Thermodynamic analysis of energy conversion from light-to-chemical, light-to-electric and electric-to-chemical is presented by the case study of water photoelectrolysis on TiO2 surface. It is demonstrated that at the current state-of-the-art energy conversion efficiency of water photoelectrolysis can be increased ∼17 times by separating the processes of solar-to-electric and electric-to-chemical energy conversion and optimizing them independently. This allows to mitigate a high overvoltage of oxygen evolution reaction with respect to thermodynamic potential as well as spectrally narrow absorbtivity of solar light by TiO2 which determine the low efficiency (∼ 1.0%) of direct light-to-chemical energy conversion. Numerical estimates are provided illustrating practical principles for optimization of the solar energy conversion and storage processes.
© 2010 OSA
In recent years series of review articles devoted to solar energy conversion and accumulation have appeared in scientific literature [1–13]. According to Fujishima et al. , the amount publications in this field have increased exponentially starting from 1990s. This points to rapidly increasing interest in solar energy utilization, whereas the abundance of reviews witnesses the need for critical evaluation and summarizing of available results in order to advance our knowledge in this area.
Photolysis or photosplitting of water into gaseous O2 and H2 as described by equation:14], however the possibilities of practical application of this process remain vague despite of numerous and comprehensive studies [1, 3–7, 9–12]. According to Ref. , various ways of modification of TiO2 particles, such as noble metal or noble metal oxide loading on TiO2 surface, metal ion or anion doping, metal ion implantation, dye sensitization and also addition of sacrificial or other components to the electrolyte lead to rather low rate of water splitting, i.e. ∼ 600 μ mol h−1 of H2 and ∼ 300 μ mol h−1 of O2 at best. In the case of unmodified TiO2 particles, only traces or even no gaseous H2 and O2 are found chromatographically .
When light-sensitive thin semiconducting layer of TiO2 is formed on titanium or some other electrically conducting substrate, its potential can be controlled electrochemically using reference electrode, whereas the usage of counter electrode, usually Pt, and external power source allows to separate anodic and cathodic processes, taking place on the surface of TiO2 oxide, and also to control their rate. In other words, photosplitting of water can be performed in photoelectrochemical cell (PEC) as photoelectrolysis of water, when evolution of hydrogen and oxygen takes place on different electrodes, i.e., O2 on TiO2 and H2 on Pt electrode. Nowotny et al. in Ref.  presented the comprehensive review of most important photoelectrochemical studies performed since 1972 till 2005. In the cases when water splitting on TiO2 surface was studied in PECs using natural or artificial full-spectrum solar illumination (∼100 mW cm−2), the values of energy conversion efficiency, ECE (see further), ranged between ∼0.1 % and ∼1.0 %. In the review devoted to vertically oriented TiO2 nanotubes , the highest reported H2 generation rate under the same conditions of illumination is 1.75 ml (Wh)−1, which should correspond to 1.75 l h−1m−2 or to photocurrent density of 0.4 mA cm−2.
The rate of hydrogen evolution can be increased significantly using artificial high power UV light sources. For instance, when UV (320 – 400 nm) illumination of ∼100 mW cm−2 power is used , the highest rate of H2 generation on TiO2 nanotubes array is 76 ml (Wh)−1, i.e. 76 l h−1 m−2 (∼ 3.39 mol H2), which should correspond to photocurrent density of 18 mA cm−2. According to authors, the quantum efficiency, QE (see further), in this case is almost 80%. The usage of high power UV light sources for H2 production is inexpedient, however the increase in H2 generation efficiency demonstrates that concentration of solar light should be very effective.
Thus, in the case of PEC, the values of ECE and H2 generation rate under natural solar illumination conditions are significantly higher, but still insufficiently high from the technological point of view. According to Ref. , there are three main reasons for low energy conversion efficiency on TiO2: (i) fast recombination of photogenerated electrons and holes, (ii) fast backward reaction, and (iii) inability to harvest visible and IR light at longer wavelengths than ∼ 400 nm.
