Preparation and Modification of Core-Shell Structured MnO2/CC as an Efficient Catalyst for Oxygen Evolution Reaction

Abstract

Oxygen evolution reaction (OER) is thought to play an essential part in electrochemical water splitting (EWS), metal-air batteries, energy storage, etc. Therefore, exploring an advanced anode material is necessary for the efficient OER process. Although manganese dioxide (MnO2) as the anode catalyst has been widely researched in recent centuries, most of the research concentrated on the specific phases of MnO2 (α-MnO2, δ-MnO2, and γ-MnO2). Very few reports were related to the ε-MnO2. In the meantime, the lower states of manganese oxides (MnOx 1 ≤ x ≤ 2) are still worth exploring. The multiple chemical states and large oxygen vacancies of MnOx will apply outstanding electrochemical performance. Moreover, the components of MnOx with other advanced materials, such as MXene, also show great catalytic activities. In addition, heat treatment was always adopted to modify the phase transformation of MnO2 samples. This dissertation mainly involved three experimental chapters: (1) ε-MnO2 was successfully covered on the carbon cloth (CC) substrate surface by electrodeposition strategy at different depositing times and temperatures. The morphological changes and OER performance of these as-prepared samples have been investigated. The results demonstrated that when the preparation duration was 30 min and the temperature was 50 °C, the as-prepared MnO2/carbon cloth (MnO2/CC) exhibited the best OER performance. From the microscopic characterization, the MnO2 was uniformly and firmly grown on the carbon cloth without binder usage. All the as-prepared samples displayed a core-shell structure, and the morphology of MnO2/CC did not have significant changes after the long-time stability test. (2) Ni-doped MnO2/CC (NMO/CC) was successfully synthesized by adding different concentrations of Ni cations into the precursor solution. Doping Ni cations increased the OER performance, which attributed to the appearance of more oxygen vacancies and the increased active surface area. In addition, MXene (MX) was used to deposit on the surface of NMO/CC to form the compounds of Ni-doped MnO2 and MXene electrode (MX-NMO/CC), which further decreased the resistance of the samples and increased their OER performance. (3) Thermal treatment was adopted to enhance the OER performance of as-prepared MnO2/CC samples. Three different heating temperatures were applied (250, 350, and 450 °C), and the duration was 1 h. The sample treated at 350 °C (MnO2/CC 350) acquired the best OER performance. The phase transition can be detected when the temperature reached 350 °C, and the effects of phase transition on OER performance were researched. This work explored a simple binder-free electrodeposition strategy to prepare different core-shell structured MnOx-based catalysts for an efficient OER process.