Enhancement of oxygen reduction activity of iron phthalocyanine electrocatalyst supported on carbon nanotubes through molecular encapsulation

Abstract

Encapsulation of dibenzo-18-crown-6 complexed with potassium ions (K⁺–DB18C6) inside single-walled carbon nanotubes (SWCNTs) enables electron doping while preserving the tubular structure. Thermoelectric measurements demonstrated that K⁺–DB18C6@SWCNTs switched from p-type (pristine SWCNTs) to n-type. Raman spectroscopy further confirmed electron doping through an upshift of the G-band and a decrease in radial breathing mode (RBM) intensity, while ultraviolet photoelectron spectroscopy (UPS) showed a decrease in work function from Φ = 4.41 eV to Φ = 4.21 eV. When iron(II) phthalocyanine (FePc) was supported on K⁺–DB18C6@SWCNTs (FePc/K⁺–DB18C6@SWCNT), the composite exhibited excellent oxygen reduction reaction (ORR) catalytic activity. Linear sweep voltammetry with a rotating ring–disk electrode (RRDE) revealed an onset potential (E_onset = 0.624 V vs. RHE) nearly identical to that of Pt/C electrodes. The enhanced ORR performance is attributed to perturbation of the FePc electronic state by the electron-doped SWCNT support, as evidenced by recovery of RBM intensity upon FePc loading. RRDE analysis further showed that the ORR followed a nearly complete four-electron pathway (n = 3.97). Durability tests by chronoamperometry at 0.4 V (vs. RHE) indicated that FePc/K⁺–DB18C6@SWCNT retained 31% of its initial current after 3 h, outperforming untreated SWCNT electrodes. This study demonstrates a novel strategy for ORR catalyst design, where molecular encapsulation within SWCNTs modulates the electronic states of supported metal complexes, offering a new route to high-performance and stable ORR electrodes.

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Introduction

To address global challenges such as climate change and abnormal weather, it is essential to shift from fossil fuels toward renewable energy sources like solar power.^1,2 Several pathways toward a sustainable energy society have been proposed. One approach is a hydrogen-based society, where hydrogen serves as a clean energy carrier that can be stored and transported like fossil fuels but emits no CO₂ when used. In such a system, fuel cells play a central role in power generation.^3–6 Another approach is to directly convert solar energy into electricity using solar cells, which requires large-capacity secondary batteries to provide power at night or under non-illuminated conditions. Metal–air batteries are attracting particular attention as promising candidates for next-generation large-scale storage.^7–9

In both fuel cells and metal–air batteries, the electrochemical oxygen reduction reaction (ORR) at the air electrode is a key process. However, the large overpotential of ORR remains a major obstacle. Therefore, the development of ORR catalyst electrodes capable of reducing this overpotential is of critical importance.

Extensive research has been conducted to improve the performance of both fuel cells and metal–air batteries.^10,11 In these devices, the electrochemical oxygen reduction reaction (ORR) at the air electrode is utilized. A major issue in these energy conversion systems is the large overpotential at the air electrode, and therefore, the development of ORR catalyst electrodes capable of lowering this overpotential is of great importance.^12–18 Recently, phthalocyanine-based molecular catalysts such as iron(II) phthalocyanine (FePc) and cobalt(II) phthalocyanine (CoPc) supported on carbon materials have attracted increasing attention as non-precious ORR catalysts, exhibiting high activity and selectivity in alkaline media.^19–23

Single-walled carbon nanotubes (SWCNTs) possess outstanding physical and chemical properties, including excellent electrical conductivity, chemical stability, and high surface area, and are actively studied for application in battery electrodes. They are already used in practice as conductive additives in lithium-ion batteries. Owing to their unique tubular structure, SWCNTs can encapsulate a wide variety of molecules inside their cavities.^24–27 Since the discovery of SWCNTs encapsulating C₆₀—so-called “peapods”—a broad range of molecules have been incorporated.²⁸ Our group has explored the use of such encapsulated molecules as active materials in batteries.²⁹ We have demonstrated that even poorly conductive molecules can function as active materials when encapsulated due to electron supply from the SWCNTs, and that even molecules soluble in electrolytes can be stabilized and utilized as electrode materials by being confined inside SWCNTs.³⁰

