Soluble sulfonated polyaniline as an aqueous catholyte for battery applications

Although polyaniline is an affordable redox-active polymer with potential applications in energy storage, its extreme low solubility in most solvents prevents it from being used in redox flow batteries (RFBs). This work reports the synthesis of water-soluble sulfonated polyaniline, poly-N-(3-sulfopropyl)aniline (PSPA), for its application as a catholyte. PSPA exhibits a solubility of 0.15 M in 1.2 M HCl (theoretical capacity of 2010 mAh·L−1). Its diffusion coefficient (1.7 × 10−6 cm2 s−1) and redox potential (0.56 V vs. Ag/AgCl) make it suitable as a catholyte. An all-aqueous battery comprising a PSPA catholyte and an anthraquinone-2-sulfonic-acid anolyte produced a voltage of 0.53 V and displayed coulombic efficiencies of 55% and 73% at 1C and 2C, respectively. The cell capacity was stable for 10 (1C) and 25 (2C) cycles. Our findings relate the capacity fading to the hydrolysis of oxidized PSPA to p-benzoquinone and the subsequent permeation of this degradation product through the membrane. In summary, this work explores a water-soluble polyaniline as a catholyte with promising electrochemical features regarding its application in redox flow batteries. Also, it identifies the stability of the pernigraniline form as the main challenge that must be addressed and discusses strategies to unleash its potential as an affordable material for RFB. [Display omitted] •PSPA, a water-soluble PANi, is proposed as a low-cost RFB catholyte.•PSPA solubility, redox potential, and diffusion coefficient are reported.•An all-aqueous PSPA/sulfonated anthraquinone cell (ΔE = 0.53 V) was assembled.•Coulombic efficiency of 73% is obtained at 2C rate.•Capacity loss after 25 cycles was ascribed to the hydrolysis of oxidated PSPA.