Synthesis, studies and fuel cell performance of “core–shell” electrocatalysts for oxygen reduction reaction based on a PtNix carbon nitride “shell” and a pyrolyzed polyketone nanoball “core”
Introduction
Fuel cells (FCs) are a family of open electrochemical energy conversion devices operating through the direct electro-oxidation of a fuel (e.g., hydrogen) at the anode and the reduction of an oxidant, usually the oxygen of air, at the cathode [1], [2], [3]. Particular attention has been attracted by a family of FCs relying on a proton exchange membrane (PEM) as the electrolyte; these systems are known as proton exchange membrane fuel cells (PEMFCs) [4]. PEMFCs are compact systems characterized by a very high energy conversion efficiency, up to 2–3 times larger in comparison with competing technologies such as internal combustion engines (ICEs) [5]. PEMFCs are particularly suitable to power light-duty electric vehicles and portable electronic devices such as audiovisual players and laptop computers [6], [7]. One of the most important bottlenecks in the operation of PEMFCs is the sluggish kinetics of the oxygen reduction reaction (ORR) [8]. Suitable electrocatalysts are required to promote the ORR kinetics in order to achieve an improved performance. As of today, the state of the art in ORR electrocatalysts for PEMFCs consists in nanocomposite materials including platinum nanocrystals supported on active carbons characterized by a large surface area and a high electronic conductivity such as XC-72R carbon black [9], [10]. These electrocatalysts afford a satisfactory performance; however, they are also very expensive owing to a significant loading of platinum (usually between 10 and 50 wt%) [11]. Furthermore, they suffer from an insufficient long-term durability [12], [13], [14]. These drawbacks are still major obstacles towards a widespread diffusion of the promising PEMFC technology. Several approaches are attempted to address the above issues. On one hand, ORR active sites showing an improved turnover frequency were prepared. In general, they include a platinum-group metal (PGM, e.g., Pt, Pd) [10], [15] alloyed with one or more first-row transition metals (e.g., Cr, Fe, Co, Ni, Cu) [16], [17], [18], [19], [20], [21], [22], [23], [24]. On the other hand, a variety of new supports were investigated, including carbon nanoparticles, graphene layers, and carbon nanotubes; some of these systems include suitable heteroatoms (e.g., N or S) to stabilize the active sites and improve the activity and durability of the electrocatalysts [25], [26], [27]. In the last decade, our research group proposed an innovative protocol to obtain electrocatalysts for application in fuel cells [28], [29]. A suitable hybrid inorganic–organic precursor is prepared, consisting of complexes of the desired metal atoms networked by an organic binder. The precursor undergoes a series of pyrolysis and activation steps, yielding the final electrocatalyst; the latter's stoichiometry is well-controlled and includes the desired concentration of heteroatoms (typically N). The obtained carbon nitride-based electrocatalysts present an improved performance and tolerance to oxidizing conditions [29], [30], [31]. The best results are reached in the electrocatalysts with a “core–shell” morphology [30], [32], [33]. In these systems, the hybrid inorganic–organic precursor is used to impregnate a suitable support before the pyrolysis and activation steps. Highly conductive XC-72R carbon nanoparticles (NPs) were adopted. In such “core–shell” morphology an improved dispersion of the active sites embedded in the carbon nitride “shell” was obtained, which resulted in a better fuel cell performance in comparison with the corresponding bulk electrocatalysts including active sites with the same stoichiometry [32].
In this paper, a new family of supports characterized by an innovative morphology and a large surface area is developed. The supports consist of nanoballs of carbon nanofibers with a diameter on the order of nanometers wrapping “core” carbon nanoparticles of ca. 30–50 nm in size. The supports are obtained in two steps: (1) polyketone (PK) fibers are grown on XC-72R carbon NPs acting as nucleation centers, giving so rise to “balls” showing a nanofibrous morphology; thus, the “support precursor” (SP) is obtained [34]; (2) SP undergoes sulfonation and multi-step pyrolysis procedures. The resulting supports are adopted as the “core” in the preparation of advanced “core–shell” ORR carbon nitride electrocatalysts. In these systems, the “core” is covered by a PtNix-CN “shell” based on PtNix alloy NPs embedded in a carbon nitride (CN) matrix. The “balls” with a nanofibrous morphology of SP are proposed as an innovative template for the modulation of: (a) the morphology and chemical composition of the supports; and (b) the activity and selectivity of the “core–shell” carbon nitride electrocatalysts, which are briefly indicated as PtNi-CNl Tf/STp (see the following discussion for more information). PtNi-CNl Tf/STp are studied in terms of composition, morphology and single-cell performance under operating conditions in order to obtain information on the most important preparation parameters to design next-generation ORR electrocatalysts characterized by an improved efficiency.
Section snippets
Reagents
Potassium tetrachloroplatinate (II), 99.9% and potassium tetracyanonickelate (II), hydrate are supplied by ABCR. D(+)-sucrose, biochemical grade is Acros reagent. EC-20 is received from ElectroChem, Inc. (nominal Pt loading: 20%) and used as the reference. In the following text, EC-20 is labeled “Pt/C reference”. Palladium (II) acetate, 98%, p-toluenesulfonic acid monohydrate, 98% and 1,3-bis(diphenylphosphino) propane are obtained from Sigma–Aldrich and in the following are labeled Pd(AcO)2,
Preparation observations
The proposed electrocatalysts are prepared in two steps, I and II, as shown in Scheme 1. In step I, the electron-conducting “core” (STp) is prepared, which is characterized by a high surface area. In detail, STp is synthesized as follows. First, the support precursor (SP) is obtained by copolymerization of CO with ethene in the presence of XC-72R carbon NPs suspended in a methanol/water solution (see the Experimental section). In this process the carbon NPs, which adsorb the polymerization
Conclusions
In this report two new STp supports, S500 and S700, are prepared by sulfonation and pyrolysis of a support precursor (SP) consisting of XC-72R carbon NPs embedded in a “ball” of PK fibers. The detailed studies carried out on the support precursors and STp “core” supports show that: (a) a successful incorporation of sulfur atoms in the chemical composition of the materials occurs; (b) the graphitization of SP becomes more evident as Tp is raised; and (c) a complex interplay takes place between
Acknowledgments
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 (53)
- et al.
Polymer electrolytes for a hydrogen economy
Int J Hydrogen Energy
(2012) - et al.
Overview of recent developments in oxygen reduction electrocatalysis
Electrochim Acta
(2012) - et al.
Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs
Appl Catal B Environ
(2005) - et al.
Effect of particle size on the activity and durability of the Pt/C electrocatalyst for proton exchange membrane fuel cells
Appl Catal B Environ
(2012) - et al.
Pd–Co carbon-nitride electrocatalysts for polymer electrolyte fuel cells
Electrochim Acta
(2007) - et al.
Oxygen reduction at Pt and Pt70Ni30 in H2SO4/CH3OH solution
Electrochim Acta
(2002) - et al.
Methanol tolerant oxygen reduction on carbon-supported Pt–Ni alloy nanoparticles
J Electroanal Chem
(2005) - et al.
Synthesis and electrocatalytic performance of MWCNT-supported Ag@Pt core-shell nanoparticles for ORR
Int J Hydrogen Energy
(2012) - et al.
Development of nano-electrocatalysts based on carbon nitride supports for the ORR processes in PEM fuel cells
Electrochim Acta
(2010) - et al.
Preparation, characterization and single-cell performance of a new class of Pd–carbon nitride electrocatalysts for oxygen reduction reaction in PEMFCs
Appl Catal B Environ
(2012)
Synthesis, characterization and X-ray structure of [Pd(dppp)(H2O)(TsO)][TsO] (dppp = 1,3-bis(diphenylphosphino)propane; TsO = p-CH3C6H4SO3), a catalytic species in COC2H4 copolymerization
Inorg Chim Acta
Influence of the reaction conditions on the productivity and on the molecular weight of the polyketone obtained by the CO-ethene copolymerisation catalysed by [Pd(TsO)(H2O)(dppp)](TsO) in MeOH
J Mol Catal A Chem
A new Pt–Rh carbon nitride electrocatalyst for the oxygen reduction reaction in polymer electrolyte membrane fuel cells: synthesis, characterization and single-cell performance
J Power Sources
New inorganic–organic proton conducting membranes based on Nafion and hydrophobic fluoroalkylated silica nanoparticles
J Power Sources
The first lithium zeolitic inorganic–organic polymer electrolyte based on PEG600, Li2PdCl4 and Li3Fe(CN)6: part I, synthesis and vibrational studies
Electrochim Acta
Interplay between structural and electrochemical properties of Pt–Rh carbon nitride electrocatalysts for the oxygen reduction reaction
Electrochim Acta
Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones
J Organomet Chem
Structure and conformational dynamics of the dicyclopropyl ketone in the ground electronic state
J Mol Struct THEOCHEM
Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information
Carbon
Catalyst layers for proton exchange membrane fuel cells prepared by electrospray deposition on Nafion membrane
J Power Sources
Investigation of oxygen gain in polymer electrolyte membrane fuel cells
J Power Sources
Vibrational assignment of in- and out-of-plane modes of some aromatic and arylaliphatic ketones
J Mol Struct
Fuel cell fundamentals
Fuel cells – from fundamentals to applications
Designing and building fuel cells
Cited by (80)
Effect of intermolecular interactions on the glass transition temperature of chemically modified alternating polyketones
2023, Materials Today ChemistryA review of g-C<inf>3</inf>N<inf>4</inf> based catalysts for direct methanol fuel cells
2022, International Journal of Hydrogen EnergyCyanogel and its derived-materials: properties, preparation methods, and electrochemical applications
2021, Materials Today EnergySystematic computational investigation of an Ni<inf>3</inf>Fe catalyst for the OER
2020, Catalysis TodayA multifunctional platform by controlling of carbon nitride in the core-shell structure: From design to construction, and catalysis applications
2019, Applied Catalysis B: EnvironmentalBoosted activity of graphene encapsulated CoFe alloys by blending with activated carbon for oxygen reduction reaction
2018, Biosensors and Bioelectronics