Developing efficient bifunctional electrocatalysts and gaining fundamental understanding of reaction mechanisms are crucial for practical water splitting. Herein, a bifunctional NiSe2 nanoparticles/carbon fiber paper (NSN/CFP) electrode is fabricated by the pyrolysis of Ni(NO3)(2) on CFP, followed by a selenization step. The as-prepared electrocatalysts exhibit superior overall water splitting behavior in 1 M KOH with low overpotentials of 145 mV and 280 mV at current densities of 10 mA cm(-2) for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) respectively, comparable to the performance of 20% Pt/C and RuO2. Detailed compositional and morphological studies reveal that the NiSe2 gradually transforms into an amorphous Ni(OH)(2)/NiOOH heterojunction during both HER and OER in alkaline medium. Based on these experimental results, an oxidation-induced self-reconstruction mechanism is proposed. Owing to the highly-oxidized Ni(OH)(2)/NiOOH active species, the self-reconstructed structure enhances the water splitting under fixed potentials for a prolonged time of 96 h with negligible current degradation. This work not only provides a facile route to fabricate efficient and stable electrocatalysts for large-scale water splitting but also reveals an underlying structural evolution mechanism, which guides the rational design of heterogeneous catalysts. (C) 2019 Elsevier Ltd. All rights reserved.