- Email: [email protected]
Electric field-assisted synthesis of Pt, carbon quantum dots-coloaded graphene hybrid for hydrogen evolution reaction
Journal of Power Sources 451 (2020) 227770
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electric field-assisted synthesis of Pt, carbon quantum dots-coloaded graphene hybrid for hydrogen evolution reaction He Xiao a, b, Jingjuan Zhang a, Man Zhao a, b, *, Jianchun Ma a, Ya Li a, Tianjun Hu a, Zhanfeng Zheng b, Jianfeng Jia a, b, **, Haishun Wu a a
Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Pt, CQDs-coloaded graphene was syn thesized with assistance of electric filed. � Obtained composite with ultrafine Pt shows much higher mass activity than Pt/C. � Voltage and electrolyte type were used to control synthesis of efficient catalyst. � Probable evolution process of Pt NPs and CQDs was revealed. � Synergetic effect between Pt NPs and CQDs contributes to the superior HER performance. A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon quantum dots Pt nanoparticles Graphene Hydrogen evolution reaction
A simple method for preparation of Pt, CQDs-coloaded graphene composite for hydrogen evolution reaction (HER) is established by electrolysis-solvothermal process. Graphene is obtained by electrochemical exfoliation of graphite in cathode, while carbon quantum dots (CQDs) and Pt nanoparticles (NPs) are in-situ generated in the electrolyte simultaneously. CQDs are evolved from propylene carbonate solvent and Pt NPs are derived from reduction of Pt intermediate species generated from anodic dissolution of Pt counter electrode during electrolysis process. Then Pt NPs and CQDs are well dispersed on graphene at subsequent solvothermal process. Work voltage and electrolyte type as the key electrolysis conditions are applied to control the concentration of CQDs and Pt NPs, which decides electrochemically active surface area and HER activity of obtained catalysts. Significantly, the obtained superior catalyst with trace amount of Pt (0.145 wt%), shows markedly enhanced catalytic HER activity with mass activity of 37.5 A mg 1 at 50 mV in acidic solution, 68.2 times more active than Pt/C. This study provides a universal and promising methodology for synthesizing Pt, CQDs coloaded hybrid with the assistance of electric field as cost-effective catalysts for fuel cells.
* Corresponding author. Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China. ** Corresponding author. Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China. E-mail addresses: [email protected] (M. Zhao), [email protected] (J. Jia). https://doi.org/10.1016/j.jpowsour.2020.227770 Received 30 November 2019; Received in revised form 10 January 2020; Accepted 16 January 2020 Available online 23 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
1. Introduction
detail. This work has an important guiding significance for facile syn thesis of Pt, CQDs-coloaded composite and other CQDs-based compos ites using electrosynthesis method.
Hydrogen production from electrocatalytic water splitting supports a promising way to replace fossil fuels in future [1,2]. Therefore devel oping efficient catalysts for hydrogen evolution reaction (HER) plays a key role in large-scale hydrogen production. Recently, noble metals and noble metal-based phosphides with high HER performance have been developed [3–9]. Among them, Pt-based materials have been proved as the most active HER catalysts [3–5], but their scarcity and high cost severely inhibit their large-scale application. Compared with traditional Pt-based catalyst (Pt/C), highly dispersed Pt (Pt single-atom or Pt nanoparticles (NPs)) loaded catalysts have great advantage with utmost utilization of Pt atoms, which can dramatically reduce the usage of noble metal Pt and decrease the cost of catalysts [3,10]. It has been shown that the highly dispersed Pt species anchored on the carrier surface such as carbon-based materials indeed show much higher catalytic performance and lower usage of expensive Pt [3,11–17]. Reduced graphene oxide (RGO), as a kind of carbon-based materials, is intensely researched as supporting materials for metal or metal oxide nanoparticles due to their distinctive properties such as high surface area, tunable surface functional groups and good conductivity [18]. Moreover, with unique structures and properties, RGO also can be introduced into various components of polymer electrolyte membrane fuel cells [19]. At present, RGO is mostly prepared from reduction of graphene oxide (GO) obtained by Hummer’s method [20]. This oxidation-reduction method produces graphene with abundant oxygen groups, which is partly benefit for anchoring nanoparticles but is detrimental to its conductivity due to irreversible structural defects [21]. In comparison, electrochemical exfoliation method, especially electrochemical cathodic exfoliation method, was applied to generate graphene with superior conductivity [22]. But the graphene prepared by electrochemical exfoliation method is more difficult to load and disperse nanoparticles than RGO because of its smooth surface. Therefore, con structing anchoring points for loading highly dispersed Pt NPs on pris tine graphene is essential to achieve high-performance electrocatalyst. Recently, Wang et al. [12] applied N-doped carbon quantum dots (NCQDs) to modify the properties of multiwall-carbon nanotube (MWCNT) for supporting Pt NPs. This Pt/NCQDs-MWCNT composite prepared by a facile hydrothermal treatment showed excellent perfor mance toward direct methanol fuel cells. The existence of NCQDs improved the dispersion and stability of Pt NPs on NCQDs-MWCNT. Moreover, Dong et al. [13] used carbon quantum dots (CQDs) as abundant nucleating and anchoring points to facilitate the formation of highly dispersed Pt NPs on RGO. They synthesized Pt-CQD/RGO by one pot reduction of CQD, GO and Pt precursor with ethylene glycol. Apart from the function of anchoring points, it’s found that CQDs as spacer also retard restacking of RGO sheets and support abundant oxygen groups to improve the antipoisonous ability of Pt. From above, it seems that CQDs could serve as anchoring point for loading highly dispersed Pt NPs on pristine graphene. As far as the synthesis of Pt, CQDs-coloaded RGO is concerned, co-reduction of Pt compound precursor such as H2PtCl6⋅6H2O and GO with existence of pre-preparation of CQDs is often applied. The preparation process is relatively time-consuming and complicated. In this work, highly efficient Pt, CQDs-coloaded graphene catalyst for HER was synthesized with an innovative and facile electrochemical method. During the first process, graphene was obtained by exfoliation of graphite in the cathode, while CQDs and Pt NPs were in-situ generated in the electrolyte simultaneously. Then both of the CQDs and Pt NPs were loaded on as-prepared graphene during the subsequent sol vothermal process. This catalyst showed superior HER performance in acidic solution with the mass activity of 37.5 A mg 1 at 50 mV, 68.2 times more active than that of Pt/C. It is known that electrolyte is affected by electrolytic conditions, thus, the influences of electrolytic conditions including work voltage and electrolyte type on the genera tion of CQDs and Pt NPs as well as HER activity were also revealed in
2. Experimental 2.1. Chemicals and materials Graphite powder (80 mesh) was purchased from Qingdao Graphite Company. Tetrabutylammonium tetrafluoroborate (TBABF4, 98%, powder), Tetrabutylammonium borohydrlde (TBABH4, 95%, powder), Tetrabutylammonium fluoridetrihydrate (TBAF, 98%, solid),Tetrabu tylammonium hydroxide (TBAOH, 98%, solid). Proprylene carbonate (PC, �99.9%) was purchased from aladdin. N, N-dimethylformamide (99.9%), and propylene carbonate (99.9%) was got from Aladdin, Anhydrous ethanol (99.5%) purchased from Woke Company. 2.2. Preparation of Pt-CQDs/graphene Graphite powder (40 mg) was put in a porous plastic tube with a platinum plate inserted as negative electrode, platinum wire served as positive electrode. Both electrodes were immersed in proprylene car bonate (PC) with tetrabutylammonium tetrafluoroborate (TBA BF4; 0.1 M) and a voltage of 30 V was applied for 12 h. After the electrochemical expansion step, the mixture was stirred for 12 h and transferred into 80 ml Teflon cup inserted in a stainless steel autoclave with an Ar flow (0.5 h) to completely remove air, then the solution was treated through solvothermal process at 160 � C under 500 round min 1 for 12 h. After that, the samples was filtered washing with PC solution for three times and ethanol for two time, then dried in a vacuum oven for 8 h at 60 � C. Finally, the samples were annealed at 400 � C to remove residue solvent in inert atmosphere for 60 min. The obtained sample was defined as PtCQDs/Gr-C400. Compared with Pt-CQDs/Gr-C400, Gr-C400 (graphite powder was first treated with electrochemical cathodic exfoliation and then calcination at 400 � C), Pt-CQDs/Graphite-C400 (graphite powder first underwent with solvothermal treatment in electrolyte solution after electrolysis and then calcination at 400 � C) and Pt-CQDs/Gr (graphite powder was first treated with electrochemical cathodic exfoliation and then solvothermal treatment at 160 � C) were prepared as show in the Table S1. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out on an X-ray diffractometer (Ultima IV-185) with Cu Ka radiation. Raman spectra were recorded on a JobinYvon LabRAM HR800 micro-Raman spec trometer using a laser excitation of 532 nm at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on Thermo Fisher SCIENTIFIC using Al Ka X-ray source. The morphologies of the samples were investigated by transmission electron microscopy (TEM, FEI Tec nai F20) and scanning electron microscopy (SEM, JSM-7001F). Fourier transform infrared (FTIR) spectra were recorded using a Bruker spec trometer. The height of the material was obtained by Atomic Force Microscope (Agilent 5500) instrument. The chemical composition (Pt) of the as-prepared samples was determined by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) method. 2.4. Electrochemical tests All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard three-electrode setup. A conventional three-electrode cell configuration was applied. Saturated Hg/Hg2Cl2 and a platinum plate were used as reference and counter electrode, respectively. A glassy carbon electrode (GCE, 3 mm in diameter) used as the support for the working electrode. Immobilization of sample was achieved by drop2
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
casting 5 μL sample (5 mg sample and 20 μL Nafion solution were dispersed in 1 ml water-ethanol solution with volume ratio of 3:1) onto GC working electrodes. 5 μL of the ink was loaded onto a rotating glassy carbon disk electrode by hand dropping. Linear sweep voltammetry (LSV) was conducted in the range of 100 to 500 mV (vs. RHE) at a sweep rate of 5 mV s 1 in 0.5 M H2SO4 solution. Prior to experiments, the solution was purged with pure H2 for at least 30 min. The over potentials were obtained at the reduction current density of 10 mA cm 2. Tafel plots were drawn from the overpotentials (η) as a function of the logarithm scales of the current density (logj). Tafel slopes (b) were obtained by fitting the linear portions of the Tafel plots according to Tafel equation (η ¼ a þ blogj). The stability measurements were per formed by cyclic voltammetry scanning 2000 cycles (CV, sweep rate, 50 mV s 1). CV method also was used to determine the electrochemical double-layer capacitances (Cdl). Electrochemically active surface area could be evaluated from the slope of the plot of the charging current versus the scan rate, which was directly proportional to Cdl. The elec trochemical impedance spectroscopy (EIS) was carried out over a fre quency range from 105 to 1 Hz with a 5 mV amplitude. Here, in the HER, SCE can be converted into the RHE by The following equation was applied ERHE ¼ ESCE þ0.059 pH þ EθSCE .Where ERHE is the converted potential versus RHE, ESCE is the experimental potential measured against the SCE, and EθSCE is the standard potential of SCE at 25 � C (246 mV). The electrochemical measurements were carried out in 0.5 M H2SO4 solution (pH ¼ 0) saturated with high purity H2 at room tem perature, thus ERHE (mV) ¼ ESCE (mV) þ 246 (mV).
aggregates with small particles on the surface. This result agrees with our previous work that electrochemical cathodic exfoliation is an effective way for exfoliating graphite into graphene [22,23]. TEM image (Fig. 2b) of Pt-CQDs/Gr-C400 reveals that small quantum dots or nanoparticles are uniformly decorated on the graphene sheets. The di ameters of these small quantum dots or nanoparticles mainly range from 1 nm to 5 nm (Fig. 2c). These dots with a light dark contrast show a lattice spacing of 0.21 nm (Fig. 2d and e), corresponding to the (100) crystal face of graphitic carbon, meaning the formation of CQDs [24]. While dots with strong dark contrast and hexagonal atomic lattice show a lattice spacing of 0.23 nm (Fig. 2d and e) [13], which are assigned to (111) crystal face of Pt [25]. Notably, many Pt NPs reside closely to CQDs. The HADDF-STEM image (Fig. 2f) for Pt-CQDs/Gr-C400 displays isolated bright dots with average diameter of about 4.1 nm (Fig. S2). These bright dots are attributable to heavy atoms on graphene, also confirming the successful anchoring of Pt NPs. The elemental mapping images reveal the homogeneous spatial distributions of platinum (Fig. 2h), carbon (Fig. 2i), oxygen (Fig. 2j) and fluorine (Fig. 2k). This result further indicates that Pt NPs generated over as-prepared gra phene. Just like Pt-CQDs/Gr-C400, Pt-CQDs/Graphite-C400 and Pt-CQDs/Gr both show formation of small quantum dots or nano particles on the graphene (Fig. S3). However, as for Gr-C400, no quan tum dots or nanoparticles are generated on the graphene. This result reveals the fact that the CQDs and Pt NPs evolved from the electrolyte solution, and combined with graphene at solvothermal process. To further characterize the chemical nature and bonding state of these prepared samples, XPS spectra for different samples were recorded [26–29]. From Fig. 3a, it can be seen that all the samples are mainly composed of C and O element, despite of some other trace amount ele ments like Pt. The Pt contents of these as-prepared catalysts were calculated and listed in Table S2. As observed, XPS data for Pt-CQDs/Gr-C400 shows that the surface Pt content is 0.393 at.%, indicating highly dispersion of enriched Pt NPs on Pt-CQDs/Gr-C400. The Pt loading in Pt-CQDs/Gr-C400, measured using ICP-OES, is 0.145 wt% (Table S2). To investigate clearly the valence state of Pt element, the high-resolution Pt 4f scan is also recorded [29]. As for Pt-CQDs/Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Graphite-C400 (Fig. 3b), their 4f7/2 and 4f5/2 peaks locate at around 71.7 and 75.0 eV, respec tively, which agrees with the values of Pt0 [30]. While for Gr-C400, which didn’t experience the same solvothermal process, the amount of Pt is below the detection limit. This result suggests that the anchoring process occurs in the solvothermal process and Pt comes from the electrolyte solution. XRD spectra (Fig. 3c) of all generated samples showed a strong peak at 2θ ¼ 26.5� corresponding to the (002) reflection peak of graphite (JCPDS No. 08-0415) [31]. Compared with graphite, these samples generated through electrochemical exfoliation like Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Gr-C400 exhibit a much more pronounced decrease of their (002) reflection peaks. This result indicates that the long-range periodicity along the c axis of graphite has been destroyed [31]. But for Pt-CQDs/Graphite-C400, the intensity of its (002) peak is
3. Results and discussion 3.1. Synthesis of Pt, CQDs-coloaded graphene composite Fig. 1a illustrates the synthesis procedure of Pt, CQDs-coloaded graphene composite (denoted as Pt-CQDs/Gr-C400) from graphite. Three consecutive processes are included: electrochemical exfoliation (or electrolysis), solvothermal treatment and calcination. Electro chemical exfoliation was conducted in a home-made setup with graphite powder as working cathode and platinum wire as anode. After exfolia tion, the expanded graphite and electrolyte were together transferred and subjected to solvothermal treatment, and then the obtained com posites were calcined at 400 � C. To investigate the function of every process, the corresponding compared samples: Gr-C400, Pt-CQDs/ Graphite-C400 and Pt-CQDs/Gr were also prepared as shown in Exper imental part and Supporting Information (Table S1). 3.2. Characterization of Pt, CQDs-coloaded graphene composite As seen from SEM image (Fig. 2a), Pt-CQDs/Gr-C400 mainly consists of thin, wrinkled tissue-like flakes. Compared samples such as Gr-C400 and Pt-CQDs/Gr (Fig. S1) prepared through the same electrochemical exfoliation process exhibit similar morphology with Pt-CQDs/Gr-C400, while the Pt-CQDs/Graphite-C400 sample (Fig. S1b) shows thick
Fig. 1. (a) Schematic of the preparation process for Pt-CQDs/Gr-C400. 3
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 2. (a) SEM images of the Pt-CQDs/Gr-C400. (b, c) Representative TEM micrographs of Pt-CQDs/Gr-C400. Scale bar is 500 nm in panel (b) and 20 nm in panel (c). Inset in panel (c) shows the nanoparticle core size histogram. (d, e) Representative HRTEM micrographs of Pt-CQDs/Gr-C400. Scale bar is 5 nm in panel (d) and 10 nm in panel (e). (f) HAADF-STEM image and (g-k) EDS maps of G-EX-ST. Scale bar is 100 nm in panel (f).
still much higher in comparison with other samples, indicating elec trochemical exfoliation process really can exfoliate bulky graphite into thin graphene nanosheets. However, no XRD reflections ascribed to Pt species can be found on these catalysts. This is mainly because of the low content and high dispersion of Pt species on graphene.
Raman spectra of different samples were shown in Fig. 3d. As for graphitic materials, three characteristic peaks at about 1350 cm 1, 1580 cm 1 and 2650 cm 1, assigned to D, G and 2D bands respectively, are often investigated. The D band is originated from edges, disorder or other defects, while G band is attributed to the zone center E2g mode 4
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 3. (a) XPS survey spectra, (b) Pt 4f XPS spectra, (c) XRD patterns and (d) Raman spectra of different samples.
corresponding to the ordered sp2 bonded carbon atom [32,33]. There fore, the ID/IG ratio (the intensities of the D (ID) band relative to the G (IG) band) allows to derive the density of Raman-sensitive defects, with a higher ID/IG ratio corresponding to a larger number of defects in the graphene lattice [34]. Compared with Gr-C400, samples like Pt-CQDs/Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Graphite-C400 which were prepared through solvothermal treatment exhibit higher ID/IG ra tios. This result suggests solvothermal process is benefit for producing defects. Among these samples, Pt-CQDs/Gr-C400 shows the most de fects, which would be beneficial for electrocatalysis of HER by providing more active sites.
electrode is almost overlaps with that using Pt as counter electrode (Fig. S4). This result further verified that it’s the preparation process decides the formation of Pt NPs over the graphene rather than the test process. Thus, the Pt used as counter electrode doesn’t affect the test results of as-prepared samples. As shown in Fig. 4b, the mass activities for Pt-CQDs/Gr-C400, PtCQDs/Gr, Pt-CQDs/Graphite-C400 and Pt/C at 50 mV are 37.5, 4.07, 7.09 and 0.55 A mg 1, respectively, in which Pt-CQDs/Gr-C400 shows the best mass HER activity, and 68.2 times more active than that of Pt/C. What’s more, as shown in Fig. 4c, Pt-CQDs/Gr-C400 shows a much smaller Tafel slope (40 mV⋅dec 1) than that of Pt-CQDs/Gr (64 mV⋅dec 1), Pt-CQDs/Graphite-C400 (83 mV⋅dec 1) and Gr-C400 (587 mV⋅dec 1), revealing more favorable electrocatalytic kinetics on the PtCQDs/Gr-C400 catalyst for HER. The electrochemical impedance spec troscopy (EIS) of all the sample electrodes were also measured in 0.5 M H2SO4 solution (Fig. 4d). The result shows Pt-CQDs/Gr-C400 exhibits extremely small charge transfer resistances and fast reaction rates compared with other reference samples. This small charge transfer resistance is originated from the intimate contraction between the generated CQDs and graphene flakes. In addition, the LSV curves remain almost unchanged before and after 2000 CV scanning (Fig. 4e), indi cating strong durability of the Pt-CQDs/Gr-C400 electrode. The composition and structure changes of Pt-CQDs/Gr-C400 catalyst after durability tests were characterized by HRTEM. As the nanoparticle core size histogram shows in the Fig. S5a, it’s seen that the diameters of these small quantum dots or nanoparticles still range from 2 nm to 6 nm after stability test undergoing 2000 cycles, almost remaining the initial size as seen in Fig. 2c. Obviously seen from the Fig. S5b and Fig. S5c, these dots with a light dark contrast show a lattice spacing of 0.21 nm corre sponding to the (100) crystal face of graphitic carbon, indicating the
3.3. Activity and stability of Pt, CQDs-coloaded graphene composite for HER The HER activities of all the samples were initially investigated in 0.5 M H2SO4 aqueous solution at room temperature. Fig. 4a shows the linear sweep voltammetry (LSV) curves of all samples with a scan rate of 5 mV s 1. As expected, these results clearly indicate that Pt-CQDs/GrC400 shows excellent HER catalytic activity. Its overpotential at the current density of 10 mA cm 2 is 38 mV, superior to that of Pt-CQDs/Gr (49.5 mV), Pt-CQDs/Graphite-C400 (59.2 mV) and Gr-C400 (242.1 mV) catalysts. To rationally compare the catalytic performance with Pt/C, their HER catalytic activity was normalized to the current/Pt loading mass (A mg 1). Notably, Pt-CQDs/Gr-C400 displays much higher HER catalytic activity than Pt-CQDs/Gr, Pt-CQDs/Graphite-C400 and com mercial Pt/C catalysts. In order to exclude the introduction of Pt from Pt counter electrode, the Pt counter electrode was replaced with a graphite rob to apply for testing the HER performance of Pt-CQDs/Gr-C400. The LSV curve of the Pt-CQDs/Gr-C400 catalyst with graphite rob as counter 5
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 4. (a) LSV (area activity), (b) Mass activity value, (c) Tafel and (d) EIS curves of Pt-CQDs/Gr-C400 and the compared samples for the HER in 0.5 M H2SO4 with a scan rate of 5 mV 1 at room temperature, respectively. (e) LSV curves of the Pt-CQDs/Gr-C400catalyst before and after 2000 CV cycles in the stability test in 0.5 M H2SO4 for the HER. (f) The electrochemically active surface area (ECSA) of Pt-CQDs/Gr-C400 and the compared samples.
formation of CQDs. While dots with strong dark contrast and hexagonal atomic lattice show a lattice spacing of 0.23 nm, which are assigned to (111) crystal face of Pt. Above result inverifies that the obtained catalyst well keeps the original structure after durability test. The Faradaic ef ficiency for HER over Pt-CQDs/Gr-C400 was also investigated. The generated hydrogen in the cathode compartment of the H-type elec trolytic cell was measured using gas chromatograph (Fig. S6 and Fig. S7). The result suggests that the amount of H2 experimentally measured is almost consistent with the theoretical value, suggesting nearly 100% Faradaic yields for Pt-CQDs/Gr-C400 (Fig. S8). Cyclic voltammetric method is employed to evaluate the electro chemically active surface area (ECSA) of Pt-CQDs/Gr-C400 and
compared catalysts. The electrochemical double-layer capacitances (EDLC) results (Fig. S9) determined by electrochemical double-layer capacitances (Cdl) suggest that Pt-CQDs/Gr-C400 possesses the largest active surface area among these generated samples (Fig. 4f), which is also beneficial for achieving better catalytic activity toward HER. By comparing Gr-C400, Pt-CQDs/Gr, Pt-CQDs/Graphite-C400 with Pt-CQDs/Gr-C400 respectively, we can conclude that these three pro cesses: electrochemical exfoliation, solvothermal treatment and calci nation all contributed to the excellent activity of the Pt-CQDs/Gr-C400 catalyst, and the solvothermal process contributed most according to the overpotential difference shown in Table S3. The previous characteriza tion results show the formation of CQDs and Pt NPs coloaded on the 6
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
graphene occurs in this solvothermal process. That’s to say, the deco ration of CQDs and Pt NPs over the graphene contributed to the excellent HER activity. Compared with Pt-CQDs/Graphite-C400, Pt-CQDs/GrC400 and Pt-CQDs/Gr both show higher HER activity due to the larger surface area of graphene than graphite, which is in favor of the disper sion of CQDs and Pt NPs, thus exposes more defect sites (active sites) as shown in Raman and higher ECSA. Due to the probable removing of residue solvent covering some active sites by means of the calcination process, Pt-CQDs/Gr-C400 shows the superior HER activity to Pt-CQDs/ Gr.
Gr-C400 prepared at 30V exhibits the smallest overpotential at the current density of 10 mA cm 2, outperformed those prepared at other voltage, which is attributed to its larger ECSA (Fig. 5a2) in comparison with other samples. Detailed observation suggest that working voltage firstly affects the current (Table S4) and further affects the electrolytic degree. With the increasement of working voltage, the color of elec trolyte solution turned from colorless into dark brown (Fig. 5a3), and the concentrations of CQDs (Fig. 5a4) and Pt NPs (Fig. 5a5) in the solution increased. Based on the relationship between overpotential and ECSA, concentrations of CQDs, concentrations of Pt NPs and loading amount of Pt (Fig. 5a5), it’s concluded that the loading of the Pt NPs and CQDs over the obtained graphene increased with the enhancement of working voltage. This further improved the ECSA of as-prepared Pt-CQDs/GrC400, which leads to superior its HER activity. Therefore, working voltage can be used to control the loading amount of Pt as well as the HER activity of obtained Pt-CQDs/Gr-C400. Just like working voltage, electrolyte type mainly affects the current (Table S5) and then affects the electrolytic degree. In order to reveal the influence of electrolyte type, TBAF (anion: F ), TBABH4 (anion: BH4 ) and TBAOH (anion: OH ) were applied to replace the TBABF4 (anion: BF4 ). As observed from Fig. 5b1, Pt-CQDs/Gr-C400 prepared using TBABF4 electrolyte shows smaller overpotential than those prepared using other electrolyte. That’s because this obtained catalyst prepared with TBABF4 electrolyte has superior ECSA to other samples (Fig. 5b2). Despite of the same working voltage (30 V), electrolytic degree was different in different electrolyte solutions. After the electrolysis, the TBABF4 electrolyte solution shows brownish black while other solutions exhibit colorless (Fig. 5b3). The concentration of CQDs (Fig. 5b4) and Pt NPs (Fig. 5b5) in the TBABF4 electrolyte solution are obviously higher than that in TBAF, TBABH4 and TBAOH electrolyte solution, which contributes to the high Pt loading of the sample and excellent HER ac tivity (Fig. 5b4). Furthermore, it’s found that participation of fluorinecontaining electrolyte (TBABF4 and TBAF electrolyte) probably facili tate the loading of Pt on graphene during the solvothermal in compar ison with fluorine-free electrolyte (TBABH4 and TBAOH electrolyte). That’s probable that fluorine-containing electrolyte support a part of fluorine source for doping graphene, which acts as anchoring points to promote the formation and dispersion of Pt NPs like CQDs. It’s seen that the electrolyte type also can be used to adjust the loading amount of Pt as well as the HER activity of obtained Pt-CQDs/Gr-C400. The result shows the improvement of catalyst is not only due to the moderate amount of Pt but also the highly dispersion of Pt, CQDs and the large ECSA of the catalyst.
3.4. Probable formation process of CQDs and Pt NPs From above discussion, it’s deduced that the loading of CQDs and Pt NPs to graphene occurred in solvothermal process, but their formation occurred during or before solvothermal process is still unclear. To reveal the fact, the solution before and after solvothermal process was analyzed detailedly. TEM and HRTEM results of two solutions (Figs. S10–S13) both show the existence of CQDs and Pt NPs, indicating that they were generated in electrolysis process (before solvothermal process). More over, the solution (1 m mol Tetrabutylammonium tetrafluoroborate (TBABF4) in propylene carbonate (PC) solvent) before solvothermal process exhibits obvious fluorescence with a strong absorption peak in the range of 250–300 nm in the UV region, suggesting that CQDs have generated after electrolysis (Fig. S14) [35,36]. In addition, to verify the existence of CQDs, we performed the comparison experiment with no graphene matrix. That is, graphite electrode is replaced with Pt wire during the electrochemical exfoliation process. The obtained electrolyte solution after electrochemical exfoliation process was further subjected to solvothermal treatment. Then the mixture was kept still overnight. The sediment was collected and characterized with Raman spectra, XRD and HRTEM. The Raman spectra of obtained sample suggests typical carbon-based materials with high ID/IG ratio (ID/IG > 1), meaning the formation of CQDs, as shown in Fig. S15. Furthermore, the characteristic peak at 26.5� in XRD spectrum (Fig. S16) and the characteristic lattice spacing in HRTEM (Fig. S17) also confirmed the formation of CQDs. The concentration of Pt in the solution before solvothermal process was also tested with neglectable concentration of 172 mg L 1, suggesting that Pt NPs formed during the electrolysis process like CQDs. These above re sults all indicate that CQDs and Pt NPs are generated in electrolysis process (before solvothermal process). According to previous reports and the results reported herein, it can be concluded that the CQDs could be obtained directly from PC solvent as previously reported synthesis of CQDs from alcohol. It’s reported that the PC solvent can be converted into alcohol by a series of reactions [37, 38], and the alcohol can be converted in CQDs with power through electrochemical carbonization [39,40]. Thus here we envision that the formation mechanism of CQDs is as follows (Fig. S18): (1) transition of propylene carbonate into alcohol with power; (2) the decomposition of alcohol to carbon substances; (3) the polymerization and growth of carbon substances resulted in CQDs. Simultaneously, it’s also deduced that anodic dissolution of Pt from the counter electrode occurs in the electrolysis process and the dissolved Pt species (Pt2þ or PtOx) then immediately were reduced into Pt atom with the help of electrolyte solution (Fig. S19) [11,41–47]. Then the Pt NPs are attached to gra phene through solvothermal process.
3.6. The structure-activity relationship of Pt, CQDs-coloaded graphene composite Here, probable reaction mechanism for HER reaction over Pt-CQDs/ Gr-C400 was proposed as shown in Scheme 1. The highly dispersion for trace amount of Pt NPs anchored on graphene contributes most to the excellent catalytic performance. As for CQDs, they not only provide abundant nucleating and anchoring points to promote the formation and dispersion of Pt NPs, but also expose high specific surface area which produces abundant sharp edge sites (active sites) for HER [48]. More over, CQDs combine with graphene via π-π bonding, which constructs a 3D conductive network and shows fast charge transfer rate, leading to better electrochemical performance [34,49–53]. Therefore, the forma tion of CQDs and Pt NPs on graphene will tremendously increase the activity of the catalyst. Besides, the formation of CQDs and Pt NPs is with the help of electrolysis. Thus the electrolysis conditions including work voltage and electrolyte type, have directly great influence on the for mation of CQDs and Pt NPs, which in turn contributes to HER activity of the catalyst.
3.5. Influence of electrolysis conditions on formation of CQDs and Pt NPs as well as HER activity As observed from above, the CQDs and Pt NPs are generated during electrolysis process. It would make sense that the electrolysis conditions such as working voltage and electrolyte type should have great influence on their formation and catalytic activity. The impact of working voltage was firstly investigated. As seen from Fig. 5a1, the HER activity of these catalysts is positively related to their applied working voltage. Pt-CQDs/ 7
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 5. LSV curves (a1), the ECSA (a2), photographs (a3), UV–Vis absorbance spectra (a4) and relationship of HER activity with content of CQDs and Pt (a5) for the PtCQDs/Gr-C400 catalysts prepared at different voltages. LSV curves (b1), the ECSA (b2), photographs (b3), UV–Vis absorbance spectra (b4) and relationship of HER activity with content of CQDs and Pt (b5) for the Pt-CQDs/Gr-C400 catalysts prepared with different electrolytes.
8
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770 [2] T.E. Mallouk, Water electrolysis: divide and conquer, Nat. Chem. 5 (2013) 362–363, https://doi.org/10.1038/nchem.1634. [3] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B. Xiao, R. Li, T. K. Sham, L.M. Liu, G.A. Botton, X. Sun, Platinum single-atom and cluster catalysis of the hydrogen evolution reaction, Nat. Commun. 7 (2016) 1–9, https://doi.org/ 10.1038/ncomms13638. [4] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N.M. Markovic, Enhancing hydrogen evolution activity in water splitting by tailoring Liþ-Ni(OH)2-Pt interfaces, Science 334 (2011) 1256–1260, https://doi.org/10.1126/science.1211934. [5] H. Yin, S. Zhao, K. Zhao, A. Muqsit, H. Tang, L. Chang, H. Zhao, Y. Gao, Z. Tang, Ultrathin platinum nanowires grown on single-layered nickel hydroxide with high hydrogen evolution activity, Nat. Commun. 6 (2015) 6430, https://doi.org/ 10.1038/ncomms7430. [6] S. Yuan, Z. Pu, H. Zhou, J. Yu, I.S. Amiinu, J. Zhu, Q. Liang, J. Yang, D. He, Z. Hu, G. Van Tendeloo, S. Mu, A universal synthesis strategy for single atom dispersed cobalt/metal clusters heterostructure boosting hydrogen evolution catalysis at all pH values, Nano Energy 59 (2019) 472–480, https://doi.org/10.1016/j. nanoen.2019.02.062. [7] W. Li, Z.Y. Hu, Z. Zhang, P. Wei, J. Zhang, Z. Pu, J. Zhu, D. He, S. Mu, G. Van Tendeloo, Nano-single crystal coalesced PtCu nanospheres as robust bifunctional catalyst for hydrogen evolution and oxygen reduction reactions, J. Catal. 375 (2019) 164–170, https://doi.org/10.1016/j.jcat.2019.05.031. [8] Z. Pu, I.S. Amiinu, Z. Kou, W. Li, S. Mu, RuP 2 -based catalysts with platinum-like activity and higher durability for the hydrogen evolution reaction at all pH values, Angew. Chem. Int. Ed. 56 (2017) 11559–11564, https://doi.org/10.1002/ anie.201704911. [9] Z. Pu, J. Zhao, I.S. Amiinu, W. Li, M. Wang, D. He, S. Mu, A universal synthesis strategy for P-rich noble metal diphosphide-based electrocatalysts for the hydrogen evolution reaction, Energy Environ. Sci. 12 (2019) 952–957, https://doi.org/ 10.1039/c9ee00197b. [10] J. Wang, F. Ma, M. Sun, Graphene, hexagonal boron nitride, and their heterostructures: properties and applications, ACS Catal. 7 (2017) 16801–16822, https://doi.org/10.1039/C7RA00260B. [11] M. Tavakkoli, N. Holmberg, R. Kronberg, H. Jiang, J. Sainio, E.I. Kauppinen, T. Kallio, K. Laasonen, Electrochemical activation of single-walled carbon nanotubes with pseudo-atomic-scale platinum for the hydrogen evolution reaction, ACS Catal. 7 (2017) 3121–3130, https://doi.org/10.1021/acscatal.7b00199. [12] J.J. Zhang, Z.B. Wang, C. Li, L. Zhao, J. Liu, L.M. Zhang, D.M. Gu, Multiwall-carbon nanotube modified by N-doped carbon quantum dots as Pt catalyst support for methanol electrooxidation, J. Power Sources 289 (2015) 63–70, https://doi.org/ 10.1016/j.jpowsour.2015.04.150. [13] T.-Z. Hong, Q. Xue, Z.-Y. Yang, Y.-P. Dong, Great-enhanced performance of Pt nanoparticles by the unique carbon quantum dot/reduced graphene oxide hybrid supports towards methanol electrochemical oxidation, J. Power Sources 303 (2016) 109–117, https://doi.org/10.1016/j.jpowsour.2015.10.092. [14] Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (2011) 776–780, https://doi.org/10.1002/ adma.201003819. [15] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, The graphene-supported Pd and Pt catalysts for highly active oxygen reduction reaction in an alkaline condition, Electrochem. Commun. 13 (2011) 182–185, https://doi.org/10.1016/j. elecom.2010.12.008. [16] K. Zhang, Q. Yue, G. Chen, Y. Zhai, L. Wang, H. Wang, J. Zhao, J. Liu, J. Jia, H. Li, Effects of acid treatment of Pt-Ni Alloy nanoparticles@graphene on the kinetics of the oxygen reduction reaction in acidic and alkaline solutions, J. Phys. Chem. C 115 (2011) 379–389, https://doi.org/10.1021/jp108305v. [17] Y. Li, Y. Li, E. Zhu, T. McLouth, C.Y. Chiu, X. Huang, Y. Huang, Stabilization of high-performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene oxide/carbon black composite, J. Am. Chem. Soc. 134 (2012) 12326–12329, https://doi.org/10.1021/ja3031449. [18] L.G. Guex, B. Sacchi, K.F. Peuvot, R.L. Andersson, A.M. Pourrahimi, V. Str€ om, S. Farris, R.T. Olsson, Experimental review: chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO) by aqueous chemistry, Nanoscale 9 (2017) 9562–9571, https://doi.org/10.1039/c7nr02943h. [19] D. He, H. Tang, Z. Kou, M. Pan, X. Sun, J. Zhang, S. Mu, Engineered graphene materials: synthesis and applications for polymer electrolyte membrane fuel cells, Adv. Mater. 29 (2017) 1–8, https://doi.org/10.1002/adma.201601741. [20] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene oxide, Carbon 64 (2013) 225–229, https://doi.org/10.1016/ j.carbon.2013.07.055. [21] F. Kim, L.J. Cote, J. Huang, Graphene oxide: surface activity and two-dimensional assembly, Adv. Mater. 22 (2010) 1954–1958, https://doi.org/10.1002/ adma.200903932. [22] M. Zhao, X.Y. Guo, O. Ambacher, C.E. Nebel, R. Hoffmann, Electrochemical generation of hydrogenated graphene flakes, Carbon 83 (2015) 128–135, https:// doi.org/10.1016/j.carbon.2014.11.033. [23] M. Zhao, X. Ma, H. Xiao, Electrochemistry Communications Regulation of the degree of hydrogenation and electrochemical properties of graphene generated by electrochemical cathodic exfoliation by using different solvents, Electrochem. Commun. 103 (2019) 77–82, https://doi.org/10.1016/j.elecom.2019.05.013. [24] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem. Soc. Rev. 44 (2015) 362–381, https://doi.org/10.1039/c4cs00269e.
Scheme 1. Schematic illustration of the proposed reaction mechanism for HER reaction over Pt-CQDs/Gr-C400.
4. Conclusion In conclusion, a facile electrolysis-solvothermal method was estab lished for the preparation of Pt and CQDs coloaded graphene catalyst for HER with the assistant of electric field. The mass activity for as-prepared superior catalyst (Pt-CQDs/Gr-C400) at 50 mV is 37.5 A mg 1, 68.2 times more active than that of Pt/C. The Pt loading of this obtained catalyst is only 0.145 wt%, which greatly reduces the usage of expensive Pt. Such high catalytic activity can be attributed to large electrochemi cally active surface area of the catalyst resulted from highly dispersed Pt NPs and defect-rich CQDs. Meanwhile CQDs combine with graphene via π-π bonding, which constructs 3D conductive network and boosts the HER performance. The electrolysis conditions including work voltage and electrolyte type can be used to adjust the formation and loading amount of Pt NPs and CQDs, which in turn contributes to electro chemically active surface area and HER activity of the catalyst. The present work provides new insights into understanding of HER activity of Pt, CQDs coloaded graphene and offers a facile approach for preparing Pt, CQDs coloaded multi-dimensional electrocatalysts. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported by the National Natural Science Foun dation of China (21571119), the Program for New Century Excellent Talents in University (NCET-12-1035), the Applied Basic Research Project of Shanxi Province (No. 201901D211393, 201901D211398), the Scientific and Technological Innovation Programs of Higher Education Institution in Shanxi (2019L0466), the Foundation of State Key Labo ratory of Coal Conversion (J19-20-605), Shanxi Normal University School Fund (ZR1707) and 1331 Engineering of Shanxi Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227770. References [1] X.P. Yin, H.J. Wang, S.F. Tang, X.L. Lu, M. Shu, R. Si, T.B. Lu, Engineering the coordination environment of single-atom platinum anchored on graphdiyne for optimizing electrocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 9382–9386, https://doi.org/10.1002/anie.201804817.
9
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
[25] Z.Y. Zhou, X. Kang, Y. Song, S. Chen, Enhancement of the electrocatalytic activity of Pt nanoparticles in oxygen reduction by chlorophenyl functionalization, Chem. Commun. 48 (2012) 3391, https://doi.org/10.1039/c2cc17945h. [26] A.M. Motin, T. Haunold, A.V. Bukhtiyarov, A. Bera, C. Rameshan, G. Rupprechter, Surface science approach to Pt/carbon model catalysts: XPS, STM and microreactor studies, Appl. Surf. Sci. 440 (2018) 680–687, https://doi.org/10.1016/j. apsusc.2018.01.148. [27] M.Y. Smirnov, A.V. Kalinkin, E.I. Vovk, P.A. Simonov, E.Y. Gerasimov, A. M. Sorokin, V.I. Bukhtiyarov, Comparative XPS study of interaction of model and real Pt/C catalysts with NO 2, Appl. Surf. Sci. 428 (2018) 972–976, https://doi. org/10.1016/j.apsusc.2017.09.205. [28] R. Vakili, E.K. Gibson, S. Chansai, S. Xu, N. Al-Janabi, P.P. Wells, C. Hardacre, A. Walton, X. Fan, Understanding the CO oxidation on Pt nanoparticles supported on MOFs by operando XPS, ChemCatChem 10 (2018) 4238–4242, https://doi.org/ 10.1002/cctc.201801067. [29] G. Wang, Z. Yang, Y. Du, Y. Yang, Programmable exposure of Pt active facets for efficient oxygen reduction, Angew. Chem. 131 (2019) 1–9, https://doi.org/ 10.1002/ange.201907322. [30] G.V. C�eline Dablemont, P. Lang, C. Mangeney, J.-Y. Piquemal, V. Petkov, F. Herbst, FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina, Langmuir 48 (2012) 3391, https://doi.org/10.1039/c2cc17945h. [31] M. Zhao, J. Zhang, H. Xiao, T. Hu, J. Jia, H. Wu, Facile in situ synthesis of a carbon quantum dot/graphene heterostructure as an efficient metal-free electrocatalyst for overall water splitting, Chem. Commun. 55 (2019) 1635–1638, https://doi.org/ 10.1039/C8CC09368G. [32] J. Shen, M. Shi, N. Li, B. Yan, H. Ma, Y. Hu, M. Ye, Facile synthesis and application of Ag-chemically converted graphene nanocomposite, Nano Res. 3 (2010) 339–349, https://doi.org/10.1007/s12274-010-1037-x. [33] J. Shen, Y. Hu, M. Shi, X. Lu, C. Qin, C. Li, M. Ye, Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets, Chem. Mater. 21 (2009) 3514–3520, https://doi.org/10.1021/cm901247t. [34] R.N. Singh, R. Awasthi, Graphene support for enhanced electrocatalytic activity of Pd for alcohol oxidation, Catal. Sci. Technol. 1 (2011) 778–783, https://doi.org/ 10.1039/c1cy00021g. [35] M. Liu, Y. Xu, F. Niu, J.J. Gooding, J. Liu, Carbon quantum dots directly generated from electrochemical oxidation of graphite electrodes in alkaline alcohols and the applications for specific ferric ion detection and cell imaging, Analyst 141 (2016) 2657–2664, https://doi.org/10.1039/c5an02231b. [36] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem. Soc. Rev. 44 (2015) 362–381, https://doi.org/10.1039/c4cs00269e. [37] E.Y. Tyunina, M.D. Chekunova, V.N. Afanasiev, Electrochemical characteristics of propylene carbonate solutions of tetraethylammonium tetrafluoroborate, Russ. J. Electrochem. 49 (2013) 453–457, https://doi.org/10.1134/s1023193513050157. [38] H. Batterien, Anodic stability of propylene carbonate electrolytes at potentials above 4 V against lithium: an on-line MS and in situ FTIR study 21 (1991) 885–894. [39] H. Li, H. Ming, Y. Liu, H. Yu, X. He, H. Huang, K. Pan, Z. Kang, S.T. Lee, Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive
[40] [41]
[42] [43] [44]
[45]
[46]
[47] [48]
[49] [50] [51] [52] [53]
10
photoluminescence properties, New J. Chem. 35 (2011) 2666–2670, https://doi. org/10.1039/c1nj20575g. J. Deng, Q. Lu, N. Mi, H. Li, M. Liu, M. Xu, L. Tan, Q. Xie, Y. Zhang, S. Yao, Electrochemical synthesis of carbon nanodots directly from alcohols, Chem. Eur J. 20 (2014) 4993–4999, https://doi.org/10.1002/chem.201304869. S. Cherevko, A.A. Topalov, A.R. Zeradjanin, G.P. Keeley, K.J.J. Mayrhofer, Temperature-dependent dissolution of polycrystalline platinum in sulfuric acid electrolyte, Electrocatalysis 5 (2014) 235–240, https://doi.org/10.1007/s12678014-0187-0. S. Cherevko, A.R. Zeradjanin, G.P. Keeley, K.J.J. Mayrhofer, A comparative study on gold and platinum dissolution in acidic and alkaline media, J. Electrochem. Soc. 161 (2014) H822–H830, https://doi.org/10.1149/2.0881412jes. S. Cherevko, A.R. Zeradjanin, A.A. Topalov, N. Kulyk, I. Katsounaros, K.J. J. Mayrhofer, Dissolution of noble metals during oxygen evolution in acidic media, ChemCatChem 6 (2014) 2219–2223, https://doi.org/10.1002/cctc.201402194. D.C. Johnson, D.T. Napp, S. Bruckenstein, A ring-disk electrode study of the current/potential behaviour of platinum in 1.0 M sulphuric and 0.1 M perchloric acids, Electrochim, Acta 15 (1970) 1493–1509, https://doi.org/10.1016/00134686(70)80070-6. D.A.J. Rand, R. Woods, A study of the dissolution of platinum, palladium, rhodium and gold electrodes in 1 m sulphuric acid by cyclic voltammetry, J. Electroanal. Chem. Interfacial Electrochem. 35 (1972) 209–218, https://doi.org/10.1016/ S0022-0728(72)80308-5. A.A. Topalov, S. Cherevko, A.R. Zeradjanin, J.C. Meier, I. Katsounaros, K.J. J. Mayrhofer, Towards a comprehensive understanding of platinum dissolution in acidic media, Chem. Sci. 5 (2014) 631–638, https://doi.org/10.1039/ C3SC52411F. L. Xing, M.A. Hossain, M. Tian, D. Beauchemin, K.T. Adjemian, G. Jerkiewicz, Platinum electro-dissolution in acidic media upon potential cycling, Electrocatalysis 5 (2014) 96–112, https://doi.org/10.1007/s12678-013-0167-9. H. Wang, X.B. Li, L. Gao, H.L. Wu, J. Yang, L. Cai, T.B. Ma, C.H. Tung, L.Z. Wu, G. Yu, Three-dimensional graphene networks with abundant sharp edge sites for efficient electrocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 192–197, https://doi.org/10.1002/anie.201709901. Y. Wang, A. Hu, Carbon quantum dots: synthesis, properties and applications, J. Mater. Chem. C. 2 (2014) 6921–6939, https://doi.org/10.1039/c4tc00988f. X. Wang, G. Sun, P. Routh, D.H. Kim, W. Huang, P. Chen, Heteroatom-doped graphene materials: syntheses, properties and applications, Chem. Soc. Rev. 43 (2014) 7067–7098, https://doi.org/10.1039/c4cs00141a. T. Bao, L. Song, S. Zhang, Synthesis of carbon quantum dot-doped NiCoP and enhanced electrocatalytic hydrogen evolution ability and mechanism, Chem. Eng. J. 351 (2018) 189–194, https://doi.org/10.1016/j.cej.2018.06.080. C.S. Lim, K. Hola, A. Ambrosi, R. Zboril, M. Pumera, Graphene and carbon quantum dots electrochemistry, Electrochem. Commun. 52 (2015) 75–79, https:// doi.org/10.1016/j.elecom.2015.01.023. R. Wang, K.Q. Lu, F. Zhang, Z.R. Tang, Y.J. Xu, 3D carbon quantum dots/graphene aerogel as a metal-free catalyst for enhanced photosensitization efficiency, Appl. Catal. B Environ. 233 (2018) 11–18, https://doi.org/10.1016/j. apcatb.2018.03.108.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electric field-assisted synthesis of Pt, carbon quantum dots-coloaded graphene hybrid for hydrogen evolution reaction He Xiao a, b, Jingjuan Zhang a, Man Zhao a, b, *, Jianchun Ma a, Ya Li a, Tianjun Hu a, Zhanfeng Zheng b, Jianfeng Jia a, b, **, Haishun Wu a a
Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Pt, CQDs-coloaded graphene was syn thesized with assistance of electric filed. � Obtained composite with ultrafine Pt shows much higher mass activity than Pt/C. � Voltage and electrolyte type were used to control synthesis of efficient catalyst. � Probable evolution process of Pt NPs and CQDs was revealed. � Synergetic effect between Pt NPs and CQDs contributes to the superior HER performance. A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon quantum dots Pt nanoparticles Graphene Hydrogen evolution reaction
A simple method for preparation of Pt, CQDs-coloaded graphene composite for hydrogen evolution reaction (HER) is established by electrolysis-solvothermal process. Graphene is obtained by electrochemical exfoliation of graphite in cathode, while carbon quantum dots (CQDs) and Pt nanoparticles (NPs) are in-situ generated in the electrolyte simultaneously. CQDs are evolved from propylene carbonate solvent and Pt NPs are derived from reduction of Pt intermediate species generated from anodic dissolution of Pt counter electrode during electrolysis process. Then Pt NPs and CQDs are well dispersed on graphene at subsequent solvothermal process. Work voltage and electrolyte type as the key electrolysis conditions are applied to control the concentration of CQDs and Pt NPs, which decides electrochemically active surface area and HER activity of obtained catalysts. Significantly, the obtained superior catalyst with trace amount of Pt (0.145 wt%), shows markedly enhanced catalytic HER activity with mass activity of 37.5 A mg 1 at 50 mV in acidic solution, 68.2 times more active than Pt/C. This study provides a universal and promising methodology for synthesizing Pt, CQDs coloaded hybrid with the assistance of electric field as cost-effective catalysts for fuel cells.
* Corresponding author. Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China. ** Corresponding author. Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, The School of Chemical and Material Science, Shanxi Normal University, Linfen, 041004, China. E-mail addresses: [email protected] (M. Zhao), [email protected] (J. Jia). https://doi.org/10.1016/j.jpowsour.2020.227770 Received 30 November 2019; Received in revised form 10 January 2020; Accepted 16 January 2020 Available online 23 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
1. Introduction
detail. This work has an important guiding significance for facile syn thesis of Pt, CQDs-coloaded composite and other CQDs-based compos ites using electrosynthesis method.
Hydrogen production from electrocatalytic water splitting supports a promising way to replace fossil fuels in future [1,2]. Therefore devel oping efficient catalysts for hydrogen evolution reaction (HER) plays a key role in large-scale hydrogen production. Recently, noble metals and noble metal-based phosphides with high HER performance have been developed [3–9]. Among them, Pt-based materials have been proved as the most active HER catalysts [3–5], but their scarcity and high cost severely inhibit their large-scale application. Compared with traditional Pt-based catalyst (Pt/C), highly dispersed Pt (Pt single-atom or Pt nanoparticles (NPs)) loaded catalysts have great advantage with utmost utilization of Pt atoms, which can dramatically reduce the usage of noble metal Pt and decrease the cost of catalysts [3,10]. It has been shown that the highly dispersed Pt species anchored on the carrier surface such as carbon-based materials indeed show much higher catalytic performance and lower usage of expensive Pt [3,11–17]. Reduced graphene oxide (RGO), as a kind of carbon-based materials, is intensely researched as supporting materials for metal or metal oxide nanoparticles due to their distinctive properties such as high surface area, tunable surface functional groups and good conductivity [18]. Moreover, with unique structures and properties, RGO also can be introduced into various components of polymer electrolyte membrane fuel cells [19]. At present, RGO is mostly prepared from reduction of graphene oxide (GO) obtained by Hummer’s method [20]. This oxidation-reduction method produces graphene with abundant oxygen groups, which is partly benefit for anchoring nanoparticles but is detrimental to its conductivity due to irreversible structural defects [21]. In comparison, electrochemical exfoliation method, especially electrochemical cathodic exfoliation method, was applied to generate graphene with superior conductivity [22]. But the graphene prepared by electrochemical exfoliation method is more difficult to load and disperse nanoparticles than RGO because of its smooth surface. Therefore, con structing anchoring points for loading highly dispersed Pt NPs on pris tine graphene is essential to achieve high-performance electrocatalyst. Recently, Wang et al. [12] applied N-doped carbon quantum dots (NCQDs) to modify the properties of multiwall-carbon nanotube (MWCNT) for supporting Pt NPs. This Pt/NCQDs-MWCNT composite prepared by a facile hydrothermal treatment showed excellent perfor mance toward direct methanol fuel cells. The existence of NCQDs improved the dispersion and stability of Pt NPs on NCQDs-MWCNT. Moreover, Dong et al. [13] used carbon quantum dots (CQDs) as abundant nucleating and anchoring points to facilitate the formation of highly dispersed Pt NPs on RGO. They synthesized Pt-CQD/RGO by one pot reduction of CQD, GO and Pt precursor with ethylene glycol. Apart from the function of anchoring points, it’s found that CQDs as spacer also retard restacking of RGO sheets and support abundant oxygen groups to improve the antipoisonous ability of Pt. From above, it seems that CQDs could serve as anchoring point for loading highly dispersed Pt NPs on pristine graphene. As far as the synthesis of Pt, CQDs-coloaded RGO is concerned, co-reduction of Pt compound precursor such as H2PtCl6⋅6H2O and GO with existence of pre-preparation of CQDs is often applied. The preparation process is relatively time-consuming and complicated. In this work, highly efficient Pt, CQDs-coloaded graphene catalyst for HER was synthesized with an innovative and facile electrochemical method. During the first process, graphene was obtained by exfoliation of graphite in the cathode, while CQDs and Pt NPs were in-situ generated in the electrolyte simultaneously. Then both of the CQDs and Pt NPs were loaded on as-prepared graphene during the subsequent sol vothermal process. This catalyst showed superior HER performance in acidic solution with the mass activity of 37.5 A mg 1 at 50 mV, 68.2 times more active than that of Pt/C. It is known that electrolyte is affected by electrolytic conditions, thus, the influences of electrolytic conditions including work voltage and electrolyte type on the genera tion of CQDs and Pt NPs as well as HER activity were also revealed in
2. Experimental 2.1. Chemicals and materials Graphite powder (80 mesh) was purchased from Qingdao Graphite Company. Tetrabutylammonium tetrafluoroborate (TBABF4, 98%, powder), Tetrabutylammonium borohydrlde (TBABH4, 95%, powder), Tetrabutylammonium fluoridetrihydrate (TBAF, 98%, solid),Tetrabu tylammonium hydroxide (TBAOH, 98%, solid). Proprylene carbonate (PC, �99.9%) was purchased from aladdin. N, N-dimethylformamide (99.9%), and propylene carbonate (99.9%) was got from Aladdin, Anhydrous ethanol (99.5%) purchased from Woke Company. 2.2. Preparation of Pt-CQDs/graphene Graphite powder (40 mg) was put in a porous plastic tube with a platinum plate inserted as negative electrode, platinum wire served as positive electrode. Both electrodes were immersed in proprylene car bonate (PC) with tetrabutylammonium tetrafluoroborate (TBA BF4; 0.1 M) and a voltage of 30 V was applied for 12 h. After the electrochemical expansion step, the mixture was stirred for 12 h and transferred into 80 ml Teflon cup inserted in a stainless steel autoclave with an Ar flow (0.5 h) to completely remove air, then the solution was treated through solvothermal process at 160 � C under 500 round min 1 for 12 h. After that, the samples was filtered washing with PC solution for three times and ethanol for two time, then dried in a vacuum oven for 8 h at 60 � C. Finally, the samples were annealed at 400 � C to remove residue solvent in inert atmosphere for 60 min. The obtained sample was defined as PtCQDs/Gr-C400. Compared with Pt-CQDs/Gr-C400, Gr-C400 (graphite powder was first treated with electrochemical cathodic exfoliation and then calcination at 400 � C), Pt-CQDs/Graphite-C400 (graphite powder first underwent with solvothermal treatment in electrolyte solution after electrolysis and then calcination at 400 � C) and Pt-CQDs/Gr (graphite powder was first treated with electrochemical cathodic exfoliation and then solvothermal treatment at 160 � C) were prepared as show in the Table S1. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out on an X-ray diffractometer (Ultima IV-185) with Cu Ka radiation. Raman spectra were recorded on a JobinYvon LabRAM HR800 micro-Raman spec trometer using a laser excitation of 532 nm at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on Thermo Fisher SCIENTIFIC using Al Ka X-ray source. The morphologies of the samples were investigated by transmission electron microscopy (TEM, FEI Tec nai F20) and scanning electron microscopy (SEM, JSM-7001F). Fourier transform infrared (FTIR) spectra were recorded using a Bruker spec trometer. The height of the material was obtained by Atomic Force Microscope (Agilent 5500) instrument. The chemical composition (Pt) of the as-prepared samples was determined by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) method. 2.4. Electrochemical tests All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard three-electrode setup. A conventional three-electrode cell configuration was applied. Saturated Hg/Hg2Cl2 and a platinum plate were used as reference and counter electrode, respectively. A glassy carbon electrode (GCE, 3 mm in diameter) used as the support for the working electrode. Immobilization of sample was achieved by drop2
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
casting 5 μL sample (5 mg sample and 20 μL Nafion solution were dispersed in 1 ml water-ethanol solution with volume ratio of 3:1) onto GC working electrodes. 5 μL of the ink was loaded onto a rotating glassy carbon disk electrode by hand dropping. Linear sweep voltammetry (LSV) was conducted in the range of 100 to 500 mV (vs. RHE) at a sweep rate of 5 mV s 1 in 0.5 M H2SO4 solution. Prior to experiments, the solution was purged with pure H2 for at least 30 min. The over potentials were obtained at the reduction current density of 10 mA cm 2. Tafel plots were drawn from the overpotentials (η) as a function of the logarithm scales of the current density (logj). Tafel slopes (b) were obtained by fitting the linear portions of the Tafel plots according to Tafel equation (η ¼ a þ blogj). The stability measurements were per formed by cyclic voltammetry scanning 2000 cycles (CV, sweep rate, 50 mV s 1). CV method also was used to determine the electrochemical double-layer capacitances (Cdl). Electrochemically active surface area could be evaluated from the slope of the plot of the charging current versus the scan rate, which was directly proportional to Cdl. The elec trochemical impedance spectroscopy (EIS) was carried out over a fre quency range from 105 to 1 Hz with a 5 mV amplitude. Here, in the HER, SCE can be converted into the RHE by The following equation was applied ERHE ¼ ESCE þ0.059 pH þ EθSCE .Where ERHE is the converted potential versus RHE, ESCE is the experimental potential measured against the SCE, and EθSCE is the standard potential of SCE at 25 � C (246 mV). The electrochemical measurements were carried out in 0.5 M H2SO4 solution (pH ¼ 0) saturated with high purity H2 at room tem perature, thus ERHE (mV) ¼ ESCE (mV) þ 246 (mV).
