3D self-supported porous vanadium-doped nickel nitride nanosheet arrays as efficient bifunctional electrocatalysts for urea electrolysis

Article abstract of DOI:10.1039/d0ta09473k

Electrocatalytic water splitting is an up-and-coming method to produce H₂, but it is mostly limited by the sluggish anodic oxygen evolution reaction (OER). Replacing the OER with the urea oxidation reaction (UOR) process with a low thermodynamic potential would significantly reduce energy consumption. Here we developed porous V-doped Ni₃N nanosheets arrays grown on nickel foam (labeled V-Ni₃N/NF) by a hydrothermal and subsequent nitridation process. As a bifunctional electrocatalyst, the V-Ni₃N/NF shows striking catalytic performances for the hydrogen evolution reaction (HER) and UOR, which require low potentials of ?83 mV and 1.361 V to drive a current density of 10 mA cm^?2, respectively. Furthermore, when applying V-Ni₃N/NF as both the cathode and anode for the HER and UOR, the assembled electrolyzer needs small cell voltages of 1.416 and 1.543 V to achieve 10 and 100 mA cm^?2, which are 180 and 243 mV lower than that in traditional water electrolysis, as well as displaying remarkable stability over 200 h. The excellent catalytic performances can be attributed to the self-supported porous nanosheet arrays on 3D NF, the intrinsic conductivity of V-Ni₃N, and the optimized Gibbs free energy of hydrogen adsorption after V doping. Our result suggests that this attractive material has promising applications in energy-saving H₂production and urea-related waste water treatment.

Full text of DOI:10.1039/d0ta09473k

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PAPER
3D self-supported porous vanadium-doped nickel
nitride nanosheet arrays as efficient bifunctional
electrocatalysts for urea electrolysis†
Cite this: J. Mater. Chem. A, 2021, 9,
4159
a
b
Qi Liu,a Yanan Zhou,
Mengjie Lu,d Jinle Hou,a Konggang Qu,a
*
*
Rui-Qing Li,
c
ce
*
Yachao Zhu and Olivier Fontaine
Electrocatalytic water splitting is an up-and-coming method to produce H2, but it is mostly limited by the
sluggish anodic oxygen evolution reaction (OER). Replacing the OER with the urea oxidation reaction (UOR)
process with a low thermodynamic potential would significantly reduce energy consumption. Here we
developed porous V-doped Ni3N nanosheets arrays grown on nickel foam (labeled V–Ni3N/NF) by
a hydrothermal and subsequent nitridation process. As a bifunctional electrocatalyst, the V–Ni3N/NF
shows striking catalytic performances for the hydrogen evolution reaction (HER) and UOR, which require
low potentials of ꢀ83 mV and 1.361 V to drive a current density of 10 mA cmꢀ2, respectively.
Furthermore, when applying V–Ni3N/NF as both the cathode and anode for the HER and UOR, the
assembled electrolyzer needs small cell voltages of 1.416 and 1.543 V to achieve 10 and 100 mA cmꢀ2
,
which are 180 and 243 mV lower than that in traditional water electrolysis, as well as displaying
remarkable stability over 200 h. The excellent catalytic performances can be attributed to the self-
supported porous nanosheet arrays on 3D NF, the intrinsic conductivity of V–Ni3N, and the optimized
Gibbs free energy of hydrogen adsorption after V doping. Our result suggests that this attractive material
has promising applications in energy-saving H2 production and urea-related waste water treatment.
Received 27th September 2020
Accepted 4th January 2021
DOI: 10.1039/d0ta09473k
rsc.li/materials-a
require additional energy to overcome the reaction barrier.4,5
Highly active electrocatalysts are therefore essential to improve
1 Introduction
the reaction rate and reduce the over-potentials of the HER and
OER processes. Currently, noble metals and their oxides, such
as Pt-based materials and RuO2 and IrO2, remain the state-of-
the-art electrode materials to catalyze the HER and OER,
respectively.6,7 However, the exorbitant price and scarcity mainly
restrict their widespread applications.
The growing concerns regarding energy depletion and envi-
ronmental issues have led to extensive investigation into
renewable and clean energy. Currently, H2, due to its environ-
mental efficiency and high energy density, is considered an
ideal alternative energy carrier for traditional fossil fuels.1
Compared to traditional H2 production with severe environ-
mental pollution, water electrolysis powered by renewable
energy sources (e.g. wind and solar) has attracted increasing
attention and is a promising approach for high purity H2
generation.2,3 Generally, water electrolysis includes the cathodic
hydrogen evolution reaction (HER) and anodic oxygen evolution
reaction (OER), both of which, especially the OER process,
As we know, compared to the HER, the OER process involves
multiple and complex proton-coupled electron-transfer steps,
which leads to sluggish kinetics and signicantly reduces the
energy conversion efficiency of water electrolysis.8,9 To avoid its
large anodic overpotential, replacing the OER with more readily
oxidizable species e.g. hydrazine, urea, and glycerol is an effective
and energy-saving method to generate H2. Among them, urea, as
one of the easily electro-oxidizable species, is mainly generated
from fertilizers and animal waste, and its oxidation reaction (UOR,
CO(NH2)2 + 6OHꢀ / N2 + CO2 + 5H2O + 6eꢀ) with lower ther-
modynamic potential (0.37 V) than that of the OER (1.23 V) is
a good process to replace the OER.10 Thus, urea electrolysis
(CO(NH2)2 + H2O / 3H2 + N2 + CO2) by integrating the UOR and
HER not only produces H2 more effectively, but treats urea-rich
water contamination.11,12 However, the UOR process shows slug-
gish kinetics because of its six-electron transfer process, which
certainly requires highly efficient catalysts to promote this reac-
tion. Moreover, the HER's kinetics under alkaline conditions is
aSchool of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng
252059, P. R. China. E-mail: liruiqing16@163.com
bSchool of Chemical Engineering, Sichuan University, Chengdu, 610065, P. R. China.
E-mail: yananzhou@163.com
c
´
Institute Charles Gerhardt Montpellier, UMR 5253, Universite Montpellier, CC 1502,
Montpellier, France. E-mail: yachao.scu@hotmail.com
dSino-Russian International Joint Laboratory for Clean Energy and Energy Conversion
Technology, College of Physics, Jilin University, Changchun 130012, P. R. China
eSchool of Energy Science and Engineering, Vidyasirimedhi Institute of Science and
Technology (VISTEC), Rayong, 21210, Thailand
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta09473k
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also more sluggish than that in acidic electrolytes due to addi- 5 mmol NH4F were dissolved in 40 mL water with magnetic
tional adsorption and dissociation of water molecules.13 There- stirring to form a homogeneous solution. Then, the solution
fore, it is urgent to explore earth-abundant and high-performance and a piece of NF were transferred into a 50 mL Teon-lined
bifunctional UOR and HER catalysts to realize urea electrolysis in stainless-steel autoclave. Aer a hydrothermal reaction at
ꢁ
the same electrolyte, which can avoid incompatibility and deteri- 120 C for 6 h, V-doped Ni(OH)2 was uniformly grown on the
oration and reduce the cost of synthesis.
surface of NF (V–Ni(OH)2/NF). Then, the V–Ni(OH)2/NF
Recently, transition metal- and carbon-based materials precursor was placed in the center of a quartz tube and calcined
including oxides, hydroxides, phosphides, nitrides, and suldes at 400 ꢁC for 2 h to synthesize V–NiO/NF under an N2 ow.
have been developed as advanced catalysts.14–16 Nitrides have Finally, the V–Ni3N/NF product was fabricated aer annealing
attracted great attention and are highly catalytic due to their the V–NiO/NF in an NH3 atmosphere at 400 ꢁC for 2 h. For
unique electronic structure, intrinsic metallicity, and high corro- comparison, the V-doped Ni3N/NF samples with different ratios
sion resistance.17,18 For instance, Ni3N catalysts have been applied of Ni and V were prepared by a similar process except for adding
in water electrolysis. Incorporating foreign metals, e.g. Co, Fe into different initial ratios (14 : 1 and 9 : 1) of Ni(NO3)2 and VCl3 and
Ni3N to form bimetallic nitrides catalysts is an effective way to named V–Ni3N/NF-1 and V–Ni3N/NF-2. The Ni3N/NF was ob-
enhance the catalytic activity, which optimizes the electronic tained using the same method without adding VCl3.
