Pt catalysts in proton exchange membrane fuel cells (PEMFCs) typically use carbon blacks such as Vulcan (Vulcan is a registered trademark of the company Cabot Corporation) based on fossil sources. Thus, an important research task is using sustainable supports in PEMFCs. Hydrothermal carbonization (HTC) converts biomasses into chars, which are possible substitutes for fossil‐based carbons. Herein, a Pt catalyst derived from HTC of coconut shells is developed for catalysis of O2 reduction in acidic media. Thermal activation enlarges the specific surface area by factor of 7 to 546 m2 g−1 and generates electrical conductivity making the material suitable for catalysis. Pt particles of 1.8 ± 0.5 nm are distributed well on the activated carbon. Cyclic and CO stripping voltammetry show an electrochemical surface area (ECSA) of 69 ± 21 m2 gPt−1, almost identical to that of the commercial catalyst using Vulcan (69 ± 6 m2 gPt−1). Although ECSAs are highly comparable, the activity for O2 reduction is lower compared with the commercial catalyst. HTC‐derived carbon has a lower degree of graphitization, less functional oxygen groups on its surface, and a lower electrical conductivity than Vulcan. This suggests different Pt–support interactions.
Chemical activation of carbons is usually assigned to an oxidative and dehydrating nature of activating agents. We herein suggest that activating agents rather act as high temperature solvents and the porosity is developed by carbon phase separation.
Hydrochars from hydrothermal carbonization of different biowaste materials (dried dandelion, sawdust, coconut shell powder) formed in the presence of aqueous salt solutions were compared to those obtained by the common method in pure water. Hydrochars with increased carbon contents, pore volume and surface areas were specifically obtained from coconut shell powder in the presence of zinc chloride. Compositional and structural changes within the hydrochar products caused by the process conditions and/or the additive were characterized by solid state 13C NMR spectroscopy, proving that cellulose and, in particular, lignin units in the biomass are more easily attacked in the presence of the salt. Under saline conditions, a distinct particle break-up led to the creation of mesoporosity, as observable from hysteresis loops in nitrogen adsorption isotherms, which were indicative of the presence of pores with diameters of about 3 to 10 nm. The obtained hydrochars were still rich in functional groups which, together with the mesoporosity, indicates the compounds have a high potential for pollutant removal. This was documented by adsorption capacities for the methylene blue and methyl orange dyes, which exceeded the values obtained for other hydrochar-based adsorbers. A subsequent physical activation of the mesoporous hydrochars in steam at different temperatures and times resulted in a further drastic increase in the surface areas, of up to about 750 m2/g; however, this increase is mainly due to micropore formation coupled with a loss of surface functionality. Consequently, the adsorption capacity for the quite large dyes does not provide any further benefit, but the uptake of smaller gas molecules is favored.
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