Fabrication of the catalytic layers in polymer electrolyte fuel cells (PEFCs) typically involves a powder-film drying process, in which material ink is prepared, coated, and then dried. The process exhibits complex phenomena and requires adjustments of a huge number of parameters to control each phenomenon. We applied Bayesian optimization to determine the temperature profile during the drying process, to obtain electrodes with smooth surfaces. The results suggest that pulsed heat treatments were more effective in smoothening of surface than in maintaining a low temperature. Further, we investigated the effect of pulsed heat treatments and discussed the mechanism of ink drying.

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Introduction For more spread of polymer electrolyte fuel cells (PEFCs), investigating the temperature distribution inside an operating PEFC is important. Last year, correlation between local through-thickness temperature gradient and the PEFC performance was investigated [1]. In this case, temperature gradient was critical to the PEFC performance under certain operating conditions. Moreover, local in-plane temperature gradient should be suggested as an important factor to the PEFC performance. Therefore, in this study, two local temperature measurement on the interface of the cathode catalyst layer (CL) and the microporous layer (MPL) in a PEFC was performed while it was operated. Experimental method Thin-Film Thermocouple (TFTC) For the local temperature measurement inside a PEFC, in-house thin-film thermocouple (TFTC) was fabricated using micro electromechanical system (MEMS). The thermocouple was made from Au and Ni. Its measurement point was about 30 µm width and about 6 µm thick, which was so fine and thin that it wouldn’t disturb a reaction inside a PEFC. Cell Configuration An active area of 1 cm2 PEFC cell was used, and is has five 1mm width gas channels and five 1mm width ribs with parallel flow configuration. 15 µm thick reinforces type PEM was used. TGP-H-060 (with MPL 2.0 mg/cm2) was used and in both anode and cathode side. TFTCs were inserted between interface of cathode CL/MPL, and two TFTCs were set on rib and channel, respectively. Operation Conditions Power generation was performed by sweeping the cell voltage from OCV to 0 V at a rate of 1 mV/s. K-type thermocouples were used to measure the bi-polar plates with gas channels and sensor terminal temperatures. An impedance analyzer was used to measure the cell resistance. O2 and air were used as a cathode supply gas, respectively. Pure H2 was used as an anode supply gas. The flow rates of anode gas / cathode gas were set in two conditions, 500/500 mL/min, and 30/75 mL/min. Results and Discussion Here, results in one operating condition is being shown, when the high flow rate has been chosen, and O2 has been chosen as a cathode supply gas. Fig.1 (a) shows the cell voltage and cell resistance transition with current density. In this case, the cell voltage decrease due to diffusion overvoltage didn’t occur. And Fig.1 (b) shows the temperature increase from the beginning operating the PEFC on the CL/MPL interface under rib and channel. The temperature on the channel was higher than that on the rib. Due to the condensed water which was generated with the PEFC operation, the effective thermal conductivity got decreased. The amount of the condensed water on the rib was larger than that on the channel. Therefore, total amount of the condensed water might affect on the effective thermal conductivity and caused the temperature difference on the rib and channel. Reference [1] K. Shigemasa, et al., ECS Trans., 92(8),161-4 (2019) Figure 1

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