GLOBAL CLIMATE AND ENERGY PROJECT | STANFORD UNIVERSITY

Insights into the electrochemical reduction of CO2 on metal surfaces

Kendra Kuhl Etosha R. Cave, David N. Abram, Toru Hatsukade

Prof. Thomas F. Jaramillo

Chemical Engineering

Stanford University

GLOBAL CHALLENGES – GLOBAL SOLUTIONS – GLOBAL OPPORTUNITIES

GCEP RESEARCH SYMPOSIUM 2012 | STANFORD, CA

Chemicals

Reduction

Goals

CxHyOz + O2 CO2 + H2O + Energy

CO2 + H2O + Energy CxHyOz + O2

2

CxHyOz + O2 CO2 + H2O + Energy

Electrochemistry

3

Water Oxidation (Anode) CO2 Reduction (Cathode)

Eο 1.23 V ~0 V

Overall Reaction:

O OHC Energy OH CO 2zyx22

)e 4(H O O2H -

22

OnH OHC )e m(H CO 2zyx

-

2

Need a Catalyst: Efficient + Selective

Fuels + Chemicals Overpotential:

-1 V

H )e 2(H 2

- 0 V Hydrogen Evolution:

CO2 Electroreduction Literature

4

Products determined by CO binding energy

X

Metals that bind CO strongly: Pt, Ni, Fe

Hydrocarbons, Alcohols

Metals that bind CO weakly: Au, Ag, Zn Intermediate CO binding: Cu

Andrew A. Peterson , Frank Abild-Pedersen , Felix Studt , Jan Rossmeisl and Jens K. Nørskov Energy Environ. Sci., 2010,3, 1311-1315

2e-, 2H+ Reduction

ne-, nH+ Reduction

Goal: better understanding of mechanism

CO2 Metal Surface

CO

H2

Electrolysis

5

Anode H2O O2 + 4(H+ + e-)

Cathode CO2 + m(H+ + e-) CxHyOz + H2O

4.5 cm2

electrode area 8 mL electrolyte volume

0.1 M KHCO3 electrolyte pH 6.8

Gas chromatograph for gas phase products

NMR for liquid phase products

Anode H2O O2 + 4(H+ + e-)

Cathode CO2 + m(H+ + e-) CxHyOz + H2O

0.1 M KHCO3 electrolyte pH 6.8

Gas chromatograph for gas phase products

NMR for liquid phase products

Transition Metals

• Cu, Ag, Au, Zn show good current efficiency for CO2 reduction

6 Increasing Overpotential

Weak CO binding

Strong CO binding

Intermediate CO binding

Metal H2 CO CH4 CH3OH C2’s HCOO-

Au

Ag

Zn

Cu

Pt

Ni

Fe

Products

• Major products match literature reports

• Hydrocarbon and/or alcohol production detected on all metals due to increased sensitivity of experimental setup

7

+

Major Products

Minor Products

Novel Products

2

6

8

10

12

14

16

18

4

Products on Cu

Reported Few Reports Novel

8

e-’s C1 C2 C3

Useful products!

2

6

8

10

12

14

16

18

4

Products on Cu

Reported Few Reports Novel

9

e-’s C1 C2 C3

Methane: main component of natural gas Ethylene: used to produce plastics

2

6

8

10

12

14

16

18

4

10

# e-’s C1 C2 C3

Acetic Acid 6.5 billion kg/yr ~$3.4 billion/yr

Acetone 6.7 billion kg/yr ~$8.0 billion/yr

Ethylene Glycol 6.7 billion kg/yr ~$8.7 billion/yr

Cu Tafel Plot

11

Energy Environ. Sci., 2012,5, 7050-7059

Methane and ethylene formed

via different pathways

C2 and C3

products share a similar formation

pathway

Increasing Overpotential

Proposed Pathway

12 Energy Environ. Sci., 2012,5, 7050-7059

Proposed Pathway

13

Dehydroxylation of enol and diol surface species responsible for C2 and C3 chemistry

Energy Environ. Sci., 2012,5, 7050-7059

Glyoxal

Acetate

Glycolaldehyde

Acetaldehyde

Ethylene

2H+, 2e- dehydroxylation

Conversion to diol

Ethylene Glycol

Ethanol

Summary

• Experimental setup allowed for sensitive product detection

• Hydrocarbon and/or alcohol production occured on all metals tested

• Suggested a pathway for C2 and C3 product formation on copper metal

http://duelingfuels.com/wp-content/uploads/2009/02/alternative-fuel.jpg 14

Basic understanding of CO2 reduction chemistry is needed to guide the design of improved catalysts

Questions?

