Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on

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Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values Chaiti Ray, Su Chan Lee, Kalimuthu Vijaya Sankar, Bingjun Jin, Jungpyo Lee, Jong Hyeok Park, and Seong Chan Jun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11192 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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ACS Applied Materials & Interfaces

Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values Chaiti Ray†, Su Chan Lee†, Kalimuthu Vijaya Sankar†, Bingjun Jin‡, Jungpyo Lee†, Jong Hyeok Park‡, Seong Chan Jun*† † Nano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea Corresponding Author *E-mail: [email protected]; Fax: +82-2-312-2159; Tel: +82-2-2123-5817

ABSTRACT The development of economical, proficient, and highly stable catalysts to substitute the expensive noble metal electrodes for electrocatalytic water splitting applications is exceedingly desirable. In this context, the most fascinating and challenging approach is rational designing of nanocomposite encompassing multiple components with unique functionalities. Herein, we describe the fabrication of a strongly catalytic and superb durable phosphorus-incorporated cobalt molybdenum sulfide electrocatalyst grown on carbon cloth (P-CoMoS/CC). The hybrid material exhibited excellent activity for hydrogen and oxygen evolution reactions over a wide 1 ACS Paragon Plus Environment


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range of pH (1–14) with extremely high stability (~90% retention of the initial current density) after 24 h electrolysis. Importantly, when P-CoMoS/CC was used as both cathode and anode for overall water splitting, a very low cell voltage of 1.54 V is required to attain the 10 mA cm−2 current density, and the hybrid material exhibited long-term stability (89.8% activity retention after 100 h). The outstanding overall water splitting performance compared to electrolyzer consisting of the noble-metal-based catalysts Pt/C and RuO2, making P-CoMoS one of the most efficient earth-abundant water splitting catalysts. Phosphorus incorporation proved to be a vital aspect for the improved charge transfer properties and catalytic durability of the P-CoMoS/CC catalyst.

KEYWORDS: Anion incorporation, electrocatalysis, water splitting, all pH values, stability.

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INTRODUCTION The gradual depletion of non-renewable energy resources and increasing energy demand leads the focus of scientific research towards development of renewable energy resources.1 Molecular hydrogen (H2) is a well-known sustainable energy source with high energy density that has the potential to meet the growing global energy demand with no environmental impact.2,3 Among the various hydrogen production methods, water electrolysis (2H2O → 2H2 + O2) is considered the most promising, secure, economical, and environment-friendly process to support hydrogen fuel. An active, durable, and affordable catalyst is essential for efficient electrolytic hydrogen generation by accelerating the kinetics.4,5 In particular, water electrolysis is a thermodynamically uphill reaction and requires extremely efficient and robust catalysts that can substantially expedite the sluggish kinetics of the two half reactions i.e., the hydrogen and oxygen evolution reactions (HER and OER, respectively).6,7 However, the scarcity and high-cost of benchmark Pt/C electrocatalysts, generally used for the hydrogen evolution reaction (HER) (H2O → H2), and RuO2 electrocatalyst, used for the anodic oxygen evolution reaction (OER) (H2O → O2), greatly hinder the development of water electrolysis technologies.8,9 Motivated by this challenge, great effort has been dedicated to developing low-cost alternative HER and OER catalysts including

carbides,10,11

nitrides,12,13

borates,14

oxides,15,16

sulfides,17,18

selenides,19,20

phosphides,21,22 phosphates,23 and many other non-precious transition metal compounds.24,25 Notably, some transition-metal-based catalysts could be used as both electrocatalysts for HER and OER, simplifying the water splitting system and reducing the product cost.26 In particular, MoS2,27 MoSx,28 CoS2,29 CoP,30 and MoP31 have engrossed significant consideration as alternate electrocatalysts for water splitting because of their excellent catalytic activity, high yield, and easy operation. Moreover, bimetallic crystal nanomaterials have gained increasing interest owing 3 ACS Paragon Plus Environment


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to their superior catalytic performance compared to the monometallic counterparts.32-34 The merger of two or more metal species supplies more active sites and improved electronic conductivity, which are advantageous for electrocatalytic applications.35 In addition, the differential combination and tunable ratio of cations in bimetallic materials offer tremendous opportunities to manipulate the physicochemical properties in terms of valence and electronic states of the metal elements.36 Various transition metal-incorporated MoS2 nanocomposites have been widely investigated as high-performance water splitting electrocatalysts.37-41 On the basis of the positive results obtained by cationic substitution, studies on a similar anionic substitution have been undertaken to evaluate its effect on the activity of transition metal-based electrocatalysts. The combination of different anions in transition metal-based catalysts have been investigated for batteries,42 transparent conducting oxides,43 and photocatalysts.44 In electrocatalysis, mixed anionic transition metal compounds such as MoS2-MoSe2,45 MoOxMoS2,46 CoFePO,47 sulfidized MoP,48 phosphidized CoS2,49 selenized Ni2P,50 NiCoPS,51 and amorphous Co-O-S52 demonstrated superior performance. In the design of highly active electrocatalysts, the impact of the nanostructure on the electrochemical reactions activity, including HER and OER, should be taken into consideration.29, 53 The major limitations of these catalysts are the deficient surface area, inferior electrical conductivity, and better exposure of the active sites. The most useful approach to resolve these concerns are enhancement of electrochemically active surface area (ECSA) by introducing hierarchical nanostructures,54,55 reduction of the charge transfer resistance by loading the catalysts on matrices with high conductivity (e.g., graphene56 and carbon nanotubes57), and creation of more defects by incorporating amorphous materials.58 Furthermore, direct growth of active materials on current collectors become advantageous for self-supported electrocatalysts 4 ACS Paragon Plus Environment

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with good electrical conductivity and excellent stability.54,59,60 For integrating all the above merits at the nano-scale, we successfully developed a synthetic strategy for the preparation of an all-in-one water splitting catalyst. Herein, we report a designed pathway for the fabrication of a proficient and stable overall watersplitting electrocatalyst consisting of phosphorus-incorporated cobalt molybdenum sulfide (PCoMoS) nanocomposites anchored on carbon cloth (CC). The electrocatalyst is designed by a two-step synthetic process: (1) controllable growth of cobalt molybdenum sulfide (CoMoS) nanocomposites on CC by a facile hydrothermal method, establishing strong interactions between the CC and the nanoparticles; (2) phosphorization to replace some sulfur atoms with phosphorus atoms, resulting in excellent catalytic activity and durability. Benefiting from the synergistic advantages of bimetallic composites, anion substitution, and unique electrode fabrication technique, the P-CoMoS electrode exhibited superb performance for both HER and OER in terms of overpotential (66 and 260 mV at 10 mA cm-2, respectively) and Tafel slope (60.1 and 72.2 mV dec-1, respectively), as well as extremely high stability with negligible change in current density even after electrolysis for 24 h.



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EXPERIMENTAL SECTION Material synthesis: The P-incorporated cobalt molybdenum sulfide (P-CoMoS/CC) nanohybrid was fabricated via a two-step synthesis protocol. In first step (synthesis of CoMoS/CC), 2.5 mmol CoCl2.6H2O, 2.5 mmol Na2MoO4.2H2O and 6.0 mmol thioacetamide (TAA) were dissolved in 50 mL DI water. The carbon cloth (CC) was first rinsed by acetone, deionized (DI) water and ethanol consecutively under sonication condition. Then CC was treated with 40% nitric acid (HNO3) solution in an ultrasound bath for 30 min. Again, DI water and ethanol were used for 5 min each to ensure that the surface of the CC was fully cleaned. Then the reaction solution and activated CC (2 cm × 1 cm) substrate were transferred to a 100 mL Teflon-lined stainless-steel autoclave for hydrothermal reaction at 120 °C for 24 h. The resulting product, CoMoS/CC was collected and subsequently rinsed with DI water and dried at 60 °C. In second step, the P-CoMoS/CC was obtained after phosphorization process. The CoMoS/CC and 0.25 g NaH2PO2.H2O were placed two separate positions in a porcelain crucible with NaH2PO2 and CoMoS/CC at the upstream and downstream side of tube furnace. The samples were heated at 400 °C for 1 h under 100 sccm Ar gas flow with a heating speed of 3 °C min-1. The samples without Mo (CoS and P-CoS) and Co (MoS and P-MoS) were prepared by following similar reaction strategy as above except the addition of Na2MoO4.2H2O and CoCl2.6H2O, respectively in the reaction vessel. Structural and surface characterization: X-ray diffraction (XRD) were recorded using a The Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) was performed under normal mode by using a K-alpha (Thermo Scientific Inc. UK) XPS system with monochromatic Al Kα X-ray. All XPS spectra are 6 ACS Paragon Plus Environment

