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Tang, J. Wang, X. Lv, X. Pan, Y. Zhao, X. Yu, H. B. Wu and R. Lu, Energy Environ. Sci., 2022, DOI:
10.1039/D2EE01463G.
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Volume 11
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April 2018
Pages 719-1000
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emission, sustainable technologies based on carbon-free energy carriers such as
hydrogen have been considered as feasible approaches to reach a carbon neutral
economic. To address the difficulties associated with the generation, storage and
transportation of green hydrogen, hydrogen-rich molecules have been proposed to
produce hydrogen or as alternative fuels for vehicular applications. In this regard,
hydrazine hydrate (N2H4·H2O), a liquid compound possessing equivalent hydrogen
content up to 8 wt.%, is foreseen as a promising hydrogen carrier. Recent
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implementations of hydrazine hydrate include onboard hydrazine decomposition and
hydrazine-assisted electrochemical hydrogen production, as well as direct hydrazine
fuel cell (DHzFC). For electrochemical utilization of hydrazine, high-performance
electrocatalysts to accelerate the hydrazine oxidation reaction (HzOR) are the key
complements to maximally extract the chemical energy from hydrazine. In this work,
we demonstrate a highly active HzOR electrocatalyst by generating few-atom vacancies
on self-supported Co arrays, which enables a prototype DHzFC with high power output
at ambient temperature.
Energy & Environmental Science Accepted Manuscript
DOI: 10.1039/D2EE01463G
Carbon neutrality has become one of the world’s most urgent missions. To reduce
CO2
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Low-Coordinated Cobalt Arrays for Efficient Hydrazine Electrooxidation
Qian Liua, Xiaobin Liaob, Yuanhao Tanga, Jianghao Wanga, Xiangzhou Lva, Xuelei Pand, Ruihu Lub,
Yan Zhaob, c, *, Xin-Yao Yue, f, *, Hao Bin Wua, *
aInstitute
for Composites Science Innovation (InCSI) and State Key Laboratory of Silicon Materials,
School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China
bState
Key Laboratory of Silicate Materials for Architectures, International School of Materials
Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China
cThe
Institute of Technological Sciences, Wuhan University, Hubei, Wuhan 430072, P. R. China
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dState
Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
International School of Materials Science and Engineering, Wuhan University of Technology,
Wuhan 430070, P. R. China
eInstitutes
of Physical Science and Information Technology, Key Laboratory of Structure and
Functional Regulation of Hybrid Materials, Anhui University, Ministry of Education, Hefei 230601,
P. R. China
fEnergy
Materials and Devices Key Lab of Anhui Province for Photoelectric Conversion, Anhui
University, Hefei 230601, P. R. China
Email: yan2000@whut.edu.cn (Y. Zhao), yuxinyao@ahu.edu.cn (X.-Y. Yu), hbwu@zju.edu.cn (H.
B. Wu)
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Abstract
DOI: 10.1039/D2EE01463G
Exploring advanced electrocatalysts for hydrazine oxidation reaction (HzOR) could expedite the
applications of direct hydrazine fuel cells (DHzFCs) for zero-carbon economics. Herein, we report a
remarkable HzOR electrocatalyst based on low-coordinated Co arrays supported on Cu foam (pCo/CF). The low-coordinated Co arrays were synthesized by the reduction of Co(OH)F precursor
with H2 plasma to induce numerous few-atom vacancies, producing low-coordinated surface Co
atoms as catalytic active sites. The as-prepared p-Co/CF electrode exhibits a low onset potential of 0.15 V (vs. RHE) and a small Tafel slope of 8.83 mV dec-1 in 0.05 M N2H4/1 M KOH electrolyte,
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which are superior to most reported electrocatalysts toward HzOR. Density functional theory (DFT)
calculations have been employed to elucidate the HzOR process on metallic Co catalysts, suggesting
that the improved catalytic activity would be attributed to the weakened adsorption of N2H3* on lowcoordinated Co sites. Benefited by the superior catalytic activity of p-Co/CF, a prototype DHzFC has
been demonstrated to deliver a high open circuit voltage of 1.1 V and a maximal power density of
186 mW cm-2 at ambient temperature. This work provides guidance for designing high-performance
non-noble metal electrocatalysts toward viable direct hydrazine fuel cells.
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Introduction
The rising attention of alternative zero-carbon fuel economies has promoted the development of
nitrogen-based fuels as energy carriers1. Among these nitrogen-based fuel technologies, direct
hydrazine fuel cells (DHzFCs) featuring high energy density (4269 Wh/L for hydrazine hydrate)2
and high voltage (N2H4 + O2→ N2 +2H2O, E0 = 1.56 V) represent a promising candidate for
vehicular and maritime applications3. The hydrazine fuel can be produced by sustainable carbon-free
methods, for example, electrochemical approaches through the oxidative homocoupling of
benzophenone imine4. The core structure of DHzFCs is formed by sandwiching an ion-exchange
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membrane between an anode and a cathode5. Both the electrodes consist of catalysts supported on
porous backing layers, allowing the facile transport of fuel and oxidant from the flow field in bipolar
plates. The cathodic and anodic reactions in DHzFCs typically occur in alkaline electrolyte are the
oxygen reduction reaction (ORR) and the hydrazine oxidation reaction (HzOR), respectively, which
can be effectively accelerated by non-noble metal catalysts2, 3, 6.
Developing anode electrocatalysts with high activity and durability is pivotal to promote the
practical applications of the DHzFC technology. In the HzOR, the hydrazine molecule is
electrochemically oxidized to nitrogen (N2) through a four-electron process. As a typical multielectron process, the potential-dependent activation energy barriers of HzOR depend on the
interaction between reaction intermediates and surface of catalysts7. Among the various explored
non-noble metal catalysts8-13, 3d transition metals such as Ni and Co have shown decent
electrocatalytic activity toward HzOR for DHzFCs, which are even superior to noble metals such as
Pt
12, 14-17.
Diverse strategies, such as forming alloys13, intermetallics18 and single-atom catalysts19
have been explored to improve the HzOR catalytic performance. Nevertheless, the overpotentials of
most catalysts are relatively large (> 230 mV) and the maximal current densities have been
compromised by the sluggish reaction kinetics, which impede their application in DHzFCs.
Moreover, in-depth understanding of the catalytic mechanism is insufficient and rational designs of
efficient HzOR catalysts remains challenging.
Defect chemistry plays an important role in designing heterogenous catalysts20. Typically, for
transition metal catalysts, d-band model offers an approximate description of the electronic
properties and illustrates the interaction between adsorbates and metal surfaces21. By manipulating
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reduced, which would tune the widths of the d band and shift the d-band center accordingly22. For
example, point defect, one of the most common defects, might produce electron delocalization to
create active sites for heterogeneous catalysis23. Atoms with low CNs on metal surface, such as stepedge and defect sites, exhibit higher d-band centers and offer moderate bindings with reaction
intermediates, which are well-known catalytic active centers22, 24-26.
In this work, we constructed low-coordinated Co arrays on Cu foam as an integrated electrode for
hydrazine electrooxidation, achieving a superior HzOR catalyst outperforming most previous reports.
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The CNs of surface Co atoms have been modulated by creating few-atom vacancies through a H2
plasma treatment. The low-coordinated Co arrays electrode (p-Co/CF) exhibited a negative onset
potential of -0.15 V (vs. RHE) and a low Tafel slope of 8.83 mV dec-1 for HzOR. The DFT
calculations revealed the up-shifted d-band center of low-coordinated Co sites, which accelerates the
N2H4 adsorption and dehydrogenation through reducing the energy barrier of the potentialdetermining step (dehydrogenation of N2H3*). Benefiting from the high HzOR activity and robust
hierarchical structure of the p-Co/CF electrode, the assembled direct hydrazine fuel cell delivered a
high open-circuit voltage of 1.1 V and a peak power output of 185.9 mW cm-2 at room temperature.
The present study demonstrates a promising strategy to develop highly efficient HzOR
electrocatalysts, which would advance the development of DHzFC and the utilization of HzOR as an
alternative anodic reaction for fuel production.
Results and discussion
Structural characterizations of low-coordinated Co arrays
A two-step self-templated process was adopted to prepare the low coordinated Co arrays as
schematically shown in Fig. 1a (see Methods for details). Needle-like Co(OH)F arrays on Cu foam
(CF) was synthesized via a hydrothermal method, and the obtained Co(OH)F/CF sample was then
transformed into defective Co arrays (denoted as p-Co/CF) by H2 plasma reduction at room
temperature. As shown in the field-emission scanning electron microscope (FESEM) image (Fig.
1b), the Co(OH)F arrays consist of needle-like building blocks, which are further assembled into a
leaf-like structure. The X-ray diffraction (XRD) pattern of Co(OH)F arrays (Supplementary Fig. 1)
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the defects in metal catalysts, the averaging coordination numbers (CNs) of metal DOI:
atoms
could be
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crystal feature as confirmed by the transmission electron microscope (TEM) image and
corresponding selected area electron diffraction (SAED) pattern (Fig. 1c). The high-resolution TEM
(HRTEM) image displays the ordered lattice fringes with 0.51 nm spacing of a Co(OH)F needle
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(Fig. 1d), which corresponds to the (200) planes of Co(OH)F.
Fig. 1 Morphological and structural characterizations of Co(OH)F/CF and p-Co/CF. a,
Schematic illustration of the synthetic procedure of Co(OH)F/CF and p-Co/CF. b, FESEM image of
Co(OH)F. c, TEM image (inset: SAED pattern) and d, HRTEM image of Co(OH)F. e, FESEM
image of p-Co. f, TEM image (inset: SAED pattern) and g, HRTEM image of p-Co.
After the plasma treatment, the needle-like structure becomes bended (Fig. 1e). Simultaneously,
Co(OH)F single-crystal needles transform into polycrystalline Co needles, which are composed of
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matches well with the standard pattern of Co(OH)F (PDF# 50-0827). Each needle exhibits
a single-
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fringes with interplanar distances of 0.20, 0.20, 0.20 and 0.17 nm, which are well assigned to (111),
(111), (111) and (200) planes, respectively, of face center cubic (fcc) Co metal. The non-uniform
lattice in the magnified HRTEM images (Supplementary Fig. 2a, b) suggests the presence of
abundant defective structures, which will be discussed shortly. Such polycrystalline array structure
has a tremendous specific surface area, which provides abundant catalytic active sites. In addition,
the open and interconnected porous structure might facilitate the transfer of hydrazine molecules and
electrons to the active sites, and also promote the release of generated gas from the electrode.10
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The low-temperature H2 plasma reduction approach27, 28 removes the OH and F moieties from the
Co(OH)F precursor to produce polycrystalline Co arrays, which would simultaneously generate
abundant point defects as schematically depicted in Fig. 2a. The change of valance state of Co was
probed by X-ray photoelectron spectroscopy (XPS). As displayed in Fig. 2b, the Co(OH)F/CF
exhibits only Co2+ signals while p-Co/CF shows dominant signals of Co0. The positions of the main
peaks agree with the Co 2p3/2 signals of Co2+ (781.1 eV) and Co0 (777.9 eV) species with unfilled 3d
states29, 30. An additional Co 2p3/2 peak in p-Co/CF at a lower binding energy (780.4 eV) originates
from the marginal surface oxidation of Co in air. The F 1s peak of Co(OH)F/CF at 684.3 eV
(Supplementary Fig. 3) is consistent with the position of F- in metal hydroxyfluoride29 while the
main O 1s peak at 531.4 eV (Fig. 2c) agrees with that from metal hydroxides (531-532 eV). After the
H2 plasma reduction process, the single F 1s shifts to a higher energy level. Such peak shift could be
related to the change of chemical environment of F, where a trace amount of remaining F bonds to
Co. The energy dispersive spectroscopy (EDS) results (Supplementary Fig. 4 and 5) indicated that
the atomic ratio of F to Co notably decreased from 0.68 in Co(OH)F to 0.18 in p-Co/CF, confirming
the removal of F during H2 plasma treatment. Meanwhile, O 1s peaks at 533.0 eV and 531.4 eV are
associated with adsorbed water molecules and residual hydroxyl groups, respectively28, 31.
To investigate the coordination environment of p-Co/CF, the X-ray absorption near edge structure
(XANES) and the extended X-ray absorption fine structure (EXAFS) characterizations were
conducted. The Co K-edge spectra of p-Co (powder of Co arrays scratched from p-Co/CF) is similar
to that of Co foil (Fig. 2d). As shown in Fig. 2e, the length of Co-Co bond for p-Co is slightly shorter
than that of Co foil (2.12 Å for p-Co vs. 2.21 Å for Co foil). Such contraction of crystal lattice might
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interconnected Co metal nanoparticles (Fig. 1f). The HRTEM image (Fig. 1g) provides
clear lattice
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Notably, the high coordination shells (dashed rectangle in Fig. 2e) of p-Co also shift to a lower R
space, suggesting the existence of coordinatively unsaturated sites. The reduced amplitude of R
space for p-Co indicates a lower CN and a higher mean-square disorder33, suggesting the presence of
a large number of Co vacancies. The fitted parameters of EXAFS are summarized in Supplementary
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Table 1.
Fig. 2 Chemical environment and atomic structure of Co(OH)F/CF and p-Co/CF. a, Schematic
showing the changes of atomic structure during H2 plasma treatment (left: Co(OH)F, right: p-Co). b,
Co 2p XPS spectra. c, O 1s XPS spectra. d, Normalized Co K-edge spectra. e, Fourier-transform
spectra from EXAFS.
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be attributed to the existence of point defects, which leads to a reduced CN of the near
Co atoms32.
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contents of free-volume holes in solid materials by measuring positron annihilation in low-density
regions around dislocations or vacancies34. Positron annihilation lifetime is proportional to the size of
defects and the relative intensity is proportional to the density of defects35. The τ1 of p-Co is much
higher than the theoretical value of monovacancy in bulk Co (~ 184 ps, estimated based on Ni with
similar crystallographic structure)35. Thus, the higher τ1 is ascribed to positrons annihilating in fewatom defects, and the size of atom vacancies in p-Co should be of 2 or 3 atoms. The p-Co shows a
longer τ1 lifetime and a higher I1/I2 ratio than that of Co np (Co nanoparticles prepared by a reduction
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method36, XRD patterns displayed in Supplementary Fig. 6), revealing abundant few-atom vacancies
in the nanocrystalline of p-Co. The p-Co and Co np show similar second lifetime components τ2,
which is attributed to positrons annihilating at interfaces or vacancy clusters at grain boundaries. The
third lifetime components τ3 (intersections of several interfaces) in Co np could be attributed to the
surface oxidation37, 38. The above results suggest that p-Co contains a large number of few-atom (2 or
3) vacancies, which is in line with the results from X-ray adsorption spectroscopy.
Table 1. Positron Lifetime Parameters of p-Co and Co np.
Sample
τ1 (ns)
I1 (%)
τ2 (ns)
I2 (%)
τ3 (ns)
I3 (%)
p-Co
0.252
73.64
0.444
26.36
0
0
Co np
0.225
62.43
0.449
32.69
1.57
4.88
Note: τ1, interfacial vacancies/vacancy complexes at grain boundary areas; τ2, vacancy clusters at the
intersections of several interfaces; τ3, open and closed micro voids in particles or intersections of
several interfaces.
