Carbon Supported Transition Metal(Co,Ni) Composites And Their Performance Of Electrocatalysis

Fossil fuels occupy the main position in current energy structure.However,the non-renewability of these fossil fuels have aroused many severe issues,such as environmental pollution,global climate change and so on.The development of new and clean renewable energy conversion and storage devices to replace the traditional fossil fuels has therefore attracted great research attention in the field of science and technology.Regenerative fuel cells,metal-air batteries and hydrogen from water splittingare promosing energy conversion and storage devices with the advantanges of clean and environmentally friendliness and have been widley considered as promising power supplies of the future.Oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?are two pivotal processes related to rechargeable Zn-air batteries and regenerative fuel cells,while hydrogen evolution reactions?HER?and OER are two main reactions for overall water splitting.Currently,the most active electrocatalysts for the ORR/OER/HER are Pt-based catalysts.These electrocatalysts,however,suffer from are high cost and the kinetic sluggishness of the ORR/OER/HER,which have strongly restricted them from widespread uses.Recent efforts have therefore been devoted to the exploitation of alternative electrocatalysts made of earth abundant elements with high efficiencies for the ORR/OER/HER active electrocatalysts.In this thesis,a series of high-performance carbon support transition-metal?Co,Ni?composites electrocatalysts have been developed and their potential applications for metal-air batteries and water splitting are investigated.The structure-property relationships of these ORR/OER/HER electrocatalysts are systematically invesitigated.Detailedly,the following aspects of research have been performed:?1?OER and ORR catalytic performance of N,S-codoped graphene supported Co S₂nanoparticles?Co S₂/NSG?.Specifically,a simple synthesis method has been developed for the synthesis of Co S₂/NSG i.e.the calcination of the hydrothermal reaction product of Co?NO₃?₂and S in the presence of morpholine ethanesulfonic acid?MEA?and graphene oxide?GO?.The investigations show there exists a strong electronic coupling between Co S₂and N,S-codoped graphene.The pyrrolic and pyridinic type nitrogen and S in the form of C-S-C in N,S-codoped graphene are demosntrated to be the main anchoring sites of the Co S₂nanoparticles.When used as a bifunctional catalyst,the Co S₂/NSG exhibits an oxygen reduction onset potential of 0.963 V vs.RHE and delivers oxygen evolution overpotential of393 m V at the current density of 10 m A cm^-2.Its oxygen reduction and evolution catalytic activities are comparable to those of the Pt/C and the state-of-art Ru O₂/C,respectively.Most impressively,the Co S₂/NSG exhibits a potential gap of 771 m V.This value is lower than those of most bifuntional catalysts reported,clearly indicating its potential use as the bifunctional catalyst to replace the noble-metal based catalysts for practical applications.The primary Zn-air battery with Co S₂/NSG shows a higher discharge peak power density than that of Pt/C.Additionally,our results also suggest a great importance to prepare a single pure phase Co S₂with good dispersity in improving the catalytic bifunctionality of the Co S₂/NSG by controlling the pyrolysis temperature.The work is therefore helpful for the design of other bifuntional catalyst with high performance.?2?Catalytic bifunctionality of Ni_xCo_?3-x?O₄nanoparticles deposited on aminated CNTs(Ni_xCo_?3-x?O₄/NH₂-CNTs)for the ORR/OER.We adoped a cation exchange reaction method for the synthesis of the Ni_xCo_?3-x?O₄/NH₂-CNTs.Briefly,Co₃O₄nanoparticles deposited on NH₂-CNTs?Co₃O₄/NH₂-CNTs?are first prepared.The synthesis of the Ni_xCo_?3-x?O₄/NH₂-CNTs are achieved through a cation exchange reaction of the Co₃O₄/NH₂-CNTs in the presence of nickel ions.The obtained Ni_xCo_?3-x?O₄/NH₂-CNTs shows an enhanced catalytic bifunctionality for the ORR and OER.Specifically,the Ni_xCo_?3-x?O₄/NH₂-CNTs can deliver ORR onset and half-wave potentials of 0.954 and 0.863 V vs.RHE,respectively,and only needs an overpotential of 386 m V to deliver a current density of 10 m A cm^-2.The ORR catalytic activity of the Ni_xCo_?3-x?O₄/NH₂-CNTs is comparable to that of the Pt/C,while its OER activity is higher than that of the Ru O₂/C.Additionally,the Ni_xCo_?3-x?O₄/NH₂-CNTs exhibits a potential difference of 0.764 V in the OER current density at 10 m A cm^-2and the ORR current density at 3 m A cm^-2,which is lower than those of most state-of-art catalysts.Most interesting,the Zn-air battery with the Ni_xCo_?3-x?O₄/NH₂-CNTs shows higher durability and performance than those of the batteries with the Pt-Ru O₂/C?an integrated catalyst consisting of the Pt/C and the Ru O₂/C?.The work present here is therefore of great interests since it provides a new method for the synthsis of bimetal oxide catalysts with high performance of the OER and ORR.?3?Tri-functional catalytic performance of Ni Co₂P_xnanoparticles on N,P codoping CNTs?Ni Co₂P_x/NPC?for the ORR,OER,and HER.Specifically,the Ni Co₂P_x/NPC is synthesized simply via the low-temperature phosphidation of Ni Co₂O₄/NH₂-CNTs.As a HER catalyst,the Ni Co₂P_x/NPC exhibits the overpotential for the HER as low as 22 m V,94 m V,and 74 m V at 10 m A cm^-2in 0.5 M H₂SO₄,1.0 M PBS,and 1.0 M KOH,respectively.Additionally,it exhibits a low overpotential of 231 m V to deliver the current density of 10 m A cm^-2for OER and onset potential of 0.939 V vs.RHE for ORR in 0.1 M KOH.The ORR catalytic activity of the Ni Co₂P_x/NPC is close to that of the Pt/C,while its OER activity is higher than that of the Ru O₂/C.In addition,the Ni Co₂P_x/NPC hybrid maintains its catalytic activity for high long-term durability over a wide p H range.The work gives a new strategy for the development of tri-functional electrocatalst with high activities toward ORR/OER/HER.