Engineered Tubular Nanocomposite Electrocatalysts Based on CuS

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Engineered Tubular Nanocomposite Electrocatalysts Based on CuS for High-Performance, Durable Glucose Fuel Cells and Their Stack G. Siva,† Md. Abdul Aziz,‡ and G. Gnana kumar*,† †

Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India Centre of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran-31261, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: Exploring the electrochemically active and robust nanocatalysts for the efficient glucose oxidation reaction (GOR) and oxygen reduction reaction (ORR) garners enormous interest in the development of high performance glucose fuel cells (GFCs). The bifunctional copper sulfide (CuS) nanotubes and their specific surface engineering modification with nickel hydroxide (Ni(OH)2) and manganese dioxide (MnO2) nanostructures evade the constrains of existing GOR and ORR catalysts, respectively. On the basis of a systematic electrochemical analysis, the fundamental intrigue on the optimization and influences of core and shell nanostructures toward GFC performances is realized. Under alkaline conditions, CuS@ Ni(OH)2 and CuS@MnO2 as GOR and ORR catalysts, respectively, demonstrate the maximum GFC power density of 1.25 mW cm−2 with 300 h of durability. Furthermore, the energy harvest from a GFC stack without any major performance loss in comparison with a single cell enunciate the excellent energetic capabilities of a stack design. These findings thus provide the compatible solutions for the thriving research areas of GOR and ORR, and by coupling the aforesaid research efforts, high performance and durable GFCs are established. KEYWORDS: Nanotubes, Core−shell, Glucose oxidation, Oxygen reduction, Fuel cell stack

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INTRODUCTION The world’s ever increasing energy demand associated with the adverse effects of existing fossil fuels directed immense research activities toward the design and development of clean and renewable energy devices, which are capable of harvesting green energy from sustainable energy sources.1−3 Recently, glucose fuel cells (GFCs) have emerged as an excellent green energy device, as it is able to offer considerable power output via the glucose oxidation reaction (GOR) and oxygen reduction reaction (ORR) at the anode and cathode, respectively.4,5 However, the development of high performance GFCs is substantially hindered by the sluggish GOR and ORR kinetics, prohibitive production cost, and limited durability, which are mostly associated with the existing anode and cathode catalysts of GFCs. Jin et al. constructed a GFC with a silver (Ag) modified gold (Au) anode and demonstrated a maximum power density of 0.022 mW cm−2 mM−1.6 Basu et al. fabricated a GFC by exploiting commercial platinum (Pt)−ruthenium (Ru)/carbon (C) and activated charcoal as GOR and ORR catalysts, respectively, and achieved a peak power density of 1.38 mW cm−2 under alkaline conditions.7 However, the limited GFC durability along with the low reserve limit, high cost, selfpoisoning, and rapid deactivation characteristics of precious metal catalysts evaded their utilization in GFCs, which prompted the engineering of active and economically viable © XXXX American Chemical Society

catalysts for high performance GFCs. Although the aforesaid constrains of precious metal catalysts are tackled with the molecular catalysis-based GOR catalyst of multiwalled carbon nanotubes (MWCNTs)/deuteroporphyrin dimethyl ester rhodium(III), it displayed a lower GFC power density of 0.18 mW cm−2 owing to the inherent steric constrains of a heterocyclic catalyst.8 Hence, it is clear that being more prone to electrode surface fouling, sluggish electron transfer kinetics, and requirement of high overpotential are the existing hurdles of GOR catalysts applicable for GFCs. ORR, the other half-cell reaction of GFCs, experiences sluggish electrokinetics, owing to the requirement of high activation energy to cleaving the strong OO bond.9,10 Apart from the extensive utilization of commercial Pt/C in GFCs, MWCNT supported cobalt phthalocyanine ORR catalyst alone was exclusively reported, and the configured GFC delivered a maximum power density of 2.3 mW cm−2 with Pt/C as the anode catalyst.11 From the aforesaid research efforts, it is clear that two different catalysts were used for the GOR and ORR processes in GFCs, which made the process relatively complex and extremely expensive. Furthermore, a lack of fundamental knowledge at the atomic/molecular level of catalytic mechaReceived: November 19, 2017 Revised: February 18, 2018

