Journal:Acta Physico-Chimica Sinica
Key Words:Oxygen reduction reaction, Carbon-based non-precious metal catalyst, Active site, Electrocatalysis, Fuel cell
Abstract:Carbon-based non-precious metal catalysts, represented by pyrolyzed Fe/N/C, are the most promising catalysts to replace platinum for the oxygen reduction reaction (ORR). Therefore, further improvement of their performance will be significant for commercialization of proton exchange membrane fuel cells. Unveiling the nature of active sites at the atomic scale paves the way for rational designs of Fe/N/C catalysts with high activity and durability. Herein, we review the advances in the active site structure of carbon-based non-precious metal catalysts. Three types of active sites are discussed in the order of their ORR activity, namely, iron/nitrogen-containing sites, nitrogen-containing sites, and carbon defects. In the iron/nitrogen-containing sites, some of iron atoms are amorphous and positioned in a porphyrin-like plane structure with single-iron-atom coordinated to nitrogen. Iron in porphyrin-like sites is believed to directly bind to dioxygen with electron transfer from eg-orbitals (dz2) of iron to antibonding orbitals of dioxygen. Factors governing the energy level of eg-orbitals (dz2) are certainly effected the ORR activity, including coordination number, atom type, axial ligand effect and electron-donating/withdrawing capability of the carbon matrix. The structures of porphyrin-like iron centers are described as four-coordinate FeN4, five-coordinate N-FeN2+2, O2-FeN4C12 and Fe-N2+2 bridging two graphene edges. Moreover, some highly active sites are proposed with basic N-group or defective carbon neighboring the Fe-N center. It is worth noting that surface probing is a powerful tool to identify porphyrin-like iron sites, as well as to estimate its density and turnover frequency. The prospect of surface probe is combined with spectroscopy techniques that will be tremendously helpful in providing further insights of pyrolyzed Fe/N/C. Besides the iron in porphyrin-like sites, other iron atoms are incorporated into crystalline iron nanoparticles and clusters, which are speculated to facilitate electron transfer from nitrogen-doped carbon to dioxygen. However, the role of crystalline iron remains uncertain, because conflicting experimental results are often observed when crystalline iron is removed. Nonetheless, it is undoubted that the iron doping highly boosts the ORR activity of carbon-based catalysts. The next category consists of the nitrogen-containing sites. Various models have been developed to describe the nitrogen-doping carbon catalysts. These include synthesis of planar nitrogen by the layer-by-layer space-confined method, controlled-synthesis of nitrogen on highly-oriented pyrolytic graphite, and selective graft of acetyl group on pyridinic-nitrogen. Strong evidences from models of nitrogen-doping carbon catalysts identify the ortho-carbon atom of the pyridinic ring is the reactive site. The last sites, dopant-free defective carbon, are also found to contribute to ORR. Exploring and summarizing the active sites of pyrolyzed Fe/N/C deepen our understanding of the structure-performance relationship and paves the way for new synthetic strategies. It is expected that the activity as well as stability of pyrolyzed Fe/N/C can be further improved, by exploring the active sites and the ORR mechanism.
Indexed by:Journal paper
Discipline:Engineering
Document Type:J
Volume:35
Issue:5
Page Number:472-485
ISSN No.:1000-6818
Translation or Not:no
CN No.:11-1891/O6
Date of Publication:2018-10-19
DOI number:10.3866/PKU.WHXB201806131
First Author:Chi CHEN
Co-author:Xiaodong YANG,Shigang SUN
Correspondence Author:Zhiyou ZHOU
Links to published journals:http://www.whxb.pku.edu.cn/EN/10.3866/PKU.WHXB201806131