Transition Metal Doped Molybdenum Diselenides with Defects and

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Energy, Environmental, and Catalysis Applications

Transition Metal Doped Molybdenum Diselenides with Defects and Abundant Active Sites for Efficient Performances of Enzymatic Biofuel Cell and Supercapacitor Applications Mani Sakthivel, Sukanya Ramaraj, Shen-Ming Chen, Tse Wei Chen, and Kuo-Chuan Ho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04884 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

Transition Metal Doped Molybdenum Diselenides with Defects and Abundant Active Sites for Efficient Performances of Enzymatic Biofuel Cell and Supercapacitor Applications Mani Sakthivela,b, Sukanya Ramarajc, , Shen-Ming Chenc*, Tse-Wei Chenc, Kuo-Chuan Hoa,b* a

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.

b

Advanced Research Center for Green Materials Science and Technology, National Taiwan University, Taipei 10617,

Taiwan. c

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National

Taipei University of Technology, Taipei 10608, Taiwan.

1 ACS Paragon Plus Environment

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ABSTRACT We have demonstrated the synthesis of defect-rich Ni doped MoSe2 nanoplates (NiMoSe2) and its application as an efficient electrocatalyst for enzymatic biofuel cell and electrochemical pseudocapacitor. In this study, a new type of interpretation is proposed that the defective surface facilitate the effective entrapment of enzymes (Glucose oxidase (GOD), laccase) for biofuel cell and additional ion diffusion for faradic charge -discharge reaction. The TEM and UV-vis spectroscopy techniques scrutinized the formation of defects/distortions and the resultant successful entrapment of enzymes. The performed electrochemical characterizations for enzyme immobilized NiMoSe2/nickel foam (NF) bio-anode (NiMoSe2/GOD/NF) and bio-cathode (NiMoSe2/laccase/NF) exhibited better direct charge conductive behaviour at the interface of enzymes and electrode material. Herein, the assembled biofuel cell exhibited the open circuit voltage (VOC = 0.6 V) and short circuit current density (JSC = 8.629 mA cm-2) with maximum power density (Pmax) of (1.2 mW cm-2). For electrochemical pseudocapacitor application, the proposed NiMoSe2/NF exhibited the excellent specific capacitance (535.74 F/g) with 86.7% of rate performance. Finally, this work suggests new insights to both the enzymatic biofuel cell and supercapacitor applications. Keywords: Molybdenum diselenides, Nickel, Enzymatic biofuel cell, Glucose oxidase, laccase, Supercapacitor. 1. INTRODUCTION The energy conversion and energy storage technologies have been focusing as a main part in industrial energy cycle sectors. For more than two decades, the enzymatic biofuel cells and electrochemical supercapacitors are becoming as the promising and existing energy conversion and storage devices due to their excellent performances1, 2. The enzymatic biofuel cells are attractive bioenergy devices, which convert the chemical reaction into the electrical energy by using the biocatalyst (enzymes). The enzymes (ex: Glucose oxidase, laccase, and bilirubin oxidase)


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are stimulating the anodic and cathodic reaction of the fuels (ex: Glucose and oxygen)3. In general, both the glucose and oxygen are so abundant in the bio-ecological system, thus the enzymatic biofuel cells can be applied as an energy system in flexible bio-implantable health monitoring devices. However, the poor stability of the enzymes obstructs the effective charge transfer between its active sites and electrode material4. It is a key problem for the commercial application of enzymatic biofuel cells and its fewer output performances. On the other hand, the storage devices including batteries, conventional capacitors, and electrochemical capacitors have been used to store the electrical energy from renewable resources. Among them, the supercapacitors exhibit more P and energy density (E) than batteries and conventional capacitors respectively. However, the E is comparatively lesser than batteries, it limits the utilization of supercapacitors in high energy storage systems5-7. Thus, many investigations have been innovated to increase the E of the supercapacitors. In general, two types of supercapacitors such as electrostatic double layer capacitors (EDLCs) and pseudocapacitors. Herein, the EDLC use the carbon allotropes and their derivatives as electrode materials and store the energy as electrostatic field. Meanwhile, the electrochemical pseudocapacitors use the metal oxides, polymers, and their nanocomposites as the electrode materials and achieved the charge storage by faradic redox reactions8-11. Among them, the overall specific capacitance of pseudocapacitors is comparatively higher due to the reversible faradic reaction. However, the metal oxides and polymers exhibit the lower surface area and conductivity than the carbon nanomaterials. Therefore, the synthesis and fabrication of efficient electrode materials with higher charge conductivity, surface area, and electrochemical activity is so essential to both the enzymatic biofuel cells and electrochemical pseudocapacitors for effective enzyme immobilization and higher specific capacitance respectively. Recently, the molybdenum diselenides have been considered as promising electrode materials in various electrochemical applications due to their interesting electronic properties12-15. Especially,



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the research on defects/distortions and resultant rich active sites on the layered structure of molybdenum diselenides becomes the interesting topics. Because the presence of active sites on surface of the layered structure offers improved electrochemical activity and electronic conductivity. Furthermore, the rich active sites due to the defects/distortions facilitate the sufficient surface for excellent immobilization of enzymes, it increases the faster charge transfer communication between the active center of enzymes and layered structure16. In new research, the transition metals doping is focusing as an encouraging way to create the defects/distortions on the surface of the layered structure. Y. Liu et al. demonstrated the atomic distortions on Mn-doped CoSe2 layered structure and achieved an enhanced electrochemical activity towards hydrogen evolution reaction (HER)17. In our previous work, we reported the defects/distortions on Mn-doped molybdenum diselenides layered structure and resultant effective entrapment of myoglobin for electrochemical determination of hydrogen peroxide from HaCaT and RAW 264.7 living cells18. Moreover, the co-occurrence of transition metals sharing their charge transfer properties together and also leads to enhance the redox reactions. For example, A. Sun et al. reported the MoS2 and CoMoS2 modified electrodes for pseudocapacitor and achieved the superior specific capacity/rate performances for CoMoS219. As mentioned in the above strategies and researches, the transition metal doping and defect engineering can become an interesting aspect for enhancing the electrochemical performances of enzymatic biofuel cells and supercapacitors. For more than decades, the Ni-based electrocatalysts have been deliberated as hopeful active materials for various electrochemical applications especially enzymatic biosensors/biofuel cells and supercapacitor applications. For example, S. Darvishi et al. achieved an excellent enhancement of glucose oxidation on Ni nanoparticle decorated reduced graphene oxide owing to the superior redox behaviour of Ni ions20. D. Tian et al. reported the Ni nanoparticle integrated G-CNFs@PANI nanocomposite and observed the significant support of Ni nanoparticle for charge storage performance, it reveals that the integration of Ni nanoparticles not only enhances the charge


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storage performances but also reduce the internal charge transfer resistance21. Therefore, we believed that the incorporation of Ni on the basal surface of MoSe2 can be able to support for the better performances of enzymatic biofuel cells and pseudocapacitors. In this work, we successfully synthesized the NiMoSe2 nanoplate by following the hydrothermal technique and subsequently applied as the electrocatalyst for enzymatic biofuel cell and electrochemical pseudocapacitor applications. Fortunately, the proposed NiMoSe2/GOD/NF and NiMoSe2/laccase/NF exhibited the superior electrochemical performance for glucose oxidation and O2 reduction reactions respectively. In addition, the assembled biofuel cell exhibited the highest VOC and JSC with Pmax values. The recorded electrochemical parameters were compared and concluded the excellent performances of the enzyme immobilized bio anode and cathode for biofuel cell application. In addition, the NiMoSe2/NF was applied as an electrocatalyst for supercapacitor application. From the results, the NiMoSe2/NF was identified as a promising electrode material for electrochemical pseudocapacitor also. 2. EXPERIMENT SECTION 2.1 Materials The sodium molybdate dihydrate (Na2MoO4·2H2O), nickel (III) nitrate (Ni(NO3)2.6H2O), and selenium powder (Se), GOD from Aspergillus niger, glucose, laccase from Trametes versicolor, poly(1,1-difluoroethane) (PVDF), carbon black (CB), and N-Methylpyrrolidinone (NMP, C5H9NO) were purchased from Sigma Aldrich. The tetrahydridodinitrogen (N2H4. xH2O) was bought from ACROS Organics. NF substrate was bought from Xiamen Technology, China. For the biofuel cell analysis, the working electrolyte (0.05 M, pH 5) was made by using the 0.05 M Na2HPO4 and NaH2PO4. The pH of the buffer was modified by changing the amount of H2SO4/NaOH. For supercapacitor application, 1.0 M KOH was used as the working solution. 2.2 Synthesis of NiMoSe2



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In this proposed method, 0.02 M of Na2MoO4 · 2H2O (0.16 g) and 0.02 M of Ni(NO3)2 · 6H2O (0.20 g) were mixed in H2O (20 mL) by following magnetic stirring for 10 min. After, 0.12 g of Se (0.04 M) contains 10 mL of N2H4 · xH2O was mixed dropwise into the stirring mixture and observed the resultant dark black precipitation. After 30 min reaction, the whole mixture solution was poured into 50 mL of hydrothermal Teflon and maintained at 180 ̊ C for twelve hour. Then, the hydrothermal reaction was terminated and allowed the reaction for cooling at room temperature. Finally, the precipitation was obtained/washed by EtOH/H2O and dried in vacuum oven for whole night. It is schematically expressed in Scheme 1.

Scheme 1. Synthesis of NiMoSe2 nanoplate 2.3 Equipments The morphology of NiMoSe2 was analyzed by performing the transmission electron microscopy (TEM, JEOL 2100F) analyses and field emission scanning electron microscopy (FESEM, Nova NanoSEM230, USA). The crystallographic properties of prepared material were characterized from X-ray diffraction (XRD, XPERT-3 diffract meter with Cu Kα radiation (K= 1.54 Å ) analysis. To confirm and measure the elemental composition of the prepared sample, the XPS (XPS:


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Thermo scientific multi-lab 2000) analysis was performed and demonstrated the spectra for Ni 2p, Mo 3d and Se 3d. The Brunauer-Emmett-Teller (BET) technique was performed by using the Micrometrics ASAP 2020M instrument, Norcross, Georgia, USA to analyze the surface area and the pore distribution of the sample. The successful immobilization of GOD and laccase was confirmed by using the UV-vis (V-770 spectrometer) analysis. The internal charge transfer resistance was estimated by performing the electrochemical impedance spectroscopy (EIS, IM6ex ZAHNER system). Both enzymatic biofuel cell and supercapacitor were analyzed by using the cyclic voltammetry (CV, CHI1205b)

and galvanostatic charge-discharge (GCD, CHI440)

electrochemical analyzers. 2.4 Electrode fabrication 2.4.1 Fabrication of electrode for enzymatic biofuel cell In this fabrication process, about 5 mg of NiMoSe2 in 1 mL of EtOH dispersion was made by using ultrasonication for 10 min. After, the 40 µL of NiMoSe2 was deposited on NF (1×1 cm2) using drop coating method and dried at ambient temperature. Then, the NiMoSe2/NF was slightly washed with H2O. To prepare the bio-anode, the NiMoSe2/NF was dipped into the GOD mixture (1 mg of GOD/1 mL of buffer pH 5) and placed at room temperature for overnight. Similarly, the bio-cathode was prepared by dipping the NiMoSe2/NF into the laccase solution (1 mg of laccase/1 mL of phosphate buffer pH 5) for the same time as above3. 2.4.2 Biofuel cell assembly Before the cell assembly, the agarose gel (AG) electrolyte was prepared by mixing of 1.5 wt% of agarose and 100 mM of glucose in 50 mL of phosphate buffer (pH 5) and maintained in the constant magnetic stirring at 90 ̊ C until the solution become gel form22. The resultant polymer gel was applied in between the +ve and -ve electrodes of cell assembly. Whereas, the NiMoSe2/GOD/NF and NiMoSe2/Laccase/NF were used as the +ve and -ve electrodes. These two electrodes were sandwiched between the cellulose acetate membrane deposited with AG



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electrolyte. After the assembly of two electrodes, the top side to the cell was covered by Scotch tape. 2.4.3 Three-electrode cell for supercapacitor In the three-electrode cell, the NiMoSe2/NF, Hg/HgO (1.0 M KOH) and Pt wire were applied as the working electrode, reference electrode and auxiliary electrode respectively, whereas 1.0 M KOH was used as working electrolyte. The CV studies were carried out in the potential window of -0.30 to 0.60 V at a scan rate of 1 - 7 mV/s. Consequently, the GCD experiment of NiMoSe2/NF was analyzed in the set of voltage from 0 to 0.55 V at 6, 7, 8, 9 and 10 A/g current densities. The Rct value was estimated by using set of the frequency range (0.1 Hz to 100000 Hz), at the applied voltage of 10 mV. Before these studies, the NiMoSe2/NF was fabricated by using the procedure. For this, the slurry containing NiMoSe2 (80 wt%), CB (15 wt%), PVDF (5 wt%) and NMP (400 µL) was made by using simple grinding process. The obtained slurry was deposited on NF (1×1 cm2) by drop coating process and kept at 100 ̊ C for overnight23. 3. RESULTS AND DISCUSSION The morphology of NiMoSe2 were observed from FESEM and TEM results (Figure. 1). The different magnified FESEM images (Figure. 1 (A-C)) reveal the homogeneously distributed nanoplate structure of NiMoSe2 with uniform size and shape. It was again estimated by using the TEM results (Figure. 1(D-F)), which exactly mimics the result of FESEM report. Especially, Figure. 1 (E, F) shows the diameter and thickness of the nanoplates of about ~ 312 and 66 nm respectively. The corresponding profile plot is demonstrated in Figure. 1G. Noticeably, Figure. 1H shows the top view of the nanoplate, which exhibits a distorted and defective surface. The high-resolution image on the edge of the plate (Figure. 1I) shows the interplanar d spacing of 0.64 nm related to (002) plane, whereas the yellow color round represents the formation of the lattice defects/distortions. In addition, the high-resolution basal surface of the nanoplate (Figure. 1J)


