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Electronic Structure Engineering of 2D Carbon Nanosheets by Evolutionary Nitrogen Modulation for Synergizing CO2 Electroreduction Tengfei Gao, Tianhui Xie, Nana Han, Shiyuan Wang, Kai Sun, Cejun Hu, Zheng Chang, Yingchun Pang, Ying Zhang, Liang Luo, Yuxin Zhao, and Xiaoming Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02176 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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ACS Applied Energy Materials

Electronic Structure Engineering of 2D Carbon Nanosheets by Evolutionary Nitrogen Modulation for Synergizing CO2 Electroreduction Tengfei Gao†, §, Tianhui Xie†, §, Nana Han†, Shiyuan Wang†, Kai Sun†, Cejun Hu†, Zheng Chang†, Yingchun Pang†, Ying Zhang#, Liang Luo*, †, Yuxin Zhao*, ‡ and Xiaoming Sun† †State

Key Laboratory of Chemical Resource Engineering, College of Energy, Beijing

Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 (P. R. China) ‡School

of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shanxi

710049, China #School

of Chemistry, Monash University, Wellington Road, Clayton 3800, VIC (Australia)

KEYWORDS: Carbon Nanosheets, Electrocatalysis, CO2 Reduction, Nitrogen Incorporation, Electronic Tailoring, Synergetic Enhancement.

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ABSTRACT: In this work, we present a supramolecular template-derived synthesis approach combined with carbon electronic structure tailoring strategy to prepare N-doped carbon (NC) nanosheets with delicately tuned nitrogen dopant levels and types for selective CO2 electrocatalytic reduction. By this method, the NC nanosheets are able to electrochemically reduce CO2 to CO with an unprecedentedly superior faradaic efficiency (FE) of 92% at a moderate overpotential as low as -0.36 V and showed great long-term stability with a remnant FECO around 85% after 10 h of electrolysis. The improved performance of NC nanosheetsis mainly attributed to the synergetic effect of electronic interaction of pyridinic nitrogen with adjacent C atoms in the interfacial nanoregime, and the concomitant pore structure, high conductivity as well as surface wettability. The capability to modulate the catalytic sites electron densities opens up further opportunities for rational reengineering carbon-based catalysts with accelerated electrocatalytic activity toward renewable energy applications and beyond.

Electrochemical CO2 reduction reaction (CO2RR), as a prospective carbon neutral route and a fundamental paradigm to explore the mechanism of multi-electron transfer processes in electrocatalysis, always requires an efficacious catalyst to achieve fast kinetics for practical applications. To date, a wide variety of available electrocatalyst candidates have been exploited, such as metals and their molecular derivatives with research focused on tuning architecture, composition1-10, grain boundaries and defects11-13. With respect to the actual implementation of CO2RR in aqueous media, these electrocatalysts suffer from one or more


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of the following issues: inherent corrosion susceptibility, low faradaic efficiencies as well as poor selectivity, high redox overpotential and engineering challenges of industrial scale. Therefore, inexpensive, stable, and selective electrocatalysts with high energy efficiency are still highly desirable to drive the development of CO2RR. Very recently, two-dimensional (2D) nitrogen-doped carbon (NC) nanomaterials have emerged as promising candidates for representing state-of-the-art electrochemical CO2RR catalysts, owing to their inherent robust nature, cost effectiveness, unique structural and electronic properties.14-17 On one hand, the distinct ultrathin 2D structure possesses abundant exposed surface atoms that can easily escape from the respective lattice to form vacancy-type defects, which can increase low-coordinated surface atoms and promote chemisorption of reactants, leading to a great enhancement in catalytic performances.18-21 On the other hand, rational control and engineering of such vacancy defects can further enable one to tailor the electronic structure and thereby gain favoring catalytic activities.22-25 Very promising results have already been demonstrated in a diversity of 2D carbon nanomaterials for energy conversion (involving hydrogen and oxygen). Additionally, the well-defined structure of 2D sp2 hybridized network can also serve as the ideal platform for studying catalytic mechanisms at the atomic level. The adequately utilization of aforementioned benefits becomes possible only when a simultaneous control over dimensionality and heteroatom sites at nanoscale precision is realized during synthesis. Up to now, substantial efforts have been made to prepare large-


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scale 2D NC nanomaterials with well controlled nanostructures through different kinds of strategies, including chemical vapor deposition (CVD)26, arc discharge27, N2 plasma28, multistep hard-template protocols and direct pyrolysis29-32. Among them, CVD approach to the synthesis of 2D N-doped carbon nanomaterials (NCs) generally requires transitional metal catalysts which may impede the mechanistic understanding of observed electrocatalytic properties due to the inevitable catalytically active metal residues. In addition, the toxicity of the nitrogen precursors (e.g., NH3 or pyridine) also limits its practical applications. In the case of arc discharge or N2 plasma, harsh conditions or special instruments are certainly required. The sacrificial hard-template procedures involving tedious procedures and hazardous corrosive solvents are time-consuming and high-cost, and not suitable for the large-scale production. The direct pyrolysis also has obvious drawbacks. Because of electrostatic interaction, strong π–π stacking and in-plane bonds within the atomic layers, this method usually gives rise to uncontrollable agglomerated 3D carbon structures other than 2Dcarbon materials. In fact, it is still an enormous challenge to realize specific component modification and structural manipulation at the atomic level for properly design and yield of 2D NCs with desired catalytic properties by a facile and environmentally synthesis method. In this work, we report a strategy based on two-step thermal annealing for converting glucose into high performance metal-free 2D NC nanosheets using low-cost industrial material melamine as both the 2D-induced template and nitrogen source. We demonstrate


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that the production of an ideal 2D carbon catalyst can be easily obtained by combining two strategies: one is the self-assembled supramolecular aggregating by utilizing planar melamine molecular structure with three-fold symmetry as building block to guide the pyrolysis, and the other is the tailoring of the electronic structure via nitrogen doping level and dopant structures modulation to achieve increased electronic conductivity, tunable surface basic sites and electron-donor affinity. The synthetic mechanism was systematically investigated by analyzing the microstructure evolution and chemical states of the precursors at different conditions. The desired structure and micromorphology of NC nanosheets could be flexibly tuned by adjusting the annealing temperature, which contributed to highly conductive pore passage connecting, abundant catalytic active sites, suitable N species and the enhanced electronic interaction within N and C atom frameworks. Benefiting from the synergistic effect between the 2D structure and the nitrogen dopant, the obtained NC materials display impressively high activity and remarkable selectivity at low overpotential regarding converting CO2 into CO.


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Scheme 1. Schematic illustration of the synthesis of nitrogen-doped carbon nanosheets and the process of electrochemical CO2 reduction reaction. The procedure for synthesizing the NCs starting from a reactive graphitic carbon source (i.e. glucose) and N rich molecules (i.e. melamine)is illustrated inscheme1.First, thermal condensation of melamine creates a layered supramolecular template, which binds the asformed aromatic carbon intermediates to its surface and finally confines their decomposition-polymerization in a cooperative process to the interlayer gaps of supramolecules, which aggregates at 550 °C during initial programming heating stage. The as-obtained products in this process possessed alamellar and silk-like structure as shown by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure 1a, b. By contrast, in the absence of melamine, only colloidal carbon blocks appeared due to the sequential dehydration and cross-linking processes of glucose during the calcinations (Figure S1). Here melamine serves as a sacrificial template for the temporary in


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situ formation of lamellar products, which aggregates by inducing glucose two-dimensional carbonization and N doping simultaneously. The distinct XRD peak (002) at 27.6° (d=0.325 nm) and peak (100) at 13.4° (d=0.675 nm) revealed in Figure 1c are indicative of the layered stacking of conjugated aromatic systems and the in-planar structural packing motif, respectively. The as-obtained products possess a2D graphitic-like sheet structure and are very similar to that of the reported g-C3N4.23,33-37 X-ray photoelectron spectroscopy (XPS) measurements show that there is no elimination of oxygen (O) atoms through the pyrolysis. The N/C atomic ratio for obtained products is0.67, much lower than the theoretical value for g-C3N4, suggesting the introduction of surface N defects according to the previous report.23 Narrow scan of C1s and N1s XPS spectra (Figure 1d and e) further indicate that the N atoms within carbon nitride framework are mainly in the form of “pyridinic” N, which is generally distributed on the edge of graphene domains and could be removed easily by annealing at higher temperature.


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Figure 1. a) Typical SEM image, b) TEM image, c) XRD pattern, d) N1s and e) C1s XPS spectra of g-C3Nx obtained after 550°C programming heating stage.


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Figure 2. SEM characterizations of a) NC-800, b) NC-900, c) NC-1000, and d) NC-1100 with EDS elemental mapping for nitrogen and their corresponding TEM images. Scale bars in SEM and TEM images are 200 nm and 20 nm, respectively. e) Hydrophobic behaviors evolution and optical pictures of NC-800, NC-900, NC-1000 and NC-1100. Since such layered graphic carbon nitride (simplified as g-C3Nx) undergoes further high temperature calcination above 800 °C under Ar atmosphere, the ultimate NCs are liberated. SEM images of NCs prepared at different temperature (Figure 2a-d) do not show obvious difference in their intrinsic sheet microstructure, but surface porosity become visible in NCs annealed above 1000 °C (denoted as NC-1000). The elemental mapping analysis of NC


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samples indicates a dramatically increased nitrogen loss with rising of the temperature of 800 °C and 1100 °C, which is likely responsible for the emerged porous morphology. TEM images (Figure 2a-d) evidence that their porous structural features become more and more apparent as the temperature raise, certainly consistent with mapping results. To further clarify the porosity evolution of NCs, the N2 sorption isotherms analysis were performed, and the results are shown in Figure S2. Hysteresis has been observed in adsorption isotherms for all kinds of N doped carbon and, generally, is attributed to adsorption in mesoporous materials with capillary condensation. This behavior is classified as Type IV in the IUPAC classification scheme. The pore analysis demonstrate the tendency of pore diameter increasing with processing temperature elevation, which is consistent with the TEM observation. Besides, it is known that carbon atoms can be endowed stronger hydrophilicity through doping more nitrogen.38-41 The contact angle measurement (Figure 2e) provides insight into the wettability evolution of the prepared microstructures, where the hydrophobicity is gradually enhanced as the annealing temperature increased. This also intuitively reflects the process of nitrogen removal from a side. Furthermore, it is well to be reminded that the surface wettability is very important for the underwater gas consumption reactions, the enhanced hydrophobicity is usually beneficial for the accessibility of reactants to the active sites according to our previous studies.42-44 As a further probe, the Raman spectra of four NC samples are presented in Figure S3. The characteristic D and G bands at around 1350 and 1600 cm-1 are related to defective/disordered hybridized carbon structure and ordered graphitic carbon structure


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with long range sp2 electronic configurations, respectively. The intensity ratio of the D/G band (ID/IG) is sensitive to structural defects as well as degree of irregularity in the NCs.45 It can be found that the ID/IG values decreased successively from 1.061 for NC-800 to 1.018 for NC-1100, indicating more highly porous but conductively ordered structures in NCs as the treat temperature increased, which is directly associated with decreased nitrogen content and the coupled edge terminations in NCs. To understand the intrinsic electrocatalytic properties of the as-prepared NCs, the catalyst surface chemical composition and state were investigated using X-ray photoelectron spectroscopy (XPS). Figures 3a-d illustrate a thorough analysis of different N species in aforementioned NC samples. The N 1s XPS spectra of all structures can be deconvoluted into four major peaks at around 398.5, 400.5, 401.6 and 403.7 eV, corresponding to pyridinic, pyrrolic,

graphitic

and

oxidized

N

atom

configurations

in

carbon

framework,

respectively.41,46-48 The relative concentrations of different nitrogen species in NCs obtained at various temperatures are summarized in Figure 3e. It can be seen that when the temperature increased from 800 to 1100 °C, the total surface N content is reduced significantly from 6.75 at% to 2.54 at%. In addition, the relative percentage of pyrrolic nitrogen decreased from 39.22% to 12.32%, the pyridinic nitrogen increased firstly from 28.12% to 34.47% and then decreased to 14.88% as the annealing temperature increased, while the graphitic nitrogen increased constantly. The incorporation of NCs appears to favor the preferential formation of thermally stable graphitic nitrogen rather than less stable


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pyridinic and pyrrolic nitrogen upon heating. This accounted for the above-mentioned N species’ change in relative quantities based on the ratio of certain nitrogen species and surface nitrogen. Previous report proposed that pyridinic N can lead to an increase of the structural defect density for providing more catalytic active sites.49-52 This implies that NC1000 may potentially have higher electrocatalytic activity than the other samples. Furthermore, as shown in Figure 3f, the carbon atomic concentrations increased from 83.02% to 88.75% while no notable differences can be seen in the proportion of oxygen species. These results confirm that the presence of porous structure is mainly assigned to the removal of unstable nitrogen species during pyrolysis process. Figure 3g displays the valleyshaped curve of surface N content and total N content (Ns/Nt) ratio with a unique bottom located at NC-1000, demonstrating that most of the bulk C-N and C=N bonds in NC-1000 have been continually broken and rearranged into exposed active surface defects. As a consequence, CO2 physisorption isotherm demonstrated that NC-1000 possesses the highest 45 cm3/g capacity for CO2 capture under low pressure conditions (Figure 3h). Such highadsorption potential for trapping CO2 molecules could result in CO2 enrichment within a local environment despite the low CO2solubility in the electrolyte solutions, and is proposed to be beneficial for the electrocatalytic application.


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Figure 3. Elemental analysis. XPS spectra of N 1s for a) NC-800, b) NC-900, c) NC-1000, and d) NC-1100. e) N contents with different chemical states and f) total atomic contents in NC800, NC-900, NC-1000, and NC-1100. g) The ratio of surface N content and total N content. h) Room-temperature CO2 adsorption isotherms. The potentiostatic CO2 electrolysis was then performed for this family of NC samples in a three-electrode setup using a CO2-saturated 0.5 M KHCO3 aqueous solution. Since the platinum herein was used as the counter electrode, here we utilized a three-electrode H-type cell coupled with an ion-exchange membrane to shield potential contribution coming from the electrochemical and chemical dissolution of Pt.53 The linear sweep voltammetry (LSV) was first conducted, and the results are shown in Figure 4a. Obviously, the NC-1000 presents the lowest onset potential of -0.95 V, and the largest cathodic current density, suggesting its highest electrocatalytic CO2 reduction activity. This may be due to the preferred binding of NC-1000 to CO2 and accelerated kinetics of CO2RR. To reveal the nature of the chemical processes occurring within our NC electrocatalysts, comprehensive product analysis using


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nuclear magnetic resonance (NMR) and gas chromatography (GC) were operated. Remarkably, no liquid products were detected, while only a mixture of CO and H2could be detected (Figure S4).The reactivity trends in faradaic efficiencies (FE) of the two principal CO2RR products using four NC catalysts at different applied overpotentials are displayed in Figure 4b and Figure S5. FEs of H2 on NC-800, NC-900 and NC-1100 remain above 40% over the entire applied potential range, showing poor selectivity towards CO2RR. By contrast, NC-1000 catalysts clearly act as highly promising catalyst for selective CO production, and the maximum FE of CO is obtained at a smaller overpotential on NC-1000 than on the other three NC electrodes. To have a fundamental mechanistic understanding of the superior CO2 catalysis performance on the NC-1000, we focus on the correlation between different N species in various NC catalysts and the CO2RR properties. As illustrated in Figure 4c, the increase of annealing temperature from 800 °C to 1000 °C is conducive to elevating the contents of pyridinic and graphitic nitrogen for the promotion of the CO selectivity. For NC800 and NC-900, FEs of CO reach maximum of 37% at -1.26 V and 54% at -1.18V, respectively. As for NC-1000, the maximum of FECO accounts for ≈92%, accompanying by the significant anodic shift onset potential of 200 mV. However, when the temperature increased up to 1100 °C, the contents of pyridinic nitrogen shows a downward tendency while graphitic nitrogen percentage continued the upward trend increasing to 57.15 wt%. We attribute that to surface rearrangement of the condensation reactions in conjunction with coupled edge termination of pyridinic units by graphitic layers. Along with this are the decline of FECO (FECO

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