Cow Dung Derived PdNPs@WO3

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9735-9748

pubs.acs.org/journal/ascecg

Cow Dung Derived PdNPs@WO3 Porous Carbon Nanodiscs as Trifunctional Catalysts for Design of Zinc−Air Batteries and Overall Water Splitting Raksha Choudhary,† Santanu Patra,† Rashmi Madhuri,*,† and Prashant K. Sharma‡ †

Department of Applied Chemistry, and ‡Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India S Supporting Information *

ABSTRACT: The main motif of this work is to fabricate a highly efficient, economic, nanodisc shaped trifunctional electrocatalyst using a tungsten trioxide modified carbon nanosheet decorated with palladium nanoparticles. The beauty of this work is that a special carbon precursor is used for the synthesis of the electrocatalyst, a waste material, i.e., cow dung. The performance of the cow dung derived nanodisc electrocatalyst (Pd@WO3-NDs) toward oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) is compared with three other electrocatalysts (derived from graphene oxide, chitosan, and graphite carbon sources) also, and it is found that Pd@ WO3-NDs show superior performance over that of the other three. The electrocatalyst exhibits the lowest onset potential (1.32 V vs NHEs), highest current density (492 mA cm−2), lowest overpotential (113 mV), and lowest Tafel slope (62.8 mV dec−1) for OER; an onset potential of 1.02 V, overpotential of 195.0 mV, and Tafel slope of 53.1 mV dec−1) for ORR; and lowest onset potential (−0.09 V), overpotential (185 mV at 10 mA cm−2), and a small Tafel slope of (58.2 mV dec−1) for HER in the same alkaline solution. In addition, the nanomaterial is successfully applied for the fabrication of rechargeable and all-solid-state zinc−air batteries, which are used to illuminate a 4.0 V light emitting diode (LED) bulb. More importantly, real air cathodes made from the trifunctional Pd@WO3-NDs demonstrated superior performance to state-of-the-art Pt/C catalysts in rechargeable zinc−air batteries. In addition, the same Zn−air battery is further used to power the laboratory-made total alkaline water electrolyzer by employing Pd@WO3-NDs as catalyst on both anode and cathode. The water electrolyzer showed comparable performance rivalling the state-of-art combination of Pt/C and RuO2, which is known to be the best of the bifunctional total-water splitting electrocatalysts reported until date. This remarkable performance of Pd@WO3-NDs indicates their future potential in energy storage and sustainable energy conversion technologies. KEYWORDS: Cow dung derived nanodics, WO3, PdNPs, Trifunctional electrocatalyst, Rechargeable and all-solid-state Zn−air battery, Water-splitting unit

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INTRODUCTION

effective rechargeable metal−air batteries using catalysts having a single property like better oxygen evolution reaction (OER) performance or oxygen reduction reaction (ORR) performance is not suitable, and we require bifunctional catalysts with low overpotential for both the OER and ORR.4 On the other hand, bifunctional electrocatalysts for OER and hydrogen evolution reaction (HER) in the same electrolyte is of practical value to accomplish overall water splitting. To date, Ir/Ru-based compounds have been recognized as the most effective OER electrocatalysts, while Pt-group metals have established their benchmark behavior as HER catalysts.5 However, their high cost, slow ORR performance, and

In this modern technological word, we are dependent on electronic gadgets and digital accessories for every task; for example, mobile phones, laptops, TV, watches, games, etc. Without electronic gadgets we cannot imagine our life for a single second. For our daily work, we need gadgets, and they require power and energy for their operation.1 Therefore, day by day the demand for energy is increasing, and we need new, low cost energy sources as well as energy storage devices with more reliability and improved efficiency.2 In recent years, watersplitting has gained the interest of a lot of researchers and become popular as an eco-friendly and renewable fuel source.3 However, metal−air batteries and fuel cells have attracted the interest of researchers as a possible energy storage/conversion solution. In order to design both of the systems, knowing the role of the catalysts is very important. Likewise, design of © 2017 American Chemical Society

Received: May 16, 2017 Revised: September 1, 2017 Published: September 29, 2017 9735

DOI: 10.1021/acssuschemeng.7b01541 ACS Sustainable Chem. Eng. 2017, 5, 9735−9748

Research Article

ACS Sustainable Chemistry & Engineering

reported in the literature. The only problem associated with such nanohybrid materials is poor integration of nanoparticles at the carbon surface. The weak catalyst−support interaction results in easy detachment, aggregation, and even dissolution of catalyst nanoparticles leading to their degraded performance. Therefore, in order to achieve good catalytic performance, a strong anchoring of catalyst nanoparticles (NPs) on the carbon supports is necessary. In the present work, for the first time, we have designed a disc-shaped trifunctional catalyst via combination of noble metal nanoparticles and TMDC decorated carbon nanosheets in an ecofriendly and economic way. First, we have prepared the combination of different origin carbon precursors (i.e., chitosan, cow dung, graphene oxide, and graphite) and TMDCs via a hydrothermal route, which later on were decorated with PdNPs using lime juice as a reducing agent. The synthesized carbon-based nanocomposites were used to explore their electrocatalytic activity toward ORR, OER, and HER. Among the four nanocomposites, the cow dung derived PdNPs@WO3 carbon nanodiscs (Pd@WO3-NDs) win the race and show better performance for all three reactions (OER, ORR, and HER) in comparison to their other colleagues. As a multifunctional electrocatalyst, the Pd@WO3-NDs exhibited an overpotential of 195 mV at current density of 10 mA cm−2 with the lowest Tafel slope of 53.1 mV dec−1 for the oxygen reduction reaction. Similarly, the low overpotential values for OER (113 mV) and HER (185 mV) with high current densities further supported their trifunctional behavior. In addition, to explore the practical applicability, the role of Pd@WO3-NDs was also explored in water splitting as well as metal−air battery fabrication. The overall study clearly shows that the proposed nanocomposite has the potential to be used in the development of storage technologies and sustainable energy conversion.

