Supercapacitors have high power density, long cycle life and excellent capacity retention compared to their battery counterparts. Unlike batteries, they can quickly store large amounts of energy and dispense it just as fast.

However, one problem with supercapacitors is their lack of energy density. While lithium-based batteries can reach an energy density of up to 265 kilowatt hours (KW/h), supercapacitors to date have only been delivering a small fraction of that; higher energy densities are needed to enable supercapacitors to serve as power sources in applications such as electric vehicles.

With an internal architecture that is more in line with basic capacitors (storing charge on metal plates or electrodes) supercapacitors have gained considerable attention from the research community of late, focusing in particular on so-called hybrid supercapacitors. These combine the features of both conventional double-layer supercapacitors and batteries, and act as high-power, high-energy-storage devices.

Here are some examples of what has been coming out of research labs very recently.

Graphene Hybrid Supercapacitors

A Technical University Munich (TUM) team has developed a novel sustainable graphene hybrid material for supercapacitors. It can serve as the positive electrode in a supercapacitor, along with a negative electrode based on titanium and carbon and us said to be able to challenge nickel metal hydride (NiMH) and lead-acid battery technologies with regard to specific energy capacity.

According to the university, the storage device attains an energy density of up to 73Wh/kg, close to the energy density of a nickel metal hydride battery and exceeding that of conventional supercapacitors which have a power density of about 16kW/kg.

With regard to lifespan, a classic lithium battery has a useful life of around 5,000 cycles. The new cell developed by the TUM researchers retains close to 90 percent capacity even after 10,000 cycles.

The materials consist of atoms-thick layers of transition metal carbides, nitrides or carbonitrides and an aqueous electrolyte. They were used to deliver ~73Wh/kg at 1kW/kg, or 32Wh/kg at 16kW/kg, compared with 30-45Wh/kg for Pb‐acid batteries and 60 – 120Wh/kg for Ni/MH batteries, according to a December 2020 paper (“Covalent Graphene‐MOF Hybrids for High‐Performance Asymmetric Supercapacitors”) published in the journal Advanced Materials.

Nickel Cobaltite Hybrid Supercapacitors

Reporting in the journal Batteries & Supercaps (Chemistry Europe), scientists at the International Advanced Research Center for Powder Metallurgy and New Materials (ARCI), affiliated with the Indian government in collaboration with IIT Hyderabad, have developed an environmentally-benign chemical reduction process to synthesize hybrid supercapacitors.

These supercaps employ electrodes made of nickel cobaltite (NiCo2O4) as an active, positive electrode material. These electrodes could be an effective alternative to the existing carbon-based electrodes, allowing new supercapacitors to achieve higher energy density.

An asymmetric supercapacitor device fabricated by the research team, using porous carbon and NiCo2O4 electrodes, is reported to have exhibited excellent capacity retention and stability. The device is said to be able to power an LED lamp and a DC fan.

Using this technology, it may be possible to create a stretchable system of micro-supercapacitors that could harvest energy from human breathing and motion for use in wearable health-monitoring devices, according to an international team of researchers led by Huanyu “Larry” Cheng, a professor in Penn State University’s Department of Engineering Science and Mechanics.

The research team, including members from Penn State and China’s Minjiang University and Nanjing University, recently published its results in the journal Nano Energy.

Micro-supercapacitors have a small footprint, high power density and the ability to charge and discharge quickly. However, according to Cheng, when fabricated for wearable devices, conventional micro-supercapacitors have a “sandwich-like” stacked geometry that displays poor flexibility, long ion diffusion distances and a complex integration process when combined with wearable electronics.

Cheng and his team found that arranging micro-supercapacitor cells in an island-bridge layout allows the configuration to stretch and bend at the bridges, while reducing deformation of the micro-supercapacitors (the islands). When combined, the structure becomes what the researchers refer to as “micro-supercapacitors arrays.”

By using non-layered, ultrathin zinc-phosphorus nanosheets and 3D laser-induced graphene foam — a highly porous, self-heating nanomaterial — to construct the island-bridge design of the cells, Cheng and his team saw drastic improvements in electric conductivity and the number of absorbed charged ions. This proved that these micro-supercapacitor arrays can charge and discharge efficiently and store the energy needed to power a wearable device.

The researchers also integrated the system with a triboelectric nanogenerator, an emerging technology that converts mechanical movement to electrical energy. This combination created a self-powered system using energy based on motion, such as bending your elbow or breathing and speaking.

MIT Scientists Investigate Nanomaterials

Scientists from the Massachusetts Institute of Technology (MIT) conducted neutron research at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) to investigate a new, highly porous nanomaterial that could function as durable, high-energy supercapacitors. The results of the study have been published in the international edition of “Angewandte Chemie” (Wiley).

To investigate the adsorption mechanism of ions in MIT’s porous, conductive metal–organic framework (MOF), the team made electrodes from the material and soaked them in a solvent containing a sodium triflate electrolyte. This enabled positively and negatively charged ions to flow freely when the researchers turned the voltage on or off and switched it to negative or positive and back again.

“MOFs typically have high porosity, but poor electrical conductivity, which limits their use in high-power applications,” said Lilin He, a neutron scattering scientist at ORNL. “This conducting MOF is a highly porous nanomaterial with an extremely large overall surface area when you factor in all of the interior pores, gaps, and surfaces.

He added: “Of equal importance to its conductivity is that this MOF showed only a 10 percent loss in capacitance and no increase in internal electrical resistance even after 10,000 cycles, which could indicate good durability for future commercial applications.”

Plant-Based Supercap Materials

Texas A&M University researchers led by Hong Liang have designed a new energy storage device that can store a charge up to 900 times greater than current state-of-the-art supercapacitors.

According to the researchers, in the near future this novel plant-based energy storage device could charge electric devices within a few minutes. To accomplish this, Liang and her team were attracted to manganese dioxide nanoparticles for designing one of the two supercapacitor electrodes.

Past research has shown that lignin, a natural polymer that glues wood fibers together, when used with metal oxides enhances the electrochemical properties of electrodes. However, Liang said, there have been few studies looking into combining manganese dioxide and lignin to leverage both of their useful properties.

To create their electrode, Liang and her team treated purified lignin with a commonly available disinfectant called potassium permanganate. They then applied high heat and pressure to initiate an oxidation reaction that results in the breaking down of potassium permanganate and the deposition of manganese dioxide on lignin. Next, they coated the lignin and manganese dioxide mixture on an aluminum plate to form the green electrode. Finally, the researchers assembled the supercapacitor by sandwiching a gel electrolyte between the lignin-manganese dioxide-aluminum electrode and another electrode made of aluminum and activated charcoal.

Upon testing their newly designed green electrode, they found that their supercapacitor had very stable electrochemical properties. In particular, the specific capacitance, or the ability of the device to store an electrical charge, changed little, even after thousands of cycles of charging and discharging. Also, for an optimal lignin-manganese dioxide ratio, the specific capacitance was observed to be up to 900 times more than what has been reported for other supercapacitors. The research is discussed in the June 2020 issue of Energy Storage (Wiley).

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