Composite Manganese Oxide Percolating Networks As a Suspension Electrode for an Asymmetric Flow Capacitor Kelsey B. Hatzell, † Lei Fan, † Majid Beidaghi, † Muhammad Boota, † Ekaterina Pomerantseva, †,§ Emin C. Kumbur,* ,‡ and Yury Gogotsi* ,† † A.J. Drexel Nanomaterials Institute, Department of Material Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ‡ Electrochemical Energy Systems Laboratory, Department of Mechanical Engineering and Mechanics, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States § Materials Electrochemistry Group, Department of Material Science and Engineering, Drexel University, 3141 Chestnut Street,Philadelphia, Pennsylvania 19104, United States * S Supporting Information ABSTRACT: In this study, we examine the use of a percolating network of metal oxide (MnO 2 ) as the active material in a suspension electrode as a way to increase the capacitance and energy density of an electrochemical flow capacitor. Amorphous manganese oxide was synthesized via a low-temperature hydrothermal approach and combined with carbon black to form composite flowable electrodes of different compositions. All suspension electrodes were tested in static configurations and consisted of an active solid material (MnO 2 or activated carbon) immersed in aqueous neutral electrolyte (1 M Na 2 SO 4 ). Increasing concentrations of carbon black led to better rate performance but at the cost of capacitance and viscosity. Furthermore, it was shown that an expanded voltage window of 1.6 V could be achieved when combining a composite MnO 2 -carbon black (cathode) and an activated carbon suspension (anode) in a charge balanced asymmetric device. The expansion of the voltage window led to a significant increase in the energy density to ∼11 Wh kg -1 at a power density of ∼50 W kg -1 . These values are ∼3.5 times and ∼2 times better than a symmetric suspension electrode based on activated carbon. KEYWORDS: asymmetric supercapacitor, electrochemical flow capacitor, flowable electrode, manganese oxide, percolating networks, suspension electrode 1. INTRODUCTION For the practical implementation of energy storage at the grid level, novel technologies based on cheap, environmentally friendly, and abundant materials are needed. 1,2 In the past couple of years, there has been a renaissance of new electrochemical energy storage technologies that have evolved to address the challenge of integrating energy storage into the electricity infrastructure. 1,3-6 Many of these systems adopt several of the key benefits derived from flow battery systems, namely, the ability to obtain both scalable energy and power densities at a lower cost. 7,8 Among, these new systems, a new type of energy storage based on flowable suspension-type electrodes has emerged as a possible method for achieving high charge capacity. 9-12 Suspension electrodes or flowable electrodes are slurries where the majority component is the electrolyte. Flowable electrodes enable the scale-up of electrochemical energy storage systems traditionally used in small-scale portable electronics in order to address a wider range of applications. Furthermore, suspension electrodes are electrically conducting in nature in contrast with traditional flow-batteries which utilize an electronically insulating redox-electrolyte for reversible charge storage. These suspensions are formed by combining nano- or microsized active materials in an electrolytic media in order to facilitate charge storage and conductivity through percolating networks. 13 The connectivity exhibited between active particles allows for continuous and scalable charge storage (Figure 1b). Electrochemical capacitor (electrochemical flow capacitor 11 ) and battery (semi-solid lithium battery 10,14 ) systems utilize this concept for grid level energy storage. In both cases, the electrolyte aids in the transportation of the active materials such that the charge can be stored in a scalable manner. The former relies on charge storage in an electric double layer, whereas the latter relies on chemical reactions for charge storage. Most recently, the idea of flowing capacitive suspension electrode (CSEs) has also been extended to desalination, as a means for scaling up capacitive deionization systems. 15-17 For high-rate grid applications, such as frequency regulation, where fluctuations of MW/min need to be accommodated, rapid charging/discharging is desirable. Thus, suspension Received: March 19, 2014 Accepted: April 23, 2014 Published: April 23, 2014 Research Article www.acsami.org © 2014 American Chemical Society 8886 dx.doi.org/10.1021/am501650q | ACS Appl. Mater. Interfaces 2014, 6, 8886-8893