Enriched active surface structure in nanosized tungsten-cobalt oxides electrocatalysts for efficient oxygen redox reactions

Applied Surface Science 513 (2020) 145831

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Enriched active surface structure in nanosized tungsten-cobalt oxides electrocatalysts for efficient oxygen redox reactions ⁎

T

⁎

Mabrook S. Amera, Prabhakarn Arunachalama, , Mohamed A. Ghanema, , Abdullah M. Al-Mayoufa, Muhammad Ali Sharb a b

Electrochemical Sciences Research Chair (ESRC), Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanoparticles Tungsten Cobalt Oxide Oxygen evolution Oxygen reduction

Nanosized tungsten-cobalt oxide (WCoO-NP) electrodes were prepared using self-assembly template approach and their bifunctional electrocatalytic behaviour for the oxygen redox reactions was investigated. The texture, morphology, specific surface area, crystallinity, and electrocatalytic activity of the WCoO-NP were strongly associated with the W and Co content. The WCoO-NP materials contains 15 mol% of tungsten showed enhanced electrocatalytic behaviour, substantial shift in the OER onset potential of 190 mV, Tafel slope (92 mV/dec), ultra-low charge-transfer resistance, and current density of 30 mA cm−2 at 1.55 VRHE, which is more efficient catalyst than bare cobalt oxide nanoparticles (Co3O4-NP) counterpart and comparable to benchmark transition metal oxide electrocatalysts. The WCoO-NP materials exhibits long-term durability and good bifunctional electrocatalytic behaviour for both the OER and ORR, having ΔE (=EOER − EORR) of only 0.92 V which could be credited to the synergistic effect, enriched specific surface area, and improved electrical conductivity upon tungsten-doping. The WCoO-NP electrocatalysts prepared from earth-abundant materials are a promising candidate for high-efficiency OER and ORR applications.

1. Introduction Over the past decades, the alkaline water electrolysis has become the most promising and environmentally friendly process for the clean, renewable, and sustainable production process of hydrogen fuel [1–3]. In alkaline media, the electrochemical water electrolysis reaction involves two vital half-cell reactions, one at the anode, i.e., the oxygen evolution reaction (OER), and one at the cathode, i.e., hydrogen evolution [4,5]. The latter is the crucial reaction in energy related devices, namely fuel cells and metal-air batteries. Furthermore, the oxygen reduction reaction (ORR) is another vital process for electrochemical energy devices [6–8]. In addition, these electrochemical reactions, particularly the ORR and OER, are energetically uphill processes requiring high overpotentials [9,10] because of the sluggish kinetics. Consequently, a large number of research works have focused on developing efficient electrocatalysts with lower overpotentials for these reactions. Although Ir- or Ru-derived materials are the standard benchmark catalysts for the OER, their performance for the ORR is very moderate, and their limited availability and production costs hamper the large-scale commercial use [11–13]. In contrast, Pt-based materials are considered to be appropriate candidates for the ORR [14,15], ⁎

although they are less active for the reverse reaction of the OER [16]. Therefore, the researchers are devoted to exploring a highly efficient, easy-to-fabricate, abundant, low cost, durable, and bifunctional (OER/ ORR) electrocatalysts to replace the noble metal materials. Recently, several materials have been employed as OER and ORR electrodes including, transition metal oxides (TMOs) [17,18], sulfides [19,20], hydroxides [21], perovskites [22], and metal-oxide-based composites such as metal oxide/graphene [23] and carbon nanotube composite materials [24]. Among the available earth-abundant TMOs, cobalt (II, III) oxides have been intensely explored as highly suitable electrocatalysts to replace precious metal catalysts in alkaline media [25,26]. Furthermore, the cost-effectiveness, high surface-to-volume ratio with highly stable chemical states and superior theoretical capacity (890 mAh g−1) make cobalt oxides suitable for use in energy storage devices [7]. Afterwards, the bimetallic catalysts based on transition metal hydroxides such as CoNiOOH and NiFeOOH have shown superior electrocatalytic activities than monometallic catalyst systems [27–30]. Particularly, the enhancing activity of the bimetallic catalysts towards OER is credited to the conductivity improvement and the creation of higher oxidation active sites [31–33]. For example, the

Corresponding authors. E-mail addresses: [email protected] (P. Arunachalam), [email protected] (M.A. Ghanem).

https://doi.org/10.1016/j.apsusc.2020.145831 Received 8 December 2019; Received in revised form 13 February 2020; Accepted 17 February 2020 Available online 18 February 2020 0169-4332/ © 2020 Published by Elsevier B.V.

