Stabilization of Hexaaminobenzene in a 2D Conductive Metal

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Stabilization of Hexaaminobenzene in a 2D Conductive Metal–Organic Framework for High Power Sodium Storage Jihye Park, Minah Lee, Dawei Feng, Zhehao Huang, Allison C. Hinckley, Andrey Yakovenko, Xiaodong Zou, Yi Cui, and Zhenan Bao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06020 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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Journal of the American Chemical Society

Stabilization of Hexaaminobenzene in a 2D Conductive Metal– Organic Framework for High Power Sodium Storage Jihye Park1,†, Minah Lee1,†, Dawei Feng1, Zhehao Huang2, Allison C. Hinckley1, Andrey Yakovenko3, Xiaodong Zou2, Yi Cui4, Zhenan Bao1,* 1

Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden 3 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA 4 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA † These authors contributed equally to this work. 2

ABSTRACT: Redox active organic materials have gained growing attention as electrodes of rechargeable batteries. However, their key limitations are the low electronic conductivity and limited chemical and structural stability under redox conditions. Herein we report a new cobalt-based 2D conductive metal–organic framework (MOF), namely Co-HAB, having stable, accessible, dense active sites for high-power energy storage device through conjugative coordination between a redox-active linker, hexaaminobenzene (HAB) and a Co(II) center. Given exceptional capability of Co-HAB for stabilizing reactive HAB, a reversible three-electron redox reaction per HAB was successfully demonstrated for the first time, thereby presenting a promising new electrode material for sodium-ion storage. Specifically, through synthetic tunability of Co-HAB, the bulk electrical conductivity of 1.57 S cm-1 was achieved, enabling an extremely high rate capability, delivering 214 mAh g-1 within 7 minutes or 152 mAh g-1 in 45 seconds. Meanwhile, almost linear increase of the areal capacity upon increasing active mass loading up to 9.6 mg cm-2 was obtained, with a trace amount of conducting agent. demonstrating 2.6 mAh cm-2

However, the addition of an inactive conductive agent de-

INTRODUCTION Organic compounds have drawn increasing interest for

creases the relative loading of the active component per unit

energy storage devices, including batteries and capacitors be-

mass and volume, thus inevitably limiting the overall energy

cause of their redox activity, ubiquity, and lighter weight than

density of the electrode. To overcome these limitations, a sim-

The synthetic tuna-

ple, stable system with dense active sites and relatively low

bility of organic compounds also enables versatile electro-

impedance is highly desired to constitute energy-dense, high-

chemistry to store various ions (e.g., Li+, Na+, K+, Mg2+), en-

power electrodes for advanced energy storage technologies.

conventional transition metal electrodes.

1,2

dowing these materials with the flexibility to satisfy the design

As a unique category of coordination polymers, metal–

requirements for next-generation energy devices.3-5 However,

organic frameworks (MOFs) present one of the simplest yet

the use of organic electrodes often faces several challenges,

most effective platforms to immobilize organic molecules

such as dissolution in electrolytes, degradation of electrochem-

through metal-ligand coordination (M-L) bonds.13 The precise

ical activity, and slow reaction kinetics, primarily due to the

arrangement of their building blocks enables crystalline and

weak stability and low electrical conductivity of the electrode

porous frameworks, providing access to built-in active sites

To alleviate the solubility issue, polymerization

for electrochemical reactions. So far, several MOFs have been

and immobilization of active molecules have been extensively

reported as potential electrode materials for energy storage,

materials.

6,7

studied, but these methods are often synthetically challenging

including lithium-ion batteries,14-24 and sodium-ion batteries.25

and suffer from limited accessibility and low density of active

Although these MOFs show moderate activities and relatively

sites.9,10 To solve the low conductivity problem, composites

stable cycling, most studies showed limited redox capabilities

with conductive nanostructures have been studied and shown

due to the sparse active sites and low electrical conductivity.21

to successfully increase the rate capability of the system.11,12

More recently, conductive MOFs have drawn gowing attention

8

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for electrochemical applications19,20,26 and a notable break-

thermal stability to 300 °C. We confirmed that the conductive

through was reported utilizing conductive MOFs for energy

Co-HAB stores nearly three Na+ and electrons per HAB in an

storage based on electrochemical double layer mechanism;

organic electrolyte (theoretical specific capacity of 312 mAh

27

however, these materials exhibited negligible redox activity,

g-1) while showing substantial pseudocapacitive contributions.

