In this work, defect-rich ordered mesoporous spinel oxides, including CoCo 2 O 4 , NiCo 2 O 4 , and ZnCo 2 O 4 , were developed as bifunctional electrocatalysts toward oxygen reduction and evolution reactions (ORR and OER, respectively). The materials are synthesized via nanocasting and modified by chemical treatment with 0.1 M NaBH 4 solution to enhance the defect concentration. The synthesized samples have metal and oxygen divacancies (V Co + V O ) as the primary defect sites, as indicated by positron annihilation lifetime spectroscopy (PALS). Cation substitution in the spinel structure induces a higher number of oxygen vacancies. The increased number of surface defects and the synergistic effect between two incorporated metals provide a high activity in both the OER and ORR in the case of NiCo 2 O 4 and ZnCo 2 O 4 . Especially, ZnCo 2 O 4 exhibits the highest OER/ORR activity. The defect engineering with 0.1 M NaBH 4 solution results in a metal-hydroxylated surface (M-OH) and enhanced the catalytic activity for the post-treated metal oxides in the ORR and OER. This fundamental investigation of the defective structure of the mixed metal oxides offers some useful insights into further development of highly active electrocatalysts through defect engineering methods.
Metal complexes with chelating ligands are known as promising precursors for the synthesis of targeted metal oxides via thermal decomposition pathways. Triethanolamine (TEA) is a versatile ligand possessing a variety of coordination modes to metal ions. Understanding the crystal structure is beneficial for the rational design of the metal complex precursors. Herein, a bis(triethanolamine)nickel (II) dinitrate (named as Ni-TEA) crystal was synthesized and thoroughly investigated. X-ray crystallography revealed that Ni(II) ions adopt a distorted octahedral geometry surrounded by two neutral TEA ligands via two N and four O coordinates. Hirshfeld surface analysis indicated the major contribution of the intermolecular hydrogen-bonding between —OH groups of TEA in the crystal packing. Moreover, several O–H stretching peaks in Fourier transformed infrared spectroscopy (FTIR) spectra emphasizes the various chemical environments of —OH groups due to the formation of the hydrogen-bonding framework. The Density-functional theory (DFT) calculation revealed the electronic properties of the crystal. Furthermore, the Ni-TEA complex is presumably useful for metal oxide synthesis via thermal decomposition at a moderate temperature (380 °C). Cyclic voltammetry indicated the possible oxidative reaction of the Ni-TEA complex at a lower potential than nickel(II) nitrate and TEA ligand, highlighting its promising utility for the synthesis of mixed valence oxides such as spinel structures.
In-situ investigations of structural transitions during the thermal-oxidative event of mixed-metal spinel oxide precursors, the so-called nickel- (NCO) and zinc-containing (ZCO) cobaltite spinel precursors, were investigated to understand the formations of the derived NiCo2O4 and ZnCo2O4 spinel oxides, respectively. In-situ XRD investigation revealed that emerged temperatures for spinel oxide phase were between 325 and 400 °C, depending on the cationic substituent. It indicated that the emerged temperature correlated with the absolute octahedral site preference energy (OSPE) of those cations that participated in the development of the spinel framework. Moreover, the incorporated nickel and zinc in the precursors was beneficial for inhibiting the occurrence of the undesired CoO phase. Time-resolved X-ray absorption spectroscopic (TRXAS) data suggested the local structure rearrangement of nickel and zinc throughout the calcination process, which differed from the behavior of single-metal cobalt system. The essential information reported herein provides a benefit to control the cationic distribution within spinel materials, leading to the tunable physical and chemical properties.
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