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Article

Kinetic Analysis of Oxygen Evolution on Spin-Coated Thin-Film Electrodes via Electrochemical Impedance Spectroscopy

by
Yu-Wei Lin
1,†,
Yi-Syuan Li
1,†,
Chun-Wei Chang
1,
Li-Cheng Huang
1,
Tai-Hsin Yin
1,
Yu-Ting Liu
1,
Dong Kyoo Park
2,
Changsik Choi
2,* and
YongMan Choi
1,*
1
College of Photonics, National Yang Ming Chiao Tung University, Tainan 71150, Taiwan
2
Clean Energy Conversion Research Center, Institute for Advanced Engineering, Yongin 17180, Republic of Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2023, 13(11), 1957; https://doi.org/10.3390/coatings13111957
Submission received: 9 October 2023 / Revised: 7 November 2023 / Accepted: 13 November 2023 / Published: 16 November 2023
(This article belongs to the Special Issue Advanced Electrochemical Surface Properties)
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Abstract

:
Sustainable and renewable energy technologies have attracted significant attention for reducing greenhouse emissions in the shift from fossil fuels. The production of green hydrogen from water electrolysis is considered an environmentally friendly strategy for a decarbonized economy. We examine the activities of the hydrogen and oxygen evolution reactions (HER and OER) using spin-coated thin-film electrodes with Pt/C and IrO2 nano-electrocatalysts under acidic conditions. The nano-electrocatalysts are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The electrocatalytic activities of nanoscale Pt/C and IrO2 are close to those of commercial Pt/C and superior to commercial IrO2, resulting in improved overall water splitting performance. Furthermore, the OER kinetics analysis using the IrO2 electrode is conducted using EIS measurements with distribution of relaxation time (DRT) analysis, resulting in a comparable exchange current density to that from the Tafel slope method (6.7 × 10−2 mA/cm2 versus 5.1 × 10−2 mA/cm2), demonstrating the validity of the kinetics analysis. This work provides a general strategy for preparing novel and highly active OER electrode materials for water electrolysis.

1. Introduction

Green hydrogen production is considered a significant solution for achieving net-zero greenhouse gas emissions, as hydrogen is the cleanest energy carrier for transportation and industry [1]. Green hydrogen is mainly produced using water electrolyzers powered by renewable energy. Because the performance of water electrolysis devices is limited by the ohmic drop and the overpotential [2,3], developing novel efficient electrocatalysts for the hydrogen and oxygen evolution reactions (HER and OER) with low ohmic drop and overpotential is pivotal [4,5,6,7,8]. Furthermore, designing novel, highly efficient electrocatalysts with acid tolerance is also vital for proton exchange membrane (PEM) water electrolysis [9]. For the HER, protons in the solution are transferred to the electrode surface to combine with electrons, producing hydrogen [6]. Although platinum-based catalysts (i.e., Pt/C) are the most active HER electrode materials in water electrolysis, their high cost and limited abundance have hindered large-scale applications. For the OER, the characteristics of the four-electron transfer and the complex electrocatalytic process [10,11,12,13] significantly affect conversion efficiencies. Iridium-based electrocatalysts (i.e., IrO2) are widely used due to their high OER activity and bifunctionality [14,15,16,17,18,19,20]. Accordingly, developing highly efficient OER electrode materials has attracted significant attention due to their sluggish reaction kinetics [21,22]. Numerous studies have been conducted to elucidate the reaction mechanisms of water splitting at electrode surfaces [23,24,25]. Simulating reaction kinetics for estimating kinetic parameters is essential for investigating the reaction process and electrocatalytic activities [26]. Typically, the Tafel extrapolation method is applied by determining the exchange current density (j0) of the OER through Tafel plots (η = a + b · log(j)) [27], where η, b, and j are the overpotential, the Tafel slope, and the current density, respectively. On the other hand, Costa et al. reported an OER kinetic analysis of a lead (Pb) electrode using electrochemical impedance spectroscopy (EIS) [28]. Lu et al. then applied the approach to obtain OER kinetic parameters with microkinetic modeling on a rhodium (Rh) electrode [26]. In this study, nanoscale IrO2 electrocatalysts were synthesized to verify their applicability in water electrolysis with nano-sized Pt/C. We applied the spin-coating method to attain reproducible electrochemical activities of the as-synthesized Pt/C and IrO2 on fluorine-doped tin oxide (FTO) glass substrates owing to their reasonable electrical conductivities and excellent stability in acidic media [29,30]. Kinetic analysis using electrochemical impedance spectroscopy (EIS) is a powerful tool to investigate the fundamental mechanisms and kinetics of electrochemical reactions at electrode/electrolyte interfaces. Then, to experimentally understand the OER kinetic process and parameters of nanoscale IrO2 electrode materials, EIS measurements with the equivalent electrical circuit (EEC) analysis were conducted [26]. Furthermore, the distribution of relaxation time (DRT) analysis was also used to investigate the contribution of the impedance of the electrochemical process evolution on the OER electrode surfaces [31,32]. In particular, we found that the exchange current density obtained from the EIS-based kinetic analysis agrees well with that from the Tafel slope method. This study supports the validity of the microkinetic model for OER processes [26].

