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Scalable Conversion of CO to N-doped Carbon Foam for Efficient Oxygen Reduction Reaction and Lithium Storage Chunxiao Xu, Song Chen, Liyong Du, Changxia Li, Xin Gao, Jianjun Liu, Liangti Qu, and Pengwan Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03542 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Sustainable Chemistry & Engineering

Scalable Conversion of CO2 to N-doped Carbon Foam for Efficient Oxygen Reduction Reaction and Lithium Storage Chunxiao Xu,†,‡ Song Chen,‡ Liyong Du,‡ Changxia Li,║ Xin Gao,† Jianjun Liu,§ Liangti Qu║ and Pengwan Chen* † †

State Key Laboratory of Explosion Science and Technology,

‡

School of Materials Science and

Engineering and ║ School of Chemistry and Chemical Engineering, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, P. R. China §

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, 15 North Third Ring Road, Chaoyang District, Beijing 100029, P. R. China * Email address of the corresponding author: [email protected] (Pengwan Chen)

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ABSTRACT With regard to CO2 usage for the sustainable production of value-added chemicals, this work provides a fast pyrolysis process for the first time that can be scaled up in directly capturing CO2 to produce N-doped carbon foam (NCF) as a functional nanomaterial. Flammable alkaline solutions of hydrazine hydrate serves as CO2 adsorbent, fuel as well as nitrogen source for the NCFs, and magnesium powders are involved into the reaction to provide complex metal ion and also energy for the self-propagating high-temperature combustion pyrolysis. The prepared NCFs exhibit efficient electrochemistry performance toward lithium-ion battery and oxygen reduction reaction benefiting from the well-formed foam structure with hierarchical pores, in situ nitrogen doping and high specific surface area. More importantly, this approach introduce a general solution system that can further integrate various additives dissolved homogeneously, thus greatly increasing process controllability and product selectivity for the thermochemical conversion of CO2. KEYWORDS. CO2 conversion, carbon foam, nitrogen doping, oxygen reduction reaction, Li-ion battery


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INTRODUCTION Carbon dioxide (CO2) emissions into the atmosphere has dramatically increased due to rising energy consumption and the demand for fossil fuels. Not only is the use of these fossil resources non-sustainable, but also the resulting increase in anthropogenic CO2 concentration is a great concern that may lead to severe global climate change.1 Therefore, development of CO2 capture, sequestration, immobilization technologies and chemical conversion of CO2 into industrially relevant products has significantly grown in recent years internationally to solve this serious problem.2,3 A variety of attractive CO2 conversion techniques have been proposed including heterogeneous catalytic hydrogenation,4,5 photocatalytic6,7 and electrochemical8,9 conversion of CO2 to hydrocarbons or oxygenates, as well as thermochemical10–12 reduction of CO2 to valuable carbon materials. Nevertheless, it remains a challenging problem for efficient and cost-effective chemical conversion of CO2 for the reason that many of these methods are quite expensive since they require either ultrahigh purity CO2 or are energy intensive, or also require the exploration of extremely active catalysts due to the awfully stability of CO2 molecules.13,14 Hence, the development of novel CO2 utilization method is still in expectation, and here, we demonstrate a general pyrolysis process for the first time that can be scaled up in directly capturing CO2 to produce N-doped carbon foam (NCF) as a useful functional nanomaterial. The pyrolysis conversion process is essentially a self-sustained thermal process combined with wet chemistry in solution in this work, which can be considered as a modified type of combustion synthesis (CS) process.15–17 During the past decade, significant progress has been achieved in synthesis of carbon nanomaterials using low cost CS methods.18 Graphene with low oxygen content was synthesized by a reaction between a refractory ceramic compound silicon carbide (SiC) and