methanol oxidation, as well as carbon monoxide poisoning, have restricted their practical applications as bifunctional catalysts. Currently, available ORR and OER electrocatalysts often show better performance in alkaline medium than acidic electrolyte.6 On the other hand, due to the inefficient dissociation of water to initiate the Volmer reaction in alkaline electrolytes, most HER electrocatalysts exhibit better performance in an acidic medium than alkaline electrolyte, which causes major problems when coupling HER and OER catalysts. Therefore, it is necessary to discover multifunctional electrocatalysts, which are stable enough to stand strongly in both strongly oxidizing and strongly reducing environments experienced during OER and ORR. Such problems are not limited to OER/ORR but exist in HER also; researchers are searching for alternative catalysts which have low cost or are made up of materials other than noble metals. In other words, development/design of multifunctional electrocatalysts with high activity and low cost has become a great challenge for the practical applications of related energy technologies.7 While searching for multifunctional electrocatalysts, the major problem faced by researchers is the requirement of different conditions for proper operation and functionality of OER, ORR, or HER, which eventually decreases the number of catalysts capable of performing all three reactions. Therefore, it is highly desirable to develop lowcost, earth-abundant, trifunctional electrocatalysts to promote ORR, OER, and HER simultaneously under the same pH environment. If this idea can be realized, it will reduce material and processing costs to large extent. Recently, it has been demonstrated that transition metal dichalcogenides (TMDCs), such as molybdenum sulfide (MoS2), tungsten disulfide (WS2), and molybdenum selenide (MoSe2), etc. in combination with other metals, metal oxides, or metal sulfides are used as suitable multifunctional electrocatalysts. For example, in our laboratory, we have prepared molybdenum/tungsten diselenide and cadmium disulfide based nanohybrid quantum dots, which shows bifunctional behavior with high catalytic activity toward OER and ORR.8 In another work, we have developed a novel, easy, and one-step synthesis of anisotropic MoSe2/MoO3 and WSe2/WO3 nanohybrids as a trifunctional catalyst for HER, OER, and ORR.9 Similarly, Zhang et al. have prepared Co/CoO nanoparticles immobilized on Co−N-doped carbon as a trifunctional electrocatalyst for ORR, OER, and HER.10 Another group has also developed nitrogen, phosphorus, and fluorine tridoped graphene which was obtained by thermal activation and used as a trifunctional catalyst for OER, ORR, and HER.11 In addition to these nonprecious metal catalysts, nowadays, carbon-based metal-free catalysts have also gained much popularity in recent years as they possess combined advantages of low cost, high efficiecy, long lifetime, and multifunctionality.12 In order to further improve the properties of carbonaceous materials in the field of energy conversion and storage, their combination with metal nanoparticles has also gained a lot of popularity. For example, Dai et al. reported N, S codoped graphitic sheets with a unique hierarchical structure consisting of stereoscopic holes over the graphitic surface which act as a trifunctional catalyst.13 Another group has developed a multiphase Fe anchoring on hierarchical N-doped graphitic carbon as a trifunctional catalyst.14 However, to the best of our knowledge, research on the synergetic effects of noble metals, carbon material, and metal oxide as tri- or bifunctional catalysts toward a combination of HER, OER, and ORR is very rarely

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EXPERIMENTAL SECTION

Reagent, instrumentation, and electrochemical measurement details are given in the Supporting Information (section S1). Ecofriendly Synthesis of Pd Nanoparticle Decorated WO3 Carbon Sheets. The electrocatalyst was prepared by a two-step process reported elsewhere, after slight modifications.15 First, the WO3 carbon sheet was prepared using four different origin carbon precursors, i.e., chitosan, graphite powder, graphene oxide, and cow dung (freshly collected from a local dairy). Prior to the use, the fresh cow dung was properly dried in sun, cleaned with water and alcohol, and carbonized at 400 °C for 2 h. After that, 100.0 mg of cow dung was mixed with KOH and a minimum volume of water with continuous stirring at 80 °C to make a homogeneous slurry. The slurry was transferred to the silica crucible and pyrolyzed at 800 °C for 2 h. The obtained product was first washed with 1.0 M HF to remove inorganic impurity from the prepare nanomaterial16 and then rinsed with distilled water, until the solution pH became neutral. The resultant product (cow dung powder) was dried overnight at 80 °C and used further. For the preparation of WO3 carbon nanosheets, 100.0 mg of graphite/chitosan/graphene oxide or cow dung powder was dispersed in distilled water by ultrasonic waves (for 1 h), followed by the addition of 0.01 g of NaWO4·2H2O. The mixture was further sonicated for 45 min, placed in a furnace at 600 °C for 4 h, and then allowed to cool at room temperature. After that, the calcined product was crushed into a fine powder, washed with water and ethanol (10 times), and stored in a vacuum desiccator. Then, Pd nanoparticles were introduced into WO3-C by the reduction of PdCl2 solution using lemon juice. First, 0.01 g of PdCl2 was dispersed in 20.0 mL distilled water by ultrasonication for 30 min. To this, fresh lemon juice (10.0 mL in 5.0 mL HCl) was added in a dropwise manner with constant stirring. The whole mixture was 9736


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Scheme 1. Fabrication of a Zn−Air Button Cell along with Photographs of Cells in Use for Illumination of a 4.0 V LED Bulb

Figure 1. XRD of (A) WO3-Chi and Pd@WO3-Chi, (B) WO3-GO and Pd@WO3-GO, (C) WO3-Gra and Pd@WO3-Gra, and (D) WO3-CNS and Pd@WO3-CD. Deconvulated XPS spectra of elements present in Pd@WO3-NDs: (E) W 4f, (F) O 1s, (G) Pd 3d, (H) C 1s, and (I) N 1s. The as prepared nanomaterials were labeled as Pd@WO3-Chi, Pd@ WO3-GO, Pd@WO3-Gra, and Pd@WO3-NDs depending on the carbon precursors used for their preparation, i.e. chitosan, graphene oxide, graphite powder, and cow dung, respectively.

further stirred for 30 min at room temperature and then kept under domestic microwave at 300 W for 30 min. The resultant material was centrifuged at 10 000 rpm, washed with ethanol and distilled water (several times) and dried in a vacuum oven for overnight at 50 °C. 9737


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Figure 2. Elemental mapping of Pd@WO3-NDs for the following: (A) C, (B) N, (C) O, (D) W, and (E) Pd. (F) FE-SEM image of Pd@WO3-NDs at higher magnification. (G) AFM image of Pd@WO3-NDs. TEM image of (H) WO3-CD (WO3 nanoparticles encircled in yellow color) and (I) Pd@WO3-NDs. (inset) Magnified single particle and SAED pattern of nanodics.

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Real-Time Applications. Fabrication of Rechargeable Zinc−Air Battery in Aqueous Phase. Here, the air cathode was prepared by coating the prepared catalyst on the carbon tape. The catalysts (12.0 mg) were dispersed in 1.0 mL of ethanol, and the resulting mixture was sonicated to form a homogeneous ink. The 250.0 μL ink was carefully dropped onto the above air cathode and kept in a vacuum container for 30 min, followed by a mild pressing procedure. The prepared air cathode with the loading of 1.0 mg cm−2 was used to assemble a rechargeable zinc−air battery. A zinc plate was used as the anode, and 0.2 M ZnCl2 in 6.0 M KOH was used as the electrolyte. Fabrication of Solid-Phase Zinc−Air Battery. For all-solid-state zinc−air battery fabrication, first, the zinc container was collected from the used battery, washed several time with ethanol, and used after vacuum drying (Scheme 1). In the container, 0.3 mL of polymer gel electrolyte [mixture of poly(vinyl alcohol) (PVA), 6.0 M KOH, and zinc acetate] was added, and it was placed in a refrigerator to make a homogeneous, stable, and thin membrane of electrolyte. The procedure was repeated twice to gelate the PVA robustly. After that the air cathode was placed above the assembly and pressed sufficiently to obtain a solid-phase zinc−air battery. Water Splitting Powered by Zinc−Air Button Cell. The trifunctional catalysts were further used in a laboratory-made water-splitting unit (for H2 and O2 generation) powered by the above-prepared Zn− air battery (Scheme S1). Here, both anode and cathode were fabricated using the same catalyst, i.e., Pd@WO3-NDs. Furthermore, to measure the amount of generated H2 and O2, after electrochemical water-splitting, the water displacement method was used (Scheme S2). For this, a water displacing unit was prepared in our laboratory using a graduated 1.0 mL syringe and the amount of collected gas was measured by the displacement of water from the syringe.

RESULTS AND DISCUSSION Characterization of Electrocatalysts. Surface Group and Composition Analysis of Electrocatalysts. For the compositional characterization of prepared nanocomposites, powder XRD analysis was performed. The powder-XRD pattern of the WO3-carbon sheets (derived from different origin carbon precursors) and Pd decorated WO3-carbon sheets (i.e., Pd@WO3-Chi, Pd@WO3-GO, Pd@WO3-Gra, and Pd@ WO3-NDs) are shown in Figure 1. In the XRD pattern of WO3Chi, the characteristic peaks of chitosan at ∼25° and ∼10° (broad) are clearly visible (Figure 1A).17 The other peaks present at 22.64, 24.68, 33.42, 34.88, 38.62, 49.37, 53.18, 56.44, 60.15, 62.23, and 72.42 can be assigned to the (001), (200), (201), (220), (310), (400), (202), (420), (421), (312), and (332) planes of tetragonal tungsten oxide (JCPDS file no 891287), which confirms the successful integration of WO3 with chitosan sheets. In contrast to the XRD pattern of WO3-Chi, the XRD pattern of WO3-Gra shows a single and small peak at around 26° corresponding to the presence of graphitic structure (002) (Figure 1B). However, the XRD pattern of WO3-GO i.e. after oxidation, the (002) peak of graphite powder disappears and an additional peak at 10.6° is observed, which is corresponding to the (001) diffraction peak of graphite oxide. In addition, the other peaks for WO3 is clearly visible in both nanocomposites (WO3-Gra and WO3-GO), supports their successful synthesis (Figure 1C). Other than these, WO3-CD (cow dung) shows the characteristic peaks of WO3 with a hump at around 20−30°, which can be ascribed to the (002) plane of 9738


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Figure 3. FE-SEM image of WO3-Chit (A), WO3-GO (B), WO3-Gra (C), WO3-CD (D: lower and E: higher magnifications), Pd@WO3-Chi (F), Pd@WO3-GO (G), Pd@WO3-Gra (H), and Pd@WO3-NDs (I).

tungsten atoms which suggests the presence of WO3.18 The XPS spectrum of O 1s is shown in Figure 1F and can be separated into two peaks. The dominant peak centered at 530.5 eV can be assigned to the oxygen atom forming the strong W O bond, and the peak at 531.3 eV is associated with the O2− ions in the oxygen-deficient regions within the matrix of WO3.19 In Figure 1G, the high-resolution XPS spectrum of the Pd shows double peaks with binding energies at 335.9 and 341.2 eV, corresponding to Pd 3d5/2 and Pd 3d3/2, respectively, which are in good agreement with the expected values for Pd(0).20 In addition, a small hump appeared at around 337 eV in the XPS spectrum of PdNPs. The hump could be assigned to the presence of Pd2+.21 The existence of Pd2+ in the composite can be ascribed to the following two reasons: (1) impurity of precursor or (2) reduction of Pd2+ may not be fully completed.21 The high-resolution spectrum of C 1s (Figure 1H) can be fitted into four peaks at 284.5, 285.4, 286.4, and 288.5 eV, which would respectively correspond to C−C, CN (N−sp2 C), C−N (N−sp3 C), and CO bonds.22 The high-resolution XPS for N 1s peak (Figure 1I) reveals the presence of pyrrolic-N (∼399.9 eV) and N−H (∼401.1 eV) groups within the resultant carbonaceous structure.23 The data were further supported by their elemental analysis obtained by

graphitic layer and confirmed the successful synthesis of carbon sheets from waste product, i.e., cow dung (Figure 1D).16 After decoration with PdNPs, the WO3-carbon nanomaterials were also characterized via XRD analysis (Figure 1). In each material, four extra peaks were identified at approximately the same position i.e. 40.11°, 46.51°, 68.24°, and 82.12° and assigned to the (111), (200), (220), and (311) plane of palladium(0) according to the JCPDS file no 88-2335. The XRD study clearly supports the synthesis of PdNPs decorated tetragonal phase WO3 modified carbon sheets. To investigate the surface composition and chemical states of the elements in the nanomaterials, XPS spectra of the Pd@ WO3-Chi, Pd@WO3-GO, Pd@WO3-Gra, and Pd@WO3-NDs were also recorded. The full scan survey spectra in Figure S1 indicates that the nanomaterial is composed of tungsten, palladium, carbon, and oxygen elements. In contrast to the other three, the presence of the nitrogen element was also identified in the XPS spectrum of Pd@WO3-NDs. Afterward, the high-resolution spectra of different elements present in Pd@WO3-NDs was also studied and shown in Figure 1E−I. The high-resolution spectra of W 4f showed peaks at 35.85 and 38.02 eV, which are assigned to W 4f7/2 and W 4f5/2, respectively, and correspond to the oxidation state +6 of 9739


