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From Sunlight to Green Energy – Controlling Photo Conversion

Postdoc-stipendium i Danmark | 02/05/2016

Conversion of light into electricity and chemical energy are the two major paths for harvesting solar energy, the latter being employed by nature through photosynthesis. The process of photo conversion is thus fundamental for the development of systems that can exploit sunlight for the production of green energy.  In molecular systems, photo conversion directly follow photon absorption; proceeding within a trillionth (10-12)  of a second in a complex cascade of electronic transitions driving structural dynamics in the absorbing molecule. The convolution of electronic and structural dynamics puts the details of photo conversion beyond the sensitivity of most established experimental techniques. This lack of experimental access to the details of photo conversion is detrimental to the functional characterisation of molecular system designed towards such applications, and thus to the rational design of new systems.

This project will establish an experimental framework for unravelling the electronic/structural dynamics of molecular photo conversion through ultrafast x-ray techniques at the world leading Solution Phase Chemistry group at Stanford University. The methodologies developed within this project will be applicable to most molecular photo converting systems currently under development, providing a foundation for making rational choices in the design of the next generations of such systems. Thus, the project holds a potential for a significant positive impact on solar energy research.

Solar Energy

Solar energy production is the process used to harvest the sun's energy and make it useable. Solar energy is an inexhaustible resource that has the potential to fully meet the global energy demand. Almost 20 years ago, the United Nations Development Programme in its 2000 World Energy Assessment found that the annual potential of solar energy was several times larger than the total world energy consumption. In spite of its significant potential, solar energy today only accounts for a few percent of the world energy supply. The large discrepancy between the potential for, and the actual production of solar energy, stems from challenges with current solar-energy-production technologies, which rely on materials that are scarce, expensive, or toxic. This research, which is supported by the Carlsberg Foundation, focuses on the development of molecular systems that holds potential for new ways of harvesting solar energy within two areas of solar energy production:

  1. The development of molecular systems based on cheap, abundant, and environmentally friendly materials for large scale application of solar power production.
  2. The development of novel molecular systems for the production of solar fuels.

The specific activities carried out within the research grant aims at developing a general experimental framework for determining the processes occurring after light absorption in the molecular systems designed the above applications.

Iron Molecules for Solar Power

Through a collaboration based at Lund University, I am involved in the development of new molecular systems for generating power from sunlight using the so-called dye-sensitised solar cells. These solar cells consist of a thin film of nanostructured titanium dioxide and a dye that captures solar energy. Today, the best solar cells of this type use dyes containing ruthenium metal – a very rare and expensive element.

“Many researchers have tried to replace ruthenium with iron, but without success. All previous attempts have resulted in molecules that convert light energy into heat instead of electrons, which is required for solar cells to generate electricity,” says Villy Sundström, Professor of Chemical Physics at Lund University.

We have now successfully produced an iron-based dye that is capable of converting light into electrons with nearly 100 percent efficiency. We have reported our results in the scientific journal Nature Chemistry [1].

Power from Dye-Sensitised Solar Cells

Solar power is generated by dye-sensitised solar cells in four major steps (illustrated in Figure 1):
  1. Light absorption by a molecular dye. 
  2. Photo induced electron transfer from the dye in to a semiconductor material (e.g. TiO2).
  3. Extraction of the photo excited electron from the semiconductor material into a circuit ending at the absorbing dye molecule. 
  4. The energised electron now has potential to drive a current before returning to the dye.

While the iron-based molecules that we have developed could represent an important step towards large scale production of very cheap solar cells, significant development is still needed to improve their efficiency. In order to optimise and improve these systems in a rational fashion, we need as much information as possible about the steps following light absorption. The most pressing question at the current stage is to understand why many of the electrons generated by the light-absorbing dyes quickly return back to the dye instead of entering the circuit and producing power. In the project supported by the Carlsberg Foundation, I am using novel x-ray techniques to determine the nature and dynamics of these unwanted decay processes so that they can be inhibited. The x-ray experiments we are carrying out allow us to follow the details of how excited electrons are returning to the iron centre of the dye molecules. We have recently shown how the technique can be applied to very similar iron centred molecules in the Scientific Journal Nature [2]. By combining the experiments with advanced computer simulations, we will be able to understand the required parameters for improving the design of the iron molecules.

