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Influence of gas-diffusion-layer current collector on electrochemical performance of Ni(OH)2 nanostructures

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  • We report the electrochemical performance of Ni(OH)2 on a gas diffusion layer (GDL). The Ni(OH)2 working electrode was successfully prepared via a simple method, and its electrochemical performance in 1 M NaOH electrolyte was investigated. The electrochemical results showed that the Ni(OH)2/GDL provided the maximum specific capacitance value (418.11 F·g−1) at 1 A·g−1. Furthermore, the Ni(OH)2 electrode delivered a high specific energy of 17.25 Wh·kg−1 at a specific power of 272.5 W·kg−1 and retained about 81% of the capacitance after 1000 cycles of galvanostatic charge–discharge (GCD) measurements. The results of scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed the occurrence of sodium deposition after long-time cycling, which caused the reduction in the specific capacitance. This study results suggest that the light-weight GDL, which can help overcome the problem of the oxide layer on metal–foam substrates, is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.
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Influence of gas-diffusion-layer current collector on electrochemical performance of Ni(OH)2 nanostructures

  • Corresponding author:

    Santi Maensiri    E-mail: santimaensiri@gmail.com;santimaensiri@g.sut.ac.th

  • 1. School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand
  • 2. SUT CoE on Advanced Functional Materials, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand
  • 3. Research Network NANOTEC-SUT on Advanced Nanomaterials and Characterization, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand

Abstract: We report the electrochemical performance of Ni(OH)2 on a gas diffusion layer (GDL). The Ni(OH)2 working electrode was successfully prepared via a simple method, and its electrochemical performance in 1 M NaOH electrolyte was investigated. The electrochemical results showed that the Ni(OH)2/GDL provided the maximum specific capacitance value (418.11 F·g−1) at 1 A·g−1. Furthermore, the Ni(OH)2 electrode delivered a high specific energy of 17.25 Wh·kg−1 at a specific power of 272.5 W·kg−1 and retained about 81% of the capacitance after 1000 cycles of galvanostatic charge–discharge (GCD) measurements. The results of scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed the occurrence of sodium deposition after long-time cycling, which caused the reduction in the specific capacitance. This study results suggest that the light-weight GDL, which can help overcome the problem of the oxide layer on metal–foam substrates, is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.

    • Electrochemical capacitors are energy storage devices consisting mainly of a current collector, separator, active material, and electrolyte [1]. Among these, the current collector determines the electrochemical performance of the capacitor. The current collector as a substrate plays significant roles in electrochemical capacitors, including the following [2]: (i) It collects and conducts electricity from the electrode to the power source; (ii) it supports the active materials or serves as a holder for the active materials. Hence, the electrochemical capacitor performance depends on both the physical and chemical properties of the current collector [34]. Nickel (Ni) foam is a widely used current collector in laboratory research because of its low cost, mechanical strength, inertness, and relatively low toxicity [5]. Generally, open-pore Ni foams are produced by coating Ni metal on a polymer-type matrix of polyurethane via chemical vapor deposition or electrochemical deposition. Therefore, an oxide layer can easily form on the Ni foam surface when it is exposed to air [6]. In one study, this layer was found to provide an overpotential that reduced the power output [5]. Also, errors in the specific capacitance value due to the presence of nickel oxide/hydroxide on the Ni foam surface have been reported [7]. To overcome this oxide layer problem, high-surface-area carbon fiber paper has been used as the current collector. Gas diffusion layers (GDLs) are carbon-based current collectors generally used in several applications such as fuel cells [811] and Li-ions batteries [12]. Generally, GDLs are composed of either woven or non-woven carbon fiber arrangements with an interconnected porous structure [13]. Due to properties such as low sheet resistance, large surface area, and light weight, GDLs have attracted much attention as a current collector for electrochemical capacitor applications [14]. For active materials, nanosized Ni(OH)2 has been considered a very promising electroactive material owing to its high capacitance, low cost, and easy processing [1516].

