Electrochemical hydrogen production in acidic water by an azadithiolate bridged synthetic hydrogenese mimic: Role of aqueous solvation in lowering overpotential

Article abstract of DOI:10.1021/cs300835a

An inherent problem stalling the development of a H₂-based global energy economy is the unavailability of efficient functional catalysts that can function in aqueous media. A Fe-Fe hydrogenase mimic is investigated for electrocatalytic hydrogen generation in aqueous medium. H₂ is produced from acid water (pH <3) by a Fe-Fe hydrogenase mimic immobilized on graphite surfaces. These catalysts are known to reduce H⁺ at very negative potentials in organic solvents. However, in aqueous medium, the H ⁺ reduction potential is shifted to much more positive values. The catalyst shows a turnover frequency of 6400 s^-1 at -0.5 V and an onset potential of -0.36 V vs NHE. Prolonged electrolysis shows that the catalyst has a turnover number ?10⁸ and a Faradaic efficiency > 95%. Even at pH 2, that is, [H]⁺ = 0.01 N, I_cat/[τ] > 400 s^-1 is obtained. The catalyst can be immobilized on cheap carbon electrodes, used in domestic Zn-Carbon dry batteries, to generate H ₂ from acid aqueous solutions.

Full text of DOI:10.1021/cs300835a

Research Article
pubs.acs.org/acscatalysis
Electrochemical Hydrogen Production in Acidic Water by an
Azadithiolate Bridged Synthetic Hydrogenese Mimic: Role of
Aqueous Solvation in Lowering Overpotential
Subal Dey, Atanu Rana, Somdatta Ghosh Dey,* and Abhishek Dey*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India 700032
S
* Supporting Information
ABSTRACT: An inherent problem stalling the development of a H2-
based global energy economy is the unavailability of efficient functional
catalysts that can function in aqueous media. A Fe−Fe hydrogenase
mimic is investigated for electrocatalytic hydrogen generation in aqueous
medium. H2 is produced from acid water (pH <3) by a Fe−Fe
hydrogenase mimic immobilized on graphite surfaces. These catalysts are
known to reduce H+ at very negative potentials in organic solvents.
However, in aqueous medium, the H+ reduction potential is shifted to
much more positive values. The catalyst shows a turnover frequency of 6400 s−1 at −0.5 V and an onset potential of −0.36 V vs
NHE. Prolonged electrolysis shows that the catalyst has a turnover number ≫108 and a Faradaic efficiency > 95%. Even at pH 2,
that is, [H]+ = 0.01 N, Icat/[τ] > 400 s−1 is obtained. The catalyst can be immobilized on cheap carbon electrodes, used in
domestic Zn-Carbon dry batteries, to generate H2 from acid aqueous solutions.
KEYWORDS: hydrogen evolution reaction, iron hydrogenase, bio-inspired catalysis
other transition metals (copper comes second in the list with an
annual production of 16 million tones).10,36−38 However, very
few Fe-complexes are known to produce H2 via H+ reduction in
the aqueous medium.39
INTRODUCTION
■
The ever increasing global energy demand and depleting fossil
fuel sources have instigated great interest in the development of
clean and sustainable alternate energy sources.1,2 Additionally,
because of increasing concerns regarding the ecological/
environmental consequences of accumulating vast amounts of
CO2 in the atmosphere, as a result of the relentless use of fossil
fuels as the chief sources of energy for centuries, a carbon
neutral energy source is of global interest.3−5 A H2 based
energy economy heralds such potential,6,7 but its success
demands the availability of efficient, durable, and inexpensive
metal catalysts that can catalyze H2 production at minimum
overpotentials with high turnover rates.8−11 Although Pt is
suitable for this purpose, its scarcity and consequently its price
has led to a global search for alternative transition metal based
catalysts.12 Over the past decade elegant Mn,13 Ni,14−18 and
Co19−27-based organometallic molecular complexes have been
evaluated for catalytic/electrocatalytic H+ reduction. Most of
these are found to be limited by their instability/insolubility in
aqueous environment, requirement of high overpotential, and
low turnover rates.28−33 A few Co or Ni-based electrocatalysts
are reported which exhibit high turnover frequencies either in
organic media or in aqueous media. Very recently, efficient Mo-
based molecular electrocatalysts have been reported to catalyze
H+ reduction in aqueous medium.34,35 Some of these catalysts
demand large overpotentials to generate hydrogen at detectable
rates. An efficient Fe-based molecular electrocatalyst is desirable
as iron is the most abundant transition metal in the lithosphere
(∼1000 times more abundant than Co) and ∼2.6 billion tones
of it is produced annually worldwide which is higher than any
Hydrogenase (H2ase) enzymes reversibly converts protons
to hydrogen with minimum over potential at physiological pHs
with turnover frequency (TOF) of 100 to 10,000 mols of H2
per mole of enzyme per second.40,41 There are three types of
H2ase, namely, binuclear Ni−Fe H2ase, binuclear Fe−Fe H2ase,
and mononuclear Fe-only H2ase, which are abundant in
nature.42,43 The Fe−Fe H2ase mainly reduces protons to H2
whereas the Ni−Fe H2ase is known for H2 oxidation while the
Fe-only H2ase is not catalytic by itself.41 These are very
sensitive to oxygen and loose their catalytic activity
permanently.44 The Fe−Fe H2ase active site contains a
Fe2S2(CO)3(CN)2 organometallic core (Figure 1A).45
A
bismethylene azadithiolate (ADT) moiety bridges between
the two Fe centers.43,46 Numerous synthetic structural and
some functional H2ase models with a basic Fe2S2(CO)6 core
are known, and their catalytic hydrogen generation in organic
solvents are reported.46,47 However these investigations,
exclusively performed in nonaqueous solvents and in
homogeneous conditions reveal that they require very low
potential and show limited TOF.48 The low over potential is
due to the fact that these catalysts, isolated in their Fe(I)Fe(I)
neutral forms, have to be reduced to the dianionic Fe(0)Fe(0)
Received: July 30, 2012
Revised: January 22, 2013
© XXXX American Chemical Society
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Research Article
bearing complex A show the presence of Fe (Figure 2A), S
(Figure 2B), N (Figure 2C), C (Figure 2D) and Br (Figure 2E)
on the electrode surface. The Fe2p ionizations were observed at
711.1 eV (2p1/2) and 724.5 eV (2p3/2) consistent with data
obtained on similar complexes.50 The binding energies of the N
(1s) and S (2p) are at 399.3 and 161.9 eV which is consistent
with those observed for tertiary aromatic amines and metal
thiolate complexes.51−53 In addition to the major peak observed
for graphite (285.5 eV, Figure 2D, dashed red), the C1s region
also shows a peak at 286.8 eV (Figure 2D, dashed blue),
characteristic of carbonyl group. These data are consistent with
the presence of complex A on the electrode.
Cyclic Voltammetry (CV). The CV of complex A in
acetonitile shows two well established quasi-reversible one
electron reductions (E1/2) at −0.61 V and −0.89 V
corresponding to the FeI-FeI/FeI-Fe0 and FeI-Fe0/Fe0-Fe0
redox couples (Figure 3, orange) consistent with a previous
report.54 The first wave at −0.61 V has a peak to peak
separation (ΔEp) of 136 mV, and the second wave at −0.89 V
has a ΔEp of 140 mV. The cyclic voltammogram of complex A
physisorbed on the EPG electrode in water shows two
reversible waves centered at −0.22 V and −0.37 V for the
FeI-FeI/FeI-Fe0 and FeI-Fe0/Fe0-Fe0 redox couples (Figure 3,
blue). Integration of the charge under these processes yields the
total number of electroactive complex A species on the
electrode to be ∼10−11 moles.55,56 The CV current increases
linearly with increase in scan speed consistent with the presence
of adsorbed species on the electrode (Supporting Information,
Figure SI−II). The ΔEp for the two consecutive reduction
waves are 200 mV and 240 mV, respectively. Thus the FeI-FeI/
FeI-Fe0 and FeI-Fe0/Fe0-Fe0 reduction potentials shift by +0.38
and +0.52 V on changing the solvent from CH3CN to H2O.
Figure 1. (A) Schematic diagram of complex A, (B) X-ray
crystallographically determined structure of complex A, and (C) the
active site of Fe−Fe hydrogenase.
state for catalysis in nonaqueous medium. Recent studies in
homogeneous hydrogen evolution by a micellar H2-ase mimic
by photochemical and electrochemical means in aqueous media
provided the information to achieve the high TOF.36,49
Here, we report, for the first time, heterogeneous electro-
chemical H2 production by reducing protons in acidic aqueous
media by a robust synthetic Fe−Fe H2‑ase active site mimic
having a Fe2S2(CO)6 core (Figure 1B, complex A) and a
bridging ADT ligand. The catalytic property of this catalyst in
aqueous medium is investigated using RDE, RRDE coulometry,
and bulk electrolysis technique.
RESULTS AND DISCUSSION
X-ray Photoelectron Spectroscopy (XPS) of Complex
A Immobilized on Electrodes. XPS analysis of the electrode
■
Figure 2. XPS study of the complex A, adsorbed on the EPG electrode. (A) Spectra of Iron-2p core level. (B) Spectra for Sulfur-2p core level. (C)
Spectra of Nitrogen-1s core level. (D) Spectra of Carbon-1s core level. Corresponding peak-fitted spectra have been overlapped with the
experimentally obtained spectra.
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oxidizing it back to H+ on a Pt ring which encircles the working
electrode when held at a constant potential of 0.7 V (at this
potential Pt oxidized H2 to H+) as described previously.16 In an
RRDE setup the H2, produced at the EPG electrode (Figure 4,
blue line) and diffused toward the Pt ring because of the
hydrodynamic flow created by the rotating shaft, gets oxidized
to H+, and an oxidation current is recorded at the ring (Figure
4, red line).16 Note that unlike RRDE of dissolved species,
where ∼20% of the product can be detected on the Pt ring,
only 1−1.5% of the hydrogen produced in the working
electrode is detected on the ring as hydrogen, being an
insoluble gas, escapes readily from the solution (see
Experimental Details section).16 The RRDE data clearly show
that the H2 oxidation current at the Pt ring is observed only
where H+ reduction current is observed at the working
electrode bearing complex A, that is, at E < −0.4 V. The
onset potential for H+ reduction (the potential where at least
50 μA of H+ reduction current is observed at the working
electrode and the corresponding H2 formed can be detected on
the Pt ring) of complex A in 0.05 N H2SO4 solution is −0.36
V.58 The thermodynamic proton reduction potential at pH 3 is
−0.18 V vs NHE. Thus complex A requires an overpotential of
only 180 mV to be functional in aqueous solutions. Note that it
has been recently shown that the overpotential can be related
to the intrinsic TON and TOF of an electrocatalyst.59,60 This
“foot-of-the-wave” analysis can also be used in this case to
obtain the kinetic parameters (vide-infra).
Figure 3. CV of a solution of complex A in acetonitrile/(nBu)4NClO4
(orange) with a glassy carbon working electrode and CV of complex A
immobilized on the EPG electrode in H2O/KPF6 (blue). Data was
obtained at a scan rate of 200 mV/s using a Pt counter electrode and
an aqueous Ag/AgCl reference and converted to NHE reference.
The positive shift in E1/2 in a polar aqueous medium is possibly
due to the enhanced stability of the reduced anionic species
(A− or A2−) in polar water medium relative to the neutral
oxidized complex (A).57 The increased solvation of the reduced
anionic species stabilizes it more relative to the neutral oxidized
species making these complexes easier to reduce, that is, more
positive reduction potential. The reduction of A− to A2− shows
a greater shift relative to A to A− because of greater negative
charge on the A2− species. Geometry optimized density
functional theory (DFT) calculations (on this complex)
indicate that the calculated formal potential for the two
electron reduction of Fe(I)Fe(I) to Fe(0)Fe(0) shifts 475 mV
positive in H2O relative to CH3CN (solvation modeled using
PCM) consistent with the positive shifts observed exper-
imentally. The large positive shifts observed in the reduction
potential in aqueous medium encouraged us to investigate the
electrocatalytic behavior of A in aqueous medium in the
presence of a cheap proton source.
