J. Phys. Chem. C 2007, 111, 16679-16685
16679
LaCrO3-VOx-YSZ Anode Catalyst for Solid Oxide Fuel Cell Using Impure Hydrogen Zhengrong Xu, Jing-Li Luo,* Karl T. Chuang, and Alan R. Sanger Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, AB, Canada, T6G 2G6 ReceiVed: June 16, 2007; In Final Form: August 26, 2007
A new, highly active and stable anode catalyst material LaCrO3-VOx-YSZ has been developed for solid oxide fuel cell systems fueled with impure hydrogen. The presence of CO in the anode feed enhanced initial fuel cell performance when compared to the use of CO-free H2 containing 5000 ppmv H2S. Impedance spectra showed that the anode polarization resistance decreased in the presence of CO. Current density of 450 mA/ cm2 at 0.6 V and maximum power density of 260 mW/cm2 (at 450 mA/cm2) were obtained at 900 °C. XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy) analyses showed that the new catalyst was chemically stable in H2S-rich syngas or hydrogen. However, when syngas was used, slow carbon deposition compromised the fuel cell performance during long tests at high temperatures.
1. Introduction A major advantage of solid oxide fuel cells (SOFCs) over low-temperature fuel cells is that SOFCs can operate on a less expensive fuel. An as-yet unrealized potential commercial application of SOFCs is for the use of synthesis gas (or syngas) manufactured by steam reforming of hydrocarbons and carbons. Syngas is an important and relatively inexpensive energy vector. However, syngas so manufactured may contain H2S derived from the sulfur content of the parent hydrocarbons, thus preventing use of the untreated syngas in fuel cells having conventional anodes such as Pt or Ni. Purification of syngas by removal of H2S to a quality appropriate for use with conventional SOFC anode catalysts is expensive, and thus it is a barrier to the development of syngas SOFC. The major combustible components of syngas are H2 and CO which are oxidized to H2O and CO2 according to the following reactions:
1 H2 + O2 ) H2O 2 ∆H ) -59.322 kcal, ∆G ) -43.766 kcal at 900 °C (1) 1 CO + O2 ) CO2 2 ∆H ) -67.322 kcal, ∆G ) -43.152 kcal at 900 °C (2) An attractive goal is development of an SOFC capable of operating on unpurified syngas as this would have lower operating costs. Several research projects have been directed toward the application of either H2S-free syngas or H2Scontaining H2 as fuel in SOFC.1-4 Other researchers used pure H2S as the fuel gas.5,6 However, there are few reports describing attempts to use H2S-containing syngas as fuel in SOFC. To use impure syngas, it is important to find a catalyst which is both stable in a H2S-rich environment and has the catalytic capability to activate both H2 and CO. To this end, we have developed new SOFC composite anode catalysts containing vanadium oxides. * Corresponding author. Tel.: +001 780 492 2232. E-mail: jingli.luo@ ualberta.ca.
Vanadium oxide-based catalysts have wide industrial applications. V2O5 is a good catalyst for oxidation. It can be used to selectively oxidize hydrocarbons7-9 and to oxidize H2S to elemental sulfur.10 It is also used for catalytic oxidation of SO2 to SO3 and for removal of SO2 and NO.11-13 There are few applications of V2O5 in SOFCs because of its low melting point (690 °C) and its tendency to be reduced in the reducing environments, though some lower-temperature SOFC applications have been investigated.14,15 Vanadium oxides have multiple available oxidation states (+2, +3, +4, +5), and the lower-oxidation-state vanadium oxide compounds have much higher melting points that make them more suitable for SOFC applications. The reducing atmosphere in the anode compartment of our fuel cell will stabilize lower oxidation states of V-based anode catalysts. Low-oxidation-state V-based catalysts have been studied for petroleum desulfurization.16 Desulfurization activity is facilitated when the oxidation state of vanadium is maintained below +3, provided that the oxide cannot be reduced further to elemental vanadium. A significant concern when using syngas as fuel in SOFCs is that CO can be reduced to form coke that will poison the catalyst surface.17-21 In order to suppress carbon deposition, LaCrO3 was admixed into the new V-based catalysts. Doped and undoped LaCrO3 are good catalysts for methane combustion or soot removal.22-25 A further potential benefit is that LaCrO3 is a good p-type electronic semiconductor, notably under oxidizing conditions at high temperatures,26,27 so it may also improve the anode electronic conductivity. LaCrO3 is very stable in a reducing environment,28 for which reason it is used as an interconnect material in SOFC stacks.29 YSZ electrolyte was widely used in the SOFC setting as a stable oxygen ions conducting electrolyte.30 2. Experimental Section To prepare the new catalyst, V2O5 first was reduced to VOx, which was then mixed with LaCrO3 and YSZ to form the anode catalysts. Fuel cell performance and impedance measurements were determined in syngas (40% H2 and 60% CO) containing 5000 ppmv H2S. We chose 5000 ppmv H2S because diesel fuel in the United States may contain up to 5000 ppm sulfur and this value also is a typical sulfur concentration in syngas derived
10.1021/jp074672o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007
16680 J. Phys. Chem. C, Vol. 111, No. 44, 2007
Xu et al.
TABLE 1: Typical Coal Gas Composition (vol %) H2
CO
CH4
CO2
H2O
sulfur
others
28-30
62-65
