Understanding the role of nitrogen-doping and surface topology in the binding of Fe(III)/Fe(II) to biobased carbon electrodes

Abstract

Low-cost and high performance electrodes are critical to advance electrochemical energy storage devices that decouple energy supply from demand. At their core, carbon is ubiquitously employed given its availability, chemical and electrochemical stability, electrical conductivity, and affordable cost. However, due to their relative inertness, carbonaceous electrodes suffer from limited wettability and kinetic activity with aqueous electrolytes. A common approach is to introduce heteroatoms, either through post-processing (thermal/acid activation) or by employing different precursors. Specifically, biobased carbons like hydrochar and biochar are rich in heteroatoms that are naturally incorporated through the production process into the electrode structure. However, achieving a fundamental understanding of the interactions between metal ions and carbon surfaces has proven elusive, leading researchers to rely on empirical approaches for heteroatom doping of carbons. To achieve a better understanding of the fundamental mechanisms, we performed density functional theory calculations of a commonly employed iron redox couple, Fe(III) and Fe(II). We investigated binding mechanisms in graphitic carbon model systems with different surface features, and explored the effect of nitrogen doping and surface topology on the binding energy, as well as the effect of ions' spin multiplicity in the carbon-metal coordination mechanisms. Our results suggest that the interactions of Fe(III) and Fe(II) ions with the nitrogen-doped carbon electrodes not only depend on the surface curvature or the nitrogen content and functionality, but also on the spin multiplicity of the metal ion. Iron ions always evolve into an open-shell electronic structure with a high number of unpaired electrons to increase their coordination sphere with the graphitic surface. We hope that our findings can assist the development of fit-for-purpose heteroatom-doped carbon electrodes with a tailored nanostructure for electrochemical devices utilizing the Fe(III)/Fe(II) redox couple.