A Porphyrinic Zirconium Metal-Organic Framework for Oxygen Reduction Reaction: Tailoring the Spacing between Active-Sites through Chain-Based Inorganic Building Units

Article abstract of DOI:10.1021/jacs.0c06329

The oxygen reduction reaction (ORR) is central in carbon-neutral energy devices. While platinum group materials have shown high activities for ORR, their practical uses are hampered by concerns over deactivation, slow kinetics, exorbitant cost, and scarce

Full text of DOI:10.1021/jacs.0c06329

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Article
A Porphyrinic Zirconium Metal-Organic Framework for
Oxygen Reduction Reaction: Tailoring the Spacing between
Active-Sites through Chain-Based Inorganic Building Units
Magdalena Ola Cichocka, Zuozhong Liang, Dawei Feng, Seoin Back, Samira
Siahrostami, Xia Wang, Laura Samperisi, Yujia Sun, Hongyi Xu, Niklas
Hedin, Haoquan Zheng, Xiaodong Zou, Hong-Cai Zhou, and Zhehao Huang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c06329 • Publication Date (Web): 10 Aug 2020
Downloaded from pubs.acs.org on August 10, 2020
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A Porphyrinic Zirconium Metal-Organic Framework for
Oxygen Reduction Reaction: Tailoring the Spacing between
Active-Sites through Chain-based Inorganic Building Units
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Magdalena Ola Cichocka,‡,† Zuozhong Liang,#,† Dawei Feng,§, Seoin Back,◦ Samira Siahrostami,⊥ Xia
Wang,‡ Laura Samperisi,‡ Yujia Sun,§ Hongyi Xu,‡ Niklas Hedin,‡ Haoquan Zheng,*,# Xiaodong Zou,‡
Hong-Cai Zhou*,§,∥ and Zhehao Huang*,‡
‡Department of Materials and Environmental Chemistry, Stockholm University, Stockholm SE-106 91, Sweden
§Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
#Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and
Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China.
◦Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107, Republic of Korea
⊥Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N1N4, Canada
∥Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003,
United States
ABSTRACT:
The oxygen reduction reaction (ORR) is central in carbon-neutral energy devices. While platinum
group materials have shown high activities for ORR, their practical uses are hampered by concerns over
deactivation, slow kinetics, exorbitant cost, and scarce nature reserve. The low-cost yet high tunablility
of metal-organic frameworks (MOFs) provide a unique platform for tailoring their characteristic
properties as new electrocatalysts. Herein, we report a new concept of design and present stable Zr-
chain-based MOFs as efficient electrocatalysts for ORR. The strategy is based on using Zr-chains to
promote high chemical and redox stability, and more importantly, tailor the immobilization and packing
of redox active-sites at an ideal density to improve the reaction kinetics. The obtained new
electrocatalyst, PCN-226, thereby shows high ORR activity. We further demonstrate PCN-226 as a
promising electrode material for practical applications in rechargeable Zn-air batteries, with a high peak
power density of 133 mW cm−2. Being one of the very few electrocatalytic MOFs for ORR, this work
provides a new concept by designing chain-based structures to enrich the diversity of efficient
electrocatalysts and MOFs.
INTRODUCTION
The increasing global demands for sustainable energy simultaneously drive the development of new
devices for energy conversion. Oxygen reduction reaction (ORR) plays a crucial role in such type of
devices including fuel cells and metal-air batteries.1,2 Platinum group materials are the predominate
class of materials that are used as electrocatalysts for ORR.3 However, their scarcity, high cost, low
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reaction kinetics, and stability prevent their widespread implementation in commercial green energy
applications. Thus, driven by the crucial importance of the ORR in energy conversion processes,
numerous systems based on non-precious elements have been developed in the search for low-cost and
stable candidates with good electrocatalytic activities.4–8 In addition, high surface area scaffolds are
paramount in the development of electrocatalysts for their implementations in various practical devices
concerning deactivation, recyclability, and water solubility.
