Journal of The Electrochemical Society, 159 (2) B195-B200 (2012) B195 0013-4651/2012/159(2)/B195/6/$28.00 © The Electrochemical Society Synthesis, Characterization and Catalytic Activity of Binary PtMn/C Alloy Catalysts towards Ethanol Oxidation Malika Ammam, z Laura E. Prest, Allen D. Pauric, and E. Bradley Easton *, z Faculty of Science, University of Ontario Institute of Technology, Oshawa ON L1H 7K4, Canada Binary PtMn/C alloy catalysts with various atomic ratios (60–90 atomic% Mn) were synthesized and tested for the activity toward ethanol oxidation. The synthesized catalysts were characterized by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD studies indicate that a Pt-Mn alloy has formed in all cases. PtMn(19:81)/C and PtMn(10:90)/C exhibited the highest activity toward ethanol oxidation compared to PtMn(39:61)/C and Pt/C. XPS analysis indicates that highest activity occurs for alloys with similar surface and bulk Mn contents. © 2011 The Electrochemical Society. [DOI: 10.1149/2.095202jes] All rights reserved. Manuscript submitted August 15, 2011; revised manuscript received November 16, 2011. Published December 21, 2011. Developing sustainable energy technologies is of great interests to reduce dependence on fossil fuels and addressing environmental issues as well as global climate changes. Fuel cells that convert chem- icals into electricity are widely studied as new eco-friendly power sources because their by-products are water or carbon dioxide. There are various kinds of fuel cells such as polymer electrolyte membrane fuel cell, phosphoric acid fuel cell, alkaline fuel cell, and solid oxide fuel cell. Among these fuel cells, a direct alcohol fuel cell (DAFC), which uses alcohols such as methanol and ethanol as anode fuels and oxygen as a cathode fuel, provides a special advantage of ease of liq- uid fuel handling. Because of this, DAFCs are considered a promising candidate for portable power applications, such as notebook comput- ers, cellular phones and electrical vehicles. 1–3 Ethanol is a hydrogen-rich liquid with a very high energy den- sity (8 kWh/kg) and low toxicity. It can be obtained from biomass through a fermentation process of renewable resources such as sugar cane, wheat, corn, or even straw. It is thus an attractive and ideal fuel for fuel cells. 4–13 Platinum is known as the best electrocatalyst for alcohol oxidation at low temperatures. However, Pt is easily poi- soned by strongly adsorbed species such as CO, which are formed by the initial dehydrogenation of the alcohol molecules. 14 In order to reduce the cost and enhance the activity, transition metals (Ru, Sn, Mo, Rh or Pb) are usually added as co-catalysts to form alloys. 15–20 Several binary Pt-based alloys systems have been explored as ethanol oxidation catalysts. In regard to this, PtSn and PtRu with optimized compositions and structures have been reported to exhibit an enhanced activity compared to other catalysts. 21–26 Jeon et al, have studied Pt- Ru-M ternary alloys (M = Mn, Mo) and have suggested that Mn has a similar surface segregation properties with that of Ru. 27 Recently, Xu et al 28 reported that electrodeposited Pt-Mn catalysts that contain only trace amounts of Mn (≤0.1 atomic% Mn) exhibited enhanced activity toward methanol and ethanol oxidation in alkaline solution. Here we report the variation in catalytic activity toward the ethanol oxidation reaction in acidic media with Mn content for a series of carbon-supported Pt-Mn alloy catalysts prepared by the chemical re- duction of pre-cursor salts. X-ray diffraction and X-ray photoelectron spectroscopy were used to monitor how the catalyst structure and sur- face composition changed with Mn content. The activity of the alloys catalysts toward ethanol oxidation is found to exhibit an improved activity compared to Pt/C and is explained in terms of alloy formation and surface composition. Experimental Catalyst synthesis.— PtMn/C with various molar ratios were synthesized following the route: H 2 PtCl 6 .6H 2 O (Aldrich) and MnCl 2 .2H 2 O (Aldrich) at the desired molar ratios are dissolved in ultrapure water (milliQ, 18.2 Mcm). After 15 min of constant stir- ∗ Electrochemical Society Active Member. z E-mail: Brad.Easton@uoit.ca; m78ammam@yahoo.fr ring Vulcan XC72R carbon black (Cabot) was added to the solution in an amount to give a total metal content of 20 wt%. PtMn nanoparticles supported on carbon were formed by reduction of the metal precursors with NaBH 4 , which was added as a solid to the mixture in a weight ra- tio of 3:1 to metals. The resulting mixture was then left under constant stirring over night and the formed supported catalysts were collected via suction filtration, washed thoroughly with ultrapure water, ethanol and acetone and finally dried over night at 80 ◦ C. Materials characterization.— Energy Dispersive X-ray Spec- troscopy (EDX) was used for a rapid, non-destructive determination of catalyst composition. EDX spectra were acquired using a JEOL JSM-7000F field emission scanning electron microscope equipped with an Oxford Systems INCA X-ray microanalyser. Inductively Coupled Plasma Optical Emission Spectrometry (ICP- OES) was utilized for quantitative determination of metal content in the catalysts. 5 mg of each catalyst was dissolved in nitric acid (>70%) and left to dissolve for at least 1 week. Afterwards, the solutions were filtered off to separate the supporting carbon from the solution and yield a clear solution for ICP-OES analyzes. Standardization was performed with three Pt and Mn solutions ranging from approximately 1–20 ppm. These standards contained approximately 2% nitric acid to ensure the complete dissolution and keep both sample and standard matrices equivalent. Power X-ray diffraction (XRD) patterns were obtained using Bruker D8 Advance powder X-ray diffractometer, with germanium monochrometer, Cu K α1 radiation. The average grain size was deter- mined from the broadening of the Pt(111) peak using the Scherrer equation. X-ray photoelectron spectroscopy (XPS) was carried out by Thermo Instruments 310-F Microlab with a monochromatic Mg K α X-ray source. Samples for XPS analyzes were first dispersed a mixture of ethanol and ultrapure water (50:50) and subsequently deposited on silicon wafer. Samples we allowed to dry for 10 minutes before they were introduced into the instrument. Transmission Electron Microscopy (TEM) images were acquired using a Philips CM 10 instrument equipped with an AMT digital camera system. Samples for TEM analysis were dispersed in ultra- pure water and applied to nickel 400 mesh formvar coated carbon reinforced grids and allowed to dry under air before they were intro- duced in the chamber. Electrochemical characterization.— The electrocatalytic activity of the catalysts toward ethanol oxidation was measured through the preparation of electrode inks, which were prepared as follows: 11 mg of the synthesized electrocatalyst was dispersed in 500 μL of a mixture of ultrapure water and 2-propanol (1:1 by volume) and the suspension was stirred in an ultrasonic bath for 15 min. 5 μL of the catalysts ink was immobilized onto the surface of a glassy carbon (GC) electrode (daim. = 3 mm, CH Instruments) and dried at 80 ◦ C Downloaded 27 Dec 2011 to 205.211.181.105. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp