Development of ACLS Electrodes for a Water Electrolysis Cell

1.Introduction Water electrolyzers are important devices for power-to-gas systems. Compared with other types of water electrolyzers, polymer electrolyte membrane (PEM) electrolyzers have the advantages of quick response, small footprint, ability to generate high gas pressure than other types of electrolyzers, and high durability against ON/OFF cycling 1 . The anode of PEM electrolyzers is conventionally an iridium oxide electrocatalyst powder combined with a platinum coating layer on a porous titanium layer (PTL) 2,3 . However, these noble metal catalysts are scarce and expensive, and reducing their use is essential for PEM electrolyzers. At Toshiba, we have developed an alternating catalyst layer structure (ACLS) catalytic electrode composed of multiple iridium oxide sheets and air gap layers without a platinum coating layer and fabricated by a sputtering process, and we are studying its application in PEM electrolyzers. In this presentation, we report the results of comparisons between powder and ACLS electrocatalysts used in PEM electrolyzers. 2.Experimental First, iridium together with pore-forming metal was sputtered onto a commercially available PTL substrate and then only pore-forming metal was sputtered, and this process was repeated to deposit multiple layers (Figure 1). Then, the pore-forming metal was removed to prepare the ACLS anode. The loading of iridium metal was adjusted by the number of iridium layers. Next, an MEA was made by hot pressing of the ACLS anode, Nafion115 ® , and the Pt/C cathode. A conventional sprayed iridium oxide powder layer on Nafion115 and a platinum-coated PTL substrate was prepared for reference. Water electrolysis performance of the MEAs was measured in a single-cell configuration by supplying water to the anode side at 80℃ and applying current to the cell under atmospheric pressure. 3 ． Results and discussion Figure 2 shows the water electrolysis cell voltage at 2 A/cm 2 for each type of anode with various iridium loadings. At a low iridium loading (0.1 mg-Ir/cm 2 ), the voltage of the ACLS electrocatalyst (1.8 V) was lower than that of the powder catalyst (2.0 V). From Tafel plot analysis, this result is attributed to the low diffusional overpotential of the ACLS anode. We assume that its optimal porous structure increases the diffusivity of water and oxygen gas in the catalyst layer. Moreover, the ACLS anode showed almost no voltage rise up to 7000 h of operation without platinum coating on the PTL. The ACLS is effective for reducing the amount of noble metals used in PEM electrolyzers. (1) K. E. Ayers et al., ECS Transactions , 41 ,15 (2012). (2) C. Rakousky et al., J. Power Sources , 326 , 120 (2016). (3) C. Rozain et al., Appl. Catal. B Environ ., 182 ,153 (2016 Figure 1