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Efficient solar hydrogen production by photocatalytic watersplitting: From fundamental study to pilot demonstration

Dengwei Jing, Liejin Guo*, Liang Zhao, Ximin Zhang, Huan Liu, Mingtao Li, Shaohua Shen,Guanjie Liu, Xiaowei Hu, Xianghui Zhang, Kai Zhang, Lijin Ma, Penghui Guo

State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an 710049, PR China

a r t i c l e i n f o

Article history:

Received 19 November 2009

Received in revised form

6 January 2010

Accepted 9 January 2010

Available online 9 February 2010

Keywords:

Solar hydrogen

Photocatalytic

Energy conversion

Water splitting

* Corresponding author.E-mail address: lj-guo@mail.xjtu.edu.cn (

0360-3199/$ – see front matter ª 2010 Profesdoi:10.1016/j.ijhydene.2010.01.030

a b s t r a c t

Photocatalytic water splitting with solar light is one of the most promising technologies for

solar hydrogen production. From a systematic point of view, whether it is photocatalyst

and reaction system development or the reactor-related design, the essentials could be

summarized as: photon transfer limitations and mass transfer limitations (in the case of

liquid phase reactions). Optimization of these two issues are therefore given special

attention throughout our study. In this review, the state of the art for the research of

photocatalytic hydrogen production, both outcomes and challenges in this field, were

briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst

preparation to reactor configuration and pilot level demonstration, were introduced,

showing the complete process of our effort for this technology to be economic viable in the

near future. Our systematic and continuous study in this field lead to the development of

a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar

rector for the first time. We have demonstrated the feasibility for efficient photocatalytic

hydrogen production under direct solar light. The exiting challenges and difficulties for this

technology to proceed from successful laboratory photocatalysis set-up up to an indus-

trially relevant scale are also proposed. These issues have been the object of our research

and would also be the direction of our study in future.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction purpose, since H2 could be obtained directly from abundant

Conventional energy resources, which are being used to meet

most of the world’s energy requirements, have been depleted

to a great extent. It is therefore necessary to produce an alter-

native fuel that should in principle be pollution-free, storable,

and economical. Hydrogen satisfies the first two conditions,

and research has been focused on fulfilling the third require-

ment in the past decades [1,2]. To be an economical and

sustainable pathway, hydrogen should be manufactured from

a renewable energy source, i.e., solar energy. Photocatalytic

water splitting is the most promising technology for the

L. Guo).sor T. Nejat Veziroglu. Pu

and renewable water and solar light from the process. If

successfully developed with economic viability, this could be

the ultimate technology that could solve both energy and

environmental problems altogether in the future [3–9].

Water splitting using light energy has been studied for

a long time using powder systems since the Honda–Fujishima

effect was reported [10,11]. Much progress has been made in

the past decades. Thermodynamically, water splitting into H2

and O2 is an uphill reaction, accompanied by a large positive

change in the Gibbs free energy (DG ¼ 238 kJ/mol). The effi-

ciency of water splitting is determined by the band gap and

blished by Elsevier Ltd. All rights reserved.

Fig. 1 – Basic principle of overall water splitting on a co-

catalyst-loaded semiconductor.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 77088

band structure of the semiconductor and the electron transfer

process, as shown in Fig. 1 [12,13]. Generally for efficient H2

production using a visible-light-driven semiconductor, the

band gap should be less than 3.0 eV (420 nm), but larger than

1.23 eV, corresponding to the water splitting potential and

a wavelength of ca. 1000 nm. Moreover, the conduction band

(CB) and valence band (VB) levels should satisfy the energy

requirements set by the reduction and oxidation potentials for

H2O, respectively. Band engineering is thus necessary for the

design of semiconductors with these combined properties.

In the past decades, efforts have been made to address the

following two important issues. One issue is the development

of efficient visible-light-driven photocatalyst which has

undergone a rapid progress especially in the past decade. The

other key issue concerns the efficient utilization of the solar

energy itself. Two major drawbacks of solar energy must be

considered: (1) the intermittent and variable manner in which

it arrives at the earth’s surface (2) efficient collection of solar

light on a useful scale. The first drawback can be resolved by

converting solar energy into storable hydrogen energy. For the

second, the solution could be the use of solar concentrator.

The strategies are schematically illustrated in Fig. 2.

As can be found in Fig. 2, whether it is photocatalyst

development or the reactor- and system-related design, the

essentials could be summarized as: photon transfer limitations

and mass transfer limitations (in the case of liquid phase

reactions)[14]. For photon transfer optimization, it concerns

Fig. 2 – Schematic illustration for the process of

photocatalytic water splitting hydrogen production under

solar light considered from a systematic point of view.

the choice of photocatalyst, the reaction media and the reactor

configuration. Here, reaction media is often the aqueous

solution containing various sacrificial agents for the elimina-

tion of photo-generated holes and for further improvement of

photocatalytic efficiency or simultaneous decomposition of

toxic organics. The photocatalytic material should efficiently

absorb photos and separate photo-generated charges. Fast

transportation of the photo-generated carriers must be guar-

anteed to avoid bulk electron/hole recombination. The sepa-

rated electrons and holes act as reducer and oxidizer,

respectively, in the water splitting reaction over semi-

conductors to produce hydrogen and oxygen. In the field of

mass transfer optimization, many reactors and reactor

configurations have been investigated for their use in photo-

catalysis. Gas–liquid two phase and gas–liquid–solid three

phase flow study in various reactor configurations, especially

tubular reactors, are important for mass transfer optimization.

Research on photocatalytic hydrogen production in China

has been initialed in nineties of the last century and we are

among the groups conducting earliest work in the field. In

2003, the project of the Basic Research of Mass Hydrogen

Production Using Solar Energy founded by National Basic

Research Program of China (973 Plan) was initiated by SKMFPE

with the participation of almost all the main teams conduct-

ing the related studies at the time. With the support of 973

Project and other financial support from the government,

SKMFPE has set their research direction to the development of

highly efficient, stable and low-cost visible-light-driven pho-

tocatalyst by various modification methods, such as doping,

sensitization, supporting and coupling methods to extend the

light responsive and performance of the photocatalyst. We

have studied the photocatalytic materials as powders for

photocatalytic reaction and as solid films for photo-

electrochemical hydrogen production as well. Various pho-

tocatalytic reactors and relevant instruments have also been

developed for photocatalytic hydrogen production, photo-

catalyst screening and evaluation, which formed a complete

platform for further in-depth study. In particular, we have

devoted to photocatalytic hydrogen under direct solar light

and have also been successful. A series of significant results

were obtained in the course of our continuous research. With

all these accomplishments, we have been supported by the

new 973 project of China which started in 2009.

In this review, the state of the art for the research of photo-

catalytic hydrogen production, both outcomes and challenges in

the field, were briefly reviewed. Research progress of our lab,

from fundamental study of photocatalyst preparation to the

issues related to reactor configuration and pilot level demon-

stration, were introduced, showing the complete process of our

effort for thistechnologytobeeconomic viable in thenear future.

2. Materials for photocatalytic/photoelectrochemical hydrogen productionunder visible light

2.1. Materials for photocatalytic hydrogen production

To split water using solar energy, semiconductor photo-

catalysts, such as TiO2, SrTiO3, Nb2O5, SiC, CdS GaP [2], etc

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 7 7089

have obtained much attention. Various modification

methods such as doping [4], sensitization [15], and hybrid

composite etc have been attempted. Up to now, over 130

materials and derivatives have been developed to photo-

catalyze the overall water splitting or produce hydrogen/

oxygen in the presence of external redox agents. Combi-

natorial method has been developed that has been

demonstrated as a convenient way for quick selection of

photocatalyst materials [16–18]. We have developed similar

instruments that have been successfully applied in our lab.

In lab-scaled photocatalytic water splitting and hydrogen

production, very high efficiencies were obtained over NiO–

La:NaTaO3 (QE ¼ 56%, pure water, 270 nm) [9], Pt–ZnS (QE ¼90%, aqueous Na2SO3 solution, 313 nm)[19], Cr/Rh-modified

GaN/ZnO (QE ¼ 5.9%, pure water, 420–440 nm) [20], and Pt-

loaded CdS (QE¼ 60%, aqueous Na2S/Na2SO3 solution, 420 nm)

[21]. However, no semiconducting material has been found to

be capable of catalyzing the overall water splitting under

visible-light with a QE larger than the commercial application

limit 30% at 600 nm [22].

It is considered that the low efficiency for the hydrogen

production of semiconductor already with appropriate band

gap is due to the following reasons: 1) quick electron/hole

recombination in the bulk or on the surface of semi-

conductor particles, 2) quick back reaction of oxygen and

hydrogen to form water on the surface of catalyst, and 3)

inability to efficiently utilize visible-light. It was often

observed that photo-generated electrons easily recombine

with holes in the semiconductor. This recombination leads

to the low quantum efficiency of photocatalysis [23]. Noble

metal loading can suppress to some extent the charge

recombination by forming a schottky barrier. More often

various sacrificial reagents such as inorganic salts and

organics were added in the reaction media, effectively

restraining the charge recombination process and improve

quantum efficiency [24]. Separation of hydrogen gas is also

required as oxygen and hydrogen are produced simulta-

neously. This could be achieved by employing

Fig. 3 – The band gap positions for various traditional sem

a photoelectron-chemical system, in which hydrogen and

oxygen are produced at different electrodes.

