Ni3N as an Active Hydrogen Oxidation Reaction Catalyst in Alkaline Medium

Article abstract of DOI:10.1002/anie.201902751

Hydroxide-exchange membrane fuel cells can potentially utilize platinum-group-metal (PGM)-free electrocatalysts, offering cost and scalability advantages over more developed proton-exchange membrane fuel cells. However, there is a lack of non-precious electrocatalysts that are active and stable for the hydrogen oxidation reaction (HOR) relevant to hydroxide-exchange membrane fuel cells. Here we report the discovery and development of Ni₃N as an active and robust HOR catalyst in alkaline medium. A supported version of the catalyst, Ni₃N/C, exhibits by far the highest mass activity and break-down potential for a PGM-free catalyst. The catalyst also exhibits Pt-like activity for hydrogen evolution reaction (HER) in alkaline medium. Spectroscopy data reveal a downshift of the Ni d band going from Ni to Ni₃N and interfacial charge transfer from Ni₃N to the carbon support. These properties weaken the binding energy of hydrogen and oxygen species, resulting in remarkable HOR activity and stability.

Full text of DOI:10.1002/anie.201902751

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Accepted Article
Title: Ni3N as an active hydrogen oxidation reaction catalyst in alkaline
medium
Authors: Weiyan Ni, Anna Krammer, Chia-Shuo Hsu, Hao Ming Chen,
Andreas Schueler, and Xile Hu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902751
Angew. Chem. 10.1002/ange.201902751
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10.1002/anie.201902751
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Ni3N as an active hydrogen oxidation reaction catalyst in alkaline
medium
Weiyan Ni[a], Anna Krammer[b], Chia-Shuo Hsu[c], Hao Ming Chen[c], Andreas Schüler[b] and Xile Hu*[a]
Abstract: Hydroxide-exchange membrane fuel cells can potentially
utilize platinum-group-metal (PGM)-free electrocatalysts, offering cost
and scalability advantages over more developed proton-exchange
membrane fuel cells. However, there is a lack of non-precious
electrocatalysts that are active and stable for the hydrogen oxidation
reaction (HOR) relevant to hydroxide-exchange membrane fuel cells.
Here we report the discovery and development of Ni3N as an active
and robust HOR catalyst in alkaline medium. A supported version of
the catalyst, Ni3N/C, exhibits by far the highest mass activity and
break-down potential for a PGM-free catalyst. The catalyst also
exhibits Pt-like activity for hydrogen evolution reaction (HER) in
alkaline medium. Spectroscopy data reveal a downshift of the Ni d
band going from Ni to Ni3N and interfacial charge transfer from Ni3N
to the carbon support. These properties weaken the binding energy of
hydrogen and oxygen species, resulting in remarkable HOR activity
and stability.
workers significantly improved the activity of Ni nanoparticles in
HOR using a nitrogen-doped carbon nanotube support.[4d] This
composite catalyst, however, is still one order of magnitude less
active than state-of-the-art Pt catalysts. Besides modest activity,
Ni also suffers from low stability at potential above ~0.1 V vs.
reversible hydrogen electrode (RHE) due to relatively strong
binding affinity towards oxygen species, which block the active
sites. To achieve considerable power output, HOR catalysts
should function at potentials up to 0.3 V vs. RHE.[1b] Thus, there
is great need for new PGM-free HOR catalysts, especially those
with enhanced oxidative stability. Here we report the discovery of
Ni3N as an active HOR catalyst. When supported on carbon, the
catalyst achieves by far the highest mass-averaged activity and
break-down potential for a PGM-free catalyst. Spectroscopy data
indicate a downshift of the Ni d band going from Ni to Ni3N, as well
as interfacial charge transfer from Ni3N to the carbon support.
These properties weaken the binding energy of hydrogen and
oxygen species, which might result in higher HOR activity and
stability than a typical Ni catalyst. For the same reason, the
catalyst also exhibits superior activity in the hydrogen evolution
reaction (HER).
Bulk and nanoparticle samples of Ni3N were prepared by
reducing an appropriate Ni precursor under a NH3 atmosphere at
an elevated temperature. The precursor for bulk Ni3N was Ni(OH)2
synthesized by a hydrothermal method and the precursor for
nanoparticles was NiO nanoparticles, synthesized via an
oleylamine-assisted method (see details in Methods and Figure
S1 and S2, Supporting Information). To prevent the aggregation
of the Ni3N nanoparticles, they were prepared on a carbon black
(Vulcan-XC 72R) support (labelled Ni3N/C). According to thermal
gravimetric analysis (TGA) in air, the loading of Ni3N in Ni3N/C
was about 51.4 mass percentage. (Figure S3). Nickel
nanoparticles were also prepared to serve as a reference sample
(denoted as Ni-Ref).[4d]
Powder X-ray diffraction (PXRD) analysis confirms the
formation of Ni3N without crystalline impurity (Figure 1a). In the X-
ray adsorption near-edge structure (XANES) spectra (Figure 1b),
the adsorption edge of Ni3N/C is positively shifted comparing to
that of Ni3N, suggesting a higher valence state of Ni in Ni3N/C.
According to extended X-ray adsorption fine structure (EXAFS)
spectra and its fitting results (Figure S4 and Table S1), due to the
interaction with nitrogen atom in the lattice, Ni-Ni bond length
were elongated in both Ni3N and Ni3N/C comparing to Ni-Ref. The
coordination number of Ni3N/C is smaller than Ni3N, indicating a
smaller particle size, which was later confirmed by electron
microscopy. In the high resolution Ni 2p3/2 X-ray photoelectron
spectra (XPS) (Figure 1c and Figure S5), the peaks with a binding
energy (BE) of about 853 eV are attributed to Ni in Ni3N and
Ni3N/C. The binding energy is similar to that of Ni in Ni2P, which
is close to metallic nickel and was previously assigned as
Ni(δ+).[5] The peaks with BE of about 856 eV are attributed to
surface oxidized Ni species, Ni(II).[6] The binding energies of the
same Ni species are higher in Ni3N/C than in Ni3N, again
suggesting a higher valence state of Ni in the former. In the N 1s
Hydrogen fuel cell technology is a key component of a low-
carbon-emission
hydrogen
economy.
Proton-exchange
membrane fuel cell (PEMFC) has advanced significantly in recent
years, and its commercial deployment in the automotive industry
has started. Nevertheless, PEMFC remains a costly power device.
Hydroxide-exchange membrane fuel cell (HEMFC) is an
alternative fuel cell technology with potential cost advantages
over PEMFC.[1] HEMFC operates in alkaline medium where non
platinum-group-metal (PGM) catalysts might be active and stable.
Moreover, HEMFC might use more economical bipolar plates, air
loop, and membranes than PEMFC.[1b, 2] However, HEMFC is at
a very early stage of development and many technical challenges
are yet to be solved.
The possibility to use PGM-free electrocatalysts is one major
advantage of HEMFC over PEMFC. Whereas PGM-free oxygen
reduction reaction (ORR) catalysts exhibit comparable activity to
PGM catalysts, PGM-free hydrogen oxidation reaction (HOR)
3]
catalysts are much inferior.[1,
Despite several decades of
research, only nickel and its metallic alloys have been shown to
be active PGM-free HOR catalysts.[3b, 3c, 4] Recently Yan and co-
[a]
W. Ni, Prof. Dr. X. Hu
Laboratory of Inorganic Synthesis and Catalysis (LSCI), Institute of
Chemical Sciences and Engineering (ISIC),
École Polytechnique Fédérale de Lausanne (EPFL),
BCH 3305, Lausanne 1015 (Switzerland)
E-mail: xile.hu@epfl.ch
Homepage: http://lsci.epfl.ch
[b]
[c]
A. Krammer, Dr. A. Schüler
Solar Energy and Building Physics Laboratory (LESO-PB), Institute
of Civil Engineering (IIC),
École Polytechnique Fédérale de Lausanne (EPFL),
GC A0 391 (Bâtiment GC), Lausanne CH 1015, Switzerland
C.S. Hsu, Prof. Dr. H. M. Chen
Department of Chemistry, National Taiwan University,
Taipei 10617, Taiwan
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201902751
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inheriting the morphology of its NiO/C precursor (Figure S2). The
size distribution of Ni3N/C is narrow (4.6 ± 1.0 nm). High resolution
TEM (HRTEM) image reveals an inter-planar spacing of ~0.20 nm
in Ni3N/C, corresponding to the (111) plane of Ni3N (Figure 2c).
Scanning transmission electron microscopy (STEM) and its
corresponding elemental mapping indicate the homogeneous
distribution of Ni and N elements (Figure S7).
The HOR activity of Ni3N and Ni3N/C was examined with a
standard three-electrode system at a scan rate of 1 mV/s in H2-
saturated 0.1 M KOH solution (See details in Methods). The
Ni3N/C sample has a much higher current density than bulk Ni3N
and Ni-Ref (Figure 3a). Its activity approaches that of 20 wt% Pt/C.
Importantly, the break-down potential of Ni3N/C is about 0.26 V vs
RHE, the highest potential achieved by an Earth-abundant
catalyst. Control experiments conducted in N2-saturated
electrolytes showed negligible anodic currents across the whole
potential range (Figure S8). In situ XANES and EXAFS
measurements (Figure S9 and Table S2) of Ni3N/C revealed a
stable catalyst at 0-0.25 V vs. RHE.
The kinetic current densities were calculated using data from
LSV curves at different rotating speeds (Figure 3b). When the
current is controlled by both reaction kinetics and H2 diffusion,
Figure 1. Characterization of Ni3N and Ni3N/C samples. (a). PXRD pattern. (b).
XANES spectra. (c). Ni 2p3/2 and (d). N 1s high-resolution XPS spectra.
Koutecky-Levich equation (eq. 1) applies:
0.5
spectra (Figure 1d), a single peak with a BE of about 398 eV,
corresponding to a nitride-type nitrogen, was observed for both
Ni3N and Ni3N/C.[6a, 7] The BE of N in Ni3N/C is also positively
shifted compared to that of Ni3N. These data indicate an
interfacial charge transfer from Ni3N nanoparticle to the carbon
support in Ni3N/C.
⁄
⁄
⁄
⁄
⁄
1 푗 = 1 푗푘 + 1 푗푑 = 1 푗푘 + 1 퐵푐0 휔
(1)
Here j is the measured current density, jk and jd are kinetic and
diffusional current density, respectively, B is the Levich constant
related to the diffusivity of H2 and kinematic viscosity of electrolyte,
c0 is the solubility of H2 in 0.1 M KOH, and ω is the rotating speed.
By fitting the data at an overpotential of 50 mV (η = 50 mV) with
eq. 1, a linear plot of ω-1/2 with j-1 was obtained (Figure S10). The
slope is 4.75 cm2/mA∙s1/2, close to the theoretical value (4.87
cm2/mA∙s1/2).[9] The kinetic current density of Ni3N/C is 3.9
mA/cm2 at η = 50 mV. The corresponding mass activity of
disk
Ni3N/C is 24.38 mA/mgNi3N at η = 50 mV, the highest among all
reported earth-abundant HOR catalysts (Table S3). The mass
activity of bulk Ni3N (1.73 mA/mgNi3N) and Ni-Ref (1.07 mA/mgNi)
are an order of magnitude lower.
Figure 2. Microscopic characterization of Ni3N and Ni3N/C composite. (a) TEM
image of bulk Ni3N. (b) TEM image of Ni3N/C. (c) HRTEM image of Ni3N/C. Inset
is the Fast-Fourier Transform (FFT) image of the composite.
