Controlled Synthesis And Catalytic Mechanisms Of Mn-based Low-cost Oxygen Reduction Electrocatalysts

With the accelerated development of society and the continuous growth of the population,there have been ever-higher demands for more energy and the transformation of the energy structure.Traditional fossil fuels are becoming increasingly unsuitable to meet the needs of today's society due to serious environmental problems such as air pollution and greenhouse effect.The world is in urgent need of new green,renewable energy and new energy storage and conversion devices.Fuel cells and metal-air batteries have broad application prospects due to their high energy density and no pollution produced to the environment.At the same time,they share a common cathode reaction—oxygen reduction reaction(ORR).The development of electrocatalysts largely determines the future development of these batteries.At present,20 wt.%or even 40 wt.%of Pt/C catalysts are commonly used commercially in the above cells and batteries.Considering the rarity and high cost of precious metal Pt,it is crucial to develop low-cost and high-efficiency catalysts.Mn is a kind of transition metal featuring abundant storage,non-toxicity,harmless and numerous valence states.The d-orbital electron of Mn can be easily regulated.More importantly,Mn is a metal that will initate no Fenton reaction and is not subject to hydrogen peroxide attack,which is an inevitable intermediate product during the ORR process.Therefore,this characteristic makes Mn theoretically a highly stable ORR catalyst with great development potential.However,the complex valence state of Mn makes its precise preparation and mechanism research very difficult.In this thesis,several Mn-based ORR catalysts have been prepared through reasonable design and controlled synthesis.Besides obtaining high performance,we have also tried to study the influence of different valence states and existence forms of Mn on the ORR performance and the reaction mechanism.In the second chapter,we have observed and investigated the in-situ valence evolution of Mn in the ORR of as prepared Mn O catalysts.Herein,a kind of nanoflower-like Mn(II)O catalyst was prepared by a simple hydrothermal-reduction two-step method,which exists in the forms of both cubic and orthorhombic phase structures.And the material contains a certain amount of oxygen vacancies.The synthesized Mn O catalytic material demonstrates an excellent ORR activity,and a half-wave potential of Mn O-600 up to 0.895 V,which is even higher than commercial Pt/C.In the long-term discharge process,we found a further increase in current density,which is believed to be caused by the production of high-valence Mn(Mn³⁺and Mn⁴⁺).Combined with spectroscopic characterization,we propose here a synergistic catalytic mechanism in which the oxygen vacancies accelerate the adsorption of O₂ molecules,Mn³⁺catalyze ORR and Mn⁴⁺catalyze peroxide decomposition in a synergetic way.This work not only broadens our horizon for constructing high-performance noble metal-free electrocatalyst by employing multivalence transition metal oxides,but also provides an in-depth mechanistic probing on the relationship between Mn valence and electrochemical performance.In Chapter 3,we have investigated the influence of the Mn existence form towards the ORR performance.Herein,glutamic acid,a unique chiral molecule,has been adopted as the template for a spiral NC matrix.Then,single-atom Mn NC catalysts were obtained after the adsorption of a manganese source,and at the same time,the introduction of polydopamine has resulted in the increased density of single atom Mn site.Among the as-prepared catalysts,the Mn NC-PDA-700 catalyst exhibits the most excellent ORR performance,and has good long-term operational stability and resistance to methanol poisoning.In addition,the Mn NC-PDA-700-equipped zinc-air battery exhibits a higher peak power density and peak current density than commercial Pt/C,and long-term charge-discharge cycle tests have confirmed its reliability.Finally,DFT theoretical calculations was employed to investigate four possible Mn N_xC_4-xstructures,which confirms that the Mn N₄ site is the origin of the superior ORR activity via a 4e^-pathway in alkaline media.This work not only delivers a high-performance noble metal-free electrocatalyst by employing multivalence manganese metal,also provides an in-depth mechanistic insight on the active sites for single atomic M-N-C catalyst in alkaline environment.In Chapter 4,we have further probed the role of Mn in the ORR process by introducing other metal species to construct composite structure.Herein,the Prussian blue structure is selected as the research object,and a hollow/porous-structured,composite Prussian blue with coexisting Mn and Fe species has been prepared by an ion-exchange method.By changing the concentration of Mn source to tune the Mn:Fe ratio in the final structure,it has been found that the Mn:Fe ratio of about 35:65,i.e.,sample 2.6Mn-HMPB,show the highest ORR activity among the samples prepared.In addition,we tested the ORR performance at different electrolyte concentrations to study the effect of ion concentration on catalytic activity.Preliminary studies have shown that the higher the electrolyte concentration is,the lower the limiting current density and internal resistance of the solution will be,but the catalyst will show greater inner layer resistance at the same time.In Chapter 5,to prepare a low-cost hydrogen evolution catalyst for promoting the development of high-purity hydrogen sources,we also carried out the research about the hydrogen production by electrolysis of water.By weakly reducing the acid-resistant tungsten trioxide,oxygen vacancies were produced for anchoring Pt atoms.This highly dispersed Pt clusters show improved utilization of Pt atoms and greatly reduced noble metal amount.Combining with the spectroscopic characterization of XPS,ESR and XAFS,we propose a synergistic mechanism between Pt and WO₃.First,Pt adsorbs H⁺in the solution to produce H*,and H*is transferred to WO₃ owing to hydrogen spillover effect.Finally hydrogen is removed from WO₃ sites through the hydrogen tungsten bronze intermediate,and the Pt sites can simultaneously adsorb H⁺,thereby ensuring the separation of hydrogen adsorption and hydrogen desorption in space and time,and finally obtaining higher HER performance.In addition,the catalyst also exhibits good ORR performance under acidic conditions.