Electroreduction of CO2to Formate on a Copper-Based Electrocatalyst at High Pressures with High Energy Conversion Efficiency

Article abstract of DOI:10.1021/jacs.0c00122

Electrocatalytic CO2 reduction (CO2RR) to valuable fuels is a promising approach to mitigate energy and environmental problems, but controlling the reaction pathways and products remains challenging. Here a novel Cu2O nanoparticle film was synthesized by square-wave (SW) electrochemical redox cycling of high-purity Cu foils. The cathode afforded up to 98percent Faradaic efficiency for electroreduction of CO2 to nearly pure formate under ≥45 atm CO2 in bicarbonate catholytes. When this cathode was paired with a newly developed NiFe hydroxide carbonate anode in KOH/borate anolyte, the resulting two-electrode high-pressure electrolysis cell achieved high energy conversion efficiencies of up to 55.8percent stably for long-term formate production. While the high-pressure conditions drastically increased the solubility of CO2 to enhance CO2 reduction and suppress hydrogen evolution, the (111)-oriented Cu2O film was found to be important to afford nearly 100percent CO2 reduction to formate. The results have implications for CO2 reduction to a single liquid product with high energy conversion efficiency.

Full text of DOI:10.1021/jacs.0c00122

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Communication
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Electroreduction of CO to formate on copper based electrocatalyst
at high pressures with high energy conversion efficiency
Jiachen Li, Yun Kuang, Yongtao Meng, Xin Tian, Wei-Hsuan Hung, Xiao Zhang, Aowen Li, Mingquan Xu,
Wu Zhou, Ching-Shun Ku, Ching-Yu Chiang, Guanzhou Zhu, Jinyu Guo, Xiaoming Sun, and Hongjie Dai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00122 • Publication Date (Web): 06 Apr 2020
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Electroreduction of CO2 to formate on copper based electrocatalyst
at high pressures with high energy conversion efficiency
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Jiachen Li1,2†, Yun Kuang1,3†, Yongtao Meng1, Xin Tian1, Wei-Hsuan Hung1,4, Xiao Zhang1,
Aowen Li5, Mingquan Xu5, Wu Zhou5, Ching-Shun Ku6, Ching-Yu Chiang6, Guanzhou Zhu1,
Jinyu Guo1, Xiaoming Sun3, Hongjie Dai1*
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Affiliations:
1 Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
2 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305,
USA.
3 State Key laboratory of Chemical Resource Engineering and Beijing Advanced Innovation
Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology,
Beijing 100029, China.
4 Institute of Materials Science and Engineering, National Central University, Taoyuan 32001,
Taiwan.
5 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049,
China.
6 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan.
† These authors contributed equally to this work.
*Correspondence to: hdai@stanford.edu.
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Abstract
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Electrocatalytic CO2 reduction (CO2RR) to valuable fuels is a promising approach to
mitigate energy and environment problems, but controlling the reaction pathways and products
remains challenging. Here, a novel Cu2O nanoparticle film is synthesized by square-wave (SW)
electrochemical redox cycling of high purity Cu foils. The cathode afforded up to 98% Faradaic
efficiency (FE) for CO2 electroreduction to nearly pure formate under 45 atm of CO2 in
bicarbonate catholytes. When paired with a newly developed NiFe hydroxide-carbonate anode in
KOH/borate anolyte, a two-electrode high-pressure electrolysis cell achieved a high energy
conversion efficiency up to 55.8% stably for long-term formate production. While the high-
pressure condition drastically increased the solubility of CO2 to enhance CO2RR and suppress
hydrogen evolution, Cu2O (111) oriented film was found important to afford near ~100% CO2
reduction to formate. The results have implications to CO2 reduction into a single liquid product
with high energy conversion efficiency.
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Intense utilization of fossil fuels in recent decades leads to continuous increases of CO2
level in atmosphere, causing serious environmental threats1. CO2RR driven by renewable energies
is a promising approach to close carbon cycle and derive fuels from CO2. Formate is one of the
valuable fuels coming from CO2RR, useful in fuel cells for direct electricity generation 2, 3. Sn4-6,
Pb7, 8, and In9-11 based electrocatalysts are commonly used for CO2RR to formate. Cu-based
electrocatalysts are well-known for producing hydrocarbons and alcohols12, 13 accompanied by
hydrogen evolution reaction (HER), with formate of low FE14-21 and mixed in various liquid
products that require separation treatments12.
