An Efficient Bifunctional Electrocatalyst for a Zinc–Air Battery Derived

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An Efficient Bifunctional Electrocatalyst for a Zinc−Air Battery Derived from Fe/N/C and Bimetallic Metal−Organic Framework Composites Mengfan Wang, Tao Qian,* Jinqiu Zhou, and Chenglin Yan* College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China S Supporting Information *

ABSTRACT: Efficient bifunctional electrocatalysts with desirable oxygen activities are closely related to practical applications of renewable energy systems including metal−air batteries, fuel cells, and water splitting. Here a composite material derived from a combination of bimetallic zeolitic imidazolate frameworks (denoted as BMZIFs) and Fe/ N/C framework was reported as an efficient bifunctional catalyst. Although BMZIF or Fe/N/C alone exhibits undesirable oxygen reaction activity, a combination of these materials shows unprecedented ORR (half-wave potential of 0.85 V as well as comparatively superior OER activities (potential@10 mA cm−2 of 1.64 V), outperforming not only a commercial Pt/C electrocatalyst but also most reported bifunctional electrocatalysts. We then tested its practical application in Zn−air batteries. The primary batteries exhibit a high peak power density of 235 mW cm−2, and the batteries are able to be operated smoothly for 100 cycles at a curent density of 10 mA cm−2. The unprecedented catalytic activity can be attritued to chemical coupling effects between Fe/N/C and BMZIF and will aid the development of highly active electrocatalysts and applications for electrochemical energy devices.

1. INTRODUCTION Owing to increasing energy demands, renewable energy systems including fuel cells,1,2 metal−air batteries3,4 and water splitting5,6 have drawn great attention because of their theoretically high energy densities, promising large-scale applications, low cost, and environmental friendliness. Among them, metal−air rechargeable batteries, especially zinc−air batteries, gained extensive attention for their fundamental advantage of being affordable, safe, and environmentally benign. However, stable and efficient bifunctional electrocatalysts for the oxygen evolution reaction (OER) during charging, and the oxygen reduction reaction (ORR) during discharging, were closely related to practical application of Zn−air battery technology, and the lack of such electrocatalysts motivated the development of low-cost but highly active catalysts to facilitate these reactions. Despite tremendous efforts, however, the relatively slow kinetics of the oxygen reactions continued to be a bottleneck because of their complicated multielectron transfer processes. Noble metals and their alloys were found to give the excellent catalytic performance. For instance, platinum and its alloys were widely used in the ORR process7,8 while ruthenium- and iridium-based nanocomposites were famous for their applications in OER catalysts.9,10 However, efforts to determine whether Pt, Ru, or Ir could exhibit outstanding ORR and OER electrocatalytic activity at the same time, let alone the long-term practical applications of these noble metal/metal © XXXX American Chemical Society

composites, were severely hampered by the prohibitive cost, scarcity, and declining activity during operation; thus, extensive studies pursuing alternatives based on nonprecious metals11,12 and metal-free materials,13,14 having abundant supply, low cost, and comparable electrocatalytic activity for oxygen reactions, were conducted. Thus far, non-noble metal−nitrogen−carbon (M−N−C, with M = Fe, Co, etc.) catalysts12,15−17 synthesized by high temperature carbonization of precursors including N, C, and particular transition metals were extensively studied due to their great potential for ORR catalysis. Although the intrinsic property of their catalytic sites was still controversial and can vary according to different M−N−C catalysts, undoubtedly their electrocatalytic performance largely depended on chemical composition, specific surface area, porous structure, and interactions between different components of the precursors.18,19 From this point of view, it was urgent to develop precursors with comparably high surface area and porosity that can ensure strong interaction among their components, thus facilitating electron transport and leading to higher ORR catalytic activity. To date, a handful of reports have described the construction of high surface area and porosity for enhancing Received: September 27, 2016 Accepted: January 19, 2017 Published: January 20, 2017 A

DOI: 10.1021/acsami.6b12197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces M−N−C catalysts’ ORR electrocatalytic activity.15,16,20 Nevertheless, these M−N−C catalysts still only delivered moderate OER performance, which was still inadequate for application in Zn−air batteries. On the other hand, cobalt-based catalysts were extensively investigated as OER catalysts due to the excellent conductivity and abundant electrochemically active sites.11,21−23 For instance, zeolitic imidazolate framework-67 (denoted as ZIF67)-derived material24−26 demonstrated promising OER catalytic activities in alkaline media due to its efficient exposure of catalytic active sites as well as free diffusion of O2 and electrolyte. Different from ZIF-67, ZIF-827 can offer high surface area with uniform nitrogen doping but low catalytic activity. Given these considerations, BMZIFs28 derived from the combination of ZIF-8 and ZIF-67 were synthesized to combine their merits to provide large surface area, uniform nitrogen doping, and high graphitization simultaneously. Although the ORR performance of BMZIFs can be improved by introducing heteroatoms, such as P, while bare BMZIF exhibits OER electrocatalytic activity better than that of a commercial carbon-supported platinum catalyst (20 wt % Pt), they still could not reach the desired performance. M−N−C and BMZIF catalysts did have advantages in one respect, in that these two materials could be integrated to compensate for each other’s defects and then, through combining the merits of both, enhance electrocatalytic activity in oxygen reactions to serve as a bifunctional catalyst. Herein we integrated Fe/N/C, which uses coordination complexes containing iron as precursor, with BMZIFs to fabricate Fe/N/ C@BMZIF composite for the first time, affording BMZIF nanocrystals homogeneously dispersed in an Fe/N/C framework. Significantly, the N-doped carbon ligaments between Fe/ N/C structures and BMZIF nanocrystals sped electron cloud migration at the Fe/N/C and BMZIF interface, revealing the combined effect between Fe/N/C and BMZIF in boosting the oxygen electrochemistry. The resultant Fe/N/C@BMZIF composite showed superior ORR and OER performance and excellent stability in 0.1 M KOH. Notably, Fe/N/C@BMZIF was also coated in an air electrode for Zn−air battery and exhibited desirable stability. This unprecedented material possesse performance far better than that of Pt/C or IrO2/C catalysts, thus presenting a novel way for developing earthabundant and low cost high-performance bifunctional electrocatalysts for metal−air batteries.

Figure 1. (a) Schematic reaction mechanism of the OER and ORR processes catalyzed by Fe/N/C@BMZIF. FESEM (b) and highmagnification FESEM (c) images of Fe/N/C@BMZIF. TEM (d) and HRTEM (e) images of Fe/N/C@BMZIF.

considerable segment of active sites, excellent electronic conductivity, and stable structure, which would be beneficial to enhance catalytic performance. The size of the individual BMZIF nanocrystals was about 200 nm, consistent with TEM data, which further revealed that BMZIF nanocrystals had a certain thickness of porous Fe/N/C decoration on the surface. High-resolution TEM (HRTEM) confirmed that BMZIF nanocrystals had a good crystalline structure and showed a crystal lattice of around 0.2 nm which suggested the existence of crystalline Co. The elemental distribution of a typical Fe/N/ C@BMZIF was further investigated by mapping as shown in Figure S1. It was obvious that Co (blue) mainly exists in the middle with spherical form while C (red), N (yellow), and Fe (green) are uniformly distributed over the whole surface. Iron was clearly present as confirmed by mapping, and if present in large quantities, nanophases should be visible in the images at this resolution, as observed for some types of Fe/N/C catalysts.29 Because no crystallinity indicative of such nanophases was detected in the TEM image, we can infer that Fe3C existed in the atomic form.30 The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area of different samples. The specific surface area and the total pore volume of Fe/N/C@BMZIF were 560 m2 g−1 and 1.05 cm3 g−1, respectively, attributed to the hierarchical porous network as shown from the above SEM and TEM images. More precisely, as shown in Figure 2a and Figure S2, these values were much larger than those of bare Fe/N/C (211 m2 g−1) or BMZIF (398 m2 g−1). Notably, Fe/N/C@ZIF-8 exhibited the largest specific surface area (670 m2 g−1), attributed to ZIF-8’s amorphous character, higher level of porosity, and nitrogen doping, while Fe/N/C@ZIF-67 only had a relatively small specific surface area (274 m2 g−1), perhaps because ZIF-67

