Enhancing the Performance of Microfluidic Fuel Cells by Modifying the Carbon-Fiber Paper Cathode by Air Annealing and Acid Oxidation

High Power Density Direct Formate Microfluidic Fuel Cells with the Different Catalyst-Free Oxidants

As micropower devices, microfluidic fuel cells (MFCs) have gained much attention due to their simple configurations and high power densities. MFCs exploit the parallel laminar flowing of two electrolytes in a microchannel with a characteristic length from 1 to 1000 μm to separate the anolyte and catholyte, without the proton exchange membranes in the traditional fuel cells. These membrane-less configurations can avoid a series of technical problems related to the membranes. To achieve an MFC with high power density and low cost, we constructed the direct formate MFCs with two catalyst-free oxidants containing FeCl₃ and Na₂S₂O₈ solutions, respectively, and compared the performance of the two MFCs. Due to Na₂S₂O₈ being an oxidant with some distinctive advantages, including its high theoretical potential, high solubility of itself and its reduction product, and environmental friendliness, the Na₂S₂O₈-based MFC showed a higher open-circuit voltage (>2.0 V) and better performance. Then, we studied the effects of oxidant concentrations, flow rates, and fuel concentrations on the performance of the Na₂S₂O₈-based MFC. The results showed the optimum performance of the Na₂S₂O₈-based MFC with the peak power density of 214.95 mW cm^-2 and the limiting current density of 700.13 mA cm^-2 under the conditions of 1.5 M HCOONa, 2 M Na₂S₂O₈, and 300 μL min^-1 at an anolyte/catholyte flow ratio of 2:1. The performance was also the highest among the direct formate MFCs reported up to now. Moreover, the Na₂S₂O₈-based MFC could stably discharge for about 4 h under a constant voltage. All of the results demonstrated that Na₂S₂O₈ was a suitable oxidant and that the Na₂S₂O₈-based MFC could realize the goals of high power density and low cost for the actual application of MFCs.

Improving the Electrochemical Properties of Carbon Paper as Cathodes for Microfluidic Fuel Cells by the Electrochemical Activation in Different Solutions

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.