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In aqueous CO2RR systems, competition with HER limits the FE of CO2 conversion. An
effective way to improve the electrocatalytic performance is to increase the supplying of CO2 to
cathodes. CO2RR occurs through active solvated/hydrated CO2 (aq) and H2CO3 species22, 23 that
show increased concentrations in electrolytes under increasing CO2 partial pressures according to
Henry’s law24. Thus, conducting CO2RR at higher CO2 pressure is promising to increase the active
carbon species in electrolytes and improve the efficiency25-31. However, Cu-based electrocatalysts
typically produce diverse CO2RR products without high selectivity under high pressures25, 27, 31, 32
.
Also, an unmet challenge under high pressure is the lack of efficient oxygen evolution (OER) using
non-precious metal electrocatalyst to pair with CO2RR for high overall energy conversion
efficiency.
This work devised a SW electrochemical redox cycling treatment of ultrapure copper foil
to produce sub-micron thick films rich in (111) oriented Cu2O nanoparticles anchored on Cu
(denoted as SW-Cu2O/Cu) for CO2RR in a high-pressure electrolyzer under 1-60 atm CO2 partial
pressures. In KHCO3 electrolytes at pressure above 45 atm, CO2RR with SW-Cu2O/Cu cathode
dominated over HER, affording up to 98% FE for selective formate production. SW-Cu2O/Cu
maintained a high formate selectivity over long-term (~20 h) CO2RR. Up to 55.8% energy
efficiency was achieved for full-cell electrolysis when paired with NiFe hydroxide-carbonate OER
catalyst33 in newly devised KOH/borate anolytes capable of maintaining basic pH environments
under high CO2 pressures. The full electrolyzer led to energy efficient formate production of useful
quantities (~0.4 g/cm2) without the need of product separation.
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Fig. 1. Synthesis and characterization of SW-Cu2O/Cu. a, Alternating potentials of -0.45 and -
0.75 V vs. SCE were added to Cu foil during 30 min SW treatment, with images showing the color
changing of the foil. b, 1 mV/s CV scan of Cu foil in synthesis electrolyte. c, SEM image of 0.5h-
SW-Cu2O/Cu, showing rough and porous nanoparticle arrays of ~50-100 nm on the surface. d,
SEM image of cross-sectioned 0.5h-SW-Cu2O/Cu, showing the ~200-300 nm Cu2O layer on
surface. e, EDX of the region marked in c, evidencing the surface oxide layer. f, XRD of 0.5h-
SW-Cu2O/Cu, with pronounced Cu2O (111) signal. g, STEM image of a single nanoparticle on
0.5h-SW-Cu2O/Cu surface. h, Enlarged STEM image of the region marked in g with
corresponding fast Fourier transform (FFT) pattern, showing dominated Cu2O (111) orientation
with d spacing of 0.248 nm.
We synthesized SW-Cu2O/Cu films by SW treatment of 99.999% Cu foil in a concentrated
KOH solution (pH 13.7) at 40 oC with 2.3 M lactate ions, through applying alternating oxidation
and reduction potentials of -0.45 and -0.75 V vs. SCE reference electrode at 5 Hz over 0.5-10 h
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(Fig. 1a). In one cycle of potential alternation from -0.45 to -0.75 and back to -0.45 V over 0.2 s,
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Cu was oxidized to Cu(OH)2 , transformed to Cu2O and partially to CuO, then reduced back to Cu.
Cyclic voltammetry (CV) showed two oxidation peaks and one reduction peak (Fig. 1b),
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corresponding to the Cu(0)→Cu(OH)2 →Cu2O/CuO→Cu(0) cycle34. Through the cycling a net
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oxidation > reduction charge was observed (Fig. S1a), which combined with mild etching to
produce a rough Cu2O nanoparticles film in 0.5 h (Fig. 1c) with 200-300 nm thick (Fig. 1d) on Cu
foil surface (Fig. 1e), and rich in (111) oriented planes based on X-ray diffraction (XRD) (Fig. 1f,
S1d). Aberration corrected scanning transmission electron microscopy (STEM) image of single
particles on 0.5h-SW-Cu2O/Cu surface (Fig. 1g) and corresponding fast Fourier transform (FFT)
pattern (Fig. 1h) clearly resolved (111) oriented planes.
