Enhanced electrochemical methanation of carbon dioxide with a dispersible nanoscale copper catalyst

Article abstract of DOI:10.1021/ja5065284

Although the vast majority of hydrocarbon fuels and products are presently derived from petroleum, there is much interest in the development of routes for synthesizing these same products by hydrogenating CO₂. The simplest hydrocarbon target is methane, which can utilize existing infrastructure for natural gas storage, distribution, and consumption. Electrochemical methods for methanizing CO₂ currently suffer from a combination of low activities and poor selectivities. We demonstrate that copper nanoparticles supported on glassy carbon (n-Cu/C) achieve up to 4 times greater methanation current densities compared to high-purity copper foil electrodes. The n-Cu/ C electrocatalyst also exhibits an average Faradaic efficiency for methanation of 80% during extended electrolysis, the highest Faradaic efficiency for room-temperature methanation reported to date. We find that the level of copper catalyst loading on the glassy carbon support has an enormous impact on the morphology of the copper under catalytic conditions and the resulting Faradaic efficiency for methane. The improved activity and Faradaic efficiency for methanation involves a mechanism that is distinct from what is generally thought to occur on copper foils. Electrochemical data indicate that the early steps of methanation on n-Cu/C involve a pre-equilibrium one-electron transfer to CO₂ to form an adsorbed radical, followed by a rate-limiting nonelectrochemical step in which the adsorbed CO₂ radical reacts with a second CO₂ molecule from solution. These nanoscale copper electrocatalysts represent a first step toward the preparation of practical methanation catalysts that can be incorporated into membrane-electrode assemblies in electrolyzers.

Full text of DOI:10.1021/ja5065284

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Article
Enhanced electrochemical methanation of carbon
dioxide with a dispersible nanoscale copper catalyst
Karthish Manthiram, Brandon J. Beberwyck, and A. Paul Alivisatos
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5065284 • Publication Date (Web): 19 Aug 2014
Downloaded from http://pubs.acs.org on September 2, 2014
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Enhanced electrochemical methanation of carbon dioxide with a dispersible nanoscale
copper catalyst
,
†
,
, ,
‡ ⊥
, ,⊥,
||
||
⊥
Karthish Manthiram,
Brandon J. Beberwyck, || and A. Paul Alivisatos* §
†
Department of Chemical and Biomolecular Engineering,
Engineering, Department of Chemistry, and Kavli Energy Nanosciences Institute, University
of California, Berkeley, California 94720, United States
‡Department of Materials Science and
§
⊥
||Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720, United States
* Corresponding author
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Abstract
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Although the vast majority of hydrocarbon fuels and products are presently derived from
petroleum, there is much interest in the development of routes for synthesizing these same
products by hydrogenating CO2. The simplest hydrocarbon target is methane, which can utilize
existing infrastructure for natural gas storage, distribution, and consumption. Electrochemical
methods for methanizing CO2 currently suffer from a combination of low activities and poor
selectivities. We demonstrate that copper nanoparticles supported on glassy carbon (nꢀCu/C)
achieve up to four times greater methanation current densities compared to highꢀpurity copper
foil electrodes. The nꢀCu/C electrocatalyst also exhibits an average Faradaic efficiency for
methanation of 80% during extended electrolysis, the highest Faradaic efficiency for roomꢀ
temperature methanation reported to date. We find that the level of copper catalyst loading on the
glassy carbon support has an enormous impact on the morphology of the copper under catalytic
conditions and the resulting Faradaic efficiency for methane. The improved activity and Faradaic
efficiency for methanation involves a mechanism that is distinct from what is generally thought
to occur on copper foils. Electrochemical data indicates that the early steps of methanation on nꢀ
Cu/C involve a preꢀequilibrium oneꢀelectron transfer to CO2 to form an adsorbed radical,
followed by a rateꢀlimiting nonꢀelectrochemical step in which the adsorbed CO2 radical reacts
with a second CO2 molecule from solution. These nanoscale copper electrocatalysts represent a
first step towards the preparation of practical methanation catalysts that can be incorporated into
membraneꢀelectrode assemblies in electrolyzers.