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originate from the polycrystalline state are by the greater number of crystallites attached to the surface of the single particles (Figure 2I, inset). The bright ring pattern showed the high density of crystallites in the material. Surface Area Analysis. High surface area and porous structure are the main characteristic of any catalyst. So, the nitrogen desorption−adsorption evaluation was done for all the prepared nanocomposites (Table S1). As displayed in the table, the PdNP decorated WO3-C possesses a higher surface area than nondecorated one, which clearly supports the role of PdNPs over the morphology and behavior of prepared nanomaterials. However, among the four nanomaterials prepared in this work, Pd@WO3-NDs (389 m2 g−1) shows a higher surface area than other three, which may be attributed to the disclike morphology of the prepared material. In addition, their pore volumes were also calculated which are also found better for nanodisc shaped nanomaterials, i.e., Pd@WO3-NDs. Electrochemical Surface Area. In addition to the BET surface area, the electrochemical surface area and roughness factor of the nanomaterials were also calculated using potassium ferrocyanide as an electrochemical probe molecule. For this, 8.0 mM of ferrocyanide solution was added to an electrochemical cell containing 1.0 M KCl as the supporting electrolyte and cyclic voltammetric (CV) runs were recorded (Figure S3). It was observed that bare pencil graphite electrode (PGE) (nonmodified) shows minimum current response for ferrocynide, which get increased after modification of PGE with different nanomaterials. First, the current response obtained with WO3-C modified PGEs were compared and found that WO3-CD modified PGE shows better performance than other three, i.e., WO3-GO, WO3-Gra, and WO3-Chi. Similar results were observed, when the current response of Pd@WO3-C were compared with Pd@WO3-Chi, Pd@WO3-GO, and Pd@WO3Gra. Pd@WO3-NDs shows 1.15, 1.35, and 1.60 fold higher current than other three. Using the CV plots, the roughness factors and electroactive surface area of nanomaterial modified electrodes were evaluated by the Randles−Sevcik equation:24

scanning electron microscopy (SEM) energy-dispersive X-ray (EDAX) spectroscopy. The EDAX spectra of WO3-carbon sheet and Pd@WO3-NDs are displayed in Figure S2. In addition, the elemental mapping was also performed and shown in Figure 2A−E. EDAX spectra clearly show the presence of W, O, and C in WO3-carbon sheet, and W, O, C, and Pd in Pd@ WO3-NDs. The element percentages of Pd, W, C, and O were found to be 2.37, 53, 16.17, and 28.49%, respectively. In addition, the elemental percentages of other prepared catalysts are shown in Figure S1B. However, the presence of nitrogen in the Pd@WO3-NDs was confirmed, after elemental mapping study. Taken together, the XRD, XPS, and elemental mapping studies clearly show the successful formation of PdNPs decorated WO3 modified carbon sheets. Morphological Analysis of Nanocomposite. Figure 3 presents the FE-SEM images of WO3-modified carbon sheets. As displayed in the figure, WO3-Chit and WO3-GO clearly possess their characteristic layered crumbled sheetlike structure (Figure 3A and B, respectively). WO3-Gra has large flakes or platelike morphology, which is entirely different than that of WO3-GO (Figure 3C). Other than these, at low magnification, WO3-CD looks like a group of small and thin flakes, but at high magnification its aggregated flake structure is clearly visible (Figure 3D and E). When the WO3-C was modified with PdNPs, the Pd@WO3-Chi, Pd@WO3-GO, and Pd@WO3-Gra nanomaterials do not show any structural or morphological changes; only some spherical nanoparticles can be visualized at their surfaces (Figure 3F, G, and H). In contrast to these, the flakelike morphology of WO3-CD gets changed to a disclike structure, after modification with PdNPs (Figure 3I). The nanodisclike structure of Pd@WO3-NDs can be easily visualized in the images, where each disc has diameter of around 20−30 nm. At higher magnification, it was observed that edges of the discs were well decorated with sphericalshaped nanoparticles that possibly can be attributed to the PdNP arrangement over the nanodiscs (Figure 2F). From the highly magnified FE-SEM image, the size of PdNPs was estimated to be less than 5 nm. Atomic force microscopy (AFM) was also used to determine the average roughness of the prepared Pd@WO3-NDs (Figure 2G). As shown in the image, the morphology of Pd@WO3-NDs has numerous troop and grooves with a maximum height of 30.0 nm. Transmission electron microscopy (TEM) was used to analyze the morphological aspects of the WO3-CD and Pd@ WO3-NDs (Figure 2H and I, respectively). In the TEM image of WO3 decorated carbon nanosheet derived from cow dung, the nanoparticles decorated monodisperse layer morphology of carbon sheet is clearly visible. The WO3 nanoparticles (encircled in yellow color) are shown in the TEM image. TEM micrographs showed that WO3 nanoparticles were successfully attached on the carbon sheet, and no nanoparticles were observed detached from GO sheets. After modification with PdNPs, the observed nanodisc shaped nanocomposite shows the average mean size of 20−30 nm (Figure 2I). In order to exhibit the PdNPs decoration over the nanodisc shaped catalyst, a single particle was magnified and shown in the inset of Figure 2I. The nanodisc clearly exhibited a two layered morphology in which the outer layer could be attributed to the presence of PdNPs. The TEM image shown in the inset of Figure 2I clearly shows the core−shell structures with different contrasts. The selected area electron diffraction (SAED) analysis showed that those continuous ring patterns which

Ip = (2.687 × 105)η3/2 ϑ1/2D1/2AC 0

(1)

Where, Ip denotes the CV current obtained for ferrocyanide on different electrodes, n is the number of electrons transfer, v denotes scan rate, D is diffusion coefficient of ferrocyanide = 0.76 × 10−5 cm2 s−1,24 A is the surface area, and C0 is the concentration of ferrocyanide used. Further, the roughness factor (Rf) was calculated as Rf = A/Ageom. Here A is the obtained surface area from above equation and Ageom = geometric surface area (0.082 cm2) of the electrode. The calculated values of surface area and roughness factor were displayed in Table S1, where it was found that Pd@WO3-NDs shows highest electrochemical surface area (0.32 cm2) and roughness factor (3.90). The higher the surface area, the better the current response, and the good roughness factor value of Pd@WO3-NDs modified PGEs could be attributed to the specific shape and morphology of the nanomaterial. In addition, the N-doping present in the carbon sheet may also involve in the higher current response of the nanomaterial, which is absent in other three.25 The performance of cow dung derived Pd@WO3-NDs was attributed to high surface area along with optimum amount of micropores and mesopores in the material. While micropores contribute to stability of nanodisc, mesopores allow efficient movement of aqueous electrolyte and thus to reach maximum surface area. Biomass can 9740


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Figure 4. (A) CV, (B) LSV, and (C) Tafel slope plots for OER in 1.0 M KOH. (D) CV, (E) LSV, and (F) Tafel slope plots for ORR in 1.0 M KOH. (G) CV, (H) LSV, and (I) Tafel slope plots for HER in 0.5 M H2SO4.

