*4.6. Discussion*

The long-term performance of our system was determined from the test record, which is compared with other DMFC systems reported in the literature as shown in Table 1.

In designing the power driving capability, although our semi-active DMFC with 4 stack modules has only rated 4 W, we added Li-ion battery buffering as well as an output regulator to boost intermittent load-driving capability to 12 W. With 12 W, it can drive high-power sensors, actuators, or a wireless device for radio communications. Therefore, the design is suitable for IoT applications. Due to the simplified components and piping, our effective system volume-based energy density reached a comparable 152 Wh/L despite a low 10% electrochemical conversion efficiency and low fuel volume-based energy density of 500 Wh/L. Moreover, due to mechanical compactness in exchange for a larger fuel tank while maintaining portability, 480 h of operating time between refueling is more than 3 times that of other systems under persistent full power demand.


**Table 1.** Performance specifications of the DMFC generating systems reported in the literature [19,31].

<sup>1</sup> Consider with M5 fuel cartridge.

This system demonstrated a stable average power generation above 3.3 W throughout the 3600 h of test. With semi-active controls, adjusting the methanol concentration from pure to 80 *v*/*v*% helped to improve the power-generation efficiency in hot summer. For the long run, operating characteristics could be updated to reflect the deterioration, or the geometric thermal conditions, so that premature flooding can be avoided and the life can be further extended. To increase power capacity, enlarging the stack for a larger reaction area, increasing the number of stack modules, and improving the reaction efficiency and stability by further optimizing the thermal balance controls are all possible. While increasing the volume energy density for portability, the space utilization of the system can be further polished.
