Imagine Charging Your Phone Within Seconds… And It Lasting For Days!

Electronic devices such as computers, cell phones, and tablets operate on batteries. Batteries are the primary energy storage devices for many systems, from small electronics to large industrial operations. Batteries store charges via chemical reactions; the more charges they store, the higher their energy.

Owing to this charge storage mechanism, it takes a long time to charge a battery. For example, it takes a few hours to fully charge a smartphone. The most advanced battery technology currently available is lithium-ion batteries, in which the chemical reactions are governed by Li ions. These type of batteries can provide energy density as high as 200 Wh/kg. However, the power density for such devices is quite low, typically less than 400 W/kg.

The energy density and power density are the two most important parameters in energy storage technology. For example, in an electric vehicle, the energy density is related to how long one can drive the car without recharging the battery, and the power density is how fast the car can go. Due to the high volume of energy storage applications and energy and power densities demands, batteries are falling short in a number of areas.

For the past several years, scientists and engineers have been working very hard to replace and/or complement batteries with a new energy storage device; as a result, electrochemical supercapacitors have been developed. Supercapacitors are being explored for a variety of applications ranging from electric vehicles to portable electronic devices due to their high power densities and long lifecycles. These devices can offer 10 to 100 times more power density than batteries. While the markets for supercapacitors are growing fast, they are small. To achieve greater market penetration, the energy densities for these devices will have to be increased.

Efforts to achieve this require new materials that possess high capacitances and wide voltage windows. Early transition-metal carbides and nitrides such as Vanadium Nitrides (VN) and Molybdenum Nitrides (Mo2N) are very attractive candidates given their high electronic conductivities and specific capacitances, and ability to be synthesized in high surface area form.

Our research efforts aim to characterize the charge storage mechanisms for these materials as part of a strategy to fully exploit their properties. We used solid-state synthesis and various physical and electrochemical measurements in combination with in-situ experimental techniques (i.e. X-ray absorption spectroscopy, neutron scattering, prompt-gamma activation analysis), in collaboration with several research scientists at Argonne and Oak Ridge National Laboratories, to understand the charge storage mechanisms and reaction pathways of these carbide and nitride materials.

So far, we have isolated the active ions involved in the charge storage mechanisms (Pande et al., Journal of Power Sources, 2012), defined the active sites for charge storage (Djire et al., Journal of Power Sources, 2015), quantified the extent of pseudocapacitance (maximum energy density) and active ion insertion (Djire et al., Journal of Batteries & Supercaps, 2018), and proposed overall charge storage mechanisms for these materials (A. Djire, Dissertation Thesis, University of Michigan, 2016). Additionally, we have demonstrated methods to remove the passivation layer, which impedes access of the electrolyte ions to the surface (Djire et al., Journal of Power Sources, 2015); and investigated the effects of crystal structure and composition on the pseudocapacitive charge storage (Djire et al., to be Submitted to the Journal of Nature Chemistry).

Combining this knowledge, we were able to increase the energy density by nearly a factor of four by using ionic liquid based electrolytes instead of aqueous electrolytes (Djire et al., Journal of Electrochemistry Communication, 2017). Recently, we demonstrate that the pseudocapacitive charge storage mechanism for high-surface area Molybdenum Nitrides (one of the best candidate material for supercapacitors) involves a proton-coupled electron transfer (2e for every H+) via micropores, which is accompanied by the reduction/oxidation of the metal (Mo).

These results suggest that capacitances in excess of 1500 Fg-1 in 1.2 V could be achieved in aqueous acidic electrolytes, which is equivalent to 10 times the energy density of currently available electrochemical supercapacitors (Djire et al., Journal of Nano Energy, 2018). The findings from our work contribute to the development of novel chemistries and provide a scientific basis for the design of high energy and power densities supercapacitors based on inexpensive carbide and nitride materials.

Ultimately, the outcome of our research would advance the science and technology of energy storage devices and allow our society to do things that otherwise would not have been possible, such as charging our phones within seconds.

These findings are in part described in the article entitled Pseudocapacitive storage via micropores in high-surface area molybdenum nitrides, recently published in the journal Nano EnergyThis work was conducted by Abdoulaye Djire, Jason B. Siegel, Olabode Ajenifujah, and Levi T. Thompson from the University of Michigan, and Lilin He from the Oak Ridge National Laboratory.

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