STATEMENT OF GOVERNMENT RIGHTS
[0001] This invention was made with government support under Grant No. CHE1308652 awarded
by the National Science Foundation. The government has certain rights in the invention.
RELATED APPLICATION DATA
FIELD
[0003] The present invention relates to the electrocatalytic reduction of CO
2 and, in particular, to the electrocatalytic reduction of CO
2 via binary alloy systems and/or oxides thereof.
BACKGROUND
[0004] Global atmospheric CO
2 concentrations have risen continuously for the past two centuries largely due to
anthropogenic activities such as fossil fuel combustion, industrial manufacturing,
and land clearing. This is cause for alarm because effects of high CO
2 levels include changes in water availability and food production capacity as well
as implications for human health. The burning of fossil fuels not only sustains high
CO
2 levels, but it also reduces our access to compounds which are critical chemical feedstocks.
The electrochemical transformation of CO
2 into chemical feedstocks and energy sources offers a potential solution to this far-reaching
problem, effectively turning a societal hindrance into practical products.
SUMMARY
[0005] Briefly, a system for providing oxygenated organic products comprises an electrochemical
cell including an electrolyte solution comprising CO
2, and a working electrode comprising a transition metal/post transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof for electrocatalytic reduction of the CO
2 to the oxygenated organic products. In some embodiments, the oxygenated organic products
comprise two or more carbon atoms and/or two or more oxygen atoms. Binary alloy of
the working electrode can be of the formula TM
xPTM
y, wherein x and y are integers independently selected from 1 to 10. As described further
herein, binary alloy of the working electrode may be in oxide form wherein one or
both of the transition metal and post transition metal are metal oxides. Oxygenated
organic products produced by the system can comprise one or more of propanol, butanol,
ethanol, oxalate, formic acid or formate and acetone. In some embodiments, oxygenated
organic products further include a single oxygen atom, including CO and methanol.
[0006] In another aspect, a system for providing oxygenated organic products comprises an
electrochemical cell including an electrolyte solution comprising CO
2, and an electrode comprising an alloy and/or mixture of metal oxides. The electrode
comprises an electrocatalytic site for reduction of CO
2 to CO, wherein CO is incorporated into the oxygenated organic products. In some embodiments,
the alloy comprises at least one of a transition metal and post-transition metal.
Moreover, metal oxides of the electrode can comprise at least one of a transition
metal oxide and post-transition metal oxide.
[0007] In another aspect, a system for providing organic products comprises an electrochemical
cell including an electrolyte solution comprising CO
2, and a working electrode comprising a transition metal/post-transition metal (TM/PTM)
binary alloy and/or oxides thereof for electrocatalytic reduction of the CO
2 to the organic products. In some embodiments, binary alloy excludes nickel and gallium.
Notably, binary metal oxides comprising nickel and gallium are not excluded. Organic
products produced by the system can comprise one carbon atom, two carbon atoms, three
carbon atoms or mixtures thereof. Additionally, the organic products can be oxygenated,
in some embodiments.
[0008] In another aspect, methods of forming oxygenated organic products are described herein.
In some embodiments, a method of forming oxygenated organic products includes providing
an electrochemical cell including an electrolyte solution comprising CO
2, and a working electrode comprising a transition metal/post transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof and electrocatalytically reducing the CO
2 to the oxygenated organic products. The oxygenated organic products can comprise
two or more carbon atoms and/or two or more oxygen atoms. Oxygenated products can
include one or more of propanol, butanol, ethanol, oxalate, formic acid or formate
and acetone. In some embodiments, oxygenated products additionally include a single
oxygen atom including CO and methanol.
[0009] In another aspect, a method of forming oxygenated organic products comprises providing
an electrochemical cell including an electrolyte solution comprising CO
2, and an electrode comprising an alloy and/or mixture of metal oxides. CO
2 is reduced to CO at an electrocatalytic site on the electrode, and the oxygenated
organic products are derived from the CO. In some embodiments, for example, the oxygenated
products comprise oxalate.
[0010] In another aspect, methods of forming organic products are described. In some embodiments,
a method of forming organic products comprises providing an electrochemical cell including
an electrolyte solution comprising CO
2, and a working electrode comprising a transition metal/post-transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof and electrocatalytically reducing the CO
2 to the organic products. In some embodiments, the binary alloy excludes combination
of nickel and gallium, without excluding binary metal oxide composition including
oxides of nickel and/or gallium. Organic products can comprise one carbon atom, two
carbon atoms, three carbon atoms or mixtures thereof. Additionally, the organic products
can be oxygenated, in some embodiments.
[0011] In a further aspect, methods of oxalate production are described. In some embodiments,
a method of oxalate production comprises providing an electrochemical cell including
an electrolyte solution comprising CO
2, and a working electrode comprising a transition metal oxide/post-transition metal
oxide composite and electrocatalytically reducing the CO
2 to oxalate via generating CO and methanol from the CO
2. In some embodiments, CO is incorporated into the oxalate product, and methanol is
excluded from the oxalate product. Moreover, oxalate can be produced at Faradaic efficiencies
of at least 60 percent. As described further herein, various aspects and/or parameters
of oxalate production methods can be altered or adjusted to achieve higher Faradaic
efficiencies, including efficiencies greater than 70 percent or greater than 80 percent.
[0012] These and other embodiments are described in more detail in the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
FIG. 1 provides powder X-ray diffraction of Ni3Al film on glassy carbon referenced to PDF 01-071-5883. Scanning electron microscopy
(SEM) images of the Ni3Al film are also provided on the right side.
FIG. 2(A) are cyclic voltammograms obtained using Ni3Al on glassy carbon under CO2 (red) and Ar (black). Voltammograms were obtained at a scan rate of 100 mV/s with
Pt mesh counter and Ag/AgCl reference electrodes in 0.1 M K2SO4 at pH 4.5.
FIG. 2(B) provides 1H-NMR of 0.1 M K2SO4 electrolyte solution following bulk electrolysis at -1.38 V, indicating the presence
of 1-propanol, methanol, and formate as major products. The broad peak at 4.66 ppm
is a suppressed water signal, and all peaks are referenced to a 1,4-dioxane internal
standard.
FIG. 2(C) provides 13C-NMR confirming the presence of ethanol.
FIG. 2(D) provides 1H-13C HSQC indicating that 13C-labelled acetone was produced.
FIG. 3(A) - Faradaic efficiencies for CO, 1-propanol, methanol, and formate are reported
at a range of applied potentials.
FIG. 3(B) - Faradaic efficiencies for H2, CO, and liquid products are plotted as a function of pH and confirm charge balance
(top); the liquid product distribution is provided in the lower portion of panel (B).
FIG. 4 quantifies the major liquid products, 1-propanol and methanol, plotted versus
the amount of charge passed using (A) CO2 and (B) CO feedstocks.
FIG. 5A presents an XRD pattern (left) of Cr2O3-Ga2O3 referenced to Cr2O3 (PDF 00-038-1479) and Ga2O3 (PDF 01-074-1610); and an SEM image (right) of Cr2O3-Ga2O3 platelets.
FIG. 5B is a fitted XPS spectra of Cr (left) and Ga (right), indicating that the surface
is primarily made up of metal oxides. Peaks were referenced to adventitious carbon
at 284.5 eV (not shown).
