Electrocatalytic Water Oxidation Based On First Row Transition Metal(Nickel,Iron,Copper,Cobalt)Catalysts

Hydrogen has been widely believed as a clean and renewable energy source with great potential for energy supply.Among with the various kinds of technology,water splitting is a highly efficient and carbon-free way to produce hydrogen with high purity(99.9%).However,as one of the half-reaction of water splitting,water oxidation has been believed as the bottleneck of water splitting reaction due to its thermodynamically uphill and kinetically sluggish.Therefore,it is critical to develop cheap and efficient water oxidation catalysts(WOCs).In this thesis,several non-noble-based WOCs were studied from perspective of the exploration of catalytic mechanism and the improvement of catalytic efficiency.Firstly,from the view of exploring mechanism,a mononuclear nickel complex based on TAML(tetraamide macrocyclic ligands)with strong electron donating capability was developed as an efficient WOC under neutral condition.In KPi buffer solution(0.1 M,pH=7.0),the catalyst can drive water oxidation at an overpotential(?)of 680 mV with a relatively high TOF value of 0.32 s-1.Mechanistically,the O-O bond formation underwent water nucleophilic attack(WNA)pathway.In the catalytic cycle,the redox non-innocent TAML ligand and HPO42-in the buffer solution participated to facilitate the proton/electron transfer and the formation of O-O bond.Secondly,to improve the intrinsic activity of WOC,Ni-based complex incorporating a long aliphatic substituent was synthesized and used as the precursor for the preparation of active and optically transparent electrodeposited NiFeOxHy thin film on the fluorine doped tin oxide(FTO)substrate with the participant of the Fe impurity in the buffer.Experimental and characterization results suggested that the dodecyl chain had great influence on the deposition of the film.The deposited film not only had highly intrinsic activity,but also had a rough morphology for exposing more catalytically active sites,which reached a current density of 10 mA cm-2 under an ? of 298 mV and remained at least over 22 h with slight decrease in 1 M KOH.A Tafel slope of 39 mV dec-1 was achieved,a high TOF of 0.7 s-1 were obtained with ?of 300 mV,and the Faraday efficiency was 98%.Besides,with the combination of BiVO4,the thin film showed a photocurrent density of 2.7 mA cm-2 under the bias of 1.23 V vs.RHE in 0.1 M NaPi(pH=9.0).More importantly,the role of the long aliphatic chain during the formation of NiFeOxHy was investigated by control experiments,which may provide some guidance for choosing the molecular precursors for preparation of metal-based material WOCs.Besides,to increase catalytic active sites and simplify the synthesis process of precursors,CuPPc(copper polymeric phthalocyanine)was in situ prepared on the surface of copper foam(CF)by one-step vapor-solid reaction at 450?.The electrode showed a low Tafel slope of 60.1 mV dec-1,and afforded the current density of 10 mA cm-2 at a low ? of 287 mV with excellent durability over 50 h in 1 M KOH.Besides,the TOF value at 1.58 V vs.RHE was calculated as 0.34 s-1 and the Faraday efficiency was estimated as 98%.Finally,to increase the activity and durability of WOC,hierarchical CoS2/Ni3S2/CoNiOx nanorods composed of porous and interconnected nanoplates were prepared by one-step hydrothermal sulfidization of CoNi foam.The as-prepared electrode had a unique morphology,thus increase the catalytic activity,achieved the current density of 10 mA cm-2 at ? of 256 mV,and exhibited a low Tafel slope of 43.4 mV dec-1.The Faraday efficiency of the catalyst was 96%.Surprisingly,the prepared electrode can remain a current density of 1 A cm-2 for one week in a 30%KOH solution with only 25 mV increment of the potential.