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Resorcinol–formaldehyde semiconducting resins as precursors for carbon spheres toward electrocatalytic oxygen reduction†

Yasuhiro Shiraishi*^ab, Keisuke Kinoshita^a, Keisuke Sakamoto^a, Koki Yoshida^a, Wataru Hiramatsu^a, Satoshi Ichikawa^c, Shunsuke Tanaka^d and Takayuki Hirai^a
aResearch Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan. E-mail: shiraishi.yasuhiro.es@osaka-u.ac.jp
^bInnovative Catalysts Science Division, Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI), Osaka University, Suita 565-0871, Japan
^cResearch Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki 567-0047, Japan
^dDepartment of Chemical, Energy and Environmental Engineering, Kansai University, Suita 564-8680, Japan

First published on 20th August 2024

The resorcinol–formaldehyde (RF) resins¹² are inexpensive common insulator polymers that have been used in many industrial materials such as adhesives, paints, and plastics.¹³ The RF resins are usually synthesized by polycondensation of resorcinol and formaldehyde at relatively low temperatures (293–373 K).¹⁴ As shown in Scheme 1a (top), they have a structure in which the benzenoid forms of resorcinol are crosslinked by methylene linkers. They have been used as precursors for carbon-based ORR catalysts,^15–17 although their activities are insufficient for practical applications. Recently, we found that a high-temperature hydrothermal (HyTh) synthesis (∼523 K) produced semiconducting RF resin spheres.¹⁸ As shown in Scheme 1b (top), the resins consist of the quinoid forms of resorcinol, which are π-conjugated with the benzenoid forms of resorcinol via methine linkers. The π-stacking interaction of the benzenoid–quinoid donor–acceptor (D–A) units hybridizes their molecular orbitals and creates n-type semiconducting band structures with relatively low bandgap energies (∼2 eV). These resins promote photocatalytic reactions under irradiation of visible light up to 700 nm.^18–21

	Scheme 1 Schematic representation for the structures of (a) insulator and (b) semiconductor RF resins and their corresponding carbons obtained by pyrolysis under different conditions.

Herein, we report that pyrolysis of the semiconducting RF resins produces the carbon spheres exhibiting enhanced ORR activity. The narrowed reticulated carbon layers, which are derived from the D–A π-stacking of the semiconducting resins, enhance electron conductivity. The N-doped porous carbon spheres can be prepared by pyrolysis under NH₃ atmosphere and show higher ORR activity.

The HyTh treatment of water containing resorcinol, formaldehyde, and NH₃ as a base (pH 8.8) at different temperatures for 12 h (x = 373, 473, and 523 K) produces RF-x_resin powders.¹⁸ The scanning electron microscopy (SEM) observations of all resins show spherical particles (Fig. S1, ESI†). The average diameter of the spheres, determined by dynamic light scattering (DLS) analysis, decreases from 990 nm to 730 nm with increasing HyTh temperature (Fig. S2, ESI†) because higher HyTh temperature enhances nucleation and facilitates the formation of smaller resin particles.¹⁹ These spheres show type III N₂ adsorption/desorption isotherms and have ∼10 m² g⁻¹ of surface areas (Fig. S3, ESI†), indicating the formation of nonporous spheres. The powder X-ray diffraction of the spheres prepared by higher-temperature HyTh synthesis shows higher-angle shift of the 002 diffraction (Fig. S4, ESI†), suggesting that they have narrower aromatic plane distances. This is because, as shown in Scheme 1b (top), the amount of quinoid (A) units increased at higher HyTh temperature leads to stronger D–A π-stacking; the benzenoid/quinoid (%/%) ratios determined by the solid-state ¹³C NMR were 83/17 (RF-373), 60/40 (RF-473), and 55/45 (RF-523), respectively.¹⁸ As shown in Fig. 1a, the electrochemical impedance spectroscopy (EIS) Nyquist plots of the resin-loaded fluorine tin oxide (FTO) electrodes indicate that the charge-transfer resistance (R_CT) of the resins decreases with increasing HyTh temperature. This suggests that the formation of the quinoid units (A) enhanced at higher HyTh temperature strengthens the D–A π-stacking and increases the conductivity of the resins.¹⁸

	Fig. 1 EIS Nyquist plots of (a) RF-x_resin monitored in O2-saturated 0.1 M Na2SO4 at a bias of −0.2 V (vs. Ag/AgCl) and (b) RF-x_Ar and RF-x_NH3 monitored in O2-saturated 0.1 M KOH at a bias of −0.4 V (vs. Ag/AgCl). The insets show the equivalent circuit models comprising ohmic resistance (RS), double-layer capacitance (CDL), charge-transfer resistance (RCT), and Warburg impedance (Zw).

The RF-x_resin spheres were pyrolyzed at 1173 K for 2 h under Ar gas flow. The obtained powders (RF-x_Ar) maintain spherical morphologies (Fig. S1, ESI†), but their sizes are shrunk by ∼10% during the pyrolysis (Fig. S2, ESI†); the average diameters of RF-523_resin and the corresponding RF-523_Ar were 726 nm and 627 nm, respectively. The RF-x_Ar spheres show type I N₂ adsorption/desorption isotherms (Fig. S3, ESI†) and have larger surface areas (∼750 m² g⁻¹) due to the formation of some micropores less than 2 nm diameters. This suggests that partial decomposition and sintering of the resin components during the pyrolysis produces porous spheres (Scheme 1, middle). Their ORR performance was evaluated in O₂-saturated 0.1 M KOH solution (pH 13.0) using a rotating ring-disk electrode (RRDE) system. Fig. 2a shows the linear sweep voltammetry (LSV) results, where the potential values are expressed relative to the reversible hydrogen electrode (RHE). The RF-x_Ar spheres prepared at higher HyTh temperature exhibit higher current density and onset potentials, suggesting that the HyTh temperature (x) strongly affects the ORR performance. Note that the electron transfer numbers per O₂ molecule (n_ET), determined based on the H₂O₂ formation (Fig. 2b), are ∼3.5 on the RF-x_Ar spheres, indicating insufficient 4e⁻ ORR selectivity, as usually observed for undoped carbons.²² The combustion elemental analysis of RF-373_Ar and RF-523_Ar (Table S1, ESI†) indicates that both contain only a trace amount of N component (∼0.5%), which originates from NH₃ used as a base during the resin synthesis. The X-ray photoelectron spectroscopy (XPS) of these spheres (Fig. S5, ESI†) shows similar C 1s spectra with similar component ratios (Fig. S6, ESI†)²³ and almost no signal in N 1s spectra (Fig. S7, ESI†). The data suggest that these RF-x_Ar spheres have similar morphology, specific surface areas, and elemental and chemical compositions, although their ORR activities are enhanced by the higher-temperature HyTh synthesis.

	Fig. 2 The electrochemical ORR performance of catalysts in O2-saturated 0.1 M KOH monitored by a RRDE system (1600 rpm). (a) LSV curves (solid line: disk current, dotted line: ring current) and (b) electron transfer numbers per O2 (nET).

The higher ORR activity of RF-523_Ar (Fig. 2a) is attributable to the higher conductivity. The Nyquist plots (Fig. 1b) indicate that R_CT of RF-523_Ar is lower than that of RF-373_Ar, which is consistent with the ORR activity. Notably, the lower R_CT of RF-523_Ar than RF-373_Ar is also consistent with lower R_CT of RF-523_resin than RF-373_resin (Fig. 1a). This indicates that the conductivity of the resins, enhanced by higher-temperature HyTh synthesis, is inherited to the corresponding carbons and critically affects their ORR activity. The high conductivity of RF-523_Ar originates from the narrower reticulated carbon layers (Scheme 1b, middle). The XRD patterns of RF-x_Ar (Fig. S4, ESI†) show two diffraction peaks at 2θ = ∼24° and ∼44° assigned to the {002} and {101} planes of graphitic carbons.²⁴ The 002 diffraction of RF-x_Ar obtained from the resins prepared by higher HyTh temperature appears at higher angles. This suggests that they have narrower reticulated carbon layers, which are consistent with narrower aromatic plane distance of the corresponding resins. This means that, as shown in Scheme 1b (top, middle), pyrolysis of the resins with narrower aromatic plane distance produces carbons with narrower carbon spacing. The Raman spectroscopy confirms this (Fig. S8, ESI†). The two bands of RF-x_Ar at 1335 cm⁻¹ and 1580 cm⁻¹ are assigned to disordered sp² hybridized carbons (D band) and crystalline graphitic carbons (G band), respectively.²⁵ Their intensity ratio (I_G/I_D) increases with increasing HyTh temperature from 0.94 (RF-373_Ar) to 0.98 (RF-523_Ar), indicating that RF-523_Ar has more graphitized structure with lower defect density.²⁶ This suggests that pyrolysis of the resins with stronger D–A π-stacking produces more graphitized carbons with narrower carbon spacing. This increases the conductivity and shows enhanced ORR activity. Therefore, semiconducting RF resins are potential as precursors for highly conductive carbons.

Next, N-doped carbon spheres were prepared by pyrolysis of the resins under the flow of a 10% NH₃/He mixed gas.²⁷ The obtained RF-373_NH₃ and RF-523_NH₃ powders are spheres (Fig. S1, ESI†) with average diameters of ∼700 nm (Fig. S2, ESI†), which are similar to those of RF-373_Ar and RF-523_Ar. However, as shown in Fig. S3 (ESI†), they have surface areas (∼2000 m² g⁻¹) much larger than those of RF-373_Ar and RF-523_Ar (∼750 m² g⁻¹). The pore size distributions revealed the formation of large volume of pores with less than 2 nm diameter, suggesting that reductive elimination of −OH, methylene, and methine groups and partial decomposition of the carbon network by NH₃ (etching) creates large volumes of micropores.²⁷ The N amounts of RF-373_NH₃ and RF-523_NH₃ are ∼7 wt% (Table S1, ESI†), which are higher than those of RF-373_Ar and RF-523_Ar (∼0.5 wt%), confirming the N doping. The N 1s XPS spectra (Fig. S7, ESI†) suggest that the doped N atoms consist of two-coordinated N (N_2c) and three-coordinated N (N_3c),²⁷ which are contained in RF-373_NH₃ (51% and 49%) and RF-523_NH₃ (54% and 46%) with similar ratios. The scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) observations (Fig. 3) indicate that the N atoms are doped homogeneously within the particles (full EDS maps and spectrum: Fig. S9, ESI†), although RF-523_Ar shows negligible N components (Fig. S10, ESI†). Therefore, pyrolysis of the resin spheres under NH₃ produces N-doped porous carbon spheres (Scheme 1, bottom).

	Fig. 3 STEM-EDS observation results of RF-523_NH3. The EDS quantitative maps (Kα line) consist of C (red) and N (green) components, where minor O components (blue) are also shown in the overlay map (right top).

RF-523_NH₃ shows ORR performance higher than that of RF-523_Ar and RF-373_NH₃ in terms of onset potential and current density (Fig. 2a). This indicates that higher-temperature HyTh synthesis and pyrolysis of the resulting resins under NH₃ cooperatively enhance the ORR activity. The high activity of RF-523_NH₃ involves (i) the formation of ORR-active C atoms by N-doping, (ii) enlarged surface area by the micropore formation, and (iii) high conductivity by the narrower reticulated carbon layers. The N atoms are more electronegative than C atoms; their doping alters the electronic configuration and creates ORR-active C atoms, which decrease the energy barrier for O₂ adsorption/reduction.²⁸ RF-523_NH₃ shows the 002 diffraction at the position identical to that of RF-523_Ar (Fig. S4, ESI†), indicating that the narrowed carbon spacing is maintained even by the pyrolysis under NH₃ (Scheme 1b, middle and bottom). In addition, RF-523_NH₃ exhibits R_CT similar to that of RF-523_Ar (Fig. 1b), indicating that it also maintains high conductivity. In contrast, RF-373_NH₃ shows wider carbon spacing (Fig. S4, ESI†) and lower conductivity (Fig. 1b), as is the case for RF-373_Ar. As shown in Fig. S8 (ESI†), the ratio of the Raman bands (I_G/I_D) of RF-373_NH₃ (0.76) is much lower than that of RF-373_Ar (0.94) because the decomposition of carbon network by NH₃ creates many defects. In contrast, the I_G/I_D ratio of RF-523_NH₃ remains relatively high (0.88) probably because the carbon networks strengthened by higher-temperature HyTh synthesis suppress the defect formation,²⁹ resulting in high conductivity (Fig. 1b). Thus, N-doping, high surface area, and high conductivity of the carbon spheres cooperatively boost ORR. Note that RF-523_NH₃ and RF-373_NH₃ show similar C/N ratios (Table S1, ESI†) and N compositions (Fig. S7, ESI†), suggesting that the active ORR sites on both catalysts are similar. The high conductivity of RF-523_NH₃ may accelerate the e⁻ transfer to O₂, thus enhancing ORR.

As shown in Fig. 2b, n_ET of RF-523_NH₃ is almost 4, whereas n_ET of RF-373_NH₃ is ∼3.5. This indicates that the 4e⁻ ORR selectivity is also enhanced and inhibits the generation of H₂O₂ that leads to a catalyst deactivation.²² Therefore, the obtained N-doped porous carbons exhibit enhanced ORR activity and high 4e⁻ ORR selectivity, although its performance is lower than that of the benchmark Pt/C (Fig. 2a) and early reported N-doped carbons^5–10 in terms of onset potential and current density. Note that the N-doped porous carbons are stable. As shown in Fig. 4a, RF-523_NH₃ exhibits long-term stability with almost no decrease in the onset potential and current density even after 3000 cycles of cyclic voltammetry (CV), whereas the Pt/C decreases the activity. In addition, the methanol poisoning test using chronoamperometry (Fig. 4b) indicates that RF-523_NH₃ maintains the current density even after the addition of 1 mL methanol, whereas Pt/C shows significant decrease in current density. Furthermore, the RF-523_NH₃ catalyst recovered after the CV cycles shows an XRD pattern similar to that of the fresh one (Fig. S4, ESI†). The results prove the long-term stability and chemical tolerance of the RF-523_NH₃ spheres.

	Fig. 4 The electrochemical performance of RF-523_NH3 and Pt/C in O2-saturated 0.1 M KOH monitored by a RRDE system (1600 rpm). (a) LSV results before and after 3000 CV cycles at 0.6–1.0 V (vs. RHE) and a scan rate of 50 mV s−1. (b) Chronoamperometric i–t data obtained at 0.67 V (vs. RHE) when adding 1 mL of methanol.

In summary, we found that pyrolysis of semiconducting RF resins, which were prepared by the high-temperature HyTh synthesis, produced carbon spheres with high conductivity. The strong D–A π-stacking of the resin led to the formation of carbon spheres with narrower reticulated carbon layers and enhanced conductivity. Pyrolysis under NH₃ atmosphere produced N-doped porous carbon spheres. The N-doping, high surface area, and high conductivity cooperatively enhance the ORR activity with long-term durability and high chemical resistance. The semiconducting RF resins are therefore potential precursors for highly active carbon-based ORR catalysts.

This work was partly supported by the Grant-in-Aid for Scientific Research on Innovative Areas (23K17346) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT).

Data availability

Conflicts of interest

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