2.1.1. CdS and CdS-based photocatalystThe most often studied photocatalysts that have suitable band

gaps for photocatalytic hydrogen production are illustrated in

Fig. 3. Among these materials, Pt-loaded CdS photocatalyst is

the earliest and most studied showing high activity for H2

production from aqueous solutions containing S2� and SO32�

ions as sacrificial electron donors, under visible-light irradia-

tion. Sacrificial electron donors that irreversibly consume

photo-generated holes may promote hydrogen evolution. If

the reaction could be turned into a practical application for the

production of hydrogen gas from byproducts such as

hydrogen sulfide and sulfur dioxide, which are emitted in

hydrogenation and flue-gas desulfurization processes at

chemical plants, it would be especially interesting in light of

current energy and environmental concerns [25,26].

It should be noted that CdS is prone to photocorrosion in

the photocatalytic reaction. In order to enhance the activity of

CdS, efforts have been made to combine CdS with other

semiconductors having different band energies (e.g., TiO2/

CdS, ZnO/CdS, ZnS/CdS [27,28], K4Nb6O17/CdS [29] or K2Ti4O9/

CdS composites [30,31]. An alternative approach to enhance

the photoactivity of CdS is to couple CdS with mesoporous

materials to form hybrid or composite photocatalysts. In these

cases, the photo-generated electrons in CdS are able to move

freely into an attached semiconductor or a framework of

porous molecules, while the photo-generated holes are trap-

ped in CdS. Therefore, high charge separation and photo

utilization would be achieved. Efforts have also to be made to

improve the stability of the metal sulfide. We have developed

a novel two-step thermal sulfuration method for the prepa-

ration of highly stable and active CdS. As shown in Fig. 4, the

surface of the CdS photocatalyst was modified with nanostep

structures which resulted in much higher hydrogen produc-

tion rate than CdS prepared by common procedures [32]. The

enrichment of Pt nanoparticles at the nanostep region are

iconductors relative to the redox potential of water.

Fig. 4 – TEM image for CdS photocatalyst prepared by two-step surfuration method and its hydrogen production. The

preparation of the photocatalyst can be found in literature [32].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 77090

considered indispensable for the much enhanced hydrogen

production. The activity of the this photocatalyst was further

improved by coupled with a zirconium titanium phosphate

mesoporous support [33]. It is expected that such composite

photocatalyst would reduce the use of noble metal and

improve that stability and efficiency of the photocatalyst. It is

also worth noting that this strategy has been successfully

applied to other sulfide such as WS2 [34].

2.1.2. Solid solution photocatlystIn recent years, solid solution photocatalysts with controlled

electronic structures has been suggested as a promising

direction. Various solid solutions such as (GaN)(ZnO) [35],

(AgIn)xZn2(1�x)S2 sulfide solution [36] and other oxide solution

[37] have been developed for photocatalytic hydrogen

production in pure water, sulfide and alcohol. ZnS with 3.6 eV-

band gap is a well-known photocatalyst for H2 evolution

though it responds to only UV. It shows high activity without

any assistance of co-catalysts such as Pt. Chen et al. reported

a nanoporous ZnS–In2S3–Ag2S solid solution synthesized by

a facile template-free method that showed high activities for

H2 evolution under visible-light irradiation in the absence of

co-catalysts. The initial rate of photocatalytic hydrogen yield

reached 3.3 mmol h�1 with 0.015 g photocatalyst employed [38].

