| With ever-increasing global consumption of fossil fuels and aggravated environmental concerns,it is paramount to develop clean and renewable energy sources.Water splitting is one of the most appealing technologies for hydrogen production,a clean and ideal substitute for non-renewable fossil fuels.However,oxygen evolution reaction（OER）,anodic half-reaction of water splitting,limits the implementation of this technology in a global-scale sustainable energy supply system due to its sluggish kinetics and high overpotential.Electrocatalysts developed so far are not up to the mark in terms of kinetic and thermodynamic requirements,which impels the development of more advanced electrocatalytic materials for OER with high efficiency and durability.Competing with the state-of-the-art high-cost noble metal electrocatalysts,cobalt oxide（Co3O4）-based materials with diverse structures and dimensions are making their place in OER catalysis due to their reasonable reactivity,low-cost,high stability,and environmental benignity.Nevertheless,the low electronic conductivity and durability in basic solutions hinder their practical application.The objective of this study is to achieve high electrocatalytic activity and stability of Co3O4-based materials towards OER.For which two distinctive approaches have been followed.The first approach attempts to enhance the intrinsic activity of catalysts by dynamic control of their active site nature via doping and catalyst/support-like effects.The second approach refers to the adopted synthetic strategies aiming at the optimization of the catalyst’s surface by controlling the catalyst’ s morphology,design of nanostructures,and porosity.Hence,this study focuses on the assimilation of the Co3O4-based materials with nitrogen-doped porous carbon（N-C）to ameliorate their OER catalytic activity.The detailed contents of the research work for developing highly effective low-cost Co3O4 and N-C-based electrocatalysts via tailoring their electrical and morphological properties are as follows:Solvothermally-prepared cobalt-pyrazolate microspheres were subjected to thermal treatment to yield N-C encapsulating uniform Co3O4 nanoparticles（NPs）.The resulting composite（Co3O4@N-C）material was evaluated as electrocatalyst for the OER in basic medium with a low onset potential of～1.52 V（vs RHE）,a very small Tafel slope of 44 mV dec-1 and overpotential of 390 mV at 10 mA cm-2 in 1.0 M KOH.The achieved superior oxygen evolution activity originated from in situ incorporation of Co3O4 into mesoporous C matrix,leading to the enhanced charge transport and conductivity,and high structural stability.An interconnected three-dimensional（3D）structure of Co3O4 NPs derived from ZIF-67 with N-C nanotubes（N-CNTs）of polypyrrole（PPy）origin was synthesized.The highly conductive N-CNTs that ran through the Co3O4 NPs not only endowed the resulting product Co3O4/N-CNTs with large active surface area and enhanced charge transfer between Co3O4 NPs,but also prevented Co3O4 NPs from aggregation.As a result,the as-synthesized Co3O4/N-CNTs electrocatalyst exhibited improved OER activity with a low onset potential of 1.37 V（vs RHE）,overpotential of only 200 mV at 10 mA cm-2 in basic media and very small Tafel slope of 40 mV dec-1.PPy with 3D flower-like structure was prepared to obtain N-C and to implant with Co3O4 via simple and cost-effective solvothermal reaction.Benefitting from the 3D flower-like porous structure,Co3O4/N-C exhibited high surface area replenish with more electrocatalytic active sites.Indeed,N-C materials can remarkably increase the stability of Co3O4 via mitigating their structural collapse tendency in the electrochemical process,as well as makeup for their poor conductivity.These exclusive structural features render amazing catalytic activity for Co3O4/N-C towards OER in basic media with an onset potential of～1.31 V（vs RHE）,low overpotential of 120 mV at 10 mA cm-2 and a Tafel slope of only 33 mV dec-1.In general,this thesis reports the shifting of the OER catalyst field from precious noble metal-based catalysts to expedient fabrication with low-cost transition metal catalysts.By picking suitable precursors and synthesis strategies,this work led the fabrication of cost-effective,non-hazardous,and efficient electrocatalytic materials with distinct morphology and advantageous electrocatalytic properties（porous structure,high electrical conductivity,and excellent stability）for the possible application of water oxidation and the full exploitation of the green potential of water splitting-based technologies.This work has the potential to break new grounds for preparing and optimizing other transition metal-based water oxidation catalysts. |
