Enhanced hydrogen evolution activity over microwave-assisted functionalized 3D structured graphene anchoring FeP nanoparticles

Electrochimica Acta 317 (2019) 242e249

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Enhanced hydrogen evolution activity over microwave-assisted functionalized 3D structured graphene anchoring FeP nanoparticles Dourong Wang a, b, Jiajia Lu a, Lin Luo a, c, *, Shengyu Jing b, f, Hanna S. Abbo c, Salam J.J. Titinchi c, Zheng Chen a, Panagiotis Tsiakaras d, e, f, *, Shibin Yin a, * a Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, State Key Laboratory of Processing for Non-Ferrous Metal & Featured Materials, Key Laboratory of Disaster Prevention and Structural Safety of China Ministry of Education, Guangxi University, Nanning, 530004, China b School of Information and Control Engineering, China University of Mining and Technology, Xuzhou, 221116, Jiangsu, China c Department of Chemistry, University of the Western Cape, 7535, Cape Town, South Africa d Laboratory of Electrochemical Devices Based on Solid Oxide Proton Electrolytes, Institute of High Temperature Electrochemistry, RAS, Yekaterinburg, 620990, Russia e Laboratory of Materials and Devices for Clean Energy, Ural Federal University, 19 Mira Street, Yekaterinburg, 620002, Russia f Laboratory of Alternative Energy Conversion Systems, Department of Mechanical Engineering, School of Engineering, University of Thessaly, Pedion Areos, 38834, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2019 Received in revised form 4 May 2019 Accepted 29 May 2019 Available online 30 May 2019

Exploring low-cost and high-performance electrocatalysts for hydrogen evolution reaction (HER) still remains a challenging issue. An efficient strategy to enhance HER activity concerns with designing suitable substrates for constructing composite materials. Herein, a new hybrid electrocatalyst, consisting of FeP (iron phosphide) nanoparticles (NPs) anchored on 3D structured graphene (3DG) is investigated towards HER. 3DG substrates are pretreated by an intermittent microwave heating (IMH) method, following the procedure 5s-ON/5s-OFF, repeated for different times. It is found that the as resulted 3DG20 substrate (submitted to 20 repetitions), is favorable for the uniform dispersion of FeP nanoparticles, which presents large specific surface area, abundant oxygen functional groups and easy electron and mass transfer. The as-prepared FeP/3DG20 electrocatalyst displays good HER activity and stability, requiring relatively small overpotentials of
113.2 and
211.4 mV to deliver current densities of
10 and
200 mA cm
2 in 0.5 M H2SO4 aqueous solution, the corresponding mechanism is discussed. Thus, this study might provide a prospective strategy to enhance HER catalytic efficiency by exploiting the synergy between electrocatalysts and support materials. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Hydrogen evolution Electrocatalysts FeP 3D structured graphene Functionalization

1. Introduction Hydrogen with the virtue of high energy density is regarded as an ideal clean energy, because its oxidation product is water [1]. It is known that hydrogen evolution reaction (HER), occurring at the cathode during electrochemical water splitting, is a promising method to economically and efficiently produce hydrogen [2]. In order to enhance HER, electrocatalysts with high catalytic activity are required to lower the overpotential and increase the reaction rate [3]. In principle, the noble metal based electrocatalysts can provide an optimal performance for HER, while scarcity increases

* Corresponding authors. E-mail addresses: [email protected] (L. Luo), [email protected] (P. Tsiakaras), [email protected] (S. Yin). https://doi.org/10.1016/j.electacta.2019.05.153 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

their costs and seriously limits their application on a large scale [4]. Therefore, considerable efforts have been motivated to explore non-noble metal substitutes made of earth-enriched elements for HER. Transition metal phosphides (TMPs) have been extensively studied as prospective HER electrocatalysts with high abundance and better electrical conductivity [5e7]. As known, Fe comprises nearly 5% of the earth's crust, which is one of the cheapest and most abundant transition metal in the earth, so FeP is especially attractive among all the TMPs [8]. Furthermore, constructing composite materials is a promising strategy to create strong chemical and electrical coupling [9], as well as overcome the drawbacks of individual materials, so that derived materials can utilize the potential synergistic effects and optimize the HER performance [10]. A suitable electrocatalyst support is also significant to affect the catalytic efficiency; it should meet the basic requirements such as

