Investigation of the anode reactions in SO-DCFCs fueled by Sn-C mixture fuels

Direct carbon fuel cells based on solid oxide electrolyte (SO-DCFCs) suffer from the poor contact between the anode surface and the carbon particles. By mixing the carbon fuel with Sn, the contact can be enhanced due to the fluidity of liquid Sn at the high operation temperature. However, Sn is an active fuel and can be oxidized electrochemically. In this study, the effect of Sn on the performance of SO-DCFCs was investigated. Four carbon fuels, including a graphite (GC), an anthracite coal (PYQ), a lignite (PCF) and an activated carbon (PAC), were mixed with Sn powder respectively and used as anode fuels. The electrochemical performances, including the polarization performance and the long-term durability, were measured at 1023-1123 K in Sn-only, Sn-C mixture and C-only mode. The morphology and elemental composition of the anodes were analyzed by SEM-EDX after the durability tests. The results show that pure Sn fueled cell degraded rapidly due to the accumulation of SnO2 in the anode. Sn-GC, Sn-PYQ and Sn-PCF displayed similar open circuit voltages (OCVs) and maximum power densities (MPDs) with pure Sn. For Sn-PAC, the OCV was as high as 1.0 V and the MPD (150.8 mW/cm(2)) was much higher than the other Sn-C mixtures. The SEM-EDX results show that SnO2 accumulation on the anode surface was the key factor affecting the cell performance. For Sn-GC, Sn-PYQ and Sn-PCF fueled cells, SnO2 reduction rate was lower than the electro-oxidation rate of Sn, leading to SnO2 accumulation on the anode surface. The cell performance was mainly governed by electro-oxidation of Sn. While for Sn-PAC fueled cell, PAC reduced SnO2 much faster than the other fuels and no SnO2 was detected in the anode pores due to its high BET surface area. The main anode reactions were CO electro-oxidation and C-CO2 gasification. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.