Galvanic Deposition of Ir-Pt-Particle As Bifunctional Catalyst for Unitized Regenerative PEM Fuel Cell
Authors: Näther Johannes , Koester Frank , Kreissig Tom , Cieluch Maximilian 
Published: July 2022

On the road to climate neutrality, hydrogen is becoming increasingly important as an energy carrier. This increase in importance is clearly illustrated, for example, by the hydrogen strategy adopted by the European Union (EU) in summer 2020, which aims to transform the EU's energy system, which is predominantly based on fossil fuels, and its sectoral value chains into a climate-neutral, integrated energy system. Regeneratively produced hydrogen is the energy carrier of the future. It is suitable for energy supply and enables the desired sector integration between the industrial, transport, energy and building sectors. [1] In order to make hydrogen fuel efficiently usable as an energy carrier of the future, electrochemical technologies are needed for the production of regenerative hydrogen as well as the reconversion of hydrogen into electricity. One possibility is polymer electrolyte membrane (PEM) technology. With PEM electrolyser cells (PEMEL), hydrogen can be produced by splitting water with electricity. For the reconversion of the hydrogen into electricity, the PEM fuel cell (PEMFC) can be used. This recombines hydrogen with oxygen to form water (with the release of electricity and heat). For the conversion chain two separate systems are required. A combination of both technologies can be realized by means of the unitized regenerative PEM fuel cell UR-PEMFC with internal reaction reversal. [2] This approach places special demands on the design of the oxygen electrode and the catalysts used there, since the reactions at this electrode represent the limiting factor for the performance of the cell. While platinum is the most suitable catalyst at the hydrogen electrode in both electrolyzer and fuel cell operation, a distinction must be made between the operating modes at the oxygen electrode. In fuel cell mode, the oxygen reduction reaction ORR takes place at the oxygen electrode. Platinum is a suitable catalyst for this. When switching to electrolyzer mode, the oxygen formation reaction OER takes place, with iridium proving to be the best catalyst, especially with regard to long-term stability. Accordingly for the oxygen electrode of a UR-PEMFC a bifunctional catalytic layer must be used. The composite of appropriately structured electrode material (titanium) and bifunctional catalyst layer is called a bifunctional oxygen electrode BOE. [3]

In this work, the electrodeposition of iridium-platinum alloy particles as a catalyst for a BOE was investigated. The deposition was first performed galvanostatically from electrolytes containing iridium and platinum ions in different concentration ratios. First, the influence of the electrolyte composition and the deposition parameters, such as current density and temperature, on the alloy composition was investigated. Electrolytes based on hexabromoiridate and diammineplatinum(II) nitrite were used to deposit alloys with iridium mass fractions between 40% and 60%, which corresponds to the typical composition from other publications, in which the catalyst particles are mixed in the slurry [3], [5], [6]. Current yields between 40% and 50% were determined for the production of closed layers. The alloy composition was characterized using X-ray fluorescence analysis and enegiedispersive X-ray spectroscopy. Examination of the surface morphology by scanning electron microscopy showed that no isolated particles could be produced on the titanium electrode by galvanostatic deposition. Rather, exposed areas are loaded with too much catalyst, while other surface sections could not be covered with catalyst material. However, a homogeneous distribution of the catalyst particles is crucial for increasing the cell conductivity.

For this reason, the electrolyte composition found was used for pulse current experiments. The pulse times were varied between 10 and 50ms. The pulse pauses were chosen so that one pulse cycle lasts 100ms. The best results were obtained with a pulse time of 25ms and a puls current density of 40mA/cm². Particles with an iridium mass fraction of 45% were deposited homogeneously on the oxygen electrode. Assuming a Faraday efficiency of 40%, the theoretical catalyst loading is 0.5mg/cm². However, by reducing the number of pulse cycles, lower loadings are also possible.

The catalytic properties of the electrode were first recorded in nitrogen-purged 0.5M H2SO4 by plotting current density-potential curves and cyclic voltammetry. Very good activity or low overvoltage with respect to the OER was observed. Measurements for the ORR in oxygen-purged H2SO4 are still pending. To gain insight into the performance of the prepared oxygen electrodes, the prepared MEAs with an electrode area of 25cm² will be investigated in a hydraulically pressed test system.

Figure 1