Revisiting the Structure and Electrochemical Performance of Poly(o-phenylenediamine) as an Organic Cathode Material

Poly(o-phenylenediamine) (PoPDA) has been recognized as a low-cost electroactive organic material and studied as a cathode for aqueous zinc batteries or as an anode for nonaqueous lithium batteries. However, there remains a lot of confusion about its synthesis, structure, and electrochemical application. Especially, the previously studied PoPDA samples were mostly synthesized at room temperature, which were proved by us to be just a dimer, that is, 2,3-diaminophenazine (DAPZ). By various characterization methods including elemental analysis and mass spectrometry, we verified that the product synthesized at high temperature, PoPDA-H, was a polymer based on DAPZ as the structural repeat unit and with some imperfect substitutes (OH and NH₃⁺CH₃COO^-). Based on the reversible redox reaction of phenazine units and the stable polymer structure within 1.3-3.8 V vs Li⁺/Li, PoPDA-H was more appropriate to be applied as a cathode rather than as an anode for lithium batteries. It achieved a high energy density of 490 Wh kg^-1 (2.12 V × 231 mAh g^-1) at 50 mA g^-1 and a high cycling stability (79%@1000th cycle) at 500 mA g^-1, both of which were comparable to previously reported expensive pyrazine- and carbonyl-based polymers. This work clarifies many misunderstandings of PoPDA, which is important to its further development toward practical application in energy-storage devices.