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Susceptibility of FeS2 hydrogen evolution performance to sulfide poisoning
Electrochemistry Communications 58 (2015) 29–32
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Susceptibility of FeS2 hydrogen evolution performance to sulfide poisoning Chun Kiang Chua, Martin Pumera ⁎ Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 22 May 2015 Accepted 27 May 2015 Available online 3 June 2015 Keywords: Hydrogen evolution reaction Iron pyrite Sulfide Poisoning
a b s t r a c t The imminent depletion of fossil fuels raises concern over the need for next-generation clean energy. Of numerous alternatives, electrochemical water splitting is a promising method to store energy in the form of hydrogen. In order to benefit from this system, technological advancement in the development of affordable and efficient electrocatalysts for hydrogen evolution reaction is necessary. Transition-metal electrocatalysts composing of earth-abundant elements, specifically natural FeS2, has demonstrated excellent performance for hydrogen evolution reaction. However, previous studies on platinum surfaces highlighted the detrimental effect toward hydrogen evolution performance upon poisoning of the active sites. In this work, we examine the susceptibility of natural FeS2 toward sulfide poisoning. Our findings showed that the degradation effect from the introduction of sulfide to natural FeS2 was not as severe as that observed on platinum. The overpotential (at a current density of 10 mA/cm2) for natural FeS2 and platinum increased by approximately 20 and 110 mV, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The development of next-generation clean energy has become increasingly important as the dependency of modern societies on fossil fuel escalates despite depleting supply. Hydrogen is seen as one of the many alternative fuels that are secure, sustainable and clean. The electrolysis of water (water splitting), either electrochemically or photoelectrochemically, is currently the most attractive route toward the production of hydrogen since harmful by-products are not generated in the process [1,2]. This has driven intense research to identify potential electrocatalysts that are applicable for hydrogen evolution reaction (HER), which at the moment, is best catalyzed by noble metals, especially platinum [3,4]. However, the high cost and availability of these precious metals prevent the large-scale application of such clean energy technologies. As such, numerous efforts have been taken to search for affordable and efficient transition-metal electrocatalysts composing of earth-abundant elements such as MoS2 [5,6], WS2 [7,8], FeP [9], CoP [10] and Ni2P [11]. Recently, cubic pyrite-phase transition-metal dichalcogenides (with the general formula of MX2, where typically M = Fe, Co, or Ni and X = S or Se) have been successfully applied as efficient HER electrocatalysts [12,13]. Moreover, these transition-metal pyrites which consist of first row transition-metals and rock-forming chalcogens are inexpensive and available in bulk quantity. In specific, the semiconducting FeS2 is not only a promising electrocatalyst for HER but is highly applicable in numerous technological systems such as dye-sensitized solar cells [14], lithium-ion batteries [15,16] and chemical/electrochemical ⁎ Corresponding author. E-mail address: [email protected] (M. Pumera).
http://dx.doi.org/10.1016/j.elecom.2015.05.016 1388-2481/© 2015 Elsevier B.V. All rights reserved.
systems including oxygen reduction reaction [17], polysulfide reduction [13] and liquefaction of coal [18]. More recently, FeS2 has been incorporated into a hybrid of Fe0.9Co0.1S2/CNT to achieve a HER overpotential of ~120 mV (at 20 mA/cm2) and a Tafel slope of ~46 mV/decade [19]. Despite the efficiency of platinum as HER electrocatalyst, such transition-metal electrocatalysts are typically susceptible to poison such as arsenide and sulfur [20,21]. For example, the rate of hydrogen evolution reaction on platinum is greatly reduced in the presence of trace amounts of poison. With respect to this, we are interested in investigating such poisoning effect on the performance of natural FeS2 for hydrogen evolution reaction in this work. In fact, the effect of sulfide (from sodium hydrosulfide, NaSH) poisoning on the performances of natural FeS2 and Pt/C for hydrogen evolution were examined by examining the polarization curve and Tafel analysis. 2. Experimental section 2.1. Materials Iron disulfide (FeS2, −325 mesh, 99.8% trace metal basis), platinum (20 wt.% on activated carbon), sodium hydrosulfide and sulfuric acid were obtained from Sigma Aldrich, Singapore. Screen printed electrode (SPE) with a diameter of 3 mm was obtained from Zensor. Milli-Q water (resistivity: 18.2 MΩ cm) was used throughout the experiments. 2.2. Equipment X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a Mg X-ray radiation source (SPECS,
30
C.K. Chua, M. Pumera / Electrochemistry Communications 58 (2015) 29–32
(a)
(b)
1 m
20 m
Fig. 1. Scanning electron microscopy images of natural FeS2 at (a) 10,000× and (b) 600× magnification.
