Information für die Wissenschaft Nr. 39 | 12. August 2011
Priority Programme “Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Tech-nological Concepts” (SPP 1613)
The Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) has announced the establishment of a new Priority Programme entitled “Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Technological Concepts” (SPP 1613). The programme is scheduled to run for six years; the present call invites proposals for the first three-year funding period.
The objective is to investigate artificial photosynthesis based on solid-state inorganic materials from a fundamental scientific perspective as well as the aspects of material science required for its technological implementation. It is expected to be able to successfully produce storable fuels in the form of H2 with high energy content using solar light, resulting in a renewable primary energy carrier that would be sustainable and secure. The artificial systems that have been identified thus far are limited with respect to conversion efficiency and lifetime, and are furthermore too expensive for technological implementation. New basic approaches are thus required that merge scientific innovation with advanced engineering strategies. Therefore, only systems with the potential of providing energy conversion efficiencies greater than ten percent will be considered.
Efficient artificial photosynthesis can only be realised by coupling a number of successive elementary processes, including broad band light absorption, optimised charge carrier generation and separation as well as an efficient electrocatalytic production of H2 and O2 from H2O in separated compartments. For technological implementation, complex device structures must be manufactured, preferably using materials that are economical, abundant and non-hazardous. In the face of the diversity of possible solutions this programme concentrates on the combination of photovoltaic converters and catalysts, both of which are highly efficient and stable, a strategy which may lead to viable solutions in the near future. Semiconductors will be used for light absorption, since they provide the best results for charge carrier generation. As potential means of generating the photo voltage needed for water-splitting, wide band gap compound semiconductors, low band gap tandem structures, and doped oxides with visible light absorption and efficient charge carrier transport properties (Janus structures) will be investigated. Nano-sized or molecularly deduced (biomimetic) metal clusters will be examined for use as electrocatalysts in subsequent multi-electron transfer. The key factors for a promising system include loss-minimised charge transfer from the photovoltaic converter to the catalyst, a highly efficient and selective catalyst, and the stability of the complete system in an aqueous solution.
The requirements cannot be met without a detailed analysis of the elementary processes involved as well as the materials and devices used in successful implementation. The Priority Programme will consequently involve two phases: during the first phase, the funded projects are to investigate selected promising model systems in order to achieve an enhanced understanding of the conditions needed for efficient light induced water-splitting. This will be done in close cooperation among subgroups using the most recent experimental characterisation techniques in combination with advanced theoretical simulation approaches.
To foster collaboration between the various research groups participating in the programme, proposals should preferably involve consortia of two to three principal investigators of complementary expertise concentrating on a specific subject. Furthermore, the consortia will develop and cultivate close mutual collaboration in order to disseminate the expertise gained in the process of their experimental or theoretical work programmes. Knowledge exchange and potential collaboration will be outlined already in each research proposal. Because of the evident need for concentration and specialisation within this Priority Programme, each research group shall be assigned to one of the research areas listed below. Proposals that in the ideal case focus on more than one of these points are highly welcome. When selecting the materials, toxicological considerations, the abundance and cost of the elements employed as well as resistance to photocorrosion must be taken into account.
Photoelectrochemical solar cells using semiconductor/electrolyte contacts have shown the best performance when used as water-splitting systems based on III-V single crystals (using a bias voltage) and multi-junction epitaxial cells (without bias voltage). However, such systems are far too expensive for any practical application. Therefore, research in this area is to focus on novel materials and material combinations. The novel systems should provide the potential for performance levels comparable to those mentioned above but represent cost-effective solutions.
Thin films of wide band gap semiconductors or of tandem or multi-junction cells as absorber materials must provide the level of photo voltage (about 1.8 eV, including overvoltage) required for the water-splitting reaction. In addition, the need to avoid corrosion reactions may require protective surface buffer layers resulting in negligible losses in photo potentials and photocurrent. Therefore, only semiconductors with realistic potential of fulfilling these conditions, or which may be part of a promising future device combination, will be promoted within this programme. Additionally, effective coupling of the photovoltaic converter to efficient electrocatalysts for the hydrogen as well as the oxygen evolution reaction (HER, OER) must be a central concern of the proposed research.
Current photocatalytic systems for water-splitting usually have two weaknesses. First, most of them are active only in the presence of UV light, which only accounts for about four percent of the sunlight reaching the earth’s surface. Second, they catalyse often only with the aid of sacrificial agents one of the redox reactions HER or OER. Doping oxides with N, S or other elements, while shifting absorption into the visible range, results often in defects in the form of recombination centres and short charge carrier diffusion lengths or lifetimes, which often drastically limits the applicability of this technique.
Thus, the requirements placed on the proposed research topics include: (1) identification and characterisation of photocatalytic systems with reduced band gaps less than 3 eV and high mobility of the charge carriers; (2) development of semiconductor systems and redox relays suitable for bridging the energy difference between the redox potentials for water-splitting; (3) well-directed vectorial separation of the electron-hole pairs and transport of the electrons and holes to co-catalyst particles ideally deposited on different sides (edges) of the semiconducting absorber particle; (4) adaptation of the diffusion length to the location of e-h pair formation; and (5) minimisation of overvoltages through effective electronic coupling of the catalyst nanoparticles to the semiconductor.
