Mechanistic Studies of Liquid Metal Anode SOFCs

Abstract

Liquid metal anode (LMA) solid oxide fuel cells (SOFCs) are a promising type of high temperature fuel cell suitable for the direct oxidation of gaseous or solid fuel. Depending upon the operating conditions they can be run in four different modes. In this first of a series of studies concerning the mechanism of reaction and species transport in LMA SOFCs, the oxidation of hydrogen fuel in a liquid tin anode has been investigated. An electrochemical model is developed based upon fast dissolution of hydrogen in a molten tin anode, slow, rate-determining homogeneous reaction of hydrogen with oxygen dissolved in the liquid tin, followed by anodic oxygen injection under diffusion control to replace the oxygen removed by reaction (so-called Chemical-Electrochemical mode or CE mode). Experimentally-generated data are used to validate the model. The model has introduced a new key parameter, z , which takes a value between zero and unity; its value is determined by geometric and convective factors in the cell as well as the partial pressure of the supplied hydrogen fuel. Current output of the cell is proportional to the value of z .

Anode Fluidization of a Liquid Tin Anode SOFC Operated on Simulated Syngas

“…But due to the sluggish nature of oxygen transport (5.3×10 -5 cm -2 s -1 @ 800 o C) in liquid Sn (16), the current limit set by mass transport of oxygen is inevitably low, resulting in a less satisfactory power density of fuel cell. Toleuova et al (17,18) introduced H 2 into the LTA to help remove oxygen content, and the H 2 fueled electrochemical system is operated at a potential between OCPs of H 2 and Sn. This high operation voltage makes SnO 2 formation thermodynamically unfavorable, but the limited over potential (<0.1 V) also left the power density inherently low.…”

Mechanistic Studies of Liquid Metal Anode SOFCs II: Development of a Coulometric Titration Technique to Aid Reactor Design

“…The previous paper (Part I) described the fundamental electrochemistry of such a LMA SOFC system. 3 It was demonstrated that the oxidation of hydrogen takes place via a Chemical-Electrochemical mechanism, (so-called CE mode), whereby hydrogen undergoes fast dissolution, then rate-determining homogeneous oxidation by oxygen dissolved in the liquid tin, followed by diffusion-controlled anodic oxygen injection to replace oxygen removed by the chemical reaction. In summary, the process involves:…”

“…potentiostatic and open-circuit), whereby electrode potential is switched between a fixed value E and open-circuit. During the open-circuit periods, hydrogenreacts with oxygen dissolved in tin according to a Chemical-Electrochemical (CE) mechanism3 . In the first regime (Regime 1) the flow of H2 into the tin is constant, whereas in the second (Regime 2) it is intermittent.…”

Direct Coal Oxidation in Modified Solid Oxide Fuel Cells

“…In molten carbonate media, the controlling anode reactions potentially include the complete oxidation of carbon [1] and/or the chemical gasification of carbon (reverse Boudouard reaction, [2]) followed by the oxidation of CO [3]. Carbonate ions are regenerated through reaction with oxygen anions [4], generated at the cathode [5] (6).…”