Synergistically enhanced performance of transition-metal doped Ni2P for supercapacitance and overall water splitting

Article abstract of DOI:10.1039/d1dt01058a

Cost-effective and readily available catalysts applicable for electrochemical conversion technologies are highly desired. Herein, we report the synthesis of dithiophosphonate complexes of the type [Ni{S₂P(OH)(4-CH₃OC₆H₄)}₂] (1), [Co{S₂P(OC₄H₉)(4-CH₃OC₆H₄)}₃] (2) and [Fe{S₂P(OH)(4-CH₃OC₆H₄)}₃] (3) and employed them to prepare Ni₂P, Co-Ni₂P and Fe-Ni₂P nanoparticles. Ni₂P was formed by a facile hot injection method by decomposing complex1in tri-octylphosphine oxide/tri-n-octylphosphine at 300 °C. The prepared Ni₂P was doped with Co and Fe employing complexes2and3, respectively, under similar experimental conditions. Doping Ni₂P with Co and Fe demonstrated synergistic improvement of Ni₂P performance as an electrocatalyst in supercapcitance, hydrogen evololution and oxygen evolution reactions in alkaline medium. Cobalt doping improved the Ni₂P charge storage capacity with a supercapacitance of 864 F g^?1at 1 A g^?1current density. Fe doped Ni₂P recorded the lowest overpotential of 259 mV to achieve a current density of 10 mA cm^?2and a Tafel slope of 80 mV dec^?1for OER, better than the undoped Ni₂P and the benchmark IrO₂. Likewise, Fe-doped Ni₂P electrode required the lowest overpotential of 68 mV with a Tafel slope of 110 mV dec^?1to attain the same current density for HER. All catalysts showed excellent stability in supercapacitance and overall water splitting reactions, indicating their practical use in energy conversion technologies.

Full text of DOI:10.1039/d1dt01058a

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Synergistically enhanced performance of
transition-metal doped Ni2P for
Cite this: DOI: 10.1039/d1dt01058a
supercapacitance and overall water splitting†
Gwaza Eric Ayom, a Malik Dilshad Khan, a,b Jonghyun Choi,c
a
Ram Krishna Gupta, c Werner E. van Zyl d and Neerish Revaprasadu
*
Cost-effective and readily available catalysts applicable for electrochemical conversion technologies are
highly desired. Herein, we report the synthesis of dithiophosphonate complexes of the type [Ni{S2P(OH)
(4-CH3OC6H4)}2] (1), [Co{S2P(OC4H9)(4-CH3OC6H4)}3] (2) and [Fe{S2P(OH)(4-CH3OC6H4)}3] (3) and
employed them to prepare Ni2P, Co-Ni2P and Fe-Ni2P nanoparticles. Ni2P was formed by a facile hot
injection method by decomposing complex 1 in tri-octylphosphine oxide/tri-n-octylphosphine at 300 °C.
The prepared Ni2P was doped with Co and Fe employing complexes 2 and 3, respectively, under similar
experimental conditions. Doping Ni2P with Co and Fe demonstrated synergistic improvement of Ni2P per-
formance as an electrocatalyst in supercapcitance, hydrogen evololution and oxygen evolution reactions
in alkaline medium. Cobalt doping improved the Ni2P charge storage capacity with a supercapacitance of
864 F g−1 at 1 A g−1 current density. Fe doped Ni2P recorded the lowest overpotential of 259 mV to
achieve a current density of 10 mA cm−2 and a Tafel slope of 80 mV dec−1 for OER, better than the
undoped Ni2P and the benchmark IrO2. Likewise, Fe-doped Ni2P electrode required the lowest overpo-
tential of 68 mV with a Tafel slope of 110 mV dec−1 to attain the same current density for HER. All catalysts
showed excellent stability in supercapacitance and overall water splitting reactions, indicating their practi-
cal use in energy conversion technologies.
Received 30th March 2021,
Accepted 29th July 2021
DOI: 10.1039/d1dt01058a
rsc.li/dalton
overcome these sluggish reactions. Supercapacitors as energy
storage devices are attracting attention due to their excellent
Introduction
The current extensive utilization of fossil fuels to generate power density and cycle life but are restricted by the usage of
energy has led to a growing concern for our planet’s sustain- noble metals as electrodes.4 The high cost and scarcity of
ability. This, in turn, has motivated a quest for zero carbon- these noble metals therefore constrain the large-scale appli-
based, readily available, and re-usable alternatives to energy cation of water splitting and supercapacitor technologies.4,5
generation and storage. Technologies that split water to gene- Cheaper and readily available alternatives to noble metals are
rate H2 and O2, as well as supercapacitors, hold considerable therefore highly sought after.
promise in the area of clean energy generation and storage.
