Tuning Single-Atom Catalysts For Oxygen Reduction And Oxygen Evolution Reaction

Facing the exhaustion of fossil fuel and environmental issues,clean,cheap and safe energy technologies have attracted worldwide attention.The core technology of these clean energy devices involves a series of electrochemical redox reactions,such as oxygen reduction reaction(ORR),and oxygen evolution reaction(OER)and hydrogen evolution reaction(HER).However,the key to achieving low-cost,large-scale production is to improve the performance of catalysts.Currently,these catalytic reactions need to be catalyzed by Pt-based materials,such as Au,Pd,RuO2 or IrO2.However,the high cost and scarcity of Pt-based catalysts call for reducing the loadings of precious metal.Singleatom catalysts(SACs)with the maximum efficiency of atom-utilization and unique structures and properties would effectively reduce the amount of metal resources used,reduce the cost of catalysts,and realize green,economic,efficient and stable synthesis of catalysts.Hence,SACs are emerging as the most promising candidate for commercial catalysts.In this thesis,we design the synthesis of catalysts at the atomic scale or molecular scale,and futher investigate the catalytic mechanism and application of ORR and OER.The main contents are as follows:(1)The secondary-atom-doped strategy was developed to synthesize atomically dispersed Fe sites on hierarchically porous carbon catalysts(Fe-N-C/FeN).This strategy can not only enhance metal loading and thus increase the number of active sites,but also improve the reactivity of the FeNx sites through fine-tuning of the coordination environment of the single Fe atoms.Upon the secondary doping,structure optimizations in terms of microstructures and hydrophobic/hydrophilic properties of catalysts can be achieved simultaneously.Due to the beneficial microstructure and abundant highly active FeN5 moieties,the obtained Fe-N-C/FeN showed excellent activity in acidic conditions with a half-wave potential of 0.81 V vs.0.83 V for Pt/C and high long-term stability,which is much better stability than Pt/C.This work demonstrates that secondary doping is an effective strategy to prepare highly active single-atom catalysts for energy conversion systems.(2)The self-assembly strategy and subsequent pyrolysis process were developed to synthesize an efficient ORR catalyst of Co-N-doped carbon nanosheet(Co-NCS-5).Combing the unique twodimensional structure and high N-doping advantages,the optimized Co-NCS-5 shows a superior ORR performance with more positive onset potential(0.96 V)and half-wave potential(0.93 V)than those of commercial Pt/C under alkaline media.Significantly,by selectively etching the active sites,the synergistic catalysis between CoNx and Co@C NPs in Co-NCS-5 was revealed.Besides,Co-NCS-5 also shows excellent ORR activity in acidic solutions,even approaching that of commercial Pt/C.After that,we used Co-NCS-5 as the air cathode of the Zn-Air Batteries(ZABs),the performance of which is better than that of the Pt-based ZABs.This discovery is of great significance to the synthesis of ORR catalysts and the exploration of their application in electrochemical energy devices.(3)The hierarchically porous single-atom iridium embedded nitrogen-doped carbon(SA-Ir/NC)was successfully prepared.SA-Ir/NC displays efficient ORR activity and stability in neutral solution.Due to its excellent performance,the theoretical calculations were carried out.The theoretical calculation results show that IrNs active sites in SA-Ir/NC display moderate free energy of adsorption to reaction intermediates,which make SA-Ir/NC possessing excellent 4 e’ ORR activity and well-enhanced H2O2 tolerance.When SA-Ir/NC is served as air cathode of ZABs,the ZABs display a large open-circuit voltage(1.42 V),a remarkable power density(90.4 mW cm-2)and excellent long-term stability.After combing ZABs with glucose oxidase,a self-power sensor system was successfully assembled.This selfpower sensor system performs sensitive detection of glucose based on a competitive response mechanism.This work has great application prospects in the field of biosensing.(4)The facile one-step reduction process was developed to synthesize 3D amorphous NiFe/Irx coreshell nanowire@nanosheets(NW@NSs)with atomically isolated Ir atoms anchored on NiFe-based core.Taking advantage of the advantage of three-dimensional amorphous core-shell structure and the synergistic effect of Ir single atom and NiFe support,the optimized NiFe/Irx NW@NSs has an excellent OER activity with low overpotentials of 200 mV and 250 mV at current densities of 10 and 100 mA cm-2 in 1 M KOH,respectively,which outperforms NiFe NW@NSs and RuO2.Additionally,NiFe/Irx NW@NSs shows excellent long-term stability at a current density of 10 mA cm-2 for 12 h.Theoretical calculations also proved that the enhancement of the intrinsic activity of NiFe/Irx NW@NSs is attributed to the synergistic effect of Ir single atom and NiFe support when Ir single atoms exist.Due to the simple synthesis process and outstanding electrochemical performance of the work,this designed nanostructure has broad applications in water splitting and other related fields.