Electrodeposited AgCu foam catalysts for enhanced reduction of CO2

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Energy, Environmental, and Catalysis Applications 2

Electrodeposited AgCu foam catalysts for enhanced reduction of CO to CO Tintula Kottakkat, Katharina Klingan, Shan Jiang, Zarko P Jovanov, Veronica H Davies, Gumaa A El-Nagar, Holger Dau, and Christina Roth ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22071 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Applied Materials & Interfaces

Electrodeposited AgCu foam catalysts for enhanced reduction of CO2 to CO Tintula Kottakkat*[a], Katharina Klingan[b], Shan Jiang[b], Zarko P. Jovanov[c], Veronica H. Davies[a], Gumaa A. M. El-Nagar[a], Holger Dau [b], Christina Roth*[a] aInstitute

of Chemistry and Biochemistry, Freie Universität Berlin,Takustr. 3, 14195 Berlin of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin cDepartment of Chemistry, Technische Universität Berlin, Straße des 17. Juni, 10623 Berlin bDepartment

*Corresponding authors: [email protected], [email protected] ABSTRACT: Selective electrochemical reduction of CO2 is an emerging field which needs more active and stable catalysts for its practicability. In this work we have studied the influence of Ag metal incorporation into Cu dendritic structures on the product distribution and selectivity of CO2 electroreduction.

Bimetallic

AgCu

foams

prepared

by

hydrogen

bubble

templated

electrodeposition shift the potentials of CO production to more positive values compared to bulk silver. The presence of Ag during the electrodeposition significantly changed the size and the shape of the dendrites in the pore walls of AgCu foams compared to Cu foam. The CO adsorption characteristics are studied by operando Raman spectroscopy. In the presence of Ag, the maximum CO adsorption is observed at a more positive potential. As a result, an improved selectivity for CO is obtained for AgCu foam catalysts at lower overpotentials compared to Cu foam catalyst, evidencing a synergistic effect between the bimetallic components. We were successful in increasing the CO mass activity with respect to the total Ag amount. AgCu foams are found to retain the CO selectivity during long term operation, and with their easily scalable electrodeposition synthesis they possess high potential for industrial application.

KEYWORDS: CO2 electroreduction, Electrocatalysis, Electrodeposited AgCu foam, Raman spectroscopy, CO formation

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INTRODUCTION The steadily rising atmospheric content of CO2 due to burning of fossil fuels generates much concern with respect to the changing global climate. As an alternative to fossil fuels and to meet our energy needs there is an increasing demand for renewable energy sources, such as wind, solar, and wave. Emphasizing the development in the renewable energy sector, CO2 conversion technologies using these renewable sources are gaining popularity due to chances of opening up a sustainable carbon neutral fuel cycle.1 Apart from that intermittently available renewable sources can be used wisely by utilizing the catalytic conversion of CO2 to store energy in the form of fuels or valuable feedstock chemicals. Among the various CO2 conversion methods2-3 electrochemical CO2 reduction especially in aqueous media is gaining considerable attention due to its environmental cleanliness, operation at ambient conditions, and possibility to tune the selectivity towards different reaction products by applying different overpotentials.4-5 However, the challenges are related to the generally low energy efficiencies due to the very large overpotentials originating from lack of suitable catalyst which can overcome the high CO2 reduction activation barriers. In addition, when operated in an aqueous medium the selected catalyst material has to impede the hydrogen evolution reaction (HER) initiated at much lower overpotentials. Even though the parasitic HER can be excluded by the use of ionic liquids as the electrolyte, development of aqueous-based CO2 reduction systems with higher activity is preferred for industrial application. Electrochemical conversion of CO2 to CO is promising given the already reported high faradaic efficiencies (FE) for CO compared to hydrocarbon products. It is further encouraged by the possibilities of coupling CO2 conversion with the industrialized Fischer–Tropsch process which uses syngas for the production of chemicals. Ag and Au are identified as the most active 2 ACS Paragon Plus Environment

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and selective catalysts for the CO2 conversion to CO due to the weak binding of the CO* intermediate.6-8 Even though metallic Au is a slightly more efficient catalyst than Ag,9-10 the comparably lower costs of Ag are of vital significance when it comes to commercialization. Hori et al. was the first to point out that CO2 can be effectively reduced on metallic silver with high efficiency.11 Recently, Hatsukade et al. has identified and quantified six reduction products including CO and hydrogen as the major ones on polycrystalline Ag.7 They showed almost 90 % efficiency for CO at -1 V vs RHE. Further improvement in activity and selectivity of the Ag catalysts is done by alloying or nanostructuring approaches. Alloying silver with other metals allows to alter the binding strength of intermediates on the catalyst surface,12 while nanostructuring of Ag has also shown to improve the performance towards CO2 reduction over bulk Ag. The advantage of nanostructuring is that a greater number of active sites are provided due to increased surface area as compared to the bulk metal, which is important in enhancing the catalytic current. Apart from that, the large number of edge sites, steps and kinks and other low coordinated sites present at the nanostructured catalyst surface offer unique catalytic properties compared to the highly coordinated sites typically found at polycrystalline catalyst surface.9, 13-14 Various nanostructured Ag catalysts in the form of free-standing and supported nanoparticles and nanoporous structures have been studied to date.15-19 For example, Lu et al. reported high CO selectivity of > 90 % at an intermediate potential of -0.6 V vs RHE for nanoporous Ag catalysts prepared by a dealloying process.20 Ham et al. studied the influence of the intensity ratio of (200) to (111) crystalline planes in dendritic Ag structures controlled by additives during the electrodeposition.21 They demonstrated higher CO production efficiency with an increase in the (200)/(111) peak intensity ratio. However, in the future and for industrial purposes, nanostructured electrodes with reduced noble metal content will be more attractive. Kim et al. 3 ACS Paragon Plus Environment


