Tuning the Catalytic Activity and Selectivity of Cu for CO2

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Tuning the catalytic activity and selectivity of Cu for the CO2 electro reduction in presence of halides Ana Sofia Varela, Wen Ju, Tobias Reier, and Peter Strasser ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02550 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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Tuning the catalytic activity and selectivity of Cu for the CO2 electro reduction in presence of halides Ana Sofia Varela ‡, Wen Ju ‡, Tobias Reier and Peter Strasser* The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin 10623, Germany

ABSTRACT: In the present study we demonstrate that the activity and selectivity of copper during the CO2 electrochemical reduction can be tuned by simply adding halides to the electrolyte. Comparing the production rate and Faradaic selectivity of the major products as a function the working potential in the presence of Cl-, Br- and I-,we show the activity and selectivity of Cu depends on the concentration and nature of the added halide. We find that the addition Cl- and Br- results in an increased CO selectivity. On the contrary, in the presence of Ithe selectivity towards CO drops down and instead methane formation isenhanced up to 6 times compared with the halide free electrolyte.Even though Br- and I- can induce morphology changes of the surface, the modification in the catalytic performance of Cu is mainly attributed to halides adsorption on the Cu surface. We hypothesizes that the adsorption of halides alters the catalytic performance of Cu by increasing the negative charge on the surface according to the following

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order: Cl-< Br-< I-. In the case of adsorbed I-, the induced negative charge has a remarkably positive effect favoring the protonation of CO. These results present an easy way to enhance CH4 production during the CO2RR on Cu. Furthermore understanding this effect can contribute to the design of new and more efficient catalyst.

KEYWORDS: CO2 reduction, electrocatalysis, electrolyte effect, halides, copper, iodide


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1. Introduction Electrocatalyticreactions hold promise as a sustainable route for energy conversion. In particular, the CO2 reduction reaction (CO2RR) is an attractive alternative for transforming the excess of electricity from renewable energies into carbon-based fuels or chemicals.1 This process would allow the direct conversion of CO2 to valuable compounds, which are conventionally obtained from fossil fuels, in a sustainable manner2 and thus would close the global carbon cycle. Given the promising advantages of this process, the CO2RR has been widely studied in recent years.3 Many of these studies have focused on the use of metallic electrodes as heterogeneous catalysts showing that the nature of the metal plays a crucial role on the selectivity of the reaction. While some metals, such as Ni and Pt have been shown to produce only minute amounts of CH4 and CH3OH,4 Cu has a unique capability of producing hydrocarbons, mainly CH4 and C2H4, in relevant amounts.5 These hydrocarbons, however, are produced at high over potentials resulting in prohibitively high efficiencylosses. Furthermore CO2RR on copper is not a selective process and it results in a variety of products such as: CO2 + 2H+ + 2e-→ CO + H2O

(1)

CO2 + 2H+ + 2e-→ HCOOH

(2)

CO2 + 8H+ + 8e- → CH4 + 2H2O

(3)

2CO2 + 12H+ + 12e- → C2H4 + 4H2O

(4)

In addition, at the potentials at which CO2 is reduced, hydrogen evolution reaction (HER) also takes place as a competing process, reducing the selectivity of the CO2RR 2H+ + 2e- → H2

(5)


