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Wettability Impact of Porous Current Supplier on Current Efficiency in Solid Polymer Water Electrolyzer
High pressure polymer electrolyte membrane water electrolyzer (PEMWE) is expected to efficiently produce high pressure hydrogen gas in direct manner. Couple of companies and research institutes continues to develop PEMWEs. PEMWEs can be utilized to fuel the hydrogen gas to FCV[1]. Also, PEMWEs with PEMFC can be a good candidate for load leveling of electricity between consumer demand and renewable energy.[2]
However, high pressure PEMWEs, as disadvantage, have large concern on its current efficiency. Produced hydrogen gas crossovers PEM from cathode to anode, and the amount of crossover hydrogen gas directly corresponds to reduction of the current efficiency. An estimation based on the permeability suggests that current efficiency decreases up to 10 % under 40 MPa [3]. Experimental result shows 23 % reduction of current efficiency [4].
Authors have succeeded to decrease the crossover and increase the current efficiency with convection effect, where water is intentionally fed to cathode channel. This additional feeding of water promotes the drainage of hydrogen gas, leading to the 4% increase of current efficiency. However, this method suggests additional cost for a pump to feed water to cathode, although the power consumption of the pump is negligible. A further simple method is expected.
This study proposes a current supplier, whose wettability is controlled so that it has hydrophobic or hydrophilic property. Carbon paper is built in a PEMWE as cathode current supplier, and experimentally evaluated if it can suppress the hydrogen gas crossover and increase the current efficiency. Also, this study conducts visualization, where the window placed in cathode separator can visualize the impact of the wettability on hydrogen gas bubble behavior.
Experimental Method
Experimental apparatus is shown in Fig. 1. In anode side, water is circulated, and the produce oxygen gas drains through gas/liquid separator. The water flow rate is adjusted so that electrolytic rate becomes 70 ppm. Anode side is operated at atmospheric pressure. Cathode piping has two cases, where water is circulated or not. The water flow rate in cathode is adjusted to be the same as anode case, and the operation pressure is controlled up to 2 MPa with back pressure valve. The flow meter located in the downstream of the valve measures the practically produced hydrogen gas amount, which is converted to current efficiency.
The cell consists of cathode separator, cathode current supplier, CCM, anode current supplier, anode separator. Two type of the current supplier in cathode are prepared. One is carbon paper (SGL, 35AA), which has hydrophobicity with the contact angle of 135. The other one is the carbon paper, which shows the hydrophilic characteristic so that a water droplet is completely absorbed. This hydrophilic carbon paper is fabricated by the impregnation of titanium oxide into carbon paper. As anode current supplier, titanium sintered compact with Pt coating is used. CCM has Iridium oxide layer as for anode and Pt/C layer for cathode sprayed and hot-pressed on Nafion membrane.
Results and discussion
Figure 2 is the wettability impact of current supplier in the case of feeding water in cathode channel. Fig (a), (b) are hydrogen current efficiency and detachment frequency of the bubble, respectively. As expected, the case of hydrophilic current supplier has slightly higher current efficiency (approximately 1%). In the hydrophilic case, the current supplier actively absorbs water and may be fully saturated. This situation enhances to detach the hydrogen gas bubble from the supplier (Fig. 2 (c)), and increase the efficiency.
Figure 3 is the wettability impact of current supplier without feeding water in cathode channel. Because of not feeding water, only the current efficiency is shown. Contrary to expectation, the hydrophobic supplier indicated higher efficiency (2%). In the case of not feeding water into cathode channel, the cathode current supplier is still impregnated with water, which is transported from anode by electro-osmosis flow. In this situation, gas path is thought to be formed in current supplier if it has hydrophobicity, and the path contributes to drain the produced hydrogen from catalyst layer to channel in cathode. Whereas, hydrophilic current supplier tend to saturate water, and the saturated water disturb the drainage of hydrogen gas from catalyst layer to channel. Thus, the hydrophobic current supplier indicated the higher efficiency.
References
[1]Eiji H, et al., Honda R&D Technical Review 2011;23:90–97.
[2]Arico AS, et al., J Appl Electrochem 2013;43:107–118.
[3]Sakai T, et al., J Electrochem Soc 1985;132:1328–1332.
[4]Bensmanna B, et al., Sundmacher K, Electrochimica Acta 2013;110:570–580.