Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal-Organic Framework Thin Film

Article abstract of DOI:10.1021/jacs.9b07084

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as

Full text of DOI:10.1021/jacs.9b07084

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Article
Electrocatalytic Hydrogen Evolution from a
Cobaloxime-based Metal-Organic Framework Thin Film
Souvik Roy, Zhehao Huang, Asamanjoy Bhunia, Ashleigh
Castner, Arvind Kumar Gupta, Xiaodong Zou, and Sascha Ott
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07084 • Publication Date (Web): 11 Sep 2019
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Electrocatalytic Hydrogen Evolution from a Cobaloxime-based
Metal-Organic Framework Thin Film
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Souvik Roy,†,§ Zhehao Huang,‡,§ Asamanjoy Bhunia,† Ashleigh Castner,† Arvind K. Gupta,† Xiaodong
Zou,‡ Sascha Ott*,†
†Department of Chemistry – Ångström Laboratory, Uppsala University, Box 523, 751 20 Uppsala, Sweden
‡Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm
University, 106 91 Stockholm, Sweden
KEYWORDS .metal-organic framework (MOF), redox-active linker, cobaloxime, electrocatalysis, hydrogen evolution,
electrochromism
ABSTRACT: Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but are
also hampered by stability concerns and practical limitations associated with their use in homogenous phase. Their
incorporation as integral linker units in metal-organic frameworks (MOFs) can remedy these shortcomings. Moreover, the
extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we
report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between
hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime
linkers promote electron transport through the film, as well as function as molecular HECs. The obtained turnover numbers
are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the
cobaloxime catalysts is maintained for at least 18 hours of electrocatalysis. Being the one of the very few hydrogen evolving
electrocatalytic MOFs based on redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity
and charge transport pathways.
aspects. Thus, the fabrication of such hybrid electrodes that
are chemically robust and combine large surface areas with
INTRODUCTION
Molecular hydrogen is the ultimate clean fuel due to its
extremely high energy density (~142 MJ Kg–1) and the fact
that its combustion produces only water as “waste” product.
However, most hydrogen is currently produced from fossil
fuels via steam-methane reforming or gasification of coal.1-3
Electrochemical water splitting to produce hydrogen from
renewable electricity represents a viable option for moving
towards a carbon-free energy economy and mitigating the
environmental effects associated with greenhouse gas
emission.4-6 Suitable electrocatalysts are required for low
kinetic barriers, and to drive the reaction at high current
densities. State-of-the-art solid electrocatalysts generally
contain precious and expensive metals, most often
platinum,4,7-9 that make their wide scale implementation
unfeasible. While earth abundant element based, often
nano-structured systems10-13 have been developed as
heterogenous alternatives, molecular catalysts14 are
somewhat disadvantaged by practical issues, mostly
associated with poor water solubility and structural
instability. On the other hand, molecular catalysts are
motivated by the high activity per metal center, as well as
the option to design tailor-made ligand structures that
allow intricate tuning of the catalysts’ redox potentials and
catalytic properties. Molecule-derived hybrid solid catalysts
have the potential to overcome the shortcomings of
molecular catalysis while maintaining their beneficial
high catalyst loadings remains a key target.
Metal-organic frameworks (MOFs) are promising materials
for catalytic application due to their porosity and modular
nature.15,16 A three-dimensional array of metal complexes
(catalysts) linked to inorganic nodes in a MOF that is grown
directly on a planar electrode could substantially boost the
areal loading of the molecular catalyst, leading to large
current densities. In contrast to a densely packed
polymerized film, a MOF thin film features a structurally
ordered porous network, which facilitates diffusion of
electrolyte and substrate molecules into the MOF to access
also interior catalytic sites. However, the implementation of
MOFs as electrocatalysts is limited by their insulating
nature and poor charge transport properties.17-20 This
deficiency can be addressed either by orbital overlap and
charge delocalization between linkers21,22 or by introducing
redox-active molecular linkers that render the MOFs
electroactive by acting as conduit for electron transfer via a
hopping mechanism.23-29 This process is expedited by the
proximity of the redox-active linkers in the MOF
architecture which, under ideal circumstances, can engage
all linkers to mediate charge transport.
