An Overview of Photocells and Photoreactors for Photoelectrochemical Water Splitting

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Review

An overview of photocells and photoreactors for

photoelectrochemical water splitting

Lorna Jeffery Minggu a,*, Wan Ramli Wan Daud a,b, Mohammad B. Kassim a,c

a Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, MalaysiabDepartment of Chemical & Process Engineering, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia,

43600 UKM Bangi, Selangor, MalaysiacSchool of Chemical Sciences & Food Technology, Faculty of Science & Technology, Universiti Kebangsaan Malaysia,

43600 UKM Bangi, Selangor, Malaysia

a r t i c l e i n f o

Article history:

Received 5 November 2009

Received in revised form

18 February 2010

Accepted 27 February 2010

Available online 8 April 2010

Keywords:

Photocell

Photoreactor

Photoelectrochemical

Water splitting

Hydrogen

a b s t r a c t

Solar hydrogen production from direct photoelectrochemical (PEC) water splitting is the

ultimate goal for a sustainable, renewable and clean hydrogen economy. While there are

numerous studies on solving the two main photoelectrode (PE) material issues i.e. effi-

ciency and stability, there is no standard photocell or photoreactor used in the study. The

main requirement for the photocell or photoreactor is to allow maximum light to reach the

PE. This paper presents an overview of the PE configurations and the possible photocell and

photoreactor design for hydrogen production by PEC water splitting.

2010 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.

1. Introduction

The interest in extracting hydrogen from water is fueled by the

need to find a renewable, sustainable and environmentally safe

alternative energy source. Hydrogen is considered as a viable

option to todaysfossilfuel based energy source especially when

it is produced from water and only sunlight as the energy input

[1]. Hydrogen is an energy carrier, when used in fuel cell which

combines it electrochemically with oxygen from air thus

producing water and energy in the process[2]. This completes

the consumption and regeneration cycle of hydrogen.

Generation: H2O energy (solar)/H2O2(PEC reactor)

Consumption: H2O2/H2O energy (Fuel cell)

Photoelectrochemical (PEC) water splitting has the poten-

tial to be an efficient and cost effective way to produce

hydrogen where the PE in PEC system absorb sunlight and

split water directly into hydrogen and oxygen. The main work

in PEC water splitting still concern increasing the efficiency

and stability of the photoactive materials [3,4]to achieve the

required efficiency target of 10% that will be viable for

commercialization [5]. At the moment stable PE materials

* Corresponding author. Tel.: 603 89216050; fax: 603 89216024.E-mail address:[email protected](L.J. Minggu).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / h e

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 ) 5 2 3 3 e5 2 4 4

0360-3199/$ e see front matter 2010 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.

doi:10.1016/j.ijhydene.2010.02.133
mailto:[email protected]://www.elsevier.com/locate/hehttp://www.elsevier.com/locate/hemailto:[email protected]


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based on metal oxide have rather low efficiency of several

percent[6e8]while high efficiency materials based on multi-

junction conventional semiconductor can achieve slightly

above 10%[9e11]but degraded within a short time.

There are various types of photocell or photoreactor used

for testing the photoactive materials that forms the PE.

Apparatus for PEC water splitting is essentially a heteroge-

neous photoreactor as the photoactive material is immobi-lized on a substrate formingthe PE. As electrodes are involved,

the apparatus are also known as cell or photocell as

commonly referred to in electrochemistry. Photocell or pho-

toreactor geometry should allow for good exposure to light

such that maximum photons can reach the PE. The irradiation

from the light source is usually normal to the photoreactor

surface[12]. Photoconversion efficiency measurement for PEC

water splitting has been described by [13e15]. Although

a standard photoreactor for studying particulate system

especially with UV light such as in environmental application

is commercially available, photoreactor for heterogeneous

film type PEC water splitting is not readily available. This

paper presents an overview of the commonly used photocellor photoreactor in PEC water splitting study.

1.1. Basic PEC water splitting set up

The basic PEC set up (Fig. 1) consists of two electrodes

immersed in an aqueouselectrolyte contained within a vessel,

where one or both of the electrodes is photoactive [1]. The

vessel containing the aqueous electrolyte is transparent to

light or fitted with an optical window that allow light to reach

the photoactive electrode or also known as photoelectrode

(PE). Water splitting will occur when the energetic require-

ments are met as shown inTable 1[3,9,16] where the practical

potential will be much higher than the minimum required toovercome overpotential and other system losses. In a lab test

situation for solar/photon to current conversion efficiency

measurement, a reference electrode is also used and this

method is commonly referred to as the three electrodescell. In

practical application and also for measurement of true solar to

hydrogen conversion efficiency, a two electrodes system is

used[17].