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exhibits the octahedral crystalline atomic arrangement (1T phase; represent by yellow hexagonal) with more defects/distortions (yellow round) due to the introduction of heterogeneous Ni atoms. The resultant defective can be ascribed due to (1) the introduction of heterogeneous Ni atoms induces the localized coulomb effect and subtle atomic distortion (2) unequal Jahn-Teller phenomena among Ni-Se and Mo-Se [17]. Figure. 1K shows the FFT pattern of the corresponding lattice plane, it clearly exhibits the octahedral atomic prismatic arrangement of NiMoSe2. The crystalline structure and formation of NiMoSe2 nanoplate were confirmed by recording the XRD result (Figure. 2A). It exhibits the peaks at 12.64 º, 33.70 º, 55.73 º for corresponding lattice plane (002), (110) and (110) respectively. It was perfectly matched to hexagonal crystal structure of fewlayered molybdenum diselenide (JCPDS no: 29-0914)24. There is no other peaks were observed for Ni, which implies that the Ni atoms are effectively dilute/integrate with the atomic lattice of MoSe2 without the presence of any other impurities.



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Figure. 1 (A-C) FESEM images with different magnification, (D-F) TEM images, (G) length profile data, (H) high magnified images, (I, J) High-resolution TEM images, (K) FFT pattern of NiMoSe2 nanoplate. In general, the XPS analysis is a widely used analytical technique for studying the elemental composition and providing the exact interpretation of the electronic state of the elements. Hence, the XPS study was carried out to NiMoSe2 nanoplate and reported the corresponding spectra for


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Ni 2p, Mo 3d and Se 3d (Figure. 2 (B-D)). Figure. 2B represents the XPS spectra of Ni 2p which exhibits the binding energies exactly matched with two spin-orbital of Ni2+ 2p1/2 and Ni2+ 2p3/2 at 875.0 and 857.4 eV respectively. Furthermore, the two shakeup satellite peaks were observed at 880.58 and 863.12 eV. Meanwhile, the XPS spectra of Mo 3d deconvoluted into three major peaks at 228.6, 232.2, and 235.5 eV corresponding electronic states of Mo4+ 3d5/2, Mo4+ 3d3/2 and Mo6+ 3d5/2 respectively. The electronic state of metal selenide was scrutinized due to the presence of the peak at 55.1 eV corresponding to the electronic state of Se 3d5/2.

Figure. 2 (A) XRD pattern, (B-D) XPS spectra, (E, F) N2 sorption measurement and pore distribution of NiMoSe2 nanoplate. The observed XPS spectra of Ni 2p, Mo 3d, and Se 3d are strongly associated with the previously reported works25, 26. Finally, the XPS result concluded that the NiMoSe2 nanoplates contain the elements including Ni, Mo, and Se with proper electronic stoichiometry. Figure. 2 (E, F) shows the N2 sorption measurement for the surface area and the pore distribution of NiMoSe2 nanoplate sample. Figure. 2E exhibits the NiMoSe2 possessing the mesoporous (Type-IV isotherm) nature with corresponding surface area (A) of 24.018 m2/g at saturated pressure (P/P0) of 769.890 mmHg. Figure. 2F shows the BJH pore profile of NiMoSe2, from which 11 ACS Paragon Plus Environment


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calculated the pore with volume (V = 10.00 cm3/g) and diameter (D = 4.18 nm). The results obtained from the BET isotherm and BJH pore distribution studies concluded the mesoporous and feasible surface area of NiMoSe2 can offer the higher surface area for enzyme immobilization and ion diffusion in faradic redox charge-discharge reaction. This supportive properties of active material accounts for gaining better electrochemical performances.

