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Co(II)1−xCo(0)x/3Mn(III)2x/3S Nanoparticles Supported on B/N-Codoped Mesoporous Nanocarbon as a Bifunctional Electrocatalyst of Oxygen Reduction/Evolution for High-Performance Zinc-Air Batteries Zilong Wang,† Shuang Xiao,† Yiming An,† Xia Long,† Xiaoli Zheng,† Xihong Lu,‡ Yexiang Tong,‡ and Shihe Yang*,† †

Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong, China School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China

‡

S Supporting Information *

ABSTRACT: Rechargeable Zn-air battery is an ideal type of energy storage device due to its high energy and power density, high safety, and economic viability. Its largescale application rests upon the availability of active, durable, low-cost electrocatalysts for the oxygen reduction reaction (ORR) in the discharge process and oxygen evolution reaction (OER) in the charge process. Herein we developed a novel ORR/ OER bifunctional electrocatalyst for rechargeable Zn-air batteries based on the codoping and hybridization strategies. The B/N-codoped mesoporous nanocarbon supported Co(II)1−xCo(0)x/3Mn(III)2x/3S nanoparticles exhibit a superior OER performance compared to that of IrO2 catalyst and comparable Zn-air battery performance to that of the Pt-based battery. The rechargeable Zn-air battery shows high discharge peak power density (over 250 mW cm−2) and current density (180 mA cm−2 at 1 V), specific capacity (∼550 mAh g−1), small charge−discharge voltage gap of ∼0.72 V at 20 mA cm−2 and even higher stability than the Pt-based battery. The advanced performance of the bifunctional catalysts highlights the beneficial role of the simultaneous formation of Mn(III) and Co(0) as well as the dispersed hybridization with the codoped nanocarbon support. KEYWORDS: bifunctional catalyst, oxygen reduction and evolution, mesoporous nanocarbon, metallic cobalt, zinc-air battery

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on commercial Pt/C.9,10 Although some bifunctional catalysts may work well in the discharge process in Zn-air batteries, they perform poorly in the charging process, and thus another catalyst must be used separately.8 Clearly, bifunctional catalysts with high and balanced ORR/OER activities will simplify the battery design and promote the overall battery performance. Metal sulfides especially cobalt sulfide were predicted to have an electrocatalytic activity on a par with Pt toward the fourelectron ORR process,11 but experiments have shown that these catalysts are still much less ORR active than Pt/C catalysts.11 To improve the catalytic activity of metal sulfides, researchers attempted metal doping, for example, with Mn atoms.12 It was found that the doped Mn atoms could enhance not only metallicity13 and thus conductivity but also the catalytic activities for both OER and ORR. The different σ*-orbital (eg) occupation and extent of oxygen covalency of transition metal atoms brought different oxygen reduction performance.14 Recent studies showed that metal atoms whose d-electron number is near 4 and 7, such as Mn(III), have a superior ORR activity.14 But it is not trivial to produce Mn(III) in doped form by traditional methods, not to

INTRODUCTION In recent years, the energy conversion and storage systems of metal-air batteries, water-splitting devices, and fuel cells have attracted much attention because of their high energy density and low cost.1,2 The commonality of these devices is their involvement of oxygen, which is ubiquitous in our atmosphere. It is well-known that rechargeable metal-air batteries have much higher energy densities than other batteries including Li-ion batteries.3,4 The heart of these batteries is oxygen-related reactions, namely, the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Specifically, the output energy capacity is mainly determined by the cathodic 4e-ORR in the discharging process, which however is kinetically sluggish resulting in voltage loss.5 In the charging process, however, the reverse anodic OER takes place. This process is not only thermodynamically uphill (E0 = 1.23 eV), but it also has high kinetic barriers inasmuch as four electrons are involved.6,7 Therefore, efficient catalysts are sorely needed for both ORR and OER, and ultimately for the development of electrically rechargeable Zn-air batteries.8 Better still, we need a catalyst that can catalyze both ORR and OER, the so-called bifunctional catalyst. Indeed, there have already been some reports about bifunctional catalysts for Zn-air batteries, but most of these bifunctional catalysts suffer from low current density, resulting in low power density of Zn-air battery compared to those based © XXXX American Chemical Society

Received: December 30, 2015 Accepted: May 10, 2016

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DOI: 10.1021/acsami.5b12803 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

In the work presented here, we designed an ORR/OER bifunctional catalyst for Zn/air batteries by supporting Mndoped cobalt sulfide (CMS) nanoparticles on a novel B/N codoped mesoporous carbon (BNC) sheets. With the Mn dopant, we modified the electronic structure of CoS via the Co2+ to Co partial conversion, which would increase the conductivity and the simultaneous formation of Mn(III) dopant, which would benefit both ORR and OER. Compared with previously reported metal sulfide catalysts or doped metal oxide catalysts, the catalysts produced by this method have well-controlled valence for the transition metal atoms, which have essentially the form of M(II)1−xM(0)x/3Mn(III)2x/3S (M = Co, Ni), and the catalytic activity was considerably improved. More specifically, compared with previous Mn-doped metal oxide catalysts, the sulfide catalysts we produced by this method have well-controlled valence of the transition metal atoms. In other words, by doping with Mn(III), we can effectively tune the valence ratio of the Co(0/II) and Ni(0/II) atoms. Such a delicate control over the valence of Mn and Co(Ni) in a single catalyst can greatly promote the catalytic activity. For instance, the metallic cobalt atoms could be produced by Mn(III) doping. The metallic cobalt atoms can help to improve the charge transfer in the oxygen reduction and evolution performance.15,17 As a side benefit, the CMS showed a much higher electrochemically active surface area and specific surface area than those of MnS and CoS. To boost the catalytic performance even further, we availed ourselves to the hybridization strategy by combining the CMS with doped carbon materials to get higher catalysis performance. After hybridization with BNC, the CMS/BNC catalyst showed significantly higher performance on ORR and OER than the control samples, including BNC, MnS, or CoS and MnS/BNC or CoS/BNC. In particular, the CMS/BNC exhibits a comparable ORR performance with Pt/C, and it shows a superior OER performance with IrO2. Moreover, the performance of the CMS/BNC catalyst was enhanced after oxygenreduction treatment, which is a natural process in a metal/air battery and thus especially advantageous for applications. We constructed for the first time a rechargeable Zn-air battery based on these bifunctional catalysts for OER and ORR, without having to use separate catalysts, and demonstrated that the performance of our purpose-built catalysts is at least comparable with that of the benchmark Pt/C catalyst. We also found that this method can be extended to other metal sulfides such as NiS, and when applied in Zn-air battery, it also showed an excellent performance.

