Enhanced Activity for CO2 Electroreduction on a Highly Active and

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Enhanced activity for CO2 electroreduction on a highly active and stable ternary Au-CDots-C3N4 electrocatalyst Siqi Zhao, Zeyuan Tang, Sijie Guo, Mumei Han, Cheng Zhu, Yunjie Zhou, Liang Bai, Jin Gao, Hui Huang, Youyong Li, Yang Liu, and Zhenhui Kang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01551 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Enhanced activity for CO2 electroreduction on a highly active and stable ternary Au-CDots-C3N4 electrocatalyst Siqi Zhao§, Zeyuan Tang§, Sijie Guo§, Mumei Han, Cheng Zhu, Yunjie Zhou, Liang Bai, Jin Gao, Hui Huang, Youyong Li*, Yang Liu*, Zhenhui Kang*

Jiangsu Key Laboratory for Carbon Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR China E-mail: [email protected]; [email protected]; [email protected].

ABSTRACT

Electrochemical reduction of CO2 to carbon-containing fuels possesses the potential to solve the environment issues caused by excess CO2 in the atmosphere. Herein, we introduce a ternary AuCDots-C3N4 electrocatalyst for efficiently reducing CO2 to CO. The ternary catalyst exhibited significantly enhanced activity and stability for CO2 electroreduction compared with pure Au NPs. The Au-CDots-C3N4 electrocatalyst demonstrates a high CO FE of ~79.8 % at -0.5 V, and a 2.8-fold enhancement of current density (with the Au loading only 4 wt %) at -1.0 V relative to pure Au NPs. The DFT calculations and experimental observations indicate that the high activity towards CO2RR originates from the synergetic effect between Au NPs, CDots and C3N4 substrate, and the capability of H+ and CO2 adsorption from CDots. The long-term stability tests

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demonstrate the electrocatalyst can be used for over 8 hours without obvious deactivations and maintained its activity over 60 days under normal condition. KETWORDS: Au nanoparticles, carbon dioxide reduction, electrocatalysis, stability, selectivity 1. INTRODUCTION Converting carbon dioxide (CO2) and water (H2O) into useful chemicals and carbon-containing fuels with electrochemical method is a desirable way to decrease the excess greenhouse gas and to achieve an artificial carbon cycle.1,2 Compared with the thermodynamic method, the electrochemical method is considered as a clean pathway at the expense of the sustainable electric energy from wind or tide. However, CO2 electroreduction is challenged for the intrinsic stability of CO2 and the low selectivity for specific product. A mass of products can be obtained by the two-, four-, six- and eight- electron pathways. Thus, it is urgent to search for a highly selective and active catalyst for CO2 reduction reaction (CO2RR). Prior researches have investigated several metallic electrodes including Au, Ag, Pd and Cu for CO2 reduction.

3–8

Cu is the only metallic electrode which is active for producing C1-C3

hydrocarbons and alcohols. However, the poor selectivity for hydrocarbons and alcohols is challenged for the later separation from the aqueous system. As an alternative, the gas phase product CO is easy to isolate and can be used directly in syngas (the mixture of CO and H2). Au, Ag and Pd are recognized as high activity catalysts for electroreduction of CO2 to CO.

9,10

It is

found that Au locates at the top of the pyramid among these noble metals for its low overpotential and high selectivity for CO2 reduction.

9,10

However, the high cost and the huge


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demand as electrocatalysts of Au are the major obstruction for its industrial application. The advances in loading metal nanoparticles on substrates as heterogeneous catalysts have overcome the limitation, which achieved a balance between the usage of Au and the lower consumption. 11– 14

Meanwhile, to keep the high activity, identification of a suitable substrate should not only

focus on its stabilization for metal nanoparticles but also the participation in the reaction.15–18 Graphitic carbon nitride (g-C3N4) is a promising candidate for its facile synthesis, low-cost and strong tolerance to acid/alkaline environments compared with other metal oxides substrates.19 The metal-free C3N4 substrate can not only stabilize the Au nanoparticles in their neutral state but also provide active sites for CO2 reduction. A mass of prior work suggest pyridinic N is the leading active site compared with common N configurations.20–23 In addition, the carbon dots feature unique advantages for designed catalysts due to their favorable electronic properties and it can greatly accelerate the electron transport in the reaction. 24–28 In this work, we demonstrate the load of carbon dots (CDots) and Au nanoparticles (NPs) on C3N4 (Au-CDots-C3N4) to form a highly active and stable electrocatalyst for CO2 electroreduction to CO. For 4 wt % Au- CDots-C3N4, it achieves a low overpotential of 190 mV and a maximum CO FE of ~79.8 % at the potential of -0.5 V (vs. RHE, all potentials reported here are with respect to this reference). The electrocatalyst demonstrates a 2.8-fold enhancement of current density compared with Au NPs at the potential of -1.0 V. Meanwhile, the Au-CDotsC3N4 electrocatalyst exhibits excellent stability which can maintain its activity over 60 days under normal conditions. Moreover, the experimental observations and DFT calculations indicate


