2020, Vol. 31 
b Analytical & Testing Center, Sichuan University, Chengdu 610064, China
Nowadays, considerable attentions have been devoted to developing clean, renewable and environmentally friendly energy sources to address the energy crisis and ameliorate the environmental pollution arisen from the overuse of fossil fuels [1, 2]. Among the reported new energies, hydrogen (H2) has been widely regarded as the most promising alternative to fossil fuels and attracted the great interest to create artificial systems for the efficient production of H2 [3, 4]. Nevertheless, industrial H2 source mainly comes from the reforming of methane steam and the coal gasification process, which has no promotion to address the source of environmental pollution and greenhouse effect of CO2 emissions [5]. Electrochemical water splitting is the most economical and sustainable method for large-scale hydrogen production because of the link to renewable electricity (solar, wind, hydropower, geothermal, etc.).
Due to the dynamically unfavorable nature of water splitting, an efficient electro-catalyst is indispensable to drive a high current density at low overpotential (η) during hydrogen evolution reaction (HER) [6]. In addition, the strongly acidic conditions in proton exchange membrane (PEM) technology used for HER usually require acid-stable catalysts [7]. Currently, Pt-based catalysts are regarded as the state-of-the-art electro-catalysts for HER in acidic media [6]. However, high cost and quantity scarcity are the barriers for its scale-up utilization in industrial deployment. There are urgent needs to not only improve catalyst activity and durability but also decrease the cost of catalyst by developing a Pt-free or with ultralow Pt loading catalyst. Over the past decade, Pt-free catalysts have been widely studied, including metal sulfides [8], phosphates [9-12], nitrides [13], selenides [14-16], carbides [17] and non-noble metal alloys [18], etc. However, there is still a significant gap in the catalytic performance between Pt-based catalyst and Pt-free catalyst [19, 20]. On the other hand, various methods are widely applied in order to reduce the Pt content in PEM technology, such as electro-deposition [21, 22], sputter deposition [23], ion-beam assisted deposition [24], inkjet printing [25], spraying [26], electrospraying [27], ultrasonic spraying [28], atomic layer deposition [29]. However, a substrate is always required in aforementioned methods, resulting in a relatively more complicated and time-consuming process. Thus, is attractive to develop a simpler and more inexpensive strategy to synthesize the catalyst with low Pt content and without substrate. Usually, Pt-based catalysts prepared for HER with solvothermal route are firstly made into ink, and then adhered on electrode surfaces with polymer binders (Nafion or PTFE) [30-33], served as the working electrode in a three-electrode system. The use of polymer binder may increase the series resistance, bury the active sites and suppress the permeability of gas, restraining the catalytic efficiency [34].
Graphene is a rapidly rising star on horizon of materials science and condensed-matter physics [35], exhibiting a high surface area, superior thermal conductivity and excellent electron transport property. Very recently, Cao demonstrated that solutioncondensed graphene oxide (GO) films can be served as a glue and electrode (after reduction) for making vertical 3D architectures [36]. In our current work, multi-walled carbon nanotubes (MWCNT) and reduced graphene oxide (rGO) supported Pt nanoparticle hydrogel (PtNP/rGO-MWCNT) catalyst was prepared by a simple one-step strategy and directly used as a 3D flexible electrode for efficient HER with ultralow Pt loading. In this strategy, the GO and MWCNT were served as a binder and architect to form the 3D architecture of the material. Non-toxic ascorbic acid (AA) was used to not only reduce GO to form 3D hydrogel but also reduce H2PtCl6 to PtNP.
The as-prepared PtNP/rGO-MWCNT exhibits extraordinary mechanical property, including high strength and excellent toughness, and can be arbitrarily tailored into any shapes, such as cubes, or strips (Fig. S1 in Supporting information). Figs. 1a and b and Figs. S2a and b (Supporting information) show the scanning electron microscopy (SEM) images of PtNP/rGO-MWCNT in different dimensions. The MWCNT were embedded in and absorbed on the sheets of rGO to form a three-dimensional network, where the Pt nanoparticles can be uniformly dispersed. Furthermore, Figs. S2c–h (Supporting information) show the SEM images of PtNP/rGO, PtNP/MWCNT, and rGO-MWCNT, respectively, indicating that the Pt nanoparticles can uniformly disperse on the layer of rGO and three-dimensional network of MWCNT with a diameter about 100–200 nm.
