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Photocatalytic Water Splitting

Energy generation from fossil fuels is a major environmental problem since it is associated with the emission of gases such as nitrogen oxides, sulfur oxides and carbon dioxide. One possibility to reduce the share of fossil fuels is the use of hydrogen as energy source generated from renewable resources.[1] Beside electrolysis of water by using renewable energy sources, water can be splitted into hydrogen and oxygen via photocatalytic processes.

Photocatalytic water splitting has been investigated extensively since its first discovery.[24] FUJISHIMA and HONDA used an n-type titanium dioxide as photoanode and a platinum counter electrode. The authors have shown that oxygen evolution occurs at the photoanode and hydrogen evolution at the cathode, when titanium dioxide is irradiated and an external or chemical bias is applied.[35]

 Allgemeine_Aufbau1

Figure 1: General function principle of photocatalytic water splitting.[5]

The first step of photocatalytic water splitting is charge separation, which is caused by irradiation of photocatalytic active semiconductor, such as TiO2 with an energy greater than the band gap of the semiconductor. Electrons and holes are generated at the photoanode:

wseq1

The generated holes can subsequently oxidizes water, so that O2 and H+ ions are generated. Afterwards H+ ions are transported through an electrolyte to the cathode.

wseq2 

 ws1

Figure 2: Solar irradiance Ee and photon fluence rate n˙Photon recieved on earth’s crust (AM 1.5 solar spectrum).[6]

The transport of electrons from the anode to the cathode via an external circuit can lead to the reduction of H+ ions at the cathode.

wseq3

The overall reaction of photocatalytic water splitting can be written as

wseq4

The overall photocatalytic reaction takes place only when the absorbed photons have equal or more energy than the band gap of the photoanode. The rutile modification of TiO2 exhibits a band gap of 3.0 eV and therefore can absorb light with a wavelength of less than 420 nm.[4] As a consequence, only UV light can be utilized to drive photocatalytic water splitting. When the use of solar light is considered, this situation is problematic since only 4 % of the total photon flux is emitted in the UV region.[6] Figure 2 illustrates the solar spectrum recieved at earth’s crust. It is evident, that the discussion of the fraction of light, which can be used for generating hydrogen becomes even more relevant, when the photon fluence rate is considered instead of the irradiance.

Thermodynamically 1.23 eV are required to split water into hydrogen and oxygen. Because of the energy losses that occur in practice, the oltage to aply to a photoelectrochemical cell (PEC) is typically above 2 V. A photoanode, which exhibits a band gap of 2 eV can absorb light up to 620 nm. This would enable the absorption of roughly 30 % of the total solar irradiation and gives the maximum energy conversion efficiency that can be achieved in practice. BOLTON et. al. discussed in detail the efficiencies of photocatalytic water splitting.[7] The authors assumed a value for unused energy per photon (Uloss) of about 0.8 eV and a realistic maximum efficiency of 17 % for a practical system.[7] Major problems of photocatalytic water splitting with conventional photo catalysts are, on the one hand the large band gap of the catalysts and on the other hand the positions of valence band (VB) and conduction band (CB).[8] In Figure 0.3 the band gap energies of some oxide materials are shown. It becomes clear, that the mentioned problems of a large band gap and the required position of the bands are similar for a great number of photocatalysts. This explains the current large research efforts on new photocatalysts with narrower band gap.[5;9]

The most commonly used and studied semiconductor photoanodes consist of the following materials: TiO2, SrTiO3, WO3 ZnO, Fe2O3, BaTiO3 and CeO2.[814] The oxidation of water on TiO2 photoanode yields oxygen. The reduction takes place at e.g. a platinum cathode, where hydrogen is produced. To increase the efficiency of a PEC, factors such as voltage, anode and cathode materials and light intensity can be varied.[15]

Another possibility for light driven water splitting is the use of two different photocatalytic active materials. The irradiation of photocatalytic active materials enables the generation of holes at the photoanode and electrons at the photocathode. This type of PEC requires a redox couple (I/IO3 ) as electron mediator.[16] In this system, a photoanode (WO3,TiO2) for the oxygen evolution reaction (OER) and a photocathode (SrTiO3 : Cr/Ta) for hydrogen evolution reaction (HER), are used. In the first step, water is reduced to hydrogen by a photoelectron at the photocathode and the mediator is oxidized. In the second step, the oxidized mediator is transported through the electrolyte to the photoanode and is reduced to oxygen by the photoinduced holes. Photocatalytic water splitting by two step photoexcitation with two different semiconductor photocatalysts has been known as so called "Z-System".[1719]

A different approach for utilizing a larger fraction of the solar light is the use of tandem cells, consisting of more than one material absorbing the light.[2022] Typically, this is realized by stacking different materials on each other so that the first layer absorbs only a fraction of the light and the remaining fraction is absorbed by the underlying layer. However, the manufacturing of such electrodes is complex and often the long term stability is not given.

 Bandluecken_diagramm

Figure 3: Band gap energies of different metal oxide materials.[8;9;11]

 

 

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