Photoelectrolysis of water: Solar hydrogen–achievements and perspectives

Kȩstutis Juodkazis; Jurga Juodkazytė; Edgaras Jelmakas; Putinas Kalinauskas; Ignas Valsiūnas; Povilas Miečinskas; Saulius Juodkazis

doi:10.1364/OE.18.00A147

1. Introduction

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. [1], 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:

H_{2} O \overset{h ν}{\to} H_{2} + 1 / 2 O_{2}

represents one of the most challenging and promising ways of solar energy accumulation, as solar hydrogen can be further used for various purposes, fuel of the future being among them. Several decades have passed since the discovery of photocatalytic water splitting on TiO₂ surface in 1972 [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. [3], 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 [3].

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. [5] 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 [8], 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 [8], 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. [3], 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 [5]. Tandem photoelectrochemical cells, consisting of n- and p-type semiconductor photoelectrodes, or PECs with solid state thin-film multi junctions have been proposed [15] 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) [5], as the standard potential of water oxidation to molecular oxygen, $E_{O_{2} / H_{2} O}^{0}$ , is 1.23 V [16]. However Grätzel [12] 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 i_a ≈ 50 mA cm⁻² and the anode-cathode voltage ranging between 150 and 180 V [22]. 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 [8].

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 [12], a layer of TiO₂ nanoparticles with light-sensitive dye molecules adsorbed on their surface is formed on the electrically and optically conducting substrate [23]. The same substrate covered with Pt nanoparticles serves as counter electrode, whereas electrolyte contains $J_{3}^{-} / J_{2}$ 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:

$E_{p h}^{o . c .}$ – open-circuit potential of Ti/TiO₂ electrode under UV illumination; measured electrometrically, i.e. with the voltmeter with very high, ∼ 10¹² Ω, input resistance;
I_ph– photocurrent, i.e. the difference between current values with and without illumination;
I_ph 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 ΔE_ph (see further), whereas ΔE_ext 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.

Fig. 1 (a) Schematics of water photoelectrolysis cell with external power source and equipment for the measurement of electric parameters. (b) The equivalent scheme of the electric circuit. WE, RE, CE - working, reference and counter electrodes, respectively, GC - gas chromatograph, ΔE_ext - voltage of external power source, ΔE_ph - voltage of photocurrent source, I_ph - photocurrent, hν- photon energy.

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Incident photon-to-electron conversion efficiency IPCE or quantum efficiency QE is calculated according to the following equation [4]:

I P C E = Q E = \frac{F l u x o f e l e c t r o n s}{F l u x o f p h o t o n s} = \frac{I_{p h} / q}{P / h ν} = \frac{I_{p h} h ν}{P q} = \frac{I_{p h} E_{h ν}}{P} \cdot 100 (%),

where hν the energy of photon, P is the power of incident light in mW cm⁻², q is the electron charge 1.602 × 10⁻¹⁹ C, I_ph is the photocurrent in mA cm⁻², and E_hν is the bandgap, in volts, over which an electron excitation takes place by direct absorption of quanta hν. The wavelength dependence of IPCE(λ) is determined via photon energy hν ≡ hc/λ, here c is the light speed and λ is the wavelength.

Another very important parameter characterizing the efficiency of light to electric energy conversion is ECE, which is understood as ratio between useful power output P_out, and total light power input P:

E C E = \frac{P_{o u t}}{P} \cdot 100 (%) .

In the case of solar cells [12] [Fig. 2(a)] ECE is calculated according to the following equation:

E C E = \frac{I_{p h}^{m p p} V_{m p p}}{P} = \frac{I_{p h}^{s c} V_{p h}^{o c} F F}{P} \cdot 100 (%),

where

I_{p h}^{m p p}

and

I_{p h}^{s c}

stand for I_ph at maximum power point and under short-circuit conditions (R = 0), respectively; V_mpp and

V_{p h}^{o c}

stand for cell voltage at maximum power point and under open-circuit conditions (R = ∞), respectively; FF is fill factor which is expressed as

F F = I_{p h}^{m p p} V_{p h}^{m p p} / I_{p h}^{s c} V_{p h}^{o c}

. The meaning of all the above parameters is explained in Fig. 2(b), which also depicts the response of I_ph and V_ph as the resistance of the circuit [Fig. 2(a)] changes from R = ∞ to R = 0.

