Exploring noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) is the key issue in hydrogen economy blueprint. Herein, bimetallic Ni2-xCoxP supported by N-doped carbon nanofibers (denoted as Ni2-xCOxP/N-C NFs) are developed via electrospinning followed by pyrolysis under an inert flow. The space confinement by in-situ formed N-doped carbon matrix produces ultrafine Ni2-xCoxP with abundant active-sites. More importantly, the engineering on Ni2-xCoxP solid-solution-alloys can vary electronic configuration and consequently optimize hydrogen binding on electrocatalyst surface, accomplishing prominent HER activity in a wide pH range. The optimal Ni2-xCOxP/N-C NFs afford low overpotentials (eta(10)) of 100, 130 and 110 mV to reach a current density of -10 mA cm(-2) in 0.5 M H2SO4, 1.0 KOH and 1.0 M PBS (phosphate buffer saline), respectively, performing among the best of noble-metal-free electrocatalysts. With a good functionality for oxygen evolution (eta(10) = 280 mV), such composite further delivers a high efficiency for overall water splitting, featuring a comparable cell voltage (1.56 V @ 10 mA cm(-2)) to a commercial IrO2/C - Pt/C couple, and remarkably better stability. Identifying electrocatalysis relying on solid-solution alloys, this work will inspire the exploration of cost-efficient electrocatalysts and the new understanding on catalytic mechanisms.