It is a general problem that most photoelectrochemical and photocatalytic systems for water cleavage do not provide a sufficient number of active sites, and hence show only low activities for the HER and OER. To overcome this drawback, additional electrocatalysts need to be imple-mented on the semiconducting materials. These should effectively catalyse the electrochemical reactions in order to yield hydrogen and oxygen and avoid high overpotentials. Well-investigated and well-defined deposition techniques such as CVD, which uses mono- and multinuclear organometallic compounds, should be employed to the end of forming appropriate electrocatalysts (e.g. monomeric species, clusters, or nanoparticles) on the surface of the semiconductors. In addition to using precious metals containing catalysts, there is a strong need for more abundant, non-toxic and cheaper materials with either a biomimetic or a quantum size effect background. The means of bonding these electrocatalysts to the carrier should facilitate an efficient charge transfer between the bulk and the catalyst. The choice of carriers should mainly focus on applicable semiconducting materials that are used in the investigations into photocatalytic and photoelectrochemical systems.
Both processes (HER and OER) are multi-electron reactions involving either two or four electrons which are not yet fully understood. Therefore, experimental testing of newly developed electrocatalysts for water cleavage should be obligatory, and be accompanied by detailed investigations of the processes resulting in charge transfer from the bulk to the electrocatalyst as well as of the hydrogen and oxygen evolution reactions in order to identify the reasons for efficiency losses.
To gain atomic-scale insights into the elementary processes occurring during the photocatalytic splitting of water using light, one or more suitable model systems need to be identified and stud-ied both by means of experiment and theory. A well-defined model system encompasses the photoabsorber for generating and separating the electron−hole pairs and the co-catalyst for promoting the oxygen evolution reaction. These should provide the basis for investigating the elementary reaction steps during the photo-induced water-splitting process: specifically the photo-induced generation of electron−hole pairs, their separation as well as migration to both electrodes, the transfer of the holes onto the co-catalyst, and finally the elementary electrocatalytic reactions occurring at each electrode (i.e. HER and OER). The goal should be to understand the processes at a microscopic level and to identify the limiting factors. Furthermore, the influence of dopants and defects on the system and on the elementary reaction steps should be investigated in order to provide guidelines for improving overall efficiency. Projects should preferably be carried out in joint collaboration between experimental and theoretical groups.
As there have been already a large number of unsuccessful research efforts in this field during the last 30 years, new research proposals will provide ample arguments for the novelty of their approach in contrast to published findings and detail the researchers’ expectations in each case as to how they intend to overcome the barriers to artificial photosynthesis encountered thus far. As stated above, new propositions within this SPP should follow an inorganic solid-state approach to artificial photosynthesis. Biological and molecular approaches are already being funded by several other research organisations and will not be considered here. The submission of proposals is also not encouraged which do not suggest novel materials and device structures and do not provide reasonable arguments as to why and how the particular approach will lead to improved solutions compared to past studies available in the literature.
Submission of proposals
Proposals for an initial three-year funding period should be submitted in English by 14 November 2011 marked with the reference code “SPP 1613”, to Deutsche Forschungsgemeinschaft, Gruppe Chemie und Verfahrenstechnik, Kennedyallee 40, 53175 Bonn.
Besides individual projects, joint proposals (involving typically two or three groups) are welcome. One printed copy (signed by all principal investigators (PIs) of the corresponding proposal) that contains as enclosures the scientific CVs of the applicants and of possible or intended scientific co-workers is required. Furthermore, an electronic version of the proposal and of all the relevant enclosures should also be provided in PDF on a CD. Please send a copy of the summary of the proposal by electronic mail to the secretary of the coordinator, Marga Lang, firstname.lastname@example.org.
The responsibilities assigned to each scientific co-worker should be evident from the work pro-gramme within the proposal, specifically the tasks to be completed by PhD students or postdocs. In the case of joint proposals, the assignment of requested funds to the individual PIs should also be evident.
Please study carefully DFG’s new publication rules.
Guidelines and Proposal Preparation Instructions are available at:
1.02e Research Grants - General Information and Guidelines for Proposals
For scientific enquiries please contact the coordinator of the Priority Programme:
Professor Dr. Wolfram Jaegermann,
Surface Science Group,
Institute for Materials Science,
Technical University Darmstadt,
phone: +49 6151 16-6304,
fax: +49 6151 16-6308
PD Dr. Bernhard Kaiser,
Center of Smart Interfaces,
Technical University Darmstadt,
phone: +49 6151 16-69664,
fax: +49 6151 16-6308
For administrative enquiries please contact the responsible programme director at the DFG:
Dr. Markus Behnke,
phone: +49 228 885-2181,
fax: +49 228 885-2777