Electrochemical oxygen reduction reaction (ORR) plays a crucial role in renewable energy applications, such as fuel cells and metal–air batteries.^1,2 For proton exchange membrane fuel cells (PEMFCs) and metal–air batteries, the typical electrocatalyst for ORR is the Pt-based precious metal catalyst. Pd is considered as a cheap alternative to the rare Pt catalyst because of its higher electrocatalytic activity and stability than that of other 3d transition metals (such as Ni, Co, and Fe) for ORR.³ Therefore, incorporating Pd NPs into N-doped carbon support matrix with high surface area and regular pore texture can result in a promising fuel cell cathode catalyst material. For this purpose, we have synthesized three types of KAPs (KAPs-Ph, KAPs-PhPh₃, and KAPs-NHC) from different monomers via a similar cross-linked reaction. These carbonized products can be used as typical metal supporting models for comparative investigation. Also, we examined Pd NPs loading on commercial Vulcan XC-72 to illustrate the superiority of such KAPs.

  We synthesized three types of KAPs (KAPs-Ph, KAPs-PhPh₃, and KAPs-NHC) from different monomers via a similar cross-linked reaction.⁴ The microporous carbon materials are obtained through directly pyrolyzing such KAPs at 800 ^oC for 4 h with a heating rate of 2 ^oC min^-1 and then cooling down to room temperature under N₂flow. The corresponding black carbon products are labeled as CKB, CKTB and CKN, respectively. The products from different temperature are also obtained for comparison.

  The surface areas and pore distributions of products are determined by nitrogen physisorption analysis. The N₂ adsorption-desorption isotherms of these samples are typical type I according to the IUPAC classification, indicating their microporous properties. The size and morphology of Pd NPs loading on three carbonized products are investigated by TEM observation. The lattice spacing of d = 0.22 nm in HRTEM images can be ascribed to the Pd(111) planes. As shown in Figure 1, the Pd NPs are well-dispersed on the surface of microporous carbon, the diameter of Pd NPs suppored on Pd/CKN is about 2 nm. Besides the reason of stable carbon skeleton, N-dopant induced defects can act as trapping sites for anchoring Pd NPs to prevent the aggregation.  

  The influences of framework and heteroatom of MOPs on the ORR catalytic activity of microporous carbonized products as well as carbonized products supported Pd composites are investigated. Among these materials, the CKN possesses the most positive half-wave potential (E_½) and highest  reaction current density because N-dopant altered the electronic property of carbon. The ORR performance of Pd loading on MOPs derived microporous carbon follows the similar order: Pd/CKN>Pd/CKTB>Pd/CKB. The highest catalytic activity is achieved in Pd/C system using CKN as support when compared with other MOPs derived microporous carbon supports (CKB, CKTB) and XC-72. The improvement of reactivity in Pd/CKN catalyst may be ascribed to the influence of N heteroatom on the electronic structure of carbon framework as well as that of metal NPs through metal/support interfacial  interactions. Meanwhile, N-doping will facilitate the charge transfer and electronic interactions between supports and metal NPs.

  In summary, we synthesized different types of MOPs via one-step ‘knitting’ of rigid aromatic building blocks with an external crosslinker and explored as novel carbon precursors to produce microporous carbon materials. The catalytic performance and durability of MOPs-derived carbon supported Pd NPs exhibit superior behaviors to those loading on commercial 5% Pd/C catalyst. Therefore, this work opens up new insights for the rational design and development of novel highly-efficient carbon for supporting metal or metal oxide for ORR by simply modulating the framework as well as electronic structure of such polymers.

  This work is supported by the National Natural Science Foundation of China (21571071 and 21473064), Hubei Provincial Natural Science Foundation of China (2015CFB313), the Fundamental Research Funds for the Central Universities (2015QN183), and the Special Fund of Harbin Technological Innovation talent (2013RFLXJ011).

References

1. J. Wu and H. Yang, Acc. Chem. Res., 46 (2013) 1848-1857.

2. F. Cheng and J. Chen, Chem. Soc. Rev., 41 (2012) 2172-2192.

3. L. Perini, C. Durante, M. Favaro, V. Perazzolo, G. Granozzi and A. Gennaro, ACS Appl. Mater & Inter., 7 (2015) 1170-1179.

4. B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan and T. Li, Adv. Mater., 24 (2012)  3390-3395.