非表示:
キーワード:
-
要旨:
In this dissertation, as-synthesized and post-treated mesostructured cobalt oxide and cobalt iron oxide catalysts, prepared by hard-templating method with silica and carbon-based templates, have been investigated for alkaline water electrolysis by focusing the oxygen evolution reaction (OER). The electrochemical in situ Raman study was also conducted to reveal the alteration of the catalyst and its true activity under OER working conditions.
The first part of the dissertation deals with the investigation of the impact of morphology and electronic structures of the spinel cobalt iron oxide nanowires for OER. For this purpose, the Co3-xFexO4 samples series were prepared by a hard-templating method by using SBA-15 silica as a hard template. The introduction of iron into spinel lattice was shown to distort the electronic structure of spinel by inducing the higher occupancy of cobalt in the tetrahedral sites, while Fe3+ mainly substitutes the cobalt in the octahedral sites. The small amount of iron was found to be beneficial to enhance the OER activity by lowering charge transfer resistance, increasing the Co2+(Td) sites, and creating open mesoporous structures. Additionally, the nanowires’ diameter and pore structure of cobalt oxides are tuned, and their impact on OER activity is investigated. Small incorporation of iron could reduce the overpotential at 10 mA/cm2 from 398 to 378 mV and increase the current density at 1.7 V from 107 to 150 mA/cm2. The improvement of the OER performance is related to the alteration of the electronic structure of spinel structure. The incorporation of iron enhances distortion around the cobalt centers and increases the ratio of Co2+ in tetrahedral sites. These Co2+(Td) sites have been proposed as the key for facilitating the adsorption of water molecules and forming the μ-OOH intermediates during the catalytic cycle.
In the second part of this dissertation, various post-treatment methods, i.e pulse laser post-processing (PLPP) and reduction post-treatment, were applied to tune the crystal and surface structures, and defects of the cobalt oxide and cobalt iron oxide. First, mesostructured cobalt oxide was treated with the single-pulse laser with varying laser intensities. The formation of rocksalt CoO, as well as structural disorder and increasing of lattice parameter, were observed with the increase in applied laser intensity, leading to an enhancement of OER activity. The lowest laser intensity was then chosen to avoid the formation of the biphase cobalt oxides, and multiple pulse laser treatment was applied. An enhancement of current density at 1.7 V vs RHE from 75 to 180 mA/cm2 and a remarkable decrease of overpotential at 10 mA/cm2 from 405 mV to 357 mV was achieved compared to pristine Co3O4 counterpart. Three low-intensity laser pulses resulted in the optimized OER electrocatalyst by introducing the structural disorder within mesostructured Co3O4 while maintaining the overall morphology and spinel-phase purity.
A reduction post-treatment was conducted on the cobalt oxide with various cobalt iron oxide nanowires. The reduction post-treatment changed the crystal structure from spinel into a rocksalt phase on the cobalt oxide. Appling this synthesis, post-treatment to the mesoporous cobalt iron oxide induced the formation of the iron oxide clusters supported on the rocksalt CoO matrix. This post-reduction enhanced the OER activity of cobalt oxide significantly by increasing a current density at 1.7 V vs RHE from 150 to 315 mA/cm2 and lowering the overpotential from 378 to 339 mV to reach 10 mA/cm2. The electrochemical in situ Raman study revealed that the rocksalt phase acted as the pre-catalyst for OER. Under the OER working potential, this rocksalt phase was transformed into distorted spinel and cobalt (oxy-)hydroxide phases and was found to be responsible to enhance the OER activity compared to the pristine spinel phase.
All in all, the results of this dissertation reveal the fundamental understanding of the OER activity enhancement on cobalt oxide and cobalt iron oxide electrocatalysts. The iron preferably occupied the octahedral sites of the spinel and governed the overall electronic structure by altering the Co2+(Td)/ Co3+(Oh) occupancy. The structural disorder and preferential defect formation on the tetrahedral sites of the Co3O4 could also be introduced by laser post-treatment. In the morphology-dependent study, a smaller nanowires diameter of mesoporous cobalt oxide could lead to the formation of an open pore structure and facilitate faster OER kinetics. A phase segregation of iron oxide on rocksalt cobalt oxide by reduction post-treatment on cobalt iron oxide played a key role in the formation of highly active oxyhydroxide intermediate active phase under OER condition. These pave a way for the development of highly active and robust cobalt iron oxide-based OER electrocatalysts.
