Hydrogen generation via sunlight-driven photoelectrochemical water splitting using hematite-based nanostructures

• Main Author: Andebet Gedamu Tamirat
• Other Authors: Bing-Joe Hwang
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/17720319012356952308

博士 === 國立臺灣科技大學 === 化學工程系 === 103 === Photoelectrochemical water splitting has been intensively studied in recent years for the production of sustainable, carbon-free hydrogen fuels. Since the first solar water splitting revelation using semiconductor titanium dioxide (TiO2) in 1972 by Honda and Fujishima, extensive efforts have been invested into improving the solar-to-hydrogen (STH) conversion efficiency and lower the production cost of photoelectrochemical devices. Various photocatalysts, however, are wide band-gap semiconductors which are active only under UV irradiation, unstable in the electrolyte and inappropriate band edge positions for overall water splitting. Among various water splitting semiconductor electrodes, hematite (α-Fe2O3) is one of few materials that favorably combines several promising properties such as; stability in aqueous solutions, visible-light absorption, non-toxicity, abundance and low cost. With an energy band gap of 2.1 eV, hematite can theoretically reach water oxidation current density as high as 12.6 mA cm-2 under air mass 1.5 global (AM 1.5G) solar irradiation; thereby potentially enabling a maximum solar-to-hydrogen conversion efficiency of 15.5%. However, the overall solar-to-hydrogen efficiency of hematite is limited by several factors such as relatively poor absorption coefficient, very short excited-state lifetime (~10-12 s), poor oxygen evolution reaction kinetics, and a short hole diffusion length (2-4 nm). In this study, to exploit iron oxide as efficient photocatalyst, we have demonstrated various approaches. First, we present a highly photoactive photoanode for solar water oxidation using three dimensional (3D) urchin-like hematite (α-Fe2O3) nanostructures modified with ultra-thin reduced graphene oxide (rGO). rGO acts as both electron conducting scaffold and surface passivation layer. By virtue of these combined effects, the composite photoanode exhibits 1.47 times higher photocurrent density (1.06 mA cm-2, at 1.23 V vs. reversible hydrogen electrode (RHE)) and two-fold enhancement in the photoconversion efficiency than that of pristine α-Fe2O3. The dual effect of rGO as both electron conducting scaffold and surface passivation layer is further evidenced from the 1.82 and 1.67 fold enhancements in charge separation and charge injection efficiencies at 1.23 and 1V vs. RHE respectively. Second, we develop an efficient photoanode that can oxidize water at low applied potential, aspired to alleviate one of the major challenges in photoelectrochemical water splitting. Consequently, a codoped (Sn, Zr) α-Fe2O3 photoanode modified with stable and earth abundant nickel oxyhydroxide (NiOOH) co-catalyst is reported. Initially, unintentional gradient monodoped (Sn) α-Fe2O3 photoanode was synthesized at controlled annealing temperature that achieved a photocurrent density of 0.86 mA cm-2 at 1.23 V vs. RHE. Further doping with optimized amount of Zr outperformed the monodoped (Sn) α-Fe2O3 photoanode providing significantly much higher photocurrent density (1.34 mA cm-2). The remarkably improved electrical conductivity and more than three times higher charge carrier density (as evidenced from electrochemical impedance spectroscopy measurements and Mott-Schottky analysis) of the codoped (Sn, Zr) α-Fe2O3 photoanode highlights the importance of codoping. The synergetic effect of codoping (Sn, Zr) led to 1.6 fold enhancement in charge separation efficiency at 1.23 V than that of the monodoped (Sn) α-Fe2O3 photoanode. The NiOOH modified codoped (Sn, Zr) α-Fe2O3 photoanode exhibited drastically lower onset potential (0.58 V) and a photocurrent density of 1.64 mA cm-2 at 1.23 V. Interestingly a 160 mV cathodic shift in photocurrent onset potential was also observed. Concomitant with this, the NiOOH modified codoped (Sn, Zr) α-Fe2O3 photoanode exhibited 1.6 to 9.5 fold enhancement in charge injection efficiency (ηinj) at kinetic control region of 0.7 to 0.9 V compared to unmodified codoped photoanode. Gas evolution measurements also showed that the NiOOH modified codoped α-Fe2O3 photoanode achieved an average Faradaic efficiency of 93%. Achieving high solar to hydrogen conversion efficiency at the lowest possible applied power is one of the challenges in solar hydrogen production from water splitting. The third and last part of this research is therefore targets on splitting water at lowest applied potential by modifying the surface of nanostructed hematite electrode sequentially with surface passivation layer and cocatalyst. Here, we demonstrate a new sequential surface treatment approach with heavily Sn doped surface passivation layer and NiOOH oxygen evolution catalyst. The Sn4+ surface treatment creates heavily doped Fe2-xSnxO3 surface passivation layer which can robustly inhibit interfacial recombination by passivating the surface states. While the NiOOH catalyst layer greatly enhances the charge transfer process across the passivated electrode/electrolyte interface. By exploiting this approach, the optimized sequentially treated photoanode (Fe2O3/Fe2-xSnxO3/NiOOH) exhibits lowest photocurrent onset potential of 0.49 V vs. RHE with an additional effect on enhancing the saturated photocurrent density as high as 2.4 mA cm-2 V at 1.5 V vs. RHE. Transient photocurrent and impedance spectroscopy measurements further reveal that the combined Fe2-xSnxO3/NiOOH layers reduce interfacial recombination and the charge transfer process across the electrode/electrolyte interface. When the NiOOH was first deposited onto Fe2O3 surface and Sn4+ treatment later as over layer to form Fe2O3/NiOOH/Sn4+ (i.e., reversed surface treatment), 200 mV anodic shift in photocurrent onset potential and 41 % decrease in water oxidation photocurrent (at 1.23 V vs. RHE) were observed. The results are convincing evidences that it is possible to address the problems of surface trap recombination and sluggish catalysis independently by employing surface passivation layers first and catalysts later sequentially. In summary, the current study show fundamental bulk and surface modifications such as: nanostructuring with various morphologies, enhancing the charge separation through doping and applying conducting scaffolds, improving poor water oxidation kinetics through co-catalysts, improving surface state recombination by applying passivation layers etc. Our results demonstrate the benefits of a noble metal free highly promising photoanode for photoelectrochemical water oxidation.