In view of practical application of photocatalytic hydrogen

production technique, cost reduction of the photocatalyst is

one of the key issues. Thus, active photocatalysts free of noble

metal like Cd1�xZnxS is valuable in this consideration. The

controllable band structure of this solid solution further adds

Fig. 5 – Conduction and valence band potentials of the C

its value for industrial application. A series of Cd1�xZnxS (x¼ 0–

0.92) photocatalysts were prepared by co-precipitation method

and were calcined at 723 K under N2 atmosphere [39]. The band

gap of the photocatalyst can be continuously adjusted by

changing the composition of the solid solution (see Fig. 5). At

the optimal composition, the solid solution showed high

activity toward hydrogen production even in the absence of

noble metal loading. However, Cd1�xZnxS prepared by

conventional co-precipitation method often shows poor crys-

tallinity. Theactivityandstability of theprepared material is far

from being satisfactory for its commercial utilization. Recently,

in our group a series of Cd1�xZnxS solid solution photocatalysts

was prepared by thermal sulfuration of corresponding oxide

precursors [40]. The band gap control of solid solution photo-

catalyst can also be achieved by varying its composition. The

final composition for all the samples prepared by thermal sul-

furation of corresponding mixed precursors is close to their

stoichiometric composition. It is found that Cd0.8Zn0.2S solid

solution with nominal x value of 0.2 showed the highest activity

toward hydrogen production as shown in Fig. 6, the quantum

efficiency achieves 9.6% at 420 nm.

For pure Cd1�xZnxS, it is assumed that the band gap of

Cd1�xZnxS would be quite large when the conduction band of

the solid solution is high enough for efficient hydrogen

production. Doping Ni2þ into Cd1�xZnxS solid solution can tune

its band structures by both solid solution and metal ion doping.

In this case, Ni2þ is expected to form a donor lever above the

valence band of Cd1�xZnxS to reduce its band gap and increase

its visible-light absorption, while still maintaining its high

d1–xZnxS photocatalysts with various Cd/Zn ratios.

Fig. 6 – Hydrogen production for Cd1LxZnxS solid solution

prepared by various methods. a) Cd0.8Zn0.2S–S, prepared by

two-step thermal sulfuration (b) Cd0.8Zn0.2S–C, prepared by

co-precipitation (c) Cd0.8Zn0.2S–N, prepared by two thermal

sulfuration under N2 atmosphere.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 7 7091

conduction band. In our study it was found that 0.1 wt% Ni2þ

doped Cd0.1Zn0.9S photocatalyst showed the highest activity

with the apparent quantum yield of 15.9% at 420 nm [41]. Band

structure of the Cd1�xZnxS solid solution was further modified

with Cu doping and high efficiency is also obtained [42].

2.1.3. Formation of hybrid or composite photocatalystCoupling of two photocatalyst has been considered effective

for improvement of photocatalytic efficiency. To extend the

light absorption of such wide band gap semiconductors as

TiO2 and Ta2O5, it is doped with cationic and anionic ions [43–

45]. In our study, nitrogen doped TiO2 was coupled with WO3

and after loaded with noble metal, high efficiency was

obtained [43]. CdS nanocrystallites have been successfully

incorporated into the mesopores of Ti-MCM-41 by a two-step

method involving ion-exchange and sulfuration forming

a CdS@Ti-MCM-41 composite photocatalyst. Owning to the

quantum confinement effect and efficient charge separation,

the activity of CdS photocatalyst has been greatly improved

[46,47]. The activity of CdS@Ti-MCM-41 was much improved

by loading Pt co-catalyst. This demonstrates that Ti-MCM-41

serves as a stable host to protect the loaded CdS particles from

photocorrosion. In our another study, CdS nanoparticles were

decorated on Na2Ti2O4(OH)2 nanotubes through partial ion-

exchange method [48]. The results showed that The high

activity of the prepared composite photocatalyst can be

attributed to the enhanced charge separation due to the one-

dimensional nanotube structure of the Na2Ti2O4(OH)2. This

further suggest that materials with special morphology or

structure are favorable for enhanced photo utilization.

2.1.4. Other novel photocatalyst developedRecently, mesoporous silicate materials involving transition-

metal ions within the mesoporous framework have opened

new possibilities in many research areas not only for catalysis

but also for various photochemical processes, such as photo-

catalytic degradation of organic pollutant. In our study,

molecular zeolite such as MCM-41 and SBA-15 was decorated

with transitional metals such as Cr,V,Ti etc to extend their

absorption [49,50]. Multi-component sulfide photocatalyst

ZnIn2S4 is the only member of the AB2X4 family semi-

conductor with a layered structure. In addition, the structure

and morphology of ZnIn2S4 could be controlled by the various

surfactants and solvents added in the hydrothermal condi-

tion. Therefore, the authors’ group conducted a systematic

investigation on the effects of solvents, surfactants and

hydrothermal time [51–53] on the photocatalytic activity of

ZnIn2S4 for hydrogen evolution under visible-light irradiation.

It was found that the assistance of CTAB not only greatly

enhanced the photocatalytic activity but also strongly affected

the crystal structure of ZnIn2S4 compared to the other

surfactant-assisted ZnIn2S4 photocatalysts. Such correlation

between the photocatalytic activity and the structure distor-

tion has so far been reported for some metal oxide photo-

catalysts. In our further research on ZnIn2S4, a series of Cu-

doped ZnIn2S4 photocatalysts was synthesized by a facile

hydrothermal method, with the copper concentration up to

2.0 wt% [54].