In addition to directly employing encapsulated molecules as active electrode materials, we have recently investigated another approach: improving electrode performance by modulating the electronic states of SWCNTs through molecular encapsulation. Specifically, when the electron-accepting molecule TCNQ is encapsulated inside SWCNTs, electron transfer occurs from the nanotubes to TCNQ, rendering the SWCNT surface positively charged. We recently reported that supporting nickel hydroxide nanosheets—an oxygen evolution reaction (OER) catalyst—on this positively charged SWCNT surface markedly enhances OER performance.³¹ Unlike conventional methods where OER catalysts are directly chemically modified, this approach enables the development of stable catalysts through indirect perturbation of catalyst electronic states from inside the nanotube. This stability arises because the encapsulated molecules remain confined within the SWCNT interior and are isolated from the reaction environment.

Encouraged by the successful enhancement of OER catalyst performance through such perturbation of SWCNT electronic states, we hypothesized that the same concept could be applied to develop novel ORR catalyst electrodes. In contrast to OER catalysts, ORR catalysts require electron-donating molecules to be encapsulated. However, many SWCNTs encapsulating electron-donating species (e.g., alkali-metal-doped SWCNTs) are unstable under ambient conditions.^32,33 Against this backdrop, we focused on a report demonstrating the synthesis of stable, negatively charged SWCNTs (K⁺–DB18C6@SWCNT) by encapsulating crown ethers complexed with alkali metal ions—specifically, dibenzo-18-crown-6 hosting K⁺ ions (K⁺–DB18C6).

In this study, we aim to develop highly efficient ORR catalyst electrodes by supporting ORR catalysts on SWCNTs negatively charged through encapsulation of K⁺–DB18C6. This novel type of ORR catalyst possesses unique features absent in conventional catalysts: the superior physicochemical properties of SWCNTs as catalyst supports are preserved, while the encapsulated molecules that perturb the electronic states are shielded from the reaction environment by the nanotube walls. This paper discusses not only the electrochemical properties of this new class of ORR catalysts but also provides, for the first time, detailed spectroscopic analyses of the electronic states involved.

Experimental

Single-walled carbon nanotubes (SWCNTs, EC2.0) were purchased from Meijo Nano Carbon Co., Ltd. A self-supporting film (buckypaper) was prepared by dispersing purified SWCNTs (treated with acid to remove impurities) in ethanol (EtOH), followed by filtration. Potassium hydroxide (KOH) and dibenzo-18-crown-6 (DB18C6) were dissolved in EtOH at a concentration of 0.1 mol L⁻¹, and the buckypaper (40 mg) was immersed in this solution for 72 h. After washing with EtOH and filtration, the sample was vacuum-dried at 110 °C for 1 h to obtain K⁺–DB18C6@SWCNT. The amount of encapsulated species was determined by elemental analysis using an Elementar Vario EL Cube.

Iron(II) phthalocyanine (FePc, Tokyo Chemical Industry Co., Ltd.) was dispersed in 30 mL EtOH at a concentration of 0.1 mg L⁻¹, to which 20 mg of K⁺–DB18C6@SWCNT was added. The mixture was ultrasonicated for 5 min in a bath-type ultrasonic apparatus (MVS-3, AS ONE Corp.), washed with EtOH, and filtered to obtain FePc/K⁺–DB18C6@SWCNT. CoPc/SWCNT was prepared by dispersing cobalt(II) phthalocyanine (CoPc, Tokyo Chemical Industry Co., Ltd.) in 30 mL of EtOH at a concentration of 0.1 mg L⁻¹, followed by the addition of 20 mg of SWCNTs. Commercial Pt/C (platinum on graphitized carbon, 10 wt% loading, Sigma-Aldrich) was used as received. The physicochemical properties of the obtained materials were characterized by scanning electron microscopy (SEM, JEOL JEM-2100F), Raman spectroscopy with an excitation wavelength of 785 nm (JASCO NRS-5500), X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000, Al Kα), and ultraviolet photoelectron spectroscopy (UPS, ULVAC-PHI VersaProbe 4, He I).

