Controlling the Product Syngas H2:CO Ratio through Pulsed-Bias

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Controlling the Product Syngas H2:CO Ratio through Pulsed-bias Electrochemical Reduction of CO2 on Copper Bijandra Kumar, Joseph P. Brian, Veerendra Atla, Sudesh Kumari, Kari A. Bertram, Robert Turner White, and Joshua M. Spurgeon ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00857 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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Controlling the Product Syngas H2:CO Ratio through Pulsedbias Electrochemical Reduction of CO2 on Copper Bijandra Kumar, Joseph P. Brian, Veerendra Atla, Sudesh Kumari, Kari A. Bertram, Robert T. White, and Joshua M. Spurgeon* Conn Center for Renewable Energy Research, University of Louisville, Louisville, Kentucky, 40292, USA.

ABSTRACT: The electrochemical reduction of CO2 is a promising method for sustainable, carbonneutral chemical synthesis as well as the storage of intermittent renewable energy in the form of energydense fuels compatible with existing infrastructure. In this work, we investigated a pulsed-bias technique for CO2 reduction on Cu, which led to a major shift in the product selectivity relative to potentiostatic electrolysis conditions. With applied voltage pulses in the millisecond time regime, syngas (CO + H2) became the only product and had a pulse-time-dependent H2:CO molar ratio, ranging from ~32:1 to 9:16 for pulse times between 10 and 80 ms, respectively, at the same applied working potential. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) data suggested that in-situ oxidation and reduction of the Cu partially caused the preference for CO formation over other carbon products on polycrystalline Cu. Significant nonfaradaic current arising from electrical double layer charging and discharging was also suspected to contribute to the desorption of key reaction intermediates and further promote CO. The results provide an electronic technique for the electrochemical production of a controllable syngas feedstock for utilization in numerous industrial applications (e.g., Fischer-Tropsch process, and hydroformylation of alkenes to aldehydes).

Keywords: Copper, Catalyst, CO2 reduction, Syngas, Selectivity, Electrochemistry ACS Paragon Plus Environment

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1. Introduction As societal energy demand continues to increase, the burning of fossil fuels to provide a majority of this energy will emit greenhouse gas CO2 at a rate that will put progressively greater strain on the global environment.1 Among various emerging CO2 mitigation technologies2 (e.g., sequestration,3 chemical conversion,4 and electrochemical reduction), the electrochemical CO2 reduction reaction (CO2RR) has advantages as a means to sustainably utilize this waste stream to produce valuable chemicals and fuels.5 For instance, electrochemical conversion is directly controllable by adjustment of the applied electrode potential, and an electrochemical system can operate near room temperature, have a relatively small footprint, require minimal chemical intake, and be easily scaled up. With the electricity supplied by intermittent renewable sources (i.e., solar and wind), CO2 reduction is also an ideal method to store energy as chemical bonds in the form of energy-dense molecules (e.g., syngas, ethanol, methane, etc.) for clean power utilities and transportation fuels.6 Because of the stability of the CO2 molecule, the cathodic reduction overpotential is significant, and thus highly active and selective electrocatalysts are necessary to efficiently promote CO2RR over hydrogen evolution. Numerous heterogeneous catalysts have been employed for CO2 electrochemical reduction, including transition metals7 and transition metal oxides,8 metal complexes,9 and heteroatomic carbon nanomaterials.10 Among these catalysts, relatively inexpensive and earth abundant copper (Cu) metal has the unusual property of producing hydrocarbons from CO2RR, and has therefore been widely studied for this reaction.11 However, unlike Ag7b, 12 and Au,7c, 7d, 7f Cu has relatively poor CO2 reduction product selectivity due to the multiple competing reaction pathways it can promote as well as a sensitivity to crystal orientation leading to variability from the heterogeneity of active sites present on the polycrystalline surface.7e,

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As many as sixteen products (e.g., methane, ethylene, formic acid, carbon monoxide, etc.)

have been identified from the electrochemical reduction of CO2 on a polycrystalline Cu film.7e Consequently, numerous efforts have been pursued to enhance the selectivity of polycrystalline Cu by finetuning the structure,14 size,15 morphology,16 and composition,13a but controlling the CO2RR reaction ACS Paragon Plus Environment