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DOI: 10.1021/acssuschemeng.7b04326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. TEM images of (a−c) CuS (inset c: corresponding SAED pattern), (d and e) CuS@Ni(OH)2(1:2) (inset e: corresponding SAED pattern), and (f and g) CuS@MnO2(1:2) nanostructures (inset g: corresponding SAED pattern). etc. were purchased from Sigma-Aldrich and used without any further purification. Carbon cloth (CC) was purchased from Electrosynthesis Co., Inc., Lancaster, NY. GC-14 (area of the electrode is 1.5 cm2) was pretreated with ethanol and water and dried in a vacuum at 100 οC for 6 h. Preparation of CuS Nanotubes. An equal volume of 0.2 M thiourea was gradually mixed with 0.1 M CuCl2·2H2O at a pH level of 9 and gently stirred at room temperature for 1 min. The obtained product was cleansed with deionized water/ethanol and dried at 90 °C for 2 h. Preparation of CuS@Ni(OH)2 Nanostructures. The prepared CuS nanotubes were impregnated in a mixture of 0.5 M NiSO4·6H2O, 0.025 M K2S2O8, and 7.5 M aqueous ammonia with continuous stirring at 80 °C for 30 min. The weight ratio maintained between CuS nanotubes and Ni2+ ions was 1:2. The resultant CuS@Ni(OH)2(1:2) product was dried at 90 °C for 2 h. CuS@Ni(OH)2 nanostructures with different weight ratios of CuS nanotubes and Ni2+ (1:1 and 1:3) were prepared by using the aforesaid procedure, and the relevant materials are referred to, respectively, as CuS@MnO2(1:1) and CuS@ MnO2(1:3). Preparation of CuS@MnO2 Nanostructures. The CuS nanotubes were soaked in a mixture of 0.5 M MnSO4·6H2O, 0.025 M K2S2O8, and 7.5 M aqueous ammonia and gently stirred for 30 min at 80 °C. The weight ratio maintained between CuS nanotubes and Mn2+ ions was 1:2. The resultant CuS@MnO2(1:2) was obtained via centrifugation, followed by washing with deionized water, and drying at 90 °C. By adopting a similar procedure, CuS@MnO2 comprising the different weight ratios of CuS nanotubes and Mn2+ ions including 1:1 and 1:3 were prepared, and the relevant samples are referred to, respectively, as CuS@Ni(OH)2(1:1) and CuS@Ni(OH)2(1:3). Characterizations. The transmission electron microscopy (TEM, FEI-F20 S-TWIX), X-ray powder diffractometer (XRD, Rigaku DMAX 2500), and X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi) were used to characterize the materials. GOR Studies. An appropriate amount of prepared catalyst (CuS/ CuS@Ni(OH)2/CuS@MnO2) and activated carbon was dispersed in 0.5% Nafion with the aid of ultrasonication. The prepared catalyst slurry was coated over the pretreated CC by using a spray gun (catalyst loading −2.5 mg cm−2) and dried at 80 °C for 30 min. GOR Electrochemical Studies. The electrochemical studies were achieved with a CHI-650E electrochemical workstation at an ambient temperature. A conventional three-electrode configuration comprising

nisms involved in GOR and ORR at the respective electrodes remains the formidable hurdle for the improvement of reaction kinetics, which enlightens the research activities to drive both the GOR and ORR by using a bifunctional catalyst. Recently, copper sulfide (CuS) has emerged as a potent bifunctional catalyst for the GOR and ORR processes. Under alkaline conditions, the surface reactivity, electrical conductivity, and stability of Cu nanostructures are improved by its chemical linkage with the sulfide (S) anion via the electronic or ligand effect.12 The preferable adsorption of glucose and oxygen over the surface active sites of CuS facilitates the electrode/ electrolyte interfacial area and provides considerable GOR and ORR activity.13−15 However, the vulnerability toward surface poisoning and Cu vacancies in the lattice sites of CuS limited its electrochemical performance toward GOR and ORR, respectively.16 It could be circumvented with a surface modification of CuS with the specific GOR and ORR catalysts in the form of core−shell architectures. Accordingly, herein we report the CuS nanotubes as a bifunctional catalyst for high performance and durable GFC and address the existing hurdles of CuS nanostructures through the decoration of a CuS core with nickel hydroxide (Ni(OH)2) and manganese dioxide (MnO2) nanoflake arrays. Furthermore, the practical applicability of GFCs in portable electronic devices is still hindered with the fluctuations occurring during the integration of single cells into the stack. Hence, this research effort is also adumbrated to harvest the maximum power performance in a GFC stack without any major performance loss from the single cells.

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EXPERIMENTAL SECTION

Materials. Aqueous ammonia (NH3·H2O, 25−29, copper(II) chloride dihydrate (CuCl2·2H2O, AR, ≥ 99%), ethanol (HPLC, ≥ 99.8%), glucose (GC, ≥ 99.5%), nickel(II) sulfate hexahydrate (NiSO4·6H2O, AR, ≥ 97%), manganese(II) sulfate monohydrate (MnSO4·H2O, AR, ≥ 98%), poly(tetrafluoroethylene) (PTFE, 60 wt % dispersed in water), potassium persulfate (K2S2O8, AR, ≥ 98%), sodium hydroxide (NaOH, AR, ≥ 97%), thiourea (Tu, AR, ≥ 97%), B


Research Article

ACS Sustainable Chemistry & Engineering the bare/modified CC as a working electrode, Ag/AgCl as a reference electrode, and a Pt wire as an auxiliary electrode was exploited for the evaluation of the electrochemical performance of prepared catalysts. The electrochemical studies were achieved according to the procedure described elsewhere.17 ORR Studies. A 6 μL aliquot of catalyst slurry was placed over the glassy carbon electrode (GCE)/disk GCE and dried under normal atmospheric conditions. The polarization experiments for ORR were achieved with a speed controlled unit equipped rotating disk electrode (RDE) at 10 mV s−1 in 0.1 M NaOH solution. The working electrode was cathodically scanned at 10 mV s−1 for the rotation rate of 200− 1000 rpm. GFC Studies. A batch-type dual chambered GFC reactor made with acrylic glass was constructed with two internal chambers, comprising a bed volume of 10 mL. The two chambers were connected with the aid of an AMI-7001 anionic exchange membrane (see Figure S1 in the Supporting Information). GFC was constructed by placing the fabricated electrodes in the respective compartments, and the compartments were filled with 0.1 M NaOH solution. 0.5 M glucose was anaerobically injected into the anodic compartment, and the cathodic solution was aerated. After reaching a steady state open circuit voltage (OCV), GFC was loaded various external resistances to evaluate the polarization and power output.18 The mean values obtained from three measurements are provided in this article.

of Ni(OH)2 and MnO2 on the host CuS matrix. Apart from the above, no significant variation in the morphological properties was observed among the different composition ratios of CuS@ Ni(OH)2 or CuS@MnO2 nanostructures. The subsequent SAED pattern (inset in Figure 1g) specifies the polycrystalline structure of CuS@MnO2(1:2). XRD Studies. The diffraction patterns of prepared CuS nanotubes exhibit the characteristic (100), (102), (103), (110), (108), and (116) reflection planes at 28.04, 29.50, 32.20, 48.11, 53.25, and 59.35° (Figure 2a(i)), respectively, specifying the

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RESULTS AND DISCUSSION Morphological Properties. The TEM images of prepared CuS nanostructures display the tubular shaped morphologies with the mean diameter and length, respectively, of 175 nm and 1.7 μm (Figure 1a and b). The nanotubular morphology of CuS is constituted by the number of 4 nm diameter sized dense nanoparticles (Figure 1c), and the voids are observed among the intraparticles of CuS nanotubes. The nanoparticles are held together via the strong interparticle interaction force of CuS nanocrystals. The single-crystalline hexagonal cubic structure of individual CuS nanoparticles in the nanotubes is substantiated from the selected area electron diffraction (SAED) pattern (Inset in Figure 1c). Highly interconnected and ultrathin Ni(OH)2 nanoflakes are homogeneously grown over the entire CuS nanotubes, and the mean diameter of the tubular matrix is increased to 190 nm (Figure 1d and e). The average thickness of Ni(OH)2 flake arrays is found to be 15 nm. The positively charged Ni2+ ions are electrostatically interacted with the negatively charged CuS nanotubes. The Ni2+ ions on the CuS surface react with OH− ions released from ammonia (NH3), forming the Ni(OH)2 nanocrystal seeds. Upon further increment in reaction time, NH3 molecules are selectively adsorbed over the Ni(OH)2 via N−H−O bonds with the N of NH3 and hydrogen of the surface hydroxyl groups of Ni(OH)2. It further proceeded with Ni(OH)2 crystallization, yielding the growth of the nanoflakes architecture along the c-axis. The polycrystalline structure of CuS@Ni(OH)2(1:2) nanostructures is explored from the SAED pattern (inset in Figure 1e). MnO2 nanoflakes with the average thickness of 10 nm uniformly envelop the CuS nanotubes in CuS@MnO2(1:2), and the average diameter of CuS@MnO2(1:2) is measured to be 188 nm (Figure 1f and g). The electrostatic interaction of Mn2+ ions with CuS of negative charge uniformly adheres the Mn2+ ions over the CuS nanotubes. The oxidation of MnSO4 is accelerated with the aid of NH3 and K2S2O8, leading to the formation of MnOOH nanostructures. With the elongation of reaction time, MnOOH is gradually transformed into the MnO2 nanoflakes and is intended to assemble with each other, yielding the uniform nanoflake patterns. The mean diameter of CuS nanotubes was slightly increased with higher concentration

Figure 2. (a) XRD patterns and (b) XPS full scan spectra of (i) CuS, (ii) CuS@Ni(OH)2(1:2), and (iii) CuS@MnO2(1:2) nanostructures.

hexagonal crystalline structure of CuS (JCPDS No. 06-0464).19 In addition to the characteristic CuS diffraction patterns, the CuS@Ni(OH) 2 (1:2) composite reveals the α-Ni(OH) 2 diffraction peaks at 25.29, 33.50, and 38.70° (Figure 2a(ii)), representing, respectively, the (002), (110), and (200) planes of hexagonal structure (JCPDS No. 22-0444).20 The CuS@ MnO2(1:2) composite exhibits the new reflection plane of (006) at 36.26° along with the CuS diffraction peaks (Figure 2a(iii)), which is related to the monoclinic γ-phase of the MnO2 nanostructure (JCPDS No. 18-0802).21 XPS Studies. The full scan XPS survey spectrum of CuS nanotubes exhibits the characteristic Cu 3p, Cu 2p, and S 2p peaks (Figure 2b(i)), whereas Ni 3p and Ni 2p and Mn 2p peaks are noticed along with the aforementioned species for CuS@Ni(OH)2(1:2) (Figure 2b(ii)) and CuS@MnO2(1:2) (Figure 2b(iii)), respectively. The high resolution core level spectra of Cu 2p of the prepared nanostructures display the distinctive peaks at 931.4 and 951.4 eV (see Figure S2a in the C


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Figure 3. (a) CVs of studied CCs in the absence and presence of 5 mM glucose in 0.1 M NaOH at a scan rate of 20 mV s−1, (b) schematic illustration of the GOR mechanism involved at CuS@Ni(OH)2(1:2)/CC, (c) CVs of CuS@Ni(OH)2(1:2)/CC in the presence of 5 mM glucose as a function of scan rates ranging from 10 to 100 mV s−1 in 0.1 M NaOH solution (inset: corresponding calibration plot of Ip vs the square root of scan rate), and (d) CVs of CuS@Ni(OH)2(1:2)/CC as a function of glucose concentrations in 0.1 M NaOH at a scan rate of 20 mV s−1.

(531.3 eV), specifying the composite formation of MnO2 with CuS.25 GOR Studies. The GOR performances of CC, CuS/CC, and CuS@Ni(OH)2/CC were investigated by using the CV technique in an alkaline medium (pH-13). Bare CC exhibits the capacitive background curves in 0.1 M NaOH (Figure 3a), specifying bare CC’s electrochemically inactive behavior under alkaline conditions. However, a well-defined redox peak is observed at CuS/CC and CuS@Ni(OH)2/CC, in which CuS@ Ni(OH)2(1:2)/CC shows the improved oxidation (Ipa) and reduction currents (Ipc). CuS@Ni(OH)2(1:2)/CC demonstrates the anodic (Epa) and cathodic peak potentials (Epc) at 0.49 and 0.39 V vs Ag/AgCl, ascribed to the Cu2+/Cu3+ and Ni2+/Ni3+ redox couple. The Epa of CuS@Ni(OH)2(1:2)/CC lies between the anodic peak potentials of Cu3+ and Ni3+ under alkaline conditions, illustrating the formation of a complex oxidation species of Ni3+/Cu3+. Under alkaline conditions, Ni2+ and Cu2+ are electrochemically oxidized into Ni3+ and Cu3+, respectively, at 0.49 vs Ag/AgCl. The higher oxidation states of the CuS@Ni(OH)2 composite are electrochemically reduced into Cu2+/Ni2+ at 0.39 V vs Ag/AgCl under negative scan. Figure S3 in the Supporting Information depicts CV responses of CuS@Ni(OH)2(1:2)/CC in 0.1 M NaOH with the scan rate of 10−100 mV s−1. Both the Ipa and Ipc are linearly scaled with the square root of scan rate (ν1/2) from 10 to 100 mV s−1 (see the inset in Figure S3 in the Supporting Information).

Supporting Information), belonging, respectively, to the Cu 2p3/2 and Cu 2p1/2 orbits. The Cu 2p doublet splitting is observed to be 20.0 eV, confirming the existence of the Cu element as Cu2+ in CuS nanotubes.22 Furthermore, the weak satellite peak at 943 eV ensures the existence of a paramagnetic chemical state of Cu2+. The satellite peaks are also observed for the CuS composites at 933.8 and 953.73 eV, originating from multiple excitations, in which a valence electron was simultaneously excited when a core electron left the atom.23 The binding energy of S 2p in CuS nanostructures is observed at 161.2 eV (see Figure S2b in the Supporting Information).22 The Ni 2p spectrum (see Figure S2c in the Supporting Information) displays two distinctive peaks at 871.9 and 854.4 eV, ascribed, respectively, to Ni 2p1/2 and Ni 2p3/2. The spinenergy separation between Ni 2p1/2 and Ni 2p3/2 is found to be 17.5 eV, representing the valence state of +2 for Ni ion.24 The satellite peaks corresponding to Ni 2p3/2 and Ni 2p1/2 are also observed, respectively, at 859.9 and 878.1 eV. The peak fitted Mn 2p spectrum shows the binding energy peaks of Mn 2p1/2 and Mn 2p3/2, respectively, at 652.9 and 641.1 eV (see Figure S2d in the Supporting Information) with a spin-energy separation of 11.8 eV. 25 The O 1s signal of CuS@ Ni(OH)2(1:2) is observed at 530.1 eV (see Figure S2e in the Supporting Information), corresponding to the hydroxide (−OH) species.26 Figure S2f in the Supporting Information shows the O 1s peak as Mn−O−Mn (530.9 eV) and Mn−OH D


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Figure 4. (a) Amperometric responses of CuS@Ni(OH)2(1:2)/CC upon the successive injection of different concentrations of glucose at an applied potential of 0.56 V vs Ag/AgCl (inset: amperometric responses of CuS@Ni(OH)2/CC toward 1−150 μM concentration of glucose), (b) corresponding calibration plot of CuS@Ni(OH)2(1:2)/CC amperometric responses as a function of glucose concentration, and (c) interference study of CuS@Ni(OH)2(1:2)/CC with the successive addition of 0.25 mM interfering species and 2.5 mM of glucose in 0.1 M NaOH at an applied potential of 0.56 V vs Ag/AgCl.

Ni(OH)2 lies in the range of 3.3−3.7, specifying that few Ni sites in this structure possess NiO2-like character that may be seen as the tetravalent Ni sites.28 The under-coordinated Ni atoms serve as good electro-acceptors, and the active material surface may improve the adsorption of glucose via its electrondonor groups of hydroxyls. The unfilled d orbitals of Ni(OH)2 generate a bond with adsorbed analyte.14 Unique hierarchical nanostructures of CuS@Ni(OH)2(1:2) provide the shortened diffusion pathways, high surface energy, and large exposed active sites, facilitating maximum glucose electrooxidation at Cu3+/Ni3+ active centers of CuS@Ni(OH)2(1:2)/CC (Figure 3b). The mass ratio between the CuS and Ni(OH)2 in CuS@ Ni(OH)2 toward efficient GOR was optimized by measuring the CV responses of CuS@Ni(OH)2 nanostructures with the different composition ratios (see Figure S4 in the Supporting Information). The obtained results clearly illustrate that CuS@ Ni(OH)2(1:2) is the optimum ratio for efficient GOR performances. The influence of scan rate toward GOR is explored in 5 mM glucose/0.1 M NaOH solution at CuS@Ni(OH)2(1:2)/CC with the scan rate of 10−100 mV s−1. Both the Ipa and Ipc are gradually enhanced with an increase in the sweep rate (Figure 3c), and Epa and Epc are positively and negatively shifted, respectively, with increase in scan rate. It is further explored that Ipas are proportional to the square root of scan rate, denoting the diffusion-controlled process (inset in Figure 3c).

