Design and Synthesis of Integrally Structured Ni3N Nanosheets/Carbon Microfibers/Ni3N Nanosheets for the Efficient Full Water Splitting Catalysis
The relatively poor electric conductivity of metal nitrides with low density of utilizable active sites always restricted their catalytic abilities toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) for the highly efficient water splitting. Herein, for the first time, we reported the design and synthesis of a novel three-dimensional (3D) hierarchically meso-macroporous and hollow tube-like architecture of Ni3N nanosheets/carbon microfibers/Ni3N nanosheets (Ni3N/CMFs/Ni3N) assembled by the porous Ni3N nanosheets (NSs) dispersed over inner and outer walls of the hollow and porous CMFs. Benefitting from the excellent electric conductivity and the high density of utilizable active sites, the Ni3N/CMFs/Ni3N revealed superior OER and HER catalytic activities compared with that of Ni3N which was supported by graphene (Gr), carbon nanotubes (CNTs) and macroporous carbons (MPCs), respectively. In O2-saturated 1.0 M KOH, the OER potential for 10 mA cm-2 [E10, 1.50 V vs. RHE] and the Tafel slope (41.54 mV dec−1) of Ni3N/CMFs/Ni3N were both lower than those of RuO2 [1.53 V vs. RHE and 43.45 mV dec−1, respectively]. As the HER E10 value of Ni3N/CMFs/Ni3N [-0.115 V (vs. RHE)] was only 40 mV larger than that of the commercial Pt/C [-0.075 V (vs. RHE)] in N2-saturated 1.0 M KOH. For full water splitting, to achieve the current density of 20 mA cm-2 (E20) in O2-saturated 1.0 M KOH, the (-) Ni3N/CMFs/Ni3N||Ni3N/CMFs/Ni3N (+) electrolysis cell just needed a cell voltage of 1.652 V, which was only 19 mV larger than that of the state-of-the-art (-) Pt||RuO2 (+) benchmark (1.633 V). In addition, due to the remarkable structural and chemical stabilities of the Ni3N/CMFs/Ni3N especially the protection of the unique ~2-3 layers of graphite carbon shells (GCSs) on the Ni3N nanoparticles (NPs) in Ni3N/CMFs/Ni3N, the Ni3N/CMFs/Ni3N based water electrolysis cell also displays excellent stability. The conceptual new double surfaces catalysis model of the Ni3N/CMFs/Ni3N which was assembled by the porous Ni3N nanosheets (NSs) dispersed over inner and outer walls of the hollow and porous CMFs opens an exciting new direction for the rational design and fabrication of porous nanomaterials for electrochemical energy and sensing applications.