Study On The Preparation And Electrochemical Application Of Carbon Supported Cobalt Nanomaterials

In order to alleviate the problems of energy and environment,it is extremely urgent to explore and research new clean energy and efficient energy conversion devices.Therefore,new energy storage and conversion technologies,such as water electrolysis for hydrogen production,fuel cells and metal air cells,have attracted more and more attention.There are three important half-cell reactions involved in these conversion processes,including oxygen reduction reaction（ORR）,oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）.However,efficient catalysts are often necessary to reduce their excessive reaction overpotential（η）due to the sluggish kinetics of these reactions.Therefore,for the large-scale application of electrochemical conversion systems,it is of great significance to develop the cheap,efficient and stable multifunctional electrochemical catalysts.To this end,we synthesized different carbon supported cobalt compounds,which was employed as ORR,OER and HER catalysts for exploring their activities and active sites,and applied as electrode in zinc-air battery（ZABS）and water electrolysis device.The main content includes the following four parts:（1）Prussian blue analogues（PBA）anchored on Cd S nanorods（Cd S NRs）pre-coated with polydopamine（PDA）were utilized as precursors to prepare ultrafine Co₄S₃ nanoparticles supported on N,S-codoped CNTs（Co₄S₃@N,S-CNT）,where Cd S NRs are served as sulfur sources and hard templates.After pyrolysis,the resulting Co₄S₃@N,S-CNT-800 shows a high specific surface area of 142.4 m² g^-1,together with the potential difference（ΔE）of merely0.780 V between the OER potential at 10 m A cm^-2 and the ORR potential at half wave potential(E_1/2).The Co₄S₃@N,S-CNT-800 based air cathode displays a higher discharge capacity of 787 m Ah g^-1 at 10 m A cm^-2,a higher output power density of 154 m W cm^-2,and better working stability,as well as a lower charge–discharge voltage gap than that of the Pt/C+Ru O₂ based air electrode at various working current density.The remarkable oxygen reversible catalytic activities are mainly attributed to the presence of a thin layer of mesoporous carbon on partial sections of the open-end N,S-CNTs,which not only shortens the mass diffusion length but also prevents N,S-CNTs from excessively bundling to maximize the exposure of Co₄S₃ nano-crystallites and graphitized carbon skeletons with N or S heteroatoms.（2）A bifunctional HER/OER catalyst,reduced graphene oxide-supported molybdenum doped cobalt phosphide nanoparticle composite（Mo-Co P/r GO）,was synthesized by low-temperature phosphating using the Prussian blue analogue（PBA）as a precursor.Due to Mo doping and the strong interaction between and Co P and r GO,the composite exhibits excellent OER/HER activity and stability with a lowηof 290/140 m V at 10 m A cm^-2(η₁₀),respectively.Benefiting from the excellent HER and OER activities,we carry out an efficient and stable alkaline water electrolysis operation using Mo-Co P/r GO as both anode and cathode,which can deliver a current density of 20 m A cm^-2 at 1.65V cell voltage and work continuously for 24 hours.This development offers a typical method to prepare bifunctional electrocatalysts in water splitting technologies.（3）Ruthenium（Ru）decorated Co nanoparticles supported by N-doped two-dimension（2D）porous carbon sheets（Ru/Co@NC）were synthesized by galvanic exchange reaction and low temperature annealing using Prussian blue analogue（PBA）and g-C₃N₄ as precursors.The resulting catalyst displays remarkable electrocatalytic activities for OER and HER,including theη₁₀ of 180 and 11m V at 10 m A cm^-2 for OER and HER,respectively,which are superior than that of commercial Ru O₂（270 m V）and Pt/C catalysts（26 m V）,respectively.The Ru/Co@NC sample also exhibits excellent ORR performance with a high E_1/2 of 0.832 V vs.RHE.Benefiting from the efficient trifunctional electrocatalytic activities,overall water splitting（OWS）has been realized on Ru/Co@NC with a small potential of 1.49 V to deliver10 m A cm^-2.And a high energy density(781 m Ah g^-1),low charge-discharge polarization and long-time cycling lives are achieved in rechargeable Zn-air battery（r-ZAB）with Ru/Co@NC as air-cathode,which overpass the benchmark Pt/C+Ru O₂ couple catalysts.The density functional theory（DFT）results exhibit that Ru-Co interfacial structure is revealed to be the main active site for HER due to its favorable H*adsorption kinetics,and the interaction between Co-Ru O₂（deriving from the oxidization of Ru on the surface in the OER process）interfacial structure contributes to the excellent OER performance.（4）The structure and morphology of carbon materials play an important role in the stability and catalytic activity of the carbon supported catalysts.3D structure constructed with1D and 2D composite is conducive to combining the advantages of 1D and 2D materials,thereby improving the stability and activity of the catalyst.Herein,low-loading Iridium（Ir）（≈2.3 wt.%）decorated Co nanoparticle-embedded nitrogen-doped carbon nanotube/porous carbon sheets（Ir Co@NCNT/PC）are prepared to serve as an OER/HER/ORR trifunctional electrocatalyst.The resulting composites demonstrate excellent OER and HER activity in both acid and alkaline electrolyte,including a lowηat 10 m A cm^-2(η₁₀)for OER（about 291and 240 m V）and HER（about 10 and 16 m V）,respectively,which are superior to benchmarking Pt/C and Ir O₂ catalysts.Hence,a low cell voltage of 1.45 and 1.51V is required at j=10 m A cm^-2for the alkalic and acidic overall water splitting（OWS）,respectively.In addition,as-prepared samples also maintain the ORR performance with a high E_1/2 of 0.83 V vs.RHE.Ir Co@NCNT/PC catalyst used as air cathodes for r-ZAB shows an outstanding energy density(810 m Ah g^-1),low charge-discharge polarizations,excellent reversibility,and along cycling lives than the commercial Pt/C+Ir O₂ couple catalysts.