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the high activity originates from (i) the synergetic effect between Au NPs, CDots and C3N4 substrate, and (ii) the capability of H+ and CO2 adsorption from CDots. 2. EXPERIMENTAL SECTION

2.1. Materials Gold (III) chloride trihydrate (HAuCl4·3H2O, 99.0 %), urea (99.0 %), ammonia water (25.0 %), trisodium citrate (99.5 %), Sodium borohydride (NaBH4, 96.0 %) and Nafion perfluorinated resin solution (5 wt. %) were purchased from Adamas-beta; graphite rods (99.99 %) was purchased from Alfa Aesar Co. Ltd.; Nafion®212 membrane was purchased from Dupont. Deionized water (purified by a Milli-Q system) was used to prepare all solutions and to rinse samples and glassware. 2.2. Instruments The crystal structure of the resultant products was characterized by X-ray diffraction (XRD) using an X‘Pert-ProMPD (Holand) D/max-γAX-ray diffractometer with Cu Kα radiation (λ=0.154178 nm). Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectroscopy were performed by a FEI-quanta 200 scanning electron microscope with an acceleration voltage of 20 kV. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were obtained with a FEI-Tecnai F20 (200 kV), respectively. Raman spectra were collected using an HR 800 Raman spectroscope (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. The spectrograph used 600 g mm-1


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gratings and a 633 nm He–Ne laser was utilized for excitation. The Fourier transform infrared (FTIR) spectrum is recorded on a FTIR spectrometer (Spectrum One, Perkin Elmer) using a standard KBr pellet technique. X-ray photoelectron spectroscopy (XPS) was obtained by using a KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with a monochromatised Mg Kα Xray source (hν = 1283.3 eV). BET surface area was determined by plotting the adsorption isotherm of N2 at liquid N2 temperature (77 K) obtained using a Micromeritics ASAP 2050 instrument. The electro-catalysis actions were tested by a Model CHI 660C workstation (CH Instruments, Chenhua, Shanghai, China). 2.3. Preparation of Au-CDots-C3N4 catalyst The CDots were synthesized by a typical electrochemical and hydrothermal method.

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For

preparation of CDots-C3N4, 10 g of urea powder was added to a 10 mL 0.1 g/L CDots solution. Then the mixture was put into an alumina crucible with a cover and heated to 550 °C in a muffle furnace and maintained at this temperature for 3 h. The resultant deep yellow powder was cooled to room temperature.

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Au-CDots-C3N4 was fabricated with a general one-step photochemical

strategy. In a typical synthesis, CDots-C3N4 (200 mg) was dispersed in 100 mL water under ultrasonic irradiation for 2 h. Then, the HAuCl4 solution (50 mM, 1 ml) was added into the CDots-C3N4 suspension under stirring. The suspension was kept stirring at room temperature and irradiated with a 300 W Xe-lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd, China). After 24 h, the Au-CDots-C3N4 product was collected by centrifugation and washed with water for several times. Then, the catalyst was dried overnight in a vacuum-oven. The Au-CDots-C3N4


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electrocatalyst with different Au loading were prepared by altering the added quantity of CDotsC3N4 substrate. 2.4. Electrochemical measurements Electrocatalytic activity. The electrocatalytic experiments were performed in a standard threeelectrode configuration. A platinum wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The working electrode was a catalyst modified glassy carbon disk electrode (GCE, 3.0 mm diameter CH Instruments). Considering Au-CDots-C3N4 electrode as an example, 3 mg of catalyst was dispersed in 1.5 mL pure water containing 0.5 wt % of Nafion to form a homogeneous ink after at least 15 minutes ultrasonic. Then 5.0 µL of the catalyst ink was loaded onto a GCE (loading ~ 0.141 mg cm-2). All potentials were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.242+ 0.059×pH) V. Linear sweep voltammetry curves were performed in 0.5 M KHCO3 solution under Ar (99.99 %) atmosphere and CO2 (99.99 %) atmosphere (solution was saturated with CO2 for at least 30 minutes), respectively. The durability test was performed in an electrolytic cell with 0.5 M KHCO3 solution (saturated with CO2) for 8 hours. During the measurement, CO2 was bubbled into the solution continuously with a constant velocity. Electrochemically Active Surface Area. The electrochemically active surface area (ECSA) of the electrocatalysts is estimated from the electrochemical double-layer capacitance (Cdl).

