Abstract
Two-dimensional molybdenum disulphide nanosheets (2D-MoS2) have proven to be an effective electrocatalyst, with particular attention being focused on their use towards increasing the efficiency of the reactions associated with hydrogen fuel cells. Whilst the majority of research has focused on the Hydrogen Evolution Reaction (HER), herein we explore the use of 2D-MoS2 as a potential electrocatalyst for the much less researched Oxygen Reduction Reaction (ORR). We stray from literature conventions and perform experiments in 0.1 M H2SO4 acidic electrolyte for the first time, evaluating the electrochemical performance of the ORR with 2D-MoS2 electrically wired/immobilised upon several carbon based electrodes (namely; Boron Doped Diamond (BDD), Edge Plane Pyrolytic Graphite (EPPG), Glassy Carbon (GC) and Screen-Printed Electrodes (SPE)) whilst exploring a range of 2D-MoS2 coverages/masses. Consequently, the findings of this study are highly applicable to real world fuel cell applications. We show that significant improvements in ORR activity can be achieved through the careful selection of the underlying/supporting carbon materials that electrically wire the 2D-MoS2 and utilisation of an optimal mass of 2D-MoS2. The ORR onset is observed to be reduced to ca. +0.10 V for EPPG, GC and SPEs at 2D-MoS2 (1524 ng cm(-2) modification), which is far closer to Pt at +0.46 V compared to bare/unmodified EPPG, GC and SPE counterparts. This report is the first to demonstrate such beneficial electrochemical responses in acidic conditions using a 2D-MoS2 based electrocatalyst material on a carbon-based substrate (SPEs in this case). Investigation of the beneficial reaction mechanism reveals the ORR to occur via a 4 electron process in specific conditions; elsewhere a 2 electron process is observed. This work offers valuable insights for those wishing to design, fabricate and/or electrochemically test 2D-nanosheet materials towards the ORR.
Highlights
Scanning electron microscope (SEM) and Transmission electron microscopy (TEM) images of the commercially sourced 2D-MoS2 are shown in Electronic supplementary information (ESI) Fig. 3 and 4.† energy-dispersive X-ray microanalysis (EDS) (ESI Fig. 3†) and X-ray photoelectron spectroscopy (XPS) (ESI Fig. 5 and 6†) confirm the presence of Mo and S at the expected ratios (0.55 Mo at% to 1.35 S and Mo to S at% concentrations at a 1 : 2.2 ratio respectively) indicating the presence of 2D-MoS2. This was supported by X-ray diffraction (XRD) analysis (ESI Fig. 7†), which shows a diffraction peak for 2D-MoS2 with a 2θ corresponding to 14.2°
Raman spectroscopy (ESI Fig. 8 and 9†) indicates that the separation between the A1g and E12g vibrational bands give a consistent value of 24.7 cm−1, which corresponds with literature to bulk MoS2.53 This implies that upon deposition of the 2D-MoS2 utilised onto the supporting electrode materials, the structural model is likely that of re-assembly, with few-layer nanosheets forming as bulk
Paper ensure that no electroactivity was observed in the region of a linear sweep voltammogram (LSV) where the oxygen reduction reaction (ORR) is expected to occur, as this would convolute the interpretation of the ORR, the results of which can be observed in ESI Fig. 10.†
Summary
The essential reactions which allow a fuel cell to produce a current are the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR).[12,13,14] The HOR occurs on the anode and typically has a negligible overpotential, whilst the ORR occurs at the cathode and has a large kinetic inhibition given the strong (di)oxygen double bond resulting in a large energy input to initiate the reaction.[12,15] This results in the ORR being the rate determining step in the production of output energy from the initial H2 fuel source. The ORR processes in alkaline and acidic media are as follows:[22,23]
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