A new study published on 29 November in the journal Science has presented a newly developed iron (Fe) molecule that can function as both a photocatalyst to produce fuel and in solar panels to generate electricity (1). The work builds upon previous studies that have presented candidate iron-based molecules for solar energy applications (2, 3).
Typical solar cells are based on metal complexes ― such as noble metals including ruthenium, osmium, and iridium ― that absorb solar rays and transform them into usable energy. In the study, a team chemists from Lund University and Uppsala University in Sweden and the University of Copenhagen in Denmark used something called advanced molecule design to create a new iron molecule with the potential to replace these more expensive and rarer metals. By comparison, iron is highly abundant and makes up six per cent of the Earth’s crust, and is, therefore, much more available.
The researchers have now shown that this iron molecule could potentially be used as a photocatalyst to produce solar fuel, either as hydrogen through water splitting or as methanol from carbon dioxide (CO2). The molecule is capable of capturing and retaining energy from solar rays for a sufficiently long period of time to allow it to react with other molecules.
Moreover, this is the first time scientists have shown that an iron-based material “exhibits strong, visible, room temperature photoluminescence.” In other words, the iron molecule can glow long enough produce light that can be seen with the naked eye at room temperature. While the current photoluminescence of the complex is still quite low, the authors suggest it would be an interesting avenue to pursue. With further improvements, the molecule could have enormous potential in light-emitting technologies, for example, as materials for light-emitting diodes (LEDs).
Solar power applications rely on something called charge-transfer (CT) excited states with a sufficient lifetime and energy to drive electron transfer and visible light emission. Abundant and environmentally inert iron complexes would be an excellent alternative to noble metal systems, however, the problem with is that their electron transfer reactivity is not efficient enough for photochemical applications and they usually exhibit a complete lack of photoluminescence. Luckily, the scientists were able to suppress the cause of this ― rapid deactivation of electrons ― by using advanced molecular design to optimise the molecular structure around the iron atom. The molecular optimisation process is generally slow and the authors were surprised to discover a suitable molecule so quickly. The process should have taken closer to ten years to optimise instead of the five years the team spent working on it.
Motivated by the need for novel technologies and approaches to generate cleaner, more renewable forms of energy, many scientists are exploring novel material design. Although the benefits of renewable forms of energy like solar power are undeniable, humans are rapidly consuming the finite sources of materials required for this type of energy production at a rapid pace. Thus, new materials are needed to truly support a sustainable future. Breakthroughs like these latest findings are crucial to the development of creative energy generation strategies.
(1) Kjær, K.S. et al. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science (2018) DOI: 10.1126/science.aau7160
(2) Bozic-Weber, B., Constable, E. C., and Housecroft, C. E. Light harvesting with Earth
abundant d-block metals: Development of sensitizers in dye-sensitized solar cells
(DSCs). Coordination Chemistry Reviews (2013). DOI:10.1016/j.ccr.2013.05.019
(3) Liu, Y. et al. Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers. Accounts of Chemical Research (2016). DOI: :10.1021/acs.accounts.6b00186