🔬 applied physics

Corrosion-resistant and conductive Ti-Nb-O coatings tailored for ultra-low Pt-loaded BPPs and PTLs in PEM electrolyzers

This study demonstrates that reactive high-power impulse magnetron sputtering (HiPIMS) of tailored Ti-Nb-O bilayer coatings on stainless steel substrates yields highly conductive and corrosion-resistant surfaces for PEM electrolyzer components, enabling ultra-low platinum loading (down to 5 nm) while meeting U.S. DOE 2026 performance targets.

Original authors: David Kolenatý, Jiří Čapek, Stanislav Haviar, Jiří Rezek, Radomír Čerstvý, Akash Kumar, Kalyani Shaji, Mariia Zhadko, Petr Zeman

Published 2026-02-09

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CC BY 4.0

Original authors: David Kolenatý, Jiří Čapek, Stanislav Haviar, Jiří Rezek, Radomír Čerstvý, Akash Kumar, Kalyani Shaji, Mariia Zhadko, Petr Zeman

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Making Green Hydrogen Cheaper

Imagine we are trying to build a machine that splits water into hydrogen and oxygen to create clean fuel. This machine is called a PEM electrolyzer. To make it work efficiently, it needs two main metal parts:

Bipolar Plates (BPPs): These are like the "walls" that separate the different rooms (cells) in the factory, keeping the electricity flowing in the right direction.
Porous Transport Layers (PTLs): These are like "sponges" that let water, gas, and electricity pass through them easily.

The Problem:
These parts need to be made of metal that doesn't rust (corrode) in the harsh, acidic environment inside the machine.

Titanium is great at not rusting, but it is expensive and hard to shape.
Stainless Steel is cheap and easy to shape, but it rusts easily. If it rusts, it poisons the machine and stops working.

To fix this, engineers usually coat these metal parts with a thick layer of Platinum (a precious metal like gold). Platinum is the "super-hero" that stops rust and conducts electricity perfectly. However, Platinum is incredibly expensive, making the whole machine too costly for regular people to use.

The Solution: A "Smart Suit" for Metal

The researchers in this paper developed a new type of "smart suit" (a coating) for the metal parts. Instead of using a thick, expensive Platinum coat, they created a thin, custom-made layer made of Titanium, Niobium, and Oxygen (Ti–Nb–O).

They used a high-tech spray method called HiPIMS (High-Power Impulse Magnetron Sputtering) to paint this suit onto stainless steel sheets. Think of this process like a very precise, high-speed airbrush that builds the coating atom by atom.

How They Made It Work

The researchers treated the coating like a recipe. They changed two main ingredients to find the perfect mix:

The Oxygen Level: They controlled how much oxygen was in the air while spraying.
The Niobium Amount: They changed how much Niobium (a metal similar to Titanium) was added.

The "Goldilocks" Zone:

If they used too much oxygen, the coating became like a dry sponge—great at stopping rust, but terrible at letting electricity through (too much resistance).
If they used too little oxygen, the coating was like a wet sponge—good at electricity, but it would rust quickly.
The Winner: They found a "Goldilocks" mix (specifically, a lower oxygen level with a moderate amount of Niobium). This created a coating that was compact (tight and dense, like a solid brick wall) and conductive (letting electricity flow easily).

The Magic Trick: The 5-Nanometer Platinum Layer

Even with their amazing new suit, the stainless steel still needed a tiny bit of Platinum to meet the strict safety standards set by the U.S. Department of Energy (DOE).

Here is the breakthrough:

Old Way: You needed a thick layer of Platinum (hundreds of nanometers) to stop the rust and keep the electricity flowing.
New Way: Because the researchers' Ti–Nb–O suit was so good at its job, they only needed to add a 5-nanometer layer of Platinum on top.

The Analogy:
Imagine you are trying to keep a house warm.

The Old Way: You wrap the house in a massive, thick wool blanket (thick Platinum). It works, but it costs a fortune.
The New Way: You first build the house with super-insulated, high-tech bricks (the Ti–Nb–O coating). Then, you just put a very thin, high-tech thermal sheet (5nm Platinum) over it. The house stays just as warm, but you used 90–99% less of the expensive material.

The Results

The researchers tested their new coating by simulating years of wear and tear in a harsh chemical bath (an "accelerated corrosion test").

Rust Resistance: The coating held up incredibly well. The amount of rust (corrosion current) was so low it was almost zero, far better than the government's safety targets.
Electricity Flow: Even after the tough test, the electricity could still flow through the metal parts easily. The contact resistance (how hard it is for electricity to jump from the metal to the next part) stayed very low.
Cost Savings: By using a Platinum layer that is 10 to 100 times thinner than what is usually used, they could drastically cut the cost of the machine.

Summary

This paper shows that by carefully mixing Titanium, Niobium, and Oxygen, scientists created a super-strong, conductive shield for metal parts in hydrogen machines. This shield is so effective that it allows them to use a microscopic amount of expensive Platinum instead of a thick layer. This makes the technology for producing green hydrogen much cheaper and more practical for the future, without sacrificing durability.

Technical Summary: Corrosion-Resistant and Conductive Ti–Nb–O Coatings for Ultra-Low Pt-Loaded PEM Electrolyzers

Problem Statement
Proton Exchange Membrane (PEM) electrolyzers are critical for green hydrogen production but face high capital costs driven by the reliance on precious metals and expensive base materials. Bipolar plates (BPPs) and porous transport layers (PTLs) constitute a significant fraction of stack weight and cost. While titanium offers excellent corrosion resistance, it is costly, difficult to machine, and prone to hydrogen embrittlement; furthermore, its native oxide layer increases interfacial contact resistance (ICR). Stainless steel is a cost-effective alternative but suffers from corrosion in the harsh anodic environment, leading to metal ion dissolution (e.g., Fe²⁺) that poisons catalysts and membranes, as well as the formation of insulating oxide layers that degrade efficiency.

