Efficient Electrocatalytic Proton Reduction with Carbon Nanotube-Supported Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.8b09521

Hydrogen production from Earth-abundant catalysts remains an important but difficult challenge. Here we report the growth of Hf₁₂-porphyrin metal-organic frameworks (MOFs) on carbon nanotubes (CNTs) for electrocatalytic proton reduction. Covale

Full text of DOI:10.1021/jacs.8b09521

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Communication
Efficient Electrocatalytic Proton Reduction with Carbon
Nanotube-Supported Metal-Organic Frameworks
Daniel Micheroni, Guangxu Lan, and Wenbin Lin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09521 • Publication Date (Web): 03 Nov 2018
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Efficient Electrocatalytic Proton Reduction with Carbon Nano-
tube-Supported Metal-Organic Frameworks
Daniel Micheroni, Guangxu Lan, and Wenbin Lin*
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Department of Chemistry, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637 United States
Supporting Information Placeholder
transfer. To improve electrical conductivity, MOFs were pyro‐
lyzed to metal nanoparticles encapsulated in carbon matrices
for electrocatalysis.19‐21 Alternatively, electrocatalysis was re‐
ported for ultra‐thin films comprised of a few MOF layers
dropcasted on or covalently tethered to the electrode sur‐
face.22‐30
ABSTRACT: Hydrogen production from Earth‐abundant cat‐
alysts remains an important but difficult challenge. Here we
report the growth of Hf12‐porphyrin metal‐organic frame‐
works (MOFs) on carbon nanotubes (CNTs) for electrocata‐
lytic proton reduction. Covalent attachment of MOF nano‐
plates to conductive CNTs improves electron transfer from the
electrode to Co‐porphyrin active sites, leading to effective pro‐
ton reduction via protonation of a CoI‐H intermediate. The
Hf12‐CoDBP/CNT assembly afforded a turnover number of
32,000 in 30 minutes with a turnover frequency of 17.7 S‐1, plac‐
ing it among the most active Co‐based molecular electrocata‐
lysts.
We recently reported a series of MOFs comprised of M12 (M
= Zr and Hf) secondary building units (SBUs) and dicarbox‐
ylate ligands that fulfill many requirements of efficient elec‐
trocatalytsts.31‐33 These MOFs are a few nanometers thick with
short inter‐ligand distances (<1 nm), leading to efficient elec‐
tron transfer through the MOF nanoplates. They are stable in
aqueous environments. Herein we report the design of Hf12‐
CoDBP comprised of Co‐metalated 5,15‐di(p‐benzoato)‐por‐
phyrin (CoDBP) bridging ligands as a HER electrocatalyst
(Figure 1). Molecular Co‐porphyrin systems have previously
been shown to effectively catalyze the HER.21, 34, 35 Covalent
tethering of Hf12‐CoDBP to multi‐walled carbon nanotubes
(CNTs) significantly improved electrical conductivity, leading
to drastically enhanced HER turnover number (TON) and
turnover frequency (TOF) for the Hf12‐CoDBP/CNT hybrid.
Growing global energy needs demand the development of
renewable and clean energy technologies. Of numerous pro‐
posed approaches, hydrogen production via catalytic water
splitting by either photochemical or electrochemical means
presents an attractive solution due to the abundance of water
and the ability of hydrogen to store energy and to transform
inert molecules into chemical fuels.1 However, most hydrogen
is currently produced by steam reforming of natural gas as
large‐scale production of hydrogen using the solar energy in‐
put remains a technical challenge.
Hydrogen evolution reaction (HER) is a key half reaction of
hydrogen production via water splitting. Typically, electrocat‐
alytic HER is mediated by inorganic materials which fall near
the top of the kinetic volcano plot based on the strength of the
metal‐hydrogen bond.1, 2 However, these materials are often
comprised of precious metals including Pt, Re, Ru, and Ir,
making their large‐scale implementation unrealistic.
To overcome the scarcity and high cost of precious metal
electrocatalysts, numerous molecular systems and semicon‐
ductors have been developed based on Earth‐abundant metals
including Co,3‐5 Ni,6‐8 Fe,9, 10 and Mo.11‐13 Tuning of organic lig‐
ands has afforded molecular HER catalysts with low overpo‐
tentials and high catalytic activities. However, these molecu‐
lar HER catalysts are often insoluble in water and their activi‐
ties are limited by diffusion to the electrodes.
