A copper single-atom catalyst towards efficient and durable oxygen reduction for fuel cells

Article abstract of DOI:10.1039/c9ta03518d

Single-atom catalysts (SACs) of transition metals have attracted enormous attention as replacements of Pt-based catalysts towards the oxygen reduction reaction (ORR). Even though Cu is nature's choice for ORR catalysis in many forms of life from bacteria to humans; however, only a few studies of Cu-based SACs for the ORR have been reported. Furthermore, the applications of Cu SACs for fuel cells have rarely been demonstrated. In this work, we prepared a Cu SAC via a simple pyrolysis method using Cu phthalocyanine as the precursor. The catalyst shows a half-wave potential of 0.81 V (vs. RHE) in alkaline medium towards the ORR, which is 30 mV lower than that of commercial Pt/C (20 wt%). It also shows high durability with only 9 mV negative shift of half-wave potential after 5000 cycles in alkaline medium. An anion exchange membrane fuel cell based on the Cu SAC exhibits a peak power density of 196 mW cm^-2 at 70 °C. The density functional theory calculation suggests that the transformation process from OOH? to O? is the rate determining step over the Cu SAC. The Cu SAC could be a promising non-noble ORR catalyst for fuel cell applications.

Full text of DOI:10.1039/c9ta03518d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C9TA03518D
ARTICLE
A copper single-atom catalyst towards efficient and durable
oxygen reduction for fuel cells
Received 00th January 20xx,
Accepted 00th January 20xx
Liting Cui, Lirui Cui, Zhengjian Li, Jin Zhang, Haining Wang*, Shanfu Lu*, Yan Xiang*
DOI: 10.1039/x0xx00000x
Single-atom catalysts (SAC) of transition metals have attracted enormous attention as replacements of Pt-based catalysts
towards oxygen reduction reaction (ORR). Even Cu is the nature’s choice for ORR catalysis in many forms of life from
bacteria to human, however, only few studies of Cu-based SAC for ORR have been reported. Furthermore, the applications
of Cu SAC for fuel cells have rarely been demonstrated. In this work, we prepared a Cu SAC via a simple pyrolysis method
using Cu phthalocyanine as precursor. The catalyst shows a half-wave potential of 0.81 V (vs. RHE) in alkaline medium
towards ORR, which is 30 mV lower than the commercial Pt/C (20 wt%). It also shows high durability with only 9 mV
negative shift of half-wave potential after 5000 cycles in alkaline medium. An anion exchange membrane fuel cell based on
Cu SAC exhibits a peak power density of 196 mW cm-2 at 70 °C. The density functional theory calculation suggests the
process from OOH* to O* is the rate determining step over Cu SAC. The Cu SAC could be a promising non-noble ORR
catalyst for fuel cell applications.
for ORR have been reported.26-28 Furthermore, the applications
of Cu SAC for fuel cells have rarely been demonstrated. For
example, Bao group reported single Cu(I)-N sites in graphene
towards efficient ORR in zinc-air batteries.27 Most recently, Wu
and Li group reported the preparation of Cu-SAC/N-C using ZIF-
8 as a metal-organic framework, which exhibited better ORR
1. Introduction
Electro-catalysts towards oxygen reduction reaction (ORR) play
a vital role in achieving highly efficient polymer electrolyte
membrane fuel cells (PEMFCs), which is one of the most
promising devices to convert chemical energy to electrical
energy1-5. Platinum-based materials are the most widely used
catalysts for ORR in PEMFCs. However, Pt is one of the major
bottlenecks for the large-scale practical applications of PEMFCs
due to the low reserves and high cost. Therefore, the
development of non-Pt catalysts is urgent for the practical
applications of PEMFCs.4, 6-12
activity (half-wave potential of 0.895
V vs. RHE) than
commercial Pt/C28. ZIF-8 is a metal-organic framework with
flexible control of structure and chemistry with unique
hydrocarbon network to provide anchoring sites for metal
single atoms28-30. In addition to ZIF-8, phthalocyanine (Pc) is a
large conjugated compound with 18 π electrons, which result
in strong coordination ability to form stable complexes with
metal elements31, 32. For example, Jiang group reported the
catalyst prepared by metal phthalocyanine (MPc, M=Co, Ni)
In recent years, single-atom catalysts (SAC) have attracted
enormous attention due to their advantages such as 100%
13-16 with excellent activity for CO2 reduction reaction31.
