Recent progress in single atomic catalysts for electrochemical N2 fixation
0 Abstract
Ammonia (NH3), as an important chemical product, is industrially produced using the traditional energy-intensive Haber-Bosch method at high temperature and pressure. Electrochemical nitrogen reduction reaction (NRR) to synthesize NH3 at ambient conditions has been considered as a promising candidate for replacing Haber-Bosch process. However, major obstacles, such as poor catalytic activity and selectivity and extensive competitive hydrogen evolution reaction, remain in NRR, which urgently need to be addressed. Single atom electrocatalysts (SACs) have attracted wide attention in view of their nearly 100% atomic utilization and outstanding catalytic performance. In this review, recent theoretical and experimental advances on novel atomically dispersed electrocatalysts for NRR are summarized and highlighted. We start with the fundamental reaction mechanism of NRR. Then, different preparation methods and the strategies of boosting catalytic performance of SACs from the aspects of coordination environment, coordination number, metal-support interaction and spatial microenvironment regulation are presented and analyzed in detail. Following this, the extensive applications of SACs in terms of noble metal based-SACs and transition metal-based SACs in NRR are discussed. Finally, we provide a perspective of the challenges of SACs for NRR, aiming to guide the rational design of advanced NRR catalysts.
Keywords
INTRODUCTION
Ammonia (NH3), as one of the most important chemical products, is widely used not only in the manufacture of fertilizers and biomedicines but also as the green energy carrier and an alternative fuel[1-3]. Up to now, the synthesis of industrial ammonia has mainly depended on the Haber-Bosch process, which is proceeding at high temperatures (300-500 °C) and high pressure (150-200 atm) with iron-based catalysts. However, this process requires high energy consumption and releases large amounts of CO2 gases (accounting for 1% of the world CO2 emission). For the sustainable development of energy and the environment, a green and economical process for the synthesis of NH3 is essential to replace the Haber-Bosch method.
Various strategies to synthesize ammonia under mild conditions have been developed in recent years, including photocatalytic nitrogen (N) fixation, electrocatalytic nitrogen fixation [nitrogen reduction reaction (NRR)] and biochemical catalytic nitrogen fixation. Among them, NRR has great potential to efficiently produce NH3 utilizing H2O as the hydrogen resource and renewable energies as the power sources. In addition, the electrochemical NRR process is able to simplify the reactor design without the employment of large equipment, thereby reducing the energy consumption during the transportation. In spite of the above advantages, the practical application of NRR is still hindered by its poor NH3 yield and Faradic efficiency (FE). Firstly, the breakage and activation of inert N≡N bond (960 kJ·mol-1) at ambient conditions is difficult[4]. Secondly, the competitive hydrogen evolution reaction (HER) always occurs preferentially in aqueous solution due to the smaller theoretical limiting potential of HER than that of NRR, which will cause the reducing of FE[5]. Therefore, the electrochemical system for NRR, such as electrocatalysts, electrolytes, and electrolytic cells, should be optimized to realize high NH3 yield and FE. As a key component, an advanced electrocatalyst with superior catalytic activity and high selectivity is essential to realize an efficient NRR process at ambient conditions.
It is well established that the homogeneous electrocatalytic process usually occurs on the surface of catalysts, making the limited active sites restricted to the surface of several atomic layers. Decreasing the size of catalysts to clusters or even single atoms can maximize the atomic utilization and thus greatly elevate the electrocatalytic performance[6] [Figure 1]. In 2011, Qiao et al. first reported the catalysts with individual Fe atomic species stabilized on the surface of iron oxides, which displayed excellent catalytic activity for CO oxidation[7]. Since then, the single atom catalysts (SACs) have attracted enormous attention and have been developed rapidly in the field of catalysis. Generally, the SACs have the following merits: (i) 100% atomic utilization and low-coordination environment make the SACs exhibit remarkable catalytic activity and selectivity toward diverse reactions; (ii) The strong force between metal active sites and support materials makes sure the atomical dispersion and stabilization of metal atoms[8]; (iii) The individual metal species enable the similar electronic structures and geometric configurations, which provide the opportunities to demonstrate the catalytic mechanism on the atomic level[9].
Figure 1. The surface atom ratios at different particle sizes[6]. Copyright 2023, ACS Publications.
In this review, we aim to provide an overview of the recent progress on the breakthroughs of the SACs. First, we demonstrate in detail the recently developed synthetic strategies to prepare SACs. Second, the effective strategies of catalytic activity enhancement for SACs in terms of coordination atoms, coordination number, metal-support interaction, and spatial microenvironment have been discussed to realize the precise design and preparation of SACs from the theoretical guidance. Third, we will summarize the breakthroughs that notable SACs have brought in NRR, including noble-metal SACs and nonnoble-metal SACs (Fe-based SACs, Mo-based SACs, and other metal-based metals). In addition, we discuss the challenges and perspectives for developing advanced SACs with desired catalytic performance toward NRR from the design principle.
FUNDAMENTAL PRINCIPLES OF ELECTROCATALYTIC NRR
Fundamentals of NRR
In the NRR process, the first bottleneck is the dissolution of N2 into an electrolyte in view of the inertness of N2 molecules, which results from the high binding energy (941 kJ·mol-1) of N≡N bond and lack of dipole. The second process is the adsorption of N2 molecules on the active sites and subsequent activation of N2, accompanied by accepting electrons from electrocatalysts. Thanks to the abundant empty d orbitals in the transition metals, they can receive the lone-pair electrons of N2 and then denote electrons into the antibonding orbitals of N2, thus activating N2 molecules. The above procedures are hindered by the adsorption of hydrogen protons in the electrolyte, which are more active than inert N2 molecules. The subsequent hydrogenation process is considered as the most difficult step due to the fact that the hydrogenation of N2 is a thermodynamic endothermic reaction (ΔH = +37.6 kJ·mol-1).
Furthermore, the severe competing HER process is an obstacle that cannot be ignored for NRR. The readily adsorption of hydrogen protons on the active sites with low energy hinders the NRR procedure at a large extent. Although the equilibrium potentials of NRR are close to HER at not only acid [Equations (1) and (2)] but also alkaline conditions [Equations (3) and (4)], the multi-proton/electron transfer mechanism for NRR largely hinders the reaction kinetics. Consequently, a majority of protons/electrons participate in the HER process. Much more negative equilibrium potential to form N2H verifies the difficulty of the first hydrogenation process. In contrast, the smaller redox potentials for the two-electron, four-electron, and six-electron reactions indicate that the subsequent hydrogenation steps are easier than the first-H addition process [Equations (5)-(8)].
