Metal Organic Frameworks – the future of solar energy?
In one and a half hours, enough solar energy hits the earth to power human civilisation for a year. The huge amount of potential energy to be harnessed is part of the reason why solar energy is an attractive option as a renewable energy source. However, the challenge has been to develop new technologies capable of converting this vast amount of light energy into electricity.
Solar cells are the most common devices for achieving this conversion. At present, most commercially available solar cells make use of silicon-based devices. These devices use two sandwiched layers of doped silicon, with one layer of n-type and another of p-type silicon. The p-type silicon is electron deficient, whereas the n-type has an excess of electrons. As such, both layers will try to lose or gain electrons until they are stable. However, at the interface between the two types of silicon, a junction is formed that acts as a barrier to the movement of any electrons between the two layers. Nonetheless, as soon as the silicon absorbs light, this gives the electrons enough energy to traverse the barrier and move around the circuit, making an electrical current flow.
Currently, silicon-based cells are leading the market in terms of the conversion efficiencies and, as they can be incorporated into thin, flexible films, have led to the production of significantly more lightweight solar panels. This, and generous tax incentives for ‘clean energy generation’, have led to their increased adoption.
However, silicon devices may not be the entire future of solar energy conversion. The silicon required for solar cells must be of a very high purity, and the refinement processes involved in making the thin wafers that ultimately end up in devices are energy intensive and produce large amounts of chemical waste that poses serious environmental issues.
Therefore, researchers, such as Dr Amanda Morris at Virginia Tech, are looking into alternative materials for next generation solar cells. Potential materials that have proved attractive candidates for both cost and efficiency reasons include dye-sensitised and polymer-based solar cells, as well as perovskites. However, Dr Morris thinks that an interesting family of compounds, known as metal organic frameworks (MOFs), might be the key to overcoming the current instability and limitations of existing solar cell devices.
MOFs in thin films
MOFs are compounds composed of metal ions or clusters, with various other chemical motifs, to make 1D, 2D or 3D structures. For example, it is possible to create MOFs that act as ‘molecular cages’, with cavities inside capable of trapping small molecules for applications such as hydrogen storage.
MOFs can be incorporated into thin film arrays as a type of ‘sensitiser’ for solar cells. Whereas in a traditional silicon solar cell, all the electrons involved in the current come from the silicon itself, in a sensitised solar cell, a material acts as a rich ‘electron source’. These electrons are then transferred to an electron acceptor, typically TiO2 – greatly increasing the overall current produced.
Dr Morris’s research has shown that MOFs are potentially a very attractive candidate for sensitisers as they are very stable and can absorb large amounts of light energy. This leads to a good efficiency in comparison to more traditional dye sensitisers.
Dr Morris’s work does not focus solely on practical applications though – her group are also working to understand the fundamental relationship between the structure (both molecular and 3D) of such MOFs and observed photophysical properties. This will help with predicting what types of compounds are likely to make good candidates for use in energy applications, as well as enhancing our understanding of the fundamental physics of energy transport.
Artificial photosynthesis and solar fuels
MOFs may have another powerful application in the world of solar energy. With the efficiency of solar cells improving year on year and decreasing manufacturing costs, solar energy generation is quickly becoming a viable alternative to fossil fuels. Now, the main limitation is finding ways to store the generated electricity, until it is required, to ensure a continuous electricity supply. Traditional battery technologies still fall far short of the capacities required.
One option is to take some inspiration from nature, and in particular, the photosynthesis process in plants. Photosynthesis is how plants convert solar energy into chemical energy to fuel their growth and drive the chemical processes necessary for their survival. Dr Morris has been investigating how this process could be done in the lab, a type of ‘artificial leaf’, and how MOFs could be used to help make this more efficient.
The biggest challenges in emulating the processes in plants is finding a way to efficiently split water into hydrogen and oxygen using light, in a process called photo-driven water splitting. The hydrogen can be stored as a ‘solar fuel’ and then combusted, in a similar fashion to current fossil fuels, when additional energy is required. The versatility of MOFs means they can be used to catalyse several steps in the complex process and can even be used to safely store the hydrogen gas produced.
Dr Morris’s group has recently shown how MOFs can be used to improve the efficiencies of several steps of the water splitting process, but there is no doubt that this is just one of many areas in which Dr Morris will continue to spearhead further crucial developments in clean energy generation.
The major component of solar cell installation cost comes from the balance of systems. This refers to the inverters, racking, cables, fuses, etc. that are also required when installing a solar module. When it comes to controlling the cost of solar installations in the short term, we need to address these costs and facilitate advances in these technologies. Additionally, the intermittent nature of the sun necessitates storage technology. Whether that solution comes from a step-change in battery capacities or methods to convert solar electricity into chemical fuels, the scientific community must solve this challenge for widespread solar energy utilisation.
Do you think the future of solar cells will involve a mixture of technologies existing simultaneously or is there the possibility of one type ‘winning out’ over the others?
History, existing manufacturing capacity, and infrastructure are predictive of dominant technology. Take the energy sector: fossil fuels dominate (85%) even though wind is cost competitive. Why? Well, if we switched completely to wind energy, new plants would need to be built, a national electric grid to transfer energy from windy areas (mountain west) to the coasts, our transportation system would need drastic changes, home heating systems would need replacement, etc. With respect to solar technology, silicon has a major stronghold in history and manufacturing capacity. New technologies will have to be drastically better to replace silicon for rooftop applications.
Are there any environmental issues associated with the use of metal organic frameworks in these types of applications?
Metal organic framework synthesis does require the use of organic solvents such as dimethyl formamide, but mechanochemical synthesis of MOFs is an emerging area and can eliminate this concern. Beyond that, the environmental concerns come down to the chosen components in the metal organic frameworks and the speciation of these components when the material is disposed of. The development of metal organic frameworks is not yet at a stage to warrant life-cycle analysis. However, the first commercial application of a MOF was recently reported, so perhaps such studies are on the horizon.
Do you think it will be possible to potentially combine the various applications of metal organic frameworks in a single device?
I firmly believe that through scientific innovation we will be able to integrate the functionality of photosynthesis into a metal organic framework array. The question for me is not if, but when. Truly, that I cannot answer, but it will be a major thrust of my research group for the foreseeable future.
Do you think it will ever be possible to perform artificial photosynthesis with similar efficiencies to photosynthesis in plants?
It depends on how one defines efficiency. If one simply considers the efficiency as solar photons to chemical fuel yield, the answer is yes. Current technology exists to drive photosynthetic chemistry at much higher efficiencies than plants – near 10%. The issue is that technology comes at a cost – another component of efficiency. Plants in comparison to platinum catalysts and semiconductors are very cheap. The challenge that remains is how to produce the efficiency of photosynthesis at a cost competitive level.
Dr Morris’ research focuses on two aspects of solar energy conversion: solar energy storage through artificial photosynthesis and next generation solar cells.
Department of Energy (DOE)
Yulia Pushkar, Purdue
Dr Amanda J. Morris, Assistant Professor
Virginia Tech – College of Science
Department of Chemistry
3109 Hahn Hall South
Blacksburg, VA 24061
E: [email protected]
T: +1 540 231 5585
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