Spinel Oxides Based Electrocatalysts for Electrochemical Oxygen Evolution Reaction

基於尖晶石結構的氧化物的電催化劑在電化學析氧反應中的應用

Student thesis: Doctoral Thesis

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Award date20 Jun 2019

Abstract

Hydrogen is believed to be a key ingredient to reaching a target of a carbon-neutral and carbon-free economy in the future. Till now, the majority of hydrogen comes from steam reforming in which as well produces carbon mon/dioxide. Exploring novel methods to produce hydrogen, therefore, has attracted much attention in industry and labs. Among various potential routes, electrochemical water splitting is one of the most promising options for hydrogen production from renewable resources. This process consists of two half reactions. One is the cathodic hydrogen evolution reaction (HER), and another one is the anodic oxygen evolution reaction (OER). Compared to two-electron involved HER, OER process with four-proton coupled electron transfer steps inherently requires a higher potential to overcome the kinetic barrier. Along the way, enormous effort has been devoted to exploring high-performance OER electrocatalysts. Noble metal-based materials, such as IrO2 and RuO2, show high activity and stability for OER in both acid and alkaline conditions. However, the scarcity and associated high-cost greatly limit their practical application. With the advantages of low-cost and rich abundance, transition metal-based electrocatalysts have been demonstrated to be promising alternatives for the OER process, but their activity and stability are still not satisfactory to some extent. In view of the interfacial reaction, rational design of surface structures using various strategies could significantly decrease the overpotential for efficient electrocatalytic water oxidation. In this thesis, we applied two methods to optimize the surface structure of transition metal-based oxides and investigated their performance for OER. Detailed researches and results are listed as follows:

(1)Surface reduction of {112} faceted Co3O4 by NaBH4 could dramatically enhance its electrocatalytic performance for OER in terms of activity and stability. Series characterizations prove the formation of oxygen vacancies on the surface after NaBH4 treatment, which leads to more exposure of Co2+ active sites. Electrochemical tests further display that the number of active sites increases along with the decrease in charge transfer resistance after surface reduction and in turn resulting in high activity for OER. Moreover, a comparison with other facets exposed samples shows the high-index faceted one outperform others. This work provides a feasible route to design the spinel oxide-based electrocatalysts for the OER process.

(2)High-valent vanadium ions have been doped into the Co3O4 matrix via a sol-gel method followed by calcination. The electrochemical tests show a significant promotion in electrocatalytic performance for OER after V-doping. Further characterization manifests that V-doping process could introduce lattice distortions and defects on the surface which in turn might reduce the surface energy and expose more active sites. DFT calculations further displayed that V-doping process will change the rate-determining step for OER and decrease the energy barriers of intermediates and products. Furthermore, similar improvement was as well observed when doping vanadium into NiFe2O4 matrix, suggesting this method could be a general approach to improve the electrocatalytic property of transition metal-based spinel oxides.

In summary, surface engineering via reduction treatment, exposing certain facet and doping high-valent ions could efficiently modulate the surface structure of spinel oxides, further lower the reaction barrier for water oxidation.

    Research areas

  • Cobalt oxide, Spinel oxide, Electrocatalysis, Oxygen evolution reaction, Oxygen vacancies, Vanadium doping