Cooperative electrochemical water oxidation by Zr nodes and Ni-porphyrin linkers of a PCN-224 MOF thin film

Article abstract of DOI:10.1039/c6ta05877a

Here, we demonstrate a new strategy for cooperative catalysis and proton abstraction via the incorporation of independent species competent in the desired reactivity into a metal-organic framework (MOF) thin film. The highly porous MOF, designated as PCN-224-Ni, is constructed by Zr-oxo nodes and nickel(ii) porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO and were found to electrochemically facilitate the water oxidation reaction at near neutral pH.

Full text of DOI:10.1039/c6ta05877a

View Article Online
View Journal
Journal of
Materials Chemistry A
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. M. Usov, S. R.
Ahrenholtz, W. A. Maza, B. M. Stratakes, C. C. Epley, M. C. Kessinger, J. Zhu and A. J. Morris, J. Mater.
Chem. A, 2016, DOI: 10.1039/C6TA05877A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/materialsA

Please do not adjust margins
Journal of Materials Chemistry A
Page 1 of 6
View Article Online
DOI: 10.1039/C6TA05877A
Journal Name
COMMUNICATION
Cooperative electrochemical water oxidation by Zr nodes and Ni-
porphyrin linkers of a PCN-224 MOF thin film
Received 00th January 20xx,
Accepted 00th January 20xx
a†
a†
a
a
a
a
a
P. M. Usov, S. R. Ahrenholtz, W. A. Maza, B. Stratakes, C. C. Epley, M. C. Kessinger, J. Zhu
a
and A. J. Morris
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here, we demonstrate a new strategy for cooperative catalysis Akin to natural photosynthesis, a current thrust in catalyst
and proton abstraction via the incorporation of independent development is to build proton management capabilities
species competent in the desired reactivity into a metal-organic about the periphery of the catalytically active site. Indeed,
framework (MOF) thin film. The highly porous MOF, designated as proton management has been used to produce impressive
PCN-224-Ni, is constructed by Zr-oxo nodes and nickel(II) enhancements in catalytic activity across all aspects of artificial
3
4-6
porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO photosynthesis – proton reduction, CO
2
reduction, and
7
and were found to electrochemically facilitate the water oxidation water oxidation. The aforementioned reactions are proton-
reaction at near neutral pH.
coupled electron transfer processes (PCET) with three possible
reaction pathways to traverse: (1) proton transfer followed by
electron transfer (PTET), (2) electron transfer followed by
proton transfer (ETPT), or concerted PCET. Manipulation of the
proton transfer pathway via the secondary coordination
sphere directly affects kinetic and thermodynamic barriers.
Therefore, at the simplest level reaction rates can increase, if
proton-transfer is rate limiting. Alternatively, the balance
between the barriers of PTET, ETPT, and PCET could shift such
that the overall reaction mechanism is altered. Herein, we
report an alternative to traditional catalyst approaches, metal-
organic frameworks (MOFs), which provide a new paradigm
for how to manipulate proton management and catalytic
activity in heterogeneous electrocatalysis.
Increasing energy demands and steadily accumulating levels of
detrimental greenhouse gases necessitates the continued
1
search for alternative, renewable energy sources. Solar
energy has the capacity to power all of human civilization and
therefore, has emerged as a leading carbon-neutral energy
2
alternative. However, two challenges remain in the
harnessing of solar energy: (1) the intermittent nature of the
sun necessitates energy storage technology, and (2) the cost of
efficient solar cell installations provides
a barrier to
widespread adoption. Artificial photosynthesis – the direct
conversion of sunlight into a stored fuel – is one possible
method by which to overcome the first barrier. At the simplest
level, an artificial photosynthetic system must (1) absorb light,
generating an excited state; (2) separate charge, resulting in
long-lived redox equivalents, and (3) catalyze desired
reactivity. The reactivity catalyzed is the oxidation of water
and the reduction of either CO2 or protons to yield a
combustible fuel, Equations (1-4).
The properties and ease of design offered by MOFs presents a
unique alternative to common water oxidation catalysts. MOFs
are porous, 3D coordination polymers commonly comprised of
8
metal-oxo clusters bridged by organic linker molecules. In this
regard, MOFs bridge the gap between molecular and material
based approaches to water oxidation. The pore size and
environment, surface area, and morphology of MOFs can be
synthetically tuned to generate ideal properties for a variety of
applications. The porosity imparted by the size and geometry
of the linkers, as well as the coordination sphere around the
metal clusters affords MOFs an increased surface area and a
higher density of accessible catalytic sites as compared to
+
-
2
2
H O
O
2
+ 4H + 4e
CH OH + H
CH + 2H
(1)
+
-
CO
2
+ 6H + 6e
3
2
O
(2)
(3)
+
-
CO
2
+ 8H + 8e
4
2
O
+
-
2
H + 2e
H
2
(4)
9
, 10
traditional catalysts.