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pathway remains a key challenge and significantly varies with respect to the applied potential. Here, we introduce a pulsed-bias electrochemical reduction process for the conversion of CO2 on polycrystalline Cu catalyst, displaying high selectivity for CO formation over other carbon products. A few previous studies have employed a square wave potential with 3 – 5 second pulses on Cu in an attempt to prevent catalyst poisoning,17 but the product distribution was only moderately affected, permitting a modest enhancement in the faradaic efficiency of CH4 and C2H4 at high cathodic potentials. These results are significantly different than investigated herein using pulses in the millisecond time regime in which transient phenomena were observed to be increasingly dominant and the CO2RR product selectivity was completely different. Interestingly, the proposed method also offers a novel route for tuning the ratio of H2 and CO (i.e., syngas) at a given applied potential by varying the pulse time. The selectivity towards syngas is attributed to the pulse-induced oxidation/reduction of the Cu surface and the constantly shifting electrical double layer, which leads to CO formation over other carbon products (e.g., methane or ethylene). The ability to translate CO2 into a tunable syngas (H2+CO) composition, which can be used in a variety of different subsequent catalytic transformation processes,4b, 18 could facilitate the industrialization of the CO2 electrochemical reduction process catalyzed by an earth abundant material. 2. Experimental A. Electrochemical measurements All experiments used polycrystalline Cu foil (99.99%, Alfa Aesar) as working electrodes. Prior to electrochemical experiments, the Cu foil was chemically cleaned with 1.0 M HCl solution to remove the surface oxide and adsorbed impurities. Cu samples were then rinsed with isopropanol and DI water. CO2 electroreduction studies were performed using a custom-made, gas-tight, two-compartment polycarbonate electrochemical cell (Fig. 1a). The cell was designed with a large active electrocatalyst area (~5 cm2) relative to a minimal electrolyte volume (7 mL catholyte), with well-dispersed, uniform CO2 reactant to maximize carbon product concentration and faradaic efficiency, which helps in the detection of gaseous and liquid products with high sensitivity as demonstrated previously.7e An anion exchange ACS Paragon Plus Environment

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membrane (Selemion AMV) separated the cathode and anode compartments to prevent oxidation of the reduction products at the anode. The cathode compartment contained a gas bubbler at the bottom, a port to position the Ag/AgCl (3.0 M KCl) reference electrode in the center, and a wide window for the catalyst working electrode. The CO2 (99.99%, Specialty Gases) was injected into the catholyte at a flow rate of 20 sccm using a calibrated mass flow controller (MKS Instruments). The anode compartment contained a Pt mesh counter electrode and was also actively bubbled with CO2 during measurements to maintain a uniform cell pH of 6.8. All CO2 reduction experiments were carried out in CO2-saturated 0.1 M KHCO3 (99.99%, Sigma Aldrich) aqueous solution at room temperature (298 K). A potentiostat (Biologic SP-200) with electrochemical impedance spectroscopy (EIS) was used for all measurements. The potentials were recorded using an Ag/AgCl (3.0 M KCl) reference electrode and reported on a reversible hydrogen electrode (RHE) scale according to VRHE = VAg/AgCl + 0.210 + 0.059*pHsoln.19 The solution pH was measured to be 6.8. Potentiostatic EIS measurements were performed before every CO2RR experiment to determine the uncompensated solution resistance, Ru, and the potentiostat subsequently compensated for 85% of Ru during electrolysis. The current densities were determined relative to geometric projected catalyst area throughout this study. B. Product analysis CO2 reduction products were measured by gas chromatography (GC, SRI 8610) and nuclear magnetic resonance (NMR, Bruker 400 MHz) for the gas and liquid products, respectively. Both instruments were calibrated with standard gases or liquid solutions. For real-time gas phase product analysis, the outlet of the cathode compartment was connected to the GC inlet. The GC system used an automatic valve injection (1 mL sample) and a thermal conductivity detector (TCD) and flame ionization detector (FID). Ultra-high purity nitrogen (99.99%, Specialty Gases) was used as the carrier gas for all experiments and was chosen to enable accurate hydrogen quantification. The first gas sample was injected after 5 min of potentiostatic CO2 reduction, followed by three additional GC measurements at 22 min intervals. In addition, NMR spectroscopy was used to analyze and quantify the products in the liquid phase by perACS Paragon Plus Environment

