Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - IRS-PEC (Elucidating the water photo-oxidation mechanism by infrared spectroscopy)

Teaser

For a society to solely depend on renewable energy sources, the fluctuating energy demand as well as the intermittent nature of renewable energies need to be addressed. A promising solution is to use the energy to produce chemicals (so-called solar fuels), either for use in...

Summary

For a society to solely depend on renewable energy sources, the fluctuating energy demand as well as the intermittent nature of renewable energies need to be addressed. A promising solution is to use the energy to produce chemicals (so-called solar fuels), either for use in the chemical industry or to act as a storage medium. A likely candidate that can serve both is hydrogen (H2). Photoelectrochemical (PEC) water splitting is a potentially effective method for direct conversion of solar power into chemical energy in the form of hydrogen and oxygen (O2). Currently, the water oxidation reaction is limiting the water splitting efficiency. Therefore, the main objective is to gain insight into the water oxidation reaction. If we know at which surface sites the oxidation reaction takes place and which are the intermediate species formed during the water splitting process, we can fabricate tailored photoelectrodes with, for example, a maximum number of catalytic sites. Thereby, the photo-oxidation process can be enhanced and, hence, the water splitting efficiency improved. To this end, a PEC set-up and samples are designed, developed, and fabricated with which reaction intermediates can be detected by means of infrared (IR) spectroscopy during actual operating conditions, the so-called infrared spectroscopy photoelectrochemical (IRS-PEC) cell. This enables us to elucidate the chemical reactions occurring at the surface of a photo-electrode during operation.

The unique combination of PEC analysis with IR spectroscopy will gain fundamental insight into the water oxidation mechanism during actual operating conditions. If we know at which surface sites the oxidation reaction takes place, we can fabricate a photoelectrode with a maximum number of those catalytic sites. Thereby, enhancing the photo-oxidation reaction and, hence, improving the water splitting efficiency. It has the potential of making water splitting an efficient, clean, and viable method to store renewable energy in the form of hydrogen and to produce hydrogen for the chemical industry.

The main objective is to gain insight into the water photo-oxidation reaction. To this end the work was divided in three specific objectives:
1. Design and fabrication of an IRS-PEC cell for operando measurements
2. Demonstration of the proof-of-principle of the IRS-PEC cell
3. Study oxidation mechanism of a novel material

Conclusion of the action:
1. Samples for IRS-PEC measurements can be fabricated and are stable (no delamination)
2. Large area and low temperature (<300 °C) fabrication routes of Fe2O3 photoanodes can be fabricated with good PEC performance
3. Surface intermediates can be detected
4. Novel photocatalyst (Ag3PO4) can be studied by IR spectroscopy

Work performed

1. A novel setup (the IRS-PEC cell) for operando (photo)electrochemical reaction studies by infrared spectroscopy has been designed, developed, and fabricated (Figure 1).
2. Samples for operando studies have been designed and fabricated; in particular, low temperature fabrication routes for hematite have been developed (Figure 2) and thin film multilayer stacks on IR suitable substrates have been developed with good adhesion and electrochemical stability properties.
3. Surface intermediates on the surface of Fe2O3 photoanode have been detected under the influence of an applied bias (Figure 3).
4. The chemical structure Ag3PO4 photocatalytic microcrystals has been studied by IR spectroscopy.

The results have been (or will be) presented at the following conference/workshops and social networking services:
• Poster presentation: PhotoElectroCatalysis at the Atomic Scale (San Sebastian, Spain, 2017)
• Poster presentation: Netherlands conference on Electrochemical Conversion & Materials (Den Haag, the Netherlands, 2018)
• Oral presentation: 2019 E-MRS Spring meeting (Nice, France, 2019)
• Invited presentation: 2019 E-MRS Fall meeting (Warsaw, Poland, 2019)

The results will be disseminated in two peer review journals; publications are in preparation.
• A novel approach to study the reaction intermediates under operando conditions by ATR-FTIR, Aafke C. Bronneberg, Erwin Zoethout, Richard C. M. van de Sanden, Anja Bieberle-Hütter
• Low temperature fabrication routes for hematite photoanodes, Aafke C. Bronneberg, Rochan Sinha, Yihui Zhao, Nitin P. Prasad, Erwin Zoethout, Kaushik Jayasayee, Ranjith K. Ramachandran, J. Dendooven, Richard C. M. van de Sanden, Anja Bieberle-Hütter

Final results

A novel setup for operando (photo)electrochemical reaction studies by infrared spectroscopy has been developed. Compared to other approaches the advantages include: (i) no IR absorption by the electrolyte, (ii) no limitations regarding the pH and amount of electrolyte, and (iii) similar cell geometry as conventional PEC cells. A sample architecture has also been developed. Using Ta adhesion and protection layers, stable and robust photoelectrodes on ATR crystals can be realized. The setup allows for a systematic approach of studying surface groups, i.e. at various bias voltages, wide pH range, and light intensity.
The (photo)electrochemical reactions that can be studied with this device are not limited to water oxidation (or reduction), also other reactions (e.g. CO2 reduction) can be studied. In addition, the setup is not limited to studying the surface of thin films in contact with liquids, but also for studying e.g. nanoparticles.

Currently, it is still not clear which reactions take place at the surface and which surface sites are involved in the water oxidation reaction. When we know this, we can tailor the photoelectrode surface to improve the water splitting efficiency. Thereby, making water splitting an efficient, clean, and viable method to store renewable energy in the form of hydrogen and to produce hydrogen for the chemical industry. Here, we studied hematite as a model system, but with the new approach other materials can be studied as well. Furthermore, identification of the surface intermediates and reaction mechanism is not only important to tailor electrode surfaces, but is also much needed input for modelling and simulation studies.

Website & more info

More info: https://www.differ.nl/research/emi.