Construction Of Highly Efficient Transition Metal Electrocatalysts For Its Device Research

Energy crisis and environmental pollution are prominent challenges that the development of the world has faced.Petroleum,natural gas and coal resources are limited and a large amount of CO₂,SO₂ and other pollutants are inevitable in the process of use.To achieve carbon reduction targets,researchers have been committed to developing clean,green and environmentally friendly renewable energy sources.Among all new energy sources,hydrogen,as a clean and sustainable energy carrier,is one of the most promising alternatives to conventional fossil fuels to address the energy crisis and severe environmental pollution.Fuel cells and water electrolysis technology are two core technologies to realize comprehensive utilization of hydrogen energy.However,some problems have not been solved for the above two energy conversion technologies,such as high catalyst cost（especially Pt-based catalysts）,low catalytic activity and durability and so on,which limits the large-scale development of hydrogen energy technology.Therefore,design and development of high-efficiency and long-term stability non-precious metal/non-Pt-based electrocatalysts is one of pivotal issues that need to be solved.This paper mainly employs cheap transition metals as the main catalysts,and firstly makes an in-depth study on design,preparation,and reaction mechanism of cathodic oxygen reduction reaction（ORR）and anodic hydrogen oxygen reaction（HOR）electrocatalysts in fuel cells and anodic oxygen evolution reaction（OER）electrocatalysts in water electrolysis.Finally,the catalysts are assembled in fuel cell and water electrolysis devices to estimate its activity and durability under real working conditions,so as to provide a theoretical guidance and practical basis for reducing the catalyst cost in the key reaction of fuel cell and water electrolysis.The main points and innovations are as follows:1.Three Fe-N-C single atomic catalysts with different O doping were successfully designed by adding metal Fe salt（containing different functional groups）during the formation of ZIF-8.The O-doped O-Fe N₄C-O catalyst of its local structures were identified through high-angle annular dark-field scanning TEM（HAADF-STEM）,X-ray absorption spectroscopy（XAS）and density functional theory（DFT）calculations.The results indicate that O-Fe N₄C-O contains two kinds of oxygen doping,i.e.,axial O ligand and second coordination shell O-doping of the Fe N₄ species.DFT calculations also reveal that the synergy of the two kinds of oxygen doping can optimize the electronic structure,and indeed enhance the binding between the O-Fe N₄C-O and OH,and therefore boost the oxygen reaction reduction（ORR）performance.The O-Fe N₄C-O exhibits excellent ORR activities in both acidic and alkaline conditions.Importantly,the O-Fe N₄C-O-based PEMFC reaches 0.88 W cm^-2,which is about 16%and 31%improvement compared to the O-Fe N₄C and Fe N₄C-based PEMFCs.Moreover,the O-Fe N₄C-O-based AEMFC also achieves ultra-high PPD of 1.24 W cm^-2 in H₂-O₂,which ranked among the top performers of NPM catalyst-based AEMFCs.2.Atomically dispersed Zn NC catalysts with Zn-Pyrrorole-N₄ active centers and rich hierarchical structure were successfully synthesized by regulated pyrolysis of polyaniline and ammonium chloride.The Zn NC-based anion exchange membrane fuel cell（AEMFC）presents an ultrahigh peak power density of 1.63 and 0.83 W cm^-2 in H₂-O₂ and H₂-air（CO₂-free）,and also exhibits long-term stability with more than 120 and 100 h for H₂-air（CO₂-free）and H₂-O₂,respectively.DFT calculations further unveil that the Zn-Pyrrolic-N₄ structure is the origin of high activity of as-synthesized Zn NC catalyst,while the Zn-Pyridinic-N₄ coordination is inactive for ORR,which successfully explain the puzzle why most Zn-MOF-derived Zn NC catalysts in previous reports did not present good ORR activity because of their Zn-Pyridinic-N₄moieties.3.By dopamine polymerization-pyrolysis synthesis method,WC_xsupported Ru nanoparticles（defined as Ru/WC_x）electrocatalyst is obtained.The experiment data demonstrate the strong electronic interaction between Ru nanoparticle and WC_x support could modulate the surrounding electronic structure of Ru sites,which in turn weaken the adsorption of*H,strengthen*OH adsorption to facilitate CO oxidation,as a result of significantly enhanced HOR activity and improved CO tolerance.The Ru/WC_x-based AEMFC not only delivers a high peak power density of 0.73 W cm^-2 in H₂-air（CO₂-free）,but also presents long-term durability with more than 210 h for H₂-air（CO₂-free）.This work can reduce the anode cost of AEMFC.4.Firstly,the stability relationship of cation-doped M-Ru O₂ solid solution of 3d transition metal M（M=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn）was calculated by DFT,and the long-term stable acidic OER catalyst was screened.According to the DFT calculation,the surface concave and anchoring caused by the doping of M increase the distance between the doped atoms and the solution interface,greatly improve the dissolution energy of Ru and doping elements in MRu O₂ solid solution,and thus improve the intrinsic stability of M-Ru O₂ solid solution.A series of M-Ru O₂ solid solutions were successfully prepared via a one-pot glucose-blowing method and were used to study the stability of acidic OER.As predicted by DFT,Zn-Ru O₂ possesses excellent OER performance.It needs an overpotential of 250 m V to drive 10 m A cm^-2 in 0.5 M H₂SO₄ and presents outstanding durability with a negligible decay observe after 15,000 CV for Zn Ru O₂.Zn-Ru O₂ displays a stable operation of>320 h under 10 m A cm^-2with no obvious degradation.It could operate steadily for 1000 h(10 m A cm^-2)in an electrolyzer assembled with Zn-Ru O₂ as an anode.The experimental results reveal that Zn is doped to the outer surface of Ru O₂ in the form of single atom,which causes the change of Ru valence state,and the lower Ru valence is not easily dissolved in OER,so as to maintain the stability of the catalysts in acidic condition.