Synthesis, studies and fuel cell performance of “core–shell” electrocatalysts for oxygen reduction reaction based on a PtNix carbon nitride “shell” and a pyrolyzed polyketone nanoball “core”

https://doi.org/10.1016/j.ijhydene.2013.08.054Get rights and content

Highlights

  • New “core” supports for fuel cell electrocatalysts are prepared from polyketone-based precursors.

  • The supports are used to prepare bimetal “core–shell” ORR electrocatalysts.

  • The “core” supports are covered by a carbon nitride “shell” embedding PtNix active sites.

  • The chemical composition and degree of graphitization of the materials are studied.

  • The interplay between the morphology and fuel cell performance is elucidated.

Abstract

This report describes a new class of “core–shell” electrocatalysts for oxygen reduction reaction (ORR) processes for application in Proton Exchange Membrane Fuel Cells (PEMFCs). The electrocatalysts are obtained by supporting a “shell” consisting of PtNix alloy nanoparticles embedded into a carbon nitride matrix (indicated as PtNix-CN) on a “core” of pyrolyzed polyketone nanoballs, labeled ‘STp’. STps are obtained by the sulfonation and pyrolysis of a precursor consisting of XC-72R carbon nanoparticles wrapped by polyketone (PK) fibers. The STps are extensively characterized in terms of the chemical composition, thermal stability, degree of graphitization and morphology. The “core–shell” ORR electrocatalysts are prepared by the pyrolysis of precursors obtained impregnating the STp “cores” with a zeolitic inorganic–organic polymer electrolyte (Z-IOPE) plastic material. The electrochemical performance of the electrocatalysts in the ORR is tested “in situ” by single fuel cell tests. The interplay between the chemical composition, the degree of graphitization of both PtNix-CN “shell” and STps “cores”, the morphology of the electrocatalysts and the fuel cell performance is elucidated. The most crucial preparation parameters for the optimization of the various features affecting the fuel cell performance of this promising class of ORR electrocatalysts are identified.

Introduction

Fuel cells (FCs) are a family of open electrochemical energy conversion devices operating through the direct electro-oxidation of a fuel (e.g., hydrogen) at the anode and the reduction of an oxidant, usually the oxygen of air, at the cathode [1], [2], [3]. Particular attention has been attracted by a family of FCs relying on a proton exchange membrane (PEM) as the electrolyte; these systems are known as proton exchange membrane fuel cells (PEMFCs) [4]. PEMFCs are compact systems characterized by a very high energy conversion efficiency, up to 2–3 times larger in comparison with competing technologies such as internal combustion engines (ICEs) [5]. PEMFCs are particularly suitable to power light-duty electric vehicles and portable electronic devices such as audiovisual players and laptop computers [6], [7]. One of the most important bottlenecks in the operation of PEMFCs is the sluggish kinetics of the oxygen reduction reaction (ORR) [8]. Suitable electrocatalysts are required to promote the ORR kinetics in order to achieve an improved performance. As of today, the state of the art in ORR electrocatalysts for PEMFCs consists in nanocomposite materials including platinum nanocrystals supported on active carbons characterized by a large surface area and a high electronic conductivity such as XC-72R carbon black [9], [10]. These electrocatalysts afford a satisfactory performance; however, they are also very expensive owing to a significant loading of platinum (usually between 10 and 50 wt%) [11]. Furthermore, they suffer from an insufficient long-term durability [12], [13], [14]. These drawbacks are still major obstacles towards a widespread diffusion of the promising PEMFC technology. Several approaches are attempted to address the above issues. On one hand, ORR active sites showing an improved turnover frequency were prepared. In general, they include a platinum-group metal (PGM, e.g., Pt, Pd) [10], [15] alloyed with one or more first-row transition metals (e.g., Cr, Fe, Co, Ni, Cu) [16], [17], [18], [19], [20], [21], [22], [23], [24]. On the other hand, a variety of new supports were investigated, including carbon nanoparticles, graphene layers, and carbon nanotubes; some of these systems include suitable heteroatoms (e.g., N or S) to stabilize the active sites and improve the activity and durability of the electrocatalysts [25], [26], [27]. In the last decade, our research group proposed an innovative protocol to obtain electrocatalysts for application in fuel cells [28], [29]. A suitable hybrid inorganic–organic precursor is prepared, consisting of complexes of the desired metal atoms networked by an organic binder. The precursor undergoes a series of pyrolysis and activation steps, yielding the final electrocatalyst; the latter's stoichiometry is well-controlled and includes the desired concentration of heteroatoms (typically N). The obtained carbon nitride-based electrocatalysts present an improved performance and tolerance to oxidizing conditions [29], [30], [31]. The best results are reached in the electrocatalysts with a “core–shell” morphology [30], [32], [33]. In these systems, the hybrid inorganic–organic precursor is used to impregnate a suitable support before the pyrolysis and activation steps. Highly conductive XC-72R carbon nanoparticles (NPs) were adopted. In such “core–shell” morphology an improved dispersion of the active sites embedded in the carbon nitride “shell” was obtained, which resulted in a better fuel cell performance in comparison with the corresponding bulk electrocatalysts including active sites with the same stoichiometry [32].

