Noble-Metal-Free MoS2 Platelets with Promising Catalytic

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Noble-Metal-Free MoS Platelets with Promising Catalytic Performance in Hydrogen Evolution Reaction for the Post Li-Ion Battery Mengmeng Deng, Meng Li, and Hyung Gyu Park ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01049 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Energy Materials

Noble-Metal-Free MoS2 Platelets with Promising Catalytic Performance in Hydrogen Evolution Reaction for the Post LiIon Battery Mengmeng Deng, Meng Li, Hyung Gyu Park* Department of Mechanical and Process Engineering Eidgenössische Technische Hochschule (ETH) Zurich Tannenstrasse 3, 8092 Zurich, Switzerland * E-mail: [email protected] KEYWORDS: Noble-metal-free, Catalyst, MoS2, Lamellar electrode, Hydrogen evolution reaction, Li-water battery

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ABSTRACT: In search of a practically alternative catalyst to costly and rare noble metals for hydrogen evolution reaction (HER), here we propose a lamellar electrode architecture out of engineered 2D MoS2 platelets that are stable in harsh electrochemical conditions, densified in the catalytically active sites, and enhanced in the interlamellar charge transfer. Energetic ion bombardment such as Ar ion beam 1 ACS Paragon Plus Environment

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milling (IBM) as a large-scale surface engineering technique can successfully create numerous edge states onto the MoS2 lamella with an observable penetration depth of ~10 nm for the first time, leading to an excellent onset overpotential of 33.3 mV at 0.1 mA/cm2. Interlamellar charge transfer enhancement by raising the metallic portion of 1T-phase MoS2 and also by inserting conductive rGO platelets in the lamella turns out to improve the HER catalytic activity remarkably. The promising results obtained in an aqueous electrolyte with rarely reported 1M LiOH imply a potential use in the seawater splitting toward energy carrier storage. Equipped in a Li-water battery cell, our engineered MoS2 lamella electrode reveals, for the first time, a remarkable discharge performance with supplying >2.2 V stably for more than 2.5 days, lending it potential adaptability to energy storage systems such as battery and fuel cells.

INTRODUCTION Hydrogen evolution reaction (HER) offers an energy storage option from renewable sources to a chemical energy carrier of hydrogen (H2) through electrochemical catalytic conversion.1 If an environmental concern such as carbon foot print in this process could be alleviated, H2 produced in this route can serve as a clean energy source. The potential of utilizing water for the clean H2 production appeals to the public attention with a vision of seawater harnessing for energy storage. Today, platinum (Pt) and Pt group metals are still standing at the top of a volcano plot for the best catalytic activities.2-3 Despite excellent performance and reliability, it is rareness and cost of these noble metal catalysts that call for the search of noble-metal-free catalysts for utilization at an 2 ACS Paragon Plus Environment

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industrial scale.4

For earth abundance, low cost, desirable tribology5-6 and great catalytic activity,2, 7-9 micrometer-sized 2D platelets of molybdenum disulfide (MoS2) have recently arisen as a promising alternative to the noble metal catalysts. Edges of the platelets are known to be responsible for the HER performance of MoS2, featured with proportionality between the edge length and the catalytic activity, thereby placing MoS2 at a close position to the Pt group metals in the volcano plot.2, 10-11 The edges of MoS2 have also been used as cocatalysts for photocatalysis.12-15 These advantages have fostered investigations of MoS2 having irregular surface morphology that permits augmented exposure of the catalytic edges to result in a great catalytic activity for HER especially when used with dopants such as conductive graphene7 and noble metal nanoparticles.16 Other than the MoS2 platelet morphology with plenty of exposed edges, introduction of defective voids on the basal plane of the MoS2 monolayer can also enhance the catalytic performance of H2 generation.9 Recently, it has been revealed that not only the edges but also the basal planes of MoS2 can serve as catalytically active sites as long as they are electrically connected to a current collector securely.8, 17 In addition to the intact basal plane of MoS2, a defective sulfur vacancy on the MoS2 plane is also capable of catalyzing the water-to-H2 conversion effectively yet at the demanding expense of 710% vacancy density and high crystallinity for an optimized performance.18-20

