Surface Area Engineering of the Anode to Eliminate Voltage Reversal in Plant-Assisted Sediment Microbial Fuel Cell Stacks

Plant-assisted sediment microbial fuel cells (SMFCs) are emerging as promising green energy technologies with potential applications in wastewater treatment and biosensing in open-environment systems. However, their practical implementation is limited by their inherently low voltage output. A common strategy to enhance the voltage involves stacking multiple unit cells in series; however, such configurations often suffer from voltage reversal (VR), which disrupts system stability and reduces the overall output voltage. Herein, a strategy is presented to mitigate VR in stacked water lettuce-assisted SMFCs (WL-SMFCs) by tuning the anodic surface area. A theoretical framework was first developed to relate electrical parameters to the anodic surface area, predicting that increasing the anodic surface area of the terminal unit would enhance the overall stack voltage. This prediction was experimentally validated by using a laboratory-scale stack of series-connected WL-SMFC units. When the anodic surface area of the terminal unit was increased to match the total anodic surface area of all other units combined, the VR was significantly reduced. In two-, three-, and four-unit stacks, VR decreased by 70%, 57%, and 54%, respectively. Electrochemical impedance spectroscopic analysis confirms the corresponding increase in anodic storage charge (C) to 318.25 ± 12.35 (670 ± 26), 453.08 ± 12.12 (964 ± 26), and 422.92 ± 9.39 (872 ± 19) from the unit value of 240.58 ± 25.65 (523 ± 55) with respective capacitance (pF) values shown in brackets. This anodic surface area tuning approach offers a technically simple, self-sustaining, and cost-effective solution for alleviating VR, thereby enhancing the feasibility of SMFCs for open-environment applications.