In particular, MOFs containing porphyrinic macrocycles as
linkers are attractive due to the reactive nature of these
1
1-14
molecules.
Porphyrin-based moieties are ubiquitous in
1
nature, capable of catalyzing a wide range of reactions.
5-18
However, MOFs incorporating free-base and metallo-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 2 of 6
View Article Online
DOI: 10.1039/C6TA05877A
COMMUNICATION
Journal Name
porphyrins are often limited by their structural stability in
polar solvents, particularly water. Recently, this limitation has
been addressed by the development of porphyrinic MOFs
A
B
4
+
3+
comprised of highly oxophilic metal ions, such as Zr and Al
e.g. PCN-222, 223, 224, 225, MOF-525, 545 and Al-PMOF),
which provide considerable improvement in structural and
(
1
9-26
chemical stabilities of the resulting frameworks.
Several of
1
0
20
30
2θ (°)
these porphyrinic MOFs have shown promise as
8, 27
1
C
2
electrocatalytic agents facilitating the reduction of CO .
The chemical properties of metal clusters that comprise MOF
structures also contribute to the overall framework reactivity.
A high diversity of available metal ions and their synthetic
tunability makes them attractive candidates as catalytic sites.
In particular, Brønsted and Lewis acid/base properties of
nodes, as well as coordinated guest molecules, such as water,
have been previously utilized for the catalysis of a number of
1
9 Å
6
µm
-
6 3 4 3 4 6 2 6 6
Figure 1. (A) Ni(II)TCPP linker and the Zr (μ -O) (μ -OH) (OH) (OH ) (COO ) secondary
2
8
organic transformations. Furthermore, additional reactive building unit of PCN-224-Ni (top) and PCN-224 structure with the Ni(II)-to-Ni(II)
20
sites can be generated within the framework by doping with distance illustrated (bottom). Hydrogen atoms have been omitted for clarity. Carbon:
2
different metal ions or through introduction of missing linker
9
gray; Oxygen, red; Nitrogen, blue; Nickel, green; Zirconium, light blue. (B) PXRD pattern
2
0
3
0
of simulated PCN-224-Ni from single crystal data (black), synthesized PCN-224-fb
red), and PCN-224-Ni (blue) powders; (C) SEM of PCN-224-Ni film.
defects at the metal node. Overall, there is a considerable
potential for engineering framework nodes and combining
their properties with the organic linkers in order to achieve
novel cooperative behavior.
(
with the non-metallated (or free-base) H TCPP ligand,
2
designated as PCN-224-fb. Films were prepared via incubation
of a FTO substrate in a solvothermal reaction containing ZrCl4
In this report a porphyrin-containing MOF, PCN-224-M (PCN =
2
+
porous coordination network, M = Ni ) constructed with
nickel(II) meso-tetrakis(4-carbxyphenyl) porphyrin, Ni(II)TCPP,
linkers is presented as an active water oxidation promoter.
and Ni(II)TCPP (for PCN-224-Ni) or H TCPP (for PCN-224-fb) in
2
the presence of formic acid as a modulator. Powder X-ray
diffraction (PXRD) was used to confirm the structure of the
PCN-224-Ni is a Zr(IV)-based MOF in which the octahedral
-
(COO )
MOF powder, which was consistent with the PCN-224 topology
20
Zr
6
(μ
3
-O)
4
(μ
3
-OH)
4
(OH)
6
(OH
2
)
6
6
secondary building units
(
Figure 1B). SEM imaging of the films shows crystallite
(SBUs) are connected by six Ni(II)TCPP ligands, forming two
features
open channels with node-to-node diameters of 23.7 and 15.1
Å, respectively, and a pore diameter of ~19 Å (Figure 1A and
consistent with the cubic PCN-224 unit cell with a film
thickness of ca. 30 μm (Figure 1C and S2-3). The film appears
as a collection of particles mechanically immobilized on the
FTO substrate. It is also possible that a self-assembled
monolayer of Ni(II)TCPP has formed on the surface of FTO
during the synthesis which would provide stronger interactions
with the MOF particles.
2
S1). The open structure and large pores afford PCN-224-Ni a
0
2
BET surface area of 2600 m g , one of the largest amongst
-1
3
-1
porphyrinic MOFs, and a total pore volume of 1.59 cm g as
2
0
calculated from N
2
adsorption isotherms. These features
O, O and electrolyte throughout
allow for rapid diffusion of H
2
2
the framework. Due to the large surface area and pore volume
of PCN-224-Ni, the high density of catalytic sites per area could
be easily accessible by water molecules, which would result in
considerably improved catalytic efficiency compared to non-
porous heterogeneous catalysts. For example, in a film one
The catalytic activity of PCN-224-Ni for water oxidation was
investigated using electrochemical techniques. Cyclic
voltammetric analysis of PCN-224-Ni under aqueous conditions
revealed a rapid increase in current at 1.0 V vs. NHE (Figure 2A,
blue). The PCN-224-Ni current was significantly larger in
magnitude compared to FTO (Figure 2A, black) and the non-
metallated PCN-224-fb film (Figure 2A, red) and consistent
with catalytic behavior. Square-wave voltammetry (SWV) of
the films (Figure 2B, blue and S4) reveals an underlying broad
electrochemical signature at 1.05 V vs. NHE attributed to the
1
3
unit cell thick, PCN-224-Ni has a density of ca. 8 × 10 catalytic
-2
sites cm , approximately 3 times greater than a monolayer of
Ni(II)TCPP.
While there have been three reports of MOFs as water
oxidation catalysts, the work reported herein demonstrates
the first example of cooperative chemistry driven by the
combination of an active, catalytic linker and proton-accepting
II
+
ꢀ
formation of a porphyrin cation radical species [Ni TCPP] ,
III
+
which is in thermal equilibrium with [Ni TCPP] . This process is
overlapped with a second electrochemical event attributed to
3
1-33
node in a MOF thin film formed in situ.
The PCN-224-Ni
thin films have been prepared by a solvothermal reaction on
fluorine-doped tin oxide (FTO) conductive substrates. The
heterogeneous MOF catalyst is active towards the oxidation of
water at near neutral pH with Faradaic efficiencies consistent
with that of the Ni(II)TCPP in solution. PCN-224-Ni films
produced significantly more oxygen compared to films
prepared
III
formation of the dication radical [Ni TCPP]
ꢀ
2+ 34, 35
.
The
oxidation of the porphyrin macrocycle to generate the initial
radical species was also observed with PCN-224-fb films
36, 37
(Figure 2B, red).
The dependence of pH on the PCN-224-Ni water oxidation
mechanism was determined by taking steady-state current
measurements at constant potential in aqueous electrolyte
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 6
Jou Pr nl e aa ls oe fd Mo an to et r ai ad lj su s Ct hm ea mr g ii sn ts ry A
View Article Online
DOI: 10.1039/C6TA05877A
Journal Name
COMMUNICATION
(
0.1 M NaClO
4
). The pH of the solution was adjusted using 0.1 Figure 3. (A) CV of PCN-224-Ni in 0.1 M TBAPF
6
/CH
3
CN as a function of scan rate; (B) A
plot of the log (Peak Current Density) vs. log (Scan Rate) gives a slope of 0.6, suggesting
diffusion limiting electrochemical behavior; (C) CV of PCN-224-Ni in 0.1
TBAPF /CH CN (black) with increasing H O concentrations; (D) Plot of Current Density
vs. [H O] demonstrates a linear relationship.
M
M
6
3
1/2
2
A
0.2
.0
0.2
0.4
0.6
0.8
1.0
B
2
0
.0
0
ꢀ
ꢀ
ꢀ
0.2
0.4
which likely occurs by a charge hopping mechanism between
the porphyrin moieties. A similar behavior has been observed
ꢀ
ꢀ
1
8, 27, 45
previously for porphyrin-based MOF electrodes.
The
ꢀ
ꢀ0.6
ꢀ
water oxidation activity of PCN-224-Ni was supported by the
increase in catalytic current with increasing water additions
ꢀ
0.8
1
.6
1.2
0.8
0.4
0.0
1.6
1.2
0.8
0.4
0.0
Potential (V vs. NHE)
Potential (V vs. NHE)
(
Figure 3C). The catalytic current density was found to increase
1
/2
2
linearly with [H O] , suggesting a first order reaction with
Figure 2. (A) Cyclic voltammetry at 100 mV/s and (B) Square wave voltammetry of FTO
black), PCN-224-fb (red), and PCN-224-Ni (blue) in aqueous 0.1 M NaClO
4
6
(
4
.
respect to water (Figure 3D and S7).