atomic efficiency and unique size quantum effects.7,
Among them, the one with transition metal/nitrogen-doped
carbon-based structure (M-N-C) exhibits good ORR
performance, where the transition metals are usually Fe, Co or
Ni.17-21 However, Cu is the nature’s choice for the catalysis of
oxygen reduction to water in many forms of life from bacteria
to human.22-24 Recently, Lu and co-workers demonstrated that
the ORR activity of Cu attributes to the high redox potential
and d orbital electron density of Cu element, which results in
weaker O-O bonds.25 Despite the nature of Cu as a promising
oxygen reduction catalyst, only few studies of Cu-based SAC
Here, we prepared a Cu SAC using Cu phthalocyanine (CuPc) as
precursor and carbon nanotubes as carrier through simple
pyrolysis method. The morphology and electronic properties of
the catalyst were characterized by aberration-corrected high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) and operando X-ray absorption
spectroscopy (XAS). The prepared Cu SAC exhibited
comparable ORR activity to the commercial Pt/C (JM, 20 wt.%).
Then, an anion exchange membrane fuel cell was assembled
with Cu SAC as catalyst in cathode, which shows a peak power
density of 196 mW cm-2. In addition, the ORR pathway on Cu
SAC was evaluated by the density functional theory (DFT)
calculation. The current work suggests Cu SAC could be a
promising non-precious metal catalyst towards ORR for
PEMFCs application.
Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of
Space and Environment, Beihang University, Beijing, 100191, China. Emails:
hwang@buaa.edu.cn, lusf@buaa.edu.cn, xiangy@buaa.edu.cn.
Electronic Supplementary Information (ESI) available: XRD patterns of CuPc. XPS
spectra of Cu SAC. LSV curve of Cu SAC in acid medium. TOFs of Cu SAC and Pt/C.
See DOI: 10.1039/x0xx00000x
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solution (0.1 mol L-1) by linear sweep voltammetry V(iLeSwVA)rtiwcleitOhnlainne
2. Experimental section
2.1 Preparation of Cu SAC
RRDE at a scan rate of 10 mV s-1.
DOI: 10.1039/C9TA03518D
The electron transfer number n of the catalyst oxygen reduction
reaction and the hydrogen peroxide yield %H2O2 were calculated by
the following equation:
The Cu SAC was prepared via a simple pyrolysis method. First, the
carbon nanotubes (purity >95%, diameter 10-20 nm, length 10-30
μm, XFNano, Inc.) were treated with concentrated hydrochloric acid
to wash away any metal ions on the samples (200 mg carbon
nanotubes with 20 mL concentrated hydrochloric acid stirring for 6
h, suction-washed and dried in an oven at 70 °C). Then, 400 mg
copper phthalocyanine (Aladdin) and 200 mg treated carbon
nanotubes were ultrasonically dispersed in Dimethylformamide
(DMF) respectively, and the two dispersions were mixed after being
uniformly dispersed. After being stirred for 12 hours at room
temperature, it was suction-washed and dried in an oven at 70 °C to
remove DMF. Next, the obtained powder was pyrolyzed at 700 °C
for 4 hours under nitrogen atmosphere. Finally, the pyrolyzed
sample was stirred in H2SO4 solution (0.5 mol L-1) at 70 °C, followed
by suction filtration and drying to obtain Cu SAC samples.
n
4 × I / (I + I /N)
d d r
=
%H2O2 200 × (I /N) / (I + I /N)
=
r
d
r
where n is the overall number of electrons transferred in the
reaction; Id and Ir are the disk current and ring current, respectively;
N is the collection efficiency of the ring electrode (0.37); %H2O2 is
the hydrogen peroxide yield. Furthermore, the number of electron
transfer was calculated from LSV curves by Koutecky–Levich
equation as:
1/2
1
1
1
(
j = jL + jK = Bω
)
1
+
jK
2/3
B = 0.62 nFCo Do γ -1/6
(
)
2.2 Physical Characterization
where j is the measured current density, jK and jL are the kinetic and
diffusion limiting current densities, respectively; ω is the angular
velocity of the disk; n is the overall number of electrons transferred
in the reaction; F is the Faraday constant (F=96485 C mol−1); Co is
the bulk concentration of O2 (1.2×10−3 mol L−1); Do is the diffusion
coefficient of oxygen (1.9×10−5 cm2 s−1); g is the kinematic viscosity
(g=0.01 cm2 s−1) of the electrolyte.