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In recent years, many research groups have focus on the CO2RR aiming to understand the fundamental factors that control the selectivity of this process. This knowledge is crucial to optimize the catalytic conditions and design new, efficient and stable catalyst. Different studies have repeatedly shown that the selectivity of CO2RR greatly depends on the detailed nature and geometry of the metallic surface. Work on polycrystalline Cu has shown that the C2H4/CH4 is strongly dependent on the surface pretreatment. Rough surfaces and oxide derived Cu favored the formation of C2 products while methane is preferred in smooth polished Cu.3b,c,g,6In addition, work on the CO2electroreduction on size-selected nanoparticles particles has revealeda characteristic particle size effect of the hydrocarbon selectivities, where smaller Cu particles showed a pronounced selectivity for hydrogen and CO rather than for hydrocarbons.7 Hori and coworkers showed that the liquidelectrolyte also plays a key role on selectivity of the reaction.8 For example when working with KH2PO4/K2HPO4 the major product was hydrogen with a Faradaic efficiency higher than 70%. On the other hand, in other electrolytes such as KClO4 the Faradaic efficiency towards hydrogen drops to 10%. Furthermore, the ratio between methane and ethylene was also affected. These results could partially been explain by a local pH effect.9Under reaction conditions, the pH near the interface is expected to rise due to proton consumption, this effect is more pronounced in non-buffered solutions such as KClO4 than in buffered solutions. The high local pH affects the product selectivity of the chemical reaction twofold: on the one hand, it inhibits the HER due to a low proton concentration. On the other hand, it has been established that the ratio between CH4 and C2H4 is affect by the local pH.Evidence has mounted that due to a decoupled proton-electron transfer step, ethylene results asthe predominant hydrocarbon in basic electrolytes, while CH4production is favored on neutral


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and acid conditions.10Thus, electrolytes with low buffer capacity typically exhibit high ethylene selectivity. The changes of the catalytic activity of copper in different electrolytes havealso been attributed to thedifferent nature of the solvatizedions in solution. Hori and co-workers showed that by changing the cationic spices (Li+, Na+, K+ and Cs+) in bicarbonate solutions they could control the selectivity of Cu during the CO2RR.11 Namely, they observed that methane formation was preferred in the presence of Na+, while selectivity towards ethylene was enhanced when the electrolyte contained Cs+. This observation wasattributed to the difference in specific adsorption of the different cations. It was argued that the smaller cations hada larger hydration number and thus adsorbed less on to the electrode surface. On the contrary, the large cations hada higher specific adsorption which causeda higher local pH inhibiting methane formation. Interestingly, CO2RR is also affected by the anionic species. Salazar-Villalpandoand co-workers observed a positive effect on the reduction currents of Cu by the addition of halides. This result suggested that the presence of halides in the solution can facilitate the reduction of CO2 on copper.12They attributed the enhanced reduction current to a covalent interaction Cu-halide that can facilitate the charge transfer between CO2/CO and the Cu electrode enhancing the CO2 reduction rate. This conclusion, however, was only made based on current response and they did not include any actual product selectivity measurements. As the CO2RR involves a variety of process including the hydrogen evolution reaction, it is crucial to quantify the reaction products to determine the selectivity of the reaction in order to determine if the current response is due to CO2RR or to HER.

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The effect of halides on selectivity has been studied for Cu-halide in

weakly acid medic media. Ogura and co-workers performed CO2RR studies on a Cu-mesh


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modified by Cu(I) halides and observed difference in selectivity depending on the added halide. In particular CuBr seemed to favor ethylene formation.14 Recent Density Functional Theory calculations have predicted Cl-, Br- and I-toadsorb on Cu at potentials in which CO2 reduction is usually carried out and therefore these anions could affect the catalytic performance of Cu.15However, an experimental verification of these computational predictions has been missing to date. This is why, in the present study, we seek to experimentally determine the role that adsorbed halides have on the CO2RR. To this end,we have studied this process in the presence of halidesions at varying concentrations (Cl-, Br-, I-). Linear sweep voltammetry and bulk electrolysis experiments have demonstrated that halides have a positive effect on catalytic performance of Cu during CO2RR. While Cl- and Br- enhance CO selectivity, the presence of Ihas a remarkable, previously overlooked effect on methane formation. At -0.9 VRHE the production rate towards methane in the presence of I- is 5 times higher than in halide free electrolyte which translates in an increase of methane selectivity of 3 times. We attribute this effect to the transfer of negative charge from the I- to the Cu surfaces, which makes it more reactive towards CO protonation. Alternatively this can be attributed to a change in the biding energy of the reaction intermediates, which also alters the catalytic activity of Cu. 13 2. Experimental section 2.1 Electrochemical measurements Electrochemical measurements were carried out in a custom made two compartment cell, in which the working electrode was separated from the counter electrode by a glass frit. This is particularly important when working in the presence of halides given than in the counter electrode the oxygen evolution reaction (OER) is accompanied by the halide oxidation forming


