Electricity Generation Of An Air-breathing Microfluidic Fuel Cell With Three-dimensional Anodes Under Different Electrolytes And Operation Modes

With the widespread application of portable electronic devices in recent years, the demand for miniature portable power sources is increasing rapidly. However, current battery technology cannot meet the demand for long standby power sources of electronic devices. Microfluidic fuel cells(MFFCs), as a kind of miniature portable power technology with great potential, have attracted much attention from scholars all over the world. With the help of co-laminar flow in the microchannel of MFFC, the fuel and catholyte solutions can be separated spontaneously without proton exchange membrane usually utilized in traditional fuel cells. As a result, we can miniaturize the MFFC more easily than proton exchange membrane fuel cell(PEMFC), and a series problems caused by proton exchange membrane can be eliminated, such as membrane degradation or pollution. Moreover, using of liquid fuels with higher energy density and simpler water management than gas fuels, MFFC is considered as a significantly promising micro power source.Currently, due to the performance limitation by structure, fuel and electrolyte, operation mode, the power output of MFFC is still not high enough to realize the commercial application. Therefore, in this study, a new structure of MFFC with non-spacers, three-dimensional anode and air-breathing cathode was presented. The electricity generation of the MFFC in acidic and alkaline electrolytes and under different operation modes was studied systematically. In addition, the effects of electrolyte concentration, fuel concentration and reactant flow rate on the cell performance were discussed. Furthermore, by the hierarchical assembly of the anode electrodes in the MFFC, the current density distribution of three-dimensional anode array was tested to analyze the mass transfer characteristics of fuel in the microchannel of air-breathing MFFC with three-dimensional anodes. The main research results of this study are listed as follows:① The air-breathing microfluidic fuel cell with three-dimensional anodes in acidic electrolyte achieves the best performance when the formic acid concentration is 0.5 mol·L-1, the electrolyte concentration 1.0 mol·L-1 and the reactant flow rate 300 μL·min-1. With a maximum power density of 49.1 m W·cm-3, the limiting current density of 231.7 m A·cm-3, the performance of the MFFC is twice as the similar MFFC with the same anode surface area and volume under the same operation conditions. With a power output of 31 m W and a maximum current of 146 m A, this MFFC exhibits better performance than the formic acid microfluidic fuel cells under electrolyte of sulfuric acid in all the previous studies. When the flow rate is small(≤ 50 μL·min-1), the fuel utilization rate of the MFFC can reach 100%.② When operated under alkaline electrolyte, the air-breathing MFFC with three-dimensional anodes achieves the best performance when the HCOONa concentration is 1.0 mol·L-1, KOH concentration is 2.0 mol·L-1 and reactant flow rate is 300 μL·min-1. The highest power density and maximum limit current density are 70.3 m W·cm-3 and 407.4 m A·cm-3, respectively. When the flow rate is 50μL·min-1 the fuel utilization rate of this MFFC can reach 100%. The MFFC operated in alkaline electrolyte can produce 1.3 times the peak power density and 1.7 times the limit current density of that in acidic electrolyte under the similar conditions.③ With the generation and discharge of CO2 bubbles under acidic electrolyte, the chronoamperometry curves of the top and bottom anode electrode of the air-breathing MFFC with three-dimensional anodes alternate periodically. At low current densities, the electricity is mainly generated by the bottom anodes, while at higher current densities, because of the fuel migration to the upper electrodes, the top anodes dominate the electricity generation of the MFFC. However, when operated under alkaline electrolyte, the electricity of this MFFC is mainly produced by the bottom anodes at all operation conditions, while the power contribution of the top anodes is less than 10% of the total output of the cell, resulting in the nonuniform distribution of current densities.④ Inclined operation mode helps to discharge CO2 bubbles generated in the air-breathing MFFC with three-dimensional anodes under acidic electrolyte. The greater the slant angle, the faster discharge speed of bubbles, which makes the MFFC achieve higher current density and more stable output performance under the inclined operation mode. However, the performance of MFFC operated under alkaline electrolyte decreases with increasing the slant angle. Current density distribution experiments show that the better performance could be achieved by the top anode electrodes at larger slant angles, while the performance of the bottom anode electrodes will decline. The inclined operation mode changed the distributions of fuel concentration and anode current density in the MFFC, which causing the performance of anode electrodes reduced gradually from top to bottom. The reversed trend is found in the MFFC operated in the horizontal mode.⑤ The air-breathing MFFC with three-dimensional anodes operated under the passive mode using gravity to driven reactants, can achieve a good performance as the MFFC operated under active mode under acidic electrolyte. However, the performance of MFFC operated in alkaline electrolyte under passive mode is slightly lower than that under active mode. Moreover, the current density distribution of the MFFC operated under passive mode is similar to that under active mode.