Figure. 3 (A, B) Absorbance spectra of modified electrodes. In the case of enzymatic biofuel cell application, the enzymes immobilization on both anode and cathode is considered as a very important characteristic. To confirm the successful immobilization of GOD and laccase on NiMoSe2 nanoplate, the UV-vis spectroscopy was performed and reported the corresponding spectra as shown in Figure. 3 (A, B). In this spectra, NiMoSe2 does not shows any signal due to its 1T octahedral prismatic structure, which exhibits the metallic nature. On the other hand, NiMoSe2/GOD shows the characteristic band at 259.4, 335.4, and 464.7 nm (Figure. 3B). Herein, the band at 259.4 nm is strongly associated to polypeptide chains of GOD, and the band at 335.4/464.7 nm are related to the flavin adenine dinucleotide (FAD) moiety27. Meanwhile, the NiMoSe2/laccase exhibits the band at 252.6 nm attributed due to the Cu Type III in the binuclear active center of the enzyme28. 3.1 Electrochemical characterizations


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3.1.1. Electrochemical performance of NiMoSe2/GOD/NF and NiMoSe2/laccase/NF electrodes The electrochemical performances of NiMoSe2/GOD/NF bio-anode were estimated by using the CV measurement and demonstrated in Figure. 4. Herein, Figure. 4A reveals the CV curve of NiMoSe2/NF, MoSe2/NF, and NF in N2 purged buffer (pH 5) at 20 mV/s. From this curve, we can observe only the capacitive response with square shape of CV curve over the voltage window from -0.6 to 0 V.

Figure. 4 CV profile of (A) electrodes without enzymes immobilization, (B) with immobilization of GOD, (C) electrochemical oxidation of glucose (1 mM) at NiMoSe2/GOD/NF in N2 purged buffer (pH 5) at 20 mV/s, and (D) schematic illustration for electrochemical oxidation of glucose at NiMoSe2/GOD/NF. Meanwhile, the CV curve of enzyme immobilized NiMoSe2/GOD/NF and MoSe2/GOD/NF is demonstrated in Figure. 4B, it implies the NiMoSe2/GOD/NF exhibits the clear anodic and



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cathodic current response at -0.27 and -0.45 V with higher peak current response. It is comparatively better redox peak current response than obtained at MoSe2/GOD/NF. The observed redox peak is strongly associated to the oxidation and reduction behaviour of the FAD active center of the enzyme. This observation is strongly confirmed the successful immobilization of GOD on NiMoSe2/NF and excellent charge transfer between the enzyme and NiMoSe2. In addition, the presence of defects on the surface of MoSe2 due to the Ni doping offers the more edge active sites/sufficient surface for superior enzyme immobilization. To study the electron transfer behaviour of NiMoSe2/GOD/NF, the CV curve was recorded by varying the scan rate of as shown in Figure. S1 (A). The resultant CV curve reveals the increasing the oxidation and reduction response by varying 10 to 40 mV/s scan rates. Figure. S1 (B) shows the calibration plot with a coefficient (R2) of 0.9995 and 0.9784 for oxidation and reduction current respectively, it indicates the excellent linearity of the modified electrode. In addition, the unchanged formal potential (E0) was observed for all the applied scan rates. It reveals that the NiMoSe2/GOD/NF followed the surface controlled reaction. The quantitative information on electrons number (n) during the redox reaction was estimated by the following equation (4)29 Ip = (nQFν)/(4RT)

(4)

where Ip is known as the peak current, Q is referred as the integrated charge of redox peak, ν is represented as the scan rate (mV/s), F and R are known as constant values of 9.6485 * 104 C mol8.314 J mol-1 K-1 respectively, and T is the room temperature (293 K). By using the equation, the number of electron (n) was detected to be 2.1, it is exactly matches to the standard value for the change of FAD/FADH2 in enzyme during the redox reaction. It is strongly associated to the previously reported literature22. According to Laviron’s theory, the charge transfer rate constant ks was calculated by the following the equation (5)30 ks = nFνm/TR


(5)

