The Interface Charge Transfer Mechanism Of Faradaic Junctions And Its Effects On Photoelectrochemical Performance

Semiconductor materials have been widely used in solar energy conversion and storage devices,such as solar water splitting cells and solar charging supercapacitors.An intermediate layer is usually introduced between semiconductor and electrolyte to improve the performance of a semiconductor photoelectrode.However,its interface charge transfer mechanism is still unclear,which hinders further improvement in its performance.In this study,we use oxide semiconductors?Fe₂O₃and Ti O₂?and Faradaic material?Ni?OH?₂?to form heterojunctions,and study interface charge transfer mechanism and its effects on the performance of solar water splitting and solar charging super capacitors.Some main results are listed as follows:?1?We propose a new concept of a semiconductor junction?Faradaic junction?.A Faradaic junction forms when a semiconductor contacts with a Farador.A Farador is an electron coupled ion transfer conductor.The interface charge transfer mechanism of a Faradaic junction is different with that of a conventional semiconductor junction?such as a Schottky junction and a p-n junction?.The charge carriers of a Faradaic junction are coupled electrons and ions.When interface charge transfer happens,the Farador will change chemically.Therefore,a Faradaic junction is a chemical junction.While the charge carriers of the traditional semiconductor junctions are electrons or holes,no chemical change will happen during interface charge transfer.Therefore,the traditional semiconductor junctions are physical junctions.The unique charge transfer process of a Faradaic junction opens up new possibility for its application in solar energy conversion and storage devices.?2?We find a new characteristic of dependence of Farador redox potential window on semiconductor band position.The same Farador?Ni?OH?₂?was deposited on different conductors?FTO,Pt and carbon cloth?and semiconductor?Fe₂O₃and Ti O₂?.The redox potential windows of Ni?OH?₂on different conductors are the same,which are independent on the fermi levels of the conductors.While the redox potential windows of Ni?OH?₂on different semiconductors are different,which are highly dependent on the conduction band positions of the semiconductors.The band theory was used to explain the adjustable potential window characteristic in a Faradaic junction.This new characteristic of a Faradaic junction can provide guidance for constructing other new type of semiconductor devices.?3?We develop a new method to improve the photoelectrochemical performance of a Faradaic junction by eliminating short circuit between Faradors and conductive substrate.Some conventional preparation methods usually cause direct contact between the Farador and the conductive substrate in a Faradaic junction.After Ni?OH?₂Farador being oxidized into Ni OOH,its electronic conductivity is enhanced by 13 orders of magnitude,which leads to short circuit contact between the Farador and the conductive substrate.And the short circuit contact remarkably decreases the performance of solar water splitting cells and solar charging supercapacitors.In this work,Ni?OH?₂was selectively deposited on the surface of semiconductor,not on the conductive substrate by photoelectrodeposition method,which eliminated the short circuit contact and improved the performances of solar water splitting cells and solar charging supercapacitors.This method can provide guidance for preparation of other efficient Faradaic junctions.