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| Fig. 1. (a) SEM image and (b) cross-section SEM image of PtNP/rGO-MWCNT. (c) TEM image, (d) HRTEM image and (e) SAED pattern taken from PtNP/rGO-MWCNT, (f) XRD patterns of PtNP/rGO-MWCNT and GO. | |
The transmission electron microscopy (TEM) image of one Pt nanoparticle on rGO and MWCNT shows that the PtNP has a rough spheroid-like morphology, thus resulting in a large surface area to enhanceits catalytic HER performance (Fig. 1c). High-resolution TEM (HRTEM) image in Fig. 1d reveals the well-resolved lattice fringes with an interplanar distance of 0.227 nm indexed to the (111) plane of Pt. The ring patterns of the selected area electron diffraction (SAED) can be assigned to the (111), (200), (220) and (311) planes of face-centered-cubic (fcc) crystal of Pt, indicating the polycrystalline nature of catalyst (Fig. 1e). The energy-dispersive X-ray (EDX) spectrum analysis confirms that PtNP/rGO-MWCNT is indeed composed of C, O and Pt (Fig. S3 in Supporting information). In addition, the content of Pt in PtNP/rGO, PtNP/MWCNT, and PtNP/rGO-MWCNT were measured by inductively coupled plasma optical emission spectrometry (ICP-OES), revealing a Pt content of 0.75 wt%, 10.25 wt% and 3.48 wt%, respectively. Fig. 1f shows the X-ray diffraction (XRD) patterns for PtNP/rGO-MWCNT and GO. The diffraction peaks featuring (111), (200), (220), (331) and (222) planes of Pt phase for PtNP/rGO-MWCNT at 39.8°, 46.2°, 67.5°, 81.2° and 85.7°, respectively, are in agreement with the results of the aforementioned SAED analysis. The peaks at 26.5° and 9.8° are indexed to the (002) plane of C phase for rGO and GO, respectively, indicating the reduction of GO to rGO with the interlayer spacing changing from 0.9 nm to 0.34 nm due to the elimination of functional groups in GO [37, 38].
X-ray photoelectron spectroscopy (XPS) was applied to characterize the surface electronic structures of PtNP/rGO-MWCNT PtNP/ MWCNT, PtNP/rGO, rGO-MWCNT and pure GO. The analytical results are summarized in Fig. 2a and Fig. S4a (Supporting information), which show the wide region survey scanning from 0 to 1200 eV, indicating Pt successfully supported on PtNP/rGO-MWCNT, PtNP/MWCNT, and PtNP/rGO. Fig. 2b and Fig. S4b (Supporting information) show the C 1s region of PtNP/rGO-MWCNT and GO, the binding energies (BEs) at 284.6 eV, 286.6 eV and 288 eV, and are assigned to C—C, epoxide and hydroxyl and carboxyl [39, 40], respectively. The ratio of C to O in as-prepared catalyst increases remarkably, and the peak areas of BEs related to epoxide, hydroxyl, and carboxyl functional groups decreased a lot compared to GO, suggesting that most of the oxygen functional groups disappeared during the reduction of GO [41, 42]. Notably, in Fig. 2b, the π-π* shake-up satellite peak, a characteristic of aromatic or conjugated systems, is available at 289.2 eV and the sharp C—C peak proves that the sp2 carbon network is restored in graphene of PtNP/rGO-MWCNT [40, 41]. Fig. 2c and Fig. S4c (Supporting information) show the O 1s region of PtNP/rGO-MWCNT and GO. The O 1s BEs of C=O groups and C—OH groups for PtNP/rGO-MWCNT is slightly shifted (531.2 eV and 533 eV) with respect to that for GO (531.4 eV and 532.5 eV), due to the influence of temperatures during the preparation of rGO [43, 44]. The arising of O=C—OH groups at 534.4 eV is corresponding to the π-π* satellite peak in the C 1s region. The photoelectron kinetic energies of O 1s are lower than those of C 1s and the O 1s sampling depth is smaller, thus the O 1s spectra are slightly more surface specific [44]. The XPS spectra of Pt 4f summarized in Fig. 2d and Fig. S4d (Supporting information) exhibit two main peaks at around 71.41 eV (Pt 4f7/2) and 74.74 eV (Pt 4f5/2), implying metallic Pt successfully formed on PtNP/rGOMWCNT, PtNP/MWCNT and PtNP/rGO, facilitating efficient HER performance [30, 45, 46].
