Low-Cost Rapid Template-Free Synthesis of Nanoscale Zinc Spinels

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Low-cost Rapid Template-free Synthesis of Nanoscale Zinc Spinels for Energy Storage and Electrocatalytic Applications Aravind Baby, Baskar Senthilkumar, and Prabeer Barpanda ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00054 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Applied Energy Materials

Low-cost Rapid Template-free Synthesis of Nanoscale Zinc Spinels for Energy Storage and Electrocatalytic Applications

Aravind Baby, Baskar Senthilkumar* and Prabeer Barpanda*

Faraday Materials Laboratory, Materials Research Centre, Indian Institute of Science, C. V. Raman Avenue, Bangalore, 560012, India.

*Corresponding Authors: [email protected] (B.Senthilkumar), [email protected] (P. Barpanda) Phone: +91-80 2293 2783; Fax: +91-80 2360 7316

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ABSTRACT

Spinels form an interesting class of compounds, finding applications in metal-ion batteries and as catalysts for metal-air batteries and fuel cells. Here, we report a fast, template-free solution combustion method to synthesize nanoscale zinc spinels for applications as cathodes in lowcost aqueous zinc-ion batteries and oxygen reduction reaction (ORR) catalysts. It leads to the formation of phase pure spinels with near spherical nanoscale morphology. Three spinelsZnCo2O4 (ZCO), ZnMn2O4 (ZMO) and ZnMnCoO4 (ZMCO) - were investigated. ZMO and, for the first time, ZMCO were observed to show reversible Zn (de)insertion involving Mn4+/Mn3+ redox couple with first discharge capacity of 109.4 mAh/g (i.e. >99% of theoretical capacity). Further, these Zn-based spinels showed appreciable oxygen reduction reaction (ORR) electrocatalytic activity. The ORR activity in alkaline solution was characterized by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using rotating disk electrode (RDE) and was observed to be comparable to Pt/C with similar chronoamperometric stability. This enhanced ORR activity can be rooted to the optimal tuning of Co3+-O bond strength due to the presence of Mn3+. This work presents a robust synthesis route to prepare ZnMnCoO4 spinel acting as an economic cathode material for large scale Zn-ion batteries for grid storage applications as well as an efficient and stable alternate ORR catalyst in alkaline solution.

Keywords: spinel, combustion synthesis, aqueous Zn-ion battery, oxygen reduction reaction, electrocatalyst.


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INTRODUCTION With the unprecedented growth in the global energy demands, there is an increased need for the development and sustainable utilization of renewable energy sources.1-3 Reliability and availability issues4 with them can be overcome by developing highly efficient storage systems like rechargeable batteries.5 Although Li-based batteries are the undisputed leaders in portable energy storage sector, it is difficult to replicate their success in large scale stationary energy storage like power grids. Economic viability and safety outweigh energy density as the deciding element in stationary applications where storage space is never the limiting factor.6 Scarcity of raw materials,6 rigorous and expensive manufacturing constraints7 and use of hazardous and flammable non-aqueous electrolytes8,9 limit the possibility of scaling up lithiumbased energy storage systems. It is under this purview that Zn-based battery systems are fast gaining popularity. From abundant and cheap raw materials to environmentally benign and safe electrolytes, zinc-based systems have numerous advantages over their lithium counterparts.10,11 Zinc anode has higher theoretical gravimetric and volumetric energy densities compared to conventional graphite anode of Li-ion batteries. However, zinc rechargeable batteries have not been commercially realized due to drawbacks like dendrite formation on zinc anode12 and unavailability of robust cathodes. A range of tunnelled and layered structures of MnO2,13-17 metal hexacyanoferrates18 analogous to the Prussian blue complexes, vanadates19,20 and spinels21-24 have been developed as potential cathodes for Zn-ion batteries. For safety and reversibility, intercalation-based mechanisms are widely explored; though activity based on conversion25 or reversible structural transformations of MnO215,16 and co-insertion of H+ and Zn2+ have also been reported recently.26 Oxidation is the most common process through which chemical energy of fuels is converted into usable forms. With the growing pollution and environmental concern, clean oxidation methods like metal-air batteries and fuel cells have garnered wide attention.27-29 3 ACS Paragon Plus Environment


Electrochemical oxygen reduction reaction (ORR) is an integral chemical process in such systems. However, due to high associated activation energies, such reactions are usually sluggish and require catalysts to enhance the kinetics.30-32 Pt-based noble metal catalysts deliver excellent ORR catalytic activity.33 However, considering material economy and enhanced stability, metal oxides are viewed as commercially viable future catalysts.34,35 Li-ion batteries have long been ruled by transition metal oxide cathodes like layered LiCoO2 and spinel LiMn2O4. The identical structure and the similar size of Zn2+ and Li+ make ZnMn2O4 spinel an intriguing candidate for Zn-ion intercalation. Although a previous report36 suggested the incompetence of an ideal spinel structure in reversible intercalation of Zn2+ due to higher electrostatic repulsion arising from its dipositive nature, Zhang et al. reported an efficient ZnMn2O4 spinel modified by cation vacancies introducing wider channels for easier intercalation.21 Recently, Wu et al. reported the reversible electrochemical activity of hollow porous ZnMn2O4 microspheres using previously synthesized carbon microsphere templates.22 In both cases, secondary carbon phases were used to enhance the electrochemical performance that reduces the overall cathode active material content and makes the process long. Spinel oxides have also been reported to show appreciable electrocatalytic activity.37,38 ZnMnCoO4 was demonstrated to show superior ORR activity than ZnMn2O4 and comparable to Pt/C.38 However, these reported methods have some prominent shortcomings. While Zhang et al21 used a large amount of secondary carbon phases to enhance electrochemical activity that reduced the active material content, the use of expensive electrolytes and precursors restricted the economic viability. The following report by Wu et al22 solved the problem with the electrolytes, but used pre-synthesized templates in a time-consuming three-step process that took 48-60 h. The first report on the ORR activity of ZnMnCoO438 also used templates in a 12 h process involving expensive acetylacetonate precursors. Here, we report an economic, fast, template-free combustion synthesis of ZnMn2O4 (ZMO), ZnCo2O4 (ZCO) and ZnMnCoO4 4 ACS Paragon Plus Environment

