Highly Efficient Electrochemical Reduction of Nitrogen to Ammonia on Surface Termination Modified Ti3C2Tx MXene Nanosheets

MXene-based catalysts exhibit extraordinary advantages for many catalysis reactions, such as the hydrogen evolution and oxygen reduction reactions. However, MXenes exhibit inadequate catalytic activity for the electrochemical nitrogen reduction reaction (NRR) because they are typically terminated with inactive functional groups, F* and OH*, which mask the active metal sites for N₂ binding. Here we modified the surface termination of MXene (Ti₃C₂T_x) nanosheets to achieve high surface catalytic reactivity for the NRR by ironing out inactive F*/OH* terminals to expose more active sites and by introducing Fe to greatly reduce the surface work function. The optimally performing catalyst (MXene/TiFeO_x-700) achieved excellent Faradaic efficiency of 25.44% and an NH₃ yield rate of 2.19 μg/cm²·h (21.9 μg/mg_cat·h), outperforming all reported MXene-based NRR catalysts. Our work provides a feasible strategy for rationally improving the surface reactivity of MXene-based catalysts for efficient electrochemical conversion of N₂ to NH₃.

Three-Phase Electrolysis by Gold Nanoparticle on Hydrophobic Interface for Enhanced Electrochemical Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) provides a facile and sustainable strategy to produce ammonia (NH3) at ambient conditions. However, the low NH3 yield and Faradaic efficiency (FE) are still the main challenges due to the competitive hydrogen evolution reaction (HER). Herein, a three-phase electrocatalyst through in situ fabrication of Au nanoparticles (NPs) located on hydrophobic carbon fiber paper (Au/o-CFP) is designed. The hydrophobic CFP surface facilitates efficient three-phase contact points (TPCPs) for N2 (gas), electrolyte (liquid), and Au NPs (solid). Thus, concentrated N2 molecules can contact the electrocatalyst surface directly, inhibiting the HER since the lowered proton concentration and overall enhancing NRR. The three-phase Au/o-CFP electrocatalyst presents an excellent NRR performance with high NH3 yield rate of 40.6 µg h-1 mg-1 at -0.30 V and great FE of 31.3% at -0.10 V versus RHE (0.1 m Na2SO4). The N2-bubble contact angle result and cyclic voltammetry analysis confirm that the hydrophobic interface has a relatively strong interaction with N2 bubble for enhanced NRR and weak electrocatalytic activity for HER. Significantly, the three-phase Au/o-CFP exhibits excellent stability with a negligible fluctuation of NH3 yield and FE in seven-cycle test. This work provides a new strategy for improving NRR and simultaneously inhibiting HER.

Computational Design of Transition Metal Single-Atom Electrocatalysts on PtS2 for Efficient Nitrogen Reduction

Electrocatalytic nitrogen reduction is promising to serve as a sustainable and environmentally friendly strategy to achieve ammonia production. Single-atom catalysts (SACs) hold great promise to convert N₂ into NH₃ because of the unique molecular catalysis property and ultrahigh atomic utilization ratio. Here, we demonstrate a universal computational design principle to assess the N₂ reduction reaction (NRR) performance of SACs anchored on a monolayer PtS₂ substrate (SACs-PtS₂). Our density functional theory simulations unveil that the barriers of the NRR limiting potential step on different SAC centers are observed to be linearly correlated to the integral of unoccupied d states (UDSs) of SACs. As a result, the Ru SAC-PtS₂ catalyst with the largest number of UDSs exhibits a much lower barrier of the limiting step than those of other SACs-PtS₂ catalysts and the Ru(0001) benchmark. Our work bridges the apparent NRR activity and intrinsic electronic structure of SAC centers and offers effective guidance to screen and design efficient SACs for the electrochemical NRR process.

Bioinspired Fe3C@C as Highly Efficient Electrocatalyst for Nitrogen Reduction Reaction under Ambient Conditions

Developing highly efficient non-precious-metal catalysts for electrochemical reduction reaction is vital for artificial nitrogen fixation under ambient conditions. Herein, we report a bioinspired Fe₃C@C composite as an efficient electrocatalyst for nitrogen reduction. The composite based on a leaf skeleton successfully replicates the natural vein structure with multichannels. The Fe₃C@C core-shell structure as the real active center contributes to selective electrocatalytic synthesis of ammonia from nitrogen with Faraday efficiency of 9.15% and production rate of 8.53 μg/(h mg_cat) or 12.80 μg/(h cm²) at a low potential of -0.2 V versus reversible hydrogen electrode (vs RHE), which is better than that of recently reported carbon- and iron-based materials, even comparable with that of noble-metal-based catalyst. Experiments with density functional theory calculations reveal that graphene-encapsulated Fe₃C nanoparticles can improve charge transfer due to core-shell interaction, beneficial for inducing active sites for N₂ adsorption and activation and thereby facilitate ammonia synthesis.

Unravelling the Reaction Mechanisms of N2 Fixation on Molybdenum Nitride: A Full DFT Study from the Pristine Surface to Heteroatom Anchoring

Transition metal nitrides (TMNs)-based materials have attracted increasing attention in electrochemical nitrogen reduction reaction (eNRR) because of their unique structures and inherent electronic properties. However, the eNRR mechanism on such nitrogen contained catalysts is still unclear, for example, which part of the catalyst act as the active sites, and how to achieve the optimal efficiency is also challenging. In this work, a comprehensive study was conducted to unravel the reaction mechanisms of N₂ fixation on molybdenum nitride by using density functional theory (DFT) calculations. The activity and selectivity of eNRR on pristine (001) and (110) Mo₅ N₆ surfaces as well as few specific numbers of heteroatom-anchored N-terminated surfaces were all evaluated and compared. It was found that the Mo and N atoms on the pristine Mo₅ N₆ surface were both active for eNRR while following different pathways in mechanism. Moreover, the eNRR catalytic performance of Mo₅ N₆ could be further boosted by specific metal atoms anchoring, such as single atom, metal dimer, and heterodiatom pair. Finally, a full map of eNRR mechanism on pristine and metal atom-decorated Mo₅ N₆ surfaces was illustrated. This work not only provides a fundamental understanding of eNRR mechanism on TMNs based materials but also offers powerful strategies towards the rational design of efficient NRR electrocatalysts.

Modulating the Oxidation State of Titanium via Dual Anions Substitution for Efficient N2 Electroreduction

The electrocatalytic nitrogen reduction reaction (NRR) is a promising approach for renewable ammonia synthesis but remains significantly challenging due to the low yield and poor selectivity. Herein, a facile N and S dual anions substitution strategy is developed to tune the Ti oxidation states of TiO₂ nanohybrid catalyst (NS-TiO₂ /C), in which anatase TiO₂ nanoplates with dense Ti³⁺ active sites are uniformly dispersed on porous carbon derived from 2D Ti₃ C₂ T_x nanosheets. The catalyst NS-TiO₂ /C exhibits a superior ambient NRR efficiency with an NH₃ yield rate of 19.97 µg h^-1 mg^-1_cat and Faradaic efficiency of 25.49% and is coupled with a remarkable 50 h long-term stability at -0.25 V versus RHE. Both experimental and theoretical results reveal that the N and S dual-substitution effectively regulate the Ti oxidation state and electronical properties of the NS-TiO₂ /C via simultaneously forming interstitial and substitutional TiS and TiN bonds in the anatase TiO₂ lattice, inducing oxygen vacancies and dense Ti³⁺ active species as well as better electronic conductivity, which substantially facilitates N₂ chemisorption and activation, and reduces the energy barrier of the rate-determining step, thereby essentially boosting NRR efficiency. This work provides a valuable approach to the rational design of advanced materials by modulating oxidation states for efficient electrocatalysis.

Microenvironment Regulation of the Ti3C2Tx MXene Surface for Enhanced Electrochemical Nitrogen Reduction

The overwhelmingly competitive hydrogen evolution reaction (HER) is a bottleneck challenge in the electrocatalytic nitrogen reduction reaction (eNRR) process. Herein, we develop a general and effective strategy to suppress the HER via covalent surface functionalization to modulate the local microenvironment of the electrocatalyst. A hydrophobic molecular layer with tunable coverage density was coated on the surface of Ti₃C₂T_x MXene, and the one with appropriate coverage density significantly improved the eNRR efficiency with an excellent faradaic efficiency (FE) of 38.01% at -0.35 V and a high NH₃ yield rate of 17.81 μg h^-1mg_cat^-1 at -0.55 V (vs RHE) in a Na₂SO₄ solution, which were 3.5-fold in FE and 6.5-fold in NH₃ yield rate higher than those of the pristine Ti₃C₂T_x. Experimental results combined with molecular dynamics (MD) simulations reveal that the hydrophobic molecular layer on the surface greatly limits the proton transfer and benefits higher exposure of active sites with enhanced N₂ chemisorption ability, which cumulatively contribute to the boosted eNRR efficiency.

Electrochemical Nitrogen Fixation for Green Ammonia: Recent Progress and Challenges

Ammonia, a key feedstock used in various industries, has been considered a sustainable fuel and energy storage option. However, NH3 production via the conventional Haber-Bosch process is costly, energy-intensive, and significantly contributing to a massive carbon footprint. An electrochemical synthetic pathway for nitrogen fixation has recently gained considerable attention as NH3 can be produced through a green process without generating harmful pollutants. This review discusses the recent progress and challenges associated with the two relevant electrochemical pathways: direct and indirect nitrogen reduction reactions. The detailed mechanisms of these reactions and highlight the recent efforts to improve the catalytic performances are discussed. Finally, various promising research strategies and remaining tasks are presented to highlight future opportunities in the electrochemical nitrogen reduction reaction.

Highly selective NH3 synthesis from N2 on electron-rich Bi0 in a pressurized electrolyzer

Electrochemical conversion of N2 into ammonia presents a sustainable pathway to produce hydrogen storage carrier but yet requires further advancement in electrocatalyst design and electrolyzer integration. This technology suffers from low selectivity and yield owing to the extremely strong N≡N bond and the exceptionally low solubility of N2 in aqueous systems. A high NH3 synthesis performance is restricted by the high activation energy of N≡N bond and the supply insufficiency of N2 to active sites. This paper describes the introduction of electron-rich Bi0 sites into Ag catalysts with a high-pressure electrolyzer that enables a dramatically enhanced Faradaic efficiency of 44.0% and yield of 28.43 μg cm-2 h-1 at 4.0 MPa. Combined with density functional theory results, in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy demonstrates that N2 reduction reaction follows an associative mechanism, in which a high coverage of N-N bond and -NH2 intermediates suggest electron-rich Bi0 boosts sound activation of N2 molecules and low hydrogenation barrier. The proposed strategy of engineering electrochemical catalysts and devices provides powerful guidelines for achieving industrial-level green ammonia production.

Electronic Perturbation of Isolated Fe Coordination Structure for Enhanced Nitrogen Fixation

Modulation of the local electronic structure of isolated coordination structures plays a critical role in electrocatalysis yet remains a grand challenge. Herein, we have achieved electron perturbation for the isolated iron coordination structure via tuning the iron spin state from a high spin state (FeN4) to a medium state (FeN2B2). The transition of spin polarization facilitates electron penetration into the antibonding π orbitals of nitrogen and effectively activates nitrogen molecules, thereby achieving an ammonia yield of 115 μg h-1 mg-1cat. and a Faradaic efficiency of 24.8%. In situ spectroscopic studies and theoretical calculations indicate that boron coordinate sites, as electron acceptors, can regulate the adsorption energy of NxHy intermediates on the Fe center. FeN2B2 sites favor the NNH* intermediate formation and reduce the energy barrier of rate-determining steps, thus accounting for excellent nitrogen fixation performance. Our strategy provides an effective approach for designing efficient electrocatalysts via precise electronic perturbation.

Toward High-Performance Electrochemical Ammonia Synthesis by Circumventing the Surface H-Mediated N2 Reduction

The rapid performance decay with potentials is a significant obstacle to achieving an efficient electrocatalytic N2 reduction reaction (eNRR), which is typically attributed to competition from hydrogen evolution. However, the potential-dependent competitive behavior and reaction mechanism are still under debate. Herein, we theoretically defined N2 adsorption, H mediation, and H2 evolution as three crucial regions along the potentials by revisiting the potential-dependent competitive adsorption between N2 and H on FeN4 and RuN4 catalysts. We revealed that the surface H-mediated mechanism makes eNRR feasible at low potentials but introduces sluggish reaction kinetics, showing a double-edged sword nature. In view of this, we proposed a new possibility to achieve high-performance NH3 synthesis by circumventing the H-mediated mechanism, where the ideal catalyst should have a wide potential interval with N2-dominated adsorption to trigger direct eNRR. Using this mechanistic insight as a new criterion, we proposed a theoretical protocol for eNRR catalyst screening, but almost none of the theoretically reported electrocatalysts passed the assessment. This work not only illustrates the intrinsic mechanism behind the low-performance dilemma of eNRR but also points out a possible direction toward designing promising catalysts with high selectivity and high current density.

Across the Board: Federico Bella on Electrochemical Nitrogen Reduction

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. F. Bella, who discusses the electrochemical reduction of nitrogen to produce ammonia through the addition of protons and electrons under mild conditions (25 °C, 1 atm). This reaction has the potential to replace the energy-intensive traditional Haber-Bosch process but faces several challenges and pitfalls.

A Robust n-n Heterojunction: CuN and BN Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with CuN and BN dual active sites is synthesized via in situ growth of a conductive metal-organic framework (MOF) [Cu3 (HITP)2 ] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu3 (HITP)2 @h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu3 (HITP)2 @h-BN shows the outstanding eNRR performance with the NH3 production of 146.2 µg h-1 mgcat -1 and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and CuN/BN dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH3 production catalyzed by the Cu3 (HITP)2 @h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.