Despite the superb catalytic activity and ongoing progress, practical applicability of 3 ACS Paragon Plus Environment


MoS2 remains elusive. In order to construct a suitable format for an electrode, particles still require a binder7, 21 that might increase the cost and raise the concern of durability. Use of an atomically thin monolayer demands delicate operation processes for transferring it to a destination substrate with no crack nor surface contamination:22-23 a practical challenge, even if the monolayer MoS2 poses a possibility of optimized performance.9 Plenty of seawater bolsters the catalytic water splitting application, and yet there is a lack of demonstration with a practical device reported in the literature that endeavors to evaluate or establish the catalyst performance at actual operational conditions.

In order to better understand the potential utility of MoS2, we propose here a lamellar architecture based on a van der Waals platelet-stacking mechanism with no need of additives or complicated transfer steps.24-25 Its freestanding nature and flexibility will offer the compatibility as practical electrode material. With envisaging an operation of a Li-water battery in seawater, we carry out our HER measurement in an aqueous electrolyte with 1M LiOH that assures a basic environment, which is rarely presented in the current literature.3 As a facile method of creating catalytically active edge states to the MoS2 platelets, we select and investigate energetic ion bombardment such as Ar ion beam milling (Ar IBM). In order to achieve charge transfer augmentation in the electrochemical catalytic process at an electrode level, we engineer the lamella composition by raising the conductive phase of MoS2 and also by inserting a small portion of conductive 2D platelets such as rGO. Among the various lamellar electrode 4 ACS Paragon Plus Environment

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design, the Ar-IBM-treated MoS2 (without any noble metal doping) reveals a very small onset overpotential of 33.3 mV at 0.1 mA/cm2 hinting a promising catalytic activity and good stability in a basic environment. Notably, it is the first time that MoS2 reveals an excellent and reliable catalytic property in a Li-water battery cell capable of offering long (2.5 days) discharge capability at >2.2 V. This cell performance result suggests that properly engineered MoS2 lamellae can offer a rational and practical catalyst for HER-involving processes and the corresponding energy transfer/storage systems such as battery and fuel cells.

Results and Discussion A freestanding, lamellar film out of MoS2 platelets is constructed via chemical exfoliation, centrifugation and vacuum filtration as reported in the previous work.25 Briefly, the exfoliated MoS2 solution after removing large agglomerates through centrifuge is vacuum-filtered with a polysulfone filter. After detached from the filter, a freestanding and flexible MoS2 film as thick as ~10 μm is obtained (Figure 1A). Topview and cross-sectional scanning electron micrographs of the membrane sample reveals a rough surface with wrinkles (Figure 1B) and a well aligned lamellar configuration (Figure 1C), respectively. X-ray photoelectron spectra (XPS) manifest the existence of Mo (Mo 3d) and S (S 2s) in the film samples. Two polymorphs of 1T and 2H MoS2 are also distinguishable with ~56% 1T phase (~44% 2H phase) for the MoS2 sample (Figure 1D). The MoS2 film with higher 1T composition (1T MoS2) of ~65% and ~35% 2H is also obtained by in-situ annealing during the exfoliation, in 5 ACS Paragon Plus Environment


agreement with the previous work (Figure 1E).8, 26

Figure 1. (A) A freestanding and flexible film of MoS2 platelets. Scanning electron micrographs of (B) the exposed membrane surface and (C) the cross section of a MoS2 film. XPS spectra of the MoS2 lamellar films synthesized (D) without and (E) with in-situ annealing.

Edges of the MoS2 platelets are known to be catalytically active sites for HER.2 Therefore, how to increase the number of such active sites will be instrumental to enhance HER. One idea of increasing the density of the catalytically active sites is to chop atoms off the basal planes of the MoS2 platelets through energetic ion bombardment. Ion beam milling as a surface engineering technique shoots energetic ions (e.g., Ar+ at ~600 eV) onto a sample surface to sputter particles off the surface.27 In order to artificially create numerous active sites and to enhance the HER performance, we employ Ar IBM for the first time in this work. Process parameters (e.g., current and duration) are varied to adjust the Ar+ dose, causing a drastic change to the surface 6 ACS Paragon Plus Environment

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texture of the lamellar MoS2 films into a rough morphology full of sags and crests, verified by AFM scanning (Figure 2A-C). Such a change in the surface morphology implies heterogeneous surface etching by Ar IBM, thereby adding a lot of catalytically active states. The STEM cross-section image clearly reveals a fault-like structure with a penetration depth of approximately 10 nm into the continuous lamellar stack (Figure 2D, orange square) that bolsters the indication of active site formation from the AFM characterization above. Additional STEM images (Figure S2) demonstrate creation of numerous fault-like edges at random locations on the MoS2 lamella. Furthermore, MoS2 platelets after Ar IBM still retain the two specific peaks of E2g and A1g in the Raman spectrum (Figure S3), similar to an untreated sample; this similarity suggests that despite the local formation of the fault-like edge states, the basal plane characteristic of the MoS2 lamellar matrix remains largely invariant under the ion bombardment conditions used in this work.

Figure 2. (A-C) Atomic force micrographs showing a drastic change in the surface texture of the lamellar MoS2 films treated with Ar IBM at various Ar+ dose (in coulomb, at an indicated ion current). (D) Scanning transmission electron micrograph of the cross section of a lamellar MoS2 film treated with Ar IBM at 72 C (300 mA). (E) LSV curves of the Ar-IBM-treated MoS2 films obtained at a scan speed of 1 mV/s. (F) Tafel plots of the Ar-IBM-treated samples in 7 ACS Paragon Plus Environment


comparison to a pristine MoS2 film.

Enhanced electrochemical activity of the lamellar MoS2 electrode caused by newly imposed defects (or edge states) on the basal plane can be characterized with linear sweep voltammetry (LSV) and Tafel plots. Compared with an untreated control, all the defect-laden MoS2 films show higher current density (Figure 2E). The fact that higher dose enhances the current density (cyan: 72 C (300 mA)) suggests that the ion dose of Ar IBM may alter the catalytic activity of the MoS2 platelets. Tafel plots of the lamellar MoS2 electrodes (Figure 2F) reveal that all Ar-IBM-treated samples exhibit less than half the onset overpotential of pristine MoS2, which suggests enhanced catalysis by defect addition and corroborates the LSV measurement. Tafel slopes of the defect imposed electrodes (58-59 mV/dec) implies a Volmer–Heyrovsky mechanism that identifies electrochemical desorption of H2 to be the rate limiting step.8, 28 Notably, an onset overpotential value of ~33.3 mV at 0.1 mA/cm2 obtained for the Ar-IBM-treated MoS2 lamella electrodes is one of the most promising results among the literature reports; further comparison is provided in Figure 3.

By way of facilitating a fast electron donation process, enhancement in charge transfer can promote HER if coupled with multiplied catalytic sites.17 We inserted conductive 2D flakes of reduced graphene oxide (rGO)29 into an MoS2 matrix by mixing colloidal graphene oxide and MoS2, stacking them with vacuum filtration and reducing the lamellar mixture. In our LSV characterization, a MoS2 lamella electrode that incorporates 5 wt% rGO exhibits the best HER performance, i.e., the highest current 8

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density and the smallest onset overpotential (Figure 3A). It is conjectured that the incorporation of an optimal amount of conductive agents such as rGO could bridge the otherwise disconnected, conductive platelets of 1T-phase MoS2 (Figure 3D). These newly built conductive pathways may serve for improved charge transfer within the film electrode. Owing to a poor catalytic property of rGO for HER, excessive inclusion of rGO such as 20 wt% and 50 wt% would actually hamper HER by substituting the catalytically active MoS2 part (Figure 3A).

