Phosphine plasma activation of α-Fe₂O₃ for high energy asymmetric supercapacitors

We report a phosphine (PH₃) plasma activation strategy for significantly boosting the electrochemical performance of supercapacitor electrodes. Using Fe₂O₃ as a demonstration, we show that the plasma activation simultaneously improves the conductivity, creates atomic-scale vacancies (defects), as well as increases active surface area, and thus leading to a greatly enhanced performance with a high areal capacitance of 340 mF cm⁻² at 1 mA cm⁻², compared to 66 mF cm⁻² of pristine Fe₂O₃. Moreover, the asymmetric supercapacitor devices based on plasma-activated Fe₂O₃ anodes and electrodeposited MnO₂ cathodes can achieve a high stack energy density of 0.42 mW h cm⁻³ at a stack power density of 10.3 mW cm⁻³ along with good stability (88% capacitance retention after 9000 cycles at 10 mA cm⁻²). Our work provides a simple yet effective strategy to greatly enhance the electrochemical performance of Fe₂O₃ anodes and to further promote their application in asymmetric supercapacitors.