Non-noble Metals Modified Electrodes Toward Electro-and Photoelectro-driven Water Splitting

The growing global demand for energy,coupled with an increasing awareness of climate change,has motivated the development of new renewable energy technologies.The splitting of water using sustainable energy sources,such as solar,is a promising pathway to achieve this goal.A key challenge is the development of highly active and cost-effective catalysts for the conversions of water to hydrogen and oxygen.In addition,efficient catalytic water oxidation to provide protons for hydrogen evolution is considered to be a challenge and one of the major obstacles for overall water splitting.The in-depth understanding of structure-activity relationship of water oxidization catalysts is crucial for developing water oxidation catalysts with higher efficiency.Meanwhile,for water splitting photoanode,the sluggish multi-electron and multi-proton processes involved in the reaction made many semiconductors having low catalytic activity towards the oxygen evolution reaction.Thus,to achieve efficient solar water splitting,it is well accepted that better water oxidization catalysts and reliable,stable and economic techniques for catalysts functionalized semiconductors are needed.The electrochemical splitting of water requires efficient functional electrodes.In this thesis,we report the fabrication of electrocatalyst consisted of an electrodeposited NiFeP alloy film which was composite plated with nano-SiO₂ on nickel foam.The structure and morphology of the film were characterized by X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy.The results showed that the surface area of this NiFeP-SiO₂ co-deposition alloy film can be significantly increased after electrochemical etching in a KOH solution.The water splitting properties of the alloy film were evaluated using electrochemistry.By using the NiFeP-SiO₂/NF?Etched?as a bifunctional electrode,total water splitting has been demonstrated in a two-electrode cell with a current density of 10 mA cm^-2 at an applied voltage of 1.57 V,which exhibited enhanced water splitting activity in comparison to the analogue cell using the pristine NiFeP/NF electrode.Photosystem II?PSII?in green plants exhibits excellent water oxidation activity with the help of oxygen evolving complex,which consists of a Mn₄CaO₅ cluster that is surrounded by electron rich ligands.Inspired by the structure and sequential proton-electron transfer?s-PET?mechanism of OEC in PS II,negatively charged carboxylate ligands?polyacrylate,PAA?containing Fe-Ni composites were deposited on nickel foam?NiFePAA/NF?.The as prepared NiFePAA/NF electrode exhibits excellent catalytic activity for water oxidation with a low overpotential of 180 mV to reach 10 mA cm^-2,a small Tafel slope of 19.8 mV dec^-11 and excellent stability in 1.0 M KOH electrolyte.Thanks to the coordination/decoordination equilibrium of carboxylates,NiFePAA/NF exhibits a pH-dependent pseudocapacitive redox phenomenon,which results in the generation of more catalytically active sites in stronger basic conditions.Kinetic studies demonstrate that NiFePAA/NF triggers a sequential proton-electron transfer water oxidation pathway,which exhibits decoupling of the proton and electron transfer,and leads to a strong pH-dependence of the reaction kinetics.In the last part,A host-guest complex self-assembled through Co²⁺cations and Cucurbit[5]uril?Co@CB[5]?is used as a water oxidation catalyst at the surface of fluorine-doped tin oxide?FTO?and BiVO₄ photoelectrodes for electrochemical-and photoelectrochemical water oxidation,respectively.Co@CB[5]on FTO exhibited an impressive turn over frequency?TOF?of 808 s^–1 at overpotential?=590 mV in a pH 9.2 borate buffer and showed good stability.When Co@CB[5]complex was applied to the three dimensional bismuth vanadate?BiVO₄?photoelectrode,the assembled Co@CB[5]/BiVO₄photoanode exhibited a low onset potential of 0.15 V?vs.RHE?and a high photocurrent of 4.8mA cm^-2 at 1.23 V?vs.RHE?under 100 mW cm^-2?AM 1.5?light illumination.The kinetic study confirmed that Co@CB[5]acts as a supramolecular water oxidation catalyst,which effectively accelerated surface charge transfer.Furthermore,the surface charge recombination of BiVO₄ was also significantly suppressed.These results demonstrate the huge potential of host-guest complexes as the supramolecular catalysts for efficient water splitting.