Theoretical Design Of Highly Efficient Electrocatalysts For Ammonia Synthesis

In addition to extensive usage in the fertilizer and chemical industries,ammonia（NH₃）is currently considered a potential alternative to carbon-based fuels and a carrier of renewable hydrogen energy for transportation worldwide.Industrially,ammonia synthesis relies on the energy-intensive Haber-Bosch process.With the intensifying energy scarcity and environmental pollution problems,developing technology of NH₃ synthesis under mild conditions to replace the Haber-Bosch method is the key to achieving green,clean and sustainable NH₃ production.Electrochemical N₂ reduction reaction（NRR）for ammonia synthesis driven by renewable energy sources with zero-carbon emission is considered a promising ammonia synthesis method.However,because of the inertness of N₂ molecule and the competitive hydrogen evolution reaction（HER）under aqueous conditions,electrochemical NRR is limited by poor catalytic activity（NH₃ yield rate）and selectivity toward NH₃ synthesis（Faradaic efficiency）.Therefore,identifying efficient NRR electrocatalysts with excellent reaction selectivity,catalytic activity and stability is the key to realizing the industrial application of NRR and is a current research hotspot in this field.Transition metals（TM）exhibit high activity toward inert N₂ molecules due to their unique electronic structures.Among these TM,Mo is widely regarded to have high NRR catalytic activity because of its half-occupied d orbitals.Meanwhile,density functional theory（DFT）calculations show that Mo is located at the top of the NRR activity volcano diagram and has a relatively higher intrinsic activity than other transition metal elements.Unfortunately,these TM sites are also easily covered by protons in solution,resulting in the deactivation of surface active sites for NRR.In this thesis,a series of NRR electrocatalysts were designed based on periodic DFT calculations,which are divided into three parts as follows.（1）Tuning the catalytic activity of a single Mo atom supported on graphene for nitrogen reduction via Se atom dopingThe atomic dispersing of metal atoms supported on an optimal substrate is considered an ideal strategy for maximizing metal utilization for catalysis,which is important for exploiting electrocatalysts with low cost and high catalytic efficiency.Meanwhile,it is well established that decreasing the dimension and size of catalysts can effectively increase the catalytic performance of the active sites as well as their surface active atom ratios.Therefore,single atomic catalysts（SAC）,with single metal atoms dispersed on various supports,are currently one of the most important catalytic systems due to their maximum atom efficiency,low coordination atom,high selectivity and identifiable reaction mechanism.Since the catalytic activity of the active atom is largely influenced by the substrate,the choice of substrate for loading the active atom is crucial.Accordingly,a model of a single Mo atom supported on defective graphene was constructed（Mo X/G）.The effect of modulating the NRR activity of Mo atom by various non-metal heteroatom（B,N,P,S,Se,etc.）doping was investigated to obtain optimal Mo-based electrocatalyst by the mean of DFT calculations.This work explores the potential of SAC in ammonia synthesis and provides a theoretical basis for the design of efficient SACs.（2）Effectively boosting selective ammonia synthesis on electron-deficient surface of Mo B₂In order to further improve the catalytic performance of catalysts,the activation mechanism of N₂ molecule during the NRR process has been investigated.Since the adsorption and activation of N₂ on the catalyst surface are mainly performed by the"acceptance-donation"mechanism,the structure of the partially occupied electronic orbitals is very important.The unoccupied electronic orbitals are conducive to the"σ-acceptance"process,which affects the bonding strength of the active site to N and determines the N₂affinity of the active site on the surface.At the same time,the number of valence electrons in the active site should be maximized because of the requirement for electron injection to proceed with the reductive hydrogenation of N₂.Herein,the B element was used to modulate the electronic structure of Mo and thus theoretically design the surface of Mo B₂ with numerous electron-deficient Mo sites(Mo^δ+).The results show that the Mo^δ+sites exhibit ultralow theoretical limiting potential for NRR（-0.34 V vs.RHE）.The positively charge Mo^δ+sites have a stronger adsorption capability for N₂,achieving preferential N₂ adsorption on the surface of Mo B₂ and thus increasing the local concentration of N₂ around the active sites.Besides,the competing HER can be evidently retard even at the NRR operating potential,indicating outstanding NRR selectivity.Our proof-of-concept experiment confirms the theoretical design,in which the Mo B₂ exhibits an excellent NRR performance(NH₃ yield rate of 40.94μg h^-1 mg^-1,FE of 30.84%).Moreover,the strategy of inducing electron deficiency on active sites provides new insight into the rational design of NRR electrocatalysts for realizing highly effective NRR.（3）Multi-site Ni₃Mo intermetallic compound effectively boosts selective ammonia synthesisThere is a scaling relation between the N₂ adsorption strength of active sites on the catalyst surface and the NRR activity.If the interaction between the catalyst and the N₂ molecule is weak,the adsorption and activation of N₂ on active sites will be limited,while if the binding is too strong,the hydrogenation of the subsequent reaction intermediate NH_z（z=1 or 2）will be hindered,which thus limiting the catalytic activity of the catalyst for NRR.Herein,Ni element with weak N₂ adsorption was utilized to regulate Mo in the NRR process for regulating the reaction intermediates of Mo.The results show that the active site of the N₂activation and the hydrogenation of NH_z intermediates could be separated on the Ni₃Mo surface.The change of adsorption configurations of intermediates on the Ni₃Mo surface breaks the scaling relations and thereby achieves exceptionally facilitated NRR with an ultralow limiting potential of-0.19 V vs.RHE.Besides,owing to the synergistic effects of Mo and Ni species,the Ni₃Mo greatly protects the active centers of NRR from competitive adsorption between H and N₂,and retards the competitive HER on the Ni₃Mo surface,thus enabling the highly selective NH₃ synthesis.This study provides an intriguing strategy to circumvent the undesirable scaling relations in designing efficient NRR electrocatalysts.