CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402759

Shewanella-mediated Biosynthesis of Manganese Oxide Micro-/Nanocubes as Efficient Electrocatalysts for the Oxygen Reduction Reaction Congcong Jiang,[a] Zhaoyan Guo,[a] Ying Zhu,*[a] Huan Liu,*[a] Meixiang Wan,[b] and Lei Jiang[b] Developing efficient electrocatalysts for the oxygen reduction reaction (ORR) is critical for promoting the widespread application of fuel cells and metal–air batteries. Here, we develop a biological low-cost, ecofriendly method for the synthesis of Mn2O3 micro-/nanocubes by calcination of MnCO3 precursors in an oxygen atmosphere. Microcubic MnCO3 precursors with an edge length of 2.5 mm were fabricated by dissimilatory metal-reducing Shewanella loihica PV-4 in the presence of MnO4 as the sole electron acceptor under anaerobic conditions. After calcining the MnCO3 precursors at 500 and 700 8C, porous Mn2O3-500 and Mn2O3-700 also showed microcubic morphology, while their edge lengths decreased to 1.8 mm due to thermal decomposition. Moreover, the surfaces of the Mn2O3 microcubes were covered by granular nanoparticles

with average diameters in the range of 18–202 nm, depending on the calcination temperatures. Electrochemical measurements demonstrated that the porous Mn2O3-500 micro-/nanocubes exhibit promising catalytic activity towards the ORR in an alkaline medium, which should be due to a synergistic effect of the overlapping molecular orbitals of oxygen/manganese and the hierarchically porous structures that are favorable for oxygen absorption. Moreover, these Mn2O3 micro-/nanocubes possess better stability than commercial Pt/C catalysts and methanol-tolerance property in alkaline solution. Thus the Shewanella-mediated biosynthesis method we provided here might be a new strategy for the preparation of various transition metal oxides as high-performance ORR electrocatalysts at low cost.

Introduction As electrochemical energy conversion devices, fuel cells and metal–air batteries have aroused considerable research attention because they offer high energy efficiency and power density coupled to zero emissions.[1] The major obstacle for the development of these energy conversion devices arises from the sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode, which limits the performance of fuel cells and metal–air batteries. So far, platinum-based materials have been identified as the most effective catalysts for ORR owing to their relatively low overpotential and high current density.[2] However, they suffer from a number of disadvantages, such as limited supply, high cost, low methanol tolerance, poisoning (by carbon monoxide), and poor operational stability.[3] To ad[a] C. Jiang,+ Z. Guo,+ Prof. Y. Zhu, Dr. H. Liu Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment Beihang University No. 37 Xueyuan Road, Beijing 100191 (PR China) E-mail: [email protected] [email protected] [b] Prof. M. Wan, Prof. L. Jiang Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing, 100190 (PR China) [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402759.

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dress these problems, non-precious-metal and even metalfree catalysts with high ORR activity and durability at low cost have recently been investigated as promising alternatives for commercial platinum-based catalysts. These alternatives include transition-metal macrocycles,[4] transition-metal oxides,[5] conductive polymers,[6] and heteroatom-doped carbon materials.[7] Among these candidates transition-metal oxides, and particularly manganese oxides, are considered among the most promising non-precious-metal electrocatalysts because they offer prominent advantages such as geological abundance, low cost, environmental friendliness, and considerable activity towards the ORR.[8] Since the first report concerning the electrocatalytic properties of manganese oxide in the early 1970s, there have been many interesting studies that evaluated and optimized manganese-based ORR electrocatalysts for fuel cells. For example, Lee and co-workers[9] reported Ketjenblack carbon-supported manganese oxide (MnOx) nanowires (MnOx NWs/KB) fabricated by a polyol method. The material displayed a significantly enhanced catalytic activity towards ORR in alkaline solutions. The authors proposed that the large surface area of MnOx NWs/KB composites, combined with its high density of surface defects, potentially offered more active sites for oxygen adsorption, and thus led to enhanced catalytic activity towards ORR. Kim and co-workers[10] exploited the spontaneous deposition of platinum nanocrystals on Mn3O4 nanoparticles (Pt/Mn3O4) by employing the galvanic replacement process between Mn3O4 ChemSusChem 0000, 00, 1 – 7

