Pyrophosphate Na₂CoP₂O₇ Polymorphs as Efficient Bifunctional Oxygen Electrocatalysts for Zinc–Air Batteries

Abstract

Developing earth-abundant low-cost bifunctional oxygen electrocatalysts is a key approach to realizing efficient energy storage and conversion. By exploring Co-based sodium battery materials, here we have unveiled nanostructured pyrophosphate Na2CoP2O7 polymorphs displaying efficient bifunctional electrocatalytic activity. While the orthorhombic polymorph (o-NCPy) has superior oxygen evolution reaction (OER) activity, the triclinic polymorph (t-NCPy) delivers better oxygen reduction reaction (ORR) activity. Simply by tuning the annealing condition, these pyrophosphate polymorphs can be easily prepared at temperatures as low as 500 °C. The electrocatalytic activity is rooted in the Co redox center with the (100) active surface and stable structural framework as per ab initio calculations. It marks the first case of phospho-anionic systems with both polymorphs showing stable bifunctional activity with low combined overpotential (ca. ∼0.7 V) comparable to that of reported state-of-the-art catalysts. These nanoscale cobalt pyrophosphates can be implemented in rechargeable zinc–air batteries.

Phosphate-based polyanionic insertion materials for oxygen electrocatalysis

“…5a) possessing more possibility of spatial rearrangements, CoO 6 which is edge shared with the P 2 O 7 unit in t-Na 2 CoP 2 O 7 showed a higher exchange current density. 75 Furthermore, from Fig. 5b, the OER activity results conducted at pH B 13 showed current densities of 2.79, 3.20 63 However, in 0.1 KOH, the current density at 1.7 V and onset potentials are quite comparable as mentioned above, while the onset potential for o-Na 2 CoP 2 O 7 outperforms at a more basic alkaline pH.…”

“…Such catalysts are enlisted with the potential (written next to the material) at a current density of À3.0 mA cm À2 vs. the respective 3d-electron in T M . Based on this potential, onset potential and Tafel slope from Tables 1 and 2, the decrease in ORR activity trend is shown here with potential in V (vs. RHE), at À3.0 mA cm À2 in brackets: 75 KCo(PO 3 ) 3 , 79 K 2 CoP 2 O 7 . 87 The electrocatalytic activity of the aforementioned catalysts as well as the spin state of the catalytic transition metal centre are detailed in Table 2.…”

“…The metal-air battery performance achieved by selected phosphate-based insertion materials as air cathodes is summarised in Table 3, where round trip efficiency was calculated from charge-discharge voltage plateaus. 65,66,75,87 The ratio of the discharge to charge voltages at the end of the cycle gives the round-trip efficiency. At current densities of 10, 1.0, 4, and 1.7 mA cm This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2024…”

“…3b. 75,76 Acting as insertion materials for the sodium-ion batteries, o -Na 2 CoP 2 O 7 , t -Na 2 CoP 2 O 7 , and β-Na 2 MnP 2 O 7 delivered a specific discharge capacity of ∼80 mA h g −1 at 3.0 V, 4.3 V, and 3.6 V, respectively. In the case of Na 2 CoP 2 O 7 , the trend for ORR exchange current density was o -Na 2 CoP 2 O 7 < t -Na 2 CoP 2 O 7 , while for OER the trend was opposite; t -Na 2 CoP 2 O 7 < o -Na 2 CoP 2 O 7 .…”

“…9,62 Data symbols are as follows: brown triangles for Mn, blue triangles for Co, red triangles for mixed transition metal, and green open circles for Fe-based phosphate-containing electrocatalysts. The data presented are replotted and were originally reported as: LiCo 0.60 Fe 0.40 PO 4 , 68 LiNi 0.75 Fe 0.25 PO 4 /rGO, 67 NaCoPO 4 , 63 NaFe(PO 3 ) 3 , 78 NaCo(PO 3 ) 3 , 78 NaCo 4 (PO 4 ) 3 , 98 Na 2 FePO 4 F, 83 Na 2 CoPO 4 F, 83 b-Na 2 MnP 2 O 7 , 76 o-Na 2 CoP 2 O 7 , 75 t-Na 2 CoP 2 O 7 ,75 KCo(PO 3 ) 3 , 79 K 2 CoP 2 O 7 87. The electrocatalytic activity of the aforementioned catalysts as well as the spin state of the catalytic transition metal centre are detailed in Table2.…”