Keywords Electrochemical Impedance Spectroscopy, Surfactants

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Modulating the Electrode-Electrolyte Interface with Cationic Surfactants in Carbon Dioxide Reduction Soumyodip Banerjee, Xu Han, and V. Sara Thoi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00449 • Publication Date (Web): 15 May 2019 Downloaded from on May 15, 2019

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Modulating the Electrode-Electrolyte Interface with Cationic Surfactants in Carbon Dioxide Reduction Soumyodip Banerjee,1 Xu Han,1 and V. Sara Thoi1* 1Department

of Chemistry, Johns Hopkin University, Baltimore, MD, USA.

*corresponding author: [email protected]

Abstract Copper is amongst the most studied electrocatalyst for CO2 conversion due to its remarkable ability to form high-order carbon products. However, controlling factors that lead to high carbon product selectivity remains a major hurdle to fundamental scientific understanding. In this work, we investigate the utility of cationic surfactants to modify the selectivity of Cu foil in electrocatalytic CO2 reduction (CO2RR). We demonstrate that cetyltrimethylammonium bromide (CTAB) significantly suppresses the hydrogen evolution reaction (HER), a competitive parallel reaction to CO2RR. In addition, high surfactant concentrations and long alkyl chain lengths enhance the selectivity for CO2RR in NaHCO3 solutions. Importantly, electrochemical impedance spectroscopy is used to monitor the evolution of the electrodeelectrolyte interface in the presence of CTAB under varying experimental conditions. Based on our extensive electrochemical characterization, we propose that cationic surfactants accumulate in the electrochemical double layer and effectively lower the available proton sources for HER. Our approach provides a facile strategy to modify surface reactivity of metal electrodes with broad implications for electrocatalysis.

Keywords Electrochemical Impedance Spectroscopy, Surfactants, Double Layer, Electrocatalysis, Copper, Hydrogen Evolution Introduction As the urgency to identify alternative carbon resources mounts, carbon dioxide is an increasingly attractive feedstock for sustainable chemical and fuel generation. The development of efficient electrocatalysts is a promising route for activating and converting this intractable molecule to value-added products. Amongst the plethora of metallic catalysts for electrocatalytic carbon dioxide reduction (CO2RR), Cu uniquely demonstrates high activity for a range of carbon products, including CO, HCO2H,

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CH4, hydrocarbons, and alcohols.1–6 However, minimizing the energy input and controlling carbon product selectivity remain as key challenges for advancing this technology to commercial relevance. Electrocatalytic CO2RR is notoriously sensitive to experimental design in Cu catalyst systems.7–10 For instance, the carbon support of porosity carbon support can influence the CO2RR selectivity on Cu particles by tuning the local environment.11,12 The identity and size of the solvated alkali metal ions also play a crucial role in maintaining the local pH and ion distribution at the electrode surface.13–16 Polyamide coatings utilize the –NH2 and –NH3+ groups to modulate carbon product distribution,17,18 while addition of ionic liquids and charged organic additives are shown to improve catalytic activity.19–21 In addition, electric field effects from solvated cations have been computationally postulated to stabilize key intermediates such as the adsorbed *CO2 and *COOH species.22 Taken together, these studies suggest that controlling ion distribution at the electrode surface offers the ability to tune product selectivity. Inspired by the ability to modulate the electrode-electrolyte interface, we examined the influence of cationic surfactants on CO2RR. Amphiphilic surfactants, which are comprised of a polar head group and long alkyl chain, have been extensively studied for anti-corrosion23,24 and electrochemical sensing.25 Cetyltrimethylammonium bromide (CTAB), a common surfactant in nanoparticle syntheses, can form hierarchal structures on metal surfaces and prevent corrosion in the presence of acidic solution.23 In Ag and In systems, this class of trimethylammonium surfactants has shown the ability to decrease HER activity,21,26 but the origin of this suppression is unknown. Herein, we demonstrate that CTAB can similarly suppress HER on Cu. By tracking the electrode-electrolyte interface using extensive electrochemical characterization, we propose the cationic surfactant plays an important role in controlling local ion concentrations. Our findings offer another strategy to modify electrode behavior and may explain how other cationic additives enhance CO2RR in previous studies.19–21,26–28 As surfactants are commonly used to stabilize nanoparticles,29 our study may also provide insights into the behavior of other nano-structured catalysts.

Experimental Materials and Instrumentation. Cu foil (99.8%), (1-hexyl)trimethylammonium bromide (98%), dodecyltrimethylammonium bromide (96%), sodium bicarbonate ( 99.998%) were purchased from Alfa Aesar. Cetyltrimethylammonium bromide (CTAB, > 99%), sodium phosphate (dibasic, >99%), and sodium










hexyl)trimethylammonium perchlorate, CTA(ClO4), was synthesized as detailed in the ESI. Dimethyl sulfoxide ( 99.9%) was purchased from Fisher Scientific and carbon dioxide ( 99.9%) was purchased

