International Journal of Energy Science (IJES) Volume 3 Issue 3, June 2013 www.ijesci.org 165 Performance Improvement of the All‐Lead Redox Flow Battery in Fluoroboric Acid Electrolyte Jie Cheng *2 , Chen Gao 1, 2 , Yue‐Hua Wen 2 , Jun‐Qing Pan 1 , Yan Xu 2 , Gao‐Ping Cao 2 1 College of Science, Beijing University of Chemical Technology, Beijing 100029, China 2 Research Institute of Chemical Defence, Beijing 100191, China *1 chengjie_chj@126.com; 2 gaochen010@sina.com Abstract The performance of the all‐lead redox flow battery was enhanced by using stainless steel as the negative electrode and tantalum carbide (TaC) as the positive electrode in an intermixture of Pb(BF4)2–HBF4 aqueous electrolyte. The results of cyclic voltammetry (CV) measurements show that stainless steel and TaC electrodes are capable of offering higher voltage efficiency than graphite electrodes at a lower scan rate. In the charge–discharge cycles, the battery was charged at current densities of 10, 20 or 40 mA·cm –2 with a charge capacity of 7.0 mAh·cm –2 and discharged down to 1.0 V at the same current density. The miniature battery offers an average discharge voltage of 1.55 V, an average coulombic efficiency of above 96%, and an energy efficiency of above 65%. The battery in 1.5 mol·L –1 of Pb(BF4)2 + 1.0 mol·L –1 of HBF4 aqueous solution is able to deliver an average energy efficiency of above 80% at the current density of 10 mA·cm –2 . Keywords All‐lead Redox Flow Battery; Tac Electrode; Lead Fluoroborate; Fluoroboric Acid Introduction In recent years, several new power storage systems have been proposed such as supercapacitors, lead– acid batteries, and the sodium–sulphur fused batteries [1, 2] . Redox flow battery is labeled as the most ideal one among the new storage systems in virtue of their special advantages [3] . A number of redox flow batteries have been fabricated, such as Cr/Fe [4–6] ,V [7, 8] , Zn/Br2 [9, 10] , polysulfide/Br2 [11] , etc. Most of the batteries require the employment of ion‐exchange membranes. However, such membranes are expensive, leading to an increase in the complexity and cost of the cell, while simultaneously introducing cross‐contamination of reactive species through the ion‐exchange membrane [12, 13] . Pletcher proposed a lead–acid redox flow battery based on the Pb 2+ /Pb and PbO2/Pb 2+ couples in methanesulfonic acid where the lead methanesulfonate is highly soluble and there is no requirement for a membrane [12–19] ; this reduces the cost of the batteries significantly. However, it is a pity that the energy efficiency is still low, only around 65% ( ~20 mAcm –2 ) [13] . On the basis of the literatures [12–19] , a novel all‐lead redox flow battery employing graphite as both positive and negative electrodes in a fluoroboric acid aqueous solution is proposed [20] , which offers an energy efficiency of above 74% ( 10~20 mAcm –2 ). In this paper, stainless steel as the negative electrode and tantalum carbide (TaC) as the positive electrode for the all‐lead redox flow battery are compared with graphite electrodes in the intermixture of HBF4– Pb(BF4)2 aqueous electrolyte, and the performance of the battery is improved. The miniature battery in 1.5 mol·L –1 Pb(BF4)2+1 mol·L –1 HBF4 can offer a maximum energy efficiency of above 80.2% at the current density of 10 mAcm –2 . Experimental All solutions were prepared with distilled water. Chemicals of analytical grade purity were used. Fluoroboric acid (40 wt%) and lead (II) oxide (99.0%) were provided by Xilong Chemistry Engineering Corporation. The lead fluoroborate was formed by using the reaction between lead (II) oxide and fluoroboric acid. In the electrolytes, the concentration of the lead (II) oxide was 0.1 mol·L –1 , 0.5 mol·L –1 , 1.0 mol·L –1 or 1.5 mol·L –1 , and the concentration of free HBF4 was kept as 1.0 mol·L –1 . The TaC was prepared as follows. The phenol‐ formaldehyde resin used here was novolac type