Combined Effect of Porosity and Surface Chemistry on the Electrochemical Reduction of Oxygen on Cellular Vitreous Carbon Foam Catalyst

Abstract

A new mechanism of O 2 reduction, which follows principles different from those generally accepted for describing ORR reduction on heteroatom-doped carbons, is suggested. It is based on the ability of oxygen to strongly adsorb in narrow hydrophobic pores. In this respect, a cellular vitreous carbon foam− graphene oxide composite was synthesized. The materials were doped with sulfur and nitrogen and/or heat-treated at 950 °C in order to modify their surface chemistry. The resultant samples presented a macro-/microporous nature and were tested as ORR catalysts. To understand the reduction process, their surfaces were extensively characterized from texture and chemistry points of view. The treatment applied markedly changed the volumes of small micropores and the surface hydrophilicity/polarity character. The results showed that the electron transfer number was between 3.87 and 3.96 and the onset potential reached 0.879 V for the best-performing sample. It is noteworthy that the best-performing sample has the highest volume of pores smaller than 0.7 nm while there was no heteroatom doping. The hydrophobicity and the strong adsorption forces provided by these pores to pull oxygen inside are the possible reasons for the observed excellent performance. A decrease in the volume of these pores resulted in a decrease in the catalytic performance. When the surface was modified with heteroatoms, the performances worsened further because of the induced hydrophilicity.

Oxygen Electroreduction on Nanoporous Carbons: Textural Features vs Nitrogen and Boron Catalytic Centers

“…Recently, Ozaki and coworkers indicated that besides B/N doping, a nanoshell structure of their catalyst might play a role in the reduction process . An indication on the role of pores and especially strong oxygen adsorption in the nanospace as an additional ORR mechanism has been recently reported by our groups ,. We found that on porous carbons practically free of N and S the electron transfer number was close to 4 (3.94).…”

“…This shows that a longer washing time enhances the porosity of the materials, especially the ultramicroporosity probably by leaching out small chemical compounds deposited on the surface. These pores might be crucial for ORR on our materials ,. From the porosity viewpoint, all tested samples are very heterogeneous and their hierarchical pore structure and a low degree of microporosity (V mic /V t between 0.30 and 0.53) might help in the transport of an electrolyte with dissolved oxygen to the catalytic centers.…”

“…These pores might be crucial for ORR on our materials. [45,46] From the porosity viewpoint, all tested samples are very heterogeneous and their hierarchical pore structure and a low degree of microporosity (V mic /V t between 0.30 and 0.53) might help in the transport of an electrolyte with dissolved oxygen to the catalytic centers. For this, especially the structure of CPSM seems to be the most favorable one.…”

“…[37] An indication on the role of pores and especially strong oxygen adsorption in the nanospace as an additional ORR mechanism has been recently reported by our groups. [45,46] We found that on porous carbons practically free of N and S the electron transfer number was close to 4 (3.94). The results suggested that small pores of hydrophobic nature withdraw oxygen from the electrolyte and the strong adsorption in these pores promotes oxygen bond splitting and electron transfer.…”

“…Since the nanopores of the catalysts are accessible for oxygen molecules but possibly not for bigger molecules like K 3 Fe(CN) 6 , the effective surface areas could not be accurately evaluated by the generally used chronocoulometry of K 3 Fe(CN) 6 method. Thus, the effective surface areas used to calculate the current density reported here are those directly calculated from the weight of the catalyst loaded on the electrode and its BET surface area measured from adsorption of nitrogen assuming that pores accessible for N 2 are also accessible for O 2 . For comparison, the experiments were also performed on the platinum‐containing graphitized carbon catalyst referred to as Pt/C (20 % Pt on Vulcan XC72)…”