Communication pubs.acs.org/JACS
Mesoporous Iron Sulfide for Highly Efficient Electrocatalytic Hydrogen Evolution Ran Miao,† Biswanath Dutta,† Sanjubala Sahoo,‡,§ Junkai He,‡ Wei Zhong,‡ Shaylin A. Cetegen,∥ Ting Jiang,∥ S. Pamir Alpay,‡,§ and Steven L. Suib*,†,‡,§,∥ †
Department of Chemistry, U-3060, University of Connecticut, Storrs, Connecticut 06269, United States Institute of Materials Science, U-3136, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Materials Science & Engineering, Unit-3136, University of Connecticut, Storrs, Connecticut 06269, United States ∥ Department of Chemical and Biomolecular Engineering, Unit-3222, University of Connecticut, Storrs, Connecticut 06269, United States ‡
S Supporting Information *
cathode material are limited by their low surface area and few active sites. Due to the volume contraction and low electronic affinity between metal and sulfur, synthesis of metal sulfides with high surface area is a major challenge.17 Meanwhile, the mesoporous materials are of great interest in electrochemical water splitting catalysis due to their abundance of accessible mesopores and exposure of active sites, which can facilitate the charge transfer and product diffusion.18 Herein, we synthesized mesoporous pyrite FeS2 nanoparticles via a facile two-step synthetic protocol including an inverse micelle sol−gel method followed by low-temperature sulfurization treatment. The structure of the resulting material is studied via multiple characterization techniques. The asprepared mesoporous FeS2 nanoparticle materials show superior performance as an HER electrocatalyst with a low overpotential of ∼96 mV at a current density of 10 mA·cm−2 with a mass loading of 0.53 mg·cm−2 and exhibit remarkable electrochemical durability for 24 h under alkaline conditions. In a typical synthesis of mesoporous FeS2 nanoparticles, amorphous Fe2O3 was first obtained via an inverse micelle sol− gel method. The synthesized material was heated to 150 °C in static air to obtain the amorphous Fe2O3 structure and to remove the adsorbed carboxyl and nitrate species on the surface. Then, H2S gas and sulfur powder were used as sulfur sources, which allowed for a facile chemical conversion from oxide to sulfide without damaging the mesoporous structure. The carboxyl and nitrate were removed in the sulfurization process. The crystalline structure of pyrite FeS2 was first confirmed by the X-ray diffraction (XRD) pattern. As shown in Figure 1a, the diffraction pattern of the mesoporous FeS2 can be indexed to the cubic phase of pyrite FeS2 (JCPDS No. 421340), suggesting successful conversion of Fe2O3 into FeS2. The selected area electron diffraction (SAED) pattern (Figure S1) further confirms the crystalline nature of mesoporous FeS2. The commercial FeS2 (comm FeS2) also exhibits the pyrite phase, but with different crystallinity degree. The much lower crystallinity of the mesoporous FeS2 may be due to the existence of the mesoporous structure, which destroyed the structural order over long distances.19 The phase purity of the
ABSTRACT: We report a facile synthetic protocol to prepare mesoporous FeS2 without the aid of hard template as an electrocatalyst for the hydrogen evolution reaction (HER). The mesoporous FeS2 materials with high surface area were successfully prepared by a sol−gel method following a sulfurization treatment in an H2S atmosphere. A remarkable HER catalytic performance was achieved with a low overpotential of 96 mV at a current density of 10 mA·cm−2 and a Tafel slope of 78 mV per decade under alkaline conditions (pH 13). The theoretical calculations indicate that the excellent catalytic activity of mesoporous FeS2 is attributed to the exposed (210) facets. The mesoporous FeS 2 material might be a promising alternative to the Pt-based electrocatalysts for water splitting.
I
ncreasing environmental issues and depletion of fossil fuels have motivated intense research on alternative clean and sustainable energy carriers.1 Hydrogen has been regarded as a promising energy carrier by virtue of its high energy density and zero-emission when burned with oxygen.2 Electrocatalytic hydrogen evolution reaction (HER) is one of the most appealing approaches to extract molecular hydrogen from water.3,4 Currently, Pt-based materials are regarded as the most efficient electrocatalysts for HER. 5 However, the low abundance and high cost limit their widespread applications.6 One challenging issue in this pursuit is the development of energy- and cost-efficient catalysts that can be applied in this process. In pursuit of an inexpensive alternative to Pt as the HER electrocatalyst, a great deal of effort has been made. Metal chalcogenides, which contain only nonprecious, transition metals, are considered as a class of very promising candidates for HER due to their high abundance, low cost, high conductivity, thermal and mechanical stability.7,8 Metal chalcogenides, MoS2,9 WS2,10 FeS2,11 CoSe2,12 have been investigated widely as HER electrocatalysts over the past decades. However, iron sulfide materials have rarely shown superior HER performance without further surface modifications.13−16 The practical applications of iron sulfide as an HER © 2017 American Chemical Society
Received: July 6, 2017 Published: September 5, 2017 13604
DOI: 10.1021/jacs.7b07044 J. Am. Chem. Soc. 2017, 139, 13604−13607
Communication
Journal of the American Chemical Society
Figure 1. (a) XRD patterns of meso FeS2, comm FeS2, and the JCPDS pattern for standard pyrite. (b) SEM image, (c−e) low- and highmagnification TEM images, and (f) high-angle annular dark-field TEM image of meso FeS2. (g−i) EDX elemental mapping images of Fe, S, and the merged image of Fe and S for the meso FeS2.
cm3·g−1, respectively. The pore size distribution of the assynthesized FeS2 was obtained by the Barrett−Joyner−Halenda (BJH) method, which exhibits a monomodal pore size (3.4 nm). In contrast, the isotherm of the comm FeS2 shows a nonporous structure with a surface area less than 1 m2·g−1. The mesoporous structure of meso FeS2 was further confirmed by low-angle XRD (Figure S8). The small diffraction angles can be an indicator of the existence of mesoporous structure due to the fact that most mesoporous materials are amorphous on the atomic scale and thus have large unit cells.23 The electrocatalytic HER performance for the mesoporous FeS2 was then carefully investigated using a three-electrode system in alkaline media (pH 13). For comparison, the HER activities of commercial FeS2, Pt/C (20%), and bare Ni foam are also measured. As depicted in Figure 2a, the mesoporous FeS2 coated electrode requires a significant small overpotential of 96 mV to achieve a current density of 10 mA·cm−2, which is comparable to most reported metal chalcogenide HER catalysts in alkaline media6,24 (Figure S10 gives a photograph showing the generation of hydrogen bubbles on the Ni foam). In contrast, bare Ni foam shows little HER activity, and the commercial FeS2 requires a large overpotential to reach a 10 mA·cm−2 current density. The Tafel slope is an important indicator of reaction kinetics and the rate-determining step in the HER process, which reveals the extra voltage required to increase current density by 10-fold. In the whole HER process, the hydrogen evolution steps involve the adsorbed H atom discharge reaction followed by electrochemical desorption of OH− and H2 (Volmer− Heyrovsky mechanism), or the direct recombination of two adsorbed H atoms to release H2 (Volmer−Tafel mechanism).25,26 In Figure 2b, the calculated value for meso FeS2 is about 78 mV·dec−1, indicating a fast Volmer−Heyrovsky mechanism. Such a mechanism involved an electrochemical desorption of hydrogen as the rate-determine step. 10 Furthermore, the exchange current density (j0) was calculated by an extrapolation method from Tafel plots (Figure S11). The exchange current density value is highly dependent on the nature of the electrode material, which suggests the intrinsic electron transfer rates between the electrode and the electrolyte. The j0 value of mesoporous FeS2 for HER is 6.3