mention studying the effect of Mn(III) in metal sulfide on ORR activity. Our strategy is to produce Mn(III) dopant by choosing a metal ion with a suitable oxidation ability to convert Mn(II) to Mn(III) in situ. In our survey experiments, we found that the Co(II) and Ni(II) ions per se in cobalt sulfide and nickel sulfide could serve as oxidants to introduce Mn(III) in the corresponding sulfide catalysts,13 accompanied by the generation of metallic cobalt and nickel atoms. In fact, there have been reports that suggest the possible benefit of metallic metal atoms in composites of metal oxide and doped carbon materials in the catalysis of oxygen reduction and evolution.15 On the one hand, our catalyst design via the in situ formation of Mn(III) and Co(Ni) in one go can avoid problems in traditional materials synthesis such as adventitious impurities and nanomaterials aggregation, because the metallic dopants were introduced not by a reducing gas at high temperatures. On the other hand, previous studies have shown that the Mn(III) dopant ion can facilitate adsorption of OH− in the ratedetermining step of OER, because of the spatial overlap of the eg orbital on Mn(III) and the O-pσ orbital on OH−, and result in a faster OER kinetics, specifically, a lower overpotential, and thus a higher OER activity than traditional catalysts containing Mn(IV).16 For nanoparticle catalysts, aggregation, poor stability, and low specific surface area and conductivity remain as stumbling blocks. The introduction of carbon support has proved to be an effective way to solve these problems.17 Compared with normal carbon materials the doped carbon materials showed superior catalysis performance, because doped atoms can provide superior conductivity and more active sites. By doping with nitrogen and/or boron, the functional role of the carbon support has been enhanced because the extra electron of N and the empty orbital of B are brought into the conjugated π-system to modulate the electronic structure or conjure new electron states that can lower the activation energy.18 Quantum mechanics calculations indicate that replacing carbon atoms with heteroatoms in carbon materials is an effective way to tailor its electron-donor properties and improve its catalytic activity. The enhanced catalytic activity of vertically aligned nitrogen-doped carbon nanotubes toward ORR was attributed to the fact that the nitrogen species, because of their high electron-accepting ability, induced net positive charges and thus enhanced oxygen adsorption and electron attraction from the anode needed for ORR.19 Auspiciously, the pyridinic and quaternary-N doping of carbon materials also creates OER active sites, but single-doped carbon materials suffered from high overpotential in OER.20 Conceivably, when doped with both N and B, one with a higher electronegativity (N: 3.04) than C (2.55) and the other with a lower one (B: 2.04), carbon nanomaterials can be made with a unique electronic structure characterized by the synergistic coupling between the heteroatoms.21 Such an electronic effect may make the dual-doped carbon catalysts much more active than singly doped ones especially in the catalytic performance for organic synthesis and fuel cells.22 For the synthesis of dualdoped carbon materials, it is commonly based on the thermal chemical vapor deposition (CVD) technology, which is expensive and, a fortiori, produces a covalent boron−carbon− nitride compound instead of the desirable B- and N-codoped carbon materials.21 On the basis of the previous work of our group,22 we developed a mesoporous nanocarbon material by in situ method without having recourse to the demanding CVD technology.

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EXPERIMENTAL SECTION

In synthesis process all the chemicals were directly used after purchase without further purification. The composition, morphology, and surface area of the resulting CoS, MnS, CMS, and CMS/BNC were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), nitrogen sorption measurements, and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was broadly evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronovoltammetry measurements in alkaline solution. The synthesis steps and structure of Zn-air battery are shown in Schemes 1 and S1. The experiment details are shown below. Synthesis of Boron/Nitrogen Codoped Mesoporous Nanocarbon. Boron/nitrogen codoped mesoporous nanocarbon was synthesized by in situ pyrolysis method. Typical synthesis of BNC: B


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crucible, heated at a rate of 2.5 °C min−1 to 550 °C, and then tempered at this temperature for 0.5 h under a flow of Ar. The material was then heated further at a rate of 3.5 °C min−1 to 1000 °C and maintained at that temperature for 1 h. The sample was then allowed to cool naturally to room temperature under the protection of Ar (99.99% from Air Products). In the synthesis, dicyandiamide worked as the carbon and nitrogen source, while boron was introduced through boric acid. Synthesis of Co−Mn Precursors. The CMS were synthesized from cobalt acetate tetrahydrate (Sigma-Aldrich, 98%) and manganese acetate tetrahydrate (Sigma-Aldrich, 98%). A mixture of metal acetate tetrahydrate (0.5 g) with different molar ratios of Co/Mn (from 5/1 to 1/5) were dissolved into 200 mL of alcohol (Sigma-Aldrich, 99.99%). The sample names are constructed in the form of AxBy with x and y as index indicating the molar ratio of Co/Mn in the starting reaction mixtures. The samples of different ratio are marked from Co5Mn1 to Co1Mn5. Then the mixture was heated to 75 °C under refluxing conditions with stirring. After reaction for 4 h, the precipitate was harvested via centrifugation before being fully dried in air at room temperature. Synthesis of B/N Codoped Mesoporous Carbon Supported Mn-Doped Cobalt Sulfide Nanoparticles. 80 mg of the above

Scheme 1. Design and Synthesis of Bifunctional CMS/BNC Electrocatalyst and the Application of the Electrocatalyst in Zn-air Battery

Dicyandiamide (4 g, Sigma-Aldrich, 99%), monohydrate glucose (0.1 g, Sigma-Aldrich, 99.5%), and boric acid (0.02 g, Sigma-Aldrich, 99.5%) were dissolved in deionized water (50 mL) and stirred at 70 °C to remove water. The resulting white powder was transferred into a

Figure 1. Morphology characterizations of the CMS/BNC catalysts. (a, c) SEM and (b, d) TEM images of BNC (a, b) and CMS/BNC hybrid (c, d), showing a crumpled and cross-linked yet still transparent thin-film morphology typical of doped carbon sheets; (e) elemental mapping of Co, Mn, S, B, N, and C in an SEM image of CMS/BNC, indicating a homogeneous distribution of the atomic species; (f) HRTEM image of the selected square in (d), showing a Mn-doped CoS nanoparticle embedded in BNC. C


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Figure 2. Structure and composition characterizations of the composite materials. (a) XRD patterns of Mn-doped CoS starting from different Co/ Mn ratios. When Co/Mn ratio is over 1/4 in the starting materials, only hexagonal cobalt sulfide phase is observed, whereas with the lower ratios, the manganese sulfide phase starts to appear; (b) Co 2p XPS spectra of Mn-doped CoS when the Co/Mn ratio is 4 and CoS; the doping leads to the Co(0) peak; (c) Mn 2p spectra of Mn-doped CoS; the doping leads to the appearance of Mn(III). (d) Nitrogen sorption analysis and specific surface area results of MnS, CoS, Mn-doped CoS, CMS/BNC, and BNC; specific surface areas are increased after Mn doping, and the high specific surface areas of CMS/BNC results from both the Mn doping and the hybridization with BNC. Co−Mn precursors was redispersed into 40 mL of ethanol, followed by addition of 0.125 g of thiourea (Sigma-Aldrich, 99%) and 50 mg of BNC. The mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 120 °C for 6 h. After centrifugation and washed with ethanol for several times, the Mn doped CoS nanoparticle embedded boron−nitrogen codoped mesoporous nanocarbon (marked as CMS/BNC) was obtained. To further improve the crystallinity, the final products were annealed under Ar atmosphere at 400 °C for 2 h. The thiourea acted as the sulfide source. General Characterizations. The morphology and microstructures of the samples were investigated by HRTEM (JEOL-2010 with an acceleration voltage of 200 kV), SEM (JEOL-6700 with an acceleration voltage of 20 kV), and XRD (PW1830 operating at 40 kV and 40 mA equipped with a nickel-filtered Cu Kα radiation, λ = 1.540 56 Å). The chemical composition was analyzed by XPS on PHI 5600. The Brunauer−Emmet−Teller (BET) model was applied to the isotherms to determine the apparent surface area. Thereby, the adsorption branch in the relative pressure range of p/p0 < 0.3 was used for surface area analysis. Preparation of Ni Electrode for Oxygen Evolution Reaction. Catalyst (1 mg) was washed and dispersed in 0.967 mL of ethanol, then 0.033 mL of polytetrafluoroethylene preparation (Sigma-Aldrich, 60 wt % dispersion in H2O) was added into it. The mixture (containing 1 mg of catalyst) was under ultrasonic treatment for 10 min. After sonication for 10 min, 1000 μL of catalyst ink was drop dried onto a 1 cm × 1 cm Ni foam (foam, thickness 0.25 mm, SigmaAldrich, 99.995%, loading 1 mg cm−2). The IrO2 (Sigma-Aldrich, 99.9%) catalyst for comparison was prepared on Ni in the same method. Preparation of Glassy Carbon Electrode for Oxygen Reduction Reaction. The preparation of a glassy carbon working electrode (Pt ring/glassy carbon disk electrode, RRDE-3A, ALS Co., Ltd., 4 mm in diameter) is as follows: prior to use the working electrode was polished mechanically with a 0.05 μm alumina slurry to obtain a mirrorlike surface, then washed with Mill-Q water and acetone, and allowed to dry. The prepared sample (5 mg) was

dissolved in a 2 mL solvent mixture of Nafion (Sigma-Aldrich, 5 wt % in a mixture of lower aliphatic alcohols and water) and alcohol (v/v ratio = 1:9) using sonication for 30 min, and then 5 μL of the resulting electrocatalyst was dropped onto the glassy carbon electrode (loading 0.10 mg cm−2 on glassy carbon electrode). The Pt/C (20 wt % on Vulcan XC72, Sigma-Aldrich) catalyst was prepared on glassy carbon electrode in the same method. Oxygen Reduction Reaction Activity Evaluation. The ORR activity was evaluated using a rotating ring disk electrode apparatus (Pt ring/glassy carbon disk electrode, RRDE-3A, ALS Co., Ltd). The linear sweep voltammetry (LSV) scans were performed using a CH instruments electrochemical workstation (model 760D) in a conventional three-electrode electrochemical cell at a scan rate of 1 mV/s in 0.1 M KOH (Sigma-Aldrich, 99.995%) solution. During measurements, oxygen (99.995% from Air Products) was continuously fed to the working electrode. Platinum wire (SVC-3, ALS Co., Ltd.) and Hg/ HgO, (RE-61AP, 1 M NaOH, ALS Co., Ltd.) electrode were used as the counter and reference electrodes, respectively. For the RRDE measurements, catalyst inks and electrodes were prepared by the same method as RDE’s, and the measurements were also performed in the same solution. The ring potential was constant at 1.5 V versus reversible hydrogen electrode (RHE). Oxygen Evolution Reaction Activity Evaluation. The OER activity was evaluated using the catalysts loaded on Ni foam as the working electrode. The LSV test was also performed using a CH instruments electrochemical workstation (model 760D) in a conventional three-electrode electrochemical cell at a scan rate of 1 mV/s and 0.1 mV/s for Tafel plots in 1 M KOH. During measurements, oxygen (99.995% from Air Products) was continuously fed to the working electrode. Platinum wire (SVC-3, ALS Co., Ltd.) and Hg/HgO (RE61AP, 1 M NaOH, ALS Co., Ltd.) electrode were used as the counter and reference electrodes, respectively. Electrochemically Active Surface Area Test. Electrochemically active surface area is evaluated by the electrochemical double-layer capacitance (Cdl). Working electrodes are same with the one in OER activity evaluation and scanned for several potential cycles until the D


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Co(II)/Co(0) in Table S3, we can find the element ratio is consistent with the formula of M(II)1−xM(0)x/3Mn(III)2x/3S (MCo). However, there is no Co(0) for nondoped CoS, showing clearly that the presence of Co(0) is due to the Mn doping. For the other samples with different Co/Mn ratios, the amount of Co(0) increased with the amount of Mn in the composite. The Mn 2p XPS spectrum of Mn-doped CoS is shown in Figure 2c, and the two peaks separated by 11.75 eV can be safely assigned to Mn(III).25 As for the MnS product without Co doping, there is a satellite feature (∼647 eV) of Mn(II) (Figure S2a). The preferential reduction of Co(II) to Co(0) as compared to the Mn counterpart can be well explained by the lower standard reduction potential of Mn(II)/ Mn(0) than Co(II)/Co(0) (Co2+ + 2e → Co, −0.277 V vs NHE; Mn2+ + 2e → Mn, −1.18 V vs NHE). The lower reduction potential means the transformation from Co(II) to Co(0) is easier than the same reaction of manganese. When Co(II) changed to Co(0) as oxidizer, Mn(II) changed to Mn(III) as reductant at the same time (Mn2+ − e → Mn3+, 1.5 V vs NHE).26 This result is similar to the NiMn composite reported previously, in which Ni(II) changed to Ni(0), while Mn(II) changed to MnxOy not Mn(0) at the same time because of the apposite reduction potential (Ni2+ + 2e → Ni, −0.257 V vs NHE).13 We verified the valence change happened in the sulfuration process because the XPS result of Co−Mn precursors showed no signals of Co(0) and Mn(III) (Figure S13), and only after sulfuration did we observe the formation of Co(0) and Mn(III) (Figure 2). Although there was a report about the valence change due to the reaction between metal ions and carbon materials, that was at a much higher temperature than the temperature of our experiment.27 Indeed the XPS result of MnS/BNC showed only Mn(II), therefore ruling out the reaction between Mn(II) and BNC. A simialr result was obtained for CoS/BNC without Mn(II); that is, no Co(0) was detected from XPS (see Figure S14). This result proves that the metal valence change comes from the reaction between Co(II) and Mn(II) leading to the formation of Co(0) and Mn(III). XPS results for N, S, and B of the product samples are shown in Figure S2. The N 1s spectrum in Figure S2b can be deconvoluted into three types of nitrogen species associated with pyridinic N, pyrollic N, and quaternary N atoms, respectively.21,28 As for the S 2p spectrum (Figure S2c), two main peaks are observed at ∼161.5 and 162.5 eV, which are characteristic of the metal−sulfur bonds.29 The B 1s peak at ∼190 eV (Figure S 2d) is indicative of the B−C bond formation and thus the successful incorporation of B atoms into the carbon lattice network.19 Next, nitrogen sorption analysis revealed the necessity of Mn doping and BNC-hybridization in the sulfuration process to form higher specific surface area materials as shown in Figure 2d. The Mn(III)/Co(0) doping helped to increase specific surface area from 23.2 (pure CoS) to 68.4 (CMS) m2/g. This result is in accord with previous reports on other Mn-doped materials.13 After it combined with the BNC, the specific surface area increased further to 545.7 (CMS/BNC) m2/g, mainly due to the high specific surface area of the B/N codoped mesoporous nanocarbon. Oxygen Reduction Reaction and Oxygen Evolution Reaction Performance of the Bifunctional Catalysts. To the best of our knowledge, the influence of Mn doping in CoS on ORR and OER has not been described previously, which will be an important aspect of this work. The strong influence of Mn doping into the CoS on the electrochemical performance

signals are stabilized, and then the data are collected with sweep rates of 10, 20, 30, 40, and 50 mV s−1. The plot of the current density (at 1.165 V vs RHE) against the scan rate has a linear relationship, and its slope is Cdl. Rechargeable Zn-Air Batteries. The batteries were tested in home-built electrochemical cells (Schemes 1 and S1). For rechargeable cells, a two-electrode configuration was used by pairing the catalysts we prepared loaded on a Ni foam electrode (1 cm2, catalyst loading 1 mg) with a Zn foil (thickness 1.0 mm, 99.99%, Sigma-Aldrich) in 30− 40 mL of 6 M KOH with 0.2 M zinc acetate (99.99%, Sigma-Aldrich) dissolved to form zincate. During battery measurements, oxygen (99.995% from Air Products) was continuously fed to the cathode. It was humidified by passing through a separate liquid water container before entering the cell. The battery internal impedance was measured to be 1.4−1.7 Ω at the open-circuit condition.

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RESULTS AND DISCUSSION Characterization of the Bifunctional Catalysts. SEM and TEM images of BNC are shown in Figure 1a,b, respectively. Clearly observed is a crumpled, cross-linked but still transparent thin-film morphology with a high specific surface area, which was confirmed by the BET analysis result (Figure 2d). A similar morphology is also apparent for the CMS/BNC sample as shown in Figure 1c,d, except for the presence of embedded CMS nanoparticles ∼5 nm in diameter (Figure 1f). Clearly, the CMS nanoparticles are uniformly dispersed in BNC with no aggregation, which is contrary to the bare CMS nanoparticles shown in Figure S1. This observation was further confirmed by the EDS elemental mappings in Figure 1e, showing that all elements are homogeneously distributed. Therefore, the electron microscopic characterizations established that the CMS/BNC hybrid features the embedding of the CMS nanoparticles in B/N codoped nanocarbon sheets and surrounded by a few carbon layers, and such a structure could enhance both activity and stability that are essential for a serviceable bifunctional electrocatalyst.23 The CMS/BNC hybrid was further characterized by powder XRD. The XRD patterns of Mn(III)-doped CoS samples prepared using different metal salt ratios are shown in Figure 2a. For samples from a high Co/Mn ratio (>1/4), only the hexagonal phase CoS (JCPDS 75−0605) is observed,24 whereas the lower ratios result in the XRD pattern with weak MnS diffraction peaks. A similar case occurred with the Mndoped cubic nickel nanostructure.13 The XRD peaks are somewhat shifted after the Mn doping of CoS. When we lower the Co/Mn ratio, the amount of Mn in the composite increases as revealed in Table S1. Moreover, EDS measurements clearly show that a Co/Mn ratio of greater than one-third in the starting material leads to the final product with low amount of manganese (Table S1). However, when decreasing the Co/Mn ratio, the amount of Mn is increased to 20 (Co1Mn4) and 25 ac% (Co1Mn5), respectively. The presence of Co and Mn was confirmed by XPS (Figure 2b,c). As manifested in Figure 2b, the two core-level signals of Mn-doped CoS located at ∼780 and 796 eV are attributed to Co (II) 2p3/2 and Co(II) 2p1/2, respectively. Co(II) is confirmed by the satellite feature at ∼786 eV. After deconvolution, the peaks at ∼782 and 797 eV are assigned to the Co(II) phase, while those at ∼778.5 and 793.0 eV correspond to Co(0). Integral data shows that the ratio of Co(II)/Co(0) is ∼10/1, meaning that most of cobalt atoms are in the form of Co(II). This result accords well with the XRD result. Table S3 lists the ratio of Co(II)/Co(0) for all samples. Combining the ratios of different element in Table S1 and E


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Figure 3. Oxygen-reduction electrocatalysis and electrochemistry impedance spectroscopy of CMS/BNC catalysts in alkaline electrolyte of 0.1 M KOH. (a) RDE voltammograms recorded of oxygen reduction for samples with different Co/Mn ratio supported on GC electrode (Co4Mn1 showed the highest catalysis performance). The electrode rotation rate was 1600 rpm; (b) RDE voltammograms recorded for CMS/BNC with Co4Mn1 at different rotation rates. (inset) RRDE voltammetric response of CMS/BNC with Co4Mn1 at a rotation rate of 1600 rpm, and from the ring and disk currents we calculated the number of electron transferred in Table S5. (c) Nyquist plots of CMS/BNC, CMS, and BNC, with corresponding charge-transfer resistance (Rct) of 10.6, 15, and 52 Ω, showing that the presence of Co(0) due to Mn-doping reduces Rct. (inset) The RS of different samples with little difference. (d) Time dependence of current density for CMS/BNC and Pt/C under a constant potential of 0.5 V (vs RHE). All of the data were recorded in O2-saturated 0.1 M KOH solution at a scan rate of 1 mV s−1.

IR-correction showed little change compared with the original data, indicating that the resistance is small. The smallness of the resistance is mainly due to the hybridization with the B/N codoped nanocarbon. To investigate the electrode kinetics and resistance under ORR process, the EIS measurements were performed from 100 MHz to 0.01 Hz (Figure 3c). The semicircles in the high- and low-frequency regions of the Nyquist plot are attributed to the charge-transfer resistance Rct and series resistance RS, respectively. Rct is related to the electrocatalytic kinetics, and a lower value of Rct corresponds to a faster reaction rate.35 Similarly, smaller Rct of 10.6 Ω is realized with CMS/BNC compared to CMS without BNC (15 Ω) and CoS (52 Ω), consistent with the higher current density and lower onset potential. This reduction of Rct mainly comes from the metallic cobalt atom brought by Mn(III)/Co(0) doping and the high conductivity of BNC. The metallic cobalt and the doped carbon materials are responsible for the high conductivity, which can reduce the charge transfer resistance so that the Rct of CMS/BNC is much smaller than either CMS or CoS. The reduced Rct of CMS/BNC helps to achieve superior performance compared with nondoped cobalt sulfide. These results show that BNC can not only provide high surface area and avoid aggregation but also increase conductivity. Besides the reduction activity, stability is another significant criterion to evaluate an advanced electrocatalyst. To probe the durability of the catalysts we prepared under basic condition, continuous ORR at 0.5 V (vs RHE) was conducted. The results of CMS/ BNC and Pt/C are shown in Figure 3d. At chosen potentials the current density exhibits negligible degradation of less than 10% even after a long period of 10 h. The outstanding stability comes from high conductivity of BNC and protected Co(0), which embedded in carbon architecture. Meanwhile the slight degradation of activity may derive from the consumption of

for the ORR is shown in Figure 3a. The electrocatalytic activities of all samples were measured on a rotating ring disk electrode (RRDE) for ORR in 0.1 M KOH solution using a typical three-electrode system. For all of the materials, the manganese doping results in lower overpotentials accompanied by a rampant increase in current density compared to the pure CoS catalyst. For CMS/BNC LSV result of their best catalyst (Co/Mn ratio is 4/1) shows an onset potential at 0.90 V. The gap of onset potential between the best-performing sample and Pt/C is less than 0.1 V with a diffusion-limited current density comparable to Pt/C. Clearly, our catalyst exhibited a comparable performance with Pt/C. Of note, nonoptimum performing samples showed waves in LSV curves (Figure 3a), which may come from the two-step two-electron process involving the formation of the HO2− intermediate.30 So the LSV results indicated two reduction waves. From the RRDE curves of CMS/BNC and NMS/BNC shown in Figure 3b, the number of electron transfers (n) at different potential is shown in Table S5. At all potentials n remained close to 4, suggesting nearly four-electron reduction of O2 in this potential region. These values such as onset potential and number of electron transfers for CMS/BNC are superior to many non-noble metal ORR catalysts in alkaline media reported previously, such as Fe3C/NG,31 tridoped nanoporous carbons,32 NiCo2O4/carbon composite,33 and CoFe-LDHs/PDAS.34 A possible explanation for the increased performance when doped with Mn could lie in the reduced cobalt, which can introduce defects and vacancies for oxygen to adsorb. The codoped nanocarbon can also help to increase the oxygen reduction performance.19 The increased specific area shown in Figure 2d could contribute to the increase of diffusion-limited current density. Compared to the original data, the variation of the IR-corrected data for CMS/ BNC is shown in Figure S3. The value of current density after F


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Figure 4. Oxygen evolution characterization and electrochemically active surface area measurement of the electrocatalysts. (a) LSV of Mn-doped CoS with different Co/Mn ratios (the sample with the Co/Mn ratio of 4/1 shows the best oxygen evolution performance). (b) LSV of R-CMS/ BNC (CMS/BNC after 30 min ORR), CMS/BNC, and CMS without BNC and BNC (R-CMS/BNC shows a much higher current density than CMS/BNC but both of them show a superior performance to IrO2; BNC shows negligible OER activity). (c) Tafel plots of CMS, CMC/BNC, and R-CMS/BNC catalyzed OER. (d) Charging current density differences plotted against scan rates. The slopes of the straight lines, equivalent to twice the double-layer capacitance, Cdl, were used to represent electrochemically active surface areas of the electrocatalysts.

OH− or the variation of valence state of metal atoms in sulfide.36 We next turn to the OER activity of these novel CMS/BNC catalysts, which was assessed in the same electrolyte as used in the ORR testing experiments (1 M KOH). As expected, the Mn(III)/Co(0) doping of CoS showed improved catalysis performance for OER. First, the OER catalytic performance was examined by LSV method. The LSV results of samples with different Co/Mn ratio showed are shown in Figure 4a. We find that the samples with best performance are the ones whose Co/ Mn ratio is 4/1. After combination with B/N codoped nanocarbon, these samples showed outstanding performance in catalyzing oxygen evolution, despite that BNC alone showed negligible OER activity (Figure 4b). The CMS/BNC showed an onset potential of 1.52 V (vs RHE) and reached 20 mA cm−2 at 1.56 V (vs RHE). These values are comparable with IrO2 and lower than many other reported OER catalysts13,17,25,37−40 (Table S6). The Tafel plots are shown in Figure 4c. The Tafel slope of CMS/BNC is ∼64 mV/dec. The value is comparable with other reports listed in Table S6. This result showed that Co(0) and BNC could indeed help to increase the conductivity and reduce the Tafel slope. The small Tafel slopes showed the catalysts we prepared are excellent for water oxidation. And the reduction mainly comes from the combination of BNC. Significantly, these catalysts are also the ones that show best performance in ORR, proving that the materials we prepared are good bifunctional catalysts for oxygen reduction and evolution. Impressively, the samples after 30 min ORR (marked as RCMS/BNC) exhibited a lower onset potential and a significantly larger current density. To see this point, we refer to the catalytic performance of R-CMS/BNC in Figure 4b. Compared with CMS/BNC, the R-CMS/BNC showed higher current density at the same potential, for example, they reached

20 mA cm−2 at 1.49 V (for R-CMS/BNC). This improvement is similar to some metal oxide and catalyst for HER and OER.15 Similarly, the Tafel slopes also decreased to 50 mV/dec after oxygen reduction, which is lower than many reports (Table S6). The additionally reduced Tafel slope of R-CMS/BNC indicates a more facile charge transfer at the catalyst solution interface, in accordance with the LSV result in Figure 4b. It is worth noting that the stability of R-CMS/BNC for oxygen evolution is also superior to CMS/BNC as shown in Figure S4. There are two main reasons for the enhanced electrocatalytic activity of R-CMS/BNC compared with CMS/BNC and other metal sulfide catalysts. First, most of the metal sulfides studied to date suffer from poor electrochemical stability in oxygen evolution reaction and charge process because of sulfide oxidation, which is also observed in our experiments.41 After 30 min of OER, some S2− in CMS/BNC changed to SO42− as can be seen from the S XPS spectrum in Figure S5a. Such a change can decrease the oxygen evolution activity because of the altered structure and morphology of metal sulfide.41 However, the R-CMS/BNC showed little change of S2− in OER, as revealed from the S XPS spectrum in Figure S5b. Note from the XPS result that after oxygen reduction and evolution, Co and Mn showed little change (see Figures S11 and S12). Clearly, it is the change of S that affected the oxygen evolution performance. Second, the B and N codoped mesoporous nanocarbon also contributed to long-term stability. Compared with traditional graphene materials, which can be easily oxidized at high potential, the mesoporous nanocarbon has proved long-term stability in both ORR and OER.1,42 We further tested the electrochemical Cdl of the catalysts by a simple cyclic voltammetry method to relate the catalytic activity with the electrochemical surface area (ECSA). As expected, the plot of the current density (at 1.165 V) against the scan rate has a linear relationship, and its slope is Cdl. As shown in Figure 4d G


Research Article


Figure 5. Characteristics of the bifunctional electrocatalyst based rechargeable Zn-air batteries. (a) A polarization curve (V vs i) and corresponding power density plot of the battery using CMS/BNC as the cathode catalyst compared with the battery using commercial Pt/C catalyst (the mass of catalysts loaded is 1 mg cm−2, and with battery discharging the current density of CMS/BNC overcome Pt/C. This result is corresponding to ORR result). (b) Typical discharge curves of Zn-air batteries with CMS/BNC as the cathode catalyst under continuous discharge until complete consumption of Zn at two different current densities. Specific capacity was normalized to the mass of consumed Zn. (c) Long-time discharge curves of the batteries using CMS/BNC as the cathode catalyst at two different current densities and for comparison, using Pt/C as the cathode catalyst. (d) Charge and discharge polarization curves of the Zn-air battery using CMS/BNC and R-CMS/BNC compared with the one using Pt/C (Battery using R-CMS/BNC showed a higher current density in charging process, and there is no obvious change in discharging process.) (e) Specific capacity and Coulombic efficiency of our Zn-air battery using R-CMS/BNC and CMS/BNC when the current density was 20 mA cm−2. (f) Cycling performance of our Zn-air battery using R-CMS/BNC and CMS/BNC at 20 mA cm−2 and a 4 h cycle period. For comparison, the battery performance using Pt/C is also shown. Note that the discharging voltage showed little difference, while the charging voltage of R-CMS/BNC is lower than others, and its charge−discharge voltage gap (Δη) is the smallest among these three batteries.

6 M KOH and 0.2 M zinc acetate. These batteries showed an open-circuit voltage of nearly 1.4 V. At the voltage of 1.0 V, it showed a high current density of ∼190 mA cm−2. The peak power density reached over 250 mW cm−2 at ∼0.70 V (Figure 5b). Both the current density at 1 V and peak power density (over 250 mW cm−2) were significantly improved over previous catalysts for Zn-air batteries (Table S7) such as carbonsupported MnOx nanowires (190 mW cm−2), Co3O4 nanoparticles decorated carbon nanofiber (125 mW cm−2), rGO-IL/ Mn3O4 (120 mW cm−2), Ag−Cu catalysts (85.8 mW cm−2), Ni-modified MnOx/C (122 mW cm−2), and FeCo-EDA (232 mW cm−2).9,10,43−46 This result is also comparable with the battery made with Pt/C ORR catalyst at the same mass loading as the result of the higher ORR activity of CMS/BNC (Figure 5a). As discharge proceeded, the Zn foil was gradually thinned. The battery eventually ceased functioning when all the Zn metal was consumed. Then the specific capacity normalized to the mass of consumed Zn was ∼550 mAh g−1 as shown in Figure 5b. Our Zn-air batteries made with CMS/BNC catalysts were very robust. When galvanostatically discharged at high current density such as 10 and 50 mA cm−2 for 16 h, no obvious voltage drop was observed (Figure 5c) owing to their high stability for ORR. After ∼24 h the voltage decreased, but simply replenishing the metal anode and electrolyte could regenerate the battery for subsequent runs at the same performance level with the same catalyst. This result proved that the Zn electrode was the limiting electrode with respect to the battery stability and that our new catalyst was rather stable and thus did not limit the battery stability performance. This is important for Znair battery application because when used in power electric vehicles or other applications it is necessary for Zn-air battery to be quickly refueled with fresh metallic Zn.8 This result again suggests a good stability of our electrocatalysts toward ORR, as

the electrochemical active area also increased after Mn doping. Cdl of CMS confirmed to be 9.3 mF cm−2, which is nearly twice that of CoS (4.7 mF cm−2). The combination with BNC increased that value to 14.3 mF cm−2. Note that the change in Cdl is in parallel with the BET result. Specifically, R-CMS/BNC has a Cdl value of 18.5 mF cm−2, which is also higher than the value of CMS/BNC (14.3 mF cm−2). The Cdl increase from CoS to Mn-doped CoS shows that Mn doping could not only increase the specific surface area but also the ECSA. This result accorded with the effect of Mn dopant reported and the performance of these samples in OER. This increase mainly comes from doped Mn(III) atoms, because OH− can adsorb and form bond readily on doped Mn atoms. The eg orbital of Mn(III) and the O-pσ orbital on OH− can overlap, and the resulting moderate bonding strength facilitates the ion exchange in the rate-determining step of the OER cycle.14 In addition, the combination with BNC can avoid the aggregation, make nanoparticles disperse better, and thus can also increase the ECSA. Rechargeable Zn-Air Batteries. Rechargeable Zn-air batteries require a bifunctional electrocatalyst to make them function efficiently, but most electrocatalysts studied so far for this purpose are good toward either ORR or OER but not both. Our low-cost and easily processed bifunctional electrocatalysts fill this gap nicely. For convenience of discussion, we propose the charge−discharge voltage gap (Δη) for appraising such Zn−air battery based on the bifunctional catalyst. For a rechargeable battery Δη = ηcharge − ηdischarge, and Δη should be as small as possible. To minimize these values, researchers often used separate catalysts each optimized for the discharging (ORR) or the charging (OER) process.7 To construct our rechargeable Zn-air batteries, we used the CMS/BNC bifunctional electrocatalysts loaded on the nickel foam electrode as the air cathode and paired it with a zinc foil in H


Research Article


Figure 6. Morphology characterization and bifunctional electrocatalytic performance of NMS/BNC in oxygen reduction and evolution as well as in a rechargeable Zn-air battery: (a) TEM images of NMS/BNC showed similar morphology with CMS/BNC in Figure 1d; (b) RDE voltammograms recorded of oxygen reduction for samples with different Ni/Mn ratio supported on GC electrode at a rotation rate of 1600 rpm (it showed that Ni4Mn1 appeared highest catalysis performance in oxygen reduction); (c) RDE voltammograms recorded for NMS/BNC with Ni4Mn1 at different rotation rates (Figure S9 shows RRDE voltammetric response of NMS/BNC with Ni4Mn1 at a rotation rate of 1600 rpm, and from this ring and disk current we calculated the number of electrons transferred, see Table S5); (d) R-NMS/BNC (NMS/BNC after 30 min ORR) compared with NMS/BNC and NMS without BNC (R-NMS/BNC showed a higher current density compared with NMS/BNC. When reached 20 mA cm−2 its overpotential also reduced to 0.29 V. This result agreed with Mn doped CoS); (e) A polarization curve (V vs i) and corresponding power density plot of the battery using NMS/BNC as the cathode catalyst compared with the battery using commercial Pt/C catalyst; (f) charge and discharge polarization curves of the Zn-air battery using NMS/BNC and R-NMS/BNC compared with the one using Pt/C (Battery using NMS/BNC showed a similar result with battery using CMS/BNC).

already demonstrated in Figure 3d. To confirm the good stability of the bifunctional catalyst, we tested a half cell on a suitable zinc foil, and the result is shown in Figure S15. After discharging test for 60 h, there is no obvious voltage change, thereby bolstering the high stability of our catalyst under the oxygen reduction operation. There have been some reports in the literature about the Zn−air batteries used for powering electric vehicles, but most of them still suffer from the low power density and stability, especially the inability to be quickly refueled with fresh metallic Zn. Our bifunctional catalysts are ideally suited for such quick refueling Zn-air batteries owing to the exceptional ORR activity and durability. As mentioned above, the charge and discharge voltage gap (Δη) should be as small as possible. To get an excellent performance, some rechargeable Zn−air batteries used one catalyst for the charge process and another catalyst for discharge process.8 In contrast, our rechargeable Zn−air battery uses a single CMS/BNC catalyst for both oxygen evolution and reduction, as the name of bifunctional catalyst suggests. The electrolyte used was 6 M KOH with 0.2 M zinc acetate to ensure reversible Zn electrochemical reactions at the anode.8 Our battery using CMS/BNC as oxygen catalyst exhibited comparable current density as the rechargeable battery made with 20 wt % Pt/C as the ORR and OER catalyst at the same potential. Moreover in charging process (OER process) the CMS/BNC and R-CMS/BNC showed a higher current density compared with Pt/C catalysts (shown in Figure 5d). This result is corresponding to the OER results in Figure 4. And the charging voltage of 20 mA cm−2 is ∼1.9 V (for battery based on R-CMS/BNC and CMS/BNC), which is lower than some other reported rechargeable Zn-air battery such as Co3O4 nanoparticles decorated carbon nanofiber (∼2.0 V)10 and Ag−Cu bifunctional electrocatalysts (over 2.0 V).44

The Coulombic efficiency of our Zn-air battery is shown in Figure 5e. After 500 cycles with current density of 20 mA cm−2, it showed little change, and this excellent stability accorded with the high stability of our catalysis in oxygen reduction and evolution. Figure 5f shows the charge and discharge polarization curves of the rechargeable Zn-air battery using different bifunctional catalysts. We can clearly find that the batteries based on R-CMS/BNC and CMS/BNC showed approximate discharge voltage. While in charging process, the batteries based on R-CMS/BNC and CNS/BNC showed lower charge voltage compared with battery based on Pt/C. The charge−discharge voltage gap of batteries we prepared at 20 mA cm−2 is ∼0.72 (R-CMS/BNC) and 0.75 V (CMS/BNC), respectively (Figure 5f). This value is much lower than the battery based on Pt/C and other reports listed in Table S8, such as MnO2/carbon nanotube (1.5 V),47 eggplant carbon sheets (0.85 V at 2 mA cm−2),48 and NiCo alloys with their oxides (0.86 V).49 Moreover this value is comparable with the battery using two separate catalysts (CoO/N-CNT as ORR catalyst and FeNi LDH as OER catalyst) whose value is ∼0.7 V at 20 mA cm−2.8 The longer-term stability of our Zn-air battery was also tested, and the result is shown in Figure S8. When repeatedly charged or discharged at 20 mA cm−2 for a total of 200 h, the battery showed high cycling stability. By extending this method to NiS, we obtained similar result, which is shown in Figures 6, S6, and S7. TEM result in Figure 6a showed Mn-doped NiS (NMS) nanoparticles dispersed on BNC uniformly, which is similar to the morphology of CMS/ BNC. The Mn doping also brought metallic Ni in NiS as the XPS result of Ni shown in Figure S6a. From EIS result in Figure S6b, we can find the Rct was also reduced after Mn doping and combined with BNC. Moreover, the XRD result in Figure S7 showed similar structure with Mn-doped CoS. At the same time the bifunctional catalysis performance of MNS/BNC I



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showed similar result compared with CMS/BNC (Figure 6b,c). And after 30 min reduction treatment the R-NMS/BNC showed an increase in oxygen evolution catalysis performance (Figure 6d). And applied in Zn-air battery it showed a peak power density of 242 mW cm−2, which is also comparable with the battery of Pt/C (Figure 6e). In Figure 6f, we can find the charging performance of Zn-air battery based on NMS/BNC and R-NMS/BNC also showed higher current density compared with Pt/C. This result is also similar to the result of CMS/BNC. These results prove that the method of Mn doping is general and can be applied in other metal sulfides with comparable results.

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.5b12803. The EDS data of various samples with different Co/Mn and Ni/Mn ratio. Co(II)/Co(0) and Ni(II)/Ni(0) ratio of various samples with different Co/Mn and Ni/Mn ratio from XPS data. Specific surface area results of samples with different Co/Mn and Ni/Mn ratio and BNC. The number of electron transfers for CNS/BNC and NMS/BNC at different potential. Summary of overpotential of various OER catalysts and their performance in Zn-air batteries in alkaline media. SEM result of Co(II)1−xCo(0)x/3Mn(III)2x/3 nanoparticles. XPS results of Mn, N, S, and B. Electrochemistry performance of NMS/BNC. Schematics of the rechargeable Zn-air battery. (PDF)

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CONCLUSION In summary, we have devised and investigated a novel type of boron/nitrogen codoped mesoporous nanocarbon supported Mn(III)/Co(0) doped CoS nanoparticles, which are highly active for both ORR and OER electrocatalysis in alkaline media. The direct use of the bifunctional electrocatalysts was demonstrated in rechargeable Zn-air batteries. The Mn doping under mild conditions was shown to assist in the partial transformation of Co(II) into metallic cobalt Co(0) accompanied by the formation of Mn(III), resulting in an improved conductivity and oxygen reduction performance. Meanwhile the doped Mn(III) also increased the ECSA and promoted both ORR and OER. Moreover, combination with B/N codoped nanocarbon further improved the ORR/OER catalytic performance, conductivity, specific surface area, and electrochemistry surface area of the electrocatalyst. The catalyst showed comparable ORR performance with Pt/C catalyst. For OER catalysis it showed superior performance than IrO2 and some other reported catalysts. After ORR process the catalyst showed increased OER performance because the reduction process can minimize the oxidation of S2− and improve electrochemical active area. The activity and durability of these materials outperformed many non-noble metal electrocatalysts and showed promise as air catalysts in aqueous rechargeable Zn-air batteries. When the CMS/BNC electrocatalyst is applied in rechargeable Zn-air batteries, we obtained an ∼1.4 V opencircuit voltage and a peak power density of 258 mW cm−2. The robustness of our catalyst allows the battery to run continuously and consistently by refueling the Zn anode and electrolyte periodically, presenting an ideal oxygen catalyst for Zn-air batteries. In the charging process, our catalyst showed a higher current density than Pt/C especially after discharging process. This device performance is in accord with the separate OER testing result. Moreover, our battery showed a low charge−discharge voltage gap of ∼0.72 V at 20 mA cm−2. This value is much lower than most previously reported bifunctional catalysts and comparable with the batteries using separate (nonbifunctional) catalysts for charging and discharging. While extending this method to NiS, we obtained a similar result, namely, the BNC supported Mn(III)/Ni(0) doped NiS is also an excellent bifunctional electrocatalyst for ORR and OER. When applied to Zn-air battery, it also showed a comparable performance with Pt/C with a peak power density of >242 mW cm−2. The strategy developed here, that is, metal doping, intermetal electron exchange, and mesoporous carbon combination, affords highly active transition metal sulfides with controllable metal valence for application in oxygen catalysis and metal-air battery.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jue Hu for her help on paper revisions and for stimulating discussions. This work was supported by the NSFC/RGC Joint Research Scheme (Grant N_HKUST610/ 14).

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REFERENCES

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