N2 Reduction Reaction Electrocatalysts Research Articles

Oxygen vacancy optimization of the titanium carbide MXene surface for enhanced electrochemical nitrogen reduction

Electrochemical N2 reduction reaction (NRR) provides a hopeful way for sustainable NH3 production. It is the key to realize efficient NRR reaction to optimize the structure and electronic configuration of catalyst materials. Increasing the number of active centers in Ti3C2Tx MXene allows for the development of effective NRR electrocatalysts. Herein, we rationally integrated defect engineering to create oxygen vacancy-rich Ti3C2-MXene (K-Ti3C2-400) as an efficient NRR catalyst with an NH3 yield of 32.25 ± 0.8 μg·h−1·mg−1cat. at − 0.55 V and a Faradaic efficiency of 12.85 % ± 0.26 % at −0.45 V versus reversible hydrogen electrode. According to the density functional theory, oxygen vacancies can prevent the hydrogen evolution reaction by delaying H adsorption, thereby activating absorbed N2 and encouraging *N2H synthesis. This research opens up new possibilities for creating MXene-based catalysts with surface reactivity and selectivity for electrochemical N2 fixation.

Tuning Surface Potential Polarization to Enhance N2 Affinity for Ammonia Electrosynthesis.

Electrocatalytic N2 reduction reaction (NRR) to synthesize ammonia is a sustainable reaction that is expected to replace Haber Bosch process. Laminated Bi2 WO6 has great potential as an NRR electrocatalyst, however, the effective activity requires that the inert substrate is fully activated. Here, for the first time, success is achieved in activating the Bi2 WO6 basal planes with NRR activity through Ti doping. The introduction of Ti successfully tunes the surface potential distribution and enhances the N2 adsorption. The subsequently strong hybrid coupling of d(Ti)-p(N) orbitals fills the electronic state of N2 antibonding molecular orbital, which greatly weakens the bonding strength of N≡N bonds. Further, in situ synchrotron radiation-based Fourier transform infrared(SR-FTIR) spectrum and theoretical calculations show that surface potential polarization enhances the adsorption of HNN* by Bi-Ti dual-metal sites, which is beneficial for the subsequent activation hydrogenation process. The Ti-Bi2 WO6 nanosheets achieve 11.44% Faradaic efficiency (-0.2V vs. RHE), a NH3 yield rate of 23.14µgmg-1 h-1 (15 N calibration), and satisfactory stability in 0.1M HCl environment. The mutual assistance of theory and experiment can help understand and develop of excellent two-dimensional (2D) materials for the NRR.

High throughput screening for electrocatalysts for nitrogen reduction reaction using metal-doped bilayer borophene: A combined approach of DFT and machine learning

NH3 plays an important role in industrial production, while it is still a challenge to develop a promising catalyst for NH3 synthesis. In particular, the metal borides have attracted much attention because of their excellent catalyst performances towards N2. In this paper, we have theoretically studied the N2 reduction reaction (NRR) by employing the transition metal (TM = Co, Cr, Cu, Fe, Mn, Mo, Ni, Sc, Ti, V, Zn, Nb, Pd, Rh, Ru or Zr) doped bilayer borophene (BB). After the screening, it is found that Sc-doped BB (Sc@BB) exhibits excellent NRR performance with the limiting potential of -0.43 V, and the hydrogen evolution reaction (HER) could be well suppressed. The limiting potential could be further improved into -0.40 V by applying 0.30 % uniaxial strains to the system. In addition, the machine learning method was employed to understand the catalytic mechanism, the results revealed that the NRR performance of TM@BB is strongly related to the inherent properties of TM atoms, such as the d-electron number, electronegativity, first ionization energy, atomic radius, and atomic number. Therefore, our paper not only proposed a promising electrocatalyst for NRR, but also further extend the application of transition metal-boron based two dimensional materials in NRR.

