Bulk Production of Nonprecious Metal Catalysts from Cheap Starch as

Research Article pubs.acs.org/journal/ascecg

Bulk Production of Nonprecious Metal Catalysts from Cheap Starch as Precursor and Their Excellent Electrochemical Activity Wei-Tang Yao,† Liang Yu,‡ Peng-Fei Yao,‡ Kang Wei,‡ Su-Ling Han,‡ Ping Chen,*,‡ and Jin-Song Xie*,§ †

Laboratory of Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China ‡ School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230601, P. R. China § Department of Chemistry and Materials Engineering, Hefei University, Hefei, Anhui 230601, P. R. China S Supporting Information *

ABSTRACT: Development of a high-performance electrocatalyst for an oxygen reduction reaction with a low-cost bulk production method is crucial for practical applications of metal−air batteries, oxygen sensors, fuel cells, and so on. Here, we developed a bulk production method for nitrogen- and ferrum-doped nanoporous carbon microspheres as a high-performance ORR catalyst. The process involved hydrothermal carbonization of starch with iron ions as catalysts to catalyze the formation of carbon spheres and annealing the carbon spheres in an NH3 atmosphere. NH3 was used not only as the nitrogen source to form the N-doped carbon material but also to etch the carbon to produce a nanoporous carbon material, which has high surface area and pore volume. As a highly available, accessible, and recyclable biomass, starch can be used to achieve a cheap electrocatalyst. The proposed approach is simple and scalable. The typical product has excellent activity in alkaline media and high activity in acidic media. The typical product is a competitive substitution for commercial Pt/C in a zinc−air battery. This work proposes a new design strategy for bulk production of the excellent nonprecious metal catalysts from cheap biomass. KEYWORDS: N- and Fe-doped carbon microsphere, Starch, Electrocatalyst, Oxygen reduction reaction, NH3 etching

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INTRODUCTION The oxygen reduction reaction (ORR) is a key process in metal−air battery, oxygen sensor, fuel cell, and other electrochemical devices. Developing a high-performance ORR electrocatalyst is crucial.1,2 Because of the scarcity of platinum, recently, many works have been done to develop nonprecious metal or metal-free electrocatalysts, which have high performance and stability.3−5 For the practical application of these electrochemical devices, the ideal ORR catalysts should be cheap and bulk produced. However, so far, it is still very difficult to prepare an excellent ORR electrocatalyst by a lowcost and bulk production method.4,6,7 Recently, the heteroatom-doped carbon materials have been hopeful nonprecious metal catalysts for ORR.8−11 Specifically, N-doped carbon materials such as carbon nanotubes,12 mesoporous carbon, graphene,13 and carbon nanotube− graphene hybrids5,14 have been prepared that exhibit high ORR activities. These carbon materials as ORR catalysts can effectively promote catalytical ability and enhance stability and methanol tolerance. Some recent researches have shown that © XXXX American Chemical Society

the co-doping of certain transition metal elements (for example, Co and Fe) and nitrogen to carbon materials can remarkably enhance the catalytic activity in alkaline or acidic conditions because these metal cations can coordinate with the nitrogen doped in the materials.7,15 Usually, annealing a mixture of carbon support and suitable precursors (containing nitrogen and transition metals) can produce an ORR catalyst based on the co-doping.16,17 However, it is still very difficult to select the ideal precursors and the carbon supports to obtain a highperformance ORR catalyst.18−21 Recently, we have focused on the development of nonprecious metal ORR electrocatalysts.7,15,22−24 We developed the interconnected N-doped carbon framework with Co/Co3O4 nanoparticles (Co/Co3O4/C-N) and N-TiO2/NG nanocomposites and found that N-doping in the carbon material and improving the conductivity of the nanocomposite contribute to Received: February 5, 2016 Revised: April 11, 2016

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DOI: 10.1021/acssuschemeng.6b00269 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. (a) Preparation of NFe-NPCS-900-4. (b) Photographs of Starch, CS, and NFe-NPCS

annealing temperature (in NH3 atmosphere) is 850, 900, and 930 °C, the products were denoted as NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4, respectively. For comparison, the ferrum-doped carbon microspheres (Fe-CS) were also prepared according to the same process as the typical NFeNPCS but in N2 at 900 °C for 3 h. Characterization. We used a field emission scanning electron microanalyzer (JEOL-6700F) to obtain the scanning electron microscopy (SEM) images. The samples were dropped and dried onto silicon substrates. We used a JEM-2100F instrument with an EDX analytical system to obtain the scanning transmission electron microscopy (STEM) images. The samples were dropped and dried onto copper grids. An X-ray photoelectron spectrometer (ESCALab MKII) was used to achieve the XPS data. A Micrometrics ASAP2020 analyzer (U.S.A.) was used to obtain the BET surface areas (P/P0 = 0.05−0.35) and pore volume. The pore volume was calculated by a desorption isotherm. The X-ray diffraction pattern (XRD) of NFeNPCS-9004 was achieved on an XD-3 X-ray diffractometer. An inViaReflex spectrometer (Renishaw) with a 532 nm laser excitation was used to obtain the Raman spectra of NFe-NPCS. Measurements of Electrochemical Activities. Electrochemical activities of all samples were determined using the CHI730E electrochemical workstation (China) in a three-electrode cell. The NFe-NPCS, Fe-CS, CS, and commercial Pt/C (20%) catalysts were tested by a rotating disk electrode (RDE) at room temperature. For each sample, the catalyst (5 mg) was dispersed in 1 mL of a 1:3 v/v isopropanol/deionized water mixed solvent with 40 uL of a Nafion solution (5 wt %).20 Then, we obtained the catalyst ink when the mixture was ultrasonicated for 30 min. We transferred and adhered the ink (8 uL) on a glassy carbon disk (PINE, 5 mm diameter), which was used as the working electrode in the cell. In the experiments, the catalyst loading is 0.200 mg/cm2. In 0.5 M H2SO4, a Pt plate was used as the counter electrode, and Ag/AgCl was used as the reference electrode. In 0.1 M KOH, a Pt plate was used as the counter electrode, and Hg/HgO was used as the reference electrode. We set the scanning rate of the working electrode at 20 mVs−1 and rotating speed from 400 to 2000 rpm. A standard three-electrode system was used to calibrate the Ag/ AgCl and Hg/HgO electrodes. In the system, the Pt wires were used as the working and counter electrodes. In addition, the Hg/HgO or Ag/AgCl electrode was used as the reference electrode. Before the measurement, electrolytes were prepurged and saturated with high purity H2. In 0.1 M KOH, E (RHE) = E (Hg/HgO) + 0.86 V,22 and in 0.5 M H2SO4, E (RHE) = E (Ag/AgCl) + 0.23 V.22,15 Zn−air batteries were measured in home-built electrochemical cells. Teflon-coated carbon fiber (1.0 cm2) was used as the gas diffusion layer. We uniformly coated the as-prepared catalyst onto the gas diffusion layer and achieved an air electrode (mass loading of 1.0 mg/ cm2), which was used as the cathode in the electrochemical cells. A Zn

the ORR activity.23,24 We also developed the NFeCo-CNT/ NC nanocomposite15 and the CMK3/graphene−N−Co nanocomposite7 and found that N-, Fe-, and (or) Co-doping can obviously improve the ORR activity.7,15 Nitrogen-doped nanoporous carbon nanosheets (NCS)22 were prepared, and we found that NH3 etching can produce a carbon material with a high surface area (BET), a number of micropores, and a high content of nitrogen atoms, which are vital to excellent ORR activity.22 Therefore, on the basis of our very recent work and conclusions, here, we developed a two-step method to prepare N- and Fe-doped nanoporous carbon microspheres (NFeNPCS). The process involved hydrothermal carbonization of starch with iron ions as catalysts to catalyze the formation of carbon spheres (CSs) and annealing of the carbon spheres in an NH3 atmosphere. In the preparation, iron ions were used as catalysts to catalyze the formation of CSs, and a small number of ferrum ions remain in the CSs. After annealing in the NH3 atmosphere, the CSs were doped by nitrogen and ferrum elements. The products have a high content of nitrogen atoms, a high surface area, and a large pore volume. The typical product has excellent activity in alkaline media and high activity in acidic media.

