Efficient CO2 Electroreduction by Highly Dense and Active Pyridinic

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Efficient CO2 Electroreduction by Highly Dense and Active Pyridinic Nitrogen on Holey Carbon Layers with Fluorine Engineering Fuping Pan, Boyang Li, Xianmei Xiang, Guofeng Wang, and Ying Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00016 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Efficient CO2 Electroreduction by Highly Dense and Active Pyridinic Nitrogen on Holey Carbon Layers with Fluorine Engineering Fuping Pan,† Boyang Li,‡ Xianmei Xiang,† Guofeng Wang,‡* Ying Li†* †

J. Mike Walker '66 Department of Mechanical Engineering, Texas A&M University, College

Station, Texas 77843, United States ‡

Department of Mechanical Engineering and Materials Science, University of Pittsburgh,

Pittsburgh, Pennsylvania 15261, United States

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Abstract: Electrocatalytic CO2 reduction by metal-free nitrogen-doped carbon (N-C) catalysts provides a solution to CO2 reuse; however, it suffers a large overpotential and poor selectivity due to the low intrinsic reactivity of N dopants. Herein, we report the promotion of CO2 reduction on N-C through the integration of increasing the numbers and inherent catalytic reactivity and selectivity of pyridinic N dopants. A novel sacrificial soft-templating approach was developed to construct two-dimensional holey carbon nanostructure to preferentially host dense edge-located pyridinic N, and electron-rich fluorine (F) was simultaneously incorporated to activate pyridinic N sites by engineering their electronic properties. Consequently, the resultant N,F-codoped holey carbon layers achieve a CO Faradaic efficiency of 90% at a low overpotential of 490 mV for 40 h without decay, significantly surpassing the F-free N-C counterpart. Density functional theory (DFT) calculations reveal that the electron donation from a nearby F atom increases the charge density and delocalizes electronic density of states of pyridinic N. These electronic benefits thus greatly promote the CO2 activation on the highly dense and active pyridinic N sites by facilitating the electron transfer and strengthening the binding interaction with *COOH intermediate. The discovery of dopant-induced synergistic interaction may create a path for manipulating catalytic CO2 reduction properties. Keywords: CO2 reduction; electrocatalysis; nitrogen-doped carbon; fluorine; density functional theory


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INTRODUCTION Electrochemical conversion of CO2 offers a promising solution to store intermittent solar and wind energy in chemical bonds while simultaneously closes the carbon cycle.1-2 However, the current electrocatalytic CO2-to-fuels process imposes great technological challenges due to the sluggish kinetics of CO2 reduction reaction (CO2RR) and the inevitable competition over hydrogen evolution reaction (HER) in the aqueous phase.3-5 Development of inexpensive, selective and stable CO2RR electrocatalysts is thus of paramount importance. Among the explored catalysts, metal-free nitrogen-doped carbon (N-C) is a new generation catalyst that shows appealing catalytic CO2RR behaviors as a consequence of the breakage of electroneutrality of sp2 C atoms induced by N modification.3, 6-7 However, N-C catalysts, in general, have poor product selectivity and low product generation rate when operated at small overpotentials,6,

8-12

demanding more research

efforts to make N-C highly efficient. Since CO2RR sites in N-C are derived from N atom modification, the activity of N-C depends closely on the concentrations and electronic structures of N atoms;6, 9, 13 the former determines the number of active sites, and the latter governs their intrinsic catalytic nature. Recent studies have demonstrated that pyridinic N (Pyri-N, N bonded to two C atoms and located at the carbon edges) and graphitic N (Grap-N, N bonded to three C atoms and embedded in the bulk of carbon) are likely effective dopants for the creation of active sites for CO2RR.6, 9, 13 It was also predicted that Grap-N can induce the adjacent C atom with positive charge as the CO2RR site, while Pyri-N itself with lone-pair electrons is the catalytic site to bond CO2, albeit their catalytic origin and reactivity 3 ACS Paragon Plus Environment

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remain elusive because of lacking of efficient methods to control the doping types of N dopants.6, 9, 13-14

