Samuel J. Rowley-Neale

3D Printed Graphene Based Energy Storage Devices

3D printing technology provides a unique platform for rapid prototyping of numerous applications due to its ability to produce low cost 3D printed platforms. Herein, a graphene-based polylactic acid filament (graphene/PLA) has been 3D printed to fabricate a range of 3D disc electrode (3DE) configurations using a conventional RepRap fused deposition moulding (FDM) 3D printer, which requires no further modification/ex-situ curing step. To provide proof-of-concept, these 3D printed electrode architectures are characterised both electrochemically and physicochemically and are advantageously applied as freestanding anodes within Li-ion batteries and as solid-state supercapacitors. These freestanding anodes neglect the requirement for a current collector, thus offering a simplistic and cheaper alternative to traditional Li-ion based setups. Additionally, the ability of these devices’ to electrochemically produce hydrogen via the hydrogen evolution reaction (HER) as an alternative to currently utilised platinum based electrodes (with in electrolysers) is also performed. The 3DE demonstrates an unexpectedly high catalytic activity towards the HER (−0.46 V vs. SCE) upon the 1000th cycle, such potential is the closest observed to the desired value of platinum at (−0.25 V vs. SCE). We subsequently suggest that 3D printing of graphene-based conductive filaments allows for the simple fabrication of energy storage devices with bespoke and conceptual designs to be realised.

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.

Mass-Producible 2D-MoS2-Impregnated Screen-Printed Electrodes That Demonstrate Efficient Electrocatalysis toward the Oxygen Reduction Reaction

Two-dimensional molybdenum disulfide (2D-MoS2) screen-printed electrodes (2D-MoS2-SPEs) have been designed, fabricated, and evaluated toward the electrochemical oxygen reduction reaction (ORR) within acidic aqueous media. A screen-printable ink has been developed that allows for the tailoring of the 2D-MoS2 content/mass used in the fabrication of the 2D-MoS2-SPEs, which critically affects the observed ORR performance. In comparison to the graphite SPEs (G-SPEs), the 2D-MoS2-SPEs are shown to exhibit an electrocatalytic behavior toward the ORR which is found, critically, to be reliant upon the percentage mass incorporation of 2D-MoS2 in the 2D-MoS2-SPEs; a greater percentage mass of 2D-MoS2 incorporated into the 2D-MoS2-SPEs results in a significantly less electronegative ORR onset potential and a greater signal output (current density). Using optimally fabricated 2D-MoS2-SPEs, an ORR onset and a peak current of approximately +0.16 V [vs saturated calomel electrode (SCE)] and -1.62 mA cm-2, respectively, are observed, which exceeds the -0.53 V (vs SCE) and -635 μA cm-2 performance of unmodified G-SPEs, indicating an electrocatalytic response toward the ORR utilizing the 2D-MoS2-SPEs. An investigation of the underlying electrochemical reaction mechanism of the ORR within acidic aqueous solutions reveals that the reaction proceeds via a direct four-electron process for all of the 2D-MoS2-SPE variants studied herein, where oxygen is electrochemically favorably reduced to water. The fabricated 2D-MoS2-SPEs are found to exhibit no degradation in the observed achievable current over the course of 1000 repeat scans. The production of such inks and the resultant mass-producible 2D-MoS2-SPEs mitigates the need to modify post hoc an electrode via the drop-casting technique that has been previously shown to result in a loss of achievable current over the course of 1000 repeat scans. The 2D-MoS2-SPEs designed, fabricated, and tested herein could have commercial viability as electrocatalytic fuel cell electrodes because of being economical as a result of their scales of economy and inherent tailorability. The technique utilized herein to produce the 2D-MoS2-SPEs could be adapted for the incorporation of different 2D nanomaterials, resulting in SPEs with the inherent advantages identified above.

Surfactant-exfoliated 2D molybdenum disulphide (2D-MoS2): the role of surfactant upon the hydrogen evolution reaction

Surfactant (sodium cholate, SC) mediated liquid (aqueous) phase exfoliation is a common approach to fabricate 2D molybdenum disulphide (2D-MoS2–SC) nanosheets since it is a facile methodology producing defect free flakes with nanometer lateral sizes. The electrocatalytic behaviour of 2D-MoS2–SC towards the Hydrogen Evolution Reaction (HER) is benchmarked within acidic media and found to exhibit inferior HER activity to an equivalent mass of pristine 2D-MoS2 (2D-MoS2 produced without a surfactant), with HER onset potentials, current densities and Tafel values of −0.61 V (vs. SCE), −2.19 mA cm−2, 141 mV dec−1 and −0.42 V (vs. SCE), −4.96 mA cm−2, 94 mV dec−1 respectively. This work demonstrates that sodium cholate has a detrimental effect upon the HER activity of 2D-MoS2. Future studies that utilise 2D materials, fabricated via liquid surfactant exfoliation, should consider the role of the surfactant in the observed electrochemical responses and perform the necessary control experiments.

Nitrogen doped nanoporous graphene: an efficient metal-free electrocatalyst for the oxygen reduction reaction

The oxygen reduction reaction (ORR) is an important half reaction, which occurs at the cathode within a fuel cell and limits their range of applications due to slow electrochemical kinetics. To overcome this issue, electrocatalysts are sought, which need to be an alternative to expensive and unsustainable metallic catalysts. Herein we report the synthesis of nitrogen doped nanoporous graphene (NPG), which is a competitive alternative to currently employed metallic catalysts. The NPG is synthesised through a chemical vapour deposition methodology followed by a chemical functionalization step introducing oxygen functional groups (carbonyl and hydroxyl groups), which is then doped with nitrogen via ortho-phenylenediamine (OPDA). The NPG is physiochemically and electrochemically characterised. The NPG demonstrates outstanding electrocatalytic activity towards the ORR in alkaline media proceeding via a favourable 4-electron pathway and is comparable to commercially available platinum–carbon (20%). We demonstrate that the electrochemical activity of the NPG is tailorable such that through increased nitrogen doping the ORR transforms from a 2-electron process to that of the favourable 4-electron process via increasing the proportion of pyridinic nitrogen while the content of graphitic nitrogen remains almost constant. The NPG exhibits excellent electrochemical performance towards the ORR in alkaline media, long-term stability and appropriate methanol crossover as benchmarked to commercialised Pt/C electrodes; this outstanding electrocatalytic activity is related to the high proportion of defects, high porosity and (pyridinic) doping.

Magnetron Sputter-Coated Nanoparticle MoS2 Supported on Nanocarbon: A Highly Efficient Electrocatalyst toward the Hydrogen Evolution Reaction

The design and fabrication of inexpensive highly efficient electrocatalysts for the production of hydrogen via the hydrogen evolution reaction (HER) underpin a plethora of emerging clean energy technologies. Herein, we report the fabrication of highly efficient electrocatalysts for the HER based on magnetron-sputtered MoS2 onto a nanocarbon support, termed MoS2/C. Magnetron sputtering time is explored as a function of its physiochemical composition and HER performance; increased sputtering times give rise to materials with differing compositions, i.e., Mo4+ to Mo6+ and associated S anions (sulfide, elemental, and sulfate), and improved HER outputs. An optimized sputtering time of 45 min was used to fabricate the MoS2/C material. This gave rise to an optimal HER performance with regard to its HER onset potential, achievable current, and Tafel value, which were −0.44 (vs saturated calomel electrode (SCE)), −1.45 mV s–1, and 43 mV dec–1, respectively, which has the highest composition of Mo4+ and sulfide (MoS2). Electrochemical testing toward the HER via drop casting MoS2/C upon screen-printed electrodes (SPEs) to electrically wire the nanomaterial is found to be mass coverage dependent, where the current density increases up to a critical mass (ca. 50 μg cm–2), after which a plateau is observed. To allow for a translation of the bespoke fabricated MoS2/C from laboratory to new industrial applications, MoS2/C was incorporated into the bulk ink utilized in the fabrication of SPEs (denoted as MoS2/C-SPE), thus allowing for improved electrical wiring to the MoS2/C and resulting in the production of scalable and reproducible electrocatalytic platforms. The MoS2/C-SPEs displayed far greater HER catalysis with a 450 mV reduction in the HER onset potential and a 1.70 mA cm–2 increase in the achievable current density (recorded at −0.75 V (vs SCE)), compared to a bare/unmodified graphitic SPE. The approach of using magnetron sputtering to modify carbon with MoS2 facilitates the production of mass-producible, stable, and effective electrode materials for possible use in electrolyzers, which are cost competitive to Pt and mitigate the need to use time-consuming and low-yield exfoliation techniques typically used to fabricate pristine MoS2.