Electrocatalytic Performance of Interfacially Synthesized Au-Polyindole Composite toward Formic Acid Oxidation Ashish Kumar, Leela Joshi, and Rajiv Prakash* School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi-221005, India ABSTRACT: The current study proposes a composite having a Au cluster embedded within polyindole flakes (Au@Pin) as a promising electrocatalyst in formic acid oxidation. The present work provides a detailed study of the Au@Pin composite and its catalytic properties compared to those of a Au commercial electrode for oxidation of formic acid and a possible mechanism. The role of a morphology controlled composite and interaction of Au in the cages of polymers flakes are discussed for effective catalytic action of the material. The electrocatalytic oxidation of formic acid is carried out over a Au@Pin composite modified glassy carbon (GC) electrode. Voltammetric and chronoamperometric measurements show that a Au@Pin composite has better CO tolerance capability than Pin modified, bare GC and Au commercial electrodes. Electrochemical impedance spectroscopy (EIS) reveals consistent results and charge transfer mainly through a diffusion controlled process. On the basis of this EIS data, an equivalent electrical circuit is proposed. The higher catalytic activity of a Au@Pin composite toward formic acid oxidation in a mixture of 1.0 M HCOOH + 0.5 M H 2 SO 4 electrolyte is observed in comparison to Pin modified and Au commercial electrodes due to the synergic effect between the Au cluster and Pin. 1. INTRODUCTION Two major drawbacks of methanol as fuel in direct methanol fuel cells limit its usage in fuel cell applications. These drawbacks are (a) sluggish methanol oxidation over the anode and (b) methanol crossover through the polymer membrane. 1,2 Therefore, formic acid (FA) as fuel has been replacing methanol nowadays, because of its environmentally friendly nature and low costs in its diluted form. 3-5 It has been reported that the electrocatalytic oxidation of FA proceeds through three mechanisms. Initially it starts with adsorption of formic acid first; then these adsorbents either directly oxidize to carbon dioxide (called the direct pathway) or dehydrogenate to a bridge- adsorbed formate (called the formate pathway) or undergo dehydration to adsorbed carbon monoxide (called the indirect pathway) as shown earlier. 6 The electrocatalytic performances of FA have been catalyzed by various catalysts. Among these metal based composite materials are state-of-the-art catalysts for promoting the oxidation of FA in electrochemical devices. In this context, several groups are studying the mechanisms of oxidation 7-11 using bimetals (e.g., Cu/Pd, Co/Pd, Pt/Cu, Pd/Au, etc.), 12-15 metal supported carbon (fiber or sheet), 16-18 and metal/ polymer composites (e.g., Pd/polypyrrole, Au/polyaniline, Pt/ polyindole, etc.) supported with carbon as electrode materi- als. 7,11,19 Mainly, their aim is to use these kinds of catalyst materials as anode in direct formic acid fuel cell application. In view of size, generally nanometer scale metallic clusters have attracted considerable attention from past years. This is due to the superior functional properties of nanometallic clusters even in their small amount compared to bulk form in various fields such as catalysis, electronic application, or sensors. 20-24 Among various nanometallics, nano-gold cluster are one of the most inert elements used for heterogeneous catalyst both in industries as well as in laboratories. However, the rising price of gold has aroused a critical problem of cost prior to their large scale usage. So dispersion of these clusters with a conducting matrix is one of the choices to reduce this issue. Heterocyclic conducting polymers are an excellent conduct- ing matrix for the dispersion purpose of nanometallics because of good stability and participation of the heteroatom during redox performance as well as capability to hold nanoclusters within the matrix. 25,26 Among heterocyclic conducting polymers, polyindole (Pin) has attracted considerable interest due to the combinational property of both poly(p-phenylene) and polypyrrole together with fairly good thermal stability, redox property, slow degradation, and better air stability. 27-29 Up to now, the nanocomposite of gold with polymers such as polypyrrole, polyaniline, polythiophene, polycarbazole, and polyindole has been synthesized by chemical and electro- chemical techniques. 30-36 In these systems gold acts as a promoter for various applications such as catalysis, sensor, memory devices, and electronic applications due to a synergic effect between constituents of the nanocomposite. 37-40 However, there is no work reporting on the electro-oxidation of FA on a Au@Pin composite. In view of the above, we tried to explain the electrocatalytic FA oxidation by a Au@Pin nanocomposite and compared with Pin alone (formed by the interfacial polymerization method) and a Au commercial electrode. The experimentations were done on Au@Pin modified GC and Pin modified GC electrodes using differential pulse voltammetry (DPV), cyclic voltammetry (CV), electro- chemical impedance spectroscopy (EIS), and chronoamper- ometry (CA). Prior to these analyses, the Au@Pin composite was thoroughly characterized by spectroscopy (XRD), X-ray photon spectroscopy (XPS), scanning electron microscopy Received: March 21, 2013 Revised: June 2, 2013 Accepted: June 11, 2013 Published: June 11, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 9374 dx.doi.org/10.1021/ie400915s | Ind. Eng. Chem. Res. 2013, 52, 9374-9380