Impacts of Catalyst Morphology on Mass Transport and

Product Selectivity for the CO2 Reduction Reaction

Stephanie Nitopi, Christopher Hahn, Jakob Kibsgaard, Thomas F. Jaramillo*

Department of Chemical Engineering, Stanford University, Stanford, CA 94305

Acknowledgements

Introduction

Conclusions & Next Steps

Catalyst Characterization

Electrochemical CO2 Reduction

(A) Current efficiencies and (B) partial current densities for each experiment demonstrate that the

product selectivity can be tuned depending on the preparation of the copper catalyst. The ethylene

to methane ratio (C2H4/CH4) increases for Experiments 1 – 5 as follows: 0.8, 2.8, 6.3, 7.2, 36.0.

Note: “Minor Products” includes acetate, glycolaldehyde, ethylene glycol, acetone, and allyl alcohol.

Product Key– Minor Products

– CO, HCOO-

– n-PrOH

– EtOH

– CH4

– C2H4

– H2

Experiment Key1 – Cu control

2 – Cu 250 ºC control

3 – Cu DG 250 ºC, 1st

4 – Cu DG 250 ºC, 2nd

5 – Cu DG 300 ºC

(A) (B)

The authors gratefully acknowledge the Global Climate and Energy Project (GCEP) at Stanford

University and the Stanford Graduate Fellowship for supporting this work, as well as the Stanford

Nano Shared Facilities (SNSF) for use of their facilities (XRD, XPS, SEM). The authors would also

like to give special thanks to Jaramillo group members for helpful scientific discussions.

The XRD pattern for calcined silica double-gyroid

templates exhibits four Bragg diffraction peaks.6

Comparing a typical DG XRD pattern (credit: Dr. J.

Kibsgaard) with those of the modified Cu samples,

it is clear that the DG structure was not achieved.

Low-angle XRD scans for silica DG

(1) C.W. Li and M.W. Kanan, JACS. 2012, 134, 7231 – 7234

(2) D. Ren et al., ACS Catalysis. 2015, 5, 2814 – 2821

(3) F. S. Roberts et al., Angew. Chem. Int. Ed., 2015, 54, 5179 – 5182

(4) Y. Hori, Modern Aspects of Electrochemistry. 2008, 89 – 189

(5) R. Kas et al., ChemElectroChem. 2015, 2, 354 – 358

SEM images after electrochemical testing

Exp 1 Exp 2 Exp 2

Exp 3 Exp 4 Exp 5

The silica double-gyroid structure was not achieved on any of the copper foils, but XRD scans still

indicated some slight degree of order. Regardless, XPS showed that these silica thin films were all

removed during electrochemical testing. The resulting morphology of the copper surface was

greatly impacted by these treatments; the three samples initially coated with double-gyroid solution

display high porosity and an abundance of cube-like structures. This is likely due to formation and

subsequent reduction of CuCl (resulting from HCl in the DG solution). A few regions on the “Exp 2”

sample also had small cubes, which may be attributed to Cl- contamination from the tube furnace or

the ref. electrode. The higher selectivity to ethylene on these surfaces can be largely explained by

their cubic structure, which suggests the surface is dominated by the (100) facet of copper and (100)

step sties.3 However, the rough nature of the treated copper foils likely causes a higher local pH, so

one can not rule out this potential contribution to the improved selectivity.3,5 Future studies will focus

on a highly ordered morphology (nanowire arrays) in an attempt to decouple these effects.

Coupling CO2 electroreduction with a renewable energy source (such as wind or solar) to

create high value fuels and chemicals is a promising strategy in ongoing efforts to achieve

a sustainable global energy economy. To date, only copper has shown a propensity to

produce further reduced products and C-C coupled products such as methane, ethylene

and ethanol. However, copper still requires a large overpotential to reduce CO2 and lacks

product selectivity.

There have been a variety of literature reports showing that modifying the structure of a

copper surface can be a path towards improved efficiency and selectivity. For example,

recent work on oxide-derived copper yielded highly nanostructured morphologies that

resulted in lower overpotential for CO2 reduction1 and improved selectivity towards

ethylene2. Increased ethylene selectivity is of particular interest, as not only is it a desirable

chemical commodity, but it also gives insight into the C-C coupling step required for

formation of multicarbon products.3 Single crystal studies have shown that that CO2

reduction product selectivity is facet-dependent; specifically, the Cu(100) surface favors

ethylene over methane formation.4 As a result, selectivity improvements on nanostructured

morphologies are often attributed to an abundance of a specific crystal surface structure.

While crystal structure certainly plays a role, this may not be the only contributor to

enhanced ethylene selectivity observed in these systems.

One study used a numerical approach to estimate the surface pH for different

concentrations of KHCO3 electrolyte:5

This correlation may provide further insight into increased ethylene selectivity in highly

nanostructured systems. Since these rough, high surface area catalysts often have higher

current densities and may also have decreased transport of OH- away from the surface, it

is likely that they have a higher local pH contributing to increased ethylene selectivity in

addition to crystal structure effects.

In this work, a mesoporous silica thin film was deposited on copper foil through an

evaporation-induced self-assembly method to serve as a diffusion barrier for reactants and

intermediates and explore mass transport effects on the CO2 reduction reaction.

CO2 + H2O + 2e- CO + 2OH-

CO2 + 6H2O + 8e- CH4 + 8OH-

2CO2 + 8H2O + 12e- C2H4 + 12OH-

2CO2 + 9H2O + 12e- C2H5OH + 12OH-

3CO2 + 13H2O + 18e- C3H7OH + 18OH-

Generation of OH-

increases the local pH

during CO2 reduction

Decreasing buffer

concentration leads

to higher local pH

and increased

ethylene selectivity

All copper foils were first mechanically polished (400G sandpaper, 3M) followed by

electropolishing in phosphoric acid (85% in H2O, Aldrich) potentiostatically at 2.1 V vs. a

graphite foil (0.13 mm thick, Alfa Aesar) counter electrode placed at a distance of 1.5 cm.

A silica double-gyroid (DG) coating solution was prepared using EO19–PO43–EO19

surfactant (Synperonic P84, Sigma-Aldrich), tetraethyl orthosilicate (TEOS), HCl, and

ethanol by following a known procedure.6 This solution was spin-coated onto the polished

copper foil and dried at a controlled relative humidity (40%–50%). After evaporation was

complete, the films were calcined in air at either 250 ºC or 300 ºC for 4 hours.

Experimental Methodology

The calcined silica templates

were analyzed using low-

angle X-ray diffraction (XRD)

and X-ray photoelectron

spectroscopy (XPS) to

confirm the thin film structure

and composition, respectively.

CO2 electroreduction performance was measured

through one hour chronoamperometry experiments at

a potential of -1.7 V vs. Ag|AgCl in 0.1 M KHCO3. Gas

chromatography (GC) and nuclear magnetic

resonance (NMR) were used to quantify the resulting

gaseous and liquid products, respectively.

After electrochemical testing, XPS was performed to

evaluate any changes in the surface composition and

SEM images were obtained to characterize the

catalyst morphology.

(6) J. Kibsgaard et al., Nat. Mater., 2012, 11, 963 – 969

Silica DG synthesis figure credit: Dr. Jakob Kibsgaard

XPS before and after electrochemical testing

The most notable difference between the XPS spectra is

the disappearance of the Si peaks after e-chem. This

was observed for all modified Cu samples, indicating

poor adhesion of the silica thin films. The presence of Cl

in these samples is also interesting, as CuCl is known to

form cubes when it precipitates in solutions above pH 4.3