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from Airgas. For electrochemical measurements, Ag/AgCl reference electrodes (3M NaCl) and graphite counter electrodes (99.9995%) were purchased from BASi and Alfa Aesar, respectively. All electrochemical measurements were performed on an Ivium n-STAT Multichannel electrochemical analyzer. Gases generated in electrolysis were quantified using Agilent Micro GC 490 equipped with thermal conductivity detector, a PoraPlot (U) column, and a Molsieve 5A column. Liquid products were analyzed by using Bruker Advance 400 MHz FT NMR spectrometer and Bruker Advance 300 MHz FT NMR spectrometer. Electrolyte Preparation. Calculated amount of sodium bicarbonate ( 99.998%) was dissolved in Milli-Q water to obtain an electrolyte concentration of 0.1 M NaHCO3. The solution was used for electrochemical measurements without further purification. Electrolysis experiments using electrolyte solutions prepared from 99.9% NaHCO3 were also conducted to demonstrate that metal ion impurities do not significantly contribute to the experimental observations (Figure S1). A METTLER TOLEDO FiveEasy Plus pH meter (FEP20) was used to track pH of both N2- and CO2-saturated electrolyte with subsequent addition of CTAB. Figure S2 shows the pH remains similar at all tested CTAB concentrations, suggesting CO2 solubility in the electrolyte is nearly unchanged in the presence of the surfactant. Electrode Preparation. Cu working electrodes were prepared just before carrying out electrochemical measurement to prevent significant surface oxidation. Cu foil was cut into square pieces and mechanically polished with 400-grit abrasive paper for at least 5 min until the surface is shiny. The foil is then sonicated in deionized water for 5 min to remove any residual particles. The Cu foil was masked with polyimide tape, exposing only 1 cm2 on one side of the foil. Linear Sweep Voltammetry (LSV). Using a Cu foil working electrode, a Ag/AgCl reference electrode, and a graphite rod counter electrode, LSV experiments were conducted in 10 ml of 0.1 M NaHCO3 solutions between -0.8 V to -2.0 V vs Ag/AgCl under an atmosphere of N2 or CO2 at a scan rate of 100 mV s-1. The surfactant concentration was maintained at 67 µM, unless otherwise noted. The electrolyte solution was purged with N2 or CO2 for at least 15 min prior to the experiment. The potential values were converted with respect to RHE by using the equation: E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + 0.0591 pH where the pH value for N2- and CO2-purged 0.1 M NaHCO3 solution is 8.3 and 6.8, respectively. Controlled Potential Electrolysis. Controlled potential electrolysis were performed in a gas-tight H-cell with a glass frit separator. The Cu foil working electrode and a Ag/AgCl reference electrode were placed in the working compartment, while the graphite rod counter electrode was in the other chamber. Both compartments were filled with 40 ml of 0.1 M NaHCO3 solution (headspace volume is 30 ml). The

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surfactant concentration was maintained at 67 µM, unless otherwise noted. Solution in both the compartments were purged with CO2 for 20 min prior to the electrolysis. Electrolysis were carried out for 2 hours in the potential region between -1.2 V to -1.5 V vs Ag/AgCl and the potentials have been reported with respect to RHE. Each set of experiments was carried out at least 3 times to establish reproducibility. The ohmic drop was measured by electrochemical impedance spectroscopy (EIS). The real component of the Nyquist plot at 104 Hz provides the value of the solution resistance, RE. IR compensation of 85% was conducted during the electrochemical experiment and the remaining 15% correction was performed manually during the data analysis. The Nyquist plots were fitted using Iviumsoft. Electrochemical Impedance Spectroscopy (EIS). EIS measurements were taken at different polarization potentials in both N2- and CO2-purged 0.1 M NaHCO3 solutions in the presence and absence of CTAB. The resulting Nyquist plots were fitted with an equivalent circuit of RE-RCT/CPE (Figure S3, S4), where RE is the solution resistance, RCT is the charge transfer resistances, and CPE is the constant phase element. The double layer capacitance, Cdl, is derived from CPE (see supporting information). The solution resistance is relatively unaffected by the addition of CTAB under our experimental conditions (Figure S5). For direct comparison of Cdl and RCT among experiments, the same Cu electrode was used to ensure similar surface roughness. Surface Roughness Measurements. A JEOL JSM-IT100 scanning electron microscope was used to qualitatively evaluate the surface roughness of the Cu foil before and after electrolysis in the presence of CTAB (Figure S6). The surface roughness factor of the Cu foil before and after electrolysis in the CTAB solution was also measured by cyclic voltammetry (Figure S7). Both measurements indicate minimal changes to the surface roughness in the presence of CTAB.

Results and Discussion We first perform linear sweep voltammetry (LSV) to investigate the effect of CTAB on the electrocatalytic activity of Cu foil towards CO2 reduction. The freshly prepared Cu foil working electrode is submerged in CO2-purged solutions of 0.1 M NaHCO3 (pH 6.8). In an electrolyte solution containing 67 µM CTAB, a notable cathodic shift of the catalytic current is observed in the voltammogram (Figure 1a). LSV experiments conducted in a N2 atmosphere also show a similar trend, suggesting that CTAB suppresses catalytic HER (Figure 1b). To compare HER and CO2RR activity at the same pH, LSV experiments in 0.1 M phosphate solution buffered at pH 6.8 reveal a comparable cathodic shift of the catalytic current in presence of CTAB (Figure S8). Taken together, these findings indicate that proton reduction is significantly impacted by the cationic surfactant under both HER and CO2RR conditions.

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Figure 1. LSV in the absence (black) and presence of 67 µM CTAB (red) under (a) CO2 and (b) N2 in 0.1 M NaHCO3 solution. A series of controlled potential electrolysis at -0.6 to -0.85 V vs. RHE (reversible hydrogen electrode) is performed with and without CTAB to identify gas and liquid products. As expected for this potential range in NaHCO3 solutions,30 the main products are H2, CO, and HCO2H (Figure 2, Figure S9). In the presence of CTAB, the H2 Faradaic efficiency drops from 75-90% to 35-52% in the presence of CTAB while the selectivity for CO and HCO2H more than doubles, with HCO2H being the major carbon product. CO Faradaic efficiency remains relatively constant as a function of potential at ~10% in CTAB solutions compared to