Controlled potential electrolysis (CPE) was performed on PCN-
4
HClO and 0.1 M NaOH. PCN-224-Ni was found to demonstrate
2
1
24-Ni in aqueous 0.1 M NaClO
500 mV vs. NHE for 60 min. PCN-224-Ni maintained a higher
4
at an applied potential of
no dependence on proton concentration (Figure S5). This
behavior is attributed to the fact that the redox-activity of the
current compared to bare FTO and PCN-224-fb films (Figure
A), as well
II
0/+
ꢀ
metalloligand is dominated by the [Ni TCPP] process, which
was reported to be pH independent for an analogous
4
as increased oxygen production (Figure 4B). The fact that the
Ni analogue generated significantly higher current than the
free base MOF indicates that the Ni(II) center is critical to the
observed electrocatalytic activity. Interestingly, the deposition
of the free base PCN-224 on FTO resulted in lower current
compared to the unmodified FTO electrode, which suggests
that PCN-224-fb forms an insulating layer and inhibits the
activity of FTO. From this behavior it can also be concluded
that the other components of the PCN-224 framework, such as
Zr-oxo nodes, are inactive in the redox processes. The Faradaic
efficiency of PCN-224-Ni was conserved with that of the
Ni(II)TCPP ligand in solution under the same conditions (Figure
S8). However, the efficiency was less than that of the similar
water-soluble catalyst due to the mechanistic importance of
35
compound in solution. The current-voltage (Tafel) behavior
of PCN-224-Ni was investigated over a 200 mV overpotential
range in 50 mV increments (Figure S6). From the onset of
linearity in the Tafel plot, the overpotential for the water
oxidation reaction was determined to be 450 mV. The Tafel
-
1
slope obtained from the linear portion was 150 mV decade
with a corresponding exchange current density (j
-
o
) of 7.7 × 10
1
0
-2
A cm , which are similar to other heterogeneous water
3
8-42
oxidation catalysts.
The Tafel slope is indicative of an EC
catalytic mechanism, consistent with electrochemical
oxidation of the porphyrin macrocycle and Ni(II) metal centers
4
3, 44
followed by a chemical step.
Under non-aqueous conditions (TBAPF
6
3
/CH CN), the CVs of
3
5
PCN-224-Ni exhibit a weak oxidation wave at 1.75 V vs. NHE,
which appears as a shoulder on the backgrounds current
phosphate buffer used in their study (vide infra). The
turnover frequency (TOF) of PCN-224-Ni was determined by
II
Figure 3A) assigned to the [Ni TCPP]
0/+
ꢀ
(
process. The plot of
3
digesting the MOF films in concentrated HNO followed by ICP
log(j) vs. log(ν) (where j = current density and ν = scan rate)
showed a linear trend with the slope of 0.6 (Figure 3B). This
relationship suggests diffusion limited electron transport
through the film,
analysis for nickel content (Table S1). The TOF was calculated
under the assumption that all Ni centers of the film are
catalytically active, which is an overestimation, and therefore,
representative of the lower limit of TOF (Equation (5)). That
-4 -1
said, the TOF for PCN-224-Ni was found to be 2 (±1) × 10 s .
A
B
1
1
1
.6
.4
.2
ꢄꢅꢆꢃꢇ
ꢉꢄꢅꢆꢃꢊꢋꢉꢌꢌꢍꢎꢏꢐꢐꢍ×ꢉꢑꢆꢑꢒꢓꢔꢅꢆꢕꢖꢋꢖꢃꢗꢘꢔꢙꢓꢋꢅꢚꢍ
ꢈ
2
0
0
ꢀꢁꢂ =ꢃ
(5)
Slope = 0.60
ꢀ
ꢀ
ꢀ
ꢀ
20
40
60
80
Given Faradaic efficiencies < 100% for O2 production by
PCN-224-Ni films, the alternative 2 electron reaction pathway
resulting in H O was considered. The immersion of indicator
2
5
1
5 mV/s
0 mV/s
00 mV/s
1.0
0.8
0.6
200 mV/s
4
8
00 mV/s
00 mV/s
2 2
strips into the electrolyte at the end of electrolysis experiment
did not detect any noticeable amount of H O , which means
2
.0
1.5
1.0
0.5
0.0
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Potential (V vs. NHE)
Log(Scan rate)
2
2
-1
C
D
160
that its concentration in solution is below 0.5 mg L . A similar
1
1
1
40
0
lack of peroxide formation was reported by Han et al. for a
20
00
ꢀ
50
water-soluble Ni(II) porphyrin electrocatalyst, suggesting that
35
O is the primary product of water oxidation in this case.
