Polybenzimidazole and polybenzimidazole/MoS2 hybrids as an active nitrogen sites: Hydrogen generation application

Article abstract of DOI:10.1039/c5ra20447j

Development of free metal or non-noble-metal catalysts for electrode materials with both excellent activity and high stability is essential for hydrogen production. In this work, nitrogen rich polybenzimidazole (PBI) networks were synthesized through a on

Full text of DOI:10.1039/c5ra20447j

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Polybenzimidazole and Polybenzimidazole/MoS Hybrids as an
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Active Nitrogen Sites: Hydrogen Generation Application
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1
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1
1
Mohammad Dinari , Afshin Nabiyan , Ali. A. Ensafi , Mehdi Jafari-Asl
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1
Department of Chemistry, Isfahan University of Technology, Isfahan 84156–83111, Islamic
Republic of Iran
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Abstract: Development free metals or nonꢀnobleꢀmetal catalysts for electrode materials with
both excellent activity and high stability is essential for hydrogen production. In this work,
high rich nitrogen polybenzimidazole (PBI) networks was synthesized through a oneꢀstep
polycondensation of 1,2,4,5ꢀtetraaminobenzene with 4,4',4''ꢀ((1,3,5ꢀtriazineꢀ2,4,6ꢀ
triyl)tris(azanediyl))tribenzoic acid. Then, organic/inorganic nanohybrids of PBI with
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amorphous MoS
2
nanoplates (PBIꢀMoS
2
) were prepared. The structural characterization and
1
morphological of these novel hybrids were studies using FTꢀIR, H NMR, elemental analysis,
thermogravimetry, transmission electron microscopy, fieldꢀemission scanning electron
microscopy and Xꢀray diffraction techniques. Electrochemical studies revealed the onset
ꢀ1
potential of only (–160 mV vs. RHE) with a small Tafel slope of 50.6 mV dec for hydrogen
ꢀ1
generation reaction in 0.5 mol L H
2 4
SO . Stability tests through long term potential cycles
confirm the excellent durability of PBIꢀMoS
2
in acid media. The outstanding hydrogen
+
generation activity is derived from the electronic penetration effect and H absorption of
pyridinicꢀN and/or pyrrolicꢀN as active sites at per repeat of the PBI matrix. It is worth noting
1
ꢀ
Corresponding author. Tel.: +98 3133913270; fax: +98 3133912351.
Eꢀmail address: dinari@cc.iut.ac.ir; mdinary@gmail.com
1

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that pure PBI and PBIꢀMoS
2
hybrids, for the first time, were used as both anode and cathode
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in twoꢀelectrode system open up new possibilities for exploring overall hydrogen generation
technology catalysts in acidic electrolyte. This development offers an attractive
electrocatalyst for largeꢀscale hydrogen generation.
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Keywords: Hydrogen generation; Polybenzimidazole; pyridinicꢀN; PyrrolicꢀN molybdenum
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disulfide.
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1
. Introduction
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The rapid growth of global energy consumption and the associated environmental
issues have triggered the urgent demand for renewable and clean energy sources.
Electrochemical water splitting driven by solar energy has been considered as an attractive
approach to produce hydrogen (H
2
) fuel, a sustainable, secure and environmentally benign
energy vector. Efficient water splitting requires highꢀperformance electrocatalysts to promote
1
ꢀ3
4
the hydrogen generation reaction. The catalysts are cornerstone of this type of reaction.
There is now enormous interest to design and develop efficient and inexpensive catalysts to
5
generate hydrogen. According to the information, which provided by the previous reports
only a few synthetic catalysts are known to operate in water and they were able to give a high
6
ꢀ9
current density at a low overpotential (Ƞ). To date, the most effective hydrogen generation
catalyst are Pt group metals, whose huge scale application has been severely limited by their
7
ꢀ10
low abundance and high cost.
Therefore, finding an inexpensive catalyst still is a serious
challenge for hydrogen generation. In earlier works, scientific researches in catalyst areas
achieve new approaches, which are used from metalꢀfree catalysts for hydrogen generation
such as 4,4′ꢀbipyridine, nitrogen doped reduce graphene oxide, metalꢀfree polymeric
11ꢀ13
photocatalyst, and etc.
A large proportion of metalꢀfree catalysts like 4,4′ꢀbipyridine and
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of nitrogen doped reduces graphene oxide, the pivotal role of the nitrogen element is
inevitable and it has crucial junction in most catalyst mechanism of hydrogen generation
reaction. Therefore, a metalꢀfree catalyst that is able to incorporate by nitrogen with hydrogen
generation mechanism like Volmer, Tafel or Heyrovsky reaction is among the most
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outstanding catalyst for hydrogen generation.
Polybenzimidazole (PBI) PBI is a basic polymer with good thermal and chemical
stability, which belongs to the class of heterocyclic polymers that contain benzimidazole
14
units. PBI is the most promising candidate polymer to use as a hydrogen generation
catalyst. There have been a numerous reasons why PBI consider or offer as a catalyst in
hydrogen generation reaction and it would explore only a few most important properties for
hydrogen generation once here. First of all, PBI has considerable proton conductivity as long
15,16
as doped or hybrid with conductive materials such as graphene and phosphoric acid.
In
some studies, the enhanced electrocatalytic activity of hydrogen generation catalyst is
attributed to pyridinicꢀN and/or pyrrolicꢀN. Therefore, PBI which is used as a membrane
materials for fuel cells, can repeat the pyridinicꢀN and/or pyrrolicꢀN in every per repeat
13,17ꢀ19
unit.
PBI show high surface areas with porous materials to enhance fast mass transport
2
0ꢀ22
of reagents. Therefore, the high surface area is able to accept PBI as eligible catalyst.
This
vital information can be confirmed that PBI as is a good catalyst candidate for hydrogen
generation.
In the past years, organic/inorganic hybrid nanocomposites (NCs) as long as the
organic part is the polymer matrix and the inorganic part is nanosized fillers have been
advanced widely since their specific properties and particular application of NCs achieve
23ꢀ25
after hybridization.
Polymer/layered inorganic NCs have attracted great consideration for
their potential in improving polymer properties such as mechanical, thermal, and physical
23ꢀ25
properties.
Molybdenum disulfide (MoS ) nanosheets are expected to become one kind of
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useful fillers for improving the properties of polymers. MoS
2
has attracted considerable
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interest over the last few years due to its extraordinary optical, thermal, and mechanical
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properties arising from its exceptional structure.
The unique sandwich structure has
potential applications in many technological fields, such as super lubricant, sensors, batteries,
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photocatalyst, hydrogen storage and NCs.
novel class of materials that combine the attractive functional properties of MoS
optical, thermal, mechanical properties, etc) with the advantages of polymers, such as low
where MoS ꢀbased polymer composites are a
2
2
(electrical,
32
cost and good processability.
Motivated by these results, in this work, we sought to explore a facile and efficient
route for preparing PBI/MoS NCs. First, 4,4',4''ꢀ((1,3,5ꢀtriazineꢀ2,4,6ꢀtriyl) tris (azanediyl))
2
tribenzoic acid (TCA) was prepared from reaction between cyanuric chloride and paraꢀ
aminobenzoic acid in glacial acetic acid. Then, PBI networks was synthesized through a
facile one step polycondensation reaction of 1,2,4,5ꢀtetraaminobenzene and TCA using
methane sulfonic acid and phosphorus pentoxide mixture as a reaction medium. MoS plate
2
was synthesized by hydrothermal method and the PBIꢀMoS NCs was achieved by ultrasonic
2
irradiation methods. Although there are numerous reports in the literature for MoS
electrocatalyst for hydrogen generation, to the best of our knowledge, there is no report which
addresses PBI as well as PBIꢀMoS hybrid as hydrogen generation electrocatalysts. The
2
as
2
electrocatalytic analysis results such as onset potentials, Tafel slopes and current densities
exhibited infinitely superior and favorable catalytic activities of PBI and PBIꢀMoS hybrids
2
electrocatalyst for hydrogen generation.
2. Experimental
2.1. Reagents
Cyanuric chloride, methanesulfonic acid, and phosphorus pentoxide were purchased
from Merck chemical Co. and 1,2,4,5ꢀbenzenetetramine tetrahydrochloride, Lꢀcysteine,
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Nafion solution (5.0 wt. % in lower aliphatic alcohols and water), 4ꢀaminobenzoic acid and
ammonium molybdate tetrahydrate, ((NH Mo O) were purchased from Sigma
97
4
)
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7
O24. 4H
2
98
Aldrich. All other chemicals used in this investigation were of analytical grade and were
purchased from Merck. Deionization water (DI) was used for preparation of all of solution.
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2.2. Instrumentation
FourierꢀTransform Infrared (FTꢀIR) spectrums of the materials were recorded using a
ꢀ1 1
JASCO 680 (Japan) spectrophotometer over the wavenumber range of 400–4000 cm . H
NMR spectra was recorded on Bruker Avance 400 MHz spectrometer operating on polymer
solution in dimethyl sulfoxideꢀd6 (DMSOꢀd6). Chemical shifts are given in the δ scale in
parts per million (ppm). Thermal properties of the compounds was performed with a STA503
o
win TA (BahrꢀThermoanalyse GmbH, Hüllhorst, Germany) at a heating rate of 10 C/min
from 25 to 800 °C under nitrogen atmosphere. The composition of the polymer (C, H, and N)
was analyzed by elemental analysis (LECO, CHNSꢀ932). The Xꢀray diffraction (XRD) was
used to characterize the crystalline structure of the polymer and NCs. XRD patterns were
collected on a Bruker, D8ADVANCE (Germany). Transmission electron microscopy (TEM)
was performed using a Philips CM120. Morphology was observed using fieldꢀemission
scanning electron microscopy (FEꢀSEM, MIRA FEꢀSEM | TESCAN, Mira 3ꢀXMU).
