ChemComm
COMMUNICATION

Cite this: Chem. Commun., 2014,
50, 3067
Received 9th January 2014,
Accepted 4th February 2014

A photofuel cell comprising titanium oxide andsilver(I/0) photocatalysts for use of acidic water
as a fuel†Yuta Ogura,a Seiji Okamoto,a Takaomi Itoi,b Yukiko Fujishima,ac Yusuke Yoshidaa
and Yasuo Izumi*a

DOI: 10.1039/c4cc00194j
www.rsc.org/chemcomm

A photofuel cell comprising two photocatalysts TiO2 and Ag–TiO2
is demonstrated. The open circuit voltage, short circuit current, andmaximum electric power of the PFC were 1.59 V, 74 lA, and 14 lW,
respectively. The electron flow was rectified due to the Schottky
barrier between TiO2 and Ag nanoparticles.

Fossil fuels have been utilized as an essential energy source forindustrialization. The 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 the greatest potential. Although silicon solar cells (Si SC)
have been commercialized, the technology that can convert solar
energy to electricity often needs subsidies to spread more widely.3
Other types of SCs4 and fuel cells (FCs) that use hydrogen fuel5
potentially obtained using solar energy6 have been extensively investigated. However, all the requisites (sustainability, durability, and anelectromotive force of 1–3 V per cell) have not been fully satisfied.
In this study, we demonstrate a new device: a photofuel cell (PFC)
utilizing two photocatalysts TiO2 and silver(0/I)-doped TiO2 on anelectrode film, both immersed in acidic solutions separated by aproton-conducting polymer (PCP). The cell mechanism of redox
reactions over the photocatalysts and the flow of electrons andprotons in the cell facilitates a theoretical electromotive force of
3 V if some reasons (charge recombination in electrodes, electrons
confined in Ag, and reverse electron flow from cathode to anode) forovervoltage are not taken into account. Moreover, the use of acidic
water as a fuel is inexpensive and sustainable.
aDepartment of Chemistry, Graduate School of Science, Chiba University,
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan. E-mail: yizumi@faculty.chiba-u.jp;
Fax: +81-43-290-2783; Tel: +81-43-290-3696
b
Department of Mechanical Engineering, Graduate School of Engineering,
Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japanc
Department of Chemistry, Faculty of Science, Tokyo University of Science,
Kagurazaka 1-3, Shinjuku-ku, Tokyo 163-8601, Japan† Electronic supplementary information (ESI) available: The derivation of eqn (2)for photocurrents of PFC, experimental methods, EXAFS curve-fit results, CV peak
positions, and supplementary EM images. See DOI: 10.1039/c4cc00194j

This journal is © The Royal Society of Chemistry 2014

Scheme 1 The energy diagram of PFC comprising two photocatalyst
electrodes.

The concept of PFC is shown in Scheme 1. The band gap values
for anatase- and rutile-type TiO2 are 3.2 and 3.0 eV,7,8 respectively,
and charges (holes, electrons) are separated by UV (and minor
visible) light irradiation. The holes in TiO2 diﬀused to the surface
to photooxidize water, while the electrons in Ag–TiO2 diffused to theTiO2 surface and then to Ag nanoparticles to photoreduce O2
molecules. Thus, in the cell, electron flow from the conduction band(CB) of TiO2 to the valence band (VB) of Ag–TiO2 is obtained. Because
the Schottky barrier is formed at the interface between TiO2 and Ag
nanoparticles, the electron flow from TiO2 to Ag is rectified.
This concept of PFC comprising two photoelectrodes is
diﬀerent from FCs comprising a photoanode and a conventional
cathode such as Pt–carbon,9–15 dye-sensitized SCs comprising adye on a semiconductor and a conventional cathode,3 andcombination of two photocatalysts separated by a PCP film to
produce O2 and H2 independently from water.16–18 Recently, a
PFC comprising WO3 photoanode and a Cu2O/Cu photocathodewas reported using organic dyes as a fuel.19 In this study, (acidic)
water is used as a fuel15 and is restored in the PFC cell.
An Ag-supported TiO2 (Ag–TiO2) sample was calcined at 673 K.
The color was light yellow, which indicated the presence of metallic
Ag0.20 However, the color changed to purple in air after 24 h(Fig. 1A). The silver K-edge extended X-ray absorption fine structure(EXAFS, Fig. 1B–a) demonstrated the dominance of Ag2O based on
the interatomic pair of Ag and O at 0.2305 nm by curve-fit analysis(Table S1a, ESI†). Scanning electron microscopy (SEM, Fig. 2A),
transmission electron microscopy (TEM, Fig. 2B) and high-resolution