Etosha R. Cave David N. Abram Toru Hatsukade Prof. Thomas F. Jaramillo

Thanks to:

And the rest of the Jaramillo lab

16

Energy Efficiency

• Energy input is determined by the difference between the anode and cathode potentials

• Thermodynamically:

• Water oxdation = 1.23 V

• CO2 reduction = 0 V

• Because of kinetics, actual applied potentials are much higher:

• Water oxidation = 1.6 V

• CO2 reduction = -1.1 V

• Energy efficiency is the ratio of thermodynamically require V / actual V:

• Thermodynamically: Vcell = 1.23

• Actually: Vcell = 2.7

• 1.23/2.7 ~ 45% efficiency

17

Possible CO2 Sources

• Fermentation

• Underground reservoirs

18

• Fossil fuel usage

• Air capture

O

OHH

HH

OHOH

H OH

H

OH

Ethanol + 2 CO2

Future Directions

• In-situ SERS to identify surface intermediates

• Effect of pH, temperature, and pressure

10 µm

• Reactivity of proposed intermediates

• Novel Materials

• Alloys

• Carbon nitrides

• Nanoparticles

?

pH C2H4

Proposed Pathway

20 Enol-like surface species responsible for C2 and C3 chemistry

Energy Environ. Sci., 2012,5, 7050-7059

Cu Surface Preparation

21

CO2 reduction on copper

22

Andrew A. Peterson , Frank Abild-Pedersen , Felix Studt , Jan Rossmeisl and Jens K. Nørskov Energy Environ. Sci., 2010,3, 1311-1315

Product Quantification/Detection

CH4 ~ 4.5 min

CO ~ 5.5 min CO2 ~ 10.5 min

CH2CH2 ~ 14 min

Gas chromatography for gas phase products

H2 ~ 1.5 min

NMR for liquid phase products 1H NMR for quantitation 600 MHz Varian Inova Water suppression

O O

H H

O

H H

O O

O-

O-

1H NMR detection limits: No protons Protons covered by water peak

13CO2 used to get higher signal for 13C NMR

23

24

NMR Stds

• Made 2-3 stds curves / product

• Error between different curves < 5%

• Acetaldehyde and propionaldehyde too volatile, used the avg m/H

Avg m % Err # protons m/H

Formate 0.19 1.2%

Allyl Alcohol 1.07 1.4% 2 0.54

Ethylene Glycol 2.10 1.1% 4 0.53

nPrOH 1.06 1.3% 2 0.53

Glycolaldehyde 0.94 0.5% 2 0.47

MeOH 1.57 3.2% 3 0.52

Acetone 3.20 1.4% 6 0.53

Hydroxy Acetone 1.41 4.1% 3 0.47

Acetate 1.64 2.9% 3 0.55

Acetaldehyde

EtOH 1.72 2.4% 3 0.57

Propionaldehyde

AVERAGE 0.52

ERR 6.9%

Peaks used to quantify

Product Detection Limitations

Low concentration

No proton signal Proton signal masked by water peak in NMR

Insoluble in water

O O

H H

Glyoxal

O

H HFormaldehyde

O O

O-

O-

Oxalate

Hydrocarbons with solubility below 100 µM

> 50 µA/cm2 for liquid phase products

> 10 µA/cm2 for gas phase products

25

Benchmarking

• Hori, Murata, Takahashi. J. Chem. Soc. Farad. Trans. 1 85:2309 1989.

• A.A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, and J.K. Norskov, Energy & Environ. Sci. 3 (2010) 1311-1315.

• K.P. Kuhl & T.F. Jaramillo

-0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2

0

10

20

30

40

50

60

70

80

Fa

rad

aic

Yie

ld (

%)

V vs. RHE

Hydrogen

Methane

Formate

CO

Ethylene

26

Current Efficiency

27

Energy Environ. Sci., 2012,5, 7050-7059

Low overpotential:

H2, CO, Formate

Intermediate overpotential

Methane and ethylene with many other minor products

High overpotential

Methane continues to increase while C2 and C3 products decrease

Tafel Plot

• H2, CO and formate pull near constant current across voltage range

• C2 and C3 products clearly rise and fall together suggesting a shared rate determining step

• Methane and ethylene slopes are different suggesting different rate determining steps

28 Kendra Kuhl - ECS 2011

CO2 Electroreduction Literature

29

• Hori, Y. (2003). CO2-reduction, catalyzed by metal electrodes. Handbook of Fuel Cells: Fundamentals,

Technology and Application. A. L. Wolf Vielstich, Hubert A. Gasteiger. Chichester, VHC-Wiley. 2: 720-

733.

CO2 CO Hydrocarbons/Alcohols Products determined by CO binding energy

Electrochemical Methods

• Hold at the desired potential for 1 hour

• Measure Products

• Determine current efficiency 30

Transition Metals

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2

-16

-12

-8

-4

0

Cu

Au

Pt

Ag

Ni

Fe

Zn

Cu

rre

nt

De

nsity (

mA

/cm

2)

V vs. RHE

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

0

20

40

60

80

100

CO

2 R

ed

uctio

n E

ffic

ien

cy

V vs. RHE

Ni

Ag

Pt

Au

Cu

Fe

Zn

Cu, Ag, Au, Zn show good current efficiency for CO2 reduction

31

Carbonyl Equilibria

Species detected by NMR are shown in blue

RH

O

R

R

R

OH

Keto-Enol Tautomerism

Aldehyde Hydration

+

Which species is produced on the electrode?