Highly Selective and Efficient Electroreduction of Carbon Dioxide to

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Highly Selective and Efficient Electroreduction of Carbon Dioxide to Carbon Monoxide with Phosphate Silver-Derived Coral-like Silver Jin Gao, Cheng Zhu, Mengmeng Zhu, Yijun Fu, Hui Huang,* Yang Liu,* and Zhenhui Kang* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, People’s Republic of China

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/26/19. For personal use only.

S Supporting Information *

ABSTRACT: Electrochemical reduction of carbon dioxide (CO2RR) to useful chemicals and fuels is one of the promising methods to reduce the accumulated greenhouse gas in the atmosphere and simultaneously satisfy sustainable energy demands. Metallic Ag has been reported in numerous studies for its excellent properties in CO2 reduction. However, most Ag catalysts require a large overpotential to realize high selectivity and the Faradaic efficiency (FE) is relatively low. Moreover, the synthetic methods of efficient Ag catalysts are usually complicated and time-consuming. In this work, a phosphate silver-derived silver (PD-Ag) electrocatalyst was fabricated by a quick facile electroreduction method, showing superior performance for the selective reduction of CO2 to CO. The maximum FE of PD-Ag could reach 97.3% with a potential of −0.7 V vs RHE, and a current density of 2.93 mA cm−2. It is demonstrated that a 19-fold increase of the electrochemically active surface area (ECSA) is obtained for PD-Ag compared to polycrystalline Ag (Ag foil). Furthermore, PD-Ag shows an excellent stability during a 10 h electrolysis with only 4% loss of the current. The enhancement of the electroactivity results from its coral-like surface, not only serving plenty of active sites, but also producing CO2− intermediate at a lower overpotential. KEYWORDS: CO2 reduction, Electrocatalyst, Phosphate silver-derived silver, CO

■

INTRODUCTION The increased emission of CO2 from the combustion of fossil fuels has risen to a dangerous level which causes great hazards to the environment like global warming and glacier melting.1,2 Therefore, decreasing the excess CO2 in the atmosphere has become a pressing issue in recent years. Numerous efforts such as thermo-, photo-, and electrochemical strategies are focused on the capture of CO2 or conversion to useful chemicals and fuels.2,3 Electrochemical conversion of CO2 through renewable energy sources (such as wind and solar energies) seems to be an attractive strategy due to its mild reaction conditions, controllable selectivity, and high energy efficiencies, showing great potential for practical industrial applications.4,5 Meanwhile, a host of challenges still remain to be solved for CO2 electroreduction. One of the challenges is the high overpotential to drive the reaction. The formation of CO2− intermediate is the main reason for the high overpotential, where a negative equilibrium potential of −1.9 V versus standard hydrogen electrode (SHE) is needed.6 The low © XXXX American Chemical Society

selectivity for diversiform products is another challenge as various kinds of carbon-containing products derived from different electronic pathways and the side reaction product H2 can decrease the selectivity.7,8 In order to mature the electroreduction technology, the most crucial essential is to develop an efficient catalyst for CO2 reduction in aqueous solution. Up to now, three transition metalsCu,7,9,10 Au,11,12 and Ag13−21have been extensively investigated for electrochemically reducing CO2 in aqueous solution. However, some shortcomings of Cu and Au materials still restrict the development of electrochemical reduction of carbon dioxide (CO2RR) technologies. For example, as reported by Hori et al.,9 Cu displays poor selectivity among its several products such as CO, CH4, HCOOH, C2H4, and C2H5OH, and up to now only a fraction of reports reported Received: November 7, 2018 Revised: January 4, 2019