polymer PTFE.19 Layered graphite materials were prepared through burning metals (e.g., Li, Mg, K and Zn) in CO2 condition for electrochemical applications.20–24 These attempts indicate that self-sustained exothermic reactions develop an effective direction for synthesis of a variety of carbon-based nanostructures. With taking advantage of its time- and energy-efficiency of CS, we present here a novel solution combustion pyrolysis process for the conversion of CO2 to NCF using flammable alkaline solution as CO2 adsorbent and magnesium (Mg) powders to support combustion. The product structure and property can be facilely tuned by controlling the combustion pyrolysis conditions such as the amount of Mg powders, the selection of different types of reaction solutions as well as the addition of modifiers dissolved in solution. Mg has been widely studied for direct reduction of gaseity CO2 to nanoscale carbonaceous materials as an efficient reducing agent in previous studies.22–24 However, these traditional gas-solid reactions show low process controllability and product selectivity, which greatly impedes their potentiality for the usage of CO2 as a practical carbon source. Thus, the key step of converting CO2 to NCF in this work is to introduce the flammable alkaline solutions into initial combustion reaction between CO2 and Mg, and form a homogeneous solution prior to the pyrolysis synthesis process. Throughout this process, the solution serves as CO2 adsorbent, magnesium ion complexing agent, fuel and also nitrogen source for the NCF products. This methodology allows the scalable usage of CO2 as feedstock for the production of carbon based materials, reduces purity requirement of CO2, and improves controllability of reaction process. Meanwhile, benefiting from the solution reaction and pyrolysis features such as homogeneous reactants mixing, high reaction temperature, short reaction duration and large quantities of gases generation,25,26 the NCF thus obtained show favorable features including high specific surface area of


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630 m2 g-1, large volume of hierarchical pores of 1.85 cm3 g-1, well-formed foam structure, and in situ nitrogen doping with high content of 2.73 at%. With respect to the synergistic effects of these features, Oxygen reduction reaction (ORR) in fuel cell and lithium-ion battery (LIB) anode are prepared to test the electrochemical performances of the synthesized NCFs, and wider application of such methodology in industry and research laboratories is foreseen, especially when functionalized carbon materials can be easily synthesized from CO2 by this one-pot approach. EXPERIMENTAL SECTION

Figure 1 and Equation 1–3 illustrate the overall fabrication of NCF using CO2 and hydrazine monohydrate (N2H4·H2O) as precursors via chemisorption, dissolution and solution combustion pyrolysis processes, and corresponding experimental photographs are displayed in Figure S1 and S2. As can be seen, the chemical sorption of emitted CO2 with hydrazine monohydrate solvent at room temperature can simply result in the formation of hydrazinoformic acid solution in large scale (Equation 1).27 Excessive Mg powders are then dissolved into the solution and automatically obtain a homogeneous gel-like phase owing to the spontaneous redox reaction between Mg and ammonium cationic group, and subsequent complexation process between Mg2+ and formate ions in solutions (Equation 2). With tungsten filament as the heat source, the ignition is induced through direct contact heating of the precursors, and the self-propagating combustion pyrolysis process is carried out for short periods of time (Equation 3). The final product is collected after etching away MgO impurities, repeated washing with deionized water and then freeze drying. The Mg reaction coefficient ϕ in Equation 3 is selected as 2 for the synthesis of NCFs in this work to support the self-sustainable combustion pyrolysis process and the thus synthesized samples are denoted as NCFN2H4. 2NH NH ∙ H O + CO → NH NHCOO + NH NH + 2H O (1) ACS Paragon Plus Environment


