We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated to nanoparticles of metal/alloy/oxide, but are instead based on Fe and Sn sub-nanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art.

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The functionalized carbon nanostructure, nitrogen and sulfur co-doped iron-containing highly porous carbon gel (obtained via carbonization of corresponding organic gel) is considered here as a non-precious metal catalyst for the electroreduction of carbon dioxide in neutral solution (potassium bicarbonate at pH = 6.8). Various electrochemical measurements (performed in distinct modes and under different conditions) have been utilized to comment about the catalyst performance and the reaction mechanism, in particular about the reaction products and the relative contribution from hydrogen evolution. At low overpotentials, the carbon dioxide reduction is favored over the hydrogen evolution reaction. Combination of conventional (stationary) stripping-type and hydrodynamic (rotating ring-disk electrode) voltammetric approaches has been demonstrated to function as an unique electroanalytical tool here. The CO 2 -reduction products can be identified as adsorbates (oxidative stripping voltammetry) or monitored continuously (electrooxidation) at the Pt ring electrode. Finally, the results of both electrochemical diagnostic and parallel gas chromatographic analytical measurements are consistent with the view that, in the examined range of the potentials, CO is the main CO 2 -reduction product. The electrochemical reduction of carbon dioxide (CO 2 ) permits, in principle, selective generation of carbon-based fuels (or syngas) and, indirectly, lowering population of one of green-house gases in the atmosphere. Due to existence of two strong C=O bonds in the CO 2 molecule, its electroreduction requires high overpotentials, and the low reaction efficiency still remains fundamental problem limiting practical applications. 1-3During recent years, various catalysts, including noble and non-noble transition metals, as well as their numerous coordination compounds, have been considered as catalysts for CO 2 electroreduction. [4][5][6][7][8][9][10] It has been demonstrated that selectivity of the process depends largely on the activating adsorptive (CO 2 ) phenomena and the affinity of catalytic centers to adsorbed carbon monoxide intermediate. 11 Depending on the strength of adsorption and according to the Sabatier principle, CO may be further protonated or hydrogenated to CHO adsorbate. This step is often considered as the rate-determining one during the hydrocarbon formation.12 It should be noted here that, for example, copper is capable of successfully catalyzing electroreduction of CO 2 to multicarbon products, 13-15 palladium induces the reaction mainly to CO and H 2 and only small amounts of formate. 2,[16][17][18][19] Another important practical issue is related to the need of lowering costs of catalysts by substituting the noble metals with inexpensive abundant elements. There has been growing interest in catalysts based on nanostructured carbons that include the metal-free Ndoped systems and the metal-containing carbons. [20][21][22][23][24][25][26] For example, the biomimetic centers containing metallic moieties in the vi...