A bifunctional catalyst for efficient dehydrogenation and electro-oxidation of hydrazine

Article abstract of DOI:10.1039/c8ta06219f

The chemical energy stored in energetic materials may often be utilized in various ways, which motivates the development of multifunctional catalysts for flexible and efficient utilization of the chemical energy. Hydrazine is a promising energy carrier due to its high energy density and high hydrogen content, which can be utilized as a chemical hydrogen storage medium or a fuel for direct fuel cells. Herein, we propose a bifunctional catalyst for efficient dehydrogenation and electro-oxidation of hydrazine. As a proof-of-concept study, a carbon-black-supported Pt_0.2Ni_0.8 nanoparticle catalyst has been developed with high activity and durability for both complete dehydrogenation (with a turnover frequency of 673 h^-1 and a H₂ generation rate of 188 L h^-1 g_metal^-1) and electro-oxidation (with a mass activity of 132 mA mg_metal^-1) of hydrazine under mild conditions, outperforming other catalysts including Pt, Ni, Pd_0.2Ni_0.8, and Au_0.2Ni_0.8 nanoparticles. Such a bifunctional catalyst can enable the utilization of hydrazine as a promising energy carrier for both on-demand hydrogen generation and electricity generation via direct hydrazine fuel cells, enhancing its flexibility for future onboard applications.

Full text of DOI:10.1039/c8ta06219f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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A Bifunctional Catalyst for Efficient Dehydrogenation and Electro-
oxidation of Hydrazine
Jun Wang,a,b Asim Khaniya,a Lin Hu,c Melanie J. Beazley,d William E. Kadena,b and Xiaofeng
Feng*a,b,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The chemical energy stored in energetic materials may often be utilized in various ways, which motivates the development
of multifunctional catalysts for flexible and efficient utilization of the chemical energy. Hydrazine is a promising energy
carrier due to its high energy density and high hydrogen content, which can be utilized as a chemical hydrogen storage
medium or a fuel for direct fuel cells. Herein, we propose a bifunctional catalyst for efficient dehydrogenation and electro-
oxidation of hydrazine. As a proof-of-concept study, a carbon-black-supported Pt0.2Ni0.8 nanoparticle catalyst has been
developed with high activity and durability for both complete dehydrogenation (with a turnover frequency of 673 h−1 and a
H2 generation rate of 188 L h−1 gmetal−1) and electro-oxidation (with a mass activity of 132 mA mgmetal−1) of hydrazine under
mild conditions, outperforming other catalysts including Pt, Ni, Pd0.2Ni0.8, and Au0.2Ni0.8 nanoparticles. Such a bifunctional
catalyst can enable the utilization of hydrazine as a promising energy carrier for both on-demand hydrogen generation and
electricity generation via direct hydrazine fuel cells, enhancing its flexibility for future onboard applications.
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bonds and released on demand, which seems promising for
future hydrogen implementation.9,10 Hydrazine monohydrate
(N2H4·H2O) is considered as one of those candidates because
Introduction
In response to the limited availability of fossil fuels and the
calls for energy conservation and emission reduction, many
governments have adopted policies and incentives to push
forward the transition from conventional internal combustion
engine vehicles to new energy vehicles in the market.1,2
Among them, hydrogen fuel cell vehicles are considered as a
promising candidate because hydrogen fuel has the
advantages of vast source, various production ways, high
energy density and efficiency, as well as zero carbon
emissions.3−5 Nevertheless, because of its low volumetric
density, hydrogen gas produced at central facilities must be
pressurized or liquefied before being dispensed to the point of
end-use, which is neither cost- nor energy-efficient and thus
of its effective hydrogen content as high as 8.0 wt%, which
meets the U.S. Department of Energy’s targets for onboard
hydrogen storage.11 N2H4·H2O is also a stable liquid with a
density similar to that of water under ambient temperature
and pressure, making it compatible with existing liquid fuel
infrastructures. Moreover, only nitrogen is produced in
addition to hydrogen if it decomposes via
dehydrogenation pathway:12−18
a
complete
N2H4(l)
→
2H2(g)
+
N2(g)
(1)
Nevertheless,
a competing decomposition pathway to
ammonia may also occur:19−24
3N2H4(l)
(2)
→
4NH3(g)
+
N2(g)
hinders its deployment at large scale.6−8 Therefore,
a
successful implementation of hydrogen as an efficient energy
carrier relies on the development of viable and effective
hydrogen storage and delivery technologies.
which will not only reduce the hydrogen yield but also require
a post-separation process to mitigate possible performance
deterioration of fuel cells caused by ammonia. Therefore, the
challenge lies in the development of highly selective catalysts
for complete dehydrogenation of hydrazine with high
hydrogen generation rate under mild conditions.
