixed with 3.33 g of untreated TiO2 (P25). The mixture
was magnetically stirred at a rate of 850 rotations per minute(rpm) and the water was distilled at 353 K. The obtained
powder was dried at 373 K for 24 h and heated in air at
673 K for 2 h. The resultant Ag-TiO2 powders contained
3.0 wt% of Ag. They were suspended in a minimum amountof water and placed on ITO-coated glass (Ag-TiO2/ITO/
Pyrex) in a manner similar to that for the TiO2/ITO/Pyrex.
1.2 Photocurrent Tests
1.2.1 Test 1: Gas-Flow Conditions

TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex electrodes were
immersed in HCl solutions (40 mL in each compartment; initial pH 4.0-2.0). The two compartments were separated by a
50-lm-thick PCP ﬁlm (Naﬁon, DuPont; acid capacity >9.2 9 104 equivalent g1). N2 (purity > 99.999%) andO2 gases (purity > 99.6%) were bubbled 30 mm away fromeach photoelectrode at a ﬂow rate of 100 mL min1(Fig. 1a). The PFC was equipped with quartz windows(/ = 80 mm) on both sides. Both the TiO2 and Ag-TiO2
photocatalysts were irradiated with UV–visible light throughthe quartz windows using a two-way branched quartz ﬁberlight guide (Model 5/-2B-1000L, San-ei Electric Co.) froma 500-W xenon arc lamp (Model SX-UID502XAM, Ushio).

The distance between the light exit (/ = 5 mm) andthe TiO2 or Ag-TiO2 ﬁlm was 46 mm. The light intensity
was 8 mW cm2 at the center of the photocatalyst ﬁlm oneach electrode.
1.2.2 Test 2: Gas-Closed Conditions

PFC comprising TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex
electrodes were prepared and immersed in HCl solutions(40 mL in each compartment; initial pH 2.0) in a similar procedure to that in Test 1. The two compartments were separated by a 50-lm-thick PCP ﬁlm. N2 and O2 gases purged
the compartments of TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex,
but the gas supply was stopped before the start of Test 2. Boththe TiO2 and Ag-TiO2 photocatalysts were irradiated with
UV–visible light in the closed compartments ﬁlled with N2
and O2, respectively.
1.2.3 Test 3: Gas-Circulating Conditions inside the PFC

An Ag-TiO2/ITO/Pyrex electrode was immersed in 50 mL of
HCl solution (initial pH 2.0), whilst TiO2/ITO/Pyrex was
immersed in 50 mL of the HCl solution phase (initial
pH 2.0) and 50 mL of hexane phase was added above theHCl solution phase (Fig. 1). All the area of the TiO2 ﬁlm
was immersed in the HCl solution phase and not in contact
with the hexane phase. N2 and O2 gases purged thecompartments of TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex,

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

856

TABLE 1
The conditions for current tests of the PFC

Light Light Light Light Light Light Light Light Light Light Light Light Light Lightoff on off on off on off on off on off on off on

N2/O2
supplyHexane@anode

Vent hole@PCP
ﬁlm

1

s

9

9

2

9

9

9

3

9

s

s

4

9

s

9

5

9

9

s

respectively, but the gas supply was stopped before the startof Test 3. Both the TiO2 and Ag-TiO2 photocatalysts were
irradiated with UV–visible light, set in the closed compartments ﬁlled with N2 and O2, respectively. The two compartments were separated by a 50-lm-thick PCP ﬁlm; however,in Test 3, gas circulation inside the PFC was enabled througha vent hole (6.0 mm 9 3.0 mm) in the upper part of the PCP
ﬁlm (Fig. 1a, b4, b5) from the anode compartment in which
O2 was formed to the cathode compartment in which O2 wasconsumed. The conditions of UV–visible light irradiation
were similar to those for Tests 1 and 2 (Tab. 1).
1.2.4 Test 4: Gas-Closed Conditions Plus Hexane@AnodeAn Ag-TiO2/ITO/Pyrex electrode was immersed in 55 mL
of HCl solution (initial pH 2.0), whilst TiO2/ITO/Pyrex
was immersed in 55 mL of the HCl solution phase (initial
pH 2.0) and 55 mL of the hexane phase above the HCl solution phase (Fig. 1). All the area of the TiO2 ﬁlm was
immersed in the HCl solution phase and not in contact withthe hexane phase. N2 and O2 gases purged the compartmentsof TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex, respectively,
but the gas supply was stopped before the start of Test 4.
Both the TiO2 and Ag-TiO2 photocatalysts were irradiatedwith UV–visible light, set in the closed compartments ﬁlledwith N2 and O2, respectively. The two compartments wereseparated by a 50-lm-thick PCP ﬁlm. The ﬁlm was complete
and did not comprise any vent holes as in Test 3. The conditions of UV–visible light irradiation were similar to those forTests 1-3 (Tab. 1).

60

Currents (µA)

Test

Test 4

Test 140

Test 3

Test 220
Test 5

0
0

2

4

6

Time elapsed (h)Figure 2
The time course of photocurrents of the PFC comprising TiO2
and Ag-TiO2 photocatalysts in Tests 1 (gas ﬂow), 2 (gasclosed), 3 (gas circulating inside the PFC), 4 (gas-closed conditions plus hexane@anode), and 5 (gas-circulating conditions
inside the PFC minus hexane@anode). The pH values of theelectrolytes in both the anode and cathode were 2.0.

the gas supply was stopped before the start of Test 5. Boththe TiO2 and Ag-TiO2 photocatalysts were irradiated with
UV–visible light, set in the closed compartments ﬁlled with
N2 and O2, respectively. The two compartments were separated by a 50-lm-thick PCP ﬁlm. The conditions of UV–
visible light irradiation were similar to those for Tests 1-4. ThisPFC had a vent hole (6.0 mm 9 3.0 mm) in the upper part of the
PCP ﬁlm (Fig. 1a, b4, b5) similar to Test 3 (Tab. 1).
For Tests 1-5, photocurrent generation (photocurrent
tests) was performed by connecting the external parallel circuit for one route via a voltmeter and another route via anammeter with a resistance of 0.5 X (Fig. 1a). Both catalysts
were exposed for 30 min to UV–visible light and then kept in
the dark for 30 min, and this process was repeated ﬁve-seventimes. The current and voltage between the two electrodes
were monitored.
1.3 Current–Voltage Dependence Tests

1.2.5 Test 5: Gas-Circulating Conditions inside the PFC Minus
Hexane@AnodeAn Ag-TiO2/ITO/Pyrex electrode was immersed in 40 mL
of HCl solution (initial pH 2.0), whilst TiO2/ITO/Pyrex
was immersed in 40 mL of the HCl solution phase (initial
pH 2.0). N2 and O2 gases purged the compartments of
TiO2/ITO/Pyrex and Ag-TiO2/ITO/Pyrex, respectively, but

The current (i)–voltage (V) characteristics were also measured for several types of PFC using a similar parallel circuit(Fig. 1a). After the photocurrent test for Figure 2 (2.5, 3.5
and 3.5 h of irradiation of UV–visible light for Tests 1, 2
and 4, respectively), the resistance was gradually decreased
from 500 kX to 0.3 X over 20 min to plot the i–V dependence. For comparison, a PFC in the conditions of Test 1

Y. Ogura et al. / Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium(Sect. 1.2.1) was also prepared and i–V characteristics werealso measured similar to the experiments described above,
but in darkness.
1.4 Product Analyses during Photocurrent Tests at theAnode1.4.1 Analysis of O2 Formed at the Anode56.2 mg of iron(III) chloride hexahydrate (Kanto Chemical,
purity  99%) was dissolved in 90 mL of HCl aqueous solution in a quartz ﬂask. The pH of this solution was adjusted to
2.0 by the addition of HCl solution. 151 mg of TiO2 powder(P25, Degussa) was suspended in this solution for 10 min.
Then, dissolved O2 in the solution was removed by N2 bubbling for 30 min. Next, N2 bubbling was stopped and theﬂask was isolated. The solution suspended with TiO2 wasirradiated with UV–visible light from model SX-UID502XAM. The distance between the light exit (/ = 60 mm)
and the center of the ﬂask was 120 mm. The sample was
exposed to the light for 15 min and kept in the dark for
15 min. This step was repeated ﬁve times. The solution
was magnetically stirred at a rate of 1 000 rpm and the temperature was 298-301 K. The formed O2 as dissolved wasmonitored in situ by a Dissolved Oxygen (DO) electrode(Model 9520-10D, Horiba).
1.4.2 Analysis of Hypochlorous Acid Formed at the Anodeby the Photooxidation of Cl