Figure 3a represents the CV responses of CC, CuS/CC, and CuS@Ni(OH)2(1:2)/CC in 5 mM glucose/0.1 M NaOH at 20 mV s−1. The inclusion of 5 mM glucose causes a negligible contribution at CC, representing the electrochemically inert behavior of bare CC toward GOR. On the contrary, CuS/CC exhibits irreversible glucose electrooxidation at 0.56 V vs Ag/ AgCl, and the GOR potential of CuS/CC closely resembled the Epa of Cu3+, implying that Cu3+ electrooxidizes the glucose into glucanolactone. The well-aligned CuS hollow nanotubular geometry provides a large active surface area, which is due to its reactive inner and outer surfaces that facilitate the effectual adsorption of an analyte. The voids between the CuS intraparticles stimulate desirable channels for rapid diffusion of sensing molecules toward active centers. After the adsorption process, the electro-active species accesses the active centers of CuS via a diffusion process. The diffused glucose is electrocatalytically oxidized at Cu3+/Cu2+ centers of CuS/CC. In the state of art CuS@Ni(OH)2(1:2)/CC, the Ipa is enhanced to 284 μA at 0.56 V vs Ag/AgCl, enumerating the synergistic effects of Ni(OH)2 and CuS. α-Ni(OH)2 is a layered double hydroxide, comprising Ni(OH)2‑x layers intercalated with various water molecules and anions. The turbostratic disorder generated from the mismatched symmetry of water molecules with Ni(OH)2‑x generates a number of structural defects, which inevitably enhance the reactivity of Ni(OH)2‑x nanostructures.27 The formal oxidation state of Ni in αE


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Figure 5. (a) CVs of (red line) CuS and (blue line) CuS@MnO2(1:2) in N2 (dashed line) and O2 saturated (solid line) 0.1 M NaOH at a scan rate of 10 mV s−1, (b) LSV curves of (red line) CuS and (blue line) CuS@MnO2(1:2) at a rotation rate of 1000 rpm with a scan rate of 10 mV s−1, (c) LSVs of CuS@MnO2(1:2) in O2 saturated 0.1 M KOH with a sweep rate of 10 mV s−1 at different rotation rates, and (d) K−L plots of ORR at CuS@MnO2(1:2) at different electrode potentials.

is established for the glucose concentration (1 μM−11 mM) (Figure 4b). Furthermore, CuS@Ni(OH)2(1:2)/CC exhibits a high sensitivity of 2171 μA mM−1 cm−2 toward GOR with a lower detection limit of 0.19 μM. Amperometric responses of CuS@Ni(OH)2(1:2)/CC are monitored by sequentially adding 2.5 mM glucose and 0.25 mM interference reagents including U, DA, AA, NaCl, CA, UA, Suc, Fru, Gal, Mal, AP, and Xyl at 0.56 V vs Ag/AgCl. As shown in Figure 4c, an obvious current response toward GOR is observed at CuS@Ni(OH)2(1:2)/CC, while insignificant responses are observed toward the other interfering species. Under the higher alkaline conditions (pH 13), CuS@ Ni(OH)2(1:2) demonstrates negative charges, owing to its isoelectric point of ≈12. On the other side, the highly electroactive UA and AA lose protons and exhibit negative charges under alkaline conditions.17 The negatively charged AA and UA molecules are inhibited to approach CuS@Ni(OH)2(1:2)/CC by the existence of the strong electrostatic repulsion forces between them, which validates the excellent selectivity of CuS@Ni(OH)2(1:2)/CC toward glucose. ORR Studies. CVs of fabricated GCEs are achieved under nitrogen (N2) or oxygen (O2) saturated 0.1 M NaOH solution at 10 mV s−1 (Figure 5a). Under N2 saturated solution, the variation in oxidation states of metallic active sites alone are observed, whereas the prominent reduction currents are observed under O2 saturated solution. CuS/GCE exhibits the considerable Ipc at −0.45 V vs Ag/AgCl, ascribed to the peculiar hollow tubular geometry of CuS nanostructures, facilitating the improved access of an analyte with the active sites of CuS. The molecular O2 with the O−O bond length of 0.12 nm31 is easily diffused and confined into the hollow

The diffusion controlled process of glucose oxidation at CuS@ Ni(OH)2(1:2)/CC is further confirmed with the log Ipa vs log ν plot (see Figure S5 in the Supporting Information). The plot of log Ip vs log ν (Figure S5) follows a linear correlation with the regression equation of log Ipa (μA) = 0.397 log ν (mV s−1) + 1.928 (R = 0.996). From a theoretical standpoint, a slope value of 0.5 represents a diffusion-controlled electrochemical process, while a slope value of 1.0 represents an adsorptioncontrolled process. Nevertheless, a slope between 0.5 and 1.0 demonstrates the control of both the diffusion and adsorption processes.29,30 The slope value of 0.396 is obtained for the glucose oxidation process at CuS@Ni(OH)2(1:2)/CC, specifying the diffusion-controlled process. The electrochemical responses of CuS@Ni(OH)2(1:2)/CC are assessed by the CV measurements with the addition of diversified glucose concentration under alkaline conditions (Figure 3d). With an increasing glucose concentration, the more evident enhancement is acquired in Ipa’s, whereas the Ipc’s responsible for the reduction of Cu3+/Ni3+ gradually become unrecognizable, which is due to the increased metallic active site utilization. The Ipa’s are gradually enhanced with an increase in glucose concentration, specifying the better electrocatalytic activity of CuS@Ni(OH)2(1:2) toward GOR in the absence of a fouling effect. The amperometric responses of CuS@Ni(OH)2(1:2)/CC toward GOR are investigated upon the consecutive addition of glucose with the different concentrations under alkaline conditions at 0.56 V vs Ag/AgCl (Figure 4a and the inset). Well-defined amperometric currents with an obvious stepwise current increment are observed with continually increased glucose concentration. It is also found that a linear relationship F