Reaction pathways of NRR
The NRR process mainly occurs at mild conditions with H2O and N2 as proton and nitrogen sources, respectively [Figure 2A and B][10,11]. Generally, the NRR process over heterogeneous catalysts is divided into two reaction pathways, including dissociative and associative pathways[12]. For a dissociative mechanism, breakage of N2 molecules first occurs, leaving isolated nitrogen atoms on the surface of catalysts. Subsequently, the nitrogen atoms are successively hydrogenated to convert into NH3 [Figure 2C]. Due to the extreme stability of N2 molecules, the dissociative reaction pathway mainly occurred in the Haber-Bosch process, requiring huge energy consumption[13,14]. Different from a dissociative pathway, two N atoms strongly joined together before the first NH3 molecule was released in the associative mechanism. Considering different sequences of a hydrogenation process, an associative reaction pathway is further separated into a distal pathway and an alternating pathway. In the distal pathway, the distal N atom preferentially undergoes the hydrogenation process for the formation of *NNH and *NNH2 intermediates successively until releasing the first NH3 molecule, and then, the same procedure happens on another N atom to form the second NH3 molecule [Figure 2D]. In contrast, for an alternating mechanism, two N atoms were alternatively pronated to form *NHNH and *NH2NH2 intermediates and release two NH3 molecules finally [Figure 2E]. Therefore, hydrazine (N2H4), as a by-product, may be generated in the alternating pathway. Moreover, the enzymatic mechanism displays a similar reaction process with an alternating mechanism except that N2 molecules are adsorbed on the electrocatalyst with a side-on configuration[15].
Figure 2. (A) The H-type cell of NRR process[10]. Copyright 2023, Elsevier; Schematic illustrate of (B) NRR process[11]. Copyright 2023, ACS Publications; and (C)-(E) NRR mechanisms, including dissociative pathway, associative distal pathway and associative alternating pathway[12]. Copyright 2023, RSC Publishing. NRR: Nitrogen reduction reaction.
Recently, an emerging Mars-van Krevelen (MvK) mechanism has also been introduced for the transition metal nitrides (TMNs) (e.g., VN, ZrN, and NbN). Unlike above-mentioned mechanisms, the surface lattice N atoms of TMNs were hydrogenated to obtain the first NH3 molecule, generating N vacancies. Then, these N vacancies were supplemented by N2 molecules in the electrolyte and resulted in the formation of the second NH3 molecule, accompanied by the decrease of lattice N atoms. Compared to the dissociative and associative mechanisms, smaller equilibrium potential is required on the N2 activation procedure in the MvK mechanism, resulting in a more beneficial NRR process.
Catalytic activity descriptors of NRR
The FE and NH3 yield rates are two important descriptors for evaluating the catalytic activity of the NRR process. The NH3 yield rate refers to the amount of NH3 produced per unit time and unit catalyst mass or area, reflecting the synthesis production rate of NH3. The FE refers to the percentage of charges applied to the NRR process and the total charge flowing, implying the selectivity of NH3 synthesis to the competitive HER process.
The NH3 yield rate and FE can be calculated by Equations (9) and (10):
where CNH3 is the concentration of NH3, V is the volume of electrolyte, t is the time of NRR potentiostatic process, m is the mass of electrocatalyst, F is Faraday constant (96,485 C·mol-1), and Q is the total charge through the electrodes.
SYNTHESIS METHODS OF SACS
Selecting appropriate supports to ensure the uniform dispersion of individual metal sites and stabilize them without agglomeration is a prerequisite but a challenge for the preparation of SACs. Developing controllable preparation methods for dispersing atomically active sites on a suitable carrier is crucial to large-scale industrial deployment of SACs. Till now, various approaches have been developed for the fabrication of SACs, such as coordinative pyrolysis, ball milling, chemical/physical deposition, electrochemical deposition, wet-chemistry, defect engineering strategy, and so on.
High temperature pyrolysis method
High temperature pyrolysis methods are widely applied to synthesize SACs by decomposing the metal precursors at various gas atmospheres (Ar, N2 or NH3)[16-18]. Due to their advantages of simple operation and low cost, most carbon-based SACs were prepared using this method from various precursors, such as polymers, metal-organic frameworks (MOFs), metal complexes, and so on. For instance, Wu et al. mixed o-phenylenediamine and SiO2 powder with Fe3+ ions and then pyrolyzed the mixture at 800 °C under Ar temperature to generate the FeN4 electrocatalysts [Figure 3A and B][19]. K-edge X-ray absorption near edge structure (XANES) spectra demonstrated that no Fe–Fe bonds existed in FeN4 SACs. Wu et al. fabricated a salt-template method by calcinating the mixture of zeolitic imidazolate framework (ZIF)-67 and molten KCl salt at high temperature to prepare the cobalt active sites supported on nitrogen (N)-doped carbon substrate (SCoNC), which achieves a considerably high mass loading of Co atoms (≈ 15.3%) [Figure 3C and D][20]. Rong et al. also used the pyrolysis method to prepare Ni-N3O SACs; then, the Ni-N3-V SAC with vacancies could also be obtained by removing the oxygen atoms at 800 °C in view of the weaker Ni-O interaction[21]. Yuan et al. reported the nitrogen and phosphorous coordinated Fe SAC (FeN3P) via calcinating the polypyrrole (PPy) hydrogel obtained by the polymerization of pyrrole with the assistance of phytic acid and FeCl3 [Figure 3E and F][22]. Besides, this method was also extended to design and prepare dual-metal atom catalysts (DACs). Wang et al. first synthesized Fe, Ni@ZIF-8 via the ion exchange strategy and then obtained the Ni, Fe-DACs/N-doped carbons (NCs) after calcinating at 920 °C for 3 h [Figure 3G and H][23]. Tamtaji et al. reported a series of CoN4, NiN4, FeN4 SACs, and FeNiN8 DACs anchored on graphene oxide (GO) nanosheets[24].
Figure 3. (A) Schematic illustration of the synthesis of FeN4 SAC; (B) FT k3-weighted χ(k)-function of the EXAFS spectra at Fe K-edge[19], Copyright 2023, Springer Nature; (C) Schematic illustration of the synthetic procedure of the SCoNC catalysts; and (D) SEM image of the SCoNC catalyst[20]. Copyright 2023, Wiley; (E) Schematic of the synthesis process of the Fe-N/P-C catalyst; (F) Enlarged image of Fe-N/P-C, partial single Fe atoms are circled in yellow and EELS atomic spectra of Fe, N, and P elements[22], Copyright 2023, ACS Publications; (G) Synthesis and morphology of Ni,Fe-SAs/NCs; and (H) BF-STEM image and corresponding HAADF-STEM image[23]. Copyright 2023, ACS Publications. SAC: Single atom electrocatalyst; EXAFS: extended X-ray absorption fine structure; SEM: scanning electron microscopy; EELS: electron energy loss spectroscopy; NCs: N-doped carbons; BF-STEM: bright field scanning transmission electron microscopy; HAADF-STEM: high-angle annular dark field scanning transmission electron microscopy.