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X2 which can react with some of the products from the CO2RR. Namely, ethylene can undergo a halogenation reaction and consequently it would be underestimated during the product analysis. The glassware was cleaned in a “nochromix” bath and afterwards in concentrated HNO3 for 1 h, respectively, rinsed and sonicated with ultra-pure water several times, and dried at T = 60 °C in a drying cabinet. The working compartment was filled with 39mL of CO2 (Air liquid 4.5) saturated electrolyte. For this study we have worked with a constant concentration of 0.1 M KHCO3(Sigma-Aldrich ≥99.95) to avoid differences in local pH.9bWe have used a halide free electrolyte as reference and test the effect of adding different concentration of potassium halide: KCl, KBr and KI (Alfa Aesar, Puratronic®) Before and during the electrochemical reaction the cell was purged continuously with CO2 (30mL/min) from the bottom of the cell and the gas atmosphere was controlled with an in-situ mass spectrometer (OmniStar GSD 301c, Pfeiffer). A platinum mesh 100 (Sigma-Aldrich 99.9%) was used as counter electrode (CE) and a leak-free Ag/AgCl electrode as reference electrode (Hugo Sachs Elektronik Harvard apparatus GmbH). The working electrode consisted of a Cu foil (Alfa Aesar 99.999%) contacted by a gold clamp. Prior each experiment Cu foil was polished mechanically using diamond pastes particle diameter down to 0.1µm, after which the electrode was sonicated in ultra-pure water-acetone-water. Every measurement was started with a linear voltammetric sweep, performed with a scan rate of −5 mV/s between E = +0.05 V/RHE and the working potential (between -0.7V and -1.0V/RHE) followed by a chronoamperometricstep for 10 minutes. All reported potentials are corrected for Ohmic drop determined by electrochemical impedance spectroscopy (EIS). EC-Lab software was used to automatically correct 85% of the Ohmicdrop, the remaining 15% was corrected


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manually. For each measurement a freshly polished copper foil and new electrolyte were used to ensure that adsorbates from previous experiments did not influence the result.

2.2 Product analysis After 10 minutes of bulk electrolysis a constant potentials a sample of the gas was analyzed by gas chromatography (Shimadzu GC 2016) to determine the production rate and Faradaic selectivity of the gaseous products. In addition an aliquot of the electrolyte was analyzed by high performance liquid chromatograph (Agilent 1200 series). 2.3 Surface characterization The morphology of the copper electrode before and after reaction was investigated by scanning electron microscopy (SEM). SEM was measured in secondary electron mode with a Jeol 7401F field emission SEM operated at 10 kV. The images were filtered. In addition the composition of the sample was analyzed by Energy dispersive X-ray spectroscopy to investigate if the halides we incorporated to the sample.

3. Results and discussion 3.1 Halide effect on the overall catalytic activity of Cu The effect of halides on the catalytic activity of Cu was studied by performing the CO2RR in the presence of different concentrations of KCl, KBr and KI, (0.05 M, 0.1 M and 0.3M). The concentration of KHCO3 (0.1M) was kept constant to act as a buffer. Working in a non-buffered solution will induce significant changes in local pH that would affect the activity and selectivity of the process leading to misinterpretations.9b


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Linear sweep voltammetry (LSV) was used as a first assessment of the catalytic performance of Cu in presence of halides (Figure 1a). When scanning towards negative potentials we observed a reduction current associated withtwo parallel competingprocesses: hydrogen evolution reaction (HER) and CO2RR. (Eq 1-5)

Figure 1: Activity measurements for polycrystalline copper in CO2 saturated electrolyte a) Linear sweep voltammetry in CO2 saturated electrolyte b) Chronoamperometric geometricreduction current density as function of applied electrode potential, averaged over the last 60s of total 600schronoamperometric steps.Standard error of the mean (68% confidence interval) is included taking into consideration 3 to 5 independent measurements. As shown in Figure 1a the presence of halides affects the overall catalytic activity of Cu. Furthermore we observed that the effect on activity is different for each individual halide. While bromide did not seem to have any significant effect on the reduction current, chloride causes a slight increase of the reduction activity. Finally, the presence of iodide has the most pronounced effect on the LSV. At low over potentials we observed inhibition of the reduction process resulting in a delay on the onset reduction potential. However, below-0.8VRHE we observed a drastic increase of the current. This observation might suggest that, while some of the reduction