Nickel phosphides continue to attract attention as catalysts
The evolution of hydrogen (HER) and oxygen (OER) require in water splitting and supercapacitance applications.6–8 The
suitable catalysts to overcome the uphill energy requirements growing attention is due to their abundant earth reserves, high
at the electrodes.1–3 To date, only noble metals such as Pt, Ru electrical conductivity, fast charge transfer, and good reaction
and Ir have been identified as efficient and durable catalysts to kinetics.9 Nickel phosphide is a binary system with multiple
compositions or phases depending on the ratio of nickel to
phosphorus in the system. Its different compositions range
aDepartment of Chemistry, University of Zululand, Private Bag X1001,
from metal-rich phases such as Ni2P, Ni12P5, Ni3P, Ni7P3 and
KwaDlangezwa 3880, South Africa. E-mail: RevaprasaduN@unizulu.ac.za
Ni5P4 to phosphide rich ones like NiP2 and NiP3.7,10
bInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-
224 Warsaw, Poland
Besides tuning phase, shape and size, doping of different
metal atoms into the metal phosphides is another facile and
useful strategy employed by researchers to improve the per-
formance of these materials as catalysts for supercapacitors
and water splitting.11 Incorporating a foreign atom in metal
phosphides has been illustrated by density functional theory
cDepartment of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA
dSchool of Chemistry and Physics, University of KwaZuluNatal, Westville Campus,
Chiltern Hills, Private Bag X54001, Durban, 4000, South Africa
†Electronic supplementary information (ESI) available: TGA of complexes, UV–
vis absorption spectra, SEM, TEM, HRTEM and XPS images of nano-materials
and comparison tables. See DOI: 10.1039/d1dt01058a
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as a route to modifying the adsorption/desorption energies of Dithiophosphonate ligands, which are phosphorus and sulfur-
reactants/products, which is crucial for the activity of a containing, are by far the less studied class of the phosphor-
catalyst,7,12 as well as modulating the density states at the 1,1-dithiolates. They have rarely been employed to prepare
Fermi level and hence improving the reaction kinetics.13 phosphides. Likewise, the preparations of nickel phosphides
Moreover, the introduction of another metal in a metal phos- are constrained by their prolonged reaction time, elevated
phide crystal lattice can cause the redistribution of valence temperatures9,22 and the toxicity of reagents usually
electrons, which in turn provide two electron donation sites employed.23 Furthermore, using precursors with similar struc-
leading to improved catalyst performance.14 For example, Liu tural features for dopants may result in easy and efficient
et al. doped Ni2P with nitrogen to improve the performance of incorporation of dopants, as all the precursors will follow a
their electrodes in supercapacitance examinations.15 Lin et al. similar kind of decomposition route.
recently fabricated an iron and oxygen co-doped nickel phos-
Herein, we have prepared three dithiophosphonate com-
phide that showed good activity in overall water splitting reac- plexes; [Ni{S2P(OH)(4-CH3OC6H4)}2] (1), [Co{S2P(OC4H9)(4-
tions.16 The performance of nickel phosphide, an excellent CH3OC6H4)}3] (2) and [Fe{S2P(OH)(4-CH3OC6H4)}3] (3) and
catalyst for water splitting7 can be improved by incorporating explored them as molecular precursors to prepare Ni2P and Co
Co into its lattice.13 For example, Qiu et al. showed by density and Fe-doped Ni2P with tri-octylphosphine (TOP) as the phos-
functional theory (DFT) analysis that incorporating Co in phorus source via the hot injection method. The electro-
nickel phosphide leads to an increase in active sites for OH− catalytic performance of nickel phosphide and the effect of
adsorption that improves OER activity.43 Co and Fe-based elec- different concentrations of Co and Fe doping on the perform-
trodes have high calculated theoretical supercapacitance ance of the prepared Ni2P was also investigated for overall
values.1717,43 The incorporation of a foreign atom into metal water splitting and supercapacitance.
phosphides has therefore been demonstrated as a strategy to
improve the electronic structure, transfer capability, and
density active sites for improved catalytic performance.11
survey of the literature, however, shows gaps in doping as a
strategy to improve Ni2P performance (especially Co and Fe) in
supercapacitors and water splitting.
Synthesis of phase pure nickel phosphide is challenging
due to the existence of nickel phosphide in different compo-
sitions. The use of molecular compounds as single-source pre-
cursors for the preparation of nanoparticles is well documen-
ted.18 The strengths of this route in nanoparticle preparation
over the use of multiple sources lie in the control of ligand
design and hence the formation of the suitable metal-phos-
phide/chalcogenide bonds suitable for the preparation of
desired nanomaterials.5 The exploration of single-source mole-
cular precursors to prepare nickel phosphides is, however,
A
Experimental details
Materials
Tri-octylphosphine (TOP) 90%, trioctylphosphine oxide (TOPO)
90%, nickel chloride hexahydrate, cobalt chloride hexahydrate,
iron chloride tetrahydrate, Celite, deionized water, chloroform,
acetone, n-butanol, phosphorus pentasulfide and anisole were
purchased from Sigma Aldrich and used without further puri-
fication. Diethyl ether and n-hexane were distilled under dini-
trogen over a Na wire with the formation of a benzophenone
ketyl indicator. Dichloromethane was distilled over P4O10.
Methanol was distilled from I2/Mg turnings.
Preparation of complexes
scarce.7,19 Lukehart and Milne employed a nickel phosphine The synthesis of complexes 1, 2 and 3 was carried out accord-
complex, tetrakis{diphenyl[2-(triethoxysilyl)-ethyl]phosphine} ing to the established literature procedure.5 In this procedure,
nickel(0), to prepare nickel phosphide nanoclusters in the first briefly, P2S5 was refluxed with excess anisole at 160 °C until all
report of the use of single-source precursors to make nickel the P2S5 dissolved using standard Schlenk techniques. The
phosphides.20 Maneeprakorn et al. in another study employed clear solution was then crystallized on cooling to form 2,4-bis
dithiophosphinates ([Ni(Se2PiPr2)2], [Ni(Se2PtBu2)2] and [Ni (4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-dithione,
(Se2PPh2)2]) as molecular precursors to prepare Ni2P and commonly referred to as Lawesson’s reagent (LR). LR was then
Ni5P4.19 Pan et al. formed the bis(triphenylphosphine)nickel stirred with stoichiometric amounts of deionized water (1 and
dichloride complex from the reaction of triphenylphosphine 3) and n-butanol (2) until the mixtures went clear. To these
and nickel chloride hexahydrate which was exploited to form clear mixtures were added NiCl2·6H2O, CoCl2·6H2O and
nickel phosphides. Habas et al. similarly employed triphenyl- FeCl2·4H2O to form immediate purple or dark-blue or yellow
phosphine and a commercially available air-stable nickel phos- precipitates which were isolated as complexes 1, 2 and 3,
phine complex [Ni(PPh3)2(CO)2] to prepare dinickel phos- respectively.
phide.21 We also recently demonstrated the solvent-less syn-
thesis of pure phase dinickel phosphide employing dithiopho-
sphonate complexes and triphenylphosphine.8
Complexes 1, 2 and 3 were confirmed by elemental analysis.
[Ni{S2P(OH)(4-CH3OC6H4)}2] (1): Calc. C: 33.82%; H: 3.24%;
S: 25.79%. Found. C: 33.78%; H: 3.30%; S: 25.65%.
The scarcity of nickel phosphides in the literature particu-
[Co{S2P(OC4H9)(4-CH3OC6H4)}3] (2): Calc. C: 44.79%; H:
larly formed via the single-source precursor route compared to 5.47%; S: 21.74%. Found. C: 44.50%; H: 5.42%; S: 21.69%.
the sulfides or oxides, is attributed to the synthetic difficulties [Fe{S2P(OH)(4-CH3OC6H4)}3] (3): C: 44.95%; H: 5.49%; S:
in the formation of desired molecular complexes. 21.81%. Found. C: 45.0%; H: 5.41%; S: 21.79%.
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Preparation of Ni2P, Co and Fe doped nanomaterials
employed as conducting acetylene black (MTI Corporation,
USA) with particle sizes ranging between 35–40 nm. Platinum
wires and saturated calomel electrodes (SCE) were utilized as
counter and reference electrodes, respectively. All the examin-
ations for supercapacitance and electrocatalysis were carried
out using 3 M and 1 M KOH electrolyte, respectively. Charge
storage capacity was measured using cyclic voltammetry (CV)
and galvanostatic charge–discharge (CD) at different scan rates
and current densities. Electrocatalytic properties of the pre-
pared electrodes were studied via cyclic voltammetry and
linear sweep voltammetry (LSV), with LSV done at a scan rate
of 2 mV s−1 for OER measurements. Electrochemical impe-
dance spectroscopic (EIS) was performed in the frequency
range of 0.05 Hz to 10 kHz at an applied AC amplitude of
10 mV.
Ni2P was prepared from the wet thermolysis of complex 1 fol-
lowing our earlier report.5 In this procedure, complex 1 (0.20 g,
0.40 mmol) dispersed in TOP (5.0 mL) was quickly injected
into TOPO (5.0 g) which had been heated to 300 °C under
nitrogen. The black precipitate formed on injection of complex
1 was further heated at 300 °C for 1 hour, cooled to room
temperature, washed with methanol/acetone solution, centri-
fuged and air-dried. 5 and 10% Co and Fe-doped Ni2P were
prepared by adding 5 and 10% moles of complexes 2 or 3 to
complex 1, agitating the solid mixtures together and then dis-
persing them in TOP followed by sonication. The dispersed
mixtures were then thermolysed as outlined above.
Instrumentation
The elemental composition of synthesized complexes was
obtained on an automated PerkinElmer 2400 series analyzer.
Thermogravimetric analyses were performed employing a
Results and discussion
Mettler-Toledo TGA/DSC at 10 mL min−1 flow rate under Synthetic protocols for the preparation of dithiophosphonate
argon. Powder X-ray diffraction was done via a Bruker D8 complexes are well reported5,24,25 and the formation of com-
Discover Diffractometer using CuKα radiation. Scanning elec- plexes 1, 2 and 3 followed these established procedures. The
tron microscopy (SEM) analyses of particles were carried out thermal stability and decomposition profile of complexes 1, 2
using Philips XL30 FEG-SEM. Energy-dispersive X-ray spec- and 3 were studied by thermographic analysis (TGA) under inert
troscopy was carried out with a DX4 detector. All analyzed par- atmosphere before thermolysis, and the results are shown in
ticles were carbon-coated by an Edwards coating system E306A the ESI (Fig. S1†). The thermogram of complex 1 indicates mul-
before the SEM analyses. Transmission electron microscopy tiple-step decomposition, which sets in at 100 °C. The initial
(TEM) and high-resolution transmission electron microscopy decomposition step of complex 1 with 18.9% weight loss corre-
(HRTEM) examinations of the prepared particles were per- lates to the calculated (21%) loss of the two hydroxyl and
formed employing a JEOL 1400 TEM and JEOL 2100 HRTEM. methoxy groups of the complex. The decomposition step of
Materials’ preparation for TEM and HRTEM were done by drip- complex 1 at 380 °C with 50% weight loss correlates to the cal-
ping a drop of the diluted particle solution on coated Formvar culated (43%) loss of all the organics leaving behind P2S4Ni.
