Exploration of cobalt phosphate as a potential catalyst for rechargeable aqueous sodium-air battery
Graphical abstract
Introduction
Due to high energy densities, rechargeable metal air batteries have been emerged as promising candidate for future energy technologies [1], [2], [3]. Lithium air batteries have been captivated scientific community attention owing to its high theoretical energy density (5200 Wh kg−1) but low Li abundance, poor cyclic stability and high overpotential makes lithium air batteries expensive and unpractical [4], [5], [6]. In recent times, sodium air batteries (referred as Na-air) have gained much attention as an alternate to lithium air batteries due to its safety, cost and higher ionic conductivity in aqueous, non-aqueous and solid electrolyte [6], [7]. However, non-aqueous Na-air batteries have been studied much but less efforts have been given to aqueous Na-air batteries. Though non-aqueous Na-air battery have disadvantage due to the formation of solid and insoluble discharge product and blockage of air electrode pores by solid discharge product which eventually reduces the performance of battery. Furthermore, aqueous Na-air batteries have ultimate advantage due to the formation of highly water soluble discharge product (sodium hydroxide) which increases the performance of the cell having low overpotential, high round trip efficiency and good cyclic stability [7], [8]. Despite these advantages, aqueous Na-air battery research has not been intensified and till date only few research articles have been published in which noble metal catalysts were used as air electrode [8], [9], [10], [11]. However, rechargeability of aqueous Na-air batteries was demonstrated recently using Pt and nanoporous gold air electrodes [10], [11]. But, cycling stability and efficiency was tested only up to 18 cycles. Improving efficiency and cycling stability of aqueous Na-air cells are still challenging, moreover, till date transition metal oxides have not been utilized as air electrode for rechargeable aqueous Na-air batteries.
A rechargeable metal-air battery can be designed by the proper selection and fabrication of air electrode which is commonly known as bifunctional electrocatalyst [12], [13]. The bifunctional electrocatalyst reduces oxygen via oxygen reduction reaction (ORR) during discharge of battery, while at charge; oxygen is evolved via oxygen evolution reaction (OER) at high applied voltage. Herein, we have prepared noble metal free Co3(PO4)2 by facile and cost-effective precipitation method followed by its electrochemical characterization and fabrication of an aqueous Na-air battery using Co3(PO4)2 as an electrocatalyst for the first time. Scheme 1 represents the schematic illustration of our designed aqueous Na-air battery with Co3(PO4)2 as an air cathode. Our motivation of this work is to utilize noble metal free air electrode and investigate the rechargeability of aqueous Na-air battery. The fabricated cell display low overpotential (0.59 V), 83% round trip efficiency and good cyclic stability which is superior to the current lithium air batteries and recent reports on aqueous Na-air batteries [4], [10], [11].
Section snippets
Synthesis of cobalt phosphate
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), ammonium dihydrogen phosphate (NH4H2PO4), sodium hydroxide pellets (NaOH) and cobalt phosphate octahydrate Co3(PO4)2·8H2O were purchased from Sigma Aldrich and used as received. For the synthesis of Co3(PO4)2, Co(NO3)2·6H2O (4.37 g) and NH4H2PO4 (1.70 g) were dissolved in distilled water followed by the addition of 1 M NaOH aqueous solution and pH was adjusted to 7. The resulting solution was stirred magnetically up to 5 h. Thereafter, the precipitate
Results and discussions
The pure phase of Co3(PO4)2 was synthesized by precipitation followed by calcination method. Fig. 1a shows the powder X-ray diffractogram (XRD) pattern of precipitate powder calcined at 700 °C. X-ray diffractogram reveals that pure Co3(PO4)2 was formed after calcination at 700 °C (Fig. 1a) which is well matched with JCPDS no. 01-0373. However, the diffractogram of precipitated powder is matched with hydrated cobalt phosphate (Co3(PO4)2·8H2O) JCPDS card no 01-0121 (Electronic Supporting
Conclusions
In summary, we have explored Co3(PO4)2 as an effective air electrode for aqueous Na-air battery for the first time which was synthesized by facile co-precipitation method. Co3(PO4)2 displays comparable electrocatalytic activity to expensive Pt/C electrode and the fabricated aqueous Na-air battery using Co3(PO4)2 air electrode delivered high round trip efficiency (83%), stable terminated discharge voltage and excellent cyclic stability. We believe that our study opens facile and cost-effective
Acknowledgement
This work was supported by the 2015 Research Fund (1.150034.01) of UNIST (Ulsan National Institute of Science and Technology) and the National Research Foundation of Korea (NRF-2014R1A2A1A11052110).
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