Despite these achievements, the great enhancement in CO2RR performance of N-C is still

hindered by the inferior intrinsic catalytic capability of N dopants towards CO2 activation and HER suppression. Activating N dopants to enhance their inherent catalytic nature is thus imperative to improve the overall CO2RR performance of N-C catalysts. One feasible approach could be the atomic-scale chemical co-doping engineering, where another suitable heteroatom is doped in N-C to drastically enhance the electrocatalytic activity through synergistic effects in modulating electronic properties of N species.15-18 Since the electronic interaction between active sites and CO2 is significantly critical for CO2 activation,19-22 we hypothesized that a Pyri-N site with enhanced charge density could facilitate CO2 activation by boosting electron transfer from Pyri-N to the molecular orbital of CO2. Considering that fluorine (F) is an electron donor, incorporating F atoms into N-C could activate Pyri-N dopants for CO2 activation by increasing their charge density. In addition, due to the edge-located nature of Pyri-N,23-25 we envisioned that constructing through-plane pores in the two-dimensional (2D) carbons could effectively increase the density of Pyri-N by creating abundant edge locations to preferentially host Pyri-N species. Inspired by these visions, it is logical that incorporating electron-rich F atoms into the 2D N-doped holey carbon nanostructure would achieve the simultaneous realization of the increased number and intrinsic nature of Pyri-N dopants for efficient CO2 reduction.


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In this work, we report a high-performance metal-free electrocatalyst for CO2 reduction featuring N and F co-doped holey carbon layers (NF-C) with high-density and active Pyri-N sites, which were verified by both experimental investigation and theoretical density functional theory (DFT) calculations. Experimentally, NF-C layers with tunable N and F contents were synthesized by a novel sacrificial soft-templating approach, which were applied as models catalysts to investigate the effects of F modulation on electrocatalytic CO2RR reactivity of N species by comparing with the F-free N-C layer counterpart. Theoretically, Gibbs free energy barriers towards CO2RR and HER on various N-doped and N,F-co-doped moieties were predicated, which were further coupled with experimental results to provide insights on the origin of CO2RR promotion, active sites, and synergistic electrocatalytic mechanism induced by highly dense N,F dual-doping. RESULTS AND DISCUSSION The method of synthesizing holey NF-C layers involves a pyrolysis-controlled sacrificial softtemplating protocol starting from dicyandiamide, sucrose, and ammonium fluoride (Figure 1), all of which play multiple roles in the formation of such a unique holey nanostructure. Specifically, dicyandiamide undergoes polycondensation into layer-structured graphitic carbon nitride (g-C3N4) at 550 °C (Figure S1),26-27 which function as 2D templated reactors to confine sucrose-derived carbon intermediates into their interlayers for the formation of carbon nanosheets. In the meantime, ammonium fluoride aggregates embedded in the carbon intermediates serve as sacrificial templates to navigate the formation of through-plane pores. The complete thermolysis of g-C3N4 and ammonium fluoride above 850 °C, therefore, yields the holey carbon layers nanostructure, during 5 ACS Paragon Plus Environment

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which the release of numerous N,F-containing gases severs as N,F sources for the incorporation of high-density N,F atoms in the carbon lattices.28-29 More importantly, the holes can provide abundant edge locations to preferentially host adequate Pyri-N while suppressing the formation of Grap-N,30-31 which also help to lower mass transportation limitations in CO2 reduction,32 beneficial for CO2 reduction. The doping levels of N,F atoms can be controlled by adjusting heating temperatures from 850, 950 to 1050 °C, and corresponding samples were labeled as NF-C-X, where X refers to the annealing temperature. For comparison, F-free N-doped carbon layer was also fabricated at 950 °C using the same procedure as for NF-C-950 except without adding ammonium fluoride (Figure S2), donated as N-C-950.

Figure 1. Schematic illustration of the processing for the synthesis of holey NF-C layers.

The morphology of NF-C was investigated by transmission electron microscopy (TEM). As depicted in the scanning TEM image (Figure 2a), NF-C-950 presented a wrinkled and entangled flake-like morphological feature. The magnified TEM and high-resolution TEM images (Figure


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2b,2c, Figure S3) clearly show the existence of through-plane pores, which randomly embedded in the graphitic layers. Energy-dispersive spectroscopy (EDS) elemental mapping images (Figure 2d) further demonstrate the uniform distribution of O, N, and F species on the carbon frameworks. By contrast, N-C-950 showed a similar crumpled sheet-like characteristic to that of NF-C-950 (Figure S4). However, no holes were observed on N-C-950, further supporting the proposed synthesis mechanism that ammonium fluoride serves as the soft template to generate through-plane pores in the NF-C layers (Figure 1).

Figure 2. (a) Scanning TEM, (b) TEM, (c) High-resolution TEM, and (d) Elemental mapping images of NF-C-950. (e) XRD patterns and (f) pore size distributions of N-C-950 and NF-C.


