We report the electrocatalytic reduction of CO_2 to the highly reduced C_2 products, ethylene and ethane, as well as to the fully reduced C_1 product, methane, on three different phases of nickel–gallium (NiGa, Ni_3Ga, and Ni_5Ga_3) films prepared by drop-casting. In aqueous bicarbonate electrolytes at neutral pH, the onset potential for methane, ethylene, and ethane production on all three phases was found to be −0.48 V versus the reversible hydrogen electrode (RHE), among the lowest onset potentials reported to date for the production of C_2 products from CO_2. Similar product distributions and onset potentials were observed for all three nickel–gallium stoichiometries tested. The onset potential for the reduction of CO_2 to C_2 products at low current densities catalyzed by nickel–gallium was >250 mV more positive than that of polycrystalline copper, and approximately equal to that of single crystals of copper, which have some of the lowest overpotentials to date for the reduction of CO_2 to C_2 products and methane. The nickel–gallium films also reduced CO to ethylene, ethane, and methane, consistent with a CO_2 reduction mechanism that first involves the reduction of CO2 to CO. Isotopic labeling experiments with ^(13)CO_2 confirmed that the detected products were produced exclusively by the reduction of CO_2

Brunschwig, Bruce S.

Crompton, J. Chance

Francis, Sonja A.

Javier, Alnald

Lewis, Nathan S.

Soriaga, Manuel P.

Thompson, Joanthan R.

Torelli, Daniel A.

Caltech Authors - Main

 S1 
Supporting Information 
Nickel-Gallium-Catalyzed Electrochemical Reduction of CO2 to Highly Reduced Products 
at Low Overpotentials 
Daniel A. Torelli¶†‡, Sonja A. Francis¶†‡£, J. Chance Crompton†‡, Alnald Javier‡, Jonathan R. 
Thompsonǁ, Bruce S. Brunschwig*§, Manuel P. Soriaga*‡, and Nathan S. Lewis*†‡§ 
†Division of Chemistry and Chemical Engineering, ‡Joint Center for Artificial Photosynthesis, 
£Resnick Sustainability Institute, ǁDivision of Engineering and Applied Sciences, and §Beckman 
Institute Molecular Materials Research Center, California Institute of Technology, Pasadena, 
California 91125, United States 
¶These authors contributed equally to this work 
 
*E-mail: bsb@caltech.edu, msoriaga@caltech.edu, nslewis@caltech.edu 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S2 
Experimental. 
 