FIG. 6 is an IR spectrum of calcium oxalate derived from Cr2O3-Ga2O3 mediated electrolysis, confirming that oxalate is a CO2 reduction product.
FIG. 7 illustrates pH dependence of Faradaic efficiencies for carbon-containing products.
Experiments were performed at -1.38 V vs. Ag/AgCl using CO2-saturated 0.1 M KCl buffered with KHCO3 (pH > 4) or adjusted with HCl (pH < 4).
FIG. 8 illustrates potential dependence of Faradaic efficiencies for carbon-containing
products. Experiments used pH 4.05, 0.1 M KCl electrolyte, and all potentials tested
were more positive than the thermodynamic potential for one-electron reduction of
CO2. Optimal oxalate production was achieved at -1.48 V vs. Ag/AgCl.
FIG. 9 illustrates Faradaic efficiencies of carbon-containing products. While oxalate
generation is suppressed at high KCl concentrations, low CO and high formate Faradaic
efficiencies contribute to decreased oxalate production at low KCl concentrations.
Electrolyses were performed at -1.48 V vs. Ag/AgCl.
FIG. 10 illustrates Faradaic efficiencies of carbon-containing products. Cr-rich stoichiometries
result in the lowest efficiencies for CO. Electrolyses were performed at -1.48 V vs.
Ag/AgCl in 0.1 M KCl (pH 4.1).
DETAILED DESCRIPTION
[0014] Embodiments described herein can be understood more readily by reference to the following
detailed description and examples and their previous and following descriptions. Elements,
apparatus and methods described herein, however, are not limited to the specific embodiments
presented in the detailed description and examples. It should be recognized that these
embodiments are merely illustrative of the principles of the present invention. Numerous
modifications and adaptations will be readily apparent to those of skill in the art
without departing from the scope of the invention.
[0015] In one aspect, systems employing binary alloys and/or oxide(s) thereof for the electrocatalytic
reduction of CO
2 to various organic products are described. Binary alloys and/or oxides thereof suitable
for electrocatalytic reduction of CO
2 comprise a transition metal and a post-transition metal (TM/PTM). In some embodiments,
the transition metal is a first row transition metal. Moreover, post-transition metals
can be selected from Groups IIB-VA of the Periodic Table. Groups of the Periodic Table
referenced herein are identified according to the CAS designation. Binary alloys,
in some embodiments are of the formula TM
xPTM
y, wherein x and y are integers independently selected from 1 to 10. Transition metal
and post-transition metal can be combined in any ratio operable for the electrocatalytic
reduction of CO
2 into various organic products. In some embodiments, for example, x is 3 and y is
1. In other embodiments, x can range from 1 to 9 and y can range from 1 to 6. Table
I provides a listing of various binary alloys operable for the electrocatalytic reduction
of CO
2 into various organic products, including oxygenated products comprising two or more
carbon atoms and/or two or more oxygen atoms.
Table I - TM
xPTM
y, wherein x = 3 and y =1
Ni-Al |
Ni-Ga |
Ni-In |
Cu-Al |
Fe-Ga |
Fe-Al |
Mn-Al |
Mn-Ga |
Co-Al |
Ni-Zn |
Co-Ga |
Cr-Al |
Cr-Ga |
Ag-Al |
Ag-Ga |
As described herein, the binary alloy can be in oxide form. For example, at least
one of the transition metal and post transition metal is a metal oxide. In some embodiments,
both the transition metal and post transition metal are metal oxides. Binary alloy
may be partially oxidized or fully oxidized. Binary metal oxides, for example, may
form surface and/or bulk regions of the material administering the electrocatalytic
reduction of CO
2. Any oxides of the binary alloy systems listed in Table I are contemplated. In some
embodiments, for example, the working electrode comprises the binary system of chromium
oxide and gallium oxide including, but not limited to, Cr
2O
3-Ga
2O
3. Moreover, stoichiometries or ratios of the transition metal and post-transition
metal can remain the same in the metal oxide as in the binary alloy. Accordingly,
the values provided for x and y above apply to metal oxide embodiments. In chromium
oxide-gallium oxide embodiments, for example, the ratio of chromium to gallium can
be 3:1.
[0016] In some embodiments, a binary alloy and/or oxides thereof contain one metal that
can bind CO
2 at the electrode interface via a Lewis acid interaction and second metal that is
moderately effective at participating in proton coupled electron transfers. In other
embodiments, binary alloys and/or associated oxides of interest for electrocatalytic
CO
2 reduction contain a d
9 valence electron count.
[0017] Binary alloy and/or oxides thereof can be provided as a thin film on the working
electrode of the electrochemical cell. The thin film of binary alloy and/or oxides
thereof can be deposited on any substrate consistent with the objectives of the present
invention. In some embodiments, for example, the thin film of alloy and/or oxide is
deposited on glassy carbon.
[0018] The electrochemical cell also comprises an electrolyte solution having CO
2 dissolved therein. Any electrolyte solution consistent with the objectives of the
present invention can be employed, including aqueous electrolyte solution. In some
embodiments, aqueous electrolyte solution comprises alkali metal salt or alkaline
earth metal salt. The electrolyte solution may also comprise CO in addition to CO
2, in some embodiments. The electrolyte solution can have neutral or acidic pH, in
some embodiments. The electrolyte solution, for example, can have pH ranging from
3-7 or from 3-6.5. In other embodiments, pH of the electrolyte solution ranges from
4-6. Oxygenated organic products formed by the electrocatalytic reduction of CO
2 at the working electrode can include one or more of propanol, butanol, ethanol, oxalate,
formic acid or formate and acetone. In some embodiments, oxygenated products include
a single oxygen atom, such as CO and methanol.
[0019] TM/PTM binary alloy and/or oxides thereof of the working electrode are operable to
form additional organic products from the electrocatalytic reduction of CO
2. These organic products can comprise one carbon atom, two carbon atoms, three carbon
atoms or mixtures thereof. Such organic products can be aliphatic or oxygenated. In
some embodiments, TM/PTM binary alloy excludes the combination of nickel and gallium
for the production of aliphatic products and oxygenated products comprising a single
carbon atom and/or single oxygen atom.
[0020] In another aspect, a system for providing oxygenated organic products comprises an
electrochemical cell including an electrolyte solution comprising CO
2, and an electrode comprising an alloy and/or mixture of metal oxides. The electrode
comprises an electrocatalytic site for reduction of CO
2 to CO, wherein CO is incorporated into the oxygenated organic products. In some embodiments,
the alloy comprises at least one of a transition metal and post-transition metal.
Moreover, metal oxides of the electrode can comprise at least one of a transition
metal oxide and post-transition metal oxide. The electrode, for example, can have
any composition and/or properties described herein. In some embodiments, the electrode
is a mixture of metal oxides, such as chromium oxide and gallium oxide. The electrocatalytic
site for reduction of CO
2 to CO may be anionic or exhibit anionic character, in some embodiments. Additionally,
the electrocatalytic site may be selective to the reduction of CO
2 to CO in that the site does not participate in other redox chemistries. The electrode
comprising an alloy and/or mixture of metal oxides may contain one or more additional
electrocatalytic sites for producing other products. In some embodiments, for example,
the electrode comprises a non-ionic site for the production of formate in addition
to the electrocatalytic site for CO
2 reduction to CO.
