  Electrocatalysts based on transition metal sulfides are drawing accelerating
concerns in renewable energy research because of their intrinsically excellent
activities towards both hydrogen evolution reaction and oxygen evolution
reaction. To date, considerable efforts are made to improve the performance of
these catalysts, but ignoring the improper synthesis strategy would incur
additional cost to the catalyst. Herein, a convenient, one-step anodization
method is developed for fast construction of cobalt sulfides. Without any
high-temperature or long-time treatment, mesoporous CoSx is self-grown on the
metal substrate in minutes. As a result, as-anodic CoSx requires overpotentials
of 102 mV for HER and 284 mV for OER to achieve a current density of 10 mA m-2
in alkaline solution. Moreover, the tandem bifunctional as-anodic CoSx exhibits
a required cell voltage of 1.64 V for overall water splitting in alkaline
solution, exceeding most of the documented Co-based electrocatalysts.
