Synthesis–structure–performance correlation for polyaniline–Me–C non-precious metal cathode catalysts for oxygen reduction in fuel cells

Abstract

In this report, we present the systematic preparation of active and durable non-precious metal catalysts (NPMCs) for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temperature, metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphology. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst's degree of graphitization through heat treatment versus the excessive loss of surface area that occurs at higher temperatures is a key to preparing an active catalyst. XPS and XAFS spectra are consistent with the presence of Me-N x structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The average speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.

Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction

“…The sample treated at 1000°C showed the highest activity in both acidic and alkaline electrolytes. Although previous studies have reported production of nitrogen-doped carbon catalysts with tunable physical and chemical properties, 35,36 here we provide the first demonstration that the diameter of carbon tubes can be manipulated by varying the TM used in the precursor mixture. Given the important role played by the TM in controlling nanocarbon morphology, we anticipate similarly strong effect on the catalytic properties of these nanocarbons.…”

Mesoporous Metal–Nitrogen-Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction

“…8 The peak at a binding energy of 398.7 eV should also include a contribution from nitrogen bound to the metal (Me-N), due to the small difference between binding energies of N-Me and pyridinic N. 8b,9 Pyridinic N (probably including Me-N) and graphitic N are generally believed to participate in the active sites. 2d,8b, [9][10] Although there was a significant amount of cobalt species (1.3-1.4 at%) in the templated C-N-Co catalysts, as indicated by XPS analysis (Table S1), we hardly observed any metal-containing nanoparticles by TEM inspection because the etching agent (hydrogen fluoride) dissolves both the inorganic templates and metal/metal oxide nanoparticles. 11 Xray diffraction (XRD) pattern further proved the absence of crystalline metal/metal oxide phases ( Figure S8).…”

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

“…This is well above the optimum content for Me-N-C catalyst precursors before pyrolysis, typically below 2-3 wt. % [7,[14][15][16]. Too large Fe or Co contents lead to the formation of highly graphitized carbon structures during pyrolysis [13,17].…”