Electrochemical performance of p-Co/CF for HzOR
The HzOR performances of the as-prepared catalysts were evaluated using a typical threeelectrode configuration with a Hg/HgO electrode as the reference electrode and a graphite rod as the
counter electrode in 1.0 M KOH with 0.05 M N2H4 mixed electrolyte. Linear sweep voltammetry
(LSV) curves (Fig. 3a) were conducted to evaluate the electrocatalytic activity toward HzOR and the
corresponding Tafel plots were presented in Fig. 3b. Cu foam as the substrate shows negligible
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Positron annihilation lifetime spectra (PALS) can supply direct evidence for the DOI:
dimension
and
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HzOR (onset at ~ 0.12 V vs. RHE). Co(OH)F/CF possesses a higher catalytic activity than Pt/C in
terms of lower overpotential (onset at ~ -0.06 V vs. RHE) and a smaller Tafel slope (15.87 vs. 17.78
mV dec-1). Remarkably, after the plasma treatment, the p-Co/CF exhibits obviously improved
catalytic activity. The onset potential shifts to a very negative value of -0.15 V and the anodic current
density rapidly raises to sub-ampere per cm2 as the overpotential increases. A very small Tafel slope
of 8.83 mV dec-1 was obtained as displayed in Fig. 3b, indicating the fast HzOR kinetics on pCo/CF. For comparison, conventional Co np are barely active for catalyzing the HzOR and only
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marginal improvement of the catalytic activity was observed after plasma treatment (denoted as p-Co
np). The superior performance of p-Co/CF implies that converting high-valance Co compounds to
metallic Co by H2 plasma treatment is the key to generate catalytic active sites for HzOR.
The HzOR activities of p-Co/CF electrode in electrolytes with different hydrazine concentrations
were compared in Fig. 3c. The LSV curves recorded in different concentrations of electrolyte
overlapped at low overpotential (-0.15 ~ -0.075 V), indicating the same reaction dynamics under
small current densities regardless of the hydrazine concentration. However, at high overpotential, the
p-Co/CF electrodes in electrolytes with higher hydrazine concentration exhibited larger current
density. Thus, the mass-transfer process limits the HzOR at high current density exceeding 200 mA
cm-2. Moreover, the comparable LSV curves in 0.2 M and 0.4 M hydrazine imply that the masstransfer limit can be largely overcome at moderate hydrazine concentration due to the open pore
structure of p-Co/CF.
To assess the intrinsic catalytic activity of the catalysts, the ECSA (electrochemical active surface
area) of different catalysts were obtained by measuring the Cdl (double layer capacitance) (Fig. 3d,
Supplementary Fig. 7). The Cdl of p-Co/CF (108.87 mF cm-2) is much higher than that of
Co(OH)F/CF (42.06 mF cm-2). Compared with catalysts in powder forms, the array structure exhibits
notable advantage in term of the ECSA. Moreover, p-Co np (H2 plasma treated Co np) also displays
a higher Cdl (2.57 mF cm-2) than Co np (0.89 mF cm-2), probably due to the increased exposure of
catalytic surface to electrolyte by H2 plasma treatment. The ECSA-normalized current density is
displayed in Supplementary Fig. 8. Among these catalysts, p-Co/CF exhibits a superior intrinsic
electrocatalytic performance towards HzOR. Thus, H2 plasma-induced vacancies and array structure
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catalytic activity, while noble metal catalyst such as Pt/C exhibits a relatively large overpotential
for
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are the keys to construct abundant highly active catalytic sites for HzOR.
Fig. 3 Electrochemical performance for hydrazine oxidation reaction. a, Polarization curves with
a scanning rate of 5 mV s-1 in 1 M KOH with 0.05 M N2H4. b, Corresponding Tafel plots. c, LSV
curves in different concentrations of N2H4. d, Capacitive current densities as a function of scan rate
to a linear regression. e, Chronopotentiometry test for p-Co/CF in 1 M KOH with 0.1 M N2H4 at 100
mA cm-2. f, Comparison of HzOR onset potential (potential at 1 mA cm-1) and Tafel slope with other
HzOR catalysts. The corresponding references of these catalysts are shown in Supplementary Table
2.
The long-term stability of p-Co/CF was further evaluated by the chronopotentiometry test at a
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the consumed hydrazine. After 29 h of measurement, the potential slightly dropped by 15 mV,
indicating the decent stability of the electrode. After long-term operation, p-Co/CF can retain the
original crystallography structure verified by XRD pattern (Supplementary Fig. 7a) as well as the
XANES and EXAFS results (Supplementary Fig. 9b, c). The needle-like structures are well retained
while some cubic particles appear (Supplementary Fig. 10), which might be related to a dissolutiondeposition process of Co and probably leads to the slight deactivation of catalyst. XPS spectra of pCo/CF after 1 and 20 h chronopotentiometry test (Supplementary Fig. 11a, b) reveal the shift of the
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Co 2p3/2 and the main O 1s peaks, which suggests minor oxidation of Co on the surface to form
Co(OH)x/CoOx during the HzOR28, 29. Notably, F complete disappears after only 1 h of electrolysis
(Supplementary Fig. 11c), which indicates that the residual F in the p-Co/CF does not contribute to
the high catalytic activity.
Compared with the reported HzOR catalysts (Fig. 3f and Supplementary Table 2), p-Co/CF
possesses the lowest Tafel slope (8.83 mV dec-1), advantageous onset potential (-0.15 V) and decent
stability (29 h) under 100 mA cm-2. In addition, the synthesis of self-supporting p-Co/CF is facile
and relatively energy-saving. Due to these distinguishing features, the p-Co/CF could serve as a
promising electrode for direct hydrazine fuel cells.
Mechanistic insights of low-coordinated Co for HzOR
We further carried out density function theory (DFT) calculations to illustrate the HzOR process
on Co-based electrocatalysts. Two pathways include alternating (path 1) and distal (path 2)
configurations of adsorbed N2H2* were considered. Cobalt (111) surface with two-atom vacancies
was built to simulate the p-Co and denoted as d-Co(111). For comparison, simulations of perfect
Co(111) and Pt(111) were also performed.
The complete free energy diagrams of the HzOR were calculated to investigate the effect of Co
vacancies on the reaction kinetics (Fig. 4a and Supplementary Fig. 12). The free-energy changes of
the elementary reactions are displayed in Supplementary Table 3 and Table 4. The results show that
the d-Co(111) prefers the alternating pathway, Pt(111) prefers the distal pathway, and Co(111) has
the same overpotential for both pathways. The dehydrogenation of N2H3* is the potential11
Energy & Environmental Science Accepted Manuscript
current density of 100 mA cm-2 (Fig. 3e). The electrolyte was refreshed twice during DOI:
the10.1039/D2EE01463G
test to refill
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respectively. For Co(111), the dehydrogenation of N2H3* and N2H2* have the same energy barrier of
0.44 eV. Notably, the adsorbed N2H3* and N2H2* are very stable at equilibrium potentials,
suggesting that the HzOR kinetics is mostly determined by the dehydrogenation of N2H3*. Thus, dCo(111) with the lowest energy barrier of the dehydrogenation of N2H3* is the most active catalyst
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toward HzOR.
Fig. 4 DFT calculations and proposed HzOR mechanism. a, Gibbs free energy diagram of HzOR
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determining step on both d-Co(111) and Pt(111) with energy barriers of 0.23DOI:
and
0.48 eV,
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and experiment measurement. c, The proposed HzOR mechanism on d-Co (purple, cobalt; blue,
nitrogen; white, hydrogen). d, DOS diagram of hydrazine molecule. e, Projected density of states
(PDOS) for d orbitals of d-Co and total DOS for N2H4 molecule. f, Detailed PDOS for N pz orbital
and Co dxz orbital before and after adsorption.
The comparison of theoretical and experimental onset potential is shown in Fig. 4b, demonstrating
the reliability of our simulation. The theoretical onset potential of d-Co(111) is -0.1 V (vs. RHE),
lower than that of both Co(111) and Pt(111), which is consistent with experimental results. The
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difference in onset potential comes from the different energy barriers of dehydrogenation of N2H3*
on the surfaces.
Comparing the configurations of intermediates on Co(111) (Supplementary Fig. 13) and dCo(111) (Fig. 4c), the adsorption configurations of N2H3* are obviously different. N2H3* bonds to
Co(111) with three chemical bonds while to d-Co(111) with only two chemical bonds, resulting in a
stronger interaction of N2H3* on Co(111) than on d-Co(111) (Supplementary Table 5). Considering
that the dehydrogenation of N2H3* is the potential-determining step of HzOR, the stronger
interaction of N2H3* with catalytic sites will lead to a higher energy barrier to the next reaction step.
In the case of d-Co(111), the two-atom vacancy on Co could regulate the adsorption configuration of
N2H3*, hence to reduce the reaction barrier.
The density of states (DOS) was performed to reveal the bonding interactions between the
hydrazine molecule and catalysts. The six peaks of isolated N2H4 molecule correspond to the
molecular orbitals (MO) comprising of ơ1, ơ1*, ơ2, ơ3, ơ4 and n orbitals (Fig. 4d and Supplementary
Fig. 14c). The highest occupied molecular orbital (HOMO) of hydrazine is a non-bonding orbital
with two lone pair electrons symbolled as n orbital. The d-band center (εd)21 of d-Co(111) is higher
than those of Co(111) and Pt(111), which confirms the superior catalytic activity of d-Co(111) (Fig.
4e and Supplementary Fig. 14a, b), in line with the d-band model for low-coordinated metals and
experimental results22,
26.
The signal of chemisorption can be seen from the delocalization of n
molecular orbital of N2H4 and its mixing with Co 3d states (Fig. 4e), which activates the N-H
bonding for subsequent dehydrogenation. The projected density of states (PDOS) (Fig. 4f) show the
disappearance of the HOMO (n) of N2H4 whereas a new state appears in the unoccupied region and
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at equilibrium potential. b, Comparison of onset potential (vs. RHE) between theoretical
calculation
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The differential charge density in Supplementary Fig. 15 confirms the formation of chemical bonds.
