Inhibited proton transfer enhances Au-catalyzed CO2-to-fuels selectivity

Anna Wuttig; Momo Yaguchi; Kenta Motobayashi; Masatoshi Osawa; Yogesh Surendranath

doi:10.1073/pnas.1602984113

Edited by Daniel G. Nocera, Harvard University, Cambridge, MA, and approved June 7, 2016 (received for review February 22, 2016)

Significance

Renewable electricity can be stored in the energy-dense bonds of carbon-based fuels via the electroreduction of CO₂. CO₂ reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H₂. Rational design of selective CO₂-to-fuels catalysts requires direct knowledge of the electrode surface structure during turnover and how electrons and protons couple to direct product selectivity. Using model Au catalysts, we uncover the complex heterogeneity in CO surface binding equilibria and the differential proton coupling requirements for CO vs. H₂ production. We assemble the spectroscopic and kinetic data to construct a mechanistic model that predicts that impeding proton transfer to the surface is an effective strategy for improving CO₂-to-fuels catalyst selectivity.

Abstract

CO₂ reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H₂. We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H₂ and CO production on polycrystalline Au electrodes. Under the conditions of CO₂ reduction catalysis, electrogenerated CO species are irreversibly bound to Au in a bridging mode at a surface coverage of ∼0.2 and act as kinetically inert spectators. Electrokinetic data are consistent with a mechanism of CO production involving rate-limiting, single-electron transfer to CO₂ with concomitant adsorption to surface active sites followed by rapid one-electron, two-proton transfer and CO liberation from the surface. In contrast, the data suggest an H₂ evolution mechanism involving rate-limiting, single-electron transfer coupled with proton transfer from bicarbonate, hydronium, and/or carbonic acid to form adsorbed H species followed by rapid one-electron, one-proton, or H recombination reactions. The disparate proton coupling requirements for CO and H₂ production establish a mechanistic basis for reaction selectivity in electrocatalytic fuel formation, and the high population of spectator CO species highlights the complex heterogeneity of electrode surfaces under conditions of fuel-forming electrocatalysis.

Product selectivity is a principal design consideration for the development of practical catalysts. Catalyst selectivity is dictated by (i) the relative free energy barriers for progress along competing reaction pathways and (ii) the relative rates of reactant delivery to active sites (1). Enzymes fine tune these parameters with exquisite precision to achieve selectivity (2). Nature augments the coordination environment of metallocofactor active sites to optimize the binding strengths of reaction partners and preorganizes reaction participants toward low-barrier pathways (3). Additionally, many active sites reside at the terminus of molecular channels that gate the coordinated delivery of substrates (4, 5) required for selective transformations. Efforts to prepare artificial catalysts with product selectivities rivaling that of nature require a detailed understanding of these factors.

Currently, our understanding of how to systematically modulate selectivity in heterogeneous catalysts remains poor (6 ⇓–8). Unlike (bio)molecular catalysts, which ideally consist of a uniform ensemble of active sites, heterogeneous catalysts consist of a nonuniform distribution of surface sites (9), requiring an understanding of which are active and which are dormant. The surface site distribution is strongly dependent on the surface nanostructure, oxidation state, and degree of restructuring (7). Superimposed on this distribution are the rate-limiting elementary reaction steps that dictate kinetic branching ratios at surface active sites (10). For catalysis mediated by electrode surfaces, these factors are subject to change as a function of the applied potential (11, 12), and the log-linear relationship between reaction rate and potential often introduces severe transport limitations that further augment the intrinsic reaction kinetics (13). Disentangling these correlated processes is essential to the development of selective electrocatalysts.

Electrocatalytic CO₂ reduction (CDR) allows for the storage of intermittent renewable electricity in energy-dense carbonaceous fuels (14 ⇓⇓–17). CDR is most practically conducted in aqueous electrolytes, in which the undesirable reduction of protons to H₂ presents a principal selectivity challenge. Because the hydrogen evolution reaction (HER) is thermodynamically accessible over the same potential range as nearly all CDR reaction products (18 ⇓–20), selective fuel formation relies on the electrode’s ability to control the relative rates of these competing pathways.

Computational investigations of CDR selectivity have largely applied the Sabatier principle, which states that an optimal catalyst is one that binds key intermediates neither too strongly nor too weakly (21). Within this framework, the relative binding energies of adsorbed H and CO, proposed intermediates along HER and CDR pathways, respectively (22 ⇓⇓⇓⇓⇓⇓⇓–30), serve as descriptors for the relative rates of each reaction. Computational studies have highlighted that H and CO display differing affinities for surface features, including terraces, edges, and corners (22, 23, 29 ⇓–31), suggesting a wide distribution of adsorbate binding energies on the polycrystalline metal surfaces that have been the subject of most CDR investigations (18, 32). Additionally, coadsorption of electrolyte ions and CO can play a dominant role in both CO (25, 29, 30, 33, 34) and H adsorption (24, 26, 29). Despite the contemporary emphasis on adsorbate binding energies as key determinants of selectivity in fuel synthesis, the surface adsorbate population has yet to be probed spectroscopically in situ under the conditions of CDR catalysis.

In electrocatalytic reactions, substrate bond activation is accompanied by electron transfer (ET) from the electrode. The barrier to this ET is augmented by the applied overpotential. Thus, the relative driving force/rate scaling factors (the transfer coefficients) for HER vs. CDR will impact selectivity, even in the limit of identical adsorption energies for key intermediates and a common active site for both reactions. However, the transfer coefficients for HER and CDR occurring concurrently have yet to be probed unambiguously because both reactions are subject to significant transport limitations that mask the activation-controlled kinetics. In particular, because the CDR and HER both consume protons, the pH local to the surface becomes more alkaline than that of the bulk (18, 35, 36). As a result, the buffer strength of the electrolyte strongly impacts the CDR product distribution (37 ⇓⇓–40). In addition, we have shown that H₂ production can be selectively suppressed relative to CO production by increasing the thickness of ordered porous gold (Au) electrodes (41). Although these studies suggest the importance of transport limitations in determining CDR catalytic selectivity, the mechanistic basis for kinetic branching remains poorly understood.

Herein, we examine the mechanistic basis for CDR vs. HER selectivity on polycrystalline Au surfaces. Because Au surfaces principally generate CO and H₂ (42, 43), they provide an ideal platform for probing reaction selectivity. To probe this bifurcation, we combine for the first time, to our knowledge, in situ surface-enhanced IR absorption spectroscopy (SEIRAS) in an attenuated total reflection (ATR) configuration (44 ⇓⇓⇓–48) with electrokinetic studies that account for transport effects to formulate a mechanistic model that describes the surface adsorbate population and rate-limiting processes for both reactions. Under conditions relevant to CDR catalysis, we observe that CO is irreversibly bound to a significant fraction of the surface, whereas (bi)carbonate desorbs from the surface. Against this backdrop, we find that CDR is gated by rate-limiting single ET to CO₂ and that the ET is not coupled to proton transfer (PT) from hydronium, bicarbonate, or carbonic acid. However, these species are competent proton donors for forming adsorbed H in the rate-limiting step of HER, establishing that the divergent proton coupling requirements for these two reactions underpin reaction selectivity.

Results and Discussion

Surface Adsorbate Population and Dynamics During CDR.

In situ surface-enhanced IR absorption (SEIRA) spectra of Au films provide insight into the surface adsorbates prevalent during CDR catalysis. A SEIRAS-active polycrystalline Au film was prepared on a silicon (Si) prism (experimental details are in SI Materials and Methods) and exhibited a roughness factor of ∼5 as determined by copper (Cu) underpotential deposition (UPD) (Fig. S1, red), consistent with previous literature reports (49). The rough metallic electrode surface amplifies IR absorption by adsorbed molecules. In line with the surface selection rule, the transition dipole moments of adsorbate vibrational modes that are observed must have components perpendicular to the electrode surface (44 ⇓⇓⇓–48). Although SEIRAS bears close resemblance to surface-enhanced Raman scattering, the latter is known to give rise to inhomogeneous amplification of Raman scattering at hot spots on surface features of high local curvature (50, 51), making it hard to discern whether spectral observables accurately reflect the adsorbate population across the entire surface. In contrast, experimental data on CO adsorption to platinum (Pt) surfaces suggest that SEIRA spectra reflect the aggregate adsorbate speciation (52), do not perturb band positions relative to those observed in IR absorption spectroscopy measurements (53), and give rise to spectral intensities that scale linearly with adsorbate coverage (54, 55). Given this literature precedent, we take our SEIRA spectra as representative of the aggregate adsorbate population and speciation on the entire electrode surface.

Fig. S1.