Sample Preparation.  
Quartz slides (VWR), gallium (III) nitrate hydrate 99.999% [69% gallium by mass] (Sigma 
Aldrich) and nickel (II) nitrate hexahydrate 99.999% (Sigma Aldrich) were used as received. 
Pyrolytic graphite plates (GraphiteStore) were cleaned in aqua regia and polished to a mirror 
finish before use. Water with a resistivity > 18 MΩ cm obtained from a Barnsted Nanopure 
system was used throughout. 
Aqueous stock solutions of 0.052 M nickel nitrate and 0.036 M gallium nitrate were prepared 
and were combined in appropriate ratios to prepare precursor solutions for NiGa, Ni5Ga3, and 
Ni3Ga. For an individual sample, 0.5 mL of the precursor solution was drop-cast onto a graphite 
plate heated to 100 °C. After drying on a hotplate for five min, the samples were placed in 
porcelain boats and then loaded into a quartz tube in a Carbolite tube furnace. The tube furnace 
was placed under a constant 4 L min-1 flow of forming gas (5% H2, 95% N2) and then heated to 
700 °C for 3 h, before cooling to room temperature. The back of the graphite plate was then 
covered in 3M 470 Electroplating Tape (Uline.com) to mask from electrical contact with the 
solution, and an alligator clip was used to make contact to the front part of the plate with the 
NixGay film. Control Ni-only samples and Ga-only samples were made by the same method but 
only drop-casting one of the precursor solutions. 
Powder X-ray diffraction (XRD) patterns were collected at room temperature on a Bruker D2 
Phaser Diffractometer with a copper Kα source (λ = 1.54184 Å) and a LynxEye-1D detector. 
Simulated XRD patterns were generated by the Crystal Maker and Crystal Diffract software 
package. 
Electrochemical Testing.  
A BioLogic SP-200 potentiostat (Biologic, Grenoble, France) was used for all electrochemical 
testing. The uncompensated cell resistance was determined from a single-point high-frequency 
impedance measurement and was compensated (85%) by the built-in positive-feedback software. 
A modified two-compartment cell was used for all electrochemical measurements. The cell 
consisted of a Pyrex weigh bottle that had been modified with ground-glass joints in its lid 
(Figure S3). The joints were used to insert electrodes and make seals during electrolysis, while 
the bottle was used so that the entire lid of the cell could be removed to accommodate larger (2-4 
cm2) working electrodes. A polyether ether ketone (PEEK) tube was modified with a detachable 
ring at the bottom that was fit with a fresh Selemion anion-exchange membrane before each 
electrolysis or set of cyclic voltammograms (CV).  The Pt mesh counter electrode was behind the 
membrane. A Ag/AgCl (3M NaCl) fritted reference electrode (CH Instruments) was used, and 
potentials were converted to values relative to a reversible hydrogen electrode (RHE) using the 
equation: E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.0591 V x pH. The working electrode 
was the Ni-Ga film supported on the graphite plate described above. Electrical contact was made 
with a stainless steel alligator clip. The electrolyte used in all cycling tests was potassium 
phosphate buffered to pH 7. No anodic cycling was performed before electrolyses. All cycling 
was performed on fresh electrodes to show their electrochemical behavior. During the first 
minute of electrolysis, the current decreased by ~5-10x as the native oxide was reduced. At these 
 S3 
potentials, a Pourbaix diagram shows that only the metallic state of Ni is stable. Bulk electrolysis 
was performed in a cell that was sealed under 1 atm CO2 and stirred at ~ 1,000 rpm. Electrolyses 
were performed at varied potentials ranging from -0.9 V to -1.8 V versus a Ag/AgCl (3 M NaCl) 
reference electrode, in 0.1 M Na2CO3 (Sigma-Aldrich ≥ 99.999% metal basis) that had been 
acidified to pH 6.8 with 1 atm CO2 creating a HCO3-/CO2 buffered system similar to the standard 
protocol.1 Na2CO3 was used instead of KHCO3 due to the much higher purity available for 
Na2CO3. Although the faradaic efficiency can be affected by the cation in the electrolyte, for a 
Cu electrode, the production of C2 products is lower for Na+ than K+.3 It is consequently 
reasonable to assume similar, if not higher, yields of C2 products with NixGay when a KHCO3 
electrolyte is used instead of NaHCO3. Electrolyses were performed until a set amount of charge, 
generally 40 C cm-2 was passed. However, for some of the lower potentials, a lower amount of 
charge was passed in the experiments. Electrolyses at -0.5 V were run for >12 h to obtain a 
significant concentration of the products. The calculated yields suggest the electrodes are stable 
on this timescale. In contrast, polycrystalline Cu has been reported to deactivate over the same 
period of time, at the same potential.6 Additionally, the 13CO2 experiment was run for > 10 h with 
a data point approximately every 2 h. No decreases in the rates of 13CH4, 13C2H4, or 13C2H6 were 
observed over this period of time.  
CO Reduction. 
For experiments where CO or N2 was required, K2HPO4 was used as the electrolyte and 
buffered to pH 7 to create similar conditions to those under CO2. The use of the CO2-free buffer 
eliminates the possibility that CO2 produced from the equilibration of HCO3- with CO2 was 
responsible for the measured response rather than being attributable to CO or N2. The CV data 
suggest that Ga slows the binding of CO rather than weakens the Ni-CO interaction. The 
potential at which CO oxidation is observed does not shift significantly (<50 mV) between the 
Ni film and the Ni5Ga3 film, suggesting that the strength of the metal-CO bond is approximately 
equivalent between the two films. However, the time it takes for CO to rebind is significantly 
longer on the Ni5Ga3 film compared to the Ni film, suggesting the presence of kinetic differences 
between the two films. Direct infrared spectroscopic detection of surface-bound CO on the Ni-
Ga systems has not been observed to date.  
Product Analysis.  
An Agilent 7890A gas chromatograph (GC), with two thermal conductivity detectors, was used 
to separate and quantify the gases in the headspace of the electrochemical cell. The oven was set 
to 50 °C for 9 min followed by ramping at a rate of 8 °C min-1 to 80 °C for a total run time of 14 
min.  For isotopic labeling experiments that involved smaller amounts of charge passed, an 
Agilent 7820A GC coupled with a 5977E MS with a heated cold quadrupole detector and a 
capillary CarbonPLOT column was used for identification and quantification of the products. 
The oven was set to 35 °C for 6.6 min and was then ramped to 150 °C at a rate of 20 °C min-1 
and held for 2 min to allow heavier molecules such as ethylene and ethane to elute. Both the GC 
and GCMS instrumentation was calibrated using tanks of 15% CH4, 10% C2H4, and 5% C2H6 
each mixed with N2. Dilutions were performed by filling a sealed 1 or 3 L round-bottom flask 
with N2 or CO2, and injecting known amounts of the calibration gas. The gaseous mixture was 
allowed to stir for ~1 min at which time aliquots were removed for GC or GCMS calibration. 1H 
NMR spectroscopy was performed on a Bruker 400 MHz Spectrometer. Standards of 10 to 100 
µM solutions of the analytes (sodium formate, methanol, etc.) were prepared by serial dilution, 
 S4 
and were used to calibrate the instrument. In general, 0.1 µL of the internal standard (dimethyl 
formamide, DMF) was added to a 2 mL aliquot of the standard solution. 0.5 mL of this solution 
was then transferred to a NMR tube that contained 200 µL of deuterated water. A water 
suppression method was used to suppress the signal of the water in the electrolyte and to allow 
visualization of the analyte peaks. The same procedure was used to quantify the liquid CO2 
reduction products, with 0.1 µL of DMF added to 2 mL of the electrolyte after electrolysis, and 
0.5 mL of this solution added in an NMR tube to 200 µL of D2O.  
Surface Analysis.  
Scanning-electron micrographs were obtained using a Nova NanoSEM 450 microscope (FEI, 
Hillsboro, OR, USA) with an accelerating voltage of 20 kV and a working distance of 5.0 mm. 
X-ray photoelectron spectroscopy (XPS) data were obtained using an AXIS Ultra DLD 
instrument (Kratos Analytical) at a background pressure of 1 × 10−9 Torr. High-intensity 
excitation was provided by monochromatic Al Kα X-rays having an energy of 1486.6 eV with an 
instrumental resolution of 0.2 eV full width at half-maximum. Photoelectrons were collected at 
0° from the surface normal at a retarding (pass) energy of 80 eV for the survey scans, whereas a 
pass energy of 20 eV was used for the high-resolution scans. The peak energies were calibrated 
against the binding energy, BE, of the adventitious C 1s peak. For quantitative analysis, the XPS 
signals were fitted using CasaXPS software (CASA Ltd., Teignmouth, United Kingdom) to 
symmetric Voigt line shapes that were composed of Gaussian (70%) and Lorentzian (30%) 
functions that employed a Shirley background. Electrochemical surface area measurements were 
preformed and electrodes were found to be only slightly roughened.  
  
 S5 
Tables and Figures. 
   
Figure S1. SEM of NixGay films after annealing showing the aggregation of microparticles into a 
cracked film, (right) zoomed out, (left) zoomed in. 
 
Table S1. Tabulated XRD reflections seen for each nickel-gallium phase. 
 
 
 
 
 
 
NiGa Ni5Ga3 Ni3Ga 
2 Theta Reflection 2 Theta Reflection 2 Theta Reflection 
44.4 110 43.2 221 43.5 111 
64.6 200 48.4 131 50.9 200 
76.7 quartz 50.9 220 (Ni3Ga) 74.8 220 
81.7 211 54.5 440 76.7 quartz 
94.1 quartz 75.2 440 90.8 311 
98 220 76.7 quartz 94.1 quartz 
  86.5 223 96.1 222 
  94.1 quartz   
50 µm 1 µm 
 S6 
 
 
Figure S2. XP spectra showing (a) the Ni 2p and (b) Ga 3p regions. In the Ni spectra the peaks 
at ~852 eV and ~870 eV correspond to Ni in the nickel-gallium films, while the peaks at ~856 
eV and ~874 eV correspond to oxidized Ni. Other peaks in the Ni spectra are satellite peaks. In 
the Ga 3p region the peaks at ~106 eV and ~109 eV correspond to oxidized Ga while the small 
peak at ~104 eV corresponds to metallic Ga.  
 
