Electrochemical Investigation of Lithium Perchlorate-Doped Polypyrrole Growing on Titanium Substrate

Abstract

Lithium perchlorate-doped polypyrrole growing on titanium substrate (LiClO₄-PPy/Ti) has been fabricated to act as electroactive electrode material for feasible electrochemical energy storage. A theoretical and experimental investigation is adopted to disclose the conductivity, electroactivity properties and interfacial interaction-dependent capacitance of LiClO₄-PPy/Ti electrode. The experimental measurement results disclose that LiClO₄-PPy/Ti reveals lower ohmic resistance (0.2226 Ω cm⁻²) and charge transfer resistance (2116 Ω cm⁻²) to exhibit higher electrochemical conductivity, a more reactive surface, and feasible ion diffusion to present higher double-layer capacitance (0.1930 mF cm⁻²) rather than LiClO₄/Ti (0.3660 Ω cm⁻², 65,250 Ω cm⁻², 0.0334 mF cm⁻²). LiClO₄-PPy/Ti reveals higher Faradaic capacitance caused by the reversible doping and dedoping process of perchlorate ion on PPy than the electrical double-layer capacitance of LiClO₄/Ti caused by the reversible adsorption and desorption process of the LiClO₄ electrolyte on Ti. Theoretical simulation calculation results prove that a more intensive electrostatic interaction of pyrrole N···Ti (2.450 Å) in LiClO₄-PPy/Ti rather than perchlorate O···Ti (3.537 Å) in LiClO₄/Ti. LiClO₄-PPy/Ti exhibits higher density of states (57.321 electrons/eV) at Fermi energy and lower HOMO-LUMO molecule orbital energy gap (0.032 eV) than LiClO₄/Ti (9.652 electrons/eV, 0.340 eV) to present the enhanced electronic conductivity. LiClO₄-PPy/Ti also exhibits a more declined interface energy (−1.461 × 10⁴) than LiClO₄/Ti (−5.202 × 10³ eV) to present the intensified interfacial interaction. LiClO₄-PPy/Ti accordingly exhibits much higher specific capacitances of 0.123~0.0122 mF cm⁻² at current densities of 0.01~0.10 mA cm⁻² rather than LiClO₄/Ti (0.010~0.0095 mF cm⁻², presenting superior electroactivity and electrochemical capacitance properties. LiClO₄-PPy/Ti could well act as the electroactive supercapacitor electrode for feasible energy storage.

1. Introduction

The electrochemical behaviors and performances of energy storage devices are related to the electrode and electrolyte materials as well as their interfacial interaction [1,2,3]. In addition, the electrode substrate materials are also related to the properties of functional electrodes. The electrochemical capacity and mechanical stability are most concerned for the electrodes of energy storage devices. These electrode materials mainly involve the various graphite carbon, conductive polymers, transition metal sulfates or oxides, and their composites [4,5,6,7]. In particular, the conductive polymers become suitable supercapacitor electrode materials for flexible energy storage [8,9]. The electrical conductive materials, such as carbon fiber, graphene, graphite carbon, titanium, and so on, have been used to prepare the energy storage electrodes [10,11,12]. They could act as the multifunctional roles, such as the electrode substrate, current collector, and electrode. However, the contradictory behaviors of these substrate materials also emerged in respect to the electrochemical activity and electrochemical stability. For example, titanium metal could resist the chemical and electrochemical corrosion in neutral, alkaline, and even acidic conditions [13]. Titanium keeps the chemical and electrochemical stability in a certain degree, and is applied to electrode material as well as electrode-supporting material [14,15,16,17]. The structure-adjusted titanium, such as titanium sheet, titanium mesh, and titanium foam with high electrochemical stability, can be used to support various electroactive materials [18,19,20,21]. On the other hand, titanium metal also behaves the relatively low electrochemical activity due to the passivation layer of titanium oxides on the surface [22]. Surface modification using conductive polymers is a useful method to Improve the electrochemical activity of titanium. The Ti metal sheet can act as the electrode substrate. The conductive polymers usually includes polyaniline, polypyrrole, and polythiophene, which have been used to act as electroactive materials for the supercapacitor electrode application [23]. The different electroactive materials can be coated on Ti substrate to form functional electrodes. The polypyrrole (pPy) has been electrodeposited on Ti substrate to form pPy/Ti electrode through cyclic voltammetry polymerization process [24]. The polyaniline (PANI) has been electrodeposited and coated on Ti substrate to form PANI/Ti electrode through a pulse potentiostatic polymerization process [25]. These conductive polymers could conduct the reversible ion doping–dedoping process to provide superior capacitance [26,27,28]. The interfacial bonding strength between the doped conductive polymers and metal substrates is related to the electrochemical energy storage performance [29]. In addition, the reactive electrolytes also influence the electrical double-layer capacitance due to the interfacial adsorption on metal substrates and the Faradaic capacitance due to the ion doping state of conductive polymers. In previous studies, the system of a lithium perchlorate electrolyte and polypyrrole has been attempted for various applications in different areas [30,31]. However, the interfacial interaction between lithium perchlorate doping polypyrrole and metal titanium substrate is still sparsely investigated [32].

This study focuses on disclosing the promotion effect of electroactive polypyrrole on the electrochemical properties of lithium perchlorate interacting with titanium substrate. The polypyrrole is used as the typical conductive polymer material, and a metal titanium sheet is used as the stable substrate material. The titanium sheet has chemical and electrochemical stability to resist the electrochemical corrosion to act as a suitable current collector. The polypyrrole growing on titanium substrate (PPy/Ti) is designed as the supercapacitor electrode in the application of electrochemical energy storage, which is prepared through the electrochemical polymerization process. The lithium perchlorate (LiClO₄) is used as a neural electrolyte to study the electrical double-layer capacitance of titanium metal substrate and the Faradaic capacitance of polypyrrole. The interfacial interaction is intensively considered between the LiClO₄ electrolyte and the PPy/Ti electrode when the electrochemical behavior is investigated for LiClO₄-Ppy/Ti. The electrochemical performances are fully evaluated through cyclic voltammetry, electrochemical impedance spectrum, and galvanostatic charge/discharge measurements. The theoretical simulation calculation has been applied to disclose the atomic charge distribution, electrostatic potential, and interfacial affinity of LiClO₄-PPy/Ti, which could lead to a better understanding of capacitance performance.

2. Results and Discussion

2.1. Microstructure and Morphology Characterization

Figure 1A–D shows SEM images of Ti and PPy/Ti. The Ti sheet substrate reveals the uniquely complete surface structure without any crack. The chemical polishing treatment could lead to forming the uniform metal surface. Comparatively, PPy/Ti revels the compact covering layer of PPy film supported on the Ti sheet. The PPy also is composed of the aggregating particles, forming uniquely complete film structure without any crack. Concerning the surface microstructure, the Ti sheet shows smooth surface and PPy/Ti shows rough surface. The insert in Figure 1D shows the cross-section SEM image of PPy/Ti. The PPy covered on the surface of the Ti substrate exhibits a close-packing film. The morphology, microstructure, and thickness of such PPy film could be adjusted by modulating the electropolymerization reaction parameters in the CV electrodeposition process. This study focuses on the interfacial interaction between PPy film and Ti substrate, which is related to the microstructure of PPy. It is believed that PPy/Ti could conduct more feasible ion diffusion and doping reaction in the presence of lithium perchlorate in comparison the ion adsorption of PPy/Ti.

Figure 2A shows the XRD pattern of the titanium sheet substrate. Concerning the titanium sheet, the distinct characteristic diffraction peaks at 2θ of 35.1°, 38.5°, 40.3°, 53.1°, 63.1°, 70.8°, 76.3°, 77.5°, and 82.3° correspond to the respective crystal planes of (100), (002), (101), (102), (110), (103), (112), (201), and (202) [33]. The titanium sheet involves the close-packed hexagonal crystal phase structure, which is acted as stable metal substrate. Figure 2B shows the Raman spectrum of the Ti substrate. The Ti substrate does not reveal any characteristic Raman peaks in full wavenumber range due to the absence of electronic transition-relaxation of crystal lattice Ti atoms at 785 nm excitation photon energy. Figure 2C shows the Raman spectrum of PPy/Ti synthesized in this study. The characteristic Raman peaks of PPy/Ti are observed at 983 and 1050 cm⁻¹, with a broad band of 1340–1410 cm⁻¹ and 1580 cm⁻¹, which are assigned to C–H ring deformation vibration, C–N stretching vibration, and C=C aromatic stretching vibration, respectively. The characteristic Raman peak position of the literature-reported PPy is located at 980, 1040, 1314–1410 cm⁻¹, and 1597 cm⁻¹ [34]. The characteristic Raman peaks of PPy are well-matched between this study-formed PPy/Ti and the literature-reported PPy. Therefore, the Raman spectrum analysis result could well confirm the formation of PPy in the as-prepared PPy/Ti electrode in this study. Figure 2D shows the FTIR spectrum of PPy/Ti. The characteristic peaks at 1540, 1480, 1190, and 1040 cm⁻¹ were ascribed to C=C double bond stretching vibration, C-C single bond stretching vibration, C-N single bond stretching vibration, and N-H in-plane bending vibration. The characteristic peaks at 912, 768, 742, and 675 cm⁻¹ are ascribed to the C-H out-of-plane bending vibration. The observation of the C=C double bond, C-C single bond, and C-N single bond could be ascribed to the formation of PPy. Accordingly, PPy fully grew on the Ti substate to form PPy/Ti through the electrochemical polymerization and deposition process.

2.2. Electrochemical Properties

The CV curves can be used to calculate the CV-based capacitance (C_CV, mF cm⁻²) using Equation (1). The GCD curves can be used to calculate the GCD-based capacitance (C_GCD, mF cm⁻²) using Equation (2).

$$C_{\mathrm{CV}}=i/v=[({\displaystyle {\int }_{V_c}^{V_a}i(\nu )}\ast d\nu )/(V_a-V_c)]/\nu =[\frac{1}{2}({\displaystyle \oint i(\nu )}\ast d\nu )/\Delta V]/\nu$$

(1)

$$C_{GCD}=\frac{I\ast t}{∆V}$$

(2)

where: V_a, upper potential; V_c, lower potential; i(v), response current density (mA cm⁻²); v, potential sweep rate (V s⁻¹); I, discharge current density (mA cm⁻²); t, discharge time (s); and ΔV, potential window (V).

Figure 3A,B shows the CV curves of LiClO₄/Ti and LiClO₄-PPy/Ti at scan rates of 5~200 mV s⁻¹ and the potential range of 0~0.8 V versus saturated calomel electrode (SCE) in 1.0 M LiClO₄ electrolyte solution. The CV curves of LiClO₄/Ti demonstrate the rectangle-like shape to present electrical double-layer capacitance due to the reversible adsorption–desorption of LiClO₄ electrolyte ion on the Ti substrate. The oxidation potential of Ti²⁺/Ti is −1.39 V versus SCE. This means that the electrochemical oxidation of Ti is unlikely to occur in the potential range of 0~0.8 V versus SCE, presenting the high electrochemical stability of the Ti substrate in LiClO₄ electrolyte. Comparatively, the CV curves of LiClO₄-PPy/Ti exhibit an obviously deviated rectangle shape in the potential range of 0~0.8 V, presenting the polarization effect. The redox potential of PPy is around 0.5 V versus SCE [35]. LiClO₄-PPy conducts reversible ClO₄⁻ doping-dedoping reaction which contributes to the Faradaic capacitance performance.

Figure 3C,D shows CV-based capacitance curves of LiClO₄/Ti and LiClO₄-PPy/Ti. When the CV cures of LiClO₄/Ti and LiClO₄-PPy/Ti conduct the scan rate increasing from 5 to 200 mV s⁻¹, their mean response current increases from 0.13 to 2.47 μA cm⁻² and from 2.28 to 9.011 μA cm⁻², respectively. The CV-based capacitance decreases from 0.0256 to 0.0124 mF cm⁻² and from 0.456–0.045 mF cm⁻², respectively. In comparison with LiClO₄/Ti, LiClO₄-PPy/Ti exhibits a much higher mean response current and specific capacitance performance at the same scan rate, presenting its superior electrochemical activity in the neutral LiClO₄ electrolyte. The oxidation potential of Ti²⁺/Ti is 1.39 V versus SCE (Ti − 2e → Ti²⁺). The oxidation potential of PPy at the doping process is approximate 0.5 V versus SCE (PPy − e + ClO₄⁻ → PPy⁺ClO₄⁻). PPy/Ti could conduct reversible doping–dedoping process of PPy rather than the electrochemical oxidation of the Ti substrate in the potential range of 0~0.8 V. Additionally, the PPy covering layer could also resist such electrochemical corrosion reaction of Ti substrate. Meanwhile, PPy could conduct the redox reaction through the reversible doping–dedoping ClO₄⁻ anion, contributing to improving the electrochemical activity and, accordingly, the Faradaic capacitance performance.