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The structure of NF-C was characterized by X-ray diffraction (XRD). The XRD patterns of all samples (Figure 2e) show two broad diffraction peaks at near 25 and 43°, which belong to (002) and (100) planes of graphitic carbon, respectively. Note that the (002) peak of NF-C-950 shifts to a smaller angle compared to N-C-950, implying that the carbon interlayer space becomes larger after doping F. In addition, the increase in annealing temperature leads to positive shifts of (002) peaks, probably due to the decrease in F contents. Surface areas and pore distributions were assessed on the basis of N2 adsorption/desorption analyses. All samples displayed well-defined hysteresis loops with type IV feature at higher N2 pressures (Figure S5), implying the mesoporous architecture. Brunauer−Emmett−Teller (BET) surface areas were calculated to be 144, 197, and 189 m2 g−1 for NF-C-850, NF-C-950, NF-C-1050, respectively, much larger than 105 m2 g−1 of NC. Moreover, NF-C showed far more porosity than N-C (Figure 2f), possessing concentrated distribution with pore sizes of 3.8 and 18 nm. The obvious increase in surface area and porosity on NF-C relative to those of N-C further manifests that the addition of ammonium fluoride in the synthetic process indeed plays key roles in evolving highly porous carbon nanostructure. The surface compositions were investigated by X-ray photoelectron spectroscopy (XPS). The survey spectra (Figure 3a) reveal that NF-C is composed of C, N, F, and O elements, with N contents being decreased from 21.4 to 14.1 and 7.7 at.% and F contents being decreased from 1.5 to 1.3 and 0.8 at.% as the heating temperature increases from 850 to 950 and 1050 °C, respectively (Table S1). The F 1s spectra of NF-C show symmetrical peaks at around 684.5 eV, which can be assigned to the covalent C−F bond (Figure 3b, Figure S 6).33 In contrast, N-C-950 exhibited an F-


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free characteristic with 10.8 at.% N, lower than that of NF-C-950 (14.1 at.% N). The N complex can be deconvoluted into three peaks centered at around 398.2, 400.6, and 403.0 eV, corresponding to Pyri-N, Grap-N, and oxidized N (Oxid-N), respectively (Figure 3c).17, 24, 34-35 It is obvious that Grap-N is dominant in N-C-950 with a portion of 52% and an atomic content of 5.7 at.% (Figure 3d), while it has a relatively low percentage in NF-C (eg., 23% and 3.2 at.% in NF-C-950). In contrast, NF-C possessed an overwhelming Pyri-N percentage (eg., 72% and 10.3 at.% in NF-C950), as compared to 40% and 4.3 at.% in N-C-950 (Figure 3d). In addition, both N-C and NF-C showed very low Oxid-N less than 8 %, which is considered to be inactive in the electrochemical reactions.

Figure 3. (a) XPS survey spectra and (b) F 1s XPS spectra of N-C-950 and NF-C-950. (c) N 1s XPS spectra of N-C and NF-C. (d) Atomic contents of total doped N and F atoms and their corresponding percentages of different species for N-C-950 and NF-C. 9 ACS Paragon Plus Environment

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Generally, N atoms prefer to locate at the edges of a graphitic layer instead of the bulk of the carbon plane because the edge-hosted carbon atoms have high reactivity and weak space resistance to facilitate the substitution of carbon atoms by nitrogen atoms.26, 30-32, 36-37 The large discrepancies of contents and types of N dopants in N-C and NF-C can thus be primarily ascribed to the differences in their textural structure. In specific, hole-deficient N-C can only load less N atoms and prefers to form Grap-N in the plane of the graphitic layer, while highly holey NF-C is able to provide plenty of edge sites to host high-density N atoms that are mainly located at the edges of holes and in the form of Pyri-N,26, 30-31 as illustrated in Figure S7. These characterization results demonstrate that both improved N contents and N,F codoping were achieved on holey carbon layers simultaneously with the assistance of this sacrificial soft-template-directing approach. The electrocatalytic CO2 reduction activity was then measured in 0.1 M KHCO3 electrolyte with a standard three-electrode configuration in a gas-tight two-compartment electrochemical cell. As shown in linear sweep voltammetry (LSV) curves (Figure 4a), the LSV curve recorded in CO2saturated solution shows a significant increase in current density and a positive shift in onset potential compared to those of the LSV curve recorded in the Ar-saturated solution, suggesting the occurrence of CO2 reduction with hydrogen evolution. The total current density at different potentials are shown in Figure S8, and it can be seen that NF-C showed much higher current density than those of N-C, implying the enhanced CO2RR rates upon F doping.