A

DOI: 10.1021/acssuschemeng.8b05776 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering high selectivity for Cu-catalyzed CO2RR.7,22 Au shows high activity and selectivity, whereas its low abundance and high cost limit industrial applications. In contrast, Ag has become a promising candidate due to its relatively low cost and high selectivity toward CO, a gas phase raw material for the Fischer−Tropsch process.23,24 Except for the good selectivity, however, the stability and activity of Ag catalysts still exist as challenges. One of the most important problems reported for Ag catalysts is high overpotential required to efficiently initiate the electroreduction of CO2. Many efforts have focused on the development of high-efficiency silver-based electrocatalysts for CO2 reduction. For instance, Lu et al. synthesized a nanoporous silver electrocatalyst which could reduce CO2 to CO by the dealloying of an Ag−Al precursor at a low overpotential (0.49 V vs RHE) with its relative Faradaic efficiency (FE) of 92%.13 Zhang et al. reported an iodidederived nanostructured silver catalyst (ID-Ag) that is able to electrochemically reduce CO2 to CO with approximately 94.5% selectivity at the potential of −0.7 V vs RHE.17 Ma et al. stated that oxide-derived Ag showed fine performance in CO2RR in which the FE could achieve 80% at low overpotential of 0.49 V (vs RHE).14 Although numerous studies have been carried out to boost the CO2RR catalytic activity of Ag-based catalysts, as of yet, all previously reported Ag catalysts including nanoporous,13 nanocoral,15,17,21 nanoparticle,18 and nanowire16,19 structures point to a conclusion that the morphology of Ag is determinative toward CO2 reduction activity. In general, the morphology of the catalyst can directly affect the activity of the catalyst with respect to the size of the catalyst and the number of surface sites available at the catalyst surface.25,26 However, current research involving the morphological engineering still suffers from some limitations, such as complex synthetic methods and difficult special morphology control. Hence, it is extremely significant to develop a facile strategy to control the precise catalyst morphology for gaining competent Ag catalysts with high efficiency, high current density, low overpotential, and excellent stability to meet the requirement of practical applications. Herein, we first propose the phosphate silver-derived Ag catalyst (PD-Ag) by a low-cost and simple electrochemical method. The PD-Ag with a coral-like surface can selectively reduce CO2 to CO, affording a current density of 2.93 mA/ cm2 with a maximum FE of 97.3% at an operating potential of −0.7 V (vs RHE). Also, the FE could reach 60.3% at a low overpotential of 0.39 V. The underpotential deposition (UPD) experiments indicate that the electrochemically active surface area (ECSA) of PD-Ag is 19-fold enhanced compared with Ag foil. Furthermore, the PD-Ag shows a stable current density and a constant CO FE (97%) in 0.5 M KHCO3 electrolyte during the 10 h electrolysis. Most remarkably, PD-Ag exhibits enhanced catalytic activity because of its nanocoral-like surface, which serves plenty of active sites. Electrokinetic studies indicate that the lower overpotential originates from the increased stabilization of the CO2− intermediate generated on the nanocoral-like surface of the PD-Ag.

■

(KHCO3, 99.5%), potassium chloride (KCl, 99.5%), and ethanol (EtOH, 99.8%) were used in this study without further purification. Carbon dioxide (99.999%) and nitrogen (99.999%) were purchased from Airgas, and Nafion perfluorinated resin solution (5 wt %) was purchased from Adamas-beta; Nafion212 membrane and Toray Carbon Paper (TGP-H-60) were purchased from Alfa Aesar. Deionized water (purified by a Milli-Q system) was used to prepare all solutions and to rinse samples and glassware. Characterization. The surface morphologies of Ag3PO4, PD-Ag, Ag nanoparticles, and Ag foil were obtained by an FEI Quanta FEG 200 scanning electron microscope with an acceleration voltage of 10 kV. The crystal structure was performed by X-ray diffraction (XRD) using an X’Pert-ProMPD (Holand) D/max-γAX-ray diffractometer with Cu Kα radiation (λ = 0.154 178 nm). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images with an FEI-Tecnai F20 (200 kV) were acquired to analyze lattice spacing. Energy dispersive X-ray (EDX) element mapping was also performed to confirm the element composition of catalysts. To further confirm the element composition and the valence state, X-ray photoelectron spectroscopy (XPS) was obtained by using a KRATOS Axis ultraDLD X-ray photoelectron spectrometer with a monochromatized Mg Kα X-ray source (hν = 1283.3 eV). The CO2 adsorption was determined by plotting the adsorption isotherm of CO2 at 25 °C obtained using a Micromeritics ASAP 2050 instrument. The electrochemical measurements were performed by a Model CHI 760C workstation (CH Instruments, Chenhua, Shanghai, China). The obtained gas phase composition and liquid products were detected by a gas chromatograph (PERSEE, G5) with a flame ionization detector (FID) and 1H nuclear magnetic resonance spectroscopy (Bruker AVANCEAV III 400) respectively. Synthesis of Ag3PO4 Cubic Nanocrystals. Ag3PO4 cubic nanocrystals were synthesized by the same strategy as reported before.27 First, 26.8 mL of deionized water and 0.3 mL of NH4NO3 (1.2 M) solution were added to a 100 mL beaker. Next, 0.54 mL of 0.6 M NaOH solution and 1.2 mL of 0.15 M AgNO3 solution were also added to the beaker. The mixed solution was stirred vigorously for 10 min to form the [Ag(NH3)2]+ complex, which can slow down the precipitation reaction to yield crystals with morphology control. Eventually, 1.2 mL of 0.3 M K2HPO4 solution was added to grow Ag3PO4 cubes. The product was collected by centrifugation (5 min, 8000 rpm) and washed with distilled H2O twice, then with ethanol once, and dried under a vacuum oven. Synthesis of the Phosphate-Derived Ag Electrocatalysts. The PD-Ag electrocatalyst was fabricated by simple electrochemical reduction of silver phosphate (Ag3PO4) through chronoamperometry at the potential of −1.2 V vs RHE for 15 min in 0.5 M KHCO3 electrolyte. Electrochemical Measurements. All of the electrochemical experiments were performed using a standard three-electrode configuration. A platinum wire and a saturated calomel electrode (SCE) were used as the auxiliary electrode and reference electrode, respectively. The working electrode was either a catalyst modified glassy carbon disk electrode (GCE; 3.0 mm diameter, CH Instruments), or a catalyst modified carbon fiber paper electrode (1 cm × 2 cm), in which the catalyst layer was prepared by 3 mg of the sample dispersing in a mixture of 500 μL of Nafion ionomer solution (0.5 wt %) with ultrasonic stirring to form a homogeneous ink. Among them, a glassy carbon (GC) disk electrode was modified by 6 μL of ink. A carbon fiber paper electrode was modified by 400 μL of ink with 200 μL of ink in each side. Linear sweep voltammetry curves were performed with a catalyst modified glassy carbon disk electrode as the working electrode in 0.5 M KHCO3 solution which is saturated with N2 and CO2 respectively. Initially, polarization curves for the modified electrode were carried out under an inert N2 (gas) atmosphere. After that, the solution was purged with CO 2 (99.999%) for at least 30 min (CO2-saturated high purity aqueous 0.5 M KHCO3) and the electrocatalytic CO2 reduction was measured. Furthermore, for product analysis, the constant-potential electrolysis experiment was performed in an airtight electrochemical H-type cell with a catalyst modified carbon fiber electrode which possesses the

