Design, preparation, and characterization of supports and catalysts for hydrogen and oxygen evolution reaction in alkaline water electrolysis

Abstract

With the ever-growing energy demand, fossil fuel consumption, and their adverse impact on the environment over the world, the advanced research complying to develop clean energy resources as an ideal alternative to fossil fuels. Electrochemical water (H2O) conversion into hydrogen and oxygen gas (2H2O  2H2 + O2), a nonfossil fuel-based technology has gained great attention around the world scientific community due to its potentials replacement of traditional fossil fuel. The water electrolysis involving two half cell reactions in the electrolyzer, one is a cathodic reaction known as hydrogen evolution reaction (HER) and another one is an anodic reaction, which known as oxygen evolution reaction (OER). However, their commercialization has been hindered by high cost and low durability of best noble metal-based catalysts such as Pt/C for HER and IrO2/RuO2 for OER in the water electrolyzer. The major drawbacks of these catalysts are rear availability, the tendency of particle agglomeration and oxidization in the oxidizing environment leads to poor activity and stability. Therefore, significant ongoing research concentrated on the developing of non-noble metal-based catalysts or reducing the noble metal usage with non-noble metal/non-metal catalysts by improving intrinsic activity and stability. To address these issues, we have carried out the research to develop new kinds of cata-lyst supports and nonprecious metal/minimal amount of precious metal-based catalysts for improving the performance and durability of HER and OER catalysts in alkaline water electrolyzer. Our advanced research acknowledges that there is still plenty of scope for the development and improvement of HER and OER elec-trocatalysts.

The work detailed in this dissertation is divided into five parts, in which the first chapter aims at providing the general introduction of electrochemical water splitting as well as the motivation, which focused on our interest in the perspective of scientific challenge. In addition, we elaborate on the reaction pathway of HER and OER in the different electrolytes with a general overview of catalyst development. The second chapter deals with the physical and electrochemical characterization techniques.

In chapter three, we present the first report on the synthesis of a new kind of conductive, robust honeycomb structured few-layer S, N-doped crystalline graphene from S-doped carbon nitride (SCN) to support the mini-mal amount of Rh clusters and its HER electrocatalytic properties in alkaline media. The introduction of ul-tra-small Rh clusters increases the active sites and the synergistic effects between Rh and S, N elements on the graphene framework. The interaction between heteroatoms (S, N) and Rh can be modulated the electron-ic structure of Rh, which increases the active sites for H adsorption and avoid the aggregation of catalyst re-sulting in favor of outstanding HER performance.

In Chapter four, ultra-small NiMo nanoparticles anchored N-doped graphene electrocatalyst is prepared for HER in alkaline media, where N-doped graphene is synthesized from nonconductive graphitic carbon nitride (CN). The optimized catalyst shows excellent HER activity and stability in alkaline media. A variety of char-acterization techniques suggest that the catalytically active sites of this unique catalyst are associated with the particle size, metal centers, and the metal coordinated to the nitrogen within the graphene framework.

Lastly, in Chapter five, we have rationally designed new kinds of cheap transitional metal (M = Co, Ni) single atom coordinated N doped graphene as electrocatalyst for OER in alkaline medium. Metal single atom coor-dinated N-doped graphene is prepared by the pyrolysis of [M(EDTA)]2- complex. It is observed from ultra-high-resolution transmission electron microscopy (UHR-TEM) that single metal atom distributed over N-doped graphene. The metal single-atom coordinated with N over graphene and act as the active sites for OER. The metal single-atom coordinated N on graphene electrocatalysts showed excellent electrocatalytic OER activity, which is better than of commercial OER catalyst. Excellent electrocatalytic OER activity and stability is mainly originated from the active sites of single atom coordinated N within graphene framework.