ABSTRACT
Development of Fe-Ni-based electrocatalysts with high efficiency and stability remains a foremost challenge in the research for oxygen evolution reaction (OER) under high-current-density. Herein, a fast reduction strategy is developed for synthesis of strongly coupled crystalline α-Ni(OH)2 with amorphous reduced FeOOH (r-FeOOH) heterostructure grown on Ni foam (r-FeOOH/α-Ni(OH)2/NF). The obtained r-FeOOH/α-Ni(OH)2 with particle sizes around ~ 10 nm is coated orderly on the 3D NF surface in this hybrid. Benefitting from the strong coupling effects between r-FeOOH and α-Ni(OH)2, low potentials of 1.62 and 1.66 V at ultra-high current densities of 1,000 and 1,500 mA cm-2, as well as a robust stability over 10 h at 1,500 mA cm-2 in alkaline electrolyte are achieved in 3D r-FeOOH/α-Ni(OH)2/NF. Such a high OER performance is almost the best among all previously reported Fe-Ni-based OER electrocatalysts. Experimental results revealed that the NiOOH species is the real OER active phase in the 3D r-FeOOH/α-Ni(OH)2/NF. Further, bifunctional 3D r-FeOOH/α-Ni(OH)2 in alkaline electrolyzer delivers low cell voltages of 2.32 and 2.78 V to attain 500 and 1,000 mA cm-2 toward the overall-water-splitting, surpassing the benchmark Pt/C-Ir/C/NF system.
ABSTRACT
Cost-effective and stable electrocatalysts with ultra-high current densities for electrochemical oxygen evolution reaction (OER) are critical to the energy crisis and environmental pollution. Herein, we report a superaerophobic three dimensional (3D) heterostructured nanowrinkles of bimetallic selenides consisting of crystalline NiSe2 and NiFe2Se4 grown on NiFe alloy (NiSe2/NiFe2Se4@NiFe) prepared by a thermal selenization procedure. In this unique 3D heterostructure, numerous nanowrinkles of NiSe2/NiFe2Se4 hybrid with a thickness of ~ 100 nm are grown on NiFe alloy in a uniform manner. Profiting by the large active surface area and high electronic conductivity, the superaerophobic NiSe2/NiFe2Se4@NiFe heterostructure exhibits excellent electrocatalytic activity and durability towards OER in alkaline media, outputting the low potentials of 1.53 and 1.54 V to achieve ultra-high current densities of 500 and 1000 mA cm-2, respectively, which is among the most active Ni/Fe-based selenides, and even superior to the benchmark Ir/C catalyst. The in-situ derived FeOOH and NiOOH species from NiSe2/NiFe2Se4@NiFe are deemed to be efficient active sites for OER.
ABSTRACT
Manganese oxides are known as one type of semiconductors, but their photocatalysis characteristics have not been deeply explored. In this study, photocatalytic degradation of phenol using several synthesized manganese oxides, i.e, acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD), were comparatively investigated. To elucidate phenol degradation mechanisms, X-ray diffraction (XRD), ICP-AES (inductively coupled plasma-atomic emission spectroscopy), TEM (transmission electronic microscope), N2 physisorption at 77 K and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed to characterize the structural, compositional, morphological, specific surface area and optical absorption properties of the manganese oxides. After 12 hr of UV-Vis irradiation, the total organic carbon (TOC) removal rate reached 62.1%, 43.1%, 25.4%, and 22.5% for cryptomelane, acidic birnessite, todorokite and alkaline birnessite, respectively. Compared to the reactions in the dark condition, UV-Vis exposure improved the TOC removal rates by 55.8%, 31.9%, 23.4% and 17.9%. This suggests a weak ability of manganese oxides to degrade phenol in the dark condition, while UV-Vis light irradiation could significantly enhance phenol degradation. The manganese minerals exhibited photocatalytic activities in the order of: CRY > BIR-H > TOD > BIR-OH. There may be three possible mechanisms for photochemical degradation: (1) direct photolysis of phenol; (2) direct oxidation of phenol by manganese oxides; (3) photocatalytic oxidation of phenol by manganese oxides. Photocatalytic oxidation of phenol appeared to be the dominant mechanism.