Germany). Natural FeS2 was homogeneously spread across on a carbon conductive tape for XPS analysis. A JEOL-7600F semi-in-lens FE-SEM, operating under SEI and GB-H modes at 2 kV, was used to acquire the SEM images. The natural FeS2 powder was adhered to a conductive carbon tape held onto a SEM holder for analyses. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed with a spectrometer from Oxford Instrument. All voltammetric experiments were performed on a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). 2.3. Procedures 2.3.1. Electrochemical measurements The electrochemical experiments were carried out in a 10 mL voltammetric cell at room temperature using a three-electrode configuration. A platinum electrode served as an auxiliary electrode and a Ag/AgCl electrode as a reference electrode; a screen-printed electrode was utilized as a working electrode. The working electrode was modified by applying 8 μL of material (1 mg mL−1 dispersed in water) and allowed to dry prior to measurements. Measurement of hydrogen evolution was performed in 0.5 M H2SO4 using linear sweep voltammetry at a scan rate of 2 mV/s. The reported results are average from triplicate measurements. 2.3.2. Poisoning of active sites The working electrode which was pre-coated with natural FeS2 or Pt/C was added one equivalence (with respect to the amount required for Pt/C, based on a 1:1 of Pt:S ratio; 8.20 × 10-9 mol) of sodium hydrosulfide solution. Upon drying, the working electrode was swirled gently in ultrapure water to remove excess or loosely adsorbed NaSH. The working electrode was ready for measurement after drying.
(b) Bulk S
S2p
N1s
800
600
(c)
O1s
C1s
1000
The poisoning effect of sulfide on natural FeS2 was examined by performing hydrogen evolution reaction analysis with linear sweep voltammetry. Similar poisoning effect was also performed on platinum for comparison. Prior to that, the morphology of FeS2 powder was investigated using scanning electron microscopy (SEM). Fig. 1 shows the SEM images at magnifications of 10,000 × and 600×. The material exists as irregular chunks with lateral dimensions up to ~90 μm and with presence of smaller sized particles which is analogous to natural pyrite samples [22]. Subsequent energy dispersive X-ray (EDX) analysis measured a composition of 39.5 at.% Fe and 60.5 at.% S for the sample. Further X-ray photoelectron spectroscopy (XPS) analysis provided an overview on the elemental composition of the natural FeS2 as shown in Fig. 2. On top of Fe and S, elements such as N, C and O were present based on the XPS survey scan. High-resolution scans of S2p and Fe2p were consequently measured and fitted with Gaussian– Lorentzian curves. The S2p spectrum was fitted with four sets of curves with each consisting of S2p3/2 and S2p1/2 (in the ratio of 2:1) peaks due to spin orbit splitting [23,24]. The peak at 162.5 eV was assigned to bulk disulfide while polysulfidic species was responsible for the peak at higher binding energy of 164.4 eV. Although present in small quantities, the set of curves at 161.7 suggested the presence of surface states including that of monosulfide and disulfide species. The peak at a much higher binding energy of 168.8 eV could result from the presence of S6+ species. All in all, the ratio of bulk S:monosulfide/disulfide:polysulfide:S6+ is 0.55:0.11:0.10:0.24. On the other hand, the high-resolution Fe2p scan showed prominent peak at 706.3 eV due to bulk Fe2+. The tailing of the peaks toward higher binding energy was contributed by surface Fe2+ and Fe3+. The effect of poisoning natural FeS2 active sites was evaluated by monitoring the performance of hydrogen evolution reaction. This was performed side-by-side with platinum (20 wt.% on activated carbon)
400
Binding Energy (eV)
200
S6+
175
Polysulfide
170
165
Binding Energy (eV)
Monosulfide/ Disulfide
160
Intensity (a.u.)