In this paper, a new family of supports characterized by an innovative morphology and a large surface area is developed. The supports consist of nanoballs of carbon nanofibers with a diameter on the order of nanometers wrapping “core” carbon nanoparticles of ca. 30–50 nm in size. The supports are obtained in two steps: (1) polyketone (PK) fibers are grown on XC-72R carbon NPs acting as nucleation centers, giving so rise to “balls” showing a nanofibrous morphology; thus, the “support precursor” (SP) is obtained [34]; (2) SP undergoes sulfonation and multi-step pyrolysis procedures. The resulting supports are adopted as the “core” in the preparation of advanced “core–shell” ORR carbon nitride electrocatalysts. In these systems, the “core” is covered by a PtNix-CN “shell” based on PtNix alloy NPs embedded in a carbon nitride (CN) matrix. The “balls” with a nanofibrous morphology of SP are proposed as an innovative template for the modulation of: (a) the morphology and chemical composition of the supports; and (b) the activity and selectivity of the “core–shell” carbon nitride electrocatalysts, which are briefly indicated as PtNi-CNl Tf/STp (see the following discussion for more information). PtNi-CNl Tf/STp are studied in terms of composition, morphology and single-cell performance under operating conditions in order to obtain information on the most important preparation parameters to design next-generation ORR electrocatalysts characterized by an improved efficiency.

Section snippets

Reagents

Potassium tetrachloroplatinate (II), 99.9% and potassium tetracyanonickelate (II), hydrate are supplied by ABCR. D(+)-sucrose, biochemical grade is Acros reagent. EC-20 is received from ElectroChem, Inc. (nominal Pt loading: 20%) and used as the reference. In the following text, EC-20 is labeled “Pt/C reference”. Palladium (II) acetate, 98%, p-toluenesulfonic acid monohydrate, 98% and 1,3-bis(diphenylphosphino) propane are obtained from Sigma–Aldrich and in the following are labeled Pd(AcO)2,

Preparation observations

The proposed electrocatalysts are prepared in two steps, I and II, as shown in Scheme 1. In step I, the electron-conducting “core” (STp) is prepared, which is characterized by a high surface area. In detail, STp is synthesized as follows. First, the support precursor (SP) is obtained by copolymerization of CO with ethene in the presence of XC-72R carbon NPs suspended in a methanol/water solution (see the Experimental section). In this process the carbon NPs, which adsorb the polymerization

Conclusions

In this report two new STp supports, S500 and S700, are prepared by sulfonation and pyrolysis of a support precursor (SP) consisting of XC-72R carbon NPs embedded in a “ball” of PK fibers. The detailed studies carried out on the support precursors and STp “core” supports show that: (a) a successful incorporation of sulfur atoms in the chemical composition of the materials occurs; (b) the graphitization of SP becomes more evident as Tp is raised; and (c) a complex interplay takes place between

Acknowledgments

This research is funded by project “PRAT 2011” financed by the University of Padova, and by the Italian MURST project PRIN2008 Prot. 2008SXASBC_002. E. Negro thanks Regione del Veneto (SMUPR n. 4148, Polo di ricerca del settore fotovoltaico) for financial support.

References (53)

  • F. Benetollo et al.

    Synthesis, characterization and X-ray structure of [Pd(dppp)(H2O)(TsO)][TsO] (dppp = 1,3-bis(diphenylphosphino)propane; TsO = p-CH3C6H4SO3), a catalytic species in COC2H4 copolymerization

    Inorg Chim Acta

    (1995)
  • A. Fabrello et al.

    Influence of the reaction conditions on the productivity and on the molecular weight of the polyketone obtained by the CO-ethene copolymerisation catalysed by [Pd(TsO)(H2O)(dppp)](TsO) in MeOH

    J Mol Catal A Chem

    (2007)
  • V. Di Noto et al.

    A new Pt–Rh carbon nitride electrocatalyst for the oxygen reduction reaction in polymer electrolyte membrane fuel cells: synthesis, characterization and single-cell performance

    J Power Sources

    (2010)
  • V. Di Noto et al.

    New inorganic–organic proton conducting membranes based on Nafion and hydrophobic fluoroalkylated silica nanoparticles

    J Power Sources

    (2010)
  • V. Di Noto et al.

    The first lithium zeolitic inorganic–organic polymer electrolyte based on PEG600, Li2PdCl4 and Li3Fe(CN)6: part I, synthesis and vibrational studies

    Electrochim Acta

    (2003)
  • V. Di Noto et al.

    Interplay between structural and electrochemical properties of Pt–Rh carbon nitride electrocatalysts for the oxygen reduction reaction

    Electrochim Acta

    (2011)
  • E. Drent et al.

    Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones

    J Organomet Chem

    (1991)
  • E.V. Rastoltseva et al.

    Structure and conformational dynamics of the dicyclopropyl ketone in the ground electronic state

    J Mol Struct THEOCHEM

    (2010)
  • A. Sadezky et al.

    Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information

    Carbon

    (2005)
  • A.M. Chaparro et al.

    Catalyst layers for proton exchange membrane fuel cells prepared by electrospray deposition on Nafion membrane

    J Power Sources

    (2011)
  • M. Prasanna et al.

    Investigation of oxygen gain in polymer electrolyte membrane fuel cells

    J Power Sources

    (2004)
  • T. Kolev

    Vibrational assignment of in- and out-of-plane modes of some aromatic and arylaliphatic ketones

    J Mol Struct

    (1995)
  • R. O'Hayre et al.

    Fuel cell fundamentals

    (2006)
  • S. Srinivasan

    Fuel cells – from fundamentals to applications

    (2006)
  • C.S. Spiegel

    Designing and building fuel cells

    (2007)
  • W. Vielstich
  • Cited by (80)

    View all citing articles on Scopus
    View full text