Elsevier

Electrochimica Acta

Volume 55, Issue 26, 1 November 2010, Pages 7564-7574
Electrochimica Acta

Development of nano-electrocatalysts based on carbon nitride supports for the ORR processes in PEM fuel cells

https://doi.org/10.1016/j.electacta.2009.11.032Get rights and content

Abstract

This report describes the development and the optimization of new synthesis routes yielding electrocatalysts for the oxygen reduction reaction (ORR) aimed at application in proton exchange membrane fuel cells (PEMFCs). The preparation protocols consist in the synthesis of two groups of hybrid inorganic–organic precursors, characterized by a different concentration of nitrogen, which subsequently undergo a high-temperature pyrolysis in inert atmosphere, washing and activation. The resulting materials show a well-controlled stoichiometry. The nitrogen incorporated in the support transforms the matrix into a supramolecular ligand, and stabilizes the electrocatalyst by coordinating the active metal clusters. The latter are composed of an “active metal” such as Pt or Pd, combined with one or more “co-catalyst” elements such as Au, Fe, Co and Ni. An extensive characterization of the carbon nitride electrocatalysts under the chemical, structural, morphological and electrochemical points of view is described, together with their use in membrane electrode assemblies (MEAs) tested in single fuel cells under operative conditions. Results indicated that the best electrocatalysts are those characterized by a “core–shell” morphology. These systems consist of metal carbon nitride materials with a low concentration of nitrogen (shell) supported on electronically conductive graphite nanoparticles (core). Promising results were obtained both in terms of ORR overpotential (η) and of mass activity (Am). Indeed, η resulted up to ∼30 mV lower with respect to reference Pt-based systems, and an Am equal to 0.3–0.4 g of Pd or Pt to achieve 1 kW was reached.

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”.

References (44)

  • H.A. Gasteiger et al.

    Appl. Catal. B: Environ.

    (2005)
  • H.A. Gasteiger et al.

    J. Power Sources

    (2004)
  • S.B. Yoon et al.
  • W.M. Wang et al.

    J. Power Sources

    (2007)
  • J. Zhang et al.

    ECS Trans.

    (2006)
  • C.W.B. Bezerra et al.

    Electrochim. Acta

    (2008)
  • F. Charreteur et al.

    Electrochim. Acta

    (2008)
  • F. Charreteur et al.

    Electrochim. Acta

    (2008)
  • V. Di Noto et al.

    Electrochim. Acta

    (2007)
  • V. Di Noto et al.

    Electrochim. Acta

    (2003)
  • E. Negro et al.

    J. Power Sources

    (2008)
  • T.J. Schmidt et al.

    J. Electroanal. Chem.

    (2001)
  • J. Larminie et al.

    Fuel Cell Systems Explained

    (2003)
  • R. O’Hayre et al.

    Fuel Cell Fundamentals

    (2006)
  • W. Vielstich
  • C. Jaffray et al.
  • R. Borup et al.

    Chem. Rev.

    (2007)
  • R. O’Hayre et al.

    Fuel Cell Fundamentals

    (2006)
  • T. Tada
  • B. Fang et al.

    Phys. Chem. Chem. Phys.

    (2009)
  • D. Thompsett
  • M.H. Shao et al.

    J. Am. Chem. Soc.

    (2006)
  • Cited by (104)

    • Carbon nitrides as catalyst support in fuel cells: Current scenario and future recommendation

      2022, Nanostructured Carbon Nitrides for Sustainable Energy and Environmental Applications
    View all citing articles on Scopus
    1

    Active ACS, ECS and ISE member.

    View full text