How nitrogen impurities and water affect atom-thick carbon electrodes inside of supercapacitors

What is it about?

Graphene is a form of carbon that is typically one- to few-layers thick. Due to its high electrical conductivity and surface area, it is a very promising electrode material for electrochemical energy storage devices, especially supercapacitors. Supercapacitors store energy in an electric field that is generated between the electrode and a (usually liquid) electrolyte. Electrolytes are typically made up of a salt in a solvent, while graphene electrodes typically include atoms non-carbon materials either on purpose (doping) or as impurities. This paper investigates how a common electrolyte solvent (water) and a common electrode dopant (nitrogen) change the overall material properties of the electrode, and how their presence influence the device's energy storage capability (capacitance).

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Photo by israel palacio on Unsplash

Photo by israel palacio on Unsplash

Why is it important?

Scientists typically employ theory and computer simulations to either (1) predict how an energy storage device made up of certain materials would behave before committing to the expense and time required for an experiment, or (2) to understand the mechanisms behind why certain designs work and others don't so experimental work becomes more targeted and fine-tuned. The problem is that when the electrode is only a single-atom thick, quantum mechanics comes into play and the models we have developed for how "bulk material" electrodes behave inside a supercapacitor break down. Importantly, we also cannot assume that graphene inside a device would have the same properties as graphene in isolation specifically because of these many-body quantum interactions with its environment. In this paper, we use large-scale quantum mechanical computer simulations to develop a new understanding of these devices and quantify their charge storage capability. This provides a toolbox for scientists to run "virtual experiments" on other combinations of electrodes and electrolytes in order to inform experimental design.