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Introduction
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The conversion of CO2 into hydrocarbons is an alternative route for synthesizing fuels
and feedstocks that are typically derived from oil or natural gas, representing one potential
strategy to store electrical energy derived from intermittent sources of clean energy, such as wind
and solar.1,2 Although electrosynthetic pathways for converting CO2 into hydrocarbon products
are not economically feasible at present,3 expected decreases in the price of electricity derived
from clean energy sources4 and policy changes regarding greenhouse gas emissions5 may alter
the economics of reducing CO2 dramatically. In fact, growing use of intermittent renewable
energy sources in certain regions has accelerated the deployment of small scale electrical energy
storage systems, including pilot plants for methanizing CO2.6 These pilot plants utilize a twoꢀstep
process, in which electrical energy is used to power an electrolyzer that splits water to produce
hydrogen and oxygen. The hydrogen is then used in the Sabatier reaction,7 in which CO2 and H2
are reacted over a heterogeneous nickel catalyst at temperatures of 250 ꢀ 400 oC and pressures of
1 ꢀ 80 bars to produce methane, which can be injected into existing natural gas networks. A
singleꢀstep electrochemical process that can directly convert CO2 to methane under conditions of
ambient pressure and temperature may represent an attractive alternative.
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Of the metals explored as catalysts for electrochemical CO2 reduction,8 the most active
and selective identified to date are gold, silver, and bismuth,9–14 which produce CO as their
terminal product. Copper is attractive in comparison, as it produces more reduced hydrocarbon
products.8,15–17 One of the hydrocarbon products formed on copper electrocatalysts is methane,
which forms through the following halfꢀreaction:
CO2 + 8H+ +8e– → CH4 + 2H2O
(1)
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Because the reaction involves eight electron transfer steps at 0.17 V (all potentials reported
versus reversible hydrogen electrode (RHE)) that can easily bifurcate to form a wide range of
products, the process exhibits poor selectivity for any single product, forming a mixture of
methane, ethylene, hydrogen, carbon monoxide, and formic acid.18,19 The highest Faradaic
efficiency for methane reported to date is 64% on a (210) copper single crystal.18,20 Although
studies conducted on highꢀpurity foils and single crystals have served as useful benchmarks and
provide fundamental insights into how copper catalyzes the reduction of CO2, these model
materials are impractical for electrolyzers as they have low surface areas, cannot be incorporated
into the membrane electrode assemblies21 that are needed to achieve high current densities with
low ionic resistances, and are expensive. From the point of view of cost and ease of
manufacturing, highly dispersed nanoparticle catalysts are much better suited for electrolyzers.22
Here, we demonstrate that wellꢀdispersed copper nanoparticles supported on glassy carbon show
high activities and Faradaic efficiencies for methanation, comparable to that of much more
expensive single crystal electrodes. Systematic studies of nanoparticle loading on the glassy
carbon support and electrochemical analysis indicate that the altered reactivity of the copper
nanoparticles is due to distinct catalytic sites present on isolated nanoparticle catalysts supported
on glassy carbon.
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Results and Discussion
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We colloidally synthesized copper nanoparticles capped with tetradecylphosphonate of
diameter 7.0 ± 0.4 nm (Figure 1a,b).23 These particles were spinꢀcoated onto glassy carbon plates
(Figure 1c), hereafter referred to as nꢀCu/C, which served as the working electrode in a threeꢀ
electrode setup containing CO2 saturated 0.1 M sodium bicarbonate electrolyte, pH 6.8. As a
control, we also used highꢀpurity copper foils as the working electrode. All current densities for
nanoparticle electrodes are surfaceꢀarea normalized.
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Morphological Evolution. During the course of electrochemical CO2 reduction, the
morphology of the copper nanoparticles changes significantly, growing in size to 23 ± 8 nm in
diameter (Figure 1d). The nanoparticles that form are highly polycrystalline, as revealed using
high resolution transmission electron microscopy (Figure 1e,f). We find that irrespective of the
initial size of the nanoparticles on glassy carbon, the particles evolve in size to form particles
which are ~25 nm in diameter, even if we begin with larger particles. For instance, if we treat the
initially cast particles (Figure 1c) with trimethylsilylchloride, the tetradecylphosphonate ligand is
stripped off of the surface of the particles, causing the particles to ripen to a diameter of 52 ± 21
nm prior to polarization (Figure 1g). These large, irregular particles then evolve in size and shape
during the course of electrochemical CO2 reduction to form smaller, uniform, roughly spherical
particles which are 25 ± 8 nm in diameter (Figure 1h). Similar changes in size are also observed
in the absence of CO2 (Figure S1). The morphological evolution observed, which may be due to
a combination of particle coalescence and dissolutionꢀredeposition, points towards the
importance of verifying if size distributions are maintained in studies of sizeꢀdependent
electrocatalysis.24
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Catalytic behavior. Although the nꢀCu/C electrodes and copper foil electrodes exhibit
comparable current densities at lower overpotentials, the current densities for nꢀCu/C electrodes
are over twice as high at more reducing potentials (Figure 2a). Of this increased current, a much
greater fraction from the nꢀCu/C electrode goes towards methane compared to the copper foil
(Figure 2b). The Faradaic efficiency for methane is improved at more reducing potentials for nꢀ
Cu/C, reaching 76% at ꢀ1.35 V. This is greater than the highest Faradaic efficiency for methane
reported in the literature (64%) on much more expensive and difficult to prepare (210) copper
single crystal electrodes. In addition, the Faradaic efficiencies for methane on nꢀCu/C are
significantly higher than the Faradaic efficiency of 44% achieved on a polycrystalline copper foil
at the same potential (Figure 2b). The combined enhancement in both the overall current density
and Faradaic efficiency for methanation on nꢀCu/C leads to partial current densities for methane
that are four times higher for nꢀCu/C compared to the copper foil at ꢀ1.35 V (Figure 2c).
Hydrogen evolution, which is undesirable since the intended reduction target is CO2, is also
suppressed on nꢀCu/C compared to the copper foil. The Faradaic efficiency for hydrogen is 13%
at ꢀ1.25 V on nꢀCu/C, half that of a polycrystalline copper foil at the same potential (Figure 2d).
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The current densities for nꢀCu/C are relatively stable at ꢀ1.25 V during extended periods
of CO2 reduction (Figure 3a), decaying only 5% over the course of 1 hour, indicating a stability
surpassing copper foil electrodes, for which the current density decays by 11% in 80 min (Figure
S2). The Faradaic efficiency for methane on nꢀCu/C does not decay during extended periods of
CO2 reduction (Figure 3b), remaining in the range of 71ꢀ90%, with an average yield of 80% over
1 hour, which is the highest Faradaic efficiency for methanation reported to date.
Continuum from nanoparticle-like to foil-like behavior. There are many structural,
morphological, and chemical differences between the copper nanoparticles supported on glassy
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carbon and copper foil electrodes, which could hypothetically be responsible for the increased
Faradaic efficiencies for methanation. Some sources of the differences include the presence of
tetradecylphosphonate ligand capping the nanoparticles and impurities present in the initial
nanoparticles, since they are prepared from a 97% pure copper precursor. In order to determine
the effect of these various factors, we utilized a distinctly different method of preparation of the
electrocatalyst, by evaporating a thin film of copper onto glassy carbon using a high purity
copper source. For a 3 nm film (Figure 4a), we find that a Faradaic efficiency for methanation of
76% can be achieved (Figure 4e), allowing us to conclusively exclude the possibility that the
presence of ligands and impurities in the starting material influence the observed improvement in
Faradaic efficiency for methanation on nꢀCu/C.
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The thickness of the initially evaporated copper film has a dramatic impact on the
Faradaic efficiency for methane. Evaporated films that are relatively thin (Figure 4a) produce
isolated nanoscale aggregates upon polarization on the electrode (Figure 4b) which somewhat
resemble the nꢀCu/C electrodes (Figure 1d); these electrodes have high methanation yields
(Figure 4e). In contrast, thicker films (Figure 4c) produce highly connected networks of fused
nanoparticles upon polarization (Figure 4d); these electrodes have low Faradaic efficiencies for
methanation (Figure 4e), as we would expect for an architecture that resembles a polycrystalline
foil. This possibly explains why previous studies of dense films of copper nanoparticles have not
observed enhanced methanation yields.25–27 A systematic study of single crystal electrodes for
CO2 reduction has put forth the possibility that the introduction of a particular stepꢀedge present
on a (210) single crystal can enhance methanation yields.20 This suggests that more isolated
nanoparticles expose catalytic sites that are more effective for methanation, which are lost as
they fuse to form dense aggregates. Structural differences have also been implicated in the
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enhanced selectivity for CO observed on copper foil catalysts which are oxidized and then
reduced.26,28 Our results demonstrate a continuum of catalytic behavior that exists between
electrodes that exhibit nanoparticleꢀlike and foilꢀlike behavior, and that this behavior can be
systematically tuned by adjusting the mass loading of copper on glassy carbon (Figure 4e).
