Electrochemical Nitrogen Reduction Research Articles

Bio-inspired catalysts for enhanced N₂ electroreduction to NH3: Architecting molecular screening interfaces via active sites microenvironment engineering

The electrochemical nitrogen reduction reaction (ENRR) promises a sustainable route for ammonia (NH3) production but faces hurdles from competing hydrogen evolution reaction (HER) and insufficient N2 concentration. Drawing inspiration from Tropical Lizards’ unique mechanism of creating a thin air layer on their hydrophobic skin, we modify the catalyst-water interface using alkyl thiols of different chain lengths on VS2 (Cx-VS2, x = 4,8,12). This novel approach suppresses HER and increases N2 availability. Among the tailored catalysts, C8-VS2 exhibits superior ENRR performance, with an ammonia yield of 58.43 μg h−1 mgcat−1 and a Faradaic efficiency of 34.69 % that surpasses unmodified VS2 (21.05 μg h−1 mgcat−1 and 8.87 %). Supporting measurements and density functional theory (DFT) calculations confirm the pivotal role of the modified interface in optimizing NH3 production. Our findings offer fresh perspectives for designing efficient electrocatalysts via tailoring interface microenvironments, with potential implications for various applications.

Research Progress on Ni-based Electrocatalysts for Electrochemical Reduction of Nitrogen to Ammonia.

The electrochemical NRRto synthesize ammoniais considered as a promising method due to its approvable advantages of zero-pollution emission, feasible reaction proceedings, good safety and easy management. The multiple efforts have been devoted to the exploration of earth-abundant-element-based nanomaterials as high-efficiency electrocatalysts for realizing their industrial applications. Among these, the Ni-based nanomaterials is prioritized as an attractive non-noble-metal electrocatalysts for catalyzing NRR because they are earth-abundance and exceedingly easy to synthesize as well as also delivers the potential of high electrocatalytic activity and durability. In this review, after briefly elucidating the underlying mechanisms of NRR during the electrochemical process, we systematically sum up the recent research progress in representative Ni-based electrocatalysts, including monometallic Ni-based nanomaterials, bimetallic Ni-based nanomaterials, polymetallic Ni-based nanomaterials, etc. In particular, we discuss the effects of physicochemical properties, such as phases, crystallinity, morphology, composition, defects, heteroatom doping, and strain engineering, on the comprehensive performance of the abovementioned electrocatalysts, with the aim of establishing the nanostructure-function relationships of the electrocatalysts. In addition, the promising directions of Ni-based electrocatalysts for NRR are also pointed out and highlighted. The generic approach in this review may expand the frontiers of NRR and provides the inspiration for developing high-efficiently Ni-based electrocatalysts.

High-Throughput screening of TM1TM2/N6–1 (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Mo) as effective electrocatalysts for nitrogen fixation

The electrochemical nitrogen reduction reaction (NRR) is an emerging nitrogen fixation method in recent years. Double-atom catalysts compensate for the disadvantage of single-atom catalysts having only one active site, improving catalytic reaction activity. Herein, using a four-step high-throughput screening strategy with density functional theory (DFT) calculations, 45 homonuclear and heteronuclear catalysts, namely, transition metal dimers anchored on nitrogen-doped graphene (TM1TM2/N6–1, TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Mo), were systematically studied to evaluate their catalytic performance for NRR. Our results showed that 16 catalysts stand out, among them, ScMo/N6–1 has an ultralow limiting potential of -0.09 V and a Faradaic efficiency of 100 %. This work provides a group of promising candidates and a reasonable high-throughput design strategy for NRR catalysts.

Synergistic effect of Bi and Fe for suppressing hydrogen evolution reaction (HER) and enhancing electrochemical nitrogen reduction reaction (eNRR) performance

