Why Can Phosphorus-Doped Nanopores Improve the Capacitance of Electrochemical Double-Layer Capacitors Using Alkaline Electrolytes? A First-Principles Study

Abstract

To investigate how phosphorus-doped carbon micropores can significantly enhance the capacitance of electrochemical double-layer capacitors (EDLCs), we performed first-principles calculations on phosphorus-doped monolayer (MGP) and bilayer graphene (BGP-x, x = 4–8), simulating macropores or mesopores and micropores, respectively. In addition, we studied the stable structure, relative capacitance, and reaction characteristics of Na+, Li+, and their hydrated ion structures {[Na­(H2O) n ]+, [Li­(H2O) n ]+, n = 1–4} on MGP, as well as their insertion behavior in BGP-x. Surprisingly, we observed a desolvation phenomenon within the phosphorus-doped carbon micropores. The calculation results suggest that the adsorption of [Na­(H2O)4]+ and [Li­(H2O)4]+ on MGP is primarily attributed to the stronger OH–π interaction compared to the Na+–π and Li+–π interactions. On the other hand, the stability of the structure after embedding [Na­(H2O) n ]+ and [Li­(H2O) n ]+ in BGP-x can be explained by the opposite scenario. The equilibrium between these two interactions and the continuous reduction in interlayer spacing facilitate the desolvation of [Na­(H2O) n ]+ and [Li­(H2O) n ]+. A decrease in the number of water molecules bound to Na+ and Li+ ions leads to a maximum increase in relative capacitance of 2.7 and 3.0 times, respectively. This indicates that desolvation can significantly enhance the capacitance of EDLCs. The complete desolvation size for Na+ is less than 4 Å, while for Li+, it is 4.3 Å. Based on these calculations, when using phosphorus-doped carbon materials as electrode materials, selecting an electrolyte containing Li+ rather than Na+ would result in a higher capacitance. The calculation results reveal the mysterious mechanism behind the significant increase in capacitance caused by phosphorus-doped carbon micropores. In addition to this, they also provide us with a magical key to optimize the perfect combination of electrolyte composition and carbon material pore size/type.