pyrite samples was further characterized by Raman spectroscopy (Figure S3). The two peaks associated with the vibrational modes of the pyrite phase at 335 and 385 cm−1 are observed for both samples. In contrast to the sharp peaks of the commercial FeS2, the Raman bands of mesoporous FeS2 are very broad, indicating the structural disorder in the mesostructure. All characteristic vibrational modes are in agreement with previous literature reported on pyrite FeS2.20,21 The composition of the mesoporous pyrite FeS2 was investigated using energy dispersive X-ray (EDX) analysis, which reveals a molar ratio of S/Fe of 2.3. In contrast to that of commercial FeS2 (1.9), the higher molar ratio is attributed to the existence of polysulfide species (Figure S4). The corresponding element mapping images show a homogeneous nature of the as-prepared material, in which Fe and S are uniformly distributed in the mapping image of FeS2 (Figure 1g−i). Scanning electron microscopy (SEM) images for the meso FeS2 sample are shown in Figure 1b. The pores are welldispersed all over the spherical material. As a comparison, the SEM imaging of commercial FeS2 material was also conducted (Figure S5). A chunky morphology is shown, indicating no porosity could be obtained in the commercial FeS2 material. Transmission electron microscopy (TEM) was used to further demonstrate the mesoporous structure and crystallinity. As shown in Figure 1c,e, the loose internal structure of FeS2 spheres confirms the existence of mesopores with a pore size in the range of 4−7 nm. The crystallite sizes of the mesoporous FeS2 spheres are estimated to be 15−20 nm, which are much smaller than that of the commercial FeS2 (103 nm). The results are in accordance with the calculated value based on the XRD patterns (Table S1). In Figure 1d, the lattice fringes show an interplanar distance of 0.31 nm corresponding to the (111) planes of pyrite FeS2. Furthermore, the high-angle annular dark-field TEM image (Figure 1f) clearly indicated a mesoporous nanostructure of meso FeS2. To estimate the pore size distribution and the surface area, nitrogen sorption measurements were performed. The isotherm of the mesoporous FeS2 reveals a type-IV adsorption isotherm (Figure S7), suggesting the existence of a mesoporous structure according to the IUPAC classification.22 The Brunauer− Emmett−Teller (BET) surface area and total pore volume of the mesoporous FeS2 were calculated to be 128 m2·g−1 and 0.23 13605
DOI: 10.1021/jacs.7b07044 J. Am. Chem. Soc. 2017, 139, 13604−13607
Communication
Journal of the American Chemical Society
bond lengths. Such changes are prominent at the surface layers compared to the interior of the slab. The γ values (eq 1 in the Supporting Information) for (100) and (210) planes are found to be 1.04 and 1.25 J·m−2, respectively. These values are similar to previous reported values.27 The lowest energy surfaces are also important for the nonequilibrium mesoporous phase. The H2O molecule is adsorbed at several high symmetry atomic sites for (100) and (210) surfaces, namely, top of S atoms, top of Fe atoms, and selected interstitial sites, labeled as A−E in Figure S14. The computations indicate that H 2 O is preferentially adsorbed at the Fe sites, forming a Fe−O bond at both surfaces. The Eads (eq 2 in Supporting Information) of H2O is found to be larger for the (210) surfaces (1.99 eV) compared to the (100) surfaces (1.67 eV). Such large Eads indicate the chemisorption of water molecule on FeS2 surfaces. The partial charges on H2O and the nearby surface atoms are shown in Figure S15 for both surfaces. There is a charge transfer from “substrate” Fe atoms to the H2O molecule where the O atoms acquire an excess of 1.34 and 1.45 e for the (210) and (100) surfaces, respectively. This is because of the large difference in the electronegativity value of O (3.44) compared to that of Fe (1.86). The variations in the O partial charges are due to the “substrate” Fe atoms (bonded to H2O) and coordination to nearby S atoms. For the (210) surfaces, the Fe atoms bound to water have four nearest neighbor S atoms forming four Fe−S bonds whereas for (100) surfaces, the Fe atoms have five nearest neighbor S atoms resulting in the formation of five Fe−S bonds. This leads to a different local charge distribution. The charge arrangement is apparent from the differential charge densities as shown in Figure S16 for both surfaces where the charge is found to be mostly localized at the interface and on the water molecule. Figure 3 shows the reaction pathways for O−H bond cleavage of water molecule on both surfaces. The activation
Figure 2. HER performance of the mesoporous FeS2 in alkaline media (pH 13). (a) Polarization curves and (b) Tafel plots of the commercial FeS2, mesoporous FeS2, 20% Pt/C, and the bare Ni foam in 0.1 M KOH at a scan rate of 10 mV/s. (c) Nyquist plots of commercial FeS2 and mesoporous FeS2 at an overpotential of 200 mV. The inset shows the fitted equivalent circuit. (d) Chronoamperometry curve of the mesoporous FeS2 at a constant overpotential of 100 mV. All the materials were loaded onto Ni foam at a mass loading of 0.53 mg· cm−2.