grids for TEM and holey carbon grids for HRTEM. These pre- According to the TGA, further decomposition of complex 1
pared particles were left to dry completely at room temperature leads to phosphorus loss at about 850 °C with only about 19%
and then viewed for TEM and HRTEM at voltages of 120 and weight residue left that corresponds to the calculated weight of
200 kV, respectively. Images were acquired images using iTEM 18.2% for NiS. TGA of complex 2 shows that it decomposes in 3
software for TEM and Gatan software for HRTEM. The optical steps beginning at 195 °C with an experimental % weight loss
properties of particles in dichloromethane were examined by a of 39. The 39% weight loss of complex 2 indicates the decompo-
UV–vis/NIR PerkinElmer Lambda 1050 spectrophotometer. sition of all the organics, with only CoS6P3 residue left (calcu-
The X-ray photoelectron spectroscopy (XPS) spectra were lated residue 40%). The second and third decomposition steps
recorded on a Kratos Axis Ultra DLD spectroscopy. The curve of complex 2 show the further loss of phosphorus and a calcu-
fitting and quantitative analysis was obtained by measuring lated theoretical % weight residue of 17.1 (Co2S3) that matches
the area under the elemental and synthesized peaks. Data ana- well with the experimental value, i.e., 17%. Complex 3 decom-
lysis was conducted using the NIST XPS database for peak poses in
a
single step, unlike complexes
1
and 2.
assignments. Decomposition sets in at 200 °C and ends at 300 °C with an
Electrochemical characterization of the prepared particles experimental % weight residue of 43% that correlates with the
was performed using Gamry Potentiostat, which employed a calculated FeP2S3 of 45%. These results indicate that complexes
three-electrode system. Sample preparation for electrochemical 1–3 are thermally stable and can thermalize cleanly to form
examinations was done by forming pastes of the particles metal sulfides or phosphides. We exploited this to form metal
using the particles (80 wt%), polyvinylidene difluoride (PVDF, phosphides or sulfides from a single molecular precursor in our
10 wt%), and acetylene black (10 wt%), which was prepared earlier report.5
using N-methyl pyrrolidinone (NMP) as a solvent. These
The hot injection decomposition of complex 1 employing
formed pastes were then applied to pre-cleaned and weighted trioctylphosphine oxide (TOPO) as a surfactant and trioctyl-
nickel foams. Dried pastes were then employed as working phosphine (TOP) as a dispersing solvent for 1 hour yielded
electrodes. Ni foams (MTI Corporation, USA), 99.99% purity, nickel phosphide, which is consistent with our previous
were used for these investigations. Commercial carbons were study.5 The formed nickel phosphide matched well to hexag-
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Fig. 1 Diffraction patterns of (a) (a) Ni2P, (b) 5% Co-doped Ni2P, (c) 10% Co-doped Ni2P, (d) 5% Fe-doped Ni2P and (e) 10% Fe-doped Ni2P. (b)
Magnification of the most intense p-XRD peak of all samples, for comparison.
onal Ni2P (ICDD# 01-089-2742). It is interesting to note that compact particles that are well distributed, with the compact-
TOP has been shown to be the phosphorus source in this for- ness of the nanoparticles decreasing as the Ni2P nanoparticles
mation of nickel phosphide despite the presence of intra- are doped with Co and Fe (Fig. 2 and S3†). For a better insight
molecular phosphorus atoms in complex 1.5 Doping Ni2P with into the morphology, transmission electron microscopy (TEM)
5 or 10% of complexes 2 (Co) and 3 (Fe) via the hot injection was used, and the images are shown in Fig. S4.† The particles
route did not change the phase of the nickel phosphide showed large irregular sheet-like morphology with smaller par-
obtained or introduce any impurity. The phase purity of all ticles anchored/stacked on them. We also employed selected
nanomaterials prepared and the effect of doping was evaluated area electron diffraction (SAED) and high-resolution trans-
by p-XRD and is shown in Fig. 1a. The sharp p-XRD diffraction mission electron microscopy (HRTEM) to further investigate
patterns of Ni2P, Co and Fe-doped Ni2P are well matched to the crystallinity and the micro-structure of as-synthesized
pure hexagonal Ni2P and are indexed as per standard reference dinickel phosphides (Fig. 2 and S4†). SAED images showed
pattern (ICDD# 01-089-2742), indicating high crystallinity. The some well-defined spots indicative of crystallinity. However,
absence of any Ni, NiS or NiO diffraction peaks in the doped the surface capping of catalysts with long alkyl chain surfac-
Ni2P indicates phase purity and successful incorporation of tants may also result in reduced crystallinity to some extent.
the dopants in Ni2P crystal structure. In all the samples, diffr- HRTEM images further confirmed the sheet-like morphology
action from the (111) plane was found to be more intense and crystallinity by showing clear lattice fringes that indexed
indicative of the significant growth direction of the hexagonal well with the d-spacing from standard reference patterns for
Ni2P. A closer examination of the most intense peak at ≈2θ = hexagonal Ni2P.
41 (Fig. 1b) shows a slight shift to a lower angle for both
The elemental composition of the prepared materials was
increased Co and Fe-doping at the (111) plane of Ni2P. This undertaken to examine the composition of Ni2P and the doped
could be due to the substitution of Ni2+ ions by the larger Co2+ Ni2P. The energy-dispersive X-ray spectroscopy (EDX) emissions
or Fe2+ ions, as supported by literature.26,27
of all prepared materials showed only Ni and P for Ni2P and Co
The introduction of dopants could result in the modifi- or Fe dopants for the doped Ni2P, which is in agreement with
cation of the electronic and optical properties of a material. the powder XRD. EDX elemental mapping of all the samples
The optical properties of the nanoparticles were therefore ana- confirms the good distribution of the elements as well as the
lyzed, and the spectra are shown in Fig. S2.† All the particles doping of Co and Fe into Ni2P crystal lattice (Fig. 2). X-ray
absorb photons within the UV–vis region of between 250 and photoelectron spectroscopy (XPS) analyses of undoped Ni2P and
300 nm. The absorption peaks of the doped Ni2P particles are doped Ni2P (Fe-Ni2P and Co-Ni2P) was performed since it can
slightly red-shifted relative to that of Ni2P, which could be provide a direct evidence for the introduction of dopants that
indicative of the Co and Fe dopants acting as auxochromes.
modulate the electronic structure of catalysts28,29 and is given in
Scanning electron microscopy (SEM) images were used to Fig. 3 and S5.† The survey spectra of the Ni2P, (Fe-Ni2P and Co-
observe the morphology of the synthesized nanoparticles. The Ni2P) samples show the presence of Ni, P, O, C, (Fe and Co)
SEM images reveal highly agglomerated spherically clustered elements as expected. The presence of oxygen and carbon in the
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Fig. 2 SEM images of (a) Ni2P (b) 10% Co-Ni2P and (c) 10% Fe-Ni2P. Distribution of Ni, P and Co or Fe in (d) Ni2P (e) 10% Co-Ni2P and (f) 10% Fe-
Ni2P. SAED of (g) Ni2P (h) 10% Co-Ni2P and (i) 10% Fe-Ni2P.
samples is attributed to surface oxidation8 and adsorption.30 Fe-Ni2P and Co-Ni2P in the P 2p3/2 window, respectively, which
Moreover the intense peak for carbon is due to the presence of are typical of the P of metal phosphides.30 The Co-Ni2P material
1
2
capping agents on the surface of nanomaterials. Survey spectra has another peak at 127.3 eV in the P 2p window assigned to
analysis of these samples indicates the incorporation of Fe and POx species which could be attributed to oxidation of the phos-
Co in the doped Ni2P which is in agreement with the p-XRD phorus in the sample.30 High resolution spectra of the Fe 2p
and SEM-EDX results. The XPS peak at 856.7 eV of the Fe-Ni2P region of Fe-Ni2P (Fig. 3) indicated two peaks at 713 and 725.8
is assigned to Ni2+ in the Ni 2p3/2 window while the satellite eV in the Fe 2p3/2 and Fe 2p1/2 windows, respectively, assigned
peak at 862.7 eV in the same window is attributed to surface oxi- to Fe3+.30 Similarly the Co 2p window of the XPS results of the
dation (Fig. 3).8 The peaks at 874.2, 875.9 and 880.4 eV are Co-Ni2P show two peaks at 783.1 and 800.6 eV in the Co 2p3/2
assigned to Ni of the Fe-Ni2P in the Ni 2p1/2 window. Similar and Co 2p1/2 windows, respectively, which may indicate pres-
results were observed for the Ni2P and the Co-Ni2P samples as ence of Co2+ and Co3+ as co-existing species.28 These XPS results
shown in Fig. S5.† The P 2p XPS regions (Fig. 3 and S5†) of confirm surface capping of nanoparticles, surface oxidation and
these samples show peaks at 133, 134.6 and 130.9 eV for Ni2P, also show the successful doping of Fe and Co into Ni2P.
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Fig. 3 XPS (a) survey spectrum of Fe-Ni2P. (b) Ni 2p, (c) P 2p and (d) Fe 2p regions of Fe-Ni2P.
Supercapacitance of Ni2P and doped-Ni2P electrodes
We observed that the area under the CV curves of all the
electrodes increases with an increase in scan rate, which is
attributed to diffusion-limited kinetics.31 Eqn (3) was
employed to gain further insight into the diffusion-limited
kinetics of the electrodes.34
The potential of synthesized materials was studied for charge
storage application and the analysis for Ni2P, 10% Co-doped
Ni2P and 10% Fe-doped Ni2P (henceforth referred to as Ni2P,
Co-Ni2P and Fe-Ni2P) are shown here, whereas, the results for
5% Co doped Ni2P and 5% Fe doped Ni2P are presented in the
supplementary data. The charge storage capacities were inves-
tigated to determine the effect of doping on Ni2P electrodes.
