Open Journal of Physical Chemistry,

Open Journal of Physical Chemistry, 2013, 3, 97-102
http://dx.doi.org/10.4236/ojpc.2013.32012 Published Online May 2013 (http://www.scirp.org/journal/ojpc)
1,4-Hydroquinone is a Hydrogen Reservoir for Fuel Cells and Recyclable via Photocatalytic Water Splitting
Thorsten Wilke, Michael Schneider, Karl Kleinermanns*
Institute of Physical Chemistry, Heinrich-Heine-University, Duesseldorf, Germany
Email: *[e-mail]
Received February 14, 2013; revised March 20, 2013; accepted April 20, 2013
Copyright © 2013 Thorsten Wilke et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Photocatalytic splitting of water was carried out in a two-phase system. Nanocrystalline titanium dioxide was used as photocatalyst and potassium hexacyanoferrate(III)/(II) as electron transporter. Generated hydrogen was chemically stored by use of a 1,4-benzoquinone/1,4-hydroquinone system, which was used as a recyclable fuel in a commercialised direct methanol fuel cell (DMFC). The electrical output of the cell was about half compared to methanol. The conver-sion process for water splitting and recombination in a fuel cell was monitored by UV-Vis spectroscopy and compared to a simulated spectrum. Products of side reactions, which lead to a decrease of the overall efficiency, were identified based on UV-Vis investigations. A proof of principle for the use of quinoide systems as a recyclable hydrogen storage system in a photocatalytic water splitting and fuel cell cyclic process was given.
Keywords: Fuel Cell; DMFC; Water Splitting; Recyclable Fue; TiO2; Chemical Hydrogen Storage; Quinones
1. Introduction
By the year 2050 the total energy consumption is expe- cted to double, as the world’s population is steadily in-creasing. The fossil fuels are not able to meet this energy demand in the long term. Therefore, renewable energy resources will come more sharply into focus. The most promising alternative is solar light, because the amount of energy that arrives on earth every hour from the sun is greater than the amount that is required by the entire hu- manity in one year [1]. Yet, there is no practical way to transform and store this huge amount of energy effi- ciently, because the widely used silicon solar cells are of limited use due to their high production costs. Therefore, it is necessary to look for less expensive and in sufficient quantities available alternatives to high-purity silicon. Storage of solar energy is possible for example by bat- teries and capacitors, but, compared to chemical bonds, these storage systems feature a low energy density. In this regard hydrogen is a good energy reservoir. It can be used as fuel for vehicles or can be converted into electri- cal energy by the use of fuel cells. The generation of hy- drogen by electrolysis requires electrical energy, which could be obtained by use of solar cells, but the effective- ness is just approximately 8% for large-scale facilities [2]. Thus, direct photolytic water splitting by the use of suit- able and inexpensive nanocrystalline semiconductors would be a promising alternative. Here the water is split with high efficiency by solar light [1,3,4].
The semiconductor titanium dioxide has a band gap of about 3.1 eV and its conduction band potential is high enough for water splitting [5]. By absorption of photons electrons can be promoted to the excited state and elec-tron-hole pairs (e− + h+) are generated, which diffuse separately on the surface of the TiO2 particles:
2TiOehh  (1)
The formed holes in the valence band are able to oxi-dize molecules, for example water: (2)
The electrons in the conduction band can reduce H+ to hydrogen as the reduction potential of TiO2 is suffi-ciently negative (−0.65 V [6]):
22H2eH (3)
For water splitting by TiO2 we finally obtain the fol-lowing overall reaction [7]: 21HO2OH2h (4)
Depending on the particle size, TiO2 nanoparticles
*Corresponding author.
