Investigation Of Structural Design Principles Of Noble-Metal-Free Two-dimensional Materials As High-performance Electrocatalysts For Oxygen Reduction/Evolution

As the crucial processes of electrochemistry in clean energy systems,oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),involving multi-electron transfer,are dependent on high-efficiency electrocatalysts.While noble metals in use nowadays possess unmatched performance,they are at a high price.Faced with the above obstacles,the plentiful platinum group metal-free based two-dimensional materials(PGMF2DMs)have gradually become a new option for oxygen electrocatalysts.On the one hand,the cost comes down owing to the free of precious metals;on the other hand,the activity is improved by virtue of the highly open ultra-thin structure and the super-large specific surface area.Despite numerous strategies to boost performance by trial and error in tradition have been developed,it is still worthy of study to screen catalysts efficiently by getting rid of cut-and-try work which is time-costing and labour-consuming,to establish design principles for structural regulation of catalysts,and to deeply understand the structure-activity relationship at the electronic levels.To overcome the above challenges and exploit remarkable ORR/OER catalysts without noble metals,four typical materials in three PGMF-2DMs systems are studied in depth through the combination of theory and experiment.A high-throughput screening approach has been developed to predict excellent oxygen electrocatalysts,and the origin of the high catalytic performance has been revealed to insightfully understand the catalytic mechanism.This thesis provides an effective way for the rational design of electrocatalysts.The main contents and innovations are as follows:(1)Starting from the transition-metal based layered double hydroxides(LDHs)system,NiFe-LDHs was used as the basic model to fulfill the enhancement of OER performance with the regulation of variable valence of Cr.All theoretical results showed that the guest Cr will facilitate the electrons accumulation around the active center(Fe)through d-orbital coupling.Meanwhile the spin-polarized electrons significantly promoted the electron transportation,especially that at the edge surface,making the theoretical overpotential reduce by 19.2%.Under the guidance of the theoretical predictions,a self-supporting NiFeCr-LDHs ultrathin array was accurately synthesized with outstanding performance.The theoretical and experimental research,validating each other,provides the inspiration to gain an insight into the synergistic effects of multiple dopants.(2)In the transition-metal single atom catalysts(SACs)system,based on NiN4/FeN4/CoN4 moieties anchored on graphene,B/P/S atom is used to modulate the ambient environment of their support,which boosts oxygen electrocatalytic performance.The bifunctional catalyst(CoN4-S8 with ORR and OER overpotentials of 0.41 V and 0.24 V,respectively),ORR catalysts(CoN4B8,NiN4-P2,NiN4-P3 and NiN4-P13,with the overpotential less than 0.40 V)and OER catalyst(CoN4-B2 with the overpotential of 0.31 V)with excellent performance and thermal stability were screened from 117 stable models.According to the energy descriptors and the electronic structure,the enhanced activity came from the change of the charge and spin density at the catalytic sites by heteroatom doping.(3)In the metal-free system,based on doped graphene nanoribbons(GR),a variety of possible strategies were proposed to resist the unfavorable effects of acidic electrolyte on the ORR performance.After systematically evaluating the correlation between the composition of the acidic electrolyte and the catalytic activity of N/B/P-GR,it was discovered that protonation and adsorption of acid group anions caused the charge redistribution at active sites,which changes the electropositivity,spin properties,adsorption of O2/intermediates and the overpotential.Graphdiyne(GDY),composed of evenly in-plane cavities in the proper size,can effectively avert the above deficiencies,in particular the invasive acidic anions.The exploitation of GDY is one of the vital and considerable directions for the development of low-cost yet highly efficient acidic ORR catalysts in future frontier research.(4)In the metal-free system,GDY,the basic model,achieved the enhancement of the ORR/OER performance by p-block element modification.Based on 31 doped GDY searched from 33 structures by cohesive energy and energy band,desirable ORR catalysts(1P/GDY-9,1 As/GDY-4 and 3N/GDY-4)and OER catalysts(2As/GDY-12,2P/GDY-12,3P/GDY-7 and 3As/GDY-7)with great stability thermodynamically were forecast on basis of the computation at 372 possible catalytic sites.Then,the dependence of the intermediates and an intrinsic descriptor were proposed,which offers a powerful tool to accelerate the process of catalysts screening.Finally,it was revealed the origin of high-activity that is the enhancement of charge transfer and spin density.Meanwhile,?GHOO*and ?GO*were tuned by changing the p,orbital center and the overlapping of covalent orbital,respectively.(5)In the metal-free system,based on the GDY model with flexibility,its ORR performance was boosted via strain engineering.By assessing the tolerance of strain and cohesive energy,30 models were identified from 56 initial structures,and their 360 active sites were simulated for ORR.Two ideal ORR catalysts with high stability were finally predicted,namely N1-GDY with-2%strain and N3-GDY with-6%strain(the corresponding overpotentials were 0.41 V and 0.42 V,respectively).Assisted by the energy descriptors,the intrinsic descriptor and their electronic characteristics,the distinguished properties were originated from the electronic effect triggered by the changes in the number of electrons and the pz orbital bonding center,and the spatial effect started by geometric distortion.This study brings about the helpful inspiration for properly utilizing strain engineering to acquire advanced carbon-based catalysts for ORR.