2. RESULTS AND DISCUSSION The design and fabrication of Fe/N/C@BMZIF materials were realized according to a modified synthetic strategy. In a typical procedure, the polypyrrole−iron coordination complex and BMZIF nanocrystals were synthesized respectively and mixed together, followed by high temperature carbonization at 900 °C with argon gas protection. To highlight the outstanding performance of Fe/N/C@BMZIF, Fe/N/C@ZIF-8 and Fe/ N/C@ZIF-67 were built by a similar method with ZIF-8 and ZIF-67 instead of BMZIF. The intrinsic mechanism of oxygen reaction processes catalyzed by Fe/N/C@BMZIF is shown in Figure 1a. The morphologies of different samples were investigated via scanning electron microscopy (SEM). The structure details were examined with transmission electron microscopy (TEM) (Figure 1b−e). The SEM images show that the ligaments were centered on BMZIF nanocrystals and were highly interconnected in a hierarchical porous network, ensuring a B

DOI: 10.1021/acsami.6b12197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) N2 adsorption−desorption isotherms. (b) XRD pattern. (c) XPS spectra. (d) High-resolution XPS spectra of N 1s of Fe/N/C@ BMZIF.

resulted in higher graphitization. The crystalline nature of Fe/ N/C@BMZIF was further characterized by powder X-ray diffraction (XRD) as shown in Figure 2b. The peak located at about 26.5° was attributed to the C (002) plane, and the small peak at about 44.3° corresponded to metallic Co. As mentioned above, Fe existed in the atomic form and cannot indicate crystallinity. However, it would be unlikely that all Fe were atomic so there might be some Fe existing in the form of particulate phases that exhibited XRD reflection (Fe3C, JCPDS no. 89-2867). Furthermore, there must be a large amount of Co3C generated during the pyrolysis process, which also exhibited strong XRD reflection at about 45° as confirmed by JCPDS no. 89-2866. Because Fe3C and Co3C both exhibited XRD reflection at about 45°, the strong signal there can be understandable. The surface chemical composition and element bonding configurations of Fe/N/C@BMZIF were accurately evaluated by X-ray photoelectron spectroscopy (XPS) as shown in Figure 2c. XPS survey scans indicate the presence of carbon, oxygen, and nitrogen. The signals of iron and cobalt were too weak to be seen from the long-range XPS spectrum maybe because these two elements were tightly covered by nitrogendoped carbon phase, so the amounts of these elements were assessed from inductively coupled plasma (ICP). The results indicated the content of iron and cobalt to be 5.47% and 3.49%, respectively, which was in good agreement with that of the ideal product. The high-resolution N 1s spectrum in Figure 2d confirmed that are five forms of nitrogen including pyridinic-N/ N−Fe at ∼398.7 eV, pyrrolic-N at ∼400 eV, graphitic-N at ∼401.2 eV, oxidized-N at ∼403.8 eV, and Fe−Nx at ∼399.2 eV, in which the peak for graphitic-N was dominant because of the high degree of graphitization of this material. Although different nitrogen species possessed different chemical/electronic

contexts for neighboring C, thus affecting catalytic performance to various degrees, all of these N species were known to speed the ORR process apart from oxidized-N, the specific role of which was not well understood. To investigate the intrinsic nature of chemical interaction between N and transition metal, the Fe 2p spectrum and the Co 2p spectrum were further deconvoluted. Figure S3a presents the deconvoluted Fe 2p spectrum, where two sets of peaks for Fe2+ (710.6 and 722.3 eV) and Fe3+ (712.9 and 725.0 eV) exist, confirming the chemical coupling between iron and N/C moieties. Furthermore, there was also a satellite peak at ∼717.8 eV, suggesting the existence of FeOx phase.17 Similarly, the peaks in the Co 2p spectrum (Figure S3b) corresponded to both Co2+ and Co3+ forms and suggested that Co species existed in the form of nitrides or oxides. Moreover, there was also a satellite peak at ∼785.80 eV corresponding to the oxide phase of Co.31 Given the above desirable characterization, cyclic voltammetry (CV) was first used to examine the ORR activity of Fe/N/ C@BMZIF in O2 versus N2-saturated alkaline solution. As shown in Figure S4d, no redox peak was observed in the black curve when the solution was saturated with N2. On the contrary, if the gas in the solution was replaced with O2, an obvious peak was observed at ∼0.76 V, suggesting the superior ORR performance. CV curves of 20 wt % Pt/C, bare Fe/N/C, and BMZIF nanocrystals were also recorded for comparison (Figure S4a−c). Remarkably, Fe/N/C@BMZIF’s peak potential was not only superior to Pt/C’s peak potential (0.75 V) but also highly exceeded that of the reference samples (0.70 V for bare Fe/N/C, 0.59 V for BMZIF nanocrystals). The ORR performance of Fe/N/C@BMZIF was further studied by linear sweep voltammetry (LSV). A series of LSVs for different samples were collected by scanning the potentials C

