More on Nomenclature and the Water Splitting Reaction Requirements

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A bewildering array of terms have been deployed in this field; thus, a few clarifying remarks appear to be in order. The term photoelectrochemical refers to any scenario wherein light is used to augment an electrochemical process. This process could be either uphill (Gibbs free energy charge being positive) or downhill (negative AG) in a thermodynamic sense. In the former case, the process is called photosynthetic (the reaction H2O ^ H2 + / O2 being an example) while the latter would be a photocata-lytic process (e.g., the oxidation of hydrocarbons at an illuminated n-TiO2/solution interface in an oxygenated medium). The term photoelectrolysis is correctly applied

Tio2 Flatband

Fig. 3. Two hybrid photoelectrolysis cell configurations.

electrolyte

Fig. 3. Two hybrid photoelectrolysis cell configurations.

to a case involving semiconductor photoelectrode(s) in an electrochemical cell. On the other hand, the term photolysis is more general and also includes the case of semiconductor suspensions. The term photoassisted splitting should be reserved for the cases wherein the excitation light energy only partially furnishes the voltage needed for the electrolysis process, the rest being accommodated by an applied external bias (see below). Finally, the term solar should be reserved for cases where sunlight (or at least simulated sunlight) was used for the semiconductor excitation. In all the cases, the more general term (or prefix) photo should be used. For example, if water is split (into H2 or O2) in a photochemical reactor containing a UV light source and semiconductor particulate suspensions, the process descriptor that is appropriate here would be: UV photolysis of water.

What photovoltage and semiconductor bandgap energy (Eg) would be minimally needed to split water in a single photosystem case (c.f., Figa. 2a or 2b)? We have seen (Chapter 2) that, to split water into H2 and O2 with both products at 1 atm, a thermodynamic potential of 1.23 V would be needed. To this value would have to be added all the losses within an operating cell mainly related to resistive (Ohmic components) and the overpotentials (kinetic losses) required to drive the HER and OER at the two electrode/electrolyte interfaces. This would translate to a semiconductor Eg value of ~ 2 eV if the splitting of water to H2 and O2 is the process objective. On the other hand, photovoltaic theory3 tells us that the photovoltage developed is nominally only ~ 60% of Eg. Taking all this into account, an Eg value around 2.5 eV would appear to be optimal.

What about a twin-photosystem configuration as in Fig. 2c? Optimal efficiency (we will define efficiency soon) is reached in such a configuration when one semiconductor has an Eg value of ~ 1.0 eV and the second ~ 1.8 eV.66 On the other hand, it has been pointed out64 that an optimal combination would be two matched electrodes of equal 0.9-eV band gap, since, in the absence of other limitations, the photo-current would have been dictated by the higher Eg electrode of a pair.

An irradiated semiconductor particle in a microheterogeneous system can be regarded as a short-circuited electrochemical cell where that particle is poised at a potential (AV) such that the anodic and cathodic current components are precisely balanced (i.e., no net current obviously is flowing through that particle.132 This photovoltage obviously has to attain a value around ~ 2 V for the water splitting reaction to be sustained. Given the need to reduce the kinetic losses and move the photovoltage value down to one around the thermodynamic (ideal) limit of 1.23 V, it is therefore not surprising that many of the studies on semiconductor particle suspensions have utilized (partially) metallized surfaces - the metals being selected to be catalytic toward the HER. The prototype here is the platinized semiconductor particle (e.g., Pt/TiO2) and the platinum islands are deposited on the oxide surface using photolysis in a medium containing the Pt precursor (e.g., PtCl62-) and a sacrificial electron donor (e.g., acetate).133,134 Obviously, the bifunctional catalyst assemblies discussed in the preceding Section, are motivated by considerations to make the HER and OER processes more facile.

Very detailed studies also have appeared on catalytic modification of semiconductor electrode surfaces to improve the HER performance; the reader is referred to the many review articles and book chapters on this topic.22,29,88,135,136,136a

Flatband Potential Semiconductor
Fig. 4. An interfacial energetic situation in a photoelectrolysis cell where the flat-band potential of the n-type semiconductor photoanode lies positive of the HER potential. Ws is the external bias potential needed in this case to drive the photoelectrolysis process.

The earlier discussion related to Fig. 1a should have indicated that it is simply not the magnitude of Eg (and the AV generated) alone that is the sole criterion for sustaining the water photosplitting process. Where the CB and VB levels lie on the energy diagram for the semiconductor at the interface is crucial. Assuming that we are dealing with thermalized electrons here (i.e., no hot carrier processes), the CB edge for the n-type semiconductor has to be higher (i.e., be located at a more negative potential) relative to the H2/H+ redox level in the solution (c.f., Fig. 1a). In the event that this is not true (see Fig. 4), an external bias potential would be needed to offset the deficit energy content of the photogenerated electrons. Other equivalent statements can be made for the requirements for an n-type semiconductor, namely that the semiconductor has to have low electron affinity or that the flat-band potential for that particular semiconductor/electrolyte interface has to be more negative than the H2/H+ redox level.

Interestingly, rutile TiÛ2 electrodes have an interfacial situation similar to that schematized in Fig. 4. Thus, the authors in the classical n-TiÛ2 water splitting study53-57 circumvented this problem via a chemical bias in their electrochemical cell by imposing a pH gradient between the photoanode and cathode chambers. On the other hand, photogenerated holes in TiO2 are generated at a very positive potential (because of its low-lying VB edge at the interface) so that they have more than enough energy to oxidize water to O2. Not too many semiconductor surfaces are stable against photocorrosion under these conditions; i.e., the photogenerated holes attack the semiconductor itself rather than a solution species such as OH- ions. Cadmium sulfide (Eg = 2.4 eV) is a particularly good example of a semiconductor that undergoes photocorrosion instead of evolving O2 from H2O. Thus, the requirements for a single photosystem for splitting water should have semiconductor energy levels that straddle the two redox levels (H2/H+ and OH-/O2), have an Eg value of ~ 2.5 eV for the semiconductor, and with a semiconductor surface that is completely immune to photocorrosion under OER (or HER) conditions. Additionally, the semiconductor surface has to be made catalytically active toward OER or HER.

Interfacial energetics in two-photosystem cells combining n- and p-type semiconductor electrodes respectively (Fig. 2c) have been discussed.67 Stability issues in photoelectrochemical energy conversion systems have been reviewed.31"

Given the above, it is hardly surprising that the search for satisfactory semiconductor candidates has continued at an unabated rate to the time of writing of this Chapter. In a historical sense, it is interesting that the shift of the research objective from initially photoelectrolysis toward regenerative photoelectrochemical cells (which generate electricity rather than a fuel such as H2) in the early years (1980s) is undoubtedly a consequence of the many challenges involved in the discovery (and optimization) of a semiconductor for the solar water splitting application. The field is now coming full circle with realization of the importance of a renewable H2 econo-my,138-141 and researchers are once again turning their attention to the use of semiconductor/electrolyte interfaces for solar H2 generation.

One approach to circumvent the semiconductor stability problem is to simply remove the photoactive junction from physical contact with the liquid. An alternate approach is to reduce the activity of water at the interface (and thus the proclivity to corrosion) by using a hydrophobic environment. In the first category, a variety of coatings have been deployed to protect the semiconductor surface (see Table 2). In the latter category, ionic liquids (such as concentrated lithium halide electrolytes) have been used173,190,191 so that instead of splitting water, compounds such as HCl or HI can be photodecomposed to H2 and Cl2 (a value-added chemical) or I2 respectively. Note that many of the examples in Table 2 are really hybrid systems where the photovoltaic junction (consisting of the semiconductor and a metal, polymer, or a transparent conducting oxide) simply biases an electrochemical interface.

Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces 177 Table 2. Types of coatings for protecting semiconductor surfaces against photocorrosion."

Entry number

Semiconductor(s)

Type of coating

Specific coating(s) employed

References

1

GaP

metal

Au

142

2

GaP, Si

metal

Pt, Pd, Ni or Cu

143

3

Si

metal

Pt

144

4

GaAs

metal

Au, Pt, Rh

145

5

Si

e.c.p.*

polypyrrole

146-148

6

Si

e.c.p.

polypyrrole

149

7

GaAs

e.c.p.

polypyrrole

150

8

CdS

e.c.p.

polypyrrole

89, 151-153

9

CdTe

e.c.p.

polypyrrole

154

10

Si

e.c.p.

polypyrrole

154a

11

Si

e.c.p.

polyacetylene

154b

12

CdS, CdSe

e.c.p./catalyst hybrid

polypyrrole/

155

polybithiophene/RuO2

13

Si

e.c.p./catalyst hybrid

Au/polypyrrole

156

14

Si

redox layer/e.c.p hybrid

ferrocene/polypyrrole

157

15

Si, InP

redox layer

ferrocene

158

16

Si, GaAs, GaP,

wide band gap scc

TiO2

159

InP and CdS

17

GaAs, GaAlAs

wide band gap

TiO2, SnO2, Nb2O3, AkOs

160

sc or insulator

or Si3N4

18

CdSe

wide band gap sc

ZnSe

161

19

CdSe

sc (photoconductor)

Se

162, 163

20

Si

insulator/catalyst hybrid

silicide/Pt

164, 165

21

Si

macrocycle film

Phthalocyanine

166

(Cu- and metal-free)

22

Si

self-assembled monolayer

alkane-thiol

167

23

GaAs

self-assembled monolayer

168

24

InP

self-assembled monolayer

"

169-171

25a

Si

transparent conducting oxide

Sn-doped indium oxide

172-174

(ITO)

26

Si

transparent conducting oxide

Sb-doped SnO2

175,176

27

Si

transparent conducting oxide

SnO2

177

28

Si

conducting oxide

Tl2O3

178

29

CdS

conducting oxide

RuO2

179,180

30

Si

redox polymer/catalyst

N,N'dialkyl-

181-184

4,4'bipyridinium polymer/

Pt or Pd

31

Si

redox polymer/catalyst

poly(benzyl viologen)/Pt

185

32

GaS

redox polymer

as in Entry 30 without the

186

metal catalyst

33

Si

redox polymer

[4,4 '-bipy ridinium] -

187

1, l'-diylmethylene-

1,2-phenylenemethylene

dibromide polymer

(poly-oXV2+) and other

viologen-based polymers

34

GaAs

redox polymer

polystyrene with pendant

188

Ru(bpy)32+ complex

(bpy=2,2'-bipyridyl ligand) 35 GaAs redox polymer/catalyst polymer as in Entry 34/ 189

"Not all these cited studies have focused on photodriven HER and OER applications for the coated semiconductor/electrolyte interfaces. *e.c.p.= electronically-conducting polymer. csc = semiconductor

(bpy=2,2'-bipyridyl ligand) 35 GaAs redox polymer/catalyst polymer as in Entry 34/ 189

"Not all these cited studies have focused on photodriven HER and OER applications for the coated semiconductor/electrolyte interfaces. *e.c.p.= electronically-conducting polymer. csc = semiconductor

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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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Responses

  • estella
    Why surface potential changes in with H2?
    9 years ago
  • yasuko ryan
    What is photocorrosion?
    9 years ago
  • roy
    What is meant by flat band potential?
    8 years ago

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