In particular, by means of a few low molecular weight
chemicals, hydrogen could be stored in the form of chemical
In addition, hydrazine itself is a high-energy fuel with an
energy density of 5.40 kWh L−1,25 which is about twice of that
of liquid hydrogen (2.54 kWh L−1). A direct hydrazine fuel cell
has a theoretical voltage of 1.56 V,26,27 also higher than that of
a hydrogen fuel cell (1.23 V). Similarly, the anodic oxidation of
hydrazine only gives rise to environmentally benign products,
nitrogen and water:28
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N2H4(l)
(3)
+
4OH−
→
N2(g)
+
4H2O(l)
+
4e− temperature for 1 h, and then cooled down to room
temperature. Subsequently, freshly prepared aqueous
a
These advantages have stimulated substantial interests to solution (30 mL) containing NaBH4 (150 mg) was added into
develop efficient electrocatalysts for the hydrazine oxidation the mixture dropwise under vigorous stirring. After 1 h, the
reaction (HzOR).29−31 To date, some noble metal catalysts, catalyst slurry was filtered and washed with water and then
particularly Pt, have showed high electrocatalytic activities for dried in vacuum at room temperature, producing Pt0.2Ni0.8
HzOR, but their applications in fuel cells may be limited by nanoparticles dispersed on carbon black support (Pt0.2Ni0.8/C).
their high cost and scarcity.32−35 Instead, many efforts turn to Control samples of Pt/C, Ni/C, Pd0.2Ni0.8/C, and Au0.2Ni0.8/C
developing low-cost transition metal or metal-free catalysts, nanocatalysts were prepared using the same procedure with
whereas most of them exhibit compromised electrocatalytic their corresponding metal precursors (1 mmol H2PtCl6, 1 mmol
activities.36−40 Alternatively, it is reasonable to propose a NiCl2·6H2O, 0.2 mmol PdCl2 + 0.8 mmol NiCl2·6H2O, 0.2 mmol
bimetallic catalyst with a low content of noble metal to obtain HAuCl4·3H2O + 0.8 mmol NiCl2·6H2O).
a
commensurate catalytic performance, mitigating the
divergence between the cost and efficacy.
Here, we demonstrate a carbon-black-supported Pt0.2Ni0.8 using
For comparison, Pt0.2Ni0.8 nanoparticle catalyst with larger
particle sizes (hereafter defined as Pt0.2Ni0.8-L) was prepared
a
conventional wet impregnation/NaBH4 reduction
nanoparticle (NP) catalyst with a homogeneous dispersion and method with the same metal precursors but no urea. Briefly,
small particle sizes. Remarkably, the Pt0.2Ni0.8 catalyst of 200 mg of carbon black was dispersed in 200 mL of aqueous
around 3 nm in size exhibits superior catalytic performance for solution containing 0.2 mmol H2PtCl6 and 0.8 mmol NiCl2·6H2O.
both complete dehydrogenation and electro-oxidation of After sonication and stirring for 1 h, a freshly prepared
N2H4·H2O under mild conditions. More specifically, the aqueous solution (30 mL) containing NaBH4 (150 mg) was
dehydrogenation of N2H4·H2O over the Pt0.2Ni0.8 catalyst added into the mixture dropwise under vigorous stirring. After
achieves a turnover frequency (TOF) of 673 h−1 and a H2 1 h, the Pt0.2Ni0.8-L/C nanocatalyst was collected by filtering,
generation rate of 188 L h−1 gmetal−1 in an alkaline medium at 50 washing and vacuum drying at room temperature.
oC, benefiting from a low apparent activation energy of 45.7 kJ
mol−1. When used for HzOR in 0.1 M phosphate buffer solution
Physical characterizations
(PBS) containing 0.5 M N2H4·H2O, the Pt0.2Ni0.8 catalyst Transmission electron microscopy (TEM) images were acquired
achieves a current density of 44 mA cm−2 at 0.5 V vs. the using a FEI Tecnai F30 TEM with a field emission gun operated
reversible hydrogen electrode (RHE), corresponding to a mass at 200 kV. All samples for TEM characterizations were
−1
activity of 132 mA mgmetal and a TOF of 2.9
×
10−2 s−1. As a prepared by dispersing the particles in ethanol with sonication
proof of concept, the bifunctionality of the Pt0.2Ni0.8 for 30 min to obtain a dilute homogenous dispersion, and then
nanocatalyst may enable the use of hydrazine as a versatile drop-casting and drying the solution on a TEM grid. X-ray
energy carrier for on-demand H2 generation and direct diffraction (XRD) patterns were collected by a PANalytical
hydrazine fuel cells in a more flexible manner (Fig. 1).
Empyrean diffractometer with a 1.8 kW copper X-ray tube
using powder diffraction mode. X-ray photoelectron
spectroscopy (XPS) data were obtained using a SPECS Electron
Spectrometer with a PHOIBOS 100 Hemispherical Energy
Analyzer and XR 50 Al Kα X-ray source (1486.67 eV). XPS
binding energies were calibrated using both the Fermi Edge
and the known peak positions of metallic Ag(111) and Au(111).
Argon ion sputtering (2 kV, Ar pressure = 2 × 10−6 Torr, 10 min)
was used to remove surface oxide formed due to air exposure
for specified samples.
Inductively coupled plasma mass spectrometry
The metal contents in the samples were accurately quantified
by inductively coupled plasma mass spectrometry (ICP-MS).
The metal nanoparticles were first dissolved in aqua regia, and
then the dissolved Pt, Ni, Au, and Pd elements were measured
using a Thermo Fisher Scientific iCap Qc inductively coupled
plasma mass spectrometer with QCell technology and
operated in kinetic energy discrimination (KED) mode of
analysis with helium as the collision gas. Calibration, internal,
and quality control standards (Inorganic Ventures) were
prepared with 2% trace metal grade nitric acid (Fisher
Scientific). Cysteine (0.5% w/v) was added to the standards
and rinse to eliminate adsorption and memory effects.
Fig. 1. Schematic illustration of the carbon-black-supported Pt0.2Ni0.8 nanoparticles as a
bifunctional catalyst for hydrazine dehydrogenation and electro-oxidation.
Experimental section
Synthesis of carbon-black-supported Pt0.2Ni0.8 nanocatalysts
Carbon-black-supported Pt0.2Ni0.8 nanoparticles were prepared
using
a
urea-assisted precipitation-reduction method.
Typically, 200 mg of carbon black was dispersed in 200 mL of
aqueous solution containing 0.2 mmol H2PtCl6, 0.8 mmol
NiCl2·6H2O, and 10 mmol urea. After sonication and stirring for
o
1 h, the resulting mixture was heated to 95 C and kept at the
2 | J. Name., 2012, 00, 1-3
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Holmium and bismuth were used as internal references in both electrolyte was 100 mL of PBS containing N2H4 with different
standards and samples.
concentrations. Linear sweep voltammetry (LSV) scans ware
performed at a scan rate of 10 mV s−1. The applied potentials
were iR-compensated and the presented current densities
Catalytic dehydrogenation of N2H4
The catalytic dehydrogenation of N2H4 was carried out in a were normalized to geometric surface areas. The measured
two-necked round-bottom flask which was connected to a gas potentials vs. SCE were converted to the RHE scale based on
burette filled with water. 30 mg of the carbon-black-supported the Nernst equation (ERHE = ESCE + 0.0591 × pH + 0.241 V).
metal catalysts were dispersed in a flask with 5 mL of NaOH
Calculations
solution (1 M) under stirring. A water bath was used to control
the reaction temperature (30−70 oC). The catalytic reaction The selectivity toward the catalytic dehydrogenation of N2H4
was initiated by injecting 0.1 mL of N2H4·H2O into the flask by a was assumed to be x. Therefore, the whole catalytic reaction
syringe. The volume of the generated gas was recorded by the can be formulated as:
displacement of water in the gas burette under atmospheric 3 N2H4 → (1+2x) N2 + 6x H2 + 4(1–x) NH3
pressure.
The generated gas was analyzed using
Thus we can have:
3λ − 1
8
in which λ = n(H2 + N2)/n(N2H4) can be experimentally
a
gas
ꢀ =
chromatograph (SRI Multiple Gas Analyzer #5) with Ar as the
carrier gas. In addition, the generated gas was passed through
an acid trap containing 0.05 M H2SO4 to capture the possibly
produced ammonia. After the reaction, the possible ammonia
was determined using the indophenol blue method.41
Typically, 2 mL of the acid solution was first pipetted from the
trap. Then, 2 mL of a 1 M NaOH solution containing 5 wt %
salicylic acid and 5 wt % sodium citrate was added, followed by
addition of 1 mL of NaClO solution (0.05 M) and 0.2 mL of
sodium nitroferricyanide solution (1 wt %). After 2 h, UV-vis
absorption spectra were acquired using a Thermo Scientific
Evolution 201 UV-Vis Spectrophotometer. The formed
indophenol blue color was determined by the absorbance at λ
= 653 nm.
determined.
The turnover frequency (TOF) for H2 generation is
calculated based on the total molar number of metal atoms in
the catalyst:
2
ꢁ ꢂꢃꢄꢅ
ꢆ
ꢇꢃꢈ/ꢉꢊ
3
TOF =
ꢋ
ꢅꢌꢄꢃꢍ ∙ ꢎ
where Patm = 101.325 kPa; Vgas is the total volume of the
generated gas at the time of the half-completion of N2H4
dehydrogenation; R = 8.314 J mol−1 K−1; T = 298 K; nmetal is the
total molar number of metal atoms in the catalyst; t is the time
of the half-completion of N2H4 dehydrogenation.
The H2 generation rate (RH2) is calculated based on the
total mass of metal in the catalyst:
The complete decomposition of N2H4 was also verified by
ꢆꢏ
ꢐ
quantifying remaining N2H4 in the solution using
spectrophotometric method developed by Watt and Chrisp.42
First, color reagent was prepared by mixing 4-
a
ꢉꢏ
=
ꢐ
ꢑꢅꢌꢄꢃꢍ ∙ ꢎ
where VH2 is volume of the evolved H2 at the time of the half-
completion of N2H4 dehydrogenation; mmetal is the total mass
of metal in the catalyst; t is the time of the half-completion of
the N2H4 dehydrogenation.
a
dimethylaminobenzaldehyde (4 g), concentrated HCl (20 mL),
and ethanol (200 mL) together. Second, the solid catalysts
were separated from the suspension after catalytic reaction,
and 1 mL of the supernatant was diluted to 10 mL with water.
Then, 5 mL of the diluted solution was mixed with 5 mL of the
color reagent. After 30 min, the formation of a yellow resultant
of 4-dimethylaminobenzaldehyde was measured by the
absorbance at λ = 458 nm to quantify possible N2H4 in the
solution.
For the electrochemical oxidation of N2H4, the TOF of a
catalyst is defined as the conversion number of N2H4 per metal
atom per s:
ꢒ
TOF =
4ꢓ ∙ ꢋꢅꢌꢄꢃꢍ
where j is the measured current density; F = 96485 C mol−1;
nmetal is the total molar number of metal atoms in the catalyst.
Electrocatalytic oxidation of N2H4
The working electrodes were prepared as follows. First, 8 mg
of carbon-black-supported metal catalyst was dispersed in
diluted Nafion alcohol solution containing 1.5 mL ethanol and
60 μL Nafion, which was then sonicated for 1 h to form a
homogeneous suspension. Afterwards, the suspension was
drop-casted onto a piece of carbon paper (1 × 1 cm2) until a
total mass loading of 1 mg was reached. After drying at room
temperature, the electrode can be used for electrochemical
HzOR. Electrochemical measurements were performed using a
CH Instruments 760E Potentiostat with a three-electrode
system. A piece of Pt gauze and a saturated calomel electrode
(SCE, filled with saturated KCl) were used as the counter
electrode and reference electrode, respectively. The
Results and discussion
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the Pt0.2Ni0.8 relative to the Pt0.2Ni0.8-L sample may help explain
the formation of smaller NPs, as such sites may promote
homogeneous adsorption and anchoring of metal precipitates
during urea-assisted precipitation-reduction process. The XPS
spectra show metallic Pt0 in both samples (Fig. S3), and Ni2+ on
the sample surfaces, with the latter most likely being
attributable to surface-bound Ni reactions with air during
transfer into the vacuum XPS system. In accordance with this
rationalization, Ni2+ signal attenuates and Ni0 begins to appear
as the surface layer of the particles is slowly removed during
mild Ar+ sputtering (Fig. S3). While the data are consistent with
our assumption that a nickel-oxide layer forms at the surface
of the metallic particles following exposure to air, it is
important to note that a similar effect might also be expected
even if the particles were completely oxidized due to the
increased sputtering efficiency of O relative to Ni under our
conditions. However, if this were the case, we would expect to
see stronger evidence of NiO-like structures in our XRD data.