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Fig. 2. CO2RR under high CO2 pressures. a, Schematic of the home-designed high-pressure
CO2RR system. b, Calculation of active carbon species concentrations and pH in 0.5 M KHCO3
under different pressures at 25 C. c, Current densities of 0.5h-SW-Cu2O/Cu in 0.5 M KHCO3
during 1 h CO2RR at -0.84 V vs. RHE under 1-60 atm. The current density decreased with pressure
increasing from 1-45 atm, but remained unchanged from 45-60 atm. d, Product distributions of 1
h CO2RR under 1 atm at various potentials.
o
We performed three-electrode CO2RR in purified 0.5 M KHCO3 electrolytes saturated with
1-60 atm CO2 in a home-designed system (Fig. 2a) with Nafion membrane separating
cathodic/anodic zones to prevent crossover of liquid reduction products. According to Henry’s
law24 and dissolution equilibria of H2CO3, concentration of active CO2 (aq) and H2CO3 species in
the electrolyte increased linearly with increasing CO2 partial pressure (Fig. 2b, S2). Calculation
based on the concentration changes of these species showed a positive shift on the equilibrium
potential of CO2 being reduced to formate (details in SI). Under CO2 pressures of 1, 25, 45, and
60 atm respectively, linear sweep voltammetry (LSV) revealed a decreasing trend in onset
potentials under elevated CO2 pressures (Fig. S3), suggesting enhanced CO2RR.
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Chronoamperometry measurements with SW-Cu2O/Cu cathode showed a decreasing trend
of reduction current density under increasing CO2 partial pressures from 1-45 atm due to
suppression of HER. The effect leveled off under 45-60 atm (Fig. 2c). At 1 atm, cathodic reduction
was dominated by HER with FE of hydrogen formation >60%, and FE for CO2RR was lower than
30% (Fig. 2d). In strong contrast, electroreduction at 45 atm afforded a drastically suppressed HER
FE to ~2.9% at -0.64 V vs. RHE. Formate was the dominant product under all overpotentials (Fig.
3a), with an FE reaching 97.7% under a partial current density of 5.2 mA/cm2 at -0.64 V. At more
negative potentials than -1.04 V, formate FE decreased, with those of C2+ liquid products
increased (Fig. 3a). Noted that the formate FE on SW-Cu2O/Cu cathode was much higher than on
metallic Cu foil (~53%, Fig. S9b, S11) under a same CO2RR condition, indicating the importance
of Cu2O for formate production.
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Under 60 atm CO2 pressure (Fig. 3b) at -0.64 V, the measured FE of CO2RR to formate
was 98.2% with a partial current ~5.7 mA/cm2 while hydrogen was below the detection limit. At
a higher current density of 26.6 mA/cm2 (i.e. -1.04 V), formate FE was still ~81.2% with barely
any other liquid products generated. A maximum partial current density of 34.1 mA/cm2 for
formate was achieved at -1.24 V with ~55.0% FE (Fig. 3c). Systematic investigations revealed a
trend that HER FE decreased with increasing CO2 partial pressure at -0.64 V from ~64% at 1 atm
to ~0% at 60 atm, accompanied by formate FE increasing from ~27% to ~98% (Fig. 3d, S5).
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Fig. 3. CO2RR with 0.5h-SW-Cu2O/Cu cathodes under high pressures. Product distribution
afforded by 0.5h-SW-Cu2O/Cu after 1 h CO2RR at various potentials under a, 45 b, 60 atm. c,
Partial current density of formate produced by 1 h CO2RR in 0.5 M KHCO3, under 1, 45, and 60
atm respectively. d, Dependence of formate and hydrogen formation FE on CO2 partial pressure
at -0.64 V vs. RHE, showing formate increase and HER decrease with pressure increasing to 45
atm.
Previous density functional theory (DFT) simulations35, 36 suggested that the production of
formate by CO2RR is favored over other products like CO on Cu2O (111) surface (details in SI).
In a control experiment we performed SW treatment of Cu foil in a pH = 8 electrolyte and obtained
a Cu2O nanoparticles film exhibiting more (100) oriented planes relative to (111) (Fig. S6b). The
resulting cathode afforded ~87% FE of formate under 45 atm at -0.64 V over 30 min CO2RR (Fig.
S6c), obviously lower than ~98% by SW-Cu2O/Cu enriched with (111). We also synthesized
electrodeposited Cu2O/Cu with (111) and (100) orientations37, as well as directional growth Cu2O
nanoparticle/C electrodes with (111) and (100) facets38 to further glean the importance of Cu2O
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(111) surface to formate selectivity. At -0.64 V under 45 atm, both of the comparison showed
higher formate FE with (111) dominated catalysts (Fig. S10).
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We observed that SW-Cu2O/Cu cathode exhibited slow phase changes through CO2RR
(Fig. S4, S7). When extending CO2RR time to 20 h, the remaining Cu2O phase on surface
decreased with formate FE decreasing accordingly. Nevertheless, reduction of SW-Cu2O/Cu was
slow, with formate FE remaining as high as ~84% when only ~20% Cu2O was left after 20 h.