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To detect products selectivity, the gas-phase products and electrolyte after stable constantpotential electrolysis were investigated by an on-line gas chromatography (GC) and a 1H nuclear magnetic resonance (NMR), respectively. We found that H2 and CO are primary products with total FE more than 99%, and no liquid products were detected. On N-C-950, CO2RR starts at −0.5 V with a CO FE of 16% (Figure 4b), meaning an onset overpotential of 390 mV with reference to the standard equilibrium potential of CO2/CO of −0.11 V.38 The CO FE then reaches a maximum value of 64% at an overpotential of 690 mV. As for NF-C-950, CO2RR starts at an extremely low overpotential of 190 mV with a CO FE of 18%. Remarkably, a maximum CO FE of 90% was achieved on NF-C-950 at an overpotential of 490 mV, which is 26% larger compared to that of NC-950 accompanied with 200 mV decrease in overpotential to reach the highest FE. As the potential sweeps more negatively from −0.6 to −1.0 V, CO FEs decrease gradually which is mainly caused by the rapid rising of the HER activity (Figure S9).33 In addition, CO partial currents of NF-C-950 are far beyond than those of N-C-950 at the all potential ranges (Figure 4c). For instance, at −0.6 V, NF-C-950 delivered a CO current of 1.9 mA cm−2, which is about 15 times higher than that of N-C-950 (0.13 mA cm−2). The significant current enhancements are also observable when normalizing CO partial currents to BET surface areas and total N contents (Table S1). These results clearly demonstrate that additional F doping indeed improves the catalytic capability of N-C in lowering CO2RR overpotentials, speeding up reaction rates, and suppressing the competitive HER, which might be due to the synergistic effects derived from N,F dual-doping for CO2 activation.


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Figure 4. Electrocatalytic CO2RR performance over N-C and NF-C. (a) LSV curves of NF-C-950 in Ar- and CO2-saturated 0.1 M KHCO3 electrolyte. (b) CO FEs, (c) CO partial current densities of N-C-950 and NF-C. (d) The relation between normalized CO partial currents at −0.6 V and atomic contents of N,F species in various NF-C. (e) Tafel plots of CO production for N-C-950 and NF-C. (f) Electrochemical stability of NF-C at −0.6 V. 12 ACS Paragon Plus Environment

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The pyrolysis temperature plays a key role in governing the electrocatalytic CO2RR performance of carbon catalysts because it influences contents/configurations of dopants and textural properties of carbons. Here, we found that NF-C-950 synthesized at 950 °C gave the best activity and selectivity for CO generation (CO FE of 90% and partial current of 1.9 mA cm−2 at −0.6 V) (Figure 4b,4c, Table S1). Among series, NF-C-850 having the highest total N,F contents showed the lowest CO FE of 69% and a moderate CO current of 1.6 mA cm−2 at −0.6 V. In addition, NF-C-1050 exhibited a moderate CO FE of 80% and the lowest CO current density of 0.8 mA cm−2 at −0.6 V despite it possessed the lowest content of N,F-dopants as compared to others. Note that the surface area of NF-C increases gradually when annealed from 850 to 1050 °C, thus the effects of surface area on the overall activity cannot be completely excluded because it affects the exposure and availability of N,F species. To accurately compare the intrinsic activity of N,F dopants and determine the CO2RR active sites over NF-C, CO partial currents at −0.6 V were normalized by the BET surface areas of the NF-C catalysts and plotted as a function the concentrations of N,F dopants. The results display that the normalized CO currents increase with the increasing of N and F contents (Figure 4d), suggesting that the overall reaction rates are strongly dependent on the contents of dopants that determine the number of active sites. Furthermore, the CO current increases linearly with the content of C−F or Pyri-N but not with the content of Grap-N, implying that the F modification might have stronger impacts in activating Pyri-N than Grap-N, rendering high-density Pyri-N as the primary active site.


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The kinetics of CO2 reduction was analyzed by Tafel slopes. As depicted in Figure 4e, NF-C showed Tafel slopes around 118 mV dec−1, indicating the initial single electron transfer to CO2 as the rate-determining step.33 Notably, NF-C-950 possessed a significantly lower Tafel slope than N-C-950 (119 vs. 189 mV dec−1), further signifying more favorable kinetic rates induced by F modification. Finally, the electrochemical stability of NF-C-950 was examined; we found that NFC-950 exhibited the impressive durability without decay in current and CO FEs in a 40 h continuous test at −0.6 V (Figure 4f). To the best of our knowledge, the performance of this NFC outperforms most of the reported metal-free catalysts under similar conditions (Table S2),6, 8-12 rendering NF-C one of the most active catalysts among metal-free carbon community. To gain an in-depth understanding of the active sites and catalytic mechanisms, DFT calculations were performed. In consistent with the observed holey carbon architecture and N,F bonding configurations, edge-hosted Pyri-N located at the hole of a graphitic layer and bulk-hosted Grap-N embedded in a carbon layer were proposed to be possible N species. Note that the bestperforming NF-C-950 has a Pyri-N : Grap-N atomic ratio of 10 : 3 and a total N : F atomic ratio of 10 : 1. Considering that it is possible that these N,F atoms may exist in NF-C with various arrangements in the graphene domain, we thus built several moieties with the number of doped N atoms varying from 1, 3, 6, 8 to 10, where the Pyri-N and Grap-N were also separately and collectively doped in the carbon lattices to simulate the distribution of N atoms. The structures of N-doped complexes are shown in Figure S11 and labeled as Pyri-xN+Grap-yN, where x and y represent the number of N atoms at different sites and the sum of x and y is the total number of N