The catalytic activity for the oxygen reduction reaction (ORR) was evaluated by linear sweep voltammetry (LSV) using a rotating ring-disk electrode (RRDE, AFMSRCE, Pine Research) at a scan rate of 10 mV s⁻¹ and a rotation speed of 1600 rpm. Catalyst inks were prepared by dispersing 1 mg of catalyst in 50 µL Nafion® solution (FUJIFILM Wako Pure Chemical Corp., DE521) and 950 µL EtOH, followed by overnight stirring and 30 min of ultrasonication. Then, 8 µL of the ink was drop-cast onto a glassy carbon disk electrode (5 mm diameter, Pine Research) and dried to obtain the working electrode. An Hg/HgO electrode was used as the reference electrode and a platinum mesh served as the counter electrode. The electrolyte was 0.1 mol L⁻¹ KOH solution (pH 13). For background current correction, LSV measurements were first carried out after bubbling with Ar for 30 min, and then after bubbling with O₂ for 30 min. The final LSV curves were obtained by subtracting the baseline measured under Ar bubbling from that under O₂ bubbling. Potentials vs. Hg/HgO were converted to the reversible hydrogen electrode (RHE) scale using the following equation:

E_RHE = E_Hg/HgO + 0.098 + 0.059 × pH

The number of electrons (n) involved in the ORR was calculated according to the following equation:

where I_Disk and I_Ring are the current densities at the disk and ring electrodes, respectively, and N is the experimentally determined collection efficiency (0.37 in this study)

Results & discussion

Raman spectrum of the SWCNT sample shows that the defect-related D band at around 1300 cm⁻¹ is very weak, indicating high crystallinity of the SWCNTs. In the low-frequency region, radial breathing mode (RBM) peaks characteristic of SWCNTs are clearly observed. The strongest RBM feature appears around 100 cm⁻¹, which corresponds to a diameter of approximately 2.5 nm based on the commonly used relation between RBM frequency and nanotube diameter.^34,35 However, it should be noted that Raman scattering in SWCNTs is strongly affected by resonance effects, and thus an average diameter cannot be discussed from a single excitation wavelength. This is because only tubes whose inter-van Hove Singularity gaps match the excitation photon energy mainly contribute to the observed Raman spectra.³⁶

Upon encapsulation of DB18C6, the Raman spectra show little change compared with pristine SWCNTs (Fig. S1). In contrast, encapsulation of K⁺–DB18C6 induces several distinct spectral changes (Fig. 1). First, the G-band peak around 1600 cm⁻¹ shifts to higher wavenumbers. Such a G-band upshift is well known to occur upon carrier doping in carbon materials,³⁷ suggesting that SWCNTs are carrier-doped in this case. A second key change is the decrease in RBM peak intensity. This reduction is attributed to electron doping, in which electrons occupy conduction band states, blocking optical transitions between van Hove singularities and thereby suppressing the resonance Raman effect (Fig. S2).^38,39

Fig. 1 Raman spectra of pristine SWCNT, DB18C6@SWCNT, and K⁺–DB18C6@SWCNT.

