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Cite this: Dalton Trans., 2021, 50,12870

Received 14th July 2021,Accepted 17th August 2021

DOI: 10.1039/d1dt02343h

rsc.li/dalton

Solid-state synthesis of single-phase nickelmonophosphosulfide for the oxygen evolutionreaction†

Miao Wang,a,b Ali Saad,a,b Xiaoguang Li, a Tao Peng,a,b Qi-Tao Zhang,c

Mohan Kumara,b and Wei Zhao *a

High-performance and cost-effective nonprecious-metal catalysts are essential for the next-generation

oxygen evolution reaction (OER). However, the electrocatalysis of the OER during water splitting is often

carried out by using noble metal catalysts, such as RuO2 or IrO2 with high-cost and limited stability.

Herein, we reported a successful synthesis of a ternary nickel monophosphosulfide (NiPS) compound via

a simple solid-state route and further investigated its electrocatalytic performances for water oxidation. It

is found that the NiPS electrocatalyst exhibits good OER performance in 1.0 M KOH solution, i.e., achiev-

ing a current density of 20 mA cm−2 at an overpotential of 400 mV and a Tafel slope of 126 mV dec−1,

comparable to commercial benchmark RuO2. The ternary NiPS electrocatalyst for the OER is superior to

its binary counterparts, i.e., Ni2P and NiS. Density functional theory (DFT) calculations combined with ex

situ XPS were performed to obtain further insights into the intrinsic catalytic mechanism of NiPS, and their

results clearly revealed that the instability of the NivO intermediate during the OH* → O* process and

the easy oxidation of the (PS)3− anion favoring the formation of hydroxyl-based species (i.e., Ni(OH)2/

NiOOH) on the surface of the catalyst, which plays a crucial role in facilitating the OER activity.

Furthermore, we creatively extended this method to the fabrication of heteroatom substituted catalysts

and a new quaternary CoNiP2S2 compound was successfully synthesized for the first time in the same

way. The structural properties and electrocatalytic performance towards the OER for CoNiP2S2 (e.g.,

20 mA cm−2 at 376 mV) are also systematically investigated in this work.

Introduction

Hydrogen (H2) as a promising renewable energy source hasattracted extensive attention in replacing finite reserves offossil fuels.1–3 Hydrogen with high purity could be producedby electrocatalytic water splitting which consists of two mainpathways such as the OER (oxygen evolution reaction) at theanode and the HER (hydrogen evolution reaction) at thecathode. However, when considering the kinetics factors, theOER-pathway via multi-step proton-coupled electron transfer ismore sluggish than the HER-pathway via a two-electron trans-fer reaction. Therefore, it is crucially needed to control the

factors that can accelerate the OER-pathway.4–6 At present, theelectrocatalytic techniques for the OER process in water split-ting mainly rely on precious metal electrocatalysts (i.e., IrO2

and RuO2).7,8 Motivated by this challenge, it is desirable to

investigate new electrocatalysts with cost-effectiveness, eco-friendliness, and high catalytic activities for practical use.Numerous endeavors have been made in order to solve thisproblem.

Transition metal oxides,9,10 chalcogenides,11,12

phosphides,13–15 and carbides,16–18 together with numerousmetals like Mn, Fe, Co, and Ni have been explored for watersplitting reactions. Recently, some studies have shown that thesynergistic effects between transition-metal phosphides(TMPs) and transition-metal chalcogenides (TMCs) can effec-tively improve the electrocatalytic performances.19–22 Forinstance, density functional theory (DFT) calculations demon-strated that substituting the S element with the less electrone-gative P element in CoS2 could tune the electronic structure formore thermo-neutral hydrogen adsorption (ΔGH*) at the activesites and thus improve the electrocatalytic activity.19,20 Weknow that high-valence transition metal ions (e.g., Ni3+) are

†Electronic supplementary information (ESI) available. See DOI: 10.1039/d1dt02343h

aInstitute for Advanced Study, Shenzhen University, Shenzhen 518060, China.

