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

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

Defining the origins of electron transfer at screen-printed graphene-like and graphite electrodes: MoO2 nanowire fabrication on edge plane sites reveals electrochemical insights

© 2016 The Royal Society of Chemistry .Molybdenum (di)oxide (MoO2) nanowires are fabricated onto graphene-like and graphite screen-printed electrodes (SPEs) for the first time, revealing crucial insights into the electrochemical properties of carbon/graphitic based materials. Distinctive patterns observed in the electrochemical process of nanowire decoration show that electron transfer occurs predominantly on edge plane sites when utilising SPEs fabricated/comprised of graphitic materials. Nanowire fabrication along the edge plane sites (and on edge plane like-sites/defects) of graphene/graphite is confirmed with Cyclic Voltammetry, Scanning Electron Microscopy (SEM) and Raman Spectroscopy. Comparison of the heterogeneous electron transfer (HET) rate constants (k°) at unmodified and nanowire coated SPEs show a reduction in the electrochemical reactivity of SPEs when the edge plane sites are effectively blocked/coated with MoO2. Throughout the process, the basal plane sites of the graphene/graphite electrodes remain relatively uncovered; except when the available edge plane sites have been utilised, in which case MoO2 deposition grows from the edge sites covering the entire surface of the electrode. This work clearly illustrates the distinct electron transfer properties of edge and basal plane sites on graphitic materials, indicating favourable electrochemical reactivity at the edge planes in contrast to limited reactivity at the basal plane sites. In addition to providing fundamental insights into the electron transfer properties of graphite and graphene-like SPEs, the reported simple, scalable, and cost effective formation of unique and intriguing MoO2 nanowires realised herein is of significant interest for use in both academic and commercial applications

Recent advances in 2D hexagonal boron nitride (2D-hBN) applied as the basis of electrochemical sensing platforms

2D hexagonal boron nitride (2D-hBN) is a lesser utilised material than other 2D counterparts in electrochemistry due to initial reports of it being non-conductive. As we will demonstrate in this review, this common misconception is being challenged, and researchers are starting to utilise 2D-hBN in the field of electrochemistry, particularly as the basis of electroanalytical sensing platforms. In this critical review, we overview the use of 2D-hBN as an electroanalytical sensing platform summarising recent developments and trends and highlight future developments of this interesting, often overlooked, 2D material

Enhancing the efficiency of the hydrogen evolution reaction utilising Fe3P bulk modified screen-printed electrodes via the application of a magnetic field

We report the fabrication and optimisation of Fe3P bulk modified screen-printed electrochemical platforms (SPEs) for the hydrogen evolution reaction (HER) within acidic media. We optimise the achievable current density towards the HER of the Fe3P SPEs by utilising ball-milled Fe3P variants and increasing the mass percentage of Fe3P incorporated into the SPEs. Additionally, the synergy of the application of a variable weak (constant) external magnetic field (330 mT to 40 mT) beneficially augments the current density output by 56%. This paper not only highlights the benefits of physical catalyst optimisation but also demonstrates a methodology to further enhance the cathodic efficiency of the HER with the facile application of a weak (constant) magnetic field

Mass-producible 2D-MoSe2 bulk modified screen-printed electrodes provide significant electrocatalytic performances towards the hydrogen evolution reaction

We demonstrate a facile, low cost and reproducible methodology for the production of electrocatalytic 2D-MoSe2 incorporated/bulk modified screen-printed electrodes (MoSe2-SPEs). The MoSe2-SPEs outperform traditional carbon based electrodes, in terms of their electrochemical activity, towards the Hydrogen Evolution Reaction (HER). The electrocatalytic behaviour towards the HER of the 2D-MoSe2 within the fabricated electrodes is found to be mass dependent, with an optimal mass ratio of 10% 2D-MoSe2 to 90% carbon ink. MoSe2-SPEs with this optimised ratio exhibit a HER onset, Tafel value and a turn over frequency of ca. −460 mV (vs. SCE), 47 mV dec−1 and 1.48 respectively. These values far exceed the HER performance of graphite (unmodified) SPEs, that exhibit a greater electronegative HER onset and Tafel value of ca. −880 mV and 120 mV dec−1 respectively. It is clear that impregnation of 2D-MoSe2 into the MoSe2-SPEs bulk ink/structure significantly increases the performance of SPEs with respect to their electrocatalytic activity towards the HER. When compared to SPEs that have been modified via a drop-casting technique, the fabricated MoSe2-SPEs exhibit excellent cycling stability. After 1000 repeat scans, a 10% modified MoSe2-SPE displayed no change in its HER onset potential of −450 mV (vs. SCE) and an increase of 31.6% in achievable current density. Conversely, a SPE modified via drop-casting with 400 mg cm−2 of 2D-MoSe2 maintained its HER onset potential of −480 mV (vs. SCE), however exhibited a 27.4% decrease in its achievable current density after 1000 scans. In addition to the clear performance benefits, the production of MoSe2-SPEs mitigates the need to post hoc modify an electrode via the drop-casting technique. We anticipate that this facile production method will serve as a powerful tool for future studies seeking to utilise 2D materials in order to mass-produce SPEs/surfaces with unique electrochemical properties whilst providing substantial stability improvements over the traditionally utilised technique of drop-casting

Screen-printed electrodes: Transitioning the laboratory in-to-the field

This short article overviews the use of screen-printed electrodes (SPEs) in the field of electroanalysis and compares their application against traditional laboratory based analytical techniques. Electroanalysis coupled with SPEs can offer low-cost, precise, sensitive, rapid, quantitative information and laboratory equivalent results. The combined use of SPEs and electroanalysis reduces the need of sample transportation and preparation to a centralised laboratory allowing experimentalists to perform the measurements where they are needed the most. We first introduce the basic concepts and principles of analytical techniques to the reader, with particular attention to electroanalysis, and then discuss the application of SPEs to common analytical targets such as food, environmental, forensics, cancer biomarkers and pathogenic monitoring and sensing

MoS2-graphene-CuNi2S4 nanocomposite an efficient electrocatalyst for the hydrogen evolution reaction

We present a facile methodology for the synthesis of a novel 2D-MoS2, graphene and CuNi2S4 (MoS2-g-CuNi2S4) nanocomposite that displays highly efficient electrocatalytic activity towards the production of hydrogen. The intrinsic hydrogen evolution reaction (HER) activity of MoS2 nanosheets was significantly enhanced by increasing the affinity of the active edge sites towards Hþ adsorption using transition metal (Cu and Ni2) dopants, whilst also increasing the edge sites exposure by anchoring them to a graphene frame- work. Detailed XPS analysis reveals a higher percentage of surface exposed S at 17.04%, of which 48.83% is metal bonded S (sulfide). The resultant MoS2-g-CuNi2S4 nanocomposites are immobilized upon screen-printed electrodes (SPEs) and exhibit a HER onset potential and Tafel slope value of -0.05 V (vs. RHE) and 29.3 mV dec-1, respectively. These values are close to that of the polycrystalline Pt electrode (near zero potential (vs. RHE) and 21.0 mV dec-1, respectively) and enhanced over a bare/unmodified SPE (-0.43 V (vs. RHE) and 149.1 mV dec-1, respectively). Given the efficient, HER activity displayed by the novel MoS2-g-CuNi2S4/SPE electrochemical platform and the comparatively low associated cost of production for this nanocomposite, it has potential to be a cost-effective alternative to Pt within electrolyser technologies

Mass-producible 2D-WS2 bulk modified screen printed electrodes towards the hydrogen evolution reaction

© The Royal Society of Chemistry 2019. A screen-printable ink that contained varying percentage mass incorporations of two dimensional tungsten disulphide (2D-WS2) was produced and utilized to fabricate bespoke printed electrodes (2D-WS2-SPEs). These WS2-SPEs were then rigorously tested towards the Hydrogen Evolution Reaction (HER) within an acidic media. The mass incorporation of 2D-WS2 into the 2D-WS2-SPEs was found to critically affect the observed HER catalysis with the larger mass incorporations resulting in more beneficial catalysis. The optimal (largest possible mass of 2D-WS2 incorporation) was the 2D-WS2-SPE40%, which displayed a HER onset potential, Tafel slope value and Turn over Frequency (ToF) of -214 mV (vs. RHE), 51.1 mV dec-1 and 2.20, respectively. These values significantly exceeded the HER catalysis of a bare/unmodified SPE, which had a HER onset and Tafel slope value of -459 mV (vs. RHE) and 118 mV dec-1, respectively. Clearly, indicating a strong electrocatalytic response from the 2D-WS2-SPEs. An investigation of the signal stability of the 2D-WS2-SPEs was conducted by performing 1000 repeat cyclic voltammograms (CVs) using a 2D-WS2-SPE10% as a representative example. The 2D-WS2-SPE10% displayed remarkable stability with no variance in the HER onset potential of ca. -268 mV (vs. RHE) and a 44.4% increase in the achievable current over the duration of the 1000 CVs. The technique utilized to fabricate these 2D-WS2-SPEs can be implemented for a plethora of different materials in order to produce large numbers of uniform and highly reproducible electrodes with bespoke electrochemical signal outputs

2D‐Hexagonal Boron Nitride Screen‐Printed Bulk‐Modified Electrochemical Platforms Explored towards Oxygen Reduction Reactions

A low‐cost, scalable and reproducible approach for the mass production of screen‐printed electrode (SPE) platforms that have varying percentage mass incorporations of 2D hexagonal boron nitride (2D‐hBN) (2D‐hBN/SPEs) is demonstrated herein. These novel 2D‐hBN/SPEs are explored as a potential metal‐free electrocatalysts towards oxygen reduction reactions (ORRs) within acidic media where their performance is evaluated. A 5% mass incorporation of 2D‐hBN into the SPEs resulted in the most beneficial ORR catalysis, reducing the ORR onset potential by ca. 200 mV in comparison to bare/unmodified SPEs. Furthermore, an increase in the achievable current of 83% is also exhibited upon the utilisation of a 2D‐hBN/SPE in comparison to its unmodified equivalent. The screen‐printed fabrication approach replaces the less‐reproducible and time‐consuming dropcasting technique of 2D‐hBN and provides an alternative approach for the large‐scale manufacture of novel electrode platforms that can be utilised in a variety of application

Correction to: MoO2 Nanowire Electrochemically Decorated Graphene Additively Manufactured Supercapacitor Platforms (Adv. Energy Mater., (2021), 11, (2100433), 10.1002/10.1002/aenm.202100433)

Adv. Energy Mater. 2021, 11, 2100433 DOI: 10.1002/aenm.202100433 Figure 4 in the originally published article is incorrect in the original manuscript. The correct figure is displayed below. 4 Figure (Figure presented.) SEMs of A,B,C) MoO2-G/AME and D) G/AME. Electrochemical decoration parameters: −1.4 V, 600 s. This error does not affect the conclusions of the report. The authors apologize for any inconvenience caused