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Mechanism. In order to glean mechanistic insights regarding the altered catalytic
behavior of nꢀCu/C compared to copper foils, we measured the Tafel slope of the nꢀCu/C
catalyst. In the region of Tafel linearity, the Tafel slope for methanation is 60 ± 4.2 mV/decade
for nꢀCu/C (Figure 5a), close to a value of 59 mV/decade, indicative of a oneꢀelectron preꢀ
equilibrium step prior to a rate limiting nonꢀelectrochemical step.11,12,29,30 Tafel slopes for
methanation on copper foils vary widely depending on surface preparation, ranging from as low
as 86 ± 4.6 mV/decade (Figure 5a) to as high as 175 mV/decade.31 It is generally thought that the
rateꢀlimiting step for methanation on copper foils involves a single electron transfer to CO2 on
copper foils,18 which would correspond to a Tafel slope of 120 mV/decade. The reduced Tafel
slope on nꢀCu/C is advantageous because smaller excursions in potential are needed to drive
logarithmic gains in methanation current.
While the Tafel slope provides general insights into the nature of the possible rateꢀ
limiting and preꢀequilibrium steps, the order dependence of the methanation current on reactants
provides a more detailed picture of the rateꢀlimiting step. We find an unusual secondꢀorder (2.03
± 0.08) dependence of methanation current on CO2 partial pressure for the nꢀCu/C catalyst
(Figure 5b).32 Based on work on other metals, such as gold9 and mercury,33 it is often assumed
that CO2 reduction on copper foils proceeds with a firstꢀorder dependence on CO2 in aqueous
electrolytes,34,35 although we observe an illꢀdefined order (Figure 5b). The methanation current
on the nꢀCu/C catalyst exhibits no clear order dependence on sodium bicarbonate concentration
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(Figure S3), although optimization of the buffer concentration can further enhance Faradaic
efficiencies for methanation by approximately 10% (Supporting Information).
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Combining the insights provided by the preceding electrochemical analysis, we propose
early steps in a possible mechanism that could lead to methane formation (Figure 5c). In this
proposed mechanism, the CO2 1 reacts in a oneꢀelectron transfer preꢀequilibrium step to form a
surface adsorbed CO2 radical 2. The CO2 radical has been experimentally observed on other
metals, and it is inferred that it also forms on copper.18,33,36,37 If we assume that the surface
coverage of the CO2 radical θ << 1, consistent with the observation that copper surfaces are
predominantly covered in CO under CO2 reduction conditions,38–40 then θ is related to the
overpotential η and CO2 partial pressure pCO2 as:
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θ = ꢀꢁꢂꢃꢄꢅ exp ꢆꢉꢇꢈꢊꢋ
(2)
where K1 is the equilibrium constant for the conversion of 1 to 2, F is Faraday’s constant, R is
the gas constant, and T is temperature (Supporting Information).29 Following the preꢀequilibrium
step, the adsorbed CO2 radical 2 is proposed to undergo rateꢀlimiting carbonꢀoxygen coupling
with a Lewis acid CO2 molecule from solution to form a CO2ꢀCO2¯• adduct 3, which could be
either a transition state or a genuine intermediate. Such an adduct has been postulated for
electrochemical reduction of CO2 in nonꢀaqueous solvents on other metals.37,41–43 The rate of the
CO2ꢀCO2¯• adduct formation step, expressed as a methanation current, is:
ꢌꢃꢍ = ꢏꢐꢑꢒꢂꢃꢄꢅꢓ
(3)
ꢎ
where n is the total number of electron transfers needed to convert CO2 1 to methane 5 and ꢑꢒ is
the rate constant for the conversion of 2 to 3. Combining equations (2) and (3), we obtain:
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ꢌꢃꢍ = ꢏꢐꢑꢒꢀꢁꢂꢒ ꢅ exp ꢆꢉꢇꢈꢊꢋ
(4)
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This rate law is consistent with the secondꢀorder dependence on CO2 partial pressure that we
experimentally observe. The Tafel slope is given by the partial derivative of the overpotential η
with respect to the logarithm of current,29 which we apply to equation (4), yielding:
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ꢕꢈ
ꢠꢡ
ꢔꢕ ꢖꢗꢘꢙ
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(5)
ꢇ
ꢚꢛ
ꢎ
ꢝ
ꢚꢞ
ꢅ
Hence, the theoretical Tafel slope of 59 mV/decade for the proposed mechanism is consistent
with our experimental measurement of a 60 ± 4.2 mV/decade (Figure 5a). The preceding analysis
involving the Tafel slope and orderꢀdependence on pCO2 has also allowed us to exclude several
alternative mechanisms, such as the selfꢀcoupling of two CO2¯• and rateꢀlimiting electron
transfer to CO2 (Supporting Information).
