Photocatalytic water splitting

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The Photocatalytic water splitting is the process in which photons are used directly to water electrochemically into its constituent hydrogen and oxygen to separate. The reaction is part of artificial photosynthesis and can be described simply as follows:

Methods for producing hydrogen will become more important with a transition to a hydrogen economy based on renewable energies . The advantage of photocatalysis over other techniques such as electrolysis is that charge separation and splitting of the water from a material can be carried out at the same interface , whereby transmission losses can be minimized and material can be saved.

Since the discovery of the photocatalytic splitting of water on the semiconductor TiO 2 in 1972 by Akira Fujishima and Kenichi Honda , a large number of scientists have tried to develop suitable materials. In addition to the synthesis of hydrogen, more storage-friendly energy carriers with higher volume energy density, such as. B. examined hydrogen peroxide.

Current research is aimed at shifting the required energy of the photons, which in A. Fujishima and K. Honda was in the UV range, into the range of visible light . Above all, losses that occur at the interfaces between catalyst and water must be reduced . Another major problem is the decomposition of the catalyst under lighting.

Types of photocatalysis cells

The splitting of water by means of sunlight can be realized through different cell structures. A voltage of at least 1.23 V must be built up between anode and cathode . This corresponds to the energy that has to be expended to separate a hydrogen atom from an oxygen atom. In practice, however, the voltage required is higher, typically in the range from 1.6 to 2.4 V, which can be attributed to a strong bond to the catalyst during the reaction and to conduction losses.

n-type photoanode

Energy scheme of an n-semiconductor photoanode with a metal cathode in contact with an electrolyte solution.

In this photocatalysis cell, the necessary voltage is generated by the excitation of an electron-hole pair by photons at the band gap of an n- type semiconductor . By aligning the Fermi level of the n-semiconductor with the quasi-Fermi level of the electrolyte , the band is bent, which causes an active charge separation between the electron and the hole. The hole is used for the reaction of the H 2 O to O 2 . In order to raise the Fermi edge of the metal above the potential from which H + reacts to H 2 , an external voltage may have to be applied with this cell arrangement.

p / n-type photoanode / photocathode

Energy scheme of an n-semiconductor photoanode connected to a p-semiconductor photocathode in contact with an electrolyte solution.

In this cell configuration, a p-semiconductor is connected to an n-semiconductor via an ohmic contact. In contrast to the n-type photoanode, the charge separation of electron and hole takes place both on the photoanode and on the photocathode. The bending of the band of the p-semiconductor leads to the electrons migrating to the interface and driving the reaction H + to H 2 there . The holes are driven to the interface with the n-semiconductor, where they recombine with the electrons . So two photons are needed to generate an electron and a hole for the reaction. However, these can each have a lower energy than with a one-photon process, whereby the spectrum of the sun can be better used.

Suspended Particle Photocatalysis

Here the water is split with the help of particles that are suspended in the water. The function of the particles is light absorption , charge separation and water electrolysis .

Electrolysis of water with suspended particles

Due to the extremely corrosive conditions during the water oxidation, the most functional particles are made of metal oxides constructed with metal or metal oxide electrocatalysts are connected. Examples include La / KTaO 3 with NiO electrocatalyst and GaN: ZnO with Cr / RhOx electrocatalyst. Particle-based systems are potentially less expensive than traditional photo-electrochemical systems. However, because of the fast electron-hole recombination, the energy efficiency of the particle systems is still too low for commercial applications. There are also safety concerns as hydrogen and oxygen form an explosive gas mixture. Research approaches for improving energy efficiency consist in the development of new semiconductor materials , in the use of nanoparticles , and in the application of surface modifications to stabilize the excited state.

Definition of efficiency

The efficiency of the photocatalytic water splitting is defined by the ratio of irradiated solar energy to the produced, usable chemical energy. For the exact definition of the usable energy per converted electron (hydrogen atom), however, there are several approaches, the calorific value or Gibbs free energy can be used. At 1.23 eV per reduced proton, the latter value is the more conservative value and is therefore generally used. The prognoses from which efficiency - assuming stability - the process could become economically interesting are still vague and assume 5–10% for the approach with suspended particles and at least 15% for high-efficiency systems.

