Sulfide-Derived Copper for Electrochemical Conversion of CO2 to

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Sulfide-Derived Copper for Electrochemical Conversion of CO2 to Formic Acid Katherine Reece Phillips, Yu Katayama, Jonathan Hwang, and Yang Shao-Horn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01601 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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The Journal of Physical Chemistry Letters

Sulfide-derived Copper for Electrochemical Conversion of CO2 to Formic Acid Katherine R. Phillips*, Yu Katayama, Jonathan Hwang, and Yang Shao-Horn* Electrochemical Energy Lab, Massachusetts Institute of Technology, Cambridge, MA 02139 Corresponding Author *to whom correspondence should be addressed: [email protected], [email protected]

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Electroreduction of CO2 is a promising direction to convert this greenhouse gas into carbon-containing products such as plastic and liquid fuels, which are currently derived primarily from petroleum. Using CO2 as a carbon source for plastics and other chemicals allows CO2 to be sequestered into existing products; alternatively, CO2-derived liquid fuels can store renewable energy such as solar and wind, by using these sources to drive the chemical conversion of CO2 into a high energy-density product that can be used directly in our existing energy system. Regardless of the product, this conversion process generally requires a catalyst. Catalysts for the electrochemical CO2 reduction reaction (CO2RR) have been widely investigated, but both selectivity (i.e. produce only one product) and efficiency (i.e. low overpotential required) remain a challenge.1,2 Copper has been widely studied as an electrocatalyst for this reaction, as it is the only transition metal that produces more than trace amounts of hydrocarbons.3 Other transition metals either produce CO, formic acid, or hydrogen under aqueous CO2RR conditions,4 which are all two-electron processes and thus are less kinetically hindered than the multielectron processes required for hydrocarbon production. Many alloy/nonmetal catalysts have also been developed, with promising results for CO and HCOOH production, and formate is reported as a potential first commercial product.1,5 Lead, oxide-derived lead, and tin electrocatalysts have selectivity for HCOO- over CO,1,6,7 with more recent work finding CuS as an active, selective, cheap catalyst for formate.8,9 For C2 and greater hydrocarbon and alcohol products, recent work has identified that oxidizing copper to CuO and Cu2O can lower the required overpotential to produce hydrocarbons: Kanan et al have reported the high activity of thermally treated copper towards electrochemical



reduction of CO210 and CO11. Conversely, sulfur-doped copper catalysts produce formate as an almost exclusive CO2RR product.8,9 In order to investigate the origin of this difference in selectivity for different copperbased catalysts, we investigated the mechanism for the selective conversion of carbon dioxide into formate. In particular, we synthesized a sulfide-derived copper (SD-Cu) catalyst for the electroreduction of CO2 into formate. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), we find evidence that formate production is through direct hydrogenation of CO2,23 and we propose that this selectivity is due to stronger binding of CO. In these experiments, copper sulfide was deposited on copper foil using a pulsed electrodeposition method described elsewhere.24 Microscopic and macroscopic images are shown in Figure 1A-C, showing the nanowire morphology of the copper sulfide. Two different deposition times were compared, 30 min (CuS-1) and 2 h (CuS-2), which both resulted in similar nanowire morphologies but different amounts deposited.24 During electrolysis, copper sulfide is reduced in situ, forming sulfide-derived copper (SD-Cu), and resulting in a loss of the nanowire morphology but remaining roughness for both CuS-1 and CuS-2 (Figure 1E-F). Capacitance measurements after electrolysis provided an estimate of the roughness factor of 8.4 for SD-Cu1 and 7.8 for SD-Cu2 by normalizing the capacitance values by that of flat copper foil (Supplemental Figure S1), which are equivalent within experimental error despite the difference in deposition time. Thus, we concluded that SD-Cu1’s and SD-Cu2’s accessible surface areas are equivalent, and we focused on SD-Cu1 for subsequent characterization.