Nevertheless, titania and titania based materials are considered to be the most promising candidates for photoelectrodes for solar hydrogen, since they exhibit outstanding resistance to corrosion and photocorrosion in aqueous environments [3, 5], whereas other valence and small band gap materials are not suitable for this purpose due to very short life time and prohibitive cost . Tandem photoelectrochemical cells, consisting of n- and p-type semiconductor photoelectrodes, or PECs with solid state thin-film multi junctions have been proposed  as an option which excludes usage of external current source.
In theory, infrared light with photon energy of 1.23 eV could induce reaction Eq. (1) , as the standard potential of water oxidation to molecular oxygen, , is 1.23 V . However Grätzel  has reported that numerous attempts to shift the spectral response of TiO2 into the visible range or to develop alternative oxides affording water splitting by visible light have been unsuccessful so far, as can be illustrated by Refs. [15,17–21]. Only UV illumination with much higher photon energy (hν ≥ 3.2 eV) is capable of inducing water cleavage [3,5–7,12]. Thorough understanding of the mechanism of oxygen evolution reaction (OER) on TiO2 electrode surface can help to find a clue to this problem.
The aim of this study is to present comprehensive analysis of the processes involved in water photoelectrolysis and to discuss detailed steps and mechanism of oxygen evolution on TiO2 surface based on the currently available scientific information. The presented analysis based on thermodynamical principles and energy conservation reveals the reasons of the low energy conversion efficiency of water photo-splitting on TiO2 surface. The energy conversion efficiencies for the light-to-electrical, electrical-to-chemical, and the light-to-chemical are presented and general expressions are provided for the investigated system of water splitting. Possibility to increase light-to-chemical energy conversion efficiency ECE are evaluated and numerical examples are provided for the current state-of-the-art in solar cell technology.
2. Experimental aspects
As this study represents an analytic overview of our published results and literature data, this section is devoted to the short characterization of the procedures used for preparation of TiO2 photoelectrodes, investigation of their photoelectrochemical properties and evaluation of energy conversion efficiency for the cases of light-to-electric, electric-to-chemical, or directly light-to-chemical energy conversion.
Photosensitive TiO2 layers of submicron thickness were usually formed on titanium substrate by means of anodizing Ti plate in the solution of 0.5 M H2SO4 for 5 min at anodic current density ia ≈ 50 mA cm−2 and the anode-cathode voltage ranging between 150 and 180 V . The anodization procedure was followed by heat treatment for 90 min at 400°C in air.
Layers of vertically oriented TiO2 nanotubes on titanium surface were formed by means of anodizing Ti plate in the solution of 0.5 M H3PO4 + 0.15 M HF. Fabrication, properties as well as application areas of such nanotubes are described in detail in review .
Various vacuum techniques are also used to deposit light sensitive TiO2 layers on glass substrate covered with transparent conducting tin oxide modified with indium oxide (ITO) or with F− ions (FTO). In the case of dye-sensitized solar cells (DSSC), or the so-called Grätzel cells , a layer of TiO2 nanoparticles with light-sensitive dye molecules adsorbed on their surface is formed on the electrically and optically conducting substrate . The same substrate covered with Pt nanoparticles serves as counter electrode, whereas electrolyte contains redox couple.
Photoelectrochemical properties of light sensitive TiO2 layers in the reaction of water splitting were investigated in a three-electrode electrochemical cell supplied with quartz window [1, 5, 8, 24]. The main parameters and characteristics that were measured are as follows:
- – open-circuit potential of Ti/TiO2 electrode under UV illumination; measured electrometrically, i.e. with the voltmeter with very high, ∼ 1012 Ω, input resistance;
- Iph– photocurrent, i.e. the difference between current values with and without illumination;
- Iph vs. E – dependence of photocurrent on the photopotential of Ti/TiO2 electrode, i.e. photovoltammogram (VA).
Schematic representation of water photoelectrolysis cell with external power source and the equipment used for the measurement of the above-indicated parameters is shown in Fig. 1(a). External power source, the voltage of which can be varied within a wide range, is necessary for the realization of water splitting. The scheme, shown in Fig. 1(a), can be realized using potentiostat working in galvanostatic mode. The equivalent circuit of the scheme shown in Fig. 1(a) should correspond to that of scheme shown in Fig. 1(b), where Ti/TiO2 photoelectrode serves also as photocurrent source. In Fig. 1(b) the difference between the potentials of holes and photoelectrons captured in TiO2 surface is denoted as ΔEph (see further), whereas ΔEext denotes the potential difference between the poles of external power source or, in other words, the potential difference between the anode and cathode of PEC.
