**Performance Enhancement of Vanadium Redox Flow Battery by Treated Carbon Felt Electrodes of Polyacrylonitrile Using Atmospheric Pressure Plasma**

#### **Chien-Hong Lin 1,\*, Yu-De Zhuang <sup>1</sup> , Ding-Guey Tsai <sup>1</sup> , Hwa-Jou Wei <sup>1</sup> and Ting-Yu Liu 2,\***


Received: 31 May 2020; Accepted: 16 June 2020; Published: 18 June 2020

**Abstract:** A high-performance carbon felt electrode for all-vanadium redox flow battery (VRFB) systems is prepared via low-temperature atmospheric pressure plasma treatment in air to improve the hydrophilicity and surface area of bare carbon felt of polyacrylonitrile and increase the contact potential between vanadium ions, so as to reduce the overpotential generated by the electrochemical reaction gap. Brunauer-Emmett-Teller (BET) surface area of the modified carbon felt is, significantly, five times higher than that of the pristine felt. The modified carbon felt exhibits higher energy efficiency (EE) and voltage efficiency (VE) in a single cell VRFB test at the constant current density of 160 mA cm−<sup>2</sup> , and also maintains good performance at low temperatures. Moreover, the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis results show that the resistance between electrolyte and carbon felt electrode decreased. As a result, owing to the increased reactivity of the vanadium ion on the treated carbon felt, the efficiency of the VRFB with the plasma-modified carbon felt is much higher and demonstrates better capacity under a 100-cycle constant current charge-discharge test.

**Keywords:** vanadium redox flow battery; carbon felt; atmospheric plasma; polyacrylonitrile