Most recently, Domen et al. showed [55] that an abundant

material, polymeric carbon nitride, can produce hydrogen

from water under visible-light irradiation in the presence of

a sacrificial donor. Contrary to other conducting polymer

semiconductors, carbon nitride is chemically and thermally

stable and does not rely on complicated manufacturing

device. The results represent an important step towards

photosynthesis in general where artificial conjugated polymer

semiconductors can be used as energy transducers. In another

interesting study, Demuth et al. found TiSi2 can be used for

overall water splitting with simultaneous hydrogen and

oxygen production. It is also worth noting that this material

has the ability for hydrogen storage [56]. All the above finding

indicates that both traditional semiconductor material with

improved properties and semiconductor of new compositions

are promising candidate materials for future application.

Nevertheless, stability and cost is still the priority for the

choice and design of the new photocatalyst from a practical

point of view.

2.2. Materials for photoelectrochemical hydrogenproduction

For photoelectrochemical decomposition of water which

takes place in photoelectrochemical cells (PECs), hydrogen

and oxygen are separately generated on the surface of photo

cathode and photoanode. During the past decades many oxide

photoelectrodes such as TiO2, WO3, and SrTiO3 have been

extensively studied for hydrogen production [57–59].

However, due to their wide band gaps, these oxides can

respond only to ultraviolet (UV) light. Materials that have

visible-light response and can be readily prepared as film form

are considered. This field is also the interest of our team. We

build up an automated photocurrent spectroscopy system to

evaluate the response or the material to different wavelength

of light. The wavelength region of solar spectrum contributing

to the water splitting reaction can be determined. Systematic

Fig. 7 – Instruments for ultrasonic spray pyrolysis (USP)

film preparation developed in our Lab.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 77092

study of the intensity modulated photocurrent spectroscopy

system (IMPS) and the intensity modulated photovoltage

spectroscopy system (IMVS) have also been carried out.

Various semiconductor photoelectrodes, especially in thin

film form, have been investigated. Traditional TiO2 photo-

electrode only respond to UV light which occupies only 4% of

the whole solar energy. Development of new PEC materials or

the new preparation method for high quality film is thus of

importance. We have designed an ultrasonic spray pyrolysis

(USP) instrument for film preparation, as shown in Fig. 7.

ZnIn2S4, BiVO4 and WO3 films with different crystal structure

were deposited by USP. For BiVO4 film, the material with

different Bi/V ratio in each deposition precursor has been

studied to explore their potential application in hydrogen

production by photoelectrochemical water splitting. The effect

of Bi/V ratio on structure and photoelectrochemical properties

were also studied which showed that USP is a reliable instru-

ment for the preparation of high quality film. ZnIn2S4 thin films

can be prepared on ITO conductive glass by spray pyrolysis

from a mixed aqueous solution [60]. The deposited film is

endowed with a cubic spinel structure with strong absorption

in visible-light. As a photoanode, the deposited film also

exhibits a good photo response. In 0.1 mol/L Na2SO3þ 0.1 mol/L

Na2S mixed solution, the IPCE amounts to over 30% at

a potential of 0.3 V vs. SCE in 400 nm irradiation wavelength.

WO3 films were also prepared by us with ultrasonic spray

pyrolysis using precursor obtained by dissolving tungsten acid

in hydrogen peroxide aqueous solution in 363K water bath. The

effect on the structure and photoelectrochemical properties of

WO3 films by varying amount of hydrogen peroxide added and

concentration of precursor was investigated.

As mentioned above, semiconductor with small band gap

can absorb more light and excite more electron-hole pairs, but

if these excited electron-hole pairs are not separated and

transport to anode or cathode in time, recombination will

occur subsequently. Along with reducing the band gap, elec-

tron separation and transport improvements are also impor-

tant ways to improve conversion efficiency. One dimensional

Nano-structure such as nanotube, nano-wire and nano-

column arrays are excellent electron percolation pathways for

charge transfer and offer a large internal surface area. We

proposed that low band gap semiconductor films with nano-

structure can be a promising approach to efficient photo-

electrochemical water splitting.

3. Instruments and reactors forphotocatalytic hydrogen production

3.1. General introduction

Our group has also developed series instruments for photo-

catalytic hydrogen production, photocatalyst screening and

activity evaluation. We have developed small closed circula-

tion reactor, photocatalytic hydrogen production reactor with

simulated solar light and direct solar photocatalytic hydrogen

production reactor with compound parabolic collector (CPC).