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Figure 6. (a) Polarization plots, (b) Nyquist plots, and (c) durability performances of (pink inverted triangle) CuS/CC(a|c)CC, (brown triangle) CuS/CC(a|c)CuS/CC, (blue star) CuS@Ni(OH)2/CC(a|c)CuS/CC, and (red circle) CuS@Ni(OH)2/CC(a|c)CuS@MnO2/CC equipped GFCs.

the effectual ORR was optimized by measuring the CV responses of CuS@MnO2 nanostructures with the different mass ratios (see Figure S6 in the Supporting Information). The obtained CV results clearly demonstrate that CuS@MnO2(1:2) is the optimum ratio for efficient ORR performance. The linear sweep voltammograms (LSV) of fabricated disk GCEs were evaluated in O2 saturated 0.1 M NaOH solution at 10 mV s−1 and a fixed rotation rate of 1000 rpm (Figure 5b). CuS@MnO2(1:2)/GCE demonstrates the substantial electrocatalytic activity toward ORR as supported from the positive onset potential and half-wave potential than that of CuS. The half-wave potential of CuS@MnO2(1:2) is also 125 mV higher than that of CuS (−0.475 vs Ag/AgCl). Furthermore, an improvement in the ORR limiting current density is examined at CuS@MnO2(1:2), elucidating the superior ORR performances of CuS@MnO2(1:2)/GCE. The RDE voltammograms at Cus@MnO2(1:2)/disk GCE with the different rotation rate from 200 to 1000 rpm at a scan rate of 10 mV s−1 are given in Figure 5c. The current densities are gradually enhanced with increased rotation rate at Cus@ MnO2(1:2)/GCE, owing to the better mass transport and shortened diffusion distance. The observed fine linearity and close parallelism of fitting lines at diverse potentials stipulate the first order reaction. The number of electrons (n) transferred per oxygen molecule in ORR was determined from Koutecky− Levich plots (K−L) (j−1 vs. w−1/2) at various electrode

interior cavity of CuS nanotubes via the periphery of tubular channels with an open end cavity size of 150 nm. The diffused O2 molecules are electrochemically reduced at the uniform active sites of CuS. The ORR process is further improved at CuS@MnO2(1:2)/GCE as substantiated from the positively shifted ORR potential of −0.36 V vs Ag/AgCl and higher Ipc’s than those of GCE and CuS/GCE. The Mn4+/Mn3+ redox couple serves as an electrochemical mediator and assists the effectual charge transfer to the molecular and adsorbed O2 during the ORR. The initial step of ORR on MnO 2 nanostructures involves the conversion of MnO2 to manganese oxyhydroxide (MnOOH) via a proton insertion process, which is significant in the formation of active Mn sites for O2 binding. The closely packed oxygen layers of MnO2 facilitate the proficient proton insertion process. The MnO2 nanoflakes exhibit the monoclinic γ-phase of MnO2, which is featured by 1 × 1 tunnels of pyrolusite and 2 × 1 tunnels of ramsdellite with the construction of [MnO6] octahedron units with the edges and corner sharing in its structural units.32 Owing to the dorbital coupling of Cu and Mn, the Gibbs free energy of electron transition for ORR process is significantly lowered, which promotes ORR kinetics. Consequently, CuS@MnO2 nanostructures dissociate the adsorbed O2, and the dissociative O migrates from Mn to Cu sites, which lowers the polarization effect toward ORR. The composition ratio between the core (CuS) and shell (MnO2) architecture of CuS@MnO2 toward G


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ACS Sustainable Chemistry & Engineering Table 1. Comparative Profile of GFC Performances anode PtRu/C/CPb PtAu/C/CPb Pt/Co/Ni Ag-modified Au film anode AuAg/MWCNTsc Au/MnO2/CPb Au/C Au/PANId Au/MWCNTs Au nanocoral/CPb Ni foam CuS@Ni(OH)2 (1:2)/CCe a

cathode activated charcoal activated charcoal

Pt/C activated charcoal Pt/C Pt/C Pt/C 10% Pt−C/CPb 10% Pt−C CuS@MnO2(1:2)/CCe

OCVa (V)

maximum power density (mW cm−2)

maximum current density (mA cm−2)

0.91 0.90 0.78 0.65 0.83 0.86 0.59 0.51 0.70 0.64 0.65 1.10

1.38 0.72 0.37 0.022f 1.6 1.1 0.5 0.18 0.28 0.847 0.62 1.25

∼3.9 ∼3.3 2.4 ∼0.078g 9.5 ∼4.0 ∼1.84 ∼0.82 ∼1.2 3.83 5.03 4.05

ref 35 7 36 6 37 38 39 40 41 42 this work

Open circuit voltage. bCarbon paper. cMultiwalled carbon nanotubes. dPolyaniline. eCarbon cloth. fmW cm−2 mM−1. gmA cm−2 mM−1.

Figure 7. (a) Schematic illustration of the constructed GFC stack and (b) polarization plots, and (c) durability performances of the CuS@Ni(OH)2/ CC(a|c)CuS@MnO2/CC equipped GFC stack.