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Figure 3. (a) ORR polarization curves of 20 wt % Pt/C, bare Fe/N/C, BMZIF, and Fe/N/C@BMZIF in O2-saturated 0.1 M KOH at room temperature with a sweep rate of 10 mV s−1 at a rotation speed of 1600 rpm. (b) Tafel plots of 20 wt % Pt/C, bare Fe/N/C, BMZIF, and Fe/N/C@ BMZIF. (c) ORR polarization curves of 20 wt % Pt/C, Fe/N/C@ZIF-8, Fe/N/C@ZIF-67, and Fe/N/C@BMZIF in O2-saturated 0.1 M KOH at room temperature with a sweep rate of 10 mV s−1 at a rotation speed of 1600 rpm. (d) Tafel plots of 20 wt % Pt/C, Fe/N/C@ZIF-8, Fe/N/C@ ZIF-67, and Fe/N/C@BMZIF.

Figure 4. (a) LSV curves for Fe/N/C@BMZIF in an O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm. (b) K-L plots. (c) Comparison of the electron transfer number of the synthesized catalysts. (d) ORR current−time chronoamperometric response of Fe/N/C@BMZIF and Pt/C in O2saturated 0.1 M KOH solution. D

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Figure 5. (a) LSV curves for different samples in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm. (b) OER durability of Fe/N/C@BMZIF. (c) The overall LSV curves of different samples.

at a scan rate of 10 mV s−1 at 1600 rpm. As shown in Figure 3a, the bare Fe/N/C catalyst with low surface area showed the poorest ORR activity while the BMZIF catalyst, even though exhibiting a higher limiting current, still cannot outperform a commercial Pt/C catalyst whether in terms of onset potential (Eonset = 0.96 V) or half-wave potential (E1/2 = 0.82 V). As expected, although bare Fe/N/C and BMZIF showed unsatisfactory ORR performance by themselves, the combination of these two materials did boost the electrocatalytic activity with Eonset of 0.99 V, E1/2 of 0.85 V, and a limiting current of 5.51 mA cm−2. For comparison purposes, materials derived from ZIF-8 and ZIF-67 were also tested as shown in Figure 3c and exhibited activities apparently worse than that of Fe/N/ C@BMZIF with Eonset of 0.95 V, 0.87 V, E1/2 of 0.79 V, 0.78 V, and a limiting current of 5.66, 5.38 mA cm−2, respectively, suggesting excellent ORR activity of Fe/N/C@BMZIF, which is due to its relatively high surface area, porous structure, and large exposure extent of catalytic active sites. This superior performance of Fe/N/C@BMZIF was further verified by comparing the Tafel slopes of different samples. It turned out that the Tafel slope for Fe/N/C@BMZIF (59.4 mV per decade) was smaller than that of Pt/C (72.1 mV per decade) as well as other counterparts (Figure 3b,d). The much smaller Tafel slope corresponded to a more favorable ORR kinetics, reconfirming the high electrocatalytic activity toward ORR. Another critical parameter for assessing ORR catalytic activity was the electron transfer number per oxygen molecule

(n), so more detailed LSV curves at different rotation rates (0− 2500 rpm, Figure 4a and Figure S5) were recorded to quantitatively study the materials. Through the Koutechy− Levich (K-L) equation, the link between the limiting diffusion current density (j) and the rotating speed (ω) can be calculated from LSVs and compared at different potentials. Figure 4b and Figure S6 showed linear relationships of different samples between j−1 and ω−1/2. When the potential ranged between 0.5 and 0.7 V, the value of n for Fe/N/C@BMZIF was figured to be 3.85−4.00, which approached Pt/C’s theoretical value of 4.00 measured in the same environment, from the slopes of K-L plots, indicating an efficient 4e dominant process, with water as the product. Furthermore, other samples’ electron transfer numbers were also compared at 0.4 V in Figure 4c, and BMZIF also exhibited a high electron transfer number of 4.01. Interestingly, bare Fe/N/C only possessed an n of 3.57, demonstrating that oxygen reduction was dominated by a twoelectron process, which may be due to the incapacity of oxygen absorption on bare Fe/N/C, further demonstrating the importance of the combined effect between Fe/N/C and BMZIF. Furthermore, electrochemical impedance spectroscopy (EIS) measurements for the ORR process of different samples were conducted (Figure S7). The Nyquist plots demonstrated that bare Fe/N/C and BMZIF exhibited a relatively worse charge transfer resistance while Fe/N/C@BMZIF possessed the smallest charge transfer resistance which outperformed that of Pt/C, reflecting the contribution from its coupling effects. E

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Figure 6. (a) Discharge voltage curve (v−i) and the corresponding power density plot. (b) Discharge/charge cycling curves of two-electrode rechargeable Zn−air batteries at a current density of 10 mA cm−2 using the Fe/N/C@BMZIF and Pt/C air electrode. (c) Optical image of a green LED screen powered by two button Zn−air batteries with the Fe/N/C@BMZIF air electrode connected in series.

of 10 mA cm−2, which was the value expected for a 10% efficient solar water-splitting device.34 The OER stability of the catalyst was also assessed through the chronoamperometric measurement at a constant potential of 1.64 V (Ej=10) in Figure 5b. The OER curve of Fe/N/C@BMZIF showed an insignificant anodic current attenuation of 84.1% within 60 000 s. All above-mentioned results suggested that Fe/N/ C@BMZIF was a desirable bifunctional catalyst for reversible oxygen reactions. The overall oxygen electrode activity of Fe/N/C@BMZIF was investigated and compared with bare Fe/N/C, BMZIF, Fe/ N/C@ZIF-8, Fe/N/C@ZIF-67, and Pt/C (Figure 5c), which can be assessed by the overvoltage between ORR and OER. More specifically, the OER activity was computed as the potential required to deliver 10 mA cm−2, while the ORR performance of the catalysts was compared at the half-potential during oxygen reduction, namely ΔE = Ej=10 − E1/2.35 The smaller the ΔE, the more desirable the catalyst as an ideal bifunctional catalyst. Remarkably, Fe/N/C@BMZIF gave a ΔE of 0.79 V, which outperformed most reported bifunctional materials, including nonmetallic materials (e.g., N, S, O-carbon nanosheets, ΔE = 0.88 V; N, P-carbon paper, ΔE = 0.96 V),13,35 noble-metal electrocatalysts (e.g.Ir/C, ΔE = 0.92 V; Pt/ C, ΔE = 1.08 V),35 and transition-metal materials (e.g.MnxOy/ N-carbon, ΔE = 0.87 V; Co3O4/2.7Co2MnO4, ΔE = 1.09 V)22,36 (Table S1). All the above-mentioned results suggested that the Fe/N/C@BMZIF material possessed remarkable electrocatalytic activity, as well as excellent electrochemical stability, making it highly promising as a superior bifunctional electrocatalyst for Zn−air batteries.