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The

Cdl was determined by measuring the CV curves at different sacn rate (5, 10, 25, 50, 100, 200 mV/s) under a non-Faradaic potential range. The non-Faradaic potential range was identified


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from CV and the non-Faradaic region is typically a 0.1 V window centered at open-circuit potential. All measured current in this region is assumed to be owing to a double-layer charging. 2.5. Products analysis For the products analysis, the measurements were performed in an airtight electrochemical Htype cell with two chambers separated with a slice of proton exchange membrane (Nafion® 212). The cathode chamber includes a working electrode and a reference electrode, while the anode compartment is composed of a platinum wire as the counter electrode. 70 µL of the catalyst (dissolved in 0.5 wt. % nafion) modified carbon fiber paper (0.7 cm×0.7 cm) (loading ~0.141 mg cm-2) was used as working electrode. Each of the chambers loaded 75.0 mL 0.5 M KHCO3 (saturated with CO2) and 40.0 mL CO2 in the headspace. The carbon-contained gas products (CO, CH4, C2H4, and C2H6) were tested by a flame ionization detector (FID) and a TDX-1 chromatographic column with a methane converter. The major by-product (H2) was tested by a thermal conductivity detector (TCD) and a molecular sieve 5A packed column. The liquid phase products were qualified by a NMR (Bruker AVANCEAV III 400) spectroscopy, in which 0.5 mL electrolyte was mixed with 0.1 mL D2O (deuterated water) and 0.05 µL dimethyl sulfoxide (DMSO, Sigma, 99.99%) was added as an internal standard. Calculation of Faradaic efficiency: For CO, ‫ܧܨ‬஼ை =

2‫݊ × ܨ‬஼ை × 100% ‫ݐ×ܫ‬

For H2,


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‫ܧܨ‬ுమ =

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2‫݊ × ܨ‬ுమ × 100% ‫ݐ×ܫ‬

where FE is the Faraday efficiency, ݊஼ை is the mole of produced CO, and ݊ுమ for the produced H2. 2.6. Gas adsorption measurements All of the gas adsorption was examined on Micromeritics ASAP 2050 instrument. For N2, the catalysts were degassed at 180 °C for 2 h before analysis. Isotherms were analyzed using ASIQwin software. The surface area of the samples was determined from nitrogen sorption isotherm at 77 K. Pore-size distributions of the samples were calculated from the nitrogen sorption isotherms using nonlocal density functional theory (NLDFT). CO2 adsorption was determined by plotting the adsorption isotherm of CO2 at 25 °C. The samples were degassed for two cycles before analysis. 2.7. H+ adsorption measurements The proton (H+) adsorption capacity of 4 wt % Au-C3N4 and Au-CDots-C3N4 was measured with a dialysis method. 0.05 g of catalyst (4 wt % Au-C3N4 or Au-CDots-C3N4) was added to 25 mL HCl solution (5 mM). Then, the 4 wt % Au-C3N4 or Au-CDots-C3N4 solution was dialyzed using a semi-permeable membrane (MWCO 1000) in a beaker with 500 mL 2.5 mM HCl in it. 2 mL HCl solution was taken out and titrated with 2.5 mM NaOH solution before dialysis. After stirring on a shaker for predetermined time intervals, 2 mL of dialysate was taken out and the concentration of HCl solution was still determined by titrating with 2.5 mM NaOH solution. The concentration of HCl solution vs. time can be obtained.


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2.8. DFT calculations All density functional theory (DFT) calculations were carried out using the Atomic Simulation Environment (ASE)32 in connection with the Vienna Ab Initio Simulation (VASP)33,34 code. The interaction between the valence electrons and the ions were described by the Project-AugmentedWave (PAW)35 scheme, with a planewave energy cutoff of 400 eV. The exchange-correlation interactions were treated within the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE)36 functional. The van der Waals interactions were described using the empirical correction in Grimme’s scheme (DFT-D3)37. As g-C3N4 is semiconductor, the Gaussian smearing with a width of 50 meV was used for the occupation of electronic levels. To simulate the hybrid CDots-C3N4 composites, CDots (C24H12) are stacked on a (3x3) stoichiometric g-C3N4 surface slab. A Γ-centered 1x1x1 Monkhorst–Pack grid for the Brillouin zone sampling. 20 K-points along each high-symmetry line in the Brillouin zone were used to obtain band structures. The ground-state atomic geometries are obtained by minimizing the Hellman–Feynman forces with the conjugate-gradient algorithm until the total force on each ion is below 0.03 eV/ Å. The convergence threshold of self-consistent filed iterations was set to 10–4 eV.

3. RESULTS AND DISCUSSION

3.1. Characterizations of the Au-CDots-C3N4 electrocatalyst.


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Figure 1. (a) TEM image of of 4 wt % Au-CDots-C3N4; (b) HRTEM image of 4 wt % AuCDots-C3N4 and corresponding FFT pattern of the crystallite Au (inset); (c) The large-angle XRD patterns of CDots (gray trace), C3N4 (brown trace), CDots-C3N4 (blue trace), 4 wt % AuCDots-C3N4 (red trace) and Au (black trace). (d) The N2 adsorption-desorption isotherms and pore size distribution of 4 wt % Au-CDots-C3N4 (inset).


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In our experiments, the amine group functionalized carbon dots were fabricated with typical electrochemical and hydrothermal methods reported in prior work19,29 and named as CDots for short. The TEM image of CDots is presented in Figure S1, indicating that the CDots are evenly distributed with diameters in the range of 2-7 nm (size distribution shown in Figure S2). The HRTEM image of CDots (inset of Figure S1) reveals that the lattice spacing of 0.21 nm is consistent with (100) plane of graphitic carbon. The surface functional groups of CDots are characterized by FTIR spectroscopy and shown in Figure S3. As previous work reported, the amine group can efficiently increase the adsorption ability of CO2,

25,38–40

which plays an

important effect on CO2 reduction. The Au-CDots-C3N4 electrocatalyst was fabricated with Au loading 1 wt %, 4 wt % and 8 wt %, respectively. It is convenient that each catalyst was named by the loading weight percentage of Au and the elementary characterizations reported here were all from the 4 wt % Au-CDots-C3N4 (the EDX spectrum showed in Figure S4). The typical TEM image (Figure 1a) reveals that the Au nanoparticles are uniformly distributed on the surface of CDots-C3N4 matrix. The HRTEM image of 4 wt % Au-CDots-C3N4 shows two dominant dspacing (Figure 1b). The d-spacing of 0.23 nm is consistent with the (111) lattice plane of Au,41 while the d-spacing of 0.21 nm is assigned to (100) lattice plane of CDots.42 The 2D fast Fourier transform (FFT) pattern of the Au nanoparticle (inset in Figure 1b) exhibits the polycrystalline structure of the Au NPs, which is consistent with the orientation of 0.23 nm. The XRD pattern (Figure 1c) of CDots-C3N4 shows two diffraction peaks at 2θ = 13.1o and 27.4o, which match well with the (100) and (002) crystal planes of g-C3N4, respectively. The peak belongs to CDots


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is located at 2θ = 26o but it is not obvious in the XRD pattern because of the low content of CDots. In comparison with CDots-C3N4, the 4 wt % Au-CDots-C3N4 composite shows additional peaks at 2θ = 38.3 o, 44.5 o, 64.8 o and 77.7 o, which can all be assigned to the (111), (200), (220) and (311) crystal planes of Au (JCPDS No. 65-2870). The N2 adsorption and the surface area of 4 wt % Au-CDots-C3N4 are investigated by the Brunauer-Emmett-Teller (BET). Figure 1d shows N2 adsorption-desorption isotherms of 4 wt % Au-CDots-C3N4 which is consistent with H3typed hysteresis loop of typical type IV isotherm,43 indicating the formation of mesoporous in the matrix. A large catalyst surface area of 117 m2/g is observed, which originates from the porous structure of C3N4 matrix. The pore size distribution obtained by BJH method is shown in the inset of Figure 1d.


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Figure 2. XPS spectra of 4 wt % Au-CDots-C3N4: (a) survey spectrum, (b) C 1s, (c) N 1s and (d) Au 4f high-resolution spectrums.

The X-ray photoelectron spectroscopy (XPS) is performed to study the elemental composition and the bonding of the 4 wt % Au-CDots-C3N4. The survey spectrum (Figure 2a) indicates the C, N, Au and O peaks in 4 wt % Au-CDots-C3N4 electrocatalyst. The O 1s peak (Figure S5) at 531.7 eV can be ascribed to the small amount absorbed water and the C-O stemming. The peak at 533.2 eV belongs to the C=O groups.44 The C 1s spectrum (Figure 2b) shows three peaks at 288.1, 284.6 and 286.2 eV, corresponding to the sp2-bonded carbon (N-C=N), graphic carbon


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(C-C) and residual C-O, respectively.45 The N 1s spectrum (Figure 2c) can be fitted into four peaks at 398.8, 399.6, 401.1 and 404.8 eV. The major peak at 398.8 eV is assigned to the sp2hybridized aromatic N (C=N-C). The peak at 399.6 eV corresponds to tertiary N bonded to carbon atoms [N-(C)3]. The peak at 401.1 eV is related to N–H side groups, and the weak peak at 404.8 eV is attributed to the π-excitation.46,47 The two peaks in Au 4f (Figure 2d) at 84.0 eV and 87.7 eV originate from 4 f7/2 and Au 4f5/2 electrons of metallic gold, respectively.41 Additional characterizations of the 4 wt % Au-CDots-C3N4 include STEM, EDX mapping (Figure S6) and SEM (Figure S7).


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Figure 3. (a) The LSVs of 4 % Au-CDots-C3N4 measured in CO2-saturated (red) and Arsaturated (blue) 0.5 M KHCO3 electrolyte, respectively; (b) The FEs of CO and H2 vs. the applied potential, catalyzed by 4 wt % Au-CDots-C3N4; (c) The partial current density for CO2 reduction (jCO, red trace) and for HER (jH2, black trace) vs. the applied potential, catalyzed by 4 wt % Au-CDots-C3N4; (d) The stability performance of 4 wt % Au-CDots-C3N4 for CO2 reduction operated at potentiostatic potential of -0.5 V (red) and -0.8 V (black) vs. RHE for 8 h.

3.2. Electrocatalytic activity for Au-CDots-C3N4.


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In typical experiments, the activities of 4 wt % Au-CDots-C3N4 towards electroreduction of CO2 were performed in a standard three-electrode configuration. The LSVs for 4 wt % Au-CDotsC3N4 are shown in Figure 3a. The trace (blue) measured in Ar-saturated 0.5 M KHCO3 (pH=8.5) shows an onset potential at -0.4 V, indicating a high overpotential of 0.4 V for HER. The trace (red) measured in CO2-satuated 0.5 M KHCO3 (pH=7.2) shows an obvious enhancement of current density which is donated by the CO2 reduction. The consequence is further evidenced by detecting the electrolytic products after short-term potentiostatic electrolysis. The gas-phase products are detected using a gas chromatography (GC) and the liquid-phase products are detected by 1H NMR and the DMSO is added as an internal standard. Only CO and H2 are observed during the overall electrolysis. Thus, the more positive onset potential at -0.3 V and a corresponding unprecedented overpotential of 0.19 V for CO2 reduction to CO are obtained. The FEs of CO (Figure 3b) for 4 wt % Au-CDots-C3N4 present a sharply incremental tendency at the applied potential range from -0.3 V to -0.5 V and reach a plateau of ~79.8 % at the potential of 0.5 V. Then, when the applied potential sweeps more negatively, it moderately declines to ~58.6 % at the potential of -1.0 V. The FEs for H2 show an opposite geometric linear trend, which decrease from 47.2 % to 13.2 % at the potential range from -0.3 V to -0.5 V. Then it rises again to 34.8 % with the increase of applied potential up to -1.0 V. Figure 3c shows the partial current densities of CO (jCO, black trace) and H2 (jH2, red trace) vs. different potentials applied in CO2saturated 0.5 M KHCO3 electrolyte. The traces present the variation tendencies of jCO and jH2 as the changes of the applied potentials. At -1.0 V, the jCO reaches 4.8 mA/cm2 based on the


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geometric area and 0.55 mA/cm2 based on the electrochemical surface area (Figure S8), respectively. The ECSA for the containing Au NPs in the Au-CDots-C3N4 is measured with Cu underpotential deposited method (Figure S9 and Table S1). The corresponding current density (A/g) versus the applied potential is shown in Figure S10. A high current density of 34.1 A/g is obtained at the potential of -1.0 V by the 4 wt % Au-CDots-C3N4 electrocatalyst. The stability performance of 4 wt % Au-CDots-C3N4 for electroreduction CO2 to CO at the potential of -0.5 V (red trace) and -0.8 V (black trace) were operated for 8 h (Figure 3d). No obvious deactivations for current densities are observed during the 8-h stability test. After the long-term test, we characterized the morphology of the Au-CDots-C3N4 composite by TEM. It was found that the Au-CDots-C3N4 keeps its initial morphology without obvious aggregation for Au NPs (Figure S11). The different Au loading electrocatalyst including 1 wt % Au-CDots-C3N4 and 8 wt % AuCDots-C3N4 were fabricated by altering the added quality of CDots-C3N4 and applied in CO2 reduction (Figure S12, 13). The corresponding characterization including XRD, EDX, TEM and XPS are shown in Figure S14-16. According to Figure S13, with the decrease/increase of the CDots-C3N4 quantity in the reaction system (compared to 4 wt % Au-CDots-C3N4), the distribution density of Au nanoparticles increases/decreases, yet both have a negative influence on their activities for CO2 electroreduction. In other word, electrocatalytic activity did not show a linear fitting with the increase of Au loading content, among which the loading content of 4 wt % is the most effective and active for CO2 reduction. The result indicates that Au NPs is not the only part contributing to the high activity towards CO2 reduction. Therefore, an outstanding


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ternary electrocatalyst not only requires active individual components but also appropriate synergetic effect among them.

