Exploiting Asymmetric Co States in a Co-N-C Catalyst for an Efficient Oxygen Reduction Reaction

Abstract

Co-NC catalysts have attracted extensive concerns derived from their high oxygen reduction reaction (ORR) activity, but the catalytic mechanism of Co species with different forms remains controversial. Herein, we prepare Co-NC catalysts with a cobalt nanoparticle-supported and nitrogen-doped carbon structure using the ZIF-67 precursor, in which the Co states in the catalyst present an asymmetric state of an exposed carbon coating (Asy-Co) and a symmetric state of buried carbon (Sy-Co). The acid etching process removed the exposed asymmetric cobalt nanoparticles on the surface. The specific role of cobalt nanoparticles with different forms in the Co-NC catalysts was comprehensively clarified through analyzing the chemical coordination environment by XPS and XAFS. The half-wave potential (E_1/2 = 0.83 V) and onset potential (E_on = 1.04 V) of the Co-NC catalysts obtained after acid etching decreased significantly. Thus, the cobalt species removed by the acid etching process offered confirmed contributions to the catalytic activity. This work puts forward an important reference for the design and exploitation of non-noble metal catalysts using symmetry-derived motifs.

1. Introduction

Owing to their highly efficient energy conversion and virtually no emissions, fuel cells and zinc batteries have received intensive research and development attention [1]. The cathodic occurrence of an oxygen reduction reaction (ORR) exhibits slow reaction kinetics, which limits the dynamic of the catalytic reaction. Noble metal catalysts including Pt-based metals hold excellent ORR activity, but their disadvantages such as a high cost, poor stability and poisoning seriously hinder their large-scale application in practical production [2,3,4,5]. In recent years, transition metals (i.e., Co) have developed a solid research foundation in ORRs attributed to their low cost, high catalytic activity and high stability, being one of the promising substitutes to noble metal catalysts [6,7,8].

Cobalt (Co) is considered to be a promising transition metal in ORR due to its highly delocalized d-band electron configuration (3d⁷ 4s²). In the development from metal macrocyclic compounds to transition metal carbon-based catalysts, the catalytic activity and stability of Co-NC have been continuously improved through considerable modification of the preparation methods. For Co-NC catalysts, metallic cobalt species exist in two forms: exposed Co on the surface and encapsulated Co within the carbon layers. It has been recognized that Co-NPs encased in carbon layers can effectively activate the inert electronic structure of carbon layers through the electron shuttle effect and act as the active sites of reaction. In most preparation methods for Co-NC, the Co-NPs exposed on the surface are usually removed by pickling [9,10]. It was found that bare Co-NPs may significantly reduce the utilization of a metal’s active components and are not conducive to the enhanced catalytic activity [11]. In fact, whether exposed Co-NPs are profitable to the enhanced catalytic activity still needs to be clarified. Metal–organic framework (MOF)-derived nitrogen-doped carbon composites have recently been widely taken as ORR catalysts [8,12,13,14,15,16]. The unique structural advantages are conducive to supporting metal sites with different forms on a support, which can be used as model catalysts to pinpoint the mechanism of different metal components in a catalytic system.

Herein, we use ZIF-67 as a precursor to synthesize ZIF800H-Anneal catalysts with a nitrogen-doped carbon skeleton structure supported by metal cobalt nanoparticles. The pickling and secondary pyrolysis processes remove cobalt species from the catalyst surface and further repair the damaged carbon structure. The annealing temperature plays an important effect on determining the existence form of cobalt species, and the optimal carbonization temperature is 800 °C. Various experimental characterization and electrochemical tests have verified the mechanism of asymmetric and symmetric cobalt states in the catalyst on the catalytic activity. It provides new ideas for and insights into the development and exploitation of catalysts with high activity and stability.

2. Results and Discussion

Typical ZIF-67 was prepared and annealed at different temperatures. The preparation methods are exhibited in Supporting Information. As shown in Figure S1, ZIF-67 precursor was annealed at 800 °C in an Ar atmosphere (ZIF800), etched by acid and annealed again to eliminate the surface functional groups introduced by the acid etching. As shown in Figure S2a, the ZIF-67 precursor exhibited a rhombic dodecahedron structure. After high-temperature annealing, the catalyst surface shrank, and a large number of nanoparticles was generated, as shown in Figure S2b–e. With the increased annealing temperature, the grain size of the nanoparticles on the surface gradually increased and agglomerated, and the degree of skeleton collapse became obvious gradually. The cobalt nanoparticles seriously reunited at higher temperature (1000 °C), and the skeleton broke down (Figure S2e).

Transmission electron microscopy (TEM) was utilized to further analyze the microstructure and phase information of the catalysts. Figure 1a,d shows typical low-magnification TEM images of the ZIF800 and ZIF800H-Anneal. The grain size of the ZIF800H-Anneal was smaller than that of the ZIF800, which also confirmed the porous structure of the ZIF800H-Anneal after annealing. Specifically, cobalt species can be divided into larger-sized asymmetric (exposed and buried by carbon) Co (Asy-Co) species (~40 nm) and smaller-sized symmetric (fully buried by carbon) Co (Sy-Co) species (~15 nm). These graphitic layer-coated Sy-Co nanoparticles are inaccessible to hydrochloric acid because they are still encapsulated completely in the graphitized carbon shell. In addition, high-resolution TEM (HRTEM) images were used to characterize the phase structures for the cobalt species (Figure 1b for the ZIF800 and Figure 1e for the ZIF800H-Anneal). Figure 1b shows that the Sy-Co nanoparticles by carbon coating had lattice stripes corresponding to the Co (111) plane and an interplanar spacing of 0.211 nm, which is well matched to the (111) plane of Co (PDF #15-0806) in the Fm-3m space group [17]. As shown in Figure 1c, the EDS spectrum demonstrated a uniform distribution of Co, N and C. After acid etching and annealing, the crystal plane spacing of the ZIF800H-Anneal was reduced. For example, the crystal plane spacing of Co (111) reduced from 0.211 to 0.204 nm. It might be due to the oxidation of a small number of Asy-Co particles near the surface, which were exposed to oxygen and partly oxidized into cobalt oxide during the first annealing. However, the acid etched the exposed Asy-Co species and cobalt oxide species, but the acid could not contact the carbon-coated Sy-Co species. Figure 1f shows the EDS elemental mapping of the ZIF800H-Anneal, and Co, N and C were distributed uniformly.

Figure S2f shows SEM image of the ZIF800H-Anneal. A large number of Asy-Co species was removed from the ZIF800, leaving only a small amount of Sy-Co species embedded in the graphitic layers after acid treatment. The morphology of the ZIF800H-Anneal samples obtained by a second annealing treatment remained the same as that before the pickling treatment, which proved that the second annealing treatment was conducive to the further repair of damaged carbon structures (Figure S2c). The phase structures for the samples was investigated by the X-ray diffraction (XRD) measurements. The XRD patterns of the ZIF700, ZIF800, ZIF800H-Anneal, ZIF900 and ZIF1000 in Figure S3 exhibited obvious diffraction peaks at 44.5°, 51.8° and 76.1°, which could be ascribed to the 111, 200 and 220 planes of Co (PDF#15-0806), respectively. In particular, the diffraction peak of graphite (PDF#75-1621) appeared in the ZIF800H-Anneal. This may be due to functional groups produced by etching carbon surface during the secondary annealing process, leading to the transition from amorphous carbon to graphite, which was consistent with Figure 1e. EDS analysis in SEM (Figures S4 and S5, Tables S1 and S2) of the ZIF800 and ZIF800H-Anneal showed that the content of Co species in the ZIF800 decreased from 22.71 wt.% to 6.96 wt.% in the ZIF800H-Anneal. EDS analysis in TEM (Figures S6 and S7 and Tables S3 and S4) showed that the content of Co species in the ZIF800 decreased from 34.9 wt.% to 5.5 wt.% in the ZIF800H-Anneal.

The coordination environment of the cobalt species of the ZIF800 and ZIF800H-Anneal were further investigated using the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques [18]. Figure 2a exhibited that Co species existed in the oxidation state. After pickling and the second annealing treatment, the near-edge absorption position shifted to the right, which proved that the valence of the Co species had been improved. As shown in Figure 2b,c, the Co-Co spacing became smaller, which was consistent with the HRTEM results. It might be due to the removal of cobalt species on the surface after acid etching. In the annealing process, the formation of CNTs produces compression stress, which results in a decrease in the Co-Co spacing [19].

X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemical state of C, N and Co. The Co 2p spectra in Figure 2d,g can be divided into Co 2p^1/2, Co 2p^3/2 and the corresponding satellite peaks. It proved that a certain oxidation state of metallic cobalt existed in the catalyst. Compared with the ZIF800, a chemical shift occurred in the ZIF800H-Anneal after treatment, indicating that the Co valence state increased. After etching, the content of Co was reduced from 1.1% to 0.5% (Figure S8, Tables S5 and S6). It meant that etching removed the exposed Co-NPs from the surface. The catalysts were found to contain the following species in the N 1s spectra: pyridine N (398.4 eV), Co-N (399.6 eV), pyrrolic N (400.2 eV), graphitic N (401.2 eV) and oxide N (403.5 eV) [20], as shown in Figure 2e,h. After pickling and secondary pyrolysis, the content of the Co-N species increased, while the content of the pyridine N was significantly decreased. It might be because by using XPS spectroscopy, one can detect the species on the catalyst’s surface. The C 1s spectrum in Figure 2f,i also confirmed the existence of four carbon-containing species such as C=C (284.7 eV), C-C (285.2 eV), C-O (285.8 eV) and C=O (287.5 eV) in the catalyst [9,21,22]. The sp² C of the ZIF800H-Anneal increased after acid etching and annealing, which may be due to the fact that the second annealing increased the graphitization degree of the catalysts, thus promoting the efficient electron transfer in the catalytic process.

To evaluate the ORR performance, linear sweep voltammetry (LSV) experiments were conducted in a 0.1 M KOH aqueous solution at 25 °C. The ORR activity of the catalysts obtained at different annealing temperatures (700–1000 °C) was analyzed in Figure 3a, which was completed at 1600 rpm. It is obvious that the ZIF800 samples unveiled the highest limiting current density of 4.28 mA cm⁻², while the onset potential and half-wave potential were 1.04 V and 0.83 V (vs. RHE), respectively. With the annealing temperature increasing from 600 °C to 800 °C, the ORR activity showed a gradual increasing trend. It was due to the high-temperature annealing forming more active cobalt species. When the temperature continued to increase to 1000 °C, the ORR activity of the catalyst showed a significant downward trend. Severe collapse of the carbon carrier and greater aggregation of the cobalt nanoparticles resulted in a decrease of active sites. The half-wave potential of the ZIF800H-Anneal catalyst obtained after pickling and a second annealing treatment significantly decreased in Figure 3b, which proved that the surface cobalt species removed during the pickling process contributed to the catalytic activity. The LSV curves of different speeds in Figure 3c proved that the catalytic reaction had first-order kinetics related to the diffusion process. Figure 3d showed the corresponding Koutecky–Levich (K–L) plots derived from the LSV curves. The results manifested that the reaction path of the ZIF800 was a direct four-electron reaction path. Next, the durability and resistance to poisoning of the catalyst were analyzed, as shown in Figure 3e,f. In the I-t stability test, it was found that the current density of the ZIF800 only decreased by 6.8% after 35,000 s of testing. It proved that the catalyst exhibited good stability in an alkaline system. By TEM (Figure S9), HRTEM (Figure S10) and XPS (Figure S8) analyses of the cycled catalysts, we found that the surface Asy-Co species were partially transformed into CoOOH species, which may continue to act as the reactive site to maintain catalytic activity and stability. The Sy-Co species coated with graphitic layers avoided direct contact with the external electrolytes, thus maintaining high stability. In the I-t test, the current density did not change significantly after 10 mL of methanol was dropped in, indicating that the ZIF800 had a good ability to resist methanol poisoning.

3. Conclusions

In summary, we utilized ZIF-67 as a precursor to prepare the ZIF800H-Anneal catalyst by high temperature annealing and acid etching. Electron microscopy combined with spectral analysis was used to analyze the catalytic effects of Co species with different forms in the catalysts. As the temperature continued to rise, cobalt atoms were aggregated, and the ZIF-67 structure was destroyed. Electrochemical tests proved that the Asy-Co species on the surface removed by pickling also played a positive effect on the catalytic activity. The ZIF structure-derived carbon support possessed a rich pore structure, which was conducive to accommodating abundant active sites and promoting the catalytic mass transfer process.