Graphitic carbon nitride supported trimeric metal clusters as electrocatalysts for N2 reduction reaction

As a cleaner and more energy-efficient alternative, the electrochemical reduction of N2 to NH3 under mild conditions has emerged as a promising avenue for environmental protection. However, developing catalysts with both high activity and selectivity has been a challenging task. The advent of atomically dispersed catalysts is revolutionizing the field of catalysis. Drawing inspiration from the successful fabrication of Ru trimer on graphitic carbon nitride (g-C3N4) nanosheets, herein, we adopted a series of trimeric metal clusters anchored on g-C3N4 monolayer (TM3@g-C3N4, TM = 3d, 4d and 5d transition metal atom), as well as utilized the criteria range to describe the stability, activity, energy cost and selectivity of supported trimers for N2 reduction reaction based on ab initio calculations combined with multi-level screening strategies. Finally, 5 candidate TM3@g-C3N4 (TM = Rh, Ru, V, Mo, and Re) exhibits decent catalytic activity with limiting potential ranging of –0.50 – –0.02 V. These spatially confined triatomic metal centers demonstrated proficiency in activating N2 molecules while exhibiting tunable surficial reactivity through modulation of the metal elements and interfacial electron coupling between trimers and underlying substrates. More importantly, a structure–activity relationship was established, which provides crucial insights for precise design of highly efficient atomically-precise catalysts at an atomic level for hydrogen fuel storage.

Dual-Atomic Porphyry Molecular Systems as Efficient Electrocatalysts for N2 Reduction Reaction: a Theoretical Investigation

Dual-Atomic Porphyry Molecular Systems as Efficient Electrocatalysts for N2 Reduction Reaction: a Theoretical Investigation

Transition metal doped and strain engineered VS−MoS2 as electrocatalysts for N2 electroreduction: A DFT study

S vacancy activated MoS2 (VS−MoS2) materials are greatly attractive as electrocatalysts in electrocatalytic N2 reduction reaction (NRR). Herein, we investigate the electrocatalytic NRR performance of transition metal (Cr, Mn, V, W) doped and strain engineered VS−MoS2 using first-principles calculations. For all the designed MoS2s, VS is easy to form due to the small defect formation energy (≤3.48 eV) and determined as active site for N2 adsorption and electrocatalytic NRR. Calculation results show that doping and strain regulate the electrocatalytic activity of VS−MoS2 by altering its electronic properties. When strain is absent, Cr-, Mn-, and V-VS−MoS2 exhibit better electrocatalytic NRR activity than undoped VS−MoS2, while W-VS−MoS2 shows slightly worse activity. However, the above situation changes with applying strain. Moreover, the catalytic activity of all the catalysts firstly decreases and then increases with increasing strain. Further comprehensive considering limiting potential (UL) and Faraday efficiency (FE), Cr-VS−MoS2 under 7 % strain is identified as the best NRR electrocatalyst with both excellent activity (UL = -0.18 V) and high selectivity (FE = 99.44 %). In addition, 5 % and 6 % strained Mn-VS−MoS2 are also good electrocatalysts for NRR with low UL and (-0.45 V and -0.23 V, respectively) and substantial FE (37.69 % and 19.29 %, respectively). Therefore, the electrocatalytic NRR performance of VS−MoS2 could be optimized through transition metal doping and strain engineering. Relevant experimental explores will be encouraged based on our theoretical findings.

Charge transfer and vacancy engineering of Fe2O3 nanoparticle catalysts for highly selective N2 reduction towards NH3 synthesis

The development of electrocatalysts for N2 reduction reaction (NRR) is significant for scalable and renewable NH3 synthesis, but calls for a technology innovation to overcome the specific problems of low efficiency and poor selectivity. Herein, we prepare a core–shell nanostructure by coating polypyrrole (PPy) onto sulfur-doped iron oxide nanoparticles (denoted as S-Fe2O3@PPy) as the highly selective and durable electrocatalysts for NRR under ambient conditions. Sulfur doping and PPy coating remarkably improve the charge transfer efficiency of S-Fe2O3@PPy, and the interactions between PPy and Fe2O3 nanoparticles produce abundant oxygen vacancies as active sites for NRR. This catalyst achieves an NH3 production rate of 22.1 μg h−1 mgcat−1 and a very-high Faradic efficiency of 24.6%, surpassing other Fe2O3 based NRR catalysts. Density functional theory calculations show that the S-coordinated iron site can successfully activate the N2 molecule and optimize the energy barrier during the reduction process, resulting in a small theoretical limiting potential.

