Gas Phase Electrolysis of Carbon Dioxide to Carbon Monoxide Using

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Gas Phase Electrolysis of Carbon Dioxide to Carbon Monoxide Using Nickel Nitride as the Carbon Enrichment Catalyst Pengfei Hou,†,∥ Xiuping Wang,‡ Zhuo Wang,† and Peng Kang*,†,‡,§,∥

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/25/18. For personal use only.

†

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Carbon Energy Technology Co. Ltd., Yancun Industrial Park, Fangshan, Beijing 102412, China § School of Chemical Engineering and Technology, Tianjin University, 135 Yaguan Rd, Tianjin 300350, China ∥ University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China S Supporting Information *

ABSTRACT: Nickel nitride was employed as the carbon enrichment electrocatalyst to reduce CO2 both in the aqueous phase and at the gas− solid interface. In an aqueous electrolyte, the CO Faradaic efficiency reached 85.7% at −0.90 V versus reversible hydrogen electrode with a partial current density of 6.3 mA cm−2. When gaseous CO2 was used as a reactant in a flow cell, the CO Faradaic efficiency increased to 92.5% and current density reached 23.3 mA cm−2. By contrast, metallic Ni and NiO generated predominantly H2. The increased amount of strong base sites in the Ni3N catalyst could enrich CO2 at the catalyst surface, and the utilization of gas phase electrolysis, has cooperatively enhanced reactivity.

KEYWORDS: nickel nitride, electrochemical reduction of carbon dioxide, carbon monoxide, gas phase electrolysis, carbon enrichment

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INTRODUCTION Increasing the carbon dioxide content in the atmosphere has caused a global climate change. A promising solution is to utilize carbon dioxide as a renewable carbon source for producing chemicals.1−4 Electrochemical CO2 reduction reaction (CO2RR) using renewable energy to produce fuels and chemicals could provide a carbon-neutral solution.5−7 Carbon monoxide (CO) is a key C1 feedstock for bulk chemical productions, also a commonly reported CO2RR product.2−4,6−14 Although CO2RR shows a promising future, issues such as large overpotential, low current density, and product selectivity still hinder its potential application.4,15,16 It is necessary to develop catalysts, electrolyzers, and electrolytic process for converting CO2RR at high efficiency and selectivity. Strategies such as modulating the size, shape, morphology, and the crystalline boundary of nanomaterial catalysts have been reported.2,4,7,8,11,14,17−23 However, low solubility of CO2 in the aqueous phase (ca. 30 mM) significantly limits the catalytic current density and product selectivity. Surface enrichment of CO2 can be an effective strategy to increase local CO2 concentration to improve selectivity and efficiency, yet this strategy has not been much explored. Meyer et al. used polyethylenimine (PEI) as a co-catalyst with nitrogen-doped carbon nanotubes for efficient conversion of CO2 to formate, and PEI could concentrate CO2 on the surface and stabilize the reduced intermediate.24 We reported © XXXX American Chemical Society

that N-doping into tin oxide and tantalum oxide could enhance CO2RR with increased CO2 selectivity, and N-doping was postulated to increase the catalyst base sites.25,26 In parallel, the CO2 enrichment strategy has been reported in CO2 hydrogenation to formate27 and methanol.28 However, in the aqueous phase, water can competitively adsorb onto the catalyst surface, making CO2 enrichment less effective than desired. Gas phase CO2RR can relieve the water competition and the CO2 low solubility issues. Newman et al. achieved high current densities by gas phase electrolysis using Ag nanoparticles.29 Ionic liquid and bipolar membranes were also used in gas phase electrolysis to improve efficiency of CO2RR.30−34 Sargent reported that a copper electrocatalyst reduced CO2 to ethylene using a gas diffusion electrode at the gas−solid−liquid interface.35 Combining CO2 surface enrichment and gas phase electrolysis strategy could lead to highly efficient and selective CO2RR. Noble metals such as Au, Ag, Pd, and earth abundant metals such as Sn, Zn, and Cu showed decent CO2RR Faradaic efficiencies (FEs) and current densities.7,15,36−41 However, catalysts based on group VIII elements Fe, Co, and Ni were less reported.42−46 Among Ni-based materials, nickel nitride (Ni3N) has been used for oxygen evolution and hydrogen Received: July 18, 2018 Accepted: October 12, 2018

A

DOI: 10.1021/acsami.8b11942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces evolution reaction (HER),47−51 although the use of Ni3N in CO2RR has been absent. Ni3N can be a potential candidate for CO2RR because of the increased surface basicity. We report herein gas phase CO2RR to CO with high efficiency and selectivity using Ni3N as the electrocatalyst.

