  Black holes possess trapping regions which lead to intriguing dynamical
effects. By properly scattering test fields off a black hole, one can extract
energy from it, leading to the growth of the amplitude of the test field in
expense of the black hole's energy. Such a dynamical phenomenon is called
superradiance. Here, we study charged-scalar-field instabilities of
Reissner-Nordstr\"{o}m black holes immersed in a $d-$dimensional de Sitter
Universe. By performing a thorough frequency-domain analysis we compute the
unstable quasinormal resonances and link their presence with a novel family of
quasinormal modes associated with the existence and timescale of the
cosmological horizon of pure de Sitter spacetime. Our results indicate that
such an instability is caused by superradiance, while the increment of
dimensions amplifies the growth rate and enlarges the region of the parameter
space where, both massless and massive, test fields are unstable.
