| It is imperative to find a sustainable energy source as the main energy to replace fossil fuel, which has increasing consumption and brings more and more pollution to environment. Hydrogen is promising to replace fossil fuel, because it has high utilization efficiency, convenient transportation and clean product after combustion, and it is easy to be converted to other energies. It is very likely to make hydrogen sustainable by converting solar energy to hydrogen energy through a suitable photoelectrode. Actuallly, photoelectrodes based on some materials like III-V semiconductors have been made with solar-to-chemical efficiencies exceeding 12%. However, all attempts to pratical application have been hindered by at least one of the following three limitations:(1) toxicity of semiconductors,(2) high cost of their constituent elements due to the low storage in nature, and(3) high preparation cost the semiconductors. On the contrary, SnS is non-toxic, having earth-abundant constituent elements, and is easy to be made by low-cost methods. Furthermore, it also has following three advantages for photo-conversion:(1) it usually has a direct energy band gap ranging from 1.3 to 1.5 eV indicating it can absorb visible light of all wavelengths;(2) it has a high optical absorption coefficient(>104 cm-1);(3) its theoretical photo-conversion efficiency is up to 24%. So in the present thesis we studied PEC hydrogen production through phtoelectrodes based on SnS films, which were fabricated by low-cost chemical bath deposition(CBD) and hydrothermal methods. Because it is difficult to realize complete water splitting on SnS and some other high-efficiency photoelectrocatalyst, here we studied electrooxidation of organic molecules under low voltage(1V) for hydrogen production. So that it may be helpful to PEC hydrogen production without bias volatge.Firstly, the weak photo-conversion behavior of reported solar cells based on SnS films was analyzed. So far the reported highest efficiency of SnS based solar cell is 2.46%, although it showed a pretty high short-circuit current of about 20mA/cm2. The relatively negative conduction band(CB) and valence band(VB) of SnS are thought to be the reason for its low open-circuit voltage(Voc) and small efficiency. Besides by using Zn(O,S) as a buffer layer to form a heterogeneous junction with SnS, it would be better to get a buffer layer of n-type SnS by doping to form a homogeneous junction with p-type SnS film. So that the junction can be favorable to separate charge carriers better and also improve its light-absorption efficiency. In and Sb doping were found to be realized in SnS, and also made a tendency to convert the conduction type of SnS from p type to n type. Sb doped SnS film was deposited on undoped p-type SnS, whose PEC property was improved by its deposition. With the increasing amount of doping elements, the electric resitivity decreased firstly and then increased. So the photoanodic current increased fast at first and then decreased.Secondly, SnS was found to be subject to photo-corrosion during its PEC process of hydrogen production. To avoid that corrosion, atomic layer deposited TiO2 was used to protect SnS. To improve its photo-conversion efficiency, CdS worked as a buffer layer to separate photo-excited electron-hole pairs. 2 nm Pt nanoparticles were deposited as the electro-catalyst for hydrogen production. Consequently, the structure of SnS/CdS/TiO2/Pt was designed, fabricated, and also realized stable photocurrent of around-2.7 mA/cm2 for at least 2 hours, IPCE of about 12% for light of wavelength from 350 nm to 600 nm and also bout 12% IQE for light of wavelength from 350 nm to 900 nm under 0V(vs. RHE). Its Faradaic efficiency is ~90%.In practical situation, PEC hydrogen production must be operated under zero bias. But a solar cell based on a single layer of light absorber won’t give a Voc bigger than 1.23 V. So we bring forward a way to produce hydrogen under low-voltage(1V) by using organic matters rich in wastewater as oxidizable sacrificial reagent. We investigated electro-oxidation behavior of these organic molecules, and also optimized the experimental conditions. Under acidic conditions(pH 2) on a platinum electrode, ethanol oxidation gave the highest currents of the organics tested. At pH 7, citric acid had the highest electrooxidation activity followed by methanol and then ethanol. Under alkaline(pH 12) conditions, glucose was a superb reductant followed in reactivity by glycerol and methanol. These results imply that alcohol hydroxylic or organic acid group moieties commonly found in organic wastewater streams are suitable for low voltage electrooxidation. Gold was proved to be the most active electrocatalyst for electrooxidation of 0.5 M glucose at pH of 13. While water is a relatively cheap resource, expensive concentrated hydroxide is required to perform alkaline water electrolysis. Here we show that relatively low pH 12 is required for alkaline glucose electrolysis while more than 3 mA/cm2 current density can be obtained via electrooxidation of glucose under 1 V applied voltage at steady state conditions. Lower pH values resulted in lower currents for the electrooxidation of glucose. For the electrooxidation of glucose on a gold electrode the current increases as a function of temperature; however, at higher temperatures, non-electrochemical reactions proceed which inhibit sustained current production, resulting in a maximum steady-state current at 40 oC. The Faradaic efficiency of hydrogen production achieved was approximately 70% for glucose on the Au electrode. As a sacrificial reagent, glucose aqueous solution was used to improve the the PEC behavior. Wastewater was also confirmed to be helpful to produce hydrogen under no bias. |
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