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Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration



Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity1,2,3,4,5,6,7. Unfortunately, the reaction suffers from slow kinetics7,8 owing to the low local concentration of CO2 surrounding typical CO2 reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species9,10, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO2 adsorption11, but this comes at the cost of increased hydrogen (H2) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO2 close to the active CO2 reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO2 reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at −0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxide-derived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at −0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.

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Figure 1: Thermodynamic barriers for the CO2-to-CO reduction reaction on Au surface under conditions with and without K+.
Figure 2: Computed electric field, K+ concentration and current density near the tip of an electrode as a function of tip radius.
Figure 3: Physical characterization of Au tips, rods and particles.
Figure 4: CO2 reduction performances on Au needles, rods and particles in 0.5 M KHCO3, pH 7.2.


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This work was supported by the Ontario Research Fund: Research Excellence programme, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the CIFAR Bio-Inspired Solar Energy programme and a University of Toronto Connaught grant. B.Z. acknowledges funding from Shanghai Municipal Natural Science Foundation (14ZR1410200) and the National Natural Science Foundation of China (21503079). We thank X. Lan, P. Kanjanaboos, G. Walters, L. Levina, R. Wolowiec, D. Kopilovic, E. Palmiano, T. Burdyny and J. Tam from the University of Toronto for Au electron beam deposition, liquid products testing, AFM testing, TEM EELS measurements, discussions and additional aids during the course of study, Y. Tian from the King Abdullah University of Science and Technology for electrode preparation assistance, and M. Bajdich, L. D. Chen and K. Chan from Stanford University for advice on DFT calculations. This work has also benefited from the Spherical Grating Monochromator beamlines at the Canadian Light Source. DFT calculations were performed on the IBM BlueGene Q supercomputer with support from the Southern Ontario Smart Computing Innovation Platform (SOSCIP).

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Authors and Affiliations



E.H.S., S.O.K. and D.S. supervised the project. M.L., Y.P. and B.Z. designed and carried out all the experiments and COMSOL simulations. P.D.L. and O.V. carried out the DFT simulation. All authors discussed the results and assisted during manuscript preparation.

Corresponding author

Correspondence to Edward H. Sargent.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Optimized structure for Au facets and data calculated with or without K+.

ad, Optimized structures. a, Au(111) facet. b, Au(100) facet. c, Au(110) facet. d, Au(211) facet. Included are the optimized positions of the adsorbates COOH and CO without and with the presence of an adsorbed K+ (purple). e, Volume slice of calculated charge densities. Bader partial atomic charges are indicated in black with and without K+. In the presence of K+ the Bader partial atomic charge on the carbon of COOH* has increased from 1.3 to 1.59 suggesting higher electron density and thus a stronger C–Au bond. f, Calculated average mean square displacement of CO2 on Au(111) surface with and without K+ in the system. This ensemble average shows CO2 is more diffuse without a K+ cation to facilitate CO2 surface binding. g, Mean square displacement of CO2 on Au(111), Au(110), Au(100) and Au(211) surface in the presence of K+. It was found that regardless of facet the mean square displacement of CO2 converges to about 2.5 Å2. h, Calculated C–Au radial distribution function under the conditions with or without K+. The radial distribution function of CO2 to Au(111) from an ensemble average of 25 ab initio molecular dynamics simulations (5 ps) shows CO2 is closer to the surface of gold on average in the presence of K+ than without K+. i, Calculated interaction energy of CO2 vary with C–Au distance under the conditions with or without K+. The interaction energy is consistently less in the presence of an adsorbed K+ (red) than without K+ (black).

Extended Data Figure 2 Electrochemical simulation model and results.

a, Schematic of the Gouy–Chapman–Stern electrical double layer model. b, Field-induced surface K+ ion concentration as a function of bulk K+ ion concentration. c, Field-induced surface K+ ion concentration as a function of electrode potential (versus RHE). d, Required CO2–Au bonding time versus electric field. With concentrated K+, CO2 quickly (in 0.5 ps) stabilizes on the Au sharp features and remains there for the remainder of the simulation run. e, Current density distributions on the surface of Au structures. The tip radius is 5 nm. The tip radius of the structure in each panel is: 5 nm (top), 60 nm (middle) and 140 nm (bottom). Arrows are magnified 2× in the middle panel and 4× in the bottom panel for the purpose of clarity. f, Simulated Tafel plots for needles (tip radius 5 nm), rods (tip radius 60 nm), particles (tip radius 140 nm). Simulated data was fitted to the experimental data with fitting parameter cathodic charge transfer coefficient being 0.95 (needles), 0.49 (rods), and 0.43 (particles).

Extended Data Figure 3 Additional physical characterization, CO2 reduction and kinetic analyses of Au samples.

a, Morphologies for Au tips (left), rods (middle) and particles (right) imaged by SEM. b, c, ECSA measurement. b, Cyclic voltammograms in 50 mM H2SO4. Scan rate 50 mV s−1. c, Underpotential Cu deposition and anodic stripping waves. The electrolyte solution was 100 mM CuSO4 in 0.50 M H2SO4. Scan rate 50 mV s−1. d, X-ray diffraction patterns for all of the electrodes exhibited peaks at the expected positions for an ideal Au lattice, indicating no uniform expansion or compression of the unit cell. e, X-ray photoelectron spectroscopy exhibited the expected peaks for Au0 but no peaks attributable to an oxide, indicating that reduction of HAuCl4 precursor was complete within the detection limits of this technique. f, Current–voltage curves on the tips of single Au needle, rod and particle. The radii for the Au needle, rod and particle are 5 nm, 60 nm and 140 nm, respectively. g, Charge transfer resistance analyses. Nyquist plots in 0.5 M KHCO3 aqueous electrolyte. h, i, CO2 reduction performances in 0.5 M KHCO3, pH 7.2 at −0.30 V (h) and −0.20 V (i) versus RHE. j, k, CO2 reduction current densities in 0.5 M KHCO3, pH 7.2, normalized by (j) geometric area and (k) ECSA. ln, Activation energy analyses. The polarization curves of Au particles (l), Au rods (m), and Au needles (n) in 0.5 M KHCO3 aqueous electrolyte at 0–25 °C. Insets are the Arrhenius plots for the dependence of reaction rate for CO2 reduction on temperature.

Extended Data Figure 4 Collective control experiments to confirm that the reactivity of Au nanoneedles cannot be simply explained by oxides or adatoms.

a, b, O 2p core-level X-ray photoelectron spectroscopy spectra (a) and O K-edge X-ray absorption spectra (b) for Au needles, pure Au and oxidized Au needles. The O 2p core-level and O K-edge X-ray absorption spectra of Au needles are similar to those of pure Au and are different from that of oxidized Au needles, indicating the different Au states in Au needles and oxidized Au. c, High-resolution TEM image of Au needle tip, indicating that there is no obvious facet and adatoms. d, Electron energy loss spectroscopy (EELS) spectra on Au needle tip. No oxide can be detected on Au needle tip, indicating that reduction of the HAuCl4 precursor was complete within the detection limits of this technique. e, Low-magnification SEM image of oxidized Au needles. f, High-magnification SEM image of oxidized Au needles. g, TEM image of oxidized Au needles. Amorphous Au oxide can be observed on the surface of Au. h, X-ray photoelectron spectroscopy spectra of oxidized Au needles and primary Au needles. i, Cyclic voltammograms collected for Au needles, and oxidized Au needles. j, CO2RR performance on oxidized Au needles.

Extended Data Figure 5 Collective control experiments to confirm the FIRC effects.

a, Morphology, crystal structure and composition for Au needles after reaction. Left, SEM image, middle, X-ray diffraction pattern, and right, X-ray photoelectron spectroscopy spectrum for Au needles after long term CO2RR. b, Left, SEM image of Au needles covered by 10-nm Au by electron bean deposition, right, CO2 reduction activity of Au needles, rods and particles at −0.35 V versus RHE. c, Left, SEM image of Au needles at 140 °C after annealing, right, CO2 reduction activity of Au needles at −0.35 V versus RHE after annealing. d, Left, SEM image of Au needles after surface etching. The Au nanoneedles were immersed in a vial containing 15 ml of CuCl2 solution (5 mM). The vial was then heated to 70 °C using an oil bath and kept at that temperature for 1 h. The etched Au nanoneedles obtained were washed with a copious amount of water and dried at room temperature27. Right, CO2 reduction activity of Au needles at −0.35 V versus RHE after surface etching. e, Left, SEM image of Au needles after surface plasma bombard (50 W, argon atmosphere, 1 h). Right, CO2 reduction activity of Au needles at −0.35 V versus RHE after surface plasma bombard. f, SEM image of Au particles (left), Au rods (middle), and Au needles (right) with secondarily deposited Au particles. g, Cyclic voltammograms collected for Au needles in 50 mM H2SO4 for ECSA measurements. h, CO2RR performances of Au needles and Au needles/Au at −0.35 V versus RHE.

Extended Data Figure 6 Morphology, electric field and CO2 RR performance of dendritic Au leaves.

a, b, SEM images of Au leaves. c, TEM image of Au leaves. d, Electric field distribution deduced using Kelvin probe atomic force microscopy. e, CO2 reduction activity of Au leaves at −0.35 V versus RHE.

Extended Data Figure 7 CO2RR performances of Au nanoneedles in various electrolyte condition.

a, Current densities and Faradaic efficiencies versus K+ concentrations on Au needles at −0.35 V versus RHE. b, Current densities and Faradaic efficiencies versus K+ concentrations on planar Au at −0.4 V versus RHE. c, CO2 reduction performance of Au needles in saturated KHCO3 solution. d, CO2 reduction performance of Au needles in NH4HCO3 solution. e, CO2 reduction performance of Au needles in water.

Extended Data Figure 8 CO2 reduction reaction performances on Pd needles, rods and particles.

ac, SEM images of Pd needles, rods and particles, respectively. d, Total current density versus time for CO2 RR on Pd needles, rods and particles in 0.5 M KHCO3 solution at −0.2 V versus RHE. e, Average Faradaic efficiency for formate production versus time on Pd needles, rods and particles in 0.5 M KHCO3 solution at −0.2 V versus RHE.

Extended Data Table 1 Summary of simulation parameters as calculated from DFT
Extended Data Table 2 Summary of ECSA, activation energies and charge transfer resistances on different Au electrodes
Extended Data Table 3 Summary of CO2RR performances on different Au and Pd electrodes in aqueous solution with inorganic electrolyte

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Liu, M., Pang, Y., Zhang, B. et al. Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration. Nature 537, 382–386 (2016).

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