NiRh nanosponges with highly efficient electrocatalytic performance for hydrogen evolution reaction

Article abstract of DOI:10.1016/j.jallcom.2019.03.003

The development of Ni-based nanoalloys has been extensively explored due to their low cost, high activity, and durability in the hydrogen evolution reaction (HER). Structural design of Ni-based nanoalloys is a promising approach to attain excellent electrocatalytic activity as well as stability of these catalysts. This study reports a facile method to prepare Ni_xRh_y nanosponges while using NaBH₄ as a reducing agent without any surfactant. The ratio of Ni and Rh has been changed to obtain various Ni_xRh_y electrocatalysts (Ni, Ni₃Rh₁, Ni₁Rh₁, Ni₁Rh₃, and Rh), respectively. Among the prepared catalysts, Ni₁Rh₃ electrocatalyst exhibits a high electrocatalytic activity with an overpotential of 48 mV at a current density of ?10 mA cm^?2 and a superior stability even after 2000 cycles of CV in 0.5 M H₂SO₄ electrolyte. In addition, the Ni₁Rh₃ electrocatalyst shows good electrocatalytic activity in 1.0 M KOH electrolyte compared to commercial Pt/C (20 wt% of Pt) (c-Pt/C). Moreover, the overall water splitting system of the Ni₁Rh₃ cathode and the commercial RuO₂ anode is evaluated, not only exhibiting excellent performance with a potential of 1.52 V at a current density of 10 mA cm^?2, but also proving very stable operation for over 100,000 s in 1.0 M KOH electrolyte.

Full text of DOI:10.1016/j.jallcom.2019.03.003

Accepted Manuscript
NiRh nanosponges with highly efficient electrocatalytic performance for hydrogen
evolution reaction
Ngoc-Anh Nguyen, Van-Toan Nguyen, Sangho Shin, Ho-Suk Choi
PII:
S0925-8388(19)30807-2
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.003
JALCOM 49767
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 21 December 2018
Revised Date: 24 February 2019
Accepted Date: 1 March 2019
Please cite this article as: N.-A. Nguyen, V.-T. Nguyen, S. Shin, H.-S. Choi, NiRh nanosponges with
highly efficient electrocatalytic performance for hydrogen evolution reaction, Journal of Alloys and
Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.003.
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ACCEPTED MANUSCRIPT
Graphical abstract
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NiRh Nanosponges with Highly Efficient Electrocatalytic Performance for
Hydrogen Evolution Reaction
Ngoc-Anh Nguyen, Van-Toan Nguyen, Sangho Shin, Ho-Suk Choi*
Department of Chemical Engineering and Applied Chemistry, College of Engineering,
Chungnam National University, 99 Daehak-ro, Yuseong-Gu, Daejeon, 34134, Republic of
Korea
Corresponding email address: * hchoi@cnu.ac.kr
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Abstract
The development of Ni-based nanoalloys has been extensively explored due to their low cost,
high activity, and durability in the hydrogen evolution reaction (HER). Structural design of
Ni-based nanoalloys is a promising approach to attain excellent electrocatalytic activity as
well as stability of these catalysts. This study reports a facile method to prepare NixRhy
nanosponges while using NaBH4 as a reducing agent without any surfactant. The ratio of Ni
and Rh has been changed to obtain various NixRhy electrocatalysts (Ni, Ni3Rh1, Ni1Rh1,
Ni1Rh3, and Rh), respectively. Among the prepared catalysts, Ni1Rh3 electrocatalyst exhibits
a high electrocatalytic activity with an overpotential of 48 mV at a current density of -10
mA.cm-2 and a superior stability even after 2000 cycles of CV in 0.5 M H2SO4 electrolyte. In
addition, the Ni1Rh3 electrocatalyst shows good electrocatalytic activity in 1.0 M KOH
electrolyte compared to commercial Pt/C (20 wt% of Pt) (c-Pt/C). Moreover, the overall
water splitting system of the Ni1Rh3 cathode and the commercial RuO2 anode is evaluated,
not only exhibiting excellent performance with a potential of 1.52 V at a current density of 10
mA.cm-2, but also proving very stable operation for over 100,000 seconds in 1.0 M KOH
electrolyte.
Keywords: NiRh alloys, Nanosponges, Electrocatalyst, Hydrogen Evolution Reaction,
Overall water splitting.
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1. Introduction
Hydrogen gas (H2) is a clean and efficient energy carrier for fuel cells and a promising
candidate to overcome future fossil fuel crisis, air pollution, and renewable resources
problems [1, 2]. Among the various methods of producing hydrogen, the electrolysis of water,
which is composed of two parts, a hydrogen evolution reaction (HER) and an oxygen
evolution reaction (OER), is considered a promising method of producing H2 [3, 4]. To
improve the HER performance, it is important to develop catalysts with high efficiency and
long-term stability. Platinum (Pt) and platinum-based alloys show the most efficient and
stable activity in both acidic and basic electrolytes, but high cost and unstable supply limit
their large-scale use [4]. Therefore, there is an urgent need to develop a non-platinum catalyst
applicable to the HER. Among the various noble metals, rhodium (Rh) is known as a good
catalyst for the HER. However, Rh, like Pt, is difficult to use as a HER catalyst because of its
high cost and unstable supply [5]. Therefore, various studies have been attempted to develop
an Rh-based catalyst capable of exhibiting high HER performance while having a low Rh
content [6-8]. Recently, Huang et al. investigated the application of Rh-Ag and Rh-Ni alloys
to the hydrolysis of KBH4 for hydrogen production [5, 9]. As a result, it has been reported
that the activity of Rh-based catalysts can be enhanced by controlling the electronic structure
of Rh through alloying of Rh and other metals. It should be noted, however, that these alloy
catalysts have not been applied to the hydrogen evolution reaction through water electrolysis.
Recently, Ni-based alloys, one of the transition metal alloys, have been extensively studied as
catalysts having low cost and stability for the HER. The previously studied Ni-based alloys
and their HER performance are summarized in Table 1 [10-23]. These studies showed
improved HER performance in either the acidic or the alkaline electrolyte, but did not
demonstrate HER performance in both acidic and alkaline electrolytes or in the overall water-
splitting system. In contrast to noble metals, transition metals exhibit very poor
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electrocatalytic performance and fast attenuation in long-term stability tests at HER.
Combinations of two or more metals, such as binary or ternary metals, have been explored to
improve the HER performance [13, 19]. In addition, various modified Ni-based alloys such as
carbon-wrapped, phosphor-doped, or sulfur-doped hybrids have been investigated for
enhancing the HER performance [24-28]. Among the several strategies to improve the HER
performance, the most efficient way is to expose more active sites by increasing the surface
area of the catalyst. Among strategies to improve catalytic activity while using the same
material, a strategy of synthesizing a three-dimensional (3D) nanosponge structure is
attractive. Catalysts with a 3D nanosponge structure not only provide a large surface area for
the reaction, but also facilitate mass transport and electron transfer on the electrode, thereby
enhancing catalytic activity [29, 30]. In our previous work, we have developed a 3D
nanosponge PtNi alloys that demonstrate excellent oxygen reduction reaction (ORR)
performance as a result of long-term structural and chemical stability due to excellent
porosity, flexibility, specific surface area, and interconnectivity. [31]. Although surfactants
can be used to synthesize 3D alloy nanowire network structures, they can degrade the
catalytic activity because they are difficult to separate after synthesis [32-34]. Thus, the
design of obtaining a 3D nanosponge structure without a surfactant in the catalyst is very
important. To the best of our knowledge, there are no reports of NiRh electrocatalysts for
HER applications in water electrolysis. Therefore, it is very desirable and necessary to obtain
3D NiRh nanosponges with various compositions and to examine their HER performance.
This study presents a facile method for synthesizing NiRh nanosponges using NaBH4 as a
reducing agent, without using a surfactant. The structural and chemical properties of the
obtained NixRhy nanosponges are investigated and the HER performance of these materials is
tested in 0.5 M H2SO4 and 1.0 M KOH electrolytes. In addition, the performance and the
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stability of the full system using the Ni1Rh3 cathode and the commercial RuO2 anode were
evaluated in 1.0 M KOH for the overall water splitting process.
2. Experimental
2.1 Materials
Nickel (II) chloride hexahydrate (NiCl2.6H2O, 99,999% trace metal basic), Rhodium(III)
chloride(RhCl3.xH2O, ≥99.9% trace metals basis), Ruthenium (IV) oxide (RuO2, 99.9% trace
metals basis, and sodium borohydride (NaBH4 powder, ≥98.0%) were purchased from Sigma
Aldrich (USA). A commercial Pt/C catalyst (20 wt% of Pt) (c-Pt/C) was obtained from Alfa
Aesar (USA). The Nafion D521 solution (5 wt%) was bought from Dupont (USA). Ni foam
(≥99.5%) was supplied from Invisible Inc., Korea.
2.2 Synthesis of NixRhy
The synthesis process of NixRhy is fully described in a scheme, which was given in Figure
1(a). For preparing Ni3Rh1, the following steps have been conducted. First, 10 mL of 0.1 M
NaBH4 is put in a three-neck flask. A mixture of 2 mL of 0.1 M NiCl2 and RhCl3 (mole ratio
of Ni:Rh to be 3:1) is then quickly added to 10 mL of 0.1 M NaBH4 solution above. After a
15 minute reaction, a black product is obtained through centrifuging and washing it three
times in distilled water and three times in ethanol. This black product is then dried in an oven
at a temperature of 80ºC for 12 hours to obtain the final product. To synthesize other samples
such as Ni, Rh, Ni1Rh1, and Ni1Rh3, the same methods are applied except using different
mole ratios of Ni and Rh: 1:0, 0:1, 1:1, and 1:3, respectively.
2.3 Characterizations of NixRhy
The structure of the prepared samples is determined by Powder X-ray diffraction (XRD)
measurement with a Cu target (Cu Kα1 = 1.541 Å), (Japan). The measurements are
conducted from 20◦ to 80◦ with steps of 0.02◦. The morphology is observed by scanning
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electron microscopy (SEM, JSM-7000F, JEOL, Japan) and transmission electron microscopy
(TEM, JEM-2100F, JEOL, Japan)). X-ray photoelectron spectroscopy (XPS) characterization
is performed on a Sigma Probe Thermo Fisher VG Scientific spectrometer (MULTILAB
2000, Thermo Scientific, USA) equipped with a monochromatic Al Kα X-ray source.
2.4 Preparation of electrodes
The working electrodes are fabricated on a glassy carbon rotating disk electrode (RDE)
(diameter: 3 mm; area: ~0.071 cm2) from ALS Co., Ltd. (Japan). A glassy carbon rotating
disk electrode is polished with a 1 µm polishing diamond suspension and then with a 0.05 µm
Al2O3 particle suspension on a moistened polishing cloth. The polished electrode is rinsed
with DI, sonicated with DI for 5 min, and rinsed again with DI. This electrode is then dried in
air at room temperature for 30 min. To prepare the testing samples, 4 mg of each prepared
catalyst of NixRhy is dispersed into 1 mL of distilled water and then this solution is sonicated
for 30 min to form homogeneous inks. Then, 3 µL of these inks is dropped onto the surface
of the glassy carbon electrode and dried at 60◦C. After that, 3 µL of 0.05 wt% nafion is
covered on the catalyst and dried. Finally, these prepared electrodes are used for
electrocatalytic measurements. For comparison, the c-Pt/C is used as a working electrode.
The preparation of the c-Pt/C electrode is similar to the preparation of the NixRhy electrodes
above. All electrodes are fabricated with the same loading level of ~ 169 µg. cm-2.
For the overall water splitting test, Ni1Rh3 is coated on a 1 cm x 1 cm piece of Ni foam (NF)
with a loading level of 1 mg.cm-2. First, 1 mg of Ni1Rh3 nanosponge or commercial RuO2 is
put into 1mL of distilled water and sonicated for 30 min to obtain a homogeneous ink. The
ink is then drop-casted on NF and dried at 60oC. After that, each of the NFs is covered by 200
mL of Nafion (0.05 wt%) and dried to obtain Ni1Rh3@NF and RuO2@NF electrodes as a
cathode and an anode, respectively. For comparison, the c-Pt/C sample is also prepared via
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the same method as used for the Ni1Rh3@NF electrode above with a loading level of 1
mg.cm-2.
2.5 Eletrocatalytic measurements
Electrochemical measurements are conducted with a potentiostat (IviumStat electrochemical
analyzer) and a rotator (RRDE-3A, ALS). All measurements are carried out in 0.5 M H2SO4
or 1.0 M KOH (99.99%, Aldrich) solution at room temperature using a three-electrode
electrochemical cell. The Pt wire (or graphite rod) and Ag/AgCl (NaCl, 3.0 M) are employed
as a counter electrode and a reference electrode, respectively. The Hg/HgO (NaOH, 1.0 M)
electrode is used as a reference electrode in alkaline media. Before the electrochemical
measurement, two electrolytes (0.5 M H2SO4 and 1.0 M KOH) are degassed by bubbling N2
for 30 minutes. It should be noted that all samples are cycled at a scan rate of 200 mV.s-1 in
an N2-saturated 0.5 M H2SO4 or 1.0 M KOH solution for electrochemical cleaning. Potentials
are converted to reversible hydrogen electrode (RHE) according to the Nernst
equation: ꢀꢁꢂꢃ = ꢀꢄꢅ/ꢄꢅꢆꢇ + 0.197 + 0.059 x ꢈꢉ. The overpotential η is calculated from the
ꢋ ꢍ
formula: ꢊ ꢌ = 0ꢋꢌꢍ − ꢀꢁꢂꢃ. The Tafel slope is modeled by the empirical Tafel equation: η
= a + b x log|j|, where η is the overpotential, j is the measured current density, b is the Tafel
slope, and a is the constant. The scan rate of all linear sweep voltammetry is kept at 5 mV.s-1
from 0.1 to -0.6 V (vs. RHE). During the linear sweep, the RDE is continuously rotated at
1600 rpm to remove the generated bubbles. The onset potential is measured at a current
density of -0.5 mA.cm-2. The electrochemical double layer capacitances (Cdl) are determined
by measuring the CVs at multiple scan rates in a potential region from 0.3 to 0.4 V (vs. RHE).
In this work, CVs are measured at scan rates of 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100
mV.s−1. Electrochemical impedance spectroscopy (EIS) is performed at -0.2 V (vs. RHE) in a
frequency range from 100 kHz to 5 mHz. Stability tests are performed for the Ni1Rh3 sample
by continuous cyclic voltammetry (CV) for 2000 sweeps with a potential scan range from
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−0.30 to 0.20 V (vs. RHE) at 100 mV.s−1 in both electrolytes of 0.5 M H2SO4 and 1.0 M
KOH. Chronoamperometry evaluates the potential of -48 mV (vs. RHE) for the Ni1Rh3
sample and -45 mV (vs. RHE) for c-Pt/C in 0.5 M H2SO4 electrolyte. Both the Ni1Rh3 sample
and c-Pt/C are analyzed at a current density of -10 mA.cm-2 in 1.0 M KOH electrolyte.
For electrochemical measurements of overall water splitting in 1.0 M KOH electrolyte, all
linear sweep voltammetry is evaluated at 5 mV.s-1 from 1.0 V to 1.8 V (vs. RHE) for all full
systems of Ni1Rh3@NF//RuO2@NF, c-Pt/C@NF//RuO2@NF, and NF//NF. The stability of
the Ni1Rh3@NF//RuO2@NF system is demonstrated using chronoamperometry at a constant
applied overpotential of 1.52 V (vs. RHE) for over 100,000 seconds.
3. Results and discussion
NixRhy catalysts are obtained without using any surfactant, as seen in Figure 1 (a). The
morphology of the prepared NixRhy materials is obtained by SEM, as displayed in Figure 1.
Specifically, low- and high-magnification SEM images of Ni, Ni3Rh1, Ni1Rh1, and Ni1Rh3
samples are described in Figure 1 (b-f) and (b1-f1), respectively. The images show that all
samples are sponge-like and very porous, resulting from a facile and quick synthesis method.
As seen from the reaction below, hydrogen bubbles that form during the rapid reduction of
Ni2+ and Rh3+ ions to Ni0 and Rh0 zero-valent metals, using NaBH4 as a reducing agent, play
a major role in the formation of sponge-like products[35-37]. It should be noted that metal
nanoparticles are formed instead of porous nanostructures when the concentration of
precursors and reducing agent is low [36]. Herein, the related reactions are as follows [35]:
NaBH4 + 2H2O ꢀ NaBO2 + 4H2
(1)
(2)
(3)
4RhCl3 + 3NaBH4 + 6H2O ꢀ 4Rh + 3NaBO2 + 12HCl + 6H2
2NiCl2 + NaBH4 + 2H2O ꢀ 2Ni + NaBO2 + 4HCl + 2H2
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In addition, the high-resolution SEM images of Ni, Ni3Rh1, Ni1Rh1, and Ni1Rh3 samples are
showed in Figure 1 (b1-f1), respectively. According to these SEM images, all samples are
composed of porous and interconnected networks.
The TEM images reveal that all prepared NixRhy nanosponge samples are composed of fused
nanoparticles with diameter ranging from 10 to 20 nm (Figure 2 (a-e) and (a1-e1) and Figure
S1.). This further confirms that the nanosponge structure for all samples obtained is in
accordance with the conclusion drawn from the SEM images above. In particular, the
amorphous feature of the Ni nanosponge is clearly revealed in the high-resolution TEM
(HRTEM) image, in which no lattice fringe can be seen in Figure 2 (a2)[37]. In contrast with
the Ni nanosponge, the Rh nanosponge shows a high degree of crystallinity with continuous
fringes, composed of a d-spacing of 0.2191 ±0.0002 nm in the same orientation,
corresponding to the (111) plane in the HRTEM image (as seen in Figure2 (e2)) [6]. Samples
prepared with the mixing of Ni and Rh are meanwhile very similar to the Rh sample. The
HRTEM images of the Ni3Rh1, Ni1Rh1, and Ni1Rh3 nanosponges also exhibit lattice fringes
with d-spacing of 0.2123±0.0004 nm, 0.2169±0.0004 nm, and 0.2172±0.0003 nm for the
(111) plane, as seen in Figure 2 (b2-d2), respectively. The obtained results suggest that Ni
and Rh are successfully alloyed together in the prepared samples. Interestingly, a thin
amorphous layer with a thickness of about 1 nm appears on the surface of Ni/Rh alloys such
as the Ni3Rh1 and Ni1Rh1 and notably contains a considerable amount of Ni, as seen in Figure
2 (b1-c1) and Figure 2 (b2-c2). In contrast, the thin amorphous layer disappears on the
surface of the Ni1Rh3 alloy containing a small amount of Ni and that of pure Rh containing
no Ni, as seen in Figure 2 (d1-e1 and d2-e2. This observation supports the observation that
the Ni1Rh3 sample has the highest degree of crystallinity among all Ni/Rh alloys tested in the
next XRD analysis. It also infers that exposure of the active sites can play a key role in the
HER electrocatalyst and subsequently affect HER performance[4].
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To investigate the structure of the prepared NixRhy nanosponges, the XRD spectra are
obtained, as seen in Figure 2 (f). Herein, the Ni nanosponge shows an amorphous structure
with only one peak at 2 theta of 45.60 ± 0.02o, which is consistent with TEM results seen
above and previous work [37]. Unlike Ni, the Rh nanosponge exhibits a crystalline state with
two characteristic peaks of (111) and (200) at 41.23±0.03o and 46.95±0.05o, respectively.
This result is consistent with the standard values of the face-centered cubic (fcc) Rh (JCPDS
No. 05-0658)[7, 8]. The XRD spectra of the Ni3Rh1, Ni1Rh1, and Ni1Rh3 nanosponges clearly
show that their structures are basically similar to the Rh structure, except for negligible shifts
of the typical peak position of the (111) plane towards a higher angle, evidencing alloy
formation between Rh and Ni. Specifically, the XRD spectra of the Ni3Rh1, Ni1Rh1, and
Ni1Rh3 nanosponges clearly indicate the (111) peaks at 42.35±0.04o, 41.46±0.01o,
41.41±0.03o and the (200) peaks at 47.44±0.04o, 47.52±0.05o 47.62±0.03o. In addition, the
degree of crystallinity increases when the amount of Rh increases in the Ni/Rh alloys. Ni1Rh3
nanosponges show the highest integrated intensity as well as sharpness of the (111) peak,
unlike the Ni3Rh1 and Ni1Rh1 nanosponges. All crystalline parameters from XRD are
summarized in Table S1. The average crystallite size of Rh, Ni1Rh3, Ni1Rh1, and Ni3Rh1 is
calculated by using the Debye-Scherrer formula, as seen in the Supporting Information. The
calculated crystallite size of Rh, Ni1Rh3, Ni1Rh1, and Ni3Rh1 is thus 3.032, 2.880, 3.207, and
4.693 nm, respectively. The Ni1Rh3 nanosponge exhibits the smallest crystallite size. On the
other hand, we calculate the d-spacing of the Rh, Ni1Rh3, Ni1Rh1, and Ni3Rh1 nanosponges by
using Bragg’s Law, as seen in the Supporting Information. The calculated values of the d-
spacing are 0.2187, 0.2179, 0.2176, and 0.2132 nm for the Rh, Ni1Rh3, Ni1Rh1, and Ni3Rh1
nanosponges, respectively, which are close to the results from HRTEM measurements (see
Table S1).
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The element mapping of the Ni1Rh3 nanosponge is investigated as presented in Figure 3.
These images suggest that Ni and Rh are uniformly dispersed in the Ni1Rh3 nanosponge. In
addition, an energy-dispersive X-ray spectroscopy (EDS) analysis of the Ni1Rh3 nanosponge
confirms that Ni and Rh exist in the Ni1Rh3 nanosponge, as seen in Figure S2.
In order to understand the chemical states and surface electronic structures of the
prepared catalysts, XPS spectra are collected (as seen in Figure S3 in the Supporting
Information). It is clear that no Ni peaks appear in the XPS spectra of pure Rh and no Rh
peaks appear in the XPS of pure Ni. Furthermore, XPS spectra of Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3,
and Rh nanosponges are shown in Figures 4 (a) and (b). In more detail, Ni 2p3/2 spectra are
displayed in Figure 4 (a1). Herein, the binding energy (BE) of the Ni 2p3/2 spectra of various
Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3 nanosponges are located from 856.39 to 856.84 (eV). Thus, with
a decreasing amount of Ni, the Ni 2p spectra are slightly shifted to the higher BE direction. In
addition, Rh 3d5/2 spectra are exhibited in Figure 4 (b1). Specifically, the BE of the Rh 3d5/2
spectra of Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh nanosponges are located from 307.12 to 307.35
(eV). Thus, with a decreasing amount of Rh, the Rh 3d5/2 spectra are shifted to the lower BE
direction. These results imply that a surface electronic combination between Ni and Rh
occurs in Ni/Rh alloys. The electronic coupling between Ni and Rh is formed due to the
electron transfer from Ni to Rh [9]. These XPS results combined with XRD and TEM data
suggest that Ni and Rh form the fcc-structured solid solution. In addition, the XPS spectra of
the Ni1Rh3 nanosponge are fitted, as seen in Figures 4 (c) and (d). Specifically, the high-
resolution XPS spectrum of the Ni 2p1/2 and Ni 2p3/2 peaks is showed in Figure 4 (c), which
discloses the presence of metallic Ni (853.34 and 869.79 eV), oxidized Ni (856.31 and
874.37 eV), and satellite (861.68 and 880.11 eV) peaks[37]. Moreover, Figure 4 (d) displays
the high-resolution XPS spectrum of Rh 3d3/2 and Rh 3d5/2. According to these peaks, the
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presence of metallic Rh (311.91 and 307.18 eV) and oxidized Rh (307.94 and 312.81 eV)
peaks is confirmed [5, 38, 39].
HER performance of NixRhy nanosponges is tested in a 0.5 M N2-degassed H2SO4 electrolyte,
as seen in Figure 5. In more detail, the linear sweep voltammetry (LSV) curves using a
typical three-electrode system are obtained at a scan rate of 5 mV.s-1. The polarization curves
of various catalysts (glassy carbon, Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh, and c-Pt/C) are exhibited
in Figure 5 (a). According to these polarization curves, glassy carbon exhibits nearly
negligible HER activity. In contrast, the onset potential of all Ni3Rh1, Ni1Rh1, Ni1Rh3 samples
is almost close 0 mV, to that of the c-Pt/C sample. In contrast with the NiRh alloys, pure Ni
and pure Rh nanosponges show higher onset potential values of -255 mV and -19 mV,
respectively. The Ni1Rh3 nanosponge possesses the highest HER activity with the lowest
overpotential to reach a current density of -10 mA.cm-2. Specifically, the overpotential of the
Ni1Rh3 sample is 48 mV, which is the lowest value (compared to the Ni, Ni3Rh1, Ni1Rh1, and
Rh nanosponges, which yield values of 403 mV, 75 mV, 64 mV and 120 mV, respectively).
Furthermore, it is comparable to the value of the c-Pt/C sample (45 mV) (as seen in Figure 5
(b)). The good HER performance may come from the nanosponge structure with the high
surface area of the synthesized catalysts. On the other hand, alloys of Ni3Rh1, Ni1Rh1, Ni1Rh3
nanosponges display higher HER activity than that of pure Ni and pure Rh, which is
consistent with the XPS results. The changed electronic structure due to the combination
between Ni and Rh atoms increased the HER catalytic activity [5, 9, 13]. Among the NiRh
alloys above, the Ni1Rh3 nanosponge exhibited the highest HER performance due to having
the highest degree of crystallinity and no thin amorphous cover on its surface, which leads to
more active sites compared to the Ni3Rh1 and Ni1Rh1 nanosponges.
