Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia Zhoutai Shang†, Bin Song†, Hongbao Li†, Hong Zhang, Fan Feng, Jacob Kaelin, Wenli Zhang, Beibei Xie, Yingwen Cheng, Ke Lu and Qianwang Chen Zhoutai Shang† Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui Graphene Engineering Laboratory, Anhui University, Hefei, Anhui 230601 Hefei National Laboratory for Physical Sciences at the Microscale, Hefei, Anhui 230026 †Z. Shang, B. Song, and H. Li contributed equally to this work.Google Scholar More articles by this author , Bin Song† Laboratory of Nanoscale Biochemical Analysis, Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, Jiangsu 215123 †Z. Shang, B. Song, and H. Li contributed equally to this work.Google Scholar More articles by this author , Hongbao Li† Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui Graphene Engineering Laboratory, Anhui University, Hefei, Anhui 230601 †Z. Shang, B. Song, and H. Li contributed equally to this work.Google Scholar More articles by this author , Hong Zhang School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author , Fan Feng School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author , Jacob Kaelin Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115 Google Scholar More articles by this author , Wenli Zhang School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006 Google Scholar More articles by this author , Beibei Xie State Key Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau, Taipa, Macau 999078 Google Scholar More articles by this author , Yingwen Cheng Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115 Google Scholar More articles by this author , Ke Lu *Corresponding author: E-mail Address: [email protected] Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui Graphene Engineering Laboratory, Anhui University, Hefei, Anhui 230601 Hefei National Laboratory for Physical Sciences at the Microscale, Hefei, Anhui 230026 Google Scholar More articles by this author and Qianwang Chen Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui Graphene Engineering Laboratory, Anhui University, Hefei, Anhui 230601 Hefei National Laboratory for Physical Sciences at the Microscale, Hefei, Anhui 230026 Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101106 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ambient electrochemical nitrogen fixation is a promising and environmentally benign route for producing sustainable ammonia, but has been limited by the poor performance of existing catalysts that promote the balanced chemisorption of N2 and subsequent electrochemical activation and hydrogenation. Herein, we describe the highly selective and efficient electrohydrogenation of nitrogen to ammonia using a hollow nanorod-based hierarchically graphitic carbon electrocatalyst with abundant atomically dispersed Mn sites. We discovered that the electron interactions strengthen the interfacial binding between nitrogen and active Mn Lewis acidic hotspots. The Lewis acid–base interactions promote the chemisorption and lock up nitrogen on the active sites and suppress proton adsorption. The proton-coupled electron transfer cleavage of the nitrogen triple bond through an associative mechanism was confirmed under lower overpotential, which delivered high ammonia yield of 67.5 μg h−1 mgcat.−1 and Faradaic efficiency of 13.7% at −0.25 V versus the reversible hydrogen electrode, along with ∼100% selectivity and significantly enhanced electrochemical stability (about 88.8% current retention over 50 h potentiostatic test) under mild conditions. Our strategy is versatile to tailor the nitrogen fixation performance of single-atom catalysts with atomic accuracy. Download figure Download PowerPoint Introduction Ammonia is an essential platform chemical for the global economy and plays key roles in industry, agriculture, and pharmaceutical chemistry.1–4 It is also a preferred carbon-neutral energy carrier for sustainable energy storage with a 17.6 wt % hydrogen content and a high energy density of 4.3 kW h−1.5–7 Dinitrogen reduction for ammonia is a vital step in the natural nitrogen cycle. Currently, approximately 200 million tons of ammonia is produced annually using the Harber–Bosch process. However, the harsh reaction conditions (typically conducted at 400–500 °C and 100–350 atm) of this incumbent energy-intensive process (N2 + 3H2 → 2NH3, ΔfH0 = −45.9 kJ mol−1) impede its flexible modular production.8–11 Further, other drawbacks, such as process