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(ZMCO) nanostructured spinels. Without any template, it develops homogeneous nanoscale morphology. For the first time, reversible Zn2+ (de)intercalation has been observed for ZMCO cathode using a low cost ZnSO4 electrolyte with MnSO4 additive to suppress Mn dissolution from cathode. Further, the electrocatalytic ORR performance of ZMCO was also found to be comparable to existing literature. While ZMO also exhibit both Zn2+ (de)intercalation and ORR activity under similar conditions, ZCO exhibited only the ORR activity. The nanoscale particle morphology is believed to be responsible for the improved rate performance due to reduced diffusion lengths associated with Zn2+ intercalation. For ZMCO, the enhanced catalytic performance is attributed to the highly active Co4+/Co3+ redox couple whose accessibility is increased by the lattice expansion due to the incorporation of a larger Mn3+ ion in the lattice.

EXPERIMENTAL SECTION Synthesis of nanoscale zinc spinels. All spinel compounds were prepared by solution combustion method using Zn(NO3)2.6H2O (SDFine Chemicals, ≥ 98 %), Mn(CH3COO)2 (Sigma Aldrich, ≥ 99%) and Co(NO3)2.6H2O (SDFine Chemicals, ≥ 99%) as oxidants and ascorbic acid (C6H8O6, Sigma Aldrich, ≥ 98%) as fuel. Typical solution combustion synthesis involves the dissolution of (oxidant) precursors and a hydrocarbon fuel, which initiates the exothermic combustion reaction in a suitable solvent followed by thermal activation. For example, ZnMn2O4 (ZMO) was prepared by dissolving 5 mmol of Zn(NO3)2.6H2O, 10 mmol of Mn(CH3COO)2 and 2 g of ascorbic acid in minimal amount of water followed by uniform stirring and heating to 150 oC to form a gel, which underwent spontaneous ignition with copious evolution of gases to form a grey fluffy powder. This powder was then ground in an agate mortar pestle, pressed into pellets and was calcined at 600 oC for 6 h (in air) to yield the final product. In a similar fashion, ZnMnCoO4 (ZMCO) and ZnCo2O4 (ZCO) were prepared by taking stoichiometric amount of Mn(CH3COO)2 and Co(NO3)2.6H2O. However, ZnCo2O4



was produced by low temperature calcination at 400 oC for 10 h to minimize the formation of ZnO impurity.

Structural Characterization. Powder X-ray diffraction (XRD) patterns of as-prepared spinels were obtained using a PANalytical X’Pert Pro diffractometer equipped with a Cu-Kα target (operating at 40 kV/ 30 mA) of monochromatic wavelength (λ=1.5404 Å). Typical diffractograms were acquired in the 2θ range of 10-90° with a scanning step of 0.02626o in Bragg-Brentano geometry under ambient temperature conditions. Microstructural features were analyzed by an FEI Inspect F50 field emission scanning electron microscope (operating at 10 kV). High-resolution transmission electron microscope (TEM) and selected area diffraction (SAED) patterns were collected using an FEI Tecnai T 20 U-Twin microscope (operating at 200 kV). X-ray photoelectron spectra were acquired with a Kratos Axis Ultra DLD unit with an incident monochromated X-ray beam from Al target (operating at 13 kV/ 9 mA). Shift correction was conducted using carbon reference (binding energy = 284.6 eV).

Galvanostatic analysis of aqueous zinc-ion battery. The active material (ZMO or ZMCO), conductive carbon black (Alfa Aesar) and polyvinylidene difluoride (Sigma Aldrich) were intimately mixed in the ratio 8:1:1 in a few drops of N-methyl-2-pyrrolidone (NMP) to form a slurry. This slurry was coated on stainless steel current collectors ( = 16 mm) followed by vacuum drying at 80C for 12 h to form the cathode. Polished zinc metal (Alfa Aesar) was used as anode. A polymeric separator was used soaked with an aqueous electrolyte (2 M ZnSO4 + 0.1 M MnSO4) to assemble CR2032 type coin cells. Following a rest period of 6 h, galvanostatic charge-discharge analysis was performed in the voltage window of 0.8-1.95 V with a Bio-logic BCS-805/810 battery cycler of 40 μV resolution and 2 ms acquisition time.


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Electrocatalytic analysis of zinc spinels. The catalyst ink was prepared by the sonication of 10 mg of active material and 5 mg conductive carbon black, dispersed in a mixture of 250 μL isopropyl alcohol and 750 μL DI water, for 5 min, followed by addition of 10 μL of Nafion solution and further sonication for 30 min. Then, 10 μL of the ink was drop cast on a clean glassy carbon electrode of 4 mm diameter followed by drying under an infrared lamp for 30 min. A three-electrode configuration consisting of a saturated Hg/HgO reference electrode, glassy carbon rod counter electrode and catalyst-loaded RDE working electrode in 0.1 M KOH electrolyte was assembled. The electrochemical characterization was conducted with a CH Instruments 7001 E electrochemical workstation. All data were converted to the reversible hydrogen electrode (RHE) reference.45 Linear sweep voltammetry (LSV) experiments were performed at a constant voltage scan rate of 10 mV/s. Chronoamperometric measurements were carried out keeping a constant potential of -0.3 V (v/s Hg/HgO) for 36,000 s for comparing Pt/C and ZnMnCoO4.

RESULTS AND DISCUSSION Combustion synthesis is a versatile route to produce suites of inorganic materials. Exothermic combustion triggers rapid reaction to form nanoscale particles with restricted grain growth. It yielded phase-pure ZnMn2O4 spinel product (Fig. 1a).39 ZnMn2O4 is a normal spinel with a tetragonal unit cell (Fig. 1b) with lattice parameters a=b=5.7204 Å and c=9.2450 Å (JCPDS 24-1133). In a normal spinel, the dipositive Zn2+ ions occupy 1/8th of the tetrahedral voids and the tripositive Mn3+/Co3+ occupy 1/2 of octahedral voids. Combustion synthesis forms normal spinel ZnCo2O4 assuming cubic structure with lattice parameters a=b=c=8.083 Å (JCPDS 23-1390), albeit with trace amount of ZnO impurity (Fig. 1c). The mixed metal ZnMnCoO4 spinel was found to be a derivative of ZnCo2O4 structure, isostructural to basic Co3O4 [CoIICoIII2O4 spinel]40 (JCPDS 74-2120). Replacing Co2+ with Zn2+forms ZnCo2O4 and



further replacing one Co3+ with Mn3+ leads to ZnMnCoO4 spinel. As Mn3+ (79 pm) is larger than Co3+ (75 pm),41 the same pattern as Co3O4 is expected in ZnMnCoO4 with all peaks shifted slightly to lower angles (Fig. 1c). ZnMnCoO4 has a normal cubic (Fig. 1d) spinel structure with a slightly larger unit cell with lattice parameters a=b=c= 8.2549 Å. In all spinels, broadening of diffraction peaks can be ascribed to the nanoscale particle size. The ease of Zn2+ (de)intercalation in spinel frameworks depends on both the size of channels and the redox nature of the tripositive cation.

Figure 1. (a) XRD pattern of combustion prepared ZnMn2O4 along with ICSD standard, (b) corresponding tetragonal spinel framework built from ZnO6 octahedra (grey) and MnO6 octahedra (pink), (c) Comparative diffraction patterns of ZnCo2O4 and ZnMnCoO4 spinels isostructural to cubic spinel Co3O4, (d) corresponding cubic spinel structure having ZnO6 octahedra (grey) and Co/MnO6 octahedra (blue). 8 ACS Paragon Plus Environment

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As combustion synthesis triggers exothermic reaction leading to rapid product formation, it restricts grain growth (or Ostwald ripening) to form nanoscale particles. Microscopy analysis of samples after calcination at 600 C revealed the formation of homogeneous, near spherical particles of ZnMn2O4 and ZnMnCoO4 spinels owing to rapid evolution of gaseous species during combustion reaction (Fig. 2a, d). Presence of excess carbon from the fuel and its subsequent oxidation during calcination led to restricted grain growth. Near spherical morphology with uniform particle sizes (20-30 nm) were observed even in the absence of any template, marking combustion route as an economic and rapid approach to form zinc spinel nanoparticles. TEM study further established nanometric morphology (Fig.2b, e). The dspacing of ZnMn2O4 spinel matches with the interplanar spacing of (211) planes (Fig.2c). As expected, there is a slight increase in spacing of (111) planes in ZnMnCoO4 (0.4863 nm) with respect to pure ZnCo2O4 phase (0.4666 nm). Presence of multiple misoriented crystalline regions were marked, indicating a high concentration of grain boundaries (Fig. S1). The rapid combustion reaction led to the high concentration of defects. Presence of excess structural defects can lead to the formation of more accessible redox active sites and wider molecular channels for cationic diffusion.42 This, along with the nanoscale particle size, is expected to reduce the diffusion barrier of Zn2+ cations to facilitate efficient Zn2+ (de)intercalation.



Figure 2. Morphology of combustion prepared zinc-based nanoscale spinels. Representative SEM images of combustion synthesized ZnMn2O4 (a) and ZnMnCoO4 (d) showing homogeneous near spherical particles in the size range of 20-30 nm. Respective TEM micrographs of ZnMn2O4 (b) and ZnMnCoO4 (e) (Inset: particle size distribution). Corresponding HRTEM images of ZnMn2O4 (c) and ZnMnCoO4 (f) showing the matching planes and their measured d-spacing (Inset: SAED patterns). The purity of spinel samples was further probed by X-ray photoelectron spectroscopy (Fig. 3). The wide spectra (Fig. S2) consist of characteristic peaks of all constituent elements. The Zn 2p peaks are between 1020-1050 eV, Mn 2p between 640-655 eV and O 1s between 525535 eV. These peaks get shifted towards slightly lower binding energies in case of ZnMnCoO4 denoting an altered bonding environment upon the addition of Co3+. Between 770-810 eV, peaks were observed only in the ZnMnCoO4 sample, denoting the presence of Co. Slight amounts of higher oxidation states (4+) were detected for Mn and Co similar to previous reports.22 Additionally, the possible deconvolution of O 2p peak into two and three constituents in ZnMn2O4 and ZnMnCoO4 respectively (Fig.3d) is consistent with the number of possible metal-oxygen bonds, as the latter has an additional Co-O linkage. 10 ACS Paragon Plus Environment

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Figure 3. Comparative XPS spectra of combustion synthesized ZnMn2O4 (ZMO) and ZnMnCoO4 (ZMCO) spinels depicting the spectra of constituent (a) Zn 2p3/2 and 2p1/2, (b) Mn 2p3/2 and 2p1/2, (c) Co 2p3/2 and 2p1/2 and (d) O 1s. The galvanostatic electrochemical analysis was conducted by assembling coin-type half-cells using spinels (coated on stainless steel) as cathode and polished zinc metal anode in a 2 M aqueous solution of ZnSO4 with 0.1 M MnSO4 redox additive acting as electrolyte. The presence of MnSO4 has been reported to decrease the dissolution and subsequent disproportionation of Mn3+ in ZnMn2O4 into Mn4+ and Mn2+ by Le Chatelier’s principle.22,25 Without any further cathode optimization, these spinels delivered reversible Zn2+ (de)insertion leading to a discharge capacity approaching 100 mAh/g (Fig. 4). In general, an initial increase in capacity was observed reaching a maximum value followed by a constant decrease (Fig. S3). At 50 mA/g, an initial discharge capacity of 93.5 mAh/g was obtained that increased to 102.4 mAh/g in the 5th cycle followed by slight drop to 100.9 mAh/g in the 10th cycle (Fig. 4a). The 11 ACS Paragon Plus Environment


discharge profile has two distinct regions separated by a small notch in the curve, which is characteristic of the coinsertion mechanism26 where H+ ions enter the structure (denoted by the pre-notch discharge plateau) slightly widening the channels, followed by the insertion of Zn2+ (post notch discharge). Spinel structures have comparatively narrower one-dimensional intercalation channels.20 Upon activation, more accessible sites are created as more Zn2+ ions dissolve into the electrolyte and deposit on the anode, thereby increasing the capacity. However, after a particular point, the structure starts to get destabilized leading to capacity fading. Simultaneously as the accessibility increases, possibility of Mn loss through disproportionation of Mn3+ from the spinel also increases. This mechanism has been documented as the primary reason for capacity loss of Mn3+ in Mn based cathodes.25 In case of ZnMn2O4, reasonable cycling stability was noticed with Coulombic efficiency over 95%. On the other hand, ZnCo2O4 spinel was found to be completely inactive in the specified voltage window, as Co4+/Co3+ redox couple is active only at higher potentials. Following, we probed the feasibility of Zn2+ (de)insertion in mixed-metal ZnMnCoO4 spinel. To the best of our knowledge, this is the first report on electrochemical activity of ZnMnCoO4 in aqueous Zn-ion batteries. Upon charging, an increase in the binding energy of Mn-2p peaks were observed, indicating the oxidation of Mn3+ to Mn4+ and a resultant extraction of Zn2+ from the spinel structure (Fig. S4). From the CV of the assembled coin cell (Fig. 4d, inset), two anodic peaks at around 1.57 V and 1.64 V indicated the stepwise deintercalation of Zn2+. Further, two cathodic peaks at 1.20 V and 1.40 V appeared corresponding to the insertion of Zn2+ upon discharge. A gradual activation of the cathode was observed similar to some other Mn-based cathodes as the CV stabilizes after the 5th cycle.13,16 In contrast with ZnMn2O4, there was no initial increase in capacity (Fig. 4b) as the first cycle consistently delivered the highest capacity for all current rates (Fig. S5). The highest capacity of 109.4 mAh/g was obtained at 50 mA/g current. Though the initial discharge capacities were different (Fig. 4c), the magnitude 12 ACS Paragon Plus Environment

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of capacity fade decreased with time and after a few cycles, they all tend to converge to the same value of about 40 mAh/g (Fig. S5, Fig. 4d), suggesting that long term cycling performance is progressively rate-independent. Extraction of Zn2+ is difficult in the first cycle due to the repulsion from Mn cations in the neighboring octahedral sites (8d).21,22 However, the small structural changes decreases the polarization and increases the reversibility as seen from the overlapping CV curves from the 5th cycle onwards (Fig. 4d inset). Presence of the notch suggests a similar

coinsertion mechanism as in ZnMn2O4. With excellent Coulombic efficiency, the capacity stabilized at 39 mAh/g after 120 cycles (at 200 mA/g) (Fig. 4d).

Figure 4. Galvanostatic voltage-capacity profiles of (a) ZnMn2O4 and (b) ZnMnCoO4 spinels for the first 10 cycles (current rate = 50 mA/g). (c) Comparative 2nd cycle (dis)charge profiles of ZnMnCoO4 spinel cathode at various current rates. (d) Long-term cycling performance of ZnMnCoO4 cycled at a rate of 200 mA/g. (Inset: Cyclic voltammogram of ZnMnCoO4 used in the assembled coin cells at 0.2 mV/s in the voltage window of 0.8-2.0 V). 13 ACS Paragon Plus Environment


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For all Zn-ion battery study, 2 M ZnSO4 with 0.1 M MnSO4 as redox additive was employed as a low-cost electrolyte. During battery cycling, deintercalation of Zn2+ ions from the spinel structure occurs followed by their deposition at the zinc anode upon charging.20,21 The general redox reaction involving Zn2+ (de)intercalation can be expressed as: xZn2+ + Zn1-xABO4 + 2xe-

ZnABO4

(1)

where A=B=Mn in ZnMn2O4 and A=Mn, B=Co in ZnMnCoO4. The maximum expected theoretical capacity for ZnMn2O4 is 224 mAh/g for x=1. However, Zn2+ deinsertion is limited by the smaller tunnels and the structural instability during the process. Reversible structural transformations from layered to spinel structure have been observed in cathode compounds like γ‑MnO2.20 However, as the current study employs spinels as starting host for Zn2+ (de)insertion, relatively lesser degree of structural transition was observed as evident by ex situ XRD study (Fig S6). Negligible peak shifts were observed indicating no major structural transformation. After the first discharge, the presence of Zn(OH)2 indicates some irreversibility in Zn-stripping and deposition25 and presence of Mn-oxides (MnO2 and Mn2O3) indicates the possible loss of Mn2+ from the solution that can hamper the capacity stabilizing effects of the MnSO4 additive. From the galvanostatic study, the maximum possible value of x was found to be 0.46 for ZnMn2O4 post which, structural destabilization led to capacity fading. Combustion synthesis yielded the initial spinel structure with high degree of disorderness, which slowly got reorganized in the first few cycles. The initial increase in capacity can be attributed to the widening of diffusion channels upon Zn2+ deintercalation and slight dissolution of Mn3+. However, the discharge behaviour of ZnMn2O4 became tedious at higher currents restricting the initial discharge capacity as low as 31 mAh/g. Maximum capacity was obtained after ~40 cycles, but the subsequent fading was quite sharp and did not stabilize even after 120 cycles


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(Fig. S3b). The presence of a larger Mn3+ ion in 1/4th of the total octahedral sites in ZMCO resulted in a slightly expanded lattice as evident from HRTEM. It increased the radii of diffusion channels resulting in a better accessibility of Zn2+ sites from the first cycle itself, eliminating the initial capacity increase. The absence of Co4+/Co3+ redox activity in ZnMnCoO4 spinels was confirmed by the discharge curves being similar to those of ZnMn2O4 with a plateau at 1.54 V arising from Mn4+/Mn3+ redox couple. ZnMnCoO4 exhibited a maximum capacity of 109.4 mAh/g (at 50 mA/g) i.e. close to its theoretical capacity of 110 mAh/g. Thus, the mechanism of widening of channels was irrelevant in ZnMnCoO4 as the first discharge itself delivered close to theoretical capacity. However, inactivity of Co meant that only half of total Zn2+ could be removed from the structure resulting in the formation of Zn0.5Mn4+Co3+O4. The saturation of discharge capacity occurs because the redox inactive Co present in ZnMnCoO4 maintains the framework better than in the case of ZnMn2O4 where only Mn is present, thus slowing down the structural destabilization. Apart from Zn-ion batteries, spinel-based materials are well known to act as efficient electrocatalysts.43 It inspired us to gauge the electrocatalytic activity of combustion made Znbased spinels. The oxygen reduction activity was characterized by cyclic voltammetry, linear sweep voltammetry and chronoamperometry techniques using an RRDE setup with a glassy carbon electrode. CV and LSV curves were recorded in the range 0.266-1.166 V v/s SHE in 0.1 M aq. KOH. CV curves indicated the candidate materials to be active catalysts for oxygen reduction with ZnMnCoO4 showing a distinct cathodic peak in alkaline solution saturated with O2 (Fig. S7). The ideal reaction for complete reduction of oxygen can be expressed as: O2 + 2H2O + 4e-  4OH-

(2)

which is a four-electron transfer process that can be barely identified from the CV, owing to the overlapping of the cathodic reduction peaks. However, using the value of ring current for



evaluating parasitic reactions, the number of electrons transferred could be calculated by the equation:

n4

Id Id  Ir / N

(3)

where Id= disk current, Ir= ring current and N=current collection efficiency of Pt ring (=0.41). Previous investigations had suggested that manganese spinels would have a poor electrocatalytic performance due to disproportionation of Mn3+.44 However, an appreciable performance was obtained in our trials (Fig. S8a). Diffusion limited current density was defined as current density at the minimum potential (0.266 V) and onset potential was defined as the point where the current density started to decrease, denoted by a sharp increase in the first derivative of the LSV curve. The onset potential at 1600 rpm was significantly improved to 0.86 V with the diffusion limited current density close to 3.20 mA/cm2. The half wave potential was also improved to 0.64 V (Fig. S8a).38 ZnCo2O4 exhibited slightly lower parameters, with onset and half wave potentials of 0.83 V and 0.62 V respectively, and diffusion limited current density of 2.97 mA/cm2 (Fig. S8c). However, the best performance was obtained for ZnMnCoO4 with an onset potential of 0.94 V which was only 7% less than the 20% Pt/C catalyst that yielded a corresponding value of 1.01 V. The diffusion limited current density of 5.22 mA/cm2 is close to 5.93 mA/cm2 exhibited by Pt/C (Fig. 5a, b). However, the half wave potential needed improvement, at 0.74 V against 0.85 V of Pt/C. Calcination under an inert atmosphere incorporates residual carbon from the combustion fuel into the particles which can affect the catalytic activity by enhancing the conductivity. However, our trial showed that conductivity is not the limiting factor for the catalytic performance as both limiting current density and onset potential values dropped (Fig. S9). All oxides exhibited linear dependency in Koutecky-Levich plots which denote characteristic first order reduction kinetics (Fig. 5c, S8b and d). From equation (3), the electron 16 ACS Paragon Plus Environment

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transfer numbers were calculated to be 3.98, 3.99, and 3.99 respectively for ZnMn2O4, ZnCo2O4 and ZnMnCoO4, which are all very close to the ideal value of 4. Stability in alkaline medium was compared between the best performing spinel (ZnMnCoO4) and Pt/C. The relative current profile was observed to be very similar, with Pt/C retaining 69% and ZnMnCoO4 retaining 70% of the initial activity after 10 h (Fig. 5d).

Figure 5. ORR electrocatalytic activity: (a) LSV curves of Pt/C, ZnMn2O4, ZnCo2O4 and ZnMnCoO4 at 1600 rpm in O2 saturated 0.1 M KOH, (b) LSV for ZnMnCoO4 at various rpms. (c) Koutecky-Levich plot (j−1 versus ω−1/2, j is the current density in mAcm-2 and ω is the angular velocity of rotation) for ZnMnCoO4. (d) Chronoamperometric measurements comparing the stability of ZnMnCoO4 with 20% Pt/C.



The catalytic activity of a general AB2O4 spinel depends upon the spin state of B3+ and bond strength of B3+-O2.38,45 From the commercial Co3O4 catalyst, replacement of Co2+ by Zn2+ increases the Zn2+-O bond strength and weakens the Co3+-O. The lattice expansion due to substitution of Co3+ by Mn3+ further contributes to this weakening. However, if the interaction is too weak, like in ZnMn2O4, the incoming O2 species may bind weakly or not bind at all to the B3+centre. Thus, there is an optimum B3+-O bond length that exhibits the maximum catalytic performance and the corresponding value for ZnMnCoO4 is very close. As the catalytic activity is strongly dependent on available surface area, the overall performance of both spinels can be attributed to the nanometer size of particles, spherical nature and the high grain boundary concentration characteristic of the combustion method.

CONCLUSIONS Nanoscale homogeneous Zn-based spinels were produced by a rapid, low cost and template-free combustion synthesis. The formation of spinel products was verified by X-ray based measurements (XRD and XPS). Both ZnMn2O4 and ZnMnCoO4 exhibited appreciable electrochemical activity albeit with the issue of capacity fading, which is common for zinc-ion systems. However, the close-to-theoretical initial capacity and the progressive capacity fading make ZnMnCoO4 an interesting material to study. If the initial capacity fading can be decelerated, ZnMnCoO4 can become a robust cathode for large scale zinc-ion aqueous battery systems. Though the ORR electrocatalytic activity of ZnMnCoO4 has been previously documented, our method presents a distinct advantage due to reasons already mentioned. Under similar experimental conditions, the activity and stability exhibited by ZnMnCoO4 spinel were comparable to the expensive commercial Pt/C catalyst. The electron transference number is almost ideal, suggesting that it can be used for primary metal-air batteries and fuel cell systems. Given that they are undoped phases, the activity exhibited by ZnMn2O4 and ZnCo2O4 are also 18 ACS Paragon Plus Environment

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remarkable. Thus, Zn-based spinels form a robust cathode for aqueous Zn-ion batteries as well as act as robust electrocatalysts. Spinel family can be put on anvil to design multifunctional materials with efficient electrochemical and electrocatalytic activities.

ASSOCIATED CONTENT Supporting Information Available: HRTEM images, pristine XPS wide spectra, discharge capacity and Coulombic efficiency plots, ex situ XPS (Zn, Mn) spectra and ex situ XRD patterns of ZnMnCoO4, ORR CV curves, LSV and KL plots.

ACKNOWLEDGMENTS The current work is financially supported by the Shell Technology Centre (STC) Bangalore. AB sincerely acknowledges Kishore Vaigyanik Protsahan Yojana (KVPY) for a student fellowship (SA-1210052). BS thanks Science and Engineering Research Board

(SERB,

Govt.

of

India)

for

a

National

Postdoctoral

Fellowship

(PDF/2015/00217). PB is grateful to SERB for an Early Career Research Award (ECR/2015/000525). Structural illustrations were performed using the VESTA software.46



References (1) Armaroli, N.; Balzani, V. The future of energy supply: Challenges and opportunities. Angew. Chem. Int. Ed. 2007, 46, 52–66. (2) Smalley, R. E. Future global energy prosperity: The terawatt challenge. MRS Bull. 2005, 30, 412–417. (3) Ibrahim, H.; Ilinca, A.; Perron, J. Energy storage systems- Characteristics and comparisons. Renew. Sust. Energy Rev. 2008, 12, 1221-1250. (4) Vesborg, P. C. K.; Jaramillo, T. F. Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy. RSC Adv. 2012, 2, 7933-7947. (5) Winter, M.; Brodd, R. J. What are batteries, fuel cells, and supercapacitors? Chem. Rev. 2004, 104, 4245-4269. (6) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652-657. (7) Wanger, T. C. The Lithium future-resources, recycling, and the environment. Conserv. Lett. 2011, 4, 202-206. (8) Roth, E. P.; Orendorff, C. J. How electrolytes influence battery safety. Electrochem. Soc. Interface 2012, 21, 45-49. (9) Jacoby, M. Safer lithium-ion batteries.Chem. Eng. News 2013, 91, 33-37. (10)

Xu, C.; Li, B.; Du, H.; Kang, F. Energetic zinc ion chemistry: The rechargeable

zinc ion battery. Angew. Chem. Int. Ed. 2012, 51, 933-935. (11)

McLarnon, F. R.; Cairns, E. J. The secondary alkaline zinc electrode. J.

Electrochem. Soc. 1991, 138, 645-656. (12)

Arouete, S.; Blurton. K. F.; Oswin, H. G. Controlled current deposition of zinc

from alkaline solution. J. Electrochem. Soc. 1969, 116, 166–169.


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13)

Lee, B.; Lee, H. R.; Kim, H.; Chung, K. Y.; Cho, B. W.; Oh, S. H. Elucidating

the intercalation mechanism of zinc ions into α-MnO2 for rechargeable zinc batteries. Chem. Commun. 2015, 51, 9265-9268. (14)

Lee, J.; Ju, J. B.; Cho, W. I.; Cho, B. W.; Oh, S. H. Todorokite-type MnO2 as a

zinc-ion intercalating material. Electrochim. Acta2013,112, 138– 143. (15)

Xu, C.; Du, H.; Li, B.; Kang, F.; Zeng, Y. Reversible insertion properties of zinc

ion into manganese dioxide and its application for energy storage. Electrochem. Solid State Lett. 2009, 12, A61-A65. (16)

Alfaruqi, M. H.; Mathew, V.; Gim, J.; Kim, S.; Song, J.; Baboo, J. P.; Choi, S.

H.; Kim, J. Electrochemically induced structural transformation in a γ-MnO2cathode of a high capacity zinc-ion battery system. Chem. Mater. 2015, 27, 3609−3620. (17)

Lee, B.; Yoon, C. S.; Lee, H. R.; Chung, K. Y.; Cho, B. W.; Oh, S. H.

Electrochemically-induced reversible transition from the tunnelled to layered polymorphs of manganese dioxide. Sci. Rep. 2014, 4, 1-8. (18)

Jia, Z.; Wang, B.; Wang, Y. Copper hexacyanoferrate with a well-defined open

framework as a positive electrode for aqueous zinc ion batteries. Mater. Chem. Phys. 2015, 149-150, 601-606. (19)

Kundu, D.; Adams, B. D.; Duffort, V.; Vajargah, S. H.; Nazar, L. F. A high-

capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode. Nat. Energy. 2016, 1, 16119. (20)

Alfaruqi, M. H.; Mathew, V.; Song, J.; Kim, S.; Islam, S.; Pham, D. T.; Jo, J.;

Kim, S.; Baboo, J. P.; Xiu, Z.; Lee, K. S.; Sun, Y. K.; Kim, J. Electrochemical zinc intercalation in lithium vanadium oxide: A high-capacity zinc-ion battery cathode. Chem. Mater. 2017, 29, 1684−1694.



(21)

Zhang, N.; Cheng, F.; Liu, Y.; Zhao, Q.; Lei, K.; Chen, C.; Liu, X.; Chen, J.

Cation-deficient spinel ZnMn2O4 cathode in Zn(CF3SO3)2 electrolyte for rechargeable aqueous Zn-ion battery. J. Am. Chem. Soc. 2016, 138, 12894−12901. (22)

Wu, X.; Xiang, Y.; Peng, Q.; Wu, X.; Li, Y.; Tang, F.; Song, R.; Liu, Z.; He,

Z.; Wu, X. Green-low-cost rechargeable aqueous zinc-ion batteries using hollow porous spinel ZnMn2O4 as the cathode material. J. Mater. Chem. A 2017, 5, 1799017997. (23)

Dang, W.; Wang, F.; Ding, Y.; Feng, C.; Guo, Z. Synthesis and electrochemical

properties of ZnMn2O4 microspheres for lithium-ion battery application. J. Alloys and Compd. 2017, 690, 72-79. (24)

Pan, C.; Nuzzo, R. G.; Gewirth, A. A. ZnAlxCo2-xO4 spinels as cathode materials

for non-aqueous Zn batteries with an open circuit voltage of ≤2 V. Chem. Mater. 2017, 29, 9351-9359. (25)

Pan, H.; Shao, Y.; Yan, P.; Cheng, Y.; Han, K. S.; Nie, Z.; Wang, C.; Yang, J.;