1.2. Photoelectrode configurations

Semiconductor is the main photoactive material used for the

PE. The semiconductors for PEC water splitting can be gener-

ally classified as metal oxide and conventional photovoltaic

(PV) material. The semiconductor PE can be n-type (Fig. 2a),

p-type (Fig. 2b) or coupling of n-type and p-type (Fig. 2c). This

can be a single photosystem as in the n-type (TiO2) [8] or

p-type (InP)[18], but for the coupled n- & p-types involve two

photosystems (n-GaAs/p-InP)[19e21]. Several n-types can be

layered together so their band gaps cover most part of the

usable solar spectrum or several p-types can also be done the

same way (Fig. 2d). Another way is to combine different layersof n- and p-types as discussed in the later section under

internal biased.

When involving more than one photosystem, it is impor-

tant to match the currents generated by the different layers to

obtain better efficiency and this is achieved by aligning

complimentary band gaps and controlling the thickness or

active area [22]. In PEC water splitting, metal oxide and

conventional PV material or their combination are used. The

anode and cathode are usually physically separated, but can

be combined into a monolithic structure [23] either using

a metal substrate by depositing the anode on one side and

cathode on theother and sealing the edges(Fig. 2e) or stacking

the anode on its own substrate with the cathode on its ownsubstrate and providing an electrical connection between the

two (Fig. 2f).

1.3. Biased and non-biased systems

1.3.1. Zero bias

The ideal PEC hydrogen production from water is the direct

water splitting without any other external energy supply but

the light energy itself where the semiconductor has the right

band gap and band edges to split water (Fig. 1). Therefore no

additional potential is required. However, so far no single

junction (single band gap) semiconductor is able to achieve

this in a satisfactory manner where the efficiency remainsvery low.

1.3.2. Biased systems

The single photosystem configuration (n-type or p-type) is

a single junction or single band gap arrangement and is nor-

mally not efficient due to the inappropriate band gap or non-

matching band edges. Additional voltage or bias, is required

to either increase the rate by reducing electron-hole recom-

bination in the bulk semiconductor when using wide band gap

semiconductor or if it is not energetic enough when the band

edges do not overlap water splitting potential. The bias can be

external or internal as discussed in the following sections. The

external bias will make the system performs better but notFig. 1e

Schematic of PEC water splitting.

Table 1 eMain PEC water splitting requirements

Conditions Main requirements

PEC water splitting H2O(liquid) 2hy/O2(gas)H2(gas)

Minimum potential required EH2O(25 C)min 1.229 eV

Practical potential

(overpotential & losses)

EH2O(25 C)prac 1.6e2.0 eV

Ebandgap>

E

H2O

For efficient utilization

of sunlight

UV> hy(Vis)> IR

hy Ebandgap

Band edges requirement Cbandedge< EH2/HVbandedge> E

O2/H2O



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necessarily increase its solar energy conversion efficiency.

However, internal biasing with properly matched current and

potential will contribute to the solar energy conversion effi-

ciency as they utilized the same solar radiation.

1.3.2.1. Electrical bias (grid) e fossil based. Using electricity

from thegridto bias thePEC water splitting is not an attractive

option because fossil based energy source is commonly used

to generate the electricity (Fig. 3a). Some may come from

sources such as thermal, hydro, wind, nuclear or solar, but

this is usually a very small percentage.

1.3.2.2. Chemical biase pH. Chemical bias is basically using

electrolyte with different pH in the anode and cathode cham-

bers separated by ion exchange membrane (Fig. 3b). Each unit

pH difference between the electrolytes chambers provides

0.06 V[24]. Normally acid electrolyte is used on one side of thechamber and alkaline electrolyte on the other [25e27].

However, the energy source used to manufacture the acid or

alkaliis normallyfossil basedand rawmaterials arerequiredto

produce the acid or alkali. The bias will decrease with progress

of the PEC reaction as H (acid) and OH (alkali) will be

consumedand the pH on both sides tendtowards equilibrium.

Hence, the acid and alkali have to be constantly supplied to

maintain the sufficient bias. This method of providing the

additional voltage is notfavorable becausethe systemrequires

additional input of chemicals besides sunlight.

