Sangchai
Sarawutanukul
,
Nutthaphon
Phattharasupakun
,
Juthaporn
Wutthiprom
and
Montree
Sawangphruk
*
Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science, and Technology, Rayong 21210, Thailand. E-mail: montree.s@vistec.ac.th
First published on 12th April 2018
In this study, a graphene oxide (GO) carbocatalyst was synthesized as a thin film on a 3D Ni foam substrate (GO@Ni) by oxidative chemical vapour deposition (CVD) using methanol and water as precursors. The GO@Ni was used as a collaborative electrocatalyst towards the hydrogen evolution reaction (HER) in an acidic electrolyte. Notably, pure Ni metal cannot be used as an electrocatalyst in acidic media due to corrosion. The amount of water in the methanol was finely tuned to optimize the electrochemical activity of the GO@Ni for HER. The optimized GO@Ni catalyst, with a sheet resistivity of 133.48 Ω square−1, an optical band gap of 1.52 eV, and a C:O ratio of 3.89 produced using 90:10 V% of methanol:water, exhibits excellent and stable HER activity with an overpotential of 137 mV vs. RHE, reaching a high current density of 10 mA cm−2 and prominent electrochemical stability for up to 12 h in 0.5 M H2SO4. In addition, the CVD GO layer on the Ni foam acts as an anti-corrosion material protecting the Ni HER catalyst in the acidic environment. As a result, the GO@Ni may be a promising electrode for HER, replacing expensive Pt catalysts.
Nickel-based (Ni-based) electrocatalysts have received great attention in industrial scale hydrogen production, owing to their cost-efficiency and stability in alkaline electrolytes.12 However, Ni catalysts cannot be sustained in an acidic solution because of their chemical instability (corrosion).13 A typical method to prevent the corrosion problem is the fabrication of a thin layer coating of carbon on the metal surface. In addition, several carbon-based materials have also been recently used as electrocatalysts for HER due to their tuneable molecular structures, low cost, natural abundance, and good stability under acid/alkaline environments.14,15 Among carbon-based materials, graphene, a two-dimensional sp2-hybridized carbon nanosheet, exhibits many unique properties, such as a high specific surface area, high mobility of charge carriers, and high stability.16 Ultrathin graphene layers not only improve the anti-corrosion properties of metals, but also promote catalytic reactions by interfacial charge transfer.17 However, pristine graphene sheets are hydrophobic in nature and are therefore unsuitable for use as electrocatalysts in the HER. On the other hand, graphene oxide (GO) is hydrophilic and well known as a carbocatalyst that exhibits a remarkable electrocatalytic activity for water splitting.18 The oxygen-containing functional groups of GO can act as an alternative path for H2 production through the recombination of the spilled Hads from Ni to graphene.19 As a result, in this work, we introduced a new process called “oxidative chemical vapour deposition (CVD) of GO on a Ni foam substrate”, producing a high-efficiency collaborative electrocatalyst, namely GO@Ni, for HER. In this CVD process, methanol diluted with water was used for the first time as the GO precursor. The water content was also finely tuned. The optimized GO@Ni catalyst, produced using 90:10 V% methanol:water, exhibits excellent and stable HER activity with an overpotential of 137 mV (vs. RHE), reaching a high current density of 10 mA cm−2 and excellent electrochemical stability for up to 12 h in 0.5 M H2SO4.
The configuration of the CVD process is illustrated in Fig. 1. The inlet gas was split into 2 paths. The first line (W1 line) was used to anneal the sample during the heating and cooling down of the CVD system. The second line (W2 line) was used to supply gas for the GO growth reaction. Firstly, the treated Ni foam was put in an alumina boat and transferred to the quartz tube furnace under an argon environment with a constant gas flow rate of 100 sccm. Subsequently, the sample was heated from room temperature (25 °C) to 1000 °C, with a heating rate of 10 °C min−1, and held for 50 min to anneal the surface of the treated Ni foam. After that, for 10 min, argon gas was flowed through a 250 ml container of methanol with water at different volume ratios in order to produce the precursor vapour for the GO growth reaction. The sample was then cooled down to ambient temperature under an argon atmosphere. To investigate the effect of the water content in the methanol source on the CVD process, different amounts of DI water were mixed with the methanol with % V/V of 0, 0.1, 1, 5, 10, 25, and 50. The obtained products were labelled as G, G-1, G-2, G-3, G-4, G-5, and G-6, respectively.