Intensity (a.u.)
Fe2p
Intensity (a.u.)
(a)
3. Results and discussion
730
725
720
715
710
Binding Energy (eV)
Fig. 2. X-ray photoelectron spectroscopy spectra of natural FeS2. (a) Survey scan; and high resolution scans for (b) S2p and (c) Fe2p.
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C.K. Chua, M. Pumera / Electrochemistry Communications 58 (2015) 29–32
(b)
0
-5
-10
Bare SPE Pt/C Pt/C-NaSH FeS2
-15
Overpotential (Vvs. RHE)
Current Density (mA/cm2)
(a)
31
0.8
0.6
0.4
Bare SPE Pt/C Pt/C-NaSH
0.2
FeS2 FeS2-NaSH
FeS2-NaSH
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
-1.0
0.4
-0.5
0.0
0.5
1.0
1.5
Log Current Density (mA/cm2)
Potential (V vs. RHE)
Fig. 3. Electrochemical characterization of bare screen printed electrode, Pt/C, poisoned Pt/C, FeS2 and poisoned FeS2 toward hydrogen evolution reaction. (a) Polarization curves of current density against potential. (b) Tafel analysis of overpotential against the log of current density.
and bare SPE for comparison. As shown in the polarization curves in Fig. 3a, the overpotential of natural FeS2 was 890 mV at a current density of 10 mA/cm2. The presence of cathodic peaks at approximately − 500 mV could indicate the possible inherent electrochemistry of FeS2, most likely due to surface decomposition of the material and subsequent reduction of surface ferrous ions [25–28]. On the other hand, the overpotential for bare SPE and Pt/C were 970 and 100 mV, respectively. A lower overpotential indicates a material as more catalytic for the evolution of hydrogen. Consequently, the addition of sodium hydrosulfide (NaSH) poison was expected to block the active sites of the materials, which is possibly at the disulfide-terminated edges in the case of FeS2, resulting in a reduced performance for hydrogen evolution [19–21]. Thereafter, the overpotential for FeS2 and Pt/C were 910 and 210 mV, respectively. The poisoning effect was more apparent on Pt/C than on FeS2, in which a 110 mV (110%) increment in overpotential was observed for the former while only 20 mV (2%) was observed for the latter. Further Tafel analyses on the polarization curves of bare SPE, Pt/C and FeS2 were performed as seen in Fig. 3b. It was noted that the Tafel slope observed on bare SPE at 248 mV/decade was unusually larger than typical glassy carbon electrode (~126 mV/decade) due to the proprietary carbon electrode material of the bare SPE. In contrast, Pt/C and FeS2 showed lower Tafel slopes. As expected from the best electrocatalyst for HER, Pt/C showed a Tafel slope of 30 mV/decade and suggested a typical Volmer–Tafel mechanism. However, natural FeS2 with a Tafel slope of 215 mV/decade indicated a possibly slow Volmer adsorption step. It should be noted that several works on FeS2 produced using various synthesis methods reported contrasting Tafel slopes (e.g. 50 to 70 mV/decade) for hydrogen evolution reaction [13,27]. The slow Volmer adsorption step in this study could suggest possible electrochemical decomposition of the active sites on natural FeS2 which thus prevented the spontaneous adsorption of H+ on its surfaces. Nevertheless, upon the introduction of sodium hydrosulfide, Pt/C and natural FeS2 revealed Tafel slopes of 76 and 194 mV/decade, respectively. The Tafel slope of Pt/C increased upon sulfide poisoning while that of FeS2 did not register any major changes. In the case of Pt/C, the adsorption of sulfide species reduced the availability of active sites for the electrocatalysis of the hydrogen evolution reaction. As for natural FeS2, effects from the introduction of sulfide were not as detrimental as that of platinum. The possibility of electrochemical decomposition and the coordination of sulfide to natural FeS2, likewise with the case of cysteine [28], may render the material less active toward the hydrogen evolution reaction. 