Li, X.; Bhattacharya, P.; Mueller, K. T.; Liu, J. Reversible aqueous zinc/manganese oxide energy storage from conversion reactions. Nat. Energy 1, 2016, 16039. (26)

Sun, W.; Wang, F.; Hou, S.; Yang, C.; Fan, X.; Ma, Z.; Gao, T.; Han, F.; Hu,

R.; Zhu, M.; Wang. C. Zn/MnO2 battery chemistry with H+ and Zn2+ coinsertion. J. Am. Chem. Soc. 2017, 139, 9775−9778. (27)

Kim, G.; Eom, K.; Kim, M.; Yoo, S. J.; Jang, J. H.; Kim, H. J.; Cho, E. Design

of an advanced membrane electrode assembly employing a double-layered cathode for a PEM fuel cell. ACS Appl. Mater. Interfaces. 2015, 7, 27581–27585. (28)

Wang, Z. L.; Xu, D.; Xu, J. J.; Zhang, X. B. Oxygen electrocatalysts in metal–

air batteries: from aqueous to nonaqueous electrolytes. Chem. Soc. Rev., 2014, 43, 7746-7786.


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29)

Rahman, M. A.; Wang, X.; Wen. C. High energy density metal-air batteries: A

review. J. Electrochem. Soc. 2013, 160, A1759-A1771. (30)

Suntivich, J.; Gasteiger, A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.;

Horn, Y. S. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal–air batteries. Nat. Chem. 2011, 3, 546-550. (31)

Cao, R.; Lee, J. S.; Liu, M.; Cho, J. Recent progress in non-precious catalysts

for metal-air batteries. Adv. Energy Mater. 2012, 2, 816–829. (32)

Senthilkumar, B.; Khan, Z.; Park, S.; Seo, I.; Ko, H.; Kim, Y. Exploration of

cobalt phosphate as a potential catalyst for rechargeable aqueous sodium-air battery. J. Power Sources. 2016, 311, 29-34. (33)

Nie, Y.; Li, L.; Wei, Z. Recent advancements in Pt and Pt-free catalysts for

oxygen reduction reaction. Chem. Soc. Rev. 2015, 44, 2168–2201. (34)

Hamdani, M.; Singh, R. N.; Chartier, P. Co3O4 and Co-based spinel oxides

bifunctional oxygen electrodes. Int. J. Electrochem. Sci. 2010, 5, 556–577. (35)

Wang, D.; Chen, X.; Evans, D. G.; Yang, W. Well-dispersed Co3O4/Co2MnO4

nanocomposites as a synergistic bifunctional catalyst for oxygen reduction and oxygen evolution reactions. Nanoscale 2013, 5, 5312–5315. (36)

Knight, J. C.; Therese, S.; Manthiram, A. Chemical extraction of Zn from

ZnMn2O4-based spinels. J. Mater. Chem. A 2015, 3, 21077-21082. (37)

Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.;Chen, J. Rapid room-

temperature synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts. Nat. Chem. 2011, 3, 79–84. (38)

Wang, H.; Liu, R.; Li, Y.; Lu, X.; Wang, Q.; Zhao, S.; Yuan, K.; Cui, Z.; Li,

X.; Xin, S.; Zhang, R.; Lei, M.; Lin, Z. Durable and efficient hollow porous oxide spinel microspheres for oxygen reduction. Joule 2018, 2, 1–12.



(39)

Sinha, A. P. B.; Sanjana, N. R.; Biswas, A. B. On the structure of some

manganites. Acta Cryst. 1957, 10, 439-440. (40)

Kotousova, I. S.; Polyakov, S. M. Electron-diffraction study of Co3O4. Sov.

Phys. Crystallogr. 1972, 17, 661-663. (41)

Shannon, R. D. Revised effective ionic radii and systematic studies of

interatomic distances in halides and chalcogenides. Acta Crystallogr. A. 1976, 32, 751– 767. (42)

Julien, C. M.; Mauger, A. Nanostructured MnO2 as electrode materials for

energy storage. Nanomaterials 2017, 7, 396-436. (43)

Zhao, Q.; Yan, Z.; Chen, C.; Chen. J. Spinels: Controlled preparation, oxygen

reduction/ evolution reaction, application, and beyond. Chem. Rev. 2017, 117, 1012110211. (44)

Xiao, J.; Chen, X.; Sushko, P. V.; Sushko, M. L.; Kovarik, L.; Feng, J.; Deng,

Z.; Zheng, J.; Graff, G. L.; Nie, Z. High‐performance LiNi0.5Mn1.5O4 spinel controlled by Mn3+ concentration and site disorder. Adv. Mater. 2012, 24, 2109–2116. (45)

Stephens, I. E.; Bondarenko, A. S.; Grønbjerg, U.; Rossmeisl, J.; Chorkendorff,

I. Understanding the electrocatalysis of oxygen reduction on platinum and its alloys. Energ. Environ. Sci. 2012, 5, 6744–6762. (46)

Momma, K.; Izumi, F. VESTA 3 for Three-dimensional visualization of crystal,

volumetric and morphology data. J. Appl. Crystallogr., 2011, 44, 1272-1276.


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Low-Cost Rapid Template-Free Synthesis of Nanoscale Zinc Spinels

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