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dopants. The addition of an F atom thus forms N,F-co-doped models, denoted as F+Pyri-xN+GrapyN (Figure S12), which have varying N:F ratios ranging from 1:1 to 10:1. Assuming CO2RR pathway involving the formation of *COOH and *CO intermediate via the proton-coupled electron transfer process,3, 33, 39 the optimization of *COOH adsorption on these sites indicates that the carbon atom adjacent to Grap-N is the active site on Grap-N and F+GrapN (Figure 5a-5f, Figure S13), whereas the N atom itself is the active site over Pyri-N and F+PyriN, in agreement with other studies.13 Furthermore, the free energy of *CO is the same as that of CO because the CO adsorption is so weak that CO is either physisorbed or cannot adsorb on the catalyst surface. The weak CO adsorption results in the facile desorption of CO from the catalyst surface and thus permits high CO selectivity without being further transformed into C2+ products. These results are in good agreement with the reported DFT finding on N-doped carbon models for the reduction of CO2 to CO.13


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Figure 5. The *COOH adsorption geometry on proposed N,F-codoped models: (a) F+Grap-1N, (b) F+Pyri-1N, (c) F+Pyri-3N, (d) F+Pyri-6N, (e) F+Pyri-6N+Grap-2N, (f) F+Pyri-8N+Grap-2N. C, N, F, O, and H atoms are represented by gray, blue, cyan, red, and white spheres. (g-l) Free energy diagrams for CO2RR to CO and HER to H2 at electrode potential U=0 V on various Grap-N and Pyri-N sites with/without F atom.


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Regarding single N-doped moieties, the calculated Gibbs free energies indicate that the formation of *COOH requires a high energy barrier of 1.26 and 1.54 eV on Grap-1N and Pyri-1N, explaining the large overpotential on N-doped carbon. When incorporating F on Grap-1N, F+Grap-1N shows a free energy barrier of 1.49 eV, which is 0.23 eV higher than that of Grap-1N, implying that F slightly passivates Grap-N. Notably, the F modification drastically decreases the free energy barrier of *COOH formation over Pyri-1N, which is reduced to a low value of 0.42 eV on F+Pyri-1N, more than 1 eV lower relative to that of F-free Pyri-1N. This computational result suggests that the F atom greatly improves the intrinsic capability of Pyri-N, while Grap-N cannot be activated. These computational studies in combination with the experimental findings suggest that NF-C activity is linearly correlated with both Pyri-N and F contents. On the other hand, pyrrolic N (Pyrr-N) is supposed to be another type of N dopants.40 We thus theoretically predicted the effects of F modification on the catalytic ability of Pyrr-N. As shown in Figure S14, we found that the free energy barrier decreases from 0.64 eV on Pyrr-N to 0.4 eV on F+Pyrr-N, suggesting that F can also slightly activate Pyrr-N. However, our XPS analyses (Figure 3c) show that no peak at around 399.5 eV related to Pyrr-N was observed, meaning that Pyrr-N might not be formed or its content was very low in our catalysts. This agrees with literature that usually Pyrr-N cannot be generated in N-C materials at the heating temperature above 800 oC due to the poor thermostability.17, 24, 34-35 Thereby, the impact of Pyrr-N on the overall activity of our N-C and NF-C catalysts is negligible.


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The Gibbs free energy diagrams of other models with more than one N atoms are depicted in Figure 5i-5l, and the corresponding CO2RR barriers are summarized in Figure 6a. It is obvious that the barriers decrease from 1.54 eV on Pyri-1N, to 1.48 eV on Pyri-3N, 1.05 eV on Pyri-6N, 0.72 eV on Pyri-6N+Grap-2N, and 0.71 eV on Pyri-8N+Grap-2N, indicating that the catalytic ability of Pyri-N can also be promoted by increasing the numbers of neighboring N atoms. This implies that the concentrated N atoms on NF-C-950 also partially contribute to its enhanced CO2RR activity as compared to N-C-950 with less total doped N atoms (14.1 at.% vs. 10.8 at.%). Furthermore, it can be notably seen that the CO2RR energy barriers on these multi-N-doped moieties can be significantly reduced by adding an F atom. For instance, the energy barrier decreases from 1.48 eV of Pyri-3N to 0.22 eV of F+Pyri-3N and from 0.71 eV of Pyri-8N+Grap2N to 0.38 eV of F+Pyri-8N+Grap-2N. In addition, we studied the effects of distance between N and F to investigate whether F could have long-range interaction to impact the catalytic nature of the N atom. As shown in Figure S15, the N atoms labeled as site 1 and site 2 were selected as the adsorption sites for *COOH intermediate, in which site 1 is near the F atom while site 2 is relatively far from the F atom. It can be seen that adding an F atom lowers the CO2RR energy barrier on both site 1 and site 2, despite that site 1 shows a much lower barrier than site 2. This indicates that F has long-range interaction to influence N atoms. Based on these calculations, it is inferred that the F atom can activate different types of N-induced species with N : F ratios ranging from 1:1 to 10:1.


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Figure 6. (a) Gibbs free energy barriers for CO2 reduction and (b) difference in limiting potentials for CO2 reduction and H2 evolution on various proposed models.

We further calculated the competing HER to unveil the influence of the F addition on catalytic CO2RR selectivity of N-C. As shown in Figure 5h,5g,5l, it was found that introducing F lowers the energy barrier of *H adsorption on Grap-1N. However, F modification makes *H strongly binding with all Pyri-N sites, which is unfavorable for hydrogen releases and could result in a suppressed HER selectivity in the CO2-saturated solution.33 Previous studies proposed that the difference between thermodynamic limiting potentials for CO2RR and HER (denoted as UL(CO2) 19 ACS Paragon Plus Environment

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− UL(H2), where UL= −ΔG0/e) can act as a descriptor to reflect the catalytic CO2 reduction selectivity of the active sites.41 A larger positive UL(CO2) − UL(H2) value means a better selectivity toward CO2 reduction. As shown in Figure 6b, all F-modified Pyri-N sites exhibit more positive UL(CO2) − UL(H2) values than the F-free counterparts, suggesting that F modification indeed plays a crucial role in boosting the selectivity for CO generation. It is thus believed that the highly concentrated Pyri-N with enhanced F-engineered reactivity and selectivity might function as the main catalytic centers, giving rise to high-rate and selective CO production on NF-C catalysts. To find out why F can activate Pyri-N instead of Grap-N, we further calculated the charge distribution. Taking F+Grap-1N and F+Pyri-1N sites as an example, the electron transfer between the active sites and *COOH occurs upon the formation of adsorbed *COOH intermediate. Regarding the active C site (labeled as Cg) of Grap-1N and F+Grap-1N (Figure S11a,S12a), it was found that the charge density of Cg increases upon binding with *COOH (Figure 7a), suggesting that electrons may transfer from *COOH to the so-called positively charged Cg sites via an electron back-donation process. As for the N sites (labeled as Np) of Pyri-1N and F+Pyri-1N (Figure S11b,S12b), decreases in their charge density suggest the occurrence of electrons transfer from Ng sites to *COOH. In this condition, a more positively charged Cg site could facilitate *COOH formation, while a more negatively charged Ng site is more desirable. Due to the electrondonating nature of F atoms, the addition of F in N-C increases the charge density for both the Cg site next to Grap-N from 3.69 to 3.71 ‫׀‬e‫ ׀‬and the Np site of Pyri-N from 5.91 to 5.98 ‫׀‬e‫( ׀‬Figure 7a,7b), respectively. This supports that it is the Pyri-N rather than Grap-N that can be activated by


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F modification. The electronic structure analyses show that the 𝑝𝑥 orbital of Pyri-N becomes more delocalized and is much closer to the Fermi level upon F doping, which helps to boost the electron transfer from Pyri-N to the antibonding orbitals of CO2 molecules for CO2 activation.19 This change is supposed to be able to create a beneficial electronic environment to strengthen the binding interaction with intermediate on NF-C.42 Consequently, both strong CO2 activation and *COOH adsorption can be achieved on Pyri-N sites of NF-C.

Figure 7. (a) The charge density values of Cg and Np sites before and after binding with *COOH on Grap-1N and Pyri-1N with and without F modification. (b) The charge density difference on F+Pyri-1N, which is computed as ρ(F+Pyri-1N)-ρ(Pyri-1N)-ρ(F). Cyan and yellow represent charge accumulation and depletion in the region; the isosurface value is 0.001 e A−3. (c) The partial density of states (pDOS) of the 𝑝𝑥 orbital of the Np site of Pyri-1N and F+ Pyri-1N.

