Synthesis And Investigation Of Nickel-based ORR Electrocatalysts

From the first industrial revolution to the present,the consumption of non-renewable fossil fuels has been continuously increasing.However,the reserves of fossil energy are limited,and the use of fossil energy inevitably generates large amounts of greenhouse gases and environmental pollution gases,which cause irreversible effects on the ecological environment of the earth.To build a sustainable and clean energy system,exploring new means of energy utilization,developing efficient energy conversion technologies,and finding energy storage technologies with greater capacity are necessary.Among energy conversion and storage technologies,the direct conversion of chemical energy into electrical energy is always one of the most researched energy conversion methods.Based on the requirements of energy density and safety of the battery,fuel cell is undoubtedly the most favorate clean energy conversion technology.The cathodic oxygen reduction reaction(ORR)is the key link in its industrial application,which reaction rate directly determines the efficiency of fuel cells.The best way to increase the rate of ORR is to coat the electrode with a large amount of catalyst.Currently,the most efficient class of electrocatalysts is mainly those containing precious metals with very low crustal content such as Pt.However,such catalysts are too expensive,insufficiently stable and less resistant to poisoning,and these drawbacks limit the large-scale application of fuel cell technology and metal-air cell technology.Therefore,the development of low-cost,easy-to-mine,high catalytic performance and electrochemically stable transition metal-based ORR catalysts has been one of the main goals of the fuel cell and metal-air cell industries.In recent years,transition metal(e.g.,Mo,Fe,Co,Ni,Mn,Cu,etc.)-based materials have become efficient electrocatalysts of great potential.Among these,nickel-containing non-precious metal electrocatalysts have attracted our research interest due to their simple synthesis,low cost,high catalytic activity and excellent stability and anti-toxicity properties.In this paper,the influence of composition,structure and morphology of nickel-containing non-precious metal electrocatalysts on electrocatalysis is discussed through the exploration and study of raw materials and synthesis methods.Firstly,the catalytic process,reaction kinetics,catalytic active sites and other elements of ORR are thoroughly investigated.In this concern,the means and methods to improve the catalytic ability are obtained.Secondly,by discerning the reaction mechanism and active sites,suitable electrocatalyst structures are discovered.Efficient and stable nickel-containing non-precious metal electrocatalytic materials with special compositions and morphologies can thus be prepared.These catalysts are subjected to physical characterizations and electrochemical tests.Finally,the reaction routes and mechanisms of ORR of several different non-precious metal electrocatalytic materials are explored by in situ electrochemical surface-enhanced Raman(SERS)techniques.The intrinsic connection between the catalytic activity and reaction mechanisms of these catalysts are discussed.In this paper,efficient and inexpensive energy conversion electrocatalysts are obtained.The catalytic mechanisms of nickel-containing electrocatalytic materials are discussed.The research in this paper is summarized as follows:(1)Two-dimensional MoS₂ has been proposed to be an effective low-cost electrocatalyst for the oxygen reduction reaction(ORR)due to its appropriate electronic configuration.However,the catalytic activity of pristine MoS₂ is still far from what was expected.Herein,we rationally synthesize heterostructured Ni₃S₂/MoS₂ ultrathin nanosheets for a direct 4-electron transfer ORR.A half-wave potential of 0.885 V(versus the reversible hydrogen electrode(RHE))is achieved in 0.1 M KOH,which is22 m V more positive than that of commercial 20 wt%Pt/C.The good activity can be attributed to the abundant heterointerfaces in the Ni₃S₂/MoS_2,which provide numerous highly active Mo edges and Mo-Ni-S sites,accelerate charge transport and enable the binding of more oxygen-containing intermediates.This work is expected to inspire the design of advanced MoS₂-based materials for diverse applications.(2)Graphene-like two-dimensional lamellae material(Ni₃S₂/Ni-N/C)composed of Ni₃S₂/Ni-nitrogen-doped carbon is prepared as efficient ORR electrochemical catalysts.The catalyst shows an onset potential of 0.996 V and a limiting current density of 5.32 m A cm^-2 with an excellent catalytic stability even after being oxidized.The special lamellar structure,the synergistic effect of metal,sulfide and nitrogen-carbon bonds,together with the mesoporous nitrogen-carbon substrate ensure the excellent ORR catalytic activity.(3)Theoretically,nickel-nitrogen codoped carbon(Ni-N/C)is a promising oxygen reduction reaction(ORR)catalyst.However,precisely controllable synthesis of Ni-N/C nanostructures with large surface area and high volume of mesopores remains insurmountable challenges.In this work,a mesoporous nickel-nitrogen codoped carbon nanocubes(denoted as Ni-N/C-NCs)catalyst with high specific surface area of 1085m² g^-1 is illustrated.The high volume of mesopores are confirmed to not only allow the great exposure of highly active C-N sites,but also facilitate the mass transfer and gas diffusion,resulting in promotion ORR activity and kinetics.In 0.1M KOH,a diffusion-limited current density of 6.61 m A cm^-2 is achieved,which is 20%higher than that of commercial 20 wt%Pt/C and superior to most catalysts reported to date.The Ni-N/C-NCs prepared by the new synthetic route not only show excellent ORR performance,but also exhibit high structural thermostability,which makes it a promising catalyst for industrial applications.(4)Engineering nonprecious metal into nitrogen-doped carbon is significant to advanced catalysis towards oxygen reduction reaction(ORR).In the present study,we synthesized transition metals(Co,Ni)and nitrogen co-doped carbon nanocube,which is expected to be an electrocatalyst for oxygen reduction reaction(ORR).The catalyst exhibits a half-wave potential of 0.895 V(vs.RHE)with a diffusion-limiting current density of 6.1 m A cm^-2 for ORR in 0.1 M KOH.This ORR behavior is comparable to that of the commercial 20 wt%Pt/C.(5)In situ electrochemical surface-enhanced Raman(in situ SERS)method is examplified to probe the catalytic mechanism of the nickel-containing non-precious metal ORR catalysts in alkaline environment.The Ni-based catalysts and a Co-doped nitrogen-doped carbon(Co-N/C)are used as the experimental group for in situ SERS testing.The decision steps and reaction mechanism of each catalyst are deduces according to the products and reaction intermediates.Based on the diverse catalytic mechanisms of these catalysts,the performance enhancement of nickel-containing non-precious metal ORR catalysts is explicated.