(b). [H]+ Dependence. H+ reduction is investigated over a
large range of proton concentration of 0.001−0.5 N (i.e., pH
3−0.3) by rotating disk electrochemistry (RDE). With
increasing acid concentrations the electrocatalytic hydrogen
production rate increases gradually till it saturates at 0.5 N
(Figure 5A). No further enhancement of catalytic current is
observed on increasing the H+ concentration. The nature of the
catalytic current suggests that it is not substrate diffusion
limited as is the case for most O2 reduction electrocatalysts
which are immobilized on EPG electrodes. Substrate diffusion
limited catalytic current is independent of the potential applied
on the working electrode. This is not the case here. Thus a
normal Koutecky−Levitch analysis could not be performed to
obtain the kcat. However the intrinsic second order rate (kcat) of
the catalyst could be determined from the foot of the wave
analysis of the LSV data obtained in 0.5 N H2SO4 solution, as
described in the Supporting Information. However, the ratio of
the catalytic current (Icat, converted to moles of H2 produced
per second) and the number of moles of electroactive complex
A present in the electrode ([τ], obtained from the integration
of the CV current) can be used as a measure of catalytic
efficiency (Icat/[τ], this has an unit of s−1). The catalytic
efficiency is estimated to be 6400 s−1 in 0.5 N H2SO4 at −0.5 V,
that is, 6400 mols of electrons are dissipated from the electrode
per second per mole of the catalyst. Consecutive RDE scans
(up to 200 scans) in 0.5 N H2SO4 show no noticeable decrease
in the catalytic current (Figure 5B). This implies that the
catalyst is quite stable under the experimental conditions. Note
that even at [H]+ < 0.01, that is, at pH 2, Icat/[τ] > 400 s−1 is
detected (Figure 5A, inset).
Electrocatalytic H+ Reduction. (a). Rotating Ring Disc
Electrochemistry (RRDE). In a linear sweep voltammetry (LSV)
scan of the complex A immobilized on the EPG electrode
immersed in 0.01 N H2SO4 solution (pH 2) a large
electrocatalytic current (Figure 3, blue) is observed as the
potential of the working electrode is lowered. Almost no
catalytic current is observed if the catalyst is not immobilized
on the electrode (Figure 4, green line). Concomitant bubble
formation is observed on the electrode surface suggesting the
formation of H2 gas. The production of H2 is confirmed by
An overlay of the CV data in the absence of [H+] with the
LVS data in the presence of increasing amount of H+ (Figure 6)
reveals that the potential at which H+ reduction starts varies
with [H]+ such that the H+ reduction occurs at much lower
potentials at lower [H+]. This implies that the catalytic
mechanism is [H]+ dependent, and at lower [H]+ higher
Figure 4. RRDE with complex A on the EPG disc working electrode
(blue line, indicating H2 generation) which is swept from 0.2 V to
−0.8 V and the corresponding Pt-ring current (red line, indicating H2
detection) with the ring held at a constant potential of 0.7 V in
aqueous 0.01 N H2SO4.
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Figure 5. (A) Plot of Icat/[τ] at varying acid concentrations. Icat values were recorded at −0.5 V vs NHE at each acid concentration. (B) Plot of Icat/
[τ] vs number of scan (N).
Figure 6. RDE for electrochemical hydrogen generation with increasing acid concentration (Current vs Potential). (A) Catalytic current for low acid
concentrations (0.001 N−0.5 N H2SO4). Data were obtained using a Pt-counter electrode, referred to an NHE reference electrode and complex A
adsorbed EPG working electrode and H2O-KPF6 electrolyte. H2SO4 was added externally.
driving forces are required to obtain measurable rates of H+
reduction. At [H]+ > 0.1 N the electrocatalytic current
coincides with the reduction of the complex to its mixed
valent Fe(I)Fe(0) state. Thus the Fe(I)Fe(0) state is the active
form of the catalyst at these acid concentrations. However at
[H]+ <0.02 N, the H+ reduction coincides with the reduction of
the cluster to its Fe(0)Fe(0) state (Figure 6). Thus at these
acid concentrations the Fe(0)Fe(0) state is active. This may
reflect the relative pKa’s of the Fe(I)Fe(0) and the Fe(0)Fe(0)
states (assuming that the H+ reduction occurs via protonation
of the catalyst). The former has lesser electron density than the
latter, and hence its protonation may occur at higher acid
concentrations than the latter.
Controlled Potential Coulometry (Bulk Electrolysis).
For estimating the Faradaic efficiency of the catalyst A,
Figure 7. Controlled potential bulk electrolysis voltamogram at −0.5
V vs NHE in aqueous 0.5 N H2SO4. The red line indicates the charge
controlled potential coulometry (CPE) is performed at 0.5 N
H2SO4 for a period of 8 h at −0.5 V in a two compartment cell.
In these experiments consumption of 1.10 × 102 C charge (i.e.,
1.10 × 102/F = ∼1.14 × 10−3 moles of electrons) from the
electrode (Figure 7, red) was recorded. A bare EPG electrode
dissipated only 0.14 C charge when held at −0.5 V (Figure 7,
green). The H2 gas formed in the process was trapped in an
inverted burette setup and was measured to be 12 mL at 1 atm.