Incident photon-to-electron conversion efficiency IPCE or quantum efficiency QE is calculated according to the following equation :
Another very important parameter characterizing the efficiency of light to electric energy conversion is ECE, which is understood as ratio between useful power output Pout, and total light power input P:12] [Fig. 2(a)] ECE is calculated according to the following equation: Fig. 2(b), which also depicts the response of Iph and Vph as the resistance of the circuit [Fig. 2(a)] changes from R = ∞ to R = 0.Eq. (6), the ECE depends on the value of Iph solely.
The efficiency of electrolysis, i.e. the electric energy conversion to chemical energy, can be evaluated according to the following equation:Eq. (4), Eq. (6) and Eq. (7) represents energy conversion efficiencies of light-to-electrical, light-to-chemical, and electric-to-chemical, respectively. By direct multiplication of Eq. (4) and Eq. (7) one would arrive to Eq. (6), since and Vmpp = Ea – Ec.
In the case of water splitting in PEC, Ea – Ec would be equal to ΔEph + ΔEext [Fig. 1(b)]. Equation analogous to Eq. (7) was proposed in ref. . The volume of H2 evolved during electrolysis can be evaluated as described below:
In the review  Nowotny et al. presented the characteristics of various light sources used for investigation of photoelectrochemical phenomena. One can see that spectral characteristics and illumination intensity of light sources used differ greatly. The results presented further in this study pertain to natural or simulated solar illumination, which is referred in literature as full spectrum sunlight illumination, air mass AM 1.5, with ∼ 100 mV cm−2 or ∼ 1 kW m−2 intensity.
3. Results and discussion
3.1. Mechanism of water photoelectrolysis
The open-circuit potential of Ti/TiO2 electrode, , measured with respect to reference hydrogen electrode, RHE (ERHE = −0.059 pH), is about 0.6 ± 0.1 V irrespective of solution pH . Under UV illumination the potential of Ti/TiO2 electrode in oxygen-free solution of 0.1 M KOH shifts negatively to ∼ 0.2 V, which is insufficiently low for hydrogen evolution to take place on the electrode surface. Therefore, in order to attain the potential of H2 evolution, i.e. EPt ≤ 0 V (SHE), minimum voltage of external power source, ΔEext, should be 0.2 – 0.25 V  and positive pole of external power source should be connected to Ti/TiO2 electrode (Fig. 1).
Under UV illumination oppositely charged zones form in the surface layer of semiconducting n-type titania. The formation of positively and negatively charged zones on TiO2 surface could be envisaged as photochemical reaction:27] where growth of Pt spheroidal nano-particles forms micro-patterns governed by Turing-type instabilities similar to vegetation in semi-arid regions.
Positively charged nano-regions participate in the anodic oxidation of H2O molecules or the co-called forward reaction (region 2 in Fig. 3), whereas negatively charged ones participate in cathodic reduction of soluble H2O oxidation products, e.g. H2O2 and O2 , i.e. backward reaction (region 3 in Fig. 3). Only the potential of negative zones can be measured experimentally [Fig. 1(a)], as these zones have the electronic contact with Ti phase. Positively charged zones, which are situated mainly on the same surface of TiO2 electrode, are, in fact, isolated one from another and are galvanically connected with negative zones and Ti phase only through the solution phase. The potential of positive zones (holes) is more positive than that of negative ones (photoelectrons) by the value of ΔEph (Fig. 3). This potential cannot be measured directly, but this E value can be attained by means of polarizing Ti/TiO2 electrode anodically. Under open circuit conditions, the value of is determined by the balance of all possible anodic and cathodic reactions on TiO2 surface, similarly to mixed potential in the case of metal corrosion processes .
