Defect engineering of nonprecious metal based catalysts for oxygen evolution reaction

Abstract

As a branch of catalysis, electrocatalytic energy conversion reactions have been extensively studied and widely applied in industry. Hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the most common electrocatalytic reactions, which have been utilized in water splitting, fuel cells, and zinc-air batteries, and so on. The ideal benchmark catalysts for these reactions are noble metal based catalysts, like platinum (Pt), ruthenium (Ru) and iridium (Ir). However, their high cost and instable catalytic performance during long-term usage make it impractical to use them on a massive scale. To develop a commercially affordable electrocatalyst with a promising activity, a series of catalysts originated from the defective two-dimensional (2D) nanomaterials emerged, such as carbon, transition metal oxides and transition metal dichalcogenides. Their atomic thickness, large lateral size, high surface-to-volume atom ratio and large specific surface area render them promising for numerous applications, such as (electro)catalysis, electronics, sensors, energy storage and conversion, and so on. Meanwhile, the defects existed on these 2D materials can significantly tailor their intrinsic physical properties even in an extremely low concentration, and thus great efforts have been devoted into the artificially surface defect engineering. But there are three major challenges that need to be addressed in the application of defects for improving the performance of the 2D materials, including the control of defect type and concentration, defect stabilization and active site structure characterization. This thesis focuses on developing the high-performance nonprecious metal catalysts through defect engineering for OER. The studies include the preparation of active 2D nonprecious metal nanosheets for OER catalysis through different novel synthesis strategies, and the investigation of the effect of the various defects, such as oxygen vacancies (Vo) and selenium vacancies (Vse), on the OER activities of the corresponding materials. It aims to explore the efficient strategies for defect engineering on various 2D nanomaterials, and pave a route for developing new high-performance OER catalysts based on nonprecious metals.In the first part of the experimental chapters, a facile solution reduction method using sodium borohydride as reductant is developed to prepare the amorphous iron-cobalt oxide nanosheets (FexCoy-ONS) with a large specific surface area (up to 261.1 m2 g-1), ultrathin thickness (1.2 nm) and, importantly, abundant Vo. Particularly, the mass activity of Fe1Co1-ONS are clearly superior to those of commercial RuO2 and crystalline iron-cobalt oxide nanoparticles. The promising OER catalytic activity of Fe1Co1-ONS can be attributed to its specific structure. Its ultrathin ONS could facilitate the mass diffusion/transport of OH- ions and provide more active sites for OER catalysis, while the Vo could enhance the electronic conductivity and promote the adsorption of H2O onto the nearby Co3+ sites. But as these Vo are in-situ created during the preparation process, their density cannot be controlled.Therefore, the second part focuses on the controllable tuning of Vo density on the 2D iron-cobalt oxide (Fe1Co1Ox-origin) via hydrogen thermal treatment at different temperatures and hydrogen pressures. Notably, the hydrogen annealed iron-cobalt oxide at an optimized condition of 200 oC and 2.0 MPa exhibits a remarkably improved OER activity in 1.0 M KOH, 1.9 times that of Fe1Co1Ox-origin at an overpotential of 350 mV. The results reveal that the optimal Vo density on the 2D Fe1Co1Ox via hydrogenation can improve the electronic conductivity and promote the OH- adsorption onto nearby low-coordinated Co3+ sites, resulting in a significantly enhanced OER activity.To further stabilize the Vo during the highly oxidizing OER conditions, in the third part of the experimental chapters, the atomically distributed non-metallic elements, including sulfur (S), nitrogen (N), and phosphorus (P), were separately introduced onto the defective iron-cobalt oxide (FeCoOx-Vo) surface. S atoms can not only effectively stabilize the Vo, but also create some extra Vo on the nearby Co sites, modulating the electronic structure of the oxide material to exhibit promising OER activity. When paired with the commercial 20% Pt/C, an overall water splitting current density up to 245.0 mA cm-2 can be achieved at the cell voltage of 2.0 V in 1.0 M KOH, which can meet with the requirement of industrial water splitting.The developed defect engineering strategies (vacancy creation and heteroatom doping) are applied in other nonprecious metal catalysts to prove their universality in enhancing the electrocatalytic activities of the materials. In the fourth part, Vse are firstly created on CoSe2-x via Ar plasma, then single Pt atoms are loaded on CoSe2-x through photoreduction to construct the atomically coordinated Pt-Co-Se moieties. Owing to the filling of single Pt atoms, CoSe2-x-Pt shows much better OER performance than single Ni and even Ru atomic species filled CoSe2-x. Mechanism studies unravel that the single Pt can induce much higher electronic distribution asymmetry degree than both single Ni and Ru, and benefit the interaction between the Co sites and adsorbates (OH*, O*, and OOH*) during OER process, leading to a better OER activity.