which limits their energy density. Integration of both high

With high active loading up to 90 wt% in the electrode, Co-

density of redox-active centers and high conductivity into a

HAB delivers 228 mAh g-1 at 1 A g-1 and 151 mAh g-1 at 12 A

MOF-based platform is a promising albeit challenging route to

g-1, outperforming many other organic and inorganic electrode

fast and energy-dense electrodes.

materials for high-power Na+ storage devices. To the best of

Herein we report a 2D conductive MOF consisting of a

our knowledge, Co-HAB is the first conductive MOF to

redox-active hexaaminobenzene (HAB) and Co(II) ion node,

demonstrate storage of sodium and moreover, to do so rapidly

namely Co-HAB. With systematic control of the synthetic

and stably. Co-HAB is thus an excellent candidate for a high-

conditions to increase particle size and crystallinity, our ob-

power electrode material to build fast Na-ion batteries or hy-

tained Co-HAB exhibited bulk electrical conductivity as high

brid capacitors.

-1

as 1.57 S cm . Meanwhile, the Co-HAB displayed outstanding chemical stability in both aqueous and organic media, and

Figure 1 | a, Synthetic scheme of Co-HAB. b, Proposed three-electron reversible reaction in Co-HAB. c, The calculated structure of Co-HAB in 3D. Na+ and PF6- ions are illustrated with size information for comparison. Hydrogens are omitted for clarity. d, HRTEM images of Co-HAB along [001] showing a hexagonal pore packing. Inset: fast Fourier transform of the image. Scale bar = 10 nm (left). HRTEM image in the red box. Scale bar = 1 nm (top right). Symmetry-imposed and lattice-averaged image calculated from the HRTEM image. The eclipsed structure model of Co-HAB is overlaid (bottom right). e, Synchrotron PXRD and Pawley fitted patterns of Co-HAB. f, N2 sorption isotherm of Co-HAB.

RESULTS AND DISCUSSION


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Journal of the American Chemical Society The imine functional group can undergo redox processes

species. Nevertheless, we hypothesized that cobalt conductive

at the C=N bond, and thus imine compounds can provide a

MOF may be an interesting system to explore the design of

6

high theoretical specific capacity due to its small mass unit.

anode materials because of its weaker electron affinity than

However, the imine species often undergo irreversible self-

Cu(II) and Ni(II).39

condensation/polymerization or other side reactions during the redox processes, precluding reversible charge storage in energy devices.6,28,29 One possible solution to stabilize these reactive intermediates is to form a coordination complex with transition metals wherein the negative charge formed upon reduction can be delocalized through d-π conjugation.30 Particularly, when such d-π conjugation is extended to 2D structures, the system often yields an electrically conductive network which can be classified as a conductive MOF.31-33 Combining electrical conductivity and redox activity in a MOF enables fast electron transport to the redox centers, and thus offers an ideal system that can fully utilize imine redox chemistry for re-

In the reported synthesis of conductive MOFs with amino group-containing linkers, the proposed mechanism includes the deprotonation and subsequent partial oxidation of the linkers.40 This is typically achieved through an excess amount of base (e.g., NH4OH) which is mixed with the metal salts prior to the addition of the linker.31,33 However, perhaps due to the stronger affinity of Co(II) to OH- compared to Cu(II) and Ni(II), we found this synthetic route produced a considerable amount of Co(OH)2 as an impurity (Supporting Information Figure S1). To avoid the hydroxide formation, HAB and Co(II) salts were first mixed in water to allow complexation between them. NH4OH was subsequently added in open air

chargeable batteries with high power density.

and the reaction was stirred until dark navy-colored precipiBearing these features in mind, we selected hexaamino-

tates of Co-HAB-A (A stands for aqueous) were obtained

benzene (HAB) as the organic linker to construct the 2D con-

(Figure 1a). To validate the proposed 2D structure, density

ductive MOF (Figure 1a). Since the fully oxidized form of

functional theory (DFT) calculations were carried out. First, a

HAB can be considered a dense assembly of six imine groups,

structural model with an eclipsed packing (Figure 1c) was

it can, in theory, undergo up to six-electron redox reactions.