The efficiencies achieved so far in the laboratory depend heavily on the material system used and range from 5% for metal oxide systems to 14% for the more expensive III-V compound semiconductors . An efficiency of over 12% using perovskite was documented in 2014, which is a very high value compared to the overall efficiency of photovoltaic electrolysis with hydrogen as a long-term energy storage device. At the same time, these systems represent an attractive option for entry into a hydrogen-based energy infrastructure, as they promise high economic efficiency through compact generation centers with access to high solar power and salt water.

Individual evidence

  1. ^ A. Fujishima, K. Honda: Electrochemical Photolysis of Water at a Semiconductor Electrode In: Nature . Vol. 238, 1972, pp. 37-38.
  2. ↑ Shunichi Fukuzumi, Yusuke Yamada, Masaki Yoneda, Kentaro Mase: Seawater usable for production and consumption of hydrogen peroxide as a solar fuel . In: Nature Communications . tape 7 , May 4, 2016, ISSN  2041-1723 , p. 11470 , doi : 10.1038 / ncomms11470 ( nature.com [accessed August 17, 2019]).
  3. ^ MG Walter, EL Warren, JR McKone, SW Boettcher, Q. Mi, EA Santori, and NS Lewis: Solar Water Splitting Cells In: Chem. Rev. 110, 2010, pp. 6446-6473.
  4. ^ MG Walter, EL Warren, JR McKone, SW Boettcher, Q. Mi, EA Santori, and NS Lewis: Solar Water Splitting Cells In: Chem. Rev. 110, 2010, p. 6448 chap. 2.
  5. ↑ CA Grimes Light, Water, Hydrogen - The Solar Generation of Hydrogen by Water Photoelectrolysis Chap. 3.
  6. ^ MG Walter, EL Warren, JR McKone, SW Boettcher, Q. Mi, EA Santori, and NS Lewis: Solar Water Splitting Cells In: Chem. Rev. 110, 2010, p. 6452 chap. 2.3.
  7. ↑ H. Kato, K. Asakura and A. Kudo, Highly Efficient Water Splitting into H-2 and O-2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure, J. Am. Chem. Soc., 2003, 125 (10), 3082-3089.
  8. ↑ K. Maeda, K. Teramura, DL Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Characterization of Rh-Cr mixed-oxide nanoparticles dispersed on (Ga1-xZnx) (N1-xOx) as a cocatalyst for visible-light-driven overall water splitting, J. Phys. Chem. B, 2006, 110 (28), 13753-13758.
  9. ^ FE Osterloh, Nanoscale Effects in Water Splitting Photocatalysis, Top. Curr. Chem., 2015, doi : 10.1007 / 128_2015_633 .
  10. ↑ Z. Chen, TF Jaramillo, TG Deutsch, A. Kleiman-Shwarsctein, AJ Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, EW McFarland, K. Domen, EL Miller, JA Turner and HN Dinh: Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols In: J. Mater. Res. 25, 2010, pp. 3-16, doi : 10.1557 / JMR.2010.0020 .
  11. ↑ BA Pinaud, JD Benck, LC Seitz, AJ Forman, Z. Chen, TG Deutsch, BD James, KN ​​Baum, GN Baum, S. Ardo, H. Wang, E. Miller and TF Jaramillo: Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry In: Energy Environ. Sci. 6, 2013, pp. 1983-2002, doi : 10.1039 / C3EE40831K .
  12. ^ FF Abdi, L. Han, AHM Smets, M. Zeman, B. Dam, R. van de Krol: Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode . In: Nature Communications . 4, July 29, 2013. doi : 10.1038 / ncomms3195 . Retrieved December 25, 2015.
  13. ↑ MM May, H.-J. Lewerenz, D. Lackner, F. Dimroth, T. Hannappel: Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure . In: Nature Communications . 6, September 15, 2015. doi : 10.1038 / ncomms9286 . Retrieved December 25, 2015.
  14. ↑ Michael Graetzel, Hong Jin Fan, S. David Tilley, Nam-Gyu Park, Mohammad Khaja Nazeeruddin: Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts . In: Science . tape 345 , no. 6204 , September 26, 2014, ISSN  0036-8075 , p. 1593–1596 , doi : 10.1126 / science.1258307 , PMID 25258076 ( sciencemag.org [accessed August 17, 2019]).