5.3.1 Theoretical considerations

Identifying the factors that determine the overall solar energy conversion efficiency is critically important in evaluating the overall cost benefit of the PEC water-splitting process. The key commercial decisions (i.e., longterm investment on research and development) are always associated with device efficiency, fabrication cost, material availability/scarcity and others. The conversion of solar energy into useful forms can be classified into thermal and photonic in general. Solar thermal processes are determined by the Carnot efficiency whereas solar photonic processes are limited by fundamental considerations of optical excitation at molecular level (for organic light harvesters) and/or bandgap excitation (for inorganic light harvesters).

Shockley and Queisser presented the concept of ideal photovoltaic cell (for a p-n junction) and showed that the maximum conversion efficiency is 31% under AM 0 illumination (Shockley and Queisser, 1961). Ross and Kelvin generalized the Shockley and Queisser treatment to all photochemical energy conversion systems (Ross and Calvin, 1967). Taking into account Ross and Calvin treatment further, Bolton and co-workers considered solar photonic converter configurations and limiting efficien- cies (Bolton, 1978, 1996; Bolton et al., 1985). A few recent papers have also discussed PEC cell configurations in terms of overall solar to hydro- gen conversion efficiency (Rajeshwar, 2007; Chen et al, 2010; Walter et al, 2010). The efficiency (η) of conversion of incident solar energy to chemical (Gibbs) energy is given by

[5.8]

where J is the absorbed photon flux (photons s⁻¹ m⁻²,μ_ex is the excess chemical potential generated by light absorption, ϕ_conv is the quantum yield for absorbed photons to chemical product generation, and S is the total incident solar irradiance in Wm⁻². ϕ_conv is a function of light harvesting, charge transport, charge collection and interfacial charge transfer efficiencies.

Bolton's classification is based on the number of photosystems (i.e., S for a single photosystem and D for dual photosystems) employed in each device configuration by considering a different light absorption threshold for each photosystem. The scheme classification is generalized by taking into account the minimum number of photons required to generate solar production of any reduced fuel by taking the minimum number of photons required to generate reducing equivalent of one molecule of hydrogen. If the device is designed based on a single semiconductor material with a band gap of 1.6 eV (light absorption threshold ~ 775 nm), the maximum solar energy conversion efficiency under standard AM1.5 illumination (assuming that hydrogen evolution takes place under standard conditions) is 30.7% (Bolton et al., 1985). The calculation considers that the device only needs two photons to generate one molecule of hydrogen (S2). However, if the device requires four photons to produce one mole of hydrogen (S4), the ideal maximum solar energy conversion efficiency under standard AM1.5 illumination would be 30.6%. Similarly, the terms D2 and D4 are assigned for devices that utilize dual photosystems (Bolton et al, 1985).

5.3.2 Experimentally assessing the solar to hydrogen conversion efficiency

The energy conversion efficiency of practical devices is significantly lower than the theoretical maximum conversion efficiency due to a number of energy losses occurring in real devices under operational conditions. Hence, all energy losses need to be accounted for in order to estimate the practically attainable efficiency for a given device configuration that utilizes a particular material combination and a specific illumination profile. Nonradiative recombination and loss occurring due to overpotentials (that are required to drive the OER and HER) are some of the losses in real devices under operational conditions. The next section deals with the determination of solar energy conversion efficiency of practical photoelectrolysis cells.

In terms of solar energy conversion efficiency of practical PEC water- splitting systems, the published literature to date generally deals with (i) mainstream reporting (bench marking efficiency), (ii) specific material performance reporting (diagnostic efficiency), (iii) fundamental aspects report- ing (charge generation, transfer, collection, interfacial charge transfer and mechanistic aspects that limit the efficiency) and new device configurations reporting (introducing new device structures to maximize the overall efficiency) (Chen et al., 2010). A detailed discussion on different PEC water- splitting cell configurations/systems and the solar to hydrogen conversion efficiency will not be attempted here as it is beyond the scope of this chapter. Instead, a brief outline of methods of experimentally assessing the solar to hydrogen conversion efficiency is presented below. Recent reports have highlighted the necessity of the correct measurement conditions in order to determine the solar-to-hydrogen conversion efficiency accurately. Generally, there are two ways of determining the overall solar-to-hydrogen conversion efficiency of PEC water-splitting systems practically. In the first method, it is calculated by taking into account the rate of hydrogen generation (Equation [5.9]). In the second method, the efficiency is determined by taking into consideration the short-circuit photocurrent density (Equation [5.10]). In order to do that the device is illuminated with Air Mass 1.5 Global (AM 1.5 G) spectra. Often the device is in two-electrode configuration where OER and HER take place in each electrode only under illumination and no external voltage is applied. Two electrodes immersed in the aqueous elec- trolyte are required to be separated by a proton exchange membrane in order to facilitate the ionic transport across (i.e., no pH gradient between anodic and cathodic side) and to prevent the exothermic reaction between the overall reaction products, oxygen and hydrogen. Connection of the two electrodes externally makes it possible to record the photocurrent density. Knowing the rate of hydrogen generation, the solar-to-hydrogen conversion efficiency (η) can be determined by the relation