4. Conclusion In summary, the susceptibility of active sites poisoning by sulfide species on natural FeS2 was examined based on its performance
for hydrogen evolution reaction. The morphological and structural characterizations of the natural FeS2 were performed. The addition of sulfide species resulted in a slight increment of overpotential (at current density of 10 mA/cm2) while minimal effect to its Tafel slope. In contrast to platinum, the poisoning of the active sites of natural FeS2 was not as drastic. Extension of this work to other transition-metal electrocatalysts will be crucial since the poisoning of electrode active sites may result in higher catalyst operational costs in the long run. Conflict of interest There is no conflict of interests. Acknowledgment M.P. acknowledges Tier 2 grant (MOE2013-T2–1–056; ARC 35/13) from Ministry of Education - Singapore. References [1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q.X. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [2] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972–974. [3] J. Greeley, T.F. Jaramillo, J. Bonde, I.B. Chorkendorff, J.K. Norskov, Computational high-throughput screening of electrocatalytic materials for hydrogen evolution, Nat. Mater. 5 (2006) 909–913. [4] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (2009) 244–260. [5] H.T. Wang, Z.Y. Lu, S.C. Xu, D.S. Kong, J.J. Cha, G.Y. Zheng, P.C. Hsu, K. Yan, D. Bradshaw, F.B. Prinz, Y. Cui, Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction, Proc. Natl. Acad. Sci. U. S. A. 110 (2013) 19701–19706. [6] M.A. Lukowski, A.S. Daniel, F. Meng, A. Forticaux, L.S. Li, S. Jin, Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets, J. Am. Chem. Soc. 135 (2013) 10274–10277. [7] D. Voiry, H. Yamaguchi, J.W. Li, R. Silva, D.C.B. Alves, T. Fujita, M.W. Chen, T. Asefa, V.B. Shenoy, G. Eda, M. Chhowalla, Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution, Nat. Mater. 12 (2013) 850–855. [8] M.A. Lukowski, A.S. Daniel, C.R. English, F. Meng, A. Forticaux, R.J. Hamers, S. Jin, Highly active hydrogen evolution catalysis from metallic WS2 nanosheets, Energy Environ. Sci. 7 (2014) 2608–2613. [9] Y. Xu, R. Wu, J.F. Zhang, Y.M. Shi, B. Zhang, Anion-exchange synthesis of nanoporous FeP nanosheets as electrocatalysts for hydrogen evolution reaction, Chem. Commun. 49 (2013) 6656–6658. [10] E.J. Popczun, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide panoparticles, Angew. Chem. Int. Ed. 53 (2014) 5427–5430. [11] E.J. Popczun, J.R. McKone, C.G. Read, A.J. Biacchi, A.M. Wiltrout, N.S. Lewis, R.E. Schaak, Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 135 (2013) 9267–9270. [12] D.S. Kong, J.J. Cha, H.T. Wang, H.R. Lee, Y. Cui, First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction, Energy Environ. Sci. 6 (2013) 3553–3558. [13] M.S. Faber, M.A. Lukowski, Q. Ding, N.S. Kaiser, S. Jin, Earth-abundant metal pyrites (FeS2, CoS2, NiS2, and their alloys) for highly efficient hydrogen evolution and polysulfide reduction electrocatalysis, J. Phys. Chem. C 118 (2014) 21347–21356.