In addition, the chronoamperometric method was conducted on both Fe/N/C@BMZIF and Pt/C catalysts in alkaline medium at 0.56 V to test their stability. As shown in Figure 4d, up to 98.6% of the original current density was retained when using Fe/N/C@BMZIF electrode over 60 000 s of continuous operation, which can be attributed to its structural and chemical stability in the alkaline medium. Under the same conditions, the Pt/C electrode showed a 20.9% drop in current density, which may be caused by the catalyst’s regression after some period of time through surface oxidation, migration, or aggregation of the Pt nanoparticles, particularly in the alkaline electrolytes,11,32,33 confirming the better ORR stability of the former. To demonstrate OER performance, LSVs of the catalysts are shown in Figure 5a at 10 mV s−1 with a rotation speed of 1600 rpm. As shown in the LSVs, Fe/N/C@BMZIF gave a current density of 10 mA cm−2 at a potential of 1.64 V, which was not only much better than bare Fe/N/C and BMZIF alone but also compared favorably to other reported bifunctional catalysts (Table S1). To generate a current density of 10 mA cm−2, the potential for Pt/C should be 1.90 V, about ∼260 mV higher than that of Fe/N/C@BMZIF. This OER performance was further compared with ZIF-8- and ZIF-67-derived materials. It was noteworthy that, among the three ZIF-derived materials, Fe/N/C@ZIF-67 exhibited the best OER performance because it required the smallest potential to generate the same current density, such as 10 mA cm−2. Although Fe/N/C@ZIF-67 gave greater current and earlier onset potential compared to other ZIF-derived catalysts, which confirmed the critical role of cobalt element in enhancing OER performance, Fe/N/C@BMZIF showed no less activity in terms of generating a current density F

DOI: 10.1021/acsami.6b12197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces To evaluate the practical utility of Fe/N/C@BMZIF in Zn− air batteries, we tested it in a primary Zn−air battery first. The cathode was obtained by dispersing Fe/N/C@BMZIF on carbon fiber paper, and a polished Zn plate was used as the anode electrode. When KOH was used as the electrolyte, the battery’s open circuit potential (OCP) was determined to be 1.48 V. The polarization curve is shown in Figure 6a, and the power density plot was obtained according to the curve. It was noteworthy that large current densities of 56 and 174 mA cm−2 can be obtained at a potential of 1.20 and 1.0 V, respectively. The peak power density was 235 mW cm−2 when the current density was about 307 mA cm−2. This excellent performance of Fe/N/C@BMZIF was derived from its porous structure and strong combined effect between Fe/N/C and BMZIF, which facilitated the spread of O2 gas and their contact with the active sites, and ensured excellent electrocatalytic activity for Fe/N/ C@BMZIF. We then used a button battery with an air vent to develop a rechargeable battery and meanwhile adding 0.2 M Zn(CH3COO)2 into the electrolyte to maintain reversible zinc electrochemical reactions at the anode.37 For an active bifunctional catalyst, low charging voltage, high discharging voltage, and minimal fluctuation of these two voltages were required for a rechargeable battery.38 As expected, when maintaining current rate at 10 mA cm−2 during the cycling, the Zn−air battery reached a charge voltage of 1.95 V and discharge voltage of 1.13 V, thus achieving a small voltage gap of 0.82 V (Figure 6b). After 100 cycles (about 17 h), no obvious potential drop was observed and the final values were 1.96 and 1.11 V, respectively, with a virtually negligible loss of 0.03 V, indicating an excellent durability of Fe/N/C@BMZIF for oxygen reactions, which resulted from its stable porous structure and strong combined effect between bare Fe/N/C and BMZIF nanocrystals. In contrast, although the Pt/C cathode delivered a slightly lower charge voltage during the first cycle, its performance noticeably deteriorated after the fifth cycle.39 Notably, the battery could be regenerated and maintained at the same performance level for subsequent runs by simply replenishing the Zn anode and electrolyte with the same Fe/N/C@BMZIF air cathode, again suggesting its great durability. Multiple Zn−air batteries with the Fe/N/C@ BMZIF air−cathode can be connected to enhance specific energy/power performance to be applied in various practical applications.40 As shown in Figure 6c, two Zn−air batteries using Fe/N/C@BMZIF air−cathode were connected to generate a stable OCP of ∼2.8 V and were applied to a green light-emitting diode (LED) screen, fully demonstrating its potential for application in an air environment.

for the development of a new generation of energy storage and conversion techniques.