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forming 1H NMR experiments. Samples were prepared by mixing D2O and electrolyte aliquots in a 1:1 volume ratio. Dimethyl sulfoxide (DMSO) was added at a known low concentration for internal calibration. Faradaic efficiency (F.E.) was calculated for the chronoamperometric measurements without pulsed bias by determining the charge required to produce the measured product concentration and dividing by the total charge passed during the time the sample underwent electrolysis. To ensure a steady state concentration in the output stream during product quantification, the first GC measurement (at 5 min) was excluded, and the following three GC measurements (at 27, 49, and 71 min) were used to determine an average faradaic efficiency. C. Catalyst characterization A scanning electron microscope (FEI NOVA nano-SEM 600) was used to study the surface morphology of the Cu foil. The catalyst surface properties before and after measurement were characterized with Xray photoelectron spectroscopy (XPS) with a VG Scientific MultiLab 3000 custom-built UHV system (with Cu-Kα radiation). X-ray diffraction analysis was done by using HR XRD Bruker D8 instrument. 3. Results A. CO2 reduction without pulsed bias The chemical surface cleaning step used before every experiment was a simple and effective method to remove the oxide layer and other impurities present on the Cu surface (Figs. S1 and S2). SEM analysis of cleaned Cu films showed featureless surface morphology and a lack of physically adsorbed particles. Further elemental analysis by XPS also confirmed the absence of any metal impurities within the XPS detection limit (Fig. S2). Figure 1b shows the current density vs. potential (J-E) polarization curve for CO2RR obtained by sweeping the potential between +0.6 and -1.2 V vs. RHE at a scan rate of 20 mV s1

. A similar polarization curve was obtained in Ar-saturated electrolyte, in which case Cu yields only H2

product (Fig. S4). The observed CO2 reduction onset potential (-0.63 V vs. RHE) and measured current density (~17 mA cm-2) at -1.2 V vs. RHE are consistent with the current density reported previously for polycrystalline Cu in CO2-saturated 0.1 M KHCO3 solution.7e Chronoamperometry (CA) experiments 5 ACS Paragon Plus Environment

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were performed at potentials varying from -0.65 to -1.15 V vs. RHE at 50 mV intervals for 60 min at each potential (Fig. 1c). Figure 1d shows the average faradic efficiencies (F.E.) for all identified major products as a function of applied potential (shown in Table S1). At low potential (positive of -0.90 V vs. RHE) H2, CO, and HCOOH were the major products. In this region, hydrogen evolution is kinetically dominant, and the faradaic efficiency of H2 formation remained high, varying from 96.4% to 53.5% at 0.65 V and -0.90 V vs. RHE, respectively. At higher applied potential (negative of -0.90 V vs. RHE), the total current attributable to CO2 reduction increased significantly, with the faradaic efficiency of CO2RR higher than hydrogen evolution in the studied potential range. The faradaic efficiency of H2 formation reached a minimum of 26.3% at -1.05 V vs. RHE, where CH4 and C2H4 were the major carbonaceous products. The observed values and trend in the product selectivity match well with previously reported results on Cu foil catalyst in aqueous electrolyte within the limited cathodic potential range.7e, 20

Figure 1. CO2 reduction performance of Cu catalyst without pulsing. (a) Schematic diagram of customized electrochemical cell. (b) Current density vs. potential (J-E) curve for Cu in CO2-saturated 0.1 M KHCO3 solution. (c) Chronoamperometry (CA) analysis at different applied potentials (vs. RHE) and (d) the corresponding measured faradaic efficiency (F.E.) for each product.