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| Fig. 2. XPS spectra of PtNP/rGO-MWCNT (a) wide region, (b) C 1s, (c) O 1s, and (d) Pt 4f regions. | |
Raman spectroscopy has been proved as an essential tool to characterize carbon-based materials and is strongly dependent upon electronic structures. Usually, carbon-based materials are characterized by two main features, the G E2g associated with the first order scattering of the E2g phonon of sp2 carbon (1570-1595 cm-1) and the D E2g associated with the breathing E2g k-point photons of A2g symmetry (1320-1334 cm-1) are related to disordered conformations and defects of the sp2 carbon lattice of graphene. Thus, the ratio of the intensity of D and G bands (ID/IG) could be a measurement of the relative disorder present in graphitic structures [47-49].
Fig. 3a shows the Raman spectra of pure GO, rGO, pure MWCNT, rGO-MWCNT, PtNP/rGO, PtNP/MWCNT, PtNP/rGO-MWCNT. The ratios of ID/IG are 1.73, 1.55, 0.99, 2.14, 1.24, 0.99 and 1.15, respectively. In comparison to pure GO, the ID/IG ratio of rGO decreased, indicating the removal of oxygen moieties and restoration of the sp2 network during the reduction process, in consistent with the XPS results. It is interesting that the ID/IG ratios of Pt-containing materials are obviously lower than that of non-Pt materials, suggesting that Pt can facilitate the reduction process.
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| Fig. 3. (a) Raman spectra of pure GO, rGO, pure MWCNT, rGO-MWCNT, PtNP/rGO, PtNP/MWCNT and PtNP/rGO-MWCNT. (b) FT-IR spectra of pure GO and PtNP/rGOMWCNT. | |
Fig. 3b shows the Fourier transform infrared spectroscopy (FT-IR) spectra of pure GO and PtNP/rGO-MWCNT. The broad peak at around 3442 cm-1 and a sharp peak at 1396 cm-1 are related to the stretching vibrations and deformation vibration of O—H on both spectra, respectively. The peaks at around 1720 cm-1, 1633 cm-1, 1230 cm-1 and 1041 cm-1 are assigned to the C=O stretching vibration, remaining sp2 skeletal vibration of C=C, C—OH stretching vibrations and C—O stretching vibration, respectively. However, all these characteristic peaks are weakened and even vanished completely for PtNP/rGO-MWCNT. While, the two peaks at 2922 cm-1 and 2853 cm-1 assigned to —CH2 and —CH stretching vibrations are stronger. This may be the absorption of AA on the surface of graphene [47-52]. All these characterization results confirm the removal of oxygen functionalities from the surface of GO as well as the formation of atomic frame of sp2 carbon in PtNP/rGO-MWCNT, thus proving the successful preparation of PtNP/rGO-MWCNT.
The electrocatalytic performance of PtNP/rGO-MWCNT (with a Pt content of 3.48 wt%) on HER was directed with a three-electrode system in 0.5 mol/L H2SO4 solution at a scan rate (v) of 2 mV/s. PtNP/rGO, PtNP/MWCNT, rGO-MWCNT and commercial Pt/C were also applied for comparison. The ohmic potential drop (iR) losses from electrolyte resistance were utilized to initial data [53]. Fig. 4a and Fig. S5a (Supporting information) show the linear sweep voltammetry (LSV) curves for catalysts with and without iR correction. The rGO-MWCNT had not any competitive superiority towards HER with a current density of 10 mA/cm2 at an overpotential of even 197 mV after iR correction, which was 278 mV initially. PtNP/rGO, PtNP/MWCNT and commercial Pt/C exhibited a relatively lower overpotential of 38 mV, 7 mV and 3 mV (135 mV, 40 mV and 11 mV before iR correction) with the same current density. PtNP/rGO-MWCNT shows a superior catalytic activity towards HER, achieving a minimum overpotential of 11 mV (26 mV before iR correction) to drive a current density of 10 mA/cm2 (Table S1 in Supporting information).
