Surface functional groups constitute major electroactive components in pyrogenic carbon. However, the electrochemical properties of pyrogenic carbon matrices and the kinetic preference of functional groups or carbon matrices for electron transfer remain unknown. Here we show that environmentally relevant pyrogenic carbon with average H/C and O/C ratios of less than 0.35 and 0.09 can directly transfer electrons more than three times faster than the charging and discharging cycles of surface functional groups and have a 1.5 V potential range for biogeochemical reactions that invoke electron transfer processes. Surface functional groups contribute to the overall electron flux of pyrogenic carbon to a lesser extent with greater pyrolysis temperature due to lower charging and discharging capacities, although the charging and discharging kinetics remain unchanged. This study could spur the development of a new generation of biogeochemical electron flux models that focus on the bacteria–carbon–mineral conductive network.

Maintaining high performance after extensive use remains a key challenge for low-Pt proton exchange membrane fuel cells for transportation applications. Strategically improving catalyst durability requires better understanding of the relationship between degradation mechanisms and catalyst structure. To investigate the effects of the carbon support morphology, we compare the electrochemical performance and durability of membrane electrode assemblies (MEAs) using Pt and PtCo_x catalysts with a range of porous, solid, and intermediate carbon supports (HSC, Vulcan, and acetylene black). We find that electrochemical surface area (ECSA) retention after a catalyst-targeted durability test tends to improve with increasing support porosity. Using electron microscopy, we investigate microstructural changes in the catalysts and reveal the underlying degradation mechanisms in MEA specimens. Pt migration to the membrane and catalyst coarsening, measured microscopically, together were quantitatively consistent with the ECSA loss, indicating that these were the only two significant degradation pathways. Changes in catalyst particle size, morphology, and PtCo core-shell structure indicate that Ostwald ripening is a significant coarsening mechanism for catalysts on all carbons, while particle coalescence is only significant on the more solid carbon supports. Porous carbon supports thus appear to protect against particle coalescence, providing an effective strategy for mitigating catalyst coarsening.

The capacity of soil as a carbon (C) sink is mediated by interactions between organic matter and mineral phases. However, previously proposed layered accumulation of organic matter within aggregate organo–mineral microstructures has not yet been confirmed by direct visualization at the necessary nanometer-scale spatial resolution. Here, we identify disordered micrometer-size organic phases rather than previously reported ordered gradients in C functional groups. Using cryo-electron microscopy with electron energy loss spectroscopy (EELS), we show organo–organic interfaces in contrast to exclusively organo–mineral interfaces. Single-digit nanometer-size layers of C forms were detected at the organo–organic interface, showing alkyl C and nitrogen (N) enrichment (by 4 and 7%, respectively). At the organo–mineral interface, 88% (72–92%) and 33% (16–53%) enrichment of N and oxidized C, respectively, indicate different stabilization processes than at organo–organic interfaces. However, N enrichment at both interface types points towards the importance of N-rich residues for greater C sequestration.