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B. CO2 reduction with pulsed bias Figure 2a generically illustrates the potential waveform applied during the pulsed-bias electrolysis experiments. In this method, the applied bias was alternated between an anodic rest potential, Ea, at +0.60 V vs. RHE (0.0 V vs. Ag/AgCl), for a time ta, and a cathodic working potential, Ea, negative of the CO2 onset potential (-0.63 V vs. RHE) for a time tc. Other than the applied potential pulse mode, experimental conditions were identical to previous experiments. Due to the discrete, step-like switch between Ea and Ec, sharp transient current spikes occurred immediately after the application of both Ea and Ec throughout the pulsed potential waveform (Fig. S7). Deconvolution of the faradaic reaction current from

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Figure 2. CO2 reduction performance of Cu catalyst with pulsing. (a) Illustrative pulse in which the applied potential was alternated between an anodic rest potential, Ea (0.6 V vs. RHE), for a time period of ta, and the cathodic working potential, Ec, for a time period of tc. (b – d) Reduction products in percentage (%) of charge required at different applied potentials (Ec), (b) -1.05 V, (c) -1.0 V, and (d) -0.95 V vs. RHE, as a function of pulse time, tc. For (b – d), ta = tc for each measurement, and CA denotes chronoamperometric conditions at the cathodic working potential without pulsed bias.


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the nonfaradaic transient current processes during pulsed electrolysis was not straightforward and deemed too unreliable for accurately reporting faradaic efficiencies of products. CO2 reduction product selectivity was therefore reported for each species as the charge required to create that amount of product as a percentage of the total charge required to create all detected products (rather than the total charge passed during the experiment). Figure 2b-d shows the variation in product selectivity at Ec of 1.05, -1.00, and -0.95 V vs. RHE while varying pulse time tc from 0 to 100 ms with equivalent time periods of ta. Without pulsed bias, CH4, C2H4, and H2 were the main products in this potential window (Fig. 1d), but the application of the pulsed potential strongly favored the direction of charge to CO and H2 in a ratio dependent on the pulse time. At 100 ms pulse times some hydrocarbon products were still detected, but at pulse times < 50 ms, CH4 and C2H4 products were completely suppressed with CO as the only detectable carbonaceous product. Under these conditions, CO formation peaked at 50 ms pulse times with a ~1:1 molar ratio of H2 to CO. Shorter pulse times led to H2 production increasingly being favored over CO formation, reaching ~97% H2 for 10 ms pulse periods. For all pulsed-bias conditions tested, no liquid phase products were detected (Fig. S8) and these species are therefore omitted from the graphs for CO2 reduction under pulsed bias.

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Figure 3. Selectivity dependence on ta. (a) Product charge percentage at -1.0 V vs. RHE with tc = 50 ms and different ta pulse times. (b) Product formation rate with varying ta pulse times.


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ying ta from 0 to 140 ms at a cathodic working potential, Ec, of -1.0 V vs. RHE. For a short rest pulse of ta = 10 ms, the GC measured ~97% H2 and ~3% CO while CH4 and C2H4 were out of the detection range. The introduction of this short pulse thus strongly reduced the rate of CO2 reduction. However, with longer pulses of ta > 10 ms, CO as a percentage of the product increased and attained a maximum value of ~64% at ta = 80 ms, and remained consistent up through ta = 130 ms. By varying the pulse times for a given cathodic working potential (i.e., -1.0 V vs. RHE), CO production was thus promoted relative to H2 production from a molar ratio of ~1:10 under potentiostatic conditions to 16:9 (CO:H2) with tc = 50 ms and ta = 80 ms. The CO and H2 formation rates with varying ta pulse time are given in Figure 3b. As expected, as the rest pulse time ta became longer with respect to the working pulse time tc at sufficiently cathodic potentials for reduction, the overall product formation rate decreased. Although the total reduction rate decreased modestly by ~20% between potentiostatic and peak CO pulsing conditions (tc = 50 ms, ta = 80 ms), CO formation demonstrated the opposite trend and increased from ~0.5 nmol s-1 cm-2 to ~5.3 nmol s-1 cm-2 at the same conditions. 4. Discussion The chronoamperometric experiments without a pulsed bias show the well-established CO2RR product distribution as a function of applied potential on a Cu foil electrocatalyst in aqueous electrolyte (Fig. 1d).7e Theoretical and experimental studies have elucidated numerous reaction mechanisms and pathways to account for the product selectivity behavior on Cu surfaces.7e, 13c, 20-21 The CO2 electroreduction mechanism on Cu is a complex phenomenon with multiple proton-coupled electron-transfer steps and possible reaction pathways dependent upon the formation of various intermediate species, involving both electrochemical as well as non-electrochemical steps (e.g., adsorption, dimerization, etc.).21a, 21d It is, however, generally accepted that the unique ability of Cu electrocatalysts to form significant amounts of hydrocarbon products is due to a surface binding energy which leads to near-optimal stability of the adsorbed CO intermediate species which is a precursor to the formation of higher carbon products (e.g.,