As a new class of tunable molecular materials, metal‐or‐
ganic frameworks (MOFs) have provided a unique platform to
design single‐site solid catalysts.14 The regularity of MOF
structures affords high densities of catalytic sites, their high
porosity allows for rapid mass transport, and their periodicity
facilitates the characterization of catalytic centers. Although
MOFs have been explored as photocatalysts for HER,15‐18 they
have not found much use as electrocatalysts due to their elec‐
trically insulating nature and sluggish inter‐ligand electron
Figure 1. a) Covalent attachment of Hf12‐CoDBP to CNTs en‐
hances electrocatalytic HER at CoDBP centers. b) The HER is
proposed to proceed through a CoIII‐H intermediate. Orange
= Hf, purple = Co, black = C, red= O, green = N, and white =
H.
Hf12‐CoDBP with the ideal composition of Hf12O8(µ3‐
OH)8(µ2‐OH)6(CoDBP)9 was prepared via a solvothermal re‐
action between H2(CoDBP) and HfCl4 in DMF at 85 C for 72h.
Alternatively, Hf12‐CoDBP was prepared by metalation of
known Hf12‐H2DBP with CoCl2 in DMF at 80 C.36 Both proce‐
dures afforded the same MOF structure with four major PXRD
peaks at 2θ = 3.8, 6.55, 7.57, and 9.99  corresponding to hk0
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indices; other PXRD peaks are unobservable due to inherent
Nitrogen sorption studies revealed a Brunauer‐Emmett‐
Teller (BET) surface area of 115.4 4 m2/g for Hf12‐CoDBP/CNT,
which is significantly higher than that of CNTs (SBET = 78.12
m2/g, Figure S8), due to the deposition of highly porous Hf12‐
CoDBP (SBET = 509.3 m2/g) on CNTs. The pore sizes of Hf12‐
CoDBP/CNT ranged from 2 to 3 nm for the MOF lattice and 4
to 10 nm for the natural porosity of the CNTs. SBET and TGA
results thus suggest that Hf12‐CoDBP comprises 20‐25 % of the
overall mass of Hf12‐CoDBP/CNT. The high loading of Hf12‐
CoDBP is also evident in TEM images.
The structural and chemical properties of Hf12‐CoDBP lend
itself toward electrochemical applications when covalently at‐
tached to CNTs. Direct tethering of Hf12‐CoDBP to a conduc‐
tive surface increases the rate of electron transfer to CoDBP
catalytic sites. The nanoplate morphology of Hf12‐CoDBP
places active sites in close proximity to the conductive sup‐
port, the electrolyte, and the substrate simultaneously. These
features not only overcome the diffusion constraint of the ac‐
tive catalyst, but also prevent detrimental bimolecular deacti‐
vation pathways as a result of active site isolation. The depo‐
sition of highly porous Hf12‐CoDBP on CNTs also increases the
number of active sites on the electrode surface.
defects along the c‐direction.33 Hf12‐CoDBP has a diameter of
30‐100 nm (Figures 2 and S1) by TEM and a thickness ranging
of 10‐40 nm by atomic force microscopy (AFM), correlating to
the stacking of 3 to 12 unit cells along the c‐axis (Figure 2a).
The thickness of Hf12‐CoDBP is thus smaller than that of the
electric double layer, making it a potential candidate for elec‐
trocatalysis.
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Figure 2. a) AFM analysis of Hf12‐CoDBP shows 20 and 25 nm
thick nanoplates. b-d) TEM images of Hf12‐CoDBP (b) and Hf12‐
CoDBP/CNT (c, d) show the retention of crystalline MOF na‐
noplate morphology when covalently bound to CNTs. The
Fast Fourier Transform in inset of (d) shows expected 6‐fold
symmetry along the c‐axis.
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Hf12‐CoDBP/CNT was synthesized by heating a DMF solu‐
tion of CoDBP and HfCl4 in the presence of carboxylated
multi‐walled CNTs at 85 C for 72 h. The carboxylate groups
on the CNTs allow for direct attachment to the MOF via the
SBU, affording a densely packed Hf12‐CoDBP/CNT hetero‐
structure with the unchanged plate‐like morphology of Hf12‐
CoDBP (Figure 2c, d). PXRD studies indicated that Hf12‐
CoDBP/CNT adopts the same crystalline structure as Hf12‐
CoDBP (Figure S3), while high resolution TEM (HRTEM) im‐
ages showed lattice points with inter‐SBU distances of 2.7 nm.
The Fast Fourier Transform (FFT) showed six‐fold symmetry
which is consistent with the projection of Hf‐CoDBP structure
along the (001) direction (Figure 2d).
