A 1.8 V Aqueous Supercapacitor with a Bipolar Assembly of Ion-Exchange Membranes as the Separator

Abstract

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...

A Crosslinked Soybean Protein Isolate Gel Polymer Electrolyte Based on Neutral Aqueous Electrolyte for a High-Energy-Density Supercapacitor

“…However, organic liquid electrolyte has several disadvantages due to its strict package and high flammability [15]. In comparison, aqueous electrolytes, in particular neutral aqueous electrolytes, have numerous advantages, such as a non-flammable nature, better corrosion resistance, greater safety, and they are also cheaper and more conductive [16,17,18]. However, to construct the solid-state supercapacitors used in miniaturized or flexible electronic devices, polymer electrolyte could be a unique replacement for conventional liquid electrolyte.…”

High‐Voltage Rechargeable Alkali–Acid Zn–PbO2 Hybrid Battery

“…It's thus highly desirable to explore high‐performance energy storage technology to meet the demands, special interests have been devoted to the aqueous batteries that hold merits of high energy and power density, good safety, and low cost [1] . However, the aqueous batteries suffer its limited thermodynamic voltage window of 1.23 V between the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), resulting in a rather low voltage and energy density than those of non‐aqueous batteries [2] . Although the voltage of aqueous battery can be enlarged by increasing the overpotentials of HER and OER without decomposing the electrolyte, it still remains a grand challenge to expand the operating voltage beyond 2 V [3] .…”

“…As the potentials of OER and HER are heavily dependent on pH value (E OER = 1.23 VÀ0.059pH V and E HER = À0.059pH V), the voltage window of alkali-acid electrochemical system can be possibly adjusted by tuning the pH gradient between anolyte and catholyte, [2] which have been demonstrated in our previous works that a variety of electrochemical devices can significantly improve the performance by developing an alkali-acid hybrid cell that shows potential in harvesting the electrochemical neutralization energy (ENE), [4] in this manner, the operating voltage window of one aqueous battery can be remarkably enlarged. A variety of alkali-acid hybrid batteries [5] have been reported recently by coupling alkaline Zn anode with specific acid cathode using either poorly reversible oxidants (e.g., KMnO 4 ) or reversible quinone as active materials, suggesting the working voltage can be significantly enhanced in an alkali-acid battery.…”

High‐Voltage Electrolytes for Aqueous Energy Storage Devices

“…In addition, by designing the dual electrolytes, the stable window of aqueous electrolytes is widened. The Nernst equation suggests that the electrode potential of HER in the alkaline solution (pH=14) is about 826 mV and in the acidic solution (pH=0), the electrode potential of OER is about 1230 mV, respectively . Therefore, by coupling the two electrolytes together by a cation selective Nafion separator, the water decomposition voltage is up to about 2 V theoretically (Figure c), which is much higher than the thermodynamic limit of 1.23 V. For example, by developing the dual electrolytes with the 0.5 M KOH (pH=13.2) electrolyte and 1 M Na 2 SO 4 electrolyte (pH=6.6), the full cell based on such electrolytes is able to operate at 1.62 V without the water decomposition .…”