Facile Synthesis of Iron and Nitrogen Doped Porous Carbon for

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Facile Synthesis of Iron and Nitrogen Doped Porous Carbon for Selective CO2 Electroreduction Jun-Jie Shi, Xin-Ming Hu, Monica Rohde Madsen, Paolo Lamagni, Emil Tveden Bjerglund, Steen Uttrup Pedersen, Troels Skrydstrup, and Kim Daasbjerg ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00755 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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ACS Applied Nano Materials

Facile Synthesis of Iron and Nitrogen Doped Porous Carbon for Selective CO2 Electroreduction Jun-Jie Shi, Xin-Ming Hu,* Monica Rohde Madsen, Paolo Lamagni, Emil Tveden Bjerglund, Steen Uttrup Pedersen, Troels Skrydstrup, and Kim Daasbjerg*

Carbon Dioxide Activation Center (CADIAC), Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University Gustav Wieds Vej 14, 8000 Aarhus C, Denmark E-mail: [email protected]; [email protected]

Abstract A general and simple solvent-free procedure using direct heating of a ball-milled mixture of L-histidine-Fe2O3-FeCl3 is developed for the synthesis of iron and nitrogen doped porous carbon electrocatalysts. Through adjustment of the reactant ratios and the pyrolysis temperature, a series of electrocatalysts are easily obtained with varying activities for electrochemical CO2 reduction reaction (CO2RR). The electrocatalyst synthesized from L-histidine-Fe2O3-FeCl3

at a 4:1:0.25 component ratio at 1000 °C exhibits the highest faradaic

efficiency of 83% for CO2-to-CO conversion at a small overpotential (360 mV) in aqueous media. The use of a number of characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy, and nitrogen sorption experiments, reveals that both Fe2O3 and FeCl3 contribute to the iron doping and formation of porosity. As a result, they are both crucial to produce the optimal CO2RR electrocatalyst. Correlation of the CO2RR activity with the carbon structure suggests that the degree of graphitization of the carbon electrocatalysts plays an important role in their CO2RR performance.

Keywords CO2 reduction; porous carbon, iron doping; electrocatalysis; graphitization

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Introduction The conversion of CO2 to value-added chemicals through electrochemical reduction reaction (CO2RR) has attracted large scientific attention in recent years.1,2 This process is not only able to mitigate the atmospheric CO2 concentration but also to store the renewable electrical energy in chemical bonds. However, CO2 is inert and usually a high overpotential is required for the direct conversion of CO2 at an electrode. Thus, highly efficient electrocatalysts should be pursued to activate the CO2RR. Porous carbon stands out among the reported materials for electrocatalysis, due to its high surface area and good conductivity.3 The abundant micropores and/or mesopores in the carbon platform contribute to an increased number of accessible active sites, while the macropores are beneficial for the mass transport.4 In addition, the high conductivity of carbon facilitates the electron transfer from a current collector to the reactant.5 For these reasons, carbon has been demonstrated to be able to enhance the activity of various metal complexes for CO2RR.6–8

Noteworthy, the carbon itself shows no catalytic activity, meaning that it is only

after functionalization or doping with heteroatoms, such as nitrogen and/or transition metals, that it exhibits high activity and selectivity for CO2RR.9–12 In this context, increasing efforts are being devoted to develop heteroatom (i.e. Fe, Co, and Ni) doped porous carbon electrocatalysts.13–16 Up to date, several procedures were explored for the synthesis of porous carbon. Since the use of hard SiO2 template was introduced by Han et al. in 1999,17 the way of employing nanostructured SiO2 as template has been widely considered.18,19 The advantage of this method is the ease by which the carbon can be formed on the template followed by the removal of the latter. Unfortunately, hazardous hydrofluoric acid is usually required to completely remove the SiO2 template.20 To avoid this problem, the silica can be replaced with amphiphilic polymers serving as a soft template for the porous carbon synthesis.21,22 Yet, this template is less tempting for large-scale synthesis because of the high cost of amphiphilic polymers. Hydrothermal carbonization presents another effective way of synthesizing porous carbon from cost-effective precursors, while avoiding the use of any templating agent.23,24 However, the conductivity of the resulting carbon is comparatively poorer. Recently, pyrolysis of porous organic polymers25 and metal-organic frameworks26,27 has been proved to be another 2