Fig. 2 (a) Schematics of Si solar cell with electric scheme for cell voltage V_ph and photocurrent I_ph measurement. (b) Typical photocurrent vs. photovoltage response when electric circuit resistance R varies from R = ∞ (open-circuit) to R = 0 (short circuit). Definition of the fill factor, FF, is schematically shown.

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In the case of photoelectrochemical solar cells with external power source following equation is used for calculation of ECE [8, 18, 25]:

E C E = \frac{(I_{p h} E_{O_{2} / H_{2} O}^{0} - I_{p h} Δ E_{e x t})}{P} \cdot 100 (%),

where

E_{O_{2} / H_{2} O}^{0} = 1.23 V

. In the latter case ECE depends on the value of photoelectrode potential, which can be changed with the help of external power source. However, in general case, when ΔE_ext = 0, total thermodynamically useful efficiency of light to chemical energy conversion should be calculated according to the equation as follows:

E C E = \frac{I_{p h} E_{O_{2} / H_{2} O}^{0}}{P} \cdot 100 (%) .

One can see that according to Eq. (6), the ECE depends on the value of I_ph solely.

The efficiency of electrolysis, i.e. the electric energy conversion to chemical energy, can be evaluated according to the following equation:

θ = \frac{E_{O_{2} / H_{2} O}^{0}}{E_{a} - E_{c}} \cdot 100 (%),

where E_a – E_c is the difference between the potentials of the anode and cathode of electrolysis cell, i.e. the voltage of the cell. It is noteworthy, that set of 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

I_{p h}^{m p p} = I_{p h}

and V_mpp = E_a – E_c.

In the case of water splitting in PEC, E_a – E_c would be equal to ΔE_ph + ΔE_ext [Fig. 1(b)]. Equation analogous to Eq. (7) was proposed in ref. [26]. The volume of H₂ evolved during electrolysis can be evaluated as described below:

V_{H_{2}} = \frac{22.4 I t}{53.6} = 0.418 I t,

where V_H₂ is volume of hydrogen gas in dm³, I – electrolysis current in amperes, t is time in hours, 22.4 is volume of mole of gas in dm³, 53.6 is charge required to produce one mole of H₂ in Ah mol⁻¹.

In the review [5] 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, $E_{T i / T i O_{2}}^{o c}$ , measured with respect to reference hydrogen electrode, RHE (E_RHE = −0.059 pH), is about 0.6 ± 0.1 V irrespective of solution pH [22]. 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. E_Pt ≤ 0 V (SHE), minimum voltage of external power source, ΔE_ext, should be 0.2 – 0.25 V [9] 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:

2 {TiO}_{2} \overset{h ν}{\to} {TiO}_{2}^{+} + {TiO}_{2}^{-}

where

{TiO}_{2}^{+}

and

{TiO}_{2}^{-}

represent holes and photoelectrons captured in TiO₂ phase, respectively. The intensity of UV illumination should determine the surface concentration of these particles, the rate of their formation and recombination and also the value of

E_{T i / T i O_{2}}^{o c, h ν}

. Thus, the semiconducting nature of TiO₂ oxide leads to the formation of oppositely charged zones, on the surface of which different electrochemical reactions can take place, i.e. the cathodic reaction on negatively charges zones and anodic reaction on positively charges ones. This type of patch formation on the uniformly illuminated surface of titania can be illustrated by Pt deposition from solution [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 [28], 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 ΔE_ph (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 $E_{T i / T i O_{2}}^{o c, h ν}$ 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 [29].

Fig. 3 (a) Photo-processes on an illuminated TiO₂ electrode at anodic bias. (b) Schematic potential diagram of TiO₂ surface state under UV illumination, open-circuit (I_ph = 0) and photoelectrolysis (I_ph ≠ 0) conditions. ΔE_ext - voltage of external power source, ΔE_ph - voltage of photocurrent source, $_{p} E_{F}^{h ν}$ and $_{n} E_{F}^{h ν}$ - potential values corresponding to Fermi levels of holes and photoelectrons in UV-illuminated TiO₂ surface under open-circuit conditions; E_a and E_c - potentials of anode and cathode under water photoelectrolysis conditions in PEC; η_a and η_c - overvoltage or polarization of anodic and cathodic processes: η_a,c = E_i – E_i₌₀; E_CB and E_VB - potential values corresponding to TiO₂ conductive and valence bands, $E_{B G}^{T i O_{2}}$ - voltage corresponding to TiO₂ band gap, HER and OER - hydrogen and oxygen evolution reactions, respectively; voltammograms: 1 - HER on Pt electrode, 2 - real OER on TiO₂ surface, 3 - visually observed OER on Ti/TiO₂ electrode (see text for details). I_ph can vary from ∼ 0.01 to ∼ 10 mA cm⁻² depending on the experimental conditions.

