Propane Fuel Cells Using Phosphoric-Acid-Doped Polybenzimidazole Membranes Chin Kui Cheng, Jing Li Luo,* Karl T. Chuang, and Alan R. Sanger Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed: December 27, 2004; In Final Form: May 4, 2005 Propane fuel cells using H 3 PO 4 -doped polybenzimidazole polymer membranes produce low and unsustainable current densities at temperatures up to 250 °C under anhydrous conditions. Stable intermediate species blocked the surface of noble metal anode catalysts, and the intermediate species could not react further into desorbable final products. In contrast, when water was introduced by light humidification (S r 0.08%) of the propane stream, sustainable and higher current densities were achieved. Water participated in the reaction sequence to form surface-bound hydrocarbon and then oxygen-containing intermediates and thereby generated CO and CO 2 as the only carbon-containing products. Background Conventionally, the chemical energy of hydrocarbon fuels is exploited as heat through combustion to CO 2 and H 2 O. Combustion of propane yields 2044.0 kJ mol -1 at standard temperature and pressure conditions. 1 Electrical energy can be generated by either deep or partial oxidation of hydrocarbons in a fuel cell. Hydrocarbon fuel cells offer several advantages when compared to conventional heat engines, in particular, efficient recovery of high-grade electrical energy rather than heat and better control of reaction rate and product selectivity. 2 For fuel cell applications, hydrocarbon fuels can be reformed to H 2 and CO 2 , either in a prereactor or within the anode chamber, after which the H 2 is consumed in a H 2 -O 2 fuel cell. 3,4 Alternatively, the fuel can be oxidized directly to CO 2 and H 2 O by reaction with oxide ions at the anode of an oxide-ion- conducting membrane electrode assembly, for example, in a solid-oxide fuel cell (SOFC). The objective of this study is to convert propane to propylene and electrical power. To avoid deep oxidation of propane, it is necessary to use proton-conducting membranes. This type of membrane will allow the partial oxidation of propane to propylene at the anode without risk of further oxidation to CO 2 . High-Molecular-Weight Polybenzimidazole-H 3 PO 4 Proton- Conducting Membranes. We have shown that membranes prepared by doping high-molecular-weight (MW) polybenz- imidazole (PBI) with 500-600 mol % phosphoric acid are thermally very stable and have low gas permeability. 5 The membranes have good ionic conductivity (up to 0.032 S cm -1 ) that was stable during operation of H 2 -O 2 fuel cells over several days at temperatures up to at least 250 °C. Conversion of Propane to Propylene. Propane can be dehydrogenated to propylene and hydrogen in a thermal equilibrium reaction (eq 1) or cracked to form elemental carbon (eq 2). Hydrogen so-generated is then available for use in H 2 - O 2 fuel cells. Each of these reactions is highly endothermic. The latter reaction typically is carried out at temperatures above 800 °C and results in undesirable carbon deposits on the catalyst surface. 6 However, the equilibrium concentration of hydrogen in reaction 1 is low, below 10% at temperatures up to 250 °C. Thus, to utilize reaction 1 effectively for cogeneration of propylene and electrical power in a fuel cell, hydrogen must be removed rapidly from the anode-catalyst sites as protons so as to overcome the equilibrium limitation of the reaction. Further- more, severe conditions cannot be used, or else carbon is generated according to reaction 2, and the anode catalyst is deactivated. In principle, it is possible to generate electricity through electrochemical oxidative dehydrogenation of propane to pro- pylene (eqs 3-5). Water is a byproduct of the reaction (eq 4). Propane in the presence of moisture may also lead to a variety of alternative partial oxidation and deep oxidation products over anode catalysts. Herein, we will describe the use of high-MW PBI-H 3 PO 4 membranes with Pt anodes and cathode catalysts in fuel cells for the conversion of propane. We will show that humidification is required for high activity and that, under these conditions, propane is converted to carbon oxides rather than propylene at temperatures up to 250 °C. To date, no combination of anode catalyst and proton- conducting membrane has been identified for efficient cogen- eration of propylene and electrical power from propane. Hydrocarbon Fuel Cells. High-temperature hydrocarbon fuel cells are described in U.S. patents 5,747,185 and 3,718,506. 7,8 These systems operate in the temperature range 500-1200 °C * Author to whom correspondence should be addressed. Phone: (780) 492-2232. Fax: (780) 492-2881. E-mail: jingli.luo@ualberta.ca C 3 H 8 (g) f C 3 H 6 (g) + H 2 (g) (1) C 3 H 8 (g) f 3C + 4H 2 (g) (2) 2C 3 H 8 (g) f 2C 3 H 6 (g) + 4H + + 4e - (anode) (3) 4H + + 4e - + O 2 (g) f 2H 2 O (g) (cathode) (4) 2C 3 H 8 (g) + O 2 (g) f 2C 3 H 6 (g) + 2H 2 O (g) (overall reaction) (5) 13036 J. Phys. Chem. B 2005, 109, 13036-13042 10.1021/jp044107a CCC: $30.25 © 2005 American Chemical Society Published on Web 06/11/2005