Anion-kinetics-selective graphene anode and cation-energy-selective MXene cathode for high-performance capacitive deionization

Capacitive deionization (CDI) is one of the most promising energy-efficient technologies for water desalination, however its industrial translation is slow and impeded by limited electrosorption capacity, slow electrosorption rate and poor cycling stability. Herein we address the above challenges by developing and validating a new concept of unimpeded and selective full-cell anion-cation separation using custom-designed anion-kinetics-selective anode and cation-energy-selective cathode. Nanoporous graphene anode enables the selective anion kinetics, while cation selectivity on the functionalized MXene cathode is due to the difference in ion adsorption energy. These mechanisms are validated by electrochemical quartz crystal microbalance measurements. The finely-tuned balance between ion transport and adsorption causes unimpeded ion diffusion, leading to the higher electrosorption rates. These effects are guided by the atomistic and quantum chemistry simulations, and also confirmed experimentally by tuning the pore size of graphene anode and the functionalization of MXene cathode. The fabricated asymmetric CDI cell exhibits superior electrosorption capacity of 49 mg g−1 and high electrosorption rate of 2.92 mg g−1 min−1 in 5000 mg L−1 NaCl solution as well as excellent cycling stability (100 cycles), which are among the best of the current state-of-the-art CDI studies. These results demonstrate the design of advanced electrode materials for the effective control of ion kinetics and energies to achieve selective ion transport, thus providing new insights for a broader range of energy-related applications.