The potential diagram reflecting the state of TiO2 surface under UV illumination and open-circuit or photoelectrolysis conditions is shown in Fig. 3. Formally, the formation of oppositely charged zones can be understood as splitting of TiO2 Fermi level into two energetic levels and , which are close to conductive and valence band levels ɛCB and ɛVB, respectively . All these energy levels correspond to the potentials , , ECB and EVB, since E = ɛ/F . The voltage of photo-power source , whereas the potential difference corresponding to semiconductor band gap (BG) EBG = EVB – ECB. The value of EBG is higher than ΔEph, i.e. EBG > ΔEph . The value of corresponds to the potential of Ti/TiO2 electrode, whereas the value of corresponds to the potential of holes, i.e. positively charged zones of TiO2 surface, where oxygen evolution actually takes place.
The oxygen evolution on TiO2 surface takes place at E ≥ 3. 0 V (curve 2 in Fig. 3), i.e. in the range of potentials corresponding to TiO2 valence band. However, due to the fact that photopotential of Ti/TiO2 electrode is determined by photoelectrons, an illusion is created that oxygen evolution occurs at potentials corresponding to TiO2 conductive band (CB). Thus, it appears that OER is observed at (curve 3 in Fig. 3) or, in other words, at under-potential conditions. In reality, however, OER is neither depolarized by ΔEph, nor catalyzed. In the case of water photoelectrolysis, the measured voltage between the Ti/ TiO2 anode and Pt cathode is Ea – Ec = ΔEext, but the real voltage between TiO2 surface, where OER takes place, and Pt electrode, where hydrogen evolution reaction (HER) proceeds (curve 1 in Fig. 3), is equal to sum ΔEext + ΔEph [Fig. 3, Fig. 1(b)]. Overvoltage, or to be more exact, polarization of O2 evolution, , is significantly higher than the overvoltage of H2 evolution, ηc, and makes the major constituent of the external voltage: (Fig. 3). As ηa increases, the surface concentration of photoelectrons fixed on TiO2 surface decreases, as also does the probability of their recombination with holes, what leads to increasing effectiveness of charge carriers separation and minimization of backward cathodic reactions on TiO2 surface discussed above. As the potential , which corresponds to the potential , turns more positive, also does the potential corresponding to the level , which is the actual potential of O2 evolution (Fig. 3).
3.2. Oxygen evolution reaction on TiO2 surface
It is well-known to electrochemists that the standard potential of H2O oxidation to molecular oxygen, i.e. oxygen evolution reaction:29, 31]. This potential can be found in every energy diagram illustrating water splitting on TiO2 surface [1,5,30]. However, one should keep in mind that: (i) this potential indicates just the thermodynamic possibility of reaction Eq. (10); (ii) this reaction always takes place on the electrodes covered with oxides  and represents the sum of various elementary steps of OER; (iii) this process really occurs at , depending on the nature of the main step through which the OER proceeds on the surface of various oxide electrodes. Those steps of reaction Eq. (10) in the case of TiO2 electrode are discussed below.
It is known from the electrochemistry of titanium , that under standard conditions, TiO2 layer forms on the surface of Ti in the anodic range of potentials at E > 0 V (SHE), leading to complete passivation of the electrode. When the thickness of TiO2 layer is 20 to 25 nm, the transpassivation of Ti/TiO2 electrode, i.e. the oxidation of TiO2 to TiO3 and H2O to O2 begins at 1.8 – 2.0 V, whereas at higher TiO2 layer thickness these processes shift towards more positive E values, i.e. up to ∼ 3.0 V . No oxygen evolution is observed on TiO2 surface at . As it is shown in Fig. 3, O2 evolution on TiO2 surface begins at E ≈ 3.0 V, i.e. at E value higher than , which is the standard potential of water oxidation to OH radicals:Fig. 3).