Application of this strategy has so far been mostly limited to
porphyrin-based three-dimensional (3D) MOFs that have
been successfully utilized for electrocatalytic CO2 reduction
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reaction (CO2RR),30-33 oxygen reduction reaction (ORR),34 geometry around the six-coordinate CoIII center with the
nitrite oxidation,35 and very recently, hydrogen evolution36
where the linkers serve dual roles as charge transport
mediators and catalysts. Other reported examples of
hydrogen evolving electrocatalysts based on coordination
polymers describe either 2D MOFs and metal-organic
surfaces (MOSs) with metal-dithiolene/dithiolene-diamine
linkers37-40 or MOFs loaded with amorphous heterogeneous
electrocatalysts with the MOF only serving as a porous and
electrochemically silent scaffold.41,42 Designing a three-
dimensional architecture composed entirely of hydrogen
evolution catalysts (HECs) as metallo-linkers has remained
an challenge in the field.
two dioxime ligands occupying the equatorial plane and the
two chloride ligands in the axial positions. The phenyl rings
that bear the carboxyl groups are twisted out of the CoN4
plane due to steric factors with dihedral angles of 47.9° and
42.7° for the two sets of phenyl rings (Figure S2).
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Herein, we report a new 3D MOF that consists exclusively of
cobaloxime HEC linkers coordinated to inorganic nodes of
zirconium-oxo clusters. The structure of the new MOF, UU-
100(Co) (UU = Uppsala University), is determined by
microcrystal electron diffraction, known as MicroED24,43 or
more precisely continuous rotation electron diffraction (cRED)
methods.44 Spectroelectrochemical studies on UU-100 grown
as thin films on FTO (fluorine doped tin oxide) electrodes allow
the quantification of electrochemically accessible cobaloxime
units (~57%). When grown on glassy carbon, UU-100(Co) acts
as an electrochemical HEC over 18 hours with a constant
current density of 1.7 mA cm–2. Post-electrolysis studies show
that the molecular integrity of the cobaloxime linkers remains
intact.
Figure 1. (A) Structure of the cobaloxime linker in UU-100(Co),
and (B) structural model of UU-100(Co) MOF viewed along
[001]
Crystalline MOF material was synthesized by a ‘controlled
secondary building unit (SBU)’ approach that involves pre-
assembly of the zirconium-oxo cluster by reacting ZrCl4
with acetic acid at elevated temperature.46,47 Subsequent
addition of the cobaloxime linker leads to an exchange of the
SBU-coordinated acetates by the tetracarboxylate ligands,
and the formation of the 3D framework. Crystallinity of the
resulting MOF, UU-100(Co), was confirmed by powder X-
ray diffraction (PXRD), which shows intense peaks in the
low angle range, characteristic of porous materials (Figure
2A). The morphology of the MOF particles was examined by
scanning electron microscopy (SEM), which shows that the
material consists of rod-shaped crystallites ~1-3 µm in
length (Figure 2B). Energy dispersive X-ray (EDX)
elemental mapping of UU-100(Co) shows uniform
distribution of cobalt and zirconium throughout the
material (Figure 2B and S8) with an average Zr:Co ratio of
2.4±0.3 (Table S2 and Figure S9). The metal content of the
MOF was further quantified by ion-coupled plasma optical
emission spectroscopy (ICP-OES) which gave a Zr:Co ratio
of 2.8±0.3.
RESULTS AND DISCUSSION
Synthesis and characterization. Cobaloximes are
amongst the most widely studied molecular HECs45 and
were employed here as metallo-linkers for MOF fabrication.
They are known for their high catalytic activity, but their
use in homogenous phase is limited by low structural
stability that is caused by the fragile ligand framework
around the Co center where the individual oximes are only
held together by weak hydrogen bonding. We hypothesized
that this design weakness can be overcome by
incorporation of the catalyst into a MOF. For this purpose, a
tetranucleating cobaloxime linker with carboxylate
anchors, [Co(dcpgH)(dcpgH2)]Cl2 (Figure 1A), was
synthesized by metalation of the dioxime, 4,4'-(1,2-
bis(hydroxyimino)ethane-1,2-diyl)dibenzoic acid, under
aerobic conditions. The solid-state molecular structure of
the cobaloxime was determined by X-ray crystallography
(Figure S1, Table S1) which shows the expected octahedral
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Figure 2. (A) Pawley fit (red) of powder X-ray diffraction pattern (λ = 1.5418 Å) for UU-100(Co) against the experimental
PXRD pattern (black), showing good agreement factor (weighted-profile R factor Rwp = 0.0770 and unweighted-profile R
factor Rp = 0.0585 after convergence). (B) SEM image of UU-100(Co) with corresponding energy dispersive X-ray
spectroscopy elemental maps of Zr-Lα, Co-Kα, Cl-Kα and O-Kα. The scalebar in EDX maps represent 1 μm. (C) HRTEM image
of UU-100(Co) along [1-10] that shows lattice fringes with d001= 19.1 Å representing the packing of Zr clusters (dark features).
(D) Fourier transform of the image showing the 00l reflections. (E) Enlarged HRTEM image of the region marked by a blue
square in C; Two Zr cluster columns are marked by red circles. (F) N2 sorption isotherm at 77 K (closed and open circle denote
adsorption and desorption, respectively) and (G) DFT pore size distribution of UU-100(Co).
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The structural constitution of the framework was analyzed
by transmission electron microscopy (TEM). A continuous
rotation electron diffraction (cRED) method44 revealed a
tetragonal unit cell (a = b = 27.3 Å, and c = 19.6 Å) with
P4/mbm as the likely space group (Figure S10, see ESI for
more details). The unit cell dimensions were further refined
by Pawley fitting of the PXRD data (a = b = 28.05(5) Å, and c
= 19.07(3)Å) (Figure 2A and S11, Table S3). The position of
the zirconium clusters within the unit-cell was determined
from the cRED data by applying Patterson method. Based on
the unit cell, space group and the positions of zirconium
clusters, a structural model of UU-100(Co) was generated
using the molecular cobaloxime as linkers and hexanuclear
Zr6 - clusters as SBUs and optimized by density functional
theory (DFT). The resulting structure has a molecular
formula [Zr6(µ3-O)8(OH)8(Cobaloxime)2], with eight of the
twelve octahedral edges of the nodal cluster connected to
cobaloxime linkers (Figure 1). The calculated Zr:Co ratio of
3:1 is roughly consistent with ICP-OES and SEM-EDX
analysis of UU-100(Co), although slightly lower
experimental Zr:Co ratios may suggest missing linker or
cluster disorder in the MOF. The ATR-IR spectrum of the
MOF contains Zr-µ3-O stretches at 659 cm-1 that originate
from the Zr6 nodes (Figure S12), in line with the proposed
model. The modelled structure of UU-100(Co) reveals
rectangular channels with elliptical pores caused by the
slightly bent structure of the cobaloxime linkers. The
distances between two cobalt centers across the channel
are 16.3 Å and 20.9 Å, yielding accessible pores with 9.0 Å
in width and 13.1 Å in length. HRTEM images along [110]
(Figure 2C-E) of UU-100(Co) display clearly lattice fringes
which correspond to the (001) planes (d001 = 19.1 Å) of the
structural model. The cobaloxime linker in UU-100(Co) is
structurally similar to the pyrene-based linker of NU-901,
and the two MOFs thus display certain structural
similarities.27
The structural integrity of the cobaloxime linker in UU-
100(Co) was confirmed by 1H NMR spectroscopy of
digested samples which exhibit peaks at chemical shifts
matching those of pristine [Co(dcpgH)(dcpgH2)]Cl2 (Figure
S13). It is very likely that the cobaloxime linkers undergo Cl–
/solvent ligand-exchange in the axial position during the
solvothermal synthesis and consequently, the cobaloxime
units in UU-100 (Co) can have a molecular formula of either
[Co(dcpgH)(dcpgH2)]Cl2 or [Co(dcpgH)2](Cl)(solvent).
Thermal stability of the MOF was investigated by
thermogravimetric analysis (TGA). The initial weight loss
below 100 °C is assigned to removal of guest solvent
molecules (Figure S14). Subsequent loss at ~250 °C is
attributed to the dehydroxylation of the Zr6 nodes. Above
400 °C, loss of the cobaloxime linker is observed that leads
to degradation of the framework and the formation of metal
oxide.
