Carbon Supports for Non-Precious Metal Oxygen Reducing Catalysts

Abstract

Porous carbon materials with varying structural and compositional properties were studied for their impact on the nitrogen content and activity of metal-nitrogen-carbon (MNC) oxygen reduction catalysts prepared using high-pressure pyrolysis. The carbon materials and resulting catalysts were characterized morphologically using nitrogen physisorption, coupled with non-local density functional theory (NLDFT) analysis to calculate pore size distributions. Graphiticity was assessed via X-ray Diffraction (XRD), bulk nitrogen content was observed using CHN combustion analysis and iron content by Inductively Coupled Plasma (ICP). The catalysts were characterized electrochemically using rotating ring-disk measurements. The results indicate that substrates adsorbing the most nitrogen and iron show the highest activity. Furthermore, a relationship found between mesoporosity and nitrogen adsorption indicate the importance of transport of precursors to potential active sites.The cost of precious metals has driven the search for lower-cost alternatives to catalyze the oxygen reduction reaction (ORR) and accelerate commercialization of low-temperature fuel cells. An important class of non-precious metal catalysts for oxygen reduction is pyrolyzed metal/nitrogen/carbon (MNC) compounds. 1-3 These compounds involve the association of metal atoms with nitrogen moieties immobilized in a conductive carbon matrix. Recent research has lead to increased understanding of key aspects of catalyst activation, function, and stability. However, the nature of the active site is not yet clear because of the complexities introduced by pyrolysis on high surface-area supports. Such lack of understanding has hindered the engineering and implementation of these catalysts in fuel cell applications.Dodelet et al. suggest that MNC catalytic sites occur where metal atoms are bridge-bonded to nitrogen sites and span micropores of width less than 2 nm. 4,5 Lefevre et al. also hypothesize that high temperature ammonia etching of micropores during pyrolysis creates potential catalytic site hosts. 6 Although microporosity can have a significant impact on activity, species transport to reactive sites occurs most efficiently through mesopores. This suggests that, for geometrically complex materials, mesoporosity will have a significant impact on the apparent catalyst activity. 7-9

Integration of Platinum Group Metal‐Free Catalysts and Bilirubin Oxidase into a Hybrid Material for Oxygen Reduction: Interplay of Chemistry and Morphology

“…[64] Pore-size distributions were obtained from the isotherms using the BJH [65] and nonlocal DFT approaches. [63,66,67] BJH calculations were performed using the desorption branch of the isotherm, whereas DFT used the adsorption branch. All samples are smoothed using Accelerated Surface Area and Porosimetry System (ASAP) 2020 software for BJH analysis.…”

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

“…When ammonia was used as a N-source, a high amount of disordered carbon was shown to be beneficial besides the micropores [ 22 , 24 ]. On the contrary, Leonard et al postulated that in addition, mesoporosity can have an impact on nitrogen incorporation and catalytic activity [ 25 ]. The influence of further physical properties of carbon support concerning undesired particle formation during the Fe-N-C synthesis was not reported in these studies.…”

Influence of Transition Metal on Oxygen Reduction Activity of Metal-Nitrogen-Carbon Electrocatalyst

“…Melamine is used as a nitrogen precursor due to the fact that it has been shown to have superior activity for use in catalysts (11). Additionally, Ketjenblack was used as a carbon source due to its optimum micro-and meso-porosity and electronic conductivity in the catalyst (12). To optimize the iron loading, catalysts were prepared with various loadings of iron, namely 0.9, 1.2, 1.7, 2.3 and 4.6 wt% with respect to carbon.…”