[0021] These embodiments are further illustrated in the following non-limiting examples.
EXAMPLE 1 - Ni3Al Electrocatalytic Reduction of CO2 to Oxygenated Organics
[0022] Here it is shown that a Ni
3Al thin film electrocatalyst supported on glassy carbon can generate reduced C
1, C
2, and C
3 products from CO
2 with good performance, stability, and reproducibility at modest overpotential. Competing
copper-based electrocatalysts were first reported to carry out the reduction of CO
2 to C
2 and C
3 products in 1988. For the first time it is demonstrated that metal alloys can generate
C
3 products, electrocatalytic activity that, until now, has been uniquely associated
with copper-based electrode systems. Further, the data presented here suggest that
the Ni
3Al system is more stable than copper-based systems.
[0023] Ni
3Al thin film alloys were synthesized on glassy carbon substrates by adapting a drop-casting
and furnace reduction procedure employed by
Torelli et al., ACS Catal. 2016, 6, 2100-2104. As shown in FIG. 1, powder X-ray diffraction of the alloys confirmed the successful
generation of the cubic Ni
3Al composition as indicated by the (111) and (211) planes; energy-dispersive X-ray
spectroscopy supported the compositional analysis. X-ray photoelectron spectra (XPS)
of a virgin (i.e. not utilized) electrode showed the presence of three Ni species:
Ni(OH)
2, NiO, and Ni metal, with the Ni
2+ components making up a majority of the composition, while all surface Al adopted
the oxidized Al
2O
3 form. Thin films exhibited macroscopic surface areas of approximately 0.75 cm
2, while imaging by scanning electron microscopy, shown in FIG. 1, indicated that the
films were comprised of micro-scale platelets uniformly distributed across the glassy
carbon surface. In preliminary electrochemical experiments, cyclic voltammetry scans
performed in aqueous electrolyte under CO
2 versus Ar saturation resulted in relatively featureless traces, although current
enhancement was observed at more negative potentials leading into a proton reduction
wave (FIG. 2A).
[0024] Bulk electrolysis experiments using the Ni
3Al thin film on glassy carbon as the working electrode and a Pt mesh counter electrode
were performed at an applied potential of -1.38 V vs. Ag/AgCl in a sealed two-compartment
cell containing 0.1 M K
2SO
4 electrolyte solution saturated with CO
2 (pH 4.5). Total current densities of -2.1 ± 0.4 mA/cm
2 were recorded and could be maintained for a period of several hours.
1H-NMR spectra obtained as a function of electrolysis time (see FIG. 2B) indicated
the growth of largely isolated peaks characteristic of 1-propanol (triplet; 0.73 ppm),
acetone (singlet; 2.07 ppm), ethanol (triplet; 1.03 ppm), methanol (singlet; 3.20
ppm), and formate (singlet; 8.29 ppm) over time.
[0025] Several analytical methods were employed to confirm the identities of these C
1-C
3 products and to support the assertion that they were, in fact, derived from the CO
2 starting material. Electrolysis experiments performed using
13CO
2 yielded
1H-NMR traces for 1-propanol, ethanol, methanol, and formate exhibiting the peak splitting
expected for
13C-coupling. However, a large doublet signal of 2-propanol (believed to be generated
from acetone reduction) obscured the ethanol triplet, and acetone splitting was inconclusive
using simple
1H-NMR. To resolve these ambiguities
13C-NMR spectra were obtained, resulting in the observation of a clear set of ethanol
peaks (FIG. 2C). In addition, a
1H-
13C heteronuclear single quantum correlation (HSQC) NMR experiment was performed to
confirm the presence of
13C-labelled acetone (FIG. 2D). Moreover, peak splitting within the
13C-NMR spectrum, particularly in the C
2 and C
3 products, implied that all product carbon atoms were derived from the original
13CO
2 material. Mass spectrometry further supported the NMR results.
[0026] Faradaic efficiencies of 1.9 ± 0.3% for 1-propanol, 1.0 ± 0.2% for methanol, and
0.75 ± 0.03% for formate were observed, with lesser contributions from ethanol and
trace amounts of acetone. As shown in FIG. 3A, the potential dependence of CO
2 electroreduction to these liquid products confirms -1.38 V vs. Ag/AgCl as the optimal
potential at which to operate the electrochemical cell. The electrolyte pH was also
found to strongly impact the overall CO
2 conversion efficiency and the distribution of products as summarized in FIG. 3B.
Highest product yields were observed at pH~4.5. It is worth noting that the solution
pH changes at most a few tenths of a unit from the beginning to the end of an electrolysis
experiment. Given that all electrolysis experiments utilized efficient and continuous
stirring of the electrolyte, the constant bulk value of the electrolyte pH suggests
that any pH variation at the electrode surface was slight and, according to FIG. 3B,
did not drastically alter the product distribution.
[0027] An understanding of the mechanism facilitated by the Ni
3Al film would allow for future work in optimizing the generation of select C
1, C
2, or C
3 products. As such, the headspace of the electrochemical cell following electrolysis
was examined, and it was discovered that CO was the only gaseous CO
2 reduction product generated. Like the liquid products, CO production was maximized
at -1.38 V vs. Ag/AgCl, at which point Faradaic efficiencies of 33 ± 3% were attained.
As reported in FIG. 3B, the remainder of the gas generated was H
2. The fact that maximum liquid product and CO yields are achieved at the same applied
potential suggests that CO might be an intermediate in CO
2 electroreduction to major products such as 1-propanol and methanol. To test this
hypothesis, CO
2 was replaced with CO as the feedstock in electrolysis and examined the resulting
reduction products by
1H- NMR. Ni
3Al thin films on glassy carbon reduced CO to methanol as well as the C
2 and C
3 liquid products achieved using a CO
2 feedstock and in the same relative quantities. Electrolysis experiments utilizing
a mixed
13CO
2/
12CO feedstock indicated that 1-propanol, methanol, ethanol, and acetone were preferentially
generated from CO, with only small contributions from
13CO
2. In these experiments, trace or no formate was produced.
[0028] Furthermore, plotting 1-propanol and methanol quantities obtained during CO-feedstock
trials against the amount of charge passed during an experiment, as shown in FIGS.
4A and 4B, illuminated differences in rate of product generation. Specifically, when
the electrochemical cell was saturated with CO
2, both 1-propanol and methanol increased linearly with charge passed. However, when
the feedstock was switched to CO, though 1-propanol retained its linearity (but had
a slope that was greater by two orders of magnitude than that of the CO
2-derived analog), methanol formation adopted an exponential growth curve.
[0029] Accordingly, it appears that CO is, in fact, an intermediate leading to Ni
3Al's generation of methanol, C
2, and C
3 products from CO
2. It is suggested that the reduction of CO
2 to CO is the limiting process in this electroreduction, leading to preferential use
of CO as the reactant when both CO and CO
2 are present, as well as linear product generation curves when CO
2 is the available species being reduced. Only one CO
2 molecule, and therefore one CO molecule, must be present to produce methanol, so
when the system is supplied with CO as the feedstock it generates methanol relatively
easily, leading to an exponential production curve. The generation of 1-propanol necessitates
the presence of three carbon atoms, so even if the limiting step is removed by providing
CO as the reactant, accumulation of three CO molecules near one another on the thin
film surface is still required. Thus, the linear trend for 1-propanol remains, though
its slope increases because the CO
2 to CO conversion step has been eliminated. This sort of mechanistic analysis, though
preliminary, will help to improve the design and optimization of future alloy catalysts.