Transmission electron microscopy (TEM) revealed the
morphology of Ni3N and Ni3N/C. Bulk Ni3N (Figure 2a) contains
large, sintered aggregates with a wide size distribution (42.2 ±
18.5 nm, Figure S6). The large size is likely inherited from its
precursor, while the large size range might be due to Ostwald
ripening and high diffusivity of nickel atoms at an elevated
temperature.[8] Thanks to the carbon support, the Ni3N
nanoparticles in Ni3N/C are evenly distributed (Figure 2b),
Figure 3. Electrochemical HOR performance. (a) HOR polarization curves of
Ni3N/C, Ni3N, Ni-Ref, and 20 wt% Pt/C in H2-saturated 0.1 M KOH with a rotating
speed of 2500 r.p.m. at a scan rate of 1 mV/s. (b) Polarization curves of Ni3N/C
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at different rotating speeds. (c) HER/HOR Tafel plots of kinetic current density
on Ni3N/C and Ni3N. (d) Accelerated durability test results of Ni3N/C. The
durability test was performed by doing cyclic voltammetry scans in H2-saturated
0.1 M KOH from -0.05 V to 0.1 V vs RHE with a scan rate of 150 mV/s. The LSV
curves were recorded after 0, 1000, 3000, 5000 scans. The catalyst loading was
0.28 mg/cm2 for Ni3N and Ni-Ref and 0.16 mgNi3N/cm2 for Ni3N/C.
measured in a 24-hour electrolysis at a constant current density
of -10 mA/cm2. The potential remained nearly constant, indicating
good stability (Figure 4c).
The kinetic current density from Eq. 1 (Figure 3c) was further fitted
to the Butler-Volmer equation (eq. 2) to obtain the exchange
current density j0:
⁄
⁄
푅푇
푅푇
푗푘 = 푗0(푒훼퐹휂
− 푒(1−훼)퐹휂
)
(2)
Here α is charge transfer coefficient, F is Faraday constant (96485
A/mol), R is universal gas constant (8.314 J/mol ‧ K), η is
overpotential and T is temperature. The exchange current
densities were then normalized by electrochemical surface area
(ECSA; Figure S11) as well as the catalyst loading (Table S3).
Comparison with Ni-ref and other state-of-the-art non-precious
HOR catalysts reveals the origin of the remarkable mass activity
of Ni3N/C.[3b, 4d] The specific activity, that is, the ECSA-normalized
exchange current density of Ni3N/C and Ni3N is similar and more
than 7 times higher than that of Ni-ref. But this specific activity is
slightly lower than that of most active hybrid Ni catalysts.[3b, 4d]
Recently a Ni/Ni3N/nickel foam catalyst was reported to have very
high geometric activity in HOR.[10] The active site was proposed
as the interfacial sites of Ni and Ni3N, instead of Ni3N itself. The
direct comparison to that catalyst is difficult due to the use of
different measurement methods. A rough estimate of ECSA-
averaged exchange current density was 0.003 mA/cm2, lower
than the present Ni3N catalysts. The mass-normalized exchange
current density of Ni3N/C is 10 times higher than that of bulk Ni3N
and about 4 times higher than most active hybrid Ni catalysts.[3b,
Figure 4. Electrochemical HER performance. (a) HER polarization curves of
Ni3N/C, Ni3N, Ni-Ref and commercial 20 wt% Pt/C in N2-saturated 1 M KOH with
a scan rate of 1 mV/s. All the catalysts were casted onto a 5 mm glassy carbon
electrode with an overall loading of 0.28 mg/cm2 except that Ni3N/C had a
loading of 0.31 mg/cm2 (loading includes the carbon support). (b) HER Tafel
slopes measured according to the HER performance in (a). (c) Durability test
for Ni3N/C. To prevent the catalyst detachment, hydrophilic carbon cloth was
used as working electrode here.