      In this work, we provide a simple method for preparing the working electrode, in which a light-weight carbon fiber of GDL was used as a current collector for electrochemical studies of Ni(OH)2. The electrochemical properties of Ni(OH)2 on GDL substrate were investigated via a three-electrode system in 1 M NaOH aqueous electrolyte. The prepared Ni(OH)2 electrode reached the maximum specific capacitance value of 418.11 F·g−1 at a current density of 1 A·g−1, and 81% of the capacitance was retained after 1000 cycles. The cause of the reduction in the specific capacitance value after 1000 cycles is also discussed in this paper.

    2.   Experimental
    • All the chemicals were of analytical grade and were used without further purification. Nanostructures of Ni(OH)2 were synthesized via a hydrothermal method as reported in a previous work [17]. Briefly, NaOH solution was dropped into a nickel chloride solution, and the mixed solution was washed with deionized (DI) water. Afterward, NaOH solution was directly added into the resulting precipitate, and the mixture was then transferred to a Teflon-lined stainless-steel autoclave. The autoclave was heated at 160°C for 20 h. The obtained green precipitate of Ni(OH)2 was finally washed with DI water and subsequently dried in a vacuum oven at 70°C.

    • In the preparation of the working electrode, an active material of Ni(OH)2, a conducting agent of carbon black (CB), and a binder of polyvinylidene difluoride (PVDF) in a given mass ratio of 80:10:10 were hand-mixed using an agate mortar and pestle. Then 200 μL of N-methyl pyrrolidone (NMP) solution was added into the mixed powder. To obtain a homogeneous slurry, the mixture was then shaken for 6 h using a mini-shaker. Then 1 μL of the obtained slurry was dropped onto both sides of the GDL substrate (MGL 190, AvCarb Material Solutions), which was placed on a hotplate of ~90°C, within an active area of 1 cm × 1 cm (Fig. 1). Finally, the dried working electrode was naturally cooled down to room temperature, and the electrochemical performance was further investigated.

      Figure 1.  Schematic representation for the electrode preparation of Ni(OH)2 on GDL substrate.

    • The crystal structures of bare GDL substrate, Ni(OH)2 powder, and the prepared working electrode were characterized via X-ray diffraction (XRD) on Bruker D2 using Cu Kα radiation at step time of 0.5 s and a scanning rate of 0.02°/min in the 2θ range from 10° to 80°. The surface morphologies and their corresponding energy-dispersive X-ray spectroscopy (EDS) spectra were obtained via field-emission scanning electron microscopy (FE-SEM). The electrochemical tests of Ni(OH)2 on GDL substrate were performed in 1 M NaOH electrolyte using a three-electrode system. A platinum plate was used as the reference electrode, and Ag/AgCl saturated in 3 M KCl solution was used as the counter electrode. The electrochemical performances were evaluated using the Metrohm Autolab PGSTAT 302N potentiostat/galvanostat via three techniques: cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS). Before the electrochemical measurement, the working electrode was soaked in 1 M NaOH for 12 h. The specific capacitance (C), energy density (E), and power density (P) of the Ni(OH)2/GDL were calculated using the following equations [18]:

      where C is specific capacitance (F·g−1) obtained from the GCD measurement, I is the discharge current (A), m is the mass of active material (g), Δt is the discharge time (s), ΔV is the operating voltage (V), E is the energy density (Wh·kg−1), and P is the power density (W·kg−1).

    3.   Results and discussion
    • The XRD patterns of the bare GDL substrate, Ni(OH)2 powder, and the prepared working electrode are presented in Fig. 2. As seen from the figure, the XRD pattern of the prepared working electrode shows a series of sharp peaks within the 2θ range from 10° to 80°. The diffracted peaks at 2θ ≈ 19.3°, 33.0°, 38.5°, 52.2°, 59.0°, 62.6°, 69.3°, 70.5°, and 72.7° correspond to (001), (100), (101), (102), (110), (111), (200), (103), and (201) planes of β-Ni(OH)2 with hexagonal crystal structure (JCPDS file No. 14-0117), respectively. The standard diffraction peaks are indicated by blue lines. Moreover, two diffraction peaks at 2θ ≈ 26° and 55° in the XRD pattern of the prepared working electrode, which are the characteristic peaks of (002) and (004) planes of graphite structure, resulted from the GDL substrate [1920]. By comparing the XRD pattern of the prepared working electrode with those of the bare GDL substrate and the synthesized Ni(OH)2 powder, we can conclude that the working electrode was successfully prepared.