In addition, we conducted accelerated durability testing. LSV was
first performed at a sweep rate of 10 mV s-1 and a stirring rate of
1600 rpm in 0.1 mol L-1 KOH solution and O2 atmosphere. Catalyst
aging was promoted by circulating the catalyst in 0.1 mol L-1 KOH
solution saturated with N2 at a scan rate of 100 mV s-1 from 0.6 V to
1.0 V for 5000 cycles. At the end of the cycle, a linear sweep
voltammetry test was again performed under O2 saturation.
The X-ray powder diffraction (XRD) patterns were determined by a
D8 ADVANCE with DAVINCI design (Cu K1, 40 kV and 200 mA) at
the rate of 0.1 step s−1 from 10 to 90°. The morphology of the Cu
SAC was obtained by an Aberration-corrected high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM
HF5000). X-ray photoelectron spectroscopy was performed using a
Thermo escalab 250Xi (American Thermoelectric) spectrometer
using monochromatic Al K1 radiation (1486.6 eV) with a power of
150 W. The binding energy of the C 1s peak at 284.8 eV was used as
a reference. The Cu content in Cu SACs was measured by ICP-OES
(Agilent ICPOES730, USA). The transmission power is 1.0 kW and
the carrier gas is argon. X-ray absorption spectroscopy (XAS) was
performed on the 1W1B beamline at BSRF (Beijing Synchrotron
Radiation Facility) at room temperature. Fluorescence mode Cu K
edge XAFS data for Cu was collected over the range of 8871-9778
eV, using a 100% Ar-filled Lytle ion chamber detector with Mn X-ray
filters and Soller slits, while absorption was collected in
transmission mode Reference is made to the spectrum of copper (II)
phthalocyanine (CuPc). The monochromator energy was calibrated
using a Cu foil. Reduction and analysis of all XAFS data were
performed using IFEFFIT.
2.4 Fuel cell assembly and test conditions
The H2-O2 fuel cell performance was tested by a Greenlight G20 test
system, and the electrochemical data was performed by a Princeton
P4000+ electrochemical workstation. The voltage scan range of the
polarization curve test is from the open circuit voltage (OCV) to 0.1
V, scan speed is 10 mV s-1. Cathode: Cu SAC with Cu loading of 2.0
mg cm-2; anode: Pt/C with Pt loading of 0.5 mg cm-2; temperature:
70 °C.
2.3 Electrochemical characterization
2.5 Theoretical calculation
All the electrochemical tests were carried out in a CHI760E
instrument (Shanghai Chen Hua Instrument Co. Ltd.). The
electrochemical measurements were performed in a traditional
three-electrode cell system with a graphite rod and reversible
hydrogen electrode (RHE, pH=13) as the counter electrode and the
reference electrode respectively. In addition, a rotating ring disk
electrode (RRDE) was deposited with catalysts as the working
electrode. The catalyst ink was made by mixing 5.0 mg homemade
catalyst and 1.0 mL 0.2 wt.% Nafion solution. The cyclic
voltammetry (CV) curves of the catalysts were measured in nitrogen
saturated 0.1 mol L-1 KOH solution or HCIO4 solution (0.1 mol L-1) at
a scan rate of 50 mV s-1. The ORR activity of the catalysts was
performed in oxygen saturated KOH solution (0.1 mol L-1) or HCIO4
The density functional theory (DFT) calculations were performed
with the Vienna Ab Initio Simulation Package (VASP)33-36. In the
calculation, we adopted a catalyst model with single metal atoms
and graphene fragment where four C atoms are replaced by N
atoms in a supercell of 12.3×12.3×15.0 Å for all catalyst models37.
The supercell was large enough to reduce the interaction between
metal atoms in adjacent cells. During the calculation, the projector-
augmented wave (PAW) formalism implementing the generalized
gradient approximation (GGA) as parameterized by Perdew et al.
(PBE)38 was used. The plan-wave kinetic energy cutoff was 400 eV,
and the k-points meshes were set as 4×4×1. All structures were
fully relaxed until the convergence in energy and force reached 1.0
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transformed by 0.3. (d)EXAFS fitting curve for Cu SACs. Wavelet transforms for the k3-
× 10-4 eV and 1.0 × 10-3 eV/Å respectively. The equation of
adsorption energy and G is obtained according to the reference37.