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EXPERIMENTAL SECTION

Chemicals. NH3 (99.9%) was bought from the Nanjing ShangYuan Company (China). The reagents used in this work were of analytical purity. (NH4)2Fe(SO4)2·6H2O and KOH were bought from the Aladdin Industrial Corporation (China). Starch soluble was bought from the Sinopharm Chemical Reagent Co. Ltd. (China). Pt/C (20%) was bought from Johnson Matthey. Preparation of Nitrogen- and Ferrum-Doped Nanoporous Carbon Microspheres (NFe-NPCS). Typically, 65.0 mL of distilled water and 8.0 g of starch were put into a beaker. Then, 4.0 g of (NH4)2Fe(SO4)2·6H2O was added with stirring. A homogeneous solution was obtained and transferred into a autoclave (100 mL). After the hydrothermal process (180 °C, 10 h), a black carbonaceous product was produced. After being washed with distilled water by filtration several times and freeze-drying (8 h), a carbon sphere (CS) was achieved. Finally, in order to obtain the N- and Fe-doped nanoporous carbon microsphere (NFe-NPCS), CS was annealed in an NH3 atmosphere (at 850, 900, and 930 °C) for 3 h. In the annealing, the heating rate and cooling rate are 5 °C/min. For simplicity, in the preparation (at 900 °C), when 1.0, 4.0, and 8.0 g of (NH4)2Fe(SO4)2· 6H2O were added, respectively, the products were denoted as NFeNPCS-900-1, NFe-NPCS-900-4, and NFe-NPCS-900-8. When the B


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ACS Sustainable Chemistry & Engineering

Figure 1. (a, b) SEM and STEM images of NFe-NPCS-900-4 and (c−f) C, O, N, and Fe element mappings. foil was used as the anode.46,47 The electrolyte was 6.0 M KOH aqueous solution. Zn−air battery performance was investigated using an IM6e electrochemical workstation (Zahner-Electrik, Germany).

homogeneously distributed in NFe-NPCS-900-4. Elemental analysis results (Figure S1, Supporting Information) also confirm the existence of C, O, N, and Fe elements in NFeNPCS-900-4. The SEM images of the carbon sphere (CS) and Fe-CS are shown in Figure 2a and b. CS is made of carbon microspheres with diameters from 3.5 to 7.5 μm. Fe-CS is made of carbon microspheres with diameters from 2.0 to 6.0 μm. In order to examine the influence of the (NH4)2Fe(SO4)2·6H2O content in the hydrothermal carbonization and annealing temperature (in

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RESULTS AND DISCUSSION Preparation. We developed a two-step method to prepare NFe-NPCS. The preliminary formation mechanism of typical NFe-NPCS (NFe-NPCS-900-4) is proposed in Scheme. 1a. The process involved hydrothermal carbonization of starch with iron ions as catalysts to catalyze the formation of carbon spheres (CSs) and annealing of the carbon spheres in an NH3 atmosphere. Commercial soluble starch (amylase) was chosen as the carbon source, and iron ions were used as catalysts to catalyze the hydrothermal carbonization of starch. After carbonization, CSs containing the Fe element were formed. Then, the CSs were annealed in an NH3 atmosphere at 900 °C for 3 h. In the annealing, carbonization continued, and NH3 acted as the nitrogen source to dope the CSs to form the Nand Fe-doped carbon spheres. Meanwhile, many nanopores (including micropores) were formed because of NH3 etching of the carbon. According to the literatures,22,25,26 the main possible reactions of the etching are as follows: 2NH3 → N2 + 3H 2

(1)

C + 2H 2 → CH4

(2)

Finally, NFe-NPCS from the starch was formed. Scheme. 1b shows the photographs of the starch, CS and NFe-NPCS. In the typical preparation, 8.0 g of starch was used to produce 3.5 g of CS, and finally, 1.5 g of NFe-NPCS was obtained. Characterization. The SEM image (Figure 1a) and STEM image (Figure 1b) clearly show that NFe-NPCS-900-4 is made of carbon microspheres with diameters from 2 to 6 μm. Some microspheres contain obvious pores. Figure 1c−f shows the C, O, N, and Fe element mappings, which indicate the existence of C, N, O, and Fe elements in NFe-NPCS-900-4. These images also demonstrate that C, N, O, and Fe elements are

Figure 2. (a−f) Representative SEM images of CS, Fe-CS, NFeNPCS-850-4, NFe-NPCS-930-4, NFe-NPCS-900-1, and NFe-NPCS900-8. C


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Figure 3. (a, b) High-resolution C 1s and N 1s XPS spectra of NFe-NPCS-900-4. (c) X-ray diffraction of NFe-NPCS-900-4. (d) Raman spectra of NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4.

