Amorphous Rhenium Disulfide Nanosheets: A Methanol-Tolerant

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Amorphous Rhenium Disulfide Nanosheets: A Methanol-Tolerant Transition Metal Dichalcogenide Catalyst for Oxygen Reduction Reaction Thulasi Radhakrishnan,† Madathil Palliyalil Aparna,‡ Raghu Chatanathodi,‡ and Neelakandapillai Sandhyarani*,†

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†

Nanoscience Research Laboratory, School of Materials Science and Engineering, National Institute of Technology Calicut, Calicut, Kerala 673601, India ‡ Department of Physics, National Institute of Technology Calicut, Calicut, Kerala 673601, India S Supporting Information *

ABSTRACT: A methanol tolerant catalyst for the reduction of oxygen is important for the effective utilization of direct methanol fuel cells (DMFCs). Herein, the synthesis of a layered transition metal dichalcogenide rhenium disulfide (ReS2) and its utilization as an effective novel precious metal free, methanol tolerant oxygen reduction catalyst is reported. A single step wet chemical synthesis procedure is used to synthesize ReS2, and morphological analysis revealed the formation of ultrathin nanosheets of thickness 1.6 nm. The catalytic activity of ReS2 toward the oxygen reduction reaction (ORR) was monitored in 0.1 M H2SO4, which showed higher current density and excellent stability comparable with the commercially available carbon supported platinum catalyst. The catalyst exhibited selective 4 electron transfer kinetics (n = 3.9) and has remarkable tolerance to methanol in contrast to platinum supported on carbon, which makes it promising as a Pt-free methanol tolerant ORR catalyst. KEYWORDS: direct methanol fuel cells, DMFC, oxygen reduction reaction, ORR, rhenium disulfide, transition metal dichalcogenides, TMDs, methanol tolerant catalyst, electrocatalyst

1. INTRODUCTION Progress in energy technologies that rely on electrochemical theories, especially fuel cells, water electrolyzers, and metal−air batteries, are vital to address the current issues associated with energy scarcity and environmental pollution.1−3 The electrochemical oxygen reduction reaction (ORR) is one of the significant reactions to achieve better energy conversion technology. The ORR is the most essential reaction that plays a crucial part in improving the overall performance of fuel cells and other electrochemical energy devices.4,5 Platinum group metal based electrocatalysts are widely used in direct methanol fuel cells (DMFCs) that operate at low temperature and are employed in powering portable devices.6,7 The expensive platinum based catalysts limit the practical application of DMFCs due to higher cost, inadequacy, sluggish kinetics, and methanol crossover. This has motivated researchers to perform extensive investigation on the development of efficient, durable, and inexpensive alternatives8,9 for ORR. In DMFCs, a small amount of methanol can be transported from the anode to the cathode chamber through the proton exchange membrane, a process known as methanol cross over, and will react directly with the Pt based cathode catalyst resulting in reduced fuel cell efficiency.10 The most active and competitive field in direct methanol fuel cells is to develop a methanol tolerant, cost-effective cathode catalyst sustaining the high performance of platinum.11,12 The major target in this area is to develop economical, long lasting, and © XXXX American Chemical Society

effective catalysts toward ORR and to replace Pt-based catalysts. Development of active catalyst for ORR in an acidic environment is vital for low temperature fuel cells.13 The metal catalyst often leaches out in the presence of acid;14 hence it is crucial to develop a highly active catalyst with minimum degradation during long-term operation in these media. Nonprecious-metal catalysts usually reduce oxygen through a 4 electron transfer reaction in alkaline media. Among the nonprecious-metal-based catalysts, metal−nitrogen−carbon (M− N−C) catalysts are considered to be promising in acidic environments.15 Efforts are being made by researchers to synthesize carbon based materials as effective catalysts for ORR.16,17 Among such classes of catalysts, carbon doped with heteroatoms is promising and allows tuning of the electronic properties of the catalyst thereby enhancing the electrocatalytic properties.18−20 It was reported that a small amount of iron in the carbon based catalyst improved its activity in acidic media.21 Recently researchers are more focused on metal− organic frameworks (MOFs) as a new platform for synthesis of carbon composites. Porous carbon, metal sulfides, and metal oxides were selectively synthesized using zeolitic imidazolate frameworks (ZIFs) and were used for various electrocatalytic Received: May 8, 2019 Accepted: June 11, 2019 Published: June 11, 2019 A

DOI: 10.1021/acsanm.9b00867 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 1. (A) FESEM image of ReS2, (B, C) TEM images, and (D) HRTEM image of ReS2 revealing the layered structure and (E) SAED pattern of ReS2.

applications.22 Cobalt based nitrogen doped carbon nanosheets were developed using MOFs and are reported to have higher catalytic activity toward ORR.23−25 Non-precious transition metal dichalcogenides entailing a metal atom and chalcogen atoms, namely S, Se, and Te were intensely explored as catalysts due to the low-cost and high durability in acidic and alkaline media.26,27 Metal dichalcogenides are more stable in the acidic electrolyte than metal oxides.28 Hence in this aspect, it is advantageous to develop an efficient metal dichalcogenide catalyst for ORR that is active in acidic media. One of the most explored transition metal dichalcogenides (TMDs) for ORR is Ru and Co based catalysts.29 The other catalysts used for ORR are Ni, Cu, and Mo based chalcogenides.26,27,30 MoS2 is the widely employed metal dichalcogenide catalyst for ORR, which reduces oxygen through 4 electron transfer reaction in alkaline media.31,32 However, there are only few reports available for MoS2 based catalyst for ORR in an acidic environment.33 The advent of ReS2 attracted scientific attention in the field of catalysis, mainly in hydrogen evolution reactions.34 Its layer independent properties are advantageous over MoS2 in catalytic applications. However, no reports are available regarding its ORR catalytic activity. Herein we report the synthesis of layered rhenium disulfide and its application as a precious metal free, methanol tolerant catalyst for ORR. The performance was compared with the widely explored transition metal dichalcogenide MoS2 and commercial Pt/C catalysts. Other metal chalcogenides and dichalcogenides, namely, WS2, CoS2, and NiS, were also employed for the initial electrochemical comparison studies. Though the overpotential is larger for ReS2 than Pt/C, the achieved current density and stability are high compared with Pt/C. Highlights of the catalyst are its higher methanol tolerance than Pt/C and its better activity in acidic electrolyte compared to other metal dichalcogenides. The overpotential is smaller for ReS2 compared to all other metal dichalcogenides used in the current work.

2. EXPERIMENTAL METHODS 2.1. Materials and Chemicals. Ammonium perrhenate and sodium sulfide were acquired from Sigma-Aldrich. Ethylene glycol, sulfuric acid, methanol, and perchloric acid were procured from Merck, India. High purity nitrogen was supplied to maintain the inert atmosphere during the synthesis. Activity of synthesized catalyst was compared to the commercially available Pt (20%) supported on carbon black obtained from Alfa Aesar and MoS2 bought from SigmaAldrich. All synthesis and measurements were carried out with deionized water from Milli-Q, Millipore ultrafiltration unit. 2.1.1. Synthesis of Rhenium Disulfide. A schematic representation of the synthesis of rhenium disulfide is given in Figure S1. ReS2 was synthesized by dissolving 25 mg of ammonium perrhenate in 10 mL of ethylene glycol under N2 atmosphere. After the dissolution, 100 mg of sodium sulfide was added to the above solution and stirred for further 15 min. Perchloric acid (0.4 mL) was added dropwise to this solution. Immediately the colorless solution changed to black color, and the solution was kept for 1 h stirring in N2 atmosphere. The precipitate was washed thrice by centrifugation at 12000 rpm using DI water. The obtained product was dried at a temperature of 70 °C overnight. 2.1.2. Synthesis of Molybdenum Disulfide. Bulk MoS2, 1 mg/mL in DMF solution, was initially prepared and sonicated at room temperature for continuous 4 h to form a black dispersion. This dispersion was further centrifuged at 1000 rpm for 15 min and the supernatant was collected. The thin layered MoS2 was collected from the supernatant by centrifuging at 8000 rpm for 15 min. Thus obtained exfoliated MoS2 was dried at a temperature of 70 °C overnight. The SEM image of corresponding MoS2 is given in Figure S2. The synthesis procedures for WS2, CoS2 and NiS are mentioned in detail in the Supporting Information. 2.2. Instrumentation. Scanning electron microscopy (SEM) images of the samples were collected using Hitachi SU6600 FESEM or Zeiss Sigma FESEM equipped for energy dispersive X-ray spectra (EDS) (Bruker). Transmission electron microscopy (TEM) images of the catalysts were analyzed by means of Jeol/JEM 2100. The thickness of the layered ReS2 was monitored using atomic force microscopy (AFM) with Park systems XE100. Crystallographic details of the samples were collected using Rigaku-smart lab diffractometer using Cu Kα (λ = 1. 5406 Å) radiation operating at 200 kV and 45 mA from 10° to 70°. Elemental composition was confirmed with B