This yields a Faradaic yield (FY) of 94% (12.8 mL = 100% FY).
To ensure the integrity of the complex of the electrode, FTIR
data as well as XPS data are collected before and after the
electrolysis experiments. The IR spectrum of the catalyst A has
been recorded after grafting it onto the EPG (Figure 8, blue).
After continuous electrolysis at −0.5 V in 0.5 N H2SO4 solution
the FTIR spectrum is recorded again (Figure 8, red). The data
accumulated by the catalyst absorbed electrode, and the green line
represents the charge accumulated without catalyst.
show minimum changes in energies and relative intensities of
the vibrations observed in the CO and C−H stretching regions.
Similarly very little change in the relative intensity of the XPS
spectra is detected (Supporting Information, Figure SI−IV).
Thus the catalyst retains its integrity during the electrolysis
process. This is different from the results obtained on HOPG
and Au electrodes covalently modified with similar catalysts
where rapid degradation of these catalysts was observed. A key
difference between these experiments and the one performed
before is the reaction medium. While here acidic aqueous
medium is used, in all of the above cases CH3CN was used as
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Figure 8. IR spectra of the catalyst A onto the EPG surface before (blue) and after electrolysis (red). (A) C−O vibration region; (B) C−H vibration
region. (X-axis in cm−1).
the solvent and HBF4 or some other organic acid was used as
the proton donor.50 These catalysts, once immobilized on
graphite surfaces, seem to be more stable in acidic aqueous
medium relative to acidic organic medium.
Turnover Frequency (TOF) and Turnover Number
(TON). The CV experiments indicate that there are ∼10−11
moles of catalyst immobilized on the EPG electrode which
could be obtained by integration of the current density under
the oxidation and reduction waves. The controlled potential
Figure 9. Control Potential Coulometry at −0.5 V vs NHE showing
electrolysis experiments at −0.5 V indicate that ∼1.1 × 10−3
electrochemical H2 generation from 0.5 N H2SO4 using a cheap
moles of H+ is reduced without any decay in the catalytic
graphite electrode.
activity in 8 h. Thus, the TON for the catalyst can be estimated
to be ≫108. The TOF determined from the Icat/[τ] ratio is
species (i.e., the reduced complex A) in an aqueous medium
6400 s−1 in 0.5 N H2SO4. Alternatively, during bulk electrolysis
raises the thermodynamic reduction potential which allows H2
1.1 × 10−3 turnovers were detected in 8 h by 10−11 moles of
production from H2O at reasonable potentials. FTIR data of
the catalyst, immobilized on the electrode, before and after the
electrocatalytic processes indicates that the complex is quite
catalyst. The average TOF during the bulk electrolysis
experiment is estimated to be ∼6 × 103 s−1 (108 turnovers
over 8 h = ∼6000 turnovers per second) at −0.5 V in 0.5 N
H2SO4, which is comparable to the value obtained from LSV. A
foot of the wave analysis (Supporting Information, Figure SI−
VI) of the LSV data has been performed to determine the
intrinsic second order rate (kcat) of this catalyst to be ∼33000
M−1 s−1.59,60 A plot of log(TOF) and overpotential (η)
obtained from this analysis agrees reasonably well with the
estimate obtained from bulk electrolysis experiments. Consid-
ering the fact that ∼10−11 moles of complex A produce 12 mL,
that is, 0.6 × 10−3 moles of hydrogen over a period of 8 h in 0.5
N H2SO4 at −0.5 V, one can estimate the yield of H2 per mole
of the catalyst to be ∼4.4 × 104 liter/sec per mole of complex
A. This is equivalent to ∼83 L/sec/gram of catalyst. To the best
of our knowledge this is possibly the highest electrocatalytic H2
generation rate reported so far34,36,61 and is comparable to the
highest activity reported for the native enzymes.
H2 Generation from Domestic Carbon Electrodes.