EXPERIMENTAL SECTION

Materials. Silver foil (0.025 mm thick, 99.998%), Ag nanoparticles (60−120 nm, 99.5%), ammonium nitrate (NH4NO3, 99.0%), silver nitrate (AgNO3, 99.8%), potassium hydrogen phosphate (K2HPO4, 99.0%), sodium hydroxide (NaOH, 96.0%), potassium bicarbonate B


Research Article

ACS Sustainable Chemistry & Engineering advantages of high catalyst loading and excellent uniformity of catalyst dispersion. There is 30 mL of electrolyte (0.5 M KHCO3 saturated with CO2) in each of the two compartments (each part volume is 50 mL), along with 20 mL headspace above the electrolyte. The working and counter electrodes were separated by a slice of proton exchange membrane (Nafion212 membrane). In addition, the stability test was measured in an open electrolytic cell with 0.5 M KHCO3 solution (saturated with CO2) for 10 h. All electrochemical data was collected vs SCE reference and converted to the reversible hydrogen electrode (RHE) by eq1. The pH here of the electrolyte after CO2 saturation was measured to be 7.2.

VRHE = VSCE + 0.2412 + 0.059pH

(1)

Product Analysis. The electrolysis experiment was further run chronoamperometrically in an airtight electrochemical H-type cell with 30 mL of 0.5 M KHCO3 electrolyte in each chamber. The electrolysis process was conducted at potentials of −0.3, −0.4, −0.5, −0.6, −0.7, −0.8, −0.9, −1.0, −1.1, and −1.2 V vs RHE respectively at room temperature and ambient pressure. The carbon-containing gas products such as CO, CH4, C2H4, and C2H6 from the working electrode compartment were analyzed by gas chromatography (GC; PERSEE G5) equipped with a TDX-1 chromatographic column and a methane converter as well as a flame ionization detector (FID) (nitrogen as the carrier gas). A thermal conductivity detector (TCD) and a molecular sieve 5 Å packed column were utilized to detect the byproduct hydrogen with nitrogen as the carrier gas. The GC was calibrated using standard gases. In addition, the liquid products were collected from the cathode chambers after electrolysis and qualified by NMR spectroscopy (Bruker AVANCEAV III 400), in which 0.5 mL of electrolyte was mixed with 0.1 mL of D2O (deuterated water) and 0.05 μL of dimethyl sulfoxide (DMSO; Sigma, 99.99%) was added as an internal standard. The Faradaic efficiencies of CO and H2 were calculated according to the following equations: for CO:

FECO =

znCOF × 100% Q

(2)

for H2:

FE H2 =

zn H2F Q

× 100%

Figure 1. SEM images of (a) synthesized Ag3PO4 with cube shape and (b) PD-Ag with coral-like surface. (c) TEM image and HRTEM image (inset) of PD-Ag. (d1−d4) HAADF-STEM image of PD-Ag and corresponding elemental mapping. (e1−e6) HAADF-STEM image and corresponding elemental mappings of Ag3PO4.

(3)

where z is the number of electrons exchanged; for example, z = 2 for reduction of CO2 to CO. F is the Faraday constant (F = 96485 C/ mol), nCO is the number of moles for produced CO, and nH2 is the number of moles for the produced H2, which were measured by GC. Q is the total charge passed. Electrochemically Active Surface Area (ECSA) Measurement. The electrochemically active surface area (ECSA) of PD-Ag, Ag nanoparticles, and pristine Ag-foil electrodes was estimated by cyclic voltammetry (CV) according to the procedure reported previously.18,28,29 The experiment was performed without removal of oxygen in a 50 mL beaker containing 5 mM Pb(NO3)2, 10 mM HNO3, and 10 mM KCl. The cyclic voltammogram was obtained at 10 mV/s between −0.1 and −0.5 V vs SCE. The ECSA was calculated by dividing the area of the peak from submonolayer Pb deposition during UPD by the scan rate and then multiplying by the constant for Pb deposited on Ag. The corresponding constant is 1.67 × 10−3 cm2/ μC for Pb deposited on Ag.