Another way to augment the charge transfer within the MoS2 lamella electrode is to increase the metallic 1T-phase portion of the MoS2 platelets. The heated exfoliation process is known to be able to increase the portion of 1T MoS2. Indeed, the MoS2 lamella electrode made out of an increased 1T portion shows a current density of 0.027 A/cm2 at -0.35 V (Figure 3B), a result that surpasses the pristine MoS2 lamella that contains higher 2H composition. It is because the increased portion of the 1T-phase MoS2 platelets existing in the lamella would link the otherwise disconnected conductive pathways in the pristine MoS2 lamellar configuration, as illustrated in Figure 3D. Even if the 1T-phase portion is raised, internal resistance of the electrode might still be large if the size of individual platelets are too small to touch one another easily (Figure 3D). In an attempt to facilitate further the internal electrode charge transfer, we added 5 wt% rGO in the same way as above to the 1T-phase augmented MoS2 lamella. Remarkably, the result is higher current density than that of the 1T-phase-raised MoS2 lamella alone.

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However, the 1T-MoS2/rGO lamella treated with Ar IBM initially thought to augment the HER performance turns out to be actually inferior to the 1T-MoS2 counterpart with a small current density of 0.022 A/cm2. This result is ascribable to two possibilities: (a) Ar IBM breaks apart the lamella everywhere to delink the charge transfer pathways; and (b) Ar IBM exposes the embedded rGO to the lamella surface to cover up the active sites of MoS2 platelets and deteriorate the catalytic performance at the electrode surface. Despite the deleterious effect, this sample is still superior to the pristine MoS2 lamella in terms of the LSV performance.

To sum up, among the aforementioned lamella electrode architectures, the superior LSV performance is obtained by both the 1T MoS2 lamella mixed with 5 wt% rGO flakes and the pristine MoS2 lamella undergone Ar IBM (Figure 3C). Comparing with reports from the literature, these two architectures display the smallest onset overpotentials (33.3 mV and 37.9 mV at 0.1 A/cm2, respectively) and the parity Tafel slopes (55.6~64.6 mV/dec) with the state-of-the-art electrodes (Figure S5), lending promising adaptability to catalyst materials for HER (Figure 3E).


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Figure 3. Effects of (A) incorporating rGO to MoS2 films and (B) employing 1T MoS2 along with various treatments on the LSV performance (scan rate: 1 mV/s). (C) A comparison the LSV performance (scan rate: 1 mV/s) among lamellar film electrodes out of pristine MoS2 (black), MoS2 with augmented 1T phase (blue), MoS2 with increased conductivity through 1T phase augmentation and rGO incorporation (olive), and pristine MoS2 with defect incorporation by Ar IBM (cyan: repeated presentation from Figure 2F (cyan)). (D) Schematic of the charge transfer facilitation via incorporation of conductive rGO to the lamellar MoS2 film electrode. (E) A diagram of Tafel slope and onset overpotential that compares the electrode performance of this work and the literature reports.

In order to verify the material’s catalytic performance in a practical condition, we operated a Li water battery cell. Figure 4A demonstrates a Li water battery setup composed of the Li metal anode and the MoS2 lamella cathode. A dual-electrolyte design will offer the organic electrolyte (non-aqueous) to protect Li metal from moisture and the aqueous electrolyte to dissolve the discharge product of LiOH. In between two electrolytes, a ceramic film LISICON as separator would stop water from crossing over to the Li anode side and maintain the ionic conductivity. From the superior catalytic property, we selected Ar-IBM-treated MoS2 as the cathode material attached to a current collector of glassy carbon (GC). 11 ACS Paragon Plus Environment


For a discharge process, the Li metal anode will release electrons to external circuit by forming Li+ ion which will transfer from the organic electrolyte through LISICON to the aqueous electrolyte. Meanwhile, H2 will be generated on MoS2 cathode side. As a result, this capability of producing pure H2 gives a unique characteristic to Li battery system while continuously offering electricity. By varying the current (Figure 4B), Li water battery with MoS2 as the cathode material can always offer a stable voltage for more than one day of discharging. In a prolonged discharge process (Figure 4C), we could obtain >2.2 V continuously for more than 60 hours (approximately two and a half days), hinting a potential stability and reliability of the cell equipped with MoS2 on the cathode side for the first time. The structure of the MoS2 lamellar electrode after longtime discharging did not change significantly according to SEM, XRD, and XPS characterizations (Figure S6), manifesting a potential reliability of this HER electrode.

