Synthesis & characterization of modified mesoporous LiMn2-xCoxO4 thin films as water oxidation electrocatalysts

The lithiated transition metal oxides (LMO) are important group of materials in energy applications, particularly as water oxidation electrocatalysts. The mesoporous LiMn2-xCoxO4 thin film has been synthesized by using molten-salt assisted self assembly (MASA) method with a high surface area. Homogenous ethanol solution of nitrate salts (lithium, manganese(II) and cobalt(II)) and surfactants (CTAB and P123) in the presence of a small amount of HNO3 is coated over a glass substrate by spin-coating to form lyotropic liquid crystal (LLC) mesophase that is calcined at elevated temperature to synthesize disordered mesoporous LiMn2-xCoxO4 thin film. The mesophases display diffraction line(s) at small angles, indicating an ordered structure. The cobalt amount (x) has been varied from 0 to 2, keeping the same mesoporous and crystal structures. The films were characterized using XRD, SEM, EDX, TEM, N2 adsorption-desorption techniques. The XRD provided that the end products have a spinel structure with very similar unit cell parameters in all compositions. The surface areas of the films vary from 98 to 144 m2/g with increasing cobalt amount in the films. The SEM images showed that the thin films are uniform with a thickness of around 200-500 nm. The LLC mesophases have been also coated over FTO glass to fabricate electrode for oxygen evolution reaction (OER) and also for electrochemical characterizations. The electrodes prepared from all composition performed as good electrocatalysts, however the Tafel slope decreased from 124 to 66 mV dec-1 going from LiMn2O4 to LiMnCoO4. The overpotential also dropped from 491 mV to 294 mV at 1 mA cm-2 in water oxidation reaction. The LiMn2O4 is the worst electrocatalyst tested in this thesis. It has high Tafel slope, which is not desired and also not stable during electrochemical test. The stability improves with increasing cobalt in the films. The LiMnCoO4 has been reported to be one of the most efficient and stable electrocatalyst even if it is used 120 mA cm-2 current densities. Therefore, the electrode with this composition has been investigated in detail in an alkali media. The mesoporous LiMn2O4 thin film is modified by successive ionic layer adsorption and reaction (SILAR) method to improve its activity and stability. This electrode is dipped into a 1 M cobalt (II) solution, then washed several time to ensure a single layer of cobalt species on the surface, the modified electrode is calcined to produce cobalt rich LiMn2-xCoxO4 surface. Eventhough, the amount of cobalt in the modified electrode is smaller than 1 %, the modification decreased the Tafel slope from 127 to 80 mV dec-1, but the electrode was unstable during water oxidation process in alkali media. A range of LiMn2-xCoxO4 (x = 0 to 0.4) compounds were modified by the SILAR method and tested for OER. The mesoporous LiMn1.6Co0.4O4 (20 % cobalt and 80 % manganese) was used as the substrate and the SILAR method was employed 5-times, the Tafel slope of this electrode decreased from 64 to 46 mV dec-1 with an overpotential decrease from 304 to 265 mV at 1mA cm-2 and 826 to 546 mV at 10 mA cm-2 by modification and displayed a robust property in water oxidation process.