Preparation of Cu atom doped LaFeO3/activated porous carbon composites and their electrocatalytic nitrogen reduction performance

The electrocatalytic N2 reduction reaction (NRR) under ambient conditions is a green and sustainable method for ammonia (NH3) synthesis, and the development of efficient electrocatalysts for NRR is a top priority. In recent years, LaFeO3 has been widely used in the field of catalysis because of its high stability, low cost, and green advantages. Through strategies such as heteroatom doping and carbon loading, we can effectively increase the content of oxygen vacancies and improve the electrical conductivity of the material to produce composites with unique electronic structures and excellent catalytic properties. In the present work, we prepared single-atom doped LaFeO3/activated porous carbon composites (LFC/AC) for electrocatalytic NRR. The NH3 yield and Faraday efficiency of LFC/AC were the highest at 23.876 μg h−1 mg−1 and 6.53% in 0.1 M Na2SO4 electrolyte solution, both of which were higher than those of LFC. A series of characterizations and tests have shown that LFC/AC has excellent stability, electrical conductivity, and electrocatalytic properties. The density flooding theory (DFT) simulations were performed to explore the main mechanisms to improve the NRR performance of the materials.

Cooperative electrocatalytic N2 reduction based on WS2-covered Co9S8 hexagonal heterostructures derived from ZIF-67@POMs

Electrochemical N2 reduction reaction (NRR) to produce NH3 at ambient conditions is a promising route to replace the energy-intensive Haber–Bosch process, but electrochemical NRR with satisfied efficiency remains challenging. Therefore, a number of efforts have been committed to develop high-performance NRR electrocatalysts for highly efficient NH3 synthesis. Herein, Co9S8 nanosheets with a WS2 outer layer (Co9S8@WS2) are designed and prepared from ZIF-67@polyoxometalates (polyoxometalates , H3PW12O40·xH2O) and used for NRR at ambient temperature and pressure. The successful introduction of S-vacancies can adjust the electronic structure of the material by forming a electron-deficient environment, can make N2 molecules easily adsorbed and activated on the W active site of the catalyst surface, and thus, can promote the electrocatalytic NRR activity of the catalyst. In 0.1 M KOH, the optimized Co9S8@WS2-900 exhibits enhanced Faradaic efficiency (46.81%) and good NH3 yield (50.25 μg h−1 mgcat.−1) at −0.4 V vs. a reversible hydrogen electrode, exceeding many recently reported cobalt- or tungsten-based materials. The superior electrocatalytic activity is attributed to the unique composite structures and synergistic effect of electronic couplings between Co9S8 and sulfur-rich vacancies WS2. A density functional theory calculation indicates that the inert N2 can be activated by bare W atoms on the rim of the S-vacancies in Co9S8@WS2. The protonation of N2 to form N–NH∗ species is the potential-limiting step (ΔG = 1.05 eV). Therefore, the unique bimetallic sulfide clad architecture and rich S vacancies of Co9S8@WS2 provide the reference and inspiration for better designing of high-efficiency nitrogen-fixing catalysts.

Defective two-dimensional Al2Si2O5(OH)4 as a promising nitrogen reduction reaction electrocatalyst

Two-dimensional (2D) materials are attracting increasing attention for the electrocatalytic nitrogen (N2) reduction reaction (NRR) owing to their unique properties; however, the development of low-cost 2D NRR electrocatalysts still constitutes a challenge. Cheap kaolinite (Al2Si2O5(OH)4) showing the repetition of silica sheets and gibbsite-type sheets can be potentially stripped to obtain a 2D material. In this study, on the basis of density functional theory calculations, the formation of monolayer Al2Si2O5(OH)4 (m-Al2Si2O5(OH)4) having oxygen (O) vacancies was established, and the rationality for forming 2D structure and O-vacancies was studied. The introduction of O-vacancies was found to improve the conductivity, and enabled it to accommodate the lone pair electrons of N2 for an enhanced N2 capture. Importantly, m-Al2Si2O5(OH)4 containing O-vacancies showed a low Gibbs free-energy change of 1.07 eV (overpotential of 0.91 V) at the potential-determined step and high NRR selectivity. This work demonstrates the potential high-performance and low-cost 2D NRR electrocatalyst.

