Cobalt and Nickel Phosphates as Multifunctional Air-Cathodes for

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Cobalt and Nickel Phosphates as Multifunctional Air-Cathodes for Rechargeable Hybrid Sodium-air Battery Applications Baskar Senthilkumar, Ahamed Irshad, and Prabeer Barpanda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09090 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Applied Materials & Interfaces

Cobalt and Nickel Phosphates as Multifunctional Air-Cathodes for Rechargeable Hybrid Sodium-air Battery Applications Baskar Senthilkumar,a* Ahamed Irshad,b* and Prabeer Barpandac* a Laboratoire

de Réactivité et Chimie des Solides (LRCS), CNRS UMR 7314, Université de Picardie Jules Verne, 33 Rue Saint Leu, 80039 Amiens Cedex, France. b Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States. c Faraday Materials Laboratory, Materials Research Centre, Indian Institute of Science, C.V. Raman Avenue, Bangalore 560012, India. * E-mail: [email protected], [email protected], [email protected]

Abstract Noble-metal-free bifunctional electrocatalysts are indispensable to realize low-cost and energy efficient rechargeable metal-air batteries. In addition, power density, energy density and cycle life of these metal-air batteries can be improved further by utilizing the fast faradaic reactions of metal ions in the catalyst layer together with oxygen evolution/reduction reactions for the charge storage. In this work, we propose mixed metal phosphates of nickel and cobalt, NixCo3-x(PO4)2 (x=0,1,1.5,2 and 3) as multifunctional air-cathodes exhibiting bifunctional electrocatalytic activity and reversible metal redox reaction (M3+/2+, M = Ni and Co). Submicron sized NixCo3-x(PO4)2 particles were synthesized by a solution combustion synthesis technique with urea acting as fuel. Electrocatalytic activity towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in 0.1 M NaOH were systematically tuned by varying the Ni to Co ratio. The synthesized NixCo3-x(PO4)2 with x=1.5 (NCP11) showed superior bifunctional catalytic activity compared to other samples. Moreover, the catalyst material delivered a specific capacity of ~110 mAh g-1 by the redox reactions of its metal sites. Hybrid Na-air battery fabricated using NCP11 catalyst loaded air-cathode exhibited low overpotential, stable cycling performance and round-trip energy efficiency exceeding 78 % in 0.1 M NaOH aqueous electrolyte.

Keywords: Nickel cobalt phosphate. Hybrid battery. Bifunctional electrocatalyst. Na-air battery. Aqueous electrolyte. NASICON. Sodium super ionic conductor.

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1. Introduction To realize gigawatt scale sustainable energy storage in the 21st century, various post Liion battery technologies are widely being explored. One such futuristic storage technology is rechargeable metal-air batteries.1-3 In pursuit of metal-air batteries, Li-air batteries have attracted primary attention owing to their very high theoretical energy density (5200 Wh kg-1). Nevertheless, their practical application is hindered by low elemental abundance of Li, poor cyclic stability and high operational overpotential.4-6 On the other hand, sodium-air (Na-air) batteries have emerged as promising alternatives to Li-air batteries due to high natural abundance of Na, low-cost and environmental-friendliness.6,7 Additionally, there are several advantages associated with sodium-air batteries such as low overpotential and enhanced power density stemming from superior ionic conductivity of NASICON class of solid electrolytes (e.g. Na3Zr2Si2PO12) and high solubility of by-products formed during discharge (i.e. NaOH vis-à-vis LiOH in aqueous alkaline electrolytes).8-10 In general, sodium-air batteries can be broadly divided into two principal categories based on cell design and electrolyte type: non-aqueous and hybrid (aqueous) batteries.6 There are several reports available in the literature on non-aqueous Na-air batteries.7 However, in nonaqueous systems, the discharge products such as Na2O2 and Na2O are insoluble in the electrolyte and they gradually clog the pores of the air electrode thereby resulting in poor power density, diminished cycling stability and large overpotential.7,8 To avoid such issues, it is recommended to use aqueous electrolyte at the cathode side by protecting reactive Na anode using a solid electrolyte. Non-aqueous electrolyte can then be included between the solid electrolyte and the negative electrode. Off late, there are several reports showing the merits of hybrid sodium-air batteries in comparison to non-aqueous storage systems. These merits include reduced overpotential, enhanced rate capability, high energy efficiency and better power density.10,11 Despite these advantages, there are only few reports on hybrid Na-air battery employing noble metal catalysts in the air electrodes.11-13 The noble metal catalysts help to 2 ACS Paragon Plus Environment