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Figure 3. a) The CV curves of the homogenous species (black),
Hf12‐CoDBP (Red), and Hf12‐CoDBP/CNT (Blue) in 0.1 M
[TBA]PF6 dissolved in acetonitrile. b,c) The CV curves of
H2(CoDBP) (b) and Hf12-CoDBP (c) with (blue) and without (red)
TFA (0.026 M) show similar hydrogen production behavior. d) Dif-
ferential pulse voltammetry of Hf12‐CoDBP in acetonitrile with
(blue) and without (red) TFA suggest the HER process occurs
via protonation of a CoI-H intermediate.
X‐ray photoelectron spectroscopy (XPS) indicated the pres‐
ence of Co3+ and Hf4+ centers in Hf12‐CoDBP/CNT (Figure S4).
Integration of XPS Co2P3/2 and Hf4d3/2 peaks gave a Hf:Co ra‐
tio of 1.78 for Hf12‐DBP‐Co/CNT. Thermogravimetric analysis
(TGA) gave a Hf:porphyrin ratio of 1.46 ‐ 1.66 (Figure S5) and
inductively coupled plasma‐mass spectrometry (ICP‐MS) gave
a Hf:Co ratio of 1.57:1 for Hf12‐DBP‐Co/CNT. The Hf:CoDBP
ratios from these analyses gave an average Hf:CoDBP ratio of
1.62, which deviates from 1.33 expected for the idealized Hf12‐
CoDBP structure. The MOF structure of Hf12‐CoDBP/CNT is
thus highly defected along the c‐axis and has an empirical for‐
mula of Hf12O8(µ3‐OH)8(µ2‐OH)6(CoDBP)7.4. Minimal Co3+
ions leached from CoDBP during the MOF growth as shown
by UV‐Vis spectroscopy (Figures S6 and S7).
Cyclic voltammetry measurements of Hf12‐CoDBP, Hf12‐
CoDBP/CNT, and homogenous CoDBP showed a reversible
one electron peak at ‐0.485 V vs the NHE in acetonitrile for
the CoII/I reduction of CoDBP (Figure 3).37 Upon addition of
0.026 M trifluoroacetic acid (TFA) as a proton source, the CoI/II
anodic peak disappeared with a concurrent increase in current
density attributable to catalytic proton reduction. The similar
behavior seen in Hf12‐CoDBP, Hf12‐CoDBP/CNT, and CoDBP
indicates an identical catalytic process, likely through the
Heyrovsky pathway via a CoI‐H intermediate, as the physical
constraints of the MOF and the thermodynamics of the sys‐
tem ensure the process proceeds via the unimolecular path‐
way through protonation to a CoI species.
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Integration of the CoII/CoI anodic peak for the Hf12‐
time lengths of electrolysis at a  of 715 mV, producing hydro‐
gen with a TON of 3.2×104 after thirty minutes of electrolysis
and a TOF= 17.7 s‐1 (Figure 3c). Hydrogen gas was detected at
a  of 315 mV at pH = 1 and Hf12‐CoDBP/CNT/Nafion showed
moderate activity in aqueous solutions up to pH = 5 (Figure
S15). In comparison, bare glassy carbon electrode, bare CNTs,
and Hf12‐H2DBP/CNT need an overpotential of ≥515 mV to
detect any trace of hydrogen, most likely from nanoparticle
formation 38, 39 at a rate that is at least one order of magnitude
lower than that of Hf12‐CoDBP/CNT/Nafion (Table S1).
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CoDBP/CNT species in acetonitrile revealed that 31.9% of total
Co species (0.947 nmol by ICP‐MS) on the electrode surface
were electrochemically active. This value represents a 114
times increase over that of Hf12‐CoDBP; dropcasting 0.1 mg of
Hf12‐CoDBP/CNT on a 0.0625 cm2 glassy carbon electrode
yields a catalytically active surface area of 49.6 cm2 (Figure
S10). A linear relationship for the log of the current vs the log
of the scan rate with a slope of 0.54 was found for Hf12‐
CoDBP/CNT, indicating the redox process occurs via a charge
hopping mechanism (Figure S11). In comparison, Hf12‐CoDBP
synthesized with non‐carboxylated CNTs showed only 0.29%
electrochemically active Co sites (Figure S12d). These results
demonstrate the importance of covalent tethering of the MOF
to the CNT in achieving good electrocatalytic activity by in‐
creasing the overall surface area of the electrode to enhance
electron injection into the catalytic sites.
Hf12‐CoDBP/CNT greatly outperformed Hf12‐CoDBP and
Hf12‐CoDBP/non‐carboxylated CNT in electrocatalytic HER in
aqueous media (Table S2). Dropcasted Hf12‐CoDBP/CNT ex‐
hibited a remarkable electrocatalytic HER activity in a pH = 1
perchloric acid solution, with a current density of 10 mA/cm2
at an overpotential () of 650 mV. This corresponds to a Tafel
slope of 178 mV/dec, which is consistent with a rate limiting
step involving the adsorption of a proton to the catalytic site
within the Volmer portion of the HER (Figure 4a,b). The im‐
portance of the CNT tethering was further supported by the
increased current densities, TONs, and Faradaic efficiencies
for Hf12‐CoDBP/CNT samples with higher CNT loadings (Fig‐
ure S13). However, long term stability of the Hf12‐CoDBP/CNT
system remained an issue as stirring and hydrogen bubble for‐
mation tended to shear the catalyst off the electrode surface.
9
Hf12‐CoDBP/CNT/Nafion showed good efficacy of electro‐
catalytic HER at varying potentials (Figure 4c). At  > 415 mV,
the Faradaic efficiency averaged at 92.4%. The Faradaic effi‐
ciency was only 51.2% at  = 315 mV, likely due to dominance
of CNT reduction and other side reactions over HER at low
overpotentials. Above  > 415, the TON showed a roughly lin‐
ear increase with ovepotential; a slight tailing off was observed
at higher potentials indicative of saturation of active sites with
protons. Hf12‐CoDBP/CNT/Nafion also showed very good sta‐
bility with consistent hydrogen production across varying po‐
tentials for at least seven hours (Figure 3d). Hf12‐CoDBP/CNT
is thus competitive with other water stable porphyrin and Co
HER catalysts in terms of onset potential and TON (Table S3).
In addition to enhancing stability, Nafion also slows down
the transport of larger ions (such as Co2+) to the electrode sur‐
face to mitigate the formation of nanoparticles, which can also
catalyze proton reduction, thus ensuring that HER by Hf12‐
CoDBP/CNT is entirely molecular in nature. In agreement
with this conjecture, Hf12‐CoDBP/CNT/Nafion showed good
stability upon 10000 CV cycles and no significant change in
the CV curve before and after electrocatalytic HER (Figure
S17). In addition, ICP‐MS analyses showed negligible leaching
of Co (<0.2%) and Hf (<0.1 %) from Hf12‐CoDBP/CNT/Nafion
after electrolysis at =715 mV for one hour based on the metal
loadings on the electrode surface. PXRD patterns (Figure S3)
and UV‐Vis spectra (Figure S17c) of Hf12‐CoDBP/CNT/Nafion
remained unchanged after electrolysis at =715 mV for 18
hours. TEM imaging showed no Co nanoparticle formation
during electrolysis (Figure S17d).
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In conclusion, we have synthesized Co‐porphyrin MOFs
supported on CNT for efficient electrocatalytic proton reduc‐
tion. Covalent attachment of Co‐porphyrin MOFs to CNTs sig‐
nificantly increases the number of catalytically active sites by
increasing both the surface areas of conductive supports and
the percentage of active sites. The MOF/CNT hybrid is highly
active for HER in acidic media with an onset potential of 315
mV and TOFs of over 17.7 s‐1. This straightforward synthetic
strategy should be amenable to the design of other MOF/CNT
heterostructures for electrochemical applications.
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Figure 4. a) CV curves of Hf12‐CoDBP/CNT and Hf12‐
CoDBP/CNT/Nafion compared to their controls in 0.1 M aqueous
perchloric acid and b) their corresponding Tafel curves. c) TON
(black) and Faradaic efficiency (red) measurements of Hf12‐
CoDBP/CNT/Nafion after 30 minutes of electrolysis at varying
overpotenials at pH = 1. d) Time‐dependent current densities
of Hf12‐CoDBP/CNT/Nafion at  = 515 and 715 mV, showing
sustained HER for >7 hours.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org. Synthesis and characterization
of MOF and MOF/CNT samples, cyclic voltammetry, and elec‐
trocatalytic proton reduction.
AUTHOR INFORMATION
Corresponding Author
Proton conducting Nafion was added to the Hf12‐
CoDBP/CNT suspension to improve thin film stability. Hf12‐
CoDBP/CNT/Nafion maintained stable currents for varying
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*wenbinlin@uchicago.edu
Notes
This work was partially supported by National Science Foun‐
dation (DMR‐1308229).
1
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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