Fe3+ is effective for improving water splitting and decreasing voltage drop of bipolar membranes (BPMs), but such systems suffer from Fe3+ loss during electrolysis. In the present study, functionalized multi-walled carbon nanotubes (MWCNTs) were used to modify BPMs for reduced Fe3+ loss. The results of such modification indicate that the hydrophilicity, mechanical properties, and amount of residual Fe3+ obviously increased, while the Fe3+ loss and voltage drop obviously decreased after BPM modification. The amounts of residual Fe3+ contents in the BPMs before and after modification were 17.44 and 82.22 mu g/g of BPM, respectively. After electrolysis of the BPMs for 600 min at the current density 60 mA/cm(2), the voltage drop of the unmodified BPM increased from 2.25 to 3.56 V due to Fe3+ loss (a 1.31 V difference), while this value increased from 0.59 to 0.85 V for the BPM modified with functional MWCNTs (an increase of 0.26 V). The Fe3+ residual amount has a negative correlation with the voltages drop (R = -0.964, P = 0.036). These results indicate that MWCNT doping can effectively decrease Fe3+ loss and voltage drop. The present MWCNT doping technique has a certain reference function for the preparation of new BPMs. (C) 2017 Published by Elsevier B.V.