The electrochemical nitrogen reduction reaction (eNRR) displays significant potential for the synthesis of ammonia. However, eNRR electrocatalysts with high catalytic activity and selectivity still need to be developed. In this study, a Bi-doped La0.9Bi0.1FeO3-δ (LBiF) perovskite was fabricated as a potential cathode catalyst and incorporated into a protonic ceramic electrolysis cell (PCEC) for eNRR. Electrochemical tests demonstrated that the LBiF cathode reveals higher eNRR catalytic activity and enhanced hydrogen evolution reaction (HER) inhibition compared to undoped LaFeO3 (LF). Characterization of the LBiF cathode following 90 h of NH3 synthesis revealed surface reconstruction of the catalyst during eNRR. Particularly, Bi3+ is partially reduced to Bi2+ and metallic Bi, resulting in A-site defects and an increased concentration of oxygen vacancies (OVs) and Fe4+ in accordance with the charge neutrality principle. Both the OVs and Fe4+ can accept electron pairs provided by N2 due to their unsaturated electronic structures. Furthermore, Bi doping inhibits the HER because Bi preferentially binds to N instead of H. Bi doping creates new Bi3+/Bi2+/Bi0 redox electron pairs, establishing an electron transfer path that improves perovskite conductivity and eNRR electrocatalytic activity. This study sets a foundation for designing eNRR catalysts aimed at suppressing the HER.

Main-Group Elements Enhance Electrochemical Nitrogen Reduction Reaction of Vanadium-Based Single Atom Catalysts Through d-p Orbital Hybridization.

Developing active sites with flexibility and diversity is crucial for single atom catalysts (SACs) towards sustainable nitrogen fixation at ambient conditions. Herein, the effects of doping main group metal elements (MGM) on the stability, catalytic activity, and selectivity of vanadium-based SACs is systematically investigated based on density functional theory calculations. It is found that the catalytic activity of V site can be significantly enhanced by the synergistic effect between MGM and vanadium atoms. More importantly, a volcano curve between the catalytic activity and the adsorption free energy of NNH* can be established, in which V-Pb dimer embedded on N-coordinated graphene (VPb-NG) exhibits optimal NRR activity due to its location at the top of volcano. Further analysis of electronic structures reveals that the unoccupancy ratio (eg/t2g) of V site is dramatically increased by the strong d-p orbital hybridization between V and Pb atoms, subsequently, N2 is activated to a larger extent. These interesting findings may provide a new path for designing active sites in SACs with excellent performance.

Modulating the Leverage Relationship in Nitrogen Fixation Through Hydrogen‐Bond‐Regulated Proton Transfer

In the electrochemical nitrogen reduction reaction (NRR), a leverage relationship exists between NH3‐producing activity and selectivity because of the competing hydrogen evolution reaction (HER), which means that high activity with strong protons adsorption causes low product selectivity. Herein, we design a novel metal‐organic hydrogen bonding framework (MOHBF) material to modulate this leverage relationship by a hydrogen‐bond‐regulated proton transfer pathway. The MOHBF material was composited with reduced graphene oxide (rGO) to form a Ni‐N2O2 molecular catalyst (Ni‐N2O2/rGO). The unique structure of O atoms in Ni‐O‐C and N‐O‐H could form hydrogen bonds with H2O molecules to interfere with protons being directly adsorbed onto Ni active sites, thus regulating the proton transfer mechanism and slowing the HER kinetics, thereby modulating the leverage relationship. Moreover, this catalyst has abundant Ni‐single‐atom sites enriched with Ni‐N/O coordination, conducive to the adsorption and activation of N2. The Ni‐N2O2/rGO exhibits simultaneously enhanced activity and selectivity of NH3 production with a maximum NH3 yield rate of 209.7 μg h−1 mgcat.−1 and a Faradaic efficiency of 45.7%, outperforming other reported single‐atom NRR catalysts.

In Situ Characterization Techniques for Electrochemical Nitrogen Reduction Reaction.

The electrochemical reduction of nitrogen to produce ammonia is pivotal in modern society due to its environmental friendliness and the substantial influence that ammonia has on food, chemicals, and energy. However, the current electrochemical nitrogen reduction reaction (NRR) mechanism is still imperfect, which seriously impedes the development of NRR. In situ characterization techniques offer insight into the alterations taking place at the electrode/electrolyte interface throughout the NRR process, thereby helping us to explore the NRR mechanism in-depth and ultimately promote the development of efficient catalytic systems for NRR. Herein, we introduce the popular theories and mechanisms of the electrochemical NRR and provide an extensive overview on the application of various in situ characterization approaches for on-site detection of reaction intermediates and catalyst transformations during electrocatalytic NRR processes, including different optical techniques, X-ray-based techniques, electron microscopy, and scanning probe microscopy. Finally, some major challenges and future directions of these in situ techniques are proposed.