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methane, ethylene, and methanol). Thus at increasingly reductive potentials, CO2RR on Cu promotes hydrocarbon products over CO formation (Fig. 1d).7e As seen in Figure 2, the application of a pulsed bias rather than a potentiostatic bias led to a drastically altered CO2RR product distribution at a Cu electrode. The pulses switched between the reductive working potential and a rest potential below the thermodynamic potential for CO2 reduction or hydrogen evolution. The introduction of the pulsed potential waveform changed the reaction mechanism, directing more charge at the tested working potentials to CO formation and suppressing other reduction products. In particular, for pulse durations of tc = ta ≥ 50 ms, the percent of charge passed to produce H2 decreased and the CO2 reduction products increased relative to the potentiostatic condition, peaking at a ~1:1 ratio at 50 ms pulses. This result is consistent with the pulsed-bias approach easing the effects of masstransport limitations on the reaction. In potentiostatic mode, diffusion of dissolved CO2 to the catalyst surface limits CO2RR and convolutes the kinetic competition with hydrogen evolution, promoting H2. Pulsed mode on the other hand, provides additional time for CO2 diffusion. During periods at the Ec potential, CO2 reduction can proceed in kinetic competition with hydrogen evolution. During periods at the Ea potential below either thermodynamic reaction potential, neither reduction reaction would be predicted to occur, providing time for CO2 from the bulk electrolyte to diffuse to the catalyst surface for reaction during the next Ec pulse. This type of pulse voltammetry to mitigate mass-transport limitations in a reaction has been successfully applied in other electrochemical systems, for instance as a technique to enable conformal electrodeposition on microstructured high-aspect ratio surfaces.22 Although the pulsed-bias effect on mass-transport limitations explains the promotion of CO2 reduction over H2 formation, this theory does not account for the observed mechanistic change to favor CO over other carbonaceous products, nor the decrease in CO production at pulse times < 50 ms. Further investigation was necessary to elucidate the mechanistic shift in CO2RR under millisecond potential pulses. One hypothesized reason for the selective formation of CO under pulsed-bias conditions was that hydrocarbons formed at reductive conditions during the Ec pulse could be oxidized from transiACS Paragon Plus Environment

