Efficient energy generation and utilization, especially from available fossil fuel sources, has emerged as a principal challenge in the 21^st century. It has provided impetus for worldwide research revolving around ‘energy' over the past two decades [1]. Robust electrochemical energy conversion processes form key segment for the fruitful execution of sustainable energy sources [2]. Water-Splitting systems are essential for clean energy production. The oxygen reduction reaction (ORR) is a key reaction involved in water splitting, which requires a catalyst. In case of electrocatalysis, there exists significant performance difference between transition metal oxides and platinum based noble metal catalysts. To bridge this gap, it is pivotal to search for alternative electrocatalysts [3]. To date, platinum-based materials are the best ORR catalysts in acidic and alkaline media. But, the kinetics of these reactions are sluggish due to the four-electron transfer involvement, which can be enhanced by the application of suitable catalysts [4,5]. However, their high cost, inadequate resources, and insufficient durability restrain their widespread applications. Therefore, enormous research efforts have been focused towards the search for alternative cost-effective non-Pt catalysts.

Recent reports demonstrate the phosphate based materials could be very competent catalysts for OER and ORR. Very recently, sodium metal phosphates NaCoPO₄ and Na₂CoP₂O₇ were studied as electrocatalysts for ORR/OER [7]. 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. Recent report on OER catalytic performance of Mn₃(PO₄)₂ in neutral solution showed the positive effect of polyanion [8]. 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.

The current work explores the possible application of sodium and potassium iron phosphates (AFePO₄, A = Na, and K) as electrocatalysts for ORR activity. Also, the better bifunctional activity of the cobalt based phosphates namely NaCoPO₄ and Na₂CoP₂O₇. These iron and cobalt based phosphates were synthesized by solution combustion synthesis. The crystal structure was analyzed by Rietveld refinement. The formation of carbon coating was identified by Raman spectroscopy for AFePO₄ System. Electrocatalytic properties of AFePO_4, NaCoPO₄ and Na₂CoP₂O₇ were investigated in alkali electrolytes for the first time by using linear sweep voltammetry with rotating disk electrode (RDE). The oxygen reduction reaction (ORR) activities of these alkali iron phosphates are comparable to that of Pt/C system. Bifunctional activity of cobalt-based polyanionic phosphates, were basically functioning better than Vulcan carbon black and also can work as low-cost alternatives to Pt/C catalyst (Fig. 1). The Tafel slope and electron transfer number of the alkali iron phosphates were calculated. The ORR activity of NaFePO₄ was found to be better than KFePO₄ and FePO₄. In terms of NaCoPO₄ and Na₂CoP₂O₇ polyanionic phosphates, pyrophosphate have shown better activity compare to phosphate. This work demonstrates alkali iron, Cobalt phosphates and pyrophosphates as alternate cost-effective, novel electrocatalysts for productive ORR and Bi-Functional activity in alkaline solution.

Figure 1. ORR catalysis of KFePO₄ and NaFePO₄. (a) Cyclic voltammograms of the KFePO₄ and NaFePO₄ catalysts compared with the Pt/C and FePO₄ catalysts (b) ORR polarizations for both catalysts and the corresponding results for the 20% Pt/C and FePO₄ catalysts are displayed for comparison. The electrolyte was O₂ saturated 0.1 M KOH. Rotating speed: 1,600 r.p.m., scan rate: 10 mV s^-1

Acknowledgement

Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship.

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