Defect-Rich High-Entropy Oxide Nanosheets for Efficient 5-Hydroxymethylfurfural Electrooxidation

Article abstract of DOI:10.1002/anie.202107390

High-entropy oxides (HEOs), a new concept of entropy stabilization, exhibit unique structures and fascinating properties, and are thus important class of materials with significant technological potential. However, the conventional high-temperature synthesis techniques tend to afford micron-scale HEOs with low surface area, and the catalytic activity of available HEOs is still far from satisfactory because of their limited exposed active sites and poor intrinsic activity. Here we report a low-temperature plasma strategy for preparing defect-rich HEOs nanosheets with high surface area, and for the first time employ them for 5-hydroxymethylfurfural (HMF) electrooxidation. Owing to the nanosheets structure, abundant oxygen vacancies, and high surface area, the quinary (FeCrCoNiCu)₃O₄ nanosheets deliver improved activity for HMF oxidation with lower onset potential and faster kinetics, outperforming that of HEOs prepared by high-temperature method. Our method opens new opportunities for synthesizing nanostructured HEOs with great potential applications.

Full text of DOI:10.1002/anie.202107390

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How to cite:
International Edition: doi.org/10.1002/anie.202107390
German Edition: doi.org/10.1002/ange.202107390
Electrocatalysis
Defect-Rich High-Entropy Oxide Nanosheets for Efficient 5-
Hydroxymethylfurfural Electrooxidation
Kaizhi Gu+, Dongdong Wang+, Chao Xie, Tehua Wang, Gen Huang, Yanbo Liu, Yuqin Zou,*
Li Tao,* and Shuangyin Wang*
Abstract: High-entropy oxides (HEOs), a new concept of
entropy stabilization, exhibit unique structures and fascinating
properties, and are thus important class of materials with
significant technological potential. However, the conventional
high-temperature synthesis techniques tend to afford micron-
scale HEOs with low surface area, and the catalytic activity of
available HEOs is still far from satisfactory because of their
limited exposed active sites and poor intrinsic activity. Here we
report a low-temperature plasma strategy for preparing defect-
rich HEOs nanosheets with high surface area, and for the first
time employ them for 5-hydroxymethylfurfural (HMF) elec-
trooxidation. Owing to the nanosheets structure, abundant
oxygen vacancies, and high surface area, the quinary (FeCr-
CoNiCu)3O4 nanosheets deliver improved activity for HMF
oxidation with lower onset potential and faster kinetics,
outperforming that of HEOs prepared by high-temperature
method. Our method opens new opportunities for synthesizing
nanostructured HEOs with great potential applications.
or near-equimolar ratios, randomly distributed throughout
a crystal structure.[10–13] Based on the discovery of entropy-
driven phase-stabilization effect, Rost et al. reported the first
high-entropy oxides (HEOs) and expanded the library of
HEMs.[14] Afterwards, several kinds of HEOs with different
crystal structures, such as rocksalt, spinel, perovskite, and
fluorite, were exploited and technologically important in
many applications, including electronics, catalysis, and energy
storage and conversion.[15–20] At present, the conventional
synthetic routes toward HEOs are generally dominated by
high-temperature approaches (T ꢀ9008C). However, these
methods offer limited control over shape and size, and tend to
form micron-scale materials with low surface area (Fig-
ure 1a).[20–24] The catalytic activity of HEOs is still far from
satisfactory because of their limited exposed active sites and
poor intrinsic activity.
Nanostructuring represents
a promising strategy to
enhance the activity of electrocatalysts, since nanostructured
catalysts with high surface area can effectively increase the
density of the exposed active sites and enhance mass diffusion
efficiency, leading to improved electrocatalytic perfor-
mance.[25–28] However, one of the major obstacles encountered
with the fabrication of nanostructured HEOs is the incom-
patible requirements of achieving high configurational
entropy, which is driven by high-temperature processes, and
preventing particle coarsening, which is facilitated by low-
temperature reactions.[29] In addition to expose more active
surfaces, defect engineering such as introduction of oxygen
vacancies is another effective strategy to promote the
electrocatalytic activity.[30–34] However, capabilities for the
rational design and synthesis of nanostructured HEOs,
especially two-dimensional (2D) HEOs nanosheets with
oxygen vacancies under low temperature yet remain chal-
lenging.
Here, we report a low-temperature plasma strategy
towards the synthesis of HEOs nanosheets with rich oxygen
vacancies (Figure 1b). Plasma is a promising technology for
the synthesis and modification of nanomaterials.[35,36] High-
energy electrons collide inelastically with oxygen molecules
and transfer their energy to the latter, which leads to the
production of excited oxygen species with a significantly
higher chemical activity than molecular oxygen. High-
entropy layered double hydroxides (HE-LDHs)[37] as precur-
sors can be oxidized by high active oxygen species and
converted into single-phase, spinel-type HEOs under mild
conditions. Low-temperature plasma technique endows the
as-synthesized HEOs with the nanosheets structure, abundant
oxygen vacancies, and high surface area, which is beneficial
for the improvement of the electrocatalytic active. As a proof
E
lectrocatalytic conversion of biomass-derived molecules
provides an attractive approach to obtain high value-added
chemicals using renewable energy.[1–4] For instance, the
electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-
furandicarboxylic acid (FDCA) has drawn tremendous atten-
tion because FDCA is a promising alternative of petroleum-
derived terephthalic acid for the production of biorenewable
polymers. Furthermore, electrochemical oxidation of HMF is
considered as a clean and environment-friendly route because
this transformation is driven by electrons at the anode without
the use of toxic chemical oxidants. In recent years, various
metal-based electrocatalysts, such as sulfides, phosphides, and
nitrides, have been explored for the HMF electrooxida-
tion.[5–9] Therefore, the development of robust, stable, and
inexpensive electrocatalysts for efficient HMF oxidation to
FDCA under mild conditions is highly desirable.
High-entropy materials (HEMs) are crystalline solid
solutions that comprise five or more elements, in equimolar
[*] Dr. K. Gu,[+] D. Wang,[+] Dr. C. Xie, Dr. T. Wang, G. Huang, Y. Liu,
Y. Zou, Dr. L. Tao, Prof. Dr. S. Wang
State Key Laboratory of Chemo/Bio-Sensing and Chemometrics,
College of Chemistry and Chemical Engineering, Hunan University
Changsha, 410082 (China)
E-mail: yuqin_zou@hnu.edu.cn
taoli@hnu.edu.cn
shuangyinwang@hnu.edu.cn
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202107390.
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Figure 1. a) The traditional synthesis method and b) low-temperature plasma strategy for HEOs. c) XRD patterns, d) Raman spectra, and e) N2
adsorption-desorption isotherm of P-HEOs and C-HEOs. f and i) TEM, g and j) HRTEM, and h and k) EDX mapping of P-HEOs and C-HEOs,
respectively.
of concept, we use the representative quinary HEOs nano-
sheets as electrocatalysts for HMF electrooxidation. The
(FeCrCoNiCu)3O4 nanosheets exhibit superior electrocata-
lytic activity for HMF oxidation with lower onset potential
and faster kinetics, providing a promising platform for
efficient electrochemical HMF oxidation.
C-HEOs, we noticed that P-HEOs show a relatively
decreased crystallinity due to the presence of structural
defects. Meanwhile, the Raman spectrum of P-HEOs shows
three features of Eg (350 cmÀ1), F2g (515 cmÀ1), and A1g
(675 cmÀ1) attributed to the characteristic vibrational modes
of Fd-3 m space group (Figure 1d).[22,38] It should be noted
that the entropy-stabilized oxides contain Fe, Cr, Co, Ni, and
Cu elements and their molar ratios are approximately 1: 1: 1:
1: 1, which is confirmed by energy dispersive X-ray (EDX)
spectroscopy and inductively coupled plasma optical emission
spectroscopy (ICP-OES) (Figure S1, Table S1 and S2). These
results jointly validate that we have successfully synthesized
spinel HEOs by the low-temperature plasma strategy. The
formation of high-entropy phase is mainly driven by the high
configurational entropy, which originates from the mixing of
the multiple elements. The P-HEOs display a typical type-IV
isotherm with a clear adsorption-desorption hysteresis loop,
and the Brunauer-Emmett-Teller (BET) specific surface area
of P-HEOs and C-HEOs are 48.6 and 3.7 m2 gÀ1, respectively
(Figure 1e). The high surface area of P-HEOs can exposure of
more active sites.[39–41] The pore size distributions reveal that,
compared with C-HEOs, the existence of more abundant
To demonstrate low-temperature plasma strategy, we first
take quinary HEOs as an example for fabricating uniformly
dispersed, single-phase, solid-solution nanosheets. Our
method for the production of HEOs nanosheets follows
a two-step process (Figure 1b). First, five metal salts (Fe, Cr,
Co, Ni, and Cu) were mixed and formed HE-LDHs by the
solvothermal method, which is a rational pathway for
randomly assembling various metal atoms into 2D nano-
structures.[37] Then, we exposed the HE-LDHs to low-temper-
ature oxygen plasma to form HEOs denoted as P-HEOs. For
comparison, we also synthesized the control sample (C-
HEOs) via reverse co-precipitation method at 10008C.[24]
As shown in Figure 1c, XRD patterns of both P-HEOs
and C-HEOs show eight major diffraction peaks correspond-
ing to (111), (220), (311), (222), (400), (422), (511), and (440)
planes of spinel phase structure (Fd-3 m). Comparing with the
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mesoporous structures in the P-HEOs (Figure S2), which
facilitates the efficient mass transfer in biomass conversion.
The microstructure of HEOs was further evidenced by
transmission electron microscopy (TEM). The TEM image
shows the morphology of P-HEOs is nanosheets with size of
ca. 20 nm (Figure 1 f). By contrast, the C-HEOs show the
particle with diameter of about 500 nm because the cohesive
energy drives codomain formation and sintering at 10008C
(Figure 1i). In contrast with the relatively smooth and
compact surface of C-HEOs, P-HEOs nanosheets exhibit
a rather rough and defective structure derived from the
etching effect of plasma, which would promote electrocata-
lytic activity. The high-resolution TEM (HRTEM) images of
P-HEOs and C-HEOs display lattice fringes with an inter-
planar distance of 0.294 nm correspond to the (220) planes of
spinel (Figures 1g and j), in line with the XRD results. The
EDX mappings illustrate that Fe, Cr, Co, Ni, Cu, and O are all
present and uniformly distributed, indicative of the formation
of the high-entropy phase (Figures 1h and k).
The X-ray photoelectron spectroscopy (XPS) was
employed to investigate the surface chemical compositions
and the valence of elements. The high-resolution XPS spectra
indicate Fe and Cu cations are mainly present as Fe3+ and
Cu2+, respectively. The Cr 2p spectra suggest the existence of
Cr3+ and Cr6+. The Co 2p and Ni 2p spectra show that there
are the corresponding divalent and trivalent species (Fig-
ure S3). Notably, the O 1 s core level spectra can be
deconvoluted into three peaks at 529.8, 531.4, and 532.8 eV,
corresponding to oxygen atoms bound to metals (O1), defect
sites with a low oxygen coordination (O2), and hydroxyl
species or surface-adsorbed oxygen (O3), respectively (Fig-
ure 2a).[42] The much higher O2 ratio of P-HEOs indicates
that P-HEOs possess higher concentration of oxygen vacan-
cies than C-HEOs. Additionally, we used the electron para-
magnetic resonance (EPR) spectroscopy to further identify
the oxygen vacancies, which is an effective tool to detect
unpaired electrons in materials.[43] The P-HEOs showcase an
obviously stronger EPR signal at g = 2.003 (Figure 2b). The
signal intensity illustrates P-HEOs have more oxygen vacan-
cies that is in good agreement with the XPS results. It is
anticipated that oxygen vacancies engineering provides
a promising opportunity to regulate the electronic structures
of oxides to effectively boost the electrocatalytic activ-
ity.[8,44,45]
Our method can synthesize a series of HEOs at low-
temperature, illustrating the reliability and generality of the
plasma strategy. As expected, XRD patterns of the synthe-
sized HEOs reveal that the diffraction peaks are indexed to
the single-phase spinel structure (Figure S4). TEM images
confirm a typical nanosheets structure with the sizes from
about 20 to 100 nm (Figure S5). The elemental mappings
suggest the homogeneous dispersion of each element within
these HEOs nanosheets without phase separation (Figur-
es 3a–f), consistent with the results of EDX and ICP-OES
(Figure S6, Table S1 and S2). XPS spectra further confirm
different HEOs contain corresponding metal and oxygen
elements, and O 1 s spectra disclose these HEOs possess rich
oxygen vacancies (Figures S7–12). All these results demon-
strate that our method is able to extend to more complex
Figure 2. a) O 1 s XPS and b) EPR spectra of P-HEOs and C-HEOs.
HEOs nanosheets with abundant oxygen vacancies (defects)
by tailoring the elemental composition.
As a proof of concept, we applied the P-HEOs nanosheets
loaded on glassy carbon electrode for HMF oxidation. The P-
HEOs exhibit the onset potential of 1.50 V vs. RHE for the
OER (Figure S13). After introducing HMF, the P-HEOs
electrode shows higher current response, and the onset
potential decrease to 1.35 V vs. RHE, which is much lower
than that of C-HEOs (Figure 4a), implying the excellent
electrocatalytic activity on the P-HEOs electrode. Further-
more, the electrocatalytic activity of P-HEOs is superior to
that of oxides with fewer elements (Figure S14). The Tafel
slope of P-HEOs and C-HEOs is calculated to be 97.8 and
279.5 mV decÀ1, respectively (Figure 4b). The smaller slope of
P-HEOs indicates significantly improved kinetics towards
HMF electrooxidation. Electrochemically active surface area
(ECSA) was evaluated by double-layer capacitance (Cdl). The
Cdl of P-HEOs (1.22 mFcmÀ2) is higher than that of C-HEOs
(0.34 mFcmÀ2) (Figure S15), suggesting the nanosheets struc-
ture of P-HEOs could increase the ECSA and expose more
active sites. The current density normalized by ECSA and
BET at 1.50 V vs. RHE of P-HEOs is about 7-fold and 2-fold
higher than that of C-HEOs (Figure 4c and S16), verifying the
surface abundant oxygen vacancies can enhance the intrinsic
activity of P-HEOs.[30]
Electrochemical impedance spectroscopy (EIS) was used
to probe the interfacial electrochemical behavior between
electrode and electrolyte. The equivalent circuit and the
parameters of each component were presented by fitting the
Nyquist plots (Figure S17 and Table S3). It is evident that the
Rct of P-HEOs (113.9 W) is smaller than that of C-HEOs
(396.5 W), demonstrating P-HEOs has faster charge transfer
rate (Figure 4d). Furthermore, in situ EIS at various poten-
tials was used to investigate the charge-transfer resistance
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Figure 3. EDX elemental mapping of HEOs: a) (FeAlCoNiZn)3O4, b) (FeCrCoNiZn)3O4, c) (FeAlCrCoNiZn)3O4, d) (FeAlCoNiZnMg)3O4, e) (FeAlCr-
CoNiZnMg)3O4, and f) (FeAlCrCoNiZnMgCu)3O4.
change and catalytic kinetics of P-HEOs electrode during
HMF oxidation processes. At the low potential of 1.05–1.30 V
vs. RHE, Nyquist plots show steep straight lines, which
indicate the infinite charge-transfer resistance. When the
applied potential exceeds 1.35 V, an apparent semicircle
appears, which indicates the HMF electro-oxidation has
occurred (Figure 4e). The Bode phase plots depict the trend
of phase angle variation with frequency. Generally, the peaks
in low frequency and high frequency region are regarded as
the interface reaction charge transfer and the electron
transfer process associated with the formation of surface
oxidative species, respectively.[46,47] As shown in Figure 4 f,
when the potential window is 1.35–1.55 V, the decreasing of
peak at low frequency, corresponding to the accelerating of
interface reaction charge transfer, indicates the faster kinetics
of HMF electrooxidation. In high frequency, the peak
position regularly shifts to higher frequency with potential
from 1.35 to 1.55 V, mainly attributed to the accelerating of
surface metal oxidation, which is closely related to the
increasing of HMF oxidation current.
oxidation products with the reaction progressed was calcu-
lated by HPLC quantitatively (Figures 4g and S19). As
depicted in Figure 4h, the concentration of HMF decreased
and the level of FDCA increased with the increasing charge,
indicating the successful conversion of HMF to FDCA. Note
that the concentration of 2,5-diformylfuran (DFF) is almost
absent, while the concentration changes of 5-hydroxymethyl-
2-furancarboxylic acid (HMFCA) and 2-formyl-5-furancar-
boxylic acid (FFCA) were detectable, suggesting FDCA is
likely generated following the route of HMFCA intermediate
(Figure S20). In six successive electrolysis cycles, no obvious
decay was observed in FDCA yield and Faradaic efficiency
(Figure 4i), strongly highlighting the cycling durability of the
P-HEOs electrode. The P-HEOs after stability tests were
investigated by TEM, EDX, and XPS, and the results
corroborate that there are no obvious changes in architecture,
composition, and structure (Figure S21 and S22). This high
stability results from favorable high-entropy effect. We finally
investigated the electrocatalytic performance of P-HEOs for
other substrates, such as ethanol, benzyl alcohol, furfuryl
alcohol, furfural, glycerol, and glucose. The P-HEOs elec-
trode exhibits low onset potential and rapid current rise
(Figure S23), manifesting the P-HEOs nanosheets has the
potential for electrochemical biomass conversion.
To realize high current density and conversion efficiency,
the P-HEOs were dispersed onto carbon paper as the
electrode for HMF oxidation. P-HEOs yield a higher current
density and a lower onset potential than C-HEOs (Fig-
ure S18). The chronoamperometric tests were carried out at
1.50 V vs. RHE and the concentration of HMF and its
In summary, we have successfully developed a versatile
and efficient strategy for the synthesis of nanostructured
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Figure 4. a) LSV curves and b) Tafel plots of P-HEOs and C-HEOs in 1.0 M KOH with 50 mM HMF. c) Current density normalized by ECSA at
1.50 V vs. RHE. d) Nyquist plots at 1.50 V vs. RHE. e) Nyquist and f) Bode phase plots of P-HEOs at various voltages vs. RHE. g) HPLC traces of
HMF oxidation catalyzed by P-HEOs at 1.50 V vs. RHE in 1.0 M KOH with 10 mM HMF. h) Concentration changes of HMF and its oxidation
products during HMF electrooxidation. i) FDCA yield and Faradaic efficiency of P-HEOs under six successive cycles.
Changsha Municipal Natural Science Foundation (Grant
HEOs at low-temperature. The proposed plasma strategy
No.: kq2007009).
ensures the nanosheets structure, abundant surface oxygen
vacancies, and high surface area of the obtained HEOs,
efficiently promoting the electrocatalytic activity. As a proof
of concept, the quinary HEOs nanosheets exhibit superior
electrocatalytic performance for HMF oxidation with lower
onset potential and faster kinetics, providing a promising
platform for efficient electrochemical biomass upgrading.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: biomass upgrading · defect chemistry ·
electrocatalysis · high-entropy oxides · plasma technology
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Manuscript received: June 2, 2021
Accepted manuscript online: June 25, 2021
Version of record online: && &&, &&&&
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Electrocatalysis
Defect-rich high-entropy oxides (HEOs)
nanosheets with high surface area are
synthesized through a plasma strategy
under low-temperature. The unique
structural and compositional advantages
endow the HEOs nanosheets with supe-
rior electrocatalytic activity for 5-hydrox-
ymethylfurfural oxidation.
K. Gu, D. Wang, C. Xie, T. Wang,
G. Huang, Y. Liu, Y. Zou,* L. Tao,*
S. Wang*
&&&& — &&&&
Defect-Rich High-Entropy Oxide
Nanosheets for Efficient 5-
Hydroxymethylfurfural Electrooxidation
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
7
These are not the final page numbers!