Lately, the energy density was further improved by
extending the operating voltage utilizing the strategy of
designing the asymmetric system by coupling different
positive and negative electrode materials with wellseparated
potential windows to obtain a high operation
voltage [15,17,18,22,23]. In fact, regardless of a higher
energy density has thus been achieved, the stability of the
supercapacitors remains a major problem [24]. Additionally,
the preparation of two different electrode materials
increases the cost of production and inhibits their widespread
application.