Nickel-carbon-B2O3 nanocomposites (Ni@C-B(2)O(3)s) were synthesized from a single-step reaction of CO2 with NaBH4 at 1 bar and investigated as catalysts for methane decomposition. These catalysts exhibited an unprecedented activity for producing hydrogen and graphitic carbon nanoonions (CNOs) without any generation of carbon oxides (COx) despite of the absence of a catalyst reduction step by hydrogen. Around 90% of methane conversion with 61 mmol min(-1) g(Ni)(-1) was achieved using 13 wt% nickel-containing Ni@C-B2O3 (13Ni@C-B2O3) at 850 degrees C under 46.15 L-CH4 h(-1) g(Ni)(-1). The uncompromising activity of the catalyst comes from the two major contributions: the absence of nickel oxide eliminates the formation of carbon oxides and the amorphous core-shell structure of Ni@C-B(2)O(3)s facilitates nano-sized nickel cores to escape the CNOs and directly decompose methane. This was verified by various microscopic and spectroscopic investigations. The generated CNOs on 13Ni@C-B2O3 were partially oxidized by CO2 as a mild oxidant with the production of CO. The catalytic activity of 13Ni@C-B2O3 was maintained for 15 cycles, providing a potential of enabling the cyclic process of CH4 decomposition/CO2 regeneration. By synthesizing Ni@C-B(2)O(3)s from CO2 and using them to the cyclic operation, not only was CO2 converted to the valuable catalyst, but hydrogen, CNOs, and CO were also produced from the greenhouse gases of methane and CO2. (C) 2015 Elsevier B.V. All rights reserved.