Controlling phase transformation of La/Ni oxides by switching annealing conditions and their resultant pseudocapacitive behaviour

Abstract

Annealing step is a critical step in the hydrothermal assisted synthesis of La/Ni oxides such as LaNiO3−δ (LNOA) and La2NiO4+δ (LNON). In the current study, we have discovered the interlink between the atmosphere and temperature conditions which dictate the product formed. La/Ni nitrate precursors were subjected to a hydrothermal synthesis followed by an annealing step at appropriate temperature and varying atmosphere resulting in the synthesis of the corresponding oxides. The annealing temperature was varied in the range between 650 °C and 800 °C and also the annealing was carried out either in pure N2 atmosphere or air. From the x-ray diffraction analysis, it was inferred that annealing in air invariably resulted in the rhombohedral LaNiO3 (LNOA) perovskite phase, while annealing in N2 atmosphere resulted in an orthorhombic Ruddlesden–Popper phase La2NiO4 (LNON), a layered oxide containing traces of NiO phase. Typically, iodometric titrations substantiate the presence of Ni (III) which further can be correlated to the presence of oxygen vacancies (δ). Iodometric test results demarcated the difference between the two phases with absolutely minimal I2 liberated from the LNON samples proving that negligible amount of Ni (III) was present in LNON Scanning electron microscopy (SEM) images showed an agglomeration of particles annealed at higher temperatures irrespective of the atmosphere. Temperature dependent oxygen non-stoichiometry (Δδ) was analyzed through thermogravimetric analysis, wherein Δδ was inversely proportional to the annealing temperature for all of the LNOA samples. Considering that large δ values favor pseudocapacitive behaviour, it was observed LNOA oxides showed excellent pseudocapacitive behaviour compared to the LNON oxides. Dunn deconvolution of the cyclic voltammograms of LNOA 800 °C at 5 m Vs−1 indicated that diffusive contribution (66%) was predominant over capacitive contributions. The LNOA sample annealed at 800 °C displayed the highest specific capacitance of 100.3 F g−1 at 1 A g−1 current density.