NH3) on the morphologies and ORR activities, we prepared different NFe-NPCS. When CS was annealed in an NH3 atmosphere (at 850, 900, and 930 °C) for 3 h, the products were denoted as NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4, respectively. In the preparation, when 1.0, 4.0, and 8.0 g of (NH4)2Fe(SO4)2·6H2O was added, the products were denoted as NFe-NPCS-900-1, NFe-NPCS-9004, and NFe-NPCS-900-8, respectively. The SEM images of NFe-NPCS-850-4, NFe-NPCS-930-4, NFe-NPCS-900-1, and NFe-NPCS-900-8 are shown in Figure 2c−f. These products are made of carbon microspheres. For NFe-NPCS-9304, some microspheres contain obvious pores. According to the XPS data of NFe-NPCS-900-4, the atomic percentage of C (87.63 at. %), N (6.08 at. %), O (5.95 at. %), and Fe (0.35 at. %) are contained in the product. The high resolution of the C 1s spectra of NFe-NPCS-900-4 (Figure 3a), indicates a main peak at 284.8 eV and two weak peaks at 285.9 and 287.6 eV. According to the reports, the main peak at 284.8 eV belongs to the sp2-hybridized graphitic carbon. In addition, the peaks at 287.6 and 285.9 eV eV are attributed to CO and C−OH configurations.27−29 From Figure 3b, pyridinic, pyrrolic, and graphitic nitrogen were doped in the product. The peak at 398.3 eV can be ascribed to the pyridinic N species, the peak at 399.7 eV to the pyrrolic N species, and the peak at 401.0 eV to the graphitic N species.5,30 Figure S2 provides high-resolution Fe 2p XPS spectra of NFe-NPCS-900-4. The peaks at 710.3 can be assigned to 2p 3/2 of Fe3+.21 From the XPS data of Fe-CS, the atomic percentage of C (90.82 at. %), N (1.62 at. %), O (7.33 at. %), and Fe (0.23 at. %) are contained in Fe-CS. From the XPS results of NFe-NPCS-900-4 and Fe-CS, NH 3

treatment can dope the nitrogen in the carbon spheres. In addition, element contents (C, O, N, and Fe) of NFe-NPCS850-4, NFe-NPCS-930-4, NFe-NPCS-900-1, NFe-NPCS-9008, CS, and Fe-CS are shown in Table S1 of the Supporting Information. Figure 3c shows the XRD result of NFe-NPCS900-4. Obviously, this result revealed that NFe-NPCS-900-4 has amorphous features of carbon. The Raman spectra of NFeNPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4 are shown in Figure 3d, which indicate the D-band peaked at 1340 cm−1, G-band peaked at 1589 cm−1, and the broader 2D-band peaked at about 2880 cm−1. Figure 4 shows the N2 adsorption−desorption isotherms and pore size distribution of NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4. Table S1 of the Supporting Information provides the data of the surface area, total pore volume, and micropore volume of the products. NFe-NPCS-930-4 exhibits the highest surface areas of 1087.7 m2 g−1 and pore volumes of 0.61 cm3 g−1. NFe-NPCS-900 exhibits high surface areas of 976.6 m2 g−1 and total pore volumes of 0.58 cm3 g−1. From Figure 4d and Table S1, NFe-NPCS-900-4 contains a large number of micropores, and the micropore volume was 0.36 cm3 g−1. For the ORR catalysts, nanoscale porosity is highly desirable. Specifically, micropores are useful for good ORR catalysts. From the BET results of the NFe-NPCS products and FeCS, NH3 etching of the carbon can produce many nanopores (including micropores) and cause high surface areas. Usually, carbon materials with a high surface area were produced by a templating method or (chemical or physical) activation.31−33 However, in this work, the high surface area results from NH3 D


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Figure 4. Nitrogen adsorption−desorption isotherm of (a) NFe-NPCS-850-4, (c) NFe-NPCS-900-4, and (e) NFe-NPCS-930-4. Pore size distribution of (b) NFe-NPCS-850-4, (d) NFe-NPCS-900-4, and (f) NFe-NPCS-930-4.

mV, respectively. Therefore, the onset potential and half-wave potential of NFe-NPCS-900-4 is 23 and 8 mV positive than that of Pt/C, respectively. Figure 5b shows the RDE voltammograms of the NFeNPCS-900-4 electrode with different rotation speeds. According to the data in Figure 5b, we can achieve the Koutecky− Levich plots at different electrode potentials. We can obtain the number of electrons transferred (n) (by calculating the slopes of their best linear fit lines) according to the Koutecky−Levich equation:21,35

etching of the carbon. In our recent work, the N-doped carbon nanosheets using NH3 etching have the highest surface areas of 898 m2 g−1 and total pore volume of 0.52 cm3 g−1.22 However, in this work, NFe-NPCS by the NH3 treatment has the highest surface area of 1087.7 m2 g−1 and pore volume of 0.61 cm3 g−1. Carbon materials containing a very high surface area are significant for many fields such as removal of pollutant, gas uptake, sensors, electrocatalysts, energy storage, and so on.31,34 Electrochemical Activities. In this work, the ORR activities in the alkaline and acidic conditions were investigated. The state-of-the-art Pt/C was also measured. Figure S3a shows the cyclic voltammetry (CV) of NFe-NPCS-900-4. Figure 5 shows the results of the ORR activities in O2-saturated 0.1 M KOH. According to the results in Figure 5a, for Pt/C and NFeNPCS-900-4, the value of onset potential is 932 and 955 mV, respectively. We defined the onset potential as the potentia, at which the current density is −0.2 mA cm−2. The value of halfwave potential of NFe-NPCS-900-4 and Pt/C is 836 and 828

1 1 1 = + j jk B ·ω1/2

(3)

where j is the measured current (mAcm−2), jk is the kineticlimiting current (mAcm−2), and ω is the electrode rotation rate. The theoretical value of the Levich slope (B) was achieved from the following formula:21,35 E


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Figure 5. (a) RDE voltammograms in 0.1 M KOH (sweep rate, 20 mVs −1; rotation speed, 1600 rpm) for Pt/C, Fe-CS, CS, and NFe-NPCS-900-4. (b) RDE voltammograms for NFe-NPCS-900-4 at different rotation speeds. (c) RDE voltammograms in 0.1 M KOH for Pt/C and NFe-NPCS-9004 with or without methanol. (d) Current−time chronoamperometric response of the NFe-NPCS-900-4 and Pt/C electrodes at 760 mV in 0.1 M KOH (rotation rate, 800 rpm). (e) RDE voltammograms in 0.1 M KOH for NFe-NPCS-900-1, NFe-NPCS-900-4, and NFe-NPCS-900-8. (f) RDE voltammograms in 0.1 M KOH for NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4.

B = 0.62·n·F ·CO2·DO2/3 ·ν−1/6 2

900-4 (Figure 5c). It was obvious that NFe-NPCS-900-4 exhibits little loss in activity, which implied excellent methanol tolerance. Durability of the electrocatalyst should be regarded as one of the most important aspects in the research and application of fuel cells.40−42 Current−time chronoamperometric durability tests were performed. Figure 5d shows the chronoamperometric response of the NFe-NPCS-900-4 and Pt/C electrodes at 0.80 V. From Figure 5d, after 15,000 s, Pt/C had a 21.2% decrease in current density. Fortunately, NFeNPCS-900-4 showed only an 18.0% decrease, which reveals that the durability of NFe-NPCS-900-4 is superior to that of Pt/C. The effect of the (NH4)2Fe(SO4)2·6H2O content on the hydrothermal carbonization and annealing temperature (in NH3) of the ORR activities in 0.1 M KOH were investigated. Clearly, the (NH4)2Fe(SO4)2·6H2O content affects the onset potential, half-wave potential, and reduction current (Figure

(4)

where n is the total number of transferred electrons, F stands for the Faradaic constant (96485 C/mol), CO2 is the oxygen concentration in 0.1 M KOH (1.2 × 10−6 mol cm−3), DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.90 × 10−5 cm2 s−1), and v stands for the kinematic viscosity of the 0.1 M KOH (0.01 cm2 s−1).22,36,37 From the obtained K-L plot, the data (shown in Figure S3b) exhibit good linearity, and the n is 3.78−3.96 at the potential from 400 to 800 mV.38 This implied a four-electron pathway for ORR on the typical NFe-NPCS electrode. It is well known that reduction of O2 by the four-electron pathway to achieve the highest energy capacity is highly desirable.4,12,39s In the practical application, ORR catalysts should have good tolerance to fuel molecules.15,20,23 We measured the ability to endure methanol (in 0.1 M KOH) for Pt/C and NFe-NPCSF


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Figure 6. (a) RDE voltammograms in 0.5 M H2SO4 (sweep rate, 20 mVs −1; rotation speed, 1600 rpm) for the Fe-CS, Pt/C, and NFe-NPCS-900-4. (b) RDE voltammograms for NFe-NPCS-900-4 at different rotation speeds. (c) RDE voltammograms for NFe-NPCS-900-4 with or without methanol. (d) Current−time chronoamperometric response of the NFe-NPCS-900-4 and Pt/C electrodes at 730 mV (rotation rate, 800 rpm). (e) RDE voltammograms in 0.5 M H2SO4 for NFe-NPCS-900-1, NFe-NPCS-900-4, and NFe-NPCS-900-8. (f) RDE voltammograms in 0.5 M H2SO4 for NFe-NPCS-850-4, NFe-NPCS-900-4, and NFe-NPCS-930-4.

900-4 and Pt/C, the onset potential was 840 and 930 mV, respectively. As far as the onset potential is concerned, NFeNPCS-900-4 is only 90 mV more negative than Pt/C. RDE voltammograms on the NFe-NPCS electrode at different rotation speeds (400−2000 rpm) are shown in Figure 6b. We measured the tolerance to methanol for NFe-NPCS-900-4. The results are shown in Figure 6c. Not only the onset potential but also the half-wave potential had no obvious change when exposed in 1.5 M methanol. From Figure 6c, NFe-NPCS-900-4 has excellent tolerance to methanol. We also assessed the durability of NFe-NPCS-900-4 and Pt/C. Figure 6d shows the

5e). The material exhibited the best activity when the (NH4)2Fe(SO4)2·6H2O content was 4.0 g in the typical preparation. From Figure 5f, the optimal annealing temperature in NH3 is found to be 900 °C. The ORR electrocatalyst in the acidic condition is essential in electrochemical devices (e.g., proton exchange membrane fuel cell), which only can work in an acidic condition.43 The designing and preparation of ORR catalysts with high performance and stability in acidic conditions remains a great challenge.44,45 Therefore, we also investigated the activities in O2-saturated 0.5 M H2SO4. From Figure 6a, for NFe-NPCSG


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Figure 7. Zn−air battery performance of NFe-NPCS-900-4 and Pt/C. (a) Curve of open circuit voltage for NFe-NPCS-900-4. (b) Typical galvanostatic discharge curves of NFe-NPCS-900-4 and Pt/C (current density, 1 mAcm−2). (c) Galvanostatic discharge curves of NFe-NPCS-900-4 and Pt/C (current density, 50 mAcm−2) until complete consumption of Zn anode.

NFe-NPCS-900-4 in a practical Zn−air battery. Therefore, we also characterized the performance of the Zn−air battery, in which NFe-NPCS-900-4 was used as the cathode. We selected Zn foil as the anode. Figure S5a and b shows the photographs of the electrochemical cell. For comparison, the Pt/C (20%) catalyst is also tested. Figure 7a shows the curve of open circuit voltage for NFeNPCS-900-4, and the open circuit voltage is 1.54 V. From Figure 7b and c, NFe-NPCS-900-4 shows very high voltages of 1.37 V (current density, 1 mAcm−2) and 1.01 V (current density, 50 mAcm−2). Normalized to the mass of the consumed Zn during the galvanostatic discharge process (50 mAcm−2), the specific capacity of NFe-NPCS-900-4 was 590 mAhg−1. For the Pt/C catalyst, the voltages of 1.34 and 0.95 V were obtained when the current densities were 1 and 50 mAcm−2, respectively. During the galvanostatic discharge process (50 mAcm−2), the specific capacity of the Pt/C catalyst was 592 mAhg−1. Compared with Pt/C, NFe-NPCS-900-4 is a competitive alternative to commercial Pt/C in the Zn−air battery.

chronoamperometric response of the NFe-NPCS-900-4 and Pt/C electrodes at 730 mV (versus RHE). After 20,000 s, the typical product maintains the 87.5% current density, while Pt/C maintains the 72.2% current density, which reveals that the durability of NFe-NPCS-900-4 exceeds that of Pt/C in acidic media. The effect of the (NH4)2Fe(SO4)2·6H2O content, temperature, and time (annealing in NH3) on the ORR activities in 0.5 M H2SO4 was measured. From Figure 6e, the catalyst shows the best performance with the (NH4)2Fe(SO4)2·6H2O content of 4.0 g in the typical preparation. The optimal temperature in NH3 is 900 °C (Figure 6f), and the optimal time in NH3 is 3.0 h (Figure S4, Supporting Information) . From the above discussion, NFe-NPCS-900-4 exhibits excellent ORR activity in alkaline conditions and high ORR activity in acidic conditions. Probably, there are three main reasons for the excellent activities. First, the nitrogen element (including pyridinic-, pyrrolic-, and graphitic-nitrogen) can be doped in the carbon spheres. The pyridinic- and graphiticnitrogen are the main components, which contributeto the high ORR activity.5,15 Second, the race Fe elements are doped in the carbon spheres, and they probably coordinate with the doped nitrogen. The co-doping of the N and Fe elements promotes the ORR activity.15,41 Finally, typical NFe-NPCS not only has a high surface area (976.6 m2 g−1), but also has a large total pore volume (0.58 cm3 g−1) and micropore volume (0.36 cm3 g−1). These can be helpful to achieve more active sites for ORR and effectively accelerate the reactant, ion, and electron transport.7,15,22 A Zn−air battery is an ideal energy device with high energy density and high safety.46−48 Although NFe-NPCS-900-4 exhibits excellent ORR activity in alkaline media, the RDE measurement is insufficient in predicting the performance of

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CONCLUSIONS We have proposed a bulk production method for nonprecious metal ORR catalysts from cheap starch as the precursor. The preparation includes hydrothermal carbonization of starch with iron ions as catalysts to catalyze the formation of carbon spheres and annealing the carbon spheres in an NH 3 atmosphere. Iron ions are used as catalysts to catalyze the formation of CSs, and a small number of ferrum ions remain in the CSs. After annealing in the NH3 atmosphere, NFe-NPCS is doped by nitrogen and ferrum elements. NH3 is used not only as the nitrogen source to form the N- and Fe-doped carbon sphere but also to etch the carbon to produce many nanopores (including micropores). The proposed approach is a readily H


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scalable. Starch is a highly available, accessible, and recyclable biomass, and therefore, it is suitable for bulk preparation of cheap catalysts. The typical product exhibits excellent activity in alkaline solution and high activity in acidic solution. The typical product also exhibits superior tolerance to the methanol molecule and stability to Pt/C. Compared with Pt/C, NFeNPCS-900-4 is a competitive alternative to Pt/C in a Zn−air battery. This work proposes a new design strategy for the bulk production of excellent nonprecious metal catalysts from cheap biomass.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00269. Elemental analysis image, high-resolution Fe 2p XPS spectra, CV and K-L plots of NFe-NPCS-900-4; RDE voltammograms in 0.5 M H2SO4 for NFe-NPCS products with different annealing times; element percentage, BET surface area, total pore volume, and micropore volume of NFe-NPCS-850-4, NFe-NPCS900-4, NFe-NPCS-930-4, NFe-NPCS-900-1, NFeNPCS-900-8, CS, and Fe-CS; photographs of the electrochemical cell. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P. Chen). *E-mail: [email protected] (J.-S. Xie). Author Contributions

Wei-Tang Yao, Liang Yu, and Peng-Fei Yao contributed equally to this paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the funding support from the National Natural Science Foundation of China (21271005), Foundation of Anhui University (02303203-0054), College Students’ Innovation and Training Foundation (J18515258, J18515020), Foundation of Anhui Province (1308085QB35), and Foundation of Education Department of Anhui Province (KJ2015A232).

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REFERENCES

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Bulk Production of Nonprecious Metal Catalysts from Cheap Starch as

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