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Figure 1. Imaging of catalyst surfaces, (A-C) before, and (D-F) after electroreduction of CO2. (A, D) Copper foil, (B, E) copper sulfide (CuS) electrodeposited for 30 min (CuS1), and (C, F) CuS electrodeposited for 2 h (CuS-2). The CuS samples convert to sulfidederived copper (SD-Cu) after electrolysis. Top scale bars are 250 nm, bottom scale bars are 10 µm for each panel. Insets are photographs of the samples.

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed to analyze the catalyst surface before and after CO2RR. The X-ray diffractogram of an



as-deposited CuS film contains peaks attributed to copper sulfides and cuprous oxide below 2 Theta of ~40 (inset, Figure 2A), in addition to copper peaks attributed to the underlying copper foil (Figure 2A). After electrolysis, only Cu peaks and one Cu2O peak remain (we attribute the presence of cuprous oxide to reaction of high surface-area copper surfaces with air after exposure to electrolyte). While these curves indicate that copper is likely reduced, XRD does not provide conclusive evidence for surface changes despite using grazing-incidence XRD to enhance the signal from the surface. XPS results for the Cu 2p3/2 region are also inconclusive due to overlap of Cu0 and Cu+ peaks24 as well as oxidation of copper under ambient conditions (Figure 2B). Alternatively, XPS results for the S 2p region provide evidence for remaining sulfur on SD-Cu. For the as-deposited CuS, a sulfur 2p3/2 peak at 163.0 eV is visible (Figure 2C), which matches literature values.9 Low levels of sulfur are also visible in the S 2p region of SD-Cu shown in Figure 2C, as well as in energy dispersive X-ray spectroscopy (EDS) maps (Supplemental Figure S2). Additionally, a peak shift to lower binding energies is visible, which indicates a lower oxidation state of copper.25,26 Inductively coupled plasma (ICP) results suggest there is sulfur in the electrolyte after in-situ reduction (Supplemental Figure S3).


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overpotential, and the faradaic efficiency is higher, for SD-Cu than for pristine copper. There was no meaningful difference between SD-Cu1 and SD-Cu2, despite the difference in deposition time.

Figure 3. Analysis of CO2 electroreduction products after 3 h potentiostatic electrolysis. (A) Calculated faradaic efficiencies for all detected products. (B) Partial current densities for hydrogen and formate. At each potential, copper foil (“Cu foil”) is compared to two copper sulfide samples, one deposited for 30 m (“SD-Cu1”), and one deposited for 2 h (“SD-Cu2”), which convert to sulfide-derived copper during electrolysis.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) was used to probe the surface species during electrolysis. Cu foil shows weak peaks corresponding to adsorbed CO molecules and adsorbed carbonate27 (Figure 4A). Conversely, SD-Cu has a strong adsorbed CO (COads) peak appearing as the potential sweeps more negative than -0.3 V (vs RHE) (Figure 4B). The current density during these measurements is shown in


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Supplemental Figure S5. Interestingly, no CO product was detected during electrolysis of SD-Cu, suggesting that the COads is tightly bound to the surface and does not desorb, unlike in the Cu foil case. Absence of HCOOads on the surface suggests that the formation of formate does not require a HCOOads intermediate, implying it is instead formed by the one-step direct hydrogenation via physisorbed CO2 reacting with Hads (CO2 + Hads + e- ! HCOO-).23 Unfortunately, hydrogen evolution has fast kinetics28 on Cu at these potentials, so SEIRAS cannot track the corresponding intermediates such as Hads at this scan rate. However, the analysis of CO2 electroreduction products on SD-Cu (Figure 3A) clearly shows hydrogen production, implying that short-lived Hads is able to co-exist on the surface with tightly bound COads. Negative interfacial water (H2Oint) peaks are present on the Cu foil sample, which we attribute to potential-dependent water reorganization.