4. EXPERIMENTAL SECTION 4.1. Synthesis of Polypyrrole−Iron Coordination Complex. The preparation of polypyrrole−iron coordination complex was based on reported procedures with some modifications.41,42 In a typical synthesis, pyrrole was first dispersed in deionized water, followed by adding ferrous chloride and then adding an excess of hydrogen peroxide to the solution dropwise. After that, pyrrole polymerization was initiated and lasted for several hours until the color of the solution turned clear yellow and transparent. The solution was transferred to the vacuum drying oven and dried until a brown product was obtained prior to use. 4.2. Synthesis of ZIF-8 Nanocrystals. Generally, zinc nitrate and 2-methylimidazole were separately dissolved in MeOH and stirred for 0.5 h. After that, the two solutions were blended together and vigorously stirred for 20 h. The sample was obtained by centrifugation and dried under vacuum for 24 h, followed by carbonization to obtain ZIF-8 nanocrystals prior to use. 4.3. Synthesis of ZIF-67 Nanocrystals. Generally, cobalt nitrate and 2-methylimidazole were separately dissolved in MeOH and stirred for 0.5 h. After that, the two solutions were blended together and vigorously stirred for 6 h. The sample was collected by centrifugation and dried under vacuum for 24 h, followed by carbonization at 900 °C for 2 h with argon gas protection to obtain ZIF-67 nanocrystals prior to use. 4.4. Synthesis of BMZIF Nanocrystals. In a typical synthesis, a mixture of zinc nitrate and cobalt nitrate with a theoretical molar ratio of Zn2+/Co2+(20) and 2-methylimidazole were separately dissolved in MeOH and stirred for 0.5 h. After that, the two solutions were blended together and vigorously stirred for 20 h. The product was collected by centrifugation and dried under vacuum for 24 h, followed by carbonization at 900 °C for 2 h with argon gas protection, and cooled to room temperature to obtain BMZIF nanocrystals prior to use. 4.5. Synthesis of Fe/N/C@ZIF-8, Fe/N/C@ZIF-67, and Fe/N/ C@BMZIF. In a typical synthesis, as-prepared PPy-Fe coordination complex and ZIF-8 powder were separately dissolved in deionized water and ultrasonicated for 0.5 h. Then the two suspensions were mixed into a flat bottle flask with vigorous stirring for 12 h, dried at 100 °C, carbonized at 900 °C for 2 h with argon gas protection, and cooled to room temperature to obtain Fe/N/C@ZIF-8 prior to use. Fe/N/C@ZIF-67 and Fe/N/C@BMZIF were built in the same way using ZIF-67 and BMZIF instead of ZIF-8. 4.6. Physical Characterization. The morphologies and structures were observed with a field emission scanning electron microscope (FESEM, SU8010, Japan) and a field emission transmission electron microscope (FETEM, Tecnai G2 F20, Hong Kong). XRD tests were performed on a Rigaku D/max-2000PC diffractometer with Cu Kr radiation. XPS tests were performed on a X-ray photoelectron spectrometer (Kratos Axis Ultra Dld, Japan). Elemental analysis tests were performed on an inductively coupled plasma−optical emission spectrometer (OPTOMA 8000, PerkinElmer Inc., Ltd.). N2-sorption and BET tests were conducted using an accelerated surface area and porosimetry instrument (ASAP 2020, Micromeritics) 4.7. Electrochemical Measurements. An evaluation of the catalytic performance was conducted in a three-electrode system on a WaveDriver 20 bipotentiostat (Pine Instrument Company, Grove City, PA). A platinum wire electrode and a silver/silver chloride electrode served as counter and reference electrodes, respectively. The working electrode adopted a rotating disk electrode (RDE) with a glassy carbon disk (5.0 mm diameter). Amounts of 12.5 mg of catalysts and 3.125 mg of acetylene black were dissolved in 350 μL of ethanol and 95 μL of Nafion solution and ultrasonicated for 60 min to achieve a homogenized ink. To obtain a loading of 1.5 mg cm−2, 7 μL of ink was drop-coated on the glass carbon surface. For comparison purposes, the catalytic performance of Pt/C was also tested. One milligram of Pt/C was dissolved in 250 μL of ethanol and 10 μL of

