Effect of mixed additives on lead–acid battery electrolyte Arup Bhattacharya, Indra Narayan Basumallick * Electrochemical Laboratory, Department of Chemistry, University of Visva-Bharati, Santiniketan 731235, India Abstract This paper describes the corrosion behaviour of the positive and negative electrodes of a lead–acid battery in 5 M H 2 SO 4 with binary additives such as mixtures of phosphoric acid and boric acid, phosphoric acid and tin sulphate, and phosphoric acid and picric acid. The effect of these additives is examined from the Tafel polarisation curves, double layer capacitance and percentage of inhibition efficiency. A lead salt battery has been fabricated replacing the binary mixture with an alternative electrolyte and the above electrochemical parameters have been evaluated for this lead salt battery. The results are explained in terms of H þ ion transport and the morphological change of the PbSO 4 layer. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Corrosion; Picric acid; Phosphoric acid; Boric acid; Tin sulphate; Lead–acid battery 1. Introduction During the last two decades the lead–acid battery has been widely used in battery driven vehicles and for storing electrical energy from non-conventional sources. In spite of rapid improvement in its performance and design, there remain some problems of the battery which are yet to be solved. These problems have drawn the attention of the battery scientists which has resulted in an annual pub- lication of more than 150 papers in the scientific journals and a good number of patents. The use of additives in the electrolyte is one of the approaches which offers improvement of the battery without much alteration of other factors. The major problem lies with selecting a suitable additive which is chemically, thermally and electrochemically stable in highly corrosive environment. Among the additives used so far the most widely investigated is H 3 PO 4 [1,2] which has been reported as a beneficial additive in terms of improving cycle life, decreasing self discharge and increasing the oxygen over potential on the positive electrode. Among the other addi- tives, H 3 BO 3 [3] and SnSO 4 [4] are also prominent. In the present research, an attempt has been made to use a mixture of additives (instead of single additive as studied earlier) to the electrolyte and to examine the performances of the electrode and the battery in the presence of these additives. The mixed additives used in the present study are: (a) H 3 PO 4 and H 3 BO 3 , (b) H 3 PO 4 and SnSO 4 , (c) H 3 PO 4 and picric acid (C 6 H 3 N 3 O 7 ). It is expected that these additives will improve the electrochemical behaviour of the individual electrodes and the battery as a whole. In this study, a lead salt battery is also investigated. In three different types of lead salt battery we used: (i) (NH 4 ) 2 SO 4 alone, (ii) hydrogel (agar agar) with (NH 4 ) 2 SO 4 , and (iii) U-foam soaked with (NH 4 ) 2 SO 4 instead of 5 M H 2 SO 4 as electrolyte. 2. Experimental The electrochemical performance of the electrodes and the electrolyte (5 M H 2 SO 4 , as blank), with and without mixed additives, has been examined from Open Circuit Potential (OCP) data, and polarisation, cyclovoltammetric and galvanotransient studies. These studies have been car- ried out using conventional techniques with a potentiostat/ galvanostat (Vibrant, Model VSMCS 30, Lab India) and a multimeter. The detailed experimental set-up has been described in our earlier paper [5]. In all these studies a Hg/Hg 2 SO 4 reference electrode in H 2 SO 4 of the same molarity (5 M) and a Pt foil counter electrode are used. The working electrode was either pure Pb (99.28% pure, Johnson Mathey) or PbO 2 (electrochemically prepared by anodic oxidation using standard techniques). 3. Results and discussions Many reports have been published on the use of H 3 PO 4 as additive to the electrolyte. In our study with mixed additives Journal of Power Sources 113 (2003) 382–387 * Corresponding author. 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00552-9