Preparation And Electrocatalytic Performances Of Ultrasmall Nanodots Based On Interlayer Confienment

The rapid development of the social economy,the environmental problems and energy crisis caused by the excessive consumption of fossil fuels have been becoming more and more serious.Therefore,developing green sustainable clean energy technology is of great importance to allivate those problems.Energy conversion and storage are key to the development of renewable energy.Among various secondary energy sources,hydrogen is considered to be one of the most promising energy sources and clean fuels due to its high energy storage density and low pollution.Electrocatalytic water splitting,including OER and HER,provides an effective way for high-purity large-scale hydrogen production,and has attracted extensive attention.Numerous studies have demonstrated that excellent electrocatalysical performance are highly dependent on the nano-size effects of the electrocatlysts.The electrocatalytic performance is remarkably enhanced,when the size is reduced to a few nanometers,such as in the case of nanoscale electrocatalysts range from 1 to 10 nm,and,in particular,those based on ultra-small nanoparticles.Therefore,the reduction of the electrocatalysts to nanoscale is critical to improving the electrocatalytic activity and stability of water-splitting catalysts.Nowerdays,the commercial Pt and IrO2 nanomaterials are the best metal catalysts for electrocatalytic water decomposition.However,their OER electrocatalytic performances as a bifunctional electrocatalyst is still unsatisfactory,and their high cost and low stabilit seriously hamperstheir large-scale application.Therefore,it is crucial to explore highly efficient and stable catalysts to advance hydrogen fuel cells towards the large-scale pratical applications.In this thesis,carbon-supported ultra-small nano-dots composites(Ir/S-C/rGO and NiS2/CoS2/C)were rationally desingned and prepared by virture of the interlayer confinement of layered double hydroxide precursors.The electrocatalytical performances of the composites used as electrocatalysts were also investigated in terms of OER,HER,and overall water splitting.The thesis includes the following two main aspects.(1)Ir/S-C/rGO electrocatalyst.The graphene-supported Ir nanodots coated within mesoporous carbon(Ir/S-C/rGO)composite catalyst material was prepared via the reduction of a SDS/Na2IrCl6 co-intercalated MgAl-layered double hydroxide/graphene followed by an acid treatment.The Ir/S-C/rGO composite has a very ultrasmall particle size and uniform size distribution(ca.1.7±0.2 nm),which can increase active sites and ensure sufficient exposure of the abundant active sites,thereby further improving the catalytic activity.Electrocatalytic evaluation shows that the composite electrode required low overpotentials of 280 mV and-20 mV for the OER and HER at 10 mA cm-2 in a 1.0 M KOH solution,respectively,and in particular,exhibited an excellent long-term durability of 50 h that is superior to that of the state-of-the-art IrO2 and Pt/C electrocatalyst.Furthermore,the Ir/S-C/rGO//Ir/S-C/rGO assembled overall water splitting electrolyzer is generated a decent cell voltage of 1.51(10 mA cm-2),and a good durability.(2)NiS2/COS2/C electrocatalyst.The flower-like NiS2/CoS2/C composite catalyst was prepared by a temperature programmed calcination method using anion(NiEDTA-2Na)-intercalated Co(OH)2 as precursor.The preparation method has the following advantages:the dual role in forming the carbon source and the nickel source of the intercalated NiEDTA-2Na anion;the unfirom size distribution of the nanodots confined by the interlayer galleries of Co(OH)2;the as-prepared carbon capable of enhancing the conductivity of the catalyst;all of which are ecpected to boost the catalytic activity and electrocatalytical stability.The electrocatalytical testing shows that the enhanced performance of composites achieved low overpotentials of 310 mV(20 mA cm-2)and 165 mV(10 mA cm-2)for OER and HER,respectively.In particular,the overall water splitting electrolyzer assembled byusing NiS2/CoS2/C as both anode and cathode exhibited a decent cell voltage of 1.61 V to output 10 mA cm-2 with an excellent stability.Our results promise an effective strategy for designing and preparing various nanodots-decorated microstructures as highly efficient overall water splitting electrocatalysts for energy conversion devices.