Towards Replacing Titanium with Copper in the Bipolar Plates for Proton Exchange Membrane Water Electrolysis

Abstract

For proton exchange membrane water electrolysis (PEMWE) to become competitive, the cost of stack components, such as bipolar plates (BPP), needs to be reduced. This can be achieved by using coated low-cost materials, such as copper as alternative to titanium. Herein we report on highly corrosion-resistant copper BPP coated with niobium. All investigated samples showed excellent corrosion resistance properties, with corrosion currents lower than 0.1 µA cm^-2 in a simulated PEM electrolyzer environment at two different pH values. The physico-chemical properties of the Nb coatings are thoroughly characterized by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). A 30 µm thick Nb coating fully protects the Cu against corrosion due to the formation of a passive oxide layer on its surface, predominantly composed of Nb₂O₅. The thickness of the passive oxide layer determined by both EIS and XPS is in the range of 10 nm. The results reported here demonstrate the effectiveness of Nb for protecting Cu against corrosion, opening the possibility to use it for the manufacturing of BPP for PEMWE. The latter was confirmed by its successful implementation in a single cell PEMWE based on hydraulic compression technology.

Keywords: PEMWE; bipolar plate; coatings; corrosion resistance; cost reduction; water electrolysis.

Figure 1

FE-SEM images of Nb coatings on Cu: (a–c) surface before corrosion, (d–f) surface and (g–i) cross-section after corrosion test in 0.05 M H₂SO₄ + 0.1 ppm F⁻ (pH = 1.4).

Figure 2

Interfacial contact resistance of Cu substrate and Nb-coated Cu pole plate at different compaction forces. Shaded area on the X-axis corresponds to the pressure range used for assembling commercial PEM electrolyzer stacks. Inset shows the experimental setup for ICR measurements.

Figure 3

Potentiodynamic polarization curves (v = 1 mV s⁻¹) measured in O₂-saturated solutions at 90 °C for metallic Nb and Nb coatings on Cu: (a) before and (c) after AST in 0.005 M H₂SO₄ + 0.1 ppm F⁻ (pH = 2); (b) before and (d) after AST in 0.05 M H₂SO₄ + 0.1 ppm F⁻ (pH = 1.4); current transients during AST in the test solutions at (e) pH = 2 and (f) pH = 1.4.

Figure 3

Potentiodynamic polarization curves (v = 1 mV s⁻¹) measured in O₂-saturated solutions at 90 °C for metallic Nb and Nb coatings on Cu: (a) before and (c) after AST in 0.005 M H₂SO₄ + 0.1 ppm F⁻ (pH = 2); (b) before and (d) after AST in 0.05 M H₂SO₄ + 0.1 ppm F⁻ (pH = 1.4); current transients during AST in the test solutions at (e) pH = 2 and (f) pH = 1.4.

Figure 4

Comparison of corrosion rates of Nb coatings on Cu at pH = 2 and pH = 1.4, before and after polarization at 2 V for 6 h.

Figure 5

Electrochemical impedance spectra of Nb coatings on Cu in O₂-saturated 0.05 M H₂SO₄ + 0.1 ppm F⁻ (pH = 1.4) solution at open circuit potential, at 90 °C: (a) Nyquist and (b) Bode plots before and (c) after polarization at constant potential E = 2 V for 6 h; (d) single time constant EEC and (e) two time constants EEC. Symbols are experimental data and continuous lines are simulated by fitting to the EEC.

Figure 6

XPS depths profile of the oxide layer: The oxide layer of the Niobium coating is etched/reduced under Ar⁺ ion beam until steadiness is reached after ~90 s. Some areas of the grainy porous surface are hidden from the beam and lead to a remaining oxide signal. High resolution XPS spectrum after etching times of: (a) 0 s; (b) 15 s and (c) 90 s.

Figure 7

AFM measurements after corrosion tests of NbCu8L: (a) height of operated BPP, (b) electronic current of operated BPP, (c) height profiles of (a,d) as indicated by the red lines, (d) height of operated BPP (oxide layer removed), (e) electronic current of operated BPP (oxide layer removed) and (f) height profile of (b,e) as indicated by the red lines.

Figure 8

Polarization curve obtained using Nb-coated Cu pole plates, recorded up to 2 A cm⁻² at 80 °C and 8 bar hydraulic pressure. The left inset shows the PEM electrolyzer cell produced by ProPuls used for the test. The right inset shows the Nb-coated Cu pole plate used in the cell.