Activation of UU-100(Co) by solvent removal either under
high vacuum or by solvent exchange with acetone affects
the crystallinity of the MOF as evidenced by an altered PXRD
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pattern (Figure S6). The altered material exhibits three
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sharp reflections at 4.7°, 9.1°, and 18.0°, while several other
reflections that are present in the as-synthesized MOF
undergo broadening and decrease in intensity, suggesting
partial collapse of the structure. Retention of the reflections
corresponding to (001), (002) and (004) planes indicate
that the long-range order along the c-axis is preserved. The
gas sorption data of the activated material exhibits a
reversible type II behavior with a saturated N2 uptake of
285 cm3 g–1. The calculated Brunauer−Emmett−Teller
(BET) surface area of the MOF was 902±15 m2 g–1 (Figure
1F and S15) with a total pore volume of 0.43 cm3 g-1 as
determined by single-point method. Even though this
surface area is obtained for the partially collapsed material,
it is significantly higher that of a recently reported HER
catalyst based on a porphyrin-based MOF on carbon-
nanotubes (Hf12-CoDBP/CNT).36 The theoretical surface
area of UU-100 (Co) based on the model structure is 2867.9
m2 g–1, substantially larger than that of the material that is
obtained after solvent removal. Density functional theory
(DFT) fitting of the N2 adsorption data showed a pore width
distribution in the range of 12 to 18 Å (Figure 1G), which is
broadly consistent with the modelled structure of UU-
100(Co) as described above.
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Electrochemical characterization of UU-100(Co)|FTO.
UU-100(Co) was synthesized directly on fluorine-doped tin
oxide (FTO) to deduce the basic electrochemical properties
of the material. For this purpose, a self-assembled-
monolayer (SAM) of the cobaloxime linker on the FTO
substrate was first formed by soaking the FTO electrodes
overnight in a 1 mM solution of Co(dcpgH)(dcpgH2)Cl2 in
DMF. The SAM coated FTO slides were subsequently heated
in a mixture of ZrCl4, cobaloxime linker, DMF and acetic acid
at 80 °C for 5 days. The PXRD pattern of the as-prepared UU-
100(Co)|FTO thin film (Figure 3A) is consistent with that of
the bulk material and the SEM images (Figure 3B and S17)
show identical particle morphology. SEM-EDX (Figure S20)
and ICP-OES analyses of the UU-100(Co)|FTO film reveal
Zr:Co ratio of 2.7±0.1 and 3.1±0.2, respectively, consistent
with the calculated structure. The film thickness was
estimated to be ~1 µm from the images obtained by cross-
sectional SEM (Figure S18). Similar to the bulk UU-100(Co),
DMF removal by overnight soaking of the thin-film
electrodes in acetone leads to the altered PXRD pattern
(Figure S23).
Figure 3. (A) PXRD of UU-100(Co) thin films (λ = 1.5418 Å). SEM
images of (B) UU-100(Co)|FTO and (C) UU-100(Co)|GC. Figure
(B) shows the rod-shaped UU-100(Co) crystals on bare FTO
surface, while the glassy carbon surface in (C) is coated with
the smaller particles of UU-100(Co).
The cyclic voltammogram (CV) of UU-100(Co)|FTO shows a
reversible redox wave with an E1/2 = –1.28 V (Ep,c = –1.40 V;
Ep,c = –1.16 V; ν = 0.1 V s-1) that is assigned to the CoII/I couple
(Figure 4A). The potential of the CoII/I couple in UU-
100(Co)|FTO is shifted cathodically by ~150 mV relative to
that of the CoII/I couple in the free cobaloxime reference
(Figure S24, see ESI for details), consistent with the greater
negative charge of the tetra-deprotonated linker in the
MOF. Reductive features associated with the CoIII/II couple
are poorly defined in the first CV scan of UU-100(Co)|FTO
and completely absent in all subsequent scans. This
behavior stems most likely from the partial exchange of the
axial chloride ligands with solvent molecules during the
solvothermal synthesis, giving rise to mixtures of
cobaloxime linkers that differ in the axial ligands. Upon
reduction of the cobaloximes from CoIII to CoI in the first
reductive scan, axial ligands are completely displaced by
solvent molecules which pushes the CoIII/II couple outside
the scanned potential window (Figure S26, see ESI for
details). As a result, all subsequent CV scans of UU-
100(Co)|FTO show exclusively the CoII/I couple (Figure 4A).
