Vacancy and defect engineering in N, S co-doped carbon Nanosheets: From waste valorization to superior desalination via membrane capacitive deionization

Membrane capacitive deionization (MCDI) shows potential for brackish water desalination, but is limited by inefficient electrode materials. We developed a porous carbon electrode by co-pyrolyzing exhausted reverse osmosis membranes (EROM) with organic potassium salts (PIPES-K2). Our findings show that the PIPES-K2 doping ratio impacts desalination performance more than the co-pyrolysis temperature. At a 3:1 PIPES-K2-to-EROM ratio, the NS-EROMC3 electrode achieved a salt adsorption capacity (SAC) of 32.76 mg g −1 , showing the highest performance. Surface analysis confirmed that interconnected hierarchical pores and defective edge-N/S sites in NS-EROMC3 were key to its efficiency. NS-EROMC3 demonstrated exceptional cycling stability (< 20 % capacity loss over 60 cycles) and adaptability to diluted saline-alkali soil drainage. In situ Raman and Density Functional Theory (DFT) calculations attributed this performance to carbon vacancies and enriched edge-N/S defects. The PIPES-K2 co-pyrolysis strategy was extended to other carbon precursors. This study provides a scalable method for producing high-performance MCDI electrodes while repurposing wasteful resources.