(Invited) Molecular Dynamics of Dipole Layer Formation at High-k/SiO₂Interface

Abstract

Origin of the electric dipole at high-k/SiO2 interfaces was investigated by classical molecular dynamics simulation. Both directions of the dipole are successfully reproduced with a simple two-body rigid ion model. The direction and magnitude of the dipole are determined by two opposing tendencies; oxygen ion migration from higher density oxide side to lower one, and the migration of metal cations in high-k toward SiO 2 side to form a silicate layer at the interface. The driving force of oxygen ion migration is the core-to-core repulsive interaction between oxygen ions. This is agreeing with the oxygen-density-differenceaccommodation model proposed by Kita and Toriumi. The driving force of the cation migration is the energy gain to form a stable silicate phase. Thus, the migration of metal cations must be taken into account as well as the migration of oxygen ions to comprehensively explain the mechanics of the dipole layer formation.

The Effect of Ambient Carbon Dioxide on Anion‐Exchange Membrane Fuel Cells

“…The self-purging mechanism is the process by which HCO 3 À / CO 3 2À can be removed from the AEMFC under high current conditions, which may help to improvec ell performance, as demonstrated in several studies. [8,13,16,17,72] The process may be explained as follows:a tr elativelyh igh current densities, a large amount of OH À anionsa re formed asaresult of the high rate of ORR in the cathode [Eq. (7)].T he existing charge, which includes HCO 3 À and CO 3…”

“…In addition, CO 2 formation and release in the anode exhaust was showni ns everal experimental studies. [10,13,43,74] For example, by flowing the anode product stream through as olution of Ca(OH) 2 ,U nlu et al observed the formationo fam ilky precipitate (CaCO 3 )s econds after the start of fuel cell operation (by using O 2 + CO 2 in the cathode at av olume ratio of 1:2). [43] They showedt hat the amount of CaCO 3 precipitate increased with time as the cell operated.O ther works used analytical methods, such as IR spectroscopy, [69,72,75] MS, [13,74] and GC [10] to measuret he concentration of CO 2 in the anode exhaust gas of an AEMFC in operation.…”

“…[60] Similarly,K imura and Yamazaki measured an increase in ex situ conductivity of an AEM exposed to CO 2 by applying an external current in ap ure O 2 environment. [29] Several theoretical [16] and experimental [13,69,72] studies have suggested that, during fuel cell operation, an equilibrium state of HCO 3 À /CO 3 2À concentration exists in the AEM;t he equilibrium state is determined by the current density.D uring fuel cell operation with ambienta ir,i fthe current density is increased, there is at emporary increasei nt he CO 2 concentration in the anode exhaust( Figure 9). Accordingly, the amount of CO 2 released after ac hange in current density is the differencei n CO 3 2À contento ft he AEM between the two equilibrium states and, at constantc urrent, the amount of CO 2 adsorbed in the cathodeise qual to the amount released in the anode.…”

“…Accordingly, the amount of CO 2 released after ac hange in current density is the differencei n CO 3 2À contento ft he AEM between the two equilibrium states and, at constantc urrent, the amount of CO 2 adsorbed in the cathodeise qual to the amount released in the anode. [13,72] Based on the data above,itc an be suggested that as tart-up protocoli nt he operation of the fuel cell should include operating at high current density/lowv oltage (i.e.,0 .1-0.2 V) for a brief period, [14] to purge HCO 3 À /CO 3 2À anionsf rom the AEM, and begin the operation with an AEM in the OH À form to yield higher performance. As imilar protocol was also successfully used to "activate" AEMFCs operated with air containing low concentrations of CO 2 (filtered air).…”

“…[11,12] The reduction in AEM effective anion conductivity may be due to several factors, among them are the larger size of the carbonate anions relative to the size of the hydroxide anion, as well as different mechanismst hat are availablef or carbonate versush ydroxide anion transport in the presenceo fw ater.F or instance, in dilute aqueous solutions,H CO 3 À /CO 3 2À have about one-fifthoft he ionic conductivity of OH À . [13] Due to this challenge, most current research into AEMs involvesfuel cells operating on pure O 2 or CO 2 -free air as the oxidizer in the cathode;t his avoids the carbonation issue. In the past couple of years,s ome researchers have been able to achieve high AEMFCp erformance with cells operated on oxygen [14] and air; [4] however,p erformance data with ambient air represents only 3% of the cell performance data reported for AEMFCs, as recently reviewed.…”