N-doped carbon, a promising alternative to Pt catalyst for oxygen reduction reactions (ORRs) in acidic media, is modified in order to increase its catalytic activity through the additional doping of B and P at the carbon growth step. This additional doping alters the electrical, physical, and morphological properties of the carbon. The B-doping reinforces the sp(2)-structure of graphite and increases the portion of pyridinic-N sites in the carbon lattice, whereas P-doping enhances the charge delocalization of the carbon atoms and produces carbon structures with many edge sites. These electrical and physical alternations of the N-doped carbon are more favorable for the reduction of the oxygen on the carbon surface. Compared with N-doped carbon, B,N-doped or P,N-doped carbon shows 1.2 or 2.1 times higher ORR activity at 0.6 V (vs RHE) in acidic media. The most active catalyst in the reaction is the ternary-doped carbon (B,P,N-doped carbon), which records -6.0 mA/mg of mass activity at 0.6 V (vs RHE), and it is 2.3 times higher than that of the N-doped carbon. These results imply that the binary or ternary doping of B and P with N into carbon induces remarkable performance enhancements, and the charge delocalization of the carbon atoms or number of edge sites of the carbon is a significant factor in deciding the oxygen reduction activity in carbon-based catalysts.

Maximum atom efficiency as well as distinct chemoselectivity is expected for electrocatalysis on atomically dispersed (or single site) metal centres, but its realization remains challenging so far, because carbon, as the most widely used electrocatalyst support, cannot effectively stabilize them. Here we report that a sulfur-doped zeolite-templated carbon, simultaneously exhibiting large sulfur content (17 wt% S), as well as a unique carbon structure (that is, highly curved three-dimensional networks of graphene nanoribbons), can stabilize a relatively high loading of platinum (5 wt%) in the form of highly dispersed species including site isolated atoms. In the oxygen reduction reaction, this catalyst does not follow a conventional four-electron pathway producing H2O, but selectively produces H2O2 even over extended times without significant degradation of the activity. Thus, this approach constitutes a potentially promising route for producing important fine chemical H2O2, and also offers opportunities for tuning the selectivity of other electrochemical reactions on various metal catalysts.

Exposing Fe–N–C catalysts to H2O2-byproduct leaves their catalytic sites untouched but decreases the turnover frequency via oxidation of the carbon surface.

Fundamental understanding of non-precious metal catalysts for the oxygen reduction reaction (ORR) is the nub for the successful replacement of noble Pt in fuel cells and, therefore, of central importance for a technological breakthrough. Herein, the degradation mechanisms of a model high-performance Fe-N-C catalyst have been studied with online inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectroscopy (DEMS) coupled to a modified scanning flow cell (SFC) system. We demonstrate that Fe leaching from iron particles occurs at low potential (<0.7 V) without a direct adverse effect on the ORR activity, while carbon oxidation occurs at high potential (>0.9 V) with a destruction of active sites such as FeNx Cy species. Operando techniques combined with identical location-scanning transmission electron spectroscopy (IL-STEM) identify that the latter mechanism leads to a major ORR activity decay, depending on the upper potential limit and electrolyte temperature. Stable operando potential windows and operational strategies are suggested for avoiding degradation of Fe-N-C catalysts in acidic medium.

For a successful replacement of Pt, tremendous efforts have hitherto been made to develop high-performing Fe-N-C catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In comparison to the remarkable progress in activity, the stability of Fe-N-C catalysts still remains critical, however. Fe demetallation in acidic medium is hypothesized to be one critical factor for the overall lifetime. In contrast to the general belief, we herein demonstrate using an operando spectroscopic analysis that catalytically inactive Fe particles exposed to acid electrolytes cannot be fully removed by acid washing due to a relatively high open circuit potential (ca. 0.9 VRHE) leading to the formation of insoluble ferric species, whereas these particles dissolve under PEMFC operating conditions (E cathode < 0.7 VRHE) due to operando reduction to soluble ferrous cations. To overcome this issue, we demonstrate two approaches: (i) synthesis of Fe-N-C catalysts free of Fe particles and (ii) postsynthesis removal of exposed Fe particles through the control of potential using an external potentiostat or an internal reducing agent (i.e., SnCl2). Operando spectroscopic analyses verified that Fe demetallation during a given voltammetric protocol was dramatically decreased for both synthetically and postsynthetically modified Fe-N-C catalysts, while the initial ORR activity did not significantly change. However, all of these catalysts showed similar performance decay over short-term PEMFC durability tests, demonstrating the lack of a role played by ferrous cations leached from inactive Fe particles on catalyst deactivation. This supports the view that the activity is mainly imparted by FeN x C y moieties. Nevertheless, the presented guidelines are generally applicable to the whole class of Fe-N-C catalysts in order to minimize Fe demetallation in PEMFCs, which provides important advances for the future design of stable electrocatalytic systems for long-term operation.

Graphene has been highlighted recently as a promising material for energy conversion due to its unique properties deriving from a two-dimensional layered structure of sp 2 -hybridized carbon. Herein, N-doped graphene (NGr) is developed for its application in oxygen reduction reactions (ORRs) in acidic media, and additional doping of B or P into the NGr is attempted to enhance the ORR performance. The NGr exhibits an onset potential of 0.84 V and a mass activity of 0.45 mA mg À1 at 0.75 V. However, the B, N-(BNGr) and P, N-doped graphene (PNGr) show onset potentials of 0.86 and 0.87 V, and mass activities of 0.53 and 0.80 mA mg À1 , respectively, which are correspondingly 1.2 and 1.8 times higher than those of the NGr. Moreover, an additional doping of B or P effectively reduces the production of H 2 O 2 in the ORRs, and shows much higher stability than that of Pt/C in acidic media. It is proposed that the improvement in the ORR activity results from the enhanced asymmetry of the spin density or electron transfer on the basal plane of the graphene, and the decrease in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphene through additional doping of B or P.

Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2 . However, successful CO2 reduction still suffers from the lack of affordable catalyst design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx /Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2 (.-) intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi-branched conifer-like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 μmol h(-1)  cm(-2) ) at -1.36 VRHE without any considerable catalytic degradation over 18 h of operation.

Continuous on-site electrochemical production of hydrogen peroxide (H 2 O 2 ) can provide an attractive alternative to the present anthraquinone-based H 2 O 2 production technology. A major challenge in the electrocatalyst design for H 2 O 2 production is that O 2 adsorption on the Pt surface thermodynamically favors "side-on" configuration over "end-on" configuration, which leads to a dissociation of O−O bond via dominant 4-electron pathway. This prefers H 2 O production rather than H 2 O 2 production during the electrochemical oxygen reduction reaction (ORR). In the present work, we demonstrate that controlled coating of Pt catalysts with amorphous carbon layers can induce selective end-on adsorption of O 2 on the Pt surface by eliminating accessible Pt ensemble sites, which allows significantly enhanced H 2 O 2 production selectivity in the ORR. Experimental results and theoretical modeling reveal that 4-electron pathway is strongly suppressed in the course of ORR due to a thermodynamically unfavored end-on adsorption of O 2 (the first electron transfer step) with 0.54 V overpotential. As a result, the carbon-coated Pt catalysts show an onset potential of ∼0.7 V for ORR and remarkably enhanced H 2 O 2 selectivity up to 41%. Notably, the produced H 2 O 2 cannot access the Pt surface due to the steric hindrance of the coated carbon layers, and thus no significant H 2 O 2 decomposition via disproportionation/ reduction reactions is observed. Furthermore, the catalyst shows superior stability without considerable performance degradation because the amorphous carbon layers protect Pt catalysts against the leaching and ripening in acidic operating conditions.