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Three-Dimensional Graphene-Supported Ni3Fe/Co9S8 Composites: Rational Design and Active for Oxygen Reversible Electrocatalysis Xuejiao Hu,‡ Tan Huang,† Yawen Tang,*,‡ Gengtao Fu,*,† and Jong-Min Lee*,† †
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
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ABSTRACT: The development of low-cost and efficient electrocatalysts with a bicomponent active surface for reversible oxygen electrode reactions is highly desirable and challenging. Herein, we develop an effective calcinationhydrothermal approach to fabricate graphene aerogelanchored Ni3Fe−Co9S8 bifunctional electrocatalyst (Ni3Fe− Co9S8/rGO). The mutually beneficial Ni3Fe−Co9S8 bifunctional active components efficiently balance the performance of oxygen reduction and oxygen evolution reactions (ORR/ OER), in which Co9S8 promotes the ORR and Ni3Fe facilitates the OER. This balance behavior has an obvious advantage over that of monocomponent Ni3Fe/rGO and Co9S8/rGO catalysts. Meanwhile, the additional synergy between porous rGO aerogels and Ni3Fe−Co9S8 endows the composite with more exposed active sites, faster electrons/ions transport rate, and better structural stability. Benefiting from the reasonable material selection and structural design, the Ni3Fe−Co9S8/rGO exhibits not only outstanding ORR activity with the high onset- and half-wave potentials (Eonset = 0.91 V and E1/2 = 0.80 V) but also satisfactory OER activity with a low overpotential at 10 mA cm−2 (0.39 V). Moreover, rechargeable Zn−air cells equipped with Ni3Fe−Co9S8/rGO exhibit excellent rechargeability and a fast dynamic response. KEYWORDS: Ni3Fe−Co9S8 composite, graphene, bifunctional electrocatalyst, synergetic effects, Zn−air batteries
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potential in ORR,40−47 because of high chemical stability and prominent structure advantages, better than other metal chalcogenides. Theoretical studies also predicted the ORR activity of cobalt chalcogenides similar to that of Pt through a 4-electron path.48 Therefore, the synergistic integration of the NiFe phase with cobalt chalcogenides should be a rational strategy to achieve the effective bifunctional catalysts. The precondition is that there is a suitable synthetic method that allows two types of materials to be perfectly combined. A common strategy is to physically combine the aforementioned ORR and OER catalysts.49,50 Nevertheless, owing to poor compatibility of the different components, such approach is often neither effective nor reliable.1 Such being the case, it is still challenging, but highly desirable, for exploring an effective approach to combine the two functionalities into a single bifunctional electrocatalyst for the reversible oxygen redox. Inspired by the aforementioned ideas, we design a composite bifunctional oxygen electrocatalyst obtained by a controlled two-step synthesis of ultrathin Ni3Fe particles and Co9S8 nanosheets on three-dimensional (3D) reduced graphene oxide (Ni3Fe−Co9S8/rGO). This method involves the initial formation of Ni3Fe/rGO through a novel hydrogel strategy
INTRODUCTION The metal-air batteries have attracted wide attention recently as an energy storage technology on account of their high energy density (energy per unit volume), low cost, and ecofriendly operation.1−5 The overall charge−discharge efficiency of metal−air batteries greatly depends on the reversibility of reversible oxygen reactions, which in turn is strongly associated with the activity of bifunctional catalysts.6−10 The carbon-supported Pt and RuO2 or IrO2 catalysts are the most efficient materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, but they are expensive and rare.11−17 Recently, the Fe-doped Nibased materials, such as alloys,18−20 nitrides,21−25 oxides,26−28 and (oxy)hydroxides,11,29−35 have been reported as one of the most effective OER electrocatalysts. Corrigan and Bendert first attributed the increased activity to the improved conductivity after Fe introduction,36 but the detailed mechanism remains unclear. Unfortunately, they cannot drive the reverse reactions efficiently owing to an unsatisfactory ORR activity. To compensate for such defects, various types of nanocarbons have been incorporated by different synthetic methods.20,21,37−39 However, the carbon oxidation or corrosion under the harsh OER condition36,37 has been identified as one of the main issues that vastly affects the catalysts’ stability.11,38 As viable alternatives, the cobalt chalcogenides (such as Co1−xS, CoS, Co3S4, Co9S8) had been reported to have great © XXXX American Chemical Society
Received: November 13, 2018 Accepted: January 7, 2019
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DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the preparation of Ni3Fe−Co9S8/rGO; (b) schematic interaction between PVA and GO; (c) XRD patterns of Ni3Fe−Co9S8/rGO, Ni3Fe/rGO, and Co9S8/rGO; (d) Raman spectra of Ni3Fe−Co9S8/rGO, Ni3Fe/rGO, and Co9S8/rGO.
