Quantification of internal resistance of microfluidic photosynthetic power cells

The micro-photosynthetic power cell (µPSC), also known as a bio-photovoltaic cell, represents an emerging green energy technology capable of addressing carbon emissions. This technology exploits photosynthetic microorganisms to convert light energy into electricity through water splitting reactions. In this study, we examined a µPSC with a 4.84 cm² electrode surface area, resulting in an open circuit voltage (V_oc) ranging from 0.7 to 0.9 V and a short circuit current (I_sc) between 0.6 to 1 mA, leading to power outputs ranging from 0.16 mW to 0.2 mW. However, the power density of µPSCs remains low due to various factors, one such major factor is the internal resistance. To address this concern, our work aimed to analyze and quantify the internal resistance components of µPSCs, simplifying their resistive modeling. We found that the total internal resistance (TIR) of the µPSC consists of activation internal resistance (AIR), concentration internal resistance (CIR), and ohmic internal resistance (OIR). Specifically, at short circuit conditions (I_sc), AIR contributed 99.65%, while CIR and OIR contributed 0.005% and 0.3436%, respectively. Notably, AIR was the major contributor to µPSC's internal resistance, accounting for approximately 99.650% of TIR. Among the challenges faced, reducing concentration loss proved to be difficult. Nevertheless, this issue could be alleviated by employing micro-sized µPSCs with smaller volumes, which reduces biofilm formation and enhances proton diffusion through proton exchange membranes compared to larger-scale counterparts. Additionally, optimizing the anode's design and using materials with lower internal resistance, such as carbon sheets and thin proton exchange membranes, could effectively reduce ohmic internal resistance. To address activation internal resistance, we proposed strategies such as minimizing the distance between the anode and cathode electrodes and fabricating electrodes on the proton exchange membrane’s surface.