generated and the geometry was optimized by DFT calcula-

Thus, HAB could have one of the highest densities of redox

tions. The unit cell parameters of the model were further re-

centers among all possible molecular structures, which satis-

fined by applying Pawley fit against the synchrotron powder

fies the primary design principle for energy-dense electrode

X-ray diffraction (PXRD, λ = 0.45212 Å, Figure 1e). The re-

materials.

sulting hexagonal unit cell parameters (a = b = 13.361(3) Å and c = 3.082(1) Å, corresponding to the interlayer spacing)

The bridging metal also plays an important role in conductive MOFs. It governs the electronic structure, conductivity, and chemical and thermal stability of the MOF on the basis of orbital interaction with the linker.34,35 Previously reported conductive MOFs typically used Cu(II) and Ni(II) as the metal nodes due to these ions’ preferred D4h coordination symmetry when interacting with strong field ligands (e.g., -NH2, SH).26,33,36-38 The D4h coordination often yields a (2,3)connected honeycomb 2D lattice when connected to a hexadentate ligand with D3h symmetry. Such structure can lead to a highly conjugated system because of the effective π orbital overlap between the metal center and the ligand. Co(II) has been relatively rarely studied in bulk synthesis of conductive MOFs, presumably due to the synthetic challenges associated with the less preferred D4h coordination symmetry of Co(II)

are in good agreement with the initial DFT-calculated structure (Supporting Information Table S1), as the d-spacings of d001 = 3.08 Å and d010 = 11.57 Å from PXRD match well with the initial eclipsed model (d001 = 3.19 Å and d010 = 11.36 Å). To further confirm the structure, high-resolution TEM (HRTEM) images of the Co-HAB-A and an optimized product, Co-HAB-D (discussed later) were obtained. The HRTEM images (Figure 1d) clearly show hexagonal pores, smaller than 1 nm with a honeycomb arrangement along [001] with d010 = 11.7 Å, which corresponds well with the eclipsed model (calculated d010 = 11.6 Å). Meanwhile, 1D channels along the rodlike crystals can also be observed, which match well with our eclipsed structural model. Furthermore, the N2 adsorption isotherm of Co-HAB-D shows a BET surface area of ~240 m2g-1 with sub-nanometer pores (Figure 1f), which cannot be



Page 4 of 12

achieved with the nearly non-porous staggered structure.41

a low yield (Figure 2c). The filtrate of this reaction was

This result, in corroboration with the well-defined pores seen

strongly colored dark blue, which is indicative of unconverted

by HRTEM, validates the 2D eclipsed honeycomb structure.

HAB linkers (Supporting Information Figure S2). The yield and crystallinity of the products improved progressively with

During the synthetic optimization, we found that the stoichiometry of the base with respect to HAB plays a critical role in determining the Co-HAB growth. For instance, when less than 3 equivalents (with respect to HAB) of NH4OH was

increasing base amount up to 10 equivalents, Co-HAB-A10 (Figure 2c). Scanning electron microscopy (SEM) shows that the better crystalline products tend to exhibit larger rod-like crystallites (Supporting Information Figure S3).

used, an amorphous product, Co-HAB-A3, was obtained with

Figure 2 | a, HRTEM images of Co-HAB-A10 (left) and Co-HAB-D (right). b, SEM images of Co-HAB-A10 (left) and Co-HAB-D (right). c, PXRD patterns of Co-HABs synthesized from different conditions. d, Full width at half maximum (FWHM) obtained from PXRD diffractions, which corresponds to [020] using a Gaussian peak-shape function. e-f, Electrical conductivity measurement of CoHAB in bulk as a function of particle size (size parmeter was set by multiplication of short side and long side of the particles of each sample from SEM images) and FWHM, respectively. g, N2 sorption isotherms and h-i, PXRD patterns of Co-HAB after treatments in different chemical environments.

Another structural confirmation was obtained using X-

creases, which coincides with the improving crystallinity of

ray photoelectron spectroscopy (XPS). The high-resolution Co

these materials. The high-resolution N 1s spectra can be de-

2p spectra displays satellite peaks in both the Co 2P3/2 and Co

convoluted into two peaks; the binding energies at 398.3 eV

2P1/2 regions, typical features of the cobalt ion in the +2 oxida-

and 400.0 eV are attributed to the quinoid imine (=N-) and

tion state in square planar geometry.

42,43

We also noticed a

small shoulder at 781.1 eV and 793.0 eV, in which we attrib-

benzoid amine (-NH-),44 consistent with the expected complex structure45 in Co-HAB (Figures 5e,f).

ute to structural disorder – a deviation from the ideal coordination of the neutral complex in the MOF. The disordered feature in the spectrum decreases as the amount of the base in-

Having confirmed the physical structure, we further examined the electronic structure and electrical conductivity of Co-HAB using UV-Vis-NIR spectroscopy and the four-point


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probe conductivity measurement, respectively. The Co-HAB

While conductive MOFs have drawn growing attention in

film on a quartz substrate shows a broad absorption band in

various energy applications, studies of their chemical stability

near-IR region which corresponds to an absorption edge of

have been somewhat limited. However, the chemical stability

0.87 eV, suggesting a high degree of conjugation arising from

of electrode materials is a critical parameter to evaluate their

the sufficient orbital overlap between HAB and Co(II) (Sup-

practical potential as it dictates their handling process, testing

porting Information Figure S5). The electrical conductivity

conditions, lifetime, and recyclability.48 Among the reported

was measured on a pressed pellet of Co-HAB. We hypothe-

MOFs, Co-HAB exhibits superior chemical stability both in

sized that the crystallinity and particle size would correlate

aqueous and in organic media. PXRD and N2 sorption iso-

with the bulk conductivity as we expect the conductivity to be

therms indicate Co-HAB maintains its framework integrity

maximized within a crystal domain. As expected, the electrical

after treatments with concentrated NH4OH, boiling water, and

conductivity increases with the improved crystallinity and

0.1 M acidic and basic solutions of HCl and KOH, respective-

larger particle size (Figures 2c-f). Specifically, a value of 0.48

ly (Figures 2g,h). The chemical stability tests of Co-HAB

-1

S cm was measured for the Co-HAB-A10 sample, which was

were also carried out in the organic electrolytes [1 M LiPF6 in

more than 6 times higher than that of Co-HAB-A4 sample

ethylene carbonate/diethyl carbonate (EC/DEC) and 1 M

(Supporting Information Figure S6).

NaPF6 in diethylene glycol dimethyl ether (DEGDME)] that are typically used for electrochemical studies and showed

Encouraged by this crystallinity-conductivity relationship, we further optimized the synthesis to obtain a more crystalline product for higher electrical conductivity. For the modified synthesis, N,N-dimethylformamide (DMF) was chosen as a co-solvent because of its poorer ability to solvate the reactants relative to water. It was thus expected to slow the MOF growth, allowing for a more crystalline product than the fully aqueous Co-HAB-A series. Indeed, the resulting product (CoHAB-D) synthesized from DMF/water mixture (1:1, v/v) showed much-improved crystallinity and a bulk conductivity of 1.57 S cm-1 (Figures 2e,f). It is worth noting that this is one of the highest values, comparable with other conductive HABand benzenehexathiolate (BHT)-based

MOFs.32,38,40,41,46,47

well-retained crystallinity of Co-HAB (Figure 2i). In addition to its remarkable chemical stability, Co-HAB also exhibits an outstanding thermal stability; its PXRD patterns and N2 adsorption isotherms remain nearly unaltered after heat treatments at different temperatures (100–300 ºC) (Supporting Information Figure S8). Such exceptional stability can be attributed to the robust coordination bonds between HAB and Co(II) as well as the strong chelating effect resulting from the hexadenticity on the smallest benzene unit.49 These features allow Co-HAB to be coupled with a wide range of electrolytes and to run at elevated temperatures, enabling rigorous optimizations of the operating conditions in energy devices.