Cu UPD of various Au substrates used in this study. Cyclic voltammograms of Au RCE (black), SEIRAS-active Au film (red), and Au foil (blue) collected at 50-mV s^–1 scan rate with negative-going direction of scan in 0.5 M H₂SO₄/7.4 mM CuSO₄ electrolyte. The total electrochemically active surface area (ECSA) of the film was quantified by integration of the voltammetric stripping feature of Cu UPD (80). Using the Cu UPD data, all currents are reported per the ECSA of Au (milliampere centimeter^–2_Au).

SEIRA spectra reveal the potential-dependent adsorption dynamics of the principal electrolyte species (bi)carbonate and water. In CO₂-saturated 0.1 M NaHCO₃ electrolyte, CDR catalysis onsets at −0.80 V [all potentials are reported vs. the standard hydrogen electrode (SHE)] on polycrystalline Au electrodes (indicated by yellow shading in Fig. 1) (43, 56). The background spectrum was recorded at 0.80 V in CO₂-saturated 0.1 M NaHCO₃ (Fig. 1A), and after polarizing to 0.60 V, a positive band at 1,456 cm^–1 is observed, attributed to adsorbed bicarbonate. Scanning the potential to negative values gives rise to a bleach at 1,460 cm^–1 and a rising band at 1,614 cm^–1 in the corresponding SEIRA spectrum (Fig. 1B). Both the rise and bleach are reversed upon scanning back to positive potentials, indicating that these processes are reversible. Based on literature precedent (57, 58), we assign the peak at 1,460 cm^–1 to (bi)carbonate that is adsorbed to the Au surface at positive potentials. As expected, when negatively polarized, the electrode repels this negatively charged anion, causing its desorption and the corresponding bleach at the same band position. We assign the peak at 1,614 cm^–1 to the δHOH bending mode of adsorbed water (59 ⇓–61), and its intensity is anticorrelated to the surface population of adsorbed bicarbonate, suggesting that water adsorption takes the place of adsorbed carbonate under the reducing potentials of CDR catalysis. Interestingly, the δHOH mode of adsorbed water is red-shifted by 31 cm^–1 relative to that of bulk water (∼1,645 cm^–1), attributed to a decrease in hydrogen bonding between water molecules arranged at the surface (59).

Fig. 1.

Potential-dependent adsorption dynamics of principal electrolyte species and CO. Cyclic voltammograms obtained at 10 mV s^–1 in (A) CO₂-saturated 0.1 M NaHCO₃, (B) simultaneously acquired SEIRA spectra, and corresponding integrated band intensities for adsorbed (C) CO_bridge and (D) CO_atop species. Cyclic voltammograms obtained at 10 mV s^–1 in (E) CO- and CO₂-saturated 0.1 M NaHCO₃, (F) simultaneously acquired SEIRA spectra, and corresponding integrated band intensities for adsorbed (G) CO_bridge and (H) CO_atop species. Background spectra were recorded at 0.80 V for both conditions. The potential regime where cathodic catalytic current is observed is highlighted in yellow for clarity.

In situ SEIRA spectra also reveal that CO is bound to the Au electrode surface during CDR catalysis. At −0.40 V, bands are observed at 2,046 and 1,977 cm^–1, attributed to electrogenerated CO bound on the Au surface in two distinct environments. In line with literature reports and our own experimental data (Fig. S2), the 2,046- and 1,977-cm^–1 bands display Stark shifts of 50 and 74 cm^–1 V^–1, respectively, with bands red-shifting with polarization to more negative potentials. Accounting for the Stark shift (62 ⇓–64), these band positions are in line with experimental vibrational frequency ranges that have been assigned to terminally (CO_atop) and bridge-bonded CO (CO_bridge) on Au surfaces, respectively (49, 65 ⇓⇓–68). Although Stark effects prevent a direct comparison of the absolute peak positions observed experimentally under polarization with those computed in the absence of an interfacial field, computed frequency separations of 80–152 cm^–1 (69 ⇓⇓⇓–73) between these two binding modes are in line with the 80- to 90-cm^–1 separation observed here. The stretching frequencies of CO_bridge on Au surfaces are higher than the 1,850–1,700 cm^–1 range typical of μ²-CO species found in discrete coordination/cluster compounds (74), suggesting that these surface species retain an intermediate bond order between two and three and may exist in a semibridging mode. We note that the geometrical preference for CO on Au is highly dependent on the electrolyte conditions (68, 73, 75), and these data provide the first insights, to our knowledge, into CO adsorption at the intermediate pH conditions relevant to CDR catalysis.

Fig. S2.

The Stark tuning slopes for adsorbed CO_bridge and CO_atop in CO- and CO₂-saturated 0.1 M NaHCO₃ for the (A and B) negative- and (C and D) positive-going potential sweeps shown in Fig. 1.

Atop adsorption of CO occurs reversibly on Au surfaces under the conditions of CDR catalysis. Under CO₂/CO cosaturation conditions (0.1 M NaHCO₃ electrolyte saturated with 0.5 atm CO and 0.5 atm CO₂), the CO_atop band rises and falls over the same potential range (−0.42 to 0.20 V) in the forward and reverse traces (Fig. 1H), indicating reversible CO_atop adsorption to Au. Data presented in Fig. S3 additionally support the reversibility of CO_atop adsorption. Under CO-free conditions, after the electrode is rapidly switched back to the open circuit potential (0.36 V) after polarization at a potential where CO is evolved (−1.00 V) (Fig. S3A), the integrated CO_atop peak intensity declines rapidly over the first three SEIRA spectra (over 1.7 s) (Fig. S3B). This decline is attributed to CO_atop desorption from the Au surface as the equilibrium concentration of electrogenerated CO in the vicinity of the electrode diminishes because of diffusion and convection away from the surface. Importantly, this decline in CO_atop is not accompanied by a rise in CO_bridge population (Fig. S3B), supporting the notion that CO_atop desorbs as gaseous CO (CO_g) rather than by its interconversion to CO_bridge. This observation, taken together with the reversibility of CO_atop adsorption between the forward and reverse potential scans (Fig. 1H), suggests that the decline in CO_atop peak intensity observed at potentials more negative than 0.00 V is also caused by CO_atop desorption as CO_g rather than its conversion to CO_bridge. If CO_atop and CO_bridge species were interconverting on the timescale of the measurement, the observed hysteresis in the latter (see below) (Fig. 1G) would give rise to a similar hysteresis in the former, which we do not observe (Fig. 1H). To the best of our knowledge, the observed desorption of CO_atop at negative potentials is unprecedented and motivates additional investigation to identify its origin. Nonetheless, the potential regime where CO_atop is bound to the surface is positive of the thermodynamic potentials for HER (E⁰ = −0.42 V) and CDR (E⁰ = –0.51 V) under these conditions. Together, our SEIRAS data indicate that CO_atop species (i) bind reversibly to the electrode surface and (ii) exist in low surface population at the potentials relevant to CDR catalysis.

Fig. S3.

Selected SEIRA spectra of a Au thin film electrode in CO₂-saturated 0.1 M NaHCO₃ (A) and corresponding time-dependent integrated band intensities of CO_bridge and CO_atop (B). The potential was switched from the open circuit (OC) potential to −1.00 V vs. SHE for 30 s and then, back to OC. The spectral background was collected at OC (0.36 V vs. SHE).

In contrast, bridge adsorption of CO occurs irreversibly on Au surfaces under the conditions of CDR catalysis. Fig. 1 C and G reveal dramatic hystereses in the CO_bridge population between negative- and positive-going scans, in line with irreversible adsorption. Data presented in Fig. S3 additionally support this irreversibility. Under CO-free conditions, after the electrode is rapidly switched back to open circuit (0.36 V) after polarization at a potential relevant to CDR catalysis (−1.00 V) (Fig. S3A), the CO_bridge peak remains present indefinitely (Fig. S3B). Indeed, whether CO is electrogenerated (Fig. 1C) or codosed (Fig. 1G), CO_bridge remains on the Au surface up until potentials are sufficiently positive to induce its two-electron oxidative desorption as CO₂ (see the following paragraph) (66, 67, 75 ⇓⇓–78). Notably, the irreversibility of CO_bridge adsorption promotes its formation 510 mV positive of the thermodynamic E⁰ of CDR to CO_g (−0.51 V) under these conditions (Fig. 1C). This 510-mV difference places a lower limit on the CO adsorption energy of these irreversibly bound species. We note that a minority population of CO_atop is also electrogenerated at 0.51 V (Fig. 1D), but it is only observed on the initial negative-going scan of the electrode, implying that the surfaces are subject to reconstruction during CDR catalysis. Nonetheless, on polarization to potentials relevant to CDR catalysis, we observe (i) that electrogenerated CO_bridge is the predominant surface species and (ii) that it is irreversibly bound to the surface.

