Abstract
Electrospinning of polyacrylonitrile/DMF dopes containing salts of nickel, cobalt, zirconium, cerium, gadolinium, and samarium, makes it possible to obtain precursor nanofiber mats which can be subsequently converted into carbon nanofiber (CNF) composites by pyrolysis at 1000–1200 °C. Inorganic additives were found to be uniformly distributed in CNFs. Metal states were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). According to XPS in CNF/Zr/Ni/Gd composites pyrolyzed at 1000 °C, nickel exists as Ni0 and as Ni2+, gadolinium as Gd3+, and zirconium as Zr4+. If CNF/Zr/Ni/Gd is pyrolyzed at 1200 °C, nickel exists only as Ni0. For CNF/Sm/Co composite, samarium is in Sm3+ form when cobalt is not found on a surface. For CNF/Zr/Ni/Ce composite, cerium exists both as Ce4+ and as Ce3+. Composite CNF mats were platinized and tested as cathodes in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Such approach allows to introduce Pt–M and Pt–MOx into CNF, which are more durable compared to carbon black under HT-PEMFC operation. For CNF/Zr/Ni/Gd composite cathode, higher performance in the HT-PEMFC at I >1.2 A cm-2 is achieved due to elimination of mass transfer losses in gas-diffusion electrode compared to commercial Celtec®P1000.
Highlights
Development of new electrocatalytic systems which are capable of performing oxygen reduction reaction (ORR) with low overpotentials is of great importance for further progress in polymer electrolyte membrane (PEM) fuel cells (FCs)
In this study we show the possibility for Pt–M and Pt–MOx electrocatalytic systems to be incorporated into carbon nanofiber (CNF) mats instead of carbon black by using the electrospinning method
We show the advantages of these CNF materials when used in HT-PEMFC at high current densities due to the fact of the elimination of mass transport losses
Summary
Development of new electrocatalytic systems which are capable of performing oxygen reduction reaction (ORR) with low overpotentials is of great importance for further progress in polymer electrolyte membrane (PEM) fuel cells (FCs). Increased ORR activity of PtNi alloys has been researched in many works [1]. Polymers 2020, 12, 1340 is the most active for ORR in low-temperature (LT-PEMFC) and high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) membrane electrode assemblies (MEAs) [2]. The most important of them is the shortening of Pt–Pt interatomic spacings [3], the downshift of the d-band center [4], when alloying modifies Pt electronic structure [5], and better chemisorption of OH groups onto transition metal sites [6]. Alloying inhibits chemisorption of oxygen intermediates, as it correlates to Pt–Pt bond shortening [3]. When the metal binds the oxygen species strongly, the ORR is inhibited by O and OH species present on the surface. When the metal binds oxygen weakly, the ORR is inhibited by slower electron and proton transfer to the adsorbed oxygen. The ORR electrocatalyst design should be optimized [1,6]
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