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

Abstract

It has been recently suggested that hydroxide ions can be formed in the electrolyte of molten carbonate fuel cells when water vapor is present. The hydroxide can replace carbonate in transporting electrons across the electrolyte, thereby reducing the CO 2 separation efficiency of the fuel cell although still producing electricity. In 3 ð1Þ Anode : H 2 + CO 2− 3 ! H 2 O + CO 2 + 2e − ð2Þ However, recent experiments have revealed a parallel oxygen reduction mechanism when operating at low CO 2 gas-phase Jeffrey M. Young and Anirban Mondal contributed equally to this work.

First-Principles Modeling of Transport Mechanisms in Carbonate–Hydroxide Electrolytes

“…These results reaffirm the observation made in our previous work based on chemical reaction equilibrium simulations. 9 Overall, we found the following order in the average lifetime of the species present in the melt: OH − > CO 3 2− > CO 2 > C 2 O 5 2− > H 2 O > HCO 3 − . Surprisingly, CO 2 exhibits a higher magnitude of average lifetime (2.9 ps) than that of water (1.0 ps), although the concentration of the later dominated over the former for most of the simulation.…”

“…In our earlier work on chemical reaction equilibrium simulations using classical force fields, we showed that the concentration of dissolved water is over 2 orders of magnitude higher than that of CO 2 in a [Li 0.6 K 0.4 ] 3 CO 3 OH electrolyte over a wide range of CO 2 gas phase partial pressures. 9 The contrast in the dissolved concentrations was primarily justified as water being a polar molecule and having the ability to form hydrogen bonds, thus being more stable in the melt of oxidic ions. The present study also reveals a similar distinction in the concentration of water and CO 2 in the [Li 0.6 K 0.4 ] 3 CO 3 OH melt based on an ab initio molecular dynamics approach.…”

“…If the system is at equilibrium, the dissolved CO 2 concentration used here would correspond to unrealistically high gas phase partial pressure, as found in our earlier work based on chemical reaction equilibrium simulations. 9 The reason for imposing such a high initial CO 2 concentration was to improve the statistical accuracy of different quantities measured in this work, as these depend on the reaction of CO 2 with other molecules in the system. The temperature was set to 923.15 K, the experimentally measured melting point of the eutectic mixture, [Li 0.6 K 0.4 ] 3 CO 3 OH.…”

“…19,20 Details of the force field parameters and protocols are described elsewhere. 9,21,22 The equilibrated supercell from the classical MD trajectory was geometry optimized within the density functional theory (DFT) framework at the same level of theory as described below. The gradients on the wave functions and on the nuclear positions were optimized with convergence criteria of 10 −6 and 10 −3 au, respectively.…”

Experimental and Modeling Investigation of CO3=/OH– Equilibrium Effects on Molten Carbonate Fuel Cell Performance in Carbon Capture Applications

“…This equilibrium conversion between hydroxide and carbonate is also evident in experimental observations on molten hydroxide fuel cells in which CO 2 is poisonous to the cell because it converts hydroxide into carbonate (Hemmes and Cassir, 2011;Frangini and Masi, 2016;Xing et al, 2017). Furthermore, a recent molecular dynamics study (Young et al, 2020) reported that at a gas phase H 2 O/CO 2 molar ratio of 10, a 40/60 Li/K molten carbonate eutectic comprises ∼5/95 OH − /CO 2− 3 at 923 K. This experimental observation directly proves that the MCFC electrolyte composition is not 100% carbonate under carbon capture conditions. Instead, the electrolyte melt also contains hydroxide ions in accordance with the carbonate/hydroxide equilibrium.…”