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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 $ɛ_{F}^{T i O_{2}}$ into two energetic levels $_{n} ɛ_{F}^{h ν}$ and $_{p} ɛ_{F}^{h ν}$ , which are close to conductive and valence band levels ɛ_CB and ɛ_VB, respectively [30]. All these energy levels correspond to the potentials $_{n} ɛ_{F}^{h ν}$ , $_{p} ɛ_{F}^{h ν}$ , E_CB and E_VB, since E = ɛ/F [30]. The voltage of photo-power source $Δ E_{p h} =_{p} E_{F}^{h ν} -_{n} E_{F}^{h ν}$ , whereas the potential difference corresponding to semiconductor band gap (BG) E_BG = E_VB – E_CB. The value of E_BG is higher than ΔE_ph, i.e. E_BG > ΔE_ph [30]. The value of $_{n} E_{F}^{h ν}$ corresponds to the potential $E_{T i / T i O_{2}}^{o c, h ν}$ of Ti/TiO₂ electrode, whereas the value of $_{p} E_{F}^{h ν}$ 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 $E_{a} ≪ E_{O_{2} / H_{2} O}^{0} = 1.23 V$ (curve 3 in Fig. 3) or, in other words, at under-potential conditions. In reality, however, OER is neither depolarized by ΔE_ph, nor catalyzed. In the case of water photoelectrolysis, the measured voltage between the Ti/ TiO₂ anode and Pt cathode is E_a – E_c = ΔE_ext, 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 ΔE_ext + ΔE_ph [Fig. 3, Fig. 1(b)]. Overvoltage, or to be more exact, polarization of O₂ evolution, $η_{a} = E_{a} - E_{T i / T i O_{2}}^{o c, h ν}$ , is significantly higher than the overvoltage of H₂ evolution, η_c, and makes the major constituent of the external voltage: $Δ E_{e x t} = E_{a} - E_{c} = Δ E_{e x t}^{o c} + η_{a} + η_{c}$ (Fig. 3). As η_a 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 $_{n} E_{F}^{h ν}$ , which corresponds to the potential $E_{T i / T i O_{2}}^{o c, h ν}$ , turns more positive, also does the potential corresponding to the level $_{p} E_{F}^{h ν}$ , 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:

2 H_{2} O - 4 e^{-} \Leftrightarrow O_{2} + 5 H^{+}

is

E_{O_{2} / H_{2} O}^{0} = 1.23 V

[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 [32] and represents the sum of various elementary steps of OER; (iii) this process really occurs at

E ≫ E_{O_{2} / H_{2} O}^{0}

, 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 [31], 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 [33]. No oxygen evolution is observed on TiO₂ surface at $E_{O_{2} / H_{2} O}^{0} = 1.23 V$ . 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 $E_{O H / H_{2} O}^{0} = 2.8 V$ [34], which is the standard potential of water oxidation to OH radicals:

H_{2} O - e^{-} \Leftrightarrow • OH + H^{+} .

The latter E value is also close to

_{p} E_{F}^{h ν}

and E_VB of UV light-excited TiO₂ (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 $O_{2}^{2 -}$ and H₂O₂ formation [1, 28]. In Ref. [10] Tachikawa et al. have indicated following standard potential values of species which play important role in O₂ evolution process: $h_{V B}^{+}$ at 2.96 V (pH= 2), $h_{t r}^{+}$ (trapped) at (1.6 – 1.7) V; ^• OH_free at 2.72 V, + and ^•OH_ads over (1.5 – 1.6) V. In terms of energy, the state of trapped hole $h_{t r}^{+}$ and adsorbed OH radical ^•OH_ads are close to the state of Ti(6+) oxide - TiO₃ [31]. 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:

{TiO}_{2} \overset{h ν, E_{a}}{\to} {TiO}_{2}^{+} + e^{-}, (h ν \approx 3.2 eV)

{TiO}_{2}^{+} + H_{2} O \to {TiO}_{2} {(• OH)}_{ads} + H^{+}, (E_{OH / H_{2} O}^{0} = 2.8 V)

{TiO}_{2} {(• OH)}_{ads} \to {TiO}_{3} + H^{+} + e^{-}, (E_{O / H_{2} O}^{0} = 2.4 V)