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effective way for the synthesis of carbon materials. The porous frameworks act as templates and/or precursors, leading to carbon with a high porosity inherited from the frameworks. However, this way may involve the complicated pre-synthesis of the porous frameworks. Therefore, it is still necessary to develop a general and simple way to synthesize porous carbon with desired properties from cost-effective precursors. In this work, we report a facile solvent-free strategy to prepare iron and nitrogen doped porous carbon. This is achieved by direct pyrolysis of a ball-milled mixture consisting of L-histidine

(His), Fe2O3 nanopowder, and FeCl3. These precursors are all commercially

available and can be purchased at low cost. A series of carbon electrocatalysts of varying CO2RR activities can be obtained by adjusting the ratio of the three reactants and the pyrolysis temperature. Notably, the carbon electrocatalyst prepared from His-Fe2O3-FeCl3 at a 4:1:0.25 component ratio at 1000 °C shows the highest faradaic efficiency (FE) for CO2-to-CO conversion in water. Further studies reveal that the graphitization degree of the carbon significantly affects the CO2RR activity, thus adding to the understanding of the relationship between this activity and the carbon structure.

Experimental section Material synthesis L-histidine

(His, Sigma-Aldrich), Fe2O3 (size < 50 nm, Sigma-Aldrich), and FeCl3 (Fisher

Scientific) were commercially available and used as received. The amounts of His, Fe2O3, and FeCl3 required to achieve a given component ratio were added to a tungsten carbide crucible containing 6 tungsten carbide balls of 8 mm diameter (see Table S1). The crucible was placed in a planetary mono ball-miller (Pulverisette 6, Fritsch) to undergo 3 cycles of 10 min ball-milling at a milling speed of 350 rpm. The ball-milled mixture was heated to 1000 °C at a heating rate of 20 °C min−1 and kept at this temperature for 1 h under an Ar atmosphere. The resulting material was first washed in 40 mL 0.5 M sulfuric acid solution at room temperature overnight, then at 90 ℃ for 4 h, and finally at room temperature overnight. At each washing, the material was filtered off and flushed with deionized water until the filtrate was neutral. Afterwards the material was stirred in deionized water for 2 h, filtered, and dried in the fume hood overnight. The last step was a 3



second pyrolysis carried out under the same conditions as the first one, yielding the target carbon electrocatalyst. For comparison, carbon electrocatalysts were also synthesized either at varied temperature (700, 800, and 900 °C) or in the absence of Fe2O3 or FeCl3, while keeping all other conditions unchanged.

Materials characterization X-ray photoelectron spectroscopy (XPS) was performed with a Kratos Axis Ultra-DLD instrument using a monochromatic Al Kα X-Ray source at a power of 150 W. The CasaXPS software was used for fitting with binding energy of C 1s at 285.0 eV for calibration. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements were carried out using a ION-TOF (GmbH) instrument equipped with a Bi liquid metal ion source. A 25 keV Bi3+ was employed as primary ion beam operating in the high current bunched mode. The total ion dose was less than 1 × 1013 ions cm−2 per analysis with a cycle time of 100 µs. Inductively coupled plasma–optical emission spectroscopy (ICP-OES) was performed on an AMETEK Spectro Arcos FHS 12 instrument. Powder X-ray diffraction (PXRD) was conducted on a Rigaku SmartLab X-ray diffractometer along with monochromatic Cu Kα1 radiation (λ = 1.54056 Å). Raman Spectroscopy was performed on a Renishaw inVia Raman microscope. An Ar-ion laser excitation at 514 nm was applied using a power of 5 mW. The N2 sorption experiments were carried out on an Autosorp iQ (Quantachrome Instruments) with a liquid nitrogen bath at 77 K. Specific surface areas were calculated using Brunauer–Emmett– Teller (BET) theory at P/P0 = 0.05–0.2. The adsorption branch with the quenched solid density functional theory (QSDFT) approach was used to analyze the pore size distribution. Bright field images and energy-dispersive X-ray spectroscopy (EDX) mapping were acquired using FEI Talos F200X analytical scanning transmission electron microscopy (STEM) with a 200 kV XFEG electron gun. The carbon electrocatalyst was sonicated in methanol and then drop casted onto a lacey carbon support film on copper grids and dried in oven. Scanning electron microscopy (SEM) images were acquired with a FEI Nova 600 instrument equipped with a through-the-lens detector. More details of the characterizations are described in our recently published paper.28 4