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Where m is associated with the value of peak to peak separation. By using the equation ks was calculated to be 15.21 s-1, it is relatively higher as compared to reported work31. In order to understand the electrocatalytic activity of NiMoSe2/GOD/NF for the oxidation of glucose, the CV study was carried out with the addition of 1 mM of glucose as shown in Figure. 4C. The obtained CV curve shows the increasing oxidation current due to the effective oxidation of glucose at the NiMoSe2/GOD/NF, which exhibits comparatively better oxidation current response than that of NiMoSe2/NF without immobilization of GOD. In addition, the charge transfer at the enzyme and electrode material interface is a direct electron transfer owing to the absence of mediator. The proposed electrochemical oxidation of glucose can be written as, Glucose + GOD (FAD) → Gluconolactone + GOD (FADH2) GOD (FADH2) → GOD (FAD) + 2H+ + 2e-

(6) (7)

The overall mechanism of the electro-oxidation of glucose at NiMoSe2/GOD/NF is schematically represented in Figure. 4D. On the other hand, the investigation of electrocatalytic behaviours of the bio-cathode NiMoSe2/laccase/NF was carried out by using the CV technique and reported in Figure. 5. As similar to above studies, the NiMoSe2/NF and MoSe2/NF without enzyme shows only the capacitive behaviour with quasi-rectangular shape CV curve over the voltage window of -0.4 to 0.8 V (Figure. 5A). In this same working condition, the NiMoSe2/laccase/NF and MoSe2/laccase/NF were performed and corresponding CV curve is displayed in Figure. 5B. In this obtained CV curve, the NiMoSe2/laccase/NF exhibits the redox peak at 0.16 and 0.11 V owing to the redox center of the enzyme. It confirms the successful attachment of enzyme and the electron conduction at the interface of enzyme/electrode material. Furthermore, the surface controlled reversible process of NiMoSe2/laccase/NF was understood from the scan rate analysis.



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Figure. 5 CV curve of (A) different modified electrodes without enzymes immobilization, (B) with immobilization of laccase, (C) electrochemical reduction of O2 at NiMoSe2/laccase/NF in N2 purged buffer (pH 5) at 20 mV/s, and (D) schematic illustration for electrochemical oxidation of glucose at NiMoSe2/laccase/NF. As shown in Figure. S2, the scan rate variation from 10 to 50 mV/s exhibits the linearly increasing redox current response. It is displayed in the linear plot with the correlation coefficient (R2) of about 0.9878 and 0.9823 for oxidation and reduction peaks respectively. In addition, we investigated the electrochemical reduction of O2 at NiMoSe2/laccase/NF modified electrode in O2 saturated buffer (pH 5). The corresponding CV curve (Figure. 5C) shows the rapidly increasing reduction current response related to the O2 reduction reaction. It confirms the efficient electrocatalytic activity of the proposed electrode for cathodic O2 reduction reaction. The observed reduction reaction at proposed bio-cathode can be written as,


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Laccase (ox) + 4e- + 4H+ → Laccase (red)

(8)

Laccase (red) + O2 → Laccase (ox) + 2H2O

(9)

3.1.2. Characterization of biofuel cell The biofuel cell was fabricated by sandwiching NiMoSe2/GOD/NF bio-anode and NiMoSe2/laccase/NF bio-cathode filled with AG electrolyte; the photographic images are displayed in Figure. 6A. To estimate the electrochemical behaviour of assembled biofuel cell, the power density and polarization curves were investigated as displayed in Figure. 6B.

Figure. 6 (A) Photographical image, and (B) polarization curve of assembled enzymatic biofuel cell. From this polarization curve, the maximum VOC was detected to be 0.6 V with the JSC of 8.629 mA cm-2, it is almost equal to the theoretical potential difference the glucose/gluconolactone and O2/H2O couples. Moreover, the power density curve exhibited the Pmax of 1.2 mW cm-2, which is comparatively higher than the previously reported glucose/O2 enzymatic biofuel cells (Table 1). Finally, we concluded that the proposed NiMoSe2 active electrode material is a promising electrocatalyst for the enzymatic biofuel cell application owing to the presence of abundant active edge sites and multiple electronic/redox behaviour. To estimate the stability of fabricated biofuel cell, the Pmax of the cell at 0.6 V for three days at every 12 hr time interval was measured and presented in Figure. S3A. The result shows that the 17 ACS Paragon Plus Environment


