The electroreduction of carbon dioxide is considered a key reaction for the valorization of CO 2 emitted in industrial processes or even present in the environment. Cobalt−nitrogen co-doped carbon materials featuring atomically dispersed Co−N sites have been shown to display superior activities and selectivities for the reduction of carbon dioxide to CO, which, in combination with H 2 (i.e., as syngas), is regarded as an added-value CO 2 -reduction product. Such catalysts can be synthesized using heat treatment steps that imply the carbonization of Co−N-containing precursors, but the detailed effects of the synthesis conditions and corresponding materials' composition on their catalytic activities have not been rigorously studied. To this end, in the present work, we synthesized cobalt−nitrogen co-doped carbon materials with different heat treatment temperatures and studied the relation among their surface-and Co-speciation and their CO 2 -to-CO electroreduction activity. Our results reveal that atomically dispersed cobalt−nitrogen sites are responsible for CO generation while suggesting that this CO-selectivity improves when these atomic Co−N centers are hosted in the carbon layers that cover the Co nanoparticles featured in the catalysts synthesized at higher heat treatment temperatures.

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR by-product. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs’ activity than the demetallation occurring at low potentials.

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The complex nature of liquid water saturation of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs) greatly affects the device performance. To investigate this problem, we present a method to quantify the presence of liquid water in a PEFC CL using small-angle X-ray scattering (SAXS). This method leverages the differences in electron densities between the solid catalyst matrix and the liquid water filled pores of the CL under both dry and wet conditions. This approach is validated using ex situ wetting experiments, which aid the study of the transient saturation of a CL in a flow cell configuration in situ. The azimuthally integrated scattering data are fitted using 3D morphology models of the CL under dry conditions. Different wetting scenarios are realized in silico, and the corresponding SAXS data are numerically simulated by a direct 3D Fourier transformation. The simulated SAXS profiles of the different wetting scenarios are used to interpret the measured SAXS data which allows the derivation of the most probable wetting mechanism within a flow cell electrode.

A new highly active family of Co‐promoted N‐doped carbon aerogel catalysts is prepared by a supercritical CO2‐assisted technique combined with NH3 treatment. The active sites are created during the complex transformation of an organic aerogel to a carbon aerogel in the presence of cobalt by NH3 as the source of nitrogen and pyrolysis medium. The mass activity for oxygen reduction reaction (ORR) is highest for the catalyst pyrolyzed at 800 °C. The catalyst displays similar activity for ORR as a commercial Pt/C catalyst. Moreover, the catalyst exhibits nearly double the mass activity of N‐doped carbon aerogel pyrolyzed at the same temperature. The nature of the active sites for ORR is also investigated by selectively adding or removing various species (i.e., Co–Nx and Co–O) via post acid–base treatments. After removing the Co species from the catalyst, the same activity as the parent catalyst is achieved suggesting that the Co species in the catalyst are spectators in ORR catalysis and they serve only to promote the formation of catalytically active graphitic N sites during pyrolysis in NH3. A strong correlation is found between the relative distribution of graphitic N species and the mass activity.