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Title: The temperature dependent performance of tin-doped hematite, titanium oxide and strontium titanate photo-anodes for photo-electrochemical water splitting
Author: Alhersh, Faye
ISNI:       0000 0004 9350 6376
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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In order for photo-electrochemical (PEC) cells to become a competitive technology in the sustainable energy market, considerable thought must be directed towards system deployment and operation of scaled up devices. Deployed systems will be subject to the daily and yearly cycles causing transient temperatures and solar irradiation. Therefore, isothermal operation may be difficult to achieve as opposed to lab scale devices. In this study, the temperature dependent performance of Sn-doped Fe2O3, TiO2 and SrTiO3 photo-anodes were reported. The cyclic voltammograms of all three semiconductors in the light showed a potential dependent temperature effect on the photo-current densities. At low applied electrode potentials, there was a perceptible decrease of current densities, while at higher electrode potentials, the saturated photo-current density increased with temperature. The dark current also increased with temperature. For the Sn-doped Fe2O3, upon the addition of 0.25 M glycerol as hole scavenger, a gradual increase in photo-current with temperature was observed. At a potential of 1.23 V vs the reversible hydrogen electrode (RHE), the photo-current densities obtained at 75°C, corresponded to an overall increase by 71.2% compared to that at 25°C. These results indicate that in the absence of the hole scavenger, the limiting factor is surface recombination. The fundamental mechanism competition between mobility, kinetics of transfer and kinetics of recombination is proposed to explain the photo-response of hematite with temperature. The increase in the dark current with temperature within the cycle is associated to faster kinetics. For the TiO2 photo-electrodes, at 1.23 vs RHE, the photo-current density at 75 ̊C represented an overall increase by 53% compared to that at 25°C. Repeating the experiment on the same sample has shown a similar response in addition to an irreversible improvement in the photo-current at each temperature. The dark current however, decreased from one temperature cycle to the next. Analysis of the results have shown that the increase in temperature results in a possible increase in the kinetics of charge transfer and mobility and an irreversible decrease in the doping density. The decrease in the doping density leads to an increase in the space charge layer, allowing a greater number of the excitations to be successfully collected. This change would also explain the decrease in the dark current from one cycle to the next as the electrons have to travel a greater distance to tunnel through. A similar response was observed for the SrTiO3 samples with temperature. Analysis of the results have shown that the increase in temperature possibly results in an increase in the kinetics of charge transfer and an irreversible change in the surface morphology due to photo-corrosion which may have resulted in an irreversible increase in the surface area of the photo-electrode.
Supervisor: Haque, Saif Sponsor: Government of the United Arab Emirates
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral