Interplay between morphology and electrochemical 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.2013.08.053Get rights and content

Highlights

  • New “core–shell” ORR electrocatalysts of the type PtNi–CNl Tf/STp are characterized by HR-TEM and the CV-TF-RRDE method.

  • The PtNi–CNl Tf/STp electrocatalysts include a rough CN “shell” matrix embedding PtNix alloy NPs of two main sizes.

  • The role of the conducting STp “core” support and its interactions with the active sites dispersed in the PtNix-CN “shell” is elucidated.

  • The interplay between morphology and ORR performance is discussed.

Abstract

The interplay between morphology and electrochemical performance of a new class of “core–shell” electrocatalysts for the oxygen reduction reaction (ORR) is studied. The electrocatalysts, labelled PtNi–CNl Tf/STp, consist of a “core” of pyrolyzed polyketone nanoballs (indicated as STp “core” support) covered by a carbon nitride (CN) “shell” matrix embedding PtNix alloy NPs (indicated as PtNix-CN). The electrocatalysts are characterized by means of: (a) high-resolution transmission electron microscopy (HR-TEM); and (b) cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) method. The structure of the STp “core” supports and the details of the preparation procedure, such as pyrolysis temperature, Tf, and treatment with H2O2, play a crucial role on modulating: (a) the morphology; and (b) the ORR performance of the electrocatalysts. In particular, the best results are achieved for PtNi–CNl Tf/STp systems: (a) including a STp “core” support with a high porosity; and (b) obtained at Tf = 600 °C. It is demonstrated that in general, the treatment with H2O2 of electrocatalysts is detrimental for the ORR performance. Nevertheless, in particular conditions, the treatment in H2O2 improves the ORR performance of PtNi-CNl Tf/STp. The results presented in this work allow to elucidate the complex correlation existing between: (a) the composition; (b) the interactions in PtNix-CN; (c) the morphology of STp and PtNi–CNl Tf/STp; and (d) the ORR performance of the electrocatalysts.

Introduction

The development of innovative electrocatalysts is one of the most important aspects of the modern research in the field of proton exchange membrane fuel cells (PEMFCs) [1]. This latter breakthrough technology shows great potential to play a major role in a variety of disparate applications, ranging from the electrification of surface transport to the efficient exploitation of the energy derived from renewable sources [2]. However, significant drawbacks are still to be addressed before PEMFCs can be applied commercially on a large scale. In particular, traditional PEMFCs operate at T < 120 °C; furthermore, they adopt electrolytes generating a highly acidic environment at the electrodes, e.g., perfluorinated ionomers and sulfonated aromatic polymers [3], [4], [5]. In these conditions, the redox process responsible of the operation of PEMFC electrodes require suitable electrocatalysts to achieve a performance level compatible with the applications [6]. In particular, the kinetics of the oxygen reduction reaction (ORR) is very sluggish, and is one of the most important bottlenecks in the overall performance of the PEMFC [6], [7]. The best ORR activity in the operating conditions typical of PEMFCs is afforded by active sites based on platinum-group metals such as platinum or palladium [6], [8]. The abundance of these elements in the Earth's crust is very limited, giving rise to high costs and a significant risk of supply bottlenecks [9], [10]. Despite these shortcomings, as of today the only viable ORR electrocatalysts for application in PEMFCs include active sites based on platinum-group metals [6], [8]. In state-of-the-art ORR electrocatalysts, the loading of platinum-group metals is reduced by dispersing the active sites on a conductive support characterized by a large surface area (e.g., XC-72R carbon black nanoparticles) [8], [11]. However, there is still a significant need to improve the performance and durability of the electrocatalysts devised following this general approach [1], [2], [12]. On one hand, the intrinsic turnover frequency of the active sites may be improved above the level showed by systems including only pristine platinum-group metals [1], [13]. This result is generally accomplished by alloying the platinum-group metal with a first-row transition metal such as Fe, Co, Ni, Cu and others [13], [14], [15], [16], [17]. On the other hand, the durability of the electrocatalysts should be enhanced. In particular: (a) the aggregation of the platinum-group metal nanoparticles bearing the active sites should be prevented; and (b) the degradation of the interfaces between the support and the platinum-group metal nanoparticles should be inhibited [12]. In principle, there is the possibility to achieve the above-described targets by synthesizing the ORR electrocatalysts according to a new procedure developed over the last years [18], [19]. This procedure is unique, and is completely different with respect to the well-assessed synthetic procedures reported in the literature [8], [11], [20]. In summary, the electrocatalysts are obtained by the pyrolysis of a hybrid inorganic-organic precursor, followed by suitable activation processes [18], [19]. The proposed procedure is extremely flexible, since it allows to obtain active sites characterized by a well-controlled chemical composition [21], [22], [23]. Furthermore, heteroatoms such as nitrogen can be introduced in the electrocatalysts where desired. Thus, graphitic-like carbon nitride-based matrices (CN) are obtained, with N-based ligands coordinating the metal alloy nanoparticles [24]. It was demonstrated that a high concentration of nitrogen in the CN matrix improves the tolerance of the electrocatalyst to the decomposition in oxidizing conditions [25]. Furthermore, significant efforts are still needed to optimize this preparation procedure to yield ORR electrocatalysts improved with respect to the state of the art. Recently, most efforts were devoted to improve the morphology of the electrocatalysts [26], [27]. It was proposed that “core–shell” morphologies of CN-based electrocatalysts are of crucial importance to improve the ORR performance and to obtain a high dispersion of the active sites. In the “core–shell” morphology, the carbon nitride (CN) “shell” matrix embedding the metal nanoparticles bearing the active sites is supported on a “core” of conducting nanoparticles [26], [27].

