The Design And Synthesis Of Layered Double Hydroxides Electrocatalysts Toward Enhanced Oxygen Evolution Reaction

Electrocatalytic water splitting (2H2O?2H2 + O2) is a promising approach towards renewable energy conversion and storage. The water oxidation half reaction requires high energy to overcome the kinetic barrier owing to the four-electron and proton transfer steps. The development of effective and stable oxygen evolution reaction (OER) electrocatalysts is highly urgent to accelerate the reaction and promote the evolution kinetics, and thus improve the energy-conversion effciency. Recently, layered double hydroxide (LDH)-based materials have gathered much attention for the green and highly-efficient electrocatalytic reactions, owing to their versatile tunability in metal cation of laminate, interlayer anions, and hierarchical nanostructure. In order to achieve transition metal-based LDH materials with high electrocatalytic OER activity, it is necessary to optimize the surface morphology with highly-disperse active sites or modify the electronic structure with superior electron transfer capability. In this dissertation, transiton-metal LDHs act as active components of water oxidation, and the technological strategies were employed for the enhancement of OER performance, via metal cation doping and active sites dispersion. The influence of the electronic structure,interlayer spacing and morphology of LDHs materials was further explored to improve their electrocatalytic performances in oxygen evolution reaction. More details are as follows:1. Modulation on host layer of transiton-metal LDHs and their electrochemical performance toward OERA facile coprecipitation method was employed to introduce hybridized atoms on the host layers of NiFe-LDH. It is found that transition metal doping(Ti4+, V3+, Cr3+, Mn2+, Co2+, Cu2+ and Zn2+) demonstrates significant influence on the OER performance of NiFe-LDH; while main group elements (Mg2+ and Al3+)show no acceleration. The promotion of transition metals can be classified to the following two aspects: (?) activating the nature of water oxidation reaction through the effective electron transfer from Fe3+ in NiFe-LDH laminate to the doping metal, such as Co2+, V3+ and Cr3+; (?) promoting the electron transfer during OER by modifying the electronic structure of NiFe-LDH materials, like V3+, Ti4+ and Mn2+. This work proposes a strategy of the metal cations doping in LDHs for the enhancement of OER electrochemical activity. Based on the above mentioned observations and the consequent optimizations, a very-low onset overpotential (241 mV) and Tafel slope value (53.7 mV dec-1) for NiFeV-LDH nanoplates were achieved.2. Modulation on interlayer anions of transition-metal LDHs and their oxygen evolution activityLayer-by-layer assembly technique was employed to fabricate the ultrathin films (UTFs) via the electrostatic attraction of positive single-layer CoNi-LDH nanosheets (NSs) and negative Fe-porphyrin (Fe-PP) molecules. It shows an excellent OER performance with a remarkably small onset potential of 230 mV and Tafel slope of 37.6 mV dec-1 as well as a remarkably high current density of 30.8 mA cm-2 at an overpotential of 300 mV, much superior to the performance of pristine LDH NSs and the commercial IrO2 catalyst. The electron transfers from transition metal in CoNi-LDH NSs to the conjugate ring of Fe-PP leads to the increase of Co3+ content, which is identified as the active sites of water oxidation. Furthermore, Fe-PP molecule acts as electron acceptor, giving rise to more positively-charged Co species in CoNi-LDH NSs, which benefits the process of OH- adsorption. In addition, this method can be extended to the preparation of other UTFs, such as CoMn-LDH NS/Fe-PP, CoFe-LDH NS/Fe-PP and ZnCo-LDH NS/Fe-PP, which show significantly enhanced OER performance relative to pristine LDH NSs.3. Design of structured transiton-metal LDHs materials for enhanced electrochemical oxygen evolutionNiFe-LDH hollow microspheres (HMS) were prepared via a one-step in situ growth technique by using SiO2 as a sacrificial template. The monodisperse NiFe-LDH@SiO2 microspheres with a narrow size distribution were synthesized by hydrothermal reaction in the initial stage. During the subsequent aging process,the SiO2 microspheres dissolved in the alkaline environment to produce the NiFe-LDH HMS. The as-synthesized NiFe-LDH HMS electrocatalyst gives a current density of 71.69 mA cm-2 at ? = 300 mV, which is ?3.75- and 5.36-fold than that of NiFe-LDH nanoparticles (NPs) and 20 wt % Ir/C commercial catalyst,respectively. Furthermore, the current density (? = 300 mV) is rather stable and even increases by 4.5% after 40000 s of testing, indicating a prominent durability.The superior OER performances can be ascribed to the highly-dispersed nanoplatelets with a full exposure of active species and a facile electron/ion transport kinetics, confirmed by various electrochemical investigations. In addition, this preparation strategy is also demonstrated in other three LDHs materials (CoFe-, CoNi-, and NiAl-LDH) with satisfactory OER performance. A prototype electrolyzer cell is also fabricated by using the NiFe-LDH HMS modified Ni foam as the anode and Pt wire as the cathode, which achieves the production of both oxygen and hydrogen by using a 1.5 V AA battery as the power source.