Boosting Formate Production in Electrocatalytic CO2 Reduction over

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Boosting Formate Production in Electrocatalytic CO Reduction over Wide Potential Window on Pd Surfaces Bei Jiang, Xia-Guang Zhang, Kun Jiang, De-Yin Wu, and Wen-Bin Cai J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12506 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Journal of the American Chemical Society

Boosting Formate Production in Electrocatalytic CO2 Reduction over Wide Potential Window on Pd Surfaces Bei Jiang†, Xia-Guang Zhang‡, Kun Jiang§, De-Yin Wu‡,* and Wen-Bin Cai†,*

† Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China § Rowland Institute, Harvard University, Cambridge, MA 02142, United States.

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ABSTRACT Facile interconversion between CO2 and formate/formic acid (FA) is of broad interest in the energy storage and conversion and the neutral carbon emission. Historically, electrochemical CO2 reduction reaction to formate on Pd surfaces was limited to a narrow potential range positive of -0.25 V (vs. RHE). Herein, a boron-doped Pd catalyst (Pd-B/C), with a high CO tolerance to facilitate dehydrogenation of FA/formate to CO2, is initially explored for electrochemical CO2 reduction over the potential range of -0.2 V to -1.0 V (vs. RHE), with reference to Pd/C. The experimental results demonstrate that the faradaic efficiency for formate (ηHCOO-) reaches ca. 70% over 2 h of electrolysis in CO2 saturated 0.1 M KHCO3 at -0.5 V (vs. RHE) on Pd-B/C, that is ca. 12 times as high as that on homemade or commercial Pd/C, leading to a formate concentration of ca. 234 mM mg-1Pd, or ca. 18 times as high as that on Pd/C, without optimization of the catalyst layer and the electrolyte. Furthermore, the competitive selectivity ηHCOO-/ηCO on Pd-B/C is always significantly higher than that on Pd/C despite a decreases of ηHCOO- and an increases of the CO faradaic efficiency (ηCO) at potentials negative of -0.5 V. The density functional theory (DFT) calculations on energetic aspects of CO2 reduction reaction on modelled Pd(111) surfaces with and without H-adsorbate reveal that the B-doping in the Pd subsurface favors the formation of the adsorbed HCOO*, an intermediate for the FA pathway, more than that of *COOH, an intermediate for the CO pathway. The present study confers Pd-B/C a unique dual functional catalyst for the HCOOH ↔ CO2 interconversion.

KEYWORDS: Carbon dioxide reduction, Pd catalysts, Boron-doping, Electrocatalysis, Competitive selectivity, Density functional theory calculations.

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1. INTRODUCTION Controlled facile interconversion between CO2 and formic acid (FA)/formate is of great significance in energy conversion and storage as well as carbon emission control.1-6 The relevant applications include but are not limited to direct FA/formate fuel cell via electrochemical dehydrogenation of FA/formate to CO2, and fuel regeneration based on electrochemical hydrogenation of CO2 to formate/FA with a projected neutral CO2 emission.7-10 Despite the fact that some new catalysts are recently developed with high performances in converting CO2 to formate, including reduced nano-SnO2,2 Cu@SnO2,11 oxide-deried Pb12 and partially oxidized atomic cobalt layers,13 Pd is still the most promising catalytic metal that enables the facile interconversion between CO2 and FA/formate. In fact, by using a Pd-based anode catalyst, a high power density comparable to that of a H2-O2 fuel cell could be achieved in direct formic acid fuel cells,5,14-17 and by using a Pd cathode catalyst, smallest overpotentials are needed to enable the CO2RR to formate due to the unique property of H adsorption/absorption on/in Pd.18,19 Nevertheless, a major challenge commonly facing such an interconversion on Pd surfaces is that the parallel CO-pathway competes strongly with or even deactivates the desired pathway.18,20-25 So far, the study of CO2RR on Pd-based catalysts was chiefly targeted to achieve high faradaic efficiencies (FE, η) and partial currents (i) on either formate or CO by tuning the catalyst particle size, shape and composition.5,18,21,26,27 Notably, the formate faradaic efficiency (ηHCOO-) and the CO faradaic efficiency (ηCO) are highly dependent on applied potential, pH and concentration of electrolyte, and electrolysis time. Despite a large discrepancy in the reported ηHCOO- and iHCOO- values owing to different measurement conditions for CO2RR on Pd electrodes, it seems to reach a consensus that a higher selectivity toward HCOO- against CO is limited to a very narrow range of potentials negative of ca. 0 V (vs. RHE) in KHCO3 solution. Specifically, Wang’s group mentioned that ηCO on Pd/C between -0.6 and -1.2 V (vs. RHE) was close to 100% in the most commonly used CO2-saturated 0.1 M KHCO3.21,26,28,29 Kanan’s group reported ηHCOO- values were higher than 90% for CO2RR on Pd/C dispersed on Ti in CO2 –saturated 0.5 M and 2.8 M KHCO3 at potentials positive of -0.25 V (vs. RHE) but decayed due to surface CO accumulation.18 A recent detailed study by Wang’s group further revealed that either ηHCOO- or ηCO higher than 90% could be attained in 1 M KHCO3 at potentials 3



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positive of -0.2 V or negative of -0.5 V (vs. RHE).19 Notably, the high ηHCOO- at a small overpotential in principle can be counter-balanced by a low total current, and the resulting iHCOO- is not necessarily high.3,18,30,31 On the other hand, using Pd/C as a benchmark to screen a series of PdxPt1-x/C for CO2RR in CO2-saturated 0.1 M K2HPO4/KH2PO4 (PBS), Koper’s group detected a ηHCOO- of ca. 10 ~ 17% for Pd/C at -0.5 V (vs. RHE) where partial formate current is highest, depending on the electrolysis time.5 Despite these contributions, unfortunately, least effort has been devoted to promote the formate production in the electrochemical CO2RR on Pd catalysts over a wide potential range, especially when they are targeted for the interconversion of FA/formate to and from CO2. Pd-B/C, a boron-doped Pd-based electrocatalyst, was first developed in our group.32 B insertion in the Pd lattice leads to the formation of interstitial Pd-B alloy. Our previous works indicate that as compared to Pd/C, Pd-B/C enhances the (electro)catalytic dehydrogenation of FA with significantly inhibited CO accumulation, promising for the anode catalyst of direct formic acid fuel cells as well as the heterogeneous catalyst of hydrogen production in FA/formate solution.22,23 With this in mind, it is enticing to explore Pd-B/C as a new catalyst for the reverse electro-hydrogenation process, with an expectation to tune up the ratio ηHCOO-/ηCO and boost the formate production at extended potentials, in an effort to ultimately close the carbon cycle between CO2 and formate/FA. In the present work, the CO2RR on Pd-B/C and Pd/C catalysts in 0.1 M KHCO3 over the potential range of -0.2 to -1.0 V (vs. RHE) is comparatively investigated by electrochemical measurement, on-line gas chromatography and NMR analysis, revealing significantly higher

ηHCOO-/ηCO ratio and formate production rate on Pd-B/C than on Pd/C under otherwise the same conditions. To understand the mechanistic aspect at a molecular level, density functional theory (DFT) calculations are applied to elucidate the competition for the FA and CO pathways during CO2RR on H-adsorbed and unadsorbed Pd(111) surfaces without and with B-doping in the subsurface. The present work signifies that Pd-B/C is a unique bifunctional catalyst for promoting the HCOOH ↔ CO2 interconversion.

2. EXPERIMENTAL & CALCULATION METHODS 2.1 Catalyst Synthesis and Characterization. Carbon supported boron-doped palladium 4


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catalyst (Pd-B/C with 18 wt.% Pd and 6.3 at.% B doping in Pd as determined by ICP-AES) was prepared through an aqueous chemical synthesis with dimethylamine borane (DMAB) as the reducing agent, based on our previous reports.22,32 The B doping content is confined to ca. 6-7 at. % by such a synthesis. Further increasing the B-doping content may be possible via an organic phase synthesis but may cause a complication in surface cleaning and a possible structural transition,33 and thus was not attempted in this work. Pd/C (ca. 40 wt. %) from BASF Fuel Cell Inc. (Lot No. F0301009), noted as Pd/C (com) and homemade Pd/C (ca. 19 wt.%), noted as Pd/C (home) with sodium borohydride (NaBH4) as the reducing agent in an aqeuous solution synthesis32 were used for comparison. The morphology and dispersion of Pd-B/C and Pd/C catalysts were characterized by A JEOL JEM-2100F tunneling electron microscope (TEM). An ESCALAB 250 XI X-ray photoelectron spectroscopy (XPS) of Thermo Scientific was used to analyze the surface electronic structures, using a monochromatic Al Kα radiation (1486.6 eV). The binding energies were calibrated with reference to the C 1s peak at 284.6 eV. The X-ray diffractometer (XRD) of a Bruker D8 advance was used to examine the crystalline structures. The chemical composition of Pd-B/C was analyzed with the aid of inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Thermo Elemental IRIS Intrepid). 2.2. Electrochemical Measurement. The electrochemical measurement on CO2RR was carried out at room temperature in an H-type cell with a three-electrode system of which the cathode and anode compartments respectively containing 45 mL of the electrolyte were separated by a Nafion 117 membrane. An L-type glassy carbon (GC, φ = 5 mm) electrode covered with a Pd/C or Pd-B/C catalyst layer served as the working electrode. A catalyst ink was prepared by ultrasonically dispersing 5 mg of Pd/C (home), 5 mg of Pd-B/C and 2.5 mg of Pd/C (com) respectively in 1 mL of isopropanol solvent mixed with 120 µL of 5 wt. % Nafion isopropanol solution (Alfa Aesar). Around 22 µL of this ink was transferred and drop-coated onto the GC electrode with a Pd loading of 100 µg cm-2 via a pipette, and dried at room temperature. Before electrochemical measurement, the catalyst layer-covered GC electrode was thoroughly rinsed with a copious amount of ultrapure Milli-Q water. A CHI 660B electrochemistry workstation (Shanghai Chenhua Instruments) was used for potential control and current measurement. A saturated calomel electrode (SCE) and a Pt foil served as the reference electrode and the counter electrode, respectively. Nevertheless, all 5



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potentials reported were converted with reference to a reversible hydrogen electrode (RHE). 2.3. Detection and Quantification of Products. The products dissolved in solution were analyzed with NMR spectroscopy by a 500 MHz nuclear magnetic resonance (NMR) from Bruker Company.34 Phenol was used as the internal standard for quantification of the dissolved products. The gas products were analyzed by an on-line gas chromatography (GC) with a Gas Chromatograph 2060 from Ramiin Instruments equipped with a flame ionization detector (FID) coupled with a methanizer and a thermal conductivity detector (TCD). The detection limits of the FID and TCD detectors are 0.5 ppm and 5 ppm, respectively. The FID was used to quantify hydrocarbons, CO and CO2 and the TCD to quantify H2. The data were read out every 15 min after the gas products were brought in the GC system at desired times of electrolysis. 2.4. Computational methods. Density functional theory (DFT) calculations were performed by using the Perdew-Burke-Ernzerhof (PBE) functional of generalized gradient approximation (GGA)35 in Vienna Ab Initio Simulation Package (VASP).36 The projector-augmented wave (PAW) method was applied to describe the electron-ion interactions.37 Energy cutoff of 500 eV and Methfessel-Paxton method with a broadening factor of 0.1 eV were used. The calculated lattice constant of Pd bulk was 3.96 Å, which agrees the experimental value of 3.89 Å (error is ca. 1.6 %).38 In this work, a vacuum space of 15 Å was used for all the calculations. The Γ-centered k-point sampling grid of 4×4×1 was adopted for all single crystal facets. For calculations of density of states (DOS), the k-point sampling grid was increased to 12×12×1. The Pd(111) surface was simulated with a 2×2 five-layer slab with the top three layers relaxed and the bottom two layers fixed in all calculations. On the top of the optimized structures, vibrational frequencies of the free and adsorbed molecules were calculated on the basis of the finite difference method where all metal atoms and boron atoms were frozen.The Bader charge analysis was performed using the code from Henkelman group.39 Adsorption energies (Eads) of reactants, intermediates, and products were estimated by

Eads = Etot − Emetal − Emol

(1)

where Etot, Emetal and Emol are total electronic energies of molecules adsorbed on a slab, clean metal slab and free molecules, respectively. In this work, all thermodynamic properties were further calculated using the Atomic Simulation Environment suite of programs.40 The Gibbs 6


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free energies were calculated at 298.15 K and 1 atm as outlined below,

G=H − TS = EDFT + EZPE + ∫

298K

0

CvdT − TS

(2)

where EDFT is the total electronic energy obtained from DFT geometry optimization, EZPE is the zero-point vibrational energy (ZPE) obtained from calculated vibrational frequencies, the thermal energy

∫

298K

0

CvdT is calculated from the heat capacity, T is the temperature, and S is

the entropy of studied systems. For the molecules of CO2, H2, CO, HCOOH and H2O, the ideal gas approximation was used, and for the adsorbates the harmonic approximation was used. The electrode potential related to the Gibbs free energy of a half electrochemical reaction could be calculated by the computational hydrogen electrode (CHE) model for CO2RR to HCOOH41. The CHE model was also used to refer the chemical potential of proton-electron pairs and hydrogen gas at the standard equilibrium condition, H + ( aq ) + e − ↔

1 H 2( g ) 2

(3)

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterizations. Typical TEM images for the Pd-B/C, Pd/C (home) and Pd/C (com) samples are shown in Figure 1A-C. The statistics of 300 nanoparticles from different TEM images of each sample reveals that the sizes of Pd-B, Pd (home) and Pd (com) nanoparticles are ca. 4.1 ± 0.5, 4.0 ± 0.3 and 3.8 ± 0.5 nm, respectively.22,23 The ICP-AES analysis indicates an insignificant amount of B-dopant (0~2 at. % B) in Pd nanoparticles for the Pd/C samples if any, and 6.3 at. % B content in Pd-B nanoparticles for the Pd-B/C sample. According to the ball model proposed by Benfield42, the coverage of B doped in the Pd subsurface of a 4-nm Pd-B nanoparticle can be calculated to be higher than 75%. For simplification, the full coverage of B in Pd(111) subsurface (denoted as Pd-4B, representing 4 B atoms in a 2×2 five-layer slab model) will be used in the following DFT calculations to explain the experimental results. The XRD patterns in Figure 1D show the diffraction peaks featuring Pd(111), Pd(200) and Pd(220) planes for Pd-B/C shift negatively, as compared to those for Pd/C (home) and Pd/C (com), suggestive of an enlarged Pd-Pd lattice by B doping in the interstices. The core-level XPS spectra in Figure 1E after deconvolution show that the Pd(0) core level binding energy is 7



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slightly blue-shifted for Pd-B/C with respect to that for Pd/C (home) or Pd/C (com), indicative of a partial electron transfer between Pd and B as well as a downshifted d-band center of surface Pd atoms. The partial charge transfer is also consistent with the Bader charge analysis in the following DFT calculation (see Table S2 in the SI). A higher surface Pd(ΙΙ) component for Pd/C (com) may arise from an aging exposure to the air, and can be readily reduced to Pd(0) at the CO2RR potentials. The TEM, ICP-AES, XRD and XPS results are largely consistent with those in our previous works for electrocatalytic oxidation of FA,15,22,43 ensuring the reliable comparison of the CO2RR properties on Pd-B/C and Pd/C in the following.

Figure 1. (A-C) TEM images and the size distribution histograms (inserts) for Pd-B/C, Pd/C (home) and Pd/C (com), respectively. (D) XRD patterns for Pd-B/C, Pd/C (home) and Pd/C 8


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(com), standard XRD peaks are indicated with vertical bars, according to JCPDS file 65-6174. (E) XPS spectra for Pd 3d regions. The XRD, TEM and XPS characterizations of the catalysts subjected to the CO2RR measurement in CO2-saturated 0.1 M KHCO3 at –0.5 V for 2 h were also carried out, and the results are shown in Figure S1 in the Supporting Information (SI). B-doping in the Pd lattice interstices largely remained for Pd-B/C even after the electrolysis, as indicated from the negative shift of the feature peak in the XRD pattern and the positive shift of Pd core level in the XPS spectrum, with respect to the reference values for Pd/C. Furthermore, Pd-B/C exhibited a higher resistance to the nanoparticle size growth at a hydrogen evolution potential, as compared to Pd/C. See the SI for more details. 3.2 CO2RR properties on Pd-B/C and Pd/C

Figure 2. Negative going linear sweep voltammograms of (A) unsupported carbon black, (B) Pd-B/C, (C) Pd/C (home) and (D) Pd/C (com) recorded at a scan rate of 50 mV s-1 in N2-saturated (black) or CO2-saturated (red) 0.1 M KHCO3 electrolyte. Figure 2 shows the linear sweep voltammograms (LSVs) recorded at 50 mV s-1 for Pd/C 9



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(home made and commercial) and Pd-B/C catalyst layers respectively casted on GC electrode in either N2- or CO2-saturated 0.1 M KHCO3. The baseline LSVs for Pd-free carbon black layer casted on GC eldctrode in the same electrolytes are also presented, showing a very small cathodic current mainly attributable to the hydrogen evolution reaction. The significant current responses on Pd/C and Pd-B/C in CO2 saturated 0.1 M KHCO3 with pH6.8 may be divided into three potential regions. At potentials positive of ca. 0.00 V, the apparent cathodic currents are very small and nearly overlapped in N2- and CO2-saturated 0.1 M KHCO3, suggestive of a tiny CO2RR current if any, consistent with an expected standard equilibrium potential of ca. -0.02 V for the CO2/formate redox in CO2-saturated 0.1 M KHCO3.8 With the potential negatively shifted, the apparent cathodic current increases with a hump peak ranging from -0.05 V to -0.65 V in CO2-saturated 0.1 M KHCO3, indicating that appreciable CO2RR does occur on each catalyst at sufficiently negative potentials. At potentials negative of ca. -0.70 V, the apparent reduction current in CO2-saturated 0.1 M KHCO3 turns even lower than that in N2-saturated 0.1 M KHCO3 on both Pd/C and Pd-B/C. One possible explanation is that the H2 evolution-dominant process in the N2-saturated 0.1 M KHCO3 is inhibited by the competitive CO2RR process in the CO2-saturated 0.1 M KHCO3 with the inhibition effect being more pronounced with further decreasing potential.25,44 Potential dependent CO and formate FEs and relative selectivity will be further discussed next following the analytic determination of CO and formate in the gas and liquid products by using GC and NMR methods respectively. The GC chromatograms, potential dependent H2 FEs plots and the NMR spectrum of a liquid product sample are shown in Figures S2, S3 and S4 in the SI, respectively. 3.3 Analysis of Gas and Liquid CO2RR Products. Figure 3 shows the FEs for the gaseous product CO (Figure 3A) and liquid product HCOO- (Figure 3B) during the electrocatalytic CO2RR process. The FEs were calculated according to the following equation.

FE =

N × c ×V × F Q

(4)

where N is the number of transferred electrons (here N = 2), V is the total volume of liquid or gas (mL), F is the Faradaic constant (96485 C mol-1), Q is the overall charge (C) over the period of 2 h, c is the product concentration as measured by GC or NMR (mol mL-1). To avoid relatively large errors in measuring the gas products by GC at lower 10


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overpotentials where the total current is quite small in CO2-saturated 0.1 M KHCO3, only the

ηCO values at potentials from -0.4 to -1.0 V are presented and compared in Figure 3A. It is clear that the CO yield increases with negatively going potential on all the Pd-based catalysts. The larger ηCO on Pd/C at higher overpotentials is in agreement with previous reports.21,27,29 Nevertheless, ηCO on Pd-B/C is much lower than that on both Pd/C (home) and Pd/C (com) at all given potentials of interest. On the other hand, since the concentration of the liquid product formate can be determined more accurately by NMR at potentials up to -0.05 V, and ηHCOO- and the formate concentration per mg of Pd (CHCOO-) over 2-h electrolysis are shown as a function of potential from -0.2 to -1.0 V in Figures 3B and 3C, respectively. The variation of ηHCOOwith potential on the two Pd/C catalysts demonstrates a profile similar to that reported by Koper’s group.5 More importantly, at any given potential, Pd-B/C always outperforms Pd/C in term of ηHCOO-. The highest ηHCOO- can reach ca. 70% at -0.5 V on Pd-B/C over a period of 2 h’s electrolysis, ca. 12 times of that on Pd/C, with the resulting formate concentration of ca. 234 mM mg-1Pd on Pd-B/C, ca.18 times as high as that on Pd/C. Besides, to our best knowledge the

ηHCOO- value exceeds all reported ones for the CO2RR on Pd-based catalysts in CO2-saturated 0.1 M KHCO3,26,28,29 and the Pd mass activity in terms of formate concentration is even a bit higher than that obtained on optimally screened Pd7Pt3/C at -0.5 V in CO2-saturated 0.1 M PBS solution with pH6.7 and smaller volume.5 Notably, in the traditional CO-formation potential region (

Boosting Formate Production in Electrocatalytic CO2 Reduction over

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