DOI: 10.1002/celc.201700873 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
Articles
Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phosphates in Alkaline Solution Ritambhara Gond,[a] Krishnakanth Sada,[a] Baskar Senthilkumar,*[a] and Prabeer Barpanda*[a] Noble-metal-free, bifunctional catalysts are essential to develop high-performance, cost-effective water-splitting systems. The current work explores the bifunctional electrocatalytic behavior of two sodium cobalt phosphate systems, namely, NaCoPO4 and Na2CoP2O7, prepared by solution combustion synthesis (SCS) for the first time. Formation of phase-pure orthorhombic maricite NaCoPO4 (space group: Pnma) and orthorhombic Na2CoP2O7 (space group: Pna21) was confirmed by Rietveld refinement. The electrocatalytic activity of maricite NaCoPO4 and Na2CoP2O7 was investigated by using linear sweep voltammetry with a rotating disk electrode (RDE). The oxygen reduction reaction (ORR) activities of these sodium cobalt phosphates are comparable to
Vulcan carbon black and Pt/C. Oxygen evolution reaction (OER) activity of Na2CoP2O7 is dominant compared to NaCoPO4 and Pt/C. The Tafel slope, electron-transfer number, and stability of the sodium-metal phosphates were calculated in different concentrations of Na + -containing aqueous electrolyte. The bifunctional activity and good stability of the sodium cobalt phosphates stem from cobalt ions and stabilization of the catalytic centers by the phosphate frameworks. The present work demonstrates sodium cobalt phosphates as alternate costeffective, novel electrocatalysts for efficient OER/ORR activity in alkaline solution.
1. Introduction
stable OER activity with low onset potential in strong alkaline medium. The electrocatalytic OER performance of the Co3 (PO4)2@NC is superior compared to state-of-the-art noble metal catalysts such as IrO2.[25] Very recently, sodium metal phosphates NaCoPO4 and Na2CoP2O7 were studied as electrocatalyst for OER.[26] The Na2CoP2O7 with distorted cobalt tetrahedral geometry exhibited enhanced activity and stability relative to cobalt phosphate. This report demonstrates that surface reorganization by the pyrophosphate ligand induces distorted cobalt tetrahedral geometry. This result favors binding of water molecules and it leads to low overpotential (~ 0.42 V) for water oxidation.[26] Particularly, polyanions affect the OER and ORR performance of the catalysts.[14,26,27] Recent report on OER catalytic performance of Mn3(PO4)2 in neutral solution showed the positive effect of polyanion.[14] The phosphate framework was found to stabilize the Mn3 + active sites much better than in manganese oxides. Due to these advantages of phosphate frameworks, our principal objective is to investigate the ORR and OER activity of new phosphate-based systems namely NaCoPO4 and Na2CoP2O7. To the best of our knowledge, no work has been reported on electrocatalytic ORR activity of the maricite phase NaCoPO4 and Na2CoP2O7 in alkaline solution. In the present work, sodium cobalt phosphates (NaCoPO4 and Na2CoP2O7) were synthesized by solution combustion technique. Crystal structure of the NaCoPO4 and Na2CoP2O7 was investigated using XRD Rietveld refinement technique. The electrocatalytic properties of sodium cobalt phosphates were examined by using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) techniques with RDE in presence of Na + ion containing aqueous electrolyte (0.1 M and 1 M NaOH). In the current work, the electrocatalytic activities of sodium cobalt phosphates and possible effects of phosphate frameworks have been explored.
Electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for water electrolysis, artificial photosynthesis and rechargeable metal-air batteries.[1–3] In rechargeable metal-air batteries, OER take place during charge and the ORR in discharge process, which are expected to occur reversibly.[4,5] However, the kinetics of these OER/ORR reactions are slow and sluggish. Catalysts are needed to overcome these sluggish kinetics.[6–8] Particularly, bifunctional catalysts are eminent to promote oxygen reduction and oxygen evolution reactions and to attain high performance in metal-air batteries.[7,8] Noble metals like Pt-based materials are well known catalysts for ORR due to their highest activity,[9,10] whereas Ru- and Ir-based materials are highly active catalysts for OER.[11,12] However these noble metal catalysts are expensive and show poor stability limiting their large-scale applications. Next to the noble-metal catalysts, metal oxides are also studied for OER/ORR in acidic and alkaline solutions exhibiting good activity.[13–16] Among the metal oxides, the Co-based oxides showed better ORR activity[17,18] whereas the Ni-based oxides delivered superior OER activity.[19–21] Recent reports demonstrate the phosphate based materials could be very competent catalysts for OER and ORR.[22–24] For example, cobalt phosphate nanoparticles decorated with nitrogen-doped carbon (Co3(PO4)2@NC) exhibited efficient and [a] R. Gond, K. Sada, Dr. B. Senthilkumar, Prof. P. Barpanda Faraday Materials Laboratory, Materials Research Centre Indian Institute of Science C.V. Raman Avenue, Bangalore, 560012, India Phone: + 91-80 2293 2783 Fax: + 91-80 2360 7316 E-mail:
[email protected] [email protected] Supporting information for this article is available on the WWW under https://doi.org/10.1002/celc.201700873
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2. Results and Discussion 2.1. Structure and Morphology of NaCoPO4 and Na2CoP2O7 Both the Co-based phosphate end-members were synthesized by solution combustion synthesis route, as it has the advantages of short reaction duration and possible formation of porous morphology.[28,33] Thermogravimetric analysis reveals progressive dehydration and decomposition of combustion ash in the temperature range of 200–450 8C, leading to the final product (Figure S1). Taking hint from TGA study, the final annealing of NaCoPO4 system was conducted at 400, 600 and 900 8C as shown in Figure S2. Phase-pure NaCoPO4 and Na2CoP2O7 were successfully prepared by annealing at a minimum temperature of 600 8C; higher annealing temperature leading to more crystallinity. Therefore, the 600 8C synthesized samples were utilized for all further characterizations. NaCoPO4 and Na2CoP2O7 crystallize into orthorhombic framework with different space groups [Pnma (#62) and Pna21 (#33) respectively], which arises due to different coordination of O and Co-transition metal as well as varied coordination of PO4 tetrahedra around the CoO6/CoO4 polyhedral units.[26] NaCoPO4, assuming Pnma symmetry, was first reported by Hammond and Barbier[34] in 1996 (PDF 82-752). Na2CoP2O7 exists in three distinct polymorphs with possible tetrahedral (CoO4) and octahedral (CoO6) coordination of Co: orthorhombic [Pna21 (#33)], tetragonal [P42/mnm (#136)] and triclinic [P-1 (#2)] polymorphs.[28] Rietveld refinement of XRD patterns of combustion synthesized end-members are shown in Figure 1. The corresponding crystal structures are depicted in the inset images. It confirmed the formation of desired NaCoPO4 and Na2CoP2O7 end products. From refinement, structural parameters were calculated to be: a = 8.9020(16) A˚, b = 6.8119(12) A˚, ˚ 3 for NaCoPO4 c = 5.0426(9) A˚, unit cell volume = 305.78(16) A ˚ ˚ ˚ , unit and a = 15.3896(08) A, b = 10.2788(59) A, c = 7.6991(31) A 3 ˚ cell volume = 1218.90(70) A for Na2CoP2O7. The detailed crystallographic parameters for NaCoPO4 and Na2CoP2O7, as derived from Rietveld refinement, are summarized in Table S1. Metal phosphate based materials, having general formula AMPO4, form three different kinds of structures namely olivine, maricite and zeolite-type structure. For AMPO4 class of compounds, while Li-based analogue stabilizes into olivine structure, the Nabased analogue thermodynamically favors the maricite structure. The maricite framework is similar to olivine structure except that alkali cation (Na) occupies the M(2) site while transition metal occupies the M(1) sites. The basic difference between phosphate and pyrophosphate structure lies in CoO coordination and local network of PO4 polyhedra. NaCoPO4 consists of CoO6 octahedra, which are linked by PO4 ligands by corner-sharing fashion. Whereas Na2CoP2O7 is built from CoO4 tetrahedra, which are isolated by pyrophosphate (PO4-PO4) groups.[26] While Co has a specific crystallite site in NaCoPO4, it occupies two distinct crystallographic sites in Na2CoP2O7. Microstructure of these catalysts were observed by scanning electron microscopy (SEM) as shown in Figure 2 (a and d). The SEM image of NaCoPO4 (Figure 2a) shows the formation of homogeneous sub-micrometric particles. In contrast, Na2CoP2O7 ChemElectroChem 2018, 5, 153 – 158
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Figure 1. Rietveld refinement of a) maricite NaCoPO4 with the space group of “Pnma” (Rp = 1.30, Rwp = 2.28, c2 = 1.61) and b) Na2CoP2O7 with the space group of “Pna21” (Rp = 1.72, Rwp = 3.38, c2 = 2.12) prepared by using the combustion synthesis technique. The observed data points (red), calculated pattern (black), their difference (blue), and Bragg diffraction positions (pink bar) are shown in each case. Structural illustration (inset) of sodium cobalt phosphates built from CoO4/CoO6 polyhedra (blue), PO4 tetrahedra (grey), and Na atoms (yellow).
Figure 2. Morphology of sodium cobalt phosphates. SEM, TEM, HRTEM image, and SAED pattern of NaCoPO4 [a–c and inset of (c)] and Na2CoP2O7 [d–f and inset of (f)] electrocatalysts prepared through solution combustion synthesis involving final annealing at 600 8C for 5 h.
forms larger particles having layered morphology of variable sizes (Figure 2d). Transmission electron microscopy (TEM) (Figure 2b and e) revealed agglomeration of nanometric particles in
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case of NaCoPO4 and formation of larger sheets folded independently in case of Na2CoP2O7. High-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns are shown in Figure 2c and f supporting the crystallinity in both cases. The lattice fringe d-spacing was determined to be 0.371 nm corresponding to (210) lattice planes for NaCoPO4 and 0.616 nm corresponding to (011) lattice planes for Na2CoP2O7.
2.2. Electrochemical ORR Analysis Electrocatalytic oxygen reduction behavior of the sodium metal phosphates was investigated by using CV, LSV and CA techniques with RDE. We have examined our combustion synthesized cobalt phosphate and pyrophosphate having ORR activity approaching that of commercial Pt/C. The CV curves of bare graphite and sodium metal phosphates, recorded in the potential range of 0.25–0.95 V vs. RHE at O2 saturated atmosphere, are shown in Figure 3a. The noticeable oxygen
Figure 3. ORR catalysis (a): ORR cyclic voltammetry of NaCoPO4 (1 M and 0.1 M NaOH displayed as 1) red and 3) brown, respectively) and Na2CoP2O7 (1 M and 0.1 M NaOH displayed as 2) blue and 4) green, respectively) compared with the bare electrode in 1 M NaOH. ORR polarizations (b) for both of the catalysts (the corresponding results for the Vulcan carbon black 5) grey in color are displayed for comparison). The electrolytes were O2saturated 1 M and 0.1 M NaOH. Rotating speed: 1,600 r.p.m., scan rate: 10 mV s1. c) Tafel plots derived from the data provided in plot (b). d) Stability of NaCoPO4 and Na2CoP2O7 catalysts monitored for 40,000 sec at high pH.
reduction peaks for respective samples in the O2-saturated alkali solutions of different concentrations were observed, whereas no perceptible voltammetric current was observed for bare electrode. Interestingly, by comparing the CV curves of NaCoPO4 and Na2CoP2O7, it is evident that electrocatalytic reduction of oxygen on the electrode is more efficient in Na2CoP2O7 system, which delivers higher cathodic current than NaCoPO4 in both 0.1 M as well 1 M NaOH aqueous electrolyte. The electrocatalytic activities were further studied using linear sweep voltammetry (LSV) curves obtained with a rotating ring disk electrode (RRDE) in O2-saturated 0.1 M and 1 M NaOH ChemElectroChem 2018, 5, 153 – 158
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solution with a scan rate of 10 mV s1 and a rotating rate of 1600 rpm (Figure 3b). Remarkably, from LSV curve, Na2CoP2O7 system showed more positive ORR onset potential (~ 0.78 and ~ 0.86 V relative to the RHE in 0.1 M and 1 M NaOH respectively) with higher cathodic currents than NaCoPO4 (~ 0.77 and ~ 0.85 V relative to the RHE in 0.1 M and 1 M NaOH respectively) approaching close to Vulcan carbon black XC 72r (~ 0.85 V). The results were also compared with 20 % commercial Pt/C (0.918 V) displayed in Figure S3. While the half-wave potentials of NaCoPO4 and Na2CoP2O7 are found to be 0.731 V and 0.744 V in 1 M NaOH respectively, it is slightly smaller than that of Pt/C (0.804 V).[26] Tafel slope for the phosphate were calculated to be 87 and 109 mV decade1 in 1 M and 0.1 M NaOH respectively. Similarly for pyrophosphate systems it was found to be 96 and 115 mV decade1 in 1 M and 0.1 M NaOH respectively (Figure 3c), which is comparable with the commercially available Vulcan carbon black value of 98 mV decade1.[32] Relative stability of the NaCoPO4 and Na2CoP2O7 catalysts, tested in strong alkaline solution over the ~ 40,000 s, is represented in Figure 3d. Current loss of phosphate is about 24 % whereas pyrophosphate exhibits more stable current with high retention (ca. ~ 100 %). In Na2CoP2O7, tetrahedral CoO4 networks with higher point symmetry are isolated from other tetrahedra by pyrophosphate units, which keeps framework more intact than NaCoPO4 system where continuous edge-shared CoO6 octahedra exist. It could be one of the reason behind excellent stability of Na2CoP2O7.[26] Also in NaCoPO4 system, poor stability is coming from the CoO6 units, which is susceptible to JahnTeller (JT) distortion and each such unit is edge-shared with other CoO6 units. The Jahn-Teller distortion affects the ORR activity as it alters the electronic structure of the catalysts, which further cannot bind with the oxygen species.[27,35] Considering above mentioned results (Figure 3), it is clear that the Na2CoP2O7 pyrophosphate offers superior electrocatalytic activity than the NaCoPO4 phosphate. Although the electrocatalytic performance of these cobalt phosphates are not superior to Pt/C, they can be advantageous for the economic large-scale application. Furthermore, to investigate the mechanistic aspects of these catalysts, a well-known KouteckyLevich (KL) plot has been derived using RDE measurements. The LSV and KL plots are shown for NaCoPO4 (Figure 4a, b, e and f) and Na2CoP2O7 (Figure 4c, d, g and h). The LSV curves recorded at different rpm (400-2025 rpm) and corresponding KL plot as sloppy lines in general reveals an electron transfer number. KL plots obtained at a potential of 0.3–0.6 V delivered straight lines that suggest characteristic first order dependence of O2 kinetics.[36,37] The electron transfer number (n) was calculated by using Equation (1): n¼4
Id Id þ Ir =N
ð1Þ
where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.41. The electron transfer number per oxygen molecule (n) for ORR is ~ 3.76 and ~ 3.58 for Na2CoP2O7
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rate of 10 mV s1 (as shown in Figure S5). A well-defined two pairs of redox peaks were observed in the CV curves of both cobalt phosphates hinting at the involvement of Co redox reactions in the OER that is associated with Co3 + /Co2 + and Co4 + /Co3 + conversion as observed in various Co-based OER catalysts.[34,40] The peak current values of Na2CoP2O7 catalyst are higher than the NaCoPO4 for both 0.1 as well as 1 M of NaOH, which is due to the open framework structure of pyrophosphate. Na2CoP2O7 as well as NaCoPO4 catalyst are performing better in 1 M NaOH than 0.1 M NaOH with slight shift in onset potentials towards higher value and higher current. The polarization curves of the sodium metal phosphates recorded in 1.0 M NaOH electrolyte (with 1600 rpm) are shown in Figure 5a.
Figure 4. LSV for NaCoPO4 (a, e) and Na2CoP2O7 (c, g) at different rotating speeds in O2-saturated 1 M and 0.1 M NaOH electrolytes and the corresponding Koutecky-Levich plots (J1 versus w1/2, where J is the current density in A m2 and w is the rotating speed in rad s1). NaCoPO4 (b, f) and Na2CoP2O7 (d, h) are for the potential window between 0.30 and 0.60 V.
and NaCoPO4 catalyst respectively (Figure S3), which is close to four electron transfer pathways for ORR suggesting at reversible reaction 2H2O + O2 + 4e$ 4OH(aq). The catalytic mechanism occurs due to the formation of di m-oxo bridge.[38,39] It is predicted that in cobalt based compounds, water molecules, cations and anions occupy the space generated after coordination of cobalt ions with di m-oxo bridge. Herein we explored the ORR catalytic performance of two Co-based phosphate materials and found that Na2CoP2O7 is superior than NaCoPO4 due to its crystal chemistry and slight enhanced electron transfer number (n) per oxygen molecule (as shown in Figure S4), which is approaching the ideal n value of 4.