E-mail: weizhao@szu.edu.cnbCollege of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen

518060, ChinacInternational Collaborative Laboratory of 2D Materials for Optoelectronics Science

and Technology of Ministry of Education, Institute of Microscale Optoelectronics,

Shenzhen University, Shenzhen 518060, China

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less electronegative due to a lack of 3d orbital electrons com-pared to neutral or low-valence metal ions, facilitating theelectrocatalytic process (e.g., 4OH− → O2 + 2H2O + 4e−, OER)in alkaline medium and leading to enhanced electrochemicalperformances.23–25 Hence, ternary transition-metal phosphide-chalcogenides (TMPCs), a class of double-anion materials, arepromising electrocatalysts for water splitting. Very recently,Peng et al. reported a successful fabrication of Ni1−xFexPS via ahigh-pressure and high-temperature (HPHT) route withefficient OER activity for the first time.26 Zhuo et al. found thatthe incorporation of Se into NiP2 can achieve double-anionNiP1.93Se0.07 that significantly enhanced the HER owing to themodulation of the electronic structure of the catalyst.22 Atpresent, many synthetic pathways such as the chemical vaportransport method (CVT), hydrothermal method, high pressuremethod (HP), etc., were utilized to fabricate energetic nano-materials, while these synthesis routes have shown someshortcomings, for example harsh reaction conditions, a lack ofphase purity or difficulty in tuning the atomic ratio of the finalsample.19,20,26–31

On the other hand, recent investigations indicated thatcombining two or more metallic species together is attractingincreasing attention in the field of energy materials.26–30,32 Forinstance, Jiang et al. proposed the combination of Fe and Nicomponents in ternary FeNiS2 nanosheets with an excellentelectrocatalytic performance toward the OER;27 Wang et al.reported ternary NiCoP nanoparticles with good electro-catalytic activity and stability for water splitting;32 A bimetallicCo1−xFexP/CNT hybrid as a highly-efficient OER electrocatalystin alkaline electrolytes was also reported by Zhang et al.28 Notethat Du et al. recently reported a 113-type of metal iron-dopedNi1−xFexPS3@MXene nanohybrid which showed a good electro-catalytic performance for water splitting.30 To the best of ourknowledge, there is no any relevant report available on cobalt-doped 111-types of nickle phosphosulfides (i.e. NiPS) to date.

In this paper, a ternary NiPS compound was successfullygrown in an evacuated quartz tube via a simple high-tempera-ture reaction of a 1 : 1 : 1 stoichiometric mixture of high-puri-fied Ni, P, and S powders (see Scheme 1 and Fig. S1 in theESI,† and also verified through the Rietveld refinement ana-lysis of the XRD pattern. This kind of confined reaction systemutilized here plays a crucial role in the formation of high-purity NiPS samples due to a strong inhibition effect on theloss of volatile P and S elements. The Rietveld refinementresult of NiPS further reveals an ordered distribution of pnicto-

gen and chalcogen atoms with the Ni–P and Ni–S bonding of2.32(5) Å in the NiPS crystal, respectively, which is differentfrom the CoPS crystal (a typical pyrite-like structure). Theelectrochemical characterization towards the water splitting ofthe as-obtained NiPS catalyst shows an overpotential of400 mV achieved at 20 mA cm−2 with a Tafel slope of 126 mVdec−1 in alkaline medium, comparable to those of commercialRuO2 and superior to binary compounds (i.e., Ni2P and NiS).Furthermore, we extended this method to the synthesis to Nisite-substituted catalysts and a new quaternary Co0.5Ni0.5PS (orCoNiP2S2) compound was successfully grown for the first timevia the same way, as displayed in Fig. S(22–26) and Table S2.†The relevant electrochemical characterization demonstratedthat CoNiP2S2 shows a superior performance towards the OER(Fig. S27†), making CoNiP2S2 another promising candidate forthe further development of new practical non-precious metalelectrocatalysts.

Experimental sectionMaterials

Nickle powder [99.5% purity], cobalt powder [99.9% purity],red phosphorous [98.5% purity] and sublimate sulphur[99.95% purity] were purchased from Aladdin Company. Thesolvent of ethanol was obtained from Shanghai ChemicalReagents Company, China. All reagents were used as receivedwithout further purification.

