Pyrrolidinium-based protic ionic liquid electrolytes for high performance RuO2 micro-supercapacitors

Advances in smart technologies and the Internet of Things (IoT) require compact and efficient energy storage devices. Among microscale systems, RuO2-based micro-supercapacitors (MSCs) can deliver high-power pulses, but the limited stability window of their aqueous electrolytes restricts their energy density. This study investigated the use of pyrrolidinium-based protic ionic liquids (PILs) with different alkyl chain lengths and anion types to achieve pseudocapacitive charge storage in RuO2 MSCs with an extended cell voltage of up to 1.5 V. Of the tested electrolytes, 1-propyl pyrrolidinium trifluoroacetate [Pyr3H]⁺[TFA]⁻ exhibited excellent energy density and cycling performance, and 1-methyl pyrrolidinium tetrafluoroborate [Pyr1H]⁺[BF4]⁻ demonstrated low equivalent series resistance and superior power retention. The experimental findings were consistent with Reactive Force Field (ReaxFF) molecular dynamics simulations which revealed proton exchange and ion diffusion mechanisms in both ionic liquids. Ionogel-based MSCs demonstrated long-term cycling stability, retaining performance over 5000 charge/discharge cycles. These results highlight the potential of using pyrrolidinium-based PILs in on-chip, solid-state MSCs to power microelectronics.