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anion-exchange resin solution (TWEDAI, Tianwei Membrane, Shandong, China), and the mixture was sonicated for 15 min. The cathode ink was sprayed onto an anion-exchange membrane with 5 cm2 active area and was then cold-pressed at 0.6 MPa for 2 min. An anode catalyst ink was made by mixing 25 mg iridium black nanoparticles (Hesen, Shanghai, China) with 5 mL n-propyl alcohol and 0.1 mL 5 wt % Nafion solution (DuPont, USA), and the mixture was sonicated for 5 min. The anode catalyst ink was spray-painted onto the above anion-exchange membrane with 5 cm2 active area and then was cold-pressed at 0.6 MPa for 2 min. The electrochemical flow cell was assembled by sandwiching the MEA with gaskets for sealing and electrical insulation. The MEA was assembled into fuel cell-like hardware with 5 cm2 active area and graphite serpentine flow channels. A titanium mesh was placed between the catalyst layer and graphite as the current collector. In the electrolysis, DI water was fed to the anode, and the flow rate was controlled at 40 mL min−1 by a peristaltic pump (BT100M, Easypump, BaoDing, China). CO2 was humidified at room temperature and fed to the cathode, and the flow rate was controlled at 30 mL min−1 by a mass flow controller (D07, Sevenstar, Beijing, China), and the content of water vapor in CO2 was measured by a hygrometer (DWS-T5, DiHui, Beijing, China). An electrochemical study was performed on CHI1140C electrochemical station (CHI Instruments, Inc.) in a two electrode electrochemical system. Cell voltage between 2.6 and 3.4 V was applied to the cell and the cathode potential was measured through a salt bridge connected to an SCE and recorded by a second electrochemical station throughout the experiment. The cathode outlet gas was analyzed by SRI Gas Analyzer 8610C GC (SRI, USA) equipped with a HID. The GC used helium carrier gas and was calibrated using external gas standard. Physical Characterizations. X-ray diffraction analyses (XRD) were collected on Bruker DAVINCI D8 ADVANCE diffractometer by using a Cu Kα source (λ = 1.54184 nm). The XRD spectra were obtained for 2θ values from 10 to 90° with a step length of 0.1°. The loadings of the catalysts were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). Transmission electron microscopy (TEM) and high resolution TEM (HR-TEM) images were obtained on a JEOL JEM-2100 and a JEOL JEM-2100F electron microscope, respectively. They were operated at 200 kV with supplied software for automated electron tomography. For TEM measurements, a drop of the Ni3N/C ink was dispensed onto a 3 mm carboncoated copper grid. Excessive ink was removed by an absorbent paper, and the sample was dried under ambient conditions. X-ray photoelectron spectra (XPS) were obtained on a Kratos Analytical Axis UltraDLD spectrometer with monochromatized X-ray Al Kα radiation (1486.6 eV). A survey scan was performed with a step size of 1 eV, a pass energy of 80 eV, and a dwell time of 200 ms. Highresolution scans were then taken for each element present with a step size of 0.1 eV and a pass energy of 20 eV. The binding energy for all peaks was referenced to the C 1s peak at 284.8 eV. CO2 adsorption− desorption isotherm was measured on a Quantachrome Autosorb-1C-TCD instrument.