were first evaluated by recording the CV runs in 1.0 M KOH. All the nanomaterials show almost similar behavior, however their current density was varied from material to material (Figure 4A). The maximum current density of 492.0 mA cm−2 was observed for Pd@WO3-NDs. Similarly, the OER activity of Pd@WO3-NDs was also compared with that of RuO2 (20 wt %), Pd@WO3-Chi, Pd@WO3-GO, and Pd@WO3-Gra by recording the LSV plots. As shown in Figure 4B and Figure S7, the Pd@WO3-NDs exhibited a low onset potential of 1.32 V (vs NHE), lower than the values reported for commercial RuO2 catalyst (1.50 V vs RHE)26 with high current density. It is worth mentioning that the current density of 10 mA cm−2 is widely used as an essential parameter to evaluate the OER activity of the catalysts. Here, to attain the current density of 10 mA cm−2, the prepared catalyst Pd@WO3-NDs required a very small overpotential (η) of 113 mV, which is even lower than that of the commercial RuO2 catalyst (370 mV) and IrO2 catalyst (390 mV).27 However, to attain the same current density Pd@WO3-GO, Pd@WO3-Chi, and Pd@WO3-Gra required higher over potentials than that of Pd@WO3-NDs (Figure 4B). As given in Figure 4C, the OER kinetics of the above catalysts was also estimated by their corresponding Tafel slopes. The Tafel slope of Pd@WO3-NDs was only 62.8 mV

significantly reduce the overall production cost and simplify the process by offering the advantage of intrinsically high nitrogen content without the need for its external introduction. Cow dung contains many organic compounds including crude protein, crude fat, and a variety of amino acids. These organic compounds can be used as the precursors for synthesizing nitrogen-containing carbon. Such prepared carbonaceous materials are becoming very popular in the field of energy owing to their high surface area, good conductivity, and high stability. For example, Bhattacharjya and Yu reported that cow dung-derived carbon exhibited high specific capacitance and durability.16 While, Zhang et al. have reported the use of cow dung as an extremely low cost precursor for preparing an Ncontaining carbon as ORR catalyst.25 Similarly, herein also all of the results clearly demonstrate that highly porous activated carbon can be easily synthesized from biological waste cow dung and could be used as an electrode material. Electrochemical Activity as OER Catalyst. Prior to the electrochemical measurement, optimization of analytical parameters for OER, ORR, and HER study was performed and given in the Supporting Information (section S2 and Figure S4S−S6). The water oxidation catalytic properties of Pd@ WO3-Chi, Pd@WO3-GO, Pd@WO3-Gra, and Pd@WO3-NDs 9741


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ACS Sustainable Chemistry & Engineering dec−1, much smaller than that of all other catalysts including the commercial RuO2 catalyst (77.0 mV dec−1),5 indicative of its more effective kinetics of water oxidation. Meanwhile, the Pd@ WO3-NDs modified electrode exhibits a smaller Tafel slope than that of Pt/C (129.1 mV dec−1), suggesting a better kinetic process (Figure 4C). While comparing the current density and overpotential values of prepared catalyst with other earlier reported metal− carbon based catalyst, we have found that this is the lowest value of overpotential reported so far (Table S2) and current density is comparable to or even outperforms other reported metal−carbon based catalysts. Zhang et al. has prepared sandwiched carbon sheets@Ni−Mn nanoparticles for enhanced OER. The reported overpotential (0.25 V vs RHE) and onset potential (1.48) was found higher than [email protected] Similarly, Ci et al. has prepared mesoporous NiFe-alloy-based hybrids and checked their OER activity. The overpotential (0.22 V), onset potential (1.45 V), and Tafel slope (73.6 mV dec−1) was found higher than our fabricated electrocatalyst.28 The result of some other reported electrocatalysts toward OER is summarized in Tables S2 and 1. As shown in Table S2, the proposed catalyst is found to be superior to reported earth abundant and cost-effective catalysts. To prove the effect of the structure and components of the catalysts on the OER process, electrochemical impedance spectra (EIS) were measured for Pd@WO3-Chi, Pd@WO3GO, Pd@WO3-Gra, and Pd@WO3-NDs in 1.0 M KOH. As shown in the Nyquist plots in Figure 5A, the Pd@WO3-NDs exhibits the smallest charge transfer resistance among all the electrodes, suggesting superior electron mobility, which is good agreement with the result of the Tafel slope. In addition, the charge transfer resistance of nondecorated and Pd-decorated WO3-CD (derived from cow dung) was also compared in the Nyquist plot. It is found that, after Pd decoration, the resistance has decreased to a large extent, which supports the role of PdNPs over the materials’ electrochemical properties. Electrochemical Performance as ORR Catalyst. To assess the ORR catalytic activity of the nanomaterials, cyclic voltammetry runs were taken in O2 versus N2-saturated 1.0 M KOH (Figure 4D). As shown in the figure, a characteristic oxygen reduction peak at ∼0.73 V was observed for the Pd@ WO3-NDs in O2-saturated aqueous KOH but not for the N2saturated electrolyte, indicating the ORR activity of the Pd@ WO3-NDs. Furthermore, in comparison to other nanocomposites, more current at less potential was observed for Pd@WO3-NDs, which shows their better oxygen evolution capability. The LSV measurements were further performed to explore the ORR activity of the different nanomaterials and displayed in Figure 4E. As seen in the figure, prepared nanomaterials exhibited better or improved onset potential (1.02 V), half-wave potential (0.678 V), and current density (328.0 mA cm−2) as those of the Pt/C catalyst, indicating comparable activities for all the catalysts. However, compared to others, Pd@WO3-NDs exhibited a positive shift in the onset potential, indicating their superior ORR performance (Table S3, Table 1). The obtained values were also compared with some of the reported electrocatalyst toward ORR. Xue et al. reported the fabrication of Pd, Co, and carbon based electrocatalyst and studied their efficiency toward ORR.29 They have found an onset potential of −0.082 V (Ag/AgCl) with a current density of 5.2 mA cm−2. Similarly, Liu et al. reported the synthesis of transition metal/nitrogen dual-doped mesoporous graphene like carbon nanosheets as a bifunctional

Table 1. Comparison in Trifunctional Behavior with Earlier Reported Trifunctional Electrocatalysts and Pd@WO3-NDs SN

catalyst

1

MoSe2/ MoO3−2

2

a

3

SHG

4

A-PBCCF-H

5

CoO(OH)

6

PPy/ FeTCPP/ Co

7

GO-CuMOF

8

N/Co-PCP/ NRGO

9

Co0.8Se@NC

10

Pd@WO3NDs

Co/CoO@ Co−N−C

reaction OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER OER ORR HER

η/onset potential (V)

Tafel slope (mV dec−1)