Electrocatalytic nitrogen reduction in continuous-flow cell via water oxidation at ambient conditions: Promising for ammonia or diazene?

Electrochemical nitrogen reduction reaction (eNRR) is recognized as an alternative green approach to the traditional energy-demanding and fossil-based catalytic processes (e.g. Haber Bosch). In this study, we implement eNRR in a proton exchange membrane (PEM) water electrolyzer in which nitrogen (N2) is fed in the cathode. This operation mode has been suggested as a way to overcome mass transfer limitations, however, there is a lack of developed evaluation protocols for appropriate product identification. Herein, we exemplify the spirit of the evaluation protocols for gas phase operation at the device level with a combination of online product analysis and isotopic labeling. Our protocol involves control experiments by replacing the cathodic N2 feed with (i) inert gas (i.e. Ar) and (ii) isotopic labeled 15N2 and by replacing the anodic water feed with isotopic labeled D2O. Taking advantage of the gas phase operation in the cathode product analysis is realized with online techniques i.e. quadrupole mass-spectrometer (QMS) and Fourier transform infrared (FTIR) spectrometer. This allows us to verify the production of diazene (N2H2) resulted from genuine N2 reduction, rather than from nitrogen-containing contaminants. Our methodology provides a pathway for how the false positive results can be eliminated in the gas phase study and a platform for follow-up studies using promising or exotic catalysts in the cathode, especially to validate the eNRR products or discover more products.

Main‐Group Metal‐Nonmetal Dynamic Proton Bridges Enhance Ammonia Electrosynthesis

The electrochemical nitrogen reduction reaction is a crucial process for the sustainable production of ammonia for energy and agriculture applications. However, the reaction’s efficiency is highly dependent on the activation of the inert N≡N bond, which is hindered by the electron back‐donation to the π* orbitals of the N≡N bond, resulting in low eNRR capacity. Herein, we report a main‐group metal‐non‐metal (O‐In‐S) eNRR catalyst featuring a dynamic proton bridge, with In‐S serving as the polarization pair and O functioning as the dynamic electron pool. In‐situ spectroscopic analysis and theoretical calculations reveal that the In‐S polarization pair acts as asymmetric dual‐sites, polarizing the N≡N bond by concurrently back‐donating electrons to both the πx* and πy* orbitals of N2, thereby overcoming the significant band gap limitations, while inhibiting the competitive hydrogen evolution reaction. Meanwhile, the O dynamic electron pool acts as a “repository” for electron storage and donation to the In‐S polarization pair. As a result, the O‐In‐S dynamic proton bridge exhibits exceptional NH3 yield rates and Faradaic efficiencies (FEs) across a wide potential window of 0.3 V, with an optimal NH3 yield of 80.07 ± 4.25 μg h‐1 mg‐1 and an FE of 38.01 ± 2.02%, outperforming most previously reported catalysts.

A high-performance electrocatalyst for sustainable ammonia synthesis via single-atom Ru-embedded Fe2O3 nitrogen-DOPED carbon

Ammonia (NH3) is an important industrial chemical, but the production of NH3 by the conventional Haber-Bosch process consumes large amounts of energy and has a serious environmental impact. Electrochemical nitrogen reduction reaction (NRR) offers a promising green alternative, but challenges remain in developing efficient catalysts. Here, we report the synthesis of single-atom Ru-embedded metal–organic framework-derived Fe2O3 nitrogen-doped carbon (Ru/Fe2O3-NC) materials via plasma-enhanced chemical vapour deposition. Under ambient conditions, the Ru/Fe2O3-NC catalyst exhibited a high NH3 yield rate of 141.18 μg h−1 mgcat.−1 in 0.1 M K2SO4 with a Faraday efficiency (FE) of 48.89 %, which was superior to the pristine Fe2O3-NC catalyst. Online differential electrochemical mass spectrometry (DEMS) analysis confirmed the selective generation of NH3 and no by-product N2H4 was detected. The Ru/Fe2O3-NC catalyst also showed remarkable stability during prolonged electrolysis. This work demonstrates the synergistic effect of incorporating single-atomic Ru in the Fe2O3-NC framework and provides a new platform for the development of advanced catalysts for the efficient and sustainable electrochemical synthesis of NH3.