The cobaloxime SAM-modified electrode (SAM|FTO) and
UU-100(Co)|FTO were further characterized by X-ray
photoelectron spectroscopy (XPS). The SAM of the
cobaloxime linker on FTO was confirmed by the Co 2p and
N 1s peaks (Figure S21 and S22). The spectra of UU-
100(Co)|FTO exhibit four additional zirconium peaks, two
at 332.5 and 346.5 corresponding to Zr 3p3/2 and 3p1/2, and
The amount of electroactive cobaloxime linkers was
determined by chronoamperometry. Potential stepping
from –0.05 to –1.5 V, and from –1.5 to –0.05 V revealed a
surface concentration of 7.010-8 mol cm-2 and 6.810-8
mol cm-2 (Figure S27 and S28), respectively. Compared to
the total cobalt content of 12.210–8 mol cm–2 as
determined by ICP-OES, 54±15% (average over three
electrodes) of the cobaloxime linkers in the MOF are thus
electrochemically addressable, which compares favorably
to the 31.9% electroactive catalyst centers in a recently
reported porphyrin-based MOF on carbon-nanotubes.36
two more at 181.5 and 183.8 for Zr 3d5/2 and 3d3/2
,
respectively. Two sets of peaks are observed in the cobalt
region of UU-100(Co)|FTO, with binding energies of ∼781
and ∼795 eV, which correspond to the 2p3/2 and 2p1/2 levels
in the expected 2:1 ratio.48 Absence of peaks below 779 eV
excludes the presence of metallic cobalt on the surface.
Deconvolution of the C 1s region generates three peaks at
283.8, 285.2, and 287.6 eV that are attributed to C=C/C-C,
C=N, and C=O bonds, respectively (Figure S22).
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Figure 4. (A) Cyclic voltammograms of UU-100(Co)|FTO electrodes at different scan rate in DMF containing 0.1 M LiClO4 (10,
20, 30, 40, 60, 80, and 100 mV s-1); inset shows the linear dependency of the peak current (CoII/CoI couple) on the scan rate
(ν) at scan rates under 100 mV s–1. (B) UV-Vis spectroelectrochemical data on UU-100(Co)|FTO thin-film electrodes, showing
the steady-state relative absorbance at different applied potentials. (C) Optical transmittance kinetic curve of the UU-100(Co)
thin-films measured at 520 and 670 nm by switching the potential from –0.05 to –1.5 V (electrode held at the potential for 60
s). (D) XPS core-level spectra of UU-100(Co)|FTO films showing Co 2p region before (black) and after (red) electrochromic
switching tests. Blue and purple filled peaks represent the peak-fits corresponding to Co 2p1/2 and 2p3/2, respectively. (E)
Photographs of the UU-100(Co)|FTO thin films at –0.05 and –1.5 V (vs Fc+/0).
Exploiting the electrochromism of molecular cobaloximes,49
UV-Vis spectroelectrochemistry can be used to probe the
kinetics of electron transport within the film. Under an
applied potential of –1.50 V in DMF (0.1 M LiClO4), freshly
prepared UU-100(Co)|FTO electrodes undergo a colour
change from pale yellow to blue with the appearance of new
absorption bands at ~520 and 670 nm that arise from
[CoI(dcpgH)2] (Figure 4B and 4E). Re-oxidation of the film
by stepping the potential back to –0.05 V results in a colour
change from blue to red. The colour change is accompanied
by a bleach of the 670 nm band and a slight red-shift (~1-2
nm) of the 520 nm peak in the UV/Vis spectrum. This
behavior indicates the formation of [CoII(dcpgH)2] linkers in
the MOF film at –0.05 V and is consistent with the CV results
described above in that the CoIII/II couple is at a more
positive potential once the film has been reduced and the
chloride ligands have detached form the cobaloximes. The
Co centers in UU-100(Co)|FTO can reversibly be cycled
between CoII and CoI as shown by repetitive electrochromic
switching experiments, monitoring the absorbance changes
at 520 and 670 nm (Figure 4C). After ten such cycles, PXRD,
XPS, and SEM/EDX (Figure 4D, S19 and S23) indicate that
the structural integrity of the MOF films is maintained. All
spectral assignments are corroborated by the analogous
experiments on the homogenous cobaloxime linker (Figure
S32).