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processes occurring at loweroverpotential, such as HER, are suppressed by the addition of I,other reduction processespredominantly taking place at higheroverpotential are enhanced. Linear sweep voltammetry (LSV) reveals performance under non-stationary catalytic conditions. To evaluate the stationary catalytic performance, we performed chronoamperometricbulk electrolysis measurements for 10 minutes.Figure 1b shows the average current densityover the last 60 s of the 10 min chronoamperometric stepsas a function of applied potential. Consistent with LSV, we observed that the stationary reduction currentswere affected by the nature of the halide andfollowed the trend: Br-Br>I-. Thus, reductive Cu+electrodepositionwith no preferential orientation on the surface during the negative potentials of CO2RRis more pronounced in case of Br- and Cl-.CuI crystals, on the other hand,with their poor solubility are more likely to undergo a competing hydrolysis process toCu2O21 according to 2CuX + 2OH-→ 2X- +Cu2O + H2O

(7)

As reported in numerous earlier studies, the resulting Cu2O will subsequentlyreduce and decompose intometallic nanostructures with preferential (001) facets, that is cubic shapes.18,21a,22What is interesting in our experiments is the fact that the resulting Cu/Cu2O cubes in the presence of I-favor the formation of methane over ethylene. This is because the behavior of our Cu2O-derived cubesappears distinctly differentfrom earlier Cu single crystal studies where, in absence of halides, metallic Cu (100) facets on cubic shaped nanoparticles clearly favored


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ethylene over methane.23The enhanced selectivity for ethylene of metallic Cu (100) facets was supportedby reports by Nilsson and co-workers on in-situ form Cu-cubes.18Therefore we conclude that our highmethane selectivitycannot originate from any kind ofcubic morphology effect.Instead,our data suggest that the enhanced methane selectivity israther associated with the presence of iodide either as surface-adsorbed iodide or interfacial iodide-containing species (e.g. CuI or Cu2O). 3.4 Morphologyversushalide effects In order to concludewhether the improved catalytic activity and enhanced methane selectivity were a result of the formed cubic features on the Cu surface or whetherit was rather aconsequence of the presence ofiodidein solution and double layer, as suggested in the previous section, we subsequently tested the catalytic activity of the in-situ formed “roughened cubic” Cu surface in I- free electrolyte. For these experiments we first carried out the bulk electrolysis measurements in an electrolyte consisting of 0.1 M KHCO3 + 0.3 M KI.Thereafter,the “roughened cubic” Cu catalystfoil was sonicated several times in ultrapure water prior toundergoing renewed CO2RR, but this time in pure halide-free 0.1M KHCO3. As Figure 5 evidences, we observed a clear difference in catalytic activity and selectivityon the “roughened cubic” Cu catalyst when tested in I--free (orange trace in Figure 5) compared to I--containing electrolyte (blue trace in Figure 5), demonstrating that solvated I-or adsorbed I-ions have an effect on the catalytic performance.As a blank measurement we also performed a second reaction onthe in-situ formed roughened cubic Cu in0.1 M KHCO3 + 0.3 M KI and observed the same behavior as on the first reaction in I- containing electrolyte(Figure S9).


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Figure 5:Comparison of the catalytic performance of the roughenedcubic Cu in I- containing and I- free electrolyte. a) Linear sweep voltammetry at 5mV/s. b) Faradaic selectivity of the gaseous products after 10 minutes of bulk electrolysis at a constant potential of 0.95 VRHE.Including the SEM images of the surface after reaction (see figure S10). For error estimation see caption of Figure 1.