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Electrode preparation Glassy carbon plates (1 × 2 cm2, Sigradur G, HTW) connected to a silver wire were used as the support material for the working electrodes. They were cleaned before use by sonication in water, acetone, and pentane for 10 min, respectively. Typically, 6 mg catalyst powder was dispersed in 450 µL Milli-Q water, 90 µL isopropanol, and 60 µL Nafion® perfluorinated resin solution (5 wt. % in a mixture of lower aliphatic alcohols and water; Sigma-Aldrich), and sonicated for 15 min. From the resulting suspension, 50 µL was drop casted on each side (1 × 1 cm2, 2 cm2 in total) of a glassy carbon plate and dried in a fume hood, providing a working electrode coated with the carbon electrocatalyst (concentration = 0.5 mg cm−2). Cyclic Voltammetry Cyclic voltammetric experiments were performed using a CH Instrument 601D potentiostat connected to a three-electrode setup. A catalyst coated glassy carbon plate served as the working electrode, while a platinum mesh and Ag/AgCl (ElectroCell LF-1) were used as the counter and reference electrodes, respectively. Cyclic voltammograms were recorded at a sweep rate of 0.01 V s−1 in aqueous 0.5 M KHCO3 solution. The potential measured against Ag/AgCl was converted to the reversible hydrogen electrode (RHE) using E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + RT/F × ln10 × pH = E (vs Ag/AgCl) + 0.63 V, where the pH of a CO2 saturated 0.5 M KHCO3 solution is 7.3. Controlled Potential Electrolysis and Gas Detection Controlled Potential Electrolysis (15 min) Controlled potential electrolysis was carried out in a custom-made H-cell using the same potentiostat as for cyclic voltammetry. The cathodic compartment contained a working electrode covered with the carbon electrocatalyst of interest and a Ag/AgCl reference electrode. The anodic compartment had a platinum mesh counter electrode. The electrolyte was aqueous 0.5 M KHCO3 solution, which was saturated with CO2 before electrolysis. For control experiment, electrolysis was carried out in Ar-saturated electrolyte under otherwise the same condition. For the poisoning experiments, 20 mM KCN was added to the catholyte. To analyze the gaseous products, the gas in the headspace of the H-cell was manually injected 5



with a 250 µL gas-tight syringe (SGE Analytical Science) to a gas chromatography system (Agilent Technologies 7890B) equipped with a thermal conductivity detector (TCD). The product in the liquid phase after electrolysis was analyzed on a Thermo Scientific Dionex ICS-1100 ion chromatography equipped with a Dionex IonPac AS10 column for anion separation, a suppressor for suppressing the conductivity of the eluent, and a DS6 conductivity cell for detection. The catholyte was filtered and diluted by a factor of 10 before being loaded for quantification. Controlled Potential Electrolysis (12 h) The setup was the same as for the 15 min electrolysis experiment, the exception being that CO2 was continuously purged to the catholyte at a rate of 10 mL min−1 during electrolysis. The gas phase in the headspace of the H-cell was automatically sampled every 30 min by an Agilent gas sampling valve connected to the same Agilent gas chromatograph as described above.

Results and Discussion Synthesis of carbon electrocatalysts Figure 1 presents the synthesis protocol for the carbon electrocatalysts. The mixture of L-histidine

(His), Fe2O3, and FeCl3 with a preset ratio was ball-milled and pyrolyzed under

inert atmosphere, leading to the formation of iron and nitrogen doped carbon containing Fe2O3, iron particles, and iron carbide. After the removal of the iron based particles by acid washing, a second pyrolysis was applied to get rid of oxygen functionalities on the surface,28 yielding the target carbon electrocatalyst. In the entire synthesis process, no organic solvent is used, and all the reactants are cost-effective and environmental friendly. Thus, it represents a sustainable way to prepare carbon electrocatalysts. The as-synthesized catalysts were named after the feed ratio of the three reactants (His-Fe2O3-FeCl3) and pyrolysis temperature (Table S1). Decent yields of carbon electrocatalysts (mostly > 20%) were obtained, calculated relative to the amount of L-histidine used. This reveals a good recovery of the carbon given that the theoretical carbon content in 6