[5.9]

where the [r_H2 ] is the rate of hydrogen generation in mol/s, ΔG is the Gibbs free energy change for the generation of one mole of H₂ by splitting water under standard conditions (237.2 kJ/mol), P is the power density of illumi- nation intensity in W/m² and A is the illuminated area of the electrode. It is generally expected that the rate of hydrogen generation at the cathode (or photocathode) is twice the rate of oxygen generation at the anodic (or photoanode) (i.e., the Faradaic efficiency of the overall process is 100%). This can be verified by conducting a quantitative analysis of gas generated at each electrode (i.e., by determining the gas type and rate of evolution). Recording the photocurrent in the external circuit allows determination of the solar-to-hydrogen conversion efficiency by using

[5.10]

where the J_sc is the photocurrent density in A/m² and P is the power density of illumination intensity in W/m². (In some reports, an extra term (η_F) is incorporated in Equation [5.9] in order to compensate the losses if HER and/or OER are not 100% Faradaic. However, the ηF may not be constant as the HER and OER progress when the cell operates, due to accumulation of undesirable products as reaction intermediates undergo side reactions in cathodic/anodic or both electrodes.) When using Equation [5.10], it is important to establish that the recorded photocurrent density corresponds to the generation of hydrogen and oxygen in cathode and anode, respec- tively (i.e., Faradaic efficiency of both HER and OER are 100%).

Stable semiconductor materials with desirable band energy positions are not currently available that would allow the design of PEC water-splitting devices that can operate without an external bias. Due to this situation, many device configurations reported to date use an externally applied voltage to assist the overall water-splitting process (Walter et al, 2010). In some cases, an internal chemical bias has been generated by employing electrolyte solutions with an appropriate pH gradient between the anode and cathode com- partments of the cell, in order to achieve the same objective (Fujishima and Honda, 1972; Aspnes and Heller, 1983). The overall efficiency of such systems needs to be evaluated with extra care as the HER and OER dynamics may alter the pH difference between the cathode and anode compartments when the device operates. As for devices that operate under an external applied voltage, the magnitude of the required voltage depends on the band energy positions in the electrolyte concerned. Equation [5.11] provides a convenient way of calculating the efficiency of PEC water-splitting devices under condi- tions of applied bias. It is clear from this equation that the externally applied voltage (Vapp) cannot exceed 1.23 V under standard operational conditions if the PEC water-splitting cell is to have a non-zero efficiency:

[5.11]

where J is the external photocurrent density in A/m², P is the power density of illumination intensity in W/m². In this case, it is assumed that the Faradaic efficiency for water splitting is 100%.

5.4.1 Introduction to materials challenges

Photoelectrolysis cells require semiconductor materials that harvest sun- light efficiently and sustain the water-splitting reactions under illumination without external assistance (bias voltage) for prolonged periods at a fast enough rate without substantial kinetic losses. Hence, semiconductors play a central role from laboratory scale research to industrial scale device design, development and operation. Although the thermodynamic scheme given in Fig. 5.6 acts as a rough guide in the search for suitable semiconductor materials, work conducted over the last few decades shows that finding materials with efficient PEC performance is far from easy. This section considers pho- toanode and photocathode semiconductor materials and their key proper- ties such as light harvesting, energetics, interfacial kinetics and stability.

5.4.2 Light-harvesting properties

The band gap of a prospective semiconductor photoanode material must be small enough to harvest sufficient amount of sunlight, but large enough to generate sufficient photovoltage to drive HER and OER under illumination. Hence, as discussed earlier, a trade-off between the light absorption thresh- old and optical band gap needs to be considered when identifying potential semiconductors. As the light penetration depth is inversely proportional to the optical absorption coefficient, semiconductors with poor light-harvest- ing properties must have good charge transport and low recombination characteristics to be able to collect minority carriers at the semiconductor/electrolyte interface (Equation [5.12]). For example, the light absorption coefficient of α-Fe₂O₃ at 450 nm is 2.3 × 10⁵ cm⁻¹ (Glasscock et al, 2008; Saremi-Yarahmadi et al., 2009a) (i.e., light penetration depth ~ 43.5 nm). However, due the short hole diffusion length of α-Fe₂O₃ (~ 2~4 nm) associated with low mobility and short lifetime of holes (Equation [5.12]), only a fraction of photogenerated holes reach the interface (Prakasam et al, 2006) and the implication is that the compact α-Fe₂O₃ electrodes are very ineffective in charge separation despite the material being a reasonably good light absorber. Thus, the collection of holes is almost entirely restricted to the space charge region created at the oxide electrolyte interface in α-Fe₂O₃. These problems have been addressed by nanostructuring the α-Fe₂O₃ to increase the chances of holes reaching the oxide/electrolyte interface (Kay et al, 2006; Tahir et al., 2009; Le Formal et al, 2010). The nanostructuring strategy has been adapted to many other semiconductors in order to enhance the minority carrier collection efficiency at the semiconductor/elec- trolyte interface (Wang et al., 2008; Tahir and Wijayantha, 2010; Boettcher et al., 2011).

