CoP Film on Ni Foam for Efficient

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One-step Electrodeposition of Co/CoP Film on Ni Foam for Efficient Hydrogen Evolution in Alkaline Solution Ningning Bai, Qing Li, Daoyong Mao, Daikun Li, and Hongzhou Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07785 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Applied Materials & Interfaces

One-step Electrodeposition of Co/CoP Film on Ni Foam for Efficient Hydrogen Evolution in Alkaline Solution Ningning Bai, Qing Li,* Daoyong Mao, Daikun Li, Hongzhou Dong School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P. R. China ABSTRACT: The development of high efficiency catalysts for hydrogen evolution via water splitting has been an effective strategy to solve the energy environmental problems and energy crisis. The abundant-reserving transition-metals and their phosphides are becoming attractive Pt alternatives for hydrogen evolution reaction (HER). Herein, a crystalline/amorphous Co/CoP film was facilely prepared on nickel foam (NF) by one-step electrodeposition technique at room temperature, named as Co/CoP-NF. The as-prepared Co/CoP-NF electrocatalyst exhibits excellent electrocatalytic activity for HER, on par with Pt/C, showing a low overpotential of 35 mV at a current density of 10 mA·cm-2 and small Tafel slope of 71 mV·dec-1 in 1.0 M NaOH solution. More importantly, the Co/CoP-NF catalyst presents well long-term durability at a overpotential of 60 mV. Moreover, the influence of the electrodeposition parameters on the catalytic performance of the catalyst was discussed. This study offers an effective strategy to develop a non-noble metal HER catalyst for industrial production of hydrogen. KEYWORDS: cobalt; cobalt phosphide; electrodeposition; hydrogen evolution reaction; alkaline solution

1. INTRODUCTION With the increasing scarcity of fossil fuels and climate change, the development of efficient and clean new energy sources has become an urgent need.1,2 Hydrogen, as the cleanest fuel with the highest specific energy density, became one of the most promising energy sources to replace the present fossil-fuel-based energy system.3,4 In

ACS Paragon Plus Environment


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the present methods of hydrogen production, electrolysis of water to produce hydrogen with a simple process, little pollution and high product purity has attracted great attention.5-8 However, the high overpotential of the cathode increases the electrolytic bath voltage and electric energy consumption, which is a big restriction in large-scale industrial application.9 The noble metals, especially platinum, are excellent hydrogen evolution reaction (HER) electrocatalysts with low overpotential and good stability in both alkaline and acidic electrolytes.10 Nevertheless, owing to the scarcity and high cost of Pt, searching and developing high-performance of noble-metal-free catalysts for HER is very important for the development of hydrogen energy in the future. Recently, great efforts have been made in the exploration of efficient catalysts for hydrogen evolution. And so far a variety of Pt alternative catalysts have been obtained, such as transition-metal sulfides,11-15 selenium compounds,16-19 carbon-based composites,20,21 phosphides

22-25

etc. The majority of these catalysts possess good

stable and catalytic performance toward HER in an acidic environment, and only few of them exhibit catalytic activity and stability in alkaline conditions. However, hydrogen production by electrolysis in alkaline environment is the trend of industrial development. For one reason, electrolysis equipment working under acidic conditions is very difficult to meet the long-term stable production of the industrialization;26 for another reason, HER, a half reaction of water splitting, is required to conduct in alkaline media for adapting to basic catalytic environment of low-cost oxygen evolution reaction (OER) catalysts.27 Therefore, developing efficiency catalytic HER


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catalysts based on earth abundant materials in alkaline media with simple fabrication process has become an urgent need. The Co-P system, as a kind of transition metal phosphide, has been studied intensively.23-25, 28-33 For instance, Pu et al. obtained CoP nanosheet arrays on a Ti plate by electrochemical deposition and low-temperature phosphidation. 23 Callejas et al. prepared hollow Co2P and CoP nanoparticles on Ti based on the reaction of Co nanoparticles with tri-n-octylphosphine. Ma et al. prepared CoP by phosphating and annealing Co3O4 at 250 °C. 28 These CoP HER catalysts show good HER performance in acidic aqueous solutions, as shown in Table S1.

30-33

Nevertheless, the HER

catalytic performance and stability in alkaline media for Co-P system still needs to be improved. Furthermore, the preparation process should be simplified to reduce costs. As is well known, when constituent elements of catalysts are the same, the crystallization and real surface area of catalysts are the main factors to affect the catalytic efficiency of hydrogen evolution. 2 Taking these two aspects into account, as well

as

combining

the

advantages

of

Co-P

system,

we

prepared

a

crystalline/amorphous Co/CoP film on nickel foam (NF) as the HER electrocatalysts in the alkaline media. Electrodeposition is a simple, rapid and cheap method to construct the nanostructures. 34 In this paper, unlike most of the reported preparation methods, we fabricated a crystalline/amorphous Co/CoP film on nickel foam (NF) by one-step electrodeposition technique at room temperature. There are two reasons for the use of NF as substrate; on the one hand, nickel metal shows high electrocatalytic activity



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among non-noble materials; on the other hand, NF, as a three-dimensional network structure, has a large surface area, so it would be beneficial to further improve the catalytic activity of the catalyst. The catalytic film obtained by electrodeposition has high bonding strength with substrate, which can reduce interface resistance and increase the catalytic activity effectively. Additionally, the crystalline/amorphous Co/CoP-NF catalysts with different electrochemical surface area (ECSA) were prepared successfully through adjusting the electrodeposition parameters simply. The HER catalytic performance and stability of the Co/CoP-NF catalyst were tested in 1.0 M NaOH solution. An overpotential of 35 mV at a current density of 10 mA·cm-2 and the Tafel slope of 71 mV·dec-1 were obtained, indicating the Co/CoP-NF catalyst with the highly effective catalytic activity under the alkaline environment. The research of this paper conforms to the needs of industrial development of HER catalytic electrode, which show great significance to the development of hydrogen energy. 2. EXPERIMENTAL SECTION 2.1 Materials and Reagents. Ni foams (areal density, 280 g/m2) were obtained from Kunshan dessco Electronics Co., Ltd. Alcohol (CH3CH2OH, 99.7%), sulfuric acid (H2SO4, 98.0%) and sodium hydroxide (NaOH, 98.0%), were from China Chongqing Chuandong chemical Ltd. Cobalt sulfate (CoSO4·7H2O, 99.5%), sodium hypophosphite (NaH2PO2·H2O, 99.0%) and sodium acetate (CH3COONa, 99.0%) were purchased from Chengdu Kelong Chemical Reagent Factory. All chemicals were of reagent grade and used without further purification.


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2.2. Preparation of Co/CoP-NF Electrode. The Co/CoP-NF electrode was prepared by galvanostatic electrodeposition in a standard three-electrode system. A platinum sheet, a saturated calomel electrode (SCE) and 1.0 cm2 of NF were used as counter, reference and work electrode, respectively. Before electrodeposition, the NF was cleaned with ethanol by ultrasonic cleaner for 10 minutes to remove contaminants, and ultrasonic clean for 5 minutes with H2SO4 (1.0 M) solution subsequently to get rid of the oxide on the NF surface. After that, the NF was rinsed with deionized water for further use. The plating bath is consist of CoSO4·7H2O (0.005 M), NaH2PO2·H2O (0.025 M) and CH3COONa (0.005 M). The electrodeposition experiments were carried out in different current densities (-0.2, -0.5, -0.8 A/cm2, respectively) under stirring at room temperature for 10 minutes. All the as-prepared Co/CoP in this study is obtained at the current density of -0.5 A/cm2 unless otherwise explicitly specified.

2.3 Characterization. The morphology of samples was characterized by scanning electron microscope (SEM; S-4800, Hitachi, Japan) in 10 KV. The phase of the samples was analyzed by high resolution transmission electron microscopy (HRTEM; JEM-2100F, Japan) in 200 KV and -ray diffractometer (XRD; Purkinje General Instrument XD-3) using Cu KR radiation (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250) was used for studying the chemical composition of as-prepared samples. The source gun type of the XPS was Al Kα and the binding energy is calibrated with C 1s (284.9 eV).



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2.4. Electrochemical Measurements. All the electrochemical measurements were conducted by using a conventional three-electrode system on an Electrochemical Workstation (CS350, Wuhan Kesite Instrument Co., Ltd.). The as-prepared samples, a saturated calomel electrode (SCE) and platinum sheet were used as work, reference and counter electrode, respectively. All potentiodynamic polarization experiments were conducted at the voltage vs. the reversible hydrogen electrode (RHE) from open circuit voltages to -1.6 V. The stability of HER catalytic electrode was carried out using cyclic voltammetry (CV) sweeps between -0.1 and -0.4 V vs. RHE at a scan rate of 100 mV·s−1. All potentials in this study were converted to the values with reference to RHE (ERHE = ESCE + 0.241 + 0.059 pH). 3. RESULTS AND DISCUSSION 3.1 Composition and microstructure of the Co/CoP-NF catalyst. The HRTEM, XRD and XPS were used to characterize the composition of the Co/CoP-NF catalyst. Figure. 1A,B presented the 2p spectra in the Co (2p) and P (2p) regions for the as-prepared catalyst. The binding energies (BE) at 778.3 eV of Co 2p3/2 and 129.8 eV of P 2p3/2 are ascribed to Co and P species in CoP. 23,35 The Co 2p1/2 region and P 2p1/2 region show at the peak of 793.5 eV and 130.5 eV, respectively.23 The peaks at 784.1 eV and 133.2 eV in the Co (2p) and P (2p) regions are attributed to oxidation of Co and P species in CoP, respectively.36,37 According to previous reports, the presence of oxygen species is produced in the inescapable contact between the air and catalyst surface.38 In addition, the BE at 778.1 eV is due to the presence of


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metallic Co according to the previous reports.23,25 The HRTEM image and XRD pattern of the as-prepared catalyst further prove the presence of metallic Co. Figure. 1C showed that the catalyst prepared here was composed of amorphous substance with doped crystals. The adjacent-plane distance of 2.17 Å and 1.92 Å correspond well to the (100) plane and (101) plane of metallic Co (JCPDS no. 01-1278). The XRD patterns exhibited weak crystalline peaks of Ni foam and the metallic Co, and there were no crystallization peaks of CoP, indicating CoP film is amorphous (Figure. 1D). So the as-prepared catalyst was comprised of crystalline Co and amorphous CoP. Due to the dispersion of metallic Co in amorphous CoP films, metallic Co shows weak crystalline peaks in the XRD pattern. While the HRTEM could choose to shoot sample in the dense crystalline areas, so the crystal with clear crystalline surface can be obtained from HRTEM images. Figure. 2 showed the microstructure of the as-prepared Co/CoP-NF catalyst. It could be observed that the Co/CoP-NF exhibits bumpy surface consisting of nodular ‘grains’, as shown in Figure. 2A and 2C. These nodular ‘grains’ are very rough owing to the presence of ball-cone-like nano particles (as shown in Figure. 2B). This kind of morphology possessed a large specific surface area comparing with relatively flat and smooth bare NF, as shown in Figure. S1, which probably provided more active sites towards HER. In addition, 3D network structure of Co/CoP-NF catalyst is advantageous to the rapid evacuation of the bubble generated on electrode surface, 39 increasing the HER activity of the electrode. The energy dispersive spectrometer (EDS) mapping analysis (Figure. 2C) indicated that Co and P elements distributed



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uniformly in the Co/CoP film.

3.2 Electrocatalytic activity and stability evaluation. The electrocatalytic HER activities of the as-prepared Co/CoP-NF catalyst were tested in 1.0 M NaOH solution at room temperature. For purposes of comparison, bare NF and commercial Pt/C (10 wt. %) deposited on NF were also measured under the same conditions. Figure. 3A showed the polarization curves for the corresponding catalysts. Obviously, Pt/C electrode presented best electrocatalytic activity towards HER due to its the lowest overpotentials at current density of 100 mA cm-2. Bare NF showed the worst HER performance with an overpotential of 430 mV at the current density of 100 mA·cm-2. In sharp comparison, the as-prepared Co/CoP-NF electrocatalyst exhibited excellent electrocatalytic activity for HER, showing a low overpotential of 35 mV and 130 mV at a current density of 10 mA·cm-2 and 100 mA cm-2, respectively. That is to say, the HER catalytic performance of the as-prepared Co/CoP-NF electrocatalyst was nearly on a par with Pt/C electrode, which indicated the Co/CoP-NF electrocatalyst had a high HER activity. The Tafel plots for bare NF, Pt/C electrode and the as-prepared Co/CoP-NF electrocatalyst are exhibited in Figure. 3B. The linear regions were fitted for the Tafel equation: η = b log j + a, where η is the overpotential, j is the current density, b is the Tafel slope and a is the cathodic intercept related to the exchange current density. Based on the Tafel equation, the Tafel plots of NF, Pt/C and the as-prepared Co/CoP-NF electrocatalyst are obtained which specific values of 198, 54 and 71 mV dec-1, respectively. Generally, cathodic HER has followed normally two principal


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steps in alkaline solutions, which are Volmer–Heyrovsky or Volmer–Tafel mechanisms: 28 Volmer: H2O + e- + *↔ Had + OH -

(1)

Heyrovsky: Had + H2O + e- ↔ H2 + OH - + *

(2)

Tafel: 2Had ↔ H2 + 2*

(3)

where * is the hydrogen adsorption sites. Tafel slopes are 120, 40, and 30 mV dec-1 for the Volmer, Heyrovsky, and Tafel step, respectively.26 Therefore, catalysis of Co/CoP-NF towards HER follows the Volmer−Heyrovsky mechanism, indicating Heyrovsky process (electrochemical hydrogen desorption) was the rate determining step.40 Good stability is of great significance to the practical application of HER catalysts. And the stability of Co/CoP-NF electrocatalyst was further investigated through constant voltage technique. Figure. 3C exhibited the changes of current density with different electrolytic time at a fixed overpotential of 60 mV. It is apparent that the electrolytic current almost has no change for at least 18 h. The continuous CV sweeps between -0.1 V and -0.4 V at a scan rate of 100 mV s-1 is also used for evaluating the stability of as-prepared electrocatalysts in 1.0 M NaOH solution, Figure. 3D showed the polarization curves of the Co/CoP-NF electrocatalysts before and after CV scanning for 1500 cycle. After cycling, there is little loss of the current density in the polarization curve, which can be negligible. The CV graph of 1st cycle and 1500th cycle were presented in Figure. S2. It could be found that both the current density of CV scanning in the 1st cycle and 1500th cycle were small, which may indicate the CV



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test had almost no impact on both morphology and or size of Co/CoP-NF, and the SEM and TEM images of Co/CoP-NF further demonstrated this. At the CV scan of 1st cycle, the Co/CoP-NF surface is covered with a number of rough nodular ‘grains’, (Figure. S3A and B), while, the nodular ‘grains’ still existed after the CV scan of 1500th cycle (Figure. S3D and E), and even the particle size has not changed comparing with the TEM images of CV scanning in the 1st cycle and 1500th (Figure. S3C and F), verifying Co/CoP-NF HER electrocatalyst had a relative stable structure. All the stability tests indicated that the Co/CoP-NF HER electrocatalyst had the good long-term stability in 1.0 M NaOH solution, which provides a great possibility for Co/CoP-NF electrocatalysts in industrial applications.

3.3 Influence of electrodeposition current density on HER performance. The electrodeposition parameters, especially the deposition current density, has a significant influence on the HER electrocatalytic activity of the as-prepared Co/CoP-NF catalyst. So in controlled experiments, several Co/CoP-NF catalysts were prepared at deposition current density of -0.2, -0.5, -0.8 A·cm-2, which were labeled as Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8 respectively in this section. Figure. 4 showed the polarization curves and Tafel slops of Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8. Obviously, the Co/CoP-NF-5 catalyst exhibits the best catalytic activity towards HER, and Co/CoP-NF-8 presented the middle HER performance, while Co/CoP-NF-2 held the lowest HER activity among three of them. The exchange current density (j0) and accurate experimental values of representing HER performance were given in Table 1. And it is clear that Co/CoP-NF-5 catalyst had the


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highest j0 of 3.10 mA cm-2, which still indicated that Co/CoP-NF-5 possessed the best HER catalytic efficiency. In order to explain the differences in HER performance among these Co/CoP-NF catalysts, the composition and crystallinity of these Co/CoP-NF catalysts have been learned, as shown in Figure. S4. It is obvious that these Co/CoP-NF catalysts almost showed the same crystalline and components, which proved that the Co/CoP-NF-2 and Co/CoP-NF-8 were also comprised of crystalline Co and amorphous CoP, just like Co/CoP-NF-5 discussed in the section 3.1. Moreover, the atom ratio of Co and P of Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8 was 2.5:1, 2.4:1 and 2.4:1 respectively by the XPS results, which almost showed the same composition (Co1.5/(CoP)1-NF-2, Co1.4/(CoP)1-NF-5 Co1.4/(CoP)1-NF-8). The unique crystalline/ amorphous structure facilitated the adsorption and desorption of hydrogen according to the previous reports, 41,42 which may be one of the reasons for the high catalytic activities of these Co/CoP-NF catalysts. In order to further investigate the reason for the different HER catalytic activity, the ECSA was calculated based on measuring double-layer capacitance of these Co/CoP-NF catalysts according to the method reported previously, calculation of ECAS is provided in ‘Supplementary Method’.

34

and the

As a result, the ECSA

of Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8 was calculated to be about 21.25 cm2, 29.25 cm2 and 36.75 cm2, respectively at the geometric surface area of 0.7 cm2. Such high ECSA explained why these Co/CoP-NF catalysts exhibited high HER catalytic activity. However, the HER performance tests showed that the Co/CoP-NF-5



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presented the best HER electrocatalytic activity rather than the Co/CoP-NF-8 with largest ECSA (Figure. 4). In order to explore the reasons, the microstructure and film thickness of these as-prepared catalysts are characterized. As shown in Figure. 5, all Co/CoP-NF catalysts displayed bumpy surface consisting of nodular ‘grains’ (Figure. 5A-C). Moreover, the nodular ‘grains’ of Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8 are aggregates of nanoparticles (see the inserts of Figure. 5A-C). The EDS mapping analysis (Figure. S5) indicated that Co and P elements distributed uniformly in the Co/CoP-2 films and Co/CoP-NF-8 films. Figure 5D-F were showed the cross section SEM images of Co/CoP-NF-2, Co/CoP-NF-5 and Co/CoP-NF-8, respectively. Obviously, with the increase of the deposition current density, the Co/CoP-NF films become thicker and the NF skeletons are filled more densely. It is well known that macroporous structure is conducive to the dissipation of H2 bubbles, 39,43 which would promote HER process. Among these Co/CoP-NF catalysts, Co/CoP-NF-2 exposed the most porous structure but the ECSA was the smallest, and Co/CoP-NF-8 exhibited the largest ECSA while the porous structure was the least, however, Co/CoP-NF-5 had larger ECSA and more porous structure compared with Co/CoP-NF-2 and Co/CoP-NF-8, respectively. Large ECSA offered catalytic sites and appropriate porous structure provides bubbles evacuation channel, so Co/CoP-NF-5 performed the best HER catalytic activity. In addition, Co/CoP-NF-8 showed a higher HER activity than Co/CoP-NF-2, which indicated that large ECSA is important role to improve the catalytic performance towards HER. To further prove the above conclusion, the Co/CoP films were deposited on Ni plate


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(NP), labeled as Co/CoP-NP, under the same conditions. As shown in Figure. 6, the bare NP showed an overpotential of 550 mV at the current density of 100 mA·cm-2, which was obviously higher than that of NF (430 mV, Figure. 3A). Furthermore, these Co/CoP-NF catalysts exhibited higher catalytic activity than Co/CoP-NP catalysts. These results illustrated large ECSA can improve the HER catalytic activity because apparently NF and Co/CoP-NF have larger ECSA than NP and Co/CoP-NF, respectively. Unlike the Co/CoP-NF, the Co/CoP-NP did not have porous structure, so they showed higher catalytic activity with the increase of the current density just due to they had the larger ECSA.

3.4 Comparison of the performance basic HER catalysts. As well known, high efficiency for noble HER catalysts is an important challenge to their developments. Many studies on basic HER catalysts, containing multicomponent alloy or metallic oxide, have been reported in recent years. And the HER performances of the as-prepared Co/CoP-NF electrocatalysts are compared with other basic HER catalysts, as shown in Table 2. Obviously, the Co/CoP-NF electrocatalysts in this study shows lower overpotential at 10 mA cm-2 or 100 mA cm-2, which indicates the as-prepared Co/CoP-NF electrocatalysts have a better catalytic performance towards HER than those oxides of nickel and cobalt,41,44 carbide and nickel-base alloy.45-50 The high activity of Co/CoP-NF electrocatalysts can be ascribed to the following four-aspect factors: (a) The unique crystalline/amorphous structure of Co/CoP-NF electrocatalysts was beneficial for hydrogen adsorption and desorption, which lowered overpotential; (b)



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large ECSA of Co/CoP-NF electrocatalysts provided more active sites; (c) the porous structure of Co/CoP-NF electrocatalysts provides excellent bubbles evacuation ability and conductive network effectively transfers electrons to the active substances; (d) using the binder-free electrodeposition method promotes interfacial electron transfer between Co/CoP film and bare NF, reducing the resistance of Co/CoP-NF electrode and improving catalytic efficiency. In addition, the Ni–Mo–N alloy exhibits favorably HER performance with Co/CoP-NF in this work.46 However, the preparation process of the Ni–Mo–N catalyst involves high temperature and high pressure operation, bringing the increase of energy consumption and production cost, which is not conducive to large-scale industrial production. Compared with the preparation process of Ni–Mo–N catalyst, the simple electrodeposition used here is so fast and low cost. Considering comprehensively, the Co/CoP-NF catalyst prepared here may provide an effective strategy for hydrogen production in alkaline solutions by the electrochemical water splitting. 4. CONCLUSION In summary, a highly efficient catalytic hydrogen evolution electrode is fabricated successfully via electrodepositing crystalline/amorphous Co/CoP-NF film onto macro-porous 3D NF substrates in the absence of chemical binders. The influence of the electrodeposition parameters on the catalytic performance of the catalyst is discussed. The as-prepared Co/CoP-NF catalytic electrode provides large ECSA for quick transport of electron and ion in the catalytic electrode. The Co/CoP-NF


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electrode shows a low overpotential of 35 mV at a current density of 10 mA·cm-2 and small Tafel slope of 71 mV·dec-1 in 1.0 M NaOH, suggesting excellent electrocatalytic activity for HER, even on par with Pt/C electrode. Stability tests indicate that the Co/CoP-NF electrode can catalyze continuously for at least 18 hours, showing good stability in alkaline solution. Our study offers an effective strategy to develop a non-noble metal HER catalyst for industrial large-scale production of hydrogen by electrolysis of water.

■ASSOCIATED CONTENT * Supporting Information Additional table and XRD, SEM, TEM, XPS, elemental mappings, CV curves results; the methods of Calculating ECSA.

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86 023 68254000. Tel: +86 023 68252360.

Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENTS The authors specially thank for the ﬁnancial support of this work from the National Natural Science Foundation of China (51103120).

■ REFERENCES (1) Chu, S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy



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Figures:

Figure 1. (A)XPS spectra in the Co 2p regions and (B) P 2p regions of Co/CoP-NF catalyst; (C) HRTEM images of Co/CoP-NF; (D) XRD pattern of Co/CoP-NF.

Figure 2. (A) Low and (B) high magnification SEM images of Co/CoP-NF. (C) EDS


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elemental mapping images of elemental Co and P in Co/CoP-NF catalyst.

Figure 3. HER electrocatalytic performance of Co/CoP-NF catalyst: (A) Polarization curves of bare Ni foam, Co/CoP-NF catalyst and Pt/C electrode in 1.0 M NaOH solution at a scan rate of 2 mV·s−1. (B) The corresponding Tafel plots of the bare Ni foam, Co/CoP-NF catalyst and Pt/C electrode. (C) Time dependence of catalytic current density in 1.0 M NaOH solution under an overpotential of 60 mV. (D) Polarization curves of Co/CoP-NF catalyst before and after the current stability test.

Figure 4. HER electrocatalytic performance of Co/CoP-NF catalysts in 1.0 M NaOH solution at a scan rate of 2 mV·s−1:(A) Polarization curves and (B) Tafel plots of the



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catalysts obtained at the deposition current density of -0.2, -0.5, -0.8 A·cm-2, respectively.

Figure 5. The SEM images of (A) Co/CoP-NF-2, (B) Co/CoP-NF-5 and (C) Co/CoP-NF-8; the cross section SEM images of (D) Co/CoP-NF-2, (E) Co/CoP-NF-5 and (F) Co/CoP-NF-8. The inserts are corresponding magnifying images.

Figure 6. HER electrocatalytic performance of Co/CoP films deposited on Ni plate (NP) in 1.0 M NaOH solution at a scan rate of 2 mV·s−1: Polarization curves CoP/NP catalysts obtained at the deposition current density of -0.2, -0.5, -0.8 A·cm-2, respectively. The insert table showed the specific values.


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Tables: Table 1. The HER performances of the as-prepared Co/CoP-NF catalysts obtained at different deposition current density. Deposition current density (A)

a

η10 (mV)

η100 (mV)

b (mV decade-1)

j0 a(mA cm-2)

-0.2

76

213

95

1.74

-0.5

35

130

71

3.10

-0.8

40

165

97

2.84

j0 is obtained from Tafel plots by using extrapolation methods.

Table 2. Comparison of the HER performances of the electrodes in alkaline solutions reported recently.

Catalysts

Electrolyte

η10 (mV)

η100 (mV)

Tafel plot (mV·dec-1)

Ref

Co/Co3O4

1.0 M NaOH

90

~240

44

41

Ni/NiO

1.0 M NaOH

145

~255

43

42

Cr2O3/NiO/Ni

1.0 M NaOH

~45

~150

—

44

1.0 M NaOH

280

—

115

45

Ni–Mo–N

1.0 M NaOH

~35

—

40

46

Ni−Mo

1.0 M NaOH

~55

—

—

47

Ni3N/NF

1.0 M NaOH

121

254

109

48

Pt-3DNi

30 wt % KOH

40

—

136

49

C-CWC

1.0 M KOH

73

—

25

50

Co/CoP-NF

1.0 M NaOH

35

130

71

This work

N-C@Ni-Al2 O3@GO



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Graphical Abstract


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CoP Film on Ni Foam for Efficient

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