[0030] The fact that Ni
3Al generates quantifiable amounts of C
3 products, alongside useful C
1 and C
2 products, is interesting because of the thin film's stability, reproducibility, and
modest overpotential. It is worth noting that, upon cursory examination of the related
intermetallic NiAl, significantly diminished Faradaic efficiencies were achieved for
the products described herein. The previously reported Ni-Ga thin film system plated
on a highly oriented pyrolytic graphite substrate achieved maximum Faradaic efficiencies
for C2 products of approximately 1.7% and 0.4% for ethane and ethylene, respectively,
with no indication of C3 product formation. Methane was also observed. Faradaic efficiency
of 1.9 ± 0.3% for the C
3 product 1-propanol indicates a heterogeneous synthetic route to higher order organic
compounds that has not previously been reported at alloy electrode interfaces.
[0031] The inventors have recently suggested a catalytic efficiency parameter that serves
to summarize both the overpotential and turnover frequency (catalytic current) of
an electrocatalytic reaction independent of mechanistic details. This single parameter
allows one to compare a variety of catalysts that transform a given substrate to the
same product. Ni
3Al catalytic efficiency parameter for 1-propanol generation is calculated to be 0.5
± 0.1%. This is comparable to the catalytic efficiency parameter for Torelli
et al.'s Ni-Ga thin film in the generation of ethane (0.44%; based on maximum Faradaic efficiency),
their major C2 product.
[0032] Furthermore, it is well established that copper electrodes suffer from instability
in solution and excessive overpotential requirements for the formation of higher order
organic products, making their usage to-date impractical. Ni
3Al, on the other hand, is stable in aqueous solution over the time scale explored
here. This work shows that Ni
3Al generates electroreduced products from CO
2 continuously over a period of four to five days. Scanning electron microscopy confirms
that the thin film is robust and, as demonstrated by the small amount of material
loss observed, withstands exposure to electrochemical conditions while maintaining
initial efficiencies for CO
2 reduction. This finding is supported by post-electrolysis XPS analysis demonstrating
that the electrode surface composition remains unchanged during electrochemical CO
2 reduction.
[0033] The Ni
3Al thin film on glassy carbon reported here is the first copper-free, heterogeneous
electrocatalyst capable of generating C
3 products, including 1-propanol and acetone, from CO
2 starting material, and its Faradaic efficiencies for 1-propanol generation are competitive
with those achieved on most copper electrodes. Ultimately, these significant factors
suggest that heterogeneous catalysts comprised of metals other than copper may generate
highly reduced products from CO
2 whose identities, Faradaic efficiencies, selectivities, or overpotentials rival or
exceed those achieved on copper catalysts.
Methods
[0034] Thin film Ni
3Al alloys were synthesized as previously described.
19 Briefly, aqueous solutions of 0.052 M nickel(II) nitrate hexahydrate and 0.036 M
aluminum(III) nitrate nonahydrate were combined in appropriate ratios to achieve the
Ni
3Al stoichiometry. In 0.1-mL increments, 0.5-mL portions of the nickel-aluminum nitrate
solution were drop-casted onto glassy carbon pieces that had been set on a hot plate
and heated to 150 °C. After drop-casting, the substrates remained on the hot plate
for 15 min until the solution completely evaporated, revealing green surface films.
The substrates were then placed in alumina boats and loaded into either a Lindberg/Blue
M or Carbolite Quartz Tube Furnace under 95% Ar/5% H
2 gas flow. The furnace was ramped at a rate of 3 °C/min to 700 °C, where it rested
for 5 h.
[0035] Electrodes were prepared by affixing a coiled copper wire to the glassy carbon substrate
using conducting silver epoxy, extending the length of copper wire through a glass
tube, and sealing both ends of the tube using insulating epoxy. It was critical that
the insulating epoxy was also used to completely cover the silver epoxy and copper
wire attached to the substrate. In some experiments, the top of a film-deposited substrate
was wrapped in copper tape and held using an alligator clip attached to copper wire
similarly threaded through a glass tube sealed with insulating epoxy. Comparable amounts
of charge were passed in electrochemical experiments featuring the two types of electrode
preparations.
[0036] Electrochemical experiments were performed using CH Instruments 760 and 1140 potentiostats.
Cyclic voltammetry experiments were completed in a three-neck round-bottom flask using
the Ni
3Al film on glassy carbon as the working electrode referenced to Ag/AgCl and a Pt mesh
counter electrode in 0.1 M K
2SO
4 at pH 4.5. Bulk electrolysis experiments were undertaken in the same electrolyte
solution (with the exception of pH dependence experiments, which utilized K
2SO
4 buffered with KHCO
3/CO
2) using custom electrolysis cells with gas-tight ports for the above electrodes. In
these experiments, the Pt mesh counter electrode was situated in a fritted gas dispersion
tube to separate the reduction reaction at the cathode from oxidation processes at
the anode, and a stir bar was employed. The reaction solutions were purged with CO
2, CO, or Ar for 20 min prior to experimental or control trials; experiments using
13CO
2 were not completely purged with the starting material, resulting in a small amount
of
12CO
2 contamination that could be quantified by
1H-NMR. Bulk electrolysis experiments were performed over intervals of at least 4 h,
during which time the headspace was sampled every 20 min and the electrochemical solution
was sampled every 60 min. During and after bulk electrolysis experiments, both the
solution and headspace were sampled for products using
1H- or
13C-NMR (referenced to 1,4-dioxane internal standard) and gas chromatography, respectively.
EXAMPLE 2 - Cr2O3-Ga2O3 Electrocatalytic Reduction of CO2 to Oxalate
[0037] In this example, an electrode composed of a chromium oxide-gallium oxide thin film
on glassy carbon is employed to transform CO
2 to oxalate in water. To our knowledge, this is the first heterogeneous electrocatalyst
system capable of transforming CO
2 to oxalate in water, introducing new possibilities for catalyst discovery and tangible
opportunities for the energy efficient conversion of CO
2 to a chemical feedstock containing more than one carbon.
[0038] Thin films of Cr-Ga (3:1 ratio) on glassy carbon solid supports were synthesized
using a drop-casting and thermal reduction method adapted from
Torelli et al, Nickel-gallium-catalyzed electrochemical reduction of CO2 to highly
reduced products at low overpotentials. ACS Catal. 6, 2100-2104 (2016). Powder X-ray diffraction (XRD; FIG. 5A) coupled with energy-dispersive X-ray spectroscopy
suggested that the bulk films were comprised of Cr
2O
3 and Ga
2O
3 in the desired 3:1 stoichiometry. Surface compositions were analyzed by X-ray photoelectron
spectroscopy (FIG. 5B), which pointed to an oxidized surface comprised of mostly Cr(III),
matching the bulk, and Ga oxides. Scanning electron microscopy (FIG. 1A) indicated
that Cr
2O
3-Ga
2O
3 films were comprised of discontinuous platelets scattered across the glassy carbon
surface, not unlike alternative bimetallic systems similarly synthesized.
