Transport Mechanism of Hydroxide Ions Focused on the Vehicle Mechanism Near the Cation Sites in Anion Exchange Membranes
Authors: Ono Ryusei , Suzuki Hiroto , Miyatake Kenji , Tokumasu Takashi 
Published: December 2024

Recently, interest in hydrogen usage and production has surged to achieve a sustainable society, with fuel cells and electrolyzers coming into focus. Fuel cells exhaust only water, offering an eco-friendly alternative. Electrolyzers, utilizing renewable sources like solar and wind, generate hydrogen from water, reducing environmental impact. We explore anion exchange membranes (AEM) used in the fuel cells and electrolyzers. While proton exchange membranes (PEM) are currently more common, AEMs offer advantages such as cost reduction through non-precious metal catalysts, operations with less corrosion, and a variety of fuel options, potentially expanding applications. However, a major performance degradation cause in AEMs is their inferior ion transport properties. Enhancing AEM ion conductivity, particularly by understanding the hydroxide ion transport mechanism, remains crucial. Thus, we aim to clarify the hydroxide ion transport mechanism in AEMs.

In AEMs under low hydration conditions, the vehicular mechanism generally prevails over the Grotthuss mechanism. Recent studies on AEM materials with rigid side chains have shown that the proximity and rigidity of cationic sites cause overlapping of cationic solvation shells, suggesting an increased contribution of the vehicular mechanism (1). Specifically, hydroxide ions are transported via the vehicular mechanism along with surrounding water molecules, implying that the overlapping of cationic solvation shells facilitates smooth movement of hydroxide ions around the cationic sites within AEMs. These insights into the importance of vehicular mechanism prompt our study to use classical molecular dynamics simulations to clarify its impact on hydroxide ion transport in AEMs.

We focus on the QPAF-4 polymer (2) distinguished by its high hydroxide ion conductivity and stability under alkaline conditions, in simulations conducted on LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). The simulation cell is composed of 35 QPAF-4 polymers, featuring 210 cationic groups (NH3
+), and 210 hydroxide ions to maintain charge balance. The number of water molecule depends on hydration level (λ), the ratio of water molecule to cationic group. Simulation parameters include a 1.0 fs timestep, three-dimensional periodic boundary, sampling from 1.0×103 to 1.0×107 steps, 10 ns simulation time, temps of 300 K and 353 K, and λ = 3, 4, 6, 9, 12.

This study reveals the relationship between hydration level (λ) and equilibrium density in QPAF-4 polymers. As λ increases, the equilibrium density increases until λ = 5, beyond which it decreases. This behavior aligns with phenomena observed in previous research using AEM materials with quaternary ammonium cationic groups (1). In earlier studies, under low λ conditions, polymer voids filled with water, leading to increase density, whereas at higher λ, polymer water absorption and expansion cause a decrease in density. These findings suggest that a similar phenomenon may occur in QPAF-4 polymers.

RDF analysis at 300 K evaluated distances between nitrogen atoms of cationic groups of QPAF-4 polymer and hydroxide ion oxygen atoms (gN-Oh(r)), depicted in Fig.1(a), and water molecule oxygen atoms (gN–Ow(r)), illustrated in Fig.1(b), under varying λ conditions. First peaks of gN–Oh(r) and gN–Ow(r) existed at approximately 3.4 Å, with first minima at approximately 4.7 Å and 5.3 Å, respectively, defining the first solvation shell. The greater first minimum value for gN–Ow(r) suggests hydroxide ion transport within the first solvation shell of cation group. The ratio of the number of hydroxide ions and water molecules in the first solvation shell of a cationic group to the total number of these molecules in the system are 96.4 % and 87.6 %, respectively. The analysis of distances between cationic group nitrogen atoms (gN–N(r)), illustrated in Fig.1(c), under each λ condition indicates a structural distance of 6.0 Å at λ = 6. Given that the radius of the first solvation shells is approximately 5.3 Å, this implies overlapping first solvation shells around cationic groups, suggesting continuous solvation shell formation. As λ increases, cationic group distances expand, but distances to water and hydroxide ions stay constant, underscoring the need to study the impact of reduced overlapping regions for ion transport.

Future research will explore ion transport in AEMs under different λ and temperatures to find optimal conditions for hydroxide ion transport. This could aid in developing new AEM materials. The study emphasizes the essential hydroxide ion transport mechanism for electrolyte membrane performance.

References

(1)Chen, Chen, et al. "Hydroxide Solvation and Transport in Anion Exchange Membranes," Journal
of the American Chemical Society, 2016, pp. 991-1000.

(2)Ono, Hideaki, et al. "Robust anion conductive polymers containing perfluoroalkylene and pendant ammonium groups for high performance fuel cells." Journal of Materials Chemistry A, 2017, pp. 24804–24812.

Figure 1