Towards Implantable Bio-Supercapacitors: Pseudocapacitance of Ruthenium Oxide Nanoparticles and Nanosheets in Acids, Buffered Solutions, and Bioelectrolytes

Abstract

Metal oxides, in particular ruthenium-based oxides, are promising electrode materials for aqueous pseudocapacitors. Strong acids or bases are favored over neutral electrolytes owing to the higher capacitance. Here we explore the pseudocapacitive behavior of ruthenium oxide nanoparticles and nanosheets in near neutral pH as an environmentally benign electrolyte. The pseudocapacitive charge storage in poorly-crystalline hydrous RuO 2 nanoparticles, and highly-crystalline RuO 2 nanosheets were investigated in acetic acid-lithium acetate (AcOH-AcOLi) buffered solutions. It is shown that capacitance values as high as 1,038 F g −1 can be achieved in AcOH-AcOLi buffered solutions with RuO 2 nanosheets, which is 44% higher than the benchmark RuO 2 · nH 2 O in H 2 SO 4 electrolyte (720 F g −1 ). Furthermore, comparable performance was obtained in phosphate buffered saline and fetal bovine serum. The mechanism of the pseudocapacitive properties is discussed based on the difference in the surface redox behavior of different RuO 2 nanomaterials in acid, neutral, buffered solutions, and in weak acid. Electrochemical capacitors (also known as supercapacitors) are energy harvesting devices capable of charge and discharging within a few seconds, and cycle life in the order of thousands of cycles. Some metal-oxides are known to provide high capacitance in aqueous electrolytes, owing to the combination of the non-faradaic electrical double layer charging and the faradaic surface or near surface confined redox capacitance (psuedocapacitance).1 Pseudocapacitance is a phenomenon generally observed in aqueous electrolytes.2 Acidic or basic electrolytes such as H 2 SO 4 and KOH are favorable in terms of power density owing to the high conductivity. RuO 2 is one of the rare oxides that is stable in both acidic and basic conditions. Hydrous RuO 2 nanoparticles (RuO 2 · nH 2 O; where n is typically 0.5) offers capacitance of ∼700 F g −1 in H 2 SO 4 electrolyte and can be cycled for thousands of cycles with practically no decay, thus is often used for performance benchmarking of new electrode materials. Although studies on the asymmetric systems and applicability of non-aqueous electrolytes to oxide electrodes in order to widen the operating voltage window have recently been initiated, non-aqueous electrolytes have yet to surpass aqueous electrolytes in terms of specific capacitance. [3][4][5][6][7][8] Electrolytes near neutral pH are selected for materials that are not as corrosion-resistant in acids and base, for example manganese oxide.9-12 Neutral electrolytes are more environmentally benign and its low corrosiveness allows a wider range in choice for periphery material, such as current collectors and packaging.13 Despite the RuO 2 -based material being the model pseudocapacitive material, studies on the electrochemical capacitor behavior in neutral electrolytes are scarce compared to the more popular acidic or basic electrolytes. One of the reasons is that the capacitance of RuO 2 in neutral electrolytes is generally 1/2 of that in s...

Recent Progress in Ruthenium Oxide‐Based Composites for Supercapacitor Applications

“…Thus, the higher specific capacitance in the former is ascribed to enhanced pseudo‐capacitance, owing to the open‐framework morphology of the nanosheets that facilitates smooth penetration of ions into the porous channels ( Figure (c)). Moreover, de‐convolution of the total capacitance observed at different pH media reported highest specific capacitance in acidic electrolytes owing to maximum contribution from proton‐involved surface Faradaic processes which is less prominent in alkaline and neutral electrolyte (Figure (d)) …”

“…Moreover, de-convolution of the total capacitance observed at different pH media reported highest specific capacitance in acidic electrolytes owing to maximum contribution from proton-involved surface Faradaic processes which is less prominent in alkaline and neutral electrolyte ( Figure 9(d)). [108] Besides the above discussion related to tuning and optimization of aspect ratio, other important factors such as porosity, film thickness, texture and crystallite size, interconnected network structural aspects are equally important to optimize all the parameters leading to expected capacitance values.…”

Design Considerations for Unconventional Electrochemical Energy Storage Architectures

“…Yet pioneering advances are being already provided, for example, towards implantable bio-supercapacitors. [ 769 ] Another issue is the relatively low specifi c energy (high molecular weight and low working potentials) that can be stored in such biomolecular systems. However, interfacing such technologies with the human body to draw, store and deliver electrical energy when needed for example, to power a wearable electronic device, via a tattooed or skin integrated devices could be regarded now as a science fi ction vision although the potential and interest are massive.…”

Conducting Nanosheets and Nanoparticles for Supercapacitors and Fuel Cell Electrocatalysts

“…16 Further studies revealed that more environmentally benign electrolytes such as acetic acid-lithium acetate (AcOH-AcOLi) buffered solutions with near neutral pH (pH 5.8) could also be used as electrolytes affording high pseudocapacitance. 41,42 Astonishingly, the achievable maximum capacitance was actually higher for AcOH-AcOLi than H 2 SO 4 electrolyte with comparable rate performance. Bio-electrolytes such as phosphate buffered saline and fetal bovine serum also showed specific capacitance of 837 and 772 F g ¹1 , respectively, opening the possibility of using these electrode materials for safe and bio-compatible implantable power sources.…”

“…Bio-electrolytes such as phosphate buffered saline and fetal bovine serum also showed specific capacitance of 837 and 772 F g ¹1 , respectively, opening the possibility of using these electrode materials for safe and bio-compatible implantable power sources. 42…”