complexity, high energy consumption, and excessive CO2 emission, increase the need for a new sustainable path to produce NH3.12–14 To this end, ambient electrochemical synthesis of ammonia from water and nitrogen under mild conditions is a promising alternative route, especially since it can also deliver about 20% more thermodynamic efficiency compared with the industrial Haber–Bosch process.6,15,16 Since the electrochemical N2-to-NH3 conversion was first reported in 1807,17 many electrocatalysts, including noble metal,18,19 metal oxide,20 metal carbide,9 metal phosphide,21 metal sulfide,22 and metal-free carbon-based materials,23 have been examined to manipulate the absorption and conversion behavior of nitrogen. Unfortunately, the electrochemical nitrogen redox is still plagued by the challenges of low NH3 selectivity and sluggish conversion kinetics.6,18,22,24,25 In addition, this reaction faces strong competition with proton H+ reduction in aqueous electrolytes, which results in very low selectivity and efficiency of ammonia formation.20,26 In this regard, preoccupation of “locked” nitrogen on active sites instead of undesirable hydrogen coverage is expected to strengthen N2 bonding and promote its subsequent electrocatalytic hydrogenation.26 In particular, one of the key approaches to dissociate the strong triple bond (dissociation energy, 942 kJ mol−1) and relieve its electrochemical activation-related kinetics is the use of transition metal-based catalysts with available d-orbital electrons for their favorable geometric and electronic structures.20,23,27,28 Atomically dispersed transition metal active sites with a local coordination unsaturation environment provide an exciting pathway for activating nitrogen.23,29–32 Considering the weak Lewis base character of nitrogen molecules, metallic Lewis acidic active sites anchored on a catalytic framework could enhance the chemisorption between N2 and active binding sites.23,26,33 In addition, the incorporation of a hierarchically porous graphitic carbon matrix could maximize the accessibility between reactant and active hotspots, and simultaneously facilitate efficient mass and electron transfer.2,23,29,34 Accordingly, versatile atomically dispersed metal catalysts are highly desirable to modulate kinetics and boost nitrogen reduction. Compared to bulk and nanoparticle counterparts, single-atom heterogeneous catalysts possess facile reaction thermodynamics and exhibit improved catalytic performance toward hydrogenation catalysis, and the atomically dispersed binding sites could possibly accelerate N2 chemisorption and hydrogenation to produce ammonia. In this contribution, we outline atomically dispersed Lewis acidic Mn sites in a nanorod carbon framework for highly efficient nitrogen conversion. The Lewis acidic Mn–N–C (LA-MnNC) catalysts have abundant and diverse N-coordinated metal hotspots and hierarchically graphitic porous structure. Mesopores especially (total pore volume: 0.47 cm3 g−1, mesopore volume: 0.35 cm3 g−1) have greatly enhanced the chemisorption of N2 through Lewis acid–base interactions. The adsorbed nitrogen in active sites have stronger substrate-reactant binding that favors subsequent N≡N bond activation. 15N isotopic labeling experiments confirmed that the nitrogen come from the reduction of feeding gas, rather than the electrochemical decomposition of the catalyst. The LA-MnNC catalyst achieved a high ammonia yield rate of 67.5 μg h−1 mgcat.−1 with the Faradaic efficiency of 13.7% at −0.25 V versus the reversible hydrogen electrode (RHE) through the associative electrohydrogenation pathway, and exhibited excellent stability in aqueous electrolytes under ambient conditions, holding high promise for further deployable electrochemical nitrogen reduction. Experimental Methods Synthesis of MnO2 nanowire In a typical procedure, 2.028 g MnSO4 (Sigma-Aldrich, Shanghai, China) was added into 300 mL 0.014 M HCl (Thermo Fisher Scientific, Cleveland, OH, USA) solution.35 1.264 g of KMnO4 (Sigma-Aldrich) was dissolved into the 100 mL deionized (DI) water. The above solutions were mixed together under stirring and stirred for another 2 h. Then the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 120 °C for 12 h. The resultant powder was washed with DI water and ethanol and then vacuum dried. Synthesis of LA-MnNC catalyst Typically, 400 mg of as-prepared MnO2 powder was added into 100 mL H2O. 3 mmol aniline (Sigma-Aldrich), 1.5 mmol pyrrole (Sigma-Aldrich) monomer, and 4 mL 0.5 M H2SO4 (Sigma-Aldrich) were added into the 100 mL ethanol (Aladdin, Shanghai, China) and water (v∶v = 1∶1) solution. After cooling down to about 4 °C, the aniline–pyrrole solution was added dropwise under stirring and kept at 4 °C for 4 h. The MnO2@copolymer products were washed with a large amount of DI water and vacuum-dried, followed by annealing at 900 °C for 1 h with a heating rate of 7 °C min−1 under Ar atmosphere. Followed by an acid washing step for leaching out the metal oxide/metal clusters, the adsorbed Mn2+ ions were confined within the carbon channel and open porous structures of the Mn–N–C samples. The acid leaching treatment was conducted in 0.5 M H2SO4 solution at 80 °C for 8 h, and about 100 mL acid solution was used for 100 mg powder. The resultant powder was heated to 900 °C under Ar flow for 2 h (heating ramp rate, 5 °C min−1) to repair the carbon structure and increase the Mn–Nx content.3 The final catalysts were achieved by a second pyrolysis and labelled as LA-2MnNC (2 represents the molar ratio between aniline and pyrrole monomers). As control samples, LA-1MnNC, LA-0.5MnNC, LA-aMnNC, and LA-yMnNC samples were prepared through the same procedures by adjusting the aniline and pyrrole molar ratio. LA-1MnNC, 2.25 mmol aniline and 2.25 mmol pyrrole were used; LA-0.5MnNC, 1.5 mmol aniline and 2 mmol pyrrole were used; LA-aMnNC, 4.5 mmol aniline was used; LA-yMnNC, 4.5 mmol pyrrole was used, respectively. N-doped porous carbon (NC) was also prepared using the same experimental conditions, and metal oxide was replaced with ammonium persulfate (APS; Sigma-Aldrich) as the initiator. Materials characterization Powder X-ray diffraction (PXRD) patterns were recorded using a Miniflex 600 rotation anode X-ray diffractometer (Rigaku, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250 X-ray photoelectron spectrometer (Thermo Scientific, New York, USA), and the binding energies were calibrated by assigning the C 1s peak at 284.5 eV. The Raman spectra were collected on LabRAM HR 800 system (Horiba Jobin Yvon, Tokyo, Japan) with a 514 nm laser. Temperature-programmed desorption (TPD) was performed on an Autochem II chemisorption analyser (Micromeritics, Norcross, GA) using N2 as the probe molecule. The N2 sorption experiments were carried out using Micromeritics ASAP 2020 system. Before measurements, samples were degassed at 100 °C for 10 h under vacuum. The specific surface areas and pore size distributions were calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The acquired Mn K-edge extended X-ray absorption fine structure (EXAFS) data were collected in fluorescence mode and processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. In situ Fourier transform infrared (FT-IR) spectra of the electrochemical cell were collected on a Nicolet 8700 FT-IR spectrometer (Thermo Fisher) equipped with an EverGlo IR source. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were conducted on an Optima 7300DV (ICP) spectrometer (Perkin-Elmer, Waltham, MA). Electrochemical studies An electrochemical workstation (Interface 1010E, Gamry, USA) was employed to perform electrochemical measurements in two-compartment H-cell at room temperature, in which a saturated calomel reference electrode (SCE) and a graphite rod were used as the reference and counter electrodes, respectively. Before nitrogen (99.99%) reduction reaction (NRR) tests, the Nafion 211 membrane (DuPont, USA) was pretreated by heating it in H2O2 5% aqueous solution at 80 °C for 1 h, ultrapure water at 80 °C for 1 h, 0.5 M H2SO4 at 80 °C for 3 h, and finally in ultrapure water at 80 °C for another 4 h. In a typical preparation procedure of the working electrode, 10 mg sample and 40 μL Nafion solution (5 wt %) were dispersed in 960 μL water–isopropanol solution with a volume ratio of 1:3 by sonicating it for 30 min to form a homogeneous ink. Then, 30 μL of the ink was loaded onto a carbon paper (1 × 1 cm2) to prepare a working electrode. The mass loading was 0.3 mg cm−2. The obtained data were calibrated with respect to the RHE by E(vs RHE) = E(vs SCE) + 0.242 V + 0.0591 × pH. The potentiostat tests were performed at different potentials including 0, −0.10, −0.20, −0.25, −0.30, −0.40, −0.50 V versus RHE. Ammonia detection The concentrations of NH3 (NH4+) produced in 0.1 M HCl solutions were determined via a widely used indophenol blue method.36 The calibration curve was established using standard NH4Cl stock solution in the concentrations of 0, 1, 5, 10, 30, 50, and 100 μM, respectively. In each test, after the electrochemical reduction reaction test, the following were added together: 3.0 mL of the standard solution or electrolyte, 0.12 mL phenol solution (20.0 g phenol dissolved in 100 mL ethanol), 0.12 mL sodium nitroprusside (C5FeN6Na2O) solution (1.0 g C5FeN6Na2O in 200 mL H2O), and 0.3 mL oxidizing solution. This solution was kept at room temperature for 2 h in the dark for the formation of indophenol blue. The oxidizing mixture solution containing 2.5 mL sodium hyphchlorite (NaClO) solution and 10 mL alkaline reagent (100 g sodium citrate and 5 g sodium hydroxide dissolved in 500 mL H2O). The UV–vis absorption spectrum of each solution was measured, and the absorbance value at the wavelength of 650 nm was obtained. Hydrazine hydrate detection The concentration of the hydrazine present in the electrolyte was determined by the Watt and Chrisp method.37 The calibration curve was established using standard N2H4 stock solution, in the concentrations of 0, 1, 2, 5, 8, and 10 μM. The p-dimethylaminobenzaldehyde (p-C9H11NO, 5.99 g), HCl (30 mL), and ethanol (300 mL) were first mixed as a color reagent. In detail, 1.0 mL of standard solution or electrolyte after NRR was mixed with 1.0 mL of the coloring reagent solution with rapid stirring for 10 min at room temperature. Then, the absorbance of the mixture was measured at a wavelength of 455 nm. 15N isotopic labeling experiment A mixture of 15N2 (99 atom % 15N, Wuhan Newradar Special Gas Co. Ltd., Wuhan, China) and 14N2 mixture at a molar ration of 1∶2 was used as the feeding gas in the labeling experiment. And a low velocity gas flow system was adopted (∼5 mL min−1). After electrolysis at −0.25 V versus RHE for 6 h, the resultant electrolyte (20 mL) was concentrated to 4 mL, and 0.9 mL of the solution was taken out, followed by adding 0.1 mL of D2O as an internal standard. The produced 15NH4+ was identified using 1H nuclear magnetic resonance measurements (Bruker DRX600, Karlsruhe, Germany). Faraday efficiency calculation The Faraday efficiency (FE) and mass-normalized yield rate of NH3 were calculated as below: FE ( NH 3 ) = [ 3 F × c ( NH 3 ) × V ] / Q Yield rate mass ( NH 3 ) = [ 17 × c ( NH 3 ) × V ] / ( t × m ) where F is the Faraday constant (96,485 C mol−1), t is the electrolysis time, m is the loading mass of the catalyst (0.3 mg), Q is the total charge passed through the electrode, V is the volume of the electrolyte, and c(NH3) is the measured ammonia concentration. Computational methods To identify the catalysis site and explore the NRR reaction mechanism, first-principles calculations were performed by using the Gaussian 16 software package. A nanotube characterized by (n, m) (n = 3 and m = 2) was built, including 76 carbon atoms and a 3.5 Å diameter. The gas-phase conformers were optimized at the M062X/6-311+G (d, p) level to better describe the weak interactions. Since no clear structural differences were observed between nanotubes with two ends hydrogenated or not, only the hydrogenated nanotube was explored as a model to reduce the computational burden. Four carbon atoms in a middle benzene ring were replaced by nitrogen, on which one manganese (Mn) atom was absorbed with them. The chemical potential of proton and electron pairs (H+ + e−) was equal to half of that of the gaseous H2 molecule.38,39 Results and Discussion Structure characterization of single-atom dispersed Mn catalyst Figure 1a illustrates the synthesis of atomically dispersed and N-coordinated Mn sites in a porous graphitic carbon framework. The general procedure includes four steps: (1) in situ copolymerization of aniline and pyrrole; (2) Mn-assisted thermal pyrolytic carbonization; (3) acid etching; and (4) second thermal treatment. The α-MnO2 nanowires were synthesized by a facile hydrothermal method.35 The synthesized nanowires were highly crystalline and had well-defined nanowire morphology, with the diameter of ∼50 nm and the length of several micrometers ( Supporting Information Figure S1). The MnO2 nanowires were used as sacrificial template, and their gradual dissolution initiated the simultaneous interfacial polymerization of pyrrole and aniline, generating coaxial crosslinked [email protected]2 ([email protected]2) nanostructure with uniform polymer wrapping shells (see Figure 1b and Supporting Information Figure S2).40–42 The redox reaction between oxidant and monomer can be expressed by the following: MnO 2 + 4 H + + 2 e − → Mn 2 + + 2 H 2 O ( 1.23 V vs RHE ) (1) n C 4 H 4 NH or n C 6 H 5 NH 2 → ( C 4 H 2 NH ) n or ( C 6 H 4 NH ) n + 2 n H + + 2 n e − ( ∼ 0.7 V vs RHE ) (2) Figure 1 | Schematic and morphological characterization of single-atom dispersed LA-MnNC catalyst. (a) Schematic illustration of the synthetic process of atomically dispersed and nitrogen-coordinated Mn sites in porous graphitic carbon framework and its corresponding atomic structure model. (b) TEM image of the core–shell structured MnO2@copolymer precursor. (c and d) TEM and STEM images showing the hollow nanorod morphology of MnNC catalyst and the corresponding elemental mapping of Mn, C, and N. (e) Atomic resolution HAADF-STEM image identifying atomically dispersed Mn atoms in porous carbon substrate. (f) The corresponding height profile of the Mn single atoms. Download figure Download PowerPoint The Mn4+/Mn2+ redox couple had higher redox potential than the polymerization potential of monomers, indicating the spontaneously interfacial polymerization. The released Mn2+ ions quickly chelated with the N-containing functional groups in three-dimensional (3D) interconnected polymer network, producing the desired Mn-doped 2PANi-PPy precursor (where 2 refers to the aniline/pyrrole molar ratio). The coaxial polymer was then pyrolyzed at 900 °C, and transmission electron microscopy (TEM) analysis confirmed that the MnO2 template was etched ( Supporting Information Figures S3 and S4). The presence of Mn2+ cations could catalyze the graphitization as discussed in the literature where metal cations (e.g., Fe3+, Mn2+) embedded in carbon precursor could facilitate the catalytic graphitization process.41,43–45 The resulting carbothermal reaction rendered the formation of expanded nanographite and multipores with the simultaneous incorporation of heteroatom and Lewis acid doping of N and Mn into the carbon matrix.41,46 The pyrolyzed product was washed with acid to remove loosely attached Mn components including MnOx and Mn clusters, and a second thermal activation process was employed to repair the structure of MnN4 into atomically dispersed single-atom catalysts and increase the Mn-doping content, producing the best-performance LA-2MnNC hollow nanorod catalysts (as above, 2 refers to the molar ratio between aniline and pyrrole monomers).45 The produced LA-2MnNC catalyst shows abundant micropores and nanographite domains with expanded d-spacing of 0.38–0.40 nm (Figure 1c). The selected area electron diffraction (SAED) pattern verifies the amorphous nature of LA-2MnNC with the absence of crystalline clusters in the catalyst ( Supporting Information Figure S5), which agrees well with XRD measurements. High-angle annular dark field scanning TEM (HAADF-STEM) images and elemental mapping of Mn and N revealed that the dopants were homogeneously distributed over the entire carbon framework (Figure 1d). The isolated atomic Mn sites (bright dots) can clearly be identified in the aberration-corrected HAADF-STEM images with sub-Å resolution (Figure 1e). The average diameter of the bright dots was 0.17 nm (Figure 1f), clearly showing the atomically dispersed nature of Mn sites. The atomic-dispersion of Mn speciation was further verified from synchrotron X-ray absorption spectroscopy (XAS) measurements. The Mn K-edge X-ray absorption near-edge structure (XANES) spectrum of the LA-2MnNC catalyst exhibited a threshold energy between Mn(II)O and Mn(IV)O2 and was very close to that of the well-defined MnN4 structure in MnPc (Figure 2a), which is in line with TEM results and suggests the strong coupling Mn–N interaction with Mn close to the 2+ oxidation state.41,45 Figure 2b compares the Fourier transform (FT) k2-weighted EXAFS spectra of the LA-2MnNC catalyst with standard references. The strongest contribution and a hump are presented at ca. 1.5 and 2.4 Å, respectively, which are indicative of Mn–N and Mn–C bonding motifs in the MnNC catalyst.45 Meanwhile, the absence of a Mn–Mn peak in the MnNC spectra further confirms the absence of Mn clusters and hence the atomic dispersion nature of Mn metal atoms. The Mn–N distance was estimated as 1.45 Å, which is very similar to the 1.44 Å of Mn–N in well-defined MnN4 chemical structure, characterized by a well-dispersed planar Mn–N4 coordination.41,45 Importantly, the microstructure and composition of a hollow nanorod catalyst can be tuned by varying the molar ratios between monomer precursors in the interfacial copolymerization step. The labels of 2, 1, and 0.5 refer to the molar ratios of aniline to pyrrole during the interfacial polymerization procedure. And a and y mean just aniline and pyrrole monomers were added during the synthesis of MnO2@copolymer precursors. Figure 2 | Structural characterization of atomically dispersed MnN4 sites in hierarchically porous carbon framework. (a) Mn K-edge XANES spectra and (b) FT-EXAFS spectra from the LA-2MnNC catalyst and standard MnO, MnPc, and MnO2 reference samples. High-resolution (c) N1s and (d) Mn2p XPS spectra. (e) Raman spectra and (f) N2 TPD curves for different MnNC samples as noted. (g) The optimized configurations of nitrogen adsorbed on the surface of the (1) pristine CNT, (2) N-doped CNT, and (3) MnN4 decorated CNT, respectively. (h) Calculated million charge of Mn and N elements in MnN4 center (1) before and (2) after nitrogen adsorption. Download figure Download PowerPoint Figures 2c and 2d present the high-resolution XPS spectra of N 1s and Mn 2p. The peak at 399.2 eV from the N 1s spectra can be assigned to the MnN4 motif in the catalyst, and its content reached as high as 15.9% in the LA-2MnNC catalyst ( Supporting Information Table S1). In addition, no metallic Mn peak was observed for all catalysts (Figure 2d), agreeing with the above result that suggests atomic dispersion of Mn sites. Furthermore, the slightly positive shift (∼0.21 eV) with higher Mn content may be due to charge transfer between adjacent Mn sites and electronic coupling.45 ICP-AES analysis was performed to better quantify Mn dopant concentration as a function of the polymer precursor, and the results are summarized in Supporting Information Table S2. The addition of pyrrole enhanced Mn doping contents, probably due to the formation of more disordered polymer networks, and the highest Mn content was identified as 3.17 wt % (0.71 atom %) in the LA-2MnNC catalyst. The specific surface area and porosity of the catalysts increased with higher Mn–N4 contents. A very high specific surface area of 645 m2 g−1 with abundant mesopores was measured ( Supporting Information Figure S6). At the same time, limited surface area and porosity were retained with the metal-free PANi-PPy derived N-doped carbon (126 m2 g−1). A continuous increase of surface area and favorable porosity, especially in mesopores, was observed with the concurrence of aniline and pyrrole monomers. This is evidence of the increased doping level and/or defect sites.43,45 Copolymerization of aniline and pyrrole facilitated the formation of crosslinked and folded open frameworks, which highlights the importance of the selected precursors. The hierarchically porous hollow structural features were favorable for nitrogen physical adsorption and provided enough accessible three-phase-contact areas.23 Raman spectra of catalysts with different degrees of graphitization are recorded and compared in Figure 2e. Compared LA-2MnNC catalysts with its counterparts, the lower ID/IG value and negative shift of G peak confirmed the higher degree of graphitization and lower defects ( Supporting Information Figure S7), which are crucial for stablizing nitrogen fixation.23,47 TPD of nitrogen was performed to investigate N2 adsorption behavior (Figure 2f). A direct correlation between chemisorption and MnN4 sites a content was observed.47 For LA-1MnNC and LA-0.5MnNC samples, the surface areas are 573 and 522 m2 g−1, respectively. However, the LA-0.5MnNC catalyst exhibited inferior chemisorption capability due to its lower Mn-doping content in the carbon framework (0.48 vs 0.58 atom %). Thus, the MnN4 spots functioned as the active centers to “lock up” dissolved nitrogen for subsequent triple-bond dissociation. Computational results in Figure 2g reveal the origin of the higher nitrogen adsorption capability of MnNC catalyst. As the key step in the electrochemical nitrogen reduction process, the decreased bond length (2.37 Å) and lower adsorption energy (−0.24 eV) indicate the spontaneously strong interaction between nitrogen and MnN4 hotspots. The favorable Lewis acid–base charge transfer/exchange (as indicated in the charge redistribution) stabilized adsorbed reactant and facilitated nitrogen activation/hydrogenation (Figure 2h and Supporting Information Figure S8). The physical and chemical cotrapped nitrogen molecule in the porous catalyst three-phase interface would be more easily activated for subsequent triple-bond dissociation. Comparatively, the adsorption energy of pristin