surface and a mean size of approximately 400 nm in Figure 1a. It is obvious that the surface of PD-Ag becomes very rough, which is like coral after the reduction of Ag3PO4 as Figure 1b displays. Moreover, the TEM image (Figure 1c) of PD-Ag clearly shows their coral structure with protrusions, while the HRTEM image (the inset of Figure 1c) exhibits the d-spacing of 0.24 nm consistent with the (111) lattice plane of Ag. Furthermore, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and corresponding elemental mappings of PD-Ag and Ag3PO4 are given in Figure 1d,e, from which the chemical elements of PDAg and Ag3PO4 are determined to be Ag and Ag, P, and O, respectively. X-ray diffraction (XRD) patterns of the as-prepared samples (Ag foil, Ag3PO4, and PD-Ag) are given in Figure 2a. The Ag3PO4 XRD pattern shows the same peaks with the standard pattern of Ag3PO4 (JCPDS No. 6-505) as Figure S1 presents. It is noted that the PD-Ag exhibits a similar pattern as the Ag foil which shows 2θ peaks at 38.1, 44.3, 64.4, 77.5, and 81.5° matched well with the (111), (200), (220), (311), and (222)

■

RESULTS AND DISCUSSION Characterization of Ag3PO4-Derived Ag (PD-Ag). PDAg was fabricated from Ag3PO4 by rapid electrochemical reduction. Parts a and b of Figure 1 show the morphologies of the precursor Ag3PO4 and PD-Ag, respectively, which were characterized by scanning electron microscopy (SEM). Ag3PO4 presents a nanocube shape with a relatively smooth C


Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. (a) XRD patterns of Ag foil (black trace), Ag3PO4 (red trace), PD-Ag (blue trace), and the standard pattern of crystal Ag (pink trace). (b) High-resolution XPS of Ag 3d peaks for Ag foil (black trace), Ag3PO4 (red trace), and PD-Ag (blue trace).

Figure 3. (a) LSVs for PD-Ag in N2-saturated (black trace) and CO2-saturated (red trace) 0.5 M KHCO3 electrolyte with a scan rate of 20 mV/s. (b) LSVs for Ag foil (black trace), Ag nanoparticles (blue trace), and PD-Ag (red trace) in CO2-saturated 0.5 M KHCO3 electrolyte with a scan rate of 20 mV/s. (c) CV curves of underpotential deposition (UPD) and bulk deposition of Pb onto Ag foil, Ag nanoparticles, and PD-Ag working electrodes, respectively. (d) Pb-UPD experiments operated at selected time points during the 10 h electrolysis.

elements of Ag3PO4, the spectra of O 1s and P 2p are presented in Figure S3c,d. Notably, for PD-Ag, the Ag 3d peaks of PD-Ag were at the same positions as those of Ag foil, implying that only metallic Ag was detected on the surface. Under the detection limit of the characterization techniques, the obtained results from XPS analysis were consistent with the results of the XRD patterns, further implying the complete electroreduction of Ag3PO4 to metallic Ag. Electrocatalytic Performance. In typical experiments, the electrocatalytic performances of untreated Ag foil, Ag nanoparticles, and PD-Ag were evaluated in a standard threeelectrode system employing electrolyte of high purity aqueous 0.5 M KHCO3. Figure 3a presents the linear sweep voltammetry (LSV) which is performed in 0.5 M KHCO3 saturated with CO2 and N2, respectively, for PD-Ag. It can be seen that the LSV curve measured in N2-saturated 0.5 M

crystal planes of Ag (JCPDS No. 4-783), indicating that the Ag3PO4 precursor is completely reduced to Ag0. To further confirm the surface elemental compositions of the samples, X-ray photoelectron spectroscopy (XPS) measurements were performed. Full XPS scan spectra of PDAg (Figure S2) and the precursor Ag3PO4 (Figure S3a) indicate that the main element is Ag for PD-Ag while the main elements are Ag, P, O for Ag3PO4. Trace C element found in the samples may come from the XPS equipment itself.30,31 As discerned from the XPS Ag 3d spectrum (Figure 2b), the Ag 3d3/2 peak at 374.4 eV and Ag 3d5/2 peak at 368.4 eV are observed for Ag foil. The two individual peaks of the synthesized Ag3PO4 are located at 373.8 and 367.8 eV attributed to Ag 3d3/2 and Ag 3d5/2 respectively (Figure S3b), corresponding to Ag3PO4 according to the Ag 3d peak analysis for Ag3PO4 in previous works.32−34 For the rest of the D


Research Article


Figure 4. (a) Total current density at various potentials on Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (b) Current density for CO at various potentials on Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (c) CO Faradaic efficiencies versus applied potentials catalyzed by Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (d) Stability performance of PDAg for CO2 reduction operated at potentiostatic potential of −0.7 V vs RHE for 10 h. Total current density vs time (left axis) and FE for CO vs time (right axis).