Nitrogen reduction reaction mechanism on Fe-doped TiO2 from theoretical perspective: A kinetic and electronic structure study

TiO2 has been recognized as a promising electrocatalyst for N2 reduction reaction (NRR), the strategies to construct oxygen vacancies and doping of iron atoms have been implemented to improve its performance. Nevertheless, the inherent NRR reaction kinetics on the modified TiO2 is rarely reported. In this study, selected key NRR pathways governing its reaction mechanisms on Fe-atom doped-TiO2 (Fe-TiO2) and oxygen vacancy TiO2 (101) surface (OV-TiO2) were systematically studied by density functional theory (DFT) calculation. It demonstrates that both models experience the same NRR reaction path (distal). Fe-TiO2 (101) delivers significantly lower energy barriers in some key reaction steps, resulting in promoted NRR kinetics compared with OV-TiO2 (101). In addition, Fe-TiO2 (101) also shows better NRR selectivity with hydrogen evolution reaction than OV-TiO2 (101). The enhanced NRR reaction on Fe-TiO2 (101) is found contributed to its unique electronic structure. That is, the doped Fe atom acts as effective electron-rich active site for efficient N2 adsorption. Meanwhile, the doped-Fe atom would affect the electron distribution of the adsorbed N2 molecule, and more electrons tend to accumulate around the outward N, which promotes the following protonation process and accelerate the sequential NRR steps. These results provide new insights on NRR mechanism on TiO2-based catalysts.

V (Nb) Single Atoms Anchored by the Edge of a Graphene Armchair Nanoribbon for Efficient Electrocatalytic Nitrogen Reduction: A Theoretical Study.

Efficient and low-cost electrocatalysts are urgently required for the electrocatalytic N2 reduction reaction (NRR) to produce valuable NH3. Single-atom catalysts (SACs) represent one class of promising candidates. Besides the defects on the basal plane, very recently, the one-dimensional edge universally existing in the finite graphene or carbon sheet has gained attention as the anchoring site for SACs, which may enable unique catalytic mechanism. Herein, using first-principles calculations, we systematically investigated the NRR over SACs of transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Nb, Mo, and W) anchored by the N-modified edge of the graphene armchair nanoribbon (denoted as TM@GNR). Three criteria were employed to screen the best candidate from all the TM@GNR, including the high stability of TM@GNR, the preferable adsorption of N2 compared with H, and the lower applied potential for the first protonation of N2 compared with that of the active site. Accordingly, V(Nb)@GNR were theoretically demonstrated to be promising NRR electrocatalyst toward NH3 with low limiting potentials of -0.65 (-0.52) V, excellent selectivity of ∼100% (97%), and good stability. Particularly, NRR on the V@GNR and Nb@GNR precedes through a novel reaction mechanism with three spectator N2 molecules. Further analysis reveals that the strong capture and activation of N2 molecules by the edge-anchored V (Nb) atoms derives from their localized spin moment and atomic orbitals. Our studies emphasize the great potential of the edge of carbon materials to synthesize SACs for NRR and other reactions, and further reveal a novel NRR reaction mechanism on SACs.

Synergize curvature and confinement effects for Fe-, Co-, Ni- N2 sites on graphene nanobuds towards eNRR

Exploring efficient and highly selective ammonia synthesis electrocatalysts can advance the development of both modern agriculture and industry. Graphene nanobuds (GN) feature both fullerene and graphene characteristics which inspires new design ideas of electrocatalysts for N2 reduction reaction (NRR) to ammonia based on the novel GN with traditional Fe-, Co-, Ni-N2 sites. Among these structures, CoN2-GN is investigated as the most promising candidate with a limiting potential of -0.83 V through a distal pathway. The excellent activity and selectivity for the eNRR can be attributed to synergistic effects originated from both confinement space of buds (fullerene fragment) and curvature of the junctions. Especially, acting as an H-activator to N2, the ‘neck’ C atoms in the GNs improve the selectivity significantly for eNRR and facilitate the subsequent processes. This research would greatly contribute to develop the novel, efficient, multi-component graphene-based eNRR catalysts.