8
6
0
0
ꢀ
ꢀ
ꢀ
100
150
200
0 M
2
1
3
.93 M
.73 M
40
Since the nickel center of the Ni(II)TCPP linkers serves as the
active catalytic site within the MOF, the mechanism for water
oxidation by PCN-224-Ni is expected to proceed via similar
reaction steps to that of a homogeneous Ni(II) porphyrin in
solution. Due to the rigid nature of the PCN-224-Ni MOF and
5.41 M
2
0
0
7
8
.00 M
.47 M
1
.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
/2 1/2
1
Potential (V vs. NHE)
[H O] (M
)
2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 4 of 6
View Article Online
DOI: 10.1039/C6TA05877A
COMMUNICATION
Journal Name
the fact that the distances between Ni(II)-centers within the providing a new class of secondary proton management
framework are ~19 Å between co-facial porphyrins lining the possibilities. The distance between the water molecule on the
same channel and ~14 Å between orthogonal porphyrins Ni center and Zr-oxo cluster is 10.7 Å (Figure S9) which means
(Figure S1), it is unlikely that the catalytic mechanism involves that three water molecules would be required to form a
interactions between neighboring Ni(II) porphyrins. It has been continuous H-bonded network capable of facilitating proton
transport. To our knowledge, this
A
B
15
0
.0
O
2
N
N
N
ꢀ0.2
ꢀ0.4
ꢀ0.6
ꢀ0.8
ꢀ1.0
ꢀ1.2
ꢀ1.4
Ni2+
1
2
9
6
3
0
OO
N
e-
-
N
N
N
-e
-
N
N
N
Ni2+
Ni2+
N
N
H
2
O
e- -H+
OOH
-
OH2
H
2
O
HOH
Ni2+
H
O
HO
O
OH
O
Zr
Zr
N
N
N
N
N
O
0
600 1200 1800 2400 3000 3600
Time (sec)
0
600 1200 1800 2400 3000 3600
Time (sec)
Ni2+
N
N
N
HO
O
ZrZr OH O
O
O
O
Zr
H
Zr
-
e-
HO
O
OH
-H+
-
H+
Figure 4. (A) Controlled potential electrolysis (CPE) at 1500 mV vs. NHE in aqueous
.1 M NaClO and (B) Oxygen evolved during CPE experiment of FTO (black), PCN-224-
OH
2
2
H O
OH
HOOH
Ni2+
0
4
N
N
N
N
N
N
Ni3+
fb (red), and PCN-224-Ni (blue).
N
2
H O
N
-H+
O
N
N
N
Ni3+
proposed previously by Han et al. for a cationic, water soluble
Ni(II) tetrakis(4-N-methylpyridil)porphyrin molecular catalyst
that the first step of the catalytic cycle is the oxidation of the
N
Scheme 1. Proposed mechanism for the electrochemical water oxidation reaction
35
•+
35
catalyzed by PCN-224-Ni. Some carboxylate groups have been omitted from the Zr
node for clarity.
porphyrin to generate the radical cation species, Por -Ni(II).
This mechanistic step is supported by the Tafel analysis, which
suggests that an electrochemical reaction occurs as the
necessary first step. The porphyrin oxidation is followed by
binding of a water molecule to the Ni(II) metal center (Scheme
is the first example of cooperative linker-node interactions
that promote chemistry not possible under different
2
constructs. The coordinated water (Zr–OH ) and hydroxyl (Zr–
•
+
1
) to generate Por -Ni(II)-OH
2
. Sequential proton and electron
49
OH) groups typically originate at missing-linker defect sites,
however, Zr-oxo clusters inside PCN-224-Ni already contain 6
terminal oxygen atoms, each of which could potentially accept
protons from the water oxidation reaction. The number of
available Zr-oxo nodes present in the framework is limited
transfer reactions result in the deprotonation of the
coordinated water molecule with concomitant oxidation of the
•
+
Ni(II) center to give the hydroxide bound species, Por -Ni(III)-
OH. The intramolecular electron transfer from the hydroxide
to the porphyrin ring coupled with the abstraction of a proton
(node:linker ratio is 2:3) and is only sufficient to complete one
•
gives rise to Por-Ni(III)-O . Following the O–O bond formation
electrochemical cycle, assuming that each node can accept 6
protons and each linker generates 4 protons per cycle. This
limitation would account for the low TON observed (0.72)
during the electrolysis experiment. It is also possible that Zr-
oxo clusters merely serve as conduits and the proton is
transferred to the bulk water. The exact role of the nodes in
the electrochemical oxidation of water by Ni(II)TCPP remains
elusive and studies are underway to gain deeper insights into
the nature of proton transfer in this MOF. That said, the
prospect of cooperative behavior between redox-active linkers
and Brønsted basic SBUs could lead to the development of
novel MOF-based electrocatalysts.
step, the peroxide bound species, Por-Ni(II)-OOH, is generated,
which undergoes another proton-coupled electron transfer
•
reaction to give Por-Ni(II)-OO . The final oxidation process
2
results in the release of O and the regeneration of the starting
3
5
Por-Ni(II) complex.
Proton management is required for catalysts to operate at
4
7
neutral pH. At pH 7, water is a very weak base and present at
-7
too low of a concentration (1 x 10 M) to effectively fill the
role of proton acceptor. In this sense, the mechanism of PCN-
224-Ni and the homogeneous catalyst differ dramatically.
Computational modeling of the homogeneous catalyst
performed by Han et al. indicated that the O–O bond
formation and subsequent deprotonation proceeds via a low
energy route when mediated by an added molecular base,
such as phosphate or acetate. Both of these weak bases are
incompatible with the PCN-224-Ni framework. However, with
the Ni(II) porphyrin situated inside the framework another
possibility presents itself. It was recently reported by Hupp et
al. that Zr-oxo nodes in related PCN and UiO MOFs could act as
Thorough stability investigations are crucial in establishing that
the MOF is indeed an active electrocatalytic species and not
merely acting as a precursor for its formation, as metal ions
often undergo considerable rearrangements of their
coordination spheres during the redox cycling. This has been
demonstrated in the Co-based ZIF-67, which is comprised of
redox-active metal nodes that underwent irreversible
50
decomposition upon anodic potentials. Therefore, certain
proton acceptors, particularly coordinated water (Zr–OH
2
) and
porphyrin-containing frameworks,
such
as
hydroxyl (Zr–OH) species for which pK values were measured
a
[
Pb (H TCPP)]·4DMF·H O, despite showing electrocatalytic
2
2
2
4
8
in 5.71-6.80 and 8.11-8.32 ranges, respectively. Therefore,
we propose that the Zr-oxo nodes serve as the proton
acceptors in a cooperative fashion with the linker catalyst,
activity towards water oxidation, are less likely to retain their
31
structure during the electrolysis. To the best of our
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 6
Jou Pr nl e aa ls oe fd Mo an to et r ai ad lj su s Ct hm ea mr g ii sn ts ry A
View Article Online
DOI: 10.1039/C6TA05877A
Journal Name
knowledge, only 3 examples of intrinsic MOF water oxidation
COMMUNICATION
Notes and references
3
1-33
electrocatalysts were reported in the literature to date.
1
2
.
.
N. S. Lewis, Chem. Rev., 2015, 115, 12631-12632.
N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,
006, 103, 15729-15735.
C. H. Lee, D. K. Dogutan and D. G. Nocera, J. Am. Chem.
Soc., 2011, 133, 8775-8777.
Within these reports, however, extensive characterization of
the framework pre- and post-electrolysis is lacking. In the case
of nickel-based systems, electrocatalytically-active oxide and
hydroxide films have been formed from soluble nickel
2
3
.
complexes at the oxidative potentials used for water oxidation 4.
1, 52
C. Costentin, S. Drouet, M. Robert and J.-M. Savéant,
Science, 2012, 338, 90-94.
5
experiments.
As a result, the present work provides a
detailed characterization of the MOF and demonstrates the 5.
D. L. DuBois, Inorg. Chem., 2014, 53, 3935-3960.
J. W. Raebiger, J. W. Turner, B. C. Noll, C. J. Curtis, A.
Miedaner, B. Cox and D. L. DuBois, Organometallics, 2006,
6
7
.
.
stability of the PCN-224-Ni films towards electrolysis
conditions using several characterization techniques.
2
5, 3345-3351.
X-ray photoelectron spectroscopy (XPS) of the MOF film
revealed that the position and the shape of Ni2p and N1s
signals remained largely unchanged before and after
electrolysis (Figure S10A-B), indicating that the Ni(II)TCPP core
did not experience any significant coordination geometry
rearrangements and/or redox state changes. In addition, SEM
Y. Zhao, J. R. Swierk, J. D. Megiatto, Jr., B. Sherman, W. J.
Youngblood, D. Qin, D. M. Lentz, A. L. Moore, T. A. Moore,
D. Gust and T. E. Mallouk, Proc. Natl. Acad. Sci. U. S. A.,
2
012, 109, 15612-15616, S15612/15611-S15612/15618.
8
.
M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M.
O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469-472.
D. Farrusseng, S. Aguado and C. Pinel, Angew. Chem. Int.
Ed., 2009, 48, 7502-7513.
images (Figure S10C-D) showed that MOF particles retained 9.
their size and morphology, suggesting that they did not
1
0.
J. Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. B. T.
Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450-
undergo any solid-solid transformations or dissolution. Further
analysis on digested PCN-224-Ni films after electrolysis via ICP
found the Zr:Ni ratio to be 4.71:1, which is comparable to the
ratio found in the as-synthesized framework (the theoretical
Zr:Ni ratio should be 4:1 in based on the molecular formula of
1
459.
Q. Z. Zha, X. Rui, T. T. Wei and Y. S. Xie, Crystengcomm,
014, 16, 7371-7384.
W. Y. Gao, M. Chrzanowski and S. Q. Ma, Chem. Soc. Rev.,
014, 43, 5841-5866.
Q. Zha, X. Rui, T. Wei and Y. Xie, Crystengcomm, 2014, 16,
371-7384.
1
1
1
1.
2.
3.
2
2
0
PCN-224-Ni, C144
the MOF remaining on FTO after electrolysis is indeed PCN-
24-Ni. Additionally, the amounts of Ni and Zr detected in
72 12 3
H N O64Ni Zr12). These results suggest that
2
2
7
solution post-electrolysis represent 7.2% and 1.8% of the total 14.
contents of the MOF films, respectively (Table S1). This result
could be attributed to the detachment of mechanically-bound 15.
Z. Guo and B. Chen, Dalton Trans., 2015, 44, 14574-
14583.
I. Bhugun, D. Lexa and J.-M. Saveant, J. Am. Chem. Soc.,
1
996, 118, 1769-1776.
MOF particles from FTO substrate into solution.
1
1
1
1
6.
7.
8.
9.
M. W. Grinstaff, M. G. Hill, J. A. Labinger and H. B. Gray,
Science, 1994, 264, 1311-1313.
K. Kadish, Guilard, R., Smith, K., The Porphyrin Handbook,
Academic Press, 2003.
I. Hod, M. D. Sampson, P. Deria, C. P. Kubiak, O. K. Farha
and J. T. Hupp, ACS Catal., 2015, 5, 6302-6309.
D. Feng, Z. Y. Gu, J. R. Li, H. L. Jiang, Z. Wei and H. C. Zhou,
Angew. Chem., 2012, 124, 10453-10456.
To summarize, the Zr(IV)-based MOF with Ni(II)TCPP linkers,
PCN-224-Ni, was grown solvothermally onto conductive FTO
substrates and shown to electrochemically oxidize water. The
mechanism of water oxidation was found to be best described
by a rate-determining electrochemical step followed by a
chemical reaction with Ni(II)TCPP playing the role of the active
catalytic sites and Zr nodes acting as proton abstractors. The
work brings to light the ability to promote unique reactivity 20.
within MOFs through the dual design of both the linker and
node. The structure of the PCN-224-Ni was found to remain
D. Feng, W.-C. Chung, Z. Wei, Z.-Y. Gu, H.-L. Jiang, Y.-P.
Chen, D. J. Darensbourg and H.-C. Zhou, J. Am. Chem. Soc.,
2013, 135, 17105-17110.
2
2
2
1.
2.
3.
B. J. Deibert and J. Li, Chem. Commun., 2014, 50, 9636-
intact during the electrochemical reaction. As such,
incorporation of active molecular catalysts into highly robust
Zr-based scaffolds can be used as a viable strategy for the
design of electrocatalytic frameworks. Application of MOFs in
the electrocatalytic oxidation of water is still in its early stages
with a lot of room for improvements in catalyst design and this
study serves as an important milestone towards better
understanding of these systems.
9
639.
J. Zheng, M. Wu, F. Jiang, W. Su and M. Hong, Chem. Sci.,
015, 6, 3466-3470.
2
A. Fateeva, P. A. Chater, C. P. Ireland, A. A. Tahir, Y. Z.
Khimyak, P. V. Wiper, J. R. Darwent and M. J. Rosseinsky,
Angew. Chem., 2012, 124, 7558-7562.
D. Feng, Z.-Y. Gu, Y.-P. Chen, J. Park, Z. Wei, Y. Sun, M.
Bosch, S. Yuan and H.-C. Zhou, J. Am. Chem. Soc., 2014,
2
2
2
4.
5.
6.
1
36, 17714-17717.
W. Morris, B. Volosskiy, S. Demir, F. Gándara, P. L.
McGrier, H. Furukawa, D. Cascio, J. F. Stoddart and O. M.
Yaghi, Inorg. Chem., 2012, 51, 6443-6445.
