An air-breathing microfluidic formic acid fuel cell with a porous planar anode: experimental and numerical investigations

Abstract

Abstract. This paper reports the fabrication, characterization and numerical simulation of an air-breathing membraneless laminar flow-based fuel cell (LFFC) with carbon-fiber-based paper as anode. The fuel cell uses 1 M formic acid as the fuel. Parameters from experimental results were used to establish a three-dimensional numerical model with COMSOL Multiphysics. The simulation predicts the mass transport and electrochemical reactions of the tested fuel cell using the same geometry and operating conditions. Simulation results predict that the oxygen concentration over air-breathing cathode is almost constant for different flow rates of fuel and electrolyte. In contrast, growth of depletion boundary layer of fuel over anode can be the major reason for low current density and low fuel utilization. At a low flow rate of 10 µl/min, simulation results show a severe fuel diffusion to the cathode side, which is the main reason for the degradation of open-circuit potential from 0.78 V at 500 µl/min to 0.58 V at 10 µl/min as observed in experiments. Decreasing the total flow rate fifty times from 500 µl/min to 10 µl/min only reduces the maximum power density approximately two times from 7.9 mW/cm 2 to 3.9 mW/cm 2 , while fuel utilization increases from 1.03% to 38.9% indicating a higher fuel utilization at low flow rates. Numerical simulation can be used for further optimization, to find a compromise between power density and fuel utilization.

Fabrication and Test of an Air-Breathing Microfluidic Fuel Cell

“…The fuel oxidation reaction at the anode and the air reduction reaction at the cathode in the direct formic acid microfluidic fuel cell and their standard potential are expressed, respectively, as follows [15]: Figure 3 shows the performance comparison between the present air-breathing microfluidic fuel cell, which operated with 1.0-M HCOOH at 0.5 mL/min, and other published data that used either 1.0-M methanol or 1.0-M formic acid as fuel and had an anode catalyst loading of 8-10 mg/cm 2 , akin to the present study. The results in Figure 3 showed that the direct formic acid microfluidic fuel cell [13][14][15][16] exhibited a higher open circuit voltage (≥0.8 V) than a direct methanol microfluidic fuel cell [12].…”

“…The results in Figure 3 showed that the direct formic acid microfluidic fuel cell [13][14][15][16] exhibited a higher open circuit voltage (≥0.8 V) than a direct methanol microfluidic fuel cell [12]. Additionally, the worst cell performance measured by Shaegh et al [15] was likely because the anode catalyst was Pt/Ru for formic acid oxidization, instead of Pd. The highest cell output presently measured in Figure 3 showed that both anode electrode preparation and the microfluidic fuel cell fabrication were comparable to those fuel cells published in similar field.…”

“…In addition, a power density in air-breathing laminar flow fuel cells of 26 mW/cm 2 was obtained. Because of the marked performance improvement of the air-breathing microfluidic fuel cells, several studies investigating the various effects on the performance of similar fuel cells reacting with either methanol or formic acid were carried out [11][12][13][14][15][16]. In order to have a notable microfluidic fuel cell performance, the anodic gas diffusion electrodes of those fuel cells were always heavily loaded with a catalyst, ranging from 7 mg/cm 2 to 10 mg/cm 2 .…”

“…Besides electricity, a gaseous product, CO2, is also produced in the anode based on the electrochemical reaction. Based on a careful examination of the related literature [10][11][12][13][14][15][16], it can be found that the anode catalyst loading of an air-breathing microfluidic fuel cell is usually so high (>8 mg/cm 2 ) that the bubbles are most likely to be presented on the electrode due to the high cell current output.…”

Experimental and One-Dimensional Mathematical Modeling of Different Operating Parameters in Direct Formic Acid Fuel Cells

“…Gao et al explain the formic acid oxidation mechanisms on Pt(111) electrodes under electrochemical methods using DFT calculations compared with the analogous gas-phase reaction for DFAFC [14]. Shaegh et al proposed a three-dimensional numerical model to predict the cell performance of formic acid in a microfluidic fuel cell [15]. Wang et al investigated formic acid electro-oxidation on a Pt/C catalyst and developed an impedance model by incorporating kinetic reactions to simulate the experimental impedance response [16].…”