First Principles Computational Screening of Highly Active Pyrites

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First Principles Computational Screening of Highly Active Pyrites Catalysts for Hydrogen Evolution Reaction Through a Universal Relation with a Thermodynamic Variable Joonhee Kang, Jeemin Hwang, and Byungchan Han J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09294 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry

First Principles Computational Screening of Highly Active Pyrites Catalysts for Hydrogen Evolution Reaction through a Universal Relation with a Thermodynamic Variable

Joonhee Kang, Jeemin Hwang, and Byungchan Han*

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Korea

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (B. C. H.) Tel: +82-2-2123-5759 Fax: +82-2-312-6401 Address: Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hydrogen gas has been regarded as a promising fuel for securing energy and environmental sustainability of our society. Accordingly, efficient and large scale production of hydrogen is central issue due to high activation barrier unless costly transition metal catalysts are used. Here, we screen optimum catalysts toward hydrogen evolution among cheap pyrites using first principles density functional theory calculations and rigorous thermodynamic approach. A key thermodynamic state variable accurately describes the catalytic activity, of which mechanism is unveiled by a universal linear correlation between kinetic exchange current density in hydrogen evolution reaction and thermodynamic adsorption energy of hydrogen atom over various pyrites. Based on the results we propose a design principle for substantial tuning the catalytic performance.


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1. Introduction Hydrogen gas has been considered as ultimate fuel to secure both energy and environmental sustainability of our society.1 Its efficient and large-scale production was, however, hindered due to substantial amounts of activation energy in most of electrochemical or chemical reactions, unless costly transition metals were utilized as catalysts. For example, over the last several decades, water electrolysis was focused as an attractive process for the production of both hydrogen and oxygen fuels but was known not to be practical for industrial scale because of expensive transition metal catalysts and weak electrochemical stability such as nanoscale Pt.2 Thus, it is of importance to identify an alternative catalyst to the conventional materials, for which understanding new design principles should be the first step.3-6 Pyrite materials composed of the first-row transition metals and di-chalcogenide ligands (MX2, M = Fe, Co or Ni, and X = S or Se) were recently tested for hydrogen evolution reaction (HER). For instance, catalytic performances of binary (FeS2, CoS2, NiS2, CoSe2, NiSe2)7-15 and ternary alloy compounds (Fe0.9Co0.1S2 and Fe0.7Co0.3Se2)16-17 were measured as active for HER. And overpotential of cobalt phosphosulfide (CoPS) toward HER was reported as low as 48 mV at the current density of 10 mA cm-2.18-19 Electrocatalytic activity of a catalyst is, in general, experimentally characterized by its exchange current density for a given reaction. Theoretical and computational approach pursue to understand underlying mechanism using electronic structures of the catalyst on fundamental aspect or with combined statistical thermodynamics with micro-kinetic models.20-26 As far as authors realize first principles density functional theory (DFT) level calculations and rigorous thermodynamic studies have not been carried out for pyrites even 3 ACS Paragon Plus Environment


though the materials can be good candidates substituting for conventional noble transition metals for HER with careful screen and optimization. Herein, we applied the methods aiming at identifying an atomic level descriptor on experimentally measured exchange current densities in HER over various pyrites, screening optimum catalyst and proposing a design principle for further tuning of the catalytic activity.

2. Computational details In bulk pyrite MX2 compound (space group,

Pa3 )

the transition metal M chemically bonds

with adjacent six ligands of chalcogenide X (typically S or Se). Thermodynamically the surface is terminated by the most stable {001} plane as [X-M-X] configurational patterns, whereas the five-fold coordinated M is locally located at the square pyramidal structure.27 Fig. 1 illustrates our slab model system for a pyrite consisting of four (eight) M and eight (16) X in a bulk ({001} surface). A unit cell of the bulk and slab with 12 atomic layers were in periodic boundary condition, and 15 Å thick vacuum space along c-axis to preclude unnecessary interaction with its images. We found that atoms in the half of the bottommost layers (6 layers of total 12 layers) did little change their positions and electronic structures, and thus we fixed their locations over our calculations, but we did fully relaxed all other atoms and adsorbates. We utilized first principles DFT calculations as implemented in Vienna Ab-initio Simulation Package (VASP)28 with revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functionals29 and the projector-augmented wave (PAW)30 approach. Kohn-Sham equation was calculated with plane waves with a cutoff energy 400 eV and dipole moment correction. All of the calculations were allowed to relax the total energy convergence below 10−5 eV and the maximum atomic forces smaller than 0.05 eV/Å . 4 ACS Paragon Plus Environment

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The first Brillouin zone in the reciprocal space was integrated with 5 × 5 × 1 k-points for structural and energy optimization and 8 × 8 × 1 k-points for density of states (DOS) calculations of electrons in the slab.

3. Results and discussion With insight proposed by the Sabatier principle20-21 and recently by Nørskov22 we attempted to find optimum thermodynamic adsorption free energy of H atom (ΔGH*) in pyrites maximizing exchange current density for HER. We defined the thermodynamic adsorption energy of H (ΔEH*) in a pyrite as equation (1),

1 n  EH *   E  surf  nH   E  surf   E  H 2   n 2 

(1)

where n is the number of H atoms adsorbed in the pyrite surface. We setup two different models to investigate effect of H coverage on ΔE H*: 0.5 (n = 1) and 1.0 (n = 2) monolayer (ML). Then, the Gibbs free of H adsorption, ΔGH*, was calculated according to equation (2),

GH *  EH *  EZPE  T S H

(2)

where ΔEZPE, ΔSH are the difference in zero-point energy, entropic change from a gas phase to an adsorbed hydrogen (H*), and T means absolute temperature, respectively. We converted ΔEH* into ΔGH* scale using previously proposed relation18 as equation (3),

GH *  EH *  0.29eV

(3)



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We calculated ΔEH* for the S-based pyrites (FeS2, CoS2, NiS2, and CuS2) to decouple any effect on the catalytic activity for HER from the d-band center theory for the transition metal. Our results indicated that except for CuS 2 a H preferentially adsorbed in transition metals of FeS2, CoS2 and NiS2. For the CuS2 a H favored S than Cu as much as 0.20 eV at 0.5 ML and 0.43 eV at 1.0 ML. This situation is unfavorable for HER since it drives a phase transition generating toxic gas such as H2S or H2Se. Thus, we calculated HER catalytic activity only at transition metals Fe, Co, and Ni. Adsorption Gibbs free energy of ΔG H* in the transition metals at 0.5 ML were calculated as 0.43 eV, 0.30 eV, and 0.73 eV for FeS 2, CoS2, and NiS2, respectively, as represented in Table 1. We found that this order did not change at the coverage of 1.0 ML. Changes of ΔGH* can be ascribed to differences in electronic structures21 of the pyrites: the closer d-band center energy (ɛd) to Fermi level (ɛF) of the transition metal the less filling of anti-bonding states leading to stronger bonding with an adsorbate H. There is one more thing to consider, however, the local symmetry breaking caused by unequal interaction of the transition metal with neighboring six ligands. It splits the energies of initially degenerated d orbitals into two different group, t2g (bonding) and eg (anti-bonding) levels as the crystal field theory dictates. Since the transition metal on {001} surface is located at a square pyramidal geometry (five coordination), orbitals of d xz , d yz , and d z are slightly 2

more stabilized, while d xy and d x  y are relatively destabilized (Fig. 2) by the ligand field. 2

2

We plot calculated projected density of states (PDOS) of various surface of pyrites in Fig. 3 and Fig. S1. Computed d-band centers of the five pyrites (MS2, M = Fe, Co, Ni; MSe2, M = Co, Ni) were represented in Table 1. Interestingly, it indicates that the FeS 2 has the 6 ACS Paragon Plus Environment

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highest ɛd (-0.83 eV), followed by CoS2 (-1.49 eV), and NiS2 (-1.57 eV), even though a H the most strongly adsorbs in CoS2 (0.30 eV). It can be explained by the peculiar local symmetry of the five-coordinated Fe. Energy difference between the split d orbitals of d xy and d z in the low spin state Fe is 0.35 eV, which is much smaller than the energy cost for 2

electron pairing 1.6 eV.27 It implies that the four valence electrons would firstly fill up the lowest energy states of d xz and d yz and then, the rest two electrons occupy each of the d xy and d z orbitals since the energy penalty for electron pairing in the orbitals at the same lower 2

energy level (about 1.6 eV) is much larger27 than occupation of some electrons in orbitals located in higher antibonding energy level (i.e., the high spin electron configuration). This is why adsorption energy strength of H in FeS2 is weaker than other pyrites. We also analyzed adsorption behaviors of H in pyrites with Se ligands. Calculated ɛd of a CoSe2 was higher than that of a NiSe2, implying that the former is the strongest adsorbant for H. We did not consider FeSe2 because it was known to easily transform into a marcasite structure.31 Our results represented in Table 1 indicate that ΔGH* is enhanced with Se than S. It originates from different interaction energies between transition metals with the two ligands. To quantitatively characterize subtle different features of the ligand S and Se in their interactions with the metals we utilized the Bader charge analysis32 and added electronic charge density difference (Δρ) calculated using equation (4) to Fig. 4.    MX 2   M   X

(4)

Here,  MX ,  M , and  X mean charge densities of bulk MX2 pyrite, transition metal M and 2

ligand X, respectively. As illustrated in Fig. 4 the ligand S interacts more strongly with 7 ACS Paragon Plus Environment


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transition metal M than Se does, which is also supported by longer bond lengths between M-Se than M-S (about 0.11 Å ). The positive value indicate that there is electron transfer from metal to its ligand. Our results conspicuously show that the subtle difference of the two ligands that the transition metal donates more charges to S rather than Se, denoting stronger interaction with the ligand S than Se. The weaker interaction of MSe2 than MS2 was caused by their different electronegativities, an important factor influencing the crystal field stabilization energy (CFSE): the weaker electronegativity the ligand X has, the narrower split of the orbital energy levels of M. Since Se (2.55) has lower electronegativity than S (2.58),33 the d-band of MSe2 should be narrower than MS2, by which d-band center of MSe2 will upshift into Fermi energy. Thus, the anti-bonding states are less occupied strengthening the bond.34 It is the reason that MSe2 shows higher adsorption energy of H than MS2 does. We also calculated the charge density difference between adsorbed hydrogen and pyrite surface (Figure S2). The charge density difference shows that electrons in the

d z2

orbital in metal (M) transfer to an adsorbed hydrogen (H). It means that hydrogen

bonding strength is described by the number of electrons in the

d z2

orbital.

In Fig. 5 we plot experimentally measured exchange current densities 7-14 in HER over various pyrites as a function of adsorption free energy of H (ΔGH*). We derived the exchange current density in endothermic regime using a micro-kinetic model20 as equation (5),

i0  ek0

exp( GH * / kT ) 1  exp( GH * / kT )

(5)


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where k0 is the pre-exponential factor (typically about 200 s-1 site-1).20 Also, e, k, α and T are elementary charge, Boltzmann constant, transfer coefficient and absolute temperature, respectively. As adsorption energy of H is close to zero the transfer coefficient approaches to 1.0, implying stronger influence of applied potential, which is consistent with previous results.22 We calculated thermodynamic free energy diagrams for HER with varying pyrites as denoted in the inset of Fig. 5. The transition metal affects the hydrogen adsorption energy more significantly than ligand does since the metal provides active sites for hydrogen evolution reaction correlated with its electronic structures in the d-band. Ligands, on the other hand, slightly affects the d-orbital of metal according to our results. It seems that a large change of the hydrogen adsorption energy can be achieved by properly choosing the transition metal, but adjusting the ligand is better way for fine tuning of the adsorption energy. It is clearly shown that the exchange current densities in HER by pyrite catalysts show a universal linearity with the adsorption energy of H: the stronger adsorption the higher exchange current density. It demonstrates the well-known Sabatier principle. To evaluate doping effects on the HER catalytic activity, we constructed surface models for Fe0.5Co0.5S2 and Fe0.5Co0.5Se2 as denoted in Fig. S3. Hydrogen adsorbed on the metal sites with high adsorption energies of 0.22 (0.26) eV for Fe (Co) in the Fe 0.5Co0.5S2, and 0.21 eV for both Fe and Co in the Fe0.5Co0.5Se2. The reason enhancing the hydrogen adsorption energy in the doped materials (Fe0.5Co0.5S2) is ascribed to electronic structure changes of PDOS in Fe and Co compared to pyrites without the doping (FeS2 and CoS2) as shown in Figure S4 (a) and (b). The electrons of Fe occupy bonding states with the downspin configurations and the d-band center energy of Co is upshifted, leading to the stronger hydrogen adsorption. The upshift of d-band center energy also appears in 9 ACS Paragon Plus Environment


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Fe0.5Co0.5Se2 in Figure S4 (c). Our results propose a design concept to further tune HER catalytic activity of pyrites: enhance the adsorption energy of H. It is at the same ground with previous attempts to dope foreign atoms to pyrites, for instance, Fe 0.9Co0.1S2 (transition metal doping)16 and CoPS (ligand doping)18-19. In practice, in addition to the high catalytic activity prolonged electrochemical stability should be sustained as well, otherwise the reaction will stop via material disintegration. We evaluated stability of a pyrite in acidic solution by calculating a defect formation energy at the transition metal site (ΔEDM) and the ligand (ΔEDX) site, as defined by equation (6) and (7):

EDM  E  surf  defect   E  M   E  surf



EDX  E  surf  defect   E  H 2 X   E  surf   E  H 2 

(6)

(7)

where E(surf+defect), E(M), and E(H2X) are energies of defected surface, bulk transition metal M, and H2X gas, respectively. Table 2 represents defect formation energies for various pyrites. Our results indicate that defect formation in the pyrite surfaces is not thermodynamically feasible due to strong chemical bonding with nearby ligands. It confirms that the transition metals in the pyrites are at least thermodynamically stable against electrochemical degradation. Ligands S and Se in CuX2, however, easily form vacancies to produce toxic H2X gas. On the other hand, strong adsorption of H in transition metal can suppress the formation of toxic gas H2X and enhance electrocatalytic activity, which was experimentally observed.9


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Thus, our results propose that pyrites considered in our study can be promising catalysts on the aspect of both catalytic activity and electrochemical stability toward HER. And the universal relation of the catalytic activity with thermodynamic state variable opens ways to further tune the performance.

4. Conclusions We demonstrated that Gibbs free energy of H adsorption in MS 2 and MSe2 type pyrites is a key descriptor on the catalytic activity for HER, and has a strong linear correlation with experimentally measured exchange current density. Underlying mechanism was elucidated by interplay of two electronic structures of the transition metal: i) location of d-band center energy, ii) competition between orbital energy split by the ligand field and energy penalty for electron pairing in the d-orbital at the same energy level. Based on our results we proposed a design concept for developing highly active HER catalyst: enhancement of adsorption strength of H at the transition metal in pyrites. Several frontier works previously attempted are at the same root with our guideline.

ACKNOWLEDGMENT This research was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078882).



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Table 1. Calculated Gibbs free energy for hydrogen adsorption, d band center (εd), and measured exchange current density (i0) in pyrite {001} surface.

FeS2

CoS2

CoSe2

NiS2

NiSe2

ΔGH* (1/2 ML) / eV

ΔGH* (1 ML) / eV

εd / eV

log(i0 / A cm-2)

Ref

0.43

0.48

-0.83

-6.042

7

-6.842

8

-5.664

14

-5.452

8

-5.706

9

-5.500

14

-5.076

12

-5.310

13

-7.776

7

-7.719

8

-7.398

10

-7.509

11

0.30

0.27

0.73

0.66

0.31

-1.49

0.26

-1.29

0.72

-1.57

0.66

-1.53



Table 2. Defect formation energy of transition metal and ligands in pyrite {001} surface. ΔGDM / eV

ΔGDX / eV

FeS2

1.09

0.79

CoS2

0.90

0.68

NiS2

0.90

0.34

CuS2

1.02

0.11

CoSe2

0.46

1.01

NiSe2

0.72

0.68

CuSe2

0.79

0.43


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Figure 1. Model systems for bulk and {001} surface of a pyrite. Yellow and gray mean ligand (S or Se) and transition metal (Fe, Co, Ni and Cu), respectively.



Figure 2. Schematic diagram of transition metal d orbital with different ligand configurations (Octahedral and square pyramidal)


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Figure 3. Projected density of states (PDOS) for d orbitals of transition metals in pyrite {001} surfaces.



Figure 4. Charge density difference plot, Bader charge analysis, and bond length between transition metal and ligand in the pyrites.


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Figure 5. Experimentally measured exchange current density, log(i0), for HER over different pyrites versus calculated Gibbs free energy of H adsorption. The inset shows the thermodynamic free energy diagram for HER in different MS2 and MSe2 type pyrites.



TOC Graphic


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Figure 1. Model systems for bulk and {001} surface of a pyrite. Yellow and gray mean ligand (S or Se) and transition metal (Fe, Co, Ni and Cu), respectively. 250x239mm (96 x 96 DPI)

ACS Paragon Plus Environment


Figure 2. Schematic diagram of transition metal d orbital with different ligand configurations (Octahedral and square pyramidal) 220x190mm (96 x 96 DPI)


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Figure 3. Projected density of states (PDOS) for d orbitals of transition metals in pyrite {001} surfaces. 300x600mm (96 x 96 DPI)



Figure 4. Charge density difference plot, Bader charge analysis, and bond length between transition metal and ligand in the pyrites. 250x300mm (96 x 96 DPI)


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Figure 5. Experimentally measured exchange current density, log(i0), for HER over different pyrites versus calculated Gibbs free energy of H adsorption. The inset shows the thermodynamic free energy diagram for HER in different MS2 and MSe2 type pyrites. 540x379mm (96 x 96 DPI)


First Principles Computational Screening of Highly Active Pyrites

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