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Article

The NiCo Bimetal Catalyst Loaded on Polyvinylidene Fluoride Coated on the Self-Supporting Silk Electrode as an Advanced Electrocatalyst for the Hydrogen Evolution Reaction

by
Haoyu Wang
1,2,
Chunyong Zhang
1,2,*,
Zhe Li
1,
Yinpin Wen
2 and
Li Shu
2
1
School of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou 213001, China
2
Jiangsu Key Laboratory of Precious Metal Chemistry and Technology, Jiangsu University of Technology, Changzhou 213001, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(6), 987; https://doi.org/10.3390/catal13060987
Submission received: 30 April 2023 / Revised: 26 May 2023 / Accepted: 5 June 2023 / Published: 9 June 2023
(This article belongs to the Special Issue Electrocatalytic Hydrogen Evolution Reaction through Water Splitting)
Browse Figures
Versions Notes

Abstract

:
In this work, a NixCox/Silk-PVDF bimetallic catalyst electrode was prepared for the hydrogen evolution reaction (HER) in hydropower. This cheap, durable, and efficient electrode has good practical application prospects. Green natural silk, which will pollute the environment. The electrodes (obtained by varying the Ni:Co ratio and hydrothermal times) were prepared hydrothermally. Ni and Co elements were revealed by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Ni2Co2/silk-PVDF was identified as an effective catalyst in 1 M KOH alkaline electrolyte with an overpotential of 89.4 mV at 20 mA cm−2 and a Tafel slope of 47.46 mv dec−1. It showed low resistance and a high specific surface area in EIS and CV tests, respectively, proving its superior HER performance. Finally, the stability and durability of the electrode coated with PVDF were demonstrated via testing at a voltage of −0.1 V over 24 h. This work provides an environmentally friendly and simple method to load metal on a self-supporting electrode to be used in the hydrogen evolution reaction.

1. Introduction

With the development of science and technology and the study of basic theories, the James Webb Space Telescope is a milestone in the human exploration of deep space. In order to launch it into orbit, the rocket carrying it is extremely important. Today, all rockets worldwide, without exception, use chemical fuel, which is generally composed of liquid oxygen and liquid hydrogen. Due to the highly active chemical properties of hydrogen, liquid hydrogen has a large calorific value. In addition, it has a higher specific impulse, which is an indicator that measures the combustion efficiency of a rocket; the higher the specific impulse value, the greater the impulse generated by consuming the same mass of fuel and the greater the acceleration of the rocket. The combination of liquid hydrogen and oxygen has a specific impulse of over 400 [1,2]. For highly pure hydrogen, the industry usually uses hydroelectrolysis (hydrogen evolution), a green and economical method of converting electrical energy into chemical energy [3].
Nowadays, some precious metal-based materials and their alloys are considered to be ideal electrocatalysts for the HER process [4]. However, due to the low output and high preparation cost of these rare metals, they have been unable to be used as the most cost-effective electrocatalysts. Therefore, it is necessary to find electrocatalysts with high activity and low cost that are environmentally sound and rich in earth to replace the above precious metals. Thus far, a variety of low-cost alternatives to catalytic HER have been studied, including metal oxides, chalcogenides, phosphates, and perovskites [5,6,7,8,9,10,11]. Atchudan’s study used Anacardium occidentale nut’s skin to product the porous carbon material. The Anacardium occidentale nut’s skin is convenient to get at a low cost [12]. Many authors have utilized natural green biowastes/biomass for the synthesis of carbon materials [13,14].
The method of hydrogen production via the electrolysis of water has developed rapidly in recent years, and the technology is now more mature than traditional methods. Its raw material is water, the reaction process is clean and environmentally friendly, and there is no carbon dioxide in the product. Water electrolysis will not aggravate the greenhouse effect, and the entire process is very much in line with the concept of sustainable development. Therefore, it is a feasible and mainstream method for carrying out HER from electrolytic water to prepare hydrogen. The electrolytic water reaction is divided into two semi-reactions: the hydrogen evolution reaction and the oxygen evolution reaction [15,16,17,18,19,20]. The cathode HER for the production of high-purity hydrogen is a reaction based on a two-electron transfer process. In the alkaline solution, HER is divided into Volmer, Heyrovsky, and Tafel steps, through which hydrogen atoms are adsorbed or desorbed [21,22]. The specific reactions are as follows:
H2O + e− → Hads + OH− (Volmer)
Hads + H2O + e− → H2 + OH− (Heyrovsky)
Hads + Hads → H2 (Tafel)
Thus far, it has been reported that the number of HER catalysts in an alkaline medium is less than that in an acidic medium. In an alkaline medium, HER commercial catalysts mainly use platinum groups, such as Pt, Pd, etc. [23]. However, these are precious metals, which are not only a scarce resource but also expensive; therefore, the use of transition metals instead of precious metals is now an important replacement for commercial catalysts. Compared with the platinum group, transition metals, for example Ni, Co, Fe, Cu, etc. [20,24], may have good hydrogen spillover properties under certain conditions [25]. In addition, studies have shown that bimetallic compounds have stronger electrochemical performance than single metal compounds, which can bring better electrochemical performance to HER. In recent years, because of the synergistic effect between the catalytic properties of Ni and Co, Ni has become known for its low HER overpotential, whereas Co has become known for its high hydrogen adsorption. Research on Ni and Co has steadily increased because their hydrogen production activity is equivalent to that of precious metal catalysts [26,27,28,29].
In ancient China, silk is made into yarn and textiles on a large scale in the textile industry. The Silk Road was a land-based trade route from China through Central Asia to South and West Asia, as well as Europe and North Africa. Silk has good mechanical properties, controlled biodegradability, and novel dielectric properties due to the development of manufacturing technology and research on the structure of silk. Silk is a well-known natural biomaterial. It has been processed into various materials such as nanofibers, flexible films, hydrogels, and 3D porous aerogels. Therefore, this study used silk as a self-supporting electrode.
Here, a NiCo bimetallic catalyst was synthesized using a simple and green solvothermal route. This study used waste silk as the self-supporting electrode on which the NiCo bimetal was loaded. PVDF was used to coat the substrate of the silk electrode, which increases the mechanical strength and durability for working stability in electrochemistry. The Ni and Co metals were loaded directly on silk via hydrothermal synthesis twice and grown uniformly on the silk surface to ensure maximum electrochemically active surface area. Under the same metal load mass and conditions, metals Ni, Co, and NiCo were loaded on four different electrodes separately as the control groups. Further, the four different electrodes were used as four different electrocatalysts for HER in the alkaline solution (1 M KOH). The crystal structure, physicochemical properties, and electrochemical performance of the prepared samples were tested using various analytical instruments. The HER reaction was monitored by cyclic voltammetry, electrochemically active surface area, electrochemical impedance spectroscopy, linear sweep voltammetry, long-term stability and Tafel methods. Their applicability as HER electrocatalysts was studied.

2. Results and Discussion

2.1. Physicochemical Properties of the NixCox/Silk-PVDF Electrodes

The strategy used for the synthesis of NixCox/Silk-PVDF electrodes is illustrated in Figure 1a. The waste silk was yellowish brown after being wrapped with PVDF. Subsequently, the final sample prepared by hydrothermal synthesis appeared brown and had purple powder on the surface. In Figure 1b,c the SEM images show the overlapping of the metal elements, where octahedral particles were attached to the surface of the silk. In Figure 1c–e, the metal Ni was uniformly distributed on the surface of the silk, while the metal Co was attached to the surface of the silk in an octahedral form (Figure 1d,e show the distribution of the Ni and Co elements, respectively). The molar mass ratio of Ni and Co used in the experiment was 50:50. According to Figure 1f, the mass percentage of Ni and Co was close to 50:50, which conforms to the experimental rule.
The morphology and fine structure of Ni2Co2/Silk-PVDF were further observed using TEM images (Figure 1g). The TEM images show that the lattice fringes of the Co (111) layer were clear, and the spacing of the lattice fringe was 0.2046 nm [30], which is very close to the properties of metal Co. The lattice fringes of (111) Ni were also clear [31], and the spacing of the lattice fringe was about 0.2034 nm, which is close to the value characteristic of metal Ni.
Figure 2 shows the XRD patterns of the NixCox/Silk-PVDF electrodes synthesized through the hydrothermal process. All the NixCox/Silk-PVDF sample electrodes showed a distinct diffraction peak similar to Silk-PVDF between 18° and 26°; the XRD patterns of silk fibroin were determined in previous studies to have the specific silk II peak at 20.2° and silk I peak at 24.5° [32,33,34]. This is due to the fact that the amorphous diffraction peak of the silk and the four diffraction peaks of α-PVDF are formed by overlapping each other; due to the minimal amount of PVDF wrapping, the diffraction peaks of α-PVDF were not obvious [35]. In general, the primary diffraction peaks of Co and Ni appear at 2θ = 44.4°, 51.6°, and 76.1° (PDF#00-001-1255 for Co and PDF#00-001-1258 for Ni) [36,37,38]. Next, the Co/Silk-PVDF, Ni/Silk-PVDF, Ni1Co1/Silk-PVDF, and Ni2Co2/Silk-PVDF samples were compared with the Silk-PVDF sample. The difference was that there were (111), (200), and (220) diffraction peaks corresponding to the two metals. In the four cases of the NixCox/Silk-PVDF electrodes, the corresponding diffraction peaks of the NiCo bimetal were almost the same as those of the single Ni or Co metal. Lastly, when the Co/Silk-PVDF and Co/Silk-PVDF samples were compared with the Ni1Co1/Silk-PVDF and Ni2Co2/Silk-PVDF samples, the diffraction peak intensity of the bimetal synthesis was obviously due to that of the single metal synthesis. Furthermore, in the case of the same quality of loaded metal, the corresponding diffraction peaks of the NixCox/Silk-PVDF electrocatalysts became sharp and narrow with an increasing number of syntheses.
Figure 3a presents the XPS survey spectrum for Ni2Co2/Silk-PVDF, also confirming the presence of the Ni and Co elements. Figure 3b–d show the XPS spectra in the C 1s, Ni 2p, and Co 2p regions. As shown in Figure 3b, the C 1s spectrum presents three peaks located at 284.58 eV, 285.88 eV, and 287.78 eV, corresponding to the binding energy (BE) of C-C, C-O, and O-C=O, respectively [39]. Figure 3c shows that for Ni 2p, the two peaks of 855.68 eV and 873.18 eV were consistent with the BEs of Ni 2p3/2 and Co 2p1/2, respectively, confirming the presence of the Ni element in the form of metallic Ni. The BEs at 861.18 eV and 879.68 eV with two satellites also corresponded to metallic Ni [40,41]. The high-resolution Co 2p spectrum was fitted into four peaks (Figure 3d). The doublet peaks at 776.28 eV and 789.98 eV were ascribed to the Co0 2p3/2 and Co0 2p1/2, while the doublet peaks at 781.28 eV and 797.68 eV were attributed to the Co2+ 2p3/2 and Co2+ 2p1/2, respectively. The BEs at 785.58 eV and 803.78 eV with two satellite peaks indicated the existence of Co [41,42,43]. When the peaks of the BEs were combined with EDS (Figure 1d–f), the ratio of Ni and Co was about 50:50 as a whole, suggesting that the loading of metallic NiCo was successful.

2.2. Electrochemical Properties of the NixCox/Silk-PVDF Electrodes

The silk self-supporting electrode was used as the working electrode. The activity of HER was measured with the traditional three-electrode system structure, and a Pt electrode was used as the counter electrode. Their catalytic activities in a 1 M KOH solution were determined by using linear sweep voltammetry (LSV) [44]. Figure 4a shows the LSV curves of different catalysts in the potential range of −0.4 V–0 V (vs. RHE) with a scanning rate of 5 mV s−1. The overpotentials of the Ni1Co1/Silk-PVDF and the Ni2Co2/Silk-PVDF electrodes reached 179.4 mV and 161.7 mV at a current density of 100 mA cm−2, respectively. Meanwhile, the electrodes loaded with the single metals Ni and Co showed insignificant catalytic activity, and the current density did not reach 100 mA cm−2. In summary, the Ni2Co2/Silk-PVDF electrocatalysts showed the best HER performance among all electrocatalysts synthesized in this study.
Figure 4b shows the Tafel slopes. The Tafel slope is determined according to the η (overpotential) and j (current density) values in the LSV curve, which reflect the change in the steady-state current under different overpotentials. The linear portion of the Tafel plot corresponds to the equation η = a + b × log j, where b is the Tafel slope and a is the constant of the material [20,45]. Tafel data can be verified by LSV and converted from LSV’s ordinate to Tafel’s abscissa through log |current density| calculation. The smaller the Tafel is, the higher the current density that can be achieved with a lower overpotential; thus, the Tafel slope is related to the mechanism of HER [46]. The Tafel slopes of the Co/Silk-PVDF, Ni/Silk-PVDF, Ni1Co1/Silk-PVDF, and Ni2Co2/Silk-PVDF electrodes were 219.67 mV dec−1, 121.14 mV dec−1, 93.65 mV dec−1, and 67.46 mV dec−1, respectively, indicating that the Ni2Co2/Silk-PVDF bimetal exhibited higher catalytic activity than the other three HER catalysts. Consequently, all the samples loaded with the same quality metal were tested to confirm that the best catalytic performance was provided by the Ni2Co2/Silk-PVDF electrode. The Tafel slope of the Ni2Co2/Silk-PVDF electrode (67.46 mV dec−1) was slightly larger than that of the commercial Pt/C electrode (42.6 mV dec−1) [47]; however, it had better performance than most NiCo catalysts reported to date (Table 1) [20,48,49].
Furthermore, electrochemical impedance spectroscopy (EIS) was used to explore the mechanical process kinetics of HER at the electrode interface. The Randles equivalent circuit is used in EIS analysis and includes Rs (solution resistance), Rct (charge transfer resistance), and Cdl (electrochemical double-layer capacitance) [58,59]. Usually, the semicircle radius of impedance in the low-frequency region of the EIS diagram is directly related to the charge transfer resistance. Therefore, a smaller semicircle induces better electrode electrical conductivity. The Rct values of the Co/Silk-PVDF, Ni/Silk-PVDF, Ni1Co1/Silk-PVDF, and Ni2Co2/Silk-PVDF electrodes were 8.61 Ω, 8.032 Ω, 4.629 Ω, and 3.847 Ω, respectively. Obviously, the impedance semicircle radius of the Ni2Co2/Silk-PVDF electrode catalyst was smaller than that of the other three electrodes. The recorded EIS Nyquist plot was perfectly fitted with the proposed equivalent circuit, as shown in the inset of Figure 5a. In particular, the Ni2Co2/Silk-PVDF electrode showed the lowest resistance, which meant that the dynamics of the hydrogen evolution reaction of the Ni2Co2/Silk-PVDF were faster; thus, it had the best charge transfer efficiency.
The double-layer capacitance (Cdl) is shown in Figure 5b,c. A double-layer capacitor was used to estimate the electrochemically active surface area (ECSA) [60], as shown in Figure 5b, at a scan rate of 120 mV s−1 and a voltage window of 0.18 V–0.28 V. The value of the ECSA directly affects the performance of HER. The width of the electrode CV curve increased sharply with the addition of the NiCo bimetal. In addition, the Ni2Co2/Silk-PVDF electrode displayed symmetrical CV curves with no obvious pseudocapacitive behavior at a voltage window of 0.18 V–0.28 V and scan rates of 20 mV s−1, 40 mV s−1, 60 mV s−1, 80 mV s−1, and 120 mV s−1 (Figure 5c) [61], indicating that this electrode had a low charge-transfer resistance. The Cdl value trend lines were calculated from the slopes of the CV curve, and the value of the Ni2Co2/Silk-PVDF electrode was 133.04 mF cm−2 (Figure 5d) [62]. Obviously, the Ni2Co2/Silk-PVDF electrode exhibited the highest Cdl value. Thus, the Ni2Co2/Silk-PVDF electrode was expected to exhibit excellent electrocatalytic activity because of its high electrical capacity.
Long-term stability is a very important indicator for commercial use. As shown in Figure 6a, we observed the change in current density at a constant potential difference of −0.1 V. The current density of the Ni2Co2/Silk-PVDF electrode decreased very little within 90,000 s, whereas that of the Ni2Co2/Silk electrode rapidly changed in a relatively short period of time. Figure 6b, 6c show the morphology of the Ni2Co2/Silk electrode and the Ni2Co2/Silk-PVDF electrode during electrochemical stability tests for 7000 s and 90,000 s, respectively. It was obvious that the PVDF still retained the original electrode shape after the electrode was tested for 90,000 s, which proves that PVDF can improve the durability of silk electrodes.

3. Experimental Section

3.1. Reagents and Materials

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, GR, 99%, CAS: 13478-00-7) was commercially available from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China). Cobaltous nitrate hexahydrate (Co(NO3)2·6H2O, AR, 99%, CAS: 10026-22-9) was purchased from the Chengdu Kelong Chemical Reagent Company (Chengdu, China). N,N-dimethylformamide (DMF, GC, 99.9%, CAS: 68-12-2), 2-methylimidazole (HMlm, 98%, CAS: 693-98-1), polyvinylidene difluoride (PVDF, average Mw: 2.75 × 105, average Mn: 1.07 × 105, CAS: 24937-79-9), and polyvinyl pyrrolidone (PVP, average Mw: 5.5 × 104, CAS: 9003-39-8) were provided by Shanghai Civi Chemical Technology Co., Ltd. (Shanghai, China). Methanol (AR, 99.5%, CAS: 67-56-1) and potassium hydroxide (KOH, AR, 99%, CAS: 1310-58-3) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China)

3.2. Preparation of Silk-PVDF

Waste silk was collected from a recycling station. The waste silk was cut into 1 × 2 cm pieces [63,64]. A total of 5 g of PVDF powder, 0.33 g of PVP, and 5 g of HMlm were added to 28 mL of DMF solvent and dissolved via magnetic stirring in an oil bath at 60 °C for 12 h to obtain a uniform brown, transparent solution. The obtained solution was then scraped evenly on the silk surface before being transferred to an air oven at 60 °C for 8 h to dry and obtain the Silk-PVDF sample.

3.3. Electrode Preparation

Briefly, the dried Silk-PVDF was laid at the bottom of a hydrothermal kettle; then, Ni(NO3)2·6H2O and Co(NO3)2·6H2O were added to a beaker containing DMF and HMlm, respectively. At this time, the molar mass ratios of Ni:Co were adjusted to 0:100, 100:0, and 50:50, corresponding to samples Co/Silk-PVDF, Ni/Silk-PVDF, and Ni1Co1/Silk-PVDF. A subsequent separate weighing of Ni and Co with a molar mass ratio of 50:50 was half that of Ni1Co1/Silk-PVDF as the first hydrothermal condition of Ni2Co2/Silk-PVDF. The samples were thoroughly stirred and added to the hydrothermal kettle. The hydrothermal kettle was then transferred to the air oven, where the temperature was raised to 140 °C for 2 h, and the reaction was allowed to progress for 48 h. After the reaction, the filter paper at the bottom of the kettle was removed, cleaned with DMF and methanol, and dried in an air oven at 60 °C for 8 h. Finally, according to the above Ni2Co2/Silk-PVDF experimental conditions, the secondary hydrothermal process was carried out. There are two schemes for the synthesis of bimetallic NiCo, ‘Ni1Co1/Silk-PVDF’ and ‘Ni2Co2/Silk-PVDF’. For example, the Ni1Co1/Silk-PVDF uses 0.2 g Ni(NO3)2·6H2O and 0.2 g Co(NO3)2·6H2O for one solvothermal synthesis. The Ni2Co2/Silk-PVDF uses 0.1 g Ni(NO3)2·6H2O and 0.1 g Co(NO3)2·6H2O for first solvothermal synthesis. After the completion of first solvothermal synthesis, 0.1 g Ni(NO3)2·6H2O and 0.1 g Co(NO3)2·6H2O were used for second solvothermal synthesis. It means that the total quality of Ni(NO3)2·6H2O and Co(NO3)2·6H2O was 0.4 g for Ni1Co1/Silk-PVDF and Ni2Co2/Silk-PVDF. The Ni2Co2/Silk-PVDF electrode was obtained after rinsing and drying for further analysis (Figure 1).

3.4. Physicochemical Characterization of Electrocatalysts

The NixCox/Silk-PVDF electrodes were analyzed via X-ray diffraction (XRD, PW3040/60, Philips, Tokyo, Japan). The XRD patterns of a catalyst were recorded over the scanning range of 2θ (5° to 90°) with Cu Ka radiation, the target voltage was 40 kV and the current was 30 mA. By comparing the diffraction peaks of the measured XRD spectrum with a standard, the composition and crystal structure of the sample can be determined. The morphology, size, and distribution of the Ni2Co2/Silk-PVDF electrodes was observed through the use of scanning electron microscopy (SEM, Sigma 300, Zeiss, Oberkochen, Germany) and energy-dispersive X-ray spectroscopy (EDS). The SEM tests were performed with a 10 kV electron beam. The SEM images was taken with an InLens at magnifications ranging from 20,000× to 50,000×. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Nexsa, Waltham, MA, USA) was used to determine the chemical composition and valence of the elements in the samples. All XPS spectra were corrected to the binding energy (BE) of C 1 s = 284.58 eV. The ultrastructure of the samples was observed using an ultra-high-resolution transmission electron microscope (TEM, JEM 2100, JEOL, Tokyo, Japan).

3.5. Evaluation of Electrochemical Properties

All electrochemical tests were conducted at an electrochemical workstation (CHI760E, Shanghai, China). The cyclic voltammetry (CV), electrochemical impedance spectrum (EIS), linear sweep voltammetry (LSV), and long-term stability were tested using a standard three-electrode system (NixCox/Silk-PVDF (working electrode area: 1 × 1 cm2), a Pt sheet, and a saturated calomel electrode (RHE, ERHE = EHg/Hg2Cl2 + 0.0592 × pH + 0.24 V)) reacted in 1 M KOH electrolyte [65]. The electrochemical double-layer capacitance (Cdl) of the NixCox/Silk-PVDF electrodes was obtained via CV measurements at scan rates of 20, 40, 60, 80, 100, and 120 mV s−1 over the non-Faradaic region. The electrode polarization curves were recorded by performing LSV measurements. The Tafel slopes of electrodes were calculated from the LSV curves. A frequency ranging from 0.1 Hz to 100 kHz with a scan rate of 5 mV s−1 at open circuit potential was used to test EIS. Long-term stability under the same conditions was set at 90,000 s (over 24 h). This was used to assess the durability and stability of the electrode.

4. Conclusions

In this study, NixCox/Silk-PVDF bimetal catalysts were designed via PVDF wrapping and hydrothermal synthesis. Silk, as a self-supporting electrode, was used as the working electrode. By varying the Ni and Co ratio, four different samples were prepared: Co/Silk-PVDF, Ni/Silk-PVDF, Ni1Co1/Silk-PVDF, and Ni2Co2/Silk-PVDF. The results of XRD, XPS, and TEM revealed the existence of the Ni and Co elements. The XRD diffraction peaks of bimetallic electrodes were significantly stronger than those of monometallic electrodes. In particular, Ni2Co2/Silk-PVDF had the highest intensity of diffraction peaks. The XPS revealed the presence of elements and the intensity of their binding energy. The TEM images showed that the lattice fringes of the metal Co and Ni. The EDS images showed that the Ni and Co elements were evenly distributed. The ratio of Ni and Co was in line with our expectations. As shown by electrochemistry, when the current density was 10 mA cm−2, the overpotential of the Ni2Co2/Silk-PVDF was 89.4 mV. Furthermore, the Tafel slope of the Ni2Co2/Silk-PVDF was just 67.46 mV dec−1. The Nyquist plots showed its Rct value was 3.847 Ω. At a scan rate of 120 mV s−1, the electrode showed the widest CV curves, indicating that its ECSA was the largest. Meanwhile, the electrode’s Cdl value was 133.04 mF cm2. After stability and long-term testing, there was a significant difference in the morphology of silk before and after wrapping with PVDF. Therefore, the proposed route can provide a reference for the preparation of electrochemical catalysts in the field of green, renewable biomass.

Author Contributions

Conceptualization, H.W. and C.Z.; methodology, H.W.; software, H.W.; validation, Y.W., Z.L. and L.S.; formal analysis, H.W.; investigation, H.W.; data curation, H.W.; writing—original draft preparation, H.W.; writing—review and editing, H.W.; visualization, H.W.; supervision, C.Z.; project administration, C.Z.; funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [Postgraduate Research & Practice Innovation Program of Jiangsu Province] grant number [SJCX22_1456] and [SJCX22_1459], [the National Natural Science Foundation of China] grant number [31800495], [Natural Science Foundation of Jiangsu Province] grant number [BK20181040].

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00987 g001 550
Figure 1. Synthesis process of the Ni2Co2/Silk-PVDF electrode by simple solvothermal and their eco-friendly energy applications (a), SEM image (b), overlapping of all the metal elements (c), EDS element mappings of the Ni2Co2/Silk-PVDF electrode (d–f), and TEM images of the Ni2Co2/Silk-PVDF electrode (g).
Figure 1. Synthesis process of the Ni2Co2/Silk-PVDF electrode by simple solvothermal and their eco-friendly energy applications (a), SEM image (b), overlapping of all the metal elements (c), EDS element mappings of the Ni2Co2/Silk-PVDF electrode (d–f), and TEM images of the Ni2Co2/Silk-PVDF electrode (g).
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Catalysts 13 00987 g002 550
Figure 2. XRD patterns of the NixCox/Silk-PVDF electrodes.
Figure 2. XRD patterns of the NixCox/Silk-PVDF electrodes.
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Catalysts 13 00987 g003 550
Figure 3. XPS survey for Ni2Co2/Silk-PVDF in the (a) XPS survey, (b) C 1s, (c) Ni 2p, and (d) Co 2p.
Figure 3. XPS survey for Ni2Co2/Silk-PVDF in the (a) XPS survey, (b) C 1s, (c) Ni 2p, and (d) Co 2p.
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Catalysts 13 00987 g004 550
Figure 4. LSV curves (a) and Tafel plots (b) for the NixCox/Silk−PVDF electrodes.
Figure 4. LSV curves (a) and Tafel plots (b) for the NixCox/Silk−PVDF electrodes.
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Catalysts 13 00987 g005 550
Figure 5. EIS (a), ECSA (b), and electrochemical double-layer capacitance (Cdl) (c) of the NixCox/Silk-PVDF electrodes. (d) Nyquist plots for the NixCox/Silk-PVDF electrodes.
Figure 5. EIS (a), ECSA (b), and electrochemical double-layer capacitance (Cdl) (c) of the NixCox/Silk-PVDF electrodes. (d) Nyquist plots for the NixCox/Silk-PVDF electrodes.
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Catalysts 13 00987 g006 550
Figure 6. Time-dependent current density changes for various electrodes under a constant potential of −0.1 V (vs RHE) (a), the morphology of the Ni2Co2/Silk electrode after testing (b), and the morphology of the Ni2Co2/Silk-PVDF electrode after testing (c).
Figure 6. Time-dependent current density changes for various electrodes under a constant potential of −0.1 V (vs RHE) (a), the morphology of the Ni2Co2/Silk electrode after testing (b), and the morphology of the Ni2Co2/Silk-PVDF electrode after testing (c).
Catalysts 13 00987 g006
Table
Table 1. The comparative set up showing HER activities of the present work with reported data.
Table 1. The comparative set up showing HER activities of the present work with reported data.
CatalystsElectrolyteOverpotential/(mV)Tafel Slope (mV dec−1)J
(mA cm−2)		Ref.
NiFeCo1.0 M KOH646410[50]
Co0.5Zn0.5MoO41.0 M KOH201162.710[51]
Ti3C2Tx@ZIF-81.0 M KOH1077720[52]
NiSSe1.0 M KOH15412510[53]
Fe-NiS20.5 M H2SO41984210[54]
NiTe20.5 M H2SO45604410[55]
Ni95Fe5/CP1.0 M KOH13095.420[20]
NiCl21.0 M KOH1764110[56]
10% F/BCN0.5 M H2SO42228710[57]
Ni2Co2/Silk-PVDF1.0 M KOH89.467.4620This work
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Wang, H.; Zhang, C.; Li, Z.; Wen, Y.; Shu, L. The NiCo Bimetal Catalyst Loaded on Polyvinylidene Fluoride Coated on the Self-Supporting Silk Electrode as an Advanced Electrocatalyst for the Hydrogen Evolution Reaction. Catalysts 2023, 13, 987. https://doi.org/10.3390/catal13060987

AMA Style

Wang H, Zhang C, Li Z, Wen Y, Shu L. The NiCo Bimetal Catalyst Loaded on Polyvinylidene Fluoride Coated on the Self-Supporting Silk Electrode as an Advanced Electrocatalyst for the Hydrogen Evolution Reaction. Catalysts. 2023; 13(6):987. https://doi.org/10.3390/catal13060987

Chicago/Turabian Style

Wang, Haoyu, Chunyong Zhang, Zhe Li, Yinpin Wen, and Li Shu. 2023. "The NiCo Bimetal Catalyst Loaded on Polyvinylidene Fluoride Coated on the Self-Supporting Silk Electrode as an Advanced Electrocatalyst for the Hydrogen Evolution Reaction" Catalysts 13, no. 6: 987. https://doi.org/10.3390/catal13060987

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