Encouraged by the bulk electrolysis results on EPG electrodes,
H2 production was also investigated by adsorbing the catalyst
on the surface of very cheap carbon electrodes conventionally
used in domestic 1.5 V Zinc−Carbon dry batteries. Profuse
evolution of H2 gas was observed on the cathode (Figure 9, also
see Supporting Information video) and in parallel O2 gas
liberation was observed in the Pt mesh cathode when a
potential of −0.5 V was applied to the working electrode. Thus
complex A can in fact be immobilized even on very cheap
electrodes to generate H2 from acidic water at −0.5 V vs NHE.
stable under these conditions. The onset potential, TON, and
TOF obtained, when compared to that of other catalysts
reported in the literature (Table 1), suggests that this is an
excellent electrocatalyst for H2 generation. While some Mo
based catalysts show reasonable H2 generation at pH 7, this Fe
based catalyst has superior TOF and low overpotential. This
catalyst has the highest TON (≫108) and TOF in aqueous
medium of all the catalysts reported to date (Table 1). In
addition to that, its reactivity is retained when immobilized on
cheap carbon electrodes, which is of significant practical
interest.
EXPERIMENTAL DETAILS
■
General Procedures. Compound A was synthesized using
the standard Schlenk technique. Anaerobic experiments were
performed within an MBRAUN glovebox. The solvents used
were purchased from Merck Specialties, Pvt. Ltd. (India),
Spectrochem Pvt. Ltd. (India), RFCL Ltd. (India), Avra
synthesis Pvt. Ltd. (India). p-Bromoaniline and thionyl chloride
(SOCl2) were purchased from Spectrochem Pvt. Ltd. (India),
p-formaldehyde and superhydride (LiBEt3H, 1 M in THF)
were bought from Sigma-Aldrich chemicals Pvt. Ltd., and
anhydrous Na2SO4 and sulfuric acid (98%) were purchased
from Merck Specialties, Pvt. Ltd. (India). SiO2 (60−120) was
purchased from SISCO Pvt. Ltd. (India). Triple distilled
deionized water has been used for all electrochemical
experiments. Column chromatography was performed on
SiO2 (60−100 mesh). Edge Plane Graphite (EPG) discs were
purchased from Pine Instruments. The FT-IR data are
measured on the Shimadzu FTIR 8400S instrument. All the
NMR spectra were recorded on the Bruker DPX-300 or DPX-
500 spectrometer at room temperature. The mass spectra are
CONCLUSION AND COMMENT
■
In summary, we report, for the first time, H2 formation from
acidic H2O (pH < 3) by a synthetic mimic of FeFe H2ase
bearing an ADT bridge. Enhanced solvation of the active
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Table 1. Comparison of H2 Evolving Electrocatalysts
performance
TOF
a
b
catalyst
Co nanoparticle
onset E (V) Vs NHE
pH
TON
ref
−0.65 V
−1.07 V
−0.65 V
−0.93 V
−0.40 V
−0.40 V
−0.90 V
>−0.7 V
−0.3 V
pH 7
pH 7
pH 3
pH 7
pH 2.2
pH 2
pH 2
pH 3
pH 2−1
26
Co pentapyridine
∼104
∼103
∼105
23
0.35 s−1
280 s−1
2.4 s−1
27
MoS2 catalyst
35
Mo Oxo catalyst
34
Co tetraaza macrocycle
Co tetraimine catalyst
Co bis iminopyridine
Fe2S2Ph(CO)6-SDS surfactant
complex A
25
∼105−6
56
2.2 h−1
2600 s−1
6400 s−1 kcat ∼3.3 × 104 M−1 s−1
33
52
36
≫108
this work
a
b
Moles of H2 per mole of catalyst. Generally calculated at 100−300 mV below the onset potential.
recorded by a QTOF Micro YA263 instrument. X-ray single
crystal data was collected at 120 K using radiation on a SMART
APEX diffractometer equipped with CCD area detector. Data
collection, data reduction, and structure solution refinement
were carried out using the software package of APEX II. The
structure was solved by the direct method and refined in a
routine manner. The nonhydrogen atoms were treated
anisotropically. All the hydrogen atoms were located on a
difference Fourier map and refined.
Synthetic Procedure for Complex A, BrC6H4N-
(CH2S)2Fe2(CO)6. Complex A was synthesized according to
the literature procedure using a standard Schlenk technique.54
Fe2S2(CO)654,62(438 mg, 1 mmol) was dissolved in 20 mL of
dry tetrahydrofuran (THF). Two milliliers of LiBEt3H (1 M in
THF) was added dropwise to the red solution at −78 °C. The
solution immediately turned to deep green. The ligand p-
BrC6H4N(CH2)2Cl2, synthesized by reacting with p-
BrC6H4NH2, p-formaldehyde, and thionyl chloride,62 was
added to the reaction mixture at −15 °C. The color
immediately changed to dark red. The reaction mixture was
stirred for further 1 h at room temperature. Removal of the
solvent produces a dark brown precipitate. A crystalline red
solid product was obtained after column chromatography with
hexane-toluene (4:1) mixture. Single crystal was grown by slow
diffusion of the hexane in toluene.
The aqueous aqueous cyclic voltamogram was recorded with
the same instrument and same software in 0.1 M KPF6- water
solution with the complex A adsorbed EPG discs (5 mm outer
diameter) held within a shaft (Pine Instruments, AFE6MB). An
anaerobic environment was maintained within a water jacketed
electrochemical cell (Pine Instrument, RRPG138) by purging
with Ar gas for 1 h. The aqueous Ag/AgCl reference (Pine
instruments, RREF0021) and Pt counter (AFCTR5) electrodes
were attached to the cell through airtight joints. All potentials
reported here are versus aqueous NHE (in saturated KCl)
reference electrode.
Construction of Graphite Electrodes with Compound
A (Fe2S2(CH2)2NC6H4Br). On Edge Plane Graphite (EPG). A
50 μL portion of a dilute solution (∼10 mM, in CH2Cl2) is
uniformly distributed on a freshly polished EPG disc (5 mm
outer diameter). After the CH2Cl2 has evaporated, the surface is
washed with CHCl3 thoroughly and sonicated in ethanol to
remove any loosely bound catalyst on the EPG surface and
washed with triple distilled water.
Rotating Disc Electrochemistry. The RDE/RRDE
measurements are performed on a CHI 700D bipotentiostat.
An EPG disc is used as the working electrode which is mounted
inside of a Pt ring assembly (Pine Instrument, AFE6RIP) which
is mounted at the tip of a shaft (Pine Instruments, AFE6MB)
which in turn is fitted to a MRS rotator. A water jacketed
electrochemical cell (Pine Instrument, RRPG138) is obtained
where the rotor was inserted through a taper plug assembly
(AC01TPA6M) which allows free rotation of the vertical rotor
while maintaining an airtight seal. An aqueous Ag/AgCl
reference (Pine instruments, RREF0021) and Pt counter
(AFCTR5) electrodes are attached to the cell through airtight
joints. The graphite surface is cleaned by polishing it uniformly
on a Silicon carbide grinding paper followed by sonication in
triple distilled water. The complex is then physisorbed on the
disc as described above.
Electrochemical Hydrogen Production by Complex A
Adsorbed on EPG in Aqueous H2SO4. The shaft bearing the
EPG electrode and the Pt ring is vertically inserted to the water
jacket cell, and the cell is purged with N2 for 30−60 min.
Hydrogen generation experiments have been investigated in
thoroughly degassed solutions (free-pump-thaw) to eliminate
any electrocatalytic O2 reduction. Background scans are
performed under the same conditions with bare electrodes.
Linear sweep voltammetry (LSV) is performed at 300 rpm
(rotations per minute) to investigate the electrocatalytic H2
generation. Pt counter electrodes may be corroded under acidic
conditions when large anodic potentials are applied.63 Thus the
electrocatalytic current was also measured using a glassy carbon
1H NMR (CDCl3): 7.50−6.52 (4H, dd), 4.27 (s, 4H); IR
(CH3CN): 2074 cm−1(s), 2038 cm−1(s), 2000 cm−1(s); ESI+
Mass(TOF) [M + H]+ = 540.3941; Calculated [m/z] =
539.772.
ATR-FTIR Spectroscopy. FT-IR spectra of the Complex A
adsorbed EPG surface was recorded by a Perkin-Elmer Frontier
FT-IR spectrometer. A total of 3000 scans were collected at
room temperature before and after electrolysis using the EPG
disc electrode.
X-ray Photoelectron Spectroscopy. XPS (XPS, Omi-
cron, model: 1712-62-11) measurement was done using a high
resolution monochromatic Mg−Kα radiation source at 1253.6
eV under 15 kV voltage and 5 mA current condition.
Electrochemical Measurements. Cyclic Voltammetry.
The cyclic voltammograms are recorded on a CH Instrument
bipotentiostat model 720D. A glassy carbon electrode and a Pt
wire is used as the working and the counter electrode,
respectively. The measurements were made against an aqueous
Ag/AgCl reference electrode and corrected to NHE. CV
experiment on a 1 mM solution of complex A and 0.1 N n-
Bu4ClO4 in acetonitrile was performed in a Glove Box at room
temperature. The working glassy carbon electrode was washed
and polished before use.
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dx.doi.org/10.1021/cs300835a | ACS Catal. 2013, 3, 429−436

ACS Catalysis
Research Article
counter electrode. Very similar results were obtained
(Supporting Information, Figure S1).
MnO2 present. This was followed by washing with conc. HNO3
and then with copious amounts of water. Then the electrode is
dried overnight in a oven at 80 °C. Complex A is physisorbed
on the graphite electrode by dipping it into a CH2Cl2 solution
of the complex. Then the electrode is washed with DCM,
ethanol, and water successively. The electrolysis is then
performed using a Pt mesh counter electrode and a normal
Ag/AgCl reference electrode.
DFT Calculations. The geometry optimizations of the
complex were carried out using a 6-31g* basis set in Gaussian
03 using the BP86 functional.64−66 The final electronic
structure and energy calculations were carried out using a 6-
311+g* basis set on all atoms. Frequency calculations were
performed on the optimized structures to ensure a global
minimum, that is, no negative frequencies. Solvation
corrections were performed using a PCM model and using
CH3CN and H2O as solvents.67
Detection of H2. H2 is generally detected by head space gas
analysis by a GC fitted with appropriate detector. Such facilities
are not accessible to the authors. Fontecave et.al. reported an
electrochemical method for detecting H2 using rotating ring
disc electrochemistry (RRDE).16 In an RRDE experiment a Pt-
ring encircling the working EPG electrode is held at a constant
potential of 0.7 V. At this potential Pt electrode oxidizes H2,
generated at the working electrode and radially diffused
outward to the Pt electrode because of the hydrodynamic
current produced by the rotating shaft back to H+ generating an
oxidation current. Thus a reduction current in observed in the
working electrode and an oxidation current is observed in the
Pt electrode. As a control experiment, in a standard three
electrode cell, a Pt working electrode was held at a constant
potential of 0.7 V and H2 was introduced into the solution by
gentle bubbling. The H2 to H+ current was immediately
generated on the Pt electrode (held at a constant potential of
0.7 V), and the current persisted as long as the H2 gas was
bubbled (Figure 10). The current decayed as the H2 gas
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic details in CIF format. Further details are given
in Figures SI-I to SI-VI as well as in discussions of the
electrochemical measurements and the wave analysis. Video of
profuse evolution of H2 gas on the cathode. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: icad@iacs.res.in (A.D.); icsgd@iacs.res.in (S.G.D.).
Notes
The authors declare no competing financial interest.
All the potential values mentioned here are with respect to
NHE, but the experiments were performed using aqueous Ag/
AgCl reference and corrected to NHE references.
Figure 10. H2 oxidation current on a Pt electrode held at a constant
potential of 0.7 V.
ACKNOWLEDGMENTS
■
bubbling was stopped. This process could be repeated several
times suggesting that oxidation of H2 back to H+ on a Pt
electrode held at a constant potential of 0.7 V is a reliable
method for detecting formation of H2 electrochemically in situ.
Thus H2 produced on the working electrodes was detected by
oxidizing it back to H+ on the Pt ring in a RRDE setup. Note
that the Pt ring electrode held at 0.7 V can not reduce H+ to
H2, that is, none of the H2 produced can derive from Pt.
Controlled Potential Coulometry and Faradaic Yield.
The controlled potential bulk coulometry experiments are
performed in the same water jacket electrochemical cell that is
used for the LSV. These experiments are performed in degassed
0.5 N H2SO4 solutions at −0.5 V vs NHE for 8 h. The shaft
bearing the working electrode is rotated at 900−1100 rpm to
disperse H2 bubbles formed into the bulk solution. During
electrolysis the H2 generated was collected through an outlet of
the cell and collected using an inverted buret setup. The ratio of
the charge dispersed and the moles of H+ reduced in the
process (estimated from the volume of H2 collected) is
reported at the FY.
This research is funded by BRNS grant number 2011/36/12-
BRNS/, DAE, India. S.D. acknowledges a CSIR-SRF fellowship.
A.R. acknowledges the IACS-integrated Ph.D. funding. The
IACS XPS facility funded by DST unit of Nano-science is
gratefully acknowledged. Dr. Tushar Ghosh is gratefully
acknowledged for XPS measurements.
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