Characterization of platinum-coated titanium bipolar plate for polymer electrolyte membrane water electrolyzer

Abstract

Polymer Electrolyte Membrane Water Electrolyzer (PEMWE) are electrochemical energy conversion devices that split water into its constituent elements of hydrogen and oxygen. Bipolar plate (BPP) is one of the key components in PEMWE systems due to their essential rules in the operation of PEMWE for supporting the electrode and membrane structure mechanically and conducting electrons between each cell. To date, titanium (Ti) is the most promising candidate for metal-based BPP due to its strength, resistance to heat, water and salt, also the fact that it is lightweight. However, pure Ti has insufficient long-term stability and performance due to the continuous formation of the Ti-oxide layer during longer operations leading to an increase in Interfacial Contact Resistance (ICR) and deteriorating overall performance. Therefore, the use of an additional coating layer on the Ti surface is an effective method to prevent the formation of an oxide layer as well as increase the corrosion resistance of the base material. This study aims to develop a uniform and reliable protective coating for Ti BPP using platinum (Pt) as the coating layer with the electrodeposition technique. The objectives of this study is to examine various parameters that influence the quality of Pt coating including pre-treatment strategies, plating current density and heat treatment. The effect of applying acid pickling/Ni-strike pre-treatment combined in the same working bath was investigated on the morphological structure and electrochemical properties of the Pt coating. The intermediate Nickel (Ni) layer effectively protected the Ti surface from re-oxidation, evidenced by a significant 91% reduction in ICR for PNC sample compared to pure Ti. Additionally, the coating morphology of the pre-treated samples showed a more uniform coating layer with fewer defects/holes compared to the sample without pre-treatment. As the next step, the effects of varying plating current densities and heat treatments were further studied in this research. Current densities of 10, 20, 30, and 40 mA/cm2 were tested, and it was found that the Pt layer produced at a current density of 30 mA/cm2 had a uniform coating layer with no visible defects/holes and consistent Pt crystal size distribution and a thickness of 2 μm. Furthermore, the sample coated at 30 mA/cm2 had the lowest polarization resistance at 24.1 KΩ among other samples. In addition, it was determined that the heat treatment tested at 400, 600 and 800 ºC had no clear advantage on the coating morphology and electrochemical properties over untreated samples. Moreover, a long-term durability test of the Ptcoated layer using the optimized coating procedures in a PEMWE simulation environment was investigated. An extended chronoamperometric (CA) test for 80 hours was performed and the electrochemical properties and morphology of the sample were tested and compared before and after the durability test. The sample coated using the optimized coating procedures showed a steady current density output at 5 mA, indicating that the cell performance is constant with no degradation at a voltage of 2 V. Additionally, the ICR and polarization resistance of the sample were measured before and after the durability test and no significant increase was observed. Therefore, this work has demonstrated that Pt-coated Ti fabricated using an electrodeposition technique at 30 mA/cm2 and having a thickness of 2 μm, coupled with acid pickling/Nistrike pre-treatment is a promising method to produce a uniform, reliable protective Pt coating on Ti surface for PEMWE BPP application.