Figure 4. In situ surface-enhanced infrared absorption spectroscopy (SEIRAS) of (A) Cu foil, and (B) SD-Cu. Difference spectra were taken in series as the potential was scanned from 0.05 V (vs RHE; bottom) to -0.9 V (top) using a reference obtained at 0.05 V.

SD-Cu has selectivity towards formic acid as an almost exclusive CO2RR product, unlike Cu foil and OD-Cu. This marked change in product formation compared to the Cu foil indicates that sulfur is playing a large role in the SD-Cu activity. Additionally, ODCu catalysts have been found to have a strong CO binding site, using temperatureprogrammed desorption to compare various versions of OD-Cu.16 The presence of metastable copper cations in OD-Cu catalysts has been the subject of some debate,18,20,21 and the change in selectivity of SD-Cu compared to OD-Cu suggests that the increased activity of OD-Cu may not be derived from copper cations. XPS confirms the presence of trace sulfur on SD-Cu even after CO2RR, presumably leading to the change in activity. Additional detailed characterization performed previously also found remaining sulfur for bulk CuS under CO2RR conditions.9 The presence of sub-surface sulfur in SD-Cu supports the theory that oxygen remains in the OD-Cu catalysts18,19 despite the reducing conditions. Residual oxygen is otherwise challenging to conclusively identify, due to formation of a native oxide layer when exposed to air or even to the electrolyte, whereas subsurface sulfur avoids these problems. Of course, remaining sub-surface sulfur in SDCu does not directly implicate remaining subsurface oxygen in OD-Cu due to sulfur’s larger size compared to oxygen; additionally, CuS and Cu2S are more stable under reducing conditions than CuO and Cu2O.29 Reaction with surface sulfur atoms also cannot be eliminated, particularly considering that metal sulfides have previously been


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found active for the hydrogen evolution reaction (HER), particularly nanostructured or amorphous sulfides.30 Remaining subsurface sulfur could lead to larger binding energy for CO on SD-Cu than on Cu and OD-Cu. Subsurface oxygen was previously proposed to cause stronger CO binding on OD-Cu than Cu based on DFT calculations.18 Several transition metals have stronger CO binding than copper, and they typically produce primarily H2,31,32 either due to enhanced H adsorption or poisoning by CO (or both). This trend indicates that if SDCu has even stronger CO binding than OD-Cu, it could also be poisoned by CO. Indeed, SEIRA spectra provide evidence that CO is bound to the SD-Cu surface (Figure 4), and the lack of detected CO product suggests it is bound strongly to the surface. In order to provide more insight into the role of sub-surface sulfur, additional probes of the CO binding energy will be required, for example using TPD, CV studies, or DFT. Nonetheless, we expect a surface coverage of bound CO, where the CO poisoning does not completely block the surface due to only some remaining Cu sites with sub-surface sulfur. The presence of remaining sulfur atoms is further supported by the ICP results (Supplemental Figure S3), which show ~1-2 µmol sulfur in the electrolyte after CO2RR for SD-Cu (estimated as 100 µM from ICP results in 15 mL electrolyte), which is similar to the amount of sulfur expected based on integrating the measured reduction currents (9 µmol total, or ~1-5 µmol attributable to catalyst reduction only).