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corrected using the C 1s line at 284.6 eV followed by curve fitting and background subtraction. JEOL-7001F field emission scanning electron microscopy (FESEM) was used to characterize the morphologies. Transmission electron microscopy (TEM) images were taken using a JEOL JEM2010 electron microscope. HRTEM and STEM analysis were performed in JEM-ARM200F Atomic Resolution analytical Microscope. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted using PerkinElmer Optima 8300. The nitrogen gas adsorption and desorption study is performed using a Quantachrome Autosorb-iQ 2ST/MP automated gas sorption analyzer. Electrochemical studies: The electrochemical characterization and catalytic activity of asobtained different electrodes were investigated using a three electrode cell in Ivium-n-Stat multichannel electrochemical analyzer, taking as-fabricated electrodes (1 cm2) as a working electrode, a platinum wire with a diameter of 1 mm as a counter electrode and a saturated calomel electrode (SCE) as reference. For the investigation both HER and OER performance of various electrocatalysts Linear sweep voltammetry (LSV) was conducted within the potential range from 0.05 to -0.3 V (vs. RHE) at a scan rate of 2 mVs−1 in the electrolyte with different pH values ranging from 1 to 14. Electrochemical impedance spectroscopy (EIS) was performed in a frequency range of 0.01Hz – 105 Hz with 5 mV amplitude at a bias potential of -0.2 and 1.50 V (vs. RHE) for HER and OER, respectively. Chronoamperometry curves were also recorded at 0.1 and 1.55 V (vs. RHE). All potentials reported in this paper were normalized with respect to the reversible hydrogen electrode (RHE) by adding a value of (0.242 + 0.059 × pH) V. Overall water splitting was performed in a two-electrode cell using P-CoMoS/CC electrodes as the cathode and anode in 1.0 M KOH. The stability of the electrocatalyst in two-electrode fuel cell



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system for full water splitting was evaluated by using 1.0 M KOH electrolyte at an applied potential (1.54 V) to reach initial catalytic current density of 10 mA cm-2. RESULTS AND DISCUSSION The two steps synthetic protocol for the fabrication of phosphorus-incorporated cobalt molybdenum sulfide on carbon cloth (P-CoMoS/CC) is depicted in Figure 1a. Firstly, cobalt molybdenum sulfide (CoMoS) nanocomposite was exclusively grown on activated carbon cloth (CC) by hydrothermal reaction at 120 °C. Herein, thioacetamide (TAA) was used as S source, which in reaction with water slowly generate the H2S reactant. The second step featured a high temperature solid/gas-phase reaction for the introduction P into the CoMoS nanostructure. In this work, NaH2PO2.H2O was employed for production of PH3 gas by undergoing thermal decomposition, which reacted with the CoMoS/CC to obtain the final P-CoMoS/CC hybrid material. The crystal structures of the as-synthesized nanohybrid material and its precursor after hydrothermal sulfurization were characterized by X-ray diffraction (XRD) technique. A comparison of the XRD patterns of CoMoS and P-CoMoS (Figure 1b) indicates that all the diffraction peaks of both the materials correspond to only CoMoS3.13 (JCPDS No. 16-0439)61 but not pure CoS2, MoS2 or their simple mixture. However, no obvious diffraction of cobalt or molybdenum phosphide was found in the diffractogram of P-CoMoS indicating unchanged crystal structure of CoMoS after P-atom introduction. The magnified view of the XRD profiles revealed a small up-shift of the dominant peaks of CoMoS3.13 (2θ = 30.73 and 32.94°) after P incorporation (2θ = 30.9 and 33.1°) for P-CoMoS, respectively) (Figure 1c). This is related to the shrinkage of the CoMoS3.13 unit cell upon replacement of some S atoms by P.51,55 However, 8 ACS Paragon Plus Environment