Additional samples have been prepared for comparative
studies, including Pt, Ni, Pd0.2Ni0.8, and Au0.2Ni0.8 nanoparticles,
which were synthesized by the urea-assisted precipitation-
reduction method as well. Representative TEM images of the
samples are provided in Fig. S4. The molar loadings of metal
Fig. 2. Characterization of the prepared NPs. TEM images of the carbon-black-
supported (a, b) Pt0.2Ni0.8 NPs and (c) Pt0.2Ni0.8-L NPs. Inset (a and c): corresponding
particle size distributions; inset (b): high-resolution TEM image. (d) XRD patterns of the precursors were kept the same for all samples (1 mmol metals
carbon-black-supported Pt0.2Ni0.8 NPs and Pt0.2Ni0.8-L NPs.
in total over 200 mg carbon black) to facilitate comparisons,
and the actual metal contents by mass were determined by
The morphology of the obtained NPs was characterized by
ICP-MS and summarized in Table S1.
transmission electron microscopy (TEM). As shown in Fig. 2a,
The catalytic activities of the samples for N2H4·H2O
the Pt0.2Ni0.8 NPs are well-dispersed on carbon support without
dehydrogenation were evaluated in an alkaline medium at a
obvious agglomeration. The NPs are mainly in the size range of
mild temperature (50 oC). Of note, an alkaline medium is
2−5 nm with an average value of around 2.7 nm, as further
favorable for the dehydrogenation of N2H4·H2O because it can:
demonstrated by the magnified TEM image in Fig. 2b. In
contrast, the Pt0.2Ni0.8-L NPs prepared with an identical metal
content have a broader size range of 3−10 nm and a mean size
of around 6 nm (Fig. 2c and S1), leading to a lower surface area
per unit mass and thus fewer active sites. A high-resolution
TEM image in Fig. 2b inset reveals the crystalline nature of
Pt0.2Ni0.8 NPs, and the lattice plane spacing is determined to be
0.21 nm, which is smaller than that of the (111) plane of face-
centered cubic (fcc) Pt (0.23 nm), indicating the incorporation
of Ni into Pt lattice to form an alloyed structure. X-ray
diffraction (XRD) patterns of the Pt0.2Ni0.8 and Pt0.2Ni0.8-L
samples are shown in Fig. 2d, both showing four main
diffraction peaks of (111), (200), (220), and (311) planes of the
fcc phase. These peaks are located between the diffraction
peaks of fcc Pt and Ni, implying the formation of PtNi alloy, in
agreement with the high-resolution TEM image. In addition,
compared to Pt0.2Ni0.8-L, the Pt0.2Ni0.8 sample shows broader
diffraction peaks, further confirming its smaller particle sizes.
X-ray photoelectron spectroscopy (XPS) was used to
interrogate the chemical composition of the samples. As
shown in Fig. S2, the carbon support of the Pt0.2Ni0.8 sample
exhibits a greater abundance of carbon atoms in an oxidation
state consistent with C-O-C bonding, which are readily
removed following mild Ar+ sputtering (relative to the C-C peak
intensity). Tentatively assigning them to O-containing
carbonaceous species, the higher intensity of this feature on
+
(a) enable the conversion of N2H5 to N2H4 in an aqueous
solution; (b) promote the reaction kinetics by accelerate the
first deprotonation step; (c) increase the H2 selectivity by
restraining the formation of basic NH3 in view of the chemical
equilibrium.15,19,23 Fig. 3a shows the catalytic activities of the
five samples prepared using the same precipitation-reduction
method. The same mass loading was used for each sample (30
mg of the metal/carbon mixture), which was dispersed into 5
mL of NaOH solution (1 M) under stirring, with 0.1 mL of
N2H4·H2O injected later to initiate the reaction. Among them,
the Pt0.2Ni0.8 catalyst exhibits the highest catalytic activity and
H2 selectivity, which accomplishes a complete release of 3.0
equivalents of gas in only 6.4 min, indicating a H2 selectivity of
100%. In comparison, the Pd0.2Ni0.8 and Au0.2Ni0.8 NP catalysts
can only generate 2.5 and 2.0 equivalents of gas in 48 and 160
min, corresponding to a H2 selectivity of 82% and 63%,
respectively. For the monometallic Pt and Ni nanocatalysts,
only a small amount of gas production is detected even after
an extended reaction time of 180 min. These observations
suggest that the dehydrogenation of N2H4·H2O strongly
depends on the chemical nature of the catalysts. In addition,
the Pt0.2Ni0.8-L catalyst also achieves
a
complete
dehydrogenation, but uses a longer time of 11 min (Fig. 3b),
indicating a lower activity than the Pt0.2Ni0.8 catalyst due to the
larger particle sizes.