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Heyrovsky-type mechanism, it is possible that tuning the selectivity of SD-Cu further from H2 and towards HCOOH could be enabled by higher pH or higher CO2 concentrations, which merits further study. In addition to the presence of sulfur, SD-Cu has a rougher surface morphology compared to pristine copper. Surface roughness increases the available surface area of an electrocatalyst, leading to higher geometric current densities (as seen here in Figure 3), and can also influence catalyst selectivity.21 The current density can instead be normalized to the electroactive area using roughness factor estimates from capacitance measurements (Supplemental Figure S1), providing insight into the intrinsic surface activity. After normalizing by the roughness factor, the current density is more than two times lower for both SD-Cu samples than for pristine copper, indicating that it is not simply a roughness effect (Supplemental Figure S4). This lower intrinsic activity also agrees with our proposed mechanism where CO blocks some of the surface sites. In summary, CuS was found to reduce to SD-Cu in CO2RR conditions, with selectivity towards formate and hydrogen production. These findings provide mechanistic insight into an earth-abundant CO2RR catalyst,8,9 and we expect that the selectivity towards formate can be further enhanced on SD-Cu by increasing the concentration of dissolved CO2, for example by increasing the concentration of bicarbonate electrolyte. In addition to having a relatively high FE for formate, these results provide further insight into the activity of a related catalyst, OD-Cu, providing support for the hypothesis that subsurface oxygen remains in the OD-Cu, improving the activity over that of bare copper. Other copper complexes may also be able to further tune the binding energy of CO, potentially allowing the ability to further tune the product selectivity.



Experimental Methods Fabrication of electrodes. The working electrodes were prepared by sanding ~0.5 cm x 1.5 cm Cu foil (Alfa Aesar Puratronic, 0.05 mm thick, 99.9999%) and sonicating in water, followed by electropolishing in 85% H3PO4 (Aldrich, 85 wt% in H2O, 99.99% trace metals basis) for 1 min at 3V and 10 min at ~1.5-2V (based on the oxidation peak from a CV measurement) vs a C paper counter electrode. For the SD-Cu samples, CuS was subsequently deposited via pulsed electrodeposition using a literature procedure.24 Briefly, the layer was deposited by cycling from 0V Ag/AgCl x20 ms to -0.85V Ag/AgCl x10 ms for a total time of 30 min (“CuS-1”) or 2 h (“CuS-2”), in a solution of 4.0 mM thiourea (Sigma Aldrich, >99.999%) and 1.0 mM copper (II) sulfate pentahydrate (Aldrich, 99.999%) at pH 1.8 (adjusted with concentrated HCl). The active SD-Cu layer was formed in situ during the first ~15 minutes of electrolysis. All samples were contacted with Cu wire (Aldrich, 0.64 mm diameter, 99.995%). For in situ SEIRA measurement, the Pt working electrode was composed of a thin (ca. 50 nm) Pt film deposited on the Si plate (radius 22 mm, thickness 1 mm, Pier optics) by an electroless deposition method.33 First, the surface of the Si plate was given a hydrophilic treatment by contacting with 40% NH4F solution for a minute. Then Palladium seeds were deposited on the base plane with 1% HF–1 mM PdCl2 for 5 min at room temperature. After rinsing with water, platinum electroless deposition was carried out by contacting with a Pt plating solution at 50 °C for ca. 12 minutes. The Pt plating solution was prepared by mixing LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd), LECTROLESS Pt 100 reducing solution (0.6 mL), 28% NH3


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solution, and ultrapure water. Magnetron sputtering was used to prepare all Cu electrocatalysts. Copper was sputtered on the electroless deposited Pt layer at a deposition rate of ~1 Angstrom/s for a total thickness of 50 nm. Sulfide-derived Cu was prepared by in situ reduction of CuS layer deposited on a similarly prepared Cu SEIRAS sample by cycling from 0V Ag/AgCl x20 ms to -0.85V Ag/AgCl x10 ms for a total time of 10 min, in the aforementioned solution. Electrochemical experiments. Electroreduction of CO2 was performed in a 2compartment H-cell, with the Pt mesh counter electrode separated from the catholyte by a Nafion 117 membrane (Fuelcellstore). Both compartments contained 15 mL of CO2saturated 0.1M KHCO3 electrolyte, which was prepared by vigorously bubbling 0.1M KOH (Millipore Suprapur, >99.995%) with CO2 at least 20 min, and the pH was confirmed to be 6.8 before use. The headspace of the cathodic chamber was continuously flushed with CO2 into the sample loop of the gas chromatograph (GC), allowing for online gas-phase product analysis. Variations between GC runs were used for the error bars shown in Figure 3A. For each 3 h run, we averaged the GC products and current measurements from the last ~2.5 h of electrolysis in order for the catalyst to fully reduce and the headspace to fill with products. Liquid-phase products were analyzed after electrolysis using 1H-NMR based on a literature procedure,34 using DMSO and phenol as an internal standard (Supplemental Figure S6), and the experimental variation was used for the error bars shown in Figure 3A. A Biologic SP-200 potentiostat was used for all experiments. Potentials were measured against an Ag/AgCl reference electrode (LowProfile electrode from Pine Research) and converted to RHE using: E (vs RHE) = E (vs Ag/AgCl) + 0.21V + 0.0591V x pH