Sample synthesis

The NiPS sample was grown via a conventional solid-state reac-tion in an evacuated silica tube. Note that the silica tube wasfirst heated to 1273 K to remove any water. In typical synthesis,stoichiometric amounts of Ni powder, red phosphorus andsublimated sulphur were thoroughly mixed together andsealed in a quartz ampoule. Then the mixture was heated to973 K at a ramping rate of 10 °C min−1 in the muffle furnaceand maintained for 3 days. For a comparative study, Ni2P, NiSand CoNiP2S2 samples were also prepared in a similar way.The stoichiometric amounts of Co powder, Ni powder, redphosphorus or sublimated sulphur were thoroughly mixedtogether in a quartz ampoule, and heated at 973 K for 2 days.After cooling down to room temperature naturally, the darkgray sample was washed with absolute alcohol and DI waterseveral times, and then dried in a vacuum oven at 60 °C.

Structural characterization

The phase and purity of the products were identified by usingthe X-ray diffraction (XRD) patterns collected on a PhilipsX’pert X-ray diffractometer with graphite-monochromatized CuKα radiation (λ = 1.5418 Å). The scanning electron microscopy(SEM) images were examined by using a JEOL-JSM-6700F field-emitting (FE) scanning electron microscope at an accelerationvoltage of 100 kV. The transmission electron microscopy(TEM) images and high-resolution TEM (HRTEM) images weretaken on a JEOL-2010 HRTEM at an acceleration voltage of 200

Scheme 1 Graphical illustration for the synthesis of the single-phaseNiPS sample.

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kV. X-ray photoelectron spectroscopy (XPS) was performed onThermo ESCALAB 250. The elemental ratios of the sampleswere analyzed from XPS results. N2 adsorption–desorptionmeasurements were carried out on an ASAP2460 instrument,Micromeritics company.

Electrochemical characterization

The carbon cloth was firstly washed with acetone, alcohol andwater, and then dried in a vacuum oven for subsequent OERtests. The catalyst ink was prepared by dispersing 6 mg of thecatalyst in 1 mL of the mixed solvent of DI water : ethanol(1 : 1), and 30 µL of Nafion followed by ultrasonication formore than 30 minutes. After that, 30 μL of the catalyst ink wasloaded onto a carbon cloth (0.5 cm × 0.5 cm) followed bydrying in an oven at 50 °C. The OER performance was evalu-ated by using a CHI660E electrochemical workstation (CHInstruments, Inc., Shanghai, China) in a standard three-elec-trode system, in which Ag/AgCl (saturated KCl solution) wasthe reference electrode, the graphite rod electrode was thecounter electrode, and the modified carbon cloth electrodewas the working electrode. All the achieved linear sweep vol-tammetry (LSV) curves were recorded at 5 mV s−1. The LSVcurves before and after the iR-correction almost overlap. Theamperometric i-t curve measurement was performed based ona current density of 20 mA cm−2 (0.61 V vs. Ag/AgCl). Thecyclic voltammetry (CV) results were tested from 0.2 to 0.3 V at2, 4, 6, 8, and 10 mV s−1. Electrochemical impedence spectro-scopic (EIS) measurements were performed at frequenciesranging from 100 000 to 0.01 Hz at 0.6 V. The equation E(RHE) = E (Ag/AgCl) + 0.0592 × pH + 0.197 was utilized fortransforming the reference electrode to reversible hydrogenelectrode (RHE) in this work.

Computational details

All the first-principles spin-polarized calculations were per-formed using the Vienna ab initio simulation package(VASP).33,34 Projector augmented wave (PAW) potentials wereemployed to describe the core-valence interactions.35 The elec-tronic exchange and correlation were treated by the generalizedgradient approximation with the Perdew, Burke, and Ernzerhof(PBE) functional36 with onsite Hubbard U corrections,37 inwhich the U value for Ni was chosen as 5.5 eV. The energycutoff for the plane-wave basis was set as 400 eV. The conver-gence criterion was set as 0.01 eV Å−1 for maximal forces and10−5 eV for energies. The Brillouin zone was sampled by usinga 9 × 9 × 9 Monkhorst–Pack k-point mesh for the relaxation ofthe NiPS crystal, while a 13 × 13 × 13 Monkhorst–Pack k-pointmesh was chosen for self-consistent field calculations and theanalysis of density of states. All the structures were shown byVESTA.38 To understand the adsorption of OH* and O* inter-mediates in NiPS, we cleaved the (200) surface of the NiPScrystal, which was clearly observed in the HRTEM image inFig. 2d. This slab contains 5 layers of the NiPS unit, with achemical formula of Ni20P20S20. The vacuum layer is 15 Å. A 5× 5 × 1 Monkhorst–Pack k-point mesh was employed for theslab calculations.

Results and discussion

The Rietveld refinement data of NiPS are displayed in Fig. 1b.The Bragg peaks in the X-ray diffraction pattern can be wellindexed to a cubic structure with the space group of P213.Specifically, the diffraction peaks at 28.1°, 32.5°, 36.4°, 40.0°,46.5°, 55.1°, 57.9°, 60.3°, and 62.8° can be indexed well to the(111), (200), (210), (211), (220), (311), (222), (023), and (321)planes of cubic NiPS, respectively. No obvious impurity phasecan be observed in the pattern. The atomic equivalent posi-tions of NiPS are Ni: 4a (0, 0, 0), P: 4a (0.6118, 0.6118, 0.6118),and S: 4a (0.3882, 0.3882, 0.3882), and all the lattice constantsa, b, and c are 5.5473 Å, as shown in Table 1. Fig. 1a illustratesthe crystal structure of NiPS. In this compound, the pnictogenand chalcogen atoms show an ordered distribution, and the

Fig. 1 (a) The schematic view of the crystal structure of NiPS. (b)Measured (crosses) and fitted (red solid line) XRD patterns for NiPS.Bragg peak positions are indicated by short vertical bars. The bottom ofthe figure shows the differences between the measured and calculatedintensities.

Table 1 Crystallographic parameters from the powder XRD refinementof the NiPS sample at 300 K

Atom Site x y z Occup. Uiso (fixed)

Ni 4a 0 0 0 1.00 e−5

P 4a 0.6118 0.6118 0.6118 1.00 2 × e−5

S 4a 0.3882 0.3882 0.3882 1.00 2 × e−5

Space group: P213; a = b = c = 5.5473 Å, V = 170.7045 Å3, Rwp = 0.0610,Rp = 0.0466, and χ2 = 1.063.

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lengths of both the Ni–P and Ni–S bonds in the NiPS crystalare 2.32(5) Å. The XRD patterns of the as-synthesized Ni2P andNiS samples are shown in Fig. S2 and S3 in the ESI,†respectively.

The morphology of the as-synthesized NiPS sample is firstlymonitored by using the SEM (scanning electron microscopy)images as shown in Fig. 2a and Fig. S4.† The NiPS sampleshows an unfixed plate-like aggregation. To further identifythe assembly of the NiPS sample, TEM (transmission electronmicroscopy) images are also collected, as shown in Fig. 1(band c). The high-resolution TEM (HR-TEM) image of NiPSshows clear crystal lattices with a d-spacing of 0.275 nm, whichcorresponds well to the (200) facet of the NiPS crystal (Fig. 2d).The elemental distribution of the as-synthesized NiPS isfurther investigated by Scanning TEM-energy dispersive X-ray(STEM-EDX) analysis. The results indicated that Ni, P, and Selements are uniformly distributed in the NiPS sample as dis-played in Fig. 2(e–h). Both of the Ni2P and NiS samples havean irregular morphology (Fig. S5 and S6†). The BET surfacearea and pore structure of the sample were evaluated usingnitrogen adsorption. Fig. S7 and S8† display the nitrogenadsorption–desorption isotherms and the corresponding pore

size distribution curves of the as-synthesized NiPS compound,respectively. The N2 sorption isotherms of NiPS are of type IIwith a distinct hysteresis loop (Fig. S7†). The pore size distri-butions of NiPS are mainly located at about 3.8 nm, indicatingthe existence of mesoporous materials in the sample, asshown in Fig. S8.†

The surface analysis of the as-synthesized NiPS sample ismonitored by the X-ray photoelectron spectroscopy (XPS) tech-nique. The high-resolution XPS spectra in Fig. 3a show thatthe Ni 2p spectrum is well fitted with two spin–orbit doubletsaccompanied by two obvious shakeup satellites (863.0 eV and881.6 eV), assigned to Ni 2p1/2 and Ni 2p3/2 signals. The peaksaround 854.2 eV and 871.5 eV are attributed to the 2p3/2 and2p1/2 levels of Ni

2+. Meanwhile, the two peaks around 857.9 eVand 875.7 eV indicated the presence of Ni3+ in NiPS.39,40 TheXPS spectrum of the P 2p core level (Fig. 3b) shows the spin–orbit doublet peaks of 2p3/2 and 2p1/2 at 131.8 eV and 132.6 eV,respectively, and the peak observed at 134.5 eV is possibly gen-erated by the slight oxidation of P on the surface of the NiPSsample.41 In addition, Fig. 3c displays the S 2p spectrum,where S (2p3/2) and S (2p1/2) are located at 162.3 eV and 163.5eV, respectively.42 Fig. S9† shows the comparison of the Ni 2p

Fig. 2 (a) SEM image of the as-synthesized NiPS sample. (b) Low-magnified, and (c) high-magnified TEM images of the NiPS. (d) HR-TEM image ofthe NiPS. (e–h) HAADF-STEM and corresponding EDX mapping images of the NiPS.

Fig. 3 X-Ray photoelectron spectral regions for the (a) Ni 2p (b) P 2p and (c) S 2p levels of the as-synthesized NiPS sample.

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levels among all the as-synthesized samples. A positive shift ofthe binding energy observed in the NiPS sample clearlydemonstrates a slightly higher valence of Ni than those of itsbinary counterparts (Ni2P and NiS), boosting the OERperformance.

The electrocatalytic performance of the as-synthesizedsamples towards the OER-pathway is investigated in 1.0 MKOH solution (Fig. 4 and Fig. S10†). The results indicated thehighest activity of NiPS which exhibited the lowest overpoten-tial of 400 mV at a current density of 20 mA cm−2 compared tothose of the Ni2P (450 mV), NiS (449 mV), and blank carboncloth samples, while comparable to those commercial RuO2 asshown in Fig. 4a and Fig. S10.† In addition, based on the LSVcurves, the corresponding Tafel plots are constructed(Fig. S11†). The Tafel plot is a useful method to evaluate thecatalytic activity and an indicator of the reactionmechanism.43,44 Impressively, the NiPS catalyst shows thesmallest Tafel slope (126 mV dec−1) among all the as-preparedsamples which is lower than that of commercial RuO2 (104 mVdec−1), implying a favorable reaction kinetics for the NiPS cata-lyst during the OER-pathway. Stability is another importantcharacteristic for the evaluation of catalytic performance.Hence, the stability performance of the NiPS catalyst is alsomonitored after continuous 5000 CV-cycles in 1.0 M KOH solu-tion at a scan rate of 50 mV s−1, which indicates no obviousactivity change, suggesting a good stability of the NiPS catalystfor the long-term electrochemical process (Fig. 4b). Inaddition, the long-term durability of the NiPS catalyst is moni-tored via a time-dependent current density plot for 20 h asshown in Fig. 4c, in which the upward trend observed before10 h could be attributed to an internal activation process of

the catalyst. The proposed OER catalytic mechanism of NiPS inalkaline solution involves five elementary steps as follows:26

