HDCFC Performance as a Function of Anode Atmosphere (N₂-CO₂)

Abstract

The mechanism for the oxidation of solid carbon in a fuel cell made as a hybrid between a molten carbonate fuel cell and a solid oxide fuel cell, known as a hybrid direct carbon fuel cell (HDCFC), was investigated. Fuel cell performance was measured by electrochemical impedance spectroscopy (EIS) and using current-potential-power density curves (I-V-P) between 700 to 800 • C. The impacts of the gas species introduced at the cathode (air vs. pure O 2 ) and at the anode (pure N 2 , pure CO 2 , and mixed N 2 -CO 2 ) were investigated, as well as the influences of temperature and anode gas flow rate. The majority of the impedance data could be modeled using an equivalent circuit consisting of a resistor (Rs) in series with three resistor-constant phase element units (RQ, in parallel), depending on anode gas atmosphere. An explanation was proposed for each impedance element, and the literature relating to impedance data acquired for carbon-carbonate mixture in a DCFC anode were discussed. By varying of the anode gas mixtures between pure N 2 and pure CO 2 , together with variations in their flow rates, it was suggested that CO 2 is a chemically active species which is not electrochemically active, a chemical intermediate in the oxidation of solid carbon in such a HDCFC.

Hybrid direct carbon fuel cell anode processes investigated using a 3-electrode half-cell setup

“…We have previously reported on the effects of variable % CO 2 in mixed N 2 eCO 2 environments on carbon black fueled HDCFC performance, both in full-cell [2] and half-cell [4] configurations. Previously reported half-cell tests showed that current measured at fixed overpotential increased, and Rp values decreased, as the % CO 2 was increased.…”

“…Briefly, there appear to be two prevailing theories on the dominant electrochemical reaction occurring in molten carbon-carbonate slurry DCFCs (co-electrolyte, MCFC-type DCFCs and HDCFCs): the complete oxidation of carbon (4 electron process) [36] and CO oxidation (2 electron process) [31]. We have previously suggested that the CO oxidation process likely predominates at the anode of planar HDCFCs [2]. Although non-coincident OCP and DE ¼ 0 values seen in our half-cell experiments in mixed N 2 eCO 2 (Fig.…”

“…As such, the RDS of this sequence is a previous step (Steps AeH), as the addition of more CO 2 facilitates these processes. Further, as EIS data acquired on both full-cell [2] and half-cell setups (Figs. 2 and 3) [4,5] are dominated by the LF contributions associated with mass transport at the anode/WE [7], the RDS is more likely to be a chemical/ physical process (Steps AeG) prior to the electrochemical reaction (Step H).…”

“…Hybridization of these two DCFC concepts aims to overcome the shortcomings while maintaining the advantages of each. A hybrid DCFC (HDCFC) consists of an SOFC with a molten mixture of carbon and alkali carbonate included in the anode chamber, such that infrastructural advantages (reduced corrosion issues at, and air supply only to, the cathode) and fuel/electrolyte contact enhancement are simultaneously achieved [2,3].…”

“…DCFC performance is typically limited by processes occurring at the anode [2]. The presence of the carbon- This study a Composition of the molten carbonate electrolyte, written as (mol% alkali metals) 2 CO 3 (e.g., (62e38 mol% LieK) 2 CO 3 .…”