Design Of Nitrogen-enriched Carbons And Their Metal Composites For Electrocatalysis

For decades,with the worldwide emphasis on environmental protection and sustainable energy development,electrochemical water splitting,as a green energy device for hydrogen production,has attracted a lot of attention from both industry and academia.The lack of efficient,cheap,and stable catalysts is still a rock hindering the further industrial expansion of electrochemical water splitting.In recent years,nanomaterials and nanotechnology have been developed rapidly.Among them,carbon nanomaterials（graphene,carbon nanotubes,etc.）with unique nano-size effect and excellent physical and chemical properties are promising materials for electrocatalytic reactions and are widely used as both support materials for metal catalysts and as metal free catalysts with heteroatom doping.Catalysts with unique structures can have enhanced activity,selectivity,and stability,moreover,those catalysts can reduce the amount of metals used,cut down the cost and be more environmentally friendly.In this thesis,a series of nanocarbon complex catalysts with special structures were designed and prepared.They exhibited excellent catalytic performance when applied as electrocatalysts for oxygen evolution reaction and hydrogen evolution reaction.Meanwhile,the catalytic reaction mechanism was studied,and the interactions between individual components and the relationship between the material structure and the catalytic performance were explored.The specific research contents are as follows:（1）Synthesis of ultrathin two-dimensional cobalt-based conductive MOF as highly active electrocatalyst for oxygen evolution reaction.Ultrathin two-dimensional conductive MOF nanosheets（Co-HAB-NSs）with an average thickness of 4.5 nm were prepared by a simple and green soft chemical method using Co（II）ions and hexaaminobenzene trihydrochloride.The poor conductivity and stability of the two-dimensional organic systems were improved by constructingπ-d conjugated conductive networks.The morphology of the conductive MOFs can be effectively controlled by changing the ratio of reactants,temperature,and solution p H.Five kinds of conductive MOFs with different shapes and size were synthesized:hexagonal nanosheets,bulk,sheets,particles and ultra-thin nanosheets.Compared with conductive MOF catalysts with other morphologies,ultrathin nanosheets of Co-HAB-NSs have the highest conductivity,largest electrochemical active surface area,and highest electrocatalysis for OER in 1 M KOH electolyte.It exhibited an excellent activity with an overpotential of 310 m V and a Tafel slope of 56 m V dec^-1.After coordinating with cobalt atoms,the electronic structure of carbon atoms was changed,which improved the adsorption strength of carbon atoms to the intermediate species and further enhanced the catalytic activity of this catalyst.（2）Synthesis of cobalt-nickel bimetallic conductive MOF as self-supported electrode for electrocatalytic oxygen evolution reaction.Co（OH）₂ was electrodeposited on carbon fiber papers and acted as both template and metal precursor to let bimetallic Co Ni-HAB conductive MOF grown in situ on it via a simple hydrothermal method.The bimetallic conductive MOF was uniformly distributed on Co（OH）₂sheets,and a self-supported electrode with hierarchical structure was obtained.The obtained electrode exhibited a low overpotential of 219 m V in1 M KOH,a small Tafel slope of 42 m V dec^-1,and a high current density retention of 91.3%after 24 hours’durability test,showing higher electrochemical catalytic activity,faster reaction kinetics and better durability compared with monometallic conductive MOFs and bimetallic conductive MOFs in a powder form.The active component was in situ grown on the substrate material,which effectively prevented the catalyst exfoliation caused by the violent evolution of oxygen.Besides,the self-supported electrode without any binders avoided the obstruction of electron transfer and ion diffusion caused by the binder.In addition,density functional theory calculations revealed that the synergistic effect of Co and Ni plays an important role in enhancing the intrinsic activity of electrocatalytic oxygen evolution reaction.（3）Synthesis of Pt Ru alloy nanoparticles on C₂N matrix for electrocatalytic hydrogen evolution reaction.C₂N was synthesized by the condensation of hexaaminobenzene trihydrochloride and hexanone cyclohexane octahydrate.Then Pt（II）ions and Ru（III）ions were electrostatically adsorbed on C₂N and subsequently reduced to metal oxides,which were further pyrolyzed into alloy nanoparticles.The Pt Ru alloy nanoparticles are uniformly distributed in the C₂N networks.C₂N nanosheets acted as a conductive network that can effectively facilitate electron transfer and a stable substrate that has strong interaction with the Pt Ru alloy nanoparticles,which can prevent corrosion of the metal components during electrolysis.The catalysts showed efficient electrocatalytic performance for hydrogen evolution reaction under both acidic（0.5 M H₂SO₄）and alkaline conditions（1 M KOH）,exhibited overpotentials of 52 m V and 59 m V,respectively,and a negligible overpotential increase of only 7 m V and 5 m V after 10,000 cyclic voltammetry cycles,respectively.Theoretical calculations showed that introducing ruthenium atoms into the platinum lattice changed the electronic structure of platinum and optimized the bonding strength between the surface of catalyst and hydrogen,accelerating the hydrogen desorption process and promoting electrocatalysis performance.（4）Preparation of nickel,iron single-atom on oxygen-rich carbon nitrides and their electrocatalytic performance for oxygen evolution reaction.The oxygen-rich carbon nitrides（CNOs）were synthesized via salt-templating method,using calcium chloride dihydrate as salt template,adenine,and ribose as precursors.Calcium chloride dihydrate acted as not only porogen,in addition,it also formed supermolecules with ribose via hydrogen bond and provided a homogeneous environment for the polycondensation and carbonization process.The nitrogen and oxygen contents of these CNOs were up to 20 wt%and 12 wt%,respectively,even when the temperature of one-step pyrolysis was as high as800℃.The structure and elemental composition of these CNOs were very stable,which enabled them excellent substrate for loading single atoms.A large number of mesopores with unique graphitic rings were seen in the CNOs flakes.Compared to the CNOs loaded with iron atoms,the CNOs loaded with nickel atoms exhibited a better catalytic performance on OER in0.1 M KOH.Furthermore,the addition of iron atoms into nickel based CNOs displayed an enhancement in electrocatalytic activity.The main reason for this enhancement is the synergistic effect between nickel and iron.Finally,the catalyst exhibited excellent durability.