structure and reduces the reaction energy barriers.19–21 As one of
the earth-abundant elements, vanadium shows more valence- 2.3 Materials characterization
state diversity and it has been applied in electrochemical
The morphological structure and composition were character-
ized using a scanning electron microscope (SEM, FIB Helios G4)
operated at 10 kV and a transmission electron microscope
elds.22,23 The vanadium-doped catalytic materials including NiV
LDH, NiFeV LDH, and V–Co4N exhibited improved catalytic
activities for the OER and HER, which stems from the promoted
(TEM, FEI Tecnai-F20) equipped with an energy dispersive X-ray
catalytic dynamics by electron interactions.24–26 Nevertheless, as
(EDX) spectrometer at an acceleration voltage of 200 kV. The X-
ray diffraction (XRD) patterns were recorded using an X-ray
far as we know, the electronic structure and catalytic perfor-
mances of Ni3N with V modication are still rarely explored. On
diffractometer using Cu Ka radiation (Bruker D8AXS). The
the other hand, the catalytic activities of materials also depend on
chemical states of samples were characterized using an X-ray
photoelectron spectrometer (XPS, ESCALAB 250) with Al Ka
radiation.
the morphology and dimensionality.27,28 The uniform nanosheet
structure can expose more catalytically active sites, promote fast
diffusion of electrolyte ions, and generate gas bubbling.29,30
Furthermore, in situ growth of catalysts on a substrate can
2.4 Electrochemical measurements
increase the dispersion of catalytically active sites and avoid
All electrochemical measurements were carried out in a three-
possible mechanical shedding and catalyst degradation.31,32
electrode electrochemical cell using an electrochemical work-
Because of the above points, it is worthwhile to develop self-
station (CHI 760E) with a 1 M KOH electrolyte. The as-
supported Ni–V-based nitride nanosheet array electrodes as
synthesized electrode, Ag/AgCl, and a graphite rod were used
promising water and urea electrolysis catalysts.
as the working, reference, and counter electrodes, respectively.
Herein we developed 3D self-supported porous V-doped Ni3N
Polarization curves were obtained using linear sweep voltam-
nanosheet arrays on nickel foam (denoted as V–Ni3N/NF) via
hydrothermal and subsequent nitridation methods. Beneting
from high conductivity, self-supported porous nanosheets on NF,
metry (LSV) with a scan rate of 2 mV sꢀ1 and were corrected for
iR compensation. The electrochemical impedance spectra (EIS)
were obtained by applying an AC voltage of 10 mV amplitude
and the modied electronic structure, the V–Ni3N/NF electrode
with frequency from 100 kHz to 0.1 Hz. Measurements of elec-
displays remarkable bifunctional catalytic activities for the HER
trochemical double-layer capacitances were performed using
cyclic voltammetry between 0.20 and 0.30 V. All potentials ob-
and UOR. Notably, with the V–Ni3N/NF electrode as both the anode
and cathode, the assembled electrolyzer exhibits excellent catalytic
tained were converted to the corresponding values based on the
performance and great stability for urea electrolysis.
reversible hydrogen electrode (RHE) using the following equa-
tion: ERHE ¼ EAg/AgCl + 0.0591 ꢂ pH + 0.21 V. For the water and
2 Experimental
2.1 Chemicals and materials
urea electrolysis tests, the V–Ni3N/NF was used as both the
cathode and anode in a two-electrode electrolyzer.
Ni(NO3)2$6H2O, VCl3, and NH4F were purchased from Sino-
pharm Chemical Reagent Co. Ltd. KOH, and urea was provided
All the DFT calculations were carried out using the projector
by Adamas-beta®. Pt/C (20 wt% Pt on Vulcan XC-72R) and RuO2
were provided by Sigma-Aldrich. All reagents were utilized as
received without further treatment.
