Core-Shell Nanocomposites Based on Gold Nanoparticle@Zinc-Iron-Embedded Porous Carbons Derived from Metal-Organic Frameworks as Efficient Dual Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions

Article abstract of DOI:10.1021/acscatal.5b02302

Core-shell nanocomposites based on Au nanoparticle@zinc-iron-embedded porous carbons (Au@Zn-Fe-C) derived from metal-organic frameworks were prepared as bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). A single Au nanoparticle of 50-100 nm in diameter was encapsulated within a porous carbon shell embedded with Zn-Fe compounds. The resulting Au@Zn-Fe-C hybrids exhibited apparent catalytic activity for ORR in 0.1 M KOH (with an onset potential of +0.94 V vs RHE, excellent stability and methanol tolerance) and for HER as well, which was evidenced by a low onset potential of -0.08 V vs RHE and a stable current density of 10 mA cm^-2 at only -0.123 V vs RHE in 0.5 M H₂SO₄. The encapsulated Au nanoparticles played an important role in determining the electrocatalytic activity for ORR and HER by promoting electron transfer to the zinc-iron-embedded porous carbon layer, and the electrocatalytic activity was found to vary with both the loading of the gold nanoparticle cores and the thickness of the metal-carbon shells. The experimental results suggested that metal-embedded porous carbons derived from metal-organic frameworks might be viable alternative catalysts for both ORR and HER.

Full text of DOI:10.1021/acscatal.5b02302

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Article
Core-shell Nanocomposites Based on Gold Nanoparticle@Zinc-
Iron-Embedded Porous Carbons Derived from Metal-
Organic Frameworks as Efficient Dual Catalysts for
Oxygen Reduction and Hydrogen Evolution Reactions
Jia Lu, Weijia Zhou, Likai Wang, Jin Jia, Yunting Ke, Linjing Yang,
Kai Zhou, Xiaojun Liu, Zhenghua Tang, Ligui Li, and Shaowei Chen
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02302 • Publication Date (Web): 04 Jan 2016
Downloaded from http://pubs.acs.org on January 11, 2016
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CoreꢀShell Nanocomposites Based on Gold
Nanoparticle@ZincꢀIronꢀEmbedded Porous Carbons
Derived from MetalꢀOrganic Frameworks as
Efficient Dual Catalysts for Oxygen Reduction and
Hydrogen Evolution Reactions
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Jia Lua, Weijia Zhoua*, Likai Wanga, Jin Jiaa, Yunting Kea, Linjing Yanga, Kai Zhoua, Xiaojun
Liua, Zhenghua Tanga, Ligui Lia, Shaowei Chena,b*
a New Energy Research Institute, School of Environment and Energy, South China University of
Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006,
China
b Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa
Cruz, California 95064, United States
ABSTRACT: Coreꢀshell nanocomposites based on Au nanoparticle@zincꢀironꢀembedded
porous carbons (Au@ZnꢀFeꢀC) derived from metalꢀorganic frameworks were prepared as
bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution
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reaction (HER). A single Au nanoparticle of 50ꢀ100 nm in diameter was encapsulated within a
porous carbon shell embedded with ZnꢀFe compounds. The resulting Au@ZnꢀFeꢀC hybrids
exhibited apparent catalytic activity for ORR in 0.1 M KOH (with an onset potential of +0.94 V
vs. RHE, excellent stability and methanol tolerance) and for HER as well which was evidenced
by a low onset potential of −0.08 V vs. RHE and a stable current density of 10 mA cm−2 at only
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−0.123 V vs. RHE in 0.5 M H2SO4. The encapsulated Au nanoparticles played an important role
in determining the electrocatalytic activity for ORR and HER by promoting electron transfer to
the zincꢀironꢀembedded porous carbon layer, and the electrocatalytic activity was found to vary
with both the loading of the gold nanoparticle cores and the thickness of the metalꢀcarbon shells.
The experimental results suggested that metalꢀembedded porous carbons derived from
metalꢀorganic frameworks might be viable alternative catalysts for both ORR and HER.
KEYWORDS: coreꢀshell structure · metalꢀorganic frameworks · oxygen reduction reaction ·
hydrogen evolution reaction · electron transfer
Introduction
Development of renewable and green energy sources has been attracting extensive research
interest lately.1 Of this, ORR has been found to play key roles in fuel cells and zincꢀair batteries,
while HER is important in producing clean and sustainable energy. Hence, the development of
highly efficient electrocatalysts for both ORR and HER is crucial. Until now, platinumꢀbased
materials have been used extensively as the electrocatalysts of choice due to their excellent
catalytic activity toward ORR2 and HER3ꢀ4. However, the low abundance and high price of
platinum greatly limit their commercial applications.5ꢀ6 Thus, developing costꢀeffective
electrocatalysts with extraordinary ORR and HER activity is urgently needed. In fact, extensive
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research efforts have been devoted to the development of alternative materials of low costs and
rich earth abundance that may eventually replace platinumꢀbased catalysts. Towards this end, a
variety of materials, including carbon nanostructures,5, 7ꢀ14 transition metals6, 15ꢀ18 and their
oxides,19ꢀ21 carbides,22 and disulfides23ꢀ25 have been prepared and examined as electrocatalysts
for ORR or HER. Nevertheless, it remains a key challenge to synthesize lowꢀcost, highly active
and stable catalysts for both ORR and HER.
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Metalꢀorganic frameworks (MOFs), materials made of metal centers and functional organic
ligands linked by coordination chemistry,26 have been widely used in various fields, including
gas storage/separation,27ꢀ29 catalysis,30ꢀ32 sensing,33ꢀ34 and drug delivery.35ꢀ36 MOFs have also
been used as precursors and templates for the preparation of metal nanoparticles encapsulated in
porous carbons with controlled morphologies, high surface area and uniform porosity.37ꢀ41 For
instance, Hong et al.40 synthesized nitrogenꢀdoped porous carbon nanopolyhedra by simple
carbonization of a zeolitic imidazolate framework (ZIF), which showed a remarkable
performance for ORR, with an onset potential of −0.017 V vs. Ag/AgCl, excellent methanol
tolerance and robust stability. In another study, Wu et al.42 prepared porous molybdenum carbide
nanooctahedra by calcination of a mixture of Cuꢀbased MOFs and Moꢀbased precursors, which
exhibited efficient electrocatalytic activity for hydrogen production in both acidic and alkaline
solutions, with an overpotential of −0.142 V in 0.5 M H2SO4 and −0.151 V in 1 M KOH at a
cathodic current density of 10 mA cm−2. In these electrocatalysts, the significant improvement of
the catalytic activity was attributed to the porous structure and exposure of active sites after
calcination of the MOFs.
Indeed, metalꢀcarbon composites have been widely employed as efficient electrocatalysts in
alkaline solutions for ORR. However, for HER in acid solutions, the leach of metals from the
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catalyst surface significantly decreases the electrocatalytic activity and stability. One effective
strategy that will mitigate such a problem is to encapsulate metal nanoparticles into carbon
nanotubes or carbon shells so as to prevent the corrosion of the metal nanoparticles in acidic
media, and more importantly, to promote catalytic reactions on the carbon surface by electron
transfer from the encapsulated metal nanoparticles.6, 43ꢀ44 Such coreꢀshell structures can be
readily prepared by calcination of MOFs at controlled temperatures, where MOFs are used as the
metal and carbon sources. However, only a few studies have been reported so far in the literature
where metalꢀembedded porous carbons derived from MOFs are used as HER catalysts in acid
media.1, 42 In these studies, it has been argued that the formation of an coreꢀshell structure is
likely an effective strategy to produce hybrid catalysts with a remarkable catalytic activity, as
compared to singleꢀcomponent counterparts, because of the synergistic effects between the core
and shell where the inside core may have an impact on the electron density of the outside
shell.45ꢀ46 For ORR, the coreꢀshell structure may improve the performance and concurrently
increase methanol tolerance and stability of the catalysts;2, 47ꢀ48 and for HER, the coreꢀshell
structure is anticipated to not only enhance the corrosionꢀresistance of the metals in acidic
electrolytes but also improve the HER activity via the synergistic contributions from both the
core and shell.49 Indeed, a variety of coreꢀshell structures have been prepared and used for both
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ORR and HER catalysis, such as Au@Co3O4 and Co@Nꢀdoped carbon43. Note that in these
studies, the core metal nanoparticles alone exhibit only minimal catalytic activity; yet the
formation of a coreꢀshell hybrid structure markedly enhances the catalytic performance of the
outer shells. This is the primary motivation of the present study where gold nanoparticles are
used as the cores that are coated with a porous carbon shell.
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Herein, we prepared a multilevel coreꢀshell structure of Au nanoparticle@zincꢀironꢀembedded
porous carbons (Au@ZnꢀFeꢀC) and used it as a bifunctional electrocatalyst for both ORR and
HER. Experimentally, a single Au nanoparticle was wrapped by a porous carbon shell embedded
with ZnꢀFe compounds by using ZnꢀFeꢀbased MOFs as the metal and carbon sources. The
structures of the coreꢀshell Au@ZnꢀFeꢀC hybrids were characterized by SEM, TEM, XPS, and
XRD measurements. Electrochemical studies showed that Au@ZnꢀFeꢀC exhibited an apparent
electrocatalytic activity for both ORR in 0.1 M KOH and HER in 0.5 M H2SO4.
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Experimental Section
Materials. All reagents were of analytical grade and used without further purification.
Tetrachloroauric acid (HAuCl4), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), iron(III)
acetylacetonate (Fe(acac)3), 1,4ꢀbenzenedicarboxylic acid (H2BDC), poly(vinylpyrrolidone)
(PVP, M.W. 30,000), N,Nꢀdimethylformamide (DMF), ethanol, 10 wt% Pt/C, and 20 wt% Pt/C
were purchased from Sinopharm Chemical Reagents Beijing Co. Deionized water was supplied
with a Barnstead Nanopure Water System (18.3 Mꢁ •cm).
Synthesis of Fe, Zn-embedded porous carbon nanocomposites. In a typical reaction, 60 mg of
Fe(acac)3, 46.4 mg of Zn(NO3)2·6H2O, and 9.6 mg of H2BDC were dissolved in 25.6 mL of a
DMFꢀethanol (v:v = 5:3) mixed solution under magnetic stirring for 30 min. The resulting
solution was transferred to a 40 mL Teflonꢀlined stainlessꢀsteel autoclave and then heated at
100 °C for 6 h in an oven before it was cooled to room temperature. The products were separated
via centrifugation at 11,000 rpm for 15 min and further purified with DMF and ethanol for
several times. Subsequently, the products were dried at 80 °C for 24 h. The dried sample were
placed in a tube furnace and heated at 600 °C for 2 h under an Ar atmosphere at a heating rate of
2 °C min−1 (note that this temperature was determined by TGA measurements, as at higher
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temperatures the samples remained thermally stable, Figure S1). The obtained powders were
denoted as ZnꢀFeꢀC. Porous carbons embedded with Zn only (denoted as ZnꢀC) was prepared by
using the same procedure but without the addition of Fe(acac)3.
