Towards high-energy-density pseudocapacitive flowable electrodes by the incorporation of hydroquinone.

This study reports an investigation of hydroquinone (HQ) as a multielectron organic redox molecule to enhance the performance of flowable electrodes. Two different methods to produce high-performance pseudocapacitive flowable electrodes were investigated for electrochemical flow capacitors. First, HQ molecules were deposited on carbon spheres (CSs) by a self-assembly approach using various HQ loadings. In the second approach, HQ was used as a redox-mediating agent in the electrolyte. Flowable electrodes composed of HQ showed a capacitance of 342 F g(-1), which is >200 % higher than that of flowable electrodes based on nontreated CSs (160 F g(-1)), and outperformed (in gravimetric performance) many reported film electrodes. A similar trend in capacitance was observed if HQ was used as a redox agent in the electrolyte; however, its poor cycle life restricted further consideration. In addition, a twofold increase in capacitance was observed under flow conditions compared to that of previous studies.

In situ distributed diagnostics of flowable electrode systems: resolving spatial and temporal limitations.

In this study, we have developed an in situ distributed diagnostics tool to investigate spatial and temporal effects in electrochemical systems based on flowable electrodes. Specifically, an experimental approach was developed that enables spatially-resolved voltage measurements to be obtained in situ, in real-time. To extract additional data from these distributed measurements, an experimentally-parameterized equivalent circuit model with a new 'flow capacitor' circuit element was developed to predict the distributions of various system parameters during operation. As a case study, this approach was applied to investigate the behavior of the suspension electrodes used in an electrochemical flow capacitor under flowing and static conditions. The volumetric capacitance is reduced from 15.6 F ml(-1) to 1.1 F ml(-1) under flowing conditions. Results indicate that the majority of the charging in suspension electrodes occurs within ∼750 µm of the current collectors during flow, which gives rise to significant state-of-charge gradients across the cell, as well as underutilization of the available active material. The underlying cause of this observation is attributed to the relatively high electrical resistance of the slurry coupled with a stratified charging regime and insufficient residence time. The observations highlight the need to develop more conductive slurries and to design cells with reduced charge transport lengths.