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[14] J. Xu, H.T. Xue, X. Yang, H.X. Wei, W.Y. Li, Z.P. Li, W.J. Zhang, C.S. Lee, Synthesis of honeycomb-like mesoporous pyrite FeS2 microspheres as efficient counter electrode in quantum dots sensitized solar cells, Small 10 (2014) 4754–4759. [15] T.S. Yoder, M. Tussing, J.E. Cloud, Y.A. Yang, Resilient carbon encapsulation of iron pyrite (FeS2) cathodes in lithium ion batteries, J. Power Sources 274 (2015) 685–692. [16] W.D. Qiu, J. Xia, H.M. Zhong, S.X. He, S.H. Lai, L.P. Chen, L-Cysteine-assisted synthesis of cubic pyrite/nitrogen-doped graphene composite as anode material for lithiumion batteries, Electrochim. Acta 137 (2014) 197–205. [17] D. Susac, L. Zhu, M. Teo, A. Sode, K.C. Wong, P.C. Wong, R.R. Parsons, D. Bizzotto, K.A.R. Mitchell, S.A. Campbell, Characterization of FeS2-based thin films as model catalysts for the oxygen reduction reaction, J. Phys. Chem. C 111 (2007) 18715–18723. [18] S. Vasireddy, B. Morreale, A. Cugini, C. Song, J.J. Spivey, Clean liquid fuels from direct coal liquefaction: chemistry, catalysis, technological status and challenges, Energy Environ. Sci. 4 (2011) 311–345. [19] D.Y. Wang, M. Gong, H.L. Chou, C.J. Pan, H.A. Chen, Y.P. Wu, M.C. Lin, M.Y. Guan, J. Yang, C.W. Chen, Y.L. Wang, B.J. Hwang, C.C. Chen, H.J. Dai, Highly active and stable hybrid catalyst of cobalt-doped FeS2 nanosheets-carbon nanotubes for hydrogen evolution reaction, J. Am. Chem. Soc. 137 (2015) 1587–1592. [20] J.O'M. Bockris, B.E. Conway, Studies in hydrogen overpotential. The effect of catalytic poisons at platinized platinum and nickel, Trans. Faraday Soc. 45 (1949) 989–999.
[21] E. Protopopoff, P. Marcus, Poisoning of the cathodic hydrogen evolution reaction by sulfur chemisorbed on platinum (110), J. Electrochem. Soc. 135 (1988) 3073–3075. [22] Z.T. Yang, X.J. Liu, X.L. Feng, Y.X. Cui, X.W. Yang, Hydrothermal synthesized micro/ nano-sized pyrite used as cathode material to improve the electrochemical performance of thermal battery, J. Appl. Electrochem. 44 (2014) 1075–1080. [23] H.W. Nesbitt, G.M. Bancroft, A.R. Pratt, M.J. Scaini, Sulfur and iron surface states on fractured pyrite surfaces, Am. Mineral. 83 (1998) 1067–1076. [24] Chapter 3 physico-chemical properties of mechanically activated minerals, in: P. Balaz (Ed.), Volume 10, Elsevier 2000, pp. 35–80. [25] B.E. Conway, J.C.H. Ku, F.C. Ho, The electrochemical surface reactivity of iron sulfide, FeS2, J. Colloid Interface Sci. 75 (1980) 357–371. [26] E. Peters, H. Majima, Electrochemical reactions of pyrite in acid perchlorate solutions, Can. Metall. Q. 7 (1968) 111–117. [27] F.M. Doyle, A.H. Mirza, Electrochemical oxidation of pyrite samples with known composition and electrical properties, Proceedings of the Fourth International Symposium on Electrochemistry in Mineral and Metal Processing, 96-6 1996, pp. 203–214. [28] A.M.A. El-Halim, N. Alonso-Vante, H. Tributsch, Iron/sulphur centre mediated photoinduced charge transfer at (100) oriented pyrite surfaces, J. Electroanal. Chem. 399 (1995) 29–39.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Susceptibility of FeS2 hydrogen evolution performance to sulfide poisoning Chun Kiang Chua, Martin Pumera ⁎ Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
a r t i c l e
i n f o
Article history: Received 1 May 2015 Received in revised form 22 May 2015 Accepted 27 May 2015 Available online 3 June 2015 Keywords: Hydrogen evolution reaction Iron pyrite Sulfide Poisoning