MOF-derived SACs
MOFs, as porous crystalline materials assembled by metal nodes and organic ligands, have boomed in the last decades due to their large surface area, definite crystalline structure, chemical tunability, and abundant metal sites[25-28]. After high temperature pyrolysis using MOFs as precursors, carbon-based SACs can be obtained, and the characteristics of MOFs are also inherited. Especially, the nitrogen elements from the parent MOF enable the formation of strong metal-nitrogen coordination bonds during calcination, thus effectively preventing the aggregation of metal atoms[29,30]. Undoubtedly, the MOF-derived nitrogen porous carbon can be applied as an ideal support to stabilize the atomic metal species. Zhao et al. prepared the NC supported Ni single atoms (Ni SAs/NC) via the ionic exchange between Zn nodes and the adsorbed Ni ions within the ZIF-8 [Figure 4A][31]. During the carbonization process, the Ni nodes were reduced to Ni SACs, accompanied by the evaporation of low-vaporizing Zn elements. The coordination bonds between NC and metal atoms ensure the tight fixation of metal atoms. Subsequently, using a similar strategy, Fe, Ni single-atom pairs immobilized on NC (Fe1-Ni1-NC) have been synthesized by Jiao et al. [Figure 4B][32]. In the
Figure 4. (A) Scheme of the formation of Ni SAs/N-C[31]. Copyright 2023, ACS Publications; (B) Schematic Illustration for Construction of Fe1-Ni1-N-C with Neighboring Fe and Ni Single-Atoms Based on ZIF-8[32]. Copyright 2023, ACS Publications; (C) Schematic illustration of synthetic route of Pd1@ZrO2[33]. Copyright 2023, ACS Publications; (D) Schematic illustration showing Ni2+ decoration in UiO-66-NH2 via an efficient microwave-assisted method and the subsequent coordination environment modulation of single Ni Atoms to afford Ni1-X/MOF[34]. Copyright 2023, ACS Publications; (E) Schematic illustration showing the synthesis of Al-TCPP-Pt for photocatalytic hydrogen production[35]. Copyright 2023, Wiley. SAs: Single atoms; ZIF: zeolitic imidazolate framework; MOF: metal-organic framework.
Apart from ZIF, other MOFs are also employed as expected platforms to stabilize single atoms. Zhao et al. designed a two-step strategy to prepare SACs with the single Pd sites anchored on a 3D zirconium oxide nanonet, denoted as Pd1@ ZrO2[33]. First, the NC supported single Pd sites and ZrO2 nanoparticles were obtained by calcination of Zr-based Uio-66-NH2 under Ar atmosphere. Then, carbonaceous skeletons etched by air and ZrO2 were welded into a porous nanonet with rich oxygen defects, which is beneficial for capturing the Pd1 species [Figure 4C]. Ma et al. reported a microwave method to fabricate Ni SACs with large metal loading (> 4 wt%) and tunable coordination environment, which were also expanded to Co and Cu SACs, respectively[34]. With the help of uncoordinated -NH2, the Ni atoms were stably immobilized on the Uio-66-NH2. By varying the treatment procedures, the coordination environments of SACs are well optimized to obtain the Ni1-X/Uio-66-NH2 (X means S, O, or Sox) [Figure 4D]. Among those, the sulfur-coordinated Ni centers exhibit the best photocatalytic activity due to their reductive oxidation state. The Pt active sites were also successfully stabilized in Al-TCPP via the tight bonding between the pyrrolic N of Al-TCPP and Pt atoms [Figure 4E]. The obtained Pt SACs give rise to a H2 turnover frequency (TOF) of 35 h-1, which is 30 times that of Pt nanoparticles stabilized by the same Al-TCPP. The confinement of Pt atoms into MOFs, which offers convenient electron transfer pathways and enhanced hydrogen binding energy, can account for the outstanding photocatalytic performance[35]. MOF-based SACs combined the advantages of MOF and SACs, exhibiting outstanding performance in various catalytic reactions. Nevertheless, developing a facile and economic synthesis procedure for MOF-based SACs and further increasing the mass loading of SACs remains challenging.
Impregnation and coprecipitation strategy
The impregnation and coprecipitation strategy have been the widespread method for synthesizing SACs due to its simpleness, high repetition, and no requirement on special equipment. The typical coprecipitation method generally includes the following steps: (i) mixing anions and cations in the solution; (ii) nucleation and growth; (iii) aggregation and coprecipitation. The obvious advantage of this method is that two or more cations can be simultaneously precipitated during the chemical reactions, and one or more metal atoms can be uniformly distributed in the catalysts. Nevertheless, during the precipitation process, the aggregation of metal atoms usually occurs, which is influenced by preparation conditions, including pH, temperature and cation concentration[36,37]. Therefore, ensuring the metal atomic dispersion remains a challenge. Yang et al. prepared the atomically dispersed Ru decorated on two-dimensional NiFe LDH nanosheets (SARu/NiFe LDH) through wet impregnation[38]. In the process, the RuCl3 as a metal precursor and NaBH4 as a reduction agent were mixed with the NiFe LDH. It is confirmed that Ru single active sites were stabilized on NiFe LDH through the Ru–O coordinated bond. Using this strategy, the Au/Ce0.5Zr0.5O2 catalysts were also prepared. In a typical process, Ce0.5Zr0.5O2 support was impregnated in the HAuCl4 solution with adjusting the pH to 7 by the Na2CO3, and HAuCl4 was precipitated and loaded onto the Ce0.5Zr0.5O2 support[39].
Single atom alloys
Single atom alloys (SAAs), with the individual metal sites anchored on another metal host, have emerged as a new hotspot because they simultaneously have the merits of SACs and metal alloys[40,41]. Besides that, the alloying effect of SAAs results in strong metal-support interaction, enabling the regulation of the d-band structure of active centers and the balancing of the dissociation and adsorption of intermediates, leading to expected catalytic activity and high selectivity[6,42]. More importantly, the tenacious alloy bonding between isolated metal atoms and the metal host would effectively inhibit the agglomeration of metal atoms during the catalysis process and allow for powerful control over the individually atomic dispersion.
Using this strategy, a series of SAAs catalysts applied in various electrocatalytic reactions have been fabricated in recent years[43-46]. Wang et al. prepared bimetallic Pd@Au electrocatalysts by decorating Au nanoparticles with controlled Pd atoms via the reduction of PdCl2 with 5% H2/N2 at room temperature
Figure 5. (A) Illustration of the synthetic scheme for the Pd@Au nanoparticles with control over the dose of Pd[46]. Copyright 2023, ACS Publications; (B) Schematic illustration of the fabrication procedures of atom-pair structured Cu anchored on Pd10Te3 nanowires[48]. Copyright 2023, Springer Nature; (C) Atomic-resolution HAADF-STEM image. Au atoms are uniformly distributed throughout the particle. Scale bar: 1 nm[47]. Copyright 2023, Wiley; (D) HAADF-STEM and EDX-mapping images of Pd-CeO2 nanorod[49]. Copyright 2021, Springer Nature. HAADF-STEM: High-angle annular dark field scanning transmission electron microscopy.
Electrochemical deposition
Electrochemical deposition strategy has been widely employed to fabricate SACs on support materials due to its simple and clean characteristics. Zhang et al. proposed a universal electrochemical deposition strategy to synthesize SACs on different matrices, including metal, metal hydroxide, or metal sulfide. Through controlling the deposition times and cycles, the concentration of metals can be varied[50]. Zhang et al. prepared NiFe LDH on the surface of Ti foil first, which was subsequently employed as the working electrode in the HAuCl4 solution. After stepping the potential to -0.6 V for 5 s and to -0.2 V for 5 s repeated for five cycles, isolated Au atoms were anchored on NiFe LDH[51]. Xu et al. reported the Cu single atoms immobilized on sulfur sites of graphite foam via an underpotential deposition strategy [Figure 6A]. By varying the electrodeposition potentials and the sulfur contents of graphite foam, the loading amount of Cu SAs can be regulated from 0.043% to 0.3% [Figure 6B][52]. In addition, the counter electrode containing the target metals can also directly act as the counter electrode for the deposition. For example, Mo2TiC2Tx MXene was employed as the platform to deposition Pt with the Pt foil as the counter electrode. During the process, MXene is originally exfoliated to nanosheets with the Mo atoms leaving the MXene, and the Pt atoms that detached from the Pt foil simultaneously moved to the working electrode and filled into the Mo vacancies [Figure 6C and D][53]. Using the same method, Jiang et al. successfully removed the Co atoms from the np-Co0.85Se to form Co vacancies with Pt foil as the counter electrode in 0.5 M H2SO4 solution which act as a trapper for embedding the Pt atoms[54].
