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

The development of an electrocatalyst with high bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances using non-precious-metal/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.