Ideal Positioning of a Pt-Interlayer for H2-O2-Recombination in Polymer Electrolyte Membrane Water Electrolysis

Abstract

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for the production of hydrogen in future energy scenarios.[1] The main electric voltage loss in PEM electrolysers is caused by the ohmic losses of the polymer electrolyte membrane. However, the reduction of the membrane thickness not only reduces the cell voltage, but also leads to lower faradaic efficiencies, which can be accompanied by severe safety issues. The crossover of the produced hydrogen gas through the membrane into the anodic compartment can lead to explosive gas mixtures (lower explosion limit (LEL) of H2 in O2 is ~ 4 % [2]).[3, 4] Typically, PEM electrolysers operate only up to 2 % H2 in O2 (50 % of LEL). An elevation of the hydrogen pressure during electrolysis enhances the crossover of hydrogen even more.[5] For realizing high market penetrations of PEMWE, a safe operation is essential.In order to meet the safety requirements, mitigation strategies for the reduction of the hydrogen in oxygen content are necessary. When increasing the membrane thickness and reducing the operation temperature are not desired, other mitigation strategies need to be evaluated. The integration of a Pt-interlayer in the polymer electrolyte membrane is one possibility.[6] When hydrogen and oxygen permeate through the membrane, they recombine to water at the platinum particles.Klose et al. proposed that the method of incorporating a recombination layer can be optimized further by evaluating its position.[7] The concentration of oxygen at the interlayer must be high enough to let a sufficient amount of hydrogen recombine. However, the oxygen concentration towards the cathode is much lower than a full recombination of hydrogen requires, which is mainly due to the lower oxygen permeation rate (lower diffusivity, lower gradient). Therefore, it is assumed that a Pt-interlayer close to the anode catalyst layer yields higher recombination rates than an interlayer close to the cathode.In this contribution, the influence of the Pt-interlayer position on the H2 in O2 content was investigated. Catalyst coated membranes (CCMs) based on spray-coated Nafion with Pt-interlayers with a loading of 0.01 mg/cm² at three positions (close to anode, centred, close to cathode, see Figure 1a)) and a reference type (without interlayer) were fabricated. The CCMs were examined with regard to their polarisation behaviour and hydrogen crossover characteristics in a 4 cm² cell. The hydrogen crossover was investigated during electrolysis operation at ambient pressure and at elevated cathode pressures (p c = 10 bar).The resulting membrane thicknesses of the CCMs were ~ 100 µm (in dry state). Since commercial Nafion membranes for water electrolysis applications are available in thicknesses between 25 and 183 µm (N211 and N117, respectively), the present membranes correspond to the state of the art. The electrochemical polarisation behaviour of the CCMs shows that the integration of a Pt-interlayer does not lead to additional voltage losses. At current densities as high as 3.5 A/cm², cell voltages below 2 V were achieved.With regards to H2 crossover, each CCM with interlayer reduced the measurable anodic H2 content in comparison to the reference CCM (Figure 1b)). Additionally, the position of the interlayer affected the resulting H2 content. The H2 content decreased with positioning the interlayer closer to the anode. Thus, a higher recombination rate of hydrogen and oxygen is observed when placing the interlayer close to the anode. Since the available oxygen amount in the membrane has obviously to decrease towards the cathode catalyst layer, the experimental findings reflect the aforementioned theoretical situation.In summary, the results of this study confirm that Pt-interlayers are a suitable mitigation strategy to reduce the H2 in O2 content in PEMWE. Further, it was shown that the positioning of the interlayer affects the results. Since the available amounts of hydrogen and oxygen in the membrane depend on the location in the membrane and on the experimental conditions (e. g. operating pressure), the integration of a recombination interlayer at an optimized position within the membrane offers an effective means to safe PEMWE operation with reduced membrane thickness. This improvement will help to consolidate PEMWE as a key technology for hydrogen generation.Literature:[1] K. Ayers, Annu. Rev. Chem. Biomol. Eng., 2019, 10.[2] H. Janssen, Int. J. Hydorgen Energ., 2004, 29.[3] H. Ito, Int. J. Hydorgen Energ., 2016, 41.[4] S.A. Grigoriev, Int. J. Hydorgen Energ., 2009, 34.[5] P. Trinke, Int. J. Hydorgen Energ., 2017, 42.[6] P. Trinke, Int. J. Hydorgen Energ., 2018, 165.[7] C. Klose, J. Elec. Soc., 2018, 165. Figure 1