Figure 2. (a,b) SEM images of Ni3Fe−Co9S8/rGO at different magnifications; (c) N2 adsorption−desorption isotherms of Ni3Fe−Co9S8/rGO; inset shows the corresponding pore distribution curve; (d−g) TEM images of Ni3Fe−Co9S8/rGO at different magnifications; HRTEM image of (h) Co9S8 nanosheet and (i) Ni3Fe nanoparticle, respectively.
response, outperforming those of conventional Pt/C + RuO2 mixture.
and subsequent hydrothermal growth of Co9S8 nanosheets on the Ni3Fe/rGO surface. The electrocatalytic characterizations demonstrate that the mutually beneficial Ni3Fe−Co9S8 bifunctional active components can endow Ni3Fe−Co9S8/rGO with high activity propelling O2 redox, better than themselves alone for ORR or OER. Moreover, the strong interface coupling of Ni3Fe−Co9S8 with 3D rGO aerogels not only promotes the accessibility of active sites but also is conducive to rapid mass and electron transfers through the whole system. Benefiting from the collaborative advantages, the assembled Zn−air cells based on the Ni3Fe−Co9S8/rGO present outstanding battery performance with a high power density (125 mW cm−2), a long-term cycling life over 4000 min, and a fast dynamic
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RESULTS AND DISCUSSION Preparation and Characterization of Catalysts. The Ni3Fe−Co9S8/rGO was fabricated via a two-step method combining thermodynamic calcination with hydrothermal reaction, as shown in Figure 1a. The Ni3Fe/rGO was first synthesized by simple polymerization of Poly (vinyl alcohol) (PVA), GO, and Ni/Fe precursors (Figure S1), followed by freeze-drying and calcination. In this step, the hydrogel formation depends on the hydrogen bonding between PVA and GO (Figure 1b),51−53 as confirmed by Fourier transform B
DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) TGA curve of Ni3Fe−Co9S8/rGO; (b) full XPS spectrum of Ni3Fe−Co9S8/rGO; (c) atom contents of Ni, Fe, Co, and S recorded from the panel (b); high-resolution XPS spectra of Ni3Fe−Co9S8/rGO at (d) Ni 2p, (e) Fe 2p, (f) Co 2p, and (g) S 2p region, respectively; XPS spectra of (h) Ni 2p and (i) Fe 2p in Ni3Fe−Co9S8/rGO and Ni3Fe/rGO.
(Figure 1d). The D and G bands could be observed at about 1348 and 1596 cm−1, respectively. Relative to Co9S8/rGO, both Ni3Fe−Co9S8/rGO and Ni3Fe/rGO samples exhibit larger graphitic degree, as confirmed from lower peak intensity of D to G band.56 This result indicates that the graphitic crystallinity of catalysts strongly depends on the reduction method of GO. Figure S3 shows the typical scanning-/transmission-electron microscopy (SEM/TEM) images of the Ni3Fe/rGO sample. An interconnected 3D porous structure with random open pores constructed from the rGO sheets was observed. The Ni3Fe particles are deposited on the rGO surface uniformly, with an ultrathin particle size (ca. 5 nm). The ultrathin size is ascribed to the confinement of graphene network and the existence of PVA,57 suppressing the growth and agglomeration of Ni3Fe particles. These results demonstrate that the present Mn+/GO-PVA hydrogel-derived method can effectively control both the structure of carbon supports and the particle size of catalysts. After decorating of Co9S8, Ni3Fe−Co9S8/rGO was observed to manifest the akin 3D interconnected architecture to the Ni3Fe/rGO (Figure 2a), whereas the sheet-like subunits of Co9S8 were formed on the Ni3Fe/rGO surface (Figure 2b). The porosity of Ni3Fe−Co9S8/rGO was investigated in detail by N2 adsorption−desorption experiment. The isotherm curves (Figure 2c) present a classical H3-type hysteresis loop,58,59 indicating the characteristics of mesoporous materials. The Brunauer−Emmett−Teller surface area was determined to be 137.5 m2 g−1. Ni3Fe−Co9S8/rGO displays a wide pore-size distribution from 20 to 300 nm. The porosity of Ni3Fe−Co9S8/rGO would be beneficial for the electrolyte