Regulating the interfacial charge transfer and constructing symmetry‐breaking sites for the enhanced N2 electroreduction activity

AbstractThe Haber–Bosch process for industrial NH3 production is energy‐intensive with heavy CO2 emissions. Electrochemical N2 reduction reaction (NRR) is an attractive carbon‐neutral alternative for NH3 synthesis, while the challenge associated with N2 activation highlights the demand for efficient electrocatalysts. Herein, we demonstrate that PdCu nanoparticles with different Pd/Cu ratios anchored on boron nanosheet (PdCu/B) behave as efficient NRR electrocatalysts toward NH3 synthesis. Theoretical and experimental results confirm that the highly efficient NH3 synthesis can be achieved by regulating the charge transfer between interfaces and forming a symmetry‐breaking site, which not only alleviates the hydrogen evolution but also changes the adsorption configuration of N2 and thus optimizes the reaction pathway of NRR over the separated Pd sites. Compared with monometallic Pd/B and Cu/B, the PdCu/B with the optimized Pd/Cu ratio of 1 exhibits superior activity and selectivity for NH3 synthesis. This study provides new insight into developing efficient catalysts for small energy molecule catalytic conversion via regulating the charge transfer between interfaces and constructing symmetry‐breaking sites.

Atomically Dispersed Uranium Enables an Unprecedentedly High NH3 Yield Rate

The low NH3 yield rate is a grand challenge for electrocatalytic N2 reduction to NH3. Herein, we report the first uranium single-atom catalyst (SAC) capable of catalyzing the electrochemical N2 reduction reaction (NRR). The uranium SAC features a low limiting potential (<0.5 V) and near-zero free energy changes for N2 adsorption and NH3 desorption. The integration of these merits enables the uranium SAC to afford an unprecedentedly high NH3 yield rate, 3-4 orders of magnitude higher than that of the Ru(0001) surface, which is widely recognized as an excellent NRR electrocatalyst. Further theoretical analysis reveals that the N2 reduction catalyzed by the uranium SAC is synergistically regulated by the d and f electrons of atomic uranium. This work proposes a promising solution (that is, atomically dispersed uranium) to the daunting challenge associated with the low NH3 yield rate, thus enabling the scalable production of NH3 via electrochemical N2 reduction.

Theoretical study of transition metal doped α-borophene nanosheet as promising electrocatalyst for electrochemical reduction of N2

Ammonia synthesis is an essential industrial production process. Through first principle calculations, this work investigated 3d/4d transition metal (TM) embedded α-borophene monolayer (BM-α) as efficient electrocatalyst for N2 reduction reaction (NRR). The computational results show that V, Cr, and Mo@BM-α exhibit excellent catalytic activity for ammonia synthesis via the enzymatic pathway. Analyses reveal that the unoccupied d orbitals of TM (V, C, and Mo) accept lone pair electrons from N2 while the anti-bonding orbitals of N2 accept a large amount of negative charge from TM, which lead to the elongation of the NN triple bond, thus effectively activating the nitrogen. In addition, V@BM-α and Mo@BM-α show a strong suppressive effect on hydrogen evolution reaction (HER) and overcome the possible oxidation on the catalyst surface. Our results demonstrate the α-borophene is a superior substrate for single atom catalysts which can be designed as high performance NRR electrocatalysts.

Density functional theory study of nitrogen-doped black phosphorene doped with monatomic transition metals as high performance electrocatalysts for N2 reduction reaction

Ammonia (NH3) is an essential resource in human production and living activities, and its demand has been rising in recent years. The catalytic synthesis of NH3 from N2 under mild conditions, inspired by biological nitrogen fixation, has piqued the interest of researchers. In this paper, density functional theory (DFT) calculations were used to investigate the catalytic activity, mechanism, and selectivity of the TM embedded nitrogen-doped phosphorene as high-performance nitrogen reduction reaction (NRR) electrocatalysts in depth. The results show that Nb- and Mo-doped catalysts present excellent catalytic performance, with low limiting potentials of −0.41 and −0.18 V, respectively. The Mo–N3–BP catalyst, for example, not only has an extremely low overpotential (−0.02 V), but also presents superior selectivity to effectively inhibit the HER competition reaction. A deeper look into the catalytic mechanism reveals a volcano relationship between the d-band center and the catalytic activity (Mo and Nb are located near the peak of the volcano-type curve). The d-band center and charge of the metal center can be regarded as effective descriptors for NRR activity on TM embedded nitrogen-doped phosphorene electrocatalysts, which hope to serve as a guiding principle for the design of high performance NRR single-atom catalyst in the future.

