| Hydrogen production by photoelectrochemical water splitting is a cheap and environmentally friendly way to get hydrogen energy. There are two advantages: firstly, the method of using clean solar energy to support hydrogen produce can avoid the mineral energy using; Secondly, the water produced by hydrogen application can be recycled, as the raw material of hydrogen production at the next rycle. However, hydrogen production efficiency is not high, the reason is the difficulties of water oxidation in thermodynamics and kinetics that make oxygen evolution reaction slow. Compared with hydrogen production, reaction rate constant of water oxidation is three orders of magnitude lower, it also suggests that the charge transfer resistance between electrode and electrolyte is huge. So enhancing charge transfer in the electrode/electrolyte interface is the key to improve photoanode activity. Suitable cocatalyst can lower the barrier of water oxidation reaction, reduce the charge transfer resistance in the electrode/electrolyte interface. However, compared with the pure photoanode, cocatalyst can really solve the problem of slow charge transport between the electrode and the electrolyte, and also introduced a new interface:the interface between the photocatalyst and cocatalyst. The charge transport property in this interface is related to activity of photoanode closely. Studing the interfacial charge transport properties has very important significance for interpretation of photoanode activity, the cocatalyst selection and construction of high efficient photoanode.In this thesis, we selected Fe2O3 as the photoanode, and deposited different cocatalyst on its surface for PEC water oxidation. A Combination of electrochemical test method and surface photovoltage technology is used to study charge transport of two interfacial in different cocatalyst/catalyst system. And we discussed the relationship between the interfacial charge transport properties and activity of photoanode. The thesis contents three parts as following:1. The interface charge transport of Ni(OH)2/Fe2O3 photoanode and its application in photoelectrochemical catalysis:Ni(OH)2/Fe2O3, as photoanode, was used in PEC water oxidation. Results show that activity of photoanode is improved firstly and then reduced along with the increasing amount of cocatalyst. The results of surface photovoltage demwnstrate that with the increase of Ni(OH)2, more holes transfer to Ni(OH)2 from Fe2O3, which is not the reason for the reduced activity for water oxidation. And PEC urea oxidation shows:with the increase of Ni(OH)2, the photocurrent of urea oxidation enhances. It suggests that the easier charge transport between cocatalyst and photocatalyst, the higher activity of cocatalyst/photocatalyst photoanode has. In addition, we guess that the reason for reduced activity for water oxidation is the increasing resistence of charge transport in the cocatalyst bulk by the thickening cocatalyst layer.2. Interfacial charge transfer between cocatalyst and the electrolyte in FeNi-LDHs/Fe2O3 and its application in photoelectrochemical water splitting:The result of first part let us reflect on the question:how to choose a cocatalyst? In this part, the electrochemical oxygen evolution catalyst FeNi-LDHs was used as cocatalyst in PEC water splitting. The dark current and charge injection efficiency of photoanode were measured to study the relationship between oxygen evolution activity and interface charge injection efficiency of cocatalyst FeNi-LDHs with different Fe content. It found that for different photoanodes, there is the same trend in interfacial charge injection efficiency, O2 evolution activity and the photocurrent. This demonsrates that the hole injection efficiency determines the activity.3. The enhanced photocurrent of CoPi/Fe2O3 photoanode by suppressing interface charge recombination:By the conclusion of the first two parts:the smooth interfacial charge transfer is the reason for the enhancing activity of photoanode. As well-known, surface states of Fe2O3 can trap the holes, resulting in slow hole transport in the interface, which reduce the activity of photoanode. To enhance the photocurrent of CoPi/Fe2O3, Al2O3 as the passivation material was inserted between CoPi and Fe2O3. The experimental results show that Al2O3 as a passivation material, passivate the surface state of Fe2O3, which makes hole captured by CoPi directly, not by surface state, thus increasing the PEC activity. |
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