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Metal-organic frameworks (MOFs) are porous inorganic-organic hybrid materials with extraordinary
high surface areas9,10 With high accessibility to the active-sites, MOFs take the dual roles acting as both
catalysts and porous supports in heterogeneous catalysis. Notably, immobilizing catalytic sites in the
well-defined structures, it can effectively limit deactivation pathways.11,12 An important advantage of
MOFs compared to carbon and metal oxides based materials is the possibility to easily and
systematically design and alter the pores size and functionality at a molecular level by applying reticular
chemistry.13 Compared to noble metals, which are mostly non-porous, the fast mass transport inside
crystals, as well as their low cost endow MOFs as a promising class of materials in the energy storage
and conversion field.14–20 To further tackle their charge transport ability, MOFs have been designed
either by orbital overlapping and charge delocalization between linkers,21–26 or by introducing redox-
active molecular linkers27–29. Metalloporphyrin complexes exhibit strong interactions with O2, and offer
a good opportunity by serving as a channel for electron transfer via a hopping mechanism.30–32
Moreover, the rich chemistry and the high electrochemical stability of metalloporphyrin complexes
endow porphyrinic MOFs as designable and durable electrocatalysts. Recently, such MOFs have been
successfully utilized for the electrocatalytic hydrogen evolution reaction (HER),33,34 CO2 reduction
reaction (CO2RR),35–37 oxygen evolution reaction (OER),38–40 and oxygen reduction reaction (ORR),41–
44 etc. By efficiently limiting the deactivation pathways related to the electrocatalytic sites, these MOFs
show electrochemical activities and relatively stable cycling. However, the structural and chemical
stabilities of MOFs, and more importantly, the sparse active-sites in most studied MOFs are the major
drawbacks, which have rendered such MOFs with limited redox capabilities.
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Herein, we report the design of a new MOF, denoted PCN-226, which is built by linking redox active
metalloporphyrins and Zr(+4) cations. Because of using earth abundant elements, it significantly
reduces the producing cost compared to noble metal-based materials. Moreover, according to the Hard
Soft Acid Base (HSAB) theory, Zr(+4) cations have strong affinity with carboxyl-functionalized
metalloporphyrins, and are able to produce stable MOFs.45 In addition to the stable, accessible and
redox active-sites for ORR, the MOF was designed uniquely based on an infinitive zigzag Zr-oxide
chain structure. The chain-based structure not only enhances the overall stability of the MOF but also
allows the redox active-sites at a close space to give rise to high reaction kinetics and catalyst loadings.
The density of catalytically active-sites of PCN-226 is the highest among the reported porphyrinic Zr-
MOFs. We prepared PCN-226 nanocrystals to improve the diffusion kinetics of adsorbed and reacting
molecules. The structure of PCN-226 was determined by continuous rotation electron diffraction
(cRED)46,47 or microcrystal electron diffraction (MicroED)48. As an electrocatalyst for ORR, PCN-226
showed high activity and reaction kinetics, especially compared to cluster-based MOFs. When applied
as electrodes for practical application in a rechargeable Zn-air battery, PCN-226 exhibited a high peak
power density of 133 mW cm−2. The integrity of PCN-226 remained intact after ORR.
