Controllable Synthesis And Properties Of Novel Non-precious Metal Electrocatalysts

To face the current energy exhaustion and environment pollution problems,it is urgently needed to develop clean and regenerative energy conversion and storage technique.As a device that can efficiently convert chemical energy into electric energy,regenerative H2-O2 fuel cells have attracted considerable attention because it is clean that without the release of pollutants and possess very high energy densities,showing great promise to apply in future electric vehicles or realizing the combined to electric grid.Except for the problems in electrolyte separators and engineering technologies,the key challenge for large-scale commercial application of regenerative H2-O2 fuel cells lies on finding low-cost,highly active and stable electrocatalysts to improve the sluggish kinetics of cathodic oxygen reduction reaction(ORR)in fuel cell part and anodic oxygen evolution reaction(OER)in the water electrolytic cell part,and reduce their overpotentials.In view of the current research status of electrocatalysts in regenerative fuel cells,we want to develop the solvothermal,topological chemical transformation and controllable thermal annealing strategies for the synthesis of some transition metal chalcogenides,nitrides,oxides and metal@metal-nitrogen-carbon catalysts and exploration of their applications in electrocatalytic ORR and OER.Besides the growth process,the catalytic performances of those catalysts in alkaline medium have been systematically studied and their related origins of catalytic activity have been discussed based on theoretical calculation or a series of spectroscopic and electrochemical analyses.Some primary results have been achieved and the details are given below:(1)The preparation,characterization and catalytic properties studies of ultrathin two-dimensional(2D)CuCo2S4 nanosheets(NSs).The 2D CuCo2S4 nanosheets NSs have been successfully fabricated for the first time via a "leveling metal reactivity and structure-directed one-pot sulfurization" strategy,i.e.,using acetylactone as the chelating ligand to level the reactivities of Cu and Co ions and then employing 1-dodecanethiol(DDT)as the sulfur resource and structure-directing reagent to mediate the nucleation and growth process.The thickness of the obtained CuCo2S4 NSs is approximately 10-13 nm.Such CuCo2S4 NSs exhibit excellent bifunctional catalytic properties toward oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)in alkaline media.For the ORR,their onset reduction potential and half-wave potential are 0.90 and 0.74 V(vs.RHE),respectively.As for the OER,their onset oxidation potential is 1.5 V(vs.RHE)and a very small overpotential(0.337 V)is observed at the current density approaching 10 mA cm-2.According to the potential drop between the OER current density at 10 mA cm-2 and the ORR half-wave potential,the bifunctional catalytic performances of such CuCo2S4 NSs are superior to binary metal sulfides,CuCo2S4 nanoparticles,commercial Pt/C,and recently reported some bifunctional oxygen electrocatalysts.The related microstructure and theoretical calculation analyses demonstrate that the excellent bifunctional catalytic activity of such CuCo2S4 NSs originates from their exposed(022)and(004)planes,which have different exposed surface atoms and suface charge distribution that have distinct priority for ORR and OER,respectively.This work discloses that the CuCo2S4 NSs may be served as a promising bifunctional electrocatalysts to apply in regenerative H2-O2 fuel cells or other clean energy technology fields.(2)The preparation,characterization and catalytic properties studies of defect-rich(DR)Ni3FeN/N-doped grapheme(N-G)nanohybrids(DR-Ni3FeN/N-G NHs).The DR-Ni3FeN/N-G NHs and nearly defect-free Ni3FeN/N-G NHs have been synthesized through temperature-programmed annealing and nitridation treatment of NiFe-layered double hydroxides(LDH)/graphene oxide(GO)precursors under Ar and NH3 atmosphere.In the obtained NHs,the DR-Ni3FeN nanocrystals(NCs)are well-dispersed on N-G nanosheet,and mainly show twin crystals defects or grain boundaries except for 10%of stacking defaults.Corresponding electrocatalytic tests demonstrate that such DR-Ni3FeN/N-G NHs can efficiently catalyze OER in alkaline media with a high turnover frequency(0.46 s-1).Additionally,their overpotential for attaining a current density of 10 mA cm-2 is only 0.25 V,outperforming to the nearly defect-free Ni3FeN/N-G NHs,pure Ni3FeN NCs and N-G nanosheets,NiFe-LDH/GO precursors,commercial IrO2,and other reported OER catalysts.Moreover,such NHs also showed better electrocatalytic stability than their counterparts.Combined with microstructural,spectroscopic and electrochemical analyses,the greatly enhanced catalytic performance of such DR-Ni3FeN/N-G NHs arises from the abundant defects in Ni3FeN NCs and the strong synergistic effect of their constituents,which not only improve the interfacial electron transfer kinetics and offer more available catalytic active sites but also efficiently deplete the stress and stabilize the interfacial structure of the NHs.This work reveals that combination of defects and hybridization strategies can develop low-cost,high-efficiency and robust electrocatalysts for applying in water oxidation or other renewable energy option.(3)The preparation,characterization and catalytic properties studies of AB2O4/C nanohybrids(NHs).A scries of AB2O4/C NHs,including MnCo2O4/C,CoFe2O4/C,MnFe2O4/C,NiCo2O4/C and NiFe2O4/C,have been synthesized by directly refluxing of bimetallic precursors and carboxylic-functionalized Vulcan XC-72C in 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone(DMPU).Such synthetic protocol is simple,rapid,versatile and easy for scalable production.In those NHs,all the AB2O4 nanocrystals are uniform and ultrafine(-5 nm),which are well-dispersed or partially embedded into C support to form strongly coupled hybrid nanostructures.Electrochemical experiments reveal that those NHs can efficiently catalyze ORR in alkaline media.Among them,MnCo2O4/C NHs exhibit the highest catalytic activity,whose onset reduction potential is 0.92 V(vs.RHE).Related electrocatalytic dynamic tests reveal that their ORR mechanism follows the direct "4e-" pathway and only 6?16%H2O2 is generated during the whole ORR process.As revealed by microstructural and spectroscopic analyses,the superior catalytic performance of MnCo2O4/C NHs results from their higher specific surface area and faster conductivity in relative to other AB2O4/C ones,which can offer more available active sites for ORR.This work demonstrate that combination of component engineering and hybridization strategy can screen and create out cost-effective and high-performance ORR electrocatalysts,showing promising to use in regenerative H2-O2 fuel cells or other renewable energy option.(4)The preparation,characterization and catalytic properties studies of Co@Co-N-C-A nanohybrids(Co@Co-N-C-A NHs).The novel cobalt nanoparticles(NPs)encapsulated by or incorporated into three-dimensional(3D)self-supported Co,N-codoped carbon(Co-N-C)mesoporous network nanohybrids(Co@Co-N-C-A NHs),have been successfully fabricated by molten-salt-assisted high temperature pyrolysis of pre-synthesized Co-folic acid(FA)complex,and subsequently etching large-amount of loosely encapsulated or freely bound Co NPs with dilute acid.The Co@Co-N-C-A NHs exhibit outstanding electrocatalytic activity toward ORR with a half-wave potential(E1/2)of 0.85 V(vs.RHE)in alkaline solution,comparable or superior to Pt/C and recently reported some ORR catalysts.Most importantly,after continuously working for 5000 cycles,their E1/2 value is only reduced 15 mV,indicating that they possess excellent electrocatalytic stability or durability yet.To clarify the role of metallic Co and single-atom Co in Co@Co-N-C-A NHs and understand the influence of their microstructure on catalytic performance.pure Co-N-C.Co/C and N-C nanosheets as well as the Co@Co-N-C-w NHs that synthesized in the absence of NaCl template,were also obtained and used as control catalysts.Deduced from a series of contrast catalysis,blockage of sing-atom active site using NaSCN and rotating ring-disk electrocatalytic dynamic tests,the excellent electrocatalytic performance of Co@Co-N-C-A NHs mainly originates from the synergistic catalytic effects of sing-atom Co in self-supported Co-N-C network and their adjacent small amout of metallic Co NPs.As for the sing-atom Co,it can offer more available catalytic active sites that beneficial to improve ORR activity.While the small amount of metallic Co NPs not only enhance the interfacial electron transfer or penetration but also can mediate the electrocatalytic kinetic process and reduce the yield of imtermediate peroxide species.Moreover,the unique 3D mesoporous structure of Co@Co-N-C-A can increase the specific area of the NHs and facilitate to mass transfer and proton/electron transport process,which may offer another positive contribution for ORR.This work demonstrates that efficiently combining different oxide state species of the same transition metal and fabricating multiple active site hybrid nanostructures may be a promising strategy for developing high-efficient and robust electrocatalysts to apply in regenerative H2-O2 fuel cells or other clean energy option.