To estimate the surface coverage of irreversibly adsorbed electrogenerated CO_bridge species observed in Fig. 1 B and C, we performed CO oxidative stripping analysis of a Au rotating cone electrode (RCE). An RCE (79) was used so that CO released from the surface would be swept away by rotation. In Fig. 2, solid line, the first cyclic voltammogram (CV) scan initiated after CDR catalysis on the RCE at −0.80 V displays a broad oxidation wave spanning −0.20 to 0.80 V, in line with reported potentials for two-electron CO oxidation to CO₂ on Au surfaces (66, 67, 75 ⇓⇓–78). On subsequent cycling (Fig. 2, dotted line), the broad wave is replaced by diminished background waves attributed to (bi)carbonate adsorption to the Au surface. Importantly, the onset potential of CO oxidation, −0.20 V, observed in the CV (Fig. 2) is the same potential at which we observe a decline in CO_bridge peak intensity in SEIRA spectra (Fig. 1C). In addition, this oxidative CV feature was still observed after a significant time delay (>1 min) after CDR catalysis, suggesting that CO_bridge, unlike CO_atop, is only removed through oxidative conversion to CO₂ rather than desorption as CO_g. Together, these data lead us to assign this wave solely to the two-electron oxidation of CO_bridge species to CO₂. Using the second scan as a baseline, we quantified the excess charge passed in the initial oxidative feature through relative integration of the CV waves (details are in SI Materials and Methods and Fig. S4) and estimated a surface concentration, Γ_CO-bridge, of 0.4 nmol cm^–2_Au. Full monolayer adsorption of CO to Au(110) − (1 × 2) has been shown to give rise to a surface concentration of Γ_CO,max = 1.8 nmol cm^–2 (68), suggesting that polycrystalline Au surfaces have a CO_bridge surface coverage of θ_CObridge = Γ_CO-bridge/Γ_CO,max ∼ 0.2 under CDR catalysis. In Fig. 2, we note the presence of an oxidative feature more positive than 0.90 V vs. SHE that is correlated with the reductive feature spanning 0.50 to 0.90 V vs. SHE. These features are attributed to Au surface oxide formation and back reduction, and they do not affect the behavior of irreversible stripping of adsorbed CO (Fig. S4B).

Fig. 2.

The first (solid line) and second (dotted line) CV cycles recorded after 7 min of CDR catalysis at −0.80 V in CO₂-saturated 0.1 M NaHCO₃ at 1,000 rpm on a Au RCE.

Fig. S4.

The first (black) and second (red) CV cycles recorded immediately after (∼1-s time delay) 7 min of CDR catalysis at −0.80 V in CO₂-saturated 0.1 M NaHCO₃ at 1,000 rpm on a Au RCE at differing potential ranges. A shows potential ranges from −0.40 to 1.00 V vs. SHE, and B shows potential ranges from −0.40 to 0.80 V vs. SHE. The broad feature spanning −0.20 to 0.80 V was integrated by drawing a horizontal line between −0.20 and 0.10 V as the baseline (region of integration denoted by gray shading in A). An authentic baseline between −0.20 and 0.10 V could not be obtained, because scanning to potentials more negative than 0.10 V leads to reformation of electrogenerated CO_bridge species as observed in SEIRA spectra (Fig. 1C).

CO Evolution Rate Is Independent of the Proton Donor Environment.

The relationship between potential and the rate of CO evolution (j_CO) was probed over the potential range, −0.80 to −1.00 V, corresponding to overpotentials (η) between 290 and 490 mV for CDR to CO. These data, taken together with studies of the reaction order with respect to bicarbonate concentration ([HCO₃^–]), pH, CO₂ partial pressure (P_CO2), and CO partial pressure (P_CO), provide the basis for mechanistic interpretation of CO production catalyzed by polycrystalline Au surfaces. Au electrodes used for kinetic studies exhibited similar surface faceting, Tafel slopes, and j_CO relative to SEIRAS-active Au films (Fig. S5), suggesting that the interfacial adsorbate structures described above are also present on the Au electrodes used for the electrokinetic studies described here; j_CO values were extracted from the total measured current using in-line GC analyses (full experimental details are in SI Materials and Methods) and were normalized to the electrochemically active surface area (Fig. S1, blue) as determined by Cu UPD (80). Fig. S6 shows that j_CO remains unaltered when the rotation rate of the Au electrode increases, suggesting that j_CO is not inhibited by mass transport under the conditions examined in this study. Thus, we take the data collected in Fig. 3 as direct measurements of activation-controlled rates for CO production.

Fig. 3.

Electrokinetic data for CO evolution catalysis. (A) CO partial current density–voltage (Tafel) behavior of Au foil in CO₂-saturated 0.1 M NaHCO₃ electrolyte. (B) P_CO2 dependence of the CO partial current density at −0.80 V (dark blue), −0.90 V (teal), and −1.00 V (sky blue) recorded in 0.1 M NaHCO₃ electrolyte. (C) Bicarbonate concentration, (D) [NaHCO₃]/[NaDCO₃], and (E) P_CO dependences of CO partial current density at −0.80 V (dark blue), −0.90 V (teal), and −1.00 V (sky blue) recorded in CO₂-saturated bicarbonate electrolyte.

Fig. S5.

(A) Surface faceting of Au rotating cone electrode (RCE), foil, and SEIRAS-active Au film used in this study. Cyclic voltammograms of Au RCE (black), SEIRAS-active Au film (red), and foil (blue) in 0.1 M NaOH/ 0.1 mM Pb(NO₃)₂ recorded at 50 mV s^–1 with a negative direction of scan. The CVs display two deposition and stripping features centered at 0.44 and 0.56 V vs. the RHE. The less positive wave is diagnostic of lead deposition/stripping from exposed Au(111) facets, whereas the latter corresponds to that from Au(110) facets (105 ⇓⇓–108). In our studies, Pb UPD analyses show that the Au surfaces used to probe interfacial surface structure (SEIRAS-active films) and electrokinetics (RCEs and foils) exhibit similar populations of low-index facets. Integration of the facet-dependent Pb UPD stripping features of these materials indicates that the relative population of Au(110) to Au(111) facets, RP_Au, is comparable across the materials examined, with RCE RP_Au = 1.5, foil RP_Au = 1.3, and film RP_Au = 1.2. The materials used in this study display higher surface populations of Au(110) facets relative to Au(111), suggesting that the interfacial structures probed by SEIRAS on the Au thin films are representative of the Au electrodes used for electrokinetic studies and that any facet-dependent catalytic activities are similar. (B) CO partial current density–voltage (Tafel) behavior of SEIRAS-active Au film on Si in CO₂-saturated 0.1 M NaHCO₃ electrolyte.

Fig. S6.

KL plots of CO partial current density operated in CO₂-saturated 0.1 M NaHCO₃ electrolyte on a rotating Au cone electrode at applied potentials of −0.80 (black), −0.90 (red), and −1.00 V (blue) vs. SHE.

The current–voltage (Tafel) data combined with the reaction order in P_CO2 support a mechanistic sequence involving rate-limiting single ET to CO₂. The Eyring equation defines an exponential relationship between the activation barrier and reaction rate. Polarizing the electrode to greater η directly reduces the activation barrier for the reaction, leading to an exponential increase in the rate of CO evolution as observed in Fig. 3A. For a mechanistic sequence involving a single rate-limiting ET from the catalyst resting state, the current is proportional to exp(βηF/RT) (81) (i.e., a limiting case of the Butler–Volmer equation at high η, where β is the symmetry factor for the reaction and the other constants take their usual meaning). For outer-sphere interfacial ET reactions, Marcus theory provides a direct estimate for β as a function of driving force and reorganization energy, λ (82). In particular, a β-value equal to 0.5 is expected for outer-sphere ET reactions for which λ is large relative to η. Although a Marcus-type model does not explicitly treat the case of inner-sphere ET reactions at interfaces, β-values of 0.5 have been observed in surface electrocatalysis (13). At modest η, the Tafel data in Fig. 3A display linearity and a slope of 140 mV per decade (dec), corresponding to a β-value of 0.4, in line with that expected for a rate-limiting single ET. The Tafel slopes observed here are similar to those observed previously for Au-catalyzed CDR (43, 56, 83 ⇓–85). In addition, j_CO is approximately first order in P_CO2 across the same potential range of linear Tafel data collection (Fig. 3B), consistent with previous reports in bicarbonate (43) and phosphate (42) electrolytes. These order data taken together with the Tafel slope suggest that CO production proceeds via rate-limiting single ET to CO₂.

