Structural Design And Electronic Regulation Of Transition Metal/oxide Based Electrocatalytic Materials

The development of inexpensive,efficient and stable catalysts to drive kinetically slow electrochemical reactions including hydrogen precipitation reaction(HER),hydrogen oxidation reaction(HOR),oxygen precipitation reaction(OER)and oxygen reduction reaction(ORR)is essential for the development of hydrogen energy conversion technologies such as fuel cells and hydrolysis cells.In this thesis,a series of transition metal-based catalysts with high activity and stability are developed by rational design of catalytic material structures using the electronmodulation interactions between different metals.They are summarized as follows.(1)Mimic-sunflower NiFe-poly(dopamine)(PDANF)thin film OER electrocatalyst:PDANF thin film with Fe ligated poly(dopamine)network(mimic sunflower disc)restricted domain NiO quantum dots(mimic sunflower seed)was prepared by self-polymerization and partial pyrolysis of Ni2+,Fe3+ ligated dopamine.During the electrochemical reaction,the dopamine-ligated Fe3+ migrated to the NiOOH quantum dots converted from NiO and was oxidized to Fe4+,which was reconstituted in situ to form the Fe4+-NiOOH quantum dot structure.the abundant Fe-Ni bimetallic sites on the NiOOH surface promoted the O*intermediate generation and O-O coupling,which in turn effectively improved the OER reaction activity.the PDANF mass activity was 32.3 and 168.9 times higher than that of NiFe layered double hydroxide and RuO2,respectively.Meanwhile,the abundant functional groups such as phenolic hydroxyl and amino groups in the polydopamine network have ligand trapping effect on Fe ions in the film,which effectively alleviates the catalyst activity degradation due to Fe ion loss.(2)Multilevel structures of Ni,Co,Fe single-atom dispersed carbon nanotubes coupled with trimetallic oxide(NCF)O/A@SACNT)for OER ORRand Zn-air battery:The(NCF)O/A@SACNT catalyst was prepared by calcination of NiCoFeMOF nanosheets in acetonitrile vapour followed by low temperature oxidation.In this catalyst,the trimetallic oxide at the top of carbon nanotubes allow full exposure of Ni-Co-Fe synergistic catalytic sites to improve the OER reaction kinetics.The array structure of carbon nanotubes accelerates the escape of O2 bubbles during the OER process and avoids the covering of the catalytic sites by the aggregation of O2 bubbles.The nitrogen-coordinated metal single atoms highly dispersed on the carbon nanotube wall can fully contact the oxygen molecules in the electrolyte,ensuring high active site density and effective mass transfer efficiency.The catalyst exhibits excellent bifunctional OER and ORR electrocatalytic activity(OER overpotential of only 224 mV@10 mA cm-2 and ORR half-wave potential of 0.81 V).Moreover,(NCF)O/A@SACNT exhibits excellent cyclic charge-discharge stability(145 h,400 cycles)in Zn-air battery cathodes.(3)Single-atom Co site-stabilized ultrasmall RuCo clusters/carbon nanosheets(RuCo/SANC NS)bifunctional HER/HOR catalysts:Co,N-doped carbon nanosheets were obtained by carbonizing ultrathin Co-MOF followed by acid washing to remove the agglomerated Co nanoparticles.Atomically dispersed Co in carbon nanosheets induce the formation of ultrasmall RuCo clusters(about 1.2 nm)and allow excellent stability of RuCo clusters.Moreover,metallic bonding between single-atom Co in carbon nanosheets and RuCo clusters facilitates the electron transfer between the carbon substrate and the clusters.The electronic modification of Ru by Co atoms in RuCo clusters effectively balances adsorption energy of Ru sites for hydrogen and hydroxyl,thus reducing the reaction energy barrier of the rate-determining Volmer step and accelerating the HER/HOR kinetics.The RuCo/SANC NS catalysts exhibits excellent mass activity for HER and HOR,36.7 and 48.1 times higher than that of Pt/C,respectively.In addition,the electron transfer from Co to Ru atoms in RuCo clusters increases the electron density of Ru 4d orbitals,leading to a lower binding energy of CO on Ru sites and improving the resistant CO poisoning of the catalyst.