Unraveling the Hydroxide Ion Transportation Mechanism along the Surface of Two-Dimensional Layered Double Hydroxide Nanosheets

Abstract

Traditional polymeric anion exchange membranes (AEMs) suffer from longstanding issues such as low ionic conductivities, poor stability, and high toxic preparation procedures. Recent experiments demonstrated that exfoliated two-dimensional layered double hydroxide (2D-LDH) could provide a super high hydroxide ion conductivity of about 0.1 S/cm, which is 1 to 2 orders of magnitude higher than that of commercial AEMs. However, the hydroxide ion conduction mechanism of this material is still unclear. Our ab initio molecular dynamics (AIMD) simulation results reveal that the positively charged 2D-LDH slab (Mg2Al­(OH)6 +) can induce a quasi-two-dimensional hydroxide ion transportation behavior along the surface of 2D-LDH with a diffusivity comparable with that in the bulk water environment. When restacking 2D-LDH nanosheets, the spatial confinement will destroy the connectivity of the hydrogen bonding network and hamper the hydroxide ion conduction capability. The hydroxyl functional groups on 2D-LDH can mediate the hydroxide ion transportation process between non-adjacent water molecules with a relatively high energy barrier, which can only be activated in low humidity conditions. Our results shed light on future designs of stable anion exchange membranes based on inorganic 2D materials.

Partial Exchange between Inorganic and Organic Anions in MgAl Layered Double Hydroxide Nanosheets for Humidity Sensing

“…This result suggests that the main charge carriers in the LDH-NO 3 2− -BDC hybrids were hydroxyl ions rather than protons, 39,40 consistent with other LDH-based materials reported previously. 26,41 Because OH − ions are the main charge carriers in LDH nanosheets and BDC is a weak acid, the intercalation of BDC molecules into LDHs can reduce the amount of OH − ions available for charge conduction. Therefore, a BDC content that is too high could cause a reduction in the ion conductivity.…”

Charge-induced proton penetration across two-dimensional clay materials

“…The presence of negative surface charges effectively confined the movement of protons, which is similar to the OH − transportation behaviour near the positively charged surface of 2D layered double hydroxide (LDH). 54–56…”

“…The presence of negative surface charges effectively confined the movement of protons, which is similar to the OH − transportation behaviour near the positively charged surface of 2D layered double hydroxide (LDH). [54][55][56] Proton penetration across two-dimensional clay nanosheets Throughout all the above unbiased AIMD simulations, the proton tends to exist in the aqueous phase and no spontaneous proton penetration behaviour across the 2D clay nanosheets can be observed. To evaluate the possibility of proton penetration, we performed ab initio metadynamics simulations to calculate the proton penetration across energy barriers.…”

NiMn-Layered Double Hydroxide Porous Nanoarchitectures as a Bifunctional Material for Accelerated p-Nitrophenol Reduction and Freestanding Supercapacitor Electrodes

“…Alongside, in LDHs, it has been observed that ion transport kinetics remains sluggish due to the absence of sufficient transport channels leading to high ion migration resistance. , This will lead to underutilization of surface and bulk active sites, and thus, redox kinetics remains low. In aqueous electrolytes, H + and OH – follow a Grotthuss transport phenomenon via hopping in H 2 O molecule chains, exhibiting a much higher transport rate than other hydrated ions. , Such transports are benefitted by electrode materials with layered structures where the electrode also paves channeled pathways. Thus, LDHs become important from that ions’ accumulation perspective too .…”

“…In aqueous electrolytes, H + and OH − follow a Grotthuss transport phenomenon via hopping in H 2 O molecule chains, exhibiting a much higher transport rate than other hydrated ions. 35,36 Such transports are benefitted by electrode materials with layered structures where the electrode also paves channeled pathways. Thus, LDHs become important from that ions' accumulation perspective too.…”