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CHEMSUSCHEM FULL PAPERS and PtCl42 complexes. The Pt/Mn3O4 nanocomposites showed highly specific catalytic activity and durability compared to commercial Pt/C catalysts. Considering the aforementioned application potential of manganese oxides, various innovative strategies are needed to synthesize them.[11] Recently, there has been increased interest in microbe-mediated synthesis of various nanomaterials as a reliable and ecofriendly fabrication method. This usually takes place at ambient temperatures and pressures in moderate pH environments.[12] Recently, Shewanella, a Gram-negative, dissimilatory metal-reducing bacterium, has received much attention because of its ability to reduce metal ions and produce various nanoscale minerals with hierarchical structures and unique properties through its respiration.[13] The process of metal reduction is associated with extracellular electron transfer,[14] in which abundant c-type cytochromes of the outer membrane transfer respiratory electrons to outer membrane surfaces. The electrons eventually arrive at solid-state acceptors directly or through mediation by electroactive metabolites. Jiang and coworkers[15] reported that Shewanella sp. HN-41 enabled the biological synthesis of goethite nanowires, resulting from the reduction of poorly crystalline FeIII-oxyhydroxide under anaerobic conditions. Lee and co-workers[16] developed a biosynthesis approach to fabricate the filamentous arsenic–sulfide nanotubes, which could behave either as metals or as semiconductors in terms of electrical and photoconductive properties, via the combined reduction of thiosulfate and arsenate by Shewanella sp. HN-41. Ho and co-workers[17] demonstrated that selenium nanowires and nanoribbons can be assembled from biogenic amorphous selenium nanospheres by Shewanella sp. strain HN41 in 80 % dimethyl sulfoxide with bacterial pellets. The formation of selenium nanospheres by bacteria was likely mediated by a respiratory reductase that reduces dissolved selenate or selenite to elemental selenium. To the best of our knowledge, however, there is no report on a Shewanella-mediated method for the biosynthesis of micro-/nanostructured manganese oxides that can function as effective active components in ORR catalysts. In the present study, we report a facile route to fabricate precursor manganese carbonate (MnCO3) microcubes by Shewanella loihica PV-4 (S. loihica PV-4), in the presence of MnO4 as sole electron acceptor and lactate as an electron donor under anaerobic conditions. The concentration of S. loihica PV4 plays an important role in the formation of MnCO3 microcubes with well-defined crystal structures. After calcination under an O2 atmosphere at 500 and 700 8C, the shape of MnCO3 is maintained to form porous Mn2O3 microcubes (denoted as Mn2O3-500 and Mn2O3-700). The Mn2O3 microcubes have an average edge length of 1.8 mm, comprised of nanoparticles with sizes of 18–202 nm in average diameter (dependent on the calcination temperature), thus exhibiting a hierarchical three-dimensional micro-/nanostructure. Importantly, Mn2O3500 micro-/nanocubes demonstrate relatively good electrocatalytic activity towards ORR in alkaline solution, owing to the unique molecular orbits of Mn2O3 and the hierarchically porous structure, which are favorable for molecular adsorption and transport of O2. Besides, the ORR as catalyzed by Mn2O3  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org micro-/nanocubes shows long-term durability and is free from methanol crossover effects. The Shewanella-mediated biosynthesis might provide a simple and feasible manufacturing method for micro-/nanoscale transition-metal-oxide ORR catalysts that ultimately can be applied in electrochemical energy transition devices.