Figure 6. (A) Schematic illustration of electrochemical synthesis/analysis of Cu SACs; (B) HAADF-STEM images of Cu SAs and Cu nanoparticles by adjusting the deposition potential and concentration of Cu precursors[52]. Copyright 2023, Elsevier; (C) HAADF-STEM images of Mo2TiC2Tx-PtSA and corresponding simulated image; (D) Schematic of the electrochemical exfoliation process of MXene with immobilized single Pt atoms[53]. Copyright 2023, Springer Nature. SACs: Single atom electrocatalysts; HAADF-STEM: high-angle annular dark field scanning transmission electron microscopy; SAs: single atoms.
Atomic layer deposition
Atomic layer deposition (ALD) is a recently emerging method to prepare SACs, which needs the target metal precursors in the gas state and to be carried in the substrate in the reaction chamber. The size and thickness of deposition layers could be well tuned by regulating the deposition times and cycles[55-57]. Song et al. prepared individually Ir modified Pt nanoparticles supported on the NC nanotube (IrSA-Pt/NCNT) via an ALD approach. The IrSA-Pt catalyst, with the combination of SA and nanoparticle (NP) cooperation structure, shows promoted catalytic activity and stability toward both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)[58]. Using this strategy, atomically dispersed Co on Pt catalysts were also prepared by Zhang et al.[59]. Li et al. anchored atomically dispersed Pt on C3N4 nanosheets with ALD strategy. By adjusting the deposition cycles, they controlled the size of Pt to obtain Pt SAs, Pt clusters and Pt nanoparticles, respectively[57]. Besides that, the ALD strategy can also be used to synthesize the 2D materials[60,61]. Yan et al. fabricated two atomic layer thickness of Pt layers onto the graphene by depositing the second atomic layer on the former first atomic layer[61].
HOW TO REGULATE THE CATALYTIC PERFORMANCE OF SACS
The microenvironments of SACs, such as coordination environment, band structure, and geometric construction, are decisive on regulating chemical properties and optimizing catalytic performance of SACs. Therefore, constructing SAC catalysts with well-controlled configuration and homogeneous dispersion is essential to exploring the relationship between the intrinsic activity of the catalysts and the structure characterization.
Coordination components
It is well known that the catalytic activity of catalysts is largely decided by the local metal coordination environment[62-64]. Clarifying the structure-function relationship between the coordination environment of active metal sites and the catalytic activity is crucial to achieving high performance SAC catalysts and demonstrating the reaction mechanism. The active metal sites were generally coordinated by nonmetal atoms, such as N, O, S, P, etc.[22,65-67]. Therefore, regulating and optimizing the coordination nonmetal atoms and coordination number will reconstruct the coordination environment and thus adjust the electronic and band structure of metal sites. As a result, the catalytic activity would be effectively improved by precisely designing the environment atoms of SACs.
Based on above theoretical guidance, researchers have denoted enormous efforts in verifying the role of micro coordination environment in the electrochemical performance from the experiment point[68]. Zhang et al. have introduced N species into the isolated Ni atoms[69] [Figure 7A and B]. They reported that the Ni-N coordination has lower Fermi level and stronger adsorption of intermediates compared with Ni-C coordination [Figure 7C]. Jiang et al. also reported a similar phenomenon in the FeN4 SACs, in which the
Figure 7. (A) The atomic resolution HAADF-STEM image of HCM@Ni-N; (B) Ni K-edge EXAFS spectra and Fourier transform of the experimental EXAFS spectrum in K space of HCM@Ni-N; (C) Schematic band diagrams of HCM@Ni and HCM@Ni-N[69]. Copyright 2023, Wiley; (D) Differential charge densities of CuN2O2 and CuN4; (E) The limiting potentials of the products of ECR and HER on the CuN2O2, CuN4, and Cu(111)[71]. Copyright 2023, Springer Nature; (F) Schematic diagram of the preparation process of Ni-B/N-C; (G) The FT EXAFS spectra of Ni foil, NiO, NiPc, Ni-B/N-C, and Ni-N-C; (H) Coordination environment of Ni-B1N3 moiety; (I) Gibbs free energy diagram for ORR process of the Ni-N4 and Ni-B1N3 moiety[72]. Copyright 2023, Wiley; (J) The scaling relationships for the adsorption energy of NH*-N2H* and Linear correlations between the adsorption energy of N2H* (EN2H*) and limiting potential of the NRR[73]. Copyright 2023, RSC Publishing. HAADF-STEM: High-angle annular dark field scanning transmission electron microscopy; EXAFS: extended X-ray absorption fine structure; ECR: CO2 reduction reaction; HER: hydrogen evolution reaction; ORR: oxygen reduction reaction; NRR: nitrogen reduction reaction.
Coordination number of active sites
Besides, regulating the coordination number of isolated active centers has also been a feasible means of modulating the intrinsic activity of SACs[21,74-76]. Liu et al. have synthesized dispersed Fe active sites with different coordination numbers (FeNx, x = 4-6) by controlling the treatment temperature [Figure 8A-C][74]. It is demonstrated that the medium-spin FeIIIN5 affords the highest TOF for oxidation of C-H, which is one order of magnitude of the FeIIIN6 structures and three times more active than the FeIIN4 structure, respectively. Rong et al. also fabricated vacancy-defect Ni-N3-V sites anchored on the GO nanosheets
Figure 8. (A) HAADF-STEM images of Fe-N-C; (B) The TOF of different sites; (C) The normalized XANES spectra at the Fe K-edge of different samples[74]. Copyright 2023, ACS Publications; (D) Specific current density of CO for Ni-N3V, NiN4 and NC; (E) The optimized structures of Ni SACs and the proposed reaction pathways of Ni-N3-V for CO2 electroreduction to CO[21]. Copyright 2023, Wiley; (F) Energy profiles for the discharging process from S8 to Li2S on the five catalyst models; (G) Energy profiles for the reduction of Li2S on Ni-N5/C[77]. Copyright 2023, ACS Publications. HAADF-STEM: High-angle annular dark field scanning transmission electron microscopy; TOF: turnover frequency; XANES: X-ray absorption near edge structure; SACs: single atom electrocatalysts.