were successful in increasing the mass activity for CO production by synthesizing Ag nanoparticles dispersed on carbon support using cysteamine anchoring agents.22 They reported a 300 mV decrease in the overpotential at 1 mA cm-2 compared to Ag foil with a four-fold increase in the CO Faradaic efficiency at -0.75 V vs RHE. Their DFT studies revealed an enhanced intermediate stabilization resulting from the surface localization of the unpaired electron induced by Ag-S interaction. Cu is well known for its ability to reduce CO2 to CO and formate at lower overpotentials and hydrocarbon products at intermediate overpotentials. Considering the cost effectiveness of Cu, it will be interesting to study Cu based catalysts doped with Ag. Most of the reports in literature deal with the modification of Cu with Au by alloying or metal overlays, which leads to improvement or change in the product distribution,23-26 whereas studies of nanostructuring Cu with Ag for CO2 reduction purposes are scarce. A recent report on electrodeposited CuAg alloy with 6 % Ag demonstrated CO2 electroreduction performance, with high Faradaic efficiencies for C2H4 (60 %) and C2H5OH (25 %) at -0.7 V vs RHE with a total current density of ∼ 300 mA cm-2.17 Chang et al.27 prepared bimetallic AgCu by galvanostatically displacing Cu with Ag from an electrodeposited Cu and reported a volcano-like curve for the faradaic efficiency of C2 products with a maximum of 14 % at -0.75 V vs RHE. However, many other researchers including us were able to report higher values on similar Cu structures without Ag.28-29 AgCu foams prepared by Lee et al. using a similar synthetic route via galvanostatically displacing electrodeposited Cu with Ag showed the highest faradaic efficiency of 17.8 % for CO at -0.66 V vs RHE in 0.1 M KHCO3 solution.30 The CO FEs are proposed to be proportional to the Ag(220)/Ag(111) ratio, which is controlled using various additives. When Cu is electrodeposited


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together with Ag, the electrodeposited Ag57Cu43 outperformed the electrodeposited Ag under similar conditions by more than 2 times in terms of mass activity at -0.83 V vs RHE.31 In this study, we prepared AgCu catalysts by electrodeposition under hydrogen evolution to generate high surface area catalysts with low overall loading of Ag. Negative hydrogen bubble templated electrodeposition is proven to be a facile method to prepare highly porous electrodes with reproducible structures in a single step process.29, 32 The as prepared AgCu foam catalysts are able to reduce CO2 to CO with high current density at lower overpotentials (300 mV at 1 mA cm-2) compared to Cu foam and metallic Ag. The amount of Ag in the deposition bath has helped to increase the surface area from the base structure of Cu. Operando Raman spectroscopy is used to validate the improved activity observed for CO production. More importantly, AgCu catalysts demonstrated an exceptionally stable activity and selectivity compared to polycrystalline Ag.

EXPERIMENTAL SECTION Material and Method Electrodeposition of AgCu and Cu foams was conducted in a three electrode electrochemical cell with polished and etched (10 % HNO3) Cu foil (0.1 mm thickness, 99.999 %, Alfa Aesar) as the working electrode, Pt (99.99+%, Goodfellow) as the counter electrode, and Ag/AgCl (Biologic) as the reference electrode. During deposition, the distance between the working and counter electrodes was kept constant. AgCu foams were electrodeposited at a constant current of 1 A cm-2 for 10 s from an aqueous electrolyte containing specific amounts of AgNO3 (10 – 50 mM) in 0.2 M CuSO4 and 1.5 M H2SO4. Immediately after deposition the catalyst films were removed from the electrolytic bath, rinsed with milli-Q water, followed by blow drying with N2 gas. The porous AgCu foams prepared from 10 mM and 50 mM AgNO3 in the electrolyte are named as 5 ACS Paragon Plus Environment