Fig. 1 Schematic diagram of the oxidative chemical vapour deposition (CVD) setup for GO growth using methanol with water as the GO precursor. |
Fig. 2 FESEM images of (a) bare Ni foam, (b) G, (c) G-1, (d) G-2, (e) G-3, (f) G-4, (g) G-5, and (f) G-6. |
The elemental compositions of the Ni foam and G-4 were studied by EDS mapping, showing the uniform and continuous dispersion of C and O throughout the Ni foam network after the CVD process, indicating that graphene and GO films were entirely formed on the Ni substrates, as shown in Fig. 3a–f. The AFM image of G-4 (Fig. 3g) shows that its surface is quite smooth, with a few nanometres in thickness on the surface corresponding to a few layers of graphene. In addition, a four-point probe technique was used to measure the sheet resistivity (Rs) of each sample. The measurement was carried out 3 times at a current of 1 μA to get average values. The resistances of G, G-1, G-2, G-3, G-4, G-5, and G-6 are 126.36, 126.59, 127.18, 133.48, 136.25, 137.92, and 140.77 Ω square−1, respectively. The obvious trend of increasing Rs with increasing water content in the oxidative CVD process can be observed as shown in Fig. 3h. This is due to the increasing oxygen-containing content in the GO samples.
Fig. 3 EDS mapping of (a–c) Ni foam and (d–f) G-4, (g) AFM image of G-4 surface, and (h) the sheet resistances of the as-prepared samples. |
Fig. 4a shows the XRD patterns of the as-prepared materials exhibiting dominant diffraction peaks around 44.4, 51.9, and 76.3°, corresponding to the (111), (200), and (220) Ni crystallographic planes. The orderly and grain-boundary-free surface of Ni (111) provides a smooth surface for uniform graphene and GO formation. In contrast, the rough surface of polycrystalline Ni (200) with its abundant grain boundaries facilitates the formation of multilayer graphene.22 The extremely weak intensity peak of C (002) confirms that the ultrathin graphene and GO sheets were formed on the surface of the Ni foam. The FTIR spectra in Fig. 4b present a broad peak at 1350–1600 cm−1, due to the stretching vibration mode of sp2 carbon (CC), and the peak at 1200–1250 cm−1, owing to the stretching vibration of alcohol (C–O). In addition, the peak at 1720–1740 cm−1 can be assigned to the CO stretching vibration of the carboxyl groups.23 As a result, it can be concluded that the oxygen functional groups were successfully generated at the defect sites of the graphene sheets. Fig. 4c shows the Raman spectra of GO growth with G to G-6 conditions.
The Raman spectra exhibit the characteristic peaks of a carbon-based material, but the Ni foam peak does not appear, indicating that the graphene sheets entirely covered the surface of the Ni foam substrate.24 The defect density is indicated by a D band (disordered) at 1350 cm−1 from the vibration mode of sp3 carbon atoms chemically bonded to oxygen-containing groups and carbon on the edge of the graphitic sheets.25 Meanwhile, the G band at 1580 cm−1 represents the vibration of sp2 graphitic carbon. The intensity ratios of the D to G band (ID/IG) of the G to G-6 samples are 0.042, 0.057, 0.1125, 0.3656, 0.4867, 0.5873, and 1.0301, respectively, as shown in Fig. 4d. The increased ID/IG represents a higher number of defects in the carbon basal planes when the water content in the carbon source (methanol) is increased. In addition, the theoretically calculated grain sizes relating to ID/IG (ref. 26) for the G to G-6 samples are 456.4, 333.9, 170.9, 52.6, 39.5, 32.7, and 18.7 nm, respectively.
Fig. 5a shows the absorbance spectra for GO with an absorption in the visible to near-IR light region. The theoretical band structure analysis shows that the optical band gap of GO is changed as a function of the increasing oxidation level.27 This is related to the change of the highest valence band state from the bonding π orbital to the oxygen 2p orbital.28 To determine the band gap energy for the direct band gap transition, the relation of the square root of the absorption energy against the photon energy was plotted. From the approximated linear extrapolation of the G to G-6 samples, Fig. 6b shows band gap energies of 1.37, 1.43, 1.48, 1.49, 1.52, 1.48, and 1.48 eV, respectively. The GO@Ni samples show narrow band gap energies, which are the intermediate states between fully oxidized GO, which has a large band gap energy of 2.4–4 eV (ref. 28), and pristine graphene, which has zero band gap energy.29 Therefore, the oxidation level of the GO samples can be adjusted to have the proper electronic properties for HER.
Fig. 5 (a) Optical absorption spectra of the GO@Ni samples (G to G-6) and (b) Tau plots of (αE)2 against the photon energy. |
Fig. 6 (a) XPS wide scan spectra, (b) narrow XPS scan of C 1s, (c) narrow XPS scan of O 1s of G-4, and (d) C:O ratio of GO@Ni. |
The surface compositions of the GO@Ni samples were investigated by XPS. Wide scan XPS spectra (Fig. 6a) show the expected photoelectron peaks of Ni, C, and O in the GO@Ni samples. The narrow scan of the C 1s of G-4 (Fig. 6b) presents typical GO characteristic peaks consisting of four deconvoluted peaks at 283.8, 284.7, 285.6, and 286.7 eV, attributed to C–Ni (5.92%), C–C (43.63%), C–O (37.25%), and epoxide (CO, 13.20%) groups,30 respectively. The appearance of oxygen species can be clearly observed, owing to the oxygen-containing groups being chemically bonded to the graphene nanosheets. Fig. 6c shows the O 1s spectra of G-4 corresponding to the peaks of O–CO (531.41 eV, 25.68%), CO (532.49 eV, 58.33%), and C–O (534.33 eV, 15.97%).23 The C:O ratios of the G to G-6 samples are 6.45, 6.34, 5.91, 4.31, 3.89, 3.82, and 3.31, respectively (Fig. 6d), in agreement with FTIR, Raman, sheet resistance, and optical absorption results. As a result, it can be concluded that the oxidative CVD in this work can provide GO films with finely tuned oxygen content and functional groups on the Ni foam HER electrocatalyst. The oxygen functional groups, including hydroxyl and carbonyl groups on the Ni foam, can assist the H spillover from its surface. In other words, they act as an acceptor of H atoms. These phenomena will facilitate the formation of free active sites on the Ni phase, which is necessary for the HER to proceed. Also, they can act as an additional path for H2 production via the recombination of the spilled Hads from the Ni foam with GO. This action is enabled by the unique reactivity of the oxygen groups of GO.19 Thus, efficient HER activity can be expected due to the oxygen incorporation on the collaborative GO@Ni electrocatalyst with several disorder structures.
Furthermore, the linear portions of the polarization curves were fitted to the Tafel equation (eqn (1)). The Tafel equation can be used to describe the current–potential relationship at significant η:
η = blogj + a | (1) |
Discharge reaction (Volmer reaction):
H3O+ + e− + cat → cat-H + H2O | (2) |
Combination reaction (Tafel reaction):
cat-H + cat-H → 2cat + H2 | (3) |
Ion + atom reaction (Heyrovsky reaction):
H3O+ + e− + cat-H →cat + H2 + H2O | (4) |
According to previous reports, a Tafel slope of 30 mV dec−1 in a low overpotential range indicates that the mechanism proceeds through the Volmer–Tafel mechanism and the recombination step is a rate-limiting step.35 Meanwhile, a Tafel slope of 40 mV dec−1 suggests that H2 is produced via the Volmer–Heyrovsky mechanism and the electrochemical desorption step is a rate-limiting step.36 In addition, a Tafel slope of 120 mV dec−1 may arise from various reaction pathways depending on the surface coverage of adsorbed H2.35 The measured values from the Tafel slopes using the GO@Ni foams and the bare Ni foam are listed in Table 1.