{TiO}_{3} \Leftrightarrow {TiOO}_{2} (Ti (4 +) peroxide)

{TiOO}_{2} + H_{2} O \to {TiO}_{2} + H_{2} O_{2}

H_{2} O_{2} \to O_{2} + 2 H^{+} + 2 e^{-} (E_{O_{2} / H_{2} O_{2}}^{0} = 0.68 V)

2 H_{2} O_{2} \to 2 H_{2} O + O_{2} (disproportionation) .

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 $E_{O H / H_{2} O}^{0} = 2.8 V$ , 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 [31]. 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 $O_{2}^{2 -}$ 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):

2 H_{2} O + 4 h^{+} \to_{h ν = 3.2 e V}^{E \geq 2.8 V} O_{2} + 4 H^{+} .

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 $E_{O_{2} / H_{2} O}^{0} = 1.23 V$ , 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 [5]. If the maximum photocurrent density I_ph makes just ∼1 mA cm⁻² at full spectrum AM1.5 illumination as reported in Ref. [8], 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. I_ph ≈ 1 mA cm⁻², is achieved using external power source, the voltage ΔE_ext of which is almost equal to the voltage of photocurrent source ΔE_ph. 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 [44]. 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 [29]. 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 [5]. 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.

Fig. 4 (a) Schematic representation of water electrolyzer using solar cells connected in series and in parallel as source of photocurrent. (b) Optimal voltammograms of anodic and cathodic processes of water electrolysis in alkaline electrolyte. (c) Comparison of average ECE in visible spectral region for light energy conversion using Ti/TiO₂ photoelectrode, DSSC, Si [43] and tandem GaInP/GaAs solar cells.

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Plasmonic light field enhancement [55] 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 [56]. According to Eq. (2), $I P C E (λ) \equiv \frac{I_{p h} (λ) E_{h ν} (λ)}{P}$ , only the photons absorbed at the wavelength λ are contributing to the useful current I_ph. 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 [57] and to control the enhancement and polarization at the nano-particle-substrate interface [58] 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 $I_{p h}^{m p p}$ , the cell voltage, i.e. $V_{p h}^{m p p}$ 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 [43], tandem solar cells [59] and also photoelectrochemical solar cells (PESC), like Grätzel cells [12], 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 [60] 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), I^mpp = 32.5 mA cm⁻², V^mpp = 505 mV, P^mpp = 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. [59] 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%, $I_{p h}^{s c} = 14.0 {mA cm}^{- 2}$ , V_oc = 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 [61]. The electric parameters of such cells are: P^mpp = 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 [61]. 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 [8] are low cost, can be made transparent from both sides, and a large-area (25 × 25 cm²) cell production is possible [61].

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 [13]. 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 [62]. 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. [5], 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 [5], 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 [40].

4. Conclusions

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: $2 H_{2} {O + 4 h}^{+} \to_{h ν = 3.2 e V}^{E \geq 2.8 V} O_{2} + 4 H^{+}$ , 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: $E C E = \frac{I_{p h} E_{O_{2} / H_{2} O}^{0}}{P} \cdot 100 (%)$ , where $E_{O_{2} / H_{2} O}^{0} 1.23 V$ , I_ph 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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Photoelectrolysis of water: Solar hydrogen–achievements and perspectives

Abstract

1. Introduction

2. Experimental aspects

3. Results and discussion

3.1. Mechanism of water photoelectrolysis

3.2. Oxygen evolution reaction on TiO₂ surface

3.3. Technological perspectives for solar hydrogen

3.3.1. Optimization of light-to-chemical energy conversion process

3.3.2. Technical solutions at the current state-of-the-art

3.3.3. Optimal conditions for water splitting: numerical estimates

4. Conclusions

References and links

Cited By

Figures (4)

Equations (19)

Optics Express

Abstract

1. Introduction

2. Experimental aspects

3. Results and discussion

3.1. Mechanism of water photoelectrolysis

3.2. Oxygen evolution reaction on TiO2 surface

3.3. Technological perspectives for solar hydrogen

3.3.1. Optimization of light-to-chemical energy conversion process

3.3.2. Technical solutions at the current state-of-the-art

3.3.3. Optimal conditions for water splitting: numerical estimates

4. Conclusions

References and links

Cited By

Figures (4)

Equations (19)

Optics Express

3.2. Oxygen evolution reaction on TiO₂ surface