To further understand the electrocatalytic performance of the Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh
catalysts, the related Tafel plots are obtained according to the Tafel equation (η= a + b log j,
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where η, a, b, and j are the overpotential, the intercept, Tafel slope, and the current density,
respectively), by using the linear portions of their HER polarization curves. Generally, there
are three steps that are accepted for the HER in acidic media, which are delineated by
equations (4), (5), and (6) as follows [3, 40]:
H3O+ + e-
Hads + H3O+ + e- ꢀ H2 + H2O (Heyrovsky reaction, b ≈ 40 mV.dec-1)
Hads + Hads ꢀ H2
(Tafel reaction, b ≈ 30 mV.dec-1)
ꢀ Hads + H2O
(Volmer reaction, b ≈ 120 mV.dec-1)
(4)
(5)
(6)
The rate-limiting step determines the HER mechanism, which can be determined from the
calculated Tafel slope. As known, Pt has very high Hads coverage (θH≈1). The hydrogen
evolution reaction on a Pt surface is known as the Volmer-Tafel mechanism (equations (4)
and (6)). The recombination step is the rate-limiting step at low overpotentials, as attested by
measuring the Tafel slope of 30 mV.dec-1 [13, 19]. In our work, the Tafel slope values of Ni,
Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh, and c-Pt/C are 152, 48, 46, 31, 92, and 30 mV.dec-1, as seen in
Figure 5 (c), respectively. Among them, the Ni1Rh3 nanosponge shows the lowest Tafel slope
of 31 mV.dec-1, which means that the HER mechanism of the Ni1Rh3 nanosponge is similar
to that of c-Pt/C based on the Volmer-Tafel mechanism. Moreover, the exchange current
density (j0) from the Tafel curve was determined by using an extrapolation method. The
exchange current density (j0) of Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh, and c-Pt/C catalyst is
calculated to be 0.022, 0.418, 0.692, 0.855, 0.190, and 0.863 mA.cm−2, respectively (as seen
in Table 2). In general, when the catalysts have higher j0 values, they possess higher HER
activity [40]. Herein, pure Ni and pure Rh exhibit low j0 values compared to those of Ni3Rh1,
Ni1Rh1, Ni1Rh3, and c-Pt/C. This implies that the HER activities are improved by alloying Ni
and Rh. Among all NiRh nanosponges, the Ni1Rh3 nanosponge exhibits the highest j0 value,
which reveals that it possesses the highest activity for HER. Figure 5 (d) shows that the mass
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activity in the HER of the Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh, and c-Pt/C catalysts at a potential
of -0.1 V is 4 x 10-4, 0.569, 0.781, 1.559, 0.271, and 0.980 A.mg-1, respectively. Herein, we
have used a method following a previous work of Cheng et al. [41]. Remarkably, the mass
activity of the Ni1Rh3 nanosponge is 1.6 times higher than that of the c-Pt/C catalyst. These
results indicate that the HER activities are enhanced by alloying Ni and Rh, where the Ni1Rh3
nanosponge possesses the highest HER activity.
Generally, to determine the relative difference in electrochemically active surface areas and
roughness of various catalysts, the electrochemical double layer capacitances (Cdl) are
measured by a cyclic voltammetry (CV) method [13, 37, 40]. Herein, the cyclic
voltammograms of Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh nanosponges are obtained at several
scan rates (10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mV.s-1) from 0.3 to 0.4 V (vs. RHE). The
Cdl for these catalysts can be calculated by plotting ꢀj at E = 0.35 V (vs. RHE) against the
scan rates. As seen in Figure 5 (e), the Cdl of the Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh
nanosponges is 0.86, 1.74, 2.16, 2.65, and 1.39 µF.cm-2, respectively. The Cdl of Ni1Rh3 is the
highest value, suggesting that this catalyst has more active sites than the others. In addition,
electrochemical impedance spectroscopy is carried out to investigate the electrode kinetics
under the HER process at -0.2 V (vs. RHE). As seen in Figure 5 (f), the Nyquist plot data are
fitted to the equivalent circuit, as inserted in Figure 5 (f). The enlarged high-frequency region
of the Nyquist plots is also inserted in Figure 5 (f). After fitting the data with the circuit
model, the charge-transfer resistances (Rct) of Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh
nanosponges are identified to be 395, 46, 37, 32, and 81 ꢁ, respectively. Among them, the
Ni1Rh3 catalyst exhibits the lowest value of Rct, indicating that this catalyst has a faster
electron transfer rate in the HER process. These results are consistent with the HER
performance results obtained previously.
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Figures 6 (a) and (b) show the electrocatalytic stability of the Ni1Rh3 nanosponge in the 0.5 M
H2SO4 electrolyte. In more detail, as seen in Figure 6 (a), after 2000 cycles between -0.3 to
0.2 V (vs. RHE), there is a negligible change from the polarization curve of the Ni1Rh3
nanosponge before and after 2000 cycles, which suggests that the durability of Ni1Rh3 is
excellent. Furthermore, chronoamperometric measurement (i-t) is also carried out to further
investigate the stability of Ni1Rh3. Figure 6 (b) shows the time dependence of the current
density at a constant static potential of -48 mV for the Ni1Rh3 nanosponge and -45 mV for the
c-Pt/C. The Ni1Rh3 nanosponge exhibits excellent stability with retained current density of
92%, which is higher than that (87%) of c-Pt/C for 25,000 s in the 0.5 M H2SO4 electrolyte.
These results presenting the HER performance for all samples are summarized in Table 2,
and they suggest that the Ni1Rh3 nanosponge is a potential candidate as a catalyst for the
production of H2 using water electrolysis.
To further investigate the HER performance in an alkaline electrolyte, the Ni1Rh3 nanosponge
was used for the HER process in 1.0 M KOH. Figure 7 displays the HER performance of
Ni1Rh3 nanosponge in 1.0 M KOH. Surprisingly, the polarization curve of the Ni1Rh3 catalyst
exhibits higher HER activity compared to c-Pt/C, as seen in Figure 7 (a). Specifically, Figure
7 (b) shows the overpotential values of Ni1Rh3 and c-Pt/C are 107 and 119 mV, respectively.
The mechanism of hydrogen evolution in an alkaline electrolyte is similar to the mechanism
of hydrogen evolution in an acidic electrolyte. In particular, the mechanism of hydrogen
evolution in an alkaline electrolyte entails three steps, as seen in the following three equations
(7), (8), and (9) [3, 10]:
H2O + e-
Hads + H2O + e- ꢀ H2 + OH- (Heyrovsky reaction, b ≈ 40 mV.dec-1)
Hads + Hads ꢀ H2
(Tafel reaction, b ≈ 30 mV.dec-1)
ꢀ Hads + OH-
(Volmer reaction, b ≈ 120 mV.dec-1)
(7)
(8)
(9)
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Additionally, the Tafel slope values are calculated for both Ni1Rh3 and c-Pt/C, and they are
75 and 76 mV.dec-1, respectively, as seen in Figure 7 (c). The resulting values suggest that
Ni1Rh3 possesses higher HER activity than that of c-Pt/C in the alkaline electrolyte. The Tafel
slope value of the Ni1Rh3 nanosponge lies within a range of 40 to 120 mV dec-1. This
indicates a Volmer–Heyrovsky mechanism for the HER on the electrode surface, wherein the
adsorption of a hydrogen atom is the rate-determining step.
Moreover, the durability of the Ni1Rh3 nanosponge is evaluated for 2000 cycles between -0.3
to 0.2 V (vs. RHE) in 1.0 M KOH solution. Figure 7 (d) exhibits the polarization curves of
the Ni1Rh3 nanosponge before and after 2000 cycles. It displays a negligible change of the
current density, suggesting the durability of Ni1Rh3 in 1.0 M KOH electrolyte. In addition,
chronopotentiometric measurement is also carried out for both the Ni1Rh3 nanosponge and c-
Pt/C at -10 mA cm-2 for over 40,000 s, as inserted in Figure 7 (d). The figure shows that the
overpotential increases negligibly and the Ni1Rh3 nanosponge presents better durability in the
alkaline electrolyte compared to c-Pt/C one. This indicates the excellent stability of Ni1Rh3 in
an alkaline electrolyte.
To further demonstrate the superiority of the Ni1Rh3 nanosponge, a full cell is assembled and
tested for its overall water splitting performance in a 1.0 M KOH solution using a Ni foam-
supported Ni1Rh3 nanosponge as a cathode and commercial RuO2 as an anode. Commercial
RuO2 is first tested for its OER performance, as seen in Figure S4, and it exhibits a high
current density of 20 mA.cm-2 at a potential of 1.6 V (vs. RHE) in the 1.0 M KOH electrolyte.
In comparison with (NF//NF) and c-Pt/C@NF//RuO2@NF pairs, the Ni1Rh3@NF//RuO2@NF
pair shows the best electrocatalytic activity in a potential range of 1.0–1.8 V. Specifically, the
polarization curves in the overall water splitting process of the NF//NF,
Ni1Rh3@NF//RuO2@NF, and c-Pt/C@NF//RuO2@NF pairs are seen in Figure 8 (a). The
overpotential value of Ni1Rh3@NF//RuO2@NF pair is 1.52 V (vs. RHE) at a current density
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of 10 mA.cm-2. Figure 8 (b) clearly reveals that bubble hydrogen gas and bubble oxygen gas
are released from the Ni1Rh3@NF//RuO2@NF pair. Moreover, the chronoamperometry
measurement (i-t) is also conducted for the pair of Ni1Rh3@NF//RuO2@NF at a constant
potential of 1.52 V (vs. RHE), as seen in Figure 8 (c). The current density initially starts at a
value of 10 mA.cm-2 and eventually tends to increase slightly during the long-term test for
over 100,000 s. Figure 8(d) shows a HRTEM image of Ni1Rh3 after the durability test. The
nanosponge structure of Ni1Rh3 is still maintained even after over 100,000 s. Furthermore, the
HRTEM image (as seen in Figure S5) of Ni1Rh3 after the durability test shows that the d-
spacing of the (111) plane is not changed, remaining at around 0.2172±0.0002 nm. Thus, the
Ni1Rh3 catalyst is a very stable and active cathode which can be applied in electrolysis of
water to produce hydrogen.
4. Conclusions
NixRhy nanosponges with various Ni/Rh ratios have been successfully synthesized through a
facile method without any surfactant. Due to a strong synergistic effect and efficient
electronic coupling, the NixRhy nanosponges show greatly enhanced electrocatalytic HER
activity and stability compared to that of pure Ni and pure Rh nanosponges. Among the
obtained NixRhy nanosponges, the Ni1Rh3 nanosponge, possessing the highest degree of
crystallinity and no thin amorphous layer on its surface, exhibits the highest HER activity
with a Tafel slope of ~31 mV.dec-1, and an overpotential of 48 mV (vs. RHE) to reach a
current density of -10 mA.cm-2 in the 0.5 M H2SO4 electrolyte. In addition, the durability of
the Ni1Rh3 nanosponge is exceptional. Moreover, in an alkaline electrolyte, the Ni1Rh3
nanosponge also shows excellent HER activity with a Tafel slope of ~75 mV.dec-1 and an
overpotential of 107 mV (vs. RHE) to reach a current density of -10 mA.cm-2, which is
higher than that of c-Pt/C (a Tafel slope of ~76 mV.dec-1 and an overpotential of 109 mV (vs.