During especially long-term CO2RR, the active electrocatalyst was a mixed phase of partially
reduced Cu oxides, likely involving the interface of Cu2O and reduced Cu (Fig. S8, S11). We
varied the SW treatment time from 0.5-10 h and found 5h-SW-Cu2O/Cu showing the highest Cu2O
amount, with maximum ~84% formate FE (partial current density ~27 mA/cm2).
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We found the purity of copper is also an important factor. 0.5h-SW-Cu2O/Cu synthesized
using 99.98% Cu foil afforded 67.9% formate FE at -0.64 V under 45 atm, much lower than
~97.7% afforded by cathodes using 99.999% Cu foil at the same condition (Fig. S9c). Among the
impurities revealed by ICP-MS, Co was most likely causing higher HER FE and lower formate
FE.
SW-Cu2O/Cu operating at high CO2 pressures showed higher formate selectivity than
previous Cu-based cathodes (Table S1), and compare favorably to known electrocatalysts for
selective CO2RR to formate including In, Pb and Sn in terms of electrode area, partial current
density or reaction duration/stability (Fig. S12-15, Table S2). We conclude that our Cu-based
electrocatalysts can afford lower overpotentials for selective high-pressure CO2RR to formate, in
addition to advantages of higher abundance and lower cost.
High-pressure full-cell CO2RR with high energy conversion efficiency has been
challenging and rarely tackled previously. We devised 2 M KOH + 0.8 M H3BO3 as a new anolyte
for OER under high-pressure CO2 environments, with borate ions being unique in maintaining
neutral or slightly basic anolyte pH. We monitored electrolyte pH by calibrating the shift of
Ni(III)/Ni(II) redox peaks using Nernst equation (Fig. 4a) and found that 2 M KOH + 0.8 M H3BO3
anolyte pH was about neutral after equilibrating under high CO2 pressures (Fig. 4b), while 2 M
KHCO3 catholyte was acidic (Fig. S16). We employed a non-precious metal based OER catalyst
that were highly active and stable in neutral and slightly basic electrolytes, nickel-iron hydroxide
carbonate (NiFe-HC)33 as the anode to pair with 5h-SW-Cu2O/Cu cathode for full-cell CO2RR-
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OER electrolysis (Fig. 4c). Three-electrode CO2RR under 60 atm at -1.04 V vs. RHE using 5h-
SW-Cu2O/Cu cathode over 20 h showed a stable formate production with FE of ~80% and partial
current density of ~26 mA/cm2 (Fig. 4d). Subsequently, two-electrode electrolysis cell with
separated anodic/cathodic zones was tested at 60 atm under constant current densities of 6 mA/cm2
and 30 mA/cm2 for 20 h respectively (Fig. S17-18). During the long-term 6 mA/cm2 electrolysis,
the full cell voltage stabilized at ~2.37 V with 92.7% formate FE (Fig. S17) and 55.8% energy
efficiency. For 30 mA/cm2 electrolysis, stable ~2.85 V was recorded with ~79.9% formate FE
(Fig. 4e) and 40.0% energy efficiency. These performances compared favorably over previous
reports (Table S3)39-43. We obtained a formate yield of ~20.6 mg/h/cm2 (0.412 g/cm2 in 20 h),
producing a useful amount of formate with >99.9% purity (molar ratio) among liquid products in
electrolytes. The novel two-electrode high-pressure electrolyzer opens up a new opportunity to
derive valuable liquid fuels from CO2.
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Fig. 4. A stable, energy efficient two-electrode CO2RR-OER full electrolyzer under high CO2
pressures. a, CV curve of NiFe hydroxide carbonate in 2 M KOH + 0.8 M H3BO3 under 60 atm
CO2, with 5 mV/s scan rate. b, Anolyte pH under different equilibrium times at high pressures. c,
Schematic of two-electrode high-pressure full cell. d, 20 h CO2RR using 5h-SW-Cu2O/Cu at -1.04
V under 60 atm, showing the long-term sustainability of formate production with ~80% FE and
~27 mA/cm2 current density. e, 20 h CO2RR-OER full-cell test using 5h-SW-Cu2O/Cu cathode
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and NiFe-HC anode at 30 mA/cm2 under 60 atm, showing the long-term sustainability of formate
production with ~80% FE and ~24 mA/cm2 current density.
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Supplementary Information
The Supporting Information is available free of charge on the ACS Publications website.
Experimental procedures and supporting figures with additional data.
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Acknowledgments
This work was supported by the Deng family gift fund and the Stanford Bits and Watts Program.
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