Carbon 200 (2022) 227–235 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon Controlling the electrochemical hydrogen generation and storage in graphene oxide by in-situ Raman spectroscopy Adrián Pinilla-Sánchez a, Emigdio Chávez-Angel b, Sebastián Murcia-López c, Nina M. Carretero c, Sidney M. Palardonio a, Peng Xiao b, d, Daniel Rueda-García b, Clivia M. Sotomayor Torres b, e, Pedro Gómez-Romero b, Jordi Martorell a, f, Carles Ros a, * a ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860, Castelldefels, Spain b ICN2 - Catalan Institute of Nanoscience and Nanotechnology, CSIC and The Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, 08193, Barcelona, Spain c IREC - Catalonia Institute for Energy Research, Jardins de les Dones de Negre 1, 08930, Sant Adrià de Besòs, Spain d Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, 08193, Barcelona, Spain e ICREA, Passeig Lluis Companys 23, 08010, Barcelona, Spain f Departament de Física, Universitat Politècnica de Catalunya, 08222, Terrassa, Spain A R T I C L E I N F O A B S T R A C T Keywords: Hydrogen, generated from water splitting, is postulated as one of the most promising alternatives to fossil fuels. Hydrogen storage In this context, direct hydrogen generation by electrolysis and fixation to graphene oxide in an aqueous sus­ Energy storage pension could overcome storage and distribution problems of gaseous hydrogen. This study presents time- Electrochemical reduction resolved determination of the electrochemical hydrogenation of GO by in-situ Raman spectroscopy, simulta­ Hydrogenation neous to original functional groups elimination. Graphene oxide Reduced graphene oxide Hydrogenation is found favoured by dynamic modulation of the electrochemical environment compared to fixed applied potentials, with a 160% increase of C–H bond formation. Epoxide groups suppression and gener­ ated hydroxide groups point at these epoxide groups being one of the key sites where hydrogenation was possible. FTIR revealed characteristic symmetric and asymmetric stretching vibrations of C–H bonds in CH2 and CH3 groups. This shows that hydrogenation is significantly also occurring in defective sites and edges of the graphene basal plane, rather than H-Csp3 groups as graphane. We also determined a − 0.05 VRHE reduction starting potential in alkaline electrolytes and a 150 mV cathodic delay in acid electrolytes. The identified key parameters role, together with observed diverse C-Hx groups formation, points at future research directions for large-scale hydrogen storage in graphene. 1. Introduction Graphene oxide (GO) has emerged as one of the best alternatives to produce graphene-derived materials on large-scale due to the lower cost In a world that demands alternative energy sources and energy – synthesized directly by exfoliation and oxidation of graphite – and its vectors to Fossil Fuels, hydrogen generated from electrochemical [1,2] hydrophilicity given by the oxygen functional groups (OFG) present in or photoelectrochemical [3] water splitting is postulated as one of the the carbon matrix. These OFG can be removed using different method­ most promising alternatives. Despite the existence of a large global ologies [6–9] to obtain reduced graphene oxide (rGO), with properties market for hydrogen gas, which is widely used in industry (oil refining, ranging in-between GO and graphene. The main application of graphene ammonia, methanol and steel production) [4], most of it is still gener­ oxide is as rGO precursor, but during the last decade its use for energy ated from fossil fuels. Along with safety concerns [5], an optimized storage has also been considered. Hydrogenation of GO has already been hydrogen storage and distribution remains a significant challenge. En­ achieved by different methodologies [10,11], and detrapping of the ergy losses from the gas pressurization, storage and distribution still hydrogen has already been reported [12–16]. Recently, electrochemical limits a wider application. water splitting on its surface and direct hydrogen bonding in solid-state * Corresponding author. Institut de Ciències Fotòniques, Organic Nanostructured Photovoltaics group, Castelldefels, Spain. E-mail address: carles.ros@icfo.eu (C. Ros). https://doi.org/10.1016/j.carbon.2022.08.055 Received 17 May 2022; Received in revised form 1 August 2022; Accepted 15 August 2022 Available online 20 August 2022 0008-6223/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 was demonstrated [17], which would solve some of the major problems by diluting K2HPO4 or KH2PO4 in ultrapure miliQ® water and adjusting of hydrogen gas storage and distribution. Computational simulations the pH with either H2SO4 or KOH. have predicted OFG local curvature and irregularities to favour C–H bond formation by inducing a sp2-to-sp3 lattice transition [18]. 