Preparation and Properties of Oxygen Reduction and Oxygen Evolution Electrocatalysts

Author:Sun Zuo

Supervisor:yin ge ping


Degree Year:2019





Oxygen electrode reaction universally exists in many important electrochemical energy conversion and storage devices,such as water electrolyzer,fuel cell,metal-air battery etc.Oxygen electrode reaction includes oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).Electrocatalysts are always required to boost the sluggish kinetics of oxygen electrode.Currently,the most widely used ORR and OER catalysts with among the best catalytic activity are noble metal Pt and IrO2/RuO2,but their high price and poor stability limit their commercial application.In order to address this challenge,on the one hand,developing advanced catalysts with improved catalytic activity and stability is promising.On the other hand,inovation in non-noble metal catalysts to replace the Pt group metal catalysts has been proposed.In this thesis,the representative Pt based,FeNi based and Co based electrocatalysts with high activity and stability were designed and prepared,and their reaction mechanism and structure-activity relationship were studied and revealled.First,the Pt/N-NbO2/C catalyst was prepared by using hybrid composite of N-doped NbO2 and carbon with good electrochemical stability as the catalyst support for Pt nanoparticles.The positive effect of N and NbO2 on Pt electrocatalytic behavior was studied.It was found that the incorporation of N in N-NbO2 lead to uniform dispersion of both N-NbO2 and carbon support,and significantly improved the dispersibility of Pt due to the anchoring effect of N to Pt.XPS results showed that both NbO2 and N could tune the electronic structure on the surface of Pt particles:Pt 4f peak and d-band center remarkably moved postively.This effect increases the repulsive affinity of Pt to OHads,and accelerates the dissociation rate of OHads,leading to enhanced ORR activity.Compared with ORR,OER has a higher operating potential and the Pt surface is easily oxidized to PtO2,thus reducing OER activity.interestingly,the addition of N-NbO2 can to some extent inhibit the oxidation of Pt at high potential and improve its OER activity.Moreover,due to the high electrochemical corrosion resistance of N-NbO2,it can effectively alleviate the Pt detachment and agglomeration caused by the instable support,so that the Pt/N-NbO2/C catalyst shows good electrocatalytic stability.In addition,a three-dimensional porous FeNi3-LDH/Fe-N-CNFs composite catalyst was designed and constructed.In the process of synthesizing FeNi3-LDH,two-dimensional structure(FeNi3-LDH)catalyst was converted to three-dimensional structure(FeNi3-LDH/Fe-N-CNFs)by adding Fe and N co-doped carbon fibers(Fe-N-CNFs).Due to the abundant-OH and-C=O functional groups on the fiber surface,the positively charged Fe3+and Ni2+ions are easily attracted by electrostatic adsorption,which provides more growth site for FeNi-LDH and realizes in-situ synthesis of three-dimensional composite catalysts.Electrochemical tests show that the addition of fiber not only improves the electrical conductivity of the composite materials,but also changes the porosity of catalyst.The high specific surface area(596 m2 g-1)and abundant mesoporous structure increase the number of active sites,accelerate the mass transfer process,and enhance OER activity of FeNi-LDH/Fe-N-CNFs,leading to the superior overpotential of only 263 mV at 10 mA cm-2.Moreover,the introduction of Fe and N atoms not only improves the content of Ni3+(primary active sites for OER)but also increases the efficient contact area between the layers and the fibers,which is beneficial for maintaining the three-dimensional structure and thus showing good OER stability.Co/N-C catalysts were prepared from zeolite imidazole metal-organic framework(ZIF-67)with good periodic structure.The effects of Co/Zn/MeIm(2-methylimidazole)ratios on the ZIF-67 structure and electrocatalytic properties of the derived catalysts were systematically discussed.At the same time,the type of Co source,reaction time,pyrolysis temperature and other synthesis conditions are also discussed and analyzed in depth,revealing the regularity of obtaining Co single atom catalyst and providing a theoretical basis for the synthesis of monatomic catalyst.It was found that Co/N-C-1000 catalyst had excellent ORR activity and ultra-high electrochemical stability.The half-wave potential was increased up to 0.856 V,higher than commercial Pt/C(0.84 V)in alkaline media.Moreover,after up to 30,000 cycles,the half-wave potential is decreased by only 3 mV.It can be confirmed that Co-Nx is the main active center of ORR through XANES,EXAFS and electrochemical tests.