Enhanced Reduction of CO2 to CO over Cu–In Electrocatalysts

Aug 11, 2016 - Copper–indium catalysts have recently shown promising performance for the selective electrochemical reduction of CO2 to CO. In this w...
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Enhanced Reduction of CO2 to CO over Cu−In Electrocatalysts: Catalyst Evolution Is the Key Gastón O. Larrazábal,† Antonio J. Martín,† Sharon Mitchell,† Roland Hauert,‡ and Javier Pérez-Ramírez*,† †

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland ‡ EMPA, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, CH-8600 Dübendorf, Switzerland S Supporting Information *

ABSTRACT: Copper−indium catalysts have recently shown promising performance for the selective electrochemical reduction of CO2 to CO. In this work, we prepared Cu−In nanoalloys by the in situ reduction of CuInO2 and In2O3supported Cu nanoparticles and found that the structure of these nanoalloys evolves substantially over several electrocatalytic cycles, in parallel with an increase in the activity and selectivity for CO evolution. By combining electrochemical measurements with ex situ characterization techniques, such as XRD, STEM, elemental mapping, and XPS, we show that this behavior is caused by the segregation of copper and indium in these materials, resulting in the formation of a heterogeneous nanostructure of Cu-rich cores embedded within an In(OH)3 shelllike matrix. The evolved catalysts show high electrocatalytic performance at moderate overpotential (i.e., jCO > 1.5 mA cm−2 at −0.6 V vs RHE). We found that the removal of In(OH)3 from these heterogeneous nanostructures decreases the performance of the evolved catalysts, particularly in terms of the selectivity toward CO, which then recovers with the reappearance of the hydroxide following the re-equilibration of the material. On the other hand, an In(OH)3-supported Cu catalyst exhibits a current efficiency for CO comparable to that of the evolved nanoalloys without the need for an equilibration stage, indicating that In(OH)3 plays a crucial role in favoring the production of CO over Cu−In electrocatalysts. These findings shed light on the link between the architecture of these materials and their performance and underscore the potential of nonreducible hydroxides to act as promoters in CO2 reduction electrocatalysis. KEYWORDS: CO2 reduction, Cu−In electrocatalyst, copper, indium, indium hydroxide, core−shell structure

1. INTRODUCTION In the context of carbon capture and utilization (CCU) technologies, combining the electrochemical reduction of CO2 (eCO2RR) with renewable energy sources is an attractive approach for recycling carbon emissions.1−3 In particular, reducing CO2 to CO would provide a versatile compound for the production of liquid fuels and plastics by well-established processes in industry. However, a key challenge for the eCO2RR on its way toward technological viability is the development of highly active electrocatalysts capable of targeting a single CO2 reduction product (to minimize the complexity of downstream processing) and of inhibiting the competing hydrogen evolution reaction (HER) in aqueous media. Several electrocatalytic systems are known to favor the reduction of CO2 to CO, such as bulk Au and Ag electrodes,4−7 oxide-derived Au and Cu electrodes,8−13 some metal nanoparticles,14−19 and systems where ionic liquids are used as cocatalysts;20−22 however, their performance is still too low for practical use.23 Theoretical studies suggest that the activity for this reaction would be maximized by favoring the stabilization of the COOH intermediate over that of adsorbed CO.24−26 © 2016 American Chemical Society

However, the relationship between the CO and COOH binding energies, which are the key activity descriptors, is suboptimal in monometallic surfaces.24,27 The emergence of synergistic effects in multicomponent systems, such as bimetallic catalysts28−31 and supported metal nanoparticles,32−34 opens new perspectives for the development of advanced generations of eCO2RR catalysts. Copper has been widely studied due to its low cost and its unique ability to appreciably reduce CO2 to hydrocarbons and oxygenates at high overpotentials.35,36 At milder overpotentials, CO2 is preferably reduced to CO and HCOOH over polycrystalline Cu electrodes. However, the parasitic HER outcompetes the eCO2RR in such cases, resulting in poor overall selectivity. In contrast, electrodes prepared from the reduction of thick oxide films (oxide-derived Cu) show a much higher current efficiency (CE) for CO.9,10 In a recent study, Rasul et al. showed that electrodepositing indium over an oxide-derived Cu electrode further improved the selectivity for CO at moderate overpotentials (ca. 85% CE at −0.6 V vs Received: July 22, 2016 Published: August 11, 2016 6265

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ACS Catalysis RHE), a finding that was ascribed to the formation of Cu−In alloys on the surface and the inhibiting effect on In on the HER.37 In a follow-up publication, the in situ reduction of the CuInO2 delafossite was introduced as an alternative strategy for generating Cu−In alloy electrocatalysts that were also selective toward CO production.38 While these results clearly highlight the potential of the Cu−In system for this reaction, a deeper understanding of the structure of these materials is needed to derive property−activity relationships that can push forward the design of better catalysts. In a previous study, we evaluated a broad set of Ag−In electrocatalysts for the eCO2RR.33 Although the catalytic performance of bulk Ag−In alloys was rather poor, the synergistic interaction between Ag nanoparticles and a nonreducible In(OH)3 support increased significantly the selectivity for CO at moderate overpotential. In this contribution, we adopted a similar approach toward the Cu− In system by evaluating the interaction of Cu nanoparticles with In2O3 and In(OH)3 supports and revisiting the previously reported CuInO2 catalyst,38 with an emphasis on studying how the structure of these materials evolves over successive electrocatalytic cycles. This approach enabled us to uncover that the Cu−In nanoalloys generated upon the reduction of Cu/In2O3 and CuInO2 converge toward a common heterogeneous nanostructure in which Cu-rich regions are surrounded by an In(OH)3 shell-like matrix. The evolution of the catalysts toward this structure is accompanied by an increase in the selectivity and activity for CO evolution. In addition, we show that the presence of In(OH)3 in these evolved catalysts is crucial for their performance, suggesting that the hydroxide, and not only the metallic component, plays a key role in the eCO2RR mechanism over Cu−In catalysts.

catalyst was collected by centrifugation, washed three times with deionized water, and dried overnight at 80 °C in a vacuum oven. The catalysts are coded as Cu/X where X denotes the support material as specified previously (X = C, IH, IO). The CuInO2 delafossite catalyst was prepared as described by Jedidi et al.38 Briefly, an equimolar mixture of In2O3 (3.25 mmol, ABCR, 99.99%) and Na2CO3 (Merck, 99.9%) was pressed into a pellet and treated in a tube furnace at 1000 °C for 12 h (5 °C min−1) under flowing air to obtain the NaInO2 precursor. Then, an equimolar mixture of CuCl (3.5 mmol, Acros Organics, 99%) and the prepared NaInO2 powder was pressed into a pellet and treated in a tube furnace at 400 °C for 12 h (5 °C min−1) under flowing nitrogen. The resulting material was washed and centrifuged three times with deionized water to remove NaCl and then dried overnight at 80 °C in a vacuum oven to obtain the final CuInO2 product. 2.2. Electrode Preparation. Electrodes were prepared by airbrushing a catalyst ink on a gas diffusion layer (GDL). The ink was prepared by ultrasonically dispersing the catalyst (50 mg) for 15 min in a mixture of ultrapure water (5 cm3), 2propanol (5 cm3, Sigma-Aldrich, 99.8%), and Nafion solution (50 μL, 5 wt %, Sigma-Aldrich). This dispersion was then painted with an airbrush (Iwata Eclipse HP-SBS) on the microporous layer of a GDL (Sigracet 35BC, SGL Group, 12 cm2 cross-sectional area) which had been mounted on a hot plate at a temperature of 90 °C. A catalyst loading of ca. 1.5 mg cm−2 was typically achieved in this manner. Finally, the electrodes were produced by cutting the GDL into L-shaped pieces and attaching them through the protrusion to a flat silver contact soldered to a copper wire. The resulting joint and the wire were covered with plenty of PTFE tape to avoid contact with the electrolyte, yielding square electrodes with an area of ca. 2.25 cm2. The electrodes were photographed and their total geometric area determined with ImageJ software (Wayne Rasband, National Institutes of Health). Current densities reported in this work are referred to the geometric area of each electrode. In addition to the airbrushed catalyst electrodes, a bulk bimetallic Cu−In electrode was prepared by the reaction of a liquid indium film with a copper substrate followed by thermal annealing, as these two metals are known to readily react under such conditions to form intermetallic compounds.40 The detailed procedure for the preparation of this electrode is provided in the Supporting Information. 2.3. Characterization of Catalysts and Electrodes. Xray diffraction (XRD) patterns of the powder catalysts, as well as of the fresh and used electrodes, were obtained with a PANalytical X’Pert PRO-MPD diffractometer with Bragg− Brentano geometry using Ni-filtered Cu Kα radiation (λ = 0.1541 nm). The instrument was operated at 40 mA and 45 kV, and the patterns were recorded in the 10−70° 2θ range with an angular step size of 0.05° and a counting time of 180 s per step. After the electrolysis, the electrodes were recovered by carefully detaching them from the silver contact, rinsing copiously with ultrapure water, and drying in a vacuum desiccator at ca. 3 mbar for at least 30 min prior to the measurement. In this way, the electrodes could be employed in subsequent electrocatalytic cycles without a loss of comparability. X-ray photoelectron spectroscopy (XPS) analyses of fresh and used electrodes were carried out on a Physical Electronics (PHI) Quantum 2000 photoelectron spectrometer using monochromated Al Kα radiation (1486.6 eV) generated from an electron beam operated at 15 kV and 32.3 W and a hemispherical capacitor electron energy analyzer equipped with a channel plate and a