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ent anodic currents during the Ea pulse. To examine this, we performed control experiments by introducing CH4 at 20 sccm without CO2 under pulsed-bias reaction conditions. The resulting absence of any CO peak in the GC spectra suggested that CO formation was only due to CO2 reduction rather than the conversion of any hydrocarbons formed in-situ. We cannot, however, exclude the possibility of the oxidation of surface adsorbed hydrocarbon intermediates during pulsed-bias electrolysis conditions. A possible change to the Cu catalyst surface induced by pulsed-bias conditions could also be responsible for the changed CO2RR product branching ratio. Figure 4a shows the Cu catalyst surface after 70 min of electrolysis under pulsed-bias conditions with a cathodic working potential of -1.0 V vs. RHE. The catalyst surface displayed increased nanoscopic roughness compared to a freshly cleaned Cu foil electrode (Figs. S1, S10). Figure 4b shows the surface chemical analysis by XPS of the O1s spectra for pristine Cu, a Cu foil after potentiostatic electrolysis, and a Cu foil after pulsed-bias electrolysis. The additional peak present at 530 eV after pulsed-bias electrolysis for 70 min, which was observed for each set of pulse times tested and ending on either potential, corresponds to CuOx (Fig. S11).23 The results are further supported by XRD analysis where characteristic peaks of CuOx are clearly visible (Fig. 4c). The pulsed-bias condition is thus interpreted to cause the in-situ formation of CuO and Cu2O surface layers, perhaps due to anodic current flow during the Ea pulse. Subsequent electrochemical reduction of the oxide layer during the Ec pulse likely leads to the surface roughening, similar to the intentional nanostructuring that arises in oxide-derived Cu catalysts for CO2RR.24 Furthermore, oxide-derived Cu as either foils24 or highly dense nanowires14 has shown high CO2RR selectivity towards CO formation as compared to standard polycrystalline Cu. The enhanced CO faradaic efficiency was attributed to the improved stabilization of the CO2˙− intermediate on the high surface area of oxide-derived Cu nanowire arrays.14 It should also be noted that the presence of an oxide layer during the Ec pulse likely further contributes to the CO selectivity. Copper oxide surfaces have been found in some cases to favor CO formation in comparison to other carbon products (e.g., CH4 and C2H4).14, 24-25 For instance, Lan, et al., reported CO as the main product on Cu(core)/CuO(shell) structures even at high applied potential (-1.73 V vs. Ag/AgCl).26 The morphological and chemical transformation of the Cu catalyst surface under the ACS Paragon Plus Environment

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transient conditions of pulsed-bias electrolysis thus lead to desorption of the CO species before it can undergo further reduction to hydrocarbon products. Thus, while the kinetics for CO2 conversion to CO are sufficiently fast for an appreciable reaction rate during the millisecond pulse time regime, sluggish kinetics for further reduction of CO to CH4 could be inhibiting hydrocarbon formation under these pulse conditions. On the millisecond time scale for pulses used in this work, transient current spikes after the step-change in potential were significant and prevented the establishment of a steady-state reduction current within a given pulse. This type of transient electrochemical current is commonly associated with a nonfaradaic process, usually due to a capacitive charging current rather than faradaic reaction current. The capacitive behavior primarily arises from the charging and discharging of the electrical double layer, or Helmholtz layer, at the electrode surface.19 We speculate that the changing local electric field near the catalyst surface during the charging and discharging of the electrical double layer could have a significant effect on the binding energy experienced by adsorbed reaction intermediates. A non-ideal binding energy during the transient charging current could thus lead to desorption of the critical CO intermediate before further reduction to hydrocarbon species can occur. The changing Helmholtz layer effect is consistent with the near-total directing of CO2 reduction current to CO product under pulsed bias. Furthermore, this effect is consistent with the observed behavior at shorter pulse times (i.e., < 50 ms), in which hydrogen evolution was increasingly promoted over CO formation (Fig. 2). At shorter pulse times, the nonfaradaic capacitive current became increasingly dominant, and the rapidly changing electrical double layer may inhibit the binding of CO2 before it can even be reduced to CO. Figure 4d illustrates the changes to the mechanistic pathway of CO2RR on Cu between potentiostatic and pulsed-bias electrolysis due to the combined effects of a surface oxide and a shifting electrical double layer as proposed in this work.


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Figure 4. Catalyst surface effects during pulsed-bias electrolysis. (a) SEM image of a Cu foil after pulsed-biased conditions at a cathodic working potential of -1.0 V vs. RHE. (b) XPS spectra on a Cu foil electrode that was pristine (Cu), tested under chronoamperometric conditions (Cu CA), and tested under pulsed-bias conditions with 50 ms pulses (Cu 50 ms) and corresponding (c) XRD spectra. (d) Schematic diagram representing the proposed mechanistic effects on CO2 reduction from the pulsed-bias method as compared to potentiostatic electrolysis. Under potentiostatic conditions, bound CO* can desorb to form CO, further reduce to CH4, or react with an adjacent CO* and reduce to C2H4. Under pulsed-bias conditions in this time regime, it is proposed that anodic current during the Ea pulse oxidizes the surface with water which is subsequently reduced back to a nanoscopically roughened metal surface as in (a) from cathodic current flow during the Ec pulse. Adsorbed CO2 during the Ec pulse is reduced to CO*, which is proposed to primarily desorb to CO owing to both the changed catalyst surface active sites relative to the pure polycrystalline Cu as well as the changing electric field due to the double layer charging/discharging.