× 10−1 mA·cm−2, which is 7 times that of commercial FeS2 (8.4 × 10−2 mA·cm−2). Electrochemical impedance spectroscopy (EIS) was applied to investigate the electrode kinetics under HER operating conditions (Figure 2c). The semicircle in the high-frequency range of the Nyquist plot revealed that mesoporous FeS2 (7 Ω) has a much lower charge transfer resistance (Rct) than that of commercial FeS2 (238 Ω). The small Rct value indicates a more favorable HER kinetics on the mesoporous FeS2 electrode. This further suggests that the mesoporosity not only provides more active sites but also facilitates the charge and mass transfer efficiency.19 The Nyquist plots in Figure S12 also exhibit the change in Rct values in the potential range of 0 to −0.6 V, which is consistent with the polarization plots of meso FeS2 in Figure 2a. Durability is another important criterion to evaluate a good electrocatalyst. The superior robustness of the mesoporous FeS2 material is confirmed by the chronoamperometric curve in Figure 2d. The initial current density maintained without an obvious decrease throughout 24 h under continuous operation at a constant overpotential of 100 mV, indicating the superior durability of the as-prepared mesoporous FeS2. To better understand the surface structure of mesoporous FeS2 and the hydrogen evolution reaction mechanism at the atomic level, we performed quantum chemical calculations based on density functional theory (DFT) for the mesoporous pyrite FeS2 structure where a comparison of catalytic activity between the (100) and (210) surfaces are investigated for the hydrogen evolution reaction (HER). The XRD analysis shows that the relative intensity of (210) planes over other high-index planes is enhanced in the mesoporous FeS2 samples. Therefore, we have used the (100) surfaces [parallel to (200)] and the (210) surfaces for our theoretical investigation. The geometrical optimization of (100) and (210) surfaces leads to surface reconstruction and rearrangement of the bulk
Figure 3. Reaction pathways for O−H bond breaking of H2O molecule on (210) (red line) and (100) (blue line) surfaces. The optimized geometries of the complex, transition state and product with the O−H and Fe−O bond distances (as numbers) are shown in Å. The energy differences are calculated with respect to the energy of the corresponding reactants (ESurface (210/100)+EH2O).
barriers EA (eq 3 in Supporting Information) are found to be 0.89 and 1.46 eV for (210) and (100) surfaces. This suggests that (210) surfaces are more reactive for O−H bond cleavage relative to the (100) surfaces making (210) planes suitable surfaces for the water splitting reaction. This conclusion is further supported by the low value of transition state energies 13606
DOI: 10.1021/jacs.7b07044 J. Am. Chem. Soc. 2017, 139, 13604−13607
Communication
Journal of the American Chemical Society
The authors thank Dr. Xuanhao Sun his assistance in SEM imaging, Xiaojiao Xu for his help with graphic design, and Dr. Frank Galasso for helpful discussions.
ETS which are calculated (eq 4 in Supporting Information) as −1.09 and −0.21 eV for (210) and (100) surfaces, respectively. The HER on the (210) surfaces is found to be exothermic where the product is formed by release of energy of 0.55 eV in contrast to that of the (100) surfaces, which show endothermic behavior. Our results are further supported by the strong binding of H2O molecules on (210) surfaces, which is 0.31 eV larger compared to the (100) surfaces. These findings provide evidence toward an atomic level understanding of the experimental results for mesoporous FeS2 that the exposed (210) surfaces have a higher preference for the HER compared to the relatively lower energy (100) surfaces. In summary, the mesoporous FeS2 materials with high surface area and abundant accessible active sites were synthesized via a two-step synthetic protocol, without the aid of a hard template. The as-synthesized mesoporous FeS2 has a much higher surface area (128 m2·g−1) than the commercial FeS2 (