OH•� þ * ! OH*þ e� ð1Þ

OH* ! Hþ þ O*þ e� ð2Þ

OH•� þ O* ! OOH*þ e� ð3Þ

OOH* ! Hþ þ O2 � þe� ð4Þ

O2* ! *þ O2 ð5ÞThe electrochemical active surface area (ECSA) of the cata-

lysts is also measured to obtain further insights into theinternal electrocatalytic performance.45,46 The CV curves ofNiPS, Ni2P, and NiS are tested from 0.2 to 0.3 V, and then thedifferences between the cathode and anode current densitiesat 0.25 V plotted versus the corresponding scan rates tomeasure the double-layer capacitance (Cdl). The Cdl values of7.6 mF cm−2, 5.9 mF cm−2, and 5.8 mF cm−2 are shown by theNiPS, Ni2P, and NiS electrocatalysts, respectively, which revealsthat the NiPS catalyst is able to offer more electrocatalyticactive sites, thus leading to enhanced catalytic performances(Fig. S12†). In order to exclude the influence of the surfacearea on the intrinsic activity, the LSV current density is nor-malized to the electrochemical surface areas, as displayed inFig. S13.† The ECSA can be calculated from the Cdl valuesaccording to the formula: ESCA = Cdl/Cs, where Cs is thespecific capacitance, and is chosen to be Cs = 0.040 mF·cm−2

in 1 M KOH based on the reported values (see Table S1†). NiPSpossesses an ECSA of 190 cm2 with an intrinsic catalyst activity(0.107 mA cm−2 ESCA at η = 430 mV) which is superior to

Fig. 4 (a) LSV curves of the NiPS, commercial RuO2 and blank carbon cloth in 1.0 M KOH solution (scan rate 5 mV s−1) for the OER. (b) Polarizationcurves of the NiPS before and after 5000 CV cycles. (c) Time-dependent current density curve of NiPS for the OER in 1.0 M KOH solution. (d)Calculated PDOS and partial charge density range from −1 eV to 0 eV relative to the Fermi energy of the NiPS model.

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those of single nickel phosphide (0.082 mA cm−2 ESCA) andnickel sulfide (0.065 mA cm−2 ESCA). These results reveal thatNiPS exhibits excellent intrinsic activity, and further offersmore active sites generated toward the OER activity. This con-tributes to the superior OER activity of NiPS when comparedto those of other electrocatalysts.

The EIS measurements are also performed for NiPS, Ni2P,and NiS to evaluate the kinetics of the electron transfer duringthe electrocatalytic process, as shown in Fig. S13.† The chargetransfer resistance (Rct) is related to the semicircle observed inthe Nyquist curve.47,48 It is found that the smallest semicircleis obtained for NiPS, suggesting the smallest value of Rct, andthus the best OER kinetics among the tested catalysts.Fig. S14† shows the results of the hydrogen peroxide (H2O2)rapid test card of the catalysts synthesized in this work beforeand after the OER, indicating that no signals of H2O2 (the 2e−

process) were detected. Fig. S15† gives the SEM images of NiPSafter the OER, showing a similar plate-like aggregation.

To obtain further insights into the mechanism related to theOER process, the XPS spectra of the NiPS catalyst of the post-OER are studied. In Fig. S16,† the two major peaks observed at873.3 eV and 855.7 eV in the Ni 2p spectrum are attributed tothe Ni 2p1/2 and Ni 2p3/2 signals, respectively, demonstratingthe main formation of a high-valence (e.g., Ni3+) intermediate inthe sample.49–51 The peaks positioned at 880.0 eV and 861.1 eVare ascribed to the two shakeup satellites in the Ni 2p spectrum.The O 2p spectra of the NiPS catalyst before and after the OERtest are shown in Fig. S17 and S18,† respectively. It is clearlyindicated that the generation of hydroxyl-based intermediates(i.e., Ni(OH)2/NiOOH) during the OER process due to theobserved peak of the surface hydroxyl groups at 530.9 eV(Fig. S18†),52 and the formed Ni(OH)2/NiOOH species improvethe OH− adsorption process and reduce the Gibbs free energyof the electrochemical intermediate during the OER, thusleading to a good OER performance for the NiPS electrode.53,54