2. Experimental Details

2.1. Electrocatalyst Synthesis and Fabrication of Thin-Film Electrodes

PtCl4 (99.9%, Alfa Aesar, Ward Hill, MA, USA), (NH4)2(IrCl6) (99%, Sigma-Aldrich, Burlington, MA, USA), NaNO3 (99.5%, Alfa Aesar, Ward Hill, MA, USA), 0.1 M HClO4 (Acros Organics BVBA, Geel, Belgium), 5 wt.% Nafion solution (Sigma-Aldrich, Burlington, MA, USA), and Vulcan XC-72 (Cabot Corporation, Boston, MA, USA) were used without further purification. Fuel-cell-grade Pt/C (20 wt.%, Fuel Cell Store, Bryan, TX, USA) and IrO2 (99%, Alfa Aesar, Ward Hill, MA, USA) were used as active benchmark electrocatalysts for the HER and OER. Carbon-supported Pt electrocatalysts (20 wt.% Pt/C) were synthesized based on a previous approach [33]. The precursor Pt salt (50.5 mg of PtCl4) was mixed with carbon blacks (Vulcan XC-72) in 50 mL of acetone (99.5%, Anaqua Chemical Supply, Houston, TX, USA) and stirred for 1 h, followed by a 30 min ultrasonication and drying at 50 °C on a hot place with stirring. Then, dried samples were annealed at 600 °C in 15% H2/Ar for 2 h. Also, nanoscale IrO2 powders were prepared using a modified Adams fusion method [34]. An amount of 206.7 mg of (NH4)2(IrCl6) was dissolved in 6.7 mL of isopropyl alcohol (IPA) (99.5%, Anaqua Chemical Supply, Houston, TX, USA) and stirred for 30 min, followed by adding NaNO3 powders to the solution with an additional 30 min stirring with a weight ratio of NaNO3/(NH4)2(IrCl6)~10 [34]. The effective removal of potential contaminants (i.e., NaCl) was verified in previous studies [34,35].
Then, samples were filtered using Isopore membrane filters (Millipore, MA, USA) with a pore size of 0.4 μm. Collected samples were dried in an oven for 30 min at 110 °C and calcined in a tube furnace (Thermo Scientific, Lindberg Blue M, Waltham, MA, USA) for 2 h at 350 °C in air. The collected samples were then washed via filtration with ~1.5 L of deionized (DI) water to remove impurities (i.e., unreacted precursor salts). IrO2 nanoparticles were dried in an oven for 2 h at 100 °C before characterization. As shown in Figure 1, thin-film electrodes were prepared via the spin-coating method on fluorine-doped tin oxide glass substrates (FTO, ~400 nm and 1 cm × 2 cm). FTO substrates were first consecutively cleaned in an ultrasonic bath of acetone, IPA, ethanol, and DI water for 5 min, then dried in an oven and subjected to UV ozone treatment for 20 min. A catalyst ink was prepared by mixing 3 mg of catalyst powders with 300 μL of IPA solution (volume ratio of DI water/IPA = 3) and 50 μL of 5 wt.% Nafion solution, followed by a 40 min ultrasonication in an ice bath. An amount of 10 µL catalyst ink was placed on an FTO substrate at 500 rpm for 30 s using a spin coater in a fume hood. An active area of 5 mm × 5 mm was set using a chemical-tolerant tape. The thin-film samples were dried in an oven at 70 °C for 30 min and then air-dried in a fume hood for 1 h.