These reactors ensure the evaluation of the developed pho-

tocatalyst from lab scale to out-door demonstration scale.

As for material development, a system for quick prepara-

tion and selection of photocatalyst has been designed in our

group, based on the development of a novel hydrogen gas

sensor that can quantitatively determines the hydrogen

concentration in a precise way. On the other hand, a novel

multi-channel photocatalyst evaluation system has also been

developed that can simultaneously evaluate six groups of

photocatalysts in a precise, convenient and in-situ manner.

To the best of our knowledge, no similar system has been

reported or patented in other groups. The set-up of all these

photocatalyst evaluation system provided a powerful support

for the relative research in SKMFPE and lead to the finding of

many active photocatalysts.

As for the new reaction system, most traditional rector can

only operate in a batch mode. And the sacrificial agent is often

irreversibly consumed. We have designed a double bed pho-

tocatalytic system, with photocatalytic hydrogen production

occurs on one bed and the sacrificial agent re-generated on

another bed. This design leads to the formation of a contin-

uous reaction system with very stable hydrogen production

rate. For the realization of mass production of hydrogen,

development of efficient solar light concentrator is one of the

key issues in regard that solar light is diffuse and has low

energy intensity. We have selected (CPC) to construct

sunlight-driven hydrogen production system (Fig. 8).

Sustainable solar light water splitting hydrogen production on

a relatively lager scale has been realized by coupling CPC with

an inner-circulated reactor. In the following part, our

consideration for the design of solar photocatalytic hydrogen

production reactor and the preliminary results will be briefly

introduced [61].

3.2. Design of solar photocatalytic hydrogen productionreactor

Although various solar reactors have been developed, it is

surprising that most of them aim only at photocatalytic

detoxification [62–64]. To the best of our knowledge no solar

reactor designed for photocatalytic hydrogen production have

been reported in literatures.

The object of our work is to explore the possibility of mass

solar hydrogen production by coupling photocatalytic reactors

with solar light concentrators by fully considering the similarity

and dissimilarity between photocatalytic hydrogen production

and photocatalytic detoxification process, both based on the

existing technologies, literatures and on our theoretical and

Fig. 8 – Photocatalytic hydrogen production reactors developed in our Lab. a) small closed circulation reactor; b) photocatalytic

hydrogen production reactor with simulated solar light; c) direct solar photocatalytic hydrogen production reactor.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 7 7093

experimental study. Following this consideration, special

requirement for photocatalytic hydrogen production process

must be addressed for the design of an efficient solar photo-

catalytic hydrogen production reactor (SPHR).

An East–West alignment with small half acceptance angle

has been chosen. For the maximum acceptance of solar radi-

ation, the aperture of the CPC should be perpendicular to the

incident light as far as possible. Thus the CPC aperture should

be tilted at the angle close to the latitude of the local site facing

south, to maximize the available solar irradiation. In our case

tilt of 35�degrees was chosen which is the latitude of Xi’an city.

It is necessary that uniform flow be maintained at all times in

the tubular reactor, since non-uniform flows causes non-

uniform residence times that can lower efficiency compared to

the ideal conditions [65]. In the case of the heterogeneous

process with photocatalyst powder in suspension, sedimen-

tation and depositing of the catalyst along the hydraulic circuit

should be avoided and turbulent flow in the reactor must be

guaranteed. As has been demonstrated [66] Reynolds’s number

varying between 10 000–50 000 ensures fully turbulent flow and

avoids the settlement of TiO2 particles in the tubes. This choice

of Reynolds’s number can thus be extended to our design.

Furthermore, every photoreactor design must guarantee that

all the useful incoming photons are used and do not escape

without having intercepted a particle in the reactor. For these

reasons, and also from a practical point of view, diameters of

less than 12.5 mm are not feasible.

3.3. Preliminary results for the designed SPHR

The prototype SPHR includes the following components: solar

collector, Pyrex photoreactor tubes, reflective surface, flow

meter; fitting, pipes and tanks; pump and sensors (pH, temper-

ature, oxygen and pyranometer). The adopted CPC parameters

in our case are: the maximum half incident angle for CPC is 14�,

for cost reduction, the half acceptance after truncation 30�.

Three consecutive clear days form June 16–18 2006 in Xi’an city

was chosen for the test of the optical properties of the designed

CPC. For the case of our local site it is found that the solar radi-

ation shows an initial increase from 12:00 to 13:00 where it

reached a maximum of 527 W/m2. For our CPC based solar

reactor, the concentration factor at this point was determined to

be 4.76. Thereafter, the solar radiation undergoes a rapid

decrease to 369 W/m2 at 15:30, where the concentration is also

the smallest. Our results indicated that for a CPC based solar

reactor to function efficiently, such an operation site is neces-

sary where strong solar radiation are available.

