A Single-Crystal Open-Capsule Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.9b02417

Micro-/nanocapsules have received substantial attention due to various potential applications for storage, catalysis, and drug delivery. However, their conventional enclosed non-/polycrystalline walls pose huge obstacles for rapid loading and mass diffusion. Here, we present a new single-crystal capsular-MOF with openings on the wall, which is carefully designed at the molecular level and constructed from a crystal-structure transformation. This rare open-capsule MOF can easily load the largest amounts of sulfur and iodine among known MOFs. In addition, derived from capsular-MOF and melamine through pyrolysis-phosphidation, we fabricated a nitrogen-doped capsular carbon-based framework with iron-nickel phosphide nanoparticles immobilized on capsular carbons interconnected by plentiful carbon nanotubes. Benefiting from synergistic effects between the carbon framework and highly surface-exposed phosphide sites, the material exhibits efficient multifunctional electrocatalysis for oxygen evolution, hydrogen evolution, and oxygen reduction, achieving well-qualified assemblies of an overall water splitting (low potential of 1.59 V at 10 mA·cm^-2) and a rechargeable Zn-air battery (high peak power density of 250 mW·cm^-2 and excellent stability for 500 h), which afford remarkably practical prospects over previously known electrocatalysts.

Full text of DOI:10.1021/jacs.9b02417

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
A Single-Crystal Open-Capsule Metal−Organic Framework
Yong-Sheng Wei,† Mei Zhang,† Mitsunori Kitta,‡ Zheng Liu,§ Satoshi Horike,†,∥
and Qiang Xu*,†,‡
†AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced
Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501, Japan
‡Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda,
Osaka 563-8577, Japan
§Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
2266-98 Anagahora, Shimoshidami, Moriyamaku, Nagoya, Aichi 463-8560, Japan
∥Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, Kyoto University, Sakyo-ku, Kyoto
606-8501, Japan
S
* Supporting Information
ABSTRACT: Micro-/nanocapsules have received substantial atten-
tion due to various potential applications for storage, catalysis, and
drug delivery. However, their conventional enclosed non-/poly-
crystalline walls pose huge obstacles for rapid loading and mass
diffusion. Here, we present a new single-crystal capsular-MOF with
openings on the wall, which is carefully designed at the molecular
level and constructed from a crystal-structure transformation. This
rare open-capsule MOF can easily load the largest amounts of sulfur
and iodine among known MOFs. In addition, derived from capsular-
MOF and melamine through pyrolysis−phosphidation, we fabricated
a nitrogen-doped capsular carbon-based framework with iron−nickel
phosphide nanoparticles immobilized on capsular carbons inter-
connected by plentiful carbon nanotubes. Benefiting from synergistic
effects between the carbon framework and highly surface-exposed phosphide sites, the material exhibits efficient multifunctional
electrocatalysis for oxygen evolution, hydrogen evolution, and oxygen reduction, achieving well-qualified assemblies of an
overall water splitting (low potential of 1.59 V at 10 mA·cm−2) and a rechargeable Zn−air battery (high peak power density of
250 mW·cm−2 and excellent stability for 500 h), which afford remarkably practical prospects over previously known
electrocatalysts.
morphologies.18−21 It is a big challenge to rationally design and
construct capsular MOF nanocrystals with openings on the wall,
let alone present single crystallinity.
INTRODUCTION
■
Micro-/nanocapsules have gained considerable attention in
various promising fields1 including storage/encapsulation,2−4
Kinetic control of MOF crystal recrystallization (dissolution−
regrowth) has a significant effect on crystal engineering and
morphology.22,23 Especially when the structures of parent and
shell crystals are well-matched, shell crystals will uniformly grow
on outer surfaces of parent crystals, the cores of which
simultaneously dissolve and diffuse out. In other words, this
core-to-surface mass transfer will probably lead to the formation
of single-crystal capsular structures and leave plentiful openings
on the wall, although the shape-controlled regrowth of single-
crystal capsular MOFs from crystal-structure transformations
are rarely achieved.
catalysis,5−7 and drug delivery8 by virtue of their high exposed
surface, modifiable shell, and large accommodation space.
However, to our knowledge, most of the capsules have enclosed
structures, which would seriously hinder mass diffusion and
exposure of inner surfaces.9 Thus, designing and constructing
new capsules with openings is highly desired but rarely reported
so far.10 In this context, metal−organic frameworks (MOFs),
constructed by metal ions and organic ligands,11−14 have shown
great potential in the synthesis of capsular crystals, which are
generally prepared by prefabricating a core−shell intermediate
with an inactive solid as inner template.15 Nevertheless, this
method usually suffers from their complicated time-consuming
processes and harsh conditions of template removal.16,17
Recently, self-templated strategies that do not require removal
of the cores have been developed to prepare hollow MOF
crystals, while most of them afford undesired polycrystalline
hollows close-packed by small MOF crystals with enclosed
Herein, we developed a self-templated strategy to successfully
construct an iron−nickel-based single-crystal open capsular-
MOF, which is carefully designed and constructed from crystal-
Received: March 4, 2019
© XXXX American Chemical Society
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Figure 1. (a) Schematic illustration for the formation of capsular-MOF from FeNi-MIL-88B. (b) TEM images of FeNi-MIL-88B. (c) SEM image of
capsular-MOF. (d) HAADF-STEM image and elemental maps of capsular-MOF. (e) SAED pattern of the selected area of inserted capsular-MOF
nanocrystal. Its regular diffraction spots can be indexed to the ⟨100⟩ zone axis. (f) N2 adsorption (solid) and desorption (empty) isotherms (77 K) of
capsular-MOF, solid-MOF, and FeNi-MIL-88B. The inset shows their corresponding pore size distribution curves. (g, h) Single-crystal X-ray
diffraction structures along the c-axis of Fe-MIL-88B (corresponding to FeNi-MIL-88B) and Fe-MIL-88B-tpy (corresponding to capsular-MOF),
respectively. Hydrogen atoms and terminal ligands of trinuclear clusters are omitted for clearity. (i, j) Difference electron density maps for Fe-MIL-88B
(corresponding to FeNi-MIL-88B) and Fe-MIL-88B-tpy (corresponding to capsular-MOF), respectively, showing the presence of tpy.
structure transformations at the molecular level. This rare open-
capsule MOF can easily encapsulate sulfur and iodine with the
highest capacities among known MOFs. Moreover, through
pyrolysis−phosphidation with melamine, capsular nanocrystals
can evolve into a nitrogen-doped carbon hollow (NCH)
framework with iron−nickel phosphide (FeNiP) nanoparticles
embedded in open capsular carbons interconnected by plentiful
carbon nanotubes (FeNiP/NCH). Coupled with its superior
electrocatalysis for oxygen evolution, hydrogen evolution, and
oxygen reduction, we successfully use FeNiP/NCH as the only
electrocatalyst to multifunctionally assemble an overall water
splitting (low potential of 1.59 V at 10 mA·cm−2) and a
rechargeable Zn−air battery (high peak power density of 250
mW·cm−2 and excellent stability for 500 h). As far as we know,
this is the first design and construction of an open-capsule
nanostructure that offers excellent material encapsulation and
multicatalytic performances with greatly potential applications
in practice.
RESULTS AND DISCUSSION
■
Fabrication of Open Capsular-MOF Nanocrystals and
Encapsulation of Sulfur and Iodine. With addition of ligand
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Figure 2. (a) Schematic representation of experimental equipment for loading sulfur and iodine into capsular-MOF, respectively. (b, c) Photographs
and SEM images of S@capsular-MOF (top) and I2@capsular-MOF (bottom). The original capsular morphology of MOF crystals was preserved,
without any observable sulfur and iodine species on the crystal surface. (d) N2 adsorption (solid) and desorption (empty) isotherms of S@capsular-
MOF, I2@capsular-MOF, and pristine capsular-MOF. (e) TGA of S@capsular-MOF and I2@capsular-MOF. (f) Correlation between S and I2 uptake
capacities and BET for capsular-MOF and the other reported MOFs (orange for I2 and violet for S).
2,4,6-tris(4-pyridyl)pyridine (tpy), uniform capsular-MOF
nanocrystals (elongated hexagonal dipyramid) can be facilely
prepared by using nonhollow crystals of FeNi-MIL-88B24 as the
template in hot N,N-dimethylformamide (DMF) (Figures 1a,
S1, and S2). Scanning electron microscopy (SEM) and high-
angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) reveal, unlike the nonhollow
nanocrystals of FeNi-MIL-88B (Figures 1b, S3, and S4), an
extremely rare capsular morphology with openings on the wall
(Figures 1c, S5, and S6). Energy-dispersive X-ray spectroscopy
(EDS) maps show its uniform distribution of iron and nickel
(Figures 1d and S7). Surprisingly, the selected area electron
diffraction (SAED) pattern reveals that the open capsular-MOFs
are single crystals (Figure 1e). Compared with FeNi-MIL-88B
(P63/mmc, a = 12.8849(9) Å, c = 18.3064(2) Å, V = 2632.1(4)
Å3, Figures S8 and S9), capsular-MOF shows a distinctly
different powder X-ray diffraction (PXRD) pattern, which can
be well indexed in the same space group by Pawley refinement
(Figure S8). Its derived unit-cell parameters vary greatly (a =
16.8736(9) Å, c = 14.8709(7) Å, V = 3666.8(5) Å3, Figure S10
and Table S1), implying that tpy may successfully participate in
constructing the capsular structure in combination with the
results of infrared spectroscopy (IR) (Figures S11 and S12). For
comparison, a conventional synthesis gives solid-MOF (non-
hollow MOF) nanocrystals by directly using the same starting
materials. A series of characterizations including PXRD, IR,
SEM, and TEM shows that solid-MOF is isostructural to
capsular-MOF (Figures S2, S8, S10, S12, and S13 and Table S1)
but has a different uniform crystal external shape (fat hexagonal
dipyramid) (Figures S2, S14, and S15), indicating that the
capsular-MOF should grow from the parent crystal (FeNi-MIL-
88B). In particular, capsular-MOF and solid-MOF present
identical Type-I N2 adsorption isotherms and pore size
distributions (Figure 1f) with corresponding Brunauer−
Emmett−Teller (BET) surface areas of 1157 and 1010 m2 g−1
and pore volumes of 0.728 and 0.558 cm3 g−1, respectively
(Table S2). Notably, a remarkable desorption hysteresis loop
was solely observed in capsular-MOF, sufficiently indicating the
mesostructured voids caused by the capsular nanostructure.25 In
contrast, guest-free FeNi-MIL-88B showed a type-II isotherm
with a negligible uptake because its shrunken framework is
hardly opened by nitrogen molecules.26,27
To reveal structural details for the growth mechanism of the
open capsule, we carried out successive characterizations for
time-dependent intermediates every 2 h during synthesis of
capsular-MOF, including PXRD, BET surface areas, SEM, and
TEM. PXRD analyses (Figure S16a) confirm that the crystals at
the fourth hour possess mixed crystal structures of FeNi-MIL-
88B and capsular-MOF, indicating that the crystal of FeNi-MIL-
88B was partially transformed into capsular-MOF, which was
verified by an obvious increase in BET surface area (Figure S16b
and Table S3). Further increasing the reaction time resulted in
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Figure 3. (a) Schematic illustration of the preparation process of FeNiP/NCH. (b) PXRD patterns, (c) SEM images, (d) TEM images, and (e) SAED
of FeNiP/NCH. (f) HAADF-STEM image and elemental maps of FeNiP/NCH.
unchanged PXRD patterns and N2 adsorption isotherms.
Nevertheless, the desorption hysteresis loop gradually became
larger and larger, implying the growth of capsular nanostruc-
tures. SEM and TEM images at the fourth hour revealed plenty
of concave defects on all crystal faces of parent FeNi-MIL-88B
crystals, which can undergo severe surface etches and release
expected, the unit-cell parameters of Fe-MIL-88B-tpy and Fe-
MIL-88B are almost identical to those obtained from capsular-
MOF and FeNi-MIL-88B with small differences of the axes
(<0.3%), respectively, manifesting that they are isostructural
MOFs (Tables S1 and S4). Careful structural analysis of Fe-
MIL-88B-tpy supports that, besides the 6-connected acs
networks similar to that of Fe-MIL-88B, each tpy ligand
coordinates with the three Fe(III) ions by its three pyridyl ends,
giving a rigid framework (Figures 1g,h and S19).26,27 In
particular, the electron-density distribution of tpy can be clearly
observed in the X-ray difference Fourier maps (Figures 1i,j and
S20). In other words, when tpy is ignored, capsular-MOF
actually has an isoreticular backbone with FeNi-MIL-88B. It
should be noted that the capsular structure cannot be
constructed via similar treatment of Fe-MIL-88B with tpy
(Figures S21 and S22). Coupled with the fact that introduction
of additional size-matching ligands into MOFs can drastically
enhance the structural stabilities,26,27 a dissolution-regrowth
mechanism for capsular-MOF is naturally proposed.28 That is, in
hot DMF, crystal faces of parent FeNi-MIL-88B crystals first
undergo severe surface etches by the hot solution and tpy is
anchored on the uncoordinated Fe(III)/Ni(II) sites of the
parent crystal surface. Then the inner core of the crystal is
Fe3+, Ni2+, and bdc-NH2 into the solution (Figures S17 and
2−
S18). When the sixth hour was reached, the ravined surface of
the crystals was partially refilled, indicating that the crystal
surface may be constructed by Fe3+, Ni2+, bdc-NH22−, and tpy
from the solution. As time goes on (the eighth through twelfth
hour), the shape of overall crystal changed slightly, while the
inner core of crystal was seemingly dissolved to release metal
ions and ligands into the solution mainly through openings on
the walls. Obviously, the origination of openings should come
from some big early formed concave defects on the parent
crystals. In addition, as shown in Figures S17, the density of
openings in the nanocrystal walls decreases with increasing the
reaction time.
To understand more details at the molecular level for capsular
growth, we also carried out single-crystal X-ray diffraction
(SCXRD) analyses. In order to obtain crystals suitable for
SCXRD and for the convenience of subsequent analysis, we used
only FeCl3 and H2bdc-NH2 with/without tpy to successfully
synthesize large crystals of Fe-MIL-88B-tpy/Fe-MIL-88B. As
simultaneously dissolved to release Fe3+, Ni2+, and bdc-NH2
2−
into the solution. Considering the identical orientations of
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crystal faces calculated from relevant single-crystal structures
(Figure S23), these released ions will diffuse out of the crystal
through these openings and then directly participate in
constructing the new structure with tpy on the parent crystal
surface. After that, the outer layer continues to grow with
dissolution of the inner core, finally generating well-defined
capsular nanocrystals with openings on the wall (Figure S24).
Within this context, this capsular-MOF with openings on the
wall therefore attracts considerable interests because of its great
promising application in storage. To address this issue, we use
capsular-MOF to store sulfur (S) and iodine (I2) (Figure 2),
both of which have been exploited as new-type cathodes for
rechargeable lithium−sulfur/iodine batteries with high theoreti-
cal energy density/discharge capacities.29−31 It is believed that
the higher loadings of S and I2 will greatly enhance the battery
performances. Sulfur and iodine were introduced into the MOF
capsules through melt-diffusion and I2 vapor adsorption
methods, respectively (Figures 2a and S25). After loading, the
color of I2@capsular-MOF changed apparently from gray to
brown (black for solid-MOF), while the S@MOF remains
almost unchanged (Figures 2b and S26). Furthermore, both
their original morphologies and crystallinities of these MOFs
were well-maintained as evidenced by SEM and PXRD patterns,
respectively (Figures 2c and S27−30). The successful loadings
of sulfur and iodine into the micropore of the single-crystalline
wall were distinctly reflected in the drastic decrease in N2
uptakes of these samples with maximum permitted loads in
comparison to the pristine MOFs (Figures 2d and S31). Their
exact loadings were analyzed quantitatively by thermal-
gravimetric analysis (TGA) (Figures 2e and S32). As expected,
capsular-MOF represents the highest loadings of 2.00 g/g for
sulfur and 2.57g/g for iodine among all known MOFs including
the solid-MOF (0.72 and 1.38 g/g, respectively) (Figure 2f and
Tables S5 and S6). Considering that nonhollow MOF crystals
are hard to be fully filled by S and I2 molecules in a short time, we
believe the unique openings on the wall provide a shortcut to the
inner surface of capsular MOF, thus effectively enhancing the
mass transfer and storage.
Preparation of Capsular Carbon-Based Network
Derived from the Open Capsular-MOF. Recently, MOF-
derived carbon composites, possessing excellent chemical
stabilities and conductivities,32−36 have shown great potential
applications in energy storage and conversion.9,37−45 However,
nonhollow MOF-derived nanocomposites usually exhibit
complete microporosity and undesired aggregation of metal-
based nanoparticles, which are considered unfavorable for ion
and electron transport. On the basis of these concerns, hollow
architectures have recently emerged as an effective platform to
significantly improve catalytic efficiency of metal nanocompo-
sites because their shell nanostructures can endow materials
with a higher density of surface-exposed active sites.43,44,46 More
importantly, compared with those widely reported nonhollow
MOF crystal-derived hollow carbons,43,46 directly using hollow
MOF crystals as precursors can effectively reduce the
aggregation of metal active sites during pyrolysis, although
harsh annealing usually causes a partial or even complete
collapse of MOF morphologies. Fortunately, through pyrolysis−
phosphidation with melamine, capsular-MOF successfully
evolved into a capsular carbon embedded with iron−nickel
phosphide nanoparticles (Figures 3 and S33). For comparison,
FeNiP/NCS from solid (nonhollow)-MOF and FeNiP/C from
FeNi-MIL-88B were also prepared. PXRD of FeNiP/NCH
shows a weak peak located at 26.4° assigned to the (002) plane
of graphitic carbon (Figure 3b), which is further verified by the
IR and Raman spectra with a G-band at 1586 cm−1 (Figure S34).
Besides, the D-band of Raman spectra at 1355 cm−1 manifests
the presence of a disordered/defect carbon. Additionally, other
peaks appearing at 32.2, 36.8, 48.0, and 56.1° can be indexed to
(−202), (021), (220), and (310) planes of NiP2, and peaks at
31.2, 35.2, 40.2, 44.1, and 47.6° refer to (101), (200), (111),
(201), and (210) planes of Fe2P, respectively. Remarkably, all of
the diffraction peaks are significantly broadened, which might be
due to ultrafine sizes of NiP2 and Fe2P nanoparticles confirmed
by subsequent TEM and SAED measurements. However, for
FeNiP/C and FeNiP/NCS, these peaks become more sharp,
and nonphosphatized Fe0.64Ni0.36 alloys (PDF no. 47-1405) are
obviously observed (Figure S35). SEM and TEM images
demonstrate that FeNiP/NCH retains capsular morphology
with larger openings compared with capsular-MOF (Figure
3c,d). As a secondary nitrogen and carbon source, coexistence of
melamine results in the growth of numerous carbon nanotubes
(CNTs) on the surface of capsular carbons,47,48 which are
successfully connected with each other to form networks
(Figures S36 and S37). HRTEM, SAED, and HAADF-STEM
images and elemental maps of FeNiP/NCH (Figures 3f, S37,
and S38) reveal that Fe, Ni, and P elements are uniformly
distributed over the entire capsule. Obviously, by virtue of
spatially homogeneous separation of Fe(III) and Ni(II) ions in
the thin walls of precursor capsular nanocrystals, the NiP2 and
Fe2P nanoparticles are homogeneously distributed over the
entire capsule nanostructures with extremely small sizes.
Compared with FeNiP/NCH, as suggested by PXRD and
SAED, besides larger nanoparticles of NiP2, Fe2P, and
Fe0.64Ni0.36 alloy is found in FeNiP/NCS and FeNiP/C (Figures
S39−S43), which can be possibly attributed to the partial
phosphorization of large nanoparticles of Fe0.64Ni0.36 alloy in
FeNiP/NCS and FeNiP/C. Their chemical status and elemental
composition are further studied by X-ray photoelectron
spectroscopic (XPS) analysis. A survey scan of FeNiP/NCH
reveals the presence of C, O, N, P, Fe, and Ni (Figure S44). The
high-resolution spectrum of N 1s can be divided into three peaks
at 400.9, 399.9, and 398.7 eV, corresponding to graphitic,
pyrrolic, and pyridinic-N, respectively. Significantly, the signal
peak of P 2p (133.5 eV) can be definitely detected, and similar
results are obtained in FeNiP/NCS and FeNiP/C, demonstrat-
ing their successful phosphorization (Figure S45). Notably, only
in XPS of FeNiP/NCS and FeNiP/C, signal peaks of metallic Fe
2p3/2 (707.0 eV) and Ni 2p3/2 (852.7 eV) can be detected
(Figure S46), in good agreement with PXRD, HRTEM, and
SAED. N2 sorption measurements reveal that FeNiP/NCH
shows a high BET surface area of 142 m2·g−1 and pore volume of
0.549 cm3·g−1, larger than those of FeNiP/NCS (118 m2·g−1,
0.367 cm3·g−1) and FeNiP/C (79 m2·g−1, 0.226 cm3·g−1)
(Figure S47a and Table S2). Pore size distribution curves
(Figure S47b) illustrate that FeNiP/NCH possesses obviously
mesoporous structures (∼10 nm). Moreover, as also suggested
by TEM (Figures 3d, S37, S41, and S42), FeNiP/NCH, FeNiP/
NCS, and FeNiP/C have rich macropores. These unique
hierarchical pores will be significantly beneficial to mass and
charge transport for electrocatalysis.
Oxygen Evolution Reaction. The above combined
analytical measurements show that FeNiP/NCH possesses a
capsular carbon-based framework and abundant Fe2P and NiP2
nanoparticles, making it a promising electrocatalyst for oxygen
evolution reaction (OER), which serves as an essential reaction
for water electrolysis devices and rechargeable metal-air
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Figure 4. (a) OER polarization and (b) corresponding Tafel curves of FeNiP/NCH and control samples in 1 M KOH on glassy carbon electrode
(GCE) with a scan rate of 10 mV·s−1. (c) Faradaic efficiency testing of FeNiP/NCH for OER using RRDE measurements (shown as an inset); ring
current curve on a RRDE (1500 rpm) in Ar-saturated 1 M KOH solution (ring potential: 0.40 V) when a constant disk current is applied. (d) Time-
dependent potential curve for FeNiP/NCH at a constant current density of 10 mA cm−2. Inset: LSV curves before and after the stability test.
batteries technologies. As shown in Figure 4a, FeNiP/NCH
exhibits the lowest overpotential of 250 mV at 10 mA cm−2,
superior to those of FeNiP/C (293 mV), FeNiP/NCS (314
mV), IrO2 (387 mV), and even among all reported non-noble
metal phosphide catalysts on GCE (glassy carbon electrode)
(Figures S48 and S49 and Tables S7 and S8). Additionally, the
Tafel slope (68 mV dec−1) of FeNiP/NCH is smaller than those
of FeNiP/C (89 mV dec−1) FeNiP/NCS (107 mV dec−1) and
IrO2 (117 mV dec−1), signifying its superior reaction kinetics
(Figure 4b). Furthermore, the FeNi/NCH offers active sites 1.8
and 1.9 times that of FeNiP/C and FeNiP/NCS, respectively,
and accordingly, the TOF of FeNiP/NCH (0.25 s−1 at 1.53 V) is
remarkably larger than those of FeNiP/C (0.15 s−1) and FeNiP/
NCS (0.084 s−1) (Figure S50 and Table S9), which is almost the
lowest value reported for bimetallic (phosphides) catalysts, only
behind one known NiCo-MOF nanosheet electrocatalyst
(Figures S48 and S49 and Tables S7 and S8).49
To ensure that the observed oxidation current was really from
oxygen evolution, we used a rotating ring disk electrode (RRDE)
method to detect electron-transfer numbers and Faradaic
efficiency during OER processes.50,51 First, when oxygen
generated on the disk electrode and the Pt ring electrode was
held at the peroxide oxidation potential of 1.50 V, a negligible
current was detected on the Pt ring (μA scale) (Figure S51),
which is 3 orders of magnitude lower than the disk current (mA
scale), indicating no production of peroxide. Then the
associated Faradaic efficiency was investigated by maintaining
a ring potential of 0.4 V for continuous reduction of oxygen
molecules generated and diffused from the disk electrode. As a
result, when a constant disk current (325 μA) was applied for
OER, a ring current of ∼68 μA was detected (collection
efficiency 0.42) (Figure 4c), which actually verified that the
observed catalytic current of FeNiP/NCH originated exclusively
from OER with a high Faradaic efficiency of 99.5%.
Strong durability for electrocatalysts is of great importance for
energy conversion and storage systems. As depicted in the inset
of Figure 4d, a negligible decay can be observed by comparing
the LSV curves before and after 5000 cycles. Subsequently, its
catalytic activities could be well-retained for another 72 h
(Figure 4d). To determine the possible origins of such an
extraordinary performance, it is essential to further elucidate
structure and chemical composition for post-OER sample. As
shown in Figure S52, the morphology of FeNiP/NCH basically
remains after the OER test. PXRD confirms that a mixture of
iron oxyhydroxides (FeOOH) and nickel hydroxide hydrate
(Ni(OH)2·2/3H2O) evolves from Fe2P and NiP2 after OER
testing, respectively (Figure 5a), which are verified by XPS
analysis, judging from the disappearance of P and reservation of
Fe and Ni signals (Figure S53).52 Considering that iron
oxyhydroxides and nickel hydroxide hydrate should be
simultaneously in situ generated from the mixture of Fe2P and
NiP2, hybridization with each other at a trace amount level is
hardly avoided as reported previously.53 As a result, these
introduced impurities have been experimentally proven to
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Figure 5. (a) HER polarization of FeNiP/NCH and control samples in 1 M KOH with a scan rate of 10 mV·s−1. Inset: corresponding Tafel curves. (b)
Polarization curve of FeNiP/NCH||FeNiP/NCH electrode in a two-electrode configuration for water splitting. Inset: representation of water-splitting
device. (c) Catalytic stability at different current densities for around 40 h. Inset: the enlarged region of the curves for 100 mA cm−2. Notably, in each
increment of current density, the voltage could increase accordingly and then stabilize rapidly, whereafter, the cell voltage was also completely restored
when the current density finally decreased to 10 mA·cm−2. (d) ORR polarization of FeNiP/NCH and control samples in O2-saturated 0.1 M KOH
solution with RDE rotation rate of 1600 rpm. (e) Representation of Zn−air battery (GDF: gas diffusion film). (f) Battery voltage and power density of
Zn−air batteries. (g) Galvanostatic discharge−charge cycling curves of rechargeable Zn−air batteries with FeNiP/NCH and Pt/C catalysts on Ni
foam, respectively.
greatly boost the catalytic activity,54−57 which is responsible for
the superior catalytic performance toward water oxidation.
In addition, the electrochemically active surface area is
another important contributor to facilitate the catalytic activity
of OER catalysts. Thus, the electrochemical surface areas
(ECSAs) of FeNiP/NCH catalyst were investigated by
measuring the double-layer capacitance (Cdl) (Figure S54).
The Cdl of FeNiP/NCH catalyst (8.1 mF·cm−2) is much higher
than those of the FeNiP/C (5.0 mF cm−2) and FeNiP/NCS (3.6
mF cm−2), suggesting that FeNiP/NCH has the largest ECSA
among these materials (Figure S54e). The lower charge-transfer
resistance (Rct, 1.174 Ω) of FeNiP/NCH compared to 2.829 Ω
of FeNiP/C and 3.225 Ω of FeNiP/NCS also suggests that its
unique capsular carbon networks effectively reduce ionic
transport resistance and significantly facilitate the kinetic
electron transfer under OER operations (Table S10).58 In
brief, such an outstanding catalytic performance of FeNiP/NCH
should be attributed to its unique capsular carbon-based
network nanostructure, which endows the material with a
highly electrochemically active surface area and efficient
electron/charge transfer from the electrode.
Hydrogen Evolution and Overall Water Splitting.
Motivated by the outstanding OER performance of FeNiP/
NCH, electrocatalytic hydrogen evolution reaction (HER) was
also investigated. Distinctly, FeNiP/NCH obtains 10 mA cm−2
at a small overpotential of 216 mV, which is superior to the
FeNiP/C (258 mV) and FeNiP/NCS (315 mV) catalysts. The
corresponding Tafel slope of the FeNiP/NCH catalyst (125 mV
dec−1) is also smaller than those of FeNiP/C (144 mV dec−1),
FeNiP/NCS (177 mV dec−1) (Figure 5a), and many other
reported bifunctional catalysts (Table S11), revealing more
favorable electrocatalytic kinetics on the FeNiP/NCH catalyst
for HER. It should be noted that the Tafel slope of FeNiP/NCH
catalyst (125 mV dec−1) suggests that its HER process
undergoes a Volmer−Heyrovesky pathway.59,60 Notably, LSV
scanning for 5000 cycles and long-term chronoamperometry
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(>68 h) measurements demonstrate its good stability during
HER (Figures S55 and S56). Moreover, the structure and
chemical components of FeNiP/NCH can be preserved very
well after HER tests (Figures S57 and S58). Considering the
electrocatalytic activities of FeNiP/NCH for HER and OER
with excellent stabilities, an electrolyzer was assembled using
FeNiP/NCH as both cathode and anode for overall water
splitting. As revealed in LSV (Figures 5b and S59), FeNiP/NCH
exhibits outstanding splitting performance with a small cell
voltage of 1.59 and 1.91 V to achieve 10 and 100 mA cm−2,
respectively, superior to those of many reported non-noble-
metal electrocatalysts (Table S12).61 In addition, the water
electrolyzer durability was well confirmed by carrying out the
reaction at different current densities from 10 to 100 mA cm−2
for over 40 h (Figures 5c and S60), proving the prominent
stability of FeNiP/NCH catalyst for overall water splitting.
Oxygen Reduction and Rechargeable Zn−Air Battery.
The inherent high OER activity and excellent water electrolysis
performance of FeNiP/NCH further prompted us to investigate
its catalysis for oxygen reduction reaction (ORR) and even
assemble it into a rechargeable Zn−air battery device to evaluate
its practical application for energy conversion.39,62,63 As shown
in the LSV curves (Figure 5d), FeNiP/NCH exhibits a higher
positive half-wave potential (0.75 V vs RHE, reverse hydrogen
electrode64) and larger diffusion-limiting current density (5.02
mA cm−2, close to that of Pt/C) than those of FeNiP/C (0.71 V,
3.22 mA cm−2) and FeNiP/NCS (0.73 V, 3.06 mA cm−2) at 0.6
V (Figures S61 and S67). Moreover, the structure and chemical
components of FeNiP/NCH can also be preserved very well
after ORR tests (Figures S57 and S58). In addition, the ECSAs
of FeNiP/NCH catalyst in 0.1 M KOH were also investigated by
Cdl (Figure S68). The Cdl of FeNiP/NCH catalyst (8.6 mF·
cm−2) is much higher than those of the FeNiP/C (4.2 mF cm−2)
and FeNiP/NCS (6.0 mF cm−2), indicating that the unique
structure of FeNiP/NCH has a great influence on the
electrochemically active surface areas. Actually, FeNiP/NCH
can serve as one of the best ORR electrocatalysts among various
metal phosphides, being comparable with other reported bi-/
trifunctional catalysts (Table S12). Considering the prominent
catalytic activities of FeNiP/NCH for OER and ORR, a
homemade Zn−air battery is assembled to further evaluate the
performance under real operation conditions (Figure 5e).
Entertainingly, this battery can work stably with an open-circuit
voltage (OCV) of 1.48 V for more than 25 h (1.38 V for Pt/C)
(Figure S69).65,66 Besides this, discharge polarization curve and
peak power density of single battery clearly manifest that the
FeNiP/NCH catalyst exhibits an extraordinary high power
density (250 mW cm−2) that surpasses 131 mW cm−2 of Pt/C-
based battery and most of other recently reported state-of-the art
electrode materials (Figure 5f and Table S13).67,68 The charge/
discharge potentials of the Zn−air battery based on FeNiP/
NCH were 1.89 and 1.23 V with a small voltage gap of 0.66 V
when cycling at 5 mA cm−2, far better than the charge/discharge
potentials of Pt/C (Figure 5g). More significantly, it displays
remarkable stability with no obvious voltage gap change over
2100 cycles during 500 h when operating at different current
densities. Obviously, FeNiP/NCH performs far better than Pt/
C including not only the peak power density of discharge but
also the durability of operation. More impressively, two primary
Zn−air batteries based on FeNiP/NCH connected in series can
provide a working voltage of more than 2.95 V, which is
expectedly sufficient to power the above water electrolyzer
assembled by FeNiP/NCH (Figure S70).
CONCLUSION
■
In summary, a new single-crystal capsular-MOF with openings
on the wall is carefully designed at the molecular level and
constructed from a rare crystal-structure transformation from a
well-known three-dimensional MOF for the first time.
Interestingly, the open-capsular MOF can be used as a nanoscale
storage material for sulfur and iodine (cathodes for lithium
batteries). Open-capsular MOF can also be upgraded to a
capsular carbon-based network, which exhibits efficient multi-
functional electrocatalysis for oxygen evolution, hydrogen
evolution, and oxygen reduction, achieving well-qualified
assemblies of an overall water splitting and a rechargeable
Zn−air battery, which affords remarkably practical prospects
over previously reported electrocatalysts. The open-capsule
nanostructures have been demonstrated to be vastly beneficial
for easily loading I2 and S into an MOF capsule and full exposure
of the inner surfaces during catalysis with the carbon-based
capsule, which could greatly endow microcapsules with better
performance than traditional nonhollow nanomaterials includ-
ing nanosized MOFs and sheetlike crystals. Considering the rich
diversities of the reported ligands and MOFs with open metal
sites,69,70 our strategy will be broadly applicable for design and
construction of various novel MOF capsules. The synthetic
strategies also point out a new way for preparing hierarchical
carbon-based nanostructures with adjustable morphologies and
compositions.
EXPERIMENTAL SECTION
■
Synthesis of FeNi-MIL-88B. A mixture of iron(III) chloride
hexahydrate (FeCl3·6H2O, 3.6 g, 13.3 mmol), nickel(II) nitrate
hexahydrate (Ni(NO3)2·6H2O, 2.0 g, 6.7 mmol), and 2-amino-
terephthalic acid (H2bdc-NH2, 3.6 g, 20 mmol) was dissolved in
DMF (200 mL) and sonicated for 20 min. Subsequently, aqueous
sodium hydroxide solution (8 mL, 1 M) was dropwise added into the
above solution under stirring. The resulted solution was sonicated for
40 min and then transferred into a 500 mL vial sealed with a screw cap.
The mixture was heated at 100 °C for 12 h. After that, brown solid was
collected by centrifuge and washed by DMF and methanol (MeOH)
several times. The obtained sample was dried in the air at room
temperature overnight (yield 4.2 g, ∼80%). Large single crystals for Fe-
MIL-88B: a mixture of FeCl3·6H2O (2.7 g, 10 mmol), H2bdc-NH2 (1.8
g, 10 mmol), hydrofluoric acid (3 mL), and DMF (100 mL) was stirred
for 30 min, transferred, and sealed into a 500 mL vial with a screw cap,
which was heated at 100 °C for 3 days, finally giving orange crystals of
Fe-MIL-88B ([Fe3(μ3-O)(bdc-NH2)3X(H2O)2], X = OH−/Cl−/F−).
Synthesis of Capsular-MOF. A mixture of FeNi-MIL-88B (200
mg) and tpy (100 mg) was transferred into 10 mL of DMF using a 25
mL Teflon-lined autoclave, which was heated at 190 °C for 14 h. After
being cooled to room temperature, the brown solid was collected by
centrifuge and washed with DMF and MeOH several times. Finally, the
sample was dried under vacuum at 100 °C for 4 h (yield 200 mg,
∼67%). Large single crystals for Fe-MIL-88B-tpy: a mixture of FeCl3·
6H2O (27 mg, 0.1 mmol), H2bdc-NH2 (18 mg, 10 mmol), tpy (10 mg,
0.033 mmol), hydrofluoric acid (10 μL), and DMF (1 mL) was stirred
in a 9 mL vial for 10 min and then transferred and sealed into a 25 mL
Teflon-lined autoclave, which was heated at 140 °C for 3 days, finally
giving orange crystals of Fe-MIL-88B-tpy ([Fe3(μ3-O)(bdc-
NH2)3(tpy)]X, X = OH−/Cl−/F−).
Synthesis of Solid-MOF. This compound was prepared by a
procedure similar to that for capsular-MOF, except that FeNi-MIL-88B
was replaced by a mixture of FeCl3·6H2O (108 mg, 0.4 mmol),
Ni(NO3)2·6H2O (60 mg, 0.2 mmol), and H2bdc-NH2 (108 mg, 0.6
mmol). Brown solid was dried under vacuum at 120 °C for 4 h (yield
150 mg, ∼70%).
Loading Sulfur into MOFs. Degassed MOF crystals (100 mg)
were first mixed with sulfur (100, 200, and 300 mg) by hand-milling for
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30 min and then transferred into Teflon-lined autoclaves. These
autoclaves were placed into an oven and heated at 135 °C for 12 h. After
that, the obtained samples were collected and analyzed by TG.
Iodine Vapor Adsorption. Degassed MOF crystals (50 mg) in a
small beaker and iodine (1.5 g) were placed in a Teflon-lined autoclave
and then transferred into an oven heated at 100 °C for 2.5 h. After that,
the obtained samples were collected and analyzed by TG.
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Ariga, K. Bioactive Flake−Shell Capsules: Soft Silica Nanoparticles for
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W.; Wang, X. Zirconium−Porphyrin-Based Metal−Organic Frame-
work Hollow Nanotubes for Immobilization of Noble-Metal Single
Atoms. Angew. Chem., Int. Ed. 2018, 57, 3493.
Single-Crystal X-ray Diffraction and Structure Analysis.
SCXRD measurements were carried out using a Rigaku AFC10
diffractometer with a Rigaku PILATUS 200 K system equipped with a
MicroMax-007 HF/Varimax rotating-anode X-ray generator with
confocal monochromated Mo Kα radiation. Crystal structures were
solved by the direct method and refined by full matrix least-squares
refinement using SHELXTL-2013 package. Fourier difference maps (F0
− Fc) were computed with the WinGX program system. Detailed
structure refinement parameters and crystallographic data are given in
Table S4.
Fabrication of a Zn−Air Battery. The rechargeable Zn−air
battery test was conducted in a home-built electrochemical cell where
FeNiP/NCH catalysts were loaded on Ni foam (1 mg cm−2) as the air
cathode and Zn plate as anode in 6.0 M KOH. For the rechargeable
zinc−air batteries, 6.0 M KOH and 0.2 M ZnCl2 were employed as the
electrolyte to ensure reversible Zn electrochemical reactions at the Zn
plate. During discharge−charge cycling tests, the electrolyte was
replenished once a day. Measurements were carried out at room
temperature with a CHI 708EY electrochemical workstation and a
Hokuto Denko HJ1001SD8C charge−discharge testing system.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b02417.
■
Organic Frameworks. Chem. Rev. 2012, 112, 673.
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Gas Adsorption Transitions. Nature 2016, 532, 348.
Experimental procedures, SCXRD, PXRD, IR, TGA, XPS,
and additional electrocatalysis measurements (PDF)
X-ray data for Fe-MIL-88B (CIF)
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X-ray data for Fe-MIL-88B-tpy (CIF)
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Geometry of Periodic Knots, Polycatenanes and Weaving from a
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AUTHOR INFORMATION
Corresponding Author
*q.xu@aist.go.jp
ORCID
Mitsunori Kitta: 0000-0001-9800-7371
Zheng Liu: 0000-0001-9095-7647
Satoshi Horike: 0000-0001-8530-6364
Qiang Xu: 0000-0001-5385-9650
■
Notes
The authors declare no competing financial interest.
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W.; Wang, X. Zirconium−Porphyrin-Based Metal−Organic Frame-
work Hollow Nanotubes for Immobilization of Noble-Metal Single
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Zhao, Z.; Tsung, C.-K. Yolk−Shell Nanocrystal@ZIF-8 Nanostructures
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ACKNOWLEDGMENTS
■
We acknowledge the reviewers for valuable suggestions and
AIST for financial support. We thank Ms. Nanae Shimanaka of
KUIAS-iCeMS, Kyoto University, for single-crystal X-ray
diffraction measurements.
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