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overcome the torpid kinetics of both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrode. However, cost and poor abundance of noble metals make such batteries prohibitively expensive and unsuitable for large scale applications. It provides impetus to develop stable and highly active noble metal free bifunctional electrocatalysts capable of operating in aqueous electrolytes with air acting as the sole dioxygen source.12,13 There are several nickel based catalysts available in the literature for the OER.14,15 Similarly, cobalt based catalytic materials are well explored for the ORR activity.16,17 Hence, it is anticipated that the mixed metal catalysts comprising of both nickel and cobalt metal centres can exhibit both ORR and OER activity at moderate overpotential. Recent reports by Nocera et al. on the electrocatalysis of OER suggest that the phosphate moiety is crucial for the high catalytic activity and stability.18,19 Thus, by judiciously incorporating the desirable properties of nickel, cobalt and phosphate in a single material, it is possible to design a highly durable and active bifunctional electrocatalyst for the air electrode, which would enable to obtain superior capacity, excellent (round-trip) energy efficiency and durable cycling in hybrid rechargeable sodium-air batteries.12,13 Here, we present the preparation of sub-micron sized particles of NixCo3-x(PO4)2 (x = 0, 1, 1.5, 2 and 3) by facile combustion synthesis route and their electrocatalytic activity towards OER and ORR in aqueous alkaline electrolyte. The catalyst with optimized composition, namely, Ni1.5Co1.5(PO4)2 (NCP11) exhibited higher bifunctional catalytic activity than Pt/C catalyst and monometallic phosphates [Co3(PO4)2 and Ni3(PO4)2]. Therefore, the NCP11 material was tested as bifunctional electrocatalyst at the air electrode of a rechargeable hybrid sodium-air battery. Interestingly the catalyst itself delivered a specific capacity of ~110 mAh g-1 by the redox reactions of its metal sites. The battery also exhibited relatively low overpotential (0.78 V), over 78% (round trip) energy efficiency and stable cycling performance up to 50 cycles at a current density of 10 A cm-2.



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2. Experimental section 2.1. Synthesis of mixed metal phosphates [NixCo3-x(PO4)2] NixCo3-x(PO4)2 materials of different composition were prepared using solution combustion synthesis.20,21 Nickel nitrate hexahydrate [Ni(NO3)2.6H2O], cobalt nitrate hexahydrate [Co(NO3)2.6H2O], ammonium dihydrogen phosphate (NH4H2PO4) and urea (NH2CONH2) precursors were used as oxidants and fuel, respectively. In a typical synthesis of Ni1.5Co1.5(PO4)2, Ni(NO3)2.6H2O (4.36 g), Co(NO3)2.6H2O (4.37 g), NH4H2PO4 (2.30 g) and urea (1.80 g) precursors were dissolved in 20 mL distilled water with steady magnetic stirring to obtain a homogenous solution. The resulting solution was placed on hot plate at 200 LC for complete dehydration. The remaining solid was transferred to a muffle furnace and was heattreated at 300 LC for 10 minutes leading to the decomposition with gradual release of gaseous species. The foamy product so formed was collected and ground well in an agate mortar and pestle. The resulting powder was further calcined at 800 LC for 5 h in air. A similar procedure was performed for the synthesis of NixCo3-x(PO4)2 (x= 0, 1, 1.5, 2 and 3). The synthesized samples were named as CP, NCP12, NCP11, NCP21 and NP corresponding to x=0, 1, 1.5, 2 and 3, respectively.