The peaks at ∼532 eV in Fig. S17 and S18† correspond to thelattice oxygen. It is well known that the Ni(OH)2/NiOOH moietyis described as a fundamental part for facilitating the OERprocess in alkaline electrolytes.55–59 For instance, the Kim grouphas confirmed that the formation of NiOOH species on a Nisubstrate greatly enhanced the OER activities;55 the Lin groupalso proved that the transformation of NiS2 to the new phase ofNi(OH)2/NiOOH during the OER process is good for theimprovement of the OER, due to the generation of new electro-catalytically active sites.56 We noticed that the metal-based oxy-genated intermediates during the OER-pathway are also pro-posed to be beneficial for improving the catalytic activity as veri-fied via theoretical simulations.60,61 Therefore, density func-tional theory (DFT) calculations for the NiPS model are carriedout in order to better understand the surface oxidation mecha-nism during the OER process, as discussed in Fig. S(19 and20)† in detail. As shown in Fig. S19,† the hydroxyl group (–OH)can be adsorbed on the Ni surface to form a Ni-OH species.However, once the –OH group becomes –O, then the possibilityfor the formation of the NivO intermediate decreases, as theO-group is attached to the Ni–P bond to form the Ni–O–P–S

unit, which suggests that the (PS)3− anion is oxidized after OH*→ O* transformation. Hence, it can be anticipated that theinstability of the NivO intermediate and the easy oxidation ofthe (PS)3− anion might account for the surface oxidation.Furthermore, projected density of state (PDOS) measurementsare performed to understand the phenomenon behind the oxi-dation of the (PS)3− ligand during the OER process. Fig. 4dshows that Ni hybridized with P and S around the Fermi level,suggesting the relatively weak Ni–P and Ni–S bonds. The partialcharge density in the energy range from −1 eV to 0 eV withrespect to Fermi energy indicates that the oxidized P and S con-tribute significantly to dominate the surface reactions duringthe OER. Therefore, we can conclude that the inability of Ni tostabilize the vO group and the easy oxidation of P and S con-tribute to the surface oxidation of NiPS during the OER.

Herein, two novel high-purified NiPS and CoNiP2S2 sampleswere successfully obtained by a conventional solid-phase routeand their electrocatalytic activities towards the OER werefurther investigated. We propose that the catalytic perform-ances could be significantly improved by many promising waysas follows: (i) The design and preparation of the nanosizedmaterial can be a highly-efficient way to remarkably enhancethe electrocatalytic performances;49,62 (ii) the incorporation ofa conductive layer (e.g., graphene) into the pristine material isdemonstrated to be advantageous for enhancing the electro-catalytic activity and reducing the OER overpotential.63,64 Webelieve that this work will attract extensive attention in thedevelopment of new cost-effective and highly-active catalystsfor practical applications.

Conclusions

In summary, we reported a successful synthesis of a ternaryhigh-purity NiPS compound through a simple solid-statemethod for a promising OER electrocatalyst. When tested inan alkaline medium, an overpotential of 400 mV and a Tafelslope of 126 mV dec−1 are achieved at a current density of20 mA cm−2. The DFT calculations and ex situ XPS results indi-cated that the instability of the NivO intermediate and theeasy oxidation of the (PS)3− groups benefit the surface oxi-dation of the catalyst, and the generation of hydroxyl-basedspecies (i.e., Ni(OH)2/NiOOH) plays an important role inimproving the OER performances. In addition, a new quatern-ary CoNiP2S2 compound was successfully obtained for the firsttime in this work, and it produced a current density of 20 mAcm−2 at 376 mV. Hence, the as-synthesized ternary NiPS andquaternary CoNiP2S2 as highly-efficient and cost-effective elec-trocatalysts toward water splitting have great potential to sub-stitute precious metal catalysts for energy storage and conver-sion devices.

Conflicts of interest

The authors declare no competing financial interest.

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Acknowledgements

We acknowledge support from the Natural Science Foundationof China (21972096) and the Shenzhen Science andTechnology Program (JCYJ20190808150615285).

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