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assembled cell keeps 89.5% of its initial performance after three days. In addition, the stability performance of NiMoSe2/NF was compared with NiMoSe2/GOD/NF. Thus, the CV technique was performed for NiMoSe2/NF (Figure. S3B) and NiMoSe2/GOD/NF (Figure. S3C) with presence of glucose (1 mM), subsequently recorded the oxidation current response before and after 24 hr electrode storage in electrolyte solution. The obtained CV curves show only a small current difference for both NiMoSe2/NF and NiMoSe2/GOD/NF electrodes even after the storage process. The overall studies confirm the feasible stability of the individual electrodes and fabricated biofuel cell. Table 1. Electrochemical performance of proposed enzymatic biofuel cell Anodic enzyme GOD Glucose dehydrogenase GOD GOD GOD GOD GOD Shewanella oneidensis GOD

Cathodic enzyme Bilirubin oxidase laccase

Pmax (mW cm-2) 0.97 0.60

Ref 32 33

Bilirubin oxidase Bilirubin oxidase laccase Bilirubin oxidase Bilirubin oxidase laccase

0.03 0.0009 1.12 0.085 0.14 0.2

34 35 36 37 38 39

laccase

1.2

This work

3.1.3. Electrochemical testing of NiMoSe2/NF for electrochemical pseudocapacitor The charge storage behaviour of NiMoSe2/NF were analysed by using GCD, CV, and EIS techniques and the corresponding results are displayed in Figure. 7. Figure. 7A reveals the CV curve of different electrodes such as NF, MoSe2/NF, and NiMoSe2/NF at 10 mV/s. From the CV result, the NiMoSe2/NF shows both the almost rectangular shape related to the EDLC behaviour of MoSe2 and reversible redox reaction associated with the faradic behaviour of Mo active atoms and Ni2+/Ni3+. In this obtained CV curves, NiMoSe2/NF exhibits the higher EDLC and faradic behaviour with the resultant integrated area of 0.0036 AV, it is 2 and 23 fold higher than MoSe2 (0.0012 AV), and NF (0.00015 AV) respectively. It preliminarily confirms the enhancement of 18 ACS Paragon Plus Environment

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electrochemical performance of MoSe2 due to the integration of Ni atoms. In addition, the rate performances of NiMoSe2/NF were estimated by varying the scan rate of 4, 6, 8, and 10 mV/s as given in Figure. S4. The obtained CV result for different scan rates indicate the excellent mass transport of ions and reversibility due to less peak potential shift and no change in the shape of the curve for all scan rates. Therefore, the overall result from the CV curves confirmed that the charge transfer reaction at NiMoSe2/NF electrode is following the diffusion based reaction and does not cause any physical alteration at the interface of electrolyte/electrode. Subsequently, the GCD technique was performed for NF, MoSe2/NF and NiMoSe2/NF electrodes and reported the corresponding results in Figure. 7B.

Figure. 7 (A) CV curve, (B) GCD of different modified electrodes, (C) GCD curve of NiMoSe2/NF for different current densities, (D) linear plot for capacitance vs. current density, (E) Nyquist plot for different modified electrodes (insert) bar diagram for corresponding Rct values, and (F) maximized image of EIS curve (insert) corresponding Randle’s circuit. As the result, NiMoSe2/NF delivered the largest triangular symmetry GCD curve with the higher specific capacitance of 535.74 F/g, it is 1.23 fold higher than obtained at MoSe2/NF (240 F/g) respectively. It indicates that the presence of defective surface due to the integration of Ni atoms



Page 20 of 27

facilitates the additional active sites for the ion diffusion in faradic redox reaction. The specific capacitance (Cs) of electrodes was calculated from the GCD curve by using equation (1)40 Cs = (Δt)I/(ΔV)m

(10)

Where Δt is referred to time for discharge process, I is known as current density, m is represented as the mass of NiMoSe2 on NF and ΔV is known as voltage window used for the GCD studies. Furthermore, the capacitive behaviour of NiMoSe2/NF was studied under different current densities. Figure. 7C displays the GCD curve of NiMoSe2/NF electrode at 6, 7, 8, 9 and 10 A/g. From this curve, the specific capacitances were detected to be 535.74, 519.14, 497.45, 486.49, and 464.90 F/g at 6, 7, 8, 9 and 10 A/g respectively. The calibration plot between capacitance vs. current density is given in Figure. 7D. In this GCD result, the capacitance value reduces by increasing the current density; it reveals the deficient contribution of electrode material at higher current density. Fortunately, the NiMoSe2/NF exhibits 86.7% of its starting capacitance. The mechanism of faradic reaction is schematically expressed in the Scheme 2.