2.2. Materials’ Characterization

Surface morphologies of the Pt/C thin film spin-coated on FTO were observed using scanning electron microscopy (SEM, Zeiss Gemini 450, Jena, Germany) operated at an acceleration voltage of 10 keV and atomic force microscopy (AFM, Bruker Innova, MA, USA) in tapping mode. X-ray diffraction (XRD, Brucker D8 DISCOVER, Billerica, MA, USA) with Cu Kα radiation (λ = 0.154184 nm) was applied to analyze as-prepared electrocatalysts (Pt/C and IrO2). Electrochemical measurements were conducted using a potentiostat (SP-150e, BioLogic, Seyssinet-Pariset, France) with a typical three-electrode system in a 0.1 M HClO4 electrolyte. In this study, an Ag/AgCl (3 M NaCl) reference electrode was used to measure potentials, which were converted to the reversible hydrogen electrode (RHE) scale using VRHE = V(Ag/AgCl) + pH × 0.059 + 0.209, where V(Ag/AgCl) is the measured potential against the Ag/AgCl reference electrode [36]. A graphite rod was used as a counter electrode to avoid the influence of HER on the Pt coil owing to the electrochemical dissolution–deposition process of Pt [37]. Electrocatalysts were always conditioned using cyclic voltammetry (CV) cycling at 20 mV/s until they turned stable. Also, we evaluated the double-layer capacitance (Cdl) using scanning CVs at different scan rates (10, 25, 50, 75, and 100 mV/s) in a non-Faradaic OER potential range. The electrolyte solution was H2- and O2-saturated for HER and OER measurements to remove the excess gas from the electrolyte. iR-corrected polarization curves [38] were obtained using linear sweep voltammetry (LSV) at a scan rate of 5 mV/s. To examine the OER kinetic process, electrochemical impedance spectroscopy (EIS) was conducted from 1 MHz to 50 mHz with 5 mV sinusoidal amplitude with six data points per frequency decade at current densities of 2, 4, 6, 8, and 10 mA/cm2.

3. Results and Discussion

3.1. Characterization of Thin-Film Electrodes

Figure 2a,b demonstrate surface morphologies of the Pt/C thin film using SEM. The thin-film morphology confirms that the spin-coating method provides a good dispersion of Pt/C on FTO, exhibiting a microporous structure. Figure 2c shows an AFM image of the Pt/C thin film whose morphology is similar to that produced using SEM (Figure 2b). The roughness value (Rq = 65.2 nm) illustrates a high active area of the thin-film electrode, potentially leading to good HER activities, as reported [39,40]. The surface wettability of electrocatalysts was examined using a homemade contact angle measurement tool with a high-resolution camera by dropping 5 μL of DI water on the thin-film electrodes (0.5 cm × 0.5 cm) [41]. As displayed in Figure 2d, the contact angles of 134.2° (IrO2) and 137.9° (Pt/C) indicate the hydrophobic behavior of the electrode materials on FTO [40], ensuring the low interaction of the generated gas bubbles [42,43]. Figure 2e shows the XRD patterns of the as-synthesized IrO2 and Pt/C nanoparticles. The XRD analysis of IrO2 verifies a rutile structure with an amorphous phase [44] without metallic iridium impurities. The diffraction peaks of 39.7°, 46.2°, and 67.6° of Pt/C are assigned to the (111), (200), and (220) planes of the face-centered cubic (fcc) [45,46,47,48]. The Pt/C sample annealed at 600 °C in 15% H2/Ar exhibits sharp and intense peaks caused by increased crystallization [49]. The peak observed around 24.5° in the XRD patterns is attributed to the graphitic (002) plane of Vulcan XC-72 [50]. The average crystallite sizes (D) of Pt and IrO2 were calculated to be 13.0 and 2.2 nm using the Scherrer equation (Equation (1)) [34].
D = K λ β c o s θ
where β is the full-width half maximum value, θ is the Bragg angle, K is the Scherrer constant (0.9), and λ is the wavelength of the incident X-ray. Nanosized electrode materials improve electrochemical activities due to their large surface area and high electrochemical surface area (ECSA) [51,52,53]. It is noted that according to recent studies [54,55], structural parameters (i.e., dislocation density and lattice strain) can be correlated with the particle size, affecting catalytic activities.

3.2. Electrochemical Properties of Thin-Film Electrodes

As described, the electrocatalyst inks of Pt/C and IrO2 were spin-coated on FTO to evaluate the HER and OER activities in 0.1 M HClO4. As mentioned, homogeneous and wettable electrode surfaces led to reproducible electrochemical measurements. Shown in Figure 3a are iR-compensated LSV measurements for the HER using the commercial and as-prepared Pt/C. Figure 3a demonstrates that the geometrical activity for Pt/C is close to that of commercial Pt/C, resulting in overpotentials of 71.3 and 73.8 mV at a current density of 10 mA/cm2. As summarized in Figure 3b, their corresponding Tafel slopes are 39.9 and 40.9 mV/dec. Also, the overpotentials and the measuring environment are compared with those in the literature (Table S1 [56,57,58,59,60,61,62,63,64]). It is noted that most Pt exists with the oxidation state of Pt0 under the (HER) conditions. Previous XPS studies demonstrated that active Pt nano-electrocatalysts can also exist as Pt2+ (PtO or Pt(OH)2) and Pt4+ (PtO2) [65,66]. Then, IrO2 electrode materials were investigated under the same acidic conditions [67]. Vulcan carbon (XC-72) was used to prepare IrO2 catalytic inks with 20 wt.% loading to enhance the conductivity. Figure 3c shows the OER polarization curves of IrO2-based electrodes. The overpotentials of the as-prepared and commercial IrO2 at 10 mA/cm2 are 371.7 and 483.8 mV. Their corresponding Tafel slopes are 178.4 and 202.7 mV/dec (Figure 3d), demonstrating the superior OER performance of the as-prepared IrO2 compared with that of commercial IrO2 due to its hydrous amorphous structure of nano-sized metal oxide [34]. Similar to the Pt-based electrode materials, the recently reported values in the literature for IrO2-based catalysts are summarized in Table S2 [68,69,70,71,72,73,74,75,76,77]. Then, as shown in Figure 3e, the overall water splitting was conducted using the as-prepared nano-sized Pt/C and IrO2 with an overpotential of 0.44 V at 10 mA/cm2.

3.3. Kinetic Analysis of Oxygen Evolution on IrO2 Using Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is one of the most powerful tools that characterizes the impedance response of electrode interfaces. To examine the reaction kinetic process, an accurate electrical equivalent circuit (EEC) is required for extracting the kinetic parameters and different overpotentials applied to the electrode. The rate-determining step (RDS) is studied by evaluating the electrode impedance response values during electrochemical reactions [78]. As is well known, IrO2 and RuO2 are the most efficient OER electrocatalysts under acid conditions. However, RuO2 is easily degraded under harsh conditions [79]. Thus, as described in Figure 3c, in this study, IrO2 was selected to conduct the kinetics analysis of oxygen evolution for exploring the capacitance and impedance effects among adsorbate, electrolyte, and catalytic materials. A modified EEC, as shown in Figure 4a, is applied to explain multiple adsorbed intermediates on electrode surfaces, and the EEC model comprises two RC circuits based on the Randles circuit [26,80]. In this study, we used the OER process under acidic conditions, where OHad and Oad are the intermediates adsorbed on the IrO2 electrode surface [26,28]. The EEC contains an electrolyte resistance (Rs) in series with the capacitance of the electrode double layer (Cdl) in parallel with a pair of the impedances of the intermediates generated during the adsorption process, which involves an adsorption resistance (Rads) and an adsorption capacitance (Cads) in series with the resistance of the charge transfer process (Rct), including an electron transfer process both within the electrode and at the electrode/electrolyte interface. The EEC model can also be presented as Rs + Cdl/(Rct + Cads/Rads), where Cads/Rads is the impedance of the intermediate species on the IrO2 electrode (i.e., oxygen species in this study) for modeling charge relaxation [81]. Figure 4a shows Nyquist plots for IrO2 at different potentials, revealing the domain Faradaic resistance. As summarized in Table 1, Rads values significantly decrease with increasing overpotential, which originates from the accelerated diffusion and transfer of the intermediates adsorbed on the electrode surface. The impedance resistance arc decreases [26] as the overpotential increases. Accordingly, the relaxation of the adsorbed intermediates is generally considered the RDS of OER processes [26,82]. Intermediates adsorbed on electrode surfaces may not be the same using different electrocatalysts. The RDS can be effectively investigated by evaluating the significant capacitance associated with surface intermediates [82,83]. The relationship between the current density (j) and the electrode potential is precisely expressed by the Bulter–Volmer equation. According to a previous study [26], the reaction admittance (Y) describes how freely a circuit allows a current flow. j is the current density of the electrode, j0 is the exchange current density, and T is the temperature.
Y = (∂j/∂E)T
The total reaction resistance of OER, then, is defined as R = Y−1 [28]. It can be modified to a Tafel-like linear relationship [84].
log(R−1) = log(R*−1) + αapa F(E − E0)/2.303RgT
R* = RgT/αapa Fj0(apa)
α a p a = 2 R T b 4 F
where F is the Faraday constant, αapa is the apparent charge transfer coefficient of the rate-determining step, Rg is the gas constant, and E − E0 is the overpotential. Accordingly, the charge transfer coefficient (αapa) is accurately calculated from its slope, while the exchange current density (j0) can be obtained through its intercept. Then, the crucial OER kinetic parameters are extracted using the kinetic model. Experimentally, Rads values were measured as a function of potentials for the OER kinetic study using the EIS approach (Figure 4b) [26]. The EIS study produced the charge transfer coefficient of IrO2 (0.10) by applying the EEC shown in Figure 4a. We then obtained the exchange current density of IrO2 (6.7 × 10−2 mA/cm2), which agrees with that from the Tafel slope method (5.1 × 10−2 mA/cm2). The kinetic method determines the exchange current density using resistance (Rads) (Figure 4c) to describe the OER mechanism compared with the Tafel slope analysis [26]. Figure 4d,e indicate the Bode plots of the IrO2 electrode prepared on FTO measured at cell densities 2, 4, 6, 8, and 10 mA/cm2. From the Bode phase plot, it can be noticed that there is a stable peak in the high-frequency (HF) region and a peak change in the low-frequency (LF) region, followed by applied voltage (current density) increases. The peak in the HF region can be attributed to the characteristic frequency of the material (103.28 Hz for IrO2), which is defined by the frequency of the highest impedance value of the imaginary part of the semicircle in an HF region [85]. Furthermore, the Bode phase plot in the LF region exhibits a peak frequency shifted towards the HF region as the applied current density increases, which can be explained by the faster OER kinetics [86]. To further understand the time constant of high current density in two semicircles of time and the impedance distribution, a distribution of relaxation times (DRT) analysis [87] was performed using the impedance data measured at different current densities [87,88,89] by deconvoluting the EIS data. Figure 5a,b show that IrO2 has stable impedance values at a time constant (τ) of 1.0 × 10−5 s in the HF region. This can be interpreted as a charge transfer process at the electrode material/electrolyte interface with the contact impedance [90]. Several peak values between 1.0 × 10−4 s and 1.0 × 10−2 s can be observed due to the OER process. A time constant of 1 × 10−1 s in the LF region decreases with the increase in current density and with the decrease in the time constant because of the capacitance response of the electrode surface and O-intermediate coverage on the catalytic active sites. Meanwhile, as the applied overpotential increases, the time constant changes because of the accelerated diffusion and transfer process of the O intermediate [26], which is consistent with the results of the Nyquist plot in the large semicircle in the LF region and the characteristic frequency of the Bode phase plot [91]. We plan to perform more systematic studies for designing novel high-performance OER electrode materials using EIS-based kinetic studies, which align with the Tafel slope analysis.

