  The coexistence of metallicity and ferroelectricity has been an intriguing
and controversial phenomenon as these two material properties are considered
incompatible in bulk. We clarify the concept of ferroelectric metal by
revisiting the original definitions for ferroelectric and metal. We propose
that a two-dimensional (2D) ferroelectric with out-of-plane polarization can be
engineered via layer stacking to a genuine ferroelectric metal characterized by
switchable polarization and non-zero density of states at the Fermi level.
Using multilayer $\alpha$-In$_2X_3$ ($X$ = S, Se, Te) as model systems, we
demonstrate that 2D ferroelectric metals serve as electrically-tunable,
high-quality electrocatalysts for hydrogen evolution and oxygen reduction
reactions.