D. Wang et al. / Electrochimica Acta 317 (2019) 242e249

relatively high specific surface area, efficient electron and/or mass transport, favorable loading of active phases, high corrosion residence in the electrolyte and so on. Taking these into account, carbon material has captured growing attention in improving the electrocatalytic performance. Carbon supports such as carbon cloth, Vulcan XC-72, carbon nanotubes, 2D graphene etc. are widely used. While with the drawbacks of relatively small specific surface area, low graphitization and porosity, high cost etc. [8,11,12], there still have large space to optimize the physicochemical properties for further improving the catalytic activity. 3D structured graphene (3DG) has been intensively investigated due to its high conductivity and the pore structure, which is favorable for mass transfer [11,13,14]. However, because of its high graphitization degree, it cannot provide effective anchor sites for directly coupling other functional metal-based species [9]. In order to solve these problems, while simultaneously obtain more exposed active sites, it is of great significance to modify the substrates with defects, curvatures or functional groups so as to enhance the electrocatalytic activity of the composites [15]. In fact, plenty of studies have been devoted to the functional pretreatment process (functionalization), especially the surface oxidation with concentrated H2SO4/HNO3 mixture is the most adopted one [16]. Nevertheless, it is a harsh, complex and time-consuming method, also with harmful and dangerous chemicals, thus an environmentally friendly method is needed [12]. Intermittent microwave heating (IMH) method has the unique advantage of high heating efficiency; it is a green, efficient and simple treatment procedure for the synthesis of nanomaterials [16e20]. In this work, a hybrid electrocatalyst with the FeP nanoparticles (NPs) anchored on 3DG was synthesized and displayed high HER activity. The 3DG substrate with high specific surface area, after pretreated, following the IMH-assisted H2O2 method, is functionalized with oxygen functional groups and increased mesoporous structure. Specifically, by the aid of IMH-assisted H2O2 solution treatment, repeated for 20 times, the as obtained 3DG20 enables the uniformly dispersion of FeP NPs, facilitating the mass transfer. The as-obtained FeP/3DG20 possesses good catalytic activity and stability for HER in 0.5M H2SO4 electrolyte.

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Scheme 1. Illustration of the synthesis procedure of FeP/3DG20.

method [21], using 40 wt% of Fe and 60 wt% of support materials. Typically, 119.9 mg of iron (III) chloride hexahydrate (FeCl3$6H2O) and 100.0 mg of 3DG were added into 200.0 ml deionized water, followed by ultrasonication and stirring for 30 min. Subsequently, 338.9 mg of sodium borohydride (NaBH4) was slowly added into the above mixture with magnetic stirring. After stirring for 6 h, the precipitate was collected by filtering, and then washed and dried at 80  C under vacuum for 10 h. Finally, the as-prepared samples and NaH2PO2$H2O in two different porcelain boats were placed side by side at the furnace, and the NaH2PO2$H2O was at the upstream position of airflow. The samples were heated at 300  C under a steady N2 atmosphere with a heating rate of 3  C min
1, and maintained for 2 h. The as-obtained samples are denoted as FeP/ 3DG10, FeP/3DG20, FeP/3DG30 and FeP/C20, respectively. For comparison, the samples in absence of P-doping (denoted as Fe3O4/ 3DG20) and in absence of metal salt (denoted as P-3DG20) were also prepared.

