RESEARCH ARTICLE www.small-journal.com EnhancedHydrogenEvolutionCatalysisofPentlanditedue

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RESEARCH ARTICLE

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Enhanced Hydrogen Evolution Catalysis of Pentlandite due

to the Increases in Coordination Number and Sulfur

Vacancy during Cubic-Hexagonal Phase Transition

Yuegao Liu, Chao Cai, Shengcai Zhu, Zhi Zheng, Guowu Li, Haiyan Chen, Chao Li,

Haiyan Sun, I-Ming Chou, Yanan Yu, Shenghua Mei,* and Liping Wang*

The search for new phases is an important direction in materials science. The

phase transition of sulﬁdes results in signiﬁcant changes in catalytic

performance, such as MoS2and WS2. Cubic pentlandite [cPn, (Fe, Ni)9S8]can

be a functional material in batteries, solar cells, and catalytic ﬁelds. However,

no report about the material properties of other phases of pentlandite exists.

In this study, the unit-cell parameters of a new phase of pentlandite,

sulfur-vacancy enriched hexagonal pentlandite (hPn), and the phase boundary

between cPn and hPn are determined for the ﬁrst time. Compared to cPn, the

hPn shows a high coordination number, more sulfur vacancies, and high

conductivity, which result in signiﬁcantly higher hydrogen evolution

performance of hPn than that of cPn and make the non-nano rock catalyst

hPn superior to other most known nanosulﬁde catalysts. The increase of

sulfur vacancies during phase transition provides a new approach to

designing functional materials.

Y. Liu, Z. Zheng, H. Sun, I-M. Chou, S. Mei

Institute of Deep-sea Science and Engineering

Chinese Academy of Sciences

Sanya 572000, China

E-mail: mei@idsse.ac.cn

C. Cai

College of Engineering

Southern University of Science and Technology

Shenzhen 518055, China

S. Zhu

School of Materials

Sun Yat-sen University

Guangzhou 510275, China

G. Li

Crystal Structure Laboratory

Science Research Institute

China University of Geosciences (Beijing)

Beijing 100083, China

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/smll.202311161

open access article under the terms of the Creative Commons Attribution

License, which permits use, distribution and reproduction in any

medium, provided the original work is properly cited.

DOI: 10.1002/smll.202311161

1. Introduction

The development and utilization of hydro-

gen energy are important ways to reduce

fossil fuel dependence and achieve carbon

neutrality. Platinum group metals and their

derivatives play a dominant role in the H2

evolution reaction (HER) and allow the fast

production of H2at low overpotentials.[1–3]

The low natural abundance and high price,

however, impede platinum’s sustainability

in the hydrogen economy. Therefore, many

non-noble metal HER catalysts with high

HER activity have been developed to sub-

stitute Pt-based materials, such as ultrathin

metallic Fe-Ni sulﬁde nanosheets,[4]nano

Mo sulﬁde,[5–7]and nano WS2.[8]Although

these materials are very eﬀective, the need

H. Chen

Mineral Physics Institute

Stony Brook University

Stony Brook, New York 11794–2100, USA

H. Chen

Argonne National Laboratory

Chicago 60439, USA

C. Li

Instrumental Analysis Center

Xi’an Jiaotong University

Xi’an 710049, China

Y. Yu

Sichuan Energy Internet Research Institute

Tsinghua University

Chengdu 610042, China

L. Wang

Academy for Advanced Interdisciplinary Studies

Southern University of Science and Technology

Shenzhen 518055, China

E-mail: wanglp3@sustech.edu.cn

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Figure 1. Crystal structure and phase diagram information of pentlandite. a) The crystal structure of cubic pentlandite (cPn). b) Phase diagram of

pentlandite, blue squares =cPn, red hexagons =hexagonal pentlandite (hPn). c) The ideal crystal structure of hPn. d) Synchrotron X-ray characterization

of the reversible phase transition of cPn to hPn at 2.1 GPa (white X-ray beam). e) Electron paramagnetic resonance (EPR) spectra of cPn and hPn.

for speciﬁc nanometer shapes required render those materials

noneconomical. The natural ore pentlandite [(Fe, Ni)9S8], as a di-

rect “rock” electrocatalyst without the need for further surface

modiﬁcations for the HER, shows an overpotential of 280 mV at

10 mA cm−2under acidic conditions and high current densities

(≤650 mA cm−2) without any loss in activity for ≈170 h.[9]In ad-

dition, pentlandite is a functional material in Li-ion batteries,[10]

Li-S batteries,[11]solar cells,[12]and diamond synthesis.[13]Most

of the world’s nickel metal comes from the natural sulﬁde pent-

landite (Fe4.5Ni4.5S8), which is one of the most common sulﬁdes

in magmatic nickel deposits.[14]It provides an important ma-

terial foundation for the development of clean energy. The de-

velopment of pentlandite’s material properties can reduce envi-

ronmental pollution during extracting nickel metals from sul-

ﬁdes. Previous studies focused on the performance of cubic pent-

landite (cPn) with a symmetry group Fm ̄

3m(Figure 1a).[15–20]

The hydrogen evolution performance of pentlandite with diﬀer-

ent compositions have been tested,[9,16,21–23], and the cubic pent-

landite with a Fe:Ni ratio of 1:1 exhibits the best HER catalytic

activity.[15]Former researchers have revealed the positive eﬀects

of Fe-Ni bimetallic heterostructures[24]and sulfur vacancies [19,25]

on the eﬃcient hydrogen evolution performance of pentlandite.

The phase transition of some catalysts will greatly improve their

catalytic performance,[26]such as the phase transitions from 2H-

MoS2to 1T-MoS2[6,27,28]and from 2H-WS2to 1T-WS2.[8]In high-

pressure experiments (1‒7 GPa), we synthesized a new phase

of pentlandite (Fe4.5Ni4.5S8), sulfur-vacancy enriched hexagonal

pentlandite (hPn). To date, no reports about the crystal structure

and cell parameters of hexagonal pentlandite exist. In addition,

the formation P‒Tconditions of hPn and a comparison between

the HER performance of hPn and cPn were unknown.

In this study, we determined the phase boundaries between

cPn and hPn based on synchrotron X-ray data. The crystal struc-

ture of hPn was determined for the ﬁrst time in the world using

powder and single crystal X-ray diﬀraction (XRD), transmission

electron microscopy (TEM), and extended X-ray absorption ﬁne

structure (EXAFS). Through a comparison between the crystal

structures of cPn and hPn, combined with the theoretical cal-

culation of the density of states and the free energy of hydro-

gen adsorption, we found that the increase in the coordination

number and sulfur vacancy during the phase transition from

cubic to hexagonal pentlandite rock provided outstanding en-

hanced hydrogen evolution catalysis. Vacancies are fundamen-

tally important because they are closely related to many physico-

chemical properties, such as electric, mechanical, thermal, opti-

cal, magnetic, and catalytic properties.[29–31]The sulfur vacancies

of the catalyst rocks in this study were caused by a phase transi-

tion by changing the P‒Tconditions without any plasma, reduc-

tion gas, reagents, or solvent treatment. Our new hPn rock cata-

lysts and novel method of sulfur vacancies introduction by phase

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transition method address some obstacles for the industrial ap-

plication of hydrogen evolution catalysts and establish new av-

enues for designing catalysts.

2. Results and Discussion

2.1. Determination of the Phase Boundary of cPn and hPn

The molecular formula of cPn in this study is

Fe4.37–4.48Ni4.59–4.66Co0.05–0.06S8with minor Cu, Te, and Se (Table

S1, Supporting Information).Here, we regarded this natural

mineral as perfect pentlandite, Fe4.5Ni4.5S8. Synchrotron radi-

ation X-ray testing showed that the cPn-hPn phase transitions

occurred at 550, 450, 258, and 200 °C and at 1.0, 2.1, 4.0, and

6.2 GPa, respectively, and the phase boundary could be rep-

resented by the following: T(°C) =660e−2.203P,wherePis in

GPa (Figure 1b; raw data in Table S2, Supporting Information).

During the step heating in the synchrotron radiation X-ray exper-

iment at 2.1 GPa, each temperature was maintained for 5 min,

and cPn was transformed to hPn at 450 °C; during the cooling

process from 800 °C to room temperature with an average rate

of 30 °Cmin

−1, hPn was reversed to cPn at the same transition

temperature (450 °C) (Figure 1d); therefore, the phase transition

is an enantiotropic transformation under a slow cooling process.

However, if the sample was quenched from high temperature,

the hPn could maintain its crystal form at room P‒T.

2.2. Production of hPn

We applied a high P‒Tenvironment to the cPn powder samples

through a multi-anvil press (Figure S1, Supporting Information;

ref. [32]); the samples were kept at 4 GPa/550 °Cfor5h,and

then quenched. We obtained hPn samples with a diameter of

≈3mm(FigureS2a, Supporting Information). The map analy-

ses of the synthesized hexagonal pentlandite using scanning elec-

tron microscopy showed that the samples were homogeneous in

Fe, Ni, and S composition and had not been separated into two

or more minerals (Figure S2, Supporting Information). To pre-

vent mineral oxidation as much as possible, both hPn and cPn

were ground to 0.2–2 μm in a glove box with an agate mortar

(Figures S3 and S4, Supporting Information) for XRD, TEM, EX-

AFS, electron paramagnetic resonance (EPR), X-ray photoelec-

tron spectroscopy (XPS), and electrochemical measurements.

2.3. The Unit Cell Parameters of hPn

The accurate composition of the synthetic hPn was consistent

with the X-ray photoelectron spectroscopy (XPS) analysis (Figure

S5 and Table S3, Supporting Information). According to the re-

ﬁnement from powder XRD data (Figure S6 and Table S4,Sup-

porting Information), the space group of the new phase pent-

landite is P63/mmc (𝛼=𝛽=90°,𝛾=120°). The TEM images

also show hexagonal features (Figure S7b,c, Supporting Informa-

tion). Based on the single crystal diﬀraction test, the unit cell pa-

rameters of hPn are as follows: a =b=3.428 Å and c =5.394 Å

(Figure 1c). In this crystal structure, S, Fe, and Ni are all six co-

ordinated. Some researchers[33]believe that rhombohedral pent-

landite exists with cell parameters of a =b=0.69062 nm, c =

1.72095 nm, and V =0.71085 nm3. If the Ni/Fe-S system is a

rhombohedral structure (a triangular crystal system), then Ni and

Fe would have a 5-coordinate structure, such as that of Ni in NiS

(CIF: 1011038).[34]However, the EXAFS data (Table S5 and Figure

S8, Supporting Information) show that S, Fe, and Ni are all six co-

ordinated, providing supporting evidence of its hexagonal crystal

structure.

In the ideal crystal structure of hPn (Figure 1c), one unit cell

(FeNiS2) contains two S atoms and two metal atoms, and the

number ratio of S to metal atoms is 1:1, which is diﬀerent from

that of cPn with a ratio of 8:9. Although this change in ele-

ment proportion often occurs during the phase transition, we still

would like to understand the imperfection of this structure. cPn

is believed to have 0.275 sulfur vacancies per unit cell.[35]The S

signal observed at ≈3515 G in the electron paramagnetic reso-

nance (EPR) spectra indicates the existence of sulfur vacancies; a

larger magnitude corresponds to more sulfur vacancies.[36]It is

clear that hPn has more sulfur vacancies than cPn (Figure 1e).

The calculation of phase transition mechanism also shows that

the NiS6polyhedron is distorted and the S site in hPn is distorted

or even defective (Figure 2). The discontinuous crystal stripes

on high-resolution transmission electron microscopy (HRTEM)

ﬁgure of sulﬁdes usually indicate the presence of abundant sul-

fur vacancy defects.[7]The HRTEM of hPn shows clear discon-

tinuous crystal stripes (Figure S9a,c, Supporting Information),

but it is not obvious in cPn (Figure S9d, Supporting Informa-

tion). Thus, some sulfur vacancies formed during the phase tran-

sition process from cPn to hPn. However, the composition of hPn

barely changes (Table S1, Supporting Information); that is, the

number ratio of S to metal atoms of hPn should still be 8:9. We

hypothesize that four and a half unit cells (Fe4.5Ni4.5S9)withone

sulfur vacancy is the approximate structure of hPn (Fe4.5Ni4.5S8).

