Abstract
Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, were used as precursor materials for the creation of N-doped carbon electrodes with varying degrees of amorphization. Incorporation of N-sites was achieved via nitrogen plasma treatments which resulted in both surface functionalization and amorphization of the carbon electrode materials. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to monitor composition and carbon organization: results indicate incorporation of predominantly pyrrolic-N sites after relatively short treatment cycles (5 min or less), accompanied by an initial etching of amorphous regions followed by a slower process of amorphization of graphitized clusters. By leveraging the difference in the rate of these two processes it was possible to investigate the effects of chemical N-sites and C-defect sites on their electrochemical response. The materials were tested as metal-free electrocatalysts in the oxygen reduction reaction (ORR) in alkaline conditions. We find that the introduction of predominantly pyrrolic-N sites via plasma modification results in improvements in selectivity in the ORR, relative to the nitrogen-free precursor material. Introduction of defects through prolonged plasma exposure has a more pronounced and beneficial effect on ORR descriptors than introduction of N-sites alone, leading to both increased onset potentials, and reduced hydroperoxide yields relative to the nitrogen-free carbon material. Our results suggest that increased structural disorder/heterogeneity results in the introduction of carbon sites that might either serve as main activity sites, or that enhance the effects of N-functionalities in the ORR via synergistic effects.
Highlights
We propose that increased structural disorder/heterogeneity results in the introduction of carbon sites that could serve as the main sites or enhance the activating effects of N-functionalities in the oxygen reduction reaction (ORR)
Films were subsequently annealed under N2 atmosphere for 1 h at 900◦C resulting in graphitized amorphous carbon electrodes (Behan et al, 2018), that were used as the precursor material for nitrogen functionalization
The N 1s peak is absent in the case of amorphous carbon electrodes (anC), the N-free precursor material, as discussed in detail in previous work from our group (Behan et al, 2018), it is clearly detectable after only 5 min exposure to the N2-plasma indicating that this treatment leads to functionalization of the carbon electrode with N-sites
Summary
Carbon electrode materials are ubiquitous in energy applications due to several important desirable characteristics, namely, their earth-abundance, low cost, good scalability and processability, high conductivity, versatile morphology, tunable microstructure and crystallinity, and good mechanical properties (Georgakilas et al, 2015; Salanne et al, 2016; Dou et al, 2019). The importance of improving our understanding and control over the interplay among disorder, functional sites, and electrochemical performance has been highlighted in the recent literature as one of the frontiers in the development of new carbon-based materials for energy technologies (Legrain et al, 2015; Zhang et al, 2016; Bommier et al, 2019; Dou et al, 2019; Saurel et al, 2019). There is great interest in developing approaches for achieving control over nanostructuring and chemical functionality to enable a better understanding of structure-function correlations, and novel strategies for the synthesis of smart carbons with tailored ad-/chemisorption sites at heteroatom and carbon defect centers (Salanne et al, 2016; Dou et al, 2019)
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