Modulation of Electronic Structure in Transition Metal Based Composites for Enhancing Catalyzed Water Electrolysis


Student thesis: Doctoral Thesis

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Awarding Institution
Award date14 Aug 2023


Water electrolysis has been deemed as a clean and efficient pathway to produce hydrogen (H2), which effectively avoids the carbon footprint from steam methane reforming for H2 evolution, thus gaining considerable attention. However, the efficiency of water splitting is greatly limited by not only the sluggish kinetics and substantial overpotential of the oxygen evolution reaction (OER) at the anode but also the unfavorable water dissociation process from hydrogen evolution reaction (HER) at the cathode. To date, the Pt-based materials and Ir-based oxides have been regarded as benchmark catalysts for HER and OER, respectively, whereas their large-scale application were overwhelmingly hindered by their low abundance and high cost. Therefore, developing catalysts with high efficiency and long durability is of great importance and necessity, but remains a huge challenge. This thesis presents several strategies based on the modulation of electronic structure in different materials, with the aim of improving the activity and stability of electrocatalysts for water splitting.

In Chapter 2, a method is presented based on regulating the electron structure of Ni and Fe sites in the ultrathin nickel-iron oxide (Mn-Ni-Fe-O) nanosheets using Mn doping, and the as-prepared Mn-Ni-Fe-O nanosheets perform enhanced OER activity as compared to the pristine Ni-Fe-O nanosheets. Experimental characterizations show that Mn dopant induces the newly generated highly active Ni3+ sites and an increased oxygen vacancy concentration in the Mn-Ni-Fe-O nanosheets. During the OER process, the high-valence Ni3+ sites can accelerate the process from OOH* to O2*, while the oxygen vacancies as catalytic sites can reduce the -OH adsorption energy.

In Chapter 3, RuO2 nanosheets with well-defined amorphous-crystalline boundaries were prepared on carbon cloth (a/c-RuO2/CC) to effectively catalyze pH-universal water oxidation. The as-made a/c-RuO2/CC enables remarkably improved OER activity and stablity relative to c-RuO2/CC and a-RuO2/CC, especially in the case of the acidic conditions. By combining experimental and theoretical studies, we clear revealed that the Ru-O covalency within the amorphous-crystalline region was weakened relative to the crystalline region, which limited the leaching of Ru species from the crystalline phase and thus enhanced the durability. Moreover, the upshift of d-band center in a/c-RuO2/CC compared with a-RuO2/CC reduced the energy barrier for formation of OOH* and significantly improved activity.

In Chapter 4, the Cr-doped Fe3C@carbon core-shell structure grown on a N-doped graphitic framework (Cr-Fe3C@C-NGF) was synthesized as an electrocatalyst to realize efficient HER in an alkaline medium. The Cr dopants occupy not only the Fe sites of metallic Fe-Fe bond but also the Fe sites of covalent Fe-C bond. Such Cr substitution contributes to the electron transfer from Fe2+ to electron deficient Cr3+ (eg0 t2g3), which leads to the decreased electron density of Fe sites, thus facilitating the H2 desorption from Fe sites. The density function theory (DFT) calculations reveal that the Cr doping allows the downshift of the d-band center and more electron occupation of antibonding states, which balances the hydrogen adsorption and desorption at Fe sites, thereby boosting HER activity.