Development of nitrogen-doped carbon for selective metal ion capture

Nitrogen-doped carbons are promising materials for applications in capacitive deionization, electrosorption and aqueous ion separation, as nitrogen atoms doped into a carbon matrix are active for the selective capture of metal ions. However, much remains unknown about the preferential interaction behavior between metal ions and nitrogen-containing active sites, which retards the development of high-performance capacitive materials for practical applications. Here, we developed two types of nitrogen-doped carbon: pyridinic N-dominated graphene (N-6-G) and pyrrolic N-dominated graphene (N-5-G). The pyridinic-N was found to prefer binding “hard” ions (H+ and Na+), while pyrrolic-N showed enhanced performance in capturing “soft” ions (Pb2+). Compared to pristine reduced graphene oxide, the specific capacitance of N-6-G in Na2SO4 electrolyte increased by 50.0% (239.7 F g−1), while an increase of only 22.1% was found for N-5-G (195.2 F g−1), which demonstrate the superior capture capacity of pyridinic N toward Na+. However, for the electrosorption of Pb2+ ion, the maximum adsorption capacity per surface area of N-5-G was 2.43 mg m−2, which is 3.3 times as high as N-6-G (0.73 mg m−2). Furthermore, the distribution coefficient (Kd) value of N-5-G is ∼50 times higher than N-6-G. It is proposed that soft-soft and hard-hard interactions exist between nitrogen-containing groups and metal ions. These findings set a new benchmark for the capture of aqueous metal ions and suggest the great potential of N-doped carbon for challenging toxic metal pollution issues.