Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors

Abstract

The experimental development of catalytically ever-more active platinum group metal (PGM)-free materials for the oxygen reduction reaction (ORR) at fuel cell cathodes has been until recently a rather empirical iteration of synthesis and testing. Here, we present how kinetic reactivity maps based on kinetic descriptors of PGM-free single-metal-site ORR electrocatalysts can help to better understand the origin of catalytic reactivity and help to derive rational synthetic guidelines toward improved catalysts. Key in our analysis are the catalytic surface site density (SD) and the catalytic turnover frequency (TOF) in their role as controlling kinetic parameters for the ORR reactivity of PGM-free nitrogen-coordinated single-metal M-site carbon (MNC) catalysts. SD–TOF plots establish two-dimensional reactivity maps. We also consider the ratio between SD and the total number of single-metal sites in the bulk, referred to as the site utilization factor, which we propose as another guiding parameter for optimizing the synthesis of MNC catalysts. Exemplified by two sets of FeNC, CoNC, and SnNC catalysts prepared using two distinctly different N- and C-precursor material classes (Zn-based zeolitic imidazolate frameworks and covalent polyaniline), we comparatively diagnose the intrinsic kinetic ORR parameters as well as structural, morphological, and chemical properties. From there, we derive and discuss possible synthetic guidelines for further improvements. Our approach can be extended to other families of catalysts and may involve kinetic performance data of idealized liquid-electrolyte cells as well as gas diffusion layer-type flow cells.

FeNC Oxygen Reduction Electrocatalyst with High Utilisation Penta-coordinated sites

“…[35] This gives a lower bound active site density compared to CO cryo-chemisorption, which can probe electrochemically inaccessible sites and can also derive another utilisation factor. [28,64] A high initial SDnitrite of 2.54×10 19 sites gFeNC -1 is observed for TAP 900@Fe (Figure 6c). Kumar et al recently found in situ nitrite stripping also probes iron oxides that formed in situ as a result of degradation of FeNx sites.…”

Axial Coordination Effect on the Oxygen Reduction Reaction of FeN4 Electrocatalysts Based on Grand Canonical Density Functional Theory

“…Among the studied FeN 4 −Ls, FeN 4 − NH 2 with the highest intrinsic U onset exhibits the optimal ORR performance. Furthermore, it is found that axial ligand/metal coordination affects the electronic structure of Fe sites, especially the d z 2…”

“…Electrocatalysis of oxygen reduction reaction (ORR) is essential for fuel cell technology, which is promising for mitigating the ever-growing energy demands. Although the fuel cell technology has come a long way, its critical dependency on platinum-based catalysts makes such devices cost-prohibitive for large-scale commercialization. − The single atom catalysts (SACs) with tunable electronic structures and 100% metal atom utilization have received considerable interest as a potential economical alternative to the traditional platinum-based electrocatalysts. − Among the investigated metal SACs, the Fe–N–C catalyst has attracted extensive attention and has been massively synthesized to catalyze the oxygen reduction reaction, benefiting from its high activity that rivals that of the state-or-the-art Pt-based electrocatalysts. ,− The Fe–N–C catalyst can efficiently facilitate 4-electron transfer for H 2 O formation, yet the intrinsic catalytic site of Fe–N–C underlying its outstanding electrocatalytic activity is still ambiguous. Therefore, identifying the actual structure of Fe–N–C catalyst and increasing its activity are of realistic significance for boosting the catalytic performance of SACs.…”

Fe–N/C catalysts with tunable mesoporous structures and carbon layer numbers reveal the role of interlayer O2 activation

“…5). 27,31,32,39,55,60,61,63 Notably, the TOF of Meso-Fe-N/C_2.0 under acidic conditions exceeds the reported TOFs of Fe-N/C catalysts evaluated by the cryo-CO adsorption method. Notably, its value is even 1.6 times greater than that of the PANI-CM catalyst, 32,59 which is one of the highperforming Fe-N/C catalysts (Table S8, ESI †).…”

“…Synthetic advances in M-N/C catalysts that increase the density of accessible M-N x sites have enabled remarkable enhancement of their ORR activity; [32][33][34][35][36][37][38][39][40][41][42][43][44][45] effective suppression of undesirable H 2 O 2 formation has boosted long-term durability; [46][47][48][49] molecular-level synthetic control and precise speciation of M-N x sites have helped to identify active sites; [50][51][52][53][54] the exploration of non-d-block compositions has uncovered new p-and s-block-element-based M-N/C catalysts; [55][56][57][58] and the standardization of active site quantification and turnover frequency (TOF) calculations has made the comparison of catalytic activity data from multiple laboratories on fair ground. [59][60][61][62][63][64] One of the outstanding issues in heterogeneous M-N/C catalysts is the realization of a structural analog of the enzymatic ORR catalyst cytochrome c oxidase (CcO). CcO contains a bimetallic center of Fe and Cu and catalyzes the ORR with excellent efficiency under ambient conditions.…”