infrared spectroscopy (FT-IR) spectrum (Figure S2). Meanwhile, the electronegative oxygenated group of GO can efficiently bind metal ions (Mn+), resulting in the formation of Mn+/GO-PVA composite hydrogel.53,54 As a result, the Ni3Fe nanoparticles can be in situ anchored on the 3D porous rGO scaffold uniformly after freeze-drying and calcination. Subsequently, Co9S8 nanosheets were grown on the Ni3Fe/ rGO surface via simply adding CoCl2, ethylene glycol (EG), and thioacetamide (TAA) as the reaction sources under the hydrothermal condition. When the solvothermal reaction occurs, the Co2+ cations first react with EG to form Cobased glycolate complexes. For the Ni3Fe alloy, it is difficult to react with the EG. Moreover, the formed glycolate complexes can easily react with S2− ions (which come from the hydrolysis of TAA: CH3CSNH2 + H2O ↔ CH3CONH2 + H2S).55 Therefore, S2− ions would preferentially react with Co2+ ions rather than with Ni3Fe metal, resulting in the formation of Co9S8. The crystal structure of Ni3Fe−Co9S8/rGO was confirmed by X-ray diffraction (XRD, Figure 1c), from which two main crystalline phases were observed, attributing to the Ni3Fe phase (JCPDS card no. 65-3244) and the Co9S8 phase (JCPDS card no. 65-6801), respectively. Apart from two main crystalline phases (Ni3Fe and Co9S8), a broad diffraction peak at 25.1° is attributed to the (002) reflection of carbon. For comparison, the monocomponent Ni3Fe/rGO and Co9S8/ rGO catalysts were prepared, and their crystal structures were also confirmed by XRD (Figure 1c). Although three catalysts (i.e., Ni3Fe−Co9S8/rGO, Ni3Fe/rGO, and Co9S8/rGO) use the rGO as the carbon support, there is a great difference in the degree of graphitization, as verified via Raman spectroscopy C
DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) ORR polarization curves in O2-saturated 0.1 M KOH (rotation rate: 1600 rpm; sweep rate: 5 mV s−1) of Ni3Fe−Co9S8/rGO, Ni3Fe/ rGO, Co9S8/rGO, and Pt/C catalysts; (b) bar plots of the Eonset and E1/2; (c) percentage of peroxide with respect to the total oxygen reduction products and corresponding electron transfer number n at different potentials; (d) bar plots of the average electron transfer number and percentage of peroxide.
accessibility and expose more active sites.24,60 Scanning TEM and TEM images (Figures S4 and 2d) further validate the porosity and active particles uniformly distribute on the rGO surface. Magnified TEM images (Figure 2e−g) indicate the coexistence of Ni3Fe particles and Co9S8 nanosheets. The highresolution TEM (HRTEM) images of Ni3Fe−Co9S8/rGO in Figure 2h,i show two kinds of clear lattice fringes with interplanar distances of approximately 0.35 and 0.21 nm, which correspond to the (220) plane of Co9S8 and (111) plane of Ni3Fe, respectively. Furthermore, the Ni, Fe, Co, and S atoms have been well distributed on the surfaces of rGO according to the energy-dispersive X-ray spectroscopy (EDX) element mappings (Figure S5). The corresponding EDX spectrum (Figure S6) confirms that the molar ratio of Ni/Fe and Co/S was around 2.5/1 and 9/12.8, respectively. The Co/S molar ratio (9/12.8) is found to be much smaller than that of the theoretical Co/S value in Co9S8 (9/8), which clearly demonstrates that S atoms are also incorporated into other components besides Co.61 All the above results demonstrated the coexistence of an interconnected 3D porous network and well-dispersed two-phase active components in Ni3Fe−Co9S8/ rGO. To determine the content of Ni3Fe and Co9S8 within the rGO aerogels, thermogravimetric analysis (TGA) was carried out (Figure 3a). After the TGA, Ni3Fe and Co9S8 were oxidized to NiO, Fe3O4, and Co3O4 completely (Figure S7), and the content of the oxides is about 30.1%. On the basis of the EDX analysis, the atomic contents of Ni, Fe, and Co in Ni3Fe−Co9S8/rGO also can be obtained. By combining EDX analysis and the content of oxides, the contents of Ni3Fe and Co9S8 in Ni3Fe−Co9S8/rGO are easier to deduce from reaction equilibrium. It was calculated to be about 16.1 wt % for Ni3Fe content and 8.6 wt % for Co9S8 content in Ni3Fe− Co9S8/rGO. The surface properties of the as-prepared Ni3Fe−