METHODS

Material Preparation. All the solvents and chemicals were used as received without further purification. All aqueous electrolytes were prepared with ultrapure water with a resistivity of 18.2 MΩ·cm−1. The Ni3N/C catalyst was synthesized by ammonolysis of a nickel precursor supported on Vulcan XC-72R (carbon black, denoted as C). The nickel precursor was prepared by a simple impregnation method. Typically, 4 mmol nickel nitrate (XiLong Chemical Co., Ltd, Ni(NO3)2·6H2O, ≥98.0%) and 1.0 g C were mixed and dispersed in 100 mL ethanol (Beijing Chemical, ≥99.7%) in a flask equipped with a stir bar, followed by agitation in air for 48 h at room temperature. The catalyst precursor was obtained after evaporating the resulting mixture at 378 K and dried in vacuum overnight. The catalyst precursor was loaded into a tubular furnace. The temperature was increased by the temperature-programmed reaction (TPR). The process was under flowing NH3 at 100 mL/min. The sample was heated to the reaction temperature from room temperature to 973 K at 5 K/min and kept for 3 h at 973 K, then cooled down to 573 K at 3.3 K/min, and naturally cooled to room temperature. As control experiments, the Ni3N/C material was treated with 1 M H2SO4 for 2 h, washed with deionized (DI) water and dried, noted as the SA-Ni/C sample. The NiO/C catalyst was prepared from the same Ni precursor by TPR in air. The catalyst precursor was heated to the reaction temperature from room temperature to 673 K at 5 K/min and kept for 2 h at 673 K, then cooled down to 573 K at 3.3 K/min, followed by natural cooling to room temperature. The Ni/C catalyst was prepared by TPR under Ar atmosphere. The catalyst precursor was heated to 973 K at 5 K/min under flowing argon and then maintained for 3 h at this temperature. Finally, the sample was cooled down to 573 K at 3.3 K/min, followed by natural cooling to room temperature. Electrochemical Measurements. The electrochemical measurements were carried out in a standard three-electrode glass cell with Nafion 117 as a separator connected to a CHI660E potentiostat (Chenhua, Shanghai, China). The Pt wire and saturated calomel electrode (SCE) were used as counter and reference electrode. The working electrode was prepared as following: 10 mg Ni3N/C catalyst and 50 μL of 5 wt % Nafion solution were dispersed in 1 mL ethanol by sonication for 1 h to form a homogeneous ink. Five microliters of the ink was loaded onto a 3 mm diameter glassy carbon electrode, and dried at room temperature. Cyclic voltammetry measurements were recorded between −1.2 and 0 V at 100 mV s−1, in stationary Ar or CO2 saturated 0.5 M NaHCO3 or NaCl solution. All electrochemical experiments were carried out in room temperature. The potentials were measured against the SCE electrode and converted to versus reversible hydrogen electrode (RHE) by using E (vs RHE) = E (vs SCE) + 0.245 V + 0.0591pH. All the potentials were reported versus RHE unless otherwise noted. Before controlled potential electrolysis, CO2 (99.999%) was purged into the cell for over 20 min to remove the air from electrolyte, then the electrolysis was conducted for 1−6 h under sealed conditions. The gas mixture in the headspace of the working compartment of the electrolysis cell was analyzed by using gas chromatography (SRI 8610C, USA) equipped with a helium ionization detector (HID) and a thermal conductivity detector (TCD). He (99.999%) was used as a carrier gas. The liquid phase was analyzed by nuclear magnetic resonance on a Bruker AVANCE 400 instrument. N,N-Dimethylformamide was used as the internal standard. The membrane-electrode assembly (MEA) consisted of an anionexchange membrane (TWEDAI, Tianwei Membrane, Shandong, China), cathode and anode catalyst layers, and was prepared as follows: cathode ink was made by mixing 25 mg Ni3N/C nanoparticles with 5 mL n-propyl alcohol and 0.1 mL 5 wt %

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RESULTS AND DISCUSSION Ni3N/C catalysts were prepared by nitriding at elevated temperatures using NH3 as the N source. In XRD spectra (Figure 1e), all the diffraction peaks of Ni3N/C can be indexed to hexagonal Ni3N (JCPDS no. 10-0280).44,48,49 Other impurities such as Ni or NiO were not detected, compared to prepared Ni/C (JCPDS no. 65-0380) and NiO/C (JCPDS no. 65-5745) (Figure 1e). Although higher nitriding temperature increased the crystallinity of Ni3N/C (Figure S1), at temperatures higher than 973 K, a small amount of elemental nickel was found as the cubic Ni phase (JCPDS no. 65-0380), because of thermal decomposition of Ni3N.47 The Ni3N loading in Ni3N/C was determined as 7.1% w/w using ICPAES measurement. B

DOI: 10.1021/acsami.8b11942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. TEM images of Ni3N/C of different loadings (w/w) as determined by ICP-AES. (a) 1.86; (b) 3.73; (c) 5.50; (d) 7.05%.