Improving the Catalytic Efficiency of Electrochemical Nitrogen Reduction Reaction (NRR) through Surface Modification and Nanostructured Bimetallic Catalysts

Electrochemical nitrogen reduction is recognized as a promising and sustainable alternative to the conventional Haber-Bosch method to produce ammonia. However, there are many challenges that pose significant barriers to the practical application of the nitrogen reduction reaction (NRR). These challenges are primarily attributed to the strong triple bond in the N2 molecule and the competing hydrogen evolution reaction (HER). This work investigates the synergy between alloys and nanostructured bimetallic systems and the application of molecular overlayers to improve efficiency and production rate of ammonia on the catalyst surface. We studied the combination of a noble metal catalyst scaffold modified with active metals (Li/Mg) that is known to cleave the N-N bond. Solvent effects were also tested in both aqueous and non-aqueous media to see changes in catalyst effects. Additionally, various experimental controls were performed including argon controls, isotope labeling, and assessing NOX contaminants to ensure that the reaction was interacting with our catalyst. Overall, this work demonstrates the use of molecular overlayers on nanostructured bimetallic systems to control hydrophobicity of catalyst surface and improve faradaic efficiency, ammonia production rate, catalyst durability and current density. Relationships between these parameters helped understand NRR mechanism and will help guide future NRR catalyst design.

Multiple Strategies for Producing and Utilizing Ammonia Using Electrochemical Methods Using Transition Metal Based Electrocatalysts

Ammonia is one of the most important chemicals in fertilizer and industrial chemical production, stands as the world’s second-largest synthetic chemical (> 200 million tons). Additionally, its potential as a clean energy source is gaining attentions due to its carbon neutrality, high energy density, and ease of transport. Electrochemical ammonia production emerges as a 'green' method which minimize the carbon footprint. However, the development of effective systems to produce and utilize 'green' ammonia remains a significant challenge [1,2].To enhance the efficiency of electrochemical systems, modifications in both catalyst and system design are mandatory. This presentation will introduce the electrochemical nitrogen reduction reaction (eNRR) using Cu-based catalysts [3]. For catalyst modification, we addressed copper’s challenge of its low affinity and activation potential for stable dinitrogen (N2) molecules. This was achieved through phosphorus activation, which enhance the adsorption and activation of N2 by creating electron-deficient Cu sites on the catalyst surface. System-wise, the poor solubility of nitrogen in aqueous electrolytes limits it’s the performance of the system. Addressing this, we applied a gas diffusion electrode (GDE) coated with polytetrafluoroethylene (PTFE) into the eNRR system. This setup establishes an efficient three-phase boundary among liquid water, gaseous N2, and the solid catalyst, thus facilitating nitrogen access to the catalytic sites. Our novel catalyst in a flow-type cell attains a Faradaic efficiency of 13.15% and an ammonia production rate of 7.69 μg h-1 cm-2 at -0.2 VRHE. These figures represent 3.56- and 59.2-times improvements, respectively, over a pristine Cu electrode in a conventional electrolytic cell.Additionally, the presentation will provide a brief introduction to the amorphous Ni-Cu electrocatalyst for electrochemical ammonia oxidation, which is an essential reaction for ammonia utilization [4]. We highlight its doubled ammonia removal efficiency compared to the Ni-Cu LDH electrocatalyst, underlining the potential of these developments.[1] Y. Zeng, C. Priest, G. Wang, G. Wu, Restoring the Nitrogen Cycle by Electrochemical Reduction of Nitrate: Progress and Prospects, Small Methods, 4 (2020) 2000672.[2] B. Yang, W. Ding, H. Zhang, S. Zhang, Recent Progress in Electrochemical Synthesis of Ammonia from Nitrogen: Strategies to Improve the Catalytic Activity and Selectivity, Energy Environ. Sci., 14 (2021) 672-687.[3] J. Kim, C.H. Lee, Y.H. Moon, M.H. Lee, E.H. Kim, S.H. Choi, Y.J. Jang, J.S. Lee, Enhancing Ammonia Production Rates from Electrochemical Nitrogen Reduction by Engineering Three-Phase Boundary with Phosphorus-Activated Cu Catalysts, J. Energy Chem., 84 (2023) 394-401.[4] J.H. Jang, S.Y. Park, D.H. Youn, Y.J. Jang. Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction.Catalysts., 13 (2023) 803.

