N-doped MnO₂ with abundant oxygen vacancies achieves high-capacity and stable ammonium ion capture by capacitive deionization

Following the concept of sustainable development and circular economy, MnO2 is used as a capacitive deionization (CDI) electrode to remove ammonium (NH4+), which not only prevents water pollution and eutrophication, but also achieves the purpose of recycling precious resources. However, the structural instability and low electron conductivity of MnO2 are the major limitations to obtain excellent deammonium performance. Here, a vacancy-doping modification strategy was proposed to prepare N-doped MnO2 with abundant oxygen vacancies and high specific surface area (N-MnO2-x) by a simple one-step hydrothermal method. The synergistic effect of high N doping, abundant oxygen vacancies and porous structure in N-MnO2-x can effectively inhibit the JahnTeller distortion, improve the conductivity, boost ion diffusion and electron conduction, accelerate the charge-transfer reaction kinetics. Consequently, the N-MnO2-x presents an ultrahigh salt adsorption capacity (SAC) (134.3 mg g-1) and displays a superior cyclic stability over 100 cycles, exceeding that of most NH4+ removal materials. Interestedly, N-MnO2-x delivers a high selectivity for NH4+ in multi-ion mixed solution. The mechanism of NH4+ removal and selective strengthening were revealed by spectroscopic studies and theoretical calculations. The present study proposes a novel approach for the development of an efficient electrode capable of capturing NH4+ and facilitating the recycling of ammonia resources.