Enhancing ammonia production rates from electrochemical nitrogen reduction by engineering three-phase boundary with phosphorus-activated Cu catalysts

Electrochemical N_(2) reduction reaction(eNRR) over Cu-based catalysts suffers from an intrinsically low activity of Cu for activation of stable N_(2) molecules and the limited supply of N_(2) to the catalyst due to its low solubility in aqueous electrolytes.Herein,we propose phosphorus-activated Cu electrocatalysts to generate electron-deficient Cu sites on the catalyst surface to promote the adsorption of N_(2) molecules.The eNRR system is further modified using a gas diffusion electrode(GDE) coated with polytetrafluoroethylene(PTFE) to form an effective three-phase boundary of liquid water-gas N_(2)-solid catalyst to facilitate easy access of N_(2) to the catalytic sites.As a result,the new catalyst in the flow-type cell records a Faradaic efficiency of 13.15% and an NH_(3) production rate of 7.69 μg h^(-1) cm^(-2) at-0.2 V_(RHE),which represent 3.56 and 59.2 times increases from those obtained with a pristine Cu electrode in a typical electrolytic cell.This work represents a successful demonstration of dual modification strategies;catalyst modification and N_(2) supplying system engineering,and the results would provide a useful platform for further developments of electrocatalysts and reaction systems.

Bi-Atom Electrocatalyst for Electrochemical Nitrogen Reduction Reactions

The electrochemical nitrogen reduction reaction(NRR)to directly produce NH3 from N_(2) and H_(2)O under ambient conditions has attracted significant attention due to its ecofriendliness.Nevertheless,the electrochemical NRR presents several practical challenges,including sluggish reaction and low selectivity.Here,bi-atom catalysts have been proposed to achieve excellent activity and high selectivity toward the electrochemical NRR by Ma and his co-workers.It could accelerate the kinetics of N_(2)-to-NH_(3) electrochemical conversion and possess better electrochemical NRR selectivity.This work sheds light on the introduction of bi-atom catalysts to enhance the performance of the electrochemical NRR.

CeO_(x)-supported monodispersed MoO_(3) clusters for high-efficiency electrochemical nitrogen reduction under ambient condition

Developing efficient and low-cost electrocatalysts is essential for the electroreduction of N_(2) to NH_(3).Here,highly monodispersed MoO_(3) clusters loaded on a coral-like CeO_(x)compound with abundant oxygen vacancies are successfully prepared by an impregnation-reduction method.The MoO_(3) clusters with small sizes of 2.6±0.5 nm are induced and anchored by the oxygen vacancies of CeO_(x),resulting in excellent nitrogen reduction reaction(NRR)performance.Additionally,the synergistic effects between MoO_(3) and CeO_(x)lead to a further improvement of the electrochemical performance.The as-prepared MoO_(3)-CeO_(x)catalyst shows an NH_(3) yield rate of 32.2 μg h^(-1) mg^(-1) cat and a faradaic efficiency of 7.04%at-0.75 V(vs.reversible hydrogen electrode)in 0.01 M Dulbecco’s Phosphate Buffered Saline.Moreover,it displays decent electrochemical stability over 30,000 s.Besides,the electrochemical NRR mechanism for MoO_(3)-CeO_(x)is investigated by in-situ Fourier transform infrared spectroscopy.N-H stretching,H-N-H bending,and N-N stretching are detected during the reaction,suggesting that an associative pathway is followed.This work provides an approach to designing and synthesizing potential electrocatalysts for NRR.

A combinatorial descriptor for volcano relationships of electrochemical nitrogen reduction reaction

Though touted as a potential way to realize clean ammonia synthesis,electrochemical ammonia synthesis is currently limited by its catalytic efficiency.Great effort has been made to find catalysts with improved activity toward electrochemical nitrogen reduction reaction(eNRR).Rational screening of catalysts can be facilitated using the volcano relationship between catalytic activity and adsorption energy of an intermediate,namely,the activity descriptor.In this work,we proposeΔG^(*)_(NH_(2))+ΔG^(*)_(NNH)as a combinatorial descriptor,which shows better predictive power than traditional descriptors using the adsorption free energies of single intermediates.The volcano plots based on the combinatorial descriptor exhibits peak activity fixedly at the descriptor value corresponding to the formation free energy of NH3,regardless of the catalyst types;while the descriptor values correspond to the top activities for eNRR on volcano plots based on single descriptors usually vary with the types of catalysts.

The role of central heteroatom in electrochemical nitrogen reduction catalyzed by polyoxometalate-supported single-atom catalyst

Single-atom catalysts(SACs)have recently emerged as stars in boosting the synthesis of NH3 from N_(2),as the catalytic performance of the supported single atoms can be modulated by their coordination environment.In this work,we propose a new strategy,based on comprehensive density functional theory calculations,whereby the coordination environment of a single Mo atom can be tuned by a central heteroatom(X=Fe,Co,Ni,Cu,Zn,Ga,Ge,and As)in the Kegging-type polyoxometalate(POM,(XW12O40)n−)substrate to catalyze the electrochemical nitrogen reduction reactions(NRR).Firstly,we demonstrate that the single Mo atom binds strongly to the POM surface oxygen hollow sites without aggregation.Secondly,the adsorption of*N_(2)on the POM-supported Mo atom is investigated and the reactivity is assessed by calculating the thermodynamics of the NRR.The results show that the POM(X=Co and As)supported Mo atom has high NRR activity with low limiting potentials.Finally,we reveal the origin of the NRR activity by analyzing the electronic structure.The results show that the charge on the O atoms of oxygen hollow sites is affected by the central heteroatom.Due to such effect,it can be found that more d electrons are transferred from Mo supported by POM(X=Co and As)to*N_(2),thus the N≡N triple bond is activated.This strategy of coordination environment tuning proposed in this work provides a useful guide for the design of efficient catalysts for electrocatalysis.

Strategies to achieve effective nitrogen activation

Ammonia serves as a crucial chemical raw material and hydrogen energy carrier.Aqueous electrocatalytic nitrogen reduction reaction(NRR),powered by renewable energy,has attracted tremendous interest during the past few years.Although some achievements have been revealed in aqueous NRR,significant challenges have also been identified.The activity and selectivity are fundamentally limited by nitrogen activation and competitive hydrogen evolution.This review focuses on the hurdles of nitrogen activation and delves into complementary strategies,including materials design and system optimization(reactor,electrolyte,and mediator).Then,it introduces advanced interdisciplinary technologies that have recently emerged for nitrogen activation using high-energy physics such as plasma and triboelectrification.With a better understanding of the corresponding reaction mechanisms in the coming years,these technologies have the potential to be extended in further applications.This review provides further insight into the reaction mechanisms of selectivity and stability of different reaction systems.We then recommend a rigorous and detailed protocol for investigating NRR performance and also highlight several potential research directions in this exciting field,coupling with advanced interdisciplinary applications,in situ/operando characterizations,and theoretical calculations.

Tuning Double Layer Structure of WO3 Nanobelt for Promoting the Electrochemical Nitrogen Reduction Reaction in Water

Electrochemical fixation of nitrogen to ammonia with highly active,highly selective and low cost electrocatalysts is a sustainable alternative to the extremely energy-and capital-intensive Haber-Bosch process.Herein,we demonstrate a near electroneutral WO3 nanobelt catalyst to be a promising electrocatalyst for selective and efficient nitrogen reduction.The concept of near electroneutral interface is demonstrated by fabricating WO3 nanobelts with small zeta potential value on carbon fiber paper,which ensures a loose double layer structure of the electrode/electrolyte interface and allows nitrogen molecules access the active sites more easily and regulates proton transfer to increase the catalytic selectivity.The WO3/CFP electrode with optimal surface charge achieves a NH3 yield rate of 4.3μg·h-1·mg-1 and a faradaic efficiency of 37.3%at-0.3 V vs.RHE,rivalling the performance of the state-of-the-art nitrogen reduction reaction electrocatalysts.The result reveals that an unobstructed gas-diffusion pathway for continually supplying enough nitrogen to the active catalytic sites is of great importance to the overall catalytic performance.

Interpretable Machine Learning-Assisted High-Throughput Screening for Understanding NRR Electrocatalyst Performance Modulation between Active Center and C-N Coordination

Understanding the correlation between the fundamental descriptors and catalytic performance is meaningful to guide the design of high-performance electrochemical catalysts.However,exploring key factors that affect catalytic performance in the vast catalyst space remains challenging for people.Herein,to accurately identify the factors that affect the performance of N2 reduction,we apply interpretable machine learning(ML)to analyze high-throughput screening results,which is also suited to other surface reactions in catalysis.To expound on the paradigm,33 promising catalysts are screened from 168 carbon-supported candidates,specifically single-atom catalysts(SACs)supported by a BC_(3)monolayer(TM@V_(B/C)-N_(n)=_(0-3)-BC_(3))via high-throughput screening.Subsequently,the hybrid sampling method and XGBoost model are selected to classify eligible and non-eligible catalysts.Through feature interpretation using Shapley Additive Explanations(SHAP)analysis,two crucial features,that is,the number of valence electrons(N_(v))and nitrogen substitution(N_(n)),are screened out.Combining SHAP analysis and electronic structure calculations,the synergistic effect between an active center with low valence electron numbers and reasonable C-N coordination(a medium fraction of nitrogen substitution)can exhibit high catalytic performance.Finally,six superior catalysts with a limiting potential lower than-0.4 V are predicted.Our workflow offers a rational approach to obtaining key information on catalytic performance from high-throughput screening results to design efficient catalysts that can be applied to other materials and reactions.

Anchoring Mo on C_(9)N_(4) monolayers as an efficient single atom catalyst for nitrogen fixation

Electrochemical nitrogen fixation via a convenient and sustainable manner,exhibits an intriguing prospect for ammonia generation under ambient conditions.Currently,the design and development of high-efficiency and low-cost electrocatalysts remains the major challenge confronting nitrogen reduction reaction(NRR).Herein,anchoring the single Mo atom on the C_(9)N_(4) substrate(Mo@C_(9)N_(4)) to form an efficient single-atom catalyst(SAC) is proposed for the conversion of N2 to NH3.By employing density functional theory(DFT) calculations,we demonstrated that gas phase N2 can be sufficiently activated and efficiently reduced to NH3 on the surface of Mo@C_(9)N_(4).Meanwhile,we found that the NRR dominantly occurred on the Mo center via a preferred distal pathway with favorable limiting potential of 0.40 V.Importantly,the as-established Mo@C_(9)N_(4) catalyst exhibits an outstanding structural stability and good selectivity toward NRR.These findings provide a promising platform for designing Mo-based SACs for electrochemical N2 fixation.

Unveiling the Underlying Mechanism of Transition Metal Atoms Anchored Square Tetracyanoquinodimethane Monolayers as Electrocatalysts for N_(2) Fixation

We for the first time systematically studied the structures and electrochemical nitrogen reduction reaction properties of two-dimensional single transition-metal anchored square tetracyanoquinodimethane monolayers(labeled as:TM-sTCNQ,TM=3d,4d,5d series transition metals)by employing density functional theory method.Through highthroughput screenings and full reaction path researches,two promising electrochemical nitrogen reduction reaction catalysts Nb-sTCNQ and MosTCNQ have been obtained.The nitrogen reduction reaction onset potential on Nb-sTCNQ is as low as−0.48 V.Furthermore,the Nb-sTCNQ catalyst can quickly desorb NH3 produced with a free energy of 0.65 eV,giving Nb-sTCNQ excellent catalytic cycle performance.The high catalytic activity of the two materials might be attributed to the effective charge transfer between the active center and adsorbed N_(2),which enables the active center to adsorb and activate inert N_(2) molecules well,and the reduction processes require small energy input(i.e.,the maximum free energy changes are small).This work provides insights for finding highly efficient,stable,and low-cost nitrogen reduction reaction electrocatalysts.We hope our results can promote further experimental and theoretical research of this field.