2.2. Electrode preparation and hybrid Na-air battery fabrication The electrode slurry was prepared by mixing the catalyst material (NCP), conductive carbon black Super-P (TIMCAL) and polyvinylidene fluoride (PVDF, Sigma Aldrich) in 8:1:1 (w/w/w) ratio. The slurry was coated on circular discs of teflon treated Toray carbon fibre paper (area: 2 cm2) and was dried at 100 oC for 12 h in vacuum. A thin ceramic pellet of NASICONtype Na3Zr2Si2PO12 (thickness = 0.8 mm, area = 2 cm2) was used as the solid electrolyte. Na3Zr2Si2PO12 layer separated aqueous and non-aqueous electrolyte compartments while allowing Na+ ions to pass through without mixing two electrolytes. The reported value of ionic


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conductivity of Na3Zr2Si2PO12 is > 7

10-4 S cm-1.22 Sodium (Na) metal anode was attached

with a Ni mesh and a long Ni tab was used for the electrical connection. The anode compartment was filled with organic electrolyte of 1 M NaClO4 in propylene carbonate (PC, Sigma Aldrich) and was tightly sealed with NASICON. In the air electrode compartment, 0.1 M NaOH was used as the electrolyte. The cell assembly was performed inside an MBraun GmbH Ar-filled glove box maintaining moisture level less than 1 ppm. The final configuration of the assembled cells was (-) Ni mesh | metallic Na | organic electrolyte | NASICON | 0.1 M NaOH | NCP | Ti mesh (+). The Na-air battery cell fabrication was reported elsewhere in detail.11-13

2.3. Materials Characterization Thermogravimetric analysis (TGA) was carried out with a PerkinElmer (STA 6000) unit in the temperature window of 25–900 oC (heating rate = 5 oC/min) in static air. The powder X-ray diffraction (XRD) patterns were collected using the PANalytical QR diffractometer having a

Pro

9ST source U = 1.5404 Å) operating at 40 kV and 30 mA. Rietveld

refinement was performed by GSAS program with the EXPGUI front-end. The morphology of the synthesized materials was captured by scanning electron microscopy (SEM) and the particles were further analysed with an FEI Tecnai T20 U-Twin transmission electron microscope (TEM) operating at 200 kV. Raman spectroscopy was performed using a LabRAM HR (Horiba Jobin Yvon) having a 532 nm (green) laser source. XPS analysis was carried out by a Kratos Axis Ultra DLD with an incident monochromated X-ray beam from the Al target (accelerating voltage = 13 kV, emission current = 9 mA). Shift corrections were performed using carbon reference (binding energy = 284.6 eV). The electrochemical properties were studied using a CH Instruments CHI7001E electrochemical workstation in a three-electrode cell configuration with mercury/mercuric oxide (Hg/HgO, 0.1 M KOH) reference electrode, a



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rotating disk electrode (RDE) or rotating ring disk electrode (RRDE) loaded with various catalysts acting as the working electrode and 0.1 M NaOH electrolyte (at 25 oC). All potential values were converted to the reversible hydrogen electrode (RHE) scale. Linear sweep voltammograms (LSV) were obtained at a scan rate of 10 mV s-1. In case of Tafel plot, iR compensation was applied to account for the potential drop between the working and reference electrodes due to solution resistance.

3. Results and discussion Sub-micron sized NCP particles were prepared by combustion method taking nitrate precursors as oxidants and urea as fuel. The combustion synthesis technique involves short reaction time and possible formation of nanoparticles.20,21 Themogravimetric analysis (TGA) curve for mixed precursors of Ni3(PO4)2 is shown in Fig.S1 (Electronic supporting information). It shows two weight loss processes: the first weight loss between 100 and 200 oC is due to removal of water molecules, and corresponding endothermic peak is observed at 180 oC.

The second weight loss is due to the exothermic combustion reactions involving emission

of CO2 and NO2 gases. The exothermic peak centred at ~ 300 oC stem from combustion process. After that, there is no significant weight loss which demonstrates the stable product formation. However, cobalt phosphate forms hydrated structure up to 700 oC. Hence, the combustion synthesized foamy powders were calcined at 800 oC for 5h to get fully dehydrated phase. Phase purity, crystal structure and crystallinity of the combustion prepared NCP samples were analysed by X-ray diffraction technique. The observed XRD patterns are shown in Fig.1a. The diffraction peaks of CP is well matched with the JCPDS pattern (JCPDS no: 01-0373) and confirms the formation of phase pure monoclinic Co3(PO4)2 with the space group of P21/a.23,24 Similarly, XRD pattern of mixed nickel cobalt phosphates (NCP) are well matched with ICSD PDF 30-801 and crystallize into monoclinic framework with the space group of P21/a. Rietveld