 S7 
Table S2. Comparison of the stoichiometric ratios of Ga 2p and Ni 2p from XPS data to the 
target values.  
Phase Ga 2p Ni 2p 
Observed 
Ni:Ga Target Ni:Ga 
NiGa 51 49 0.95 1 
Ni3Ga 28 72 2.53 3 
Ni5Ga3 43 57 1.35 1.667 
 
 
 
 
 
Figure S3. Diagram of the cell design employed in this study.  
 S8 
  
Figure S4. Electrochemical surface area measurements showing a slightly roughened film by 
lower (a) capacitative current on carbon substrate compared to (b) capacitative current on Ni3Ga.  
 S9 
 
 
Figure S5.  Potential-dependent Faradaic efficiencies (solid lines) and current densities (dotted 
lines) for CO2 reduction in 0.1M Na2CO3 (aq) acidified to pH 6.8 with 1 atm CO2 (g) to methane 
(triangles), ethylene (squares) and ethane (exes). (a) NiGa and (b) Ni3Ga show similar behavior 
to Ni5Ga3 (See Figure 2a) with respect to product distributions at different potentials.  
 
 S10 
 
Figure S6. Representative example of a gas chromatogram for the products of CO2 reduction at a 
nickel-gallium film. The example shows specifically results from electrolysis with Ni5Ga3 at 
−0.68 V vs. RHE in 0.1M Na2CO3 (aq) acidified to pH 6.8 with 1 atm CO2 (g) where 150 C was 
passed. Given the poor separation between the C2H4 and CO2 peaks at these concentrations, GC-
MS rather than GC was used to quantify the F.E. for C2H4. Inset shows the full-scale 
chromatogram. Relative peak heights for O2, N2 and CO2 are representative for both control 
experiments and bulk electrolyses.  
 
 S11 
   
 
Figure S7. Histograms showing (a) Faradaic efficiency and (b) partial current density towards 
hydrocarbon products for the nickel-gallium films under CO2 or under CO.  
 
  
 S12 
 
 
 
 
 
 
Figure S8. GC/MS data obtained during a 13CO2 electroreduction. (a) m/z=17 corresponding to 
13CH4, (b) m/z=29: first peak corresponds to 13C2H4 and second peak to 13C2H6, (c) m/z=31 
corresponding to 13C2H6.  The number of Coulombs (C) of charge passed during the 
electrocatalysis when the products were analyzed is color-coded as indicated in the legends. 
 S13 
  
 
Figure S9. Example GC/MS chromatograms for each m/z fragment corresponding to CH4, C2H4, 
and C2H6. (a) The signals corresponding to CH4 at 4.7 min are m/z=16 (CH4+) and m/z= 15 
(CH3+). Signals for m/z=14, 13, and 12 (CH2+, CH+. and C+) are roughly 5-6 times less intense 
than m/z=16 and 15 and therefore are not observed.  The signal at 4.3 min corresponds to O2 
where m/z=16 (O+) and N2 where m/z = 15 (15N-14N+). (b) The signals corresponding to C2H4 at 
10.1 min are m/z=27 (C2H3+), m/z= 26 (C2H2+) and m/z=25 (C2H+). Because of the substantial 
N2+ background at m/z=28 (C2H4+) is not observed. The signals corresponding to C2H6 at 11.8 
min are m/z=30 (C2H6+), m/z=29 (C2H5+), m/z=27 (C2H3+), m/z= 26 (C2H2+) and m/z=25 (C2H+); 
again m/z=28 (C2H4+) is not observed because of N2+ background. 
 
 S14 
 
Figure S10. NMR spectra of the 0.1M Na2CO3 (aq) acidified to pH 6.8 with 1 atm CO2 (g) after 
an electrolysis with NiGa at -1.2 V vs. RHE. The peak at 8.44 ppm corresponds to formate, the 
peaks at 7.92, 3.00, and 2.85 ppm correspond to the DMF internal standard at 65 µM, and the 
peak at 3.34 ppm corresponds to methanol. The peak at 1.90 ppm corresponds to acetate which is 
formed upon bubbling CO2 into the solution. The large peak at 4.8 ppm corresponds to water. 
 
 
 
 
  
 


Nickel−Gallium-Catalyzed Electrochemical Reduction of CO_2 to Highly Reduced Products at Low Overpotentials

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Nickel−Gallium-Catalyzed Electrochemical Reduction of CO_2 to Highly Reduced Products at Low Overpotentials

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