0.23/1.1 −/0.93 −0.27/−0.085 0.64/− −/−0.05

45.0 32 27

−/1.49

71.0

13

−/−0.23 0.41/1.55 −/0.87

112 99

39

42 55.0

40

/1.6 /0.25 /−0.8 1.61/− −/1.01 −0.24/− −/1.19 −/0.29 −/−0.087 1.66/− −/0.93 −0.229/−0.058 1.55/1.49 −/0.912 −0.23/-0.14 0.113/1.32 0.195/1.02 0.185/−0.09

ref 9

10

87.0 65.0 69.0 84.0 65.0 69.0 84.0 292.0 85.0 126.0 75.0 125 62.8 53.1 58.2

41

42

43

44

this work

a

Potential is reported with reference to Ag/AgCl electrode. Co−N− C: cobalt, nitrogen doped carbon. SHG: stereoscopic holes over the graphitic surface. PBCCF: PrBa0.8Ca0.2Co1.5Fe0.5O5+δ. FeTCCP: iron tetrakis(4-carboxyphenyl)porphyrin. GO: graphene oxide. MOF: metal organic framework. N/Co-PCP/NRGO: nitrogen-doped graphene/cobalt embedded porous carbon polyhedron. NC: nitrogen doped carbon.

catalyst.30 As of their study toward ORR, they have reported an onset potential of −0.075 V (Ag/AgCl) with a Tafel slope of 48 mV dec−1. From the comparative study, it may be concluded that our fabricated material is superior in terms of onset potential toward the reported electrocatalysts and have a comparable value for Tafel slope. The favorable ORR activity of nanomaterials can also be revealed from their Tafel plots (Figure 4F). The Pd@WO3NDs show a small Tafel slope of 53.1 mV dec−1, which is found slightly lower to the value of Pt/C (69.4 mV dec−1, Figure 4F). This Tafel slope value is close to the theoretical value of −2.303RT/3F, where R is the universal gas constant, F is the Faraday constant, and T is absolute temperature.31 The low Tafel slope, comparable to that of benchmark Pt/C in this study, indicates good intrinsic electrocatalytic activity of the prepared nanomaterial (Pd@WO3-NDs), which is desirable for the electrochemical applications. This improved performance of the nanomaterial can be credited to their large surface area and special disc like morphology, which is important for oxygen diffusion and makes simple bonds between the catalytic site and oxygen molecule lead to high electrical conductivity and easier 9742


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Figure 5. (A) EIS spectra for Pd@WO3-Chi, Pd@WO3-GO, Pd@WO3-Gra, Pd@WO3-NDs, and WO3-CD in 1.0 M KOH. (B) Current−time chronoamperometric response for ORR on Pd@WO3-NDs in O2-saturated 1.0 M KOH solution. (C) FE-SEM images showing the change in surface morphology after the electrochemical study with current−time chronoamperometric response of Pd@WO3-NDs for OER in O2-saturated 1.0 M KOH solution (embedded). Current−time chronoamperometric response of Pd@WO3-NDs for ORR, before and after addition of (D) 3.0 M methanol and (E) 10% CO. (F) Schematic configuration of the rechargeable two-electrode Zn−air battery using Pd@WO3-NDs as air cathode and Zn as an anode. (G) Polarization curve (V vs i) and corresponding power density plot of the battery prepared with Pd@WO3-NDs as the cathode catalyst compared with the battery prepared using commercial Pt/C catalyst. Discharge/charge cycling curves of two-electrode rechargeable Zn−air batteries at a current density of 1.0 mA cm−2 using the Pd@WO3-NDs air electrode for (H) 1500 and (I) 7000 min.

Tafel plot is used to study the mechanism and the inherent properties of the materials for HER. As shown in Figure 4I, the Tafel plots obtained by plotting the logarithm of the kinetic current density derived from the HER polarization curves in Figure 4H. The Tafel slope of Pd@WO3-NDs was calculated to be 58.2 mV dec−1, a value lower than 120 mV dec−1, indicating a Volmer−Heyrovsky mechanism for HER, i.e., the HER rate is determined by the electrochemical desorption of H+ from the catalyst surface to form hydrogen.34 The obtained values were also compared with some of the reported electrocatalyst toward HER (Table S4, Table 1). Mandegarzad et al. reported the fabrication of Cu, Pt, and carbon based electrocatalyst and studied their efficiency toward HER.35 They have found an onset potential of −0.01 V vs RHE with a current density of −2.75 mA cm−2. Similarly, Dai et al. reported the synthesis of cobalt encapsulated in bamboolike and nitrogen-rich carbonitride nanotubes for hydrogen evolution reaction.36 As of their study toward HER, they have reported an onset potential of

electron transfer during the oxygen reduction at the catalytic site.32 Electrochemical Performance as HER Catalyst. The electrocatalytic HER performance of nanomaterials was investigated in 0.5 M H2SO4 as electrolyte together with Pt/ C for comparison. In the CV runs, the better current response was obtained with Pd@WO3-NDs (Figure 4G), and a similar one was observed in LSV runs also (Figure 4H). As shown in the polarization curves, Pd@WO3-NDs exhibited the highest HER activity compared with the other three nanomaterials and comparable results with Pt/C. The overpotential for driving a current density of 10 mA cm−2 is a useful parameter for comparing catalysts in solar hydrogen production also.33 Therefore, the corresponding overpotential at currents of 10 and 20 mA cm−2 was calculated and found as −0.275 V and −0.355 V . The overpotential of other three nanomaterials, i.e., Pd@WO3-GO, Pd@WO3-Chi, and Pd@WO3-Gra are found to be −0.329, −0.351, and −0.376 V at 10 mA cm−2, respectively. 9743