Oxygen evolution on TiO2 surface under UV illumination has been thoroughly studied [1, 5, 9, 28, 35, 36]. There is no doubt already that the process goes through the stages of OH radicals and also peroxide group and H2O2 formation [1, 28]. In Ref.  Tachikawa et al. have indicated following standard potential values of species which play important role in O2 evolution process: at 2.96 V (pH= 2), (trapped) at (1.6 – 1.7) V; • OHfree at 2.72 V, + and •OHads over (1.5 – 1.6) V. In terms of energy, the state of trapped hole and adsorbed OH radical •OHads are close to the state of Ti(6+) oxide - TiO3 . By analogy with results presented in refs. [37–40], where OER on Ti/RuO2 and Ni/NiO electrodes in the potential range of their transpassivation was studied, the mechanism of oxygen evolution on TiO2 surface under UV illumination or in E range of its transpassivation could be represented by the sequence of the following main steps:
The sum of reactions Eqs. (12)–(17) will give the overall reaction Eq. (10). Step Eq. (13), which, in fact, is oxidation of H2O molecule to OH radical with , determines the potential of OER onset on TiO2 surface. OH radical, formed in step Eq. (13), is a particle with extremely high oxidizing energy and is capable of oxidizing TiO2 to TiO3 [Eq. (14)]. The latter oxide should be stable on TiO2 surface at E > 1.8 V . It is highly probable that, similarly to electrocatalysis of OER by RuO2 and NiOOH [37, 38, 40], oxidation of oxide ion O2− to peroxide ion takes place within TiO3 molecule, which undergoes internal restructuring to Ti(4+) peroxide TiOO2 [Eq. (15)]. Hydrogen peroxide formed in chemical reaction Eq. (16) can either be further oxidized electrochemically on TiO2 surface [Eq. (17)], or disproportionate to O2 and H2O in the solution [Eq. (18)]. Several alternative mechanisms of O2 evolution initiated also by OH radicals can be found in literature [28, 35, 41, 42]. So the overall equation of photoelectrochemical oxidation of H2O molecules to O2 on TiO2 surface should be expressed by the following reaction, which is more adequate to reality than Eq. (10):
Since the energy of visible light is too low to induce the formation of OH radicals, all attempts to shift the absorption of TiO2 into visible region so that water splitting would be possible under visible light illumination were not and cannot be successful. Therefore oxygen evolution on TiO2 surface is possible either at very high overvoltage with respect to , i.e. in transpassivation region of Ti/TiO2 electrode, or under UV illumination (hν ≈ 3. 2 eV) only. Due to its chemical and electrochemical stability TiO2 does not posses any catalytic properties towards OER. Unmodified titanium anodes are not used for OER due to their passivation. Since in the case of water photoelectrolysis anodic and cathodic processes are interrelated, the low rate of OER determines the low rate of hydrogen evolution reaction. The way how to overcome this limitation is discussed in the next section.
3.3. Technological perspectives for solar hydrogen
3.3.1. Optimization of light-to-chemical energy conversion process
Despite the optimism expressed in Refs. [3,5–7] regarding the perspectives to produce solar hydrogen on TiO2 surface, a major breakthrough in this area of research is hardly probable. The portion of UV spectrum absorbed by titania makes just ∼4% of the whole solar spectrum . If the maximum photocurrent density Iph makes just ∼1 mA cm−2 at full spectrum AM1.5 illumination as reported in Ref. , then the light to chemical (H2 and O2 gases) energy conversion efficiency (ECE) according to Eq. (6) would make just ∼1%, though quantum efficiency, QE calculated according to Eq. (2) for UV portion of the spectrum is rather high, i.e. ∼80%. It should also be kept in mind that such result, i.e. Iph ≈ 1 mA cm−2, is achieved using external power source, the voltage ΔEext of which is almost equal to the voltage of photocurrent source ΔEph. Thus it is evident that in order to improve the situation essentially and to increase ECE significantly the photosensitive material should absorb up to 100% of solar illumination (i); the voltage of photocurrent source should be high enough so that external current source would not be necessary (ii) and the process of electrolysis should be facile and fast, i.e. with minimum overvoltage of OER (iii). All these requirements can be fulfilled by means of separating the processes of light to electric and electric to chemical energy conversion. Such configuration would make it possible to choose the photocurrent source with maximum light absorption parameters and also to make optimal conditions for water electrolysis by using OER catalyzing anodes. It would also exclude any problems related with corrosion, photocorrosion and energy losses, which are inevitably brought about by the electrochemical charge transfer processes on the surface of semiconductor . Moreover, such realization of water photoelectrolysis is not only much more efficient, but also simpler from the technological point of view than photoelectrolysis in PEC. In fact, this would mean abandoning the idea of direct water photoelectrolysis on TiO2 surface and replacing it by water electrolysis with a photocurrent source. Such concept of indirect water photoelectrolysis has been demonstrated in Refs. [26,45] and is schematically shown in Fig. 4(a). In the case of alkaline medium, OER catalyzing anodes such as Ti/RuO2, Ti/RuO2·NiO, Ni/NiO [37, 38, 40] and Pt cathode for HER should be used, as hydrogen evolution is the most facile on platinum . This would allow reducing the voltage of electrochemical water splitting down to 1.4 – 1.5 V [Fig. 4(b)] instead of 3.0 V (Fig. 3), what would formally correspond to light quantum energy of 1.4 – 1.5 eV and would cover ∼64% of solar spectrum . This would open up possibilities to use various photocurrent sources and to employ plasmonic effects [46–54], which may find application in a next generation of solar cells.