In this work, innovative “core” supports based on pyrolyzed polyketone nanoballs are used in the preparation of a family of PtNix alloy “core–shell” carbon nitride-based electrocatalysts as described elsewhere [28]. These electrocatalysts present a formula PtNi–CNl Tf/STp, where PtNi–CNl Tf refers to the “shell”, and STp denotes the electrolcatalysts' “core”. The electrocatalysts' morphology and ORR electrochemistry are investigated extensively by means of HR-TEM and the CV-TF-RRDE method, respectively. The aim of the paper is to achieve a detailed insight of the complex interplay between the electrochemical mechanism and “ex situ” performance and the: (a) features of the conducting STp “core” supports; (b) parameters of the preparation procedure; (c) morphology of the final electrocatalysts; (d) chemical composition of both the PtNix-CN “shell” and the STp “core” supports; (e) interactions between the CN matrix and the PtNix alloy nanoparticles (NPs).

Section snippets

Preparation of the PtNi–CNl Tf/STp electrocatalysts

The electrocatalysts investigated in this work are obtained as described elsewhere [28] using a procedure which involves two main steps: (a) preparation of the STp “core” supports; and (b) synthesis of the electrocatalysts. The STp “core” supports are obtained by growing polyketone (PK) fibers on XC-72R carbon nanoparticles; subsequently, the samples are treated with conc. H2SO4 and undergo a series of pyrolysis steps under vacuum. Two STp “core” supports are obtained, labelled “S500” and

HR-TEM investigations

The main features of the morphology of the PtNi–CNl Tf/STp electrocatalysts are shown in Fig. 1. Highly polydisperse dark spots associated with PtNix alloy NPs are scattered throughout a light matrix (see Fig. 1(a)). The very small PtNix alloy NPs present a particle size d < 10 nm (see Fig. 1(a and b)), while the large submicrometric NPs show d > 50 nm. Fig. 1(c) discloses the morphology of: (a) XC-72R NPs introduced in the first phase of the preparation of the STp “core” supports; (b) the

Conclusions

In this paper, a family of innovative “core–shell” PtNi–CNl Tf/STp electrocatalysts prepared as described elsewhere [28] is extensively investigated to elucidate the interplay between the morphology, the preparation parameters, and the electrochemical performance as determined by the CV-TF-RRDE “ex situ” method. The PtNi–CNl Tf/STp electrocatalysts include a very rough and porous CN matrix “shell”, derived by the pyrolysis of both the STp “core” supports and the supported fraction of the

Acknowledgements

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 (42)

  • T.J. Schmidt et al.

    The oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of chloride anions

    J Electroanal Chem

    (2001)
  • A. Pozio et al.

    Comparison of high surface Pt/C catalysts by cyclic voltammetry

    J Power Sources

    (2002)
  • H. Yang et al.

    Methanol tolerant oxygen reduction on carbon-supported Pt–Ni alloy nanoparticles

    J Electroanal Chem

    (2005)
  • H.A. Gasteiger et al.

    Just a dream or future reality?

    Science

    (2009)
  • S. Srinivasan

    Fuel cells – from fundamentals to applications

    (2006)
  • C.S. Spiegel

    Designing and building fuel cells

    (2007)
  • J.X. Wang et al.

    Intrinsic kinetic equation for oxygen reduction reaction in acidic media: the double Tafel slope and fuel cell applications

    Faraday Discuss

    (2008)
  • A.J. Appleby

    Electrochemical engine for vehicles

    Sci Am

    (1999)
  • A. Morozan et al.

    Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes

    Energy Environ Sci

    (2011)
  • R. Borup et al.

    Scientific aspects of polymer electrolyte fuel cell durability and degradation

    Chem Rev

    (2007)
  • J. Greeley et al.

    Alloys of platinum and early transition metals as oxygen reduction electrocatalysts

    Nat Chem

    (2009)
  • Cited by (63)

    • Three-dimensional hierarchically porous iridium oxide-nitrogen doped carbon hybrid: An efficient bifunctional catalyst for oxygen evolution and hydrogen evolution reaction in acid

      2020, International Journal of Hydrogen Energy
      Citation Excerpt :

      This proves that there is a strong synergistic interaction between IrO2 and N@C in the 3D-IrO2/N@C hybrid. The presence of nitrogen in N-doped carbon support also helps to enhance the OER/HER activity of the 3D-IrO2/N@C catalyst [67,90]. The direct growth of 3D-IrO2/N@C in conducting Ti-foil enhances electron transport and strong bonding between 3D-IrO2/N@C and Ti-foil gives strong structural stability for long-term stability.

    View all citing articles on Scopus
    View full text