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Synthesis of Au Nanoparticles. 0.7284 g of PVP was dissolved in 8 mL of a DMFꢀethanol (v:v
= 5:3) mixed solution in a 15 mL autoclave. 100 ꢂL of a HAuCl4 solution that was preꢀprepared
by dissolving 1 g of HAuCl4 in 40 mL of pure water was then added into the autoclave. The
autoclave was transferred to an oven and heated at 140 °C for 3 h. After cooling to room
temperature, the products were centrifuged and washed with ethanol for several times before
being reꢀdispersed in ethanol.
Synthesis of Au@Zn-Fe-C and Au@Zn-C core-shell hybrids. To prepare Au@ZnꢀFeꢀC, in a
typical reaction, 0.2 g of PVP were dissolved in 23.6 mL of a DMFꢀethanol (v:v = 5:3) mixed
solution, into which was injected 1 mL of the Au nanoparticle solution obtained above under
magnetic stirring for 30 min, where the final concentration of the Au nanoparticles was
controlled at 0.17 M. Then, 60 mg of Fe(acac)3, 46.4 mg of Zn(NO3)2·6H2O, and 9.6 mg of
H2BDC were added into the above solution under magnetic stirring for another 30 min
(corresponding to a Au:Fe molar ratio of 1:1). The resulting solution was transferred to a 40 mL
Teflonꢀlined stainlessꢀsteel autoclave. The sealed vessel was then heated at 100 °C for 6 h before
being cooled down to room temperature. The products were separated via centrifugation at
11,000 rpm for 15 min and further purified with DMF and ethanol for several times.
Subsequently, the product was dried at 80 °C for 24 h. The dried samples were placed in a tube
furnace and heated at 600 °C for 2 h under an Ar atmosphere at a heating rate of 2 °C min−1. The
obtained powders were denoted as Au@ZnꢀFeꢀC. Au@ZnꢀC was prepared in a similar manner
but by using ZnꢀC instead of ZnꢀFeꢀC.
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For comparison, composites were also synthesized by the same procedure but with the
addition of a different amount of Au nanoparticles, which were denoted as Au@ZnꢀFeꢀCꢀL,
Au@ZnꢀFeꢀCꢀH, in which the molar ratio of the Au and Fe precursors added in the synthesis
process was 1:2 and 2:1, respectively.
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Characterization. Fieldꢀemission scanning electron microscopic (FESEM, NOVA NANOSEM
430, FEI) measurements were employed to characterize the morphologies of the asꢀprepared
samples. Transmission electron microscopic (TEM) measurements were carried out with a
Tecnai G220 FEI microscope. Powder Xꢀray diffraction (XRD) patterns of the samples were
recorded on a Bruker D8 Advance powder Xꢀray diffractometer with Cu Kα (λ = 0.15406 nm)
radiation. Xꢀray photoelectron spectroscopic (XPS) measurements were performed using a PHI
Xꢀtool instrument. Raman spectra were acquired on a RENISHAW invia instrument with an Ar
laser source of 488 nm in a macroscopic configuration. Nitrogen adsorptionꢀdesorption analysis
was conducted with an ASAP 2020 instrument to evaluate the specific surface areas of the
samples. TGA analysis was carried out with a TGA/DSC 1 (METTLER TOLEDO) instrument.
Electrochemistry. Electrochemical measurements of ORR activity were performed with a CHI
750E electrochemical workstation (CH Instruments Inc.) in a 0.1 M KOH aqueous solution. A
rotating ringꢀdisk electrode (RRDE) with a glassy carbon disk and gold ring was used as the
working electrode. A Ag/AgCl electrode (3 M KCl) and a platinum foil were used as the
reference and counter electrode, respectively. 4 mg of the catalyst powders was dispersed in 1
mL of 1:4 (v:v) ethanol/water mixed solvents along with 80 ꢂL of a Nafion solution (5% Nafion
in ethanol), and the mixture was sonicated for 30 min. Then, 10 ꢂL of the above solution was
dropcast onto the surface of the glassy carbon disk (GCE) at the catalyst loading of 0.204 mg
cm−2 and dried at room temperature. The polarization curves were acquired at 10 mV s−1 from 0
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to −1.0 V (vs. Ag/AgCl) at different electrode rotation rates (100 to 2500 rpm) in O2ꢀsaturated
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0.1 M KOH. CV tests were performed at 50 mV s−1 from 0 to −1.0 V (vs. Ag/AgCl) in N2 and
O2ꢀsaturated 0.1 M KOH. The numbers of electron transfer were calculated from RRDE data
using the equation, n = 4ID/(ID + IR/N), where ID and IR are the disk current and the ring current,
respectively, and N is the current collection efficiency (0.37) of the Au ring.
Chronoamperometric measurements were performed at +0.47 V vs. RHE for 30,000 s in
O2ꢀsaturated 0.1 M KOH.
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HER measurements were also performed with the CHI 760C electrochemical workstation but
in a 0.5 M H2SO4 aqueous solution. A Hg/Hg2Cl2 electrode and a carbon rod were used as the
reference and counter electrode, respectively. 4 mg of the catalyst powders was dispersed in 1
mL of 4:1 (v:v) water/ethanol mixed solvents along with 80 ꢂL of a Nafion solution (5% Nafion
in ethanol), and the mixture was sonicated for 30 min. Then, 5 ꢂL of the above solution was
dropꢀcast onto the surface of the glassy carbon disk (or carbon cloth) at a catalyst loading of
0.285 mg cm−2. The asꢀprepared catalyst film was dried at room temperature. Polarization curves
were acquired by sweeping electrode potentials from 0 to −0.8 V (vs. Hg/Hg2Cl2) at a potential
sweep rate of 5 mV s−1. Accelerated stability tests were performed at room temperature by
potential cycling between 0 and −0.5 V (vs. Hg/Hg2Cl2) at a sweep rate of 100 mV s−1 for 1,000
cycles. Currentꢀtime responses were monitored by chronoamperometric measurements at −0.15
V vs. RHE for 12 h.
Results and Discussion
The fabrication procedure of coreꢀshell Au@ZnꢀFeꢀC hydrids was schematically depicted in
Figure 1a. The synthetic process mainly included three steps: (i) Au nanoparticles were
synthesized by chemical reduction of HAuCl4; (ii) a ZnꢀFeꢀMOF shell was grown on the
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nanoparticle surface, and (iii) the ZnꢀFeꢀMOFs shell was converted to porous carbons embedded
with zincꢀiron compounds (ZnꢀFeꢀC) through a simple calcination process. Experimentally, Au
nanoparticles were prepared by reducing HAuCl4 at 140 °C with DMF both as a solvent and as a
reducing reagent in the presence of PVP which stabilized the Au nanoparticles, a procedure
described previously by Tang et al.51 Then, a ZnꢀFeꢀMOF shell was grown on the Au
nanoparticle surface by using Fe(acac)3 as the iron source, Zn(NO3)2 as the zinc source (and an
activating reagent for pore formation), H2BDC as organic linkers, and a DMFꢀethanol mixed
solution as the solvent. PVP was also added in the above mixture as a stabilizing reagent for the
formation of a uniform ZnꢀFeꢀMOF shell. Finally, the ZnꢀFeꢀMOF shell was converted to porous
carbons embedded with zincꢀiron compounds by a simple calcination process in an Ar
atmosphere, forming coreꢀshell Au@ZnꢀFeꢀC hybrids.
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Figure 1. (a) Schematic illustration of the synthetic process of Au@Zn-Fe-C core-shell nanostructures. (b,
c) Representative SEM images of Au@Zn-Fe-C, (d) XRD patterns and (e) Raman spectra of Zn-Fe-C and
Au@Zn-Fe-C hybrids.
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SEM measurements were then carried out to confirm the successful preparation of
Au@ZnꢀFeꢀC coreꢀshell nanostructures. From Figure 1b and c, we can see that Au@ZnꢀFeꢀC
inherited the octahedral structure of ZnꢀFeꢀMOFs (Figure S2),52 although with a somewhat
roughened surface, and the sample indeed exhibited a wellꢀdefined coreꢀshell structure and a
rather uniform size and shape. The crystal structures of Au@ZnꢀFeꢀC were further investigated
by XRD measurements. From Figure 1d, it can be seen that a series of diffraction peaks emerged
at 2θ = 38.2° for Au(111), 44.6° for Au(200), 64.6° for Au(220), and 77.8° for Au(311) (JCPDS
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card no. 001ꢀ1172); the peaks at 2θ = 35.6°, 43.1°, 56.9°, 62.7° are consistent with the (222),
(400), (110), and (103) planes of ZnFe2O4 (JCPDS card no. 077ꢀ0011); and those at 2θ = 32.0°,
34.7°, and 36.5° arose from the (100), (002), and (101) diffractions of ZnO (JCPDS card no.
075ꢀ0576). Raman measurements of ZnꢀFeꢀC and Au@ZnꢀFeꢀC (Figure 1e) further confirm the
structural composition, where the characteristic D and G bands of graphitic carbons can be seen
at 1340 cm−1 and 1589 cm−1 in both ZnꢀFeꢀC and Au@ZnꢀFeꢀC, which arose from the disordered
carbon atoms and sp2 hybridized graphitic carbon atoms, respectively.53 With the encapsulation
of Au nanoparticles, the intensities of the D and G bands in Au@ZnꢀFeꢀC became much stronger
than those of ZnꢀFeꢀC (about 10 times), likely due to the surface enhanced Raman scattering
(SERS) effects of Au nanoparticles in the core;54ꢀ55 and the ratio of the D and G band intensities
(ID/IG) decreased slightly from 1.05 to 1.02, implying an increasing degree of graphitization of
the carbon, though with abundant defects in both samples. The small variation of ID/IG also
indicates that Fe and Zn, not the Au cores, played a dominant role in determining the
graphitization (and defects) of the carbon shells.56
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Figure 2. (a, b) Representative TEM, (c) HRTEM images, (d) the corresponding SAED patterns, and (e)
EDX maps of Au@Zn-Fe-C hybrids.
TEM measurements were then performed to further characterize the morphology and
structure of the Au@ZnꢀFeꢀC hybrids. Figure 2a shows a typical TEM image of Au@ZnꢀFeꢀC,
which exhibited an average diameter of 214.5 nm (Figure S3). At higher magnification (Figure
2b), one can clearly see a darkꢀcontrast object of about 50ꢀ100 nm located at the center that was
most likely a Au nanoparticle; and the gold nanoparticle was wrapped with a uniform,
lightꢀcontrast ZnꢀFeꢀC shell of 30ꢀ60 nm. Note that only shortꢀrange crystalline lattices can be
observed in the ZnꢀFeꢀC shell, whereas for the Au nanoparticle, one can see very wellꢀdefined
lattice fringes with an interplanar spacing of 0.235 nm that is in good agreement with the spacing
of the (111) planes of fcc Au (Figure 2c). Consistent behaviors were observed in selected area
electron diffraction (SAED) measurements (Figure 2d), where the patterns suggested
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polycrystalline zincꢀiron compounds, disordered carbon shells, and singleꢀcrystalline Au cores,
as manifested by the scattered dots (Au) and diffraction rings (ZnꢀFeꢀC). EDX mapping of a
selected Au@ZnꢀFeꢀC shows that the Au core was indeed surrounded by a uniform
ZnꢀFeꢀembedded porous carbon shell (Figure 2e). The BET surface area of Au@ZnꢀFeꢀC was
then quantified by nitrogen adsorptionꢀdesorption measurements, which was 8.7 m2 g−1, smaller
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than that (19.4 m2 g−1) of the ZnꢀFeꢀC counterparts, due to the high mass density of the Au
nanoparticles (Figure S4).
Figure 3. (a) XPS survey spectrum and high-resolution scans of (b) C1s, (c) Fe2p, and (d) Au4f electrons
of the Au@Zn-Fe-C hybrids. The spectrum of the Au4f electrons of pure Au nanoparticles is also included
in panel (d). Black curves are experimental data and colored curves are deconvolution fits.
XPS measurements were then carried out to investigate the chemical composition as well as
elemental valence states. The survey spectrum in Figure 3a confirmed the presence of C, Zn, Fe,
and Au elements in Au@ZnꢀFeꢀC. From the highꢀresolution scan in Figure 3b, the C1s spectrum
may be deconvoluted into three peaks, sp2ꢀhybridized graphiteꢀlike carbon (C=C) at 284.6 eV,
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sp3ꢀhybridized diamondꢀlike carbon (C−C) at 285.5 eV, and C−O−C bond at 288.5 eV.40 For
Fe2p electrons (Figure 3c), two distinct peaks can be identified at 711.3 eV and 724.5 eV, which
can be assigned to the Fe2p3/2 and Fe2p1/2 electrons, respectively. In addition, the binding
energies of Au4f7/2 and Au4f5/2 for Au@ZnꢀFeꢀC (top curve, Figure 3d) can be identified at 84.2
eV and 88.0 eV, in comparison with those of pure Au nanoparticles (Au4f7/2 82.8 eV and Au4f5/2
86.6 eV, bottom curve). The positive shift of ca. 1.4 eV suggested that electron transfer likely
occurred from the encapsulated Au nanoparticles into the ZnꢀFeꢀC shell. Furthermore, on the
basis of the integrated peak areas, the atomic contents of Au and Fe in Au@ZnꢀFeꢀC were
estimated to be 3.2 at.% and 5.9 at.%, respectively (Table S1).
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Figure 4. (a) RRDE voltammograms of Au nanoparticles, Zn-Fe-C, Au@Zn-Fe-C, and 10 wt% Pt/C at a
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rotation rate of 1600 rpm in an O2-saturated 0.1 M KOH solution at 10 mV s− . (b) RRDE voltammograms
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of Au@Zn-Fe-C at different rotation rates in an O2-saturated 0.1 M KOH solution at 10 mV s− . The ring
potential was set at +1.5 V. (c) Koutecky-Levich plots of Au@Zn-Fe-C derived from RRDE data at different
rotation rates in panel (b). (d) Number of electron transfer of Au nanoparticles, Zn-Fe-C, Au@Zn-Fe-C and
10 wt% Pt/C as a function of electrode potential. (e) Chronoamperometric responses of Au@Zn-Fe-C and
10 wt% Pt/C at +0.47 V vs. RHE in an O2-saturated 0.1 M KOH solution (900 rpm). (f)
Chronoamperometric responses of Au@Zn-Fe-C and 10 wt% Pt/C at +0.466 V vs. RHE in O2-saturated