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RESULTS AND DISCUSSION
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Synthesis and Characterization. The Zr-chain building units have firstly been reported in the MIL-140
series.49 Owning to the high connectivity of Zr and robustness of the oxide chain unit, these structures
show high mechanical, thermal, and hydrothermal stability compared to the UiO series Zr-MOFs based
on the Zr6-oxo cluster.49 Yet, such Zr-oxide chain structure has been found so far only in MIL-140 and
MIL-163 and their derivatives,49,50 presumably due to the lack of rationale for synthesis of such building
unit. In particular, in the reported porphyrinic Zr-MOFs, the Zr6-oxo cluster (ZrO4(OH)4) is dominating
and adopts versatile connecting modes and yields different structures with outstanding stabilities.51–58
Due to the large size of the porphyrnic linker, the metalloporphyrin centers in these structures are
isolated by the Zr6-oxo nodes, preventing collaborative catalytic processes in the frameworks. To avoid
the formation of the Zr6-oxo phases, we herein adopt a thermodynamically controlled synthesis to
enforce the formation of the Zr-oxide chain structure. Specifically, we conducted the reaction at a higher
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temperature (140 C) than that in a typical Zr-MOF synthesis and added a high excess of benzoic acid
(BA, BA/Zr = 67, molar ratio) as a competing reagent. As the BA can competitively coordinate to Zr
atoms to control both the kinetics and thermodynamics of the Zr-MOF formation (Scheme S2), a large
amount of BA can inhibit the formation of Zr6-oxo containing structures with lower thermodynamic
stability. Meanwhile, higher reaction temperatures can facilitate the formation of Zr chains by
overcoming the associated higher energetic barrier, as compared to the formation of kinetically favored
Zr6 clusters in typical Zr-MOFs. As expected, we obtained a Zr-oxide chain-based framework PCN-226.
By applying the same strategy, isostructural MOFs with uncoordinated porphyrin and different
metalloporphyrins, containing ions of metals such as Cu, Co, Ni, Fe, and Zn, have been successfully
obtained, creating a new series of ultra-stable porphyrinic Zr-MOFs.
The resulting MOF, PCN-226(Cu), was characterized by powder X-ray diffraction (PXRD), which
shows the high crystallinity of the MOF with intense and sharp peaks (Figure 1a). Moreover, the
features in the PXRD patterns (Figure S1) show that a series of isostructural PCN-226s could be
synthesized with the same structure and topology. Scanning electron microscopy (SEM) images show a
plank-like crystal morphology of the MOF with 1-20 μm in length and ca. 500 nm in thickness (Figure
S2). Continuous rotation electron diffraction (cRED) was applied for ab initio structure determination of
PCN-226(Cu) and PCN-226(Co), respectively. The unit cell parameters obtained from the cRED data
were refined against PXRD data by using the Pawley method, and the structural models were refined
against cRED data (Figures 1b-g, S3, S4, and Table S1-3, see Supporting Information for more details).
The refinement results further confirm that PCN-226s have the same structure. As expected from our
synthetic strategy, the three-dimensional (3D) structure is constructed by linking infinite zigzag ZrO7
chains with metalloporphyrin molecules (Figure 2). Interestingly, besides the carboxylates from
metalloporphyrin, BAs also connected to the Zr-oxo chains (Figure S5). This connection indicates the
importance of using excessive BAs in the synthesis, where BAs inhibit the formation of Zr6-oxo clusters
and favor the formation of Zr-oxide chains by coordinating to the Zr(+4) cations. To the best of our
knowledge, the unique chain structure endows PCN-226 with a novel topology that has never been
reported. The metal loadings in the porphyrins of PCN-226(Cu) and PCN-226(Co) were 79.4%, and
87.2%, respectively, which was calculated from inductively coupled plasma optical emission
spectroscopy (ICP-OES) (Table S4).
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Figure 1. (a) Pawley fit of the experimental PXRD pattern (λ = 1.5406 Å) of PCN-226, showing a good
agreement. (b and d) Reconstructed 3D reciprocal lattice of PCN-226(Cu) and PCN-226(Co), respectively. (c and
e) The particles from which the cRED data was collected. (f and g) The hk0 reciprocal planes of PCN-226(Cu)
and PCN-226(Co), respectively, showing a similar intensity distribution. (h) HRTEM image of PCN-226(Cu)
along the [010] direction that shows a rectangular pore packing with d200 = 18.32 Å and d002 = 16.18 Å (calculated
d200 = 18.53 Å and d002 = 16.25 Å). Inset: Fourier transform of the image. (i) Symmetry imposed HRTEM image
from the region highlighted by the red square in h). The structural model of PCN-226(Cu) is superimposed in the
image showing the good agreement.