The possibility of the photooxidation of Cl ions at theanode of the PFC instead of the photooxidation of water
was checked. If Cl2 is formed by the photoxidation at theanode (Eq. 2), it should immediately decompose into hypochlorous acid and hydrochloric acid in the HCl solution of
the anode (Eq. 3):
2Cl þ 2hþ ! Cl2

ð2Þ

Cl2 þ H2 O ! HCl þ HClO

ð3ÞThe HClO formed was evaluated by the N,N-Diethyl-pPhenyleneDiamine (DPD) method [22].
20 mg of DPD reagent (0.8 mg of DPD sulfate salt
(H2SO4) and 19.2 mg of Na2SO4) were dissolved in
1.0 mL phosphate buffer solution (pH 6.5). This solution
was mixed with 5.0 mL of 0-39.0 lmol L1 of NaOCl aqueous solutions and used as a standard solution. The mixed
solutions (6.0 mL) were used to obtain a linear regression
relationship between the absorbance at 515 nm and theconcentration of ClO ions.
Next, 20 mg of DPD reagent were dissolved in 4.0 mL
phosphate buffer solution (pH 6.5). This solution was mixed

857

with 2.0 mL of sample solution (HCl aqueous solution) after
a photocurrent Test 1 using a PFC (Fig. 1) for 3 h at pH 2.0.
The UV–visible spectrum of the mixed solution (6.0 mL)
was measured by UV–visible spectrometer (Model V650,
JASCO). The concentration of HClO in the sample wasdetermined based on the absorbance at 515 nm and the relationship obtained using NaOCl standard solutions. The reason for the volume difference for the phosphate buffer andsample solutions was to adjust the pH within 6.4 and 6.6
for mixed solutions (6.0 mL).
2 RESULTS AND DISCUSSION
2.1 Photocurrent Tests
The time courses of the photocurrent tests are summarized in
Figure 2. For Test 1, in response to the UV–visible irradiation,
photocurrents gradually increased from 43.7 to 60.5 lA in ﬁve
cycles at pH 2.0. In contrast, the photocurrents increased andconverged to a constant value (26.7 lA) in the test at pH 3.0(not shown) [11]. Thus, the active metallic Ag0 species for photoreduction of O2 at the cathode should gradually increase,transformed from Ag2O by the reaction with protons, in the ﬁve
cycles at pH 2.0. The transformation of the active Ag species(Ag0, Ag2O and AgCl) was reported based on an extended
X-ray absorption ﬁne-structure study [11].
At pH 4.0, a PFC current of 3 lA was obtained(not shown) for 2.5 h irradiated by UV–visible light(0.28 lmol-e generated in total) [11]. The pH values starting from 4.0 for the HCl solution (40 mL, 4.0 lmol-H+)
remained constant after the PFC test at both the anode andcathode. The pH change due to each half reaction was calculated as ±0.03. As this value is nearly the accuracy of pH
monitoring under these conditions, we cannot judge thatthe photooxidation of water (at the anode) and photoreduction of O2 (at the cathode) exclusively took place. The possibility of a by-reaction, e.g. the consumption of acid,remained. For the test at pH 2.0, the initial proton concentration was 400 lmol in 40 mL of electrolyte solution at boththe anode and cathode, while the PFC current for 2.5 h
was 4.7 lmol-e in total. The pH change due to each half
reaction is calculated as ±0.005. It was even more difﬁcult
to judge the possibility of photooxidation of water and/oranions of acid under the conditions.
In Test 2, the photocurrent in the ﬁrst cycle (34.7 lA) was
similar to that in Test 1 (43.7 lA). In response to theUV–visible irradiation, photocurrents gradually increased
to 35.8 lA in three cycles. In contrast to the gradual increasein photocurrents throughout 5 h of the study in Test 1, thephotocurrents peaked at 35.8 lA in the third cycle andgradually decreased to 32.2 lA in the seventh cycle in Test 2(Fig. 2). This decrease by 3.6 lA suggested partial

Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

deactivation of the TiO2 photocatalyst at the anode. One of the
major reasons is the reverse reaction of produced O2 which
remained dissolved in the HCl solution to water (Eq. 1).
In contrast to the gas-ﬂowing Test 1, the anode compartmentof TiO2/ITO/Pyrex was purged with N2 gas and produced
O2 remained in the compartment for Test 2 (Tab. 1).
The photocurrent in the ﬁrst cycle (54.8-40.3 lA) was
clearly higher in Test 3 than that in Tests 1 and 2. The photocurrent decreased to 36.5 lA at the end of the second cycle(Fig. 2) and continued to decrease gradually to 31.0 lA at
the end of the seventh cycle. The photocurrents in the ﬁrst
cycle were probably higher because N2 was purged in theanode compartment more completely than in Tests 1 and 2
by the effect of hexane to preferably dissolve a contamination level of O2 (~0.04 ppb). This evaluation was based on
the impurity of O2 (< 1 ppm) in the N2 gas used and the equilibrium of dissolution. Conversely, the decrease in the photocurrent in seven cycles suggested an even smaller
amount of formed O2 remained in the anode and inhibited
the PFC performance considerably, and also unexpected
reverse diffusion of O2 from the cathode into the anodewas not excluded through the vent hole.
In Test 4, the photocurrent in the ﬁrst cycle (69.7 lA) was
clearly higher than that in Tests 1-3. However, in response to
the UV–visible irradiation, photocurrents gradually
decreased to 51.9 lA in seven cycles (Fig. 2). The photocurrent in Test 4 was always higher than that in Tests 2 and 3
throughout the photocurrent test. Thus, the positive effectsof hexane on PFC were conﬁrmed (Tab. 1), but unexpectedly, the vent hole in Test 3 contributed negatively to thephotocurrent generation. Furthermore, the linear decreaseof the photocurrent as a function of time that elapsed during
Test 4 was more gradual than the exponential-like decrease
as a function of time in Test 3.
In Test 5, the photocurrent in the ﬁrst cycle (41.0-33.0 lA)
was similar to that in Test 2 (Fig. 2). Then, it decreased to29.2 lA in the second cycle. Later than the third cycle, thephotocurrents became stable at 28.4 lA. This stabilized current
was lowest among the Tests 1–5, as the result of produced
O2 which remained dissolved in the HCl solution to water,
and the unexpected reverse diffusion of O2 in the cathode into
the anode through the vent hole, as in Test 3 (Tab. 1).
In Test 2, 1.11 lmol of O2 were generated during the 7-hphotocurrent test based on the total photocurrent for 3.5 h
and assuming the next reaction (Eq. 1 is the reverse reactionof Eq. 4):
2H2 O þ 4hþ ! O2 þ 4Hþð4Þ

All of them were dissolved in the anode electrolyte basedon the O2 solubility. Thus, the O2 concentration in the anodeelectrolyte was 27.8 lmol L1. On the other hand, in Test 4,
2.01 lmol of O2 were generated after the 7-h photocurrent

80
70

Test 1 (Gas flow)60

Currents (µA)

858

50
40
30

Test 4 (Gas-closed conditions
plus hexane@anode)

20
10
Test 1’ (Gas flowin darkness)
00

Test 2 (Gas closed)
500

1 000

1 500

Voltage (mV)

Figure 3
i–V characteristics of the PFC comprising TiO2 and Ag-TiO2
photocatalysts in Test 1 (gas ﬂow), Test 2 (gas closed), Test 4(gas-closed conditions plus hexane@anode), and control Test 1’
in darkness (gas ﬂow).

test based on the total photocurrent for 3.5 h and assuming
reaction 4. In this test, the O2 should be distributed between
the HCl solution and hexane. Based on the O2 solubility
values in the literature to hydrochloric acid solution(1.27 mmol L1) and to hexane (15.0 mmol L1) [23], thedistribution of O2 to hexane was calculated as:
0:055 L  15 mmol L1¼ 0:9220:055 L  1:27 mmol L1 þ 0:055 L  15 mmol L1ð5Þ

Thus, 1.85 and 0.157 lmol of O2 should be distributed to thehexane and HCl solutions, respectively, in equilibrium. Thus,
the O2 concentration in the anode electrolyte (HCl solution)
was calculated as 2.85 lmol L1. Hence, the O2 concentration
at the end of Test 4 was only 10% of that at the end of Test 2.
This difference accounted for the higher current density in
Test 4 by a factor of two as compared with Test 2 (Fig. 2).
2.2 i–V Characteristic Tests
The obtained i–V characteristics are shown in Figure 3. Furthermore, the properties of PFC calculated based on i–V data
in Tests 1, 2 and 4 are summarized in Table 2. In Test 1, the ivalue gradually increased as the cell voltage decreased, starting from open circuit voltage (VOC) at 1.59 V, which is a similar trend to the i–V dependence of the SC [24] or the Ohmic

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Y. Ogura et al. / Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium

TABLE 2
PFC characteristics deﬁned by i–V characteristics in Tests 1 (gas ﬂow), 2 (gas closed), and 3 (gas circulating inside the PFC)
Test

ISC (lA)

VOC (V)

PMax (lW)

1 (Gas ﬂow)

73.7

1.59

14.0

2 (Gas closed)

29.8

1.23

5.5

4 (Gas-closed conditions plus hexane@anode)

29.2

1.18

6.1

1’ (Gas ﬂow; control in darkness)< 0.1< 0.002< 0.0002

2.3 Product Analyses during Photocurrent Tests
at the Anode2.3.1 Analysis of O2 Formed at the AnodeThe time course of dissolved O2 concentrations is shown in
Figure 4. When TiO2 was irradiated with UV–visible light,

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120
100

O2 (µmol L–1)

loss region of a Polymer Electrolyte Fuel Cell (PEFC).
When the voltage became less than 0.45 V, the current
increased linearly from 20 to 74 lA (short-circuit current,
ISC). This increasing trend was different from that in a SCor the transportation limit region of a PEFC [7, 8].
In Test 2, the shape of the i–V characteristic curve was similar to that for Test 1. However, the increase in currents in thelower voltage region ( 0.45 V) was less steep than the slope
in Test 1. The values of ISC (29.8 lA), VOC (1.23 V) andPMax (5.51 lW) were 40, 77 and 39% of the corresponding
values obtained in Test 1. The lower photocurrents in Test 2
suggested that the reverse reaction 1 that occurred in the anodeseriously affected the performance of the PFC.
In Test 4, the curve of i–V characteristics was similar tothat for Test 2; the increase in photocurrents in the lowvoltage region ( 0.45 V) was less steep than the slope in
Test 1. The values of ISC (29.2 lA) and VOC (1.18 V) wereslightly lower than the corresponding values in Test 2, whilst
the value of PMax (6.10 lW) was clearly higher than that in
Test 2 (5.51 lW). This low performance (Fig. 3) was inconsistent with the better performance in photocurrent Test 4
than that in Tests 1 and 2 (Fig. 2). This trend cannot be rationalized based on the remaining amount of produced O2 dissolved in the anode. One of the possibilities is thepolarization of photoelectrodes during the i–V measurements
at high voltage to modify the band bending (charge balanceat the surface) at the surface of photocatalysts [25-29].
In a control Test 1’ in darkness, the value of the current
was always less than the detection limit by changing theresistance from 500 kX to 0.3 X. The corresponding voltage
was less than 0.002 V. Therefore, the values of ISC, VOC andPMax were less than 0.1 lA, 0.002 V and 0.0002 lW, respectively. This indicated electricity of the PFC was generated
exclusively by irradiation with UV–visible light.

80
60
40
20
0

0

0.5

1.0

1.5

2.0

2.5

Time elapsed (h)Figure 4
The time course of dissolved O2 concentrations during thephotoirradiation test on TiO2. UV–visible light andFeCl36H2O (2.31 mmol L1) were used as excitation lightand a sacriﬁcial reagent, respectively. The initial pH value
was 2.0.

the dissolved O2 concentration linearly increased immediately in all ﬁve cycles. For 1.25 h irradiation, dissolved O2
was increased from 7.8 lmol L1 to 119 lmol L1. If theirradiation was stopped, dissolved O2 decreased graduallyin all cycles. This indicated that the formed O2 dissolvedin acidic water and was transported to the gas phase in thequartz ﬂask. In this photoreaction test, 151 mg of TiO2
was used, signiﬁcantly greater than the 5 mg for the photocurrent test (Sect. 2.1, 2.2). Accordingly, a greater amount of
O2 was generated from the water (63.1 lmol L1 per 0.25 h)in this photoreaction test than in photocurrent Test 2(27.8 lmol L1 per 3.5 h). Thus, excessive O2 greater thanthe dissolution amount would be transported to the gas
phase.
When the photocatalyst was irradiated, the rate would be
the balance between O2 formation and the transportation toOil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 70 (2015), No. 5

the gas phase. The increase in dissolved O2 in the ﬁrst cycle
was the quickest among all the cycles. The increase in dissolved O2 was 63.1 lmol L1 per 0.25 h. The slope of the
dissolved O2 gradually became less steep as the cycles wererepeated. As a result, in the ﬁfth cycle, it was 36.2 lmol L1
per 0.25 h. This trend is contradicted by the gradual increasein the photocurrent in Figure 2, Test 1 as the cycles repeated.
We suspect that the major reason was that the transportationof dissolved O2 to the gas phase became faster as the concentration of dissolved O2 increased, as the cycles were repeated(Fig. 4).
The increase rate of dissolved O2 in the ﬁrst cyclewas:1

63:1 lmol L  0:090 L=0:25 h=0:151¼ 151 lmol-O2 h1 g1
cat

0.6
0.5

Absorbance

860

0.4
0.3
0.2
0.1
0

Absorbance = 0.0166 [HClO] (µmol L-1) + 0.007
R2 = 0.970

0

5

gcat

10

15

20

25

30

35

-1

[HClO] (µmol L )ð6Þ

In comparison, the total photocurrent in Test 2 was average current:

Figure 5
Absorbance at 515 nm for standard mixed solutions as a function of the concentration of HClO standard solutions.

34:0 lA  3 600 s h1 =9:65  104 A s mol-e1 =0:0050 gcat
¼ 253 lmol-e h1 g1
ð7Þcat
The electron ﬂow rate corresponded to the O2 generation
rate of 63 lmol-O2 h1 gcat1. The net O2 generation rate in
Figure 4 should be even higher because the O2 transported to
the gas phase was not included for the evaluation in Figure 4.
This difference in the amount based on the PFC current was
not inconsistent. In the presence of a sacriﬁcial oxidation
agent, Fe3+, photoexcited electrons to the conduction bandof TiO2 easily diffuse with sufﬁcient Fe3+ and react incontrast so that the electrons need to travel to the cathodeto react with holes at the valence band of [Ag-]TiO2 in the
PFC. The concentration of holes was determined by the balance of charge separation by light and the cathode reaction
rate of O2 reduction [11, 21].