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| Fig. 4. (a) Polarization curves of PtNP/rGO-MWCNT, Pt/C, PtNP/MWCNT, PtNP/rGO and rGO-MWCNT in 0.5 mol/L H2SO4 at a scan rate of 2 mV/s with iR correction. (b) Current densities at -0.05 V vs. RHE with iR correction. (c) Tafel plots of PtNP/rGO-MWCNT, Pt/C, PtNP/MWCNT, PtNP/rGO and rGO-MWCNT. (d) The multi-step chronopotentiometric curve of PtNP/rGO-MWCNT measured at the current densities between 20 mA/cm2 and 110 mA/cm2 with an increment of 10 mA/cm2 per 500 s. (e) Polarization curves of PtNP/rGO-MWCNTobtained before and after 1000-cycle CV in 0.5 mol/L H2SO4 with iR correction. (f) Time-dependent current density curve for PtNP/rGO-MWCNT at -45 mV (vs. RHE) in 0.5 mol/L H2SO4. | |
Fig. 4b shows the comparison of current densities obtained with different electro-catalysts at the same overpotential of 50 mV. The current densities for rGO-MWCNT, PtNP/rGO, PtNP/MWCNT and Pt/C were 1.28, 12.72, 23.86 and 37.3 mA/cm2, respectively. In sharp contrast, PtNP/rGO-MWCNT exhibits an excellent current density of 98.62 mA/cm2, which is ca. 8, 4 and 3 times of those obtained with PtNP/rGO, PtNP/MWCNT and Pt/C, respectively. Furthermore, at an overpotential of 100 mV without iR correction, there is still a superior current density on PtNP/rGO-MWCNT compare to those obtained with other catalysts, as shown in Fig. S5b (Supporting information). It needs to be noticed that PtNP/MWCNT and Pt/C may slowly fall off from the glassy carbon electrode because of the bubbling during the reaction, thus the destabilization of the LSV curves. Fig. S6 (Supporting information) shows the mass-normalized HER performances of PtNP/rGO-MWCNT, with the mass activity of 10 mA/μgPt at an overpotential of 20 mV. The electrochemical impedance spectroscopy (EIS) is further executed (Fig. S7 in Supporting information). The EIS data for PtNP/rGOMWCNT, Pt/C, PtNP/MWCNT, PtNP/rGO and rGO-MWCNT were 9.284, 7.838, 8.482, 39.12, 30.77 ohm, respectively, indicating a much smaller semicircle radius of PtNP/rGO-MWCNT than those of the PtNP/rGO and rGO-MWCNT, resulting in a higher chargetransfer rate and more rapid catalytic kinetics. Our homemade catalyst performs excellent catalytic activityon HER and better than most of Pt-based HER catalysts (Table S2 in Supporting information). The LSV curves of PtNP/rGO-MWCNT in 0.5 mol/L H2SO4 with different amount of H2PtCl6 (Fig. S8 in Supporting information) suggest that the addition of 1 mL H2PtCl6 is enough for HER, which is equal to a Pt loading of 3.48 wt%.