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incorporation of high-valence W6+ dopant ions to modify the OH− adsorption energy on the cobalt oxides surface has been used to overcome sluggish kinetics towards the OER activity [34,35]. Over the past decades, the porous form of TMOs has recently emerged as suitable electrode materials for electrocatalytic applications [36]. These porous forms of the metal-oxide catalysts tend to have a great surface-to-volume ratio, large specific surface areas (SSA) and enormous pore volume, and this can result in a large number of catalytic active sites, which assists the rapid diffusion of reactive species as well as products and improves the electrocatalytic activity [36,37]. However, the fabrication of a porous form of cobalt oxide via conventional methods is challenging owing to its weak inorganic-surfactant interactions [38]. Therefore, a numerous templating approaches with different surfactants and precursors have been reported for the synthesis of TMOs with different architectures and morphologies [39]. Within this context, a soft-template approach, the so-called evaporation-induced self-assembly (EISA) route, have been recognized as an innovative approach to fabricate crystalline porous materials. In the EISA approach, a blend of volatile solvent, surfactants/block copolymers, and metal precursors is carefully evaporated under controlled conditions. This evaporation drives the co-assembly of surfactant molecules and metal precursors. Subsequently, a calcination process results in the development of ordered metal-oxide mesostructures [40,41]. The EISA approach has a few advantages compared to the hydrothermal process because it offers short synthesis time, easiness of controlling template/metal ratio and the possibility of fabrication of a wide range of mesoporous catalysts with various compositions [37]. For example, Feng et al. demonstrated an EISA approach to fabricate a series of porous TMOs with high SSA and closely packed mesostructures [42]. Recently, we have demonstrated the fabrication of low-symmetry, hexagonal mesoporous TiO2 electrocatalysts for the electrocatalytic OER using the EISA approaches [43,44]. Through this EISA route, a wide range of metal oxides and bimetal oxides materials also can be fabricated easily and employed as electrode materials for the OER and ORR. Subsequently, the ORR/OER behaviour of the bimetal oxide still needs further advancement to develop highly efficient and durable, with respect to expensive noblemetal catalysts, and appropriate for industrial applications. Although considerable advances have been made in EISA approach to fabricate porous TMOs, the development of a widespread method to fabricate thermally stable and crystalline porous cobalt-based bimetal oxides is still a great challenge. Based on the above-mentioned considerations, the cobalt-based materials reported in the past always revealed a single catalytic activity towards the ORR or OER, while cobalt-based bimetal oxides (especially W-doped) porous materials as dual-functional catalysts were rarely reported. Herein, we describe a one-step soft-templating approach via the EISA method to fabricate nanoparticles of tungsten-cobalt oxide (WCoO-NP) having different W wt% for use as dual-functional catalytic materials for the OER and ORR. In the EISA process, the WCoO-NP samples were prepared from Pluronic P123 surfactant, cobalt and tungsten precursors, which were treated in the air by heating at 150 for 4 h at a rate of 2 °C/min then further second stage heating for 2 h at 250, 350, or 450 °C. Further, the structural and morphological examination of the fabricated WCoO-NP materials was performed via FTIR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy, and N2 sorption Brunauer–Emmett–Teller (BET) measurements. Notably, the dual function and highly durable electrocatalytic activity of the WCoO-NP catalysts for the OER and ORR were examined in basic media are presented. WCoO-NP with 15 mol% possess a maximum SSA of 142 m2/g and exhibited an overpotential of 270 mV for OER (at 10 mA/cm2), which is superior in comparison with the state-of-art IrO2 catalysts and the cobalt-based catalysts reported so far. The prepared WCoO-NP catalysts were found to be more efficient with respect to pristine Co3O4 nanoparticles (Co3O4-NP) and other commercially available catalysts.

2. Materials and methods 2.1. Chemicals Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥98.0%), tungsten hexachloride (WCl6, ≥99.9%), citric acid (≥97.0%), anhydrous, 1butanol (99.8%), PEO20–PPO70–PEO20 (Pluronic® P123), and nitric acid (68–70% HNO3) were acquired from Sigma–Aldrich. Potassium hydroxide (KOH, pure) was obtained from BDH group. All chemicals were used as received without any further purification. 2.2. Synthesis of mesoporous tungsten cobalt oxide (WCoO-NP) The WCoO-NP and bare Co3O4 nanoparticles (Co3O4-NP) catalysts were prepared as reported by Qiao et al. [37] using the EISA approach in butanol/Pluronic® P123/HNO3/citric acid/Co(NO3)2·6H2O solution followed by stepwise pyrolysis in air. Typically, the bare Co3O4-NP catalyst was prepared by dissolving the P123 surfactant (1.36 × 10−4 mol), citric acid (5 mmol), and concentrated. HNO3 (16 mmol) in 1-butanol (70 mmol) solution with magnetic stirring, and, then, Co(NO3)2·6H2O (2.5 mmol) was introduced to the template mixture. Afterwards, the obtained mixture was agitated continuously for 10 h at normal temperature to obtain a clear transparent sol. Subsequently, the obtained sol was transported to a Petri dish for the evaporation of the solvent by heating at 120 °C in an air oven for 4 h. Successively, the as-synthesized gel/powder product was scraped away from the dish and annealed in air through stepwise pyrolysis process at 150 °C for 4 h (2 °C/min), 250 °C for 2 h, and 350 °C for 3 h. After each heating step the catalyst was allowed to cool down in an open atmosphere to room temperature yielding the crystalline Co3O4-NP product. To prepare the WCoO-NP electrodes, a defined amount of WCl6 was introduced to the solution at different metal loading ratios following the same procedure as stated earlier. In particular, the content of WCl6 was 0.052, 0.112, 0.175 and 0.247 g which equivalent to 5, 10, 15, and 20 mol% with related to its metal precursors. The obtained catalysts are denoted as WCoO-NP-X, where X is 5, 10, 15, or 20 and indicates the tungsten loading percentage. 2.3. Physicochemical characterizations of catalysts XRD data were recorded via Rigaku Miniflex 600 X-ray diffractometer using Cu K-alpha radiation (40 kV, 15 mA) and the FTIR spectra of the fabricated mesoporous catalysts were obtained using a Bruker TENSOR 27 spectrometer. Field-emission SEM (FE-SEM, HITACHI S4800) images and high-resolution transmission electron microscopy (HR-TEM, JEOL-2100F) images were documented to inspect the morphological nature of the fabricated mesoporous materials. The surface area of the fabricated cobalt-based samples was assessed by means of BET measurements using a sorption isotherm analyzer (NOVA 2200e, Japan). Analysis of the chemical states in the mesoporous cobalt-based catalysts was carried out using XPS (Escalab 250 spectrometer, Thermo Fisher) with a monochromatic MgKα X-ray source. 2.4. Electrochemical measurements The electrochemical characterization of the Co3O4-NP and WCoONP catalysts were recorded in a classical three-electrode assembly on an electrochemical system (Bio-Logic). The cell consisted of a Glassy carbon (GC, ALS Co., Ltd, TOKYO JAPAN) as a working electrode of the mesoporous catalyst, and a saturated calomel electrode (SCE) and Pt wire acted as the reference and counter electrodes, correspondingly. The fabricated catalyst ink was obtained by dispersing the 5 mg of the samples in 1 mL of isopropanol/water mixture and then subjected to the adding of 10 µL of 5 wt% Nafion solution. The oxygen redox reactions were carried out in 1.0 M KOH at 25 °C versus RHE. Further, RDE measurements were used in the linear sweep voltammetry (LSV) 2

