*2.1. DMFC Stack System*

The DMFC stack system consists of a fuel-cell stack, BOP components and piping for air or liquid delivery. The DMFC stack is the core for the electrochemical reaction, and the BOP components assist in maintaining appropriate reaction conditions, supplying appropriate amount of reactants, and discharging by-products.

Figure 1 is a comparison of the BOP components in an active and our semi-active DMFC stack system. The whole block diagram represents an active system architecture, while the semi-active system consists of only a fuel tank plus the members encircled by the red dashed line. All four members enclosed in the dashed line can be constructed using only miniaturized components such that the complex active DMFC stack system can be slimmed down to a compact stack module. In addition, the semi-active DMFC stack module is made of only two separate sets of planar stack and flow field plates without the need for precise channel alignment and pressurized spacing, thereby greatly reducing the production difficulties. With a sufficient air contact area, an air blower instead of an air pump is enough and one piezoelectric dosing pump is enough to supply fuel. Thus, miniature BOPs can be integrated into the stack module.

**Figure 1.** Comparing full balance of plant (BOP) components in an active direct methanol fuel cell (DMFC) stack system with that of a semi-active system marked in red, where a DMFC stack module is enclosed by the red dashed line.

Simplifying the active architecture, the semi-active stack module does not have active temperature, humidity control, fuel buffer tank, nor check valve, therefore, supporting measures are needed. The feedback control on the fuel-supplying dosing pump was implemented to compensate the influence of changing the fuel level height on the flow rate. Fuel tank was relocated below the DMFC stack modules to prevent leakage due to either gravity or pressure imbalances. Due to the passive water recovery mechanism, the air supply flow rate was reduced and the reaction temperature was also decreased from the electrochemical optimum of 75 ◦C to 45 ◦C to reduce excessive water evaporative loss.

Figure 2 shows the photo of the semi-active DMFC stack module proposed. The stack module has a sandwich structure, with planar DMFC stacks on both sides with the metal electrodes protruding on the top. The flow field plate is in the middle with two air blowers on the right edge blowing air supplying oxygen. The fuel dosing pump is mounted at the lower right corner with an inlet connected to the fuel tank through a hose. It has membrane electrode assembly (MEA) of 61.6 cm2, 8 cells series, operating at 2.8 V to generate 1 W nominally. Its dimension is 120 <sup>×</sup> 60 <sup>×</sup> 15 mm<sup>3</sup> and it weighs less than 200 g.

**Figure 2.** Semi-active DMFC stack module.
