4.4 Conclusion and Outlook
6.1.4 Photoelectrochemical Water Splitting
This process is divided into two half-cell reactions, HER and OER, depending on the pH:[203]
Acidic conditions:
Water splitting is not only implemented electrocatalytically but also photocatalysis and photoelectrocatalysis are used to achieve overall water splitting or one of the half reactions, OER or HER.[201,204] To run reactions by solar energy, semiconductors are applied as photocatalysts inducing electron-hole-separation under irradiation of certain light wavelengths according to the band gap. The remaining holes in the valence band as well as the excited electrons in the conduction band contribute to OER and HER.[202] The combination of photoactive electrodes with water electrolysis results in photoelectrocatalytic water splitting driven by photogenerated electron-hole pairs at one or both electrodes.[204]
6.1.4 Photoelectrochemical Water Splitting
In 1972, Honda and Fujishima first accomplished direct photoelectrochemical (PEC) water splitting by applying a semiconducting n-type titanium oxide (TiO2) electrode and a platinum counter electrode.[205] Photoelectrochemical water splitting attracted attention as a promising approach for carbon-free hydrogen production powered by solar energy. In general, PEC cells are composed of a light-harvesting semiconductor as working electrode, which is connected to a metal counter electrode and immersed in a suitable aqueous electrolyte.[206] Figure 46 depicts
two possible setups for PEC cells. In the first case, a n-type semiconductor is applied as photoanode in combination with a common cathode. Under irradiation, electrons get excited, and the generated holes in the valence band (VB) migrate to the surface of the electrode where water oxidation takes place. Meanwhile, excited electrons are transferred via the external circuit to the corresponding cathode, where the electrons enable the hydrogen evolution reaction (HER). For the second setup, electrons and holes are photogenerated in the semiconductive p-type cathode. The excited electrons in the conduction band (CB) contribute to the hydrogen reduction on the interface of photocathode and electrolyte, as the holes in the valence band drive the external circuit, whereby oxygen evolution (OER) is performed at the connected anode.[207]
Apart from these described systems, even more complex PEC cells have been developed.
Ideally, a suitable photocathode as well as photoanode are combined and applied in one cell for PEC water splitting.[206,207]
Figure 46: Schematic setup of PEC cell consisting of n-type or p-type semiconductor.[207]
Semiconductors play a key role in operating PEC cells as solar energy is converted into an electrical current, which drives the catalytic water splitting. An ideal material for photoelectrodes should provide certain requirements such as efficient light-harvesting properties as well as facile electron-hole separation and efficient charge transfer.[208] The conductivity of a semiconductor is enhanced by introducing dopants with electron donating or accepting properties, to change the electronic properties. For n-type semiconductors the better conductivity is based on an excess of electrons due to doping with donor groups within the materials, whereas p-type semiconductor are doped with extra electron acceptor groups leading to a surplus of holes in the valence band.[209] Besides conductivity, the band gap (Eg) and the positions of valence and conduction band need to be suitable in respect to the oxidation and
reduction potential of water. At best, the redox potential of water is positioned between the edges of conductive and valence band: ECB > Ered, water and EVB < Eox, water.[204,210] The size of the band gap determines the range of wavelengths, which is required to overcome the energetic step between valence and conduction band. In case the band gap is too large, efficient light-harvesting becomes more difficult as the required intensity of light cannot be absorbed.[211]
Although the theoretical band gap that is necessary to achieve water splitting is 1.23 eV, the perfect size of the band gap is considered to be between 1.6 and 2.2 eV for efficient water splitting experimentally.[207] To further optimize the catalytic system, the introduction of suitable co-catalysts is a possible way to enhance surface reaction kinetics as well as a decline in overpotential.[206,212]
6.2 Objective
For many industrial applications such as energy storage devices, optical coatings or membranes and sensors, thin layers of material are more favorable than powdery materials.[213] In this respect, polymer films are favored to broaden the usability of microporous polymers.
Especially, carbazole-based materials offer this possibility as carbazole is not only polymerizable via oxidative reactions, but also by applying electropolymerization. The advantage of an electrochemically deposited material is that a homogeneous and transparent film is formed within minutes without any metal catalyst applied.[200] Besides, film thickness is easily controlled and therefore it can be precisely tailored for the desired application.
The electropolymerization of carbazole-equipped molecules opens the possibility to design a variety of monomers for embedding organic functionalities into polymer films, which can be applied in catalysis. This offers an option to process films containing phosphine and bipyridine functionalities homogenously on conductive substrates enhancing the applicability of the material for electrocatalysis.
Carbazole films can be polymerized on various substrates, entailing a simple approach of modifying electrodes for electro- or photoelectro-catalysis. This paves the way for applying carbazole films in electrocatalysis as well as using them as photoelectrodes for water reduction.
By incorporation of chelating ligands within the polymer films even coordination of co-catalyst via post-synthetic modification is possible to tailor the material properties. For that purpose, bipyridine-containing carbazole monomers are electropolymerized on FTO and semiconductors to study their applicability in electrocatalytic OER and as photocathode in PEC cells.