Acridinium salts as metal-free electrocatalyst for hydrogen evolution reaction

Article abstract of DOI:10.1016/j.elecom.2016.04.015

Here we report the purely organic metal-free electrocatalyst - acridinium salt 9-phenyl-10-methylacridinium iodide for hydrogen evolution reaction. A controlled potential electrolysis experiment in present 9-phenyl-10-methylacridinium iodide with a simult

Full text of DOI:10.1016/j.elecom.2016.04.015

Acridinium salts as metal-free electrocatalyst for hydrogen evolution reaction
A.V. Dolganov, B.S. Tanaseichuk, D.N. Moiseeva, V.Y. Yurova, J.R.
Sakanyan, N.S. Schmelkova, V.V. Lobanov
PII:
DOI:
Reference:
S1388-2481(16)30097-2
doi: 10.1016/j.elecom.2016.04.015
ELECOM 5688
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
11 April 2016
22 April 2016
26 April 2016
Please cite this article as: A.V. Dolganov, B.S. Tanaseichuk, D.N. Moiseeva, V.Y. Yurova,
J.R. Sakanyan, N.S. Schmelkova, V.V. Lobanov, Acridinium salts as metal-free electro-
catalyst for hydrogen evolution reaction, Electrochemistry Communications (2016), doi:
10.1016/j.elecom.2016.04.015
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Acridinium salts as metal-free electrocatalyst for hydrogen
evolution reaction
A.V. Dolganov1*, B.S. Tanaseichuk1, D.N. Moiseeva1, V.Y. Yurova1,
J.R. Sakanyan1, N.S. Schmelkova1, V.V. Lobanov1
1Department of Chemistry, Mordovian Ogarev State University, Saransk 430005,
Russia
*Corresponding author. Phone: +7(8342) 29-05-48. Fax: +7 (8342) 242444
E-mail address: dolganov_sasha@mail.ru (A.V. Dolganov)
Abstract
Here we report the purely organic metal-free electrocatalyst − acridinium salt 9-
phenyl-10-methylacridinium iodide for hydrogen evolution reaction. A controlled
potential electrolysis experiment in present 9-phenyl-10-methylacridinium iodide
with a simultaneous gas chromatographic analysis confirmed the catalytic
production of molecular hydrogen. The behavior of the catalytic wave is typical for
a “total catalysis”. A mechanism involving initial reduction of PhAcr+/PhAcr˙ and
subsequent protonation is proposed.
Keywords: Hydrogen evolution reaction, electrocatalytic reduction, metal-free
electrocatalyst, total catalysis, acridinium salt
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1. Introduction
Besides finding use in many industries such as petroleum refining, metal
treatment and chemical production, molecular hydrogen has been declared a clean
fuel of the future [1]. If produced by water electrolysis [2] powered from
renewables, such as wind and solar [3], it might offer a solution to the increasing
energy demands of an ever-growing human population and environmental
sustainability.
One of the key obstacles towards making hydrogen production by water
electrolysis economically feasible is the high cost associated with the catalysts
used in industrial applications; these are often based on platinum, a precious metal
of low abundance [4]. This triggered the search for new catalysts formed by earth-
abundant elements [5], mostly by learning from natural enzymes, iron and/or
nickel hydrogenases [6-7], that produce molecular hydrogen from water under
ambient conditions with turnover frequencies reaching 9000 mole of H2 per mole
of catalyst per second [8]. Although hydrogenases themselves are so efficient in
terms of high activity and low overpotentials, they are extremely sensitive to
environmental conditions (temperature, pH, presence of oxygen) and are difficult
to obtain in sufficient quantities for industrial purposes [9].
Most of these issues of high cost and low stability would be solved by
designing a metal-free molecular catalyst ultimately made from earth-abundant
elements (C, H, N, O). Note that hydrogenases were initially thought to be metal-
free enzymes [10]; even though it turned out otherwise, this inspired further search
for pure organic catalysts. Recently it has also been shown that some
polysaccharides, such as chitosan, catalyze hydrogen evolution [11,12]. However,
purely organic molecular compounds have never been viewed as electrocatalysts
for hydrogen production.
Here we report metal-free electrocatalysts for generating hydrogen from
acidic organic media. A controlled potential electrolysis experiment at −0.850 V
versus Fc/Fc+ confirms high reactivity and high fidelity in a 9-phenyl-10-
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methylacridinium iodide in hydrogen evolution reaction from acidic organic
media. The electrocatalyst is one of a family of acridinium compounds with a
phenyl substituent at the 9 position (Scheme 1), which do not suffer from the
radical dimerization common for the unsubstituted acridinium [13]. The findings
provide clear evidence that, like precious metals, well-designed metal-free
counterparts have great potential for highly efficient electrocatalytic HER, thus
opening a new avenue towards replacing noble metals by alternatives in a wide
variety of applications.
2. Material and methods
2.1. Synthesis
9-phenyl-10-methylacridinium iodide (PhAcrI) was obtained as described
previously [13] by quaternization of 9-phenylacridine with methyl iodide in a
sealed tube for 10 hours. The purity of the compound, assessed by NMR and
elemental analysis, was estimated as more than 99.6%.
2.2. Cyclic voltammetry
Cyclic voltammetry experiments were carried out in acetonitrile solutions (with
0.1 M [n-Bu4N][BF4] as a supporting electrolyte) using a model Parstat 2273
potentiostat with a conventional one-compartment three-electrode cell (10 ml of
solution). A glassy carbon electrode (GCE) with an active surface area of
0.125 cm2 was used as a working electrode. The electrode was thoroughly polished
and rinsed before and between the measurements. A platinum counter electrode
and a standard Ag/AgCl reference electrode were used. Conversion to other
reference electrodes is as follows: the ferrocene Fc+/Fc couple lies at 0.41 V versus
Ag/AgCl in acetonitrile. All the solutions were thoroughly deaerated by passing
argon through them before the cyclic voltammetry experiments and above them
during the measurements.
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2.3. Controlled potential electrolysis in acetonitrile.
10 ml of acetonitrile solution of HClO4 (150 mM), PhAcrI (1 mM) and [n-
Bu4N][BF4] (0.1 M) was electrolyzed for 60 min at –850 mV (related to Fc+/Fc ).
A glassy carbon electrode (GCE) with an active surface area S= 0.125 cm2 was
used as a working electrode. The production of molecular hydrogen was confirmed
by gas chromatography.
2.4. Gas chromatography
Gas chromatography analysis of the gases evolved during the electrolysis was
performed with a Varian 450 gas chromatograph equipped with a pulsed discharge
helium ionization detector D-4-I-VA38-R. Hydrogen production was quantitatively
detected using a stainless steel column 30 m in length with inside diameter 250 µm
at 120°C for the detector and at 80°C for the oven. The carrier gas was helium
flowing at a rate of 40 ml min–1. The injections (250 μL) were performed via a
sampling loop. The retention time of gaseous H2 was 2.48 min.
3. Results and discussion
Cyclic voltammograms for PhAcrI in acetonitrile (Figure 1) at a glassy
carbon electrode (GCE) show a reversible one-electron reduction wave at –0.86 V
versus Fc/Fc+, and this is assigned to a redox couple PhAcr+/PhAcr˙ [13]. Note that
the values reported for cyclic voltammograms are the experimental peak potentials;
the potentials at half of the catalytic current are used below to calculate
overpotentials [14] and to perform electrolysis experiments.
Addition of perchloric acid to this solution results in an increase in the
reduction current and in the disappearance of a reverse wave PhAcr˙/PhAcr+
(Figure 1), suggesting catalytic reduction of protons to molecular hydrogen. The
behavior of the catalytic wave at different acid-to-catalyst concentration ratios is
typical for a “total catalysis” [15]: at a lower ratio, it occurs at potentials more
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positive than that of the couple PhAcr+/PhAcr˙ and shifts to more negative
potentials as this ratio becomes higher. Cyclic voltammetry experiments performed
in the absence of PhAcrI show no signs of electrocatalysis; introducing a “used”
GCE into a fresh electrolyte without PhAcrI also does not result in any
electrocatalytic activity. This implies a truly homogeneous catalysis with no
significant adsorption or decomposition of the catalyst on the surface of an
electrode. Note that the shape of all the cyclic voltammograms does not reach a
plateau even at scan rates up to 10 V s-1 or at high concentrations of both
perchloric acid (up to 0.1 M) and PhAcrI (up to 12.5 mM), which is a sign of a fast
catalytic process limited by the diffusion of the substrate to the electrode surface.