Plasmonic light field enhancement  causes a strong band bending at the surface of solar cell substrate (e.g., Si, TiO2) providing electron-hole separation and injection at the wavelength, λ, which is defined by particle size and shape and can be tailored for extinction in visible or IR spectral range . According to Eq. (2), , only the photons absorbed at the wavelength λ are contributing to the useful current Iph. This presumes a low efficiency of solar cell at the wavelength longer than that of the bandgap since the photo-current is due to electrons from the defects which, also, increase recombination losses at the direct absorption wavelengths where efficiency of solar cell has its maximum. Possibility to fabricate large areas of uniform plasmonic nanoparticles  and to control the enhancement and polarization at the nano-particle-substrate interface  are promising for practical solar cell applications especially at wavelengths where IPCE is not small.
3.3.2. Technical solutions at the current state-of-the-art
In order to perform the water electrolysis at a maximum rate , the cell voltage, i.e. should be higher than ∼1.5 V due to overvoltages of OER and HER and also ohmic drop IR due to resistance of electrolyte [Fig. 4(b)]. In industrial electrolyzers this voltage amounts to 2.0 – 2.5 V. Currently produced solar cells of any type , tandem solar cells  and also photoelectrochemical solar cells (PESC), like Grätzel cells , can serve as photocurrent source [Fig. 4c]. In order to ensure required power of current source, i.e. certain values of current and voltage, these cells can be interconnected either in series or in parallel. For instance, solar cells XOB17-12-1 (IXYS Corporation, USA), based on monocrystalline Si  with bondable solar cell dies, are characterized by excellent light absorbance (QE ≥ 90%) not only in visible, but also in infrared range of the spectrum. The main parameters of solar cell are as follows: dimensions 22 × 7 × 1.4 (mm3), Impp = 32.5 mA cm−2, Vmpp = 505 mV, Pmpp = 16.6 mW cm−2, FF = 75%, ECE = 17%. If such cells were connected in series by 5 and then in parallel into 1 m2 solar panel, this would produce current of 50 A at voltage of 2.5 V, what corresponds to the power of 125 Wm−2, under AM1.5 full spectrum illumination. This would, in fact, correspond to light to electric energy conversion efficiency of ∼12.5%, whereas the total light to chemical energy conversion efficiency according to Eq. (6) would be: 50 A m−2 ·1.23 V/1000 W m−2 = 6.15%. One can see that the latter ECE value makes just half of the former. This should be attributed to the energy losses during electric to chemical energy conversion and mainly to a thermodynamic irreversibility of O2 evolution process [Fig. 4(b)].
The ECE values given above are not the highest possible. Bertness et al.  have reported that in the case of GaInP/GaAs tandem solar cells, which are produced since 1994, the light to electric energy conversion efficiency achieves ∼29.5%. Other parameters are as follows: QE ≈ 90%, , Voc = 2.385 V, FF = 88.5%. In this case, the power of 1 m2 solar panel would make 295.5 W m−2 and the light-to-chemical energy conversion efficiency according to Eq. (6) would be ∼ 17.2%, whereas the rate of H2 evolution according to Eq. (8) would be ∼58.5 l h−1m−2 (or 2.6 mol h−1m−2). This corresponds to an approximately 17 times improvement of the light-to-chemical energy conversion as compared to that achievable on a TiO2 electrode. As shown above, the efficient technology of water photoelectrolysis is technically possible yet expensive and therefore not commercially viable so far.
It has recently been reported about a possibility to produce large-area photoelectrochemical Grätzel type solar cells . The electric parameters of such cells are: Pmpp = 221.4 mW, ECE = 8.4% at aperture area of 26.47 cm2. This would correspond to power of ∼ 90 W m−2 and current density of 35 A m−2 . Though, from the standpoint of indirect water photoelectrolysis, the above parameters are almost 3 times worse, it is very important that PESC of Grätzel type  are low cost, can be made transparent from both sides, and a large-area (25 × 25 cm2) cell production is possible .
All the above parameters were determined under natural or artificial AM1.5 solar illumination. In reality, however, the intensity of solar illumination depends on many factors, which were thoroughly discussed in . When all these factors are taken into account, the average power of solar illumination during 24 hours makes just ∼ 200 W m−2, whereas in northern regions it is even lower.
3.3.3. Optimal conditions for water splitting: numerical estimates
Returning once again to the maximum parameters of water photoelectrolysis mentioned above, it should be pointed that solar panel of 20 m2 area with average power of ∼1.2 kW (0.2 kW m−2 ·20 m2 ·0.3 = 0.5 kA·2.4 V) is capable of producing about 30 kWh of electric energy or about 5.0 m3 of hydrogen gas during 24 hours. To put it differently, such panel is capable of accumulating electric charge of ∼ 12.0 kAh or ∼15.0 kWh of chemical energy in the form of H2 and O2 gases, which is equal to ∼54 MJ or ∼ 13 ·103 kcal or ∼ 2.0 l of gasoline . This production of hydrogen will also yield in 2.5 m3 O2 and would consume 4 l (or ∼ 220 mol) of water. Such amounts of energy can suit household needs, but are far too low for industrial purposes. According to Ref. , the amount of solar hydrogen equivalent to 1 GJ of energy should cost about 28 USD including production equipment and maintenance costs, whereas the production of the same amount of hydrogen from natural gas costs just 6 USD. Thus solar hydrogen is too expensive alternative so far for hydrogen produced by conventional ways, including water electrolysis. On the other hand, restriction of ecological requirements regarding CO2 emission , and price reduction of solar electricity can change the situation in the near future. It is also beneficial that no transformations of solar electricity are necessary for water electrolysis and even alkalized seawater can serve as an electrolyte .
The presented analysis of photoelectrolysis of water on TiO2 surface explains the low efficiency of the direct light-to-chemical energy conversion. Based on thermodynamic arguments of solar-hydrogen generation, strategies for the most efficient solar energy conversion are outlined. At the current development of solar cells, it is possible to improve solar hydrogen generation by at least 17 times as compared to the attempts of direct solar-to-chemical energy conversion on a TiO2 electrode.
More specifically, it is shown that water splitting on titania, i.e., oxidation of water molecules to molecular oxygen on the surface of semiconducting TiO2 oxide proceeds via the stages of OH radicals and, most likely, titanium peroxide TiO3 formation. The process is feasible only at E ≈ 3.0 V (SHE), what corresponds to Ti/TiO2 electrode transpassivation region or to the potential of holes in UV-excited titania. The overall equation of photoelectrochemical oxidation of H2O molecules to O2 on TiO2 surface can be expressed by the following reaction: , revealing the role of photo-generated holes.
Thermodynamic efficiency of solar to chemical energy conversion is defined by the useful energy which can be harnessed by reaction of oxygen and hydrogen and is given by: , where , Iph is the density of photocurrent of water electrolysis, mA cm−2, P is the power of incident light, mW cm−2.
It is demonstrated that by separating and optimizing processes of light-to-electric and electric-to-chemical energy conversion, the overall ECE could be increased from ∼1% up to ∼17% by using solar cells with maximum, i.e. ∼30%, light-to-electric energy conversion efficiency as well as OER catalyzing anodes for water electrolysis. Then, a solar panel of 20 m2 area would be capable of producing ∼ 5 m3 of H2 and ∼ 2.5 m3 of O2 during 24 hours, what is equivalent to accumulation of 12 kAh of electric charge or to ∼ 15 kWh, 54 MJ, 13 Mcal of chemical energy or to ∼ 2.0 l of gasoline. This would consume ∼ 4 l (or ∼ 220 mol) of water.
Generic energy storage and conversion formulas suitable for different solar energy harvesting applications are presented and justified by analysis of the water splitting mechanism. The used basic principles can be applied to other solar energy conversion systems.
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