Crystallites of Co-HAB-D exhibit an even larger size (~75 nm,

With its optimized conductivity and exceptional chemi-

on the long side) than Co-HAB-A10 (Figures 2a,b). The con-

cal/thermal stability, Co-HAB-D was tested as an electrode

ductivity of these materials is many orders of magnitude high-

material to store sodium ions. To evaluate the redox capability

er than that of the nanosheets of the previously reported 2D

and cycle stability of Co-HAB, we tested electrodes composed

coordination polymers prepared from similar starting materials

of 90 wt% Co-HAB as an active material in half-cells versus

32

via a very different synthetic route. The correlation between

sodium metal. In a voltage range of 0.5–3.0 V versus Na+/Na

crystallinity and conductivity provides a rationale for these

at 50 mA g-1, a reversible specific capacity as high as 291 mA

findings, given the significantly higher crystallinity of our

g-1 was obtained, corresponding to 2.8 Na+ per HAB (Figure

MOF materials relative to these coordination polymers. The

3a). Considering partially oxidized HAB in the neutral Co-

high conductivity of Co-HAB represents one of the highest

HAB platform can theoretically store 3 Na+ (312 mAh g-1),

values among the reported conductive MOFs measured in

this result demonstrates nearly full utilization of redox activity

26

Thus, we expect that Co-HAB would provide fast

of HAB was realized in the MOF for the first time. After the

electron transport within the electrode, which is highly desira-

formation of the solid/electrolyte interface (SEI) layer in the

ble to achieve high power energy devices.

first discharge, negligible changes in the voltage profiles were

pellets.



Page 6 of 12

observed during repeated cycles, which suggests that the elec-

Na+/Na, only 34% of the specific capacity was retained after

trochemical reaction is highly reproducible. Figure 3b shows

40 cycles in a Na cell (Supporting Information Figure S9).

the cycle stability of the Co-HAB, wherein a capacity of 226

These results imply that the ultralow potential during the dis-

-1

-1

mAh g at a current rate of 500 mA g was maintained with a

charge may further reduce Co(II), causing irreversible redox

coulombic efficiency close to 100 % for more than 50 cycles

processes, likely involving the destruction of the framework

(Figure 3b). We noticed that further reduction of Co-HAB

(Supporting Information Scheme 1c). To maintain the struc-

makes the MOF substantially less stable, as indicated by poor

tural stability and the cycle performance, the voltage window

cycle retention. According to the voltage profiles obtained

was thus fixed to be above 0.5 V versus Na+/Na.

with a lower discharge cut-off potential at 0.05 V versus

Figure 3 | a, Voltage profiles of Co-HAB-D in a Na cell at a current of 50 mA g-1 in the voltage window of 0.5–3.0 V. b, Cycle retention of Co-HAB-D in the voltage window of 0.5–3.0 V. c, Rate capability of Co-HAB-D in comparison with that of Co-HAB-A4 and CoHAB-A10. d, Discharge profiles of Co-HAB-D at various current densities. e-f, Capacity retention of Co-HAB-D at high current densities.

To ascertain the impact of electron transport in Co-HAB

(Figures 3e,f), Co-HAB-D has outstanding tolerance to re-

on electrochemical energy storage, we compared the rate per-

peated fast sodiation/desodiation processes. It should be em-

formances of Co-HAB-D with Co-HAB-A4 and Co-HAB-

phasized that such high rate capability obtained with 90 wt%

A10 in Na cells (Figure 3c). Notably, the Co-HAB-D elec-

active loading was primarily due to the improved electrical

trode yielded much higher specific capacities at the elevated

conductivity of Co-HAB-D. We further confirmed that car-

-1

-1

current densities of 0.5 A g and 1 A g , which can be at+

bon-free electrode can still exhibit 76% of the theoretical ca-

tributed to (i) ideal crystallographic pathways for Na diffu-

pacity (Supporting Information Figure S10). Considering most

sion and (ii) a higher electrical conductivity compared to that

organic compounds require a large amount of conductive addi-

of Co-HAB-A electrodes. Figure 3d shows the discharge pro-

tive to compensate for the lack of intrinsic electrical conduc-

-1

files of Co-HAB-D at various current densities from 0.1 A g

tivity,11,12 our results highlight the advantage of the intrinsic

to 12 A g-1. Notably, Co-HAB-D still demonstrates a specific

conductivity in the redox-active MOF platform.

