Development of Nickel and Cobalt Based Electrocatalysts for Efficient Water Splitting

Ganguli, Sagar (2019) Development of Nickel and Cobalt Based Electrocatalysts for Efficient Water Splitting. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

For a sustainable future, the increasing energy demands with the advancement of civilization must be addressed through environment-friendly technologies. Despite their popularity, fossil fuels are known to be extremely harmful for the ecology and therefore, the exploration of alternative fuel based technologies is indispensable. In this regard, hydrogen as fuel is being increasingly envisaged as a promising alternative due to its high energy density and environmentally benign properties. Unfortunately, the industrial methods, such as steam reforming of natural gas, partial oxidation of coal and other hydrocarbons, etc. for hydrogen production also produce green-house gases as toxic by-products. This makes hydrogen production via splitting of water in an electrochemical cell (electrolyzer) a suitable alternative. However, the involvement of multiple electrons in this process makes the reaction kinetics sluggish and therefore electrocatalysts are generally employed to facilitate the reaction. Currently, the high-cost and scarcity of the state-of-the-art electrocatalysts, such as Pt, IrO2, etc. are acting as bottlenecks for the commercialization of this technology. This has enthused us to develop electrocatalysts from comparatively cheaper transition metals, in particular, nickel and cobalt based materials for electrocatalytic water splitting. The entire work has been divided into five chapters. Chapter 1 provides a general introduction to the field of electrocatalytic water splitting for energy storage. For example, the historical background of fuels, electrochemical setups, parameters for performance evaluation and the current status of transition metal based electrocatalysts, in particular, nickel and cobalt based materials have been discussed in detail. Chapter 2 illustrates the prudent choice of various amines and solvents to synthesize different pure phases of cobalt oxides/hydroxides and discusses the corresponding formation mechanisms. The paradoxical behaviour of “intrinsic” and “geometric” activities of different phases towards oxygen evolution reaction has been investigated. The results revealed that materials that show higher geometric activity may not possess inherently more active catalyst centers and therefore evaluation of materials only on the basis of their geometric activity is not an appropriate method to compare electrocatalysts. Chapter 3 focuses on the inception of molybdate as a pore-forming additive to enhance the bifunctional electrocatalytic activity of nickel and cobalt based mixed hydroxides for overall water splitting. The role of molybdate to produce materials with enhanced textural properties and its etching-induced pore formation in materials during electrocatalysis has been analyzed in detail. Chapter 4 delves into the discovery of structural and chemical differences between nickel cobalt oxy-sulfides and nickel cobalt oxides to understand factors that make oxy-sulfides better electrocatalysts compared to oxides. The factors were found to be (i) faster catalyst activation due to lower metal-sulfur bond energy compared to metal-oxygen bond energy, (ii) robust nature of sulphur incorporated flakes, (iii) higher pore diameter that enables faster diffusion of reactants and products in metal oxy-sulfides, (iv) higher electrochemically active surface area upon sulfur incorporation and (v) lower charge transfer resistance of nickel cobalt oxy-sulfides over nickel cobalt oxides. Chapter 5 discusses the development of a one-pot strategy to synthesize pure phase nickel cobalt selenide (NiCo₂Se₄) and cobalt selenide (Co₃Se₄) with enhanced electrocatalytic properties towards oxygen evolution reaction. In-depth analyses has been carried out to comprehend the role of each reactants involved in the synthesis and this gave important insights into the formation mechanism. Despite, similar electrocatalytic performance from geometric perspective, the intrinsic activity of NiCo₂Se₄ was found to be higher compared to that of Co₃Se₄. This suggested that the substitution of cobalt centers by nickel promotes the potency of the catalyst centers in metal selenides.

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