11.32%, respectively, suggesting that the oxygen content on the surface of Ni3N/C is low and may not contribute significantly to the catalysis. The XPS results confirmed the formation of Ni3N nanomaterials. X-ray adsorption fine structure (XAFS) analyses were used to investigate the valence and structural information. In X-ray absorption near-edge structure (XANES) spectra (Figure 3a), the edge in Ni3N/C shifts to higher energy for ca. 3.79 eV compared to Ni foil and Ni/C, indicating that Ni atoms in Ni3N/C are at a higher valence state. The pattern of Ni3N/C in extended X-ray absorption fine structure spectroscopy (EXAFS) is significantly different from that of the Ni foil and

Figure 1. TEM and HR-TEM images (a,b), high-resolution Ni 2p (c) and N 1s (d) XPS spectra of Ni3N/C, and XRD spectra (e) [the peak at 2θ = 25° was from Vulcan XC-72R] of Ni/C (black), Ni3N/C (red), and NiO/C (blue).

The morphology of Ni3N/C was examined using TEM (Figure 1a). Ni3N nanoparticles were spherical with sizes of 16−30 nm, uniformly deposited on the surface of the carbon black. The HR-TEM image (Figure 1b) showed lattice fringes of Ni3N with an average lattice spacing of ca 0.2143 nm, corresponding to the (002) plane (0.2144 nm) of the hexagonal phase Ni3N, consistent with XRD analysis. The asprepared Ni/C and NiO/C nanoparticles were also consistent with the XRD assignment from the lattice spacing measurement (Figure S3). When the Ni3N loading was varied from 1.86 to 7.05% (Figure 2), the nanoparticles kept the spherical shape of similar sizes, and no significant aggregation was found at higher loading. The surface composition of Ni3N/C was characterized by XPS. As shown in the high-resolution Ni 2p XPS spectrum (Figure 1c), peaks at 853.3 and 870.5 eV can be assigned to Ni 2p3/2 and 2p1/2, corresponding to the Ni+ cation in Ni3N.49,50 Meanwhile, the binding energies of satellite peaks at 856.1 and 873.4 eV were assigned to Ni 2p3/2 and Ni 2p1/2 in NiO or Ni(OH)2, probably because of spontaneous air oxidation.48,49,51 The N 1s XPS spectrum in Figure 1d showed two main peaks at 398.5 and 400.3 eV in binding energy, assigned to Ni−N, pyridonic N in carbon, respectively.50,51 For comparison, the XPS spectrum (Figure S4c) of XC-72R carbon black after the same ammonia treatment showed predominantly one peak at 400.1 eV, assigned to pyrrolic nitrogen (C−N). The survey and high-resolution spectra of O 1s from Ni3N/C and Ni/C were shown in Figure S13. The oxygen atom ratios of Ni3N/C and Ni/C were 5.84 and

Figure 3. XAFS characterizations. Ni K-edge XANES spectra (a). Fourier transforms of Ni K-edge EXAFS oscillation k3χ(k) of Ni3N/C, Ni/C, and Ni foil (b). Corresponding Fourier transform plots (c). Geometrical structure (d) of the Ni3N crystal (N atoms: blue; Ni atoms: cyan). C

