- Email: [email protected]
Engineering inner-porous cobalt phosphide nanowire based on controllable phosphating for efficient hydrogen evolution in both acidic and alkaline conditions
Applied Surface Science 481 (2019) 1524–1531
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Engineering inner-porous cobalt phosphide nanowire based on controllable phosphating for efficient hydrogen evolution in both acidic and alkaline conditions ⁎
T
⁎
Shucong Zhang, Ting Xiong, Xiaofan Tang, Qiuyi Ma, Feilong Hu , Yan Mi
Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University for Nationalities, Nanning, 530006, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inner-porous Cobalt phosphide Hydrogen evolution Electrocatalytic
Cobalt phosphide has been extensively studied as non-precious metal-based robust electrocatalyst for hydrogen evolution reaction. Innovation breakthrough is still highly desired to enhance its catalytic efficiency. In this work, through rational engineering strategy, a controlled inner-porous CoP nanowire array, which is vertical to the carbon fiber (ip-CoP/CF) was in-situ topotactically synthesized. The resulting ip-CoP/CF demonstrates improved hydrogen evolving efficiency in both acidic and alkaline conditions. At a current density of 10 mA∙cm−2, which with small Tafel slopes of 47 mV·dec−1 and 93 mV·dec−1 in acidic and alkaline solution, respectively. Such engineering strategy opens a diverting route to construct controllable inner-porous materials for high efficient electrocatalysts.
1. Introduction While meeting the increasing demands on environmental purification and energy supply, great efforts have been devoted to pursue and utilize sustainable alternative clean energy conversion technologies [1–5]. As a clean, high energy density and sustainable energy carrier, hydrogen possesses excellent metrics as a fuel, made it to be a great promising candidate in the future [6,7]. Producing hydrogen via water electrolysis is an appealing approach [5,7–12]. However, significant challenge remains if achieving high efficiency of electrolytic hydrogen production on electrolyze. At present, the state-of-art active catalysts are Pt based, however, suffering from the high cost and scarcity, it is indispensable to develop inexpensive and earth-abundant alternatives for the possibility of commercialization. In the past decade, as transition-metal phosphides (TMPs) have been emerged to be promising non-noble metal catalysts for hydrogen evolution reaction (HER), tremendous efforts have been dedicated to exploiting efficient TMPs related HER catalysts [10,13–19]. In particular, cobalt phosphide has acquired intensive attention due to its characteristic electronic structure endowed good catalytic performance and fine stability in both acidic and alkaline media [20–34]. In spite of this, innovation breakthrough is still highly desired to further improve its HER performance. Based on the theoretical calculation, it is aware of that the phosphorous in Co&+—P&- moieties is the active site for HER process. The morphologies of TMPs greatly impact their electronic ⁎
structures [9,32,35,36]. Therefore, it is highly possible to achieve excellent catalytic activity though rational morphology design. Among the developed nanostructures, the more interfaces exposure of nanoporous structure could be beneficial for fast interfacial charge transfer to decrease the reaction energy barrier [37–41]. Meanwhile, the poly- and low-crystallinity of most nanoporous catalysts possess disorder and grain boundary, which may result in abundant active sites and miraculous charge transport ability [42,43]. However, the overall porous structure of cobalt phosphide might suffer structure-collapse during electrochemical process, which is hardly satisfy the needs of industry. Therefore, it is attractive and meaningful to construct a morphology of cobalt phosphide with the benefit of nanoporous structure to achieve excellent electrocatalytic activity in multi-fields, while maintaining its morphology. Herein, a controlled inner-porous CoP nanowire array, which is directly grow on the carbon fiber was in-situ topotactically synthesized by rational engineering strategies: a convenient hydrothermal process combining with a controlled phosphorization, and a following acidtreating process. The as-constructed ip-CoP/CF demonstrates improved hydrogen evolving efficiency in both acidic and alkaline conditions. At a current density of 10 mA∙cm−2, which with small Tafel slopes of 47 mV·dec−1 and 93 mV·dec−1 in acidic and alkaline solution, respectively. Meanwhile, it also reveals a brilliant oxygen evolution property under alkaline condition. Such engineering strategy opens a diverting route to construct controllable inner-porous materials for high efficient
Corresponding authors. E-mail addresses: hfl[email protected] (F. Hu), [email protected] (Y. Mi).
https://doi.org/10.1016/j.apsusc.2019.03.250 Received 21 December 2018; Received in revised form 16 February 2019; Accepted 23 March 2019 Available online 24 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
2.5. Electrochemical measurements
electrocatalysts.
Electrochemical experiments of the as-prepared samples were performed on a CHI 660E electrochemical workstation (CH Instrument, Inc.) with a three-electrode. A saturated calomel electrode (SCE) and a carbon rod were used as the reference and counter electrodes, respectively. The as-prepared samples directly used as the working electrode. All the electrochemical measurements were proceeded at room temperature. The HER performance was studied in 0.5 M H2SO4 and 1 M KOH electrolytes. The OER performance was operated at 1 M KOH electrolyte. Before the start of tests, the N2 gas was used to purge the electrolyte for 30 min to remove the dissolved oxygen. The catalytic performances of samples were investigated by Linear sweep voltammetry (LSV) measurements which performed at a scan rate of 1 mV·s−1. The stability of samples was tested at a scan rate of 100 mV·s−1. The Tafel calculations under acidic condition for HER [14], under alkaline condition for HER [20] and OER [43] were based on previous works. The electrochemical impedance spectroscopy (EIS) was performed using a VMP2 multi-potentiostat (Princeton Applied Research, USA) controlled by EC-Lab (V9.24) software (Bio-Logic SA) with the same conditions at different overpotentials ranging from 105 Hz to 10−2 Hz with an AC voltage of 10 mV. The RHE potential reported in our study was calculated as ERHE = ESCE + 0.244 V. The iR compensation was applied for all data in this work.
2. Experimental 2.1. Materials Cobalt(II) nitrate hexahydrate, urea, ammonium fluoride, and sodium hypophosphite were purchased from Aladdin Industrial Co. Commercial carbon fiber (CF) was purchased from DongLi (Japan). The commercial Pt/C (platinum, nominally 20 wt% on carbon black) and RuO2 catalysts were purchased from Alfa Aesar Co. All of the materials were of analytical regent grade and directly used without additional purification. Deionized water was used during the whole experiment.
2.2. Synthesis of Co-precursor/CF Co-precursor/CF was prepared via a typical process: Co (NO3)2•6H2O (4 mmol), urea (5 mmol), and NH4F (2 mmol) were dissolved in 20 mL of deionized water by ultrasonic process forming a aqueous solution. A piece of CF (1 cm × 2 cm) which was separately rinsed by hydrochloric acid, acetone, ethanol and deionized water under ultrasonic treatment for 15 min was put into a Teflon-lined stainless autoclave (25 mL volume). Then the prepared aqueous solution was also transferred in. After that, the autoclave was maintained at 120 °C for 12 h. Subsequently, the autoclave was cooled down to room temperature naturally, and then the Co-precursor on CF was taken out and washed with ethanol, deionized water to remove residues, and finally dried at 60 °C for 10 h.
3. Results and discussion The generation of ip-CoP/CF was schematically illustrated in Fig. 1a. In brief, the inner-porous nanowire structure of CoP on CF was made by partial phosphorization of Co-precursor nanowire on carbon fiber skeleton to form CoP coated Co-precursor on CF, followed by a hot-acid etching of the non-phosphate Co-precursor. Firstly, the crystal structures of the as-controlled products were investigated by XRD. As can be seen in Fig. s1(a), three broad peaks at around 26°, 43° and 54° belong to the carbon, which were consistent with those obtained in the literature [21]. The other peaks located at 2θ values of 31.6°, 36.3°, 46.2° and 56.0° are matched well with the (011), (111), (112) and (020) crystal planes of orthorhombic CoP (JCPDS card No. 29–0497), respectively [21,22]. It is demonstrated that Co-precursor is successfully converted to CoP after the phosphorization and the subsequent hot-acid treatment. Meanwhile, the XRD patterns of the Co-precursor and which after phosphorization were also studied. As shown in Fig. s1b, the peaks at around 33.8°, 39.5°, 59.8° can be assigned to the (221), (231) and (412) crystal planes of Co(CO3)0.5(OH)•0.11H2O (JCPDS card No. 48–0083) [22]. After the annealing process, most of the precursor was converted to CoP, there have same signals belong to CoeO (Fig. s1c). That means the precursor was partially transferred into CoP. The appearances of the as-prepared samples were investigated by FE-SEM. Fig. 1b and c depict typical FE-SEM images of the obtained Co (CO3)0.5(OH)•0.11H2O/CF at low and high magnification. As can be seen, they show that the entire surface of the carbon fiber was uniformly and densely covered by vertically grown of Co(CO3)0.5(OH) •0.11H2O nanowires. With the controlled phosphorization process, the as-prepared Co-precursor was partially converted to CoP (Fig. 1d, Fig. s2). After the further hot-acid treatment process, obtained the pure CoP on CF (Fig. 1e). The nanowire appearance still maintained. To further study its morphology and structure characteristics, TEM investigation was performed. The TEM image in Fig. 1f reveals the inner-porous structure with many cavities inside the nanowire. They small cavities also could be found in the high-resolution TEM in Fig. 1g (some of the them are marked with red circles). And the BET analysis was used to further identify the inner-porous structure. The nitrogen adsorption/ desorption isotherm (Fig. s3) of as-prepared samples suggest the larger BET surface area of the ip-CoP to that of the CoP nanowires. And the BJH pore-size distribution curve (inset in Fig. s3) shows a peak at around 2.1 nm, confirming the nanoporous nature of ip-CoP nanowires.
2.3. Synthesis of innerporous CoP/CF Innerporous CoP/CF (ip-CoP/CF) was prepared by a controlled thermal phosphorization and following acid treatment process. In brief, sodium hypophosphite was located at the upstream side of a tube furnace and a piece of synthesized Co-precursor/CF was placed at the downstream side. Then the system was firstly calcined at 300 °C for 2 h with a heating speed of 2 °C min−1 under N2 atmosphere, then cooled naturally to ambient temperature with N2. Afterwards, the as-prepared CoP/CF was immersed into 0.5 M H2SO4, and maintained at 60 °C, followed by washing with ethanol and then drying at 60 °C. The ip-CoP/ CF was obtained. Meanwhile, the influence parameters like phosphorization time, phosphorization temperature, and acid-treatment time for the catalytic performance were investigated.
2.4. Characterization The field emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 55 Sapphire), which equipped with energy-dispersive X-ray spectrometry (EDS) with an acceleration voltage of 30 kV was used to obtain the morphology and element distribution information of the asprepared samples. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore-size distribution of samples were detected by Tristar3000 via N2 physisorption at 77 K. Thermo-gravimetric analysis (TGA, Mettler-Toledo TGA1) was characterized with a heating rate of 10 °C min−1 from 200 to 900 °C in air. Transmission electron microscopy (TEM) and the relative high resolution transmission electron microscopy (HRTEM) images of the samples were achieved at the FEI Tecnai G2 F20 S-TWIN under an accelerating voltage of 200 kV. The structure information of the as-prepared samples were characterized by a X-ray powder diffraction (XRD, Rigaku MiniFlex 600) equipped with Cu Ka radiation (λ = 0.15406 nm). And the chemical states of the samples were measured by a X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS UltraDLD. 1525
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 1. (a) Schematic illustration of the synthesis procedure of ip-CoP/CF. (b-c) SEM images of Co-precusor/CF at different magnifications. (d) SEM image of CoP/CF. (e) SEM, (f) TEM and (g) HRTEM images of ip-CoP/CF; and (h-i) EDS elemental mapping results of the as-prepared ip-CoP/CF.
Fig. 2. XPS spectra of (a) Co2p and (b) P 2p for the as-prepared ip-CoP/CF sample.
The surface chemical environmental of the as-prepared sample was further investigated with X-ray photoelectron spectroscopy (XPS). Fig. s6 shows the signals of Co, P, O and C, being in accordance with elemental mapping results (Fig. 1i). The high-resolution Co 2p spectrum (Fig. 2a) shows that the binding energy of Co 2p3/2 at 779.1 eV is positively shifted [17,21]. From the high-resolution P 2p spectrum (Fig. 2b), it could see the binding energies of P 2p3/2 and 2p1/2 at 129.8 and 130.6 eV, respectively. The negatively shifted binding energy of P and positively shifted binding energy of Co suggesting that the transfer of electron density from Co to P in ip-CoP/CF sample [17,21]. The peaks at 781.7 and 134.6 eV corresponding to the oxidized states of Co and P, which should resulting from superficial oxidation of CoP because of air contact [17,21].
The phosphorization time is of prime importance for the construction of ip-CoP/CF. Too short phosphating time (15 min, Fig. s4a) or too long phosphating time (6 h, Fig. s4b) would lead to collapsing or solid of the Co-precursor nanowire after their further hot-acid etching, respectively, without the formation of inner porous structure. TGA experiment (Fig. s5) shows that ip-CoP/CF has a CoP content of 54 wt% [20]. The HRTEM image of ip-CoP/CF (Fig. 1g) shows a good crystallinity characteristic with lattice fringe spacing of 0.283 nm, corresponding to the (011) plane of orthorhombic CoP [26]. The SAED pattern (inset in Fig. 1 g) shows the polycrystalline structure of the product. And the further elemental-mapping images (Fig. 1h–i) show that Co and P elements are uniformly dispersed along the CoP nanowires on carbon fiber.
1526
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 3. Electrocatalytic hydrogen evolution reaction in 0.5 M H2SO4. (a) The HER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The time-dependant curve for ip-CoP/CF under static overpotential of 75 mV for 24 h. (e) Nyquist plots of different catalysts. (f) A plot of (Ja-Jc) against scan rate.
(Fig. s7). It is demonstrating the optimal phosphating time and temperature, and acid-treating time parameters are 2 h, 300 °C and 6 h for ip-CoP/CF, respectively. The Tafel plots of the detected samples were shown in Fig. 3b. As can be seen, the Tafel slope of Pt/C is only 28 mV∙dec−1, which agrees with the previously reported value [26]. The constructed ip-CoP/CF has a Tafel slope of 47 mV∙dec−1, which is smaller than that of the comparison CoP/CF (55 mV∙dec−1). As well known, the Tafel slopes of 116, 38 and 29 mV∙dec−1 are corresponding to Volmer, Heyrovsky and Tafel steps, respectively. The results revealing that the HER for the ip-CoP/CF proceeds via Volmer-Heyrovsky mechanism [44], while the Volmer reaction as the rate-determination step. The HER stability of the ipCoP/CF in acidic media was also investigated. As presented in Fig. 3c, after 2000 cycles of continuous cyclic voltammetry scanning with rate of 100 mV∙s−1, there has no obvious decay of HER catalytic activity, indicating its superior stability. Meanwhile, the long-term stability test of the ip-CoP/CF for HER in 0.5 M H2SO4 solution at a constant voltage
The electrocatalytic activities of the samples were intensively investigated. Firstly, the catalytic HER activity of samples in 0.5 M H2SO4 electrolyte was evaluated using a typical three-electrode system. The commercial Pt/C and the blank carbon fiber were also examined for comparison. As the polarization curves shown in Fig. 3a, the commercial Pt/C shows superior HER performance. But the pristine CF shows almost zero current density in the scanning range from 0 to −400 mV, demonstrating no HER catalytic activity. The comparison CoP/CF presents that it can achieve a current densities of 10 mA∙cm2 and 50 mA∙cm2 at an overpotential of 85 mV and 138 mV, respectively. In comparison, the constructed ip-CoP/CF shows an overpotential of only 68 mV and 101 mV to achieve current densities of 10 mA∙cm2 and 50 mA∙cm2, respectively. The results suggest that the constructed ipCoP/CF catalyst acts as a highly efficient cathode for the HER process in acidic condition. In addition, the samples obtained by controlling the reaction time of phosphating, the temperature of phosphating and the reaction time of acid-treating show inferior HER catalytic activities 1527
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 4. Electrocatalytic hydrogen evolution reaction in 1 M KOH. (a) The HER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The time-dependant curve for ip-CoP/CF under static overpotential of 90 mV for 24 h. (e) Nyquist plots of different catalysts. (f) A plot of (Ja-Jc) against scan rate.
capacitances of electrodes were calculated by the average values of anodic and cathodic current at 0.094 V for each of CV curves, and further plotted as a function of different scan rates according to the equation of: (Ja - Jc)/2 = Cdlv [21]. It is clear that the obtained electrochemical surface area of ip-CoP/CF (136 mF∙cm−2) is larger than that of CoP/CF (87 mF∙cm−2)(Fig. 3f). Thus, the excellent HER catalytic activity of ip-CoP/CF might be explained by the good electron transfer performance and the large electrocatalyst/electrolyte interface. The above mentioned results show the excellent HER catalytic activity of the as-constructed ip-CoP/CF under acidic condition. However, the alkaline environmental is more widely exist in industrial application. Therefore, we further determined the HER catalytic performance of the as-prepared samples under alkaline condition. As shown in Fig. 4a, even in 1 M KOH electrolyte, the Pt/C electrode still has the lowest onset potential, as well as the lowest overpotential to achieve current densities of 10 mA∙cm2. Similarly, the as-constructed ip-CoP/CF
of 75 mV versus RHE for 24 h presented in Fig. 3d, and the no change of appearance (Fig. s8a-b) further indicate its superior HER catalytic stability. In addition, in order to determine the reason of activity differences of the as-present samples, the electron transfer resistance of samples were checked by the electrochemical impendance Nyquist plots. As depicted in Fig. 3e, comparing with CF and CoP/CF, the ipCoP/CF shows the lowest semicircle under the same condition. This might be attribute to the inner-pore structure of CoP on carbon fiber, which decreases the charge transfer resistance at the electrode/electrolyte interface. Meanwhile, the electrochemical surface area of CoP/ CF and ip-CoP/CF electrodes were further examined by evaluating the double-layer capacitance. A cyclic voltammogram of samples were performed at scan rates of 10, 20, 40, 60, 80 and 100 mV∙s−1 in a potential regime (0.04–0.14 V vs RHE). As shown in Fig. s9a–b, there have no observable faradic reaction occurs, suggesting the only capacitive current was measured in the scan window. The double-layer
1528
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 5. Electrocatalytic oxygen evolution reaction in 1 M KOH. (a) The OER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The inset is the time-dependant curve for ip-CoP/CF under static overpotential of 1.54 V for 24 h.
CF. They only need an overpotential of 300 mV to drive the current density of 10 mA∙cm−2. Although from the typical LSV curves seem like the CoP/CF has a similar OER activity to that of ip-CoP/CF, the Tafel plots in Fig. 5b shows it has a higher Tafel slope (109 mV∙dec−1) to the ip-CoP/CF (91 mV∙dec−1), indicating the better OER activity of the ipCoP/CF electrode in alkaline media. The much higher Tafel slope of RuO2 (144 mV∙dec−1) further reveals the superior OER catalytic activity of the as-constructed ip-CoP/CF electrode. The LSV curves of the ip-CoP/CF electrode before and after 2000 cycles of scanning were used to check its stability. As depicted in Fig. 5c, it has an inconspicuous decay of OER catalytic activity. And its durability was further tested by fixed at an overpotential of 300 mV for 24 h (Fig. 5d), which demonstrates that the ip-CoP/CF electrode held its OER catalytic activity over a long period. It was further confirmed by the no noticeable change in appearance (Fig. s8e–f). All of the results verify that the ip-CoP/CF is a brilliant OER catalyst. Herein, it is clear that the ip-CoP/CF is an advanced electrocatalyst for both of the HER and the OER in alkaline electrolyte. In order to evaluate its electrochemical water splitting property, a two-electrode system which use the ip-CoP/CF as both the anode and the cathode electrodes was constructed. As shown in Fig. s10, the current density of 10 mA cm−2 can be achieved at a cell voltage as low as 1.62 V (Fig. s10a), while the polarization curve recorded at 1.0 mV s−1. Furthermore, at the setting voltage of 1.62 V, the long-term testing (Fig. s10b) shows only a slight deactivation after 50 h, indicating the good stability of electrodes. And the evolution of H2 and O2 gas bubbles could be clearly observed (inset in Fig. s10b). According to the above mentioned results, we inferred that the excellent electrocatalytic activity of the as-constructed ip-CoP/CF is might owe to the following reasons: (1) The inner-porous structure makes some parts to be a thin shell to contact with the electrolyte, which enables the fast charge transfer; (2) the crisscrossing inside of the innerporous CoP enables a larger electrochemical active area; and (3) the
also demonstrates a brilliant HER catalytic activity. It has an overpotential of 76 and 135 mV to reach current densities of 10 and 50 mA∙cm2, respectively, much lower than that of CoP/CF (162 and 223 mV to reach current densities of 10 and 50 mA∙cm2, separately). Meanwhile, the Tafel slope of the as-constructed ip-CoP/CF is 93 mV∙dec−1. Although it is higher than that of Pt/C (72 mV∙dec−1) electrode, it is lower than that of CoP/CF (110 mV∙dec−1) electrode in alkaline condition. Moreover, the almost no change of HER catalytic activity before and after 2000 cycles of cyclic voltammetry scanning with rate of 100 mV∙s−1 (Fig. 4c), the maintained HER catalytic activity under static overpotential of 90 mV for 24 h (Fig. 4d), and the almost unchanged morphology (Fig. s8c–d) identify the superior stability of the as-constructed ip-CoP/CF electrode under alkaline condition. The EIS results of samples in Fig. 4e show the smaller diameter of the Nyquist plot of the ip-CoP/CF, revealing it possesses faster charge transfer kinetics and lower contact resistance in comparison with the CoP/CF. Furthermore, the electrochemical surface area of electrodes under alkaline condition, which evaluated by the double-layer capacitance shows a larger value of ip-CoP/CF (103 mF∙cm−2) to CoP/CF (81 mF∙cm−2). Above all, no matter in acidic nor alkaline electrolytes, the as-constructed ip-CoP/CF electrode shows superior HER catalytic activity and stability, and it is better than that of the CoP/CF electrode. Accordingly, it might deduce that the ip-CoP/CF possessing great promise for HER in industrial application. For electrochemical water splitting catalyst, it is more challenging to have an efficient OER catalytic activity. Because the breaking of the OeH band, and in the meantime, the formation of the O]O band needs four-electron transfer process. All of them are complex and sluggish kinetics process [43,45]. Thus, besides the HER performance, the OER performance of as-prepared samples in 1 M KOH were further studied. The typical LSV curves of pristine CF, CoP/CF, ip-CoP/CF and RuO2 are presented in Fig. 5a. As can be found, the ip-CoP/CF and CoP/CF show much more better OER catalytic activity than that of RuO2 and pristine 1529
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
direct growth of inner-porous CoP on carbon fiber guarantees its good electrical conductivity, leading to the high charge transfer.
633–644, https://doi.org/10.1016/j.scib.2016.12.011. [14] F. Lai, D. Yong, X. Ning, B. Pan, Y.-E. Miao, T. Liu, Bionanofiber assisted decoration of few-layered MoSe2 nanosheets on 3D conductive networks for efficient hydrogen evolution, Small. 13 (2017) 1602866, , https://doi.org/10.1002/smll.201602866. [15] X. Zhu, L. Mo, Y. Wu, F. Lai, X. Han, X.Y. Ling, T. Liu, Self-supported MoS2@NHCF fiber-in-tube composites with tunable voids for efficient hydrogen evolution reaction, Compos. Commun. 9 (2018) 86–91, https://doi.org/10.1016/j.coco.2018.06. 010. [16] X. Xiao, L. Tao, M. Li, X. Lv, D. Huang, X. Jiang, H. Pan, M. Wang, Y. Shen, Electronic modulation of transition metal phosphide via doping as efficient and pHuniversal electrocatalysts for hydrogen evolution reaction, Chem. Sci. 9 (2018) 1970–1975, https://doi.org/10.1039/c7sc04849a. [17] Y. Cheng, F. Liao, W. Shen, L. Liu, B. Jiang, Y. Li, M. Shao, Carbon cloth supported cobalt phosphide as multifunctional catalysts for efficient overall water splitting and zinc-air batteries, Nanoscale 9 (2017) 18977–18982, https://doi.org/10.1039/ c7nr06859j. [18] W. Li, S. Zhang, Q. Fan, F. Zhang, S. Xu, Hierarchically scaffolded CoP/CoP2 nanoparticles: controllable synthesis and their application as a well-matched bifunctional electrocatalyst for overall water splitting, Nanoscale 9 (2017) 5677–5685, https://doi.org/10.1039/c7nr01017f. [19] Y. Lin, J. Zhang, Y. Pan, Y. Liu, Nickel phosphide nanoparticles decorated nitrogen and phosphorus co-doped porous carbon as efficient hybrid catalyst for hydrogen evolution, Appl. Surf. Sci. 422 (2017) 828–837, https://doi.org/10.1016/j.apsusc. 2017.06.102. [20] H. Lu, W. Fan, Y. Huang, T. Liu, Lotus root-like porous carbon nanofiber anchored with CoP nanoparticles as all-pH hydrogen evolution electrocatalysts, Nano Res. 11 (2018) 1274–1284, https://doi.org/10.1007/s12274-017-1741-x. [21] S.H. Yu, D.H.C. Chua, Toward high-performance and low-cost hydrogen evolution reaction electrocatalysts: nanostructuring cobalt phosphide (CoP) particles on carbon fiber paper, ACS Appl. Mater. Interfaces 10 (2018) 14777–14785, https:// doi.org/10.1021/acsami.8b02755. [22] W. Yuan, X. Wang, X. Zhong, C.M. Li, CoP nanoparticles in situ grown in threedimensional hierarchical nanoporous carbons as superior electrocatalysts for hydrogen evolution, ACS Appl. Mater. Interfaces 8 (2016) 20720–20729, https://doi. org/10.1021/acsami.6b05304. [23] Z.-S. Cai, Y. Shi, S.-S. Bao, Y. Shen, X.-H. Xia, L.-M. Zheng, Bioinspired engineering of cobalt-phosphonate nanosheets for robust hydrogen evolution reaction, ACS Catal. 8 (2018) 3895–3902, https://doi.org/10.1021/acscatal.7b04276. [24] Y. Zhang, L. Gao, E.J.M. Hensen, J.P. Hofmann, Evaluating the stability of Co2P electrocatalysts in the hydrogen evolution reaction for both acidic and alkaline electrolytes, ACS Energy. Lett. 3 (2018) 1360–1365, https://doi.org/10.1021/ acsenergylett.8b00514. [25] H. Wang, S. Min, Q. Wang, D. Li, G. Casillas, C. Ma, Y. Li, Z. Liu, L.J. Li, J. Yuan, M. Antonietti, T. Wu, Nitrogen-doped nanoporous carbon membranes with co/CoP janus-type nanocrystals as hydrogen evolution electrode in both acidic and alkaline environments, ACS Nano 11 (2017) 4358–4364, https://doi.org/10.1021/acsnano. 7b01946. [26] J. Tian, Q. Liu, A.M. Asiri, X. Sun, Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogen-evolving cathode over the wide range of pH 0-14, J. Am. Chem. Soc. 136 (2014) 7587–7590, https://doi.org/10.1021/ ja503372r. [27] Y. Pan, Y. Lin, Y. Chen, Y. Liu, C. Liu, Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution, J. Mater. Chem. A 4 (2016) 4745–4754, https://doi.org/10.1039/C6TA00575F. [28] J. Wang, Z. Liu, Y. Zheng, L. Cui, W. Yang, J. Liu, Recent advances in cobalt phosphide based materials for energy-related applications, J. Mater. Chem. A 5 (2017) 22913–22932, https://doi.org/10.1039/C7TA08386F. [29] T. Wu, M. Pi, D. Zhang, S. Chen, 3D structured porous CoP3 nanoneedle arrays as an efficient bifunctional electrocatalyst for the evolution reaction of hydrogen and oxygen, J. Mater. Chem. A 4 (2016) 14539–14544, https://doi.org/10.1039/ C6TA05838H. [30] D. Zhou, L. He, W. Zhu, X. Hou, K. Wang, G. Du, C. Zheng, X. Sun, A.M. Asiri, Interconnected urchin-like cobalt phosphide microspheres film for highly efficient electrochemical hydrogen evolution in both acidic and basic media, J. Mater. Chem. A 4 (2016) 10114–10117, https://doi.org/10.1039/C6TA03628G. [31] F.H. Saadi, A.I. Carim, E. Verlage, J.C. Hemminger, N.S. Lewis, M.P. Soriaga, CoP as an acid-stable active electrocatalyst for the hydrogen-evolution reaction: electrochemical synthesis, interfacial characterization and performance evaluation, J. Phys. Chem. C 118 (2014) 29294–29300, https://doi.org/10.1021/jp5054452. [32] X.-Y. Yan, S. Devaramani, J. Chen, D.-l. Shan, D.-D. Qin, Q. Ma, X.-Q. Lu, Selfsupported rectangular CoP nanosheet arrays grown on a carbon cloth as an efficient electrocatalyst for the hydrogen evolution reaction over a variety of pH values, New J. Chem. 41 (2017) 2436–2442, https://doi.org/10.1039/C6NJ03887E. [33] G. Hu, Q. Tang, D.-E. Jiang, CoP for hydrogen evolution: implications from hydrogen adsorption, Phys. Chem. Chem. Phys. 18 (2016) 23864–23871, https://doi. org/10.1039/C6CP04011J. [34] T. Wang, Y. Jiang, Y. Zhou, Y. Du, C. Wang, In situ electrodeposition of CoP nanoparticles on carbon nanomaterial doped polyphenylene sulfide flexible electrode for electrochemical hydrogen evolution, Appl. Surf. Sci. 442 (2018) 1–11, https:// doi.org/10.1016/j.apsusc.2018.02.133. [35] Z. Jin, P. Li, D. Xiao, Metallic Co2P ultrathin nanowires distinguished from CoP as robust electrocatalysts for overall water-splitting, Green Chem. 18 (2016) 1459–1464, https://doi.org/10.1039/C5GC02462E. [36] Y. Shao, X. Shi, H. Pan, Electronic, magnetic, and catalytic properties of thermodynamically stable two-dimensional transition-metal phosphides, Chem. Mater. 29 (2017) 8892–8900, https://doi.org/10.1021/acs.chemmater.7b03832.
4. Conclusion In summary, a inner-porous CoP on carbon fiber was in-situ topotactically constructed via a convenient hydrothermal combined with subsequent phosphorization and acid-treating processes. The as-constructed ip-CoP/CF gained an excellent HER catalytic activity under both of acidic and alkaline conditions, and also a brilliant OER catalytic performance in alkaline media. This work may not only features a good performance of dual-functional water splitting catalyst, but also sever as a universal route to construct various inner-porous transition metal phosphides growth on supporter. Acknowledgments The authors thank the National Natural Science Foundation of China (Grant Nos. 21761004, 21701035), Guangxi Collaborative Innovation Center of Forest Chemistry and Engineering (Grant Nos. 201605, 201608), Specific research project of Guangxi for research bases and talents19 486 (AD18126005, AD18126002), Talent Introduction Start-Up Foundation of Guangxi University for Nationalities (2016MDQD004, 2016MDQD005) and the 100 Talents Program for Introducing Overseas High-level Talents into Universities of Guangxi for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.250. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (2001) 332, https://doi.org/10.1038/35104599. [2] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. 103 (2006) 15729–15735, https://doi.org/10. 1073/pnas.0603395103. [3] J. Turner, G. Sverdrup, M.K. Mann, P.-C. Maness, B. Kroposki, M. Ghirardi, R.J. Evans, D. Blake, Renewable hydrogen production, Int. J. Energy Res. 32 (2008) 379–407, https://doi.org/10.1002/er.1372. [4] Y. Mi, L. Wen, Z. Wang, D. Cao, Y. Guo, Y. Lei, Building of anti-restack 3D BiOCl hierarchitecture by ultrathin nanosheets towards enhanced photocatalytic activity, Appl. Catal. B Environ. 176-177 (2015) 331–337, https://doi.org/10.1016/j. apcatb.2015.04.013. [5] M.S. Faber, S. Jin, Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications, Energy Environ. Sci. 7 (2014) 3519–3542, https://doi.org/10.1039/C4EE01760A. [6] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972–974, https://doi.org/10.1126/science.1103197. [7] D. Kong, J.J. Cha, H. Wang, H.R. Lee, Y. Cui, First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction, Energy Environ. Sci. 6 (2013) 3553–3558, https://doi.org/10.1039/C3EE42413H. [8] H. Zhu, L. Gu, D. Yu, Y. Sun, M. Wan, M. Zhang, L. Wang, L. Wang, W. Wu, J. Yao, M. Du, S. Guo, The marriage and integration of nanostructures with different dimensions for synergistic electrocatalysis, Energy Environ. Sci. 10 (2017) 321–330, https://doi.org/10.1039/C6EE03054H. [9] H. Zhu, G. Gao, M. Du, J. Zhou, K. Wang, W. Wu, X. Chen, Y. Li, P. Ma, W. Dong, F. Duan, M. Chen, G. Wu, J. Wu, H. Yang, S. Guo, Atomic-scale core/shell structure engineering induces precise tensile strain to boost hydrogen evolution catalysis, Adv. Mater. 30 (2018) 1707301, , https://doi.org/10.1002/adma.201707301. [10] M.-I. Jamesh, X.M. Sun, Recent progress on earth abundant electrocatalysts for hydrogen evolution reaction (HER) in alkaline medium to achieve efficient water splitting – a review, J. Energy Chem. 34 (2019) 111–160, https://doi.org/10.1016/ j.jechem.2018.09.016. [11] Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction, Chem. Soc. Rev. 45 (2016) 1529–1541, https://doi.org/10.1039/C5CS00434A. [12] I.K. Mishra, H. Zhou, J. Sun, F. Qin, K. Dahal, J. Bao, S. Chen, Z. Ren, Hierarchical CoP/Ni5P4/CoP microsheet arrays as a robust pH-universal electrocatalyst for efficient hydrogen generation, Energy Environ. Sci. 11 (2018) 2246–2252, https:// doi.org/10.1039/C8EE01270A. [13] J. Su, J. Zhou, L. Wang, C. Liu, Y. Chen, Synthesis and application of transition metal phosphides as electrocatalyst for water splitting, Sci. Bull. 62 (2017)
1530
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
[37] Y. Zeng, Y. Wang, G. Huang, C. Chen, L. Huang, R. Chen, S. Wang, Porous CoP nanosheets converted from layered double hydroxides with superior electrochemical activity for hydrogen evolution reactions at wide pH ranges, Chem. Commun. 54 (2018) 1465–1468, https://doi.org/10.1039/c7cc08838h. [38] Y.E. Miao, F. Li, Y. Zhou, F. Lai, H. Lu, T. Liu, Engineering a nanotubular mesoporous cobalt phosphide electrocatalyst by the Kirkendall effect towards highly efficient hydrogen evolution reactions, Nanoscale 9 (2017) 16313–16320, https:// doi.org/10.1039/c7nr05825j. [39] X. Yu, S. Zhang, C. Li, C. Zhu, Y. Chen, P. Gao, L. Qi, X. Zhang, Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting, Nanoscale 8 (2016) 10902–10907, https://doi.org/10. 1039/c6nr01867j. [40] M. Xu, L. Han, Y. Han, Y. Yu, J. Zhai, S. Dong, Porous CoP concave polyhedron electrocatalysts synthesized from metal–organic frameworks with enhanced electrochemical properties for hydrogen evolution, J. Mater. Chem. A 3 (2015) 21471–21477, https://doi.org/10.1039/C5TA05018A. [41] Y. Du, H. Qu, Y. Liu, Y. Han, L. Wang, B. Dong, Bimetallic CoFeP hollow microspheres as highly efficient bifunctional electrocatalysts for overall water splitting in
[42]
[43]
[44]
[45]
1531
alkaline media, Appl. Surf. Sci. 465 (2019) 816–823, https://doi.org/10.1016/j. apsusc.2018.09.231. F.H. Saadi, A.I. Carim, W.S. Drisdell, S. Gul, J.H. Baricuatro, J. Yano, M.P. Soriaga, N.S. Lewis, Operando spectroscopic analysis of CoP films electrocatalyzing the hydrogen-evolution reaction, J. Am. Chem. Soc. 139 (2017) 12927–12930, https:// doi.org/10.1021/jacs.7b07606. K. Xu, H. Cheng, L. Liu, H. Lv, X. Wu, C. Wu, Y. Xie, Promoting active species generation by electrochemical activation in alkaline Mmedia for efficient electrocatalytic oxygen evolution in neutral media, Nano Lett. 17 (2017) 578–583, https://doi.org/10.1021/acs.nanolett.6b04732. B.E. Conway, B.V. Tilak, Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H, Electrochim. Acta 47 (2002) 3571–3594, https://doi.org/10.1016/S0013-4686(02)00329-8. W. Li, X. Gao, D. Xiong, F. Xia, J. Liu, W.G. Song, J. Xu, S.M. Thalluri, M.F. Cerqueira, X. Fu, L. Liu, Vapor-solid synthesis of monolithic single-crystalline CoP nanowire electrodes for efficient and robust water electrolysis, Chem. Sci. 8 (2017) 2952–2958, https://doi.org/10.1039/c6sc05167g.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Engineering inner-porous cobalt phosphide nanowire based on controllable phosphating for efficient hydrogen evolution in both acidic and alkaline conditions ⁎
T
⁎
Shucong Zhang, Ting Xiong, Xiaofan Tang, Qiuyi Ma, Feilong Hu , Yan Mi
Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University for Nationalities, Nanning, 530006, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inner-porous Cobalt phosphide Hydrogen evolution Electrocatalytic
Cobalt phosphide has been extensively studied as non-precious metal-based robust electrocatalyst for hydrogen evolution reaction. Innovation breakthrough is still highly desired to enhance its catalytic efficiency. In this work, through rational engineering strategy, a controlled inner-porous CoP nanowire array, which is vertical to the carbon fiber (ip-CoP/CF) was in-situ topotactically synthesized. The resulting ip-CoP/CF demonstrates improved hydrogen evolving efficiency in both acidic and alkaline conditions. At a current density of 10 mA∙cm−2, which with small Tafel slopes of 47 mV·dec−1 and 93 mV·dec−1 in acidic and alkaline solution, respectively. Such engineering strategy opens a diverting route to construct controllable inner-porous materials for high efficient electrocatalysts.
1. Introduction While meeting the increasing demands on environmental purification and energy supply, great efforts have been devoted to pursue and utilize sustainable alternative clean energy conversion technologies [1–5]. As a clean, high energy density and sustainable energy carrier, hydrogen possesses excellent metrics as a fuel, made it to be a great promising candidate in the future [6,7]. Producing hydrogen via water electrolysis is an appealing approach [5,7–12]. However, significant challenge remains if achieving high efficiency of electrolytic hydrogen production on electrolyze. At present, the state-of-art active catalysts are Pt based, however, suffering from the high cost and scarcity, it is indispensable to develop inexpensive and earth-abundant alternatives for the possibility of commercialization. In the past decade, as transition-metal phosphides (TMPs) have been emerged to be promising non-noble metal catalysts for hydrogen evolution reaction (HER), tremendous efforts have been dedicated to exploiting efficient TMPs related HER catalysts [10,13–19]. In particular, cobalt phosphide has acquired intensive attention due to its characteristic electronic structure endowed good catalytic performance and fine stability in both acidic and alkaline media [20–34]. In spite of this, innovation breakthrough is still highly desired to further improve its HER performance. Based on the theoretical calculation, it is aware of that the phosphorous in Co&+—P&- moieties is the active site for HER process. The morphologies of TMPs greatly impact their electronic ⁎
structures [9,32,35,36]. Therefore, it is highly possible to achieve excellent catalytic activity though rational morphology design. Among the developed nanostructures, the more interfaces exposure of nanoporous structure could be beneficial for fast interfacial charge transfer to decrease the reaction energy barrier [37–41]. Meanwhile, the poly- and low-crystallinity of most nanoporous catalysts possess disorder and grain boundary, which may result in abundant active sites and miraculous charge transport ability [42,43]. However, the overall porous structure of cobalt phosphide might suffer structure-collapse during electrochemical process, which is hardly satisfy the needs of industry. Therefore, it is attractive and meaningful to construct a morphology of cobalt phosphide with the benefit of nanoporous structure to achieve excellent electrocatalytic activity in multi-fields, while maintaining its morphology. Herein, a controlled inner-porous CoP nanowire array, which is directly grow on the carbon fiber was in-situ topotactically synthesized by rational engineering strategies: a convenient hydrothermal process combining with a controlled phosphorization, and a following acidtreating process. The as-constructed ip-CoP/CF demonstrates improved hydrogen evolving efficiency in both acidic and alkaline conditions. At a current density of 10 mA∙cm−2, which with small Tafel slopes of 47 mV·dec−1 and 93 mV·dec−1 in acidic and alkaline solution, respectively. Meanwhile, it also reveals a brilliant oxygen evolution property under alkaline condition. Such engineering strategy opens a diverting route to construct controllable inner-porous materials for high efficient
Corresponding authors. E-mail addresses: hfl[email protected] (F. Hu), [email protected] (Y. Mi).
https://doi.org/10.1016/j.apsusc.2019.03.250 Received 21 December 2018; Received in revised form 16 February 2019; Accepted 23 March 2019 Available online 24 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
2.5. Electrochemical measurements
electrocatalysts.
Electrochemical experiments of the as-prepared samples were performed on a CHI 660E electrochemical workstation (CH Instrument, Inc.) with a three-electrode. A saturated calomel electrode (SCE) and a carbon rod were used as the reference and counter electrodes, respectively. The as-prepared samples directly used as the working electrode. All the electrochemical measurements were proceeded at room temperature. The HER performance was studied in 0.5 M H2SO4 and 1 M KOH electrolytes. The OER performance was operated at 1 M KOH electrolyte. Before the start of tests, the N2 gas was used to purge the electrolyte for 30 min to remove the dissolved oxygen. The catalytic performances of samples were investigated by Linear sweep voltammetry (LSV) measurements which performed at a scan rate of 1 mV·s−1. The stability of samples was tested at a scan rate of 100 mV·s−1. The Tafel calculations under acidic condition for HER [14], under alkaline condition for HER [20] and OER [43] were based on previous works. The electrochemical impedance spectroscopy (EIS) was performed using a VMP2 multi-potentiostat (Princeton Applied Research, USA) controlled by EC-Lab (V9.24) software (Bio-Logic SA) with the same conditions at different overpotentials ranging from 105 Hz to 10−2 Hz with an AC voltage of 10 mV. The RHE potential reported in our study was calculated as ERHE = ESCE + 0.244 V. The iR compensation was applied for all data in this work.
2. Experimental 2.1. Materials Cobalt(II) nitrate hexahydrate, urea, ammonium fluoride, and sodium hypophosphite were purchased from Aladdin Industrial Co. Commercial carbon fiber (CF) was purchased from DongLi (Japan). The commercial Pt/C (platinum, nominally 20 wt% on carbon black) and RuO2 catalysts were purchased from Alfa Aesar Co. All of the materials were of analytical regent grade and directly used without additional purification. Deionized water was used during the whole experiment.
2.2. Synthesis of Co-precursor/CF Co-precursor/CF was prepared via a typical process: Co (NO3)2•6H2O (4 mmol), urea (5 mmol), and NH4F (2 mmol) were dissolved in 20 mL of deionized water by ultrasonic process forming a aqueous solution. A piece of CF (1 cm × 2 cm) which was separately rinsed by hydrochloric acid, acetone, ethanol and deionized water under ultrasonic treatment for 15 min was put into a Teflon-lined stainless autoclave (25 mL volume). Then the prepared aqueous solution was also transferred in. After that, the autoclave was maintained at 120 °C for 12 h. Subsequently, the autoclave was cooled down to room temperature naturally, and then the Co-precursor on CF was taken out and washed with ethanol, deionized water to remove residues, and finally dried at 60 °C for 10 h.
3. Results and discussion The generation of ip-CoP/CF was schematically illustrated in Fig. 1a. In brief, the inner-porous nanowire structure of CoP on CF was made by partial phosphorization of Co-precursor nanowire on carbon fiber skeleton to form CoP coated Co-precursor on CF, followed by a hot-acid etching of the non-phosphate Co-precursor. Firstly, the crystal structures of the as-controlled products were investigated by XRD. As can be seen in Fig. s1(a), three broad peaks at around 26°, 43° and 54° belong to the carbon, which were consistent with those obtained in the literature [21]. The other peaks located at 2θ values of 31.6°, 36.3°, 46.2° and 56.0° are matched well with the (011), (111), (112) and (020) crystal planes of orthorhombic CoP (JCPDS card No. 29–0497), respectively [21,22]. It is demonstrated that Co-precursor is successfully converted to CoP after the phosphorization and the subsequent hot-acid treatment. Meanwhile, the XRD patterns of the Co-precursor and which after phosphorization were also studied. As shown in Fig. s1b, the peaks at around 33.8°, 39.5°, 59.8° can be assigned to the (221), (231) and (412) crystal planes of Co(CO3)0.5(OH)•0.11H2O (JCPDS card No. 48–0083) [22]. After the annealing process, most of the precursor was converted to CoP, there have same signals belong to CoeO (Fig. s1c). That means the precursor was partially transferred into CoP. The appearances of the as-prepared samples were investigated by FE-SEM. Fig. 1b and c depict typical FE-SEM images of the obtained Co (CO3)0.5(OH)•0.11H2O/CF at low and high magnification. As can be seen, they show that the entire surface of the carbon fiber was uniformly and densely covered by vertically grown of Co(CO3)0.5(OH) •0.11H2O nanowires. With the controlled phosphorization process, the as-prepared Co-precursor was partially converted to CoP (Fig. 1d, Fig. s2). After the further hot-acid treatment process, obtained the pure CoP on CF (Fig. 1e). The nanowire appearance still maintained. To further study its morphology and structure characteristics, TEM investigation was performed. The TEM image in Fig. 1f reveals the inner-porous structure with many cavities inside the nanowire. They small cavities also could be found in the high-resolution TEM in Fig. 1g (some of the them are marked with red circles). And the BET analysis was used to further identify the inner-porous structure. The nitrogen adsorption/ desorption isotherm (Fig. s3) of as-prepared samples suggest the larger BET surface area of the ip-CoP to that of the CoP nanowires. And the BJH pore-size distribution curve (inset in Fig. s3) shows a peak at around 2.1 nm, confirming the nanoporous nature of ip-CoP nanowires.
2.3. Synthesis of innerporous CoP/CF Innerporous CoP/CF (ip-CoP/CF) was prepared by a controlled thermal phosphorization and following acid treatment process. In brief, sodium hypophosphite was located at the upstream side of a tube furnace and a piece of synthesized Co-precursor/CF was placed at the downstream side. Then the system was firstly calcined at 300 °C for 2 h with a heating speed of 2 °C min−1 under N2 atmosphere, then cooled naturally to ambient temperature with N2. Afterwards, the as-prepared CoP/CF was immersed into 0.5 M H2SO4, and maintained at 60 °C, followed by washing with ethanol and then drying at 60 °C. The ip-CoP/ CF was obtained. Meanwhile, the influence parameters like phosphorization time, phosphorization temperature, and acid-treatment time for the catalytic performance were investigated.
2.4. Characterization The field emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 55 Sapphire), which equipped with energy-dispersive X-ray spectrometry (EDS) with an acceleration voltage of 30 kV was used to obtain the morphology and element distribution information of the asprepared samples. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore-size distribution of samples were detected by Tristar3000 via N2 physisorption at 77 K. Thermo-gravimetric analysis (TGA, Mettler-Toledo TGA1) was characterized with a heating rate of 10 °C min−1 from 200 to 900 °C in air. Transmission electron microscopy (TEM) and the relative high resolution transmission electron microscopy (HRTEM) images of the samples were achieved at the FEI Tecnai G2 F20 S-TWIN under an accelerating voltage of 200 kV. The structure information of the as-prepared samples were characterized by a X-ray powder diffraction (XRD, Rigaku MiniFlex 600) equipped with Cu Ka radiation (λ = 0.15406 nm). And the chemical states of the samples were measured by a X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS UltraDLD. 1525
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 1. (a) Schematic illustration of the synthesis procedure of ip-CoP/CF. (b-c) SEM images of Co-precusor/CF at different magnifications. (d) SEM image of CoP/CF. (e) SEM, (f) TEM and (g) HRTEM images of ip-CoP/CF; and (h-i) EDS elemental mapping results of the as-prepared ip-CoP/CF.
Fig. 2. XPS spectra of (a) Co2p and (b) P 2p for the as-prepared ip-CoP/CF sample.
The surface chemical environmental of the as-prepared sample was further investigated with X-ray photoelectron spectroscopy (XPS). Fig. s6 shows the signals of Co, P, O and C, being in accordance with elemental mapping results (Fig. 1i). The high-resolution Co 2p spectrum (Fig. 2a) shows that the binding energy of Co 2p3/2 at 779.1 eV is positively shifted [17,21]. From the high-resolution P 2p spectrum (Fig. 2b), it could see the binding energies of P 2p3/2 and 2p1/2 at 129.8 and 130.6 eV, respectively. The negatively shifted binding energy of P and positively shifted binding energy of Co suggesting that the transfer of electron density from Co to P in ip-CoP/CF sample [17,21]. The peaks at 781.7 and 134.6 eV corresponding to the oxidized states of Co and P, which should resulting from superficial oxidation of CoP because of air contact [17,21].
The phosphorization time is of prime importance for the construction of ip-CoP/CF. Too short phosphating time (15 min, Fig. s4a) or too long phosphating time (6 h, Fig. s4b) would lead to collapsing or solid of the Co-precursor nanowire after their further hot-acid etching, respectively, without the formation of inner porous structure. TGA experiment (Fig. s5) shows that ip-CoP/CF has a CoP content of 54 wt% [20]. The HRTEM image of ip-CoP/CF (Fig. 1g) shows a good crystallinity characteristic with lattice fringe spacing of 0.283 nm, corresponding to the (011) plane of orthorhombic CoP [26]. The SAED pattern (inset in Fig. 1 g) shows the polycrystalline structure of the product. And the further elemental-mapping images (Fig. 1h–i) show that Co and P elements are uniformly dispersed along the CoP nanowires on carbon fiber.
1526
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 3. Electrocatalytic hydrogen evolution reaction in 0.5 M H2SO4. (a) The HER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The time-dependant curve for ip-CoP/CF under static overpotential of 75 mV for 24 h. (e) Nyquist plots of different catalysts. (f) A plot of (Ja-Jc) against scan rate.
(Fig. s7). It is demonstrating the optimal phosphating time and temperature, and acid-treating time parameters are 2 h, 300 °C and 6 h for ip-CoP/CF, respectively. The Tafel plots of the detected samples were shown in Fig. 3b. As can be seen, the Tafel slope of Pt/C is only 28 mV∙dec−1, which agrees with the previously reported value [26]. The constructed ip-CoP/CF has a Tafel slope of 47 mV∙dec−1, which is smaller than that of the comparison CoP/CF (55 mV∙dec−1). As well known, the Tafel slopes of 116, 38 and 29 mV∙dec−1 are corresponding to Volmer, Heyrovsky and Tafel steps, respectively. The results revealing that the HER for the ip-CoP/CF proceeds via Volmer-Heyrovsky mechanism [44], while the Volmer reaction as the rate-determination step. The HER stability of the ipCoP/CF in acidic media was also investigated. As presented in Fig. 3c, after 2000 cycles of continuous cyclic voltammetry scanning with rate of 100 mV∙s−1, there has no obvious decay of HER catalytic activity, indicating its superior stability. Meanwhile, the long-term stability test of the ip-CoP/CF for HER in 0.5 M H2SO4 solution at a constant voltage
The electrocatalytic activities of the samples were intensively investigated. Firstly, the catalytic HER activity of samples in 0.5 M H2SO4 electrolyte was evaluated using a typical three-electrode system. The commercial Pt/C and the blank carbon fiber were also examined for comparison. As the polarization curves shown in Fig. 3a, the commercial Pt/C shows superior HER performance. But the pristine CF shows almost zero current density in the scanning range from 0 to −400 mV, demonstrating no HER catalytic activity. The comparison CoP/CF presents that it can achieve a current densities of 10 mA∙cm2 and 50 mA∙cm2 at an overpotential of 85 mV and 138 mV, respectively. In comparison, the constructed ip-CoP/CF shows an overpotential of only 68 mV and 101 mV to achieve current densities of 10 mA∙cm2 and 50 mA∙cm2, respectively. The results suggest that the constructed ipCoP/CF catalyst acts as a highly efficient cathode for the HER process in acidic condition. In addition, the samples obtained by controlling the reaction time of phosphating, the temperature of phosphating and the reaction time of acid-treating show inferior HER catalytic activities 1527
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 4. Electrocatalytic hydrogen evolution reaction in 1 M KOH. (a) The HER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The time-dependant curve for ip-CoP/CF under static overpotential of 90 mV for 24 h. (e) Nyquist plots of different catalysts. (f) A plot of (Ja-Jc) against scan rate.
capacitances of electrodes were calculated by the average values of anodic and cathodic current at 0.094 V for each of CV curves, and further plotted as a function of different scan rates according to the equation of: (Ja - Jc)/2 = Cdlv [21]. It is clear that the obtained electrochemical surface area of ip-CoP/CF (136 mF∙cm−2) is larger than that of CoP/CF (87 mF∙cm−2)(Fig. 3f). Thus, the excellent HER catalytic activity of ip-CoP/CF might be explained by the good electron transfer performance and the large electrocatalyst/electrolyte interface. The above mentioned results show the excellent HER catalytic activity of the as-constructed ip-CoP/CF under acidic condition. However, the alkaline environmental is more widely exist in industrial application. Therefore, we further determined the HER catalytic performance of the as-prepared samples under alkaline condition. As shown in Fig. 4a, even in 1 M KOH electrolyte, the Pt/C electrode still has the lowest onset potential, as well as the lowest overpotential to achieve current densities of 10 mA∙cm2. Similarly, the as-constructed ip-CoP/CF
of 75 mV versus RHE for 24 h presented in Fig. 3d, and the no change of appearance (Fig. s8a-b) further indicate its superior HER catalytic stability. In addition, in order to determine the reason of activity differences of the as-present samples, the electron transfer resistance of samples were checked by the electrochemical impendance Nyquist plots. As depicted in Fig. 3e, comparing with CF and CoP/CF, the ipCoP/CF shows the lowest semicircle under the same condition. This might be attribute to the inner-pore structure of CoP on carbon fiber, which decreases the charge transfer resistance at the electrode/electrolyte interface. Meanwhile, the electrochemical surface area of CoP/ CF and ip-CoP/CF electrodes were further examined by evaluating the double-layer capacitance. A cyclic voltammogram of samples were performed at scan rates of 10, 20, 40, 60, 80 and 100 mV∙s−1 in a potential regime (0.04–0.14 V vs RHE). As shown in Fig. s9a–b, there have no observable faradic reaction occurs, suggesting the only capacitive current was measured in the scan window. The double-layer
1528
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
Fig. 5. Electrocatalytic oxygen evolution reaction in 1 M KOH. (a) The OER polarization curves of different catalysts. (b) The relevant Tafel plots. (c) Polarization curves before and after 2000 cycling test. (d) The inset is the time-dependant curve for ip-CoP/CF under static overpotential of 1.54 V for 24 h.
CF. They only need an overpotential of 300 mV to drive the current density of 10 mA∙cm−2. Although from the typical LSV curves seem like the CoP/CF has a similar OER activity to that of ip-CoP/CF, the Tafel plots in Fig. 5b shows it has a higher Tafel slope (109 mV∙dec−1) to the ip-CoP/CF (91 mV∙dec−1), indicating the better OER activity of the ipCoP/CF electrode in alkaline media. The much higher Tafel slope of RuO2 (144 mV∙dec−1) further reveals the superior OER catalytic activity of the as-constructed ip-CoP/CF electrode. The LSV curves of the ip-CoP/CF electrode before and after 2000 cycles of scanning were used to check its stability. As depicted in Fig. 5c, it has an inconspicuous decay of OER catalytic activity. And its durability was further tested by fixed at an overpotential of 300 mV for 24 h (Fig. 5d), which demonstrates that the ip-CoP/CF electrode held its OER catalytic activity over a long period. It was further confirmed by the no noticeable change in appearance (Fig. s8e–f). All of the results verify that the ip-CoP/CF is a brilliant OER catalyst. Herein, it is clear that the ip-CoP/CF is an advanced electrocatalyst for both of the HER and the OER in alkaline electrolyte. In order to evaluate its electrochemical water splitting property, a two-electrode system which use the ip-CoP/CF as both the anode and the cathode electrodes was constructed. As shown in Fig. s10, the current density of 10 mA cm−2 can be achieved at a cell voltage as low as 1.62 V (Fig. s10a), while the polarization curve recorded at 1.0 mV s−1. Furthermore, at the setting voltage of 1.62 V, the long-term testing (Fig. s10b) shows only a slight deactivation after 50 h, indicating the good stability of electrodes. And the evolution of H2 and O2 gas bubbles could be clearly observed (inset in Fig. s10b). According to the above mentioned results, we inferred that the excellent electrocatalytic activity of the as-constructed ip-CoP/CF is might owe to the following reasons: (1) The inner-porous structure makes some parts to be a thin shell to contact with the electrolyte, which enables the fast charge transfer; (2) the crisscrossing inside of the innerporous CoP enables a larger electrochemical active area; and (3) the
also demonstrates a brilliant HER catalytic activity. It has an overpotential of 76 and 135 mV to reach current densities of 10 and 50 mA∙cm2, respectively, much lower than that of CoP/CF (162 and 223 mV to reach current densities of 10 and 50 mA∙cm2, separately). Meanwhile, the Tafel slope of the as-constructed ip-CoP/CF is 93 mV∙dec−1. Although it is higher than that of Pt/C (72 mV∙dec−1) electrode, it is lower than that of CoP/CF (110 mV∙dec−1) electrode in alkaline condition. Moreover, the almost no change of HER catalytic activity before and after 2000 cycles of cyclic voltammetry scanning with rate of 100 mV∙s−1 (Fig. 4c), the maintained HER catalytic activity under static overpotential of 90 mV for 24 h (Fig. 4d), and the almost unchanged morphology (Fig. s8c–d) identify the superior stability of the as-constructed ip-CoP/CF electrode under alkaline condition. The EIS results of samples in Fig. 4e show the smaller diameter of the Nyquist plot of the ip-CoP/CF, revealing it possesses faster charge transfer kinetics and lower contact resistance in comparison with the CoP/CF. Furthermore, the electrochemical surface area of electrodes under alkaline condition, which evaluated by the double-layer capacitance shows a larger value of ip-CoP/CF (103 mF∙cm−2) to CoP/CF (81 mF∙cm−2). Above all, no matter in acidic nor alkaline electrolytes, the as-constructed ip-CoP/CF electrode shows superior HER catalytic activity and stability, and it is better than that of the CoP/CF electrode. Accordingly, it might deduce that the ip-CoP/CF possessing great promise for HER in industrial application. For electrochemical water splitting catalyst, it is more challenging to have an efficient OER catalytic activity. Because the breaking of the OeH band, and in the meantime, the formation of the O]O band needs four-electron transfer process. All of them are complex and sluggish kinetics process [43,45]. Thus, besides the HER performance, the OER performance of as-prepared samples in 1 M KOH were further studied. The typical LSV curves of pristine CF, CoP/CF, ip-CoP/CF and RuO2 are presented in Fig. 5a. As can be found, the ip-CoP/CF and CoP/CF show much more better OER catalytic activity than that of RuO2 and pristine 1529
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
direct growth of inner-porous CoP on carbon fiber guarantees its good electrical conductivity, leading to the high charge transfer.
633–644, https://doi.org/10.1016/j.scib.2016.12.011. [14] F. Lai, D. Yong, X. Ning, B. Pan, Y.-E. Miao, T. Liu, Bionanofiber assisted decoration of few-layered MoSe2 nanosheets on 3D conductive networks for efficient hydrogen evolution, Small. 13 (2017) 1602866, , https://doi.org/10.1002/smll.201602866. [15] X. Zhu, L. Mo, Y. Wu, F. Lai, X. Han, X.Y. Ling, T. Liu, Self-supported MoS2@NHCF fiber-in-tube composites with tunable voids for efficient hydrogen evolution reaction, Compos. Commun. 9 (2018) 86–91, https://doi.org/10.1016/j.coco.2018.06. 010. [16] X. Xiao, L. Tao, M. Li, X. Lv, D. Huang, X. Jiang, H. Pan, M. Wang, Y. Shen, Electronic modulation of transition metal phosphide via doping as efficient and pHuniversal electrocatalysts for hydrogen evolution reaction, Chem. Sci. 9 (2018) 1970–1975, https://doi.org/10.1039/c7sc04849a. [17] Y. Cheng, F. Liao, W. Shen, L. Liu, B. Jiang, Y. Li, M. Shao, Carbon cloth supported cobalt phosphide as multifunctional catalysts for efficient overall water splitting and zinc-air batteries, Nanoscale 9 (2017) 18977–18982, https://doi.org/10.1039/ c7nr06859j. [18] W. Li, S. Zhang, Q. Fan, F. Zhang, S. Xu, Hierarchically scaffolded CoP/CoP2 nanoparticles: controllable synthesis and their application as a well-matched bifunctional electrocatalyst for overall water splitting, Nanoscale 9 (2017) 5677–5685, https://doi.org/10.1039/c7nr01017f. [19] Y. Lin, J. Zhang, Y. Pan, Y. Liu, Nickel phosphide nanoparticles decorated nitrogen and phosphorus co-doped porous carbon as efficient hybrid catalyst for hydrogen evolution, Appl. Surf. Sci. 422 (2017) 828–837, https://doi.org/10.1016/j.apsusc. 2017.06.102. [20] H. Lu, W. Fan, Y. Huang, T. Liu, Lotus root-like porous carbon nanofiber anchored with CoP nanoparticles as all-pH hydrogen evolution electrocatalysts, Nano Res. 11 (2018) 1274–1284, https://doi.org/10.1007/s12274-017-1741-x. [21] S.H. Yu, D.H.C. Chua, Toward high-performance and low-cost hydrogen evolution reaction electrocatalysts: nanostructuring cobalt phosphide (CoP) particles on carbon fiber paper, ACS Appl. Mater. Interfaces 10 (2018) 14777–14785, https:// doi.org/10.1021/acsami.8b02755. [22] W. Yuan, X. Wang, X. Zhong, C.M. Li, CoP nanoparticles in situ grown in threedimensional hierarchical nanoporous carbons as superior electrocatalysts for hydrogen evolution, ACS Appl. Mater. Interfaces 8 (2016) 20720–20729, https://doi. org/10.1021/acsami.6b05304. [23] Z.-S. Cai, Y. Shi, S.-S. Bao, Y. Shen, X.-H. Xia, L.-M. Zheng, Bioinspired engineering of cobalt-phosphonate nanosheets for robust hydrogen evolution reaction, ACS Catal. 8 (2018) 3895–3902, https://doi.org/10.1021/acscatal.7b04276. [24] Y. Zhang, L. Gao, E.J.M. Hensen, J.P. Hofmann, Evaluating the stability of Co2P electrocatalysts in the hydrogen evolution reaction for both acidic and alkaline electrolytes, ACS Energy. Lett. 3 (2018) 1360–1365, https://doi.org/10.1021/ acsenergylett.8b00514. [25] H. Wang, S. Min, Q. Wang, D. Li, G. Casillas, C. Ma, Y. Li, Z. Liu, L.J. Li, J. Yuan, M. Antonietti, T. Wu, Nitrogen-doped nanoporous carbon membranes with co/CoP janus-type nanocrystals as hydrogen evolution electrode in both acidic and alkaline environments, ACS Nano 11 (2017) 4358–4364, https://doi.org/10.1021/acsnano. 7b01946. [26] J. Tian, Q. Liu, A.M. Asiri, X. Sun, Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogen-evolving cathode over the wide range of pH 0-14, J. Am. Chem. Soc. 136 (2014) 7587–7590, https://doi.org/10.1021/ ja503372r. [27] Y. Pan, Y. Lin, Y. Chen, Y. Liu, C. Liu, Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution, J. Mater. Chem. A 4 (2016) 4745–4754, https://doi.org/10.1039/C6TA00575F. [28] J. Wang, Z. Liu, Y. Zheng, L. Cui, W. Yang, J. Liu, Recent advances in cobalt phosphide based materials for energy-related applications, J. Mater. Chem. A 5 (2017) 22913–22932, https://doi.org/10.1039/C7TA08386F. [29] T. Wu, M. Pi, D. Zhang, S. Chen, 3D structured porous CoP3 nanoneedle arrays as an efficient bifunctional electrocatalyst for the evolution reaction of hydrogen and oxygen, J. Mater. Chem. A 4 (2016) 14539–14544, https://doi.org/10.1039/ C6TA05838H. [30] D. Zhou, L. He, W. Zhu, X. Hou, K. Wang, G. Du, C. Zheng, X. Sun, A.M. Asiri, Interconnected urchin-like cobalt phosphide microspheres film for highly efficient electrochemical hydrogen evolution in both acidic and basic media, J. Mater. Chem. A 4 (2016) 10114–10117, https://doi.org/10.1039/C6TA03628G. [31] F.H. Saadi, A.I. Carim, E. Verlage, J.C. Hemminger, N.S. Lewis, M.P. Soriaga, CoP as an acid-stable active electrocatalyst for the hydrogen-evolution reaction: electrochemical synthesis, interfacial characterization and performance evaluation, J. Phys. Chem. C 118 (2014) 29294–29300, https://doi.org/10.1021/jp5054452. [32] X.-Y. Yan, S. Devaramani, J. Chen, D.-l. Shan, D.-D. Qin, Q. Ma, X.-Q. Lu, Selfsupported rectangular CoP nanosheet arrays grown on a carbon cloth as an efficient electrocatalyst for the hydrogen evolution reaction over a variety of pH values, New J. Chem. 41 (2017) 2436–2442, https://doi.org/10.1039/C6NJ03887E. [33] G. Hu, Q. Tang, D.-E. Jiang, CoP for hydrogen evolution: implications from hydrogen adsorption, Phys. Chem. Chem. Phys. 18 (2016) 23864–23871, https://doi. org/10.1039/C6CP04011J. [34] T. Wang, Y. Jiang, Y. Zhou, Y. Du, C. Wang, In situ electrodeposition of CoP nanoparticles on carbon nanomaterial doped polyphenylene sulfide flexible electrode for electrochemical hydrogen evolution, Appl. Surf. Sci. 442 (2018) 1–11, https:// doi.org/10.1016/j.apsusc.2018.02.133. [35] Z. Jin, P. Li, D. Xiao, Metallic Co2P ultrathin nanowires distinguished from CoP as robust electrocatalysts for overall water-splitting, Green Chem. 18 (2016) 1459–1464, https://doi.org/10.1039/C5GC02462E. [36] Y. Shao, X. Shi, H. Pan, Electronic, magnetic, and catalytic properties of thermodynamically stable two-dimensional transition-metal phosphides, Chem. Mater. 29 (2017) 8892–8900, https://doi.org/10.1021/acs.chemmater.7b03832.
4. Conclusion In summary, a inner-porous CoP on carbon fiber was in-situ topotactically constructed via a convenient hydrothermal combined with subsequent phosphorization and acid-treating processes. The as-constructed ip-CoP/CF gained an excellent HER catalytic activity under both of acidic and alkaline conditions, and also a brilliant OER catalytic performance in alkaline media. This work may not only features a good performance of dual-functional water splitting catalyst, but also sever as a universal route to construct various inner-porous transition metal phosphides growth on supporter. Acknowledgments The authors thank the National Natural Science Foundation of China (Grant Nos. 21761004, 21701035), Guangxi Collaborative Innovation Center of Forest Chemistry and Engineering (Grant Nos. 201605, 201608), Specific research project of Guangxi for research bases and talents19 486 (AD18126005, AD18126002), Talent Introduction Start-Up Foundation of Guangxi University for Nationalities (2016MDQD004, 2016MDQD005) and the 100 Talents Program for Introducing Overseas High-level Talents into Universities of Guangxi for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.250. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (2001) 332, https://doi.org/10.1038/35104599. [2] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. 103 (2006) 15729–15735, https://doi.org/10. 1073/pnas.0603395103. [3] J. Turner, G. Sverdrup, M.K. Mann, P.-C. Maness, B. Kroposki, M. Ghirardi, R.J. Evans, D. Blake, Renewable hydrogen production, Int. J. Energy Res. 32 (2008) 379–407, https://doi.org/10.1002/er.1372. [4] Y. Mi, L. Wen, Z. Wang, D. Cao, Y. Guo, Y. Lei, Building of anti-restack 3D BiOCl hierarchitecture by ultrathin nanosheets towards enhanced photocatalytic activity, Appl. Catal. B Environ. 176-177 (2015) 331–337, https://doi.org/10.1016/j. apcatb.2015.04.013. [5] M.S. Faber, S. Jin, Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications, Energy Environ. Sci. 7 (2014) 3519–3542, https://doi.org/10.1039/C4EE01760A. [6] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972–974, https://doi.org/10.1126/science.1103197. [7] D. Kong, J.J. Cha, H. Wang, H.R. Lee, Y. Cui, First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction, Energy Environ. Sci. 6 (2013) 3553–3558, https://doi.org/10.1039/C3EE42413H. [8] H. Zhu, L. Gu, D. Yu, Y. Sun, M. Wan, M. Zhang, L. Wang, L. Wang, W. Wu, J. Yao, M. Du, S. Guo, The marriage and integration of nanostructures with different dimensions for synergistic electrocatalysis, Energy Environ. Sci. 10 (2017) 321–330, https://doi.org/10.1039/C6EE03054H. [9] H. Zhu, G. Gao, M. Du, J. Zhou, K. Wang, W. Wu, X. Chen, Y. Li, P. Ma, W. Dong, F. Duan, M. Chen, G. Wu, J. Wu, H. Yang, S. Guo, Atomic-scale core/shell structure engineering induces precise tensile strain to boost hydrogen evolution catalysis, Adv. Mater. 30 (2018) 1707301, , https://doi.org/10.1002/adma.201707301. [10] M.-I. Jamesh, X.M. Sun, Recent progress on earth abundant electrocatalysts for hydrogen evolution reaction (HER) in alkaline medium to achieve efficient water splitting – a review, J. Energy Chem. 34 (2019) 111–160, https://doi.org/10.1016/ j.jechem.2018.09.016. [11] Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction, Chem. Soc. Rev. 45 (2016) 1529–1541, https://doi.org/10.1039/C5CS00434A. [12] I.K. Mishra, H. Zhou, J. Sun, F. Qin, K. Dahal, J. Bao, S. Chen, Z. Ren, Hierarchical CoP/Ni5P4/CoP microsheet arrays as a robust pH-universal electrocatalyst for efficient hydrogen generation, Energy Environ. Sci. 11 (2018) 2246–2252, https:// doi.org/10.1039/C8EE01270A. [13] J. Su, J. Zhou, L. Wang, C. Liu, Y. Chen, Synthesis and application of transition metal phosphides as electrocatalyst for water splitting, Sci. Bull. 62 (2017)
1530
Applied Surface Science 481 (2019) 1524–1531
S. Zhang, et al.
[37] Y. Zeng, Y. Wang, G. Huang, C. Chen, L. Huang, R. Chen, S. Wang, Porous CoP nanosheets converted from layered double hydroxides with superior electrochemical activity for hydrogen evolution reactions at wide pH ranges, Chem. Commun. 54 (2018) 1465–1468, https://doi.org/10.1039/c7cc08838h. [38] Y.E. Miao, F. Li, Y. Zhou, F. Lai, H. Lu, T. Liu, Engineering a nanotubular mesoporous cobalt phosphide electrocatalyst by the Kirkendall effect towards highly efficient hydrogen evolution reactions, Nanoscale 9 (2017) 16313–16320, https:// doi.org/10.1039/c7nr05825j. [39] X. Yu, S. Zhang, C. Li, C. Zhu, Y. Chen, P. Gao, L. Qi, X. Zhang, Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting, Nanoscale 8 (2016) 10902–10907, https://doi.org/10. 1039/c6nr01867j. [40] M. Xu, L. Han, Y. Han, Y. Yu, J. Zhai, S. Dong, Porous CoP concave polyhedron electrocatalysts synthesized from metal–organic frameworks with enhanced electrochemical properties for hydrogen evolution, J. Mater. Chem. A 3 (2015) 21471–21477, https://doi.org/10.1039/C5TA05018A. [41] Y. Du, H. Qu, Y. Liu, Y. Han, L. Wang, B. Dong, Bimetallic CoFeP hollow microspheres as highly efficient bifunctional electrocatalysts for overall water splitting in
[42]
[43]
[44]
[45]
1531
alkaline media, Appl. Surf. Sci. 465 (2019) 816–823, https://doi.org/10.1016/j. apsusc.2018.09.231. F.H. Saadi, A.I. Carim, W.S. Drisdell, S. Gul, J.H. Baricuatro, J. Yano, M.P. Soriaga, N.S. Lewis, Operando spectroscopic analysis of CoP films electrocatalyzing the hydrogen-evolution reaction, J. Am. Chem. Soc. 139 (2017) 12927–12930, https:// doi.org/10.1021/jacs.7b07606. K. Xu, H. Cheng, L. Liu, H. Lv, X. Wu, C. Wu, Y. Xie, Promoting active species generation by electrochemical activation in alkaline Mmedia for efficient electrocatalytic oxygen evolution in neutral media, Nano Lett. 17 (2017) 578–583, https://doi.org/10.1021/acs.nanolett.6b04732. B.E. Conway, B.V. Tilak, Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H, Electrochim. Acta 47 (2002) 3571–3594, https://doi.org/10.1016/S0013-4686(02)00329-8. W. Li, X. Gao, D. Xiong, F. Xia, J. Liu, W.G. Song, J. Xu, S.M. Thalluri, M.F. Cerqueira, X. Fu, L. Liu, Vapor-solid synthesis of monolithic single-crystalline CoP nanowire electrodes for efficient and robust water electrolysis, Chem. Sci. 8 (2017) 2952–2958, https://doi.org/10.1039/c6sc05167g.