4. Conclusions

Nanoscale Pt/C and IrO2 electrocatalysts were synthesized, and their thin-film electrodes were fabricated on FTO using the spin-coating method to validate their electrochemical efficiencies in water splitting. The electrocatalytic activities of Pt/C and IrO2 on FTO were close to those of commercial Pt/C and superior to those of commercial IrO2, exhibiting improved overall water splitting performance. Furthermore, an OER kinetics analysis of the IrO2 nano-electrocatalysts was conducted based on the EEC model using EIS measurements. It was observed that the exchange current density of the IrO2 electrode was in good agreement with that of the Tafel slope method (6.7 × 10−2 mA/cm2 versus 5.1 × 10−2 mA/cm2), indicating the validity of the microkinetic models used for the kinetic analysis. Systematic in situ characterization of the OER electrocatalysts using Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) can improve kinetics analyses [54]. In addition, advanced surface morphological and elemental analyses using high-resolution SEM and TEM may provide a better understanding of active sites in kinetic analysis. The EIS-based kinetic method could be applied to propose novel and highly active electrode materials for electrocatalytic and photoelectrocatalytic water electrolysis and novel energy materials.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/coatings13111957/s1, Table S1: Comparison of HER activities of Pt/C, Table S2: Comparison of OER activities of IrO2-based electrocatalysts.