DOI: 10.1021/acsami.8b11942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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reaching 68 mA cm−2 at −1.2 V versus RHE, suggesting high catalytic efficiency among these Ni materials. The HER of Ni-based materials were also examined under Ar atmosphere in 0.5 M NaCl solution for additional insights. The current densities of Ni3N/C and Ni/C were comparably high at 188.2 mA cm−2 and 186.9 mA cm−2 at −1.2 V, ca. 6 times of NiO/C (32.2 mA cm−2) (Figure S5a), confirming that NiO/C is the least capable catalyst for reductive catalysis. The Tafel slopes of Ni/C and Ni3N/C were comparable, at 153 and 157 mV dec−1 (Figure S5b), respectively. Yet, the exchange current density j0 of Ni3N/C at 1.47 × 10−2 mA cm−2 was ca. 6 times of Ni/C (2.34 × 10−3 mA cm−2), suggesting a reduced kinetic barrier for HER. From the above results, Ni3N/C was the most reactive Ni material for HER, in line with literature reports.49,51 The N-doping strategy also enhanced HER of Ni3N, because of added surface-basic sites and increased water adsorption. The water adsorption and HER could potentially compete with CO2RR in the aqueous phase. Controlled potential electrolyses (CPEs) were initially performed in the aqueous phase under CO2 atmosphere to investigate the product distribution. The current densities of Ni3N/C increased with further applied potentials between −0.80 and −1.05 V versus RHE in CO2 saturated 0.5 M NaCl solution (pH = 3.7). CO was the main product, and FE for CO reached the maximum of 85.7% at −0.90 V versus RHE (Figure 4b). The rest was H2 with no other product detected. By contrast, NiO/C predominantly yielded H2 with CO less than 5% (Figure 4c), and Ni/C yielded CO of 16.8% at maximum (Figure 4d). The Ni3N/C catalyst attained CO partial current density of 8.7 mA cm−2 at −1.0 V (Figure 4e), significantly higher than that of Ni/C (0.73 mA cm−2). The Ni3N/C catalyst showed decent stability during electrolysis.16 In CPE for 6 h at −0.90 V (Figure S7), the FEs for CO were more than 84% during the electrolysis. The XRD of Ni3N/C after the electrolysis (Figure S6) showed hexagonal Ni3N (JCPDS no. 10-0280) and no other peaks were detected, suggesting the high stability of the catalyst. The choice of electrolyte also influenced CO2RR by Ni3N/ C.13,52,53 When switched to CO2-saturated 0.5 M NaHCO3 electrolyte (pH = 7.2), hydrogen evolution increased as compared to the NaCl electrolyte. CO FE dropped to 66.7% at −0.80 V (Figure S8d), lower than 85.7% in NaCl. A similar electrolyte effect has been reported using Zn and Ag catalysts.39,54 We postulate that HCO3− could bind to the Ni sites and inhibit the CO2 surface adsorption.52,54 In addition, HCO3− anion could serve as a local proton source, thus buffers local pH value and promotes H2 evolution. The origin of the catalytic activity was also explored using various control experiments. N-doped XC-72R with no Ni loaded yielded CO for less than 5%, indicated that the Ndoped carbon has quite low CO2RR activity (Figure S9). As the Ni3N loading increased, the CO selectivity also increased (Figure 5). The maximum CO selectivities were 47.8, 74.5, 81.4, and 85.7% for Ni loading of 1.86, 3.73, 5.50, and 7.05%, suggesting that the CO selectivity herein must involve Ni atoms. It is possible that single Ni atoms could contribute to catalysis. SA-Ni/C was prepared by acid removal of Ni3N particles from Ni3N/C, and the absence of Ni3N particles was confirmed by TEM and XRD (Figure S10a,b). SA-Ni/C showed slightly higher CO selectivity (80−90%) than that of Ni3N/C within the same applied potential, but the CO partial current density was ca 3-fold lower (3.5 mA/cm2, Figure S10c,d). This indicated that both Ni3N and SA-Ni could be

Ni/C, suggesting different local atomic arrangements in Ni3N/ C.45,48,51 The corresponding Fourier transform plot of Ni3N/C in Figure 3c presented two peaks in the range of 1−3 Å. The one at 1.89 Å was fit as Ni−N correlation and the other at 2.65 Å as Ni−Ni correlation.48 The coordination number of the first nearest N shell is close to 2 and that of the Ni−Ni pair is 12 in Ni3N/C (Table S1), consistent with the crystal structure of Ni3N. The Ni−Ni distance in Ni3N/C is longer than in Ni/C (2.49 Å) because of the change in the lattice upon N atom doping. The aqueous CO2RR activity of Ni3N/C was evaluated in CO2 saturated 0.5 M NaCl solution. The cathodic linear sweep voltammetry scans (LSVs) of Ni3N/C (Figure 4a showed that

Figure 4. Electrocatalysis using Ni3N/C, Ni/C, and NiO/C materials in CO2-saturated 0.5 M NaCl solution. (a) LSVs at 100 mV s−1 using N-doped carbon black (black), Ni3N/C (blue), Ni/C (red), and NiO/C (pink). (b) FEs for CO (blue), and H2 (red) with Ni3N/C. FEs for CO and H2 with Ni/C (c). FEs for CO and H2 with NiO/C (d). CO partial current densities of Ni3N/C and Ni/C (e). Tafel plot of Ni3N/C and Ni/C using CO partial current densities (f).