[0039] Initial bulk electrolysis experiments were conducted using a Pt mesh counter electrode
and 0.1 M KCl electrolyte (pH 4.1 after CO
2 purging). Applying a potential of -1.38 V vs. Ag/AgCl to an electrochemical cell
purged with
13CO
2 induced generation of CO and H
2, sampled by gas chromatography, as well as oxalate, formate and methanol, detected
in the liquid phase by
1H and
13C-NMR. A high-intensity peak at 161 ppm overshadowed formate, methanol, and residual
CO
2 signals and was assigned to oxalate. To confirm this product assignment, a sample
of the electrolyzed solvent which had been treated with HCl to remove any carbonate
present was mixed with calcium bromide, causing precipitation of a white solid which
was isolated by vacuum filtration and examined by infrared (IR) spectroscopy (FIG.
6). Only IR transitions associated with calcium oxalate were observed.
[0040] In order to optimize the Cr
2O
3-Ga
2O
3 system for oxalate production, pH, electrolyte, potential dependence, and stoichiometric
studies were undertaken. Gravimetric determination of oxalate is well established
and was found to be quantitative in the present study when a 1 M calcium bromide solution
was utilized on post electrolysis samples. Standard curves were employed for quantifying
CO/H
2 and formate/methanol using gas chromatography and
1H-NMR, respectively.
[0041] All pH-varying experiments were conducted at an applied potential of -1.38 V vs.
Ag/AgCl and used CO
2-saturated KCl electrolyte (buffered with KHCO
3 for pH > 4; adjusted with HCl for pH < 4; 0.1 M concentration). As shown in FIG.
7, the Cr
2O
3-Ga
2O
3 system is only slightly sensitive to solution pH, since statistically equivalent
oxalate Faradaic efficiencies were achieved at pH 4.1 and 5.1, while pH 6.1 yielded
slightly inferior results. Markedly lower Faradaic efficiencies at pH 7.1 suggest
that carbonate is not involved in oxalate production, as these experiments contained
the highest original concentration of KHCO
3 buffer. Charge balance was achieved in all cases by H
2 generation.
[0042] In separate experiments, the electrolyte anion was varied (i.e., KCl, KBr, and KI
were compared), since other researchers have reported that CO
2 reduction product selectivity can be highly electrolyte dependent. However, in the
Cr
2O
3-Ga
2O
3 system, carbon-containing products did not exhibit this dependence. Subsequent experiments
therefore utilized CO
2-saturated, pH 4.0 KCl, because this pH maximized total Faradaic efficiency for carbon-containing
products compared to H
2. Furthermore, bulk solution pH consistently rose to 4.5-5.0 by the end of electrolysis
(when initial pH = 4.1), and as shown in Fig. 7, maximum oxalate Faradaic efficiencies
would still be achieved at these final pH conditions.
[0043] Subsequently, potential dependence experiments were conducted using the optimized
electrolyte conditions. Notably, all potentials examined resulted in some oxalate
generation, despite being significantly more positive than the thermodynamic potential
required for one-electron reduction of CO
2 to CO
2·-, the intermediate that has historically been invoked for the conversion of CO
2 to oxalate. The resulting Faradaic efficiencies, displayed in Fig. 8, suggest that
the Cr
2O
3-Ga
2O
3 system is more sensitive to applied potential than pH. Additionally, the two major
carbon-containing products, oxalate and CO, reached maximum efficiencies at different
potentials. An electrode potential of -1.48 V vs. Ag/AgCl (630 mV more positive than
E° for reducing CO
2 to CO
2·-) was determined to be the optimal potential for oxalate production.
[0044] A cell employing 0.1 M KCl (pH 4.0) at a potential of -1.48 V vs. Ag/AgCl, generated
Faradaic efficiencies for oxalate, CO, formate, and methanol of 59 ± 3%, 8.1 ± 0.7%,
0.16 ± 0.02%, and 0.15 ± 0.02%, respectively. Materials characterization post-electrolysis
suggested that Cr
2O
3-Ga
2O
3 system continued to be chemically and physically stable. XPS analysis revealed only
subtle changes in surface composition. Surface Cr remained more than 99% Cr(III),
in agreement with the Cr Pourbaix diagram. Still, Ga metal did not make up the majority
of the sample, but its XPS spectrum largely resembled its pre-electrolysis analog,
confirming a stable surface. SEM imaging indicated that the thin film incurred only
slight erosion at platelets' edges during electrolysis, while EDX showed that the
3:1 Cr:Ga stoichiometry was maintained. A single Cr
2O
3-Ga
2O
3/glassy carbon electrode could transform CO
2 continuously for more than 10 days (the longest time period studied), suggesting
an attractive catalytic lifetime.
Determination of reaction intermediates en route to oxalate
[0045] The Cr
2O
3-Ga
2O
3 film on glassy carbon is a promising catalyst due to its high oxalate Faradaic efficiency,
good stability, and, perhaps most interestingly, its ability to perform the electrochemical
transformation in water. At the applied potentials studied here, ranging from 530
to 930 mV more positive than the E° required for CO
2·- generation, CO
2 reduction to oxalate cannot occur through a CO
2·- intermediate, which means a pathway as-yet unreported for the electrochemical CO
2-to-oxalate transformation must be at play. While prior studies often rely on the
supposition of a CO
2·- intermediate, calculations have been performed evaluating a homogeneous catalytic
system, consisting of a dinuclear Cu complex in acetonitrile solvent, that explicitly
refute a CO
2·--dependent pathway in that case. To determine whether any of the alternative CO
2 reduction products serve as intermediates en route to oxalate, a series of electrolysis
experiments were performed, which replaced the CO
2 feedstock with CO, formate, methanol, or combinations of these carbon-containing
compounds.
[0046] Ultimately, during electrolysis experiments conducted using the optimized pH, electrolyte,
and potential plus
13CO and methanol (rather than CO
2), oxalate was produced, as confirmed by precipitation with calcium bromide as well
as
13C-NMR. This
13CO experiment, which used
12C-methanol, also verified that CO was incorporated into the oxalate product. The opposite
labeling experiment, using
12CO and
13C-methanol, was also undertaken, resulting in no
13C-NMR signal even though calcium oxalate was precipitated out of solution. Therefore,
methanol is not incorporated into the product. Significantly, supplying Cr
2O
3-Ga
2O
3 with either CO or methanol, rather than both, does not result in an oxalate end product;
both species are required even though only the CO ends up in the reaction product.
Rather than a CO
2·--dependent pathway, Cr
2O
3-Ga
2O
3 production of oxalate therefore appears to rely on CO and methanol, which it can
first generate from CO
2.
[0047] The incorporation of CO and use of short-chain alcohols in oxalate generation is
not unprecedented, although it has not previously been accomplished using a CO
2 starting material. Large-scale manufacture of oxalate is frequently accomplished
by oxidative carbonylation of small alcohols to achieve diesters of oxalic acid, followed
by hydrolysis to attain the oxalate product. This reaction consumes O
2 to re-oxidize the catalyst, which can be a two-component metal system, such as Pd
plus FeCl
2 or CuCl
2.
[0048] FIG 9 illustrates Faradaic efficiencies of carbon-containing products. While oxalate
generation is generally suppressed at high KCl concentrations, low CO and high formate
Faradaic efficiencies contribute to decreased oxalate production at low KCl concentrations.