2NH NH + 2NH NHCOO + Mg → (NH NHCOO) Mg + 2NH NH + H (2) (NH NHCOO) Mg + 2NH NH + ϕMg + (ϕ/2+2)O → 2C + ϕMgO + 4N + 8H O (3) Conversion of CO2 to NCFs using hydrazine monohydrate as solvent. In a typical synthesis process, CO2 gas passes through a solution of hydrazine monohydrate with a concentration of 98% to form an ethyl hydrozino-formate solution. The absorption equilibrium was achieved within 10 minutes and the saturated CO2 adsorption capacity was 70 mol% corresponding to the reaction ratio displayed in Equation 1. Then, 10 g CO2-saturated hydrazine hydrate solution and excessive magnesium powders (2.5 g, ϕ=2) were added to a quartz crucible to form homogeneous solution spontaneously. Top ignition of the solution with tungsten filament contact heating and the self-propagating combustion pyrolysis process was carried out under ambient condition for ca. 10 seconds. Large quantities of gases, big flame and spark slop over of the crucible during combustion and thus the exterior environment should be noncombustible for safety. The grey powder was then neutralized in 1 M HCl to thoroughly etching away the MgO products. The final NCF product was collected after filtration, repeated washing with deionized water and vacuum freeze drying. Extension of the approach to fabricate NCFs with ethanediamine solvent. To certify the universality and investigate the structural changes of NCF under different adsorption solvents, ethanediamine (C2H8N2, EDA) solution is also employed and the thus synthesized sample is denoted as NCFEDA correspondingly. CO2 gas passes through the solution of EDA with a concentration of 98% to form an carbaminate solution respectively. 10 g stable carbaminate solution was then added to a quartz crucible with addition of excessive magnesium powders (2.2 g, ϕ=2). By subsequently heating the mixture of Mg powders and carbaminate solution above an electric stove (90°C) for 10 minutes, a uniform mixed reactant was shaped. Ignite the reactants through tungsten


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filament contact heating and the final NCF products are collected after etching away MgO impurities, repeated washing with deionized water and freeze drying. Combustion temperature measurement and calculation The temperature change was detected using a K-type thermocouple (OMEGA) during the combustion pyrolysis process. To view the comparison with experimentally measured maximum temperature, the calculation of adiabatic temperature (Tad) was conducted through equation S1 using the “Thermo” software package. The “Thermo” software contained properties of more than 2,500 compounds and its thermodynamic calculation was according to the minimization of thermodynamic potential.28 The reaction equation S1 was generated with simultaneous system of equations 1–3, which was considered for the rough thermodynamic calculation. The evolved gases content per mole of product was also calculated based on “Thermo” software and equation S1. Oxygen reduction reaction (ORR) measurement. The 4 mg NCFs was first ultrasonically dispersed in 1ml ethanol with 50 µl of Nafion solution (5%), and the mixed suspensions (~10 µl) were then dropwise added onto a glass carbon electrode (GC, ca. 0.25 cm2) to form a working electrode (mass load, ca. 0.16 mg·cm2). Measurements were carried out on a rotating ring-disk electrode (RRDE) using an MSRX electrode rotator (Pine Instrument) and a CHI 760D potentiostat. 0.1 M KOH was selected as the electrolyte. The counter and reference electrodes were Pt wire and Ag/AgCl, respectively. Nitrogen and Oxygen were used to give the N2-saturated and O2-saturated electrolyte solution, respectively. The transferred electron number (n) per O2 involved in ORR is determined by the Koutecky-Levich (K-L) equation. Current density j is related to the rotation rate ω of the electrode according to

!

= + =

(4)

+

where jk is the kinetic current and B is Levich slope which is given by



B = 0.2%&'( ()( )

! ! + *

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(5)

Here n is the number of electrons transferred in the reduction of one O2 molecule, F is Faraday constant (F = 96485 C mol-1), CO2 is the concentration of O2 in 0.1 M KOH (CO2 = 1.2 × 10–6 mol cm-3), DO2 is diffusion coefficient of O2 in the solution (DO2 =1.9 × 10–5 cm2 s–1) and ν is kinematics viscosity (v = 0.01 cm2 s–1). A constant of 0.2 is adopted when expressing the rotation speed in rpm. The n was also calculated by the equation as followed based on the RRDE data. (6)

% = 4 ,- ⁄(,- + ,. ⁄/ ) The peroxide percentage (% HO2¯) was calculated based on the equation: %23 = 200,. ⁄/ /(,- + ,. ⁄/)

(7)

Here Id is the disk current and Ir is ring current. N is the current collection efficiency of Pt ring that determined to be 0.37. Lithium ion battery (LIB) test. The working electrodes were fabricated by dispersing 80 wt.% synthesized NCFs materials, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) binder in 1-methyl-2-pyrrolidinone (NMP), and then the resultant slurry was uniformly coated on Cu foil current collector and dried at 100 °C overnight under vacuum condition. The CR2025 type coin cells were assembled using metal lithium foil as the reference and counter electrodes, and Celgard 2400 membranes as separators, respectively. The electrolyte was 1 M LiPF6 dissolved into an ethylene carbonate (EC) and dimethyl carbonate (DMC) (VEC:VDMC=1:1) mixture. The galvanostatic charge/discharge tests at different current densities were carried out at room temperature using a LAND CT-2001A cell test system in the voltage between 0.01 and 3 V (vs Li+/Li). Cyclic voltammetry (CV) was conducted on an IM6e electrochemical workstation in the potential range from 0.01 to 3 V at a scan rate of 0.1 mV s-1.


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Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 100 kHz to 0.01 Hz. Characterization. The morphology of the prepared samples was investigated by scanning (SEM, Hitachi S-4800) and transmission (TEM, JEM 2010) electron microscope. The elemental mappings were performed on a scanning transmission electron microscope (STEM) unit with high-angle annular dark-field (HAADF) detector (FEI Tecnai G2 F30) operating at 30 kV. The structures were characterized through X-ray diffraction (XRD) patterns by using a Netherlands X'pert pro MPD with a Cu Kα irradiation source (λ = 1.54 Å). Nitrogen adsorption-desorption isotherms were measured using a Quantachrome Autosorb-IQ-MP apparatus at 77 K, and the degassing process was conducted in vacuum at 473 K for 10 h. The pore size distribution and the specific surface areas (SSAs) of the samples were obtained by Brunauer Emmett Teller (BET) and Density Functional Theory (DFT) analyses of their adsorption isotherms. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab 250 Xi electron spectrometer from Thermo Fisher Scientific using 300 W AlKα radiation. Raman spectra were recorded using a LabRAM Aramis Microscopic Confocal Raman Spectrometer (Horiba Jobin Yvon, France) with a 532 nm laser.

RESULTS AND DICUSSION The morphology of NCFN2H4 product after chemical leaching is confirmed by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) characterization. As displayed in Figure 2a and b, well-defined three-dimensional foam of ultrathin carbon with porous structure can be discerned, and therein the interconnected nanoholes are clearly shown with a uniform diameter of ca. 3-50 nm (Figure S3, see Supporting Information). Figure. 2c and d show the corresponding TEM images of NCFN2H4. As can be seen, the NCFN2H4 displays a dense pore structure



with highly curved and wrinkled surface which corresponding to the SEM observation. The selected area electron diffraction (SAED) pattern (Inset in Figure 2c) gives a typical multiple ring feature with d-spacing of 0.338, 0.213 and 0.123 nm respectively, which correspond to (002), (100) and (110) face of graphite, indicating the random arrangement of NCFN2H4. Moreover, high resolution TEM (HRTEM) images of the suspended edge of the samples show around 10 layers of carbon composed of partially graphitized structure with an interplanar spacing of 0.34 nm, corresponding to the d-spacing of (002) crystal plane of bulk graphite with slight distortion. Consistent with the HRTEM results, the XRD profiles of the NCFN2H4 reveal two broad peaks at approximately 26° and 44°, which are indexed to the (002) and (101) planes of graphite (Figure 2e). Brunauer-Emmett-Teller (BET) analyses based on low-temperature nitrogen adsorption measurements show type-IV isotherms (IUPAC classification) with a typical mesopore hysteresis loop (Figure 2f) and NCFN2H4 has a specific surface area (SSA) of 375 m2 g-1, slightly larger than 362 m2 g-1 of newly developed 3D graphene foam.29 Furthermore, the NCFN2H4 expose wide pore size distribution with diameters ranging from 1 to 100 nm with a total pore volume of 1.40 cm3 g-1 (inset in Figure 2f). To probe the doped nitrogen atoms in the synthesized NCFN2H4 structure, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) measurements are carried out. As can be seen in Figure 2g, XPS shows a pronounced C 1s peak for NCFN2H4 at 284 eV, along with a weak O 1s peak and a relatively weaker N 1s peak at ca. 400 eV. Meanwhile, N/C atomic ratio of the NCFN2H4 sample is calculated to be 2.22% based on the atomic sensitivity factor and areas of C 1s and N 1s peaks (Table S1). The high-resolution N 1s spectrum inset in Figure 2g reveals the presence of pyridine (398.4 eV), pyrrolic (399.8 eV), and graphitic (401.3 eV) like N species, indicating that the N heteroatom has been doped into the NCF backbones.30,31 EDS quantifes the atomic percentages


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of C, N and O in NCFN2H4 are 92.95%, 4.80% and 2.24%, respectively (Figure S4, Table S2). Furthermore, EDS elemental mapping reveals the homogeneous distribution of C and N throughout the whole area of the NCFN2H4 samples (Figure 2h). Accordingly, NCF derived from CO2 is successfully prepared using N2H4·H2O as solvent. The pyrolysis approach is extended further to synthesize various porous nanostructures with different CO2 adsorption solvents, and NCF can be synthesized when altering the solvent from N2H4·H2O to EDA in this work. XRD pattern of the NCFEDA shows strong peaks located at approximately 26° and 44° indexed to the typical (002) and (101) planes of graphite (Figure S5). Further insight into the morphologies is performed with Raman spectroscopy (Figure 3a), which represents four typical bands indexed at ca. 1349, 1589, 2686 and 2943 cm−1, corresponding to sp3 defects (D band), the pristine sp2 lattice (G band), 2D band and D+G band. The strong D band with ID/IG ratio of 1.01 observed in NCFEDA indicates the presence of massive topological defects generated during pyrolysis synthesis and N-doping process.32 XPS measurements reveal pronounced C 1s, O 1s, and N 1s peaks (Figure 3b). The corresponding atomic contents of C, N, and O are 90.49%, 2.73%, and 6.78%, respectively, which are consistent with the contents confirmed by EDS (Table S2). The high-resolution N 1s spectrum given in Figure 3b reveals the presence of pyridine (398.4 eV), pyrrolic (399.8 eV), and graphitic (401.3 eV) like N species within graphene backbones.30,31 which is highly needed for electrochemical activity and could play great roles in the ORR and Li+ storage process.33 SEM images show a porous foam morphology of NCFEDA with a large size of about 10 µm (Figure 3c,d). TEM and HRTEM images reveal the presence of mesoporous structures of NCFEDA with the pore size around 9 nm (Figure 3e). N2 adsorption and desorption measurements show that the NCFEDA exhibites the mesoporous feature of the pore-size



distribution (Figure 3f), and the SSA is 630 m2 g-1. Furthermore, the total pore volume of NCFEDA is found to be 1.85 cm3 g-1, with the mesopore volume 1.35 cm3 g-1 (73%) and macropore volume 0.50 cm3 g-1 (27%) (Table S3). The size of the mesopores are centered mainly at 3.2 and 9.0 nm (inset in Figure 3f), which is consistent with the TEM results. Heteroatom-doped carbon material, particularly hierarchical porous carbon with thin layers, has been considered as the promising metal-free catalyst in electrochemical field.34–38 Therefore, the ORR performances of the NCFs are extensively investigated to demonstrate their electrocatalytic activity. Correspondingly, the cyclic voltammogram (CV) curves are recorded in nitrogen or oxygen saturated 0.1 M KOH solution by using Ag/AgCl as the reference electrode. As shown in Figure 4a, no redox features can be observed in the absence of oxygen. On the contrary, significant cathodic peaks are detected in O2-saturated electrolytes, implying electrocatalytic activity of the NCFs. Moreover, NCFEDA exhibits a cathodic peak potential at ca. –0.15 V, which is about 0.08 V more positive than that of NCFN2H4. To further assess the ORR performance of the NCFs, the linear sweep voltammograms (LSVs) measured on a rotating ring-disk electrode (RRED) is performed with a rotation rate of 1600 rpm (Figure 4b). In terms of the half-wave potential and diffusion-limited current density, the ORR performances are as follows: NCFEDA (–0.22 V, 3.2 mA cm−2) > NCFN2H4 (–0.28 V, 4.0 mA cm−2). Furthermore, the NCFEDA has a high onset potential of ca. -0.09 V, which is among the best results of recently reported typical porous carbon-based materials (Table S4). The LSV curves of NCFs are also recorded at different electrode rotation rate (Figure 4c, Figure S6). With increasing the rate of rotation, the diffusion current densities of the NCFEDA catalyst gradually increase. The transferred electron number (n) can be analyzed based on the Koutecky-Levich (K-L) equation (Equation 4 and 5), and the K–L plots as well as n at various potentials are displayed in


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Figure 4c inset and Figure S6b. Furthermore, ring and disk currents obtained from the RRDE curves are futher employed to confirm n and the H2O2 yield on NCFs electrode (Equation 6 and 7). As shown in Figure 4d, the mean n of NCFEDA is calculated about 3.7, which is consistent with the n calculate by K–L measurements, thus indicating a nearly four-electron pathway for the ORR. The corresponding H2O2 yield obtained from the ring current is below 20% over the entire potential range, which further confirms the nearly four-electron ORR process of the NCFEDA electrode. These observations demonstrate the NCFs derived from pyrolysis conversion of CO2 is indeed an efficient metal-free catalyst for ORR in an alkaline solution. Between these two samples, the greater performance of NCFEDA could be attributed to its hierarchically porous structure with an higher surface area, which ensures that electrocatalytic active sites are sufficiently exposed. The carbon-based material with a hierarchical porous architecture and electron-rich N doping is suitable as a high-performance electrode material for energy storage,39,40 and the electrochemical performance of the synthesized NCFN2H4 is also investigated as a LIB anode. Figure 5a shows the charging/ discharging behavior profiles of NCFN2H4 for the 1st, 2nd, 20th, 50th and 100th cycles at a current density of 100 mA g-1 with a voltage range from 0.01 to 3.0 V. It is observed that an initial discharge capacity of 1667 mAh g-1 is achieved, and the reversible capacity maintains a high value of ca. 856 mAh g-1, 2.3 times as compared to the theoretical capacity of graphite (372 mAh g-1) and also among the best values reported previously (Table S5). The capacity becomes stable and reversible after the first cycle, and the capacity loss of ca. 49% is due to the formation of a solid electrolyte interphase (SEI) layer and irreversible trapping of lithium ions in the SEI film and/or in the lattice, as well as electrolyte decomposition.41,42 Cyclic voltammograms (CV) are further studied in detail to track the Li-ion storage process. As shown is Figure 5b, an apparent peak centered at 0.46 V is



observed in the first discharge of NCFN2H4, which behavior could be attributed to irreversible reactions leading to the formation of SEI layer.43 The peak disappears completely in the following cycles, and the almost overlapped CV curves between the third and fifth cycles indicate that a stable state of the NCFN2H4 is attained quickly. In the process of Li-extraction, four main peaks located at ca. 0.1 V, 1.2 V, 1.9 V and 2.4 V appear simultaneously, implying the multiple available lithium storage sites. Of these, the peak at 0.1 V is associated with lithium extraction from the graphitic layers, while thats between 0.8 and 2.0 V are due to lithium extraction from the pore structures and defects, and the relatively weak peak between 2 and 3 V arises from Li binding with heteroatoms on the anode surface. 44–46 The three kinds of lithium storage locations could be ascribed to the high surface area, hierarchically porous structure with reasonable distribution of micro-, meso-, and macropores, and the reversible binding at heteroatoms, which may co-contribute ultimately to the high capacity of the NCFN2H4 electrode. Figure 5c displays the cycling performance and Coulombic efficiency of NCFN2H4 at a current density of 100 mA g-1. As can be seen, the reversible capacity is gradually stabilized at ca. 881 mAh g-1 over 100 cycles, and the Coulombic efficiency retains around 100%. In addition, the capacity is observed to increase upon subsequent cycling, which may be attributed to the activation process of its hierarchical porous structure. Importantly, the NCFN2H4 exhibits a satisfactory rate capacity under variable rates from 100 to 3000 mA g-1 (Figure 5d). The reversible capacities are ca. 770, 656, 597, 491, 426, and 408 mA h g-1 at 100, 200, 300, 500, 1000 and 2000 mA g-1, respectively. Even at a high current density of 3000 mA g-1, the reversible capacity (380 mAh g-1) of NCFN2H4 is still larger than that of theoretical capacity of graphite. Moreover, the high value of 875 mAh g-1 is recovered when the current density is lowered back to 100 mA g-1 after cycling at different rates, implying


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highly stable cycling performance and good reversibility of NCFN2H4 anode. Electrochemical impedance spectroscopy (EIS) analysis are further performed at 100 mA g-1 when before and after 100 cycles (Figure S7). The high-frequency semicircle is due to the SEI resistance and its intercept on Z' axis represents the resistance of electrolyte. The intermediate-frequency semicircle is associated with the charge-transfer resistance while the low-frequency straight line is corresponding to the Warburg impedance of lithium ions diffusion. According to the fitting results, the coin battery of NCFN2H4 after cycling presents an obvious depressed semicircle than that of the initial battery, indicating the improved electronic conductivity and high mobility for Li+ ion diffusion between the interfaces of electrolyte and the NCFN2H4 electrode materials, thus enhance the discharge–charge performance and rate capability of the resultant batteries. The pyrolysis process is demonstrated to be an effective method for the production of NCF materials based on the above analysis, to penetrate into the formation and evolution of NCF morphology, other NCF samples without etching away MgO are also analysed through XRD, BET and TEM images (Figure 6, S8, S9). Since MgO is widely used as hard template to produce porous or thin layers of carbon structure owing to its structural, compositional, and thermal stabilities,24,47 it is reasonable to assume that the synthesized MgO in our combution pyrolysis system exerts a crucial effect on the final morphologies of NCFs. XRD pattern verify that MgO are synthesized in-situ with high crystalline degree (Figure S8), and BET analysis exhibit a SSA of 85.11 m2 g-1 for NCFN2H4 sample before etching treatment (Figure S9). The TEM images show that the holes of NCFN2H4 are filled by MgO nanoparticles with a size less than 50 nm, which demonstrate that MgO nanoparticles work as templates for the porous carbon structure (Figure 6a–d). In addition, NCFEDA sample emerges large size MgO sheets completely covering on the porous carbon, suggesting that MgO at



this time acts as templates both to provide suffcient locations for carbon growth and to the formation of nanoholes (Figure 6e–h). Measurement and theoretical calculation of adiabatic temperature (Tad) also helps to understand and control the combustion pyrolysis process. For this self-sustained reaction shown in Table S6, Tad of NCFN2H4 and NCFEDA samples are estimated as 3095.66 K and 3002.40 K, respectively (Figure S10a). Meanwhile, the calculation results show large amount of gas evolution during combustion, which could dissipates the heat and promotes the porosity of NCFs (Table S6). As expected, the measured maximum temperature of NCFN2H4 is always lower than Tad with a value of 1400K (Figure S10b). The discrepancy can be attribute to the nonadiabatic experimental conditions as well as the possible delay and threshold of the thermocouple signal. Nevertheless, the trends of Tad keep consistent with the measured maximum temperature, that is the Tad show no such rise by increasing the amount of Mg. On the basis of the above theories, we speculate that the formation of hierarchical pore structure in the NCFs is due to the release of H2O and N2 gases arising from the thermal pyrolysis that generate the relatively large pore structures (macropores and mesopores), as well as left small pore structures (mesopores) after etching away MgO particles. CONCLUSIONS In summary, we develop a new controllable and cost effective pyrolysis process to produce N-doped carbon foam directly from captured CO2 in flammable alkaline solutions, which may allow the widespread usage of greenhouse gas CO2 as a feedstock for the large-scale production of highly valued form of carbon materials. Throughout this process, the solution is used as CO2 adsorbent, fuel as well as nitrogen source for the NCFs, and Mg powders are directly involved into the reaction to provide complex metal ion and also energy for self-propagating high-temperature combustion


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pyrolysis. The as-prepared NCFs exhibit outstanding electrochemistry performance toward ORR and lithium-ion battery benefiting from well-formed foam structure, in situ nitrogen doping, ultrahigh specific surface area and hierarchical porous structure. More importantly, our approach introduce a general system that can further integrate various additives dissolved in reaction solutions homogeneously, thus opening a new avenue of constructing carbon-based funequactional materials with controlled morphology and modified property derived from CO2 for promising applications in future fuel cells, supercapacitors, lithium ion batteries, and sensors areas.



ASSOCIATED CONTENT Supporting Information Experimental details and data. Figure S1-S10 and Table S1-S6. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors thank the financial support from National Natural Science Foundation of China under Grant No. 11521062.


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Figure 1. Overall process of solution combustion pyrolysis conversion of CO2 to NCFN2H4. (i) Chemical sorption of CO2 with hydrazine hydrate to form hydrazinoformic acid solution. (ii) Dissolution of the added Mg powders for the formation of homogeneous organic salt of magnesium. (iii) Generation of NCFN2H4 under pyrolysis reaction and the following acid etching treatment to remove impurities.


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Figure 2. (a) Low- and (b) high-magnification SEM images of as-prepared NCFN2H4. (c) TEM image and (d) the corresponding high-resolution TEM image of the edge of NCFN2H4. Inset in (c) is the selected area electron diffraction pattern. (e) XRD pattern of NCFN2H4 sample. (f) The nitrogen adsorption-desorption isotherm of NCFN2H4, inset of (f) is the corresponding pore size distribution curves obtained by DFT method from nitrogen adsorption isotherm. (g) XPS spectrum of NCFN2H4 and the corresponding high-resolution N 1s peak. (h) STEM image and (i) C- and N- elemental mappings for nanohole regions of NCFN2H4.



Figure 3. (a) Raman spectra and (b) XPS spectrum of NCFEDA sample. The inset of (b) is the corresponding high-resolution N 1s peak. (c,d) SEM and (e) TEM images of NCFEDA, inset of (e) is the corresponding HRTEM images. (f) The nitrogen adsorption-desorption isotherm of as-prepared NCFEDA sample, inset of (f) is the corresponding pore size distribution curves obtained by using DFT method from nitrogen adsorption isotherm.


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Figure 4. (a) CV curves of the obtained NCFs (NCFN2H4 and NCFEDA) in an O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s–1. (b) LSV of various electrocatalysts on a RRDE (1600 rpm) in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s–1. (c) LSV curves of the NCFEDA in O2-saturated 0.1 M KOH solution at different rotating speeds. The inset shows the Koutecky-Levich plots derived from LSV curves at different potentials. (d) HO2- production and the corresponding n of the NCFEDA derived from RRED measurement.



Figure 5. (a) Galvanostatic discharge/charge voltage profiles of NCFN2H4 electrode for the initial two cycles, the 20th, 50th and 100th cycles between 0.01 and 3.00 V at a current density of 100 mA g-1. (b) CV curves of NCFN2H4 electrode in the initial five cycles. (c) Capacity vs cycle number and the corresponding Coulombic efficiency of the NCFN2H4 electrode at 100 mA g-1 for 100 cycles. (d) Rate capacity of NCFN2H4 electrode, and the corresponding Coulombic efficiency electrode from 100 to 3000 mA g-1.


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Figure 6. TEM images of as-prepared (a-d) NCFN2H4 and (e-h) NCFEDA samples before etching (a,b,e,f) and after etching (c,d,g,h) treatment correspondingly.



Table of Content (TOC) graphic and synopsis

This work provides a general pyrolysis reaction for the first time that can be scaled up in directly capturing CO2 to produce N-doped carbon foam as a functional nanomaterial.


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Scalable Conversion of CO2 to NâDoped Carbon ... - ACS Publications

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