Bifunctional bamboo-like CoSe₂ arrays for high-performance asymmetric supercapacitor and electrocatalytic oxygen evolution

Bifunctional bamboo-like CoSe₂ arrays are synthesized by thermal annealing of Co(CO₃)_0.5OH grown on carbon cloth in Se atmosphere. The CoSe₂ arrays obtained have excellent electrical conductivity, larger electrochemical active surface areas, and can directly serve as a binder-free electrode for supercapacitors and the oxygen evolution reaction (OER). When tested as a supercapacitor electrode, the CoSe₂ delivers a higher specific capacitance (544.6 F g^-1 at current density of 1 mA cm^-2) compared with CoO (308.2 F g^-1) or Co₃O₄ (201.4 F g^-1). In addition, the CoSe₂ electrode possesses excellent cycling stability. An asymmetric supercapacitor (ASC) is also assembled based on bamboo-like CoSe₂ as a positive electrode and active carbon as a negative electrode in a 3.0 M KOH aqueous electrolyte. Owing to the unique stucture and good electrochemical performance of bamboo-like CoSe₂, the as-assembled ACS can achieve a maximum operating voltage window of 1.7 V, a high energy density of 20.2 Wh kg^-1 at a power density of 144.1 W kg^-1, and an outstanding cyclic stability. As the catalyst for the OER, the CoSe₂ exhibits a lower potential of 1.55 V (versus RHE) at current density of 10 mA cm^-2, a smaller Tafel slope of 62.5 mV dec^-1 and an also outstanding stability.