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Figure 2. (A) XRD pattern (the corresponding JCPDS values of ReS2 are marked in dotted lines) and (B) nitrogen adsorption/desorption isotherm of ReS2. XPS spectra in the region of (C) Re 4f and (D) S 2p of ReS2. electrolyte was purged with high quality N2/O2 for 20 min before performing any measurements. The electrocatalytic activity of the ReS2 was also monitored in alkaline media. The catalyst did not exhibit notable activity in the alkaline media compared with the bare electrode in same condition. Linear sweep voltammograms (LSVs), cyclic voltammogram (CVs) and electrochemical impedance spectra (EIS) were used to investigate the catalytic activity of the samples. The LSVs were recorded at different rpm in O2 saturated and N2 saturated 0.1 M H2SO4 to perform the ORR measurements. The durability assessment was performed using chronoamperometry for 30000 s. The durability of the catalysts was monitored in 0.1 M H2SO4 at a potential that exhibited same current density by Pt/C and ReS2 to accurately compare the decay in current density with respect to time. The methanol tolerance of the catalyst was evaluated by chronoamperometry. Initially, the data was recorded for 2000 s in nitrogen saturated 0.1 M H2SO4, and after 2000 s, oxygen was purged to the electrolyte, which resulted in oxygen reduction until 4000 s. At 4000 s, 1 M methanol was added to the electrolyte, and the influence of methanol in ORR performance of the catalysts was monitored.

Thermo Scientific X-ray photoelectron spectrometer (XPS) using twin anode Mg/Al (300/400W) as X-ray source. BET surface area analysis was performed using a BET surface area analyzer Gemini 2375, Micromeritics, USA. The samples were degassed at 150 °C for 2 h. 2.3. Density Functional Theory Calculations. DFT calculation for determination of active site for O2 adsorption on ReS2 was performed with plane-wave basis VASP codes.35,36 Perdew−Burke− Ernzerhof (PBE)37 exchange-correlation and DFT-D2 corrections38 for van der Waals interactions were used. A 2 × 3 sheet of ReS2 was built to model the ReS2 sheet. Plane wave cut off of 500 eV and kpoint sampling using a 4 × 4 × 1 grid is used in these calculations, giving lattice constants a = 6.39 Å and b = 6.47 Å for ReS2 and a band gap within 10% error of the experimentally measured value. 2.4. Electrochemical Measurements. The electrochemical studies were performed in a three-electrode cell using CHI 760E electrochemical workstation (CH instruments, Texas, Austin) that is attached to a rotating disc electrode (RDE) system (ALS Instruments, Tokyo, Japan). Ag/AgCl and Pt coiled wire were used as reference and auxiliary electrodes. Rotating disc electrode (RDE) or rotating ring disc electrode (RRDE) was used as working electrode for ORR measurements after modifying the electrode with the catalysts. Catalyst dispersion was prepared by adding 1 mg of electrocatalyst in 1 mL of DI water and sonicating for 20 min to achieve uniform dispersion. Six microliters of thus obtained suspension was loaded onto a RDE of 3 mm diameter maintaining a loading of 0.08 mg/cm2. A glassy carbon disc with a polycrystalline Pt ring electrode is used for RRDE measurements, and linear sweep voltammogram (LSV) was collected at 1600 rpm. H2SO4 (0.1 M) was employed as the supporting electrolyte for all electrochemical measurements. The

3. RESULTS AND DISCUSSION Morphology of ReS2 was monitored by SEM and TEM. The FESEM image given in Figure 1A reveals the formation of layered architecture of ReS2. The low magnification TEM micrograph shown in Figure 1B further confirmed that the material exhibited nanosheet morphology with the dimension approximately 100−200 nm for each ReS2 layers. The high magnification TEM (Figure 1C) and the HRTEM image given in Figure 1D clearly authenticate the formation of thin and C


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Figure 3. (A) Cyclic voltammogram of ReS2, MoS2, and Pt/C in N2 (dotted) and O2 (solid) saturated 0.1 M H2SO4 and (B) Nyquist plot of ReS2, MoS2, and Pt/C in 0.1 M H2SO4. The corresponding equivalent circuit is given in the inset.