Overall, DFT calculations suggest that the low-coordinated Co atoms are catalytic active sites for
HzOR. The strong adsorption of key intermediate, namely N2H3*, on the d-Co(111) with two-atom
vacancies is reduced in comparison with the situation on pristine Co(111). As a result, the
subsequent dehydrogenation of N2H3*, which is the potential-determining step for HzOR, could be
notable accelerated.
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Assembly and Performance of DHzFC
Direct hydrazine fuel cell (DHzFC) has been considered as one of the most promising lowtemperature fuel cells because of the convenient storage and transport of the liquid fuel, high
theoretical open-circuit voltage (OCV) of 1.56 V (oxygen as oxidant) and high energy density of
hydrazine hydrate (4269 Wh/L)2. Schematic illustration of the DHzFC is shown in Fig. 5a.
Comparing with the well-developed hydrogen fuel cell technology, DHzFC possesses certain
advantages in terms of the easy transportation/refilling and high volumetric energy density of
hydrazine fuel, as well as the high OCV (Fig. 5b).
The potential use of the p-Co/CF electrode for DHzFC was demonstrated in a homemade DHzFC
composed of p-Co/CF, anion exchange membrane and commercial Pt/C gas-diffusion electrode
(Supplementary Fig. 16). For comparison, we also employed Pt/C coated Cu foam (PtC/CF) and
Co(OH)F/CF electrodes as anodes for HzOR. The DHzFC assembled with p-Co/CF exhibited a high
OCV of 1.1 V and a maximal power density (Pmax) of 186 mW cm-2 was achieved at a current
density of 357 mA cm-2 (Fig. 5c). In contrast, the control DHzFCs using PtC/CF and Co(OH)F/CF as
the anode only exhibited a low OCV of 0.49 and 0.93 V, and delivered a peak power of only 45 and
29 mW cm-2, respectively. Owing to the high-power output at ambient condition and the easy storage
of fuel, the DHzFC would also be suitable to power small portable devices. For example, Fig. 5d
shows a small size prototype DHzFC powering a mini electric fan. After 7 h of operation at a
constant current density of 120 mA cm-2, the corresponding working potential of the prototype
DHzFC dropped by 0.13 V (Fig. 5e). The voltage loss might be attributed to the
deactivation/detachment of catalysts or accumulation of gas bubbles in the electrodes, which demand
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Energy & Environmental Science Accepted Manuscript
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hybridizes with the Co dxz states, indicating the bonding between N2H4 and the d-Co(111)
surface.
further optimization of the electrocatalysts and device configuration.
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Compared with literature reports of DHzFCs operated under similar ambient conditions (Fig. 5f),
the DHzFC assembled with p-Co/CF electrode demonstrates excellent performance in terms of OCV
and maximal power output. Specifically, the high Pmax of 186 mW cm-2 achieved by our prototype
DHzFC is about 3~4 times that of previous reported DHzFCs at ambient temperature, and even
higher than that of a DHzFC operated at an elevated temperature and with concentrated hydrazine
fuel. We anticipate further performance improvement by accelerating the reaction kinetics and
transport dynamics (e.g., increasing temperature/backpressure/fuel concentration, etc.), which would
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push the development of practical DHzFCs for portable and vehicular applications.
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hydrazine fuel cells (DHzFC). b, Comparison of hydrogen and direct hydrazine fuel cells2, 14, 39. c,
Polarization and power density curves of DHzFCs using different anodes. d, Optical images of the
homemade DHzFC using p-Co/CF and a fan powered by the DHzFC. e, Stability test of DHzFC. f,
Performance comparison of our DHzFC with literature reports (the corresponding references are
summarized in Supplementary Table 6).
Conclusions
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In this work, we report an integrated electrode based on low-coordinated Co arrays with
remarkable electrocatalytic activity for hydrazine oxidation reaction (HzOR). Few-atom vacancies
are induced by H2 plasma reduction of Co(OH)F into metallic Co, creating abundant low-coordinated
surface Co atoms. The as-prepared p-Co/CF electrode exhibits a low onset potential of -0.15 V (vs.
RHE) and a small Tafel slope of 8.83 mV dec-1 in 0.05 M N2H4/1 M KOH electrolyte, which are
superior to most reported HzOR electrocatalysts. When assembled into DHzFC, the device delivers a
high open circuit voltage of 1.1 V and a maximal power density of 186 mW cm-2 at ambient
temperature. Combining explicit experimental and computational analyses, we demonstrate that the
low-coordinated Co atoms are the catalytic active sites for HzOR while the integrated array electrode
maximizes the active surface area. Moreover, our DFT calculations elucidates the HzOR process on
metallic Co catalysts and suggests that the weakened adsorption of N2H3* intermediate on lowcoordinated Co sites is accounted for the improved catalytic activity. This work reveals the
mechanism of hydrazine electrooxidation on defective Co surface and provides guidance for
designing high-performance non-noble metal electrocatalysts toward viable direct hydrazine fuel
cells.
Methods
Materials.
