ABSTRACT
The kinetic process of a slow oxygen evolution reaction (OER) always
constrains the efficiency of overall water electrolysis for
H2 production. In particular, nonprecious metal
electrodes for the OER have difficulty simultaneously possessing
excellent electrocatalytic activity and stability in pH-universal media.
In this work, urea is first used as a pore-forming agent and active C/N
source to fabricate a nanoporous NiFeCoCN medium-entropy alloy (MEA) by
high-temperature sintering based on the nanoscale Kirkendall effect. The
NiFeCoCN MEA achieves an overpotential of 432 mV at a current density of
10 mA cm-2 and a lower Tafel slope of 52.4 mV
dec-1 compared to the IrO2/Ti
electrode (58.6 mV dec-1) in a 0.5 M
H2SO4 solution.
In a 1 M KOH solution,
the NiFeCoCN MEA obtains an
overpotential of 177 mV for 10 mA cm-2 and a Tafel
slope of 36.1 mV dec-1, which is better than
IrO2/Ni foam. This work proves a novel strategy to
design and prepare nanoporous MEA materials with desirable C/N species,
which provides promising prospects for the industrial production of
H2 energy.
KEYWORDS: Oxygen evolution reaction; Medium-entropy alloy;
Urea; Electrocatalytic activity; Stability
INTRODUCTION
Hydrogen energy is an important component of the future energy system,
and it is an important carrier to achieve green and low-carbon
development [1, 2]. The
oxygen evolution reaction (OER)
as an anode reaction during water decomposition is a four-electron
transfer process, which requires a higher overpotential than the
hydrogen evolution reaction (HER) in a two-electron transfer process
[3, 4]. This phenomenon severely limits the efficiency of
electrolyzed water to hydrogen. In industrial applications, the
IrO2 electrocatalyst has good electrocatalytic activity
and stability, especially in acidic solution [5, 6], but the
scarcity of iridium and extremely high cost make it difficult to apply
on a large scale in the industry of H2 production [7,
8]. Since the anode in the proton exchange membrane (PEM) water
electrolytic cell is performed in a strongly acidic environment, the
anodic electrocatalyst is usually defined as IrO2 [9,
10]. Moreover, the current electrocatalysts for the alkalious OER must
further decrease the overpotential and increase the Faraday efficiency.
Therefore, research on
nonprecious metal-based
electrocatalysts with high activity performance and long-term stability
for pH-universal OER is significant [11-14].
High-entropy alloys (HEAs) can be used as OER electrocatalysts because
of the synergistic effect among different elements in the solid
solution. However, HEAs requires all atoms to exist in similar
proportions to ensure an entropy of 1.5 R or higher, which has
fundamental limitations in optimizing the composition of
electrocatalysts [15-18]. Medium-entropy alloys (MEAs) are composed
of three or four main elements, and their entropy is smaller than that
of HEAs but higher than that of traditional alloys. Compared with HEAs,
MEAs are easier to control the composition of the alloy to maximize
catalytic activity while keeping the long-term stability, and due to the
lattice distortion and synergy effects, MEAs also has excellent
electrocatalytic activity [15, 19]. Meanwhile, the single phase and
low free enthalpy of MEAs contribute to excellent corrosion resistance
[20]. However, the application of MEAs in the field of OER
electrocatalysis has rarely been reported. Han et al. prepared
three-dimensional porous medium-entropy alloy aerogels (MEAAs) of
Ni50Co15Fe30Cu5by using a self-propagating sintering method, which simultaneously
achieved excellent methanol oxygen reaction (MOR) and HER
electrocatalytic properties [21].
Compounds of Ni, Fe and Co have higher catalytic activity than Ir in
OER, which is due to they are the first-row transition metals in the
periodic table, have lower crystal field stabilization energy and
smaller d orbitals than Ir [22-24]. The bulk Ni in the elemental
Ni-based oxide for the OER is considered the active centre of the OER,
and oxygen is released through the NiOO- precursor.
The incorporation of Fe in electrocatalysts usually enables the OER to
occur at lower overpotentials because new highly active sites are
generated on the surface [25, 26]. The incorporation of Co in
electrocatalysts can improve the conductivity of the surface
(oxy)hydroxide layer based on the synergistic effect of multiple metal
species and further promoting proton migration and charge transportation
on the catalyst surface [27, 28]. Recently, studies have shown that
the electrocatalyst with C and N elements produces higher OER activity
than the pristine one by adjusting the binding energies of the
adsorbed/desorbed intermediate
(*O, *OH and *OOH) on the active sites [29-32]. For
example, our previous work proves
that C/N/S doping in Ni-based alloy electrodes improved the OER activity
via the nitrocarburizing and sulfonitrocarburizing technology [33].
In this work, urea (CH4N2O) with
theoretical carbon and nitrogen contents of 20 at.% and 46 at.%,
respectively [34, 35], is employed as a pore-forming agent and
active C/N source to fabricate a nanoporous NiFeCoCN medium-entropy
alloy (MEA) based on the nanoscale Kirkendall effect via
high-temperature sintering The NiFeCoCN MEA exhibits excellent
electrocatalytic activity and long-term stability in pH-universal media.
EXPERIMENTAL SECTION
Preparation of bulk NiFeCoCN MEA
Iron (12.7 μm, > 99.5%), cobalt (13.7 μm, >
99.5%) and nickel (17.2 μm, > 99.7%) powders and urea
(molecular biology grade, ≥ 99.5%) were purchased from Search Bureau
Chemicals (Shanghai, China). First, Ni, Fe and Co powders were weighed
with an atomic percent of 56
at.%, 33 at.%, 11 at.%, then, uniformly ground urea powders were
added in different mass percentages (0-30 wt.%). For comparison, pure
Ni, pure Fe, pure Co, NiFe, NiCo, FeCo,
NiFeCo, NiFeCoCN, NiFeCoCN-1 and
NiFeCoCN-2 were fabricated, and all as-prepared samples were described
in Table S1. According to the Linear Sweep Voltammetry (LSV) data of all
as-prepared samples (see Figure S1), the optimal composition of NiFeCoCN
was identified for the preparation of nanoporous compound electrodes. A
planetary ball mill (Nanjing University Instrument Factory XQM-2A) was
used to mix all metal and urea at a rotational speed of 260 r/min for 6
hours, and ball grinding beads were not placed in the ball grinding tank
to avoid element pollution. The mixed powder was pressed into a
cylindrical compact of φ12 mm × 6 mm under 580 MPa. The tube furnace
(Hefei Kejing Materials Technology Corp.,OTF-1200X) was employed to
sinter the cylindrical compact at a high temperature of 900 °C for 1
hour. Argon gas was poured into evacuate the air in the tube for 20
minutes before sintering. The entire sintering process was performed in
a high-purity argon atmosphere.
Materials
characterizations
The phase analysis of the
as-prepared samples was performed using XRD (Bruker D8 ADVANCE). The
surface morphology, microstructure and element distribution of the
electrode were determined by FESEM (FEI Nova Nano SEM450). Transmission
electron microscopy (TEM) attached with an energy dispersive X-ray
spectrometer (EDS), high-resolution TEM (HRTEM, FEI Talos F200X)
equipped with selected area electron diffraction (SAED) were used to
verify the crystal structure and element distribution. XPS (KRATOS
ANALYTICAL LTD Axis Ultra DLD) was used to further determine the change
in element valence state before and after the electrochemical ageing of
the electrode. To determine the specific surface area and average pore
size, BET (Micromeritics ASAP 2460) was tested by using
N2 as the adsorbed gas. An infrared C-S analyzer (LECO
CS-844) and O-N-H joint analyzer (LECO ONH-836) were used to quantify
the contents of C and N in the sample, respectively.
Electrochemical measurements
Using a standard three-electrode system at an electrochemical
workstation (CHI-660C, Shanghai CHI Instruments, Inc.) to finished all
electrochemical tests in this work and electrolyte is 0.5 M
H2SO4 and 1.0 M KOH. The as-prepared
samples were connected with copper wires and subsequently sealed to
become working electrodes (1 cm-2). A platinum sheet
and a graphite rod were used as auxiliary electrodes in the acidic
solution and alkalinous solution, respectively. Two calibrated saturated
calomel electrodes (SCEs) were used as reference electrodes under
different solutions of 1 M KOH [33] and 0.5 M
H2SO4 [36], respectively. The
polarization curve of the OER was determined by linear sweep voltammetry
(LSV) with a scan rate of 1 mV s-1. Tafel slopes withiRs correction were obtained by the LSV curves,
where Rs is the solution resistance obtained by
EIS. Electrochemical impedance spectroscopy (EIS) was measured at 0.01
Hz-100 kHz with an amplitude of 5 mV. The ECSA was obtained by CV
measurements of the Cdl capacitance at
non-Faradaic potentials and different scan rates of 10-100 mV
s-1 in a 1 M KOH solution, 100 to 1000 mV
s-1 in a 0.5 M H2SO4solution. Electrode stability was measured at a constant step current
density of 10, 50, 100 and 200 mA cm-2. Long-term
stability for 28 hours was achieved using a two-electrode cell at a
current density of 100 mA cm-2 in 0.5 M
H2SO4 solution. Faraday efficiency
formula placed in supplementary materials.
DFT calculations
The data of density functional theory (DFT) are calculated by Vienna ab
initio simulation package (VASP). The generalized gradient approximation
(GGA) is used to calculate the exchange correlation energy in the
Perdew-Burke-Ernzerhof (PBE) functional framework, and the projector
augmented wave (PAW) method is used to describe the electron-ion
interaction. The energy cutoff for the plane wave expansions is set to
600 eV, while the energy and force for the convergence threshold are set
to 10-5 eV and 0.03 eV Å-1,
respectively. A vacuum space of 10 Å is increased to avoid the
interaction between periodic images generated during the calculation of
the flat plate model. In addition, the dipolar correction was adopted
for slab models with the symmetrization switching off. The brillouin
zone was sampled with 2 × 2 × 1 k-points.
Both EDS and XPS data (see Table S2 and Table S3) show that Ni in the
as-prepared electrodes will dissolve in 0.5 M
H2SO4 solution, so the
γ-Fe2O3 after OER ageing is selected as
the main model. Initially, constructing a (110) slab model of
γ-Fe2O3 consisting of four layers to
facilitate comparison. To conform with XPS data, randomly substituted
some Fe atoms in the structure with Co and Ni atoms while maintaining
the appropriate ratios of Fe, Co, and Ni atoms. Subsequently, employing
the Ovito method in the Atomsk software to amorphize the first layer of
the slab, resulting in an amorphous model.
RESULTS AND DISCUSSION
Figure 1a shows the electrode fabrication process of nanoporous NiFeCoCN
MEAs with different urea contents. Interconnected nanopores and nanogaps
are formed based on the nanoscale Kirkendall effect due to the different
diffusion rate of atoms/vacancies between the diffusion couples (e.g.,
Ni/Co, Ni/Fe and Fe/Co) during the high-temperature sintering process
[36, 37]. Urea is first isomerized into isocyanic acid (HNCO) during
the heating process via a boiling reaction at 380 °C-400 °C. When the
temperature increases to 900 °C, urea volatilizes to form pores, and
part of the decomposed active C and N form solid solutions with Ni, Fe
and Co [38-40]. Figure 1b shows the X-ray diffraction (XRD) curves
of NiFeCoCN. The three peaks of NiFeCoCN at 44.3°, 51.5° and 75.8°
correspond to the (111), (200), and (220) facets of the γ-Ni solid
solution of the face-centred cubic (FCC) structure, respectively. This
consequence is consistent with the MEA composed of NiFeCo elements
analysed by XRD in other papers [41, 42]. The diffraction peaks of
NiFeCo, NiFeCoCN, NiFeCoCN-1 and NiFeCoCN-2 were basically unchanged
(see Figure S2), which indicates the urea content had little effect on
the MEA structure. However, compared with the standard PDF card
(PDF#04-0850) of pure Ni, the diffraction peaks of the NiFeCoCN exhibit
an obvious negative shift, which should be ascribed to the incorporation
of Fe, Co, C and N in the γ-Ni solid solution. Hence, nanoporous
NiFeCoCN with a single γ-Ni solid solution structure is confirmed in
combination with the latter TEM analysis in Figure 1e.
Figures 1c and d show the FESEM images of NiFeCoCN. As shown in the
figures, NiFeCoCN has an interconnected porous structure with a
nanosheet morphology of approximately 10-20 nm (see Figure S3). Figure
S4 and Table S4 show that NiFeCoCN has a BET surface area of 0.63
m2 g-1 with an average pore diameter
of 10.21 nm. This result indicates that urea plays a critical role in
the formation of nanoporous structures of NiFeCoCN and favourably forms
abundant active sites for OER. As shown in Figure 1e, the 0.211 nm and
0.182 nm interplanar spacings of NiFeCoCN correspond to the (111) and
(200) facets of the γ-Ni solid solution, respectively. The selective
diffraction pattern (SAED) also shows the (111), (200), and (220) facets
of the γ-Ni solid solution, which is consistent with the XRD analysis.
Figure 1f is the HRTEM taken from Figure 1e and clearly shows that the
crystal lattice has many defects. Figure 1g is obtained by performing an
inverse fast fourier transformation (IFFT) on Figure 1f. As shown in the
figure, the dislocations can also demonstrate lattice distortion occured
in NiFeCoCN. The high-angle annular dark-field scanning-transmission
electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX)
elemental mappings in Figures 1h-n show that Fe, Co, Ni and C, N, O
atoms are uniformly dispersed in the nanoporous NiFeCoCN bulk. The
atomic percentages of C and N in NiFeCoCN are 1.1 at.% and 0.25 at.%,
respectively (see Table S5). These results prove the incorporation of
C/N in the NiFeCoCN bulk and show that urea can be used as a C/N source.
Based on the above analysis, NiFeCoCN is indeed a MEA with the
nonmetallic incorporation of C/N.