The Relationship between Activity and Durability of Surface Area and Catalyst Loading of IrOx Anode Catalyst in Proton Exchange Membrane Water Electrolysis (PEMWE)

Abstract

The R&D of highly active and durable anode catalysts for the oxygen evolution reaction (OER) is very important for high efficiency Proton Exchange Membrane Water Electrolysis (PEMWE). The reason is that the high overpotential of OER reaction is the main factor of the energy loss. At present, the use of highly active and durable iridium catalyst is one of the resolutions for reducing overpotential. From the perspective of iridium production, not only is limited the producing area, but production amount is limited because iridium is by-product of PGM mine in South Africa. Therefore, in order to popularize hydrogen production by large-scale PEMWE, it is necessary to to achieve the high performance with the minimum amount of iridium material. In this study, we report the activity and durability of catalysts coated membranes (CCMs) based on the IrOx catalyst surface area and loadings. High surface area IrOx catalyst (SA100) and a low surface area IrOx catalyst (SA5) from Tanaka Kikinzoku Kogyo K.K. were used for OER catalysts in this study. These catalysts were characterized by ICP-AES analysis, powder XRD, N2-BET, and SEM images. CCMs were prepared by spray-coating and hot-press with IrOx catalysts at the anode and carbon supported platinum at the cathode (TEC10E50E, Tanaka Kikinzoku Kogyo K.K.). Single cell electrochemical analysis including initial polarization curves, cyclic voltammetry, and durability tests (at 50℃, 2A/cm2) and SEM cross section images was studied for all CCMs. The SEM images of the CCMs microstructure near the catalyst layer were shown in Figure 1-A and 1-B. The particle size of IrOx (SA100) was found to be ca.5nm. For IrOx (SA5), the particle size was less than 2nm. The cell voltages focused on the iridium loadings were confirmed in Figure 2-A and 2-B. Although there was significant difference in SEM cross section images of catalyst layers, the trend obtained for the SA100 was consistent with the trend in the SA5. The IrOx loaded from 0.1 to 0.3 mg/cm2 range exhibited a gradual increase in cell potential. Furthermore, the catalytic durability and CV studies using single cells at 2A/cm2, 50℃ will be discussed. This report will be one of the valuable source of information for CCM design for general PEMWE. Figure 1

Durability Testing of Low-Iridium PEM Water Electrolysis Membrane Electrode Assemblies

“…An increase in Tafel slope is also observed for the Benchmark catalyst, namely from 52 to 72 mV/dec, which may be indicative of the gradual transformation of the initially partially amorphous iridium oxide into predominantly crystalline iridium oxide over extended times at high anodic potentials. 54,55 In order to obtain a better resolution of the time evolution of the OER mass activities, the weekly recorded polarization curves were also subjected to the same type of Tafel analysis. The results are shown in Fig.…”

“…Considering that the OER activity is highest for hydrous iridium oxide and lowest for crystalline iridium oxide, 40 we believe that the rather fast OER mass activity decay of the P2X catalyst in the first 1000 h observed here is due to a gradual conversion of the hydrous iridium oxide phase at the high anode potential during electrolysis to an amorphous iridium oxide phase, which over longer times becomes more crystalline. 54,55 On the other hand, the Benchmark catalyst that starts out with a roughly 50/50 mixture of amorphous and crystalline iridium oxide, 38 experiences a much slower OER activity decay, presumably due to an intrinsically slower conversion of amorphous to crystalline iridium oxide. Over much longer times than the 3700 h examined here, we would thus expect that the intrinsic OER activities of the two catalysts should become identical, which in terms of OER mass activity would imply that the P2X catalyst should retain a long-term OER mass activity benefit over the Benchmark catalyst by a factor that corresponds to the iridium utilization ratio, namely ≈2.1 (see Section 3.1).…”