Cell performance of passive paper-based microfluidic fuel cells with mini-hole array in the anode flow channel

Abstract

Paper-based microfluidic fuel cells (PMFCs) exploit the capillary flow in paper porous media to enable passive transport of reactants, and also use the co-laminar flow nature to separate the fuel and oxidant, thereby eliminating the conventional proton exchange membrane and micro-pumps. PMFCs have been regarded as promising micro-power sources for next-generation paper-based electrochemical microfluidic assay for biomedical diagnosis. The PMFC performance is usually limited by fuel transport, failing to meet the power requirement. At present, the paper-based microfluidic fuel cells have been reported to improve the performance mainly by changing the operating parameters, the physical parameters of the paper channel and the absorption pad, and the cell structure. However, few studies have been reported to improve the cell performance by enhancing fuel transfer. In this paper, a new anode structure is proposed to enhance fuel transfer. That is, a mini-hole array is constructed in the paper-based channel on the anode to make part of the catalyst layer of the anode contact with the mini-hole array. It was expected that the fluid in the mini-hole array can not only enhance the convective fuel transport, but also improve the diffusive fuel transport due to the direct contact between the fluid in the paper-based flow channel and anode catalyst layer. Performance comparison was firstly conducted between paper-based microfluidic fuel cells with conventional flow channel and mini-hole array in the anode flow channel. Moreover, effects of operation conditions including anode/cathode interspace, electrode length, fuel and electrolyte concentration on the cell performance were investigated and discussed. The experimental results confirmed that the mini-hole array in the anode flow channel can enhance fuel transport and ion conduction. As compared to the PMFC with the conventional flow channel, the PMFC with mini-hole array in the anode flow channel yielded 41.2% higher maximum power density. In addition, the experimental data also indicated that the ion transfer resistance can be reduced by diminishing the anode/cathode interspace, leading to an improved cell performance. No obvious fuel crossover was observed even at the interspace of 1.0 mm, because the flow rate of catholyte was higher than that of the anolyte, impeding the fuel crossover towards the cathode. The maximum power density of the cell increased with the decrease in the electrode length. This was mainly due to electrochemical reaction consumption and slow diffusion on the anode surface, which would reduce the fuel concentration on the anode surface along the flow direction, forming a concentration boundary layer and limiting the cell performance. In addition, upon the increasement of fuel or electrolyte concentration, the cell performance was enhanced at relatively low concentration but then dropped at high concentration. The fuel transfer was enhanced at high concentration and the ion transfer resistance can be reduced by increasing the electrolyte concentration, leading to an improved cell performance. However, the viscosity and density of the fuel and electrolyte increase with the electrolyte concentration, which led to the decrease of the flow rate of the reactants, further resulting in fuel mass transfer limitation and performance reduction. The optimal power density reached 29.7 mW/cm2 by the proposed PMFC at the anode/cathode interspace of 1.0 mm, the electrode length of 5.0 mm, and the fuel and electrolyte concentrations of 2.0 mol/L.