(Invited) Breaking the Bottlenecks in Electrochemical Nitrogen Reduction

There is a burgeoning interest in the development of a green method of ammonia synthesis; ammonia, already critical for fertilisers in the agricultural industry, is also being touted as a possible future energy vector or carbon-free fuel. The current method of production - the Haber Bosch process - is environmentally damaging and energy intensive but to date no viable alternative has been demonstrated. An electrochemical method operating under ambient conditions would be particularly attractive, as it would enable ammonia to be produced on a decentralised basis on-site and on-demand.1 Thus far, amongst solid electrodes, only lithium and calcium based electrodes in organic electrolytes can unequivocally reduce nitrogen to ammonia.2-4 Even so, at present, there is ample room for improvement.To investigate this reaction, we use a combination of electrochemical experiments, cryo-microscopy, infrared spectroscopy, electrochemistry mass spectrometry, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy and density functional theory. By drawing from the adjacent fields of enzymatic nitrogen reduction and battery science, we will aim to build a holistic picture of the factors controlling nitrogen reduction.I will explore the role of the individual electrolyte components, including the cation, the salt, solvent and proton carrier.5- 7 On the basis of the insight, I will propose avenues towards higher rates and higher efficiencies in electrochemical nitrogen reduction.1 Westhead, O., Barrio, J., Bagger, A., Murray, J. W., Rossmeisl, J., Titirici, M.-M., Jervis, R., Fantuzzi, A., Ashley, A. & Stephens, I. E. L. Nature Reviews Chemistry 7, 184, (2023).2 Andersen, S. Z., Colic, V., Yang, S., Schwalbe, J. A., Nielander, A. C., McEnaney, J. M., Enemark-Rasmussen, K., Baker, J. G., Singh, A. R., Rohr, B. A., Statt, M. J., Blair, S. J., Mezzavilla, S., Kibsgaard, J., Vesborg, P. C. K., Cargnello, M., Bent, S. F., Jaramillo, T. F., Stephens, I. E. L., Norskov, J. K. & Chorkendorff, I. Nature 570, 504, (2019).3 Westhead, O., Jervis, R. & Stephens, I. E. L. Science 372, 1149, (2021).4 Fu, X., Niemann, V. A., Zhou, Y., Li, S., Zhang, K., Pedersen, J. B., Saccoccio, M., Andersen, S. Z., Enemark-Rasmussen, K., Benedek, P., Xu, A., Deissler, N. H., Mygind, J. B. V., Nielander, A. C., Kibsgaard, J., Vesborg, P. C. K., Nørskov, J. K., Jaramillo, T. F. & Chorkendorff, I. Nature Materials , (2023).5 Bagger, A., Wan, H., Stephens, I. E. L. & Rossmeisl, J. ACS Catalysis 11, 6596, (2021).6 Spry, M., Westhead, O., Tort, R., Moss, B., Katayama, Y., Titirici, M.-M., Stephens, I. E. L. & Bagger, A. ACS Energy Letters , 1230, (2023).7 Westhead, O., Spry, M., Bagger, A., Shen, Z., Yadegari, H., Favero, S., Tort, R., Titirici, M., Ryan, M. P., Jervis, R., Katayama, Y., Aguadero, A., Regoutz, A., Grimaud, A. & Stephens, I. E. L. Journal of Materials Chemistry A , (2023).

Nature's Catalyst: Investigating the Mechanisms behind Nitrogenase's N2rr Activity

Electrochemical nitrogen reduction reaction (eNRR), one of the most studied problems in catalysis, has the potential to replace the century-old Haber-Bosch process for ammonia synthesis. However, a low-cost, highly selective and efficient catalyst for eNRR is yet to be discovered. Nitrogenase, a naturally occurring biocatalyst responsible for nitrogen conversion to ammonia under ambient conditions, is the most efficient catalyst for eNRR. The Iron-Molybdenum cofactor (FeMo-co) in the enzyme is identified as the catalytic part with bridging irons being active sites. Despite decades of work the exact mechanism of its operation and various intermediates along the pathway are still debated. We revisit eNRR on FeMo-co and decipher the key mechanisms at play.Most studies on FeMo-co tend to ignore the magnetic nature of transition metals (TMs) in the cofactor. Proper treatment of these TMs (Fe and Mo) is necessary to pinpoint the intermediates and potential limiting step along the eNRR pathway. We start with an iterative optimization of the Hubbard U parameter to capture the correct magnetic structure of the cofactor. We also explore the effect of local chemical environment including, H-coverage and various ligands attached to the cofactor, on its activity and selectivity. The potential limiting step is found to be sensitive to the magnetic orientation of the TMs in the cofactor and it is thus imperative to provide the U correction to capture the true bottleneck of eNRR. The cofactor also shows an affinity towards a medium H-coverage under eNRR operating conditions. These findings provide a comprehensive and elaborate view of nitrogen reduction on FeMo-co and insights from this study paves the path towards designing heterogeneous counterparts to the catalyst.Acknowledgments:This work was supported by the U.S. Department of Energy, via Grant DE-FOA-0002676 to the SUNCAT Center for Interface Science and Catalysis.

