Gold nanorod-polyaniline composites: Synthesis and evaluation as anode electrocatalysts for direct borohydride fuel cells Jadranka Miliki c a , Una Stamenovi c b , Vesna Vodnik b , Scott P. Ahrenkiel c , Biljana Sljuki c a, * a Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12e16, 11158, Belgrade, Serbia b Vinca Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522,11001, Belgrade, Serbia c South Dakota School of Mines and Technology, 501 E. Saint Joseph St., Rapid City, SD, 57701, USA article info Article history: Received 15 July 2019 Received in revised form 17 October 2019 Accepted 18 October 2019 Available online 22 October 2019 Keywords: Borohydride oxidation reaction Direct borohydride fuel cell Gold Polyaniline Nanostructures abstract Two gold nanorod-polyaniline (Au-PANI) composites with different contents of Au were prepared by two methods. An ex situ method, in the presence of preformed gold nanorods (AuNRs) and in situ one, when an AuNRs and PANI matrix is produced simultaneously, were used. Both methods were performed in immiscible water/toluene biphasic system as a simple interfacial polymerization process. Optical, structural and morphological characteristics of the formed nanocomposites were identified. It was found that AuNRs are embedded in the conducting emeraldine salt form of PANI. Nanocomposites containing 2.0 and 28.9 wt% of Au were subsequently systematically studied for borohydride oxidation reaction (BOR) for potential application in direct borohydride-peroxide fuel cell (DBPFC). Reaction parameters: number of electrons exchanged, order of reaction and activation energy, were evaluated. Both Au-PANI nanocomposites showed activity for BOR. A laboratory DBPFC was tested reaching specific peak power density of 184 Wg -1 at 65 C with Au-PANI 1 nanocomposite (containing only 2.0 wt% of Au) as anode. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Ongoing world’s energy crisis established search for renewable energy sources as one of the imperatives of the modern society. Furthermore, due to the ozone layer depletion and consequent human health and environment endangerment, these sources must also be environmentally friendly and unthreatening (benign) to human health. Sodium borohydride (NaBH 4 ) has been indicated as alternative fuel to be used in direct borohydride fuel cells (DBFCs) as energy sources fulfilling the mentioned requirements [1]. DBFCs are low-temperature fuel cells, which strictly operate with highly alkaline anolyte and with oxygen (O 2 ) or hydrogen peroxide (H 2 O 2 ) (direct borohydride peroxide fuel cells, DBPFCs) as oxidant in acidic or alkaline catholyte [2]. DBPFC/DBFCs offer several benefits as they operate at high theoretical voltage of 3.01 V (acidic catholyte) and 1.64 V (due to the low thermodynamic potential of the BOR of 1.24 V vs SHE), with high theoretical power density of 17 and 9.3 kW h kg 1 in case of DBPFC and DBFC, respectively [3]. Borohydride (BH 4 ) oxidation reaction (BOR) in highly alkaline media (pH 14) theoretically occurs by exchange of 8 e (Eq. (1))[4]. BH 4 þ 8 OH / BO 2 þ 6H 2 O þ 8e E 0 ¼ 1.24 V vs. SHE (1) The incomplete (partial) oxidation of BH 4 proceeding with ex- change of only 4 e is also possible (Eq. (2)). BH 4 þ 4 OH / BO 2 þ2H 2 þ 2H 2 O þ 4e (2) Efficiency of BOR is further reduced by BH 4 ions hydrolysis (Eq. (3)) occurring simultaneously with their oxidation and by their crossover to the cathode part [5]. BH 4 þ H 2 O / BH 3 OH þ H 2 (3) Efficiency of DBFC/DBPFC depends on the pH of the anolyte solution, concentration of NaBH 4 , operating temperature as well as on the electrodes material [6e13]. The overall BOR depending on the anode materials can be represented by Eq. (4). BH 4 þ x OH / B(OH) 4 - þ (x - 4) H 2 O þ (4 e x/2) H 2 þ xe (4) * Corresponding author. E-mail address: biljka@ffh.bg.ac.rs (B. Sljukic). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2019.135115 0013-4686/© 2019 Elsevier Ltd. All rights reserved. Electrochimica Acta 328 (2019) 135115