2. Experimental 2.3. Electrocatalysts characterization 2.1. Support materials pretreatment 3D structured graphene (3DG) (nitrogen-self-doped) was provided by Guangxi Beibu Gulf Graphene Industrial Technology Development Co., Ltd, and prepared by the procedure reported previously [13]. Functionalization process of 3DG was achieved according to IMH-assisted H2O2 solution treatment [17]. The steps are described as follows: The as-received 3DG (0.5 g) was immersed in 30.0 wt% H2O2 solution (50 mL), and then a well-dispersed mixture was obtained after stirring and ultrasonication for 15 min. Thereafter, the as obtained slurry was heated in a homemade program-controlled microwave oven by the aid of IMH method (5s-ON/5s-OFF), repeated for 10, 20, 30 times and denoted as 3DG10, 3DG20 and 3DG30 respectively; then washed by deionized water, dried under vacuum at 80  C, and ready to be used. Moreover, a commercially available Vulcan XC-72 carbon IMH treated by the same method for 20 repetitions was denoted as C20. A brief schematic illustration of the synthesis procedure is shown in Scheme 1. 2.2. Electrocatalysts preparation All used samples were synthesized through a one-pot reduction

Fourier transform infrared (FTIR) spectra were obtained with an FTIR spectrometer (Nicolet iS50, Thermo Scientific, USA). Raman measurements were performed by the aid of a Raman spectrometer (Horiba Jobin Yvon Inc., France), using a 473 nm He/Ne laser excitation source. The specific surface area and pore size distribution of the substrates were obtained using an ASAP 2420 (Micrometeritics Co., USA), by analyzing N2 adsorption/desorption data at 77 K. An Xray diffractometer (SmartLab, Rigaku Corp., Japan) equipped with Cu-Ka radiation (l ¼ 0.15406 nm) was used to characterize the samples at 40 kV and 30 mA, with a scan rate of 5 min
1. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 Xi (Thermo Fisher Scientific, USA) instrument with a monochromatic Al X-ray radiation source. The morphologies of the samples were characterized by scanning electron microscopy (SEM) (FESEM SU8220 - Hitachi Corp., Japan). Scanning transmission electron microscopy (STEM) images and energy-dispersive spectroscopy (EDS) were carried out on a spectrometer (Titan ETEM G2 80-300 - FEI Co., USA), equipped with a high-angle annular dark field (HAADF) detector at 300 kV. Electrochemical measurements were performed using a computer controlled electrochemical workstation (Zahner IM6e, Germany) in a standard three-electrode system at 25  C. The chronopotentiometry test was carried out on a PINE workstation

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(PINE, USA). A glassy carbon (GC) electrode of 5.0 mm in diameter decorated with the electrocatalyst film, a carbon rod, and a saturated calomel electrode (SCE) were employed as working, counter and reference electrodes, respectively. In all experiments, the SCE reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE) [22]. For the preparation of the electrocatalyst film, a mixture containing 5.0 mg of the as-prepared electrocatalysts, 20.0 mL of 5.0 wt% Nafion solution and 980.0 mL of ethanol was ultrasonicated to create a homogeneous ink. Then, 20.0 mL of this slurry was dropped, by using a micropipette, on the GC electrode that was dried in ambient conditions (loading amount: ~0.51 mg cm
2). For comparison, the Pt/C electrocatalyst (20 wt% Pt/C, Johnson Matthey) was similarly prepared with 10.0 mL of slurry loaded on the GC electrode. After the initial stabilization by CV scanning from 0.1 to
0.5 V in the N2-saturated 0.5 M H2SO4 aqueous solution for 50 cycles with a scan rate of 100 mV s
1, the linear sweep voltammetry (LSV) was carried out at a scan rate of 5 mV s
1 on a rotating disk electrode with a rotating speed of 1,600 rpm. The electrochemical impedance spectroscopy (EIS) measurements were performed in potentiostatic mode of operation at
0.15 V from 100 kHz to 100 mHz with an AC amplitude of 5 mV. For electrochemical stability tests, the chronoamperometry curve was obtained by holding the current density at
10 mA cm
2 in 0.5 M H2SO4 aqueous solution. It should be noted that in this work all polarization curves were presented with iR compensation.

3. Results and discussion 3.1. Physicochemical characterization In order to investigate the changes of oxygen functional groups on the support surface (3DG) after the IMH-H2O2 pretreatment, the

3DG samples treated for different procedures (5s-ON/5s-OFF) are studied by FTIR. The peaks at 1220 cm
1, seen in Fig. 1a, are attributed to the stretching of CeO in the phenolic group. The bands of C]O groups that appear at 1582 and 1730 cm
1 are assigned to the bending and stretching vibration of the COOH group. The broad bands observed at 3406 cm
1 are related to the OeH stretching [17,23]. Compared with the untreated 3DG, it is obviously that the absorption intensity of oxygen functional groups increase as the pretreatment repetitions increase. This is due to the strong oxidizing property of H2O2 solution, especially under rapid microwave heating operation, which can lead to a higher temperature after repeated treatments (the more repetitions the higher the temperature). In addition, the intensity of the peak corresponding to 1522 cm
1 (sp2 C]N and/or C]C) decreases and is gradually overlapped by the C]O peak [24]. The results proved that the original N-doped graphite structure was functionalized by the pretreatment. The edge effect caused by violent destruction connects oxygen functional groups to the carbon layer, which could increase the hydrophilicity of 3DG and provide more anchor sites for directly coupling with metal particles. The Raman spectra shown in Fig. 1b are used to identify the defects and disorder degree of 3DG support materials. The two major bands such as D-line (due to out-of-plane vibrations of disordered graphite) and G-line (due to in-plane vibrations of sp2hybridized carbon atoms) appear at ~1338 cm
1 and ~1580 cm
1, respectively. The intensity ratios of G-line and D-line (IG/ID) can be used as an indicator to obtain the evident ordered/disordered information in the carbon material with and without treatments [25]. The IG/ID value of 3DG is 1.03 due to the insertion of N atoms into the graphene lattices [26]; however, it is still relatively higher than that of the other samples. Based on the Raman spectra, the calculated value of IG/ID are 1.01, 0.98 and 0.96 for 3DG10, 3DG20 and

Fig. 1. (a) FTIR spectra, (b) Raman spectra, (c) Nitrogen adsorption/desorption isotherms and (d) DFT pore-size distribution curves of 3DG, 3DG10, 3DG20 and 3DG30.

D. Wang et al. / Electrochimica Acta 317 (2019) 242e249

3DG30, respectively. The decreasing trend can be explained by the fact that the aggressive oxidation process results in a more severe edge effect, which destroys parts of the graphite structure and increases the disorder degree [27]. According to the nitrogen adsorption/desorption isotherms, shown in Fig. 1c, the 3DG exhibits combined characteristics of type II/IV with a high BrunauereEmmetteTeller (BET) specific surface area of 1432 m2 g
1. As the pretreatment repetitions increased from 10, 20 to 30, the BET specific surface areas decline to 1181, 895 and 591 m2 g
1, respectively. Besides, the isotherms of 3DG20 and 3DG30 accompanied by larger H4 hysteresis loops illustrate that the capillary condensation may occur, since the pore width exceeds a certain critical value; under this test conditions, hysteresis starts to occur for pores wider than ~ 4 nm [28]. As demonstrated in Fig. 1d, the pore-size characteristics further reveal the effect of different pretreatment on the substrates. The average pore sizes of 3DG, 3DG10, 3DG20 and 3DG30 are 3.08, 3.21, 3.84, and 4.43 nm, respectively. While, as the treatments repetitions increased, the micropores amount decreases and the mesoporous feature is dominant. This may be due to the expansion of the pores in carbon wall during high temperature oxidation process, which leads to the closure and/or enlargement of micropores. It should be noted that abundant mesoporous structure can

245

accelerate the mass transfer rate of reactants and products during the reaction process, thereby improving the HER activity. Additionally, the excessive oxidation treatment also leads to the structural damage and gradual decrease of the specific surface area, which is consistent with the SEM characterization as displayed in Fig. S3. Compared with the untreated 3DG, the 3D honeycomb structures of 3DG10 and 3DG20 almost have no visible damage, while 3DG30 has a significant collapse, which is not good for the well dispersion of active phases. The morphologies of 3DG20 with and without FeP NPs are clearly displayed in Figs. 2a & 2b. It can be seen that the metal loading process does not evidently damage the original 3D interconnection cellular network structure of the substrate. From Fig. 2c, the FeP NPs with small size are homogeneously and densely distributed on the 3DG20 support. However, as shown in Figs. S4a & S4b, FeP particles deposited on the 3DG10 support appear a sparse distribution while aggregate on 3DG30. According to Figs. 1a & 1c, this is probably due to the fact that 3DG10 does not have sufficient oxygen functional groups to anchor the metal particles, while 3DG30 with a smaller specific surface area and more oxygen functional groups, the overloading of metals is facile to agglomeration. The elements content of the samples (Table S1) manifest a coincident description of this result.

Fig. 2. SEM images of (a) 3DG20 and (b) FeP/3DG20, (c, d) TEM images of FeP/3DG20 and the HRTEM image of FeP (inset of fig. d), (e)-(i) EDS mapping images of (c).

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Fig. 3. XRD patterns of FeP/3DG20 and Fe3O4/3DG20.

Fig. 2d (an enlargement of Fig. 2c) and its inset (HRTEM) clearly display the microstructure of FeP/3DG20; the primary crystal plane of FeP(011) can be also observed. EDS spectra are further taken to characterize the elemental mapping of FeP/3DG20 (Figs. 2e-2i). The analysis results indicate a homogeneous distribution of Fe, P and N elements over the 3DG20 support. As can be seen from Fig. 3, the XRD pattern of Fe3O4/3DG20 hybrid displays broad and low

intensity peaks assigning to 30.36 (220), 35.76 (311), 43.47 (400), 57.51 (511) and 63.17 (440) planes of Fe3O4 (PDF#75-0449). After phosphatation, by thermal decomposition of NaH2PO2$H2O, all the identified peaks with phase conversion are assigned to 32.75 (011), 37.15 (111), 46.30 (121), 46.96 (220), 48.31 (211) and 56.08 (221) planes of FeP (PDF#71-2262). The sharp peaks at 26 are ascribed to graphite (002) derived from 3DG20 [29]. The broadening diffraction peaks illustrate that the metal NPs are small in size, which is coincident with TEM images (Figs. 2c & 2d) [12]. Detailed elemental compositions and valence states of FeP/ 3DG20 are determined by XPS. The similar wide survey spectra of FeP/3DG10, FeP/3DG20 and FeP/3DG30, shown in Fig. S6, indicate that the components of the three samples are uniform, and the nitrogen peaks, originated from the self-doped 3DG, are overlapped because of its low content. For accuracy, all the data are aligned by C1s peak at 284.8 eV. In the C1s region (Fig. 4a), five different peaks at the binding energy of 284.8, 285.3, 286.1, 287.1 and 289.2 eV can be indexed to C]C (sp2 C), CeP, CeN, CeO groups and C]O, respectively [23,25,30]. These observations of all oxygen related peaks imply that the oxygen groups are incorporated into 3DG during the pretreatment process. As shown in Fig. 4b, distinct peaks containing two oxidized Fe species of Fe3þ and Fe2þ are observed in the Fe2p spectrum, whose binding energies are 728.5 and 714.4, 725.4 and 711.7 eV, respectively. The peaks located at 720.5 and 707.6 eV are assigned to the Fe2p1/2 and Fe2p3/2 in FeP [31]. Similarly in the high-resolution P2p spectrum (Fig. 4c), the doublet peaks at 130.7 and 129.8 eV reflect the P2p1/2 and P2p3/2 in FeP. While the peaks fitted to two pronounced peaks at 134.5 and 133.7 eV correspond to the PeO and PeC species [30,31]. The presence of oxidized Fe and P species are

Fig. 4. XPS spectra of (a) C1s, (b) Fe2p, (c) P2p and (d) N1s for FeP/3DG20.

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unavoidable arising from the nature of nanosized materials, which are easily oxidized on the surface as they are exposed in air [32]. Meanwhile, three broad peaks can be found in the high-resolution N1s spectrum (Fig. 4d) at 401.1, 400.0 and 398.9 eV, corresponding to graphitic N, pyrrolic N and pyridinic N, respectively [26]. The results above could verify the formation of FeP and the existence of heteroatoms (N and P) in the substrate. 3.2. Electrochemical characterization The HER activities of the as-prepared electrocatalysts were evaluated in N2-saturated 0.5 M H2SO4 using a typical threeelectrode electrochemical system. For comparison, the commercial Pt/C was also assessed at the same conditions. As shown in Fig. 5a, FeP/3DG20 exhibits an optimal HER catalytic activity excepting the one obtained over the commercial Pt/C. Specifically, the FeP/3DG20 electrode demands small overpotentials of 113.2 and 211.4 mV to deliver cathodic current densities of
10 and
200 mA cm
2 respectively, which is remarkably outperformed the FeP/C20 (h10 ¼ 174.7 mV) and the P-3DG20 (h10 ¼ 336.0 mV). In addition, the HER kinetic parameters are calculated by fitting the linear part to Tafel slopes. As can be seen from Fig. 5b, the Tafel slope for FeP/3DG20 is approximately 65.84 mV dec
1, much lower than that of FeP/C20 (87.21 mV dec
1) and of P-3DG20 (192.94 mV dec
1), demonstrating that the HER reaction pathway fits the VolmereHeyrovský mechanism [21]. It should be noted that the Tafel slope value of Pt/C (20.35 mV dec
1) is smaller than the theoretical result (29 mV dec
1), probably due to the reduced mass transport resistance of electrolyte owed to RDE operation at a higher rotating speed (1,600 rpm) [3]. The above

247

findings imply that by replacing commercial carbon powder with 3DG20, as a substrate for FeP, a higher electrocatalytic HER activity is achieved. The relatively higher specific surface area of 3DG20 than XC-72 carbon (256.5 m2 g
1) and higher graphitization degree, could be the origin of the outstanding catalytic activity exhibited by FeP/3DG20 [33]. In addition, supports with different treatments were evaluated for HER. In contrast, the HER activities of FeP/3DG (h10 ¼ 171.1 mV), FeP/3DG10 (h10 ¼ 138.2 mV) and FeP/3DG30 (h10 ¼ 143.5 mV) are inferior to that of FeP/3DG20 (Fig. S1a). Similarly, Fig. S1b demonstrates that the Tafel slopes of FeP/3DG, FeP/3DG10 and FeP/3DG30 are 103.31, 73.54 and 71.22 mV dec
1, which are higher than FeP/ 3DG20 (65.84 mV dec
1). The reasons could be as follows: 3DG and 3DG10 with a slight degree of oxidation result in a low amount of oxygen functional groups, which increases the difficulty (negative effect) of FeP NPs anchoring on these substrates. Combined with the analysis of pore-size distribution (Fig. 1d), the substrates with a lower amount of mesopores are unfavorable to the mass transfer. While, the morphology of the over-treated 3DG30 is violently destroyed (due to high temperature reached), leading to the decrease of specific surface area. Besides, excessive oxygen groups give rise to the overloading of FeP that easily results in the aggregation of active particles, thus, affecting HER activity. The element contents of the samples listed in Table S1, are in agreement with the above descriptions. To provide a better insight into the kinetics of the HER process at the electrode/electrolyte interface, Fig. 5c and Fig. S2 show the Nyquist plots of the experimental and fitted data for both electrocatalysts at
0.15 V, employing the one-time constant model equivalent circuit (inset of Fig. 5c). The high-frequency intersection

Fig. 5. Electrochemical measurements of the as-prepared samples for hydrogen evolution in 0.5 M H2SO4 aqueous solution. (a) Polarization curves, (b) Tafel plots, (c) the Nyquist plots of experimental (symbols) and fitted data (solid lines) in frequency values from 105 to 10
1 Hz at
0.15 V by the equivalent circuit (inset c) and (d) chronopotentiometry curve.