Fe-Single-Atom Catalysts on Nitrogen-Doped Carbon Nanosheets for Electrochemical Conversion of Nitrogen to Ammonia

Electrochemical nitrogen reduction reaction (NRR) has been established as a promising and sustainable alternative to the Haber–Bosch process, which requires intensive energy to produce ammonia. Unfortunately, NRR is constrained by the high adsorption/activation of the N2 energy barrier and the competing hydrogen evolution reaction, resulting in low faradic efficiency. Herein, a well-dispersed iron single-atom catalyst was successfully immobilized on nitrogen-doped carbon nanosheets (FeSAC-N-C) synthesized from pre-hydrothermally derived Fe-doped carbon quantum dots with an average particle size of 2.36 nm and used for efficient electrochemical N2 fixation at ambient conditions. The as-synthesized FeSAC-N-C catalyst records an onset potential of 0.12 VRHE, exhibiting a considerable faradic efficiency of 23.7% and an NH3 yield rate of 3.47 μg h–1 cm–2 in aqueous 0.1 M KOH electrolyte at a potential of −0.1 VRHE under continuous N2 feeding conditions. The control experiments assert that the produced NH3 molecules only emerge from the dissolved N2-gas, reflecting the remarkable stability of the nitrogen–carbon framework during electrolysis. The DFT calculations showed the FeSAC-N-C catalyst to demonstrate a lower energy barrier during the rate-limiting step of the NRR process, consistent with the observed high activity of the catalyst. This study highlights the exceptional potential of single-atom catalysts for electrochemical NRR and offers a comprehensive understanding of the catalytic mechanisms involved. Ultimately, this work provides a facile synthesis strategy of FeSAC-N-C nano-sheets with high atomic-dispersion, creating a novel design avenue of FeSAC-N-C that can vividly have a potential applicability in the large spectrum of electrocatalytic applications.

In-Situ Spectroelectrochemistry for Elucidating the Stable Mars-Van Krevelen Catalytic Cycle for Green Ammonia Production on Nitride MXene

The electrochemical nitrogen reduction reaction (NRR) provides an opportunity to convert nitrogen (N2) to ammonia (NH3) at ambient conditions. Protons (H+), which stem from water oxidation, are necessary for this process, but result in the hydrogen evolution reaction (HER) reducing overall selectivity. Another cause for low selectivity in the NRR is the high energy required to activate the N≡N bond during typical adsorption-desorption mechanisms. An avenue to mitigate these challenges altogether is available on transition metal nitride materials through the Mars-van Krevelen (MvK) mechanism, where sub-lattice nitrogen atoms are protonated to form NH3, which then desorb to form nitrogen vacancies that require less energy to weaken the N≡N. Recently, we have provided preliminary evidence that the Ti2NTx MXene operates through this MvK mechanism. To further corroborate this mechanism, a series of electrochemical and in-situ spectroscopic techniques, including X-ray absorption spectroscopy (XAS) and Fourier-transform infrared (FTIR) spectroscopy, were utilized. We use XAS to track the Ti oxidation state shift as a function of applied potential to understand the role of the Ti in the MvK mechanism and to understand the dynamic change in the Ti-N-Ti bond network. Furthermore, we use FTIR to elucidate in real-time the adsorption of NRR reactive species and intermediates on the MXene surface, especially with regards to the periodicity of the -NH2 and N-N spectroscopic intensities, which allows for the MvK mechanism to be differentiated from the typical adsorption-desorption mechanisms. Finally, we use DFT to corroborate the experimental work and further enhance our understanding of the mechanisms of NRR. These findings serve as the foundation to produce green ammonia using earth abundant resources.

