Voltage was recorded between two RE (Ag/AgCl reference electrodes). Thus, the current and voltage drop across the membrane was collected. Linear sweeping voltammetry (LSV) was applied with a scan rate of 3 mV s -1 to collect the polarization curve. The BPM showed similar potential of the onset of water dissociation (~ 0.83 V) and current density at various pH couples (Fig. ). Energy efficiency penalty when using different pH pairs (Fig. )