View Article Online
weighted EXAFS signals of (e) Cu foil and (f) Cu SAC, respectively.
DOI: 10.1039/C9TA03518D
(404.5 eV). Therefore, Cu+/Cu2+ bonding with N atoms to form CuN2
or CuN4 might be the structure of Cu SAC. The ICP-OES results
reveal the mass content of Cu in Cu SAC was only 0.2 wt.%.
To further understand the structure of Cu SAC, the Cu-K edge X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) of Cu SAC are obtained and
compared with those of standard Cu foils, as shown in Fig. 2b-d. The
absorption edge and the main transition energy of Cu SAC are
compared with those of Cu foils, indicating partial oxidation of Cu
atoms in Cu SAC. This agrees well with the XPS results of Cu+ or Cu2+
bonded to N atoms. The Fourier transform of the EXAFS analysis
showed the presence of Cu-N bond with a distance of 1.5 Å in Cu
SAC. The peak of Cu-Cu bond with a distance of 2.3 Å in Cu SAC is
very weak as shown in Fig. 2c, which indicates no Cu-Cu bond
exists.28 Fig. 2d shows the coordination number is consistent with
the fitting result of CuPc molecule, indicating that the CuN4
structure of Cu SAC. The wavelet transforms for the k3-weighted
EXAFS signals confirm the obvious Cu-Cu strength peak around 2.3
Å of Cu foil, while Cu SAC does not have such a strength peak, as
shown in Fig. 2e and 2f respectively. The results indicate that Cu
atoms are isolated from each other and bonded to N atoms in Cu
SAC, which further confirms the successful preparation of Cu single
atoms.
Fig. 1 (a) TEM and (b) aberration-corrected HAADF-STEM images of Cu SAC. (c)
Magnified HAADF-STEM image of Cu SAC. The Cu single atoms are highlighted by red
circles. The inset of (c) shows the line-scanning intensity profile from the area indicated
with black line in (c).
3. Results and discussion
3.1 Characterization of Cu SAC
The preparation details of SAC could be found in the experimental
section. From the TEM image in Fig. 1a, there is no obvious Cu
nanoparticles (NPs) on the surface of the carbon nanotubes.
Meanwhile, the aberration-corrected HAADF-STEM images reveal
the presence of Cu single atoms dispersed on the surface of carbon
nanotubes, as shown in Fig. 1b and 1c. The line-scanning intensity
profile indicates the isolated Cu atoms were separated of ~0.6 nm
(Fig. 1c inset).
Fig. 2a shows the X-ray diffraction (XRD) pattern of Cu SAC. The
broad diffraction peak around 26° could be assigned to graphitic
carbon. There is no diffraction peak of Cu crystals, which further
confirms there is no Cu NPs or CuPc (Fig. S1). The XPS spectrum of
Cu SAC is shown in Fig. S2. The peaks at binding energies of 932.5
and 952.5 eV are assigned to the Cu+ species while the shoulder
signals at 935.2 and 956.1 eV are assigned to the Cu2+ species. The
peak at 944.3 eV in the Cu2p region refers to the satellite peak of
Cu2+.39 For the N 1s spectrum (Fig. S3), the N 1s signal band could be
split into four kinds of nitrogen configurations, as pyridinic-N (398.2
eV), pyrrolic-N (399.1 eV), graphitic-N (401.6 eV), and oxidized N
3.2 Electrochemical Characterization
The cyclic voltammograms (CV) curves of the Cu SAC in oxygen or
nitrogen saturated 0.1 mol L-1 KOH solution are shown in Fig. 3a.
The results present the obvious reduction peak in oxygen saturated
solution, indicating the ORR activity of Cu SAC. The linear sweep
voltammograms (LSV) curve of Cu SAC was measured in oxygen
saturated 0.1 mol L-1 KOH solution at a scan rate of 10 mV s-1 and a
stirring rate of 1600 rpm. As shown in Fig. 3b, the ORR activity of Cu
SAC is comparable to the commercial Pt/C (JM, 20 wt.%). The half-
wave potentials of Cu SAC and Pt/C are 0.81 eV and 0.84 eV
respectively, and the onset potential are 0.97 V and 1.00 V (vs. RHE)
Fig. 3 (a) CV curve of Cu SAC in nitrogen or oxygen saturated 0.1 mol L-1 KOH solution at
a scan rate of 50 mV s-1. (b) LSV of Cu SAC in oxygen saturated 0.1 mol L-1 KOH solution
Fig. 2 (a) XRD pattern and (b) Cu Kꢀedge X-ray absorption near-edge structure (XANES)
spectra. (c) K3-weighted χ(k) function of EXAFS spectra. Note that the Cu foil data are
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at a scan rate of 10 mV s-1 and a stirring rate of 1600 rpm in comparison to that of
commercial Pt/C (JM, 20 wt.%) and bare RRED electrode. (c) Electron transfer number
of ORR in 0.1 mol L-1 KOH solution at a scan rate of 10 mV s-1 and a stirring rate of 1600
rpm. (d) H2O2 yield at a ring potential of 1.3 V vs. RHE.
View Article Online
DOI: 10.1039/C9TA03518D
Fig. 5 (a) ORR stability of Cu SAC in comparison with that of Pt/C over 1000 s. (b) ORR
stability of Cu SAC over 10,000 s. Test conditions: constant potential of 0.8 V (vs. RHE),
1600 rpm.
Fig. 4 (a) LSV curves of Cu SAC at different rotational speeds. (b) The corresponding
transferred electrons number (n) under different potentials. The inset shows the
corresponding Koutecky-Levich plots.
respectively. The turnover frequency (TOF) values of Cu SAC are
1.03 s-1 and 0.11 s-1 at potential of 0.8 V and 0.85 V respectively as
shown in Table S1, and the TOF values of commercial Pt/C are 0.46
s-1 and 0.24 s-1 respectively. In addition, the electron transfer
number of oxygen reduction reaction and the hydrogen peroxide
yield (%H2O2) were obtained from the ring and disk currents in
rotating ring-disk electrode (RRDE) tests. As shown in Fig. 3c, the
electron transfer number (n) of Cu SAC is about 3.99 from 0 V to 0.6
V (vs. RHE), which is slightly higher than that of commercial Pt/C.
The corresponding hydrogen peroxide yield is shown in Fig. 3d,
which shows Cu SAC achieved much lower amount of hydrogen
peroxide (about 0.078%) than commercial Pt/C at varied potentials.
Furthermore, LSV curves of Cu SAC at different rotational speeds
were shown in Fig. 4a. According to the LSV curves at different
speeds, the electron transfer number can be calculated using the
Koutecky–Levich equation (Fig. 4b), which indicates similar results
as Fig. 3c. The results reveal Cu SAC shows promising ORR activity in
alkaline medium, however, the ORR activity of Cu SAC needs to be
further improved under acidic conditions (Fig. S4).
Fig. 6 Accelerated durability testing (ADT) measurements of Cu SAC before and after
5000 cycles in 0.1 mol L-1 KOH solution.
unchanged), and there was no obvious difference of ORR activity
before and after the treatment (Fig. S5b). Therefore, it could be
speculated that Cu SAC is mainly in a four-coordinate structure27.
Combined with the previous results, it is proposed that the Cu+-N4
structure is crucial for ORR process.
In addition, the chronoamperometry test (Fig. 5a) shows that the
stability of Cu SAC is much better than that of commercial Pt/C. Cu
SAC was further tested for 10,000 s and the stability was still good
(Fig. 5b). In addition, we conducted accelerated durability testing to
further evaluate the ORR performance. As shown in Fig. 6, there
was almost no obvious change of the LSV curve (with only 9 mV
negative shift) before and after 5000 cycles, indicating the good
durability of Cu SAC. In order to better evaluate the feasibility of the
catalyst, we assembled an anion exchange membrane fuel cell with
Cu SAC as cathode catalysts.
In order to further distinguish the contributions of Cu(I) and Cu(II)
in the catalysts, Cu SAC was treated in 1.0 mol L-1 HNO3 solution at
3.3 Fuel cell performance
70 °C for 12 hours to obtain a sample with Cu+ converted to Cu2+ 27
.