1.3.2.3. PV cell bias.The solar PV cell is connected directly to

the PEC water splitting device without going to the grid first

(Fig. 3c). The other conventionalsolarhydrogen is using a solar

PV panel converting sunlight into electricity and transferring

this electricityto an electrolyzer to perform water electrolysis.

However, for PV cell biased PEC water splitting, instead ofgoing to an electrolyzer,the PV panel generated current goes to

the PEC cell. The PV cell normally consists of multi-junction

semiconductor layers of various n- and p-combinations.

When the PV cell is illuminated by sunlight, current is gener-

ated by the creation, separation and movement of the

electron-hole pairs (compare to DSSC below). The PEC cell can

potentially have lower cost than an electrolyzer [13,28].ThePV

and PEC cells can be integrated into a single unit as described

in later section under internal biased.

1.3.2.4. Dye sensitized solar cell (DSSC) bias. This is a new

generation of solar cell based on a cheap material TiO2. The

principle is similar to the PV cell biased system (Fig. 3c).Instead of the conventional solar cell, a DSSC is used to

capture sunlight and convert it to current which then flow to

the PEC cell where water splitting occurs [6]. The DSSC itself is

a PEC system because it also involves electrolyte which in this

case can be regenerated. The conventional solar PV cell on the

other hand is a solid state device. In DSSC, TiO2which forms

the photoanode is sensitized by incorporation of dye that acts

as antenna to capture visible light and a Pt coated substrate

forms the cathode. The electrolyte consists of I2/KI redox

couple. Upon irradiation of the DSSC, the dye molecule (D)

becomes excited (D*) and injectselectron into TiO2 conduction

band. The reduced dye (D) is regenerated by oxidizing the I2/

KI electrolyte. The electron from TiO2 conduction band then

Fig. 2 e Types of PE for PEC water splitting (SC-semiconductor; M-metal).

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 ) 5 2 3 3 e5 2 4 4 5235


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flows out to do work and connected to the counter electrode

where the I2/KI electrolyte is reduced. The DSSC is capable of

generating current continuously with light irradiation [29].

The DSSC and PEC cells can also be integrated into a single

unit as described below.

1.3.2.5. Internal bias. The internal bias refers to the biasing

potential produced on the PE itself (Fig. 3d). This is normally

known by its structure as layered or stacked or hybrid which

involves arranging several different semiconductors films on

top of each other such that the total band gap is large enough

for water splitting but also small enough to absorb visible

portion of the light spectrum which has high photonsconcentration. This also allows the arrangement of the

resultant band edges to overlap water splitting redox poten-

tial. The internal biased PE structure can be of several type

such as PV/PEC (PVwa-SiGe, PECwWO3) [30], PV/PV

(PV1wGaInP, PV2wGaAs) [10] and PEC/PEC (PEC1wDSSC,

PEC2wWO3)[31]. These are the general arrangements used for

the PE in the later section under photoreactor. There are

numerous possibilities of combinations to achieve the

appropriate band gap and band edges for direct water splitting

by internal biasing but the drawback is this system can

become rather complex [22]. As mentioned in previous section

it is critical to match the potentialand current for each layer of

the PV/PEC, PV/PV or PEC/PEC structure in order to achieve

improved efficiency as this is based on the total active area of

solar collection. Since these systems are capable of splitting

water directly, therefore internally biased structure is essen-

tially a zero bias system.

1.4. Photoelectrode assembly

In the more basic apparatus, the PE is usually made by

depositing a thin layer of the semiconductor on top of the

conductive side of the substrate. The substrate is normally

Fig. 3 e The different methods of biasing for PEC water splitting.

Fig. 4e

Preparation of PE.



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a transparent conductive glass or metal foil. A small area

usually a strip at the top or side of the conductive substrate is

left uncoated to allow for electrical connection. The ohmic

contact is normally made by attaching copper wire to the

exposed part with silver conductive glue and covering the

connection with epoxy resin to provide insulation and also to

strengthen the connection (Fig. 4). If the substrate is metal,

then the ohmic contact is usually made on the back side. Thecopper wire can be further protectedby inserting it into a glass

tube which also functions as a handle for the PE. All the four

edges and thebackof thePE are then covered with epoxy resin

for insulation [32,33]. A transparent epoxy can be used for

back side illumination with glass substrate provided the epoxy

layer is very thin or only the sides are covered by epoxy

leaving exposed area on the front and back.

The area of the exposed PE which will be in contact with

the electrolyte produced in this way is non-defined. The area

can be estimated using the method by Kelly & Gibson [34],

where a piece of paper with known area and weight is used as

a standard. The surface of the PE is photocopied and the

exposed area is carefully cut along the outline. The weight ofthe cutout of the exposed PE area is compared with the weight

of a standard piece of paper (1 cm 1 cm) and using the

weight/area ratio to get the required area. Where unknown PE

areaweight of standard paper (known standard paper

areaw1 cm2)/(weight of PE cutout). Other method to estimate

the area is by tracing the image of the exposed PE area onto

a predefined grid and then estimates the area by counting the

squares in the grid. The PE produced in this method is

completely immersed in the electrolyte in PEC water splitting

study.

2. Photocell and photoreactor types

The photoreactors reported for PEC water splitting have

various shapes and configurations. The apparatus used range

from simple vessel to more complicated assembly. There are

numerous variations to the PEC water splitting set up reported

in the literature and some of the designs have been patented.

Some articles show just the schematic and others show the

apparatus 3D drawing or actual picture. The actual pictures

are normally only available through their websites. Some of

the more sophisticated apparatus show the arrangement of

the PE and the gases separation system which usually require

electrolyte separation.

2.1. Single chamber vessel

2.1.1. Open vessel

The most basic set up of a photocell for PEC water splitting is

an open vessel that is transparent which allows light to reach

the PE and sufficient to immerse all the electrodes (working

electrode-WE/PE, reference electrode-RE, counter electrode-

CE) (Fig. 5b). The simplest is a square quartz vessel [34]with

a flat surface area as optical window to allow most of the light

spectrum including UV to pass through (Fig. 5a). A flat surface

is important during measurement to avoid distortion of the

impinging light. A simple beaker has also been used to

demonstrate PEC water splitting using a monolithic PE[35,36],

where the curved surface of the beaker provides light focusing

(Fig. 5c). As light is concentrated, more photons will reach the

PE surface for the required reaction. The light focusing effect

can also be achieved by using round or semi-round trans-

parent container[37]. To simulate higher light concentration

in lab setting, a higher powered light source was used to

provide the higher intensity.

2.1.2. With ports

Other vessels have separate ports for the PE, counterelectrode

and reference electrode (Fig. 5d and e). The advantage of

having separate ports is to ensure consistence distance

between the electrodes for each measurement. One example

is a circular disc made from quartz with ports at the top and

on both sides[38,39]and a rectangular vessel [40]which also

allow illumination from front and back surfaces.

2.1.3. With optical window

Quartzvessels arevery expensive andtherefore normally used

in a very small size and thickness, hence it tends to be more

fragile. An alternative to this is to have an opening where a flatquartz piece can be attached. For glass material, the design is

easier using a horizontal cylindrical vessel, with ports at the

top,wherebothoroneendisopentoattachthequartzpiecefor

the optical window. The open end of the glass vessel can be

melted around theedgeto fuse it with thequartz pieceto form

sealing so that theelectrolyte does not leak out [14] (Fig. 6a), or

the opening isin the form ofscrew typewhere the quartzpiece

can be screwon, usually with o-ringto forma leakproof seal. It

is known that quartz and glass does not fused easily.

The vessel can also be made from other inert and more

robust material such as Perspex and Teflon or other plastic

material instead of glass. This allows both cylindrical and

rectangular shapes to be made easily. Optical window isprovided by making an opening on the vessel at the illumi-

nated side. The flat quartz piece can be fitted to the side of the

vessel with the opening by sandwiching it with a holder plate

that holds the quartz with o-ring or silicone gasket to form

a tight seal (Fig. 6b) using nuts and bolts. The top of the

Fig. 5 e (a) Simple square transparent open vessel, (b)

Schematic of PEC water splitting set up, (c) Simple open

beaker, (d) Single chamber circular vessel (adapted from

[39]), (e) Single chamber rectangular vessel (adapted from

[40]).



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Perspex[41,42]or Teflon vessel can have various ports drilled

easily into the vessel for the electrodes and other require-

ments. The PE can be inserted through the port and immersed

in the electrolyte. The vessel can be sealed completely and

have additional outlets for collecting the evolved gases.

However, as the efficiency of most PE material for water

splitting is low, thegas produced is in theorder of several ml/h

for a few cm2 test area.

2.2. Vessel with gas separation

2.2.1. With electrolyte separation

In PEC water splitting system, it is important to separate H2and O2as a mixture of the two gases are potentially explosive.