The dependence of the electrolysis product selectivity on the pulse time periods provides an electronic technique for tuning the H2 to CO ratio at a specified working potential. The production of syngas of varying composition can therefore be controlled without changing the applied working voltage, and hence the overpotential, of the reaction. As Figures 2 and 3 demonstrate, syngas can be produced without significant byproducts with CO ranging from ~3% to ~64% by changing the pulse times within a range of 10 – 80 ms. The syngas production rate, however, decreased as the anodic rest potential Ea time became a greater fraction of the overall potential waveform (Fig. 3b). The decline in total product formation can be attributed to the drop in the fraction of time (i.e., tc/(tc + ta)) responsible for CO2 electro13 ACS Paragon Plus Environment

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reduction. Because the applied potential Ea during ta is positive of the thermodynamic potential for both CO2 reduction and hydrogen evolution, syngas production does not occur within this period. Pulsed-bias electrochemical CO2 reduction thus holds promise for industrial applications requiring syngas. Syngas can either be used directly as a fuel or as a feedstock for the synthesis of numerous, energy-dense chemicals including synthetic natural gas, liquid hydrocarbons via the Fischer-Tropsch process, and valueadded chemicals through the hydroformylation process.18 In each syngas conversion process, a specific ratio of H2 to CO is required in the process feed gas to optimize the product yield. The pulsed-bias technique could be applied to correct and optimize the syngas composition in real time without degrading the electrolysis efficiency. The pulsing method could also provide versatility to a centralized CO2 reduction system meant to supply syngas to a variety of chemical processes. One current commercial technology for adjusting the H2:CO syngas ratio involves a system of specialized membranes with flow control valves.27 The pulsed-bias electrolysis method, in comparison, has the advantage of tuning the syngas ratio at the source without the need for any capital-intensive hardware or additional energy expenditures. 5. Conclusion Electrochemical CO2 reduction at Cu electrodes was characterized with full product quantification in aqueous electrolyte under both potentiostatic and pulsed-bias conditions. The introduction of a squarewave applied potential to the Cu catalyst drastically altered the CO2RR product selectivity, producing only H2 and CO in ratios strongly dependent on the pulse time. The product syngas H2:CO molar ratio varied between ~32:1 and 9:16 with working pulse times of 10 and 50 ms, respectively. Several complex phenomena are likely interacting during the transient electrochemical process to create the observed behavior. Increased CO2 reduction relative to hydrogen evolution at longer pulse times is evidence that the pulse waveform enhanced the mass-transport of dissolved CO2 molecules to the electrode surface. In situ oxidation and reduction of the Cu surface during transient anodic/cathodic currents is suspected to create a changing surface of nanostructured Cu oxide and oxide-derived Cu, both of which have been demonstrated to favor CO production over hydrocarbons. The transient, capacitive charging ACS Paragon Plus Environment

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and discharging of the electrical double layer at the electrode surface is also proposed to lead to an unstable local electric field at the catalyst surface on the millisecond scale that inhibits the binding of key intermediate species and prevents the subsequent reduction steps that lead to hydrocarbon products. The results highlight a unique strategy for tunable syngas production on cost-effective Cu catalysts. The pulsed waveform dependency of the H2 to CO ratio permits the syngas product composition to be easily adjusted in real time by changing the pulse duration and/or cathodic working potential. This technique provides versatility to integrating electrochemical CO2 reduction into industrial processes for the synthesis of fuels or value-added chemicals at a large scale. ASSOCIATED CONTENT Supporting Information. Additional information about the morphological, elemental, and electrochemical characterization results are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Dr. Joshua M. Spurgeon, Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY 40292, email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge financial support from the Kentucky Department for Energy Development and Independence grant #PON2 127 1500002410. The authors also acknowledge support from the Conn Center for Renewable Energy Research at the University of Louisville.


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Controlling the Product Syngas H2:CO Ratio through Pulsed-Bias

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