Ultraviolet photoemission spectroscopy (UPS) was applied to
study the electronic structure of Ni3N/C, Ni3N, and Ni-Ref. As
shown in Figure 5a and Fig. S14, all the materials have electronic
bands crossing the Fermi level. The presense of such a Fermi
edge in the UPS spectra indicates a metallic nature of these
materails.[11] This result is consistent with previous studies which
showed Ni3N as an intrinsic metallic material.[12] The reporetd
electronic electronic resistivity was about 1.09 × 10−4 ·m at 300
K.[12a] For Ni3N and Ni3N/C, the signals located at 5-10 eV are
tentatively attributed to hybridized Ni 3d and N 2p emissions, as
observed in similar regions for other transition metal nitrides such
as TiN and VN.[13] The peaks located at 0-1 eV arise from nickel’s
3d band.[14] The Ni 3d band reaches it maximum at 0.305 eV, 0.65
eV and 0.805 eV for Ni-Ref, Ni3N, and Ni3N/C, respectively. These
data indicate that from Ni to Ni3N and to Ni3N/C, the 3d band is
pushed away from the Fermi level. According to the d-band theory,
the interaction of adsorbate with transition metal catalysts can be
simplified as electronic coupling between the adsorbate’s valence
states and the catalyst’s d band, which results in bonding and
antibonding states. A higher d-band center (closer to the Fermi
level) leads to more empty antibonding states above the Fermi
level, hence a stronger adsorption.[15] Thus, the absorption energy
should have the order of Ni > Ni3N > Ni3N/C.
4d]
Thus, dispersion of Ni3N nanoparticles on a carbon support
leads to a high surface area/mass ratio, which contributes to its
exceedingly high mass activity.
An accelerated long-term durability test (ADT) for Ni3N/C was
conducted by running repetitive CV from -0.05 V to 0.1 V vs RHE
at a scan rate of 150 mV/s in 0.1 M H2-saturated KOH. To prevent
Pt contamination, the working and counter electrodes were
separated by a frit. After 1000, 3000, 5000 CVs, the exchange
current density of the electrode decreased to 81.0 %, 71.7 % and
59.8 % of the initial value, respectively (Figure 3d). TEM image
(Figure S12) shows that after the measurement the catalyst
retains its morphology. Size distribution analysis indicates a
slightly increased particle size (4.7 ± 1.2 nm). HRTEM shows no
formation of an extra phase at the surface. Some catalyst fell off
from the electrode after 5000 CVs, which led to a decreased
ECSA (Figure S13). The ECSA-normalized activity decreased to
74.4 % of the initial value after 5000 CVs, indicating good stability
of Ni3N/C (Table S4, Supporting Information).
Since HER is the reverse reaction of HOR, the good HOR
activity of Ni3N/C promoted us to examine its HER activity. The
experiments were conducted 1 M N2-saturated KOH solutions.
Ni3N/C exhibits remarkable HER activity, close to the benchmark
20 wt% Pt/C catalyst. The overpotential to reach 10 mA/cm2 is
only 64 mV for Ni3N/C, among the lowest for earth-abundant HER
catalysts in alkaline media (Table S5). The geometric activity of
Ni3N/C is much higher than that of Ni3N and Ni-ref. The Tafel
slopes are 32, 48, 122, 88 mV/decade for Pt/C, Ni3N/C, Ni3N and
Ni-Ref, respectively. The durability of Ni3N/C in HER was
The mechanism of HOR on PGM catalysts in alkaline medium
is studied only in recent years.[16] A prevailing theory is that
hydrogen bonding energy (HBE) determine the rate of HOR.[16a-c,
16i] However, there is a vivid debate on whether and how surface-
adsorbed OH species influence HOR kinetics.[16c-h, 17] In addition,
the pH-dependent potential of zero point charge was proposed to
affect the transportation of OH- through the double layer region
and hence the HOR rate.[16h] Dedicated mechanistic studies of
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non-PGM HOR catalysts are rare,[1a, 1c, 18] although the modest
activity of Ni in HOR was commonly attributed to a too high
HBE.[3b, 19] Accordingly, the higher specific activity of Ni3N over Ni
might be explained by a weakening of HBE. Going from Ni3N to
Ni3N/C, the mass activity is much higher due to the much higher
surface area per mass, but the specific activity is slightly lower.
The decrease of specific activity might be due to an overcorrection
of the hydrogen binding strength to an unfavorable weak region.
It should be pointed out that the hydroxide bonding energy
(OHBE) changed in the same direction as HBE for these samples
(see below), thus, the improved activity might be due to more
optimal OHBE, or both HBE and OHBE. A recent study indicated
that both HBE and OHBE (from density functional theory (DFT)
computations) were important parameters in describing the HOR
activity of Ni alloys.[16e]
carbon support, we obtain a Ni3N/C catalyst with the highest mass
activity and break-down potential in HOR for a base metal catalyst.
The catalyst also exhibits excellent HER activity in alkaline
medium, comparable to the benchmark Pt/C. Spectroscopy data
reveal a downshift of the Ni d band and interfacial charge transfer
from Ni3N to the carbon support as factors that lead to weak
binding of hydrogen and oxygen species in Ni3N/C. This weak
binding might be the origin of its remarkable HOR activity and
stability.
Acknowledgements
This work is supported by the EPFL and the Ministry of Science
and Technology, Taiwan (Contract No. MOST 107-2628-M-002-
015-RSP). We thank Swiss-Norwegian beam line at the European
Synchrotron Radiation Facility and Taiwan beamline of National
Synchrotron Radiation Research Center (NSRRC) for provision of
beam time. We also thank Lichen (Edward) Bai (EPFL) for
microscopic measurements and Dr. Jun-Ho Yum (EPFL) for
discussions.
Keywords: Hydrogen oxidation reaction • Ni3N • alkaline fuel
cell • electrocatalysis • hydrogen evolution
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Figure 5. (a) UPS spectra of Ni3N, Ni3N/C and Ni-Ref. The excitation photon
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The higher break-down potential in HOR for Ni3N and Ni3N/C
compared to Ni-Ref can be, at least partially, explained by
attenuated adsorbate binding. It was reported that catalysts with
high affinity to oxygen species, especially those based on Ru and
20]
Ni, deactivate readily at positive potentials vs RHE.[3b,
The
[3]
deactivation was attributed to the oxidative adsorption of oxygen
species from the electrolyte. In Pt-based ORR catalysts, a
downshift d-band decreases the binding strength of O species.[21]
The UPS data above show that nickel’s d-band is downshifted
from metallic Ni to Ni3N to Ni3N/C. Thus, the oxygen binding
strength should follow the order Ni > Ni3N > Ni3N/C. The
decreased oxygen binding in Ni3N/C results in its high break-down
potential. To further corroborate this hypothesis, the OH- oxidative
adsorption peaks in the three catalysts are compared (Figure 5b).
Indeed Ni3N/C has the most positive peak. Besides, it is noted
that XANES and XPS data show an interfacial charge transfer
from Ni3N to the carbon support in Ni3N/C. This charge transfer
decreases the electron density of Ni3N, making it more difficult to
be oxidized. Enhanced oxidative resistance due to interfacial
[4]
[5]
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charge transfer observed by XPS was reported for other
[22]
supported catalysts such as Cu/WO2.72
.
Whether the charge
transfer is the cause of the downshift of the d-band, or works in
parallel to it, remains unclear.
In summary, we discover the significant activity of Ni3N for HOR
in alkaline medium. By dispersing Ni3N nanoparticles onto a
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10.1002/anie.201902751
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Weiyan Ni, Anna Krammer, Chia-Shuo
Hsu, Hao Ming Chen, Andreas Schüler
and Xile Hu*
Ni3N was discovered as a new
hydrogen oxidation reaction catalyst
with superior activity and robust
stability.
Page No. – Page No.
Ni3N as an active hydrogen oxidation
reaction catalyst in alkaline medium
This article is protected by copyright. All rights reserved.