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Water splitting by solar energy: From lab-scale to prototype devices

Periodic Reporting for period 1 - SolHyPro (Water splitting by solar energy: From lab-scale to prototype devices)

Reporting period: 2015-06-01 to 2017-05-31

Sunlight is the most abundant sustainable source of energy and there is no doubt that it will play a major role in future energy production. One of the main challenges for renewable energy technologies is its intermitted nature. Accommodating the different cycles (e.g. day-night, clear-stormy weather) and seasonal variations demands intermediate energy storage on a massive scale. An attractive solution to this storage problem is to produce hydrogen from water using sunlight as a power source.
Hematite (alpha-Fe2O3) is a promising photoanode material for using solar energy by splitting water into hydrogen and oxygen. It has a favourable bandgap energy (2.1 eV), good catalytic activity for water oxidation, low cost, is chemically stable in alkaline solutions and environmentally friendly. However, its water splitting efficiency is limited by fast electron-hole recombination and it produces a below threshold photovoltage. The key to increasing the lifetime of photo-generated charge carriers is supressing defects such as grain boundaries or surface defects.
The hematite films as well as the adjacent layers were optimized in terms of their microstructure, chemical composition and defect chemistry in order to achieve the goal of an enhanced photocurrent at the reversible water oxidation potential (1.23 VRHE). This was addressed in the first two objectives. In the third objective, the idea was to couple photoelectrolytic cell to a photovoltaic cell. This was achieved by a wavelength-selective dielectric mirror, so-called a distributed Bragg reflector (DBR). Depending on the required deposition conditions for the best-performing photoanode, the fabrication sequence needs to be adjusted, which was done in the fourth objective. Finally, a scaled-up device was planned to be fabricated with the optimized thin film structure in the fifth objective. The last objective was only partially fulfilled as explained below.
Objective 1: minimize bulk recombination: suppress microscopic point defects such as vacancies and macroscopic defects such as grain boundaries in order to increase the lifetime of minority charge carriers: With an optimized deposition condition the defect structure could be drastically reduced and heteroepitaxial film stack was developed.[1] This resulted in pinhole free films as evidenced by the ability to make measurements in H2O2 containing electrolyte. The electrochemical measurements were all consistent with one another and the optics of the system could be well described by calculation, showing that Al2O3(0001)/Pt(111)/alpha-Fe2O3(0001) heteroepitaxial photoanodes serve as an excellent model system for investigation of hematite properties.
Objective 2: minimizing failures: reduce defects such as pinholes and cracks within the individual thin films and the whole stack: With improved deposition condition and post-deposition thermal treatment the performance and stability was increased.[2–5] Crack free thin films can be deposited by lowering the deposition temperature. Thermal treatment improves photoelectrochemical performance in hematite thin films both in low onset potential and high photocurrent density. In addition the underneath lying substrate was optimized to reduce the defect density and roughness drastically.[3,6–8] This also leads to high specular reflectivity.
Objective 3: optimizing the coupling between the photoelectrolytic cell and the photovoltaic cell in the tandem-cell: First steps to couple the photoelectrochemical cell with a photovoltaic cell was performed by investigating wavelength-selective dielectric mirrors (distributed Bragg reflectors, DBR).[9] Different multilayer stacks were simulated and the selected structures were fabricated and tested. Thin film alpha-Fe2O3 photoanodes deposited on DBR stacks were found to show a photocurrent enhancement compared to similar photoanodes on transparent substrates.
Objective 4: optimizing the fabrication by avoiding incompatible processing conditions: This allowed implementation into a flip-over process to increase the specular reflectivity and therefore the device performance.[4] The film transfer process was invented to allow high temperature hematite processing while avoiding tarnishing the metallic back reflector.[3,6–8] Through this flip transfer process an absorbed photocurrent of more than 9 mA/cm2 was achieved for an ultra-thin hematite film. Our work shows the high potential of hematite as photoanode material in photoelectrolysis using thin film technology.
Objective 5: production of a prototype water splitting device: scale up from 1x1cm3 to 10x10cm3 devices:
For scale up ultrasonic spray pyrolysis and sputtering deposition techniques were investigated as in-house pulsed laser deposition only allows the deposition of samples with 2 cm2.[2] For both methods suitable deposition conditions were found, which lead to similar performances as the PLD deposited films. With ultrasonic spray pyrolysis (USP) a 10x10 cm2 cell was fabricated.

[1] D. A. Grave, H. Dotan, Y. Levy, Y. Piekner, B. Scherrer, K. D. Malviya, A. Rothschild, J. Mater. Chem. A 2015, 0, 1.
[2] D. Shai Ben, S. Barbara, R. Avner, The effect of thermal treatment on the performance of the hematite photoanodes; 2017.
[3] B. Scherrer, A. Key, Y. Piekner, K. D. Malviya, D. Grave, H. Dotan, A. Rothschild, Prep. 2017.
[4] K. D. Malviya, B. Scherrer, D. Shlenkevich, A. Tsyganok, H. Mor, H. Dotan, A. Rothschild, Prep. 2017.
[5] B. Scherrer, T. Li, B. Gupta, M. Doebeli, K. D. Malviya, B. Gault, O. Kasian, N. Maman, I. Visoly-Fischer, Raabe, Dierk, A. Rothschild, Prep. 2017.
[6] A. Kay, M. Leben, K. D. Malviya, H. Dotan, A. Rothschild, B. Scherrer, In Gordon Research Conferences: Renewable Energy: Solar Fuels; 2016.
[7] A. Kay, M. Leben, K. D. Malviya, H. Dotan, A. Rothschild, B. Scherrer, In Material Research Society Spring 2016; 2016.
[8] B. Scherrer, A. Kay, H. Dotan, A. Rothschild,
Following the results of this work students in the group of Prof. Avner Rothschild will investigate further the topics reported here. The optimized fabrication of hematite thin films is the new standard used in our research group. The focus of further investigations lie on the large scale hematite photoanode fabrication, which will be used in photoelectrolysis-photovoltaic tandem cells. This work is the first step towards the foundation of a start-up company or other industrial activity aiming at photoelectrochemical hydrogen production using renewable and abundant resources (water and sunlight). Also the work on DBR is carried on and broadened to additional photoactive materials. More important, environmental benefits for the society will rise with a viable option for sustainable energy production and storage. This would be a big step towards truly renewable fuel and would support the European society to reach the goal for 2020 to increase renewable energy contribution to 20%.
Improved deposition conditions to avoid defects in hematite photoanode thin films while maintaining
Film transfer process sequence[6–8]
Scaling up of hematite thin films deposited on FTO glass by USP.
Defected film transfer sample. Left inlens micrograph, right EDX map, where Fe, O, Al and Ag is blue
Maintaining reflectivity with to film transfer process to improve light trapping effect in hematite