Electrolyses were performed at -1.48 V vs. Ag/AgCl. In feedstock experiments, formate
was shown to be a competitor of, rather than intermediate contributing to, oxalate
production.
[0049] Cr
2O
3-Ga
2O
3 methods of generating oxalate exhibit critical mechanistic differences with oxidative
carbonylation processes, which could make Cr
2O
3-Ga
2O
3 a more attractive option for oxalate synthesis. Ultimately, the Cr
3Ga catalyst introduces a new and practical means of generating oxalate from CO
2, but it also demonstrates that electrochemical routes excluding a CO
2·- intermediate are not only possible but can operate both in aqueous environments and
at much lower applied potentials than previously thought.
[0050] Such a departure from the previously accepted mode of CO
2 reduction to oxalate invites a question about the roles of the metals within the
Cr
2O
3-Ga
2O
3 catalyst. To answer that question, the pure metal films as electrocatalysts were
first examined. At -1.48 V vs. Ag/AgCl (pH 4.0 KCl), films of Cr on glassy carbon
generated modest amounts of CO, formate, and methanol from CO
2, while the activity of Ga films was dominated by CO production at around 40% Faradaic
efficiency. Regardless, plain Ga thin films converted about 20% of the CO
2 in the system to carbonate, observable on the surface post-electrolysis by XPS, while
Cr's carbonate production was more limited. As thin films, neither metal alone could
produce oxalate from CO
2, confirming the importance of having both metals present.
[0051] A clear need for both Cr and Ga implies that an optimal Cr:Ga stoichiometry exists
for maximizing oxalate production. To determine this stoichiometry and gain additional
insight into the role of each metal, a range of stoichiometries spanning from 100%
Cr to 100% Ga were synthesized as thin films on glassy carbon and analyzed for their
performance as CO
2 reduction electrocatalysts. All experiments were conducted at -1.48 V vs. Ag/AgCl
in 0.1 M KCl (pH 4.05) for comparison to the optimized oxalate outcome for Cr
2O
3-Ga
2O
3.
[0052] The trends in carbon-containing product generation based on Cr:Ga stoichiometry are
displayed in Fig. 10. Cr
2O
3-rich stoichiometries corresponded to lower overall quantities of non-oxalate products
compared to Ga-rich variants, but, most notably, Faradaic efficiencies for CO (essential
for oxalate generation) reached their lowest values at the highest percentages of
Cr. The highest Faradaic efficiency for oxalate was obtained at the original 3Cr:lGa
stoichiometry. When considering the product trends in FIG. 10, it seems that optimal
oxalate generation requires ideal combinations of CO/methanol production and formate
suppression. Some critical quantity of Cr is needed to ensure that oxalate is produced
efficiently, but excess Cr prevents adequate generation of CO, which decreases oxalate
Faradaic efficiencies. Complementarily, Ga seems to be the major CO-generating component
of the system. In combination, these factors make Cr
2O
3-Ga
2O
3 one preferred embodiment for oxalate production, with the recognition that oxalate
production can be achieved with other electrode compositions, systems and methods
described herein. Other alloys and/or oxides thereof exhibiting the mechanistic requirements
described herein may be employed for oxalate production.
[0053] The ability of the Cr
2O
3-Ga
2O
3 thin film on glassy carbon to generate oxalate from CO
2 in water makes it a landmark example of heterogeneous CO
2 electroreduction, as previous studies of this conversion were confined to use of
nonaqueous electrolytes and applied potentials reflective of a CO
2·- intermediate. With optimal electrolysis conditions of pH 4.0 aqueous KCl and -1.48
V vs. Ag/AgCl, the pathway used to generate oxalate by this system must not include
CO
2·- coupling. Instead, CO
2 is reduced to CO and methanol, which are then used to produce oxalate.
[0054] Oxalate Faradaic efficiencies of 59 ± 3% and initial lifetime studies exceeding 10
days of continuous use show the potential for Cr
2O
3-Ga
2O
3 as a candidate catalyst for a new industrial oxalate process, especially because
it achieves the desired end product using aqueous solution, atmospheric pressure,
and CO
2 starting material.
Nature of Cr-Ga Surface Sites Active in CO2 Reduction
[0055] Notably, the reactant experiments that initially pointed to oxalate-generating roles
for CO and methanol did not implicate an important role for formate, which has been
indicated as a competitor of oxalate production in the literature. With a Cr-Ga electrode,
use of a formate feedstock resulted in only trace amounts of methanol and failed to
generate oxalate. This result suggested that distinct active sites may exist for generation
of formate and CO-derived products, including oxalate. Further support for this prediction
was provided by electrolyte dependence studies. While experiments varying the electrolyte
anion (i.e., KCl, KBr, KI, K
2SO
4, and KH
2PO
4) failed to exhibit significant differences in the distribution of products, a stark
dependence was noted when varying the electrolyte cation.
[0056] Use of LiCl or NH
4Cl electrolytes (0.1 M) and -1.48 V vs. Ag/AgCl applied potential resulted in similar
product distributions and efficiencies as those recorded for KCl. However, analogous
experiments using CsCl, (CH
3)
4NCl ((TMA)Cl), and CaCl
2 electrolytes failed to generate any detectable quantities of oxalate, and CO Faradaic
efficiencies were also reduced. (TMA)Cl supporting electrolyte increased the Faradaic
efficiency of formate to 7.7 ± 0.4%, compared to the 0.16 ± 0.02% value achieved using
optimal oxalate-generating conditions (0.1 M KCl). These cation-dependence results
are summarized in Table 2.

Furthermore, Cr-Ga electrodes previously used in (TMA)Cl experiments did not regain
their oxalate-generating ability when re-introduced into a KCl-containing electrolyte.
This KCl electrolyte was subjected to
1H-NMR after electrolysis, and the resultant spectrum exhibited an overwhelming signal
from TMA
+, which must have come from the Cr-Ga surface. TMA
+ had therefore chemisorbed to the catalyst during prior electrolyses, likely contributing
to inhibition of oxalate generation in those and subsequent experiments. Formate generation
remained higher than usual in these trials.
[0057] Exacerbation of formate production when oxalate generation is suppressed further
supports the proposal that at least two surface active sites are present in the Cr-Ga
catalyst: one for CO and CO-derived productions and a second for formate. Moreover,
chemisorption of TMA
+ onto the Cr-Ga surface suggests that a surface anion is present, while lack of oxalate
production in the presence of cations having few waters of hydration (TMA
+ and Cs
+) or strong anion-binding capacity (Ca
2+) hints that this surface anion is critical to CO/oxalate generation. To probe this
theory, the Cr-Ga system (0.1 M KCl, -1.48 V vs. Ag/AgCl) was treated with 15 mM NaCN
prior to electrolysis, anticipating that the Lewis-basic, anionic CN
- ligand would bind specifically to a Lewis-acidic, non-anionic surface site. Indeed,
after performing electrolysis with this modified system, CO, oxalate, and methanol
were detected in typical yields, while no formate was produced. Thus, it appears that
the CN
- ligates the formate-generating active site, simultaneously demonstrating that this
site is (A) chemically distinct from the CO-generating site and (B) not anionic in
character. This experiment, combined with the cation-dependence data, strongly suggests
that Cr-Ga contains two types of electrocatalytic surface sites for CO
2 reduction: an anionic site leading to CO-derived products and a non-anionic site
that produces formate.
Materials and Methods
Materials
[0058] Chromium(III) nitrate nonahydrate (>_ 99.99%), gallium(III) nitrate hydrate (99.9%),
KHCO
3 (99.7%), oxalic acid (>_ 99%), NH
4Cl (99.998%), (CH
3)
4NCl ((TMA)Cl; ≥ 98%), NaCN (97%), methanol (≥ 99.9%),
13C-methanol (99 at%
13C), formic acid (>_ 98%), 1,4-dioxane (99.8%), acetonitrile (99.8%), ethanol (≥ 99.8%),
isopropanol (≥ 99.7%),
13CO
2 (99 at%
13C),
12CO (
13C-depleted), and
13CO (99 at%
13C) were obtained from Sigma-Aldrich. KCl, KBr, KI, K
2CO
3, K
2SO
4, KH
2PO
4, LiCl, CsCl, CaCl
2, and HCl, all ACS grade, were purchased from EMD Chemicals, and calcium bromide (99.5%)
was obtained from Alfa Aesar. Ar, CO
2, CO, 95% Ar/5% H
2, and 50% CO/50% H
2 gases and mixtures were ordered from AirGas. Glassy carbon plates (GLAS11; 25 x 25
x 3 mm; Structure Probe Inc.) were cut in half lengthwise prior to use. Conducting
silver and Loctite Hysol insulating epoxies were purchased from Epo-Tek and Grainger,
respectively. All chemicals were used as received except for methanol and formic acid
for standard curves, 1,4-dioxane for NMR internal standards, and HCl, all of which
were diluted prior to use.
Methods
[0059] Synthetic procedures to create Cr-Ga thin films of various stoichiometries are described.
Aqueous solutions of 0.052 M chromium(III) nitrate nonahydrate and 0.036 M gallium(III)
nitrate hydrate were mixed to achieve the desired Cr:Ga ratio. Glassy carbon pieces
were heated to ~120 °C on a hotplate, and 0.1-mL samples of the Cr-Ga nitrate solution
were drop-casted onto them. After the solution evaporated completely, the glassy carbon
pieces were placed in an alumina boat and loaded into either a Lindberg/Blue M or
Carbolite Quartz Tube Furnace. The furnace was ramped at a rate of 3 °C/min to 700
°C under 95% Ar/5% H
2 gas flow; it rested at this state for 5 h prior to cooling to room temperature at
a rate of -3 °C/min. Resulting Cr-Ga films were olive green in color, with Cr-rich
stoichiometries tending toward kelly green and Ga-rich stoichiometries tending toward
gray.
[0060] Electrodes were prepared in one of two fashions. One electrode configuration involved
connecting copper wire to the glassy carbon support using conducting silver epoxy,
feeding the wire through a glass tube, and covering both ends of the tube (including
any exposed copper or silver) with insulating epoxy. The second configuration featured
the same general setup, but the copper wire was attached to an alligator clip, which
could then be used to reversibly hold glassy carbon pieces whose tops had been wrapped
in copper tape. Experiments using both electrode configurations yielded identical
results, both in terms of charge passage and product distribution.
[0061] Electrochemical experiments were conducted using CH Instruments 760 and 1140 potentiostats.
Bulk electrolysis experiments utilized custom electrochemical cells with gas-tight
ports for the working, Pt mesh counter (situated in a gas dispersion tube), and Ag/AgCl
reference electrodes. The electrolyte was continuously stirred. Unless otherwise noted,
0.1 M KCl was used as the electrolyte, and it was buffered with KHCO
3 to achieve CO
2-saturated pH values > 4 or adjusted with 0.01 M HCl for values < 4. Electrolyte solutions
were purged with CO
2 for 30 min prior to experimentation. In experiments without CO
2 (i.e., CO, formic acid, methanol, or combinatorial feedstocks), the pH was adjusted
to the appropriate, CO
2-analogous value. The majority of electrolyses were conducted at pH 4.1, and post-electrolysis
measurements indicated that the final solution pH was consistently between 4.5 and
5.0. Experiments using
13CO
2,
13CO, and
12CO (
13C-depleted) were not completely purged with the respective gas.
[0062] Electrolysis experiments were performed until 30-40°C charge had passed, unless the
experiment was meant to determine catalyst lifetime. The solution and headspace of
electrochemical cells were sampled for liquid and gaseous products by
1H-NMR (referenced to 1,4-dioxane internal standard) and gas chromatography, respectively,
both during and after bulk electrolysis. Oxalate was detected by
13C-NMR and quantified by precipitation of the calcium salt.
Cr-Ga Thin Film Characterization
[0063] The compositions and morphologies of Cr-Ga films were analyzed by a variety of materials
characterization techniques. Powder X-ray diffraction was performed using a Bruker
D8 Advance diffractometer with 0.083° step size and CuKα radiation. XRD samples either
remained on the glassy carbon support or were scraped from the surface; resulting
patterns were identical, except that scraped samples exhibited significantly less
carbon intrusion and were therefore selected for presentation herein. Thin film morphology
and additional bulk composition data were obtained using a FEI XL30 FEG-SEM equipped
with EVEX EDS detector. SEM images and EDX spectra were obtained using a 5 or 10 keV
electron beam with a 10-15 mm working distance. XPS spectra were collected using a
ThermoFisher K-Alpha X-Ray Photoelectron Spectrometer set to 20 eV pass energy and
50 ms dwell time. Resulting data were analyzed using the Thermo Scientific Avantage
Data System and CasaXPS software. Materials characterization was conducted before
and after electrochemistry in designated experiments.
Product Analysis
[0064] Formate and methanol were detected by
1H-NMR after combining 530 µL electrolyte with 60 µl D
2O and 10 µL 1,4-dioxane (10 mM); the latter served as an internal standard. In
13C-NMR experiments (used primarily to detect oxalate), only 1 µL 1,4-dioxane (10 mM)
was added. A Bruker Avance III 500 MHz NMR Spectrometer with cryoprobe detector was
used for all NMR experiments, and the experiments incorporated a custom water suppression
method to permit sampling of aqueous electrolyte solutions. Formate and methanol were
quantified using 5-point calibration curves for
1H-NMR, while oxalate was visualized qualitatively by a large
13C-NMR signal (in experiments utilizing
13C-labeling) in the 160-170 ppm range.
[0065] Oxalate was quantified by first treating a sample of the electrolysis solution with
1 M HCl (to remove any carbonate byproduct) and then adding 1 M calcium bromide solution,
which resulted in the precipitation of calcium oxalate. The calcium oxalate sample
was dried in an oven at 105 °C overnight and then massed; this mass was used to calculate
the total quantity of oxalate. IR spectra of calcium oxalate samples were obtained
using a Thermo Diamond Smart Orbit IR Spectrometer set at 1 cm
-1 resolution. The carbonate byproduct could be quantified by finding the difference
in mass between two electrolysis samples, one treated with HCl and the other untreated
prior to calcium bromide addition; the difference in mass was attributed to calcium
carbonate, which was then calculated as a percentage of the total CO
2 in solution (based on the electrolyte volume unique to each experiment). Calcium
carbonate was also examined by IR spectroscopy. Experimental calcium oxalate and calcium
carbonate samples were compared to control compounds made by combining calcium bromide
and either oxalic acid or K
2CO
3 in aqueous solution.
