Electrochemical Property of Ni-Ba(Zr,Y)O3 Anode for Direct Ammonia-Fueled SOFC

Abstract

Hydrogen carriers have attracted much attention as alternative fuels to hydrogen. This is because the hydrogen fuel has technological difficulties in transportation and storage, which impedes the wide application. Among various hydrogen carriers, ammonia is one of the promising candidates because of various advantages such as ease in liquefaction, high energy density, large hydrogen content, and zero emission of CO2during reforming or decomposition. In addition, the infrastructure for ammonia utilization has been established because ammonia has been used as a fertilizer and commodity chemicals worldwide. In the case of solid oxide fuel cells (SOFCs) directly supplied with ammonia fuel, ammonia decomposes into hydrogen and nitrogen on the anode and subsequently hydrogen is electrochemically oxidized. Therefore, the anode materials for ammonia-fueled SOFCs need to be active for the electrochemical hydrogen oxidation as well as the catalytic ammonia decomposition. The mixture of nickel and Ba(Ce,Y)O3−δ (BCY), which is one of the promising proton conductors, has been already reported as a highly active catalyst for ammonia decomposition[1].In this study, then, the activity tests for ammonia decomposition were performed for various nickel catalysts supported on proton conductors. Ammonia decomposition proceeded at lower temperature over Ni−proton conductor cermets compared with the conventional anode of Ni−yttria-stabilized zirconia (YSZ) cermet. Ammonia decomposed completely over Ni−proton conductor cermets at ca. 600ºC, while a complete ammonia decomposition was accomplished over Ni−YSZ at higher temperature. In particular, the Ni−Ba(Zr,Y)O3−δ (BZY) catalyst exhibited superior activity. In order to evaluate the anode performance of ammonia-fueled SOFC, a single anode-supported cell composed of Ni−BZY|BZY|Pt was fabricated. Electrochemical measurements were conducted at 600−700ºC by using ammonia or hydrogen as a fuel and oxygen as an oxidant. The partial pressure and flow rate of hydrogen in the hydrogen-containing fuel gas were controlled to be the same as those after the complete decomposition of ammonia in the ammonia-containing one. At each temperature, the open circuit voltages (OCVs) under both fuel gas flows were almost the same. From this result, it can be tentatively concluded that ammonia was oxidized via the two-step reaction on the anode as mentioned above. At 700ºC, the current density at the terminal voltage of 0.5 V under hydrogen and ammonia fuel conditions were 280 mA cm−2 and 250 mA cm−2, respectively. The difference in performance between ammonia and hydrogen-containing fuels was small. Consequently, the Ni−BZY catalyst is a promising anode for ammonia-fueled SOFCs. [1] J. Yang, A.F.S. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi. ACS Appl. Mater. Inter., 7, 7406 (2015).