Fabrication And Performance Study Of Carbon-based Electrocatalysts Based On Shrimp Shell Derived Carbon Nanodots

In recent years,with the increasing prominence of energy and environmental issues,people pay more and more attention to the development of clean and renewable energy technologies.Fuel cells and electrocatalytic water splitting to generate hydrogen can undoubtedly meet energy and environmental requirements of human society,but high performance,low-cost and source abundant electrocatalysts have become the biggest limitation to their large-scale applications.At present,the most effective electrochemical catalyst for fuel cells and electrocatalytic water splitting is Pt-based noble metal material,however,the Pt-based materials are expensive and scarce resources,which would undoubtedly restrict their large-scale applications;therefore,search and development of abundant,low cost and high electrocatalytic active non-precious metal catalysts have been the focus of attention of researchers.In this thesis,we try to use cheap and rich natural biomass(e.g.,shrimp shell)derived N-doped carbon nanodots as precursor to fabricate series of N-doped carbon-based electrocatalysts for the ORR,OER and HER applications.The shrimp-shell derived N-doped carbon nanodots have the following characteristics:firstly,the nanodot sizes are below 10 nm,favorable for the further assembly of nanostructured materials;secondly,the nanodots surface contains rich oxygen and nitrogen functional groups(-COOH,-OH,-NH2,etc.),which is conducive to the functional modification of the materials;thirdly,the nature of natural heteroatom(e.g.,N)doping in carbon nanodot structure makes the carbon nanodots promising for fabrication of N-doped carbon-based materials for improved electrocatalysis performance.Owing to the above characteristics,three-dimensional(3D)N-doped porous carbon and Co or Mo contained N-doped carbon materials have been developed in this thesis work using shrimp-shell derived N-doped carbon nanodots as carbon and nitrogen sources for electrocatalysis applications.According to the oxygen reduction reaction(ORR)polarization curve,a simple method for characterizing the ORR activity of the catalyst was developed to effectively compare the ORR activity of different electrocatalysts.The main content can be summarized as follows:1.Three-dimensional(3D)N-doped porous carbon(NPC)materials have been successfully developed by a simple template-assisted(e.g.SiO2 spheres)high temperature pyrolysis approach using shrimp-shell derived N-doped carbon nanodots(N-CNs)as carbon and nitrogen source obtained through a facile hydrothermal method.The catalyst(NPC-800)prepared at 800 ℃ has a large specific surface area(360.2 m2/g)with porous structure containing microporous,mesoporous and macroporous pores.Electrochemical measurements showed that the ORR onset potential,half-wave potential and limiting current of the NPC-800 catalyst in the oxygen-saturated 0.1 M KOH solution are-0.06 V,-0.21 V(vs.Ag/AgCl)and 5.3 mA/cm2(-0.4 V vs.Ag/AgCl,1600 rpm),comparable to that of commercial Pt/C catalyst.In addition,the 3D porous carbon material also exhibits higher resistance to methanol overcross and stability than the commercial Pt/C catalyst.The experimental results demonstrated that the presence of nitrogen in the form of pyridine nitrogen and graphite nitrogen plays an important role in improving the electrocatalytic active sites and electrical conductivity of catalyst.Moreover,the larger specific surface area and porous structure will favor the contact of the reactants with the active sites of the catalyst,thus enhancing its ORR activity.2.Utilizing shrimp-shell derived N-doped carbon nanodots as carbon and nitrogen sources,we first synthesize Co9S8@N-doped porous carbon(Co9S8@NC)in the presence of CoSO4 by one-step molten-salt calcination method.Following by low-temperature phosphorization in the presence of NaH2PO2,Co9S8@N,P-doped porous carbon(Co9S8@NPC)was finally obtained using Co9S8@NC as precursor.Electrocatalyst measurements showed that Co9Ss@NC exhibits bifunctional catalytic activity of the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)in alkaline media.In addition,the introduction of P into Co9S8/NC would be helpful to improve the catalytic activity of HER,but has little effect on the catalytic activity of OER.Among all catalysts investigated,Co9S8@NPC-10(mass ratio of NaH2PO2 to Co9S8@NC=10:1)displays the best HER activity with overpotential of 261 mV at 10 mA cm-2 in alkaline media.Owing to the bifunctional catalytic activities towards both HER and OER,the fabricated Co9Ss@NPC-10 was simultaneously used as anode and cathode material to generate O2 and H2 from the overall water splitting in alkaline media,exhibiting a nearly 100%Faradaic yield.3.In order to further study the electrochemical catalysts prepared by shrimp-shell derived N-doped carbon nanodots and transition metal salts,the molybdenum-containing carbon-based catalysts were prepared with ammonium molybdate and sodium sulfate under the molten salt calcination conditions.By adjusting the molar ratio of S and Mo in the feedstock,a series of carbon-based catalysts containing MoC and MoS2 components can be obtained and their performance as water-splitting cathode catalyst materials were studied in detail.Electrochemical tests showed that the carbon-based catalysts containing MoC and MoS2 exhibit higher HER catalytic activity than the carbon-based catalysts only containing MoC or MoS2 in the 0.5 M H2SO4 electrolyte.When the molar ratio of S and Mo in the raw material is 1.0,the catalyst indicates the highest HER catalytic activity with an onset overpotential of 109 mV and high stability.The results demonstrate that the synergistic effect of different electrocatalytic active components can effectively improve the catalytic activity of electrocatalyst.4.On the basis of the ORR polarization curve of electrocatalyst,a new method for characterizing the catalytic activity of ORR catalyst has been developed in this work.The electron transfer process in ORR polarization curve can be equivalent to a pure resistance behaviour.The total resistance(R)of the process is calculated from the ORR polarization curve and the ohm law,and the relationship between R values and the corresponding limiting currents(IS)under different rotation rates was studied using the commercial Pt/C and lab-made Co-containing carbon based catalysts as examples.It was found that there is a hyperbolic relationship between R and Is,which satisfies the equation:R = R0 + k/Is.Where,R0 is a physical quantity related to the catalyst loading,which decreases with the increase of the catalyst loading amount and then achieve a stable status,suggesting that the process of ORR reaction is controlled by the intrinsic characteristic of the catalyst when the catalyst loading is lower,while the catalyst loading is higher,the ORR reaction process is controlled by the mass transfer process;in addition,k is a physical quantity independent of the catalyst loading amount,which reflects the minimum applied voltage at which the ORR reaction occurs,the value of which is close to the absolute value of the ORR onset potential of the catalyst,representing the thermodynamic property of an electrocatalyst.