Author Contributions

Conceptualization, Y.C.; methodology, Y.C.; investigation, Y.-W.L., Y.-S.L., C.-W.C. and T.-H.Y.; data curation, Y.-W.L., Y.-S.L., C.-W.C. and Y.-T.L.; visualization, Y.-W.L. and L.-C.H.; writing—original draft preparation, Y.-W.L., Y.-S.L. and C.-W.C.; writing—review and editing, D.K.P., C.C. and Y.C.; supervision, Y.C.; project administration, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Council of Taiwan (NSTC grant No. 110-2221-E-A49-017-MY3). C.C. acknowledges the Ministry of Trade, Industry, and Energy (MTIE) of Korea for the Net-Zero Demand Management Program (KETEP 20212010200110).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We acknowledge the Higher Education Sprout Project of the National Yang Ming Chiao Tung University and Ministry of Education (MOE), Taiwan.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 01957 g001
Figure 1. Schematic of the preparation process of thin-film electrodes on an FTO-coated glass substrate using spin-coating.
Figure 1. Schematic of the preparation process of thin-film electrodes on an FTO-coated glass substrate using spin-coating.
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Figure 2. (a,b) SEM images at different magnifications and (c) AFM image of the Pt/C thin-film electrode on FTO. (d) Contact angles of IrO2 and Pt/C on FTO. (e) XRD patterns of IrO2 and Pt/C powders.
Figure 2. (a,b) SEM images at different magnifications and (c) AFM image of the Pt/C thin-film electrode on FTO. (d) Contact angles of IrO2 and Pt/C on FTO. (e) XRD patterns of IrO2 and Pt/C powders.
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Figure 3. (a) HER polarization curves and (b) Tafel slopes of commercial and as-prepared Pt/C on FTO. (c) OER polarization curves and (d) Tafel slopes of commercial and as-prepared IrO2 on FTO. (e) Polarization curve of overall water splitting of Pt/C versus IrO2. The measurements were conducted in 0.1 M HClO4 at a scan rate of 5 mV/s.
Figure 3. (a) HER polarization curves and (b) Tafel slopes of commercial and as-prepared Pt/C on FTO. (c) OER polarization curves and (d) Tafel slopes of commercial and as-prepared IrO2 on FTO. (e) Polarization curve of overall water splitting of Pt/C versus IrO2. The measurements were conducted in 0.1 M HClO4 at a scan rate of 5 mV/s.
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Figure 4. (a) Nyquist plots, (b) overpotential-dependent Rads variation in the kinetic analysis using EIS, and (c) log(1/Rads) as a function of the overpotentials of IrO2 under OER conditions. An illustration of the electrical equivalent circuit (EEC) model used for the EIS analyses is shown [26]. Bode plots of (d) impedance magnitude versus frequency and (e) phase angle versus frequency.
Figure 4. (a) Nyquist plots, (b) overpotential-dependent Rads variation in the kinetic analysis using EIS, and (c) log(1/Rads) as a function of the overpotentials of IrO2 under OER conditions. An illustration of the electrical equivalent circuit (EEC) model used for the EIS analyses is shown [26]. Bode plots of (d) impedance magnitude versus frequency and (e) phase angle versus frequency.
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Figure 5. The (a) 2D and (b) 3D DRT plots of the IrO2 electrode on FTO.
Figure 5. The (a) 2D and (b) 3D DRT plots of the IrO2 electrode on FTO.
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Table 1. EIS fitting parameters of IrO2 nano-electrocatalysts deposited on FTO.
Table 1. EIS fitting parameters of IrO2 nano-electrocatalysts deposited on FTO.
Overpotential (V) 1Rs (Ω)Cdl × 10−6 (F) Rct (Ω)Cads × 10−3 (F)Rads (Ω)log(1/Rads)
0.3024.74.1425.04.5346.6–1.67
0.3325.14.0225.74.1825.5–1.41
0.3525.23.7525.33.4117.5–1.24
0.3725.44.1925.73.7713.5–1.13
0.4025.54.2826.93.8610.3–1.01
1 V versus RHE.
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Lin, Y.-W.; Li, Y.-S.; Chang, C.-W.; Huang, L.-C.; Yin, T.-H.; Liu, Y.-T.; Park, D.K.; Choi, C.; Choi, Y. Kinetic Analysis of Oxygen Evolution on Spin-Coated Thin-Film Electrodes via Electrochemical Impedance Spectroscopy. Coatings 2023, 13, 1957. https://doi.org/10.3390/coatings13111957

AMA Style

Lin Y-W, Li Y-S, Chang C-W, Huang L-C, Yin T-H, Liu Y-T, Park DK, Choi C, Choi Y. Kinetic Analysis of Oxygen Evolution on Spin-Coated Thin-Film Electrodes via Electrochemical Impedance Spectroscopy. Coatings. 2023; 13(11):1957. https://doi.org/10.3390/coatings13111957

Chicago/Turabian Style

Lin, Yu-Wei, Yi-Syuan Li, Chun-Wei Chang, Li-Cheng Huang, Tai-Hsin Yin, Yu-Ting Liu, Dong Kyoo Park, Changsik Choi, and YongMan Choi. 2023. "Kinetic Analysis of Oxygen Evolution on Spin-Coated Thin-Film Electrodes via Electrochemical Impedance Spectroscopy" Coatings 13, no. 11: 1957. https://doi.org/10.3390/coatings13111957

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