KHCO3 shows an onset potential of −0.5 V (vs RHE), which can be ascribed to the hydrogen evolution reaction (HER) with a relatively high overpotential of 0.5 V, whereas the LSV curve measured in CO2-saturated 0.5 M KHCO3 shows an onset potential of −0.4 V and an obvious increase of current density (from 0.80 to 5.35 mA/cm2 at −0.8 V, normalized by the geometrical surface area). The significant enhancement of current density not only results from the improvement of HER caused by the increased H+ density but also by the occurred CO2RR. Additionally, LSVs for Ag foil and Ag nanoparticles in N2-saturated and CO2-saturated 0.5 M KHCO3 electrolyte are shown in Figure S4. Comparing PD-Ag with Ag foil and Ag nanoparticles (Figure 3b), the electrocatalytic activity for PDAg is much better than those of Ag foil and Ag nanoparticles with respect to the lower overpotential of PD-Ag. The lower overpotential and the higher current density on PD-Ag in CO2RR may stem from the coral-like structure that has increased specific surface area and active sites compared to others as is illustrated in Figure S5. Furthermore, the electrochemical surface area (ECSA) of the catalysts was measured by Pb underpotential deposition (UPD) method to confirm the high activity of PD-Ag. As presented in Figure 3c and Table S1, the calculated specific active surface area of PDAg is 43.21 cm2, exhibiting a 19-fold enhancement and a 3-fold enhancement compared with Ag foil and Ag nanoparticles, respectively. The boosted ECSA of PD-Ag is responsible for the enhanced current densities compared to those of Ag foil and Ag nanoparticles during electrolysis. Moreover, the ECSA data of PD-Ag during the electrolysis at different time points are displayed in Figure 3d. It is distinct that the ECSA of PDAg remains almost the same after long time electrocatalysis, meaning a high stability of the active sites. Besides, controlled potential electrolysis of CO2 was tested in a gastight two-

compartment cell with a magnetic stirrer bar for investigating the current densities and FEs of the products. After the shorttime electrolysis at different potentials, only CO and H2 were observed during the overall electrolysis. The gas phase composition was analyzed by gas chromatography, and the liquid phase products were detected by 1H nuclear magnetic resonance spectroscopy (NMR) with DMSO as an internal standard. The total current density and the CO partial current density (normalized to the geometric electrode area) are presented in Figure 4a,b. It can be seen no matter the total current density or CO partial current density, the current density of PD-Ag is higher than those of Ag foil and Ag nanoparticles. Moreover, the CO FE for PD-Ag is higher than those for Ag foil and Ag nanoparticles through all the potentials as displayed in Figure 4c. The CO FEs were measured for PD-Ag, Ag foil, and Ag nanoparticles across the potential ranging from −0.3 to −1.2 V (vs RHE). The onset potential of −0.3 V (vs RHE) corresponds to a low overpotential of 0.19 V with a CO FE of 8.0% for PD-Ag. In other words, CO2RR began to occur since −0.3 V (vs RHE) with the overpotential of 0.19 V. Note PD-Ag displays its maximum CO FE of 97.3% at the potential of −0.7 V vs RHE and slightly decreased CO FEs of 91.1 and 71.8% at the potentials of −0.8 V vs RHE and −0.6 V vs RHE, accordingly. When the applied potential is more negative, the CO FE moderately decreases to 63.1% at −1.2 V vs RHE. In stark contrast, the Ag foil presents a very low CO FE (2.3− 62.7%) and high H2 FE (40.2−95.0%) from −0.6 to −1.2 V (vs RHE) with low current density. Ag nanoparticles show a relatively low CO FE (6−81.7%) and high H2 FE (11.9− 80.2%) from −0.4 to −1.2 V (vs RHE) with low current density. The specific CO and H2 current densities versus the applied potentials are extracted from the potentiostatic E


Research Article


Figure 5. (a) Tafel plots (overpotential versus CO partial current density) on PD-Ag, Ag foil, and Ag nanoparticles. (b) Influence of bicarbonate concentration on CO partial current density at constant potential of −0.7 V vs RHE for PD-Ag.

overpotential with a transfer coefficient of 0.74, meaning that the reaction kinetics on the PD-Ag is faster than that on Ag foil and Ag nanoparticles. However, the Tafel slope of the PD-Ag at higher overpotential could reach 254 mV/dec. This may be caused by the mass transport limitation on the coral-like nanostructured Ag.35,36 Based on the Tafel plot, the mechanism insight into two-electron reduction of CO2 to CO on the Ag surface can be summarized in the following steps. In the first step, a single electron is transferred to adsorbed CO2 and “CO2−” intermediate is formed:

electrolysis data and provided in Figure S6, where the different activities and the selectivities of PD-Ag, Ag foil, and Ag nanoparticles may come from the mass-transfer effect induced by their different surface morphologies (Figure 1 and Figure S5).20 To further examine the CO2RR activity and stability, the i−t curve of PD-Ag was tested at −0.7 V (vs RHE) for 10 h as shown in Figure 4d. A current density of ∼3.0 mA/cm2 was obtained on the PD-Ag throughout the test, suggesting a longterm stability. The CO FE is maintained at about 97% throughout the entire process of the 4 h electrolysis, during which there is almost no loss. Although the CO FE on PD-Ag decreased to 92.7% after the 10 h electrolysis, the loss is only 4%. As a comparison, Ag foil manifests worse stability than PDAg concerning the current density and the Faradaic efficiency (Figure S7), suggesting that PD-Ag is more stable for CO2RR than Ag foil. The XRD patterns and XPS spectra gained at different time points during the electrolysis can prove the stability of PD-Ag as well (Figure S8a,b). Moreover, the SEM image of PD-Ag after 10 h of testing in Figure S8c indicates that the morphology of PD-Ag after the stability test is almost the same as the morphology before the stability test, which further demonstrates the stability. Apart from the current density and physical structure, the durability was also confirmed by ECSA of PD-Ag during the electrolysis at six different time points. It is obvious that the obtained ECSA of PD-Ag after different times of electrolysis almost maintained the same values (Figure 3d). All the results above have clearly verified that PD-Ag is a highly efficient and stable electrocatalyst. Mechanism Study. To obtain insights into the electrokinetic mechanism for CO2 reduction using the catalysts, Tafel plots (a plot of overpotentials versus log of the partial current density for CO production) for PD-Ag, Ag foil, and Ag nanoparticles were tested and are displayed in Figure 5a, where the slope and the transfer coefficient are consistent with a ratedetermining initial electron transfer to CO2. The plot of the Ag foil is linear over the range of overpotentials from 0.49 to 0.79 V with a slope of 129 mV/dec, corresponding to a transfer coefficient of 0.46 (Supporting Information, eq S1). As for Ag nanoparticles, the plot of the Ag nanoparticles is linear over the range of overpotentials from 0.29 to 0.44 V with a slope of 93 mV/dec, corresponding to a transfer coefficient of 0.64. In contrast, the plot of the PD-Ag is linear over the range of overpotentials from 0.19 to 0.34 V. It is noteworthy that the Tafel slope of the PD-Ag was only 80 mV/dec at low

CO2 + e− → CO2,ads•−

(4)

In subsequent steps, the “CO2−” intermediate takes two protons and another one electron to form a CO molecule and a H2O molecule. CO2,ads•− + HCO3− → COOHads + CO32 −

(5)

COOHads + HCO3− + e− → COads + H 2O + CO32 − (6)

COads → CO

(7)

It was believed that the standard reduction potential for the first step is much more negative (E0 = −1.9 V vs SHE) than the following steps.37,38 The Tafel study has suggested that the reaction kinetics on the coral-like PD-Ag surface is faster than that on a flat Ag foil surface. In order to confirm this mechanism, constant-potential electrolysis was performed in different HCO3− solutions with concentrations from 0.5 to 0.1 M. As is shown in Figure 5b, the CO partial current density shows zero-order dependence on CO2 concentration, implying the donation of a proton from HCO3− is not a ratedetermining step for CO2 reduction on Ag surfaces. The proton from H2O is not considered because its pKa is much higher than that of HCO3−. Therefore, the first step is the ratedetermining step for the whole process, which is consistent with the most classic mechanism proposed by Hori.39 A possible reason for the excellent properties is that the coral-like Ag surface with abundant active sites might stabilize the CO2− intermediate to improve intrinsic activity.

■

CONCLUSIONS We design an efficient nanocoral-like PD-Ag catalyst by a lowcost and relatively simple electrochemical method for electrochemical CO2 reduction. The major product CO on the PDF


