Segmented Fuel Cell for Verification of Variable Temperature Flow Field Concept

Abstract

Variable temperature flow field concept allows maintaining close to 100% relative humidity along the entire flow field of the anode and the cathode side without external humidification. In previously reported research the concept of variable temperature was successfully applied to the cathode channel only. However, with appropriate temperature and flow distribution it may be possible to achieve water transfer to the anode side and keep hydrogen close to 100% relative humidity too. In order to prove this an interactive combination of CFD modeling and experimental verification is applied. For experimental verification a 5-segment fuel cell is designed, constructed and tested. It consists of 5 linearly placed molded graphite segments 40×40×5 mm, insulated electrically and thermally from the neighboring segments by a polycarbonate matrix. The five parallel, straight channels with rectangular 1×1 mm cross-section are machined in the polycarbonate matrix and the molded graphite plates with 1 mm spacing, thus forming a 250 mm long and 10 mm wide flow field (25 cm2). The molded graphite segments are electrically connected in parallel by copper current collectors. The connecting rods of the current collectors are of equal length in order to achieve similar ohmic losses of each segment. Current shunts are soldered on each connecting rod and calibrated to enable spatially resolved in-situ current density monitoring on each segment. Peltier thermoelements are individually attached to each segment of copper current collector. Each Peltier thermoelement has an attached aluminium heat sink and a fan for improved heat removal rates. The temperature of each Peltier thermoelement can be prescribed independently via in-house built PID control in LabVIEW®software. In order to ensure equal pressure force on each segment, thus equal contact resistance, specially designed pressure rods are installed which also serve to fix each Peltier thermoelement, heat sink and the fan to the construction. The structural components are fixed between two stainless steel end plates, actually frames, with 12 nuts and bolts. Figure 1 shows the exploded view of the segmented fuel cell. Relative humidity and temperature sensors are placed between the neighboring segments in order to enable insertion of specially designed fittings with RH and temperature sensors directly inside the reactant channels. The overall experimental setup assembly enables: i) measurement of the relative humidity and temperature profiles directly inside the channels before and after each segment in total of 12 points, 6 along each side, ii) measurements of temperature on each segmented current collector terminal, total of 10 measurements, 5 on each side of the cell, iii) measurements of the current density distribution along the cell via 5 current shunts, iv) regulation of the temperature of each segment separately along the anode and cathode side via total of 10 Peltier thermoelements driven by the in-house built relay PID control, 5 on each side of the cell, v) establishment of desired temperature distribution along the cell to the actual water vapor saturation temperature profile, vi) calculation of net local water transport through the membrane for each segment. The segmented fuel cell is tested at several operating conditions, namely at isothermal conditions 60°C with fully and partially humidified reactants, and with a prescribed variable temperature flow field with dry reactants. Air and hydrogen are in counterflow. Very good agreement between modelled and experimentally obtained data is achieved at all operating conditions. The results indicate that it is possible to achieve 100% RH along the entire flow field with dry reactants on both sides at the cell inlet. Such operation along the cathode side can be achieved by prescribing the variable temperature flow field profile extracted from Mollier's h-x chart along the cathode side. By selecting an appropriate membrane thickness, thereby manipulating the back-diffusion water flux, it is also possible to humidify hydrogen on the anode side. The first segment on each side, where the dry reactants enter, is humidified from the opposite side of the membrane, keeping the membrane hydrated in those regions as well. Figure 1