Among the studied GFCs, CuS@Ni(OH)2(1:2)(a|c)CuS@ MnO2(1:2) demonstrates the maximum power density of 1.25 mW cm−2 under alkaline conditions. The covalent bond that exists between the Cu and S increases the ability of Cu atoms to form a Cu−Cu bond (due to the lower formal charge), resulting in the improved chemisorption of glucose molecules on the active metal surfaces.15 Furthermore, the existence of unpaired d-electrons of Ni3+ enhances the adsorption of glucose molecules via competent bond formation.33 The effectual interaction of Ni(OH)2 with CuS nanotubes facilitates the electron transportation between analyte and electrode, resulting in the improved GOR. Owing to the formation of CuS@MnO2 heterogeneous components, the crystal lattice vacancies promote the O2 adsorption, and the well-organized novel architecture provides the effectual interaction with the active species, leading to rapid ORR kinetics. The uniform catalytic active sites in CuS@Ni(OH)2 and CuS@MnO2 nanostructures

potentials (Figure 5d). The electron transfer number (n) for the aforesaid reaction at CuS@MnO2(1:2)/GCE is found to be 3.5, specifying that CuS@MnO2(1:2) demonstrates a 4electron pathway dominated ORR process. GFC Studies. The electrochemical influences of synthesized nanostructures toward GFC performances were evaluated for the batch-type GFCs equipped with bare CC as anode and cathode (CC(a||c), CuS/CC anode and bare CC cathode (CuS/CC(a|c)CC), CuS/CC anode and cathode (CuS/CC(a|| c)), CuS@Ni(OH)2(1:2)/CC anode and CuS/CC cathode (CuS@Ni(OH) 2 (1:2)/CC(a|c)CuS/CC), and CuS@Ni(OH)2(1:2)/CC anode and CuS@MnO2(1:2)/CC cathode (CuS@Ni(OH)2(1:2)/CC (a|c) CuS@MnO2(1:2)/CC) (Figure 6a). Owing to the inferior electrochemical activity of CC, CC(a||c) has not demonstrated the significant GFC power output, and the data are not shown. H


Research Article


applied pressure of 10 kg cm−2. Although the Ni mesh was used to increase the electrical conductivity of a carbon paper, the weaker interfacial contact between them disrupted the continuous electron transfer, and the delamination observed with the electrodes over time deteriorated the durability of the constructed GFCs.7,35 Apart from the decrement in the utilization amount of Pt catalyst in GFCs via the alloy of Pt with Co−Ni,36 any significant improvement in GFC power performance was not achieved. The systematic electrochemical analysis of Pt/Ni/Co toward GOR was also not realized by using cyclic voltammetry, which hinders the fundamental understanding of the GOR process at the prepared catalysts.36 Furthermore, the Pt based catalysts exhibited severe surfacepoisoning effects, which drastically limited the durability performances of GFCs.7,35,36 Although the GFC with Agmodified Au film as an anode material demonstrated an appreciable power performance, the morphological properties of prepared nanostructures and their influences toward GFC power and durability performances were not detailed, which has not authenticated the reliability of the constructed system.6 The durability performances of GFCs equipped with Au−Ag/ MWCNTs,37 Au/MnO2,38 Au/C,39 and Au/PANI39 anode catalysts were not analyzed, and the lifespan of Au/ MWCNTs40 and Au nanocoral41 were inferior, which hindered the scale-up processes of GFCs. Furthermore, the aforementioned precious metal nanocatalysts significantly increased the cost of an entire GFC device, which faded the utilization of GFCs in large scale applications.6,7,35−41 Although the metal nanocatalyst free Ni-foam was demonstrated as an anode material in GFCs,42 the lack of efficient active sites and limited electrical conductivity of Ni foam prohibited its maximum GFC performance. In this article, CuS and its core−shell nanostructures were synthesized via simple, facile, and cost and time efficient preparation techniques, which effectively overwhelmed the preparation disadvantages of nanocatalysts associated with previous GFC reports. This is the first ever report of the utilization of CuS nanotubes as a bifunctional catalyst in GFCs. The catalytic activities of CuS were increased via the core−shell formation of CuS with Ni(OH)2 and MnO2, respectively. The systematic electrochemical analysis achieved on the prepared nanostructures detailed the exact mechanisms involved in GOR and ORR processes, which increases further research opportunities in this area. With the combinative efforts of enhanced GOR and ORR kinetics, GFCs with CuS@ Ni(OH)2(1:2)/CC(a|c)CuS@MnO2(1:2)/CC exhibited the maximum GFC power-output and durability performances than those of other reported GFCs. The original concept of a GFC stack addressed in this article broadens the horizons in demonstrating its feasibility for powering the portable electronics. The life span of implantable pacemakers conventionally operated with batteries is limited, owing to the drainage of batteries over time. It compels the replacement or recharge of batteries via pacemaker implants, leading to unavoidable bleeding and infections, patient discomfort, and painful experiences, and is not economical to the health care system, which could be effectively avoided with the utilization of GFCs.43 The ubiquitous availability of glucose (concentration in the range of 3−5 mM) and oxygen (concentration in the range of 45 μM) in the body fluids (e.g., blood and interstitial fluid)44 can be taken advantage of for the sustainable supply of fuels for GFCs. It can afford the patient a nearly lifetime power source to drive implantable medical devices without any

effectually oxidize and reduce the glucose and O2, respectively, which collectively maximizes the GFC performances. The electrochemical impedance spectroscopy (EIS) properties of constructed GFCs were evaluated with 300 kHz−0.001 kHz frequency under open circuit conditions (Figure 6b). Among the studied GFCs, CuS@Ni(OH)2(1:2)(a|c)CuS@ MnO2(1:2) demonstrates a lower charge resistance (Rct) of 85 Ω that is minimal over the CuS(a|c)CuS (206 Ω), CuS(a| c)CuS (660 Ω), and CuS@Ni(OH)2(1:2)(a|c)CuS (893 Ω). The improved electronic transport is facilitated through the core and shell components of CuS@Ni(OH)2, which reduces the Rct of an entire system. It significantly governs the effectual oxidation and reduction of the fuel and oxidant, respectively, and facilitates an optimal GFC performance of CuS@ Ni(OH)2(1:2)(a|c)CuS@MnO2(1:2). The durability performances of constructed GFCs were continuously monitored by measuring the voltages in a quiescent state at an external resistance, which generated the maximum power output (Figure 6c). When the volatges of the studied GFCs dropped to 0.05 V, the fresh anolyte and catholyte were replaced. CuS@Ni(OH)2(1:2)/CC(a|c)CuS@ MnO2(1:2)/CC demonstrates excellent charge and discharge processes than those of other studied GFCs (Figure 6c). The effectual combination of S with Cu promotes the structural stability of CuS under an alkaline environment.34 The strong interaction of CuS with Ni(OH)2/MnO2 stabilizes the nanostructures, preserving intact tubular morphology and avoids particles drifting and agglomeration under harsh fuel cell environments. CuS@Ni(OH) 2 (1:2)/CC(a|c)CuS@ MnO2(1:2)/CC facilitates diffusion and transportation of an electrolyte and limits volume expansion. By the effectual core− shell composite concept, CuS@Ni(OH)2(1:2)/CC(a|c)CuS@ MnO2(1:2)/CC exhibits concrete durability for more than 300 h without any major performance loss. In comparison with the previous GFC reports (Table 1), CuS@Ni(OH)2(1:2)(a| c)CuS@MnO2(1:2) equipped GFC reported in this study exhibits excellent power generation and durability performance, which authenticates its viable applications in high performance GFCs. GFC Stack Performance. The GFC stack was constructed with the aid of acrylic glass, and anode/cathode compartments were evenly divided into 10 units with the capacity of 10 mL for each unit. The fabricated anode and cathode compartments were connected with the aid of an AMI-7001 anionic exchange membrane and gasket (Figure 7a). The identically fabricated CuS@Ni(OH)2(1:2)/CC and CuS@MnO2(1:2)/CC were placed, respectively, in anode and cathode compartments, and the electrodes were connected in parallel circuit mode. The anolyte and catholyte used in a single GFC cell were filled in the respective compartments. The GFC stack connected with 10 units demonstrates the maximum power density of 9.8 mW cm−2 (Figure 7b), which is 7.8-fold higher than those of absolute power density of the relevant GFC single cell. It clearly specifies that the major power loss is not obtained in the GFC stack. Furthermore, the constructed GFC stack exhibits good durability performance (Figure 7c), which is comparable to the durability performance of a single cell, illustrating the excellent energetic capability and stability of the constructed GFC stack. Basu et al. constructed GFCs with Pt−Au/C35 or Pt−Ru/C7 and activated charcoal as GOR and ORR catalysts, respectively, and observed significant GFC power performances. However, the fabrication of aforementioned anodes was achieved by hot pressing the catalyst coated carbon paper on a Ni mesh at an I



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ACKNOWLEDGMENTS This study was supported by the Science and Engineering Research Board (SERB), New Delhi, India, with Major Project Grant No. EMR/2015/000912.

necessity of battery replacement. The presence of significant level of glucose in human sweat (30 and 100 μM)45 can offer an opportunity to develop GFC based skin-worn energy devices. The extracted energy could be used to power smart phone connected miniature body-interface devices and low powered electronics including light-emitting diodes (LEDs), wireless sensors, smart watches, and bluetooth radios. The endorsement of flexible electrodes and stretchable electronic foundations can bring forth the development of GFCs for powering flexible smart phones and body-bound personal wearable health monitoring devices with the ability to withstand harsh deformation and bent conditions. The photosynthetic processes of trees involve the conversion of atmospheric carbon dioxide (CO2) to sugar (glucose) and O2,46 which can be used as natural fuel sources for GFCs. The implantation of GFCs into living trees could increase the possibilities of direct and continuous green energy generation from photosynthesis processes. The harvested sustainable energy from trees can be used to power street lights and environmental monitoring sensors.

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CONCLUSIONS In summary, a convenient and cost-efficient strategy has been developed to realize the influences of core−shell composites toward GFC performances operated under room temperature. The electrochemical properties substantiated the bifunctional characteristics of CuS nanotubes toward GOR and ORR, and their performances were improved with the explicit shell engineering of Ni(OH)2 and MnO2, respectively, with the CuS nanotubes. By the combined efforts of improved GOR and ORR kinetics, CuS@Ni(OH) 2 (1:2)/CC(a|c)CuS@ MnO2(1:2)/CC exhibited a maximum GFC power density, which is superior among the reports filed on GFCs. The strong interaction between the core and shell matrix protected the tubular morphology and increased their resistivity toward the surface poisoning of chemisorbed intermediates and guaranteed its analytical stability for more than 300 h. The self-designed GFC stack remarkably harvested the clean energy associated with long durability without any performance losses from the single cells. With the build up of rapid GOR and ORR kinetics at the specifically engineered nonprecious catalytic nanostructures, this article provides a versatile avenue for high performance and durable GFCs, paving the futuristic dimensions for its large scale applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04326. GFC single cell schematic, high-resolution XPS spectra, CV responses, and calibration plot (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel:91-9585752997. E-mail: [email protected]. ORCID

G. Gnana kumar: 0000-0001-7011-3498 Notes

The authors declare no competing financial interest. J


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