Facile one-pot synthesis of CoFe alloy nanoparticles decorated N

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Facile one-pot synthesis of CoFe alloy nanoparticles decorated Ndoped carbon for high-performance rechargeable zinc–air battery stacks Tao An, Xiaoming Ge, Nguk Neng Tham, Afriyanti Sumboja, Zhaolin Liu, and Yun Zong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00657 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Sustainable Chemistry & Engineering

Facile one-pot synthesis of CoFe alloy nanoparticles decorated N-doped carbon for high-performance rechargeable zinc–air battery stacks Tao An†,‡, Xiaoming Ge†,‡, Nguk Neng Tham†, Afriyanti Sumboja†, Zhaolin Liu†,*, and Yun Zong†,* †

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science,

Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore. *(Z.L.) Email: [email protected], *(Y.Z.) E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: We report facile one-pot synthesis of CoFe alloy nanoparticles decorated N-doped carbon (CoFe/N-C) from commercial Vulcan carbon black and transition metal salts. With ~ 8.0 wt.% of metal in the composite catalyst, efficient catalysis towards oxygen reduction reaction (ORR) can be achieved with a pseudo four-electron transfer per oxygen molecule, outperforming the benchmark catalyst, Pt/C, by a more positive onset potential, smaller half-wave potential and Tafel slope. A rechargeable battery stack built from five zinc-air cells in series gives a power output of up to 28.5 W. Benefitting from its low internal resistance the battery stack can deliver a high current of 8.4 A and a stable discharge voltage above 5.4 V over 200 cycles, showing great potential as basic unit for grid-scale energy storage. KEYWORDS: electrocatalysis, oxygen reduction reaction, oxygen evolution reaction, carbon black, nitrogen-doping, zinc-air battery stack


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INTRODUCTION Maximizing the renewable energy utilization is a key strategy in sustainable development of human society.1 Due to their intermittent nature,2-4 the renewable energies are generally more accessible if coupled to grid energy storage.5-8 A limiting factor herein is the battery technology, with safety, cost, and durability as key considerations.9-10 Zn-air battery, thanks to its merits of high energy density, low cost, environmental benignity and pronounced good safety, is emerging as a promising option of unit cells for the grid.11-16 The performance of a rechargeable Zn-air cell is dependent on multiple factors, among which one is the kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring at its air cathode as the cell is discharged or charged, respectively.11 These reactions with multiple electron transfer processes are sluggish by nature, requiring efficient electrocatalysts on air cathode to expedite the reaction kinetics.17-18 Pt-based catalysts are known for their excellent ORR electrocatalytic activity.19 The skeptics on their scarcity and high cost have triggered intensive research on non-noble metal or even metal-free electrocatalysts.20-22 Among the development a promising family of high-performance yet low-cost electrocatalysts are the nanostructured porous carbonaceous materials23-25 and their composites with transition metal oxides.26-30 For instance, nitrogen-doped graphene loaded with iron (Fe) was found to boost the ORR activity,28-29 and cobalt (Co) embedded N-doped carbon showed high capability of smoothening the OER.30 More recently, CoFe nanoalloys have been loaded onto N-doped carbon nanotube (N-CNT) via pyrolysis of selected precursors followed by acidic treatments.31-32 Despite abundant reports on the design and synthesis of various efficient bifunctional catalysts, the reported desirable features of these catalysts often cannot be retained in mass production required for large-scale deployment, e.g. in grid energy storage. Graphene and CNT-based catalysts are not cheap,33 while the activity of commercial carbon black-based


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catalysts are not satisfactory.34 Therefore, Zn-air battery stacks are only demonstrated35-36 on catalytic air cathode of common metal oxides, with some carbon-based catalysts being evaluated in half-cells37-38 or small Zn-air cells.39-41 Herein, we report one-step synthesis of CoFe alloy nanoparticles loaded N-doped carbon from commercial Vulcan carbon black and air stable organic cobalt and iron salts. The acetylacetonate salts of Iron (III) and cobalt (III) were homogeneously mixed into carbon black, and pyrolyzed under inert atmosphere to produce the catalyst. The beauty of these salts is that they decompose easily to yield desirable metal or metal oxide particles while leaving no disturbing ion residues in the final product. The as-prepared CoFe/N-C composite catalyst was fully evaluated and used at air-cathode to fabricate large-area Zn-air cells. A Zn-air battery stack built from five of such cells is proven to be compact, robust and highly efficient, showing enormous potential for grid scale energy storage applications.

EXPERIMENTAL METHODS Synthesis of electrocatalysts. The CoFe/N-C catalyst was synthesized via a facile one-pot direct pyrolysis, with all reagents placed in a single reactor and no separation or purification process is needed. In a typical synthesis process, 1 g of Vulcan XC-72 carbon black (VC, Cabot Corporation) was mixed thoroughly with 2 g of urea (99%, Sigma-Aldrich), 0.85 mmol of iron (III) acetylacetonate (99.9%, Sigma-Aldrich) and 0.85 mmol of cobalt (III) acetylacetonate (99.99%, Sigma-Aldrich) in an agate mortar. Aliquots of ethanol was added to facilitate the grinding, and the fine ground samples were left to dry naturally. The dried mixture was then transferred into an alumina tray and calcined in a tube furnace at 900 °C for 1 h under nitrogen (N2) atmosphere with a constant flow of N2 gas at 50 ml⋅min-1, producing about 1.2 g of the


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catalyst (nearly the theoretical yield). As control, catalysts with the similar loading of each single metal nanoparticles on nitrogen-doped carbon (N-C), i.e. Co/N-C and Fe/N-C, were synthesized from the mixtures of 1 g of Vulcan carbon, 2 g of urea and 1.7 mmol of respective Co or Fe acetylacetonate salt. All the reaction and treatment conditions were kept the same. Materials characterization. Thermogravimetric analysis was carried out using a TGA Q500 analyzer (TA Instruments). X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance Diffractometer with Cu Kα X-rays generated at 40 kV and 40 mA. The morphologies were studied on a field emission scanning electron microscope (FE SEM, JEOL-7600F) and a field emission transmission electron microscope (FE TEM, JEOL 2100F). X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB 200i-XL XPS system with Al Kα radiation, and the collected data were corrected with the binding energy of C 1s line set to 285.0 eV. Electrochemical studies. The electrocatalytic activities of CoFe/N-C toward ORR and OER were assessed with a three-electrode half-cell setup comprising a rotating disk electrode (RDE) as working electrode, Pt foil as counter electrode and Ag/AgCl in 3 M KCl solution as reference electrode, which was subsequently calibrated and converted to the reversible hydrogen electrode (RHE) as ERHE (V) = EAg/AgCl + 0.059 pH + 0.197.42,43 For each catalyst, the ink was made by mixing 5 mg of the catalyst with 25 µl of Nafion solution (5 wt.%, Aldrich, USA) in 923 µl of ethanol followed by 1h of sonication. The working electrode was then prepared by applying 10 µl of the respective catalyst ink onto the glassy carbon disk (5 mm in diameter), which was dried naturally in air to achieve a loading of 0.25 mg⋅cm-2. Similarly, a rotating ring disk electrode (RRDE) with similar mass loading of catalyst was prepared by applying 11 µl of the respective catalyst ink onto glassy carbon disks (5.25 mm in diameter) and dried naturally, and an E7R9 AFE7R9GCPT tip (Pine Instruments) was used. The Pt ring potential of the RRDE was set for


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0.5 V, with the collection efficiency (N) calibrated to be 38.8 ± 0.2%.43 The electrodes were immersed into a 0.1 M KOH aqueous solution saturated with O2, and the I-V data were collected from an Autolab PGSTAT302N at a scan rate of 5 mV⋅s-1 under a constant flow of oxygen gas. The benchmark catalyst, Pt/C (20 wt.% Pt supported on Vulcan XC72, Sigma-Aldrich), and the control catalysts, Co/N-C and Fe/N-C, were studied under the same experimental conditions for comparison. Battery stack assembly and testing. Battery cycling tests were conducted with small homebuilt Zn-air single cells26 to compare the electrocatalytic activities and durability of CoFe/N-C and benchmark Pt/C. The CoFe/N-C catalyst was further used to prepare the air cathodes for the large-area Zn-air single cells and the battery stack, which have a planar design with the details described elsewhere.44 The battery tests were carried out at 25 °C on a Maccor 4200 battery tester, with the battery discharged and then charged at constant current rates for each testing cycle.

RESULTS AND DISCUSSION The XRD pattern of the CoFe/N-C catalyst is shown in Figure 1a. Major diffraction peaks are observed at 44.8°, 65.2°, and 82.6°, which can be assigned to the (101), (200), and (211) planes of cubic CoFe,45 respectively, with lattice parameter a calculated to be 2.858 Å. The broad peak at 25° is attributed to the amorphous Vulcan carbon.46 The XRD peaks suggest successful formation of metallic Co and Fe in the form of CoFe alloy from the pyrolysis synthesis. TGA data gave the Fe2O3 and Co3O4 residuals as ~11.0 wt.% of the original mass (Figure 1b), from which the metallic loading in CoFe/N-C was calculated to be ~8.0 wt.%, slightly lower than the initial ratio of 8.9 wt.% in the starting materials (0.85 mmol each of Co and Fe in 1 g Vulcan


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Figure 1. (a) The XRD pattern of CoFe/N-C catalyst, showing the co-existence of cubic CoFe alloy and amorphous Vulcan carbon. (b) TGA of CoFe/N-C catalyst in air. The burning-off of carbonaceous content was accompanied by the metal oxidization at temperature between 400 and 600 °C, and the residue of 11 wt.% at 900 °C is the oxides of cobalt and iron. (c,d) XPS spectra of the CoFe/N-C catalyst. (c) Survey scans. Peaks of C 1s, N 1s, Co 2p3/2, Fe 2p3/2, and residual O 1s are clearly identifiable. (d) Highresolution N 1s spectra fitted with pyridinic, pyrrolic, graphitic nitrogen and pyridine-N-oxide.

carbon). XPS survey scan was performed to inspect the elemental composition of CoFe/N-C catalyst. Figure 1c shows the N 1s peak which locates at binding energy of 399.0 eV, with its content in the sample determined to be 3.0 at.%. Despite a decent loading (8.0 wt.%) of Co and Fe in the catalyst, only weak peaks were found at binding energies of 781.1 and 710.9 eV for Co 2p3/2 and Fe 2p3/2, respectively.47 This likely suggests that the metals are mostly buried under N-


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doped carbons, as the XPS is a highly surface sensitive technique with a typical probing depth of less than 10 nm from the sample surface.42 Fitted high-resolution spectra of N 1s (Figure 1d) revealed the co-existence of pyridinic (398.7 eV), pyrrolic (399.6 eV), graphitic (401.5 eV) N 48 and pyridine-N-oxide (404.9 eV) 49,50 in CoFe/N-C, confirming successful introduction of active nitrogen by doping into the Vulcan carbon matrix. Oxygen signal in the XPS survey scan may have also received contributions from other oxygen-containing groups (e.g. CO) produced by the decomposition of acetylacetonate salts and adsorbed onto the Vulcan carbon surfaces.

Figure 2. (a) A typical bright field TEM image of the CoFe/N-C catalyst and its elemental mapping of (b) Co and (c) Fe, and (d) high resolution TEM image of a metallic particle on the N-doped Vulcan carbon.

Figure 2a shows a bright field TEM image with the metallic CoFe particles loaded on N-doped Vulcan carbon. While thicker areas of carbon gave a darker color in the image, the metallic particles appear the darkest due to their notably higher atomic numbers with greater diffracting power to electron beams.51 Apart from small metallic particles of a few nanometers, some larger ones beyond 10 nm are also visible. This can be a drawback of this simple synthetic approach, if compared to the uniform particles on N-doped carbons obtained via hydrothermal synthesis.26


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Elemental mappings of the inspected area suggest the homogenous alloy nature of CoFe metallic particles (Figure 2b and 2c), with an interplanar spacing of ~2 Å given by high resolution TEM image (Figure 2d) which corresponds to the (101) planes of cubic CoFe phase. It is not surprising to witness the formation of metal particles rather than their oxides here, because a high reducing power may be achievable from a collective effect of urea, some decomposition intermediates of acetylacetonate salts, and Vulcan nanocarbons at elevated temperature. The preparation of metal nanoparticles from thermal decomposition of acetylacetonate salts has also been reported recently.52,53 Hence, the obtained catalyst, CoFe/N-C, should be a composite of nanosized CoFe particles loaded on N-doped carbons in which the metal content is about 8.0 wt.%. The ORR and OER activities of CoFe/N-C were assessed using RDE technique, with Co/N-C, Fe/N-C and Pt/C being studied under the same experimental conditions for comparison (Table 1). Impressively, CoFe/N-C even displays an onset potential of 20 mV more positive than that of Pt/C (Figure 3a), with its half-wave potential being more positive (0.821 vs. 0.817 V) and thus better, too. The onset potentials of Fe/N-C (0.95 V) and Co/N-C (0.91 V) are inferior to those of CoFe/N-C and Pt/C (Figure 3a, inset), and their half-wave potentials are also less positive. The notably improved ORR activity of CoFe/N-C as compared to Co/N-C and Fe/N-C suggests a synergetic coupling effect between Co and Fe in their alloy metallic nanoparticles supported on N-doped Vulcan carbon. The Tafel slope of CoFe/N-C catalyst is about 86 mV⋅dec-1 (Figure 3b), which is smaller than that of the Pt/C (93 mV⋅dec-1). This indicates that CoFe/N-C catalyst can deliver a higher current density at the same overpotential and hence has greater electrocatalytic activities.


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Table 1. Electrocatalytic data of onset potential (Eonset), Tafel slope, half-wave potential (E1/2, for ORR) and overpotential at current density of 10 mA⋅cm-2 (E@10 mA⋅cm-2, for OER). ORR Materials

OER E @ 10

Eonset (V

E1/2 (V

Tafel slope

Eonset (V

vs. RHE)

vs. RHE)

(mV⋅dec-1)

vs. RHE)

mA⋅cm-2 (V vs. RHE)

Tafel slope (mV⋅dec-1)

CoFe/N-C

1.03

0.821

86

1.525

1.665

249

Pt/C

1.01

0.817

93

1.596

1.823

934

Fe/N-C

0.95

0.764

80

1.628

1.798

309

Co/N-C

0.91

0.776

54

1.555

1.684

321

In term of the OER activity, CoFe/N-C beats Co/N-C (Figure 3c) in both onset potential and overpotential (at 10 mA⋅cm-2) by 30 and 19 mV, respectively, and both catalysts were notably better than Fe/N-C and Pt/C. The superior OER activity of CoFe/N-C is also shown in its smaller Tafel slope of 249 mV⋅dec-1, as compared to Fe/N-C, Co/N-C and Pt/C with the Tafel slopes of 309, 321 and 934 mV⋅dec-1, respectively (Figure 3d). The superb ORR and OER activities make CoFe/N-C an excellent choice of bifunctional catalysts for rechargeable Zn-air batteries. To investigate the mechanism of ORR catalysis by CoFe/N-C, RRDE experiment was performed to determine the electron transfer number, n, according to the equation below: n = 4 ID / (ID + IR / N)

(1)

where ID and IR are the recorded disk and ring current, respectively. The yields of HO2− are found below 10% over the potential range of 0 to 0.9 V (Figure 3e), giving an n value in the range of 3.41 to 3.92 (Figure 3f). Obviously, the ORR catalysis by CoFe/N-C follows a pseudo 4-electron transfer pathway which mitigates the production of detrimental HO2− ions at the airelectrode and thus would favor long-term cycling stability of the Zn-air batteries.


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Figure 3. (a) ORR and (c) OER voltammetric curves of CoFe/N-C, Pt/C, Fe/N-C and Co/N-C. The insets show how the onset potentials were derived for (a) ORR (where current density increases sharply)43 and (c) OER (points with an OER current of 0.5 mA⋅cm-2)54,55. Tafel plots from (b) ORR and (d) OER of CoFe/N-C, Pt/C, Fe/N-C and Co/N-C. All ORR and OER voltammetric curves were obtained at a rotation rate of 1600 rpm and the Tafel plots are fitted in linear-least-squares. (e) The RRDE voltammogram of CoFe/N-C obtained at rotation speed of 1600 rpm and (f) the electron transfer number, n, as calculated from the measured disk and ring currents.


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Figure 4. (a) ~ 70 g of CoFe/N-C catalyst obtained from a few batches. (b) A blank piece of 8 cm × 8 cm SGL 10BC carbon paper (left) and its counterpart loaded with CoFe/N-C catalyst (right) under optical microscope at 2.5× magnification. (c) A representative SEM image of the air cathode surface with CoFe/N-C loading density of 1 mg⋅cm-2.