For photocatalytic hydrogen production, total volume of

water in our SPHR is 11.4 L. To optimize the design of the

SPHR, such parameters as tube radius, flow velocity, photo-

catalyst and sacrificial agent concentrations, were investi-

gated. In all these tests, water used is distilled water avoiding

the possible influence from the inorganic salts on the photo-

catalytic reaction. As shown in Table 1, the maximum

hydrogen production rate amounted to 1.88 L/h for our

designed SPHR at optimum conditions of case 7, correspond-

ing to a hydrogen production rate of 0.164 L/h per unit volume

of reaction solution. While for lab scale photocatalytic

hydrogen production under visible-light irradiation (lS430), it

is 0.126 L/h. The higher hydrogen production rate per unit

volume may be attributed to the design of tubular reactor well

illuminated by CPC on one hand. This combination enables

solar rays to illuminate the complete perimeter of the round

receiver, rather than just the ‘‘front’’ of it, which may greatly

Table 1 – Investigation of various parameters for SPHR.

No. Tuberadius(mm)

Flowvelocity(cm/s)

CatalystConcentration

(g/L)

SacrificialAgent(mol/L)

HydrogenproductionRate (L/h)

Solarintensity

W/m2

h%

1 11 11.7 0.5 0.05,0.06 0.45 453 0.12

2 11 15.4 0.5 0.05,0.06 0.71 473 0.18

3 11 20.3 0.5 0.05,0.06 0.94 419 0.27

4 11 25.6 0.5 0.05,0.06 1.50 518 0.35

5 11 25.6 0.8 0.05,0.06 1.71 491 0.42

6 11 25.6 1.0 0.05,0.06 1.49 410 0.44

7 11 25.6 1.0 0.1,0.1 1.88 490 0.47

8 15 15.4 0.5 0.05,0.06 0.29 447 0.08

9 15 25.6 1.0 0.05,0.06 0.86 491 0.21

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 77094

reduce the available solar rays. On the other hand, efficient

utilization of UV part of solar light by CPC may be another

important reason. As is discussed previously, though having

limited number, photos in UV range are much more efficient

in driving the photocatalytic hydrogen production.

4. Numerical investigations of catalyst–liquid slurry flow in the photocatalytic reactor

To investigate the operation of a photocatalytic reactor with

high photocatalytic efficiency, research on photocatalyst, ray

transfer, local absorption of photons has been carried out

frequently [67,68] However, up to now almost all the relevant

investigations have ignored the influence of different flow

regions or catalyst distributions on the photocatalysis in the

reactor. In fact, the distribution characteristics of catalyst

particle determine the radiation distribution and photon

absorption in the reactor. So the investigations on the

Fig. 9 – Catalyst volume fraction distribution contours and profi

volume fraction.

catalyst-liquid two-phase flow in the reactor play an impor-

tant part in the reactor design and its operation.

The research on the solid–liquid two-phase flow has been

carried out widely including pressure drop, flow region,

particle deposition velocity, which have been obtained by

experiments and theoretical analysis [69,70]. With the devel-

opment of numerical simulation, relevant prediction models

have also been established. For example, the particle trajec-

tory and collisions in two-phase flow has been simulated by

Lagrangian method. And the Eulerian method based on

kinetic theory has been widely used in the conditions of dense

particles. In our study, we focused on the effects of solid–

liquid two-phase flow characteristics on the photocatalytic

process during hydrogen production by numerical simulation,

especially the effects of catalyst distribution coupling with ray

transfer. Considering the characteristics of catalyst–liquid

slurry flow, an algebraic slip mixture (ASM) model was

selected in our study.

Slurry pressure gradient is an important parameter in

practice, reflecting the operation conditions of the reactor.

les under different mean slurry velocities with catalyst

Fig. 10 – Slip velocity distributions along the vertical center

line of the cross section under different catalyst volume

fractions with mean slurry velocity 1.5 m/s.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 7 7095

According to Wasp et al. [71], the pressure drop in solid–liquid

two-phase flow included two parts: pressure drop due to

vehicle (homogeneous distribution) and excess pressure drop

due to bed formation (heterogeneous distribution). Our results

indicate that the mean slurry pressure gradient increases with

the mean slurry velocity, and the mean slurry pressure

gradient also increases with the catalyst volume fraction [72].