Figure 4. (a) The LSVs of C3N4 (black trace), CDots-C3N4 (pink trace), CDots (brown trace), 4 wt % Au-C3N4 (blue trace), Au NPs (green trace) and 4 % Au-CDots-C3N4 (red trace) measured in CO2-saturated 0.5 M KHCO3 electrolyte; (b) The FEs of CO vs. the applied potential, catalyzed by C3N4 (black trace), CDots (brown trace), CDots-C3N4 (pink trace), 4 wt % Au-C3N4 (blue trace), Au NPs (green trace) and 4 % Au-CDots-C3N4 (red trace); (c) The mass CO FEs of C3N4 (black), CDots-C3N4 (pink), 4 wt % Au-C3N4 (blue), Au NPs (green) and 4 % Au-CDots-


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C3N4 (red); (d) The partial current density for CO2 reduction (jCO) vs. the applied potential, catalyzed by C3N4 (black trace), CDots-C3N4 (pink trace), 4 wt % Au-C3N4 (blue trace), Au NPs (green trace) and 4 % Au-CDots-C3N4 (red trace).

3.3 Catalytic activity of controlled components catalysts.

As control experiments, C3N4, CDots, CDots-C3N4, Au NPs and Au-C3N4 (TEM images and XPS showed in Figure S17, 18) were performed for CO2RR under the same experimental condition (Figure S19) to distinguish the Au NPs effects as well as CDots and C3N4 effects and to differentiate their contributions for CO2RR. Figure 4a shows the LSV results of C3N4, CDots, CDots-C3N4, Au NPs, 4 wt % Au-C3N4 and 4 wt % Au-CDots-C3N4. The pure C3N4 shows a negligible current density duo to its nonconductive nature (-0.1 mA/cm2 at -0.7 V). However, its electrochemical activity is much-enhanced when combined with CDots (-0.38 mA/cm2 at -0.7 V). The 4 wt % Au-CDots-C3N4 composite shows an appropriately 4.0-fold, 2.1-fold and 1.2-fold enhancement of current density compared with CDots-C3N4 and Au-C3N4 and Au NPs at -0.7 V, respectively. Moreover, the Au-CDots-C3N4 exhibits a more positive onsite potential relative to other samples. To confirm the increased current density and the promoted kinetics process are from CO2RR rather than HER, the short-time potentiostatic electrolysis was performed under a potential range from -0.3 V to -1.0 V. Figure 4b shows the CO FEs over the potential range from -0.3 V to -1.0 V for C3N4, CDots, CDots-C3N4, Au NPs, 4 wt % Au-C3N4 and 4 wt % Au-CDotsC3N4, which inspired a definite consequence of the insight into the mechanism for CO2RR on


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Au-CDots-C3N4. The pure C3N4 exhibits a poor CO selectivity (4.2 % at -0.8 V) towards CO2RR, while the combination of CDots in C3N4 much improved the catalytic activity and achieved a maximum FE of 8.9 % at -0.7 V. However, the CDots generate H2 only at the whole potential range from -0.5 V to -1.0 V. It indicates that the CDots are not the direct active sites for CO2 electroreduction to CO, while the combination of CDots with C3N4 can effectively promote the CO2 electroreduction. The Au NPs, as a recognized CO2 reduction catalyst, can effectively convert CO2 to CO. Here, the synthesized Au NPs present a moderate selectivity (56.3 % at -0.7 V) for CO. Thus, both the substrate (CDots-C3N4) and the Au NPs are active towards CO2RR. For 4 wt % Au-CDots-C3N4 composite, a maximum CO FE of 79.8 % at -0.5 V was observed. The much-enhanced activity (relative to Au NPs and CDots-C3N4) demonstrates the significant synergistic effect among Au NPs, CDots and C3N4. The comparison of Au NPs, Au-C3N4 and Au-CDots-C3N4 can further confirm the consequence. The 4 wt % Au-C3N4 and Au NPs show their highest CO FE of 63.6 % and 56.3 % at the potential of -0.6 V and -0.7 V, respectively, which are much lower than the CO FE of 79.8 % (-0.5 V) for 4 wt % Au-CDots-C3N4 composite (Figure 4c). The CO current density (jCO) for C3N4, CDots, CDots-C3N4, Au NPs, 4 wt % AuC3N4 and 4 wt % Au-CDots-C3N4 are shown in Figure 4d. Notably, although the 4 wt % AuC3N4 shows a comparably higher CO FE compared with Au NPs, the jCO shows a 1.7-fold reduction compared with Au NPs. The result indicates that combination of Au NPs and C3N4 can efficiently promote CO2RR but harmed by the poor conductivity of C3N4. By coupling the CDots with Au-C3N4, the Au-CDots-C3N4 with only 4 wt % Au loading delivers more current


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relative to the Au-C3N4 under the CO2 atmosphere. At -1.0 V, the CO current density for AuCDots-C3N4 shows a 4.8-fold and 2.8-fold enhancement compared with 4 wt % Au-C3N4 and Au NPs, respectively. Consequently, we can observe that CDots play important roles in the ternary Au-CDots-C3N4 electrocatalyst.

3.4. The proposed mechanism for CO2RR.

We propose the high activity and high selectivity of the ternary Au-CDots-C3N4 is originated from not only the significantly synergistic effect between the Au NPs, CDots and C3N4 but also the excellent ability of H+ and CO2 adsorption for CDots. Meantime, the combination of CDots with Au-C3N4 can exactly accelerate the charge transfer rate in the reaction. The DFT calculations and experiments observations further confirm the consequence.


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Figure 5. (a) Band structure of pure g-C3N4 (black), CDots-C3N4 composite (red) and AuCDots-C3N4 composite (blue)； (b) The projected density of states on pure C3N4 (black), CDotsC3N4 hybrid (red) and Au-CDots-C3N4 composite (blue); Interfacial electrons transfer in (c) CDots-C3N4 composite and (d) Au-CDots-C3N4 composite; Yellow and cyan iso-surface represent electron accumulation and electron depletion, respectively; the iso-surface value is 0.0003 eÅ-3.


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The density functional theory (DFT) calculations show the electronic-coupling effect among the CDots-C3N4 and Au-CDots-C3N4 (Figure 5). Contrary to the pure g-C3N4 (1.92 eV), CDots-C3N4 shows a narrower band gap (1.35 eV) due to the new introduced valence band maximum (Figure 5a) and the downshift PDOS of C3N4 (Figure 5b). And the band gap of Au-CDots-C3N4 (0.32 eV) decreases further. Moreover, it exhibits apparent charge redistribution in CDots-C3N4 (Figure 5c) and Au-CDots-C3N4 (Figure 5d). The electron-depletion and electron-accumulation regions are marked as cyan and yellow iso-surfaces, respectively. Notably, the charge redistribution between Au, CDots and C3N4 and the change in projected density of states between pure g-C3N4, CDotsC3N4 and Au-CDots-C3N4 indicates enhanced electron mobility in the composite, which is signiﬁcant for the electrocatalytic CO2 reduction. Meanwhile, the introduction of Au also enhances the electrocatalytic performance of CO2 reduction on C3N4. The loading of Au makes the structure of C3N4 become more corrugated (Figure S20) and the corrugation of C3N4 could lead to a lower energy barrier for CO2 reduction based on previous DFT calculations. 48


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Figure 6. (a) The EIS Nyquist plots of C3N4 (black points), CDots-C3N4 (pink points), 4 wt % Au-C3N4 (blue points) and 4 wt % Au-CDots-C3N4 (red points) electrodes in CO2-saturated 0.5 M KHCO3; inset shows the equivalent circuit impedance model; data were collected under -0.3 V vs. RHE; (b) The time-course adsorption of H+ (based on HCl) by the 4 wt % Au-CDots-C3N4 (black trace) and 4 wt % Au-C3N4 (red trace) (top); CO2 adsorption isotherm of 4 wt % AuCDots-C3N4 (black trace) and 4 wt % Au-C3N4 (red trace) (down).

The existence of CDots actually promotes the CO2 electroreduction as previously mentioned. The electrochemical impedance spectroscopy (EIS) for C3N4, CDots-C3N4, Au-C3N4 and AuCDots-C3N4 were explored (Figure 6a) to investigate the charge-transfer process in CO2 reduction. The apparent smaller radius of CDots-C3N4 relative to C3N4 and Au-CDots-C3N4 relative to Au-C3N4 are observed which originates from its increased conductivity because of the excellent charge-transfer capability for CDots. Moreover, the CO2 and H+ adsorption of 4 wt % Au-C3N4 and 4 wt % Au-CDots-C3N4 were measured and shown in Figure 6b. 4 wt % AuCDots-C3N4 adsorbs larger amount of CO2 (~0.34 mmol/g at 1.2 atm) compared with 4 wt % Au-C3N4 (~0.22 mmol/g at 1.2 atm), indicating that the CDots possesses admirable CO2 adsorption capability. Moreover, the incorporation of CDots in 4 wt % Au-C3N4 increases a mass amount of H+ adsorption from 0.18 g/g to 0.37 g/g (based on the quality of HCl). Thus, the CDots in the ternary Au-CDots-C3N4 electrocatalyst plays a leading role in facilitating CO production for their excellent ability of CO2 and H+ adsorption.