In a small scale lab photocell this does not pose a significant

concern. However for large scale practical hydrogen produc-

tion, it is essential to separate the gases. Traditionally, for an

electrochemical reaction study with two different electrolytes,

two separate vessels containing the electrolytes are con-

nected by a salt bridge. In a PEC water splitting photoreactor,

this can be made from two separate vessels held together bynuts and bolts with a membrane acting as a separator for the

electrolytes. Proton exchange membrane (Nafion) commonly

used in fuel cell is also used in PEC water splitting study to

separate the anode and cathode compartments [43](Fig. 7a).

The nafion membrane does not allow H2 or O2 gas to crossover

but only allow H to pass through.

In PEC water splitting reaction in acidic medium, ion

conduction is achieved by H movement, therefore it is quite

well suited to use this type of membrane. However, the nafion

membrane is not suitable to be used in electrolyte that

contains cation (eg. Na) as this will replace the H in the

membrane and therefore limit the movement of H across.

Hence, less H

ions are available to be reduced to H2 at

cathode. Since the proton is consumed equimolarly with

electrons at the cathode, this will increase the pH at the

cathode [44]. The membrane divider also allows different

electrolytes to be used for the anode and cathode chambers.

Besides nafion membrane, glass frit and diaphragm can

also be used as the electrolyte separator (Fig. 7b). The photo-

reactor can also be made into a single vessel whereglass frit is

used as a divider to separate the two liquids but still allow ionmovement[14]. As pointed out previously, if the electrolytes

in the two compartments have different pH then this will lead

to potential difference which can contribute to additional

voltage to the system.

2.2.2. H-type

The H-type vessel allows gas separation without a membrane

separator. The construction of the vessel is quite simple in

which two or more vertical glass tubes are connected by

smaller horizontal tube[45e48](Fig. 8). The connection allows

for ion movement and as the electrodes are placed above the

connection, and since the gases evolved will flow upward,

therefore the gases will be separated naturally. Ion permeabledivider such as glass frit can be placed in the connection tube

to allow different electrolytes to be used in the compartments.

However, it is quite difficult to get a flat surface for the optical

window on a tube or attaching a flat quartz piece to the tube.

2.3. Photocell with fixed area photoelectrode

Instead of immersing the PE in the vessel, it can also be placed

outside the vessel where it is fixed to an opening that allows

a certain area of the PE to be in contact with the electrolyte

[49,50]. In a front attachment configuration, thePE can then be

glued to a holder plate that has an opening with a predefinedarea (Fig. 9a). The PE can also be sandwiched between the

holder plate and the vessel (Fig. 9b) with o-ring to provide

sealing. The PE active area for both front attachment and

sandwich assemblies is fixed or defined by the opening of the

holder plate, the o-ring or gasket. The holder plate can be

attached using screws and bolts to the vessel which has an

opening that allow the electrolyte to reach the PE. Care must

be taken when assembling the PE to the vessel to ensure no

electrolyte leak outside the predefined area, as the leak will

increase the actual test area and this leads to higher current

density.

The PE on glass substrate functions as the optical window

if illuminated from the back side. On the other hand,

Fig. 6 e Vessel with optical window. (a) Cylindrical

(adapted from[14]), (b) Cubic (adapted from [41]).

Fig. 7e

Vessel with electrolyte/gas separation. (a) Separate vessels (adapted from[43]), (b) Single vessel (adapted from[14]).



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illumination from the front side through the electrolyte

requires a piece of glass or quartz used as the optical window.

Comparison of the results from the back side and front side

illumination has to be done by considering the distance

between the PE, in both cases, from the electrolyte and the

light source since the front side illumination can produce

a different result due to interference by electrolyte and also

changes in the light intensity.

3. Potential for large scale application

The shape and geometry of PEC water splitting photoreactor

depend much on the PE assembly. The PE is always flat to

ensure maximum exposure to light therefore the simplest

photoreactor design is based on plate-type configuration. For

PE that does not require bias, metal substrate will make

a simpler PE because the counter electrode can be deposited

on the opposite side and form a monolithic device asmentioned in earlier section. Hence, wiring is not required

and the monolithic device can be completely enclosed.