Fig. 9 shows catalyst volume fraction distributions on the

outlet of reactor pipe and its vertical center line respectively,

under different mean slurry velocities with catalyst volume

fraction 0.10. Due to the density difference, some catalyst

particles begin to deposit, causing a higher volume fraction of

catalyst at the bottom. And when the mean slurry velocity

increases, the variation gradient of catalyst volume fraction

on the outlet cross section and its center line will be smaller,

which means the catalyst distribution is more homogeneous.

It can be found that the catalyst distribution is asymmetric

due to the deposit along the vertical direction. The volume

fraction near the bottom is larger than that of the top. Due to

higher concentration at the bottom, the catalyst particles may

contact or assemble, causing less effective surface area where

photocatalysis reaction occurs. That is not favorable for the

photocatalysis in the reactor. Slip velocity is the velocity

difference between solid phase and liquid phase, which is

determined by catalyst volume fraction and particle diameter.

The slip velocity between phases along the vertical center line

of the cross section under different catalyst volume fractions

is shown in Fig. 10. The slip velocity is much smaller than

mean slurry velocity. And the slip velocity near the bottom is

smaller than that in the other area relatively. And with the

increase of catalyst volume fraction, the slip velocities

become smaller. The effects of slurry velocity on slip velocity

have also investigated. Theoretical analysis also indicates the

slip velocity along the vertical center line of the cross section

under different mean slurry velocities. The slip velocities

under different mean slurry velocities are almost in the same

magnitude. The slip velocity with higher mean slurry velocity

is a litter larger.

5. Conclusion and remarks

The state of the art for the research of photocatalytic

hydrogen production, both outcomes and challenges in this

field, were briefly reviewed. Research progress in our lab, from

fundamental study of photocatalyst preparation to the issues

in reactor configuration and pilot level demonstration, were

introduced, showing the complete process of our effort for this

technology to be economic viable in the near future.

As the quantum efficiency obtained over the present pho-

tocatalysts is not yet satisfactory for practical application,

more efficient visible-light-driven photocatalysts should be

developed. Thus, it is necessary to narrow the band gap of

photocatalysts to harvestvisible-light in the longer wavelength

region and enhance the photo-generated charge separation in

photocatalysis. High-efficiency and cost-effective water split-

ting systems based on these photocatalyst should also be

constructed. The factors such as electronic properties, chem-

ical composition, structure and crystallinity, surface states and

morphology, determining the photocatalytic activity of mate-

rials have to be further elucidated in sufficient detail. Lower

cost alternative co-catalysts, such as non-noble metals and

derived metal compounds, should be tested for possible

substitution for the most frequently used noble metals such as

Pt, which is very efficient but expensive. Photocatalyst free of of

noble metal is also highly preferred considering that such

photocatalyst can be readily used in a more economic way.

Additionally, new insights into the water splitting mecha-

nism are needed, particularly with regards to identification of

thermodynamic and kinetic bottlenecks, in order to facilitate

design of the most effective photocatalytic water splitting

systems. On the other hand, in order to achieve enhanced and

sustainable hydrogen production, continual addition of electron

donors is required to make up half of the water splitting reaction

to reduce H2O to H2, as sacrificial electron donors can irreversibly

consume photo-generated holes to prohibit charge recombina-

tion. Takinginto account the lowering cost for solar-to-H2 energy

conversion, pollutant byproducts from industries and low-cost

renewable biomass from animals or plants are preferred to be

used as sacrificial electron donors in water splitting systems.

Our systematic study and accomplishments lead to the

development of a Compound Parabolic Concentrator (CPC)

based photocatalytic hydrogen production solar rector for the

first time. We have demonstrated the feasibility for efficient

photocatalytic hydrogen production under direct solar light. It

is anticipated that this demonstration of concentrator-based

solar photocatalytic hydrogen production would draw atten-

tion for further studies in this promising direction. It is also

considered that the current lack of industrial applications of

this technology is mainly due to two reasons: the low photo-

catalytic efficiency, and related to that the lack of agreement

on how to quantify this efficiency, in particular with respect to

the photocatalyst preparation and reactor configuration;

Nevertheless, both for material and reactor design, reduction

of cost have to be given special priority. The other challenge is

the lack of examples where the successful laboratory photo-

catalysis set-up has been scaled up to an industrially relevant

scale. These two issues have been the object of our research

and would also be the direction of our study in future.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 0 8 7 – 7 0 9 77096

Acknowledgement

The work was financially supported by the National Natural

Science Foundation of China (Contracted No. 90610022,

50821064) and the National Basic Research Program of China

(Contracted No. 2009CB220000).

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