Development of cobalt-based catalysts for water electrolysis

The present thesis investigates the synthesis, structural modification, and electrochemical characterisation of cobalt based catalysts for alkaline and acidic water splitting. The focus is on the impact of iron (Fe) and manganese (Mn) doping on cobalt phosphate and cobalt oxide electrocatalysts, particularly their influence on structure and morphological changes and knock-out effects on the catalytic performance. The thesis is divided into eight chapters, beginning with an introduction to the research context, followed by research objectives and methodology, the main results contained in four chapters covering material synthesis, characterization and electrochemical testing, and finishing with the main conclusions of this work. Chapter 1 This chapter establishes the motivation behind the study by discussing the global need for sustainable hydrogen production. It introduces electrochemical water splitting, emphasizing the role of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. The fundamental principles of electrocatalysis in alkaline and acidic environments are reviewed, followed by a discussion on the limitations of conventional electrocatalysts. The chapter concludes with an overview of cobalt-based materials, highlighting their tunability via doping and structural modifications. Chapter 4 This chapter explores the impact of morphology and Mn doping on the OER activity of cobalt oxide (Co3O4) in acidic conditions. Co3O4 catalysts with different morphologies, including sheets, flowers, and cubes, were synthesized and evaluated for their OER performance. Sheets morphology shows superior catalytic performance due to a greater number of exposed active sites and improved charge transfer kinetics. Mn-doped Co3O4 sheets were then prepared to investigate the role of Mn in enhancing catalytic activity. It was reported in literature that Mn doping was found to enhance stability by strengthening the Mn-O bond, reducing cobalt dissolution, and potentially activating self-healing mechanisms. The work in this chapter highlights the potential of Mn as a strategic dopant for improving the durability and efficiency of platinum group metal free (PGM-free) OER catalysts in acidic environments. Chapter 5 This chapter focuses on the synthesis and electrochemical evaluation of ammonium cobalt phosphate (NCP) catalysts for OER and HER in alkaline conditions. Cobalt phosphate is identified as a promising electrocatalyst due to its structural stability, high electronegativity of phosphate ions, and variable cobalt oxidation states that facilitate electron transfer. The study explores the role of morphology, composition, and doping strategies in optimizing catalytic performance. NCP nanosheets were synthesized via a surfactant-free co-precipitation method, with controlled size and thickness achieved by varying solvent viscosity. The impact of Fe and Mn doping on catalytic activity was investigated, leveraging their ability to introduce structural defects, alter electronic properties, and enhance charge transfer. Fe doping was found to lower the overpotential for OER, while Mn doping stabilized reaction intermediates, improving HER performance. Chapter 6 Inspired by Chapter 4, this study investigates Fe-doped Co3O4 sheets for alkaline OER, using a systematic low Fe doping approach to optimize performance. Catalysts were tested in a three-electrode system, including temperature dependence studies and 100 h stability test. It was reported in literature that Fe3+ doping modifies the Co-O bond structure, enhancing electrical conductivity and optimizing O* and OOH* adsorption energies, reducing overpotentials. However, excessive Fe leads to FeOOH formation, negatively impacting conductivity. In this study, we aimed to improve the catalytic behaviour of previously synthesised Co3O4 catalysts by introducing a dopant element to modify their crystal structure and chemical composition slightly. To address the intrinsic slow reaction kinetics at the anode, we have developed Fe doped cobalt oxide electrocatalyst to facilitate the OER process. The incorporation of iron atoms modifies the electronic framework of pristine cobalt oxide, thereby catalyzing an enhancement in OER performance. Iron doped cobalt oxide catalysts were synthesized using varying Fe ratios via the hydrothermal approach and subsequently calcined at 450°C. The results demonstrate that even small concentrations of Fe significantly impact the catalytic performance for the OER. Moreover, while higher Fe concentrations enhance performance, there is a saturation point beyond which further doping leads to the formation of cobalt ferrite (CoFe2O4) instead of maintaining the Fe-doped Co3O4 structure. Chapter 7 As an extension of Chapter 6, this chapter evaluates Fe-doped Co3O4 sheets. The catalysts were integrated into an anion exchange membrane water electrolysis (AEMWE) cell to assess their performance under practical operating conditions. The PGM-free AEMWE cell demonstrated excellent performance, reaching 1000 mA·cm-2 at 1.92 V, approaching industrial-scale operation requirements. The PGM-free electrolyser exhibited promising performance at current densities up to 2000 mA·cm-2, maintaining low cell voltages and exceptional durability. Over 100 hours of operation at 1000 mA·cm-2, the average degradation rate was 1.3 mV·h-1, which later decreased to 0.4 mV·h-1 as the initial membrane-related degradation stabilized. Additionally, X-ray photoelectron spectroscopy (XPS) and post-mortem analysis were conducted to investigate changes in the electronic structure and surface composition after electrochemical testing. These analyses provided deeper insights into catalyst stability and the role of systematic low-level Fe doping in maintaining long-term OER activity. Chapter 8 This chapter summarises the main outcomes of the work carried out in this thesis, highlighting the influence of morphology and elemental doping on the performance of cobalt-based catalysts across different water-splitting environments. It also discusses the broader significance of these findings, assesses the practical applicability of the developed materials, and presents perspectives for future research.

Publisher

University of Galway

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CC BY-NC-ND

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University of Galway Theses (PhD Theses)