Research Article


(6) Lu, Q.; Jiao, F. Electrochemical CO2 Reduction: Electrocatalyst, Reaction Mechanism, and Process Engineering. Nano Energy 2016, 29, 439−456. (7) Guo, S. J.; Zhao, S. Q.; Gao, J.; Zhu, C.; Wu, X. Q.; Fu, Y. J.; Huang, H.; Liu, Y.; Kang, Z. H. Cu-CDots Nanocorals as Electrocatalyst for Highly Efficient CO2 Reduction to Formate. Nanoscale 2017, 9 (1), 298−304. (8) Jhong, H.-R. M.; Ma, S.; Kenis, P. J. A. Electrochemical Conversion of CO2 to Useful Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2 (2), 191−199. (9) Hori, Y.; Murata, A.; Takahashi, R. Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution. J. Chem. Soc., Faraday Trans. 1 1989, 85 (8), 2309−2326. (10) Nie, X. W.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. 2013, 125 (9), 2519−2522. (11) Chen, Y. H.; Li, C. W.; Kanan, M. W. Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles. J. Am. Chem. Soc. 2012, 134 (49), 19969−19972. (12) Zhu, W. L.; Michalsky, R.; Metin, Ö .; Lv, H.; Guo, S. J.; Wright, C. J.; Sun, X. L.; Peterson, A. A.; Sun, S. H. Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO2 to CO. J. Am. Chem. Soc. 2013, 135 (45), 16833−16836. (13) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. A Selective and Efficient Electrocatalyst for Carbon Dioxide Reduction. Nat. Commun. 2014, 5 (1), 3242−3247. (14) Ma, M.; Trześniewski, B. J.; Xie, J.; Smith, W. A. Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide-Derived Nanostructured Silver Electrocatalysts. Angew. Chem., Int. Ed. 2016, 55 (33), 9748−9752. (15) Peng, X.; Karakalos, S. G.; Mustain, W. E. Preferentially Oriented Ag Nanocrystals with Extremely High Activity and Faradaic Efficiency for CO2 Electrochemical Reduction to CO. ACS Appl. Mater. Interfaces 2018, 10 (2), 1734−1742. (16) Qiu, W. B.; Liang, R. P.; Luo, Y. L.; Cui, G. W.; Qiu, J. D.; Sun, X. P. A Br− Anion Adsorbed Porous Ag Nanowire Film: In Situ Electrochemical Preparation and Application toward Efficient CO2 Electroreduction to CO with High Selectivity. Inorg. Chem. Front. 2018, 5 (9), 2238−2241. (17) Zhang, Y.; Ji, L.; Qiu, W. B.; Shi, X. F.; Asiri, A. M.; Sun, X. P. Iodide-Derived Nanostructured Silver Promotes Selective and Efficient Carbon Dioxide Conversion into Carbon Monoxide. Chem. Commun. 2018, 54 (21), 2666−2669. (18) Ma, S. C.; Lan, Y. C.; Perez, G. M. J.; Moniri, S.; Kenis, P. J. A. Silver Supported on Titania as an Active Catalyst for Electrochemical Carbon Dioxide Reduction. ChemSusChem 2014, 7 (3), 866−874. (19) Luan, C. H.; Shao, Y.; Lu, Q.; Gao, S. H.; Huang, K.; Wu, H.; Yao, K. High-Performance Carbon Dioxide Electrocatalytic Reduction by Easily Fabricated Large-Scale Silver Nanowire Arrays. ACS Appl. Mater. Interfaces 2018, 10 (21), 17950−17956. (20) Jee, M. S.; Jeon, H. S.; Kim, C.; Lee, H.; Koh, J. H.; Cho, J.; Min, B. K.; Hwang, Y. J. Enhancement in Carbon Dioxide Activity and Stability on Nanostructured Silver Electrode and the Role of Oxygen. Appl. Catal., B 2016, 180, 372−378. (21) Hsieh, Y.-C.; Betancourt, L. E.; Senanayake, S. D.; Hu, E.; Zhang, Y.; Xu, W.; Polyansky, D. E. Modification of CO2 Reduction Activity of Nanostructured Silver Electrocatalysts by Surface Halide Anions. ACS Appl. Energy Mater. 2018, DOI: 10.1021/acsaem.8b01692. (22) Manthiram, K.; Beberwyck, B. J.; Alivisatos, A. P. Enhanced Electrochemical Methanation of Carbon Dioxide with a Dispersible Nanoscale Copper Catalyst. J. Am. Chem. Soc. 2014, 136 (38), 13319−13325. (23) Guo, S. J.; Zhao, S. Q.; Wu, X. Q.; Li, H.; Zhou, Y. J.; Zhu, C.; Yang, N. Y.; Jiang, X.; Gao, J.; Bai, L.; et al. A Co3O4-CDots-C3N4 Three Component Electrocatalyst Design Concept for Efficient and Tunable CO2 Reduction to Syngas. Nat. Commun. 2017, 8 (1), 1828.

Ag catalyst could achieve a high FE of 97% at a moderate potential of −0.7 V with a current density of 2.93 mA/cm2. Furthermore, the PD-Ag catalyst’s stability is good as it can serve a working lifetime without obvious deactivation for at least 10 h. The excellent performance is due to the increased active sites on the surface of PD-Ag, which can improve the CO2 adsorption. This assumption of the reaction mechanism is supported by ECSA measurements and Tafel plot study, which demonstrate the increased electrochemical active surface area and the promoted reaction rate of the intermediates, respectively.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05776.

■

Additional material characterizations including XRD patterns, XPS spectra, electrochemical measurements, and SEM images; electrochemically active surface areas (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.H.). *E-mail: [email protected] (Y.L.). *E-mail: [email protected] (Z.K.). ORCID

Hui Huang: 0000-0002-9053-9426 Zhenhui Kang: 0000-0001-6989-5840 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National MCF Energy R&D Program (2018YFE0306105), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project, the National Natural Science Foundation of China (51725204, 51572179, 21471106, 21771132), and the Natural Science Foundation of Jiangsu Province (BK20161216). We also thank BSRF for the support of XAS experiments.

