Shaowu Zha

Sulfur Poisoning and Regeneration of Ni-Based Anodes in Solid Oxide Fuel Cells

The effect of hydrogen sulfide (H 2 S) on the performance of nickel/yttria-stabilized zirconia (YSZ) cermet anode for solid oxide fuel cells has been studied under various operating conditions. In all cases, a small amount of H 2 S (ppm level) causes a sharp drop in cell performance within the first few minutes of exposure, followed by a gradual but persistent deterioration in performance for several days. The extent of anode degradation caused by sulfur poisoning increases with increasing H 2 S concentration, increasing cell voltage (i.e., closer to open-circuit voltage or decreasing cell current density), or decreasing cell operating temperature. The initial sharp degradation in performance is attributed to rapid adsorption of sulfur onto the Ni surface, which blocks the active sites for hydrogen adsorption and oxidation. However, the mechanism for the subsequent slow degradation is still not clear. Upon removal of H 2 S from the fuel stream, the anode performance can be recovered fully or partially, depending on operating conditions and duration of H 2 S exposure. The rate of the recovery process increases with operating temperature and cell current density.

GDC-Based Low-Temperature SOFCs Powered by Hydrocarbon Fuels

The critical issues facing the development of economically competitive solid oxide fuel cell (SOFC) systems include lowering the operation temperature and creating novel anode materials and microstructures capable of efficiently utilizing hydrocarbon fuels. In this paper, we report our recent progress in developing more efficient anodes for direct utilization of methane and propane in low-temperature SOFCs. Anode-supported SOFCs with an electrolyte of 20 μm thick Gd-doped ceria (GDC) were fabricated by copressing, and both Ni- and Cu-based anodes were prepared by a solution impregnation process. Results indicate that both microstructure and composition of the anodes, as fabricated using a solution impregnation technique, greatly influence fuel cell performance. At 600°C, SOFCs fueled with humidified H 2 , methane, and propane reach peak power densities of 602, 519, and 433 mW/cm 2 , respectively.

Stability of materials as candidates for sulfur-resistant anodes of solid oxide fuel cells

Development of sulfur-resistant anode materials for solid oxide fuel cells (SOFCs) is critical to cost reduction of SOFC technologies. In this paper, we report the thermodynamic stability of various candidate materials for sulfur-resistant anodes of SOFCs, including metal carbides, borides, nitrides, silicides, perovskite-structured oxides, and transition metal sulfides. The most probable reaction between each candidate material and hydrogen sulfide (H 2 S)-containing fuels under SOFC operating conditions (i.e., high temperature, reducing atmosphere with significant concentration of water vapor) were predicted and compared with experimental observations. It was found that proper thermodynamic analysis provided accurate prediction of the stability for a wide range of materials and significantly reduced the number of candidate materials for further in-depth study.

A solid oxide fuel cell running on H2S/CH4 fuel mixtures

Solid oxide fuel cells (SOFCs) that were capable of running on sour natural gas were fabricated and tested. The electrolyte of the SOFCs was yttria-stabilized zirconia (YSZ), the anode material was a strontium-doped lanthanum vanadate with a nominal composition of La 0 . 7 Sr 0 . 3 VO 3 (LSV), and the cathode was a porous composite of La 0 . 8 5 Sr 0 . 1 5 MnO 3 (LSM) and YSZ. The observed peak power density at 950°C was 280 mW/cm 2 when 5% H 2 S/95% CH 4 gas mixture was used as the fuel and ambient air as the oxidant. Analysis of the effluent gas from the SOFC indicated that, apart from being oxidized electrochemically to sulfur and SO 2 , a significant portion of the H 2 S was transformed to carbon disulfide (CS 2 ), an important solvent currently used to dissolve heavy hydrocarbon deposits. The results indicated that SOFCs with a LSV anode might be used for the processing of sour natural gas (containing CH 4 , H 2 S, etc.) in which both electricity and high value chemicals (i.e., sulfur and CS 2 ) are produced simultaneously.

Sulfur-tolerant materials for the hydrogen sulfide SOFC

This study examined sulfur tolerant materials for solid oxide fuel cells (SOFCs) operating on H 2 S and H 2 S containing fuels, focusing on the stability and electrochemical performance of perovskite-type materials in a H 2 S atmosphere under SOFC operating conditions (P = 1 atm, T > 800°C). Preliminary results indicate anodes of the general form La x Sr y VO 3 - δ are stable and active toward the electrochemical oxidation of H 2 S. In particular, an SOFC using La 0 . 7 Sr 0 . 3 VO 3 as the anode has shown good performance at H 2 S levels of 10%, over 5000 times greater than the H 2 S tolerance level of contemporary Ni-based systems. The results are promising due to the drastic improvement in sulfur tolerance compared to the current generation of SOFC anode materials.