The air cathode was prepared by casting a slurry comprising 90 wt.% of CoFe/N-C (Figure 4a) and 10 wt.% of Nafion binder onto carbon paper (SGL 10BC, Germany) to achieve a loading density of 1 mg⋅cm-2 (Figure 4b). The topology of the air-electrode surface was revealed by its SEM image (Figure 4c), in which the CoFe/N-C catalyst on carbon paper formed a large number of spheres in the size of ~100 nm. Similar air cathode was made with commercial Pt/C catalyst applied at the same loading of 1 mg·cm-2. Both CoFe/N-C and Pt/C air cathodes were used in a home-built prototype frame to couple with metallic Zn anode and use an electrolyte of 6 M KOH with added 0.4 mol⋅L-1 zinc acetate to form their respective Zn-air cells.26 Battery cycling tests were performed with constant current discharge at 26 mA (6.5 mA·cm-2) for 5 min followed by constant current charge at 13 mA (3.25 mA·cm-2) for 10 min in each cycle, and the performance data are presented in Figure 5. The initial discharge voltage was slightly lower for the CoFe/N-C cell (first cycle: 1.18 V vs. 1.26 V for the Pt/C cell), but caught up quickly and outperformed the latter from the 4th cycle (1.13 V vs. 1.12 V) onwards (Figure 5a). In terms of the charge voltage


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CoFe/N-C cell was better from the first cycle (2.12 V vs. 2.16 V for Pt/C cell) due to its higher OER catalytic activity as established by RDE. As the cycling test went on, the charge voltage of the CoFe/N-C cell even dropped to 2.06 V (possibly some activation process had taken place) while that of Pt/C rises to about 2.53 V. This has led to much narrower charge-discharge voltage gap for CoFe/N-C cell, suggesting its higher energy efficiency. In addition, the Pt/C cell ceased its operation at 76th cycle in prolonged battery test, while the CoFe/N-C cell survived over 500 cycles with a discharge voltage of 1.0 V and a charge voltage of 2.13 V (Figure 5b). The failure of Pt/C at a much earlier stage could be due to its low OER activity, requiring a notably higher voltage in the charging process when the oxidation of carbon in alkaline electrolyte was more readily to take place. This shows the importance of OER activity to a durable rechargeable Zn-air battery, confirming CoFe/N-C as a notably better choice of the air-cathode catalysts compared to the benchmark Pt/C due to its superior efficiency and durability.

Figure 5. Battery cycling performance data of home-built Zn-air single cells with CoFe/N-C and commercial Pt/C catalyst for (a) initial 15 cycles and (b) 500 cycles (for the CoFe/N-C cell). It should be noted that battery failure for Pt/C cell occurred at 76th cycle.


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To further validate the high performance of CoFe/N-C catalyst, a large-area single cell was assembled using the 8 cm × 8 cm air cathode (Figure 4b) following the structure shown in Figure 6a, with metallic Zn as anode and a plenum frame used to construct the electrolyte reservoir (6 M aqueous KOH containing 0.4 mol⋅L-1 zinc acetate). A peristaltic pump, whenever necessary, is used to drive the flow of electrolyte through the inlet/outlet of the plenum frame, which promotes the diffusion of zinc ions to suppress dendritic growth at the anode.56 Linear sweep voltammetry polarization curves were recorded at a scan rate of 0.1 V⋅s-1. The maximum power output was found to be 6.3 W (power density: 128.6 mW⋅cm-2), achieved at the point when the cell delivered a current of 10 A (current density: 204.1 mA⋅cm-2) with a discharge voltage of 0.63 V (Figure 6b). Five of such Zn-air single cells were connected in series to form a battery stack (structure shown in Figure 6c), with a stainless steel bipolar plate backing the air cathode in each single cell to enhance the electrical conduction to their adjacent cells (or external load) and facilitate the air supply or release through the pre-engineered grooved channels. The battery stack was assembled using a screw-fastening compression mechanism for compactness and the ease of maintenance, with each screw tightened to 0.6 N⋅m by a torque spanner. LSV polarization curve was recorded at the same scan rate of 0.1 V⋅s-1. It shows that the battery stack was able to deliver a power up to 28.5 W (or 5.7 W per cell in average, power density: 116.3 mW cm-2), achieved at a current of 8.4 A (current density: 171.4 mA⋅cm-2) with a discharge voltage of 3.4 V (Figure 6d). The slightly compromised power density as compared to single cells was due to the increased contact resistance from cell interconnections. This can be seen clearly from the impedance data recorded at 5 V polarization (Figure S1), with the ohmic and polarization impedance as 285 and 40 mΩ, respectively. Excluding the impedance of external circuits, the average ohmic and polarization impedance per cell were merely 57 and 8 mΩ, respectively. When the five-cell battery stack was


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Figure 6. (a) Illustrations of an exploded view of the Zn-air single cell. The Zn anode (and bipolar plate) that is adjacent to the end plate has a protruded flag for the ease of electrical connection to external circuit. (b) The voltage and output power of a single cell at discharge. (c) An assembled battery stack consists of five individual cells. (d) The voltage and output power of the five-cell battery stack at discharge, and (e) the data of first 15 cycles at constant-current charge-discharge.


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cycled at constant currents with 5 min of discharge at 0.32 A and 10 min of charge at 0.16 A in each cycle, the charge voltage increased by 0.06 V (9.79 to 9.85 V) over the first 15 cycles which was merely about 0.04% per cycle (Figure 6e). The drop in the discharge voltage was 0.54 V (6.24 to 5.70 V) over the same duration, giving a degradation rate of 0.58% per cycle. The broadening of the charge-discharge voltage gap over the initial 15 cycles leads to a round trip efficiency drop of 5.8% (63.7 to 57.9%) for the 5-cell battery pack. Interestingly, such drop seems to be a stabilization process, as the discharge voltage remains essentially unchanged (~ 5.5 V) after 30 cycles (Figure S2). The good cycling stability with a reasonable round-trip efficiency makes the system an attractive choice for the further scale-up tests which can be eventually used in grid-scale energy storage applications. It is noteworthy that no large dendrites were observed on zinc anode after the cycling of the batteries (Figure S3). The suppression of dendrite growth56 in this case is likely due to the enhanced Zn2+ diffusion by the flow electrolyte and a lower charging current of 0.16A. Some corrosion of zinc anode was seen by a darker (rougher) surface, which could be addressed by anode engineering, including alloying zinc with other metals,57,58 incorporating additives into the anode,59,60 or applying protective coatings on zinc.61,62

CONCLUSIONS A highly efficient bifunctional catalyst, CoFe/N-C, has been synthesized via a one-step direct pyrolysis using commercial Vulcan carbon black as the base material. This metallic CoFe alloy nanoparticles decorated nitrogen-doped carbon catalyst delivers an ORR activity superior to Pt/C in terms of onset potentials, half-wave potentials and Tafel slopes, and a decent OER activity. The superior electrocatalytic performance of CoFe/N-C as compared to its counterparts loaded with nanoparticles of a single metal (Co or Fe), i.e. Co/N-C or Fe/N-C, suggests a synergetic


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coupling between the two metals as the origin of further catalytic performance enhancement in CoFe/N-C. This has enabled a high performance CoFe/N-C Zn-air single cell that outperformed its counterpart with benchmark Pt/C catalyst in the air-cathode in terms of energy efficiency and durability. The subsequently assembled 5-cell battery stack with a power output up to 28.5 W has shown reasonably good cycling stability, and was able to provide a stable voltage output above 5.5 V beyond 180 cycles. The use of flow electrolyte in the battery pack promoted Zn2+ diffusion and thus suppressed the formation of dendrites at Zn anode. This high-performance CoFe/N-C catalyst, prepared from facile and scalable synthesis at low cost, may play an important role in the large-scale manufacturing of rechargeable metal-air batteries for the advancement of grid-scale energy storage technologies to enable more efficient renewable energy utilization.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.XXXXXXX: impedance data and cycling performances. (PDF)

AUTHOR INFORMATION Corresponding Author *

[email protected] (Z. Liu)

*

[email protected] (Y. Zong)

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the last version of the manuscript. ‡These authors contributed equally. Notes The authors have no conflict of interest to declare.

ACKNOWLEDGMENT This work was supported by the projects IMRE/12-2P0503 and IMRE/12-2P0504 under the SERC Advanced Energy Storage Research Programme, and Institute of Material Research and Engineering (IMRE), A*STAR, Singapore. We thank Seng Hwee Leng Debbie (IMRE) for XPS and Dr Yanan Fang (Nanyang Technological University, Singapore) for TEM.

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SYNOPSIS This paper reports a facile one-pot synthesis of a highly efficient electrocatalyst at low cost for rechargeable Zn-air battery, a promising energy storage device for renewable electricity. TOC Graphic: For Table of Contents Use Only


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Facile one-pot synthesis of CoFe alloy nanoparticles decorated N

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