Preparation Of Cobalt Based Composite Catalysts By Electrospinning Technology And Their Applications In Water Electrolysis

The development of technologies for storage and conversion of renewable energy,such as fuel cells,metal-air batteries,and electrolyzed water splitting,is currently the most promising alternative to fossil fuels.These energy storage and conversion devices involve oxygen evolution reaction?OER?,oxygen reduction reaction?ORR?and hydrogen evolution reaction?HER?.However,sluggish kinetics and high overpotential of the electrochemical reactions limit the whole efficiency of the devices.Therefore,it is vital importance to develop non-precious metal catalysts with high activities and stabilities.Compared with the commonly used chemical methods,electrospinning as a physical method has the advantages of simple operation,less solvent usage and large scale preparation,especially in the preparation of one-dimensional nano-materials,such as nanotubes,nanofibers and so on.In this thesis,a series of cobalt-based composite bifunctional catalysts for HER and OER were prepared by electrospinning technology.The as-prepared catalysts have exhibited excellent catalytic activities and stabilities in electrolytic water.The thesis includes the following two works:?1?Ni-Co-PVP fibers containing cobalt nitrate,polyvinylpyrrolidone?PVP?and nickel nitrate were prepared by electrospinning technology.Nickel-cobalt oxide nanofibers?Ni₂CoO_x?were prepared by calcination the Ni-Co-PVP fibers in air.The final composite nickel-cobalt sulfide porous nanofibers?Ni₂CoS_x?were prepared by sulfuration of Ni₂CoO_x through hydrothermal reaction.The as-prepared catalysts have shown excellent bifunctional electrocatalytic performances for HER and OER.Especially,the optimized catalyst Ni₂CoS-125 has exhibited the superior catalytic performance,and only 106/108 mV overpotential and 83/87 mV·dec^-1Tafel slope is needed for HER in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.The Ni₂CoS-125 also has shown high catalytic activity for OER,and only 212 mV of overpotential is needed at 10 mA·cm²with a Tafel slope of 84 mV·dec^-1,which is superior to that of commercial RuO₂.Furthermore,when the catalyst was employed as a bifunctional catalyst for overall water splitting,only 1.53 V cell voltage is needed to reach a current density of 10 mA·cm²,which is lower than 1.61 V for Pt/C-RuO₂ based water electrolyzer.?2?Polyacrylonitrile?PAN?nanofibers were prepared by electrospinning technology.The polypyrrole was then wrapped on the surface of PAN nanofibers by in-situ polymerization to form core-shell PAN/PPy nanofibers.ZIF-67 was grown in situ on the surface of PAN/PPy to obtain a core-shell structure material?PAN/PPy/ZIF-67?.CoP/N-doped carbon hollows sphere/core shell N-doped carbon nanofibers composite catalysts?CoP/NCF-X?were obtained by pyrolysis and phosphorization.The as-prepared catalysts have shown excellent bifunctional electrocatalytic performances for HER and OER.Especially,the optimized catalyst CoP/NCF-200 has exhibited the superior catalytic performance,and only 86/141 mV overpotential(at 10 mA·cm^-2)and 55/84 mV·dec^-11 Tafel slope is needed for HER in 0.5M H₂SO₄ and 1.0 M KOH,respectively.The CoP/NCF-200 also has shown high catalytic activity for OER,and only 288 mV of overpotential is needed at 10 mA·cm²with a Tafel slope of 60 mV·dec^-11 in 1.0 M KOH.Furthermore,when the catalyst was employed as a bifunctional catalyst for overall water splitting,only 1.64 V cell voltage is needed to reach a current density of 10 mA·cm²,which is competitive in analogous catalysts reported.