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Growth of radial microspheres of Ni-Co-O at porous Ti and its phosphorization for high efficient hydrogen evolution
Electrochimica Acta 259 (2018) 329e337
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Growth of radial microspheres of Ni-Co-O at porous Ti and its phosphorization for high efficient hydrogen evolution Mao Zhou 1, Ya Liu 1, Dejuan Fa 1, Lihong Qian 1, Yuqing Miao* University of Shanghai for Science and Technology, Shanghai, 200093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2017 Received in revised form 31 October 2017 Accepted 1 November 2017 Available online 2 November 2017
The phosphorized radial microspheres of Ni-Co-O on a porous Ti plate (Ni-Co-O/Ti) were fabricated for the electrocatalysis of hydrogen evolution reaction (HER). They are characterized by SEM, TEM, XRD, XPS and BET. The Ni-Co-P/Ti exhibited the low onset potential of 100 mV, achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 20 and 80 mV, respectively. The Tafel slope is approximately 43 mV/dec, which is close to that of commercial Pt catalyst and lower than many previous reports. On the other hand, the Ni-Co-O/Ti exhibited high electrocatalytic oxidation of glucose with a relatively low onset potential at 0.29 V, where glucose was introduced to decrease the cell potential of water splitting. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Ni-Co-P Ni-Co-P Radial microspheres Phosphorization
1. Introduction The depletion of fossil fuels and increased environmental concerns have resulted in an urgent demand for clean and sustainable replacement of energy sources [1e4]. As an ideal candidate, hydrogen produced through water splitting is especially attractive. The platinum group metals or their compounds possess relatively high catalytic activity for hydrogen evolution reaction (HER) in acidic media [5], but the elemental scarcity and the high cost hinder their widespread applications [6e8]. It is therefore urgently demanded to design and develop cost-effective, active, and durable HER catalysts made of earth-abundant elements [9e11]. The past few years have witnessed the rapid development of transition metal phosphides (TMPs) as efficient HER catalysts in acid media [12e15]. As an important class of compounds with metalloid characteristics and good electrical conductivity [16], various TMPs have emerged as promising catalysts of HER including NiP [17], CoP [11,18], MoP [19], FeP [16], FexCo1exP and so on [20]. However, further exploration has been still desired to improve their activity and durability to connect the gap of HER electrocatalytic performance between TMPs and noble metal based
catalysts. It has been reported that there are many approaches to improve the performance of non-noble-metal catalysts, for example, synthesizing the catalysts of high specific area with the super-small/ fine particle/wire size, ultrathin thickness, porosity or mesoporosity, or other special morphologies [1,7,21,22]. Furthermore, the enhanced activity can be obtained by heteroatom doping or addition of other elements to prepare multinary compounds/ nanocomposites [8]. Frequently, TMPs were hybridized with other nanomaterials like carbon nanotube [23], graphene, graphite or carbon etc. The increased stability can be obtained by using carbon cloth and Ti plate as the catalytic substrates [1,18,24]. A Mn-doped CoP nanosheets array was fabricated on Ti mesh as an efficient 3D HER electrocatalyst with good stability at all pH values [25]. Herein, the radial microspheres of Ni-Co-O composed of nanoneedle subunits were in site synthesized and deposited at porous Ti. After phosphorization, they were employed for HER electrocatalysis. The catalysts were characterized with their morphology, composition and structure. Their electrocatalytic performance was studied in detail. 2. Experiments
* Corresponding author. E-mail address: [email protected] (Y. Miao). 1 Mao Zhou, Ya Liu, Dejuan Fa and Lihong Qian contributed equally to this work and should be considered as co-first authors. https://doi.org/10.1016/j.electacta.2017.11.001 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
2.1. In site growth of the radial microspheres of Ni-Co-O at porous Ti plate The porous Ti plate was obtained by ultrasonic corrosion of a
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Fig. 1. SEM images of porous Ti plate (A), Ni-Co-O/Ti (B,C) and Ni-Co-P/Ti (DeF). EDXS of Ni-Co-O/Ti (G) and Ni-Co-P/Ti (H).
powder pressed sintering Ti plate (1 cm 4 cm 1 mm) with open-cell rate of 70% and porosity of 60% in 14.4 M concentrated HNO3 for 60 min and then cleaned through ultrasonic process in
alcohol and ultra-purified water sequentially each for 5 min, finally dried for use. 20 mL of 50 mM NiNO3, 10 mL of 100 mM CoCl2, 10 mL of 50 mM
Fig. 2. Formation mechanism of Ni-Co-O.
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urea were mixed under continuously stirred at room temperature for 1 h to form a homogeneous solution. The pink solution was then introduced to a 60 mL Teflon-lined stainless-steel autoclave with a piece of cleaned porous Ti plate inside, sealed and subsequently heated to 180 C for 24 h in an oven. After cooling down to room temperature naturally. The color of porous Ti plate turned from grey to bluish violet. The porous Ti plate deposited with Ni-Co-O was washed for several times with deionized water and absolute ethanol to remove the remaining reactants. Finally, it was dried at 50 C under vacuum for 12 h to obtain the sample of Ni-Co-O/Ti.
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system (PerkinElmer). The N2 adsorption isotherms were monitored with a Surface Area and Porosity Analyzer of ASAP 2460 (Micromeritics). For the N2 adsorption-desorption measurements of Ni-Co-O and the Ni-Co-P, they were scratched gently from the porous Ti substrates. Electrochemical analysis was performed using an electrochemical workstation of CHI 760D (CH Instruments) with a Ni-Co-O/Ti or Ni-Co-P/Ti as working electrode, an Ag/AgCl electrode as reference electrode and a platinum wire as counter electrode. 3. Results and discussion
2.2. Phosphorization of the Ni-Co-O/Ti 200 mg hypophosphite (NaH2PO2) was placed at the upstream of the corundum crucible with the Ni-Co-O/Ti at the downstream. The temperature of the tube furnace was raised from 25 C to 300 C with a heating rate of 5 C/min and held at 300 C for 2 h. After then, the furnace was allowed to cool down to room temperature under N2 atmosphere. Finally, the black product of Ni-CoP/Ti was obtained. As contrast, a Ni-Co-O/Ti was phosphorized at 350 C to obtain the Ni-Co-P’/Ti. 2.3. Instruments The morphologies and compositions of samples were characterized using a scanning electron microscope (SEM) of MIRA3 XMU/ XMH (TESCAN) and an energy dispersive X-ray spectrometer (EDXS) of SYSTEM 7 (Thermo Fisher). A JEOL JEM-2010F Transmission electron microscope (TEM) with selected area electron diffraction (SAED) pattern was used to investigate the samples. Xray diffraction (XRD) patterns were collected by using a D8 Advance diffractometer (Bruker). X-ray photoelectron spectroscopy (XPS) analysis were performed by a RBD upgraded PHI-5000C ESCA
A powder pressed sintering Ti plate was firstly corroded in concentrated HNO3 to obtain the porous Ti plate with high surface area as catalyst support. As shown in Fig. 1A, the yielded porous Ti plate exhibits the 3D, porous and honeycombed structures. By hydrothermal reaction, the nanoneedle array of Ni-Co-O grows and deposits on the surface of the porous Ti plate. Interestingly, two kinds of morphologies are observed in Fig. 1B. At the local Ti substrate, the nanoneedle array of Ni-Co-OI distribute uniformly on the surface. It may be due to the fact that the nucleus formation of NiCo-OI occurs at the sites of Ti substrate. Because of the presence of steric hindrance, the nucleus growth extends along the direction away from the Ti substrate. The formation mechanism of the Ni-CoOI nanoneedle array is described in Fig. 2. Within the cavity of the porous Ti plate, the radial and urchin-like microspheres of Ni-Co-OII were observed and they connect each other. The microspheres were composed of sharp nanoneedles radially grown from the center with the diameter about 8 mm. As seen from Fig. 2, the nucleus formation of Ni-Co-OII occurs in solution. Due to the homogeneity of solution in every direction, the nucleus growth extends freely and radially. Then the growing Ni-Co-OII radial microspheres accumulates within the cavity of the porous Ti plate and connects
Fig. 3. EDXS elemental mapping images of the radial microspheres of Ni-Co-PII.
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each other. Fig. 1DeF shows the morphology of Ni-Co-P/Ti, the phosphorized Ni-Co-O/Ti. No obvious differences are observed. It means that the temperature of 300 C and the phosphorization process do not affect their morphology. EDXS was performed to further investigate the composition and the element distribution. As shown in Fig. 1G, several peaks correspond to the elements of Ni, Co and O. The atom ratio of Ti:Ni:Co:O are about 23.42:5.98:5.33:64.27. The peaks from Ti, Ni, Co, and P in Fig. 1H confirm the presence of these elements in the sample of Ni-Co-P/Ti. The atom ratio of Ti:Ni:Co:P:O are about 20.17:6.62:6.56:6.34:60.31, revealing the ratio of Ni-Co and P in the sample to be close to 2: 1. To identify the composition distribution of the Ni-Co-PII radial microspheres, their elemental mappings were examined by EDS. As presented in Fig. 3, the EDXS mapping images clearly illustrate that Ni, Co, O and P uniformly distribute throughout the entire microspheres. The results verify the successful chemical conversion of NiCo-O/Ti into Ni-Co-P/Ti via the high-temperature phosphorization. TEM images in Fig. 4A confirm the morphology of radial microsphere for Ni-Co-PII. It is composed of many radial nanoneedles with a width about 36 nm. As shown in Fig. 4B, the highresolution TEM (HRTEM) images taken from the Ni-Co-PII radial microspheres display two lattice fringes with the interplanar spacing of 0.36 nm and 0.19 nm, corresponding to the [001] plane of Ni2P and the [002] plane of Co2P, respectively [17]. It has been well known that with phosphorus resource at the high temperature,
typically above 300 C, often lead to the formation of phosphide [9,13,26,27]. The discrete dots in the selected area electron diffraction (SAED) pattern of Fig. 4C and the sharp peak shape in the XRD pattern of Fig. 4D confirm the crystalline property of the NiCo-P. The XRD pattern are indexed to the multiphase of hexagonal Ni2P (PDF#65-9706) and Co2P (PDF#54-0413) [12,28,29], in accord with the result of EDS in Fig. 1H where the atomic ratio of NiCo to P is close to 2:1. Fig.S1 also shows the sharp peak shape, exhibiting the crystalline property of the Ni-Co-O. Its XRD pattern can be indexed to the multiphase of Co1$29Ni1$71O4 (PDF#40-1191) and CoO (PDF#42-1300). The overall spectrum of XPS covers the signals of Ni 2p, Co 2p, P 2p and O 1s regions of the Ni-Co-P (not shown). Fig. 5A exhibits two main peaks at 874.5 eV for NiII 2p1/2 and 856.9 for NiII 2p3/2 with a split spin-energy of 17.6 eV, accompanied by the satellite peaks at 880.5 and 862.2 respectively [12]. A small peak at 853.6 eV is attributed to zero valence state Ni0. Similarly, the Co 2p spectrum in Fig. 5B also shows two main peaks at 798.2 eV and 782 eV with the respective satellite peaks at 786.3 eV and 803.2 eV. They can be assigned to CoII 2p1/2 and CoII 2p3/2, respectively. In Fig. 5C, two peaks of P 2p spectrum are observed at 134.1 eV and 129.5 eV. The former is ascribed to oxided P (phosphate) from the superficial oxidation of phosphide due to the exposure of the sample to air. The latter is assigned to the reduced P in phosphide [30,31]. The O 1s peak in Fig. 5D is located at 531.6 eV, corresponding to the surface oxides or phosphate.
Fig. 4. TEM images (A), HRTEM image (B) and SAED pattern (C) of the Ni-Co-PII radial microspheres. The diffraction spots were marked in (C). XRD pattern (D) of the Ni-Co-P.
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Fig. 5. High resolution XPS spectra of the Ni-Co-P.
N2 adsorption-desorption measurements were performed to study the textural properties of the Ni-Co-O and the Ni-Co-P. The N2 adsorption-desorption isotherms in Fig. 6 reveal their property of mixed type of II and IV. The S shape isotherms is ascribed to the nonporous property of radial Ni-Co-O/Ni-Co-P microspheres. The hysteresis loop is usually described as type IV, a typical characteristic of mesoporous material. However, for radial Ni-Co-O/Ni-Co-P
microspheres, the mesoporosity-like property exists at the core area of the microspheres, resulting in a narrow distribution of pore diameter, as shown in the inset of Fig. 6A and B. From Fig. 6A, the surface area, total pore volume and average pore diameter of the Ni-Co-O are obtained with the values of 68.7 m2/g, 21.8 cm3/g and 12.7 nm, respectively. Phosphorization of the Ni-Co-O at 300 C leads to the slight collapse of the radial structure at the core area,
Fig. 6. N2 adsorption-desorption isotherm and BJH Desorption dV/dD (inset) of the Ni-Co-O (A) and the Ni-Co-P (B).
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Fig. 7. (A) Polarization curves of the porous Ti plate, the Ni-Co-O/Ti, the Ni-Co-P’/Ti and the Ni-Co-P/Ti in 0.5 M H2SO4. (B) Tafel plots of the Ni-Co-O/Ti and the Ni-Co-P/Ti in 0.5 M H2SO4. (C) The amount of H2 theoretically calculated (black curve) and experimentally measured (red curve) versus time for the Ni-Co-P/Ti under the overpotential of 0.2 V. (D)Time-dependent current density curve of the Ni-Co-P/Ti under the overpotential of 0.15 V for 80000 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
activity than the Ni-Co-O/Ti and the Ni-Co-P’/Ti. It means that phosphorizing the Ni-Co-O/Ti results in the increased HER activity and the temperature of 300 C for phosphorization is preferred to that of 350 C. The Ni-Co-P/Ti exhibits the lowest onset potential of 100 mV, achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 20 and 80 mV, respectively. These values of the Ni-Co-P/Ti compare favorably to many previously reported
and thus brings out the change of values with the surface area of 19.9 m2/g, the total pore volume of 15.0 cm3/g and the average pore diameter of 30 nm, for the Ni-Co-P. The HER electrocatalysis of the Ni-Co-P/Ti was studied by comparing with the porous Ti plate, the Ni-Co-O/Ti and the Ni-CoP’/Ti. As shown in Fig. 7A, the blank porous Ti plate doesn't show obvious HER activity, while the Ni-Co-P/Ti exhibits far higher HER
Table 1 HER electrocatalysis performance of some typical catalysts reported previously. Catalysts /electrode
Onset potential (mV)
Overpotential (mV)
Current density (mA/cm2)
Tafel (mV/dec)
radial microspheres of Ni-Co-P on porous Ti plate
100
herein
55
61
[32]
Pt
24
30
[32]
Ni-Mo-S/carbon fiber cloth MoSx-carbon nanotube/GCE NiS2-graphite/graphite NiSx/fluorinated tin oxide Fe-Ni-S/GCE CoP nanotubes
132 75 110 330
10 50 10 50 10 50 10 10 10 10 10 10 20 100
43
Ni-Mo-S/Ti
20 80 30 86 18 57 200 110 190 440 105 108 127 173
85.3 40 80 62 40 59
[24] [23] [33] [34] [35] [7]
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Fig. 8. CV curves at the Ni-Co-O/Ti in 1 M NaOH without (a) and with (b) 0.01 M glucose with the scan rate of 0.1 V/s. (B) CV curves at the platinum wire cathode with the Ni-Co-O/Ti as anode in 1 M NaOH with (a) and without (b) 0.01 M glucose at the scan rate of 0.1 V/s.
non-noble-metal HER catalysts, as shown in Table 1. Fig. 7B shows the Tafel plots of the Ni-Co-O/Ti and the Ni-Co-P/ Ti. They are fit to the Tafel equation where h ¼ b log j þ a and j is the current density. The Tafel slopes of b are measured to be approximately 73 mV/dec for the Ni-Co-O/Ti and 43 mV/dec for the Ni-CoP/Ti, while the latter is close to that of commercial Pt catalyst and lower than many previous reports in Table 1. Under the applied overpotential of 0.2 V, gas bubbling of H2 evolved continuously at the Ni-Co-P/Ti cathode and that of O2 at the Pt anode. The amount of the produced H2 was collected and measured during 4000 s of H2O electrolysis in Fig. 7C. The theoretically calculated amount of H2 and the experimentally measured one coincide well versus time, indicating high Faraday efficiency of the Ni-Co-P/Ti due to its high electrocatalytic efficiency of HER. Stability is one of key factors for practical applications of HER electrocatalysts, for which the durability of the Ni-Co-P/Ti was evaluated by long-term electrolytic tests in 0.5 M H2SO4. As shown in Fig. 7D, the electrocatalytic signal of HER at the Ni-Co-P/Ti under the overpotential of 0.15 V is relatively constant and no obvious decrease can be observed during the range from 20000 s to 80000 s. The slight decrease within the first 20000 s can be due to the transient state before stabilization. The tested stability duration is much longer than most of previously reports [23,24,32,33]. Observed from the SEM images in Fig.S2, the morphology of Ni-CoP radial microspheres changes slightly where the slight adhesion of their blades or needles occurs. The practical application of H2 generation through H2O electrolysis has been limited by its high energy consuming. In order to decrease cell potential, the anodic oxygen evolution reaction of H2O can be replaced with that of more readily oxidizable, hydrogencontaining small molecules like ammonia [36], methanol [37,38], ethanol [39], glycerol [40,41], ammonia borane [42], and urea [43,44] et al. Here, glucose was introduced to be oxidized for decreasing the cell potential of water splitting. It is found that the Ni-Co-O/Ti exhibited far higher electrocatalytic oxidation of glucose than the Ni-Co-P/Ti (not shown). In the presence of glucose (Fig. 8A), the oxidation current increases sharply with a relatively low onset potential at 0.29 V vs Ag/AgCl that is far lower than that for oxygen evolution from H2O. The signal increase between 0.29 V and 0.55 V could be attributed to the mediated electrocatalysis by the redox couples of NiII/NiIII and CoII/CoIII [45,46]: MII þ OH e % MIII þ H2O
MIII þ glucose / MII þ intermediate products where M means Ni or Co. When the applied potential is higher than 0.55 V, the steeper increase of oxidation current is observed, which is due to the direct electrocatalysis of NiIII and CoIII [46e51]: MIII þ C6H12O6 þ 24OH / MIII(C6H12O6)ads / MIII þ 6CO2[ þ 18H2O þ 24e In the process of electrocatalytic oxidation toward glucose, gas bubbles occurred violently at both the anode and cathode. The gas bubbles at anode is CO2 from glucose oxidation, while that at cathode is H2 from HER: 24H2O þ 24e / 12H2[ þ 24OH The overall cell reaction is summarized as follows: C6H12O6 þ 6H2O / 6CO2[ þ 12H2[ which was further confirmed by exchanging the working electrode and the counter electrode as used in Fig. 8A. As a result, the platinum wire works as working electrode and the Ni-Co-O/Ti as counter electrode in Fig. 8B. In the same way, glucose oxidation occurs at Ni-Co-O/Ti anode and HER does at the platinum wire cathode. In the presence of glucose, HER occurs at a less negative onset potential of 1.03 V vs Ag/AgCl (or 0.179 V vs RHE), achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 150 and 400 mV.
4. Conclusions Herein, we report a straightforward approach for preparing radial microspheres of Ni-Co-O composed of nanoneedles subunits at porous Ti as a novel electrocatalyst for HER. Firstly, a porous Ti plate was obtained by ultrasonic corrosion of Ti plate in concentrated HNO3, where the radial microspheres of Ni-Co-O in site grew during hydrothermal condition. The phosphorized product of NiCo-P/Ti was obtained by calcining the Ni-Co-O/Ti at 300 C. Owing to the radial nanoneedle configuration and the leading large specific surface area, the Ni-Co-P/Ti exhibited remarkable HER catalytic performance, such as low onset potential, small Tafel
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slope, and excellent durability. In the presence of glucose as the substitution of H2O to be oxidized, the Ni-Co-P/Ti showed high electrocatalysis for it and H2 was produced at low potentials for both anode and cathode.
[21]
[22]
Acknowledgments [23]
The authors greatly appreciate the support from National Natural Science Foundation of China (21305090).
[24]
Appendix A. Supplementary data [25]
Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.11.001. [26]
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Growth of radial microspheres of Ni-Co-O at porous Ti and its phosphorization for high efficient hydrogen evolution Mao Zhou 1, Ya Liu 1, Dejuan Fa 1, Lihong Qian 1, Yuqing Miao* University of Shanghai for Science and Technology, Shanghai, 200093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2017 Received in revised form 31 October 2017 Accepted 1 November 2017 Available online 2 November 2017
The phosphorized radial microspheres of Ni-Co-O on a porous Ti plate (Ni-Co-O/Ti) were fabricated for the electrocatalysis of hydrogen evolution reaction (HER). They are characterized by SEM, TEM, XRD, XPS and BET. The Ni-Co-P/Ti exhibited the low onset potential of 100 mV, achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 20 and 80 mV, respectively. The Tafel slope is approximately 43 mV/dec, which is close to that of commercial Pt catalyst and lower than many previous reports. On the other hand, the Ni-Co-O/Ti exhibited high electrocatalytic oxidation of glucose with a relatively low onset potential at 0.29 V, where glucose was introduced to decrease the cell potential of water splitting. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Ni-Co-P Ni-Co-P Radial microspheres Phosphorization
1. Introduction The depletion of fossil fuels and increased environmental concerns have resulted in an urgent demand for clean and sustainable replacement of energy sources [1e4]. As an ideal candidate, hydrogen produced through water splitting is especially attractive. The platinum group metals or their compounds possess relatively high catalytic activity for hydrogen evolution reaction (HER) in acidic media [5], but the elemental scarcity and the high cost hinder their widespread applications [6e8]. It is therefore urgently demanded to design and develop cost-effective, active, and durable HER catalysts made of earth-abundant elements [9e11]. The past few years have witnessed the rapid development of transition metal phosphides (TMPs) as efficient HER catalysts in acid media [12e15]. As an important class of compounds with metalloid characteristics and good electrical conductivity [16], various TMPs have emerged as promising catalysts of HER including NiP [17], CoP [11,18], MoP [19], FeP [16], FexCo1exP and so on [20]. However, further exploration has been still desired to improve their activity and durability to connect the gap of HER electrocatalytic performance between TMPs and noble metal based
catalysts. It has been reported that there are many approaches to improve the performance of non-noble-metal catalysts, for example, synthesizing the catalysts of high specific area with the super-small/ fine particle/wire size, ultrathin thickness, porosity or mesoporosity, or other special morphologies [1,7,21,22]. Furthermore, the enhanced activity can be obtained by heteroatom doping or addition of other elements to prepare multinary compounds/ nanocomposites [8]. Frequently, TMPs were hybridized with other nanomaterials like carbon nanotube [23], graphene, graphite or carbon etc. The increased stability can be obtained by using carbon cloth and Ti plate as the catalytic substrates [1,18,24]. A Mn-doped CoP nanosheets array was fabricated on Ti mesh as an efficient 3D HER electrocatalyst with good stability at all pH values [25]. Herein, the radial microspheres of Ni-Co-O composed of nanoneedle subunits were in site synthesized and deposited at porous Ti. After phosphorization, they were employed for HER electrocatalysis. The catalysts were characterized with their morphology, composition and structure. Their electrocatalytic performance was studied in detail. 2. Experiments
* Corresponding author. E-mail address: [email protected] (Y. Miao). 1 Mao Zhou, Ya Liu, Dejuan Fa and Lihong Qian contributed equally to this work and should be considered as co-first authors. https://doi.org/10.1016/j.electacta.2017.11.001 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
2.1. In site growth of the radial microspheres of Ni-Co-O at porous Ti plate The porous Ti plate was obtained by ultrasonic corrosion of a
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Fig. 1. SEM images of porous Ti plate (A), Ni-Co-O/Ti (B,C) and Ni-Co-P/Ti (DeF). EDXS of Ni-Co-O/Ti (G) and Ni-Co-P/Ti (H).
powder pressed sintering Ti plate (1 cm 4 cm 1 mm) with open-cell rate of 70% and porosity of 60% in 14.4 M concentrated HNO3 for 60 min and then cleaned through ultrasonic process in
alcohol and ultra-purified water sequentially each for 5 min, finally dried for use. 20 mL of 50 mM NiNO3, 10 mL of 100 mM CoCl2, 10 mL of 50 mM
Fig. 2. Formation mechanism of Ni-Co-O.
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urea were mixed under continuously stirred at room temperature for 1 h to form a homogeneous solution. The pink solution was then introduced to a 60 mL Teflon-lined stainless-steel autoclave with a piece of cleaned porous Ti plate inside, sealed and subsequently heated to 180 C for 24 h in an oven. After cooling down to room temperature naturally. The color of porous Ti plate turned from grey to bluish violet. The porous Ti plate deposited with Ni-Co-O was washed for several times with deionized water and absolute ethanol to remove the remaining reactants. Finally, it was dried at 50 C under vacuum for 12 h to obtain the sample of Ni-Co-O/Ti.
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system (PerkinElmer). The N2 adsorption isotherms were monitored with a Surface Area and Porosity Analyzer of ASAP 2460 (Micromeritics). For the N2 adsorption-desorption measurements of Ni-Co-O and the Ni-Co-P, they were scratched gently from the porous Ti substrates. Electrochemical analysis was performed using an electrochemical workstation of CHI 760D (CH Instruments) with a Ni-Co-O/Ti or Ni-Co-P/Ti as working electrode, an Ag/AgCl electrode as reference electrode and a platinum wire as counter electrode. 3. Results and discussion
2.2. Phosphorization of the Ni-Co-O/Ti 200 mg hypophosphite (NaH2PO2) was placed at the upstream of the corundum crucible with the Ni-Co-O/Ti at the downstream. The temperature of the tube furnace was raised from 25 C to 300 C with a heating rate of 5 C/min and held at 300 C for 2 h. After then, the furnace was allowed to cool down to room temperature under N2 atmosphere. Finally, the black product of Ni-CoP/Ti was obtained. As contrast, a Ni-Co-O/Ti was phosphorized at 350 C to obtain the Ni-Co-P’/Ti. 2.3. Instruments The morphologies and compositions of samples were characterized using a scanning electron microscope (SEM) of MIRA3 XMU/ XMH (TESCAN) and an energy dispersive X-ray spectrometer (EDXS) of SYSTEM 7 (Thermo Fisher). A JEOL JEM-2010F Transmission electron microscope (TEM) with selected area electron diffraction (SAED) pattern was used to investigate the samples. Xray diffraction (XRD) patterns were collected by using a D8 Advance diffractometer (Bruker). X-ray photoelectron spectroscopy (XPS) analysis were performed by a RBD upgraded PHI-5000C ESCA
A powder pressed sintering Ti plate was firstly corroded in concentrated HNO3 to obtain the porous Ti plate with high surface area as catalyst support. As shown in Fig. 1A, the yielded porous Ti plate exhibits the 3D, porous and honeycombed structures. By hydrothermal reaction, the nanoneedle array of Ni-Co-O grows and deposits on the surface of the porous Ti plate. Interestingly, two kinds of morphologies are observed in Fig. 1B. At the local Ti substrate, the nanoneedle array of Ni-Co-OI distribute uniformly on the surface. It may be due to the fact that the nucleus formation of NiCo-OI occurs at the sites of Ti substrate. Because of the presence of steric hindrance, the nucleus growth extends along the direction away from the Ti substrate. The formation mechanism of the Ni-CoOI nanoneedle array is described in Fig. 2. Within the cavity of the porous Ti plate, the radial and urchin-like microspheres of Ni-Co-OII were observed and they connect each other. The microspheres were composed of sharp nanoneedles radially grown from the center with the diameter about 8 mm. As seen from Fig. 2, the nucleus formation of Ni-Co-OII occurs in solution. Due to the homogeneity of solution in every direction, the nucleus growth extends freely and radially. Then the growing Ni-Co-OII radial microspheres accumulates within the cavity of the porous Ti plate and connects
Fig. 3. EDXS elemental mapping images of the radial microspheres of Ni-Co-PII.
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each other. Fig. 1DeF shows the morphology of Ni-Co-P/Ti, the phosphorized Ni-Co-O/Ti. No obvious differences are observed. It means that the temperature of 300 C and the phosphorization process do not affect their morphology. EDXS was performed to further investigate the composition and the element distribution. As shown in Fig. 1G, several peaks correspond to the elements of Ni, Co and O. The atom ratio of Ti:Ni:Co:O are about 23.42:5.98:5.33:64.27. The peaks from Ti, Ni, Co, and P in Fig. 1H confirm the presence of these elements in the sample of Ni-Co-P/Ti. The atom ratio of Ti:Ni:Co:P:O are about 20.17:6.62:6.56:6.34:60.31, revealing the ratio of Ni-Co and P in the sample to be close to 2: 1. To identify the composition distribution of the Ni-Co-PII radial microspheres, their elemental mappings were examined by EDS. As presented in Fig. 3, the EDXS mapping images clearly illustrate that Ni, Co, O and P uniformly distribute throughout the entire microspheres. The results verify the successful chemical conversion of NiCo-O/Ti into Ni-Co-P/Ti via the high-temperature phosphorization. TEM images in Fig. 4A confirm the morphology of radial microsphere for Ni-Co-PII. It is composed of many radial nanoneedles with a width about 36 nm. As shown in Fig. 4B, the highresolution TEM (HRTEM) images taken from the Ni-Co-PII radial microspheres display two lattice fringes with the interplanar spacing of 0.36 nm and 0.19 nm, corresponding to the [001] plane of Ni2P and the [002] plane of Co2P, respectively [17]. It has been well known that with phosphorus resource at the high temperature,
typically above 300 C, often lead to the formation of phosphide [9,13,26,27]. The discrete dots in the selected area electron diffraction (SAED) pattern of Fig. 4C and the sharp peak shape in the XRD pattern of Fig. 4D confirm the crystalline property of the NiCo-P. The XRD pattern are indexed to the multiphase of hexagonal Ni2P (PDF#65-9706) and Co2P (PDF#54-0413) [12,28,29], in accord with the result of EDS in Fig. 1H where the atomic ratio of NiCo to P is close to 2:1. Fig.S1 also shows the sharp peak shape, exhibiting the crystalline property of the Ni-Co-O. Its XRD pattern can be indexed to the multiphase of Co1$29Ni1$71O4 (PDF#40-1191) and CoO (PDF#42-1300). The overall spectrum of XPS covers the signals of Ni 2p, Co 2p, P 2p and O 1s regions of the Ni-Co-P (not shown). Fig. 5A exhibits two main peaks at 874.5 eV for NiII 2p1/2 and 856.9 for NiII 2p3/2 with a split spin-energy of 17.6 eV, accompanied by the satellite peaks at 880.5 and 862.2 respectively [12]. A small peak at 853.6 eV is attributed to zero valence state Ni0. Similarly, the Co 2p spectrum in Fig. 5B also shows two main peaks at 798.2 eV and 782 eV with the respective satellite peaks at 786.3 eV and 803.2 eV. They can be assigned to CoII 2p1/2 and CoII 2p3/2, respectively. In Fig. 5C, two peaks of P 2p spectrum are observed at 134.1 eV and 129.5 eV. The former is ascribed to oxided P (phosphate) from the superficial oxidation of phosphide due to the exposure of the sample to air. The latter is assigned to the reduced P in phosphide [30,31]. The O 1s peak in Fig. 5D is located at 531.6 eV, corresponding to the surface oxides or phosphate.
Fig. 4. TEM images (A), HRTEM image (B) and SAED pattern (C) of the Ni-Co-PII radial microspheres. The diffraction spots were marked in (C). XRD pattern (D) of the Ni-Co-P.
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Fig. 5. High resolution XPS spectra of the Ni-Co-P.
N2 adsorption-desorption measurements were performed to study the textural properties of the Ni-Co-O and the Ni-Co-P. The N2 adsorption-desorption isotherms in Fig. 6 reveal their property of mixed type of II and IV. The S shape isotherms is ascribed to the nonporous property of radial Ni-Co-O/Ni-Co-P microspheres. The hysteresis loop is usually described as type IV, a typical characteristic of mesoporous material. However, for radial Ni-Co-O/Ni-Co-P
microspheres, the mesoporosity-like property exists at the core area of the microspheres, resulting in a narrow distribution of pore diameter, as shown in the inset of Fig. 6A and B. From Fig. 6A, the surface area, total pore volume and average pore diameter of the Ni-Co-O are obtained with the values of 68.7 m2/g, 21.8 cm3/g and 12.7 nm, respectively. Phosphorization of the Ni-Co-O at 300 C leads to the slight collapse of the radial structure at the core area,
Fig. 6. N2 adsorption-desorption isotherm and BJH Desorption dV/dD (inset) of the Ni-Co-O (A) and the Ni-Co-P (B).
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Fig. 7. (A) Polarization curves of the porous Ti plate, the Ni-Co-O/Ti, the Ni-Co-P’/Ti and the Ni-Co-P/Ti in 0.5 M H2SO4. (B) Tafel plots of the Ni-Co-O/Ti and the Ni-Co-P/Ti in 0.5 M H2SO4. (C) The amount of H2 theoretically calculated (black curve) and experimentally measured (red curve) versus time for the Ni-Co-P/Ti under the overpotential of 0.2 V. (D)Time-dependent current density curve of the Ni-Co-P/Ti under the overpotential of 0.15 V for 80000 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
activity than the Ni-Co-O/Ti and the Ni-Co-P’/Ti. It means that phosphorizing the Ni-Co-O/Ti results in the increased HER activity and the temperature of 300 C for phosphorization is preferred to that of 350 C. The Ni-Co-P/Ti exhibits the lowest onset potential of 100 mV, achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 20 and 80 mV, respectively. These values of the Ni-Co-P/Ti compare favorably to many previously reported
and thus brings out the change of values with the surface area of 19.9 m2/g, the total pore volume of 15.0 cm3/g and the average pore diameter of 30 nm, for the Ni-Co-P. The HER electrocatalysis of the Ni-Co-P/Ti was studied by comparing with the porous Ti plate, the Ni-Co-O/Ti and the Ni-CoP’/Ti. As shown in Fig. 7A, the blank porous Ti plate doesn't show obvious HER activity, while the Ni-Co-P/Ti exhibits far higher HER
Table 1 HER electrocatalysis performance of some typical catalysts reported previously. Catalysts /electrode
Onset potential (mV)
Overpotential (mV)
Current density (mA/cm2)
Tafel (mV/dec)
radial microspheres of Ni-Co-P on porous Ti plate
100
herein
55
61
[32]
Pt
24
30
[32]
Ni-Mo-S/carbon fiber cloth MoSx-carbon nanotube/GCE NiS2-graphite/graphite NiSx/fluorinated tin oxide Fe-Ni-S/GCE CoP nanotubes
132 75 110 330
10 50 10 50 10 50 10 10 10 10 10 10 20 100
43
Ni-Mo-S/Ti
20 80 30 86 18 57 200 110 190 440 105 108 127 173
85.3 40 80 62 40 59
[24] [23] [33] [34] [35] [7]
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Fig. 8. CV curves at the Ni-Co-O/Ti in 1 M NaOH without (a) and with (b) 0.01 M glucose with the scan rate of 0.1 V/s. (B) CV curves at the platinum wire cathode with the Ni-Co-O/Ti as anode in 1 M NaOH with (a) and without (b) 0.01 M glucose at the scan rate of 0.1 V/s.
non-noble-metal HER catalysts, as shown in Table 1. Fig. 7B shows the Tafel plots of the Ni-Co-O/Ti and the Ni-Co-P/ Ti. They are fit to the Tafel equation where h ¼ b log j þ a and j is the current density. The Tafel slopes of b are measured to be approximately 73 mV/dec for the Ni-Co-O/Ti and 43 mV/dec for the Ni-CoP/Ti, while the latter is close to that of commercial Pt catalyst and lower than many previous reports in Table 1. Under the applied overpotential of 0.2 V, gas bubbling of H2 evolved continuously at the Ni-Co-P/Ti cathode and that of O2 at the Pt anode. The amount of the produced H2 was collected and measured during 4000 s of H2O electrolysis in Fig. 7C. The theoretically calculated amount of H2 and the experimentally measured one coincide well versus time, indicating high Faraday efficiency of the Ni-Co-P/Ti due to its high electrocatalytic efficiency of HER. Stability is one of key factors for practical applications of HER electrocatalysts, for which the durability of the Ni-Co-P/Ti was evaluated by long-term electrolytic tests in 0.5 M H2SO4. As shown in Fig. 7D, the electrocatalytic signal of HER at the Ni-Co-P/Ti under the overpotential of 0.15 V is relatively constant and no obvious decrease can be observed during the range from 20000 s to 80000 s. The slight decrease within the first 20000 s can be due to the transient state before stabilization. The tested stability duration is much longer than most of previously reports [23,24,32,33]. Observed from the SEM images in Fig.S2, the morphology of Ni-CoP radial microspheres changes slightly where the slight adhesion of their blades or needles occurs. The practical application of H2 generation through H2O electrolysis has been limited by its high energy consuming. In order to decrease cell potential, the anodic oxygen evolution reaction of H2O can be replaced with that of more readily oxidizable, hydrogencontaining small molecules like ammonia [36], methanol [37,38], ethanol [39], glycerol [40,41], ammonia borane [42], and urea [43,44] et al. Here, glucose was introduced to be oxidized for decreasing the cell potential of water splitting. It is found that the Ni-Co-O/Ti exhibited far higher electrocatalytic oxidation of glucose than the Ni-Co-P/Ti (not shown). In the presence of glucose (Fig. 8A), the oxidation current increases sharply with a relatively low onset potential at 0.29 V vs Ag/AgCl that is far lower than that for oxygen evolution from H2O. The signal increase between 0.29 V and 0.55 V could be attributed to the mediated electrocatalysis by the redox couples of NiII/NiIII and CoII/CoIII [45,46]: MII þ OH e % MIII þ H2O
MIII þ glucose / MII þ intermediate products where M means Ni or Co. When the applied potential is higher than 0.55 V, the steeper increase of oxidation current is observed, which is due to the direct electrocatalysis of NiIII and CoIII [46e51]: MIII þ C6H12O6 þ 24OH / MIII(C6H12O6)ads / MIII þ 6CO2[ þ 18H2O þ 24e In the process of electrocatalytic oxidation toward glucose, gas bubbles occurred violently at both the anode and cathode. The gas bubbles at anode is CO2 from glucose oxidation, while that at cathode is H2 from HER: 24H2O þ 24e / 12H2[ þ 24OH The overall cell reaction is summarized as follows: C6H12O6 þ 6H2O / 6CO2[ þ 12H2[ which was further confirmed by exchanging the working electrode and the counter electrode as used in Fig. 8A. As a result, the platinum wire works as working electrode and the Ni-Co-O/Ti as counter electrode in Fig. 8B. In the same way, glucose oxidation occurs at Ni-Co-O/Ti anode and HER does at the platinum wire cathode. In the presence of glucose, HER occurs at a less negative onset potential of 1.03 V vs Ag/AgCl (or 0.179 V vs RHE), achieving the current densities of 10 and 50 mA/cm2 at the overpotentials of 150 and 400 mV.
4. Conclusions Herein, we report a straightforward approach for preparing radial microspheres of Ni-Co-O composed of nanoneedles subunits at porous Ti as a novel electrocatalyst for HER. Firstly, a porous Ti plate was obtained by ultrasonic corrosion of Ti plate in concentrated HNO3, where the radial microspheres of Ni-Co-O in site grew during hydrothermal condition. The phosphorized product of NiCo-P/Ti was obtained by calcining the Ni-Co-O/Ti at 300 C. Owing to the radial nanoneedle configuration and the leading large specific surface area, the Ni-Co-P/Ti exhibited remarkable HER catalytic performance, such as low onset potential, small Tafel
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slope, and excellent durability. In the presence of glucose as the substitution of H2O to be oxidized, the Ni-Co-P/Ti showed high electrocatalysis for it and H2 was produced at low potentials for both anode and cathode.
[21]
[22]
Acknowledgments [23]
The authors greatly appreciate the support from National Natural Science Foundation of China (21305090).
[24]
Appendix A. Supplementary data [25]
Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.11.001. [26]
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