aggregates with small particles on the surface. This result agrees with our previous work that electrochemical cathodic exfoliation is an effective way for exfoliating graphite into graphene [22,23]. TEM image (Fig. 2b) of Pt-CQDs/Gr-C400 reveals that small quantum dots or nanoparticles are uniformly decorated on the graphene sheets. The di ameters of these small quantum dots or nanoparticles mainly range from 1 nm to 5 nm (Fig. 2c). These dots with a light dark contrast show a lattice spacing of 0.21 nm (Fig. 2d and e), corresponding to the (100) crystal face of graphitic carbon, meaning the formation of CQDs [24]. While dots with strong dark contrast and hexagonal atomic lattice show a lattice spacing of 0.23 nm (Fig. 2d and e) [13], which are assigned to (111) crystal face of Pt [25]. Notably, many Pt NPs reside closely to CQDs. The HADDF-STEM image (Fig. 2f) for Pt-CQDs/Gr-C400 displays isolated bright dots with average diameter of about 4.1 nm (Fig. S2). These bright dots are attributable to heavy atoms on graphene, also confirming the successful anchoring of Pt NPs. The elemental mapping images reveal the homogeneous spatial distributions of platinum (Fig. 2h), carbon (Fig. 2i), oxygen (Fig. 2j) and fluorine (Fig. 2k). This result further indicates that Pt NPs generated over as-prepared gra phene. Just like Pt-CQDs/Gr-C400, Pt-CQDs/Graphite-C400 and Pt-CQDs/Gr both show formation of small quantum dots or nano particles on the graphene (Fig. S3). However, as for Gr-C400, no quan tum dots or nanoparticles are generated on the graphene. This result reveals the fact that the CQDs and Pt NPs evolved from the electrolyte solution, and combined with graphene at solvothermal process. To further characterize the chemical nature and bonding state of these prepared samples, XPS spectra for different samples were recorded [26–29]. From Fig. 3a, it can be seen that all the samples are mainly composed of C and O element, despite of some other trace amount ele ments like Pt. The Pt contents of these as-prepared catalysts were calculated and listed in Table S2. As observed, XPS data for Pt-CQDs/Gr-C400 shows that the surface Pt content is 0.393 at.%, indicating highly dispersion of enriched Pt NPs on Pt-CQDs/Gr-C400. The Pt loading in Pt-CQDs/Gr-C400, measured using ICP-OES, is 0.145 wt% (Table S2). To investigate clearly the valence state of Pt element, the high-resolution Pt 4f scan is also recorded [29]. As for Pt-CQDs/Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Graphite-C400 (Fig. 3b), their 4f7/2 and 4f5/2 peaks locate at around 71.7 and 75.0 eV, respec tively, which agrees with the values of Pt0 [30]. While for Gr-C400, which didn’t experience the same solvothermal process, the amount of Pt is below the detection limit. This result suggests that the anchoring process occurs in the solvothermal process and Pt comes from the electrolyte solution. XRD spectra (Fig. 3c) of all generated samples showed a strong peak at 2θ ¼ 26.5� corresponding to the (002) reflection peak of graphite (JCPDS No. 08-0415) [31]. Compared with graphite, these samples generated through electrochemical exfoliation like Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Gr-C400 exhibit a much more pronounced decrease of their (002) reflection peaks. This result indicates that the long-range periodicity along the c axis of graphite has been destroyed [31]. But for Pt-CQDs/Graphite-C400, the intensity of its (002) peak is
3. Results and discussion 3.1. Synthesis of Pt, CQDs-coloaded graphene composite Fig. 1a illustrates the synthesis procedure of Pt, CQDs-coloaded graphene composite (denoted as Pt-CQDs/Gr-C400) from graphite. Three consecutive processes are included: electrochemical exfoliation (or electrolysis), solvothermal treatment and calcination. Electro chemical exfoliation was conducted in a home-made setup with graphite powder as working cathode and platinum wire as anode. After exfolia tion, the expanded graphite and electrolyte were together transferred and subjected to solvothermal treatment, and then the obtained com posites were calcined at 400 � C. To investigate the function of every process, the corresponding compared samples: Gr-C400, Pt-CQDs/ Graphite-C400 and Pt-CQDs/Gr were also prepared as shown in Exper imental part and Supporting Information (Table S1). 3.2. Characterization of Pt, CQDs-coloaded graphene composite As seen from SEM image (Fig. 2a), Pt-CQDs/Gr-C400 mainly consists of thin, wrinkled tissue-like flakes. Compared samples such as Gr-C400 and Pt-CQDs/Gr (Fig. S1) prepared through the same electrochemical exfoliation process exhibit similar morphology with Pt-CQDs/Gr-C400, while the Pt-CQDs/Graphite-C400 sample (Fig. S1b) shows thick
Fig. 1. (a) Schematic of the preparation process for Pt-CQDs/Gr-C400. 3
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 2. (a) SEM images of the Pt-CQDs/Gr-C400. (b, c) Representative TEM micrographs of Pt-CQDs/Gr-C400. Scale bar is 500 nm in panel (b) and 20 nm in panel (c). Inset in panel (c) shows the nanoparticle core size histogram. (d, e) Representative HRTEM micrographs of Pt-CQDs/Gr-C400. Scale bar is 5 nm in panel (d) and 10 nm in panel (e). (f) HAADF-STEM image and (g-k) EDS maps of G-EX-ST. Scale bar is 100 nm in panel (f).
still much higher in comparison with other samples, indicating elec trochemical exfoliation process really can exfoliate bulky graphite into thin graphene nanosheets. However, no XRD reflections ascribed to Pt species can be found on these catalysts. This is mainly because of the low content and high dispersion of Pt species on graphene.
Raman spectra of different samples were shown in Fig. 3d. As for graphitic materials, three characteristic peaks at about 1350 cm 1, 1580 cm 1 and 2650 cm 1, assigned to D, G and 2D bands respectively, are often investigated. The D band is originated from edges, disorder or other defects, while G band is attributed to the zone center E2g mode 4
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 3. (a) XPS survey spectra, (b) Pt 4f XPS spectra, (c) XRD patterns and (d) Raman spectra of different samples.
corresponding to the ordered sp2 bonded carbon atom [32,33]. There fore, the ID/IG ratio (the intensities of the D (ID) band relative to the G (IG) band) allows to derive the density of Raman-sensitive defects, with a higher ID/IG ratio corresponding to a larger number of defects in the graphene lattice [34]. Compared with Gr-C400, samples like Pt-CQDs/Gr-C400, Pt-CQDs/Gr and Pt-CQDs/Graphite-C400 which were prepared through solvothermal treatment exhibit higher ID/IG ra tios. This result suggests solvothermal process is benefit for producing defects. Among these samples, Pt-CQDs/Gr-C400 shows the most de fects, which would be beneficial for electrocatalysis of HER by providing more active sites.
electrode is almost overlaps with that using Pt as counter electrode (Fig. S4). This result further verified that it’s the preparation process decides the formation of Pt NPs over the graphene rather than the test process. Thus, the Pt used as counter electrode doesn’t affect the test results of as-prepared samples. As shown in Fig. 4b, the mass activities for Pt-CQDs/Gr-C400, PtCQDs/Gr, Pt-CQDs/Graphite-C400 and Pt/C at 50 mV are 37.5, 4.07, 7.09 and 0.55 A mg 1, respectively, in which Pt-CQDs/Gr-C400 shows the best mass HER activity, and 68.2 times more active than that of Pt/C. What’s more, as shown in Fig. 4c, Pt-CQDs/Gr-C400 shows a much smaller Tafel slope (40 mV⋅dec 1) than that of Pt-CQDs/Gr (64 mV⋅dec 1), Pt-CQDs/Graphite-C400 (83 mV⋅dec 1) and Gr-C400 (587 mV⋅dec 1), revealing more favorable electrocatalytic kinetics on the PtCQDs/Gr-C400 catalyst for HER. The electrochemical impedance spec troscopy (EIS) of all the sample electrodes were also measured in 0.5 M H2SO4 solution (Fig. 4d). The result shows Pt-CQDs/Gr-C400 exhibits extremely small charge transfer resistances and fast reaction rates compared with other reference samples. This small charge transfer resistance is originated from the intimate contraction between the generated CQDs and graphene flakes. In addition, the LSV curves remain almost unchanged before and after 2000 CV scanning (Fig. 4e), indi cating strong durability of the Pt-CQDs/Gr-C400 electrode. The composition and structure changes of Pt-CQDs/Gr-C400 catalyst after durability tests were characterized by HRTEM. As the nanoparticle core size histogram shows in the Fig. S5a, it’s seen that the diameters of these small quantum dots or nanoparticles still range from 2 nm to 6 nm after stability test undergoing 2000 cycles, almost remaining the initial size as seen in Fig. 2c. Obviously seen from the Fig. S5b and Fig. S5c, these dots with a light dark contrast show a lattice spacing of 0.21 nm corre sponding to the (100) crystal face of graphitic carbon, indicating the
3.3. Activity and stability of Pt, CQDs-coloaded graphene composite for HER The HER activities of all the samples were initially investigated in 0.5 M H2SO4 aqueous solution at room temperature. Fig. 4a shows the linear sweep voltammetry (LSV) curves of all samples with a scan rate of 5 mV s 1. As expected, these results clearly indicate that Pt-CQDs/GrC400 shows excellent HER catalytic activity. Its overpotential at the current density of 10 mA cm 2 is 38 mV, superior to that of Pt-CQDs/Gr (49.5 mV), Pt-CQDs/Graphite-C400 (59.2 mV) and Gr-C400 (242.1 mV) catalysts. To rationally compare the catalytic performance with Pt/C, their HER catalytic activity was normalized to the current/Pt loading mass (A mg 1). Notably, Pt-CQDs/Gr-C400 displays much higher HER catalytic activity than Pt-CQDs/Gr, Pt-CQDs/Graphite-C400 and com mercial Pt/C catalysts. In order to exclude the introduction of Pt from Pt counter electrode, the Pt counter electrode was replaced with a graphite rob to apply for testing the HER performance of Pt-CQDs/Gr-C400. The LSV curve of the Pt-CQDs/Gr-C400 catalyst with graphite rob as counter 5
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 4. (a) LSV (area activity), (b) Mass activity value, (c) Tafel and (d) EIS curves of Pt-CQDs/Gr-C400 and the compared samples for the HER in 0.5 M H2SO4 with a scan rate of 5 mV 1 at room temperature, respectively. (e) LSV curves of the Pt-CQDs/Gr-C400catalyst before and after 2000 CV cycles in the stability test in 0.5 M H2SO4 for the HER. (f) The electrochemically active surface area (ECSA) of Pt-CQDs/Gr-C400 and the compared samples.
formation of CQDs. While dots with strong dark contrast and hexagonal atomic lattice show a lattice spacing of 0.23 nm, which are assigned to (111) crystal face of Pt. Above result inverifies that the obtained catalyst well keeps the original structure after durability test. The Faradaic ef ficiency for HER over Pt-CQDs/Gr-C400 was also investigated. The generated hydrogen in the cathode compartment of the H-type elec trolytic cell was measured using gas chromatograph (Fig. S6 and Fig. S7). The result suggests that the amount of H2 experimentally measured is almost consistent with the theoretical value, suggesting nearly 100% Faradaic yields for Pt-CQDs/Gr-C400 (Fig. S8). Cyclic voltammetric method is employed to evaluate the electro chemically active surface area (ECSA) of Pt-CQDs/Gr-C400 and
compared catalysts. The electrochemical double-layer capacitances (EDLC) results (Fig. S9) determined by electrochemical double-layer capacitances (Cdl) suggest that Pt-CQDs/Gr-C400 possesses the largest active surface area among these generated samples (Fig. 4f), which is also beneficial for achieving better catalytic activity toward HER. By comparing Gr-C400, Pt-CQDs/Gr, Pt-CQDs/Graphite-C400 with Pt-CQDs/Gr-C400 respectively, we can conclude that these three pro cesses: electrochemical exfoliation, solvothermal treatment and calci nation all contributed to the excellent activity of the Pt-CQDs/Gr-C400 catalyst, and the solvothermal process contributed most according to the overpotential difference shown in Table S3. The previous characteriza tion results show the formation of CQDs and Pt NPs coloaded on the 6
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
graphene occurs in this solvothermal process. That’s to say, the deco ration of CQDs and Pt NPs over the graphene contributed to the excellent HER activity. Compared with Pt-CQDs/Graphite-C400, Pt-CQDs/GrC400 and Pt-CQDs/Gr both show higher HER activity due to the larger surface area of graphene than graphite, which is in favor of the disper sion of CQDs and Pt NPs, thus exposes more defect sites (active sites) as shown in Raman and higher ECSA. Due to the probable removing of residue solvent covering some active sites by means of the calcination process, Pt-CQDs/Gr-C400 shows the superior HER activity to Pt-CQDs/ Gr.
Gr-C400 prepared at 30V exhibits the smallest overpotential at the current density of 10 mA cm 2, outperformed those prepared at other voltage, which is attributed to its larger ECSA (Fig. 5a2) in comparison with other samples. Detailed observation suggest that working voltage firstly affects the current (Table S4) and further affects the electrolytic degree. With the increasement of working voltage, the color of elec trolyte solution turned from colorless into dark brown (Fig. 5a3), and the concentrations of CQDs (Fig. 5a4) and Pt NPs (Fig. 5a5) in the solution increased. Based on the relationship between overpotential and ECSA, concentrations of CQDs, concentrations of Pt NPs and loading amount of Pt (Fig. 5a5), it’s concluded that the loading of the Pt NPs and CQDs over the obtained graphene increased with the enhancement of working voltage. This further improved the ECSA of as-prepared Pt-CQDs/GrC400, which leads to superior its HER activity. Therefore, working voltage can be used to control the loading amount of Pt as well as the HER activity of obtained Pt-CQDs/Gr-C400. Just like working voltage, electrolyte type mainly affects the current (Table S5) and then affects the electrolytic degree. In order to reveal the influence of electrolyte type, TBAF (anion: F ), TBABH4 (anion: BH4 ) and TBAOH (anion: OH ) were applied to replace the TBABF4 (anion: BF4 ). As observed from Fig. 5b1, Pt-CQDs/Gr-C400 prepared using TBABF4 electrolyte shows smaller overpotential than those prepared using other electrolyte. That’s because this obtained catalyst prepared with TBABF4 electrolyte has superior ECSA to other samples (Fig. 5b2). Despite of the same working voltage (30 V), electrolytic degree was different in different electrolyte solutions. After the electrolysis, the TBABF4 electrolyte solution shows brownish black while other solutions exhibit colorless (Fig. 5b3). The concentration of CQDs (Fig. 5b4) and Pt NPs (Fig. 5b5) in the TBABF4 electrolyte solution are obviously higher than that in TBAF, TBABH4 and TBAOH electrolyte solution, which contributes to the high Pt loading of the sample and excellent HER ac tivity (Fig. 5b4). Furthermore, it’s found that participation of fluorinecontaining electrolyte (TBABF4 and TBAF electrolyte) probably facili tate the loading of Pt on graphene during the solvothermal in compar ison with fluorine-free electrolyte (TBABH4 and TBAOH electrolyte). That’s probable that fluorine-containing electrolyte support a part of fluorine source for doping graphene, which acts as anchoring points to promote the formation and dispersion of Pt NPs like CQDs. It’s seen that the electrolyte type also can be used to adjust the loading amount of Pt as well as the HER activity of obtained Pt-CQDs/Gr-C400. The result shows the improvement of catalyst is not only due to the moderate amount of Pt but also the highly dispersion of Pt, CQDs and the large ECSA of the catalyst.
3.4. Probable formation process of CQDs and Pt NPs From above discussion, it’s deduced that the loading of CQDs and Pt NPs to graphene occurred in solvothermal process, but their formation occurred during or before solvothermal process is still unclear. To reveal the fact, the solution before and after solvothermal process was analyzed detailedly. TEM and HRTEM results of two solutions (Figs. S10–S13) both show the existence of CQDs and Pt NPs, indicating that they were generated in electrolysis process (before solvothermal process). More over, the solution (1 m mol Tetrabutylammonium tetrafluoroborate (TBABF4) in propylene carbonate (PC) solvent) before solvothermal process exhibits obvious fluorescence with a strong absorption peak in the range of 250–300 nm in the UV region, suggesting that CQDs have generated after electrolysis (Fig. S14) [35,36]. In addition, to verify the existence of CQDs, we performed the comparison experiment with no graphene matrix. That is, graphite electrode is replaced with Pt wire during the electrochemical exfoliation process. The obtained electrolyte solution after electrochemical exfoliation process was further subjected to solvothermal treatment. Then the mixture was kept still overnight. The sediment was collected and characterized with Raman spectra, XRD and HRTEM. The Raman spectra of obtained sample suggests typical carbon-based materials with high ID/IG ratio (ID/IG > 1), meaning the formation of CQDs, as shown in Fig. S15. Furthermore, the characteristic peak at 26.5� in XRD spectrum (Fig. S16) and the characteristic lattice spacing in HRTEM (Fig. S17) also confirmed the formation of CQDs. The concentration of Pt in the solution before solvothermal process was also tested with neglectable concentration of 172 mg L 1, suggesting that Pt NPs formed during the electrolysis process like CQDs. These above re sults all indicate that CQDs and Pt NPs are generated in electrolysis process (before solvothermal process). According to previous reports and the results reported herein, it can be concluded that the CQDs could be obtained directly from PC solvent as previously reported synthesis of CQDs from alcohol. It’s reported that the PC solvent can be converted into alcohol by a series of reactions [37, 38], and the alcohol can be converted in CQDs with power through electrochemical carbonization [39,40]. Thus here we envision that the formation mechanism of CQDs is as follows (Fig. S18): (1) transition of propylene carbonate into alcohol with power; (2) the decomposition of alcohol to carbon substances; (3) the polymerization and growth of carbon substances resulted in CQDs. Simultaneously, it’s also deduced that anodic dissolution of Pt from the counter electrode occurs in the electrolysis process and the dissolved Pt species (Pt2þ or PtOx) then immediately were reduced into Pt atom with the help of electrolyte solution (Fig. S19) [11,41–47]. Then the Pt NPs are attached to gra phene through solvothermal process.
3.6. The structure-activity relationship of Pt, CQDs-coloaded graphene composite Here, probable reaction mechanism for HER reaction over Pt-CQDs/ Gr-C400 was proposed as shown in Scheme 1. The highly dispersion for trace amount of Pt NPs anchored on graphene contributes most to the excellent catalytic performance. As for CQDs, they not only provide abundant nucleating and anchoring points to promote the formation and dispersion of Pt NPs, but also expose high specific surface area which produces abundant sharp edge sites (active sites) for HER [48]. More over, CQDs combine with graphene via π-π bonding, which constructs a 3D conductive network and shows fast charge transfer rate, leading to better electrochemical performance [34,49–53]. Therefore, the forma tion of CQDs and Pt NPs on graphene will tremendously increase the activity of the catalyst. Besides, the formation of CQDs and Pt NPs is with the help of electrolysis. Thus the electrolysis conditions including work voltage and electrolyte type, have directly great influence on the for mation of CQDs and Pt NPs, which in turn contributes to HER activity of the catalyst.
3.5. Influence of electrolysis conditions on formation of CQDs and Pt NPs as well as HER activity As observed from above, the CQDs and Pt NPs are generated during electrolysis process. It would make sense that the electrolysis conditions such as working voltage and electrolyte type should have great influence on their formation and catalytic activity. The impact of working voltage was firstly investigated. As seen from Fig. 5a1, the HER activity of these catalysts is positively related to their applied working voltage. Pt-CQDs/ 7
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
Fig. 5. LSV curves (a1), the ECSA (a2), photographs (a3), UV–Vis absorbance spectra (a4) and relationship of HER activity with content of CQDs and Pt (a5) for the PtCQDs/Gr-C400 catalysts prepared at different voltages. LSV curves (b1), the ECSA (b2), photographs (b3), UV–Vis absorbance spectra (b4) and relationship of HER activity with content of CQDs and Pt (b5) for the Pt-CQDs/Gr-C400 catalysts prepared with different electrolytes.
8
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770 [2] T.E. Mallouk, Water electrolysis: divide and conquer, Nat. Chem. 5 (2013) 362–363, https://doi.org/10.1038/nchem.1634. [3] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B. Xiao, R. Li, T. K. Sham, L.M. Liu, G.A. Botton, X. Sun, Platinum single-atom and cluster catalysis of the hydrogen evolution reaction, Nat. Commun. 7 (2016) 1–9, https://doi.org/ 10.1038/ncomms13638. [4] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N.M. Markovic, Enhancing hydrogen evolution activity in water splitting by tailoring Liþ-Ni(OH)2-Pt interfaces, Science 334 (2011) 1256–1260, https://doi.org/10.1126/science.1211934. [5] H. Yin, S. Zhao, K. Zhao, A. Muqsit, H. Tang, L. Chang, H. Zhao, Y. Gao, Z. Tang, Ultrathin platinum nanowires grown on single-layered nickel hydroxide with high hydrogen evolution activity, Nat. Commun. 6 (2015) 6430, https://doi.org/ 10.1038/ncomms7430. [6] S. Yuan, Z. Pu, H. Zhou, J. Yu, I.S. Amiinu, J. Zhu, Q. Liang, J. Yang, D. He, Z. Hu, G. Van Tendeloo, S. Mu, A universal synthesis strategy for single atom dispersed cobalt/metal clusters heterostructure boosting hydrogen evolution catalysis at all pH values, Nano Energy 59 (2019) 472–480, https://doi.org/10.1016/j. nanoen.2019.02.062. [7] W. Li, Z.Y. Hu, Z. Zhang, P. Wei, J. Zhang, Z. Pu, J. Zhu, D. He, S. Mu, G. Van Tendeloo, Nano-single crystal coalesced PtCu nanospheres as robust bifunctional catalyst for hydrogen evolution and oxygen reduction reactions, J. Catal. 375 (2019) 164–170, https://doi.org/10.1016/j.jcat.2019.05.031. [8] Z. Pu, I.S. Amiinu, Z. Kou, W. Li, S. Mu, RuP 2 -based catalysts with platinum-like activity and higher durability for the hydrogen evolution reaction at all pH values, Angew. Chem. Int. Ed. 56 (2017) 11559–11564, https://doi.org/10.1002/ anie.201704911. [9] Z. Pu, J. Zhao, I.S. Amiinu, W. Li, M. Wang, D. He, S. Mu, A universal synthesis strategy for P-rich noble metal diphosphide-based electrocatalysts for the hydrogen evolution reaction, Energy Environ. Sci. 12 (2019) 952–957, https://doi.org/ 10.1039/c9ee00197b. [10] J. Wang, F. Ma, M. Sun, Graphene, hexagonal boron nitride, and their heterostructures: properties and applications, ACS Catal. 7 (2017) 16801–16822, https://doi.org/10.1039/C7RA00260B. [11] M. Tavakkoli, N. Holmberg, R. Kronberg, H. Jiang, J. Sainio, E.I. Kauppinen, T. Kallio, K. Laasonen, Electrochemical activation of single-walled carbon nanotubes with pseudo-atomic-scale platinum for the hydrogen evolution reaction, ACS Catal. 7 (2017) 3121–3130, https://doi.org/10.1021/acscatal.7b00199. [12] J.J. Zhang, Z.B. Wang, C. Li, L. Zhao, J. Liu, L.M. Zhang, D.M. Gu, Multiwall-carbon nanotube modified by N-doped carbon quantum dots as Pt catalyst support for methanol electrooxidation, J. Power Sources 289 (2015) 63–70, https://doi.org/ 10.1016/j.jpowsour.2015.04.150. [13] T.-Z. Hong, Q. Xue, Z.-Y. Yang, Y.-P. Dong, Great-enhanced performance of Pt nanoparticles by the unique carbon quantum dot/reduced graphene oxide hybrid supports towards methanol electrochemical oxidation, J. Power Sources 303 (2016) 109–117, https://doi.org/10.1016/j.jpowsour.2015.10.092. [14] Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (2011) 776–780, https://doi.org/10.1002/ adma.201003819. [15] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, The graphene-supported Pd and Pt catalysts for highly active oxygen reduction reaction in an alkaline condition, Electrochem. Commun. 13 (2011) 182–185, https://doi.org/10.1016/j. elecom.2010.12.008. [16] K. Zhang, Q. Yue, G. Chen, Y. Zhai, L. Wang, H. Wang, J. Zhao, J. Liu, J. Jia, H. Li, Effects of acid treatment of Pt-Ni Alloy nanoparticles@graphene on the kinetics of the oxygen reduction reaction in acidic and alkaline solutions, J. Phys. Chem. C 115 (2011) 379–389, https://doi.org/10.1021/jp108305v. [17] Y. Li, Y. Li, E. Zhu, T. McLouth, C.Y. Chiu, X. Huang, Y. Huang, Stabilization of high-performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene oxide/carbon black composite, J. Am. Chem. Soc. 134 (2012) 12326–12329, https://doi.org/10.1021/ja3031449. [18] L.G. Guex, B. Sacchi, K.F. Peuvot, R.L. Andersson, A.M. Pourrahimi, V. Str€ om, S. Farris, R.T. Olsson, Experimental review: chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO) by aqueous chemistry, Nanoscale 9 (2017) 9562–9571, https://doi.org/10.1039/c7nr02943h. [19] D. He, H. Tang, Z. Kou, M. Pan, X. Sun, J. Zhang, S. Mu, Engineered graphene materials: synthesis and applications for polymer electrolyte membrane fuel cells, Adv. Mater. 29 (2017) 1–8, https://doi.org/10.1002/adma.201601741. [20] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene oxide, Carbon 64 (2013) 225–229, https://doi.org/10.1016/ j.carbon.2013.07.055. [21] F. Kim, L.J. Cote, J. Huang, Graphene oxide: surface activity and two-dimensional assembly, Adv. Mater. 22 (2010) 1954–1958, https://doi.org/10.1002/ adma.200903932. [22] M. Zhao, X.Y. Guo, O. Ambacher, C.E. Nebel, R. Hoffmann, Electrochemical generation of hydrogenated graphene flakes, Carbon 83 (2015) 128–135, https:// doi.org/10.1016/j.carbon.2014.11.033. [23] M. Zhao, X. Ma, H. Xiao, Electrochemistry Communications Regulation of the degree of hydrogenation and electrochemical properties of graphene generated by electrochemical cathodic exfoliation by using different solvents, Electrochem. Commun. 103 (2019) 77–82, https://doi.org/10.1016/j.elecom.2019.05.013. [24] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem. Soc. Rev. 44 (2015) 362–381, https://doi.org/10.1039/c4cs00269e.
Scheme 1. Schematic illustration of the proposed reaction mechanism for HER reaction over Pt-CQDs/Gr-C400.
4. Conclusion In conclusion, a facile electrolysis-solvothermal method was estab lished for the preparation of Pt and CQDs coloaded graphene catalyst for HER with the assistant of electric field. The mass activity for as-prepared superior catalyst (Pt-CQDs/Gr-C400) at 50 mV is 37.5 A mg 1, 68.2 times more active than that of Pt/C. The Pt loading of this obtained catalyst is only 0.145 wt%, which greatly reduces the usage of expensive Pt. Such high catalytic activity can be attributed to large electrochemi cally active surface area of the catalyst resulted from highly dispersed Pt NPs and defect-rich CQDs. Meanwhile CQDs combine with graphene via π-π bonding, which constructs 3D conductive network and boosts the HER performance. The electrolysis conditions including work voltage and electrolyte type can be used to adjust the formation and loading amount of Pt NPs and CQDs, which in turn contributes to electro chemically active surface area and HER activity of the catalyst. The present work provides new insights into understanding of HER activity of Pt, CQDs coloaded graphene and offers a facile approach for preparing Pt, CQDs coloaded multi-dimensional electrocatalysts. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported by the National Natural Science Foun dation of China (21571119), the Program for New Century Excellent Talents in University (NCET-12-1035), the Applied Basic Research Project of Shanxi Province (No. 201901D211393, 201901D211398), the Scientific and Technological Innovation Programs of Higher Education Institution in Shanxi (2019L0466), the Foundation of State Key Labo ratory of Coal Conversion (J19-20-605), Shanxi Normal University School Fund (ZR1707) and 1331 Engineering of Shanxi Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227770. References [1] X.P. Yin, H.J. Wang, S.F. Tang, X.L. Lu, M. Shu, R. Si, T.B. Lu, Engineering the coordination environment of single-atom platinum anchored on graphdiyne for optimizing electrocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 9382–9386, https://doi.org/10.1002/anie.201804817.
9
H. Xiao et al.
Journal of Power Sources 451 (2020) 227770
[25] Z.Y. Zhou, X. Kang, Y. Song, S. Chen, Enhancement of the electrocatalytic activity of Pt nanoparticles in oxygen reduction by chlorophenyl functionalization, Chem. Commun. 48 (2012) 3391, https://doi.org/10.1039/c2cc17945h. [26] A.M. Motin, T. Haunold, A.V. Bukhtiyarov, A. Bera, C. Rameshan, G. Rupprechter, Surface science approach to Pt/carbon model catalysts: XPS, STM and microreactor studies, Appl. Surf. Sci. 440 (2018) 680–687, https://doi.org/10.1016/j. apsusc.2018.01.148. [27] M.Y. Smirnov, A.V. Kalinkin, E.I. Vovk, P.A. Simonov, E.Y. Gerasimov, A. M. Sorokin, V.I. Bukhtiyarov, Comparative XPS study of interaction of model and real Pt/C catalysts with NO 2, Appl. Surf. Sci. 428 (2018) 972–976, https://doi. org/10.1016/j.apsusc.2017.09.205. [28] R. Vakili, E.K. Gibson, S. Chansai, S. Xu, N. Al-Janabi, P.P. Wells, C. Hardacre, A. Walton, X. Fan, Understanding the CO oxidation on Pt nanoparticles supported on MOFs by operando XPS, ChemCatChem 10 (2018) 4238–4242, https://doi.org/ 10.1002/cctc.201801067. [29] G. Wang, Z. Yang, Y. Du, Y. Yang, Programmable exposure of Pt active facets for efficient oxygen reduction, Angew. Chem. 131 (2019) 1–9, https://doi.org/ 10.1002/ange.201907322. [30] G.V. C�eline Dablemont, P. Lang, C. Mangeney, J.-Y. Piquemal, V. Petkov, F. Herbst, FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina, Langmuir 48 (2012) 3391, https://doi.org/10.1039/c2cc17945h. [31] M. Zhao, J. Zhang, H. Xiao, T. Hu, J. Jia, H. Wu, Facile in situ synthesis of a carbon quantum dot/graphene heterostructure as an efficient metal-free electrocatalyst for overall water splitting, Chem. Commun. 55 (2019) 1635–1638, https://doi.org/ 10.1039/C8CC09368G. [32] J. Shen, M. Shi, N. Li, B. Yan, H. Ma, Y. Hu, M. Ye, Facile synthesis and application of Ag-chemically converted graphene nanocomposite, Nano Res. 3 (2010) 339–349, https://doi.org/10.1007/s12274-010-1037-x. [33] J. Shen, Y. Hu, M. Shi, X. Lu, C. Qin, C. Li, M. Ye, Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets, Chem. Mater. 21 (2009) 3514–3520, https://doi.org/10.1021/cm901247t. [34] R.N. Singh, R. Awasthi, Graphene support for enhanced electrocatalytic activity of Pd for alcohol oxidation, Catal. Sci. Technol. 1 (2011) 778–783, https://doi.org/ 10.1039/c1cy00021g. [35] M. Liu, Y. Xu, F. Niu, J.J. Gooding, J. Liu, Carbon quantum dots directly generated from electrochemical oxidation of graphite electrodes in alkaline alcohols and the applications for specific ferric ion detection and cell imaging, Analyst 141 (2016) 2657–2664, https://doi.org/10.1039/c5an02231b. [36] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem. Soc. Rev. 44 (2015) 362–381, https://doi.org/10.1039/c4cs00269e. [37] E.Y. Tyunina, M.D. Chekunova, V.N. Afanasiev, Electrochemical characteristics of propylene carbonate solutions of tetraethylammonium tetrafluoroborate, Russ. J. Electrochem. 49 (2013) 453–457, https://doi.org/10.1134/s1023193513050157. [38] H. Batterien, Anodic stability of propylene carbonate electrolytes at potentials above 4 V against lithium: an on-line MS and in situ FTIR study 21 (1991) 885–894. [39] H. Li, H. Ming, Y. Liu, H. Yu, X. He, H. Huang, K. Pan, Z. Kang, S.T. Lee, Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive
[40] [41]
[42] [43] [44]
[45]
[46]
[47] [48]
[49] [50] [51] [52] [53]
10
photoluminescence properties, New J. Chem. 35 (2011) 2666–2670, https://doi. org/10.1039/c1nj20575g. J. Deng, Q. Lu, N. Mi, H. Li, M. Liu, M. Xu, L. Tan, Q. Xie, Y. Zhang, S. Yao, Electrochemical synthesis of carbon nanodots directly from alcohols, Chem. Eur J. 20 (2014) 4993–4999, https://doi.org/10.1002/chem.201304869. S. Cherevko, A.A. Topalov, A.R. Zeradjanin, G.P. Keeley, K.J.J. Mayrhofer, Temperature-dependent dissolution of polycrystalline platinum in sulfuric acid electrolyte, Electrocatalysis 5 (2014) 235–240, https://doi.org/10.1007/s12678014-0187-0. S. Cherevko, A.R. Zeradjanin, G.P. Keeley, K.J.J. Mayrhofer, A comparative study on gold and platinum dissolution in acidic and alkaline media, J. Electrochem. Soc. 161 (2014) H822–H830, https://doi.org/10.1149/2.0881412jes. S. Cherevko, A.R. Zeradjanin, A.A. Topalov, N. Kulyk, I. Katsounaros, K.J. J. Mayrhofer, Dissolution of noble metals during oxygen evolution in acidic media, ChemCatChem 6 (2014) 2219–2223, https://doi.org/10.1002/cctc.201402194. D.C. Johnson, D.T. Napp, S. Bruckenstein, A ring-disk electrode study of the current/potential behaviour of platinum in 1.0 M sulphuric and 0.1 M perchloric acids, Electrochim, Acta 15 (1970) 1493–1509, https://doi.org/10.1016/00134686(70)80070-6. D.A.J. Rand, R. Woods, A study of the dissolution of platinum, palladium, rhodium and gold electrodes in 1 m sulphuric acid by cyclic voltammetry, J. Electroanal. Chem. Interfacial Electrochem. 35 (1972) 209–218, https://doi.org/10.1016/ S0022-0728(72)80308-5. A.A. Topalov, S. Cherevko, A.R. Zeradjanin, J.C. Meier, I. Katsounaros, K.J. J. Mayrhofer, Towards a comprehensive understanding of platinum dissolution in acidic media, Chem. Sci. 5 (2014) 631–638, https://doi.org/10.1039/ C3SC52411F. L. Xing, M.A. Hossain, M. Tian, D. Beauchemin, K.T. Adjemian, G. Jerkiewicz, Platinum electro-dissolution in acidic media upon potential cycling, Electrocatalysis 5 (2014) 96–112, https://doi.org/10.1007/s12678-013-0167-9. H. Wang, X.B. Li, L. Gao, H.L. Wu, J. Yang, L. Cai, T.B. Ma, C.H. Tung, L.Z. Wu, G. Yu, Three-dimensional graphene networks with abundant sharp edge sites for efficient electrocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 192–197, https://doi.org/10.1002/anie.201709901. Y. Wang, A. Hu, Carbon quantum dots: synthesis, properties and applications, J. Mater. Chem. C. 2 (2014) 6921–6939, https://doi.org/10.1039/c4tc00988f. X. Wang, G. Sun, P. Routh, D.H. Kim, W. Huang, P. Chen, Heteroatom-doped graphene materials: syntheses, properties and applications, Chem. Soc. Rev. 43 (2014) 7067–7098, https://doi.org/10.1039/c4cs00141a. T. Bao, L. Song, S. Zhang, Synthesis of carbon quantum dot-doped NiCoP and enhanced electrocatalytic hydrogen evolution ability and mechanism, Chem. Eng. J. 351 (2018) 189–194, https://doi.org/10.1016/j.cej.2018.06.080. C.S. Lim, K. Hola, A. Ambrosi, R. Zboril, M. Pumera, Graphene and carbon quantum dots electrochemistry, Electrochem. Commun. 52 (2015) 75–79, https:// doi.org/10.1016/j.elecom.2015.01.023. R. Wang, K.Q. Lu, F. Zhang, Z.R. Tang, Y.J. Xu, 3D carbon quantum dots/graphene aerogel as a metal-free catalyst for enhanced photosensitization efficiency, Appl. Catal. B Environ. 233 (2018) 11–18, https://doi.org/10.1016/j. apcatb.2018.03.108.