Rational Fabrication,Theoretical Investigation,and Device Application Of Non-Precious Metal Group-based Oxygen Electrocatalysts

As the next-generation energy storage and conversion devices in the future,fuel cells and metal-air batteries with the advantages of low cost,high energy density,and environmental friendliness hold great prospects in many energy-related fields.Up to now,in order to enable such devices with accelerated oxygen reaction kinetics,precious metal group(PGM)compounds have usually been employed as the efficient electrocatalysts,of which platinum(Pt)and ruthenium dioxide(Ru O₂)electrocatalysts have been widely applied to oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),respectively.However,due to the limiting resources and high cost,the large-scale commercial application of such devices with PGM catalysts is greatly restricted.In this regard,designing and processing the non-PGM candidates with abundant resources,cost effective,and high performance even surpassing PGM electrocatalysts can not only improve these above shortcomings of PGM compounds,but also significantly broaden the material database of electrocatalyst selection.This work is based on three question:i)how to design and process non-PGM catalysts including heteroatom-doped carbon,atomically dispersed iron-nitrogen-carbon,and Mott-Schottky heterojunction with excellent activity for oxygen electrocatalysis;ii)how to identify the active centers and oxygen reaction pathway during the catalytic process,and further increase the content of active sites;iii)how to fabricate the catalyst with built-in electric fields via a mechanochemical method and expand its further application in metal-air battery devices.The research details are as follows:(1)Nitrogen(N)-doped carbon materials such as graphene and carbon nanotubes can be used as electrocatalysts for oxygen reduction,and their activity and long-term durability are comparable or even superior to commercial Pt/C under alkaline conditions.However,the preparation of graphene and carbon nanotubes with high quality usually consumes a great number of environmentally harmful chemicals,which often results in the high production cost.Moreover,such catalysts usually need to introduce some external nitrogen sources such as urea,melamine,etc.,thus greatly restricting their large-scale and universal fabrication.To solve these above problems,biomass gelatin with sustainable and cheap properties and sodium chloride particles have been adopted as precursors and sacrificial templates,respectively.Then,a nitrogen self-doped carbon aerogel with three-dimensional interconnected structure and high specific surface area has been synthesized via a facile sol-gel strategy.Such structure is helpful to accelerate the mass transfer during the catalytic process.Gelatin,which can be extracted from marine fish and mammalian's bone tissues,contains a large number of nitrogen-containing functional groups,therefore,it is unnecessary to introduce external nitrogen sources during the pyrolysis.Additionally,using sodium chloride as the template can also avoid the corrosion of traditional KOH activation to the pyrolysis equipment.The temperature optimization results demonstrate that the electrocatalyst prepared at 800°C possesses the remarkable ORR performance,in which onset potential,half-wave potential and limiting current density are comparable to those of commercial Pt/C.More impressively,the as-fabricated electrocatalyst remains 95.7%of oxygen reduction current after 4 hrs of continuous operation,which is better than commercial Pt/C(74.3%).(2)In recent years,atomically dispersed iron(Fe)-based catalysts have achieved great progress in the identification of active sites and the exploration of reaction mechanism,owing to their well-defined coordination environment and maximum atom utilization efficiency.Unfortunately,the preparation of Fe-based single atom catalysts(SACs)with high metal-loading amounts is still a grand challenge due to the serious migration and agglomeration of Fe species during the pyrolysis.In addition,because of various metal atom coordination environments and electrode structures,the ORR mechanism of the active sites in Fe SACs under acidic and alkaline conditions is not clear.Therefore,in this research,a nitrogen-doped porous carbon electrocatalyst with Fe SACs has been successfully synthesized by using agarose as complexing agents and carbon precursor.X-ray absorption near edge structure and extended X-ray absorption fine structure characterizations have confirmed the Fe-N₄configuration in the catalyst.The actual loading of atomic Fe species in the catalyst is up to 2.17 wt.%.Benefitting from such high metal loading of atomically dispersed Fe-N₄ sites,the as-synthesized catalyst exhibits remarkable oxygen reduction activity,long-term durability,and methanol tolerability in acidic,neutral,and alkaline media.Moreover,the formation mechanism for atomically dispersed Fe-N₄ sites during the preparation has been proposed,and the ORR mechanism of as-synthesized electrocatalyst in both acidic and alkaline media has also been elucidated by density functional theory.(3)Transition metal phosphides(TMPs)as non-PGM bifunctional oxygen electrocatalysts display excellent catalytic activities for oxygen electrocatalysis.However,in the actual metal-air battery devices,TMPs compounds are still facing two obstacles:mass production with high safety and intrinsic semiconductor characteristics.Firstly,most of TMPs are prepared with sodium hypophosphite as phosphorus source,which will inevitably produce highly toxic and flammable phosphine gas during the annealing process.Secondly,the semiconductor properties of TMPs enable them with low electronic conductivity,which greatly restricts their electrocatalytic activity.Therefore,in this research,a novel Mott-Schottky electrocatalyst with core@shell structure has been successfully fabricated via a mechanochemical-pyrolysis strategy,in which Co/Co₂P Schottky heterojunction is used as core,and N,P co-doped carbon nanotube(NPCNT)is used as shell.Notably,this cost-effective mechanochemical method can initiate the solid-phase complexation reaction between Co²⁺and melamine without using any solvents,so it is an economic,sustainable,and environmentally friendly strategy.It is demonstrated that the Mott-Schottky Co/Co₂P heterojunction nanoparticles and NPCNTs can synergistically improve the overall electronic conductivity and bifunctional electrocatalytic activity,whereas the hollow NPCNTs can enhance the mass transfer of oxygenated species.As a result,the as-prepared electrocatalyst can be used as an excellent bifunctional electrocatalyst in alkaline medium.More impressively,the bifunctional electrocatalyst possesses better performance than PGM catalysts in home-made zinc-air battery devices,and its long-term cycle stability can even exceed 200 hrs.Density functional theory(DFT)calculations show that the spontaneous electron transport from the Co side to the Co₂P side can be generated at the Co/Co₂P heterojunction interface,thus forming a built-in electric field.This phenomenon can result in the up-shifted d-band center of Co/Co₂P heterojunction,which is conducive to the adsorption of intermediates during the electrocatalytic process.