Surface/Interface Structure Modulation Of Regenerative Fuel Cell Electrocatalysts And Their Performance Studies

Hydrogen energy is regarded as the most promising clean energy in the 21st century because of its cleanliness,wide source,high calorific value,and sustainability.Water electrolysis to produce green hydrogen using renewable energy and efficient utilization of hydrogen through hydrogen-oxygen fuel cells are two crucial technologies in the development of hydrogen energy,which play an important role in energy storage and conversion.Regenerative fuel cells,which combine the water electrolysis and fuel cells,are considered to be an efficient energy storage/conversion system with good energy conversion and environmentally friendly features.The cores of water electrolysis and fuel cells are hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,oxygen reduction reaction（ORR）,and hydrogen oxidation reaction（HOR）on the cathode and anode,respectively.To facilitate these four reactions,it is often necessary to use Pt group metal（PGM）catalysts and apply a large overpotential,which results in high energy consumption and limited output power density of cells.Moreover,PGMs are limited in resources and prices.Electrocatalysts play an important role in these four reactions.At present,the electrocatalysts applied to the above four reactions face several key problems in terms of high PGM loading,activity to be improved,poor durability,and limited mass transfer.For reducing PGM loading,researchers have pay attention to the low PGM loading catalysts and even non-PGM catalysts with high activity.To improve the activity of electrocatalysts,increasing the intrinsic activity of active sites and the number（density,utilization）of active sites is crucial.The durability of electrocatalyst can be improved from both the active substance itself and the support.All of the above four reactions involve gaseous reactants or products,so the formation of the three-phase interfaces and the transport of materials in the catalyst layer play a crucial role in the overall performance of the catalysts.The performance of electrocatalysts is closely related to their structures.The improvement of the overall performance of electrocatalysts depends on the rational design and regulation of electrocatalyst structures based on a deep understanding of the reaction mechanism.The main strategies for electrocatalyst modulation include not only the structural modulation of the active substance itself（size,facet,component control,well-defined structure,etc.）by using size effect,facet effect,strain effect,ligand effect,confinement effect,etc.,but also the surface/interface modulation（surface modification,surface defects,support usage,interfacial modulation,etc.）.Surface/interface modulation is one of the feasible and effective strategies to improve the performance of electrocatalysts.Electrochemical reactions usually occur on the surface or interface of the electrocatalyst.The adsorption strength and configuration of the reaction intermediates are closely related to the physical structural characteristics（atomic arrangement,electronic structure,etc.）of the catalyst surface/interface.Therefore,the rational design and regulation of the surface/interface of electrocatalysts are necessary to improve the electrocatalytic performance.At present,how to ensure the high activity of the active material and maintain its high dispersion and stability,how to select a suitable catalyst support and perform surface modification,and how to endow the catalyst with functionalization effect to improve its application in devices are still key scientific problems and difficulties to be studied and solved in the development of electrocatalysts.Based on the review of the mechanism of several electrochemical reactions and the research progress of electrocatalysts,this paper focuses on the issues of improving the performance of water electrocatalysts and hydrogen-oxygen fuel cells in regenerative fuel cells and reducing the loading of precious metals.In this dissertation,several low PGM loading/non-PGM electrocatalysts with high activity and high durability were prepared for HER,HOR,OER,and ORR by various surface/interface modulation strategies,such as metal-support interaction,heterojunction structure,support usage,surface modification,with the starting points of exposing more active sites,improving intrinsic catalytic activities per active site,and promoting the mass transfer in the catalyst layer.The"structure-property relationship" between the structure properties and electrocatalytic performance of the catalysts was investigated in depth by combining various physical characterization techniques and electrochemical testing methods.And the performance of the catalysts was also verified in the corresponding application devices.The main contents and results of this paper are as follows:（1）To improve the HER/HOR activity of Pd-based catalysts,a Pd/CeO₂/C catalyst with high Pd-CeO₂ interface density was prepared by adopting the impregnationadsorption-reduction method using CeO₂/C with rich oxygen vacancy as support.A variety of physical characterization techniques have been used to analyze the physical structure characteristics of the catalyst.Electrochemical test results showed that the catalyst has superior HER and HOR performance than Pt/C,with a HER mass activity reaching 33 times that of Pd/C.The improved catalyst performance can be attributed to the abundant metal-metal oxide interfaces,the electronic interactions between metalmetal oxide support,and the CeO₂ support with rich oxygen vacancy defects.Anionexchange membrane fuel cells（AEMFC）single-cell tests showed that its maximum power density is twice as high as that of Pt/C under the same conditions.（2）Regarding the easy stacking characteristics of Co（OH）₂ and its limited OER activity,carbon nanoarrays loaded with MnO₂ nanosheets were used as composite substrate materials here,and carbon nanoarrays loaded with Co（OH）₂@MnO₂ heterojunctions（Co（OH）₂@MnO₂-CNAs）were prepared by solvothermal method for OER.The structure of the catalysts was investigated by a variety of physical characterization techniques.Emphasis was placed on the capacitive properties of the catalysts.The catalyst exhibits better OER activity superior to RuO₂ and good stability in alkaline environment.Electrochemical measurements and theoretical calculations indicate that the charge transfer at the interface of the Co（OH）₂@MnO₂ heterojunction and the increased charge storage capacity caused by the introduction of MnO₂ are the main reasons for the enhanced catalytic activity.（3）In order to improve the ORR activity and stability of Pt,a Pt@Co-NC catalyst was prepared here for ORR by partially replacing the surface Co atoms with Pt on the Co nanoparticles through an electrochemical replacement method.In this work,the zeolitic imidazolate framework（ZIF）derived Co-NC obtained from pyrolysis was played as a support.Electron microscope characterization showed that PtCo nanoparticles were supported on Co-NC containing Co single atoms.Pt@Co-NC displayed a higher ORR activity than Pt/C with a half-wave potential at 0.92 V（vs.RHE）in 0.1 M HClO₄ solution,and its mass activity was 5.6 times that of Pt/C.The improved ORR activity mainly stems from the synergistic effect between PtCo and CoNC.The results of single-cell tests on proton exchange membrane fuel cells（PEMFC）also reflect the positive effects of surface Pt atom exposure and Co-NC mesoporous structure on promoting oxygen transfer.（4）Considering the low utilization of active sites and the limited mass transfer of metal-nitrogen-carbon ORR catalysts,Fe-N-C_（N）modified with amino groups and TAPPCo-QA covalent organoskeletal materials（COF）modified with quaternary ammonium groups were prepared here for ORR by surface modification strategy,respectively.Physical characterizations showed that the successful preparation of FeN-C_（N）and TAPPCo-QA COFs as well as the electronic structure and coordination environment of the metal active centers of the catalysts before and after the modifications did not change significantly.Fe-N-C_（N）showed improved oxygen mass transfer in PEMFC single-cell tests.The improved ORR activity and AEMFC performance of TAPPCo-QA COF can be attributed to the modification of quaternary ammonium groups and the construction of an ordered COF structure,which improves the utilization of active sites and promotes mass transfer.