The electron doping of SWCNTs by K⁺–DB18C6 encapsulation was further confirmed by thermoelectric power measurements. As shown in Fig. 2, pristine SWCNTs display p-type characteristics with a Seebeck coefficient of +42 µV K⁻¹, and encapsulation of DB18C6 does not change this behavior. On the other hand, K⁺–DB18C6@SWCNTs exhibit n-type characteristics with a Seebeck coefficient of −43 µV K⁻¹. Although the detailed mechanism of electron donation remains unclear, previous reports suggest that encapsulation of K⁺–DB18C6 induces electron transfer from counter-anions to the SWCNTs.⁴⁰ Nonoguchi et al. proposed that coordination of DB18C6 to K⁺ forms a [K(DB18C6)]⁺ complex, which generates a highly reducing “naked anion”. Electron donation from this naked anion to the SWCNT framework is considered responsible for the observed n-type behavior, accompanied by an upward shift of the Fermi level.

Fig. 2 Thermoelectric measurements of pristine SWCNT, DB18C6@SWCNT, and K⁺–DB18C6@SWCNT.

This result is also supported by ultraviolet photoelectron spectroscopy (UPS). Fig. 3 shows UPS spectra of SWCNT and K⁺–DB18C6@SWCNT samples which reflects their valence bands. The binding energy is referenced to the Fermi level, which is set to zero. The work function was determined from the cutoff energy (E_cutoff) of the secondary electron edge using the equation Φ (eV) = 21.22 − E_cutoff, where the photon energy of the He I line is taken as 21.22 eV. Using this method, the work function was evaluated to be Φ = 4.41 eV for pristine SWCNTs, while it decreased to Φ = 4.21 eV for K⁺–DB18C6@SWCNTs. These results indicate that encapsulation of K⁺–DB18C6 leads to electron doping of the SWCNTs, resulting in an upward shift of the Fermi level.

Fig. 3 UPS spectra of pristine SWCNT and K⁺–DB18C6@SWCNT. The binding energy in the spectra is referenced to the Fermi level set at zero.

Next, FePc molecules were deposited on K⁺–DB18C6@SWCNTs to prepare FePc/K⁺–DB18C6@SWCNT samples. The presence of Fe was confirmed by SEM–EDS and XPS measurements (Fig. 4 and 5). Furthermore, SEM and high-resolution TEM (HRTEM) images of FePc/K⁺–DB18C6@SWCNT are presented in Fig. S3 and S4. Nitrogen elemental mapping in the TEM analysis shows that the N signal from FePc overlaps with the SWCNT framework, suggesting that FePc is present on the nanotube surface. No significant difference in the elemental concentration ratio of C to K (C/K) was observed before and after the deposition (Table S1). Taking the ∼2 nm molecular size of FePc into account, encapsulation inside the nanotubes is unlikely, and FePc is considered to be supported on the external surfaces of SWCNTs. The molecular state of FePc on SWCNTs was investigated by XPS N 1s spectra. While metal phthalocyanines typically show split peaks in N 1s spectra due to different nitrogen environments, supported FePc exhibits a single N 1s peak (Fig. 5(c)). This is explained by strong π–π interactions between the FePc and the SWCNT surface, which induce bending distortions of the macrocycle, bringing all nitrogens into similar environments.⁴¹ This observation is consistent with previous reports, supporting the presence of FePc as a thin molecular layer on SWCNT surfaces.

Fig. 4 (a) SEM image, (b) elemental mapping data, (c) EDS spectrum of K⁺–DB18C6@SWCNT.

Fig. 5 (a) XPS survey spectrum, (b) Fe 2p, (c) N 1s XPS peaks of FePc/K⁺–DB18C6@SWCNT.

Raman spectra further reveal that FePc loading significantly alters K⁺–DB18C6@SWCNT. As previously discussed, electron doping by K⁺–DB18C6 shifts the G-band peak to higher frequency and reduces RBM intensity. After FePc deposition, the G-band peak shifts back toward the pristine SWCNT position and RBM intensity recovers (Fig. 6). These changes indicate partial electron transfer from K⁺–DB18C6@SWCNT to FePc molecules supported on the surface. Table S2 summarizes the G-band peak positions and full width at half maximum (FWHM) together with the Raman intensity (I_D/I_G ratio, I_RBM) for pristine SWCNT, DB18C6@SWCNT, K⁺–DB18C6@SWCNT and FePc/K⁺–DB18C6@SWCNT.

Fig. 6 Raman spectra of K⁺–DB18C6@SWCNT, and FePc/K⁺–DB18C6@SWCNT.

The ORR catalytic performance of FePc/K⁺–DB18C6@SWCNT electrodes was evaluated by RRDE measurements (Fig. 7). A Pt mesh was used as the counter electrode. The validity of using the Pt mesh as the counter electrode was confirmed by conducting experiments employing a Ti mesh counter electrode (Fig. S5 and Table S3). Compared with other SWCNT-based samples, FePc/K⁺–DB18C6@SWCNT exhibits excellent ORR catalytic activity. The onset potential (E_onset), defined as the potential corresponding to 10 µA cm⁻² above the baseline, was as high as 0.624 V vs. RHE—comparable to that of Pt/C electrodes (Table 1). Tafel plots show a smaller Tafel slope for FePc/K⁺–DB18C6@SWCNT than other SWCNT-based samples (Fig. S6), indicating more favorable ORR kinetics. This superior ORR performance is attributed to electron donation from K⁺–DB18C6@SWCNT to FePc. Interestingly, XPS Fe 2p spectra showed binding energies of Fe 2p_3/2 at ∼709.8 eV for FePc/K⁺–DB18C6@SWCNT and ∼709.6 eV for FePc/DB18C6@SWCNT, suggesting little change in core-level energies (Fig. 5(b)). Given that the Fe/K atomic ratio is 0.56 in FePc/K⁺–DB18C6@SWCNTs (K > Fe), Thus, electron donation appears to occur to FePc outer orbitals, enhancing O₂ activation without significantly affecting core levels. The ORR proceeded with an electron number close to 4 (n = 3.97), indicating a nearly complete four-electron reduction pathway (Fig. 8). Since pristine SWCNTs exhibit n < 3, the results demonstrate that the ORR predominantly occurs on FePc surfaces, with minimal contribution from SWCNT surfaces.

Fig. 7 RRDE LSV curves of pristine SWCNT, FePc/DB18C6@SWCNT, FePc/K⁺–DB18C6@SWCNT, CoPc/SWCNT and Pt/C.

Table 1 Experimentally measured values of s of prepared electrocatalysts

Catalyst	E onset (V vs. RHE)	E 1/2 (V vs. RHE)	n (RRDE)
Pristine SWCNT	0.617	0.521	2.89
FePc/DB18C6@SWCNT	0.863	0.760	3.94
FePc/K^+–DB18C6@SWCNT	0.873	0.768	3.97
CoPc/SWCNT	0.731	0.606	2.86
Pt/C	0.887	0.763	3.93

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Fig. 8 Electron number plots of pristine SWCNT and FePc/K⁺–DB18C6@SWCNT.

Durability tests were performed by chronoamperometry at 0.4 V vs. RHE under 1600 rpm for 3 h (Fig. 9)⁴². The FePc/K⁺–DB18C6@SWCNT electrode retained 31% of its initial current, compared with only 17% retention for untreated SWCNTs. The presence of K was still detected in the FePc/K⁺–DB18C6@SWCNT electrode after the durability test (Fig. S7). These results indicate that electron donation from encapsulated K⁺–DB18C6 is more stable than in control systems, though further optimization of durability is required. In addition, Raman spectra observed after 3 h ORR experiment (Fig. S8) exhibit no significant changes in the G-band of the SWCNTs compared with the as-prepared electrode, suggesting that the SWCNT framework and the FePc coordination environment are largely preserved during reaction.

Fig. 9 Durability tests of pristine SWCNT and FePc/K⁺–DB18C6@SWCNT.

Conclusion

By encapsulating dibenzo-18-crown-6 complexed with potassium ions (K⁺–DB18C6) inside single-walled carbon nanotubes (SWCNTs), we achieved electron doping of SWCNTs while maintaining their tubular structure. This electron doping was confirmed by thermoelectric power measurements, Raman spectroscopy, and ultraviolet photoelectron spectroscopy (UPS). Thermoelectric measurements showed that K⁺–DB18C6@SWCNTs switched from p-type (pristine SWCNTs) to n-type upon molecular encapsulation. Raman spectra further confirmed electron doping, as evidenced by the upshift of the G-band peak position and the decrease in radial breathing mode (RBM) intensity. UPS measurements revealed that the work function of SWCNTs decreased from Φ = 4.41 eV to Φ = 4.21 eV after K⁺–DB18C6 encapsulation, again consistent with electron doping.

When iron(II) phthalocyanine (FePc) was supported on the outer walls of K⁺–DB18C6@SWCNT (FePc/K⁺–DB18C6@SWCNT), the composite exhibited excellent oxygen reduction reaction (ORR) catalytic activity, as demonstrated by linear sweep voltammetry with a rotating ring–disk electrode (RRDE). The presence of FePc on the SWCNT surface was confirmed by direct SEM observations and XPS N 1s spectral analysis. The onset potential of FePc/K⁺–DB18C6@SWCNT electrodes (E_onset = 0.624 V vs. RHE) was nearly identical to that of Pt-loaded carbon electrodes. The high ORR performance is attributed to perturbation of the electronic state of FePc by electron-doped SWCNTs. This perturbation was corroborated by Raman measurements, where the RBM intensity of K⁺–DB18C6@SWCNT—reduced upon encapsulation—recovered after FePc loading, indicating partial charge transfer from the SWCNTs to FePc.

RRDE results further revealed that the ORR on FePc/K⁺–DB18C6@SWCNT proceeds predominantly via a four-electron pathway (n = 3.97). Durability tests by chronoamperometry at 0.4 V (vs. RHE) demonstrated that the catalytic current retained 31% of its initial value after 3 h, showing higher stability compared with untreated SWCNT electrodes. The enhanced ORR efficiency of FePc/K⁺–DB18C6@SWCNT can be understood in terms of a cooperative interaction between the encapsulated K⁺–DB18C6 and the FePc molecules on the nanotube surface. K⁺–DB18C6 confined inside the SWCNTs serves as an internal electron donor, raising the Fermi level of the nanotube framework, while FePc is immobilized on the outer wall via π–π interactions. Electron donation from the n-doped SWCNTs to the FePc is considered to increase the electron density around the Fe centers, thereby facilitating O₂ activation and supporting an efficient four-electron ORR pathway.

Thus, our FePc/K⁺–DB18C6@SWCNT catalyst adopts an inside–outside architecture, in which encapsulated K⁺–DB18C6 tunes the electronic state of FePc on the outer surface via charge transfer from inside the nanotubes. This nano space mediated tuning of external catalytic sites represents a new strategy for exploiting the inner space of SWCNTs and offers promising potential for fuel-cell and sensing applications.^43–45 This study demonstrates a novel strategy for ORR catalyst design, where the electronic structure of supported metal complexes is modulated through molecular encapsulation inside SWCNTs. This approach represents a fundamentally new route to developing ORR catalyst electrodes and offers valuable guidance for next-generation electrode catalyst design.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

The data supporting this article have been included as part of the supplementary information (SI). The supplementary information includes additional Raman spectra, Density of states model, SEM and HRTEM images, elemental mapping and EDS data, Tafel plots, RRDE LSV curves and supplementary tables summarizing structural and electrochemical parameters of the catalysts. See DOI: https://doi.org/10.1039/d5cp04139b.

Acknowledgements

This work was financially supported by JSPS KAKENHI grant number 23K23448.

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