1. G ENERAL I NTRODUCTION
1.3. M–N/C E LECTROCATALYSTS
1.3.2. Preparation of M–N/C Electrocatalysts
General preparation methods for Fe−N/C catalysts involve high-temperature (600−1100 °C) pyrolysis of a mixture of Fe, N, and C precursors because pyrolysis has been proven to be essential for endowing conductivity and structural integrity to Fe−N/C catalysts.14,67,71 However, numerous Fe atoms aggregate into less active, large Fe-based particles during pyrolysis, thus decreasing the mass activity. Furthermore, these Fe-based particles are frequently encapsulated by graphitic carbon shells generated in situ via an Fe particle-catalyzed Fischer−Tropsch reaction, rendering them resistant against acid etching.75 Hence, the development of synthetic routes toward Fe−N/C catalysts with predominant Fe−Nx sites has remained a significant challenge. A notable method is based on the thermal conversion of metal−organic frameworks (MOFs); Li and co-workers employed a zeolitic imidazolate framework (ZIF-8) with a cavity diameter of 11.6 Å and a pore diameter of 3.4 Å as a molecular-scale cage.40 They systematically designed a Zn/Co bimetallic MOF with a homogeneous distribution of Zn and Co considering the same sodalite coordination of Co2+ and Zn2+ with 2‐methylimidazole (Figure 1.13a). The intentional addition of Zn2+ replaces a certain proportion of Co2+ sites and serves as a “fence” to further expand the adjacent distances of Co atoms. Owing to the low boiling point (bp 907 °C), Zn atoms are evaporated at high temperatures over 800 °C, and Co nodes are reduced in situ by carbonization of the organic linker. When the molar ratio of the added
16
Zn/Co is higher than 1:1, Co single atom/nitrogen‐doped porous carbon (Co SA/N‐C) are successfully synthesized. The ORR onset potential and half-wave potential of Co SAs/N‐C(900) were better than those of Co NPs/N‐C and commercial Pt/C catalysts (Figure 1.13b).
Figure 1.13. (a) Schematic of preparation of Co SAs/N–C catalyst in presence of Co–Nx sites derived from ZIF-67. (b) ORR polarization curves of Co SAs/N–C, Co NPs/N–C, and Pt/C in 0.1 M KOH. (c) Schematic of CNT/PC catalyst fabrication using silica-protective-layer-assisted strategy. (d) ORR polarization curves of CNT/PC measured in 0.1 M KOH and 0.1 M HClO4. Reproduced with permission from refs. 40 and 38, respectively. Copyright © 2016 John Wiley & Sons, Inc. Copyright
© 2016 American Chemical Society.
The “silica coating” strategy has been commonly used in catalysis for mitigating the sintering of catalytic particles under high-temperature or harsh reaction conditions; however, Joo and co- workers first proposed a “silica-protective-layer-assisted” strategy that could preferentially generate catalytically active Fe−Nx sites.38 The catalyst synthesis involved adsorption of a porphyrinic precursor on CNTs, silica layer overcoating, high-temperature pyrolysis, and silica layer etching, yielding the nanocomposite structure of CNTs coated with a thin layer of a porphyrinic carbon (CNT/PC) (Figure 1.13c). XAS and Mössbauer spectroscopy results confirmed that the CNT/PC catalyst contained a higher density of active Fe−Nx sites than that without silica coating (CNT/PC_w/o SiO2). The CNT/PC catalyst outperformed CNT/PC_w/o SiO2 in both acidic and alkaline electrolytes, indicating that the silica protective layer plays a critical role in enhancing the
17
ORR activity. The kinetic parameters of CNT/PC were higher than those of most catalysts previously reported in the literature.38 These results suggest that promoting the formation of Fe−Nx sites while suppressing the formation of Fe-based particles is a key factor for enhancing the ORR activity.
The addition of heteroatoms has also been proven effective in tuning the ORR activity of M–
N/C catalysts. Nitrogen atoms in Fe–Nx structures act as anchoring sites to stabilize the metal center and play an important role in modulating the electronic structures of the active sites.76 Therefore, the electrocatalytic activity can be finely tuned by simply altering the electronic structures of the active sites. In particular, doping heteroatoms, such as sulfur and phosphorus, in the central structures can form different coordination environments, which can significantly affect the electronic structure of the central metal atoms and further improve the electrocatalytic activity.77−85 For instance, Lee et al.
demonstrated a strategy for tuning and enhancing the kinetic activity of a single Fe–N4 site by controlling the electron-withdrawing/donating properties of a carbon plane with the incorporation of sulfur functionalities.82 Electron-withdrawing oxidized S functionalities induced an increase in the ORR activity of the Fe–N4 sites, whereas electron-donating thiophene-like S functionalities causes a decrease in the ORR activity. The change in the ORR activity was found to originate from the electronic effect of the sulfur functionality, instead of from the change in the carbon crystallinity, number of active sites, or catalytic site structure. Furthermore, the Wang group distinguished structural differences between Fe-centered single-atom catalysts (Fe-SAs/NSC) and Co-centered/Ni- centered single-atom catalysts (Co-SAs/NSC and Ni-SAs/NSC) based on the different trends the metal ions in forming complexes with the N, S-containing precursor during the initial synthesis process.83 The Fe-SAs/NSC mainly consisted of a well-dispersed Fe–N4S2 center site where the S atoms form bonds with the N atoms. In comparison, the S atoms in Co-SAs/NSC and Ni-SAs/NSC form metal−S bonds, resulting in Co–N3S1 and Ni–N3S1 center sites. DFT and experimental results indicated that the Fe–N4S2 center site was more active than the Co–N3S1 and Ni–N3S1 sites owing to the higher charge density, lower energy barriers of the intermediates, and products involved.
Studies to identify new active Fe–N/C catalysts in the last decades have relied mainly on empirical approaches involving systematic changes in both the elemental precursors and synthetic conditions and their correlation with the resulting kinetic current density (Jkin) of the catalysts.86–89 This approach achieved some success in the early stages of Fe–N/C catalysts development; however, it now appears to have saturated. Novel and more rational approaches are needed to clarify the contributions of different Fe-based active sites to the overall activity and durability of Fe–N/C catalysts. Specifically, the methods must identify the most active or most durable sites, and synthetic strategies must be developed to optimize the number of such sites selectively.90,91 The first step toward this objective is the development of experimental methods that evaluate the number of Fe-based catalytic sites located at the catalysts surface (site density, SD). The SD can subsequently be combined with Jkin and the elemental electric charge, e, to extract the average intrinsic turn over
18
frequency (TOF) of the Fe-based active sites in a given Fe–N/C catalyst.34
(1) The TOF and the SD are fundamental descriptors of catalytic reactivity and can provide guidelines for synthesizing new active catalysts. Efforts to improve the overall activity of a catalyst based on equation (1) can focus on synthetic strategies to increase the SD or the intrinsic TOF of the active sites. To accurately calculate the SD of Fe–N/C experimentally, Strasser, Kucernak, and Choi groups reported experimental ex situ and in situ Fe quantification methods using probe materials such as CO, NO2−, and CN−.34,92–95 Strasser and co-workers demonstrated the Fe site utilization factor derived from both experimental CO and NO2−-based SD values.94 Site utilization factors can be used as guidelines for synthetic optimization of catalyst morphologies.