-1

-1

capacity of 214 mAh g and 152 mAh g even at the extremely high current densities of 2 A g-1 and 12 A g-1, respectively. As demonstrated by its stable capacity retention at high rates

We further tested the electrochemical properties of CoHAB-D electrodes having higher mass loadings to increase the


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areal capacity which is one of critical metrics for practical

trode was still exhibited in the 9.6 mg cm-2 electrode (Support-

applications. As shown in Figures 4a and b, the areal capacity

ing Information Figure S11). The areal capacity of 2.6 mAh

-2

can be increased almost linearly from 0.35 mAh cm to 2.6

cm-2 with negligible polarization confirms remarkable charge

mAh cm-2 with the increase of active mass loading from 1.3 to

transport kinetics in the Co-HAB-D electrode which only con-

-2

9.6 mg cm . The voltage profile of the electrode with the

tains 5% of carbon black. For the 4.4 mg cm-2 electrode, the

highest active mass loading of 9.6 mg cm-2 is almost identical

areal capacity of 0.85 mAh cm-2 can be achieved at the current

to that of 1.3 mg cm-2 at the same rate of 50 mA g-1. More im-

density of 4.4 mA cm-2, further supporting the exceptional

portantly, 90% of specific capacity of the 1.3 mg cm-2 elec-

high-rate performance of Co-HAB-D (Figure 4c).

Figure 4 | a, Voltage profiles comparison of Co-HAB-D electrodes with various active mass loadings. b, Areal capacity of Co-HAB-D electrodes as a function of active mass loading. The red line indicates the theoretical values. c, Rate capability of Co-HAB-D with active mass loading of 4.4 mg cm-2. d, Areal capacity comparison of Co-HAB-D with representative anode materials for fast Na-ion storage in previous study.38,50-59

We compared the areal performance of Co-HAB-D with

high electrical conductivity and ordered sub-nano 1D channels,

other sodium electrodes, primarily anodes showing high rate

affords high mass loading electrodes to have proper electrical

performances.

38,50-58

Among the reported sodium anodes deliv-

and structural integrity. Considering its areal capacity, cycle

ering the capacity within 20 minutes to date, Co-HAB-D

stability, rate capability, and high active loading, Co-HAB

demonstrated the highest areal capacity (Figure 4d). In previ-

represents a superb electrode material for ultrafast Na-ion bat-

ous studies, the areal capacities were barely assessed at high

teries and Na-ion hybrid capacitors.

rates because their high rate capabilities were attained predominantly by using nanostructured electrodes with small mass loadings and limited packing densities. (Supporting Information Table S3). In contrast, our MOF platform, exhibiting

To understand the electrochemical processes for Na-ion storage, cyclic voltammetry (CV) was carried out at sweep rates ranging from 0.375 mV s−1 to 3 mV s−1. The cyclic volt-



ammogram displays two quasi-reversible reduction/oxidation +

processes (Figure 5a). As Co-HAB can store nearly 3 Na per +

Page 8 of 12

unit in Co-HAB (Supporting Information Scheme S1b). The resulting negative charges on Co-HAB can be balanced with

HAB which corresponds to 2 Na per coordination unit during

incoming Na+ ions, thereby sustaining a reversible reaction

the galvanostatic cycling, these features represent successive

mechanism in the cation host for energy storage.

reduction by two electrons, yielding a di-anionic coordination

Figure 5 | a, Cyclic voltammograms of Co-HAB at different sweep rates. b, b-value determination of the cathodic currents assuming that the current obeys a power-law relationship. The inset shows peak currents versus scan rate in logarithm and the fitted lines at 1.25 V (red) and 0.5 V (blue) during reduction. c, Capacitive and diffusion currents contributed to the charge storage of Co-HAB at the rate of 0.375 mV s−1. d, A Self-discharge profile of Co-HAB in a half-cell versus Na metal. The inset shows voltage profiles before and after storage for the self-discharge test. e-f, ex situ XPS spectra of N 1s and Co 2p regions during discharge and recharge processes in a potential window of 0.5 V to 3.0 V.

We further performed quantitative analyses on the

charged cell for 10 days. The result shows that Co-HAB ex-

charge-storage behaviors in Co-HAB, which showed substan-

hibits remarkably low self-discharge rate of less than 0.003 V

tial pseudocapacitive contributions. By plotting the peak cur-

h-1. This is significantly lower than most other capacitors that

rents (݅ሻ and sweep rates (‫ )ݒ‬in logarithm, we can determine

typically lose a half of the maximum voltage within few

the b-value of the cathodic currents (Figure 5b) with an as-

hours.62 Furthermore, after 10 days of the storage, more than

sumption that the current obeys a power-law relationship,

85% of original capacity was still confirmed, thus proving its

݅=ܽ ‫ ݒ‬௕ ( ܽ and ܾ are adjustable values).