Atomic Molybdenum for Synthesis of Ammonia with 50% Faradic Efficiency.

The electrochemical dinitrogen (N2 ) reduction reaction (NRR) under ambient conditions has gained significant interest as an environmentally friendly alternative to the traditional Haber-Bosch process for the synthesis of ammonia (NH3 ). However, up to now, most of the reported NRR electrocatalysts with satisfactory catalytic activities have been hindered by the large overpotential in N2 activation. The preparation of highly efficient Mo-based NRR electrocatalyst in acidic electrolytes under ambient conditions is demonstrated here, consisting of stabilized single Mo atoms anchored on holey nitrogen-doped graphene synthesized through a convenient potassium-salt-assisted activation method. At -0.05V versus a reversible hydrogen electrode (RHE), an electrode consisting of the resultant electrocatalyst immobilized on carbon fiber paper can attain an exceptional Faradaic efficiency of 50.2% and a NH3 yield rate of 3.6µg h-1 mgcat-1 with low overpotentials. Density functional theory calculations further unveil that compared to the original graphene without holes, the edge coordinated Mo atoms and the existence of vacancies on holey graphene lower the overpotential of N2 reduction, thereby promoting the NRR catalytic activity. This work could provide new guidelines for future designs in single-atom catalysis that would be beneficial to ambient N2 fixation, and replacement of classical synthesis processes that are very energy-intensive.

In situ localization of BiVO4 onto two-dimensional MXene promoting photoelectrochemical nitrogen reduction to ammonia

The existing industrial ammonia synthesis usually adopts the Haber-Bosch process, which requires harsh conditions of high temperature and high pressure, and consumes high energy. Under this circumstance, photoelectrochemical (PEC) catalysis is regarded as a promising method for N2 reduction reaction (NRR), but bears problems of low efficiency and yield. Thus, exploring active catalysts remains highly desirable. In this work, BiVO4@MXene hybrids have been facilely synthesized by a hydrothermal route. The heterojunctions by the in situ growth of BiVO4 onto two-dimensional (2D) MXene greatly increase the NRR efficiency: under photoelectric conditions, the optimized NH3 yield is 27.25 µg h − 1 cm−2, and the Faraday efficiency achieves 17.54% at −0.8 V relative to the reversible hydrogen electrode (RHE), which are higher than most state-of-the-art NRR (photo) electrocatalysts. The mechanism speculation shows the enhanced light absorption range and the heterojunction formation largely promote the separation and the transfer efficiency of photogenerated carriers, thereby improving the PEC catalytic ability. Therefore, this work provides a hybrid route to combine the advantages of photo and electric catalysis for effective artificial nitrogen fixation.

In situ modification of the d-band in the core-shell structure for efficient hydrogen storage via electrocatalytic N2 fixation.

The electrochemical N2 reduction reaction (NRR) into NH3, especially powered by clean and renewable electricity, is a promising alternative to the capital- and energy-intensive Haber-Bosch process. However, the inert N[triple bond, length as m-dash]N bond and the frantic competition of the hydrogen evolution reaction lead to a poor NH3 yield rate and faradaic efficiency (FE). Here, we in situ construct a series of two-dimension core/shell V2O3/VN nanomeshes with a gradient nitride-layer thickness. Among them, V2O3/VN-2 exhibits the highest FE of 34.9%, an excellent NH3 yield rate of 59.7 μg h-1 mgcat. -1, and outstanding cycle stability, exceeding those of most of the NRR electrocatalysts reported to date. First-principles calculations reveal that the d-band center of VN shifts up in a nearly linear manner with the decrease of nitride-layer thickness, and V2O3/VN-2 with a d-band center closer to the Fermi level can strengthen the d-2π* coupling between the catalyst and N2 molecule, notably facilitating the N2-into-NH3 conversion.