0.1 M KOH solution with the addition of 3 M methanol (900 rpm).
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The electrocatalytic activity of Au@ZnꢀFeꢀC for ORR was then evaluated in an O2ꢀsaturated
0.1 M KOH solution using RRDE voltammetry. The potentials were referenced to a reversible
hydrogen electrode (RHE). Pure Au nanoparticles, ZnꢀFeꢀC, and 10 wt% Pt/C were also tested as
controls (note that ZnꢀFeꢀC prepared by pyrolysis at temperatures higher than 600 °C did not
show any apparent difference of the ORR performance, Figure S5). As shown in Figure 4a,
Au@ZnꢀFeꢀC (green curve) showed an excellent electrocatalytic activity, with an onset potential
of +0.94 V vs. RHE, comparable to that of 10 wt% Pt/C (+0.94 V, blue curve), and a higher
diffusion limited current density (2.63 mA cm−2 at +0.4 V) than that of 10 wt% Pt/C (2.27 mA
cm−2). Nevertheless, the ORR activity of Au@ZnꢀFeꢀC was somewhat lower than that of 20 wt%
Pt/C, which exhibited an onset potential at +1.01 V and a diffusion limited current density of 5.0
mA cm−2 at +0.4 V (Figure S6). Cyclic voltammetric measurements of Au@ZnꢀFeꢀC tested in
O2ꢀsaturated 0.1 M KOH also showed that apparent cathodic currents started to emerge at about
+0.94 V (Figure S7). The fact that the onset potential of Au@ZnꢀFeꢀC was markedly more
positive than that of ZnꢀFeꢀC (+0.84 V vs. RHE, red curve) signifies the important role of the
encapsulated Au nanoparticles in enhancing the ORR activity. However, pure Au nanoparticles
(black curve, with a representative TEM image shown in Figure S8) exhibited only a poor ORR
performance with the onset potential at +0.80 V. These results imply a synergistic interaction
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between the Au core and the ZnꢀFeꢀC shell that played a crucial role in enhancing the
electrocatalytic activity of Au@ZnꢀFeꢀC for ORR. In addition, to distinguish the roles of Zn and
Fe in the catalytic reactions, we also prepared ZnꢀC and Au@ZnꢀC and examined their
electrocatalytic activity. Voltammetric studies (Figure S9) showed that the ORR activity of both
samples was markedly lower than that of Au@ZnꢀFeꢀC, implying that Fe, not Zn, was
responsible for promoting the ORR activity of carbon.
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RRDE measurements were also conducted at different rotation rates to further investigate the
electronꢀtransfer kinetics of Au@ZnꢀFeꢀC and the results are shown in Figure 4b. The
corresponding KouteckyꢀLevich (KꢀL) plots within the potential range of +0.4 V to +0.7 V are
included in Figure 4c. The good linearity with a rather consistent slope of Au@ZnꢀFeꢀC suggests
firstꢀorder reaction kinetics for ORR with respect to oxygen concentration in the solution. The
difference in ORR performance is also obvious in the number of electron transfer (n) involved in
ORR. As depicted in Figure 4d, the n values for Au@ZnꢀFeꢀC were between 3.26 and 3.82
within the potential range of 0 to +0.8 V, signifying that ORR largely followed a fourꢀelectron
pathway, similar to that of 10 wt% Pt/C (3.19 to 3.82). In contrast, the n values at both pure Au
nanoparticles and ZnꢀFeꢀC were significantly lower at only 0.22 to 3.28 and 0.65 to 3.48,
respectively. Significantly, the ORR activity of Au@ZnꢀFeꢀC (onset potential +0.94 V vs. RHE,
n = 3.26 – 3.82) was better than or at least comparable to those of other catalysts derived from
MOFs that have been reported in recent literature (Table S2), such as carbon polyhedra derived
from ZIFꢀ67 (onset potential +0.86 V vs. RHE, n = 3.7),57 nitrogenꢀdoped porous carbons
derived from ZIFꢀ7 (onset potential +0.87 V vs. RHE, n = 3.68 – 3.8),58 and highly graphitized
nitrogenꢀdoped porous carbon nanopolyhrdra derived from ZIFꢀ8 (onset potential +0.95 V vs.
RHE, n = 3.8 – 3.9)40.
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The durability of Au@ZnꢀFeꢀC was tested by chronoamperometric measurements at 900 rpm
in an O2ꢀsaturated 0.1 M KOH solution, as shown in Figure 4e. It can be seen that in addition to
a high activity, Au@ZnꢀFeꢀC also exhibited robust stability. After continuous operation at +0.47
V for 30,000 s, about 93.4 % of the current density was retained. In contrast, the current density
of commercial 10 wt% Pt/C catalyst exhibited a sharp drop to 73.6%. It was worth mentioning
that Au@ZnꢀFeꢀC also exhibited greater methanol tolerance than commercial 10 wt% Pt/C
catalysts (Figure 4f), which was important for practical applications. A sharp decrease in the
current density was observed for the 10 wt% Pt/C catalyst upon the addition of 3 M methanol. In
contrast, the current density of Au@ZnꢀFeꢀC even increased slightly with the addition of 3 M
methanol.
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Figure 5. Representative TEM images of (a) Au@Zn-Fe-C-L (1:2) and (b) Au@Zn-Fe-C-H (2:1).
High-resolution XPS scans of (c) Fe2p and (d) Au4f electrons of Au@Zn-Fe-C at different Au/Fe molar
ratios.
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To examine the effects of gold contents on the ORR activity, two additional samples were
prepared by the same procedure but at the Au/Fe molar ratios of 1:2 and 2:1 (as confirmed by
XPS measurements, Table S1), which were denoted as Au@ZnꢀFeꢀCꢀL and Au@ZnꢀFeꢀCꢀH,
respectively. From Figure 5, coreꢀshell structures can be clearly seen in both samples, similar to
Au@ZnꢀFeꢀC (Figure 2). However, one can see that the thickness of the ZnꢀFeꢀC shells
decreased with increasing Au/Fe molar ratios. Electrochemical studies were then carried out to
assess and compare their ORR activity. From Figure S10, it can be seen that Au@ZnꢀFeꢀCꢀH
showed a slightly lower ORR activity (onset potential +0.92 V vs. RHE) than Au@ZnꢀFeꢀC.
