Metallosupramolecular Polymer Precursor Design for Multi-Element Co-Doped Carbon Shells with Improved Oxygen Reduction Reaction Catalytic Activity

Abstract

Heteroatom-doped carbon materials have been extensively studied in the field of electrochemical catalysis to solve the challenges of energy shortage. In particular, there is vigorous research activity in the design of multi-element co-doped carbon materials for the improvement of electrochemical performance. Herein, we developed a supramolecular approach to construct metallosupramolecular polymer hollow spheres, which could be used as precursors for the generation of carbon shells co-doped with B, N, F and Fe elements. The metallosupramolecular polymer hollow spheres were fabricated through a simple route based on the Kirkendall effect. The in situ reaction between the boronate polymer spheres and Fe³⁺ could easily control the component and shell thickness of the precursors. The as-prepared multi-element co-doped carbon shells showed excellent catalytic activity in an oxygen reduction reaction, with onset potential (E_onset) 0.91 V and half-wave (E_half-wave) 0.82 V vs reversible hydrogen electrode (RHE). The fluorine element in the carbon matrix was important for the improvement of oxygen reduction reaction (ORR) activity performance through designing the control experiment. This supramolecular approach may afford a new route to explore good activity and a low-cost catalyst for ORR.

1. Introduction

During the commercialization process of hydrogen fuel cells, exploring electrocatalysts with high oxygen reduction reaction (ORR) activity and outstanding stability is the primary task [1,2,3]. At present, precious metals incorporating carbon materials, such as commercial Pt/C, have been successfully used as ORR catalysts [4,5]. However, their expensive cost and scarcity greatly limit their broader application [6,7,8]. Therefore, fabricating non-precious metal composite materials with low-cost, and a catalytic performance comparable or better than commercial Pt/C toward ORR, is of great significance to commercial applications [9,10].

Pure carbon materials have many advantages, including excellent electrical transport properties, a highly active surface area, chemical stability and superior thermal stability. They have thus become an ideal choice for electrochemical energy storage materials [11,12,13,14,15]. However, pure carbon materials have a highly hydrophobic surface and limited active sites, which bring many problems for the application in hydrogen fuel cells. Heteroatom-doping has been recognized as an effective approach to increase the electrochemical activity and surface wettability of carbon materials [16,17]. The chemical elements used for doping carbon materials include nitrogen, phosphorus, boron, fluorine and non-precious metals [18,19,20,21,22]. Notably, N, B co-doped carbon materials have shown excellent ORR performance, because they possess large amounts of defects and active sites [23,24]. Except for the heteroatom-doping materials, carbon materials incorporated with metal and N elements (M–N–C) have been considered as one of the promising candidates. Most of the M–N–C catalysts were prepared via heat-treating the carbon materials with an N-containing compound, as well as metal salts, or obtained from the simple pyrolysis of transition metal macrocyclic polymers [25,26,27]. Nevertheless, these approaches have shortcomings, such as inhomogeneity of the carbon materials, the aggregation of the active sites, complicated synthesis, as well as high-cost [28,29]. Hence, there exists a need to develop a facile and effective route to fabricate polymeric precursors, containing multiple elements like B, P, S, F, Co and Fe. This is of great importance to incorporate multi-elements into the carbon materials.

We have developed a supramolecular approach, in which the condensation reaction between boronic and catechol monomers is accompanied with the formation of B–N dative bonds, and can organize as formed boronate polymers into nanospheres with controllable sizes [30]. The catechol moiety has a high coordination efficiency with transition metal ions and the Kirkendall effect occurs during the reaction between boronate polymer nanospheres and transition metal ions, thus resulting in the formation of metallosupramolecular polymer hollow spheres [31]. In this work, we extended this approach through the design of the building blocks of the boronate polymers, and therefore metallosupramolecular polymer hollow sphere precursors, containing B, N, F and Fe elements, could be fabricated. Carbonation of the metallosupramolecular polymer hollow sphere precursor at 650 °C afforded carbon shells co-doped with B, N, F and Fe elements. We focused on the control over the thickness of the shell, the influence of the shell thickness and doping elements (B, N, F and Fe) on the ORR performance of the carbon materials.

2. Results and Discussion

2.1. Morphology Evolution

The synthetic process of carbon spheres (CSs) is shown in Scheme 1. We firstly prepared the boronate polymer nanospheres through a simple condensation reaction between 4,4′-((1E,1′E)-(((((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))bis(4,1-phenylene))bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diol) (DFC) and (((1E,1′E,1′′E)-((nitrilotris(benzene-4,1-diyl)tris(azanylylidene))tris(methanylylidene))tris(benzene-4,1-diyl))triboronic acid (TBB). The catechol moiety has a high coordination ability to transition metal ions. This coordination interaction is of particular interest in the fabrication of supramolecular polymers, assemblies, smart hydrogels and metal–organic frameworks [32,33,34]. Since the boronate moiety is dynamic, there probably existed free catechol groups in our boronate polymer nanospheres. Thus, we intended to introduce Fe³⁺ into the boronate polymer nanoparticles through the catechol–Fe³⁺ coordination, thereby forming metallosupramolecular polymers, which comprise Fe, N, B and F elements. After carbonization, carbon materials co-doped with Fe, N, B and F could be obtained.

With the assistance of the B–N coordination, a cross-linking reaction between DFC and TBB adopted a typical cooperative polymerization mechanism, and therefore, mono-dispersed boronate polymer nanospheres (BPN) could be formed in one-step (Figure 1a,b) [30]. The coordination reaction between catechol and Fe³⁺ could evidently change the morphology of the boronate polymer nanospheres, which accorded well with a previous report [31]. The metallosupramolecular polymer could be easily carbonized to afford CSs with hollow structures.

Figure 1c–e show the TEM images of the CSs prepared from precursors derived from different reaction times. Small pores were created in the interior of the nanospheres within a 6 h reaction time (Figure 1c). When the reaction time was 12 h, hollow nanospheres with shell thickness ~70 nm were obtained (Figure 1d). However, too long of a reaction time was likely to destroy the nanospheres, because many of the obtained particles collapsed (Figure 1e). As demonstrated by the previous report, hollow structural metallosupramolecular polymer precursors were generated according to the Kirkendall effect during the reaction between boronate polymer nanospheres and Fe³⁺ [31]. This process greatly relied on the removal of the boronic acid component. Too long of a reaction time between the boronate polymer nanospheres and Fe³⁺ can result in excessive removal of the boronic acid component, therefore leading to the collapse of the particles. Dark-field TEM imaging of carbon particles obtained from a 12 h reaction time between the boronate polymer nanospheres and Fe³⁺ confirmed the hollow structure (Figure 1f). Energy-dispersive X-ray (EDX) mapping of representative hollow particles indicated the coexistence of C, N, B, F, O and Fe elements (Figure 1g–l). The carbonization temperature likely had no evident effect on the morphology of the carbon materials, as the TEM images of CS_12-550 and CS_12-750 (Figure S1), display similar morphology to CS_12-650.

2.2. Composition and Structure Characterization

Figure 2a gives the Raman spectra of the CSs. The two prominent peaks at 1340 and 1571 cm⁻¹ represent the D band of disordered graphitic structure and the G band of ordered carbon matrix, respectively. The calculated intensity ratio from the D band and the G band (I_D/I_G) was applied to evaluate the disorder degree of the carbon materials. The calculated I_D/I_G value of CS_6-650, CS_12-650 and CS_18-650 were 1.22, 1.26 and 1.29, respectively. Probably, the content defect site in the carbon materials increased with the increasing reaction between BPN and Fe³⁺. The calculated I_D/I_G value of CS_12-550 and CS_12-750 were 0.94 and 1.19, respectively, which were lower than that of CS_12-650.

The crystalline structures of CSs were characterized through XRD. As shown in Figure 2b, an evident broad diffraction peak located at about 25° could be attributed to the (002) plane of ordered graphitic structure. The sharp peak at about 45° was the (110) plane of iron, reduced by the hydrogen–argon mixture gas. Two broad diffraction peaks at about 35° and 43° were derived from the (311) and (222) planes of Fe₃O₄ [35]. With the elongation of reaction time between BPN and Fe³⁺, the characteristic peaks of both iron and Fe₃O₄ were obviously enhanced. It is likely that penetration of Fe³⁺ into BPN and formation of metallosupramolecular polymers was time dependent. The effect of carbonization temperature on the crystalline structure of CSs was studied by XRD. CSs obtained from different carbonization temperatures generally had similar peak positions. However, with the carbonization temperature increased from 550 °C to 750 °C, the shape, width and intensity of the peak at 25° changed. Therefore, the carbonization temperature could affect the carbon matrix crystalline structure of CSs. It was observed that CS_12-650 prepared by the carbonization temperature of 650 °C had the best crystallinity.

The pore character of CSs was tested by physisorption of nitrogen at 77 K. As shown in Figure 3, CSs comprise both microporous and mesoporous structures. Detailed Brunauer–Emmett–Teller (BET) data is also listed in

Table 1. Surface area, porosity of CSs.

Samples	SBET a [m2 g−1]	Smicro b [m2 g−1]	Smeso + macro c [m2 g−1]	Vtotal d [cm3 g−1]
CS6-650	361.62	149.54	212.08	0.34
CS12-550	394.38	182.54	211.84	0.34
CS12-650	439.47	180.79	258.68	0.40
CS12-750	445.20	178.97	266.23	0.35
CS18-650	451.13	261.13	190.00	0.36

^a Specific surface area obtained from BET, ^b Surface area of micropores calculated by the t-plot method, ^c Surface area of mesopores and macropores calculated by the t-plot method, ^d Total pore volume.

. The surface areas of CS_6-650, CS_12-650 and CS_18-650 are 361.62, 439.47 and 451.13 m² g⁻¹, with relative pore volumes of 0.34, 0.40 and 0.36 cm³ g⁻¹, respectively. Obviously, their surface areas mainly resulted from the microporous- and mesoporous-pore structures. The specific surface area of the CSs increased gradually with the increase of reaction for the generation of the precursor. The mesoporous-pore volumes of CS_6-650, CS_12-650 and CS_18-650 were 212.08, 258.68 and 190.00 m² g⁻¹, respectively. Apparently, CS_12-650 had the maximum mesoporous-pore compared with CS_6-650 and CS_18-650. The mesoporous-pore is important and beneficial for ORR. The surface areas of CS_12-550 and CS_12-750 were 394.38 and 445.20 m² g⁻¹, with relative pore volumes of 0.34 and 0.35 cm³ g⁻¹ (Table 1), respectively, which were lower than that of CS_12-650 and CS_18-650. This result indicated that the precursor might be not completely carbonized at 550 °C, and a carbonization temperature of 750 °C was not helpful for the development of the pore structure.

The X-ray photoelectron spectroscopy (XPS) survey spectra of CS_6-650, CS_12-650 and CS_18-650 are shown in Figure 4a. In addition, the high-resolution XPS spectra of C 1s, N 1s, B 1s, F 1s and Fe 2p of CS_12-650 were characterized (Figure 4b–f). The C 1s signal can be split into four representative peaks at about 288.2 (C=O), 285.4 (C–O, C–N), 284.4 (C–C, C=C) and 283.6 eV (C–B) (Figure 4b). The six peaks of N 1s spectrum shown in Figure 4c at 403.2, 401.3, 400.3, 399.4, 398.3 and 397.7 eV are attributed to oxidized N, graphitic N, pyrrolic N, amine, pyridinic N, as well as N–B [23]. Notably, pyridinic N was recognized to produce excellent oxygen reduction reaction activity [24]. Three typical peaks of B 1s at 192.1, 190.8 and 189.2 eV were assigned to B–O, B–N, B–C (Figure 4d), separately [36]. The F 1s signal is displayed in Figure 4e. This indicated that fluorine remains in the carbon matrix. The weak peak at 685.7 eV was assigned to the F⁻ anions. The existence of the F element had the ability to accelerate the oxygen reduction reaction process of carbon catalysts [37]. For the Fe 2p spectrum shown in Figure 4f, the peak at 723.2 eV was assigned to the binding energy of Fe²⁺, and the peak for Fe³⁺ was detected at 725.4 eV for the 2p_1/2 band. Another two peaks at 714.4 and 710.5 eV could be respectively attributed to the binding energies of the 2p_3/2 orbitals of Fe³⁺ and Fe²⁺ species [38]. The Fe 2p_3/2 peak at approximately 703.5 eV corresponded to Fe⁰. The last peak at 719.6 eV was a satellite peak. The XPS results, in combination with the XRD results, clearly confirmed that only a small amount of iron element was transformed into Fe₃O₄, and most of iron element was changed into zero-valent iron, which may be of potential in improving the electrical conductivity of the carbon materials.

The XPS survey spectra of CS_12-550 and CS_12-750 are shown in Figure S2. CS_12-550 obtained at 550 °C had F content of 0.94%. However, at carbonization temperature of 750 °C, the prepared CS_12-750 comprised no fluorine element (Table S1). Because the C–F bond in the precursor accorded to a regular fracture, the temperature of 650 °C was the highest temperature that could retain the fluorine element in the carbon matrix, as well as endow the materials with adequate carbonization.

2.3. ORR Performances

We then tested the electrochemical performance of the CSs to evaluate their potential as ORR electrocatalysts. The cyclic voltammetry (CV) curves of CS_6-650, CS_12-650 and CS_18-650 were tested in Ar- or O₂-saturated 0.1 M KOH solution (Figure 5a). All samples only displayed an obvious reduction peak in O₂-saturated solution. The linear sweep voltammetry (LSV) curves of CSs are shown in Figure 5b–d. The onset potential (E_onset) of CS_6-650, CS_12-650 and CS_18-650 for the oxygen reduction reaction were 0.83 V, 0.91 V and 0.78 V, respectively. The half-wave (E_half-wave) potentials of CS_6-650, CS_12-650 and CS_18-650 for the oxygen reduction reaction were 0.79 V, 0.82 V and 0.76 V. This result was also confirmed by a LSV test in the O₂-saturated 0.1 M KOH solution, at a rotation rate of 1600 rpm, with a scan rate of 10 mV/s (Figure 5e). On the other hand, the catalytic activity of the commercial 20 wt% Pt/C was tested under identical experimental conditions (Figure 5e). In comparison, the E_onset and E_half-wave of CS_12-650 were 0.91 V and 0.82 V, respectively, which were close to that of the commercial 20 wt% Pt/C catalyst with the onset and half-wave potentials of 0.95 V and 0.85 V vs. RHE.

The evidently improved electrocatalytic performance of CS_12-650 could be explained by the following two reasons. First, CS_12-650 had relatively higher specific surface area compared with CS_6-650 as proved by BET results, thus leading to the exposure of more active sites. CS_12-650 also had a higher content of mesoporous-pore than CS_6-650 and CS_18-650, and the mesoporous-pore was beneficial for ORR. Second, CS_12-650 had relatively thin shell thickness and a regular hollow morphology, which was beneficial for the transfer of water and oxygen during the catalytic reaction. As illustrated by the LSV results, the second platform of the LSV curves of CS_12-650 became very flat in comparison with CS_18-650, indicating the equilibrium of the catalytic reaction. As the morphology of CS_18-650 was completely damaged and collapsed, the irregular structure could have increased the specific surface area of CS_18-650 to some extent, but also could have prevented the generation and loading of active sites during carbonization. The activity of CS_18-650 was thus reduced. Therefore, we consider that a synergistic effect between surface area, shell thickness, morphology and enough available active sites, directly led to the optimization of ORR catalytic activity of CS_12-650.

The LSV curves at different rotating rates were tested to study the reaction kinetics of the ORR catalyzed by CSs. The current density of CS_6-650, CS_12-650 and CS_18-650 increased when increasing the rotating rate from 400 to 2500 rpm, as shown in Figure 5b–d. This could be attributed to the decrease of the diffusion distance. The Kouteckye–Levich (K–L) plots were calculated from the above LSV curves, which revealed a clear linear relationship and quite similar slopes at the corresponding potential, ranging from 0.3 V to 0.5 V (Figure S3a–c). Obviously, the ORR catalyzed by CSs accorded classic first-order reaction kinetics. By calculating underlying scope shown in Figure 5f, the electron transfer numbers (n) derived from the corresponding slopes of Kouteckye–Levich (K–L) plots were approximately 3.94 for CS_6-650, 3.96 for CS_12-650 and 3.88 for CS_18-650, verifying an atypical four-electron transfer process. This is the same as the many reported B and N elements co-doped carbon materials [23,24,39].

The CV curves of CS_12-550 and CS_12-750 were also tested in Ar- or O₂-saturated 0.1 M KOH solution (Figure 6a), and the corresponding LSV curves of CS_12-550 and CS_12-750 are shown in Figure 6b,c. The onset potential (E_onset) of CS_12-550 and CS_12-750 were 0.85 V and 0.86 V. The half-wave (E_half-wave) potentials of CS_12-550 and CS_12-750 were 0.74 V and 0.77 V. In comparison, the E_onset and E_half-wave of CS_12-650 were 0.91 V and 0.82 V, respectively, which were much better than the CS_12-550 and CS_12-750. The electron transfer numbers (n) derived from the corresponding slopes of Kouteckye–Levich (K–L) plots (Figure S3d–f) were approximately 3.26 for CS_12-550 and 3.56 for CS_12-750, verifying a typical four-electron transfer process.

Since they were derived from the same precursor, we considered that the carbonization temperature could evidently affect the ORR performance of the CSs. First, CS_12-650 had higher I_D/I_G value than CS_12-550 and CS_12-750, implying more defect sites in the carbon matrix. Second, CS_12-650 had relatively higher specific surface area relative to CS_12-550, as proved by BET results, and thus lead to the exposure of more active sites. The specific surface area of CS_12-650 and CS_12-750 was nearly the same, so CS_12-650 and CS_12-750 had the same utilization towards the active site in the same condition. Third, comparing CS_12-650 and CS_12-750, the existence of the fluorine element in CS_12-650 was important for the improved ORR performance of CS_12-650. Therefore, we considered that the fluorine element in the carbon matrix could promote the generation of defect sites, which was beneficial and important for the ORR activity.

The durability of CS_12-650 was tested at 1600 rpm in O₂-saturated 0.1 M KOH aqueous solution after 1000 cycles of CV curves (Figure 7a). The onset and half-wave potentials of CS_12-650 were kept at 0.90 V and 0.80 V. No evident current decrease in the onset and half-wave potentials was observed. The attenuation of electrocatalytic activity was less than 5%, which was much better than the commercial 20 wt% Pt/C catalysts (as shown in Figure S4). Thus, CSs had preferable durability for ORR in the alkaline condition. The relevant crossover effects test was also carried out by taking CS_12-650 as an example through a chronoamperometric experiment. After adding 3.0 M methanol, the methanol oxidation reaction made the current density of the commercial Pt/C decrease immediately (Figure 7b). However, the current density of CS_12-650 did not display an evident change. These results directly confirmed that CS_12-650 has good catalytic selectivity for the oxygen reduction reaction.

3. Materials and Methods

3.1. Materials

Iron(III) chloride hexahydrate, 4-formylphenylboronic acid and 3,4-dihydroxybenzaldehyde were purchased from Aladdin Company (Shanghai, China) and directly used as received. Tris(4-aminophenyl)amine was obtained from J&K Chemical (Beijing, China). KOH, anhydrous methanol and anhydrous ethanol were supplied by Shanghai Chemical Reagent Industry (Shanghai, China). Nafion (5 wt%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Boronic monomer TBB was synthesized through our reported method [23]. The catechol monomer DFC was directly synthesized through the Schiff base formation reaction between 4,4′-(((perfluoropropane-2,2-diyl)bis(4,1-phenylene))bis(oxy))dianiline and 3,4-dihydroxybenzaldehyde (detailed procedure is described in Supporting Information).

3.2. Catalyst Preparation

DFC and TBB were dissolved in methanol to afford a concentration of 1.0 mg/mL. TBB solution (6 mL, 0.087 mmol) was added into DFC solution (10 mL, 0.132 mmol) dropwise under N₂ atmosphere with vigorous stirring. The mixture solution became deep orange and a suspension of boronate polymer nanospheres (BPN) formed. BPN powder was obtained by centrifugation and washed with anhydrous methanol three times.

BPN was redispersed in 16 mL of methanol to get 2.0 mg/mL of particle suspension. Then 2.37 mL of methanol solution of FeCl₃ (10 mg/mL, 0.037 mmol/mL) was added to the suspension of BPN. After different reaction times, such as 6, 12 and 18 h, the resultant hollow particles were collected by centrifugation, washed by anhydrous methanol, and then dried in vacuum oven at 50 °C overnight. The hollow particles were firstly carbonized at 650 °C for 2 h in an argon atmosphere with a fixed heating rate of 5 °C /min. The second carbonation was performed at 650 °C for 1 h in a mixture gas containing 5% hydrogen and 95% argon with a fixed heating rate of 10 °C /min to reduce Fe³⁺ into Fe, thus forming B, N, F and Fe co-doped hollow carbon nanospheres (denoted as CSs). CS_6-650, CS_12-650 and CS_18-650 represent carbon shells prepared from 6, 12 and 18 h reaction times between BPN and FeCl₃, respectively. To evaluate the carbonization temperature on the electrochemical properties of carbon shells, control experiments were performed at pyrolysis temperatures of 550 and 750 °C, and the prepared samples were denoted as CS_12-550 and CS_12-750.

3.3. Characterization

The scanning electron microscopy (SEM) were characterized using an SU-70 microscope (HITACHI, Tokyo, Japan). The morphology of the samples was tested by transmission electron microscopy (JEM-2100) (JEOL, Tokyo, Japan). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, elemental energy-dispersive X-ray spectroscopy (EDX) mapping and line scanning analyses were acquired on a FEI TECNAI F20 microscope (Hillsboro, OR, USA) tested at 200 kV. The Raman spectra were tested on a Labram HR800 Evolution (Horiba, Lille, France). The powder X-ray diffraction (XRD) patterns were obtained through a desktop X-ray Diffractometer using Cu (600 W) Kα radiation (Rigaku, Tokyo, Japan) to characterize the crystallographic structure of the samples. The Brunauer–Emmett–Teller (BET) surface area and pore volume of the samples were tested through an ASAP 2460 system (Norcross, GA, USA). Before measurement, all of the samples were uniformly degassed at 120 °C for 12 h under vacuum before the test. The X-ray photoelectron spectroscopy (XPS) was tested by PHI Quantum-2000 photoelectron spectrometer (Physical Electronics, Inc., Chanhassen, MN, USA) through a monochromatic Al X-ray source of Kα radiation (1486.6 eV), and all of the spectra had been calibrated with the C 1s peak at 284.6 eV as an internal standard.

3.4. Electrochemical Measurements

The electrochemical properties of the samples were measured on an electrochemical workstation (CHI 760E), through a conventional three-electrode system. Before testing, 5.0 mg of CSs was dispersed in 1.0 mL of a mixed solvent containing anhydrous ethanol (500 μL), H₂O (450 μL) and 5 wt% Nafion (50 μL). The above slurry (4.5 μL) was dropped onto a glassy carbon electrode and used as working electrode. The quality of the commercial 20 wt% Pt/C was half of the CSs catalyst. ORR performance was tested in freshly made KOH aqueous solution (0.1 M) at room temperature. Pt foil and an Ag/AgCl (KCl saturation) electrode were used as the counter electrode and the reference electrode, respectively. The potential in this article was relative to the Ag/AgCl electrode.

4. Conclusions

In summary, a new type of carbon shell co-doped with multi-element including B, N, F and Fe was designed and synthesized. The morphology and the content of the Fe element of the carbon shell could be easily tuned by changing the reaction time between the boronate polymer and Fe³⁺. The CS_12-650 catalyst showed excellent catalytic activity toward ORR (E_onset = 0.91 V, E_half-wave = 0.82 V vs. RHE), which were comparable to commercial Pt/C in an alkaline system. We verified that the existence of the fluorine element in the carbon matrix was important for the improvement of ORR performance. From the prospective of methodology, we consider that the Kirkendall effect-based route to metallosupramolecular polymer hollow spheres may be of great interest in the design of precursors incorporated with transition metal elements, thereby generating high-performance doped carbon materials.