Smart Interfacing between Co-Fe Layered Double Hydroxide and Graphitic Carbon Nitride for High-efficiency Electrocatalytic Nitrogen Reduction

Bimetallic compounds such as hydrotalcite-type layered double hydroxides(LDHs)are promising electrocatalysts owing to their unique electronic structures.However,their abilities toward nitrogen adsorption and reduction are undermined since the surface-mantled,electronegative-OH groups hinder the charge transfer between transition metal atoms and nitrogen molecules.Herein,a smart interfacing strategy is proposed to construct a coupled heterointerface between LDH and 2D g-C_(3)N_(4),which is proven by density functional theory(DFT)investigations to be favorable for nitrogen adsorption and ammonia desorption compared with neat LDH surface.The interfaced LDH and g-C_(3)N_(4) is further hybridized with a self-standing TiO_(2) nanofibrous membrane(NM)to maximize the interfacial effect owing to its high porosity and large surface area.Profited from the synergistic superiorities of the three components,the LDH@C_(3)N_(4)@TiO_(2) NM delivers superior ammonia yield(2.07×10^(−9) mol s^(−1) cm^(−2))and Faradaic efficiency(25.3%),making it a high-efficiency,noble-metal-free catalyst system toward electrocatalytic nitrogen reduction.

The pitfalls in electrocatalytic nitrogen reduction for ammonia synthesis

Ammonia synthesis by electrocatalytic nitrogen reduction reaction(EC-NRR)has gained momentum in recent years fueled by its potential to operate at ambient conditions,unlike the highly energyintensive yet long-standing Haber-Bosch process.However,the large disparity of the yields and Faradic efficiencies reported for EC-NRR raises serious concerns about the reliability of the experimental results.In this perspective,we elaborate on the potential sources of error when assessing EC-NRR and update the testing protocols to circumvent them,and more importantly,we pose a general call for consensus on ammonia production analysis and reporting to lay the solid foundations that this burgeoning field requires to thrive.

Recent progress and prospects of electrolytes for electrocatalytic nitrogen reduction toward ammonia

Electrochemical nitrogen reduction reaction(ENRR) provides a promising strategy to achieve sustainable synthesis of ammonia. However, despite great efforts devoted to this research field, the problems such as low energy efficiency and weak selectivity still impede its practical implementation. Most of the research to date has been concentrated on creating sophisticated electrocatalysts, and adequate knowledge of electrolytes is still lacking. Herein, the recent progress in electrolytes for ENRR, including alkaline, neutral,acidic, water-in-salt, organic, ionic liquid, and mixed water-organic electrolytes, is thoroughly reviewed to obtain an in-depth understanding of their effects on electrocatalytic performance. Recently developed representative electrocatalysts in various types of electrolytes are also introduced, and future research priorities of different electrolytes are proposed to develop new and efficient ENRR systems.

Microfluidic platform serves as controllable fabrication of binary Mo/Ir nanodots/carbon hetero-material for efficient electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction reaction(NRR)is regarded as a potential routine to achieve environment-friendly ammonia production,because of its abundant nitrogen resources,clean energy utilization and flexible operation.However,it is hindered by low activity and selectivity,in which con-dition well-designed catalysts are urgently in need.In this work,a binary Mo/Ir nanodots/carbon(Mo/Ir/C)hetero-material is efficiently constructed via microfluidic strategy,of which the nanodots are ho-mogeneously distributed on the carbon skeleton and the average size is approximately 1 nm.Excellent performance for NRR is obtained in 1 mol L^(-1) KOH,of which the optimized ammonia yield and faradic efficiency are 7.27μg h^(-1) cm^(-2) and 2.31%respectively.Moreover,the optimized ammonia yield of 6.20μg h-1 cm-2 and faradic efficiency of 10.59%are also obtained in 0.005 mol L^(-1) H_(2)SO_(4).This work achieves the continuous-flow synthesis and controllable adjustment of hetero-materials for favorable morphologies,which provides an innovative pathway for catalyst design and further promotes the development of ammonia production field.

Mesoporous MnO_(2)nanosheets for efficient electrocatalytic nitrogen reduction via high spin polarization induced by oxygen vacancy

The electrochemical N_(2)reduction reaction(NRR)represents a green and sustainable route for NH_(3)synthesis under ambient conditions.However,the mechanism of N_(2)activation in the electrocatalytic NRR remains unclear.Herein,we found that the high spin state Mn^(3+)-Mn^(3+)pairs induced by oxygen vacancy in MnO_(2)nanosheets greatly enhance the catalytic activities.The strong electron transfer between d orbital of Mn and orbital of N2 forces the N_(2)to be of radical nature,which activates the hydrogenation process and weakens the N≡N bond.Based on the density functional theory(DFT)calculation results,we precisely designed mesoporous MnO_(2)nanosheets with rich oxygen vacancies via using methyltriphenylphosphonium bromide(MPB)to induce more Mn^(3+)-Mn^(3+)pairs(Mn^(3-3)-MnO_(2)),which can achieve a fairly high ammonia yield of up to 147.2μg·h^(−1)·mgcat−1.at−0.75 V vs.reversible hydrogen electrode(RHE)and a high Faradaic efficiency(FE)of 11%.Furthermore,these mesoporous MnO_(2)nanosheets exhibit the superior durability with negligible changes in both NH3 yield and FE after a consecutive 6-recycle test and the current density electrolyzed over a 24-hour period.Our findings offer an approach to designing highly active transition metal catalysts for electrocatalytic nitrogen reduction.

Carbon Fiber Supported Binary Metal Sulfide Catalysts with Multi-Dimensional Structures for Electrocatalytic Nitrogen Reduction Reactions Over a Wide pH Range

Green and environmentally friendly electrocatalytic nitrogen(N_(2))fixation to synthesize ammonia(NH3)is recognized as an effective method to replace the traditional Haber-Bosch process.However,the difficulties in N_(2) adsorption and fracture of hard N≡N bond still remain major challenges in electrocatalytic N_(2) reduction reactions(NRR).From the perspectives of enhancing N_(2) adsorption and providing more catalytic sites,two-dimensional(2D)FeS_(2) nanosheets and three-dimensional(3D)metal organic framework-derived ZnS embedded within N-doped carbon polyhedras are grown on the carbon cloth(CC)template in this work.Thus,a composite NRR catalyst with multi-dimensional structures,which is signed as FeS_(2)/ZnS-NC@CC,is obtained for using over a wide pH range.The uniform distribution of hollow ZnS-NC frameworks and FeS_(2) nanosheets on the surface of CC largely increase the N_(2) enrichment efficiency and offer more active sites,while the CC skeleton acts as an independent conductive substrate and S-doping helps promote the fracture of N≡N bond during the NRR reaction.As a result,the FeS_(2)/ZnS-NC@CC electrode achieves a high Faraday efficiency of 46.84%and NH3 yield of 58.52μg h^(−1) mg^(−1) at-0.5 V vs.Ag/AgCl in 0.1 M KOH.Furthermore,the FeS_(2)/ZnS-NC@CC electrode displays excellent NRR catalytic activity in acidic and neutral electrolytes as well,which outperforms most previously reported electrocatalysts including noble metals.Therefore,this work provides a new way for the design of multi-dimensional electrocatalysts with excellent electrocatalytic efficiency and stability for NRR applications.

Insights into electrochemical nitrogen reduction reaction mechanisms:Combined effect of single transition-metal and boron atom

Developing single-atom catalysts(SACs) for electrochemical devices is a frontier in energy conversion.The comparison of stability,activity and selectivity between various single atoms is one of the main research focuses in SACs.However,the in-depth understanding of the role that the coordination atoms of single atom play in the catalytic process is lacking.Herein,we proposed a graphene-like boroncarbon-nitride(BCN) monolayer as the support of single metal atom.The electrocatalytic nitrogen reduction reaction(eNRR) performances of 3 d,4 d transition metal(TM) atoms embedded in defective BCN were systematically investigated by means of density functional theory(DFT) computations.Our study shows that the TM-to-N and B-to-N π-back bonding can contribute to the activation of N_(2).Importantly,a combined effect is revealed between single TM atom and boron atom on eNRR:TM atom enhances the nitrogen reduction process especially in facilitating the N_(2) adsorption and the NH3 desorption,while boron atom modulates the bonding strength of key intermediates by balancing the charged species.Furthermore,Nb@BN3 possesses the highest electrocata lytic activity with limiting potential of-0.49 V,and exhibits a high selectivity for nitrogen reduction reaction(NRR) to ammonia compared with hydrogen evolution reaction(HER).As such,this work can stimulate a research doorway for designing multi-active sites of the anchored single atoms and the innate atoms of substrate based on the mechanistic insights to guide future eNRR research.

FeTe_(2) as an earth-abundant metal telluride catalyst for electrocatalytic nitrogen fixation

Design of cost-effective,yet highly active electrocatalysts for nitrogen reduction reaction(NRR)is of vital significance for sustainable electrochemical NH_(3) synthesis.Herein,we have demonstrated,from both computational and experimental perspectives,that FeTe_(2) can be an efficient and durable NRR catalyst.Theoretical computations unveil that FeTe_(2) possesses abundant surface-terminated and low-coordinate Fe sites that can activate the NRR with a low limiting potential(-0.84 V)and currently impede the competing hydrogen evolution reaction.As a proof-of-concept prototype,we synthesized FeTe_(2) nanoparticles supported on reduced graphene oxide(FeTe_(2)/RGO),which exhibited a high NRR activity with the exceptional combination of NH_(3) yield(39.2 lg h^(-1) mg^(-1))and Faradaic efficiency(18.1%),thus demonstrating the feasibility of using FeTe_(2) and other earth-abundant metal tellurides for electrocatalytic N_(2) fixation.

Accelerated Discovery of Single-Atom Catalysts for Nitrogen Fixation via Machine Learning

Developing high-performance catalysts using traditional trial-and-error methods is generally time consuming and inefficient.Here,by combining machine learning techniques and first-principle calculations,we are able to discover novel graphene-supported single-atom catalysts for nitrogen reduction reaction in a rapid way.Successfully,45 promising catalysts with highly efficient catalytic performance are screened out from 1626 candidates.Furthermore,based on the optimal feature sets,new catalytic descriptors are constructed via symbolic regression,which can be directly used to predict single-atom catalysts with good accuracy and good generalizability.This study not only provides dozens of promising catalysts and new descriptors for nitrogen reduction reaction but also offers a potential way for rapid screening of new electrocatalysts.

Perspectives on electrochemical nitrogen fixation catalyzed by two-dimensional MXenes

Ammonia is the most basic raw material in industrial and agricultural production.The current industrial production of ammonia relies on the Haber-Bosch process with high energy consumption.To overcome this shortcoming,the development of electrocatalytic ammonia synthesis under moderate conditions is considered as a potential alternative technology.The two-dimensional(2D)MXenes family has been proved promising as electrocatalysts,but from the currently available literature,it is hard to find a systematic review on MXenes-catalyzed ammonia synthesis.So in the present review,we summarize the key perspectives on that topic in recent years as well as outline,from a prospective view,strategies of catalyst design.We analyze in detail the methods for preparing high performance MXenes-based catalysts and the corresponding underlying mechanisms,and also discuss the criteria and potential challenges,expecting to provide inspiration for the development of efficient MXenes-based route to electrochemical ammonia fixation.

Polyoxometalates-derived ternary metal oxides electrocatalyst for N_(2) reduction under ambient conditions

Accelerating the breaking of the nitrogen nonpolar bond(N≡N)is an important factor to improve the effi ciency of the electro-catalytic nitrogen reduction reaction(e-NRR).In this work,polyoxometalates-derived FeMo-based ternary oxide materials MoO_(2)-Mo_(4)O_(11)-FeMoO_(4)@X(abbreviated as MoFeO@X,X represents the synthesis temperature of 650,750 and 850℃)were designed and synthesized for e-NRR under ambient conditions.The scanning electron microscopy images of MoFeO@750 show an ellipsoidal-like structure(0.86×0.6μm).The relatively large specifi c surface area,formation of multiple interfaces,together with the synergistic eff ect of iron and molybdenum bimetals,would make MoFeO@X catalyst more easily absorb and activate N_(2)in the e-NRR.Expectedly,the synthesized MoFeO@750 shows an optimal NH_(3)production rate of 16.57μg·h^(−1)·mg cat.^(−1)and Faradaic effi ciency of 12.33%at−0.3 V versus reversible hydrogen electrode(νs.RHE),with outstanding electrochemical and structural stability.

Sulfur-deficient Bi_(2)S_(3-x) synergistically coupling Ti_(3)C_(2)T_(x)-MXene for boosting electrocatalytic N_(2) reduction

Electrocatalytic nitrogen reduction reaction(NRR)is an appealing route for the sustainable NH_(3)synthesis,while developing efficientand durable NRR catalysts remains at the heart of achieving high-efficiency N_(2)-to-NH_(3)electrocatalysis.Herein,we rationally combine vacancy and interface engineering to design sulfur-deficient Bi2S3 nanoparticles decorated Ti_(3)C_(2)T_(x)-MXene as an effective NRR catalyst.The developed Bi2S3 nanoparticles decorated Ti_(3)C_(2)T_(x)-MXene(Bi2S3-xTi_(3)C_(2)T_(x))naturally contained abundant S-vacancies and exhibited a dramatically boosted NRR activity with an NH_(3)yield of 68.3μg·h^(-1)mg^(-1)(-0.6 V)and a Faradaic efficiency of 22.5%(-0.4 V),far superior to pure Bi_(2)S_(3)and Ti_(3)C_(2)T_(x)and surpassing almost all ever reported Bi-and MXene-based NRR catalysts.Theoretical investigations unveiled that the exceptional NRR activity of Bi_(2)S_(3-x)/Ti_(3)C_(2)T_(x)stemmed from its dual-active-center system involving both S-vacancies and interfacial-Bi sites,which could synergistically promote N_(2)adsorption and*N_(2)H formation to result in an energetic-favorable NRR process.

Hydrogen-assisted activation of N_(2)molecules on atomic steps of ZnSe nanorods

Electrochemical reduction reaction of nitrogen(NRR)offers a promising pathway to produce ammonia(NH_(3))from renewable energy.However,the development of such process has been hindered by the chemical inertness of N_(2).It is recently proposed that hydrogen species formed on the surface of electrocatalysts can greatly enhance NRR.However,there is still a lack of atomiclevel connection between the hydrogenation behavior of electrocatalysts and their NRR performance.Here,we report an atomistic understanding of the hydrogenation behavior of a highly twinned ZnSe(T-ZnSe)nanorod with a large density of surface atomic steps and the activation of N_(2)molecules adsorbed on its surface.Our theoretical calculations and in situ infrared spectroscopic characterizations suggest that the atomic steps are essential for the hydrogenation of T-ZnSe,which greatly reduces its work function and efficiently activates adsorbed N_(2)molecules.Moreover,the liquid-like and free water over T-ZnSe promotes its hydrogenation.As a result,T-ZnSe nanorods exhibit significantly enhanced Faradaic efficiency and NH3 production rate compared with the pristine ZnSe nanorod.This work paves a promising way for engineering electrocatalysts for green and sustainable NH3 production.

Single-atom catalysts based on two-dimensional metalloporphyrin monolayers for ammonia synthesis under ambient conditions

We systematically investigated the catalytic performance of 3d,4d,and 5d transition metals anchored onto two-dimensional extended porphyrin(PP)substrates as nitrogen reduction reaction(NRR)electrocatalysts,employing density functional theory(DFT)calculations and four-step high-throughput screening.Four novel metalloporphyrin(MPP,M=Zr,Nb,Hf,and Re)single-atom catalyst candidates have been identified due to their excellent catalytic performance(low onset potential,high stability,and selectivity).Through comprehensive reaction path search,the maximum Gibbs free energy changes for NRR on the ZrPP(enzymatic-consecutive hybrid path),NbPP(consecutive path),HfPP(enzymatic-consecutive hybrid path),and RePP(distal path)catalysts are 0.38,0.41,0.53,and 0.53 eV,respectively.Band structures,projected density of states,and charge/spin distributions show that the high catalytic activity is due to significant orbital hybridizations and charge transfer between N_(2) and MPP catalysts.We hope our work will promote experimental synthesis of these NRR electrocatalysts and provide new opportunities to the electrochemical conversion of N_(2) to NH_(3) under ambient conditions.

FeNi@CNS nanocomposite as an efficient electrochemical catalyst for N_(2)-to-NH_(3) conversion under ambient conditions

The electrocatalytic nitrogen reduction reaction(NRR)has emerged as a promising renewable energy source and a feasible strategy as an alternative to Haber-Bosch ammonia(NH_(3))synthesis.However,finding an efficient and cost-effective robust catalyst to activate and cleave the extremely strong triple bond in nitrogen(N_(2))for electrocatalytic NRR is still a challenge.Herein,a FeNi@CNS nanocomposite as an efficient catalyst for N_(2) fixation under ambient conditions is designed.This FeNi@CNS nanocomposite was prepared by a simple water bath process and post-calcination.The FeNi@CNS is demonstrated to be a highly efficient NRR catalyst,which exhibits better NRR performance with exceptional Faradaic efficiency of 9.83%and an NH_(3) yield of 16.52μg h^(−1) cm^(−2) in 0.1 M Na_(2)SO_(4) aqueous solution.Besides,high stability and reproducibility with consecutive 6 cycles for two hours are also demonstrated throughout the NRR electrocatalytic process for 12 h.Meanwhile,the FeNi@CNS catalyst encourages N_(2) adsorption and activation as well as effectively suppressing competitive HER.Therefore,this earth-abundant FeNi@CNS catalyst with a subtle balance of activity and stability has excellent potential in NRR industrial applications.