CDR catalysis is zeroth order (Fig. 3 C and D) in all other electrolyte constituents over the entire range of linearity of the Tafel data. The rate of CO production is insensitive to the [HCO₃^–] (Fig. 3C). At constant P_CO2, a change in log[HCO₃^–] also alters the pH by 1 unit (18); thus, this experiment alone does not rule out the possibility that the bicarbonate order is positive, whereas the pH order is negative and of the same magnitude, canceling each other out. However, the pH also declines with increasing P_CO2 at fixed [HCO₃^–] (18). Thus, any intrinsic negative order in pH would be reflected in an increase in the CO₂ order to a value greater than one, which we do not observe (Fig. 3B). We, therefore, take the data in Fig. 3C as evidence that j_CO is zeroth order in both pH and bicarbonate. These results are consistent with studies in phosphate buffer (42), in which no pH dependence was observed for Au-catalyzed CDR. In addition, the j_CO does not depend on the ratio of hydrogen/deuterium (H/D) in the electrolyte (Fig. 3D). We note that changing the H/D ratio can alter the pK_a of the proton donors, but because we observe no dependence on [HCO₃^–] or pH, neither of these effects convolute the observed absence of an H/D kinetic isotope effect (KIE). Together, the order data in Fig. 3 C and D suggest that the principal electrolyte constituents are not involved in the mechanistic sequence up to and including the rate-limiting step of catalysis.

CDR catalysis on Au generates CO, which may be expected to inhibit its production, particularly given the high population of adsorbed CO observed by SEIRAS (Fig. 1). To probe whether CO is a component of the rate expression for CDR, we examined j_CO as a function of P_CO. However, this measurement requires an alternative method for product detection because electrogenerated CO is dwarfed by the CO stream and therefore, cannot be quantified directly by in-line GC detection of CO. Because Au electrodes are known to predominantly generate CO and H₂ with >95% cumulative Faradaic efficiency and because the CO and H₂ Faradaic efficiencies add to near unity under our experimental conditions, the H₂ partial current (j_H2) can be subtracted from the total observed current (j_TOT) to indirectly measure j_CO. The pH of the solution was kept at 7.1 by maintaining the P_CO2 at 0.5 atm, whereas the P_CO was varied by Ar dilution. We observe that indirectly measured j_CO is zeroth order in P_CO (Fig. 3E), indicating that minor equilibrium CO dissociation from the surface does not precede CO₂ activation under these conditions.

The data in Fig. 3 are consistent with an electrochemical rate law described by the following expression (also shown schematically in Fig. 4A):jCO=k1θCDRPCO2expβEFRT,[1]where k₁ is a potential-independent constant. The rate expression shows the first-order dependence in P_CO2 as well as the zeroth-order dependence in pH, [HCO₃^–], and P_CO. This rate law also includes an exponential term that captures the potential-dependent rate of an irreversible single ET step. In Eq. 1, θ_CDR is the surface coverage of CDR active sites, which is defined as the ratio of the surface concentration of CDR active sites, Γ_CDR, to the total number of Au surface sites, Γ_Au. Thus, this value must be between zero and one.

Fig. 4.

Proposed mechanisms for (A) CO evolution (blue) and (B) concurrent H₂ evolution (red) on polycrystalline Au surfaces. Proposed rate-limiting steps (RLS) indicated for both reactions.

The foregoing rate expression is consistent with a mechanistic model for CDR catalysis that involves rate-limiting ET to CO₂ to form an adsorbed Au-COO^– species. This species then undergoes rapid one ET, two PT to liberate water and generate CO, which rapidly dissociates from the surface to regenerate the active site for subsequent turnover (Fig. 4A). Importantly, our mechanistic model invokes that reductive adsorption of CO₂ is not concerted with PT as previously postulated (18, 43, 56, 83). The lack of dependence on pH and bicarbonate explicitly excludes a PT/ET sequence, in which the PT resides in quasiequilibrium, as well as a concerted proton electron transfer (CPET) sequence involving either one of the proton donors. However, water can be an effective CPET donor or acceptor (86 ⇓–88), and we cannot rule out its participation explicitly, especially if H-bonding of water and CO₂ induces an early CPET transition state that may not give rise to an appreciable KIE. CPET sequences have been shown to give rise to appreciable H/D KIEs in surface electrochemical reactions (89 ⇓⇓–92), which we do not observe for CDR catalysis on Au (Fig. 3D). Instead, over the potential range probed in this study, we postulate that CO₂ activation on Au proceeds by an ET/PT mechanism with rate-limiting ET. Because the thermodynamic potential for outer-sphere reduction of CO₂ to its radial anion, CO₂^•–, is at −1.90 V in water (93), which is at least 1.10 V more negative than the top end of the linear Tafel range, we postulate that surface adsorption to Au imparts dramatic stabilization of the adsorbed ET intermediate and enables an ET/PT pathway. Whereas computational studies typically probe the energetics for adsorption of the neutral COOH intermediate (22, 27, 28), our findings suggest that the adsorption energy of COO^– is a better descriptor of the rate of CO₂ activation on Au.

The population of surface active sites for CDR (θ_CDR) on Au is fractional. An estimate of θ_CDR is provided by quantifying the population of CO_bridge species on the Au surface under catalytic conditions (Fig. 2). Because these CO species are bound irreversibly, we postulate that they act as inert spectators during CDR catalysis. Analyses of the stripping charge associated with CO oxidation, described above, reveal a surface coverage of CO_bridge of ∼0.2. This high coverage suggests that CO_bridge species may decorate a combination of surface defects, basal planes, and/or reconstructed surface regions. The remaining surface fraction, ∼0.8, not bound by CO_bridge places an upper limit on θ_CDR. Although these studies provide no direct insight into the active sites responsible for CDR catalysis, they are consistent with the emerging notion that CDR is mediated by minority surface features existent at Au step edges (22, 30, 31) or grain boundaries (56). Because we observe CO_atop adsorption in equilibrium with CO_g, we postulate that CO_g production proceeds through the intermediacy of CO_atop species, which rapidly desorb from the surface (Fig. 4A).

H₂ Production Is Dependent on the Proton Donor Environment.

The relationship between potential and the rate of H₂ evolution (j_H2) was probed over the same potential range as CDR, −0.80 to −1.00 V, corresponding to η between 0.80 and 1.00 V for HER. These data taken together with studies of reaction orders with respect to [HCO₃^–], pH, and P_CO provide the basis for mechanistic interpretation of H₂ evolution occurring simultaneously with CDR on polycrystalline Au surfaces. Fig. S7 shows that j_H2 is significantly affected by the rotation rate of the working electrode at applied potentials of −0.80, −0.90, and −1.00 V in CO₂-saturated 0.1 M NaHCO₃, showing that j_H2 is convoluted by mass transport limitations under these conditions. For the same reaction occurring in 1 M NaHCO₃, however, the rotation rate dependence is greatly suppressed (Fig. S7). Thus, data taken in CO₂-saturated 0.1 M NaHCO₃ are interpreted after accounting for mass transfer effects by extrapolating Koutecký–Levich (KL) plots (Fig. S7A) of j_H2^–1 vs. ω^–1/2 to the y-intercept, representing infinite rotation rate, to extract activation-controlled j_H2 values. Conversely, data recorded in CO₂-saturated 1 M NaHCO₃ at 2,000 rpm are taken as direct measurements of activation-controlled HER and overlaid with data analyzed using KL extrapolation (Fig. S8).

Fig. S7.

(A) KL plots of H₂ partial current density (j_H2) operated in CO₂-saturated 0.1 M NaHCO₃ electrolyte on a rotating Au cone electrode at applied potentials of −0.80 (black), −0.90 (red), and −1.00 V (blue) vs. SHE. (B) KL plots of j_H2 operated in CO₂-saturated 1 M NaHCO₃ electrolyte on a rotating Au cone electrode at applied potentials of −0.85 (black), −0.95 (red), and −1.05 V (blue) vs. SHE. (C) Two runs of KL plots of j_H2 operated in CO₂-saturated 0.1 M NaHCO₃ at −0.80 V vs. SHE (teal and blue) and CO₂-saturated 1 M NaHCO₃ at −0.85 V vs. SHE (black and red). (D) Two runs of KL plots of j_H2 operated in CO₂-saturated 0.1 M NaHCO₃ at −0.90 V vs. SHE (teal and blue) and CO₂-saturated 1 M NaHCO₃ at −0.95 V vs. SHE (black and red).

Fig. S8.

Tafel plots of H₂ partial current density operated in CO₂-saturated 1 M NaHCO₃ electrolyte on a rotating Au cone electrode at 2,000 rpm (blue) and using extrapolation of KL plots in Fig. S7B (red). The error bars correspond to the error in the y-intercept obtained from least-squares fitting of the data in Fig. S7B.

The Tafel behavior for HER is highly dependent on the proton donor environment. In Fig. 5A, we observe that the KL-extrapolated Tafel slope of 380 mV/dec for j_H2 in CO₂-saturated 0.1 M NaHCO₃ is 170 mV/dec higher than the 210-mV/dec slope observed in CO₂-saturated 1 M NaHCO₃. The high Tafel slope measured in CO₂-saturated 0.1 M NaHCO₃, where incremental excursions in η do not significantly increase the rate, implies that rate-limiting chemical steps gate HER rather than ET (81). The lower Tafel slope observed in CO₂-saturated 1 M NaHCO₃ suggests a mechanism operative by rate-limiting single ET, with β equaling 0.3 (see above discussion for j_CO). Although Marcus theory would predict β = 0.5 for rate-limiting, outer-sphere, single-ET, transfer coefficients as low as 0.3 have been observed for inner-sphere electrochemical reactions involving rate-limiting ET (40, 94) and have also been predicted theoretically (95, 96). The observed changes in Tafel slope with bicarbonate concentration are further supported by the observation that j_H2 is invariant with η at low [HCO₃^–] (0.03 mM), whereas j_H2 becomes highly dependent on η at higher concentrations (1 M) (Fig. 5B). We note that this experiment, however, is not a direct probe of the explicit bicarbonate order on j_H2, because under CO₂ saturation conditions, varying the bicarbonate concentration by 1 dec changes the bulk pH by 1 unit (18). Thus, extracting the explicit bicarbonate order requires the ability to hold two of three parameters in the CO₂–bicarbonate–pH equilibrium constant, which cannot be achieved experimentally while maintaining this equilibrium. Given this convolution, we take the values of the bicarbonate order as approximate. We stress that this convolution was simplified when examining the dependence of j_CO on bicarbonate concentration (Fig. 3C), because j_CO is zeroth order in both bicarbonate and pH. Nevertheless, HER is forward order in bicarbonate at −0.80 and −0.90 V (Fig. 5B), suggesting that the anion is a viable proton donor for the reaction. In contrast, the explicit dependence of HER catalysis on pH can be obtained, assuming that CO₂ is not a direct reaction participant in HER. Under fixed [HCO₃^–], we varied the pH by modulating the P_CO2 and found that j_H2 exhibits an inverse order dependence on the pH of the bulk electrolyte (Fig. 5C), suggesting that hydronium ions, as has been previously reported (97), or carbonic acid, which could be formed in minor equilibrium (98, 99), are also viable proton donors for this reaction. Together, the data suggest that hydronium, carbonic acid, and bicarbonate are viable proton donors and show that the rate of HER is highly dependent on the buffering strength.

Fig. 5.

Electrokinetic data for H₂ evolution catalysis. (A) KL-corrected Tafel plot for HER in CO₂-saturated 0.1 M NaHCO₃ and raw Tafel plot for HER in CO₂-saturated 1 M NaHCO₃. The error bars correspond to the errors in the y-intercept obtained from least squares fitting of the data in Fig. S7A. (B) Bicarbonate concentration dependence of HER partial catalytic current density at −0.80 V (dark red), −0.90 V (orange), and −1.00 V (light orange). (C) pH and (D) P_CO dependence of HER on a Au RCE (2,000 rpm) at constant potentials of −0.85 V (red), −0.95 V (brown), and −1.05 V (dark yellow) in CO₂-saturated 1 M NaHCO₃ electrolyte.

The rate of HER is invariant with P_CO. In Fig. 5D, we observe that j_H2 is independent of P_CO, suggesting that equilibrium CO dissociation does not precede H adsorption. Given the precedent that a variety of Au surface facets can support HER catalysis in the absence of CO (100), we expect that surface sites that are not bound by CO under these conditions remain competent for HER, provided that the proton donor concentrations are sufficiently high. The order data imply that j_H2, like CO production, is not gated by equilibrium dissociation of CO.

The data in Fig. 5 imply an electrochemical rate law described by the following equation (also shown schematically in Fig. 4B):jH2=k2θHER[BH]x expβEFRT+k3[BH]x,[2]where k₂ and k₃ are potential-independent constants. The rate expression shows forward-order dependence in proton donor, but we refrain from assigning the proton donor as bicarbonate, carbonic acid, or hydronium, because they may all be competent donors depending on the buffer strength and overpotential. Thus, we denote these viable donors cumulatively as BH. Although we note that minority proton donors, such as carbonic acid and hydronium, are expected to be depleted in the diffusion layer during steady-state catalysis, we stress that they can be readily replenished by rapid PT from majority buffer constituents or autoionization of water. Additionally, although we observe reaction orders in BH, the value of the order scales between 0.05 and 0.55 for bicarbonate and between −0.19 and −0.70 for pH depending on the applied potential. Despite the high population of CO on the surface (see above), the rate expression for HER is zeroth order in CO. This rate law also includes an exponential term that captures the potential-dependent rate of an irreversible single ET step. At low bicarbonate concentration, we observe a higher Tafel slope, leading us to include a second term in the rate expression that carries no potential dependence and is, instead, gated by a rate-limiting chemical step that still carries BH dependence. In Eq. 2, θ_HER is defined as the surface coverage of HER active sites responsible for the first term in the rate expression.

The rate expression shown in Eq. 2 is consistent with a mechanistic model for HER catalysis occurring simultaneously with CDR that involves rate-limiting ET to hydronium, carbonic acid, or bicarbonate to form an adsorbed Au–H species at high bicarbonate concentrations. This Au–H intermediate undergoes rapid one ET, one PT or recombination with another Au–H to generate H₂ and regenerate the surface site for subsequent turnover (Fig. 4B). This mechanism is consistent with the fact that Au–H species have not been spectroscopically observed to date, suggesting a low population of this intermediate in contrast to HER on Pt, which exhibits a high H coverage during catalysis (101). At lower bicarbonate concentrations, the second term dominates, limiting the rate of HER, which we postulate to be related to slow CO₂–bicarbonate equilibration steps (98, 99) that become competitive with the rate of PT to the surface at lower buffering strength and overpotentials. Nonetheless, the ensemble rate of HER is strongly dependent on the proton donor environment in a way that CDR catalysis is not.

SI Materials and Methods

Materials.

Pb(NO₃)₂ (99.999% purity), Na₂CO₃ (≥99.9999%; TraceSELECT), sulfuric acid (99.999%), NaClO₄•H₂O (99.99%), and NaOH (99.99%) were purchased from Sigma Aldrich. CuSO₄•5H₂O (99.999%) was purchased from Strem Chemicals. CO₂ (research grade), Ar (ultrahigh purity), and CO (ultrahigh purity) were purchased from Air Water or Airgas. Na[AuCl₄]•2H₂O (95.0%), NH₄Cl (99.0%), Na₂SO₃ (97.0%), Na₂S₂O₃•5H₂O (99.0%), Pb(NO₃)₂ (99.5%), and CuSO₄•5H₂O (99.9%) were purchased from Wako Pure Chemicals. Sulfuric acid (96% Suprapur) was purchased from Merck. All chemicals were used without additional purification. Millipore Type 1 Water (18.2 MΩ) was used throughout the study. H/D studies were carried using 99% D₂O (Cambridge Isotope Laboratories). In all cases, electrolytes were purified using regenerated Chelex100 (Bio-Rad) according to manufacturer’s protocol with slight modifications (104). Bicarbonate electrolytes of varying concentrations (0.03, 0.1, 0.3, and 1 M) were prepared by sparging CO₂ through a purified Na₂CO₃ solution with a concentration one-half of the desired final bicarbonate concentration.

Electrode Preparation: Au Films for in Situ Surface-Enhanced IR Spectroscopy Analyses.

A thin Au film (∼75 nm thick) was chemically deposited on the reflecting plane of a triangular Si prism (Pier Optics) by an electroless plating technique (49). Before each measurement, the Au electrode surface was electrochemically cleaned by cycling between 0.20 and 1.50 V vs. Ag/AgCl in 0.1 M H₂SO₄ until a stable cyclic voltammogram (∼5 CV cycles) was obtained. The 0.1 M H₂SO₄ was emptied out, and the film was rinsed with Millipore Water five times. Subsequently, CO₂-sparged bicarbonate electrolyte was carefully added to the electrochemical cell before the initiation of SEIRAS measurements. All data reported in Fig. 1 and Figs. S2 and S3 made use of the same SEIRAS-active Au film electrode. A new thin Au film was prepared on a doped Si wafer substrate [University Wafer ID 444 Si (100) P/B] using the same deposition procedure to evaluate the Tafel behavior of SEIRAS-active films (Fig. S5B). The Au on Si substrate was assembled into an electrode by connecting it to a Ti wire and cleaned by the same electrochemical treatment in H₂SO₄ as used for SEIRAS substrates. The electrode was assembled into an H cell to measure its activity, and Cu UPD was used to measure the electroactive surface area as detailed below.

Electrode Preparation: Au Foil Electrodes for Electrokinetic Analyses.

Au foil (2 cm²;99.999%; Alfa Aesar) was welded to a Au wire (99.999%; Alfa Aesar). The as-received Au foil was etched by dipping in aqua regia for 30 s. Before each measurement, the Au electrode surface was electrochemically cleaned by cycling five times without pause between 0.20 and 1.50 V vs. Ag/AgCl in 0.1 M H₂SO₄ electrolyte. The electrode was then rinsed with Millipore Water and transferred into an H cell for electrokinetic analyses using a protective water droplet.

Electrode Preparation: Au RCEs for Electrokinetic Analyses.

A rotating cone (Au; r = 0.25 cm; 45° cone angle; custom milled; PINE Research Instrumentation) was used as the working electrode at various rotation rates. Electrode rotation was controlled with a Metrohm Autolab B.V. Rotator that formed an air-tight seal with the working compartment of the H cell. The electrodes were polished sequentially using 1 and 0.3 µm alumina (Precision Surfaces International) on a polishing pad (Buehler) for 3 min each and sonicated using a bath sonicator for 5 min. Before each measurement, the Au electrode surface was electrochemically cleaned by cycling five times without pause between 0.20 and 1.50 V vs. Ag/AgCl in 0.1 M H₂SO₄ electrolyte. The electrode was rinsed with Millipore Water and transferred into the H cell for electrokinetic analyses using a protective water droplet.

Electrochemical Methods: In Situ Surface-Enhanced IR Spectroscopy Analyses.

All electrochemical experiments were conducted using an EG&G PAR Model 263A Potentiostat, a leakless Ag/AgCl electrode (eDAQ), and a high-surface area Pt mesh counterelectrode (99.997%; Alfa Aesar). Ag/AgCl reference electrodes were stored in Millipore Water between measurements and periodically checked relative to pristine reference electrodes to ensure against potential drift. All experiments were performed at ambient temperature. Electrode potentials were converted to the reversible hydrogen electrode (RHE) scale or SHE scale using E_RHE = E_Ag/AgCl + 0.197 V + 0.059(pH) or E_SHE = E_Ag/AgCl + 0.197 V. Potentials were corrected for the uncompensated ohmic loss (iR_u) in situ by positive feedback. R_u was measured using the R_u test function in the Model 270/250 Research Electrochemistry Software 4.11. All current density values are reported relative to the electrochemically active surface area of the working electrode measured by Cu UPD as described below. The three-compartment spectroelectrochemical cell held ∼35 mL electrolyte. The details of ATR-SEIRAS cell configuration have been described elsewhere (45 ⇓⇓–48, 59, 103). Before assembly, the cell was cleaned for at least 1 h in a concentrated H₂SO₄/HNO₃ mixture, rinsed with Millipore Water thoroughly, sonicated for at least 1 h in Millipore Water, and stored in fresh Millipore Water before all experiments. The Au-coated Si prism was assembled to the cell by a Kalrez vacuum O ring (i.d. = 15 mm). The Pt mesh was housed in a countercompartment separated from the working chamber by a porous glass frit. The reference electrode was separated from the working electrode by a Luggin capillary. Before experiments, the Au film was held at 0.60 V vs. Ag/AgCl (Fig. 1) or 0.18 V vs. Ag/AgCl (Fig. S3). Before all experiments, the electrolyte was sparged with CO₂ at 40 standard cubic centimeters per minute (sccm) for 15 min (Fig. 1 A–D and Fig. S3) or CO₂/CO at 20 sccm each (Fig. 1 E–H and Fig. S2). During all experiments, the electrolyte was sparged continuously with CO₂ at 20 sccm (Fig. 1 A–D and Fig. S3) or CO₂/CO at 20 sccm each (Fig. 1 E–H and Fig. S2).

Electrochemical Methods: Electrokinetic Analyses.

All electrochemical experiments were conducted using a Gamry REF 600 Potentiostat, a leakless Ag/AgCl electrode (eDAQ) or a Hg/HgSO₄ electrode (CH Instruments), and a high-surface area Pt mesh counterelectrode (99.997%; Alfa Aesar). Ag/AgCl and Hg/HgSO₄ reference electrodes were stored in Millipore Water and saturated K₂SO₄, respectively, between measurements and periodically checked relative to pristine reference electrodes to ensure against potential drift. All experiments were performed at ambient temperature. Electrode potentials were converted to the RHE or SHE scale using E_SHE = E_Ag/AgCl + 0.197 V + 0.059(pH) or E_SHE = E_Ag/AgCl + 0.197 V. Potentials were corrected for the uncompensated Ohmic loss (iR_u) in situ by positive feedback. R_u was measured using the R_u test function in the Gamry Framework software. All current density values are reported relative to the electrochemically active surface area of the working electrode measured by Cu UPD as described below. All electrolyte solutions were used as both the catholyte and the anolyte with stirring of the working chamber at a constant rate of 700 rpm during all experiments. Experiments were conducted in an airtight H cell with 25 mL catholyte and 20 mL anolyte separated by an anion exchange membrane (AGC Selemion Membrane). The H cell was cleaned for at least 1 h in concentrated H₂SO₄/HNO₃ mixture, sonicated for at least 15 min in Millipore Water, and stored in fresh Millipore Water before each experiment. Before all measurements, the electrolyte was sparged with CO₂ at 40 sccm for 1 h followed by the desired gas mixture (CO₂ only, CO₂/Ar, or CO₂/CO/Ar) for at least 1 h. During all experiments, the catholyte was sparged continuously with CO₂ at 10 sccm (Figs. 2, 3 A–C, 5 A and B, and 6 and Figs. S4, S5B, S6, S7, and S8), varying amounts of CO₂/Ar at a total of 10 sccm (Figs. 3B and 5C), or varying amounts of CO₂/CO/Ar at a total of 10 sccm (Figs. 3E and 5D), whereas the anolyte was sparged continuously with N₂. A mass flow meter was placed downstream of the cell before and after each measurement to ensure that no leaks were introduced into the system on cell assembly. The pH of the solution was measured after each experiment to ensure against pH drift.

Pb UPD Measurements.

Surface terminations of Au foils, RCEs, and SEIRAS-active films were analyzed using Pb UPD (105 ⇓⇓–108). The samples were cycled from −0.20 to −0.70 V vs. Ag/AgCl at a scan rate of 50 mV s^–1 in N₂-saturated 0.1 M NaOH electrolyte containing 0.1 mM Pb(NO₃)₂ to obtain the data shown in Fig. S5A. For all samples, the third CV cycle is reported.

Cu UPD Measurements.

Electroactive surface areas of Au foils, RCEs, and SEIRAS-active films were determined by Cu UPD (80). The Au samples were cycled from 0.00 to −0.45 V vs. Hg/HgSO₄ at a scan rate of 50 mV s^–1 in 0.5 M H₂SO₄ containing 7.4 mM CuSO₄. The broad anodic wave at ∼−0.15 V vs. Hg/HgSO₄ was integrated and divided by 92.4 C/cm² to obtain the electroactive surface area (56) (Fig. S1). The surface area values for each electrode were used to normalize the measured total current density and partial current densities for CO and H₂ evolution for the films and rotating cones/foils, respectively, for Figs. 1, 2, 3, and 5 and Figs. S4, S5, S6, S7, and S8.

SEIRAS.

SEIRA spectra were recorded in the Kretschmann ATR configuration using a Bio-Rad FTS-60A/896 FTIR Spectrometer equipped with an HgCdTe (MCT) detector and a homemade single-reflection accessory (incident angle of 60°). The spectrometer was operated in kinetic mode (40 kHz). Spectra were sequentially acquired with a spectral resolution of 4 cm^–1 at every 0.76-s interval for cyclic voltammetry measurements (Fig. 1) or 0.58-s interval (Fig. S3) for chronoamperometry measurements. A single-beam spectrum collected at the starting potential was used as the reference spectrum. All ATR-SEIRA spectra are reported in absorbance units defined as A = −log (I/I₀), where I and I₀ stand for the sample and reference single-beam spectra, respectively.

CO Stripping Analysis.

Stripping voltammetry data were collected without IR compensation on rotating Au electrodes at 1,000 rpm using a Metrohm Autolab B.V. Rotator. First, the electrode was poised at −0.80 V vs. SHE for 7 min to initiate sustained CDR catalysis in CO₂-saturated 0.1 M NaHCO₃. Second, CVs were recorded at a 50-mV s^–1 scan rate and initiated at −0.40 V vs. SHE immediately after (∼1-s time delay) the conclusion of electrolysis (Fig. 2 and Fig. S4). The broad feature spanning −0.20 to 0.80 V was integrated relative to the second CV scan (Fig. S4, red) by drawing a horizontal line between −0.20 and 0.10 V as the baseline for the integration of the first CV cycle for this 300-mV potential range. An authentic baseline between −0.20 and 0.10 V could not be obtained, because scanning to potentials more negative than 0.10 V leads to reformation of electrogenerated CO_bridge species as observed in SEIRA spectra (Fig. 1C). The raw integrated charge was divided by the Au surface area determined by Cu UPD (see above) to yield a surface area-normalized CO stripping charge value of 75 ± 15 μA cm^–2_Au. This value is the average and SD of three independent experiments. The data reported in Fig. 2 and Fig. S4 were smoothed using a Savitzy–Golay second-order polynomial to remove noise caused by solution stirring and CO₂ bubbling during the measurement.

In-Line Product Detection and Analysis.

Product distribution was measured using an in-line gas chromatograph (Multi-Gas Analyzer #3; SRI Instruments) equipped with a thermal conductivity detector, methanizer, and flame ionization detector in series after Molsieve 13× and Hayesep D columns. Before each experiment, the uncompensated cell resistance was measured and typically ranged from 11 to 120 Ω. Electrodes were polarized, and GC traces were collected every 20 min to allow for equilibration of the headspace and ensure that a steady state was achieved. The partial current density (j_p) for each CDR product, p, was calculated using the following relationship: j_p = [p] × flow rate × nFP/RT × 1/area; [p] is the parts per million value of the product measured by GC using an independent calibration standard gas mixture, n is the number of electrons transferred per equivalent of product, P is the pressure in the electrochemical cell headspace (1.1 atm), T is the temperature, and F is Faraday’s constant. The partial current density for a given product is divided by the total current density (averaged over a 35-s span immediately before each GC run) to determine its partial Faradaic efficiency.

Potentiostatic Tafel Data Collection.

Current–potential data were obtained by conducting controlled potential electrolysis in purified CO₂-saturated NaHCO₃ electrolyte at a variety of potentials using a Au RCE at 2,000 rpm (Fig. 5A, 0.1 and 1 M NaHCO₃ and Fig. S8, 1 M NaHCO₃) or a stationary Au foil (Fig. 3A, 0.1 M NaHCO₃) in a stirred electrolyte solution. Steady-state current was measured at applied potentials that ascended from −0.80 to −1.05 or −1.15 V vs. SHE proceeding in 50-mV steps. For a given applied potential, a GC trace was taken after steady state was reached, and the generated gases were equilibrated in the headspace (after 20 min). Before each reductive potential step, the Au surface was polarized at 0.80 V vs. SHE for 10 s to liberate adsorbed CO. Partial current densities for CO and H₂ production were determined by in-line GC as described above.

P_CO2 Dependence.

Partial current densities for H₂ and CO production were measured at applied potentials of −0.80, −0.90, and −1.00 V vs. SHE at P_CO2 values of 0.03, 0.1, 0.3, and 1 atm. P_CO2 was controlled by changing the relative flow rates of CO₂ and Ar and maintaining the total flow rate at 10 sccm. For a given applied potential, a GC trace was taken after steady state was reached, and the generated gases were equilibrated in the headspace (after 20 min). Before each reductive potential step, the Au surface was polarized at 0.80 V vs. SHE for 10 s to liberate adsorbed CO. After obtaining current–voltage data at a given P_CO2, the cell was disassembled, both the cell and electrode were cleaned as described above, the gas line was purged with N₂, and then, the next partial pressure was measured. Data collected in Fig. 3B were collected in CO₂-saturated 0.1 M NaHCO₃ on a Au foil, and data collected in Fig. 5C were collected on a rotating Au electrode operated at 2,000 rpm in CO₂-saturated 1 M NaHCO₃ at applied potentials of −0.85, −0.95, and −1.05 V vs. SHE.

Bicarbonate Concentration Dependence.

Partial current densities for H₂ and CO production were measured at applied potentials of −0.80, −0.90, and −1.00 V vs. SHE at bicarbonate concentrations spanning 0.03, 0.1, 0.3, and 1 M at a constant 1 atm CO₂ saturation. For each concentration, inert excess perchlorate (0.5 M NaClO₄) in addition to the varying bicarbonate were used to preserve solution conductivity and rule out diffuse double-layer effects. For a given applied potential, a GC trace was taken after steady state was reached, and the generated gases were equilibrated in the headspace (after 20 min). Before each reductive potential step, the Au surface was polarized at 0.80 V vs. SHE for 10 s to liberate adsorbed CO. After obtaining current–voltage data at a given bicarbonate concentration, the cell was disassembled, both the cell and electrode were cleaned (procedure described above), the gas line was purged with N₂, and then, the next bicarbonate concentration was measured. Data collected in Figs. 3C, 5 B and D, and 6 were collected on a Au foil.

H/D Dependence.

Partial current densities for H₂ and CO production were measured at applied potentials of −0.80, −0.90, and −1.00 V vs. SHE at H/D ratios of 0.188, 0.375, 0.65, and 0.925 under constant 1 atm CO₂ saturation. Ratios were tuned by adjusting the relative ratios of 0.1 M NaHCO₃ electrolyte to 0.1 M NaDCO₃ electrolyte; 0.1 M NaDCO₃ electrolyte was prepared by dissolving 0.478 g Na₂CO₃ in 100 g D₂O, saturating with CO₂, and then, purifying using Chelex Resin regenerated in 1 M NaOD instead of 1 M NaOH. For a given applied potential, a GC trace was taken after steady state was reached, and the generated gases were equilibrated in the headspace (after 20 min). Before each reductive potential step, the Au surface was polarized at 0.80 V vs. SHE for 10 s to liberate adsorbed CO. After obtaining current–voltage data at a given H/D ratio, the cell was disassembled, both the cell and electrode were cleaned as described above, the gas line was purged with N₂, and then, the next H/D ratio was measured. Data collected in Fig. 3D were collected on a Au foil.

P_CO Dependence.

Partial current densities for H₂ production were measured at applied potentials of −0.80, −0.90, and −1.00 V vs. SHE at P_CO values of 0.06, 0.13, and 0.25 under constant 0.5 atm CO₂ saturation. The P_CO was controlled by changing the relative flow rates of CO to Ar and maintaining the total flow rate of CO, Ar, and CO₂ at 10 sccm. For a given applied potential, a GC trace was taken after steady state was reached, and the generated gases were equilibrated in the headspace (after 20 min). Before each reductive potential step, the Au surface was polarized at 0.80 V vs. SHE for 10 s to liberate adsorbed CO. After obtaining current–voltage data at a given P_CO, the cell was disassembled, both the cell and electrode were cleaned as described above, the gas line was purged with N₂, and then, the next partial pressure was measured. Data collected in Fig. 3E were collected on a Au foil in 0.1 M NaHCO₃, and data collected in Fig. 5D were collected on a Au RCE at 2,000 rpm in 1 M NaHCO₃ at applied potentials of −0.85, −0.95, and −1.05 V vs. SHE.

KL Analyses.

For a given rotation rate, a GC trace was taken after the total current reached a steady state and the generated gases were equilibrated with the headspace (after 20 min). The rotation rate was varied in the following order (2,063, 976, 3,460, 730, and 1,371 rpm) to ensure against systematic errors caused by electrode history. The rotation sequence was performed twice, and the reported data in Figs. S6 and S7 are the average of the two sequences. This procedure was repeated at applied potentials of −0.80, −0.90, and −1.00 V vs. SHE for electrolysis conducted in CO₂-saturated 0.1 M NaHCO₃ and at applied potentials of −0.85, −0.95, and −1.05 V vs. SHE for electrolysis conducted in CO₂-saturated 1 M NaHCO₃. Partial current densities for CO and H₂ production were determined by in-line GC as described above. KL plots (Fig. S7 A and B) were extrapolated to infinite rotation rate to construct Tafel data in Fig. 5A, 0.1 M NaHCO₃ and Fig. S8, 1 M NaHCO₃. The error bars correspond to the error in the y-intercept obtained from least-squares fitting of the data in Fig. S7 A and B.

Concluding Remarks

CDR catalysis at a metal electrode is an ensemble phenomenon that arises from the complex dynamics of surface adsorbates, intrinsic reaction kinetics, proton-coupled ET dynamics, and concentration gradients that develop at the interface. Herein, we have used a combination of electrochemical kinetics and in situ IR spectroscopy to parse these effects, and we have explicitly accounted for diffusional gradients by comparing experimental data collected on static and rotating electrodes. We find, for CDR and HER conducted on polycrystalline Au in CO₂-saturated bicarbonate electrolytes, that (i) the surfaces of Au electrodes consist of a high population of spectator CO_bridge species, (ii) CDR is gated by rate-limiting ET to CO₂ and is not dependent on the proton donor environment, (iii) HER is strongly dependent on the proton donor environment, (iv) the observed transfer coefficients for CDR are higher than those for HER, and (v) the intrinsic activation-controlled kinetics of CDR and HER are augmented by interfacial diffusional gradients that serve to suppress HER preferentially relative to CDR.

Together, these factors determine the critical parameters—proton donor concentration and applied overpotential (η)—that dictate catalyst selectivity. The effects of these two parameters on reaction selectivity are illustrated specifically in Fig. 6. Because of the intrinsically lower Tafel slope for CDR vs. HER, higher η favors CO (Fig. 6, blue) over H₂ evolution (Fig. 6, red) at a given bicarbonate concentration. In addition, accelerated rates of proton consumption with increased η serve to further decelerate the rate of HER as the proton donor concentration is decreased proximate to the surface. Because CDR is insensitive to the proton donor environment, changes in the interfacial proton activity do not augment the rate of CO production. Furthermore, CO₂ equilibration with bicarbonate is hindered by slow CO₂ hydration kinetics [24-s half-life (99)], preserving a high interfacial CO₂ concentration, despite an elevated local pH. As a result, at modest η (Fig. 6, ■), similar rates of CDR and HER are observed irrespective of bicarbonate concentrations, whereas at higher η (Fig. 6, ● and ▲), lower bicarbonate concentrations serve to significantly suppress HER, enhancing selectivity. These mechanistic insights predict that CDR selectivity can be enhanced over a wider potential range by amplifying proton depletion effects within a porous electrode, a notion supported by our recent studies on ordered porous Au inverse opals (41). Interestingly, similar suppression of HER has been shown to enhance CDR selectivity in molecular catalytic systems (102).

Fig. 6.

CO (blue) vs. H₂ (red) Faradaic efficiency at −0.80 V (■), −0.90 V (●), and −1.00 V (▲) as a function of bicarbonate electrolyte concentration.

Our results establish the value of combining electrokinetic experiments with in situ IR studies to gain a molecular-level understanding of complex reactions, such as CDR. Spectroscopic data reveal that spectator CO_bridge species occupy a high population of the Au surface, consistent with the notion that catalysis proceeds at a minority fraction of surface sites. Taken together with electrochemical kinetic data, this work establishes a comprehensive model of the interfacial structure and reactivity of Au under catalytically relevant conditions, which highlights the divergent proton coupling requirements of CDR and HER as key drivers of selectivity in fuel formation.

Materials and Methods

Au films for in situ SEIRAS analyses were synthesized using an electroless plating technique (49). Handling procedures for the Au film, foil, and RCE are reported in SI Materials and Methods, and their electrochemically active surface area and surface faceting are shown in Figs. S1 and S5, respectively.

SEIRAS measurements were conducted using an EG&G PAR Model 263A Potentiostat and a Bio-Rad FTS-60A/896 FTIR Spectrometer equipped with an HgCdTe (MCT) Detector and a homemade single-reflection accessory. The details of the ATR-SEIRAS cell configuration have been previously reported (45 ⇓⇓–48, 59, 103). CO₂ or CO₂/CO mixtures were continuously delivered to a three-compartment spectroelectrochemical cell containing purified (104) 100 mM NaHCO₃ during measurements. Spectra were sequentially acquired with a spectral resolution of 4 cm^–1 at every 0.76-s interval for cyclic voltammetry measurements (Fig. 1) or 0.58-s interval (Fig. S3) for chronoamperometry measurements. A single-beam spectrum collected at the starting potential was used as the reference spectrum. Details of electrochemical methods used are further described in SI Materials and Methods.

Electrokinetic measurements were conducted using a Gamry REF 600 Potentiostat and an in-line gas chromatograph (Multi-Gas Analyzer #3; SRI Instruments) equipped with a thermal conductivity detector, methanizer, and flame ionization detector in series after Molsieve 13× and Hayesep D Columns. CO₂, CO₂/Ar, or CO₂/Ar/CO mixtures were continuously delivered to an airtight H cell containing purified (104) NaHCO₃ or NaDCO₃ (preparation is described in SI Materials and Methods) during measurements. GC traces were collected every 20 min, and the evolved CO or H₂ gases were converted to partial current densities as described in SI Materials and Methods.

Acknowledgments

We thank Anthony Shoji Hall for helpful discussions and Tomohiro Fukushima for facilitating spectroscopic studies conducted at Hokkaido University (HU). A.W. and Y.S. thank the Massachusetts Institute of Technology (MIT) International Science and Technology Initiatives and the Hayashi Seed Grant for Travel Funds to HU. This research was supported by Air Force Office of Scientific Research under AFOSR Award FA9550-15-1-0135 and by the MIT Department of Chemistry through junior faculty funds (Y.S.). Work at HU was partially supported by the New Energy and Industrial Technology Development Organization. A.W. is supported by a Graduate Research Fellowship from the National Science Foundation.

Footnotes

↵¹Present address: Department of Engineering Physics, Electronics, and Mechanics, Nagoya Institute of Technology, Nagoya 466-8555, Japan.
↵²To whom correspondence should be addressed. Email: yogi{at}mit.edu.

Author contributions: A.W. and Y.S. designed research; A.W. and M.Y. performed research; K.M. and M.O. contributed new reagents/analytic tools; A.W., M.Y., M.O., and Y.S. analyzed data; and A.W., M.Y., M.O., and Y.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1602984113/-/DCSupplemental.

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Inhibited proton transfer enhances Au-catalyzed CO₂-to-fuels selectivity

Significance

Abstract

Results and Discussion

Surface Adsorbate Population and Dynamics During CDR.

CO Evolution Rate Is Independent of the Proton Donor Environment.

H₂ Production Is Dependent on the Proton Donor Environment.

SI Materials and Methods

Materials.

Electrode Preparation: Au Films for in Situ Surface-Enhanced IR Spectroscopy Analyses.

Electrode Preparation: Au Foil Electrodes for Electrokinetic Analyses.

Electrode Preparation: Au RCEs for Electrokinetic Analyses.

Electrochemical Methods: In Situ Surface-Enhanced IR Spectroscopy Analyses.

Electrochemical Methods: Electrokinetic Analyses.

Pb UPD Measurements.

Cu UPD Measurements.

SEIRAS.

CO Stripping Analysis.

In-Line Product Detection and Analysis.

Potentiostatic Tafel Data Collection.

P_CO2 Dependence.

Bicarbonate Concentration Dependence.

H/D Dependence.

P_CO Dependence.

KL Analyses.

Concluding Remarks

Materials and Methods

Acknowledgments

Footnotes

References

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Significance

Abstract

Results and Discussion

Surface Adsorbate Population and Dynamics During CDR.

CO Evolution Rate Is Independent of the Proton Donor Environment.

H2 Production Is Dependent on the Proton Donor Environment.

SI Materials and Methods

Materials.

Electrode Preparation: Au Films for in Situ Surface-Enhanced IR Spectroscopy Analyses.

Electrode Preparation: Au Foil Electrodes for Electrokinetic Analyses.

Electrode Preparation: Au RCEs for Electrokinetic Analyses.

Electrochemical Methods: In Situ Surface-Enhanced IR Spectroscopy Analyses.

Electrochemical Methods: Electrokinetic Analyses.

Pb UPD Measurements.

Cu UPD Measurements.

SEIRAS.

CO Stripping Analysis.

In-Line Product Detection and Analysis.

Potentiostatic Tafel Data Collection.

PCO2 Dependence.

Bicarbonate Concentration Dependence.

H/D Dependence.

PCO Dependence.

KL Analyses.

Concluding Remarks

Materials and Methods

Acknowledgments

Footnotes

References

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H₂ Production Is Dependent on the Proton Donor Environment.

P_CO2 Dependence.

P_CO Dependence.