Scheme 2. Schematic sketch for electrochemical ion diffusion on NiMoSe2 nanoplates for pseudocapacitor application


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The obtained Cs of the proposed electrode was compared with previously reported works (Table 2). It specifies the superior rate capability of the NiMoSe2/NF during the faradic reaction. The possible faradic redox reaction mechanism at NiMoSe2/NF can be written as below, NiMoSe2 + 2OH- ↔ NiSeOH + MoSeOH + 2eMoSeOH+ OH- ↔ MoSeO + H2O + eNiSe+ OH- ↔ NiSeOH + e-

(11) (12) (13)

In general, the internal Rct at the interface of electrolyte/electrode is considered as one of the important factors for an efficient faradaic charge-discharge reaction. The EIS analysis can provide a quantitative parameter of Rct. Therefore, the EIS experiment was recorded for NF, MoSe2/NF and NiMoSe2/NF electrodes and demonstrated the obtained Nyquist plots in Figure. 7(E, F). From this Nyquist plots, it can be seen that the NiMoSe2/NF delivered the smallest circle as compared to MoSe2/NF and NF electrodes. The detected Rct values of different modified electrodes are clearly shown in Figure. 7E (insert). The lowest Rct of NiMoSe2/NF implies its higher internal charge conductivity at the electrolyte/electrode interface. Therefore, the introduction of Ni atoms not only enhances the electrocatalytic activity but also significantly reduces the internal charge transfer resistance. It is more favourable for the faster ion diffusion in the faradic reaction. The equivalent Randle’s circuit is demonstrated in Figure. 7F (insert). Table 2: Capacitance of various supercapacitor electrodes



Electrode

Substrate used

Electrolyte

MoSe2 MoS2/CMG

Stainless steel Ti foil

MoS2/G

Page 22 of 27

Cell Voltage (V) 0-0.8

Capacitance (F/g)

Ref

1M H2SO4

Current Density (A/g) 5

199

41

1M Na2SO4

15

-0.4-0.8

268

42

carbon

1M Na2SO4

0.1

-0.7-0.5

270

43

NiSe@MoSe2//NPMCN MoSe2/rGO

Ni Foam

3M KOH

1

223

44

carbon

6M H2SO4

5

-0.10.65 -0.5-0.4

211

45

MoS2

3M TEABF4 1M Na2SO4

0.75

-3.0-3.0

14.75

46

1

-1-0

429

47

MoS2/CNTs-MnO2

Stainless steel Carbon cloth Ni Foam

2M Na2SO4

8

0-4

365.6

48

1T MoS2x Se2(1-x)

Graphite

6M KOH

0.5

-0.8-0.8

36

49

NiMoSe2

Ni Foam

1M KOH

6

0-0.55

535.74

This work

MoS2

4. CONCLUSION In summary, we have successfully developed the NiMoSe2 based electrocatalyst for enzymatic biofuel cell and electrochemical pseudocapacitor application. We concluded that the existence of defective surface on NiMoSe2 delivers the additional active sites for enzyme immobilization and enhances both the electrochemical activity/charge conductivity. Thus, the developed biofuel cell contains NiMoSe2/GOD/NF and NiMoSe2/laccase/NF electrodes with AG electrolyte reaches the highest VOC (0.6 V) and JSC (8.629 mA cm-2) with a Pmax of 1.2 mW cm-2. In the case of the supercapacitor, the NiMoSe2/NF showed excellent specific capacitance and rate performance with lower internal charge transfer resistance. The achieved capacitance is substantially higher than the already reported molybdenum based nanocomposites. Finally, this study proposed that the new material and concepts for enzymatic biofuel cell and supercapacitor applications.


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ACKNOWLEDGEMENTS

Financial supports of this work by the Ministry of Science and Technology, Taiwan (MOST 1072113-M-027-005-MY3 to SMC) is gratefully acknowledged. This work was partially supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 105-2221-E-002-229-MY3, 107-2811-E-002-559, and 107-3017-F-002-001). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX-XXXX. The CV curve for NiMoSe2/GOD/NF and NiMoSe2/laccase/NF with corresponding linear plot, calibration linear plot and CV results for stability test, CV curve of NiMoSe2/NF supercapacitor electrode for different scan rates.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (S.M.Chen) *E-mail: [email protected] (K.C.Ho) NOTES

The authors declare no competing financial interest



Page 24 of 27

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TOC


Transition Metal Doped Molybdenum Diselenides with Defects and

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