•Enhanced CuHCF performance due to MWCNTs, improving charge transfer.•Ion selectivity sequence determined: K+ > Ca2+ > Na+ > Mg2+.•Voltage control modulates CuHCF@CNT ion selectivity factor. This study utilizes a simple co-precipitation method to synthesize copper hexacyanoferrate (CuHCF) composites reinforced with carbon nanotubes (CNTs) (denoted as CuHCF@CNTs) serving as superior electrode materials for the electrochemical deionization (ECDI) application, a versatile method for brackish water deionization, water softening, and heavy metal removal. The enhanced performance of CuHCF, one of the Prussian blue analogues (PBAs) with typical Faradaic cation-capturing capability, is attributed to the increased charge transfer and a large surface area provided by multi-wall carbon nanotubes (MWCNTs) for the promoting the utilization of CuHCF units within the composites. The ion selectivity of electrode materials is investigated using both batch and microfluidic reactors, revealing a selectivity sequence of K+ > Ca2+ > Na+ > Mg2+. Additionally, this study introduces a novel approach for controlling the ion selectivity of CuHCF@CNT from the operating voltage. By varying the cell voltage, the selectivity factor, βK/Mg, can be increased from 1.7 at 1.2 V to 4.5 at 0.6 V. Our research offers a new perspective on the ion-capturing selectivity of ECDI systems using Faradaic materials in deionization and ion-concentrating applications.