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refinement of combustion synthesized NCP11 sample is shown in Fig.1b. The respective crystal structure is provided in the inset of Fig. 1b. From refinement, structural parameters of NCP11 were found to be: a=10.205(2) Å, b=4.715(1) Å, c=5.872(2) Å, X = 91.07(5)° unit cell volume=282.50(2) Å3. The detailed crystallographic parameters for NCP11 obtained by Rietveld analysis are given in Table S1. The structure of NCP consists of hexagonally close packed layers of oxygen along (100) direction.24,25 The Co and Ni atoms are in two types of CoO6 and NiO6 octahedral sites. These distorted octahedra sharing edges with PO4 tetrahedra are shown in Fig. 1b (inset). Further, symmetric and asymmetric stretching vibration modes of phosphate oxy-anions (PO4) were revealed by the band peaks (900-1200 cm-1) observed in the Raman spectrum of NCP11 (Fig. S2).26 SEM analysis suggests the formation of grain-like cobalt phosphate structure (Fig. 2a) with size range of 4~10 m. The Ni doping changes the shape and size of the particles (Fig. 2b-e). The grain-like structures turned in to rod-like structure while increasing the Ni content. The particles were highly agglomerated due to calcination. Further energy dispersive X-ray spectroscopy (EDS) analysis revealed uniform distribution of Co, Ni, P and O elements (Fig. 2f and Fig. S3, S4). Representative TEM images of NCP11 sample confirmed the formation of sub-micron sized particles (Fig. 3a,b). Highly crystalline nature of the sample was further endorsed by HRTEM (Fig. 3c) and dot-like SAED pattern (Fig. 3d). X-ray photo-electron spectroscopy (XPS) was conducted to find the bonding state in the mixed nickel cobalt phosphate (NCP11) (Fig. 4). The survey spectrum of NCP11 confirmed the presence of constituent elements (Co, Ni, P and O) (Fig. S5). The high resolution spectrum of Co 2p (Fig. 4a) shows a doublet of Co 2p3/2 and Co 2p1/2 involving spin-orbit interaction located at 796 and 781 eV.27 Similarly, the high resolution spectrum of Ni 2p (Fig. 4b) has two main peaks at 855 and 873 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, respectively.28 The strong satellite feature in the Co 2p and Ni 2p core level suggest oxidation state of +2 in the as-prepared materials.27



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The 133 eV peak in the high resolution spectrum of P 2p (Fig 4c) is in sync with metal phosphate bonding.29 The electrocatalytic activity of the synthesized NixCo3-x(PO4)2 samples towards OER and ORR were studied in an aqueous 0.1 M NaOH (Fig. 5 and 6) electrolyte. In general, OER is the primary charging reaction in the air electrode of metal-air batteries. However, reaction is kinetically sluggish due to the involvement of multi-electron steps with several intermediates (OH*, O* and OOH*).30 Therefore, OER usually requires large overpotential, which necessitates the use of highly active catalysts. Catalyst helps to reduce the required overpotential and thus improve voltaic and round trip efficiency of the battery. Catalyst should also be stable in the electrolyte under harsh oxidative potential of OER to provide sufficient battery life time. It is reported the adsorption/desorption energies of the reaction intermediates largely determine the OER kinetic overpotentials.30 Ideal catalyst has optimum binding energy for the intermediates, which is neither too strong nor too week, resulting in a volcano-shape correlation between activity and binding energy.30 Doping of foreign metal ions is a generally used strategy to finely tune the electronic properties, morphologies and adsorption energies to improve the OER kinetics.31 Hence, it is anticipated that the bimetallic catalyst with optimum metal ion ratio can offer high electronic conductivity, surface area, ideal value for the adsorption energy etc., which will result in high OER activity. In the present study, cobalt phosphate (CP) material shows the least OER catalytic activity with 493 mV overpotential (overpotential, Z = E-1.23 V where E is the applied potential for the specified current density) for 4.8 mA cm-2 current density (Fig. 5a, Table S2). Nickel phosphate shows slightly higher activity and requires 26 mV less overpotential for the same current. As it is seen in Fig. 5a and Table S2, all bimetallic phosphate catalysts (NCP) exhibit superior performance compared to monometallic catalysts. The OER activity follows the order NCP11>NCP12>NCP21>NP>CP. The best catalyst NCP11 requires overpotential of 340 mV at 4.8 mA cm-2, which is 153 mV