The cyclic voltammetry (CV) and galvanostatic charge–dis-
charge (GCD) measurements of the five electrodes (Ni2P, Co-
Ni2P, Fe-Ni2P, and 5% doped Co and 5% Fe doped Ni2P) were
obtained within the potential range of 0–0.6 (V, Hg/HgO) using
3 M KOH electrolyte. The CV test, which is the relationship
between the current density and the electric potentials, of the
electrodes for Ni2P, Co-Ni2P and Fe-Ni2P electrodes is given in
Fig. 4 whereas for 5% dopants i.e. 5% Co-Ni2P and 5% Fe-Ni2P
are shown in Fig. S6,† at scan rates ranging from 2 mV s−1 to
300 mV s−1. The CV plots for all the electrodes are indicative of
the electrodes’ pseudo-capacitive behavior.31 The formation of
these peaks could be due to surface faradaic reactions, as
shown in eqn (1) and (2).32,33
i ¼ avb
ð3Þ
In eqn (3), i is the peak current, v is the scan rate (mV s−1),
while a and b are variable parameters. The charge storage
mechanism of electrodes can be separated by capacitive and
diffusion-limited contributions. The charge storage mecha-
nism of an electrode is therefore based on parameter b in eqn
(3). The charge storage mechanism could be diffusion-limited
when b = 0.5 or capacitive when b = 1. The b values of 0.51,
0.54, and 0.51 were obtained for Ni2P, Co-Ni2P, and Fe-Ni2P
electrodes, respectively, suggesting that diffusion-limited far-
adaic reaction dominates over the capacitive process in con-
tribution to the charge storage capacity for all the electrodes
(Fig. 5a). The dominance of diffusion-limited faradiac
process suggests that the synthesized materials behave more
like a battery, rather than the capacitance materials. To esti-
mate the diffusion-controlled and capacitive contributions to
Ni2þ þ 2OHꢀ ! NiðOHÞ2
ð1Þ
NiðOHÞ2 þ OHꢀ ! NiOOH þ H2O þ eꢀ
ð2Þ the total energy storage of all the electrodes, a low and high
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Fig. 4 CV curves of (a) Ni2P, (b) Co-Ni2P, and (c) Fe-Ni2P electrodes at various scan rates.
Fig. 5 (a) Log (current) versus log (scan rate) plot. (b) Capacitance and diffusion contributions at 10 mV s−1 scan rate and (c) at 100 mV s−1 scan rate.
(d) Variation of specific capacitance as a function of scan rate for all samples.
scan rate of 10 and 100 mV s−1 was evaluated employing that of the capacitive process (k1v) is 14, 11, 5% for Ni2P, Co-
eqn (4),34
Ni2P and Fe-Ni2P, respectively (Fig. 5b). At a higher scan rate
of 100 mV s−1, the diffusion-limited contribution of 66, 72,
86% and the capacitive contribution of 34, 28 and 14% were
observed for Ni2P, Co-Ni2P, Fe-Ni2P, respectively (Fig. 5c).
i ¼ k1v þ k2v1=2
ð4Þ
where k1 and k2 are suitable values and k1v and k2v1/2 are Fig. 5d gives the variation of specific capacitance as a func-
capacitive and diffusion-limited contributions to total tion of the scan rate for Ni2P, Co-Ni2P and Fe-Ni2P electro-
storage at a fixed potential, respectively. The total specific des, which indicates excellent specific capacitance mainten-
capacitance of an electrode is, therefore, a combination of ance at different scan rates. These results suggest that the
the capacitive effect (k1v) and the diffusion-controlled (k2v1/ charge was mainly stored by a diffusion-controlled process
2) process. At 10 mV s−1 scan rate, the contribution of for these electrodes, charge storage is maintained at higher
diffusion-limited (k2v1/2) process is 86, 89 and 95%, while scan rates, and that doping Ni2P with Co or Fe synergisti-
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Fig. 6 GCD curves of (a) Ni2P, (b) Co-Ni2P, (c) Fe-Ni2P electrodes at various current densities and (d) variation of specific capacitance as a function
of current density.
cally improves the excellent storage capacity of the Ni2P g−1), 45% lower than Co-Ni2P (864 F g−1), and 44% lower than
electrode.
our Fe-Ni2P (856 F g−1) electrodes, respectively at 1A g−1
Fig. 6a–c and S6† give the galvanostatic charge–discharge current density. A comparison of the supercapacitance of our
graphs of the five electrodes indicating the potential vs. time electrodes to other similar nickel phosphide materials shows
charge–discharge characteristics of the electrodes at different that our materials outperform other electrodes under similar
current densities. As shown in all the graphs, the plateau conditions (Table S1†). The specific capacitance of an elec-
region was observed after a sharp potential drop. These trode generally decreases progressively with current density
plateau regions are attributed to the faradaic redox reactions, due to increasing limitation or infiltration of electrons and
which corresponds to the CV results.32 The Ni2P, 5% Co-Ni2P, ions into the electrode surface. We, consequently, plotted the
and 5% Fe-Ni2P electrodes recorded the specific capacitance of calculated specific capacitance values as a function of the
674, 864, 856 F g−1 at 1A g−1 current density, respectively, current density, which is given in Fig. 6d and S6.† The specific
based on the galvanostatic charge–discharge profiles. The capacitance of 417, 554, and 530 F g−1 was recorded for our
recorded specific capacitance shows that doping Ni2P with Co Ni2P, Co-Ni2P, and Fe-Ni2P electrodes at a high current density
and Fe improves its supercapacitance properties. The superior of 30 A g−1. This result does not only indicate excellent stabi-
performance of the Co-doped Ni2P electrode compared to that lity of charge storage capacities for our electrodes but also
of the Fe-doped Ni2P electrode may be due to the superior shows that doping Ni2P with Co and Fe improves the Ni2P elec-
theoretical specific capacitance of Co-based electrodes com- trodes’ charge retention capacity.
pared to that of Fe-based ones. Lu et al. employed an organic-
We also measured the long-term stability of the Ni2P, Co-
phase strategy to synthesize Ni2P electrodes that recorded a Ni2P, and Fe-Ni2P electrodes in 3000 charge–discharge cycles,
specific capacitance of 418 F, which is 40% lower than our as shown in Fig. 7. The electrodes showed a high retention
Ni2P electrode (674 F g−1) at the same 1A g−1 current density.33 capacity of 89, 85 and 74% of the initial cycles after 3000
The synthesized Ni2P electrodes in that study were fabricated cycles, along with 99% coulombic efficiency for Ni2P, Co-Ni2P
into a composite by coating with Ni, which improved their and Fe-Ni2P electrodes, respectively, suggesting good cyclic
specific capacitance to 581 F g−1 compared to ours, where the stability. The CV and GCD measurements show that doping
doped Ni2P electrodes had 864 F g−1 (Co-Ni2P) and 856 F g−1 our Ni2P electrodes with Co and Fe dramatically improved its
(Fe-Ni2P), respectively. Liu et al. recently prepared a series of energy storage properties.
Ni2P doped polypyrrole composites and tested their electro-
chemical application as supercapacitors.35 Their 30% Ni2P
Water splitting of Ni2P and TM doped-Ni2P electrocatalysts
doped polypyrrole composite had the best performance of The electrocatalytic performance of Ni2P, Co and Fe-doped
476.5 F g−1, which is 29% lower than our prepared Ni2P (674 F materials for OER was measured in a standard 3 electrode
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Fig. 7 Capacitance retention and coulombic efficiency of (a) Ni2P, (b) Co-Ni2P, (c) Fe-Ni2P for 3000 cycles.
system using 1M KOH electrolyte. Fig. 8a, b and S7† shows the ively. The 5% Co-Ni2P and 5% Fe-Ni2P electrodes required an
OER polarization curve along with the Tafel slopes for Ni2P, overpotential of 298 and 355 mV, respectively to achieve the
Co-Ni2P and Fe-Ni2P samples. The performance of an OER same current density. Furthermore, the Tafel slope of Ni2P was
catalyst is evaluated conventionally by measuring the overpo- measured as 103 mV dec−1, but after Co and Fe doping, the
tential required to produce a current density of 10 mA cm−2 36
.
slope of 98, 91, 130 and 80 mV dec−1 for 5% Co-Ni2P, 10% Co-
Our Ni2P catalyst required an overpotential of 340 mV, while Ni2P, 5% Fe-Ni2P and 10% Fe-Ni2P, respectively was measured,
320 and 259 mV was required for Co-Ni2P and Fe-Ni2P suggesting faster reaction kinetics after Co and Fe doping.37 It
materials to deliver a current density of 10 mA cm−2, respect- can be observed from these results that the higher concen-
Fig. 8 (a) Polarization curves, (b) Tafel slopes of all samples for OER process, and (c) Nyquist plot at 0.6 V (V vs. SCE).
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tration of iron i.e. 10% Fe-Ni2P (259 mV) decreased the overpo- electrochemical impedance spectroscopy (EIS) measure-
tential more as compared to all the samples. Density func- ments were employed to further study the effect of doping
tional theory analysis has shown that doping the nickel phos- Co and Fe into the Ni2P electrode. These measurements
phide system with Co leads to the substitution of the nickel (Nyquist plots), which are typically used to explore the
metal with Co, which provides more active sites for charge transfer processes at the electrode, were obtained
OH−adsorption and subsequent OH− oxidation to form O2.13 within the frequency range of 0.05 Hz to 10 kHz with applied
The better performance of Fe-Ni2P catalyst compared to Co- AC amplitude of 10 mV at 0.6 V (V vs. SCE) and are given in
Ni2P in OER could be due to better oxygen binding energy of Fig. 8c and S7.† In the obtained Nyquist plots, two arcs are
Fe compared to Co in doped-Ni2P catalyst. Fe-Ni2P could there- seen in Ni2P, 5% Co-Ni2P, Co-Ni2P and 5% Fe-Ni2P electrodes
fore generate more moderately charged O anions to balance and only one semicircle was observed in the Fe-Ni2P elec-
the binding energies among O*, HO* and HOO* species trode. One arc at the high frequency represents the charge
leading to lesser OER overpotential compared to Co-Ni2P.38 transfer resistance; the other arc at the low frequency rep-
Lower Tafel slope values are associated with better OER cata- resents the mass transfer resistance.40 The diameter of the
lysts, and it was observed that higher doping concentrations of arc or semicircle is a function of the charge transfer resis-
Co and Fe into Ni2P lowered the Tafel slopes significantly. It is tance.36 The smaller diameter of the arc at the high fre-
interesting to note that, except 5% doped Fe-Ni2P, all prepared quency for two of our electrodes indicates less charge trans-
catalysts had better Tafel slopes than that of the benchmark fer resistance and thus better charge transfer in the order Fe-
OER catalyst IrO2 of 124.5 mV dec−1. Doping a foreign atom Ni2P > Co-Ni2P > Ni2P. The 5% Co-Ni2P electrode had less
into a crystal lattice can synergistically modulate its local charge transfer resistance compared to the 5% Fe-Ni2P
coordination and electronic structure leading to improved though both of them did not improve the charge transfer re-
OER activity.39 The good performance of our Ni2P electrode as sistance of the Ni2P electrode. In the case of arcs at low fre-
an OER catalyst is further improved by doping, likely due the quency, there is none for the Fe-Ni2P graph but one in 5%
new environment created in it by incorporating Co and Fe into Co-Ni2P, Co-Ni2P, 5% Fe-Ni2P and Ni2P graphs. The smaller
its crystal lattice.