2
.3. Electrochemical characterizations
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–
1
The electrochemical measurements were performed in 0.5 mol L H
2
4
SO solution at
room temperature (RT). The electrode potential was controlled by an Autolab electrochemical
analyzer, Model PGSTAT 30 potentiostat /galvanostat (EcoꢀChemie, The Netherlands). Data
were acquired and processed (background correction) using the GPSE and FARA computrace
software 4.9.007. A standard threeꢀelectrode cell containing a platinum wire auxiliary
2
electrode, a saturated Ag/AgCl reference electrode and PBI/MoS and/or PBI modified glasy
carbon electrode as a working electrode.
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22
2
.4. Preparation of the working electrodes
The glassy carbon electrode (GCE) was polished to a mirror finish using alumina
powder. After that, the electrode was washed several times with DI water and ethanol. Then,
0 mL of catalyst ink (PBI/MoS and PBI, that prepared by dispersing of 2.5 mg of the as
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1
2
prepared catalysts in 1 mL of ethanol/water (1:1)) was dropꢀcoated on the polished GCE
electrode surface. After drying, 5 mL of 5.0 wt. % Nafion solution in lower aliphatic alcohols
and water was coated on the catalyst layer to ensure better adhesion of the catalyst on the
glassy carbon substrate. This electrode was then dried under room temperature.
2.5. Preparation of 4,4',4''-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid
4,4',4''ꢀ((1,3,5ꢀtriazineꢀ2,4,6ꢀtriyl) tris(azanediyl))tribenzoic acid (TCA) as a three
33
acid monomer was prepared according to the reported procedure. Briefly, cyanuric chloride
3.68 g, 20 mmol) was added in one portion to a stirred solution of a 4ꢀaminobenzoic acid (33
(
mmol) in 150 mL of glacial acetic acid and the mixture was refluxed for 12 h. The products
were precipitated from the mixture as white solids and were recovered by filtration. The solid
products were washed with boiling water to neutral pH and dried at 90°C in air.
2.6. Preparation of star polybenzimidazole
Star PBI was synthesized by polycondensation method. In a typical procedure, a
threeꢀnecked flask equipped with a mechanical stirrer and N inlet, was charged with 3.0 g of
2
o
phosphorus pentoxide and 20 mL of methanesulfonic acid. The mixture was stirred at 50 C
under a nitrogen flow until phosphorus pentoxide dissolved. Then, TCA (0.69 g) and 1,2,4,5ꢀ
tetraaminobenzene (0.29 g) were added and the temperature was increased and the mixture
o
o
o
o
was allowed to polymerize at 80 C for 2 h, 100 C for 1 h, 120 C for 1 h ,and 140 C for 3 h.
The resulting mixture was poured into ice water and then collected by filtration. The solid
was washed with ammonia solution and then with deionized water to neutrality and remove
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o
the possibly unreacted monomers. Final, the perception was dried at 100 C under vacuum to
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constant weight. Yield: 90%.
2.7. Preparation of molybdenum disulfide
3
4
MoS was synthesized according to the reported procedure. Briefly, 1.5 mmol of
2
(
(NH ) Mo O 4H O was dissolved in 30 ml deionized water, and then a given amount of Lꢀ
4 6 7 24 2
cysteine was added into the solution under violent stirring. After stirring for 10 min,
approximately, 12 mol/L HCl was added into the above solution mixture drop by drop under
stirring to adjust the pH value to less than 1. Finally, the solution was transferred into a 100
ml Teflonꢀlined stainless steel autoclave and heated at 240 °C for 24 h. After cooled to room
temperature naturally, the black precipitates of MoS were collected, washed with distilled
2
water and absolute ethanol for several times, and then dried in vacuum at 60 °C for 12 h.
2.8. Preparation of polybenzimidazole/molybdenum disulfide (PBI/MoS
2
) hybrids
For preparations of PBIꢀMoS hybrid, 0.1 g of the PBI was dispersed in 10 mL of
2
DMSO and a uniform colloidal dispersion was obtained after sonication for 2h at room
temperature. Then the suspension was mixed with the different amounts of MoS (1, 5 and
2
1
0%wt) to produced electrocatalyst followed by sonication for 2 h and stirred for 3 day at 110
o
C. The solvent was removed and the powders obtained were dried under vacuum for 24 h at
o
6
0 C. The NCs are named as PBI/MoS
2
NC1%, PBI/MoS
2
NC5% and PBI/MoS
2
NC10%,
where the percentage given in the genetic abbreviations is the weight percentage.
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3
. Results and discussion
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3.1 Synthesis, structural characterization and morphological investigation
The synthesis of the triazine based monomer is presented in Fig 1. TCA was prepared
through a successive procedure of aryl amination of cyanuric chloride with paraꢀ
aminobenzoic acid in glacial acetic acid. The synthesis route to polybenzimidazole networks
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is outlined in Fig. 1. PBI was respectively prepared by TCA and 1,2,4,5ꢀtetraaminobenzene
via a oneꢀstep polycondensation in a solution of phosphorus pentoxide in methanesulfonic
acid i.e. Eaton's reagent. Eaton's reagent has been reported for the preparation of linear PBIs,
and it has been found that the polymerization system has, a high reaction temperature and a
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low viscosity, less reaction time and high catalytic efficiency. To clarify, it was observed
here that the target polymerizations proceeded clearly and could be completed within several
o
hours at a temperature below 140 C. The obtained light brown products are insoluble in
many common solvents. "Figure 1 Here"
Fig.2 shows FTIR spectra of the triazine based monomer (TCA), MoS , neat PBI and
2
PBI NCs with different amount of MoS . For TCA monomer, the FTIR spectrum shows the
2
ꢀ1
broad bond around 2600ꢀ3500 cm for acid functional groups. Absorbance of the NH group
ꢀ1
was appeared around 3550 cm . Peaks correspond to C=N in cyanuric ring is shown at 1500ꢀ
ꢀ1
ꢀ1
1
600 cm . Also, the carbonyl groups of TCA appeared around 1706 cm (Fig. 2a). After
polymerization of TCA with tetraamine monomer, new absorbance bond corresponded to the
ꢀ1
PBI was appearing and broad bond in the range of 2600ꢀ3500 cm was disappeared to
ꢀ1
confirm the formation of the polymer (Fig. 2b). The board band from 3450 to 3400 cm is
attributed to the isolated NꢀH stretching of the imidazole, whereas the broad band near 3250ꢀ
ꢀ1
2
900 cm is assigned to the selfꢀassociated NꢀH bond. This band is broader in the presence of
ꢀ1
moisture (ꢀOH). The C=C and C=N stretching appear in the region of 1630ꢀ1500 cm . A
ꢀ1
strong band at 1400ꢀ1390 cm must be attributed to the deformation of benzimidazole rings
ꢀ1
36,37
and inplane CꢀH deformations appear at 1235ꢀ1225 cm in PBI.
Moreover, it is clear that
the FTIR spectrum of PBI/MoS NCs shows the same characteristic signals with pure PBI.
2
All characteristic peaks related to the PBI such as NH, C=N, C=C, CꢀN, CꢀC and CꢀH were
appearing in the FTIR spectra of the NC materials. The FTIR results indicate that PBI/MoS2
NCs have been prepared successfully. "Figure 2 Here"
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The formation of PBI polymer was confirmed by the H NMR spectra as shown in
Fig. 3. In this Fig., the peak of the imidazole proton was observed at 12.74 ppm, and
secondary amine (aromatic NꢀH) was observed at 9.75 ppm as well as all of the aromatic
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protons were appeared at 7ꢀ9 ppm and confirmed the formation of the PBI.
Figure 3 Here
"
The composition of the C, H, and N in the PBI structure was analyzed by elemental
analysis, as shown in Table 1. It is known that PBI is very hygroscopic. During the sample
preparation and handling for elemental analysis, the PBI absorbed water from air. The
residual percentage due to the oxygen element in the structure is caused by absorbed water
throughout the analysis. This absorption justifies decrease in the C and N percentages and
also increase in the H percentage of the experimental results. The elemental analysis on dry
base was performed several times and the same percentages values were obtained for each
3
9
experiment. "Table 1 Here"
Thermal properties of the pure PBI and PBI/MoS were studied by means of TGA at a
2
heating rate of 20 °C/min under a nitrogen atmosphere. The PBI network exhibited
remarkably high thermal stability. TGA data for PBI and all the PBIꢀMoS
2
with different
percentage loadings of MoS
2
are shown in Fig. 4. All samples show weight loss at around
o
5
30ꢀ600 C. These weight losses are due to the degradation of the polymer backbone. Table 2
summarizes the corresponding thermoanalysis data, including the temperatures at which 5%
T ) as well as 10% (T ) degradation occurs. Char yield is at 800 ºC and also limiting oxygen
(
5
10
4
0
index (LOI) is based on Van Krevelen and Hoftyzer equation.
LOI = 17.5 + 0.4 CR
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220
Where CR = char yield. From these data, it is clear that neat PBI and its NCs are stable at
about 530 ºC, owing to the existence of heterocyclic benzimidazole ring. All samples had
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LOI values higher than 40, calculated based on their char yield at 800 °C. On the basis of
LOI values, such NCs can be classified as selfꢀextinguishing materials. The obtained TGA
data clearly indicates that the thermal stability of the PBI improved significantly after NCs
formation with MoS and the stability increased with increasing loading of MoS nanoplates
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2
in PBI matrix. Interfacial interaction between PBI and MoS has a very significant function in
2
the thermal degradation of polymeric NCs. A suitable interfacial interaction permits MoS
nanoplates to act as a thermal barrier in the PBI matrix. The shielding ability depends on the
dispersion patterns of the MoS nanoplates in the polymer matrix. This aspect is related to
2
2
both the nature and morphological features of the MoS nanoplates in the PBI matrix. Since,
2
in the current NCs, the morphological and structural feature changes with the MoS loading
2
(
as seen in Fig. 4), we obtained a different level of thermal stability. This clearly proves the
effect of morphologies in the thermal stability. In addition, as a typical layered inorganic
material, MoS is expected to disperse and exfoliate in the polymers and results in the
2
physical barrier effects, which inhibit the diffusion of heat and the decomposition products of
the polymer. Moreover, the transition metal element, Mo, promotes the formation of the
charred layer acting as a physical barrier, which could slow down heat and mass transfer
during the burning. So it is reasonable that MoS
resistance of polymerꢀbased composites just like MMT, LDHs and grapheme.
Figure 4 and Table 2 Here"
The morphological information of MoS , PBI, and PBIꢀMoS with 5 and 10% w/w of
2
may improve the thermal stability and fire
26,41
"
2
2
6
the MoS
the MoS
electroneutral MoS
2
characterized by FEꢀSEM are show in Fig. 5. According to the previous study, in
2
nanoplate, Mo atoms are covalently bonded to S atoms in two adjacent S layers; the
2
slabs are held together by van der Waals interactions. Due to the weak
interaction between the SꢀMoꢀS layers, the slabs can be easily separated from each other,
6
leading to a plateꢀlike morphology with visible edges. In this study, as shown in Figs. 5a and
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5
b, MoS
2
that obtained using Lꢀcysteine in the layered structure, show plateꢀlike morphology.
obtained through the proposed method consists of largeꢀscale sheets
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In the other word, MoS
2
that are tightly stacked together. Pure PBI shows two different types of morphology such as
semi plateꢀlike morphology and nanorod (Figs. 5, b and f). From these FEꢀSEM images (b
and f), it is clearly observed that synthesized PBI is matrix with different distribution for
morphology's elements. The obtained FEꢀSEM images, beside two different percent of PBIꢀ
MoS
2
2
(5 and 10 % w/w), are shown in Fig. 5 (cꢀh). According to these images in the PBIꢀ
hybrids, the micrograph exhibits a good dispersion of MoS into polymer matrix. It
MoS
2
seems that the particles are distributed uniformly in the polymer matrix with both of the
plateꢀlike and nanorod morphology. "Figure 5 Here"
The hybrid structure were further characterized using transmission electron
microscopy (TEM) and TEM images of MoS
2
sheets beside PBIꢀMoS
2
(5% w/w) with two
, the
different magnifications are shown in Fig. 6. As shown in Figs. 6a and 6b, for MoS
2
nanoparticles are actually in the form of irregularly sized nanopltate, and each MoS2
nanosheet is well stacked to other plateꢀlike MoS . On the other hand, in the case of PBIꢀ
2
2 2
MoS (5% w/w), a mixture of PBI and MoS nanosheets can be observed, in which the PBI
2
simply serve as surface with uniform hole and MoS nanosheets displays themselves by
discrete connection. "Figure 6 Here"
XRD analysis of the synthesized samples (PBI, MoS , and PBIꢀMoS ) is presented in
2
2
Fig. 7. Figure 7 also shows XRD patterns of commercial MoS and the synthesized MoS
2
2
2
nanoparticles. It appears that the commercial MoS has strong crystallinity and a layered
structure with an interlamellar distance of 6.15 Å (0.615 nm), as seen from the high order
diffraction peaks (004), (006) and (008) as shown in Fig. 7. This highly crystalline material is
very difficult to exfoliate and disperse in polymer matrices without special procedures
However, MoS , synthesized by the above described method is unambiguously amorphous as
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shown by the lack of crystalline or highꢀorder basal peaks in the XRD diffractograms and is
therefore presumably very dispersible in various polymer matrices. The absence of the (002)
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288
o
reflection at 6.15 Å (2θ=14.5 ) and a broad feature indicating the absence of crystalline longꢀ
range order, strongly suggests a large extent of destacking in the synthesized MoS . Fig. 7
2
also shows the XRD pattern of PBIꢀMoS NCs with 1% and 10 w/w of MoS as compared to
2
2
the MoS
this polymer is member of the crystal polymers. The XRD patterns of the PBIꢀMoS
with 1% and 10 w/w of MoS are characterized. AS shown in this figure, because of the low
amounts of MoS , the peaks of amorphous MoS could not be find. This complete
2
and pure PBI. For neat PBI, several crystalline peaks were observed, indicating that
2
NCs
2
2
2
disappearance of MoS peaks may be due to the partial exfoliated structures.
2
"Figure 7 Here"
3.2. Electrochemical measurements
Electrochemical measurements were performed in a conventional threeꢀelectrode
single cell at room temperature. A glassy carbon electrode (GCE) with a geometric surface
2
area of 0.0314 cm was used to prepare the modified working electrode. A Pt wire and
Ag/AgCl were used as the counter and reference electrodes, respectively. Cyclic
voltammograms (CVs) and liner sweep voltammetry (LSV) of the electrocatalysts were
2 4
measured in a 0.5 M H SO aqueous solution, to determine the hydrogen generation activity.
2
89
3
.3 Electrocatalytic Activity of PBI and PBI-MoS2
The catalytic activities of the PBI and PBIꢀMoS
1%, 5%, and 10% w/w) for hydrogen generation were studied by electrochemical methods.
The electrochemical activities of PBI and PBIꢀMoS , and MoS were also studied and
290
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295
2
with different percentage of MoS
2
(
2
2
compared. Generally, an optimal hydrogen generation catalyst is a material that could give
the highest current at the lowest overpotential, as well as a low hydrogen generation onset
1
2

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potential (i.e., the potential at which hydrogen generation activity begins) comparable to that
of Pt catalyst. The Tafel slope, which can be deduced from Tafel equation (η = b log (j) + a,
where η is the overpotential, j is the current density and b is the Tafel slope), is always
correlated with reaction pathway and the adsorption type.
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311
312
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315
316
317
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319
320
First of all, the electrochemical activity of PBI was studied and compared with MoS2
(
Fig. 8A). The LSV curve of PBI shows an onset potential of –40 mV, while the curve of
MoS is more positive with a higher onset potential of about –60 mV. Moreover, PBI exhibits
a current density of 10 mA cm at overpotential of –40 mV, which is much smaller than
2
ꢀ2
ꢀ2
MoS (–8 mA cm at –1.0 V). Possible reasons for different hydrogen generation activities of
2
PBI and MoS can be deduced from the morphological difference (TEM and FEꢀSEM) and
2
hydrogen generation active sites. PBI has much more active sites (such as pyridinicꢀN and
pyrrolicꢀN) than MoS
aggregates than MoS
2
and according to the morphological analysis, PBI has several less
2
, which cause the open structure for easy electron transfer. In this
regard, PBI shows more uniform distribution of nanosheets and nanorods that exposed much
more active edges, thus leading to the highest hydrogen generation catalytic activity.
In the second part, to developing new electrocatalyst for hydrogen generation, the
electrocatalytic activity of the PBIꢀMoS
0% w/w) were investigated. In this regards, liner sweep voltammograms of the
electrocatalysts were recorded in a 0.5 M H SO aqueous solution, to determine the hydrogen
2 2
with different percentage of MoS (1%, 5%, and
1
2
4
generation activity. The results of LSV studied of PBIꢀMoS (1%, 5%, and 10% w/w)
2
catalysts are shown in Fig. 8B. As can be seen in the polarization curves, the different
percentage of MoS
0% w/w PBIꢀMoS
PBI, the onset potential of hybrid catalyst shifts to ~240 mV. Especially, upon the addition of
0% w/w of MoS , the overpotential value is close to ~160 mV. Thus, it demonstrates that
2
exhibits almost favorable hydrogen generation activity, particularly for
1
2
. However, when only 10% w/w of MoS nanosheets is added to pure
2
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MoS
2
nanosheets are the core catalyst with abundant active edges for hydrogen generation,
sheets provide a conductive active sites and smaller sizes substrate affording
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345
while MoS
2
2
more active edge sites for them. Moreover, with the further addition of MoS , the current
density also is improved. "Figure 8 Here".
Undoubtedly, identifying the most active site(s) is critical to design and developed
improved catalytic materials. In this study, electrocatalytic behaviors suggest that PBI has
6
already carried an adequate load of active MoS
2
nanosheets. According to pervious study,
although bulk form of MoS
2
has a poor activity as a hydrogen generation catalyst, but
nanoparticulate MoS has high activity for this purpose. Due to achieving the excellent
2
nanoparticulate properties of MoS in PBI matrix, PBIꢀMoS hybrid catalysts improved
2
2
electronic contact between the active sites MoS
2
with PBI, thus exhibiting an enhanced their
hydrogen generation activities. To clarify, PBI matrix contribute the MoS
2
sheets and
nanoparticulates to reach their own exfoliated properties.
