Understanding the Roles of Nitrogen Configurations in Hydrogen Evolution: Trace Atomic Cobalt Boosts the Activity of Planar Nitrogen-Doped Graphene

Abstract

Revealing the exact roles of nitrogen configurations and precise control of the nitrogen configuration in nitrogen-doped graphene (NG) are extremely important for realizing its advanced functions in clean energy technologies. Herein, for the first time, we established that the hydrogen evolution reaction (HER) activities of NG display definite trends due to its nitrogen configurations, which were selectively generated by using layer-structured montmorillonite (MMT) with different layer distances and functions modulated by Co2+, Ni2+, Na+, and H+ ions. We found that among the three types of N, i.e., pyridine, pyrrolic, and quaternary N, quaternary N is the most active one for HER in a metal-free NG catalyst, whereas with an introduction of trace atomic cobalt, the planar (pyridine and pyrrolic) N becomes the better one. In contrast, when trace atomic Ni is involved to replace the Co, the former results in heavily depressed HER activities for the NG catalyst. Density functional theory calculations further revealed that (i) the carbon atoms are highly activated for HER by the nearby quaternary N but not by planar N and (ii) nickel blocks but cobalt promotes the hydrogen adsorption after coordination with planar N, leading to an excellent HER performance.