Physically-Based Descriptors of the Nitrogen Reduction Reaction on Transition Metal Nitrides

Due to the high energy intensity and consequent carbon footprint of the Haber-Bosch process for ammonia production, researchers have been intensely studying the electrochemical nitrogen reduction reaction (NRR) as a means to directly produce ammonia using clean energy with lower emissions of carbon by-products. One exciting class of materials for NRR catalysis are transition metal nitrides (TMNs), as they have been shown to have good stability, flexible electronic structures, and decent electrochemical activity [1]. Previous studies using density functional theory (DFT) have explored the energetics of known NRR pathways (associative, dissociative, and Mars-van Krevelen) on the mononitrides of all naturally occurring d-block metals in rock salt and zincblende structures [2]. The most favorable NRR mechanism (determined to be the pathway that contains the smallest maximum potential change across all steps of the reaction) on these materials was shown to be the Mars-van Krevelen mechanism. Uncovering electronic, structural, and dynamic descriptors that correlate well with the energetics of this favorable NRR mechanism on TMNs will be useful in mitigating the need to run full DFT calculations when expanding the search space of potential catalyst candidates, such as ternary transition metal nitrides (TTMNs) and perovskite transition metal oxynitrides (TMONs). Previous literature has shown that the proton adsorption energy is a good descriptor for catalytic NRR activity on TMNs [3], oxygen vacancy content [4] and the oxygen p-band center of bulk perovskites [5] are good descriptors for catalytic oxygen evolution reaction performance, and the metal d-band center of pure transition metal catalysts is a good descriptor for reactivity and adsorption energies [6]. Inspired by these results, this work uses DFT on a set of TMNs to calculate a variety of potential physics-based descriptors, including: metal d and nitrogen p-band centers, hybridization between nitrogen p and metal d orbitals, nitrogen vacancy content, surface Bader charges, and phonon band centers. Since the energetics of the NRR can be modulated by the properties of either the atoms in the bulk or in the top surface layers, we calculate all of our descriptors from atoms in the TMN bulk form as well as from surface and subsurface atoms in the TMN slab form.Our results reveal several correlations between the descriptors and material attributes (such as adsorption energies and vacancy formation energies), which in turn govern the energetics of the NRR on the TMNs. We find that the N2 adsorption energy is negatively correlated with the metal d-band center, the N adsorption energy is positively correlated with the nitrogen p-band center, and both the proton adsorption energy and nitrogen vacancy formation energy are negatively correlated with nitrogen p and metal d orbital hybridization. Interestingly, we also find that for some attributes, the subsurface layer atoms dictate the attribute values and not the surface layer atoms. This suggests that heterogeneous catalysis reactivity may be modulated by atomic properties not purely from the bulk or purely from the surface, but rather from multiple subsurface layers as well.Overall, the insights gained from our descriptor analysis can be used to constrain the search and design space of future potential NRR catalysts, such as TTMNs and TMONs. In doing so, the descriptors can be used in future works in either a high-throughput screening or machine learning framework to allow for the inverse material design of optimal NRR catalysts. These optimal catalysts can in turn lead to the emergence of electrochemical ammonia synthesis in industrial production.References Qiao Z, Johnson D and Djire A. Cell Rep Phys Sci 2021; 2.Abghoui Y and Skúlason E. Catal Today 2017; 286: 69-77.Abghoui Y, Garden AL, Hlynsson VF, Björgvinsdóttir S, Ólafsdóttir H and Skúlason E. Physical Chemistry Chemical Physics 2015; 17: 4909-4918.Marelli E, Gazquez J, Poghosyan E, Müller E, Gawryluk DJ, Pomjakushina E, Sheptyakov D, Piamonteze C, Aegerter D, Schmidt TJ, Medarde M and Fabbri E. Angewandte Chemie - International Edition 2021; 60: 14609-14619.Jacobs R, Hwang J, Shao-Horn Y and Morgan D. Chemistry of Materials 2019; 31: 785-797.Nørskov JK, Abild-Pedersen F, Studt F and Bligaard T. Proc Natl Acad Sci U S A 2011; 108: 937-943.