[5.12]

where L is the minority carrier diffusion length, D is diffusion coefficient and τ is lifetime of minority careers. Although photoelectrolysis cells have been successfully made by using single-crystal semiconductors to prove the concept, photoelectrolysis will not be competitive with other hydrogen gen- eration technologies in cost terms unless advances are made in constructing devices with affordable semiconductors. As shown in Fig. 5.1, if photoelectrolysis cells are designed to employ only a single semiconductor, the con- duction and valence band edge positions must straddle the electrochemical potentials of H⁺/H₂ and O₂/H₂O if photoelectrolysis is to be achieved without an external voltage. In some cases, alternative hybrid multi-junction water-splitting systems have also been proposed (Miller et al, 2005). Then, the band edge positions of each material should be appropriately aligned to meet the criteria of such hybrid multi-junction water-splitting systems. Figure 5.6 illustrates the conduction and valence band edge positions of some common semiconductors in contact with aqueous electrolyte (pH = 0). The band energies of only a very few semiconductors (CdS, ZnS, SiC, GaN, SrTiO. and KTaO.) are favourable for unassisted PEC water splitting (Memming, 1978; Kudo and Miseki, 2009). Due to the poor stability of chalcogenide-, carbide- and nitride-based semiconductors in aqueous electrolytes (i.e., photocorrosion), metal oxide anodic semiconductors are generally favoured to drive the OER. In fact, anodic oxide semiconductor surfaces are known to be reasonably stable to drive water oxidation reac- tion (Schiller et al., 2009; Walter et al., 2010). The OER is associated with multiple charge transfer processes at the interface, such that four holes are needed to oxidize four OH- ions to dioxygen and water in alkaline solution. Therefore, other redox-active transition metal oxides such as CeO₂, ZrO₂, TiO₂, NbO₂ and Ta₂O₅ can be considered as potential photoanodic materials (Walter et al, 2010).TiO₂ has been extensively investigated as a model mate- rial for photoelectrolysis since the concept was first demonstrated in 1972 (Fujishima and Honda, 1972). WO₃ and α-Fe₂O₃ are other commonly studied metal oxide photoanode materials (Santato et al, 2001; Solarska et al, 2004; Duret and Grätzel, 2005; Kay et al, 2006; Tahir et al, 2009; Macdonald et al., 2010).The valence band of metal oxides generally consists of O 2p orbitals. An unwelcome consequence is that the potential of the E_vb edge of many metal oxide semiconductors stays highly positive at 3.0 ± 0.5 V vs NHE. As the conduction band is generally formed by the valence orbitals of metal(s), the Ecb edge normally stays close to the H⁺/H₂ energy level (Scaife, 1980). The fact that the OER potential is at 1.23 V vs NHE means approximately 1.5 eV of absorbed photon energy by many oxide semiconductors is wasted by thermal relaxation (Cheng et al, 2010). The implication of this situation is that many metal oxide semiconductors known to operate as photoanodes in photoelectrolysis cells are very inefficient due to (i) their wide band gap and (ii) the need of a bias voltage to drive the HER on the cathode.

5.4.3 Interfacial energetics

Most photoanode materials (and multiple band gap photoelectrodes) can- not be operated in photoelectrolysis cells without applying a bias internally or externally (Kennedy and Frese, 1978; Sartoretti et al, 2003; Miller et al, 2004; Duret and Grätzel, 2005; Cesar et al, 2006; Kay et al, 2006; Glasscock et al, 2007; van de Krol et al, 2008; Saremi-Yarahmadi et al, 2009b; Hisatomi et al, 2010; Kleiman-Shwarsctein et al, 2010). This situation has led to studies of methods that are capable of controlling the band positions at semicon- ductor/electrolyte interfaces in order to realign them in favour of unassisted photoelectrolysis. Addressing the energetic disparity between the band edges of known semiconductor materials and the HER and OER redox energy levels by engineering the semiconductor/electrolyte interface is a demand- ing task. In principle, the band edges of oxide semiconductors can be con- trolled by using their Nernstian pH dependence. However, any pH changes in the electrolyte will not alter the overall energetics at the metal oxide semiconductor/electrolyte interface as the H⁺/H₂ and O₂/H₂O energy levels generally show the same dependence on pH. Attempts have therefore been made to alter band alignments by other methods. Surface derivatization is a well-known method for a long time to control band energetics (Wrighton, 1983). For example, Vilan et al. were able to modify the band alignment at the interface by adsorbing tailor-made small molecules on the surface (Vilan et al., 2000). Covalent chemical modification of the surface is an alter- native approach which has been used to control the band energies at the semiconductor/electrolyte interface and eliminate the pH dependence of the band edges, allowing independent pH control of the electrolyte redox energies (Bookbinder et al, 1980; Kocha and Turner, 1995; Hamann and Lewis, 2006).

Recently we have adapted an alternative approach to band edge align- ment which involves manipulating the preferential crystallographic ori- entation of the semiconductor electrode. In energetic terms, the flat-band potential (V_FB) of a semiconductor is an indication of the position of the conduction and valence band energies relative to a reference energy scale or to a redox energy level in the electrolyte. V_FB can be changed by altering the surface dipole potential, for example as a consequence of acid/base equilibria or ionic adsorption (Santhanm and Sharon, 1988; Peter et al., 2003). The interaction between a semiconductor and an electrolyte depends on the chemical nature of the semiconductor surface, which depends in part on crystallographic termination (Gerischer, 1983; Zhong et al, 2009). In order to test that, we have chosen α-Fe₂O₃ as the photoanodic electrode material of interest. We have found that the preferential orientation of α-Fe₂O₃ can be controlled in order to optimize the interfacial band alignment.

The energetics of water splitting in a α-Fe₂O₃ PEC cell with and without external bias are illustrated in Fig. 5.7. The Gibbs energy change driving interfacial electron transfer during water photo-oxidation is determined by the energy difference between the quasi-Fermi level of holes (_pE_F) in the semiconductor and the redox energy level corresponding to the oxygen reduction potential. At sufficiently high illumination intensities, _pE_F approaches the valence band energy (Nozik, 1978; Fajardo and Lewis, 1996). In the absence of external bias voltage, the Gibbs energy change for the reduction of protons corresponds to the difference between nE_F, the Fermi level of electrons in the anodic semiconductor (and hence Fermi level in the cathode), and the proton reduction potential. On the electrochemical energy scale, _nE_F corresponds to the flatband potential V_FB of the anodic semiconductor in contact with the electrolyte. Since the flatband potential of α-Fe₂O₃ is typically 0.4 V positive than the hydrogen redox potential, an external bias voltage is required for photoelectrolysis. This bias voltage can be reduced if VFB is made more negative, which corresponds to mov- ing the conduction band energy towards the vacuum level. We have discovered that VFB of α-Fe₂O₃ films can be varied systematically by varying their preferential crystallographic orientation by controlling the deposition conditions. Hence, the required external bias voltage for α-Fe₂O₃ was sub- stantially reduced (by 0.32 V), favouring the water splitting in a photoelectrolysis cell.

α-Fe₂O₃ crystallizes in the rhombohedral crystal system with space group R-3c. The crystal structure of α-Fe₂O₃ features distorted hexagonal close packing (a = 5.043 Å and c = 13.748 Å) of oxygen atoms, with 2/3 octahedral interstices occupied by Fe. Each Fe and oxygen atom has six oxygen and four Fe neighbours, respectively. Polycrystalline α-Fe₂O₃ layers are generally randomly oriented in directions such as [110], [104], [300], [012] and [024]. The surface that is preferentially oriented in the [110] direction is dominated by Fe(III) stoichiometric termination, whereas it is predominantly composed of O²⁻ groups when the preferential orientation is in the [104] direction. Figure 5.8 shows the crystal structure of α-Fe₂O₃ showing the [110] direction and [104] direction.

Figure 5.9 shows a family of XRD patterns obtained for a series of α-Fe₂O₃ photoelectrodes prepared by aerosol-assisted chemical vapour deposition at different deposition temperatures. All peaks not assigned to the SnO₂ substrate such as [110], [104], [300], [012] and [024] were identified as hae- matite (JCPDS 33–0664). The [110], [104] and [300] are more pronounced than the other peaks. On the whole, the α-Fe₂O₃ XRD patterns are strong compared to the SnO₂ patterns, indicating a high degree of crystallinity in the haematite films. The [104] peak is dominant for electrodes deposited at low temperatures suggesting that the α-Fe₂O₃ crystalline structure is pref- erentially oriented along the [104] direction. As the deposition temperature was gradually raised above 350°C while keeping all other parameter unchanged, the [104] peak diminished, and simultaneously the [110] peak became pronounced. This indicates that the preferential orientation of α-Fe₂O₃ in the [110] direction is favoured at high deposition temperatures. Figure 5.10 shows how the relative intensity ratio of [110]/([110]+[104]) of normalized XRD patterns varies with deposition temperature. Analysis of XRD patterns suggests that (i) electrodes deposited at low temperatures are oriented in such a way that the basal plane is preferentially aligned parallel to the FTO substrate, (ii) electrodes deposited at high temperatures are preferentially oriented, so that the c- axis is vertical to the substrate, (iii) there is a critical deposition temperature around 490°C where the relative ratio of [110]/([110]+[104]) reaches its maximum.

Figure 5.11 shows how VFB of α-Fe₂O₃ in 1 M NaOH varies as the relative intensity ratio of [110]/([110]+[104]) is changed by altering the α-Fe₂O₃ deposition temperature. At low deposition temperatures (~ 350°C), the rel- atively small [110]/([110]+[104]) intensity ratio suggests that the α-Fe₂O₃ surface is dominated by oxygen termination. As the deposition temperature is increased above 350°C, the [110]/([110]+[104]) intensity ratio gradually rises towards a maximum at 490°C, suggesting that the surface termination becomes dominated by Fe(III) groups. The observation of the most negative VFB when the [110]/([110]+[104]) intensity ratio is at its maximum is consistent with higher affinity of the Fe(III) surface for OH⁻. Figure 5.11 mirrors Fig. 5.10 suggesting that the negative shift of VFB is a direct result of the increased preferential orientation of α-Fe₂O₃ in the [110] direction. The magnitude of the negative shift is estimated to be about 0.32 V. Such a large negative shift in VFB. which corresponds to an upward movement of the conduction band relative to the hydrogen redox level, should reduce the external bias required to overcome the energetic mismatch at the α-Fe₂O₃/electrolyte interface. Indeed, this is the case, as revealed by PEC studies of these α-Fe₂O₃ electrodes.

Plate III (see colour section between pages 238 and 239) compares the steady-state current–voltage characteristics of the series of α-Fe₂O₃ electrodes, and the inset shows their incident photon to electron conversion efficiency (IPCE%) spectra recorded at 0.23 V vs Ag/AgCl/3M KCl (− 1.23 V vs RHE). The J-V plot with the most negative photocurrent onset (Vonset) corresponds to the electrode that had the highest relative [110]/([110]+[104]) intensity ratio. Conversely, the J-V plot with the most positive Vonset is recorded for the electrode with lowest [110]/([110]+[104]) intensity ratio. When the α-Fe₂O₃ surface is preferentially oriented in this direction (i.e., the [110]/([110]+[104] ratio is lowest), the photocurrent transients for chopped illumination exhibited negative and positive spike characteristics for strong surface recombination. The photocurrent transients for electrodes preferen- tially oriented in the [110] direction did not show such spikes, suggesting the suppression of recombination. The absence of characteristic spikes in photocurrent transients and the negative shift of Vonset as the [110]/([110]+[104]) ratio is increased once again highlight the importance of the energetics of the semiconductor/electrolyte interface for photo-induced charge transfer kinetics. The negative shift of V_onset (approx. –0.32 V) closely follows the negative shift of VFB and is accompanied by an improved fill factor in the J-V plots suggesting low surface recombination. The other noticeable feature in Fig. 5.11 is the shift of dark-current onset potential of the α-Fe₂O₃ electrodes in the opposite direction to that of V_onset. The most negative dark-current onset corresponds to the α-Fe₂O₃ electrode that had the lowest [110]/([110]+[104]) ratio, whereas the most positive dark-current onset was observed for the electrode with the highest [110]/([110]+[104]) ratio. This may reflect the density of surface or defect states that can promote water oxidation under inversion conditions where electron transfer involves tunnelling across the space charge region in the a-Fe₂O₃. Water photo-oxidation and oxidation reactions have been studied on different surface terminations of rutile TiO₂, and a significant variation of overpotential was predicted as the chemical environment of rutile surface is changed (Valdés et al., 2008). The incident photon-to-electron conversion efficiency (IPCE) at 0.23 V vs RHE also provides a useful insight into the competing processes at the a-Fe₂O₃ surface. As shown in the inset of Plate III, the IPCE increases as the photocurrent onset shifts in the negative direction accompanied by increased fill factor. The maximum IPCE was recorded for the electrode that had the highest [110]/([110]+[104]) ratio and hence the most negative VFB. The IPCE maximum recorded in this work (− 40%) is the highest value reported for an unmodified a-Fe₂O₃Fe₂O₃ electrode and indicates the potential of this approach. In principle, the same strategy can be used to manipulate energetics of other semiconductor/electrolyte interfaces to optimize the HER and OER under illumination in order to split water without an external bias.

5.6.1 New photoelectrode materials

Until the last decade, the studies of photoelectrolysis had been confined to a very few semiconductor material classes including metal oxides and III-V materials. For example, modifying wide band gap semiconductor oxides by doping has been traditionally a very popular route to extend the light absorption threshold to the visible part of the solar spectra (Jordan et al, 1972; D'Amico et al, 1984). The failure to develop efficient and sustainable photoelectrolysis cells from such semiconductors led to the extension of studies to new semiconductor material classes including binary, ternary and quaternary metal oxide compounds as well as non-oxide semiconductors. The energetics of many metal oxide semiconductors are poorly aligned with respect to the HER and OER potentials such that Ecb is generally below the H +/H₂ energy level. Binary metal oxides such as metal titanates (i.e., SrTiO₃, BaTiO₃) deviate from that trend as the incorporation of second metal cat- ions (i.e., Sr.⁺, Ba^2 +) results in a perovskite structure. This shifts the Ecb upwards and realigns the interfacial energetics in favour of photoelectrolysis (Walter et al., 2010). Modification of the perovskite materials such as titan- ates, niobates and tantalates has been conducted to incorporate O₂- layers, and La₂Ti₂O₇, K₂La₂Ti₃O₁₀, Ba₅Nb₄O₁₅, Sr₂Ta₂O₇ and Ba₅Ta₄O₁₅ come under that category. It has been reported that such materials promote the OER through formation of catalytic centres (Kim, 2005). Another strategy was the incorporation of suitable elements with N2p and S3p orbitals into the crystal structure. The objective is to form a new energy level negative to Evb and extend the optical absorption threshold into the red region. BaTaO₂N is an example for such semiconductors (Maeda, 2007). The commonly prac- ticed analogous approach is doping metal oxides with nitrogen, sulphur or carbon (Asahi, 2001). Metal oxides with d. 0 electronic configuration have also been looked at as another class of semiconductors suitable for photo- electrolysis due to their higher electronegativity (Sato, 2001, 2003, 2004). Metal ferrites are another class of semiconductor photoelectrode materials and studies have been conducted to investigate their optical and ener- getic properties with the focus of employing them in photoelectrolysis cells. Recent studies have identified ZnFe₂O₄ and CaFe₂O₄ as potential photo- anode and photocathode materials (Ida et al., 2010; Tahir and Wijayantha, 2010). Tungstates are another class of semiconductors recently investigated for photoelectrolysis (Chang et al., 2011). Woodhouse and Parkinson have developed a high-throughput combinatorial search strategy for the identification of multicomponent metal oxide materials with suitable bandgaps and band positions for water photoelectrolysis operating as either a photoanode or a photocathode (Woodhouse et al., 2005). Therefore, both synthesis of specifically designed new materials and combinatorial material discovery have opened up a completely new dimension to ongoing work on finding desirable semiconductors for photoelectrolysis in recent years (Woodhouse et al, 2005; Osterloh, 2008; Osterloh and Parkinson, 2011).

5.6.2 Tandem cells for photoelectrolysis

Cascade electrodes and tandem cells have been actively pursued to over- come light-harvesting limitations in photoelectrodes and energetic disparity between the band edges of known semiconductor materials and the HER and OER redox energy levels, respectively, in photoelectrolysis (Gratzel, 1989, 2001; Nozik, 1996; Khaselev and Turner, 1998; Wijayantha and Auty, 2005). Generally, photoelectrolysis tandem cells employ more than one photosystem that complement one another optically and energetically. For example, dual-photoelectrode tandem photoelectrolysis cell configuration (photoanode/electrolyte/photocathode) has been widely investigated since the 1980s to drive OER and HER on separate photoelectrodes (Hamnett, 1982). The energetics of a desirable dual photoelectrode tandem photo- electrolysis cell constructed by combining photoanodic and photocathodic junctions are given in Fig. 5.12. In this case, the photoanodic and photoca- thodic materials are chosen on the basis that their optical band gaps complement each other (to maximize the light-harvesting properties) and that the photocurrent density under operational condition in each electrode match the other.

An alternative tandem photoelectrolysis cell configuration is the combination of a photoanode/electrolyte interface with an optically comple- mentary solar cell. An analogous tandem cell can also be constructed by combining a photocathode/electrolyte interface with an optically comple- mentary solar cell (Nozik, 1996). The selection of appropriate semiconduc- tor photoelectrode and photovoltaic cell is primarily done on the basis of the optical absorption threshold of the photoelectrode in the photoelectrol- ysis cell and PV cell such that they complement each other (i.e., estimated attainable photocurrent density in each photosystem under operational condition matches as well as the bias voltage provided by PV cell is suffi- cient for alignment of the energetics of the semiconductor/electrolyte interface with HER and OER redox energy levels to drive photoelectrolysis). The energetics of such a tandem cell based on a photoanode are illustrated in Fig. 5.13 considering the energetics of n-WO₃/electrolyte interface and a PV cell (Grätzel, 1989, 2001; Wijayantha and Auty, 2005). The illumination profile of this tandem cell in optically series and optically parallel configu- rations is shown in Figs 5.14 and 5.15, respectively. In the optically series configuration (Fig. 5.14), the bias voltage is provided by the solar cell which is illuminated by the spectral region unused by the photoelectrolysis cell (i.e., filtered light), hence no external bias is required. In contrast, each photosystem is illuminated by the entire solar spectra in the optically parallel configuration. The downside is the requirement of a large illuminated area to drive the same current density in the tandem cell. This needs to be taken into account in device efficiency calculation of the optically parallel configuration. Khaselev and Turner incorporated a GaAs PV cell at the back of a cathodic GaInP₂/electrolyte junction in order to provide the necessary bias voltage to boost the free energy of photogenerated electrons to drive the HER at the interface (Khaselev and Turner, 1998). Similar approaches were also taken by others (Licht et al, 2000; Mauk et al, 2001).

In principle, the reactions occurring in natural photosynthesis are analogues to the processes in tandem photoelectrolysis cells. However, the work conducted to date to construct photoelectrolysis prototype devices shows that the situation is not as simple as have been portrayed. The key components in photosynthesis are membranes, enzymes, etc., whereas the elements in tandem photoelectrolysis cell are semiconductors, liquid electrolytes and catalysts, etc. The energy conversion efficiency of photosynthesis is relatively low (~< 1% under natural light conditions) whereas it has to be significantly high in tandem cells. The photosynthetic reaction occurs at membranes of mono- or bi-layer (thickness of a few nanometres), allowing separation of oxidative and reductive products to prevent reverse reaction (Gerischer, 1977; Scheuring and Sturgis, 2006). The direct consequence of this is the weak light absorption, an issue that nature has been addressed by employing lamellar membrane stacks. A similar approach has been adopted in the DSC PEC cell to increase the light-harvesting efficiency (O'Regan and Grätzel, 1991). Further, one might say that nature understood the fact that all organic molecules undergo photodecomposition reactions sooner or later following the light absorption, and hence facilitated continuous regeneration of the photosynthetic system. Mimicking such a regeneration mechanism in a photoelectrolysis tandem cell to replenish photodecomposed materials is a very challenging and complex process. Another key conceptual differ- ence between natural photosynthesis and tandem photoelectrolysis cell is the absence of charge transport over a long distance in the photosynthesis. Unlike in photosynthesis, tandem photoelectrolysis cells operate on the basis that photogenerated electron–hole pairs are separated by transport through the semiconductor(s) in opposite directions before undergoing HER and OER reaction at the interfaces.

5.8.1 Hydrogen evolution reaction and oxygen evolution reaction catalysts

The previous section suggested that even if semiconductor materials with desirable energetics are used in PEC reactions, the slow interfacial kinetics may still limit the overall efficiency. The overpotentials required to overcome the kinetic limitations in OER and HER are significantly responsible for the low efficiency in photoelectrolysis. The previous section also highlighted the non-ideal nature of illuminated semiconductor/electrolyte interface in general and showed the need of incorporating catalysts on semiconductor surface to reduce the overpotential (Wijayantha et al, 2011). In order to act as a successful catalyst, it requires satisfying at least two basic requirements (it must be highly active toward the reaction(s) of interest and robust enough to maintain catalytic activity over sufficiently long period of time). As OER is a four-electron transfer process, it is thought to be the main contributor to the loss associated with kinetic overpotentials in photoelectrolysis. Heller showed that the recombination velocity of photogenerated electrons at the semiconductor-catalyst interface must be low (Heller, 1983). For electrocat- alytic water oxidation, the typical overpotential on Co₃O₄ incorporated Co electrodes in basic electrolytes are reported to be between 350 and 400 mV at 10 mAcm-2 current density. On the other hand, the desired target overpo- tential for OER at illuminated anodic semiconductor/electrolyte interface is in the order of 100–200 mV for the same magnitude of photocurrent density (Chou et al, 2011). This highlights the challenge in developing efficient photoanodic half-cells to drive OER RuO₂, MnOx, IrO₂, Co₃O₄, NiCO₂O₄, Pt, Pd, Ru–Co, Ru–Ir, Ru–Cu are some well-known metal oxide, metallic and bi-metallic electrocatalysts that already used for this purpose at moderate overpotential and relatively high current densities (Trasatti and Lodi, 1981; Forgie et al, 2010). Lewerenz et al. recently reported that solar-to-hydrogen conversion efficiency reach 12.1% for homoepitaxial InP thin-films covered with Rh nanoislands (Lewerenz et al, 2010). Despite the extensive work on binary and ternary electrocatalysts, Pt has been the popular choice of OER catalyst on the surface of many semiconductor photoanodes (Harris and Kamat, 2010; Rosseler et al., 2010). Nocera and co-workers have recently reported a new class of electrocatalysts based on cobalt phosphate and borate to drive OER, and the current density as high as 100 mA/cm² has been achieved at 350 mV overpotential by employing them (Esswein et al, 2011). The potential of such catalysts has been demonstrated by incorpo- rating them on n-Si surface to drive OER at a significantly low overpotential (Young et al, 2011). As for the HER catalysts, there are two main types used to date: namely metal composites/alloys and non-metallic compounds. Among catalysts investigated, nickel and Ni-containing binary and ternary metallic catalysts have shown the performance to be as good as that of Pt (Conway et al, 1983; Wendt and Plzak, 1983; Endoh et al, 1987; Raj and Vasu, 1990; Raj, 1993; Hu et al, 1995; Walter et al, 2010). Such catalysts have been incorporated to reduce the HER overpotential at illuminated p-Si junction to achieve cathodic photocurrent densities as high as 12 mAcm⁻² in pH 4.5 by Lewis and Gray group in Caltech (McKone et al, 2011).

5.8.2 Stability of photoelectrodes

One of the serious challenges in development of practical PEC cells based on the semiconductor/electrolyte interface (i.e., regenerate PEC cells and photoelectrolysis cells) is, retaining the electrode stability over a prolonged operation. Anodic oxidation and cathodic reduction are well- known routes of electrode material deterioration and may occur under illumination or external bias in the dark (Gerischer, 1977). The driving force for these electrochemical processes is the free energies of holes and electrons, respectively. As illustrated in Fig. 5.2, these free energies are represented by _pE_F and _nE_p under illumination. If _pE_p is positive to the anodic decomposition (oxidation) potential of a particular material, it oxidizes under illumination. Similarly, if nE_F is negative to the cathodic decomposition (reduction) potential it under- goes reduction. Semiconductor materials degradation/dissolution influenced by illumination are commonly referred to as photocorrosion. CdS, ZnO, Cu₂O, Si and GaAs are well-known semiconductor photoelectrode materials that undergo photocorrosion (Morrison, 1980; Meissner et al, 1987; Zhang et al, 2009). Studies have been conducted to circumvent both photo-oxidation and photoreduction by surface modifications, protective coatings and cata- lytic incorporation (Hodes et al, 1983; Pijpersa et al, 2011).

5.8.3 New frontiers for photoelectrolysis cells

It has been 40 years since photoelectrolysis was first demonstrated (Fujishima and Honda, 1972). Since then, numerous efforts have been made not only by chemists, physicists, engineers and materials scientists in technology development, but also by economists, investors, policy makers, entrepreneurs, environmental lobbyists and enthusiasts in promoting the concept. Despite that, we are still waiting to see a photoelectrolysis device that is efficient, affordable, stable for a prolonged operation, scalable and hence, commercially attractive for long-term and large industrial scale invest- ments. Nevertheless, tremendous progress has been made in many areas directly or indirectly related to photoelectrolysis. In terms of the science and technology of semiconductor photoelectrolysis, the work conducted up to now suggests that the key challenges are associated with finding desira- ble semiconductor materials, understanding the fundamental processes in photoelectrolysis that matters in device operation and addressing the com- patibility issues of key components in device construction. Besides, a significant increase of the demand for hydrogen is essential to trigger large-scale investments for semiconductor photoelectrolysis. This final section provides an overview of the future research directions and emerging new frontiers in semiconductor photoelectrolysis.

As already highlighted, finding desirable semiconductor photoelectrode materials that meet the stringent criteria for photoelectrolysis has always been a key challenge. Generally, a few routes are available to pursue and meet this challenge (i.e., continue current efforts to design and discover new materials, attempt to alter the interfacial energetics and charge transfer properties of known semiconductors). A brief summary was given in Section 5.4 with a few notable examples of recent advances in finding new materials with appreciable PEC properties. Such efforts need to be continued with the focus not only on light-harvesting properties but also on interfacial energetics and charge transfer properties of targeted new materials. Many complex oxides are known for their low charge mobility and short minority carrier lifetime. Therefore, as already discussed, construction of electrodes with such materials in textured form (i.e., nanostructuring) may circumvent the charge recombination in bulk as well as within the space charge region (Tahir and Wijayantha, 2010). However, the use of very fine nanoscale features containing photoelectrodes can also be introduced some unwanted complications for the photoelectrolysis device operation. For example, driv- ing HER or OER with a decent photocurrent density (i.e., 10 mAcm⁻²) on a nanostructured electrode may build a steep local pH gradient within the high porous matrix due to mass transport limitations in the liquid phase which could eventually cause corrosion/dissolution of semiconductor elec- trode material as well as alter the energetics at the interface with respect to redox potentials in bulk electrolyte. Therefore, the average feature size (of porous electrodes) should be carefully controlled during electrode prepa- ration by considering such effects. Construction of textured semiconductor electrodes has been mainly achieved to date by following well-documented bottom-up protocols (Hodes et al, 1992; Rothenberger et al, 1992; Barbé et al. 1997; Goh et al, 2005; van de Krol et al, 2008; Sivula et al, 2010). Surprisingly, very little attention has been paid to date for texturing semiconductor electrodes for PEC applications by top-down approaches (Keis et al, 1999; Pacholski et al, 2002; Peng et al, 2007). Electrodes prepared by top- down methods have shown novel properties and found applications in many devices (Sun et al., 2006; Kislyuk and Dimitriev, 2008). In fact, top-down methods offer unique advantages over bottom-up methods particularly in semiconductor photoelectrode preparation with desired PEC properties (i.e., ability to etch off specific surface facets selectively that promote inter- facial recombination, flexibility of starting with flat films of crystalline materials which may have better charge mobility) (Hollan et al, 1979). Therefore, more attention should be devoted to the preparation of textured electrodes for photoelectrolysis by top-down techniques.

The work conducted by us and others show that engineering energetics of known semiconductor materials in favour of photoelectrolysis by manipu- lating preferential orientation is a promising method (Section 5.4.3). Such efforts may be continued and extended to unravel the true potential of known semiconductors which have left out due to the polycrystalline nature of photoelectrodes tested to date. Along with such efforts, other parallel strategies such as construction of smaller band gap heterojunction photo- electrodes needs to be continued to align interfacial energetics in favour of unassisted photoelectrolysis, provided that key semiconductor properties such as energetics, light absorption and stability complement each other. Surface derivatization by covalent chemical modification to control the band energies at the semiconductor/electrolyte interface, and then eliminate the pH dependence of the band edges, is an area that requires further work. Also, the protection of the surface of electrode materials known for corro- sion by incorporating ultra-thin-films of stable oxides is another emerging area of interest following the recent work conducted on Cu₂O and α-Fe₂O₃ to increase PEC properties and stability (Le Formal et al, 2011; Paracchino et al, 2011). The work conducted to study materials such as α-Fe₂O₃ shows that slow interfacial kinetics and accumulation of surface charge is associated with Fermi level pinning and changes in surface composition that must be avoided to improve the PEC properties at the semiconductor/electro- lyte interface (Section 5.7). As discussed in Section 5.8, this can be achieved by incorporating HER and OER catalysts on the semiconductor surface. Although Pt has been the widely used OER catalyst on many semiconduc- tor photoanodes, a significant overpotential is still required even after Pt incorporation (Bockris and Huq, 1956; Rosseler et al, 2010). Further work is required to find new electrocatalysts that are stable (resistive to corro- sion and poisoning and retain the catalyst properties without undergoing compositional or morphological changes) and that can be incorporated on the semiconductor surface. In order to achieve this, an understanding of reaction mechanisms is required (i.e., awareness of the rate determining step(s), intermediates and suitable catalysts to overcome the kinetics). This requires further work and the recent efforts made by us and others need to be continued (Cowan et al, 2011; Cummings et al, 2012a, 2012b; Pendlebury et al, 2011; Peter et al. 2011; Wijayantha et al, 2011).

As already discussed, metal oxides such as α-Fe₂O₃ and WO3 have been extensively studied as photoanodic electrode materials for water splitting. In order to use them in water splitting cells an external bias voltage is required. Tandem water splitting cell configuration offers a solution for this as such an optically parallel or series mounted PV cell provides the necessary bias voltage. Section 5.6 provided a brief overview of photo- electrolysis tandem cell configurations. It is an attractive way of splitting water using the visible light. In fact, photoelectrolysis by visible light has been a challenge for many decades (Wrighton, 1979; Bard and Fox, 1995). Some work has already been done in order to construct and test industrial-scale tandem cells for photoelectrolysis. For example, a UK-based company Hydrogensolar Ltd successfully constructed prototype photoelectroly- sis devices based on the tandem concept (see Fig. 5.25). Further work is required to understand the fundamental limitation of these devices in order to make them robust, stable and the individual device components are chemically compatible. Dye sensitization is an alternative way of using visible light for photoelectrolysis and the concept has been already proven (Youngblood et al, 2009; Sun et al, 2010). Although the performance reported to date for the photoelectrolysis cells based on dye-sensitization is still poor, the advances made in understanding photoinduced electron transfer reactions in molecular donor-acceptor dyads and in sensitizer- semiconductor dyads could be very useful to make further advances in that area. The low efficiency reported for such systems is mainly due to the degradation of the oxidized dye molecules as the kinetic pathways of water oxidation are very slow. The band energies of wide band gap semi- conductors such as SiC, SrTiO₃ and KTaO₃ are favourable for unassisted PEC water splitting, they could be useful materials for sensitization with appropriate dye molecules.

Organic molecules and semiconductors undergo photodecomposition reactions sooner or later following the light absorption. In order to over- come this, nature facilitates continuous regeneration of the photosynthetic system. Mimicking such regeneration mechanism in photoelectrolysis to replenish photodecomposed materials is a very challenging and complex problem to solve. Inspired by the photosynthesis performed by plants, Nocera and his team at the Massachusetts Institute of Technology have combined a catalyst with photovoltaic cells to achieve nearly 100% elec- trolysis (Esswein and Nocera, 2007; Surendranath et al, 2010; Esswein et al, 2011). This is a major breakthrough in the recent history towards mimicking photosynthesis efficiently to generate hydrogen. A detailed account of his work is beyond the scope of this chapter. However, the efforts by Nocera and co-workers could be a catalyst for further advancements for the generation of hydrogen from sunlight at an affordable cost in the near future.

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