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XRD patterns of P-CoS and P-MoS/CC matched perfectly with CoS (JCPDS No. 65-8977) and MoS2 (JCPDS No. 73-1508), as shown in Figure S1. Thus, it can be concluded that for CoMoS presence of both Co and Mo leads to formation single phase CoMoS3.13 material instead of CoS2 and MoS2 mixture.

a

c

b

Figure 1. (a) Schematic of the synthetic route for P-CoMoS nanohybrid on CC. (b) Comparison of XRD patterns of CoMoS and P-CoMoS with CoMoS3.13 (JCPDS No. 16-0439). (c) Enlarged view of the XRD patterns. The black dotted line indicates the standard XRD peak of CoMoS3.13. The X-ray photoelectron spectroscopy (XPS) measurements were carried out for the detail understanding about the surface composition and oxidation state of P-CoMoS. In Figure 2a, the Co 2p3/2 and 2p1/2 peaks of CoMoS/CC electrode positioned at 779.1 eV and 794.2 eV, 9 ACS Paragon Plus Environment


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respectively indicate the formation of Co-S phase.62 After phosphorization, the shift of Co 2p peaks towards higher binding energy suggested an electron density transfer from Co towards P leading to mostly positively charged Co sites (Coδ+) in the P-CoMoS/CC electrode.51 Moreover, the exchange of Co atoms by the more electronegative Mo atoms (electronegativity of Co = 1.88 and Mo = 2.16) persuaded an electron transfer from Co to Mo and as a result more positively charged Co species is generated compared to CoS2 for which the Co 2p peaks appear at 778.8 eV and 794.0 eV.1 Thus, a high intrinsic catalytic activity could be expected for P-CoMoS/CC based on the hydrogenase-like mechanism. Mo 3d peaks (228.6 and 231.8 eV) of CoMoS are shifted to lower binding energy in comparison to MoS262 (229.3 and 232.5 eV), confirming a decrease in Mo valence in CoMoS due to electron transfer from Co (Figure 2b). As can be seen from Figure 2b, P incorporation also leads to a little red shift of the Mo 3d binding energies in the PCoMoS/CC electrode. In case of CoMoS/CC the Mo 3d spectrum consists of two extra peaks (related to Mo6+ of MoO3) along with Mo4+ peaks. Interestingly, no Mo6+ peaks were detected in case of the P-CoMoS/CC electrode suggesting that P-doping provided chemical inertness to the catalyst under ambient conditions. For both the CoMoS/CC and P-CoMoS/CC samples, the presence of sulfur at the surface was revealed by a small S 2s peak, close to the Mo 3d5/2 peak, and a stronger S 2p peak, which could be deconvoluted into two distinct doublets (2p3/2, 2p1/2): one (161.7 eV and 162.9 eV) arising from the S2− species and the other (162.6 eV and 163.7 eV) indicating the thiolate nature of the sulfur atoms (Figure 2c).48 These sulfur species were the same as those observed for the phosphosulfides in the hydrodesulfurization reaction.63 In the case of CoMoS/CC, an additional broad peak at 168.6 eV was detected, indicating a surface SO42− species, possibly originated due to the exposure of catalyst to air. Amusingly, no SO42− species were observed for P-CoMoS/CC again suggests that P-incorporation increase robustness towards 10 ACS Paragon Plus Environment

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surface oxidation. The deconvolution of the P 2p signals in the XPS spectrum of P-CoMoS/CC gave two peaks at 129.6 eV and 130.7 eV corresponds to 2p3/2 and 2p1/2, respectively (Figure 2d) suggest the presence of low valence phosphorus species, Pδ- (δ

Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on

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