Current solutions often involve thick precious metal coatings (e.g., hundreds of nanometers of Pt, Au, or Ag) to ensure low ICR and corrosion resistance, which significantly increases system costs. The U.S. Department of Energy (DOE) sets stringent targets for 2026, requiring ICR below 10 mΩ·cm² and corrosion current densities ( $J_{corr}$ ) below 1 µA/cm² after accelerated corrosion testing. The challenge lies in developing coatings that meet these durability and conductivity targets while drastically reducing the loading of precious metals.

Methodology
The authors developed and evaluated 200 nm-thick bilayer Ti–Nb–O coatings deposited on AISI 316L stainless steel substrates using reactive High-Power Impulse Magnetron Sputtering (HiPIMS).

Deposition Process: The process involved a bottom layer deposited in pure Ar to ensure adhesion, followed by an upper layer where oxygen partial pressure ( $p_{ox}$ ) was precisely controlled (0, 3, 5, and 8 mPa) and Niobium (Nb) content was varied by adjusting the sample position relative to a Nb strip on the Ti target.
Characterization: Elemental composition was analyzed via Wavelength Dispersive Spectroscopy (WDS), and crystal structure via X-ray Diffraction (XRD). Electrical resistivity was measured using a four-point probe.
Performance Testing: Samples underwent accelerated corrosion testing consisting of potentiostatic polarization at 2 V vs. SHE (2.3 V vs. RHE) for 1 hour in H₂SO₄ electrolyte (pH 5.0) at 60°C.
ICR Measurement: Interfacial Contact Resistance was measured before and after corrosion testing using a carbon gas diffusion layer (GDL) and gold-coated copper electrodes under compaction pressures of 0.3–2 MPa.
Pt Overlayer Strategy: To evaluate the potential for reducing precious metal usage, a 5 nm Pt overlayer was applied via RF sputtering to optimized coatings, and performance was re-evaluated against DOE targets.

Key Results

Composition and Structure: The oxygen content in the coatings was directly controlled by $p_{ox}$ . At $p_{ox} = 0$ mPa, the coatings were dominated by hexagonal $\alpha$ -Ti. At $p_{ox} = 3$ mPa, a substoichiometric cubic TiO $_x$ phase formed alongside $\alpha$ -Ti. Higher oxygen pressures (5 and 8 mPa) resulted in predominantly cubic TiO $_x$ with increased amorphous character. Nb was found to be substitutionally dissolved in the crystal lattices of both phases.
Electrical and Corrosion Properties:
- Resistivity: The lowest electrical resistivity ( $\sim 10^{-4}$ Ω·cm) was achieved at $p_{ox} = 0$ and 3 mPa. Higher oxygen pressures increased resistivity by an order of magnitude.
- Corrosion Resistance: All coatings exhibited exceptional corrosion resistance, with $J_{corr}$ values ranging from 0.01 to 0.08 µA/cm² after accelerated testing. These values are more than an order of magnitude lower than the DOE target. Coatings with $p_{ox} = 3$ mPa showed a decrease in $J_{corr}$ post-testing due to surface passivation, whereas higher $p_{ox}$ coatings showed slight increases, likely due to defect formation.
- ICR Performance: Without a Pt overlayer, optimized coatings (specifically those with intermediate Nb content and $p_{ox} = 3$ mPa) achieved initial ICR values below 7 mΩ·cm². However, after accelerated corrosion, ICR values for bare coatings increased significantly (exceeding 500 mΩ·cm²) due to surface oxidation.
Impact of Ultra-Low Pt Loading:
- Applying a 5 nm Pt overlayer to the optimized Ti–Nb–O coatings (specifically the Ti–O layer at $p_{ox} = 3$ mPa) reduced the post-corrosion ICR to 5.6 mΩ·cm², successfully meeting the DOE target of <10 mΩ·cm².
- Increasing the Pt thickness beyond 5 nm (e.g., to 20 nm) yielded no further improvement in ICR.
- The 5 nm Pt layer effectively maintained high surface conductivity and prevented the ICR degradation typically seen in bare coatings, while the underlying Ti–Nb–O layer provided the necessary corrosion barrier.

Significance and Claims
The paper claims that the developed Ti–Nb–O bilayer coatings, tailored via HiPIMS, enable the simultaneous achievement of high electrical conductivity and extreme corrosion resistance. The primary significance of this work is the demonstration that the DOE ICR target can be met after accelerated corrosion testing using a Pt overlayer as thin as 5 nm. This represents a reduction in Pt loading by up to two orders of magnitude compared to conventional approaches that typically require hundreds of nanometers of precious metal.

The authors emphasize that while the underlying coating alone cannot maintain low ICR after long-term oxidation, the synergy between the compact, conductive Ti–Nb–O substrate and the ultra-thin Pt layer offers a durable, cost-effective solution for PEM electrolyzers. This approach significantly reduces reliance on precious metals without compromising the 80,000-hour operational lifespan targets. The study notes that while results were obtained on flat stainless steel sheets, the coating protocol is designed to be adaptable for complex BPP and PTL geometries, though further validation on actual component architectures is required.