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Integrating molecular water oxidation catalysts with semiconductors for solar fuels generation

Periodic Reporting for period 1 - WO for solar fuels (Integrating molecular water oxidation catalysts with semiconductors for solar fuels generation)

Reporting period: 2015-05-01 to 2017-04-30

One of the main challenges that humankind is facing nowadays is to obtain a clean and renewable energy source. This is a major concern because it will to help reduce global warming by reducing the CO2 emissions to the atmosphere, and other type of pollution problems such as, nuclear waste. In addition, fossil fuels reserves, which are our main energy source in these days, are decreasing and their extraction is every time more difficult and contaminant. One of the most attractive alternative to fossil fuels is the use of sunlight because with the energy coming from the Sun to Earth in one hour we could obtain the energy to sustain the whole planet for a full year. Sunlight energy has been used since Earth early times in nature by green plants to obtain energy. The process is the so-called photosynthesis through which green plants produce sugar (its own energy) from sunlight, water and carbon monoxide (CO2). In other words, plants are capable of storing sunlight energy in chemical bonds. Solar fuels, or artificial photosynthesis, is the field that aims to mimic green plants to store sunlight energy into the chemical bonds of small molecules (hydrogen, methanol, …) which later on can release the accumulated energy. As in natural photosynthesis a key process is the oxygen production from water, in other words, the electron extraction from water to, later on, reduce CO2. This is a difficult process that needs first a system to capture the sunlight energy, which needs to be transferred to a catalyst to facilitate the reaction. This project was focused on the understanding of all the fundamental processes taking place in different artificial systems (from molecular to materials), from light absorption to oxygen production. With this gained knowledge, the design of new systems will be more efficient and successful.
During this project, I have been working with several state-of the art systems for light-driven water oxidation. We have studied and analysed all these systems using optic and electrochemical techniques: Transient Absorption Spectroscopy, steady state photoluminescence, PhotoInduced Absorption Spectroscopy simultaneously to Transient Photocurrent measurements. These techniques allowed me to follow different processes that take place in the systems I studied once they are irradiated: from charge transfer between the light absorbing unit and the catalyst to monitor how these charges react to produce oxygen.
In one of our studies on a complete molecular system, we have been able to demonstrate that the whole system was limited by the light capture events. This is important in the field because points out the direction to follow in the next generation of these systems, which should focus on the light capture part of the system.
On the other hand, studying solid systems we have been able to understand which the limiting factors are when a catalyst layer is added on top of the light harvesting material. In these systems the role of the catalyst nature and its interaction with the catalyst layer is key to understand the impact in the final performance.
Finally, one of the major objectives of the project was the building of a system capable to produce a fuel (such as hydrogen) from sunlight and water. To achieve this objective we have put some efforts in the development of an efficient and stable photocathode (material able to absorb light and produce hydrogen, our fuel).
All these results have been presented in several specific international and national conferences to the scientific community. Apart from the scientific community, this project highlighted the need of a good dissemination to the general public, and for this reason we have been involved in several outreach activities such as the Imperial Festival (for a general audience) and Youth and Science summer camp (for 15 year old students).
The results collected during this project will help to move forward the artificial photosynthesis field, from molecular to materials level, because some of the final results will help to design better systems. In addition, during this project we have been further developing and understanding the recent published technique PhotoInduced Absorbance Spectroscopy together with Transient Photocurrent measurements. This opens a new way to study photoelectrochemical catalytic processes to shed light into the mechanism of reaction and understanding the kinetics involved.
Transient Absorption Experiment set-up