Synthesis And Electrochemical Application Of Transition Metal Based Layered Double Hydroxides

With the consumption of traditional fossil energy, such as oil, natural gas, energy and resource problem has severely restricted the development of the society. Therefore, the production of clean, renewable resources as well as the exploitation of high, efficient energy storage and conversion devices has become a hot topic. Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) play a core role in these high technologies. Among all the OER or ORR catalysts, transition metal (Ni, Co, Fe, Mn) based layered double hydroxides (LDHs) have high intrinsic OER or ORR activity and tunable interlayer metal species to facilitate the design synthesis. Besides, compared with the noble catalysts, the synthesis of LDHs is much easier and cheaper. However, there still exist some problems referred to the usage of LDHs as electrochemical catalysts. They have poor intrinsic conductivity. Besides, most of the studies focus on the design of binary materials, leading to the lack of catalysis varieties. Moreover, the structure optimization of ternary high valence state transition metal doped LDHs and the activity changes that may cause by them have been rarely studied so far. To address the above concerns, the essay will summarized as follows:1. We introduced Ni, Co, Fe simultaneously into the interlayer using the merit of adjustable metallic species and compositions in the LDH matrix, thus granting the ternary LDH both OER and ORR active sites. We also proposed a preoxidation process to enhance the conductive of LDH by improve the valence of Co2+. It was found that, in 0.1 M KOH electrolyte, NiCoFe-LDH has a reasonable bifunctional performance, while exploiting a preoxidation treatment can significantly enhance both OER and ORR activity. We further tested the practical application of O-NiCoFe-LDH by loading the catalyst on Teflon-treated carbon fiber paper. The results showed that, in 6 M KOH electrolyte, O-NiCoFe-LDH possess the smallest AE (a potential hysteresis between OER and OER when reaching a specific current density to evaluate the bifunctional activity). For example, when the current density reached 20 mA cm"2, a small △E of 800 mV obtained, even exceeding noble-metal catalysts. Moreover, O-NiCoFe-LDH also exhibited a long-term stability. A neglectable ORR decrease was obtained for 40h when giving a constant potential to reach a current density of 20 mA cm-2. The OER performance decreased slightly after 15 h testing and when enlarging the initial current density to 100 mA cm-2, the catalyst could also sustain 10 h. Therefore, O-NiCoFe-LDH may hold great potential in new-generation rechargeable metal-air battery.2. We introduced Mn into the interlayer of NiFe-LDH using an one-pot co-precipitation method and oxidized Mn2+ by O2. The Mn4+ could adjust the electronic structure and improving the conductivity of LDH, thus improving the activity of OER. It showed that, in 1 M KOH electrolyte, a small overpotential (298 mV) was obtained when reaching a current density of 20 mA cm-2, exceeding binary NiFe-LDH, NiMn-LDH and Ir/C catalyst. Besides, ternary NiFeMn-LDH possessed the smallest Tafel slope (47 mV dec-1). Moreover, when giving a constant potential (initial current density was 20 mA cm’2) and a constant current density of 10 mA cm-2, the NiFeMn-LDH catalyst could exhibit a 15 h long-term stability. The excellent OER activity and stability made the ternary NiFeMn-LDH an outstanding electrode of water splitting and rechargeable metal-air battery.3. We further expended the binary or ternary LDHs (NiFe-LDH and NiCoFe-LDH) synthesis to non-aqueous system and obtained dehydrated LDH by solvothermal using ethanol as a solvent. The structural information has been confirmed by XRD and the following rehydration process. The interlayer spacing of the dehydrated LDH was 0.71 nm, while after a rehydration process, the spacing could expand to 0.79 nm again. This method could also be applied to synthesis Fe-based transition metal containing LDH nanoarrays, which cannot be synthesized by simple urea hydrothermal process. Combined the intrinsic high activity of NiFe-based LDH with special structural morphology, the LDH nanarrays exhibited excellent OER activity, even exceeding Ir/C catalyst. The onset potential of E-NiCoFe-LDH NPAs was 1.46 V, while the overpotential was only 295 mV when reaching a current density of 30 mA cm-2. Besides, the synthsised LDH NPAs showed an excellent stability. It could sustain 20 h without any decrease when the initial current density reaching 50 mA cm-2 and 100 mA cm-2, respectively. This work not only opened a novel synthesis system of LDHs, but also provided a new idea for oxygen-electrode design.