Editorial of the PCCP themed issue on “Basic Mechanisms in Energy Conversion”

The sustainable generation and storage of energy is one of the most important scientific and technical challenges of the 21st century. The world’s vanishing fossil fuel supplies are in dire need to be replaced by renewable resources. The negative side effects of the current energy supply system on climate, environment and health should, however, be avoided.^1,2 Among renewable energy resources, solar energy is by far the largest exploitable option. The sun provides more energy per hour to the Earth’s surface than all of the energy consumed by humans in an entire year combined. If solar energy is to be a major renewable energy source, it must be stored and dispatched on demand to the consumers. Thus the capture, storage and distribution of solar-converted energy represent the main scientific and engineering challenges that need to be solved.

Solar energy capture and conversion may be accomplished by photovoltaic technology, while solar electricity can be stored mechanically by pumped storage hydroelectricity, or electrochemically by using batteries.³ Solar thermal energy conversion technologies are presently by far the cheapest methods. However, the most promising method for the future conversion and storage of solar energy is a design borrowed from nature: in photosynthesis, carbon dioxide and water are converted into organic molecules, such as sugars, and oxygen. Although we can learn from nature, natural photosynthesis is too complex and also relatively inefficient. However, part of these processes can still be copied or retraced in artificial photosynthesis.⁴

Chemical energy conversion technologies include photovoltaic applications, hydrogen storage, photocatalytic water splitting devices, the development of fuel cells, hydrogen storage in small molecules, and the synthesis of organic molecules (methanol or methane) from fixation of atmospheric carbon dioxide. One of the most attractive methods for energy conversion and storage is the production of cheap solar fuels.⁵ However, harvesting solar energy and converting it to electricity or chemical fuels at low cost with abundantly available raw materials is still a huge challenge. New technologies, methods, reactions and materials for solar energy capture and conversion are badly needed.^3–5

This themed issue picks up recent efforts in this field. Technologies, methods, materials and reactions for energy conversion with a particular focus on mechanistic understanding are presented.

Efficient and economical water splitting is a key technological component of a hydrogen economy. The conversion of solar energy to hydrogen by means of the water splitting process is one of the most interesting ways to achieve clean and renewable energy (see Fig. 1). Metal hydrides are important intermediates in both chemical and biological hydrogen production. Hugenbruch et al. [DOI: 10.1039/C5CP07293J] studied the X-ray absorption and emission of [NiFe] hydrogenase model complexes. Iridium- and copper-based complexes are used for harvesting sunlight in the excited state and for providing electrons for water reduction and hydrogen evolution. Tschierlei et al. [DOI: 10.1039/C6CP00343E] applied time-resolved spectroscopy to investigate the excited state dynamics of iridium complexes and their changes upon immobilization onto titanium dioxide layers. Water splitting by using photovoltaic electricity potentially offers the cleanest way to produce hydrogen. Klett et al. [DOI: 10.1039/C5CP06230F] performed band engineering for efficient catalyst–substrate coupling for photoelectrochemical water splitting. Several methods of hydrogen storage are available. An interesting option is to store hydrogen in small molecules (sun fuels) or basic chemicals.⁵ Fink et al. [DOI: 10.1039/C5CP06996C] investigated the solvation energetics for hydrogen storage in the carbon dioxide/formic acid system. The role of CO in methanol synthesis via CO₂ hydrogenation over a Au/ZnO catalyst has been studied by Hartadi et al. [DOI: 10.1039/C5CP06888F].

Fig. 1 Light harvesting: photosensitizer material captures photons with different wavelengths covering the visible part of the solar spectrum. Suitable catalysts are needed for water splitting.

A design of efficient oxygen evolution catalysts is required for reducing energy losses in water electrolysers. Weidler et al. [DOI: 10.1039/C5CP05691H] provide insight into the water oxidation reaction steps. Krewald et al. [DOI: 10.1039/C5CP07213A] show that the redox potential can be tuned by redox-inactive cations in nature's water oxidizing catalyst and synthetic analogues. The charge carrier dynamics of methylammonium lead iodide including low-dimensional broadly emitting perovskites is reported by Klein et al. [DOI: 10.1039/C5CP07167D]. Schneider et al. [DOI: 10.1039/C5CP07115A] present improved charge carrier separation in barium tantalate composites by means of flash photolysis.

Efficient materials for solar energy conversion are required.^3,4 Wood et al. [DOI: 10.1039/C5CP05326A] compare dye-sensitized NiO photocathodes for solar energy conversion. Gutkowski and Schuhmann [DOI: 10.1039/C5CP07678A] present electrochemically induced sol–gel deposition of ZnO films on Pt-nanoparticle-modified FTO surfaces for the enhancement of photoelectrocatalytic energy conversion. A facile alternative to other well-known strategies for synthesizing flexible thermodynamic materials by producing thermoelectric pastes is presented Andrei et al. [DOI: 10.1039/C5CP06828B].

A detailed understanding of processes and the formation of intermediates can improve energy conversion. Schwager et al. [DOI: 10.1039/C5CP07145C] show the formation of reactive oxygen species in organic lithium–oxygen batteries. The adsorption processes at the surface of Pt(331) model electrocatalysts in acidic aqueous media is elucidated by Pohl et al. [DOI: 10.1039/C5CP08000B]. Pougin et al. [DOI: 10.1039/C5CP07148H] identify and exclude intermediates of photocatalytic CO₂ reduction on TiO₂ under conditions of highest purity.

These examples demonstrate that “Energy conversion” represents an interdisciplinary field, and requires combined efforts from chemistry, physics, biology, engineering and materials sciences. It is the aim of this themed issue to highlight some of the ongoing activities in the exciting and interdisciplinary field. This understanding at short time and small length scales is a prerequisite for designing new materials, tuning reactions and improving their efficiency.

We would like to thank all contributors to this themed issue and the Editorial Team for their help and patience.

References

1. N. Armaroli and V. Balzani, Energy for a Sustainable World, Wiley-VCH, Weinheim, 1st edn, 2011.
2. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. Nocera, Chem. Rev., 2010, 110, 6474.
3. X. Chen, C. Li, M. Grätzel, R. Kostecki and S. S. Mao, Chem. Soc. Rev., 2012, 41, 7909.
4. F. E. Osterloh, Chem. Mater., 2008, 20, 35.
5. D. Mellmann, E. Barsch, M. Bauer, K. Grabow, A. Boddien, A. Kammer, P. Sponholz, U. Bentrup, R. Jackstell, H. Junge, G. Laurenczy, R. Ludwig and M. Beller, Chem. – Eur. J., 2014, 20, 13589.

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