Synthesis of mesoporous LiMn2O4 and LiMn2−xCoxO4 thin films using the MASA approach as efficient water oxidation electrocatalysts

Abstract

Mesoporous, highly active, robust, and cost-effective thin films are in big demand for water splitting by electrocatalysis. Molten-salt assisted self-assembly (MASA) is an effective method to synthesize mesoporous thin films. Transparent clear solutions of salts (LiNO₃ and [Mn(H₂O)₆](NO₃)₂), acid (HNO₃), and surfactants (CTAB and P123) can be spin-coated over substrates as liquid crystalline (LC) films and calcined to obtain mesoporous high quality transparent thin films. A mixture of three salts (LiNO₃, [Mn(H₂O)₆](NO₃)₂, and [Co(H₂O)₆](NO₃)₂) also forms LC mesophases that can be calcined to produce mesoporous nanocrystalline mixed metal lithiates (meso-LiMn_2−xCo_xO₄) with surface areas as large as 144 m² g⁻¹ (for LiMn_1.5Co_0.5O₄). The synergic effects of these salts improve the pore-size of the final products; the pore size drops from around 11 nm (in the meso-LiMn₂O₄) to 6–7 nm in the meso-LiMn_1−xCo_xO₄. The meso-LiMn_2−xCo_xO₄ films were tested at pH 13.6 as water oxidation electrocatalysts over a broad range of x. While meso-LiMn₂O₄ shows a low activity towards water oxidation, the catalytic activity increases with the increasing Co(III) content of the films. The highest mass activity per cobalt, 1744 A g⁻¹, is obtained for meso-LiMnCoO₄, which remains as a robust and efficient film even at a current density of 120 mA cm⁻².