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Furthermore, a series of PtxNi(1-x) catalysts (0
ARTICLE
≤ x ≤ 1) were
examined to reveal the effect of the Pt/Ni ratio on the catalytic
activity. As shown in Fig. S5, a complete dehydrogenation is
achieved with x (Pt ratio) between 0.2 and 0.6, accompanied
by a slightly improved catalytic activity at a higher Pt ratio.
Further increase of the Pt ratio in the catalyst results in a
deterioration of the activity and the H2 selectivity. Thus, the
Pt0.2Ni0.8 catalyst was selected as the focus in this study due to
its low Pt content but a relatively high activity, which may
mitigate the divergence between the cost and efficacy. The
activity of the Pt0.2Ni0.8 catalyst is temperature-dependent (Fig.
S6). For example, the reaction time for
a complete
dehydrogenation of N2H4·H2O over the Pt0.2Ni0.8 catalyst
decreases from 28.3 to 2.0 min as the temperature increases
o
from 30 to 70 C, while the H2 selectivity remains unaffected.
We mainly compare the results obtained at 50 oC as a
relatively mild temperature for the reaction.
The TOF for H2 generation and the H2 generation rate can
be calculated using the total metal amount determined by ICP-
MS. As shown in Fig. 3c, a TOF of ~673 h−1 and a H2 generation
rate of ~188 L h−1 gmetal−1 are achieved at 50 oC for the Pt0.2Ni0.8
catalyst, which are much higher than those for the Pt0.2Ni0.8-L
catalyst (~354 h−1 and ~98 L h−1 gmetal−1). As a result, the
Pt0.2Ni0.8 catalyst is capable of providing H2 output with a
power density of 254 W h−1 gmetal for energy generation. An
−1
Fig. 3. Dehydrogenation of hydrazine over the catalysts. (a) Time-course plots for the
catalytic decomposition of N2H4·H2O on different catalysts in 1 M NaOH solution at 50
oC. (b) Comparison of the N2H4·H2O decomposition activities on the Pt0.2Ni0.8 and
Pt0.2Ni0.8-L catalysts in 1 M NaOH solution at 50 oC. The same mass loading (30 mg) was
used for each catalyst. (c) Calculated TOF values for H2 generation and H2 generation
rates over the Pt0.2Ni0.8 and Pt0.2Ni0.8-L catalysts at different temperatures, and (d)
corresponding Arrhenius plots (ln(TOF) vs. 1/T) for the two samples. (e) Gas
chromatogram of the generated gas from N2H4·H2O decomposition over the Pt0.2Ni0.8
catalyst. (f) UV-vis spectra for the detection of possible NH3 in the generated gas (upper
frame) and residual N2H4 in the solution (lower frame) using spectrophotometric
method after N2H4·H2O decomposition over the Pt0.2Ni0.8 catalyst. Error bars were
derived from the standard deviations of measurements over separately prepared
samples under the same conditions.
Arrhenius plot of ln(TOF) vs. 1/T is used to determine the
activation energy of the catalysts for N2H4·H2O
dehydrogenation. As shown in Fig. 3d, the Pt0.2Ni0.8 catalyst
yields an apparent activation energy of 45.7 kJ mol−1, which is
lower than that of the Pt0.2Ni0.8-L catalyst (53.2 kJ mol−1).
Interestingly, in spite of the same elemental composition, the
two catalysts show different activation energies due to their
different particle sizes, most probably because smaller
nanoparticles provide more low-coordinated surface sites for
the decomposition of N2H4.43 Note that the performance of the
Pt0.2Ni0.8 catalyst may rival many of those reported catalysts
for N2H4·H2O dehydrogenation under similar conditions (Table
S2).
