Preparation And Oxygen Electrocatalytic Properties Of Transition Metal-based Carbon Materials Derived From MOFs/GO Composites

The consumption of fossil fuels has spurred the development of green and renewable energy storage and conversion devices.Zinc-Air Batteries(ZABs)have been targeted as the most promising energy conversion devices due to their environmental-friendliness,high efficiency,long-term operating capacity as well as remarkable energy density.However,the main challenge facing ZABs in widespread application and commercialization is still the bottleneck of low energy conversion efficiency and poor stability caused by sluggish electron transfer dynamics in ORR and OER.Heretofore,it is well known that Pt-based catalysts and RuO2-based catalysts are the most effective catalysts for ORR and OER,respectively.Unfortunately,their wide application are severely restricted by the high price and terrestrial scarcity.Hence,the design of efficient and low-cost noble-metal-free ORR/OER electrocatalysts is of great significance,which will promote the development of the ZABs.In this paper,a series of transition metal-nitrogen-carbon(M-N-C)nano-catalysts were prepared by means of coordination assembly and in-situ growth using graphene as catalyst carrier and metal-organic framework as metal source.In addition,their catalytic abilities of ORR and OER were investigated.The specific research content is as follows:(1)Graphene oxide(GO)was selected as a catalytic carrier,the formation of Fe-TA@GO is driven by the coordination assembly between GO,Fe3+and tannic acid(TA)under hydrothermal conditions.Further,a nano-Fe/C catalyst(Fe/Fe3C@NC-Gs)with a core-shell structure is prepared through a one-step carbonization method using urea with high nitrogen content as the nitrogen source.Fe/Fe3C@NC-Gs has a unique core-shell structure,abundant mesoporous structure,large surface area and high efficiency nitrogen doping.The close contact between the iron core and the graphite shell is beneficial to adjust the electron density and promote the transfer of electrons from the iron core to the carbon shell.Furthermore,the carbon shell also plays a role of protecting and confining the metal species Fe/Fe3C,which effectively prevents the alkaline medium from corroding the active species,and also helps to avoid the agglomeration of metal species during the high-temperature carbonization process.The electrochemical test under alkaline conditions shows that Fe/Fe3C@NC-G-2 has an onset potential of 0.97 V and a half-wave potential of 0.88 V,both of which are higher than those of commercial Pt/C catalysts.In addition,when Fe/Fe3C@NC-G-2 is used as an ORR catalyst,its stability and methanol resistance are better than commercial Pt/C catalyst.Compared with Fe/Fe3C@C-G-2,which is not doped with nitrogen,the electrocatalytic performance of Fe/Fe3C@NC-G-2 has been significantly improved.(2)The catalyst precursor ZIF-67/GO was synthesized by using graphene oxide as the catalytic carrier in the liquid phase environment.The nitrogen-doped graphene decorated Co/N co-doped hollow carbon nanocapsules(Co@N-HCCs@NG)were prepared by high temperature carbonization and acid etching.The obtained material has the following advantages:(i)Co@N-HCCs@NG simultaneously integrates Co nanoparticles(0D),hollow carbon nanocapsules(0D)and graphene(2D)into a catalytic entity,enriched catalytic active species and interface characteristics;(ii)Hollow carbon nanocapsules provide large surface area(up to 618.12 m2·g-1)for the whole catalyst due to their unique inner surface;(iii)The introduction of heteroatom N into the carbon skeleton can increase the conductivity and electron affinity of the carbon substrate,thereby generating more active centers to improvethe electrocatalytic performance.Concurrently,the introduction of N atoms can promote the reactivity of carbon nanocapsules and O2-related species,and strengthen the coupling between the various ingredients.The electrochemical test under alkaline conditions shows that Co@N-HCCs@NG has excellent ORR/OER bifunctional catalytic activity.For ORR,Co@N-HCCs@NG exhibited a high onset potential of 0.98 V and half-wave potential of 0.86 V For OER,Co@N-HCCs@NG only requires a potential of 1.528 V to reach a current density of 10 mA·cm-2,which is also better than the commercial RuO2 catalyst(1.583 V).Moreover,Co@N-HCCs@NG also possessed excellent catalytic stability.(3)A dicyanamide-based Co-MOF was successfully synthesized by a simple mechanical coordination self-assembly method.It was then compounded with graphene oxide,and finally the nitrogen-doped graphene decorated three-dimensional coral-like carbon nanotube assembly(CNTAs-NG)was prepared by high temperature carbonization and acid etching process.In this tactic,dicyanamide ligand on the Co-MOF not only was instrumental in the introduction of nitrogen but also acted as the inducer of CNTs.In this structure,Co is encapsulated in the carbon layer.Graphene oxide(GO)is chosen as a matrix to bridge the CNTs and ensure the uniform distribution of CNTs.The obtained CNTAs-NG structure possesses 3D open porous texture,abundant defects,desired nitrogen bonding type and high specific surface area,providing them with excellent ORR and OER properties.The electrochemical test under alkaline conditions shows that the optimized CNTAs-NG sample shows a high onset potential(Eonset=0.97 V)and half-wave potential(E1/2=0.85 V)for ORR as well as an overpotential of 340 mV at 10 mA·cm-2 for OER.Simultaneously,CNTAs-NG has higher stability than commercial noble metal-based catalysts(Pt/C,RuO2).