Magnesium-Aluminum Layered Double Hydroxide Nanosheet-Stacked Membranes as Solid Electrolytes for Fuel Cells

Layered double hydroxides (LDHs) have emerged as promising solid state hydroxide ion (OH^-) conductors owing to their tunable composition, 2D layered architecture, and high anion exchange capability. However, their practical implementation as electrolytes in fuel cells has long been impeded by the difficulty of fabricating dense membranes, which leads to severe hydrogen crossover and a markedly reduced open circuit voltage (OCV) of only ∼0.8 V, far below the theoretical value. Here, we demonstrate for the first time a dense, free-standing nanosheet-stacked membrane of magnesium aluminum layered double hydroxide (MgAl-LDH) prepared via an exfoliation-reassembly strategy. The membrane exhibits an out-of-plane hydroxide ion conductivity of 0.0065 S cm^-1 at 80°C and 100% RH, while its in-plane conductivity reaches 0.22 S cm^-1, approximately 30-fold higher, underscoring the intrinsic anisotropy of ion transport in LDH nanosheets. Remarkably, when employed in direct ethanol fuel cells, the MgAl-LDH nanosheet membrane achieves an OCV of ∼1.10 V, indicating substantially suppressed fuel crossover compared with conventional LDH-based electrolytes. This work establishes LDH nanosheet membranes as a new class of dense, high-performance hydroxide ion conductors, overcoming a long standing barrier in LDH-based electrolytes and paving the way for nanosheet-stacked solid electrolyte fuel cells.