ARTICLE pubs.acs.org/JPCC
Graphene-Supported PtAu Alloy Nanoparticles: A Highly Efficient Anode for Direct Formic Acid Fuel Cells Chitturi Venkateswara Rao,* Carlos R. Cabrera, and Yasuyuki Ishikawa* Department of Chemistry and the Chemical Physics Program, University of Puerto Rico, P.O. Box 23346, San Juan, Puerto Rico 00931-3346, United States
bS Supporting Information ABSTRACT:
Graphene-supported Pt and PtAu alloy electrocatalysts are prepared by ethylene glycol reduction method and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). XRD reveals the face-centered cubic structure of Pt in the materials. SEM and TEM images show the good spatial distribution of metal nanoparticles on layered graphene sheets. EDX reveals that the average composition of elements in the PtAu alloy catalyst is approximately 1:1. Electrocatalytic performance of the prepared materials toward formic acid oxidation (FAO) is investigated using cyclic voltammetry. FAO activity of the PtAu/graphene is found to be ten times higher than that of Pt/graphene. The prepared electrocatalysts are used as anode in a direct formic acid fuel cell and tested at 303 and 333 K. An increase in the performance with increasing temperature is observed. A maximum power density of 185, 70, and 53 mW/cm2 is observed with PtAu/graphene, Pt/graphene, and commercial Pt/C anodes, respectively, at 333 K. The high electrocatalytic performance of PtAu/graphene is attributed to the change in the electronic structure of Pt by the presence of alloying element, Au.
1. INTRODUCTION Direct formic acid fuel cells (DFAFCs) are considered as one of the promising energy sources for stationary and portable electronic device applications.14 These fuel cells are characterized by the oxidation of formic acid at the anode and reduction of oxygen at the cathode according to the following equations: Anode : HCOOH f CO2 þ 2Hþ þ 2e ; E° ¼ 0:25 V vs NHE 1 Cathode : O2 þ 2Hþ þ 2e f H2 O; 2 E° ¼ þ 1:23 V vs NHE 1 O2 f CO2 þ H2 O; 2 OCV ¼ þ 1:48 V
(1.18 V) fuel cells.4,5 Hence, it is desired to achieve high power density under ideal conditions. However, the fuel cell performance and efficiency are hindered due to various limiting factors under the operating conditions.6 One of the issues is the insufficient activity and durability of electrocatalyst used for formic acid oxidation at the anode. Among the investigated materials, PtAu bimetallic catalysts seem to be effective in promoting the adsorption/dissociation of formic acid in acid media. Several approaches such as the optimization in size, shape, structure and composition, different preparation methods, and use of new carbon supports have been attempted to tailor PtAu electrocatalysts to improve their activity and durability.719 The results indicate that the electrochemical properties of PtAu are much affected by the preparation method as well as the nature of supporting carbon material.
Overall : HCOOH þ
The theoretical open circuit potential of DFAFC (1.48 V) is higher than hydrogenoxygen (1.23 V) and methanoloxygen r 2011 American Chemical Society
Received: March 17, 2011 Revised: September 22, 2011 Published: September 30, 2011 21963
dx.doi.org/10.1021/jp202561n | J. Phys. Chem. C 2011, 115, 21963–21970
The Journal of Physical Chemistry C In recent years, graphene has received tremendous scientific and technological interests with potential applications in various fields such as electronic devices, energy storage and conversion devices, solar cells, biosciences, and biotechnologies.20,21 The unique properties of graphene, especially good electronic conductivity, surface area, mechanical and/or thermal stability, and durability makes it a promising electrode/electrode support material for fuel cell applications.21,22 Several investigations have been carried out to produce graphene nanosheets in bulk quantity by chemical reduction of graphite oxide (GOx) in solution.2326 Since abundant functional groups on the surfaces of GOx can be used as anchoring sites for metal nanoparticles,27 it is possible to use them as a support to produce graphenenanoparticle hybrids. Graphene has been used as a support for metal nanoparticles and its activity and durability toward various electrochemical reactions have been investigated.28,29,22 The results indicate more enhanced performance of graphene supported metal electrodes than that of amorphous carbon-supported electrodes. The enhanced performance is attributed to the intrinsic graphitization degree of graphene and the enhanced metalsupport interaction. On the basis of the above aspects, the combination of PtAu alloy nanoparticles and graphene as an electrode support may exhibit high power density in DFAFCs. Recently, Sheng Zhang et al.17 reported the FAO activity of graphene-supported PtAu alloy nanoparticles (20 wt.%), prepared by a NaBH4 reduction method. However, the fuel cell device performance was not studied. In the present work, graphene-supported PtAu alloy nanoparticles with 40 wt.% metal loading are prepared by ethylene glycol (EG) reduction method and the anodic performance of PtAu/graphene is compared with commercial Pt/C (E-TEK) under DFAFC operating conditions.
2. EXPERIMENTAL SECTION 2.1. Preparation of Graphite Oxide (GOx). GOx was synthesized from graphite powder by a modified Hummers method.30 The composition (wt.%) was determined to be C: 61.3, O: 35.7, H: 2.4, and N:
