Nanoarchitectonic Mesoporous Ni1–xMnxO Electrodes: Charge Capacity and Oxygen Evolution Reaction Electrocatalysis in Alkaline Media

Abstract

Stable electroactive mesoporous Ni_1–xMn_xO thin-film electrodes are fabricated over FTO and graphite rods using the molten-salt-assisted self-assembly (MASA) method. Ethanol solutions of two salts ([Mn(H₂O)₄](NO₃)₂ and [Ni(H₂O)₆](NO₃)₂ with varying Ni(II)/Mn(II) mole ratios, 1.0 to 0.1) and two surfactants (C₁₂H₂₅(OCH₂CH₂)₁₀OH, C₁₂E₁₀ and C₁₆H₃₃N(CH₃)₃Br, CTAB) are coated over a conducting substrate (FTO and graphite rod) to assemble the salt–surfactant lyotropic liquid crystalline (LLC) mesophase that is calcined to obtain a mesoporous Ni_1–xMn_xO thin-film electrode. Ni_1–xMn_xO is a solid solution up to x of 0.7, but it transforms the NiMnO₃, Mn₃O₄, and Mn₂O₃ phases in the samples with x values of 0.5 and higher at higher annealing temperatures. FTO and graphite-coated (F-Ni_1–xMn_xO and G-Ni_1–xMn_xO) electrodes have a high charge capacity, but the FTO-coated electrodes are unstable and undergo degradation. They display an increasing charge capacity during early CV cycles (or consecutive GCD measurements) but decay in capacity over long-term experiments. The G-Ni_1–xMn_xO electrodes are more robust and display high charge capacities (958 C/g in pure NiO and 720 C/g in Ni_0.9Mn_0.1O, close to the theoretical values). During the electrochemical tests, both pure NiO and Ni_1–xMn_xO electrodes transform to core-NiO/shell-Ni(OH)₂ and core-Ni_1–xMn_xO/shell-Ni(OH)₂ structures on the pore walls, respectively. The shell thickness decreases from 2.0 nm in pure NiO to 1.1 nm with 10% Mn(II) addition in Ni_0.9Mn_0.1O at 350 °C. Moreover, the shell thickness is also dependent on the pore-wall thickness that increases exponentially with annealing temperature (from 4.4 to 27.1 nm in pure NiO and 4.0 to 12 nm in Ni_0.9Mn_0.1O by increasing the temperature from 350 to 500 °C, respectively). It increases from 2.0 to 4.5 nm in pure NiO and 1.1 to 1.5 nm in the Ni_0.9Mn_0.1O electrodes at those temperatures, respectively, and determines the charge capacity of the electrodes. The addition of manganese significantly improves the stabilities of the electrodes but almost has no effect on the overpotential of the electrodes. Even though the charge capacity depends on the annealing temperature, OER performance almost shows no effect on the annealing temperature.