Evaluation of Corrosion Resistance of Au-Plated Titanium Bipolar Plates for Polymer Electrolyte Membrane Water Electrolysis
Authors: Minoura Ayumu , Kumagai Masanobu , Yashiro Hitoshi , Murase Masatsugu 
Published: December 2024

Polymer Electrolyte Membrane Water Electrolysis (PEMWE) is one of the most promising on-site hydrogen production methods using renewable energy because of its fast start-up and higher current density compared to alkaline water electrolysis (AWE). The urgent issues related to the widespread use of PEMWE are cost reduction of components and durability. Currently, titanium (Ti) is the most common material for bipolar plates for PEMWE and is coated with gold (Au) or platinum (Pt) to reduce contact resistance and improve corrosion resistance. Therefore, it is a very expensive component, accounting for about 50% of the PEMWE stack cost. Optimizing the amount of noble metal used in the coating is one way to reduce the cost of bipolar plates. In this study, the corrosion resistance and contact resistance of Ti bipolar plates with different Au plating thicknesses are compared to determine the optimal amount of Au used in coatings for bipolar plates.

Commercial pure Ti plate (Ti 99.5%) was used as a test specimen. The Ti plate with 0.1 mm thickness was machined into a rectangular shape (20 mm x 50 mm) and electroplated with Au of different thicknesses (plating thickness: 0, 0.1, 0.3, 1.0 μm). The corrosion resistance of Au-plated Ti bipolar plates was evaluated by potentiostatic polarization at 2.0 V (vs. RHE) for 24 hours in a sulfuric acid solution (H2SO4, pH 3.0) at 353 K simulating the PEMWE anode environment. After the polarization test, the concentration of Ti in the test solution was determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The contact resistance was measured according to the four-terminal method between Au-plated Ti bipolar plates and the porous transport layer (PTL) before and after the potentiostatic polarization test. Pt-plated titanium sintered fiber (2GDL-06N-030-PT05, Becart Co. Ltd.) with a Pt-plating thickness of 0.5 μm was used as the PTL. The surfaces of the specimens before and after the potentiostatic polarization were observed using the Ultra-Low Voltage Scanning Electron Microscope (ULV-SEM) to calculate the coverage of Au plating. Elemental qualitative analysis of the specimen surfaces was also carried out by Energy Dispersive X-ray Spectroscopy (EDX).

The Ti concentration determined by ICP-MS after the potentiostatic polarization test was 4 μg dm-3 for the unplated Ti and 2 μg dm-3 for the Au plated-Ti (0.1 μm), while it was below the limit of quantification (<1 μg dm-3) when the Au plating thickness was 0.3 μm or higher. The contact resistance of unplated Ti increased by 22.0 mΩ cm2 at 2.0 MPa after polarization. On the other hand, the increase in contact resistance of Au-plated Ti was less than 0.1 mΩ cm2, regardless of the Au-plating thickness. Fig.1 shows SEM images (a) and EDX spectra (b)-(c) of the surface of the Au-plated Ti(0.1 μm) after polarization. It can be seen that there is a plating defect with a diameter of about 1 μm on the surface. EDX analysis results revealed that almost the entire Ti surface was coated with a straight Au plating layer, with only the defective areas of the underlying Ti exposed. The coverage of the Au-plated Ti (0.1 μm) was 99.86%, calculated from an area of 1 mm2 by binarizing the multiple backscattered electron images. The coverage of the Au-plated Ti (0.3 μm) is 99.98%, which indicates fewer plating defects than the Au-plated Ti (0.1 μm). Therefore, it is considered that the plating defects in the Au-plated Ti (0.1 μm) preferentially corroded and Ti was leached out. On the other hand, the contact resistance is almost the same regardless of the Au plating thickness or coverage and does not increase immediately provided that most the Ti surface is covered with Au plating. These results indicate that thinner Au coating with higher coverage on Ti bipolar plates for PEMWE can reduce the amount of Au used in the coating.

Figure 1