Fig. 7 shows the polarization curve of an anion exchange membrane
fuel cell based on Cu SAC as cathode catalysts and Pt/C as anode
catalysts. The open circuit voltage of the cell is about 0.85 V and the
The LSV curves of Cu SAC before and after HNO3 treatment were
compared with that of CuPc molecules (Fig. S5a). With the
increased ratio of Cu2+, the ORR activity decreased, which indicates peak power density could reach 196 mW cm-2. For a comparison,
Cu+ is the main active sites for ORR. Furthermore, a small amount of
disodium salt of ethylenediaminetetraacetic acid (EDTA) was added
in electrolyte as ligands to complex Cu2+ (the pH value was basically
the fuel cell shows an open circuit voltage of 1.03 V and a peak
power density of 236 mW cm-2 when Pt/C is used for both anode
and cathode catalysts. The relatively low open circuit voltage might
attribute to the TOF value as shown in Table S1. A comparison of Cu
SAC with other Cu-based catalysts is shown in Table 1.
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Reference
Catalyst
Cu SAC
Eonset
0.97
0.97
0.89
0.92
0.89
E1/2
0.81
n
P
DOI: 10.1039/C9TA03518D
3.99
4.0
196
N/A
N/A
N/A
N/A
This work
Nat. Catal. 2018, 1,
Cu SAs/N-C
Cu-N@C-60
Cu-N-C
0.895
0.78
781.
Energ. Environ.Sci.
2016, 9, 3736.
3.96
3.97
3.87
Energy Environ. Sci.,
2018, 11, 2263
0.869
0.70
J. Phys. Chem. C 2018,
122 , 18468.
N-rGO/Cu3N
Fig. 7 Polarization curve of an anion exchange membrane fuel cell based on Cu SAC in
comparison to that of Pt/C. Anode: Pt/C with Pt loading of 0.5 mg cm-2; cathode: Cu
SAC with loading of 2.0 mg cm-2, or Pt/C with Pt loading of 0.5 mg cm-2; temperature:
70 °C; RH: 100%.
Eonset: Onset potential (V vs. RHE);
E1/2: Half-wave potential (V vs. RHE);
n: Electron transfer number of ORR;
P: Peak power density of H2-O2 fuel cell (mW cm-2).
Conclusions
In summary, Cu single-atom catalysts (Cu SAC) were prepared via a
simple pyrolysis method using Cu phthalocyanine as precursor. The
as-prepared Cu SAC exhibits comparable ORR activity and much
improved ORR stability in comparison to the commercial Pt/C in
alkaline medium. An anion exchange membrane fuel cell based on
Cu SAC exhibits a peak power density of 196 mW cm-2 at 70 °C. The
DFT calculation suggests the process from OOH* to O* is the rate
determining step of ORR on Cu SAC. The Cu SAC could be a
promising non-noble ORR catalyst for fuel cell applications.
Fig. 8 DFT calculation of ORR on Cu SAC. The adsorption model of (a) O, (b) OH and (c)
OOH on Cu SAC. Grey, blue, red, white and dark orange balls are C, N, O, H and Cu Conflicts of interest
atoms, respectively. (d) Free energy diagram at 0 V for oxygen reduction over Cu SAC
and Pt (111) surface respectively.
The authors declare no conflict of interest.
Acknowledgements
3.4 Theoretical calculation
The authors thank the financial support by grants from
National Natural Science Foundation of China (No. 21872003,
21722601), the Key Research and Development Program of
Beijing (Z171100000917011), and the Fundamental Research
Funds for the Central Universities. The authors would like to
thank Beijing Synchrotron Radiation Facility, Institute of High
Energy Physics, Chinese Academy of Sciences, Beijing, China
for the synchrotron beam time.
To better understand the oxygen reduction process over Cu SAC,
the density function theory (DFT) calculation was performed40. Fig.
8a-c shows the adsorption of O, OH, OOH on single Cu atom in CuN4
structure, respectively. The free energy diagram at potential of 0 V
(vs. RHE) for oxygen reduction over Cu SAC is shown in Fig. 8d. At
the potential of 0 V, all steps show downhill process. The maximum
free energy change (G) comes from the process from OOH* to O*
(-0.30 eV). As a comparison, the maximum G over Pt (111) surface
is -0.33 eV. According to Cheng’s work37, the negative value of
maximum G could be used to represent the onset potential of the
catalysts. As a result, Cu SAC shows a slightly lower onset potential
than that of Pt, which is consistent with the LSV curves as shown in
Fig. 3b.
Notes and references
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3
4
Table 1. Comparison of the ORR activity between Cu SAC and
other Cu-based catalysts in 0.1 moI L-1 KOH solution.
5
K. L. M. Gang Wu, Christina M. Johnston, Piotr Zelenay, Science,
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6 | J. Name., 2012, 00, 1-3
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