The Effect of Particle Motion on Charging Behavior Observed in a Microfluidic Flow Capacitor

Electrochemical flow capacitors (EFCs) are promising energy storage devices because of their large storage capacity, low fatigue rates, and fast charge/discharge rates. These devices employ the electric double layer of high-surface-area porous carbon particles in a flowable slurry, i.e., a ‘flowable electrode’, to store ionic charges from an electrolyte solution. In the charging process, continuous electrical paths between carbon particles and the stationary electrode have to be established in the flowing slurry. The slurry can then be pumped into storage reservoirs. To extract the stored electricity, the flow is reversed, and the stored charges can be released to the electrodes again relying on particle-electrode and particle-particle interactions. Therefore, the flow characteristics of the carbon slurry and the particle interactions could play an important role in the charging and discharging performance of the device. Despite this, there are very few studies that directly observe the particle motion inside the charging channels. Most studies consider the EFC from an electrochemical perspective, so slurry components, such as electrolyte selection, carbon particle porosity, and carbon loading percentages, are tested to achieve the best possible charging performance. This approach however does not allow the researcher to probe the effect of slurry flow characteristics on charging performance. To address this issue, we have developed a microfluidic electrochemical flow capacitor (MEFC) that is compatible with optical imaging. The device is fabricated using transparent indium tin oxide (ITO) electrodes, a transparent polycarbonate track etch (PCTE) separator membrane, and polydimethylsiloxane (PDMS) channels for the slurry to flow through. This platform allows us to investigate the influence of various slurry flow behaviors on the charging performance of the flowable electrode during the operation of the device using an inverted microscope with a high-speed camera. We have used this setup to directly observe the effect of slurry flow rate on the particle-electrode and particle-particle interactions and consequently the charging/discharging performance. We will report on the fabrication of the MEFC, the experimental setup, and the charging/discharging results under different slurry flow rates.