Trends in the Catalytic Activity of Hydrogen Evolution during CO2

Jan 17, 2018 - ABSTRACT: During CO2 electroreduction (CO2R), the hydrogen evolution reaction (HER) is a competing reaction. We present a combined expe...
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Letter Cite This: ACS Catal. 2018, 8, 3035−3040

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Trends in the Catalytic Activity of Hydrogen Evolution during CO2 Electroreduction on Transition Metals Etosha R. Cave, Chuan Shi, Kendra P. Kuhl, Toru Hatsukade, David N. Abram, Christopher Hahn, Karen Chan,* and Thomas F. Jaramillo* Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States S Supporting Information *

ABSTRACT: During CO2 electroreduction (CO2R), the hydrogen evolution reaction (HER) is a competing reaction. We present a combined experimental and theoretical investigation of the HER activity of transition metals under CO2R conditions. Experimental HER polarization curves were measured for six polycrystalline metal surfaces (Au, Ag, Cu, Ni, Pt, and Fe) in the presence of CO2 gas. We found that the HER activity of the transition metals is significantly shifted, relative to the CO2-free case. Density functional theory (DFT) calculations suggest that this shift arises from adsorbate−adsorbate interactions between *CO and *H on intermediate and strong binding metals, which weakens the *H binding energy. Using a simple model for the effect of *CO on the *H binding energy, we construct an activity volcano for HER in the presence of CO2 gas that is consistent with experimental trends. The significant changes in HER activity in the presence of CO2 gas is an important consideration in catalyst design and could help develop catalysts that are more selective for CO2R than the HER. KEYWORDS: CO2 electroreduction, hydrogen evolution reaction, electrocatalysis, volcano plot, adsorbate−adsorbate interactions

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INTRODUCTION Carbon-based chemicals derived from petroleum products form the foundation of our energy and specialty chemicals infrastructure. Finding cost-effective, sustainable pathways to produce these compounds could be beneficial to the long-term growth of these industries and reduce the accumulation of CO2 in the atmosphere.1 The use of CO2 as a feedstock for the synthesis of specialty chemicals and fuels has been proposed as one such pathway.2,3 With renewable electricity as the energy input, the electroreduction of CO2 and water into higher-value chemicals would create a sustainable supply of industrial chemicals with a concomitant decrease of CO2 entering the atmosphere.4 Efficient, selective, and robust catalysts are required to create this industrial carbon cycle.5 Advances in the fundamental understanding of electrochemical CO2 reduction (CO2R) are needed to develop better catalysts. One major challenge is to decrease the selectivity toward the hydrogen evolution reaction (HER), which is ubiquitous under aqueous conditions. Several strategies have been explored to suppress the HER and enhance CO2R, which include ionic liquids,6−8 an aqueous pH buffer region,9,10 oxide-derived surfaces,11,12 cations,13 nanostructuring,14 and nitrogenous cocatalysts.15 We seek to understand how the HER activity is suppressed or promoted under CO2R conditions. © XXXX American Chemical Society

In this study, we show the trends in HER activity on six transition metals in the presence of CO2 gas. We present a combined experimental and theoretical investigation of HER activity in the presence of CO2 on the polycrystalline transition metals, which have a broad range of hydrogen [*Eb(H)] and CO binding energies [*Eb(CO)]. Importantly, the activity and selectivity toward HER are quantified by analysis of all major and minor products. With the aid of density functional theory (DFT) calculations, we propose a simple model for the effect of *CO on *Eb(H) and construct an activity volcano based on experimentally determined HER potentials under CO2R conditions. Our analysis shows a significant shift in the trends in HER activity in the presence of CO2 gas, and our model provides a simple way to account for this effect in future catalyst screening studies.

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RESULTS AND DISCUSSION Y-Axis Descriptor. To identify the y-axis descriptor for the HER-under-CO2R-conditions volcano, we examined the average H2 partial current densities, as a function of potential for the six polycrystalline transition metals (Figure 1). The total Received: November 7, 2017 Revised: January 17, 2018

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Figure 1. Hydrogen partial current densities under CO2R conditions for six polycrystalline transition metals. The potential at 1 mA/cm2 was used as an activity metric for the metals.

Figure 2. *Eb(CO) vs *Eb(H) for the stepped (fcc metals - (211), bcc metals - (310)) and flat surfaces (fcc metals - (111), bcc metals (110)) of various transition metals.

current, current efficiencies (also called Faradaic efficiencies), and partial current densities for the metals tested in this study can be found in the Supporting Information. The partial current densities to H2 increase as a function of potential for all the tested metals, and do not appear to have reached mass transport limitations that would have caused the partial current densities to plateau. Given the reducing potentials applied, we expect all the metals tested to be in their reduced form. The metals tested within this study encompass a wide range of HER activities, and their order of activity under CO2R conditions is clearly delineated as Fe > Pt > Ni ≈ Au > Cu > Ag. To establish a quantitative activity metric for the HER during CO2R, we chose the potential to reach a H2 partial current density of 1 mA/cm2 as a common metric among all the metals. A zoomed-in plot of the activities near 1 mA/cm2 can be found in the Supporting Information. There is a clear discrepancy in the order of activity, when compared to typical HER conditions, since Fe is the most active in this case, whereas Pt is considered one of the most active metals for evolving H2 in the presence of an inert gas.16−18 The leading hypothesis for this behavior is that the presence of *CO, from the reduction of CO2, changes *Eb(H) and suppresses hydrogen evolution on Pt. X-Axis Descriptor. It has previously been established that *Eb(H) is a suitable descriptor of HER activity.17,19 It has also been previously reported that the effect of spectator species can be considerable on the activity of HER.16 Under CO2R conditions, metals with strong *Eb(CO) are expected to have a stable coverage of *CO formed from CO2, which block sites as well as alter the *Eb(H) through adsorbate−adsorbate interactions.20,21 These interactions significantly shift the *Eb(H) and, subsequently, the HER activity among the transition metals. To create the x-axis descriptor for the HER under CO2R conditions volcano, we quantified the effect of coadsorbed carbon monoxide (*CO) on *Eb(H). Figure 2 shows *Eb(H) vs *Eb(CO) on bare high and low Miller index (stepped and flat, respectively) surfaces as noted. The binding energies of these two adsorbates scale linearly, which is consistent with previous reports.22 We assume that the steady-state coverages, under CO2R conditions, are largely determined by the equilibrium coverage

of *CO. This assumption is consistent with a recent kinetic model of CO reduction, which suggests the dominance of *CO vs *H coverage on transition metals.23 Furthermore, Fourier transform infrared (FTIR) spectroscopy of metals surfaces during CO2 and CO electroreduction show a large coverage of CO and a very low coverage, if any, of other stable reaction intermediates.24−27 Based on the experimental current densities,28 the CO head pressure was estimated to be 1 mbar.22 At this pressure, *CO is expected to adsorb under equilibrium conditions on metals with *Eb(CO) energies more negative than −0.5 eV, which is the range to the left of the vertical line in Figure 2. Therefore, we expect that, under CO2R conditions, Cu and any metal with stronger *Eb(CO) to its left have some coverage of *CO, while those to its right are not expected to bind CO. Based on their *Eb(CO), we classify the metals here into three categories: strong CO binding (Ir to Pt), intermediate CO binding (Cu), and weak CO binding (Au to Pb). Effect of CO Coverage on H binding. To calculate the effect of coadsorbed CO on *Eb(H), we first determined the expected *CO coverage for each category of metals. Figure 3 shows the differential binding free energy as a function of coverage on Pt(111) at a CO pressure of 1 mbar, which suggests an equilibrium *CO coverage of ∼0.6 ML. For simplicity, we assume this equilibrium coverage for all of the other strong binding metals, since the slopes of their average CO binding energies as a function of coverage on (111) surfaces are similar. Thus, their differential binding free energies, as a function of coverage, would be similar as well, indicating that the equilibrium coverage of *CO on Pt(111) can be used as a proxy.29 Previous work has suggested that binding energies for various adsorbates shift linearly with coverage between a threshold coverage of ∼0.5 to 1.30 We determined the corresponding shift in *Eb(H) at 0.6 ML of *CO to be +0.57 eV (Table S3 in the Supporting Information) via linear interpolation of previously reported data.22 Thus, this +0.57 eV shift was added to *Eb(H) for all strong binding metals to account for the *CO. Metals with weak *Eb(CO), i.e., with a binding energy more positive than −0.5 eV, are not expected to bind CO; therefore, no shift in *Eb(H) is expected in the presence of CO2 or CO gas. For 3036

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Figure 4. Experimental volcano of HER under CO2R conditions. The dashed line is drawn to guide the eye. Figure 3. Differential ΔG for *CO, as a function of CO coverage (θ) on Pt(111) at a CO pressure of 1 mbar. Data for this figure were first published in a previous article22 and is shown here with alterations.

was shown to enhance the HER activity on Mo while suppressing the HER activity on Cu due to the weakening of *Eb(H).21 The analysis in this letter is consistent with the previous study and has a wider range of metals with more comprehensive product detection. High selectivity for the HER reduces the energy efficiency of a catalyst to desirable carbon-based products. To understand the interactions between CO2R and the competing HER, we have shown that it is necessary to determine how *CO affects the *Eb(H) of metals. These insights could aid in establishing a more comprehensive view of competing reactions during aqueous CO2R and are applicable to future catalyst screening studies to develop catalysts that are highly selective to carbonbased products.

Cu, which binds at intermediate strength, we have assumed an intermediate *CO coverage to be 0.5 ML, which gives a corresponding *Eb(H) shift of +0.26 eV.22 The experimental trends in HER activity corroborate this intermediate coverage, since a Cu *CO coverage of 0.5 ML has a resulting *Eb(H) between that of Ag and Au. Previous experiments showing the suppression of HER on Cu in the presence of CO gas31 or CO2 gas20 further support the hypothesis of an intermediate coverage, since a coverage of 0.4 ML or lower would have an estimated correction to *Eb(H) of nearly zero within our assumptions. These results demonstrate a simple method to account for the effect of coadsorbed CO on the HER activity under CO2R conditions by adding a correction to *Eb(H) based on the expected coverage of CO. Constructing the Volcano Plot. Based on the experimental onset potentials and the corresponding *Eb(H) free energies at the estimated equilibrium *CO coverage, a volcano plot was constructed for HER under CO2R conditions with both stepped and flat surfaces (see Figure 4). Overall, a clear correlation is observed between the H2 partial current densities and *Eb(H), indicating that the experimental and theoretical trends in activity agree. Note that the dashed line in Figure 4 is drawn to guide the eye to the overall trend and is not a linear regression. The slight scatter in the data is on the order of the accuracy of DFT calculations (∼0.2 eV). A few key observations can be made with the data. Most notably, Pt, which is widely known for being the most active HER catalyst, as shown in previously generated volcano plots,19,32−34 is no longer near the ideal *Eb(H), because of the weakening effect of *CO on *Eb(H). Furthermore, this effect causes all of the strong *Eb(CO)/*Eb(H) metals, i.e., Fe, Pt, Ni, to shift to the weak *Eb(H) side of the volcano, while a weaker *Eb(H) causes Cu to shift further to the right. In effect, the weakening of *Eb(H) brings Fe closer to an optimal ΔG value of zero, which promotes HER activity, while Pt, Ni, and Cu move further away, which suppresses HER activity. The dramatic shift in the activity of transition metals is consistent with a previous analysis of Mo and Cu under CO2R conditions, where *CO

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CONCLUSIONS In summary, we have investigated the HER activity for six polycrystalline transition-metal surfaces under CO2R conditions. We found that the order of HER activity for the transition metals is significantly altered, relative to the CO2-free case, with Fe performing the best out of all the metals tested. From DFT analysis, we demonstrate that this behavior is a result of adsorbate−adsorbate interactions on metals with strong to intermediate *Eb(CO). Specifically, metals with a strong or intermediate *Eb(CO) have a coverage of *CO that leads to a substantial weakening in *Eb(H). Weak binding metals are unaffected, since no *CO coverage is expected. Using a simple model for the effect of coadsorbed CO on *Eb(H), we constructed a volcano for HER in the presence of CO2, which accurately describes the experimental trends. Most notably, Fe is shifted near the top of the volcano, while Pt shifts down the weak *Eb(H) side and, thus, has lower activity. This ultimately shifts the order of activity of these metals, when compared to HER when CO2 is absent. The effect of CO2 gas on HER activity is an important consideration in catalyst design. Future catalyst screening studies could apply the simple model presented to estimate the shift in *Eb(H) and corresponding HER activity. Future work will consider a larger range of transition metals and extend the current simple thermodynamic picture to include explicit barriers and kinetics, as well as adsorbate−adsorbate interaction models. 3037

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with H+ + e− at U = 0 vs RHE, the adsorption free energy we report is equivalent to the adsorption free energy of H* at U = 0 V vs RHE. The adsorption free energy for *H at any potential vs RHE can be obtained via ΔG = ΔG° + eURHE.46

METHODS We previously tested these transition-metal catalysts, analyzed their CO2R activity, and determined their product selectivity with high sensitivity to minor products.28,35−37 Using experimentally determined onset potentials, volcano plots for CO gas evolution and further reduced carbon products were constructed using a theory-guided understanding of the role of adsorbed carbon monoxide on CO2R activity.28 A thorough description of CO2R testing conditions has been given in previous publications.35−39 Briefly, experiments were conducted in a two-compartment electrolysis cell at room temperature and atmospheric pressure. Each metal was purchased as a polycrystalline foil from Alfa-Aesar; see the Supporting Information for the lot and product numbers of the metals purchased. Several 1 h chronoamperometry experiments were averaged to represent the performance of the metals as a function of each potential; see Figure S1 for representative total current versus time data. Gas chromatography was used to measure the gaseous products of each sample, with a thermal conductivity detector to measure the H2 and a flame ionization detection device to measure carbon products. Measurements from the gas chromatograph were used to calculate the H2 partial current density. Because the experiments were done potentiostatically across a range of potentials, often an exact measurement at 1 mA/cm2 was not recorded. Thus, an exponential curve was fit to the data to find the potential where an expected current density of 1 mA/cm2 would be measured. Figure S2 in the Supporting Information contains the HER partial current density (∼1 mA/cm2) for each metal and their corresponding exponential fit. Table S1 in the Supporting Information has the equations used to fit the data, as well as their corresponding R2 values. Other partial current density values were evaluated (0.25, 0.5, and 5 mA/cm2) and produced similar trends in activity. Details for DFT calculations were given in previous publications.40,22 Briefly, DFT calculations were performed using the DACAPO plane-wave DFT code, ultrasoft pseudopotentials, and the RPBE41 exchange−correlation functional were used with a planewave cutoff of 340 eV, a density cutoff of 500 eV, and an extrapolated electronic temperature of 0 K and a 0.1 eV. Fermi smearing was applied. To represent terraces and steps, for fcc structures, we considered (111) and (211) facets, and for bcc structures, we considered (100) and (310) facets. Spin-polarized calculations were performed on Fe and Ni. We considered (3 × 3) computational cells for all facets except for bcc (310), where (3 × 4) cells were applied. We applied three layers of atoms for the (211) surfaces, and four layers for all others, and, in all cases, the bottom two layers were fixed at the bulk lattice constant, (4 × 4 × 1) and (4,3,1). Monkhorst− Pack42 k-point grids were used to sample the Brillouin zone for the (3 × 3) and (3 × 4) cells, respectively. Using the atomic simulation environment interface,43 geometric optimization of the topmost two layers of the cell and adsorbates was performed using the BFGS line search method, until all force components were below 0.05 eV/Å. All adsorption sites were considered on the aforementioned facets and adsorption energies were referenced to gas-phase molecules CO and H2 at pressures of 0.05 and 1 bar, respectively; the CO pressure was estimated based on the partial pressure corresponding to Faradaic yields.44,45 To construct the HER volcano, free-energy corrections to the electronic adsorption energies were applied as outlined in previous work.40,45 Since 1/2H2 is in equilibrium

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03807. The current efficiency of the metals tested in this study, a zoomed-in plot of H2 partial current density with curve fitting equations, linear interpolation of *Eb(H), a table of calculated binding energies for hydrogen and carbon monoxide for various bcc and fcc metals including the ones used in this study; a table of information on the metals purchased for this study; a table of the chronoamperametry data; and a graph of the standard error for the metals tested. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K. Chan). *E-mail: [email protected] (T. F. Jaramillo). ORCID

Christopher Hahn: 0000-0002-2772-6341 Karen Chan: 0000-0002-6897-1108 Thomas F. Jaramillo: 0000-0001-9900-0622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub. The electrochemistry data analysis and density functional theory calculations were supported by the Office of Science of the U.S. Department of Energy under Award No. DESC0004993. The development of electrochemical testing was supported by the Global Climate Energy Project (GCEP) at Stanford University. K.P.K. and E.R.C. acknowledge support by the National Science Foundation Graduate Research Fellowship, and E.R.C. acknowledges support from the Ford Foundation. D.N.A. acknowledges support from a Stanford Graduate Fellowship.

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