3. CONCLUSIONS In summary, although BMZIF or Fe/N/C alone had undesirable ORR and OER performance, their composite materials showed unprecedented ORR and comparatively excellent OER activities in alkaline solution, not only outperforming a commercial Pt/C electrocatalyst but also showing much better performance than most reported bifunctional electrocatalysts, which were attributed to their considerable segment of active sites, unique porous architecture, and excellent charge-transfer ability. The synthesis strategy reported here can be applied for the preparation of many other Fe/N/C-related bifunctional materials and allows new avenues for synthesizing highly active Fe/N/C-related ORR and OER electrocatalysts, which would be highly promising candidates G

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Nafion solution (5 wt %) and ultrasonicated for 60 min to form a homogenized ink. To achieve a loading of 0.20 mg cm−2, 10 μL of Pt/ C ink was drop-coated on the GC surface. Before each experiment, electrolyte was saturated with N2/O2 by gassing with N2/O2 for at least 30 min. CV tests were performed in N2- and O2-saturated alkaline solution at 50 mV s−1. LSV tests were performed in O2-saturated alkaline solution at 400, 625, 900, 1225, 1600, 2025, and 2500 rpm at 10 mV s−1. The Nernst equation was used as the conversion formula for converting potentials from Ag/AgCl (4 M KCl) to the reversible hydrogen electrode (RHE) scale44 ° E RHE = EAg/AgCl + 0.059pH + EAg/AgCl

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tao Qian: 0000-0001-7252-8224 Chenglin Yan: 0000-0003-4467-9441 Notes

The authors declare no competing financial interest.

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(1)

ACKNOWLEDGMENTS We acknowledge the support from the National Natural Science Foundation of China (no. 51402202, no. 51622208), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

■

(2)

where η was the overpotential, j was the measured current density, and b was the Tafel slope. Electron transfer numbers were obtained according to the Koutecky−Levich equations44

1/j = 1/jk + 1/jL = 1/jk + 1/(Bω1/2)

(3)

B = 0.62nFC0(D0)2/3 v1/6

(4)

REFERENCES

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where j, jK, and JL were the measured current density, the kinetic current density, and the diffusion-limited current density, respectively, ω was the angular velocity (ω = 2πN, N was the linear rotation speed), F was the Faraday constant (F = 96485 C mol−1), C0 was the bulk O2 concentration (1.2 × 10−6 mol cm−3), D0 was the O2 diffusion coefficient (1.9 × 10−5 cm2 s−1), and v was the kinetic viscosity (0.01 cm2 s−1). 4.8. Zn−Air Battery Assembly. Amounts of 12.5 mg of catalysts and 3.125 mg of acetylene black were dissolved in 700 μL of ethanol and 190 μL of Nafion solution and ultrasonicated for 60 min to achieve a homogenized ink. To obtain a loading of 1 mg cm−2, the ink was uniformly drop-coated onto Teflon-treated carbon fiber paper (AvCarb P75T, Fuel Cell Store). A Zn plate was first polished to serve as the anode. The Zn anode and air cathode were assembled into a button battery with an air vent. Six molar KOH was used as electrolyte for the primary Zn−air batteries. Zn(CH3COO)2 (0.2 M) was added into the electrolyte to be applied to the rechargeable batteries. 4.9. Zn−Air Battery Test. Primary Zn−air batteries were tested to obtain a polarization curve of the material. A home-built container was used to test the rechargeable Zn−air batteries (Figure S9). The galvanostatic discharge−charge cycling (5 min discharge and then 5 min charge and that cycle repeated) was carried out with a LAND testing system. The green light-emitting diodes (LEDs) were commercially available.

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where EAg/AgCl was the experimentally measured potential using Ag/ AgCl as the reference electrode and E°Ag/AgCl was 0.196 V. Unless noted otherwise, all the potentials mentioned in this paper were versus RHE. Tafel slopes were obtained according to the Tafel equation43 η = a + b log j

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12197. Mapping images of Fe/N/C@BMZIF, N2 adsorption− desorption isotherms, high-resolution Fe 2p and Co 2p spectrum, CV curves, LSV curves at different sweeping speeds, K-L plots of different materials, impedance data for different samples, comparison of catalytic performance with different loadings, optical image of button Zn− air battery, and electrocatalytic properties for different samples (PDF) H

DOI: 10.1021/acsami.6b12197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b12197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX