Regulating the electrolyte ion types and exposed crystal facets for pseudocapacitive energy storage of transition metal nitrides

Pseudocapacitance is a charge storage behavior dominated by the surface kinetics of electrode materials, and it can differentiate into adsorption pseudocapacitance associated with surface redox reactions and intercalation pseudocapacitance associated with ion insertion in bulk without structural change. Despite major efforts to develop pseudocapacitive materials of transition metal nitrides (TMNs) and understand their energy storage mechanism, they are limited to identifying the specific pseudocapacitance that the target TMNs occurs, a probe into the internal matching relationship between the pseudocapacitance of TMNs and electrolyte ions as well as exposed crystal facets is still lacking. Here, we use mesoporous Mo2N nanowires as a subject of study to reveal the influences of the ion type of aqueous electrolytes and exposed crystal facets on the pseudocapacitive behavior of TMNs. Our results show that almost only solvated cations occur intercalation pseudocapacitance in acid and neutral electrolytes; solvated cations and anions both contribute energy in alkaline electrolyte by means of intercalation pseudocapacitance and adsorption pseudocapacitance, respectively. Crystal facet regulation further elucidates that the (111) crystal facet can provide larger interstitial sites for cation intercalation and more active terminations for anion adsorption compared with the (200) counterpart, thereby achieving a higher capacity. These findings offer a new insight on the pseudocapacitive energy storage, and for the first time emphasize the importance of crystal facet regulation for the rational design of pseudocapacitive materials.