Synthesis Of Metal-organic Frameworks And Its Derivatives For Electrocatalytic Applications

Designing highl y efficient electrocatal ysts is the core for promoting the performance of novel energy conversion systems such as metal-air batter y and water splitting.Though the noble metal based catal ysts(platinum(Pt)and ruthenium(Ru))possess superior catal ytic activity,the high cost and poor stability seriousl y limit their large-scale application.Therefore,construction of high efficient and cheap noble metal-free catal ysts is ver y important for the pratical application and commercialization of these novel energy conversion devices.In this thesis,a series of efficient metal-organic frameworks based noble metal-free catal ysts have been prepared b y p yrolysis,controlling dimension and adjusting metal composition of metal-organic framework.Due to the large special surface areas,uniform pore size and periodic molecular structure of metal-organic framework,the as-prepared metal-organic framework based noble metal-free catalysts exhibit excellent electrocatal ytic performance,which is better than those commercial noble metal catalysts such as Pt/C or Ru O2,and thus possessed huge application potential and commercial value.More important,the involved reaction mechanisms on surfaces of the as-prepared catal ysts have also been clearly elucidated b y combination of the X-ra y absorption spectra(XAS)experiment and density functional theor y(DFT)calculation.The details are listed as following:Iron metal-organic frameworks with uniform nanoscale size and morphology were synthesized by using the hydrothermal method.Fe doped carbon materials with porous structure were then obtained by carbonizing Fe based metal-organic framework,and the as-prepared carbon materials maintain the framework and morphology of the precussors very well.Then,the electrocatalytic properties of carbonized metal-organic framework materials for oxygen reduction reaction were studied.It was found that the as-prepared carbonized metal-organic framework materials,displayed high electrocatalytic activity,excellent methonal tolerence and good stability.Besides,the effect size of the precussors and carbonization temperature on its electrocatalytic activitywere investigated,indicating that size and suitable carbonization temperature were the key factors for improving its ORR performance.Most important,the output power density of the direct methanol fuel cells(DMFCs)equipped with the as-prepared catalysts is 22.2 m W/cm2,1.7 times higher than that equipped with Pt/C cathode under the same condition.A series of ultrathin metal organic-framework nanosheets with uniform structure and thickness(~ 3.1 nm)were synthesized by the ultrasonication method at the room temperature,and their electrocatalytic activity for oxygen evolution reaction was studied.The results indicate that the bi-metal Ni Co-UMOFNs exhibit the superior electrocatalytic activity,high Faradaic efficiency and ultra-long term stability for OER.It should be stressed that both activity and stability of bi-metal Ni Co-UMOFNs are better than those of commercial Ru O2 catalysts,exhibting huge practical application potential.Furthermore,the relationships between structure and catalytic activity in Ni Co-UMOFNs were studied by combination of the XAS experiments and density functional theory(DFT)calculations,indicating that there are lots of unsaturated metal sites on the surface of Ni Co-UMOFNs.A series of metal sites controllable metal-organic frameworks-74(MOFs-74)were prepared and used as electrocatalysts for oxygen evolution reaction.By accurately controlling the metal ratio of the bi-metal MOFs-74 system,Ni1Co1-MOFs-74 catalyst showed low overpotential and excellent stability during OER process.Moreover,active center and determining-rate step of the OER were confirmed by combining DFT calculations,and the optimizing process of Gibbs freedom during catalysis process was clearly presented.Finally,a freestanding oxygen electrode was fabricated by coating the catalysts onto carbon paper,revealing high water splitting activity and ultra-long team stability(maintaining 96 % after 100 h test).