Nanostructured Nickel/Ruthenium/Ruthenium‐Oxide Supercapacitor Displaying Exceptional High Frequency Response

Abstract

The lower performance of pseudocapacitive supercapacitors in high‐frequency applications such as alternating current (AC) line filtering has been ascribed to presumed slow kinetics of redox processes compared to ion diffusion in electric double layer capacitors. A nickel‐deposited ruthenium/ruthenium‐oxide symmetric supercapacitor exhibiting remarkable electrochemical properties, particularly very high frequency response (>1 kHz) is developed. The electrodes are prepared via a simple process consisting of electrochemical reduction of ruthenium chloride on commercially available nickel foil as the current collector. A symmetric supercapacitor comprising nickel/ruthenium/ruthenium‐oxide electrodes and a polystyrene‐based thin spacer exhibits particularly fast scan rates, high power density of 1500 mW cm−2 (88 kW cm−3) with a maximum energy density of 0.58 µWh cm−2 (34 mWh cm−3), and excellent capacitance retention. Notably, supercapacitors prepared by the same synthetic method albeit using conventional gold substrate instead of nickel exhibit significantly lower frequency response. The exceptional electrochemical properties of the nickel/ruthenium/ruthenium‐oxide supercapacitor and simple electrode synthesis point to promising applicability in AC line filtering and power conditioning. In a broader context, this work demonstrates that, contrary to the widely held presumption, the kinetics of redox reactions at the active layers of pseudocapacitors may not be the primary barriers to high‐frequency applications.

Nickel Alloying Significantly Enhances the Power Density of Ruthenium‐Based Supercapacitors

“…Importantly, the dendritic pattern, which has not been previously reported for nickel or ruthenium alloys, endows high electroactive surface area, significantly more extensive compared to electrodes prepared through electrodeposition of only ruthenium upon nickel foil. [29] Accordingly, the STEM images in Figure 1B-C underscore the important contribution of the nickel constituent in the Ni/Ru parent solution to the structural and electrochemical properties of the electrode. Notably, the STEM images and electron dispersion spectroscopy (EDS) elemental maps in Figure 1B-C indicate that the final electro-chemical oxidation step did not disrupt the dendritic film morphology, nor modified the uniform distribution of the nickel and ruthenium within the protruding nanostructures.…”

“…[42] Notably, the calculated phase angle of À 76°at 120 Hz is higher than angles reported for varied high capacitance devices, constructed from CNTs, [31] graphene, [39] reduced graphene oxide (rGO), [9] MXene, [27] and Ru/RuO 2 . [29] Notably, Figure 4B shows that a phase angle of À 45°, which indicates an equal capacitor and resistor behavior, was attained at a frequency of 966 Hz, underscoring an exceptionally high frequency in which the device can still operate. The impedance as a function of the frequency, calculated from both the imaginary and real impedance components, is presented in Figure 4B (blue circles).…”

“…[26] Recently, however, we and others have shown that varied pseudocapacitive-based materials can operate at high frequencies and that redox reaction dynamics do not hinder supercapacitor performance. [27][28][29] Indeed, it has been demonstrated that the interface between the active material and current collector plays a significant role in ion diffusion and electronic transport, thereby affecting the electrochemical performance of the device. [29][30][31] In particular, electrode structure and surface morphology have been shown to exhibit critical roles in shaping device properties.…”

“…[27][28][29] Indeed, it has been demonstrated that the interface between the active material and current collector plays a significant role in ion diffusion and electronic transport, thereby affecting the electrochemical performance of the device. [29][30][31] In particular, electrode structure and surface morphology have been shown to exhibit critical roles in shaping device properties. A recent study reported, for example, showed that a polymer/MXene film fabricated via filtration displayed a pronounced void structure.…”

Ultrahigh‐Rate Supercapacitor Based on Carbon Nano‐Onion/Graphene Hybrid Structure toward Compact Alternating Current Filter

“…As can be seen in Figure 5j, with a superior or comparable phase angle at 120 Hz, EDLC(I/II/A) shows the highest up-limit of scan rate (5000 V s −1 ) for good linearity among the capacitors reported to date, more than two orders of magnitude higher than highpower devices, [65,75] and electrodes such as RGO, [39,40] and PiCB, [13] and over one order of magnitude higher than supercapacitors based on different carbon nanomaterials applied for AC filtering, including CNT film, [8,31] ERGO, [9] onion-like carbon, [43] carbon black, [34] and CNT-Graphene hybrid structure. [3] Also, this up-limit value outperforms the supercapacitors based on different materials with ultrahigh scan rates for linearity, such as PEDOT:PSS/EG (2000 V s −1 ), [16] graphene nanomesh (2000 V s −1 ), [32] carbon black (2000 V s −1 ), [15] Ru/ RuO 2 (2000 V s −1 ), [74] SWNT film (1000 V s −1 ), [30] VOGNs (1000 V s −1 ), [29] T 3 C 2 /PEDOT:PSS (1000 V s −1 ), [10] PEDOT film (1000 V s −1 ), [18,19] and TTF-TCNQ/graphene (1000 V s −1 ). [69] These indicate that the CNO-graphene hybrid-structured film has a significant advantage over other electrode materials for rapid ion transportation and high power capability, possibly benefiting from the fast ion migration on outer surface of CNOs and high conductivity of the hybrid structure.…”