Study On μDMFC Flow Field With Extreme-wettability Micro Structures And Its Gas-liquid Separating Performance

Micro direct methanol fuel cell (μDMFC) has been considered as a promising power source candidate due to its remarkable advantages such as high energy efficiency, environmental friendliness, room-temperature operation, instantaneous refueling, low noise and so on. Gas-liquid two-phase flow, which may induce the CO2 bubbles block in the anode and water flooding in the cathode, is one of the key factors that impact on the μDMFC performance. In this paper, aiming to achieve gas-liquid separating transportation, the microstructured auxiliary channels with extreme surface wettability (superhydrophilic /superhydrophobic) were designed and manufactured. The characteristics of gas-liquid separating transportation in new flow fields and its effects on μDMFC performances were studied.The micro-nano hierarchical structures on Ti substrates were prepared by single-step anodization, which obtained superhydrophilic surface with a water contact angle (WCA) of 0°, The effects of different process parameters on superhydrophilic property were investigated. Results show that higher anodization voltage could yield stronger superhydrophilicity on Ti surface, but the impact of electrolyte temperatures on superhydrophilicity at different voltage ranges had different performance. At 20-40V anodization voltages, the increase of electrolyte temperature can improve the surface superhydrophilicity, but this trend would be reversed when the voltages rise to 60-80V. After fluoroalkylsilane modification, the anodized Ti surface became superhydrophobic, and the WCA, rolling angle and contact angle hysteresis on the as-prepared superhydrophobic Ti surface were measured as 160°,2° and 1.7° respectively. In this work, the extreme surface wettability showed good stability in acidic, neutral and alkaline aqueous solutions as well as in air conditions.Two kinds of novel flow field plates were designed and fabricated, one was new serpentine anode flow field with micro superhydrophobic gas channels, and another was new perforated structure cathode flow field with micro superhydrophilic liquid channels. Simulation with ANSYS software was used to determine the reasonable depth of micro channels. The channel width and other structural parameters were determined by flow field open ratio and flow field pressure drop. The micro machining process of Ti flow field plate with extreme surface wettability were determined by experiments. Combined with lithography and wet etching techniques, the superhydrophilic auxiliary channels were obtained by anodization, then fluoroalkylsilane modification was utilized to get superhydrophobic gas channels. After that, the main channels for fuel or oxygen were developed by second lithography and wet etching, and the novel flow field plates were successfully prepared.A dummy polycarbonate (PC) flow field plates with nested arrangement of hydrophilic fuel channels and superhydrophobic gas channels was designed and fabricated. The H2O2 decomposition reaction was used to imitate gas-liquid two-phase flow. It was found that a 37% pressure drop decrease can be obtained in the new serpentine flow field compared with that of the conventional one, and the visualization test demonstrated that the auxiliary super-hydrophobic gas channels can speed up the discharge of the gas bubbles.The uDMFC using novel anode flow field plates were assembled to evaluate its gas-liquid transportion performance. Results show that the trends of pressure drop changing and in situ images of bubbles’ behavior in the anode flow field were respectively in good agreement with that in dummy flow field experiment at the same current density. When novel anode flow field with 40 and 80μm deep superhydrophobic gas channels were respectively employed, the peak power density rose by 33.3% and 41.4% compared with that of the conventional cell can be achieved. This paper also reports on the effects of novel cathode flow fields on μDMFC performances. After longterm operation, it was observed that the reduction extent of the peak power density of regular perforated flow field is 2.3-fold higher than that of the novel cathode flow field with 80μm deep superhydrophilic liquid channels, indicating that the novel flow field can significantly improve the water management performance in the μDMFC cathode.