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. In(OH)3 and In2O3 were prepared as described previously.39 Briefly, a solution of ethanol (54 cm3, Sigma-Aldrich, 99.8%) and deionized water (18 cm3) containing InCl3 (3.6 g, Acros Organics, 99.995%) was rapidly mixed at room temperature with 72 cm3 of a second solution (3/1 ratio by volume) of ethanol and NH4OH (Sigma-Aldrich, 25%). The resulting suspension was placed immediately in a preheated water bath at 80 °C for 10 min with magnetic stirring. The precipitate was collected by centrifugation, washed three times with water, and dried overnight at 80 °C in a vacuum oven. The dried precursor powder was ground in a mortar and thermally treated for 3 h in static air at 185 °C to obtain the In(OH)3 support (denoted IH) and at 250 °C to obtain the In2O3 sample (denoted IO). Apart from the prepared In(OH)3 and In2O3 powders, carbon black (Vulcan XC-72, Carbon Corporation) was employed as a reference support. The supported catalysts with a target Cu loading of 20 wt % were prepared by the reduction of Cu2+ with NaBH4 in a suspension of the support with citrate as a stabilizer. In a typical synthesis, the support (205 mg) was added to a solution of CuCl2 (0.8 mmol, 20 cm3, Aldrich, 99.99%) to form a suspension and magnetically stirred for 30 min. Afterward, an aqueous solution of trisodium citrate (2.65 mmol, 20 cm3, Sigma-Aldrich, 99%) was added dropwise to the suspension, which was then placed in an ice bath. A freshly prepared NaBH4 solution (1.65 mmol, 25 cm3, SigmaAldrich, 96%) was added dropwise with magnetic stirring (500 rpm). The reduction resulted in the formation of a gray slurry, which was then gently stirred (200 rpm) for 30 min before the 6266

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of the cell flowed continuously through the sample loop of an SRI 8610C gas chromatograph (Multi-Gas #3 configuration) operating with Ar as carrier gas at a head pressure of 2.3 bar and equipped with HayeSep D and Molecular Sieve 13X packed columns. Gas samples were injected and analyzed 10 min after the start of the electrolysis and thereafter every 15 min. Following the electrolysis, a sample of the catholyte was taken for liquid-phase analysis. The concentration of formate was determined by high-performance liquid chromatography (HPLC) in a Merck LaChrom system equipped with a BioRad Aminex HPX-87H column heated at 35 °C and a refractive index detector (Hitachi Chromaster 5450) set at 30 °C. An aqueous solution of H2SO4 (0.005 M, flowing at 0.6 cm3 min−1) served as the eluent. Finally, the current efficiency for formate was calculated by relating the amount produced to the total charge passed during the electrolysis.

position-sensitive detector. The binding energy (BE) scale was calibrated with the Au 4f signal being at 84.0 ± 0.1 eV. Compensation of surface charging during spectra acquisition was obtained by simultaneous operation of an electron and an argon ion neutralizer. Since all C 1s signals were found to be in the range of 284.5 eV, no further charging compensation was applied. Depth profiles were recorded by employing alternating cycles of XPS analysis and sputtering with a focused 2 kV Ar+ beam rastered over an area of 4 mm2. The sputter yield was calibrated to be 8.7 nm min−1 on a 100 nm Ta2O5 reference film. The sputter rate on the tested samples was semiquantitatively estimated by adopting the reference value for indium tin oxide (ITO), which is etched 1.4 times faster than Ta2O5,41 yielding a sputter rate of ca. 12.5 nm min−1. Scanning transmission electron (STEM) micrographs in bright field (BF) and high-angle annular dark field (HAADF) modes, as well as energy-dispersive X-ray spectroscopy (EDX) element maps of the electrocatalysts, were acquired on a FEI Talos instrument operated at 200 kV. The fresh catalysts were dispersed as dry powders onto lacey-carbon-coated nickel grids. For the analysis of the used electrocatalysts, the catalyst layer was scratched off the electrode with a utility knife and the residue was dispersed in a few drops of ethanol by ultrasonication for 20 min. Thereafter, 10 μL of the dispersion was cast on the TEM grid. Standard scanning electron microscopy (SEM) was carried out in a FEI Quanta 200F instrument operated at 20 kV. The Cu content of the catalysts was determined following digestion with concentrated HNO3 by inductively coupled plasma optical emission spectrometry (ICP-OES) in a Horiba Ultima 2 instrument. Continuous-wave electron paramagnetic resonance (EPR) spectroscopy of the Cu/IO and Cu/IH catalysts was carried out with a Bruker Elexsys E500 spectrometer equipped with a Super High Q (SHQ) resonator at a microwave frequency of 9.88 GHz (X-band). 2.4. Electrochemical Tests. A custom gastight glass cell with two compartments separated by a Nafion 212 membrane (Alfa Aesar, 0.05 mm thickness) was employed for all electrochemical experiments. A 0.1 M KHCO3 solution (Sigma-Aldrich, 99.95% trace metals basis) prepared with 18.2 MΩ cm ultrapure water was used as the electrolyte. Each compartment contained 45 cm3 of the electrolyte that was saturated with CO2 (Messer, purity 4.8) for at least 20 min prior to the start of the electrolysis, with a resulting pH of 6.75, and CO2 was continuously bubbled into the catholyte during the electrolysis at a flow rate of 20 cm3 min−1. All electrochemical measurements were carried out at room temperature with an Autolab PGSTAT302N potentiostat, using a platinum wire as the counter electrode and a Ag/ AgCl reference electrode (3 M NaCl, Model RE-1B, ALS). All potentials reported in this work are referenced to the reversible hydrogen electrode (RHE) scale. The potentiostatic electrolyses had a duration of 2 h and were carried out with the iR compensation function set at 85% of the uncompensated resistance Ru, which was determined before the start of the electrolysis and updated every 7.5 min by electrochemical impedance spectroscopy measurements at the electrolysis potential. The recorded potentials were converted to the RHE scale following the electrolysis after manually correcting for the remaining uncorrected Ru in a manner similar to that of Kuhl et al.36 Following this correction, the applied potentials were within 10 mV of the target potential of the electrolysis. 2.5. Product Analysis. For the analysis of the gas-phase reaction products, the outlet gas of the cathodic compartment

