Engineering of Complex Metal Oxide for Electrooxidation of Ethanol: A Modern Practical Solution

Abstract

The rapid expansion of industrialization and the rising global population have significantly increased the consumption and depletion of fossil fuels such as coal, oil, and natural gas. This heavy reliance on these energy sources has greatly elevated greenhouse gas emissions, which are contributing to global climate change. To tackle the escalating energy needs and the exhaustion of fossil fuels, there is an urgent need to develop sustainable, clean, and renewable energy sources. Recently, alcohol-based fuel cells have attracted considerable attention as a promising clean energy solution. Among the various alcohols, ethanol is considered as the most effective choice for direct alcohol fuel cells due to its high energy density, ease of storage, safety in transport, and eco-friendly nature, as it can be easily derived from biomass. This alternative is considered effective in mitigating the significant environmental impacts of traditional fossil fuels, including their contribution to greenhouse gas emissions and other forms of pollution. By transitioning to this cleaner energy option, it is possible to lessen the overall environmental footprint and promote a more sustainable approach to energy consumption. In addition to its use in fuel cells, ethanol oxidation plays a significant role in biomass conversion processes for producing commodity chemicals. It can also be integrated with hydrogen evolution reactions to replace the slower oxygen evolution reaction in hybrid water electrolyzers, thereby facilitating the production of hydrogen, a clean energy source. Platinum (Pt) and Palladium (Pd) is the most common noble electrocatalysts used in direct ethanol fuel cells. The high cost, limited supply, and the problem of catalyst deactivation due to CO adsorption limit their practical application towards ethanol oxidation. To address these issues, there is an urgent need for the development of non-precious metal-based electrocatalysts that are affordable, durable, and highly effective. Such alternatives could enhance the viability of ethanol-based technologies and support the wider adoption of sustainable energy solutions. Non-noble metal oxide-based electrocatalysts offer a promising alternative to platinum (Pt) and palladium (Pd) for ethanol oxidation, addressing key issues related to cost, stability, and tolerance to carbon monoxide (CO). Unlike Pt and Pd, which are expensive and suffer from limited stability and CO poisoning, non-noble metal oxides are cost-effective and exhibit better resistance to CO, enhancing their practicality for large-scale applications. The present thesis xxiii investigates the engineering of complex mixed metal oxides for use in electrochemical ethanol oxidation reactions (EOR). The first chapter summarizes the rising global energy demands and associated challenges, focusing on the role of electrocatalysts in creating sustainable solutions, particularly through ethanol-based fuel cells. It explores ethanol oxidation as a practical pathway, highlighting how converting waste into energy not only meets energy requirements but also aids in environmental remediation. This thesis underscores the potential of waste recycling in advancing eco-friendly energy technologies to address future energy needs effectively. The first study focuses on synthesizing low-cost Ca and Fe-based metal oxides, particularly CaFe2O4, for electrochemical ethanol oxidation. Calcium and iron are selected for their natural abundance and cost-effectiveness. The chapter outlines the synthesis process, evaluates the performance of these oxides, and examines their potential as sustainable catalysts. The synthesized CaFe2O4 nanoparticle demonstrates an anodic peak current density of 1.01 mA cm⁻² at 1.50 V, a Tafel slope of 112 mV/decade, and superior CO tolerance and durability, indicating its effectiveness for ethanol oxidation. Additionally, CaFe2O4 was developed from repurposed waste materials, such as A4 sheets and stapler pins, addressing economic and environmental concerns. This waste-derived CaFe2O4 also exhibited nanoparticle morphology and shows catalytic performance comparable to commercially produced catalyst, highlighting the advantages of sustainable practices in catalyst development and waste reduction. The first chapter establishes CaFe2O4 as a potent electrocatalyst for ethanol oxidation. Expanding upon this, the second work involves the synthesizes of SrFeO3 to evaluate and contrast various iron based oxides, undertaking a systematic exploration of perovskite structures to refine electrochemical efficiency and propel innovations in sustainable energy solutions. The SrFeO3 oxide was synthesized through sol-gel method resulting in formation of nanoplates. The unusual Fe4+ oxidation state in SrFeO3 plays a crucial role in enhancing ethanol oxidation. The SrFeO3 catalyst demonstrates an anodic peak current density of 4.83 mA cm⁻² at 1.50 V, a Tafel slope of 104 mV/decade, and exhibits excellent CO tolerance and durability, reflecting its high efficiency. The synthesis of SrFeO3 via recycling steel industry waste, resulting in performance comparable to SrFeO3 produced from commercial salts. This approach reutilizes industrial by-products, promoting environmental sustainability and cost-effective catalyst development. Expanding on the effectiveness of Fe-based oxides in electrocatalysis, xxiv the study transitions to Mn-based oxides, which provide distinct redox characteristics, improved stability, and greater versatility across various reactions. In the third work, MnCo2O4 was explored as an electrocatalyst for electrochemical ethanol oxidation. The MnCo2O4 was synthesize using CTAB as surfactant result in nanorods formation. The MnCo2O4 catalyst achieves an anodic peak current density of 4.52 mA cm⁻² at 1.53 V, featuring a low Tafel slope of 81 mV/decade and demonstrating improved CO tolerance and durability, indicating high efficiency. MnCo2O4 was synthesized through advanced hydrometallurgical processing of spent batteries, recovering valuable metals from electronic waste. This method supports environmental sustainability by reducing landfill waste and offers a cost-effective solution for catalyst development. In the fourth study, CuMn2O4 was examined as an electrocatalyst for ethanol oxidation, leveraging the synergistic effects of copper and manganese along with Jahn Teller distortion. This CuMn2O4 nanoparticle achieves an anodic peak current density of 10.10 mA cm⁻² at 1.56 V, features a low Tafel slope of 132 mV/decade, and demonstrates excellent CO tolerance and durability, underscoring its effectiveness. The synthesis of CuMn2O4 involved advanced hydrometallurgical processing of spent batteries, which recovers valuable metals from electronic waste. The final study investigated Ni6MnO8, a catalyst that performs exceptionally well in ethanol oxidation due to the active role of nickel. The Ni6MnO8 nanoparticles demonstrates excellent stability and CO tolerance, with an anodic peak current density of 9.06 mA cm⁻² at 1.53 V. It was also synthesized using advanced hydrometallurgical treatment of spent batteries for material reclamation and for practical and eco-friendly solution for recycling electronic waste. This thesis delves into the study of diverse mixed metal oxide nanostructures, assessing their potential for ethanol electro-oxidation and showcasing their efficacy in optimizing electrocatalytic performance.