a b s t r a c t The imminent depletion of fossil fuels raises concern over the need for next-generation clean energy. Of numerous alternatives, electrochemical water splitting is a promising method to store energy in the form of hydrogen. In order to benefit from this system, technological advancement in the development of affordable and efficient electrocatalysts for hydrogen evolution reaction is necessary. Transition-metal electrocatalysts composing of earth-abundant elements, specifically natural FeS2, has demonstrated excellent performance for hydrogen evolution reaction. However, previous studies on platinum surfaces highlighted the detrimental effect toward hydrogen evolution performance upon poisoning of the active sites. In this work, we examine the susceptibility of natural FeS2 toward sulfide poisoning. Our findings showed that the degradation effect from the introduction of sulfide to natural FeS2 was not as severe as that observed on platinum. The overpotential (at a current density of 10 mA/cm2) for natural FeS2 and platinum increased by approximately 20 and 110 mV, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The development of next-generation clean energy has become increasingly important as the dependency of modern societies on fossil fuel escalates despite depleting supply. Hydrogen is seen as one of the many alternative fuels that are secure, sustainable and clean. The electrolysis of water (water splitting), either electrochemically or photoelectrochemically, is currently the most attractive route toward the production of hydrogen since harmful by-products are not generated in the process [1,2]. This has driven intense research to identify potential electrocatalysts that are applicable for hydrogen evolution reaction (HER), which at the moment, is best catalyzed by noble metals, especially platinum [3,4]. However, the high cost and availability of these precious metals prevent the large-scale application of such clean energy technologies. As such, numerous efforts have been taken to search for affordable and efficient transition-metal electrocatalysts composing of earth-abundant elements such as MoS2 [5,6], WS2 [7,8], FeP [9], CoP [10] and Ni2P [11]. Recently, cubic pyrite-phase transition-metal dichalcogenides (with the general formula of MX2, where typically M = Fe, Co, or Ni and X = S or Se) have been successfully applied as efficient HER electrocatalysts [12,13]. Moreover, these transition-metal pyrites which consist of first row transition-metals and rock-forming chalcogens are inexpensive and available in bulk quantity. In specific, the semiconducting FeS2 is not only a promising electrocatalyst for HER but is highly applicable in numerous technological systems such as dye-sensitized solar cells [14], lithium-ion batteries [15,16] and chemical/electrochemical ⁎ Corresponding author. E-mail address: [email protected] (M. Pumera).
http://dx.doi.org/10.1016/j.elecom.2015.05.016 1388-2481/© 2015 Elsevier B.V. All rights reserved.
systems including oxygen reduction reaction [17], polysulfide reduction [13] and liquefaction of coal [18]. More recently, FeS2 has been incorporated into a hybrid of Fe0.9Co0.1S2/CNT to achieve a HER overpotential of ~120 mV (at 20 mA/cm2) and a Tafel slope of ~46 mV/decade [19]. Despite the efficiency of platinum as HER electrocatalyst, such transition-metal electrocatalysts are typically susceptible to poison such as arsenide and sulfur [20,21]. For example, the rate of hydrogen evolution reaction on platinum is greatly reduced in the presence of trace amounts of poison. With respect to this, we are interested in investigating such poisoning effect on the performance of natural FeS2 for hydrogen evolution reaction in this work. In fact, the effect of sulfide (from sodium hydrosulfide, NaSH) poisoning on the performances of natural FeS2 and Pt/C for hydrogen evolution were examined by examining the polarization curve and Tafel analysis. 2. Experimental section 2.1. Materials Iron disulfide (FeS2, −325 mesh, 99.8% trace metal basis), platinum (20 wt.% on activated carbon), sodium hydrosulfide and sulfuric acid were obtained from Sigma Aldrich, Singapore. Screen printed electrode (SPE) with a diameter of 3 mm was obtained from Zensor. Milli-Q water (resistivity: 18.2 MΩ cm) was used throughout the experiments. 2.2. Equipment X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a Mg X-ray radiation source (SPECS,
30
C.K. Chua, M. Pumera / Electrochemistry Communications 58 (2015) 29–32
(a)
(b)
1 m
20 m
Fig. 1. Scanning electron microscopy images of natural FeS2 at (a) 10,000× and (b) 600× magnification.