3.1. Photoreactore non-biased monolithic

photoelectrode

The photoreactor is a plate-type rectangular container

enclosing the monolithic PE forming two separate compart-

ments filled with aqueous electrolyte (acid or alkaline) as

demonstrated by Deng and Xu [51] (Fig. 10a). For the PE, onone

side of the metal substrate, multi layers of PV-based semi-

conductor material (double or triple-junction Si) is deposited

and the outer layer that is in contact with the aqueous elec-trolyte is covered by a protective material which is trans-

parent, conductive and corrosion-resistant (doped-metal

oxide or doped-polymer). As mentioned earlier it is important

to match the current and potential of each photoactive layer

in order to achieve higher solar energy conversion efficiency.

On the other side of the metal substrate, hydrogen evolution

catalyst is deposited (carbon-Pt). Other group has developed

monolithic PE that is suitable for use in this set up[22,52]. The

top compartment has an optical window for light to reach the

PE. Here, the placement of the ion exchange membrane is

such that theareathat is exposedto sunlight is utilized for the

photon reaction and not for non-photon related function such

as the membrane. This design allows interconnected

chambers to reduce ohmic losses when the current collection

area becomes large.

3.2. Photoreactore PV-biased monolithic photoelectrode

In another design [53,54], the PV-based semiconductor PE is

totally separated from the aqueous electrolyte. The photo-reactor is also a rectangular shaped plate-type configuration

(Fig. 10b). Stainless steel is used as the substrate and PV

semiconductor material (triple-junction Si) is deposited on

one side and hydrogen evolving catalyst is deposited on the

other (CoMo). The electrons generated from the PV-based PE

move through the stainless steel substrate to reach the elec-

trolyte to reduce H to H2and the holes are collected on the

surface of the PV by interconnect and joined to the other

substrate deposited with oxygen evolving catalyst (Fe:NiOx)

where H2O is oxidized to O2. The membrane for the gas

separation is placed in the middle on a porous support

between H2evolution chamber and the O2 evolution chamber.

In the same publication, the hydrogen and oxygen evolvingsubstrates (anode and cathode) are placed side by side and the

membrane is placed perpendicular to the electrodes thus

separating the two compartments which are similar to

Section3.4under bi-photoelectrodes. This design also allows

interconnected chambers and other similar PE assembly

suitable for this set up is by[55].

3.3. Photoreactore DSSC-biased monolithic

photoelectrode

This tandem photosystem enables the use of the wide band

gap but stable and cheap metal oxide coupled to a relatively

cheap DSSC also based on metal oxide alternative to the PVsolar cell. However, water splitting only occurs on one of the

celltermed photolysis cell. The other cell, DSSC, only generate

electricity to provide bias potential for the photolysis cell. The

function of the DSSC is similar to a conventional solar cell.

However, the DSSC is based on PEC reaction to generate

electricity as described previously in Section 1.3.2 under

DSSC-biased. The DSSC used for biasing the photolysis cell

can be placed separately or combined into a monolithic unit as

shown by Gratzel [56]. It is important to match thecurrentand

voltage generated by the DSSC and the photolysis cell to

obtain better efficiency and this is achieved by controlling the

active area[57]. More than one DSSC connected in series may

be required to supply enough voltage for water splitting as thevoltage for one DSSC is about 0.7 V[58].

The apparatus is made from glass container where the

front compartment is the photolysis cell and the glass based

DSSC is placed directly behind it. In another layout, still made

from glass container, the photolysis cell is separated into two

compartments so that the evolved gases are separated [31].

The front compartment is the oxygen evolution chamber fol-

lowed by theDSSC and thebackcompartment is thehydrogen

evolution chamber (Fig. 11a). The hydrogen evolution

compartmentis placed on the back so that it does not interfere

with light reaching the PEof thephotolysis and PEof the DSSC.

The electrolyte in the front compartment is connected with

the back compartment by a membrane to allow ion

Fig. 8e H-type vessels. (a) With three compartments

(adapted from[46]), (b) With two compartments (adapted

from[45]).



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movement. The tandem DSSC design has been licensed to

Hydrogen Solar[59].

As both photolysis cell and the DSSC have PEs that absorbs

different part of solar spectrum, to achieve higher efficiency,

they can be placed in front of each other and ensuring the

current and potential are properly matched. Therefore the

effective area is reduced by half compared to when they are

placedsideby side instead ofstacked together,as theefficiency

calculation involves dividing by the effective area. Hence the

first PE absorbs the shorter wavelength and the second PE will

absorb the longer wavelength. The arrangement will require

a transparent PE and substrate so that light can reach the

second PE placed behind the first. To connect these two

systems electrically wire is used as in Gratzel[57]system. To

eliminate wiring, a bi-polar system using metal substrate can

beusedasshownbyParkandBard[56].ThePEofthephotolysis

cell is sharing the substrate with the CE of theDSSCand the PE

of DSSC is sharing the substrate with the photolysis cell CE

(Fig. 11b). Therefore, the electron can pass through the metal

substrate directly from one photosystem to the other.

However, the advantage of eliminating the wiring might be

outweighed by the disadvantage of less active area as the PE is

slanted to allow both PEs to receive light. This DSSC also has

less efficiency due to the bigger electrodes distance and also

lower redox electrolyte concentration is used so that light can

pass through the colored electrolyte to reach the second PE.

3.4. Photoreactor with bi-photoelectrodes

This PEC water splitting apparatus where both electrodes are

photoactive have two PEs arranged side by side where both

PEs can face the light. One PE is the photoanode based on

n-type semiconductor and the other is the photocathode

based on p-type semiconductor. To eliminate wiring, metal

substrate can be used and both the PEs semiconductor

materials can be deposited on the surface of the metal

substrate side by side. To separate the gases evolved, an

arrangement to separate the compartment is required and ion

exchange route is provided as shown by Aroutiounian et al.

[60] (Fig. 12). In this design, n-type PE(n-doped TiO2 or n-Fe2O3)

and p-type PE (p-doped TiO2 or p-Cu2O) [60,61] are placed

towards the top of the reactor, therefore they are only

submerged slightly in the electrolyte. The underlying metal

substrate back side has to be covered with insulating material

so the current doesnt leak to the electrolyte. The two PEs are

separated by placing a divider along the length of the middle

of the two PEs separating them into their own compartment,

hence separating the evolved gases. Along the divider,

a membrane is placed to allow ion movement. Light concen-

trator parabola and focusing optical window are also placed

on top of the photoreactor.

For n-type and p-type system, water splitting occurs

spontaneously without bias if the right conditions are fulfilled

Fig. 9 e Defined area PEs. (a) Front attachment, (b) Sandwich assembly.

Fig. 10e

Monolithic photoreactors. (a) Non-biased (adapted from [51]), (b) PV-biased (adapted from [53]).



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[3]. However, the PEs used by Aroutiounian et al. [60,61]are

based on metal oxides that are not very efficient in utilizing

the solar spectrum. Hence, the system has very low efficiency.

To increase the rate, they biased their system by using elec-

trolyte with different pH for the two compartments. This

difference in pH provides the additional potential however, as

described in earlier section pH biasing needs chemical input

as well as sunlight and therefore is not a suitable method for

practical hydrogen production. They have also biased their

system with external PV cell(Si) but as discussed in Section 3.2

the n- and p-type PEs can also be coupled internally with the

PV-biased system.In term of efficiency calculation, the side by

side configuration has lower efficiency because the solar

collection area is larger instead of reduced by half as in the

stacked arrangement.

3.5. Dual bed photoreactor

Another system using two PEs is the dual bed system [62e65]

(Fig. 13) which is basically a two photosystem where two PEC

cells are linked together and each cell has half the electro-

chemical potential for water splitting. The apparatus consist of

two shallow, flat sealed containers where photocatalytic parti-

cles are immobilizedon thebedat thebottomof thecontaineror

grids. An aqueous solution (alkaline) containing the redox

mediator (M IO3/I) is pumped between thetwo chambers. In

large scale application, to eliminate the need to pump large

volume of solution between the two chambers, an alternative is

tousesomeformofmembranebetweenthetwobedsthatallow

the ions of the mediator and electrolyte to crossover. Since the

two beds are now isolated, particles can be suspended in thesolution instead of immobilized on the bed[66].

In this system, both oxidation and reduction reactions occur

on each PE, compared with only the oxidation half reaction on

one PE and reduction on the other PE in the previously

described system. For water oxidation, acceptor (M) is used to

accept the electron from PE1 (TiO2 or Ir-TiO2) while the hole

created by PE1is used to oxidize water to O2. The acceptor (M)

is then reduced (M)and becomes thedonor for theH reduction

on the second PE2 (InP or TiO2-Pt). H is reduced by electron

produced by PE2and the donor will donate electron (M) to the

hole on PE1. As such, the electron is moved through the donor/

acceptor. As the potential for each cell is smaller (smaller band

gap), it can absorb broader range of the solar spectrum.

However, the numberof photons requiredis also more per unit

of H2produced. This system has the drawback of difficulty in

ensuring cyclic operation of the donor/acceptor and need

further development of the redox mediator[67].