This may be ascribed to incomplete coating of the excessive Au nanoparticles by the carbon
shells. Au@ZnꢀFeꢀCꢀL also exhibited a more negative ORR onset potential (+0.86 V vs. RHE)
and a lower current density (1.38 mA cm−2 at +0.4 V) than Au@ZnꢀFeꢀC. This is probably
because of the limited contact between the Au nanoparticle and the carbon shell. That is, within
the present experimental context, Au@ZnꢀFeꢀC at the Au/Fe molar ratio of 1:1 represented the
best catalysts for ORR among the series. Taken together, these results strongly suggest that the
intimate electronic interactions between the Au core and the carbon shell played an important
role in catalyzing the fourꢀelectron reduction of oxygen. Similar behaviors have been observed in
the literature with, for instance, Au@Co3O4 for OER,50 Fe@carbon nanotubes for ORR59 and
CoNi@carbon for HER.6
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Figure 6. (a) Polarization curves and (b) corresponding Tafel plots of Au nanoparticles, Zn-Fe-C,
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Au@Zn-Fe-C-L, Au@Zn-Fe-C, Au@Zn-Fe-C-H and 20 wt% Pt/C at 5 mV s− in a 0.5 M H2SO4 solution
(iR-corrected). (c) Electrochemical impedance spectra of various samples for HER at −0.3 V. (d)
Electrochemical impedance spectra of Au@Zn-Fe-C at various overpotentials for HER in 0.5 M H2SO4.
Inset shows the equivalent circuit, where Rs is the (uncompensated) electrode series resistance, CPE is
the constant-phase element and Rct is the charge-transfer resistance. (e) Polarization curves of
Au@Zn-Fe-C before and after 1,000 cycles in stability test. (f) Time dependence of the HER current
density of Au@Zn-Fe-C loaded on carbon cloth at −0.15 V vs. RHE (iR-uncorrected).
The electrocatalytic activity of Au@ZnꢀFeꢀC for HER was then investigated in 0.5 M H2SO4
in a threeꢀelectrode configuration. Figure 6a shows the polarization curves of the different
samples in a 0.5 M H2SO4 aqueous solution. It can be seen that Au@ZnꢀFeꢀC (blue curve)
exhibited a low onset potential of −0.08 V vs. RHE, which was much more positive than those
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for Au (−0.225 V, black curve) and ZnꢀFeꢀC (−0.292 V, red curve) but slightly more negative
than that of commercial 20 wt% Pt/C (−0.006 V, magenta curve) at the same catalyst loading.
This suggests synergistic effects between the Au core and the ZnꢀFeꢀC shell in enhancing the
HER activity of Au@ZnꢀFeꢀC. Moreover, at Au@ZnꢀFeꢀC, an overpotential of only −0.123 V
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was needed to reach a current density of 10 mA cm−2. Just like tests conducted in ORR, the
polarization curves of the Au@ZnꢀFeꢀC samples at different Au/Fe molar ratios were also
acquired to examine the impacts of the content of Au nanoparticles on the HER activity. It can be
seen that the Au@ZnꢀFeꢀC possessed the highest HER activity among all samples, suggesting
that a proper thickness of the ZnꢀFeꢀC shell was also important in enhancing the HER activity.
The Tafel slopes of the different samples were then compared (Figure 6b), which were
obtained by fitting the linear portions of the polarization curves with the Tafel equation (η = blog
j + a), where j was the current density and b was the Tafel slope. The Tafel slope of
Au@ZnꢀFeꢀC was 130 mV dec−1, which was lower than those of other samples, for example, Au
(167 mV dec−1), ZnꢀFeꢀC (271 mV dec−1), Au@ZnꢀFeꢀCꢀL (267 mV dec−1) and Au@ZnꢀFeꢀCꢀH
(160 mV dec−1), but much higher than that of the 20 wt% Pt/C (42 mV dec−1). A lower Tafel
slope corresponds to more favorable HER kinetics, further confirming a synergistic contribution
of the Au nanoparticle core and ZnꢀFeꢀC shell. Mechanistically, there are three major reaction
steps for HER in acidic electrolytes, including a primary discharge step (Volmer reaction, with a
Tafel slope of 120 mV decꢀ1) followed by an electrochemical desorption step (Heyrovsky
reaction, with a Tafel slope of 40 mV decꢀ1) or a recombination step of hydrogen adsorption on
catalyst surfaces (Tafel reaction, with a Tafel slope of 30 mV decꢀ1). The results obtained above
indicate that HER at Au@ZnꢀFeꢀC likely proceeded through a VolmerꢀHeyrovsky mechanism
and the electrochemical discharge process was the rateꢀlimiting step.
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It should be noted that such an HER performance of Au@ZnꢀFeꢀC (onset potential −0.08 V vs.
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RHE, −0.123 V vs. RHE at 10 mA cm−2, and Tafel slope of 130 mV dec−1) was markedly better
than or at least comparable to those of the most active HER electrocatalysts in acidic solutions
that have been reported in recent literature (Table S3), for instance, nitrogenꢀdoped
graphene/Coꢀembedded porous carbon polyhedron hybrids derived from CoꢀMOFs (onset
potential at −0.058 V vs. RHE, −0.229 V vs. RHE at 10 mA cm−2, and Tafel slope of 126 mV
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dec−1),53 graphene oxide and copperꢀcentered MOF composite (onset potential at −0.087 V vs.
RHE, and Tafel slope of 84 mV dec−1),1 porous molybdenum carbide nanooctahedra derived
from MOFs (onset potential at −0.025 V vs. RHE, −0.142 V vs. RHE at 10 mA cm−2, and Tafel
slope of 53 mV dec−1),42 Coꢀembedded nitrogenꢀrich carbon nanotubes (onset potential −0.05 V,
−0.26 V vs. RHE at 10 mA cm−2, and Tafel slope of 69 mV dec−1),43 and FeCoꢀalloy into
nitrogenꢀdoped carbon nanotubes (onset potential at −0.07 V vs. RHE, −0.275 V vs. RHE at 10
mA cm−2, and Tafel slope of 74 mV dec−1)60.
