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Carbon paper has been widely used as a flow-through electrode to deliver the reactants through the electrodes in microfluidic fuel cells (MFCs). This electrode architecture can utilize the interior area in porous electrodes. Considering its disadvantages, such as a few functional groups, low specific surface area, and poor wetting property, it is necessary to actively treat the carbon paper. We study how the two activation methods, i.e., air annealing and acid oxidation, affect the contents of surface functionality, redox activity of iron ions, cathode resistance, and performance of MFCs. Our conclusion is that the two methods should be coupled together to achieve optimal surface physical and electrochemical properties: 303.23 ± 3.09 mW cm–3 in volumetric power density and 1541.75 ± 134.17 mA cm–3 in volumetric limiting current density, which are 1.57 and 1.75 times higher than that of a MFC with an untreated carbon paper cathode, respectively.

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Membraneless microfluidic fuel cells (MFCs) have garnered tremendous interest as micropower devices, which exploit the colaminar nature of two aqueous electrolytes to separate the anode and cathode and avoid the membrane usually used in a fuel cell. Our previous research shows that the performance of FeCl3-based MFCs with catalyst-free cathodes is mainly limited by the cathode. To improve the power output of these MFCs, we activated the carbon paper cathode by an electrochemical method in the three solutions (Na2SO4, NaOH, and H2SO4) to improve the electrochemical characteristics of the carbon paper cathode. The surface functionalities and defects, reduction activation of iron ions as the oxidant, cathode resistance, and performance of FeCl3-based MFCs were measured and compared. Our work shows that the electrochemical activation of the carbon paper in different solutions is a simple and effective method to enhance the electrochemical characteristics of the carbon paper cathode and improve the performance of the FeCl3-based MFC. Also, the MFC with the carbon paper cathode activated in the H2SO4 solution reaches the optimum performance: 235.6 mW cm–3 in volumetric power density and 1063.33 mA cm–3 in volumetric limiting current density, which are 1.58 and 1.52 times as much as that of a MFC with an untreated carbon paper cathode, respectively. This best performance can be attributed to the cathode activated in the H2SO4 solution with the largest number of oxygen-containing functional groups, the largest electrochemical active surface area, strongest reduction of iron ions, and least resistance of the cathode.