Generally, the sulfur vacancies are introduced by Ar

plasma,[37]hydrogen plasma,[38]nitrogen plasma,[39]Ar/H2

annealing,[40]chemical reduction,[41]calcination under a ni-

trogen atmosphere,[42]liquid-ammonia-assisted lithiation,[36]

acid treatment,[43]etching by H2O2,[44]and electrochemical

desulfurization.[45]These methods are eﬀective for multilayer or

monolayer sulﬁdes supported on substrates or nanosheets, but

they are not needed in the rock catalyst. The sulfur vacancies

of the pentlandite rocks were introduced during the phase

transition by changing the P‒Tconditions without any plasma,

reduction gas, reagent, or solvent treatment; this provides new

insights for designing electrocatalysts.

2.4. Potential Energy Surface (PES) of the Phase Transition

To further investigate the phase transition mechanism, we uti-

lized the SSW pathway sampling method to explore the poten-

tial energy surface (PES) of the phase transition. In this work,

the cPn and hPn have diﬀerent stoichiometric ratios, namely

Ni9S8and Ni8S8for cPn and hPn. To simplify the calculation,

we delete one Ni in the 17-atoms cubic phase cell (Figure 2a)to

form a defective 16-atoms cubic phase (defected cPn, Figure 2b).

Then, the lowest energy barrier pathway is obtained by using

the SSW pathway sampling method, which has successfully been

used to resolve the phase transition mechanism of AlPO4.[46]As

Figure 2b,c shows, after removing one Ni atom, the defected-cPn

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Figure 2. Phase transition mechanism from cPn to hPn.

cell is distorted while the framework is kept. By an exhaustive

sampling (Figure 2c,f), we ﬁnd the lowest energy barrier path-

way is a two-steps path with an energy barrier 0.13 eV/f.u. (f.u. =

NiS) (Figure 2g). Such a low energy barrier supports the phase

transition from cPn to hPn is reasonable. The NiS4tetrahedron

in the cubic phase diﬀuses into the adjoining octahedron inter-

stice to form NiS6, while the two NiS6polyhedrons in the cubic

phase are kept. Interestingly, the S sublattice transit from fcc to

hcp by shearing the subgroup lattice. As a result, the orientation

relation of this path is (111)c//(001)h+[1̄

10]c//[100]h (Note: Here

c and h represent cubic and hexagonal phases, respectively). The

animation of the phase transition process was listed as Movie S1

(Supporting Information). Since the Ni atom diﬀusion, the NiS6

polyhedron would be distorted. As a result, the S site in hPn is

distorted or even defective. This is consistent with the fact that

the hPn shows more sulfur vacancies compared to cPn.

2.5. Electrochemical Hydrogen Evolution Performance

The HER activities of cPn and hPn powders with diameters of

0.2–2 um that are grinded by hand in glove box were tested

using a standard three-electrode conﬁguration. The hPn ex-

hibits a higher HER activity with an overpotential of −60 mV

at 10 mA cm−2, in comparison to -168 mV for cPn in 0.5 M

H2SO4solution (Figure 3a). The Rct of hPn in the acidic so-

lution is 203.4 Ω, which is signiﬁcantly lower than that of cPn

(407.5 Ω)(Figure3b). The Tafel slope of cPn in acidic solution is

98 mV dec−1. This value is slightly higher than the 72 mV dec−1of

synthetic Ni4.5Fe4.5S8“rocks” in ref. [9]. But the Tafel slope of hPn

in the acidic solution is much lower (34 mV dec−1)(Figure3c),

suggesting a highly promoted charge transport eﬃciency. In ad-

dition, hPn shows high stabilities at 300 mV and high current

densities (≤205.8 mA cm−2) without any loss in activity for ≈50 h

in the acidic solution (Figure 3d), which is much higher than

that of cPn (26 hrs). The electrochemical double-layer capaci-

tance (Cdl) of the hPn (0.27 mF cm−2) using cyclic voltamme-

try is much higher that of cPn (0.14 mF cm−2)(Figure3e). The

EPR of the catalysis was invested (Figures 1e and 3f). The S sig-

nalobservedat≈3515 G indicates the existence of sulfur vacan-

cies, and the larger the magnitude, the more sulfur vacancies

are represented.[36]The hPn has more sulfur vacancies than cPn

(Figure 1e). After the hydrogen evolution reaction, the sulfur va-

cancy of the catalysts becomes less (Figure 3f).