2.3. Electrochemical OER Analysis The oxygen evolution activity of sodium metal phosphates was also investigated, as the catalysts with bifunctional (OER/ORR) activities are prominent for the rechargeable metal-air batteries. Before polarization experiments, CV measurements were performed in the potential range of 0.95 ~ 1.55 V vs. RHE at a scan ChemElectroChem 2018, 5, 153 – 158
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Figure 5. OER catalysis of NaCoPO4 and Na2CoP2O7. OER polarizations (a) and corresponding Tafel plots (b) are derived for the catalysts in alkaline medium.
The LSV curve shows better OER activity in Na2CoP2O7 when compared to NaCoPO4 and Pt/C, having low onset potential (1.55 V) and high current density. The current associated with OER increases rapidly above 1.55 V (vs. RHE) for Na2CoP2O7. However, no such sharp increase in current was observed for Pt/C. Among the tested catalysts, Na2CoP2O7 had the most negative onset potential for the OER, followed by NaCoPO4 and Pt/C. Despite the closely matched OER onset potential at 1.50 V, Na2CoP2O7 showed more rapid increase in current density with
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potential than NaCoPO4. This rapid increase resulted in a small Tafel slope, which is desired for the OER. Tafel slopes measured in this study are shown in Figure 5b. These results confirm the competent electrocatalytic activity of sodium cobalt phosphates, which can work as economic bi-functional electrocatalysts for metal-air batteries.
3. Conclusions Sodium cobalt phosphates, NaCoPO4 and Na2CoP2O7, were successfully synthesized by solution combustion synthesis method crystallizing into orthorhombic frameworks with Pnma and Pna21 symmetry respectively. We explored the ORR abilities of phosphate and pyrophosphate-based cobalt redox-active materials for the first time. Using 0.1 and 1.0 M NaOH electrolyte, excellent catalytic activity was observed for Na2CoP2O7 system vis-a`-vis NaCoPO4, which is due to distorted cobalt tetrahedral geometry. Na2CoP2O7 showed high ORR activity comparable to NaCoPO4 along with high stability resulted from distorted cobalt tetrahedral geometry and electron transfer number per oxygen molecule which is approaching the ideal value of 4. The superior performance of Na2CoP2O7 can stem from isolated CoO4 tetrahedra linked with PO4-PO4 pyrophosphate units unlike NaCoPO4 where edge-shared CoO6 octahedra leads to Jahn-Teller distortion and may alter the structure for long time electron transfer. Similarly, OER activity is notably superior for Na2CoP2O7 than NaCoPO4, which suggests in OER during the O2 evolution pathway structure get stabilized by actively rotating pyrophosphate groups that are linked with isolated tetrahedron. Thus, tailoring cobalt-based polyanionic phosphates, efficient catalysts can be developed with electrochemical ORR-OER bifunctional activities are basically functioning better than Vulcan carbon black and also can work as lowcost alternatives to Pt/C catalyst.
product was ground in an agate mortar pestle, pressed into pellets and was calcined at 600 8C for 5 h in a muffle furnace to obtain the final products.
Synthesis of Electrocatalysts from NaCoPO4 and Na2CoP2O7 For electrocatalytic analyses, catalysts in the form of slurry were coated on the glassy carbon disk electrodes. The slurry was prepared by mixing cobalt phosphates (NaCoPO4 and Na2CoP2O7; 10 mg), Super P carbon black (5 mg) with 0.75 mL of double distilled water and 0.25 ml isopropyl alcohol (IPA). This slurry was sonicated for 5 min. followed by addition of 20 ml of Nafion as binder followed by sonication for 30 min. Then 5 ml of slurry was coated on platinum ring glassy carbon disk electrode. The active material loading was 0.13 mg cm2. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and stability tests were carried out at room temperature using 0.1 M and 1 M NaOH as electrolyte with CH instruments bipotentiostat.
Structural Characterization Powder X-ray diffraction (XRD) patterns of as synthesized NaCoPO4 calcined at different temperatures and Na2CoP2O7 with different fuels were collected with a PANalytical X’Pert Pro diffractometer equipped with CuKa target of monochromatic wavelength (l = ˚ ) operating at 40 kV/ 30 mA. Typical room temperature 1.5404 A diffraction patterns were acquired in the 2q range of 10–908 with a scanning step of 0.026268 in Bragg-Brentano geometry. Rietveld refinement was performed using GSAS program with the EXPGUI front-end[29,30,31] for both materials, which reveal purity of the phase for each material. Microstructures of the prepared material were captured with a field emission scanning electron microscope (FESEM, FEI Inspect F 50 operating at 10 kV). High-resolution TEM and selected area diffraction patterns (SAED) of cobalt phosphate powders were collected with an FEI Tecnai T 20 U-Twin TEM microscope operating at 200 kV. Thermogravimetric (TGA) data were acquired with a TA Q-50 unit in the temperature range of 25– 900 8C at a heating rate of 5 8C/min in a static air atmosphere for NaCoPO4.
Electrochemical Characterization
Experimental Section Synthesis of NaCoPO4 and Na2CoP2O7 The materials were synthesized by solution combustion synthesis (SCS) method using NaH2PO4·H2O (Merck, 98 %), Co(NO3)2 · 6H2O (SDFine Chemicals, 99 %), NH2CONH2 (SDFine Chemicals, 99 %) and C6H8O7 (Merck, 99.5 %) precursors. Combustion synthesis employs mixture of precursors giving constituent elements (known as ‘oxidants’) and hydrocarbon complex facilitating exothermic reaction (known as ‘fuels’). NaH2PO4·H2O and Co(NO3)2 · 6H2O oxidants were dissolved in distilled water in calculated stoichiometric ratios (1 : 1 for NaCoPO4 and 2 : 1 for Na2CoP2O7) with continuous stirring in order to make homogenous solution followed by addition of 1 mole fuel (i. e. urea and/or citric acid).[28] NaCoPO4 was synthesized by using urea (NH2CONH2) as fuel, whereas Na2CoP2O7 was synthesized by using urea as well as citric acid (C6H8O7). Upon thorough dissolution of all precursors in distilled water (20 mL), this solution was heated at 120 8C for dehydration. After complete dehydration, the temperature was raised to 300 8C to propel the exothermic combustion reaction followed by gradual release of gases yielding purple foamy combustion ash at the end of reaction. This intermediate foamy
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A saturated mercury oxide electrode was used as the reference electrode in all measurements and was converted to the reversible hydrogen electrode (RHE).[32] The electrochemical properties were measured using a CH Instruments 7001 E electrochemical workstation employing three electrode cell configuration consisting of Hg/HgO electrode as the reference electrode, Pt as the counterelectrode, and rotating disk electrode (RDE) loaded with various catalysts as the working electrode in 1 M and 0.1 M NaOH electrolyte (at 25 8C). The working electrode in the form of slurry were applied onto an electrode (4 mm diameter) and then dried for more than 30 min. The scan rate was 10 mV s1 for cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. Chronoamperometry (ORR Stability) tests were carried out for 40,000 s keeping potential constant at 0.3 V (vs. reference electrode Hg/HgO) for both cobalt phosphate samples.
Acknowledgements We acknowledge financial support from the Science and Education Research Board (SERB, Govt. of India) for an Early Career Research Award (ECR/2015/000525). R.G. sincerely acknowledges
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the University Grants Commission (UGC) for a fellowship. K.S. acknowledges the Department of Science and Technology (DST, Govt. of India) for financial support under the INSPIRE fellowship scheme. B.S. thanks DST-SERB for a National Postdoctoral Fellowship (PDF/2015/00217). Structural illustrations were performed by using the VESTA software.[41]
Conflict of Interest The authors declare no conflict of interest. Keywords: Naair battery · bifunctional electrocatalyst · maricite · oxygen reduction reaction
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Manuscript received: August 21, 2017 Accepted Article published: October 4, 2017 Version of record online: October 18, 2017
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