As the need for alternative energy
becomes increasingly important,
energy research and related industries are rapidly expanding. This
lab incorporates current energy-storage research into a second-year
lab that instills real-world, industry-relevant knowledge and skills
while teaching and reinforcing physical-chemistry concepts. A manganese
oxide electrode, aqueous-Na2SO4-electrolyte
supercapacitor system is used because it has no air or water sensitivity,
unlike most battery technologies, so it is easy to implement in an
undergraduate-lab setting. Manganese oxide is an increasingly popular
supercapacitor material, and this lab introduces the concept of pseudocapacitance,
in which current flows while still being governed by the Nernst equation
(i.e., at equilibrium). Students conduct realistic and industrially
relevant electrochemical experiments; they electrodeposit manganese
oxide films and test them using cyclic voltammetry. Students compare
the manganese oxide results to those from a nonpseudocapacitive system
(i.e., a poor supercapacitor). In doing so, they learn the concepts
of charge storage and energy and power (and their important differences),
while reinforcing the physical-chemistry topics of thermodynamics
and kinetics, all within a frame of familiar electrochemical knowledge
(i.e., the Nernst equation). This lab can be completed in one 4 h
laboratory period or in a 3 h period if the solutions are provided
to the students or they prepare them a week in advance. Student interest
and engagement is heightened by their being able to see the real-world
applications and skills.
Manganese oxide pseudocapacitive materials are deposited using a novel procedure involving first depositing a heat-treated base layer followed by a hydrous top layer. The ratio of heat-treated to hydrous film is optimized to generate films that excel across a wide range of electrochemical capacitor (EC) properties and to elucidate the mechanisms underpinning the film performance. We show that a thin heat-treated base layer imparts low resistance, high energy efficiency and power-capability and enhanced film stability in a large potential window. These benefits are due to an improved oxide-current collector connection; however, if the layer is too thin (<25 nm), the stability is lost. Conversely, heat-treatment causes more parasitic oxidation reactions during initial film cycling, though these reactions are mitigated with a thick hydrous top layer. This hydrous film also affords high capacitance, capacity, coulombic efficiency and energy density due to an abundance of hydrated sites in the oxide to facilitate the cation insertion/removal needed for pseudocapacitance. The double-deposited films also show less self-discharge. We find that a dry:wet film ratio of 5:95 results in optimal film performance. While this novel dry-wet double-deposition has been demonstrated with manganese oxide, we anticipate similar performance benefits with other pseudocapacitive materials.
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