Utilizing the diagnostic absorbance of the reduced CoI linker
at 670 nm, the concentration of reduced species inside UU-
100(Co)|FTO can be directly measured as a function of time.
The obtained trace can be used to extract charge transport
kinetics by applying the modified Cottrell equation,24
퐷푎푝푝
2퐴푚푎푥
훥퐴 =
푡1/2
휋
푑푓
where Amax denotes the absorbance maximum, t time in
seconds, ΔA change in absorbance at 670 nm, and df the
thickness of the film. A fit of the data yields an apparent
diffusion coefficient (Dapp) of 4.1(±0.8) 10-8 cm2 s-1 (Figure
S34) which is several orders of magnitude higher than that
of other MOF thin film electrodes with redox-active linkers
measured in organic solvents (10–11-10–13 cm2 s−1).25,30-32
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Figure 5. (A) Cyclic voltammograms of UU-100(Co)|GC at different scan rates (0.2, 0.16, 0.14, 0.12, 0.1, 0.8, 0.06, 0.05, and
0.025 V s-1) in DMF. (B) Linear sweep voltammograms (LSV) of UU-100(Co)|GC (red) and blank GC (black) electrodes at pH 4
recorded at 20 mV s-1 (the dashed line shows the potential applied for electrolyses experiments). (C) Tafel plot derived from
the LSV; black line shows the linear fit of the data in the low-overpotential region.
Since charge transport in UU-100(Co) films occurs through
redox hopping between the redox-active cobaloxime
linkers, CVs were recorded at different scan rates to probe
the kinetics of the redox process (Figures S29-S31). At slow
scan rates (ν = 5–80 mV/s), the cathodic and anodic peak
currents of the CoII/I couple show a linear dependence on the
scan rate, which is consistent with a surface confined
process that is not limited by mass transport (Figure S30).
At faster scan rates (> 100 mV s-1), however, the peak
currents clearly deviate from linear correlation (inset in
Figure 4A) and show characteristics of a diffusion-
controlled process, as shown by their linear dependence on
ν1/2 (Figure S30). This assignment is further corroborated
by a double-logarithmic analysis of the voltammetry data
which shows that the linear fit of log(ip) versus log(ν) plot
in the slower scan rate region (< 80 mV s-1) has a slope of
0.92, whereas the corresponding slope in the faster scan
rate range is 0.51 (Figure S31). Such a switch in scan rate
dependence has previously been observed by Morris et al
on UiO-67 materials with varying degrees of incorporated
redox-active [Ru(bpy)3]2+ linkers,48 but, to the best of our
knowledge, has not been observed in one and the same MOF
simply by altering the scan rates. Such a behavior is
predicted by theory though, and is well established in the
redox polymer film literature.50 The threshold scan rate at
which the peak current responses change from that of a
surface-confined to a diffusional process can be calculated
from the diffusion coefficient Dapp and the film thickness df,
according to,
Using the values for UU-100(Co)|FTO determined above,
the switching scan rate is calculated to 100(±25) mV s–1
which is in excellent agreement with the experimental
value. This observation in a MOF with a significantly high
fraction of electrochemically accessible metallo-linkers
(>50%) is for the first time enabled by the unmatched high
Dapp in UU-100(Co)|FTO. Related MOF films with redox
active linkers (Co/Fe-porphyrins and naphthalene-diimide)
with smaller Dapp exhibit diffusion-controlled redox
processes even at slow scan rates.24-26,30,32,34
Electrocatalytic HER by UU-100(Co)|GC. As FTO is
unstable in aqueous solution at applied negative potential,
UU-100(Co) was grown on glassy carbon (GC) electrodes to
study its HER activity. The UU-100(Co)|GC electrodes were
prepared under solvothermal conditions on GC that had
been functionalized with carboxylic acids by
electrochemical diazonium grafting. PXRD, and SEM of the
UU-100(Co)|GC electrodes were identical to those of the
bulk material, confirming its successful formation (Figure 3
and S35). Concentration of the electroactive cobaloxime
linker determined from cyclic voltammetry was ~1.610-8
mol cm–2 which shows electrochemical accessibility of
cobalt centers in GC (31%) is somewhat lower than that
observed in the FTO films (Figure 5A). The catalytic
performance of UU-100(Co)|GC electrodes was determined
by linear sweep voltammetry (LSV) experiments in acetate
buffer at pH 4, which showed the evolution of a catalytic
wave with an early onset potential of ~–0.15 V vs. RHE
(Figure 5B). Tafel analysis of the LSV data gave a Tafel slope
of 250 mV dec-1 and an exchange current density of ~10–4.2
A cm-2 (Figure 5C).