Based on mechanistic understanding of CO2 reduction on other metals, we may also
propose downstream steps for the conversion of CO2ꢀCO2¯• into methane. In studies done on
mercury and lead in dimethylformamide,37,41–43 the CO2ꢀCO2¯• adduct 3 is believed to
reductively disproportionate to yield both CO32ꢀ and CO, which is a terminal product on these
electrodes. Our experimental observation of CO as a minor product on nꢀCu/C electrodes (Figure
S4) is consistent with its appearance as an intermediate in our proposed mechanism. Given CO is
known to bind to copper with an adsorption enthalpy of ~20 kcal/mol,44 we may expect it to bind
to the copper and further react. The downstream steps in which the adsorbed CO molecule 4
reacts to form methane 5 may be similar to what has been previously proposed for copper foil
electrodes based on detailed studies of their reactivity with CO.35,45
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Conclusion
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These nanoscale copper electrocatalysts represent a first step towards the development of
a dispersed electrochemical methanation catalyst that can be used in practical electrolyzers.21
Copper nanoparticles are ideal for preparing gas diffusion layers for membraneꢀelectrode
assemblies which minimize polarization losses, maximizing the energy efficiency of
electrolyzers. In addition, these colloidallyꢀprepared copper nanoparticles have catalytic
properties that rival much more expensive highꢀpurity foils and singleꢀcrystal electrodes. The
finding of improved methanation activity and Faradaic efficiency for copper nanoparticle
catalysts on glassy carbon through a unique mechanism paves the way for complementary
computational and spectroscopic studies to develop a more detailed mechanistic understanding
of the origin of the improved catalytic properties.
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Experimental Methods
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Synthesis of copper nanoparticles. Copper nanoparticles capped with
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tetradecylphosphonate and suspended in hexane were synthesized following a literature
method.23 Briefly, 10 mL of trioctylamine (Sigma Aldrich, 98%) was heated in a 25 mL three
neck flask equipped with a condenser and stir bar to 130 oC under argon for 1 hour in order to
dry the solvent. The trioctylamine was then cooled to room temperature; while keeping the flask
purged with argon,123 mg of copper (I) acetate (Sigma Aldrich, 97%) and 139 mg of nꢀ
tetradecylphosphonic acid (TCI Synthesis, Lot # 808002N09) were added to the trioctylamine.
The solution was then rapidly heated to 180 oC under argon, swirled briefly to release any
precursors adhered to the walls of the flask, and kept at this temperature for 30 minutes. The
solution was rapidly heated to 270 oC and held at that temperature for 30 minutes. After cooling
to room temperature, the copper particles were removed using airꢀfree techniques and transferred
to a glovebox. Just prior to electrode fabrication, 0.15 mL of the asꢀsynthesized particles in
trioctylamine was diluted with 0.75 mL of hexane, and then precipitated by adding 1.4 mL of
isopropanol, under airꢀfree conditions. The particles were separated by centrifuging at 8000 rpm
for 8 minutes. The solvent was decanted under ambient conditions, and the particles were
resuspended in 1 mL of hexane by vortexing. The volume of hexane in which the particles are
resuspended can be changed in order to optimize the particle loading on glassy carbon; this is
critical for achieving high methanation yields.
