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”
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.
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