Metal-support interaction
Electronic metal-support interaction is a promising approach to regulate the electronic structure of metal active sites via the covalent bond effect with the supports[78,79]. In these materials, orbital hybridization and electron transfer occur on the metal-support interfaces, which can form new bonds and effectively modulate the d-electron status of metal active sites[80-82]. The related electron configuration will enhance the chemisorption and activation of reaction species and hence lower the energy barrier in the overall reaction process, thus facilitating the catalytic performance. For instance, Hu et al. realized the stabilization of isolated Pd species on different crystals of CeO2 [Figure 9A][83]. X-ray absorption spectroscopy (XAS) results demonstrate that Pd species are well trapped by the oxygen vacancy of the (100) facet for CeO2, whereas Pd atoms would aggregate into clusters on the (111) facet [Figure 9B]. Shi et al. have immobilized Pt single atoms on various transition metal sulfides (TMDs) supports (TMDs: MoS2, WS2, MoSe2, and WSe2) to reveal the structure-activity relationship of SACs. They found that the oxidation state of Pt sites could be well controlled through the electronic metal-support interaction, delivering positive relationship with the Pt-OH interaction. Meanwhile, the alkaline HER activity showed a volcano-type relationship [Figure 9C-F][84]. Recently, Qu et al. have proposed a facile strategy to contrast SACs by trapping transition metal atoms (Fe, Co, Ni and Cu) with the dangling bonds on GO. The electron transfer between M0 and the oxygen-containing functional groups gives rise to the isolated Mδ+ (0 < δ < 3) species by forming M-O bands
Figure 9. (A) Structural models and HAADF-STEM images of Pd1/CeO2(100) and Pdn/CeO2(111); (B) Fourier transforms of k3-weighted Pd K-edge EXAFS spectra[83]. Copyright 2023, Wiley; (C) Pt 4f XPS spectra of the Pt-SAs/TMDs samples and commercial Pt/C; (D) The fitted average oxidation states and d-band hole counts of Pt from XANES spectra; (E) First-shell fitting of EXAFS spectra of Pt foil, PtO2, Pt-SAs/MoS2, and Pt-SAs/MoSe2; (F) Relationship of average oxidation state, Pt-OH interaction and alkaline HER activity of PtSAs/TMDs[84]. Copyright 2023, Springer Nature; (G) Schematic illustration for the preparation of Fe SAs/GO; (H) TEM and (I) HAADF-STEM images of Fe SAs/GO; (J) EXAFS spectra of Fe SAs/GO[85]. Copyright 2023, Wiley. HAADF-STEM: High-angle annular dark field scanning transmission electron microscopy; EXAFS: extended X-ray absorption fine structure; TMDs: transition metal sulfides; XANES: X-ray absorption near edge structure; SAs: single atoms; HER: hydrogen evolution reaction; GO: graphene oxide; TEM: transmission electron microscopy.
Spatial microenvironment regulation
Spatial microenvironment has been considered as an effective strategy to synthesize SACs with regulated catalytic activity and selectivity. Hu et al. developed a MOF-assisted spatial confinement strategy for confining Ru atoms into the N-doped porous carbon, having an important effect on the internal affinity of Li2O2 [Figure 10A][86]. Liu et al. have stabilized individually dispersed Mo into the pores of the molecular sieve ZSM-5 matrix[87]. The integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) image verifies the dispersion of Mo single species [Figure 10B and C]. The Pt atoms were also successfully anchored in Al-TCPP via the tight bonding between the pyrrolic N of Al-TCPP and Pt atoms[35]. Chen et al. have embedded the N-heterocyclic carbene (NHC)-ligated Cu SAs into UiO-67. The σ donation of NHC riches the electron density of SAs and promotes the adsorption of carbon containing intermediates during the CO2RR process [Figure 10D and E][88].
Figure 10. (A) Scheme of the formation of Ru SAs confined on NC[86]. Copyright 2023, ACS Publications; (B) A representative iDPC-STEM image of Mo/ZSM-5; (C) Zoomed-in three scenarios: empty channel, channels containing a MoO3H cluster bound at the T8 site and at the T1 site[87]. Copyright 2023, Wiley; (D) Molecular modelingof 2Bn-Cu@UiO-67; (E) Calculated free-energy diagrams for CO2RR over 2Bn-Cu@UiO-67 and Cu (111)[88]. Copyright 2023, Wiley; (F) HAADF image of A-CoPt-NC and Model of the configuration of the two metal atoms trapped in the defect[27]. Copyright 2023, ACS Publications; (G) Illusion of carbon vacancies sites and Pd single-atom-site in g-C3N4[89]. Copyright 2023, Elsevier; (H) illusion of Fe1-Nv/CN ultrathin nanosheets, and CIP Degradation curves of CN,
Defects or vacancies are also usually applied to stabilize the single atoms, which play a positive effect in optimizing the electrochemical performance of SACs. Controlled design of defects or vacancies on support is considered as an effective way to prepare SACs with regulated microenvironment. Zhang et al. have implanted atomic PtCo on the defect C/N graphene surface to generate Co-Pt-N-C coordination structure as active centers for ORR [Figure 10F][27]. Liu et al. have confined isolated Pd species on C3N4 by the abundant carbon vacancies for photocatalytic NO conversion. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and XAFS analyses confirm that the Pd atoms prefer to be trapped on carbon vacancy sites. The activity of Pd-Cv-CN is approximately 4.4 times of Pd loaded on pristine g-C3N4 [Figure 10G][89].Su et al. also confined isolated Fe sites with N vacancies and proved that the N vacancies act as electron trap sites to form highly concentrated electron density around Fe sites for improving the H2O2 conversion efficiency [Figure 10H][90]. Kumar et al. have realized the ultrahigh loading of Rh atoms (6.6 wt%) on Co3O4 nanolayers with tensile strain (S-Co3O4) obtained through liquid N2-quenching. A significantly increased migration energy barrier of RhSA on the S-Co3O4 is revealed than on pristine Co3O4, which inhibits the migration and aggregation of Rh atoms[91].
EFFICIENT SINGLE ATOM ELECTROCATALYSTS FOR NRR
SACs have displayed outstanding catalytic activity and selectivity in other electrochemical reactions, including HER[92-94], OER[95-97], ORR[98,99], CO2RR[100,101], etc., due to their 100% atomic utilization and strong interactions between active centers and matrices. Thanks to the merits, the SACs were also applied in the NRR and were supposed to adsorb and activate the inert N≡N bonds with high binding energy. Different from the bulk catalysts, the active metal centers in SACs coordinating with supporting atoms are usually partially charged, which substantially accelerate the activation of N2. In addition, the uniform active centers can be modified precisely on the atomic scale[102-104].
Generally, the electrochemical NRR process includes three steps: (i) generation and migration of protons (H+) in the electrolyte; (ii) adsorption of N2 on the surface of electrocatalyst; (iii) hydrogenation of N2 to form NH3. DFT calculations verify that the NRR on metal active sites is limited by the linear relation between the adsorption energies of *N2H and *NH2 intermediates. The outstanding NRR catalysts should satisfy the following three standards: (i) the catalysts should enable to bind the N2 molecule and subsequently activate the N≡N bond; (ii) the active sites should selectivity stabilize the *N2H intermediate for suppressing the competitive HER process; (iii) the liner scaling between *N2H and *NH2 should be broken to destabilize the *NH2 intermediate for the releasing of NH3 molecules[4,105].
On this basis, large amounts of DFT calculation research on SACs were investigated to evaluate the NRR activity on different supports[106-109]. For example, Choi et al. demonstrated the NRR performance of SACs anchored on defected graphene with various coordination structures, such as M@C3, M@C4, M@N3, and M@N4[108]. It was investigated that the adsorption of *H on SACs was inhibited, in comparison with the bulk metal surfaces. The interactions between metal atoms with defected graphene make metal active sites positively charged, enabling metal centers to bind and activate N2 more easily. The Ti-N4 and V-N4 are considered to be optimal configurations as NRR catalysts. Zhao et al. studied the NRR catalytic activity over transition metal atoms on boron nitride (BN) monolayers with rich B vacancies[109]. The N2 adsorption on transition metal (i.e., Ti, V, Mn, Fe, Mo, Ru, and Rh)-doped BN is an exothermic process, indicating energetically favorable adsorption of N2 molecules on these active sites. Among all the investigated materials, the Mo-BN catalyst displays the best NRR performance via an enzymatic reaction pathway, requiring quite a low overpotential of 0.19 V. The breakage of liner scaling between the adsorption of *N2H and *NH2, along with the high spin-polarization of Mo, contributes to the outstanding performance of the Mo-BN catalyst. Inspired by these DFT calculation results, many researchers have designed and fabricated the SACs for NRR following the predicted structures. Table 1 shows the summary of SACs along with the corresponding materials, metal contents and their performances for NRR.
Summary of single atom catalysts for NRR application
| Electrocatalysts | Metal content (%) | Electrolyte | NH3 yield and FE | Ref. |
| Au/TiO2 | 1.54 wt% (ICP-MS) | 0.1 M HCl | 21.4 μg·h-1·mgcat-1/8.11% | [111] |
| Ru@ZrO2/NC | 0.1wt% (ICP-AES) | 0.1 M HCl | 3.665 μg·h-1·mgRu-1/21% | [112] |
| Au1/C3N4 | 0.15% (ICP-OES) | 5 M H2SO4 | 1,305 μg·h-1·mgAu-1/11.1% | [114] |
| Au-SA/FeOOH | 2.0 wt% | 0.05 M H2SO4 | 2.860 µg·h-1·mg-1/14.2% | [115] |
| Ru SAs/N-C | 2.64% | 0.05 M H2SO4 | 120.9 µg·mgcat-1·h-1/29.6% | [116] |
| PdCu/NC | Pd: 2.23 wt% Cu: 2.32 wt% (ICP-AES) | 0.05 M H2SO4 | 69.2 ± 2.5 μg·h-1·mgcat-1/24.8% ± 0.8% | [118] |
| PdFe1 | Fe: 3.91 at% | 0.5 M LiClO4 | 111.9 μg·h-1·mg-1/37.8% | [65] |
| Fe SAC | 1.51 wt% (ICP-OES) | 0.5 M KNO3 + 0.1 M K2SO4 | 0.46 mmol·h-1·cm-2/75% | [5] |
| Fe-PPy SACs | 2.38 wt% (ICP-MS) | 0.1 M KOH + 0.1 M KNO3 | 2.75 mg·h-1·cm-1 | [105] |
| Fe1-N-C | 1.71 wt% (ICP-AES) | 0.1 M HCl | 1.56 × 10-11 mol·cm-2·s-1/4.51% | [123] |
| ISAS-Fe/NC | 4.2 wt% (ICP-AES) | 0.1 M PBS | 62.9 ± 2.7 μg·h-1·mgcat-1/18.6% ± 0.8% | [125] |
| FeSA-N-C | 1.09 wt% (ICP-AES) | 0.1M KOH | 7.48 μg·h-1·mg-1/56.55% | [126] |
| Fe-SAs/LCC | 0.60 wt% (ICP-AES) | 0.1M KOH | 5,350 μg·h-1·mgFe-1/29.3% | [127] |
| Fe-MoS2 | 2.0 wt% | 0.1 M KCl | 36.1 ± 3.6 mmol·g-1·h-1/31.6% ± 2% | [131] |
| Fe-MoS2 | 5.3 at% (ICP-AES) | 0.5 M K2SO4 (pH = 3) | 8.63 μg·h-1·mgcat-1/18.8% | [132] |
| SA-Mo/NPC | 9.54% (ICP-AES) | 0.1M KOH | 34.0 ± 3.6 μg·h-1·mgcat-1/14.6% ± 1.6% | [134] |
| B-Mo2C/NC-50 | - | 0.5 M K2SO4 | 52.1 μg·h-1·mg-1/36.9% | [135] |
| Ni-Mo2C/NC | - | 0.5 M K2SO4 | 46.49 μg·h-1·mg-1/29.05% | [136] |
| MoSAs-Mo2C/ NCNTs | - | 0.005 M H2SO4 and 0.1 M K2SO4 | 16.1 µg·h-1·cmcat-2/7.1% | [137] |
| Mo/BCN | 1.50 wt% (ICP-AES) | 0.1M KOH | 37.67 μg·h-1·mgcat-1/13.27% | [140] |
| Mo/HNG | 2.4 wt% | 0.05 M H2SO4 | 3.6 μg·h-1·mgcat-1/50.2% | [141] |
| Mn-O3N1/PC | 3.85 wt% (ICP-AES) | 0.1 M HCl | 66.41 ± 4.05 μg·h-1·mgcat-1/ 4.91% ± 0.82% | [143] |
| NC-Cu SA | 5.31 wt % (ICP-MS) | 0.1 M KOH 0.1 M HCl | ~53.3 μgNH3·h-1·mgcat-1/13.8% ~49.3 μgNH3·h-1·mgcat-1/11.7% | [144] |
| Y1/NC; Sc1/NC | - | 0.1 M HCl | 23.2 µg·cm-2·h-1/12.1% 20.4 µg·cm-2·h-1/11.2% | [146] |
Noble metal-based materials
Noble metals have a crucial effect on various electrocatalytic reactions in view of their superior catalytic activity. Researchers have developed a series of Noble metal-based SACs on NRR[110-113]. Wang et al. synthesized isolated gold single atoms anchored on C3N4 nanosheets. Thanks to the high atom utilization, a NH3 yield rate of 1,305 g·h-1·mgAu-1 was obtained with a FE of 11.1%, approximately 22.5 times that for Au nanoparticles [Figure 11A-C][114]. To further improve the catalytic activity, Sun et al. fabricated the Au SACs in β-FeOOH with large surface area, yielding a 2,860 µg·h-1·mgAu-1 at -0.4 V vs. RHE, exceeding lots of reported Au-based NRR catalysts [Figure 11D and E][115]. Experiment results and DFT results reveal that the charge transfer from β-FeOOH to anchoring Au single atoms further facilitates the injection of electrons to N2 for activating the NRR process. The synergistic effect of porous β-FeOOH and high activity of isolated dispersed Au contributes to the high NRR catalytic activity. Geng et al. fabricated Ru single species on NC (Ru SAs/N-C) via calcinating the Ru-containing ZIF-8[116]. The NH3 yield rate of Ru SAs/N-C reaches at 120.9 µg·mgcat-1·h-1 at -0.2 V vs. RHE with a FE of 29.6%.
Figure 11. (A) TEM observation of Au1/C3N4; (B) NH4+ formation Faradaic efficiencies for Au1/C3N4, AuNPs/C3N4, pure C3N4 and
Although the noble-based SACs possess high NRR catalytic activity, they also show excellent HER performance, which results in severe competing HER and leads to low FE. Therefore, introducing another element that inhibits the HER activity is considered as a feasible strategy for adjusting the NRR selectivity. He et al. constructed active bimetallic sites via the combination of transition metal heteroatoms (e.g., Fe, Co, Mo, and W) and Ru through the DFT calculation. The energy barriers of the first and last hydrogenation process for bimetallic sites are much lower than those of Ru active sites [Figure 11F-I][117]. The resulting Mo-Ru species displayed an ultra-low onset potential of only 0.17 V [Figure 11G]. They revealed that the side-on adsorption mode and synergistic action of bimetal configuration moderate the adsorption energy of key intermediates and thus suppress HER side reaction. Han et al. designed a diatomic Pd-Cu site on NC for modulating the Pd sites with Cu elements[118]. It is demonstrated that the introducing of Cu can not only upshift the density states of Pd to the fermi level but also enhance the d-2* orbital hybridization between Pd and N2 molecules, leading to the promotion of N2 activation and suppressed hydrogen evolution [Figure 11J]. As a result, the PdCu/NC catalyst gives rise to a high FE of 24.8% ± 0.8%, 14.6 times and three times of Pd/NC and Cu/NC, respectively [Figure 11K]. The N15 isotopic labeling experiment confirmed that ammonia was produced from the NRR process [Figure 11L].
Non-noble metal-based SACs
Non-noble metals, especially transition metal-based SACs, are considered as the alternative of noble materials in view of their cost effectiveness, resource abundance, and comparable catalytic activity to noble metals. Recently, various transition metal-based SACs have been reported as efficient NRR catalysts under ambient conditions.
Fe-based SACs
The volcano-type relationship between the nitrogen adsorption energy and the NRR catalytic activity of different metals identified that the non-noble metals (Mo, Fe) that on the top of volcano diagrams are the most active metals. The viability of atomical Fe-based catalysts in the NRR field has been confirmed by theoretical studies[79,119-121]. For instance, DFT calculation results indicate that the outstanding NRR performance of FeN3 species on graphene was assigned to the high-spin polarization of FeN3 configuration[122]. Based on this, numerous atomically dispersed Fe SACs have been developed for NRR[19,105,123,124]. Lü et al. dispersed Fe active sites in Fe-N4 configuration on NC via calculating the Fe-doped ZIF-8 [Figure 12A][125]. Figure 12B shows the NRR performance of Fe-N4, which achieves a NH3 yield rate (62.9 μg·h-1·mgcat-1) and high FE (18.6%) in 0.1 M phosphate buffer saline (PBS) electrolyte. Long catalytic stability of the Fe/NC catalyst was revealed by electrolyzing for 24 h with negligible activity decay. Wang et al. immobilized Fe sites by modulation of the PPy-iron coordination complex to achieve a high FE of 56.55% in 0.1 M KOH solution [Figure 12C and D][126]. Zhang et al. reported Fe-SAC catalyst with
Figure 12. (A) Illustration of ISAS-Fe/NC catalyst synthesis; (B) Average j and FEs measured in N2-saturated 0.1 M PBS solution[125]. Copyright 2023, Elsevier; (C) TEM image, HAADF-STEM image and corresponding EDS mapping images of single-atom dispersed Fe-N-C; (D) NH3 yield rate and FE of Fe-N-C catalysts[126]. Copyright 2023, Springer Nature; (E) Optimized Fe-(O-C2)4 sites and N2 adsorption on Fe-(O-C2)4; (F) Dependence of RNH3 and FE on the applied potentials[127]. Copyright 2023, Wiley; (G) Polarization fields formed between Fe atoms and MoS2 boost the N2 activation; (H) and (I) NH3 yield rate and FE of Fe-MoS2 catalysts[131]. Copyright 2023, Elsevier; (J) Different charge densities of N2 adsorbed on Pd and PdFe1; (K) NH3 yield rate and FE of PdFe1 catalysts; (L) The comparison of NH3 yield rate of Pd, PdFe1 and PdFex[79]. Copyright 2023, Wiley. FE: Faradic efficiency; PBS: phosphate buffer saline; TEM: transmission electron microscopy; HAADF-STEM: high-angle annular dark field scanning transmission electron microscopy.
Immobilizing Fe atoms on other transition metal compounds or alloys to regulate the band structure of Fe species through neighboring metal-support interaction is also beneficial for improving NRR performance[73,119,123,128-130]. Li et al. immobilized the protrusion-shaped Fe atoms on MoS2 [Figure 12G-I][131]. The engendered polarization electric field accelerates the electron transfer into antibonding orbitals of N2 for adsorbing and activating N2 molecules. Moreover, this strategy was also extended to other metals, such as Co-MoS2, Cu-MoS2, Rh-MoS2, and Ru-MoS2. Su et al. also confirmed that coordination of Fe species with edge S sites of MoS2 is beneficial for suppressing the competing HER, with the FE of Fe-MoS2 six times larger than that of MoS2[132]. Li et al. stabilized isolated Fe active sites with Pd nanosheets to generate PdFe1 SAAs[79]. In this unique architecture, the Fe atoms not only act as the active sites to adsorb N2 molecules but also ensure a good balance between the N2 activation and nitrogen intermediate stabilization. Nevertheless, the Fe multimers bind too strongly with the intermediates and thus result in compromised NRR activity
Mo-based SACs
It is well known that nearly all natural N2 is fixed with the enzyme nitrogenase in bacteria, and the Mo elements act as the main active sites. Therefore, Mo-based materials (especially Mo SACs) have been considered as the most promising catalysts for NRR. Ling et al. first revealed by the DFT calculation that Mo1-N1C2 possesses expected NRR catalytic activity with an enzymatic mechanism and can release NH3 molecules with a low energy barrier, exceeding most ever reported NRR catalysts[133]. Inspired by this, Han et al. immobilized the Mo single species with nitrogen doped porous carbon (NPC) with a mass loading of 9.54% to catalyze the NRR and yielded a NH3 rate of 34.0 μg·h-1·mgcat-1 with a high FE of 14.6% [Figure 13A and B][134]. Previous research reported that Mo2C possessing abundant unoccupied d orbitals also delivers effective N2 adsorption capability[135,136]. Therefore, Ma et al. combined atomically Mo and Mo2C on NC nanotubes to prepare the MoSAs-Mo2C/NCNTs catalyst [Figure 13C]. In this structure, Mo2C is selective to NRR, while the HER selective Mo SAs enable the increase of *H coverage near the Mo2C nanoparticles [Figure 13D and E]. The synergistic mechanism of Mo SAs and Mo2C makes the NH3 catalytic activity of MoSAs-Mo2C/NCNTs catalyst four times and 4.5 times higher than that of Mo2C nanoparticles and Mo SAs, respectively[137]. Wang et al. also demonstrated that the positively charged isolated Mo species produced by denoting electrons to PC-TFPN substrate has low binding affinity to H, which is conductive to suppress the competing HER
Figure 13. (A) Illustration of SA-Mo/NPC and atomic structure model; (B) NH3 yield rate (red) and FE (blue) at each given potential of SA-Mo/NPC[134]. Copyright 2023, Wiley; (C) SEM image of MoSAs-Mo2C/NCNTs; The (D) NRR and (E) HER Gibbs free energy diagrams on the MoNC2 SAC and Mo2C (101) surface; (F) NH3 yield rate and FE of MoSAs-Mo2C/NCNTs, MoSAs/NCNTs and
Other transition metal-based SACs
Besides above-mentioned widely studied Fe and Mo atomically catalysts, other transition metal-based SACs, containing Ni, Mn, W, Co, etc., were also investigated. Atomical Ni active site coordinated with nitrogen environment was produced by Mukherjee et al., delivering a NH3 yield of 115 µg·cm-2·h-1 at -0.8 V vs. RHE with a high FE of 21% [Figure 14A]. DFT results reveal that the NiN3, as the main active sites, can catalyze NRR process completely thermodynamically favorably under a limiting potential of 0.79 V[142]. In addition, black phosphorene (BP) was also employed as the substrate to immobilize the transition metal atoms with MP3 coordinated structure (M = Fe, Mn, Cr, Mo, W, V, and Nb). Among these catalysts, W@BP delivers the highest NRR activity and selectivity, which is ascribed to the unique WP3 active sites that activate the N≡N bonds by injecting electrons to N2, thereby regulating the electron transfer between BP and nitrogen containing intermediates [Figure 14B and C][124]. Han et al. constructed Mn-O3N1 active sites on P-doped porous carbon through the local modulation of Mo-O bonding conditions [Figure 14D and E][143]. In this structure, the more electronegativity of O prompts more electrons transfer from Mo to O, making Mn sites more active than MnN4 in the N2 adsorption process (ΔEN2 is -0.83 V for Mn-O3N1 and 0.09 V for MnN4) [Figure 14F]. As a consequence, the Mn-O3N1/PC achieves a NH3 yield rate of 66.41 μg·h-1·mgcat
Figure 14. (A) Comparison of NH3 production rate and FE at different pyrolysis in 0.1 M KOH solution[142]. Copyright 2023, Wiley; (B) Charge density difference between W@BP and N2 with side-on configurations; (C) The reaction pathways on W-BP[124]. Copyright 2023, RSC Publishing; (D) TEM image and HAADF-STEM image of Mn-O3N1/PC; (E) FT-EXAFS spectra of Mn foil, Mn-O3N1/PC, and the curve fitting with the Mn-O3N1/graphene; (F) Free energy diagram of an associative distal pathway on the Mn-O3N1/graphene and the Mn-N4/graphene[143]. Copyright 2023, ACS Publication; (G) The NH3 yield rate and FE at different potentials in 0.1 M KOH and 0.1 M HCl[144]. Copyright 2023, ACS Publication; (H) Calculated active site structures of an yttrium single atom and corresponding adsorption energies and FT-EXAFS fitting in R-space of Y1/NC; (I) The NH3 yield rate and FE of Y1/NC; (J) The NRR catalytic activity of Y/Sc based nanomaterials and single atom catalysts[146]. Copyright 2023, ACS Publication. FE: Faradic efficiency; TEM: transmission electron microscopy; HAADF-STEM: high-angle annular dark field scanning transmission electron microscopy; EXAFS: extended X-ray absorption fine structure; NRR: nitrogen reduction reaction.
CONCLUSION AND PERSPECTIVE
The production of NH3 at ambient conditions has attracted great interest, but it remains challenging. The rational design of efficient and cost-effective catalysts possessing high catalytic activity and selectivity is the key to realizing an effective NRR process. SACs have gained significant breakthroughs in various electrocatalytic reactions in view of their unique size effect, regulated coordination environment, and electronic structure. In this review, we first summarize the synthesis strategy toward SACs, such as high temperature pyrolysis, MOF derived SACs, impregnation and coprecipitation strategy, SAAs, electrochemical deposition, ALD method, and so on. Then, we highlighted the in-depth theoretical analysis for rationally designing SACs from the following four aspects: coordination components, coordination number of active sites, metal-support interaction and spatial microenvironment regulation. Finally, some previously reported SACs for NRR, such as noble-metal SACs and non-noble transition metals (Fe SACs, Mo SACs, and other metal SACs), and the corresponding mechanisms are presented and demonstrated. Catalysts with atomic level active sites have displayed excellent catalytic activity, selectivity, and electrochemical stability for NRR. However, some challenges in the following aspects still should be addressed for achieving the practical applications for SACs toward NRR:
(i) The SACs with high atomic metal mass loading are still a challenge. As shown in Table 1, the amounts of metal loading in most of the SACs are still low (< 5 wt%). Therefore, exploring feasible synthesis strategies and selecting suitable supports are vital to realizing the improvement of metal active site density and high performance NRR catalytic reaction. Preparing 2D materials with large surface area and introducing defects or vacancies to anchor the metal sites are beneficial for increasing the metal sites amounts in SACs.
(ii) Most previously reported transitional-based SACs only have a promotion in either ammonia yield rate or FE, while another parameter is still unsatisfactory. Especially, the FE for most SACs is smaller than 30%, resulting from the competition HER. However, commercial catalysts usually require comprehensive optimized performance, especially high catalytic activity and selectivity. Therefore, NRR performance of SACs should still be improved for satisfying the commercial applications.
(iii) Stability of the catalyst is also an important indicator to measure whether it can be realized in commercial applications. Generally, the isolated metal active sites are usually stabilized on various supports by the heteroatoms, including N, O, S, P, and so on. However, the heterogeneous species may undergo the degradation during the catalytic reaction process, resulting in the band structure and coordination environment changes of metal species. This leads to an uncontrollable reaction pathway. Therefore, more advanced in situ characterization instruments and DFT calculations should be carried out to unveil the structural evolution of catalysts when catalyzing and thereby clarify the reaction mechanism and stability of designed catalysts. Moreover, the N elements in the N containing catalysts are also critical issues that influence the measurement of the NH3 yield rate. As a result, the fabrication of N-free SACs is urgently required.
(iv) Up to now, many theoretical computational models have been constructed to study their NRR catalytic activity, some of which have also been verified by the spectroscopies experimentally. Simultaneously, the experiment results reversely promote the new theories to clarify the reaction mechanism. However, the theoretical calculations are usually based on the ideal assumptive models with simplification and tunable parameters. It is essential to match the theoretical results with experimental data for avoiding obtaining the biased conclusions that misguide the research directions. It is believed that the feedback loop between theoretical studies and experimental results may not only effectively guide the rational design of SACs but also unveil the reaction mechanism and kinetics of active centers.
DECLARATIONS
Authors’ contributions
Drafted the manuscript: Fan B, Yuan H
Participated in the writing and literature discussion: Wang W, Liu Z, Guo J, Yuan H, Tan Y
Coordinated the writing and finalized the manuscript: Fan B, Tan Y
Availability of data and materials
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (No. 52072193 and 52101182).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
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Fan, B.; Wang W.; Liu Z.; Guo J.; Yuan H.; Tan Y. Recent progress in single atomic catalysts for electrochemical N2 fixation. Microstructures. 2024, 4, 2024025. http://dx.doi.org/10.20517/microstructures.2023.71
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