Hydrogen
economy based on electrochemical water splitting is one
of the most prospective strategies to circumvent the rapid consumption
of traditional fossil fuels. The development of efficient and durable
electrocatalysts for water splitting plays a crucial role in the energy
conversion and storage process. Nickel selenide (Ni
x
Se
y
), as a typical multifunctional
electrocatalyst, has attracted great consideration owning to its component
diversity, high conductivity, and regulated morphology/electronic
structures. In Ni
x
Se
y
, nickel has a unique valence electron configuration (3d84s2) and acts as the main catalytic activity site.
Compared with S and O, Se in Ni
x
Se
y
not only has the same valence electrons
and oxidation number but also has excellent intrinsic metal properties,
which means better electrical conductivity and electrocatalytic activity.
In addition, Ni and Se could form stoichiometric compounds (NiSe,
NiSe2, Ni3Se2, Ni3Se4) and nonstoichiometric compounds (Ni0.85Se), which
is ascribed to the electronegativity difference between Ni (1.9) and
Se (2.4). In this review, the crystal structure, preparation methods,
and practical applications of Ni
x
Se
y
-based electrocatalysts are summarized. The
merits and limitations of nickel selenide are discussed in terms of
structure and composition. Finally, the challenges and opportunities
faced by NixSey-based electrocatalysts in water splitting are discussed.