Germany). Natural FeS2 was homogeneously spread across on a carbon conductive tape for XPS analysis. A JEOL-7600F semi-in-lens FE-SEM, operating under SEI and GB-H modes at 2 kV, was used to acquire the SEM images. The natural FeS2 powder was adhered to a conductive carbon tape held onto a SEM holder for analyses. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed with a spectrometer from Oxford Instrument. All voltammetric experiments were performed on a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). 2.3. Procedures 2.3.1. Electrochemical measurements The electrochemical experiments were carried out in a 10 mL voltammetric cell at room temperature using a three-electrode configuration. A platinum electrode served as an auxiliary electrode and a Ag/AgCl electrode as a reference electrode; a screen-printed electrode was utilized as a working electrode. The working electrode was modified by applying 8 μL of material (1 mg mL−1 dispersed in water) and allowed to dry prior to measurements. Measurement of hydrogen evolution was performed in 0.5 M H2SO4 using linear sweep voltammetry at a scan rate of 2 mV/s. The reported results are average from triplicate measurements. 2.3.2. Poisoning of active sites The working electrode which was pre-coated with natural FeS2 or Pt/C was added one equivalence (with respect to the amount required for Pt/C, based on a 1:1 of Pt:S ratio; 8.20 × 10-9 mol) of sodium hydrosulfide solution. Upon drying, the working electrode was swirled gently in ultrapure water to remove excess or loosely adsorbed NaSH. The working electrode was ready for measurement after drying.
(b) Bulk S
S2p
N1s
800
600
(c)
O1s
C1s
1000
The poisoning effect of sulfide on natural FeS2 was examined by performing hydrogen evolution reaction analysis with linear sweep voltammetry. Similar poisoning effect was also performed on platinum for comparison. Prior to that, the morphology of FeS2 powder was investigated using scanning electron microscopy (SEM). Fig. 1 shows the SEM images at magnifications of 10,000 × and 600×. The material exists as irregular chunks with lateral dimensions up to ~90 μm and with presence of smaller sized particles which is analogous to natural pyrite samples [22]. Subsequent energy dispersive X-ray (EDX) analysis measured a composition of 39.5 at.% Fe and 60.5 at.% S for the sample. Further X-ray photoelectron spectroscopy (XPS) analysis provided an overview on the elemental composition of the natural FeS2 as shown in Fig. 2. On top of Fe and S, elements such as N, C and O were present based on the XPS survey scan. High-resolution scans of S2p and Fe2p were consequently measured and fitted with Gaussian– Lorentzian curves. The S2p spectrum was fitted with four sets of curves with each consisting of S2p3/2 and S2p1/2 (in the ratio of 2:1) peaks due to spin orbit splitting [23,24]. The peak at 162.5 eV was assigned to bulk disulfide while polysulfidic species was responsible for the peak at higher binding energy of 164.4 eV. Although present in small quantities, the set of curves at 161.7 suggested the presence of surface states including that of monosulfide and disulfide species. The peak at a much higher binding energy of 168.8 eV could result from the presence of S6+ species. All in all, the ratio of bulk S:monosulfide/disulfide:polysulfide:S6+ is 0.55:0.11:0.10:0.24. On the other hand, the high-resolution Fe2p scan showed prominent peak at 706.3 eV due to bulk Fe2+. The tailing of the peaks toward higher binding energy was contributed by surface Fe2+ and Fe3+. The effect of poisoning natural FeS2 active sites was evaluated by monitoring the performance of hydrogen evolution reaction. This was performed side-by-side with platinum (20 wt.% on activated carbon)
400
Binding Energy (eV)
200
S6+
175
Polysulfide
170
165
Binding Energy (eV)
Monosulfide/ Disulfide
160
Intensity (a.u.)
Intensity (a.u.)
Fe2p
Intensity (a.u.)
(a)
3. Results and discussion
730
725
720
715
710
Binding Energy (eV)
Fig. 2. X-ray photoelectron spectroscopy spectra of natural FeS2. (a) Survey scan; and high resolution scans for (b) S2p and (c) Fe2p.
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C.K. Chua, M. Pumera / Electrochemistry Communications 58 (2015) 29–32
(b)
0
-5
-10
Bare SPE Pt/C Pt/C-NaSH FeS2
-15
Overpotential (Vvs. RHE)
Current Density (mA/cm2)
(a)
31
0.8
0.6
0.4
Bare SPE Pt/C Pt/C-NaSH
0.2
FeS2 FeS2-NaSH
FeS2-NaSH
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
-1.0
0.4
-0.5
0.0
0.5
1.0
1.5
Log Current Density (mA/cm2)
Potential (V vs. RHE)
Fig. 3. Electrochemical characterization of bare screen printed electrode, Pt/C, poisoned Pt/C, FeS2 and poisoned FeS2 toward hydrogen evolution reaction. (a) Polarization curves of current density against potential. (b) Tafel analysis of overpotential against the log of current density.
and bare SPE for comparison. As shown in the polarization curves in Fig. 3a, the overpotential of natural FeS2 was 890 mV at a current density of 10 mA/cm2. The presence of cathodic peaks at approximately − 500 mV could indicate the possible inherent electrochemistry of FeS2, most likely due to surface decomposition of the material and subsequent reduction of surface ferrous ions [25–28]. On the other hand, the overpotential for bare SPE and Pt/C were 970 and 100 mV, respectively. A lower overpotential indicates a material as more catalytic for the evolution of hydrogen. Consequently, the addition of sodium hydrosulfide (NaSH) poison was expected to block the active sites of the materials, which is possibly at the disulfide-terminated edges in the case of FeS2, resulting in a reduced performance for hydrogen evolution [19–21]. Thereafter, the overpotential for FeS2 and Pt/C were 910 and 210 mV, respectively. The poisoning effect was more apparent on Pt/C than on FeS2, in which a 110 mV (110%) increment in overpotential was observed for the former while only 20 mV (2%) was observed for the latter. Further Tafel analyses on the polarization curves of bare SPE, Pt/C and FeS2 were performed as seen in Fig. 3b. It was noted that the Tafel slope observed on bare SPE at 248 mV/decade was unusually larger than typical glassy carbon electrode (~126 mV/decade) due to the proprietary carbon electrode material of the bare SPE. In contrast, Pt/C and FeS2 showed lower Tafel slopes. As expected from the best electrocatalyst for HER, Pt/C showed a Tafel slope of 30 mV/decade and suggested a typical Volmer–Tafel mechanism. However, natural FeS2 with a Tafel slope of 215 mV/decade indicated a possibly slow Volmer adsorption step. It should be noted that several works on FeS2 produced using various synthesis methods reported contrasting Tafel slopes (e.g. 50 to 70 mV/decade) for hydrogen evolution reaction [13,27]. The slow Volmer adsorption step in this study could suggest possible electrochemical decomposition of the active sites on natural FeS2 which thus prevented the spontaneous adsorption of H+ on its surfaces. Nevertheless, upon the introduction of sodium hydrosulfide, Pt/C and natural FeS2 revealed Tafel slopes of 76 and 194 mV/decade, respectively. The Tafel slope of Pt/C increased upon sulfide poisoning while that of FeS2 did not register any major changes. In the case of Pt/C, the adsorption of sulfide species reduced the availability of active sites for the electrocatalysis of the hydrogen evolution reaction. As for natural FeS2, effects from the introduction of sulfide were not as detrimental as that of platinum. The possibility of electrochemical decomposition and the coordination of sulfide to natural FeS2, likewise with the case of cysteine [28], may render the material less active toward the hydrogen evolution reaction. 4. Conclusion In summary, the susceptibility of active sites poisoning by sulfide species on natural FeS2 was examined based on its performance
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