Abstract
We investigate kinetic barriers for the oxygen evolution reaction (OER) on singly and doubly nitrogen-doped single-walled carbon nanotubes (NCNTs) using the climbing image nudged elastic band method with solvent effects represented by a 45-water-molecule droplet. The studied sites were chosen based on a previous study of the same systems utilizing a thermodynamic model which ignored both solvent effects and kinetic barriers. According to that model, the two studied sites, one on a singly nitrogen-doped CNT and the other on a doubly doped CNT, were approximately equally suitable for OER. For the four-step OER process, however, our reaction barrier calculations showed a clear difference in the rate-determining *OOH formation step between the two systems, with barrier heights differing by more than 0.4 eV. Thus, the simple thermodynamic model may alone be insufficient for identifying optimal OER sites. Of the remaining three reaction steps, the two H2O forming ones were found to be barrierless in all cases. We also performed solvent-free barrier calculations on NCNTs and undoped CNTs. Substantial differences were observed in the energies of the intermediates when the solvent was present. In general, the observed low activation energy barriers for these reactions corroborate both experimental and theoretical findings of the utility of NCNTs for OER catalysis.
Highlights
With the looming threat of climate change, impending rise in global population, and increasing energy demands, the development of alternative and sustainable energy sources becomes essential.[1−3] Because of the intermittent availability of sources such as solar and wind energy, an efficient storage system is required to realize their potential fully.[1,2] Long-term storage capability, the absence of carbon emissions during conversion, high energy density, and flexibility of use both in mobile and stationary applications make hydrogen an appealing energy vector.[4]
The barrierlessness manifested itself in that the system relaxed to the final geometry of the nudged elastic band (NEB) process directly during the geometry optimization of the initial image, even in cases where the distance between the approaching OH− and the reacting surface species was large
We have looked at the kinetic barriers for the fourstep oxygen evolution reaction (OER) on pristine and nitrogen-doped carbon nanotubes (CNTs), both with and without the solvent
Summary
With the looming threat of climate change, impending rise in global population, and increasing energy demands, the development of alternative and sustainable energy sources becomes essential.[1−3] Because of the intermittent availability of sources such as solar and wind energy, an efficient storage system is required to realize their potential fully.[1,2] Long-term storage capability, the absence of carbon emissions during conversion, high energy density, and flexibility of use both in mobile and stationary applications make hydrogen an appealing energy vector.[4]. With larger amounts of nitrogen, pyrrolic five-ring defects start to appear which facilitate the closing of the nanotube through cap formation.[27] Importantly, Received: April 6, 2018 Revised: May 21, 2018 Published: May 25, 2018 Article many of these nitrogen-doping modifications can significantly improve the OER activity of the CNT in alkaline solutions.[28,29].
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