Electrode | Required overpotential (mV, at 10 mV cm−2) | Tafel slope (mV dec−1) | Exchange current density (jo, A cm−2) |
---|---|---|---|
Ni foam | 335.7 | 73.0 | 4.75 × 10−6 |
G | 247.0 | 65.7 | 3.95 × 10−5 |
G-1 | 225.7 | 65.4 | 8.30 × 10−5 |
G-2 | 207.5 | 64.8 | 5.51 × 10−4 |
G-3 | 160.5 | 64.5 | 6.95 × 10−4 |
G-4 | 137.0 | 54.4 | 7.38 × 10−4 |
G-5 | 186.2 | 56.9 | 1.46 × 10−4 |
G-6 | 191.5 | 56.4 | 1.09 × 10−4 |
The GO (G-4) not only exhibits a superior HER activity in terms of the lowest η, but also shows a low Tafel slope of 54.4 mV dec−1, suggesting that the Volmer–Heyrovsky reaction is the rate-limiting step of the electrode. In addition, the GO can also generate a second pathway in which electrons can transfer along the layer and will be trapped by defect carbons that serve as alternative active sites for hydrogen evolution, as shown in the following reactions ((5)–(7)).37
H3O+ + e− + C(defect) → C(defect)–H + H2O | (5) |
C(defect)–H + Ni(cat) → Ni(cat)–H + C(defect) | (6) |
Ni(cat)–H + Ni(cat)–H → 2Ni(cat) + H2 | (7) |
Firstly, the reactant H3O+ molecules permeate through the GO layer, reaching the active sites via the defects or edges of GO (see reaction (5)). The formation of C–H bonding at the defect carbons will transform the H3O+ molecules into H atoms coupled with Ni metal (see reaction (6)).38 However, the C–H bond energy of pristine GO is quite weak (0.82 eV),39 causing the unstable adsorption of the H atoms. After coupling with the Ni metal, the C–H bond energy can be increased (0.96–1.15 eV),39 making the H atoms become more stable, upon which a large amount of them can adsorb on the GO surface that covers the Ni. The chemisorbed H atoms on the defect carbons with very low activation energy40 can move along the GO layer to the surface of the Ni substrate, recombining with other H atoms to form H2, which will then be easily desorbed from the Ni surface (see reaction (7)). As a result, both the GO (defect carbons) and Ni metal synergistically promote HER. Additionally, the exchange current density (jo) can be used to evaluate the activity of the catalysts toward the HER in water electrolysis.19 The obtained values in Table 1 show that G-4 has an exchange current density 18.7 times higher than that of non-oxidative graphene (G) and 155.4 times higher than that of the bare Ni foam. These results demonstrate the dramatic effect of GO in enhancing the activity of the Ni foam. It also suggests that the supported GO@Ni foam can substantially increase the catalyst conductivity and the oxygen-containing groups can provide sufficient active sites for the electrolyte reactivity. To evaluate the stability of the G-4 catalyst, the electrode was tested under a static overpotential of −0.45 V vs. Ag/AgCl (−0.21 V vs. RHE) in 0.5 M H2SO4, as shown in Fig. 7c. Its cathodic current density still remains after 12 h operation, indicating that the GO@Ni foam electrode exhibits high stability during electrocatalytic hydrogen production, even in corrosive acidic solution. Digital photographs of the bare Ni foam and GO@Ni electrodes before and after HER testing (Fig. 7d) indicate that the GO coating can strongly enhance anti-corrosion in acidic solution, while 72.98% wt. of the bare Ni foam catalyst was reduced after the HER testing. On the other hand, the slight decrease in the mass of the GO@Ni samples was also observed due to H2 gas bubbling during the HER process, causing the removal of small amounts of GO on the Ni foam surface.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00161h |
This journal is © The Royal Society of Chemistry 2018 |