Bulk and surface chemical modification of hematite photoanode for solar water splitting
Date of Issue2016
School of Materials Science and Engineering
Due to increasing demand of energy and the limited supply of the main energy sources (coal, oil, uranium etc.), scientists are looking for the alternative energy sources which are renewable, low cost, easy to manufacture, abundant, light weight and efficient. Photoelectrochemical (PEC) cell has the potential to convert and store solar energy into chemical bonds through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe2O3) is a promising photoanode material due to its chemical stability in aqueous mediums, suitable energy band gap for light harvesting, low cost and environment friendliness. However, its performance for water oxidation is limited by low carrier concentration and poor electronic properties that lead to high overpotential for water oxidation and low solar to hydrogen conversion (STH) efficiency. Nanostructuring, elemental doping, surface passivation layers and deposition of co-catalyst layers are the possible ways to improve the performance of hematite. Here, I report a comprehensive approach (nanostructuring, doping and surface passivation) to improve the performance of hematite photoanode. A substantial improvement in the photoelectrochemical (PEC) performance of hematite (α-Fe2O3) has been observed by doping with manganese (Mn). Fe2O3 nanorods sample with 5% Mn treatment show a photocurrent density of 1.6 mAcm-2, (75% higher than that of pristine Fe2O3) at 1.23 V vs. RHE and a plateau photocurrent density of 3.2 mAcm-2 at 1.8 V vs. RHE in 1M NaOH electrolyte solution (pH 13.6). Consequently, we established a simple method to passivate the surface defects of hematite photoanode for water splitting with a core-shell hematite (α-Fe2O3) nanorods system. Electrochemical impedance spectroscopy (EIS) characterization reveals passivation of the surface defects by the highly crystalline hematite shell layer, which enhances the charge injection. In pristine hematite, more holes are accumulated on the surface and the charge transfer to the electrolyte occurs through surface states, whereas in core-shell hematite photoanode, the majority of hole transfer process occurs through the valence band. As a result, the photoactivity of the core-shell nanorods: 1.2 mAcm-2, at 1.23 V vs. RHE, is twice that of pristine hematite nanorods. After establishing the Mn doped and core-shell hematite systems, we employed intensity modulated photocurrent spectroscopy (IMPS) to understand their charge dynamics for PEC water splitting. The information about the photogenerated holes transfer at the electrode/electrolyte interface for water oxidation and losses due to electron-hole recombination via surface states will help to understand the limiting factors in hematite photoanodes. We observed a two-fold increase in charge transfer rate upon Mn doping, and more than one order reduction in charge recombination rate in core-shell hematite as compared to the pristine hematite photoanode. These results indicate that both Mn doped and core shell hematite photoanodes enhance the PEC performance, although the reasons behind the enhancement are different. Single hematite photoanode even after modifications has remained elusive for overall unassisted water splitting due to stringent overpotential, photovoltage, electronic and thermodynamic requirements. A tandem cell, wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes, is a possible pathway to unassisted overall light induced water splitting. Here, we demonstrate a single solution processed organic-inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a core-shell/Mn doped Fe2O3 photoanode can achieve overall unassisted water splitting with a solar to hydrogen conversion efficiency of 2.4%. It was observed that the overall STH efficiency was limited by the hematite’s onset potential and photocurrent. To understand these limitations, we have calculated the intrinsic solar to chemical conversion efficiency of the Fe2O3 photoanodes. The total photo-potential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6V), for overall unassisted water splitting. The work presented here focused on enhancing the performance of hematite based PEC device by bulk and surface modifications and also through coupling it with perovskite based PV to provide additional voltage. These implementations resulted in improved performance; however, the reported efficiency could be further improved. Based on the observed limitation in this work, I propose strategies for improving the efficiency of hematite.