Analysis of Influence of Cathode Current Collector Wettability on Current Loss By Crossover Evaluation Both at Cathode and Anode Side
Authors: Otaguro Koichiro , Saito Ryo , Nakajima Hironori , Ito Kohei 
Published: December 2020

Hydrogen gas crossover, which increase current loss, is critical issue in high pressure PEMWE (Polymer Electrolyte Membrane Water Electrolysis). To reduce the crossover, controlling wettability of current collector (CC) at cathode side has been proposed [1]. Different wettability of CC causes different bubble behavior of hydrogen bubble produced at cathode, and possibly changes the hydrogen gas crossover to anode side. To elucidate and maximize this idea, this study tests two carbon CCs. one has high hydrophobicity (SGL 39BA) and the other one has less hydrophobicity (SGL 39AA). Here, they are named as HCC and LHCC, respectively. PEMWE embedded by the CC was operated under 2 MPaG at cathode side and under atmospheric pressure at anode side. This study carefully evaluates the hydrogen gas crossover. Evaluation of the gas crossover was carried out both at cathode and anode side. In the cathode evaluation, hydrogen gas flow rate is measured at the outlet of cathode, and it is compared with theoretical production of hydrogen gas, providing hydrogen crossover. In the anode evaluation, hydrogen concentration in oxygen gas is measured at the outlet of anode, and it is converted to hydrogen gas flow rate in anode side, providing hydrogen crossover. The CC with different hydrophobicity impacts on bubble behavior of hydrogen gas at cathode side and is thought to influence the hydrogen crossover. To understand the influence of the CC wettability, gas/liquid two phase flow model in the through plane direction in cathode CC was developed. The theoretical model can estimate gas saturation and gas phase pressure at cathode catalyst layer, which determine hydrogen gas crossover. In the two phase flow model, boundary condition of gas saturation at the interface between CC and channel is critical, which governs the calculation result based on the model (distribution of gas saturation and pressure in CC). To provide accurate boundary condition, hydrogen bubble behavior, which is possibly converted to the gas saturation at the interface between CC and channel, was visualized. A high speed camera captured the bubble behavior in several micrometer and millisecond time resolution through the window embedded in cathode separator.

Obtained Hydrogen gas crossover, evaluated at both cathode and anode side, suggested that HCC case had better performance and showed smaller crossover. Among the two kind of crossover evaluation, the evaluation at cathode side showed higher crossover rate than that at anode side. As shown in the figure, evaluation at cathode side tends to overestimate the hydrogen crossover, because the evaluation also detects hydrogen gas leakage to outside in addition to the influence of hydrogen crossover to anode side. On the other hand, evaluation at anode side tends to underestimate the crossover, because it detects the combustion possibly happened at anode catalyst layer with oxygen gas produced there. These characteristic of the evaluation is thought to cause that the evaluation at cathode side showed higher crossover rate than that at anode side. In the visualization, various size of bubble was appeared on the CC. Statistical processing for the visualization revealed that HCC case shows the large size of bubble compering with LHCC case. The bubble size difference between the HCC and LHCC case is thought to cause a different capillary pressure and saturation of hydrogen gas bubble in current corrector. Theoretical analysis based on the two phase flow model shows also that HCC case provides less crossover. It is noted that the theoretical value positioned between cathode and anode evaluation result. This indicates that the theoretical analysis has some reliability. Detail information obtained by the theoretical analysis shows that HCC case provides smaller hydrogen gas saturation and gas phase pressure at the cathode catalyst layer. However, the gas pressure difference between the two CC case was not significant. Thus, the reason why HCC show less crossover is thought to be that HCC causes large bubble formation at the interface between CC and channel and suppresses gas saturation at the cathode catalyst layer and reduces the hydrogen crossover to anode side. In conclusion, hydrogen crossover significantly occurred in high pressure PEMWE can be suppressed by controlling wettability of CC.

[1] Kohei Ito, et.al., ECS Transactions, 75 (14) 1107-1112 (2016), 75, 14, 1107-1112, 2016.10

Figure 1