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composites, distinctive IR bands around 847 cm−1 appeared (Fig. 1b), which can be credited to ν(O-W–O) [50], confirming the development of composite materials. The porous nature and pore size distribution (PSD) of the obtained Co3O4-NP and WCoO-NP catalysts were investigated using N2-sorption isotherms, and the results are presented in Fig. 1c. The isotherms of the obtained porous catalysts shown in Fig. 1c are typical type-IV sorption isotherms with H1-hysteresis loops and capillary condensation steps, which are representative of mesoporous samples per the IUPAC definition [51,52]. As W content increases in Co3O4-NP, a slight decrease of the adsorption plateau might be credited to the distortion of micropores. In addition, Fig. 1c shows distinctive capillary condensation at a relative pressure (P/P0) of 0.45–0.8, signifying a uniform, as well as narrow, mesopore size distribution. The textural parameters of the Co3O4-NP and WCoO-NP catalysts are provided in Table 1. Moreover, the BET specific surface area of the WCoO-NP composite materials was found to slightly increased (ca. 112–142 m2 g−1) with varying W amounts and superior to its corresponding bare Co3O4-NP (95 m2 g−1). The introduction of W to Co3O4 slightly improved the specific area of the catalysts and a maximum specific surface area of 142 m2 g−1 was achieved in the WCoO-NP composites. The Barrett–Joyner–Halenda (BJH) particle size distribution of the Co3O4-NP and WCoO-NP catalysts were evaluated from the adsorption isotherms, as displayed in Fig. 1d. Particularly, the obtained pore size measurements demonstrates that, as the W content increases the pore size decreases and the particle size distribution becomes broad, presumably due to the structural changes of the WCoO-NP catalysts, which is concordant with XRD results. Fig. S3 discusses the effect of annealing temperature on WCoO-NP catalysts. Particularly, Fig. S3a reveals the transformation from the amorphous to the crystalline phase with an upsurge in annealing temperature varied from 150 °C to 450 °C and its corresponding electrochemical properties are shown in Fig. S3b. The sample annealed at 250 °C displays lower crystallinity, but it showed superior electrochemical activities. At 350 °C annealing, the diffraction pattern shows well-defined distinctive diffraction peaks, which can be allotted to Co3O4 of JCPDS No. 431003, confirming the generation of a crystalline phase. Subsequently, by changing annealing conditions to 450 °C induces added crystal growth as well as enrichment in crystallinity and it might reduce the porosity of the fabricated WCoO nanoparticles. The electron microscopies measurements (FESEM/HRTEM) were employed to define the morphological and fine structure of the asprepared Co3O4-NP, and WCoO-NP catalysts. In particular, the stepwise pyrolysis in the air is shown to be advantageous in maintaining the structure of the nanoparticles. For instance, the SEM image of the prepared Co3O4-NP (Fig. 2a) that was annealed in the air clearly shows interconnected nanoparticles. The porous network created between the particles is clearly noticed in the SEM images (Fig. 2a, b). As depicted in Fig. 2b, the Co3O4-NP catalyst exhibits the morphology of assembled microblock that comprised nanoparticles aggregates with mean particle size varying from 15 to 20 nm (Fig. 2b). Further, the SEM images of the WCoO-NP-15 composites in Fig. 2c, d shows the incorporation of W into Co3O4-NP may pull apart the nanoparticles and hamper the aggregation of nanoparticles, reducing the particle size in comparison to that of bare Co3O4-NP which is agreeing with our XRD results (Fig. 1a). For instance, the SEM images of all the ratio of WCoO-NP catalysts are presented in Fig. S4. In particular, the degree of aggregation of the WCoONP composites decreases with increasing W doping amount, which results in the observed continuous increase in surface area (Table 1). For comparison, the SEM micrographs in Fig. S5 revealed the bare-WO3 particles are mostly comprised of the disordered structure of WO3 particles. Lastly, Fig. 2c, d are high-magnification SEM micrographs of WCoO-NP-15 that show the decreased nanoparticle size with respect to those of bare Co3O4-NP. Further examination of the morphology of the Co3O4-NP and WCoO-NP catalysts was performed using TEM. The TEM micrographs in Fig. 3 revealed that the bare Co3O4-NP is mostly comprised of a large

experiments with different rotations for the ORR to evaluate the number of required electrons required in the reaction by means of Koutecky–Levich (K–L) equation [37,38,45]. Chronoamperometric (CA) and chronopotentiometric characterization was executed to assess the durability of the fabricated electrode materials by fixing the potentials. Further, electrochemical impedance spectroscopic (EIS) characterization was completed in the frequency varying from 10−2 to 200 kHz with a 20 mV amplitude at a bias of 1.5 V versus RHE in a 1.0 M KOH. 3. Results and discussion 3.1. Structural, morphological, and chemical properties of W-doped Co3O4 nanoparticles The WCoO-NPs was fabricated by assembling the carboxyl-coordinated metal precursors with an amphiphilic surfactant through citric acid assisted EISA route. Fig. S1 illustrates the various stages for the fabrication of crystalline W-CoO nanoparticles. In step (1) the coassembly of the Co precursor, W precursor, PEO- PEO20PPO70PEO20 into spherical micelles with the core of hydrophobic PPO segments and the shell of PEO segments. While step (2) shows the creation of the as-made inorganic/organic composite structures by means of the evaporation of the n-butanol and thermosetting in the oven at 120 °C for further solidification. Finally, in step (3) carbonization of the inorganic and organic composites was performed by burning the templates, followed by the various phases of calcination processes in air, resulting in the formation of crystalline WCoO-NPs. Fig. 1 shows the powder XRD patterns of the produced WCoO-NP samples compared to bare Co3O4 nanoparticles catalyst. As shown in Fig. 1, the WCoO-NP and bare Co3O4 -NP both show diffraction peaks that characteristic spinel Co3O4 (JCPDS # 43-1003). The clear and sharp diffraction lines reveal the high crystallinity of the catalysts. The lower crystallinity of Co3O4-NP is shown by its comparatively wider diffraction lines, having a mean particle size of 10.63 nm. Further, the diffraction patterns of the WCoO-NP materials in Fig. 1a are similar to the standard pattern of the Co3O4-NP catalyst (PDF # 00-043-1003). Interestingly, the peak positions and intensities in the diffraction patterns do not change significantly upon the doping of tungsten with up to 15% W, whereas the WCoO-NP-20 with 20 mol% tungsten shows a diffraction peak corresponding to monoclinic WO3 (PDF # 01-0872395). However, after the incorporation of W into the Co3O4 lattice, the peak locations of the diffraction lines slightly shifted to lower values, indicating the expansion of the lattice. In particular, the calculated lattice parameters increased from 8.07 Å for Co3O4 -NP to 8.13 Å for WCoO-NP-20. Then the transformation of the Co3O4-NP crystal lattice is mostly influenced by lattice distortion, which is evidence of the effects of the introduction of W into the Co3O4 structure and the development of a W-Co oxide solid solution. Moreover, the diffraction patterns were analyzed to evaluate the mean particle sizes by means of the Scherrer equation. Notably, the mean particle size decreased with the increasing amount of doped W as shown in Table 1, where the introduction of W reduced the crystallinity of Co3O4-NP. Thus, the increased crystallographic disorder in the WCoO-NP samples resulted in smaller crystallites [46]. The FTIR spectra of the WCoO-NP and bare Co3O4-NP catalyst are presented in Fig. 1b. The broad absorption peak observed for the catalysts in the region around 3200–3660 cm−1 is credited to the water adsorbed from the atmosphere [47,48]. Additionally, the weak band at 1624 cm−1 agrees to the H–O–H stretching vibrations of adsorbed water molecules on the material surface. Further, two distinctive characteristic bands situated at 664 and 576 cm−1 are characteristic Co–O stretching vibrations in Co3O4 [49]. The FTIR spectra (Fig. 1b) of WCoO-NP-5 and WCoO-NP-10 are identical, illustrating that the basic bonding structures of all obtained materials are also similar. With increasing W dopant concentration in WCoO-NP-15 and WCoO-NP-20 3

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Fig. 1. Characterization of Co3O4 and W-CoO-NP nanoparticles (a) Powder XRD patterns of Co3O4 nanoparticles and various amounts of doping W-CoO-NP-X% (X = 5, 10, 15, 20) catalysts prepared by EISA approach. * indicates the peaks corresponds to tungsten oxide. (b) FTIR patterns of Co3O4 NP and various amounts of W-CoO-NP-X% (X = 5, 10, 15, 20) catalysts prepared by EISA approach. (c) N2 sorption isotherms, and (d) its corresponding BJH desorption pore size distributions of Co3O4 NP and tungsten loaded W-CoO-NP-X%.

[52]. The high-magnification TEM image in Fig. 3c displays the lattice fringes having d-spacings of 0.24 nm (3 1 1) and 0.47 nm (1 1 1), which match well with the X-ray diffraction data of Co3O4-NP. As depicted in the TEM images in Fig. 3d and e, the WCoO-NP-15 composite material

number of ultra-small Co3O4 nanocrystals (Fig. 3a), and numerous small pores created between the particles as evidenced in the TEM images in Fig. 3b. From the Fig. 3b, Co3O4-NP has an average particle size of ca. 10 nm, which is concordant with previously reported values

Table 1 Textural properties of the Co3O4 NP and WCoO-NP-15 catalyst derived from nitrogen adsorption and desorption data. Catalyst

BET Surface Area (m2/g)

BJH Des. Pore Volume (cm3/g)

BJH Des. Pore Diameter (nm)

Lattice Constant (Ǻ)

Scherrer Crystallite Size (nm)

X/(X + Co) atomic ratio (X = W) Atomic ratio by EDX

Co3O4-NP WCoO-NP-5 WCoO-NP-10 WCoO-NP-15 WCoO-NP-20

95 112 138 142 126

0.24 0.23 0.19 0.12 0.18

8.3 6.7 7.2 5.8 7.6

8.102 8.102 8.136 8.139 8.137

10.63 7.11 6.98 4.82 7.02

– 3.53 7.61 12.57 –

4

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(a)

(b)

100 nm

100 nm

(c)

(d)

100 nm

100 nm

Fig. 2. Morphological characterization of Co3O4 NP and W-doped CoO samples. FE-SEM micrographs of Co3O4 NP (a, b) and WCoO-NP-15 (c, d) samples obtained after two-step annealing process through citric acid assisted EISA approach.

Co3O4 particle size [46]. Fig. 3f shows the lattice distance in the HRTEM images consistent with the (3 1 1) interplanar distance of the Co3O4-NP, further demonstrating that the produced WCoO-NP-15 composite materials is highly crystalline.

has an average particle size of < 10 nm and the obtained particles are fused together forming a continuous mesoporous network. HRTEM of the WCoO-NP-15 composites revealed smaller particles than bare Co3O4-NP, attributed to W coordination with cobalt cations in reducing

(c)

(b)

(a)

0.24 nm

0.24 nm (311)

0.47 nm

(111)

0.46 nm

50 nm

10 nm

(f)

(e)

(d)

2 nm

(111) (311) 0.45 nm

(111)

50 nm

10 nm

2 nm

Fig. 3. TEM analysis of Co3O4 NP and WCoO-NP-15 samples. TEM images of crystalline Co3O4 NP samples at different magnification, (a-c) and optimized W-doped WCoO-NP-15 nanoparticles samples at different magnification (d-f) obtained through two-step annealing process via citric-acid assisted EISA approach. 5

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Fig. 4. EDX elemental maps of Co, W and O. SEM and the energy–dispersive X–ray EDX profile of Co, W and O elements of WCoO-15 NP (a-e).

WCoO-NPs, which indicates an influence of W addition on the electronic state of Co element; probably implies some of the Co ions get substituted with W ions in the lattices [54,55] Further, in the broad and asymmetrical O 1s spectrum (Fig. 5c), two main peaks having BEs of 529.9 and 530.8 eV were observed in the spectra of Co3O4-NP and WCoO-NP-15 catalysts and are attributed to the lattice oxygen in Co3O4-NP. In particular, during annealing in air, additional oxygen can be introduced to the products. The core-level high-resolution W 4f XPS spectra of WCoO-NP-15 composites is presented in Fig. 5d, and two major XPS peaks at BEs of 34.73 and 36.88 eV were observed related to W 4f 5/2 and 4f7/2, along with one satellite peak, which indicates that dopant-W has a valence of both +4 and +6 oxidation state in the composite materials, occupied the position of Co3+ ion [54,56]. These XPS results confirm that the Co3O4-NP and WCoO-NP-15 catalysts are mostly pure and, apart from carbon, there are no impurities.

The W-doping percentages in the fabricated materials were evaluated by energy-dispersive X-ray (EDX) spectroscopy measurements. The mapping analysis reveals uniform distribution of the W, Co and O elements throughout the WCoO-15 NP (Fig. 4) and bare Co3O4 catalysts (Fig. S2). Clearly, the elemental mapping analysis discloses a uniform distribution of Co and W within the nanoparticles framework. The amounts of W in the fabricated metal catalysts are 3.53 wt%, 7.61 wt%, 12.57 wt%, for WCoO-NP-5, WCoO-NP-10 and WCoO-NP-15 respectively, further the existence of W atoms in the fabricated composite electrodes are possibly accountable for the synergistic performance of the materials. The inset EDX analysis shows the elements composition of 12.57, 56.09 and 31.33 wt% for the W, Co and oxygen elements respectively in case of WCoO-15 catalyst (Fig. 4e). The atomic content of W is about 2.3 atomic % and the chemical composition can be estimated as WCo14O28 indicating on very small amount of W have been doped into Co3O4 catalyst. On the other hand, for bare Co3O4 catalyst, the EDX elemental analysis recorded 73.25 and 26.75 wt% for Co and O elements respectively that perfectly match the chemical composition of Co3O4 for bare cobalt oxide catalyst (Fig. S2d). To analyze the WCoO-NP in greater depth, the surface states and purity of the WCoO-NP-15 and Co3O4-NP catalysts were determined by XPS analysis. The XPS survey spectra and the narrow scan spectra of Co3O4-NP and the WCoO-NP-15 catalyst are depicted in Fig. 5. Particularly, comparative XPS data for the Co3O4-NP and WCoO-NP-15 catalysts are presented in Fig. 5a. Peaks corresponding to Co 2p and O 1s are present in the XPS spectra of the Co3O4-NP and WCoO-NP-15 catalysts, but peaks corresponding to W are also present in the WCoONP-15 samples and the amount of W in fabricated catalysts estimated to be around < 3 atomic %. The Co 2p XPS spectra of the Co3O4-NP and WCoO-NP-15 catalysts (Fig. 5b) contain two major peaks with binding energies (BE) of 781.4 eV (Co 2p3/2) and 796.8 eV (Co 2p1/2), along with two supporting shake-up satellite peaks positioned 6 eV from the major peaks. Moreover, the obtained peaks are characteristic of the Co3O4 phase and are consistent with the results of earlier works [47]. After deconvolution, the energy difference between the Co 2p3/2 and the Co 2p1/2 peaks was found to be ca. 15.5 eV, which is agreeing with the reported values [46,53]. Besides, after incorporation of W over Co3O4, there is a considerable shift in the position of the peaks along with the variation of the peaks half width between Co3O4-NP and

3.2. Electrocatalytic features of WCoO-NP for OER The OER performances of the Co3O4-NP and WCoO-NP catalysts having different W loadings were examined in 1.0 M KOH, and analogous polarization plots were acquired from LSV analysis at 5 mV s−1. Fig. 6a displays the anodic LSV measurements of the Co3O4-NP, WCoONP-5, WCoO-NP-10, WCoO-NP-15 and WCoO-NP-20 catalysts. From the Fig. 6a, the OER onset potential increases in the order WCoO-NP-15 (190 mV) < WCoO-NP-20 (227 mV) < WCoO-NP-10 (250) < WCoO-NP-5 (281 mV) < Co3O4-NP (326 mV) (Table S1), indicating that the addition of1 W decreases the OER overpotential and increases the thermodynamic favorability. In particular, the WCoO-NP15 sample was the best performance electrocatalyst, having the maximum current density and a significant enhancement in the current between 1.45 and 1.55 V, indicating a rapid reaction rate. Besides, a slight loss in activity was identified when 20% W was doped into the Co3O4-NP. Fig. 6b displays the comparative LSV curves of Co3O4-NP, WCoO-NP-15, bare WO3 and the state-of-art OER commercial catalyst (IrO2). In particular, the WO3 electrode does not display any significant electrocatalytic behaviors for the OER, whereas the bare Co3O4-NP catalyst has OER overpotential of 390 mV at 10 mA cm−2. Based on the OER results, the incorporation of W into Co3O4-NP greatly improves the OER performance, as illustrated by the onset potential of 190 mV and 6

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Fig. 5. Surface and chemical composition of catalysts. (a) Comparative XPS survey spectra, (b) comparative core-level Co 2p spectra, (c) O 1s spectra of fabricated Co3O4 NP and WCoO-NP-15 catalyst fabricated by EISA approaches, (d) W 4f spectra of fabricated WCoO-NP-15 catalysts.

the overpotential of 270 mV at 10 mA cm−2 for WCoO-NP-15 composite, which considerably lower in comparison with bare Co3O4-NP electrode. Moreover, the Co3O4-NP catalyst shows almost identical electrochemical performance to the benchmark IrO2 OER catalyst (Fig. 6b). Notably, the Co3O4-NP electrode reached a considerably higher current density of 100 mA cm−2 at 485 mV overpotential in comparison to the state-of-the-art IrO2 obligatory at an overpotential of 452 mV to achieve this current density, further signifying synergetic coupling nature between two components in our composites [57,58]. These values are superior to those of existing cobalt-based catalysts and a comparison between bare Co3O4-NP, WCoO-NP and other commercial catalysts are summarized in Table S2. The water oxidation kinetics was examined using Tafel plots by fitting the LSV data to the Tafel equation [59]. The Tafel slope was extracted from the LSV data, and the obtained results are presented in Fig. 6c. The WCoO-NP-15 and state-of-the-art IrO2 catalysts have Tafel slopes of 92.18 and 65.4 mV dec−1, respectively. While those of pure Co3O4-NP and WCoO-NP-5, WCoO-NP-10,

and WCoO-NP-20 are 84.2, 82.3, 78.2, and 87.9 mV dec−1, respectively. The activities of various electrocatalysts are generally assessed by relating the onset potential and overpotential necessary for attaining a current density of 10 mA cm−2, which is a metric for commercial solar fuel synthesis [59]. Fig. 6d provides further evidence of the superior activity of the fabricated WCoO-NP-15 composite electrodes with respect to the benchmark IrO2 catalyst, where the OER onset potential and overpotential necessary to obtain a current density of 10 mA cm−2 are presented. Moreover, to assess the electrocatalytic activity of the WCoO-NP composite electrodes for the OER, the mass activity and another parameter of turnover frequency (TOF) are assessed at an overpotential of 1.6 V RHE and are presented in Table S1. In particular, the smaller Tafel slope and lower overpotential of WCoO-NP indicate good catalytic activity and favorable kinetic and in particular the superior performance of WCoO-NP-15 composite electrodes compared to the bare Co3O4-NP for the OER in basic media, validating the 7

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Fig. 6. OER performances of catalysts: (a) LSV curves of Co3O4 NP, WCoO-NP-5, WCoO-NP-10, WCoO-NP-15, and WCoO-NP-20 in 1 M KOH solution at a sweep rate of 5 mV/s. (b) Comparative LSV curves of 15% W-Co with respect to Co3O4 NP, WO3 NP and IrO2 catalysts. (c) Tafel plot at low polarization for OER at meso-Co and various amount of X% W-Co and IrO2 catalysts. (d) Comparison of the fabricated catalyst OER activity with the OER onset potential, and overpotential obtained to achieve a current density of 10 mA cm−2.

presented in Table 2. The WCoO-NP-15 composite electrode has the lowermost R2 value, indicating lesser electron and charge-transfer resistances and, consequently, rapid electrode kinetics. Lastly, these results reveal that WCoO-NP-15 has superior electronic conductivity, and the OER on this electrode is more kinetically promising than that on bare Co3O4-NP electrode; these results are consistent with those obtained from cyclic voltammetry measurements. To evaluate the longterm durability of the WCoO-NP-15 composite electrode during electrolysis, the CA response was evaluated at constant potential and or constant current in alkaline media. For comparison, commercial IrO2 catalysts were assessed under identical conditions. Fig. 7b presents the CA and chronopotentiometry responses of WCoO-NP-15 and the standard IrO2 catalysts assessed for 11 h at a constant potential of 1.50 and 1.55 V vs. RHE and at 10 mA cm−2 constant current in alkaline media. The long-term stability measurements of WCoO-NP-15 electrode revealed that the current density was very stable with no apparent loss during the prolonged electrolysis process. Interestingly, by increasing the electrolysis potential by only 50 mV the current density almost doubled up and increased from 12 to 23 mA cm−2 at 1.50 and 1.55 V vs. RHE respectively. Moreover, when the electrolysis current of

synergistic coupling between Co3O4 and W is vital component for the high OER features of the composite materials. The EIS measurements were performed to evaluate the electrode reaction kinetics and charge-transfer resistance of the Co3O4 nanoparticles before and after the incorporation of tungsten. Fig. 7a shows typical Nyquist impedance plots (Z' versus Z'') of the EIS data and equivalent circuit (inset) acquired for the bare Co3O4-NP and different WCoO-NP-x catalysts at 1.5 V bias in the frequency varying from 10−2 to 200 kHz and the impedance parameter are listed in Table 2. The lowfrequency region of the plot and its corresponding equivalent circuit (inset of Fig. 7a), agrees to the charge-transfer resistance (R2) of the investigated electrode materials. Moreover, it is obvious that the radii of the arc of the Nyquist plots of the WCoO-NP-x catalysts are lower with respect Co3O4-NP, signifying that the W-doped catalysts have smaller charge-transfer resistance (R2) values, again indicating the superior electrochemical OER performance, as stated earlier. Moreover, the obtained results show that the WCoO-NP-15 samples have enhanced charge transfer kinetics and functions as an efficient OER catalyst. The R2 values for Co3O4-NP, WCoO-NP-5, WCoO-NP-10, and WCoO-NP-15 were found to be 30, 18.97, 15.53, and 7.90 Ω, respectively, as 8

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dissolved in alkaline solution at high oxidation potentials [59,60]. Finally, the electrocatalytic performance of the WCoO-NP catalysts for OER could be attributed to the incorporation of optimal W concentration into cobalt oxide improves the surface active sites for better electron transfer, adsorption, and desorption of the associated OER intermediates. Moreover, the small size of nanoparticles enhances the electroactive surface area and improves the accessibility to the catalyst active sites. After the chronoamperometry tests, the loaded materials were cautiously detached from the GC electrode and examined by means of HR-TEM (Fig. S6). The observed nanoparticles remain the same and undisturbed lattice fringes indexed to Co3O4 crystalline structure can be noticed, indicating good structural and crystal phase stability of the fabricated materials. 3.3. Electrocatalytic activity of W doped CoO-NP catalysts for ORR The ORR electrocatalytic activities of the Co3O4-NP and WCoO-NP catalysts were observed via CV analysis in 1.0 M KOH at ambient condition. Fig. 8a presents the LSV curve acquired using a GC rotating disc electrode (RDE) added with 58 μg of Co3O4-NP, WCoO-NP-5, WCoO-NP-10, WCoO-NP-15 and WCoO-NP-20 materials at a sweep rate of 5 mVs-1 and at a constant speed of 2000 rpm in alkaline media. As presented in Fig. 8a, no noticeable redox peaks were detected for WCoO-NP-15 electrodes in the N2-saturated alkaline solution (brown line). When the alkaline solution was purged with O2, a distinct cathodic peak around 0.6 V vs. RHE was clearly observed for all studied electrodes and WCoO-NP-15 electrocatalyst showed the greatest electrochemical catalytic performance for the ORR. Remarkably, the WCoO-NP-15 catalysts showed much more ORR onset potential as well as superior current density about 2.7 mA cm−2, which is considerably greater with respect to pure Co3O4-NP (1.08 mA cm−2) and other reported catalysts (Table S3), suggesting synergistic ORR catalytic features of Co3O4 and W in the composite electrodes. On the other hand, from the linear part of the Tafel plots (Fig. S8) and its corresponding slopes are shown in Table S4. All the investigated WCoO-NP catalysts display well-distinct linear ORR Tafel regions with slope varying from 120 to 84 mV/dec for ORR region. Particularly, the Tafel slope for WCoO-NP-15 was 84 mV/dec, which was lower than bare Co3O4-NP (118 mV/dec). Further, Fig. 8b shows the RDE measurements were conducted at different rotation speed to examine the electrocatalytic performances and kinetics of the WCoO-NP-15 electrocatalyst in O2saturated alkaline solution. The rotational speed of the RDE was increased from 500 to 3000 rpm for the ORR measurements and the diffusion current densities of WCoO-NP-15 improved considerably with increasing RDE speed owing to the decreased O2 diffusion distance. To evaluate the number of transferred electrons and the reaction kinetics for the ORR, Fig. 8c shows the ORR current as analyzed using the K–L equation [61]. The related K–L plots between the inverse current density (mA−1 cm2) and ω−1/2 (rpm−1/2) of WCoO-NP-15 electrode at numerous potentials indicate the first-order kinetics of the ORR in the basic medium. Further, the evaluated number of electrons transported per O2 molecule (n) in the water redox reactions was in the range of 3.2–3.6 (Fig. 8c) for the applied bias range of 0.25–0.45 V, which specifying the 4e− oxygen reduction process is favoured. This value is closer to the described value of 4.00 for the Pt/C catalyst [62]. The EIS measurements of the WCoO-NP catalysts are presented in Fig. S7. With increased W content in the electrode, the obtained diameter of the semicircle in the Nyquist plot decreased. The assessed Rct parameter follows the order Co3O4-NP > WCoO-NP-5 > WCoO-NP20 > WCoO-10 > WCoO-15, displaying an inverse correlation with activity, as verified by CV and RDE analysis. Lastly, the incorporation of W into Co3O4 is capable of promoting the ORR performance, resulting in more positive onset potentials, as well as peak potentials, superior diffusion-limiting current densities, considerable greater average electron transfer numbers, and ultra-low charge-transfer resistance (Table S3). Further, to evidence the ORR catalytic pathways of the WCoO-NP-

Fig. 7. (a) Nyquist plots derived from EIS measurements in 1 M KOH at 1.5 V vs RHE of pure Co3O4 NP and W-CoO nanoparticles based catalysts and (b) Chronoamperometry and chronopotentiometry responses WCoO-NP-15 and commercial IrO2 catalysts measured in 1.0 M KOH under a constant potential of 1.55 V vs RHE and current density of 10 mA/cm−2 respectively. Table 2 Impedance parameter values derived from the fitting to the equivalent circuit for the impedance spectra recorded in 1.0 M KOH solution. Rs = solution resistance, R1 = oxide film resistance, R2 = charge-transfer resistance, Q1 = CPE of catalyst layer, Q2 represents the double layer capacitance. Samples

Rs (ohm)

Q1 (µF)

R1 (ohm)

Q2 (µF)

R2 (ohm)

Co3O4-NP WCoO-NP-5 WCoO-NP-10 WCoO-NP-15 WCoO-NP-20

12.44 12.58 12.75 12.46 12.88

131 966 999 1284 1033

290.8 75.60 53.50 13.27 15.42

363 1100 2301 2370 1865

30.0 18.97 15.53 7.90 3.60

10 mA cm−2 was applied, no apparent potential drift was observed and potential of 1.52 V vs. RHE was sustained during 11 h electrolysis process (Fig. 7b, blue line). In particular, CA investigation of the WCoONP-15 composites revealed no evidence of corrosion, surface passivation, or degradation throughout electrolysis. These negative effects are frequently observed for simple metal oxide catalysts, which are 9

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Fig. 8. ORR performances of catalysts (a) the LSV plots of Co3O4 NP, WCoO-NP-5, WCoO-NP-10, WCoO-NP-15 and WCoO-NP-20 loaded on GC electrode at a sweep rate of 10 mV s−1 at 2000 rpm in 1.0 M KOH solution, (b) comparative cathodic LSV plots at 10 mV s−1 of optimized WCoO-NP-15 and Co3O4 NP catalysts/GC electrode in Ar and O2 saturated 1.0 M of KOH solution, (c) Koutecky-Levich (K-L) plots (at potentials of 0.25–0.45 V vs. RHE) for WCoO-NP-15 nanoparticles catalysts. LSV curve at 10 mV s−1 for WCoO-NP-15 loaded on GC at varied rotation speed in O2 saturated electrolyte of 1.0 M KOH, (d) the LSV of both OER and ORR at scan rate of 10 mV s−1 of Co3O4 NP, WCoO-NP-15, IrO2, and Pt/C catalysts loaded on GC electrode in O2-saturated 1.0 M KOH solution.

least ΔE energy of 0.92 V among the studied catalysts and the obtained value is remarkably lower than the ΔE for commercial IrO2 (0.96 V) and Pt/C catalyst (1.18 V). These electrochemical investigations revealed that the fabricated WCoO-NP catalysts are a proficient bifunctional catalyst for both the ORR and the OER. Lastly, these results validate the significance of preparing binary metal oxides and the strong synergetic role of tungsten doping to cobalt oxide enhancing the OER and ORR activities for electrochemical energy-related applications.

15 nanocomposites, we performed rotating ring-disk electrode (RRDE) investigation to observe the generation of peroxide species during the reaction (Fig. S9a) [63]. The measured HO2− yields are below ~5% for WCoO-NP nanocomposites over the potential region of 0.30–0.50 V, giving electron transfer number of ~3.7 (Fig. S9b). The obtained results are consistent with the result from the K-L plots of RDE, signifying the ORR catalyzed by WCoO based catalysts is mostly 4e− reduction. The electrocatalytic performance for OER and ORR reaction is measured by the value of voltage gap (ΔE), obtained using the overpotential required for the current density of 10 mA/cm2 for OER and 2 mA/cm2 for ORR, correspondingly [64]. Fig. 8d displays the LSV of Co3O4-NP, WCoO-NP-15 and for comparative purpose, the commercial Pt/C and benchmark IrO2 catalysts electrode in O2-saturated basic medium at a sweep rate of 10 mV s−1. The WCoO-NP-15 reveals the

4. Conclusions We have fabricated highly OER/ORR-active low-cost tungstendoped cobalt oxide catalysts via an interfacial soft-template method using a citric-acid-assisted surfactant self-assembly strategy. The effects 10

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of the degree of W-doping on the structural morphologies and electrocatalytic activity of the fabricated WCoO-NP were examined. Among the examined materials the WCoO-NP-15 catalyst (i.e., 15 mol% W doping) was a proficient electrocatalytic material for both the OER and ORR in basic medium. Moreover, the resultant WCoO-NP catalysts, as dual-function metal electrocatalysts, displayed excellent electrocatalytic activities for the OER and ORR with higher current densities, low onset potentials (190 mV for OER), and excellent stability better than the state-of-the-art IrO2 electrocatalyst. The enhanced activity of the WCoO-NP materials is credited to the good electronic conductivity and the existence of a mesoporous network, which assists rapid reactants and products diffusion and interfacial electron transport throughout the electrocatalytic process. Lastly, the synergetic coupling of composite materials contains only abundant elements making them promising catalysts for energy applications.

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CRediT authorship contribution statement Mabrook S. Amer: Data curation, Formal analysis, Investigation, Writing - original draft. Prabhakarn Arunachalam: Conceptualization, Data curation, Formal analysis, Investigation, Supervision, Writing original draft, Writing - review & editing. Mohamed A. Ghanem: Supervision, Writing - review & editing. Abdullah M. Al-Mayouf: Funding acquisition, Project administration. Muhammad Ali Shar: Investigation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful to the Deanship of Scientific Research, King Saud University for funding through the Vice Deanship of Scientific Research Chairs. The authors also thank the Deanship of Scientific Research and RSSU at King Saud University for their technical support. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145831. References [1] Y. Zhao, K. Kamiya, K. Hashimoto, S. Nakanishi, Efficient bifunctional Fe/C/N electrocatalysts for oxygen reduction and evolution reaction, J. Phys. Chem. C 119 (2015) 2583–2588. [2] N. Garg, M. Mishra, Govind, A.K. Ganguli, Electrochemical and magnetic properties of nanostructured CoMn2O4 and Co2MnO4, RSC Adv. 5 (2015) 84988–84998. [3] S. Marini, P. Salvi, P. Nelli, R. Pesenti, M. Villa, M. Berrettoni, G. Zangari, Y. Kiros, Advanced alkaline water electrolysis, Electrochim. Acta 82 (2012) 384–391. [4] D.S. Kong, J.J. Cha, H.T. Wang, H.R. Lee, Y. Cui, First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction, Energy Environ. Sci. 6 (2013) 3553–3558. [5] Z. Peng, D. Jia, A.M. Al-Enizi, A.A. Elzatahry, G.F. Zheng, Electrocatalysts: From water oxidation to reduction: Homologous Ni−Co-based nanowires as complementary water splitting electrocatalysts, Adv. Energy Mater. 5 (2015) 1402031. [6] S. Feng, C. Liu, Z. Chai, Q. Li, D. Xu, Cobalt-based hydroxide nanoparticles @ Ndoping carbonic frameworks core–shell structures as highly efficient bifunctional electrocatalysts for oxygen evolution and oxygen reduction reactions, Nano Res. 11 (2018) 1482–1489. [7] M.M. Shahid, P. Rameshkumar, W.J. Basirun, J.C. Juan, N.M. Huang, Cobalt oxide nanocubes interleaved reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction in alkaline medium, Electrochim. Acta 237 (2017) 61–68. [8] C. Karuppiah, S. Wang, R. Devasenathipathy, C.C. Yang, Dry particle coating preparation of highly conductive LaMnO3@ C composite for the oxygen reduction reaction and hydrogen peroxide sensing, J. Taiwan Inst. Chem. Eng. 93 (2018) 94–102.

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