The potential was corrected for internal resistance by measuring the resistance using potentiostatic electrochemical impedance spectroscopy, and multiplying by the average current. This value was then subtracted from the potential to give an iR-corrected potential. The electroactive area was estimated using capacitance measurements by performing cyclic voltammetry at several scan rates (10, 20, 50, 100, and 200 mV/s) in a potential range without anodic or cathodic peaks (-0.4 to -0.25 V vs Ag/AgCl, or 0.21 to 0.36 V vs RHE). In situ spectroscopy. Surface-enhanced infrared absorption spectroscopy (SEIRAS) was performed in a spectro-electrochemical three-electrode cell with an Ag/AgCl reference electrode and a platinum wire counter electrode. The SEIRA spectra were obtained on a FT-IR Vertex 70 (Bruker) equipped with a MCT detector. The optical path was fully replaced with N2 gas. The measurements were done with 4 cm–1 resolution in the 3800–1400 cm–1 spectral range; 32 scans were averaged unless otherwise noted. The SEIRA spectra were recorded using a single reflection ATR accessory (Pike Vee-Max II, Pike Technologies) with a Si plate/ZnSe prism at incident angle of 70 degree. The linear sweep voltammetry (LSV) was conducted at room temperature by using HSV-110 (Hokuto Denko). The electrolyte solutions were prepared by mixing KOH (Sigma– Aldrich, >85 wt%) with KHCO3 (Sigma–Aldrich, 99.7 wt%) and ultrapure water. Before every experiment, Argon were bubbled through the electrolyte for 15 minutes in order to remove air from the solution. CO2 were bubbled through the electrolyte for 2 hours before every experiment to saturate the solution with CO2, and during the experiments the gas was kept flowing above the solution. After deoxygenation of the electrolyte solution by


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purging Ar, the prism surface was cleaned by cycling the potential between 0.05 and 0.90 V vs. RHE. All spectra are shown in the absorbance units defined as log(I0/I), where I0 and I represent the spectra at reference and sample potentials, respectively. The reference spectrum I0 was measured at 0.05 V in the blank KOH solution. Characterization of electrodes before and after CO2RR. Scanning electron microscopy (SEM) was performed on a Zeiss Ultra Plus Field Emission SEM. Photographs were taken using an iPhone 6. X-ray photoelectron spectra were obtained with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer and fitted with Gaussian peaks using Igor Pro software. Binding energies and full width at half maximum (FWHM) values, reported in Supplemental Table S1, are consistent with literature data.35 X-ray diffraction (XRD) was performed on a Rigaku SmartLab multipurpose diffractometer in grazing-incidence mode. Additionally, inductively-coupled plasma (ICP) was used to analyze the sulfur content of the electrolyte after electrolysis using an Agilent 5100 DVD Inductively Coupled Plasma-Optical Emission spectrometer in axial mode.

ACKNOWLEDGMENT The authors thank Eni S.p.A. for their financial support of this research.

Supporting Information. Additional figures (including capacitance estimates, energydispersive X-ray spectra, and inductively-coupled plasma of the electrolyte solution) are provided in the Supporting Information, which is available free of charge. The authors declare no competing financial interests.



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Sulfide-Derived Copper for Electrochemical Conversion of CO2 to

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