Nanoscale tuning of bifunctional electrocatalysts based on cobalt for energy storage and conversion

• Main Author: Kakkarakunnel Jose Vishal
• Other Authors: Lee Jong-Min
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Chemical engineering
• Online Access: https://hdl.handle.net/10356/155108
• Institution: Nanyang Technological University

There is a high demand for alternative clean energy sources as the world faces serious environmental and energy crisis. Renewable energy sources such as solar and wind are highly intermittent and diffuse which leaves the prime option of storing the clean energy in form of chemical bonds and converting it back efficiently when required. Such techniques must be carried out using electrochemical methods in particular water splitting and rechargeable metal air batteries (rechargeable MAB). Water electrolysis has captured high attention as a green method to produce hydrogen which is the principal constituent behind a promising ‘hydrogen economy’. MABs, particularly zinc-air batteries (ZAB) are considered as potential candidates for post-lithium batteries as they exhibit much higher theoretical energy density. The water splitting electrolyser carries oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode, meanwhile, in a rechargeable ZAB cathode oxygen reduction reaction (ORR) occurs during the discharging process and OER during the recharging process. These reactions determine the efficiency of these electrochemical techniques. Since these reactions are thermodynamically uphill, they require highly active electrocatalysts to enhance the overall efficiency. Currently, the state-of-the-art electrocatalysts for OER, HER, and ORR are precious metal‐based materials. The high cost, poor stability, and scarcity of such electrocatalysts limit the industrial usage of these techniques. Since cobalt is one of the most abundant transition metals on the earth and has attracted much interest as a non-precious electrocatalyst towards all HER, OER, and ORR, they have to be analyzed for their bifunctional activities electrocatalysts (towards ORR/OER or HER/OER). Moreover, fabrication and operational cost factors can be also reduced by design bifunctional electrocatalysts. The hypothesis of our work is that the bifunctional electrocatalytic activities of Co-based materials can be enhanced through proper morphology engineering, integration of metal components with carbon supports, stabilization of single atoms, and structural engineering approaches. To realize our hypothesis, initially, earth abundant metals-based phosphide compound (NiCo2Px) with unprecedented surface morphology and shape tuning was synthesized via facile hydrothermal treatments. One of the NiCo2Px was designed to have active sharp edges (spiked) on a hollow spherical surface while the other was spherical with a smooth surface. The highly exposed, branched spikes-covered hollow structure of NiCo2Px shows remarkable performance enhancement for HER and OER in a wide range of pH solutions. The active site density and synergistic effects were tailored according to the surface morphological features of this catalyst. An alkaline electrolyzer assembled using the optimized catalyst produced 10 mAcm-2 of current density at 1.62 V without almost any decrease in this value even after the continuous run for 50 hours. To understand the effect of integrating metal components with carbon supports, the second work elaborate engineering earth abundant bimetal/metal oxide nanoparticles encapsulated in a mesoporous carbon framework for ORR and OER activity. The Fe, Co, and CoO containing electrocatalyst was developed by direct annealing of N enriched ZIF superstructures in the N2 environment. Owing to the factors like high surface area carbon framework with uniform dopant distribution, sufficient mesopore density, and presence of metal-Nx/C structures, this electrocatalyst had a comparable performance with precious metal based electrocatalysts. Moreover, this work also revealed the significance of annealing temperature in tuning the bifunctional activity of such materials. However, it was found that the optimized material was not much stable during the electrocatalytic process. To enhance the stability and activity of ZIF derived materials, in the third part, isolated single atomic sites of Fe and Co coordinated with nitrogen on carbon support (Fe,Co-SA/CS) was designed and deployed. The (Fe,Co-SA/CS) was prepared by taking the advantage of unique structure and pore characteristics of ZIF and molecular size of Ferrocene, a Fe-containing species. Fe single atomic sites neighbouring Co sites facilitated relatively easy reactant adsorption and charge transfer on Co active sites, which enhanced bifunctional activity of this material towards ORR and OER in alkaline electrolyte. This resulted in the Fe,Co-SA/CS attaining a similar oxygen electrode activity with that of commercial electrocatalysts. Finally, a fast and facile method for improving the activity through the structural engineering approach is elaborated when compared with the complex synthesis processes of previous works. Here, a cheap oxygen reduction electrocatalyst based on metal boride - N doped carbon heterointerfaces (CoB-Nx/C) was developed and analyzed. Transition metal borides are known to show superior OER performance when compared with their counterparts, however, their ORR performance was unexplored due to severe oxidation effects. Meanwhile, as prepared CoB-Nx/C outperformed the Pt/C electrocatalyst towards ORR due to its nanocrystalline-nanosheet nature, kinetic enhancement due to charge redistribution at heterointerfaces, and incorporation of the N doped carbon. Also, annealing temperature as well as the amount of carbon and nitrogen sources was optimized to tune the activity of this material. At last, a ZAB was assembled using CoB-Nx/C structures that provided an open-circuit voltage of 1.50 V with excellent cycling stability.