The overpotential of non-nano hPn rock in 0.5 M H2SO4solu-

tion is 60 mV at 10 mA cm−2, which is lower than that of most

nano Mo sulﬁde (150–250),[47]WS2nanosheets (150–200 mV),[8]

𝛽-NiS nanosheets,[4]cubic pentlandite nanoparticles,[21]and so

on (Table 1).

The Tafel slope of hPn is much lower than that of cPn, sug-

gesting highly promoted charge transport eﬃciency. The Tafel

slope of cPn is 98 mV dec−1, between 38 and 116 mV dec−1

(Figure 3c), indicating that both the Volmer step (electrochemi-

cal hydrogen adsorption, H3O++e+→Hads) and Heyrovsky step

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Figure 3. Electrochemical hydrogen evolution performance. a) The linear sweep voltammetry (LSV) curve of electrocatalysts in the acidic solution testing.

b) EIS at 300 mV of cPn and hPn in the acidic solution testing. c) Tafel plot in the acidic solution. For better comparison, cPn and hPn were examined

under similar conditions. d) Durable test of cPn and hPn at a constant potential of 300 mV in the acidic solution. e) The measured Cdl values of cPn

and hPn. f) EPR spectra of cPn and hPn pre-hydrogen evolution reaction (pre-HER) and post-hydrogen evolution reaction (post-HER) in the acidic (Aci)

environments.

(electrochemical hydrogen desorption, Hads +H3O++e+→H2

+H2O) are rate-determining steps. In contrast, the Tafel slope of

hPn is 34 mV dec−1, between 29 and 38 mV dec−1(Figure 3c),

indicating that the Heyrovsky step and Tafel step (chemical des-

orption, Hads +Hads →H2) are rate-determining steps.[48,49]

2.6. Calculation of the Gibbs Free Energy of Hydrogen Adsorption

(𝚫GH*)

To better understand the hydrogen evolution mechanism of cPn

and hPn, we calculated the Gibbs free energy of hydrogen evo-

lution adsorption (ΔGH* value). The (111) plane is chosen to

be the representation of cPn (Figure S10a, Supporting Informa-

tion) since this face is considered to be one of the most eﬃcient

planes for HER.[9,24,25,51]There are two diﬀerent terminals for

this plane: 1) the Fe/Ni atom on the surface of this plane, called

the Fe/Ni atom terminal (Figure S10b, Supporting Information),

and 2) the S atom on the surface, called the S atom terminal

(Figure S10d, Supporting Information). For the Fe/Ni atom ter-

minal (slab result-1), the ΔGH* values of the atoms Fe8, Fe18, and

Fe1 with coordination numbers (CNs) of 3, 4, and 6, are 0.812,

0.603, and 0.316 eV, respectively; the ΔGH* values of the atoms

Ni4, Ni5, and Ni10 with CNs of 3, 4, and 6, are 1.337, 1.043, and

0.431 eV, respectively (Figure 4a; Figure S10c and Table S6,Sup-

porting Information). Thus, the ΔGH* values of Fe and Ni atoms

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RESEARCHARTICLEwww.small-journal.comEnhancedHydrogenEvolutionCatalysisofPentlanditeduetotheIncreasesinCoordinationNumberandSulfurVacancyduringCubic-HexagonalPhaseTransitionYuegaoLiu,ChaoCai,ShengcaiZhu,ZhiZheng,GuowuLi,HaiyanChen,ChaoLi,HaiyanSun,I-MingChou,YananYu,ShenghuaMei,*andLipingWang*Thesear...

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