S. Yuan, T.-F. Liu, D. Feng, J. Tian, K. Wang, J. Qin, Q.
Zhang, Y.-P. Chen, M. Bosch, L. Zou, S. J. Teat, S. J.
Dalgarno and H.-C. Zhou, Chem. Sci., 2015, 6, 3926-3930.
Acknowledgments
This material is based upon work supported by U.S.
Department of Energy, Office of Basic Energy Sciences under
Award Number DE-SC0012446.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 6 of 6
View Article Online
DOI: 10.1039/C6TA05877A
COMMUNICATION
Journal Name
2
7.
N. Kornienko, Y. Zhao, C. S. Kley, C. Zhu, D. Kim, S. Lin, C. J.
Chang, O. M. Yaghi and P. Yang, J. Am. Chem. Soc., 2015,
37, 14129-14135.
A. Herbst, A. Khutia and C. Janiak, Inorg. Chem., 2014, 53,
319-7333.
M. Lammert, M. T. Wharmby, S. Smolders, B. Bueken, A.
Lieb, K. A. Lomachenko, D. D. Vos and N. Stock, Chem.
Commun., 2015, 51, 12578-12581.
1
2
8.
9.
7
2
3
0.
1.
F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde,
M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi, M.
Waroquier, V. Van Speybroeck, C. Kirschhock and D. E. De
Vos, J. Am. Chem. Soc., 2013, 135, 11465-11468.
F. Dai, W. Fan, J. Bi, P. Jiang, D. Liu, X. Zhang, H. Lin, C.
Gong, R. Wang, L. Zhang and D. Sun, Dalton Trans., 2016,
3
4
5, 61-65.
S. Wang, Y. Hou, S. Lin and X. Wang, Nanoscale, 2014, 6,
930-9934.
E. A. Flügel, V. W. h. Lau, H. Schlomberg, R. Glaum and B.
V. Lotsch, Chem. Eur. J., 2016, 22, 3676-3680.
D. Chang, T. Malinski, A. Ulman and K. M. Kadish, Inorg.
Chem., 1984, 23, 817-824.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
9
Y. Z. Han, Y. Z. Wu, W. Z. Lai and R. Cao, Inorg. Chem.,
2015, 54, 5604-5613.
K. Kalyanasundaram and M. Neumannspallart, J. Phys.
Chem., 1982, 86, 5163-5169.
M. W. Renner and J. Fajer, J. Biol. Inorg. Chem., 2001, 6,
8
23-830.
S. Dey, B. Mondal and A. Dey, Phys. Chem. Chem. Phys.,
014, 16, 12221-12227.
2
A. J. Esswein, Y. Surendranath, S. Y. Reece and D. G.
Nocera, Energy Environ. Sci., 2011, 4, 499-504.
R. D. L. Smith, M. S. Prévot, R. D. Fagan, S. Trudel and C. P.
Berlinguette, J. Am. Chem. Soc., 2013, 135, 11580-11586.
Y. Surendranath, D. K. Bediako and D. G. Nocera, Proc.
Natl. Acad. Sci., 2012, 109, 15617-15621.
Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am.
Chem. Soc., 2010, 132, 16501-16509.
4
4
3.
4.
S. Fletcher, J. Solid State Electrochem., 2009, 13, 537-549.
E. Gileadi, Electrode Kinetics for Chemists, Engineers, and
Materials Scientists, VCH Publishers, Inc., New York, 1993.
S. R. Ahrenholtz, C. C. Epley and A. J. Morris, J. Am. Chem.
Soc., 2014, 136, 2464-2472.
D. L. DuBois, A. Miedaner and R. C. Haltiwanger, J. Am.
Chem. Soc., 1991, 113, 8753-8764.
M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072-
45.
46.
47.
48.
49.
50.
1075.
R. C. Klet, Y. Liu, T. C. Wang, J. T. Hupp and O. K. Farha, J.
Mater. Chem. A, 2016, 4, 1479-1485.
C. A. Trickett, K. J. Gagnon, S. Lee, F. Gándara, H.-B. Bürgi
and O. M. Yaghi, Angew. Chem., 2015, 127, 11314-11319.
P. M. Usov, C. McDonnell-Worth, F. Zhou, D. R.
MacFarlane and D. M. D’Alessandro, Electrochim. Acta,
2015, 153, 433-438.
5
1.
2.
A. Singh, S. L. Chang, R. K. Hocking, U. Bach and L. Spiccia,
Catal. Sci. Technol., 2013, 3, 1725-1732.
A. Singh, S. L. Y. Chang, R. K. Hocking, U. Bach and L.
Spiccia, Energy Environ. Sci., 2013, 6, 579-586.
5
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins