Understanding the Roles of Sulfur Dopants in Carbonaceous Electrocatalysts for the Oxygen Reduction Reaction: The Relationship between Catalytic Activity and Work Function

Abstract

We prepared a series of hollow sulfur-doped carbons with diverse S contents through the carbonization of microporous organic networks (MONs), which were synthesized through the Sonogashira coupling of thiophene moieties with different numbers of S atoms as building blocks. This preparation method enabled the doping level to be controlled without inducing any notable differences in textural and morphological characteristics, and these S-doped carbons did not show any notable differences in the chemical properties of carbon, regardless of the sulfur content. We used these well-controlled MON-derived carbons as a model to elucidate the role of sulfur dopants in the oxygen reduction reaction (ORR) and to investigate the relationship between the activities and work functions of carbonaceous catalysts. By excluding the effect of electrical properties of the S-doped carbon catalysts using conducting agents, we could successfully verify that increasing the number of dopants led to an enhancement in the ORR activities, and the high applicability of work function as the activity descriptor was also demonstrated. We believe that our experimental observations will provide a deeper understanding of carbonaceous electrocatalysts with p-block dopants, and the investigations performed in this study are also anticipated to serve as a rational guideline in designing carbonaceous catalysts for various electrochemical reactions.

General Method for Synthesis Transition‐Metal Phosphide/Nitrogen and Phosphide Doped Carbon Materials with Yolk‐Shell Structure for Oxygen Reduction Reaction

“…Figure A shows the normalized C K‐edge XANES spectra of Fe 2 P/NPC, Co 2 P/NPC and Ni 2 P/NPC characterized with three main peaks A, B 2 and C at about 285.8 eV, 288.7 eV and 292.8 eV, respectively. The intensity feature A represents a π* transition of C=C, the broad peak C represents the σ* transitions of C−C bond . And the strong peak B 2 together with shoulder peak B 1 are considered as contributions predominantly from C−O and C=O .…”

Highly Efficient Electrochemical Reduction of Nitrogen to Ammonia on Surface Termination Modified Ti3C2Tx MXene Nanosheets

“…The work function is a critical parameter for predicting electrocatalytic performance of catalysts. A lower work function implies the density-of-states near the Fermi level and a lower energy barrier for effectively transferring electrons from the surface of catalysts to the absorbed N 2 molecule and reaction intermediates, thus expediting the kinetic activities of the NRR. ,, Besides, the charge density difference based on density functional theory (DFT) calculations in Figure d displayed the electron accumulation at the junction of Fe–MXene, indicating the strong interaction between Fe and MXene. Consequently, the negatively charged Fe atoms as active sites absorbed the N 2 molecule with the absorption energy of −0.573 eV and boosted the NRR (Figure e).…”

Rapid Inversion of Surface Charges in Heteroatom‐Doped Porous Carbon: A Route to Robust Electrochemical Desalination

“…In contrast, if the reaction takes place by electron transfers at the outer Helmholtz plane only, H 2 O 2 is generated as a consequence of the two‐electron transfer reaction. The introduction of nitrogen or sulfur dopants to carbon is known to be favorable for shifting the overall ORR to the four‐electron transfer reaction and this has been widely regarded as an efficient strategy to increase the catalytic performance of the cathode in alkaline fuel cells . Because our experimental condition was significantly different from that in the ORR in such fuel cells, wherein a concentrated base solution serves as electrolyte, total electron transfer numbers in ORR were estimated to understand the interactions between ACs and dissolved oxygen in a neutral electrolyte with low salt concentration.…”