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Figure 7. (a) The proposed reaction mechanism for CO2 reaction on Au-CDots-C3N4; (b) The comparison of LSVs measured in Ar-saturated and CO2-saturated 0.5 M KHCO3 electrolyte for the initial catalyst, after 30-days storage catalyst and after 60-days storage catalyst; (c) The stability performance of 4 wt % Au-CDots-C3N4 for CO2 reduction operated at potentiostatic potential of -0.5 V vs. RHE.

Based on above results, a proposed mechanism schematic program for electroreduction CO2 to CO on Au-CDots-C3N4 is shown in Figure 7a. Mass of CO2 and H+ are adsorbed to the surface of the catalyst by CDots, which make the reaction can be preceded with high efficiency.


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Moreover, we further measured the electrocatalytic performance of the 4 wt % Au-CDots-C3N4 after the 30-days and 60-days storage under normal conditions. It is found that the catalyst could maintain its activity over 60 days. Figure 7b shows the LSVs of 4 wt % Au-CDots-C3N4 under different storage periods (initial, 30 days, 60 days). Figure 7c shows the corresponding stability performance for 4 wt % Au-CDots-C3N4. The comparison of current density and CO FE after CO2 reduction at the potential of -0.5 V is shown in Figure S21. After 30-days storage under normal condition, the Au-CDots-C3N4 composite showed degradation for the CO FE from 79.8 % to 72.5 % and the current density dropped by approximately 9.2 % from (0.29 mA/cm2) to (0.26 mA/cm2) at the potential of -0.5 V. After 60-days storage under normal condition, the Au-CDotsC3N4 composite showed degradation for the CO FE from 79.8 % to 65.3 % and the current density dropped by approximately 18.2 % from (0.29 mA/cm2) to (0.24 mA/cm2) at the potential of -0.5 V. Consequently, we consider that the ternary Au-CDots-C3N4 is an extremely stable electrocatalyst.

4. CONCLUSIONS We fabricated a ternary Au-CDots-C3N4 electrocatalyst for efficient electroreduction CO2 to CO. For 4 wt % Au-CDots-C3N4, it can convert CO2 to CO with a maximum FE of ~79.8 % at the potential of -0.5 V and a low overpotential of 0.19 V. The 4 wt % Au-CDots-C3N4 electrocatalyst demonstrates a 2.8-fold enhancement of current density compared with Au NPs at the potential of -1.0 V. Moreover, the long term stability tests indicate that the electrocatalyst


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could be performed for 8 hours at the potential of -0.5 V and -0.8 V without obvious deactivations and can maintain its activity over 60 days under normal condition. The experimental observations and DFT calculations indicate that the obviously enhanced activity for CO2 electroreduction is attributed to the synergetic effect between Au NPs, CDots and C3N4 substrates. The combination of CDots with C3N4 can facilitate the CO2 reduction to CO which is attributed to the excellent capability of CO2 and H+ adsorption, and prominent conductivity for CDots. Thus, we propose the ternary Au-CDots-C3N4 electrocatalyst is an excellent candidate with high-activity and high-stability for selectively electroreducing CO2 to CO. Our results also suggested that CDots-C3N4 is not only a good support but also an efficient component for high efficient electrocatalyst design in CO2 reduction.

ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]; [email protected].

Author Contributions


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Z.K. fully designed and supervised the project. Y.L. and Y.L. supervised part of the project. S.Z., Z.T. and S.G. contributed equally to the conception of the experiment and co-wrote the manuscript. S.Z. and S.G. conducted the synthesis and all electrochemical measurements. Z.T. carried out the DFT calculation. M.H., C.Z., Y.Z., J.G. and L.B. performed parts of the characterization. Z.K., H.H. and Y.L. participated in the manuscript modify, data analysis and the interpretation. All authors contributed to data analysis and gave approval to the final version of the manuscript. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS This work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Natural Science Foundation of China (51422207, 51572179, 21471106, 21501126), the Natural Science Foundation of Jiangsu Province (BK20161216) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1) (2) (3)

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Enhanced Activity for CO2 Electroreduction on a Highly Active and

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