Results and Discussion A schematic diagram of the synthesis procedure of MnCO3 microcubes is given in Figure 1 a. Under anaerobic conditions, lactate as an electron donor goes through a tricarboxylic acid (TCA) cycle involving a series of enzyme-catalyzed chemical reactions, by which carbon dioxide (CO2) is produced and elec-

Figure 1. (a) Schematic procedure for the Shewanella-mediated biosynthesis of manganese carbonate (MnCO3) microcubes. (b, c) SEM images of the asprepared MnCO3 at an OD value of 0.7. (d, e) SEM images of the sample formed in the absence of bacterial cells (i.e., OD value of 0).

trons are released in the process of cellular respiration, as reported previously.[18] Then the electrons are transported to the surface of the bacterial cells by c-cytochromes on the surface of the outer membrane of the bacteria. As a result, the MnO4 as the sole electron acceptor is reduced to low-valence Mn2 + . At the same time, CO2 molecules produced by bacterial respiration react with Mn2 + instantaneously to form the MnCO3 microcubes. Figure 1 b shows a scanning electron microscopy (SEM) image of a sample obtained at an optical density (OD) value of 0.7, indicating that microcubes with an average side length of 2.5 mm were obtained. From the high-magnification SEM image in Figure 1 c, the surfaces of MnCO3 microcubes appear relatively smooth, except for small pores derived from peeled bacteria after biomineralization. Figure S1 a (Supporting Information) shows its X-ray diffraction (XRD) pattern, indicating that the diffraction peaks of the as-prepared microcubes ChemSusChem 0000, 00, 1 – 7

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CHEMSUSCHEM FULL PAPERS can be perfectly indexed to MnCO3 with a pure rhombohedral hexagonal phase, which is consistent with standard powder diffraction data reported before.[19] To elucidate the effect of Shewanella on the formation of MnCO3, three separate incubations with different cell suspension OD values were carried out in the presence of MnO4 as the sole electron acceptor under anaerobic conditions. Figure 1 d and e shows that irregular granules were obtained in the absence of S. loihica PV-4 (i.e., OD = 0). But no obvious peaks for MnCO3 were observed from the XRD pattern of the granules (Figure S1 b). SEM images of the as-prepared product formed at an OD value of 0.3 indicate that more particles aggregated to form larger irregular clusters (Supporting Information, Figure S2a and b). As the OD value of the cell suspension increased to 1.2, the shape of the as-prepared product changed dramatically, to incomplete cubes, and it seemed that there was not enough manganese available (Supporting Information, SEM images in Figure S2 c and d). XRD measurements (Figure S1 b) confirmed that two products formed at OD values of 0.3 and 1.2 could be assigned to the pure phase of MnCO3. Moreover, it is well-known that Shewanella strains secrete redox-active flavins that mediate extracellular electron transfer.[20] To investigate the effect of flavins on MnCO3 formation, the reduction of MnO4 was carried out by the addition of exogenous flavins for 24 h in the absence of bacterial cells and, simultaneously, bubbling with CO2 gas. As shown in Figure S3 a and b (Supporting Information), irregular granules were obtained (SEM image) and the XRD pattern did not show obvious peaks arising from MnCO3. Moreover, a control experiment with killed cells was performed to demonstrate that the reduction of MnO4 arises from biotic processes of S. loihica PV-4 (Supporting Information, Figure S4). In addition, comparative experiments confirmed that the CO2 for the formation of MnCO3 comes from bacterial respiration, and not from the bicarbonate (present in high concentration) in the DML medium, as presented in Figure S5 a–f (Supporting Information). When combining these findings, it becomes clear that at suitable concentrations S. loihica PV-4 plays an important role in forming MnCO3 microcubes with uniform morphology, providing proof for the formation mechanism mentioned above. Mn2O3 micro-/nanocubes with porous structures were obtained through thermal decomposition of MnCO3 at high temperature for 5 h in O2 atmosphere. Before calcination, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were performed to select appropriate calcination temperatures. The TGA curve displayed a dominant weight loss of about 31 %, that was associated to the decomposition of MnCO3 to Mn2O3 with the liberation of CO2 (Supporting Information, Figure S6). The DSC curve showed a broad endothermic peak located in the range between 350 and 450 8C, which fitted very well with the weight loss peak in the TGA curve. Based on the TGA and DSC results, 500 8C and 700 8C were chosen to ensure translation of MnCO3 to form pure Mn2O3. SEM images of the as-prepared Mn2O3 are shown in Figure 2 a–d. The Mn2O3 obtained at 500 8C (Mn2O3-500) displayed micro-/nanocubic features, and is almost identical to the corresponding MnCO3 precursor (Figure 1 a and b); howev 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. SEM images of the as-prepared (a, b) Mn2O3-500 and (c, d) Mn2O3700.

er, the average edge length of the micro-/nanocubes decreased to 1.8 mm due to thermal decomposition at high temperature. The magnified image clearly shows that the homogeneous micro-/nanocubes are rough and are densely covered by nanoparticles of about 18 nm in average diameter. This feature is attributed to the release of CO2 in the decomposition process. When the Mn2O3 was obtained at 700 8C (Mn2O3-700), the product still maintained a cubic architecture with edge lengths of 1.8 mm in (Figure 2 c and d). But the Mn2O3-700 micro-/nanocubes comprised many “peanut”-like particles with an average diameter of 202 nm, linked with each other in maze-like structure. Figure S1 a displays XRD patterns of as-prepared Mn2O3-500 and Mn2O3-700 samples. All diffraction peaks can be assigned to cubic Mn2O3, consistent with previous reports,[21] thus indicating that the chemical transformation from MnCO3 to cubic Mn2O3 was achieved. The specific surface areas for Mn2O3-500 and Mn2O3-700 were measured to be 1381  9.42 cm2 g1 and 1100  18.57 cm2 g1, respectively. The electrocatalytic activities of Mn2O3-500 and Mn2O3-700 for ORR were first investigated by cyclic voltammetry (CV) in an O2- or N2-saturated 0.1 mol L1 KOH solution at a scan rate of 50 mV s1. As a reference point, commercial 20 wt % platinum on Vulcan carbon black (Pt/C) was measured under the same conditions. The CV of Mn2O3-500 obtained in N2-saturated electrolyte did not show any obvious peaks (Figure 3 a). In the O2-saturated electrolyte, a well-defined cathodic ORR peak at 0.35  0.01 V (vs. Ag/AgCl) with a reaction current of 1.17  0.07 mA cm2 was detected, indicating the pronounced electrocatalytic activity of Mn2O3-500. The CV curve of Mn2O3700 exhibited a single cathodic peak at 0.28  0.01 V (vs. Ag/ AgCl) with a current density of 1.00  0.07 mA cm2 in O2-saturated electrolyte. Figure 3 b shows the ORR polarization curves of Mn2O3-500, Mn2O3-700, and commercial Pt/C in O2-saturated 0.1 mol L1 KOH solution at a rotation rate of 1600 rpm. The ChemSusChem 0000, 00, 1 – 7

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tentials and comparable limiting current densities, which is more favorable for the reduction of oxygen. To compare the kinetic parameters for ORR of the Mn2O3-500, Mn2O3-700, and commercial Pt/C catalysts, linear sweep voltammetry (LSV) measurements on a rotating disk electrode (RDE) were recorded at a scan rate of 10 mV s1 at different rotation rates, from 400 to 1600 rpm, as shown in the Figure 3 c–d and Figure S7 a (Supporting Information). The electron-transfer number (n) in the catalytic process was calculated by the Kotecky–Levich equation. The Koutecky–Levich plots of Mn2O3-500, Mn2O3-700, and Pt/C catalysts were plotted for different potentials, as shown in Figure 3 e and f, and Figure S7 b. The parallel and straight fitting lines of 1/j vs. 1/w0.5 imply a first-order reaction towards dissolved oxygen. The n values are 3.10 and 3.30 for Mn2O3-500 and Mn2O3-700, respectively. A comparison to the value for the commercial Pt/ C catalyst (3.7) indicates that catFigure 3. (a) Cyclic voltammetry (CV) curves for Mn2O3-500, Mn2O3-700, and commercial Pt/C catalysts, in N2alysis by Mn2O3 may proceed via (dashed line) or O2-saturated (solid line) 0.1 mol L1 KOH at a scan rate of 50 mV s1. (b) ORR polarization curves of a coexisting pathway involving 1 Mn2O3-500, Mn2O3-700, and commercial Pt/C catalysts in O2-staturated 0.1 mol L KOH at a rotation rate of both two- and four-electron re1600 rpm. (c, d) Linear sweep voltammograms (LSVs) and related K–L plots (e, f) of Mn2O3-500, Mn2O3-700 catalysts action processes. According to in O2-saturated 0.1 mol L1 KOH solution at a scan rate of 10 mV s1 at different rotation rates from 400 to 1600 rpm. the above results, the porous Mn2O3-500 and Mn2O3-700 micro-/nanocube catalysts exhibonset potential of Mn2O3-500 catalyst was 0.07  0.006 V (vs. ited improved electrocatalytic activity, especially the Mn2O3Ag/AgCl), similar to that of commercial Pt/C (0.065 V, vs. Ag/ 500 catalyst. Although the exact mechanism requires further AgCl) and slightly more positive than that of Mn2O3-700 investigation, the improved electrocatalytic activity of the (0.09  0.006 V, vs. Ag/AgCl). Generally, a positive shift of the Mn2O3 micro-/nanocubes may be rationalized as follows: firstly, onset potential indicates a decrease of the overpotential rethe s and p orbits of oxygen molecule overlapping with the dquired to initialize the ORR in the cathodes of fuel cells.[22] The orbit of the manganese ion are advantageous to the breaking of O=O bonds, which can enhance the catalytic performance half-wave potential of Mn2O3-500 was 0.24  0.003 V (vs. Ag/ of Mn2O3 to ORR, as proved by a previous report.[9] Secondly, AgCl), comparable to that of the Pt/C catalyst (0.24 V, vs. Ag/ AgCl), while the half-wave potential of Mn2O3-700 was about the high surface areas of the hierarchical Mn2O3 micro-/nano0.26  0.01 V (vs. Ag/AgCl) at 1600 rpm. The catalytic activity structures may facilitate the absorption, diffusion, and reducof Mn2O3-500 micro-/nanocubes towards ORR is higher than tion of oxygen. The durability of cathodic electrocatalysts is a common issue that of the Mn2O3-700 micro-/nanocubes owing to its relatively for fuel cells, and is crucial from an industrial point of view. To large surface area. The limiting diffusion current densities were investigate the cycle stability of the Mn2O3-500 and Mn2O3-700 3.30  0.33 mA cm2 and 2.50  0.22 mA cm2 for Mn2O3-500 and Mn2O3-700 at 0.6 V, respectively. Importantly, compared catalysts, an acceleration degradation test (ADT) was performed by continuous chronoamperometric measurements at to chemically synthesized manganese oxides catalysts[9, 23, 24] 0.3 V in 0.1 mol L1 O2-saturated KOH aqueous solution at such as a-MnOx spheres[9] or a-/b-/g-MnO2 nanowires,[24] the Mn2O3 micro/nanocubes exhibited the more positive onset poa rotation rate of 800 rpm. As shown in Figure 4 a, the com 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemsuschem.org toward the oxygen reduction reaction in alkaline solution. Importantly, the Mn2O3-500 catalyst exhibits a high onset and half-wave potential in 0.1 mol L1 KOH solution, which is close to that of the commercial Pt/C. We propose that the improved oxygen reduction reaction (ORR) activity of the Mn2O3 catalysts can be ascribed to a synergistic effect between the overlapped molecular orbits of oxygen and manganese and the hierarchical porous structures. Moreover, the Mn2O3 micro-/nanocubes display a better stability than the commercial Pt/C catalyst, and are free from methanol crossover effects in alkaline solution. Therefore, we imagine that the Shewanella-mediated biosynthesis method provides a low-cost, eco-friendly approach for the development and design of transition-metal-oxide catalysts for the ORR with high efficiency and good stability.

Experimental Section Bacteria culture

Figure 4. Current–time (i–t) chronoamperometric responses for ORR at the Mn2O3-500, Mn2O3-700, and Pt/C electrodes (vs. Ag/AgCl) in an O2-saturated 0.1 mol L1 KOH solution at 0.3 V versus Ag/AgCl. (a) Time dependence of durability evaluation at a rotation rate of 800 rpm- (b) The arrow indicates the addition of 5 mol L1 methanol in O2-saturated 0.1 mol L1 KOH solution after around 200 s.

mercial Pt/C catalyst showed a rapid current decline of 60.5 % after 40 000 s, which is indicative of its poor durability. However, the Mn2O3-500 and Mn2O3-700 catalysts exhibited excellent durability performance with only 15.3 % and 6.7 % retention of the initial current after 40 000 s, demonstrating excellent durability relative to the commercial Pt/C catalyst. Importantly, the current density of commercial Pt/C catalyst is inferior to those Mn2O3 catalysts after operation for about 13 000 s. Upon the addition of 5 mol L1 methanol, moreover, a sharp decrease in current density was observed for the commercial Pt/C catalyst. Mn2O3-500 and Mn2O3-700 catalysts exhibited a stable amperometric response after the addition of methanol (Figure 4 b), which is similar to the commercial palladium catalyst. These results show excellent stability and good tolerance to methanol crossover effects, implying the use of these materials in methanol-tolerant cathodes.

S. loihica PV-4 was cultured aerobically in 100 mL of marine broth (MB, 20 g L1) at 30 8C for 24 h with shaking. The Shewanella cells were centrifuged, and MB was replaced with a defined medium (DML: NaHCO3 (2.5 g), CaCl2·2 H2O (0.08 g), NH4Cl (1.0 g), MgCl2·6 H2O (0.2 g), NaCl (10 g), and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid HEPES (7.2 g)) per liter with lactate. The cells were further cultivated aerobically in DML at 30 8C for 24 h using lactate as a carbon source. Finally, the suspension was centrifuged and the resultant cell suspension was washed with DML three times prior to being used for synthesis of MnCO3. To understand the effect of Shewanella on the formation of MnCO3, the concentrations of cell suspension were set with optical density (OD) value of 0, 0.3, 0.7, and 1.2.

Biosynthesis of Mn2O3 micro-/nanocubes In a typical MnCO3 synthesis, 0.079 g KMnO4 was added into 100 mL DML solution in the septum vials, and bubbled with N2 for 30 min to remove dissolved O2. The resuspended cells (OD value = 0.7) were injected into the septum vials, then incubated anaerobically at 30 8C for 3 days under shaking conditions. With the reaction, the color of the solution changed from purple to light-pink. The resulting produce was collected by centrifugation at 7000 rpm for 6 min, then washed with Milli-Q water several times to remove the S. loihica PV-4 and then dried at 30 8C for 24 h. The as-prepared MnCO3 precursors were loaded in a porcelain boat and calcinated under O2 atmosphere for 6 h at 500 and 700 8C, respectively.

Conclusions

Characterization

We develop a simple, novel, and ecofriendly biological method for the synthesis of well-structured MnCO3 microcubes by the dissimilatory metal-reducing S. loihica PV-4 under anaerobic conditions, in which MnO4 is supplied as the sole electron acceptor. Mn2O3-500 and Mn2O3-700 micro-/nanocubes with porous structures are obtained by the thermal decomposition at 500 and 700 8C under O2 atmosphere. The process maintains the shape of the MnCO3 precursor. The resultant Mn2O3-500 and Mn2O3-700 micro-/nanocubes have hierarchically porous structures, and display a relatively good electrocatalytic activity

The morphologies of samples were measured by scanning electron microscopy (SEM, Quanta 250FEG). X-ray powder diffraction (XRD) patterns were recorded on a D/Max 220 PC diffractometer with graphite monochromatized CuKa radiation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out to determine the decomposition of samples with a STA-449F3 Luxx simultaneous apparatus (Netzsch Co., Selb, Germany) at a heating rate of 10 8C min1 under air atmosphere. The specific surface area of samples was measured using Burnauer–Emmett– Teller (BET) technique (Micromeritics, ASAP 2460) using N2 gas adsorption/desorption. The concentration of cell suspension was de-

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termined by measuring the optical density (OD) value of cell suspension at 600 nm in a spectrophotometer. [3]

Electrochemical characterization

[4]

Electrochemical measurements were conducted on electrochemical workstation (CHI 760C, CH Instrument, Shanghai, China) with a standard three-electrode cell. A platinum wire was used as counter-electrode and an Ag/AgCl (3 mol L1 KCl) electrode as reference. The working electrodes were prepared by applying respective catalyst inks onto the pre-polished glass carbon disk (GC) electrodes. Briefly, the catalysts were dispersed in ethanol and ultrasonicated for 20 mins to form a uniform catalyst inks (2 mg mL1). A total of 15 mL of a well-dispersed catalyst ink was placed on the GC electrode (5 mm in diameter). After drying at room temperature, 7.5 mL Nafion (0.05 wt %) ethanol solution was applied onto the surface of the catalyst, then dried at room temperature. The ORR performance of catalysts was investigated by cyclic voltammogram (CV) and linear sweep voltammogram (LSV) measurements in 0.1 mol L1 KOH solution. CVs were measured as a scan rate of 50 mV s1, and LSVs were studied at a scan rate of 10 mV s1 at different disk rotation rates of 400, 600, 900, 1200, and 1600 rpm. The electron-transfer number (n) in the ORR process was determined using the Koutecky–Levich (K–L) equation:[25]

[5]

1 1 1 ¼ þ j jk Bw0:5

ð1Þ

B ¼ 0:2nFðDO2 Þ2=3 u1=6 CO2

ð2Þ

[6] [7]

[8]

[9] [10] [11] [12] [13]

Here, jk is kinetic current and w is the rotation rate. B can be determined from the slope of K–L plots based on the Levich equation. F, n, DO2, n, and CO2 represent the Farady constant (96 485 C mol1), transferred electron number per oxygen molecule, diffusion coefficient of O2 (1.9  105 cm2 s1), the kinetic viscosity (1.1  102 cm2 s1), and the bulk concentration of O2 (1.2  106 mol cm3) in 0.1 mol L1 KOH.

[14] [15] [16]

[17] [18]

Acknowledgements

[19]

The authors acknowledge financial support by the National Natural Science Foundation of China (51273008, 51473008), the National High-tech Research and Development Program of China (2012AA030305), the National Basic Research Program of China (2012CB933200), Program for New Century Excellent Talents in University (NCET-13-0024), Fok Ying Tong Education Foundation (132008), and the Fundamental Research Funds for the Central Universities.

[20] [21]

[22] [23] [24] [25]

Keywords: biosynthesis · heterogeneous manganese · nanostructures · oxygen reduction

catalysis

·

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Received: July 28, 2014 Revised: September 1, 2014 Published online on && &&, 0000

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FULL PAPERS Shewie here tells me you’re lookin’ for manganese oxide cubes? Mn2O3 micro-/nanocubes are prepared by calcination of MnCO3 precursors, fabricated by dissimilatory metal-reducing Shewanella loihica PV-4 in the presence of MnO4 as sole electron accepter in anaerobic condition. The Mn2O3 micro-/ nanocubes, calcinated at 500 8C and 700 8C serve as intrinsic electrocatalysts, and exhibit promising catalytic activities towards the oxygen reduction reaction (ORR), outstanding methanol crossover resistance, as well as long-term operational stability.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

C. Jiang, Z. Guo, Y. Zhu,* H. Liu,* M. Wan, L. Jiang && – && Shewanella-mediated Biosynthesis of Manganese Oxide Micro-/Nanocubes as Efficient Electrocatalysts for the Oxygen Reduction Reaction

ChemSusChem 0000, 00, 1 – 7

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These are not the final page numbers! ÞÞ

nanocubes as efficient electrocatalysts for the oxygen reduction reaction.

Developing efficient electrocatalysts for the oxygen reduction reaction (ORR) is critical for promoting the widespread application of fuel cells and m...
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