      Figure 2.  XRD patterns of (a) the bare GDL substrate, (b) Ni(OH)2 powder, and (c) the prepared working electrode.

      The FE-SEM images (Fig. 3) show the surface morphologies and microstructures of the bare GDL substrate and the prepared working electrode. The bare GDL substrate was composed of a random arrangement of straight carbon fibers. This arrangement provides a large space between interconnected fibers, which is useful for electrochemical applications, as it allows for uninterrupted charge freeway networks for quick electron transfer [18]. After the slurry was dropped on the GDL substrate and the substrate was dried at ~90°C, the slurry entirely covered the GDL substrate (Fig. 3(b)). In addition, cracks occurred on the surface of the prepared working electrode, due to the solvent (NMP) evaporation. The high-magnification FE-SEM images of Ni(OH)2 in Fig. 3(d) show that the Ni(OH)2 plates were agglomerated, with pores on the electrode surface. These pores are beneficial to electrolyte ions in electrochemical applications, as they can reduce the diffusion pathway, resulting in a high specific capacitance value.

      Figure 3.  FE-SEM images of (a) bare GDL substrate and (b)–(d) the prepared working electrode with different magnifications.

    • Two substrates, the bare Ni foam and GDL, were investigated by the CV technique. Fig. 4(a) shows the comparison CV plots of the bare Ni foam and GDL substrates; the curves were obtained by measurement using a 1 M NaOH electrolyte at a scan rate of 2 mV·s−1. As seen from Fig. 4(a), the CV curves show a remarkable difference between the GDL and Ni foam substrates. The CV curve of the Ni foam presents a redox peak, whereby the oxidation and reduction peaks are related to the Faradaic reactions of Ni(OH)2 described by the following equation [21]:

      Figure 4.  Electrochemical characterizations of Ni(OH)2 electrode: (a) comparison CV curves of Ni(OH)2 on Ni foam and GDL substrates; (b) CV curves of Ni(OH)2 on GDL substrate measured at different scan rates (Iox.—Peak current for oxidation; Ired.—Peak current for reduction); (c) scan rate dependence of peak current; (d) the contributions of the capacitive effect and diffusion-controlled mechanism of Ni(OH)2 on GDL substrate.

      The CV result for the Ni foam substrate without any active materials (Fig. 4(a)) shows the existence of a redox peak. The presence of this redox peak has been reported to generate pseudocapacitance, resulting in an error in the specific capacitance evaluation [7]. Regarding the GDL substrate, Fig. 4(b) displays the CV plots of Ni(OH)2 on GDL substrate at different scan rates within the potential window of 0–0.65 V. The CV curves are non-rectangular and feature shifts of redox peaks with increasing scan rates, implying that the capacitance originates from the redox reactions [22]. The shift of these redox peaks was further used as a basis to investigate the charge storage mechanism of Ni(OH)2 on GDL substrate. In general, the peak current (ip) dependence on scan rate (v) in a CV curve can be expressed according to the power-law relation $ {i}_{\rm{p}}{}{=}{}{a}{{v}}^{{b}} $, where a and b are adjustable values [23]. The plot of lg ip versus lg v provides the slope of the b-value. Moreover, b-values of 1 and 0.5 respectively indicate that the obtained peak current is due to a capacitive and intercalation/deintercalation mechanisms [24]. As shown in Fig. 4(c), the b-values of the oxidation (box.) and reduction (bred.) process are close to 0.5, implying that the charge storage mechanism of Ni(OH)2 on GDL substrate was due to the diffusion-controlled intercalation/deintercalation mechanism. Based on the above result, the possible charge storage mechanism of Ni(OH)2 on GDL substrate measured in 1 M NaOH aqueous electrolyte can be expressed as follows [25]:

      Furthermore, the total charge storage in an electrode is the sum of contributions from the capacitive effect (k1v) and diffusion-controlled intercalation/deintercalation (k2v1/2), following the relation given below [2627]:

      where the slope and intercept of the plot of ip/v1/2 versus v1/2 provide the k1 and k2 values, respectively. Therefore, the contributions from the capacitive effect and diffusion-controlled intercalation/deintercalation process can be distinguished. Fig. 4(d) shows the capacitive and diffusion-controlled contributions obtained from the plot of ip/v1/2 versus v1/2 at scan rates of 1–10 mV·s−1. As shown in the figure, with increasing scan rates, the capacitive contribution increased, whereas the contribution from the diffusion-controlled intercalation/deintercalation process decreased, as summarized in Table 1. Decreasing the scan rate affords the electrolyte ions enough time to diffuse into the electrode matrix, whereas increasing the scan rate limits the diffusion time [28].

      Scan rate / (mV·s−1)Contribution /%
      CapacitiveDiffusion-controlled
      117.14882.852
      222.64577.355
      326.38973.611
      429.27670.724
      531.63968.361
      1039.56160.439

      Table 1.  Dependency of the capacitive effect and diffusion-controlled intercalation/deintercalation mechanisms of Ni(OH)2 on GDL substrate at different scan rates

      Fig. 5(a) shows GCD plots of Ni(OH)2 on GDL substrate at current densities of 1–10 A·g−1 within the potential window of 0–0.55 V. The GCD curves consist of two different curve profiles. The linear profile indicates that the electrode stores charge based on electrolyte ions sorption at the electrode surface, while the plateau profile or battery-type behavior profile implies that the electrode stores the charge based on a redox reaction or the intercalation/deintercalation mechanism [29]. The specific capacitance of the Ni(OH)2/GDL was calculated using Eq. (1), and the result is presented in Fig. 5(b). At a current density of 1 A·g−1, the Ni(OH)2/GDL exhibited the highest specific capacitance (418.11 F·g−1). Moreover, the specific capacitance values decreased with increasing current densities. In Table 2, the specific capacitance values of Ni(OH)2 on different substrates, corresponding to different studies [3034], are compared. It can be seen that the specific capacitance value of the prepared Ni(OH)2 electrode depends on the substrate, electrolyte, and the applied current density and voltage. The prepared Ni(OH)2/GDL in this work provided a higher specific capacitance value than the other reported substrates, including carbon-based substrates (Table 2).

      Figure 5.  Electrochemical characterization of (a) charge–discharge curves of Ni(OH)2 on GDL substrate at different current densities and (b) the calculated specific capacitances obtained from the discharge curves.

      SampleMethodSubstrateElectrolyteC / (F·g−1)Ref.
      Ni(OH)2 filmElectrodepositionTitanium substrate3 M KOH578 (at 0.0025 A)[30]
      Ni(OH)2 thin filmsChemical bath depositionStainless steel2 M KOH398 (at 5 mV·s−1)[31]
      Ni(OH)2 nanoparticlesChemical precipitationNi foam6 M KOH255.1 (at 2 mV·s−1)[32]
      Ni(OH)2 nanofibersElectrodepositionCarbon fiber paper1 M KOH277.5 (at 5 mV·s−1)[33]
      Planar Ni(OH)2 nanoflakesChemical bath depositionCarbon fibers1 M KOH275 (at 1 A·g−1)[34]
      Ni(OH)2 nanoplatesHydrothermal synthesisGas diffusion layer1 M NaOH418.11 (at 1 A·g−1)This work

      Table 2.  Comparison of the specific capacitances of Ni(OH)2 on different current collectors

      To gain insight into an electrochemical performance of Ni(OH)2 on GDL substrate, EIS was performed in the frequency range of 0.1 Hz–100 kHz at an amplitude of 0.1 V. Fig. 6 shows the Nyquist plots of the bare GDL and the Ni(OH)2/GDL (i.e., the prepared working electrode). Nyquist plots are generally divided into three regions, according to the frequency range. As seen in Fig. 6, the solution resistance (Rs) values at the intercept of the Z′-axis in the high-frequency region of the bare GDL substrate and the prepared working electrodes are estimated as 1.85 and 4.28 Ω, respectively. The Rs is associated with the resistance of the substrate and the layer thickness of electroactive material [35], and expressed as follows: $ {R_{\rm{s}}} = L/(\sigma A)$, where L is the electrode thickness, σ is the solution conductivity, and A is an area of the electrode [36]. Therefore, the higher Rs value in the prepared working electrode was due to the increased electrode thickness after the slurry was dropped on the GDL substrate. In addition, the bare GDL substrate featured a larger charge transfer resistance (Rct) in the mid-frequency region than the prepared working electrode. In general, Rct is associated with electron exchange involving the redox reaction at the electrolyte/electrode interface. It is expressed by the following equation: ${R_{{\rm{ct}}}} = RT/(nF{i^0}) $, where R is the molar gas constant, T is temperature, n is electron transfer number, F is Faraday’s constant, and i0 is the exchange current density of the reaction [37]. Based on the above equation, it may be concluded that the larger Rct value in the bare GDL substrate was due to the lack of the Ni(OH)2 electroactive material, which limited electron exchange. The vertical lines perpendicular to the Z′-axis in the low-frequency region of Fig. 6, which indicate the diffusion of the electrolyte ions in an electrochemical reaction, are estimated to be about 82.5° and 77.4° for the bare GDL and the prepared working electrode, respectively. This result implies the more capacitive behavior and low ions diffusion resistance in the bare GDL substrate, as the straight line is closer to 90° [38].

      Figure 6.  Comparison of Nyquist plots of the bare GDL and Ni(OH)2/GDL measured at the frequency range of 0.1 Hz to 100 kHz (Z′—Real part; Z″—Imaginary part). Insets show the higher frequency details of the impedance spectra.

      The long-term cyclic stability of Ni(OH)2 on GDL substrate was measured by performing 1000 cycles of GCD tests at a current density of 5 A·g−1. As seen in Fig. 7, the Ni(OH)2/GDL retained about 81% of the original capacitance after 1000 cycles. The decay of the capacitance value was due to the loose electrical contact between the active material and the current collector [39]. The coulombic efficiency (η) of the Ni(OH)2/GDL was calculated using the following equation: $ \eta = {t_{\rm{d}}}/{t_{\rm{c}}} $, where td is the discharging time, and tc is the charging time [40]. The coulombic efficiency after 1000 cycles was calculated to be about 80%.

      Figure 7.  Capacitance retentions and coulombic efficiencies of Ni(OH)2/GDL measured in 1 M NaOH at current density of 5 A·g−1 for 1000 cycles.

      To further clarify the long-term cyclic stability of the Ni(OH)2/GDL, the FE-SEM images and the corresponding EDS results before and after 1000 cycles of GCD measurements are provided in Figs. 3 and 8, respectively. The surface of the working electrode after the cyclic stability measurement consisted of two different colors of black and white and featured significant cracks (Fig. 8(b)). The surface morphology of the prepared working electrode before the cyclic test (Fig. 3(b)) featured minor cracks. The cracks became larger after the continuous charge–discharge tests for 1000 cycles.

      Figure 8.  (a)–(d) SEM images of surface morphologies and (e, f) the corresponding EDS spectra in (c) for Ni(OH)2 on GDL substrate after 1000 cycles of GCD measurements.