Figure 1. Schematic of solar power generated through light absorption in one of the iron-centred molecular systems investigated in this study. The four steps in the process are described in the fact box.

The continued development of these iron-centred systems could also advance research on solar fuels in which, like in photosynthesis of plants, sunlight is used to produce energy-rich molecules. 

“We envision that the new iron-based molecules could also drive the chemical reactions that create solar fuel,” says Kenneth Wärnmark, Professor of Organic Chemistry at Lund University.

Investigating molecules that generate solar fuels through artificial photosynthesis is the second aim of the project.

Towards Solar Fuels

Artificial photosynthesis utilises the principles of natural photosynthesis for energy production. The long-term goal of researching into artificial photosynthesis is to construct a synthetic, photochemical system that converts solar energy into a storable fuel.

The Steps of Artificial Photosynthesis

Solar fuels are generated by molecular systems in five major steps (illustrated in Figure 2):

1)  Light absorption by a molecular dye. 
2a) Photo induced electron transfer from the dye in several steps to a catalytic site capable of reductive reactions.
2b) Secondary electron transfer from another catalytic site capable of oxidative reactions, back to the molecular dye.
3a) Reduction at the reductive catalyst to produce a fuel, such as H2 or an alcohol.
3b) Oxidition at the oxidative catalyst to split water to O2, thereby extracting the electrons and protons needed for fuel generation.

Figure 2. Schematic of a solar fuel (H2) generated through light absorption in one of the iron-centred molecular systems investigated in this study. The five steps in the solar fuel production are described in the fact box above.

The research carried out within the project focuses on understanding and improving the key steps in the production of solar fuels. 

The more complex steps involved in solar fuels production (compared to solar power production) means that experiments typically aim at understanding a few, or even a single step of the full process better. 

We have recently showed how we can apply the x-ray techniques used in this project to follow electrons moving through a molecular model system for photo catalysis with unprecedented detail and precision [3]. This study also allowed us to compare different molecules in a systematic fashion, allowing us to determine that one of the systems was almost a thousand times more efficient at moving the electron from the initial point of excitation to the catalytic site of the molecule. 

”Photocatalytic systems hold a very large potential. Controlling electron transfer in light-absorbing molecules will allow us to produce fuels or electricity without loss of energy to heat. That is why the insight into the electronic movement through the molecule is so important,” says Professor Martin Meedom Nielsen, DTU Physics.

My current research at Stanford University focuses not only on the electron transfer aspects, but also on understanding the catalytic reactions happening at the catalytic site of the molecules (steps 3a and 3b). 

The Future is Bright

The project described here benefits from a series of experiments of which I have already initiated or participated in at the world’s brightest x-ray source, the Linear Coherent Light Source, located at Stanford University half a kilometre form my current office in the Solution Phase Chemistry group. By investigating molecular systems designed for solar energy applications, the project also strengthens  my unique ties to the Consortium for Artificial Photosynthesis in Scandinavia. In this way, the project aims to understand and develop some of the most promising molecular systems for solar energy applications, at the best possible site for conducting the proposed experiments. Ultimately, the methodology developed within this project will be applicable to most molecular systems currently being developed for solar energy applications, and can provide a foundation for making rational choices in the design of the next generations of such systems for efficient solar energy conversion.

In terms of Scientific Social Responsibility, the project targets the development of sustainable, low-cost, environmentally friendly technologies for solar energy production. In this way the project involves research seeking to address two of the UN sustainable development goals, namely the Affordable and Clean Energy goal as well as the Climate Action goal. Moreover the development of such new technologies will hopefully stimulate innovation and provide the clean energy needed for sustainable economic growth.
On a personal level, the support from the Carlsberg Foundation has given me a chance to build on my international network and expand on my unique research profile, both of which are strictly necessary for establishing a career in science. 

[1] http://www.nature.com/nchem/journal/v7/n11/full/nchem.2365.html

[2] http://www.nature.com/nature/journal/v509/n7500/full/nature13252.html

[3] http://www.nature.com/ncomms/2015/150302/ncomms7359/full/ncomms7359.html