Surface And Interface Control Of Electrocatalysts Toward Alkaline Hydrogen Evolution/Oxygen Evolution/Oxygen Reduction Reactions

As rising concerns on the rapid fossil fuel consumption and serious environmental pollution,it is necessary to seek the clean,sustainable and renewable energy sources.Proton-exchange membrane fuel cells（PEMFCs）,as the most promising technology for efficient transmission of electricity in transportation and mobile equipment,have received widespread attention because of their high efficiency,high energy density,and negligible emission of noxious gases.Meanwhile,water splitting to produce hydrogen（H₂）and oxygen（O₂）via electrochemical process provides a promising solution to deal with the climate change and increasing energy shortage owing to its giant advantage of abundance in resources and carbon-free emissions.Many studies of the electrolysis of water and hydrogen fuel cell are focusing on improving electrocatalytic activity of non-Pt metal catalyst to decrease the overpotentials of cathode and anode.However,it remains a major contemporary technological challenge to seek low-cost non-Pt catalysts with high electrocatalytic activity and long-term operational durability and stability.This study has focused on oxygen evolution reaction（OER）,hydrogen evolution reaction（HER）as well as oxygen reduction reaction（OER）,and four micro/nanostructure-optimized and high-performance electrocatalysts have been designed via surface and interface engineering.The electrocatalysts are flowerlike Ag-supported Ce-doped Mn₃O₄ nanosheet heterostructure（Ce-Mn O_x/Ag）,Ag nanoparticles decorated urchin-like cobalt carbonate hydroxide composites（Ag/CCHH）,Fe-modified Co₂（OH）₃Cl microspheres（Fe-Co₂（OH）₃Cl）,and flowerlike CeO₂-supported Ru nanoparticles composite（Ru/CeO₂）.Different characterization methods are employed to characterize the structure,morphology,composition and surface electron state of these electrocatalysts.Combined with lots of electrochemical tests,we also have proposed the possible relationship between micro/nanostructure and electrocatalytic performance.The main research works are as following:1.Tuning Ag surface states by metal oxides is a feasible method to enhance the oxygen reduction reaction（ORR）performance of Ag-based catalysts.Herein,we develop a heterostructural electrocatalyst of flowerlike Ag-supported nanosheets of metal oxides（Mn₃O₄ and Ce-doped Mn₃O₄）by a facile two-step solution method for a highly efficient alkaline ORR.Not only does Mn₃O₄/Ag exhibit activity comparable to that of commercial Pt/C but also small amounts of Ce³⁺（2-5 mol%）doped into Mn₃O₄ make the ORR activity and stability outperform those of Pt/C.The roles of the adjustment of Ag surface states induced by the supported Mn₃O₄ and Ce doping in the enhanced ORR performance were studied.The strong electron transfer from Ag substrates to Mn₃O₄ makes the d center of Ag shift up,accelerating the kinetics of O-O bond splitting on Ag surfaces（increasing the activity of active sites for O-O bond dissociation）.Additionally,5%Ce³⁺doping further tunes Ag surface electronic structures to improve the ORR performance of Mn₃O₄/Ag.Meanwhile,supported Mn₃O₄ can reduce the adsorption of oxygencontaining species on Ag surfaces by a spillover effect,and the moderately increased oxygen vacancies of Mn₃O₄ due to 5%Ce doping further increase the active sites on oxide/Ag surfaces and electrochemically active surface areas for the ORR.2.Herein,a novel composite of small amounts of Ag nanoparticles（NPs）decorated urchin-like cobalt carbonate hydroxide hydrate（CCHH）was developed for highly-efficient alkaline oxygen evolution reaction（OER）.Not only can Ag colloids,as template agents,modify the morphologies of urchin-like CCHH microspheres to expose more active sites available,but also the supported Ag NPs formed by Ag colloids can transfer the electron to CCHH surfaces,accelerating the transformation of surface Co^II to Co^III/Co^IV（proton-coupled electron transfer（PCET）process）.The urchin-like Ag/CCHH（0.013 mmol）precatalyst（before cyclic voltammetry（CV）activation）exhibits a better OER performance(a low overpotential of 273 mV at 10 mA cm^-2 and small Tafel slope of 65 mV dec^-1)as compared with commercial RuO₂.Furthermore,the dynamic surface self-reconstruction(surface CO₃^2-and OH^-exchange)can further enhance the activities of Ag/CCHH precatalysts.Consequently,the optimal Ag/CCHH（0.013 mmol）catalyst presents a superior activity(a lower overpotential of 267 mV at 10 mA cm^-2 and markedly reduced Tafel slope to 56 mV dec^-1)along with an excellent stability after CV cycles.The study provides a feasible strategy to fully realize the low overpotential of CCHH-based OER electrocatalysts.3.Surface self-reconstruction by the electrochemical activation is considered as an effective strategy to increase the oxygen evolution reaction（OER）performance of transition metal compounds.Herein,uniform Co₂（OH）₃Cl microspheres are developed and present an activation-enhanced OER performance caused by the etching of lattice Cl^-after 500 cyclic voltammetry（CV）cycles.Furthermore,the OER activity of Co₂（OH）₃Cl can be further enhanced after small amounts of Fe modification(Fe²⁺as precursor).Fe doping into Co₂（OH）₃Cl lattices can make the etching of surface lattice Cl^-easier and generate more surface vacancies to absorb oxygen species.Meanwhile,small amounts of Fe modification can result in a moderate surface oxygen adsorption affinity,facilitating the activation of intermediate oxygen species.Consequently,the 10%Fe-Co₂（OH）₃Cl exhibits a superior OER activity with a lower overpotential of 273 m V at 10 m A cm^-2（after 500 CV cycles）along with an excellent stability as compared with commercial RuO₂.4.In this study,flowerlike（FL）CeO₂-supported Ru nanoparticles composite（Ru/CeO₂）was prepared by a simple two-step method and applied in an efficient alkaline hydrogen evolution reaction（HER）.The Ru/CeO₂exhibited a superior HER performance with a small Ru loading amount（only 3 wt%）as compared with Pt/C（20 wt%）.The excellent HER performance of Ru/CeO₂（3 wt%）can be attributed to the synergistic effect and electronic effect between metal Ru and CeO₂ support.The electron transferring from CeO₂ to Ru can make Ru surfaces present an electron-rich state,which is advantageous to the H₂O adsorption and dissociation,meanwhile,the adsorbed OH^-(OH_ad)on Ru surfaces can be easy to spill over to the neighboring oxyphilic CeO₂,which can make Ru surfaces continually regain more active sites to adsorb/dissociate H₂O molecules and increase the electrochemical activity areas.Additionally,the surface interaction between Ru and CeO₂ results in more surface oxygen vacancies on CeO₂,which is more favorable for the adsorption of OH^-spilled over from Ru surfaces.This study provides a simple and effective strategy to design the low-cost and high-efficiency alkaline Ru-base HER catalyst.