PCN-226 contains two topologically distinct yet interconnected channels along the [010] and [001]
directions, respectively (Figures 2c, 2d and S6). The chain-based structure effectively limits the spacing
and results in a high packing density of metalloporphyrins, where the electrochemical active-sites are
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loaded. Based on the crystal structures, the packing density of PCN-226 is 1.4 – 11.0 times higher
compared to other reported porphyrinic Zr-MOFs (Table 1). HRTEM images clearly show the
rectangular pore arrangement (Figures 1h and i), which is in good agreement with the structural model.
The permanent porosity of PCN-226 was further estimated by analysis of the N2 sorption isotherm at 77
K. A N2 uptake of 10.1 mmol g−1 and a Brunauer-Emmett-Teller (BET) surface area of 800 m2 g−1 were
calculated for PCN-226(Cu) (Figure S7). The experimental surface area was slightly larger than the
theoretical value of PCN-226(Cu) of 621 m2 g−1 based on the structural model, indicating the formation
of possible defects with missing linkers. Simulation from the N2 adsorption isotherm based on non-local
density functional theory (NLDFT) gave a pore size distribution in the range of 5.3 – 7.8 Å. Other PCN-
226 MOFs with different metalloporphyrins show similar N2 uptake, surface area, and pore size
distribution (Figures S8-S12).
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Figure 2. (a) PCN-226 is formed by the linkage of infinitive zigzag zirconium chains and TCPP. The infinitive
zigzag zirconium chains have the composition [ZrO(-COO)2]∞, with Zr atoms being hepta-coordinated. (b) The
connection of PCN-226 showing a new topology, with Schlafli symbol {4.82}{42.6.82.10} for the net. (c) The
crystal structure of PCN-226 viewed along the b-axis showing the pore opening 7.2 Å × 4.8 Å, and (d) the crystal
structure viewed along the c-axis showing the 5.4 Å × 4.2 Å opening pores. Cyan capped octahedral: Zr; red
spheres: O; black spheres: C, blue spheres: N; orange spheres: Cu.
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Table 1. Comparison of the density of catalytically active-sites of PCN-226 with most reported porphyrinic Zr-MOFs.
Zr-metallo-
porphyrin MOF
PCN-226
PCN-221
MOF-525
PCN-222
MOF-545
PCN-223
PCN-224
PCN-225
PCN-228
PCN-229
PCN-230
MMPF-6
Density (108 porphyrinic
Building unit
Topology
Reference
metal sites μm−3)[b]
ZrO7 zigzag chain
Zr8(μ4-O)6 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-OH)8 cluster
Zr6(μ3-O)8 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)4(μ3-OH)4 cluster
Zr6(μ3-O)8 cluster
New[a]
ftw
ftw
csq
csq
shp
she
sqc
ftw
ftw
ftw
csq
5.91
4.04
4.11
2.29
2.28
4.47
2.10
3.72
1.84
1.37
0.53
2.28
This work
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[a]The new topology of PCN-226 with Schlafli symbol {4.82}{42.6.82.10} for the net.
[b]The density was calculated based on the structural models.
Compared to the isolated clusters, the highly connected chain structures provide enhanced stability by
resisting the attack of water and other guest species.49 We tested chemical stabilities of PCN-226 using
reported procedures in the MOF field.18,49 It exhibited remarkable stabilities of PCN-226 in aqueous
solutions in a wide pH range (1-13) for at least 7 days (Figures S13 and S14), as a result of the
combination of chain structure and strong Zr(+4)−O bonding. PCN-226 showed very little leaching (<
0.97%) in acidic and basic conditions, as calculated from the UV-Vis spectra of PCN-226 measured at
different pH values (pH 1-3 and 11-13) for 7 days (Figures S15-S18). Thermogravimetric analysis
o
(TGA) showed PCN-226(Cu) was stable up to at least 400 C (Figure S19). After the first weight loss
before 100 oC due to the removal of guest molecules, only ~2% of the total framework weight was lost
o
before the ligands starting combustion at above 400 C. This shows the chain-containing PCN-226 is
much more stable than the cluster-containing Zr-MOFs, which undergo dehydroxylation at a much
lower temperature (~250 oC).29,45
Electrocatalytic ORR of PCN-226.
PCN-226 provides an ideal example for designing MOF electrocatalysts through chain-based structures,
which could improve the chemical stability as well as increase the packing density. A close packing
could further enhance the overall charge mobility via hopping mechanism.30–32 As a proof of concept,
the electrochemical ORR catalyzed by PCN-226 was tested.
The isostructural PCN-226 series displayed excellent electroactivities towards ORR, which were
assessed by cyclic voltammogram (CV) and linear sweep voltammetry (LSV) measurements (Figures
S20, S21 and Table S5). As a 3D porous material, PCN-226 benefits from limited deactivation pathways
compared to molecular catalysts (Figure S22). Given the best activity, PCN-226(Co) showed a strong
reversible redox wave at 0.7 V (vs reversible hydrogen electrode, RHE, Figure 3a). The LSV curve of
PCN-226(Co) (Figure 3b) exhibited an onset potential of 0.83 V (Eonset) and half-wave potential of 0.75
V (E1/2), while the values of pure carbon black were 0.70 and 0.62 V, respectively (Figure S23).
Meanwhile, PCN-226(H2) shows a very low activity with E1/2 of 0.6 V (vs RHE) (Figure S24). This
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result highlights the importance of the metal cations in the porphyrin as they serve as the catalytic active
sites. Notably, the potential can be further enhanced by increasing the catalyst loading (Figure S25).
Due to the Zr-chain structure, the density of active metal sites in PCN-226 is so far the highest among
reported porphyrinic Zr-MOFs, and it is 144% and 358% denser than the well-known PCN-221/MOF-
525, and PCN-222/MOF-545, respectively (Table 1). The close packing increases the active-sites
density as well as provides a better pathway for electron hopping. As a consequence, PCN-226 showed
a better overall ORR activity than did by cluster-based MOFs PCN-221 (E1/2 = 0.70 V vs. RHE) and
PCN-222 (E1/2 = 0.69 V vs. RHE) (Figures 3b, and Table S6). In addition, PCN-226 outperformed other
commonly reported MOFs as ORR electrocatalysts, such as the MIL-88(Fe), ZIF-67(Co), MOF-5(Ni),
and ZIF-8(Zn) (Figure S26).
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Figure 3. (a) CV curves of PCN-226(Co) in 0.1 M KOH solution saturated by N2 and O2, respectively. (b) LSV
curves, (c) electron transfer numbers (n), and (d) Tafel plots of PCN-226(Co), PCN-221(Co), PCN-222(Co) and
Pt/C. (e) Durability test at 0.46 V (vs. RHE) and 1600 rpm in O2-saturated 0.1 M KOH solution, j0 is the initial
current. It shows PCN-226(Co) has the lowest current loss (18%), followed by Pt/C (20%), PCN-221(Co)
(48%), and PCN-222(Co) (50%). The MOFs were mixed with 50% carbon black.
Electrocatalytic ORR can be carried out in an alkaline solution through four electron (4e−) and two
electron (2e−) pathways. Notably, the 4e− selectivity provides higher reaction kinetics and can prolong
the durability of the cell. The electron transfer number (n) of PCN-226(Co) was calculated as ~3.3,
which indicated a mixed 4e− and 2e− pathways. Compared with the n number of Zr-cluster based PCN-
221 (2.6) and PCN-222 (2.3), as well as pure carbon black (2.1), PCN-226(Co) offers a better reaction
pathway towards the reduction of O2 to OH− (Figures 3c and S27, see Supporting Information for
more details). Although the n number of PCN-226(Co) was slightly lower than that (3.9) observed for
Pt/C, PCN-226(Co) provided a faster reaction kinetics as shown by the Tafel slopes (Figure 3d). As
expected from the close spacing of active-sites, the reaction kinetics of PCN-226 was the fastest among
the porphyrinic Zr-MOFs we tested (Figure 3d, see Supporting Information). The calculated turnover
frequency (TOF) of PCN-226(Co) at 0.2 V (vs RHE) is 0.17 e site−1 s−1, assuming all the Co atoms on
the electrode are available. This TOF is comparable with that of Pt/C (0.18 e site−1 s−1) and other
reported materials (Table S7). Furthermore, by limiting deactivation pathways, PCN-226(Co) endowed
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a superior tolerance to methanol crossover (Figure S28). PCN-226(Co) exhibited a high durability with
the lowest current loss of 18%, especially compared to the other cluster-based MOFs (Figures 3e and
S29). After the durability test, structural and compositional measurements confirmed that PCN-226(Co)
remained intact, without decomposition or generation of metal oxides (Figures S30-S32).
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We further evaluated the OER activity (Figure S33, see Supporting Information for more details), and
used PCN-226(Co) as air electrodes for practical applications in a rechargeable Zn-air battery (Figures
4a, S34 and Movie S1). By coating the catalysts on carbon cloth to further enhance electron transfer,59
the assembled battery exhibited an open-circuit voltage of 1.37 V (Figure S35), and the discharge and
charge voltage were 1.17 and 2.14 V, respectively (Figure 4b). The specific capacity of PCN-226(Co)
was 724 mAh g−1, which is close to 757 mAh g−1 using Pt/C+RuO2 catalysts (Figure 4c). We further
compared the peak power density, which is one of critical metrics for practical applications, with other
electrodes. The peak power density of PCN-226(Co) was 133 mW cm−2, which was comparable to the
150 mW cm−2 observed for Pt/C+RuO2 (Figure 4d), and exceeded those for the previously reported
state-of-the-art noble metal-free electrocatalysts, such as MOFs and MOF derived composites,60–64
carbon materials,11,65–69 and metal oxide composites70–72 (Table S8). Significantly, no noticeable
performance decrease was observed after 550 charge-discharge cycles over 160 h, indicating a high
electrocatalytic durability (Figure 4e).
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Figure 4. (a) Schematic illustration of the Zn–air battery. (b) Discharge and charge cycling curves. (c)
Discharging curves at j = 20 mA cm−2. (d) Polarization curves and corresponding power density. (e) Long-term
durability test of Zn-air batteries at j = 2 mA cm−2 assembled with PCN-226(Co) and commercial Pt/C+RuO2.
The loading of PCN-226(Co) is 6.25 times as it was used in ORR test.
Theoretical calculation.
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As we demonstrated above, a highly dense structure could improve the performance through high
catalyst loading as well as enhanced charge transfer. Single metal atom catalysts, which readily bind to
the reaction intermediates of ORR are an extreme example with infinitive distance between active-sites.
As shown below, tuning the spacing between two single metal atom active-sites becomes extremely
crucial for optimizing both the density of the active-sites and the reaction thermodynamics, which in
turn affects the reaction kinetics. The central key is to engineer the spacing so that the binding energies
of ORR intermediates, i.e., OOH*, O* and OH* are neither too strong nor too weak. This design leads
to the minimum overpotential as well as high intrinsic ORR catalytic activity.73 To demonstrate this
effect, we studied the confinement in the Co-porphyrin sites of PCN-226 using a truncated model with
the same atomic coordination (Figure 5a). PCN-226 has two inherent confined spacings between the
porphyrin units i.e., 4 Å and 7 Å. In addition to the natural spacings in PCN-226, we considered several
other spacings to investigate the effect of confinement on the ORR activity. The calculated adsorption
free energies (△G) of ORR intermediates (O*, OH*, OOH*) at different spacings are displayed in
Figure 5b. The results show that at 4 Å spacing all ORR intermediates are significantly destabilized
indicating that this spacing is less probable to contribute to the observed ORR activity. On the other
hand, all △G values approach those of an ideal catalyst for spacings larger than 6 Å. The ideal spacing
for △GOOH is between 6 Å to 8 Å. Combined with the highly dense chain structure, this spacing explains
the enhanced ORR activity observed in the case of our PCN-226 catalyst with well-defined 7 Å spacing.