Figure 5acontraststhe LSVs ofthe roughened cubic Cu in I--free (orange) and 0.3 M I--containing (blue) electrolytes in comparison with the halide-free reference experiment (black) on a polished Cu surface. Comparison of theI--free LSVs (orange and black) demonstrates the effect of the cubic roughening of Cu: The roughened cubic Cu surfacerevealed a cathodic voltammetric feature at around -0.55 V, and, past it, continued to show lower overall activity than the polished Cu reference sample. Similar cathodic voltammetric waveswere previously attributed to the reduction of CO2 to CO which blocked part of the active surface sites. For the roughened cubic Cu in the present case, it is reasonable to expect a higher ratio of undercoordinated sites which typically show a higher binding energy towards CO making this peak more evident.8 Key for our discussion is the fact that the voltammetric trace of theroughened cubic Cu surface in I-- free bicarbonate electrolyte is drastically different form that in the presence of 0.3 M I-. First,


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in I-free conditions, the roughened cubic Cudid notdisplay the suppression of catalytic current density between -0.3 V and -0.6 V,which appears to be characteristic for the presence of I-. More importantly, the roughened cubic Cuunder I-free conditions did not display the dramatic activity increase more negative of -0.8 V. Hence, these two differences are likely to originate from an interaction of I- with the catalyst surface. Figure 5b shows the corresponding Faradaic product selectivity at -0.95V for the three cases of Figure 5a. The presence of I-had a clear and pronounced effect on the Faradic selectivity on H2, CO and methane, regardless of the morphology of Cu. In particular, the enhancement of CH4 selectivity at the expense of selectivity for H2 and CO is observed exclusively in presence of I-. Contrast that to the selectivity of the roughened cubic Cu surface in absence of I-, which resembledthat ofthe polished Cu reference case. These results further suggest that the drastically improved catalytic activity in presence of I- appears to bebased on the surface electrochemistry of I- ions on the surface or in the electrochemical double layer. In order to arrive at more molecular mechanism to explain the effect of halide on the CO2RR, we link our observations to recent results of a computational Density Functional Theory (DFT) based study by Janik and co-workers.15According to their calculations,specific halide adsorption on a well-defined Cusurface occurs at negative electrode potentials vs NHE and it is increasingly favorable in the order: Cl-< Br-< I-. The exact electrode potential at which the halides chemisorb also depends on the coverage and the presence of other adsorbates. Nevertheless, based on the computational results, it is reasonable to assume that under reaction conditions there will be little to no Cl- adsorption on the Cu surface at the applied electrode potentials. This is consistent with the finding that Cl-has only a minor, if at all, effect on the catalytic CO2RR on Cu.


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Br-was theoretically predicted to show enhanced chemisorption and stronger binding to Cu surfaces compared to Cl-. This is why we assume that it is at least partially adsorbed on the experimental Cu surfaceduring the CO2RR. Adsorbed Br-will poisonactive sites translating into a lower overall activity combined witha decrease in HER and hydrocarbon production, consistent with observations. The observed enhanced CO production (see Figure 2b) at larger negative overpotentials, despite lower overall activity (see Figure 1b),suggests that the presence of Br- in the double layer affects the CO2 to CO reaction pathway either through an interaction with the reactant or intermediate or via a modification of the catalytic properties of the Cu surface. The I- ion was predicted to have the strongest interaction with Cu surfaces and desorb from Cu surface at very negative electrode potentials. Hence, it is safe to assume that I-is present at the surface as adsorbed adatom. Experimentally, the presence ofI-at electrode potentials negative of 0.8 V boostedthe overall catalytic activity. Given our selectivity analysis, we conclude that the Faradaic activity gain is mainly causedbyan enhancedreduction rate of CO to methane, adding 6 electrons per converted CO2 molecule, according to CO + 6H+ + 6e- → CH4 + H2O.

(8)

The uniquely activating effect of chemisorbed I- on the CO2RR rate can now be rationalized based on capacity of the adsorbed halides to donate or retain negative charge. Related to this, Janik and co-workers also predicted the dipole moments and partial charges ofthe three halides adsorbed on Cu.15Both calculations suggest that the retention of the negative ionic charge in the chemisorbed state increases in the order: I-

Tuning the Catalytic Activity and Selectivity of Cu for CO2

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