RHE)). Additionally, the Ni1Rh3 nanosponge exhibits high stability for a long-term test in 1.0
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M KOH electrolyte until over 40,000 s. For overall water splitting, the pair of a Ni1Rh3
nanosponge cathode and a commercial RuO2 anode (Ni1Rh3@NF//RuO2@NF) achieved a
current density of 10 mA.cm-2 at a low voltage of 1.52 V, which is excellent performance for
a symmetric two-electrode water electrolysis system. Furthermore, this system also shows
exceptional durability for a long-term test until over 100,000 s. According to these results, the
Ni1Rh3 nanosponge is a promising catalyst for the HER as well as a potential cathode for
overall water splitting by electrolysis.
Acknowledgements
This research was supported by a National Research Foundation (NRF) grant (NRF-
2017R1A2B2001911, NRF-2017R1A4A1015360) funded by the Ministry of Science and
ICT, Republic of Korea.
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Table 1. Previous works for various Ni-based alloys and their HER characteristics.
Catalyst
Onset
potential
(mV vs.
RHE)
Tafel slope
[mV.dec-1]
Exchange current
Density
Overpotential
(mV vs. RHE)
@ j=
Electrolyte
Ref.
[mA.cm-2]
-10mA.cm-2
190
Ni-La
-100
75.6
69
0.115
0.27
1 M KOH
1 M NaOH
[10]
[11]
[12]
[12]
[13]
[14]
[15]
[15]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[23]
[23]
NiZn
None
-1020
-170
~0
None
None
None
41
Ni-Sn@C
Ni-Sn@C
NiRu
62
None
1.0 M NaOH
0.5 M H2SO4
0.5 M H2SO4
30% KOH
35
None
31
0.617
NiCo
None
-65
91
0.1
None
None
None
None
None
None
None
None
None
None
300
NiMo
NiW
38
0.025
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
6.0 M NaOH
0.1 M NaOH
6.0 M KOH
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
0.1 M KOH
1.0 M NaOH
1.0 M NaOH
1.0 M NaOH
-85
43
0.01
NiFe
-257
None
None
None
None
~0
86
2 x 10-3
1.04 x 10-3
0.31
NiCu
60
NiMn
NiCr
125
150
41
None
NiPt
None
NiPd
37.1
36
None
NiAu/Au
NiAg
-7
None
None
None
None
None
None
82
None
Ni95Ce5
Ni94Pr6
Ni94Y6
9.0 x 10-2
39.9 x 10-2
3.8 x 10-3
250
81
240
108
325
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Table 2. HER performance comparison of the Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, Rh and c-Pt/C
catalysts.
Catalyst
Onset
Tafel slope
[mV.dec-1]
Exchange current
Density
[mA.cm-2]
0.022
Overpotential
Electrolyte
potential
(mV)@j=-10
mA.cm-2
(mV)
Ni
-255
-10
~0
152
48
46
31
75
92
31
76
403
75
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
1.0 M KOH
0.5 M H2SO4
0.5 M H2SO4
1.0 M KOH
Ni3Rh1
Ni1Rh1
Ni1Rh3
Ni1Rh3
Rh
0.418
0.692
64
~0
0.855
48
~0
0.302
107
109
45
-19
~0
0.190
c- Pt/C
c- Pt/C
0.863
~0
0.251
119
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Figure 1. Scheme of synthesis process and morphology of materials prepared. (a) Schematic
illustration of synthesizing NiRh nanosponges. (b-f) Low-magnification and (b1-f1) high-
magnification SEM images of Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh nanosponges, respectively.
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Figure 2. Morphology and crystal structure analysis characterization. (a-e) TEM images; (a1-
e1) Low-magnification and (a2-e2) high-magnification HRTEM images of Ni, Ni3Rh1,
Ni1Rh1, Ni1Rh3, and Rh nanosponges, respectively. (f) XRD patterns of Ni, Ni3Rh1, Ni1Rh1,
Ni1Rh3, and Rh nanosponges, respectively.
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Figure 3. Elemental analysis characterization. (a-d) HAADF-STEM image and element
mapping of Ni1Rh3 nanosponge.
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Figure 4. XPS spectra analysis. (a-a1) Comparing the Ni 2p3/2 XPS spectrum of the Ni,
Ni3Rh1, Ni1Rh1, and Ni1Rh3 nanosponges; (b-b1) comparing the Rh 3d5/2 XPS spectrum of
the Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh nanosponges; and (c) high-resolution XPS spectra of Ni,
and (d) high-resolution XPS spectra of Rh of Ni3Rh1 nanosponge.
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Figure 5. HER activity characterization in 0.5 M H2SO4. (a) Polarization curves with a scan
rate of 5 mV.s-1, (b) overpotential at current density of -10 mA.cm-2, (c) Tafel plots and Tafel
slopes derived from (a), (d) mass activity at -0.1 V (vs. RHE), of the Ni, Ni3Rh1, Ni1Rh1,
Ni1Rh3, Rh nanosponges, and the c-Pt/C catalysts in 0.5 M H2SO4 at 298K. (e) The
differences in current density at 0.35 V (vs. RHE) plotted against scan rates from 10-100
mV.s-1, and (f) Nyquist plots at -0.2 V (vs. RHE) (Inset is an enlargement of an area in (f)) of
the Ni, Ni3Rh1, Ni1Rh1, Ni1Rh3, and Rh nanosponges in 0.5 M H2SO4 at 298K.
28