Diamond Cloth-Supported Nickel-Manganese Oxide Electrodes for High-Performance Flexible Supercapacitors

Supercapacitor (SC), one of the most important classes of electrochemical energy storage devices, has attracted considerable attention due to their high power density, fast charging-discharging capabilities, superior cycling stability, and reliable performance. With the rapid advancement of portable and wearable electronics, the demand for high-performance flexible SCs has significantly intensified, thereby driving extensive research into advanced electrode materials, electrolytes, and device architecture. Among them, the development of electrode materials plays a decisive role in determining both electrochemical performance and mechanical flexibility of SCs.
Herein, three electrode systems were designed and fabricated in this thesis: flexible diamond cloth (DC), NiMnOx-coated carbon cloth (NiMnOx@CC), and NiMnOx-coated diamond cloth (NiMnOx@DC). Their electrochemical performance was further and comprehensively evaluated by means of various techniques and tools. More details ae demonstrated as follows.
The flexible DC was synthesized through the overgrowth of a boron-doped diamond (BDD) film on CC in a microwave plasma enhanced chemical vapor deposition (MWCVD) reactor. A Ti interlayer was first introduced to protect CC from plasma etching while simultaneously enhancing BDD adhesion. The DC electrode demonstrated excellent electrochemical performance, including enhanced capacitance, good rate performance, and exceptional cycling stability. A symmetric pseudocapacitor (PC) assembled from two DC electrodes delivered an energy density of 45.96 µWh cm-2 at a power density of 4.25 mW cm-2, retaining 17.64 µWh cm-2 at 67.93 mW cm-2. Moreover, the symmetric DC electric double-layer capacitor (EDLC) exhibited excellent mechanical flexibility.
Pseudocapacitance was then introduced by electrochemical synthesis of Mn-oxide films on CC. This NiMnOx@CC electrode offered better conformality, thickness control, and reproducibility compared with electrodes prepared through hydrothermal methods. The optimized Ni-doped electrode exhibited larger current responses, a wide potential window (1.5 V), and distinct redox peaks, yielding a specific capacitance of 47.1 mF cm-2 at a scan rate of 10 mV s-1 and 42.8 mF cm-2 at a current density of 1 mA cm-2 in 1.0 M Na2SO4, with > 98.5 % coulombic efficiency over 10,000 cycles. Its cycling retention was limited (≈ 16% after 10,000 cycles), constraining practical application.
CC was further replaced with DC as the substrate to produce a more stable NiMnOx@DC electrode. The optimal deposition potential of 1.0 V balanced deposition rate and side reactions. The as-fabricated NiMnOx@DC electrode exhibited pronounced redox features and high specific capacitances of 180.5 mF cm-2 at a scan rate of 10 mV s-1 and 228.1 mF cm-2 at a current density of 2 mA cm-2, coupled with exceptional coulombic efficiency approaching 100 % over 10,000 charging-discharging cycles. Furthermore, the NiMnOx@DC electrode exhibited significantly enhanced capacitance retention in comparison to the NiMnOx@CC electrode. The maximum energy density of 72.8 μWh cm-2 and maximum power density of 17.0 mW cm-2 were obtained for a symmetric NiMnOx@DC PC. Simultaneously, the quasi-solid-state device using the NiMnOx@DC electrodes showed excellent mechanical flexibility during the bending tests.
In summary, this thesis presents a systematic and stepwise route to design different SC electrodes from DC to NiMnOx@DC, effectively integrating the advantages of CC, BDD, and Ni–Mn oxides. This strategy enables the fabrication of high-performance flexible SCs, which exhibit not only high energy density and rapid power delivery but also excellent long-term stability and mechanical flexibility. These advancements in electrode materials significantly contribute to the realization of practical applications for flexible SCs in portable and wearable electronic devices.