We introduce a novel bipolar assembly of ion-exchange membranes as the separator for aqueous supercapacitors. The new bipolar separator enables the positive electrode and the negative electrode to operate in acidic electrolyte and alkaline electrolyte, separately. The bipolar separator increases the theoretically stable voltage window from 1.23 to 1.76 V for the device when pH 1 and pH 10 are selected for the positive and negative electrode, respectively, based on the pH tolerance of commercial ion exchange membranes. By considering the results from comprehensive electrolyte stability investigation, we charge the cells to 1.8 V, which increases the maximum cell discharge voltage to 1.77 V. A specific energy of 12.7 Wh/kg is achieved based on the electrodes' mass, which is twice the value of the comparative best performing single electrolyte. The new cell configuration with the three-compartment bipolar separator effectively prevents the acid/base electrolyte cross diffusion, where after 10,000 cycles, the capacitance retention is 97% with coulombic efficiency maintained above 99.6% all through cycling. Supercapacitors, also known as ultracapacitors or electrochemical capacitors (ECs), are highly complementary to batteries by providing high power density and ultra-long cycling life that the state-of-theart batteries have yet to achieve.1 These devices are indispensable in applications, including power cranking for transportation, energy recovery in heavy-duty systems 2 and potentially laser generation. In order to develop more powerful ECs, prior works have studied the impacts of carbon electrodes' properties on ECs' performance, including surface area, 3-8 pore size, 9-14 heteroatom doping, 15-19 and pseudocapacitance. [20][21][22][23][24][25][26][27] Recently, attention has also been paid to redox-active electrolytes. [28][29][30][31][32]47 As well known, most commercial ECs employ non-aqueous electrolyte rather than aqueous electrolyte despite the facts that aqueous electrolytes are typically more conductive, safer and cheaper. The overwhelming advantage of non-aqueous electrolyte is its much wider stable electrochemical potential window than its aqueous counterpart, where the latter is limited to 1.23 V, the potential gap between the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).1 Therefore, the operating voltage for aqueous ECs is normally below 1.2 V, whereas it is as high as 3.0 V for non-aqueous devices. The energy density of capacitors obeys the following equation: E = 1 8
C V2 , where C is the specific capacitance of one electrode and V is the maximum discharge voltage of a device. Although the electrode capacitance in aqueous ECs is often higher than that in nonaqueous ECs, the overall energy density of the former still falls far below what the latter could offer.In order to cultivate the advantages of aqueous supercapacitors, it is highly desirable to increase its operating voltage without decomposing its electrolyte. One example is lead-acid batteries that operate at a voltage...