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with the X-axis refers to the uncompensated ohmic electrolyte resistances (Rs), which mainly arise from the electrolyte and testing system contacts. The values used for iR-correction are 4.88, 5.36, 6.43, 5.11, 5.36 and 5.51 U, corresponding to Pt/C, FeP/3DG, FeP/ 3DG10, FeP/3DG20, FeP/3DG30 and FeP/C20, respectively. The charge transfer resistance Rct reflects the intrinsic activity of electrocatalysts and its lower value corresponds to faster kinetics, which can be acquired by the semicircle in the low-frequency zone of EIS [10]. Compared with the Rct values of Pt/C (9.43 U), FeP/3DG (17.42 U), FeP/3DG10 (16.36 U), FeP/3DG30 (18.60 U) and FeP/C20 (38.80 U), the smaller Rct (11.50 U) for FeP/3DG20 among all the non-precious hybrid samples suggests its faster electrons and mass transfer [34,35]. This may be associated with the following reasons: i) 3DG20 with abundant mesoporous structure has high interface contact area, which can shorten the diffusion path of ions and accelerate the mass transfer [31]; ii) the stronger interaction in the hybrid between the nanosized FeP and 3DG20 can adjust the intrinsic electronic properties; iii) the stereotaxically constructed graphene with N, P co-doping can further increase the active sites and enhance the electron transfer capacity. The long-term stability is another key parameter for HER, therefore, the durability of FeP/3DG20 was tested at a fixed current density of
10 mA cm
2 for more than 25 h. As shown in Fig. 5d, the chronopotentiometry curve exhibits a steeper decrease initially, which then (after 10 h of operation) remains relatively constant. In addition, Fig. S7 reveals the negatively change in polarization curves and Rct of FeP/3DG20 before and after 25 h durability tests, indicating a partial decrease in HER activity [36]. After the accelerated stability tests, FeP/3DG20 is further characterized by TEM and XPS to understand the change of active species [37]. It can be distinguished a certain aggregation of FeP (as seen in Fig. S5). XPS characterization (Fig. S8) illustrated the elemental peaks that is similar to Fig. 4. As listed in Table S2, the atomic percentage of Fe and P are lower after stability tests, which seems an inevitable corrosion when surrounded by acid environment. Moreover, the peak area ratio between FeP and oxidized Fe increased significantly, indicating that FeP dissolution is relatively slight, means a good stability in acid solutions. Furthermore, we compared this work with other reported TMPs-based materials as HER electrocatalysts in acid media (Fig. 6) [38e46]. Clearly, the FeP/3DG20 obtained in this work shows a better HER activity, which may be ascribed to the following reasons: i) the support with suitable pretreatment possesses high

specific surface area and abundant oxygen functional groups, which is beneficial to enable the dispersion and interactions between the active materials and substances; ii) the stereotaxically constructed graphene with well-developed mesoporous structure magnifies the reactive contact area, mitigates the diffusion restriction of reactants and facilitates the electron and mass transfer process [14]; iii) the doping of N and P in graphene tunes the electronic structure of carbon backbone and raises the defect density, which could serve as Hþ adsorption sites [47]. 4. Conclusions This work successfully prepared a novel composite of FeP grafted onto a modified 3DG, and the effect of substrates with different microwave-assisted pretreatments is discussed. The 3DG20 with large specific surface area, high porosity and strong conductivity is essential for strengthening the interactions between the metal particles and support materials, exposing more active sites, promoting electrons and mass transfer process. The as-prepared FeP/ 3DG20 electrocatalyst displays excellent HER activity requiring small overpotentials of
113.2 and
211.4 mV to deliver current densities of
10 and
200 mA cm
2, which is superior to the recently reported TMPs-based electrocatalysts in 0.5 M H2SO4 aqueous solution. This study paves the way of designing suitable substrates for preparing composite materials, which is highly desirable for enhancing the catalytic activity towards the reaction of hydrogen evolution. Acknowledgement This work was supported by the Natural Science Foundation of China (21872040), the National Basic Research Program of China (2017YFB0103000), the Natural Science Foundation of Guangxi (2016GXNSFCB380002), the Nanning Science and Technology Project (20171107), the Hundred Talents Program of Guangxi Universities, and the Key Laboratory of Disaster Prevention and Structural Safety of China Ministry of Education. Prof. Tsiakaras thankfully acknowledges co-financing of European Union and Greek National Funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH e CREATE e INNOVATE (Project code: T1EDK-02442). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.05.153. References

Fig. 6. Comparison of HER activity for the as-prepared samples with TMPs-based electrocatalysts in 0.5 M H2SO4 aqueous solution.

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