Electrochemical impedance spectroscopy (EIS) measurements were then carried out to further
probe the electronꢀtransfer kinetics involved. Figure 6c depicts the Nyquist plots acquired at
−0.30 V for a glassyꢀcarbon electrode modified with the various catalyst samples. We can see
that Au@ZnꢀFeꢀC exhibited a smaller charge transfer resistance (Rct) than other samples, as
manifested by the smallest semicircle, consistent with the best HER performance. Additionally,
Rct actually diminished markedly with increasing overpotential, from 792 ꢁ at −250 mV to 316
ꢁ at −300 mV, as depicted in Figure 6d.
In addition to excellent catalytic activity, Au@ZnꢀFeꢀC also exhibited extraordinary HER
stability in acid solution. Figure 6e depicts the polarization curves of Au@ZnꢀFeꢀC before and
after the stability tests, where a slight increase in the current can be observed after 1,000 cycles
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of potential scans. Moreover, over 12 h’s continuous operation at the applied potential of −0.15
V (vs. RHE), the current density of Au@ZnꢀFeꢀC loaded on carbon cloth remained virtually
invariant, once again confirming the remarkable stability of Au@ZnꢀFeꢀC for HER in 0.5 M
H2SO4 solution (Figure 6f). Note that using carbon cloth as the working electrode minimized the
accumulation of H2 bubbles on the electrode surface during the stability test, as compared to
glassy carbon electrode (Figure S11).
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Based on the above results and in conjunction with recent theoretical calculations of
metal@carbon coreꢀshell nanostructures, the remarkable bifunctional properties of Au@ZnꢀFeꢀC
might be ascribed to the encapsulation of Au nanoparticles within the ZnꢀFeꢀC shell where
effective electron transfer from the Au core to the ZnꢀFeꢀC shell not only decreased the local
work function of the carbon surface, but also changed the carbon electronic density of states (for
instance, Figure 3d). This resulted in the generation of new catalytically active sites on the
carbon surface,50 as the synergistic manipulation of the electronic structure of the porous carbons
likely enhanced the adsorption of oxygen intermediates (in ORR) and protons (in HER). In fact,
the unique advantage of a Au@ZnꢀFeꢀC coreꢀshell structure toward ORR and HER was also
confirmed by the observation that a simple mixture of Au nanoparticles and ZnꢀFeꢀC (denoted as
Au/ZnꢀFeꢀC) exhibited a much lower catalytic activity (Figure S12). Interestingly, the
electrocatalytic activity did not seem to be very sensitive to the core metals. For instance, when
Pt@ZnꢀFeꢀC was prepared in a similar fashion, the ORR and HER performances (Figure S13)
were actually similar to those of Au@ZnꢀFeꢀC described above. Additional contributions might
arise from the ZnꢀFe compounds embedded within the porous carbons where the intimate
electronic interactions between ZnꢀFe and C have been found to also contribute to the composite
catalytic activity.53 Taken together, the remarkable features of high activity, favorable kinetics,
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and strong durability suggest that the strategy for the preparation of coreꢀshell Au@ZnꢀFeꢀC
hybrids may be a viable route in the synthesis and engineering of effective multifunctional
electrocatalysts for both ORR and HER.
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Summary
In the present study, multilevel coreꢀshell Au@ZnꢀFeꢀC hybrids were prepared by a facile
pyrolysis procedure using MOFs as functional precursors. The nanocomposite hybrids were
synthesized by direct pyrolysis of a uniform ZnꢀFeꢀMOFs shell coated on a Au nanoparticle in
an inert atmosphere, and exhibited excellent electrocatalytic activities for ORR (a positive onset
potential at +0.94 V vs. RHE, excellent stability and methanol tolerance) and HER (a low onset
overpotential of −0.08 V and a stable current density of 10 mA cm−2 at −0.123 V in 0.5 M
H2SO4). This was accounted for the conducting shells of ZnꢀFeꢀC for rapid electron transport and
formation of active sites for electrocatalytic reactions promoted by the encapsulated Au
nanoparticle, where the electrocatalytic activity was found to vary with both the loading of the
Au nanoparticle cores and the thickness of the metalꢀcarbon shells. These results may open up a
new avenue for the rational design and engineering of coreꢀshell hybrid structures based on metal
nanoparticles and MOFs as multifunctional electrocatalysts for energy conversion applications.
ASSOCIATED CONTENT
Supporting Information Available. Additional experimental data in Figures S1ꢀS13 and Tables
S1ꢀS3. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* Eꢀmail: eszhouwj@scut.edu.cn (W. J. Z.); shaowei@ucsc.edu (S. W. C.)
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ACKNOWLEDGMENTS
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This work was supported by the National Recruitment Program of Global Experts and the
National Natural Science Foundation of China (21528301 and 51502096), the PhD Startꢀup
Funds of the Natural Science Foundation of Guangdong Province (S2013040016465), Project of
Public Interest Research and Capacity Building of Guangdong Province (2014A010106005), and
the Fundamental Research Funds for Central Universities (D2153880). S. W. C. acknowledges
partial support from the National Science Foundation (CHEꢀ1265635 and DMRꢀ1409396).
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