Redox Deposition of Nanoscale Metal Oxides on Carbon for Next-Generation Electrochemical Capacitors

Abstract

Transition metal oxides that mix electronic and ionic conductivity are essential active components of many electrochemical charge-storage devices, ranging from primary alkaline cells to more advanced rechargeable Li-ion batteries. In these devices, charge storage occurs via cation-insertion/deinsertion mechanisms in conjunction with the reduction/oxidation of metal sites in the oxide. Batteries that incorporate such metal oxides are typically designed for high specific energy, but not necessarily for high specific power. Electrochemical capacitors (ECs), which are typically composed of symmetric high-surface-area carbon electrodes that store charge via double-layer capacitance, deliver their energy in time scales of seconds, but at much lower specific energy than batteries. The fast, reversible faradaic reactions (typically described as "pseudocapacitance") of particular nanoscale metal oxides (e.g., ruthenium and manganese oxides) provide a strategy for bridging the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage capacity to boost specific energy, while maintaining the few-second timescale of the charge-discharge response of carbon-based ECs. In this Account, we describe three examples of redox-based deposition of EC-relevant metal oxides (MnO2, FeOx, and RuO2) and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures. Expressing the metal oxide in a nanoscale form often enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge. The "wiring" of the metal oxide, in terms of both electron and ion transport, when fabricated into a practical electrode architecture, is also a critical design parameter for achieving characteristic EC charge-discharge timescales. For example, conductive carbon must often be combined with the poorly conductive metal oxides to provide long-range electron pathways through the electrode. However, the ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of nanostructured carbons, with the carbon surface acting as a sacrificial reductant when exposed to a solution-phase, oxidizing precursor of the desired metal oxide (e.g., MnO4(-) for MnO2). These redox deposition methods can be applied to advanced carbon nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion transport to the nanoscale metal oxide domains within the electrode architecture, which further enhances high-rate operation.

Effect of visible light and electrode wetting on the capacitive performance of S- and N-doped nanoporous carbons: Importance of surface chemistry

“…Efficiently here means with a high gravimetric and volumetric capacitances and with high energy and power densities. During the last decade numerous original research papers and reviews have been published analyzing the factor governing these features [1][2][3][4][5][6][7][8][9][10]. Generally two types of materials have been considered: carbon based supercapacitors [1][2][3][4][5][6][7][8] and metal oxides based redox type capacitors [10,11].…”

“…Generally two types of materials have been considered: carbon based supercapacitors [1][2][3][4][5][6][7][8] and metal oxides based redox type capacitors [10,11]. To increase the performance the composites of both types of materials have been also considered [9][10][11][12].…”

Nickel/Iron Oxide Nanocrystals with a Nonequilibrium Phase: Controlling Size, Shape, and Composition

“…Metal oxides and mixed metal oxides (MMOs) are of significant interest as electrode materials in a broad range of device applications, including Li-ion batteries, 1−3 electrochemical capacitors, 2,4,5 fuel cells, 6,7 gas sensors, 8,9 thin film transistors 10−12 and electrochemical water oxidation catalysts. 13−16 Many synthetic methods have been reported for the production of high-purity MMOs, including the mechanical mixing of metal precursors, 17−19 spray pyrolysis, 20,21 sol−gel decomposition, 11,22−24 hydrothermal processing, 24−27 polymerbased processes, 28,29 and solution-phase thermal decomposition of metal−organic precursors.…”

Extending Electrochemical Quartz Crystal Microbalance Techniques to Macroscale Electrodes: Insights on Pseudocapacitance Mechanisms in MnOx-Coated Carbon Nanofoams

“…[8][9][10][11] The electrochemical characteristics of MnOx have been extensively demonstrated, yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate. Spectroscopic methods have been used to confirm that pseudocapacitive charge storage is supported by reversible toggling of the Mn oxidation state (ranging between +3 and +4, but typically <1 e -per Mn site) during electrochemical cycling in aqueous electrolytes.…”

“…11,28,31 The incorporated metal oxides provide pseudocapacitance that significantly enhances the charge-storage capacity of the nanofoam beyond its inherent double-layer capacitance, while the nanoscale dimensions of the oxide coatings and readily available supply of ions within the electrolyte-filled pore structure of the nanofoam support high-rate charge-discharge response. [27][28][29][30][31] This class of electrode materials has proved an effective platform with which to investigate the interplay of structure and electrochemical performance for pseudocapacitive oxides 27,31 and also exhibits "plug-and-play" function for ready incorporation into EC devices.…”