Dengke Zhao

Volatilizable template-assisted scalable preparation of honeycomb-like porous carbons for efficient oxygen electroreduction

The electrocatalytic activity of nitrogen-doped carbons towards the oxygen reduction reaction is largely determined by the concentration of active nitrogen dopants and the electrochemically accessible surface area. Herein we report a novel, facile route for the preparation of N-doped carbons based on direct pyrolysis of polypyrrole nanosheet precursors synthesized by confining the polymerization on the surface of NaCl crystals using FeCl3 as both the initiator and dopant. In the heating-up process of pyrolysis, a large amount of homogeneously distributed FeCl3 dopant and its derivatives gradually evolved into volatile nanoparticles which helped to generate abundant hierarchical macro- and meso-pores, resulting in honeycomb-like porous carbons with a high content of nitrogen dopants ranging from 7 to 18 at%, a large surface area, and an ORR activity superior to that of commercial Pt/C in alkaline electrolytes. Significantly, by using the best sample that was prepared at 800 °C (HPC-800) as the air electrode, a Zn–air battery was found to display a specific capacity of 647 mA h g−1 at 10 mA cm−2 and a negligible loss of voltage even after continuous operation for 110 h, a performance markedly better than that with Pt/C as the air cathode. The results not only highlight the significance of precursor engineering in the synthesis of highly efficient nitrogen-doped carbon catalysts for oxygen electroreduction, but also suggest the high potential of the interfacially confined polymerization method in the scalable preparation of cost-effective, highly porous carbons for electrochemical energy storage and conversion devices.

Graphene Composites with Cobalt Sulfide: Efficient Trifunctional Electrocatalysts for Oxygen Reversible Catalysis and Hydrogen Production in the Same Electrolyte

Nitrogen and sulfur-codoped graphene composites with Co9 S8 (NS/rGO-Co) are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere. Significantly, in 0.1 m KOH aqueous solution the best sample exhibits an oxygen evolution reaction (OER) activity that is superior to that of benchmark RuO2 catalysts, an oxygen reduction reaction (ORR) activity that is comparable to that of commercial Pt/C, and an overpotential of only -0.193 V to reach 10 mA cm-2 for hydrogen evolution reaction (HER). With this single catalyst for oxygen reversible electrocatalysis, a potential difference of only 0.700 V is observed in 0.1 m KOH solution between the half-wave potential in ORR and the potential to reach 10 mA cm-2 in OER; in addition, an overpotential of only 450 mV is needed to reach 10 mA cm-2 for full water splitting in the same electrolyte. The present trifunctional catalytic activities are markedly better than leading results reported in recent literature, where the remarkable trifunctional activity is attributed to the synergetic effects between N,S-codoped rGO, and Co9 S8 nanoparticles. These results highlight the significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts for versatile electrochemical reactions.

Conducting Polymers Crosslinked with Sulfur as Cathode Materials for High-Rate, Ultralong-Life Lithium–Sulfur Batteries

Low electrical conductivity and a lack of chemical confinement are two major factors that limit the rate performances and cycling stabilities of cathode materials in lithium-sulfur (Li-S) batteries. Herein, sulfur is copolymerized with poly(m-aminothiophenol) (PMAT) nanoplates through inverse vulcanization to form the highly crosslinked copolymer cp(S-PMAT) in which approximately 80u2005wtu2009% of the feed sulfur is bonded chemically to the thiol groups of PMAT. A cp(S-PMAT)/C-based cathode exhibits a high discharge capacity of 1240u2005mAhu2009g-1 at 0.1u2009C and remarkable rate capacities of 880u2005mAhu2009g-1 at 1u2009C and 600u2005mAhu2009g-1 at 5u2009C. Moreover, it can retain a capacity of 495u2005mAhu2009g-1 after 1000 deep discharge-charge cycles at 2u2009C; this corresponds to a retention of 66.9u2009% and a decay rate of only 0.040u2009% per cycle. Such a remarkable rate performance is attributed to the highly conductive pathways of PMAT nanoplates, and the excellent cycling stability is ascribed mainly to the chemical confinement of sulfur through a large number of stable covalent bonds between sulfur and the thiol groups of PMAT. The results suggest that this strategy is a viable paradigm for the design and engineering of conducting polymers with reactive functional groups as effective electrode materials for high-performance Li-S batteries.