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Mesoporous WO3 photoanodes for hydrogen

production by water splitting and PhotoFuelCell

operation

Stavroula Sfaelou  a, Lucian-Cristian Pop  a, Olivier Monfort  a,1,Vassilios Dracopoulos  b, Panagiotis Lianos  a,b,* 

a Department of Chemical Engineering, University of Patras, 26500 Patras, Greeceb FORTH/ICE-HT, P.O. Box 1414, 26504 Patras, Greece

a r t i c l e i n f o

Article history:

Received 6 November 2015

Received in revised form

15 January 2016

Accepted 15 February 2016

Available online 21 March 2016

Keywords:

Tungsten trioxide

Water splitting 

PhotoFuelCells

Hydrogen production

a b s t r a c t

WO3 photoanodes have been constructed by a simple soft chemistry procedure that pro-

duced efficient mesoporous nanocrystalline films. These photoanodes absorbed visible

light and could be efficiently employed for photoelectrochemical hydrogen production

under electric bias. The current increased and the rate of hydrogen production more than

tripled in the presence of a small quantity of ethanol showing that such photoanodes may

be successfully used in alternative photoelectrochemical installations for solar fuel pro-

duction by consumption of organic wastes.

Copyright  © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

WO3  is one of the most popular metal oxide semiconductor

photocatalysts. It is being studied for several decades [1e4] as

an alternative to the UVA absorbing titania. WO3   has abandgap of 2.5e2.8 eV, therefore, it absorbs visible light up to

500 nm, which accounts for 12% of the solar radiation on the

surface of the earth   [4]. WO3   is an n-type indirect semi-

conductor. It is relatively easy to synthesize and deposit on

electrodes, it has a moderate hole-diffusion length (~150 nm

[4]), it exhibits resistance against photocorrosion and it

demonstrates satisfactory chemical stability at relatively low

pH values. For this reason, WO3  has been studied as a pho-

toanode material for photoelectrochemical water splitting 

applications [1e7]. Its valence band is located approximately

at þ2.8 V vs  NHE, therefore, it is well placed for water oxida-

tion. Its conduction band is located at positive potentials(approximately  þ0.2   tο  þ0.3 V   vs   NHE), therefore, a bias is

necessary in order to guide photogenerated electrons to the

counter electrode and produce hydrogen by water or proton

reduction. The expected theoretical solar to hydrogen effi-

ciency for WO3 photoanodes is about 4.8%, based on its range

of light absorption [7] (the corresponding value is only 2.2% for

*   Corresponding author. Department of Chemical Engineering, University of Patras, 26500 Patras, Greece.

E-mail address:  lianos@upatras.gr (P. Lianos).1 Permanent address: Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynska

Dolina, 84215 Bratislava, Slovakia.

 Available online at www.sciencedirect.com

ScienceDirect 

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http://dx.doi.org/10.1016/j.ijhydene.2016.02.063

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compartments were separated by a silica frit (ROBU,Germany,

porosity SGQ 5, diameter 25 mm, thickness 2 mm). Hydrogen

was monitored on line by using Ar as carrier inert gas and by

applying a bias measured  vs  Ag/AgCl.

Illumination was made in all cases using a Xe lamp

providing an intensity of 100 mW cm2 at the position of the

photoanode.

Hydrogen was detected by using an SRI 8610C gas chro-matograph. Calibration of the chromatograph signal was

accomplished by comparison with a standard of 0.25% H2 in

Ar. Application of electric bias and currentevoltage curves

were traced with the help of an Autolab potentiostat

PGSTAT128N.

Morpho-structural characterization of the WO3  films

UVeVis diffuse reflectance spectra (DRS) were recorded with a

Shimadzu model 2600 spectrophotometer and IPCE spectra

with a home-made apparatus using a Xe lamp and a series of 

interference filters. The surface morphology and particle size

of the samples was observed with Field-Emission Scanning Electron Microscopy (FESEM, Zeiss SUPRA 35 VP). XRD mea-

surements were carried out with a D8 ADVANCE (Bruker AXS)

diffractometer (Source/CuKa: 1.54A,Power 1.6 W) operating in

Bragg-Brentano   q /q   geometry and BET measurements were

made with a Micromeritics Tristar 3000 apparatus.

Results and discussion

Characterization of the WO3  films

Mesoporous tungsten trioxide films produced by the method

described in subsection   Preparation of electrodes   had the

structure shown in the FESEM image of  Fig. 1. Nanoparticles

had polydispersed sizes ranging between 20 and 50 nm. BET

specific surface area SSA was 25.4 m2 g 1. This SSA value is

rather low, compared with, for example, commercial nano-

particulate titania P25, which is around 50 m2 g 1. X-Ray dif-

fractograms revealed the formation of monoclinic WO3

crystallites (Fig. 2) of size about 25 nm as calculated by using 

Scherrer's formula. Obviously, this value matches the data

obtained by FESEM measurements.

The absorption spectrum of the presently made WO3photoanode (Fig. 3) had a threshold at 465 nm, which corre-

sponds to an energy gap of 2.7 eV.

Current density-voltage characteristics of the WO3 photoanodes

The photocurrent produced by WO3  photoanodes reached a

substantial value, which almost doubled in the presence of 

ethanol. Indeed, as seen in Fig. 4, when the voltage was 1.6 V

vs  Ag/AgCl, i.e. just before water electrolytic oxidation onset

(1.7 V), the current density in the absence of ethanol reached

3.5 mA cm2 but became 6.3 mA cm2 in its presence. Obvi-

ously, a strong bias is necessary to reach current density

Fig. 1 e  FESEM image of a typical WO3 photoanode used in

the present work. The scale bar is 200 nm.

Fig. 2  e  X-ray diffractogram of monoclinic WO3nanocrystals on the FTO background.

Fig. 3 e  Diffuse reflectance absorption spectrum and IPCE%

 values for a WO3 photoanode. IPCE data were recorded

under bias of 1.6 V  vs  Ag/AgCl in the presence of 0.5 M

aqueous NaClO4 .

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Hydrogen production by photoelectrochemical water

splitting and ethanol oxidation

Photoelectrochemical cells comprising mesoporous WO3photoanodes can be used to produce hydrogen by photo-

electrocatalytic reductive reactions at the cathode electrode.

Hydrogen production was indeed monitored by applying an

external bias of 1.6 V   vs   Ag/AgCl, i.e. just before the onsetpotential for electrocatalytic water oxidation. Hydrogen pro-

duction was monitored both in the absence or the presence of 

ethanol in the anode compartment. As seen in  Fig. 7, a sub-

stantial quantity of hydrogen was produced by water splitting 

but it became much larger when ethanol was present. Thus

the cell can be efficiently used both for water splitting and for

PhotoFuelCell operation producing hydrogen. Ethanol was

presently used as model fuel, however, other organic sub-

stances or mixtures of substances that create wastes can be

used as fuel with obvious environmental benefit. Hydrogen

production rate in Fig. 7 demonstrated a fast rising part and

then dropped a little to attain a very stable value for several

hours. The reason for this small drop is not clear but it is not

important either. Apparently, the studied system demon-

strated a remarkable stability.

The above data may be subject to skepticism concerning 

the relatively high bias applied to produce hydrogen.

Hydrogen can, of course, be also produced at lower bias but, asexpected, it is substantially lower, as can be seen by the data

of  Fig. 7. Indeed, hydrogen production rate dropped by more

than 50% when the applied bias was 1.0 V   vs  Ag/AgCl. The

choiceof a biasof 1.6 V vs Ag/AgCl is, of course,(slightly) lower

than the onset for water electrocatalytic oxidation, which

appears at 1.7 V vs Ag/AgCl. Furthermore, both current values

and hydrogen production rates became zero when the light

was turned off.  The process of hydrogen production is then purely

 photoelectrocatalytic. Therefore, it makes no harm to exploit the

whole potential range. In addition, it must be taken into ac-

count that water electrocatalytic splitting necessitates the

employment of expensive Pt electrocatalysts on both anode

and cathode electrode. At least in the present case, one of thetwo electrodes, i.e. the photoanode, was constructed by using 

an inexpensive photocatalyst. Furthermore, a favorable situ-

ation would prescribe water splitting at smaller biases than

the theoretical value of 1.229 V. Unfortunately, real electro-

catalytic water splitting cases so far reported were supported

by electric biases much higher than this theoretical value

[24,25]. Such data, justify why high bias may also be necessary

to make the present WO3 photoanodes effective.

Conclusions

Mesoporous WO3   films on FTO electrodes make efficient

visible-light-responsive photoanodes, which can be used for

photoelectrochemical hydrogen production. WO3   photo-

anodes can approach the maximum current density value

expected for their light absorption range only under strong 

bias, while in the presence of ethanol, employed as model

fuel, the current density was doubled, indicating a clear-cut

current doubling phenomenon. WO3   demonstrated a large

advantage vs titania photoanodes, obviously due to its visible

light response.

Acknowledgments

This project is implemented under the   “ARISTEIA”  Action of 

the   “OPERATIONAL PROGRAMME EDUCATION AND LIFELONG

LEARNING” and is co-funded by the European Social Fund and

National Resources (Project No.2275).

Olivier Monfort wishes to acknowledge a grant provided by

the Scientific Grant Agency of the Slovak Republic (Project

VEGA 1/0276/15) and the National Scholarship Program of the

Slovak Republic managed by SAIA n.o. and funded by the

Ministry of Education, Sport, Science and Research of the

Slovak Republic, which allowed his stay in the University of 

Patras.

Fig. 7 e Photoelectrochemical hydrogen production rate (A)

and cumulative hydrogen production (B) in the absence

and in the presence of ethanol using a WO3 photoanode.

The applied bias was measured  vs  Ag/AgCl.

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Finally, we would like to thank Prof.E.Stathatos for his help

with the BET measurements.

r e f e r e n c e s

[1]  Hodes G, Cahen D, Manassen J. Tungsten trioxide as a

photoanode for a photoelectrochemical cell (PEC). Nature

1976;260:312e3.

[2]   Ashokkumar M, Maruthamuthu P. Photocatalytic hydrogen

production with semiconductor particulate systems: an

effort to enhance the efficiency. Int J Hydrogen Energy

1991;16(9):591e5.

[3]   Alexander BD, Kulesza PJ, Rutkowska I, Solarska R,

Augustynski J. Metal oxide photoanodes for solar hydrogen

production. J Mater Chem 2008;18:2298e303.

[4]   Gan J, Lu X, Tong Y. Towards highly efficient photoanodes:

boosting sunlight-driven semiconductor nanomaterials for

water oxidation. Nanoscale 2014;6:7142e64.

[5]   Zhu T, Chong MN, Chan ES. Nanostructured tungsten

trioxide thin films synthesized for photoelectrocatalyticwater oxidation: a review. Chem Sus Chem 2014;7:2974e97.

[6]   Janaky C, Rajeshwar K, de Tacconi NR, Chanmanee W,

Huda MN. Tungsten-based oxide semiconductors for solar

hydrogen generation. Catal Today 2013;199:53e64.

[7]   Liu X, Wang F, Wang Q. Nanostructure-based WO3photoanodes for photoelectrochemical water splitting. Phys

Chem Chem Phys 2012;14:7894e911.

[8]   Li Z, Luo W, Zhang M, Feng J, Zou Z.

Photoelectrochemical cells for solar hydrogen production:

current state of promising photoelectrodes, methods to

improve their properties, and outlook. Energy Environ Sci

2013;6:347e70.

[9]  Li W, Li J, Wang X, Ma J, Chen Q. Photoelectrochemical and

physical properties of WO3 films obtained by the polymeric

precursor method. Int J Hydrogen Energy 2010;35:13137e

45.[10]   Jiao Z, Wang J, Ke L, Sun XW, Demir HV. Morphology-tailored

synthesis of tungsten trioxide (Hydrate) thin films and their

photocatalytic properties. Appl Mater Interfaces

2011;3:229e36.

[11]  Cristino V, Caramori S, Argazzi R, Meda L, Marra GL,

Bignozzi CA. Efficient photoelectrochemical water splitting 

by anodically grown WO3  electrodes. Langmuir

2011;27:7276e84.

[12]   Lianos P. Production of electricity and hydrogen by

photocatalytic degradation of organic wastes in a

photoelectrochemical cell the concept of the PhotoFuelCell: a

review of a re-emerging research field. J Hazard Mater

2011;185:575e90.

[13]   Michal R, Sfaelou S, Lianos P. Photocatalysis for renewable

energy production using PhotoFuelCells. Molecules

2014;19:19732e50.

[14]   Antoniadou M, Lianos P. Near ultraviolet and visible lightphotoelectrochemical degradation of organic substances

producing electricity and hydrogen. J Photochem Photobiol

2009;204:69e74.

[15]   Murau PC. Dissolution of tungsten by hydrogen peroxide.

Anal Chem 1961;33:1125e6.

[16]   Orel B, Krasovec UO, Groselj N, Kosec M, Drazi G, Reisfeld R.

Gasochromic behavior of sol-gel derived Pd doped

peroxopolytungstic acid (W-PTA) nano-composite films. J

Sol-Gel Sci Technol 1999;14:291e308.

[17]   Kudo T. A new heteropolyacid with carbon as a heteroatom

in a Keggin-like structure. Nature 1984;312:537e8.

[18]  Morisson SR, Freund T. Chemical role of holes and electrons

in ZnO photocatalysis. J Chem Phys 1967;47:1543e51.

[19]  Maeda Y, Fujishima A, Honda K. The investigation of current

doubling reactions on semiconductor photoelectrodes bytemperature change measurements. J Electrochem Soc

1981;178:1731e4.

[20]   Ohno T, Izumi S, Fujihara K, Masaki Y, Matsumura M.

Vanishing of current-doubling effect in photooxidation of 2-

propanol on TiO2  in solutions containing Fe(III) ions. J Phys

Chem B 2000;104:6801e3.

[21]   Kalamaras E, Lianos P. Current doubling effect revisited:

current multiplication in a PhotoFuelCell. J Electroanal Chem

2015;751:37e42.

[22]   Pop L-C, Sfaelou S, Lianos P. Cation adsorption by

mesoporous titania photoanodes and its effect on the

current-voltage characteristics of photoelectrochemical

cells. Electrochim Acta 2015;156:223e7.

[23]  Kanai F, Kurita S, Sugioka S, Li M, Mita Y. Optical

characteristics of W03 electrochromic cells under heavy Liion injection. J Electrochem Soc 1982;129:2633e5.

[24]   Liu Q, Gu S, Li CM. Electrodeposition of nickel-phosphorus

nanoparticles film as a Janus electrocatalyst for electro-

splitting of water. J Power Sources 2015;299:342e6.

[25]   Carmo M, Fritz DL, Mergel J, Stolten D. A comprehensive

review on PEM water electrolysis. Int J Hydrogen Energy

2013;38:4901e34.

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