Figure 4A,B shows the GCD curves of LiClO₄/Ti and LiClO₄-PPy/Ti in the potential range of 0~0.8 V (vs. SCE) and 1.0 M LiClO₄ electrolyte solution. The GCD curves of LiClO₄/Ti show complete symmetric characteristics. This shows the good reversible properties of LiClO₄ ion adsorption–desorption during the charge–discharge process. Such LiClO₄/Ti exhibits ideal electrical double-layer capacitance behavior. Comparatively, the GCD curves of LiClO₄-PPy/Ti shows the sloping plateau. The less-symmetric characteristic is ascribed to the redox reaction in the charge–discharge process. Such LiClO₄-PPy/Ti exhibits Faradaic capacitance behavior. Therefore, LiClO₄-PPy/Ti demonstrates much higher capacitance performance than LiClO₄/Ti in the neutral LiClO₄ electrolyte. Furthermore, LiClO₄-PPy/Ti conducts the doping–dedoping reaction of PPy, and LiClO₄/Ti conducts the ion adsorption–desorption process of Ti. The GCD curve shape of LiClO₄-PPy/Ti shows fewer identical characteristics than that of LiClO₄/Ti. Figure 4C,D shows GCD-based capacitance curves of LiClO₄/Ti and LiClO₄-PPy/Ti. According to the GCD curves at current densities of 0.01~0.10 mA cm⁻², the corresponding GCD-based specific capacitances achieve 0.010~0.0095 mF cm⁻² and 0.123~0.0122 mF cm⁻². Therefore, LiClO₄-PPy/Ti exhibits much higher capacitance than LiClO₄/Ti, since electroactive LiClO₄-PPy mostly contributes to Faradaic capacitance rather than electrical double layer capacitance of Ti substrate in LiClO₄ electrolyte. These GCD measurement results agree with the CV measurement results. Furthermore, this LiClO₄-PPy/Ti electrode keeps the similar capacitance performance in comparison with the ion doped PPy reported in the literature [36,37]. Figure 4E shows the cycling capacity retention curve of LiClO₄-PPy/Ti. The capacity retention achieves 76.1% after 1000 cycles during the cycling charge–discharge process. Therefore, the PPy/Ti electrode exhibits the reasonable cycling stability in the electrochemical Faradaic reaction process.

An electrochemical impedance spectroscopy (EIS) is adopted to investigate the electrolyte ion diffusion and interfacial charge transfer properties of electrode materials. Figure 5A,B shows the Nyquist plots of the EIS spectra of LiClO₄/Ti and LiClO₄-PPy/Ti. Figure 5C shows equivalent circuit model for fitting curves of EIS spectra. The equivalent circuit includes charge-transfer resistance (R_ct), ohmic resistance (R_o), double layer capacitance (C_dl), constant phase element (CPE), and CPE exponent (n). n = 1 indicates ideal capacitor behavior, and n = 0 indicates pure resistor behavior.

Table 1. Fitting parameter values of elements used in equivalent circuit model of LiClO₄/Ti and LiClO₄-PPy/Ti.

Substance	Cdl (mF cm−2)	Rct (Ω cm−2)	Ro (Ω cm−2)	CPE (S sn cm−2)	n
LiClO4/Ti	0.0334	65,250	0.3660	0.00263	0.746
LiClO4-PPy/Ti	0.1930	2116	0.2226	0.03427	0.912

lists the fitting parameter values of these elements used in an equivalent circuit model. In comparison with LiClO₄/Ti, the slightly lowered R_o value of LiClO₄-PPy/Ti indicates higher electronic conductivity. The ohm resistance of LiClO₄-PPy/Ti is mostly dependent on the Ti substrate. The conducting LiClO₄-PPy layer prevents the formation of semi-conducting titanium oxide. The quite lowered R_ct value indicates the more electroactive interface of LiClO₄-PPy for the charge transfer. Its higher C_dl value indicates higher double-layer capacitance due to the rougher surface of PPy/Ti rather than Ti substate. However, its higher CPE value indicates the higher Warburg impedance in view of electrolyte diffusion. The Warburg impedance of Ti substrate keeps relatively low value. The lithium ion has relatively high diffusion coefficient and plays a more predominant role than the electrode materials in the LiClO₄ electrolyte. LiClO₄/Ti carries out a shorter ion diffusion length than LiClO₄-PPy/Ti. Therefore, LiClO₄/Ti reveals a relatively lower CPE value than LiClO₄-PPy/Ti. Moreover, LiClO₄-PPy/Ti exhibits a larger CPE exponent (n value) than LiClO₄/Ti. This indicates that LiClO₄-PPy/Ti more likely approaches capacitor behavior rather than resistance behavior. The LiClO₄-PPy layer provides more active interface for electrolyte ion adsorption and charge transfer. Therefore, LiClO₄-PPy/Ti exhibits higher electroactivity than LiClO₄/Ti, contributing to a higher specific capacitance in neutral LiClO₄ electrolyte solution.