It is known that the apparent activity for a doped carbon-based catalyst is generally governed by both the number of exposed available active sites and their catalytic reactivity, which are determined by extrinsic physicochemical properties of carbon support and intrinsic electronic 21 ACS Paragon Plus Environment

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structure of dopants. Taking into account the structures and compositions of the model catalysts, the superior CO2RR performance of NF-C is suggested to be cooperatively governed by the following key aspects. First, the large surface area of NF-C enables high exposure of N,F species and make them fully accessible, thus providing the high chance for CO2 to contact with active sites. Meanwhile, the holey carbon architecture affords smooth transportation channels for CO2 and CO, resulting in a low mass transfer limitation. Second, the high content of doped N atoms with dominant Pyri-N on NF-C could provide a great amount of densely distributed N dopants, which increase the number of Pyri-N centers. Third, the F modulation alters electronic properties (eg., charge density, density of states) of Pyri-N, which further activates Pyri-N dopants and makes them as the highly active and selective sites in the reduction of CO2 to CO. Thereby, it might be the synergistic merits derived from favorable structural and compositional properties that make great contributions to the enhanced CO2RR activity over NF-C as compared to N-C. CONCLUSIONS In summary, we demonstrated the great enhancement in electrocatalytic CO2 reduction over N-C through incorporating electron-rich F atoms into high-density pyridinic N-doped holey carbon layers. The CO2RR activity of NF-C reached stable CO FEs of 90% at a low overpotential of 490 mV in a 40-h test, much superior to the performance of the N-C counterpart and most of metalfree carbon catalysts reported in literature. The improved performance is ascribed to the synergistic benefits induced by the high surface area with holey architecture and dense N,F dual doping. The abundant pyridinic N provides a large number of highly active and selective CO2RR sites and the 22 ACS Paragon Plus Environment

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high surface area makes these active sites fully accessible; the holey structure also facilitates the rapid mass transportation. Theoretical calculations reveal that the electron donation from F atoms improves the intrinsic catalytic reactivity and selectivity of pyridinic N by increasing charge density and delocalizing electronic density of states. The F-modulated pyridinic N sites enable strong CO2 activation through boosting electron transfer and strengthening the binding interaction with *COOH intermediate while greatly suppressing the competitive hydrogen evolution reaction. This study advances the understanding of the synergistic CO2 electrocatalysis and thus enables useful design concepts for the development of advanced carbon catalysts for CO2 reduction and other energy-related electrochemical processes. EXPERIMENTAL SECTION Catalysts synthesis NF-C was prepared via a simple solid-phase pyrolysis route. In a typical synthesis, sucrose (0.2 g), ammonium fluoride (0.2 mg), and dicyandiamide (3 g) were dissolved in deionized water sequentially. After stirring for 2 h, the solution was dried at 40 ºC under vacuum furnace to remove water. The solid was then ground into a uniform powder, which was put in a combustion boat and transferred into a tube furnace. Then, the temperature of the furnace was increased to 550 ºC for 2 h and further raised to 950 °C for 1 h at a heating rate of 3 °C min−1 under argon flow. After cooling down to room temperature, the final powder sample was collected. For optimizing the electrocatalytic performance, a series of NF-C catalysts were prepared by adjusting the pyrolysis


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temperatures ranging from 850 to 1050 °C. F-free N-C was also synthesized at 950 °C using the same procedure as for NF-C except without adding ammonium fluoride. Electrochemical CO2RR activity measurements Electrocatalytic CO2RR activity was evaluated in a two-compartment H-type electrochemical cell using a standard three-electrode configuration in CO2-saturated 0.1 M KHCO3 electrolyte (pH = 6.8). A Pt foil and an Ag/AgCl (3M KCl) were used as the counter electrode and reference electrode, respectively. The measured potentials after iR compensation were rescaled to the reversible hydrogen electrode by E (RHE) = E (Ag/AgCl) + 0.210 V + 0.0591V×pH. The working electrode was prepared by drop casting catalyst ink onto a carbon paper (1 cm2) with mass loading of 0.5 mg cm−2. The ink was prepared by dispersing 3 mg catalysts in a mixture solution of 200 µL DI-water, 370 µL ethanol, and 30 µL 5% Nafion solution via sonication for 3 h. The counter electrode was placed in the anode chamber, while the working and reference electrodes were placed in the cathode chamber. The two chambers were separated by a piece of Nafion 115 proton exchange membrane (PEM) (Figure S16). The use of the PEM-separated cell has two advantages as follows. For one hand, this configuration can avoid the transfer of CO2RR-generated products from the cathode chamber to the anode chamber and being re-oxidized at Pt foil.43 On the other hand, this configuration can exclude the potential interferences of Pt dissolution on the quality of electrochemical measurements. In the traditional one-compartment cell, the dissolved Pt species from the electrochemical or chemical dissolution of Pt may impact the recorded activity of catalysts because Pt is very active toward hydrogen evolution.44-45 In the two-chambered cell, the 24 ACS Paragon Plus Environment

