Date of Award

2022-12-01

Degree Name

Doctor of Philosophy

Department

Environmental Science and Engineering

Advisor(s)

Sreeprasad T. Sreenivasan

Second Advisor

Luis Echegoyen

Abstract

Due to their superior conversion efficiency, high power density, and green nature, sustainable energy technologies such as Fuel cells and Zinc-air batteries, once installed to produce green hydrogen fuel as well as energy storage devices, are expected to play an essential role in reducing the environmental effect of human transportation by replacing fossil fuels. However, the current bottleneck for the low-temperature polymer membrane fuel cells that dominate the fuel cell vehicle market is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. The slow ORR kinetics significantly reduce the efficiency of chemical-electrical energy conversion. Similarly, hydrogen production (to be used in the anode of fuel cells) through water splitting in electrolyzers also suffers from the inferior kinetics of oxygen evolution reaction (OER). The same kinetic limits exist in Zinc-air batteries, where ORR and OER are critical for their charging and recharging cycles. Typically, devices utilize catalysts to increase the reaction kinetics, and Pt/C and other platinum-group-metal (PGM) catalysts are the current benchmarks for oxygen electrocatalysis. However, using expensive and low-abundance noble metals decreases the commercial viability of electrochemical energy systems. Moreover, current benchmark catalysts also suffer voltage-dependent area loss, poor methanol tolerance, and CO poisoning, limiting their applicability. Therefore, it is critical to discover durable, abundant, active, and inexpensive catalysts that are PGM or noble metal-free. While dedicated efforts lead to many novel catalyst systems with considerable efficacies, most of the catalysts still lag behind the benchmarks and hence fail to achieve the performance targets the U.S. Department of Energy (DOE) set for low-temperature fuel cells and Zin-air batteries. Due to the large surface area and programmable structure, chemically synthesized nanosystems, molecular materials, and network systems emerged as the front runners to replace benchmark catalysts. Despite progress, the activities of such alternate candidates still lag behind the benchmarks, and significant advancements are necessary before such catalysts can be commercially applied. Thus, the thesis focuses on formulating novel PGM-free ORR and OER catalysts that can rival benchmark PGM catalysts utilizing the fundamental chemical approaches. We hypothesize that chemical tailoring the electronic distribution and density of states of the active sites in nanomaterials, molecular systems, and metal-organic frameworks can lead to superior electrocatalytic activity by optimizing the adsorption-desorption energy of key intermediates. We test and prove the validity of our hypothesis in pristine systems, heterostructures, and on free-standing electrodes. We also investigate the effect of superior electrocatalytic activity on the performance of Fuel cells and Zn-air batteries that employ our developed catalysts as the electrodes and compare their performance to devices that use benchmarks to illustrate their superiority. Further, we unravel the fundamentals behind the observed activities through a comprehensive and highly symbiotic experimental-theoretical exploration of the processes. The thesis is divided into four chapters based on the systems investigated and the reactions explored. Below I present a brief description of the chapters, content, and novel findings. Chapter 1 introduces work and briefly discusses the importance of Fuel cells and Zinc-air batteries technologies. Additionally, it explains how chemical tailoring of materials can generate superior electrochemical activities and elevate the electrochemical renewable energy platform to address the impending energy crisis. Chapter 2: This chapter demonstrates how pure systems can be chemically engineered to derive superior ORR activity by taking two specific examples: Metal-Organic Framework (MOFs) and higher-order fullerene-based molecular systems. Platinum (Pt)-based-nanomaterials are currently the most successful catalysts for the oxygen reduction reaction (ORR) in electrochemical energy conversion devices such as fuel cells and metal-air batteries. Nonetheless, Pt catalysts have serious drawbacks, including low abundance, stability/catalyst poisoning, and very high costs, which limit their practical applications. Due to high atomic utilization and conjugations, MOFs have recently been explored as potential electrocatalysts. Our initial efforts focus on controlling the intermetallic electron transfer in well-known molecular and supramolecular MOFs with highly abundant metals such as Co and Cu. We selected a highly studied Hong Kong University of Science and Technology (HKUST) MOF and modified its structure and composition to derive superior activity. Initially, we investigated the HKUST MOF as an oxygen electrochemistry catalyst, where it performed moderately. Then we employed bi-linkers and added a second metal, cobalt, to boost the catalytic activity of the HKUST MOF, resulting in performance superior to benchmarks in terms of activity and stability. Further, in the second part of the chapter, for the first time, we demonstrate Cylindrical C96 Fullertubes are highly active metal-free ORR catalysts. We