Since the production of ammonia-free H2 is crucial for fuel
cell applications, the gas generated over the Pt0.2Ni0.8 catalyst
was delivered into a gas chromatograph (GC) for compositional
analysis. In the GC spectrum (Fig. 3e), H2 and N2 with a molar
ratio of 2 was determined, consistent with the stoichiometric
ratio for the complete dehydrogenation of N2H4 (Fig. 3a). To
have another verification, the generated gas was passed
through an acid trap (0.05 M H2SO4) to absorb the possibly
produced ammonia, if any. After the reaction, the possible
ammonia in the acid trap was determined by the indophenol
blue method using UV-vis absorption spectroscopy.41 As shown
in Fig. 3f, in the spectrum region where the indophenol blue
shows a characteristic absorbance at
λ = 653 nm, the
measured spectrum of the acid trap after reaction is the same
as that of the acid background, indicating no ammonia
production from the reaction. Furthermore, the complete
depletion of the N2H4·H2O over the Pt0.2Ni0.8 catalyst after the
reaction was also verified by a spectrophotometric method.42
Compared with the background and the standard solution, no
residual N2H4·H2O was detected in the post-reaction solution,
which is also in agreement with the measured volume of the
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generated gas. All the above results have confirmed
Journal Name
a
complete decomposition of hydrazine to H2 and N2 on the
Pt0.2Ni0.8 catalyst with a high efficiency. Furthermore, the
durability of the Pt0.2Ni0.8 catalyst for the dehydrogenation
reaction was evaluated by cycling experiments, and only a
slight decrease in activity was observed after five cycles (Fig.
S7), indicating a high durability of the Pt0.2Ni0.8 catalyst for the
dehydrogenation of hydrazine.
The electrocatalytic HzOR activity was evaluated at room
temperature using a typical three-electrode cell with carbon-
paper-supported metal catalysts as the working electrode, a
piece of Pt gauze as the counter electrode, and a saturated
calomel electrode as the reference electrode. The electrolyte
used in this study is 0.1 M PBS in light of the high activity of
the catalysts for N2H4 decomposition in an alkaline medium,
which is a spontaneous non-faradaic process and can cause a
decrease of the fuel efficiency.28 All the potentials were iR-
compensated and reported based on the RHE scale. The pH
value of the electrolyte changes slightly with different N2H4
concentrations, as shown in Table S3, which was included in
our calculation of potentials. Fig. 4a shows LSV curves of the
Pt0.2Ni0.8 catalyst for the HzOR with various N2H4
concentrations in 0.1 M PBS. No apparent anodic current is
measured in a PBS electrolyte without N2H4, indicating a
negligible background current. When 0.1 M N2H4 is present in
the PBS electrolyte, a sharp rise of the current density with the
Fig. 4. Electro-oxidation of hydrazine over the catalysts. (a) LSV curves of the Pt0.2Ni0.8
catalyst for HzOR with various N2H4 concentrations in 0.1 M PBS at a scan rate of 10 mV
potential is observed, starting at ~0 V, which is attributed to s−1. (b) LSV curves of different catalysts for HzOR in 0.1 M PBS containing 0.5 M N2H4 at
a scan rate of 10 mV s−1. (c) TOFs and mass activities of different catalysts for HzOR
the HzOR. The current density increases when the N2H4
measured at 0.5 V vs. RHE. (d) LSV curves of the Pt0.2Ni0.8 catalyst for HzOR in 0.1 M PBS
concentration increases from 0.1 to 0.5 M. Fig. 4b compares
containing 0.5 M N2H4 at different scan rates (inset: current density against scan rate
the electrocatalytic HzOR activities of the catalysts in PBS
plot at 0.5 V vs. RHE). (e) Multi-potential step measurement of the Pt0.2Ni0.8 catalyst for
electrolyte containing 0.5 M N2H4. Both Au0.2Ni0.8 and Ni
HzOR in 0.1 M PBS containing 0.5 M N2H4. (f) Chronoamperometric curves of the
catalysts show poor activities for the HzOR in the potential Pt0.2Ni0.8 and Pt0.2Ni0.8-L catalysts for HzOR at 0.5 V vs. RHE in 0.1 M PBS containing 0.5
M N2H4. Error bars were derived from the standard deviations of measurements over
range. The monometallic Pt catalyst exhibits the highest
separately prepared samples under the same conditions.
current density at potentials < 0.46 V, beyond which the
Pt0.2Ni0.8 catalyst shows a greater current density. At 0.5 V vs.
