Understanding the phase formation and compositions of barium carbonate modified NiO-yttria stabilized zirconia for fuel cell applications

Highlights

• BaCO₃ reacts with YSZ to produce a BaZr_1−xY_xO_3−δ (BZY) phase.

• BaCO₃ reacts easily with NiO to form a mixture of BaNiO₃-BaNiO_2.36 at 900 °C.

• The BaNiO₃-BaNiO₂ phases are unstable at elevated temperatures or under a reducing atmosphere.

Abstract

A simple modification of a conventional nickel-yttria stabilized zirconia (Ni-YSZ) anode for a solid oxide fuel cell (SOFC) by BaCO₃ can enhance the coking resistance of the anode in carbon-containing fuels. In this study, X-ray diffraction, Raman spectroscopy, and thermo-gravimetric analysis are used to investigate the chemical reactions among BaCO₃, NiO, and YSZ to understand the phase formation and compositions during fabrication process. Our results indicate that BaCO₃ reacts readily with YSZ to produce a BaZr_1−xY_xO_3−δ (BZY) phase at temperatures higher than 900 °C. For the mixture of BaCO₃ and NiO, both BaNiO₃ and BaNiO_2.36 are formed at 900 °C, which are unstable at elevated temperatures, in the presence of YSZ, or under a reducing atmosphere. The results imply that a BZY/YSZ-BaO/Ni anode can be easily created by a simple heat-treatment (e.g., during cell fabrication) and a subsequent reducing process (e.g., exposure to the fuel) of a mixture consisting of BaCO₃, NiO, and YSZ.

Introduction

Solid oxide fuel cells (SOFCs) run on hydrocarbon fuels have attracted much attention because direct use of hydrocarbon fuels (rather than pure hydrogen) can reduce the costs of the SOFC systems by eliminating or minimizing the additional reforming and purification process [1], [2], [3], [4], [5], [6], [7], [8]. Furthermore, the direct use of hydrocarbon fuels to generate electrical power has the potential to substantially accelerate the use of fuel cells in transportation and distributed power generation [1], [4], [5]. However, one of the major challenges for direct utilization of hydrocarbon fuels in SOFC is the deactivation of traditional nickel-yttria stabilized zirconia (Ni-YSZ) anodes by coking under carbon contained fuels [1], [2], [3], [4], [5], [6], [7], [8].

To improve the coking resistance of Ni-YSZ anodes, substantial efforts have been devoted to the development of alternative Ni-free anode materials or modification of traditional Ni-based anodes. The search for alternative materials to Ni-YSZ has been mainly focused on Cu-based cermets [9], [10] and perovskite anodes, including LaSr_0.25Cr_0.5Mn_0.5O_3-δ [11], Sr₂Mg_1−xMn_xMoO_6−δ [12], doped (La,Sr) (Ti)O₃ [13], [14], and La_0.4Sr_0.6Ti_1−xMn_xO_3-δ [15]. However, practical application of Cu-based cermets is hindered by their poor performance while the critical issues for perovskite anodes are poor electro-catalytic activity and limited chemical and thermal compatibility with YSZ electrolyte at high temperatures required for fabrication. On the other hand, some catalysts such as SDC [5], [16], SrZr_0.95Y_0.05O_3-δ [17], [18], and Ru-CeO₂ [1], [19], [20], [21] have been applied to modify the Ni surface to promote reforming of hydrocarbon fuels before they reach Ni surfaces, which indeed showed different degrees of improved coking tolerance to various hydrocarbon fuels. It should be noted, however, that the performances of fuel cells run on hydrocarbon fuels are still much lower than those on H₂ fuel [21]. When a heavy hydrocarbon fuel such as iso-octane was used, a larger amount of CO₂ and/or O₂ must be co-fed to prevent coking [1], [5], [16], reducing the efficiency and increasing the complexity of the system [4]. In addition, the high cost of Ru [1], [19], [20], [21] may limit the practical application of SOFCs.

In recent years, surface-modified Ni-YSZ anodes have shown excellent coking resistance when they were directly exposed to hydrocarbon fuels, including methane, propane, and even octane fuels [2], [3], [4], [22], [23]. For example, a Ni-BaZr_0.1Ce_0.7Y_0.2−xYb_xO_3−δ (BZCYYb) anode was reported to demonstrate superior tolerance to coking and sulfur poisoning due to strong water adsorption capability of the mixed ion conductor BZCYYb [2]. Later study indicated that the enhanced coking resistance might result from the BZCYYb phase modification of Ni surface during high temperature treatment [22]. Li et al. proposed that BZCYYb perovskite is a promising alternative anode material for direct hydrocarbon SOFCs with high activity and excellent carbon cracking resistance [24], [25]. In addition, a new anode with nanostructured barium oxide/nickel (BaO/Ni) interfaces via vapor deposition of BaO into Ni-YSZ was designed and investigated [3]. The obtained results indicated that these BaO/Ni interfaces indeed promoted a water-mediated carbon removal process and enhanced Ni resistance to carbon buildup and deactivation under carbon contained fuels [3], [26]. The cells with the novel anode demonstrated high power density and stability in C₃H₈ and gasified carbon fuels at 750 °C. Recently, we adopted a simpler and cost-effective modification to fabricate a multi-functional composite anodes, derived from a conventional NiO-YSZ anode mixed with small amount of BaCO₃ [4]. The cell with BaCO₃ modified Ni-YSZ anode were operated on a transportation fuel (iso-octane) without the addition of excess amount of CO₂ and/or O₂, demonstrating a peak power density of 0.6 W/cm² at 750 °C. More importantly, at 750 °C the cell showed a stable power output at a cell voltage of 0.7 V with wet iso-octane (6.5% in Ar) as the fuel over 100 h, indicating the great potential of BaCO₃ modified NiO-YSZ anode for direct utilization of transportation fuels. The unique properties were attributed to the formation of a unique microstructure: discrete nano-particles of BaO on Ni grain surface and a conformal coating of BaZ_1−xY_xO_3−δ (BZY) BZY on YSZ grain surface, during the processes for fabricating the novel cells.

In those studies, different phases of Ba species have been detected in Ba modified NiO-YSZ anodes at different stages of the preparation and testing processes, including BaNiO₂ and BaNiO₃ on the NiO surface [3], BaO on Ni surface [3], and BZY on YSZ surface [4]. However, the details of the phase composition of a Ba modified NiO/YSZ anode during fabrication process are yet to be understood to gain deeper understanding of the coking resistance mechanism of the Ba modified Ni-YSZ anode. In the present work, we systematically study the chemical reactions in the system of BaCO₃-NiO-YSZ using temperature-dependent X-ray diffraction (XRD), post-micro-Raman spectroscopy, and thermo-gravimetric analysis (TGA).