Copyright © 2013 SciRes. OJPC
98 T. WILKE ET AL.
[8]. Larger particles show smaller band gaps, thus the absorption is red-shifted. For an efficient and safe
ed hydrogen quinoid systems are suitable. They mi- mic natural processes, e.g., photosynthesis, which also use quinoid systems like plastoquinone for hydrogen transfer [7]. Substituted
nzoquinone (DDQ) are known from literature as good hydrogen acceptors [9] and were already investigated by our group in the past [4]. 1,4-benzoquinone is less effi-cient compared to substituted quinoides, but in contrary to DDQ it can be converted in a direct methanol fuel cell (DMFC), because of its relatively good resistance to wa- ter. Benzoquinone and hydroquinone are well distin-guishable by UV-VIS spectroscopy allowing rather easy quantitative analysis. Our group presente
stem for splitting water by use of semiconductor nano- particles [4], which is a further development of an ex- perimental setup introduced by Matsumura et al. in 1999 [9]. The experimental setup consists of a two-phase sys- tem. The photocatalytic water splitting takes place in the aqueous phase containing the photocatalyst and the elec- tron transporter. It is covered with a solution of quinone in n-butyronitrile, which forms the organic layer and ser- ves as hydrogen storage system. Hydroquinone is formed by reduct
d acceptance of two protons. BQ2e2H
HQ rm quinhydrone
mplexes [BQ·HQ]. BQ
HQ[BQHQ] charge-transfer,
hich is moderately soluble in n-butyronitrile. Dissolved quinhydrone undergoes a consecutive reduction to hy- droquinone.
[BQHQ]2e2H2HQ n be used as
el for fuel cells. Hydroquinone is converted to benzo- quinone in an air-breathing fuel cell normally used with methanol. Benzoquinone is converted back to hydro- quinone by photocatalytic water splitting. Commercialised direct methanol fuel cells (DMFC) consist of a polymer electrolyte ion exchange membrane embedded between the anode and the cathode. Both electrodes are composed of three layers: a catalytic, diffusion and a backing layer, mostly based on Pt or Pt Ru as catalyst. For a successful transport of oxygen to the surface of the catalyst, a mix- ture of carbon and polytetrafluoroethylene is used as dif- fusion layer [10]. In air-breathing DMFCs atmospheric oxygen is used without active blowing components by diffusion through open holes of the cathode [11]. The following platinum catalysed reactions take place [12-14] anode:
32CHOH
2HOCO6H6e
221.5O3HO
6H6e
32CHOH1.5O
2CO2HO it voltage is al-
ays lower than the theoretical value, because of over- potential effects at both electrodes. The electrooxidation of hydroqui
ribed in similar way [14] anode:
2HQ2
BQ4H4e 
22O2HO
4H4e
22HQ0.5OBQ
HO ects the observed open circuit
oltage is lower than the theoretical value.
Titanium nanopowd
1,4-benzoquinone (99.5%, Aldrich), potassium hexa- cyanoferrate(III) (99%, Aldrich), n-butyronitrile (purum, ≥99.0%, Fluka) and 1,4-hydroquinone (≥99%,
Aldrich) were used without purification or other treat-ment.
The water splitting experiments were c
quartz cuvette of 45 × 12.5 × 12.5 mm3 size. 30 mg TiO2 nanopowder was dispersed in 16 mL of an aqueous po- tassium hexacyanoferrate(III) solution (8.0 mM). 2.29 ml of the dispersion was placed in the cuvette and was care- fully covered with 0.71 mL of a 1,4-benzoquinone solu- tion in n-butyronitrile (1.9 mM) to form the organic phase. In order to avoid an evaporation of the organic solvent, the cuvette was sealed with a PTFE plug. To avert heat- ing of the reaction system during irradiation and to re- duce diffusion of BQ into the aqueous phase, the vessel was cooled to 15°C by a water flushed aluminium block, connected to the cuvette by a heat-conductive paste. The irradiation was carried out for 90 minutes b
W mercury-vapor lamp (Oriel, Germany), which was mildly focused to give 100 mW/cm2. To prevent irradiation of the organi
gradation of BQ and HQ an aluminium mask of ade-
Copyright © 2013 SciRes. OJPC
T. WILKE ET AL. 99
quate size was used. To exclude contribu
t due to the process of water splitting, a reference solu- tion of the organic phase was kept in the dark without contact to the aqueous phase during irradiation. After 90 min of irradiation a sample of the o
ase was taken, diluted 1:25 with n-butyronitrile and analysed by UV-Vis spectroscopy.
1,4-Benzoquinone in a Direct MethanFuel Cell (DMFC) reaction was carried o
fuel cell purchased from H-TEC Education GmbH with an electrode area of 4 cm2 and a maximal power output of 20 mW. The cell was operated as an air-breathing fuel cell. Oxygen was obtained from the atmosphere by diffu- sion and convection. To verify the given
methanol (99.9%, Aldrich) in water was used. A 3% by weight solution of 1,4-hydroquinone
r was poured into the cell until the electrode was soaked. The electrical output of the cell was recorded by a VC-840 digital multimeter (Voltcraft, Germany) for 180 minutes reaction time. Afterwards a
ted with water and analysed by UV-Vis spectroscopy. Furthermore, UV-Vis spectra of aqueous solutions o
4-benzoquinone and 1,4-hydroquinone in a 1:3 concen- tration value (BQ:HQ) were taken. A simulated UV-Vis spectrum of a 1:3 mixture of benzoquinone and hydro- quinone in water was obtained by mathematical addition of the recorded spectra.
To verify the given properties of t
transmission electron microscopy (TEM) images were taken. The particle size distribution was investigated by trans-
ission electron microscopy (TEM) measurements, which were performed with a HITACHI TEM 7500 microscope equipped with a Mega View II camera (Soft Imaging Sys-tem) at the Max-Planck institute for coal research (MPI Mülheim a.d. Ruhr). The TEM image of
Figure 1. An average particle size of 21 nm and a homogeneous size distribution have been found. O

النتائج (العربية) 1:

فتح دفتر اليومية الكيمياء الفيزيائية، بحلول عام 2013، 3, 97-102http://dx.doi.org/10.4236/ojpc.2013.32012 المنشورة على الإنترنت مايو 2013 (http://www.scirp.org/journal/ojpc)1، 4-الهيدروكينون هو "خزان الهيدروجين" لخلايا الوقود، وريسيكلابل عن طريق "تقسيم المياه بهوتوكاتاليتيك"تورستن يلك، Michael شنايدر، كارل كلينيرمانس *معهد الكيمياء الفيزيائية، هاينريش هاينه جامعة، دوسلدورف، ألمانياالبريد الإلكتروني: * [e-mail]وتلقى 14 فبراير 2013؛ المنقحة 20 مارس 2013؛ قبول 20 أبريل 2013حقوق الطبع والنشر © 2013 تورستن يلك وآخرون. هذا هو مقال وصول مفتوح وزعت بموجب "الإبداعي لجنة الترخيص الإسناد"، استخدام تصاريح غير مقيد وتوزيعها والاستنساخ في أي وسيط، الذي قدم هو ذكر العمل الأصلي بشكل صحيح.خلاصةPhotocatalytic splitting of water was carried out in a two-phase system. Nanocrystalline titanium dioxide was used as photocatalyst and potassium hexacyanoferrate(III)/(II) as electron transporter. Generated hydrogen was chemically stored by use of a 1,4-benzoquinone/1,4-hydroquinone system, which was used as a recyclable fuel in a commercialised direct methanol fuel cell (DMFC). The electrical output of the cell was about half compared to methanol. The conver-sion process for water splitting and recombination in a fuel cell was monitored by UV-Vis spectroscopy and compared to a simulated spectrum. Products of side reactions, which lead to a decrease of the overall efficiency, were identified based on UV-Vis investigations. A proof of principle for the use of quinoide systems as a recyclable hydrogen storage system in a photocatalytic water splitting and fuel cell cyclic process was given.Keywords: Fuel Cell; DMFC; Water Splitting; Recyclable Fue; TiO2; Chemical Hydrogen Storage; Quinones1. IntroductionBy the year 2050 the total energy consumption is expe- cted to double, as the world’s population is steadily in-creasing. The fossil fuels are not able to meet this energy demand in the long term. Therefore, renewable energy resources will come more sharply into focus. The most promising alternative is solar light, because the amount of energy that arrives on earth every hour from the sun is greater than the amount that is required by the entire hu- manity in one year [1]. Yet, there is no practical way to transform and store this huge amount of energy effi- ciently, because the widely used silicon solar cells are of limited use due to their high production costs. Therefore, it is necessary to look for less expensive and in sufficient quantities available alternatives to high-purity silicon. Storage of solar energy is possible for example by bat- teries and capacitors, but, compared to chemical bonds, these storage systems feature a low energy density. In this regard hydrogen is a good energy reservoir. It can be used as fuel for vehicles or can be converted into electri- cal energy by the use of fuel cells. The generation of hy- drogen by electrolysis requires electrical energy, which could be obtained by use of solar cells, but the effective- ness is just approximately 8% for large-scale facilities [2]. Thus, direct photolytic water splitting by the use of suit- able and inexpensive nanocrystalline semiconductors would be a promising alternative. Here the water is split with high efficiency by solar light [1,3,4].The semiconductor titanium dioxide has a band gap of about 3.1 eV and its conduction band potential is high enough for water splitting [5]. By absorption of photons electrons can be promoted to the excited state and elec-tron-hole pairs (e− + h+) are generated, which diffuse separately on the surface of the TiO2 particles:2TiOehh  (1)The formed holes in the valence band are able to oxi-dize molecules, for example water: (2)The electrons in the conduction band can reduce H+ to hydrogen as the reduction potential of TiO2 is suffi-ciently negative (−0.65 V [6]):22H2eH (3)For water splitting by TiO2 we finally obtain the fol-lowing overall reaction [7]: 21HO2OH2h (4)Depending on the particle size, TiO2 nanoparticles*Corresponding author.Copyright © 2013 SciRes. OJPC98 T. WILKE ET AL.[8]. Larger particles show smaller band gaps, thus the absorption is red-shifted. For an efficient and safeed hydrogen quinoid systems are suitable. They mi- mic natural processes, e.g., photosynthesis, which also use quinoid systems like plastoquinone for hydrogen transfer [7]. Substitutednzoquinone (DDQ) are known from literature as good hydrogen acceptors [9] and were already investigated by our group in the past [4]. 1,4-benzoquinone is less effi-cient compared to substituted quinoides, but in contrary to DDQ it can be converted in a direct methanol fuel cell (DMFC), because of its relatively good resistance to wa- ter. Benzoquinone and hydroquinone are well distin-guishable by UV-VIS spectroscopy allowing rather easy quantitative analysis. Our group presentestem for splitting water by use of semiconductor nano- particles [4], which is a further development of an ex- perimental setup introduced by Matsumura et al. in 1999 [9]. The experimental setup consists of a two-phase sys- tem. The photocatalytic water splitting takes place in the aqueous phase containing the photocatalyst and the elec- tron transporter. It is covered with a solution of quinone in n-butyronitrile, which forms the organic layer and ser- ves as hydrogen storage system. Hydroquinone is formed by reductd acceptance of two protons. BQ2e2HHQ rm quinhydronemplexes [BQ·HQ]. BQHQ[BQHQ] charge-transfer,hich is moderately soluble in n-butyronitrile. Dissolved quinhydrone undergoes a consecutive reduction to hy- droquinone.[BQHQ]2e2H2HQ n be used asel for fuel cells. Hydroquinone is converted to benzo- quinone in an air-breathing fuel cell normally used with methanol. Benzoquinone is converted back to hydro- quinone by photocatalytic water splitting. Commercialised direct methanol fuel cells (DMFC) consist of a polymer electrolyte ion exchange membrane embedded between the anode and the cathode. Both electrodes are composed of three layers: a catalytic, diffusion and a backing layer, mostly based on Pt or Pt Ru as catalyst. For a successful transport of oxygen to the surface of the catalyst, a mix- ture of carbon and polytetrafluoroethylene is used as dif- fusion layer [10]. In air-breathing DMFCs atmospheric oxygen is used without active blowing components by diffusion through open holes of the cathode [11]. The following platinum catalysed reactions take place [12-14] anode:32CHOH
2HOCO6H6e
221.5O3HO
6H6e
32CHOH1.5O
2CO2HO it voltage is al-
ays lower than the theoretical value, because of over- potential effects at both electrodes. The electrooxidation of hydroqui
ribed in similar way [14] anode:
2HQ2
BQ4H4e 
22O2HO
4H4e
22HQ0.5OBQ
HO ects the observed open circuit
oltage is lower than the theoretical value.
Titanium nanopowd
1,4-benzoquinone (99.5%, Aldrich), potassium hexa- cyanoferrate(III) (99%, Aldrich), n-butyronitrile (purum, ≥99.0%, Fluka) and 1,4-hydroquinone (≥99%,
Aldrich) were used without purification or other treat-ment.