2.3. DFT Simulation Calculation of LiClO₄/Ti and LiClO₄-PPy/Ti

The LiClO₄ molecule conducts the adsorption–desorption to establish interfacial interaction between LiClO₄ and Ti substrate through Van der Waals forces in the electrochemical process. The LiClO₄ molecule also conducts the doping–dedoping process to establish electrostatic interaction between LiClO₄-PPy and Ti substrate. The atomic charge distribution could affect the interaction of LiClO₄/Ti and LiClO₄-PPy/Ti. The DFT simulation calculation is used to investigate electronic properties of LiClO₄/Ti and LiClO₄-PPy/Ti [38].

Figure 6A,B shows molecule the structure models of LiClO₄/Ti and LiClO₄-PPy/Ti. LiClO₄/Ti involves an interfacial electrostatic interaction between the LiClO₄ and Ti substrate. LiClO₄-PPy/Ti involves an interfacial electrostatic interaction between the LiClO₄-PPy and Ti substrate. LiClO₄-PPy presents the perchlorate ion doping state of PPy.

Figure 6C,D shows the molecular electrostatic potentials of LiClO₄/Ti and LiClO₄-PPy/Ti. Concerning LiClO₄/Ti, the negative charge of ClO₄⁻ ion could induce the charge dissociation of the Ti substrate, which leads to forming positive and negative crystal lattice Ti atoms. Concerning LiClO₄-PPy/Ti, the doping state of LiClO₄-PPy reveals a positive charge of hydrogen atoms and negative charge of conjugated pyrrole rings, which leads to the forming of a positive and negative crystal lattice of Ti atoms. The intermolecular interaction distance is determined to be 3.537 Å for perchlorate O…Ti of LiClO₄/Ti and 2.450 Å for pyrrole N…Ti of LiClO₄-PPy/Ti, respectively. Therefore, LiClO₄-PPy/Ti presents stronger electrostatic interaction than LiClO₄/Ti. The electrostatic interaction is allowed to occur for LiClO₄/Ti and LiClO₄-PPy/Ti. The intensified interaction has been established between the LiClO₄-PPy molecule chain and the Ti substrate, contributing to the activation of the LiClO₄-PPy/Ti electrode.

Figure 6E,F shows the density of states curves of LiClO₄/Ti and LiClO₄-PPy/Ti. Both LiClO₄/Ti and LiClO₄-PPy/Ti demonstrate metal conductivity characteristics, since the characteristic density of states at Fermi energy 0 eV cross from the valance band to the conduction band. Comparatively, LiClO₄-PPy/Ti demonstrates more orbit energy levels than the LiClO₄/Ti at valence band and conduction band regions, which are ascribed to the interfacial interaction between the LiClO₄-PPy and the Ti substrate. Furthermore, LiClO₄-PPy/Ti shows a higher DOS level (57.321 electrons/eV) than LiClO₄/Ti (9.652 electrons/eV) at Fermi energy 0 eV. LiClO₄-PPy/Ti exhibits higher electron states and higher electronic conductivity.

Figure 6G,H shows the band gap structure curves of LiClO₄/Ti and LiClO₄-PPy/Ti. The band gaps of LiClO₄/Ti and LiClO₄-PPy/Ti approach 0.03 and 0 eV, presenting similar superior conductivity of metal substrates. Comparatively, LiClO₄-PPy/Ti demonstrates much greater energy levels at the valance band and conduction band regions than LiClO₄/Ti, which presents superior electrical conductivity of LiClO₄-PPy/Ti. Therefore, the band gap analysis results agree with the DOS analysis results, proving the superior electronic conductivity of LiClO₄-PPy/Ti rather than LiClO₄/Ti. The declined interfacial energies are −5.202 × 10³ eV for LiClO₄/Ti and −1.461 × 10⁴ for LiClO₄-PPy/Ti, which presents the intensified interfacial interaction of LiClO₄-PPy/Ti. Therefore, LiClO₄-PPy/Ti shows higher electroactivity than LiClO₄/Ti. LiClO₄-PPy has more electronic distribution than LiClO₄-PPy/Ti, which leads to stronger interaction, lower interfacial energy, and, accordingly, higher capacitance performance.

Figure 6I shows the HOMO and LUMO HOMO and LUMO frontier molecular orbital charge distribution of LiClO₄/Ti and LiClO₄-PPy/Ti. Concerning HOMO and LUMO electron distribution in LiClO₄/Ti and LiClO₄-PPy/Ti, their molecular orbitals are mostly dependent on Ti rather than LiClO₄ and LiClO₄-PPy. In Ti atoms, there occurs an obvious electronic transition, which causes heterogeneous electron distribution. Comparatively, different O and Cl atoms in LiClO₄ occur insignificant electronic transition due to weak electron transfer. Similarly, the electron transition of different atoms in LiClO₄-PPy also becomes relatively neglective because of weak electronic transfer. The HOMO-LUMO molecule orbital energy gaps of LiClO₄/Ti and LiClO₄-PPy/Ti are determined to be 0.340 eV and 0.032 eV, respectively. Therefore, LiClO₄-PPy/Ti with a low HOMO-LUMO gap presents its high conductivity and high electroactivity, also. The HOMO-LUMO gap analysis results agree with the DOS and band gap analysis results. The DFT simulation calculation results support the CV and GCD measurement results, proving the higher capacitance of LiClO₄-PPy/Ti.

Figure 7 shows the Mulliken charge population of C, O, N, Cl, and Ti atoms of LiClO₄/Ti and LiClO₄-PPy/Ti. The interfacial interaction in LiClO₄/Ti and LiClO₄-PPy/Ti leads to the center charge offset of the Ti lattice atoms and the negative charge density distribution of oxygen atoms as well as nitrogen atoms. Accordingly, Ti atoms involve the dissociation distribution of negative and positive charge in LiClO₄/Ti and LiClO₄-PPy/Ti.

Table 2. Mulliken charge population of C, O, N, Cl, and Ti atoms in LiClO₄/Ti and LiClO₄-PPy/Ti.

Element	LiClO4-Ti	LiClO4-PPy-Ti
O1	−0.78	−0.76
O2	−0.80	−0.77
O3	−0.82	−0.78
O4	−0.95	−0.90
Cl	+2.45	+2.47
C1	/	+0.03
C2	/	+0.04
N	/	(−0.58~−0.47)
Ti	(−0.23~−0.04)	(−0.05~−0.01)
	(+0.13~+0.05)	(+0.62~+0.08)

lists the Mulliken charges of C, O, N, Cl, and Ti atoms in LiClO₄/Ti and LiClO₄-PPy/Ti. The negative charge densities of oxygen atoms are changed from LiClO₄/Ti (−0.78, −0.80, −0.82, −0.95) to LiClO₄-PPy/Ti (−0.76, −0.77, −0.78, −0.90), indicating a slightly decreased negative charge. The dissociated positive and negative charge densities of Ti substrate are (+0.13~+0.05)/(−0.23~−0.04) for LiClO₄/Ti and (+0.62~+0.08)/(−0.05~−0.01) for LiClO₄-PPy/Ti, respectively. Therefore, the electrostatic interaction is formed between negative-charged perchlorate O and positive-charged Ti. Additionally, the negative charge densities of nitrogen atom are (−0.58~−0.47). Obviously, the strong electrostatic interaction is further formed between the negative-charged pyrrole N and the positive-charged Ti in LiClO₄-PPy/Ti. Herein, LiClO₄ exhibits the surface adsorption state of Ti in LiClO₄/Ti and the doping state of PPy in LiClO₄-PPy/Ti. The Mulliken charge population analysis proves the shorter intermolecular interaction distance in LiClO₄-PPy/Ti in comparison with LiClO₄/Ti. Accordingly, LiClO₄-PPy/Ti achieves higher electroactivity than LiClO₄/Ti. The DFT simulation calculation results agree with the experimental measurement results in confirming the superior capacitance of LiClO₄-PPy/Ti.

3. Experimental Section

3.1. Preparation of LiClO₄-PPy/Ti

The titanium sheet (Ti, purity 99.5%, size 10 × 50 mm, thickness 0.2 mm.) was washed using acetone, ethanol, and then deionized water to remove surface stains. The Ti sheet then conducted the chemical polishing treatment in HF-HNO₃ solution to dissolve the surface-covered oxide layer. The chemical cleaned Ti sheet was obtained. The Ti sheet conducted the electrostatic adsorption in lithium perchlorate solution form LiClO₄/Ti. A normal pulse voltammetry process was employed to synthetize LiClO₄-PPy/Ti using a three-electrode reaction system, which included the fresh Ti sheet working electrode, Pt foil counter electrode, the Hg/Hg₂Cl₂ reference electrode, and acetonitrile solvent solution of 0.1 M LiClO₄ electrolyte and 0.15 M pyrrole. The reaction parameters included a pulse potential range of 0.7~1.1 V and pulse potential increment of 0.001 V.

3.2. Characterization and Measurement

The surface microstructure characterization of LiClO₄-PPy/Ti and LiClO₄/Ti was conducted using a scanning electron microscope (SEM, ZEISS Ultra Plus, Germany). A Raman spectrum measurement of LiClO₄-PPy/Ti was conducted using a Raman spectroscope (Thermo Fisher DXR3, He-Ne laser, Emitting wavelength of 785 nm). A crystal phase structure characterization of titanium substrate was conducted using an X-ray diffraction (XRD, Bruker D8-Discover, Wavelength of X-ray source, 0.154 nm) apparatus. Electrochemical capacitive measurements of LiClO₄-PPy/Ti and LiClO₄/Ti were performed using an electrochemical workstation (CHI 760D, CH Instruments Co., Ltd., China), which includes galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV). The electrochemical impedance spectrum (EIS) measurement was carried out at the frequency range of 10⁻²~10⁵ Hz. The 1.0 M LiClO₄ was used as the doping ion source and the electrolyte solution, contributing to maintaining the electrochemical stability of the titanium substrate in LiClO₄-PPy/Ti. electrode. The total surface area was used to evaluate electrochemical capacitance.

3.3. Theoretical Simulation Calculation

Density functional theory (DFT) simulation calculation was carried out to investigate electronic properties and atomic charge distribution of LiClO₄ and LiClO₄-PPy interaction at the interface of Ti substrate [38]. The total density of states (DOS), Mulliken charge population, interfacial binding energy, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and electrostatic potential were determined and discussed for better understanding of the electrostatic interfacial interaction of LiClO₄-PPy/Ti and LiClO₄/Ti. The DMol3 package was applied to calculate HOMO-LUMO energy gaps, and the CASTEP package was applied to calculate electronic energy gap and charge density difference and DOS value.

4. Conclusions

The PPy/Ti has been prepared through an electro-polymerization deposition process which is applied as supercapacitor electrode for electrochemical energy storage. Theoretical and experimental investigations are adopted to disclose the conductivity and interfacial interaction-dependent capacitance of LiClO₄-PPy/Ti. LiClO₄/Ti reveals ideal electrical double-layer capacitance due to the reversible adsorption/desorption of LiClO₄ electrolyte on Ti substrate in the potential range of 0~0.8 V. LiClO₄-PPy/Ti reveals Faradaic capacitance due to reversible doping/dedoping perchlorate ion in PPy. LiClO₄-PPy/Ti reveals lower charge transfer resistance, lower ohmic resistance, and lower Warburg resistance to present more active interface, higher electronic conductivity, and more feasible ion diffusion properties than LiClO₄/Ti. LiClO₄/Ti and LiClO₄-PPy/Ti achieve the mean response current of 0.13~2.47 μA cm⁻² and 2.28~9.011 μA cm⁻² at 5~200 mV s⁻¹. The corresponding specific capacitances are 0.010~0.0095 mF cm⁻² and 0.123~0.0122 mF cm⁻² at 0.01~0.10 mA cm⁻². LiClO₄-PPy/Ti keeps much higher electrochemical conductivity and higher specific capacitance than LiClO₄/Ti. Theoretical calculation results prove that interfacial electrostatic adsorption of LiClO₄/Ti and ClO₄⁻ anion doping of LiClO₄-PPy/Ti could change atomic charge density distribution. LiClO₄-PPy/Ti exhibits higher electronic conductivity and lower interface energy than LiClO₄/Ti. LiClO₄-PPy/Ti can act as a superior supercapacitor electrode to exhibit electroactivity and capacitance for the promising energy storage application.