AgCu-10 and AgCu-50, respectively. For comparison, Cu foam and Ag foam were fabricated under similar conditions applying a current of 1 A cm-2 for 10 s. The prepared catalyst films were also compared to polycrystalline silver (0.1 mm thickness, 99.999 %, Alfa Aesar). The as received Ag foil was mechanically polished, etched with sulphuric acid and washed with milli-Q water. The geometric area for all the catalyst films was 1 cm2. Structural Characterization Powder X-ray diffraction (XRD) patterns were recorded in transmission geometry, using a STOE STADI P, operating with CuKα radiation, after scratching off the electrodeposited films from the substrates. SEM images of samples were acquired using a Hitachi SU 8030 equipped with cold field emission cathode and Oxford X-Max 80 mm2 EDX detector. The measurements were done at room temperature at 10 mA with 10 or 15 kV. The amount of deposited Cu and Ag was identified with total X-ray fluorescence analysis (TXRF). TXRF measurements were performed using a PicoTAX (Bruker) spectrometer operating at 50 kV with a Si-drift detector (Rontec). Either Cs standard (1000 mg/L, 2 % HNO3, ROTISTAR) for quantification from L-edge peak areas of Ag or Ga standard (1000 mg/L, 5 % HNO3, TraceCert) for K-edge quantification from K-edge peak areas of Cu were chosen. Cu K-edge ex-situ XAS was performed at beamline KMC3 at Helmholtz-Zentrum-Berlin (BESSY II) at a temperature of 20 K. Spectra were collected in fluorescence mode with a 13-element windowless Germanium detector (Canberra). The deposited catalysts were frozen in liquid N2 directly after running three CVs. Raman spectra were collected with a Renishaw inVia Raman spectrometer coupled with a Leica microscope. The herein shown averaged XAS spectra contain four individual scans on two duplicate samples in each case. Calibration was done using a silicon wafer standard (521 cm-1). A 633 nm laser (He-Ne laser, 0.95 mW power) focusing on a line (~ 100 μm length) served as an excitation source. A homebuilt spectroelectrochemical cell made of PTFE was interfaced with the Raman microscope for 6 ACS Paragon Plus Environment

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spectroscopic measurements. The desired potential was applied for 2 min before collecting the spectra to ensure steady-state conditions. The acquisition time for each spectrum was 30 s. All spectra were smoothed and baseline corrected by Renishaw software. Fitting of the baseline is done after SERS peaks were automatically subtracted from the raw data spectra. The baseline consisted of a polynomial function with degrees between 5 and 11, and noise tolerance level around 1.4 - 1.6. The degree of the polynomial function depended strongly on the amount of SERS peaks present. Normalization was done by averaging the intensity of the background curve and this averaged intensity was used to divide the background subtracted and smoothed spectra. Further experimental details can be found in ref. 33.33 Electrochemical Characterization and Product Analysis Gamry Reference 600 potentiostat was used for all CO2 reduction experiments. The electrolysis was performed under room temperature in a gas-tight two compartment cell separated by a Nafion 211 membrane. Aqueous 0.1 M KHCO3 solution was used as the electrolyte after saturating with CO2 gas (99.995 vol.%, Air Liquide) for 20 min, pH 6.8. During electrolysis, the electrolyte in the working electrode (WE) compartment was purged with CO2 from the bottom of the cell at a flow rate of 20 mL min-1 using a calibrated mass flow controller. The electrolyte in the WE compartment was stirred continuously to achieve maximum mass transport of CO2 to the electrode surface. All the potentials measured were compensated for iR drop by current interrupt method and represented with respect to reversible hydrogen electrode (RHE). An Ag/AgCl reference electrode (Biologic) was placed in front of the working electrode of 1 cm² area. A Pt coil (Biologic) was used as the counter electrode and CO2 was also purged to the counter electrode compartment to maintain a similar pH value of 6.8. To investigate the active potential range for CO2 reduction, linear sweep voltammogramms (LSV) are measured on selected AgCu catalysts along with electrodeposited Cu foam, Ag foam and Ag foil as the control experiment. 7 ACS Paragon Plus Environment


Before the LSV measurements, CVs are carried out to reduce any surface oxide, which may be unstable in the process. The gaseous products of CO2 reduction reaction were identified and quantified using an online gas chromatograph (GC) (Shimadzu, GC-2014) coupled with the two compartment electrochemical cell. The product gases were analyzed at every 20 min and 1 h of electrolysis at a specified potential. The gaseous products H2, CO, CH4, C2−C3 hydrocarbons and CO2 were separated in HayeSep capillary columns connected in series (HayeSep Q + HayeSep R, 80/100 mesh, 2 + 2 m × 1/8 in.). The GC is equipped with a thermal conductivity detector (TCD) for the detection of H2. A flame ionization detector (FID) is used for detecting CO (in form of CH4 after passing through a methanizer unit before FID) and hydrocarbons. Ar of grade 5 is used as the carrier gas. The electrolyte was collected after 1 h of electrolysis at each potential and analyzed for liquid products. A gas chromatograph (Shimadzu GC-2010-Plus) with AOC 150i (autosampling), column of type SH-Stabilwax for separation and a FID for detection and quantification of the volatile aldehydes and alcohols in the electrolyte was used. A high performance liquid chromatograph (Agilent Technologies 1200 Series) was used for detection and quantification of the carboxylic acids. Separation is done in a Organi-Acid Resin column flushed with 0.005 M H2SO4 at 1 mL min-1 flow. Detection is achieved via a refraction index detector. The relative faradaic selectivity (FS) of the products was determined by dividing its partial currents by the sum of the partial current of all detected products. 𝐼𝑖

(1)