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Figure 6. (A) Controlled potential electrolysis using UU-100(Co)|GC electrode at –0.45 V vs RHE in acetate buffer at pH 4. The black
trace represents geometric current density, and the red circles denote Faradaic efficiency for H2 evolution. (B) PXRD patterns of the
as-synthesized electrodes (black), after solvent exchange with acetone for 24 h (blue), and after 5 hours of electrocatalysis (red),
demonstrate that the MOF retains its crystalline structure after electrolysis (see Figure S6 and S7 for PXRD of solvent exchanged UU-
100 (Co)). (C) SEM images of the UU-100(Co)|GC after electrolysis and (D) corresponding EDX line scan showing retention of rod-
like morphology and uniform distribution of Zr and Co in the MOF crystal.
Controlled potential electrolysis of UU-100(Co)|GC
electrodes in NaClO4 (0.1 M)/ acetate (0.2 M) buffer at pH 4
and at an applied potential of −0.45 V vs. RHE consumes 29
C cm-2 charge over 5 hours (Figure 6A) with a stable current
density of ~ –1.7 mA cm-2. Analysis of the gas mixture in the
headspace of the working compartment of the electrolysis
cell by gas chromatography confirmed evolution of 66 µmol
H2 (geometric surface area of electrode = 0.5 cm2) with a
Faradaic yield of 84 ± 5%. This corresponds to a turnover
number (TONCo) of 8250 based on the amount of
electroactive cobalt in the film, with an average formal
turnover frequency (TOFCo) of 1650 h-1. Unmodified GC
electrodes display much lower H2-evolution activity under
the same condition (Figure S44). The MOF films show high
durability as demonstrated by their sustained activity over
the course of 18 h electrolysis at pH 4 (Figure S37) during
which 309 µmol H2 cm–2 was generated with 79 ± 3%
Faradaic efficiency (FEH2). A total TONCo of 20875 was
obtained after 18 h and an average TOFCo of 1171 h-1. The
relatively Lower TONCo value obtained from long-term
electrolysis is likely caused by H2 leakage leading to low
FEH2. Post-catalysis structural integrity of the framework
was probed by PXRD analysis of UU-100(Co)|GC electrodes
which shows that the two major reflections at ~4.5° and
~9° were present even after 18 h electrolysis (Figure 6B
and S38). This is consistent with the presence of the
partially collapsed structure that is also obtained by DMF
solvent removal from the as-prepared UU-100 (Co) without
any applied bias. SEM-EDX analysis of the MOF film after
electrolysis showed the largely unchanged rod-like
morphology. Additionally, the elemental composition of UU-
100(Co) remained intact as demonstrated by EDX line-
profile of the MOF particles on the electrode (Figure S39-
S43) and a similar Zr:Co ratio of 2.4±0.2 (Figure 6 and S43).
However, prolonged electrolysis (18 h) damages the
smooth exterior of the crystals significantly and causes
flaking of the MOF nanosheets, leading to a rough surface as
shown by SEM image (Figure S44).
The cobaloxime linkers are coordination compounds, and
as the crystallinity and morphology of UU-100(Co) is
preserved during electrolysis, the linkers must have
remained intact. With structurally uncompromised HECs,
there is no reason to suspect that catalysis is not molecular
in nature. The performance of UU-100(Co)|GC as a HEC is
vastly superior to that of the most efficient electrocatalytic
materials with analogous molecular cobaloxime catalysts.
Such systems have been immobilized on carbon nanotubes
(CNT) and CNT-polymer composites, and are reported to
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support 120 and 420 turnovers, respectively, at near
the free coordination sites of the SBUs.38 Such developments
will further close the gap between molecular and materials
catalysis, potentially even by including higher coordination
sphere active site concepts from bioinorganic chemistry.
neutral pH.51,52 The molecular cobaloxime linker itself
displayed a TON of 10 under homogeneous condition – 2 h
electrolysis in mildly acidic aqueous solution (Figure S46).
These TONs are substantially lower than the TONCo
observed for UU-100(Co) (more than 20000) which
highlights the stabilization of cobaloxime-core provided by
the rigid 3D architecture of the framework. In comparison
to the only other electrocatalytic MOF for hydrogen
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ASSOCIATED CONTENT
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
General methods and instrumentation, synthetic details,
characterization of materials, electrode fabrication and
additional electrochemical data. Crystallographic data for the
cobaloxime metallolinker deposited at the Cambridge
Crystallographic Data Centre under deposition nos. CCDC
1910684.
evolution, (Hf12-CoDBP/CNT), UU-100(Co) displays
a
9
slower rate, but operates under milder conditions (pH 4
versus pH 0) and without any nafion binder to maintain
stable current densities.36 From a more fundamental
perspective, an important distinction between the two
electrocatalytic MOFs is the necessity of growing the MOF
on carboxylated CNTs for realizing the HER activity in the
case of Hf12-CoDPB, which emphasizes the excellent
‘molecular-wiring’ in UU-100(Co) imparted by the
cobaloxime linkers.
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AUTHOR INFORMATION
Corresponding Author
*sascha.ott@kemi.uu.se
Author Contributions
§ S.R. and Z.H. contributed equally.
CONCLUSION
The present work describes electrocatalytic hydrogen
evolution by a MOF that exclusively consists of molecular
HECs, specifically cobaloximes, as metallo-linkers. Structure
elucidation by cRED reveals that UU-100(Co) contains
rectangular channels with elliptical pores. Zr6(µ3-O)8(OH)8
clusters constitute the SBUs with eight of the twelve
octahedral edges being connected to the cobaloxime
linkers. UU-100(Co) can be grown directly on conducting
substrates, which allows for in-depth study of the materials’
electrochemical and charge transport properties. The
individual cobaloxime units in UU-100(Co) function as both
redox mediator and catalyst. The CV response of the films is
not limited by diffusion, indicating fast electron transport
kinetics which is further corroborated by direct
spectroelectrochemical methods on UU-100(Co)|FTO. The
apparent diffusion coefficient that takes account of
contributions from both electron hopping as well as
electrolyte diffusion was determined to 4.1 (±0.8) 10-8
cm2 s-1, which is orders of magnitude higher than that of
related porphyrin-based MOF thin films.30-33 The structural
integrity of the molecular cobaloxime catalyst is greatly
enhanced by MOF incorporation, and consequently,
electrocatalytic hydrogen evolution from water at pH = 4 by
UU-100(Co)|GC is highly durable for at least 18 hours.
Orcid
Souvik Roy: 0000-0003-0146-5283
Xiaodong Zou: 0000-0001-6748-6656
Sascha Ott: 0000-0002-1691-729X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support from the Wenner-Gren Foundations
(postdoctoral stipend to S.R.), the European Research Council
(ERC-CoG2015-681895_MOFcat), the Swedish Research
Council, and the Swedish Energy Agency is gratefully
acknowledged. Z.H. and X.Z. acknowledge the support by the
CATSS project (2016.0072) from the Knut and Alice
Wallenberg Foundation (KAW) and the Swedish Research
Council (VR, 2017-04321). Dr. Jean Pettersson is
acknowledged for ICP-OES measurements. Mr. Ben Johnson
and Dr. Brian McCarthy are acknowledged for valuable
discussions.
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