2.5 DFT calculations
augmented wave (PAW) approach to represent ion–electron
interaction, as implemented in the Vienna ab initio Simulation
Package (VASP).33–35 The generalized gradient approximation
(GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was
2.2 Sample synthesis of V–Ni3N/NF
used to describe the electronic exchange and correlation
V–Ni3N/NF was synthesized through the following process. effects.36,37 The kinetic energy termination was set to 500 eV. The
Typically, 1.9 mmol Ni(NO3)2, 0.1 mmol VCl3, 10 mmol urea and convergence criteria for force and energy were set to 10ꢀ5 eV and
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10ꢀ2 eV Aꢀ1, respectively. The Brillouin zones were sampled at 5 pattern of V–Ni3N displays ordered discrete spots and these
˚
ꢂ 5 ꢂ 1 using Monkhorst–Pack mesh.38 A vacuum region of 15 A diffraction rings are assigned to the (110), (111) and (202)
˚
was set to avoid interactions between adjacent images. To planes, further indicating the successful formation of nitride.
describe the van der Waals interaction, the semi-empirical The EDX spectrum conrms the presence of Ni, V, and N
dispersion-corrected DFT-D3 scheme proposed by Grimme elements in nitride (Fig. S2†). Furthermore, the high angle
was used.39 To understand the HER mechanism well, the Gibbs annular dark-eld scanning TEM (HAADF-STEM) and EDX
free energy of hydrogen adsorption (DGH*) was calculated using elemental mapping images (Fig. 1i) exhibit a uniform distri-
the following equation: DGH* ¼ DEH* + DEZPE ꢀ T ꢂ DSH*, where bution of Ni, V, and N elements in the product, signifying the
DEH*, DEZPE, and DSH* are the hydrogen adsorption energy, the uniform doping of the V element.
difference in zero point energy between the adsorbed hydrogen
To further analyze the surface composition and element
and hydrogen in the gas phase, and the entropy difference states of the obtained V–Ni3N/NF, XPS measurements were
between the adsorbed state and the gas phase, respectively.
performed (Fig. 2). The survey spectrum (Fig. 2a) shows the
presence of Ni, V, and N elements. The atomic ratio of Ni : V : N
elements is calculated to be 19.4 : 1.0 : 6.1 in V–Ni3N, which
matches the feed ratio of Ni and V, indicating the highly
controlled incorporation of V. In the high-resolution spectrum
of Ni 2p (Fig. 2b), the peaks located at 853.3 and 854.5 eV are
3 Results and discussion
The preparation process of 3D self-supported porous V–Ni3N
nanosheet arrays on NF is illustrated in Scheme 1. First, the NiV
hydroxide precursor (denoted as V–Ni(OH)2) with a uniform
nanosheet-like structure was directly grown on NF via a facile
hydrothermal reaction. The SEM images (Fig. 1a and b) show
that the V–Ni(OH)2 with uniform interconnected nanosheet
arrays and a smooth surface directly grows on the surface of the
NF skeleton, and the corresponding crystalline structure is ob-
tained by XRD (Fig. 1d), in which the main characteristic
diffraction peaks can be indexed to Ni(OH)2 (JCPDS no. 73-1520)
and Ni(OH)2 $ 0.75H2O (JCPDS no. 38-0715). Then, aer calci-
nation in a N2 atmosphere, the precursor was successfully
converted to V–NiO (JCPDS no. 71-1179). Moreover, it still
retains the uniform nanosheets arrays with smooth surfaces, as
presented in Fig. 1c and S1.† Aer thermal ammonolysis, the
prominent characteristic diffraction peaks of the product
correspond to Ni3N (JCPDS no. 89-5144). The morphology of V–
Ni3N/NF was characterized using a SEM and TEM. In Fig. 1e, the
V–Ni3N nanosheet arrays are well preserved. Notably, close
observation reveals that V–Ni3N nanosheets display porous
features, which may be caused by the corrosion of NH3 and is
consistent with previous reports.40 The porous nanosheet-like
structure was further conrmed using a TEM, as shown in
Fig. 1f. The high-resolution TEM (HRTEM) image of V–Ni3N
shows well lattice fringes with an interplanar distance of
0.214 nm, which is corresponds well to the (002) plane of Ni3N
(Fig. 1g). In Fig. 1h, the selected area electron diffraction (SAED)
ascribed to Ni 2p3/2, whereas the peaks at 871.2 and 872.4 eV are
41
assigned to Ni 2p1/2
.
Meanwhile, two satellite peaks at 860.0
and 877.9 eV can be observed. The V 2p spectrum shown in
Fig. 2c displays two peaks at 513.6 and 514.4 eV assigned to the
V–N and V–O bonds, respectively. The generation of V–O bonds
may stem from surface oxidation.42 The N 1s spectrum shows
the formation of Ni–N and V–N bonds (Fig. 2d). Also, the peak
located at 401.3 eV can be attributed to the N–H group, sug-
gesting the presence of abundant H species on the surface of the
catalyst, which is benecial for improving catalytic
performance.43
The electrocatalytic performances of the obtained electrodes
were analyzed in an alkaline electrolyte. Fig. 3a depicts the
linear sweep voltammetry (LSV) curves for the HER. It is found
that the commercial Pt/C has excellent catalytic HER perfor-
mance, while the Ni3N/NF displays moderate catalytic HER
activity with an overpotential of 111 mV at 10 mA cmꢀ2. It is of
importance to point out that the V–Ni3N/NF electrode (Fig. 3a
and c) shows the best catalytic HER activity with a low over-
potential of 83 mV to achieve 10 mA cmꢀ2, which surpasses that
of all other synthesized V-doped Ni3N electrodes, indicating
that the suitable introduction of V is of great importance to
enhancing the catalytic performance. Remarkably, the V–Ni3N/
NF electrode can afford large current densities of 100 and 300
mA cmꢀ2 at low overpotentials of 172 and 260 mV, respectively.
Moreover, the V–Ni3N/NF shows outstanding catalytic activity
for the HER with a low overpotential of 90 mV to reach 10 mA
cmꢀ2 in a neutral electrolyte (1 M PBS). To reveal the catalytic
mechanism for the HER, the Tafel slope obtained by the linear
tting of Tafel plots was analyzed.44,45 In Fig. 3b and c, the Tafel
slope of V–Ni3N/NF is 45 mV decꢀ1, which is smaller than that of
Ni3N/NF (67 mV decꢀ1), V–Ni3N/NF-1 (52 mV decꢀ1), and V–
Ni3N/NF-2 (76 mV decꢀ1), revealing that the V–Ni3N/NF is more
favorable for facilitating HER reaction kinetics and undergoes
the Volmer–Heyrovsky mechanism. The catalytic performances
are superior to those of most of the recently reported state-of-
the-art HER catalysts (Table S1†). To better understand the
origin of catalytic activity, the electrochemically active surface
area (ECSA) was investigated according to the electrical double-
layer capacitance tests using cyclic voltammetry (CV) methods
Scheme 1 Schematic illustration of the preparation process for the V–
Ni3N/NF electrode.
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Fig. 1 SEM images of (a and b) V–Ni(OH)2/NF and (c) V–NiO/NF. (d) XRD patterns of V–Ni(OH)2/NF, V–NiO/NF and V–Ni3N/NF. (e) SEM, (f) TEM,
and (g) HRTEM images and (h) SAED pattern of V–Ni3N/NF. (i) HAADF-STEM image and the corresponding elemental mapping images of Ni, V,
and N elements for V–Ni3N.
Ni3N/NF with the highest catalytic activity exhibits the smallest
charge transfer resistance (RCT) in these electrodes, suggesting
its rapid electron transfer for V-doped Ni3N to accelerate the
turnover of protons and adsorbed water molecules.47 Addition-
ally, the intrinsic ohmic resistance (the rst intercept along the
real axis) of V–Ni3N/NF is smaller than that of other electrodes
(Fig. 3e), indicating that the electrical conductivity of the Ni3N is
improved aer V doping. The catalytic durability is another key
parameter to evaluate the catalyst. In Fig. 3f, the chro-
noamperometry tests of V–Ni3N/NF with continuous electrolysis
at 10 mA cmꢀ2 for over 25 h exhibit excellent long-term dura-
bility in both alkaline and neutral media. The SEM images
(Fig. S4a and b†) show that the porous nanosheet-like structures
of V–Ni3N/NF are well maintained aer the stability tests
demonstrating the magnicent robustness of the porous
nanosheet array electrode in alkaline and neutral electrolytes.
Moreover, the XRD phase matches well with the initial one
Fig. 2 (a) XPS survey and high-resolution spectra of the (b) Ni 2p, (c) V
(Fig. S5†), indicating the great stability of the crystallographic
2p, and (d) N 1s of the V–Ni3N/NF electrode.
structure aer the HER stability test.
To further understand the high HER activity of V–Ni3N/NF,
DFT calculations were performed. The optimized structural
representations of Ni3N (111), V–Ni3N (111), Ni3N (002) and V–
at various scan rates (Fig. S3†). In Fig. 3d, the double-layer
capacitance value of the V–Ni3N/NF electrode is 8.0 mF cmꢀ2
,
Ni3N (002) are presented in Fig. S6a–d.† As is well known, DGH*
is a key indicator of the HER activity for a given catalytic site.48
which is larger than that of the Ni3N/NF (5.0 mF cmꢀ2), V–Ni3N/
NF-1 (6.5 mF cmꢀ2), and V–Ni3N/NF-2 (2.7 mF cmꢀ2), indicating
the highest exposure of the electrochemically active surface area
and accessible active sites.46 To further explore the reactive
kinetics, electrochemical impedance spectroscopy tests were
conducted, and the Nyquist plots are shown in Fig. 3e. The V–
A
smaller absolute value indicates a greater catalytic activity, and
the value of DGH* of a good HER catalyst should be close to 0 to
benet the subsequent H2 generation. We calculated DGH*
values based on the (111) and (002) surfaces of Ni3N and V–
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Fig. 3 (a) HER polarization curves and (b) Tafel slopes of Ni3N/NF, V–Ni3N/NF, V–Ni3N/NF-1, V–Ni3N/NF-2, and Pt/C catalysts. (c) Comparison
of the overpotentials at 10 mA cmꢀ2 and Tafel plots of these catalysts. (d) The double-layer capacitance and (e) EIS of Ni3N/NF, V–Ni3N/NF, V–
Ni3N/NF-1, and V–Ni3N/NF-2. (f) Chronoamperometric curves of V–Ni3N/NF at 10 mA cmꢀ2 in 1 M KOH and 1 M PBS electrolytes.
Ni3N, respectively. As illustrated in the calculated free energy the catalyst decreases with the 3d downshiing, which is
diagram (Fig. 4a), the DGH* values of clean Ni3N (111) and Ni3N consistent with recent reported results.49,50
(002) surfaces are ꢀ0.48 and ꢀ0.64 eV, respectively, suggesting
We further evaluated the catalytic OER performances of the
its relatively high energy and that it is adverse to hydrogen V–Ni3N/NF electrode in alkaline media. As shown in Fig. 5a, the
desorption from the surfaces of the Ni3N during the HER V–Ni3N/NF exhibits high catalytic OER activity, achieving
process. With the introduction of V, the DGH* values of V–Ni3N a current density of 10 mA cmꢀ2 at a low potential of 1.519 V,
(111) and V–Ni3N (002) are reduced and shied to ꢀ0.12 and which is smaller than the values for Ni3N/NF (1.547 V), V–Ni3N/
ꢀ0.18 eV, respectively, indicating that the introduction of V is NF-1 (1.526 V), V–Ni3N/NF-2 (1.522 V), and RuO2 (1.504 V). The
benecial for optimizing the free energy of hydrogen adsorption catalytic performance of V–Ni3N/NF exceeds that of RuO2 when
and it can regulate the hydrogen adsorption that is neither too the current density is beyond 74 mA cmꢀ2, and it delivers high
strong nor too weak on the surface, thereby enhancing the current densities of 200 and 400 mA cmꢀ2 at low potentials of
subsequent H2 generation.
1.634 and 1.677 V, respectively. Generally, the anodic OER
Furthermore, the d-band center as a descriptor for the HER process is environment friendly. However, the high over-
activity was also investigated. We calculated the density of states potential increases the energy consumption. Substituting the
(DOS) of the (111) and (002) surfaces of Ni3N and V–Ni3N, OER with other smart anodic reactions to perform hybrid water
respectively. In Fig. 4b, the d-band center values (3d) of pure electrolysis can reduce the overall energy cost. As one of the
Ni3N (111) and Ni3N (002) are ꢀ1.81 and ꢀ1.78 eV, respectively. common molecules, the urea molecule has a satisfying ther-
Aer V doping, the 3d of V–Ni3N (111) and V–Ni3N (002) shi to modynamic oxidation potential (0.37 V) and is an ideal candi-
more negative values (ꢀ1.93 and ꢀ2.02 eV), meaning that the date to energy-efficiently generate H2.51 Accordingly, the UOR
antibonding states shi to lower energy levels and become more activity of V–Ni3N/NF was evaluated in a 1 M KOH electrolyte
occupied, which leads to a decrease in the binding energy for with 0.5 M urea. As depicted in Fig. 5a, the V–Ni3N/NF exhibits
hydrogen atoms. Thus, the interaction between hydrogen and a greatly improved anodic catalytic activity, and it only requires
a low potential of 1.361 V to achieve 10 mA cmꢀ2, which is lower
than the values for Ni3N/NF (1.371 V), V–Ni3N/NF-1 (1.365 V), V–
Ni3N/NF-2 (1.382 V), and RuO2 (1.356 V). Moreover, the required
potentials decrease to 1.437 and 1.510 V at large current
densities of 200 and 400 mA cmꢀ2, respectively. In addition, the
Tafel slope of V–Ni3N/NF is lower than that of Ni3N/NF, V–Ni3N/
NF-1, and V–Ni3N/NF-2, implying its enhanced UOR reaction
kinetics (Fig. S7†). Such catalytic activities surpass those of most
recently reported non-precious metal based UOR catalysts
(Table S2†), suggesting that the V–Ni3N/NF is a promising UOR
catalyst. Then, the long-term UOR stability of V–Ni3N/NF was
evaluated by a chronoamperometry test. In Fig. 5b, the chro-
noamperometry test maintains the excellent durability at a xed
(111), Ni3N (002), and V–Ni3N (002). The Fermi level is set at zero.
Fig. 4 (a) Calculated HER free–energy diagram on the surface of Ni3N
and V–Ni3N. (b) Total electronic DOS calculated for Ni3N (111), V–Ni3N
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Fig. 6 (a) Schematic illustration of the urea electrolyzer using V–Ni3N/
NF as the bifunctional electrocatalyst for both the HER and UOR. (b)
Polarization curves of V–Ni3N/NF for urea electrolysis and water
electrolysis. (c) Long-term stability test performed at a constant
current density of 10 mA cmꢀ2
.
1 M KOH with a 0.5 M urea electrolyte (Fig. 6a). In Fig. 6b, the
urea electrolyser needs low cell voltages of 1.416 and 1.543 to
achieve 10 and 100 mA cmꢀ2, respectively, whereas traditional
water electrolysis requires higher values of 1.596 and 1.786 V at
identical current densities, suggesting the signicant advan-
tages of thermodynamics for urea electrolysis. The high cata-
lytic activity is superior to that of recently reported catalysts
(Table S3†). Furthermore, the urea electrolyzer shows excellent
long-term stability upon 200 h of continuous electrolysis at 10
mA cmꢀ2 (Fig. 6c). All the above results demonstrate that the V–
Ni3N/NF electrode is promising for large-scale urea electrolysis
for energy-efficient H2 production.
Based on the above results, the marvelous catalytic activities
of V–Ni3N/NF can be ascribed to the following factors. First, the
porous sheet arrays grown on NF not only provide a large elec-
trochemically active surface area and abundant catalytically
active sites, but also facilitate the diffusion of electrolyte ions
and the release of generated gas from the surface of the catalyst
during the reaction process. Second, the V ion doping improves
the intrinsic electrical conductivity of Ni3N, which is benecial
for facilitating electron transport during the catalytic process.
As we know, cation doping is an effective strategy to regulate the
electronic structure of materials, which is benecial for opti-
mizing the DGH*. Accordingly, the incorporation of V modies
the electronic structure of Ni3N, bringing the DGH* of V–Ni3N
closer to the thermal neutral than that of Ni3N, and provides
more catalytically active sites.55 In addition, the formed
hydroxides during the UOR process are considered to be the real
catalytically active species and are favorable for water molecule
adsorption for the UOR, and the V-doped Ni3N has lower water
dissociation energy than Ni3N, similar to previous reports.56
Finally, the self-supported electrode, due to directly integrating
the V–Ni3N on NF, avoids the use of a binder, which can block
Fig. 5 (a) Polarization curves of the OER and UOR for different cata-
lysts. (b) Chronoamperometric curve of V–Ni3N/NF at 10 mA cmꢀ2 in
1 M KOH with a 0.5 M urea electrolyte.
current density of 10 mA cmꢀ2. We further analyzed the change
of morphology and chemical composition of V–Ni3N/NF aer
the stability measurement. In Fig. S8,† the SEM image of V–
Ni3N/NF shows that the morphology of porous nanosheet arrays
is almost retained aer the long-term UOR test. The XRD
pattern of V–Ni3N/NF (Fig. S9†) shows two new crystalline
phases of NiOOH and VO(OH)2 due to the partial surface
oxidation during the UOR test. Additionally, the composition on
the surface was observed by XPS. The content of high-valent Ni
(854.5 and 872.4 eV) in the Ni 2p spectra (Fig. S10a†) increases
compared with that before the stability test, indicating the
partial oxidation to form NiOOH at high oxidation potential,
which is consistent with the XRD results and considered to be
the real surface active sites.52,53 In the V 2p spectra (Fig. S10b†),
the ratio of the V–O bond to V–N bond increases, indicating that
the vanadium ion in V–Ni3N is oxidized to the high-valence
hydroxide species during the UOR process, which is favorable
for improving the catalytic activity.54 In Fig. S10c and d,† the
peak intensity of N 1s declines further demonstrating the
surface oxidation of metal nitrides.
Based on the excellent HER and UOR performance of V–
Ni3N/NF, a two-electrode urea electrolyzer was constructed
using V–Ni3N/NF as both the anode and cathode electrodes in
4164 | J. Mater. Chem. A, 2021, 9, 4159–4166
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Journal of Materials Chemistry A
the active sites and decrease the catalytic activity. All the above 11 Z. Yu, C. Lang, M. Gao, Y. Chen, Q. Fu, Y. Duan and S. Yu,
merits contribute to enhancing the catalytic performance of V–
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4 Conclusions
In summary, porous V doped Ni3N nanosheet arrays on NF were
synthesized by a hydrothermal and subsequent nitridation
process. Beneting from the abundant catalytically active sites,
high electrical conductivity, and optimized DGH*, the V–Ni3N/
NF electrode exhibits outstanding catalytic activity for the
HER with low overpotentials of 83 and 90 mV at 10 mA cmꢀ2 in
an alkaline and neutral electrolyte, respectively, and it also
displays striking catalytic activity for the UOR. Based on the
prominent bifunctional performance of the V–Ni3N/NF elec-
trode, a two-electrode urea electrolyzer was constructed which
needs a low cell voltage of 1.416 V to afford 10 mA cmꢀ2, which
is 0.180 V smaller than that required in water electrolysis. This
work creates an avenue to rationally design and fabricate the
promising 3D self-supported transition-metal nitride electrode
for urea-assisted energy-efficient H2 production.
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Conflicts of interest
There are no conicts to declare.
23 Q. Liu, Z. Li, Y. Liu, H. Zhang, Y. Ren, C. Sun, W. Lu, Y. Zhou,
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Acknowledgements
This work was nancially supported by the Doctoral Program of
Liaocheng University (No. 318051947) and the development
project of Youth Innovation Team in Shandong Colleges and
Universities (No. 2019KJC031).
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