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Partial phosphorization of porous Co–Ni–B for efficient hydrogen evolution electrocatalysis
international journal of hydrogen energy xxx (xxxx) xxx
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Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis Dongqi Dong a,1, Xiuling Xu b,1, Chuanli Ma a, Liangyu Gong a, Linghao Su a, Jie Wang a,* a
College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao, 266109, People’s Republic of China b Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shanxi, 712100, People’s Republic of China
highlights
graphical abstract
Simple two-step preparation of partial phosphorization of porous CoeNieB composite. Morphology
can
be
tuned
by
introduction of Ni species. Porous architecture can be tuned during phosphorization process. Coordination effects between the metal and non-metal species result in the excellent HER performance.
article info
abstract
Article history:
Design of cost-effective and high-efficient electrocatalysts for hydrogen evolution reaction
Received 25 September 2019
(HER) is of vital significance for the current renewable energy devices d fuel cells. Herein,
Received in revised form
we report a facile strategy to prepare partial phosphorization of CoeNieB material with
8 December 2019
porous structure via a water-bath boronizing and subsequent phosphorization process at
Accepted 11 December 2019
moderate temperature. The optimal atomic proportion of Co to Ni is investigated via
Available online xxx
physical and electrochemical characterization. As a result, Co9eNi1eBeP exhibits the best HER activity, which require an lower overpotential of ~192 mV to deliver a current density
Keywords:
value of 10 mA cm2 and a smaller Tafel slope of 94 mV dec1 in alkaline media, relative to
CoeNieB material
P-free CoeNieB catalysts, Co9eNi1eBeP with other Co: Ni proportion and mono metallic
Porous architecture
borides The excellent electrocatalytic performance of Co9eNi1eBeP is mainly ascribed to
Partial phosphorization
the three-dimensional (3D) porous structure and the coordinate functionalization between
Quaternary composites
the borides and phosphides. This work provides a promising strategy for the exploration of
Hydrogen evolution reaction
quaternary composites as efficient and cost-effective electrocatalysts for HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J. Wang). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.12.066 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Introduction With the depletion of traditional fossil energy sources and the increasing environmental pollution issues, hydrogen, a renewable energy carrier, has caught much attention in newenergy vehicles [1e4]. Relative to traditional hydrogen production methods (pyrolysis natural gas method and coal gasification at high-temperature), electrochemical and photo assisted electrochemical water-splitting features environmental friendly, mild preparation conditions, and highefficiency [5e7]. At present, platinum shows to be the most effective catalyst for HER, which possess near theoretical overpotential value (nearly zero overpotential in acidic media) [8e12]. However, the scarcity and high cost situation seriously hindered their large-scale commercialization applications. Therefore, it is urgent to explore earth-abundant and costeffective electrocatalysts for the HER aiming to enhance the electrocatalytic efficiency and minimize the practical reaction resistance [13e16]. Till now, earth-abundant and non-precious metal-based catalysts have been extensively pursued for catalyzing the electrochemical HER, in which inorganic compounds are one of the important categories, especially the transition-metal (Fe, Co, Ni, and Mo)-based materials [17e24]. Specifically, the mono-metal borides (e.g. NieB [25] and MoXB [26,27]) have achieved wide attention in hydrogen generation. It is worth noting that most of the metal boride-based materials synthesized recently possess unsatisfied catalytic performance due to the large particle size distribution and low surface area, which are mainly attributed to the synthetic methods. Compared with solid nanocomposites, catalysts with hierarchical construction possess unique structural traits, the micrometer and nanometer-scaled building blocks joint features unique properties on electrocatalytic hydrogen generation [28e30]. As for HER, hierarchical porous structure could effectively facilitate the rapid mass rate and increase the electrochemical interface area, which are beneficial for highperformance electrocatalytic activity and stability. Tai et al. [26] reported an ultrathin hexagonal Mo3B films with 6.48 nm in thickness by chemical vapour deposition, which can be scalable production. The ultrathin film features metallic property, which exhibits excellent stability and activity. Cui et al. [31] demonstrated a porous Ni3B catalyst for efficient HER electrocatalysis via a simple one-step sintering method. Among them, Ni3B sintered at 850 C showed the best HER activity in acidic media due to the unique porous structure. It should be noted that the specific electrocatalytic mechanisms of improved HER activity by introduction of a second metal in the mono-metallic borides has been rarely elucidated [32,33]. Meanwhile, metal phosphide hybrids have greatly improve the activity of HER, and it has been investigated extensively, such as CoP [19,32,34], FeP [23], Ni2P [18,35], and MoP [36,37]. Therefore, tailoring the chemical composition of metal borides by doping or partial phosphorization with P sources would be a rational strategy to design efficient catalysts with higher HER performance through the coordinative effects of metal borides and metal phosphides. Along with compositional tailoring, controllable of the micro-/nano-structure to derive porous structure with high surface area is also an
important strategy to improve the electrocatalytic activity [38]. In this paper, we report a 3D porous Co9eNi1eBeP compound as a highly efficient and stable catalyst for HER in 1 M KOH media though a facile two-step approach. Firstly, CoeNieB compound was prepared by water-bath reduction of Co2þand Ni2þ using polyvinyl pyrrolidone (PVP) as a surfactant and NaBH4 as a reducing agent. Secondly, Co9eNi1eBeP composite with 3D porous structure was obtained by phosphorization at 300 C for 2 h in nitrogen-saturate atmosphere. As an electrocatalyst, the Co9eNi1eBeP composite exhibited excellent HER activity and long-term stability due to the coordinate functionalization between the borides and phosphides and the 3D porous structure derived by the addition of small quantity of Ni composite.
Experimental part Materials and reagents All the chemical reagents used in the sample preparation including CoCl2 6H2O (AR, ~99.0%), NiCl2 6H2O (AR, ~98.0%), polyvinyl pyrrolidone (PVP) and NaBH4 (AR, ~98.0%) were obtained from Sinopharm Chem. reagent Co. Ltd, China. Deionized water was obtained from an ultra-pure purifier (Ulupure, China, resistivity 18.2 MU).
Samples preparation The NieCoeB samples were fabricated by a rapid and facile reducing reaction. Firstly, 0.8 mmol of CoCl2 6H2O and NiCl2 6H2O with different Co: Ni molar ratios of 6:1, 9:1 and 12:1, respectively, together with 800 mg of polyvinyl Pyrrolidone (PVP) were added into a flask containing 200 mL deionized water and magnetic stirred in N2-saturated atmosphere for 30 min at room temperature. Subsequently, with continuous stirring and aerating condition, 80 mL of deionized water with 100 mg NaBH4 was quickly added into the above solution. At last, the resulting black products was obtained through vacuum filtration and washed several times with deionized water and alcohol in sequence. The obtained catalysts were denoted as Co6eNi1eB, Co9eNi1eB and Co12eNi1eB, respectively. The CoeB and NieB samples were synthesized with the same process accordingly. Phosphorous doped CoeNieB (CoeNieBeP) samples were prepared using a low-temperature phosphorization method. Different mass ratio of CoeNieB and NaH2PO2 (1:10; 1:15; 1:20) were added into two separated porcelain boats, and then annealed with a heating rate of 2 C min1 to 300 C for 2 h under N2 atmosphere in a tube furnace.
Physical characterization Morphologies of the composite were characterized by scanning electron microscopy (SEM, JSM-7500F). Transmission electron microscopy (TEM) and scanning TEM (STEM) images was performed on JSM-2100F equipment. Powder X-ray diffraction (XRD) data were collected on a Bruker D8, and diffraction patterns were collected using Cu Ka (l ¼ 1.5406 A)
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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radiation at a scanning rate of 8 min1. X-ray photoelectron spectroscopy (XPS) was conducted on AXIS-ULTRA DLD to investigate the chemical valence and composition. The specific surface area was calculated follows the BrunauereEmmetteTeller (BET) method by using Quantachrome (NOVA 2200e) equipment.
Electrochemical characterization The electrochemical measurement for the HER was carried out via a typical three-electrode system in 1 M KOH solution using a CHI 760 E potentiostat. The active material depositing Ni foam, carbon rod and reversible hydrogen electrode (RHE) were applied as working electrode, counter electrode and reference electrode, respectively. Before preparing the working electrode, the Ni foam was processed with diluted HCl solution, ethanol and deionized water several times before utilization. The prepared catalysts with carbon black and polyvinylidene fluoride (7:2:1, mass ratio) were dispersed into n-methyl-pyrrolidone (NMP) to form homogenous ink. And then measure 100 mL and deposited onto the Ni foam (working area 1.5 cm2), dried at 80 C for 12 h (the mass loading was 0.46 mg cm2). Comparison of linear sweeping voltammograms (LSV) curves of Co9eNi1eBeP via Ni foam and rotating disc electrode (RDE) in 1 M KOH solution at 1600 rpm rotation speed was conducted (Fig. S5). Negligible change can be clarified, indicating that the two kind of electrochemical measurements can be comparable. Taking account of the practical application of water splitting, the catalysts loaded on the Ni foam were used in this paper. The cyclic voltammograms (CVs) for the determination of double layer capacitance (Cdl) values of the catalysts were measured at scan rate of 20, 40, 60, 80, 100, 120 mV s1 from 0.1 to 0.2 V. The linear sweeping voltammograms (LSV) was conducted from 0.05 to 0.6 V at a scanning rate of 5 mV s1. The long-term durability of the asprepared electrocatalysts was analyzed by chronoamperometry measurements. Multi-step chronoamperometric curve of Co9eNi1eBeP was obtained at overpotentials as follows: 0.12, 0.16, 0.19, 0.21, 0.23, 0.21, 0.19, 0.16 and 0.12 V, respectively. Long-term stability of Co9eNi1eBeP was performed via chronoamperometry measurement at a constant potential of 0.19 V.
Results and discussion Microscopic characterization The physical properties, especially the morphological and structural parameters of the as-prepared catalysts were characterized via SEM and TEM techniques. As can be seen from the SEM image in Fig. 1a, the CoeB sample featured porous flake-like structure, while by increasing the Ni proportion in the synthetic approach, the flake-like structure gradually disappeared and derived smaller pore structure from Co12eNi1eB to Co6eNi1eB (Fig. 1bed). However, the NieB sample showed negligible pore structure, but composed of spherical aggregates (Fig. 1e). To confirm the SEM results, N2 adsorption/desorption measurements (Fig. 1f) was carried out to further study the specific surface physical structures.
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According to the N2 adsorption/desorption curves, the isotherm showed obvious hysteresis loop, which belongs to type IV and is typical for mesoporous materials [38]. The BET specific surface area of the as-prepared CoeB, Co9eNi1eB and NieB samples are 97.8, 112.8 and 34.7 m2 g1, respectively, indicating that the addition of Ni in the Co-based architecture efficiently increased the surface area, which would provide more electrochemical interfaces to enhance the HER performance. While after partial phosphorization step, the Co9eNi1eBeP sample presents more porous architecture according to the upwards-magnifying SEM images (Fig. 2aec) compared with the Co9eNi1eB (Fig. 1c). The high-resolution TEM image in Fig. 2d showed the enlarged magnification of Co9eNi1eBeP with discontinuous lattice fringes, indicating a poor crystallization of the sample. The inter-planar spacing for the lattice fringes is calculated to be 0.22 nm, corresponding to the (111) plane of Co2P. The high-angle annular dark field scanning TEM image (HAADF-STEM, Fig. 2e) and the corresponding elemental mapping originate from the energy dispersive spectrum (EDS, Fig. 2eeh and Fig. S1) showed that Co, Ni and P elements are evenly distributed in the local area, which further demonstrates the successfully phosphorization of the CoeNieB sample. Moreover, as can be seen from Fig. S2, obvious hysteresis loop can be clearly classified for the two samples, while, the hysteresis loop of Co9eNi1eBeP sample distributed close to the higher P/P0 area, indicating that the pore size distribution would be changed on the trend from mesoporous to macroporous by phosphorization process [39]. Moreover, the specific surface area of the two samples showed no obvious change. The pore distribution change and phosphorization reaction would benefit to improve the electrochemical interfaces and increase the ion transport rate during electrochemical process.
Structural measurements The crystal structure of Co9eNi1eBeP sample was further investigated by X-ray diffraction (XRD) technique (Fig. 3a). A single broad peak centred at 28 is corresponding to the amorphous state of the obtained composite. Recent studies have been demonstrated that amorphous multi-metallic materials often display higher electrochemical activity than the crystalline material [40]. Besides, the diffraction peaks located at 40.8 , 44.9 , 48.4 and 55.5 are corresponding to the (111), (021), (120) and (030) planes of Co2P, which is consistent to the standard diffraction pattern of Co2P (PDF No. 00-054-0413). Compared with XRD pattern of Co9eNi1eB, partial phosphorization process effectively improved the crystallinity of the material in some extent. The surface property of Co9eNi1eBeP sample was investigated via X-ray photoelectron spectroscopy (XPS) survey scan spectrum, in which the characteristic peaks of Ni, Co, B, P elements are shown in Fig. 3b. The high-resolution spectrum of the existing components were characterized and analyzed. For the P 2p energy level, two fitted peaks located at 129.3 and 133.5 eV in binding energies can be assigned to M P (M related to Co or Ni elements) and PeO species (Fig. 3c) [41]. Meanwhile, the two peaks at 186.9 and 192.3 eV for the B 1s spectrum are ascribed to the M B and BeO species (Fig. 3d) [42,43]. The appearance
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 1 e Upwards-magnifying SEM images of (a) CoeB, (b) Co12eNi1eB, (c) Co9eNi1eB, (d) Co6eNi1eB and (e) NieB; The scale bar in the SEM images is 200 nm. (f) The nitrogen adsorptionedesorption isotherm and the specific surface area of Co9eNi1eB, CoeB and NieB.
of PeO and BeO species may be attributed to the surface oxidation when prepared materials were exposed to air [44,45]. The P 2p and B 1s spectra both demonstrate the coexistence of metal boride and phosphide in the sample. In the high-resolution Co 2p spectrum (Fig. 3e), the peaks located at 778.0 eV and 781.6 eV are representing to CoeB and CoeP
bonds, respectively [33,46]. Besides, the other peak located at a higher binding energy of 785.4 eV is derived from the oxidation of cobalt species. The high-resolution Ni 2p spectrum (Fig. 3f) shows three characteristic peaks at 852.8 eV, 854.8 eV and 857.0 eV, which are ascribed to be attributed to Ni in Ni 2p, oxidized Ni species and the satellite of the Ni 2p3/2
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 2 e (aec) Upwards-magnifying SEM images of Co9eNi1eBeP sample. (d) HRTEM image of Co9eNi1eBeP. (eeh) The HAADF-STEM image and the corresponding EDS mapping of Co, Ni and P.
peak, respectively [41,47]. According to the XPS analysis, it can be concluded that the Co9eNi1eB was successfully experienced partial phosphated.
Electrochemical performance The electrocatalytic properties of the as-prepared catalysts were investigated in 1.0 M KOH solution via a typical threeelectrode system. According to the previous reports, an efficient electrocatalyst should possess relative low overpotential to deliver a specific current density during the LSV measurements [48]. The electronic structures can be improved to just the reaction kinetics by the introduction of different metal components, which is beneficial to lower the kinetic energy barriers of electrocatalytic reactions [49,50]. To investigate the coordination effect of Co and Ni, the catalysts were optimized by investigating the Co and Ni ratios. As demonstrated in Fig. 4a obviously, the Co9eNi1eB delivers the lowest overpotential (340 mV) at 100 mA cm2 relative to the Co6eNi1e B (365 mV) and Co12eNi1eB (353 mV) samples. The Tafel curves derived from the polarization curves (Fig. 4b) showed calculated Tafel slopes to investigate the reaction kinetics of the CoeNieB compound. It can be seen that the obtained Co9eNi1eB exhibited the lowest Tafel slope value of 96 mV dec1 compared with Co6eNi1eB (107 mV dec1) and Co12eNi1eB (97 mV dec1), indicating the fastest hydrogen production rate of Co9eNi1eB. Detailed comparisons of the HER for the varying
ratios of Co and Ni precursors are provided from the histogram in Fig. 4c. Although the Co9eNi1eB present excellent performance for the HER, they still fall behind the precious metal-based catalysts. As is well known that metal phosphides are highly active for electrocatalysis in hydrogen production [32,51]. Thus, low-temperature phosphorization was further investigated for the target of improving the HER.
Performance As can be seen from the polarization curves in Fig. 4d, the overpotential evidently decreased from 340 mV for Co9eNi1eB to 291 mV for Co9eNi1eBeP (1 : 15, mass ratio of M: P) to drive the same current density of 100 mA cm2. Fig. 4e and f clearly exhibit that among all of the phosphorization samples, the catalyst obtained at a M: P mass ratio of 1:15 exhibited the best HER activity for the HER with the lowest Tafel slope of 94 mV dec1 relative to 1:10 (105 mV dec1) and 1:20 (104 mV dec1), and the lowest overpotential of 192 mV at 10 mA cm2 relative to 1:10 (237 mV) and 1:20 (200 mV), which demonstrated that different degrees of phosphorization condition result in distinct activities. Therefore, the partial phosphorization for the obtained bimetal borides is efficient to improve the electrocatalytic properties towards the HER. The HER performance of the porous Co9eNi1eBeP composite was studied systematically, especially the reaction activity, kinetics and long-term durability. It can be seen from
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 3 e (a) XRD patterns of Co9eNi1eBeP and Co9eNi1eB samples. (b) Full-range XPS spectrum of Co9eNi1eBeP sample and the corresponding high-resolution fitting spectrum of P2p (c), B1s (d), Co2p (e) and Ni2p (f).
Fig. 4 e (a) The polarization curves of Co12eNi1eB, Co9eNi1eB and Co6eNi1eB catalysts in 1.0 M KOH solution at scanning rate of 5 mV s¡1, the corresponding Tafel slopes (b) and the corresponding histograms of overpotentials at 10 mA cm¡2 and Tafel slopes (c). (d) The polarization curves of Co9eNi1eB with different P source proportion in 1.0 M KOH solution at scanning rate of 5 mV s¡1, the corresponding Tafel slopes (e) and the corresponding histograms of overpotentials at 10 mA cm¡2 and Tafel slopes (f). Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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the polarization curves in Fig. 5a, the Co9eNi1eBeP showed higher HER activity with the lowest overpotential compared with the mono metal borides and Co9eNi1eB, except for the commercial Pt/C. Compared with the recent reports, the HER activity of Co9eNi1eBeP ranked in the front (Table 1). As is well known, the electrochemically active surface area (ECSA) is linearly proportional to the electrochemical double layer capacitance (EDLC, Cdl). According to the capacitive current at 0.15 V as a function of scan rate derived from the CV curves at
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different scanning rates (Fig. S3), the Cdl value of Co9eNi1eBeP was calculated to be 10.0 mF dec1 (Fig. 5b), higher than Co9eNi1eB (9.0 mF dec1), CoeB (2.8 mF dec1) and NieB (1.6 mF dec1), demonstrating that the partial phosphorization and the coexistence of Ni and Co effectively enlarged the electrochemical active interfaces, which is beneficial for enhancing the catalytic activity. The Cdl value decreasing trend of Co9eNi1eB, CoeB and NieB samples is fully consistent with the nitrogen adsorptionedesorption isotherm results in Fig. 1f.
Fig. 5 e (a) The polarization curves of NieB, CoeB, Co9eNi1eB, Co9eNi1eBeP and Pt/C in 1.0 M KOH solution at a scanning rate of 5 mV s¡1. (b) The Cdl value of NieB, CoeB, Co9eNi1eB and Co9eNi1eBeP. (c) The Tafel plots corresponding to the catalysts in a. (d) The j0 value of NieB, CoeB, Co9eNi1eB and Co9eNi1eBeP obtained via extrapolating the Tafel plots and Histograms of j0 value and Tafel slopes for the four samples (inset). (e) Multi-step chronoamperometric curve of Co9eNi1eBeP at different overpotentials. (f) Long-term stability of Co9eNi1eBeP via chronoamperometry measurement at a constant potential of ¡0.19 V. Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Table 1 e Comparison of HER activities for Co9eNi1eBeP and other similar electrocatalysts reported at current density of 10 mA cm¡2. Catalysts
morphology
overpotential/mV at 10 mA cm2
Electrolyte
1 2 3 4 5 6 7 8 9 10
Fe10Co40Ni40P Coe50NieB/CC CoP@BCN CoNiSe/C/NF Co/NBC Co9S8eNi3S2 NAs/NF Co4Ni1P Co2P Coe30NieB CoS2/RGO-CNT
68 80 87 90 117 120 129 130 133 142
1 M KOH 1.0 M KOH 0.5 M H2SO4 1 M KOH 1.0 M KOH 1.0 M KOH 1.0 M KOH 0.5 M H2SO4 1 M NaOH 0.5 M H2SO4
[53] [54] [55] [56] [57] [58] [59] [60] [61] [62]
11 12 13 14 15 16 17 18 19
NieCoeMoS2 CoFe LDH-F Co9eNi1eBeP CoFe/NF Ni65Co29Y6 Co9S8@C Co2P Co9S8 Co and N Co-Doped Tungsten Carbide
nanosheets network-like porous structure nanotubes nanostructure microspheres nanosheets nanotubes nanorods spherical nanoparticles porous CNT network nanoboxes nanosheets porous architecture nanosheets / nanoparticles / Nanotubes /
155 166 192 220 230 245 247 290 450
0.5 m H2SO4 1 M KOH 1 M KOH 1 M KOH 1 M NaOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH
[63] [64] This work [65] [66] [67] [68] [69] [70]
Number
The Tafel slope fitted from the polarization curves in Fig. 5a was also calculated to investigate the reaction kinetics of the as-obtained catalysts (Fig. 5c). Despite the lowest Tafel slope value of 53 mV dec1 for Pt/C catalyst, the Co9eNi1eBeP exhibited the smaller Tafel slope value of 94 mV dec1, relative to Co9eNi1eB (96 mV dec1), CoeB (109 mV dec1) and NieB (113 mV dec1), demonstrating that the Co9eNi1eBeP follows a VolmereHeyrovsky reaction process [52]. Moreover, the lowest Tafel slope of Co9eNi1eBeP composite indicates the fastest hydrogen production rate relative to other samples prepared via the same method. Exchange. Current density (j0) is also a key kinetic parameter for examining the intrinsic electrocatalytic properties of the obtained catalysts, which can be obtained by extrapolating the Tafel plots to 0 V (Fig. 5d). The j0 values of the catalysts are shown from the inset of Fig. 5d, in which the Co9eNi1eBeP exhibits the largest j0 value relative to Co9eNi1eB and mono metal borides, indicating the fastest electron transfer rate of HER. Such results might be ascribed by both the morphology of pore structure and particle size influence.
Stability tests The multi-step chronoamperometric responses of Co9eNi1eBeP were also conducted to evaluate the mass transport properties and durability. As shown in Fig. 5e, the current density dropped with the increasing of overpotential, and in return, the current density elevated with the decreasing of overpotential, indicating the excellent reversible capability and stability of the Co9eNi1eBeP composite. Chronoamperometry measurement was further conducted (Fig. 5f), and demonstrating that the current density showed slightly decrease at the initial 3 h and then tend to be constant even after 11 h under a applied overpotential of 0.19 V,
Refs
confirmed the excellent durability of Co9eNi1eBeP composite. Besides, further accelerate CV measurements were applied to investigate the stability of Co9eNi1eBeP. as shown from the LSV curves (Fig. S4), only a slight potential decay after 1000 potential cycles, further suggesting the high stability of the Co9eNi1eBeP for HER. As discussed above, the as-prepared Co9eNi1eBeP possesses outstanding catalytic performance for the HER. The improved electrocatalytic performance of the Co9eNi1eBeP nanocomposites would be ascribed to two aspects: (i) the coordination effects between metal borides and metal phosphides effectively accelerate the electrocatalytic rate by lowering the reaction energy [32,33]. (ii) The increased pore size distribution derived from the phosphorization process endowed a rapid mass transport and provided more electrochemical interfaces, which is beneficial for the electrocatalytic activity.
Conclusions In summary, the partial phosphorization of porous CoeNieB composite was successfully prepared via a simple two-step strategy, which results in excellent electrocatalytic activity for the HER. The enhanced HER activity was ascribed to the coordination effects between the metal and non-metal species, which effectively accelerate the electrocatalytic rate by lowering the reaction energy. Besides, the specific porous architecture derived by bit of Ni exposes more electrochemical interfaces, which facilitate the catalytic activity. Moreover, the as-prepared catalyst exhibited excellent mass transport rate and long-term durability. This work paves a novel and efficient strategy to develop non-precious and cost-effective metal-based electrocatalysts with porous structure for scalable energy-related applications.
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Acknowledgements This work was supported by the Research Foundation for Distinguished Scholars of Qingdao Agricultural University (665-1119008). The authors thank the Central Laboratory of Qingdao Agriculture University for the physical characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.066.
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enhancing HER catalytic activity in alkaline solution. Adv Mater 2017;29:1606980. [69] Jin L, Lv C, Wang J, Xia H, Zhao Y, Huang Z. Co9S8 nanotubes as an efficient catalyst for hydrogen evolution reaction in alkaline electrolyte. Am J Anal Chem 2016;7:210. [70] Bukola S, Merzougui B, Akinpelu A, Zeama M. Cobalt and nitrogen co-doped tungsten carbide catalyst for oxygen reduction and hydrogen evolution reactions. Electrochim Acta 2016;190:1113e23.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis Dongqi Dong a,1, Xiuling Xu b,1, Chuanli Ma a, Liangyu Gong a, Linghao Su a, Jie Wang a,* a
College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao, 266109, People’s Republic of China b Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shanxi, 712100, People’s Republic of China
highlights
graphical abstract
Simple two-step preparation of partial phosphorization of porous CoeNieB composite. Morphology
can
be
tuned
by
introduction of Ni species. Porous architecture can be tuned during phosphorization process. Coordination effects between the metal and non-metal species result in the excellent HER performance.
article info
abstract
Article history:
Design of cost-effective and high-efficient electrocatalysts for hydrogen evolution reaction
Received 25 September 2019
(HER) is of vital significance for the current renewable energy devices d fuel cells. Herein,
Received in revised form
we report a facile strategy to prepare partial phosphorization of CoeNieB material with
8 December 2019
porous structure via a water-bath boronizing and subsequent phosphorization process at
Accepted 11 December 2019
moderate temperature. The optimal atomic proportion of Co to Ni is investigated via
Available online xxx
physical and electrochemical characterization. As a result, Co9eNi1eBeP exhibits the best HER activity, which require an lower overpotential of ~192 mV to deliver a current density
Keywords:
value of 10 mA cm2 and a smaller Tafel slope of 94 mV dec1 in alkaline media, relative to
CoeNieB material
P-free CoeNieB catalysts, Co9eNi1eBeP with other Co: Ni proportion and mono metallic
Porous architecture
borides The excellent electrocatalytic performance of Co9eNi1eBeP is mainly ascribed to
Partial phosphorization
the three-dimensional (3D) porous structure and the coordinate functionalization between
Quaternary composites
the borides and phosphides. This work provides a promising strategy for the exploration of
Hydrogen evolution reaction
quaternary composites as efficient and cost-effective electrocatalysts for HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J. Wang). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.12.066 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Introduction With the depletion of traditional fossil energy sources and the increasing environmental pollution issues, hydrogen, a renewable energy carrier, has caught much attention in newenergy vehicles [1e4]. Relative to traditional hydrogen production methods (pyrolysis natural gas method and coal gasification at high-temperature), electrochemical and photo assisted electrochemical water-splitting features environmental friendly, mild preparation conditions, and highefficiency [5e7]. At present, platinum shows to be the most effective catalyst for HER, which possess near theoretical overpotential value (nearly zero overpotential in acidic media) [8e12]. However, the scarcity and high cost situation seriously hindered their large-scale commercialization applications. Therefore, it is urgent to explore earth-abundant and costeffective electrocatalysts for the HER aiming to enhance the electrocatalytic efficiency and minimize the practical reaction resistance [13e16]. Till now, earth-abundant and non-precious metal-based catalysts have been extensively pursued for catalyzing the electrochemical HER, in which inorganic compounds are one of the important categories, especially the transition-metal (Fe, Co, Ni, and Mo)-based materials [17e24]. Specifically, the mono-metal borides (e.g. NieB [25] and MoXB [26,27]) have achieved wide attention in hydrogen generation. It is worth noting that most of the metal boride-based materials synthesized recently possess unsatisfied catalytic performance due to the large particle size distribution and low surface area, which are mainly attributed to the synthetic methods. Compared with solid nanocomposites, catalysts with hierarchical construction possess unique structural traits, the micrometer and nanometer-scaled building blocks joint features unique properties on electrocatalytic hydrogen generation [28e30]. As for HER, hierarchical porous structure could effectively facilitate the rapid mass rate and increase the electrochemical interface area, which are beneficial for highperformance electrocatalytic activity and stability. Tai et al. [26] reported an ultrathin hexagonal Mo3B films with 6.48 nm in thickness by chemical vapour deposition, which can be scalable production. The ultrathin film features metallic property, which exhibits excellent stability and activity. Cui et al. [31] demonstrated a porous Ni3B catalyst for efficient HER electrocatalysis via a simple one-step sintering method. Among them, Ni3B sintered at 850 C showed the best HER activity in acidic media due to the unique porous structure. It should be noted that the specific electrocatalytic mechanisms of improved HER activity by introduction of a second metal in the mono-metallic borides has been rarely elucidated [32,33]. Meanwhile, metal phosphide hybrids have greatly improve the activity of HER, and it has been investigated extensively, such as CoP [19,32,34], FeP [23], Ni2P [18,35], and MoP [36,37]. Therefore, tailoring the chemical composition of metal borides by doping or partial phosphorization with P sources would be a rational strategy to design efficient catalysts with higher HER performance through the coordinative effects of metal borides and metal phosphides. Along with compositional tailoring, controllable of the micro-/nano-structure to derive porous structure with high surface area is also an
important strategy to improve the electrocatalytic activity [38]. In this paper, we report a 3D porous Co9eNi1eBeP compound as a highly efficient and stable catalyst for HER in 1 M KOH media though a facile two-step approach. Firstly, CoeNieB compound was prepared by water-bath reduction of Co2þand Ni2þ using polyvinyl pyrrolidone (PVP) as a surfactant and NaBH4 as a reducing agent. Secondly, Co9eNi1eBeP composite with 3D porous structure was obtained by phosphorization at 300 C for 2 h in nitrogen-saturate atmosphere. As an electrocatalyst, the Co9eNi1eBeP composite exhibited excellent HER activity and long-term stability due to the coordinate functionalization between the borides and phosphides and the 3D porous structure derived by the addition of small quantity of Ni composite.
Experimental part Materials and reagents All the chemical reagents used in the sample preparation including CoCl2 6H2O (AR, ~99.0%), NiCl2 6H2O (AR, ~98.0%), polyvinyl pyrrolidone (PVP) and NaBH4 (AR, ~98.0%) were obtained from Sinopharm Chem. reagent Co. Ltd, China. Deionized water was obtained from an ultra-pure purifier (Ulupure, China, resistivity 18.2 MU).
Samples preparation The NieCoeB samples were fabricated by a rapid and facile reducing reaction. Firstly, 0.8 mmol of CoCl2 6H2O and NiCl2 6H2O with different Co: Ni molar ratios of 6:1, 9:1 and 12:1, respectively, together with 800 mg of polyvinyl Pyrrolidone (PVP) were added into a flask containing 200 mL deionized water and magnetic stirred in N2-saturated atmosphere for 30 min at room temperature. Subsequently, with continuous stirring and aerating condition, 80 mL of deionized water with 100 mg NaBH4 was quickly added into the above solution. At last, the resulting black products was obtained through vacuum filtration and washed several times with deionized water and alcohol in sequence. The obtained catalysts were denoted as Co6eNi1eB, Co9eNi1eB and Co12eNi1eB, respectively. The CoeB and NieB samples were synthesized with the same process accordingly. Phosphorous doped CoeNieB (CoeNieBeP) samples were prepared using a low-temperature phosphorization method. Different mass ratio of CoeNieB and NaH2PO2 (1:10; 1:15; 1:20) were added into two separated porcelain boats, and then annealed with a heating rate of 2 C min1 to 300 C for 2 h under N2 atmosphere in a tube furnace.
Physical characterization Morphologies of the composite were characterized by scanning electron microscopy (SEM, JSM-7500F). Transmission electron microscopy (TEM) and scanning TEM (STEM) images was performed on JSM-2100F equipment. Powder X-ray diffraction (XRD) data were collected on a Bruker D8, and diffraction patterns were collected using Cu Ka (l ¼ 1.5406 A)
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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radiation at a scanning rate of 8 min1. X-ray photoelectron spectroscopy (XPS) was conducted on AXIS-ULTRA DLD to investigate the chemical valence and composition. The specific surface area was calculated follows the BrunauereEmmetteTeller (BET) method by using Quantachrome (NOVA 2200e) equipment.
Electrochemical characterization The electrochemical measurement for the HER was carried out via a typical three-electrode system in 1 M KOH solution using a CHI 760 E potentiostat. The active material depositing Ni foam, carbon rod and reversible hydrogen electrode (RHE) were applied as working electrode, counter electrode and reference electrode, respectively. Before preparing the working electrode, the Ni foam was processed with diluted HCl solution, ethanol and deionized water several times before utilization. The prepared catalysts with carbon black and polyvinylidene fluoride (7:2:1, mass ratio) were dispersed into n-methyl-pyrrolidone (NMP) to form homogenous ink. And then measure 100 mL and deposited onto the Ni foam (working area 1.5 cm2), dried at 80 C for 12 h (the mass loading was 0.46 mg cm2). Comparison of linear sweeping voltammograms (LSV) curves of Co9eNi1eBeP via Ni foam and rotating disc electrode (RDE) in 1 M KOH solution at 1600 rpm rotation speed was conducted (Fig. S5). Negligible change can be clarified, indicating that the two kind of electrochemical measurements can be comparable. Taking account of the practical application of water splitting, the catalysts loaded on the Ni foam were used in this paper. The cyclic voltammograms (CVs) for the determination of double layer capacitance (Cdl) values of the catalysts were measured at scan rate of 20, 40, 60, 80, 100, 120 mV s1 from 0.1 to 0.2 V. The linear sweeping voltammograms (LSV) was conducted from 0.05 to 0.6 V at a scanning rate of 5 mV s1. The long-term durability of the asprepared electrocatalysts was analyzed by chronoamperometry measurements. Multi-step chronoamperometric curve of Co9eNi1eBeP was obtained at overpotentials as follows: 0.12, 0.16, 0.19, 0.21, 0.23, 0.21, 0.19, 0.16 and 0.12 V, respectively. Long-term stability of Co9eNi1eBeP was performed via chronoamperometry measurement at a constant potential of 0.19 V.
Results and discussion Microscopic characterization The physical properties, especially the morphological and structural parameters of the as-prepared catalysts were characterized via SEM and TEM techniques. As can be seen from the SEM image in Fig. 1a, the CoeB sample featured porous flake-like structure, while by increasing the Ni proportion in the synthetic approach, the flake-like structure gradually disappeared and derived smaller pore structure from Co12eNi1eB to Co6eNi1eB (Fig. 1bed). However, the NieB sample showed negligible pore structure, but composed of spherical aggregates (Fig. 1e). To confirm the SEM results, N2 adsorption/desorption measurements (Fig. 1f) was carried out to further study the specific surface physical structures.
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According to the N2 adsorption/desorption curves, the isotherm showed obvious hysteresis loop, which belongs to type IV and is typical for mesoporous materials [38]. The BET specific surface area of the as-prepared CoeB, Co9eNi1eB and NieB samples are 97.8, 112.8 and 34.7 m2 g1, respectively, indicating that the addition of Ni in the Co-based architecture efficiently increased the surface area, which would provide more electrochemical interfaces to enhance the HER performance. While after partial phosphorization step, the Co9eNi1eBeP sample presents more porous architecture according to the upwards-magnifying SEM images (Fig. 2aec) compared with the Co9eNi1eB (Fig. 1c). The high-resolution TEM image in Fig. 2d showed the enlarged magnification of Co9eNi1eBeP with discontinuous lattice fringes, indicating a poor crystallization of the sample. The inter-planar spacing for the lattice fringes is calculated to be 0.22 nm, corresponding to the (111) plane of Co2P. The high-angle annular dark field scanning TEM image (HAADF-STEM, Fig. 2e) and the corresponding elemental mapping originate from the energy dispersive spectrum (EDS, Fig. 2eeh and Fig. S1) showed that Co, Ni and P elements are evenly distributed in the local area, which further demonstrates the successfully phosphorization of the CoeNieB sample. Moreover, as can be seen from Fig. S2, obvious hysteresis loop can be clearly classified for the two samples, while, the hysteresis loop of Co9eNi1eBeP sample distributed close to the higher P/P0 area, indicating that the pore size distribution would be changed on the trend from mesoporous to macroporous by phosphorization process [39]. Moreover, the specific surface area of the two samples showed no obvious change. The pore distribution change and phosphorization reaction would benefit to improve the electrochemical interfaces and increase the ion transport rate during electrochemical process.
Structural measurements The crystal structure of Co9eNi1eBeP sample was further investigated by X-ray diffraction (XRD) technique (Fig. 3a). A single broad peak centred at 28 is corresponding to the amorphous state of the obtained composite. Recent studies have been demonstrated that amorphous multi-metallic materials often display higher electrochemical activity than the crystalline material [40]. Besides, the diffraction peaks located at 40.8 , 44.9 , 48.4 and 55.5 are corresponding to the (111), (021), (120) and (030) planes of Co2P, which is consistent to the standard diffraction pattern of Co2P (PDF No. 00-054-0413). Compared with XRD pattern of Co9eNi1eB, partial phosphorization process effectively improved the crystallinity of the material in some extent. The surface property of Co9eNi1eBeP sample was investigated via X-ray photoelectron spectroscopy (XPS) survey scan spectrum, in which the characteristic peaks of Ni, Co, B, P elements are shown in Fig. 3b. The high-resolution spectrum of the existing components were characterized and analyzed. For the P 2p energy level, two fitted peaks located at 129.3 and 133.5 eV in binding energies can be assigned to M P (M related to Co or Ni elements) and PeO species (Fig. 3c) [41]. Meanwhile, the two peaks at 186.9 and 192.3 eV for the B 1s spectrum are ascribed to the M B and BeO species (Fig. 3d) [42,43]. The appearance
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 1 e Upwards-magnifying SEM images of (a) CoeB, (b) Co12eNi1eB, (c) Co9eNi1eB, (d) Co6eNi1eB and (e) NieB; The scale bar in the SEM images is 200 nm. (f) The nitrogen adsorptionedesorption isotherm and the specific surface area of Co9eNi1eB, CoeB and NieB.
of PeO and BeO species may be attributed to the surface oxidation when prepared materials were exposed to air [44,45]. The P 2p and B 1s spectra both demonstrate the coexistence of metal boride and phosphide in the sample. In the high-resolution Co 2p spectrum (Fig. 3e), the peaks located at 778.0 eV and 781.6 eV are representing to CoeB and CoeP
bonds, respectively [33,46]. Besides, the other peak located at a higher binding energy of 785.4 eV is derived from the oxidation of cobalt species. The high-resolution Ni 2p spectrum (Fig. 3f) shows three characteristic peaks at 852.8 eV, 854.8 eV and 857.0 eV, which are ascribed to be attributed to Ni in Ni 2p, oxidized Ni species and the satellite of the Ni 2p3/2
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 2 e (aec) Upwards-magnifying SEM images of Co9eNi1eBeP sample. (d) HRTEM image of Co9eNi1eBeP. (eeh) The HAADF-STEM image and the corresponding EDS mapping of Co, Ni and P.
peak, respectively [41,47]. According to the XPS analysis, it can be concluded that the Co9eNi1eB was successfully experienced partial phosphated.
Electrochemical performance The electrocatalytic properties of the as-prepared catalysts were investigated in 1.0 M KOH solution via a typical threeelectrode system. According to the previous reports, an efficient electrocatalyst should possess relative low overpotential to deliver a specific current density during the LSV measurements [48]. The electronic structures can be improved to just the reaction kinetics by the introduction of different metal components, which is beneficial to lower the kinetic energy barriers of electrocatalytic reactions [49,50]. To investigate the coordination effect of Co and Ni, the catalysts were optimized by investigating the Co and Ni ratios. As demonstrated in Fig. 4a obviously, the Co9eNi1eB delivers the lowest overpotential (340 mV) at 100 mA cm2 relative to the Co6eNi1e B (365 mV) and Co12eNi1eB (353 mV) samples. The Tafel curves derived from the polarization curves (Fig. 4b) showed calculated Tafel slopes to investigate the reaction kinetics of the CoeNieB compound. It can be seen that the obtained Co9eNi1eB exhibited the lowest Tafel slope value of 96 mV dec1 compared with Co6eNi1eB (107 mV dec1) and Co12eNi1eB (97 mV dec1), indicating the fastest hydrogen production rate of Co9eNi1eB. Detailed comparisons of the HER for the varying
ratios of Co and Ni precursors are provided from the histogram in Fig. 4c. Although the Co9eNi1eB present excellent performance for the HER, they still fall behind the precious metal-based catalysts. As is well known that metal phosphides are highly active for electrocatalysis in hydrogen production [32,51]. Thus, low-temperature phosphorization was further investigated for the target of improving the HER.
Performance As can be seen from the polarization curves in Fig. 4d, the overpotential evidently decreased from 340 mV for Co9eNi1eB to 291 mV for Co9eNi1eBeP (1 : 15, mass ratio of M: P) to drive the same current density of 100 mA cm2. Fig. 4e and f clearly exhibit that among all of the phosphorization samples, the catalyst obtained at a M: P mass ratio of 1:15 exhibited the best HER activity for the HER with the lowest Tafel slope of 94 mV dec1 relative to 1:10 (105 mV dec1) and 1:20 (104 mV dec1), and the lowest overpotential of 192 mV at 10 mA cm2 relative to 1:10 (237 mV) and 1:20 (200 mV), which demonstrated that different degrees of phosphorization condition result in distinct activities. Therefore, the partial phosphorization for the obtained bimetal borides is efficient to improve the electrocatalytic properties towards the HER. The HER performance of the porous Co9eNi1eBeP composite was studied systematically, especially the reaction activity, kinetics and long-term durability. It can be seen from
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Fig. 3 e (a) XRD patterns of Co9eNi1eBeP and Co9eNi1eB samples. (b) Full-range XPS spectrum of Co9eNi1eBeP sample and the corresponding high-resolution fitting spectrum of P2p (c), B1s (d), Co2p (e) and Ni2p (f).
Fig. 4 e (a) The polarization curves of Co12eNi1eB, Co9eNi1eB and Co6eNi1eB catalysts in 1.0 M KOH solution at scanning rate of 5 mV s¡1, the corresponding Tafel slopes (b) and the corresponding histograms of overpotentials at 10 mA cm¡2 and Tafel slopes (c). (d) The polarization curves of Co9eNi1eB with different P source proportion in 1.0 M KOH solution at scanning rate of 5 mV s¡1, the corresponding Tafel slopes (e) and the corresponding histograms of overpotentials at 10 mA cm¡2 and Tafel slopes (f). Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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the polarization curves in Fig. 5a, the Co9eNi1eBeP showed higher HER activity with the lowest overpotential compared with the mono metal borides and Co9eNi1eB, except for the commercial Pt/C. Compared with the recent reports, the HER activity of Co9eNi1eBeP ranked in the front (Table 1). As is well known, the electrochemically active surface area (ECSA) is linearly proportional to the electrochemical double layer capacitance (EDLC, Cdl). According to the capacitive current at 0.15 V as a function of scan rate derived from the CV curves at
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different scanning rates (Fig. S3), the Cdl value of Co9eNi1eBeP was calculated to be 10.0 mF dec1 (Fig. 5b), higher than Co9eNi1eB (9.0 mF dec1), CoeB (2.8 mF dec1) and NieB (1.6 mF dec1), demonstrating that the partial phosphorization and the coexistence of Ni and Co effectively enlarged the electrochemical active interfaces, which is beneficial for enhancing the catalytic activity. The Cdl value decreasing trend of Co9eNi1eB, CoeB and NieB samples is fully consistent with the nitrogen adsorptionedesorption isotherm results in Fig. 1f.
Fig. 5 e (a) The polarization curves of NieB, CoeB, Co9eNi1eB, Co9eNi1eBeP and Pt/C in 1.0 M KOH solution at a scanning rate of 5 mV s¡1. (b) The Cdl value of NieB, CoeB, Co9eNi1eB and Co9eNi1eBeP. (c) The Tafel plots corresponding to the catalysts in a. (d) The j0 value of NieB, CoeB, Co9eNi1eB and Co9eNi1eBeP obtained via extrapolating the Tafel plots and Histograms of j0 value and Tafel slopes for the four samples (inset). (e) Multi-step chronoamperometric curve of Co9eNi1eBeP at different overpotentials. (f) Long-term stability of Co9eNi1eBeP via chronoamperometry measurement at a constant potential of ¡0.19 V. Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Table 1 e Comparison of HER activities for Co9eNi1eBeP and other similar electrocatalysts reported at current density of 10 mA cm¡2. Catalysts
morphology
overpotential/mV at 10 mA cm2
Electrolyte
1 2 3 4 5 6 7 8 9 10
Fe10Co40Ni40P Coe50NieB/CC CoP@BCN CoNiSe/C/NF Co/NBC Co9S8eNi3S2 NAs/NF Co4Ni1P Co2P Coe30NieB CoS2/RGO-CNT
68 80 87 90 117 120 129 130 133 142
1 M KOH 1.0 M KOH 0.5 M H2SO4 1 M KOH 1.0 M KOH 1.0 M KOH 1.0 M KOH 0.5 M H2SO4 1 M NaOH 0.5 M H2SO4
[53] [54] [55] [56] [57] [58] [59] [60] [61] [62]
11 12 13 14 15 16 17 18 19
NieCoeMoS2 CoFe LDH-F Co9eNi1eBeP CoFe/NF Ni65Co29Y6 Co9S8@C Co2P Co9S8 Co and N Co-Doped Tungsten Carbide
nanosheets network-like porous structure nanotubes nanostructure microspheres nanosheets nanotubes nanorods spherical nanoparticles porous CNT network nanoboxes nanosheets porous architecture nanosheets / nanoparticles / Nanotubes /
155 166 192 220 230 245 247 290 450
0.5 m H2SO4 1 M KOH 1 M KOH 1 M KOH 1 M NaOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH
[63] [64] This work [65] [66] [67] [68] [69] [70]
Number
The Tafel slope fitted from the polarization curves in Fig. 5a was also calculated to investigate the reaction kinetics of the as-obtained catalysts (Fig. 5c). Despite the lowest Tafel slope value of 53 mV dec1 for Pt/C catalyst, the Co9eNi1eBeP exhibited the smaller Tafel slope value of 94 mV dec1, relative to Co9eNi1eB (96 mV dec1), CoeB (109 mV dec1) and NieB (113 mV dec1), demonstrating that the Co9eNi1eBeP follows a VolmereHeyrovsky reaction process [52]. Moreover, the lowest Tafel slope of Co9eNi1eBeP composite indicates the fastest hydrogen production rate relative to other samples prepared via the same method. Exchange. Current density (j0) is also a key kinetic parameter for examining the intrinsic electrocatalytic properties of the obtained catalysts, which can be obtained by extrapolating the Tafel plots to 0 V (Fig. 5d). The j0 values of the catalysts are shown from the inset of Fig. 5d, in which the Co9eNi1eBeP exhibits the largest j0 value relative to Co9eNi1eB and mono metal borides, indicating the fastest electron transfer rate of HER. Such results might be ascribed by both the morphology of pore structure and particle size influence.
Stability tests The multi-step chronoamperometric responses of Co9eNi1eBeP were also conducted to evaluate the mass transport properties and durability. As shown in Fig. 5e, the current density dropped with the increasing of overpotential, and in return, the current density elevated with the decreasing of overpotential, indicating the excellent reversible capability and stability of the Co9eNi1eBeP composite. Chronoamperometry measurement was further conducted (Fig. 5f), and demonstrating that the current density showed slightly decrease at the initial 3 h and then tend to be constant even after 11 h under a applied overpotential of 0.19 V,
Refs
confirmed the excellent durability of Co9eNi1eBeP composite. Besides, further accelerate CV measurements were applied to investigate the stability of Co9eNi1eBeP. as shown from the LSV curves (Fig. S4), only a slight potential decay after 1000 potential cycles, further suggesting the high stability of the Co9eNi1eBeP for HER. As discussed above, the as-prepared Co9eNi1eBeP possesses outstanding catalytic performance for the HER. The improved electrocatalytic performance of the Co9eNi1eBeP nanocomposites would be ascribed to two aspects: (i) the coordination effects between metal borides and metal phosphides effectively accelerate the electrocatalytic rate by lowering the reaction energy [32,33]. (ii) The increased pore size distribution derived from the phosphorization process endowed a rapid mass transport and provided more electrochemical interfaces, which is beneficial for the electrocatalytic activity.
Conclusions In summary, the partial phosphorization of porous CoeNieB composite was successfully prepared via a simple two-step strategy, which results in excellent electrocatalytic activity for the HER. The enhanced HER activity was ascribed to the coordination effects between the metal and non-metal species, which effectively accelerate the electrocatalytic rate by lowering the reaction energy. Besides, the specific porous architecture derived by bit of Ni exposes more electrochemical interfaces, which facilitate the catalytic activity. Moreover, the as-prepared catalyst exhibited excellent mass transport rate and long-term durability. This work paves a novel and efficient strategy to develop non-precious and cost-effective metal-based electrocatalysts with porous structure for scalable energy-related applications.
Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066
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Acknowledgements This work was supported by the Research Foundation for Distinguished Scholars of Qingdao Agricultural University (665-1119008). The authors thank the Central Laboratory of Qingdao Agriculture University for the physical characterization.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.066.
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Please cite this article as: Dong D et al., Partial phosphorization of porous CoeNieB for efficient hydrogen evolution electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.066