3. RESULTS AND DISCUSSION 3.1. Characterization of the Supports and Catalysts. In2O3 and In(OH)3 supports were prepared by precipitation of In3+ with NH4OH in an ethanolic medium, followed by calcination.39 The XRD pattern indicates that the prepared In(OH)3 powder is mostly amorphous, although the reflection from the (200) plane at 22.6° 2θ is very prominent (Figure 1),

Figure 1. XRD patterns of CuInO2 and the supported Cu catalysts (blue) and of the corresponding supports (gray). The supported catalysts show the presence of (◆) Cu2O and (◇) metallic Cu. In the delafossite catalyst (top), the unmarked reflections correspond to CuInO2, whereas (★) NaInO2 can be identified as an impurity.

in accordance with other forms of In(OH)3 reported in the literature, such as single-crystalline microcubes,42 nanospheres and nanorods,43 and amorphous thin films.44 Consequently, this peak is a valid fingerprint of the presence of In(OH)3. Copper was deposited on carbon black (C), In2O3 (IO), and In(OH)3 (IH) by reducing Cu2+ with NaBH4 in an aqueous slurry of the support. ICP-OES analysis of the thus prepared catalysts showed a copper loading of 22.7 wt % in Cu/IO, 16.3 wt % in Cu/IH, and 14.2 wt % in Cu/C. Therefore, the supported catalysts have a lower copper content than does CuInO2 (30.2 wt % Cu). The XRD patterns show that copper is mostly present as Cu2O in the supported catalysts, although reflections from metallic Cu are also evidenced in Cu/C 6267

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was expected from previous reports,33,38 the fresh Cu/IO and CuInO2 electrodes underwent an initial potentiostatic electrochemical reduction for 2 h (herein referred to as the prereduction step) in CO2-saturated 0.1 M KHCO3 (at −0.6 V for Cu/IO and −0.75 V for CuInO2, mimicking the conditions employed by Jedidi et al.).38 After this prereduction step, the CuInO2 and Cu/IO electrodes were used in successive CO2 reduction electrolyses at −0.6 V. The electrocatalytic performance in each run and the cyclic voltammograms (CV) of the Cu/IO and CuInO2 electrodes at different stages are shown in Figure 3. The CVs of the fresh and reduced electrodes show that the prereduction step caused a large change in the nature of their electrochemically active surfaces, as would be expected from the reduction of the materials (Figure 3b, left panel). The CVs of the reduced electrodes show the redox features around −0.3 V typically ascribed to the formation and reduction of a thin indium oxide film.47 Interestingly, in the case of Cu/IO it also shows the start of an oxidation wave at 0.6 V (with its corresponding reduction feature), which has been attributed to the formation of Cu2O.48 However, this feature is not present in the reduced CuInO2 electrode, suggesting that copper sites might be electrochemically more accessible in reduced Cu/IO than in reduced CuInO2. In terms of the catalytic results, the performances of Cu/IO and CuInO2 in the first electrolytic run are noticeably different, with Cu/IO showing a considerably higher current efficiency for CO but a lower total current density than CuInO2 (ca. 40% and −1.3 mA cm−2, in comparison to ca. 20% and −2.3 mA cm−2). Meanwhile, the benchmark Cu/C catalyst mostly favored hydrogen evolution at −0.6 V (i.e., CECO ca. 5%) with the high total current density (ca. −3 mA cm−2). The results from the initial runs over the reduced Cu/IO and CuInO2 electrodes confirmed that the interaction between copper and indium phases indeed leads to a suppression of hydrogen evolution in comparison to the Cu/C catalyst and to increased CO selectivity at moderate overpotential. However, it was unexpected that the current efficiency for CO over the CuInO2 electrode in the first run was significantly lower than that reported by Jedidi et al. with the same material and conditions.38 Subsequently, we found a gradual shift of the product distribution toward CO and improved activity over the Cu/IO and CuInO2 electrodes over successive electrolytic runs, so that the current efficiencies for the different products over both electrodes practically converge after several cycles. Following this equilibration process, the average current efficiency for CO over both electrodes (ca. 55%) matched the reported value over CuInO2 at the same potential.38 At this point, the Cu/IO and CuInO2 electrodes showed a practically identical mass activity for CO of ca. 3.6 mA mgCu−1. This convergence of the electrocatalytic behavior manifests itself not only in the product distribution but also in the cyclic voltammograms, as both evolved electrodes have essentially the same redox response. It is intriguing that these catalysts, which were initially very different in terms of their crystalline structures and the distribution of Cu and In, display a similar electrocatalytic behavior only after prolonged reaction times. Analysis of the XRD patterns of the Cu/IO and CuInO2 electrodes (Figure 4; full patterns and assignments are shown in Figure S2 in the Supporting Information) show that exposure of the catalysts to cathodic potentials results in the generation of Cu−In intermetallic compounds (IMCs) due to the formation and subsequent reaction of metallic Cu and In. However, CuInO2 appears to be significantly more resistant to

(Figure 1). EPR analysis of the Cu/IO and Cu/IH catalysts shows a small fraction of Cu(II) in the samples (Figure S1 in the Supporting Information). Although these observations reflect the difficulty of synthesizing supported Cu metal nanoparticles in aqueous media,45 it is expected that the oxidic copper in the fresh catalysts will be fully reduced to its metallic form at the potentials at which CO2 reduction takes place.10,46 The XRD pattern of the delafossite catalyst evidences the formation of the CuInO2 phase along with a small impurity of unreacted NaInO2 precursor, consistent with previous reports (Figure 1).38 The particle morphology and the distribution of copper and indium in the synthesized catalysts were examined by STEM coupled with EDX (Figure 2). The CuInO2 catalyst

Figure 2. HAADF-STEM micrographs and corresponding elemental maps of the fresh electrocatalysts.

is comprised by well-defined rectangular crystallites in which Cu and In are homogeneously distributed, consistent with the distribution of Cu+ and In3+ cations in the expected delafossite lattice. The elemental mapping also indicates the presence of small amounts of unreacted copper precursor from the solidstate synthesis. In contrast to CuInO2, examination of Cu/IH and Cu/IO reveals that these catalysts have a heterogeneous structure at the nanometer scale, with the support having a spongelike texture. Chemical mapping shows that copper is concentrated in large particles of 50−100 nm on the surface of the support, leading to a poorer dispersion of Cu and In in comparison to that in the CuInO2 mixed oxide. Taken together, these observations indicate marked structural differences between CuInO2 and the supported Cu catalysts prior to reaction. 3.2. Electrochemical Behavior and Structural Evolution of the Cu/IO and CuInO2 Catalysts. Since the reduction of CuInO2 and of In2O3 under eCO2RR conditions 6268

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Figure 3. (a) Current efficiency (bars), total current density (circles), and partial current density for CO (diamonds) in successive CO2 reduction electrolyses at −0.6 V vs RHE in CO2-saturated 0.1 M KHCO3 (2 h duration each) over Cu/IO (top) and CuInO2 (bottom). (b) Cyclic voltammograms (scan rate 20 mV s−1) showing the evolution of the electrochemical behavior of the catalysts. The scaled CVs of the fresh catalysts (i.e., prior to the reduction of CuInO2 and of the In2O3 support) are shown in the corresponding first panel (dashed lines).

of 40−45° 2θ, it appears that the CuIn phase is more abundant in the reduced Cu/IO catalyst, whereas the formation of Cu2In appears to be favored when CuInO2 is the starting material. This is consistent with the fact that CuInO2 has a higher Cu content than Cu/IO. It is important to remark that the assignment of the mixed phases to Cu2In and CuIn is not completely unequivocal, due to the abundance of Cu−In alloys reported in the literature (e.g., Cu7In3, Cu11In9, Cu16In9) and the complexity of the reference patterns, although the presence of two distinct IMCs in the reduced electrodes, one of which is comparatively richer in Cu, is fairly certain. Interestingly, the evolution of the XRD patterns shows that the improvement of the selectivity for CO over both electrodes, particularly in the later runs (i.e., CECO > 50%), is associated not only with the formation of CuIn and Cu2In but also with the appearance of In(OH)3, as evidenced from its characteristic reflection at 22.6° 2θ coming from the (200) plane. Since this peak was not present in either the Cu/IO and CuInO2 powders or in the fresh electrodes, it appears that In(OH)3 was gradually generated over the course of the experiments. Aiming to shed light on the role of the Cu−In IMCs and of In(OH)3 in the evolved Cu/IO and CuInO2 electrodes, we prepared a bimetallic bulk Cu−In electrode by the reaction of an In film with a Cu substrate, resulting in the formation of the Cu11In9 IMC (Figure S3 in the Supporting Information). However, the electrocatalytic performance of this electrode was poor in terms of both selectivity and activity throughout the six electrolyses at −0.6 V (Figure S3), with the electrode greatly favoring H2 evolution (i.e., CECO < 10%) with very low total current densities (typically less than −0.2 mA cm−2). Moreover, the XRD analysis showed that, unlike the case for the Cu/IO and CuInO2 catalysts, repeated exposures to CO2 reduction conditions did not result in major compositional changes or in the appearance of In(OH)3. The large differences between the bulk Cu−In and the Cu/IO and CuInO2 electrodes suggest that the evolving behavior of the catalysts and the appearance of In(OH)3, as well as their electrocatalytic performance, are not solely a consequence of the formation of Cu−In alloys upon

Figure 4. XRD patterns of the (a) Cu/IO and (b) CuInO2 electrodes showing the evolution of the catalysts following each electrocatalytic cycle. The reflections originally present in the carbon GDL are labeled with “c”. The patterns are normalized to the height of the graphitic peak at 26.6° 2θ, which retained a similar intensity throughout the runs. The inset shows in detail the region around the (200) reflection of In(OH)3, evidencing the gradual generation of the hydroxide following several CO2 reduction electrolyses.

reduction than the In2O3 support in Cu/IO, whose main reflection is absent in the pattern of the reduced electrode. In fact, the reduction of the Cu/IO catalyst results in the formation not only of Cu−In mixed phases but also of pure indium, which then disappears after subsequent CO2 reduction electrolyses. This explains the comparatively high fraction of HCOO− obtained with the Cu/IO electrode during the early runs, as indium electrodes are known to favor the production of formate over that of CO.49 On comparison of the intensity of the reflections from the Cu−In IMCs, particularly in the range 6269

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ACS Catalysis exposure to cathodic potentials, but that the structure of the material (nano vs bulk) also plays an important role. The XPS spectra of Cu/IO and CuInO2 electrodes after 1.5 min of sputtering with Ar+ (resulting in the removal of ca. 20 nm of material) are consistent with the reduction of the In2O3 support and of CuInO2 in the prereduction step, as evidenced by the appearance of a large contribution of metallic In at ca. 444 eV in the reduced electrodes (Figure 5). This result is in

Figure 6. SEM micrographs of the fresh and evolved CuInO2 catalysts, evidencing the surface roughening that leads to a “cauliflower-like” microstructure.

elemental mapping shows a clear segregation of copper and indium in the evolved catalysts, a finding that is particularly unexpected in CuInO2, considering the homogeneous nature of the starting material. More specifically, Cu-rich regions seem to be surrounded by an In-rich shell-like structure which is evidenced as a halo in the mapping. The EDX profile across a Cu-rich region in the evolved Cu/IO catalyst confirms the core−shell-like distribution of Cu and In (Figure S5 in the Supporting Information). Oxygen appears to be distributed around copper in a way similar to that for indium, strongly suggesting that the In-rich regions consist of the previously identified In(OH)3 phase. Taken together, these observations confirm that the Cu/IO and CuInO2 catalysts converge toward a common core−shell-like structure over the course of several electrocatalytic cycles. The nanostructure observed in the evolved catalysts is likely the result of the progressive segregation of Cu and In, although the mechanism for this is not immediately clear. It has been reported that some intermetallic compounds favor the enrichment of the less noble element on the surface,51 and we have previously observed a surface enrichment of indium in bulk Ag−In alloy electrodes.33 The XPS depth profiles of the reduced Cu/IO and CuInO2 electrodes suggest that (on average) a greater amount of indium is exposed on the surface of the catalyst particles (Figure 8), although the elemental mapping of individual particles of the reduced CuInO2 electrode shows some Cu-rich regions on the surface (Figure S6 in the Supporting Information). Nevertheless, it is reasonable to postulate that, following the in situ reduction of Cu/IO and CuInO2 that generates Cu−In nanoalloys, the surface of the particles becomes enriched in indium due to its lower nobility relative to copper. Consequently, it is likely that indium exposed on the surface becomes at least partially oxidized at the end of each electrolysis, possibly upon return to open circuit potential, during the anodic sweeps of the cyclic voltammograms or due to contact with air after the electrode is removed from the cell. It has been previously reported that In(OH)3 (E° = −0.177 V vs RHE at pH 6.8) is stable under the highly cathodic potentials of the eCO2RR, in contrast to In2O3.33,49 Therefore, In(OH)3 likely acts as an indium sink, so that any metallic In that is oxidized to the hydroxide after the run becomes unavailable for the re-formation of metallic Cu−In phases in subsequent runs. Formation of the hydroxide is likely, given the aqueous environment of the electrolyses. In turn, this would force the segregation of more indium from the IMCs (to replenish the surface enrichment), making a new “batch” of metallic In susceptible to oxidation following the run. The repetition of such cycles of reduction, surface enrichment, and

Figure 5. XPS spectra around the In 3d region of Ar+-sputtered Cu/ IO and CuInO2 electrodes at different stages. The dashed vertical lines indicate the binding energies of In3+ and In0.

agreement with previous reports on pure indium cathodes.49 Rather surprisingly, it appears that the contribution from metallic In is mostly absent in the XPS spectrum of the evolved electrodes, indicating that the equilibration of the catalysts leads to the generation of In3+ species at the expense of metallic In. This observation is consistent with the appearance of In(OH)3 in the XRD patterns of the electrodes as more electrolysis runs were performed, as well as with the gradual decrease of the intensity of the peaks coming from CuIn and Cu2In. In the case of copper, it is difficult to distinguish Cu2O from metallic Cu in the Cu 2p region of the XPS spectrum,50 but the peak shape in the Cu LMM Auger spectra (taken after 1.5 min of sputtering) confirms the expected formation of Cu0 following the prereduction step, which persists despite the evolution of the electrodes (Figure S4 in the Supporting Information). In summary, the XRD and XPS results indicate the presence of two different Cu−In phases along In(OH)3 in the evolved Cu/ IO and CuInO2 electrodes. The evolution of the Cu/IO and CuInO2 catalysts is even reflected in their visual appearance. For instance, whereas the fresh CuInO2 electrode is brown, the evolved electrode is dark gray with a purplish tinge. SEM analysis of the fresh and evolved CuInO2 electrodes showed that the smooth surface of the original crystallites is greatly roughened, developing a “cauliflower-like” texture (Figure 6). This surface roughening is likely a factor in the large increase of the current density observed through the equilibration stage of the electrode. In addition, in order to gain insights into the nanostructural changes undergone by the Cu/IO and CuInO2 electrodes after successive electrolytic runs, we scratched off the catalytic layer from the evolved electrodes and examined the thus-obtained powder with transmission electron microscopy and EDX (Figure 7). This analysis revealed drastic changes in both materials in comparison to the pristine catalysts. For instance, the STEM imaging of the evolved CuInO2 catalyst showed that the large well-defined crystals observed in the fresh delafossite were completely transformed into a much finer structure comprised of agglomerates of small particles, which is also present in the evolved Cu/IO catalyst. Interestingly, the 6270

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Figure 7. BF- and HAADF-STEM micrographs and elemental mapping of copper, indium, and oxygen in the evolved Cu/IO (top row) and CuInO2 (bottom row) electrocatalysts, showing the core−shell-like structure of In(OH)3 around Cu-rich regions.

Figure 8. XPS depth profile analysis of reduced Cu/IO and CuInO2 electrodes.

oxidation over several electrolytic runs is a possible mechanism for the segregation of Cu and In in the nanoalloys and the convergent evolution of the Cu/IO and CuInO2 catalysts toward the observed heterogeneous nanostructure. 3.3. Unveiling the Role of In(OH)3. The appearance of In(OH)3 in this equilibration process raises the question of to what extent this component is crucial for the high selectivity for CO observed over the evolved catalysts. To evaluate this, we removed In(OH)3 from evolved Cu/IO and CuInO2 electrodes by etching them briefly in 1 M H2SO4, followed by dipping in 0.1 M KHCO3 and copious rinsing with ultrapure water. The XRD patterns of the etched electrodes show the disappearance of the (200) reflection from In(OH)3 but the preservation of the peaks from the Cu2In and CuIn alloys (Figure 9b), confirming that the acid treatment selectively removed In(OH)3 from the electrodes without significantly corroding the metallic component. Additionally, EDX mapping of the acid-treated sample (obtained from an etched Cu/IO electrode) shows the removal of the In(OH)3 shell-like structure from the catalyst and the re-exposure of Cu-rich regions to the surface (Figure S7 in the Supporting

Figure 9. (a) Current efficiency for H2 and CO and total current density in successive CO2 reduction electrolyses following the acid etching of evolved Cu/IO and CuInO2 electrodes. The horizontal dashed lines indicate the selectivity for CO attained prior to the acid treatment. (b) XRD patterns of the evolved, etched, and reequilibrated Cu/IO and CuInO2 electrodes. The insets on the left show the region around the (200) reflection of In(OH)3 (marked with ▼).

Information). The current efficiency for CO over Cu/IO and CuInO2 electrodes after the acid etching is significantly lower than that over the corresponding evolved electrodes (Figure 9a), suggesting that this selectivity, which is similar to that of the original electrodes at an “intermediate” stage (e.g., runs 2 and 3 in Figure 3a), is characteristic of the “naked” Cu−In nanoalloys in the absence of In(OH)3. Interestingly, the equilibration process in the etched electrodes led to a steady 6271

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ACS Catalysis increase of the CECO up to pre-etching levels, which again matched the reappearance of In(OH)3 in the diffractogram. The electrocatalytic performance of the Cu/IH electrode provides further insight into the role of In(OH)3. The selectivity for CO over Cu/IH was similar to that of the evolved Cu/IO and CuInO2 catalysts over several runs without requiring an equilibration phase (Figure 10a), and the XRD

Figure 11. Current efficiency (bars) and partial current density for CO (diamonds) in 12 successive CO2 reduction electrolyses at −0.6 V vs RHE over a CuInO2 electrode, evidencing the evolution stage (runs 1−6) followed by a plateau in the electrocatalytic performance (runs 7−12).

Figure 10. (a) Current efficiency (bars), total current density (circles), and partial current density for CO (diamonds) in successive CO2 reduction electrolyses at −0.6 V vs RHE over Cu/IH. (b) XRD patterns of the fresh and used Cu/IH electrode.

run 6 to run 12 (Figure S8 in the Supporting Information), indicating that the catalyst attains a stable configuration following the initial equilibration stage. These results indicate that the evolved catalysts are not only more active toward CO evolution than Cu/IH, but also much more stable to repeated exposures to CO2 reduction conditions and confirm the crucial role of In(OH)3. Although more detailed studies would be necessary to clarify this behavior, it is possible that the equilibration process in the CuInO2 and Cu/IO catalysts results in the generation of a larger number of active sites in comparison to directly depositing Cu on In(OH)3 or that electronic conduction in the evolved catalysts is enhanced due to the generation of metallic pathways that bypass the poorly conductive In(OH)3, leading to higher activity. Moreover, the nanostructure of the evolved catalysts appears to be crucial for their stability. In this context, the interplay between composition and structure observed in CuInO2 and Cu/IO after the equilibration stage opens new avenues for the rational optimization of Cu−In catalysts for the eCO2RR.

patterns show that no Cu−In mixed phases (and only metallic Cu) are formed even after many electrolyses (Figure 10b). These results confirm that In(OH)3 indeed plays a crucial role in the selectivity of these multicomponent catalysts, possibly because the reduction of CO2 to CO is favored in the contact points of In(OH)3 with Cu (in Cu/IH) or with CuIn and Cu2In phases (in the evolved Cu/IO and CuInO2 catalysts). Nevertheless, the current density over Cu/IH is comparatively low and shows a decreasing trend with each electrolytic run, indicating that the Cu/IH catalyst is rather unstable upon repeated exposures to CO2 reduction conditions. For instance, the partial current density for CO in run 6 is only about half of the maximum reached in the early runs. For comparative purposes we evaluated the stability of the CuInO2 catalyst over 12 consecutive runs, reaching a cumulative electrolysis time of 24 h (Figure 11). In line with the results previously shown in Figure 3a, the first six runs correspond to an equilibration phase in which the increase of the current efficiency for CO is accompanied by a 5-fold enhancement of the partial current density for this product. Following this stage, the selectivity toward CO of the evolved catalyst decreases slightly, stabilizing at ca. 45% in the subsequent runs (runs 6−12). However, in contrast to the case for Cu/IH, the evolved catalyst maintains a high partial current density for CO (in the range of 1.5−2 mA cm−2). Additionally, neither the CV nor the diffractogram of the evolved CuInO2 catalyst shows substantial changes from

4. CONCLUSIONS We have studied the evolution of the electrochemical response and the structural changes over prolonged reaction times of Cu−In catalysts for the reduction of CO2. The reduction of In2O3-supported Cu nanoparticles (Cu/IO) and of CuInO2 resulted in the formation of Cu−In nanoalloys which initially showed comparatively poor performance for CO2 reduction. However, we found that the activity and selectivity for CO evolution over the Cu/IO and CuInO2 electrodes gradually increased upon successive electrocatalytic cycles, eventually converging to the same values despite the marked structural differences in the fresh catalysts. This evolution was associated with the gradual segregation of Cu and In and the formation of a core−shell-like nanostructure in which Cu-rich regions are surrounded by In(OH)3. Additional experiments with acidetched Cu/IO and CuInO2 electrodes (after the equilibration) and In(OH)3-supported Cu catalysts showed that In(OH)3 plays a key role in ensuring a high selectivity toward CO, while the formation of the heterogeneous structure from the Cu−In nanoalloys appears to be necessary for producing highly active and stable electrocatalysts. We remark that further studies employing model catalysts with more controlled interfaces would help elucidate the identity of the active sites and the exact nature of the synergistic effect between In(OH)3 and the metallic phases. Nevertheless, the structural insights from this study provide valuable guidelines for the development of 6272

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multicomponent electrocatalysts for CO2 reduction and highlight the value of adopting a comprehensive experimental approach for studying the evolution of eCO2RR catalysts beyond the short time scales frequently found in the literature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02067. Calculation of the current efficiency of the gas and liquid products, synthesis procedure of the bulk Cu−In electrode, EPR spectra of In(OH)3 and the fresh Cu/ IO and Cu/IH catalysts, electrocatalytic performance and XRD patterns of the bulk Cu−In electrode, X-ray diffractograms of the Cu/IO and CuInO2 electrodes after each electrolytic run with reference patterns of the assigned phases, Cu Auger spectra of the Cu/IO and CuInO2 electrodes at different stages, elemental profile across a section of the evolved Cu/IO catalyst, elemental map of particles from reduced CuInO2 electrode, STEM micrograph and elemental mapping of the evolved Cu/ IO catalyst after the etching, and CVs and XRD patterns of a CuInO2 electrode at different stages during extended testing (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J.P.-R.: tel, +41 44 633 7120; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by ETH Zurich (Research Grant ETH-01 14-1). The authors gratefully acknowledge Dr. Takuya Segawa for the EPR measurements and the Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich for access to its facilities.

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