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histidine is 45.6%. Notably, the yield of carbon electrocatalyst increases initially with addition of FeCl3 in the synthesis to reach a plateau at a certain point (Figure S1a). This indicates that FeCl3 works as activation agent in the pyrolysis process to catalyze the formation of carbon electrocatalyst from L-histidine.29,30 There seems to be no such correlation between the yield of carbon electrocatalyst and the amount of Fe2O3 added (Figure S1b).

Figure 1. Protocol for the synthesis of carbon electrocatalysts.

Characterization of carbon electrocatalysts The composition of the carbon electrocatalysts was characterized using X-ray photoelectron spectroscopy (XPS). In general, the elemental composition for all electrocatalysts is similar, in that they consist of mostly carbon, a small amount nitrogen (0.3−3.6 at%), and trace amount of iron (maximum 0.3 at%) (Figure 2a and Table S2). The iron content in the electrocatalysts was further confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Figure S2 and Table S3), which reveals similar results to the XPS analysis. In general, the iron content of the resulting electrocatalysts increases with the addition of FeCl3 and Fe2O3, indicating that both iron sources contribute to the iron doping. Deconvolution of the high-resolution N 1s spectra of these carbon electrocatalysts shows four types of N species, i.e. the pyridinic (398.5 eV), pyrrolic (399.6 eV), and graphitic N (401.3 eV), as well as N-oxide (402.5 eV) (Figures 2b).31 The N percentage of all types decreases with the pyrolysis temperature (Figure S3a). Notably, Fe2O3 seems to promote N depletion during synthesis since 4-1-0.25-1000 has a considerably lower N content than 4-0-0.25-1000 (without using Fe2O3). The high-resolution Fe 2p spectra of 4-1-0.25-700 and 4-1-0.25-800 both show a narrow peak at 707.5 eV and a broad peak around 711 eV (Figure 2c). The former peak is attributed to zero-valence Fe (Fe0),32 while the latter one is assigned to the Fe3+ 2p3/2 spin orbit 7



component with exactly the same binding energy as that for iron(III) porphyrin.6,33 Moreover, the binding energy and peak shape of this Fe3+ resemble the iron species in previously reported iron doped carbon materials, in which, the iron was shown by extended X-ray absorption fine structure (EXAFS) to be coordinated to N (forming Fe-Nx).28,34 In addition, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on 4-1-0.25-1000, which shows the existence of FeNC species (Figure S4). Hence, it is reasonable to infer that Fe3+ species present in the carbon electrocatalysts, most likely, are coordinated with nitrogen atoms to take a similar structure as that of iron porphyrin as suggested by many researchers.28,31,34,35 The much broader peak for Fe-Nx than for Fe3C reveals that the Fe-Nx species is dominant. In other carbon electrocatalysts, these two peaks are hardly discernible due to the low iron concentration, but, nevertheless, they are still formed (Figure S3b).

C

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Binding Energy (eV)

Figure 2. (a) Survey, (b) N 1s, and (c) Fe 2p XPS spectra of the carbon electrocatalysts.

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Figure 3a shows the powder X-ray diffraction (PXRD) patterns of the carbon electrocatalysts. They all display a sharp peak at 2θ = 26.1°, which is attributed to the diffraction from (002) planes of graphitic carbon. Noteworthy, the intensity of this peak increases when the pyrolysis temperature is increased from 700 to 1000 °C, indicating that a higher degree of graphitization occurs at higher temperature. In addition, a set of small peaks is observed, which is ascribed to the diffraction from crystalline phases of iron carbide (Fe3C, Figure S5). These Fe3C species are suggested to arise from the reduction of iron precursors during pyrolysis. In this high temperature process, the iron precursors (i.e. Fe2O3 and FeCl3) can be reduced to active metallic iron atoms, which react with nitrogen to form Fe-N and with carbon to form Fe3C.34,36 Note that no diffraction peaks of Fe0 particles is observed, which suggests that Fe0 particles are completely removed after the acid washing, while Fe3C and Fe-N remain.

a

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Figure 3. (a) PXRD patterns, (b) Raman spectra, and (c) pore size distribution of the carbon electrocatalysts. 9



Raman spectra show three strong peaks in all carbon electrocatalysts at 1350, 1590, and 2700 cm−1, corresponding to the D, G, and 2D peaks, respectively (Figure 3b). The D peak reflects defects induced by disorder, the 2D peak is the D-peak overtone, while the G peak arises from the stretching of the C-C bond in graphitic carbon.37 At higher pyrolysis temperature, the electrocatalyst exhibits a lower ratio of the intensities of D and G peaks, ID/IG, but a more intense 2D peak. These results indicate that the graphitization is considerably favored by increasing the temperature, in good agreement with the recorded PXRD patterns. Compared with 4-0-0.25-1000 (without using Fe2O3), 4-1-0.25-1000 shows considerably lower ID/IG ratio and thus less defects or disorders in the graphitic structure. This indicates that Fe2O3 enhances the graphitization of the resulting material, consistent with its effect on promoting N depletion as shown by XPS analysis. Nitrogen sorption experiments were used to characterize the porosity of the carbon electrocatalysts produced (Figure S6). In general, they exhibit similar pore size distribution, with both micropores and mesopores being present, although 4-1-0.25-700 features more mesopores (Figure 3c and Table S4). Accordingly, their surface areas are similar, all in the range of 200−315 m2 g−1, with the 527 m2 g−1 obtained for 4-1-0.25-700 being the exception. This suggests that both FeCl3 and Fe2O3 can contribute to the formation of porosity. Nevertheless, addition of more Fe2O3 results in a 1.5 folds higher surface area, from 204 m2 g−1 for 4-1-0.25-1000 to 315 m2 g−1 for 4-4-0.25-1000, while in the case of FeCl3 the surface area does not change. This indicates a templating effect of the Fe2O3 nanopowder, which is further confirmed by scanning electron microscopy (SEM) (Figure S7). Transmission electron microscopy (TEM) clearly reveals the presence of both graphene sheets and amorphous carbon domains in the electrocatalysts (Figures 4a-c). A lattice distance of 0.339 nm is observed in the graphene sheets (Figure 4c), which corresponds to the (002) plane and is consistent with the sharp peak of 2θ = 26.1° observed in PXRD patterns. Furthermore, some particles wrapped by graphitic carbon layer can be observed (Figure 4c). The lattice distance of these particles is measured to be 0.206 nm, corresponding to the (210) plane of Fe3C. This is completely consistent with the PXRD peak at 2θ = 43.8° of Fe3C (Figure S5). The presence of Fe3C particles is further confirmed by elemental mapping using energy dispersive X-ray spectroscopy (EDX), where bright Fe signals are seen (Figure 4d). 10


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Apart from this, the elemental mapping shows the uniform distribution of N and also small amount of Fe distributed in the carbon electrocatalyst.

Figure 4. TEM images of the typical structure of the 4-1-0.25-1000 carbon electrocatalyst at scale bar (a) 200 and (b) 50 nm. (c) HRTEM image of a typical iron carbide particle wrapped in graphitic layer (scale bar: 10 nm). (d) HAADF-STEM image (scale bar: 200 nm) and element mapping of the 4-1-0.25-1000 carbon electrocatalyst.

CO2RR activity on carbon electrocatalysts Next, the catalytic CO2RR activity of the carbon electrocatalysts deposited on glassy carbon plates was tested. The cyclic voltammogram recorded on 4-1-0.25-1000 shows a reduction current in aqueous 0.5 M KHCO3 solution under Ar atmosphere, which is attributed to the hydrogen evolution reaction (HER, Figure 5a). Under a CO2 atmosphere the reduction current starts increasing at a less negative potential, indicating that this material electrocatalyzes the reduction of CO2. The same phenomenon is observed for other carbon electrocatalysts, although the electrocatalytic effect in the case of e.g. 4-1-0.25-700 is not as significant (Figure S8).

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Potential (V vs RHE)

Figure 5. (a) Cyclic voltammograms recorded on 4-1-0.25-1000 as electrocatalyst at a sweep rate of 0.01 V s−1 in 0.5 M KHCO3. (b) FE and |j| of 15 min electrolysis of CO2 at −0.57 V vs RHE for various electrocatalysts in 0.5 M KHCO3.

Controlled potential electrolysis was performed at −0.57 V vs RHE to confirm the CO2RR. Gas and ion chromatography were employed to quantify the products in the gaseous and liquid phases, respectively. CO and H2 were detected as the sole gaseous products with the sum of their faradaic efficiency (FE) being close to 100% after 15 min electrolysis in CO2 saturated 0.5 M KHCO3 solution (Figure 5b). No carbon containing products were detected in the liquid phase. These results suggest that CO2 is selectively reduced to CO catalyzed by the carbon electrocatalysts. Of the various electrocatalysts pyrolyzed at different temperatures, 4-1-0.25-1000 exhibits the highest FE for CO production (FECO = 80%) with an absolute current density (|j|) of 1.15 mA cm−2 at −0.57 V vs RHE. In contrast, electrolysis in Ar saturated electrolyte using the same electrocatalyst results in a much lower |j| (= 0.29 mA cm−2) and produces H2 (FEH2 = 58%) as the main product. Notably, a small amount of CO (FECO = 9%) is also formed, which is attributed to the reduction of the small amount of CO2 available from the equilibrium of HCO3−/H2CO3/CO2 in the electrolyte, as demonstrated in our previous work.8 Lowering the pyrolysis temperature from 1000 to 700 °C results in an increase of |j| (from 1.15 to 2.93 mA cm−2), which is accompanied by a monotonic decrease of FECO from 80% to 41% while FEH2 increases. In comparison, the carbon electrocatalyst synthesized in the absence of FeCl3 (i.e. 4-1-0-1000) exhibits both lower FECO and |j| (Figure 5b). In our initial study, the amounts of 12


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His and Fe2O3 were kept constant, while the amount of FeCl3 was adjusted to optimize the synthesis. As shown in Figure S9a, addition of more FeCl3 results in an initial increase of both FECO and |j|. Further addition of FeCl3 leads to a slightly larger |j|, but at the expense of a significantly lowering of FECO. The carbon electrocatalyst prepared without using Fe2O3 (i.e. 4-0-0.25-1000) shows even lower FECO and |j| (Figure 5b). The maximum FECO is achieved for 4-1-0.25-1000, while less and more Fe2O3 feeding both leads to lower FECO, although |j| becomes higher in the latter case (Figure S9b). These results highlight the importance of adding FeCl3 as well as Fe2O3 in the synthesis because both play a role as iron source and porogen. Why do these carbon electrocatalysts exhibit so different activities? To shed light on this question, it was attempted to correlate the CO2RR activity with their structure. We employed the blocking capability of cyanide ions (CN−) toward metal centers to test the active sites in the electrocatalysts. Addition of CN− to the catholyte leads to decrease of both FECO and |j| in the electrolysis, which can be partly recovered after washing the electrode material and replacing the electrolyte with freshly prepared 0.5 M KHCO3 (Figure S10). This suggests that the N-coordinated Fe (Fe-Nx) centers are active sites for CO2RR, in agreement with literature.28,38 In addition, it has been demonstrated that Fe3C shows little activity for CO2RR and catalyzes HER in a recent report.35 The differences observed in the activities of the catalysts could then simply originate from the abundances of Fe-Nx present in the electrocatalysts. Yet, this does not seem to be the case, considering that 4-1-0.25-700 exhibits the lowest FECO (Figure 5b), although the amounts of Fe-Nx sites are much higher than the other carbon electrocatalysts produced at other temperatures in the 4-1-0.25 series (Figures 2c and S3b). Furthermore, the porosity of these carbon electrocatalysts is comparable (Figure 3c), i.e. neither this can be the reason of the activity differences seen. Rather, the increase in FECO seems to be correlated with the degree of graphitization as evidenced by the trend observed for the four 4-1-0.25 carbon electrocatalysts prepared at different temperatures. The same is observed when comparing 4-1-0-1000 and 4-0-0.25-1000, with the former one showing a higher degree of graphitization and thus higher FECO. These results point to the conclusion that the degree of graphitization (Figure 3a and b) significantly affects the CO2RR activity of the carbon electrocatalysts. This effect can be explained by the 13



higher electron transfer capability in graphitic carbon compared with the amorphous counterpart,5 which facilitates the electron transfer process for CO2RR. To test the effect of electrode potential, electrolysis was carried out on 4-1-0.25-1000 in a potential range from −0.77 to −0.47 V vs RHE (Figure 6a). The maximum FECO (= 83%) is achieved at −0.47 V, which corresponds to an overpotential of 360 mV, given that the equilibrium potential for CO2-to-CO conversion is −0.11 V vs RHE. This represents an overpotential which is among the lowest for such efficient CO2-to-CO conversion using state-of-the-art electrocatalysts.39 However, the |j| = 0.57 mA cm−2 at this overpotential is comparatively low, due to the limited number of Fe-Nx active sites. At more negative potentials, the FECO starts decreasing due to the competing HER while |j| increases. The stability of the 4-1-0.25-1000 carbon electrocatalyst was tested in long term electrolysis performed at −0.57 V vs RHE (Figure S11). As seen, |j| decreases significantly during the first 4 h of the electrolysis. After this period, the current density becomes relatively stable; the FE remains unchanged throughout the electrolysis, which testifies its durability for long term applications. H2 CO

0.3

10 |j|

b

0.2

100

8

80 6 60 4 40 2

20

log |jCO(mA cm -2)|

a

| j | (mA cm-2)

120

FE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1 0.0 -0.1 -0.2 -0.3

0

0 -0.77

-0.4

-0.67 -0.62 -0.57 -0.52 -0.47

Potential (V vs RHE)

350

400

450

500

550

600

Overpotential (mV)

Figure 6. (a) FE and |j| of 15 min electrolysis recorded on 4-1-0.25-1000 as electrocatalyst at various potentials in CO2 saturated 0.5 M KHCO3 solution. (b) Plot of log |j| vs overpotential for CO production in electrolysis at various overpotentials using 4-1-0.25-1000 as electrocatalyst in CO2 saturated 0.5 M KHCO3 solution.

To elucidate the kinetics of CO2RR, Tafel analysis of the 4-1-0.25-1000 carbon electrocatalyst was performed in a small overpotential range (≤ 560 mV, Figure 6b). The Tafel 14


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slope is determined to be 370 mV dec−1, far from the 118 mV dec−1 expected for a rate-limiting single electron transfer step.40 This indicates that the rate-limiting step is the CO2 adsorption to and/or CO desorption from the active sites,41 rather than the electron transfer step from the carbon electrocatalyst to CO2.

Conclusion We have developed a simple procedure to prepare a series of carbon electrocatalysts doped with iron and nitrogen atoms via direct pyrolysis of a ball-milled mixture of L-histidine-Fe2O3-FeCl3.

The physical characterization demonstrates the important role of

Fe2O3 as well as FeCl3 in both iron doping and formation of porosity. The Fe-Nx moieties created serve as the active site for CO2RR. Of the many carbon electrocatalysts synthesized, the 4-1-0.25-1000 material shows the highest maximum FECO (= 83%) for CO2-to-CO conversion at a small overpotential (360 mV) in water. Comparison of the structure and activity of these carbon electrocatalysts suggests that the degree of graphitization of the carbon significantly affects the final performance for CO2RR, thus advancing the insight into the structure-activity relationship.

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Characterization details (XPS, XRD, SEM, TOF-SIMS, and N2 adsorption); cyclic voltammograms; electrolysis results. Notes The authors declare no conflict of interest. Acknowledgement We appreciate the financial support from the Danish National Research Foundation (grant no. DNRF118) and Carlsberg Foundation (CF14-0506). We thank Dr. Marcel Ceccato for the help with the TOF-SIMS measurement.

15



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Facile Synthesis of Iron and Nitrogen Doped Porous Carbon for

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