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ACS Sustainable Chemistry & Engineering −0.089 V with a Tafel slope of 48 mV dec−1. From the comparative study, it may be concluded that our fabricated material is superior in terms of onset potential toward the reported electrocatalysts and have a comparable value for Tafel slope. Stability of the Prepared Electrocatalysts. In addition, the durability of Pd@WO3-NDs and commercial Pt/C catalyst was further evaluated and compared by chronoamperometry (CA) measurement at a potential of 1.60 V. As shown in the Figure 5B, ∼90% of original current density was maintained for the electrode over 48 h testing. In contrast, a decrease of ∼40% in current density was observed for the commercial Pt/C catalyst over a 48 h continuous operation testing period for ORR. Similarly, to evaluate the stability of proposed catalyst for OER, the CA measurement was performed at 0.65 V and shows very good stability in the duration of 48 h (Figure 5C). This affirms the superior stability of prepared catalyst under alkaline condition and higher ORR ability than that of commercially popular catalysts. After the electrochemical studies, the SEM image of the nanomaterials was recorded, which is found exactly similar to the SEM image of fresh catalyst (supplied as Figure 5C, inset). This implies the excellent stability of Pd@ WO3-NDs toward long-term testing. Methanol cross over effect and CO poisoning effect are known to be major disadvantages or drawbacks of state of art Pt-based catalysts. To evaluate the methanol crossover effect over the as fabricated electrocatalyst, 3.0 M methanol was added to the KOH solution, and their activities were compared in terms of CA response with respect to Pt/C catalyst (Figure 5D). It can be observed that the current density is decreased after the addition of methanol only in the case of Pt/C catalyst. But in the case of Pd@WO3-NDs, a negligible crossover effect was observed. To study the CO poisoning effect on the ORR activity 10% volume CO was introduced to the supporting electrolyte solution. As can be shown in Figure 5E that the relative current density get decreased remarkably (∼30%) for oxygen reduction at Pt/C electrode. In the case of Pd@WO3NDs, the change in current density was ∼3% under the same condition. The result indicates that Pd@WO3-NDs has good selectivity toward ORR and high resistance toward the CO poisoning and methanol crossover effect. This also suggests that Pd@WO3-NDs is highly stable and can be used as a better or superior catalyst over the commercial Pt/C for ORR. In general, HER catalysts have been proven to work well in acidic environments only. However, water splitting in an alkaline medium has been more popular in terms of commercial large-scale hydrogen production. Therefore, in order to expand the application range of the proposed catalysts, their HER activity was evaluated in basic and neutral media also. As shown in Figure S8, Pd@WO3-NDs exhibited excellent electrocatalytic activity toward HER in 1.0 M KOH solution also, with an overpotential of 184 mV required to drive a current density of 10 mA cm−2. In addition, the cyclic stability (runs after multiple cycles, until the 2000th cycle for OER/HER/ORR) of electrocatalyst (Figure S9), durability (comparison between fresh and used catalyst after several HER/OER or ORR measurements, Figure S10), and storage stability (for 4 months, Figure S11) was also performed, and details are given in SI section S3. Real-Time Applications. Generally, the bifunctional catalyst used for OER/ORR were explored for their use in the field of metal-air battery fabrication and the HER/OER combinations were used for water splitting, which is a future for

H2 and O2 generation from renewable resources. Herein, we have demonstrated the real-time application of proposed nanomaterial in fabrication of the Zn−air battery, and afterward, the same battery was used for electrochemical water splitting purposes. Bifunctional Behavior As Overall Oxygen Catalysts. The overall oxygen activity of the proposed catalyst was first evaluated by calculating the potential gap value, i.e., ΔE = EJ=10 − E1/2, where EJ=10 is the OER potential at a current density of 10 mA cm−2 and E1/2 is the ORR half-wave potential. The Pd@ WO3-NDs catalyst exhibited the small potential gap of 0.85 V, which is found lower or comparable than that of the commercially popular ORR/OER catalysts i.e. Pt/C (0.94 V) and Ir/C (0.92). While comparing with the other reported bifunctional catalyst, the Pd@WO3-NDs exhibited the lowest bifunctional activity. The superior bifunctional performance of Pd@WO3-NDs compared with the other control samples may also provide insight into the mechanisms governing the ORR and OER active sites in the proposed catalyst. First, the presence of nitrogen as a dopant in the catalyst can render a higher positive charge density on adjacent C atoms, which act as the catalytic active sites for ORR.6 In addition, the N atoms with lone pair electrons can also accept electrons from neighboring C atoms, favoring the adsorption of water oxidation intermediates,37 thus facilitating the improvement of OER activity in the catalysts. Second, The PdNPs interact with nitrogen to form intimate complex, which contribute to the generation of more efficient catalytic active sites for ORR and OER.38 Third, the hierarchical porous disclike structure is beneficial for the mass transport during the electrochemical reaction. Thus, we can demonstrate that the excellent ORR and OER catalytic activity of Pd@WO3-NDs can be attributed to the synergistic effects of nitrogen dopant, WO3/Pd decoration, and morphologically special porous structure. Role of Prepared Catalyst in Battery Fabrication. Apart from traditional electrochemical studies in aqueous electrolytes, a zinc−air cell was designed to further determine the bifunctional cathode catalyst performance under real battery operation conditions. Here, Zn powder and nanomaterials coated carbon tapes were used as anode and cathode, respectively. The setup of the battery was shown in the Figure 5F. In this configuration, using 6.0 M KOH as electrolyte, the open circuit potential was found to be ∼1.2 V. With the fabricated battery, a ∼250 mA cm−2 current density of was obtained at 0.6 V voltage. For control purposes, the Pt/C modified carbon tape as cathode was also used to make a different battery. It was found that battery made with Pd@ WO3-NDs as the catalyst shows better power and current density at 1.0 V in comparison to a battery made with the Pt/C (Figure 5G). Also, the peak power density and current density at 1.0 V of Pd@WO3-NDs was found much better in comparison to other catalysts that are reported in Table S5, which shows the potential application of proposed nanomaterial as a cathode, while designing the metal−air batteries. Figure 5H shows the charge and discharge performance of the Pd@WO3-NDs based battery at a current density of 1.0 mA cm−2 with respect to time. The corresponding columbic efficiency was also calculated and shown in Figure S12-A. Good recharge and discharge capability was shown by Pd@ WO3-NDs catalyst based battery, which can be proved by 74 charge/discharge cycles over a period of 25 h. In addition, the first 10 charge−discharge cycles curves of the battery were also recorded and shown in Figure S12−B. Notably, the 9744


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Figure 6. Two electrode test for Pd@WO3-NDs along with the comparison Pt/C (−) and RuO (+) toward (A) HER and (B) OER at a scan rate of 5.0 mV s−1. (C) The chronopotentiometric curve for water electrolysis using Pd@WO3-NDs in a two-electrode setup with a current density of 20 mA cm−2 toward HER and OER. Photograph showing the displacement of water due to the evolution of oxygen and hydrogen gas after (D) 0, (E) 2, (F) 3, (G) 4, and (H) 5 min in a lab-made water electrolysis setup. (I) Linear plot between the volume of evolved hydrogen and oxygen gas (μL) with time (min) calculated from the displacement of water.

toward both OER and HER, a water splitting electrolyzer was fabricated by applying the Pd@WO3-NDs both as anode and cathode in 1.0 M KOH solution (Scheme S1). As seen in Figure 6A, the Pd@WO3-NDs HER electrode (with the same Pd@WO3-NDs as the OER counter electrode) shows a small onset potential of −0.1 V with the current density gradually increased with increasing potential, as is the case for the Pt/C and RuO2 setup. In conjunction with the other half reaction for OER (Figure 6B), comparable behavior to that of Pt/C and RuO2 setup was found. The H2 and O2 evolution at anode and cathode was also recorded and shown in the supporting video (Video S1). The long-term stability of electrolyzer is another obstacle for practical, large scale H2 production. Therefore, the durability of the prepared electrolyzer was examined by measuring the chronopotentiometry runs in N2-saturated 1.0 M KOH solution for 48 h at 20 mA cm−2 (Figure 6C). No significant change was observed in the initially applied potential of about −0.15 V (for HER) and 1.47 V (for OER), during 48 h of testing period, which demonstrated the excellent long-term stability of prepared catalyst.

rechargeable zinc−air battery shows the highest round-trip efficiency (negligible voltage change at the end) with almost constant charge and discharge potential during the whole cycle. The prepared battery was also tested for long-term stability over the time span of 116 h, and no change in voltage was seen (Figure 5I). The result clearly indicates the long-term impact and application of the prepared catalyst in the renewable energy resource. To demonstrate potential applications in optoelectronics, we have further developed all-solid-state Zn−air batteries (Scheme 1). As shown in the scheme, five all-solid-state Zn−air batteries integrated into a series circuit could be used to power a blue light emitting diode (LED) (≈4.0 V). Series-connected allsolid-state Zn−air batteries show stable current for more than 12 h, which was confirmed through constant monitoring of intensity of the LED light. This indicates the possibilities for the all-solid-state Zn−air batteries based on the Pd@WO3-ND to be used in optoelectronic devices. Water Splitting. Since the Pd@WO3-NDs has been demonstrated to be an active and stable bifunctional catalyst 9745


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that our findings may help in an ecofriendly and economic way the fabrication of multifunctional electrocatalysts, which could be used in the future for metal−air batteries, fuel cells, and various other real-time applications.

To confirm that the measured cathodic and anodic currents were associated with overall water splitting, the amount of oxygen and hydrogen produced by a water electrolyzer with Pd@WO3-ND was through the gas-displacement method (Scheme S2). For the collection of gases formed during the water-splitting process, we inserted the Pd@WO3-NDs as both cathode and anode into the tube with volume marks and then the top end of the tube is sealed. The tube was filled with KOH electrolyte and its bottom end was immersed into the electrolyte in a beaker. When the water-displacement unit was connected with the Zn−air battery, the water starts splitting into oxygen and hydrogen in the form of bubbles from their corresponding electrodes. The produced gases, i.e., O2 and H2, were collected on the top of the water filled tubes. Meanwhile, the water level in the tube falls down due to the pressure of the internal gas formed during water displacement process. In this way the produced gas volume was measured. As can be seen in Figure 6D−H, as the gas volume increases inside the electrolyte, the surface level in the tube drops down. The amount of oxygen and hydrogen gas released by water splitting at 10 mA cm−2 current density is plotted as a function of electrolysis time and shown in Figure 6I. From the plot, a linear relationship was obtained with a slope of 195.0 and 100.5 μL min−1, for hydrogen and oxygen gas, respectively. The slope ratio of 1.94 is very close to the theoretical ratio of two, as anticipated for hydrogen and oxygen production by water splitting. Therefore, high performance and long durability of the Pd@WO3-NDs catalyst in the alkaline electrolyzer provides a new strategy for developing a nonprecious efficient catalyst for overall water splitting. After evaluating the trifunctional performance of the proposed electrocatalyst, their activities were also compared with other recently reported trifunctional electrocatalysts and portrayed in Table 1.9,10,13,39−44 We have also incorporated a comparison between precious metal-based catalyst and the proposed catalyst in Table S6. As shown in the tables, the proposed catalyst possesses the lowest onset as well as overpotential among all the reported trifunctional electrocatalysts as well as precious metal-based catalysts. The value of tafel slope is also found to be comparable or better than many reported catalysts. The comparative study clearly shows the significance, potential, and advancement over earlier reported trifunctional catalysts, which are very rare in the literature.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01541. Schematic representation of water splitting setup and water displacement unit; XPS survey spectra and elemental composition of catalysts; comparative table for properties of catalysts; Electrocatalytic study bare and other nanomaterials modified PGEs; optimization of loading mass, scan rate, and supporting electrolyte for OER, ORR, and HER; onset potential of prepared electrocatalysts and RuO2; comparative table for Pd@ WO3-NDs performance with other reported electrocatalysts toward OER, ORR, and HER; HER activity of Pd@WO3-NDs and Pt/C in KOH and K2SO4; stability of Pd@WO3-NDs after different cycles, after all electrochemical measurements and various storage time toward HER, OER and ORR; comparative table for peak power density of primary Zn−air batteries; Coulombic efficiency; enlarged charge−discharge cycles; comparison of proposed catalyst with precious metal-based catalysts. (PDF) H2 and O2 gas evolution at respective electrodes in twoelectrode system (AVI)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.M.). Tel.: +91 9471191640. ORCID

Rashmi Madhuri: 0000-0003-3600-2924 Prashant K. Sharma: 0000-0001-5283-0901 Notes

The authors declare no competing financial interest.

■

■

ACKNOWLEDGMENTS The authors are thankful to DST, BRNS, and ISM for sponsoring the research projects of R.M. (ref nos.: SERB/F/ 2798/2016-17; SB/FT/CS-155/2012; FRS/43/2013-2014/ AC; 34/14/21/2014-BRNS) and P.K.S. (ref nos.: SR/FTP/ PS-157/2011; FRS/34/2012-2013/APH; 34/14/21/2014BRNS). The authors are also thankful to Dr. D. Kumar (Indian Institute of Science, Bangalore) for his kind assistance and help in recording XPS spectra.

CONCLUSION In summary, we have developed a nanodisc shaped nanocomposite based on WO 3-modified carbon nanosheets decorated with PdNPs by a simple and ecofriendly method. From the comparative study between four different origin carbon source based nanocomposites as trifunctional electrocatalysts, it was concluded that Pd@WO3-NDs has the highest efficiency toward all the three recently important reactions, i.e., HER/OER and ORR. The main credit behind the better efficiency of Pd@WO3-NDs goes to their higher surface area, porosity, special disclike morphology and nitrogen doping. The synthesized Pd@WO3-NDs was utilized for the fabrication of rechargeable Zn−air batteries (in aqueous phase) and an allsolid-state battery to power up a 4.0 V LED. Moving toward different applications of the proposed catalyst, it was also employed for overall water splitting powered by the same allsolid-state Zn−air battery. The excellent performance in terms of stability, low cost, easy preparation procedure, and discharge/charge potentials in working conditions, suggested

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REFERENCES

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Cow Dung Derived PdNPs@WO3

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