Ultrafine NiFe-Based (Oxy)Hydroxide Nanosheet Arrays with Rich Edge Planes and Superhydrophilic-Superaerophobic Characteristics for Oxygen Evolution Reaction.

NiFe-based (oxy)hydroxides are the benchmark catalysts for the oxygen evolution reaction (OER) in alkaline medium, however, it is still challenging to control their structures and compositions. Herein, molybdates (NiFe(MoO4 )x ) are applied as unique precursors to synthesize ultrafine Mo modified NiFeOx Hy (oxy)hydroxide nanosheet arrays. The electrochemical activation process enables the molybdate ions (MoO4 2- ) in the precursors gradually dissolve, and at the same time, hydroxide ions (OH- ) in the electrolyte diffuse into the precursor and react with Ni2+ and Fe3+ ions in confined space to produce ultrafine NiFeOx Hy (oxy)hydroxides nanosheets (<10 nm), which are densely arranged into microporous arrays and maintain the rod-like morphology of the precursor. Such dense ultrafine nanosheet arrays produce rich edge planes on the surface of NiFeOx Hy (oxy)hydroxides to expose more active sites. More importantly, the capillary phenomenon of microporous structures and hydrophilic hydroxyl groups induce the superhydrophilicity and the rough surface produces the superaerophobic characteristic for bubbles. With these advantages, the optimized catalyst exhibits excellent performance for OER, with a small overpotential of 182 mV at 10 mA cm-2 and long-term stability (200 h) at 200 mA cm-2 . Theoretical calculations show that the modification of Mo enhances the electron delocalization and optimizes the adsorption of intermediates.

Mediation of water-soluble oligoaniline by phenol in the aniline-persulfate system under alkaline conditions.

Although synthesis of oligoaniline (OANI) by persulfate and aniline has been investigated in the recent years, the impact of phenol on the synthesized soluble OANI is still not clear. In this study, our results indicate that phenol and pH mediate the production of the blue water-soluble OANI (OANIblue) in the reaction between sodium persulfate (SPS) and aniline under alkaline conditions, and the yields of OANIblue increase with increasing concentrations of phenol and pH values. Quenching experiments rule out the contributions of SO4˙- and ˙OH to aniline oxidation and imply that the non-radical activation of SPS is an important pathway in the formation of OANIblue. MALDI-TOF-MS analysis indicates that phenol apparently inhibits the polymerization degree of aniline in that the molecular weights of OANIblue gradually decrease from 1586.4 to 684.6 when phenol is increased from 0 to 2.0 mM. FTIR and Raman analyses confirm the structure of aniline oligomers in OANIblue and indicate that phenol inhibits the phenazine-like structure in OANIblue and facilitates the transformation of benzenoid rings to quinoid rings in the oxidation products. However, simultaneous activation of SPS by phenol and aniline is likely to occur in the reaction system with the formation of PhNH˙, as indicated by DFT calculations. The high scavenging reactivity of phenol towards both PhNH2˙+ and PhNH˙ implies that PhNH2˙+ and PhNH˙ are not the intermediates in the formation of OANIblue. DFT calculations also reveal that apart from the one-electron transfer pathway between aniline and SPS, the two-electron transfer pathway is also likely to occur in the presence of phenol, resulting in the formation of PhNH+/PhN˙˙ without producing PhNH2˙+ and PhNH˙. The produced PhNH+/PhN˙˙ intermediates then couple with aniline, PhNH+, aminophenyl sulfate and its hydrolysate to form dimers, trimers, oligomers, and eventually OANIblue. This study not only describes a novel method to prepare water-soluble OANI, but also gives new insight on the importance of phenol in the production of OANIblue.