Molten Carbonate Fuel Cell Performance for CO₂ Capture from Natural Gas Combined Cycle Flue Gas

Abstract

CO2 capture and sequestration (CCS) from stationary flue gas sources is one of the critical technologies needed as the future energy landscape shifts to low carbon intensity energy systems. Molten carbonate fuel cells (MCFCs) have the potential to capture CO2 from flue gas at higher thermal efficiency than traditional CCS technologies while simultaneously producing electricity. Herein, we present an investigation of molten carbonate fuel cell behavior at carbon capture conditions using simulated natural gas combined cycle flue gas. Measurements at these low CO2 and high current conditions reveal a lower than expected cathodic consumption of CO2 Based on the strong dependence of this deviation on water partial pressure as well as mass balances revealing a net consumption of water at the cathode, a parallel oxygen reduction mechanism is proposed. In this mechanism, water and oxygen are consumed at the cathode to produce hydroxide ions which migrate through the electrolyte to the anode. This parallel mechanism contributes to power generation but not to CO2 capture. Mass transport limitations in the molten carbonate fuel cell cathode were identified as the primary driver for this alternative mechanism which were heavily influenced by the design of the current collector and cathode interface.

Predicting chemical reaction equilibria in molten carbonate fuel cells via molecular simulations

“…At low partial pressures of CO 2 , the concentration of hydroxide in the melt can be high enough that it may significantly contribute to oxygen and electron transfer within MCFCs. This could account for some of the non‐carbonate current observed in MCFCs at low cathode CO 2 concentration 13 …”

“…This could account for some of the non-carbonate current observed in MCFCs at low cathode CO 2 concentration. 13 The equilibrium hydroxide concentration in the melt also depends on the cation composition, with smaller (harder) cations leading to more hydroxide in the molten carbonate phase. Therefore, a balance must be struck between the higher ionic conductivity from smaller cations and the decrease in CO 2 separation due to hydroxide formation.…”

“…CO 2− concentration and high current density. 13 Measurements suggest a net consumption of water at the cathode, caused by water reacting with oxygen to produce hydroxide ions.…”

“…However, recent experiments have revealed a parallel oxygen reduction mechanism when operating at low CO 2 gas‐phase concentration and high current density 13 . Measurements suggest a net consumption of water at the cathode, caused by water reacting with oxygen to produce hydroxide ions. …”

Electrochemical carbon dioxide capture to close the carbon cycle

“…again. An alternative design enables a molten carbonate fuel cell, where CO 2 is captured while H 2 is produced [169][170][171]. Involved challenges are difficult operating conditions due to the high temperatures, corrosion and sensitivity to the presence of SO x in the gaseous mixture [20,24,172].…”