As an intrinsic properties of the electrocatalyst materials, the Tafel slopes, which is
associated with the rateꢀlimiting step of the hydrogen generation, have also been driven from
the Tafel plots where their linear portions are fit well with the Tafel equations. For the PBIꢀ
ꢀ1
2
MoS 10% w/w, the Tafel slope is 50.6 mV dec which is better than for pure PBI (54 mV
ꢀ1
dec ). This is due to the fact that the conductivity of pure PBI is very small and it will be
activated and increased its catalytic when the active sites and conductive sheets of MoS2
incorporated with pure PBI's active edges. This result is well consistent with the polarization
curves. When the dosage of MoS
2
nanosheets is increased to 10% w/w, as mentioned before,
ꢀ1
the Tafel slope shifts to 50.6 mV dec which is comparable to the other MoS
2
catalysts. The
hydrogen generation activities data for the proposed nanocomposites and for several recently
reported electrocatalysts are compared in Table 3. "Table 3 Here"
3.4. Stability in the long-run
1
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High durability was another important parameter for a good electrocatalyst. To assess
the hydrogen generation stability of the catalysts, longꢀterm potential cycling of PBIꢀMoS
0% w/w catalysts is better than the other synthetic electrocatalysts, when its tested in 0.5 M
H SO at room temperature by taking a potential scan at a scan rate of 20 mV/s, continuously
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347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
2
1
2
4
for 1000 cycles. After the end of the cycles, not only any slight loss in the cathodic current
was not observed for the PBIꢀMoS 10% w/w, but also the good stability of the catalysts and
rising current density as well as reducing overpotentials was observed in acidic medium. In
other words, the almost identical curve (Fig. 9) indicates high stability of PBIꢀMoS 10%
2
2
w/w in a longꢀterm electrochemical process. Hence these materials can be excellent
electrocatalyst for hydrogen generation reaction. "Figure 9 Here"
3.5. Electrochemical impedance spectroscopy analysis
To understand the electrochemical behavior of the modified electrodes for hydrogen
generation operating circumstance, electrochemical impedance spectroscopy (EIS) tests were
conducted for pure PBI and PBIꢀMoS hybrid (10% w/w). The Nyquist plots of PBI and PBIꢀ
2
MoS hybrid measured at various negative potentials within the region corresponding to the
2
S
LSV curves, as shown in Fig. 10. The best fitting was achieved using the Randles circuit (R ,
S
CPE and Rct), and according to the solution resistance (R ) in series with two parallel
ct
components, the chargeꢀtransfer resistance (R ) and a constant phase element (CPE), which
is associated with the double layer capacitance. It can be seen that the sequence of the values
of R for the different modifiedꢀGCEs are as PBIꢀMoS < PBI. "Figure 10 Here"
ct
2
4. Conclusions
In summary, we synthesized a polycarboxyl aromatic monomers, 4,4',4''ꢀ((1,3,5ꢀ
triazineꢀ2,4,6ꢀtriyl)tris(azanediyl))tribenzoic acid, which was utilized to polymerize with
,2,4,5ꢀtetraaminobenzene to achieve a rich nitrogen polybenzimidazole network. The
polycondensations could be completed in oneꢀstep within a short reaction time and at a
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5

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moderately low temperature, using a methane sulfonic acidꢀphosphorus pentoxide mixture as
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379
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381
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383
384
385
386
387
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389
390
391
392
393
394
395
1
a medium. Their chemical structures were confirmed by FTꢀIR, H NMR spectra, elemental
analysis, and TGA. TGA results showed that PBI possess excellent thermal stability. Then, a
series of organic/inorganic nanohybrid materials consisting of PBI with amorphous MoS2
nanopalates are prepared by sonochemichal method. FTꢀIR, XRD, TEM, TGA, and FEꢀSEM
experiments are carried out to characterize the morphologies and properties of the
nanohybrids. Finally, the electrochemical behavior of the synthesized PBI and MoS
2
hybrids
were evaluated for electrochemical hydrogen generation. By further MoS in PBI matrix, the
2
catalysts improved electronic contact between the active sites MoS with PBI, thus exhibiting
2
an enhanced their hydrogen generation activities. In this regards, the electrochemical studies
showed that the obtained nanoelectrocatalys exhibited excellent hydrogen generation
activities with an onset potential ꢀ160 mV vs. RHE. Large current densities, small Tafel
slopes as well as prominent electrochemical durabilities are the main characteristic of these
compounds. As the results, the newly proposed protocol opens a potential avenue for the
development of highꢀperformance Ptꢀfree hydrogen generation catalysts.
Acknowledgements
We gratefully acknowledge the partial financial support from the Research Affairs
division Isfahan University of Technology (IUT), Isfahan.
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Table 1: The elemental analysis of the PBI
468
Elements
Theoretical
Experimental
48.90
Theoretical+16H O
2
C
N
H
O
65.6
30.6
3.6
0
48.64
22.69
5.62
21.10
5.30
24.7
23.04
4
4
69
70
2
Table 2: Thermal properties of the PBI and different PBI/MoS hybrids.
Decomposition
Char
yield
o
c
Samples
temperature ( C)
LOI
a
a
b
T
5
T
10
(%)
PBI
541
566
574
581
556
576
587
600
71
46
NC1%
NC5%
NC10%
73
75
77
46.7
47.5
48.0
a
Temperature at which 5% and 10% weight loss was recorded by TGA
o
ꢀ1
2
at heating rate of 20 C min in an N atmosphere.
b
Weight percent of the material left undecomposed after TGA at
o
maximum temperature 800 C in an N
2
atmosphere.
c
o
Limiting oxygen index (LOI) evaluated at char yield at 800 C.
4
4
4
4
4
4
4
71
72
73
74
75
76
77
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0

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78
Table 3: Collected hydrogen activity data.
Catalyst
Overpotential Exchange current density
479
Tafel slope
Reference
(
mV versus
ꢀ2
ꢀ1
(
mA cm )
(mV decade )
RHE)
MoS
2
/SnO
2
ꢀꢀ187
>ꢀ500
ꢀ120
10
10
20
10
10
10
43
70
42
43
Exfoliated MoS
2
FeS /CNT
46
44
2
MoSe
PBI
2
/rGO
ꢀ115
69
45
ꢀ400
54
This work
This work
PBIꢀMoS
2
~ꢀ160
50.6
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
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4
95
Legends for the Figures:
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
Fig. 1 Synthesis route to the TCA monomer and PBI networks.
Fig. 2 FTꢀIR of (a): 4,4',4''ꢀ((1,3,5ꢀtriazineꢀ2,4,6ꢀtriyl)tris(azanediyl))tribenzoic acid (TCA),
(
b): neat PBI, (c): PBIꢀMoS 1%w/w, (d): PBIꢀMoS 5%w/w, (e): PBIꢀMoS 10%w/w and
2 2 2
(f): pure MoS2.
1
Fig. 3 H NMR spectra of PBI. Numbers (1ꢀ26) indicate peaks arising from spinning side
bands.
Fig. 4 TGA curves of PBI, PBIꢀMoS 1%w/w, PBIꢀMoS 5%w/w and PBIꢀMoS 10%w/w.
2
2
2
Fig. 5 FEꢀSEM images of (a and e): MoS , (b and f): PBI,(c and g): PBIꢀMoS 5%w/w, (d
2
2
and h): PBIꢀMoS
2
10%w/w.
Fig. 6 TEM images of (a and b): MoS
2
sheets and (c and d): PBIꢀMoS
2
(5% w/w).
2 2 2
Fig.7 XRD pattern of: Commercial MoS , MoS Synthesized, PBI, PBIꢀMoS 1% w/w, and
PBIꢀMoS 10% w/w.
2
ꢀ
1
Fig. 8 Linear sweep voltammograms for hydrogen evaluation reaction in 0.5 mol L H SO
2
4
at (A): (a): PBI, (b): amorphous MoS
evaluation reaction in 0.5 mol L–1 H SO
and w/w, (c): PBIꢀMoS 1% w/w.
Fig. 9 Linear sweep voltammograms for longꢀterm electrochemical stability test of PBIꢀMoS2
2
, (B): Linear sweep voltammograms for hydrogen
2
4
at (a): PBIꢀMoS 10% w/w, (b): PBIꢀMoS 5%
2
2
,
2
ꢀ1
1
0%w/w in 0.5 mol L H SO at (A): 1th, 200th, 400th, 600th, 800th, 1000th cycles, with a
2 4
–
1
scan rate of 20 mV s .
Fig. 10 Nyquist plots (from electrochemical impedance spectroscopy data) at (A): PBI
–
1
modified GCE in different potential vs. Ag/AgCl. Conditions: electrolyte, 0.5 mol L H
2 4
SO .
B): Nyquist plots of PBIꢀMoS (10%w/w) modified GCE in different potential vs. Ag/AgCl.
2
ꢀ1
Conditions: electrolyte, 0.5 mol L H SO .
2
4
2
2

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5
20
5
21
Fig. 1
522
5
5
5
5
5
5
5
5
5
23
24
25
26
27
28
29
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31
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3

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5
32
5
5
33
34
Fig. 2
535
2
4

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5
36
537
538
Fig. 3
5
39
Fig. 4
540
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5
41
5
543
544
42
Fig. 5
5
45
Fig. 6
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DOI: 10.1039/C5RA20447J
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DOI: 10.1039/C5RA20447J
Graphical Abstract with text
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High active nitrogen site polybenzimidazole network was synthesized through polycondensation and it was modified
with amorphous MoS for hydrogen generation.
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