commercial application is significantly restricted due to the paucity and preciousness of Pt. Therefore, the exploration of high-efficient and earth-abundant materials is of great importance for the future of hydrogen economy, [6][7][8] and it is highly desirable to create a low-cost, robust, and high-active electrocatalyst for HER.In the last decade, abundant transition metal-based electrocatalysts have been reported as the promising alternatives to noble metals for HER, including transition metal chalcogenide, carbides, and nitrides, such as MoS 2 , [9] CoSe 2 , [10] W 2 C, [11] and MoN 4 . [12] In particular, transition metal phosphides (TMPs) have attracted considerable attention in HER, [13][14][15] where CoP has been regarded as one of the best candidates for HER. It has been demonstrated that P atoms in the TMPs can draw electrons from metal and serve as a proton-acceptor, [16,17] and the activity of this kind of catalyst severely depends on the microstructures of CoP. [18] Hence, great efforts have been made to fabricate different morphologies of CoP materials, such as nanoparticles, [19] nanowires, [20,21] nanosheets, [22,23] and nanotube. [24] However, some CoP nanorods or nanoparticles are vulnerable to agglomeration due to the large current density and long-term testing, resulting in degenerative stability and low electrocatalytic activity. [25] Accordingly, it is challengeable but more attractive to develop an electrocatalyst with robust stability and high activity.3D nanoarrays (NA) directly grown on the current collectors have been regarded as popular high-capability electrodes in the past few years. The advantages of 3D nanoarray can be mainly summarized as the following points: 1) The 3D nanostructures could accelerate the desorption of H 2 gas bubbles from catalyst surface and then improve the mass transfer. [26] 2) The 3D structures can avoid the aggregation of electrocatalysts caused by the high current and long-term testing. 3) The self-assembled electrodes possess prominent advantages than that required to be immobilized by polymer binder such as Nafion which will block active sites and inhibit diffusion. [27] In order to further enhance electrocatalytic performances and stability, an effective method is to wrap the nanorods array or nanoparticles (NPs) with carbon layers, which can protect the catalysts from the degradation and agglomeration at performance testing. [28] For Hydrogen evolution reaction (HER) is a key reaction in water splitting, and developing efficient and robust non-noble electrocatalysts for HER is still a great challenge for large-scale hydrogen production. Herein, a vertically aligned core-shell structure grown on Ti foil with CoP nanoarray as a core and N,P-doped carbon (NPC) as a shell (CoP/NPC/TF) is first reported as an efficient electrocatalyst for HER. Results indicate that CoP/NPC/TF only demands the overpotentials of 91 and 80 mV to drive the current density of 10 mA cm −2 in acidic and alkaline solutions. The electrochemical measurements and theoretical calcula...