60

Both ܾ -values for

outstanding stability in the fully charged state.

cathodic peaks at 1.25 V and 0.5 V are 0.74, which confirms pseudocapacitive charge storage in Co-HAB. By separating the diffusion-controlled process where ݅ varies as ‫ݒ‬1/2, and the capacitive process where ݅ varies as ‫ݒ‬,61 the capacitive contribution was calculated to be 47% out of the total current, at a sweep rate of 0.375 mV s−1 (Figure 5c). Given the pseudocapacitive storage mechanism, the self-discharge behavior of Co-HAB was examined (Figure 5d). The time-dependent voltage change on an open circuit was recorded with a fully-

In support of the redox mechanism, detailed studies were implemented by ex situ XPS, which provided evidence for the evolution of the oxidation state of Co-HAB in response to changes in the state of charge of the electrode. Both the cobalt node and the HAB of the cycled MOF electrode were monitored to understand the origin of the redox activity. In the N 1s spectra (Figure 5e), reversible changes of the peak intensities at 398.3 eV and 400.0 eV were observed during the discharge and recharge processes. This represents the reversible


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reduction of the quinoid imine moiety in Co-HAB to the cor-

In summary, we successfully address the stability and conduc-

responding benzoid amine species during the discharge/charge

tivity challenges associated with redox-active organic com-

cycle. In contrast, no obvious change was observed in Co 2p

pounds by designing a 2D conductive MOF, Co-HAB. Sys-

spectra (Figure 5f), indicating cobalt remains divalent in the

tematic control of the synthetic conditions allow the highly

cycled electrode during the charge/discharge. This result sug-

crystalline Co-HAB to combine multiple features of an ideal

gests that the reduction mainly occurs on the HAB instead of

electrode, including high intrinsic conductivity, high density

II

the Co(II). This is in accordance with observations of M (ophenyldiamine)2 complexes (M = Ni, Pd, Pt),

30

of redox-active sites, porosity, and excellent chemical/thermal

which con-

stability. We first time demonstrated that Co-HAB can store

cluded that the reduction from neutral to dianionic species is

three electrons and Na+ per HAB in organic electrolytes,

invariably centered on the ligand. Based on these results and

thereby exhibiting a specific capacity as high as 291 mAh g-1

the previous findings, we propose that the redox activity of

with a stable cycling life. In addition, Co-HAB shows a re-

Co-HAB is centered on the HAB linkers, while the cobalt

markable rate capability, yielding 152 mAh g-1 within 45 se-

serves as the bridge to immobilize HAB and imparts high elec-

conds, conferred by its high intrinsic conductivity and porosity.

trical conductivity to the 2D framework.

Given the chemical tunability of both the organic and inorganic building blocks in this MOF system, we envision that the

To demonstrate the broader feasibility of using the conductive MOF platform with tunable metals for sodium storage, the electrochemical property of Ni-HAB was also examined

configuration of other redox-active organic molecules in a conductive framework should afford additional high-power electrode materials.

(Supporting Information Figure S12). Under identical conditions as those used for Co-HAB, Ni-HAB exhibited similar voltage profile to that of Co-HAB with a reversible capacity of

EXPERIMANTAL SECTION

295 mAh g-1, corresponding to 2.8 Na+ per HAB. The cycle

Synthesis of Co-HAB-A series by varying the amount of

stability, however, was marginally worse than that of Co-

NH4OH. A solution of 21.0 mg (0.072 mmol) of cobalt nitrate

HAB, which would require further investigation. During the

hexahydrate [Co(NO3)2⋅6H2O] was dissolved in 6 mL of water.