Cobalt
nitrate
hexahydrate
(Co(NO3)2·6H2O),
Cobalt
chloride
hexahydrate
(CoCl2·6H2O), urea and ammonium fluoride (NH4F) were purchased from Aladdin Industrial
Corporation. Sodium borohydride (NaBH4) and trisodium citrate dihydrate (C6H5Na3O7) were
purchased from China National Medicines Corporation Ltd. Cu foam (CF) with a thickness of 1.0
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Fig. 5 Electrochemical performance of direct hydrazine fuel cells. a, Scheme
of a direct
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purchased from Shanghai He Sen Electric Co., Ltd. Commercial 20 wt% Pt/C (obtained from
Johnson Matthey) was used for comparison in three-electrode system. All of the chemicals were used
directly without further purification.
Synthesis of p-Co/CF and Co nanoparticles. Co(OH)F/CF was synthesized by hydrothermal
method. A Cu foam (3 × 3 cm2) was cleaned in deionized water and ethanol with ultrasonication, and
dried overnight in vacuum oven. The cleaned Cu foam was immersed into a 30 ml aqueous solution
containing 5 mmol Co(NO3)2·6H2O, 5mmol urea and 200 mg NH4F. The sample was transferred to a
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100 mL Teflon autoclave and then heated at 120 °C for 12 h. After naturally cooling to room
temperature, the as-obtained Co(OH)F/CF sample was washed with deionized water and ethanol for
several times, and dried at 60 °C in a vacuum oven. The plasma reduction is performed in a
conventional low-temperature RF plasma device. A piece of Co(OH)F/CF sample was fixed on a
glass slide and reduced by H2 plasma at 400W RF power, 40 Pa of gas pressure, and 20 min of
reduction time to obtain the p-Co/CF.
The cobalt nanoparticles (Co np) were synthesized based on the reduction of salt by borohydride36.
First, 6 mmol of CoCl2·6H2O and 500 mg of C6H5Na3O7 were dissolved in 14 ml DI water. Then, 20
mmol of NaBH4 ethanol solution was added in CoCl2 mixture under vigorous stirring in a drop-like
manner, at room temperature. Finally, the slurry was washed by ethanol, and deionized water and
finally washed by ethanol again to remove water. The obtained Co nanoparticles were dried at 40 ℃
for 12 h in vacuum oven.
Materials characterization. X-ray diffraction patterns were acquired by an X-ray diffractometer
(EMPYREAN PANalytical) with Cu-Kα radiation (λ=1.54 Å). The morphologies of the samples
were observed by field-emission scanning electron microscopy (SEM) (Phenom, PW-100-060) and
transmission electron microscopy (TEM, FEI Tecnai G2 F20), high-resolution transmission electron
microscopy (HRTEM), selected area electron diffraction (SAED). The surface composition and
valence state were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fischer ESCALAB
250 Xi) with Al Kα radiation. All XPS spectra were calibrated by shifting the detected adventitious
carbon C 1s peak to 284.8 eV. Positron annihilation spectroscopy (PAS) was performed on trap
based slow position beam (RGM-1/APBS-2). The X-ray absorption fine structure (XAFS)
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mm was purchased from Kunshan Guangjiayuan New Material Co., Ltd. Carbon paper
(CP) was
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Electrochemical measurements. The electrochemical measurements were performed with a CHI
760E electrochemistry workstation in a three-electrode system using an Hg/HgO electrode as
reference electrode and a carbon rod as counter electrode. All potentials are given versus reversible
hydrogen electrode (RHE) according to the equation: E (vs. RHE) = E (vs. Hg/HgO) + 0.0591 × pH
+ 0.098. The iR compensation was conducted at open circuit voltage with a step amplitude of 0.05 V.
During the linear sweep voltammetry, the workstation was set to compensate 85 % of the ohmic drop
automatically, and no further iR correction was performed. The stability test was performed without
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iR compensation. All nanoparticle samples were tested on carbon paper. The loadings of active
materials on Co(OH)F/CF and p-Co/CF were 8 and 4.8 mg cm-2, respectively. The loading of powder
catalysts is 1 mg cm-2 for three-electrode test.
The electrochemically active surface area (ECSA) was estimated from the electrochemical doublelayer capacitance (Cdl) according to ECSA = Cdl/Cs. The Cdl was measured from the scan-ratedependent CVs in the non-Faradaic region in 1 M KOH with scan rates of 20, 40, 50, 60 and 80 mV
s-1. Then the difference in current density between the anodic and cathodic sweeps (Δj = ja - jc) at the
middle potential of the non-Faradic region was plotted as a function of scan rates, yielding a straight
line with slope equal to Cdl. The general specific capacitance of 35 μF cm-2 was used to estimate the
ECSA40.
