Structural Engineering Of Cobalt Sulfides/N,S Co-doped Carbon Composites As The Bifunctional Oxygen Electrocatalysts

Rechargeable metal-air battery is a promising technology that can meet the energy needs of various applications.Oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are two important reactions in rechargeable metal-air battery.At present,precious-metal catalysts,such as,Pt/C,RuO2,IrO2,etc.,are considered as the most promising electrocatalysts for the OER/ORR,but their disadvantages of expensive price,poor stability,resistance to toxicity,and single oxygen reduction/oxygen precipitation catalytic activity have seriously hindered their application.Hence,the development of low-cost,high-efficiency,and stable bifunctional OER/ORR catalysts with excellent stability,toxicity resistance,and low cost is critical for the large-scale application of this technology.In the thesis,on the basis of the self-polymerization of dopamine(DA)molecuels and strong coordination interaction with transition metal cations,cobalt sulfides/nitrogen and sulfur co-doped carbon composite catalysts with urchin-like and double-layered tubular structures are successfully prepared via the facile pyrolysis process.The relationship between the structure and catalytic activitiy is further examined and analysed,and the OER/ORR catalytic mechanisms of cobalt sulfide are proposed.The most optimized bifunctional activity is achieved with the overvoltage of 0.707 V between the OER polarization voltage at 10 mA cm-2 and ORR half-wave potential,superior to that of most of previously reported precious-metal-free electrocatalysts and precious-metal(RuO2 and Pt/C)catalysts.This study provides a new idea for the development of an efficient non-precious metal bifunctional oxygen electrocatalyst.The main research content of this article is as follows:(1)Urchin-like Cobalt Sulfides/N,S Dual-doped Carbon nanotubes for the Oxygen Reduction Reaction and Oxygen Evolution ReactionIn this work,we successfully design and construct urchin-like non-precious-metal bifunctional oxygen electrocatalysts,where the intercalated heteroatoms(N,S)-doped carbon nanotubes frameworks are grafted onto mesoporous cobalt sulfide/nitrogen,sulfur co-doped carbon spheres via two-step pyrolysis process.In addition,we used physical characterization and electrochemical testing methods to investigate the effect of chemical composition and structure on the performance of electrocatalytic oxygen reduction and oxygen evolution.As a result,benefiting from the unique urchin-like architecture and chemical composition,the resulting catalysts demonstrate good catalytic activities.With a positive half-wave potential of 0.860 V vs.RHE for oxygen reduction reaction and low overpotential of-390 mV at the current density of 10 mA cm-2 for oxygen evolution reaction in alkaline medium,the catalysts can rank them among one of the most promising non-precious metal bifunctional oxygen electrocatalysts reported recently.(2)One-dimensional Mesoporous Double-layered Tubular Cobalt Sulfides/N,S Dual-doped Graphitic Carbon for the Oxygen Reduction Reaction and Oxygen Evolution ReactionIn the previous work,the active sites of metals that play an important role in the performance of electrocatalytic oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are not uniformly distributed,the particle size is large,and the catalytic performance of the active sites is not exhibited sufficientlly.Based on this,we have explored and discovered a method for synthesizing cobalt-polydopamine polymer(Co-PDA)with universal significance using the room-temperature chelation of dopamine and hexaammine cobalt ions.With this method,one-dimensional mesoporous double-layered tubular structure,where Co9S8 nanocrystals are embedded into nitrogen,sulfur dual-doped carbon matrix,is successfully synthesized via two-step pyrolysis process.The structure renders the characteristics of the large electrochemically active surface area and fast mass transfer ability.The chemical compositions composed of Co9S8 and nitrogen,sulfur co-doped carbon matrix construct efficient cobalt oxide/oxyhydroxide active sites and heteroatoms(N,S)-induced active sites for synergetically catalyzing the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).Hence,the resulting catalyst shows exceptionally high bifunctional electrocatalytic activities for both the ORR(E1/2 = 0.890 V vs.RHE)and OER(η10mA cm-2 = 0.367 V)with considerable stability.The overvoltage between the OER potential at a current density of 10 mA cm-2(E10)and ORR half-wave potential(E1/2)is as low as 0.707 V,which is even smaller than that(0.711 V)of the state-of-art Pt/C and RuO2 catalysts,as well as most of non-precious metal bifunctional oxygen electrocatalysts reported so far.