Preparation, characterization and single-cell performance of a new class of Pd-carbon nitride electrocatalysts for oxygen reduction reaction in PEMFCs
Graphical abstract
Highlights
► New plurimetal Pd-based carbon nitride ORR electrocatalysts are synthesized. ► For the first time the morphology, structure and electrochemical performance are investigated. ► A model is proposed to describe the chemical composition, structure and morphology. ► The electrochemical behaviour and fuel cell performance are carefully studied.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are a class of flow-through devices where the chemical energy of the reagents is converted directly into electrical power. The operation of PEMFCs does not rely on the Carnot cycle; thus, even if they run at low temperatures (T < 130 °C) they can yield an energy conversion efficiency as high as 50% or more [1]. Furthermore, PEMFCs: (a) operate very silently and do not produce NOx, SOx or fine particulate; (b) yield very high power densities in very simple devices which are characterized by the lack of any mechanical moving part. Finally, when a PEMFC is fed with hydrogen the only reaction product is water; as a consequence, no greenhouse gas is dumped in the atmosphere [2]. Several of these features make this technology very friendly to the environment. For all these reasons, in the last few decades a tremendous effort has been devoted from both private companies and public institutions to the development of viable PEMFCs [3], [4]. Notwithstanding that several PEMFC systems are now approaching commercialization in niche markets [5], [6], the large-scale diffusion of this technology is limited by issues such as cost and durability [2], [4], [7]. The heart of a PEMFC is the membrane-electrode assembly (MEA), which consists in a proton-conducting membrane covered on both sides by thin electrode layers sandwiched between porous gas-diffusion electrodes. The electrode layers promote the electrochemical reactions needed for the operation of the device [8], [9]. One of the most critical limitations suffered by PEMFCs is that at their low operating temperatures (T ≈ 80 °C) the only viable electrocatalysts require a significant loading of platinum group metals (PGMs) [10]; ca. 1.1 gPGM is reported as a requirement to obtain 1 kW [11]. In PEMFCs fuelled with hydrogen, most of the PGM loading is concentrated at the cathode, where the oxygen reduction reaction (ORR) takes place [10], [12]. Therefore, the development of new ORR electrocatalysts leading to a low mass loading of PGM in the electrode configuration is a major aim in PEMFC research [10]. State-of-the-art ORR electrocatalysts used in PEMFCs consist of PGM nanoparticles, predominantly based on Pt, supported on carbon blacks with a medium surface area such as Vulcan XC-72R (Cabot Corp. TM), Ketjenblack (AkzoNobel Chemicals™) or similar furnace blacks [10], [13], [14], [15]. Significant research efforts are currently devoted to improve the supports of this class of materials [16], [17]. However, these systems may suffer from PGM dissolution at the highest cell operating potentials and degradation of the carbon support owing to the formation of H2O2 as byproduct of the ORR [7], [18]. Furthermore, it is well-known that Pt-based systems are not very tolerant to poisoning effects arising from common contaminants such as halides and methanol (fuel in direct methanol fuel cells, DMFCs), which lead to a significant loss in performance [19], [20], [21]. In particular, methanol is known to permeate easily through the standard perfluorinated membranes mounted in DMFCs from the anode to the cathode compartment. Once methanol reaches the cathode it undergoes oxidation, giving so rise to a mixed potential which lowers the overall energy conversion efficiency of the DMFC [10], [21]. It was suggested that one possible way to address all these issues is to prepare ORR electrocatalysts characterized by a low loading of palladium coordinated on a carbon nitride (CN) support [22]. These materials are obtained through an innovative synthetic protocol which is reasonably straightforward and, by itself, cheap. Indeed, the various preparation steps may be carried out in the open atmosphere, with simple glassware, starting from common commercial reagents, and it is easy to obtain batches of electrocatalysts in the gram range. It is not necessary to add highly purified carbon supports, and the nitrogen atoms of the CN matrix coordinate the metal atoms of the active sites, improving the tolerance of the materials to degradation in oxidizing conditions [22]. Preliminary results showed clearly that this approach is feasible, demonstrating that Pt can be substituted with the somewhat cheaper Pd in ORR electrocatalysis [22], [23], [24], [25], [26]. The best results were obtained with bi-metallic electrocatalysts where a metal active in the ORR such as Pd or Pt is associated with one or more co-catalytic transition metals such as Au, Co, Ni and Fe [11], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. It was proposed that co-catalytic metals enhance the ORR by improving the desorption kinetics of the reaction products.
In this report, two electrocatalysts were prepared according to a new synthesis protocol which consists in the following three-step procedure: (1) synthesis of a homogeneous precursor; (2) pyrolysis of the precursor in an inert atmosphere; and (3) post-synthesis treatments and activation of ORR electrocatalysts [22], [32]. To improve the tolerance of the proposed electrocatalysts towards oxidizing conditions and poisoning effects arising from chloride anions and methanol, materials with a high nitrogen concentration in the carbon nitride support matrix were devised. Two electrocatalysts are prepared and studied in detail in this report, indicated as “PdNi-CNh 900” and “PdCoAu-CNh 600”, in an effort to achieve a better understanding of the structure–properties relationship of this family of innovative materials in systems as diverse from one another as possible [24], [26]. The left side of the formulas indicates the metals included in the electrocatalysts (either Pd and Ni or Pd, Co and Au). The label –CNh indicates that the carbon nitride support is endowed with a high concentration of nitrogen, >13 wt%. The last figure corresponds to the temperature of the pyrolysis process.
Section snippets
Reagents
Potassium tetrachloropalladate(II) 99%, (K2PdCl4), potassium tetrachloroaurate(III) 98%, (KAuCl4) and H2O2 36 vol.% are supplied by ABCR GmbH. Nickel(II) nitrate hexahydrate 99% (Ni(NO3)2·6H2O) is supplied by Fluka. Cobalt(II) chloride hexahydrate 98% (CoCl2·6H2O) and polyacrylonitrile, Mw 150,000, are supplied by Aldrich. Acetonitrile and dimethylformamide are supplied by Carlo Erba. All the chemicals are used as received. XC-72R carbon black, provided as a courtesy by Carbocrom s.r.l., is
Chemical composition of the electrocatalysts
The chemical analysis of the proposed electrocatalysts is reported in Table 1(a). It is observed that the preparation procedure yields electrocatalysts characterized by a nitrogen content larger than 13 wt%. With respect to PdNi-CNh 900, the chemical composition of PdCoAu-CNh 600 includes a small amount of hydrogen and more nitrogen. This evidence points to an incomplete graphitization of the carbon nitride support in PdCoAu-CNh 600, attributed to the lower temperature of the pyrolysis process
Discussion
“Ex situ” CV-TF-RDE measurements and data derived from single cell FC tests point out that the ORR overpotential of the electrocatalysts increases in the following order: Pt/C reference < PdCoAu-CNh 600 < PdNi-CNh 900. FC tests show that, with respect to PdCoAu-CNh 600, PdNi-CNh 900 is characterized by a higher Tafel slope. These evidence are coherent with a picture where the various materials are characterized by active ORR sites with a very different chemical composition. If the ORR is carried
Conclusions
In this report, an innovative synthesis protocol is described to prepare a new class of electrocatalyst for the oxygen reduction reaction (ORR) for application in PEMFC cathodes. The proposed materials, which are labeled PdNi-CNh 900 and PdCoAu-CNh 600, consist of graphite-like carbon nitride (CN) matrices with nitrogen content larger than 13 wt% supporting multi-metallic platinum-free clusters forming the active sites. Two different metal combinations in the multimetal clusters are
Acknowledgement
Research was funded by the Italian MURST project PRIN2008, “Direct polymer electrolyte membrane fuel cells: synthesis and study in prototype cells of hybrid inorganic–organic membranes and electrode materials”.
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