Simple Method to Enhance the Oxidation–Reduction Reaction Activity of a Carbon Nanotube-Based Nickel–Iron Layered Double Hydroxide as a Bifunctional Electrocatalyst

Abstract

The development of an electrocatalyst with high bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances using non-preciousmetal/metal oxides is highly important as a future energy material. NiFe-layered double hydroxides (LDH) are well-known catalysts that have high OER activity, while their ORR activity is low. We here present a method to enhance the ORR of NiFe-LDH. We first synthesized a polybenzimidazole (PBI)-coated multiwalled carbon nanotube (MWNT)-based NiFe-LDH catalyst (MWNT/PBI-NiFe-LDH), then by sulfurization of the LDH, sulfur-doped catalysts, MWNT/PBI/NiFeS-xh (x is sulfurization time) were synthesized, which were characterized by TEM, STEM, EDX, thermogravimetry, X-ray diffraction analysis, XPS, and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). After sulfurization, the nanoplate structure of MWNT/PBI-NiFe-LDH changed to nanoparticles that were composed of Ni−Fe−S composites. The valency of Ni and Fe is suggested to be +2 and +3, respectively. MWNT/PBI/NiFeS-xh provided a higher ORR (E 1/2 = 0.80 V vs RHE) than that (E 1/2 = 0.70 V vs RHE) of MWNT/PBI-NiFe-LDH with no sulfurization, while the sulfurization did not change their OER activities. This study provides an efficient and simple method for enhancing ORR of LDH catalysts, and the method is useful for designing a non-precious-metal-based ORR/OER bifunctional catalyst with high performance.

Supramolecular-Based One-Pot Separation of Highly Pure Adsorbent-Free Semiconducting Single-Walled Carbon Nanotubes and Machine Learning-Based Nanotube Solubilization

“…In this Account, we focus on supramolecular chemistry-based one-pot preparation of adsorbent-free semiconducting single-walled carbon nanotubes (sem-SWNTs). − Carbon nanotubes (CNTs) are made of rolled-up graphene sheets and are classified into three types of CNTs in terms of the number of graphene layers within a CNT, that is, SWNTs, double-walled carbon nanotubes and multiwalled carbon nanotubes. Among these, SWNTs have remarkable electronic, mechanical, optical, chemical and thermal properties, which are derived from their one-dimensional extended π-conjugated structures; − thus, they demonstrate a high potential toward the development of the next-generation nanoelectronics, , (nano)bio − and energy and environmental materials and devices. − The fundamental electronic properties − ,− (i.e., electronic densities, Fermi levels, and redox potentials) of the SWNTs are related to their atomic structures that have a specified diameter and chirality angle that is expressed by their own chiral indices ( n , m ), in which the integers ( n , m ) represent the number of unit vectors along the two directions within the honeycomb lattice structure. − When ( n–m )/3 is an integer, the SWNTs are metallic, and others are sem-SWNTs. The chiral indices ( n , m ) govern fundamental SWNT properties. − …”