Zirconia-Based Electrolyte Stability in Direct-Carbon Fuel Cells with Molten Sb Anodes

Abstract

Direct carbon fuel cells (DCFC) that use zirconia-based electrolytes and molten Sb anodes have much promise for the efficient conversion of carbonaceous solid fuels into electricity. However, etching of the electrolyte, and ultimately cell failure, has been observed during operation. In this study, we have investigated this etching phenomenon as a function of the electrolyte composition and cell operating conditions and demonstrated that it is not electrochemical in nature, but rather results from reaction between the electrolyte and Sb 2 O 3 that is produced during cell operation. Etching was also observed when a yttria-stabilized zirconia ( Electrical power generation in a solid oxide fuel cell (SOFC) directly utilizing solid, carbonaceous fuels (e.g. coal and biomass) would provide significant benefits including high energy conversion efficiency. CO 2 sequestration, if needed, is easy since the exhaust gas from the anode is highly concentrated.1 Direct utilization of solid carbonaceous fuels would eliminate the need for gasification and steam reforming steps leading to great simplification of the overall system. The transfer of oxide ions from the electrolyte to the solid fuel is often found to be the limiting step for SOFCs operating on solid fuels.3 Among various approaches to facilitate this transfer, 4-7 one of the most promising routes is to use of molten-metal electrodes. [8][9][10] In this scheme, the anode reaction occurs in two steps:The first step (Equation 1) is the electrochemical oxidation of some portion of the metal in the anode at the electrolyte-anode interface. The oxidized metal is then transported (via diffusion, natural convection or mechanical mixing) to access solid fuel where the carbon is oxidized while reducing metal oxide back to metal (Equation 2). Sn previously attracted much attention as the molten metal phase for this process. 8,9Unfortunately, there is evidence that a solid SnO 2 layer forms at the electrolyte-anode interface during fuel cell operation, inhibiting further facile transfer of the oxygen from the electrolyte, resulting in high electrode losses. 11,12 While this can be avoided by using a molten metal with high oxygen solubility that does not form a stable bulk oxide phase at SOFC operating temperatures (such as Ag), electrode impedances can still be high due to slow diffusion of oxide ions within the molten metal. 13In our opinion, the metal that has provided the most intriguing results as an anode for this type of direct carbon fuel cell (DCFC) is Sb.14-16 Both Sb (melting point 903 K) and Sb 2 O 3 (melting point 929 K) become liquid at SOFC operation temperature ranging from 973 to 1073 K. This precludes the formation of a solid metal-oxide film at the electrolyte-anode interface as in the case of Sn. Furthermore, reduction of Sb 2 O 3 by carbon is facile and has been practiced industrially for more than 100 years.17 Although the Open-Circuit Voltage (OCV) for Sb oxidation is only 0.75 V at 973 K, the energy losses associated with this low potential are largely c...

Liquid Antimony Anode Fluidization within a Tubular Direct Carbon Fuel Cell

“…Electrochemical impedance spectroscopies were taken before and after the discharge, as presented in Figure 3, the ohmic resistance of the fuel cell decreased as discharge went on. X. Zhou et al 2 have found that liquid Sb may corrode the solid oxide electrolyte and result in a reduction of ohmic resistance of the fuel cell. However, in present work, the electrolyte tube was plugged vertically into liquid anode rather than stagnating the metal flow, so the effect of etching which happens during fuel cell operation is negligible in this situation.…”

“…1 The major reaction in anode zone can be written as follows, M stands for metal. 2 M + nO 2− = MO n + 2n e −…”

A SnO2-samarium doped ceria additional anode layer in a direct carbon fuel cell

“…The low output voltages of these cells reduce the energy efficiency and limit their practical utilization. Besides, the chemical reaction between Sb 2 O 3 and the YSZ electrolyte reduces the stability of the cell [29].…”

Electrolyte Stability in Direct-Carbon Fuel Cells with Molten Sb Anodes

“…Various electrolyte compositions have been adopted and electrical current was drawn at various set potentials. Additional chemical reaction tests were carried out to clarify the cause of the electrolyte etching (2).…”