Role of the MnCoGe alloys to enhance the capacitance of flexible supercapacitors made with electrodes of recycled aluminum and carbon nanotubes

Flexible and sustainable supercapacitors (SCs) were fabricated with carbon nanotubes (CNTs) as the anode and recycled aluminium as the cathode (obtained from soda cans). Devices made with the configuration of CNT//Al had maximum energy-densities/capacitances of 97.8 Wh kg-1 / 3 13.5 F g-1 . Later, MnCoGe (MCG) alloy powders doped with boron (MCG:B) or gallium (MCG:Ga) were incorporated into the SC electrodes and the energydensity/capacitance was enhanced up to 269.5 Wh kg-1 / 8 62.3 F g-1 . This outstanding increment of the electrochemical performance was caused by the formation of multiple redox centers on the SC electrodes (oxygen vacancies, Mn 3+ /Mn 4+ , Co 2+ /Co 3+ , and Ge 4+ /Ge 2+ /Ge 0 ) as confirmed by XPS and Raman spectroscopies. The SCs made with MCG alloys were immersed in acid (pH=3)/basic (pH=11) aqueous media and electrochemically tested. Surprisingly, the capacitance retention was reduced a maximum of 0.6%, demonstrating their capacity to operate underwater. The tests of bending and charging/discharging cycles demonstrated that the SCs made with the MCG:B powder were the most stable because their capacitance retention was above 95%. In contrast, SCs made with the MCG:Ga alloy had capacitance retentions below 93% after the same tests. Overall, we demonstrated that efficient/flexible/waterproof SCs can be made with low-cost aluminium, which of interest for wearable electronics.