3.6. Photoreactor with electrolyte containment

A design that protects the PE from direct exposure to aqueous

environment is shown by Fan et al. [68](Fig. 14). The photo-

reactor shown by this group is based on plate-type. The

hydrogen and oxygen evolving electrodes are put side by side.

They used conventional solar cells in series and also a metal

oxide layer which they termed as light sensitive catalytic

layer. The structure of the hydrogen evolving electrode

(n-type) is comprised of Nafion membrane, porous metal

substrate layer, porous PV solar cell layers and an outermost

layer of metal oxide semiconductor (n-doped TiO2-C with eg.

Pt, Ni, Fe or dye) formed by mixing with Nafion solution anddeposited on the porous solar cell layer. The porous nature of

the layers allows water to permeate. Similar structure is also

constructed for the oxygen evolving electrode (p-doped TiO2-C

with eg. Pt, Ni, Fe or dye). The metallic layer serves as support

for the electrode and also as current collector.

When put side by side with the photoactive sides facing up,

the electrolyte (acidic) is contained behind the electrodes by

the membrane that is the bottom layer in the electrodes

assembly. Water will permeate through the membrane and

Fig. 11 e DSSC-biased photoreactor. (a) Tandem DSSC PEC (adapted from [31]), (b) Bi-polar (adapted from [57]).

Fig. 12e

Bi-photoelectrodes reactor (adapted from[60]).



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throughout the electrode through its porous nature to reach

the surface layer. Therefore, only thin layer of water is in the

three-phase zone. Since no electrolyte at the surface resulting

in surface tension that hinder the release of the formed gas

from the surface, the gas can escape easily. And also, since

there is no thicklayer of electrolyte,sunlight can reach the PEs

surface without absorption by the electrolyte. However, water

normally only absorb IR radiation from light therefore it does

not affect much of the other light wavelength (UV and visible)but it will reflects and disperse the light.

4. Other considerations

Direct PEC water splitting is still a very challenging task in

term of finding material that can match the energetic and

stability requirements. In some PEC studies, organic additives

had been included in the aqueous electrolyte to enhance the

H2 production. The study by [69e73] using various organic

additives (ethanol, methanol, formic acid and formaldehyde)

all indicated that the photocurrent improved with addition ofthe organic substance. The organic addictive acts as sacrificial

donor and is preferentially oxidized then water. However, the

organic additives will decompose to form CO2along side H2.

This is only attractive when used in conjunction with a rather

transparent liquid waste clean up application which provides

the source of the organic additives. Otherwise, the issue will

be similar to the pH biased system as discussed in earlier

section.

Recently a report has been produced on the initial tech-

noeconomic analysis of several possible version of PEC reactor

for hydrogen production which considered particulate system

and also photoelectrode (photocell) system [66]. The particulate

system is not considered in our review because it has beenidentified early on that it is not favorable for large scale

hydrogen production. However, based on the technoeconomic

report the model shows that particulate system is more cost

effective. Nonetheless, the concept used for the particulate

system in the report has a lot of uncertainties. The report also

indicatedthat the photoelectrode system based on PV/PV or PV/

PECsystemhasthe mostmatureconceptwith severalexamples

have been fabricated. The PV/PEC system has advantage over

the PV/PV system because the PEC face (layer) can replace the

face conductor grids which partially obscure the PV layer.

5. Conclusion

There are not many reports on photoreactor for PEC water

splitting as there is much work still needed to improve on the

efficiency and stability of the PE. Generally, the shape and

geometry of photocell andphotoreactor forPEC water splitting

depends very much on the PE assembly. PE based on multi-

junction PV with PEC layer seems promising however, the

cost prediction indicates that using this arrangement is still

not commercially viable. Discovery of new PE material has the

greatest potential in driving the PEC water splitting to achievethe set target. In the mean time improvement of current

semiconductor efficiency and stability will help this move

forward. The ideal design of the photocell and photoreactor is

such that the PE has a maximum exposure to light. The

requirement for product gases management and ion move-

ment also affect the design consideration. In most case for

bench scale testing, a vessel with optical window that can fit

all the electrodes and electrolyte is sufficient. In practical

applications, there are various configuration options to

explore and the plate-type design seems to be the simplest.

Acknowledgement

The authors gratefully acknowledge Universiti Kebangsaan

Malaysia for the financial support of this project under the

grant UKM-GUP-BTT-07-30-189.

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Documents

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