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Pt species was fully excluded to reaching the catalysts surface in the cathode chamber because Pt species cannot pass through PEM.43,

45-46

Therefore, the Pt counter electrode did not cause

interferences on electrochemical CO2 reduction performance over as-prepared carbon catalysts. The high-purity CO2 was introduced in the cathode chamber for 1 h with a flow rate of 34 ml min−1 to saturate electrolyte before starting electrolysis and maintained this flow rate during measurements, and the cathodic electrolyte was stirred at 900 rpm. The gas-phase products were analyzed via an online gas chromatograph (GC, Fuel Cell GC-2014ATF, Shimadzu) equipped with a thermal conductivity detector (TCD) and a methanizer assisted flame ionization detector (FID). The electrolyte after electrolysis was characterized by a nuclear magnetic resonance (NMR) spectrometer (Bruker Avance III 500 MHz); no liquid products were detected. Density functional theory (DFT) calculations The first principles DFT calculations were performed using the Vienna ab initio simulation package (VASP) code. Projector augmented wave (PAW) pseudopotential was employed to describe the core electrons. The cutoff energy was set as 400 eV to expand the wave functions. Electronic exchange and correction were described by generalized gradient approximation (GGA) of the revised Perdew, Burke and Ernzernhof (RPBE) functionals. The N, F co-doped graphene was chosen as the active site. The Brillouin Zone was sampled by Monkhorst 331 k-point grid for active sites Pyri-6N+Grap-2N, Pyri-8N+Grap-2N and 441 k-point grid for the other active sites. The atomic positions were optimized until the force fell below 0.01eV/ Å. The computational hydrogen electrode (CHE) was used to calculate the free energy of each intermediate state.47 All 25 ACS Paragon Plus Environment

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computations were conducted at electrode potential U=0 V, and the solvation energy correction was included.48 The free energy of a chemical reaction is calculated by ∆𝐺 = ∆𝐸𝐷𝐹𝑇 + ∆𝐸𝑍𝑃𝐸 + ∆𝐸𝑠𝑜𝑙𝑣 + ∆𝐻0 𝑡𝑜 𝑇 ― 𝑇∆𝑆 where EDFT is the energy change calculated by DFT, EZPE is the zero-point energy, Esolv is the solvation energy, H0 to T is the enthalpy change from 0 to T K, and S is entropy change. A solvation effect correction was included by following previously reported values, namely 0.25 eV stabilization of COOH*, 0.1 eV stabilization of CO*. ZPE corrections were calculated as ZPE = 1

∑2ℎ𝑣𝑖 , where h is Planck’s constant and 𝑣𝑖 is the frequency of the corresponding vibrational mode 𝑡

of binding molecules. ∆H0 to T was calculated by the vibrational heat capacity integration∫0𝐶𝑝𝑑𝑇. The entropy terms for gas phase were derived from partition functions and compared with the data from NIST Standard reference database. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional experimental section, catalysts characterizations, CO2RR performance, and DFT results (PDF). AUTHOR INFORMATION 26 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Y. Li acknowledges the support from American Chemical Society Petroleum Research Fund (ACS-PRF, grant no. 58167-ND10). The use of the Texas A&M University Materials Characterization Facility is also acknowledged. G. Wang acknowledges the computational resources provided by the computer facility at Center for Simulation and Modeling of the University of Pittsburgh and at the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation under grant number ACI-1053575. REFERENCES 1. Dinh, C.-T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.; Gabardo, C. M.; García de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De Luna, P.; Bushuyev, O. S.; Zou, C.; QuinteroBermudez, R.; Pang, Y.; Sinton, D.; Sargent, E. H., CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface. Science 2018, 360, 783-787. 2. Spurgeon, J. M.; Kumar, B., A comparative technoeconomic analysis of pathways for commercial electrochemical CO2 reduction to liquid products. Energy Environ. Sci. 2018, 11, 1536-1551. 3. Duan, X.; Xu, J.; Seh, Z. W.; Ma, J.; Guo, S.; Wang, S.; Liu, H.; Dou, S., Metal‐Free Carbon Materials for CO2 Electrochemical Reduction. Adv. Mater. 2017, 29, 1701784. 27 ACS Paragon Plus Environment

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Efficient CO2 Electroreduction by Highly Dense and Active Pyridinic

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