Co9S8/rGO were investigated with X-ray photoelectron spectroscopy (XPS). The full XPS spectrum in Figure 3b contains all expected Fe 2p, Ni 2p, Co 2p, S 2p, and C 1s signals. The presence of the O 1s signal probably results from surface oxidation of metal species or the oxygenated functionality of carbon. The surface atomic content of Ni, Fe, Co, and S was calculated as 39.6, 16.0, 18.5, and 25.9% (Figure 3c), in accordance with EDX analysis (Figure S6). Figure 3d,e show the high-resolution Ni 2p and Fe 2p XPS spectrum, respectively. The presence of Ni0 and Fe0 in the Ni3Fe−Co9S8/rGO composite was verified by the spin−orbit doublets at 853.5 eV/870.6 eV (Figure 3d) and 706.1 eV/ 720.3 eV (Figure 3e), demonstrating the formation of Ni3Fe alloy. The doublets at 856.5 eV/873.9 eV and 712.8 eV/724.9 eV should be assigned to Ni2+ and Fe3+/Fe2+ accordingly, all of which stem from the in situ oxides formed at the exposed surfaces of Ni3Fe alloy.38,62 As the XRD did not detect the oxide phases, it is possible that these surface oxides are either amorphous or clusters. The high-resolution Co 2p spectrum was fitted with two spin−orbit doublets (Figure 3f), characteristic of 2p1/2 and 2p3/2, respectively. The peaks located at 777.7, 780.8, 792.6, and 796.7 eV are attributed to Co−S bond, whereas the peaks at 786.3 and 802.4 eV correspond to Co−O species.61,63 The Co−S bond was further identified via high-resolution S 2p XPS spectrum at 160.6 and 162.0 eV (Figure 3g).61 The peak located at 168.1 eV is assigned to S− O species because of the partial oxidation of S. After decorating of Co9S8, we found that the Ni 2p peak in Ni3Fe−Co9S8/rGO negatively shifts by ∼0.55 eV (Figure 3h), whereas the Fe 2p peak positively shifts by ∼0.76 eV (Figure 3i), compared to those of Ni3Fe/rGO. This implies possible electron interaction between Ni3Fe and Co9S8 phases with the redistribution of charge, which can be explained by the different electronegativity of three elements (Fe < Co < Ni). The larger the D
DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) OER polarization curves in O2-saturated 0.1 M KOH (rotation rate: 1600 rpm; sweep rate: 5 mV s−1) of Ni3Fe−Co9S8/rGO, Ni3Fe/ rGO, Co9S8/rGO, and RuO2 catalysts; (b) current densities at a chosen potential of 1.65 V and overpotentials at a chosen current density of 10 mA cm−1; (c) overall polarization curves of catalysts; (d) comparison of the bifunctional activities in this work with representative catalysts in references (the dotted lines show the ΔE at constant values). (e) Schematic illustrations of structural advantages for Ni3Fe−Co9S8/rGO; (f) bar plots of the degradation percentage of the catalysts for the ORR and OER after the stability tests.
for the ORR than the Ni3Fe component. On the one hand, the S2‑ ions offer more adsorption sites for O2 and facilitate O−O bond scission;47,48 on the other hand, the OH− covered (202) plane of Co9S8 is active for the ORR.47,48 Even compared with Pt/C catalyst, the Ni3Fe−Co9S8/rGO catalyst has only a slightly smaller Eonset value of 0.07 V and E1/2 value of 0.05 V. Also, Ni3Fe−Co9S8/rGO exhibits the lowest Tafel slope of about 75.6 mV dec−1 among all the studied catalysts (Figure S8), indicating favorable ORR catalytic kinetics of Ni3Fe− Co9S8/rGO. To quantify the electron transfer number (n) and the yield of the peroxide intermediate (H2O2 %), the rotating ring-disk electrode measurements were further carried out (Figure S9). As indicated in Figure 4c, Ni3Fe−Co9S8/rGO exhibits higher electron transfer number and lower H2O2 yield in the potential range from 0.2 to 0.7 V than those of monocomponent Ni3Fe/rGO and Co9S8/rGO catalysts. The average n value of Ni3Fe−Co9S8/rGO catalyst was calculated as 3.71 with a low H2O2 yield of 14.3% (Figure 4d), approaching the values of Pt/C (n: 3.87 and H2O2 %: 9.8). This result suggests that the ORR process on Ni3Fe−Co9S8/ rGO was a dominant 4-electron pathway (O2 + 2H2O + 4e− → 4OH−).
electronegativity of the metal element, the stronger the ability to attract electrons, and vice versa. The electronic interaction is important for regulating the electronic environments of metal centers, which can optimize the adsorption energy of reaction intermediates.64,65 Bifunctional Electrocatalysis. Given that the interconnected 3D porous architecture (promotes the mass and electron transport) and well-dispersed two-phase active components (present two different catalytic behavior) coexist in the Ni3Fe−Co9S8/rGO, Ni3Fe−Co9S8/rGO should be very promising as the bifunctional catalyst. First, the ORR properties of Ni3Fe−Co9S8/rGO were evaluated through the rotating disk electrode technique. The Pt/C, Ni3Fe/rGO, and Co9S8/rGO catalysts were selected for comparison. Figure 4a presented the linear sweep voltammetry (LSV) of these four catalysts recorded in 0.1 M KOH. The LSV curve of Ni3Fe− Co9S8/rGO shows a high onset and half-wave potentials (Eonset = 0.91 V and E1/2 = 0.80 V vs RHE), which outperforms those of Co9S8/rGO (Eonset = 0.88 V, E1/2 = 0.73 V) and Ni3Fe/rGO (Eonset = 0.81 V, E1/2 = 0.56 V) catalysts. The detailed comparison is shown in Figure 4b. Relative to the Ni3Fe/rGO catalyst, Co9S8/rGO presents the more positive Eonset and E1/2 values, indicating that the Co9S8 component is more effective E
DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a) Open-circuit plots of Zn−air cell driven by Ni3Fe−Co9S8/rGO; inset shows the illustration of Zn−air cell; (b) discharge profiles of Ni3Fe−Co9S8/rGO-based Zn−air cell at different current densities (OCV, 10, 20, and 50 mA cm−2, and recovery to OCV); (c) charge−discharge cycling curves of Zn−air cell driven by Ni3Fe−Co9S8/rGO at a current density of 10 mA cm−2.
(ΔE(Co9S8/rGO) = 1.01 V; ΔE(Ni3Fe/rGO) = 1.12 V). This value is even comparable to that of the Pt/C + RuO2 catalyst with mass ratio of 1:1 (ΔE = 0.76 V) and many of the non-precious metal-based catalysts (Figure 5d and Table S1). By combining ORR and OER analyses, we can draw a conclusion that the synergy of Ni3Fe and Co9S8 endows Ni3Fe−Co9S8/rGO with better electrocatalytic activities than themselves alone for ORR or OER. In other words, the mutually beneficial Ni3Fe−Co9S8 bifunctional active components can efficiently balance both the ORR and the OER performance, which is schematically illustrated in Figure 5e. The electronic interaction in the Ni3Fe−Co9S8/rGO electrode (as confirmed by XPS) can modulate the adsorption energies for ORR/OER intermediates to some extent, resulting in the optimization of the catalytic reaction path.64,65 Apart from the synergetic effect between Ni3Fe and Co9S8 phases, the strong interface coupling between Ni3Fe−Co9S8 and 3D porous rGO aerogel is also important for the outstanding bifunctional performance: (i) 3D porous architecture with high surface area can guarantee more exposed active sites generated from the Ni3Fe−Co9S8 active component accessible to the electrolyte;45,66 (ii) the porous network is also favorable for the mass transport of electrolyte and intermediate species, as well as the electron mobility between Ni3Fe−Co9S8 and rGO aerogel, allowing a rapid electrocatalytic reaction.24,60 The stability of the catalysts was further investigated by chronoamperometric measurements (Figure S13). For the ORR stability, the Ni3Fe−Co9S8/rGO exhibits only slight current attenuation of around 22% after 40 000 s continuous running (Figure 5f), which is much lower than that of Co9S8/rGO (33%), Ni3Fe/ rGO (27%), and Pt/C catalysts (39%). Similarly, the Ni3Fe− Co9S8/rGO also shows outstanding OER stability, with respect to other counterparts (Figure 5f). Compared with commercial catalysts (Pt/C and RuO2), the significantly improved stabilities of Ni3Fe−Co9S8/rGO may be because of the hierarchical structure, which prevents particle aggregation,24,67