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Fabrication of n-Cu/C electrodes. Glassy carbon plates (Type 2, Alfa Aesar) with 5.2
cm2 of active surface area were polished using 1 ꢁm alpha alumina (CH Instruments) and 50 nm
gamma alumina (CH Instruments). The plates were rinsed with MilliꢀQ water, sonicated briefly,
and blown dry with nitrogen. 600 ꢁL of copper nanoparticles suspended in hexane were
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deposited on the substrate, which was then spun at 1000 rpm on a spinꢀcoater for 60 seconds.
600 ꢁL of ethanol was deposited on the substrate, which was allowed to sit for 30 seconds, and
then spun at 1000 rpm for 60 seconds. Then, for trimethylsilylchloride treated electrodes, the
substrate was covered in 600 ꢁL of 2 wt% trimethylsilylchloride in hexane and spun at 1000 rpm
for 60 seconds, which was repeated once; then, the substrate was covered in 600 ꢁL of ethanol
and spun at 1000 rpm for 60 seconds, which was also repeated once.
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Fabrication of evaporated copper electrodes. Glassy carbon plates were polished as
described above. A thermal evaporator (Edwards Auto 500, FTM7) in an argon glovebox was
used to evaporate copper films on the glassy carbon, using a highꢀpurity copper source
(99.9999%, Alfa Aesar). The films ranged in thickness from 3 nm to 21 nm, as measured using a
quartz crystal monitor, and were deposited at a rate of approximately 3 nm/min. The plates were
transferred under ambient conditions for use in the electrochemical cell.
Preparation of copper foil electrodes. High purity copper foils were wetꢀsanded using
1500 grit sandpaper (Norton Blackice), rinsed with MilliꢀQ water, dipped in 8% nitric acid for 30
seconds, rinsed again with MilliꢀQ water, and then blown dry with nitrogen.
Microscopy of electrodes. As synthesized copper nanoparticles were imaged by dropꢀ
casting on a TEM grid (Electron Microscopy Sciences, CFꢀ400ꢀCu) and acquiring images on a
200 kV Tecnai G2 20 SꢀTWIN with a Gatan SC200 CCD camera. After polarization, glassy
carbon electrodes were rinsed with MilliꢀQ water and blown dry with nitrogen. Particles were
transferred from the glassy carbon electrode to a TEM grid by placing a grid on top of the
electrode and applying pressure to the grid using a glass microscope slide for a few seconds.
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SEMs of electrodes were acquired using a Zeiss Ultra 55 Field Emission Scanning Electron
Microscope with an InLens detector, 5 kV accelerating voltage, and 5 mm working distance.
Electrochemical methods. Electrochemical measurements were conducted in a twoꢀ
compartment electrochemical flow cell fabricated from polychlorotrifluoroethylene (KelꢀF),
similar to a design used in the literature.19 The working electrode compartment and counter
electrode compartment, which each had an electrolyte volume of 5 mL and a gas headspace of ~1
mL, were separated by a Selemion membrane (AMV, AGC Engineering). A Ag/AgCl reference
electrode (BASi, REꢀ6), which was stored in saturated KCl when not in use, was used; all
measured potentials were converted to the RHE scale. The current densities measured for
nanoparticle electrodes were normalized by the copper surface area; the surface area was
determined by measuring the diameter of particles using SEM and calculating their surface area
by treating them as spheres. For copper foils, the geometric surface areas were used, such that
the reported current densities serve as upper bounds for the activity of copper foils. 0.1 M
NaHCO3 in MilliꢀQ water was used as the electrolyte, which was prepared by bubbling CO2
(Praxair, CD Mꢀ50, >99%) through a solution of half the molarity of Na2CO3 (99.9999%, Sigma
Aldrich), producing a solution of pH 6.8 after approximately 2 hours. The electrochemical cell
was continuously purged with CO2 at a flow rate of 20 mL/min and a pressure of 1.2 atm; cells
were purged for at least 5 minutes following assembly and before electrochemical polarization.
For the experiments where CO2 partial pressure was varied, the total flow rate and pressure of
CO2 were kept constant, while adding in a diluent stream of argon. Potentiostatic experiments
were conducted by stepping to the desired potential, holding at that potential for 10 minutes, and
sending a sample to the gas chromatograph (SRI Instruments, MG #3 Configuration) at the end
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of the 10 minute interval. The gas chromatograph was equipped with TCD and FID detectors, a
methanizer, and Molsieve 13x and Hayesep D columns.