Additional experiments were performed to reveal the
RHE, the current densities of the catalysts follow such an
impact of the scan rate on the current density of the Pt0.2Ni0.8
order: Pt0.2Ni0.8 (44 mA cm−2) > Pt (42 mA cm−2) > Pd0.2Ni0.8 (36
catalyst. As shown in Fig. 4d, only a slight increase in the
mA cm−2) > Pt0.2Ni0.8-L (35 mA cm−2). The intrinsic activity of
current density is observed with the scan rate increased from
the catalysts is further compared in terms of TOF and mass
10 to 100 mV s−1, indicating an efficient mass/charge transfer
activity at 0.5 V based on their total metal contents. As shown
capability of the Pt0.2Ni0.8 catalyst for the HzOR. A multi-
in Fig. 4c, the TOF of the Pt catalyst reaches ~4.5
×
10−2 s−1,
10−2
potential step measurement was also performed with the
Pt0.2Ni0.8 catalyst to evaluate its response to potential change.
Fig. 4e shows the current density with the potential changing
from 0.1 to 0.5 V with steps of 0.1 V, which shows a quick
response without any obvious hysteresis and maintains a
stable current at each step. The HzOR activity of the Pt0.2Ni0.8
catalyst is also reversible with the current density fully
recovered in the back-and-forth measurement. These results
indicate that the Pt0.2Ni0.8 catalyst can provide varied
instantaneous power densities for direct hydrazine fuel cell,
which may be crucial for future onboard applications. The
long-term durability of the catalyst was evaluated by constant
potential electrolysis at 0.5 V vs. RHE (Fig. 4f). After a
continuous electrolysis for 10 h, the Pt0.2Ni0.8 catalyst retains
89% of the initial current density, which is superior to that of
which is higher than that of the Pt0.2Ni0.8 catalyst (~2.9
s−1), but the latter is more cost-effective due to its low Pt
×
content (~20%). On the other hand, the Pt0.2Ni0.8 catalyst
−1
exerts the highest mass activity of ~132 mA mgmetal among
these catalysts, whereas the Pt catalyst shows a much lower
one of ~88 mA mgmetal−1, due to their different mass loadings
of metal contents (Table S1).
6 | J. Name., 2012, 00, 1-3
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ARTICLE
the Pt0.2Ni0.8-L catalyst (71%) and indicates a high durability of
the Pt0.2Ni0.8 catalyst for N2H4 electro-oxidation.
Acknowledgements
This work is supported by a startup fund from the University of
Central Florida (UCF). J. Wang acknowledges the partial
support from the Preeminent Postdoctoral Program (P3) at
UCF.
Since the catalytic dehydrogenation and electrocatalytic
oxidation of hydrazine have different reaction pathways,28 the
bifunctionality of the Pt0.2Ni0.8 catalyst may be attributed to
shared reaction intermediates or transition states between the
two processes. Indeed, a previous study by Strasser et al.
suggested a hydrazine surface adsorbate that serves as a
reaction intermediate for both the non-faradaic (reaction 1)
and the faradaic (reaction 3) pathway.28 Correspondingly, the
low-coordinated surface sites on the Pt0.2Ni0.8 catalyst
(presumably edge sites) may have an enhanced binding with
the hydrazine surface adsorbate that can promote both
dehydrogenation and electro-oxidation of hydrazine. These
sites can subsequently break N-H bonds in NxHy intermediates,
and further react along either the non-faradaic or the faradaic
pathway, depending on reaction conditions (alkaline vs.
neutral solution, with electrical potential or not). In particular,
we emphasize that a neutral PBS electrolyte was used in our
electro-oxidation study to minimize the spontaneous non-
faradaic dehydrogenation process, in contrast to previous
studies of N2H4 electro-oxidation in an alkaline medium that
favors the non-faradaic dehydrogenation of N2H4.28−30 Our
study can shed light on the development of bi-functional
catalysts for other energy conversion and utilization processes,
by identifying surface sites that can stabilize shared reaction
intermediates or transition states.
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Table of Contents Entry:
A Bifunctional Catalyst for Efficient Dehydrogenation and Electro-oxidation of Hydrazine
Jun Wang, Asim Khaniya, Lin Hu, Melanie J. Beazley, William E. Kaden, Xiaofeng Feng*
Keywords: hydrazine, dehydrogenation, electro-oxidation, bifunctional catalyst, energy carrier
A Pt0.2Ni0.8 nanocatalyst has been developed, which shows superior catalytic performance for
both complete dehydrogenation and electro-oxidation of hydrazine.