Thus, the amount of HClO in the anode solution was calculated as:
3:98 nmol  40 mL=2:0 mL ¼ 0:080  0:056 lmol ð9Þ
For 3 h of Test 1 (pH 2.0), total electron ﬂow was 3.92
lmol based on the integration of photocurrents. Two holes
reacted with Cl ions (Eq. 2) correspond to one HClO
formed from Cl2 (Eq. 3). As the concentration of HClO
was the impurity level for the anode solution in Test 1, theamount of HClO was nearly the detection limit. Therefore,
the upper limit of selectivity to proceed with Equation (2)for photogenerated holes was:2  0:080 lmol-HClO=3:92 lmol-hþ ¼ 0:041  0:031
ð10Þ

2.3.2 Analysis of Hypochlorous Acid Formed at the Anodefrom Cl Ions

The absorbance at 515 nm for standard mixed solutions(6.0 mL) was plotted as a function of NaOCl concentrationsin the solution (6.0 mL). Then, a standard curve was calculated as follows (Fig. 5):
Absorbance@515 nm ¼ 0:0166  ½ClO–  ðlmol L1 Þ þ 0:007
ð8Þ

20 mg of DPD reagent in 4.0 mL phosphate buffer
solution (pH 6.5) were mixed with 2.0 mL of sampled
solution from the anode solution (40 mL) of the PFC after
Test 1 for 3 h. The absorbance of this mixed solution was
0.018 ± 0.008. Therefore, the amount of HClO in the mixed
solution (6.0 mL) was 3.98 nmol based in Equation (8).

The participation of chlorine ions in the anode reaction
cannot be denied; however, the ratio was as low as 4.1%
at most.

CONCLUSIONSFirst, the anode reaction in the PFC was demonstrated exclusively to be the photooxidation of water over TiO2 based on
the O2 evolution test, and negligible (or no) photooxidationof Cl– occurred based on the detected amount of HClO.
The selectivity of photooxidation of Cl– was 4.1% at mostas compared with the photooxidation of water. Next,
a product gas-trapping PFC was demonstrated to achieve a

Y. Ogura et al. / Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium

photocurrent value of 69.7 lA per 1.3 cm2 of photocatalysts(53.6 lA cm2). Gas trapping was enabled by accommodating an organic solvent layer over the aqueous electrolyte
solution phase in the anode and utilizing the solubility difference of produced O2. However, gas circulation by a venthole in the upper part of the PCP ﬁlm due to pressure differences was not well achieved. To transfer generated O2 to an
anode, a greater pressure difference was needed. The performance of the gas-closed PFC equipped with a hexane layeron the anode (Test 4) was the best at the initial stage of thephotocurrent test due to the O2 solubility effect of hexane
rather than the other conditions. However, the photocurrents
gradually decreased during the measurements for 7 h.
The small amount of remaining O2 in the anode electrolyte(2.85 lmol L1) reduced the PFC performance considerably in Test 4. The ISC, VOC and PMax values of the gascirculating PFC were 29.2 lA (22.5 lA cm2), 1.18 V and6.10 lW (4.69 lW cm2), respectively. These values weresimilar to corresponding values for the gas-closed PFC.
The feasibility of a gas-trapping PFC was successfully
demonstrated in this study. Further, the improvement of
power efﬁciency is required by the optimization of photocatalysts and the thickness and density of photocatalyst layers
in order to be accommodated in portable electronic devices,
remote-control light gliders, and environmental cameras/
sensors.

ACKNOWLEDGMENTS
The authors are grateful for ﬁnancial support from theFeasibility Study Stage of A-STEP (AS262Z00159L,
AS251Z00906L, AS231Z01459C) from the Japan Scienceand Technology Agency, the Iwatani Naoji Foundation(2011-2012), and a Grant-in-Aid for Scientiﬁc Research C(26410204, 22550117) from the Japan Society for the
Promotion of Science. X-ray absorption experiments wereconducted under the approval of the Photon Factory
Proposal Review Committee (2013G159).

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Manuscript submitted in October 2014
Manuscript accepted in September 2015
Published online in October 2015

Cite this article as: Y. Ogura, M. Yoshiba and Y. Izumi (2015). Recyclable PhotoFuel Cell for Use of Acidic Water as a Medium,
Oil Gas Sci. Technol 70, 5, 853-862.