oa Carbon-based Electrode Materials for Biotic Treatment of Wastewater and Saline Water

In the past decades, microbial fuel cells (MFCs) have been intensively studied in order to provide sustainable and environmentally friendly wastewater treatment concurrent with energy harvesting. A highly porous, highly efficient, light-weight, and inexpensive 3D sponges consisting of interconnected carbon nanotubes (CNTs) were developed as anodes of MFCs in order to allow more efficient microbe-to-anode electron transfer that are key to the operation of MFCs. The MFCs equipped with the 3D CNT sponge anode generates high power densities of 2150 Wm^–3 (per anode volume) or 170 Wm^–3 (per anode chamber volume), comparable to those of commercial 3D carbon felt electrodes under the same conditions (1). The high performances are due to excellent charge transfer between CNTs and microbes, which is evident by the 13 times lower charge transfer resistance compared to that of carbon felt. The 3D CNT sponges produced here has low cost (∼$0.1/gCNT) and high production rate (3.6 g/hr) compared to typical production rate of 0.02 g/hr of other CNT-based materials (1). The high production rate and low cost of this highly efficient electrode material can make MFCs more feasible to be scaled up for various applications such as desalination of seawater or saline water. Also, other electrode materials were compared to the 3D CNT sponge in evaluating the efficiency of the MFC and extending the use of these electrode materials to a field of microbial desalination cell (MDC).

Once MDCs are applied to the desalination process, there are several challenges that need to be addressed. First, a pH gradient forms between anode and cathode chambers (due to proton accumulation in the anode chamber and hydroxyl ion accumulation in the cathode chamber). In addition, chloride ion accumulation inhibits the activities of electrochemically active microbes. Together these activities degrade the overall performance of the system. Recirculation of the anolyte and catholyte provides one solution to addressing this challenge. However, this approach results in lower Coulombic efficiency. Here, we studied to develop a modified three-chamber configuration where part of the anode chamber and part of the cathode chamber are directly connected through a cation exchange membrane, thus partially allowing transport of protons between the chambers, and thereby limiting the drop in pH, while still maintaining charge differences that drive Cl^– and Na⁺ ions to move from seawater to the anode and cathode chambers. Practical MDCs require continuous or batch-mode feeding of wastewater into the anode chambers of the system, thus accumulated chloride ions will be simply flushed out or diluted due to the influx of new wastewater or catholyte. This aspect will mitigate the impacts of the chlorine ion accumulation problem. Also, a pivotal performance limitation centers on the cathode catalyst layer owing to sluggish kinetics of the oxygen reduction reaction and several transport losses. On the cathode side, expensive precious metal catalysts have been used in conventional systems to overcome the slow reactions on the electrode. Platinum and Pt-based electrocatalysts, commonly used in the electrodes, not only contribute to high fuel cell cost but also lead to durability concerns in terms of Pt cathode oxidation, catalyst migration, loss of electrode active surface area, and corrosion of the carbon support. So, this study used Pt-free 3D carbon-based cathode for MDC system.

Reference

[1] Celal Erbay, Gang Yang, Paul de Figueiredo, Reza Dadr, Choongho Yu, Arum Han, “Three-dimensional porous carbon nanotube sponges for high-performance anodes of microbial fuel cells”, Journal of Power Sources, (2015), 177–183.