Expansion of the multifunctionality in off-stoichiometric manganites using post-annealing and high pressure: physical and electrochemical studies

Abstract

Prospects for the use of manganites in various areas of modern technologies require comprehensive studies of their physical and chemical properties. La_0.9Mn_1.1O₃ (LMO) ceramics have been synthesized at an annealing temperature t_ann of 1150 °C with further post-annealing at 1250, 1350, and 1450 °C. As t_ann increases, the structure symmetry changes, and both the crystallite size and chemical defects increase. The post-annealing, on one hand, leads to a dramatic reduction of the magnetocaloric effect (MCE) |−ΔS^max_M| from 3.50 to 0.75 J (kg K)⁻¹ at 2 T and a Curie temperature T_C from 227 to 113 K with increasing t_ann. On the other hand, an external hydrostatic high-pressure P works oppositely enhancing ferromagnetic interactions. The saturation of −ΔS^max_M and T_C is already achieved at a relatively low P of ≈ 0.4 GPa. LMO-1150 exhibits the best magnetocaloric characteristics compared with other studied samples. Moreover, the electrochemical characteristics of the LMO materials as electrocatalysts for overall water splitting (OER process) and features of their transformation in different 0.5 M K₂SO₄, 0.5 M K₂HPO₄, and 0.1 M K₂B₄O₇ electrolytes have been studied thoroughly. After electrocatalysis of LMO, the magnetization M decreases and T_C remains, which makes it possible to control the depletion of electrodes and predict their working time based on the magnetic measurements. All samples show the best OER activity in the 0.5 M K₂HPO₄ media. The obtained results demonstrate the ways for controlling the MCE of LMO under changing internal and external conditions, and an evaluation of the possibilities for their OER applications in electrocatalysts.