diameter of the arc at low frequency for 5% Co-Ni2P, Co-Ni2P
Table S2† gives a further comparison of our electrodes as and 5% Fe-Ni2P compared to Ni2P indicate that these electro-
OER catalysts with respect to the OER activity of some state- des have less mass-transfer resistance than the Ni2P elec-
of-the-art nickel phosphide-based electrocatalysts. The trode. These results of charge transfer resistance put
Fig. 9 (a) Chronoamperometry (CA) at 0.55 V (V vs. SCE) for all samples. OER polarization curves at different cycles for (b) Ni2P, (c) Co-Ni2P, and (d)
Fe-Ni2P.
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together show that doping our Ni2P with Co and Fe generally 10 mA cm−2 current density for HER. The 5% Fe-Ni2P elec-
improves its OER electrocatalytic activity. trode with 68 mV overpotential had the best synergistic cata-
The long-term stability of a catalyst is crucial to its practical lytic performance with the un-doped Ni2P electrode by redu-
applications. The durability of our electrodes was therefore cing its overpotential by about 56%. All the doped electrodes
investigated employing the chronoamperometry (CA) test and had a lower calculated Tafel slope with respect to the un-
polarization curve measurements, which are shown in Fig. 9a– doped Ni2P electrode suggestive that doping in this study
d and S7.† CA measurements were done at 0.55 V (V vs. SCE), improved the reaction kinetics for HER. Doping Ni2P with Co
and a stable current density was delivered for all samples over and Fe only improved slightly the catalytic and reaction kine-
35 hours. A slight fluctuation was observed in the graphs for tics of the Ni2P electrode unlike the OER and supercapaci-
all samples during the CA test due to the bubbling caused by tance. Kucernak et al. fabricated Ni2P and Ni12P5 materials on
the occurrence of oxygen gas during oxygen evolution.41
A
glassy carbon electrodes and recorded about 270 and 450 mV
measurement of the polarization curves for the 1st cycle and overpotential with Tafel slopes of 84 and 108 mV dec−1
,
the 1000 cycles showed a negligible deviation between two which are lower than the performance of our electrodes.42
curves for all the electrodes (Fig. 9b–d and S7†), indicating the Similarly, Tian et al. prepared Ni12P5 materials from a metal–
excellent stability or durability of our electrodes as OER organic framework that required about 670 mV overpotential
catalysts.
We also examined the electrocatalytic activity of our elec- 270 mV dec−1 44
to achieve 10 mA cm−2 current density with a Tafel slope of
Our as prepared catalysts compare well or
.
trodes for hydrogen evolution reactions. Fig. 10 and S8† give outperform other HER catalysts in electrocatalytic examin-
the polarization curves and Tafel slopes of all the samples for ations and these comparative results are further shown in
the HER. The Ni2P, 5% Co-Ni2P, Co-Ni2P, 5% Fe-Ni2P and Fe- Table S3.†
Ni2P electrodes recorded an overpotential of 164, 156, 158, 68
The stability and durability of our electrodes for HER were
and 202 mV to achieve a current density of 10 mA cm−2 with also investigated and are shown in Fig. 11 and S8.† A compari-
Tafel slopes of 117, 110, 113, 110 and 113 mV dec−1, respect- son of the polarization curves for all the electrodes for 1000
ively. All the doped Ni2P electrodes except Fe-Ni2P decreased cycles are well-matched, indicating excellent durability and
the overpotential of the un-doped Ni2P electrode to attain stability.
Fig. 10 (a) Polarization curves and (b) Tafel slopes of all samples for HER process.
Fig. 11 HER Polarization curves at different cycles for (a) Ni2P, (b) Co-Ni2P, and (c) Fe-Ni2P.
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the years 2020
MSCA-COFUND/2019/2.
-
2024; agreement no. 5005/H2020-
Conclusion
We have synthesized dithiophosphonate complexes of Ni, Co
and Fe, and employed them to prepare Ni2P, Co- and Fe-doped
Ni2P composites by the hot injection method. The doping of
Co and Fe into Ni2P did not change its crystal structure or
introduce any impurity and were uniformly distributed.
Doping was shown as a strategy to improve the electrochemical
performance of the prepared Ni2P with Co-Ni2P, recording a
supercapacitance of 864 F g−1 compared to Ni2P with 674 F g−1
at 1A g−1 current density. Similarly, Fe-Ni2P with an overpoten-
tial of 69 mV dec−1 to achieve a current density of 10 mA cm−2
enhanced the OER activity of Ni2P that had an overpotential of
103 mV dec−1 to achieve the same current density. The pre-
pared nickel phosphides also showed good catalytic activity in
HER examinations and excellent stability as electrochemical
catalysts, however, the doping did not significantly enhance
the HER performance. These results indicate that our phos-
phide materials show good potential as a catalyst to be
employed in water splitting and supercapacitor technologies.
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