Article pubs.acs.org/Langmuir
Polyaniline and Perfluorosulfonic Acid Co-Stabilized Metal Catalysts for Oxygen Reduction Reaction Bei Ye,† Kun Cheng,†,‡ Wenqiang Li,† Jing Liu,† Jie Zhang,† and Shichun Mu*,† †
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China ‡ Institution for Interdisciplinary Research, Jianghan University, Wuhan, 430056, China S Supporting Information *
ABSTRACT: A proton (perfluorosulfonic acid, PFSA) and electron (polyaniline, PANI) conductor polymer costabilized Pt catalyst (Pt-PFSA/C@PANI) is synthesized to improve the long-term stability of polymer electrolyte membrane fuel cells (PEMFCs). The prepared catalyst not only displays comparable oxygen reduction reaction (ORR) activity, but significantly higher electrochemical stability than commercial porous carbon nanosphere supported Pt catalysts (Pt/C). This robust electrochemical property can be due to the result of PFSA and PANI. PANI as protector inhibits carbon nanospheres from corrosion of carbon supports in harsh chemical and electrochemical conditions. Meanwhile, PFSA wrapped Pt NPs (Pt@PFSA) can also anchor Pt NPs on C@PANI to avoid aggregation and detachment of Pt NPs, due to the increased metal−support interaction caused by the strong electrostatic attraction between PANI and PFSA with corresponding positive and negative charges. Significantly, after coating PANI on carbon supports (C@PANI), almost all micropores in the surface of carbon disappear, effectively avoiding the embedding of Pt nanopaticles into micropores. Furthermore, the triple-phase boundary toward ORR catalysis can be facilitated by PFSA as proton conductor (solid electrolyte). These are of benefit to increase utilization of Pt noble metals and ORR activity of our new catalysts.
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differences in free energy).18−21 Besides, Pt NPs would migrate and coalesce on carbon supports during continual cycling.22−26 Moreover, the carbon oxidation can be accelerated under rigorous chemical and electrochemical circumstances, leading to detachment of Pt PNs from supports and decreasing the electrical conductivity of supports.27−31 Undoubtedly, all of these lead to degradation of Pt catalysts toward both ECSA and ORR.12,18,32 To allow for a better stability of catalysts, the migration, coalescence, and detachment of Pt NPs should be minimized, and the oxidation of carbon supports should be prevented to promote the ORR performance of catalysts. Currently, much effort has been devoted to solving such issues by immobilizing Pt NPs on supports. Some research introduced zirconia nanocages to encapsulate Pt NPs from migration.33 However, they have little contribution to protecting carbon supports. Previously, as reported modifying or substituting carbon nanospheres with oxidation-resistant materials such as ceramic (e.g., TiO2, TiB2, and SiC) greatly enhanced resistance to electrochemical oxidation of supports and then the durability of catalysts.34,35 However, for Pt NPs, their migration and
INTRODUCTION Polymer electrolyte exchange membrane fuel cells (PEMFCs) are one of the most potential energy conversion devices for their environment-friendliness and high energy conversion efficiency.1−4 Hitherto, in PEMFC systems the best-known commercial electrocatalyst is noble platinum (Pt) nanoparticles (NPs) with diameter of 2−5 nm supported on porous carbon Vulcan XC-72 (Pt/C), due to their outstanding high activity for ORR. However, the poor electrochemical stability of Pt/C becomes one of the most important obstacles to the large-scale implementation of PEMFCs.5−10 Some investigations of the degradation mechanism for Pt/C catalysts have been carried out, which can be attributed to the loss of the electrochemical surface area (ECSA) with loading cycling caused by the detachment of Pt NPs and migration/aggregation derived from the oxidation carbon supports or weak interaction between metal−support, and the Oswald ripening process under typical PEMFC operating conditions.11−14 Meanwhile, the porous carbon black (e.g., Vulcan XC-72), with large specific surface area and high electrical conductivity, is widely employed as support of Pt catalysts to ensure good dispersion of Pt NPs.15−17 However, highly dispersed ultrasmall metallic Pt NPs on support materials tend to dissolution and redeposition into larger grains for decreasing surface energy (the so-called Ostwald ripening phenomenon, driven by © 2017 American Chemical Society
Received: February 27, 2017 Revised: April 27, 2017 Published: May 11, 2017 5353
DOI: 10.1021/acs.langmuir.7b00642 Langmuir 2017, 33, 5353−5361
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Langmuir
Figure 1. (a) FTIR spectra of PANI, PFSA, PFSA-Pt/C, and PFSA-Pt/C@PANI. (b) The XPS spectra of PFSA-Pt/C@PANI, PFSA-Pt/C, and commercial Pt/C. (c) TEM images of Pt NPs wrapped with PFSA and (d) is the corresponding EDS pattern. The inset is the corresponding HRTEM. (e−g) AC-STEM images and corresponding EDS mappings of Pt NPs wrapped with PFSA.
Hence, to addresses the issues of both Pt degradation and carbon oxidation, we design and synthesize a proton (perfluorosulfonic acid, PFSA) and electron (polyaniline, PANI) conductor polymer costabilized Pt catalyst (Pt-PFSA/ C@PANI). PANI-coated carbon (C@PANI) as support is prepared by in situ polymerization of aniline monomers physically adsorbed onto the surface of carbon black (Vulcan XC-72) through the π−π interaction. The introduction of electron-conductive polyaniline can protect the carbon material from electrochemical oxidation and further enhance the interaction between C@PANI supports and PFSA wrapped Pt NPs. Meanwhile, PFSA serves as stabilizer mainly constrains migration, aggregation, and detachment of Pt NPs, and facilitates tripe-phase boundary. Thus, a significantly promotion toward electrochemical stability with good ORR activity is strongly expected for this new catalyst.
coalescence problems cannot be solved only by protecting supports and always at the expense of the electrical conductivity of supports. Recently, functional conducting polymers are widely concerned and developed due to their potential applications in the fields of corrosion resistance, energy conversion and storage, and sensors. Among conducting polymers, perfluorosulfonic acid (PFSA)34,36−38 and phenyl sulfonic acid,39 with high proton conductivity are used to stabilize electrocatalysts because of the enhanced interaction between metal−support. Besides, polyaniline (PANI) is a kind of conducting polymer with superior stability and high electrical conductivity in acidic environments and has been widely used in anticorrosion coatings, supercapacitors, and PEMFCs.40−42 But there are only a few studies that focus on the development of PANI as catalyst support. 5354
DOI: 10.1021/acs.langmuir.7b00642 Langmuir 2017, 33, 5353−5361
Article
Langmuir
Figure 2. (a, b) AC-STEM images and (c) corresponding EDS mappings of C@PANI.
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EXPERIMENTAL SECTION Synthesis of C@PANI Supports. A total of 100 mg of commercial carbon black (Vulcan XC-72 produced by Cabot Corp., BET = 250 m2 g−1) was dispersed in 100 mL of 0.5 M H2SO4 aqueous solution, after ultrasonic blending for 30 min, 50 μL of aniline monomer were dissolved in it. The mixture was then stirred ultrasonically at room temperature overnight to attain the uniform dispersion. After that, 50 mL of 0.5 M H2SO4 solution with 115 mg of (NH4)2S2O8 (ammonium peroxydisulfate) was introduced dropwise with vigorous stirring (the molar ratio of (NH4)2S2O8 to aniline was controlled at 1:1). The polymerization was performed in an ice bath (
