Development of Nanocatalysts for Water Splitting - A Sustainable Source of Energy

Dr. Irshad Hussain
Akhtar Munir
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Chemistry & Chemical Engineering
The demand and consumption of energy is expected to be doubled in the coming 50 years and meanwhile the energy cost is also expected to rise significantly. Fossil fuels, currently the leading source of energy, subsidize a lot of CO2 to the environment causing many environmental issues including greenhouse effect. To address these challenges, there is a dire need to develop alternative and renewable resources of energy. Water, being a renewable source, is a promising candidate in this regard because of its ability for the production of H2 (HER) and O2 (OER). OER is the 4eˉ multistep process with a demand of 240-600 mV extra energy, which makes it more unfavorable both kinetically and thermodynamically. Intense research efforts have been made to accelerate the kinetics of water splitting process at the potential reasonably close to the thermodynamics limits (1.23 V). Until now, RuO2 and IrO2 are among the best electrocatalysts for OER, while Pt based materials for HER with minimum overpotential. However, their instability in wide pH range and high cost deteriorate their large scale applications. Therefore, the design and development of cost-effective and more efficient electrocatalysts is direly needed to make the water splitting process practically viable for the sustainable production of ‘H2’. In this regard, nanostructuring of the materials, especially those of supported transition metal oxides, has garnered much attention not only due to their natural abundance but also their tunable size dependent catalytic/electronic properties and high inherent redox potential for electrochemical water splitting. More promisingly, metal nanoclusters (NCs, size ≈ 2nm) are new emerging nanoscale materials for water redox reaction. The drastic decrease in their size and intriguing size dependent catalytic properties make them more fascinating due to their high surface to volume ratio and superatoms-like behavior unlike their bulk counterparts. This research work mainly focuses on the development the cost-effective transition metals (Co, Cu and Ni) based nanostructured materials (metal oxides and carbides) to control their size, shape, and chemistry of the underlying carbon based support as electrode materials for OER and overall water splitting process.

During this research work, we have initially developed the thiol functionalized graphene oxide (G-SH) nanosheets via selective epoxidation reaction. The surface modified and exfoliated 2D G-SH nanosheets were used as support materials for the immobilization of ultrasmall meta/metal oxides NCs, and their comparative evaluation and pre-/post spectroscopic investigation for water oxidation catalysis were made (chapter 4). Motivated by the remarkable performance of ultrasmall NCs, we extended this work to develop a facile solid-state strategy for the in-situ growth of Co/Co(OH)2 NCs embedded in N-doped mesoporous carbon network (HCN) for OER (Chapter 5). These ultrasmall NCs have high electrochemical active surface area, maximum accessibility of active sites and high redox potential for OER while endorsing the fast heterogeneous electron transfer owing to the concerted synergistic effect. All the supported NCs have been found much more active than their bulk analogues and ligand stabilized NCs, and the Co/Co(OH)2 NCs outperform all the NCs with low onset potential (1.44 V) and high stability (˃ 5 days).

We extended the above solid-state strategy for the synthesis of Ni/Ni3C NPs catalyzed growth of NCNTs of different sizes as bifunctional electrocatalyst for water splitting in wide pH window. It was found that the electrocatalytic performance of these catalysts depends on the size of Ni/Ni3C NPs and thus also the size of NCNTs probably due to the internal structural strain and electronic modulation. We believe, that metal carbides have a great potential for overall water splitting owing to their appreciable mechanical strength, high conductivity, high stability in harsh condition and tunable electronic properties.

This work significantly contributes towards the novel findings at the interface of materials science and electrocatalysis through (1) the design of nanoscale materials (NCs) based electrocatalytic approaches (ii) spectroscopic investigation of structure-performance relationship (iii) and the structure and surface modification of carbon based supports for the redox reaction of water splitting in the wide pH range. These accomplishments may pave the way towards the development of more advanced materials with tunable structure-properties relationship in energy conversion science.