the catalytic onset potential (−0.51 V vs RHE) was earlier than that of Ni/C (−0.70 V), suggesting that the energetic barrier for CO2RR is decreased in Ni3N/C. By comparison, the onset potential of NiO/C was the most negative at −1.05 V, the least capable for catalysis. The LSV current densities of Ni3N/C were substantially higher than that of Ni/C and NiO/C, D

DOI: 10.1021/acsami.8b11942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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*COOH··· N + H+(aq) + e− → CO* + H 2O···N

(2)

CO* → CO(g) + *

(3)

Gas phase electrolysis was carried out in a flow cell loaded with MEA (Figure S11). Ni3N/C was loaded (0.355 mg cm−2) at the cathode side on an anion exchange membrane (AEM) and the oxygen-evolving catalyst at the anode was Ir nanoparticles. Figure S12 showed the flow diagram of the gas phase electrolysis process. The cathode flow channel was passed with 1 atm CO2 with 70% relative humidity at room temperature, and the anode was passed with DI water only. The half and overall reactions are summarized in eqs 4−7. Cathode: 3CO2 + H 2O + 2e− → CO + 2HCO3−

(4)

Anode: 2HCO3− → 1/2O2 + H 2O + 2CO2 + 2e−

(5)

Trans-membrane: HCO3−(cathode) → HCO3−(anode)

Overall

Figure 5. Product distribution by Ni3N/C catalysts of different loading: (a) 1.86; (b) 3.73; (c) 5.50; (d) 7.05%. CO (blue), and H2 (red), 0.5 M NaCl.

CO2 → CO + 1/2O2

0 ECO = 1.35 V 2

(7)

The polarization curve (Figure 6a) showed that the catalysis occurred at an applied cell voltage of 2.6 V. During constant voltage electrolysis at 2.8 V, the current density was stable throughout the process with CO FE > 90% (Figure 6d), indicating excellent catalytic stability and selectivity of the Ni3N/C catalyst. As the cell voltage increased from 2.8 to 3.4 V, CO partial current density increased from 23.3 to 37.9 mA cm−2 (Figure 6b), and the CO selectivity decreased from 92.5 to 71.7%. At 2.8 V, the cathode potentials in the cell was −0.79 V versus NHE (Figure 6c), more positive than in the aqueous phase (−1.12 V vs NHE). These results suggest that in the gas phase electrolysis, the activation energy required for the catalysis is reduced compared to that in the liquid phase, as CO2RR occurs directly at the interface between the gaseous CO2 and the catalyst surface without competition of water adsorption.34 CO2 temperature-programmed desorption (TPD) was used to investigate the base sites of the catalyst. As shown in Figure 7, at temperatures from 100 to 350 °C, the CO2-TPD profile of Ni3N/C displayed a significant desorption peak at 304 °C, assigned to strong base sites.55,56 The strong base site is probably due to doped N atoms. By comparison, Ni/C displays two weak desorption peaks at 109.2 and 339.8 °C, assigned to the weak and strong base sites, respectively. They are probably due to surface residual OH and O2− from

active sites for CO2RR, yet Ni3N particles contributed more to the reactivity as reflected by the higher current density. Comparison of various Ni CO2RR catalysts from the literature was listed in Table 1, and Ni3N/C is a quite potent catalyst with high current density and CO selectivity. The CO2RR mechanism was probed by the Tafel plot using CO partial current density in 0.5 M NaCl (Figure 4f). The Tafel slope of Ni3N/C was ca. 160 mV dec−1, close to 118 mV dec−1. It suggests that the rate-determining step is initial single electron transfer to CO2 to form a CO•− 2 radical intermediate. The Tafel slope of Ni/C was ca. 254 mV dec−1. The exchange current density j0 of 2.78 × 10−5 mA cm−2 for Ni3N/C is comparably higher than j0 of 1.54 × 10−5 mA cm−2 for Ni/C, suggesting that Ni3N/C has reduced energetic barrier in catalysis. We postulate that the doped nitrogen atoms can stabilize the CO•− 2 radical intermediate by hydrogen bonding, which can reduce the energy barrier. The COOH intermediate then accepted a proton and an electron, and formed adsorbed CO and H2O. Finally, CO was released from the active site. These steps are summarized in the mechanism in eqs 1−3 (* denotes an active site and dot represents hydrogen bonding)16,20,37,38 CO2 (g) + * + H+(aq) + e− → *COOH ···N