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and 127 mV lower than that of CP and NP, respectively (Fig. 5a, Table S2). This clearly indicates the advantage of incorporating both the metal ions in the same material. It is to be noted that the overpotential for 10 mA cm-2 is the generally used standard for comparing the OER performance of different catalysts.30,31 However, in the present study a current density of 4.8 mA cm-2 is used since mono-metallic phosphate catalysts (NP and CP) are unable to deliver 10 mA cm-2 in the selected potential region. The NCP11 catalysts has Tafel slope of 73 mV dec-1 for the OER (Fig. 5b) which is close to the reported values for the OER on nickel and cobalt based catalysts.32,33 Also, NCP11 catalyst shows higher OER activity than Pt/C (Fig. S6a) and doesn’t show any sign of degradation during constant potential electrolysis for 10,000 s (Fig. S6b). As suggested by the XPS data (Fig. 4), nickel and cobalt exist in +2 oxidation state in the as-synthesized NCP material. However, during the anodic sweep, the metal centers undergo oxidation as evidenced by the diffusion controlled current peak prior to OER in the voltammogram. Thus, it is anticipated that the coexistence of Co3+ and Ni3+ in the bimetallic phosphate promote the rate limiting step in the multi-step OER process. Phosphate plays a crucial role of stabilizing the catalytic centres by its framework relative to the less stable environment of MO6 octahedra in the oxide based catalysts.19 A better accommodation of the Jahn-teller distortion is anticipated in the presence of flexible phosphate moiety compared to rigid oxide frame work. In order to investigate the bifunctional property, catalysts were also tested for the ORR on a rotating disk electrode (RDE) by linear potential sweep voltammetry at a scan rate of 10 mV s\& at 1600 rpm (Fig. 6a). The ORR limiting current follows the order NCP12>NCP11=CP>NCP21=NP. The best ORR catalyst NCP12 provides a limiting current of -2.75 mA cm-2 whereas it is -1.8 and -1.5 mA cm-2 for the CP and NP, respectively. The electron transfer number per oxygen molecule (n) for the ORR on NCP11 was calculated to be ~4 from the KL-plot (Fig. 6c, Fig. S7) and RRDE measurements (Fig. S7). In addition, peroxide wasn’t detected in the ring electrode during RRDE measurement (Fig. S7). This suggests a



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direct 4e- transfer pathway for the ORR, and rule out the possibility of peroxide formation. Thus, as observed in the case of OER, mixed metal phosphate has higher ORR activity in comparison with monometallic phosphates. Although NCP12 shows the highest ORR activity among all the NCP catalysts, its OER performance is much inferior to NCP11 catalyst. On the other hand, NCP11 shows reasonable activity towards ORR also. In order to achieve high coulombic and voltaic efficiency in the air battery systems, it is necessary to use the catalyst material that has reasonable activity towards both ORR and OER. Thus, for the best compromise between ORR and OER activities, NCP11 catalyst was used for the hybrid Na-air battery testing (vide infra). We explored the possibility of using mixed phosphate of nickel and cobalt as a multifunctional air electrode for Na-air battery application. Rechargeable hybrid sodium-air battery with NCP11 as multifunctional air-cathode is schematically illustrated in Fig. 7a. Na-air battery was fabricated using Na metal anode and a thin ceramic plate of NASICONtype Na3Zr2Si2PO12 solid electrolyte. The solid electrolyte separated aqueous and non-aqueous electrolyte compartments, and allowed Na+ ions selectively to pass through without mixing two different solvents.14 Fig. 7b represents galvanostatic charge-discharge voltage profile of the hybrid sodium-air battery with NCP11 as multifunctional air-cathode in 0.1 M NaOH electrolyte at a current density of 10 ] cm-2. The working principle of the battery with the NCP11 is provided in the scheme (Fig. 7a) and can be expressed as:34 At anode:

Na ^ Na+ + e- ;

At air cathode:

O2 + 2H2O + 4e- ^ 4OH- ;

Net reaction:

4Na + O2 + 2H2O ^ 4 NaOH ; Eo = 3.11 V (3)

Eo = -2.71 V Eo = +0.40 V

(1) (2)


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The open circuit voltage of the Na-air cell was ~3.1 V and the cell was charged first. The cell was (dis)charged up to the limited specific capacity of 500 mAh g-1. The voltage profile during charging shows a distinct region up to the capacity of ~110 mAh g-1 (Fig. 7b). This is attributed to the oxidation of Co2+ and Ni2+ in the NCP11 catalyst accompanied by the insertion of OH- ions in order to maintain the charge neutrality. Similar observation has been reported in the literature on NiCo2O4 spinel in Zn-air battery.35 Following the metal oxidation reaction, the charge storage is based on OER that can be ascribed by the voltage plateau obtained at ~3.54 V. Similarly, during the discharge process, Co3+ and Ni3+ in the charged catalyst layer undergo reduction to give a capacity of ~110 mAh g-1 at ~3.36 V. This is followed by the ORR reaction at ~2.76 V (Fig. 7b). The results demonstrate the multifunctional electrochemical activity of the NCP material loaded on the air electrode. The battery exhibited charge (OER) and discharge (ORR) plateaus at 3.54 and 2.76 V, respectively. Thus, the cell has an overpotential gap of 0.78 V and round-trip efficiency close to 78 %. The round trip efficiency was calculated as the ratio between the terminated voltage during discharge and charge. Cycling stability of the Na-air cell was tested for 50 cycles by repeated (dis)charge processes (Fig. 8a). During cycling, the capacity contribution from metal redox reactions is increased probably due to improved surface wetting and formation of higher oxidation states of metal ions during OER.35 Terminated discharge voltage and round trip energy efficiency of the Na-air cell are shown in Fig. 8b. The terminated discharge voltage remains constant up to 50 cycles. Similarly, there is no significant variation in the round-trip efficiency during cycling after initial stabilization period up to 5 cycles, and the values are comparable to the literature reports on similar systems (Table S3). The results demonstrate high rechargeability, improved round trip efficiency and good cycling stability of a hybrid sodium-air battery using a mixed nickel-cobalt phosphate in the air electrode.



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Conclusions Noble-metal-free mixed metal phosphate, NixCo3-x(PO4)2 (NCP), was tested as a bifunctional electrocatalyst for the air electrode in rechargeable hybrid sodium-air battery. The highly crystalline sub-micron sized NCP particles were prepared by a simple solution combustion synthesis route employing urea as fuel. Electrocatalytic activity towards OER and ORR of NixCo3-x(PO4)2 materials were systematically investigated at different Ni to Co ratios. NCP11 catalyst with Ni2+ and Co2+ in 1:1 ratio exhibited high electrocatalytic activity towards both OER and ORR in 0.1 M NaOH electrolyte. The NCP11 was further explored as a potential bifunctional catalyst for the air electrode in a hybrid aqueous sodium-air battery. The fabricated sodium-air battery delivered low overpotential for the charge-discharge process with round trip energy efficiency exceeding 78 % and stable cycling performance up to 50 cycles. Moreover, NCP11 material contributed a specific capacity of ~110 mAh g-1 by the redox reactions of its metal sites. Mixed metal phosphates of nickel and cobalt are thus proposed as robust aircathodes to design efficient hybrid Na-air batteries. It is emphasized that the present study monitored the performance of the battery only for 50 cycles and long term stability test over several thousands of cycles needs to be verified for potential use in commercial batteries. The present work will inspire fundamental research on synthesis and application of various types of mixed metal phosphates of low-cost metals (preferably Fe or Mn) as multifunctional air electrodes. Nevertheless, it is also necessary to address issues associated with Na anode and electrolytes for the successful commercialization of hybrid Na-air batteries.36 It includes high reactivity of Na anode with electrolyte, formation of unstable SEI, Na dendritic growth damaging the solid electrolyte that separates aqueous and non-aqueous electrolytes, severe water evaporation and carbonization of alkaline electrolyte in the cathode compartment. It may pave way for economic hybrid Na-air batteries.


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ASSOCIATED CONTENTS Supporting Information

The supporting information is available free of charge on the ACS publication website. Synthesis of NASICON-type Na3Zr2Si2PO12, TGA-DTA curve (Fig. S1), Raman spectrum (Fig. S2), EDXA spectra of CP and NP (Fig. S3), elemental mapping (Fig. S4), XPS survey spectrum (Fig. S5), comparison of OER on Pt/C and NCP11 (Fig. S6a), constant potential electrolysis (Fig. S6b), calculation of electron transfer number (n), RRDE studies using NCP11 (Fig. S7), structural parameters of NCP11 (Table S1), comparison of OER activity (Table S2), and comparison of round trip efficiencies of various Na-air batteries (Table S3) AUTHOR INFORMATION Corresponding Authors

[email protected], www.prabeer.org; Fax: +91-80-2360 7316 [email protected] [email protected] ORCID Baskar Senthilkumar: https://orcid.org/0000-0003-2545-5020 Ahamed Irshad:

https://orcid.org/0000-0001-7107-9623

Prabeer Barpanda:

https://orcid.org/0000-0003-0902-3690

Notes. The authors declare no competing financial interest. Acknowledgements The authors would like to thank Prof. Nookala Munichandraiah for extending the facilities and support. Technical support of C. Murugesan is gratefully acknowledged. The authors sincerely acknowledge the Science and Engineering Research Board (SERB, Govt. of India) for financial support under the aegis of an Early Career Research Award (ECR/2015/000525). B.S. is grateful to the DST (SERB) for providing the National Postdoctoral Fellowship (PDF/2015/00217). Financial support from the Shell Technology Centre (STC) Bangalore is gratefully acknowledged. 13 ACS Paragon Plus Environment


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(a)

(b)

NCP21

NCP11

NCP12

× Observed

NCP11 S.G. P21/a (14)

Calculated Bragg position Obs. – Cal.

Intensity (a.u)

NP

Intensity (a.u.)

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


CP

15

20

25

30

35

40

45

50

55

60 10

20

30

2 (degree)

40

50

60

70

80

90

2 (degree)

Figure 1. (a) XRD patterns of NixCo3-x(PO4)2 samples (b) Rietveld refinement of NCP11 sample with the space group of “P21/a” (Rp =1.29, Rwp = 1.59, c2 = 1.18) prepared by combustion synthesis technique. The observed data points (red), calculated pattern (black), their difference (blue) and Bragg diffraction positions (cyan bar) are shown. (Inset) Structural illustration of mixed nickel cobalt phosphates built from CoO6 and NiO6 octahedra (cyan and green), PO4 tetrahedra (blue) and O atoms (red).



(b)

(a)

(e)

(d)

(c)

1 m 1 µm

11 µm m

11 µm m

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O

Co Ni 1 m 1 µm

1 m 1 µm

C

P

(f)

Co Ni Co Ni KeV

Figure 2. Field emission scanning electron microscopy (FESEM) images of combustion synthesized phosphates (a) CP, (b) NCP12, (c) NCP11, (d) NCP21, (e) NP showing sub-micron size particles and (f) EDX spectrum of NCP11 revealing all constituent elements.


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(a)

(b)

(c)

(d)

Figure 3. Representative (a,b) transmission electron microscopy (TEM) images, (c) HRTEM image and (d) SAED pattern of combustion synthesized NCP11 sample.



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15

(a)

(b) 1.7

10

Overpotential (V)

-2

Current density (mA cm )

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


CP NCP12 NCP11 NCP21 NP

5

73 mV/dec

1.6

1.5 0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

-3.5

-3.0

Potential (V) vs. RHE

-2.5

-2.0

-1.5

-1.0

-2

Log Current density (A cm )

Figure 5. (a) Linear sweep voltammograms of OER catalysts at 10 mV s-1 in 0.1 M NaOH, and (b) Tafel slope for NCP11 sample. iR correction was applied for the Tafel plot.




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100

3.5

4.0

3.0

th

st

50 cycle

1

1-50 cycles

2.5

Discharge voltage (V)

3.5

80

3.0

60 2.5 40 2.0

20

Discharge voltage Round trip efficiency

0

1.5

2.0 0

50 100 150 -1 Specific capacity (mAh g )

200

Round-trip efficiency (%)

(b)

(a) Cell Voltage (V)

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

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0

10

20

30

40

50

Cycle number

Figure 8. (a) Galvanostatic charge-discharge profiles, and (b) variation in the terminated discharge voltage and round trip efficiency with cycle number for the hybrid sodium-air battery with NCP11 multifunctional air-cathode in 0.1 M NaOH electrolyte up to 50 cycles.


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Cobalt and Nickel Phosphates as Multifunctional Air-Cathodes for

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