Trio strategy of harmonizing electronic structure, interface, and microenvironment on amorphous indium oxide nanofiber for selective electrochemical ammonia synthesis

Suppressing parasitic hydrogen evolution reaction (HER) remains a dilemma in developing aqueous electrochemical nitrogen reduction reaction (NRR). Nevertheless, previous studies have revealed the significant challenge of relying solely on electrocatalyst design to pursue selective NRR. Herein, we present a ‘Trio’ strategy to harmonize electronic structures of electrocatalysts, properties of interfaces, and configurations of microenvironments, thereby governing the intricate proton behaviors throughout the reaction, to suppress HER while boosting NRR. As proof-of-concept demonstration, the first designed amorphous In2O3-based nanofiber electrocatalyst, with optimized electronic state by oxygen vacancy and anchoring Mo species, is in conjunction with low-surface-energy monolayer interface and molecular-crowding microenvironment. Such rational synergy creates an advantageous catalytic configuration with decelerated proton diffusion and restricted proton transfer to active sites, thus achieving NH3 yield of 59.72 μg h−1 mg−1 and a FE of 30.60 %. We expect these findings will inspire “collaborative combat” strategies and desirable systems of NRR in the future.

Electrochemical Reduction of Nitrogen to Ammonia Using Zinc Telluride.

Electrosynthesis of ammonia (NH3), an important constituent molecule of various commercial fertilizers, is a promising and sustainable alternative strategy compared with the century-old Haber-Bosch process. Herein, zinc telluride (ZnTe) is demonstrated as an efficient electrocatalyst for reducing nitrogen (N2) under ambient conditions to NH3. In this simple chemical strategy, Zn preferentially binds N2 over hydrogen (H2), and Te, by virtue of its superior electronic properties, enhances the electrocatalytic activity of ZnTe. The analysis of the X-ray diffraction data using the Bravais-Friedel-Donnay-Harker (BFDH) theory predicted a crystal geometry with the active electrocatalytic sites predominantly confined to the (111) planes of ZnTe. The preferential binding of nitrogen (N2; adsorption energy = -0.043 eV) over hydrogen (H2, adsorption energy = -0.028 eV) to Zn on the (111) plane of ZnTe is further confirmed by density functional theory. The ZnTe catalyst is observed to be stable in the acidic medium and delivers a very high yield of NH3 (19.85 μg/h-1 mgcat -1) and a Faradaic efficiency of 6.24% at -0.6 V (versus RHE). Additional verification experiments do not reveal the formation of side products (such as NH2-NH2) during N2 reduction by ZnTe. Further, density functional theory calculations strongly predict that the electrocatalytic reduction of N2 to NH3 by ZnTe preferentially occurs via the alternate pathway.

Unraveling the Active Sites on Mesoporous CuFe2O4@N-Carbon Catalysts with Abundant Oxygen Vacancies and M-N-C Content for Boosted Nitrogen Reduction Toward Electrosynthesis of Ammonia.

Transition metal centers dispersed over nitrogen-doped carbon (M-NC) supports have been widely explored for electrocatalytic reactions; however, sparsely reported for electrochemical nitrogen reduction reaction (ENRR). Particularly, the single-atom catalysts (SACs) have shown reasonable ammonia yield rate and faradaic efficiency (FE), but their complex synthesis and low durability for long-term electrocatalysis runs restrict their use on a larger scale. Importantly, the catalytic active sites in metal nanostructured-based M-NC catalysts toward enhanced N2 adsorption and activation are still not clear as they are highly challenging to reveal. A few studies have predicted that the surface oxygen vacancies (Ovac) favor an enhanced ENRR performance. Herein, a strategy using tailored M-NC content and Ovac in a single catalyst for enhanced ammonia electrosynthesis is devised. A mesoporous bimetallic spinel oxide (CuFe2O4) supported over N-doped carbon (CuFe2O4@NC) derived from Prussian blue analog (PBA) via controlled pyrolysis possess is found to show boosted ENRR activity. Moreover, operando NH3 formation over the catalyst is observed using four electrode set up. This approach enables rapid evaluation ofelectrocatalytic efficacy and avoids false positive results. The rotating disc electrode results reveal that mass transport in acidic media and surface absorption in alkline media primarily regulate ENRR over CuFe2O4@NC electrocatalyst.