  Phosphorene, a monolayer of bulk black phosphorus, is promising for light
harvest owing to its high charge mobility and tunable direct band gap covering
a broad spectral range of light. Here, via atomic-scale first-principles
simulations, we report a ultrahigh activity of hydrogen evolution reaction
(HER) of phosphorene originated from defective activation. Quantitative
evaluation of the Gibbs free energy of the ad desorption of hydrogen (H) to
from phosphorene (delta_GH) reveals that atomic vacancies and edges play a
dominant role in activating the reaction. We find that the defective states,
empty and well-localized around the defect core, are compensated by H species.
This induces a proper hydrogen interaction complying with the thermoneutral
condition of the free energy (delta_GH=0) comparable to platinum. Our findings
of the highly activating defective states suggest the design of non-metal HER
catalysts with structural engineering of earth-abundant phosphorus structures.