A controlled potential electrolysis experiment at −0.850 V versus Fc/Fc+ for one
hour with a simultaneous gas chromatographic analysis confirmed the catalytic
production of molecular hydrogen at an overpotential of 0.82 V. The latter value is
calculated as a difference between the thermodynamic potential for proton
reduction in an acetonitrile solution of perchloric acid and the observed potential at
half of the catalytic current [16, 17]. A one-hour electrolysis of 10-6 mol PhAcrI in
acetonitrile led to the evolution of 7.6×10-7 mol of molecular hydrogen (electric
charge 0.14 C, Faradaic yield 35±5). The low Faradaic yield is probably due to
side electrochemical or chemical reactions. Indeed, according to the 1H and
13C{1H} NMR spectra obtained after one-hour electrolysis is observed the signals
starting 9-phenyl-10-methylacridinium iodide and additionally 9-phenyl-10-
methyl-9,10-dihydroacridine. The proposed catalytic cycle for the generation of H2
mediated by 9-phenyl-10-methyl-9,10-dihydroacridine is shown in Figure 2. The
first step in this mechanism includes the reduction of the cations PhAcr+ to the
radical species PhAcr˙, which was unambiguously identified using the potential of
the one-electron reduction wave (–0.86 V versus Fc/Fc+) in the cyclic
voltammograms of PhAcrI [13]. At −0.850 V (the potential used for
electrocatalytic hydrogen production), no further reduction of the resulting radicals
PhAcr˙ to the anions PhAcr- by the EEC pathway can take place, as it occurs at a
significantly more negative potential (−1.73 V versus Fc/Fc+). So the second step
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is the protonation of the radicals PhAcr˙, which we propose to involve the nitrogen
atom as the most Lewis basic site in PhAcr˙. The initial cations PhAcr+ are
quantitatively regenerated during the process so there is no need for the second
electrochemical reduction to produce molecular hydrogen. This is similar to a
classic hemolytic pathway for proton reduction catalyzed by a transition metal-
based electrocatalyst. At present, more detail is studied electrocatalytic
mechanism.
Figure 1 (inset) shows the ratio of catalytic current in the presence of perchloric
acid (icat) to the peak current for the reduction of PhAcrI in the absence of acid (ip)
as a function of the acid concentration in the experiment. At concentrations of less
than 60 equiv of perchloric acid to (relative to catalyst), the observed catalytic
current increased with acid concentration. Beyond this acid concentration, no
significant increase in catalytic current was observed (Figure 1). This type of
behavior is expected when substrate (H+) concentration is sufficiently high that it is
not depleted by the catalytic process during the course of the experiment and some
other elementary step becomes rate limiting in the catalytic cycle. Using data in
this acid independent regime, the rate constant for hydrogen evolution (kobs) can be
calculated from the expression:
icat
2
RTkobs

,
(1)
ip 0.4463
F
in which ν is the potential scan rate, n is the number of electrons transferred (two
for hydrogen evolution), R is the universal gas constant, T is the temperature, and
F is Faraday’s constant. The icat/ip values of ≈3 in the acid independent region (for
a scan rate of 100 mV s−1) indicate an acid independent rate constant of 1.12 s−1.
Repeating this analysis using independent data sets generated using different
electrochemical potential scan rates indicates that the TOF is a robust parameter in
the range 1.12 − 1.13 s−1. Note that in the absence of an extended plateau region
for jmax, eq 1 provides only an approximation of kobs in the present system [17].
Alternatively, the catalytic current-enhancement parameter, reflecting the
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efficiency of the electrocatalytic process, expressed as the icat/ip ratio with icat being
the catalytic current and ip the peak current associated with a monoelectronic wave
of the catalyst, is used as a proxy for the turnover frequency [18]. The resulting
icat/ip for PhAcrI value of ≈3 (Fig. 1 inset) is higher than that for any reported
complex, such as [Fe2(S2C3H6)-(CO)3(dppv)(NO)]+ [dppv
=
cis-1,2-
bis(diphenylphosphinoethylene)], (μ-SC6H4-2-(CO)S-μ)[Fe2(CO)6] and (μ-
pdt)[Fe2(CO5)PPyr3] [pdt = propanedithiolate; PPyr = tripyrrolylphosphine)]
(in all case icat/ip< 2), for complex μ-pdt[Fe(CO)3][Fe(CO)2IMes] [IMes =1,3-
bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene] the icat/ip ratio ≈3 [18] . The icat/ip
value exhibited by PhAcrI is, however, lower than those of some molecular Ni/Co
systems in organic solvents: [(dmgBF2)2Co(MeCN)2] [dmg = dimethylglyoxime]
(icat/ip≈30); [(PPh2NPh)2Ni]2+ (≈38); [(PtBu2NPh2)Co(MeCN)3]2+ (≈40) [18]. It
should be noted that the potentials of the electrocatalytic processes in the presence
of such complexes are significantly shifted in the cathodic region compared to
PhAcrI. So for (μ-SC6H4-2-(CO)S-μ)[Fe2(CO)6] E = –1.3 V (vs Fc/Fc+) and for (μ-
pdt)[Fe2(CO5)PPyr3], and μ-pdt[Fe(CO)3][Fe(CO)2IMes] E = –1.3 V and –1.9 V(vs
Fc/Fc+), respectively.
4. Conclusions
In summary, we have shown that a metal-free electrocatalyst, a simple acridinium
salt, demonstrates promising catalytic activity in electrochemical hydrogen
evolution from acidic organic media. The electrocatalytic activity of 9-phenyl-10-
methylacridinium iodide was studied in acetonitrile solutions: addition of HClO4 to
these solutions produced a catalytic wave at the reduction potential that is
characteristic of the electrocatalytic process of the hydrogen evolution reaction.
Further studies are in progress to optimize the reaction conditions for efficient
hydrogen production by varying the electronic properties of the 9-phenyl-10-
methylacridinium iodide catalyst.
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Scheme 1.
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Figure 1. Cyclic voltammogramsfor 1.0 mM solutions of PhAcrI in acetonitrile at a
concentration of perchloric acid (in mM) increasing from top to bottom: 0 (1), 6.05 (2), 12.1 (3),
18.15 (4), 24.2 (5), 30.25 (6), 48.4 (7), 60.5 (8), 84.7 (9), and perchloric acid (concentration of
84.7 mM) in the absence of PhAcrI (10). Conditions: scan rate 200 mV s–1, 25°C, 0.1 M [n-
Bu4N][BF4] in acetonitrile, glassy carbon working electrode. Potentials are referenced to Fc/Fc+.
Inset: Dependence of the icat/ip on the concentration of HClO4 mM at a scan rate 200 mV s–1 in
1.0 mM solutions of PhAcrI in acetonitrile.
Figure 2. Tentative catalytic mechanism for proton reduction by PhAcrI with alternative EEC
and ECEC pathways; counter-ions are omitted for clarity. The potentials are given for
acetonitrile solution and are referenced to Fc/Fc+.
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1
2
I, A
10
3
4
5
6
5
0
7
8
-5
9
10
-10
-15
-20
-25
-30
-35
-40
-45
I
/I
cat
p
3,0
2,5
2,0
1,5
1,0
0,5
0,0
+
[H ] mM
0
20
40
60
80
100
-1,2
-1,1
-1,0
-0,9
-0,8
-0,7
-0,6
-0,5
-0,4
E, V
Figure 1.Cyclic voltammogramsfor 1.0 mM solutions of PhAcrI in acetonitrile at a
concentration of perchloric acid (in mM) increasing from top to bottom: 0 (1), 6.05 (2), 12.1 (3),
18.15 (4), 24.2 (5), 30.25 (6), 48.4 (7), 60.5 (8), 84.7 (9), and perchloric acid (concentration of
84.7 mM) in the absence of PhAcrI (10). Conditions: scan rate 200 mV s–1, 25°C, 0.1 M [n-
Bu4N][BF4] in acetonitrile, glassy carbon working electrode. Potentials are referenced to Fc/Fc+.
Inset: Dependence of the icat/ip on the concentration of HClO4 mM at a scan rate 200 mV s–1in
1.0 mM solutions of PhAcrI in acetonitrile
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Figure 2. Tentative catalytic mechanism for proton reduction by PhAcrI with alternative EEC
and ECEC pathways; counter-ions are omitted for clarity. The potentials are given for
acetonitrile solution and are referenced to Fc/Fc+.
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Highlights
• Acridinium salts are electrocatalysts for H2 production.
• Tentative catalytic mechanism of proton reduction by PhAcrI is discussed.
• A mechanism involving initial reduction of PhAcr+/PhAcr˙ and subsequent
protonation is proposed.
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