(6)

(1)

Table 1. Comparison of CO2RR Activity by Various Ni-Based Catalysts catalyst

electrolyte

applied potential (V vs RHE)

current density (mA cm−2)

Faradaic efficiency (%)

refs

Ni3N/C Ni3N/C SA-Ni/N−C Ni−N−graphene Ni−N−C Ni−N4−C NiSA-N−CNT

0.5 M NaCl anion exchange membrane 0.5 M KHCO3 0.1 M KHCO3 0.1 M KHCO3 0.5 M KHCO3 0.5 M KHCO3

−0.90 −0.79a −1.00 −0.70 −0.78 −0.81 −0.70

8.70 23.0 10.48 12.5b 1.85c 28.6 23.5

85.7 92.5 70.19 95 85 98b 91.3

this work this work 44 45 46 55 56

Value versus NHE. bDerived from the chart. cThe unit is A g−1.

a

E

DOI: 10.1021/acsami.8b11942 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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catalyst operated quite positively at ca. 1.7 V versus NHE, and the over-voltage can be reduced in the future with the improved water-oxidation catalyst.

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CONCLUSIONS In summary, efficient CO2 reduction to CO was achieved using Ni3N/C as a highly selective catalyst, due to increased surface base sites, and the CO Faradaic efficiency in the aqueous phase reached 85.7%. When using gas phase electrolysis, the CO efficiency increased to 92.5% with increased CO current density of 23.3 mA cm−2. By contrast, Ni/C and NiO/C were not capable CO2 reduction catalysts. The combination of the CO2 enrichment strategy and gas phase electrolysis is promising for increased performance in CO2 electrolysis and could be broadly applied to other CO2 reduction catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11942. Figure 6. Gas phase electrolysis in a flow cell. (a) LSV at 5 mV s−1. (b) CO (red) and H2 (blue) partial current densities. (c) FEs for CO (blue) and H2 (red) (numbers above the bars are measured cathode potentials vs NHE). (d) Total current density (black) vs time at 2.8 V, FE for CO (blue), and FE for H2 (red). In the anode channel was passed DI water at a flow rate of 40 sccm, and in the cathode the CO2 flow rate was 30 sccm at 70% humidity.

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XRD spectra for Ni3N/C, TEM and HR-TEM images for Ni/C and NiO/C, XPS spectra and CPEs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peng Kang: 0000-0002-6639-8299 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Projects for Fundamental Research and Development of China (2016YFB0600901) and Key Research Program of the Chinese Academy of Sciences (ZDRW-ZS-2016-3). The XAFS experiment was supported by BSRF.

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Figure 7. CO2-TPD of Ni3N/C, Ni/C, NiO/C, and N doped carbon.

spontaneous air oxidation, and therefore the intensity of each is much lower than Ni3N/C.57,58 NiO/C showed two adsorption peaks at 206.3 and 281.8 °C, assigned to medium and strong base sites, respectively, lower than that of Ni3N/C.57,58 The CO2-TPD profile for N-doped carbon showed no obvious desorption peaks. The TPD results indicate that the Ni3N/C catalyst surface has a relatively strong adsorption of CO2 with increased amounts of base sites, which allows gaseous phase reduction at high catalytic selectivity and efficiency.49 The CO2 adsorption specific areas of Ni3N/C, Ni/C, and NiO/C (Figure S2) were fit as 170.2, 128.8, and 101.5 m2/g, respectively, indicating that the Ni3N/C has the largest physical surface area which also contributes to enhanced catalysis. Gas phase electrolysis offers many advantages to conventional aqueous phase catalysis. Compared to previous reports, no coinage metal catalyst or ionic liquid was used in our study;29,34 and the liquid intermediate layer was eliminated, simplifying the cell design.29 However, the resistance of AEM is still very large due to conduction of bicarbonate anion, and a 0.4 V cross-membrane voltage was calculated. The anode

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