Moore et al. [29] studied the performance of plate-frame architecture with porous flow in microfluidic fuel cells via electrode designs that combine the microfluidic fuel cells with those of traditional plate frame PEM fuel cells and enable vertical stacking with little dead volume (Fig. 10). However, a high ohmic resistance through the electrolyte predominantly limited the performance of this type of fuel cell. Solloum et al.[30] presented a fuel cell stack architecture for membrane-less microfluidic cells that reuses reactants from one cell in a subsequent one (Fig. 11). The overall power density of the fuel cell was found to correlate positively with the reactant flow rate and negatively with the separating electrolyte flow rate.Lee et al. [31] studied the electrochemical characteristics of vanadium redox reactions on porous carbon electrodes for microfluidic fuel cell applications. The results agreed well with the measured polarisation curves from the operation of a microfluidic vanadium redox fuel cell that employed the same material as the flow-through porous electrodes. Shaegh et al.[37] worked on air-breathing microfluidic fuel cells with fuel reservoirs (Fig. 12) and found that the ohmic losses are mini-mal because of the low anode-to-cathode spacing and improved mass transport. These changes were the result of a supply of a uniform fuel concentration over the anode and efficient bubble removal from the anode active sites. Erik et al.[75] measured the microfluidic diffusion in H-cells, and their work proved that the H-cell design allows for a fast, efficient and cheap method to analyse diffusion.
2.3Media flexibility
MMFCs can be operated in acidic conditions, alkaline condi-tions or mixed-media conditions. Choban et al. [17] explored the media flexibility of an MMFC methanol/dissolved O 2 sys-tem that is operated in acidic media, alkaline media and mixed-media conditions. They found that operating the fuel cell in alkaline conditions has positive effects on the reaction kinetics, while the mixed-media conditions increase the maximum achievable open cell potential (OCP) (Fig. 13).Brushett et al. [49] investigated the performance of air-breathing laminar flow fuel cells using five different fuels(formic acid, methanol, ethanol, hydrazine, and sodium borohydride) in both acidic and alkaline media. Their results showed that alkaline conditions significantly improve meth-anol and ethanol oxidation kinetics and stabilize sodium borohydride, while acidic conditions showed superior per-formance with formic acid when compared with methanol and ethanol.