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Fig. 2 SEM (A), TEM (B), HR-TEM (D), and HAADF-STEM images (C, E) of
as-prepared Ag–TiO2 photocatalysts stored in ambient air.

When the Ag–TiO2 on the electrode immersed in a HCl solution
at a pH of 2.0 was irradiated by UV-visible light, the yellow color
changed to ocher within 10 s (Fig. 1A, top). From the EXAFS data, it
was observed that the interatomic pair of Ag and Ag became
predominant and the distance was 0.287 nm (Fig. 1B–c2 and Table
S1c, ESI†). Therefore, all of the outer layer AgCl and a part of theinner layer Ag2O were transformed to metallic yellow Ag0 due to the
reduction by photogenerated electrons diffused from TiO2.20
In response to the UV-visible irradiation, the photocurrents
increased and stabilized within 6–9 min at pH 3.0 (Fig. 3A1). In
five light on–off cycles, the photocurrents converged to 11.8–
12.7, 17.6–18.8, and 26.0–26.7 mA using Ag–TiO2 of 0.33, 1.0,
and 3.0 wt%-Ag, respectively, on a photocathode. The converged photocurrent values were plotted as a function of the
Ag content in the Ag–TiO2 photocatalysts (Fig. 3A2), and they
increased proportionally to the cube root of the Ag content.
Next, the dependence of photocurrents on the pH of the electrolyte
solution was investigated for the PFC comprising TiO2 and Ag–TiO2(3.0 wt% of Ag) at the pH values between 2.0 and 4.0 (Fig. 3B1, 2).
Throughout this study, the pH values changed negligibly (within
the variation of 0.02) over 5 h. The photocurrents stabilized within
1–11 min of irradiation and converged to 26.0–26.7 mA at pH 3.0. The
current gradually increased from 43.7 to 60.5 mA in five cycles at pH
2.0 (Fig. 3B1); in contrast, the current gradually decreased from 3.2 to
2.6 mA at pH 4.0 (B2). The photocurrents quickly decreased to zero
over 6 min at pHs of 2.0 and 3.0 when the UV-visible light was off,
while the currents decreased more slowly (30–43 min) at pH 4.0.
This suggested limited diffusion at lower proton concentrations.
The converged PFC photocurrent values were plotted as afunction of electrolyte solution proton concentrations (Fig. 3B3).

(HR) TEM (Fig. 2D) images revealed Ag2O nanoparticles with a mean
size of 3.8 nm accompanying lattice intervals between 0.222 and0.249 nm, corresponding to the (002) lattice (a/2 = 0.2361 nm) andnot the (111) lattice (a/O3 = 0.2726 nm).21 The brightness of the highangle annular dark field (HAADF)-scanning TEM (STEM) image wasproportional to the square of atomic weight and distinguished 47Ag
and 22Ti atoms (Fig. 2C and E). Indeed, the distribution of Ag2O
nanoparticles on TiO2 was clearly observed (Fig. 2C), and the atomic
resolution image of a single Ag2O nanoparticle supported thepresence of Ag2O (Fig. 2E), as shown in the HR-TEM image(Fig. 2D). A weak peak due to the interatomic pair of Ag and Ag
because of metallic Ag0 was also observed by EXAFS (Fig. 1B–a2);
however, the population was small (B10% on atom basis) based on
the curve-fit analysis (Table S1a, ESI†).
The photocurrent generation of PFC comprising TiO2 andAg–TiO2, both placed on an indium tin oxide (ITO)-coated Pyrex
glass electrode, was tested under N2 and O2 gas flows, respectively, separated by a PCP film. When the Ag–TiO2 film on theelectrode was immersed in a HCl solution with a pH of 2.0, thecolor of the film changed to light yellow within 5 s (Fig. 1A,
right). The interatomic pair distance between Ag and Cl at
0.262 nm obtained by EXAFS (Fig. 1B–b2 and Table S1b, ESI†)
demonstrated that outer layers of Ag2O transformed to colorless Ag(I) chloride (Fig. 1A, right). The light yellow color was dueto the small number of inert Ag0 nanoparticles.

Fig. 3 (A1) The time course of photocurrents of the PFC comprising TiO2 andAg–TiO2 photocatalyst electrodes immersed in a HCl solution at pH of 3.0.
Circle (J): 3.0 wt% of Ag, square (&): 1.0 wt% of Ag, and diamond (B): 0.33 wt%
of Ag for Ag–TiO2. (A2) The correlation between converged photocurrents
within 5 h and the amount of Ag (wt% of Ag) or cube root of the amount of Ag
and the fits. (B1, 2) The time course of photocurrents in the PFC comprising TiO2
and Ag–TiO2 (3.0 wt%-Ag) photocatalyst electrodes, which are immersed in a
HCl solution at pH 2.0 (triangle, n), 3.0 (square, &) (B1), and 4.0 (circle, J) (B2).
(B3) The correlation between converged photocurrents within 5 h and the H+concentrations of acid solution in the PFC or the power of square of the H+concentrations. The fits were to linear function (lower x-axis) or to the kinetic
model equation i = A[H+]c1/2  B[H+]c1/2 (eqn (2), see text; upper x-axis). (C) i–V
dependence for PFC comprising TiO2 and Ag–TiO2 (3.0 wt%-Ag) immersed in a
HCl solution at pH 2.0.

Fig. 1 (A) Transformation of Ag active sites on the Ag–TiO2 photocatalyst. (B)(a) Silver K-edge EXAFS spectra of as-prepared Ag–TiO2 (3.0 wt%-Ag), (b) sample
a was immersed in a HCl solution of pH 2.0 and then kept under 101 kPa of N2,(c) sample b was irradiated under UV-visible light, and (d) the light was turned off
and sample c was kept under 101 kPa of O2. Note that (1) corresponds to the k3weighted EXAFS w-function and (2) corresponds to best-fit results in R-space tothe Fourier-filtered transformed data. The red line represents the experimental
values, and the blue line represents calculated values. The solid line represents
the magnitude and the dotted line represents the imaginary part in panel (2).

3068 | Chem. Commun., 2014, 50, 3067--3070

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ChemCommThe values increased proportionally to the square root of the
proton concentrations.
The current (i)–voltage (V) characteristic was also studied for the
TiO2 and Ag–TiO2 of 3.0 wt%-Ag PFC at pH 2.0 (Fig. 3C). The i value
gradually increased as the cell voltage decreased, starting from theopen circuit voltage (VOC, 1.59 V), which is similar to the i–V
dependence for the SCs.22 When the voltage was less than 0.45 V,
the current increased linearly from 20 to 74 mA (short circuit current,
ISC). The maximum electric power was 14 mW (1.1 V  13 mA) among
the three runs using the photocatalyst film area of 1.3 cm2. As acomparison test, Ag mesh (150 mm-f, 40 lines in 2.54 cm) was used
as an electrode instead of ITO-coated glass. The VOC (0.52 V) and ISC(107 mA) obtained for PFC using Ag mesh were quite different fromthat obtained using ITO-coated glass (1.59 V, 74 mA; Fig. 3C),
suggesting that the interface between photocatalysts and electrodes(In/Sn oxide, Ag) was also critical for the characteristic of PFC.
The i–V characteristic can be explained based on the equivalent
SC circuit comprising a diode, a series resistance (Rseries), and ashunt resistance (Rshunt).23 The current i can be expressed as follows eðV þ iRseries ÞV þ iRseries(1)i ¼ iphoto  idiode exp
1 
RshuntnkT
where iphoto is the photoelectric current, idiode is saturated i of thediode, n is a constant for the diode, e is elementary charge, k is
Boltzmann constant, and T is the temperature of the cell. The Rseries
and Rshunt values were calculated to be 35 and 5.9 kO cm2,
respectively, based on the tangent lines from VOC and ISC (Fig. 3C).
Compared to the general requisite for ideal Si SCs (Rseries o 1 O cm2,
Rshunt 4 1 kO cm2), the resistance in photocatalysts and that at theinterface between photocatalysts and electrodes needs to be
improved for PFCs to decrease the Rseries value.
The PFC photocurrents are generated by the balance ofphotoexcitation and charge recombination in TiO2 and Ag–TiO2
photocatalysts, and the reaction rates of water photooxidation andO2 photoreduction (Scheme 1) analogous to dye-sensitized SCs, inwhich the dye attached to TiO2 is photooxidized and the redox
mediator is reduced at the cathode.
The allowed indirect band gap electronic transition24 leads to theseparation of electrons and holes in TiO2. The equilibrium constantsof charge separation (Ka and Kc) are assumed in the photoanodic
TiO2 and photocathodic Ag–TiO2, respectively. The electrons excitedto CB for Ag–TiO2 may be favorably trapped at the Ag sites20 owing
to the Schottky barrier (work functions: 4.52–4.74 eV (Ag)25 4 4.13–
4.3 eV (TiO2)26,27). The trap was evidenced by the reduction of AgI to
Ag0 in EXAFS for Ag–TiO2 immersed in a HCl solution andirradiated by UV-visible light (Fig. 1B–c and Table S1c, ESI†).
The photooxidation over TiO2 was essentially irreversible under
the N2 flow. The rate and rate constant were denoted as rox and kox,
respectively. The O2 photoreduction at Ag–TiO2 was in equilibriumwith the product (water) and the constant is denoted as Kred. The
forward electron flow rate from TiO2 to Ag–TiO2 via the external
circuit should be proportional to both the excited electron concentration in TiO2 and the hole Ag–TiO2 concentration based on theprinciple of PFC as shown in Scheme 1. The reverse electron flowrate should be proportional to both the unreacted trapped electron
concentration at Ag–TiO2 and the unreacted hole concentration

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at TiO2. Thus, the effective electron flow rate (photocurrent i) is
formulated in eqn (2). The derivation is shown in eqn (S1)–(S4)(ESI†).
0i ¼k

0

Ka kox

 k0

14

Kc

0

ðrox Þ
14ðrox Þ Kc

12

Kred

1
4

0

12

0

18

1
1
½O2 c 8 ½Hþ c 2
(2)
1
1

1
ðkox Þ ðKred Þ ½O2 c 8 ½Hþ c 2
1
0 4

1
0 8

where [O2] and [H+] with a subscript ‘‘c’’ denote the concentrations of O2 and H+ in acidic solution around the cathode, andrate and equilibrium constants with the prime symbol denotethe constants multiplied with essentially constant concentrations for predominant species in acidic solution/photocatalysts(see ESI† for a detailed definition).
The experimental data fit of pH dependence in eqn (2) ispresented in Fig. 3B3 per x-axis. The second term was negligible in
the fit (i = 626.7 [H+]1/2  0.009977 [H+]1/2), demonstrating that thereverse electron flow from Ag to the VB of anodic TiO2 via external
circuit was minimal. Moreover, the net photocurrents were essentially proportional to [H+]1/2 (Fig. 3B3, lower x-axis; the first term ofeqn (2)). In this study, PFC was advantageous for rectifying theelectron flow direction owing to the Schottky barrier between TiO2
and Ag nanoparticles. Moreover, the suppression of the reverse
reaction at anodic TiO2 by purging the resultant O2, analogous to
the suppression of dye re-reduction by electron transfer from the CB
of TiO2 to the dye in dye-sensitized SC.22 In addition, the total
current generated as shown in Fig. 3B1 (pH 2.0) corresponded to
4.9 mmol-e versus the amount of Ag mounted on the cathode which
was 1.4 mmol. Therefore, major part of the forward electron flow to
Ag on cathode should be consumed to reduce O2.
The eﬀects of Ag (electron trap and electron transfer toO2-derived species) are related to the charge separation equilibrium
Kc (or Kc0 = Kc[Ag+]c[O2]c, ESI†) that appears in eqn (2). The weight(w) of sphere-like Ag nanoparticles (Fig. 2) is proportional to the
cube of the average radius (w = N43p(%r)3r, where N is the number of
Ag nanoparticles and r is the density). The eﬀective charges
trapped in Ag for photocatalysis should be related to the surface
2
2
area of Ag nanoparticles, N4p(%r)2 = (4Np)3(3w/r)3. The dependenceof eﬀectively trapped charges on w (2/3) takes into accountthe square root dependence of the first term in eqn (2) on
Kc 0 (2/3  1/2 = 1/3). Thus, the cube root dependence ofphotocurrents on the Ag content (Fig. 3A2) can also be explained.
In the cyclic voltammetry (CV) measurements for Ag–TiO2(3.0 wt% of Ag) in an aqueous HCl solution of pH 4.0, only the redox
reactions of AgCl (AgCl + e $ Ag0 + Cl) and H2 formation (2H+ +2e - H2) occurred under N2 in the dark (Fig. 4d and Table S2,
ESI†). When the Ag–TiO2 was irradiated by UV-visible light, the redox
reactions of Ag2O (Ag2O + 2H+ + 2e $ 2Ag0 + H2O) also occurred(Fig. 4c). Under O2, the reduction reaction peak from Ag2O to Ag0 at
0.35 V (versus SHE) was more intense, as seen in Fig. 4a, whenirradiated by UV-visible light compared to Fig. 4b, which shows the
reduction peak in the dark. This result is in accordance with theAg photoreduction as monitored using EXAFS (Fig. 1B–c). If thephotoreduction of Ag is coupled with simple oxidation of Ag by O2

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experiments were conducted under the approval of the Photon
Factory Proposal Review Committee (2013G159).

Notes and references

Fig. 4 CV for the TiO2 or Ag–TiO2 (3.0 wt% of Ag) working electrode andthe glassy carbon counter electrode in an aqueous HCl solution of pH 4.0
under an O2 flow (a, b) or N2 flow (c, d) under the irradiation by UV-visible
light (a, c) or in the dark (b, d).

(4Ag + O2 - 2Ag2O), O2 photoreduction at the cathode of the PFC
can be explained (Fig. 4a, bottom inset, chemical formula). In
contrast, no distinct peaks appeared under any of the conditions
employed for TiO2.
The feasibility of PFCs comprising two photoelectrodes wasdemonstrated using TiO2 and Ag–TiO2 immersed in a HCl solutionof pH 2, separated by a PCP film. The VOC, ISC, and maximum power
were 1.59 V, 74 mA, and 14 mW, respectively. TiO2 photooxidized
water under a N2 flow, while Ag–TiO2 photoreduced O2. The kinetic
model successfully explained the photocurrent dependences on pH
values and the amount of Ag. The quantum efficiency was evaluatedto be 20% both for photooxidation and photoreduction (ESI†) andthat of PFC should be the product (4%). The quantum efficiency
needs to be improved by the optimization of photocatalysts on thecathode and the thickness, packed density, and the serial resistancefor photocatalysts on electrodes.
The authors are grateful for the financial support from theFeasibility Study Stage of A-STEP (AS251Z00906L, AS231Z01459C)
from the Japan Science and Technology Agency, the Iwatani Naoji
Foundation (2011–2012), and the Grant-in-Aid for Scientific
Research C (22550117) from MEXT (2010–2012). X-ray absorption

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