Investigating the Integrity of Graphene towards the Electrochemical Hydrogen Evolution Reaction (HER)

Mono-, few-, and multilayer graphene is explored towards the electrochemical Hydrogen Evolution Reaction (HER). Careful physicochemical characterisation is undertaken during electrochemical perturbation revealing that the integrity of graphene is structurally compromised. Electrochemical perturbation, in the form of electrochemical potential scanning (linear sweep voltammetry), as induced when exploring the HER using monolayer graphene, creates defects upon the basal plane surface that increases the coverage of edge plane sites/defects resulting in an increase in the electrochemical reversibility of the HER process. This process of improved HER performance occurs up to a threshold, where substantial break-up of the basal sheet occurs, after which the electrochemical response decreases; this is due to the destruction of the sheet integrity and lack of electrical conductive pathways. Importantly, the severity of these changes is structurally dependent on the graphene variant utilised. This work indicates that multilayer graphene has more potential as an electrochemical platform for the HER, rather than that of mono- and few-layer graphene. There is huge potential for this knowledge to be usefully exploited within the energy sector and beyond.

. Hydrogen bubble size growth generated in-situ while performing chronoamperometry, potential held at -1.2 V vs. RHE, using monolayer graphene, which clearly shows the evolution of a single hydrogen bubble from its initial generation to its explosion. Note that the measurement of the bubble started once it was big enough to be analysed with the optical microscope, therefore we timed that initial measurement as 'time 0 seconds'.  Figure S1. Voltammograms of a non-faradaic region between +0.16 and +0.26 V analysed to calculate the data shown in Table S1. In all cases the scan rate was 100 mVs -1 (vs. RHE) and the solution composition was 0.5 M H2SO4 (degassed using nitrogen). Figure S2. Snapshot from an in-situ video recorded while performing chronoamperometry, (potential held a -1.2 V (vs. RHE)) using monolayer graphene clearly showing the evolution of hydrogen bubbles on top of the graphene electrode over the following time periods: 0 (A) (zero as initial measurement time), 1 (B), 4 (C), 14 (D), 45 (E) and 67 (F) seconds. Video recorded from the top of the graphene during the electrochemical experiment.

Physicochemical characterisation of the graphene used in this work
The physicochemical characterisation of the batch graphene samples used in this work and their characterisation is reported below.
Atomic force microscopy (AFM) characterisation of the batch graphene samples used in this work including the monolayer and multilayer graphene have been reported previously in Ref [2]. Furthermore, X-ray photoelectron spectroscopy (XPS) has previously been performed on these batch samples revealing the monolayer graphene to comprise of an O/C ratio of ca. 0.05, which is consistent with that of a low oxygen content of the graphene domain and thus is indicative of being pristine (aka pristine graphene). 1 In the case of the multilayer graphene samples, XPS reveals a O/C ratio of ca. 0.07, that is again consistent with inferences gained through Raman spectroscopy (see later) and indicates that the this material is comprised of pristine graphene.
Raman characterisation of the batch mono-, few-and multilayer graphene electrode normalised to the G peak were performed and are as depicted in Figure S3. The Raman spectra of the graphene films confirms the G (ca. 1550 cm -1 ) and 2D (ca. 2680 cm -1 ) characteristic peaks that allow us to quantify the number of graphene layers. The Raman spectra of the monolayer graphene sheets reveals that the full width at half-maximum (FWHM) of the 2D band corresponds to 34.72 cm -1 , which upon exploring the literature [3][4] indicates that our batch samples are comprised of single layer of graphene; additionally the intensity ratio G/2D of 0.72 suggests that the graphene samples are comprised of monolayer due to the lower intensity of the G band in relation to the 2D peak. The Raman spectrum of the few-layer graphene films reveals an intensity ratio of G/2D of 1.00 suggesting that the electrode is comprised of duallayer graphene, but as there are occasional multilayer islands; it is therefore named herein as "few-layer" graphene. The Raman spectrum of the multilayer graphene reveals an intensity ratio of G/2D of 1.76 suggesting that such electrode is comprised of multilayer graphene. Figure S3. Raman characterisation of mono-, few-and multilayer graphene utilised within this work. Raman is performed with a 532 nm excitation laser at a low power of 3 mW to avoid any heating effects. Spectra were recorded using a 3 seconds exposure time for 3 accumulations at each point.

Graphene housing 3D printed cell
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