The electrocatalytic property of Ni3Fe−Co9S8/rGO toward OER was also studied, as depicted in Figure 5a. The remarkable OER activity of Ni3Fe−Co9S8/rGO is exhibited via its current density being larger than that of Ni3Fe/rGO and Co9S8/rGO catalysts. At 1.65 V, the current density of Ni3Fe− Co9S8/rGO can reach 12.2 mA cm−2 (Figure 5b), larger than that of Ni3Fe/rGO (8.5 mA cm−2) and Co9S8/rGO (4.1 mA cm−2). Meanwhile, Ni3Fe−Co9S8/rGO shows a smaller overpotential (0.39 V) compared to that of Ni3Fe/rGO (0.45 V) and Co9S8/rGO (0.51 V) at 10 mA cm−2. Particularly, the current density at 1.65 V and overpotential at 10 mA cm−2 of Ni3Fe−Co9S8/rGO are even close to those of RuO2 (13.1 mA cm−2 and 0.38 V), demonstrating its superior electrocatalytic activity for the OER. The favorable OER kinetics of Ni3Fe−Co9S8/rGO was gleaned from its smaller Tafel slope of around 109.8 mV dec−1 (Figure S10) than Co9S8/rGO (112.7 mV dec−1) and Ni3Fe/rGO (141.2 mV dec−1). The intrinsic excellent OER activity of NiFe-based phase20,23 should be one of the key reasons for the high OER activity of Ni3Fe−Co9S8/rGO because of the relatively poor OER activity of Co9S8/rGO. Another possible reason for the excellent OER activity of Ni3Fe−Co9S8/rGO may be attributed to the relatively low charge transfer resistance of Ni3Fe/Co9S8/rGO because of its high graphitic degree. This is confirmed by electrochemical impedance spectroscopy (Figure S11). Furthermore, the electrochemical double-layer capacitance (Cdl) was also studied, as it is linearly proportional to the electrochemical active surface area. From Figure S12, Ni3Fe− Co9S8/rGO shows the highest Cdl value (21.6 mF cm−2), implying the existence of more abundant active surface area compared with Ni3Fe/rGO and Co9S8/rGO. The outstanding bifunctional performance of Ni3Fe−Co9S8/rGO was corroborated via overall oxygen activities evaluated by the potential difference (ΔE = EJ10 − E1/2) between the OER potential at 10 mA cm−2 (EJ10) and the E1/2 of ORR.20,56 As depicted in Figure 5c, Ni3Fe−Co9S8/rGO presents a smaller value (ΔE = 0.82 V) than that of the monocomponent catalysts F
DOI: 10.1021/acsami.8b19971 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces which can also be reflected in monocomponent Ni3Fe/rGO and Co9S8/rGO catalysts. Rechargeable Zn−Air Batteries. Benefiting from outstanding bifunctional electrocatalytic activities, we further assembled the Ni3Fe−Co9S8/rGO into the air cathode of Zn−air cell to evaluate their practical applicability (Figure S14). The assembled cell with Ni3Fe−Co9S8/rGO affords a high open-circuit voltage (OCV) of 1.45 V versus Zn/Zn2+ (Figure 6a). The cell delivers a peak power density of about 125 mW cm−2, higher than that of a controlled cell driven (112 mW cm−2) by pairing Pt/C and RuO2 air cathode (Figure S15). This value can also be comparable to many previously reported batteries (Table S2). These results highlight the excellent electrocatalytic performance of the Ni3Fe−Co9S8/ rGO air cathode. The cell catalyzed by Ni3Fe−Co9S8/rGO also exhibited excellent rate capability and a fast dynamic response. As revealed in Figure 6b, the steady voltage plateaus vary with increasing current densities, indicating that the Ni3Fe−Co9S8/ rGO-driven Zn−air battery can be operated at a wide range of current densities. With the current density again decreased, an OCV value of about 1.45 can be recovered and without obvious decrease. The rechargeability of the Ni3Fe−Co9S8/ rGO air cathode was further investigated through the charge− discharge cycling tests. As observed in Figure 6c, the Zn−air cell of Ni3Fe−Co9S8/rGO can stably run for 200 cycles (4000 min) at 10 mA cm−2 (20 min for each cycle), which is better than that of Pt/C + RuO2-based cell. After only 35 cycles running (720 min), the discharge voltage of Pt/C + RuO2based Zn−air cell degrades to