ECSAs of ReS2, MoS2, and Pt/C were measured to be 0.203, 0.035, and 0.136 (Figure S5) respectively. The details regarding the relative ECSA calculation are mentioned in the Supporting Information. The catalytic activity exhibited by ReS2 was compared with different metal dichalcogenides (MoS2, WS2, CoS2, and NiS). The corresponding linear sweep voltammogram recorded at 0 rpm in O2 saturated 0.1 M H2SO4 is given in the Supporting Information. Figure S6 indicates ReS2 as the most active TMD ORR catalyst compared to MoS2, WS2, CoS2, and NiS. It is seen that MoS2 exhibits better catalytic activity among the TMDs investigated and hence further comparative study for ReS2 was only carried out using MoS2 from TMDs and Pt/C commercial catalyst. Figure 3A shows the electrocatalytic behavior of the catalyst in N2 (dotted) and O2 saturated (solid) 0.1 M H2SO4 electrolyte at 50 mV/s. Featureless curves were obtained for all of the catalysts in N2 saturated solution whereas distinct reduction peaks were obtained in O2 saturated electrolyte with onset potential of 0.54 V for ReS2, at 0.11 V for MoS2, and 0.86 V for Pt/C. Electrochemical impedance spectroscopy (EIS) is a valuable tool to understand the effect of charge transfer resistance in catalytic activity. The charge transfer resistance (Rct) of the catalyst could be obtained from the diameter of the semicircle in the Nyquist plot, which is an indication of reaction kinetics toward ORR. Figure 3B shows the Nyquist plot resulting from the EIS measurements. The equivalent circuit used to fit the EIS data is given in the inset. Rs, Rct, and Cdl mentioned in the inset represent the solution resistance, charge transfer resistance, and the double layer capacitance. The lowest charge transfer resistance was exhibited by the commercial Pt/ C followed by ReS2 and MoS2. The decreased diameter of semicircle in ReS2 compared with MoS2 indicates the enhanced charge transfer exhibited by ReS2. The lower charge transfer resistance in ReS2 compared to MoS2 is attributed to the layer independent electrical properties of ReS2 unlike MoS2 where the electrical properties are layer dependent. The EIS result suggests better performance of ReS2 compared to MoS2, a TMD ORR catalyst widely investigated in the recent past. LSVs were recorded at different rotation rate to study the kinetics involved in ORR; LSV experiments with various electrode rotation rates were conducted on RDE in O2

transparent layered morphology of ReS2. The amorphous nature of ReS2 was revealed from the selected area electron diffraction (SAED) pattern (Figure 1E). The topographic image obtained from AFM (Figure S3A) and the corresponding height profile clearly reveal the formation of layered ReS2 exhibiting a thickness of around 1.6 nm and lateral dimension approximately 150 nm as given in Figure S3B. The EDS profile shows the presence and the homogeneous distribution of Re and S in the catalyst (Figure S4). The structural properties of ReS2 were investigated using XRD, and the corresponding peaks are marked in Figure 2A. XRD shows the characteristic diffraction peaks of 1T phase structured ReS2.39 The broad nature of the peaks points to the amorphous character of ReS2 supporting the SAED pattern. Surface area of the catalyst was analyzed by Brunauer− Emmett−Teller (BET) surface area measurement. The corresponding adsorption isotherm is given in Figure 2B. It shows H3 type hysteresis loop with a type II adsorption isotherm. Loops of this type are given by plate like particles of nonrigid aggregates.40 ReS2 exhibited a surface area of 44.93 m2/g and pore volume of 0.11 cm3/g. Figure 2C shows the X-ray photoelectron spectrum of Re corresponding to 4f7/2 and 4f5/2 with difference in the energy of 2.5 eV that matches well with the splitting of Re metal. The 4f7/2 and 4f5/2 exhibit peaks at 42 and 44.4 eV for ReS2, with a shift of ∼1.5 eV compared to the elemental Re 4f peaks centered at 40.5 and 42.9 eV. The shifts in the energy of the 4f levels are due to the bonding of sulfides to rhenium. Figure 2D shows two S 2p peaks at 162.9 eV (2p3/2) and 164.1 eV (2p1/2) corresponding to the S22− consistent with the reported literature on ReS2.41,42 The peak at 161.9 eV shows the presence of S2− that comes from Na2S, which might have intercalated between the layers of ReS2. The electrocatalytic activity exhibited by the catalyst toward ORR was evaluated in O2 and N2 saturated 0.1 M H2SO4 vs Ag/AgCl at room temperature. The potential was converted with reference to RHE as explained in Supporting Information (eq S1). For this measurement, the catalyst was coated on a precleaned RDE. The exposed active surface area of these catalysts was estimated by calculating the double layer capacitance (Cdl) of the catalysts. Cdl of a material generally reflects the relative electrochemical active surface area (ECSA).43 The relative D


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Figure 4. (A, B) Linear sweep voltammogram at different rpm of MoS2 and ReS2 (200, 400, 1000, 1600, 2000, 2600 rpm). (C, D) K-L plot of MoS2 and ReS2 at different potentials. (E) Tafel plot of MoS2, ReS2, and Pt/C acquired by mass transport correction of RDE voltammogram at 1600 rpm.

electrode resulted in enhanced diffusion of oxygen to the electrode surface, which led to the increased current density. The LSV of commercial catalyst at different rpm was also monitored for comparison. The potential window chosen for Pt/C was 1.1 to 0 V as given in Figure S7A.

saturated 0.1 M H2SO4 at a scan rate of 10 mV/s for ReS2 and MoS2. Figure 4A,B shows the LSV plots of MoS2 at a potential range of 0.4 to −0.2 V and ReS2 at a potential range of 0.6 to −0.3 V. The potential range for both catalysts for ORR study was optimized such that the oxygen evolution reaction (OER) does not take place. The increasing rotation rate of the E


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Figure 5. Disc (A) and ring (B) voltammograms of ReS2, MoS2, and Pt/C at 1600 rpm in O2 saturated 0.1 M H2SO4 and (C) peroxide yields calculated and (D) electron transfer from the RRDE measurement results.

and near parallelism of the fitting lines of K-L plot suggests first-order reaction kinetics toward the ORR. The electron transfer number (n) per oxygen molecule for the electrocatalysts was derived from K-L plot according to the equations given in Supporting Information (eqs S4 and S5). MoS2 exhibited an electron transfer number of 3.3, while the ReS2 exhibited ∼3.9 revealing the reaction mechanism to be the desirable 4 electron pathway. K-L plot for commercial Pt/ C is given in Figure S7B. The electron transfer number was calculated to be ∼3.4, which was lower than that of the ReS2. This deviation from the general 4 electron transfer in Pt/C might be due to the poisoning of Pt in the presence of sulfate ions. We also have noted 4 electron transfer reaction on Pt/C in alkaline media as reported in the literature. Comparable ORR activity of ReS2 to Pt/C was also noted from the Tafel slope derived from mass transport corrected LSV at 1600 rpm for ReS2 and Pt/C (−68.89 mV/decade and −67 mV/decade, respectively) given in Figure 4E, while MoS2 exhibited a slope of 141.9 mV/decade revealing the catalyst as less active in acidic media. To further validate the reaction pathways of the catalysts during the reduction reaction, RRDE measurements were performed to monitor the amount of peroxide species generated at the Pt ring surface. The Pt ring currents obtained

The LSV curves of ReS2, MoS2, and Pt/C at 1600 rpm are presented in Figure S8 to calculate the Eonset and E1/2 of the catalysts. The onset potential and half wave potential of MoS2 were found to be 0.236 and 0.05 V at 1600 rpm. The plateau limiting current at 1600 rpm was found to be −1.8 mA/cm2. ReS2 exhibited an onset potential and half wave potential of 0.44 and 0.241 V. The current density exhibited was 3 times (5.6 mA/cm2) higher than that of MoS2. The more positive onset potential is attributed to the higher catalytic activity exhibited by ReS2. The onset potential as well as the half wave potential exhibited by Pt/C was 0.804 and 0.648 V, which were higher compared to those for ReS2 indicating higher electrocatalytic activity exhibited by the Pt/C (Figure S8). The plateau limiting current at 1600 rpm was found to be 4.2 mA/cm2 for Pt/C. Even though ReS2 exhibited lower onset potential compared to Pt/C, the diffusion limited current density achieved for ReS2 was found to be higher than that for Pt/C. The kinetic parameters were evaluated based on RDE voltammograms at different rpm using Koutecky−Levich (KL) equation. Figure 4C,D shows the K-L plots of MoS2 and ReS2 at different potential; ω−1/2 is plotted in the unit of s1/2/ rad1/2 (by using SI eq S3). Details regarding the calculation of K-L plot are given in Supporting Information. The linearity F


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Figure 6. (A) Chonoamperometry analysis for stability (inset shows the decay for initial 250 s) and (B) methanol tolerance study of electrocatalyst performed using chronoamperometry for ReS2 and Pt/C confirming the methanol tolerance of ReS2.

was monitored in 0.1 M H2SO4. The potential was chosen such that both catalysts exhibit the same current density. As shown in Figure 6A, ReS2 showed similar durability compared to the expensive Pt/C catalyst. The inset in Figure 6A clearly shows the slow decay of current for ReS2. The long-term operation of electrocatalyst may lead to change in morphology as well as the electronic structure of the material. To monitor the operational stability of the reported catalyst, SEM and TEM images were taken after the durability experiment. Morphology of the catalyst was preserved even after long-term operation, which was confirmed from SEM and TEM analysis (Figure S10). The surface states of ReS2 were investigated by performing XPS analysis after the durability test. The core level Re 4f and S 2p revealed negligible change in chemical state after the durability test (Figure S11A,B). The peak positions of 4f7/2 and 4f5/2 of ReS2 after the durability test were at 41.9 and 44.3 eV, exhibiting a shift of 0.1 eV compared to the catalyst before the durability test. The S 2p peaks also were not changed after the durability test. For practical use of a catalyst toward ORR, it should be resistant toward methanol oxidation. The methanol tolerance of the catalyst was evaluated by performing chronoamperometry (which is shown in Figure 6B). For the initial 2000 s, the data was recorded in nitrogen saturated 0.1 M H2SO4, and oxygen was purged to the electrolyte after 2000 s, which resulted in oxygen reduction reaction that remained constant until 4000 s in both ReS2 and Pt/C. At 4000 s, 1 M methanol was added to the electrolyte, which resulted in an oxidation current in the presence of Pt/C, whereas no oxidation current was observed on rhenium disulfide by the addition of methanol indicating the methanol tolerance exhibited by ReS2 making it more suitable than Pt/C for practical ORR applications. Methanol tolerance was also evaluated in 0.1 M H2SO4 in the presence and absence of 1 M methanol using LSV (Figure S12). A well-defined methanol oxidation peak was visible in the case of the commercial Pt/C catalyst, whereas no obvious change was seen for ReS2 under the same conditions, indicating excellent methanol tolerance of ReS2. The durability and the methanol tolerance of MoS2 were also monitored and are given in Figure S13A. The chronoamperogram clearly shows the fast decay (just by 1400 s) in current, which got stabilized at 0.05 mA/cm2, whereas ReS2 exhibited a current density of 0.08 mA/cm2 at the same time interval pointing

during ORR measurement correspond to the oxidation of peroxide species generated at the disc electrode. Figure 5A shows the current exhibited by the catalysts on the disc electrode. It is clearly seen from Figure 5B that the ring current exhibited by ReS2 is very low compared to MoS2 and Pt/C making ReS2 a more suitable metal dichalcogenide catalyst for ORR in acidic media. The details of calculation of peroxide yield and number of electrons from RRDE measurements are given in Supporting Information (eqs S8 and S9). The measured peroxide yield (Figure 5C) was found to be below ∼10% for ReS2, which was lower than the Pt/C and MoS2 having a yield of ∼13% and ∼82%, respectively. The higher peroxide yield for MoS2 was further confirmed by calculating the number of electrons transferred per oxygen molecule. MoS2 exhibited an electron transfer number of ∼2.9, revealing the catalyst to undergo oxygen reduction reaction via a 2 electron pathway (Figure 5D). A higher electron transfer number of ∼3.9 was observed for ReS2 from the RRDE measurements as shown in Figure 5D making the catalyst more active than the most commonly used TMD MoS2. These electrochemical characterizations revealed the catalyst ReS2 as a better TMD cathode catalyst under acidic environment for DMFCs. The active center for ORR on the ReS2 catalyst surface was determined from plane-wave DFT calculations. It was found that the oxygen molecule preferably binds on top of Re atoms, with adsorption energies of −0.14 to −0.15 eV indicating the strongest adsorption on these sites. Equally preferred for adsorption is one of the hollow sites, which has a 3-fold rotation symmetry. In all these relaxed configurations, the O2 molecule is vertical to the ReS2 sheet. Active sites for oxygen adsorption on a ReS2 sheet are given in Figure S9. Details of these calculations are included in the Supporting Information. From the experiments and the DFT calculations, the mechanism of ORR can be suggested as the four electron transfer reaction with the surface of the ReS2 as active sites. The reaction mechanism is explained in the Supporting Information. The durability and methanol tolerance of the catalyst ReS2 were compared with those of the Pt/C catalyst. The durability assessment was performed using chronoamperometry where the decay in current density with respect to time was monitored. The durability of both catalysts, ReS2 and Pt/C, G


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toward the higher durability of the ReS2. The methanol tolerance of MoS2 was also studied using the same method. Figure S13B indicated that the addition of 1 M methanol after the completion of 4000 s has not resulted in an oxidation current. Though the lower onset potential of ReS2 than Pt/C, is a disadvantage of ReS2, the ORR mechanism with the electron transfer number close to 4 and the better methanol tolerance compared to Pt/C are advantageous for ReS2. Moreover ReS2 showed a higher current density too. These factors point to the significance of ReS2 as the promising ORR catalyst. It should be possible to alter the overpotential of ReS2 by doping or when used along with other noble metals such as Ag nanorods. These are under investigation in our laboratory.

This work was financially supported by the Kerala State Council for Science, Environment and Technology (Grant No. 003/SRSPS/2014/CSTE). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Kerala State Council for Science, Environment and Technology for financial support. We acknowledge Dr. Jayaraj, M.K. for extending the SEM facility for the characterization. SAIF, IIT Bombay is acknowledged for TEM images.

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4. CONCLUSIONS In conclusion, we successfully prepared rhenium disulfide and reported for the first time as an effective electrocatalyst toward the ORR. The catalytic activity of ReS2 was compared with MoS2 and the commercially available Pt supported on carbon black. ReS2 exhibited a current density around 3 times higher than MoS2 with a higher onset potential. The catalyst reveals excellent activity exhibiting comparable durability with Pt/C and superior tolerance to methanol than commercial catalyst in 0.1 M H2SO4 containing 1 M methanol, which makes it a promising noble metal free electrocatalyst toward ORR for direct methanol fuel cell application. This precious-metal free catalyst will have very high impending practical applications as their use will lead to substantial reduction in cost and complexity of fuel cells.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00867. Schematic representation for the synthesis of ReS2, methods for synthesis of WS2, CoS2 and NiS and SEM images of molybdenum disulfide, tungsten disulfide, cobalt disulfide, and nickel sulfide, AFM image of thin layered ReS2 and corresponding height profile, EDS spectrum and SEM image of ReS2 region used for EDS mapping and the distribution of Re and S, ECSA calculation, LSV plots of ReS2, MoS2, WS2, CoS2, and NiS in O2 saturated 0.1 M H2SO4 at 0 rpm, LSV at different rpm, K-L plot of Pt/C at different potentials, LSV plots of ReS2, MoS2, and Pt/C in O2 saturated 0.1 M H2SO4 at 1600 rpm, K-L plot, calculation of %H2O2 and n from RRDE measurements, active sites for reduction on a ReS2 sheet, SEM and TEM images of ReS2 after durability test, XPS analysis of ReS2 after durability test, methanol tolerance test and corresponding LSV plot of ReS2 and Pt/C in 0.1 M H2SO4 before and after addition of 1 M methanol, and stability performance and methanol tolerance study of MoS2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Raghu Chatanathodi: 0000-0001-9649-8285 Neelakandapillai Sandhyarani: 0000-0002-6623-3347 H


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Amorphous Rhenium Disulfide Nanosheets: A Methanol-Tolerant

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