      In general, cracks originate from the elastic mismatch between two attached surfaces and are commonly considered as defects [41]. The existence of cracks on an electrode surface for a lithium-ion battery has been attributed to the large volume change, leading to a loss of the electrical contact between individual particles, which decreases the battery capacity [42]. On the contrary, the introduction of cracks into NiO nanosponge has also been reported to lead to excellent long-term stability, because the NiO can freely expand/contract during the charge–discharge process [43]. Therefore, in the current study, the presence of cracks on the working electrode surface after 1000 cycles of GCD measurement suggests poor cyclic stability. Interestingly, the high-magnification FE-SEM image in Fig. 8(c) shows the separation of two areas with different colors, denoted as area 1 and area 2. The magnified FE-SEM image of area 2 (Fig. 8(d)) reveals densely packed hexagonal Ni(OH)2 plates. In contrast, a smooth surface was observed in area 1. Figs. 8(e) and 8(f) depict the elemental compositions of the prepared working electrode, considering different detected areas. The EDS spectrum of area 1 (Fig. 8(e)) shows a small amount of Ni; however, that of area 2 (Fig. 8(f)) shows a high Ni content. Additionally, area 1 featured a higher sodium (Na) content than area 2. This result suggests that a new phase consisting of Na or Na deposition occurred after the cyclic stability measurement. The proposed schematic representation of Na deposition is shown in Fig. 9. The decay of the specific capacitance from its initial value may be due to the Na deposition on the working electrode surface after long cyclic stability measurement, which agrees well with other studies [4445].

      To further understand the decrease in the capacitance, ex-situ XRD characterization for the structural change of the Ni(OH)2 electrode after 1000 cycles was performed, and the result is presented in Fig. 10. The low-intensity peaks of Ni(OH)2 in the XRD pattern of the prepared Ni(OH)2 electrode before cycling are almost absent in the XRD pattern of the electrode after cycling, and sodium peroxide (Na2O2) peaks are present in the latter. The possible reaction involving the Na2O2 formation is as follows [46]:

      Figure 9.  Schematic representation of Na deposition on the prepared working electrode surface after long-term stability measurement.

      Figure 10.  XRD patterns of the prepared Ni(OH)2 electrode before and after cyclic stability test. Inset shows the enlarged view of the XRD patterns in 2θ ranging from 10° to 50°.

      Therefore, the formation of Na2O2 after cycling significantly affects the cyclic stability of the Ni(OH)2 electrode. This result is consistent with the SEM–EDS results presented in Fig. 8.

      The Ragone plot is used to compare the performances of energy storage devices [47]. Fig. 11 displays the Ragone plots of the Ni(OH)2/GDL. The Ni(OH)2/GDL provided the maximum specific energy of 17.25 Wh·kg−1 at a current density of 1 A·g−1 and a specific power of 2.76 kW·kg−1 at a current density of 10 A·g−1 (Fig. 11(a)). According to the Ragone plot, electrochemical capacitors cover a wide range of specific energy (0.05–15 Wh·kg−1) and specific power (10–106 W·kg−1) [48]. Therefore, as shown in Fig. 11(b), the Ni(OH)2/GDL is suitable for electrochemical capacitor applications.

      Figure 11.  (a) Ragone plot (specific energy vs. specific power) of Ni(OH)2 on GDL substrate and (b) green dots showing the Ni(OH)2/GDL are in the range of electrochemical capacitors when compared to the Regone plot of Ref. [48].

    4.   Conclusion
    • In this work, a working electrode of Ni(OH)2 was successfully prepared on a GDL substrate via a simple method, and the electrochemical performance of the Ni(OH)2/GDL in 1 M NaOH electrolyte was investigated. Through kinetic analysis, the charge storage of the Ni(OH)2/GDL was determined to mainly originate from the diffusion-controlled intercalation/deintercalation mechanism. Within the potential window of 0–0.55 V, the Ni(OH)2/GDL displayed the highest specific capacitance (418.11 F·g−1) at a current density of 1 A·g−1. The electrode retained about 81% of the capacitance after 1000 cycles; the decay in capacitance is attributed to the presence of Na on the working electrode surface, as clearly indicated by the SEM–EDS results after repeated GCD measurements. To improve the long-term cyclic stability of the Ni(OH)2/GDL, the electrode preparation needs to be optimized in future works.

    Acknowledgements
    • This work was financially supported by the Office of the Higher Education Commission under NRU Project of Thailand and the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation (MHESI), Thailand. T. Sichumsaeng would like to thank the Science Achievement Scholarship of Thailand (SAST) for the support of her PhD study.

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