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ASSOCIATED CONTENT
Supporting Information
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Additional electrochemical data on electrode stability, order dependence, and mechanistic
insights. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
alivis@berkeley.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We would like to thank Virginia Altoe, David Barton, Alex Bell, Trevor Ewers, Eric Granlund,
Prashant Jain, Kendra Kuhl, Bryan McCloskey, Pete Nickias, Phillip Ross, Rachel Segalman,
Yogesh Surendranath, and Mark Yoshida for useful discussions. This work was supported by the
Dow Chemical Company under contract 20120984. SEM was conducted at the Molecular
Foundry, supported by the Office of Science, Basic Energy Sciences, of the U.S. Department of
Energy under contract DEꢀAC02ꢀ05CH11231. K.M. gratefully acknowledges the support of the
Department of Energy Office of Science Graduate Fellowship. A.P.A. was supported by the
United States Department of Energy under contract DEꢀAC02ꢀ05CH11231.
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Figures
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Figure 1. Morphological evolution of copper nanoparticles during the course of
electrochemical CO2 reduction. TEM of asꢀsynthesized copper nanoparticles of diameter 7.0 ±
0.4 nm at A) low magnification and B) high magnification, showing that the initial particles are
highly polycrystalline. C) SEM of nꢀCu/C electrode, consisting of copper nanoparticles
supported on glassy carbon substrate. D) SEM of nꢀCu/C electrode following polarization for 10
minutes at ꢀ1.25 V under CO2 electroreduction conditions, demonstrating that the average
particle diameter grows to 23 ± 8 nm. TEMs of copper nanoparticle transferred from glassy
carbon substrate onto TEM grid, at E) low magnification and F) high magnification, in which it
is evident that the particles that form under polarization are highly polycrystalline. G) SEM of
trimethylsilylchloride treated nꢀCu/C electrodes prior to polarization, in which particles have an
average diameter of 52 ± 21 nm, and H) SEM of the same electrode following polarization for 10
minutes at ꢀ1.25 V, in which the particles that form are 25 ± 8 nm in diameter.
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Figure 2. Comparison of current densities and Faradaic efficiencies for n-Cu/C and copper
foil electrodes. A) Total current density, demonstrating that nꢀCu/C has greater overall reduction
activity than the copper foil. B) Faradaic efficiency for methane, in which it is evident that nꢀ
Cu/C is more selective for methane than the copper foil. C) Methanation current density, in
which the combined effect of the improved current density and Faradaic efficiency on nꢀCu/C is
apparent. D) Faradaic efficiency for hydrogen as a function of potential, showing suppressed
hydrogen evolution on the nꢀCu/C catalyst.
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Figure 3. Stability of the n-Cu/C and copper foil catalysts. A) Total current density and B)
Faradaic efficiency for methanation as a function of time for both nꢀCu/C and copper foil
polarized at ꢀ1.25 V, demonstrating that the nꢀCu/C catalyst is stable.
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Figure 4. Continuum of catalytic behavior between nanoparticle-like and foil-like
electrodes. 3 nm evaporated copper film A) prior to and B) following polarization at ꢀ1.25 V for
10 minutes. 15 nm evaporated copper film C) prior to and D) following polarization at ꢀ1.25 V
for 10 minutes. E) Methanation Faradaic efficiency and gravimetric methanation current as a
function of evaporated copper film thickness, from which it is evident that thin evaporated films
behave like the nꢀCu/C electrodes while thick evaporated films behave like copper foils.
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Figure 5. Mechanistic insights from Tafel analysis. A) Tafel plot for nꢀCu/C and copper foil,
with linear fit at low current densities demonstrating that these catalysts have Tafel slopes of 60
± 4.2 mV/decade and 86 ± 4.6 mV/decade, respectively. The Tafel slope for nꢀCu/C indicates a
one electron preꢀequilibrium step precedes a nonꢀelectrochemical rate limiting step. B)
Methanation current density as a function of partial pressure of CO2 at ꢀ1.25 V, demonstrating
that methanation current density has a 2.03 ± 0.08 order dependence on the partial pressure of
CO2 at lower CO2 partial pressures. C) Proposed mechanism for the electrochemical reduction of
CO2 to methane, including the rate limiting step (RLS). This mechanism is consistent with the
gathered electrochemical data and known intermediates for CO2 reduction that have been
identified in the literature.
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