Enhanced bifunctional catalytic activities of N-doped graphene by Ni in a 3D trimodal nanoporous nanotubular network and its ultralong cycling performance in Zn-air batteries

Free-standing and fexible air electrodes with long-lasting bifunctional activities for both the oxygen reduction reaction(ORR)and the oxygen evolution reaction(OER)are crucial to the development of wearable Zn-air rechargeable batteries.In this work,we synthesize a fexible air electrode consisting of 3D nanoporous N-doped graphene with trimodal shells and Ni particles through repeated chemical vapor deposition(CVD)and acidic etching processes.Our results indicate that such trimodal graphene morphology significantly enhances the active N-dopant sites and graphene-coated Ni surface,which consequentially boosts both the ORR and OER activities,as well as catalytic durability.First-principles density functional theory(DFT)calculations reveal the synergetic effects between the Ni and the N-doped graphene;namely,the Ni nanoparticles boost the bifunctional activities of the coated N-doped graphene,and in turn the graphene-covering layers enhance the stability of Ni.Thanks to the better protection from the triple graphene shells,our trimodal N-doped graphene/Ni-based Zn-air battery can be stably discharged/recharged beyond 2500 h with low overpotentials.It is reasonable to expect that such freestanding trimodal graphene/Ni would be promising in many fexible energy conversion/storage devices.

Investigation of Sulfate Attack Resistance of Shotcrete under Dry-wet Cycles

In order to research the sulfate attack resistance of shotcrete, the sulfate attack of shotcrete in the presence and absence of steel fiber was experimentally studied by using dry-wet cycle method. Meanwhile, compared with ordinary concrete by the same mixture, the difference of sulfate attack resistance of shotcrete was studied. The experimental results showed that, with dry-wet cycles increasing, the changes of loss rate of relative dynamic elastic modulus and mass loss rate of specimens included three stages： initial descent stage, stable stage, and rapid descent stage, respectively. However, the changes of mechanical properties first increased and then decreased. Furthermore, the corrosion products of shotcrete after sulfate attack were observed by using the method of XRD, thermal analysis, and SEM, respectively, and the failure mode of shotcrete turned from ettringite destruction to ettringite-gypsum comprehensive failure. Meanwhile, the contents of ettringite and gypsum increased with increasing dry-wet cycle. Simultaneously, the stratified powders drilled from shotcrete under 150＇s dry-wet cycle were analyzed for the mineral phase composition and thermal analysis. With the drywet cycle increasing, the content of ettringite first increased and then decreased and tended to stable. However, the determination of gypsum decreased gradually and even to 0 when the depth was more than 12 mm.