+

course of our study, we noticed that reversible Li storage in 16

Ni-HAB has been recently reported. A reversible capacity of -1

10 mg of HAB⋅3HCl (0.036 mmol) in 5 mL H2O was then added into the cobalt nitrate solution in air with stirring (300

approximately 100 mAh g was demonstrated in the voltage

rpm). 60 µL of 6 M aqueous ammonium hydroxide (NH4OH)

range of 2–3.5 V vs Li+/Li which corresponds to one Li+ stor-

(0.36 mmol, 10 equiv to HAB) was then subsequently added

age per HAB. In their study, the redox reaction was observed

to yield Co-HAB-A10. The mixture was stirred for an hour at

to be centered on Ni atoms by XPS as opposed to our observa-

RT in air. The resulting dark navy solids were filtered and

tion with Co-HAB. This contrasting observation to ours in the

washed with copious amount of water and acetone, and dried

redox chemistry can be potentially due to dissimilar materials

under vacuum at 80 °C for 3 h unless otherwise noted. Like-

and electrochemical conditions which is beyond the scope of

wise, the reactions with different stoichiometry of NH4OH (3

this report. Nonetheless, the foregoing results demonstrate our

– 8 equiv.) were denoted with a number that corresponds with

concept of employing coordination chemistry between HAB

base equivalent used in the synthesis (e.g., Co-HAB-A3 – Co-

and metal centers to construct conductive MOFs can be an

HAB-A8).

effective and general approach to better exploit electrochemi-

Synthesis of Co-HAB-D using mixed solvent. A solution of

+

cal activity of organic molecules for Na storage.

105

mg (0.36

mmol) of cobalt

nitrate

hexahydrate

[Co(NO3)2⋅6 H2O] was dissolved in a mixture of 25 mL DMF and 20 mL water in a round bottom flask. The solution was preheated at 75 °C in the oil bath for 15 min. 50 mg of CONCLUSION

HAB⋅3HCl (0.18 mmol) in 5 mL H2O was then added to the cobalt nitrate solution under stirring in open air. 180 µL of 6



Page 10 of 12

acetone, and dried under vacuum at 80 °C for 3 h.

and Catalysis. A.C.H. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147474. The structural characterization by HRTEM and PXRD was supported by the Knut & Alice Wallenberg Foundation through the project grant 3DEM-NATUR and the Swedish Research Council (VR) through the MATsynCELL project of the Röntgen-Ångström Cluster.

MOF electrode preparation. To prepare the working elec-

REFERENCES

M aqueous ammonium hydroxide (NH4OH) (1.08 mmol, 6 equiv to HAB) was added immediately. The reaction mixture was stirred for 2 h at 75 °C. The resulting dark navy solids were filtered and washed with copious amount of water and

trodes, Co-HAB powders were mixed with carbon black conductive additive (Super P, TIMCAL), and polytetrafluoroethylene (PVDF) binder in the mass ratio of 90:5:5. Subsequently, N-Methyl-2-pyrrolidone (NMP) was added to the mixture and stirred for 1 h. The resulting slurry was coated on Cu foil and dried at 70 °C for overnight under vacuum. The dried samples were cut into 1 cm2 circular disks to afford a mass of 1–1.5 mg for typical electrodes. To prepare high mass loading electrodes, Co-HAB powders were mixed with polytetrafluoroethylene (PTFE) binder instead of PVDF in the same mass ratio of 90:5:5. Glass microfiber filters (Whatman, GF/D) were used as the separator. 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1, v/v) and 1 M NaPF6 in diethylene glycol dimethyl ether served as the electrolytes. Li metal and Na metal were used as the anodes for the Li and Na cells, respectively. The resulting coin cells were loaded into a battery tester (Arbin Instruments) for galvanostatic cycles and into a VSP300 potentiostat (Biologic) for cyclic voltammetry.

ASSOCIATED CONTENT Supporting Information. Characterization details and additional experimental data are in Supporting Information. This material is available free of charge via the Internet at http: //pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] (Prof. Zhenan Bao) Author Contributions

†These authors contributed equally to this work. Jihye Park & Minah Lee Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT J.P. acknowledges support from the Dreyfus Foundation Postdoctoral Fellowship for Environmental Chemistry. M.L. acknowledges partial support by the Postdoctoral Fellowship from the National Research Foundation of Korea under Grant No. NRF2017R1A6A3A03007053. D.F. acknowledges support from the U.S. Department of Energy, Office of Sciences, Office of Basic Energy Sciences, to the SUNCAT Center for Interface Science

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Table of Contents Graphic


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