Development of nano-electrocatalysts based on carbon nitride supports for the ORR processes in PEM fuel cells
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are a class of energy conversion devices which have recently drawn considerable attention from both the academic and the industrial worlds due to their advantages over other technologies such as internal combustion engines [1], [2], [3]. In particular, PEMFCs are characterized by a very high energy conversion efficiency (as high as 50% or more [3]) and by very large energy and power densities, making them particularly interesting to provide power for light-duty electric vehicles and portable electronic devices [2]. Furthermore, PEMFCs do not produce polluting agents such as fine particulate, NOx and SOx and operate at T < 130 °C [3]. In these conditions, the most effective electrocatalysts to promote the electrochemical reactions involved in PEMFC operation require a significant amount of platinum group metals (PGMs) [4]. The resulting systems are very expensive owing to the low PGM abundance [5] and to a relatively poor durability [4], [6]. One of the key electrochemical processes involved in PEMFC operation is the oxygen reduction reaction (ORR). Even on the best electrocatalysts, the ORR is very sluggish as determined from the values of its exchange current [7] and of the ORR overpotential, which is the dominant source of efficiency loss in hydrogen-fuelled PEMFCs in most operating regimes [4], [7]. For these reasons, most of the PGM loading in hydrogen-fuelled PEMFCs is performed at the cathode electrode, where the ORR takes place [4], [8]. On these bases, the development of improved electrocatalysts for the ORR characterized by a high activity, a low PGM loading and a good durability is one of the most important goals among the topics of PEMFC research. The current state of the art in ORR electrocatalysts consists in platinum nanoparticles (d ∼3–5 nm) supported on active carbons with a large surface area such as XC-72R [4], [9], [10], [11]. The efforts spent in the last 20 years to go beyond this technology were mainly concentrated along two routes. Firstly, materials which are cheaper and more active with respect to the reference systems were studied, by using platinum or palladium in combination with other transition metals to achieve electrocatalysts with improved ORR overpotentials and reduced PGM loadings. One of the most common research activity in this direction was to prepare electrocatalysts consisting of PGMx–My alloys supported on graphite nanoparticles [12], [13], [14], where PGM = Pt, Pd and M = Fe, Cr, Co, Ni and others. With respect to pure Pt, they present an ORR overpotential decreased by a few tens of mV [12]. Nevertheless, it was reported that there are serious concerns as regards the long-term stability of these latter materials [4], [15]. Another family of electrocatalysts which is currently under development is characterized by “onion-like” metal nanoparticles supported on active carbons. In this case, the ORR is catalyzed by strained monolayers of transition metals such as Pt, Pd, Ir, Re and others deposited on suitable metal alloy nanoparticles [16], [17], [18]. With respect to state-of-the-art systems, it was claimed that the electrocatalysts supporting “onion-like” metal nanoparticles may reduce the overall loading of PGMs and the ORR overpotential by a few tens of mV. However, electrocatalysts supporting “onion-like” metal nanoparticles are still in their early development phase and their long-term stability requires to be proven. The second investigation route deals with the development of PGM-free ORR electrocatalysts [4], [19], [20]. It was reported that the ORR in this latter materials takes place at the edges of aromatic graphene layers including a significant concentration of nitrogen species, possibly coordinating transition metal ions such as iron, cobalt and others [4], [19], [21]. It was claimed that the performance in the ORR of these materials is promising. Indeed, with respect to PGM-based systems, their ORR overpotential resulted up to ∼100 mV higher [21], [22]. Nevertheless, in fuel cell tests the geometric current density and the long-term durability of the PGM-free ORR electrocatalysts resulted insufficient for practical applications [4], [6], [19]. In this report, a new class of ORR electrocatalysts will be described, focusing on their preparation methods, their properties and the state of the art of their application at the cathode of PEMFCs. Taken together, these electrocatalysts are prepared by the pyrolysis processes of suitable hybrid inorganic–organic network precursors [23], [24], [25], [26], [27], [28], [29], [30]. The obtained electrocatalysts are characterized by nitrogen atoms in the graphitic support, giving so rise to a carbon nitride system (CN). The latter originates a supramolecular support capable to coordinate efficiently the metal-based active sites, thus stabilizing the resulting electrocatalysts.
Section snippets
Synthesis of the electrocatalysts
A number of different preparations of the proposed CN electrocatalysts have been described recently [23], [24], [25], [26], [27], [28], [29], [30]. In this report, we can generalize that the preparation procedure of this family of electrocatalysts involves three subsequent steps. The first step is aimed at dissolving all the reagent components in solution in the same solvent or in highly miscible solvents. In the second step, the synthesis of the hybrid inorganic–organic precursor material
Chemical composition
The chemical composition of the electrocatalysts was determined as reported elsewhere [24]. ICP-AES was used to quantify the metal assay, while microanalysis allowed to study the stoichiometry of the matrix. Typical results are reported in Table 1. It is observed that both of the proposed preparation protocols (see Section 2) lead to a good control of the metal stoichiometry of the final electrocatalysts. In materials obtained at Tf = 400 °C, some low-weight MRClxy− species are eliminated by
Structural and functional models
The structure of CN-based electrocatalysts is mainly influenced by the following preparation parameters: (a) the metal stoichiometry; (b) the concentration of nitrogen; (c) the pyrolysis temperature Tf; and (d) the structure and morphology of the support. It is observed that the size of the metal-rich phases depends on the main active metal. With respect to the Pd-based materials, Pt-based systems are characterized by smaller metal-rich phases at the same Tf (5–30 nm vs. 4–15 nm). A large
Conclusions
The protocols for the preparation of CN-based ORR electrocatalysts devised and optimized as reported in this report are based on the pyrolysis process of HIO-PN and Z-IOPE homogeneous precursors and yielded I and II materials, respectively. These synthesis protocols allow to modulate easily the stoichiometry of the materials. The best ORR kinetics has been obtained with electrocatalysts where the “active metal” is Pt or Pd in its (0) oxidation state, while the “co-catalyst” is a good Lewis acid
Acknowledgement
This research was funded by the Italian MURST project NUME of FISR2003, “Sviluppo di membrane protoniche composite e di configurazioni elettrodiche innovative per cella a combustibile con elettrolita polimerico”.
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