The water splitting experiments were c
quartz cuvette of 45 × 12.5 × 12.5 mm3 size. 30 mg TiO2 nanopowder was dispersed in 16 mL of an aqueous po- tassium hexacyanoferrate(III) solution (8.0 mM). 2.29 ml of the dispersion was placed in the cuvette and was care- fully covered with 0.71 mL of a 1,4-benzoquinone solu- tion in n-butyronitrile (1.9 mM) to form the organic phase. In order to avoid an evaporation of the organic solvent, the cuvette was sealed with a PTFE plug. To avert heat- ing of the reaction system during irradiation and to re- duce diffusion of BQ into the aqueous phase, the vessel was cooled to 15°C by a water flushed aluminium block, connected to the cuvette by a heat-conductive paste. The irradiation was carried out for 90 minutes b
W mercury-vapor lamp (Oriel, Germany), which was mildly focused to give 100 mW/cm2. To prevent irradiation of the organi
gradation of BQ and HQ an aluminium mask of ade-
Copyright © 2013 SciRes. OJPC
T. WILKE ET AL. 99
quate size was used. To exclude contribu
t due to the process of water splitting, a reference solu- tion of the organic phase was kept in the dark without contact to the aqueous phase during irradiation. After 90 min of irradiation a sample of the o
ase was taken, diluted 1:25 with n-butyronitrile and analysed by UV-Vis spectroscopy.
1,4-Benzoquinone in a Direct MethanFuel Cell (DMFC) reaction was carried o
fuel cell purchased from H-TEC Education GmbH with an electrode area of 4 cm2 and a maximal power output of 20 mW. The cell was operated as an air-breathing fuel cell. Oxygen was obtained from the atmosphere by diffu- sion and convection. To verify the given
methanol (99.9%, Aldrich) in water was used. A 3% by weight solution of 1,4-hydroquinone
r was poured into the cell until the electrode was soaked. The electrical output of the cell was recorded by a VC-840 digital multimeter (Voltcraft, Germany) for 180 minutes reaction time. Afterwards a
ted with water and analysed by UV-Vis spectroscopy. Furthermore, UV-Vis spectra of aqueous solutions o
4-benzoquinone and 1,4-hydroquinone in a 1:3 concen- tration value (BQ:HQ) were taken. A simulated UV-Vis spectrum of a 1:3 mixture of benzoquinone and hydro- quinone in water was obtained by mathematical addition of the recorded spectra.
To verify the given properties of t
transmission electron microscopy (TEM) images were taken. The particle size distribution was investigated by trans-
ission electron microscopy (TEM) measurements, which were performed with a HITACHI TEM 7500 microscope equipped with a Mega View II camera (Soft Imaging Sys-tem) at the Max-Planck institute for coal research (MPI Mülheim a.d. Ruhr). The TEM image of
Figure 1. An average particle size of 21 nm and a homogeneous size distribution have been found. O

النتائج (العربية) 3:

افتح مجلة الكيمياء الفيزيائية، ٩٧ إلى ١٠٢, 3
http://dx.doi.org/10.4236/ojpc.2013.32012 نشرت على الانترنت مايو 2013 (http: / / www.scirp. org / مجلة / ojpc)
1,4-hydroquinone هو خزان الهيدروجين خلايا الوقود وقابلة لإعادة التدوير عن طريق تقسيم المياه photocatalytic
ترستن يلك، مايكل شنايدر، كارل kleinermanns *
معهد الكيمياء الفيزيائية، جامعة هاينريش هاينه، دوسلدورف، ألمانيا
البريد الالكتروني: * kleinermanns @ جامعة دوسيليدروف. دي
تلقى 14 فبراير 2013، ونقحت في 20 مارس 2013؛ قبول 20 أبريل 2013
حقوق الطبع © 2013 تورستن يلك et al.هذا هو مفتوح وصول المادة توزيعها تحت رخصة المشاع الإبداعي، والذي يسمح للاستخدام دون قيود، والتوزيع، والتكاثر في أي وسيط، بشرط الأصل في العمل بشكل صحيح ذكرت ملخص

photocatalytic تقسيم المياه أجري في نظام من مرحلتين.نانوكريستالي ثاني أكسيد التيتانيوم هو استخدام photocatalyst والبوتاسيوم hexacyanoferrate (III) / (الثاني) كما نقل الإلكترون.توليد الهيدروجين كيميائيا المخزنة باستخدام نظام بارا بنزوكينون 1,4-hydroquinone، الذي كان بمثابة الوقود القابلة للتدوير في تسويقها خلية وقود الميثانول المباشر (dmfc).