We further examined the effect of curvature on the ORR activity by examining a flattened model of the
Co-porphyrins with 7 Å spacing, where we found negligible differences in the △G values of roughly
0.05 eV. The calculation results signify the importance of the spacing of active-sites on the performance
in ORR.
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Figure 5. (a) A truncated molecular model of PCN-226. Color code; brown: Co; blue: N; grey: C; white: H. We
varied the distance between two porphyrin motifs to investigate the effect of the spacing on ORR. (b) Adsorption
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free energy of ORR intermediates as a function of the spacing. The two inherent spacings in PCN-226 are ca. 4
and 7 Å. The horizontal dashed lines indicate △G of an ideal catalyst. (c) Free energy diagram for the 7 Å spacing
between porphyrin motifs (red) compared with the ideal catalyst (black).
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CONCLUSIONS
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We developed a new Zr oxide-chain based MOF, PCN-226. Metalloporphyrin was incorporated in the
framework, which can act as redox active catalyst. PCN-226 demonstrates an excellent chemical
stability, which enforces its compatibility in aqueous solution as electrocatalysts. We present a proof-of-
concept study that highlights the importance of chain-based 3D structure in MOF design for
electrochemical applications. In particular, chain-based MOFs not only allow the incorporation of close-
packed, high-density redox-active-sites, but also provide an opportunity to tailor the spacing between
those active-sites. Thanks to this strategy, PCN-226 shows excellent electroactivity compared with that
of cluster-based MOFs. We foresee the tunability and designability in MOF structures will promote
future development on chain-based 3D MOFs as electrocatalysts. Such developments will further close
the gap between reticular chemistry and materials catalysis, particularly in electrocatalysis, which
contribute significantly to green energy conversion and storage.
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ASSOCIATED CONTENT
Supporting Information
The crystallographic data for PCN-226(Cu) and PCN-226(Co) have been deposited at the Cambridge Crystallographic Data
Centre (CCDC, free for charge at https://www.ccdc.cam.ac.uk) under deposition number CCDC 2004306, and 2020844,
respectively. The Supporting Information is available free of charge on the ACS Publications website at DOI:.
Experimental details, PXRD, gas sorption, SEM, ICP-OES, structure refinement, XPS, electrocatalytic O2 reduction, and DFT
calculations (PDF)
Crystallographic information files for PCN-226(Cu) and PCN-226(Co) (CIF)
Movie S1: Recording of a Zn-air battery powered mini car using PCN-226 as air electrodes (MP4)
AUTHOR INFORMATION
Corresponding Author
*(H.Z.) E-mail: zhenghaoquan@snnu.edu.cn.
*(H.-C.Z.) E-mail: zhou@chem.tamu.edu.
*(Z.H.) E-mail: zhehao.huang@mmk.su.se.
Author Contributions
†M. O. Cichocka, and Z. Liang contributed equally to this work.
Notes
The authors declare no competing interests.
ACKNOWLEDGMENT
This research was supported by the Swedish Research Council (VR, 2016-04625, 2017-04321), the CATSS project from the Knut
and Alice Wallenberg Foundation (KAW, 2016.0072), and National Natural Science Foundation of China (Grant No. 21975148,
21808138, and 21601118). S.S. acknowledges the support from the University of Calgary’s Canada First Research Excellence Fund
Program, the Global Research Initiative in Sustainable Low Carbon Unconventional Resources.
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