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Development of novel catalytic solutions applied for the hydrogen evolution and oxygen reduction reactions

Rowley-Neale, Samuel James (2017) Development of novel catalytic solutions applied for the hydrogen evolution and oxygen reduction reactions. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

This thesis reports the utilisation of 2D nanomaterials, namely molybdenum disulphide (2D-MoS2) and molybdenum diselenide (2D-MoSe2), as cheap, earth abundant and effective catalytic alternatives to platinum (Pt) for hydrogen production (via the hydrogen evolution reaction (HER)) within electrolysers and energy generation (via the oxygen reduction reaction (ORR)) within proton exchange membrane fuel cells (PEMFC). Chapter 1 introduces the chemical reactions associated with electrolysers and PEMFCs, then gives an overview of the relevant fundamental electrochemical concepts utilised throughout this thesis. Subsequent to this, Chapter 2 specifically describes the equipment and fabrication techniques implemented herein, in addition to providing the full physicochemical characterisation of the 2D-MoS2 and 2D-MoSe2 utilised in later chapters. Chapter 3 demonstrates that a commonly employed surfactant (sodium cholate) used in the liquid exfoliation of 2D-MoS2 has a profound effect upon its electrocatalytic activity. It is shown that the surfactant has a negative effect upon the observed HER signal output (decreasing the current density and increasing the electronegativity of the HER onset potential) of the 2D-MoS2 compared to “pristine” 2D-MoS2 (produced without a surfactant present). This suggests that future studies utilising 2D nanomaterials should carefully consider their use of a surfactant as well as perform the necessary control experiments. Chapters 4 and 5 reveal that, in specific conditions, 2D-MoS2 nanosheets are effective at reducing the electronegativity of the HER and ORR onset potentials, increasing their achievable current density and allowing the ORR reaction mechanism to occur via the desirable 4 electron process (product: H2O). This electrocatalytic effect is reported herein for the first time. Research was undertaken by electrically wiring the 2D-MoS2 to four commonly employed commercially available carbon based electrode support materials, namely edge plane pyrolytic graphite (EPPG), glassy carbon (GC), boron-doped diamond (BDD) and screen-printed graphite electrodes (SPE). The reduction in the electronegativity of the HER and ORR onset potential is shown to be associated with each supporting electrode's individual electron transfer kinetics/properties and is thus distinct from the literature, which predominately uses just GC as a supporting electrode material. It is revealed that the ability to catalyse the HER and ORR is dependent on the mass deposited until a critical coverage of 2D-MoS2 nanosheets is achieved, after which its electrocatalytic benefits and/or surface stability curtail. In Chapter 6, 2D-MoS2 screen-printed electrodes (2D-MoS2-SPEs) are designed, fabricated and their performance is evaluated towards the electrochemical HER and ORR within acidic aqueous media. A screen-printable ink is developed, which allows for the tailoring of the 2D-MoS2 content/mass used in the fabrication of the 2D-MoS2-SPEs. The 2D-MoS2-SPEs are shown to exhibit an electrocatalytic behaviour towards the ORR, which is found, critically, to be reliant upon the percentage mass incorporation of 2D-MoS2 in the 2D-MoS2-SPEs. Chapter 7 utilises the exact methodology for electrocatalytic ink production as Chapter 6, however it incorporates 2D-MoSe2 and explores the fabricated 2D-MoSe2-SPEs towards the HER where beneficial electrochemistry is observed. Both the 2D-MoS2-SPEs and 2D-MoSe2-SPEs display remarkable stability with no degradation in their respective performances over the course of 1000 repeat scans. The electrocatalytic inks produced in these chapters and the resultant mass producible electrodes mitigate the need to post hoc modify an electrode via the drop-casting technique that has been shown to result in poor stability. This thesis reports that novel 2D nanomaterials can be implemented as beneficial electrode materials towards enhancing “green” energy generation technologies. Specifically, 2D-MoS2 is shown to be effective at lowering the onset potential and increasing the achievable current density for the HER and ORR, giving rise to further benefits when 2D-MoS2 (and 2D-MoSe2 towards the HER) are incorporated into SPEs. These novel electrodes exhibit the inherent unique electrochemical behaviour of the 2D nanomaterials incorporated and benefit from the remarkable stability attributed to the intrinsic properties of a SPE. Consequently, the findings of this thesis are highly applicable to industrial electrolyser/fuel cell applications.

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