Insight into the Mechanism of Oxygen Reduction Reaction on Micro/Mesoporous Carbons: Ultramicropores versus Nitrogen-Containing Catalytic Centers in Ordered Pore Structure

Abstract

Ordered mesoporous/microporous carbon was synthesized from sucrose. The carbon was oxidized and modified with urea at 600, 800, and 950 °C. The obtained carbons differed in the pore size distributions in the meso- and micropore ranges. The carbons, after an extensive surface characterization, were used as ORR catalysts in a KOH electrolyte. Kinetic current densities and n reached 12.5 mA cm–2 and 3.94, respectively. The results suggested that small micropores increased the efficiency of ORR. The density of surface oxygen in mesopores had also a positive effect on ORR by increasing the hydrophilicity and the electrochemically active surface area (ECSA) and, thus, the accessibility of oxygen dissolved in the electrolyte to the pores of the highest adsorption potential. From the viewpoint of the number of electron transfers, current density, and onset potential, the carbons with small pore sizes and with only traces of nitrogen performed equally, or even better, in ORR process as those containing the catalytic nitrogen sites.

Selectivity of Mixed Iron-Cobalt Spinels Deposited on a N,S-Doped Mesoporous Carbon Support in the Oxygen Reduction Reaction in Alkaline Media

“…The well-defined redox peaks at ~0-0.3 V correspond to oxidation/reduction reactions of the Some asymmetry of the anode and cathode current density between the visible redox pair, and the absence of a second reduction peak, may suggest quasi-reversibility or irreversibility of the involved redox processes [57]. Moreover, in the case of this sample, noticeably higher capacitive currents were recorded, which are consistent with the high contribution of the capacitive effects resulting from the carbon support of high specific surface area [58,59]. Along with the increase in the iron content a systematic increase of the pseudo-capacitive effects on the registered current densities was observed.…”

Exploring the Silent Aspect of Carbon Nanopores

“…This directed us to hypothesize that a hydrophobic surface (ethers are hydrophobic [92]) and small pores advance the oxygen reduction reaction by providing sites for strong adsorption of oxygen upon its withdrawal from an electrolyte, and in these pores, the reduction process is enhanced. To further support our hypothesis, we expanded the research to porous carbons containing heteroatom-based groups known to catalytically enhance ORR [54][55][56][57]. The introduction of N, S, and B to the carbon matrix was our goal, and the spatial geometry of To further support our hypothesis, we expanded the research to porous carbons containing heteroatom-based groups known to catalytically enhance ORR [54][55][56][57].…”

“…To further support our hypothesis, we expanded the research to porous carbons containing heteroatom-based groups known to catalytically enhance ORR [54][55][56][57]. The introduction of N, S, and B to the carbon matrix was our goal, and the spatial geometry of To further support our hypothesis, we expanded the research to porous carbons containing heteroatom-based groups known to catalytically enhance ORR [54][55][56][57]. The introduction of N, S, and B to the carbon matrix was our goal, and the spatial geometry of those groups and the modification means used rather excluded their existence in ultramicropores with size less than 0.7 nm.…”

Engaging nanoporous carbons in “beyond adsorption” applications: Characterization, challenges and performance

“…Quite recently, the search expanded and nanoporous carbons have been also included in that quickly advancing quest. Similarly to their other applications, there are reports in the literature indicating the importance of the porosity [83][84][85][86][87][88] and oxygen adsorption on the nanoporous carbons [89][90][91][92] for achieving an efficient ORR. The latter is usually described by the electron transfer number, kinetic current density, and onset potential.…”

“…A descriptor for active sites was based on the number of electrons occupying the outermost p orbital and electronegativity, and then correlated to the catalytic activity. Another example is the Pore Influence Factor (PIF), a descriptor of the catalytic activity of metal-free carbons for ORR proposed by Barrera et al [92]. It combines the effects of the number of dissociating groups (affecting hydrophilicity), ECSA (electrochemically active surface area) and the volume of ultramicropores (affecting the strength of O 2 adsorption) on the electron transfer number.…”