𝐹𝑆𝑖 = ∑𝐼 × 100 𝑖

For gaseous products, 𝐼𝑖 = 𝑥𝑖𝑧𝑖𝐹𝑛 , where xi , zi, F and 𝑛 are the volume fractions of detected gaseous product, number of electrons involved for a particular reduction product, Faraday constant and molar flow rate, respectively. RESULTS AND DISCUSSION 8 ACS Paragon Plus Environment

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Physical Properties Porous Cu and AgCu foams were obtained under galvanostatic conditions applying a current density of 1 A cm-2. As reported elsewhere,32 porous structures with dendritic walls were obtained as shown in Figure 1 (a, b). During the electrodeposition the generated hydrogen bubbles act as dynamic negative templates directing the porous structures, whereas the fine dendritic morphology of the metal film is determined by the nucleation and growth of the metal. Dendritic-like growth is observed for Cu deposits presumably at the diffusion limited regime.34-35 AgCu foams were electrodeposited using two different concentrations of the AgNO3 in the electrolyte bath, namely 10 mM (AgCu-10) and 50 mM (AgCu-50), keeping the concentration of Cu identical. Upon introduction of the second metal a change in the morphology with respect to the electrodeposited Cu foam was observed as shown in Figure 1. Increasing the concentration of AgNO3 from AgCu-10 to AgCu-50 also takes an influence on the porous structure. The dendrites present in the walls of the pores are shorter and smoothened down for AgCu compared to the Cu only deposit. We observe that on AgCu individual dendrites grow in a two dimensional pattern with reduced branching, while the dendrites of Cu foam are results of three dimensional growth. The macropores formed by the Ag deposit (Figure 1c) are significantly smaller than the Cu and AgCu macropores. The sidewalls of the macropores contain randomly ordered needle like structures in contrast to dendritic structures observed on Cu and AgCu catalysts. One could anticipate high surface area for the porous structure of the electrodeposited catalysts from the SEM images. Roughness factors of the foam catalysts were estimated from the double layer capacitance in the non-faradaic regions of the CVs as detailed in the supporting information (Figure S1, Table S1). Higher roughness factors are obtained for electrodeposited catalysts compared to bulk metal foil catalysts (Cu and Ag). Roughness of the deposited catalysts follows the order Ag < Cu < AgCu-10 < AgCu-50 (Table S1). Even though these roughness factors 9 ACS Paragon Plus Environment


estimated from the capacitance do not represent the absolute electrochemically active surface area, they agree well with the SEM images. As one could see from the SEM pictures, the dendrites in AgCu are smaller compared to the Cu only deposit accounting for its increased surface area. With an increase in the Ag content, the surface area is increased further due to the increased density of dendrites and their smaller dimensions in AgCu-50 in comparison to AgCu-10.

Figure 1. SEM images of Cu (a, b), Ag (c, d), AgCu-10 (e, f) and AgCu-50 (g, h) foams electrodeposited at 1 A cm-2.

Elemental analysis was carried out to determine the bulk composition of AgCu foams. Figure 2 shows the wt. % of Ag and Cu in the AgCu foams. An increase in concentration of AgNO3 in the deposition bath from 10 to 50 mM leads to an increase in the deposited Ag amount from ~11 to ~33 wt. %. Additionally, the metallic composition of AgCu-10 and AgCu-50 deposited at various deposition times ranging from 5 to 20 s is shown (Table S2). The wt. ratio of Cu and Ag remains nearly constant for a particular concentration of AgNO3 in the electrolyte irrespective of the deposition time. The determined ratios of Cu:Ag are 6:1 and 2:1 for Ag deposited from 10 mM and 50 mM AgNO3 solution, respectively. As the film thickness grows 10 ACS Paragon Plus Environment

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with deposition time, a uniform AgCu ratio is maintained keeping the bulk concentration similar. The Faradaic efficiency of the AgCu electrodeposition is less than 10 %, while the major charge density is utilized for hydrogen evolution (bubble template). The overall hydrogen evolution can be reduced via increasing the concentration of AgNO3 in the electrolyte, which in turn improves the deposition efficiency of both the metals (Figure S2). With a significant difference in HER activities between Cu and Ag,32 it is expected that the freshly deposited metal surface with relatively high concentrations of Ag would impede the hydrogen evolution compared to Cu only surface.

Figure 2. Wt. % of Ag and Cu measured by TXRF in the electrodeposited AgCu-10 and AgCu-50 catalysts at varying deposition time.

XRD measurements were carried out to investigate the crystalline structure of AgCu catalysts. Figure 3 represents the XRD patterns of AgCu-10 and AgCu-50 electrodeposited for various times. The main phases of the catalyst can be identified as Cu2O along with metallic Ag. The presence of oxides of Cu is expected in relatively high concentrations, due to air oxidation at ambient conditions during storing and scratching off

the catalyst films from the substrate

material. We have recently shown that the detected presence of Cu oxides via ex-situ techniques 11 ACS Paragon Plus Environment


is due to the relatively facile oxidation of Cu foams in their resting state under ambient conditions.28 However, porous Cu foams are completely metallic throughout the bulk and at the surface at catalytic potentials as determined via operando XAS and XPS. In both AgCu-10 and AgCu-50, the reflections at 29.5°, 36.4°, 42.3°, 61.3° and 73.5° correspond to the diffraction of (110), (111), (200), (220), and (311) planes of Cu2O, respectively. The reflections at 38.1°, 44.2°, 64.5°, and 77.4° can be indexed to (111), (200), (220), and (311) planes of Ag in the dendritic structures. All the reflections of Cu oxide and Ag refer to their face centered cubic structure. Additionally, the absence of any shift in the Ag reflections indicates the absence of any significant alloy formation with Cu. Despite both crystallizing in fcc structure, Cu and Ag are very unlikely to form alloys during deposition at room temperature. For AgCu-10 and AgCu-50 catalysts individually, the intensity of Cu2O and Ag crystalline peaks increases with the increase in deposition time from 5 s to 20 s. Remarkably, the relative peak intensity increases for both Cu and Ag while keeping a roughly constant ratio between each other, which is already observed in the TXRF results.


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Figure 3. XRD patterns of AgCu films electrodeposited at 1 A cm-2 from 0.2 M CuSO4 in 1.5 M H2SO4 containing 10 mM (AgCu-10) and 50 mM (AgCu-50) AgNO3 with an increasing deposition time from 5 s to 20 s.

Cu K-edge X-ray absorption spectroscopy (XAS) has been performed on dry catalysts in their resting state to obtain more detailed structural information. The Cu K-edge XANES spectra of the AgCu catalyst are similar to Cu foil and the Cu foam without Ag (Figure S3). In our previous work of Cu foam catalysts, we have shown that the experimental XANES can be deconvoluted into metallic Cu and Cu2O contributions. In the resting state of the foam Cu oxide contributions can straight forwardly be explained as products from air oxidation.28 Usually, a higher pre-edge peak intensity represents a higher Cu2O content. On incorporation of Ag into the porous Cu structure, the pre-edge peak is lowered in comparison to Cu foam as shown in Figure S3. The Cu structure remains the same for AgCu even after 20 minutes of electrolysis (measured on a dry catalyst frozen after the electrolysis) at -0.6 V vs RHE. This result indicates the stability of AgCu: the foams maintain their initial structure during the course of the electrolysis. To track down the changes in the metal structure, the EXAFS spectra have been simulated. Comparison of 13 ACS Paragon Plus Environment


experimental and simulated EXAFS data of Cu metal foil, the Cu foam, and the AgCu-50 catalyst can be found in Figure 4. The intensity of the FT EXAFS peaks decreases in the order Cu foil > Cu foam > AgCu-50. The AgCu-50 catalyst maintains the same interatomic distances as Cu foil, but the number of neighboring Cu atoms is reduced up to 4 times. The AgCu catalyst consists of a less ordered Cu metal phase than Cu metal foil and the Cu foam. This is indicated by the overall decreased peak intensities of the AgCu-50 film compared with the Cu foam, and Cu foil (see simulation results in the supporting information, Table S3). The lower order metallic contribution of AgCu could be due to a combination of the disorder resulting from the reduction of Cu oxides into metallic Cu phase and the presence of Ag in the system. From our experience with Cu foams28, the reduction of Cu oxides into metallic Cu usually has no significant impact on the Cu foam structure. Therefore, we favor the explanation that the presence of Ag is responsible for the lower ordered Cu phase.


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Figure 4. a) Experimental Fourier-transformed EXAFS spectra of Cu foil- grey, electrodeposited Cu- blue, AgCu50- green and their corresponding simulations (black lines). The first 4 Cu shells ( -1.2 V vs RHE. Upon incorporation of Ag into the Cu structure (AgCu catalysts) an improved carbonyl selectivity was observed at higher potentials by Clark et al.37 and Higgins et al.38 Whereas few other reports in literature show higher selectivity for C2 hydrocarbons and alcohols.17,

27

Consequently, it is not surprising that in the higher potential region different

phenomena are observed based on the structure and composition of AgCu catalysts. Our study focuses on the influence of CO activity and selectivity with Ag incorporation into the Cu in the lower potential region.

Figure 6 Faradaic selectivities of the major products CO and formate along with H2 of electrodeposited Cu, electrodeposited Ag, AgCu-10, AgCu-50, and Ag foil as a function of applied potential vs RHE. The error bars correspond to the average of two individual measurements on two different samples.

CO Selectivity Improved CO selectivity and suppression of hydrogen evolution is generally observed at lower overpotentials for both the electrodeposited AgCu catalysts in contrast to Ag foil. Similarly, the 18 ACS Paragon Plus Environment

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faradaic selectivity for formate is decreased by the presence of Ag in relation to the Cu foam. AgCu-50 showed the highest CO2 reduction performance reaching a maximum faradaic selectivity of 58.4 % at -0.6 V vs RHE. Additionally, a maximum FS of 44.1 % is observed for AgCu-10 with low Ag loading. There is a clear enhancement in the performance for AgCu compared to Ag foil (5.26 % @ -0.6 V vs RHE) in the lower overpotential region (-0.5 – -0.7 V vs RHE). Furthermore, the Cu only structure exhibits the highest FS of 20.3 % at -0.6 V vs RHE, which is reached by AgCu-50 already at a 200 mV more positive potential. By nanostructuring the Ag catalysts, a number of authors have reported an improvement in the CO selectivity at lower potentials compared to bulk Ag.9, 19-20, 22, 39-40 The prepared Ag foam catalyst delivers a selectivity of 50 % at -0.6 V vs RHE. Interestingly, AgCu catalysts exhibit similar selectivities as the pure Ag foam catalyst. When comparing the recent reports on AgCu systems with our study, the studied catalysts exhibit a significant improvement in CO selectivity especially at lower overpotentials (-0.4 – -0.7 V vs RHE).27, 30-31, 41 To better understand this interesting behavior of AgCu catalysts towards CO formation, CO partial current densities are calculated from the amount of CO detected at the potential of interest and their respective steady state current from chronoamperometry. Figure 7a shows the CO current densities plotted as a function of the overpotentials, assuming a thermodynamic reduction potential of -0.11 V vs RHE for CO2 to CO.22 All the electrodeposited catalysts exhibit enhanced catalytic current density compared to bulk Ag foil. AgCu catalysts show higher CO current densities at lower overpotentials compared with Cu foam and Ag foam. The CO current density of pure Cu catalysts drops with the increase in overpotentials due to formation of hydrocarbons. As we could see, despite the high roughness of AgCu compared with Cu foam (Table S4), the influence of the presence of Ag in AgCu is higher in decreasing the total HER and improving the CO production than the roughness effect. More specifically, at an overpotential of 19 ACS Paragon Plus Environment


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490 mV (-0.6 V vs RHE) an increase in the CO partial current densities is observed with an increase in the concentration of Ag in the Cu foam catalyst (Figure 7b). At the same time the hydrogen partial currents decrease. This means that Ag has helped to suppress the hydrogen evolution despite the high roughness of the catalysts. Higgins et al. showed that by increasing the nominal Ag composition in AgCu thin films the HER activity steadily decreases.38 As Ag has lower hydrogen binding energy than Cu, the presence of Ag surface atoms results in lower *H surface coverage and hence suppression of HER rate.

Yoon et al. demonstrated on

mesostructured Ag electrodes that roughness plays an important role in the suppression of hydrogen evolution due to alkalization at the catalyst surface (local pH increase).42 The CO selectivity trend for Ag foil and Ag foam at -0.6 V vs RHE are in agreement with this statement (see Figure S7). At a higher potential, e.g. -1 V vs RHE, we therefore would expect the local pH to have a more pronounced effect on suppressing the HER. However, at -1 V vs RHE both Ag foil and Ag foam show highest faradaic selectivity for CO (> 80 % FSCO) irrespective of the difference in roughness. To further support this fact, we have measured and compared the pH gradients for AgCu foam catalyst with Cu foam by investigating the HCO3-/CO3- equilibrium through operando Raman spectroscopy. We have recently shown that on Cu foam catalysts,28 the local pH increases from 6.8 (at open circuit potential) to 10 at an applied potential of -0.6 V vs RHE. At a potential of -0.7 V, the local pH was higher than 10. Whereas at -0.7 V on AgCu surface (see Figure S8), the local pH was 10.5, which is insignificant when considering ~ 2x roughness of AgCu to Cu foam. The additional result from the operando Raman experiments showed that the increased CO2RR/HER ratio at lower potential is not solely a roughness effect, but due to the presence of Ag in the sample which has synergic interaction with Cu in catalyzing the reaction. In addition, nanostructured Ag foam catalysts can provide low-coordinated surface Ag sites for stabilizing the COOH* intermediate through reducing the activation energy barrier of 20 ACS Paragon Plus Environment

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the initial electron transfer.18, 39 This could also be a reason for the enhanced CO activity for Ag foam and AgCu catalysts compared to planar Ag foil. These facts strongly indicate that roughness itself has not the decisive role in tuning the CO activity and selectivity in the herein investigated bimetallic AgCu catalysts. On Ag foil, large overpotentials are required to initiate the CO2 reduction and CO formation. From the data in Figure 7a, the overpotentials were compared at fixed CO current densities of 0.1, 1 and 2 mA cm-2 as shown in Figure 7c. Both AgCu catalysts showed noticeable difference in overpotentials compared with Ag foil or Ag foam. In particular, AgCu-10 and AgCu-50 showed ~ 200 mV and ~ 300 mV shift in potential at 1 mA cm-2 compared with Ag foil, respectively. A similar anodic shift for CO reduction is reported for nanostructured Ag. Interestingly at lower potentials AgCu catalysts show similar activity to carbon supported Ag nanoparticles which were studied by Kim et al. in 2015.22 Furthermore, an appreciable anodic potential shift for AgCu catalysts compared with Ag foam is observed: a potential difference of 89 mV and 187 mV at 1 mA cm-2 for AgCu-10 and AgCu-50, respectively, as shown in Figure 7c. The results indicate that the presence of Ag in AgCu plays a vital role in enhancing the activity at lower overpotentials.



Figure 7. a) CO current density as a function of overpotential for Ag foil, Cu foil and electrodeposited Cu, Ag, and AgCu catalysts; b) H2 and CO partial current densities with increasing roughness and Ag content in AgCu catalysts; c) overpotential at 0.1, 1 and 2 mA cm-2 for Ag foil, and electrodeposited Cu, Ag, and AgCu catalysts; d) CO mass activity with respect to Ag loading in AgCu-10 and AgCu-50. The error bars correspond to the average of two individual measurements on two different samples

Many researchers have shown high CO selectivity and activity for modified Ag catalysts at lower potential compared to bulk Ag catalysts.19-20 Among these, Lu et al. demonstrated the highest CO selectivity of 92 % at -0.6 V vs RHE on nanoporous Ag catalysts.20 However, the industrial potential of these catalysts will be impacted only when both the mass activity with respect to the silver metal content and the CO selectivity are sufficiently high. In order to evaluate the activity with respect to the amount of precious metal content, the CO mass activities 22 ACS Paragon Plus Environment

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of the catalysts were compared as shown in Figure 7d. The mass activity of AgCu-10 and AgCu-50 follows a different trend than the CO current density. The mass activity of AgCu-10 considerably outperforms AgCu-50. This could be due to the fact that Ag in the bulk of AgCu-50 is inaccessible for the reaction compared to AgCu-10. These mass activity values are higher than the values reported recently by Choi et al.31

CO Adsorption Behavior The adsorbed reaction intermediates and products formed during the course of CO2 reduction on AgCu catalyst surfaces were further studied by surface enhanced Raman spectroscopy (SERS). Figure 8 shows the operando Raman spectra of a freshly deposited AgCu-50 during a potential series from 0 to -0.8 V vs RHE. The reduction behavior of surface Cu oxide species inside the AgCu film is comparable with the one of Cu foam.33 Copper oxides (527 and 604 cm-1) are present until -0.4 V vs RHE. The peak around 419 cm-1 belongs to the Ag-O stretching band in AgO, which gets reduced after -0.1 V vs RHE.43 The band at 985 cm-1 observed from open circuit potential to -0.2 V vs RHE is proposed to result from a specific silver/sulphate group.44 This is most likely a contamination from the electrodeposition and is stripped off at more negative potentials (< -0.2 V vs RHE). CO2 reduction resulting in HCOO-/HCOOH species is indicated by the presence of Raman bands at 304, 2848, and 2906 cm-1. This finding is in line with previous literature results. Literature values of HCOO-/HCOOH Raman bands can be found in Table S5.



Figure 8. Operando Raman spectra of AgCu-50 in 0.1 M KHCO3, pH 6.8. Potentials are given with reference to RHE. Arrow indicates the scan direction. Spectra are background subtracted, normalized, and offset. The noise level in the spectrum of -0.8 V is enhanced due to intense gas evolution.

Intramolecular CO stretching vibrations with different adsorption geometries are observed as two main peaks at around 2040 and 2080 cm-1.33 Before the onset of effective CO2 reduction (0 V vs RHE) we observed adsorbed CO on Ag sites (partially oxidized Ag) giving rise to a band around 2138 cm-1.45 Atop binding of CO on Ag is expected to happen at around 20442051 cm-1,46 and therefore might overlap with the stretching vibrations of CO adsorbed at Cu sites. The accumulation of CO at the catalyst surface is more intense at low overpotentials in the AgCu than in pure Cu foam. The presence of Ag within the Cu foam drastically changes the 24 ACS Paragon Plus Environment

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potential dependency of the CO peak area compared with Cu foam catalyst (Cu foam data was acquired from Figure 3a in ref 33)33 as shown in Figure 9. We note that the differences in roughness between the Cu and AgCu foam will affect the Raman peak intensities, nevertheless the clear change in the potential dependency of the CO adsorption/desorption trend between Cu and AgCu can be demonstrated. The maximum CO adsorption is reached at much higher potentials (between -0.2 and -0.4 VRHE) than for the Cu foam (> -0.6 V). As a result, CO can probably desorb at more positive potentials from the AgCu surface than from Cu foam. In contrast to pure Cu foam we did not detect any Cu-CO vibrations on AgCu-50, which are expected at 280 and 360 cm-1.33 Similarly, Ag-CO vibrations are as well absent during the whole potential series (expected as two bands at around 420 and 520 cm-1

47

or at around 353 and

406 cm-1 46). This result is surprising, given the high roughness and the consequently high SERS effect of the AgCu catalyst. On the other hand, Ag is known to have a lower CO binding energy than Cu, which can give rise to more facile desorption of CO.48-49 The absence of any metal-CO bands in the AgCu catalysts could also result from the facile release of CO in AgCu foam. We note that Cu oxide species undergo a slower reduction in AgCu than Cu foam (Figure S9). However, the Cu oxide species are nearly reduced at -0.3 V vs RHE where AgCu exhibits high CO adsorption intensities. In Figure S9, the difference in peak intensities of Cu oxide species at -0.3 V vs RHE for Cu foam and AgCu are nearly the same and are negligible in relation to the peak intensities at 0 V, assuming CO adsorption mainly at metallic Cu sites. We conclude that incorporation of Ag modifies the CO adsorption behavior significantly compared to pure Cu foams resulting in intense CO accumulation at low overpotentials, peaking at -0.3 V vs RHE. Further CO2 reduction to more negative potentials is followed by a gradual decay of the CO concentration at the surface which is accompanied by an increase in CO release (with a maximum at -0.6 V vs RHE). The product characterization together with Raman studies 25 ACS Paragon Plus Environment


reveals a strong synergistic interaction between the metal components in AgCu catalysts in catalyzing the CO2 reduction to CO.

Figure 9. Potential dependence of the integrated peak area for the intramolecular CO stretch bands at 2040 and 2080 cm-1.

Catalytic Stability Durability tests were performed for the prepared AgCu-50 catalyst, Ag foam and Ag foil at potentials, where CO shows maximum selectivity. Subsequently, 5 h electrolysis was performed at -1 V vs RHE for Ag foil and Ag foam and at -0.6 V vs RHE for AgCu-50 film. The potential was chosen because at -0.6 V vs RHE the Ag foil mainly produces H2. Despite the roughness and fine structure details, the electrodeposited AgCu catalyst and Ag foam are more durable than Ag foil, as represented in Figure 10. Although all the catalysts show stable total current densities during the 5 h test, there are significant differences in the stability of the monitored CO current density. The CO current density declines dramatically for Ag foil in comparison to AgCu-50 and hence the selectivity for CO. When the drop in performance is represented in terms of faradaic 26 ACS Paragon Plus Environment

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selectivity, almost 50 % (from 73.1 % to 34.9 %) decrease in selectivity is observed for Ag foil. This decrease in CO selectivity is transferred to the H2 faradaic selectivity for Ag foil. Kim et al. observed a similar degradation in the performance for Ag foil at -1.1 V vs RHE.22 About 8 % decrease in selectivity is observed on Ag foam. However, almost 100 % of selectivity is retained for the AgCu catalyst after 5 h. After the 5 h durability test, the morphology of the AgCu catalysts was altered, despite retaining a stable performance. From the SEM images given in Figure S10, the dendrites appear smoothened and more rounded compared to freshly deposited samples. In our earlier studies a similar behavior was observed for the electrodeposited Cu dendrites.28 This could be due to the reduction of Cu and Ag oxides, which are initially formed during the electrodeposition and their air exposure, at the CO2 reduction potentials and a possible re-oxidation of the surface Cu and Ag.50 However more studies are required to understand the exact origin of the morphological changes. In general, nanostructured catalysts with high surface area exhibit high stability in performance over their bulk counterpart with planar morphology.

Figure 10. CO current densities and CO faradaic efficiencies of Ag foil (at -1 V vs RHE), electrodeposited Ag (at -1 V vs RHE), and AgCu-50 (at -0.6 V vs RHE) during 5 h long CO2 electroreduction.

CONCLUSIONS 27 ACS Paragon Plus Environment


In summary, we have demonstrated an enhanced activity and selectivity for CO production from CO2 for AgCu catalysts prepared by a reproducible and facile electrochemical method. The incorporation of Ag along with cheap metal catalysts such as Cu has not only led to an increase in the mass activity with respect to CO production, but also shifted the overpotential of the reaction to ~ 300 mV and ~ 187 mV lower value compared with bulk Ag and Ag foam, respectively. The prepared catalysts also maintained a constant selectivity during 5 hours of operation. Raman studies showed that the presence of Ag has shifted the peak intensity of the intramolecular CO band (CO surface coverage) to more positive potentials compared to Cu foam. This further elucidates the facile desorption of CO from the AgCu surface at lower overpotentials and hence enhanced activity for CO2 reduction to CO. Nanostructuring of Ag with cheap and CO2RR active Cu underline a synergy between the metal components in effectively catalyzing CO2 reduction to CO at low overpotentials with an improved stability over pure Ag catalyst.

ASSOCIATED CONTENT Supporting information Estimation of electrochemically active surface area from double layer capacitance, Efficiencies of electrochemical deposition of AgCu foams, elemental analysis data (TXRF), Cu K-edge XANES spectra, EXAFS simulation results, chronoamperometric curves for 1 h of CO2 electroreduction, CO2RR and H2 partial current densities, Faradaic selectivities of all the detected products, comparison of the CO selectivity and CO current density between Ag foil and Ag foam, potential dependence of Raman intensities for Cu oxides, SEM images of AgCu foam and Ag foam before and after electrolysis, and summary of Ag loading and electrochemical properties of Cu, Ag and AgCu foams compared with Ag foil. 28 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding authors *E-mail: [email protected], [email protected] ORCID iD Tintula Kottakkat: 0000-0002-7569-4276 Katharina Klingan: 0000-0003-2357-2397 Gumaa A. M. El-Nagar: 0000-0001-8209-4597 Holger Dau: 0000-0001-6482-7494 Christina Roth: 0000-0003-1159-2956 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was funded by the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF) under grant 03SF0523 (CO2EKAT). We thank Xingli Wang (Department of Chemistry, Technische Universität Berlin) for the help with the HPLC measurements.

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GRAPHICAL ABSTRACT


Electrodeposited AgCu foam catalysts for enhanced reduction of CO2

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