Fig. 4c shows the HER overpotential versus logarithm plots of current density on PtNP/rGO-MWCNT, PtNP/rGO, PtNP/MWCNT, rGO-MWCNT and Pt/C. All Tafel plots were fit to the equation η = a + blogj, where η is the HER overpotential, j is the corresponding current density, b is the Tafel slope and a is the Tafel constant. The Tafel slop on PtNP/rGO-MWCNT (28.6 mV/dec) is much smaller than that on PtNP/rGO (108.4 mV/dec), PtNP/MWCNT (109 mV/dec), rGO-MWCNT (239 mV/dec) and Pt/C (64.8 mV/dec), suggesting that the HER on PtNP/rGO-MWCNT went through a Volmer-Tafel mechanism pathway with the Tafel reaction as the rate-determining step (RDS) [54, 55], while the HER on the other catalysts suffered a typical Volmer-Heyrovsky pathway with the Volmer step as the RDS [56]. The smaller Tafel slope on PtNP/rGOMWCNT indicated the higher electrocatalytic activity on HER in 0.5 mol/L H2SO4. The multi-step chronopotentiometry was further applied to estimate the electrochemical performance of PtNP/rGOMWCNT during HER at current densities varying from 20 mA/cm2 to 110 mA/cm2 with anincrement of 10 mA/cm2 per 500 s (Fig. 4d). The potential of each step remains almost unchanged for 500 s, implying that the as-prepared catalyst retains high conductivity, good mechanical robustness and rapid mass transport. Long-term stability of electro-catalysts is also a critical character for practical applications. Cyclic voltammetry (CV) technology and chronoamperometry test (current-time curve) were conducted to explore PtNP/rGO-MWCNT. Continuous 1000 cycles' CV scanning between 0.20 V and -0.20 V (vs. RHE) at a v of 100 mV/s on PtNP/rGO-MWCNT results in almost no degradation in the catalytic performance on HER (Fig. 4e), suggesting the superior stable performance of PtNP/rGO-MWCNT. A 15 h long-term electrolysis was run on PtNP/rGO-MWCNT at the overpotential of 45 mV in 0.5 mol/L H2SO4, shown in Fig. 4f. The catalyst showed a stabilized current density with a slight decrease, in accordance with the superior stability. The SEM characterizations of PtNP/rGO-MWCNT subjected to the hydrolysis measurement proved that it still remained the nanoparticle nature and had a resistance to the size growth, further suggestingits superiorlong-term stability (Fig. S9 in Supporting information).
We further measured the double-layer capacitance (CDL) at the solid/liquid interfaces for rGO-MWCNT, PtNP/rGO and PtNP/rGO-MWCNT [57]. Figs. S10a–c (Supporting information) show the typical CV curves at various scan rates from 0.5 mV/s to 3 mV/s in the non-Faradaic capacitance current range from 0.291 V to 0.326 V (vs. RHE). CDL values were determined by the non-Faradaic capacitive current (ic), which was associated with double-layer charging from the v depending on CV curves. CDL was calculated by the equation ic = v × CDL, a plot of ic as a function of v yields a straight line with a slope equal to CDL as shown in Fig. S10d (Supporting information). CDL values for PtNP/rGO, rGOMWCNT and PtNP/rGO-MWCNT were 0.46, 0.51 and 2.6 m F/cm2, respectively, suggesting that PtNP/rGO-MWCNT has much higher surface roughness and thus exposes more active sites for HER.
The Faradaic efficiency (FE) was calculated by comparing the amount of experimentally quantified gas with the theoretically one calculated by the overall charge. Fig. S11 (Supporting information) shows the amount of produced H2 increased with electrolysis time in 0.5 mol/L H2SO4. The FE for HER was calculated to be approximately 93.3%.
In summary, we have demonstrated a new strategy to synthesize a flexible binder-free PtNP/rGO-MWCNT catalyst for efficient electrochemical HER. The as-prepared catalyst couldbe arbitrarily tailoredinto any shapes and directly supplied as an electrode without using any binders. This strategy not only avoided the serious resistance, but also effectively prevented the peeling off of active materials during electrolysis. Our catalyst exhibited superior activity and stability in comparison with other catalysts in acid, requiring an overpotential of only 11 mV to drive 10 mA/cm2. The synergistic effect of Pt, rGO and MWCNTmay contribute to the high HER activity of this material. To our best knowledge, in acidic medium, this new flexible binder-free PtNP/rGO-MWCNTcatalyst is one of the best Pt-based electro-catalysts for the HER, with an ultralow Pt loading. This work presents a new strategy for designing advanced binder-free catalysts and enables the widespread deployment of convenient and environmentally friendly methodology for the synthesis of 3D porous binder-free catalysts. This work is expected to be extended to construct other composite catalysts for high-performance batteries, catalysis, and supercapacitors.
Declaration of competing interestWe declare that there are no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interestof any nature orkind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
AcknowledgmentsThis work acknowledges the fund support from the National Natural Science Foundation of China (Nos. 21575092, 21622508) and the 111 project (No. B17030). The characterizations of XRD, XPS, SEM and TEM by Prof. Hui Wang, Dr. Shanling Wang and Dr. Yunfei Tian from the Analytical and Testing Center of Sichuan University are greatly appreciated.
Appendix A. Supplementary dataSupplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2019.11.014.
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