[0066] Headspace samples were analyzed by gas chromatography for gaseous products. CO was
measured using a HP6890 Gas Chromatograph fitted with a Molsieve 5A PLOT capillary
column (Agilent) and TCD. The sampling method was a 5-min, 60 °C isotherm with He
flow gas. An SRI 8610C Gas Chromatograph with Ar flow, which also used a Molsieve
column and TCD, was run for a 7-min isotherm at 80 °C to detect H
2. CO and H
2 were quantified using 30-point calibration curves having R
2 values ≥ 0.99. The headspace was also sampled following
13CO
2 electrolyses using a KBr-terminated gas cell and Nicolet iS50 FT-IR Spectrometer
with 1 cm
-1 resolution; this confirmed that the CO product was derived from CO
2. Faradaic efficiencies for all products, gaseous and liquid, were calculated based
on the charge passed during each experiment as well as the product quantities determined
by gas chromatography,
1H-NMR, or calcium bromide precipitation. Catalytic efficiencies were calculated based
on the following equation:

[0067] Various embodiments of the invention have been described in fulfillment of the various
objects of the invention. It should be recognized that these embodiments are merely
illustrative of the principles of the present invention. Numerous modifications and
adaptations thereof will be readily apparent to those skilled in the art without departing
from the scope of the invention.
[0068] The following features of the invention were claimed in the parent application and
are presented here for basis of the current claims and for possible future amendments
or divisional applications.
Feature 1. A system for providing oxygenated organic products comprising:
an electrochemical cell including an electrolyte solution comprising CO2, and a working electrode comprising a transition metal/post transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof for electrocatalytic reduction of the CO2 to the oxygenated organic products, wherein the oxygenated organic products comprise
two or more carbon atoms and/or two or more oxygen atoms.
Feature 2. The system of Feature 1, wherein the TM/PTM binary alloy is of the formula
TMxPTMy, and x and y are integers independently selected from 1 to 10.
Feature 3. The system of Feature 1, wherein the TM is a first row transition metal.
Feature 4. The system of Feature 1, wherein the PTM is selected from Group IIB or
IIIA of the Periodic Table.
Feature 5. The system of Feature 1, wherein the electrolyte solution further comprises
CO.
Feature 6. The system of Feature 1, wherein the oxygenated organic products comprise
oxalate.
Feature 7. The system of Feature 1, wherein the TM is selected from Group VIIIB and
the PTM is selected from group IIIA of the Periodic Table.
Feature 8. The system of Feature 7, wherein the TM is nickel and the PTM is aluminum.
Feature 9. The system of Feature 1, wherein the TM is chromium and the PTM is selected
from Group IIIA of the Periodic Table.
Feature 10. The system of Feature 1, wherein transition metal oxide/post transition
metal oxide comprises Cr2O3-Ga2O3.
Feature 11. The system of Feature 1, wherein the electrolyte solution has an acidic
pH.
Feature 12. A system for providing oxygenated organic products comprising:
an electrochemical cell including an electrolyte solution comprising CO2, and an electrode comprising an alloy and/or mixture of metal oxides, the electrode
having an electrocatalytic site for reduction of CO2 to a CO, wherein the CO is incorporated into the oxygenated organic products.
Feature 13. The system of Feature 12, wherein the alloy comprises at least one of
a transition metal and post-transition metal.
Feature 14. The system of Feature 12, wherein the metal oxides comprise at least one
of a transition metal oxide and post-transition metal oxide.
Feature 15. The system of Feature 14, wherein the transition metal oxide comprises
a first row transition metal.
Feature 16. The system of Feature 14, wherein the post-transition metal oxide comprises
a metal selected from group IIB or IIIA of the Periodic Table.
Feature 17. The system of Feature 12, wherein the mixture of metal oxides comprise
chromium oxide and gallium oxide.
Feature 18. The system of Feature 12, wherein the oxygenated organic products comprise
oxalate.
Feature 19. The system of Feature 12, wherein the electrolyte solution has an acidic
pH.
Feature 20. The system of Feature 12, wherein the electrocatalytic site is anionic.
Feature 21. A system for providing organic products comprising:
an electrochemical cell including an electrolyte solution comprising CO2, and a working electrode comprising a transition metal/post-transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof for electrocatalytic reduction of the CO2 to the organic products.
Feature 22. The system of Feature 21, wherein the organic products comprise one carbon
atom, two carbon atoms, three carbon atoms or mixtures thereof.
Feature 23. The system of Feature 21, wherein the organic products are oxygenated.
Feature 24. The system of Feature 21, wherein the organic products are aliphatic.
Feature 25. The system of Feature 21, wherein the TM/PTM binary alloy is of the formula
TMxPTMy, and x and y are integers independently selected from 1 to 10.
Feature 26. The system of Feature 21, wherein the TM is a first row transition metal.
Feature 27. The system of Feature 26, wherein the PTM is selected from Group IIB or
IIIA of the Periodic Table.
Feature 28. The system of Feature 21, wherein transition metal oxide/post transition
metal oxide comprises Cr2O3-Ga2O3.
Feature 29. A method of forming oxygenated organic products comprising:
providing an electrochemical cell including an electrolyte solution comprising CO2, and a working electrode comprising a transition metal/post transition metal (TM/PTM)
binary alloy and/or oxide(s) thereof; and
electrocatalytically reducing the CO2 to the oxygenated organic products, wherein the oxygenated products comprise two
or more carbon atoms and/or two or more oxygen atoms.
Feature 30. The method of Feature 29, wherein the TM/PTM binary alloy is of the formula
TMxPTMy, and x and y are integers independently selected from 1 to 10.
Feature 31. The method of Feature 29, wherein the TM is a first row transition metal.
Feature 32. The method of Feature 29, wherein the PTM is selected from Group IIB or
IIIA of the Periodic Table.
Feature 33. The method of Feature 29, wherein the oxygenated organic products comprise
oxalate.
Feature 34. The method of Feature 29, wherein transition metal oxide/post transition
metal oxide comprises Cr2O3-Ga2O3.
Feature 35. A method of forming oxygenated organic products comprising:
providing an electrochemical cell including an electrolyte solution comprising CO2, and an electrode comprising an alloy and/or mixture of metal oxides; and
reducing CO2 to CO at an electrocatalytic site on the electrode; and
deriving the oxygenated organic products from the CO.
Feature 36. The method of Feature 35, wherein the oxygenated products comprise oxalate.
Feature 37. The method of Feature 35, wherein the metal oxides comprise at least one
of a transition metal oxide and post-transition metal oxide.
Feature 38. The method of Feature 37, wherein the transition metal oxide comprises
a first row transition metal.
Feature 39. The method of Feature 37, wherein the post-transition metal oxide comprises
a metal selected from group IIB or IIIA of the Periodic Table.
Feature 40. The method of Feature 35, wherein the mixture of metal oxides comprise
chromium oxide and gallium oxide.
Feature 41. The method of Feature 35, wherein the electrocatalytic site is anionic.
Feature 42. The method of Feature 35, wherein the electrolyte solution has acidic
pH.