2.1. Graphene oxide electrodes Graphene presents a theoretical hydrogen storage capacity of up to 7.7 wt% [18,19], and percentages of ~5 wt% have already been re­ GO films on graphite (GO-graph) were prepared by drop-casting 100 1 ported [16,20,21]. However, such results were obtained in harsh μL of 4 mg mL− GO suspension into 1.2 x 1.2 × 0.0254 cm graphite chemical environments or highly energetic plasmas. For this reason, substrates and allowed to dry first at 60 ◦ C in a convection-drying oven electrochemical hydrogenation is as a promising route. It requires for 90 min and then at room temperature overnight. Graphite foil pur­ inexpensive equipment, is easily scalable and it can be performed at chased from Alfa Aesar was used as substrate. environmental conditions [22]. Several studies have been carried out in a wide range of conditions, 2.2. Electrochemical reduction leading to different results and conclusions, depending on hydrogena­ tion occurring [12,13,17,23–26] or not [27–30]. Few works have All electrochemical experiments were carried out using a home- identified the specific hydrogenation sites, differentiating the basal made 5 mL three-electrode PEEK/PTFE electrochemical cell comprised plane, defective sites, pre-existing OFG sites or edges of the GO [13,31]. of working electrode (GO-graph or GO-Cu), a counter electrode (plat­ Specific studies have reported preferential reduction on epoxide rings inum wire) and reference electrode (Ag/AgCl/KCl 1 M E◦ = 0.222 VRHE). [27,30,32], while other reports do not point any specific site. It is The working electrode set up was obtained by pressing the GO film accepted that H+ plays an essential role in the electrochemical reduction against an O-ring, sealing a small aperture in the electrochemical cell, [23,25] and two specific reduction mechanisms for the epoxide groups defining an electrode area of 0.5 cm2. Experiments were performed in have been proposed [12,13,27]. The first one is the partial reduction 0.1 M KPi, at either pH 2 or pH 12, as supporting electrolyte. The generating a hydroxyl group and one radical, which leads through a electrochemical reduction was made by cyclic voltammetry between reversible reaction to the reduction of epoxide to form a C–OH/C–H pair +0.34 and − 1 VRHE (starting and ending at +0.34 VRHE) at different scan (Eq. 1). The second mechanism corresponds to the complete reduction of rates and cycles, or by chronoamperometry at different potential values the epoxide to restore the sp2 graphene structure (Eq. 2). and times using a potentiostat model SP-300 Bio-Logic SAS controlled by However, knowledge on the conditions that favour the reduction the EC-Lab software. mechanism of different OFG and its hydrogenation is still under debate. In this work, we present a fundamental analysis to determine the in­ 2.3. Characterization techniques fluence of different parameters such as: substrate, electrolyte pH, time, potential, and cycling potentials in the electrochemical reduction and Raman spectra were obtained in a Renishaw inVia spectrometer, hydrogenation mechanisms of GO. In-situ live Raman spectroscopy has equipped with a 532 nm wavelength laser and 50x objective. In order to been used to follow the electrochemical reduction, together with ex-situ avoid sample damage or laser induced reduction [33], the laser was Fourier transform infrared (FTIR) spectroscopy and other morphological attenuated to 0.5% power, 4 s exposition time and 8 acquisitions. Raman characterization techniques. Voltammetric cycling and chronoamper­ spectra were fitted to sums of functions using Peak-o-mat 1.2.9 software, ometries were used as electrochemical hydrogenation techniques, and using three pseudo-Voigt functions (for D, G and D′ bands) and two were found to have influence on the reduction mechanism and to the Gaussian function (for D* and D′′ bands). FTIR spectra were recorded amount of C–H bonds formation. Other parameters such as pH- using a Cary 630 FTIR-ATR. The spectra were obtained after compilation dependant retardment of the OFG reduction and H2 bubbling in the of 128 scans at 4 cm− 1 resolution. The surface morphology of the films GO/substrate interface were also studied. was evaluated by field emission scanning electron microscopy (FEG-­ SEM, FEI Inspect F-EBL). X-ray diffraction (XRD) patterns of GO films 2. Materials and methods were recorded on a Regaku Alpha 1 diffractometer using Cu-Kα (λ = 1.5406 Å) radiation, in the 5◦ ≤ 2θ ≤ 60◦ range at 0.01◦ /s. A commercial suspension of 4 mg mL− 1 GO in water from Graphenea (Graphenea Headquarters, San Sebastián, Spain) was used for the elec­ 2.3.1. In-situ Raman spectroscopy trode preparation. Potassium phosphate salts (K2HPO4 and KH2PO4, A three-electrode 0.5 mL volume home-made PDMS electrochemical Sigma Aldrich), sulphuric acid (H2SO4, Sigma Aldrich) and potassium cell was used to perform the Raman in-situ measurements. The GO film hydroxide (KOH, Merk) were used to fabricate the electrolytes. GO was used as working electrode, a Pt wire as counter electrode and Ag/ aqueous suspensions with concentration 4 mg mL− 1 were prepared by AgCl 3.5 M as reference electrode. The Raman spectra were recorded by diluting the commercial mother solution with distilled water. The T64000 Raman spectrometer manufactured by HORIBA Jobin Yvon phosphate buffer solutions (KPi) 0.1 M at pH 2 and pH 12, were prepared (Chilly-Mazarin, France). It was used in single grating mode (2400 lines) 228 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 with a spectral resolution better than 0.4 cm− 1. The Raman spectra of corresponding to alcohols (1400 cm− 1) and intercalated water (1640 GO were recorded in the range 1250–1700 cm− 1 in two different spec­ cm− 1) [38]. Stretching of C– – C from sp2 domains appears at 1580 cm− 1, tral windows. A 532 nm diode laser was focused on the surface of the GO partially overlapping with intercalated water band. The other main film by using a 10x long working distance microscope objective. Raman functional group present in the GO, along with alcohols groups, are spectra were fitted to sums of functions using Peak-o-mat 1.2.9 software, epoxide rings, showing different C–O stretching bands at 970, 1050 and using three pseudo-Voigt functions (for D, G and D′ bands) and fixing D’ 1240 cm− 1 [37]. Meanwhile, the stretching vibrations from C– – O cor­ band position in 1615–1620 cm− 1. Error bars were determined from the responding to ketones and carboxylic acids appear at 1730 cm− 1 and fitting accuracy. 1800 cm− 1 respectively [37]. The different OFG can also be classified depending on its location. Mainly, alcohols and epoxides are known to 3. Results and discussion be located in the basal plane while carboxylic acids and ketones in the edges of GO sheets [37]. 3.1. GO electrode characterization The Raman spectrum exhibits the typical G (1588 cm− 1) band related to vibrations of sp2 domains and several defect-associated bands (D, D′ , Pristine GO electrodes deposited on graphite by drop-casting were D′′ and D*), whose position, width and relative intensity depend on the characterized by SEM and XRD. SEM (Fig. 1a) shows characteristic local atomic configuration. Bands D (1354 cm− 1) and D’ (1615 cm− 1) graphene nanosheets uniformly distributed and covering the whole arise when defects (such as OFG attracting C atoms out-of-plane, or substrate. The morphology of the substrate causes the generation of defective sites in the C aromatic lattice) break the periodic lattice of wrinkles and stretch marks. The XRD spectrum (Fig. 1b) shows a (001) graphene, whilst D’’ (1540 cm− 1) is related to amorphous phases and D* diffraction peak at 2θ = 10.26◦ (d = 8.61 Å). This large interlayer dis­ (1150 cm− 1) to disordered graphitic lattice due to existence of sp2-sp3 tance is attributed to OFG present in GO, which expand the space be­ bonds [39–43]. tween graphitic 2D layers. This is known as a clear sign of the oxidation The changes in Raman spectra can be interpreted in the framework of of graphite into GO [34,35]. In addition, there is a small broad peak at the two stage classification introduced by Ferrari [44] for graphene 2θ = 18.81◦ owing to a disordered component(s) generated during the materials. This classification allows us to interpret how variations in GO thermal drying process after GO electroreduction [36]. Moreover, it defect density influence Raman peaks [44–46]. ID/IG ratio was measured is also observed a (100) diffraction peak at 2θ = 44.52◦ , indicating short to be 1.57 for GO-graph (Table S1), which depending on the average range order in stacked GO layers [28]. distance between defects (LD), could correlate to low disorder degree FTIR-ATR and Raman spectroscopy were used to characterize the (Stage 1, LD > 3 nm) or to high disorder degree (Stage 2, LD < 3 nm) starting material and determine the different OFG and oxidation state (Fig. S1). In the case of Stage 2 graphene (LD < 3 nm), the ID/IG ratio (Fig. 1c–d). Spectroscopic data shows IR absorption peaks correspond­ decreases proportionally to the number of defects instead of increasing, ing to vibrations related to different OFG bonded to the carbon lattice. as it would be expected in Stage 1 graphene (LD > 3 nm) [45–47]. For The broad band at 3500-2600 cm− 1 corresponds to stretching vibrations very small LD, the D band intensity is proportional to the number of of –OH bonds, arising from alcohol groups and intercalated water be­ 6-fold rings, thus an increase in ID/IG indicates ordering and recovery of tween GO sheets [37]. Two different –OH bending bands appear, sp2 domains [44]. Fig. 1. (a) SEM top view image of a GO-graph sample. (b) XRD diffractogram corresponding to GO-graph (red) and graphite (black), where characteristic peaks of GO can be observed simultaneous to substrate’s graphitic peaks. (c) FTIR absorption spectra and (b) deconvoluted Raman spectra of a pristine GO-graph sample. All measurements were performed after electrode fabrication and prior to any modification. (A colour version of this figure can be viewed online.) 229 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 To distinguish between Stage 2 and Stage 1 region, the Raman groups a possible reason for the increase in interlayer distance. spectra was analysed in terms of full width at half-maximum (FWHM) When applying potentials of − 0.76 and − 0.17 VRHE to GO-graph [45,47]. In our case, the fitted FWHM for D and G bands were 109 and electrodes (corresponding to − 1.25 V vs OCP at pH 2 and 12, respec­ 60 cm− 1, respectively, and thus typical of Stage 2 region. Whereas for tively), stronger evidence of structural changes was observed. Fig. 2a Stage 1 region FWHM of about 18 and 15 cm− 1 would be expected, shows almost complete absence of epoxide rings (1050-850 cm− 1), but respectively [45,47]. In Stage 2 region, when decreasing the defect only a slight reduction in hydroxyl stretching band (3500-2600 cm− 1) is density, the Raman spectra evolves as follows: the G and D bands po­ observed. Although the C– – C stretching band (1580 cm− 1) disappears, sition blueshifts and redshifts, respectively, meanwhile the ID/IG ratio this does not indicate a reduction of sp2 domains but rather a recovery of increases and FWHM of both D and G band decrease. non-OFG-altered hexagonal carbon lattice, which is inactive to IR ab­ sorption [25,30]. A slight reduction of intercalated water band is observed. 3.2. Electrochemical GO reduction at fixed potentials The Raman spectrum of reduced samples for GO-graph (Fig. 2b) confirms the high reduction degree observed in FTIR. A blueshift and Initially, GO-graph electrodes were set at open circuit potential redshift is observed in G and D band (Fig. S4), along with a FWHM (OCP) to track the stabilization potential of the samples in contact with sharpening of both bands. That indicates a reduction in the number of the electrolyte, which were found to be 0.50 and 1.08 VRHE for pH 2 and defects in high disordered GO [42]. An increase of ID/IG (Fig. S3) ratio 12, respectively (Fig. S2). Then, the samples were electrochemically indicates a restoring of sp2 domains. Meanwhile, a reduction of ID’’/IG reduced for 60 min in 0.1 M KPi in acid (pH 2) and alkaline electrolytes and ID*/IG ratio point out a higher order degree and reduction of GO (pH 12), by applying fixed voltages of − 0.45 and − 1.25 V vs OCP – oxygen content, respectively [39,48]. corresponding to 0.04 and − 0.76 VRHE at pH 2 and 0.63 and − 0.17 VRHE The SEM images (Figs. S5a–b) show an increase in stretch marks at pH 12. To study the reduction mechanisms of the different OFG, the along the surface. On the other hand, X-ray diffraction pattern (Fig. S6) electrodes were characterized ex-situ by FTIR and Raman spectroscopy displays a splitting of the peak around 10◦ corresponding to the (001) (Fig. 2). graphite plane, indicating the existence of two different phases with Both at OCP and at +0.04 VRHE (− 0.45 V vs OCP) at pH 2, we did not different interlayer distances of 10.15 Å (2θ = 8.7◦ ) and 7.19 Å (2θ = detect significant changes in the infrared and Raman spectra. But at pH 12.3◦ ). These two components suggest a differential reduction of the 12, which OCP stabilized at 1.08 VRHE or under applied +0.63 VRHE surface and the inner GO film not in contact with water, where protons (− 0.45 V vs OCP), a drastic reduction of C–– O stretching bands at 1730 cannot reach [25]. and 1800 cm− 1 was observed. Two new bands appeared at 1575 and To understand the impact of substrate on the reduction of GO film, 1365 cm− 1 attributed to carboxylate group (-COO-). The presence of we repeated the experiment using copper substrate (Figs. S7 and S8) COO− groups is due to the reaction between alkaline electrolyte hy­ following previous work by Ciammaruchi et al. [17]. Spectroscopic data droxides (-OH) and carboxylic acid groups from GO (-COOH), which of GO samples reduced on copper (Fig. S8) showed similar reduction loose a proton [26]. The protonation also can explain the changes degree and OFG removal. However, the FTIR spectra did not show C–H observed in Raman spectra (Fig. S3, Fig. S4). An enhancement of ID*/IG bands, indicating that the main reduction mechanism is the reduction to is also observed, commonly associated to the increase of C/O ratio. This C–– C (Eq. 2). The absence of hydrogenation mechanism in copper sub­ enhancement is also associated to an increase of interlayer distance strates may be due to uncontrolled electrical contact problems between [48]. Moreover, the increase in ID’’/IG ratio also indicates a loss of order the copper substrate and H2 bubble-detached GO film. The appearance [39], being the repulsion between negatively charged carboxylate Fig. 2. FTIR absorption spectra (a) and Raman spectra (b) of GO-graph samples as-deposited and after applying different potentials for 60 min, at pH 2 (top) or 12 (bottom). (A colour version of this figure can be viewed online.) 230 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 of hydrogen bubbles on the copper surface beneath the GO film caused stretching bands at 3000-2800 cm− 1 is observed, along with a small its total or partial detachment (Fig. S9). The detachment hindered the reduction in –OH, C– – O and C–O stretching bands. For a more negative information related to how long/much time GO was actually polarized. potential of − 0.4 VRHE there is a slight increase of hydrogenation (C–H To further study the role of the pH and potential at which the stretching band at 2925 cm− 1) although a major reduction of the rest of reduction begins, the process was monitored by in-situ Raman spec­ OFG is observed. Therefore, GO hydrogenation level does not become troscopy. The GO-graph electrodes were electrochemically reduced by more favoured by increasing potential, but HER becomes more kineti­ applying a 50 mV stepped potential sweep (Fig. 3a and Fig. S10a). Po­ cally favoured. This represents less efficient H storage in GO and could sition of D and G bands were followed during the reduction, using a end up causing GO film detachment. 2400 grid and two different spectral windows for better resolution in To further support the in-situ Raman obtained results, different GO- frequency. graph electrodes were reduced by applying a constant potential of From stepped potential sweep on GO-graph electrodes, a major − 0.76 VRHE for 1, 4, 20 and 60 min in 0.1 M KPi pH 2, and characterized blueshift of the D-Band position is observed at − 0.2 VRHE for pH 2 and at ex-situ with FTIR and Raman (Fig. 4). As observed also with in-situ − 0.05 VRHE for pH 12, which can be attributed to the starting reduction Raman spectroscopy (Fig. 3b), hydrogenation and OFG reduction hap­ potential at each pH. Both starting reduction potentials are located pens during first 4 min (Fig. 4b–c), prior to the negative current peak in lower than thermodynamic potentials for hydrogen evolution reaction the current-time curve (Fig. 4a). Reduced GO shows an increase in ID/IG (HER). The variation of the reduction potential with different pH can be ratio, an upper and lower shift of G and D bands respectively (Fig. 4e–f), caused by the inefficient dissociation of water to initiate the Volmer indicating high reduction degree and sp2 recovery. Although, in the reaction in alkaline electrolytes, which may favour the reduction of OFG minute 4, not all the electrode’s area is yet homogeneously reduced over HER [49,50]. Similar trends have been observed in other works (Fig. S11). Some areas show complete reduction while others remain [25]. unchanged. By increasing the time to 20 min, the rest of the electrode’s To further evaluate the influence of the potential and the kinetics of surface is reduced completely, and no more changes are observed from the electrochemical reduction, acidic conditions were selected. This 20 to 60 min. favoured C–H bond formation over OFG reduction [18]. Two different Interestingly, the Raman spectra of GO-graph polarized for 1 min constant voltages of − 0.2 and − 0.4 VRHE were applied for 30 min in KPi (Fig. 4f) shows a reduction in ID/IG ratio, indicating introduction of pH 2 (Fig. 3b–d and Fig. S10b). Faster and larger D-Band blueshift, sign defects into the structure. This changes may be caused by the intro­ of GO electrochemical reduction, was observed for a more negative duction of intercalated H+ (in acid electrolytes) or water molecules in applied potential. The D and G bands position change mainly happens the GO film, observed as deformations of the graphene periodic lattice during the first 5–10 min (Fig. 3b) correlates with the decrease of [51]. negative current recorded during the experiment (Fig. 3d). From the FTIR (Fig. 3c and inset) it can be observed that both GO hydrogenation and loss of OFG in the 1700-800 cm− 1 range were active 3.3. Simultaneous C–H bond formation during electrochemical reduction at − 0.2 VRHE, indicating GO reduction (OFG elimination) takes place simultaneously to HER and C–H bonding. The appearance of C–H Figs. 3c and 4c shown the appearance of four new bands during the reductive process. We observed two strong bands at 2925 and 2850 Fig. 3. In-situ measurement of GO-graph G and D Raman shift band positions under (a) an applied stepped potential sweep or (b) a chronoamperometry at different fixed potentials. (c) FTIR and (d) intensity-time curves of samples reduced at different fixed potentials. Measurements were obtained using 0.1 M KPi pH 2 as electrolyte. (A colour version of this figure can be viewed online.) 231 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 Fig. 4. (a) Intensity-time curve obtained after applying a potential of − 0.76 VRHE to GO-graph electrode. (b) FTIR spectra of GO-graph electrodes after applying the same potential for different times. (c) FTIR delimited to C–H stretching region. (d) Raman spectra of the same samples, (e) the position of D, G and D′ bands and. (f) evolution of ID/IG ratio. Measurements were obtained using 0.1 M KPi pH 2 as electrolyte. (A colour version of this figure can be viewed online.) cm− 1 corresponding to asymmetrical and symmetrical stretching of sp3 hydrogenation as following Eq. 1, further support the hydrogenation CH2 groups, and another two at 2950 and 2880 cm− 1 from asymmetrical mechanism of epoxides to be present. and symmetrical stretching of sp3 CH3 groups [52]. This indicates hydrogen is being introduced in Csp3 forming CH2 and CH3 moieties. To better understand the changes observed, it should be taken into 3.4. Electrochemical reduction by cyclic voltammetry consideration the reduction mechanism of epoxides (Eq. 1 and 2). The presence of C–H bands observed in FTIR proves both mechanisms (i.e. In previous sections, the effect of fixed time and potential was C–H and C– – C formation) happen simultaneously, being the formation demonstrated. A further comparison of the GO hydrogenation process of C–H bands in the ~3000 cm− 1 region less pronounced than the loss of was performed between chronoamperometry [12,13,17] or cyclic vol­ signal originating from epoxide rings (1050-850 cm− 1). This points at tammetry [23–26], reported by dew works. GO-graph electrodes were epoxide rings reduction into C– – C formation (Eq. 2) being favoured over reduced by different number of cyclic voltammetries in the range of C–H binding (Eq. 1) under the constant-potential applied electro­ +0.34 to − 1 VRHE in KPi pH 2. The scan rate was varied from 5 to 20 chemical conditions. Furthermore, the low decrease of alcohol associ­ mV/s and total experiment time was kept at 60 min. The reduction was ated bands in FTIR, which are expected to be generated along with followed by in-situ Raman spectroscopy and analysed ex-situ by FTIR, XRD and SEM. Fig. 5. (a) Cyclic voltammograms of GO-graph electrode at scan rate of 5 mV/s (b) FTIR spectra of GO-graph pristine electrodes (black) and either reduced by chronoamperometry at − 0.76 VRHE during 60 min (red) or by cyclic voltammetry between 0.34 and − 1 VRHE during 55 min (green). (c) In-situ measurement of D band position and FWHM during cyclic voltammetries. The beginning of each cycle (0.0) corresponds to 0.34 VRHE, one-quarter and three-quarter cycle (0.25 and 0.75) to − 0.33 VRHE and half cycle (0.5) to − 1 VRHE. Measurements were obtained using 0.1 M KPi pH 2 as electrolyte. (A colour version of this figure can be viewed online.) 232 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 Voltammetric curves (Fig. 5a) present both a capacitive behaviour groups reduction process being a major one in the hydrogenation pro­ increase (larger difference between the forward and reverse curves in cess, although other origins for the C–OH and C–H band formation the 0.3 to − 0.2 VRHE region) and a shift of the onset potential to negative cannot be discarded. The fact that continuous potential cycling result in potentials with successive cycles. A broad reduction peak is observed at increased C–H formation can be caused by increased penetration of H+ − 0.4 VRHE, attributed to the reduction of epoxides and carbonyl OFGs ions or water molecules inside the GO film due to variant electrostatic [27]. After the second cycle, a change of tendency is observed. The forces, exposing more GO sites to the electrolyte, and to a not ionic capacitance and the cathodic current in the − 0.2 to − 1 VRHE region depletion of the local electrolyte environment as a constant potential decreases progressively and the reduction peak of OFG almost disap­ would cause. pears. It indicates an irreversible reduction of different OFG occur dur­ Clearly, C–H stretching bands can be separated into characteristic ing the first voltametric cycle in the studied range of potentials. With symmetric and asymmetric stretching bands originating from sp3- increase of scan cycles a pair of redox peaks appear at − 0.9 V, suggesting bonded CH2 and CH3 groups. Thus, this suggests hydrogen is being reversible removal and reformation of some OFGs. This is further proven introduced in out-of-plane Csp3 in the basal plane or in the edges of GO with the in-situ Raman (Fig. 5b) where it is observed that at positive flakes giving raise to CH2 and CH3 moieties. The herein presented potentials during cycling there is a recovery of the D and G peaks shift. electrochemical hydrogenation mechanism of GO results in a hydroge­ The cathodic current measured during the first cycles (mainly in the nated reduced GO (rGO:H) differing from graphane (GA) [11], corre­ − 0.2 to − 0.6 VRHE region) perfectly matches the decaying negative sponding to a single proton per carbon atom H-Csp3 moieties – showing current observed in Figs. 4d and 5a when constant potentials of − 0.2, a single FTIR absorption peak at 2850 cm− 1 [53]. − 0.4 and − 0.76 VRHE are applied. This further confirms the same OFG electrochemical reduction phenomena is happening both when applying 4. Conclusions cyclic voltammetries or fixed potential chronoamperometries, as observed by in-situ Raman spectroscopy in Fig. 4d. Graphene oxide, having various oxygen functional groups, was GO-electrodes reduced by cyclic voltammetry can be observed by selected as candidate material to study the incorporation of hydrogen. It SEM (Fig. S5c). The images show an increment of wrinkles and porosity, presented mainly alcohol, epoxides, ketones, and carboxylic acids indicating that cyclic voltammetries induce the deformation of the functional groups, which alter the stable structure of graphene and can surface and reorganization of the GO flakes. Additionally, the (001) peak help in easing hydrogen electrochemical bonding. Using a simple and in the XRD (Fig. S6) also shows two components, indicating the exis­ scalable electrochemical treatment in water-based electrolytes, tence of 2 phases, but with lower interlayer distances: 9.0 Å (2θ = 9.8◦ ) hydrogen was incorporated to reduced GO (rGO:H). Simultaneously, and 7.19 Å (2θ = 12.3◦ ). The porous and wrinkled morphology induced OFG were eliminated or reduced, obtaining highly reduced material. by cyclic voltammetry suggests an increase of water molecules pene­ The control of these two simultaneous (and in the epoxy site, competi­ tration into the film and an increase of the electrochemically active tive) reactions was demonstrated possible by using fixed reductive po­ surface and thus, amount of GO active for reduction or hydrogenation. A tentials or continuous cyclic voltammetries. This control allowed up to video has been included as Electronic Supplementary Material to visu­ 160% hydrogen storage increase. alize the GO reorganization under electrochemical cycling at negative Different pH and electrochemical conditions have been studied, potentials. aiming at understanding hydrogen incorporation and storage, chemi­ To further study the electrochemical reduction by cyclic voltamme­ cally bonded to GO, and the electrochemical reduction mechanism of try, the process was followed by in-situ Raman spectroscopy measuring GO. Epoxide groups suppression and alcohol groups generated point at at four times at each cycle (at 0.34 → − 0.33 → − 1 → − 0.33 → 0.34 epoxide groups sites being one of the sites where hydrogenation was VRHE). If analysing the position and FWHM of D and G bands (D band, possible. FTIR revealed characteristic symmetric and asymmetric Fig. 5b and G band, Fig. S14), the reduction can be divided in two stretching vibrations of C–H bonds in the form of CH2 and CH3 groups. phenomena. During the first scan towards cathodic potential (0.34→-1 This shows that hydrogenation is significantly also happening in VRHE), position and FWHM of D band drop drastically and are not defective sites and edges of the graphene basal plane, rather than H-Csp3 recovered in subsequent cycles, indicating an irreversible reduction. groups as graphane. This initial irreversible reduction is consistent with fitted G band posi­ In-situ tracking of the main D and G Raman peaks was used to tion and FWHM trend (Fig. S14). In subsequent cycles, the variations in determine the starting potentials of the electrochemical reduction of GO D positions suggest the GO might be slightly incorporating OFG when into rGO:H, and − 0.2 and − 0.05 VRHE were determined for pH2 and pH scanning towards anodic potential and reduced again when reversing 12, respectively. The significant difference is attributed to the inefficient the potential, pointing into a reversible reduction-oxidation process. dissociation of water to initiate the HER Volmer reaction step in alkaline This initial irreversible step followed by a reversible reaction is in medium, thus favouring the elimination of OFG. More reductive po­ accordance with the first mechanism proposed in Eq. 1 and is consistent tentials did not present higher hydrogenation, rather kinetically endorse with Tateishi [12] and Taniguchi’s [13] results. H2 gas generation, thus diminishing H storage in GO efficiency. Comparing the FTIR absorbance spectra of samples reduced by For constant-applied reductive potential experiments, hydrogen was chronoamperometry (CA) or cyclic voltammetry (CV) (Fig. 5c), similar revealed to be incorporated simultaneous to the observation of a peaks are observed in 800–1700 cm− 1 region, but also clear differences reductive current peak, which also corresponds to the loss of OFG. By are displayed in the 2600-3600 cm− 1 zone. Samples reduced under cy­ further increasing the applied reductive potential, no increase in the clic voltammetries present more –OH stretching (2600–3600 cm− 1) and hydrogenation was observed, it rather favoured the HER which can end bending (1400 cm− 1) together with increased C–H stretching bands than up causing film detachment. On this matter, hydrogen evolution when the ones reduced under − 0.76 VRHE constant applied potential. The in­ using a copper substrate quickly leads to GO film detachment, which crease in the C–H bands absorbance was quantified to be a 160% higher may be the reason of hydrogenation absence observed. than under constant-applied potentials. Additionally, FTIR indicate no Cyclic voltammetry has been proven to favour the hydrogenation dehydrogenation nor OFG formation occur when potential is increased mechanism over a constant-applied potential, reaching 160% more up to 0.34 VRHE. Moreover, 60 min experiments with different scan rates hydrogen trapping. This is caused by variant electrostatic forces gener­ were performed in order to evaluate its influence. It was found the ating a reorganization of the surface that increases electrolyte penetra­ slower the scan rate, the more it favours the hydrogenation mechanism tion and thus, the electrochemical active area of GO. The scanning rate is (Figs. S12 and S13). Both C–OH and C–H increased formation under stablished as a governing factor of the reaction, increasing hydrogen cyclic voltammetries point into a favoured Eq. 1 in front of constant trapping when decreasing scanning rate. In situ Raman spectroscopy potentials favouring Eq. 2. These results highly support the epoxide was used, allowing the identification of two reduction steps: a major 233 A. Pinilla-Sánchez et al. Carbon 200 (2022) 227–235 irreversible initial step followed by a minor reversible reaction. [5] R. Moradi, K.M. Groth, Hydrogen storage and delivery : review of the state of the art technologies and risk and reliability analysis, Int. J. Hydrogen Energy 44 (2019) All this shows that precise control of hydrogen incorporation is 12254–12269, https://doi.org/10.1016/j.ijhydene.2019.03.041. possible, but faces significant simultaneous functional groups elimina­ [6] J.F. Rocha, L. Hostert, M.L.M. Bejarano, R.M. Cardoso, M.D. Santos, C. tion. 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