Development of Nickel-based Electrocatalysts for Efficient Water Splitting

Athma, E. P. (2022) Development of Nickel-based Electrocatalysts for Efficient Water Splitting. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Increased energy demand as civilization progressed and the environmental consequences of fossil fuel use have stimulated research into energy generation from renewable energy sources in recent decades. However, due to the intermittent nature of renewable energy sources, storage of the surplus energy is very imminent. In this context, storage of energy in the form of chemical energy, especially in hydrogen (H₂), is receiving increased attention due to its high energy density and environmentally benign nature. As hydrogen does not occur naturally in the free form, methods like steam reforming, biomass gasification, electrochemical water splitting, and partial oxidation of hydrocarbon are developed to produce hydrogen. At present, steam reforming is the primary method to produce hydrogen, which again involves a large amount of CO₂ emission. On the other hand, hydrogen (H₂), produced from water splitting is a clean, efficient, and sustainable approach to store the excess energy from renewable sources. However, electrochemical water splitting reactions involve multiple steps, which give rise to the larger energy barriers requiring high cell potential than thermodynamic potential needed for the electrolysis. Therefore, electrocatalysts are generally employed to minimize these kinetic barriers, thereby reducing the overpotential for OER and HER. Compared to HER, the reaction dynamics of the OER are greatly constrained due to high activation energy barriers. Significant research has been conducted to discover new OER electrocatalysts for improving the properties of already known electrocatalysts to achieve superior performance. However, even in the presence of state-of-the-art noble metal catalysts (such as RuO₂, IrO₂), a substantial overpotential is required to drive the oxidation of water. Therefore, an effective strategy to replace the OER with a more thermodynamically favorable reaction at the anode is a promising concept to decrease the cell voltage for H₂ production. Recently, it has been proposed that replacing anodic OER with the oxidation of hydrazine (N₂H₄ + 4OH− → N₂+ 4H₂O + 4e−), which possesses a lower theoretical thermodynamic potential of −0.33 (V vs. RHE) compared to that of OER (1.23 V vs. RHE) is a promising approach to achieve energy-saving H₂ production. Pt-based precious metal materials are the most often used choices in this area, but their lack of availability, high cost and poor dual functionality prevent them from being widely used. This has inspired us to create electrocatalysts from less expensive transition metals, specifically nickel-based compounds for electrocatalytic water splitting. The whole thesis is separated into five chapters. Chapter 1 presents a comprehensive overview of the research works in the field of electrocatalytic water splitting for energy storage. For example, the historical background of fuels, current energy crisis and alternative energy resources, electrochemical setup, performance evaluation parameters, and the current state of nickel-based electrode materials have been discussed in detail. Chapter 2 provides new insight towards an electrochemical strategy to activate inactive materials into efficient electrocatalysts for OER. A suitable precondition strategy has been devised to transform only the surface of conductive metallic Ni nanowires into active catalytic centers. The resulting material with intimate contact between the electrically conductive core and electrocatalytically active surface showed promising “specific” and “geometric” electrocatalytic activity towards alkaline OER at different pH. Chapter 3 discusses how functional groups in ligands were found to effectively tune the energetics of nickel phosphide phases during hydrothermal synthesis and its performance towards hydrazine assisted hydrogen generation. Phase-pure Ni₂P and Ni1₂P₅ have been obtained in the presence of different ligands under identical experimental conditions. Through time-dependent experiments, we can attribute this phase-selectivity to the capability of thiol (-SH) and carboxylate (-COO-) functional groups of ligands in tuning the energetics of nickel phosphide phases during hydrothermal synthesis. Among synthesized materials, Ni₂P found to have excellent electrocatalytic activity towards hydrazine assisted hydrogen generation. Chapter 4 depicts the development of a one-pot approach for the synthesis of phosphorus incorporated NiCo₂S₄ with enhanced electrocatalytic performance for hydrazine assisted water splitting. We discovered that the formation of the NiCo₂S₄ phase was triggered by the inclusion of phosphorus. Time-dependent phase analysis and several control experiments have been carried out to apprehend the role of phosphorus in the synthesis and formation mechanism of NiCo₂S₄ phase. P-incorporated NiCo₂S₄ demonstrates excellent activity toward hydrazine oxidation compared to other as-synthesized metal sulphides such as CoS₂/NiS₂ and NiCo₂S₄. This suggested that the incorporation of P could trigger the formation of the NiCo₂S₄ phase in one step as well as enhance its electrochemical activity towards hydrazine assisted water splitting. Chapter 5 illustrates the development of a room temperature ligand-assisted strategy to synthesize NiSe and CoSe in pure phase. This is achieved by using dicarboxylic acid containing molecules in the synthesis. The role of functional group in the ligands were probed and found that functional groups played a key role in the synthesis of nickel selenides. By similar approach bimetallic NiCoSe was synthesized, and the electrochemical evaluations showed that bimetallic NiCoSe has superior electrocatalytic performance toward hydrazine oxidation and hydrogen evolution reaction compared to NiSe and CoSe.

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