investigated the ORR behavior of C60, C70 (spheroidal fullerenes), and C90, C96, and C100 (tubular fullerenes) using experimental and theoretical techniques. Our studies revealed that C96 (a metal-free catalyst) are excellent ORR activity, with an onset potential of 0.85 V and a halfway potential of 0.75 V, close to the state-of-the-art Pt/C benchmark catalyst values. Chapter 3: After exploring individual material systems, in chapter 3, we explored the designing of precise engineered heterostructures to elicit superior activity. The first part of this chapter shows that facile electron transport and intimate electronic contact at the catalyst-electrode interface in zero-dimensional-two-dimensional (0D-2D) heterostructures are critical to deriving excellent electrocatalytic performances. Further, the improved performance can be leveraged for the ideal performance of electrochemical devices such as glucose biofuel cells and biosensors. Here, through a comprehensive experimental-theoretical exploration, we demonstrate that engineering of interfacial properties, including interfacial electron dynamics, electron affinity, the electrode-catalyst-adsorbate electrical synergy, and electrochemical active surface area, can lead to highly efficient graphene-based electrochemical devices. We selected two closely related but electronically and chemically different functionalized graphene analogs-graphene acid (GA) and reduced graphene oxide (rGO)-as the model graphenic platforms. Our studies reveal that compared to rGO, GA is a superior bifunctional catalyst with high oxygen reduction reaction (an onset potential of 0.8 V) and high glucose oxidation activities. Spectroscopic and electrochemical analysis of GA and rGO indicated that the higher carboxylic acid content on GA increases its overall electron affinity and improves conductivity and band alignment, leading to GA's better electrochemical performance. The formulation of a heterostructure between GA and samarium oxide (Sm2O3) nanoparticles led to better conductivity (lower charge-transfer resistance) and glucose binding affinity, further enhancing glucose oxidation activity. The interdimensional Sm2O3/GA heterostructure, leveraging their enhanced glucose oxidation capacity, exhibited excellent nonenzymatic amperometric glucose sensing performance, with a detection limit of 107 nM and a sensitivity of 20.8 μA/μM. Further, a nonenzymatic, membrane-free glucose biofuel cell (with Sm2O3/GA heterostructure as anode and GA as biocathode) produced a power density of 3.2 μW·cm–2 (in PBS spiked with three mM glucose), which can function as self-powered glucose sensors with 70 nM limit of detection. This study establishes the potential of interfacial engineering of GA to engage it as a highly tunable substrate for a broad range of electrochemical applications, especially in future self-powered biosensors. The second part of this chapter will discuss how to improve the Fullertubes activity by forming a heterostructure with 1D carbon nanotubes. A heterostructure formed by adsorbing Fullertubes (C100) onto single-walled carbon nanotubes (SWCNTs) as an electron-acceptor molecular catalyst influenced the interfacial charge transfer with the SWCNTs and led to a novel class of PGM-free C100-SWCNT electrocatalysts. These newly advanced C100/CNTs worked as a bifunctional metal-free catalyst for oxygen electrochemistry with benchmark-close activity. Chapter 4. Design and synthesis of novel electrocatalysts with favorable hydrogen and oxygen electrochemistry are paramount for sustainable hydrogen production using electrochemical water splitting. Hence, after demonstrating the effect of electronic modulation on individual materials and heterostructures, we applied a similar strategy to fabricate hydrogel-based self-standing electrodes. The focus is to formulate a self-standing electrode, optimize the charge distribution and dynamics, and demonstrate how surface features affect the total catalytic activity. For this, we combined hydrophilic biopolymer alginate with highly conductive and catalytically active nanomaterials such as SWCNTs and CuO nanoparticles to create a bifunctional Alginate-CNT-CuO-self standing electrode with excellent activity and durability for the water splitting. Here, the hydrophilic and aerophobic character of the catalyst developed by the presence of Alginate in (AL)-CNT-CuO electrode facilitates the facile bubble release from the electrode and leads to high electrocatalytic water splitting conversion efficiency as well as increased stability. Hence, under optimal conditions, the self-standing electrode displayed outstanding bifunctional hydrogen evolution reaction (HER) and OER electrocatalytic activity in an alkaline solution, with an onset potential of 93 mV and 161 V for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. While different catalysts show appreciable HER property under acidic conditions, catalysts that can work under alkaline conditions are few. The self-standing catalyst achieved an onset potential of 93 mV and 161 V for HER and OER, respectively. Furthermore, the water electrolyzer constructed with this bifunctional electrode exhibits an overpotential voltage of 1.85 V at 100 mA. cm-2 in overall water splitting.

Language

en

Provenance

Received from ProQuest

File Size

232 p.

File Format

application/pdf

Rights Holder

Mohmed Sanad Noufal

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