Design,Synthesis and Properties of Noble Metal-free Nano-Electrocatalysts

Author:Du Cheng

Supervisor:chen wei


Degree Year:2019





With ever-increasing energy crisis and environmental issues caused by the consuming of traditional fossil fuel,developing novel sustainable energy is urgently needed.Recently,electrochemical energy techniques aroused great attention owing to their ability in energy conversion and storage,which can be driven by sustainable electrical power,such as wind energy and solar energy.However,the intrinsically sluggish kinetics of the electrochemical reactions and overuse of expensive and unstable noble metal-based catalysts have severely hampered the development of these clean energy technologies.Alternatively,noble metal-free catalysts have been developed to increase scalability and reduce costs,but they are usually subjected to low activity.Hence,it is a great challenge to design cost-effective catalysts with both improved catalytic activity and stability for the electrochemical reactions.This dissertation focuses on the design of noble metal-free catalysts with controlled structure and composition,and their application in the fields of electrocatalysis.The main findings are summarized below:1.Balancing the Micro-Mesoporosity for Activity Maximization of N-Doped Carbonaceous Electrocatalysts for the Oxygen Reduction ReactionCarbonaceous porous structures have instigated global research interests as promising low-cost electrocatalysts for numerous energy technologies.However,the rational design principle of pore structures for activity maximization is still unclear.In this work,a series of N-doped carbon(N-C)catalysts with exclusively different micro-mesoporosity are investigated for the oxygen reduction reaction(ORR).By combining the experiment results and a pioneering mathematical model,it was observed that the best catalytic activity can only be attained by balancing the micro-mesoporosity.These findings offer a definite criterion for pore structure optimization in carbon-based ORR catalysts,which is of great importance for various energy technologies.2.Confined recrystallization strategy to engineer hierarchical porous carbon-anchored Fe single-atom bifunctional catalyst for efficient flexible rechargeable Zn-air batterySingle-atom catalysts provide the most efficient metal utilization for various catalytic reactions.Meanwhile,hierarchical pore structure plays a decisive role in determining the catalytic activity of N-doped porous carbon materials,since normally micropores provide active locations and macro/mesopores serve as mass transport channels.However,it remains challenging to engineer highly dispersed single-atom catalyst(SAC)with hierarchical pore structures.Herein,we develop a facile"confined recrystallization" strategy to synthesize hierarchical porous Fe SAC on N-doped carbon(Fe-NC SAC)with an iron mass loading of 1.5 wt%,using polyvinylpyrrolidone(PVP)to restrict the recrystallization of Fe(NO3)3 serving as multifunctional self-templates to produce hierarchical pores as well as atomically dispersed Fe-Nx sites with assistance of melamine as nitrogenous source.With the Fe-NC SAC as catalyst,an excellent bifunctional ORR/OER performance and long-term durability superior to those of Pt/C and RuO2 are achieved in both liquid and solid Zn-air battery tests.Notably,in the flexible solid Zn-air battery,the Fe-NC SAC showed a high discharge voltage of 1.25 V and stable charge-discharge cycle at 1 mA cm-2 under different bending conditions.This work not only provides a novel strategy to endow SAC with hierarchical pore structure but also represents an example of how to effectively investigate the correlation between the structure and catalytic activity of catalysts.3.Fe-Cu dual-atom electrocatalyst mimicking Cytochrome c oxidase to boost the oxygen reduction reactionThe intrinsically sluggish kinetics of oxygen reduction reaction(ORR)and overuse of expensive and unstable Pt-based catalysts have severely hampered the development of clean energy technologies.Herein,for the first time,a novel electrocatalyst,with Fe-Cu dual atomic sites embedded in three-dimensional porous N-doped carbon(FeCu NC),has been fabricated to boost the ORR,by mimicking both the constituents of active centers and enzymatic microenvironment of Cytochrome c oxidase(CcO).The atomically dispersed Fe-Cu sites are clearly identified by direct aberration-corrected transmission electron microscopy imaging and systematic X-ray absorption fine structure(XAFS)analyses.The FeCu NC exhibits a superior ORR activity,with a more positive half-wave potential(E1/2)of 0.86 V than that of commercial Pt/C catalyst(0.83 V)in base,and moreover with only an E1/2 gap of 20 mV to the Pt/C in acid.Importantly,the FeCu NC displays excellent durability in both acid and base conditions,which is much superior to Pt/C.Density functional theory(DFT)calculations further demonstrate that the enhanced performance is attributed to the fact that Cu-N4 serves as the electron donor to increase the electron density of the active centers of Fe-N4 and thus to facilitate O2 activation.4.Achieving 59%Faradaic efficiency of N2 electro-reduction reaction in aqueous Zn-N2 battery by facilely regulating the surface mass transport on metallic CuAlthough the huge potential of electrocatalytic N2 reduction reaction(NRR)in replacing traditional energy-intensive Harber-Bosch(H-B)process for NH3 synthesis,the extremely low NRR selectivity induced by the intensified competition of hydrogen evolution reaction(HER)severely hampers their large-scale industrialization.Herein,for the first time,we find that the surface mass transport properties of catalysts play critical roles in determining the electrocatalytic NRR selectivity over competitive HER in aqueous media on the metallic Cu.Especially,by controlling the monolayer thickness of Cu nanoparticles with minimum mass transport barrier,the Faradaic efficiency(FE)of NRR can be dramatically enhanced to 33%in H-type cell test and even to 59%in a new-type Zn-N2 aqueous battery.The detailed NRR mechanism is further explored by density functional theory(DFT)calculations and in situ Fourier Transform infrared spectroscopy(FTIR)technology.The present work provides a general principle of tuning the surface mass transport process of electrocatalysts to improve the NRR efficiency in ambient aqueous systems.