In contrast, a discharge process using a GC electrode alone as cathode ended within 3 hours with a rapid drop of voltage to zero, indicating an inactive nature of GC for the H2 evolution. From this comparison, it seems that the Ar-IBM-treated MoS2 lamella plays the major role in catalyzing the reaction for the discharge process. Another supporting evidence can be obtained by comparing the discharge performance of the MoS2 lamella with that of an activated carbon electrode that has been commercialized as the cheap and effective electrode material widely adopted in the battery system. The discharge performance using an activated carbon mesh (Figure 4D, blue curve) reveals 12 ACS Paragon Plus Environment

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an initially nonlinear increase of voltage likely caused by an activation process. After the nonlinear increase, a sharp decline for ~6 hours is observed, which reveals a poor catalytic performance just as the case of pure GC and bolsters the unique catalytic property of the Ar-IBM-treated lamellar MoS2 electrode in the Li water battery. Although Li water battery is regarded as one of the next generation batteries with the unique characteristic of pure H2 byproduct, a rechargeable functionality remains in need of intensive study in the future. Nevertheless, such a H2 generation system could also be expected to connect with a fuel cell setup by offering H2 as the fuel. Consequently, this novel MoS2 based catalyst would be instrumental to a series of new energy conversion and storage systems and hence demand further investigation and understanding.



Figure 4. (A) Schematic of a Li-water battery cell in dual electrolytes. (B) Discharge curves for different currents. (C) A continuously prolonged discharge at the current of 2.5 μA. (D) Discharge performance comparison with GC and activated carbon mesh electrodes.

Conclusion We proposed one practical design of noble-metal-free MoS2 platelets film as an alternative HER catalyst. Both Ar IBM treatment and electrically conductive 2D materials including rGO and 1T MoS2 could efficiently improve the catalytic activity of MoS2. In particular, the very small onset overpotential of ~33.3mV and comparable Tafel slope of 55.6 mV/dec to the state-of-the-art electrodes found in the literature, achieved by Ar-IBM-treated MoS2 in a strong basic solution for the first time reveals a potential possibility of seawater splitting. The energy generation in Li-water battery cell for the first time with a reliable >2.2 V voltage supply for around 2.5 days manifests 14 ACS Paragon Plus Environment

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the immense potential of MoS2 in the future energy field. It draws further studies in the next generation battery and also the H2 utility such as fuel cell.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website Description of the materials synthesis, electrode fabrication and Li-watery battery construction. Characterization of the lamellar MoS2 platelets.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the National Research Programme "Energy Turnaround" (NRP 70) of the Swiss National Science Foundation for financial support (407040_153978). We acknowledge the SCCER Heat & Electricity Storage in collaboration with the CTI Energy Programme for provision of forum for discussion. We appreciate Prof. Katharina M. Fromm and Dr. Nam Hee Kwon (University of Fribourg, Switzerland) for active collaboration in NRP 70. Most of the work have been carried out in Binnig and Rohrer Nanotechnology Center of ETH Zurich and IBM Zurich. M.M.D. thanks 15 ACS Paragon Plus Environment


Dr. Ning Yang for the AFM characterization, Karl-Philipp Schlichting for the FIB lamellar sample preparation, and Dr. Seul Ki Youn for the TEM characterization. M.M.D. and M.L. are grateful to Dr. Jörg Patscheider (Empa, Switzerland) for support in the XPS and XRD measurements.


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