■

REFERENCES

(1) Friedlingstein, P.; Solomon, S.; Plattner, G.-K.; Knutti, R.; Ciais, P.; Raupach, M. R. Long-Term Climate Implications of Twenty-First Century Options for Carbon Dioxide Emission Mitigation. Nat. Clim. Change 2011, 1 (9), 457−461. (2) Lackner, K. S. A Guide to CO2 Sequestration. Science 2003, 300, 1677−1678. (3) Stanbury, M.; Compain, J.-D.; Chardon-Noblat, S. Electro and Photoreduction of CO2 Driven by Manganese-Carbonyl Molecular Catalysts. Coord. Chem. Rev. 2018, 361, 120−137. (4) Li, F. W.; Chen, L.; Knowles, G. P.; MacFarlane, D. R.; Zhang, J. Hierarchical Mesoporous SnO2 Nanosheets on Carbon Cloth: A Robust and Flexible Electrocatalyst for CO2 Reduction with High Efficiency and Selectivity. Angew. Chem., Int. Ed. 2017, 56 (2), 505− 509. (5) Jones, J.-P.; Prakash, G. K. S.; Olah, G. A. Electrochemical CO2 Reduction: Recent Advances and Current Trends. Isr. J. Chem. 2014, 54 (10), 1451−1466. G


Research Article

ACS Sustainable Chemistry & Engineering (24) de Klerk, A. Fischer−Tropsch Fuels Refinery Design. Energy Environ. Sci. 2011, 4 (4), 1177−1205. (25) Liao, F. L.; Huang, Y. Q.; Ge, J. W.; Zheng, W. R.; Tedsree, K.; Collier, P.; Hong, X. L.; Tsang, S. C. Morphology-Dependent Interactions of ZnO with Cu Nanoparticles at the Materials’ Interface in Selective Hydrogenation of CO2 to CH3OH. Angew. Chem., Int. Ed. 2011, 50 (9), 2162−2165. (26) Narayanan, R.; El-Sayed, M. A. Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability. J. Phys. Chem. B 2005, 109 (26), 12663−12676. (27) Hsieh, M.-S.; Su, H.-J.; Hsieh, P.-L.; Chiang, Y.-W.; Huang, M. H. Synthesis of Ag3PO4 Crystals with Tunable Shapes for FacetDependent Optical Property, Photocatalytic Activity, and Electrical Conductivity Examinations. ACS Appl. Mater. Interfaces 2017, 9 (44), 39086−39093. (28) Kim, C.; Jeon, H. S.; Eom, T.; Jee, M. S.; Kim, H.; Friend, C. M.; Min, B. K.; Hwang, Y. J. Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles. J. Am. Chem. Soc. 2015, 137 (43), 13844−13850. (29) Salehi-Khojin, A.; Jhong, H.-R. M.; Rosen, B. A.; Zhu, W.; Ma, S.; Kenis, P. J. A.; Masel, R. I. Nanoparticle Silver Catalysts That Show Enhanced Activity for Carbon Dioxide Electrolysis. J. Phys. Chem. C 2013, 117 (4), 1627−1632. (30) Niu, P.; Zhang, L. L.; Liu, G.; Cheng, H.-M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Adv. Funct. Mater. 2012, 22 (22), 4763−4770. (31) Du, X. R.; Zou, G. J.; Wang, Z. H.; Wang, X. L. A Scalable Chemical Route to Soluble Acidified Graphitic Carbon Nitride: An Ideal Precursor for Isolated Ultrathin g-C3N4 Nanosheets. Nanoscale 2015, 7 (19), 8701−8706. (32) Zhang, W. P.; Li, G. Y.; Wang, W. J.; Qin, Y. X.; An, T. C.; Xiao, X. Y.; Choi, W. Enhanced Photocatalytic Mechanism of Ag3PO4 Nano-Sheets Using MS2 (M = Mo, W)/RGO Hybrids as CoCatalysts for 4-Nitrophenol Degradation in Water. Appl. Catal., B 2018, 232, 11−18. (33) Yang, X. F.; Cui, H. Y.; Li, Y.; Qin, J. L.; Zhang, R. X.; Tang, H. Fabrication of Ag3PO4-Graphene Composites with Highly Efficient and Stable Visible Light Photocatalytic Performance. ACS Catal. 2013, 3 (3), 363−369. (34) Guan, X.; Guo, L. Cocatalytic Effect of SrTiO3 on Ag3PO4 toward Enhanced Photocatalytic Water Oxidation. ACS Catal. 2014, 4 (9), 3020−3026. (35) Rosen, J.; Hutchings, G. S.; Lu, Q.; Rivera, S.; Zhou, Y.; Vlachos, D. G.; Jiao, F. Mechanistic Insights into the Electrochemical Reduction of CO2 to CO on Nanostructured Ag Surfaces. ACS Catal. 2015, 5 (7), 4293−4299. (36) Hatsukade, T.; Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Insights into the Electrocatalytic Reduction of CO2 on Metallic Silver Surfaces. Phys. Chem. Chem. Phys. 2014, 16 (27), 13814−13819. (37) Costentin, C.; Robert, M.; Savéant, J.-M. Catalysis of the Electrochemical Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42 (6), 2423−2436. (38) Schwarz, H. A.; Dodson, R. W. Reduction Potentials of CO2− and the Alcohol Radicals. J. Phys. Chem. 1989, 93 (1), 409−414. (39) Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry; Vayenas, C., White, R., GamboaAldeco, M., Eds.; Springer: New York, 2008; Vol. 42, pp 89−189. DOI: 10.1007/978-0-387-49489-0_3.

H


Highly Selective and Efficient Electroreduction of Carbon Dioxide to

Recommend Documents