Direct hydrazine fuel cell (DHzFC) preparation and test. The synthesized p-Co/CF and
Co(OH)F/CF was investigated as an anode catalyst layer in membrane electrode assembly (MEA)
testing. 0.5mg cm-2 loading of 60 wt% Pt/C (JM, Hispec9100) on carbon paper (Toray, TGP-H-060,
includes a microporous layer) was used as the cathode catalyst layer. For fuel cell test, p-Co/CF
(catalyst loading: 4.8 mg cm-2) was used as the anode. The control PtC/CF anode layer was prepared
by 1 mg cm-2 of 20 wt% Pt/C loading on Cu foam. The anion exchanged membrane (AEM,
Fumasep, FAA-3-50) was immersed in 0.5~1.0 M KOH at room temperature for 24 h to exchange
anions completely before being applied in MEA. Then, the MEA was fabricated by pressing the pCo/CF, AEM, Pt/C carbon paper under 3 MPa for 3 min at 80 ℃. The DHzFC device was evaluated
by a CHI 760E electrochemistry workstation with a current booster under room temperature, with 1.5
M N2H4/6 M KOH as anode electrolyte and humidified O2 plunged into cathode side. The active cell
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measurements were carried out at 1W1B station in Beijing Synchrotron Radiation Facility
(BSRF).
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polarization mode, and each point was maintained for 1 min to obtain the steady voltage.
Computational details. All DFT calculations were performed using the projected augmented wave
(PAW)41 method implemented in the Vienna ab initio simulation package (VASP)42, 43. The KohnSham one-electron states were expanded using the plane wave basis set with a kinetic energy cutoff
of 500 eV. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional was employed in calculation44, 45. The Co(111) and Pt(111) slabs
with four atomic layers were constructed with 20 Å vacuum region. The p-Co model utilized was
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obtained by Co(111) slab with two Co atom vacancies at the top layer, denoted as defect Co(111) (dCo(111)). The Brillouin zone was sampled using 4 × 4 × 1 Gamma-center k-mesh for all the
surfaces. The van de Walls (vdW) correction by the Grimme method (DFT-D3)46 was included in the
interaction between molecules/atoms and substrates. All the structures were optimized until the total
energy was converged to 10-5 eV per atom and the force on each atom was less than 0.02 eV Å-1.
The four-electron pathway of N2H4OR might proceed through two possible mechanisms: the
alternating pathway(path1) and distal pathway (path2).
N2H4(g) + * → N2H3* + (H+ + e-)
(1)
N2H3* → N2H2* + (H+ + e-)
(2)
N2H2* → N2H* + (H+ + e-)
(3)
N2H* → * + N2(g) + (H+ + e-)
(4)
The asterisk (*) denotes the substrates, and N2H3*, N2H2*, N2H* were adsorbed intermediates.
The difference between alternating and distal pathway was the adsorption configurations of N2H2*.
HNNH* configuration was in alternating pathway while NNH2* configuration was in distal pathway.
The Gibbs free energy diagrams were obtained based on computational normal hydrogen electrode
(NHE) model developed by Nørskov et al.47. For each elementary step, ΔG was evaluated by the
following equation:
ΔG = ΔE + ΔZPE – TΔS + ΔGU + ΔGpH + ΔGfield
(5)
where ΔE, ΔZPE and ΔS were the energy of reaction, the zero-point energy contribution, and
entropy change, respectively. T was the temperature (set to 298.15 K). The bias effect on the free
energy of elementary steps involving electron transfer was assessed with the equation ΔGU = −neU,
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area of DHzFC was 1.4 ×1.4 cm2. The polarization data were recorded in theDOI:galvanostatic
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and n was the number of electrons transferred. ΔGpH was the correction of the H+ free energy, which
can be calculated through △GpH = kBT×ln10×pH. The kB was the Boltzmann constant and pH was set
to 0 (acidic medium). Consequently, the theoretical equilibrium potential for N2H4OR at pH = 0 was
-0.33 V (vs. NHE) and -0.33 V (vs. RHE) according to Nernst equation URHE = UNHE + 0.0591pH. It
should be noted that in our calculations, the total Gibbs free energy change of the four elementary
reactions on Pt(111) and d-Co(111) was -1.28 eV, which was inconsistent with the theoretical value
(-1.32 eV) (see Supplementary Table 4 and Table 5 ). Therefore, the calculated equilibrium potential
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for N2H4OR on Pt(111) and d-Co(111) was set to -0.32 V (-1.32 divided 4)48. The calculated
equilibrium potential on Co(111) was equal to the value of theoretical equilibrium potential (-0.33
V). ΔGfield is the free-energy correction resulted from the electrochemical double layer, which is
negligible in this study.
Author contributions
Qian Liu and Hao Bin Wu conceived the project. Qian Liu completed the original idea, carried out
the experiments and simulations, interpreted the results and wrote the manuscript. Xiaobin Liao
Interpreted the theoretical results. Yuanhao Tang, Jianghao Wang, Xiangzhou Lv, Xuelei Pan and
Ruihu Lu carried out the experiments. Yan Zhao supervised the theoretical simulations. Xin-Yao Yu
supervised the project. Hao Bin Wu conceived and supervised the project. All authors have
contributed a lot the scientific discussion of the project.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is supported by Zhejiang Provincial Natural Science Foundation (Grant No.
LR21E020003), National Natural Science Foundation of China (Grant No. 22005266) and “the
Fundamental Research Funds for the Central Universities” (2021FZZX001-09). The authors
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where U was the electrode applied potential relative to NHE, e was the elementary charge
transferred
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discussions regarding the PALS analysis and Shiyanjia Lab (www.shiyanjia.com) for the XPS and
TEM analysis.
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